Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

PUBLICATION NO. FHWA-HRT-23-110 JANUARY 2024

Alternative Backfills for Highway 

Applications: State of the Practice
FOREWORD

Alternative backfill materials are increasingly used in applications such as retaining walls, bridge 

abutments, pipes/culverts, embankments, ground improvements, and slopes. This report 

summarizes the current state of practice for nine alternative backfills. The advantages and 

disadvantages, material characterization, design requirements, design guidelines, placement and 

construction specifications, cost information, environmental considerations, lifecycle assessment, 

and case studies/performance records are presented for each material. 

The report presents the maturity level of each technology, barriers to the advancement and 

widespread use of each material, and recommendations for moving forward. The information 

presented in this report could interest practitioners, designers, and researchers working in 

roadway geotechnical applications. 

Jean Nehme, Ph.D., P.E. 

Director, Office of Infrastructure 

Research and Development 

Notice 

This document is disseminated under the sponsorship of the U.S. Department of Transportation 

(USDOT) in the interest of information exchange. The U.S. Government assumes no liability for 

the use of the information contained in this document. 

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document. 

Quality Assurance Statement 

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards 

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its 

information. FHWA periodically reviews quality issues and adjusts its programs and processes to 

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE 

1. Report No. 

FHWA-HRT-23-110 

2. Government Accession No. 3. Recipient’s Catalog No. 

4. Title and Subtitle 

Alternative Backfills for Highway Applications: State of the

Practice 

5. Report Date 

January 2024 

6. Performing Organization Code: 

HPTech-GTL 

7. Author(s) 

Erol Tutumluer (ORCID: 0000-0003-3945-167X) and Issam 

I. A. Qamhia (ORCID: 0000-0002-2020-8437) 

8. Performing Organization Report No. 

HPTech-GTL-00010520-01

9. Performing Organization Name and Address 

HPTech Inc. 

13800 Coppermine Rd., Ste 100 

Herndon, VA 20171 

10. Work Unit No. 

0001 

11. Contract or Grant No. 

693JJ319D000057/693JJ320F000160 

12. Sponsoring Agency Name and Address 

Office of Infrastructure Research and Development 

Federal Highway Administration 

6300 Georgetown Pike 

McLean, VA 22101-2296 

13. Type of Report and Period Covered 

Final Report; December 2021–October 2022 

14. Sponsoring Agency Code 

HRDI-40

15. Supplementary Notes 

Jennifer Nicks (HRDI-40; ORCID: 0000-0001-7230-3578) was the Task Order Technical Lead, and Mike Adams 

(HRDI-10; ORCID: 0000-0003-2358-4128) was the Contracting Officer Task Order Representative. 

16. Abstract 

Using alternative materials (e.g., lightweight, recycled) in geotechnical applications may be advantageous 

compared to conventional structural backfills. This report summarizes current knowledge and the state of practice 

for using alternative backfill materials in applications, including backfills for retaining walls, bridge abutments, 

and pipes/culverts, and for embankments, ground improvements, and slope stability. The researchers evaluated 

nine alternative and lightweight fill materials as follows: controlled low-strength materials (CLSM); expanded 

shale, clay, and slate (ESCS); foamed glass aggregates (FGA); lightweight cellular concrete (LCC); polystyrene 

geofoams; reclaimed asphalt pavement (RAP); recycled concrete aggregates (RCA); recycled glass aggregates; and 

tire-derived aggregates (TDA). For each material, the researchers gathered information on the advantages and 

disadvantages of using backfill in applications, available literature on material characterization, design 

requirements, design guidelines, placement and construction specifications, cost information, environmental 

considerations, lifecycle assessment studies, case studies, and performance records. Further, the researchers 

present the maturity level of each technology, the suitability of using the various geotechnical applications, barriers 

to the advancement and widespread use of each material in backfill applications, and recommendations for moving 

forward. Some alternative materials (e.g., RAP, RCA, ESCS, CLSM, and TDA) are more widely used in the 

United States for geotechnical applications, while others are still emerging. Further, some materials (e.g., LCC, 

TDA, and geofoams) have well-developed design guidelines, while others (e.g., CLSM and FGA) could require 

more research. Lastly, the researchers concluded that most of these recycled/alternative materials, while marketed 

as “green” materials, lacked quantifiable environmental impacts obtained from lifecycle assessments and 

environmental product declarations for products manufactured in the United States. 

17. Key Words 

Lightweight fill, geotechnical highway applications, 

foamed glass aggregates, recycled glass, expanded 

shale, expanded clay, expanded slate, tire-derived 

aggregates, controlled low-strength materials, 

cellular concrete, geofoams, reclaimed asphalt 

pavement, recycled concrete aggregates 

18. Distribution Statement 

No restrictions. This document is available to the public 

through the National Technical Information Service, 

Springfield, VA 22161. 

https://www.ntis.gov

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

151 

22. Price 

N/A 

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized.
ii
iii

TABLE OF CONTENTS 

EXECUTIVE SUMMARY .......................................................................................................... 1

CHAPTER 1. INTRODUCTION ................................................................................................ 3

Project Overview ..................................................................................................................... 3

Objectives................................................................................................................................. 4

Target Applications ................................................................................................................ 4

Project Tasks ........................................................................................................................... 5

Research Matrix ...................................................................................................................... 6

Report Organization ............................................................................................................... 6

CHAPTER 2. CLSM .................................................................................................................... 7

Introduction and Background ............................................................................................... 7

Advantages and Disadvantages ............................................................................................. 8

Reported Geotechnical Applications For Highways ............................................................ 9

Material Characterization...................................................................................................... 9

Physical Properties .............................................................................................................. 9

Mechanical Properties ......................................................................................................... 9

Use of Recycled and Byproduct Materials for CLSM ...................................................... 10

Design Requirements and Available Guidelines ................................................................ 16

ASTM Standards for CLSM ............................................................................................. 16

Backfill Placement and Construction Specifications ......................................................... 20

Cost Information ................................................................................................................... 21

Environmental Considerations ............................................................................................ 22

LCA Studies........................................................................................................................... 22

Case Studies ........................................................................................................................... 22

CHAPTER 3. ESCS .................................................................................................................... 25

Introduction and Background ............................................................................................. 25

Advantages and Disadvantages ........................................................................................... 26

Reported Geotechnical Applications For Highways .......................................................... 26

Material Characterization .................................................................................................... 26

Physical Properties ............................................................................................................ 26

Mechanical Properties ....................................................................................................... 30

Design Requirements and Available Guidelines ................................................................ 30

Backfill Placement and Construction Specifications ......................................................... 31

Available Cost Information .................................................................................................. 31

Environmental Considerations ............................................................................................ 31

LCA Studies........................................................................................................................... 31

Case Studies ........................................................................................................................... 33

CHAPTER 4. FGA...................................................................................................................... 35

Advantages and Disadvantages ........................................................................................... 36

Reported Geotechnical Applications For Highways .......................................................... 36

Material Characterization.................................................................................................... 37

Physical Properties ............................................................................................................ 37

Mechanical Properties ....................................................................................................... 38
iv

Backfill Placement and Construction Specifications ......................................................... 41

Available Cost Information .................................................................................................. 42

Environmental Considerations ............................................................................................ 42

LCA Studies........................................................................................................................... 42

Case Studies ........................................................................................................................... 44

CHAPTER 5. LCC...................................................................................................................... 45

Introduction and Background ............................................................................................. 45

Advantages and Disadvantages ........................................................................................... 46

Reported Geotechnical Applications For Highways .......................................................... 47

Material Characterization.................................................................................................... 47

Physical Properties ............................................................................................................ 47

Mechanical Properties ....................................................................................................... 48

Design Requirements and Available Guidelines ................................................................ 48

Backfill Placement and Construction Specifications ......................................................... 51

Transportation ................................................................................................................... 52

Mixing and Construction Equipment ................................................................................ 52

Placement and Handling ................................................................................................... 53

QA/QC Measures—Inspection, Testing, and Maintenance .............................................. 53

Available Cost Information .................................................................................................. 55

Environmental Considerations ............................................................................................ 55

LCA Studies........................................................................................................................... 55

Case Studies ........................................................................................................................... 55

CHAPTER 6. POLYSTYRENE GEOFOAMS ........................................................................ 57

Introduction and Background ............................................................................................. 57

Advantages and Disadvantages ........................................................................................... 58

Reported Geotechnical Applications For Highways.......................................................... 58

Material Characterization.................................................................................................... 58

Physical Properties ............................................................................................................ 58

Mechanical Properties ....................................................................................................... 60

Design Requirements and Available Guidelines ................................................................ 61

Backfill Placement and Construction Specifications ......................................................... 61

Available Cost Information .................................................................................................. 62

Environmental Considerations ............................................................................................ 62

LCA Studies........................................................................................................................... 62

Case Studies ........................................................................................................................... 63

CHAPTER 7. RAP ...................................................................................................................... 65

Introduction and Background ............................................................................................. 65

Advantages and Disadvantages ........................................................................................... 66

Reported Geotechnical Applications For Highways .......................................................... 66

Material Characterization .................................................................................................... 68

Physical Properties ............................................................................................................ 68

Mechanical Properties ....................................................................................................... 68

Design Requirements and Available Guidelines ................................................................ 69

Backfill Placement and Construction Specifications ......................................................... 71

Available Cost Information .................................................................................................. 72
v

Environmental Considerations ............................................................................................ 73

LCA Studies........................................................................................................................... 73

Case Studies ........................................................................................................................... 75

CHAPTER 8. RGA ..................................................................................................................... 77

Introduction and Background ............................................................................................. 77

Advantages and Disadvantages ........................................................................................... 78

Reported Geotechnical Applications For Highways.......................................................... 78

Material Characterization.................................................................................................... 78

Physical Properties ............................................................................................................ 78

Mechanical Properties ....................................................................................................... 80

Design Requirements and Available Guidelines ................................................................ 82

Backfill Placement and Construction Specifications ......................................................... 83

Available Cost Information .................................................................................................. 83

Environmental Considerations ............................................................................................ 84

LCA Studies........................................................................................................................... 84

Case Studies ........................................................................................................................... 85

CHAPTER 9. RCA ..................................................................................................................... 87

Introduction and Background ............................................................................................. 87

Advantages and Disadvantages ........................................................................................... 88

Reported Geotechnical Applications For Highways .......................................................... 88

Material Characterization .................................................................................................... 89

Physical Properties ............................................................................................................ 89

Mechanical Properties ....................................................................................................... 90

Design Requirements and Available Guidelines ................................................................ 91

Backfill Placement and Construction Specifications ......................................................... 92

Available Cost Information .................................................................................................. 93

Environmental Considerations ............................................................................................ 93

LCA Studies........................................................................................................................... 94

Case Studies ........................................................................................................................... 95

CHAPTER 10. TDA.................................................................................................................... 97

Introduction and Background ............................................................................................. 97

Advantages and Disadvantages ........................................................................................... 98

Reported Geotechnical Applications For Highways .......................................................... 98

Material Characterization .................................................................................................... 99

Physical Properties ............................................................................................................ 99

Mechanical Properties ....................................................................................................... 99

Design Requirements and Available Guidelines .............................................................. 103

Backfill Placement and Construction Specifications ....................................................... 103

Available Cost Information ................................................................................................ 104

Environmental Considerations .......................................................................................... 105

LCA Studies......................................................................................................................... 105

Case Studies ......................................................................................................................... 107

CHAPTER 11. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS ............... 109

Summary of Findings ......................................................................................................... 109
vi

Alternative Lightweight/Backfill Materials Used in Geotechnical Highway 

Applications ......................................................................................................................... 110

Conclusions .......................................................................................................................... 111

Knowledge gaps and Barriers to Widespread Use........................................................... 113

Recommended Future Work and Investigations ............................................................. 115

ACKNOWLEDGMENTS ........................................................................................................ 119

REFERENCES .......................................................................................................................... 121
vii

LIST OF FIGURES 

Figure 1. Photos. CLSM in field applications................................................................................. 7

Figure 2. Illustration. Comparison between CLSM and other cement-based materials for 

water and cement contents (Halsted 2020). .......................................................................... 8

Figure 3. Graphs. Effects of C/W and W/S ratios on the flowability and UCS of rubberized 

CLSM with 40 percent sand (Wu and Tsai 2008). .............................................................. 12

Figure 4. Graph. Effects of C/W and W/S ratios on the flowability of CLSM mixes with 

RS (Wu and Lin 2011). ....................................................................................................... 13

Figure 5. Graphs. Effects of C/W and W/S ratios and curing time on the strength of CLSM 

mixes with RS (Wu and Lin 2011). ..................................................................................... 14

Figure 6. Graphs. Shear box testing results for RS-based CLSM tested at two different 

C/W ratios (Wu and Lin 2011). ........................................................................................... 15

Figure 7. Photo. Lightweight expanded clay aggregate (Creative Commons n.d.a). ................... 25

Figure 8. Map. ESCS sources and manufacturers in the United States (Google 2022)................ 25

Figure 9. Photos. FGA. ................................................................................................................. 35

Figure 10. Graphs. Stress-strain relationships for loose and compacted FGA material tested 

at different confining pressures. .......................................................................................... 40

Figure 11. Illustration. Preparation of ready-mixed LCC (Taylor and Halsted 2021). ................ 45

Figure 12. Illustration. LCC flow chart (Chica and Alzate 2019). ............................................... 46

Figure 13. Graphs. Load-displacement behavior of full-scale MSE walls backfilled with 

various materials, including LCC (Rollins 2020). .............................................................. 51

Figure 14. Photos. Placement of geofoam blocks for various applications. ................................. 57

Figure 15. Photos. RAP materials. ................................................................................................ 65

Figure 16. Graph. Resilient modulus properties of RAP materials collected from different 

States (Soleimanbeigi et al. 2015). ...................................................................................... 66

Figure 17. Illustrations. Pavement cross sections with conventional and RPM (RAP) base 

materials (Edil 2017). .......................................................................................................... 74

Figure 18. Photos. RGAs. ............................................................................................................. 77

Figure 19. Graph. CBR versus water content for RGA, RCA, RAP, and VAs tested at the 

same gradations (Ooi et al. 2017)........................................................................................ 82

Figure 20. Photos. Recycled concrete pavements and concrete demolition waste. ...................... 87

Figure 21. Graph. Resilient modulus properties of RCA materials collected from different 

States (Soleimanbeigi et al. 2015). ...................................................................................... 91

Figure 22. Illustration. Rubber shredding procedure and outputs (Creative 

Commons n.d.c). ................................................................................................................. 97

Figure 23. Graphs. Effect of TDA particle size on shear strength and secant shear modulus 

(El Naggar, Zahran, and Moussa 2021). ........................................................................... 101

Figure 24. Graphs. Effect of shear box size on the shear strength properties of TDA 

(Zahran 2021). ................................................................................................................... 103
viii

LIST OF TABLES 

Table 1. Typical unit weight ranges for alternative lightweight materials (Tafreshi, Siabil, 

and Dawson 2020; Loux 2022). ............................................................................................ 3

Table 2. Example CLSM mix properties and the corresponding compressive strengths 

(Türkel 2007)....................................................................................................................... 10

Table 3. Direct shear test (DST) results for CLSM mixtures at 7 d (Türkel 2007). ..................... 10

Table 4. Examples of CLSM specifications by different agencies (Kaneshiro et al. 2001). ........ 18

Table 5. Design example of a pond ash-based CLSM passing both strength and flowability 

requirements (Dev and Robinson 2020). ............................................................................. 20

Table 6. Matrix of field-testing issues and applications (Folliard 2008). ..................................... 23

Table 7. Recommended physical and mechanical properties for ESCS. ...................................... 28

Table 8. Physical and chemical properties for commercially available ESCS materials 

(Zukri et al. 2018)................................................................................................................ 29

Table 9. Mechanical properties for commercially available ESCS materials 

(Zukri et al. 2018)................................................................................................................ 30

Table 10. Emissions and environmental impacts for three European expanded clay 

products. .............................................................................................................................. 32

Table 11. Energy and resource utilization for three European expanded clay products............... 32

Table 12. Engineering properties of FGAs (Arulrajah et al. 2015). ............................................. 37

Table 13. Shear strength characteristics of FGA (Swan et al. 2016). ........................................... 39

Table 14. Compaction equipment specifications (McGuire, Loux, and VandenBerge 2021)...... 42

Table 15. Available EPD information on three European FGAs.................................................. 43

Table 16. Emissions and environmental impacts for three European FGAs. ............................... 43

Table 17. Energy and resource utilization for three European FGAs. .......................................... 44

Table 18. Typical density and compressive strength of LCC mixes (Halsted 2020). .................. 48

Table 19. Mechanical and thermal properties of LCC mixes (Ni, Oyeyi, and Tighe 2020)......... 48

Table 20. Physical and mechanical properties of LCC mixes with partial cement 

replacement (Chica and Alzate 2019). ................................................................................ 49

Table 21. Sampling requirements for LCC mixes for density and compressive strength 

testing (Taylor and Halsted 2021; Ni, Oyeyi, and Tighe 2020). ......................................... 54

Table 22. Physical property requirements for EPS geofoams (ASTM 2021d; Geofoam 

International, LLC, n.d.)...................................................................................................... 59

Table 23. Physical property requirements for XPS geofoams (ASTM 2021d). ........................... 59

Table 24. Physical property requirements for EPS geofoams (ASTM 2022; Geofoam 

International, LLC, n.d.). ..................................................................................................... 60

Table 25. Emissions and environmental impacts for EPS and XPS (Quad-Lock 2023). ............. 63

Table 26. Select States allowing RAP usage in highway geotechnical applications (Ncube 

and Bobet 2021). ................................................................................................................. 67

Table 27. Physical properties of RAP—average values and typical ranges (Edil 2017). ............. 68

Table 28. Shear strength properties of RAP, RCA, and VAs (Stroup-Gardiner and 

Wattenberg-Komas 2013). .................................................................................................. 69

Table 29. General guidelines for using RAP and other backfill materials in MSE wall 

backfill applications (Stroup-Gardiner and Wattenberg-Komas 2013). ............................. 70

Table 30. Survey of transportation agency responses related to restricting use of RAP in 

highway applications (Tutumluer, Moaveni, and Qamhia 2018)........................................ 71
ix

Table 31. Ratios of nuclear moisture contents to oven-dried measurements for RAP (Rathje 

et al. 2002; Stroup-Gardiner and Wattenberg-Komas 2013). ............................................. 72

Table 32. Effects of percent RAP used on LCA results (material phase) (Ncube and Bobet 

2021; Aurangzeb et al. 2014). ............................................................................................. 75

Table 33. Physical properties of RGAs (Ooi et al. 2008). ............................................................ 79

Table 34. Physical and geotechnical properties of RGA samples (Disfani et al. 2011). .............. 80

Table 35. Mechanical properties of RGAs (Ooi et al. 2008). ....................................................... 80

Table 36. Summary of shear strength properties of RGAs (Ooi et al. 2008). .............................. 82

Table 37. Suitable glass content that best enhances respective properties 

(Perera et al. 2021). ............................................................................................................. 83

Table 38. Geotechnical properties of the limestone and glass blends for Texas projects 

(Davio n.d.). ........................................................................................................................ 86

Table 39. Physical and chemical properties of RCA compared to VAs (Stroup-Gardiner 

and Wattenberg-Komas 2013). ........................................................................................... 89

Table 40. Physical properties of RCA—average and range values (Edil 2017). .......................... 90

Table 41. Ranges of friction angles for soils and coarse aggregate materials, including 

RCA and RAP (Soleimanbeigi et al. 2016)......................................................................... 90

Table 42. Survey of DOT responses for the restrictions on using RCA in highway 

applications (Tutumluer, Moaveni, and Qamhia 2018). ..................................................... 92

Table 43. Concerns for using RCA in highway applications—agency survey responses 

(Tutumluer, Moaveni, and Qamhia 2018). .......................................................................... 94

Table 44. Typical passenger car and truck tire compositions (El Naggar and 

Iranikhah 2021). .................................................................................................................. 97

Table 45. Grain size distributions for Type A and B TDAs (Cheng 2016; ASTM 2020). ........... 99

Table 46. Specifications for Type A and B TDAs (Cheng 2016; ASTM 2020). ......................... 99

Table 47. Optimum TDA percentages in aggregate and soil mixes (El Naggar and 

Iranikhah 2021). ................................................................................................................ 100

Table 48. Summary of shear box sizes and aspect ratios (Zahran 2021). ................................... 102

Table 49. Summary of the TDA gradation parameters and grain size distribution 

(Zahran 2021). ................................................................................................................... 102

Table 50. Cost for shredding tires (El Naggar and Iranikhah 2021)........................................... 104

Table 51. Cost comparison between TDA and other fill materials for different applications 

(Cheng 2016). .................................................................................................................... 104

Table 52. Inputs and outputs of the tire grinding process (Corti and Lombardi 2004). ............. 106

Table 53. Inputs and outputs of the tire shredding process (Corti and Lombardi 2004). ........... 106

Table 54. Summary of environmental impacts from grinding and crushing 1.0 m3

of tires. ..... 106

Table 55. Key advantages of using alternative backfill materials in geotechnical 

applications. ....................................................................................................................... 110

Table 56. Alternative backfill materials and reported geotechnical applications. ...................... 111

Table 57. Technology readiness for the studied alternative backfill materials........................... 112

Table 58. Identified barriers to deployment of the studied alternative backfill materials. ......... 114

Table 59. Proposed research needs for the studied alternative backfill materials. ..................... 116
x

LIST OF ABBREVIATIONS

AC asphalt content

ACI American Concrete Institute

ACPA American Concrete Pavement Association

AMD acid mine drainage

ASCE American Society of Civil Engineers

ASR alkali-silica reactions

CaCO3 calcium carbonate

Caltrans California Department of Transportation

CBR California Bearing Ratio

CD consolidated drained

CDF controlled density fill

CFC chlorofluorocarbon

CLSM controlled low-strength material

CO2 carbon dioxide

CP Tech Center National Concrete Pavement Technology Center

CRG coarse recycled glass

CU consolidated undrained

C/W cement-to-water

CWC Clean Washington Center

DOT department of transportation

DOTD department of transportation and development

DST direct shear test

EN European Norm

EPA Environmental Protection Authority

EPD environmental product declaration

EPS expanded polystyrene

ESCS expanded shale, clay, and slate

ESCSI Expanded Shale, Clay and Slate Institute

FGA foamed glass aggregate

FGD flue gas desulfurization

FHWA Federal Highway Administration

FRG fine recycled glass

GRG gasoline-resistant geomembrane

HCFC hydrochlorofluorocarbon

HMA hot-mix asphalt

IBU Institut Bauen und Umwelt

IDOT Illinois Department of Transportation

ISO International Organization for Standardization

LA Los Angeles

LCA lifecycle assessment

LCC lightweight cellular concrete

LCCA lifecycle cost analysis

LWA lightweight aggregate

MRF material recycling facilities
xi

MRG medium recycled glass 

MSE mechanically stabilized earth 

NRMCA National Ready Mixed Concrete Association 

ODOT Ohio Department of Transportation 

ODP ozone depletion potential

OI oxygen index 

OSHA Occupational Safety and Health Administration 

PAH polycyclic aromatic hydrocarbon 

PCA Portland Cement Association 

PCC portland cement concrete 

PCR product category rule 

PI plasticity index 

QA quality assurance 

QC quality control 

RAP reclaimed asphalt pavement 

RCA recycled concrete aggregate 

RCC recycled crushed concrete 

RCM recycled concrete material 

RGA recycled glass aggregate 

RPM recycled pavement material 

RS reservoir siltation 

TCLP toxicity characteristic leaching procedure 

TDA tire-derived aggregates 

TRL Technology Readiness Level 

TxDOT Texas Department of Transportation 

UCS unconfined compressive strength 

USCS Unified Soil Classification System 

UV ultraviolet 

VA virgin aggregate 

W/C water-to-cement ratio 

W/S water-to-solid 

XPS extruded polystyrene 

1

EXECUTIVE SUMMARY

A variety of alternative backfill materials are currently processed, reclaimed, or manufactured 

and widely used in various highway applications. These alternative backfill materials may 

include controlled low-strength material (CLSM); expanded shale, clay, and slate (ESCS);

polystyrene geofoam; foamed glass aggregates (FGAs); lightweight cellular concrete; reclaimed 

asphalt pavement; recycled concrete aggregates (RCAs); recycled glass aggregates (RGAs);

tire-derived aggregates (TDAs); or blends of one or more of these materials with or without 

conventional aggregates. Some alternative backfills have a long performance history, while 

others are emerging and are at different stages of testing, trials, demonstration, and 

implementation, depending on the application. For example, RCAs have been used in highway

construction for more than 70 yr, usually as pavement base and subbase layers and as 

embankment and shoulder materials. On the other hand, FGA is a recycled aggregate with 

limited history but excellent potential. This report provides information on nine known 

alternative backfill materials for use in highway fill applications and information and literature 

on their usage, popularity, and suitability for highway geotechnical applications.

This study aimed to assess the current knowledge and state of the practice of alternative backfill 

materials, particularly emerging materials on the market, for their application as backfill in 

highway applications such as retaining walls, embankments, bridge abutments, and pipe/culvert 

covers. The researchers performed a comprehensive review of domestic and international 

literature. The literature review presents the current knowledge base, knowledge gaps, 

technology maturity level, barriers to technology advancement and implementation, needs for 

future research, and a general framework for conducting future laboratory studies and field 

demonstrations.

Ultimately, the researchers concluded that all the recycled and lightweight materials investigated 

in this study have been used (or can be successfully used) for the target highway geotechnical 

applications (i.e., bridge abutments, embankments, retaining wall fills, pipe backfills, and ground 

improvements) without major issues reported or early failures. For any design alternative, the 

design engineer should pay special attention to the design requirements of each material, 

required design aspects, and changes in properties due to the use of a prospective material, such 

as the improvement/changes in shear strength properties, including the angle of internal friction, 

and the weight/density of the fill. Further, using the appropriate construction equipment and 

closely following lift thickness requirements and proper construction guidelines/practices to 

achieve the desired performance are of the utmost importance. 

The researchers identified technology-ready aspects for each alternative lightweight and recycled 

material based on the comprehensive literature reviewed and synthesized in this report. Overall, 

these materials have been used in full-scale field construction projects and as alternative designs 

to conventional backfills, which places all the materials at a Technology Readiness Level (TRL) 

of 8 according to the guidelines of the Federal Highway Administration’s Technology Readiness 

Level Guidebook (Towery, Machek, and Thomas 2017). Despite all the materials being at a TRL 

of 8, the literature search clarified that some materials have higher market penetration and are 

more commonly used in the United States for geotechnical applications. For example, 

comprehensive field application data exist for TDA, expanded polystyrene, and CLSM for most 
2

of the aspects of interest in this report, while some materials, such as RGAs, lack comprehensive 

information, with only a few case studies. One major area in which information is missing for 

most of the materials (except ESCS and FGA) is the presence of environmental product 

declarations (EPDs) to summarize the environmental impacts and energy demands for 

commercial products. In fact, the only product category rule available in the United States is for 

ESCS, but no publicly available EPD or comprehensive lifecycle assessment studies exist for any 

of the materials used in geotechnical applications.

Based on the information provided in this report, the researchers identified several knowledge 

gaps about using recycled or lightweight alternative backfill materials for highway geotechnical 

applications. Therefore, the researchers provide a list of research need statements to promote

more commonplace uses of lightweight and recycled alternative backfill materials to take 

advantage of these technologies and associated benefits. These research need statements are 

based on the knowledge gaps identified from the comprehensive literature search and primarily 

aim to eliminate technology barriers and promote more sustainable construction practices that do 

not jeopardize performance.
3

CHAPTER 1. INTRODUCTION

PROJECT OVERVIEW

In certain project conditions, using alternative materials (e.g., lightweight, recycled) in 

geotechnical applications may be advantageous compared to more conventional structural 

backfills (e.g., open- and well-graded granular aggregates). Alternative backfills can be either 

recycled or modified materials engineered to obtain certain physical and mechanical properties 

(e.g., unit weight, shear strength). Of the currently available alternative backfills on the market, 

lightweight backfills are becoming a more common solution to address settlement concerns, 

reduce lateral earth pressures, improve thermal properties, reduce carbon emissions, and 

accelerate project construction. The engineering properties of these lightweight backfills, 

whether composed of virgin or recycled materials, are not well established, particularly regarding

how they relate to each other and the resulting lifecycle and lifecycle cost analyses (LCCA) 

impact on their geotechnical applications.

The Federal Highway Administration (FHWA) has a long-standing materials selection policy to 

consider recycled materials first because recycling and reusing can offer engineering, economic,

and environmental benefits. However, FHWA’s Recycled Materials Policy cautions that 

“material used in highway or bridge construction, be it virgin or recycled, shall not adversely 

affect the performance, safety or the environment of the highway system” (FHWA 2015). This

policy requires research, field trials, and project demonstrations on the performance of virgin and 

recycled materials before the materials can be reliably used. Table 1 lists available lightweight 

civil engineering materials and their density ranges.

Table 1. Typical unit weight ranges for alternative lightweight materials (Tafreshi, Siabil, 

and Dawson 2020; Loux 2022).

Lightweight Fill Type

Density Range

kg/m3lb/ft3

Air-cooled slag 1,100–1,500 68.7–93.6

Boiler slag 1,000–1,750 62.4–109.3

Controlled low-strength materials 1,442–1,602 90–100

Expanded shale and clay 600–1,040 37.5–65

Foamed concrete 320–961 20–60

Foamed glass aggregates 160–400 10–25

Fly ash 1,120–1,440 70–90

Geofoams 11–48 0.7–3

Lightweight cellular concrete 320–961 20–60

Recycled glass aggregates 1,762–1,922 110–120

Shredded/waste tires 384–900 24–56.2

Wood chips 320–560 20–35

Mineral soils and aggregates (reference) 1,522–2403 95–150
4

A variety of alternative backfill materials are currently processed, reclaimed, or manufactured 

and widely used in various highway applications. Alternative backfill materials may include the 

following: 

• Controlled low-strength material (CLSM).

• Expanded shale, clay, and slate (ESCS). 

• Polystyrene geofoam. 

• Foamed glass aggregates (FGAs). 

• Lightweight cellular concrete (LCC). 

• Reclaimed asphalt pavement (RAP). 

• Recycled concrete aggregates (RCAs).

• Recycled glass aggregates (RGAs).

• Tire-derived aggregates (TDAs). 

• Blends of one or more of these materials with or without conventional aggregates. 

Some alternative backfills have a long performance history, while others are emerging and are at 

different stages of testing, trials, demonstration, and implementation, depending on the 

application. For example, RCAs have been used in highway construction for more than 70 yr 

(Abukersh and Fairfield 2011). RCAs have been commonly used as pavement base and subbase 

layers and as embankment and shoulder materials. On the other hand, FGA is a recycled 

aggregate with a limited history. Though the technology was introduced in Europe more than 

25 yr ago, it did not receive much attention in the United States until 2017 (Hibbert 2016; Gibson 

2019). This report will provide information on these nine alternative backfill materials for 

highway fill applications and provide information and literature on their use and suitability. 

OBJECTIVES 

The primary objective of this study is to assess the state of the practice of alternative backfill 

materials, particularly emerging materials on the market, for their application as backfill in 

highway applications, such as retaining walls, embankments, bridge abutments, and pipe/culvert 

covers, through a comprehensive review of domestic and international literature. The literature 

review will present the current knowledge base, knowledge gaps, technology maturity level, 

barriers to technology advancement and implementation, needs for future research, and a general 

framework for conducting future laboratory experimental studies and field demonstrations. 

TARGET APPLICATIONS 

The report will mainly target the following applications for lightweight and recycled alternative 

backfill materials: 

• Retaining walls. 

• Embankments. 

• Bridge abutments. 

• Pipe/culvert covers and backfills. 

• Ground improvements. 
5

The report will also cover other geotechnical applications for highways for any respective 

material found in the literature. In addition, the researchers investigated literature on alternative 

backfill materials as a constituent in hot-mix asphalt (HMA) or portland cement concrete (PCC).

While HMA and PCC are not geotechnically focused, any testing of the studied lightweight fill 

materials conducted for such applications may potentially be applicable to the target geotechnical 

applications of interest in this study. 

PROJECT TASKS 

The researchers undertook the following specific tasks for this study: 

• Identify and describe various alternative backfills currently produced and used in 

highway applications. 

• Delineate emerging technologies that need future research and add details that facilitate 

the development of future laboratory experimental studies and field demonstrations. 

• Describe the advantages and disadvantages of each type of alternative backfill material. 

• Describe the primary highway transportation applications for the various types of 

alternative materials. 

• Identify and describe case studies. 

• Identify barriers and impediments to the advancement and widespread application of 

various alternative backfill technologies. 

• Describe any available data on alternative backfills that are important from the standpoint 

of geotechnical designs, such as gradation, unit weight, friction angle, permeability, etc. 

• Describe any needs or special provisions for testing alternative backfills compared to 

conventional aggregates that researchers and practitioners may have proposed. 

• Describe the environmental and economic lifecycle impacts of alternative backfills. 

• Identify and describe any available construction specifications and quality assurance 

(QA)/quality control (QC) procedures for placing alternative backfills in the field. 

• Identify future research needed for emerging alternative backfill technologies and develop 

a research need statement for each topic. 

• Develop a general framework for experimental laboratory studies and field 

demonstrations for emerging alternative backfill technologies. 

• Recommend eliminating or minimizing the barriers and impediments to the advancement 

and widespread application of alternative backfills. 

• Recommend any specialized testing needs for alternative backfills. 
6

RESEARCH MATRIX 

The researchers conducted a comprehensive literature search and assessment for each 

lightweight and alternative material included in the study scope to determine the current 

knowledge base on each material. The literature search encompassed the following topics: 

• Broad topics on the geotechnical applications covered in the literature. 

• Advantages and disadvantages of using each material in backfill applications.

• Data on material characterization, including physical and mechanical properties, design 

requirements, and guidelines. 

• Placement and construction specifications. 

• Cost information. 

• Environmental considerations. 

• Lifecycle assessment (LCA) studies. 

• Case studies and performance records. 

After collecting all the available information for the individual materials, the researchers 

reflected on the maturity level of each technology, suitability for the various geotechnical 

applications, barriers to the advancement and widespread use of the materials in backfill 

applications, knowledge gaps, and recommendations for moving forward and overcoming any 

barriers to widespread and effective use. 

REPORT ORGANIZATION 

This report is divided into 11 chapters and a references section, including this introductory 

chapter. Chapters 2–10 provide available information on each alternative/lightweight backfill 

material, such as applications, advantages, laboratory characterization, environmental studies, 

case studies, and construction and design requirements. Chapter 11 provides a summary of the 

findings, conclusions, and recommendations. Chapter 11 also includes a section on the suitability 

of the lightweight/alternative materials for the geotechnical applications being considered and a 

section on the gaps in knowledge and barriers to widespread use. Finally, the last section 

presents an extended list of references compiled through an exhaustive literature search of the 

available publications for each lightweight/alternative backfill material discussed in the report. 
7

CHAPTER 2. CLSM 

INTRODUCTION AND BACKGROUND 

CLSMs are self-leveling and self-consolidating cementitious fills that are typically used as an 

alternative to conventionally compacted aggregate backfills. Other common names for CLSM 

include flowable fill, unshrinkable fill, liquid dirt, controlled density fill (CDF), and various 

trademark names. CLSMs have traditionally been used in the construction industry since the 

1960s for pipeline backfill, but more recently, their applications have expanded (Wagstaff 2016). 

CLSMs are now used for a wide selection of geotechnical backfill applications. Figure 1 presents 

photos illustrating field applications of CLSM. 

© 2014 Metropolitan Transit Authority. 

A. Backfilling a trench with CLSM (Creative Commons n.d.a). 

Source: Commander, U.S. Naval Forces Europe-Africa. 

B. Using a concrete vibe to spread CLSM evenly. 

Figure 1. Photos. CLSM in field applications. 
8

CLSMs are made by mixing various amounts of portland cement, fine aggregates, coarse 

aggregates (mostly sand), and water. Some mix designs include byproduct materials to replace 

cement or aggregates such as fly ash and other industrial byproducts and admixtures to reduce 

cement content. ASTM International (ASTM) D5971 and American Concrete Institute® (ACI) 

229R define CLSM as a mixture of soil, fly ash, cement, water, and sometimes admixtures that 

hardens into a material with a higher strength than soil but less than 1,200 psi (ASTM 2010; ACI 

2013). In many applications, even lower strengths are often required to ensure the fill can be 

excavated without excessive effort, e.g., typical unconfined compressive strength (UCS) values 

of 300 psi or lower. Figure 2 presents different cement-based concrete materials and how they 

differ in terms of cement and water content. Flowable fill materials (i.e., CLSM) tend to have 

higher water contents and lower cement contents compared to other cement-based concrete 

materials. 

© 2020 Portland Cement Association. Modifications to colors by FHWA. 

Figure 2. Illustration. Comparison between CLSM and other cement-based materials for 

water and cement contents (Halsted 2020).

ADVANTAGES AND DISADVANTAGES 

Reported advantages of CLSM include the following (ACI 229R 2013; Wagstaff 2016): 

• Low shrinkage and compressibility characteristics. 

• Excavation of the fill is easy using hand equipment at any age. 

• Fast construction and strength gain: fill hardens within a short time (i.e., a few hours). 

• Compaction or tamping not required. 

• Strength and other properties not widely affected by changing moisture conditions. 

• Land utilization for dumping coal ash (fly ash) and other industrial byproducts reduced.
9

ACI 229R (2013) and Wagstaff (2016) also reported the following advantages for CLSM 

materials: readily available, easy to deliver, easy to place, versatile and adjustable mixes, 

reduced excavation costs, all-weather construction, no onsite storage required, and less 

inspection needed. 

Disadvantages of CLSM materials include the following: 

• Designed to be low strength (thus, generally not considered a disadvantage for the target 

applications).

• Could incur additional material costs. 

• Requires additional training and familiarity by contractors with its purposes, mixture 

design, testing, and installation to help ensure a quality project. 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

The researchers found the following applications for CLSM in the literature (Ling, 

Kaliyavaradhan, and Poon 2018): 

• Lightweight road bases and fills. 

• Backfill behind retaining walls. 

• Bridge-approach embankments. 

• Void and cavity filling (tunnel shafts and sewers). 

• Pipe backfilling. 

• Structural fills. 

• Insulation and isolation fills. 

• Erosion control. 

MATERIAL CHARACTERIZATION 

Physical Properties 

The physical properties of CLSM materials are expected to vary greatly based on the constituents 

used. Flowability is one of the major physical properties specified for CLSM in most design 

procedures. Flowability measures the average diameter of a disk-shaped spread when the mix is 

poured from a standard-sized cylinder as per ASTM D6103 (ASTM 2017a). According to ACI 

229R and ASTM D6103 guidelines, mix flowability should range between 200 and 300 mm 

(7.9 and 11.8 inches) to ensure the mix flows easily without segregation while pumping (ACI 

2013). Further, water-to-solid (W/S) and cement-to-water (C/W) ratios are the control 

parameters to achieve a proper mix design that satisfies flowability and strength. 

Mechanical Properties 

As a consensus, flowability and UCS are the two main properties to check for CLSM mixes. 

Strength requirements vary depending on the application. Generally, a minimum strength of 

345 kPa (50 psi) is specified (ACI 2013). The maximum strength is often limited to 
10

1.4 megapascal (MPa) (200 psi), especially for applications that require excavability; however, 

the ultimate maximum strength for CLSM is 82.7 MPa (1,200 psi), as required by ACI 229R

(ACI 2013; Pierce and Blackwell 2003).

Other mechanical properties reported for CLSM include compressibility from a one-dimensional 

consolidation test, undrained shear strength using shear box test apparatus, and California 

Bearing Ratio (CBR) for up to 7 d (Wu and Tsai 2008; Wu and Lin 2011). The angle of shearing 

resistance (friction angle) varies from 25 to 55 degrees for CLSM mixes cured for 28 d. Lower 

friction angle values are rarely reported in the literature, e.g., 11.2–12.6 degrees after 7-d curing

(Lee and Kim 2013). The cohesion or cohesion intercept typically ranges from 8.3 to 60.0 kPa 

(1.2 to 8.7 psi). The variation in shear strength properties depends on the type of aggregates and 

the mix design adopted (McGrath and Hoopes 1998; Langton, Rajendran, and Smith 1998; 

Masada and Sargand 2007; Türkel 2007). Examples of the mixture properties and mechanical 

properties of fly ash-based CLSM mixes are given in table 2 and table 3 from a study by Türkel 

(2007). The researchers measured shear strength properties using a shear box apparatus with tests 

conducted at a loading strain rate of 0.300 mm/min and having a dial gauge reading of 

0.12 mm/division.

Table 2. Example CLSM mix properties and the corresponding compressive strengths

(Türkel 2007).

Mixture

Mixture Properties

W/S Ratio

Spread

(mm)

UCS (MPa)

F/(C + FA)

W/(C +

FA) 7 d 14 d 21 d 28 d

M1 3:1 1.00 0.25 220 0.28 0.58 0.84 1.15

M2 3.5:1 1.00 0.22 210 0.24 0.38 0.65 1.06

M3 4:1 1.10 0.22 207 0.22 0.37 0.63 1.02

M4 5:1 1.25 0.21 210 0.19 0.39 0.72 0.92

M5 5.5:1 1.30 0.20 210 0.17 0.34 0.64 0.88

M6 6:1 1.40 0.20 205 0.16 0.31 0.61 0.85

F = filler (crushed limestone powder); C = cement; FA = fly ash; W = water.

Table 3. Direct shear test (DST) results for CLSM mixtures at 7 d (Türkel 2007).

Mixture M1 M2 M3 M4 M5 M6

Cohesion intercept (MPa) 0.047 0.045 0.044 0.041 0.040 0.038

Angle of shearing resistance (degrees) 54 52 51 47 45 43

Use of Recycled and Byproduct Materials for CLSM

A major disadvantage of CLSM is the high cost of materials. However, CLSM’s relatively low 

mechanical property requirements compared to those for concrete or other cementitious materials

enable CLSM production to use industrial wastes, which is a common practice to reduce cost and 

for environmental considerations. ACI 229R recommends that any available recycled granulated 

material be considered an alternative aggregate for CLSM as long as it has been tested before use

(ACI 1999, 2013). The byproduct materials used to replace cement and aggregates include fly 

ash, bottom ash as a fine aggregate, cement kiln dust, byproduct foundry sand, flue gas 
11

desulfurization (FGD), oyster shells, scrap tire rubber, recycled glass, acid mine drainage (AMD) 

sludge, blast furnace slag, recycled concrete, and clean coal ash. 

According to Do and Kim (2016), the advantages of using fly ash as a replacement for cement 

include improving flowability, reducing segregation, and reducing material costs. Examples of 

studies that investigated the use of fly ash in CLSM include those by Dockter (1998); Gabr and 

Bowders (2000); Katz and Kovler (2004); Lee et al. (2013); Nataraja and Nalanda (2008); 

Pierce, Gassman, and Richards (2002); and Siddique (2009). Examples of studies that 

investigated the use of bottom ash as a fine aggregate include those by Katz and Kovler (2004); 

Naganathan, Razak, and Hamid (2010); Naganathan, Razak, and Hamid (2012); Razak, 

Naganathan, and Hamid (2009); and Razak, Naganathan, and Hamid (2010). Cement kiln dust is 

ideal for producing CLSM at a lower cost because it contains smaller amounts of active lime and 

silica, so it can be effectively added to produce a very low-strength material. Several past 

research efforts investigated the use of cement kiln dust in CLSM, for example, Pierce, Tripathi, 

and Brown (2003); Lachemi et al. (2008); and Lachemi et al. (2010). Byproduct foundry sand 

was also used at lower cost and is abundantly available. Studies investigating the use of foundry 

sand in CLSM include those by Bhat and Lovell (1997); Siddique and Noumowe (2008); 

Tikalsky, Gaffney, and Regan (2000); and Trejo, Folliard, and Du (2004). Several research 

studies also investigated the other byproduct materials: 

• Butalia, Wolfe, and Lee (2001) studied FGD. 

• Kuo et al. (2013) investigated oyster shells. 

• Pierce and Blackwell (2003) and Wu and Tsai (2008) used scrap tire rubber in their 

studies. 

• Lachemi et al. (2010) and Muhmood, Vitta, and Venkateswaran (2009) studied blast 

furnace slag. 

• Ohlheiser (1998), Gabr and Bowders (2000), Achtemichuk et al. (2009), and Naik et al. 

(1998) studied recycled glass, AMD sludge, recycled concrete, and clean coal ash. 

Nehdi and Khan (2001) suggested benefits of using worn-out tire rubber mixed with portland 

cement include lower density, better toughness and ductility, and higher impact resistance. Wu 

and Tsai (2008) investigated using recycled crumb rubber (from scrap tires) in a CLSM for 

bridge approach backfill. Rubber fine particle sizes ranged from 0.9 to 4.76 mm (passing No. 4 

sieve), with the rubber fines classified as poorly graded sand according to the Unified Soil 

Classification System (USCS) and with an average specific gravity of 1.18. The researchers 

tested 44 rubberized CLSM mixtures. CLSM mixes with rubber fines alone were not flowable in 

the trial mixes, and reasonable strength values could not be achieved. Rubberized CLSM without 

the addition of sand exhibited heavy bleeding, and a standard flowability of 200 mm was not 

achievable. The researchers found that including 40 percent sand in rubberized CLSM resulted in 

an increase in flowability and strength. The researchers tested flowability immediately after 

mixing and compressive strength for samples cured at 1, 7, and 28 d.
12

For the rubberized CLSM with 40 percent sand, Wu and Tsai (2008) found flowability increased

with an increase in the W/S ratio (figure 3). The rate of increase became insignificant when the 

W/S ratio was greater than 0.5. A W/S ratio of 0.3 can be used as the minimum criterion for 

flowability, while a W/S ratio of 0.5 appears to be the maximum threshold. Further, samples with 

C/W ratios of 1.0 or less had a maximum 28-d UCS of only 165.5 kPa (24 psi). The use of sand 

in rubberized CLSM increased the strength considerably. Ultimately, a W/S ratio of 0.3 and a 

C/W ratio of 0.7 could satisfy both strength and flowability requirements (figure 3). 

© 2008 Wu and Tsai. Modifications by FHWA to 

the color and line labels. 

A. Effects of C/W and W/S ratios on the flowability of rubberized CLSM with 40 percent sand. 

© 2008 Wu and Tsai. Modifications by FHWA to 

the color and line labels. 

B. Effects of C/W and W/S ratios on the UCS of rubberized CLSM with 40 percent sand.

Figure 3. Graphs. Effects of C/W and W/S ratios on the flowability and UCS of rubberized 

CLSM with 40 percent sand (Wu and Tsai 2008). 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
13

According to Wu and Tsai (2008), “Based on the promising performance, rubberized CLSM was 

used to rehabilitate a bridge approach damaged by significant differential settlement .... The 

final mix design for the trial construction was 0.7 for C/W and 0.35 for W/S to ensure a design 

28-day strength of 600 kPa (87 psi). The flowability observed was about 250 mm (9.8 in.). 

Testing and monitoring for this project provided evidence that, from a geotechnical standpoint, 

rubberized CLSM can be successfully used as an alternative to mitigate the settlement under a 

bridge approach. Field trial details and construction monitoring for this project will be presented 

elsewhere.” However, no followup publications on the topic were found to include in this report. 

Wu and Lin (2011) investigated the use of CLSM with reservoir siltation (RS—silty materials 

collected from settling ponds as an aggregate material). This study used gray silty clay with a 

plasticity index (PI) of approximately 11 percent and a liquid limit of 35 percent. While using RS 

is not conventional, the mix design selection methodology can be broadly applied to any 

byproduct material and CLSM design for geotechnical highway applications, so the researchers 

evaluated the study further. Series I testing consisted of evaluating proper design mixes for 

CLSM. Tests included physical properties, flowability, and UCS. A W/S ratio of 0.8–0.9 and a 

C/W ratio of 0.4–0.5 was proposed for the passing design. Flowability increased with the 

increase of C/W ratios. For the same C/W ratio, flowability increased with the increase of the 

W/S ratio because of the presence of clayey fines in the RS material. The UCS of each trial mix 

varied as a function of W/S, C/W, and curing time. Thus, the values of UCS (i.e., Qu) increased 

with the increase of C/W and curing time but did not show a regular behavior trend for the W/S 

ratios. Further, strength improvements with curing time were more pronounced for W/S ratios 

between 0.7 and 0.8. Figure 4 summarizes the results for flowability requirements and figure 5

for strength requirements. 

© 2011 Wu and Lin. Modifications by FHWA to colors and patterns 

for accessibility. 

Figure 4. Graph. Effects of C/W and W/S ratios on the flowability of CLSM mixes with RS

(Wu and Lin 2011). 
14

© 2011 Wu and Lin. Modifications by FHWA 

to colors and patterns for accessibility. 

A. W/S = 0.7. 

© 2011 Wu and Lin. Modifications by FHWA to colors 

and patterns for accessibility. 

B. W/S = 0.8. 

© 2011 Wu and Lin. Modifications by FHWA to colors 

and patterns for accessibility. 

C. W/S = 0.9. 

Figure 5. Graphs. Effects of C/W and W/S ratios and curing time on the strength of CLSM 

mixes with RS (Wu and Lin 2011). 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
15

Wu and Lin’s (2011) series II testing involved selecting an optimum design mix formula 

(flowability and strength) for compressibility and undrained shear strength testing with two 

different C/W and W/S ratios. For compressibility testing, samples with smaller C/W ratios 

showed a slightly greater vertical strain for similar vertical stress. For the shear box testing, the 

measured friction angles were quite low (7.1 and 9.9 degrees only). However, the values for 

cohesion were on the high end, i.e., 2 MPa (288 psi) and 3.75 MPa (544 psi), indicating an 

overall high shear strength. Figure 6 presents the stress-displacement curves for the shear box 

testing. Overall, hardened RS-based CLSM behaved as heavily overconsolidated clay due to the 

strong cementation action of cement, leading to a sound shearing resistance and negligible 

compressibility. 

© 2011 Wu and Lin. Modifications by FHWA to 

color and line labels. 

A. C/W = 0.4, W/S = 0.8. 

© 2011 Wu and Lin. Modifications by FHWA to 

color and line labels. 

B. C/W = 0.5, W/S = 0.8. 

Figure 6. Graphs. Shear box testing results for RS-based CLSM tested at two different 

C/W ratios (Wu and Lin 2011).
16

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES 

As a general consensus on the control parameters for mix design, W/S ratio and C/W ratio are 

reported and controlled to achieve a proper mix. 

In-place properties and design requirements for CLSM include the following 

(Graniterock® 2023): 

• Strength: A CLSM UCS of 345 kPa to 690 kPa (50 to 100 psi) is equivalent to the 

bearing capacity of a well-compacted soil. Maintaining CLSM strengths at low levels is 

critical in projects requiring future excavation. 

• Density: Wet density of most CLSM ranges from 1,842 to 2,323 kg/m3 (115 to 

145 lb/ft3). A CLSM mixture with only fly ash, cement, and water might have a density 

range of 1,142 to 1,602 kg/m3 (90 to 100 lb/ft3). 

• Flowability: According to ACI 229R and ASTM D6103 guidelines, mix flowability 

should range between 200 and 300 mm (7.9 and 11.8 inches), which ensures the mix 

flows easily without segregation while pumping (ACI 2013; ASTM 2017a). 

• Settlement: CLSM does not settle after it hardens. 

• Shrinkage: The ultimate shrinkage of CLSM is typically in the range of 

0.02-0.05 percent. 

• Excavatability: CLSM with compressive strengths of 690–2,068 kPa (100–300 psi) can 

generally be excavated with mechanical equipment, such as backhoes. 

• Corrosivity: Electrical resistivity tests can be performed on CLSM on corrugated metal 

culvert pipe (e.g., California Department of Transportation (Caltrans) test method 643 

(Caltrans 2020)). CLSM uniformity reduces the chance of corrosion. 

ASTM Standards for CLSM 

The following ASTM standards apply to CLSM: 

• ASTM D4832: Standard Test Method for Preparation and Testing of Controlled Low 

Strength Testing Materials (CLSM) Test Cylinders (ASTM 2018). 

• ASTM D5971/D5971M: Standard Practice for Sampling Freshly Mixed Controlled LowStrength Material (ASTM 210a). 

• ASTM D6023: Standard Test Method for Density (Unit Weight), Yield, Cement Content, 

and Air Content (Gravimetric) of Controlled Low-Strength Material (CLSM) (ASTM 

2106a). 

• ASTM D6103: Standard Test Method for Flow Consistency of Controlled Low Strength 

Material (CLSM) (ASTM 2017a). For flowable fill mixtures placed on a slope, or when a 
17

stiffer mix is required, the slump cone test becomes a more useful indicator of 

consistency, according to ASTM C143/C143M Standard Test Method for Slump of 

Hydraulic-Cement Concrete (ASTM 2015a). 

• ASTM D6024/D6024M: Standard Test Method for Ball Drop on Controlled Low 

Strength Material (CLSM) to Determine Suitability for Load Application (ASTM 2017b).

Table 4 provides examples of the specifications and design requirements for CLSM, as specified 

by different agencies in the United States for many applications. Reported specifications include 

strength, aggregate size requirements, cement content, and placement time.
18

Table 4. Examples of CLSM specifications by different agencies (Kaneshiro et al. 2001). 

Agency/Date 

Strength 

(kPa) 

Max. Aggregate/ 

Percent Fines Aggregate Cement Fly Ash

Placement 

Time (h) Remarks 

ACI Committee 229R 

Guideline, 1999 

(ACI 1999)

<8,300 19 mm 1,540–1,780

kg/m3

30–119 

kg/m3

Type F <1,200 

kg/m3; Type C 

<208 kg/m3

3 

Report/ 

guideline, fly 

ash, quarry 

waste, native 

soil 

Greenbook 

Committee of Public 

Works Standards, Inc. 

“Greenbook,” 

(APWA 1997) 

—

9.5 mm/ 

SE = 20, 

<12 percent, 

nonplastic 

<30% for hand 

excavation 11.3 kg min 

<20 percent, 

Type C not 

allowed 

2.5. 

1998 regional 

supplement 

allows 

>20 percent fly 

ash

Caltrans Section 

19-3.062, (Caltrans 

1999) 

345–690 — — — — 3 

Minimum pH, 

chloride, and 

sulfide content 

specified; 

>60 kPa for 

>6 m of cover 

Building Technology 

Research, (BTI HUD 

1981) 

480–2,400 —

>74 kg/m3 for 

trial batches — — —

Types F, C, and 

N fly ash 

allowed 

City of Orange 

Environment 

Management Agency 

(California), 1993a

690–2,400 SE >60 — <160 kg/m3 Type F >1.5 —

Denver International 

Airport, 1994a 345–690 18 percent — 77 kg/m3 — —

Onsite SM 

material, cement 

increased from 

25 kg/m3; slump 

reduced to 

127 mm
19

Agency/Date 

Strength 

(kPa) 

Max. Aggregate/ 

Percent Fines Aggregate Cement Fly Ash

Placement 

Time (h) Remarks 

ISG Resources, 2000a520–1,040 

9.5 mm; 

10-12 percent, 

nonplastic 

<30–40 percent — — — 

>2,070 kPa 

specified where 

higher strength 

desirable 

Southern Nevada 

Water Authority, 

2000a

690–2,070 25–76 mm; 

nonplastic 25–76 mm Type V Type C —

Conflict to use 

Type C fly ash 

with Type V 

cement; Type F 

specified 

City of San Diego 

Capital Improvement 

Program Guidelinea

<8,300 9.5–76 mm; 

nonplastic As required As required Type F or any 

suitable fillers 3 

All verified by 

lab and field 

tests, strength as 

required 

—No data. 

aFor more information about the specifications and design requirements of these agencies, see Kaneshiro et al. 2001. 

Max. = maximum; SE = sand equivalent; BTI = Building Technology Inc.; HUD = U.S. Department of Housing and Urban Development; ISG = Industrial 

Services Group; SM = silty sands, sand-silt mixtures.
20

According to ACI 229R and ASTM D6103 guidelines, mix flowability should range between 

200 and 300 mm, which ensures the mix flows easily without segregation while pumping (ACI 

199, 2013; ASTM 2017a). Table 5 provides a design example (for determining required water 

content given other variables are predetermined) based on the work of Dev and Robinson (2020) 

for pond ash-based CLSM. The pond ash is classified as a poorly graded silty sand per the USCS 

and consists of 94 percent sand-sized particles and 6 percent fine (silt and clay) particles. The 

researchers reported the specific gravity of the pond ash as 2.21 and the pH as 7.5. The optimum 

moisture contents were 23 and 19 percent, and the maximum dry densities were 1,246 and 

1,358 kg/m3(77.8 and 84.8 pounds per cubic ft (pcf)) for the standard and modified Proctor 

compaction methods, respectively (ASTM 2021a, 2021b).

Table 5. Design example of a pond ash-based CLSM passing both strength and flowability 

requirements (Dev and Robinson 2020).

Cement 

(percent) Flowability (mm) 

Water Content 

(percent) UCS (MPa) 

2

200 41.5 0.244

300 45.5 0.175

3

200 42 0.444

300 46.5 0.359

4

200 42.5 1.048

300 47 0.722

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS 

CLSM can be transferred to the jobsite using ready-mix trucks, pumps, volumetric measuring, 

and continuous-mixing concrete equipment for jobsite mixing; dump trucks are discouraged 

(Ragan 2023). The material can be placed with a chute, conveyor, pump, or bucket. CLSM 

consolidates under its own weight and requires little spreading or compacting when at the 

appropriate slump. Thus, CLSM is typically cured in the ambient environment (ACI 1999). 

Further, CLSM does not require a cover or moisturizing equipment to properly cure it to the 

design strength, but the material might require protection from freezing. 

Typical and common field-testing requirements for CLSM include visual checks, consistency 

checks (flowability and in-place density), and strength checks. The following ASTM standards 

are commonly used for the QA/QC of in-place CLSM: 

• ASTM D6024: Standard Test Method for Ball Drop on Controlled Low Strength 

Material (CLSM) to Determine Suitability for Load Application (ASTM 2017b).

• ASTM C403/C403M: Standard Test Method for Time of Setting of Concrete Mixtures by 

Penetration Resistance (ASTM 2017c). 

• ASTM D4832: Standard Test Method for Preparation and Testing of Controlled Low 

Strength Material (CLSM) Test Cylinders (ASTM 2018). 
21

Most of the specifications allowed 2 to 2.5 h of placement time for CLSM. Some specifications 

allowed up to 3 h (Kaneshiro et al. 2001). Some specifications also indicate at least two vibrators 

should be used (if needed) according to jobsite conditions. 

Unstable soil from trench walls and floors should be removed before placing CLSM (Kaneshiro 

et al. 2001). Backfilling above CLSM is restricted until after the initial set of 8 h, or 138 kPa

strength, is achieved. Alternatively, the results of an indentation of the standard test for the ball 

drop on CLSM can be used to estimate the setting (ASTM 2017b). Pavement, or other backfill 

layers, may be placed directly over the CLSM after setting. 

Kaneshiro et al. (2001) presented specific construction specifications for when CLSM is used for 

pipeline fills to ensure proper placement and quality checks. As a start, the placement of material 

needs to be continuous. Operators should check the wet density to ensure the proper placement 

density is achieved to prevent segregation. Further, pipe flotation can be prevented by anchorage 

or backfill sequencing or ballasting, etc. Pours/lifts should be bulkheaded to prevent floatation or 

separate pours when required. Pipes should also be properly “choked” and supported with 

sandbags (or an equivalent) to allow free flow of CLSM around the pipes. 

COST INFORMATION 

The materials cost of CLSM poses a major disadvantage. CLSM’s cost comes close to that of 

ready-mix concrete from several suppliers around the country, as highlighted in the following 

three examples: 

• From Louisville Water Company Bid 16-175/Ready-Mix Concrete, the contractor priced 

1 yd3of ready-mix concrete at $127.20–$148.40 during regular hours and 

$190.80-$212.00 after hours. In comparison, flowable fill was priced at $100.70–$148.40

(Louisville Water Company 2017). 

• From GSI Concrete in Pennsylvania, 1 yd3 of ready-mix concrete is priced at $113 for the 

3,000-psi mix and $133 for the 5,000-psi mix. In comparison, 1 yd3 of flowable fill is 

priced at $120 (GSI Concrete 2022). 

• For a competitive sealed bid in Frisco, TX, the letting for bid No. 1911-023 opened in 

April 2018 and had flowable fill (CLSM) priced between $88 and $157 per cubic yard, 

depending on the delivery option (Frisco n.d.).

Note that although the price per cubic yard of flowable fill is more expensive than other backfill 

materials (e.g., crushed stone backfill material typically costs $16–$23 per ton), flowable fill 

reduces in-place costs across the board (Charleston Stone Company 2023). For example, on a 

small pipe backfill job for the City of Houston Department of Public Works, more than $12,000 

was saved by using flowable fill (CLSM) (National Ready Mixed Concrete Association 

(NRMCA) 2011). The properties of CLSM, being nonsettling, self-leveling, quick drying, 

nonsegregating, and easy to excavate, makes it cost effective overall. An NRMCA presentation 

on the use of CLSM in transportation projects compared the labor cost for placing and 

compacting a traditional granular backfill to a flowable fill (Killingsworth 2021). The costs were 
22

$226.18 versus $35.09, respectively, indicating that the higher material cost was offset during 

placement. 

ENVIRONMENTAL CONSIDERATIONS 

Folliard (2008) reported that CLSM is a consumer of various byproduct materials that are not 

typically allowed in conventional concrete, such as fly ash not meeting ASTM C618 (ASTM 

2019a). Therefore, some concern could exist about the potential for leaching of constituents in 

byproduct materials (e.g., heavy metals, organics) from CLSM and the environmental impacts. 

Folliard (2008) studied the leachate potential of three fly ashes, one bottom ash, and one foundry 

sand. Tested heavy metal concentrations included arsenic, barium, cadmium, chromium, lead, 

mercury, selenium, and silver. All the byproduct materials tested easily passed the toxicity 

characteristic leaching procedure (TCLP), meaning the heavy metal concentrations were very 

low and not of concern. 

LCA STUDIES 

The comprehensive literature review found no environmental product declarations (EPDs) or 

product category rules (PCRs) for CLSM materials in the United States or worldwide. Further, 

no comprehensive LCA studies were conducted in the United States or elsewhere. Most LCA 

studies examine traditional concrete mixes. Further, many research papers and reports focus on 

making the CLSM more “sustainable” by using recycled materials to replace cement (e.g., fly 

ash) and aggregates. Many of these studies have been reported earlier in this chapter. Thus, the 

lack of LCA studies on CLSM and the lack of PCRs that regulate the publication of EPDs for 

CLSM are major gaps in research and sustainability assessment that need to be addressed. 

CASE STUDIES 

Case studies reported for CLSM include its use in bridge approaches, utility fills, void fills, and 

other backfills: 

• Du et al. (2006) reported on using CLSM for bridge approaches in many U.S. case 

studies. According to Du et al., the Ohio Department of Transportation (ODOT) 

pioneered this application, and the Delaware DOT favors using CLSM for many of its 

bridge approaches. Further, the Oklahoma DOT constructed three new bridges on 

U.S. 177 north of Stillwater, OK, out of five approaches constructed using different 

structural fill materials. One of the designs had an approximately 2.5-m (8-ft)-deep 

CLSM bed. No differential settlement at the end of the bridges with CLSM was reported 

after paving, but data collected before paving indicated the CLSM settled 44 mm/m 

(1.6 inches/yd), and the settlement increased incrementally until approximately 10 mo 

after placement. The Iowa DOT had experience with backfilling four bridge approaches 

using flowable mortar (CLSM), and no appreciable settlement was observed between the 

bridges and the bridge approaches. 

• Seattle Public Utilities in the State of Washington used quick-setting CLSM fill to 

replace an old water main in downtown Seattle (Vanga 2013). Using CLSM greatly 

expedited the construction activity, which resulted in reducing the project duration and 
23

advancing construction activities. In a similar case study, CLSM was chosen to fill an 

abandoned pipeline under a segment of I–70 near Officers Gulch, Copper Mountain, CO 

(Vanga 2013). 

• Several agencies and institutions have also experimented with using CLSM for various 

highway backfill applications. Table 6 provides a comprehensive summary from Folliard 

(2008) on the technical issues and applications addressed by the case studies and 

full-scale test sections by the different agencies. More information can be found in the 

original study. 

Table 6. Matrix of field-testing issues and applications (Folliard 2008).

Issue/Application 

Agency or Organizationa

UT

Austin NRMCA 

Hamilton 

County 

(OH) EBMUD 

Texas 

DOT TAMU 

Technical Issue

Long-term strength 

gain/excavatability 

x x — x — x

Short-term strength 

gain/constructability 

x — x x x x

Corrosion of metals in CLSM — — x — — x

Productivity and cost — — x — — x

Resistance to freezing and thawing — — x — — —

Construction issues (i.e., pipe 

floating) 

— — — — — x

Settlement x — — x — —

Use of byproduct materials x x x x x

Environmental issues — — x — — —

CLSM Application

Backfill 

x x x x x

Utility bedding — — x — — —

Void fill — — — x — —

Bridge approach — — — — x —

—No data. 

aA field test was planned with Florida DOT, but permitting issues prevented the field test. 

UT Austin = University of Texas at Austin; EBMUD = East Bay Municipal Utility District; TAMU = Texas A&M 

University. 

25

CHAPTER 3. ESCS 

INTRODUCTION AND BACKGROUND 

ESCS are lightweight ceramic aggregates that are prepared by expanding and vitrifying select 

minerals, specifically select shales, clays, or slates, in a rotary kiln at temperatures exceeding 

982 °C (1,800 °F) (ESCS Institute (ESCSI) 2023). If shale is used, the product is called expanded 

shale. If clay or slate are used, the products are called expanded clay and expanded slate, 

respectively. The production and raw material selection processes are strictly controlled to 

ensure a uniform, high-quality, lightweight aggregate (LWA). Figure 7 shows a typical structure 

of expanded clay aggregates. Figure 8 presents a map of the sources and manufacturers of ESCS 

in the United States, as reported on the ESCSI website. 

© 2007 Lucis. 

Figure 7. Photo. Lightweight expanded clay aggregate (Creative Commons n.d.a). 

Original map © 2022 Google® MapsTM. Modifications by FHWA to show locations (see 

acknowledgments section). 

Figure 8. Map. ESCS sources and manufacturers in the United States (Google 2022). 

ESCS have low relative density due to the cellular pore system that forms during the 

manufacturing process (ESCSI 2023). Heating certain raw materials to incipient fusion form the 

cellular structure. At the incipient fusion temperature, gases evolve within the pyroplastic mass, 
26

causing expansion, which is retained upon cooling. ESCS thus have uniformly distributed pores 

that range in size from 5 to 300 μm. The system with pores is continuous, relatively crack free, 

and produces high-strength ESCS. When exposed to moisture, the permeable pores closest to the 

surface typically fill with water within a few hours to a few days. Interior pores fill up with water 

extremely slowly and may take months of submersion to approach saturation. 

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using ESCS in highway backfill applications include the following 

(ESCSI 2023): 

• Cost effective. 

• Water and acid insoluble.

• Chemically inert. 

• Highly durable. 

• Increased stability. 

• Free draining. 

• Thermal insulation. 

• Reduced overturning and lateral forces. 

• High internal friction angle and high strength. 

• Lightweight. 

• Controlled gradations. 

• Controlled settlement. 

Disadvantages of ESCS are the initial cost of plants and manufacturing facilities and the 

energy-intensive manufacturing process. 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

ESCS can be used in the following geotechnical applications for highways (ESCSI 2023): 

• Retaining wall backfill. 

• Bridge abutment fills. 

• Slope stabilizer. 

• Lightweight embankment fill. 

• Insulating backfill. 

• Underground conduits and pipelines. 

• Various landfill applications. 

MATERIAL CHARACTERIZATION 

Physical Properties 

ESCS produced by the rotary kiln process should meet the requirements of ASTM C330, 

Standard Specification for Lightweight Aggregates for Structural Concrete (ASTM 2017d). The 

following presents some of the common physical and mechanical properties of ESCS that should 
27

be met for highway geotechnical applications, with more detailed information provided in table 7

(ESCSI 2008): 

• Soundness loss: A maximum of 30 percent with four cycles of magnesium sulfate is 

recommended according to AASHTO T 104, Standard Method of Test for Soundness of 

Aggregate by Use of Sodium Sulfate or Magnesium Sulfate (AASHTO 2022a).

• Abrasion resistance: A maximum of 40 percent is recommended according to 

ASTM C131, Standard Test Method for Resistance to Degradation of Small-Size Coarse 

Aggregate by Abrasion and Impact in the Los Angeles Machine (ASTM 2010b).

• Chloride content: A maximum of 100 ppm is recommended according to 

AASHTO T 291, Standard Method of Test for Determining Water-Soluble Chloride Ion 

Content in Soil (AASHTO 2022b).

• Grading: A wide variety of grain size distributions could be made available, specified 

based on performance needs, in accordance with ASTM C136, Standard Test Method for 

Sieve Analysis of Fine and Coarse Aggregates (ASTM 2015b). Commercially available 

gradings include the following categories: 19.5–4.75 mm (3/4 inch to No. 4), 

12.7-4.75 mm (1/2 inch to No. 4), 9.5–2.38 mm (3/8 inch to No. 8), 9.5–0 mm (3/8 inch

to 0 inches), 51–19.5 mm (2 to 3/4 inch), and 51–0 mm (2–0 inches). Blends of the 

aforementioned categories can also be produced on request.
28

Table 7. Recommended physical and mechanical properties for ESCS.

Aggregate 

Property

Measuring 

Method Test Method

Commonly Used 

Specifications for 

ESCS

Typical Values for 

ESCS

Typical Design Values for 

Ordinary Fills

Soundness loss

Magnesium 

sulfate AASHTO T 104 (2022a) <30 percent <6 percent <6 percent

Abrasion 

resistance

Los Angeles 

abrasion ASTM C131 (2010b) <40 percent 20–40 percent 10–45 percent

Chloride 

content

Chloride content 

of soils AASHTO T 291 (2022b) <100 ppm 10–70 ppm —

Grading Sieve analysis ASTM C136

(2015b)

Wide variety Wide variety —

Compacted inplace bulk 

density (unit 

weight)

Density test Contact ESCS producer for 

local practices <1120 kg/m

3 640–1,040 kg/m

3

moist 1,600–2,080 kg/m

3

Stability (Φ

angle, f)

DST consolidated 

drained

triaxialconsolidated 

drained

ASTM D3080 (2012)

(>0.75 inches removed)

U.S. Army Corps of 

Engineers (USACE)

EM 1110-2-1906 

appendix X

(USACE 1970)

Depending on 

procedure, grading, 

particle angularity, 

amount of 

compaction, and 

consolidating stress

35–45 degrees 30–38 degrees

(fine sand—sand and gravel)

Loose bulk 

density (unit 

weight)

Loose ASTM C29 (2017e) Dry <50 lb/ft3

saturated <65 lb/ft3 Dry 480–800 kg/m

3

1,425–1,680 kg/m

3

pH pH meter AASHTO T 289 (2022c) 5–10 7.0–10 5–10

EM = engineer manual.

Further, Zukri et al. (2018) reported the physical and chemical properties of commercially available ESCS materials (table 8).
29

Table 8. Physical and chemical properties for commercially available ESCS materials (Zukri et al. 2018).

LWA pH

Density (kg/m3)

Specific 

Gravity

Permeability

(cm/s)

Water 

Absorption 

(percent)

Chloride 

Content

Sulphate 

Loose Compacted Content

Normal aggregate 4–6.5 1,426 2,339 2.6–3.0 1–100 <2 <100 ppm —

Argex®

No 

info.

450–635 0.62–1.30 — 7–35 <0.02% <0.80%

ESCS 5–10 720 960 1.25–1.40 >1 8 10–70 ppm —

GBC® India 6–7 380–710 — — 18 — —

Geo Leca No 

info. 424–520 2.37 — — 5–46 —

Leca® 8.05 220–325 0.41 0.1 36 0.50 percent <450 mg/L

Optiroc 8–11 280–335 0.45–0.635 0.1–5 14–45 38 mg/kg 1,700 mg/kg

LEXCA Leca 9 266 283 0.771 2.53 30 6.8 mg/L 95 mg/L

Norlite 7.4 644 729 1.25–1.55 13.4–15 — 5–46 ppm 146 ppm

Stalite® 7–9 769 881 1.45 5–15 9–12 0.60–7.0 

ppm

32 ppm

Techniclay — 321–360 — — — — —

—No information.

info = information.
30

Mechanical Properties

Zukri et al. (2018) reported on the mechanical properties of commercially available ESCS 

materials. Researchers measured angles of internal friction by DSTs, consolidated drained (CD) 

triaxial tests, or large-scale triaxial tests. The angles of internal friction ranged between 35 and 

53 degrees for the various products, indicating relatively high shear strength. Table 9

summarizes the full suite of results, and the maximum sizes of the tested particles are reported 

when available.

Table 9. Mechanical properties for commercially available ESCS materials

(Zukri et al. 2018).

LWA

Gradation/Size 

Information (mm)

Internal Friction Angle

Measuring Method Value (degrees)

Normal aggregate — Triaxial (CD) 3,642

Argex — — 35–42

ESCS 10–20 Direct shear; triaxial (CD) 35–45

GBC India — Triaxial (CD) 53

Leca 10–20 — 37

LEXCA Leca — Triaxial (CD) 35

Minto — Large-scale triaxial test 40.5

Norlite 10–20 Triaxial (CD) 42–53

Optiroc

10–20 Static triaxial 37

0–32 Static triaxial 45

Solite — Large-scale triaxial test 40

Stalite 10–20 Direct shear; triaxial (CD) 4,346

Techniclay — Direct shear; triaxial (CD) 53

—No information.

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES

ESCSI recommends consulting with the producers for precise information on aggregate grading, 

bulk density, in-place compacted density, friction angle, thermal conductivity, and placement 

method(s) (ESCSI 2018). ESCS fills are approximately half the weight of conventional fill 

materials. Design with ESCS should thus take advantage of the load reduction, coupled with the 

high internal friction angles commonly reported for the ESCS. Generally, ESCS can reduce 

vertical and lateral forces by more than one-half and thus be used to solve numerous 

geotechnical engineering problems and convert soft and unstable soil into usable property.
31

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS

No special equipment is needed on the jobsite for compacting ESCS (ESCSI 2008). The material 

can be placed in any weather with no waiting time between placing lifts. ESCS should be placed 

in uniform layers. Lift thickness and the number of passes by the compaction equipment are 

determined by the engineer, depending on project requirements (i.e., specified levels of stability, 

compaction, and density). A common practice is to limit the thickness of each uniform layer to 

300 mm (12 inches) or less in a loose state before compaction. Layers are also commonly 

compacted using vibratory compaction equipment weighing no more than 12 tons static weight. 

Vibratory plate compaction equipment should be used in confined areas (3.7-14.9 kW (5-20 hp)). 

A minimum of two passes should be applied in the case of 150-mm (6-inch) lifts with a 5-hp 

plate power, and 300-mm (12-inch) lifts with a 20-hp plate (ESCSI 2008, 2018).

Further, the contractor or site engineer should take all necessary precautions to ensure the ESCS 

material is not over-compacted in the field. Construction equipment, other than for placement 

and compaction, should not be operated on exposed ESCS lightweight fill (ESCSI 2008, 2018).

AVAILABLE COST INFORMATION

The cost of shale in the United States ranges from $20 to $60 per ton, while slate ships between 

$27 and $64 per ton (Homeguide 2022a). The production of ESCS involves using a rotary kiln, 

which is an energy-intensive production process. Thus, the expansive shales and slates are at the 

higher end of the cost range (i.e., $60 and $64 per ton for shale and slate, respectively). This cost 

is equivalent to approximately $85 per cubic yard when the lightweight density of these 

expanded aggregates is taken into consideration.

ENVIRONMENTAL CONSIDERATIONS

No health or safety issues related to using ESCS materials were cited in the literature. No study 

reported issues of toxicity with the leachate from these aggregates. The materials are known to 

be water and acid insoluble, chemically inert, and highly durable (ESCSI 2008).

LCA STUDIES 

A PCR document for preparing an EPD for ESCS LWAs is available and geared toward U.S.

production methods and applications (ASTM 2015c). The PCR is operated by ASTM, primarily 

for ESCSI. The targeted applications for ESCS in this PCR include all applications but primarily 

focus on masonry, concrete, asphalt pavement, lightweight geotechnical fills, horticulture, soil 

amendment, and water treatment applications. In the United States, the PCR expired in 2020, and 

no new amendments or newer versions were found online at the time this report was drafted. 

Despite having a PCR for ESCS in the United States, the researchers found no publicly available 

EPDs for any commercially available ESCS material or supplier in the United States. However, 

the researchers also found very few EPDs for ESCS products manufactured in Europe. All these 

EPDs were for expanded clay products and included Leca expanded clay (Norway), Argex 

expanded clay (Portugal), and Laterlite S.p.A. expanded clay (Italy) (Leca 2017; Argex 2015; 

Laterlite S.p.A. 2018). Note that many of the manufacturers had multiple EPDs for their different 

products, but the researchers chose to present one as an example for each manufacturer in this 
32

report for brevity. The functional unit of all EPDs is 1.0 m3 of materials produced. The results 

from the three EPDs are summarized for emissions and environmental impacts (table 10) and 

energy and resource consumption (table 11). For the environmental impacts, all three products 

had the same order of magnitude impacts in all categories, except for the Leca product from 

Norway, which had one order of magnitude lower ozone depletion potential (ODP). Similarly,

for energy and resource consumption, Argex expanded clay from Norway had zero reported use 

of renewable secondary fuels, much lower than the other two products, but used significantly 

higher net fresh water to produce. 

Table 10. Emissions and environmental impacts for three European expanded clay 

products.

Parameter Unit Laterite Leca Argex

GWP kg CO2-eq 5.44E+01 6.15E+01 4.56E+01

ODP kg CFC-11-eq 7.55E-06 7.47E-07 4.27E-06

AP kg SO2-eq 2.20E-01 1.98E-01 1.03E-01

EP kg (PO4)3-eq 3.22E-02 4.93E-02 1.21E-02

POCP kg ethene-eq 1.08E-02 1.19E-02 1.02E-02

ADPM kg Sb-eq 1.32E-05 1.86E-05 7.70E-05

ADPE MJ 7.21E+02 5.21E+02 3.80E+02

GWP = global warming potential; AP = acidification potential of land and water; EP = eutrophication 

potential; POCP = photochemical ozone creation potential; ADPM = abiotic depletion potential of materials; 

ADPE = abiotic depletion potential for fossil resources; CO2 = carbon dioxide; eq. = equivalent; 

CFC-11 = trichlorofluoromethane; SO2 = sulfur dioxide; PO4 = phosphate; Sb = antimony; MJ = megajoule.

Table 11. Energy and resource utilization for three European expanded clay products.

Parameter Unit Laterite Leca Argex

RPEE MJ 3.93E+01 8.28E+01 5.14E+01

RPEM MJ 0.00E+00 3.44E-02 0.00E+00

TPE MJ 3.93E+01 8.28E+01 5.14E+01

NRPE MJ 7.69E+02 5.41E+02 3.88E+02

NRPM MJ 0.00E+00 0.00E+00 0.00E+00

TRPE MJ 7.69E+02 5.41E+02 3.88E+02

SM kg 0.00E+00 0.00E+00 9.86E+00

RSF MJ 0.00E+00 0.00E+00 0.00E+00

NRSF MJ 6.22E+02 3.42E+02 0.00E+00

FW m3 4.32E-01 2.46E-01 6.26E+00

RPEE = renewable primary energy resources used as energy carrier; RPEM = renewable primary energy 

resources used as raw materials; TPE = total use of renewable primary energy resources; 

NRPE = nonrenewable primary energy resources used as energy carrier; NRPM = nonrenewable primary 

energy resources used as materials; TRPE= total use of nonrenewable primary energy resources; SM = use of 

secondary materials; RSF = use of renewable secondary fuels; NRSF = use of nonrenewable secondary fuels; 

FW = use of net fresh water.
33

CASE STUDIES 

ESCSI (2018) reported a multitude of case studies using ESCS to solve challenging highway 

geotechnical problems. The following case studies have been reported in the United States using 

ESCS in real-world applications: 

• Fill behind bridge abutment: I–90, Everett Road, Exit 5A (bridge over CSX Spur), 

Albany, NY. This project used 6,500 m3(8,500 yd3) of ESCS materials. Construction was 

performed in phases to avoid interrupting traffic flow. ESCS was placed in 300-mm

(12-inch) lifts on top of a geosynthetic fabric, which held the lifts in place. The ESCS 

backfill reduced loads on the existing poor subsoils and minimized settlements at the 

bridge approach. The lateral pressures acting on the abutments were significantly reduced 

compared to conventional sand, stone, or clay backfill due to the high angle of internal 

friction of the ESCS material. 

• Slope stability: New York Route 32 at Kenwood Avenue, Delmar, NY. This project was 

originally constructed in 1963, but after decades of use, the fill on soft soils started to 

slip, causing movement in the highway. ESCS fill was specified as a solution for this 

problem due to its lightweight nature and high friction angle. ESCS of 3,975 m3

(5,200 yd3) of 19.5 mm (3/4 inch) to 4.75 mm (No. 4)-sized aggregates were used. The 

sloped portion of the fill was covered with normal-weight riprap to prevent erosion of the 

lightweight fill. 

• Backfill behind mechanically stabilized earth (MSE) walls: Indiana “Major Moves” 

Project, U.S. 31 and 156th Street Intersection, Carmel/Westfield, IN. A new bridge and 

bridge approach design used MSE wall construction to eliminate the at-grade crossing 

with 156th Street. ESCS was used as the backfill material, and 26,000 m3 (34,000 yd3) 

were used between April and October 2013. Buried gas pipelines running under the 

intersection required the use of a lightweight fill to mitigate a vertical loading issue, and 

ESCS was chosen for this purpose. An excavator and a small rubber track dozer were 

used to place and compact 300-mm (12-inch) lifts of the LWA. A 20-hp vibratory plate 

compactor was used where the excavator could not manage. 

• Bridge approaches: Tranters Creek bridge approach, Washington, NC (ESCSI 2018; Wall 

and Castrodale n.d.). Approximately 2,750 m3(3,600 yd3) of ESCS material were used as 

the lightweight embankment material for widening and raising the existing bridge 

embankment as part of the bridge replacement. The embankment elevation was raised by 

0.3 m (1 ft), and the bridge was lengthened by 36.6 m (120 ft). The roadway embankment 

fill was underlain by clayey alluvial muck (2.7–4.9 m (9–16 ft) thick). The older 

embankment consisted of very loose to loose, silty, fine to coarse sand and was showing 

poor performance. 

• Road base: North Carolina Route 133, repairs to approaches at Lilliput Creek bridge, 

Brunswick County, NC. Originally, the bridge approach portion of the road had 

settlement problems that required an unusual amount of maintenance. The road 

approaches were excavated to 610 mm (24 inches) below the subgrade, and a geofabric 

was installed. The subgrade was then returned to elevation using 2,390 m3(3,125 yd3) of 
34

lightweight ESCS aggregate fill. The construction was completed in August 2005. The 

repaired approaches have been in service for many years without showing any further 

issues. 

• Pipeline fill: Calgary Pipeline, Calgary, Canada. This project was initiated by the City of 

Calgary. The reasoning behind using ESCS was to substantially reduce ground 

movement-induced stresses on buried pipes and structures and as an insulation layer to 

counteract frost heaving and resist adverse effects caused by freeze-thaw cycles. Using 

ESCS as a lightweight fill allowed the engineers to reduce the trench depth from 3.3 to 

2.1 m (10.8 to 7.0 ft), resulting in cost savings. The design with ESCS also reduced the 

disruption of water supply by decreasing construction time. 

• Embankment fill: Louisiana Department of Transportation and Development (DOTD) 

Test Project, Morgan City, LA. The Louisiana DOTD constructed test sections with sand 

fill, 2.9 m (9.5 ft) in depth. In one section, 0.76 m (2.5 ft) of ESCS LWA fill was used. 

The reduction in weight, coupled with the increase in long-term stability, provided the 

LWA’s reduced settlement by 40 to 60 percent compared to the all-sand fill. 

Considerable savings in highway maintenance, repairs, and replacement can be realized if 

differential settlement is reduced. 

Other case studies have also been reported by Wall and Castrodale (n.d.) in a webinar entitled 

“Expanded Shale Clay and Slate Lightweight Aggregate Geotechnical Fill”: 

• Design-build project: 11th Street Bridge replacement, Washington, DC, 2012. The 11th 

Street Bridge is a historic structure, and ESCS was used as a lightweight material to 

minimize weight on top of the stormwater drainage outfalls constructed in the 1850s. 

ESCS was also used as a backfill material for the MSE walls on the project. 

• Emergency bridge replacement: Blackburn Road over Neabsco Creek, Woodbridge, VA,

2012. Tropical Storm Lee in 2011 and heavy rainfalls led to scouring of the bridge 

abutments. ESCS was used behind the bridge abutments to reduce lateral pressure on the 

drilled shafts. 

• Rapid embankment construction: U.S. 17 bypass interchange, Myrtle Beach, SC. A 3- to

12.8-m (10- to 42-ft)-height embankment was constructed over soft compressible soil. 

ESCS was used to reduce lateral and vertical pressures due to the thick embankment 

design. 
35

CHAPTER 4. FGA

FGA (figure 9) is a lightweight fill alternative frequently used in transportation projects. 

Beneficial properties of FGA include a low unit weight (<2.4 kN/m3(15 pcf) bulk dry unit 

weight), good insulating value, high friction angle, nonreactive and nonleaching behavior, 

volume stability, and high porosity (Loux 2022; AeroAggregates® n.d.). Additionally, the 

material is made from 100 percent recycled glass (primarily glass cullet and a foaming agent). 

Dry processing and closed-cell manufacturing are appropriate for transportation infrastructure 

applications. 

According to a U.S. manufacturer of FGA, the U.S. manufacturing process uses 100 percent 

curbside recycled glass powder and mixes it with a foaming agent. The mixed powder is softened 

by heating in a kiln. During the process, FGAs are formed as the foaming agent creates bubbles 

in the mix (Loux 2022). 

© 2011 McZusatz. 

A. One FGA product (Creative Commons n.d.a).

© 2012 Hagmag. 

B. Another FGA product (Creative Commons n.d.a).

Figure 9. Photos. FGA. 
36

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using FGA in geotechnical highway applications include the following 

(Loux 2022; AeroAggregates n.d.): 

• Lightweight material with a low net surcharge due to the low unit weight. 

• High friction angle (high shear strength). 

• Excellent insulation and drainage. 

• Not flammable. 

• No rot or decay.

• Easy to place, especially in difficult-to-reach areas or confined spaces. 

• No special equipment required to construct, place, or compact. 

• Ultraviolet (UV) stable, volume stable. 

• Efficient installation—not weather sensitive. 

Disadvantages of FGA include the following: 

• Material costs can be relatively high compared to virgin aggregates (VA) and other 

lightweight materials, though this cost could be offset if hauling, labor, and placement are 

considered. 

• Environmental risks exist because inhaling glass powder can lead to respiratory issues 

and skin irritation or both.

• Breakage concerns exist because the material is brittle. Construction guidelines need to 

be strictly followed. 

• Concrete strength containing glass aggregates can be 10–20 percent less than 

conventional aggregates due to bonding issues. 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

FGA can be used in the following geotechnical applications for highways (Loux 2022): 

• Embankments. 

• Retaining wall/MSE wall backfills. 

• Bridge abutments backfill. 

• Pipeline and utility backfill. 

• Backfill material over and around tunnels and culverts. 

• Roadway widening and shoulder repair. 
37

MATERIAL CHARACTERIZATION

Physical Properties

Swan et al. (2016) conducted a laboratory study to evaluate the effect of compaction energy on 

gradation, compression, and direct shear strength of lightweight FGA produced by a U.S. FGA 

manufacturer. Dry and wet particle-size analyses were conducted before and after testing to 

evaluate particle breakage due to crushing under vibratory compaction, static compression, or 

direct shear testing. Vibratory compaction resulted in significant particle breakage, which led to 

a 1.4 times density increase to 3.2 kN/m3(20.3 pcf). Modified Proctor compaction with half the 

modified Proctor’s energy level resulted in significant breakage and 2.4 to 2.7 times density 

increases (ASTM 2021). The density increased from 227.5 kg/m3(14.2 pcf) to a range between

537 kg/m3(33.5 pcf) and 612 kg/m3(38.2 pcf). The results indicate that uniformly graded FGA 

(as produced) transforms into an increasingly well-graded material with increasing compactive 

effort, static loading, and shear conditions. In accordance, using Proctor compaction to prepare 

samples with FGA might not be an appropriate method due to significant potential changes in 

gradation and density resulting from particle breakage. Instead, a more controlled and consistent 

method should be investigated in future research.

Arulrajah et al. (2015) reported the physical and mechanical properties of FGA and how it

compares to a typical lightweight material and a conventional fill material. Table 12 provides the

comparative data. Compared to a typical lightweight material, the density, water absorption, pH, 

and Los Angeles (LA) abrasion test results all fall within the limits of other lightweight and 

conventional fill materials. The CBR (strength) of FGA is higher than that of a typical 

lightweight material, and the peak friction angles are on the higher end of the range from that

typically measured for lightweight materials.

Table 12. Engineering properties of FGAs (Arulrajah et al. 2015).

Engineering properties

Foamed

Recycled

Glass

Typical Lightweight

Material

Typical

Conventional Fill

Material

D10 (mm) 0.13 N/A 0.09

D30 (mm) 1.2 N/A 1.30

D50 (mm) 18.7 N/A 4.40

D60 (mm) 20.6 N/A 6.70

Coefficient of uniformity (Cu) 158 N/A 78.83

Coefficient of curvature (Cc) 0.53 N/A 2.97

Gravel-sized particles:

4.75–40 mm (percent)

66 <70 47.9

Sand-sized particles:

0.075–4.75 mm (percent)

32 <40 42.2

Clay and silt-sized particles: 

<0.075 mm (percent)

2 <3 9.90

Minimum dry density 

(kg/m3)

170 112–204 N/A
38

Engineering properties

Foamed 

Recycled

Glass

Typical Lightweight 

Material

Typical 

Conventional Fill 

Material

Maximum dry density 

(kg/m3) 290 204–306 N/A

Water absorption—coarse

fraction (percent) 60 50–60 6.50–6.70

Water absorption—fine 

fractions (percent) 0.3 <1.0 6.50–7.50

pH 10.48 9–12 10.20–11.40

Organic content (percent) 0 0 1.7–2.1

CBR (percent) 9-12 2–10 172

LA abrasion loss (percent) 94 80–100 29.9–31.7

Peak apparent cohesion, c′ 

(kPa)—DST 23.4 20–100 95

DST: Peak friction angle, Θ 

(degrees) 55.7 35–60 65

DST: Critical state apparent

cohesion, c′ (kPa) 22.1 20–100 80

Dn = sieve size for which n percent of material is passing, n = 10 percent, 30 percent, 50 percent, 60 percent; 

N/A = values not provided. 

Mechanical Properties 

Shear strength properties calculated for FGA using large-scale shear box testing setups (given 

the large sizes of the particles) are comparatively high. For one FGA material tested by Arulrajah 

et al. (2015), the measured cohesion and friction angles were 3.4 psi and 54.7 degrees, 

respectively. Arulrajah et al. (2015) tested geogrid-reinforced recycled foamed glass using a 

large-scale DST. The researchers found the geogrid-reinforced FGA material to be 

stress-hardening, and it exhibited smaller vertical displacement and greater dilatancy ratio than 

the unreinforced FGA material. The researchers also found the geogrid-reinforced FGA 

exhibited a stress-softening behavior. The failure envelope of the geogrid-reinforced FGA, at 

peak and critical states, yields a linear trend, likely due to the crushing of FGA particles and the 

rearrangement of crushed FGA after the peak shearing state. The interface shear strength 

coefficient, α, was approximately constant at 0.9, indicating that it can be used as the interface 

parameter for designing a reinforced embankment and MSE wall. In the case of 

geogrid-reinforced FGA, the geogrid carries the tensile forces, while FGA reduces the bearing 

stresses imposed on the in-situ soil. 

Swan et al. (2016) conducted large-scale shear box testing at different normal stresses. The shear 

box was 12 inches by 12 inches in plane and had a total depth of 6 inches The researchers 

conducted tests on an as-received FGA material with no compaction at applied normal stresses of 

14.4, 35.9, 57.5, 143.6, 287.3, and 426.1 kPa (300, 750, 1,200, 3,000, 6,000, and 8,900 psf 

respectively). The test series on a modified FGA material that had undergone modified Proctor 

compaction using ASTM D1557 with the lower compactive energy were also conducted at 

applied normal stresses of 143.6, 287.3, and 426.1 kPa (3,000, 6,000, and 8,900 psf respectively) 

(ASTM 2021a). Table 13 summarizes the results for peak friction angles and cohesion intercepts.
39

Table 13. Shear strength characteristics of FGA (Swan et al. 2016). 

Tested Material 

Range of Normal 

Stress (kPa) 

Peak Friction Angle 

(degrees) 

Peak Cohesion

(kPa) 

As-received FGA 14.5–57.2 56 2.1

As-received FGA 35.9–143.4 29 45.8

As-received FGA 143.3–426.1 27 46.0

Modified FGA 143.4–426.1 31 51.7

The authors of this report conducted triaxial tests on an FGA material (Glasopor®) produced in 

Norway and intended for use in aircraft energy-arresting system applications at the end of 

runways.0F0F

1 The authors tested both loose and compacted specimens at confining pressures 

ranging from 35 to 275 kPa (5 to 40 psi). The samples experienced a rapid stress gain at the 

low-strain levels toward the beginning of the test due to the crushing and densification of the 

particles. The behavior of the material and the shape of the stress-strain curves were found to be 

similar at the different confining pressures and compaction states. Figure 10 presents the 

stress-strain results for the loose and compacted specimens. 

1Work performed under a small technical testing agreement for a company. 
40

© 2016 Erol Tutumluer. 

1.0 psi =6.895 kPa, 1 pcf = 16.0 kg/m3. 

A. Loose specimens, density = 9.1–10.3 pcf. 

© 2016 Erol Tutumluer. 

1.0 psi =6.895 kPa, 1 pcf = 16.0 kg/m3. 

B. Compacted specimens, density = 11.1–11.4 pcf. 

Figure 10. Graphs. Stress-strain relationships for loose and compacted FGA material 

tested at different confining pressures. 

The researchers found no specific design guidelines or requirements for FGA used in highway 

geotechnical applications. Therefore, the researchers recommend consulting with producers for 

precise information on aggregate grading, bulk density, in-place compacted density, friction 

angle, and proper placement method(s). FGA design should thus take advantage of the load 
41

reduction, coupled with the high internal friction angles commonly reported for FGA, which can 

reduce the vertical and lateral forces by more than one-half. Surcharge and buoyancy 

calculations should be conducted to ensure that the aggregates do not float under submerged 

conditions. A thin cover material (e.g., conventional backfill) could be placed over the FGA to 

add any weight necessary to prevent buoyancy if that is a concern. 

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS 

Compaction of foamed glass layers is commonly performed with a tracked excavator or dozer 

exerting loads ranging from 28.7 to 47.9 kPa (600 to 1,000 psf) (Loux 2018). A maximum lift 

thickness of 24 inches is recommended, but a 300-mm (12-inch) maximum lift is specified if a 

plate compactor is used for the compaction. The FGA layers are normally compacted with two to 

four passes of the tracked excavator or dozer. Operating construction equipment on the FGA, 

other than for placement and compaction, should be avoided. Excessive compaction should also 

be avoided to minimize crushing of the LWA.

The guidelines listed in the previous paragraph, which are recommended by a U.S. manufacturer, 

are primarily obtained and modified from the following three Scandinavian installation guides 

that are available to provide direction on the proper installation of FGA (Loux 2022): 

• The Swedish Geotechnical Institute handbook recommends using tracked equipment for 

installation. According to the handbook, the pressure exerted by the equipment should be 

less than 50 kPa (1,044 psf), the lift thickness should not exceed 0.8 m (2.6 ft), and two 

passes at a minimum are needed. If a plate compactor is used, the weight should be 

between 100 kg (220 lb) and 200 kg (440 lb), the lift thickness should not exceed 0.5 m 

(1.6 ft), and a minimum of four passes are required. 

• The Norwegian Roadway Authority also recommends using tracked equipment exerting a 

force of 50 kPa (1,044 psf). The maximum lift thickness is 1 m (3.3 ft) and only 0.6 m 

(2 ft) for retaining wall backfills. 

• The National Roadway Guideline from Finland recommends using tracked equipment 

with 30–50 kPa (627–1,044 psf) force, a minimum of two passes, and a maximum of 

0.6 m (2 ft) lift thickness. For vibratory plate compactors, the weight should be between 

45 and 200 kg (100 and 440 lbs.), the maximum lift thickness should be 0.4 m (1.3 ft), 

and the minimum number of passes is two. 

McGuire, Loux, and VandenBerge (2021) conducted eight field-scale tests on FGA layers to 

answer two questions regarding the compressibility and volume reduction of FGA compacted in 

the field. The first is the expected reduction in FGA fill volume using a typical compaction 

method specification. The second is the increased volume reduction expected for additional 

passes of the compaction equipment beyond what is required by the specification. During the 

tests, compaction was performed using a dozer (two tests), two track-mounted excavators (three 

tests), a vibratory plate compactor (two tests), and a small tandem roller (one test). Each test 

involved compacting one to three lifts with multiple passes and observing reductions of the fill 

volume using a light detection and ranging scanner (LiDAR). Table 14 presents the 

specifications of the compaction equipment. The results of the study showed that a typical 
42

method specification results in compacted lifts with a volume that is 7.6 to 16.6 percent lower 

compared to the uncompacted volume after the placement of the lift. Note that a typical method 

specification (0.6 m (2-ft) lift compacted using four passes of tracked equipment for area fills) 

reduces fill volume by 7.6 to 10.7 percent. A typical 0.3-m (1-ft) lift compacted using four passes 

of a vibratory plate compactor in smaller areas reduces fill volume by 9.9 to 16.6 percent. The 

researchers found that the volume reduction versus the number of roller compactor passes 

followed a hyperbolic relationship (McGuire, Loux, and VandenBerge 2021).

Table 14. Compaction equipment specifications (McGuire, Loux, and VandenBerge 2021).

Equipment

Static 

Ground 

Pressure 

(kPa)

Contact

Width 

(mm)

Empirical Compactor

Effectiveness 

Coefficient, C

John Deere 700K XLT dozer 46.5 558 0.31

Doosan DX85R-3 excavator 38.3 451 0.79

Komatsu PC138USLC-11 excavator 32.9 710 0.48

Wacker WP 1550 vibratory plate 2.5 509 5.42

Bomag BMP 8500 roller 90.4 851 0.11

AVAILABLE COST INFORMATION

The cost for FGAs runs between $54–$61 per cubic meter ($70–$80 per cubic yard) (Gibson 

2019). Construction does not require any special equipment, and hauling costs are lower than 

conventional aggregates because of the low density.

ENVIRONMENTAL CONSIDERATIONS

Environmental assessments (pH value, organic content, and leachate concentration) showed all 

the hazardous concentrations in the leachate are lower than 100 times those of the drinking water 

standards (Arulrajah et al. 2015; Lenart and Kaynia 2019). Thus, FGA is not considered 

hazardous for highway geotechnical and pavement applications. 

LCA STUDIES

Energy savings assessment demonstrates much lower energy consumption relative to 

conventional aggregate-cement material in construction projects (Arulrajah et al. 2015). A set of 

standard references and PCR documents are available for Europe to conduct LCA studies for 

FGA. These references include European Norm (EN) 15804, the Institut Bauen und Umwelt

(IBU) Parts A and B (2012), and the International Organization for Standardization (ISO) 14025 

standard (European Committee for Standardization 2019; IBU 2012; ISO 2006). The references

have been used for preparing EPDs for FGA in the European markets. The targeted applications 

for FGA in these PCR and reference documents include highway applications.

Four publicly available EPDs were found for FGA products manufactured in Europe. In 

particular, two manufacturers (Glasopor in Norway and Hasopor in Sweden) have published 

EPDs for their products (Glasopor 2014, 2017, 2020; Hasopor 2017). Glasopor had three 
43

available EPDs for three different products, while one EPD was found for a Hasopor product. 

Two similar products, one from Norway and one from Sweden, were selected to summarize the 

environmental impacts in this report. Table 15 provides a summary of the available information 

on selected products. The results from the two selected EPDs are summarized for emissions and 

environmental impacts (table 16) and energy/resource consumption (table 17). The functional 

unit of all EPDs is 1.0 m3 of materials produced. For the environmental impacts, the products 

had varying impacts in most categories, but the difference was at most one order of magnitude 

except for the depletion of the ozone layer, where the difference is two orders of magnitude. The 

differences can be attributed to the two countries’ different manufacturing processes and energy 

sources. The impact results for the energy and resource consumption were quite similar for the 

two manufacturers. The researchers did not find any publicly available EPDs for FGA in the 

United States.

Table 15. Available EPD information on three European FGAs.

Product Producer Country 

EPD 

Validity 

System 

Boundary 

Functional 

Unit 

Reference 

Standard(s) 

and PCR(s) 

Glasopor 

10–60 mm 

Glasopor 

AS Norway 01/16/2025 A1-A5

1 m3 of

Glasopor 

10–60 mm 

EN 15804a

IBU—Parts 

A and Bb

Glasopor 

10–60 mm 

Glasopor 

AS Norway 01/31/2022 A1-A4

1 m3 of

Glasopor, 

to factory 

gate 

EN 15804a

IBU

(Parts A and 

B)b

Glasopor 

10–60 mm 

Glasopor 

AS Norway 10/27/2019 

A1-A3

(Cradleto-gate) 

1 m

3

 of 

Glasopor 

10–60 mm 

EN 15804a

IBU (Part B)b

ISO 14025c

Hasopor 

foam glass 

10–60 mm 

Hasopor

AB Sweden 12/13/2022 

A1-A3

(Cradleto-gate) 

1 m

3

 of 

Hasopor 

ready for 

transport 

EN 15804a

ISO 14025c

aEuropean Commission for Standardization 2019. 

b

IBU 2012. 

c

ISO 2006. 

Table 16. Emissions and environmental impacts for three European FGAs.

Parameter Unit Hasopor Glasopor 

GWP kg CO2-eq 7.77E+00 6.96E+00

ODP kg CFC11-eq 7.23E-08 1.01E-06

AP kg SO2-eq 2.05E-02 3.01E-03

EP kg (PO4)3-eq 2.66E-03 2.22E-02

POCP kg ethene-eq 8.33E-04 1.19E-03 

ADPM kg Sb-eq 1.70E-05 1.96E-05

ADPE MJ 7.99E+01 1.07E+02
44

Table 17. Energy and resource utilization for three European FGAs. 

Parameter Unit Hasopor Glasopor 

RPEE MJ 5.41E+02 4.38E+02 

RPEM MJ 0.00E+00 4.50E-01 

TPE MJ 5.41E+02 4.38E+02 

NRPE MJ 8.13E+01 1.08E+02 

NRPM MJ 0.00E+00 4.00E-02 

TRPE MJ 8.13E+01 1.08E+02 

SM kg 1.53E+02 1.80E+02 

RSF MJ 0.00E+00 1.97E-04 

NRSF MJ 0.00E+00 −7.22E-06 

FW m3

1.40E+00 0.00E+00

CASE STUDIES 

Case studies for all the target geotechnical applications of interest in this report, except slope 

stability, were mentioned in the literature. FGA has been used as a lightweight material for 

embankments, backfills behind retaining walls, bridge abutment fills, and backfill for pipes with 

no reported failures or adverse effects. No extensive details were found on the performance of 

the FGA in these applications. A concise list of case studies follows: 

• Case studies in Europe include New Metro Line from Jar to Kolsås, Halden Rail Yard, 

New E18 Motorway—Retvet-Knapstad, Norwegian Public Roads Administration, 

Hämeenlinna, and Highway E12 (Loux 2022). 

• Case studies in the United States include Philadelphia Navy Yard, Pennsylvania DOT; 

I-95 Cottman-Princeton Ramp F, Pennsylvania DOT; Route 7/Hackensack, New Jersey 

DOT; and Southeastern Pennsylvania Transportation Authority Media Line (Loux 2022).

• FGA was approved (as of 2018) for use in the United States by the Maryland DOT State 

Highway Administration, Virginia DOT, New York State DOT, Massachusetts DOT’s

READi (Review, Evaluate, Accelerate, Deploy, innovation) Committee, Connecticut 

DOT, New Hampshire DOT, Pennsylvania Turnpike Innovation Council. Two projects 

were conducted with Pennsylvania DOT, and the first project was conducted with New 

Jersey DOT (Loux 2022). 
45

CHAPTER 5. LCC

INTRODUCTION AND BACKGROUND 

LCC is a mixture of portland cement, water, and air created through a preformed foaming agent,

as shown in figure 11 (American Society of Civil Engineers (ASCE) Texas Section 2022). Other 

ingredients may be incorporated into the mix if they do not adversely affect the quality, size, and 

distribution of the air bubbles in the cellular concrete. Some common examples include fly ash, 

slag, silica fume, fibers, gums, accelerators, and retarders. Having low viscosity, LCC allows for 

long-distance placements and nearly self-leveling installations. As shown previously in figure 2, 

LCC typically has higher cement content and lower water content than CLSM in the original 

mix, but LCC tends to be more economical due to foaming and the inclusion of air bubbles 

(Halsted 2020). LCC is typically placed into its final location using a pump and hose. The LCC 

is fluid enough to self-consolidate, and no vibration is required. If placed in moderate 

temperatures (15 to 27 °C), LCC will set and harden within approximately 10 to 14 h.

© Aerix Industries. 

Figure 11. Illustration. Preparation of ready-mixed LCC (Taylor and Halsted 2021). 

After placing and hardening, the LCC material should be protected with a wearing course or a 

surface layer such as concrete, soil, subbase material, drainage mat, etc. No recommended 

maintenance exists for in-place LCC; once buried and protected, no additional maintenance 

would be necessary (ASCE Texas Section 2022). Thus, LCC can save construction time, money, 

and natural resources by replacing aggregates with air bubbles in numerous project applications 

(Halsted 2020). According to Chica and Alzate (2019), LCC can be classified based on different 

metrics, including pore nature, agent, production method, and curing method. Figure 12 presents 

the metrics for classification. This chapter focuses on the pores with a foamed or aerated nature. 
46

© 2019 Chica and Alzate. Modifications by FHWA to colors, shapes, and classification labels. 

Figure 12. Illustration. LCC flow chart (Chica and Alzate 2019). 

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using LCC in highway backfill applications include the following 

(Taylor and Halsted 2021): 

• LCC is a lightweight, strong, durable, and inexpensive soil or fill replacement for 

geotechnical applications. 

• LCC is self-compacting and highly fluid, with water-to-cement ratios (W/C) ranging 

from 0.35 to 0.80.

• LCC is very strong compared to the material it replaces in the geotechnical environment 

(typically soil and compacted aggregates). 

• The surface of a hose-placed LCC is relatively flat with a slight splatter pattern and 

normally does not require any additional finishing or curing compounds. 

Disadvantages of LCC include the following (Taylor and Halsted 2021): 

• Water content significantly affects many properties of LCC, especially its strength and 

viscosity. 

• Small variations in mix designs can cause great differences in the final product, possibly 

leading to unacceptable materials, failures, and unexpected expenses. 
47

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

LCC can be used in the following geotechnical applications for highways (Portland Cement 

Association (PCA) 2023): 

• Lightweight road bases and fills. 

• Bridge approach embankments. 

• Void and cavity fills.

• Pipe and culvert abandonment fills.

• Annular space tunnel grout fills.

• Foundation fills and energy-arresting systems. 

• Retaining wall backfills. 

• Landslide repair and slope stabilization fill. 

• CDF. 

MATERIAL CHARACTERIZATION 

Physical Properties 

Measured physical properties for LCC include autogenous (drying) shrinkage, permeability, 

sorption, heat of hydration, and thermal conductivity. Physical properties of interest depend on 

the state of LCC. Ramamurthy, Nambiar, and Ranjani (2009) broadly classified LCC properties 

into fresh and hardened state properties. Ni, Oyeyi, and Tighe (2020), on the other hand, reported 

that LCC properties could be divided into the fresh state, early state, and hardened state. 

In the fresh state, LCC is generally free-flowing, self-leveling, and self-compacting (highly 

workable). Stability (volumetric stability) is crucial because the separation of solids and air 

phases might cause segregation during fresh state if the LCC is unstable. Consistency (flow 

behavior), on the other hand, depends on its spreadability and flowability. Spreadability can be 

measured using the Brewer spread test and slump flow test (British Standards Institution 1998). 

Flowability is determined by measuring the time taken for the paste to flow through a Marsh 

cone (Ni, Oyeyi, and Tighe 2020). 

In LCC’s early state, two main physical characteristics are of interest: heat of hydration and rate 

of hardening. The volume of pour, cement content, and concrete density influence the heat of 

hydration. The peak temperature can decrease by 40 percent when the amount of cement is 

decreased from 600 to 300 kg/m3. The peak temperature also decreases when more than

30 percent of the cement is replaced with fly ash. For the rate of hardening, there is no standard 

test method for determining the setting time of LCC. However, the ASTM C266 test method for 

cement may be a suitable procedure to test the setting time of LCC (ASTM 2021c; Ni, Oyeyi, 

and Tighe 2020). 

LCC is quite thixotropic, making restarting the construction difficult once the concrete starts to 

harden. In the hardened state, the crucial physical and functional properties include drying 

shrinkage, density, thermal insulation/conductivity, durability properties, water absorption, 

permeability, porosity, and freeze-thaw resistance. 
48

Mechanical Properties

UCS is the most commonly measured and reported property for hardened LCC. Other reported 

mechanical properties in the hardened state include flexural strength, indirect tensile strength, 

modulus of elasticity, and Poisson’s ratio. ACI provides a table of industry-accepted values for 

maximum cast density and range of UCS properties for various LCC mixes, as shown in table 18

(Halsted 2020).

Table 18. Typical density and compressive strength of LCC mixes (Halsted 2020).

Oven-Dry Density Usual Range of UCS at 28 d

lb/ft3 kg/m3 psi MPa

20–25 320–400 70–125 0.48–0.86

25–30 400–480 125–225 0.86–1.55

30–35 480–560 225–350 1.55–2.41

35–40 560–640 350–450 2.41–3.10

40–50 640–800 450–750 3.10–5.17

Ni, Oyeyi, and Tighe (2020) reviewed the potential use of LCC in pavement applications and 

reported the mechanical properties of LCC mixes and how they vary with compressive strength.

Table 19 presents the results of that review. In general, as the compressive strength increases, the 

average drying shrinkage decreases, the modulus of elasticity increases, and thermal conductivity 

increases.

Table 19. Mechanical and thermal properties of LCC mixes (Ni, Oyeyi, and Tighe 2020).

Maximum 

Compressive

Strength (ksi)

Average Drying

Shrinkage 

(percent)

Maximum Modulus of

Elasticity (ksi)

Minimum Thermal

Conductivity (W/mK)

1.0 0.30–0.35 1,000 0.10

1.5 0.22–0.25 1,500 0.11

2.0 0.20–0.22 2,500 0.17

3.0 0.15–0.18 3,000 0.23

5.5 0.09–0.11 4,000 0.38

8.0 0.07–0.09 6,000 0.50

10.0 0.06–0.07 12,000 0.62

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES

LCC’s properties depend on the microstructure and composition, which are a function of the 

binder (cement and additives), method of pore formation, and curing (Narayanan and 

Ramamurthy 2000; Ni, Oyeyi, and Tighe 2020). According to Brady, Watts, and Jones (2001), 

no standard method exists to calculate LCC’s mix proportions. Densities for LCC mixes 

typically range from 320 to 800 kg/m3(20 to 50 pcf), with 28-d UCSs ranging from 483 kPa to 

5170 kPa (70 to 750 psi). The high desorption (50 to 200 kg/m3) of LCCs makes designing LCCs

for a target dry density challenging (Jones and McCarthy 2006). Therefore, a target plastic 

density is used as the design criterion. The target plastic density is assumed to be the sum of 
49

solids and water mix as presented in the following equation (Brady, Watts, and Jones 2001; Dhir, 

Jones, and Nicol 1999; Ni, Oyeyi, and Tighe 2020). 

(1) 

Where: 

D = target plastic density (kg/m3). 

C = cement content (kg/m3).

W = water content (kg/m3).

F = fine aggregate content (kg/m3

). 

Other design concerns for LCC include bearing capacity, hydrostatic pressure, buoyancy, 

punching, design life, seismic considerations, temperature during hydration, drainage, structural 

number, angle of friction, and pavement support design. Chica and Alzate (2019) presented the 

physical and mechanical properties of LCC mixes for mixes using fly ash and other additives for 

cement replacement. Part of their data is presented in table 20. Recycled and byproduct materials 

are allowed in LCC mixes as long as the minimum required physical and mechanical properties 

(primarily the UCS) are met. 

Table 20. Physical and mechanical properties of LCC mixes with partial cement 

replacement (Chica and Alzate 2019). 

Type of Additive 

POC: 

Binder 

Physical Properties Mechanical Properties 28 d 

Dry 

Density 

(kg/m3) 

Porosity/ 

Sorptivity 

(%, mm) 

Shrinkage 

(%) 

Thermal 

Conductivity 

(W/mk) 

Compressive 

Strength 

(MPa) 

Fly ash-class F 1:3 — 2.5 mm 0.37 — 5.5

Fly ash-class F 1:1 1,150 29% — — 19

Fly ash—class F — 1,000 31% — — 9

Fly ash-class F — 650 34% — — 4

Fly ash 1:0.25 1,000 — — — 1.4

Fly ash 1:1.5 1,000–

1,200 <10% 0.06–0.10 — 3.7–6.7

Fly ash —

1,300–

1,500 — — — 10–18.8

Fly ash 0.3 800 56% — — 3.92

Fly ash 1:0.3 1,590 150–550 — 12.1–32.1

Class F fly ash + 

peroxide 1:0.4 100–300 80.3–70.9% — 0.043–0.078 0.12–1.0

Fly ash Class F + blast 

furnace slag + 

hydrogen peroxide 

0:5 1,889–

2,106 12–25.4% — — 38.3–47.8

Fly ash + blast furnace 

slag + silica fume 1:20 1,020–1,550 — — 0.24–0.75 4.2–44.1
50

Type of Additive

POC: 

Binder 

Physical Properties Mechanical Properties 28 d

Dry 

Density 

(kg/m3) 

Porosity/ 

Sorptivity 

(%, mm) 

Shrinkage 

(%) 

Thermal 

Conductivity 

(W/mk) 

Compressive 

Strength 

(MPa) 

Fly ash + silica fume 1:1.5 1,280–

1,870 — — 0.498–0.962 19–47

Blast furnace slag 0.3–

0.7 1,300 — — — 2.2–0.5

Blast furnace slag 1:6 153–303 6.6–8.3% — 0.05–0.071 0.57–1.1

—No data. 

In a full-scale laboratory study, Wilkinson (2021) and Rollins (2020) investigated using a 

reinforced LCC backfill behind an MSE wall. The reinforcement included ribbed strips 

measuring 51 mm (2 inches) wide and 5 mm (0.2 inches) thick. The experiment was conducted 

in a large-scale box, 3 m (10 ft) tall by 3.8 m (12.5 ft) long by 3 m (10 ft) wide. The MSE wall 

panels were nominally 1.5 m (5 ft) tall by 3 m (10 ft) wide and 0.15 m (0.5 ft) thick. The study 

involved a 1:1 slope of silty sand with a sliver fill of reinforced LCC behind the MSE wall 

panels. The researchers concluded that MSE walls with LCC sliver fills adjacent to soil slopes 

can withstand significant surcharge loadings with limited axial and lateral deformations 

(figure 13). Failures and excessive displacements or both can occur when the surcharge pressures 

are increased to about half of the UCS of LCC. The researchers found the surcharge pressure 

versus axial settlement for the sliver LCC fill test to be consistent with curves from previous tests 

conducted by the researchers with a recycled crushed concrete (RCC) cantilever wall and an 

MSE wall prior to failure. The failure load in the LCC case was also somewhat higher due to the 

higher UCS for the LCC. In addition, the researchers previously tested a full-scale MSE wall 

with an unreinforced LCC backfill and concluded that the failure of the reinforced MSE wall in 

this study was more ductile compared to the brittle failure of the unreinforced LCC, which 

indicates an improved performance produced by the MSE reinforcements. 
51

© 2020 Rollins. Modifications to measurements, 

colors, and labels by FHWA. 

A. Load-axial displacement behavior. 

© 2020 Rollins. Modifications to measurements, 

colors, and labels by FHWA. 

B. Load-lateral displacement behavior. 

Figure 13. Graphs. Load-displacement behavior of full-scale MSE walls backfilled with 

various materials, including LCC (Rollins 2020).

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS 

In 2021, the PCA and the National Concrete Pavement Technology Center (CP Tech Center) at 

Iowa State University published a comprehensive report on the use of LCC in geotechnical 

applications (Taylor and Halsted 2021). The report primarily focused on the proper construction 

and QA and QC methods used to produce high-quality mixes with LCC. Most of the information 

in this section comes from that report. Overall, LCC should be carefully observed, inspected, and 

regulated with the highest QC available. Field measurements of unit weight, along with the 

known C/W ratio of the fresh LCC mixture, are the primary QC mechanisms. Wet density (cast 
52

density, or target plastic density, as defined in equation 1) is the density used in the specification 

and design of LCC projects.

LCC should not be allowed to set and then be remixed (Taylor and Halsted 2021). Instead, it 

should be kept plastic until allowed to set in its final location. Weather conditions should be 

monitored before starting LCC placement. If heavy rain is imminent, placement should be 

delayed; however, LCC mixes can still be placed in conditions of light rain. LCC placement is 

not recommended if the ambient temperature is below 32 °F (0 °C) or above 100 °F (38 °C). 

High heat can evaporate the water, causing excessive shrinkage. Cold weather, on the other hand, 

can inhibit the curing time and the quality of placed LCC.

Transportation

Cement and water slurry mix are routinely delivered to jobsites for LCC (Taylor and Halsted 

2021). Transportation of premixed LCC should always be avoided because vibrations from 

movement may displace the entrained air and alter the density of the final product. Prolonged 

periods in a ready-mixed concrete truck may also cause the properties of LCC to change.

Mixing and Construction Equipment

Mixing energy (shearing) imparted to the mixture affects the final properties and is the primary 

factor affecting production equipment selection. Taylor and Halsted (2021) reported that batch 

mixing and auger mixing (mobile volumetric mixers) are the two types of production systems 

generally used to mix cement and water in LCC.

Batch mixing is the simplest system. It provides an excellent mix quality at low production rates

and can produce 23 to 38 m3(30 to 50 yd3) of LCC per hour (Taylor and Halsted 2021). Batch 

mixers are typically mounted on a trailer for easy mobility. The mixing action must be 

aggressive enough to thoroughly disperse the cement, water, and foaming agent. When a batch 

mixer is used, operators should ensure enough mixing time to provide a uniform product with no 

cement lumps. High-shear batching mixers result in higher strength LCC due to faster mixing 

and can produce 38 to 115 m3(50 to 150 yd3) of LCC per hour.

Auger mixers (mobile volumetric mixers), on the other hand, use a rotating shaft and flange 

(auger) to blend ingredients. An operator must pay attention to the first amounts of material 

exiting the machine and make flow and rate adjustments to ingredients. Auger mixers are a

convenient and fast method for making large volumes of LCC. Production rates vary from 23 to 

382 m3(30 to 500 yd3) per hour for the largest equipment (Taylor and Halsted 2021). The 

prepared LCC may be placed directly into the final location. For other types of mixers, LCC is 

typically pumped first.

Pumping LCC can be challenging. The challenge can occur a little later during placement when 

the cement paste starts to collect on the interior walls of the pump and pumping hoses and begins 

to solidify. Operators should closely control pumping pressures with LCC, given that pressure 

makes a difference with preformed foam. Preformed foam must conform to the properties listed 

in ASTM C869 when tested following the procedures in ASTM C796 (ASTM 2016b; ASTM 

2019b). Field monitors should check density in accordance with ASTM D6023 before and after 

pumping, observing any increase or change in density (ASTM 2016a). 
53

The three pumping systems most used for LCC are progressive cavity pumps, peristaltic pumps, 

and piston pumps (Taylor and Halsted 2021). Less commonly used pump types are ball valve 

pumps, centrifugal pumps, and diaphragm pumps. Progressive cavity pumps are the most 

common for LCC. Progressive cavity pumps are extremely steady, with no pulsing required, and 

they keep themselves clean on the inside during operation. These pumps operate at relatively low 

pressures ranging between 690 and 2,070 kPa (100 and 300 psi). On the other hand, peristaltic 

pumps are squeeze pumps that can easily transport LCC. The benefit of using peristaltic pumps 

for LCC is that they separate the cementitious materials from the pumping mechanism, which 

makes them beneficial for sticky and solidifying LCC mixtures. Peristaltic pumps have lower 

repair costs and can obtain higher pressures than progressive cavity pumps. Lastly, piston pumps 

are reliable and provide sufficient power for moving fluids. Piston pumps are the third most 

common pumps for LCC and should be used with caution since a line blockage can lead to 

extremely high pressures. Piston pumps commonly use a check valve and a piston retracting 

system. 

Placement and Handling 

Most LCC placing equipment uses a progressive cavity pump because it is extremely steady and 

self-cleaning. Peristaltic pumps can also be used to transport LCC easily. Factors to consider 

during placement include the following (Taylor and Halsted 2021): 

• Metered cement content or flow rate. 

• Metered water content or flow rate, density of cement, and water slurry. 

• Density of preformed foam. 

• Density of the final product. 

• Pumping distance. 

• Metered pumping pressure. 

• Time required to fill the area. 

• Material segregation in the placement. 

• Depth of daily placement. 

• Drainage that might lead to buoyancy. 

• Temperatures that are excessively hot or cold. 

• Lumps of cement in the mix. 

• Leakage in the formwork (if present). 

• Excessively high cure heat. 

• Location of any bleed water after curing.

• Buoyancy on LCC layers placed below grade.

• Weather (i.e., avoid placement in heavy rain). 

QA/QC Measures—Inspection, Testing, and Maintenance 

ASTM testing necessary for LCC includes ASTM C495, ASTM C796, and ASTM C869 (ASTM 

2019c; ASTM 2019b; ASTM 2016b). An experienced quality engineer should be employed for 

verification of LCC’s quality. The two main QC parameters are the material density and the UCS 

of the hardened material. Slurry density checks should be performed regularly to ensure that the 

flow meters for the cement and water are operating properly. The wet density and temperature of 
54

the mix should be monitored during production and placement, with necessary adjustments made 

to density if and when required. Table 21 presents the density and compressive strength sampling 

requirements and acceptance criteria.

Table 21. Sampling requirements for LCC mixes for density and compressive strength 

testing (Taylor and Halsted 2021; Ni, Oyeyi, and Tighe 2020).

Material 

Property Frequency Acceptance Criteria

Comments/Additional 

Requirements

Density

One per batch or every 

10 m3

Every 50 m3 or once per 

20 min during

continuous production

10 percent of the 

design density

No additional comments or 

requirements

Compressive 

strength One sample per 100 m3

Meets or 

surpasses design 

strength

75 × 150 mm cylinders

Store in an unobstructed condition 

within 13.7 m of the molding area

Curing temperature of 77–86 °F for 

24-96 h

Cure in an 80–100-percent

humidity chamber at 64–81 °F

Further, LCC construction and placement have some special considerations. Excessive walking 

and driving directly on the LCC surface can cause damage and should be avoided (Taylor and 

Halsted 2021). As a rule of thumb, construction may proceed as soon as the LCC layer can be 

walked on without excessive surface penetration (up to 1.0-inch penetration is acceptable).

According to ACI Committee 523, special precautions should be taken if the ambient 

temperature is below 32 °F or above 100 °F (ACI 2013). LCC placed in moderate temperatures 

(60-80 °F) will set and harden within 10–14 h. Moisture should not evaporate off the placed LCC 

too quickly to avoid excessive shrinkage. In most applications, shrinkage cracks are only 

superficial and do not detrimentally impact service life. Excessive evaporation of water from the 

LCC can be reduced by covering the surface with a layer of plastic sheeting or insulated 

blankets. Due to the insulative properties of LCC, a relatively high thermal differential is 

possible, causing surface cracking and accelerated wear if left exposed.

Postconstruction inspection is also crucial for LCC. The LCC should be inspected the day after 

placement, and the mix should be stable enough to walk on with limited surface impressions of 

no more than 1.0 inch (Taylor and Halsted 2021). Postconstruction testing primarily involves 

testing for UCS since no strict standards have been developed for other properties. The 

procedure provided by ASTM C495 provides the most accurate laboratory results for UCS 

testing (ASTM 2019c; Siebold and Tootle 2016). Variations (e.g., oven drying or improper 

curing) can have dramatic effects on UCS test results.
55

AVAILABLE COST INFORMATION 

The cost of LCC depends on several factors, primarily the mix composition and properties,

including water concentration ratio, foam density, and cost per gallon of foam concentrate. For 

example, a 30 pcf wet-density material starting with 1 yd of neat cement and having a 0.50 W/C 

ratio would require 2,060 lb of cement, 467 kg (1,030 lb) of water, and 2.27 m3(80 ft3) of foam. 

The total yield of LCC material would be 2.9 m3 (3.75 yd3). With these calculations, the base 

slurry cost is $175 per cubic yard (delivered as a local ready mix). The cost of foam is $36 

(based on $13.2 per L ($50 per gal), 48 kg/m3

 (3 pcf) foam density, and 40:1 water concentrate 

ratio). Thus, the total material cost is $211 per yield. This cost divided by 2.9 m3(3.75 yd3) 

equals $56.26 per cubic yard of cellular concrete (Richway Industries 2022). This cost is still 

significantly less than CLSM and ready-mix concrete if comparing only material costs. 

ENVIRONMENTAL CONSIDERATIONS 

Similar to CLSM, the major environmental concern with LCC is the leaching of toxic levels of 

minerals from a byproduct material used in the mix. However, as indicated by Folliard (2008) for 

CLSM, leachate potential from three tested byproduct materials, namely three fly ashes, one 

bottom ash, and one foundry sand, was lower than tolerable levels. Heavy metal concentrations, 

including arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver, passed the 

TCLP, meaning the heavy metal concentrations were very low and not of concern. 

LCA STUDIES 

In the comprehensive literature review, the researchers found no EPDs or PCRs to date for LCC 

materials in the United States or worldwide. Further, no comprehensive LCA studies were 

conducted in the United States or elsewhere. Some research papers and reports focused on 

making the LCC more “sustainable” by using recycled materials to replace cement (e.g., fly ash). 

However, with normal cement-water mixes, the only concern for leachate and safety from LCC 

would be the type and chemical composition of the foaming agent. The lack of LCA studies on 

LCC and the lack of PCRs that regulate the publication of EPDs for LCC is a research gap that 

needs to be addressed. 

CASE STUDIES 

There have been numerous installations of LCC for geotechnical applications with excellent 

performance records across the United States. LCC is extremely stable long term and has no 

known flaws once properly designed and installed. Case study examples in the United States 

include the following: 

• Bridge approaches and retaining wall backfills (Sutmoller 2020). The Illinois DOT 

(IDOT) installed LCC in a segmental wall configuration on the Lake Shore Drive and 

I-55 Interchange. In this project, 16,820 m3(22,000 yd3) of LCC (384–480 kg/m3

(24-30 pcf)) were placed to drastically reduce lateral and vertical loads and enable the 

construction of three new ramps. In another IDOT project for the Circle Interchange, 

13,762 m3

(18,000 yd3) of LCC (384–480 kg/m3 (24–30 pcf)) were placed at a circle 

interchange for the reduction of lateral loads. 
56

• Fill for underground tanks, pipelines, abandoned mines, and conduits (Sutmoller 2020). 

LCC was used in a water main abandonment fill application as part of an Illinois Tollway 

project. High fluidity, lightweight, and elevated compressive strength made LCC an ideal 

choice for this project. A total of 9,940 m3

(13,000 yd3) of LCC (384–480 kg/m3

(24-30 pcf)) was installed into 3,962 m (13,400 LF) of abandoned water mains along 

three Illinois tollways.

• Rapid Design and Construction of an Integral Abutment Bridge with MSE Walls and 

Cellular Concrete Backfill—Manitoba, Canada. LCC was used for the Chief Peguis Trail 

Extension at the Rothesay Street Overpass. Loewen, Baril, and Eric (2012) published a 

comprehensive paper on the design and construction challenges of this project. 

Traditional structures could not achieve the project goals, so LCC was used as the fill 

behind MSE walls, abutment piles, and some embankments. The project used LCC 

backfill up to a depth of 5 m (16.5 ft) to compensate for poor soils and high anticipated 

settlement. Operators poured LCC with a unit weight of 476 kg/m3 (29.7 pcf). The 

benefits of LCC included low weight, cost savings, and rapid construction. As Loewen, 

Baril, and Eric reported, the components of the structure performed well, and the 

expansion and contraction of the structure performed as anticipated. 

• Slope stability. Reported case studies include the following: 

o Mount Hamilton Road: Caltrans emergency road repair, San Jose, CA, 2017

(Cell-Crete Corporation n.d.).

o Highway 128 (west of Winters, CA), 2015 (Cell-Crete Corporation 2023).

• Bridge abutment fills. Reported case studies include the following: 

o Tahoe Echo Summit Bridge, 2019 (Caltrans 2019). 

o Michigan DOT bridge replacement to modernize 7 mi of I–94 in the City of Detroit, 

with an estimated completion date in 2023 (Cell-Crete Corporation 2022). 
57

CHAPTER 6. POLYSTYRENE GEOFOAMS 

INTRODUCTION AND BACKGROUND 

Polystyrene geofoams are extremely lightweight blocks that have been widely used in highway 

applications to mitigate issues related to low bearing capacity; freeze-thaw and soil heaving 

issues, particularly in areas with harsh winters; and construction of pavements/structures over 

pipes and utilities. Geotechnical highway applications for polystyrene geofoams include use as a 

lightweight fill, thermal insulator, and vibration dampener, as well as for the protection of 

underground services (Mohajerani et al. 2017). Depending on the manufacturing methods, 

geofoams can be either expanded polystyrene (EPS) or extruded polystyrene (XPS). EPS 

geofoams are more common for highway applications due to their consistent properties, 

sufficient strength, and more sustainable and energy-efficient production technologies 

(Walimbe 2020). Figure 14 shows some images of EPS installations. 

© 2009 Washington State DOT. 

A. Polystyrene geofoam block (Creative Commons n.d.b). 

© 2018 Washington State DOT. 

B. Polystyrene geofoam blocks for a backfill application (Creative Commons n.d.b). 

Figure 14. Photos. Placement of geofoam blocks for various applications. 
58

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using EPS geofoams in geotechnical highway applications include the 

following (XR Geomembranes 2019; Styrene Insulation Industry 2021): 

• Accelerates construction. 

• Can be constructed in adverse weather conditions.

• Eliminates the need for heavy earthmoving equipment. 

• Provides the potential for overall project cost savings. 

• Reduces labor needs and labor costs. 

• Eliminates the need for environmental permits. 

Reported disadvantages of geofoams include the following (Worley 2016; Emergen Research 

2022): 

• EPS is a plastic material prone to flammability and burning in fire. 

• EPS is vulnerable to petroleum solvents. 

• EPS is buoyant. For example, an incident was reported for a parking structure in 

Crayford, UK, on October 9, 2016. Cars were crushed against the ceiling after 

floodwaters raised the ESP geofoams below the floor of the parking structure. 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

EPS can be used in the following geotechnical applications for highways (EPS Industry Alliance 

n.d.; Styrene Insulation Industry 2021): 

• Insulation layers. 

• Lightweight fill for highways. 

• Bridge abutment fills. 

• Slope stabilizers. 

• Retaining wall fills. 

• Pipeline and trench fill for weight reductions. 

• Temporary road construction. 

MATERIAL CHARACTERIZATION 

Physical Properties 

ASTM D6817 reports the physical property requirements for geofoams, as listed in table 22 and 

table 23 for EPS and XPS geofoams, respectively (ASTM 2021d). Physical properties commonly 

reported for geofoams include puncture strength, abrasion resistance, tensile strength, flexibility, 

toughness, and chemical resistance. EPS and XPS have different grades with different 

compressive resistances and flexural strengths based primarily on product density. Projects may 

specify a minimum grade to be used, which will primarily depend on the expected dead weight 

and live load on top of the geofoam. An oxygen index (OI) is also specified to ensure that the 

geofoam is not flammable in air. An OI of 24 (as required by ASTM D6817) entails chemically 
59

treating the polystyrene material, which is inherently flammable, to provide fire resistance and 

additional protection. 

Table 22. Physical property requirements for EPS geofoams (ASTM 2021d; Geofoam 

International, LLC, n.d.).

Grade EPS 12 EPS 15 EPS 19 EPS 22 EPS 29 EPS 39 EPS 46

Density, min. (pcf) 0.70 0.90 1.15 1.35 1.80 2.40 2.40

Compressive resistance, 

min. (psi at 1 percent) 2.2 3.6 5.8 7.3 10.9 15.0 15.0

Compressive resistance, 

min. (psi at 5 percent) 5.1 8.0 13.1 16.7 24.7 35.0 35.0

Compressive resistance, 

min. (psi at 10 percent) 5.8 10.2 16.0 19.6 29.0 40.0 40.0

Flexural strength, min. 

(psi) 10.0 25.0 30.0 35.0 50.0 60.0 60.0

OI, min. (volume percent) 24.0 24.0 24.0 24.0 24.0 24.0 24.0

1 pcf = 16 kg/m3; 1.0 psi = 6.895. 

min. = minimum 

Note: Original table is in U.S. customary units. 

Table 23. Physical property requirements for XPS geofoams (ASTM 2021d). 

Grade XPS 20 XPS 21 XPS 26 XPS 29 XPS 36 XPS 48 

Density, min. (pcf) 1.15 1.25 1.54 1.73 2.11 2.88

Compressive strength, min. (psi 

at 1 percent) 2.90 5.08 10.88 15.23 23.21 40.61

Compressive strength, min. (psi 

at 5 percent) 12.33 15.95 26.83 34.08 48.59 77.60

Compressive strength, min. (psi 

at 10 percent) 15.08 15.08 25.09 40.03 60.05 100.08

Flexural strength, min. (psi) 40.03 40.03 50.04 60.05 74.98 99.93

OI, min. (volume percent) 1.15 1.25 1.54 1.73 2.11 2.88

ASTM C578 presents detailed information on the physical property requirements for different 

grades of EPS geofoams (ASTM 2022a). These include density, thermal resistance, compressive 

strength, flexural strength, and maximum allowed water absorption (table 24).
60

Table 24. Physical property requirements for EPS geofoams (ASTM 2022a; Geofoam 

International, LLC, n.d.).

Property Type XII Type I Type VIII Type II Type IX

Normal density (pcf) 0.75 1.00 1.25 1.50 2.00

Density, minimum (pcf) 0.70 0.90 1.15 1.35 1.80

Design thermal resistance per 

1.0-inch thickness at 75 °F

(°F·ft2·h/Btu)

3.22 3.85 3.92 4.17 4.35

Design thermal resistance per 

1.0-inch thickness at 40 °F

(°F·ft2·h/Btu)

3.43 4.17 4.25 4.55 4.76

Thermal resistance, min per 

1.0-inch thickness at 75°F

(°F·ft2·h/Btu)

3.10 3.60 3.80 4.00 4.20

Thermal resistance, min per 

1.0-inch thickness at 40°F

(°F·ft2·h/Btu)

3.30 4.00 4.20 4.40 4.60

Compressive strength at 10% 

deformation, min. (psi) 5.0 10.0 13.0 15.0 25.0

Flexural strength, min. (psi) 10.0 25.0 30.0 40.0 50.0

Water vapor permeance of 

1.0-inch thickness, max., 

permeance

5.0 5.0 3.5 3.5 2.0

Water absorption1 by total 

immersion, max. (volume %)

4.0 4.0 3.0 3.0 2.0

OI, min. (volume %) 24.0 24.0 24.0 24.0 24.0

Flame spread 20 20 20 20 20

Smoke developed 150–300 150–300 150–300 150–300 150–300

Mechanical Properties

Unlike other alternative lightweight and recycled materials discussed in this report, the selection 

of geofoams for highway geotechnical applications is based on specifying a material that fits the 

physical property requirements mentioned in the previous section. However, the researchers

found extensive studies in the literature on the mechanical properties of geofoams. Kaké, 

Temesgen, and Negussey (2018) studied the influence of strain rate on the stress-strain behavior 

of EPS geofoams. Beju and Mandal (2018) studied the CBR behavior of EPS geofoams through 

experimental and numerical approaches. Gade and Dasake (2018) investigated the mechanical 

properties of EPS geofoam under various loading conditions, including cyclic uniaxial 
61

compression, accelerated creep, and pseudo-long-term tests. They noted that elastic modulus (E) 

decreases with an increase in compressive creep.

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES 

Several thorough guidelines are available for designing highway applications with geofoams. For 

pavement foundations and embankments, available design guidelines include the Swedish 

Standard by Gandahl (1988), the Norwegian Standard (Norwegian Road Research Laboratory 

1992), and U.S. guidelines for highway embankments by Stark et al. (2004a, 2004b). For 

highway culverts, Sun, Hopkins, and Beckham (2005) published guidelines for using geofoams 

to reduce the stresses in highway culvert extensions. Further, Arellano et al. (2010) published 

guidelines for geofoam applications in slope stability projects. Other general guidelines for using

geofoams in ground modification methods were published by Schaefer et al. (2016). In fact, for 

all lightweight and recycled materials investigated in this report, the guidelines and design 

requirements for EPS geofoams are the most complete. The resources mentioned in this 

paragraph are good sources of information for design guidelines and requirements.

Due to the extremely lightweight nature of geofoams, buoyancy can be a major issue. The design 

guidelines have addressed these issues by calculating the buoyancy forces and requiring

counteracting dead loads for any EPS geofoam block fill that may be subjected to submergence.

Sufficient dead-load stress on the geofoam or another physical restraint must be applied to 

counteract uplift forces (Horvath 1999). Additionally, to minimize creep and deflection, 

manufacturers recommend that EPS geofoam design loads not exceed the compressive resistance 

at 1 percent capacity. Staying below this load limit will protect the geofoam from excessive 

long-term deflections from the sustained permanent loads.

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS

EPS is resistant to alkalis, dilute inorganic acids, gypsum plaster, most alcohols, portland 

cement, silicone oil, and solvent-free bitumen. Chemical products that may damage EPS include

hydrocarbons, chlorinated hydrocarbons, organic solvents, ketones, ethers, esters, diesel and 

gasoline, concentrated acids, vegetable oils, paraffin, and animal fats and oils (Mohajerani et al. 

2017). For this reason, Caltrans specifies that for geofoams used as a structural backfill, a 

reinforced gasoline-resistant geomembrane (GRG) must be used to protect the geofoam from 

spilled liquid hydrocarbons, and other fluids (XR Geomembranes 2019). The GRG membrane 

must cover and conform to the corners of the EPS geofoam.

Any free-standing water should be drained from the site, and a proper drainage system should be 

installed to keep the water level from rising (Tafreshi, Siabil, and Dawson 2020). Except where 

the designer has considered the effect of thawing in permafrost regions, the EPS should not be 

placed directly on frozen ground. Further, any vegetation and debris should be removed. To 

avoid EPS damage from direct contact with coarse granular soil, operators should place sand in a 

0.5–1.0-inch-thick layer. Adding the sand will also help level or slope the ground surface.
62

AVAILABLE COST INFORMATION

Multiple sources and manufacturers report the cost of geofoam blocks to range between 

$40-$76 per cubic meter ($52 and $100 per cubic yard) ($40–$76 per cubic meter). XPS 

geofoams are sold at a higher price and can achieve higher strengths. The material cost is thus 

comparable to other lightweight fill materials and more expensive than traditional fills. The 

information available on the cost of installation and handling is conflicting. Some sources and 

manufacturers claim that using the geofoam blocks reduces labor and transportation costs due to 

ease of handling and their lighter weight. However, some sources claim that the cost of installing 

geofoams can be higher than other alternative backfill materials, depending on the specified 

density (grade) of the geofoam and the total dead and live load applied on top of the geofoam.

ENVIRONMENTAL CONSIDERATIONS 

EPS is considered to be nonharmful. EPS has even been used as a material for the manufacture 

of eating utensils, containers, and beverage cups (Mohajerani et al. 2017). Thus, no issues with 

leaching of chemicals or toxic metals into groundwater are expected.

EPS and XPS are inherently flammable. Flammability is quantified by the OI, which is the 

minimum percentage of oxygen required for maintaining the combustion of an ignited material. 

EPS geofoams naturally have an OI of 18 percent by volume, and since air is 21 percent oxygen, 

geofoams are deemed flammable. In the United States, however, ASTM D6817 requires a 

minimum OI of 24 percent by volume for geofoams used in highway applications (table 22 and 

table 24) (ASTM 2021d). Beads that contain fire-retardant additives are used to manufacture 

these geofoams. Even by chemically treating the polystyrene material, these flame-retardant EPS 

geofoams will still completely melt at temperatures exceeding 300 °F. The flame-retardant EPS 

is more expensive to produce, and some questions exist regarding its environmental safety 

(Mohajerani et al. 2017; Horvath 2011).

LCA STUDIES

EPS and XPS are both closed-cell foam materials, but the main ingredient, manufacturing 

process, and resulting emissions are quite different. Thus, the environmental impacts are 

completely different. Many EPDs for geofoams are available, but the applications are mostly for 

home and roof insulations. No publicly available EPDs could be found for geofoams rated for 

use in highway applications.

EPS is more popular than XPS, with similar engineering properties, because the manufacture of 

EPS has not been linked to the depletion of the earth’s ozone (Mohajerani et al. 2017). Unlike 

XPS, the manufacture of EPS does not involve the use of chlorofluorocarbon (CFC) and 

hydrochlorofluorocarbon (HCFC) and does not result in the release of formaldehyde. 

Table 25 presents data collected from a comparative lifecycle assessment on extruded and 

expanded geofoams. Though EPD data are only available for home insulation applications, one 

may infer that the use of geofoams in highway applications should be limited to EPS geofoams 

due to the huge difference in environmental impacts. XPS geofoams can provide higher strengths 

than EPS, but the ODP is approximately 42 times higher due to the CFCs and HCFCs (QuadLock 2023). Further, the global warming potential is approximately 34 times higher for XPS, 
63

indicating that the manufacturing process is more energy intensive. Due to higher costs, XPS 

geofoams are less common in geotechnical and pavement applications. 

Table 25. Emissions and environmental impacts for EPS and XPS (Quad-Lock 2023).

Impact Category Units EPS XPS Ratio XPS/EPS 

ODP kg CFC-11-eq 0.000014 0.604 41.938

Global warming kg CO2-eq 2,511 85,757 34

Eutrophication kg N-eq 0.324 2.45 7.6

Acidification mol H+-eq 414 898 2.2

Total energy MJ 64,250 79,908 1.24

Total solid waste kg 675 722 1.07

Smog formation kg O2-eq 180 178 1

Water 

consumption kg = liter 4,904 4,904 0.55

CASE STUDIES 

The literature includes extensive case studies using EPS geofoams in highway geotechnical 

applications. Arellano et al. (2018) published a multitude of case studies as part of the 

proceedings of the 5th International Conference on Geofoam Blocks. The document covered 

case studies related to the use of geofoams for lightweight fills (Norway), embankments 

(Netherlands and Turkey), bridge abutments (Norway), and pipeline and culvert fills (Norway 

and Turkey). The case studies reported in the United States and Canada include the following: 

• Road embankment: Port Mann/Highway 1 Improvement Project, Vancouver to Langley, 

BC, Canada (Arellano et al. 2018). 

• Bridge abutment fill: Norman Kill, Albany, NY (Arellano et al. 2018). 

• Utility fill: Two case studies referenced by Tafreshi, Darabi, and Dawson (2020) using 

EPS geofoam. 

• Culvert fill: Layers of rigid geofoam insulation were applied under a culvert in Manitoba, 

Canada. Moussa et al. (2019) collected the temperature data from the thermistors 

installed under the culvert and calibrated a numerical model for predicting the freezing 

and thawing regimes. The geofoam insulation layer was found to reduce the temperature 

variation under the culvert and prevent freezing so that the frost heave–related pavement 

distress could be avoided. 

• Roadway embankments: In Illinois in 2016, Contract 60W55 used geofoams. No issues 

have been identified in the project thus far (Arellano et al. 2018). Another project 

involving geofoams in Cook County, IL, for Project No. NHPP-VQ14(255) used EPS 

geofoam blocks that conformed to the minimum requirements of the ASTM C578 

standard (2022). Sand layers were placed underneath the first level of EPS geofoam 

blocks and compacted to satisfaction. No issues were raised with this installation. 

65

CHAPTER 7. RAP

INTRODUCTION AND BACKGROUND 

RAP materials are created by removing, crushing, and reprocessing existing asphalt layers. RAP 

materials typically contain aggregate particles that vary in size and are coated partially or fully 

with asphalt binder (figure 15). Particle size and shape properties, the amount of asphalt coating 

the RAP particles, and the binder content of the RAP are among the important engineering 

parameters that control the quality of this material. Since a principal constituent of RAP is its 

mineral aggregates, the overall chemical composition of RAP is similar to that of the mineral 

aggregates in the mix. RAP has traditionally been used in the United States and other parts of the 

world as granular material for fills and pavement layers, in asphalt mixes, and in concrete mixes 

(Bennert et al. 2000; Copeland 2011; Huang, Shu, and Li 2005). When RAP is used as an 

unbound aggregate, the volume of asphalt in the RAP reduces the specific gravity. The amount 

and percent coverage of asphalt binder on the aggregate surface impacts the moisture sealing of 

the mineral aggregate in RAP, which is an important consideration for the reuse associated with 

highway geotechnical applications. These characteristics result in RAP having a lower unit 

weight compared to VAs, and the amount of water needed to achieve the desired compaction 

level is often less. 

© 2021. Wikifili1989. 

A. RAP material—a closeup (Creative Commons n.d.c). 

© 2016 Moondosmile. 

B. Another photo of RAP from a different source. 

Figure 15. Photos. RAP materials. 
66

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using RAP in geotechnical highway applications include the following 

(Dager et al. 2023; Edil 2017; FHWA 1997; Richfield Blacktop 2020): 

• Cost-effective.

• Environmental impacts (emissions) reduced. 

• Energy savings from production anticipated. 

• High modulus and layer stiffness characteristics. 

• Moderate to high internal friction angle and high strength (debatable in literature). 

• Controlled gradations are possible. 

• Nonplastic, free draining, and not frost susceptible. 

Disadvantages of RAP include the following (Barzegar et al. 2023; Dager et al. 2023; Edil 2017; 

Gao et al. 2021): 

• Not lightweight. 

• Temperature sensitivity—mechanical properties vary with temperature (figure 16). 

• Poor creep performance—may have excessive settlement under load and compaction. 

• Source variability and impurities. 

© 2015 Soleimanbeigi. Modifications by FHWA to consolidate information into one graph. 

Figure 16. Graph. Resilient modulus properties of RAP materials collected from different 

States (Soleimanbeigi et al. 2015). 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

RAP can be used in the following geotechnical applications for highways (Al-Shujairi, Al-Taie, 

and Al-Mosawe 2021): 

• Road shoulders as embankments. 

• Back-filling material. 
67

• Retaining walls as a backfill material. 

• Slope protection/slope stability. 

• Landfill capping systems. 

• Pothole filler material. 

• Drainage works. 

• Aggregates in bituminous and concrete mixes. 

• Base and subbase materials for roadways.

• Soil stabilization—mixed with naturally weak soils. 

Based on the literature review, some States allow RAP in embankments or as structural backfills 

(table 26 provides a nonexhaustive list). Texas successfully used RAP for embankments, MSE 

wall backfills, and erosion control (Ncube and Bobet 2021). According to the National Asphalt 

Pavement Association, “RAP must not be considered a hazardous waste and can be used as a 

clean fill” (Ncube and Bobet 2021). 

Table 26. Select States allowing RAP usage in highway geotechnical applications (Ncube 

and Bobet 2021). 

State 

RAP 

in 

HMA 

Mixes 

RAP for 

Embankments 

and Fills

Structural 

Fills Subgrade 

Max. % 

RAP 

Allowed 

in 

Subgrade 

RAP 

for Base 

or 

Subbase 

Max. % 

RAP 

Allowed 

in Base or 

Subbase 

California ✓ — — — — ✓ 40

Colorado ✓ ✓ — ✓ — ✓ —

Florida ✓ ✓ — ✓ — ✓ —

Illinois ✓ ✓ — ✓ 40 ✓ 50

Minnesota ✓ — — — — ✓ 25

Texas ✓ ✓ ✓ — — ✓ 20

Wisconsin ✓ — — — — ✓ —

—No data. 

According to FHWA, at least nine States reported using RAP in embankments (Chesner et al. 

1997). Connecticut, Indiana, Kansas, Montana, New York, and Tennessee used RAP as an 

additive in embankment construction, while California, Connecticut, Illinois, Louisiana, and 

Tennessee used RAP directly as an embankment fill material. RAP performance in these 

applications was generally considered satisfactory to good. However, some States (e.g., Illinois) 

experienced excessive settlements in some locations where RAP was used for embankments. A 

request for proposal by the Illinois Tollway (2019) stated, “In isolated instances, excessive 

settlement has occurred, which may be attributed to the use of RAP. The excessive settlement is 

typically identified months after placement, primarily in situations where construction was 
68

performed late in the season (late fall/winter).” No report or further information detailing the 

explanations of these excessive settlements has been published or provided to date.

MATERIAL CHARACTERIZATION

Physical Properties

Based on samples obtained from eight States, including California, Colorado, Michigan, 

Minnesota, New Jersey, Ohio, Texas, and Wisconsin, the RAP and recycled pavement materials

(RPMs) had reasonably consistent properties (Edil 2017). Note that RAP and RPM are used 

interchangeably for the context of this report to indicate materials recycled from asphalt content 

(AC) layers. Table 27 shows the range of properties for gradation (e.g., fines content), 

absorption, AC, and soil classification.

Table 27. Physical properties of RAP—average values and typical ranges (Edil 2017).

Properties

RAP/RPM

Average (range)

Fines (percent) 0.92 (0.4–1.8)

Gravel (percent) 38.38 (32–51)

Coefficient of uniformity, Cu 9.80 (7–17)

Specific gravity 2.38 (2.34–2.57)

Absorption (percent) 1.84 (0.6–3.0)

AC (percent) 5.9 (4.7–7.1)

Classification

SP, SW, GW

A-1-a, A-1-b

SP = poorly graded sands, gravelly sands, little or no fines; SW = well-graded 

sands, gravelly sands, little or no fines; GW = well-graded gravels, gravel-sand 

mixtures, little or no fines.

Mechanical Properties

Stroup-Gardiner and Wattenberg-Komas (2013) reported average friction angles for RAP 

materials comparable to those of the virgin crushed stone and RCAs (table 28). Among all three, 

the friction angle of RAP was significantly lower, but the effective cohesion due to the presence 

of the binder adds to the shear strength. Edil (2017) reported the typical resilient modulus values 

of RAP materials collected from four different sources and States, as previously shown in 

figure 16. The resilient modulus values were quite similar and showed a temperature-dependent 

behavior, with lower modulus reported for higher testing temperatures. Domitrović, Rukavina, 

and Lenart (2019) conducted repeated load triaxial testing for resilient modulus and permanent 

deformation on RAP and crushed limestone mixtures, with RAP contents varying between 

0 percent, 20 percent, 35 percent, and 50 percent of dry mass. Samples exposed to 

14 freeze-thaw cycles were also tested in addition to standard samples. Freeze-thaw conditioning 
69

resulted in a decrease in resilient modulus values and an increase in permanent deformation. This 

result was most pronounced for the crushed limestone with 0 percent RAP. Mixtures with 

35 percent RAP exhibited stable, resilient behavior and the lowest change in permanent 

deformation accumulation after freeze-thaw conditioning. 

Table 28. Shear strength properties of RAP, RCA, and VAs (Stroup-Gardiner and 

Wattenberg-Komas 2013).

Measurements Control RCA RAP

Effective confining pressure range (kPa) 83–276 83–255 83–310

Effective stress friction angle (degrees) 55 54 39

Effective cohesion, c (kPa) 0 0 55.2

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES 

The literature does not contain extensive guidelines for using RAP as an embankment 

construction material. The undersized portion of crushed and screened RAP (<2 inches) may be 

blended with soil or finely graded aggregates or both (Chesner et al. 1997). The uncrushed or 

coarsely graded RAP may be used as the embankment base. The RAP utilized in embankment 

construction does not take advantage of the asphalt cement component, so it is typically not used 

for embankments unless abundant quantities are available or if the RAP quality is not suitable for 

other uses. 

The design requirements for RAP used in embankments are the same as the requirements for 

similar-sized soil-aggregate blends or conventional aggregates. The design should take into 

consideration slope stability, settlement, consolidation, and bearing capacity concerns (Chesner 

et al. 1997). Representative samples of RAP or blended materials should be tested for triaxial 

compression. The maximum particle size for the CBR test is 3/4 inch. When used for 

embankment applications, the engineering properties of interest for RAP include the following 

(Chesner et al. 1997): 

• Particle size: Maximum allowed in embankments is 610 mm (24 inches). 

• Compacted density: Typically is between 1,600 and 2,000 kg/m3

 (100 and 125 pcf). 

• Moisture content: Typically is higher than that for conventional/virgin embankment 

materials. 

• Shear strength, bearing strength, and consolidation characteristics: Consolidation is 

negligible for coarsely graded RAP. 

• Permeability: Typically is satisfactory.
70

• Durability: Is not a concern because the quality of aggregates used in HMA is better than 

those specified for fill materials. 

• Drainage characteristics and corrosivity: RAP is considered noncorrosive for steel and 

other metals (limited testing results). 

For MSE walls, critical material properties for using RAP as a backfill material include hydraulic 

conductivity (i.e., permeability), shear strength properties, interface friction, compaction 

characteristics, compressibility of compacted materials, time-dependent effects (i.e., creep), and 

corrosivity (Stroup-Gardiner and Wattenberg-Komas 2013). Corrosivity is checked for RAP in 

the proximity of steel or other metals. Rathje et al. (2001) evaluated RAP and RCA for MSE 

applications; their research concentrated primarily on the durability aspects of geosynthetic and 

metallic reinforcements. The researchers recommended polymeric reinforcement be designed to 

resist most forms of degradation and provide adequate performance with RAP and RCA 

backfills. RAP from mixes with a tendency to strip should be avoided. A pH typically around 8 

is satisfactory for MSE wall backfill applications. Further, the researchers found limited deicing 

chemical salt contents on the RAP/RCA surfaces to be a minor issue for long-term durability. 

Rathje et al. (2002) continued to evaluate RAP and RCA for MSE backfill applications. The 

researchers conducted triaxial testing to determine the cohesion and the angle of internal friction 

for RAP, RCA, and a control VA material. RAP generated a small cohesive value because of the 

asphalt particles adhering together in the compacted specimens. The internal friction angle of 

RAP was also the lowest among all three materials. Rathje et al also evaluated the corrosive 

potential of RAP by soaking materials in water, decanting the water, and using it as an 

electrolyte in a corrosion cell. No statistically different corrosive potentials were determined 

after 64 d of testing. Carley (2002) also evaluated RAP as backfill for MSE walls compared with 

crushed limestone. RAP provided adequate strength and hydraulic conductivity as a backfill 

material. Table 29 presents general guidelines and design parameters for RAP and other backfill 

materials used in MSE wall backfill applications (Stroup-Gardiner and Wattenberg-Komas 

2013). 

Table 29. General guidelines for using RAP and other backfill materials in MSE wall 

backfill applications (Stroup-Gardiner and Wattenberg-Komas 2013). 

Requirement

TxDOT 

(Type A) 

TxDOT 

(Type B)* FHWA 

Gradation: maximum size 76 mm 

(3 inches) 

152 mm 

(6 inches) 

102 mm 

(4 inches) 

Gradation: percent passing sieve 76 mm 

(3 inches) — 75–100 —

Gradation: percent passing sieve No. 40 0–60 — 0–60

Gradation: percent passing sieve No. 200 0–15 0–15 0–15

PI — — <6
71

Requirement

TxDOT 

(Type A) 

TxDOT 

(Type B)* FHWA 

Compaction: dry density 

95 percent 

(Tex-114-E 

(TxDOT 

2011)) 

Not specified 

95 percent 

(AASHTO 

T-99 (2022e)) 

Compaction: moisture content ±2% of Wopt Not specified Within 2% dry 

of Wopt

pH 5.5–10 Not specified 5–10

Resistivity (ohm-cm) >3,000 Not specified >3000 

TxDOT = Texas DOT. 

*Type B backfill that does not meet the sieve No. 200 requirement may be used if less than 25 percent passes sieve 

No. 200, PI < 6 percent, friction angle is at 95 percent dry density (Tex-114-E), and OMC > 34 degrees. 

In addition, a survey of transportation agency guidelines and practices in the United States and 

Canada by Tutumluer, Moaveni, and Qamhia (2018) has demonstrated that some agencies limit 

the use of RAP in highway applications. Table 30 summarizes the agency responses. Note that 

the study focused primarily on traditional highway applications (i.e., mostly in pavement layers).

Table 30. Survey of transportation agency responses related to restricting use of RAP in 

highway applications (Tutumluer, Moaveni, and Qamhia 2018). 

Transportation 

Agency Restriction for Using RAP, Considering Quality Concerns 

British Columbia Only for asphalt pavement 

Florida 

Minimum 4-percent AC and mix must meet Florida DOT 

specifications. Minimum 2.5-percent AC for coarse portion above 

No. 4 sieve 

Illinois Not allowed in concrete pavement 

Kentucky Only allowed as part of the aggregate blend for asphalt layers 

New Hampshire Dust-to-asphalt ratio as identified in AASHTO M 323 (2022d); 

stockpiles must be tested for gradation and AC every 1,000 tons 

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS 

To minimize/avoid agglomeration, crushed RAP should be blended as soon as possible with 

conventional aggregate (using a cold feed system) to obtain a homogeneous mixture. If the 

blended material was stockpiled for a considerable period, particularly in warm weather, the 

stockpile might harden and require recrushing and rescreening before use (Chesner et al. 1997).

Blended RAP-aggregate stockpiles should not be allowed to remain in place for extended periods 

because the stockpiled material is likely to become overly wet. 
72

The same equipment and procedures used to handle and place conventional aggregates are 

applicable for RAP. RAP blended with VAs can be placed as a conventional granular material. 

Blending at a central plant provides better material consistency. Additional care is required 

during stockpiling and handling of RAP and RAP blends to avoid segregation or 

reagglomeration (Chesner et al. 1997). Conventional granular aggregates generally do not bond 

well with RAP. Consequently, raveling may occur if thin layers of conventional aggregates are 

placed over constructed layers containing RAP. During placement of RAP or blends with RAP, 

finish grading can be difficult because of the adhesion of asphalt in the RAP. Particular attention 

should be paid to avoid postconstruction densification. Compaction improves if little or no water 

is used.

The same field test procedures and QC measures used for conventional aggregates are also 

recommended for RAP. However, QC testing needs additional sampling and testing of the 

recycled stockpiles to account for the increased material variability (Stroup-Gardiner and

Wattenberg-Komas 2013). Testing moisture content and compaction using nuclear gauges are

affected by the presence of binder in RAP. Both parameters are overestimated because of the 

presence of hydrogen ions in the binder. In particular, the nuclear gauge showed higher readings 

for moisture content than measurements obtained with oven drying (Rathje et al. 2002). 

Compaction may be carried out using a control strip to avoid such measurement issues in the 

field. Laboratory moisture checks should be completed to calibrate nuclear density gauge 

readings and create correction multipliers. Table 31 presents the ratios of moisture contents 

obtained using a nuclear gauge to those calculated from oven-drying measurements.

Table 31. Ratios of nuclear moisture contents to oven-dried measurements for RAP (Rathje 

et al. 2002; Stroup-Gardiner and Wattenberg-Komas 2013).

Statistics

Ratio of Nuclear Moisture to Oven-Drying Measurement

Control RCA RAP

Average 0.99 1.19 3.07

Standard Deviation 0.12 0.10 0.69

Min. and max. values 0.84–1.19 1.03–1.33 2.36–4.51

AVAILABLE COST INFORMATION

The price of RAP in the United States generally ranges from $10 to $20 per ton (Auburn 

Aggregates 2022; Homeguide 2022b). The price is comparable to that of VAs but is typically on 

the higher end due to the presence of asphalt binder, a precious and costly material for asphalt 

mixtures. For this reason, RAP used for fill applications should either be in abundance or not 

pass the specifications for use in asphalt mixes.
73

ENVIRONMENTAL CONSIDERATIONS 

Asphalt pavement layers consist of aggregates and petroleum-derived asphalt binders containing 

volatile and semivolatile constituents such as polycyclic aromatic hydrocarbons (PAHs)

(Recycled Materials Resource Center n.d.). Additionally, asphalt pavement roadways may 

contain surface treatments, rubberized materials, or contaminants from vehicles or other 

emissions (e.g., historically, lead). Since RAP is coming from these materials, such contaminants 

can be present at various levels and can pose environmental concerns. The environmental issues 

are different for RAP based on various beneficial uses. For unbound applications, such as use as 

a backfill material, leachability from the RAP may be a concern (Melton and Kestler 2013).

A project in Florida tested leaching volatile organic compounds, PAHs, and heavy metals in 

RAP materials (Melton and Kestler 2013). Results showed that the levels of these contaminants 

were far below the detection limit and the State’s regulatory groundwater guidance. Similarly, 

batch and column leaching tests found constituents leached were low and generally below 

European drinking water standards. Further, the University of Minnesota completed a review on 

PAHs in asphalt pavements, and the researchers concluded that PAH concentrations depended on 

the type of pavement (coal tar versus petroleum-based pavement) (Melton and Kestler 2013). 

Petroleum-based asphalt pavements contained PAHs at concentrations below Minnesota 

Pollution Control Agency human health risk clean-up levels. Based on these studies, RAP does 

not pose environmental considerations when used as an unbound backfill material. When RAP is 

mixed with soils or other conventional aggregates, the mixture would not likely exceed 

applicable limits for leaching or other environmental pollutants.

LCA STUDIES

Edil (2017) conducted comparative LCA and LCCA studies for designs using RAP/RPM as a 

base material. The researchers’ analyses concluded a 20-percent reduction in carbon dioxide

(CO2) emissions, 16-percent reduction in energy consumption, and a 21-percent savings in 

lifecycle costs due to the use of RAP/RPM in lieu of crushed aggregates in base materials. 

Figure 17 presents the pavement cross sections. The researchers assumed that the service life of 

conventional and alternative designs could be based on international roughness index predictions 

made with the AASHTOWare® Pavement ME Design software and that the rehabilitation occurs 

at the end of the predicted service life for both designs (AASHTO 2015). Thus, these 

conclusions can apply to using RAP in other geotechnical highway applications, such as fill 

materials.
74

© 2017 Edil. Modifications by FHWA 

to figure label. 

A. Conventional design. 

© 2017 Edil. Modifications by FHWA 

to figure label. 

B. Design with RPM (RAP). 

Figure 17. Illustrations. Pavement cross sections with conventional and RPM (RAP) base 

materials (Edil 2017). 

Another study by Aurangzeb et al. (2014) presented an LCA for HMA mixes with varying 

percentages of RAP ranging between 0 and 50 percent. The study assumed similar construction 

practices and maintenance activities; therefore, the reductions obtained in all energy and 

emission footprints due to the use of RAP are primarily attributed to the material phase. Thus, 

these results can be further extended to different highway applications (e.g., embankments, MSE 
75

wall backfills, and bridge abutments) for environmental impacts due to initial material use. 

Table 32 presents the results of the study.

Table 32. Effects of percent RAP used on LCA results (material phase) (Ncube and Bobet 

2021; Aurangzeb et al. 2014).

Parameter Lifecycle Phase 0% RAP 30% RAP 40% RAP 50% RAP

Energy (Btu 

millions)

Total 10,897 10,100 9,834 9,569

CO2 (lb CO2e) Total 1,528,780 1,416,499 1,379,072 1,341,645

CH4 (lb CO2e) Total 272,749 251,459 244,362 237,265

N2O (lb CO2e) Total 11,324 10,418 10,115 9,813

GHG (lb CO2e) Total 1,821,700 1,686,510 1,641,446 1,596,383

CO2e = carbon dioxide equivalent; CH4 = methane; N2O = nitrous oxide; GHG = greenhouse.

CASE STUDIES

Currently, several States and agencies allow RAP to be used in roadway embankments, structural 

backfills, and aggregate surfacing. In some cases, such as for the Illinois Tollway, RAP in 

roadway embankments and as a structural backfill is loosely specified in terms of maximum size, 

gradation, engineering properties, and acceptance (Illinois Tollway 2019). The literature is rich 

with case studies related to the use of RAP in asphalt mixes and in base and subbase layers, but 

the researchers found no well-documented case studies for the use of RAP in embankment, 

backfill, and other highway geotechnical applications.

77

CHAPTER 8. RGA 

INTRODUCTION AND BACKGROUND 

In the United States, 10 million tons of glass are disposed of every year, yet only 33 percent of 

the waste glass gets recycled (Jacoby 2019). One of the early steps of glass recycling involves 

crushing the cleaned glass to make glass cullet, as shown in figure 18. The glass is normally 

collected from municipal and industrial waste streams. The crushing process can be controlled by 

crushing the glass into cullet of a specific size (Tao 2017). Glass cullet is the main raw material 

to produce FGAs, discussed in chapter 4. Glass cullet can ultimately be used as RGAs for 

various highway and pavement applications. In fact, RGA has been used in different construction 

applications, including cement and aggregate replacement in concrete and asphalt concrete 

mixtures, a material for roadbeds and pavement, trench fill, and a drainage medium 

(Chindaprasirt and Cao 2015). Some of the glass that cannot be recycled into new containers 

may find its way into pavement applications, especially as a substitute for sand or fine aggregates 

in concrete mixes. 

© 2013 Lynn Friedman. 

A. Glass cullet, the main ingredient for RGA (Creative Commons n.d.d). 

© 2016 makamuki0. 

B. Glass cullet of different colors, shapes, and sizes.

Figure 18. Photos. RGAs.
78

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using RGA in geotechnical highway applications include the following 

(Manyara 2019; Afshinnia 2019): 

• Comes in a variety of sizes and gradations. 

• Has a high friction angle.

• Is inexpensive to produce/crush. 

• Has achieved density that is insensitive to moisture content. 

• Has favorable compaction characteristics and good workability. 

• Can be placed and effectively compacted during wet weather. 

• Has improved resistance to freeze-thaw effects. 

• Has reduced drying shrinkage and abrasion potential when used in concrete mixes and 

when blended with other virgin or recycled aggregates or both. 

Disadvantages of RGA include the following (Mike’s Trucking 2022): 

• Health effects are possible from the dust generated from dumping, leveling, and 

compacting glass aggregates. 

• Dust, though the Occupational Safety and Health Administration (OSHA) currently 

classifies glass dust as “nuisance” dust and not as hazardous dust. 

• Skid resistance lower when used in asphalt pavements (use is limited to lower speed 

roadways, <65 mph). 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

RGAs can be used in the following geotechnical applications for highways (Donlon 2019; Dodge 

2015): 

• Backfill applications. 

• Backfill for pipes and trenches. 

• Aggregate replacement: When finely crushed, RGA can replace sand. If less finely 

crushed, RGA can be used to replace gravel. RGAs are commonly used as aggregate 

substitutes in asphalt paving or fine aggregate replacement in concrete. 

MATERIAL CHARACTERIZATION 

Physical Properties 

Ooi et al. (2008) provide the physical properties of RGAs from different literature sources, 

summarized in table 33. The physical properties include soil classification, specific gravity, 

optimum moisture content, and maximum dry density. The specific gravities reported for 

sand-sized RGA mostly range between 2.45 and 2.50 for specific gravity. The maximum dry 
79

densities, calculated using the modified Proctor compactive energy, typically range between 

109 and 120 pcf, which indicates that the sand-sized materials are lighter than conventional 

aggregates but are not necessarily lightweight compared to the other lightweight materials 

discussed in this report, such as ESCS (chapter 3), FGAs (chapter 4), and geofoams (chapter 6). 

Disfani et al. (2011) investigated the physical properties of RGA using glass cullet ranging in 

size according to different gradations. The researchers tested coarse, medium, and fine RGAs 

(termed herein as coarse recycled glass (CRG), medium recycled glass (MRG), and fine recycled 

glass (FRG), respectively). The maximum particle size for CRG, MRG, and FRG was 0.5 inches, 

3/8 inches, and 0.2 inches (i.e., No. 4 sieve), respectively, and all materials were well graded. 

The study found that CRG can be highly crushable by a standard LA abrasion test. Table 34

presents the physical properties of all three aggregate gradations. 

Table 33. Physical properties of RGAs (Ooi et al. 2008). 

Description* 

USCS 

Symbol Specific Gravity OMC (%) 

Maximum Dry 

Densitya (kg/m3) 

Stoltzfus—as received SW 2.48 10.8 1,894

Stoltzfus—coarse fraction GP — 7.8 1,750

Heller—as received SW 2.49 10.8 1,810

Heller—coarse fraction GP — 9.9 1,759

Stoltzfus—as received SW 2.48 9.7 1,882

Heller—as received SW 2.49 11.2 1,800

City of Philadelphia curbside 

glass SP 2.48 8.0 1,924

CA-14 6.4 mm (1/4 inch) minus SP 2.49 5.6 1,791

WA-09 6.4 mm (1/4 inch) minus SP 2.52 5.2 1,831

WPBMRF SW 2.45–2.50 — —

ASTM No. 8 average GP 2.45–2.50 — —

ASTM No. 9 average SP 2.45–2.50 — —

ASTM No. 10 average SP 2.45–2.50 — —

Uniform—loose SP 2.50 — —

Well-graded—loose SW 2.50 — —

—Could not determine from literature. 

aBased on ASTM D1557 (ASTM 2021a).

GP = Poorly graded gravels, gravel-sand mixtures, little or no fines; WPBMRF = West Palm Beach (FL) Material 

Recycling Facility. 

*FGA materials having the same description (name) indicate tests were performed by different agencies or

researchers. More information can be found in the original reference by Ooi et al. (2008). 
80

Table 34. Physical and geotechnical properties of RGA samples (Disfani et al. 2011). 

Test FRG MRG CRG

Specific gravity 2.48 2.50 2.50

Flakiness index — 85.4 94.7

Debris level (visual method) (percent) 7 5 3

Debris level (weight method) (percent) 1.23 2.01 2.98

Organic content (percent) 1.3 0.5 0.23

pH value 9.9 10.1 9.6

Standard Proctor max. dry density 

(kN/m3) 16.7 18 N/A 

wopt from standard Proctor (percent) 12.5 9 N/A 

Modified Proctor max. dry density 

(kN/m3) 17.5 19.5 N/A 

wopt from modified Proctor (percent) 10 8.8 N/A 

LA abrasion value (percent) 24.8 25.4 27.7

Mechanical Properties 

According to Ooi et al. (2008), appropriate gradations and sufficient compaction densities of 

RGA can yield significant shear strengths because of the large friction angles observed. 

Therefore, RGA is a potential candidate for use in geotechnical and highway projects for 

foundation and ground improvement applications (e.g., compacted aggregate foundations, sand 

compaction piles, vibro-replacement, vibro-flotation) The researchers compiled data on the 

mechanical properties of RGAs from different literature sources and manufacturers. The 

researchers measured shear strength data, primarily peak friction angle, by DSTs and CD and 

consolidated undrained (CU) triaxial tests. Table 35 shows that the friction angles measured by 

DSTs were consistently higher than those measured using CD triaxial test experiments. Table 36

summarizes the shear strength properties (friction angles) for RGA of different grain size 

distributions, i.e., well-graded and poorly graded sands and poorly graded gravel. The friction 

angles range from 37 to 62 degrees, indicating considerably high shear strength properties. 

Disfani et al. (2011) reported similar trends and magnitudes for MRG and FRG. The researchers 

conducted DSTs on RGA samples compacted in three layers. The friction angles ranged from 

50 to 53 degrees for FRG and from 40 to 47 degrees for MRG samples. 

Table 35. Mechanical properties of RGAs (Ooi et al. 2008). 

Description* 

Densitya

(kN/m3) 

Maximum 

Dry Densityb

(kN/m3) 

Relative 

Compaction 

(percent) Test Type

Peak 

Friction 

Angle 

(degrees) 

Stoltzfus—as received 106.0 18.4 90.0 Direct shear 61

CD triaxial 47

Stoltzfus—coarse 

fraction 98.3 17.0 91.0 Direct shear 54

CD triaxial 45

Heller—as received 101.5 17.6 92.0 Direct shear 56

CD triaxial 46
81

Description* 

Densitya

(kN/m3) 

Maximum 

Dry Densityb

(kN/m3) 

Relative 

Compaction 

(percent) Test Type

Peak 

Friction 

Angle 

(degrees) 

Heller—coarse 

fraction 98.9 17.1 93.0 Direct shear 48

CD triaxial 44

Stoltzfus—as received 106.0 18.3 90.1 Direct shear 

61–63c

58–61

53–58

CD triaxial 48

Heller—as received 101.5 17.5 90.0 Direct shear 

59–62c

55–59

47–55

CD triaxial 47

City of Philadelphia 

curbside glass 114.3 18.7 95.0 CU triaxial 37

CA-14 (1/4 inch) minus 99.6–100.2 17.4 89.1–89.3 Direct shear 51.3

WA-09 (1/4 inch) minus 100.2–100.8 17.8 88.0–88.2 Direct shear 51.2

WPBMRF 91.8 — — Direct shear 40

104.1 — — 45

ASTM No. 8 average† 88.6 — — Direct shear 45

93.8 — — 51

ASTM No. 9 average† 91.9 — — Direct shear 37

102.2 — — 45

ASTM No. 10 average† 95.7 — — Direct shear 34

109.2 — — 46

Uniform—loose — — —

CD and CU

triaxial 31–32

Uniform—medium — — — — 35

Uniform—dense — — — — 36

Well-graded—loose — — —

CD and CU

triaxial 37–38

Well-graded—medium — — — — 39

Well-graded—dense — — — — 42–43

—Could not determine from literature. 

aAs-compacted for shear strength test. 

bBased on ASTM D1557 (ASTM 2021a). 

cThe three ranges of friction angle correspond to normal stresses of 0 to 60, 60 to 120, and 120 to 200 kPa, 

respectively. 

*FGA materials having the same description (name) indicate tests were performed by different agencies or

researchers. More information can be found in the original reference by Ooi et al. (2008). 

†Based on ASTM E11 (ASTM 2022b).
82

Table 36. Summary of shear strength properties of RGAs (Ooi et al. 2008). 

USCS 

Classification

Friction Angle (degrees) 

Direct Shear Triaxial Overall 

SW 40–62 37–48 37–62

SP 34–51 31–37 31–51

GP 45–54 44–45 44–54

Further, Ooi et al. (2008) studied the bearing strength characteristics (CBR) of three recycled 

materials (RGA, RAP, RCA) and a basaltic VA, all having similar gradations. The gradations of 

the materials were at the finer end of Hawaii DOT fill gradation. The researchers found the CBR 

(strength) of recycled glass was insensitive to moisture content and comparable to or slightly 

higher than that of fine RAP (figure 19).

© 2017 Ooi. Modifications by FHWA to line colors and patterns. 

Figure 19. Graph. CBR versus water content for RGA, RCA, RAP, and VAs tested at the 

same gradations (Ooi et al. 2017). 

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES 

Several studies investigated and reported recycled glass used as percentage replacements in 

unbound granular materials for highway applications (i.e., as a percentage replacement of RCA 

and crushed rock) and for clayey soils. Such studies include the worldwide study by Perera et al. 

(2021), the study by Arulrajah et al. (2017) for Australia, and the study by Machuca et al. (2020) 

for Peru. The researchers reported improved performance trends with certain percentage 

replacements with RGA ranging from 0.75 to 50 percent, depending on the properties that 

needed to be enhanced, as shown in table 37. 
83

Table 37. Suitable glass content that best enhances respective properties

(Perera et al. 2021).

Materials/Blends UCS (%) CBR (%)

LA Abrasion

(%)

Resilient 

Modulus

(%)

Permanent 

Deformation

(%)

Shear 

Strength

(%)

RG-CR/WR 10 15 50 15 10 15

RG-CR-R 5 5 5 5 5 3

RGRCA/RCC/CC20 10 15 10 10 15

RG-RCA-R 5 3 5 5 5 3

RG-soil-cement 10 — — — — —

RG-clay/soil 10 12 — 30 — 15

GF-clay/soil 0.75 1 — — — 0.75

GP-geopolymer 15 20 — 20 — 40

—No data.

RG = recycled glass, GF = glass fiber, CR = crushed rock, WR = waste rock.

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS

The blending of crushed glass with other aggregates in low percentages (<15 percent) will not 

noticeably affect the compaction characteristics of the major constituent of the blend (Mike’s

Trucking 2022). According to FHWA, the recommended percentages of glass cullet in the base 

and subbase layers (and other applications) are between 15 and 30 percent (Chesner et al. 2002). 

According to the Clean Washington Center (CWC) (1996), for 100-percent cullet, the apparent 

cohesion is low, and the material flows relatively freely. Therefore, CWC recommended using 

hoppers for directing flow, which has been proven efficient and successful. Some cullet particles 

are sharp and can puncture tires. Experience has indicated that solid rubber tires are a better 

alternative to compact layers with glass cullet and RGAs. Lift thickness should be between 4 and 

6 inches for manually operated equipment and between 8 and 12 inches for automatic 

compaction equipment. Vibratory compactors are effective for cullet or cullet mixtures; jumping 

jacks and walk-behind rollers can be used. Plate or slick compactors are ineffective and should 

be avoided.

AVAILABLE COST INFORMATION

According to Rossetti (2020), recycling glass costs between $70 and $90 per ton, but the cullet 

sells for only about $10 per ton. A feasibility study conducted in Iowa by Tao (2017), who 

investigated the feasibility of using RGAs in highway applications, confirmed the costs of 

recycling glass. The report indicated that a recycled materials facility was shipping glass at a cost 

as high as $40 per ton, but this cost significantly exceeds the price of RGA/glass cullet, which is 

$15 per ton. Currently in Iowa, the recycling facilities are receiving subsidies from the State to 

cover freight and operation costs (Tao 2017). Thus, RGAs are relatively inexpensive compared 

to other materials discussed in this report since RGAs are being sold at a price ranging from $10 

to $15 per ton in the United States.
84

ENVIRONMENTAL CONSIDERATIONS

OSHA currently classifies glass dust as “nuisance” dust and not as hazardous dust (Mike’s 

Trucking 2022). Imteaz, Ali, and Arulrajah (2012) found that conductivity, pH values, most 

heavy metals, and organic and inorganic material (sulfate and chloride) contents were all within 

acceptable limits for extracts into water, acid, and base. The researchers tested the leachate from 

water extract that had contact with recycled glass. The pH increased from 7.65 prior to contact to 

7.93 after the contact, which indicated insignificant leaching potential from the glass samples. 

Disfani et al. (2011) obtained a pH value of 9.9 from the leachate of FRG, which is not 

designated hazardous by the Federal regulatory limits since it falls within the range of 2.0 to 12.5 

(inclusive). Thus, RGA leachates are not considered hazardous.

According to the Environmental Protection Authority (EPA) Victoria (Australia)-specified 

limits, all the metals in the recycled glass samples are well below the threshold limits except iron 

(EPA Victoria 2007). Iron content in normal water extracts remained well under acceptable 

limits. The inorganic content (chloride and sulfate) was also within acceptable limits.

LCA STUDIES

The largest market for recycled glass is using glass cullet to produce new glass products and 

containers. Several studies have focused on the LCA of using glass cullet in the glass 

manufacturing industry to offset the need to mine new raw materials for making glass. The 

researchers found no studies in the literature for the lifecycle impacts of using glass cullet in 

geotechnical highway applications since this application has not yet gained good momentum, 

especially in the United States. The lack of glass cullet use is also clear from the limited 

applications and case studies found related to recycled glass use in embankments, slope stability, 

and backfill applications. If glass cullet is used in such applications, it is normally blended in 

small quantities with other materials.

The literature review revealed that recycling glass for highway geotechnical applications is more 

environmentally friendly and sustainable than recycling glass for other applications, particularly 

recycling glass for making new glass containers (Buczynski 2010; Strategic Materials 2023; 

Friends of Glass 2023. The main reason is that the glass recycled to make glass cullet for 

highway geotechnical applications does not need to be sorted into different colors or types, 

which saves energy. Additionally, some glass types that cannot be recycled for use as raw 

material in glassmaking can still be recycled into finely ground sand for highway pavement 

applications. Nevertheless, these impacts have not been quantified through comparative LCA 

studies. EPDs from recycling facilities in the United States or worldwide are not publicly 

available. Conducting such environmental studies remains a future need for promoting the use of 

glass cullet and getting the technology ready as a sustainable and relatively inexpensive material 

for highway geotechnical applications.
85

CASE STUDIES 

The following case studies included research publications and websites documenting the use of 

glass cullet/RGAs in various geotechnical highway applications. Other than the stated 

information, no further details were mentioned on the performance trends related to glass cullet 

or RGA applications:

• “Glassphalt” was used in the 1970s to pave the streets around Trump Tower in New York 

City. A combination of asphalt and crushed glass, glassphalt produced pavement that 

glistened and glowed in the light. The roads are still in good condition. (Mike’s Trucking 

2022).

• Fairfax County, VA, used RGAs as a utility fill over sewer pipes in 2019, marking the 

first time Fairfax County used crushed glass in a construction project (American Public 

Works Association 2022). The pipe is a 175-ft-long sanitary sewer pipe made of ductile 

iron encased in steel. The conventional approach to stabilize the pipe in place is to put the 

pipe on a 6-inch-deep bed and cover it with 4 inches of crushed stone before backfilling 

soil into the trench. In the Fairfax County project, the pipe rested on 525 tons of crushed 

glass. The Fairfax County Department of Public Works and Environmental Services 

anticipates that the crushed glass will support the sanitary sewer pipe for decades. 

• A feasibility study was conducted in Ohio to use crushed recycled glass in the 

construction of local roadways (Tao 2017). The study was initiated to assess the 

feasibility of using RGA in local roadway construction. The study concluded that glass 

processors and material recycling facilities (MRFs) in Ohio had a strong interest in using 

recycled glass in roadway construction, but the projected annual supply of recycled glass 

was not sufficient for wide-scale use, given other competitive applications in the State. 

The study recommended that leadership from ODOT and other agencies was crucial to 

encourage better coordination of MRFs, glass suppliers, and end user. 

• The City of Devine, TX, used an 80/20 blend of crushed limestone and glass cullet to 

construct a flexible base (Davio n.d.). A local company supplied 440 tons of waste glass. 

The glass and limestone were blended in the proper ratio and crushed before construction. 

The mix conformed to Texas DOT (TxDOT) specifications. Table 38 presents the 

geotechnical properties of the limestone and glass blends. The project experienced 

premature cracking due to unusually high and heavy truck traffic, exceeding three to four 

times the design traffic, due to nearby road closure. 

• The TxDOT Abilene District also carried out a project using recycled glass in a flexible 

base (Davio n.d.). Glass cullet was mixed with conventional crushed limestone at the 

jobsite. The May 1997 project involved widening a two-lane road to a five-lane road. 

Construction involved spreading 12 inches of crushed limestone and glass cullet. A 

recycler mixed the two materials prior to compaction, and then a 1.5-inch-thick HMA 

was placed on top. The eastern section of the road used a 10-percent glass cullet, while 

the western section used 15 percent. Dyess Air Force Base crushed the glass for this 

project at a cost of $9 per ton. The case study reported that the recycled glass in this 

project performed well, but no detailed test data were available. 
86

Table 38. Geotechnical properties of the limestone and glass blends for Texas projects 

(Davio n.d.). 

Test Limestone 

Limestone with 

10% Glass 

Limestone with 

15% Glass 

Sieve analysis (percent retained) 

1 3/4 inches 0 0 0

1 inch 12 17 17

7/8 inch 17 21 21

1/2 inch 34 44 44

3/8 inch 39 53 53

No. 4 55 68 68

No. 10 68 74 74

No. 40 81 84 84

Atterberg limits (percent) 

Liquid limit 22.2 30.2 23.5

PI 7.1 12.8 7.0

Standard Proctor 

Max. dry density (pcf) 134.0 136.8 135.4

Optimum moisture content (%) 7.8 7.3 6.3
87

CHAPTER 9. RCA 

INTRODUCTION AND BACKGROUND 

RCAs (figure 20), also termed recycled concrete materials (RCMs), have been used in the 

construction of highways for more than 70 yr (Cackler 2018). RCAs have been commonly used 

as pavement base and subbase layers and as embankment and shoulder materials. A more 

rigorous evaluation and assessment by industry experts and researchers determined that RCA can 

also replace VAs in concrete mixtures. Even with 70 yr of history, RCA’s use in highway 

applications is still limited. Two primary barriers to the widespread use of RCA exist. First, 

sufficient information and understanding of the properties of RCA are lacking, and second, the 

existing State specifications on the use of RCA can be rigid. FHWA, working with its 

stakeholders, has taken a leadership role in overcoming these barriers. FHWA has developed a 

series of webinars, technical briefs, guides, and reports to fill this knowledge gap. These 

collaborative efforts and increased technical information can help States revise specifications to 

allow RCA for compatible highway applications (Van Dam 2018). This chapter presents the 

current knowledge and state of practice on using RCA in geotechnical highway applications. 

© Charles Rondeau. 

A. RCA pile at a concrete yard. 

© 2019 Washington State DOT. 

B. Pile of RCA rubbelized to a uniform size (Creative Commons n.d.d). 

Figure 20. Photos. Recycled concrete pavements and concrete demolition waste. 
88

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using RCA in highway geotechnical applications include the following 

(Edil 2017; Melton and Kestler 2013; Specify Concrete 2019; Tigard Sand & Gravel 2019; ETM 

Recycling 2023): 

• Cost effective compared to conventional aggregates. 

• Environmental impacts (emissions) are less than conventional aggregates. 

• Energy savings from production are anticipated. 

• High internal friction angle and high strength are characteristic. 

• Nonplastic RCM is not susceptible to frost. 

• Temperature dependence is less compared to RAP. 

• Structurally reliable and safe for use as natural aggregate materials (debatable). 

Reported disadvantages include the following: 

• Not lightweight. 

• Source variability and presence of impurities. 

• High moisture absorption.

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

FHWA (2004) reported the following highway applications for RCA/RCM: 

• Base and subbase materials for roadways (most common use).

• Aggregate base. A common theme from Texas, Minnesota, and California was that RCA 

performed better than VA as an aggregate base. 

• Backfilling material. 

• MSE walls as a backfill material. 

• Slope protection and slope stability. 

• Riprap and erosion control applications (large-sized RCA). 

• Replacement of natural aggregates in HMA mixes. Used in Western Michigan but does 

not seem to be promising due to the high absorption of the RCA increasing binder 

demand and cost. 

• Replacement of natural aggregates in concrete mixes. Allowed in specifications 

established in Michigan, Minnesota, and Texas. Texas is the most advanced. 
89

MATERIAL CHARACTERIZATION

Physical Properties

RCA is typically highly angular. According to the American Concrete Pavement Association

(ACPA) (2008), RCA has the following characteristics compared to conventional construction 

aggregates: higher water absorption capacity, lower specific gravity, lower strength, and lower 

abrasion resistance. NCHRP Synthesis 435 by Stroup-Gardiner and Wattenberg-Komas (2013) 

compared RCA’s physical and chemical properties to that of a new VA material. Table 39

presents the range of properties and shows that RCA has a significantly higher water absorption 

capacity, lower specific gravity, and generally more variable properties due to source variability.

Melton and Kestler (2013) reported LA abrasion loss for RCA ranged from 20 to 45 percent for 

coarse aggregates, while the magnesium sulfate soundness loss was less than 4 percent for coarse 

RCA and less than 9 percent for fine RCA.

Table 39. Physical and chemical properties of RCA compared to VAs (Stroup-Gardiner 

and Wattenberg-Komas 2013).

Property New Aggregate RCA

Shape and texture Varies Angular with rough 

surface

Water absorption capacity (percent) 0.8–3.7 3.7–8.7

Specific gravity 2.4–2.9 2.1–2.4

LA abrasion (percent loss) 15–30 20–45

Sodium sulfate soundness (percent loss) 7–21 18–59

Magnesium sulfate soundness (percent

loss)

4–7 1–9

Chloride content (lb/yd3) 0–2 1–12

Based on samples obtained from eight States (i.e., California, Colorado, Michigan, Minnesota, 

New Jersey, Ohio, Texas, and Wisconsin), the RCA materials had more variable properties than 

RAP materials collected from different sites and States (Edil 2017). Table 40 presents the range 

of properties for gradation (e.g., percentage fines, referring to passing the No. 200 sieve size or 

smaller than 0.075 mm), absorption, mortar content, and classification.
90

Table 40. Physical properties of RCA—average and range values (Edil 2017).

Properties

RCA

Average (range)

Fines (percent) 5.05 (2.01–12.8)

Gravel (percent) 46.19 (32–69)

Cu 24.60 (8–45)

Specific gravity 2.31 (2.2–2.4)

Absorption (percent) 5.52 (5.5–6.9)

Mortar content (percent) 50 (37–65)

Classification

SP, GP, GW

A-1-a, A-1-b

Mechanical Properties

Melton and Kestler (2013) reported the CBR for RCA material ranges from 94 to 148 percent. 

Soleimanbeigi et al. (2016) reported the range of soil friction angles of RCA material compared 

to other coarse aggregate materials, including sand, gravel, and crushed stone (table 41). The 

ranges reported by Soleimanbeigi et al. were collected from multiple sources. Among the 

different materials, the friction angles of RCA, which range between 41 and 65 degrees, are

among the highest and are higher than those for RAP, which is mainly attributed to the higher 

angularity. Lastly, Edil (2017) reported the resilient modulus of RCA materials collected from 

three States: Colorado, Texas, and New Jersey. The resilient modulus values were quite similar 

and temperature-independent (figure 21) and were generally lower than the resilient moduli of 

RAP presented earlier.

Table 41. Ranges of friction angles for soils and coarse aggregate materials, including RCA 

and RAP (Soleimanbeigi et al. 2016).

Soil Type Range of Friction Angles (degrees)

Sand (well graded) 34.7–37.5

Gravel 38.4–45.9

Crushed stone 34.8–55.3

RCA 41.0–65.0

RAP 37.0–45.0

Foundry sand 31.0–44.0
91

© 2015 Soleimanbeigi. Modifications to colors and labels by FHWA. 

Figure 21. Graph. Resilient modulus properties of RCA materials collected from different 

States (Soleimanbeigi et al. 2015). 

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES 

The material properties that are usually of interest when using RCA materials in embankment 

applications are included in the following list (Chesner et al. 1997). The design requirements for 

RCA in embankments are the same as those for similar-sized soil-aggregate blends or 

conventional aggregates. The researchers found no comprehensive specifications covering the 

use of RCA and RCM as embankment or fill materials in the literature. However, the following 

information was documented for the properties of RCA: 

• Specific gravity is between 2.0 and 2.5, slightly lower than VAs. 

• Stability and strength are generally satisfied due to the high friction angles exceeding 40 

(table 41). For CBR, a range from 90 to 140 percent is acceptable, which is typically 

satisfied for the highly angular RCA materials. 

• Durability is proven (i.e., resistant to weathering and erosion). 

• Drainage characteristics are that the RCA material will be nonplastic and nonsusceptible 

to frost to maintain drainage. Coarse fractions of RCA are free draining. 

• Corrosivity is like those of steel and aluminum, due to RCA’s high alkalinity. 

“RCM has demonstrated satisfactory performance as an embankment or backfill material. Its use 

is covered by special provisions to specifications in a number of jurisdictions” (Chesner et al. 

1997). However, RCA materials used in embankments or backfills may not make the best use of 

the high-quality aggregates associated with RCA because the benefits are not fully taken 

advantage of in design since RCA is considered by many agencies as conventional aggregate for 

design purposes. RCA requires minimal processing to satisfy physical requirements for 

embankment/fill. The relatively lower compacted unit weight of RCA versus VAs results in 

higher yield (greater volume for the same weight), which makes it economically attractive to 
92

contractors. Further, onsite processing and recycling of RCM is likely to result in economic 

benefits due to reduced hauling costs (Chesner et al. 1997).

In addition, a survey of DOT practices in the United States by Tutumluer, Moaveni, and Qamhia

(2018) demonstrated that some States limit the use of RCA in highway applications. While the 

study focused primarily on traditional highway applications, i.e., mostly in pavement layers, only 

a few States (e.g., Pennsylvania) specifically mentioned using RCA as embankment or backfill 

material. Table 42 presents a summary of DOT responses.

Table 42. Survey of DOT responses for the restrictions on using RCA in highway 

applications (Tutumluer, Moaveni, and Qamhia 2018).

Transportation 

Agency Restrictions on Using RCA, Considering Quality Concerns

Arkansas Only used in unbound base courses.

British Columbia Only for base and subbase.

Florida Not permitted in new concrete pavement. RCA is not permitted in new 

asphalt pavement unless concrete is from a Florida DOT project.

Maryland TCLP may be required. Specific gravity and LA abrasion tests are 

performed routinely.

Nebraska Not used in PCC or AC.

New Jersey Allowed only in subbase.

North Carolina Limited to use in base applications. Not allowed to be used in concrete 

mixes.

Oklahoma Only as an unbound aggregate base course layer.

Pennsylvania Only used for subbase and fill.

Utah Not allowed for concrete use.

Virginia Not used as subbase or base when any subsurface drainage system is 

present except when cement stabilized.

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS

The same methods and equipment are used for RCA handling and storage as for conventional 

aggregates. Some jurisdictions in Canada (e.g., Ontario) may restrict stockpiling and placement

near water courses due to alkaline leachate possibilities (Chesner et al. 1997). To avoid 

segregations, Chesner et al. recommend stockpiling RCA using radial stackers and remixing 

using a front-end loader or bulldozer prior to load out.

Due to its high angularity, RCA usually requires the addition of water during the placement and 

compaction of RCA layers. For QC, the same procedures used for conventional aggregate are 

appropriate for RCA (Chesner et al. 1997).
93

AVAILABLE COST INFORMATION 

RCAs typically cost around $15 to $55 per ton in the United States, depending on the 

availability, processing, and added transport cost. Given RCA’s typical density, the final cost 

translates to $18 to $80 per cubic yard. The price also depends on the quality of the RCA 

(Civiconcepts 2022). 

ENVIRONMENTAL CONSIDERATIONS 

RCA contains water-soluble calcium hydroxide from the original cement hydration reaction. 

When water flows through RCA aggregates, some calcium hydroxide will dissolve in water. 

Eventually, it interacts with atmospheric CO2 to form calcium carbonate (CaCO3), hence 

precipitating out of solution and leaving deposits where the water flows. This reaction will result 

in issues if the precipitate clogs up pavement drainage systems, such as filter fabrics, drainage 

pipes, and outlets (Van Dam et al. 2011; ACPA 2009). In locations subject to wet conditions, 

tufa-like precipitates (CaCO3) associated with leachate may develop. Further, environmental 

considerations have focused on the leaching of high-pH solutions from concrete aggregates. The 

potential for pH and drainage issues led some States/jurisdictions to require that RCA stockpiles 

be separated a minimum distance from water sources (Melton and Kestler 2013). 

Melton and Kestler (2013) reported several case studies about using RCA aggregates in the 

United States: 

• ODOT research concluded that RCA as an aggregate base in low-lying or wet areas 

where alkaline runoff likely would occur could adversely affect the environment. 

• An Iowa report found that the high pH of the drainage water from RCA use could kill or 

impede grass growth at a drain outlet. 

• Texas completed research on using RCA in MSE berms. The study involved material 

characterization, pH measurements, and a usage evaluation. The study concluded that the 

pH and resistivity specifications for MSE wall backfill materials should be waived, and 

concrete structures that have suffered sulfate attack should not be crushed and used as 

backfill in MSE walls. Further, the study recommended that MSE walls with crushed 

concrete backfill include adequate drains and high-permittivity filter fabrics behind the 

wall to avoid potential drainage issues. 

Overall, the most encountered concerns among different agencies are often the high pH values 

and issues with leaching. Tutumluer, Moaveni, and Qamhia (2018) reported concerns about the 

number of fines from crushing and the possibility of alkali-silica reactions (ASR) (table 43). 
94

Table 43. Concerns for using RCA in highway applications—agency survey responses

(Tutumluer, Moaveni, and Qamhia 2018).

Agency

Description of Environmental and Performance 

Concerns

Alberta, British Columbia; Kentucky;

Maine; Pennsylvania; Tennessee; and 

Washington State

Concerned with potential leaching with RCA.

Arkansas Concerned with RCA in rigid or flexible pavement due 

to the number of fines from crushing.

Delaware Not allowing RCA in concrete because it may have an

ASR.

Florida and Maryland Higher pH of RCA if used in the same area as 

metallized pipe.

Michigan, Montana, and South 

Dakota

RCA cannot be used in ephemeral drainages or high

water table conditions.

Ohio

Leaching of high-pH water leaving the right-of-way 

from the use of RCA. The formation of tufa in 

underdrains due to the use of RCA.

LCA STUDIES

The vast majority of the LCA studies involving RCA calculate the environmental impacts of

RCA used in concrete mixes to justify using the recycled material for lower environmental 

impact and lower cost concrete mixes. The literature is shy on RCA used for embankments, 

backfill materials, and slope stability applications. More broadly, the literature lacks LCA studies 

for RCA used as an unbound fill material either by itself or blended with other VA or recycled 

aggregates. In the United States, there are no available PCR documents for RCA. EPDs from 

recycling yards and manufacturers are also not available publicly. This lack of information is

expected due to source variability and the low energy required to recycle concrete pavements and 

produce RCA. However, the lack of PCRs, EPDs, and comprehensive or comparative LCA 

studies for RCA in backfill applications is a research gap that requires attention and can be easily 

addressed since this material has been used in the United States for more than 70 yr. A multitude 

of data should be readily available on the production and fuel consumption rates involved in 

producing RCA materials and the fuel consumption of construction equipment used to construct 

layers with RCA. 
95

CASE STUDIES

According to Kim, Shin, and Cha (2013) and Gabr and Cameron (2012), RCA is mostly used in 

granular bases, embankments, sound barriers, and fills. Further, Van Dam et al. (2011) reported

that 65.5 percent of the RCA produced in the United States is used for aggregate bases, 

9.7 percent in asphalt concrete, and 6.5 percent in new concrete mixtures, while 7.6 percent is 

used as a fill material. Abundant case studies can also be found describing the use of RCA as an 

unbound or bound base/subbase material, which has been the largest application for RCA in the 

United States to date. RCA has been used in retaining wall backfill, slope stability, 

embankments, and pipe fill applications. However, the researchers did not find well-documented 

case studies that monitored and reported the performance trends of RCA in these applications.

97

CHAPTER 10. TDA 

INTRODUCTION AND BACKGROUND 

TDAs are made from shredded scrap tires and are used in a wide range of construction projects, 

such as retaining wall backfill, lightweight embankment fill, landslide stabilization, vibration 

mitigation, and various landfill applications. More than 500 million scrap tires are discarded in 

the United States every year (Edinçliler, Baykal, and Saygili 2010), but as little as 22 percent of 

these tires get recycled. Passenger car and truck tires are composed of synthetic rubber, fibers, 

and steel cords (El Naggar and Iranikhah 2021). Table 44 presents typical tire composition, and 

figure 22 illustrates the tire shredding procedure. 

Table 44. Typical passenger car and truck tire compositions (El Naggar and 

Iranikhah 2021). 

Material Passenger Car Tire Truck Tire

Natural rubber

(percent) 14 27

Synthetic rubber

(percent) 27 14

Carbon black (percent) 28 28

Steel (percent) 14–15 14–15

Fiber (percent) 16–17 16–17

Average weight (new) 

(kg) 11 —

—No data. 

© 2022 Michal Ďufina. Modifications by FHWA to use American English spelling. 

Figure 22. Illustration. Rubber shredding procedure and outputs (Creative 

Commons n.d.c).
98

Between 1994 and 1996, some expensive failures took place related to using TDA in highway 

applications. By 1996, reports from three projects that experienced catastrophic internal heating 

reactions stopped the use of TDA in pavement projects in the United States. Two of these 

projects were in Washington State and one was a retaining wall project in Colorado (Patenaude 

and Wright 2017). In 1997, a partnership between government and industry formed an ad hoc 

civil engineering committee to establish new design guidelines for TDA. The committee 

investigated the failures and established guidelines to minimize TDA internal heating in backfill 

applications. As a consequence, TDA has been used in highway applications since then. 

ADVANTAGES AND DISADVANTAGES 

Reported advantages of using TDA in highway applications include the following (Edinçliler, 

Baykal, and Saygili 2010; Liberty Tire Recycling 2022; CalRecycle 2023): 

• Cost effective: Less expensive than other lightweight fill materials. 

• Well performing: Lightweight and free-draining characteristics help solve engineering 

problems effectively. 

• Environmentally friendly: TDA reduces the need for mined resources such as pumice and 

gravel. 

• No special equipment needed.

• Easy to transport and handle: Transported by trucks to a project in a 40-ft walking-floor 

trailer, which makes it easy to unload the TDA material. Once delivered to the site, TDA 

is spread and compacted by standard earthwork equipment. 

• Vibration dampening. 

• Viable reuse alternative to the millions of waste tires currently disposed of in landfills. 

Reported disadvantages of TDA include that internal heating reactions may cause catastrophic 

failures (CalRecycle n.d.). New regulations overcome this issue as long as the TDA is 

constructed according to the guidelines. 

REPORTED GEOTECHNICAL APPLICATIONS FOR HIGHWAYS 

TDAs can be used in the following geotechnical applications for highways (CalRecycle 2023): 

• Retaining wall backfill. 

• Lightweight embankment fill. 

• Landslide stabilization. 

• Vibration mitigation. 

• Landfill applications. 
99

MATERIAL CHARACTERIZATION

Physical Properties

ASTM D6270 is the standard specification that regulates the size of TDA and other physical 

properties based on application (ASTM 2020). ASTM D6270 divides TDA into Types A and B. 

Type A is suitable for drainage, vibration damping, and insulation applications, while Type B is 

larger, produced by a shearing process (not a hammer mill), and is suitable for embankment fill, 

wall backfill, and landfill drainage applications. Table 45 and table 46 compare the properties of 

both types of TDA.

Table 45. Grain size distributions for Type A and B TDAs (Cheng 2016; ASTM 2020).

Sieve Opening (mm)

Sieve Opening 

(inches)

Type A 

Specification

Requirements 

(percent passing)

Type B Specification

Requirements 

(percent passing)

450 18 100 100

300 12 100 100

200 8 100 75–100

100 4 100 —

75 3 95–100 0–85

38 1.5 0–70 0–25

4.75 0.187 (No. 4) 0–5 0–1

Pan Pan 0 0

—No data.

Table 46. Specifications for Type A and B TDAs (Cheng 2016; ASTM 2020).

Property

Type A 

Specification

Requirements 

(percent passing)

Type B Specification

Requirements 

(percent passing)

Free steel 1 maximum 1 maximum

Longest shred (inches) 10 18

Percent weight of shred > 12 inches — 16 maximum

Sidewall shreds (each) 0 0

Shreds > 2 inches, wire exposed 10 maximum 10 maximum

Shreds > 1 inch, wire exposed 25 maximum 25 maximum

—No data.

Mechanical Properties

Studies have investigated the mechanical properties of TDAs used in highway geotechnical 

applications. El Naggar and Iranikhah (2021) looked at the optimum TDA percentage to 

maximize strength/performance, and El Naggar, Zahran, and Moussa (2021) investigated the 

effect of TDA particle size on mechanical properties. A third study investigated the effect of 
100

shear box size on the calculated shear strength properties (Zahran 2021). This section 

summarizes some of the essential technical content from these research efforts.

El Naggar and Iranikhah (2021) investigated the effect of the percentage of TDA in aggregate 

and soil mixes. The researchers mixed TDA with gravelly, sandy, and clayey soils to determine 

the optimum soil-TDA mixtures for each soil type. The researchers used a large-scale direct 

shear box (12 inches by 12 inches by 8.7 inches) and examined the mixtures for mechanical 

behavior trends through a series of DSTs conducted at confining pressures of 7.3, 14.3, and 

28.5 psi. Table 47 presents the results. For the gravel-TDA mixes, the optimum TDA percentage 

that maximizes the shear strength properties is 0–10 percent. On the other hand, the optimum 

TDA percentage for TDA mixed with sand and clay is 10–25 percent.

Table 47. Optimum TDA percentages in aggregate and soil mixes (El Naggar and 

Iranikhah 2021).

Mixtures

Mixture

ID

TDA by 

Weight 

(percent)

Soil by 

Weight 

(percent)

Bulk Density, 

γbulk (kN/m3)

Friction 

Angle 

(degrees)

Cohesion 

(kPa)

Gravel—TDA

GT0 0 100 127.4 44.0 3.6

GT10 10 90 118.1 45.4 2.5

GT25 25 75 94.9 43.9 2.1

GT50 50 50 65.6 42.2 2.2

GT100 100 0 43.7 23.9 2.6

Sand—TDA

ST0 0 100 120.3 37.1 0.7

ST10 10 90 113.8 38.4 1.9

ST25 25 75 101.0 38.3 2.1

ST50 50 50 77.5 31.8 2.3

ST100 100 0 43.7 23.9 2.6

Clay—TDA

CT0 0 100 131.0 18.8 3.2

CT10 10 90 121.6 32.3 4.2

CT25 25 75 98.5 25.6 4.2

CT50 50 50 70.7 25.0 2.8

CT100 100 0 43.7 23.9 2.6

El Naggar, Zahran, and Moussa (2021), who conducted shear box tests using a large-scale direct 

shear machine, studied the effect of TDA particle size on the shear properties. The researchers 

conducted the tests under three normal stresses: 49.4, 97.6, and 196 kPa (7.3, 14.3, and 28.5 psi)

using a constant shearing rate of 0.5 mm/min (0.02 inches/min). The researchers found the angle 

of internal friction of TDA to increase as the maximum particle size (Dmax) increased from 

19 mm (0.75 inches) to 102 mm (4.0 inches). The cohesion from the interlocking among the 

TDA particles was not significantly affected by the particle size (the difference was less than 

0.5 psi). Further, all Type A TDA samples exhibited contractive behavior. The smaller TDA 

aggregates were more compressible than the relatively larger TDA aggregates. Figure 23 shows 

the effect of particle size on shear stress and secant shear modulus.
101

© 2021. El Naggar, Zahran, and Moussa. Modifications 

by FHWA to color and symbols. 

A. Normal stress versus shear stress. 

© 2021 El Naggar, Zahran, and Moussa. Modifications by 

FHWA to bar patterns for accessibility. 

B. Normal stress versus secant shear modulus. 

Figure 23. Graphs. Effect of TDA particle size on shear strength and secant shear modulus 

(El Naggar, Zahran, and Moussa 2021). 

Another recent study conducted by Zahran (2021) investigated the effect of shear box size on the 

shear strength properties of TDA. The researchers used five shear boxes with different box sizes 

and aspect ratios, as summarized in table 48. Table 49 presents the properties of the 1.5-inch-mix 

TDA used for all studies, and the calculated shear strength properties are listed and summarized 

in figure 24. According to these results, the smaller shear box sizes can measure slightly higher 

friction angles. However, no clear trend was found for the cohesion/cohesion intercept.
102

Table 48. Summary of shear box sizes and aspect ratios (Zahran 2021). 

Dimensions 

Shear Box 

1 2 3 4 5

Length (mm) 305 225 150 100 60

Width (W) (mm) 305 225 150 100 60

Height (H) (mm)—

lower part 80 80 80 18 18

H (mm)—upper part 130 130 130 25 25

W/H aspect ratio 1.45 1.07 0.71 2.33 1.40

W/Dmax aspect ratio 8 6 4 2.6 1.6

Table 49. Summary of the TDA gradation parameters and grain size distribution 

(Zahran 2021). 

Gradation Parameter Value 

D10 (mm (inches)) 12 (0.47) 

D30 (mm (inches)) 15.5 (0.61) 

D50 (mm (inches)) 24.5 (0.96) 

D60 (mm (inches)) 27 (1.06) 

Cc 2.25

Cu 0.74

© 2021 Zahran. Modifications by FHWA to color and symbols. 

A. Normal stress versus shear stress. 
103

© 2021 Zahran. Modifications by FHWA to colors and symbols. 

B. Effect of shear box size on shear properties. 

Figure 24. Graphs. Effect of shear box size on the shear strength properties of TDA 

(Zahran 2021). 

DESIGN REQUIREMENTS AND AVAILABLE GUIDELINES 

ASTM D6270, entitled Standard Practice for Use of Scrap Tires in Civil Engineering 

Applications, provides general guidelines for using TDA in pavement geotechnical applications 

(ASTM 2020). Construction requirements and guidelines are fully developed, but, to date, no 

comprehensive design guidelines or detailed design requirements for TDAs have been published 

in the United States. 

BACKFILL PLACEMENT AND CONSTRUCTION SPECIFICATIONS 

ASTM D6270 indicates that TDA should be covered with sufficient soil thickness to limit 

overlying pavement deflections (ASTM 2020). Soil thicknesses of 2.6 ft or higher are suitable 

for light traffic scenarios. Further, TDA layers should not be more than 10 ft thick. Multiple 

TDA layers should have a minimum 3-ft separation layer of nonorganic soil. Lastly, a TDA layer 

should be wrapped completely in a layer of nonwoven or woven geotextile to minimize the 

infiltration of soil particles. Including this layer is essential for pavement, drainage, and retaining 

wall backfill applications. 

Certain specifications are recommended to ensure TDAs are placed and compacted properly. 

Such specifications/guidelines include the following (Patenaude and Wright 2017):

• Tire shreds should not contain any contaminants (e.g., oil, grease, gasoline, or diesel fuel, 

among other chemical substances).

• Tire shreds should not contain fragments of wood, wood chips, or any other fibrous 

organic matter and should not contain loose wire or metal fragments. 
104

• TDA material should be spread using track-mounted equipment.

• TDA lifts should be compacted using a minimum of six complete coverage passes by a 

vibratory smooth drum steel roller imposing a minimum static weight of 10 tons. 

AVAILABLE COST INFORMATION 

El Naggar and Iranikhah (2021) presented the cost of producing TDA by shredding tires 

(table 50). According to the data, the cost is lowest for larger shreds exceeding 2 inches in 

particle size but gets increasingly higher for smaller shred sizes. The process rate is also faster 

for the larger shred sizes. 

Table 50. Cost for shredding tires (El Naggar and Iranikhah 2021).

Particle Size Cost Per Ton ($) Process Rate (tons/h)

50 mm 12 10–12

<50 mm 31 7

<12.5 mm 31–68 2–3

Additionally, a third party performed a cost-benefit assessment for California’s Department of 

Resources Recycling and Recovery in 2015 for TDA use in geotechnical highway applications. 

The study evaluated six civil engineering projects that used TDA in California for four different 

applications: embankment fill projects, landfill applications, landslide repair projects, and 

light-rail vibration mitigation projects (Cheng 2016). Table 51 presents the results and lists the 

costs associated with using TDA, which are by far the least expensive among the alternative 

backfill materials. The total costs include material and transportation costs but exclude 

installation costs and the contractor’s overhead and profit. 

Table 51. Cost comparison between TDA and other fill materials for different applications 

(Cheng 2016). 

Application Location 

Material 

Option Material Total Cost ($) 

TDA 

Cost (%) 

Embankment 

Hwy 101, 

Mendocino 

County, CA

Traditional 

fill option Soil 316,358 169

Embankment Mendocino 

County, CA

Lightweight 

fill option 

Pumice rock 514,354 274

EPS 643,539 343

Expanded shale 

clay 632,716 337

Wood chips 307,138 164

Type B TDA 187,500 100

Embankment Dixon Road, 

Milpitas, CA

Traditional 

fill option Soil 562,864 169

Lightweight 

fill option 

Pumice rock 632,726 190

EPS 1,144,984 343
105

Application Location 

Material 

Option Material Total Cost ($) 

TDA 

Cost (%) 

Embankment Milpitas, CA

Expanded shale 

clay 489,829 147

Wood chips 545,226 163

Type B TDA 333,600 100

Landfill Riverside

County, CA

Traditional 

trench Crushed gravel 19,102 139

Landfill Riverside

County, CA TDA material TDA Type A 13,750 100

Landfill Sacramento 

County, CA

Traditional 

trench Crushed gravel 295,343 141

Landfill Sacramento 

County, CA TDA material TDA Type A 209,585 100

Slide repair Mendocino 

County, CA

Traditional 

fill option 2 Soil 937,612 492

Slide repair Mendocino 

County, CA

TDA material 

option 

Type B TDA 

with soil layers 190,555 100

Vibration 

attenuation 

Santa Clara 

VTA, CA

−15 dB for 

14–17 Hz

Floating concrete 

slabs 2,115,000 2951

Vibration 

attenuation 

Santa Clara 

VTA, CA

−10 dB for 

>16 Hz TDA 71,667 100

VTA = Valley Transportation Authority. 

ENVIRONMENTAL CONSIDERATIONS 

Laboratory and field testing have shown that TDA is not a hazardous waste based on TCLP 

standards. Low levels of metals and various organic compounds were detected in leachate that 

contacted the TDA, and these levels were below applicable water quality thresholds (Cheng 

2016). Chronic toxicity testing showed no adverse effects from elevated levels of iron and 

manganese in the leachate from TDA fill above the water table based on leachates collected from 

two fill sites in Maine after approximately 10 yr in place. A toxicity effect was found in leachate 

from a TDA fill that was below groundwater level, but the metals quickly formed immobile and 

insoluble particles in the subsurface soil (Sheehan et al. 2006). Field testing additionally showed 

that the concentrations of metals within a TDA fill were effectively attenuated within a few feet 

of soil. 

LCA STUDIES 

No EPDs or PCRs were found for TDAs in the United States or worldwide. Further, no 

comprehensive LCA studies exist in the United States. However, a journal paper by Corti and 

Lombardi (2004) evaluated the emissions at the end of life of tires (i.e., tire shredding and 

pulverization) for tire waste in Italy. The researchers considered two processes, a mechanical 

pulverization process and a cryogenic pulverization process. Mechanical pulverization involves 

three steps (grinding, crushing, and pulverization) of mechanical size reduction up to 0.04-inch

shred sizes. Cryogenic pulverization (freeze-milling) produces a higher quantity and quality of 
106

recycled rubber. Since highway geotechnical applications require relatively large shred sizes, all 

pulverization processes (i.e., fine grinding) were omitted from the reported environmental 

impacts. Thus, for mechanical pulverization, only the first two steps of grinding and crushing 

were included, and cryogenic pulverization was completely excluded. The assumed size for the 

tire shreds after the crushing process is 0.63 inches by 0.63 inches (16 mm). Corti and Lombardi 

(2004) reported that the pulverization process consumes around 50 percent of the energy for tire 

shredding. The analysis was performed for 1,000 kg of tires (table 52 and table 53 for the inputs 

and outputs of the process). To reasonably compare the results with the other lightweight 

materials presented in this report, a density of 45 pcf equals 720 kg/m3 was assumed for the 

TDAs, and the environmental impacts were reported with a functional unit of 1.0 m3 of crushed 

tire shreds (table 54).

Table 52. Inputs and outputs of the tire grinding process (Corti and Lombardi 2004).

Input Output Amount

Tires — 1,000 kg

Electricity — 170 MJ

Water — 150 kg

Steel — 0.230 kg

Oil — 0.011 kg

— Ground tires 966 kg

— Iron scrap 34 kg

—Not applicable.

Table 53. Inputs and outputs of the tire shredding process (Corti and Lombardi 2004).

Input Output Amount

Ground tires — 1,000 kg

Electricity — 573 MJ

Steel — 0.010 kg

— Crushed tires (16×16 mm) 750 kg

— Iron scrap 250 kg

—Not applicable.

Table 54. Summary of environmental impacts from grinding and crushing 1.0 m3 of tires.

Environmental 

Effect Units

Impacts from Mechanical Crushing and 

Grinding

Greenhouse effect kg CO2 4.39E+01

Ozone layer 

depletion g CFC11 1.51E-02

Acidification kg SO2 1.71E-01

Eutrophication g PO4 3.60E-01
107

CASE STUDIES 

The following list includes projects that used TDA for various geotechnical highway applications 

in the United States, as documented by Igwe (2021). 

• Riverside, CA, 2007: TDA was used as a lightweight fill material to exert less pressure 

behind the retaining wall 207 project, which was constructed to aid the widening of the 

I-215/Route 60/Route 90 freeway interchange (Cheng 2016). TDA served as both the 

backfill and the drainage material. 

• Sonoma County, CA, 2008: TDA was applied as the subgrade layer for Geysers Road, 

providing a lightweight fill to repair the landslide failure caused by the saturation of soil 

backfill during periods of heavy rain (Cheng 2016).

• Indiana, 2008: A mixture of TDA and soil was used as lightweight backfill for the 

widening of SR–110 in Marshall County and SR–19 in Elkhart County, where the grade 

needed to be raised over peat to minimize soil settlement (Hoppe and Oman 2013). 

• Mankato, MN, 2010: TDA was used as a lightweight fill to stabilize the peat under the 

embankment area of the Blue Earth County Road 12 bridge, which sustained cracks after 

construction (Hoppe and Oman 2013). 

California implemented a long list of successful projects using TDA (Patenaude and Wright, 

2017): 

• Dixon Landing Interchange, 2001 (first TDA project). 

• Highway 215 and Route 91 retaining wall research, 2003–2007. Joint project with 

Caltrans. 

• Valley Transit Authority vibration mitigation, 2004. 

• Marina Drive, Mendocino County landslide repair, 2007. 

• Riverside County landfill gas collection system pilot projects, 2008. 

• Caltrans Confusion Hill lightweight fill embankment, 2008. 

• Sonoma County, Geysers Road landslide repair, 2008. 

• Sonoma Mountain Road landslide repair, 2009. 

• Sacramento County, Keifer Landfill leachate recirculation, 2009.

• Santa Barbara County, Palomino Road slide repair, 2010. 

The following list includes projects described by Igwe (2021) that successfully used TDA for 

various geotechnical highway applications in Canada: 
108

• Winnipeg, Manitoba, 2000: TDA was used to build a 305-m- (1,000-ft)-long 

embankment above soft ground to act as a subgrade thermal insulator to limit frost 

penetration and prevent road damage (Hoppe and Oman 2013).

• New Brunswick, 2007: TDA was used as a lightweight fill in the first stage of 

construction of the Route 1 highway embankment after it collapsed due to underlying soft 

marine clay (Hoppe and Oman 2013). TDA helped reinforce the holding capacity of the 

foundation soil. 

• Stettler County, Alberta, 2013: The Alberta Recycling Management Authority and the 

County of Stettler used TDA as a lightweight fill in place of conventional clay fills for a 

section of Range Road 184 between Township Road 360 and 361 (Igwe 2021). TDA’s 

elasticity resulted in a less rigid fill that still provided the required structural strength. 

• Edmonton, Alberta, 2013: TDA was used as a fill to construct an 80-m- (262.5-ft)-long 

embankment connecting the Anthony Henday Drive ring road to the Edmonton Waste 

Management Centre (Meles 2014). TDA was chosen because of its light weight and 

greater permeability. 

• Halifax, Nova Scotia, 2016: TDA was used to build a high-traffic off-ramp for the 

Ragged Lake Transit Centre, saving the municipality up to $140,000 (Moore 2016). TDA 

was applied because of its effectiveness with high groundwater table and poor soil 

conditions (Igwe 2021). 
109

CHAPTER 11. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS 

SUMMARY OF FINDINGS 

This report presents findings on the potential use of alternative lightweight and recycled 

materials in geotechnical highway applications, including retaining walls, MSE walls, 

embankments, fills behind bridge abutments, pipe/culvert backfills, and ground improvements. 

Such materials may be advantageous compared to more conventional structural backfills to 

combat design challenges and reduce costs, construction time, surcharge loads, and 

environmental impacts. 

The researchers studied nine lightweight and alternative backfill materials in detail: CLSM, 

ESCS, FGA, LCC, polystyrene geofoams, RAP, RCA, RGA, and TDA. The researchers covered 

the following aspects of each material when information was available: 

• Geotechnical highway applications in the literature. 

• Advantages and disadvantages of using each material in backfill applications.

• Literature on material characterization.

• Design requirements. 

• Design guidelines. 

• Placement and construction specifications. 

• Cost information. 

• Environmental considerations. 

• LCA studies.

• Case studies and performance records. 

The report summarized the material characteristics in detail. Table 55 summarizes the key 

aspects of each investigated material that make it a suitable fill material, along with the friction 

angle and density. Six of the materials are considered lightweight alternatives (CLSM, ESCS, 

FGA, LCC, geofoams, and TDA), while the other three (RAP, RCA, and RGA) are recycled, 

low-cost alternatives. 
110

Table 55. Key advantages of using alternative backfill materials in geotechnical 

applications.

Lightweight/

Alternative 

Material

Friction 

Angle

(degrees)

Density

(kN/m3) Key Advantages as a Backfill Material

CLSM 25–55 14.14–15.71

Lightweight, easily excavated, fast strength 

gain, does not require compaction, suitable 

for complex geometries

ESCS 35–53 5.88–10.20

Lightweight, chemically inert, high 

durability, high friction angle, controlled 

gradation

FGA 27–56 1.57–3.92

Lightweight, high friction angle, inert 

(nonflammable and UV stable, low decay), 

requires no special equipment, can be 

installed in any weather

LCC — 3.14–9.42

Lightweight, high workability, does not 

require compaction, suitable for complex 

geometries

Polystyrene 

geofoams — 0.11–0.47

Extremely lightweight, thermal insulator, can 

be installed in any weather, fast construction, 

and low labor cost

RAP 39–54 15.69–19.61 Nonplastic, free draining, not susceptible to 

frost

RCA 41–65 16.67–20.10 High strength and friction angle, nonplastic, 

not susceptible to frost

RGA 37–62 17.28–18.85

Inexpensive to produce/recycle, high friction 

angle, can be installed in any weather, easy 

to compact (good workability), requires no 

special equipment

TDA 24–45 3.77–8.83

Lightweight, inexpensive to produce, free 

draining, requires no special equipment, 

vibration-dampening characteristics

—No data.

ALTERNATIVE LIGHTWEIGHT/BACKFILL MATERIALS USED IN 

GEOTECHNICAL HIGHWAY APPLICATIONS

The researchers concluded that all the recycled and lightweight materials investigated in this 

study have already been used (or can be successfully used) for the target geotechnical highway 

applications (i.e., bridge abutments, embankments, retaining wall fills, pipe backfills, and ground 

improvements) without major reported issues or early failures. For any design alternative, the 

design engineer should pay special attention to the design and construction requirement details of 

each material separately in their prescriptive environment while accounting for the changes in

physical properties, such as the improvement/changes in shear strength and angle of internal 

friction, and the weight/density of the fill. Further, closely following the proper construction 

guidelines and practices with the correct construction equipment at the specified lift thickness is 
111

paramount. Table 56 lists applications for the lightweight/recycled materials that the literature 

reports have been used successfully. If a material is not listed for a certain application, it might 

mean that it has not been attempted for that application or that the researchers did not find a 

documented case study for its use in the perspective application. 

Table 56. Alternative backfill materials and reported geotechnical applications. 

Material/Application 

Retaining Walls Embankments Bridge Abutment Fill/Bridge Approach embankment Pipe/Culvert Covers and Backfills Slope Stabilization Ground Improvements 

CLSM X X X X — X 

ESCS X X X X X X 

FGA X X X X — X 

LCC X X X X X X 

Polystyrene geofoams X X X X X X 

RAP X X — X X X 

RCA X X — X X X 

RGA — — — X — X 

TDA X X X X X X 

—No data. 

CONCLUSIONS 

Table 57 identifies and summarizes the technology-ready aspects of each alternative lightweight 

and recycled material discussed in this report based on the comprehensive literature review. 

Overall, all these materials have been used in full-scale field construction projects as an 

alternative to conventional backfills. All of these alternative backfill materials are placed at a 

TRL of 8, according to FHWA’s Technology Readiness Level Guidebook (Towery, Machek, and 

Thomas 2017). A TRL of 8 requires that the following questions be fully answered and the 

requirements met by each alternative lightweight or recycled material discussed in this report for 

use in geotechnical highway applications: 

• Are all system components form-, fit-, and function-compatible with each other and with 

the operational environment?

• Is the technology proven in an operational environment (i.e., meets target performance 

measures)? 
112

• Was a rigorous test and evaluation process completed successfully?

• Does the technology meet its stated purpose and functionality as designed? 

Note that despite all the materials having a TRL of 8, the literature search made clear that some 

materials presently have a higher market penetration and are more commonly used in the United 

States for geotechnical applications. For example, TDAs and CLSMs have comprehensive field 

application data for most of the aspects of interest in this report, while some materials, such as 

RGA, lack comprehensive information with very few case studies. One major area where 

information is missing for most of the materials (except ESCS and FGAs) is the presence of 

EPDs to summarize the environmental impacts and energy demands for commercial products. In 

fact, the only PCR available in the United States is for ESCS, but no publicly available EPDs or 

comprehensive LCA studies exist for any of the materials used in geotechnical applications. 

Table 57. Technology readiness for the studied alternative backfill materials. 

Material Technology-Ready Aspects 

CLSM 

• Extensive studies have been done on laboratory characterization, 

including using recycled and byproduct materials to reduce cost. 

• ASTM standards exist for QA/QC and postconstruction practices. 

ESCS 

• ESCSI published a PCR (expired in 2021). EPDs are available for 

some products in Europe. 

• Promising (well-performing) case studies are available, with 

abundant examples in the literature. 

• Construction procedures seem to be well developed; no special 

equipment is needed. 

• Physical and mechanical properties are well understood and well 

cited in the literature. 

• ASTM/AASHTO standards used for conventional aggregates can 

be extended to ESCS. 

FGAs 

• PCR and EPDs are available from Europe. 

• Case studies in the United States and worldwide are available 

(and promising). 

• The number of States in the United States using/accepting use is 

on the rise. 

LCC

• Construction procedures seem to be well developed.

• ASTM standards are available to control foaming material 

properties and requirements. 

• Promising (well-performing) case studies are available. 

Polystyrene 

geofoams 

• Design guidelines are available for various geotechnical 

applications. 

• Promising case studies, especially using polystyrene geofoams as 

an insulating material, are available. 

RAP

• RAP is heavily used for highway applications. Most DOTs and 

agencies have had good experience with RAP as a construction 

material. 
113

Material Technology-Ready Aspects 

RCA

• RCA is heavily used for highway applications. Most DOTs and 

agencies have had good experience with RCA as a construction 

material. 

RGA

• Laboratory testing techniques have been well developed, and 

abundant testing results are available. 

• Special requirements for construction are minimal. RGA does not 

need special equipment. 

TDA 

• Documented case studies are abundant. 

• Laboratory characterization is well documented. 

• Special equipment for construction and handling is not needed. 

• Cost saving is well documented from several projects in the 

United States. 

KNOWLEDGE GAPS AND BARRIERS TO WIDESPREAD USE 

Table 58 summarizes several knowledge gaps pertaining to the use of recycled and lightweight 

alternative backfill materials for geotechnical highway applications. For all the alternative 

backfills, barriers to widespread use are mainly a result of concerns about long-term 

performance, given a lack of local experience. The following section recommends research 

needed to adequately address these knowledge gaps and promote more successful use of these 

alternative backfill materials in the United States. 

A clear understanding of the key geotechnical properties and design considerations of each 

material (compared to conventional fills) needs to be fully studied to promote more use of the 

investigated materials in the United States. While the researchers found information about 

geotechnical properties and design considerations, to a large extent, the information was from 

laboratory studies. Quantitative information from full-scale field testing or actual construction 

projects was scarce. The researchers found few long-term performance records, especially 

comparative performance to conventional fill materials. The specifications of most U.S. DOTs 

and agencies are performance-based rather than material-based. Therefore, a full understanding 

of the long-term performance and better knowledge about the key design aspects and material 

properties, including consistent and standardized procedures to characterize these unique 

materials across different laboratories, will increase understanding of these materials’ 

performance and promote more successful use in highway geotechnical applications. The lack of 

knowledge about long-term performance and standardized laboratory test procedures are major 

barriers to implementation and gaps in knowledge that need to be addressed for lightweight 

backfill materials, particularly for emerging products, in order to disseminate their full potential 

and promote increased use. 
114

Table 58. Identified barriers to deployment of the studied alternative backfill materials. 

Material Barriers to Implementation and Gaps in Knowledge

CLSM 

• The initial cost of the material is relatively high. However, 

lower labor costs can make it cost effective. 

• Special construction requirements need special equipment and 

skilled labor.

• Postconstruction testing (e.g., QA/QC) is not fully developed 

and can be challenging. 

• Few documented case studies exist for backfill applications. 

• No PCRs, EPDs, or LCAs are available in the United States or 

elsewhere. 

• Little/no data from long-term performance are available. 

ESCS 

• Production sites in the United States are limited—high hauling 

costs for some locations.

• No EPDs or LCAs are available in the United States. 

• Little/no data from long-term performance are available. 

FGAs 

• Few ASTM/AASHTO standards exist for laboratory testing. 

• Compaction method for laboratory testing is largely based on 

the Proctor method, methods that significantly alter gradations, 

or both. 

• PCR, EPDs, and LCAs are not available in the United States. 

• Some construction recommendations are available, but no solid 

construction guidelines are available. 

• Postconstruction testing is not fully developed. 

• Little/no data from long-term performance are available. 

LCC

• No approved or standard mix design procedure is available. 

• No approved density check/measurement procedure (except for 

plastic density) is available. 

• No PCR, EPD, or LCA was found in the literature. 

• Postconstruction testing is not fully developed; only UCS 

(strength) checks are performed. 

• Little/no data from long-term performance are available. 

Polystyrene 

geofoams 

• No published PCRs, EPDs, and LCAs exist for geofoams used 

in geotechnical applications.

• Not suitable for complex geometries. 

RGAs

• PCR, EPDs, and LCA studies are not available for geotechnical 

uses. 

• Postconstruction testing (e.g., QA/QC) is not fully developed. 

• Very few documented case studies are available for backfill 

applications. 

• Some States conducted feasibility studies and found they don’t 

have enough resources to mass-produce the material. Little/no 

data from long-term performance is available. 
115

Material Barriers to Implementation and Gaps in Knowledge

TDAs 

• Material can be used elsewhere as an energy source; market 

availability is questionable. 

• No PCRs, EPDs, or LCAs are available, especially in the 

United States or elsewhere. 

• Little/no data from long-term performance are available. 

RECOMMENDED FUTURE WORK AND INVESTIGATIONS 

Based on the findings and conclusions presented in table 58, the following research needs are 

suggested to promote more commonplace uses of lightweight and recycled alternative backfill 

materials to take advantage of these technologies and associated benefits in geotechnical 

highway applications (see table 59). These research needs are recommended based on the 

knowledge gaps identified from the comprehensive literature search and primarily aim to 

eliminate technology barriers and promote more sustainable construction practices while 

ensuring good performance. 

One major research need for all the lightweight and alternative backfill materials investigated in 

this report is to establish standardized test procedures to correctly characterize the materials 

across different laboratories in the United States and obtain key design aspects for the different 

highway geotechnical applications of interest. Developing the test procedures is a major task that 

is deemed essential for these materials to be characterized properly, reveal their full potential,

and increase their utilization and market penetration. Another related research need that this 

report touched on, but that needs to be studied more comprehensively, is performing a 

nationwide survey of the current state of practice of the investigated materials by the U.S. State 

DOTs. This research should include an understanding and compilation of what materials each 

agency uses or allows, what applications these materials are used for, and any 

performance/material specification each agency has (including what test procedures are followed 

and what are the key design aspects). The best way to gather this information is for researchers to 

contact the right personnel in each agency. Such detailed information is deemed necessary to 

better understand the current knowledge gaps and gear future research toward these pressing 

needs. Lastly, research is needed to investigate which current test methods are most suitable for 

the different alternative materials and what new test methods or modifications to the current test 

methods need to be proposed. 
116

Table 59. Proposed research needs for the studied alternative backfill materials. 

Material Proposed Research Needs 

CLSM 

• Developing an ASTM-sponsored PCR for conducting LCA 

and issuing PCRs for CLSM produced in the United States. 

• Conducting cost-benefit analyses of using CLSM for 

geotechnical highway applications. 

• Conducting pilot projects and collecting further data from 

construction projects to document case studies for using 

CLSM in highway fill applications. 

• Developing postconstruction inspection and testing methods 

for CLSM used in highway fill applications. 

ESCS 

• Conducting cost-benefit analyses of using ESCS for 

geotechnical highway applications. 

• Conducting laboratory testing of various ESCS materials 

produced in the United States to obtain design-specific 

parameters for the various backfill applications. 

FGAs 

• Evaluating best practices for compacting laboratory samples 

to test FGAs.

• Developing ASTM/AASHTO standards for testing FGA 

aggregates using a shear box and triaxial shear strength 

testing. 

• Developing an ASTM-sponsored PCR for conducting LCA 

and issuing PCRs for FGA materials produced in the United 

States. 

• Conducting cost-benefit analyses of FGA materials used for 

geotechnical highway applications. 

• Conducting laboratory testing of various FGA materials 

produced and sold in the United States for obtaining designspecific parameters for the various backfill applications. 

• Developing construction guidelines for using FGA in 

highway fill applications, including postconstruction QA/QC 

methods. 

LCC

• Developing an ASTM-sponsored PCR for conducting LCA 

and issuing PCRs for LCC produced in the United States. 

• Conducting cost-benefit analyses of using LCC for 

geotechnical highway applications. 

• Developing postconstruction inspection and testing methods 

for LCC used in highway fill applications. 

Polystyrene geofoams 

• Developing an ASTM-sponsored PCR for conducting LCA 

and issuing PCRs for geofoams produced in the United States. 

• Conducting cost-benefit analyses of using geofoams for 

geotechnical highway applications. 

• Examining best practice installation methods for geofoams to 

reduce buoyancy issues. 
117

Material Proposed Research Needs 

• Developing construction requirements for geofoams in 

highway fill applications to avoid buoyancy, including 

postconstruction inspection and testing methods. 

RGAs

• Developing an ASTM-sponsored PCR for conducting LCA 

and issuing PCRs for RGA materials produced in the United 

States. 

• Conducting cost-benefit analyses of RGA materials used for 

geotechnical highway applications. 

• Conducting pilot projects and better documentation of case 

studies for using RGAs in highway fill applications.

• Developing construction guidelines for using RGAs in 

highway fill applications, including postconstruction QA/QC 

methods. 

TDAs 

• Developing an ASTM-sponsored PCR for conducting LCA 

and issuing PCRs for TDA produced in the United States. 

• Conducting laboratory testing of TDA materials available in 

the United States for obtaining design-specific parameters for 

the various backfill applications. 

119

ACKNOWLEDGMENTS 

The map in figure 8 was modified. The original map is the property of Google Maps and can be 

accessed from https://www.google.com/maps (2022). The image location markers were added by 

the authors. 

121

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Recyclable

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(Washington, DC: 2024) https://doi.org/10.21949/1521442