TECHBRIEF Lightweight Concrete: 

Mechanical Properties

FHWA Publication No.: FHWA-HRT-13-061

FHWA Contact: Ben Graybeal, HRDI-40, (202) 493-3122, benjamin.

graybeal@dot.gov.

This document is a technical summary of the Federal Highway 

Administration report, Lightweight Concrete: Mechanical 

Properties (FHWA-HRT-13-062), available through the National 

Technical Information Service at www.ntis.gov.(1)

NTIS Accession No. of the report covered in this TechBrief: 

PB2013-107688

Objective

There is a limited amount of test data on the mechanical properties of high-strength lightweight concrete (LWC) with a concrete 

unit weight (wc) between that of traditional LWC and normal 

weight concrete (NWC). Concrete with a wc in this range is also 

not covered in the American Association of State Highway and 

Traffic Officials (AASHTO) Load-and-Resistance Factor Design 

(LRFD) Bridge Design Specifications.

(2) This research program 

includes a significant number of mechanical property tests on 

this type of concrete. The results from this research project are 

included into a LWC database that covers a range of wc to determine trends for LWC as a function of wc. New design expressions 

for mechanical properties are proposed for LWC as a function of 

wc as opposed to the more common method of using concrete 

constituent materials. The design expressions represent potential revisions to the AASHTO LRFD Bridge Design Specifications

relating to the mechanical properties of LWC.(2)

Introduction

Much of the fundamental basis for the current LWC provisions 

in the AASHTO LRFD Bridge Design Specifications is built 

on research of LWC from the 1960s. (See references 2–6.) The 

LWC that was part of this research used traditional mixes of 

coarse aggregate, fine aggregate, portland cement, and water. 

Broad-based advancement in concrete technology over the 

past 50 years has led to significant advancements in concrete 

mechanical and durability performance. Research during the 

past 30 years, including the recent National Cooperative Highway Research Program (NCHRP) studies on different aspects of 

high-strength concrete, has resulted in revisions to the AASHTO 

LRFD Bridge Design Specifications to capitalize on the benefits 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

 www.fhwa.dot.gov/research
2

of high-strength NWC. However, as described 

by Russell, many of the design equations in the 

AASHTO LRFD Bridge Design Specifications are 

based on data that do not include tests of LWC 

specimens, particularly with regard to structural 

members with compressive strengths in excess 

of 6 ksi (41 MPa).(7) 

The Federal Highway Administration’s (FHWA) 

Turner-Fairbank Highway Research Center 

(TFHRC) has executed a research program 

investigating the performance of LWC with 

concrete compressive strengths in the range 

of 6 to 10 ksi (41 to 69 MPa) and equilibrium 

densities between 0.125 and 0.135 kcf (2,000 

and 2,160 kg/m3). The research program used 

LWC with three different lightweight aggregates that were intended to be representative of 

those available in North America. The program 

included tests from 27 precast/prestressed LWC 

girders to investigate topics including transfer 

length and development length of prestressing 

strand, time-dependent prestress losses, and 

shear strength of LWC. The development and 

splice length of mild steel reinforcement used 

in girders and decks made with LWC was also 

investigated using 40 reinforced concrete (RC) 

beams. While much of the research program 

focused on structural behavior, it also included 

a material characterization component wherein 

the compressive strength, elastic modulus 

(Ec), and splitting tensile strength (fct) of the 

concrete mixes used in the structural testing 

program were assessed. One key outcome of 

the research program is to recommend 

changes to the AASHTO LRFD Bridge Design 

Specifications relevant to LWC.(2)

This TechBrief summarizes the results of mechanical property testing that was conducted as 

part of the prestressed girder and RC beam 

testing. The mechanical properties of LWC 

tested in this study are included in a database 

of mechanical property tests on LWC that 

was collected from test results available in the 

literature. This TechBrief also summarizes the 

LWC database and the analysis of mechanical 

properties in the database. Design expressions 

in the current edition of the AASHTO LRFD 

Bridge Design Specifications are compared to 

the database.(2) Potential revisions to the 

AASHTO LRFD Bridge Design Specifications

relating to LWC are also presented.

LWC Mix Designs

The Expanded Shale, Clay, and Slate Institute 

assisted FHWA in obtaining LWC mixes that 

had been used in production. One of the criteria 

for this research project was to use lightweight 

aggregate sources that were geographically 

distributed across the United States. Additional 

selection criteria included mixes using a large 

percentage of the coarse aggregate as lightweight coarse aggregate, mixes using natural 

sand as the fine aggregate, and mixes with a 

target equilibrium density between 0.125 and 

0.135 kcf (2,000 and 2,160 kg/m3). The concrete 

density needed to be in the range of densities 

not currently covered by the AASHTO LRFD 

Bridge Design Specifications.

(2) 

Three mix designs were selected with a design 

compressive strength greater than or equal to 

6.0 ksi (41.4 MPa) to represent concrete that could 

be used for bridge girders. Another mix design 

was selected that had a design compressive 

strength less than 6.0 ksi (41.4 MPa) to represent 

concrete that could be used for a bridge deck. 

The selected mix designs are shown in table 1.

Table 1. Selected concrete mix designs.

Cast Date

Haydite Girder 

(HG)

Stalite Girder 

(SG)

Utelite Girder 

(UG)

Stalite Deck 

(SD)

Design 28-day strength (ksi) 6.0 10.0 7.0 4.0

Design release strength (ksi) 3.5 7.5 4.2 —

Target wc (kcf) 0.130 0.126 0.126 0.125

Water/cementitious materials ratio 0.36 0.31 0.34 0.43

— Indicates release strength not necessary for nonprestressed elements.

1 ksi = 6.89 MPa

0.001 kcf = 16.01 kg/m3
3

Each uses partial replacement of the coarse 

aggregate with lightweight aggregate to achieve 

their reduced wc. The lightweight aggregates 

in the mixes were Haydite (an expanded shale 

from Ohio), Stalite (an expanded slate from 

North Carolina), and Utelite (an expanded shale 

from Utah). The normal weight coarse aggregate was No. 67 Nova Scotia granite. Natural 

river sand was used as the fine aggregate. 

Type III portland cement was used to obtain the 

high early strengths typically required in highstrength precast girders. Admixtures included a 

water reducer, an air entrainer, and a high-range 

water reducer. 

Specimen Fabrication and Testing

The girders were fabricated at a concrete precasting plant in Mobile, AL. The fabricator was 

asked to prescriptively produce the concrete 

mixes without trying to adjust them for target 

strengths or wc. This was intended to remove 

batch-to-batch variations as a variable in the 

study. The lightweight aggregates were stored 

in three piles at the plant and watered continuously using a sprinkler on each pile as shown in 

figure 1.

Compression tests were performed on 4- by 

8-inch (102- by 203-mm) and 6- by 12-inch 

(152- by 305-mm) cylinders to determine the 

compressive strength at release of prestressing, 

at 28 days, and at girder testing. Ec was determined using one of the 4- by 8-inch (102- by 

203-mm) cylinders intended for compressive 

strength testing. The indirect tensile strength 

was measured on 4- by 8-inch (102- by 203-mm) 

cylinders using the fct test. Density measurements were made to determine the air-dry density of cylinders used for compression testing. 

They were also conducted on separate cylinders 

to determine the oven-dry density and equilibrium density. Average compressive strength, 

Ec, fct, and air-dry wc for each concrete mix are 

provided in table 2.

Summary of Specimen Test Results

The LWC test results were compared to design 

expressions for a lightweight modification factor 

and for Ec. Nearly all fct tests on all three girder 

mixes gave splitting ratios that were greater 

Figure 1. Lightweight aggregate stockpiles with 

continuous sprinklers.

Table 2. Mean concrete properties from tests on 4- by 8-inch (102- by 203-mm) cylinders.

Concrete Mix Specimen Age

Compressive

Strength

(ksi)

Air-Dry Density

(kcf)

fct

(ksi)

Ec

(ksi)

HG

Release 7.07 0.133 0.607 3,840

28 days 9.50 0.132 0.714 4,470

Test day 10.45 0.130 0.771 4,320

SG

Release 7.32 0.125 0.604 3,770

28 days 9.66 0.125 0.680 4,140

Test day 10.56 0.123 0.717 4,360

UG

Release 6.04 0.131 0.569 3,500

28 days 8.68 0.130 0.685 4,110

Test day 10.10 0.127 0.757 4,150

SD* 28 days 5.67 0.138 — —

Test day 7.59 0.137 — —

*Release strength not necessary for nonprestressed elements.

— Indicates no value was recorded.

1 ksi = 6.89 MPa

0.001 kcf = 16.01 kg/m3
4

than the splitting ratio requiring modification of 

LWC for shear and development length of mild 

steel in tension in the AASHTO LRFD Bridge 

Design Specifications.

(2) On average, Ec was 

overestimated by the AASHTO LRFD expression and underestimated by the NCHRP 12-64 

expression and the ACI 363-10 expression.(2,8,9)

TFHRC LWC Database

A thorough literature review was performed 

to find published journal papers, conference 

papers, technical reports, and university dissertations that included tests, analyses, and 

discussions of LWC. Over 500 references were 

found that mentioned LWC. These documents 

were reviewed for LWC data consisting of a 

compressive strength value and data from at 

least one other mechanical test. The citations 

for the reviewed documents are provided in 

the full report.(1) The recorded mechanical tests 

included compressive strength, Ec, fct test, modulus of rupture (fr), and Poisson’s ratio. The concrete density was also recorded. Unpublished 

test data, data in graphs, and NWC test data 

were not included in the database. 

The TFHRC LWC database consists of 3,835 data 

lines. These data were collected from 128 publications. Data lines were selected for evaluating 

material properties based on the presence of 

available data and on being within a range of 

material property values. A full list of references 

for the TFHRC LWC database and more information about the data selection criteria is included 

in the full report.(1)

Design Expressions for Ec

A total of 2,556 data lines are in the TFHRC 

subset database for Ec. To compare design 

expressions for Ec to both NWC and LWC data, 

the Ec database from NCHRP Project 12-64 was 

utilized.(8) The NWC and LWC data contain lines 

of compressive strength, Ec, and wc.

(8) For this 

evaluation, test data from concrete with a wc

greater than or equal to 0.135 kcf (2,160 kg/m3) 

(i.e., NWC data) from the NCHRP 12-64 database was combined with test data from concrete 

with a wc less than 0.135 kcf (2,160 kg/m3) 

(i.e., LWC data) from the TFHRC database.

The Ec data were compared to three designs 

expressions: (1) the expression in the AASHTO 

LRFD Bridge Design Specifications, (2) the 

expression in the NCHRP Project 12-64 final 

report, and (3) the expression in the ACI 

Committee 363 report on high-strength concrete.(2,8,9) The ratio of the tested Ec to the 

predicted Ec by the three design expressions 

is provided in table 3. A test-to-prediction ratio 

greater than unity indicates an underestimation 

of Ec, while a ratio less than unity indicates an 

overestimation of Ec. The mean test-to-prediction ratios in table 3 show that the AASHTO 

LRFD expression overestimates Ec of LWC, and 

the NCHRP 12-64 expression underestimates 

Ec of LWC. The ACI 363-10 expression closely 

predicts Ec of LWC but underestimates Ec of 

NWC. The test-to-prediction ratios using the 

AASHTO LRFD expression is compared to 

compressive strength in figure 2. This figure 

shows that the AASHTO LRFD expression 

tends to overestimate Ec at higher compressive 

strength levels for both NWC and LWC.

Optimization of Ec Equation Variables

An analysis was performed to evaluate the 

effect of different exponents on the basic form 

of the expression for Ec. The analysis was 

performed on a database consisting of the 

TFHRC LWC subset database combined with 

the NCHRP 12-64 NWC database.(1,8) The analysis was divided into three parts. In the first 

part, the exponent applied to the wc term was 

Table 3. Mean test-to-prediction ratio of Ec for LWC data from the TFHRC database and NWC data from the 

NCHRP 12-64 database.

Data Source

AASHTO 

LRFD(2)

NCHRP 

12-64(8) ACI 363(9) Proposed

TFHRC LWC and NCHRP NWC 0.957 1.087 1.056 1.000

TFHRC LWC 0.936 1.206 1.001 1.019

NCHRP NWC 0.972 1.007 1.094 0.987
5

varied and showed that an exponent of 1.5 

or 2.0 applied to wc resulted in the lowest 

coefficient of variation (COV) and a test-toprediction ratio near unity for the LWC data. In 

the second part, the exponent applied to the 

compressive strength term was varied and 

showed that the exponent applied to compressive strength should be 0.33 or 0.5 for a low 

COV without considerable overestimation of Ec

for LWC data. The third part was to vary the 

exponents applied to both wc and compressive 

strength. A new proposed expression with an 

exponent of 2.0 for wc and 0.33 for compressive strength was evaluated and had the lowest 

COV of the four expressions evaluated in the 

third part of the analysis. The proposed expression slightly underestimated the prediction of 

Ec for LWC and gave a close prediction of Ec

for NWC.

LWC Modification Factor

The AASHTO LRFD Bridge Design Specifications account for the reduced tensile strength 

of LWC in a variety of ways.(2) Article 5.8.2.2 of 

the report gives a modification for LWC that is 

applicable to the articles of the specifications 

involving sectional analysis of nominal shear 

resistance.(2) In this article, a 0.75 factor is used 

for all-lightweight concrete, and a 0.85 factor is 

used for sand-lightweight concrete. The article 

allows interpolation between the two factors 

for partial sand replacement. Article 5.11.2.1.2 

describing the development length of mild 

reinforcement in tension also includes modification factors all-lightweight concrete 

and sand-lightweight concrete and allows 

for interpolation to be used with partial sand 

replacement.(2) Unfortunately, the amount of 

sand replacement is rarely known during the 

design phase of a project. Also, a definition 

based on the proportions of constituent materials becomes more cumbersome if partial 

replacement of normal weight coarse aggregate with lightweight coarse aggregate is also 

considered. A lightweight modification factor

based on a specified mix property, such as 

concrete density, may be preferable.

Prediction of the Splitting Ratio

The ratio of fct to the square root of the compressive strength is known as the splitting 

ratio, Fsp. Early references to Fsp was made 

by Hanson and ACI Committee 318.(4,10) The 

term “splitting ratio” is no longer used in the 

AASHTO LRFD Bridge Design Specifications, 

but the definition is still a part of the modification 

factor for LWC in Articles 5.8.2.2 and 5.11.2.1.2.(2)

Concrete with a Fsp greater than 0.212 does 

not require modification of the expressions in 

Figure 2. Ec Test-to-prediction ratio compared to compressive strength for AASHTO LRFD equation.

1 ksi = 6.89 MPa

0.0 5.0 10.0 15.0 20.0 25.0

0.0 34.5 68.9 103.4 137.9 172.4

0.2

0.6

1.0

1.4

1.8

LWC

NWC (NCHRP 12-64)

Underestimation

Overestimation

Compressive Strength (ksi)

Compressive Strength (MPa)

Test-to-Predicted Modulus of Elasticity
6

Articles 5.8.2 and 5.8.3 for LWC. Fsp implied by 

the AASHTO LRFD Bridge Design Specifications

for sand-lightweight concrete and all-lightweight 

concrete are based on the 0.85 and 0.75 modification factors described in Article 5.8.2. 

The fct subset of the TFHRC LWC database was 

used to evaluate the expression for Fsp implied 

by the AASHTO LRFD Bridge Design Specifications.

(2) The database includes 954 lines of 

sand-lightweight and 311 lines of all-lightweight 

concrete. The test-to-prediction ratios for the 

sand-lightweight and all-lightweight AASHTO 

LRFD expressions for Fsp are given in table 4.

A test-to-prediction ratio greater than unity 

is an overestimation of the splitting ratio and 

indicates a conservative prediction of concrete 

tensile strength when used for calculating nominal shear resistance or development length of 

mild reinforcement. The AASHTO LRFD expression gave conservative predictions of concrete 

tensile strength for most of the data.

Linear Expressions for Fsp Using wc

An expression for predicting Fsp as a function of

wc was developed. This section describes this 

piecewise continuous function for predicting 

Fsp. Other types of expressions for Fsp are evaluated in the full report.(1) The expression consists 

of a constant predicted Fsp of 0.159 for wc ≤ 

0.100 kcf (1,600 kg/m3). The prediction then 

assumes a linearly increasing Fsp with wc to 

a limit on wc of 0.135 kcf (2,160 kg/m3). For 

wc ≥ 0.135 kcf (2,160 kg/m3), a constant predicted value of 0.212 for Fsp is used since this 

aligns with the existing provisions for NWC. 

A lower limit of 0.159 on Fsp is used because 

this value is specified in Article 5.8.2.2 as Fsp

for all-lightweight concrete (0.75 × 0.212).(2)

The test-to-prediction ratios for the proposed 

expression are shown in figure 3.

The proposed expression gave a larger predicted fct than the expression in the AASHTO 

LRFD Bridge Design Specifications for sandlightweight concrete with a wc up to 0.110 kcf 

(1,760 kg/m3).(2) For larger unit weights, the 

AASHTO LRFD expression gave a very conservative prediction of fct. 

The proposed expression for Fsp can be converted to LWC modification factor by dividing it 

by 0.212, the upper limit on Fsp. The term -factor

is used to refer to a LWC modification factor.

Modulus of Rupture

The accuracy of the fr expression is important 

for the strength, serviceability, and ductility of 

structural concrete bridges. The AASHTO LRFD 

Bridge Design Specifications have different 

expressions for fr depending on the use of the 

calculation and the type of concrete.(2) For normal weight concrete, one expression for fr is 

used to calculate the nominal shear resistance 

provided by concrete when inclined cracking 

results from combined shear and moment (Vci) 

(Article 5.8.3.4.3), and another expression for fr

is used for all other calculations such as effective 

moment of inertia, cracking control, and minimum flexural reinforcement.(2) For LWC, there 

are two different expressions for fr depending 

on the use of sand-lightweight concrete or alllightweight concrete. Unlike NWC, the AASHTO 

LRFD Bridge Design Specifications do not give 

different expressions for fr of LWC depending 

on the use of the concrete.(2) This creates varying levels of conservatism in the calculations 

of cracking control, effective moment of inertia, 

Table 4. Test-to-prediction ratios of Fsp using the AASHTO LRFD expression and proposed expression.

LWC Fsp Expression Total

wc ≤ 

0.090 kcf

0.090 

< wc ≤ 

0.100 kcf

0.100 

< wc ≤ 

0.110 kcf

0.110 

< wc ≤ 

0.120 kcf

0.120 

< wc ≤ 

0.135 kcf

Sandlightweight

AASHTO LRFD 1.222 1.011 0.920 0.992 1.181 1.279

Proposed 1.150 1.138 1.036 1.061 1.137 1.169

All-lightweight

AASHTO LRFD 1.129 0.991 1.143 1.094 1.190 1.188

Proposed 1.078 0.984 1.135 1.034 1.050 0.956

0.001 kcf = 16.01 kg/m3

λ
7

and cracking moment for Vci when used for 

members made from LWC.

Comparison of fr to fct

In this section, fr is compared to the fct in order 

to justify defining the material property fr in 

terms of another material property fct (through 

the -factor).

For this comparison, a new subset database 

was created for concrete mixes with test results 

in both the fct subset database and a wet fr

subset database. An alternate wet fr subset 

database was created to include only specimens 

that remained wet until tested due to the reduction in the tested fr of specimens allowed to dry. 

A comparison of fr and fct is shown in figure 4. 

The figure shows fr increasing proportional to 

fct, which supports the observations of previous 

research on a limited number of data points.(4)

Proposed Design Expression for fr

A new expression for fr was proposed that 

includes the LWC modification factor (λ-factor). 

The proposed expression for fr is applicable 

to the calculation of the effective moment of 

inertia, cracking control requirements, and minimum area of flexural reinforcement. 

The ratio of the tested fr from the wet fr subset 

database to the fr predicted by the AASHTO 

LRFD expressions and proposed expression 

is given in table 5. Both the proposed expression and the AASHTO LRFD expression gave 

predictions of fr that were larger than the 

tested values. 

Preliminary Recommendations

A set of preliminary recommended changes to 

the AASHTO LRFD Bridge Design Specifications

were developed in this research effort.(2) This 

TechBrief has only considered the analysis of 

tests on the mechanical properties of LWC. 

Additional analysis on the structural performance of LWC members is needed before final 

recommendations can be made. The areas needing additional analysis include the development 

of mild reinforcement in tension, the transfer 

and development length of prestressing strands, 

and the shear resistance of reinforced and 

prestressed members. The effects of the preliminary recommendations will be included in those 

further analyses.

The analysis of the TFHRC LWC database using 

the subset database for Ec and the subset 

database for fct has resulted in several new 

expressions for Ec, an LWC modification factor (λ-factor), and fr. The new expressions are 

not based on the proportions of constituent 

materials and include tests from types of

λ

Figure 3. Test-to-prediction ratios of Fsp predicted by the proposed expression. Test-to-Predicted Splitting Ratio

0.96 1.28 1.60 1.92 2.24

0.060 0.080 0.100 0.120 0.140

0.20

0.60

1.00

1.40

1.80

Unit Weight (kg/m3

 × 103)

Unit Weight (kcf)

Underestimation

Overestimation

0.001 kcf = 16.01 kg/m3
8

mix designs that are not explicitly permitted 

by the current edition of the AASHTO LRFD 

Bridge Design Specifications.

(2) These mix 

types include specified density LWC (typically a 

blend of lightweight and normal weight coarse 

aggregate) and inverted mixes (normal weight 

coarse and lightweight fine aggregate). The new 

expressions are instead based on wc and as 

a result the definitions of sand-lightweight 

concrete and all-lightweight concrete would 

no longer be needed. This section proposes a 

revised definition of LWC that does not include 

the terms sand-lightweight concrete or alllightweight concrete.

Proposed Definition for LWC

The definition for LWC in Article 5.2 of the 

AASHTO LRFD Bridge Design Specifications

limits wc for LWC to 0.120 kcf (1,920 kg/m3

) 

and includes definitions for sand-lightweight 

and all-lightweight concrete.(2) The proposed 

definition for LWC expands the range of 

wc and eliminates the definitions for terms 

relating to the constituent materials in LWC. 

The proposed definition for LWC is as follows: 

concrete containing lightweight aggregate 

and having an equilibrium density not exceeding 0.135 kcf (2,160 kg/m3

), as determined by 

ASTM C567.(11)

The term “air-dry unit weight” is used in the 

current definitions; however, this term is not 

found in ASTM C567.(11) The AASHTO LRFD 

Bridge Design Specifications term “air-dry unit 

weight” is interpreted to be equivalent to the 

ASTM C567 term “equilibrium density.”(2,11) A 

statement could be added to the commentary 

to clarify the term “air-dry unit weight” or the 

Table 5. Test-to-prediction ratios of fr using the AASHTO LRFD expression and proposed expression. 

LWC fr Expression Total

wc ≤ 

0.090 kcf

0.090 

< wc ≤ 

0.100 kcf

0.100 

< wc ≤ 

0.110 kcf

0.110 

< wc ≤ 

0.120 kcf

0.120 

< wc ≤ 

0.135 kcf

Sandlightweight

AASHTO LRFD 1.394 1.277 1.222 1.344 1.415 1.414

Proposed 1.299 1.419 1.357 1.412 1.351 1.227

All-lightweight

AASHTO LRFD 1.571 1.328 1.664 1.538 1.498 1.901

Proposed 1.409 1.254 1.571 1.387 1.253 1.428

0.001 kcf = 16.01 kg/m3

Figure 4. fr compared to fct with AASHTO LRFD expression and linear regression.

0.0 0.4 0.8 1.2

0.0 2.8 5.5 8.3

0.0

0.4

0.8

1.2

0.0

2.8

8.3

Splitting Tensile Strength (MPa)

Splitting Tensile Strength (ksi)

Modulus of Rupture (ksi)

Modulus of Rupture (MPa)

1 ksi = 6.89 MPa

wc ≤ 90 5.5

90 < wc ≤ 100

100 < wc ≤ 110

110 < wc ≤ 120

120 < wc ≤ 135

AASHTO LRFD

Linear Regression
9

term “equilibrium density” could be used in 

the definition for LWC.

Proposed Expression for Ec

The proposed new expression for Ec would 

have the same limits on wc and specified compressive strength as the current expression in 

Article 5.4.2.4.(2) The only proposed change is 

the expression for Ec itself. The proposed expression for Ec is shown in figure 5.

According to the AASHTO LRFD Bridge Design 

Specifications, in the absence of measured 

data, Ec for concrete with unit weights between 

0.090 and 0.155 kcf (1,440 and 2,480 kg/m3) 

and specified compressive strengths up to 

15.0 ksi (103 MPa) may be taken as follows:(2)

Where:

Ec = Modulus of elasticity in ksi.

K1 = Correction factor for source of aggregate.

wc = Concrete unit weight in kcf.

f ’c = Compressive strength in ksi.

Figure 6 shows the expression compared to 

the current AASHTO LRFD expression for an 

assumed wc of 0.110 kcf (1760 kg/m3) and K1

equal to unity.

Proposed Expression for LWC Modification 

Factor

The concept of including a modification factor 

for LWC in expressions for predicting nominal 

resistance is included in many articles of the 

AASHTO LRFD Bridge Design Specifications.

(2)

However, a single unified expression or LWC 

modification factor is not specified. This section 

proposes a term, the λ-factor, to quantify the 

modification in nominal resistance that could 

be included in any expression for nominal resistance. The λ-factor relates to the material properties of structural LWC so the new article for 

the definition for the λ-factor could be located 

in Article 5.4.2.(2)

Where lightweight aggregate concretes are 

used, the LWC modification factor, λ, shall 

be determined using the equation in figure 7 

where fct is specified.

Figure 7. Expression for -factor with fct specified.

λ = 4.7fct ≤ 1.0

 √f ’c 

Ec = 120,000K1wc

2.0f ’c0.33

Figure 5. Expression for Ec.

λ

Figure 6. Ec for proposed expression.

0.0 4.0 8.0 12.0 16.0

0.0 27.6 55.2 82.7 110.3

0.0

2.0

4.0

6.0

0.0

13.8

27.6

41.4

Compressive Strength (ksi)

Compressive Strength (MPa)

Modulus of Elasticity (ksi × 103

)

Modulus of Elasticity (GPa)

1 ksi = 6.89 MPa

AASHTO (wc = 0.110 kcf)

Proposed (wc = 0.110 kcf)

0 < wc ≤ 0.100

0.100 < wc ≤ 0.120

0.120 < wc ≤ 0.135
10

Where fct is not specified, λ shall be determined using the equation in figure 8.

An illustration of the proposed expression for 

the λ-factor is shown in figure 9, and the 

predicted splitting ratios (λ-factor × 0.212) are 

shown in figure 10. The λ-factors implied in 

AASHTO LRFD for sand-lightweight concrete 

and all-lightweight concrete are also shown. 

Figure 10 shows that a considerable amount of 

sand-lightweight concrete data are not defined 

in the current AASHTO LRFD Bridge Design 

Specifications.

(2)

As stated previously, the effect of using the 

λ-factor in expressions for nominal resistance 

needs to be evaluated. The proposed λ-factor

could then be included in the expression for 

nominal resistance in the AASHTO LRFD Bridge 

Design Specifications.

(2) For example, the 

λ-factor could be added directly to design 

expressions for nominal shear resistance in 

Articles 5.8.2 and 5.8.3 and would replace the 

existing modification factor for LWC.(2)

0.75 ≤ λ = 7.5wc ≤ 1.0

Figure 8. Expression for -factor with fct not 

specified.

λ

Figure 9. Proposed expression for -factor.

Proposed Expression

wc = 0.100 kcf wc ≈ 0.135 kcf

λ = 1.00

λ = 0.75

Unit Weight, wc

λ-factor (Splitting Ratio/0.212)

0.001 kcf = 16.01 kg/m3

λ

Figure 10. Splitting ratio (fct /√f ’c ) for the proposed expression (λ-factor × 0.212). Splitting Ratio (ksi0.5)

0.060 0.080 0.100 0.120 0.140

0.0

0.10

0.20

0.30

0.40

0.00

0.26

0.53

0.97

1.05

0.96 1.28 1.60 1.92 2.24

Splitting Ratio (MPa0.5)

Unit Weight (kcf)

Unit Weight 

Gap

Sand-Lightweight

All-Lightweight

Other LWC

Proposed

AASHTO Sand-Lightweight

AASHTO All-Lightweight

1 ksi = 6.89 MPa

0.001 kcf = 16.01 kg/m3

Unit Weight (kg/m3 × 103)
11

Proposed Expression for fr

The expression for fr in the AASHTO LRFD Bridge 

Design Specifications is in Article 5.4.2.6.(2)

The proposed expression for fr is as follows 

for NWC and LWC:

The proposed expression is as follows when 

used to calculate the cracking moment of a 

member in Article 5.8.3.4.3:(2)

The proposed expressions for fr include the 

proposed λ-factor and would be applicable to 

both NWC and LWC. The expression for fr used 

to calculate the cracking moment of a member 

in Article 5.8.3.4.3 (Vci) includes the proposed 

λ-factor for consistency. The fr expression for use 

with Article 5.8.3.4.3 will need to be validated 

on shear test data from LWC members available in the literature before it is proposed for 

inclusion into the AASHTO LRFD Bridge Design 

Specifications.

(2)

The ratio of the predicted fr (see figure 11) to √f ’c

is shown in figure 13 with sand-lightweight and 

all-lightweight concrete data. Figure 13 shows 

that most of the test data are above the predicted 

fr (i.e., underestimated) and that a considerable 

amount of the sand-lightweight concrete data 

are in the gap of wc not defined in the current 

AASHTO LRFD Bridge Design Specifications.

(2)

Conclusion

This TechBrief describes mechanical property 

tests on LWC, provides information about a LWC 

mechanical property database, and presents 

potential revisions to the AASHTO LRFD Bridge 

Design Specifications relating to the definition 

and mechanical properties of LWC.(2) The proposed design expressions for Ec, LWC modification factor, and fr were compared to tested 

values in a LWC database collected as part of 

this research effort. A full description of the 

database and the development and evaluation 

of prediction expressions are included in the 

full report.(1) Future phases of this research 

compilation and analysis effort will include synthesis of past work on structural performance 

of LWC. The test results will be compared 

to the prediction expressions for nominal 

resistance in the AASHTO LRFD Bridge Design 

Specifications incorporating appropriate proposed revisions for LWC mechanical properties 

as presented in this TechBrief.

fr = 0.24λ√f ’c

Figure 11. Expression for fr except when used in 

Article 5.8.3.4.3.(2)

fr = 0.20λ√f ’c

Figure 12. Expression for fr when used in 

Article 5.8.3.4.3.(2)

Figure 13. fr /√f ’c for the proposed expression (0.24 λ√f ’c ).

0.060 0.080 0.100 0.120 0.140

0.96 1.28 1.60 1.92 2.24

0.00

0.10

0.20

0.30

0.40

0.50

0.00

0.26

0.53

0.79

1.05

1.31

Unit Weight (kcf)

Unit Weight (kg/m3

 × 103)

fr /√f ’c (ksi0.5)

fr /√f ’c (MPa.5)

Sand-Lightweight

All-Lightweight

Proposed

AASHTO NWC

AASHTO Sand-Lightweight

AASHTO All-Lightweight Unit Weight 

Gap

1 ksi = 6.89 MPa

0.001 kcf = 16.01 kg/m3
12

Researchers—This study was led by Ben Graybeal at FHWA’s Turner-Fairbank Highway Research Center. It was 

conducted by Gary Greene of PSI, Inc. through laboratory support contract DTFH61 10 D 00017. For additional 

information, contact Ben at (202) 493-3122 or in the FHWA Office of Infrastructure Research and Development 

located at 6300 Georgetown Pike, McLean, VA, 22101-2296.

Distribution—The report (PB2013-107688) covered in this TechBrief is being distributed through the National 

Technical Information Service at www.ntis.gov.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center by email to report.

center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at http://www.fhwa.dot.gov/research.

Key Words—LWC, lightweight concrete, bridge design, LRFD design specifications.

Notice—This document is disseminated under the sponsorship of the U.S. Department of Transportation 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 TechBrief only because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration 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.

june 2013 FHWA-HRT-13-061

HRDi-40/06-13(250)E

References

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Lightweight Concrete: Mechanical Properties, Report No. FHWA-HRT-13-062, Federal 

Highway Administration, Washington, DC.

2. AASHTO. (2012). AASHTO LRFD Bridge 

Design Specifications, Sixth Ed., American 

Association of State Highway and 

Transportation Officials, Washington, DC.

3. ACI Committee 213. (1967). “Guide for Structural Lightweight Aggregate Concrete,” 

ACI Journal, 64(8), 433–469, American 

Concrete Institute, Farmington Hills, MI.

4. Hanson, J.A. (1961). “Tensile Strength and 

Diagonal Tension Resistance of Structural 

Lightweight Concrete,” ACI Journal, 

58(1), 1–40, American Concrete Institute, 

Farmington Hills, MI.

5. Ivey, D.L. and Buth, E. (1966). “Splitting 

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859–871.

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Institute, Farmington Hills, MI.

7. Russell, H. (2007). Synthesis of Research and 

Provisions Regarding the Use of Lightweight 

Concrete in Highway Bridges, Report 

No. FHWA-HRT-07-053, Federal Highway 

Administration, Washington, DC.

8. Rizkalla, S., Mirmiran, A., Zia, P., Russell, 

H., and Mast, R. (2007). Application of the 

LRFD Bridge Design Specifications to HighStrength Structural Concrete: Flexure and 

Compression Provisions, NCHRP Report 

595, NCHRP Project 12-64, National 

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9. ACI Committee 363. (2010). Report on 

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10. ACI Committee 318. (1962). “Building Code 

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318-56),” ACI Journal, 59(12), 1821–1848, 

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11. ASTM C567. (2005). “Standard Test Method 

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