TECHBRIEF Lightweight Concrete: 

Shear Performance 

FHWA Publication No.: FHWA-HRT-15-021

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: 

Shear Performance (FHWA-HRT-15-022).

Objective

Concrete with a unit weight between that of traditional 

lightweight concrete and normal-weight concrete 

(NWC) is not covered in the American Association of 

State Highway and Transportation Officials (AASHTO) 

Load and Resistance Factor Design (LRFD) Bridge Design 

Specifications. As part of an effort to address this and 

other perceived shortcomings in how the specification 

addresses lightweight concrete and how lightweight 

concrete is deployed in bridges, research was completed 

to assess the shear performance of these different 

density concretes. Thirty full-scale precast, prestressed 

girder tests were completed, and a database of shear 

performance results was developed that covered a wide 

range of concrete densities. Proposed revisions to 

the AASHTO LRFD Bridge Design Specifications were 

developed and are presented as part of a framework that 

addresses the performance of structural concrete as a 

function of density.

Introduction

Much of the fundamental basis for the current lightweight concrete provisions in the AASHTO LRFD Bridge 

Design Specifications is research on lightweight concrete 

from the 1960s. (See references 1 through 5.) The 

lightweight concrete 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 given 

rise to significant advancements in concrete mechanical 

and durability performance. Research during the past 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research
2

30 years, including the recent National 

Cooperative Highway Research Program 

studies on different aspects of highstrength concrete, has resulted in revisions 

to the AASHTO LRFD Bridge Design 

Specifications to capitalize on the benefits 

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 lightweight 

concrete specimens, particularly structural 

members with compressive strengths in 

excess of 6 ksi (41 MPa).(6) 

The Federal Highway Administration 

(FHWA) Turner-Fairbank Highway Research 

Center (TFHRC) has executed a research 

program investigating the performance 

of lightweight concrete with concrete 

compressive strengths in the range of 

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

densities from 0.125 kcf to 0.135 kcf 

(2,000 to 2,160 kg/m3). The research program 

used lightweight concrete with three 

different lightweight aggregates that 

are intended to be representative of 

those available in North America. The 

program included tests of 27 precast/

prestressed lightweight concrete girders to 

investigate topics such as transfer length 

and development length of prestressing 

strand, time-dependent prestress 

losses, and shear strength of lightweight 

concrete. The development and splice 

length of mild steel reinforcement used in 

girders and decks made with lightweight 

concrete 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, and splitting tensile strength 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 

lightweight concrete.

This document summarizes the results 

of shear tests conducted on prestressed 

concrete (PC) girders. The shear tests on 

lightweight concrete girders tested in this 

study are included in a database of tests 

on lightweight concrete and NWC that 

was collected from test results available in 

the literature. This document summarizes 

the database and the analysis of the database. Design expressions in the AASHTO 

LRFD Bridge Design Specifications are 

compared with the database. Potential 

revisions to the AASHTO LRFD Bridge 

Design Specifications relating to shear 

resistance are presented.

Prestressed Girder Shear Tests 

Conducted at TFHRC

Lightweight Concrete Mix Designs

The Expanded Shale, Clay, and Slate 

Institute assisted FHWA in obtaining lightweight concrete 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.(1)
3

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. The mix designs selected are shown 

in table 1. Each uses partial replacement 

of the coarse aggregate with lightweight 

aggregate to achieve their reduced unit 

weight. 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 high-strength precast, pretensioned girders. Admixtures included a 

water reducer, an air entrainer, and a highrange water reducer. 

Property Haydite Girder Stalite Girder Utelite Girder

Design 28-Day Strength (ksi) 6.0 10.0 7.0

Design Release Strength (ksi) 3.50 7.5 4.2

Target Unit Weight (kcf) 0.130 0.126 0.126

Water/Cementitious Materials Ratio 0.36 0.31 0.34

Table 1. Selected concrete mix designs.

1.0 ksi = 6.89 MPa.

0.001 kcf = 16.01 kg/m3

.

Experimental Program

The experimental program consisted of 

30 tests on 15 PC girders made using 

3 different lightweight concrete mixes. Key

test parameters included the lightweight 

aggregate, the amount of shear reinforcement, girder depth, and the use of straight 

or draped strands. Five girder designs were 

developed to evaluate the effect of the key 

parameters. The ends of each girder had 

different amounts of shear reinforcement. 

While girder designs 1–4 were used for a 

different part of the research program 

wherein development length of prestressing strand was assessed, girder 

designs 5–9 were used for the evaluation 

of shear performance. A set of five girders 

was cast for each of three different concrete 

mixes intended to represent typical lightweight concrete for girders. 

Table 2 gives the nominal details for the 

six girder end designs that were AASHTO 

Type II girders. The dead end of girder 

design 5 (5D) was designed to have the 

minimum amount of shear reinforcement 

allowed by the AASHTO LRFD Bridge Design 

Specifications (Article 5.8.2.5) at nearly the 

maximum spacing (Article 5.8.2.7). The 

dead end of girder design 7 (7D) was 

designed to have a ratio of shear stress to 

concrete compressive stress (vu/f’c) near 

the limit of 0.18 given in the AASHTO LRFD 

Bridge Design Specifications for the applicability of the sectional design method 

(Article 5.8.3.2). Girder design 6 (6D) had 

draped strands and an amount of shear 

reinforcement between the amounts used 

in 5D and 7D. 
4

Table 3 gives similar details for the four 

girder end designs that were AASHTO/

PCI Bulb Tee girders with a 54-inch (1.37-m) 

height (BT-54). The amount of shear reinforcement in girder designs 8 (8D) and 9 (9D) 

was designed to give similar vu/f’c ratios 

as 5D and 7D, respectively. This reinforcement design was chosen to investigate 

the effect that girder depth has on shear 

strength, which is commonly known as the 

“size effect.”

Girder Test†

Effective 

Shear 

Depth, dv

(inch)

DesignNormalized 

Shear 

Stress‡,

vu/f’c

Number of Strands Stirrups Design Amount 

of Stirrups, 

pvfy

Bottom Top Bar Size (ksi) Spacing

(inch)

5D 35.0 0.068 10-straight 2 3 22 0.12

5L 35.0 0.075 10-straight 2 3 15 0.18

6D 31.7 0.088 10-straight

+ 4-drape 2 4 15 0.32

6L 31.7 0.096 10-straight

+ 4-drape 2 4 12 0.40

7D 32.8 0.15 18-straight 4 4 8 0.60

7L 32.8 0.12 18-straight 4 4 12 0.40

Girder Test†

Effective 

Shear 

Depth, dv

(inch)

DesignNormalized 

Shear 

Stress‡,

vu/f’c

Number of Strands Stirrups Design Amount 

of Stirrups, 

pvfy

Bottom Top Bar Size (ksi) Spacing

(inch)

8D 51.6 0.068 16-straight 2 3 22 0.12

8L 51.6 0.076 16-straight 2 3 14 0.19

9D 47.5 0.15 28-straight 4 4 8 0.60

9L 47.5 0.14 28-straight 4 4 10 0.48

Table 2. Design details of the AASHTO Type II girders.

Table 3. Design details of the AASHTO/PCI BT-54 girders.

†

Specimen name of form #%, where: # is girder design; and % is D for dead end or L for live end. 

‡

Assumed f’c for design was 10 ksi.

1.0 inch = 25.4 mm.

1.0 ksi = 6.89 MPa.

†

Specimen name of form #%, where: # is girder design; and % is D for dead end or L for live end. 

‡

Assumed f’c for design was 10 ksi.

1.0 inch = 25.4 mm.

1.0 ksi = 6.89 MPa.
5

Material Properties

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 unit weight. 

This decision 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.

Compression tests were performed on 

4- by 8-inch (102- by 203-mm) cylinders. 

The indirect tensile strength was measured 

on 4- by 8-inch (102- by 203-mm) cylinders 

using the splitting tensile test. Density 

measurements were made to determine 

the air-dry density of cylinders used for 

compression testing. Average compressive 

strengths, splitting tensile strengths, and 

air-dry unit weights for each concrete mix 

are given in table 4.

The reinforcing bars were ASTM A615, 

Grade 60.(7) The mechanical properties 

were tested under displacement control in 

a 100-kip (445-kN) testing machine. Strain 

was measured with an 8-inch (203-mm) 

extensometer. The yield strength was determined using the 0.2-percent offset method. 

The average yield strength and the ultimate 

strength of the two bars in each size tested 

are given in table 5.

Concrete Mix Compressive 

Strength, 28 Day (ksi)

Compressive 

Strength, Test Day (ksi)

Splitting Tensile

Strength (ksi)

Air-Dry 

Density (kcf)

Haydite Girder 9.5 10.4 0.770 0.130

Stalite Girder 9.7 10.6 0.720 0.123

Utelite Girder 8.6 10.1 0.760 0.127

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

1.0 ksi = 6.89 MPa.

0.001 kcf = 16.01 kg/m3

.

Table 5. Reinforcing bar properties.

†

Calculated using 0.2-percent offset method.

1.0 inch = 25.4 mm.

1.0 in2

 = 645 mm2.

1.0 ksi = 6.89 MPa.

Nominal

and Measured 

Property

Stirrups

Girder

Design 5

Girder

Design 8

Girder

Designs 6 and 7

Girder

Design 9

Bar Size 3 3 4 4

Nominal 

Diameter (inch) 0.375 0.375 0.500 0.500

Nominal Area 

(inches2

) 0.11 0.11 0.20 0.20

Yield Strength†

 (ksi) 70.8 65.1 68.0 65.3

Ultimate Strength 

(ksi) 112.2 101.9 97.8 104.8
6

An 8-inch- (200-mm-) thick composite NWC 

deck was cast onto each lightweight concrete girder at TFHRC to move the neutral 

axis above the web and top flange. The 

concrete used in the decks had a specified 

compressive strength of 4 ksi (28 MPa). 

The decks had two orthogonal mats of reinforcing, as specified in Article 9.7.3 of the 

AASHTO LRFD Bridge Design Specifications 

for bridge decks. The deck reinforcement is 

shown in typical cross-sections in figure 1.

Shear Girder Test Procedure

Figure 2 shows a photograph of the setup 

for test C8D after the completion of the test. 

Before a test, the girder was supported by 

a roller at one end (which was being subjected to high shear) of the span and by a 

hydraulic jack at the other end of the span. 

These supports are referred to as the “roller 

support” and the “loading jack,” respectively. 

The roller support consisted of a 6-inch- 

(152-mm-) diameter steel roller and a 

2-inch- (51-mm-) thick steel bearing plate. 

The bearing plate was long enough to 

fully support the width of the girder’s bottom flange. Grout was placed between the 

girder and bearing plate to uniformly support the girder.

The girder rested directly on another 2-inch- 

(51-mm-) thick steel bearing plate at the 

loading jack. A greased Teflon sheet was 

placed between the bearing plate and a 

6-inch- (152-mm-) diameter roller between 

two grooved plates. Below the roller assembly was a loadcell with a 300-kip (1,340-kN) 

capacity and then a hydraulic jack with a 

1,000-kip (4450-kN) capacity. 

The load in the jack was controlled by a 

closed-loop servo-value system. The 

feedback for the closed loop system was 

provided by the loadcell and by a linear 

variable differential transformer with a 

10-inch (254-mm) stroke. The loading was 

applied by specifying the jack force in “loadcontrol” or by specifying the jack travel in 

“displacement-control.” 

Figure 1. NWC deck cast onto the lightweight concrete girders.

1 inch = 25.4 mm.
7

When the jack applied load to the girder, 

a heavy load frame applied the reaction 

force into the girder through a spreader 

beam, spherical bearing plates, and two 

pairs of 300-kip (1,340-kN) loadcells on the 

deck. The loadcells were mounted to 4-inch- 

(102-mm-) thick bearing plates that were 

grouted to the top of the deck.

Summary of Experimental Results

The first tested girder failed in horizontal 

shear through the concrete deck. Four of the 

remaining tests on the Type II girders failed 

in shear, and 13 girders failed in flexure. Six 

of the tests on the BT-54 girders failed in 

shear, and the remaining six girders failed 

in flexure.

In each test resulting in a shear failure, there 

was significant yielding in several of the 

stirrups (indicated by measured strains 

greater than three times the yield strain). 

Two of the Type II girder tests that failed in 

shear experienced concrete crushing in the 

web over much of the test region. The other 

two Type II girder tests failing in shear had 

concrete crushing as the diagonal compression was funneled to the support. Three 

of the BT-54 girders tests failed in shear 

after multiple stirrups ruptured. Two of the 

tests on BT-54 girders failed in shear after 

experiencing general yielding in the stirrups followed by local crushing in the web. 

This kind of failure is shown in figure 2 for 

the tests on C8D. The failure of Test A9L was 

due to concrete crushing as the diagonal 

compression was funneled to the support.

As expected, the average shear stress at 

failure increased as the amount of transverse reinforcement increased. The mean 

shear stress at failure for the Type II girders was larger than for the BT-54 girders. 

The effect of reduced shear strength with 

Figure 2. Test setup for girder test C8D as observed after completion of test.
8

increased girder depth was observed for 

three separate groups of tests. Each group 

of tests had similar percentages of longitudinal and transverse reinforcement. The 

mean test-day compressive strength was 

near 10 ksi (69 MPa) for all three girder 

mixes. No dependency on aggregate was 

observed in the average shear stress at failure.

All three design procedures in the AASHTO 

LRFD Bridge Design Specifications gave 

conservative predictions of shear resistance 

for the tests failing in shear. The two methods of the General Procedure (GP) gave 

less conservative predictions of shear resistance for the BT-54 girders than the Type II 

girders. The opposite was observed with 

the Simplified Procedure, which gave more 

conservative predictions of shear resistance for the BT-54 girders than the Type II 

girders. All of these predictions were 

made without modification for lightweight 

concrete.The high splitting tensile strength 

of most of the girders did not require 

modification for lightweight concrete 

according to Article 5.8.2.2 of the AASHTO 

LRFD Bridge Design Specifications. 

The shear force at web-shear cracking was 

conservatively predicted by all three design 

procedures in the AASHTO LRFD Bridge 

Design Specifications for all of the tests. The 

GP-equation procedure and the Simplified 

Procedure gave the most conservative and 

least conservative predictions of web-shear 

cracking for both the Type II and BT-54 girders, respectively. All three design procedures gave less conservative predictions 

of web-shear cracking for the BT-54 girders 

than for the Type II girders. 

On average, the three design procedures in the AASHTO LRFD Bridge Design 

Specifications tended to underestimate 

the web-shear crack inclination angle of 

the Type II girders (i.e., the observed crack 

angle was greater than the predicted angle) 

and overestimate the web-shear crack 

inclination angle of the BT-54 girders 

(i.e., the observed crack angle was less 

than the predicted angle). An underestimation of the inclination angle will result in 

an increase in the predicted contribution of 

the stirrups to the nominal shear resistance. 

TFHRC Shear Database

A thorough literature review was performed to find published journal papers, 

conference papers, technical reports, and 

university dissertations that included tests, 

analysis, or discussions of lightweight concrete. More than 500 references were found 

in the literature that mentioned lightweight 

concrete. These references were reviewed 

for data from tests on beam and girder 

specimens. Tests included in the database 

were limited to data from RC beams and PC 

beams that culminated in a shear failure. 

Only test data from published reports were 

included in the database. 

The TFHRC Shear Database consists of 

data from 886 tests on lightweight concrete 

specimens. More information about the 

tests in the database and a full list of references for the database is included in the 

associated report.(8)

In addition to data from tests on lightweight 

concrete, a select number of tests on NWC 

were also included in the database for 

comparison. The American Concrete 

Institute-Deutsche Ausschuss für Stahlbeton 

(ACI-DafStb) Database has data from 

928 specimens.(9,10) A subset of the ACIDafStb Database with similar concrete 

compressive strength and specimen height 

was selected for comparison to the lightweight concrete specimens. 
9

Proposed Expressions for Nominal 

Shear Resistance Including Lightweight Concrete Modification Factor

The test-to-prediction ratio is the ratio 

of the shear force at failure to the predicted shear resistance determined using 

a design expression. A slight revision was 

made to the expressions in the AASHTO 

LRFD Bridge Design Specifications to 

include the lightweight concrete modification factor. The measured shear force at 

failure was compared with the predicted 

shear resistance determined using the 

equation method of the GP-equation, the 

table method of the GP-table, and the 

Simplified Procedure for Prestressed and 

Nonprestressed Sections (Simplified-RC/

PC). Tests on RC specimens that satisfied the limits given in Article 5.8.3.4.1 

were also compared with the Simplified 

Procedure for Nonprestressed Sections 

(Simplified-RC).

The test-to-prediction ratios are given in 

table 6 for all lightweight concrete specimens and then by specimen type. For all of 

the specimens, GP-equation and GP-table 

gave similar ratios at 1.33 and 1.40, respectively. The Simplified-RC/PC procedure 

gave a more conservative prediction 

with a ratio of 2.09. The scatter in the 

test-to-prediction ratios for the three 

methods, as indicated by the coefficient of 

Specimen Group† Design

Expression Mean COV Maximum Minimum Percent < 1.0 

Percent 

< 0.8

All Specimens (326)

G.P.–Eq. 1.33 48.9% 4.13 0.43 34.0% 9.2%

G.P.–Tables 1.40 47.5% 4.19 0.46 24.8% 6.1%

S.P.–RC/PC 2.09 58.3% 7.4 0.43 5.2% 3.7%

RC Specimens 

Without Av (222)

G.P.–Eq. 1.30 47.5% 3.85 0.43 39.2% 9.5%

G.P.–Tables 1.41 46.4% 4.19 0.46 25.7% 5.0%

S.P.–RC/PC 2.21 54.0% 7.43 0.67 3.2% 2.3%

RC Specimens 

With Av (44)

G.P.–Eq. 1.12 18.9% 1.62 0.75 36.4% 4.5%

G.P.–Tables 1.14 23.7% 1.87 0.72 36.4% 4.5%

S.P.–RC/PC 1.57 22.4% 2.50 0.93 4.5% 0.0%

PC Specimens 

Without Av (27)

G.P.–Eq. 2.08 54.9% 4.13 0.56 25.9% 25.9%

G.P.–Tables 2.03 54.9% 3.89 0.54 25.9% 25.9%

S.P.–RC/PC 2.92 70.3% 6.80 0.43 25.9% 25.9%

PC Specimens 

With Av (33)

G.P.–Eq. 1.24 10.0% 1.52 0.97 3.0% 0.0%

G.P.–Tables 1.24 11.5% 1.52 0.98 3.0% 0.0%

S.P.–RC/PC 1.32 17.6% 2.00 0.99 3.0% 0.0%

Table 6. Test-to-prediction ratio of shear resistance for design expressions in AASHTO LRFD.

†

Number of specimens given in parentheses.

Av = shear reinforcement.

COV = coefficient of variation.

G.P.-Eq. = General Procedure using equations (Article 5.8.3.4.2).

PC = prestressed concrete.

RC = reinforced concrete.

S.P.-RC = Simplified Procedure for Nonprestressed Sections (Article 5.8.3.4.1).

S.P.-RC/PC = Simplified Procedure for Prestressed and Nonprestressed Sections (Article 5.8.3.4.3).
10

variation, was high at nearly 50 percent for 

the GP methods and nearly 60 percent for 

the Simplified-RC/PC procedure. All three 

methods have more conservative predictions for the RC specimens without shear 

reinforcement than for RC specimens with 

shear reinforcement. A similar trend was 

observed for PC specimens with and without shear reinforcement. All three methods gave more conservative predictions for 

PC specimens without shear reinforcement 

than for RC specimens without shear reinforcement. The two GP methods also gave 

more conservative predictions for PC specimens with shear reinforcement than for RC 

specimens with shear reinforcement. For 

the Simplified-RC/PC procedure, however, 

the prediction of RC specimens with shear 

reinforcement was more conservative than 

for PC specimens with shear reinforcement.

Test-to-prediction ratios are given by 

specimen type for the four design expressions in the AASHTO LRFD Bridge Design 

Specifications and are shown compared 

with unit weight in figure 3 through figure 6.

 The specimen types are RC without stirrups 

(“RC: no Av”), RC with stirrups (“RC: with 

Av”), PC without stirrups (“PC: no Av”), 

and PC with stirrups (“PC: with Av”). Leastsquares linear regression lines are shown 

for each specimen type. The number of 

specimens in each group is shown in parentheses after the group label.

Reliability Analysis for Lightweight 

Concrete in Shear

The mean reliability index (RQ) was determined for the shear resistance of lightweight concrete cross-sections. The RQ for 

lightweight concrete was compared with 

the target reliability index and the mean 

reliability index for NWC cross-sections.

The uncertainty due to the loads was 

evaluated using data from previous studies 

to determine the mean and variation of the 

load.(11,12) The uncertainty due to fabrication 

and materials was evaluated using the 

Monte-Carlo Simulation method of statistical sampling.(11) One RC cross-section and 

two PC cross-sections were simulated. The 

uncertainty due to analysis was evaluated 

using lightweight concrete specimens in 

the TFHRC Shear Database and selected 

NWC specimens in the ACI-DafStb Database. The combined uncertainty was used 

to determine the mean and variance 

of resistance. The reliability index was 

evaluated at a resistance factor for light

Figure 3. Test-to-predicted shear resistance compared 

with unit weight for Simplified Procedure for 

Nonprestressed Sections.

Figure 4. Test-to-predicted shear resistance compared 

with unit weight for GP-equation method.
11

weight concrete of 0.75, 0.80, 0.85, and 

0.90. The reliability index for lightweight 

concrete and NWC in shear was then determined over a range of dead-to-total load ratios.

The RQ for lightweight concrete and NWC 

was compared with the target reliability 

index. The reliability indexes determined 

for lightweight concrete were less than 

target for all cross-sections except PC specimens with shear reinforcement, regardless 

of the resistance factor (up to  = 0.90) or 

the design expression used to determine 

nominal shear resistance. The reliability 

indexes determined for NWC were less than 

target for all cross-sections except RC specimens with shear reinforcement determined 

using the Simplified-RC/PC procedure.

The RQ for lightweight concrete (LWC) was 

also compared with RQ for NWC (NWC). 

The LWC determined using the GP-equation 

method was greater than the NWC for all 

cross-sections. The Simplified RC and 

Simplified-RC/PC procedures had LWC

greater than NWC for RC specimens without shear reinforcement, and they had NWC

greater than LWC for RC specimens with 

shear reinforcement. For PC specimens 

without shear reinforcement, the 

Simplified-RC/PC procedure had NWC 

greater than LWC. For PC specimens with 

shear reinforcement, LWC determined using 

the Simplified-RC/PC procedure was greater 

than NWC.

Preliminary Recommendations 

for AASHTO LRFD Bridge Design 

Specifications

A set of preliminary recommended 

changes to the AASHTO LRFD Bridge 

Design Specifications were developed in this research effort. The first 

two recommended changes regarding 

the definition of lightweight concrete and 

the introduction of a lightweight concrete 

modification factor (-factor) were previously described in a related document concerning the mechanical properties of lightweight concrete and are 

presented again for clarity.(13,14) Additional 

recommended changes to the AASHTO 

LRFD Bridge Design Specifications are 

presented that are based on the analysis 

described in this document. These additional recommendations are built on the 

two previous recommendations.

Figure 6. Test-to-predicted shear resistance compared 

with unit weight for Simplified Procedure for RC and PC 

Sections.

Figure 5. Test-to-predicted shear resistance compared 

with unit weight for GP-table method.
12

Three types of changes are recommended 

regarding the shear performance of lightweight concrete as a result of the database 

analyses summarized in this document. 

The analysis of the TFHRC Shear Database 

included an evaluation of design expressions for nominal shear resistance in the 

AASHTO LRFD Bridge Design Specifications. 

The design expression for the minimum 

amount of transverse reinforcement was 

also evaluated using the TFHRC Shear 

Database. Lightweight concrete specimens 

in the TFHRC Shear Database and NWC 

specimens in the ACI-DafStb Database 

were used as part of a reliability analysis to 

determine the appropriate resistance factor 

for lightweight concrete.

The design expressions for nominal shear 

resistance and minimum transverse reinforcement include the recommended new 

expression for the -factor. The -factor is 

not based on the proportions of constituent 

materials and includes tests from types of 

mix designs that are not explicitly permitted by the current edition of the AASHTO 

LRFD Bridge Design Specifications.

(1)

These mix types include specified-density 

lightweight concrete (typically a blend of 

lightweight and normal-weight coarse 

aggregate) and inverted mixes (normalweight coarse and lightweight fine 

aggregate). The recommend new expression for the -factor is based on unit weight 

and splitting tensile strength, and as a result, 

the definitions of sand-lightweight concrete 

and all-lightweight concrete would no 

longer be needed.

Proposed Definition for Lightweight Concrete

The definition for lightweight concrete 

in Article 5.2 of the AASHTO LRFD 

Bridge Design Specifications limits the 

unit weight for lightweight concrete to 

0.120 kcf (1,920 kg/m3

) and includes 

definitions for sand-lightweight concrete 

and all-lightweight concrete. The proposed 

definition for lightweight concrete expands 

the range of unit weights and eliminates 

the definitions for terms relating to the 

constituent materials in lightweight 

concrete. The proposed definition for lightweight concrete is as follows:

Lightweight Concrete—Concrete 

containing lightweight aggregate 

conforming to AASHTO M 195 and 

having an equilibrium density not 

exceeding 0.135 kcf, as determined 

by ASTM C567. 

Proposed Expression for Lightweight 

Concrete Modification Factor

The concept of including a modification factor for lightweight concrete in expressions 

for predicting nominal resistance is 

included in many articles of the AASHTO 

LRFD Bridge Design Specifications. 

However, a single unified expression or 

lightweight concrete modification factor is 

not specified. This section proposes a new 

term, the -factor, to quantify the modification in nominal resistance that could 

be included in any expression for nominal 

resistance. The -factor relates to the material properties of structural lightweight 

concrete, so the new article for the 

definition for the -factor could be located 

in Article 5.4.2, “Normal Weight and 

Structural Lightweight Concrete.” The 

-factor will be referred to as Article 5.4.2.8 

in the present document. The proposed text 

for the -factor is as follows:
13

Where lightweight aggregate concretes are 

used, the lightweight concrete modification 

factor, , shall be determined using the 

equation in figure 7 where fct is specified.

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

Where:

fct = Concrete splitting tensile strength in ksi.

f’c = Compressive strength in ksi. 

wc = Concrete unit weight in kcf.

 = Lightweight concrete modification factor.

Proposed Design Expressions for Nominal 

Shear Resistance

Three recommended changes to the 

AASHTO LRFD Bridge Design Specifications 

involve adding the -factor to the three 

terms for the nominal shear resistance 

provided by the concrete (i.e., Vc, Vci, Vcw). 

The change to the Vc term is described 

first and includes the existing language for 

the nominal shear resistance (Vn). The 

changes to Vci and Vcw are presented after 

the expression for Vc. The proposed design 

expression for nominal shear resistance 

provided by tensile stresses in the concrete 

(Vc) is as follows: 

The nominal shear resistance, Vn, shall be 

determined using the equation in figure 9.

Where:

bv = Effective web width (inch). 

dv = Effective shear depth (inch).

Vc = Nominal shear resistance provided by 

tensile stresses in the concrete (kip). 

Vn = Nominal shear resistance of the 

section (kip).

Vp = Component of the effective prestressing force in the direction of the applied 

shear (kip). 

Vs = Nominal shear resistance provided by 

the shear reinforcement (kip). 

Where the procedures of Articles 5.8.3.4.1 

or 5.8.3.4.2 are used, Vc shall be determined 

using the equation in figure 10.

Where the procedures of Article 5.8.3.4.3 

are used, Vc shall be determined as the 

lesser of Vci (determined using the equation 

in figure 11) and Vcw (determined using the 

equation in figure 12).

Figure 7. Expression for -factor with fct specified.

Figure 8. Expression for -factor with fct specified not 

specified.

Figure 9. Expression for Vn.

Figure 10. Expression for Vc.
14

Where: 

fpc = Compressive stress at the centroid of 

the concrete after all prestress losses have 

occurred (ksi).

Mcre = Moment causing flexural cracking at 

the section due to externally applied loads 

(kip-inch).

Mmax = Maximum factored moment at the 

section due to externally applied loads 

(kip-inch).

Vci = Nominal shear resistance provided 

by concrete when inclined cracking results 

from combined shear and moment (kip).

Vcw = Nominal shear resistance provided 

by concrete when inclined cracking results 

from excessive principal tensions in the 

web (kip).

Vd = Shear force at the section due to unfactored dead load (kip).

Vi

 = Factored shear force at the section due 

to externally applied loads occurring simultaneously with Mmax (kip).

 = factor indicating ability of diagonally 

cracked concrete to transmit tension and 

shear.

The ratio of test-to-predicted shear resistance for lightweight concrete specimens 

in the TFHRC Shear Database is compared 

with compressive strength in figure 13 and 

figure 14. Figure 13 shows the ratios for 

RC specimens without shear reinforcement, and figure 14 shows the ratios for 

PC specimens with shear reinforcement. 

For comparison, the ratios for NWC specimens in the ACI-DafStb Database are also 

shown in each figure. The nominal shear 

resistance was determined using the GP 

with the equations for  and  given in 

Article 5.8.3.4.2 and the new expression 

-factor. Regression lines are shown for 

the lightweight concrete specimens and 

NWC specimens. The regression line for the 

lightweight concrete specimens is slightly 

greater than the regression line for NWC 

specimens for nearly all values of compressive strength.

Figure 11. Expression for Vci.

Figure 12. Expression for Vcw.
15

Figure 13. Test-to-predicted shear resistance using GP for RC members without shear reinforcement.

Figure 14. Test-to-predicted shear resistance using GP for PC members with shear reinforcement.
16

Proposed Design Expression for Minimum 

Transverse Reinforcement

A recommended change to the AASHTO 

LRFD Bridge Design Specifications involves 

adding the new expression for -factor to 

the design expression for minimum transverse reinforcement. The proposed design 

expression for the minimum transverse 

reinforcement is as follows: 

Except for segmental post-tensioned concrete box girder bridges, where transverse 

reinforcement is required, as specified in 

Article 5.8.2.4, the area of steel shall satisfy 

the equation in figure 15:

Where:

Av = Area of transverse reinforcement 

within distance s (inch2

).

s = Spacing of shear reinforcement (inch).

fy = Yield strength of transverse reinforcement (ksi).

The ratio of test-to-predicted shear resistance for lightweight concrete specimens 

in the TFHRC Shear Database is compared 

with the transverse reinforcement ratio in 

figure 16 for RC specimens and figure 17 

for PC specimens. The nominal shear 

resistance was determined using the GP 

with the equations for  and  given in 

Article 5.8.3.4.2 and the new expression for 

-factor. A vertical line indicates the minimum amount of transverse reinforcement 

specified by Article 5.8.2.5. The ratios of testto-predicted shear resistance for the few 

specimens with less transverse reinforcement than the required amount were similar to the ratios for specimens with slightly 

greater than the required amount. For comparison, NWC specimens in the ACI-DafStb 

Database are also shown in the figures.

Figure 15. Expression for minimum transverse 

reinforcement.

Figure 16. Test-to-predicted shear resistance compared with transverse reinforcement ratio using GP for RC specimens.
17

Proposed Resistance Factor for Shear and 

Torsion of Lightweight Concrete

A reliability analysis was performed to 

evaluate the resistance factor for lightweight concrete in shear. Based on the 

analysis, a change to the reduction factor 

for lightweight concrete in shear and 

torsion is recommended. The proposed 

resistance factor for lightweight concrete 

in shear and torsion is 0.90. This resistance factor is the same as that specified 

for NWC for shear and torsion.

Table 7 shows a comparison of the reliability index for lightweight concrete using 

a resistance factor of 0.90 to the reliability 

index for NWC using the resistance factor 

of 0.90 as specified in Article 5.5.4.2.1. The 

reliability indexes were determined using 

the three different methods for calculating 

shear resistance in Article 5.8.3 and the new 

expression for -factor. Table 7 shows that 

for nominal resistance determined using 

the GP, the reliability index for lightweight 

concrete with a resistance factor of 0.90 was 

greater than the reliability index for NWC.

Figure 17. Test-to-predicted shear resistance compared with transverse reinforcement ratio using GP for PC specimens.
18

Concluding Remarks

This document describes shear tests on 

lightweight concrete prestressed girders, 

summarizes a database of lightweight 

concrete and NWC shear tests, describes a 

reliability analysis, and presents potential 

revisions to the AASHTO LRFD Bridge 

Design Specifications relating to the shear 

resistance of lightweight concrete. The 

proposed design expressions for shear 

resistance were compared with test 

results in a database collected as part of 

this research effort. A full description of 

the database and the development and 

evaluation of prediction expressions is 

included in the full report.(8) Future phases 

of this research program and analysis 

effort will focus on other structural performance attributes as related to lightweight concrete. The test results will be 

compared with the prediction expressions 

for nominal resistance in the AASHTO LRFD 

Bridge Design Specifications incorporating 

appropriate proposed revisions for lightweight concrete mechanical properties.

References

1. AASHTO (2012), AASHTO LRFD Bridge 

Design Specifications, Customary U.S. 

Units, 6th Ed., Washington, DC.

2. ACI Committee 213 (1967), “Guide 

for Structural Lightweight Aggregate 

Concrete,” ACI Journal, 64(8), American 

Concrete Institute, August, pp. 433–469.

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

Diagonal Tension Resistance of Structural 

Lightweight Concrete,” ACI Journal 

Proceedings, 58(1), July, pp. 1–40.

Table 7. Reliability index for lightweight concrete and NWC in shear.

G.P.-Eq. = General Procedure using equations (Article 5.8.3.4.2).

NWC = normal-weight concrete.

PC = prestressed concrete.

RC = reinforced concrete.

S.P.-RC = Simplified Procedure for Nonprestressed Sections (Article 5.8.3.4.1).

S.P.-RC/PC = Simplified Procedure for Prestressed and Nonprestressed Sections (Article 5.8.3.4.3).

Specimen Group Design Expression

Reliability Index (RQ)

Lightweight 

Concrete 

( = 0.90) 

NWC 

( = 0.90)

RC Beam Without Shear 

Reinforcement

S.P.-RC 2.99 2.73

G.P.-Eq. 2.69 2.43

S.P.-RC/PC 2.91 2.41

RC Beam With Shear 

Reinforcement

S.P.-RC 3.07 3.43

G.P.-Eq. 3.07 2.79

S.P.-RC/PC 2.92 3.70

PC Girder Without Shear 

Reinforcementt

G.P.-Eq. 2.00 1.83

S.P.-RC/PC 1.54 3.02

PC Girder With Shear 

Reinforcement

G.P.-Eq. 4.75 3.16

S.P.-RC/PC 4.10 2.81
19

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

Tension Test of Structural Lightweight 

Concrete,” ASTM Journal of Materials, 

1(4), pp. 859–871.

5. Pauw, A. (1960), “Static Modulus of 

Elasticity of Concrete as Affected by 

Density,” ACI Journal, 57(6), December, 

pp. 679–687.

6. Russell, H. (2007), Synthesis of Research 

and Provisions Regarding the Use of 

Lightweight Concrete in Highway 

Bridges, FHWA-HRT-07-053, FHWA, 

Washington, DC.

7. ASTM A615 (2012), “Standard 

Specification for Deformed and Plain 

Carbon-Steel Bars for Concrete 

Reinforcement,” Book of Standards, 

01.04, West Conshohocken, PA.

8. Greene, G., and Graybeal, B. 

(2015), Lightweight Concrete: Shear 

Performance, FHWA-HRT-15-022, FHWA, 

Washington, DC.

9. Reineck, K.-H., Kuchma, D.A., and with 

Fitik, B. (2010), Extended Databases 

With Shear Tests on Structural Concrete 

Beams Without and With Stirrups for 

the Assessment of Shear Design Procedures, Research Report, Institut für 

Leichtbau Entwerfen und Konstruieren 

(ILEK) (Institute for Lightweight 

Structures and Conceptual Design), 

University of Stuttgart and University of 

Illinois, Champaign, Urbana, July.

10. Reineck, K.-H., Bentz, E., Fitik, B., 

Kuchma, D.A., and Bayrak, O. (2013), 

“The ACI-DAfStb Database with Shear 

Tests on Slender Reinforced Concrete 

Beams Without Stirrups,” ACI Structural 

Journal, 110(5), September–October, 

pp. 867–875.

11. Nowak, A.S., and Collins, K. (2000), 

Reliability of Structures, 2nd Ed., 

McGraw-Hill Higher Education, New 

York, NY.

12. Paczkowski, P., and Nowak, A.S. 

(2010), Reliability Models for Shear 

in Lightweight Reinforced Concrete 

Bridges, Concrete Bridge Conference, 

Phoenix, AZ.

13. Greene, G., and Graybeal, B. (2013), 

Lightweight Concrete: Mechanical 

Properties, FHWA-HRT-13-061, FHWA, 

Washington, DC.

14. Greene, G., and Graybeal, B. (2013), 

Lightweight Concrete: Mechanical 

Properties, National Technical 

Information Service Report No. 

PB2013 -107688, FHWA, Washington, DC.
20

APRIL 2015 FHWA-HRT-15-021

HRDI-40/04-15(200)E

Researchers—This study was led by Ben Graybeal at FHWA’s TFHRC. It was conducted by Gary Greene 

of PSI, Inc. through laboratory support contract DTFH61-10-D-00017. For additional information, contact 

Ben Graybeal at (202) 493-3122 or in the FHWA Office of Infrastructure Research and Development located at 6300 Georgetown Pike, McLean, VA, 22101-2296. 

Distribution—This TechBrief is being distributed according to a standard distribution. Direct distribution is being made to the Divisions and Resource Center.

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—Lightweight concrete, bridge design, AASHTO LRFD design specifications.

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