U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
2023664000
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
TECHBRIEF 
This TechBrief is an archived publication and may contain dated technical, contact, and link information 

Publication Number: FHWAHRT13061 Date: June 2013 
Publication Number:
FHWAHRT13061
Date: June 2013 
PDF files can be viewed with the Acrobat® Reader®
FHWA Publication No. of this TechBrief: FHWAHRT13061 FHWA Contact: Ben Graybeal, HRDI40, (202) 4933122, benjamin.graybeal@dot.gov. 
This document is a technical summary of the Federal Highway Administration report, Lightweight Concrete: Mechanical Properties (FHWAHRT13062), available through the National Technical Information Service at www.ntis.gov.^{(1)}
NTIS Accession No. of the report covered in this TechBrief: PB2013107688
There is a limited amount of test data on the mechanical properties of highstrength lightweight concrete (LWC) with a concrete unit weight (w_{c}) between that of traditional LWC and normal weight concrete (NWC). Concrete with a w_{c} in this range is also not covered in the American Association of State Highway and Traffic Officials (AASHTO) LoadandResistance 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 w_{c} to determine trends for LWC as a function of w_{c}. New design expressions for mechanical properties are proposed for LWC as a function of w_{c} 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)}
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. Broadbased 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 highstrength concrete, has resulted in revisions to the AASHTO LRFD Bridge Design Specifications to capitalize on the benefits of highstrength 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) TurnerFairbank 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/m^{3}). 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, timedependent 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 (E_{c}), and splitting tensile strength (f_{ct}) 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.
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/m^{3}). 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. Each uses partial replacement of the coarse aggregate with lightweight aggregate to achieve their reduced w_{c}. 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 highrange water reducer.
Table 1. Selected concrete mix designs.
Cast Date  Haydite Girder (HG) 
Stalite Girder (SG) 
Utelite Girder (UG) 
Stalite Deck (SD) 

Design 28day 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. 
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 w_{c}. This was intended to remove batchtobatch 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 8inch (102 by 203mm) and 6 by 12inch (152 by 305mm) cylinders to determine the compressive strength at release of prestressing, at 28 days, and at girder testing. E_{c} was determined using one of the 4 by 8inch (102 by 203mm) cylinders intended for compressive strength testing. The indirect tensile strength was measured on 4 by 8inch (102 by 203mm) cylinders using the f_{ct} test. Density measurements were made to determine the airdry density of cylinders used for compression testing. They were also conducted on separate cylinders to determine the ovendry density and equilibrium density. Average compressive strength, E_{c}, f_{ct}, and airdry w_{c} for each concrete mix are provided in table 2
Table 2. Mean concrete properties from tests on 4 by 8inch (102 by 203mm) cylinders.
Concrete Mix  Specimen Age 
Compressive 
AirDry Density 
f_{ct} 
E_{c} 

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. 
The LWC test results were compared to design expressions for a lightweight modification factor and for E_{c}. Nearly all f_{ct} tests on all three girder mixes gave splitting ratios that were greater 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, E_{c} was overestimated by the AASHTO LRFD expression and underestimated by the NCHRP 1264 expression and the ACI 36310 expression.^{(2,8,9)}
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, E_{c}, f_{ct} test, modulus of rupture (f_{r}), 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)}
A total of 2,556 data lines are in the TFHRC subset database for E_{c}. To compare design expressions for E_{c} to both NWC and LWC data, the E_{c} database from NCHRP Project 1264 was utilized.^{(8)} The NWC and LWC data contain lines of compressive strength, E_{c}, and w_{c}.^{(8)} For this evaluation, test data from concrete with a w_{c} greater than or equal to 0.135 kcf (2,160 kg/m^{3}) (i.e., NWC data) from the NCHRP 12 64 database was combined with test data from concrete with a w_{c} less than 0.135 kcf (2,160 kg/m^{3}) (i.e., LWC data) from the TFHRC database.
The E_{c} 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 highstrength concrete. ^{(2,8,9)} The ratio of the tested E_{c} to the predicted E_{c} by the three design expressions is provided in table 3. A testtoprediction ratio greater than unity indicates an underestimation of E_{c}, while a ratio less than unity indicates an overestimation of E_{c}. The mean testtoprediction ratios in table 3 show that the AASHTO LRFD expression overestimates E_{c} of LWC, and the NCHRP 1264 expression underestimates E_{c} of LWC. The ACI 36310 expression closely predicts E_{c} of LWC but underestimates E_{c} of NWC. The testtoprediction 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 E_{c} at higher compressive strength levels for both NWC and LWC.
Table 3. Mean testtoprediction ratio of E_{c} for LWC data from the TFHRC database and NWC data from the NCHRP 1264 database.
Data Source 
AASHTO 
NCHRP 
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 
Optimization of E_{c} Equation Variables
An analysis was performed to evaluate the effect of different exponents on the basic form of the expression for E_{c}. The analysis was performed on a database consisting of the TFHRC LWC subset database combined with the NCHRP 1264 NWC database.^{(1,8)} The analysis was divided into three parts. In the first part, the exponent applied to the w_{c} term was varied and showed that an exponent of 1.5 or 2.0 applied to w_{c} resulted in the lowest coefficient of variation (COV) and a testtoprediction 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 E_{c} for LWC data. The third part was to vary the exponents applied to both w_{c} and compressive strength. A new proposed expression with an exponent of 2.0 for w_{c} 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 E_{c} for LWC and gave a close prediction of E_{c} for NWC.
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 alllightweight concrete, and a 0.85 factor is used for sandlightweight 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 alllightweight concrete and sandlightweight 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 f_{ct} to the square root of the compressive strength is known as the splitting ratio, F_{sp}. Early references to F_{sp} 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 F_{sp} greater than 0.212 does not require modification of the expressions in Articles 5.8.2 and 5.8.3 for LWC. F_{sp} implied by the AASHTO LRFD Bridge Design Specifications for sandlightweight concrete and alllightweight concrete are based on the 0.85 and 0.75 modification factors described in Article 5.8.2.
The f_{ct} subset of the TFHRC LWC database was used to evaluate the expression for F_{sp} implied by the AASHTO LRFD Bridge Design Specifications.^{(2)} The database includes 954 lines of sandlightweight and 311 lines of alllightweight concrete. The testtoprediction ratios for the sandlightweight and alllightweight AASHTO LRFD expressions for F_{sp} are given in table 4. A testtoprediction 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.
Table 4. Testtoprediction ratios of Fsp using the AASHTO LRFD expression and proposed expression.
LWC 
F_{sp} Expression 
Total 
w_{c} ≤ 
0.090 
0.100 
0.110 
0.120 

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 

Alllightweight 
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/m^{3} 
Linear Expressions for F_{sp} Using w_{c}
An expression for predicting F_{sp} as a function of w_{c} was developed. This section describes this piecewise continuous function for predicting F_{sp}. Other types of expressions for F_{sp} are evaluated in the full report. ^{(1)} The expression consists of a constant predicted F_{sp} of 0.159 for w_{c} = 0.100 kcf (1,600 kg/m^{3}). The prediction then assumes a linearly increasing F_{sp} with w_{c} to a limit on w_{c} of 0.135 kcf (2,160 kg/mM^{3}). For w_{c} = 0.135 kcf (2,160 kg/m^{3}), a constant predicted value of 0.212 for F_{sp} is used since this aligns with the existing provisions for NWC. A lower limit of 0.159 on F_{sp} is used because this value is specified in Article 5.8.2.2 as F_{sp} for alllightweight concrete (0.75 x 0.212).^{(2)} The testtoprediction ratios for the proposed expression are shown in figure 3.
Figure 3. Graph. Testtoprediction ratios of Fsp predicted by the proposed expression.
The proposed expression gave a larger predicted f_{ct} than the expression in the AASHTO LRFD Bridge Design Specifications for sandlightweight concrete with a w_{c} up to 0.110 kcf (1,760 kg/m^{3}). ^{(2)} For larger unit weights, the AASHTO LRFD expression gave a very conservative prediction of f_{ct}.
The proposed expression for Fsp can be converted to LWC modification factor by dividing it by 0.212, the upper limit on F_{sp}. The term λfactor is used to refer to a LWC modification factor.
The accuracy of the f_{r} expression is important for the strength, serviceability, and ductility of structural concrete bridges. The AASHTO LRFD Bridge Design Specifications have different expressions for f_{r} depending on the use of the calculation and the type of concrete. ^{(2)} For normal weight concrete, one expression for f_{r} is used to calculate the nominal shear resistance provided by concrete when inclined cracking results from combined shear and moment (V_{ci}) (Article 5.8.3.4.3), and another expression for f_{r} 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 sandlightweight 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, and cracking moment for V_{ci} when used for members made from LWC.
Comparison of f_{r} to f_{ct}
In this section, f_{r} is compared to the f_{ct} in order to justify defining the material property f_{r} in terms of another material property f_{ct} (through the λfactor).
For this comparison, a new subset database was created for concrete mixes with test results in both the f_{ct} subset database and a wet f_{r} subset database. An alternate wet f_{r} subset database was created to include only specimens that remained wet until tested due to the reduction in the tested f_{r} of specimens allowed to dry. A comparison of f_{r} and f_{ct} is shown in figure 4. The figure shows fr increasing proportional to f_{ct}, which supports the observations of previous research on a limited number of data points. ^{(4)}
Proposed Design Expression for ƒ_{r}
A new expression for f_{r} was proposed that includes the LWC modification factor (λfactor). The proposed expression for f_{r} 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 f_{r} subset database to the f_{r} 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 f_{r} that were larger than the tested values.
Table 5. Testtoprediction ratios of fr using the AASHTO LRFD expression and proposed expression.
LWC 
f_{r} 
Total 
w_{c} ≤ 
0.090 
0.100 
0.110 
0.120 

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 

Alllightweight 
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/m^{3} 
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 E_{c} and the subset database for f_{ct} has resulted in several new expressions for E_{c}, an LWC modification factor (λfactor), and f_{r}. The new expressions are not based on the proportions of constituent materials and include tests from types of 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 sandlightweight concrete and alllightweight concrete would no longer be needed. This section proposes a revised definition of LWC that does not include the terms sandlightweight concrete or alllightweight concrete.
Proposed Definition for LWC
The definition for LWC in Article 5.2 of the AASHTO LRFD Bridge Design Specifications limits w_{c} for LWC to 0.120 kcf (1,920 kg/m^{3}) and includes definitions for sandlightweight and alllightweight concrete.^{(2)} The proposed definition for LWC expands the range of w_{c} 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/m^{3}), as determined by ASTM C567. ^{(11)}
The term "airdry 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 "airdry 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 "airdry unit weight" or the term "equilibrium density" could be used in the definition for LWC.
Proposed Expression for E_{c}
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 E_{c} itself. The proposed expression for E_{c} is shown in figure 5.
According to the AASHTO LRFD Bridge Design Specifications, in the absence of measured data, E_{c} for concrete with unit weights between 0.090 and 0.155 kcf (1,440 and 2,480 kg/m^{3}) and specified compressive strengths up to 15.0 ksi (103 MPa) may be taken as follows:^{(2)}
Where:
E_{c} = Modulus of elasticity in ksi.
K_{1} = Correction factor for source of aggregate.
w_{c} = 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 w_{c} of 0.110 kcf (1760 kg/m^{3}) and K_{1} equal to unity.
1 ksi = 6.89 MPa 
Figure 6. Graph. E_{c} for proposed expression.
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 λfactor, to quantify the modification in nominal resistance that could be included in any expression for nominal resistance. The λfactor relates to the material properties of structural LWC so the new article for the definition for the λfactor 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 f_{ct} is specified.
Where f_{ct} is not specified, λ shall be determined using the equation in figure 8.
An illustration of the proposed expression for the λfactor is shown in figure 9, and the predicted splitting ratios (λfactor x 0.212) are shown in figure 10. The λfactors implied in AASHTO LRFD for sandlightweight concrete and alllightweight concrete are also shown. Figure 10 shows that a considerable amount of sandlightweight concrete data are not defined in the current AASHTO LRFD Bridge Design Specifications.^{(2)}
0.001 kcf=16.01 kg/m^{3} 
Figure 9. Illustration. Proposed expression for λ factor.
1ksi=6.89 MP_{a} 
Figure 10. Graph. Splitting ratio (f_{ct} / √f'_{c} ) for the proposed expression ( λfactor x 0.212).
As stated previously, the effect of using the λfactor in expressions for nominal resistance needs to be evaluated. The proposed λfactor could then be included in the expression for nominal resistance in the AASHTO LRFD Bridge Design Specifications.^{(2)} For example, the λfactor 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)}
Proposed Expression for f_{r}
The expression for f_{r} in the AASHTO LRFD Bridge Design Specifications is in Article 5.4.2.6. ^{(2)} The proposed expression for f_{r} 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 f_{r} include the proposed λfactor 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 (V_{ci}) includes the proposed λfactor for consistency. The f_{r} 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 f_{r} (see figure 11) to √f 'c is shown in figure 13 with sandlightweight and alllightweight concrete data. Figure 13 shows that most of the test data are above the predicted f_{r} (i.e., underestimated) and that a considerable amount of the sandlightweight concrete data are in the gap of w_{c} not defined in the current AASHTO LRFD Bridge Design Specifications. ^{(2)}
1 ksi=6.89 MPa 
Figure 13. Graph f_{r} / √f '_{c} for the proposed expression (0.24 λ √f '_{c} ) .
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 E_{c}, LWC modification factor, and f_{r} 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.
Researchers —This study was led by Ben Graybeal at FHWA's TurnerFairbank 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) 4933122 or in the FHWA Office of Infrastructure Research and Development located at 6300 Georgetown Pike, McLean, VA, 221012296. Distribution —The report (PB2013107688) 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) 2392156, phone to (814) 2391160, or online at https://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 highquality 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. 