Summer 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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C R E AT I V E C O N C R E T E C O N S T R U C T I O N ASPIRE Summer 2019 | 49 L ightweight concrete may have reduced tensile strength compared to normalweight concrete. If the tensile strength is reduced, aspects of design related to tensile strength, such as shear and development length of reinforcement, are usually affected. The American A ssociation of State Highway and Transpor tation Officials' (AASHTO's) bridge design specifications have included provisions that reflect this potential reduction to the properties of lightweight concrete since at least 1983. However, a variable had not been assigned to the reduction factor. Therefore, the factor had to be defined wherever it was needed and, without a variable assigned, the factor could not be inserted into equations. The current version of the AASHTO LRFD Bridge Design Specifications 1 contains a major revision to provisions addressing lightweight concrete that was adopted in 2015. The revision defined the concrete density modification factor, λ, in Article and included the variable in all equations where it was appropriate. For normalweight concrete, λ = 1.0. In Article, two approaches are provided to determine the value of λ for lightweight concrete. The first option is based on the ratio of a specified splitting tensile strength, f ct , of lightweight concrete to the computed splitting tensile strength of normalweight concrete. This option is rarely used, but it can provide significant benefit when shear or development lengths are important factors in design. Prior to 2015, the second option for determining λ, which is used when the splitting tensile strength is not specified, was based on the type of lightweight concrete: 0.85 for sand-lightweight concrete and 0.75 for all- lightweight concrete. The 2015 revision changed this approach by defining λ using the unit weight of concrete w c , which is a great improvement for designers. 2 AASHTO LRFD specifications Eq. is: 0.75 ≤ λ = 7.5 w c ≤ 1.0 Recent tests of lightweight concrete for a range of compressive strengths and types of lightweight aggregate 3,4,5 have shown that the splitting tensile strength of lightweight concrete often exceeds the computed tensile strength of normalweight concrete, which is not defined in the specifications, but can be determined by solving AASHTO LRFD specifications Eq. (with λ = 1.0) for f ct : f ct = 1 4.7 ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ʹ f c = 0.213 ʹ f c in ksi ( ) An example of results from research is presented in Table 1, which shows splitting tensile strengths for a series of lightweight concrete bridge deck mixtures made with three types of lightweight aggregate and a normalweight control mixture. 4 Using a design compressive strength of 4.0 ksi for the deck concrete and the expression for f ct shown previously, the predicted splitting tensile strength for normalweight concrete would be 0.426 ksi. All values of splitting tensile strength for the lightweight concrete in Table 1 are comfortably above this value; only the splitting tensile strength of the normalweight concrete is close to the predicted value. For this study, constituent proportions for the mixtures were held constant and the lightweight concrete compressive strengths were less than the normalweight concrete strength. Proportions of the lightweight concrete mixtures could have been adjusted to produce compressive strengths equal to the control. As shown in the last two columns of the table, the measured tensile strengths of the lightweight concretes were greater than the predicted tensile strengths computed using measured compressive strengths, whereas the measured tensile strengths of the normalweight concrete mixture were below the predicted value. These data indicate that, for these lightweight concrete mixtures, f ct could be specified equal to the predicted splitting tensile strength using λ = 1.0. Table 1. Splitting tensile and compressive strength test results (ksi) and comparison of test results to predicted splitting tensile strength for lightweight and normalweight bridge deck concrete mixtures (test data from Ref. 4) f ct f c f ct 0.213 f c ( ) NWC 0.438 5.505 0.880 Type of LWC Sand All Sand All Sand All Slate LWA 0.490 0.461 5.135 4.685 1.021 1.005 Clay LWA 0.520 0.493 5.200 4.675 1.076 1.075 Shale LWA 0.510 0.465 4.980 4.550 1.079 1.029 Key: NWC = normalweight concrete; LWC = lightweight concrete; LWA = lightweight aggregate. The types of LWC are defined in Ref. 4. A second example of test results, in this case taken from production of prestressed lightweight concrete girders, is presented in Table 2 for three projects that all used the same mix proportions and type of lightweight aggregate. 5 As shown in the last three rows of data in the table, the measured splitting tensile strengths of lightweight concrete were greater than the predicted tensile strengths computed using specified compressive strengths. Again, these data indicate that, for this lightweight concrete mixture, f ct could be specified equal to the predicted splitting tensile strength using λ = 1.0. Specifying Splitting Tensile Strength for Lightweight Concrete to Improve and Simplify Design by Dr. Reid W. Castrodale, Castrodale Engineering Consultants PC

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