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Physical and mechanical properties

- Jan 15, 2019 -

In discussions related to the properties of FRP bars or tendons, the following points must be kept in mind. 

First, an FRP bar is anisotropic, with the longitudinal axis being the strong axis. 

Second, unlike steel, mechanical properties of FRP composites vary significantly from one product to another. 

Factors such as volume and type of fiber and resin, fiber orientation, dimensional effects, and quality control during manufacture, play a major role in establishing product characteristics. 

Furthermore, the mechanical properties of FRP composites, like all structural materials, are affected by such factors as loading history and duration, temperature, and moisture. 

While standard tests have been established to determine the properties of traditional construction materials, such as steel and concrete, the same cannot be said for FRP materials. 

This is particularly true for civil engineering applications, where the use of FRP composites is in its stage of infancy. 

It is therefore required that exact loading conditions be determined in advance and that material characteristics corresponding to those conditions be obtained in consultation with the manufacturer. 

Specific gravity—FRP bars and tendons have a specific gravity ranging from 1.5 to 2.0 as they are nearly four times lighter than steel. 

The reduced weight leads to lower transportation and storage costs and decreased handling and installation time on the job site as compared to steel reinforcing bars. 

This is an advantage that should be included in any cost analysis for product selection. Thermal expansion—Reinforced concrete itself is a composite material, where the reinforcement acts as the strengthening medium and the concrete as the matrix. 

It is therefore imperative that behavior under thermal stresses for the two materials be similar so that the differential deformations of concrete and the reinforcement are minimized. Depending on mix proportions, the linear coefficient of thermal expansion for concrete varies from 6 to 11 x 10-6 /° C (Mindess et al. 1981). 

Tensile elastic modulus—The longitudinal modulus of elasticity of GFRP bars is approximately 25 percent that of steel. 

The modulus for CFRP tendons, which usually employ stiffer fibers, is higher than that of GFRP reinforcing bars. 

Compressive strength—FRP bars are weaker in compression than in tension.

 This is the result of difficulties in accurately testing unidirectional composites in compression, and is related to gripping and aligning procedures, and also to stability effects of fibers. 

Higher compressive strengths are expected for bars with higher tensile strength. 

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