Mechanical Properties of High Strength Concrete for Prestressed Concrete Bridge Girders

This is the second of four reports that document the findings of a Texas Department of Transportation sponsored research project to evaluate the allowable stresses and resistance factors for high-strength concrete (HSC) prestressed bridge girders. HSC is widely used in prestressed concrete bridges. However, current design provisions for prestressed concrete bridge structures, such as the American Association of State Highway and Transportation Officials (AASHTO) Load and Resistance Factor Design(LRFD) Specifications, were developed based on mechanical properties of normal strength concrete (NSC). As a first step toward evaluating the applicability of current AASHTO design provisions for HSC prestressed bridge members, statistical parameters for the mechanical properties of plant-produced HSC were determined. In addition, prediction equations relating mechanical properties with the compressive strength were evaluated. HSC samples were collected in the field from precasters in Texas and tested in the laboratory at different ages for compressive strength, modulus of rupture, splitting tensile strength, and modulus of elasticity. Statistical analyses were conducted to determine the probability distribution, bias factors (actual mean-to-specified design ratios), and coefficients of variation for each mechanical property. Creep and shrinkage were also monitored and evaluated. Researchers found that for each short-term mechanical property, the mean values are not significantly different among the considered factors (precaster, age, specified strength class) or combination of these factors, regardless of the specified design compressive strength. Overall, the 28-day bias factors (mean-to-nominal ratios) decrease with an increase in specified design compressive strength due to the relative uniformity of mixture proportions provided for the specified strength range. Nevertheless, the 28-day bias factors for compressive strength are greater than those used for the calibration of the AASHTO LRFD Specifications. With few exceptions, the coefficients of variation were uniform for each mechanical property. In addition, the coefficients of variation for the compressive strength and splitting tensile strength of HSC in this project are lower than those for NSC used in the development of the AASHTO LRFD Specifications. Based on the experimental data, the creep and shrinkage of the HSC in this project are overestimated by the AASHTO equations that predict the development of creep and shrinkage with time.