| Two grades of high-performance concrete (HPC) were investigated for more than a year in order to determine their time-dependent mechanical properties as well as their time-dependent structural responses in precast prestress girders. The design compressive strengths for the two grades of HPC were 10,000 psi and 14,000 psi at 56-day age for Grade 2 and Grade 4 HPC, respectively. The corresponding minimum average 56-day compressive strengths (according to ACI 318) were 11,400 psi and 15,400 psi for Grade 2 and Grade 4 HPC, respectively.; In addition to a large number of compressive strength and modulus of elasticity tests, 20 creep and shrinkage tests were conducted. Test variables included the concrete age at time of load application (12 hours to 28 days), the applied stress ratio (0.16 to 0.6 of the ultimate strength at loading), and the initial curing method (ASTM curing and heat curing). The inaccuracy of the current creep and shrinkage models with respect to HPC was illustrated. A new model for predicting creep and shrinkage strains of HPC is proposed. The new model compared favorably with published data.; The time-dependent prestress loss and camber of four 33&feet;5 1/2&inches; long, AASHTO Type II girders and six 9 in. x 18 in. x 14 ft. beams were monitored for more than a year. Test results indicated that the current AASHTO-LRFD code and the PCI Design Handbook significantly overestimate the time-dependent prestress loss and deflection of HPC precast beams. A step-function method and a time-step analysis, using the proposed creep and shrinkage model, were found to predict the responses within ±10%. Modifications to the AASHTO-LRFD provisions on prestress losses are proposed to account for the characteristics of UPC. The modified provisions compared favorably with published data. |
