Evaluation of allowable stresses for high strength concrete prestressed bridge girders

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 150-153).Issued also on microfiche from Lange Micrographics.High strength concrete (HSC) is regularly used for prestressed bridge girders in Texas and other states. The American Concrete Institute (ACI) Committee 363 (1997) defines HSC as concrete with a compressive strength exceeding 6,000 psi (41 MPa) produced without using exotic materials or techniques. The design provisions for the design of prestressed concrete members according to the AASHTO Standard and LRFD Specifications are based on mechanical properties determined for normal strength concrete (NSC). Therefore, there is a need to evaluate these specifications based on the properties of HSC. The main objective of this research is to evaluate the allowable tensile stresses for HSC prestressed bridge girders. The effects of field curing conditions on the compressive strength and the flexural tensile strength of HSC were determined. The HSC mixtures tested in this study are representative of mixtures used in Phase 1 of this research program (Chompreda 2001). The plant-produced samples from the earlier study were lab cured after the first day (approximately 24 hours). Necessary strength adjustment factors are needed to account for the effect of field curing conditions. These factors were determined for representative HSC mixtures and applied to the compressive and flexural strength data from Phase 1 in order to evaluate the potential to increase the limiting allowable stresses. A parametric study was conducted to evaluate the impact of a modified tensile stress limit on the design of typical HSC prestressed girders. The safety of the modified tensile stress limit was evaluated by applying structural reliability theory

Mathematical model for predicting the ultrasonic pulse velocity of concrete

AbstractThis paper proposes a mathematical model approach to predict the Ultrasonic Pulse Velocity (UPV) of concrete. This paper presents the formulation, calibration, evaluation, and validation of the proposed UPV model. An experimental program was developed to calibrate and evaluate the proposed model. Furthermore, the model was validated using a separate group of measurements in the experimental program, to test its accuracy and generalization capability. Analysis of the results reveals that the proposed model provides a good fit to the experimental data and does not contain outliers or discerning pattern. The corresponding standard error and determination coefficient are 45 m/s and 0.92, respectively. Model validation demonstrates high predictability and generalization capability. The corresponding standard error and determination coefficient are 74 m/s and 0.91, respectively