Compressive Strength and Modulus of Elasticity Relationships for Alabama Prestressed Concrete Bridge Girders
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The state of Alabama constructs many of its bridges using precast, prestressed concrete bridge girders. There is a desire to more accurately predict girder cambers through the time of deck placement. Proper understanding of the short-term and long-term material properties of Alabama concretes is important information that must be obtained before consistently accurate camber predictions can be made. Common practices of girder producers were observed and compared for predictable trends. Historical records including more than 1900 girder pours from four girder producers over a six-year period were analyzed for patterns in project requirements, fresh concrete properties, measured concrete compressive strengths, and producer practices. Using strength measurements and curing temperature measurements from 435 girder pours, the authors established trends in concrete maturity at prestress transfer and in compressive strength development over time. Methods for predicting average concrete compressive strength at prestress transfer and at 28 days were developed based on historical records, and were compared to in-plant measured strengths. The compressive strength prediction methods were used to improve modulus of elasticity prediction accuracy for the 2012 AASHTO LRFD Bridge Design Specifications. Modulus of elasticity predictions were compared using five or more different prediction methods for both release and 28 days. Lastly, modulus of elasticity development over time was compared to concrete strength development over time.