The long-term camber of prestressed bridge girders are typically over-estimated at erection (typically 3 month after production of girders), especially for long-span bulb tee girders, which possibly leads to increase cost due to the more use of haunches between the bridge girder and deck, and unnecessary delay of construction. Creep and shrinkage of concrete play an important role in the long-term camber of a prestressed bridge girder. The current models used to predict the creep and shrinkage of concrete have the large difference with the actual behavior of concrete cast using local materials in Iowa. In order to improve the accuracy of prediction of the camber of prestressed bridge girders, creep and shrinkage tests of concrete using local materials were performed. Seven mixes from three precast plants were investigated in this study, in which four of them were high performance concrete (HPC) that are currently used to cast prestressed bridge girders, and three of them were normal concrete (NC) that were utilized to produce prestressed bridge girders previously. Mineral admixtures including slag and fly ash are typically added into HPC. All concrete specimens were prepared by the quality control staffs of three precast plants. Half of the specimens were sealed with Sikagard 62 to minimize the evaporation of water, and the rest were unsealed specimens. All specimens with 4 in. diameter and 8 in. height are stored in an environmentally controlled chamber for one year. Twenty-six prestressed bridge girders produced using HPC from three precast plants were monitored and the corresponding long-term cambers were measured.
It was observed that due to the early age of loading and the use of slag and fly ash HPC had the higher average creep coefficient and average shrinkage strain than NC for both sealed and unsealed specimens during 1 year. It was also found that sealed specimens represent the creep and shrinkage behavior of a full scale prestressed bridge girder much better than unsealed specimens, which agreed with the results of previous literature. It was also observed that the sealed creep coefficient and sealed shrinkage strain measured from the four HPC mixes were similar, and it was acceptable to use the average sealed creep coefficient and average sealed shrinkage strain of the four HPC mixes tested to predict long-term camber of prestressed bridge girders produced in Iowa.
Three simplified methods were applied to predict long-term camber of a prestressed bridge girder, including Tadros’s Method (2011), Naaman’s Method (2004) and incremental method. Naamans’ Method and incremental method grant similar results, and both methods yielded ±25% errors relative to measured camber of 26 prestressed bridge girders, but Tadros’s Method yielded up to ±50% errors. The calculation of Naaman’s Method was simpler than incremental method. Therefore, Naaman’s Method was the recommended method to predict the long-term camber of prestressed bridge girders produced in Iowa.
Available at: http://works.bepress.com/wenjun_he/1/