Effects of curing on shrinkage cracking in bridge deck concrete
Aamidala, Hari Shankar G
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Shrinkage cracking is a critical factor affecting the durability of concrete bridge decks. Since cracks in reinforced concrete decks provide a path for corrosive agents to enter the concrete, thereby accelerating the deterioration of the reinforcing steel. Concrete undergoes volumetric changes due to variation in temperature and moisture. When concrete is prevented by a surrounding structure from undergoing these volumetric changes freely, tensile stresses are developed. Depending on the amount of restraint applied, these tensile stresses potentially lead to cracks in the structures. This type of cracking can be reduced by providing rebars in the form of mesh to take the tensile stresses and/or the use of shrinkage-reducing admixtures (SRAs). Recent studies performed on SRAs show that the use of such admixtures is an effective method to reduce shrinkage cracking. Simulating restraint experienced in highway pavements and bridge decks is a challenge to researchers. This challenge has led to the development of different methods to assess the potential of restrained shrinkage cracking. Currently, there exists no standard test to assess cracking due to restrained shrinkage, though several researchers have simulated the effects of restraint in various ways to better understand the behavior of concrete shrinkage. The objective of this research is the evaluation of the different concrete mix designs used in bridge decks in Texas. Specifically, the influence of curing duration on the cracking potential of concrete bridge decks has been investigated based on the unrestrained (free) linear shrinkage measurements of prismatic specimens. In addition to free shrinkage tests, tests have been performed to study the weight loss, modulus of elasticity and split tensile strength of the concrete mix designs. Using the data obtained from these tests, shrinkage stresses developed due to full-restraint (100%-restraint) have been estimated and compared with the split tensile strength to estimate the age at which a first crack can be observed in the concrete. The approach presented assists in better understanding the effect of shrinkage on the cracking of bridge decks under full-restraint. Using the weight loss measurements and weight of oven-dried specimens, internal moisture content was tracked. This data is used to compare the behavior of the material to the development of shrinkage strains. Comparing the different curing durations did not indicate a significant difference in the development of either modulus of elasticity or split tensile strength. However, a favorable correlation was observed between the tensile strength of concrete and shrinkage stresses developed due to full restraint based on different curing durations. The results did not show a significant delay in the age of first crack with increase in curing duration from 4 days to 7 days. Thus, based on the research to date, one can conclude that 4 day curing suffices for durability of bridge decks from a shrinkage cracking perspective.
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