Development of concrete shrinkage performance specifications

During its service life, concrete undergoes volume changes. One of the types of deformation is shrinkage. The four main types of shrinkage associated with concrete are plastic, autogenous, carbonation, and drying shrinkage. The volume changes in concrete due to shrinkage can lead to the cracking of the concrete. In the case of reinforced concrete, the cracking may produce a direct path for chloride ions to reach the reinforcing steel. Once chloride ions reach the steel surface, the steel will corrode, which itself can cause cracking, spalling, and delamination of the concrete. The unrestrained drying shrinkage and restrained cracking tendency of concrete mixtures typically used by the Virginia Department of Transportation (VDOT) were assessed to establish an appropriate limit on drying shrinkage for use in a performance specification. Five existing shrinkage prediction models were assessed to determine the accuracy and precision of each model as it pertains to the VDOT mixtures used in this study. The five models assessed were the ACI 209 Code Model, Bazant B3 Model, CEB 90 Code Model, Gardner/Lockman Model, and Sakata Model. The CEB 90 model performed best for the portland cement concrete mixtures, while the Gardner/Lockman Model performed best for the supplemental cementitious material mixtures. Based on a comparison of the unrestrained drying shrinkage and restrained cracking tendency, it was determined that the potential for cracking could be minimized by limiting the unrestrained shrinkage of the concrete mixtures. Based on the results of this study, the recommended percentage length change specification limits are 0.0300 at 28 days and 0.0400 at 90 days for the portland cement concrete mixtures. For the supplemental cementitious material mixtures, the percentage length change specification limits were 0.0400 at 28 days and 0.0500 at 90 days.