DIMENSIONAL STABILITY OF CONCRETE SLABS ON GRADE

Abstract

Drying shrinkage is one of the major causes of cracking in concrete slabs on grade. The moisture difference between the top and bottom surface of the slabs causes a dimensional or "shrinkage" gradient to develop through the depth of the slabs. This can cause cracks and warping which result in serviceability and performance problems for concrete slabs on grade. There have been numerous analytical and experimental investigations to characterize drying shrinkage as a material property. However, there have not been significant improvements in terms of validation and calibration to provide engineers with a reliable evaluation of the strains and stresses within a concrete element subjected to moisture gradients and restrained shrinkage.

This test program characterizes the dimensional properties of selected concrete materials, evaluating their performance as real slabs-on-grade in that they are exposed to ground moisture on the bottom surface and drying conditions on the top surface. The concrete mix designs examined included low and high strength concrete (PCC and HPC), typical Portland cement using two common types of shrinkage reducing admixtures (SRA+PCC) , and Calcium sulfoaluminate cement (CSA). The data includes standard concrete material characterization tests, joint opening measurements, internal relative humidity and temperature in ½ in. increments through the depth of the slab, prism tests and compression test results. It was found that CSA is very stable, with no long term shrinkage, cracking or warping while typical PCC and HPC continue to show crack growth at over 600 days of age. Shrinkage Reducing Admixtures have a minor impact at early age but do not impact long term sectional stability. The SRA concrete exhibited shrinkage, cracking and warping nearly similar to typical PCC but slightly better than HPC.