Preventing the formation of cracks in slabs is vital to ensure the serviceability of a structure. If a slab is under restraint then it will be prone to cracking. Designers will typically specify a standard 56-day shrinkage strain as determined by AS1012.13 to control cracking in slabs.
Recent research has shown that the development of tensile stresses at early ages can be so significant as to cause premature cracking. Standard 56-day strains determined in accordance with AS1012.13 are sometimes wildly variable from specified shrinkage strains, so designers are at a loss.
Three series of experiments were carried out in The University of Queensland concrete laboratory to assess shrinkage in concrete slabs. The experiments used three cements with commercially specified 56-day shrinkage strains of 450, 650 and 800 microstrain. Each mix was cast into a slab and a box. The slab modeled the effects of restraint, whilst the box represented a free shrinkage condition. Strain gauges were inserted and strains were monitored.
The results of the experiment show that early age tensile strains can almost exceed the standard 56-day drying shrinkage strain if the concrete is not properly sealed off from the atmosphere. In relatively benign conditions, the dramatic early age tensile strains warranted the use of aliphatic alcohol to reduce evaporation within the first few hours after casting.
Moist curing was by far the most effective method of curing. Chemical curing compounds wore off quickly and exposed the slab to drying shrinkage.
Two shrinkage prediction models were evaluated against the data from the experiments. It was found that the Australian Standards draft concrete structures code is a good predictor of shrinkage, but had trouble modeling the larger strains which develop on the open face of the slab. The GL200 model was compared to the data, and was found to accurately predict average free shrinkage strains, but had difficulty incorporating restraint.