A quantitative study on plastic shrinkage cracking in concrete : influence of selected cements, aggregates and chemical admixtures available locally

Abstract

One of the age-old problems in concrete is cracking. In addition to being unsightly, early-age cracks may significantly impact the durability of concrete structures, as they act as weak planes for further distress or may accelerate the ingress of aggressive agents. Fresh concrete may be susceptible to plastic shrinkage cracking. Many researchers believe that plastic shrinkage crack development remains a concern to the concrete industry, occurring in particularly large-area pours such as low-volume quay deck casts, and therefore requires further research to understand their formation and minimization. This study seeks to find out what effect cement types, type of aggregate and the use of chemical admixtures have on the formation and/or propagation of plastic shrinkage cracks. An experimental procedure was developed where a restrained slab specimen was exposed to typical local environmental characteristics immediately after casting. After 24 hours, image analysis was used to process digital images of surface cracks. The crack data extracted from these images was then analysed. Findings indicate that Ordinary Portland Cement 42.5N reduced the formation of plastic shrinkage cracks when compared to Portland Limestone Cement Type II/A-LL 42.5R, under the same test conditions. Another trend indicates that mixes containing local aggregate resulted in lower plastic shrinkage crack formation then mixes with Montenegro aggregate. Furthermore, the addition of superplasticizers increased the formation and/or development of plastic shrinkage cracks.