Drying Shrinkage In Shotcrete Swimming Pools

This is in response to numerous requests received by Pool Engineering, Inc. for information on drying shrinkage and shrinkage related cracking in wet and dry mix shotcrete swimming pool construction.

When concrete dries, it contracts or shrinks. The reduction in volume that occurs with loss of moisture content is an inherent characteristic of Portland cement-based concrete. Drying shrinkage and the resulting drying shrinkage cracking is typically greater in wet mix shotcrete than most conventional concretes, mainly because wet mix shotcrete typically has less volume of coarse aggregate and more cementitious material and therefore requires a greater total volume of water for the same water/cement ratio. The drying shrinkage of shotcrete varies with the mixture proportion, but generally falls within the range of 0.06 and 0.10% at three months (ACI 506R-05, Guide to Shotcrete).

As concrete within a structure, such as a swimming pool left exposed to atmospheric conditions, shrinks from drying, the structure typically is restrained from shortening by friction between the structure and the soil it is embedded in. Also the geometrics of the structure often provide restraint against shortening. Tensile stresses within structures from restrained shortening results in drying shrinkage cracking. The orientation of cracking due to drying shrinkage would be most prevalent perpendicular to the length or longest dimension in a structure. Shrinkage cracks appear most frequently in pool floors and are usually irregular and intermittent in configuration. They can occur at various random points throughout the shell but may also extend beyond the floor and into the radius and walls.