Shrinkage is caused by loss of water from the concrete matrix. As the water demand of a concrete mix increases, shrinkage characteristics generally worsen (increased shrinkage). The primary factors that affect water demand are water to cement ratio and cement paste demand. Typically, cement paste demand is related to the surface area of aggregate being used. As the maximum nominal aggregate size decreases, surface area increases, and cement paste demand also increases. Water to cement ratio is related to the required strength, and durability of the mix. As required strength increases, the w:c ratio generally decreases.
Although shotcrete mixes are typically designed with low water to cement ratios, they are generally designed with a maximum nominal aggregate size of 10mm. Compared to a typical wall mix which is designed with a maximum nominal aggregate size of 20mm, a 10mm mix has a relatively higher cement paste demand. Due to the increased cement paste demand, at equal water to cement ratio’s a 10mm mix has a higher total water demand than a 20mm mix. Additionally, due to the need for shotcrete mixes to be highly cohesive, total cementicious content is further increased. Because of this, in the absence of any other changes, shotcrete mixes would generally be higher shrinkage mixes compared to conventional formed wall mixes. The shrinkage characteristics of shotcrete are moderated by the following:
- lower water to cement ratio than formed wall mixes;
- lower slump requirements;
- use of low water demand supplementary cementing materials (slag);
- use of supplementary wire mesh reinforcement;
- use of fiber reinforcement to control shrinkage;
- use of shrinkage control admixtures;
- prudent curing.
Properly proportioned, and designed, placed, and cured shotcrete mixes will have similar shrinkage characteristics to a conventional formed wall mix.