Hydration is the chemical reaction of cement and water to form a cementitious crystalline structure. When cement binds sand and coarse aggregates together around deformed (“surface ridges”) reinforcing steel it creates reinforced concrete – the primary structural component of footings, retaining walls, and many water-retaining structures. Since both dry-mix (gunite) and wet-mix (shotcrete) are simply placement methods for concrete like cast-in-place, the effect is exactly the same for all processes.
Hydration is an exothermic reaction which means that it releases heat in the process. This is called the heat of hydration and it can be damaging to the structure. In fact, the heat of hydrating concrete has been known to melt shoes and can even melt the PVC pipes passing through the concrete. Fortunately, most reinforced concrete used in water-retaining structures is relatively thin shells for floors and walls. Thin shells dissipate heat efficiently and we don’t hear too much about cracking problems due to excessive heat. However, there are problems in thicker sections and watershapes can have many problematic areas.
Consider the stair in many pools. Often, the pool is engineered as a shell and the stairs are simply shot-in without reinforcement and the stairs can require a thick section of concrete. We do strongly suggest that the stairs are reinforced as needed per the temperature and shrinkage requirements of the ACI 318 Building Code – however, that may not be enough to overcome the extreme temperatures and resulting damage from the heat of hydration. You may need to change from Type I or II cement to Type IV.
Heat of hydration can be so damaging in thick sections that a special type of cement was developed to reduce the heat. Type IV cement cures more slowly and this reduces the exothermic reaction to a more manageable temperature. Type III cement would do the opposite – high early strength would result in higher curing temperatures.
Recently, a pool builder in Southern California called me about a problem he had with multiple PVC fittings and pipes being deformed and cracked by the curing concrete. The spa was elevated and had a thick floor with 3” pipes crossing through it. The heat softened the PVC and the crushing pressure of the concrete simply cracked the pipe and fittings in multiple locations.
In a study about the heat of hydration, the temperature in the center of a 2 m (6.5 ft) thick concrete block reached 78°C (172°F) [Anura Nanayakkara, University of Moratuwa]. PVC pipe is typically rated for 73°F and higher temperatures require de-rating factors for the pressure. The de-rating factor for 140°F is 0.22 meaning that the pressure rating at 140°F is only 22% of the pressure rating at 73°F. The maximum working, or service, temperature of PVC is 140°F because the crystalline structure of the PVC starts to decompose at higher temperatures.
Since hydrating concrete temperatures can exceed the maximum rating of the PVC pipe, it should not be a surprise that the concrete can seriously damage the pipe and fittings as shown in the image below. With a little planning one quick solution is to use Type IV cement but large masses may also be done in multiple lifts.