In cold weather a vented heater should be located outside the building during placement of a topping.

Industry recommendations are that the base slab should be maintained at a temperature of at least 40 F during placement and curing of the topping to promote hydration, strength development, and avoid early-age freezing. Cooler base slabs may retard the set of the topping mix, prolonging bleed time and finishing activities. This may also make the topping more susceptible to other finishing issues such as plastic shrinkage and surface crusting. Whenever possible, we recommend heating the base slab to prevent freezing and provide acceptable curing conditions.

Cold weather topping mixtures can be designed to help offset the effects of ambient and base slab temperature on setting time. Replace slower reacting supplementary cementitious materials with straight cement, use Type III cement, and use accelerating admixtures (consider increasing dosage as the placement progresses to maintain an even setting time).

Moisture conditioning the prepared base prior to placement can be challenging in cold weather. Pre-wetting the base slab is not recommended if freezing is expected. Most toppings, however, are constructed on existing slabs where the building is constructed and enclosed. Therefore, adding heat to the area where the topping will be placed is usually less of a challenge than it is during initial construction of the superstructure and base slab.

As with pre-wetting of the base, moist curing should also be avoided if freezing is expected. However, thin bonded toppings are particularly sensitive to early drying while the bond strength is developing. If the bonded topping dries and shrinks prior to developing adequate bond strength to the base, shear forces can cause the topping to delaminate from the base. Once delamination occurs at an early age, the topping will not reestablish bond to the substrate. Therefore, preventing early drying is an important factor in the construction of bonded toppings.