DEALING WITH AMBIENT CONDITIONS WHEN PLACING CONCRETE TOPPINGS (Rain, heat, and cold can all have a negative impact on a topping slab.)
Toppings are subject to the same general industry recommendations for hot- and cold-weather concreting practices as any other concrete. Proper planning and execution is essential for minimizing the negative effects of weather extremes on topping placement, consolidation, finishing, curing, and strength development.
When planning around the effects of ambient conditions on topping construction, a key factor to consider is the mass of the existing slab. In hot and cold weather extremes, the topping and base slabs will normally be at different temperatures when placed but will come to a thermal equilibrium during curing. Typically, the base slab accounts for the bulk of the composite slab (bonded or unbonded) so conditioning of the base slab prior to construction cannot be overlooked.
Thinner toppings may be more susceptible to temperature-related issues. A cold base slab can cause finishing problems due to delayed set, retarded strength gain, or even freezing of the topping if not properly conditioned. A hot base slab can lead to rapid stiffening that negatively affects workability, consolidation, finishing, and bond.
Industry recommendations for dealing with hot and cold weather are well documented; however, concrete placements are also exposed to other weather-related risks that the industry is nearly silent on, such as rain. Weather is difficult to predict, and placements often occur when there is a chance of rain to meet project schedule demands. Timing, duration, and intensity of a rainstorm are all important variables that can affect the success of a placement.
Exposure to Rain During Placement
Under most circumstances, concrete placements exposed to rain will not be damaged if excess rainwater is removed prior to finishing. According to the Guide to Concrete Flatwork Finishes published by Cement Concrete & Aggregates Australia, if the concrete surface appears wet (similar to bleeding) the rainwater will need to be removed to allow continuation of finishing. The common concern is that rain may increase the water-cement ratio of the placement resulting in lower strength, higher shrinkage, and a weaker surface. This may be true if the water cannot or is not removed prior to finishing; however, contractors have shown that this is not the case when precautions are taken to remove excess water. The most common precautions are to either cover the concrete with plastic or allow exposure to rain and remove the excess water prior to finishing.
If possible, the placement can be covered with plastic to minimize contact with rain. While this is good practice, the application of the plastic may be difficult or impossible if laborers cannot walk on the surface, the sheets are not wide enough to cover the entire width of the placement, or reinforcement or other penetrations protrude through the topping. Some contractors also caution against the use of plastic since it retains heat and can cause the surface to set faster. Reducing the window of finishability under these circumstances can be undesirable because additional time may be needed to remove water and complete the finishing operations.