A concrete topping is a cement-based self-levelling overlay that corrects uneven floors. It is an alternative to removing and replacing the entire floor, and can be installed quickly without major disruption.
A bonded topping should be placed on a clean, prepared base slab that is structurally sound and has been thoroughly cured. This preparation should be carried out by a concrete contractor who specializes in bonded toppings. If the base is not cleaned properly before the topping is placed, contaminates such as oil, grease, and solvents can collect and affect the bond between the bonded topping and the base.
There are many ways to prepare a base for a bonded topping. However, the most common is to slurry it, which requires a heavy-duty pump machine. A slurry layer of coarse sand mixed with water, emulsified asphalt, or a cementitious admixture can be pumped into the base before a bonded topping is poured over it.
The slurry layer must be thoroughly brushed in to the surface of the base in order for the topping to bond effectively with the base. The slurry must also be completely wiped down of any dirt and debris that may have collected on it. This cleaning must be done prior to the slurry being poured.
Topping is usually installed a few months after the base slab, so preparing the base to receive the topping can save labor costs for later cleaning of the base. However, the base slab must still be kept clean to avoid contamination from any spills that may occur during the course of installation.
If the base slab is contaminated and does not have a smooth surface, the slurry layer cannot be brushed in to the surface. It is therefore necessary to apply a bonding grout that will bond the topping to the base in an effort to achieve full contact between the base and the slurry layer.
It is important to make sure that all cracks are filled or stabilized before installing a bonded topping. If cracks are not filled, they can telegraph through the bonded topping and cause hairline cracks. In some situations, the cracks can be filled with a polymer structural filler.
In other situations, the cracks can be drilled or saw cut to provide access to the crack and allow for its stabilization before the slurry layer is brushed in. This is especially important for moving cracks that may occur in the base slab, such as expansion joints.
Once the slurry layer has been brushed in to the base, it is vital that a vacuum unit is used to remove any loose debris and dust that may have collected on the slurry layer. The most effective method of vacuuming is to use a 2-inch line with a small brush attachment. Vacuum units that have high filtering systems release the accumulated dust back into the air when they are emptied, so it is essential to locate them outdoors.
Topping must be cured to ensure maximum surface strength, resistance to surface abrasion, and low impermeability. Effective curing also delays the development of drying shrinkage cracks, which can deteriorate the quality of the underlying concrete. Proper curing should be performed for at least seven days.