Sometimes concrete surfaces are given a decorative topping that adds further protection and durability to a floor or carpark. The underlying concrete will need to be prepared properly before any topping is laid on top. This will ensure that a good bond is formed between the two materials and that no cracks or chips will form over time.
When a bonded topping is being applied to a concrete surface, the concrete must be vacuumed and cleaned thoroughly first. This will remove any potential contaminants and dirt that could affect the bonding process. Even old adhesive from floor tiles that have been removed can be a potential contaminant for the new topping. Other common contaminants include oil, metal filings and paint spills.
The next step before the topping is applied to the concrete surface is filling and stabilising any existing cracks. Cracks have the potential to move and spread underneath a topping. This can obviously affect the bond between the topping and the underlying concrete. All cracks need to be filled with epoxy or stabilised with steel reinforcing bars.
Concrete surface preparation for a topping
Once the concrete surface is clean and any cracks have been eliminated, it can be prepared for the topping. The concrete must be scarified so than any coarse aggregate is exposed and the adhesive has a rough surface with which to bond. The scarification process can be carefully performed with a hammer and a chisel. Lines are chipped out of the concrete surface and any smooth sections of concrete can be lightly dimpled.
Be careful not to use too much force with the hammer and chisel as this can cause microcracking in the concrete. These hairline cracks could potentially grow into larger cracks a few years later, once the topping has already been applied. Some concrete slabs have a thick layer of mortar at their surface. In such instances, there is no need to expose underlying aggregate – just dimple the surface to create a rough area.
Second cleaning of the surface
After the scarification process, be sure to clean the concrete surface thoroughly once again. This is best done by brushing the concrete with a broom and cold water, then vacuuming any remaining dust once the concrete is dry. The dust in the concrete pores must be removed as it could affect the bond with the topping. Avoid using any chemical cleaners as they could leave an oily residue or stain the exposed, untreated concrete.
Saturated surface dry conditioning
Just as aggregates are supposed to be saturated surface dry (SSD) before being added to a concrete batch, so too should the concrete surface be SSD before the topping is applied. Many contractors believe that a dry concrete surface is necessary for a strong bond. However, an SSD concrete surface will prevent any water from being drawn out from the mortar or adhesive paste used to bond the topping to the concrete. However, be careful not to oversaturate the concrete surface as any standing pools of water in the pores will affect the bond.
Applying the bonding mortar or adhesive
In this step, it is important that the topping gets into full contact with the textured concrete base through the mortar or adhesive. Full contact ensures that the adhesive spreads evenly and thoroughly across the two surfaces, which increases the bond strength. If full contact is not achieved, the topping could peel away over time and lift, creating air bubbles underneath the surface.
The topping must also be placed before the adhesive dries out. If the bonding mortar or adhesive dries before the topping is applied, it can create a weak layer that is prone to lifting and cracking. However, do not add additional water to the mortar in an attempt to prevent it from drying as this could affect the overall strength of the bond. A plastic sheet may protect the mortar from the drying effects of wind and heat, so keep the adhesive covered until the topping is ready to be applied.
Once the topping is applied across the entire surface of the underlying concrete, leave it to dry and cure for 24 hours before walking or driving across the surface. As with concrete, the topping and adhesive will harden slowly over time. This will ensure that the topping is durable and lasts for many years to come.
LafargeHolcim is a leading building materials and solutions company that has been operating in international markets for decades. We produce cement and aggregates for construction projects, ranging from small affordable housing developments to large-scale infrastructure projects such as high-rise buildings, dams and bridges.
LafargeHolcim Tanzania has been supplying the country and neighbouring countries with our world-class Tembo cement brand for over 30 years. Our head office and fully-integrated plant are located in Mbeya, Southwest Tanzania.
At LafargeHolcim Tanzania, we believe customers come first. We listen to your specific requirements to supply and develop the best solutions for your needs. As the new leader in building materials, you can also rely on our cutting-edge research and development capabilities that have resulted in the finest materials for your construction projects, whether large or small.