Topping concrete is defined as a layer of high strength concrete placed on an old, worn out concrete surface to provide a dense, abrasion-resistant surface, to increase structural depth and strength of the base concrete.
Work Procedure of Laying Topping Concrete
Base Concrete Requirement
A preliminary assessment is required before the laying of topping concrete over an existing concrete surface. The existing concrete must be hard and strong. The base should not be weak and fragile and free of cracks as the topping concrete doesn’t bridge over the cracks of the existing concrete and by the passing time they may show up in the topping concrete too.
Preparation of Base Concrete
This is the important step in the laying of topping concrete, as this ensures the proper bonding and adhesion of old and new concrete. The concrete surface on which the topping slab is to be laid must be uniform, clean and free from dust particles or other contamination. Prior if any topping concrete is laid , it must be completely removed.
If the base concrete is finished smoothly then the surface must be made course by the help of mechanical scabbing or scarification machine , so that the course aggregate of the base slab is exposed. The loose debris, dirt and dust must be removed using a vacuum cleaner.
The base preparation must be done a day before to the placing of topping slab, in order to prevent any contamination or accumulation of dirt.
Placing of Topping Concrete
Before placing the topping concrete, existing slab nature must be studied. A small amount of water is to be poured in the base slab and the absorption rate must be checked. If the water is absorbed in few minutes then the slab must be kept wet for 4 hours before placing the topping concrete and the water must be completely removed before just before placing.
If no water absorption is seen, then the topping concrete can be laid as done in normal process.
Commonly placed topping slab is in the range of 50-100 mm. Based on the thickness and the requirement of the topping slab, steel reinforcement is provided.
The surface is first poured with the slurry made out of cement , sand and latex bonding agent. Before the slurry dries up, the concrete is placed. The concrete mix shall consists of aggregates. sand and cement depending upon the strength requirement of the topping. Minimum M20 Grade mix is to be laid.
Finishing depends on many factors such as place, usage and surface requirement. If topping concrete is placed on the existing slab, then it is given rough finish for laying of flooring material. If the concrete is placed on floor concrete , it can be made as smooth finish.
Concrete has to be effectively cured if maximum surface strength, maximum resistance to surface abrasion and low impermeability of the concrete are to be attained, and the development of drying shrinkage cracks is to be avoided or minimised.
Effective curing also reduces the effect of differential shrinkage, and therefore curling, by delaying the effect of differential drying until the concrete is better able to resist its effects. Curing must be done at least for minimum of 7 days.