Cement used in concrete does not usually contain an adhesive agent, meaning that when new concrete is added on top of old, dry concrete, the two will not stick together without a binder. This can be a problem, as it can reduce the lifespan of the area. To ensure that the two layers adhere, an adhesive agent must be applied. This is especially important for large projects, as coarse aggregates are more durable and will make the concrete stronger and longer-lasting.
When patching a concrete surface or pouring new concrete over old concrete, it is important to invest in a strong adhesive agent and a concrete patch mixture. There are many adhesives on the market that can offer effective bonding functions. It is best to repair any damage as soon as it appears to prevent it from spreading or letting too much water into the concrete. The thinnest layer of new concrete that should be applied to old concrete is about 2 to 2 ½ inches.
However, there are cases when pouring new concrete over old concrete should be avoided. Before doing so, it is important to sweep away all loose stones and dirt and rinse the concrete to prepare the surface. This will help improve adhesion with the leveling compound and seal the concrete slab underneath.