The simple answer is YES, you can pour new concrete over existing concrete. If the foundations under the slab are not solid, the new concrete could sink or develop deep potholes. The mixture of water and cement is made into grout and spread over the existing slab to form a bonding layer between the old concrete and the new concrete to be laid. Pouring on existing concrete exposed to vehicle traffic is also being done in highway projects, driveways and parking lots.
For example, if you have large working cracks in the slab and the concrete has settled to the point of being on several different levels, you need to remove it. Pouring fresh concrete over existing concrete is a common way to level old slabs and repair damage. The condition of the existing slab is the main factor that determines how long the new concrete will last. Before talking about options and how to prepare concrete for resurfacing, keep in mind that resurfacing will not solve any underlying problems that your concrete may have.
Fine sand or crushed stone is preferred for small-thickness concrete and coarse aggregate is used for thicker concrete. Check if adding a few inches of new concrete would prevent you from closing a door or obstructing a walkway before starting work. Yes, if you try to renovate a patio that looks tired, you can cover small cracks and damage with a new layer of concrete. Most concrete surfaces are suitable for a new layer, provided that it does not have large cracks or tree roots.
Any dirt, debris or impurities present on the old slab is likely to interfere with the next steps and compromise the quality of the new concrete layer. Similarly, if the roots of the trees are causing existing concrete to move, then it is better to tear off the concrete, solve root problems and start all over again.