The old saying goes: “out with the old, and with the new.” There are many instances where you cannot combine old products or ingredients with those that are new. For example, in cooking, combining old ingredients with new can mean that flavour will be lost or maybe that the old ingredient is past its expiration. In sports, it could mean that your back could break, your glove won’t catch, or your ball will not properly inflate. The old is just not necessarily always the right way to go.
However, when it comes to concrete, you may wonder if you can use new concrete over the top of old. This is a good question, as it would seem that this would have no impact on the new concrete whatsoever. After all, the old concrete is laid and would not be mixed with the new. It would simply be one slab on top of another. So, can you do this?
Of Course, You Can
Before diving into all the specifics, let’s just stated the way it is. Of course, you can use new concrete over the top of old. In fact, you will have no issues with this whatsoever. You are likely to find that on some projects there could be layers of concrete that are 50 years separated in time. The oldest slab was laid four or five decades ago, long before the new layer was poured. Yet, there is no problem with the whatsoever.
What needs to be understood is that concrete does not naturally bond to the existing slab. It is not looking to mix with the previous layer. In fact, if you want to add a new layer of concrete over the top of an old one, you have to use a bonding agent that connects the two layers together. The new concrete will not connect to the old it all, no matter how wet or dry, quick drying or whatever factor you may consider. They will not become connected without a bonding agent being applied. It is simply the way that concrete works.
As long as your previous layer of concrete is structurally sound, you should have no issues whatsoever. The problems you face or when there are issues such as tree roots coming out of the previous layer, cracks are buckling due to frost heaves, or other signs of setting. When you have issues like this, the previous layer may have structural problems that prohibit you from adding an additional layer.
This is where you may find that the use of a commercial contractor is beneficial. The last thing you want is to add your new layer of concrete and find, because of previous issues, that the new layer will not properly set or is not providing the structural integrity that you need. A contractor will understand and be able to properly assess if there are issues involved. This prevents you from having foundational instructional problems that could create a severe safety hazard later run.
Once again, to answer your question concisely, there is no issue with adding a new concrete layer to an old one. As long as the old layer is structurally sound and you have used a bonding agent to connect the two layers of concrete, you are ready to go.