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Whether it’s interior flooring or an outdoor concrete deck, concrete surfaces experience wear-and-tear and often need repairs and maintenance. One available solution to surface-repair is to add new concrete on top of the old one, but should you?
You can add new concrete to an existing concrete slab if the old concrete is in good condition. You will need to thoroughly clean the old concrete surface and use a suitable concrete bonding agent before pouring the new concrete to satisfy all structural requirements.
While it is possible to use your old concrete as the base for new concrete, many factors will determine if your concrete is strong enough to accommodate the new batch. This article aims to detail what it takes and why it might not be advisable to pour new concrete over the old one.
When Can You Use Old Concrete as a Fill for New Concrete?
The most advisable way to repair your concrete is to demolish the existing one and redo the slab. But this will require that you dig deeper into your pocket and spend a lot more time on the project.
Although it is not the best option, adding a new concrete coat on top of the old concrete remains the next best option. However, whether or not to use this option depends on the condition of the old concrete.
For example, if the floor has heaved, dropped, or if some slabs look uneven, repairing it won’t provide a long-term solution. If the floor has multiple cracks, or has sand and gravel showing, it means that it is time for a new fill.
It is easy for anyone to scrape off the old concrete and put in a new one. But concrete bonding and restructuring should only be done through consultation with a professional.
How To Bond New Concrete to Old Concrete
It’s advisable to consult an expert for this process if you intend to carry out a substantial construction. But if you are building a small part of your patio or just want to give DIY construction a try, here is the process that you should follow for solid and reliable bonding between the old and the new concrete.
Install New Rebaring
Larger projects may require new rebaring to ensure a strong foundation for the new layer and avoid future cracking and delamination. If this is the case you may want to work with a structural engineer to ensure the rebar is adequate. On the other hand, if you are just repairing your floors you do not need to install rebars. You can skip this step if the project is small and you are not demolishing and rebuilding the slab.
Clean Old Concrete
This is a crucial step. If the surface is not well cleaned, the bonding is going to fail and delaminate. Ensure no debris, oil, or dust is on the surface before you put in the new layer. The following video explains in more detail how to clean and bond concrete:
You can use chemical cleaners like TSP or a pressure washer to clean the surface thoroughly. Another way that professionals advise is to use liquid detergent or a degreaser to remove all the stubborn spots.
After cleaning, you need to leave the surface wet. A dry surface is not a good canvas to work on, as old concrete is porous. If not well moisturized, it will absorb water from the new layer, causing the bond to fail.
Set Up the Perimeter
To estimate/calculate the amount of concrete you will need, you need to know the perimeter of the area you will be working on. Assuming you already know the thickness required for the new slab, mark the exact height that you want to reach. Using these measurements, estimate how much concrete you will need.
It’s also important to install boundaries on the area to be filled to avoid spillage. Wood is the most commonly used material for such a frame.
Paint the Surface With the Bonding Agent and Pour the Primer
A suitable bonding agent is essential when repairing or restructuring your concrete. It ensures that everything is nicely bonded and merged as one. Use a suitable bonding agent and paint the old concrete as per the instructions on the manual.
After painting on the bonding agent, ensure the area is level and apply a thin layer of scratch coat (also called the first coat). A scratch coat is a mix of water and concrete and has a liquid consistency.
Add the Top Layer of Concrete
You will need coarse aggregate concrete if you intend to make a thick slab, and fine sand or crushed stone for minor floor repairs. For big projects, coarse aggregates have large gravel and are more durable, ensuring that the concrete is more sturdy and durable.
These materials come with user manuals to help you when mixing. Once you have correctly mixed these materials, pour them into the marked-out area and spread them out evenly. Ensure that all of it is poured in at once to avoid inconsistencies when drying.
Ensure that the top layer is evenly spread, thoroughly compressed, and everything is uniformly leveled. If necessary, you can finish-up by spraying a curing compound immediately after pouring in the final layer to preserve moisture in the concrete.
Will New Concrete Stick to Old Concrete?
Most cement brands that are used in the concrete do not have a bonding agent. When new concrete is added on top of dry old concrete, the two will not bond together without a binder. A binding agent acts as the glue holding together the two unrelated layers.
Therefore, before you start the process, you need to find a suitable bonding agent. This bonding agent will be painted on the old concrete to ensure that the new concrete will successfully adhere. This is necessary for both big projects and minor floor repairs.
Disadvantages of Using Old Concrete As Fill for New Concrete
Unfortunately, there are some disadvantages to pouring new concrete on top of old concrete. A big issue would be that it can lower the lifespan of the area. For example, a new concrete building on top of a new gravel foundation will ensure that the structure is sturdy and can last for up to 40 years. However, if the old concrete is not in good condition, it will significantly reduce your floors’ lifespan over time.
Let’s take a look at some other disadvantages of using old concrete as a fill for new concrete to help you decide if it is the right choice for your project.
The Bonding Strength Is Not Reliable
Whether you like it or not, bonding the old concrete with the new concrete will lead to conflicts. These conflicts will show in the form of cracks and inconsistencies. This can be avoided by keeping the new concrete from fully binding with the old concrete, allowing each to shrink and expand on its own when the temperature changes.
However, in doing so, the foundation is not in unity, and a nonuniform settling might lead to sinking of the surface.
Pouring a new slab over an old one makes the floor more vulnerable to frost heave damage and cracks. Therefore, your floor might end up needing much more of your attention and care.
Cracks and damage might appear often. It is advisable to repair these as soon as they appear to stop them from spreading or letting too much water into the concrete.
The patch or area on which you are adding new concrete will be elevated compared to the rest of the house. This is not necessarily a problem for a central part of a room, but if the area runs up to a door, then you might have a problem.
The level might not provide adequate clearance for the door to open. If it’s on a walkway or driveway, this might pose a slight safety hazard as someone might trip and fall when walking.
What if the New Concrete Does Not Stick to the Old Concrete?
The two concrete pieces will not make a perfect bond; however, following the instructions correctly and using a suitable bonding agent will make your concrete sturdy and sufficiently bonded.
Before you consider adding a new layer on top of the old concrete, ensure that:
- The old concrete is in a good condition.
- The floor is thoroughly cleaned.
- A suitable bonding agent is available.
- The new concrete materials are of good quality.
If these conditions are sufficiently satisfied, then your bonding should be sturdy enough to support a new layer of concrete.
How To Join Slabs When Pouring New Concrete Next to Old Concrete
Large areas are split into small manageable slab sizes in new constructions and filled in on different days. Sometimes, if the small sections are scheduled on different construction phases, the time difference can take months to complete.
Joining the new slab to the old slab involves creating a joint that structurally connects the two while providing a flexible point for the two concrete pieces to respond to climate and other external manipulations.
If you don’t have the budget or the time to tear out your floors and redo them, do not worry. Using new concrete over existing concrete is an efficient and economical option. All you need is to ensure that you get professional advice from a structural engineer and use quality materials for the repairs.
- Wikipedia: Concrete Recycling
- The Constructor: How to Pour Concrete Over-Existing Concrete Slab?
- Rocland: CONCRETE BONDING AGENTS: WHY YOU NEED TO USE THEM, AND WHICH TO CHOOSE.
- ChemCo Systems: Bonding Concrete
- Indiana Decorative Concrete Network: Placing New Concrete Over Old
- T&T Redi Mix LLC: Can You Pour New Concrete Over Old Concrete?
- wikiHow: How to Add Concrete to Existing Concrete
- wikiHow: How to Adhere Concrete to Concrete
- Sakrete: Bonding to Existing Concrete
- Wikipedia: Frost Heave
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Giovanni Valle is an architect, designer, internet entrepreneur, and the managing editor of various digital publications including BuilderSpace, Your Own Architect, and Interiors Place. He is the founder of BuilderSpace LLC.