Q.: I recently poured a 30x50-foot pad that has cracked very badly. The pad had no rebar and no expansion joints. Expansion joints have been added since. My question is, can I pour another pad on top of the existing pad and how, or what are my options?
A.: Although we have no way of knowing the cause of the initial cracking in your slab, when contemplating placing an overlay on an existing slab, consider the following:
Yes, you may pour a concrete pad overlay over an existing slab.
- You need to consider the added height and weight of the overlay on the existing structure.
- Overlays may include polymers, portland cement concrete, or epoxies. If you decide to use a portland cement, latex-modified, or epoxy-modified concrete overlay, carefully follow the manufacturer's instructions for preparation and execution. If the cause for your original cracking is improper subbase preparation or settlement, an overlay may not be the answer.
- You should add reinforcement to the overlay if the old cracks are active (welded wire reinforcement or #4 rebar). This will hold any reflected-through cracks together. You might also consider adding fibers to the overlay to control plastic shrinkage cracking.
- You should improve the drainage of the slab by having a good slope on the overlay.
- While an overlay may hold together underlying nonmoving cracks, the overlay will not prevent active cracks in the existing slab from reflecting through the overlay. Repair existing cracks and use a bond breaker over them to try to prevent a repeat of the same crack in the overlay.
- Cracking can occur in the overlay due to drying or plastic shrinkage. To prevent cracking in the overlay, make sure that the finish operation is timed properly to prevent drying of the overlay. If the surface dries out, you can get cracks from drying shrinkage or from tearing during finishing. Thinner overlays are more prone to this type of cracking.
- Try to avoid times of day when hot or dry conditions will be encountered. Cure the overlay by covering it for 7 days with wet burlap. Cut or tool control joints as soon as possible but absolutely within 4 to 12 hoursù4 hours in hot weather, 12 hours in cold, and try to have the temperature of the existing slab close to that of the overlay material when placing.
- In general, you wouldn't want to use a bonded overlay over an existing slab that has active cracks because the cracks will recur in the overlay.
- It is possible that you may have to remove the existing slab, prepare the supporting subbase properly, and then pour a new slab to achieve the results you desire.
You'll find a lot of useful information in two ACI reports: "Concrete Repair Guide," ACI 546, and "Guide for Concrete Floor and Slab Construction," ACI 302.1. ACI 302.1 provides steps for proper preparation of the subgrade. They are available from ACI (American Concrete Institute) by contacting www.aci-int.org, or calling 248-848-3700.