Question: Is the overlay installer responsible for cracks in the existing slab? And is the installer also responsible after the job ends if those cracks reflect through the overlay?
Answer: Yes, you are responsible for repairing any cracks in the existing concrete before resurfacing. And despite your best efforts, the cracks may eventually re-form and mirror through to the topping. If you did the repair properly, any crack that does reappear is likely to be minor. Be sure to spell out in your contract that you will take all the steps necessary to repair cracks, but can’t guarantee the results.
Types of cracks
Cracks fall into two categories: static and moving. Static cracks are hairline flaws that only affect the surface, such as craze cracks and plastic shrinkage cracks. They typically require little or no repair, so you can get by with normal surface prep.
Moving cracks, or active cracks, are more serious. They are often structural in nature and continue through the entire depth of the concrete. Causes include insufficient spacing and sequencing of control joints, not isolating new concrete from old, and improper subgrade compaction.
If the crack is equal to or wider than the width of a credit card, it’s probably a moving crack and needs to be repaired before you resurface the slab. Also, consider treating borderline cracks (not quite credit card width but slightly wider than hairline) as structural cracks. When cracks are extremely wide and unstable, I recommend completely removing and replacing the existing concrete.
Check with your material supplier for recommendations on the repair method best-suited for use with its product. And always ensure that each crack is cleaned thoroughly before filling. The following are techniques that I’ve used successfully:
Crack, chase, clean, and fill. Route out the crack using an angle grinder with a V-grooved diamond blade. Remove all debris with a compressed air or shop vac. Fill the crack, preferably with a semi-rigid material such as a polyurea-based product.
Install new control joints. After filling cracks, I strongly recommend that you saw-cut a new control joint close to the crack to help relieve expansion and contraction stresses in the slab.
Epoxy injection. Cracks are unlikely to reappear with this method, but it requires skill and can be time-consuming and costly. Mount ports in the crack at intervals equal to the concrete thickness; inject epoxy through the ports using a low-pressure pump to fill the crack.
Crack stitching. Drill holes on both sides of the crack. Span the affected area with wires or U-shaped metal strips, and grout or epoxy in place. (Some installers saw cut a wide kerf across the crack and then epoxy rebar into the area to fuse the two sides together.) This is another time-consuming, expensive, but highly effective method. You must also apply enough overlay over the repair so it doesn’t reflect through the finished surface.
Use a repair kit. Some manufacturers have crack repair kits that include all of the materials you need before applying their overlay systems.
Make cracks part of the design. Route out cracks with a V-shaped diamond blade. Apply the cementitious overlay material, letting some of it fill the routed cracks. Once the topping has reached sufficient strength, rechase the cracks. Fill cracks with a flexible or semi-rigid filler to allow them to expand and contract naturally. You can saw cut additional “faux cracks” where necessary, to mimic natural flagstone for example.
Bob Harris, founder of the Decorative Concrete Institute, Temple, Ga., and senior decorative concrete consultant for Structural Services Inc., has more than 25 years of experience in the construction industry. He conducts seminars in architectural and decorative concrete worldwide, is involved with numerous associations, and is a popular speaker at World of Concrete and other events. The information in this article is based on Bob Harris’ Guide to Concrete Overlays & Toppings, which now comes with a DVD that provides step-by-step instructions for rejuvenating floors and exterior flatwork. Visit www.decorativeconcreteinstitute.com.