Repairing Common Slab Surface Defects
Concrete contractors are often asked to address non-structural blemishes that can occur soon after a concrete slab is placed. Blemishes, such as popouts, plastic shrinkage cracks, and drying shrinkage cracks, can happen even when the flatwork contractor follows the best finishing practices.
Traditionally, most owners did not attempt any sort of repair on these nonstructural blemishes as the slab was often covered with resilient flooring or carpet. But with the increased interest in exposed concrete floors, many owners are asking contractors to turn blemishes into wearable surfaces. By employing simple repair techniques combined with the right repair material, contractors can often return surfaces to an acceptable appearance.
Contractors should be proactive when blemishes occur. Armed with an engineered approach, contractors can flip an owner’s concern into a positive customer service experience.
Using this engineered approach to address owner’s concerns starts with a detailed visual inspection of the slab. This survey begins by documenting all blemishes in a written report. The information should include pictures of problem areas, measurements of the blemishes’ positions, length and width of cracks, and recording each popout diameter.
An important aspect of this inspection is to determine if the visible cracks are dormant. Most plastic shrinkage cracks and drying shrinkage appear soon after the slab’s placement. Once formed, these non-structural cracks won’t exhibit any further movement. Surveyors can determine if a crack is dormant by measuring the width and depth on subsequent visits over a short period of time.
If there is evidence of a crack’s movement, there could be more serious problems. At this point an engineer should be contacted to perform a more thorough inspection of the project.
Popouts are a very common blemish on a slab. They are easily distinguished from cracks as they occur when a small piece of concrete breaks away from a slab’s surface. Popouts are a cosmetic blemish and typically do not affect the function or service life of the flatwork.
The blemish is caused by an unsound piece of coarse aggregate. The offending aggregate differs from the bulk of the aggregates used in the concrete mix. These unsound aggregates, located just below the slab’s surface, absorb moisture from water. The aggregate then swells, pushing upward as it follows the path of least resistance. The pushing action causes an eruption, leaving a shallow cone-shaped hole, often with the offending aggregate at the base of the popout.
Popouts are classified by diameter
Small: Ranges up to 7/16” in diameter
Medium: 7/16” to 2”
Large: Greater than 2” in diameter
Popouts can also affect adjacent concrete surfaces. The ruptured cones may weaken the slab’s crust and can cause nearby areas to delaminate beyond the initial blemish – another reason to deal with them sooner rather than later.
How To Repair Popouts
Usually, isolated popouts are tolerated or overlooked. But on interior floors, or slabs that could be subject to de-icing salts, it’s important to repair the blemish to avoid further slab damage.
The popout repair starts with good surface prep. By removing any remaining unsound rock pieces lodged in the hole, there is less chance for a repeat popout. For small popouts, it’s best to use a rotary-hammer drill to remove the unsound rock, and to create a hole with a minimum depth of 1/2″. For medium and large popouts, contractors may have to use a hammer and chisel to extract any unsound aggregate. Some contractors even use a core drill on larger popouts.
Once the offending rocks are removed, you’ll want to remove any weakened concrete at the slab’s crust around the popout depression. This final surface check can be conducted by vigorously scraping with a stiff wire brush. Each final hole should have square edges that are perpendicular to the slab’s surface. It’s best not to feather the hole’s edges.
There are a wide range of prepackaged repair materials for filling holes. They often exhibit a superior service life as compared to simple water/mortar mixes. Preblended products are resistant to shrinkage and provide better adhesion to the new hole. Many manufacturers offer different cement colors to match the existing surface.
Curing Your Repairs
The final important step of the repair process is to cure the repaired areas. You can use a wettened burlap sack or piece of cheesecloth. Some contractors will use a spray-on curing compound.
Three cautionary suggestions on this type of repair:
- It’s important to refer to the manufacturer’s recommendation on whether to or not prewet the repair area.
- Try to not place or smear the repair mortar over a larger area than necessary.
- Purchase only enough material for the project at hand. The material in leftover packages can be less effective if too much time lapses between uses.
Thin-Bonded Concrete Overlays
If the surface has too many popouts for individual repairs, or if during the repair the surface suffered delamination, contractors should consider using a thin-bonded concrete overlay. This type of repair will leave a more consistent look, desirable on exposed floors.
First address the removal of all unsound material in each popout as previously described. Then outline the entire area that will be receiving the overlay.
The area should receive a thorough pressure washing to remove all dirt, chemical residue, and loose concrete on the slab’s surface. Most overlay material manufacturers also recommend that the entire area receive an aggressive treatment, such as grinding or shotblasting that results in a uniformed roughened surface profile.
Material manufacturers often specify the exact requirement of the final prepared surface referring to the industry standard ICRI Concrete Surface Profile Guidelines. Matching the specified profile is necessary to achieve maximum bonding, and is determined by the overlay material. Refer to the manufacturer’s recommendation with respect to prewetting the repair area. And after the overlay’s placement, follow recommended curing procedures.
Cosmetic Cracking Blemishes
Plastic shrinkage and drying shrinkage cracks differ from popouts because they emerge during or soon after the slab’s hardening. Unlike popouts, these cracks are the result of improper curing that occurs as the concrete hardens. The cracks are not localized and can extend across the slab. The cracks are most often shallow and tend to be wider at the surface and narrower into the slab.
Crazing cracks occur when the surface of the concrete dries and contracts at a different rate than the underlying concrete. To reduce/prevent crazing, don’t allow the surface to be exposed to low humidity, hot air, solar heat, and similar elements. After finishing, don’t wait too long before you start your cure. If all the surface moisture evaporates before curing it can result in cracking, crazing, etc.
Often times these types of cracks are left untreated, as they normally do not affect the slab’s service life. Yet on most exterior applications, it’s best to repair these cracks to prevent water intrusion that could lead to rebar corrosion, uneven settlement, or slab movement. On interior applications, unrepaired exposed cracks can be unsightly, and sometimes lead to tripping hazards.
Repairing Static Cracks
Contractors should approach the repair of both plastic shrinkage cracks and drying shrinkage in the same manner. The surface preparation of both begins by removing any debris that may have fallen into the crack’s openings. Often curing compound, densifiers, and sealers from the slab’s placement may have filled in portions of the opening.
Some contractors may sawcut the crack to create a consistent width. This action helps with aesthetics and adhesion of the filling agent. The next step is a vigorous cleaning with a wire brush or oil free sand blasting.
When the crack’s channel is finally shaped, use a high-pressure vacuum to extract any dust and residue. Avoid using compressed air or water blasting to clean the cracks.
Since these cracks are often shallow, it’s best to fill the entire cavity with repair material. Most repair materials can be applied to crack depths from 1/8” to 1”. Do not use backer rod to reduce volume. If the channel is deep, use dry silica sand to fill the bottom of the joint. The sand prevents 3-sided adhesion.
The cracks should be overfilled and allowed to cure for approximately 1 hour and up to 24 hours before shaving flush with a floor razor scraper. The top of the repair material should be flush with the slab’s upper surface.
The most common repair material is a two-component hybrid urethane joint filler. Opt for an ultra-low viscosity material that penetrates deep into cracks, mending them back together. If necessary, contractors can mix the liquid with aggregate to make a tough mortar.
Just prior to the crack filling process, technicians should be absolutely sure to completely and thoroughly mix the two components together following the manufacturer’s requirements for mixing sequence and times. The sample should be deposited into a waste container.
With the proper tools and materials, you can execute durable, aesthetically pleasing repairs of common concrete surface blemishes. The general consensus is to deal with cracks and blemishes in a timely manner to improve aesthetics (on visible flatwork), and prevent subsequent damage. And even more importantly, keeping our customers happy often translates into repeat business and new customers through word of mouth.
For more info on related slab repair techniques: