Cracking of concrete flatwork is natural, expected, and accepted. However, cracks may affect appearance, performance, and maintenance costs. For these reasons, building owners commonly want cracks in interior and exterior slabs repaired. Before choosing a crack repair option, concrete surfaces contractors should perform a crack evaluation and establish the repair objectives.


When evaluating a slab or performing a crack survey, always measure the crack widths and determine if steel reinforcement passes through it and whether the crack is active or dormant. This information, especially reinforcement details and crack activity, is extremely important when selecting the best repair option.

Crack widths

Crack widths typically are estimated using a crack comparator card. Measuring crack widths helps to classify the severity of cracks and evaluate the effectiveness of the aggregate interlock across the crack for vertical slab alignment and load transfer. Also, many repair options are somewhat dependent on crack widths.

Unless steel reinforcement passes through a crack, vertical slab alignment and load transfer across the crack are dependent on the aggregate interlock. If the crack width is less than about 0.035 inches, aggregate particles projecting across the crack into recesses on the other side will typically provide sufficient load transfer and maintain vertical slab alignment across the crack.

For widths greater than that, maintaining vertical slab alignment may be challenging. The success of a repair technique will depend on the slab loading and the quality of the base material and soil subgrade.

Steel reinforcement

Although steel reinforcement does not prevent cracking, it does limit crack widths so aggregate interlock is maintained. Reinforcement itself also provides load transfer and helps maintain vertical slab alignment across cracks. When reinforcement passes through a crack, both crack width and future crack width growth are controlled. New cracks in unreinforced concrete will continue to grow and widths may double before crack-width growth subsides.

For cracking caused primarily by drying shrinkage, cracks will stabilize eventually. Cracks due to other causes, though, such as soil settlement or heave, may never stabilize and can continue to widen unless a significant amount of reinforcement passes through the crack. Determining the amount of reinforcement passing through a crack is important for predicting future crack behavior and choosing the best repair option.

Dormant and active cracks

Cracks that are crossed by steel reinforcement typically are stable and often are considered dormant. Of course, the stability of cracks depends on the root cause and the amount of reinforcement crossing the cracks. Dormant cracks have stable widths and can be repaired with either rigid or flexible materials.

Active cracks should be repaired only using a flexible material that can accommodate future crack movements. Usually, active cracks in slabs are actually working joints and should be treated in the same manner as joints. After all, random or out-of-joint cracks in flatwork are really self-installed joints.



Structural repairs typically involve using epoxy resins to bond or weld the concrete back together. Epoxy repairs not only seal cracks but also restore the strength and stiffness of the concrete. Use epoxy to repair only dormant cracks. Repairing an active crack with epoxy typically results in a new crack forming near the original, epoxied crack.

Epoxy injection is a common means of installing epoxy into cracks, especially for vertical and overhead surfaces. But gravity filling works well and is the most common means of installing epoxy into cracks in flatwork or horizontal surfaces.

Gravity filling consists of pouring a low-viscosity epoxy into cracks that have been sawed or routed and letting gravity pull the epoxy down into the cracks. Viscosity is the liquid’s resistance to flow and is measured in centipoises (cps). The lower the viscosity value, the thinner (runnier) the material. Grades I and II epoxies have viscosities less than 20 cps and between 20 and 100 cps, respectively.

For comparison, the viscosities of some common liquids are: water–1 cps, milk–3 cps, anti-freeze–15 cps, maple syrup–150 to 200 cps, and honey–3000 cps. Lower viscosities are required for tighter cracks. Otherwise, repair materials will be too thick to penetrate and flow into the crack.

Epoxies cure or become hard because of the chemical reaction between the resin and hardener. The chemical reaction is faster at higher temperatures. Epoxies are available for different installation or cure temperatures so the chemical reaction is not too fast or slow (Type A, below 40º F; Type B, 40º F to 60º F; and Type C, above 60º F).

For successful gravity-fill crack repairs, consider crack widths and depths, epoxy viscosity, and installation temperatures when selecting the grade and type of epoxy. Otherwise, the epoxy may be too thick or harden too fast to penetrate or it can just run through the crack and pool beneath the slab.

Sealing and filling

Sealing and filling are nonstructural repairs. However, some repair materials do bond the crack faces together and may have considerable strength that can cause material and concrete tearing if substantial crack movements occur.

Common sealants and fillers for interior repairs include semi-rigid epoxies, hybrid polyurethanes and polyureas, and polymer mortars. Polyurethanes and silicones are common sealants used for exterior repairs.

Typically, sealants are flexible or elastomeric materials; whereas, fillers are considerably harder so the repair material supports the crack edges to avoid edge spalling or breakdown under load. When floors are exposed to hard-wheeled traffic, joint fillers or special crack repair materials with a minimum Shore A Hardness of about 80 should be used.

Unfortunately, there is a tradeoff for increasing the hardness of crack repair materials. As repair materials become harder, they become less flexible and more intolerant of crack movements. For example, semi-rigid epoxies typically have a hardness value of about 90 and an elongation value of 50%; whereas, silicone sealants have a hardness value of about 5 and an elongation value of 1400%. Therefore, when selecting a repair material, consider the anticipated crack movements versus the need to support the crack edges.

For active cracks with anticipated crack movements, use elastomeric sealants or sealants with sufficient elongation properties, realizing the tradeoff will be less support for the crack edges. For cracks exposed to hard-wheeled traffic, use harder materials that support crack edges. But understand these materials are less tolerant of crack movements.

When using elastomeric sealants in active cracks, always install a sealant reservoir by sawing or routing as recommended by the manufacturer of the repair material. Without a reservoir, there will be insufficient material to tolerate future crack movements. Elongation properties reported by the manufacturers are based on the recommended reservoir size or, more specifically, the width to depth ratio of the reservoir.

Of course, sawing or routing enlarges crack widths and makes crack repairs more noticeable. For cosmetic repairs in colored or decorative flatwork where cracks are dormant or fairly stable, don’t saw or route a sealant or filler reservoir. Instead, choose a crack repair material that has a low viscosity (less than about 100 cps) and small injection tips to gravity-fill cracks.

Reservoirs are not required. But if crack movements occur, tearing of the repair material or concrete are likely due to the small material width-to-depth ratio. Also, manufacturers now are supplying many of these low-viscosity repair materials in various colors or are providing recommendations for producing colors and textures that match the surrounding concrete surface.

Before repairing your next crack, perform a crack evaluation and establish the repair objectives. Decide what type of repair is needed. Choices include a structural repair using epoxy, a route and seal repair using a flexible sealant to accommodate future crack movements, and a hard or semi-rigid filler repair to support crack edges, with or without routing. Also, establish the cosmetic requirements. After choosing the repair material and procedure, follow the manufacturer’s recommendations.

Kim Basham, PhD, PE, is president of KBE Engineering and specializes in concrete construction, troubleshooting, nondestructive forensics, and repair. Email him at [email protected].