Concrete repair is a four billion dollar a year business according to "Concrete Repair Digest" magazine. Concrete crack repair is one element of this market.

This report limits itself to repair of concrete cracks in general and specifically to cracks of structures 16” in thickness or less. Most typically, we are relating to basements, other building foundations, parking decks, swimming pools, and unique poured-wall structures such as sea walls.

These applications have in common the preferred method of repair being low pressure crack injection of a liquid polymer which hardens with time. Other applications, such as those involving very thick-walled structures (such as dams) and very long cracks (such as found on bridges and highways) may be more suited to high pressure injection.

By far the most frequent type of cracks is caused during construction by failure to provide sufficient control joints or expansion joints to accommodate drying shrinkage and thermal movement. Also common are those cracks caused by structural settlement, overload or earthquakes. Most cracks are formed in the first 30 days of the pouring of the concrete structure.

These cracks may initially be too small to be detected and to have any negative consequences at first and, at times, never grow to be any problem at all. Other cracks become visible very early and cause problems, such as water leakage, almost immediately.

Even the early undetected cracks can, in time, become larger and cause problems whether structural or, more commonly, a source of water leakage.

How this happens can be delineated as:

1. Especially in colder climates, moisture can permeate these tiny breaks in the concrete substrate and enlarge them to full-fledged leaking cracks by moisture expansion/contraction resulting from freeze/thaw cycle of the moisture.


2. In addition, as the ground around the foundation stabilizes, any movement can cause the rigid concrete substrate to separate at these tiny breaks in the concrete, enlarging the to a water- leaking size.


3. A more serious problem to solve is when the area around the foundation remains unsettled, resulting in an ongoing stress on the concrete structure. If this stress exceeds the strength of the concrete, cracks will form even where initial cracks did not exist (even after repair of these initial cracks).

The first two listed sources of crack formation and propagation are situations to which repair can readily be effective and complete. The third situation should not be addressed unless done jointly with soil stabilization, piering or mud-jacking to eliminate the cause of continuing settling.

Even the first two situations require proper applications and procedure to effectively solve the problem. The materials proven to be most effective in concrete crack repair are:

1. Polyurethane elastomeric foams are recommended. The concrete structural integrity is not a problem and the problem is only water leakage. Polyurethane foams harden very rapidly (unlike most epoxies) and are less likely to flow out the back of some cracks as epoxies may. Furthermore, polyurethane foams expand in the crack area and may reach areas that an epoxy may not if not properly injected. Polyurethane, being elastomeric, may also handle concrete movement more effectively than the more rigid epoxies (although this is a debated point and not one that this report draws conclusions on).



2. Two-component epoxies, which effectively seal a crack and at the same time reinforce the repair area to be actually stronger than the unrepaired concrete area around it. Epoxies are always the preferred material when the structural integrity of the concrete is open to question.

The secret to effective crack injection is patient, low pressure introduction of the liquid into the cracks. Low pressure (20-40 psi) allows the applicator to properly monitor the polyurethane injection process. At this pressure range, the applicator can be confident that the crack has been saturated with the liquid polymer up to that point when liquid begins to collect at an adjacent surface port. If done at higher pressure, the liquid polymer may only be filling the larger sections of the crack, leaving smaller crack sections available for future deterioration.

Low pressure injection crack repair begins with the surface sealing of the crack and the placement of the surface ports along the crack opening. The best material for this is epoxy pastes. Epoxies bond very effectively on to clean, dry roughened concrete surfaces. This is accomplished by scraping the crack area with a wire brush. This is followed by the placement of the surface ports as far apart as the wall is thick.

There are several epoxy pastes (such as E901) which harden in under three hours in a thin film such as done in surface sealing (1/8” or less on the average). Only a mercaptan based epoxy (such as E301) however, can harden in less than 30 minutes and be ready for injection. This is true even in cold weather. While this type of epoxy is preferred when expediency is important (such as in individual cracks less that 20 feet in length), these products require ventilation because of an undesirable odor before mixing. For most do-it-yourself individuals, E901 works best.

Polyurethane elastomeric foams work best for those applications involving only crack sealing (water proofing) and not structural repair. Along with their nature to be elastomeric and being able to move with slight concrete movement to keep a seal, Polyurethanes begin to harden and foam within minutes of injecting.

The rapid thickening and hardening of polyurethane foams permits the removal of the surface seal and ports within one to two hours of injection. It also reduces the chances of it flowing out of an injected crack while still in liquid form and, even if it is leaking out slowly, it still has the ability to foam to fill out the crack.

For those typical crack injection repairs of a non-structural nature, it is this report’s opinion that polyurethane foams (especially products like E102) work equally as effectively as epoxies as long as the foaming is kept to a minimum (two to three times its liquid volume). At this level the strength and elastomeric nature of the polyurethane is optimized, and the foaming process is best utilized (improves the bond by adding a mechanical nature to the chemical bond plus the foaming leads to faster hardening).

Low pressure injection of epoxies and polyurethane foams are a proven solution to the problems associated with many if not most concrete crack repair situations.