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 article limits itself to the repair of concrete cracks in general and specifically to cracks of structures 16 inches 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 - 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 (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 working 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, while at other times, never growing to be a 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 then 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, peering, 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. Two-component epoxies, which effectively seal a crack and at the same time reinforce the repair area to be actually stronger than the un-repaired concrete area around it. Epoxies are always the preferred material when the structural integrity of the concrete is open to question.

2. Polyurethane elastomeric foams, when concrete structural integrity is not a problem and 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).

The secret to effective crack injection, whether epoxies or polyurethanes, is patient, low-pressure introduction of the liquid into the cracks, Low pressure (20-40 PSI) allows the applicator to properly monitor the 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.

Traditionally, crack injection required expensive, cumbersome proportioning equipment. These remain useful where high pressure and/or very large volumes of liquid polymer need to be injected.

The development of dual cartridge dispensing, utilizing either disposable or re-usable dual cartridges or containers, has significantly simplified the equipment and power requirements. It is now possible to utilize manual dispensing tools similar to caulk guns to inject both epoxies and polyurethane systems. It is important to note that it is best to choose such equipment which utilize a spring to control injection pressure. Other manual tools, without the spring as a control, can easily cause injecting at pressure much higher than desired.

This may result in the incomplete injection of a crack, the most common reason for crack repair failure. Air-powered equipment is also available to do crack injection via dual cartridge dispensing. It is important that this equipment have means of controlling injection pressure to 20-40 PSI. Air powered equipment make it feasible to use larger containers, which may reduce the overall cost of the liquid polymer system.

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 which harden less than three hours in a thin film such as done in surface sealing (1/8 inch or less on the average). Only a mercaptan based epoxy 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 than 20 feet in length), these products require ventilation because of an undesirable odor before mixing.

Epoxies for crack injection vary in viscosities to accommodate the width of the crack. Some applicators prefer to use a low viscosity system (300-500 CPS) for all sized cracks, while others prefer to use increasing viscosity systems as the width of the cracks increase (up to 3000 cps). Some applicators will use epoxies in gel form for cracks exceeding ¼ inches. It is this article's opinion that the key is touse any viscosity which requires less than 40 PSI to inject a given crack. If there is concern about the material leaking out the back of the crack, polyurethane foam should be used.

Most epoxies require hours to harden. This is advantageous to assure time for the epoxy to flow and fill even the smallest openings of a crack. At the same time, this characteristic can have disadvantages.

For one, it is possible for the epoxy to flow out of the crack before it has hardened if the area behind the concrete has separated from the foundation. This is why it is important to re-inject the crack after the initial filling. If a substantial amount of epoxy is again injected, there is cause for concern.

Secondly, if it is necessary to remove the surface seal and ports (i.e. for aesthetic reasons) this must be done 1-3 days after injection with most systems.

To overcome these disadvantages of epoxies, polyurethanes elastomeric foams become effective alternatives 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. Some begin to foam virtually upon entering the crack and are ideal to stopping flowing water and to filling a large void (although this same characteristic keeps it from filling very small openings of a crack).

The rapid thickening and hardening of polyurethane foams permits the removal of the surface seal and ports within 1-2 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 work equally as effectively as epoxies as long as the foaming is kept to a minimum (2-3 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.