What is the better choice: polyurethane or epoxy? Epoxy is the better choice if structural strength is required to be maintained. If the crack is only repaired to prevent water leakage then either product can accomplish this task.

Cause of Cracks

Lack of joints, to accommodate drying shrinkage and thermal movement during the construction phase is one of the major causes of concrete cracks. Settlement and overloading are also common causes of cracking. Cracks usually appear within the first month after construction. These cracks can leak as soon as they are formed or might expand and leak later. Crack can grow and cause problems as they expand.

How Cracks Grow

Moisture, which is the worst enemy of homes, enters the tiny cracks in the concrete and in colder weather, freezes, enlarging the crack. Also, movement of soils surrounding your home, through settling, can also cause expansion of your cracks. In extreme cases, soils that are unstable can overcome the inherent strength of concrete causing crack and expansion.

Cracks caused by construction, heat, freezing and settlement can be corrected by injection. Cracks caused by unstable soils must have soil problem stabilized prior to attempting injection corrective methods.

Low Pressure Injection

This method of injecting polymers into the concrete is adequate for repairs of cracks not involving high volumes of water. High-pressure injection is better suited for this type of application. Low pressure is defined as injection pressures between 20 to 40 psi and can be used with either ports or surface injection. Pressures as high as 250 psi may be used.

High Pressure Injection

1,000 to 10,000 psi utilizing injection packers usually placed in holes drilled at 45 degrees to intersect the interior of the crack. This is common method of repair using polyurethane foam. High cost of packers, clean up of excess, potential stress damage and dangers of using high pressure are overcome by using low-pressure injection methods.

The use of surface ports (with one-way check valves) together with low-pressure injection of a sealed crack eliminates these problems in most situations. The injection of high-density foam is an effective water proofing method.

The best method for low-pressure crack repair is to slowly inject liquid polymer into crack until crack is completely filled, filling of adjacent surface port is visual confirmation. Using pressures above the 40-psi will allow polymer to rise and may indicate a false presence of the polymer filling voids.

Methods of Surface Sealing and Placing of Surface Ports

Surfaces are first cleaned with wire brush, and then surface ports are placed along crack at intervals equal to the thickness of the wall. The surface crack is then filled with epoxy paste which when sufficiently hardened allows filling of surface ports to begin. Hydraulic cement is used when cracks are actively ?wet?. The ports must then be anchored to prevent blowing off during injection.

Types of Epoxies

Epoxies for crack injection vary in viscosities to suit the width of crack to be filled. Whatever viscosity chosen, the applicator has to ensure that he can keep injection pressure less than 40 psi to ensure complete coverage of cracks and voids.

Injecting Polyurethane Foam

Polyurethane elastomeric foams are effective for water proofing cracks and where structural repair is not required. These products are flexible enough to move with concrete and maintain seal. These foams can be used where there is active water flow and mix with water, combined with foaming action, effectively stop water infiltration.

The rapid thickening of foam products makes it possible to remove surface seal and surface ports a couple of hours after injection. This also prevents and minimizes leakage out the back of repair area, which may not be accessible.

Polyurethane foams are classed as hydrophilic or hydrophobic. They both react with water, either in the crack or mixed prior to injection. The hydrophilic system is typically more resilient than that of the hydrophobic foams. Hydrophilic foam can lose its excess water due to dry conditions and subsequently shrink. The hydrophobic product is reputed to be impervious to shrinkage through drying.

Dual Cartridge Dispensing

This method has simplified the injection application allowing for fast inexpensive injection of cracks. Clean up is virtually eliminated and maintenance is bare minimum. This method is usually used when less than 5 gallons of material is required.