The question often arises as to which is better in concrete crack repair: epoxy or polyurethane foam. If the crack needs to be structurally repaired and the area needs to be as strong or stronger than the concrete around it, the answer is simple: epoxy.
The answer is less simple if the crack needs only to be repaired to prevent water leaking through it. Either epoxy or polyurethane can accomplish this task, and applicators typically choose the material they are most experienced with.
Epoxies have the advantage of introducing structural integrity, whether needed or not. Polyurethane foams are often more versatile when the crack is actively leaking at the time of repair, or if the area is still subject to limited movement.
Today, it is often more relevant to ask about the latest techniques, equipment possibilities and resin characteristics formulated with these materials.
This article limits itself to repair of concrete cracks of structures 8-12 inches in thickness or less. Most typically, we are referring to basements, other building foundations, parking decks, swimming pools, and unique poured-wall structures such as sea walls, and manholes. In addition, the application does not involve the stopping of high volumes of water during the repair.
Causes for Cracks in Poured Concrete
1. Moisture permeates the tiny breaks in the concrete substrate and in colder climates enlarges them to full-fledged leaking cracks by expansion/contraction resulting from freeze/thaw cycle of the moisture.
2. 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. In an extreme of the latter example the ground remains unstable and maintains a stress on the concrete sufficient to cause a crack where one did not previously exist. In other words, the movement is exceeding the strength of the concrete substrate.
The first two listed sources of crack formation and propagation are situations in which repair can readily be effective and complete. The third situation should not be addressed unless done jointly with soil stabilization, piering, mud-jacking or other appropriate solutions to eliminate the cause of continuing settling or movement.
Low Pressure Injection
The repair of the concrete structures mentioned above is suitably accomplished using low-pressure injection of the damaged areas with a liquid polymer, which hardens with time. Other applications, such as those involving very thick-walled structures (e.g. dam repair) or where a high volume of water flow must first be stopped may be better suited for high-pressure injection.
Low-pressure injection, here defined as 20-40 psi, utilizes surface ports placed directly on the surface of an otherwise sealed crack as the entry point of the liquid polymer. This technique can be utilized at up to 250 psi of injection pressure (although the author recommends keeping the pressure at 20-40 psi, as explained later in this article).
High-pressure injection at 1,000-10,000 psi utilizes injection packers which are typically placed in holes drilled at 45 degrees to intersect the interior of the crack. Until recently, this technique has traditionally been used in polyurethane foam repair as the preferred procedure. Disadvantages of this approach include the need to drill into the concrete; the cost of packers vs. surface ports ($1-2US cost differential per unit); the clean up of excess, uncontrolled foaming material flowing from the surface of the crack; the dangers associated with working at high pressure; and potential stress damage on the concrete itself.
The use of surface ports (with one-way check valves to contain the foam within the crack) together with low pressure injection of a sealed crack eliminates these problems in most situations. The resultant high-density foam attained in low-pressure injection is a typically effective permanent waterproofing repair.
The secret to effective crack injection, whether epoxy or polyurethane foam, is patient, low-pressure introduction of the liquid polymer into the crack. 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.
At injection pressures greater than 20-40 psi, the liquid polymer will flow first in the direction of least resistance, namely the larger sections of the crack. This factor and the resultant problem are best seen during the repair of a vertical crack. It is common for a crack to be wider at the front than throughout. The visual clue that a crack is filled with polymer is when liquid begins to exude out of the port above the one being injected. At pressures above 40 psi, the liquid has sufficient force to overcome gravity and rise up the crack without filling the back side of the crack, if that section is narrower than up front. Therefore, the applicator cannot confidently determine that sufficient material has been injected, eventhough material has traveled to the adjacent port. The crack repair is incomplete and subject to failure if a crack is not fully and permanently filled with liquid polymer.
Placement of Surface Ports and Surface Seal
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 preferred material for this is an epoxy paste. Epoxies bond very effectively to clean, dry, roughened concrete surfaces. Surface preparation is accomplished by scraping the crack area with a wire brush followed by the placement of the surface ports at intervals equal to the thickness of the wall.
There are several conventionally formulated epoxy pastes which harden sufficiently to a thin film in under 3 hours at which time injection of the crack can begin. Mercaptan based epoxy pastes, however, are still the best suited for those applications which require injection within 20-30 minutes after beginning the sealing of the surface of the crack and still offer the applicator and adequate working time of 7-10 minutes. Past objection to using mercaptan-based epoxies was the rotten egg sulfur odor. Newer formulations are now offered minimizing this problem without sacrificing working time.
Styrene-based car repair pastes are sometimes used for crack sealing as a more economic alternative to epoxies. These products are not as effective in sealing the crack as epoxies and material leakage is more common than with epoxy pastes. More critically, the odor of these materials is significant and the vapors are toxic, requiring good ventilation while in use.
Hydraulic cement is often used to seal when the crack is actively wet and the surface cannot be adequately dried for the epoxy paste to be applied. The ports, however, must be anchored within the crack’s surface, as the hydraulic cement cannot otherwise keep the ports from “blowing off” the surface during injection.
Using Epoxy for Injection into Concrete Cracks
Epoxies for crack injection vary in viscosities to accommodate the width of the crack. Some applicators prefer to use a low viscosity system (100-500 cps) for all sized cracks. Others prefer to use increasing viscosity for wider cracks. Some applicators will use epoxies in gel form for cracks exceeding ¼ inch. It is this author’s opinion that the key is to use any viscosity which requires less than 40 psi to inject a given crack. A polyurethane foam maybe the best candidate, if there is concern about the material leaking out the back of the crack. If an epoxy is the required material, there are now systems available at suitable viscosity which thickens within the crack in less than 5 minutes, even at a thickness less than 1/16th inch. These systems are also suitable where an epoxy repair requires removal of the surface within one hour after injection.
Using Polyurethane for Injection into Concrete Cracks
Polyurethane elastomeric foams are effective alternates to epoxies for those applications involving only crack sealing (waterproofing) and not structural repair. Properly formulated polyurethane foams are sufficiently flexible and resilient to “move” with slight concrete movement and not break the sealing substrate.
The polyurethane liquid polymer flows into the crack, mixes with any water present, and rapidly thickens as it begins to foam. Even with low pressure injection, such foam systems can stop water infiltration very rapidly against even actively flowing water in the type of structures under discussion.
The rapid thickening of polyurethane foams makes possible to remove the surface seal and ports within one to two hours of beginning injection. It also reduces the chances of liquid polymer flowing out the back of an injected crack while still in liquid form.
Even if the material is leaking out slowly, it still is capable of further foaming to fill out the crack as well as filling the void beyond the crack.
Polyurethane foams are classified as hydrophilic or hydrophobic. Both a hydrophilic and a hydrophobic react with water (typically that present in the crack, although they can also be mixed with water immediately before injection). A hydrophilic foam system will entrap any excess water present within its structure during foam formation. Both can be formulated to be flexible but a hydrophilic system is typically more resilient than its hydrophobic counterpart. The disadvantage of a hydrophilic foam is that it can lose any excess water due to evaporation under dry conditions and subsequently shrink (growing again when exposed to more water) Recently, a hydrophobic formulation has been introduced with claims of being as resilient and flexible as a hydrophilic without subsequent susceptibility to shrinkage with time.
Dual Cartridge Dispensing
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 has significantly simplified the equipment needed for many applications. It is now possible to dispense fast reacting dual-component systems with equipment which require little maintenance and virtually no cleanup. One can utilize manual dispensing tools which generate sufficient and controllable force to inject both epoxy and polyurethane foam systems. It is important to note that it is best to choose a tool with a spring attachment to control injection pressure at 20-40 psi. Other manual tools without such a spring control are capable of generating excessive pressure which can lead to incomplete filling of the crack. Air-powered tools are also available for dual cartridge dispensing and have the means of controlling injection pressure.
The question arises as to when to use dual cartridge dispensing versus plural component equipment. Even if the applicator has such equipment available, it may be more practical and/or economical to use dual cartridge dispensing in applications being discussed in this article. The advantages of mobility and minimal cleanup may overcome the cost advantage of beginning with bulk-packaged product versus dual cartridges. As a rule of thumb, if the application calls for less than three to five gallons of material usage, dual cartridges may be the more effective approach.
A Permanent Concrete Crack Repair Solution
Low pressure injection of fast-reacting dual component epoxies and polyurethane foams has proven to be effective for repairing concrete cracks. The utilization of dual-cartridge dispensing greatly simplifies the usage of these materials.
This article was originally written by Emecole founder Louis Cole and published by Concrete Repair Bulletin.