What is the Minimum Crack Width for Epoxy Injection?

Learn about minimum crack widths for epoxy injection when repairing structural cracks in concrete foundations & walls.

What is the Minimum Crack Width for Epoxy Injection?

Crack Injection epoxy resins used for structural crack repairs meet the requirements of ASTM C881,5 Type IV and have appropriate viscosities for the crack widths and degree of resin confinement. Wider cracks that cannot be sealed on all sides require a higher viscosity (paste) injection material, while most cracks that can be sufficiently sealed are injected with epoxies at viscosities of 300 to 600 cps at room temperature. For applications at temperatures below 60°F (15°C) and for fine cracks of less than 0.01 in., it is necessary to clean the crack and surrounding surface to allow the paste to adhere to solid concrete. At a minimum, the surface for gluing must be brushed with a wire brush.

Oil, grease or other contaminants must be removed from the surface to allow the paste to adhere properly. Take care not to impact any residue in the crack during cleaning. With clean, oil-free compressed air, blow the crack to remove dust, debris or standing water. The best results will be obtained if the crack is dry at the time of injection. If water seeps continuously through the crack, the flow must be stopped for the epoxy injection to produce a proper repair.

Other materials, such as polyurethane resins, may be required to repair an actively leaking crack. If a coating, sealant or paint has been applied to concrete, it must be removed before applying the epoxy for gluing. Under injection pressure, these materials can rise and cause a leak. If the surface coating covers the crack, it may be necessary to route the crack opening in a “V” shape with a grinder to overcome surface contamination. Epoxy injection is commonly used to restore the pre-cracking condition of the member without increasing its strength.

The epoxy tensile bond to the concrete substrate is stronger than the tensile strength of concrete. Future cracking can occur with the same load as that of the original seamless member, but in different locations. Reinforcement is achieved by the installation of additional reinforcements in the fault plane in combination with resin injection. Often, internal or external reinforcement is installed in combination with epoxy injection for strengthening and restoration. Crack injection can be successfully performed in cracks as narrow as 0.013 mm wide with general epoxy injection resins.

Cracks with less width can be injected with epoxy or other polymer systems that have a low viscosity of 200 cps. For slabs, designers should choose a material with suitable flexibility and hardness or rigidity properties to suit both anticipated floor traffic and future crack movements. If the crack penetrates completely through the concrete element and cannot be sealed, the application may not be suitable for injection repair. Usually, cracking caused by drying shrinkage of concrete will be active at first, but will eventually stabilize and become inactive as the moisture content of the concrete stabilizes. With transparent adhesive tape, drilled holes and a rubber-tipped mixing tube connected to a dual-cartridge hand gun, a low-pressure repair material can be injected into fine cracks. Crack injection concrete repair can be completed with compressed air guns, manually operated delivery systems, spring- or balloon-operated capsules, or single- or two-component electric or pneumatic injection pumps.

Moving to the next port as soon as the epoxy appears will allow the epoxy to travel along the widest parts of the crack to the next ports rather than forcing it into the crack before it travels to the next ports. To hold the epoxy resin in the crack until it hardens, it is good practice to seal cracks on all sides of the concrete element. If active cracks or cracks that act as shrinkage or expansion joints are injected, expect other cracks to form next to or away from the repaired crack. For active cracks, the size and form factor of the sealant reservoir are just as important as selecting a suitable sealant that can accommodate anticipated future crack movements. For active cracks, flexible sealants are preferred, but their bearing capacity and edge support are lower. Increasing width-to-depth ratio will increase sealant's ability to tolerate future moments of cracking. First, clean out dirt and debris from within cracks before filling them with low-viscosity repair material using compressed air guns, manually operated delivery systems, spring- or balloon-operated capsules or single- or two-component electric or pneumatic injection pumps.

Epoxy injection is commonly used for repairing concrete cracks in foundations, basements, beams, columns, slabs, walls and other concrete structures on horizontal surfaces such as bridge decks or tops of pillar caps. Epoxy injection can also be used for shallow concrete cracks on horizontal surfaces by gravity feeding with epoxy or high molecular weight methacrylate (HMWM). Injection ports are typically installed with a gap that is equal to or greater than measured depth of crack.

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