Epoxies are frequently used as repair materials because they bond well to almost all materials, cure rapidly, attain high strengths and exhibit good chemical resistance. Applications include use in bonding concrete (hardened to hardened, and hardened to fresh), and in patches, overlays and protective coatings. Epoxy compounds consist of resin, a curing agent or hardener and modifiers that make them suitable for specific end uses. Modifiers include accelerators that make the rate of cure depend less on temperature, diluents that reduce viscosity and make mixing easier or improve workability, and fillers that reduce cost and improve such properties as thermal expansion. Epoxy mortars employed in overlays and patches contain aggregates to reduce the maximum polymerization temperature, decrease shrinkage, lower the cost and reduce the coefficient of thermal expansion.
Epoxies are high in both tensile and compressive strength and they shrink very little during curing. Typical tensile strengths of epoxy compounds range from 500 to 5000 psi and compressive strengths from 500 to 12,000 psi. Typical ranges for modulus of elasticity at 73 degrees F are from 480,000 to 2,100,000 psi in tension and from 660,000 to 1,000,000 psi in compression.
TESTS MEASURE PERFORMANCE PROPERTIES
In the test for bond strength, a cylinder comprised of two sections of concrete glued together on a 60-degree angle are tested in compression. Shrinkage during curing is measured by casting a layer of epoxy on a glass plate, aging it and then cycling it through a number of temperature changes. If the epoxy shrinks it will fracture the glass plate. To measure thermal compatibility between concrete and epoxy resin overlays, an epoxy resin mortar is placed on a block of concrete. After curing, the specimen is cycled between 77 and minus 6 degrees F. Thermal incompatibility results in delamination of the epoxy resin mortar.