Plastics with long-chain molecules, called polymers, are used in several ways to enhance concrete properties: replacement of portland cement, incorporation in a mix as an admixture, and impregnating hardened concrete.
Polymer concretes, such as methyl methacrylate and unsaturated polyester, in which a polymer replaces portland cement may have more than double the strength and modulus of elasticity of portland-cement concrete. Creep is less and resistance to freezing and thawing cycles is higher with the polymer concretes. After curing for a very short time, for example, overnight at room temperature, polymer concretes are ready for use, whereas ordinary concrete may have to cure for about a week before exposure to service loads.
Monomers and polymers may be used as admixtures for restoring and resurfacing deteriorated concrete surfaces. Latexes of methyl methacrylate, polyester, styrene, epoxy-styrene, furans, styrene-butadiene, and vinylidene chloride have been employed for these purposes. The resulting concrete hardens more rapidly than normal concrete. A polymer admixture may also be used to improve the bonding properties of portland cement. Inserted in a mix as an emulsion for this purpose, the admixture supplies a significant amount of water to the mix, which becomes available for hydration of the cement. The release of the water also sets the emulsion.
Hence, moist curing is not desirable, inasmuch as the emulsion needs to dry to develop the desired strength. A grout or mortar containing the bonding admixture develops a higher bond strength when applied as a thin layer than as a thick one and the bond may be stronger than materials being joined.
Impregnation of concrete with polymers is sometimes used to harden surfaces exposed to heavy traffic. Strength and other properties of the impregnated concrete are similar to those of concrete in which polymers replace portland cement. Impregnation is achieved by first drying the concrete surface with heat and then soaking the surface with a monomer, such as methyl methacrylate, styrene, acrylonitrile, or tert-butyl styrene. It is subsequently cured with heat.
Slab Toppings. At least partly because of excellent adhesion, epoxies are formulated with sand and other fillers to provide surfacing materials for concrete.
Unlike standard concrete topping, epoxy-based surfacing materials can be thin.
They are especially useful for smoothing uneven, irregular surfaces. The epoxy cures quickly, allowing use of the surface in a short time.
Grout. Cracked concrete can be repaired with an epoxy grout. The grout is forced into cracks under pressure for deep penetration. Because of its good bonding strength, the epoxy grout can largely restore strength, while, at the same time, sealing the crack against penetration by liquids.
(‘‘Polymers in Concrete,’’ ACI 548; ‘‘Guide for the Use of Polymers in Concrete,’’ ACI 548.1; and ‘‘Polymer Modified Concrete,’’ SP-99, American Concrete Institute.)