In the 1960's, some radically new approaches to concrete were introduced and tested. One of the newest is polymer concrete. Polymer concrete is ordinary concrete that has been impregnated or loaded with a monomer material and then polymerized by radiation, or by heat and catalytic ingredients, or by a combination of these two techniques. The result is a composite material with dramatically improved characteristics. When compared with ordinary concrete, polymer concrete exhibits the following improvements: compressive strength increased four times; modulus of elasticity increased 80 percent; modulus of rupture improved 256 percent; freezing/thawing resistance upped 300 percent; and hardness-impact increased 73 percent. The technique used to load the concrete with the monomer also affects the results obtained. Two basic techniques were employed in these tests; performed, or impregnation after the air-entrained concrete has been formed and hardened; and premixed, or incorporation of the monomer by intermixing it while the concrete is still plastic, either as a partial replacement for the mix water or as an admixture. Loading of the polymer into the concrete ranged as high as 6.7 percent by weight of the unimpregnated concrete, or about 13 percent by volume. Preformed polymer concrete showed significantly greater improvements in performance than premixed polymer concrete. The choice of a monomer material is based on such factors as its cost, its viscosity for ease of impregnating the concrete, and its properties when used in concrete. Another factor in selecting a monomer material is the amount of radiation or heat needed to polymerize it. The main advantages of radiation over the heat-catalyst technique are the somewhat greater improvements in physical characteristics of the resulting polymer concrete and the ability to polymerize at lower temperatures. Heat-catalyst polymerization eliminates the problems inherent in radiation but introduced some problems of its own.