Conventional concrete is a conglomerate of hydraulic (Portland) cement, sand, stone, and water. It was developed approximately 150 years ago to imitate natural stone while providing less labor-intensive methods of shaping the material (i.e., casting rather than hewing and carving). As such, it was initially expected to resist only compressive loads. As highway systems developed and expanded after World War II, however, concrete started to find new uses in roads and bridges, where it was subjected to tensile-bending stresses as well.
In MSE terms, conventional concrete is a particulate-strengthened, ceramic-matrix-composite material. The sand and stone are the dispersed particles in a multiphase matrix of cement paste. Reinforced concrete can then be considered a “fiber-reinforced”composite, with the reinforcing steel bar (rebar) acting as the “fiber.”One fundamental difference, however, between conventional concrete and other engineering composites is that the composition; and hence the properties, of the cement paste do not remain constant after processing but vary with time, temperature, and relative humidity. A second difference is concrete's porosity. The pores of concrete are filled with a highly alkaline solution with a pH of between approximately 12.5 and 13.8 at normal relative humidity. This solution can be regarded as a separate phase of the microstructure and plays a major role in determining the
strength and durability of concrete.
The dimensions of the different structural features in concrete span 10 orders of magnitude, from nanometer-sized pores and gel “particles”to rebar that can be tens of meters in length, and to paste, sand, and stone particles of all sizes in between. Although the performance of concrete is affected by the properties (e.g., density and porosity) of its sand and stone components, these properties are determined by nature. Suitable aggregate must be selected from available sources. Therefore, it is the cement paste in conventional concrete that is the most important MSE systems component because it can potentially be tailored to fit the job.