An important parameter for concrete durability is its permeability, which describes the ability of gases or liquids to penetrate into and through concrete sections. The movement of gases or liquids in concrete primarily occurs in the capillary pores of the cement paste matrix. Because the volume of capillary pores increases with an increasing w/c ratio, permeability increases correspondingly. High values of permeability can be expected when the capillaries form an interconnected continuous network caused by a low degree of hydration. For w/c ratios exceeding ca. 0.7, continuity of capillaries must be assumed even after complete hydration.
The thermal conductivity of concrete increases with decreasing porosity and increasing moisture content. Crystalline aggregates give higher conductivity than amorphous aggregates. For normal concrete the thermal conductivity is 1.5 – 3.7 W m?1 K?1.
Concrete is a brittle material, but reinforcing with steel bars (reinforced concrete) or with prestressed tendons (prestressed concrete) has greatly expanded its applicability. It is now one of the most important building materials because of its structural, physical, and chemical properties and its economics. Furthermore, it can be considered an environmentally friendly material. Fresh concrete can easily be shaped, and most concrete is poured in place on the construction site. However, there is increasing use of concrete elements or concrete products that have been prefabricated in plants.
Concrete is an artificial stone in which natural stones of well-graded sizes are bound together by a cement matrix, thus providing strength. Although this principle of manufacturing building materials can be traced back at least to the Roman Empire, it was later neglected for centuries. Concrete regained importance in the 1800s, when methods for large-scale production of modern cement were developed.
