| Introduction: Since concrete is ubiquitous and its history can be traced to ancient Egypt and Rome, it is often falsely perceived as a "simple" material. Actually, the microstructure of concrete tends to be highly complex. Furthermore, the structure and the properties of this composite material can change over time. Most modern concrete structures are reinforced with steel, since concrete itself displays relatively low strength when loaded in tension. While steel reinforced concrete is obviously a widely used, cost-effective construction material, degradation of such structures has become a major problem in many parts of the world and not gone unnoticed. The basic constituents of concrete are cement, water and aggregate (and selected additives). Cement is produced by heating limestone and clay to very high temperatures in a rotating kiln. Cement is produced by grinding the resulting clinker to a fine powder. Water reacts chemically with cement to form the cement paste, which essentially acts as the "glue" (or binder) holding the aggregate together. The reaction is an exothermic hydration reaction. The water:cement ratio is an important variable that needs to be "optimized". High ratios produce relatively porous concrete of low strength, whereas too low a ratio will tend to make the mix unworkable. Aggregates are usually described as inert "filler" material of either the fine (sand) or coarse (stone) variety. The shape, size density and strength of aggregate particles can vary significantly, and can therefore influence the properties of the concrete. Aggregate tends to represent a relatively high volume percentage of concrete, to minimize costs of the material.
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