In geology, cementations and concretions occur when a process called lithification takes place, which means that loose particles of rock are bonded together by a mineral such as calcium carbonate (calcite) or iron oxide (limonite). Man has recognized this natural phenomenon and been able to duplicate it from early times for construction purposes. In approximately 200 b.c. the Romans used lime and gypsum to bond sand particles together. The resulting cement was used as mortar and paving material. By adding stones and pottery shards to the cement, they were able to create a material that was adequately hard, but because it required less cement, it was cheaper to make. The Romans also created a forerunner of Portland cement. They mixed volcanic ash with cement to create a variety of cement that could be used under water for aqueducts, drains, and bridges. Concrete and cement have been used virtually throughout history; yet, their physical properties have been fully understood for less than 300 years. The consequence of not knowing about it was a trial and error process that often had disastrous results. In 1758 John Smeaton rebuilt the Eddystone lighthouse in England. It had originally been built from rock quarried at Portland. After much experimentation, Smeaton developed a hydraulic cement to seal the lighthouse from the rough conditions of the English Channel. In 1824 Joseph Aspdin (1799-1855) patented Portland cement, named for the gray Portland limestone--such as that used in the Eddystone lighthouse--that it resembled. It was developed mainly in response to the growth of canal systems in Europe and, especially, in the United States. Portland cement has become the most widely used construction material in the world. Portland cement is made of calcium derived from limestone, chalk, marl, or shells and of silica derived from clay or shale. It is manufactured by quarrying rock and crushing it to a high fineness. It is then subjected to baking in a rotary kiln. The "klinkers" (remaining large pieces) are then reground into a fine powder that binds when mixed with water. The two main keys to the strength of concrete is its proportion of aggregate to matrix and water to matrix. Generally, the bond increases with the amount of cement matrix used. Water is the key bonding agent. The bond is not established from drying. On the contrary, the cement gets stronger the longer it sets in water. This is the reason that construction workers hose down concrete as it sets. This process is referred to as curing.
The first reinforced concrete using iron rods was developed by Frenchman Jean Louis Lambot and patented in 1867 by Joseph Monier, who used Portland cement instead of concrete. The purpose of the iron rods was to increase the tensile strength of the material, a property that cement and concrete lack. Later, steel rods replaced iron as reinforcement. Ferrocement is similar to reinforced concrete, though instead of separate reinforcing bars, a wire mesh is used to form a cement cast. The steel mesh can be formed into nearly any shape desired, making it strong and versatile. Italian engineer Pier Luigi Nerri used ferrocement in 1942 and 1943 to build a fleet of boats. Nerri referred to ferrocement as "melted stone," for it could be easily formed into statues and building adornments. Fiber reinforcement is increasingly used in concrete. Asbestos cement was developed in 1899 by Ludwig Hatschek for its insulating ability. Steel fiber cement has been used in England in the construction of motorways. In other parts of the world, such natural fibers as sisal, sugar cane, coconut, and bamboo are used because they are cheap and readily available. There has been some debate over the cost-effectiveness of concrete reinforcement. On the one hand, its greater strength reduces construction costs. This is countered by increased materials cost, especially when steel is used. It cannot be denied that it has made many ambitious construction projects possible that were previously impossible. The entire Interstate Highway System was made possible by reinforced concrete. The most recent development, which is taking construction to new heights, is prestressed concrete. First introduced by Eugene Freyssinet (1879-1962) in 1927, steel rods are placed under tension while the concrete sets around them. This creates pressure within the beam. Instead of passively carrying a load, the beam actively resists the load placed upon it. It is commonly used in bridges and domes.
Despite its simple utility, concrete and cement continue to evolve to meet demands of modern times. Special cements are produced for mortars and architectural or engineering applications, such as masonry cement or plastic cement. Cement made from the compound supercritical carbon dioxide may have a host of applications--metals could be introduced to make concretes conduct electricity, or make them radar-invisible. Bacteria have been used to make cement, as a byproduct of their breakdown of urea. This breakdown makes nearby calcium precipitate out as limestone, which can fill in the cracks in fissures in cement, from the inside out. (The bacteria die off when their nutrient urea die out.) Such cement would be useful for sealing up cracks in buildings and other structures.
Smart cementis cement embedded with sensors that can provide non-destructive damage assessment. By placing sensors like fiber optic strain gauges directly in the concrete of a structure like a bridge, a computer can directly and instantly monitor the bridge for cracks, strain, and road-salt corrosion, eliminating expensive maintenance checks. Prototype concrete containing short carbon fibers may be able to do the same thing, without embedded sensors, by monitoring its electrical response.
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