A secure residence is one of the basic human necessities. The need for housing has been satisfied via various structures in conjunction with science and technology. The first durable building material used was stone. However, transportation of stone and other heavy materials was a problem. This situation pushed mankind to seek newer structural systems. Upon discovery of binding agents such as lime and natural cement, much stronger buildings were made possible. Cement is believed to have been first employed by the Romans. The cement used today was developed during the nineteenth century. The earlier concrete produced by adding sand and gravel to the cement was vulnerable to impacts and tension. Therefore, it is now known that it is ideal to strengthen the concrete with steel rods.

After the discovery of using steel to reinforce concrete, reinforced concrete buildings became extremely popular and presented a significant solution to the housing needs of urban populations.

The composition of concrete

Concrete is a structural material formed via blending sand, gravel, cement, and water. The specifications and ratios of the materials present in the mix directly determine the quality of the concrete. Generally, this ratio is 31 sand, 46 gravel, 15 cement, and 8 water. These ratios may vary depending on the construction needs.

The mixture of sand and gravel is described as an aggregate. Usually, aggregates up to 7 mm are called sand, and aggregates between 7-70 mm are called gravel. The most important role of the aggregate as a fill material is to reduce the volumetric changes of the concrete. The dough composed of water and cement displays great changes in volume. The introduction of sand and gravel into the cement helps to lessen these changes and also saves resources, since it is cheaper than cement. In order to obtain a concrete of good quality and applicable texture, the sand and gravel grains should be as round as possible and have similar diameters to each other.

Cement is produced from grinding a mixture of clay stones and limestone (CaCO3) that are cured at high temperatures. Cement is very important; when combined with water, it helps concrete to quickly solidify. The time the mix takes to solidify is called the setting time, and it is usually between an hour and an hour and a half, depending on environmental conditions. This time is shorter on warmer days and longer on colder days. Concrete begins to gain endurance (hardening) as it solidifies. It takes 28 days for the concrete to reach an endurance of 60-90 , and a much longer time to reach 100, depending on conditions. The cement amount in a cubic meter of concrete is called the dosage. One common and incorrect perception is that concrete endurance changes with the dosage. However in a mixture of a well adjusted sand and gravel ratio, concrete endurance depends on the water-cement ratio.

The water that can be used in the concrete mixture should be drinkable water that does not contain acids and salts. It is important that the water has a pH value higher than 7 and is free of carbonic acid, manganese compounds, ammonium salts, free chlorine, mineral oils, and industrial waste. Therefore, it should not be forgotten that sea water must not be used in the concrete mixture because of the salt it contains.

The properties of steel

Iron alloys that can be processed mechanically - either through pressing or rolling - are called steel. Iron is the most abundant metal in the Earth's crust, making up nearly 4.5 of it. The most important element that specifies the property of steel is carbon. The role of carbon in steel's structure is to harden the iron alloy and prevent the shifting of iron atoms. By adjusting the amount of carbon in the alloy, steel's hardness, ductility, and endurance can be changed. Both the endurance and hardness of steel increases as the amount of carbon is enriched. However, this application increases steel's fragility, reducing some of its features, such as ductility. Therefore, a 5 carbon level in the raw iron obtained through the melting of iron ore is decreased to 0.1 0.2, enabling steel to be processed. Iron alloys (steel) composed of elements such as carbon, silicon, manganese, chromium, copper, nickel and molybdenum are utilized in building structures.

The conformity of concrete and steel as reinforced concrete

Reinforced concrete materials are used in the construction of buildings, bridges, dams, and tunnels. The use of reinforced concrete became common at the end of the nineteenth century. For the best final product, the concrete and steel should be well integrated, and both should be of high quality.

Concrete and steel are two substances with very different characteristics. However, an inseparable coupling forms by balancing one's disadvantages with the other's advantages. Concrete is a material of high pressure endurance. And even though steel also has high pressure endurance, it still faces the risk of bending. The tensile strength of concrete is weak, but it is high in steel. Concrete is fire resistant; steel is vulnerable. Concrete is durable against external impacts, whereas steel is vulnerable, with a high risk of corrosion. Concrete has a brittle, breakable structure; steel, however, has a higher level of ductility. Even though concrete and steel generally behave the opposite of each other, they compensate for each other when they are together. For example, the tendency of steel to bend disappears after it is surrounded with concrete; concrete also increases steel's resistance to fire and corrosion. Furthermore, with the steel's presence inside it, the tensile strength of concrete is enhanced.

The first of the three characteristic features of reinforced concrete buildings is the compensation of all tensile forces via steel rods; the second one is the integration of concrete and steel into each other like the adherence of flesh and bone; and finally, concrete and steel have the same thermal expansion coefficients. The thermal expansion coefficient is the value that determines the amount a material will expand or retract when impacted by heat. The thermal expansion coefficients of substances on Earth vary greatly. For instance, aluminum has a coefficient of 2,2x10-5 L/0C, copper of 1,7x10-5 L/0C, gold of 1,4x10-5 L/0C and glass of 0,85x10-5 L/0C (L= Length). It is harder for materials of different thermal expansion coefficients to have conforming movements. The most important reason for the harmonious union of concrete and steel is that their thermal coefficient values are almost the same (1,2x10-5 L/0C). If this was not the case, because of the temperature differences of inside and outside environments, the concrete and steel that make up the reinforced concrete would expand at different speeds, resulting in cracks and fractures of the load bearing elements of the building (columns, beams, flooring).

Though concrete and steel have vastly different properties, their thermal expansion coefficient values are the same and this causes them to move together during temperature changes. A similar system is put to use during the creation of cartilage, bone, and connective tissues in our body. The fibers of the connective tissue resemble the iron and steel, the cells are similar to gravel, and the intercellular matrix resembles the cement. The difference is that this system is renewed dynamically, and is flexible and strong.

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