Reinforced Concrete: the Most Important Building Material in 20th Century
Throughout history, human beings are trying to build shelters that protect them from the harshness of the climate and wild animals. However, in their continuous quest to perfect their structures, they have always been looking for new and advanced building materials that would satisfy their needs and requirements. In the past, human used many different building materials such as mud brick, stone, wood, etc. In the first century BC, the Roman invented the concrete to substituted stone work. It was a mixture of lime mortar, sand, water, and stones, which represents the contribution of the Roman civilization to the present modern world.
In early 19th century, the English Joseph Aspdin invented the Portland cement, which represented the main component of the present-day concrete. By the mid-19th century, the French Joseph Monier introduced a new way of strengthening concrete by imbedding steel mesh and later was replaced by steel bars. The combination of concrete and steel is called reinforced concrete, which is considered as one of the most important building materials until the present day. Throughout 20th century, reinforced concrete played an essential role in changing the thinking of architects and civil engineers and allowed new opportunities to create daring building masses, shapes and forms.
Reinforced concrete consists of mainly two important and essential materials, Concrete and Steel. Although reinforced concrete is inelastic material, it can be shaped to any form according to the architectural drawings. The compressive strength of concrete is high while its tensile strength is low. However, the use of steel would reinforce concrete because of its high tensile strength, specifically in the tension zones. The reinforcement is usually round steel rods, which are placed in the forms before casting the concrete. When completely surrounded by the hardened concrete mass, it forms an integral part of the structural member and the two materials statically act together as one material.
Deformed High Grade Steel
Under the effect of the different vertical loads, the structural member such as a beam will be subject to compressive stresses above the neutral axis NA and tensile stresses below it. If the beam is made of plain concrete only, its bearing capacity will be controlled by the low tensile strength of concrete. The failure will take place due to the cracking of the tension zone and the high compressive strength of the concrete is not totally used. However, in order to increase the bearing capacity of the beam, its weak tension zone is reinforced by steel bars placed near the lower extreme fiber as shown in the following diagram.
As a building material, concrete is made of number of different materials including, cement, water, sand and gravel. Cement and water are the two essential ingredients that do all the chemical reaction and the gravel and sand would give strength to the whole. One cubic meter (1m3) of finished plain concrete usually consists of:
1. Gravel 0.8 m3 (fig.a)
2. Sand 0.4 m3 (fig.b)
3. Cement 200-400 kg (fig.c)
4. Water 180-200 Liter/m3 (according to the quantity of cement)
There are number of factors that determine the properties of the concrete including, the proportions of the ingredients, the quality and granular composition of the aggregate, and the degree of compactness of concrete, as well as the method of curing to get full hardening. The main idea of curing is to keep the concrete adequately moist in order to achieve a reasonable hardening. Curing can mainly be achieved by wetting the concrete. Wetting concrete is to spray the exposed surface and the timber formwork with water two or three times daily depending on the region temperature and the degree of humidity.
The concrete allowable compressive stress ranges between 25-75 kg/cm2 according to the amount of cement used in 1 m3 concrete. Concrete’s allowable tensile stress is very small and does not represent more than 10% of its allowable compressive stress. The allowable stress in steel reinforcement ranges between 1400 kg/cm2 to 2400 kg/cm2 according to the type of the raw iron used in the production of steel.