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Reinforced concrete is concrete in which reinforcement bars ("rebars"), reinforcement grids, plates or fibers have been incorporated to strengthen the concrete in tension. It was invented by French gardener Joseph Monier in 1849 and patented in 1867. The term Ferro Concrete refers only to concrete that is reinforced with iron or steel. Other materials used to reinforce concrete can be organic and inorganic fibres as well as composites in different forms. Concrete is strong in compression, but weak in tension, thus adding reinforcement increases the strength in tension. In addition, the failure strain of concrete in tension is so low that the reinforcement has to hold the cracked sections together. For a strong, ductile and durable construction the reinforcement shall have the following properties: 9338
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• High strength
• High tensile strain
• Good bond to the concrete
• Thermal compatibility
• Durability in the concrete environment
In most cases reinforced concrete uses steel rebars that have been inserted to add strength.
1. Use in construction
Concrete is reinforced to give it extra tensile strength; without reinforcement, many concrete buildings would not have been possible.
Reinforced concrete can encompass many types of structures and components, including slabs, walls, beams, columns, foundations, frames and more.
Reinforced concrete can be classified as precast or cast in-situ concrete.
Much of the focus on reinforcing concrete is placed on floor systems. Designing and implementing the most efficient floor system is the key to creating optimal building structures. Small changes in the design of a floor system can have significant impact on material costs, construction schedule, and ultimate strength, operating costs, occupancy levels and end use of a building.
2. Behavior of reinforced concrete
2.1 Materials
Concrete is a mixture of Coarse (stone or brick chips) and Fine (generally sand) aggregates with a binder material (usually Portland cement). When mixed with a small amount of water, the cement hydrates form microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid structure. Typical concrete mixes have high resistance to compressive stresses (about 4,000 psi (28 MPa); however, any appreciable tension (e.g., due to bending) will break the microscopic rigid lattice, resulting in cracking and separation of the concrete. For this reason, typical non-reinforced concrete must be well supported to prevent the development of tension.
If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists not only compression but also bending and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel resists the tension can be made into almost any shape and size for the construction industry.
2.2 Key characteristics
Three physical characteristics give reinforced concrete its special properties.
First, the coefficient of thermal expansion of concrete is similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.
Second, when the cement paste within the concrete hardens this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel.
Third, the alkaline chemical environment provided by the alkali reserve (KOH, NaOH) and the portlandite (calcium hydroxide) contained in the hardened cement paste causes a passivating film to form on the surface of the steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions. When the cement paste exposed to the air and meteoric water reacts with the atmospheric CO2, portlandite and the Calcium Silicate Hydrate (CSH) of the hardened cement paste become progressively carbonated and the high pH gradually decreases from 13.5 – 12.5 to 8.5, the pH of water in equilibrium with calcite (calcium carbonate) and the steel is no longer passivated.