Steve D UK said:
Steve.sorry about the long posting....BUT......you did ask.....!!!!!!!!
Rebar is common steel reinforcing bar, an important component of reinforced concrete and reinforced masonry structures. It is usually formed from mild steel, and is given ridges for better frictional adhesion to the concrete.
Concrete is a material that is very strong in compression, but virtually without strength in tension. To compensate for this imbalance in concrete's behavior, rebar is formed into it to carry the tensile loads.
Masonry structures and the mortar holding them together have similar properties to concrete and also have a limited ability to carry tensile loads. Some standard masonry units like blocks and bricks are made with strategically placed voids to accommodate rebar, which is then secured in place with grout. This combination is known as reinforced masonry.
While any material with sufficient tensile strength could conceivably be used to reinforce concrete, steel and concrete have similar coefficients of thermal expansion: a concrete structural member reinforced with steel will experience minimal stress as a result of differential expansions of the two interconnected materials caused by temperature changes.
Although rebar has ridges that bind it mechanically to the concrete with friction, it can still be pulled out of the concrete under high stresses, an occurrence that often precedes a larger-scale collapse of the structure. To prevent such a failure, rebar is either deeply embedded into adjacent structural members, or bent and hooked at the ends to lock it around the concrete and other rebars. This first approach increases the friction locking the bar into place while the second makes use of the high compressive strength of concrete.
Common rebar is made of unfinished steel, making it susceptible to rusting. As rust takes up greater volume than the iron or steel from which it was formed, it causes severe internal pressure on the surrounding concrete, leading to cracking, spalling, and ultimately, structural failure. This is a particular problem where the concrete is exposed to salt water, as in bridges built in areas where salt is applied to roadways in winter, or in marine applications. Epoxy-coated rebar or stainless steel rebar may be employed in these situations at greater initial expense, but significantly lower expense over the service life of the project. Fiber-reinforced polymer rebar is now also being used in high-corrosion environments.
Most grades of steel used in rebar cannot accept welding; such as, to adjacent steel plates or as means to bind single pieces of rebar together. However, special grades of rebar steel and welding rods make welding by expert welders possible.
To prevent workers from accidentally impaling themselves, the protruding ends of steel rebar are often bent over or covered with special plastic "mushroom" caps.