Concrete is strong in compression but weak in tension, and where tension occurs it is usual to introduce steel bars to provide the tensile strength which the concrete lacks. For example, with a concrete beam or lintel, compression occurs at the top and tension at the bottom of the beam and the ends are often hooked to provide a grip. The 25 mm cover prevents rusting of the reinforcement. The steel must be free from loose mill scale, loose rust, grease, oil, paint, mud and other deleterious substances which impair the bond between the steel and concrete. The most common form of reinforcement is the mild steel bar to Bs 4449 or Bs 4482. Medium and high tensile bars are also available, and deformed bars which are twisted and/or ribbed provide a better bond and greater frictional resistance than round bars and obviate the need for hooked ends. It is important that the reinforcement is fixed securely to avoid displacement during the placing of the concrete. The bars are tied together with soft wire at intersections, and spacers of small precast concrete blocks, concrete rings or plastic fittings ensure the correct cover of concrete. In raft foundations, reinforcement may take the form of steel fabric to Bs 4483, and this consists of a grid of small diameter bars, closely spaced and welded at the joints. Contact us for your building projects or other enquiries through this e-mail, michaeltony29@yahoo.com