The importance of controlling the quality of concrete, from batching right through to curing, is fully appreciated by contractors. But what of reinforcement, the other partner in the team that makes up reinforced concrete? Not so readily apparent or detectable, perhaps, are the omissions or mistakes in reinforcement that could result in a structural failure- potentially far more damaging, possibly disastrous. In order to control the quality and performance of the steel reinforcement system he places, the concrete contractor must know something about the basic theory behind reinforcement, and why it functions as it does in combination with concrete. He should be aware of the different types of reinforcement and how to handle and place them so that the resultant structure performs a the designers planned. Concrete's strong point is its resistance to compressive force, with stresses theoretically possible up to 18,000 psi. But it is weak in tension, 550 psi being about all the pulling it can take. Reinforcing steel, on the other hand, is high in tensile strength; typical bars are useful up to 60,000 psi, but they are not suitable for usual compressive loads, except in combination with a mass of concrete. The designer's problem is to locate the concrete and steel in combination with and in relation to each other so that their respective strengths as best employed. For example, in a simple beam between a pair of supports, the tendency of the beam is to sag downward at the middle, or deflect. When this happens, the upper surface is shortened, or compressed, while the lower surface is stretched, or pulled apart. Obviously, the place for reinforcing steel is at the bottom of the beam where the pulling or stresses occur, with the concrete at the top resisting the compressive stresses. And the, somewhat oversimplified, is what reinforced concrete design is about.