Despite the many advantages of reinforced concrete, it has been recognized for many years its distinct shortcomings in terms of the efficiency with which it uses materials. Tensile stresses are passed on to the steel bars by the lengthening of the concrete member. The concrete stretches somewhat before the bars begin to act. This stretching of the concrete causes cracks to form in the base of the beam. Since concrete is strong in compression and weak in tension, the problem boiled down to devising a technique whereby all of the concrete would be in compression and, therefore, crack free. In prestressing, this is accomplished by embedding high strength steel cables in the concrete member, stretching the steel and using the force exerted by the cables trying to return to their former length and thickness to compress the concrete in the beam. Most of the cables are placed near the bottom of members because tensile stresses are concentrated there. Often, however, some cables are positioned in the middle and top to control camber and prevent any tensile stresses form developing at the top of the member before loading occurs. Prestressing work has come a long way since 1940. Although bridges dominated the prestressing market for several years, there has been an encouraging sign of its being used in many other applications. Its use in building products such as roofs and floor plank at least equal the bridge market. In piling, prestressing offers greatly enhanced characteristics in transportation and driving. Thin shell roofs have also benefitted in longer spans because of prestressing.