GB554693A - Improvements in reinforced concrete structures and in the manufacture thereof - Google Patents

Abstract

554,693. Reinforced concrete. ABELES, P. W. Jan. 12, 1942, No. 430. [Class 20 (iv)] Reinforced concrete is provided with pretensioned reinforcing members of high tensile steel having an elastic limit higher than mild steel, the pre-tension being above such limit. The pre-stressing is effected either before or after hardening of the concrete, the total stretching force being such that, when loaded, the tensile stress corresponding to a straight line stress distribution for a homogenous material is above 200 lb. per sq. in. The stretching force is considerably less than that employed in fully prestressing, the object being to create a stress distribution under load similar to that in concrete reinforced with untensioned mild steel, but with a smaller weight of steel. When tensioning is effected before hardening of precast reinforced concrete, a number of units may be arranged coaxially or side by side, the tensioned members are anchored at one end and the stretching force applied at the other end, the release of stretching force being effected by cutting the reinforcing members or by gradual reduction of the stretching force. The reinforcing members may be prevented from bonding with the concrete by means of lubricant or paper applied to the reinforcement or by laying the reinforcement in thin pipes of high-strength steel. The pipes may also be pre-stressed. The stretching may be effected by nuts engaging threaded parts, or by twisting thin wires, and the compression may be applied to the concrete by anchor plates or wedge pieces at the ends of the reinforcement. Prefabricated elements of concrete, cement, brick, clay or other plastic substance may be provided with apertures to receive reinforcement tensioned as above. A concrete beam 1 may be provided with reinforcement 2 in the tensile zone, and 3 as reinforcement for the counter-bending caused by pre-stress in the members 2. Fig. 2 shows the stress distribution where members 2 only are pre-stressed. If members 3 also are prestressed, only compressive stress occurs. Fig. 4 shows the stress distribution due to loading, assuming a homogenous material, in which some tension is permitted in the tensile zone. The tensile stress permitted is above that of weak concrete, i.e. above 200 pounds per sq. in., so that some cracks mav occur in the tensile zone. Specifications 338,864, 338,934, [both in Class 20 (iv)], and 541,835 are referred to.