GB739480A - Improvements in and relating to the production of prestressed concrete - Google Patents

Abstract

739,480. Reinforced concrete. LEE, D. H. March 18, 1954, No. 7877/54. Class 20 (4). In a pre-stressed concrete member having high-tensile steel reinforcement bars with riblike projections for increasing the bond, the ends of the bars are encircled by annular reinforcement means embedded in the concrete, to resist the forces in the concrete tending to burst the same. The rib-like projections may be of any desired arrangement, and the annular reinforcement may be a wire helix or a number of spaced wire rings. In the post-tensioning method shown in Fig. 4, one end 3 of the reinforcement bar 2 is embedded in the concrete 1, the remainder of the bar being surrounded by a sleeve 4 which is sealed at 5 to the bar, to prevent the entry of water or concrete, and is provided with grout feed and air escape passages 6, 8. The reinforcements 9 are positioned in the mould either by wire clips on the reinforcement 2, or by being suspended from a rod carried by the mould walls. When the concrete has set, the reinforcement is tensioned by a jack 13 and held in tension by a temporary anchor 14. Grout is then supplied to the sleeve 4 by the passages 6, 8 and, if desired, through the end 18 of the sleeve by the use of suitable apertured masks 19. The end anchorage may be increased, Fig. 6, by applying a collar 20 to the bar, e.g. by pressing on the collar whilst hot. In this case, the sleeve 4 is splayed to accommodate the collar and the grout in which it is finally embedded. Alternatively, as in Fig. 7, the collar may comprise two wedge pieces 22 of cast iron formed to fit over the rib-like projections on the reinforcement, and kept together by a collar 23 which is driven tighter on to the wedge pieces 22 by the tension in the reinforcement. In Fig. 8, the bar 2 is gripped by wedge-pieces 25 in a chuck 26 which is screwed internally at 32 to receive a complementary screw on the jack by which the tension is applied. The tension is held by a sleeve 30 which is screwed down on to the end face of the concrete 1. The chuck 26 may have an externally-screwed part of reduced diameter, instead of the internal thread 32, to connect with the jack, and the sleeve 30 may be merely a screwed ring which bears upon a plain sleeve or U-shaped spacer which bears against the concrete. In Fig. 9, the bar 2 extends diametrically through a hole 35 in a block 33 having transverse bars 38 to which a jack stirrup can be connected. Segment-shaped members 36, with serrated chords 37 are inserted in the hole 35 to grip the bar. The tension is held by bolts 41 screwed into a U-shaped member 40 which bears on the concrete. When the invention is used for pre-tensioning, the same technique is used except that the sleeve 4 is absent and the moulds are made strong enough to withstand the stress of tensioning. The invention may be applied to " long line " production, i.e. the arrangement of moulds in a line with common reinforcements running through them.