Patent GB-1307513-A (GB)
Title: Post-stressing of reinforced concrete structures
IPC: E04C5/12
Filed: 1968-12-18
Abstract: 1307513 Releasable fastenings PRE-STRESS PIONEERS Ltd 16 March 1970 [18 Dec 1968] 60126/68 Heading E2B A method of post-stressing the strand or strands of a cable-reinforced concrete structure, includes the steps of passing the or each strand, projecting from an end of the structure, through a flared aperture in an anchor head 21 and then through an aperture in a plate 22 mounted on the anchor head parallel to the outer face of the head and also through a segmented and tapered grip 27 seatable in the aperture to clamp the associated strand, and retained loosely in the associated aperture by the plate, the plate being freely movable a predetermined distance from the outer face of the anchor head, positioning a bearing cup 42 over the anchor head and the plate, positioning a centre-hole jack 40 on the bearing stool, anchoring the strand or strands externally of the jack by passing the strand or strands through an auxiliary anchor assembly 50, stressing the or each strand a predetermined amount by means of the jack, driving a drift wedge 70 through the bearing cup to bear laterally against the plate to cause the plate to seat the or each grip in its associated aperture in the anchor head, and releasing the stress maintained by the jack on the strand or strands. A multiple strand cable is shown, the strands 11 passing loosely through a sleeve 12 embedded in a beam 10 and emerging from the beam through a trumpet 13 connecting the sleeve 12 with a bearing plate 14 embedded in the face of the beam and having a central opening 15. The strands are anchored at the other end of the beam. The strands then pass through the anchor head 21 and plate 22 described above, these and the grip or grips 27 constituting an anchor assembly 20 contained in the aperture of the cup 42, the cup resting against the plate 14. The strands then pass through the centre hole of the jack 40 which bears against the cup 42 and then through the auxiliary anchor assembly 50 which is seated on the end of an annular piston 46 of the jack. The auxiliary anchor assembly consists of an anchor head 51 which has a plurality of apertures corresponding to the number of strands 11 each aperture flaring upwardly in the direction of a plate 53 and each aperture containing a segmented grip 58 which grips the strand. The strands also pass through apertures in the plate 53. The strands also pass through apertures of a plate 33 which is disposed between the anchor assemblies 20 and 50 in the hole of the jack and whose purpose is to align the strands with the auxiliary anchor assembly for ease of threading them through the auxiliary anchor assembly. For stressing the cable the jack is operated to move piston 46 away from the plate 14, the auxiliary anchor assembly then grips the strands and tensions the cable the strands slipping through the anchor assembly 20. When the jack piston is retracted the anchor assembly 20 grips the strands with only a small reverse movement of the strands and the auxiliary anchor assembly can then be pushed along the cable into engagement again with the jack piston. The process is repeated until the strands reach the desired tension; when this happens and before the piston is retracted the wedge 70 is passed through a slot 43 in the cup 42 to force the plate 22 towards the head 21 to cause the grip 27 to engage the strand thus preventing the normal small slippage when the jack is released and before the grips 27 fully engage the strands. The auxiliary anchorage 50, jack 40, cup 42 and plate 33 are then removed and the free end portions of the strands 11 projecting from the anchor assembly 20 are severed, the cable is grouted in sleeve 12 if desired and the anchorage is capped with concrete; plate 22 is removed before capping. The plate 22 is connected to the head 21 by bolts so that its position can be adjusted. As regards the auxiliary anchor assembly the grips are each recessed at the larger end to form a shoulder and a collar, the recessed portion being disposed in the respective aperture 57 in the plate 53 which is connected to the anchor head by nuts and bolts so that its position can be adjusted. In a modification each grip instead of having a recessed end portion is directly connected to the plate by a bolt. A modified anchor head 21a (not shown) is described which is used where it is desired to restress the strands. This differs from the anchor head 21 only in that its external surface is screw-threaded so that it can be engaged by a jack for drawing it upwardly; it is also designed to receive a sealing cap which is fixed to its face by bolts; the cap has spouts by means of which it can be filled with epoxy resin. A further modified anchor head 21b (Fig. 9 not shown) is similar to the head 21 except it has a laterally projecting annular flange 92 which defines an annular recess 93 when the head is positioned against bearing plate 14, the flange having a plurality of apertures 101 which receive grips 27a for engaging strands from a second beam 90. To secure the grips 27a to the strands 11a pairs of wedges 105 are used which are inserted into the recess 93. The wedges are either fixed to the bearing plate 14 by bolts (not shown) or are held in place by epoxy resin which fills the recess. The strands 11a are covered bv a trumpet 102 which is subsequently filled with grout.