GB773890A - An improved method of pre-stressing reinforced concrete of large area, and apparatusfor carrying out said method - Google Patents

Abstract

773,890. Reinforced concrete. UDALLS PRESTRESSED CONCRETE, Ltd. Aug. 17, 1955 [May 17, 1954], No. 14481/54. Class 20(4) A method of prestressing large areas of reinforced concrete comprises casting the concrete on a supporting bed or base and placing the reinforcement in position relative thereto, applying tension to the reinforcement, raising the slab of concrete off the supporting bed or base (so that the compression stress may extend throughout the slab) and then returning the slab on to the supporting bed or base. A large slab 11 of concrete is cast on to a supporting bed or base 12, which may be natural ground, gravel, concrete or other material. A separating membrane 39 of building paper may be placed between to prevent adhesion. Reinforcing rods 13, 14, 15 are placed in position, either in ducts before casting or threaded through the ducts after casting. Vertical holes 16 are left in the slab at suitable intervals and adjacent each hole, screw-threaded ferrules 34 are embedded at the bottom of narrow holes 33 to take bolts 31. In each hole 16, a pedestal 17 is positioned, comprising a pillar 20 between bearing plates 19, 21, the pillar 20 having convex ends 23, 25 seating in concave faces 22, 24 in the plates 19, 21, which allows slight rocking of the pillars when the-slab 11 contracts. The upper plate 21 is screw-threaded to receive bolts 26 by which it is secured to a crosshead 27, the bolts 26 passing freely therethrough and the crosshead resting on collars 30. The crosshead 27 comprises angle irons, spaced apart to allow the passage of the ram 28 of a hydraulic jack 29, and secured to the slab 11 by the bolts 31. The jack 29 is secured to each crosshead 27 by a crosshead 35 above the body of the jack having vertical arms 36 secured by easily detachable bolts 37 and nuts to the crosshead 27, so that the ram 28 presses on the bearing plate 21. The jack is operated to raise the crosshead 27 relative to the pedestal 17, thereby raising the slab 11 from the base 12 and the screw bolts 26 adjusted so that the collars 30 take the strain from the jack 29, which is then removed to be used at a further hole 16. A single jack may be used, or a row of jacks along one side of the slab to raise the slab in strips The lift is of the order of one-eighth of an inch. When the whole slab has been raised, a film or layer of oil or bitumen may be injected into the space 38 between the slab 11 and base 12 or membrane 39, through the holes 16. The slab is then lowered, with the assistance of the jack 29, and the crosshead 27 and pedestal 17 removed from each hole 16 which is then filled in with concrete. One edge of the slab may be left in contact with the base 12 to prevent undue tilting of the pillars 20.