Patent GB-746589-A (GB)
Abstract: 746,589. Pre-stressed concrete. FREYSSINET, E. Oct. 19, 1954 [Oct. 21, 1953 ; Jan. 6, 1954], No. 30091/54. Class 20(4). The concrete in a runway, road or the like is pre-stressed by dividing the runway at right angles to its length, forcing apart the edges of the division, or divisions, which force is resisted by the friction of the concrete with the ground so that the concrete is put into compression, the ends of therunway being prevented from moving unduly relatively to the ground by elastic abutments anchored to the ground. A layer, e.g., of sand is interposed between the ground and the concrete to render the friction uniform, and the concrete may be pre-stressed transversely. As shown in Fig. 1, the runway comprises sections 21, 22 ... and end abutments 1. If the abutments were intended to function as non-yielding abutments, an excessive force thereon, due for example to unexpected thermohygrometric conditions, would cause permanent displacement of the abutment and the compression in the concrete would be lost when normal conditions returned. The abutment is therefore designed to apply yieldingly a substantially constant reaction pressure to the compression forces in the concrete resulting from the force of the means tending to separate the divisions 3<SP>1</SP>, 3<SP>2</SP> &c., as modified by the friction of the concrete with the ground. The elasticity in the abutment may be provided by inflatable flat bags arranged between the abutment proper and the end face of the section 2<SP>1</SP>, the abutment proper being formed with an inspection passage beneath the bags to enable repairs to be effected, the compression in the concrete being temporarily maintained by wedges which are inserted between the abutment and the section 2<SP>1</SP>. The pressure in the bags is maintained by connection with an hydraulic accumulator or a liquid or gas pump controlled by a pressostat. The end of the section 21 rests upon a shoulder on the abutment through the intermediary of a layer of anti-friction material, e.g. pitch, to permit free action of the elastic means. In Fig. 6 the elastic connection between the abutment and the end section 2<SP>1</SP> comprises tensioned wires 12 which are about one hundred metres long and are anchored in a transverse concrete block 18. The wires are held by cones 13 and are free to move in pasages in the concrete 15 of the abutment and also in passages in the end section 2<SP>1</SP>. Rusting is prevented by filling the passages with pitch, and friction between the abutment and the section 2<SP>1</SP> is prevented by layers of pitch 14. The abutment may consist of steel piles which are driven through holes in the section 2<SP>1</SP>, some of the piles being driven obliquely with the points directed away from the runway and the others being driven obliquely with the points directed towards the runway. The flexure of the piles provides the elasticity. In Fig. 9, the abutment is integral with the section 2<SP>1</SP>, the end portion 25 of the section being pre-stressed by cables 26. The outer length of the portion 25 rests upon a rough layer 27, the high friction of which renders this length practically immovable, whilst the inner length of the portion 25 rests upon an antifriction layer 29, and constitutes the resilient part of the abutment. The abutment shown in Fig. 6 may be modified by directing the wires 12 and the encasing concrete in a downwardly curving path and anchoring the anchor 18 to a still deeper anchor to which it is joined by similar concrete-enclosed tensioned wires. In a modification, the abutment which is separated from the section 2<SP>1</SP> by any suitable resilient means, is assisted by tensioned wires which are attached to the sides of the section 2<SP>1</SP> and to ground anchors. Similar wires may be attached to the underside of the section 21 and to other anchors. If the divisions in the runway are too widely spaced, shrinkage cracks occur in the concrete sections. To avoid this, the concrete is laid in comparatively short lengths, e.g., less than one hundred metres, spaces between these lengths being left to accommodate jacks for applying to the concrete a temporary pre-stress which is maintained during the shrinkage period. The jacks are then removed and the spaces are filled with concrete. The main divisions in the runway, for accommodating the jacks for applying the final pre-stress are more widely spaced and are shown in Fig. 17. The jacks are flat metallic envelopes 34 which are filled with a liquid which ultimately hardens. They are assembled as a single block 39 having the form of a wedge so as to be readily insertable in a division space having inclined walls. The jacks are separated by steel sheets 36 and fillings of concrete 37, and the block has end plates 38. Wet and dry weather produce different shrinkage conditions in the upper and under layers of the concrete sections, so that the application of the full pre-stress may produce lifting or buckling of the sections, either the edges at the divisions or the centre parts of the sections rising from the ground and losing the essential friction therewith. This can happen only when the jacks contain liquid and, if prevented until the liquid hardens, will not occur. The buckling may be countered partly by replacing each envelope 34 of the jack unit shown in Fig. 17, by two envelopes of half the height, the upper and lower envelopes then being separately expansible to vary the effective line of thrust upon the edges of the concrete sections. The said edges may be prevented from lifting by embedding longitudinally disposed spaced metal members in each of the end regions of the concrete sections, which members have a sliding guide relationship with an member projecting upwardly from a concrete foundation on which the said end regions rest. A pitch anti-friction layer is placed between relatively movable surfaces. Lifting of the edges may, alternatively, be prevented by downward pressure applied by gantries which are connected to ground anchors through openings in the concrete sections. When the liquid in the jacks has hardened, the gantries are removed and the openings are filled up. In Fig. 26, the edges of the sections 21,22 are prevented from lifting by vertical cables 80 which are anchored in the sections and, at the bottom, in a concrete mass which has been made bulbous by the use of explosive. As the jack block 34 applies the pressure, the cables move apart with the edges of the sections, and become tensioned. Alternatively, the upper ends of the cables may be anchored in the concrete of the jack 34. Fig. 31 shows the method of laying a runway having a number of strips laid side by side and separated, for example, by a layer of paper or a strip of steel 46. The concreting machine 45 is supported for the first strip, on two rails 44, and for subsequent strips, on a rail 44 and the previously laid strip. Withdrawable mandrels 48 provide passages for the cables for transverse pre-stressing, and the temporary pre-stress to avoid shrinkage cracks is applied as soon as the concrete will stand it. Dust &c., is excluded from all divisions by flexible covers 51. If the concrete layer is very wide, transverse compression may be applied thereto by jacks in the same manner as it is applied longitudinally. In the case of concrete roads, the width is small comparedwith the length. Elastic abutments are therefore not provided in the straight lengths, but only slots at intervals for the application of compressive forces, the friction with the ground being sufficient to maintain the compression. At the bends, any variation in thermohygrometric conditions tends to move the bend section away from its centre of curvature and, since it is pre-stressed to bear upon the ends of the straight lengths, such outward movement would result in a loss of pro-stress. Elastic abutments are therefore provided in the form of tensioned cables 56, Fig. 33, which are anchored to a common block 57. If the road length has a hump which would result in the pre-stress lifting the road away from the hump, the concrete may be tied down with ground anchors. Specification 623,729 is referred to.