Patching and Repairing of Old Concrete
Patching is necessary when some of the concrete at the surface has disintegrated or become honey-combed or where spalling or crazing has occurred, or there are defective construction joints. In patch repairs all deteriorated and loose materials should be removed and the surface prepared to establish a satisfactory bond between the old and the new concrete. For patches with depths more than 10 cm, anchors or dowels may have to be used consisting of 6 mm bars embedded about 40 mm in sound concrete by drilling holes and grouting. These anchors may be spaced 40 to 60 cm apart. In addition to use wire mesh, which may be
75 mm x 75 mm, tied to the anchors and kept a distance of about 35 mm from the finished top surface and also above the old concrete surface. In case of vertical and overhead surfaces, the filler concrete should be placed in small layers and each layer thoroughly compacted before the next layer is applied. After about 12 hours setting gently hammer the concrete to test if there has been a proper bond. Remove any portion which produces a hollow sound.
Repair Work
Repair work in concrete is difficult, as the fresh concrete does not easily stick to the old set concrete. The most effective way of repairing is by guniting. Cement and sand are mixed in suitable proportion and shot on to the surface of repair by compressed air. Experiments show that unite of maximum strength and density is obtained with a certain velocity. Insufficient velocities produce reduced strength while excess velocities increase the amount of rebound.
A feel with hand would indicate concrete quality; loose concrete portions falling down with little or no efforts.
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