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Defective Concrete Removal TechniquesRemoval of deteriorated, damaged, porous or defective concrete is a must for any repair work but the extent of this removal can not usually be determined in the planning stage. The decision about how much damaged concrete is to be removed and what level of aging for existing concrete is allowed to remain in place is very tedious. This decision becomes particularly difficult because the change in properties of concrete occurs gradually along the depth of concrete and there is no hard and fast border line. One guideline for the extent of removal in medium and high strength concretes is to continue removing material until aggregate particles are being broken rather than simply removed from the cement matrix. It is not a good practice to remove material only up to the plane of reinforcement and to have a joint between new and old materials right at the same location. Even in those cases where it is decided not to encase the bars fully, it is better to expose about three-fourth of the bars diameter and to expose the corner bars fully. Blasting of damaged concrete can produce problems in the surrounding concrete and hence its use must be carefully planned. Use of impact tools may also produce small-scale cracking to the surface of the concrete left in place. The debris removal by some primary means is usually followed by using a secondary method such as chipping, sand blasting (impacting sand with high air pressure) or high pressure water jetting to clean the surface. For more precise removal of damaged concrete in small areas, saw cutting may be used but the surfaces obtained must be treated with thin layers of materials to improve the feather edge surface. When exposing narrow but deep areas for repair along cracks, the cavity is better to be undercut to lock the repair material, see Fig.1 for two alternate methods. For large areas, the edges of the area are cut back sharply perpendicular to the face of the existing concrete without any undercut. For large cavities to be filled, the top surface is preferably made slopping towards the interior for easy placement and compaction of the repair material.
Fig. 1. Shaping Exposure Hole for RepairThe concrete removal techniques used should be effective, safe and economical, and should produce minimum damage to the concrete left in place. Some removal techniques take much longer time than the others but are environment friendly. Some techniques permit a portion of the work to be accomplished without removing the structure from service. A single removal technique may not be the optimum solution for all portions of a given structure. The properties of the concrete to be removed provide important information required determining the method and cost of concrete removal and this information must be made available to contractors for bidding. After the removal, the concrete surface is checked by visual inspection and by sounding at the surface, microscopic examination or bond testing for near-surface damage and by taking cores, pulse velocity tests and pulse echo tests for subsurface condition of remaining concrete. All damaged or deteriorated concrete is to be removed and the quantities must be estimated as accurately as possible in the start.
The broad categories of concrete removal methods are blasting, cutting, impacting, milling, pre-splitting, and abrading. The details of various methods are given below: