Replacement concrete is a method of repair for defective concrete when the defects or cracks in concrete have large area. In this method of concrete repair, defective concrete is removed from the structural member and area is prepared for repairs and then replacement concrete is used. Concrete repair using this method is used when the area exceeds 1 square foot and has a depth greater than 6 inches and when the repair is of appreciable continuous area.
Replacement concrete repairs are also used for:
- Holes extending entirely through concrete sections.
- Holes in which no reinforcement is encountered, or in which the depth extends 1 inch below or behind the backside of the reinforcing steel and which are greater in area than 1 square foot and deeper than 4 inches, except where epoxy- bonded concrete replacement is required or permitted as an alternative to concrete replacement.
- Holes in reinforced concrete greater than one-half square foot and extending beyond reinforcement Replacement concrete is the most common concrete repair material and will meet the needs of a majority of all concrete repairs.
Preparation of Surface for Replacement Concrete:To obtain satisfactory results with the replacement concrete method, preparation should be as follows:
- The reinforcement in the reinforced concrete should be exposed for a minimum of 25mm around the it to use replacement concrete.
- The minimum depth of concrete removal for replacement concrete should be around 25mm and should extend till all the defective concrete has been removed. In case of a vertical member, the top of the hole should be cut in a 1:3 upward slope from the back toward the face from which the concrete will be placed. This is essential to permit vibration of the concrete without leaving air pockets at the top of the repair. In case where concrete from removed from one side to other side of the structural member, the replacement concrete should be filled completely in the hole.
- The cutting of damaged concrete should be such that there is no damage to the existing concrete and there is no spalling of concrete.
- For repairs on the surfaces subject to destructive water action and for other repairs on exposed surfaces, the outlines of areas to be repaired should be saw cut as directed to a depth of 1-1/2 inches before the defective concrete is excavated.
Materials for Replacement Concrete:
- Concrete for repair should have the same water-cement ratio as used for similar new structures or the existing structure but should not exceed 0.47, by weight.
- Aggregate of as large a maximum size and slump as low as is consistent with proper placing and thorough vibration should be used to minimize water content and consequent shrinkage.
- The concrete should contain 3 to 5 percent entrained air.
- To minimize shrinkage, the concrete should be as cool as practicable when placed, preferably at about 20 °C or lower. Materials should, therefore, be kept in shaded areas during warm weather. Use of ice in mixing water may sometimes be necessary.
- Batching of materials should be by weight; but batch boxes, if of the exact size needed, may be used. Since batches for this class of work will be small, the uniformity of the materials is important and should receive proper attention.
- The slump of concrete shall be as minimum as possible for replacement concrete and should be around than 75mm.