REPLACEMENT CONCRETE: MATERIALS AND APPLICATIONS

 

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.

Replacement concrete repairs are made by bonding new concrete to the repair areas without the use of a bonding agent or Portland cement grout. Because the defective concrete is being replaced with high quality concrete very similar to the surrounding concrete, the repair is compatible in thermal expansion and in other physical and chemical properties with the old concrete. For this reason, in many cases, the best repair method is the use of replacement concrete.

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.

Surfaces of old concrete to which new concrete is to be bonded must be clean, rough, and in a saturated surface dry condition. Extraneous material on the joint resulting from form construction must be removed prior to placement.

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.

Structural concrete placements should be started with an over-sanded mix containing about a 20mm-maximum size aggregate; a maximum water-cement ratio of 0.47, by weight; 6 percent total air, by volume of concrete; and having a maximum slump of 100 mm. This special mix should be placed several inches deep on the joint at the bottom of the placement. A mortar layer should not be used on the construction joints.

Application of Replacement Concrete:

The quality of a repair depends not only on use of low-slump concrete, but also on the thoroughness of the vibration during and after depositing the concrete. There is little danger of over-vibration. Immersion-type vibrators should be used if accessibility permits.
Immediately after the hole has been completely filled, pressure should be applied to the fill and the form vibrated. This operation should be repeated at 30-minute intervals until the concrete hardens and no longer responds to vibration. Pressure is applied by wedging or by tightening the bolts extending through the pressure cap.
Concrete replacement in open-top forms, as used for reconstruction of the tops of walls, piers, parapets, and curbs, is a comparatively simple operation. Only such materials as will make concrete of proved durability should be used. Top surfaces should be sloped to provide rapid drainage.

Forms for concrete replacement repairs usually may be removed the day after casting unless form removal would damage the green concrete, in which event stripping should be postponed another day or two.
Some replacement concrete does not require forms. Replacement of damaged or deteriorated paving or canal lining slabs, wherein the full depth of the slab is replaced, involves procedures no different from those required for best results in original construction.

Curing and Protection of Replacement Concrete:

It is very important for the replacement concrete to be cured after the forms have been removed or the concrete has hardened. If the proper curing is not done to the replacement concrete, there are chances of complete failure of the repaired concrete. Because of the relatively small volume of most repairs and the tendency of old concrete to absorb moisture from new material, water curing is a highly desirable, at least during the first 24 hours. One of the best methods of water curing is a soil- soaker hose laid beneath a plastic membrane covering the repair area.
When curing compound is used, the best curing combination is an initial water-curing period of 7 days (never less than 24 hours) followed, while the surface is still damp, by a uniform coat of the compound.