The Constructor


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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:

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: 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:

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.
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