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SECTION XII. REPAIRING

NEW CONCRETE

5-95. Although repairs are costly and time-consuming, field experience dictates that steps in the repair procedure cannot be omitted or performed carelessly without harming the serviceability of the repair work.

PRELIMINARY PROCEDURES

5-96. If they are not properly performed, repairs later loosen, crack at the edges, and allow water to permeate the structure. Preliminary procedures includes--

Inspection. Following form removal, inspect the concrete for such surface defects as rock pockets, inferior quality, ridges at form joints, bulges, bolt holes, or form-stripping damage.

Timely repair. On new work, when repairs are made immediately after form removal, while the concrete is quite green, the best bonds are developed and are more likely to be as durable and permanent as the original work. Therefore, make all repairs within 24 hours after removing forms.

REMOVING RIDGES AND BULGES

5-97. Remove objectionable ridges and bulges by carefully chipping and rubbing the surface with a grinding stone.

PATCHING NEW CONCRETE

5-98. Before placing mortar or concrete into patch holes, keep the surrounding concrete wet for several hours. Then brush a grout made from cement and water mixed to a creamy consistency into the hole surfaces before applying the patch material. Start curing the patch as soon as possible to avoid early drying. Use damp burlap, tarpaulins, or membrane curing compounds. Patches are usually darker in color than the surrounding concrete. If appearance is important, mix some white cement into the mortar or concrete and use as a patch. Make a trial mix to determine the best proportion of white and gray cements to use.

Small Patches

5-99. Do not apply a single shallow layer of mortar on top of honeycombed concrete, because moisture will form in the voids and subsequent weathering will cause the mortar to spall off. Instead, chip out small defective areas, such as rock pockets or honeycomb, down to solid concrete. Cut the edges either as straight as possible at right angles to the surface, or undercut them slightly to make a key at the edge of the patch. Keep the surfaces of the resulting holes moist for several hours before applying the mortar. Fill shallow patch holes with mortar placed in layers not more than 1/2 inch thick. Make rough scratches in each layer to improve the bond with the succeeding layer, and smooth the surface of the last layer to match the adjacent surface. Allow the mortar to set as long as possible to reduce shrinkage and make a more durable patch. If absorptive form lining was used, make the patch match the adjacent surface by pressing a piece of the form lining against the fresh patch.

5-100. Patch large or deep holes with concrete held in place by forms. Reinforce such patches and dowel them to the hardened concrete as shown in Figure 5-29.

 

Figure 5-29. Repairing large and deep holes in new concrete

Bolt and Tie-Rod Holes

5-101. When filling bolt holes, pack small amounts of mortar into place carefully. Mix the mortar as dry as possible with just enough water to compact tightly when forced into place. Fill tie-rod holes extending through the concrete with mortar, using a pressure gun similar to an automatic grease gun.

Flat Surfaces

5-102. View 1 of Figure 5-30 shows why feathered edges around a flat patch break down. First chip an area at least 1 inch deep with edges at right angles to the surface, as shown in view 2 of Figure 5-30, before filling the hole. Then screed the patch as shown in view 3 of Figure 5-30. The fresh concrete should project slightly beyond the surface of the hardened concrete. Allow it to stiffen before troweling and finishing to match the adjacent surfaces.

 

Figure 5-30. Patching flat surface in new concrete

OLD CONCRETE

5-103. When repairing old concrete, first determine how much material to remove by thoroughly inspecting the defect. Remember that it is far better to remove too much old material than not enough.

PRELIMINARY PROCEDURES

5-104. Remove all concrete of questionable quality unless nothing would be left. In this case, remove only the loose material. If the old and new concrete will join on a surface exposed to weathering or chemical attack, make sure the old concrete is perfectly sound. After removing weakened material and loose particles, thoroughly clean the cut surfaces using air or water or both. Keep the area surrounding the repair continuously wet for several hours, preferably overnight. Wetting is especially important in repairing old concrete because a good bond will not form without it.

PATCHING OLD CONCRETE

5-105. The repair depth depends on many conditions. In large structures such as walls, piers, curbs, and slabs, the repair should be at least 6 inches deep, if possible. If the old concrete contains reinforcement bars, allow a clearance of at least 1 inch around each exposed bar. Use rectangular patches on small areas, cutting 1 to 2 inches vertically into the old concrete to eliminate thin or feathered edges. Following the wetting period, place the new concrete into the hole in layers and thoroughly tamp each layer. The patch concrete should be a low slump mixture allowed to stand for awhile to reduce shrinkage. Forms may be needed to hold the patch concrete in place. The design and construction of such forms often require a lot of cleverness, but well-designed and properly constructed forms are important in concrete repair. Reinforce deep patches to the hardened concrete. Good curing is essential. Begin curing as soon as possible to avoid early drying.

David L. Heiserman, Editor

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Revised: June 06, 2015