Concrete
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Partial Depth Repairs

This document discusses partial depth repairs of a concrete surface.

How to Make Partial-Depth Concrete Repairs

Partial-depth concrete repairs can be used to repair spalls, scaling, and joints where "D" cracking and alkali reactivity have been a problem, as well as other concrete distresses. This type of patching can be very effective when properly placed and should last for as long as the remaining life of the pavement.

 

The type of material needed for such a project depends on the amount of time allowed for the repair, air temperature, size, and cost. Materials such as concrete, portland cement grout, or epoxy resin can be used as the patching materials. (When using an epoxy resin, follow the manufacturer's recommendations for placement.)

 

Preparation
To prepare the area for patching, certain procedures should be followed. First, make a 2-inch-deep saw cut around the perimeter of the patch (see Figures 1 and 2).

 

The concrete in this area should be removed using pneumatic tools until a sound surface is found (see Figures 3 and 4). When removing concrete, it is important to use equipment that is properly sized. The pneumatic hammer should not exceed 30 pounds, to keep from fracturing the concrete and ultimately causing a compression failure below the patch.

 

The patch should then be cleaned, using either air or water blasting to remove any loose or foreign materials. It is important that the surface be completely clean so that the patch will adhere to it.

 

Bonding Agents
Before placing the patching material, a bonding agent should be used to ensure that the patch will adhere to the concrete. An epoxy or cement grout mixture can be used. (When using an epoxy coating, be sure to follow the manufacturer's directions.) Epoxies are generally used when an area has to be opened to traffic in four to six hours. When using a cement grout mixture, it should consist of cement, sand (1:1 mix), and water, mixed to the consistency of a thick paint. This is usually sufficient when the patch will be protected for 24 to 72 hours. The bonding agent is then scrubbed into the surface of the opening with a stiff brush. Don't let excess bonding agent accumulate in the patch, and make sure the bonding agent is still wet at the time of placement. Don't apply the agent to any surface that the new patch should not bond to, such as at a control joint (see Figure 3).

 

Patching Material
When a concrete mix is being used as the patching material, it should be similar to the concrete already in place. If the original concrete mix is unknown, a water-cement ratio of .50 or less should be used. Where the color of the slab is important, several trial batches should be made to determine the best-suited mix. White portland blended with regular portland can also obtain the desired effect. The concrete should be allowed to cure and dry to determine the color.

 

The cement content depends on the desired concrete strength. If an early opening is needed, 3,000 psi at 24 hours should be specified. For high early strength, a rich cement content (eight bags per cubic yard) or a Type III cement is suggested. Either of these two methods will work to get the strengths needed for an early opening. If immediate use is not important, then a mix design that allows for a longer curing period should be sufficient.

 

The maximum aggregate size should be as large as possible to compensate for shrinkage. However, it should not exceed half the depth of the patch. For example, if the patch is 2 inches deep, the maximum aggregate size is 1 inch.

 

The concrete slump should be roughly 2 inches, which will reduce the amount of shrinkage. Place the concrete in horizontal layers, allowing the final layer to overflow the patch slightly. Then, strike off the surface and float. Wait until all the bleed water is off, then trowel the surface to the desired finish. Begin curing as soon as finishing is completed. When repairing a spalled joint, the joint reservoir should be maintained using roofing felt or some other material, as shown in Figure 5.

 

Curing
Good curing practices must be used on the patches to ensure a long service life. Cover the patch with wet burlap and a sheet of plastic. Insulating blankets should be used when the air temperature is below 40 degrees F.

 

Summary

  • Determine the size of the area needing patchwork.
  • Chip out the loose material and thoroughly clean the patch area.
  • Use an epoxy or cement grout bonding agent before placing the patch material.
  • Place the concrete patch material and finish it.
  • Use proper curing practices.


For more information, please contact your Lafarge sales representative or technical sales engineer. 

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