Mortar and dry pack: Preparation and application

Mortar and dry pack

Dry pack is a combination of Portland cement and sand passing a No. 16 sieve mixed with just enough water to hydrate the cement. Dry pack should be used for filling holes having a depth equal to, or great then, the least surface dimension of the repair area, for cone bolt, she holt, core holes, and grout-insert holes; for holes left by the removal of form ties; and for narrow slots cut for repair cracks. Dry pack should not be used for relatively shallow depressions where lateral restraint cannot be obtained, for filling behind reinforcement, or for filling holes that extend completely, through a concrete section.

For the dry-pack, method of concrete repair, holes should be sharp and square at the surface edges, but corners within the holes should be rounded, especially when water tightness is required. The interior surfaces of holes left by cone bolts and she bolts should be roughened to develop an effective bond; this can be done with a rough stub of 7/8-inch steel-wire rope, a notched tapered reamer, or a star drill. Other holes should be undercut slightly in several places around the perimeter. Holes for dry pack should have a minimum depth of 1 inch.

1 Preparation and application

Application of dry-pack mortar should be preceded by a careful inspection of the hole, which should be thoroughly cleaned and free from mechanically held loose pieces of aggregate. One of three methods should be used to ensure good bond of the dry-pack repair. The first method id the application of a stiff mortar ar grout bond coat immediately before applying the dry-pack mortar. The mix for the bonding grout is 1: 1 cement and fine sand mixed with water to a fluid paste consistency.

All surfaces of the hole are thoroughly brushed with the grout, and dry packing is done quickly before the bonding grout can dry. Under no circumstances should be bonding coat be so wet or applied so heavily that the dry-pack material becomes more has slightly rubbery.

When a grout bond coat is used, the hole to be repaired can be dry. Presoaking the hole overnight with wet rags or burlap prior to dry packing may sometimes give better results by reducing the loss of hydration water, but there must be no free surface water in the hole when the bonding grout is applied.

The second method of ensuring good bond starts with presoaking the hole overnight with wet rags or burlap. The hole is left slightly wet with a small amount of free water on the inside surfaces. The surfaces have been covered and the free water absorbed. Any dry cement in the hole should be removed using a jet of air before packing begins.

The hole should not be painted with neat cement grout because it could make the dry-pack material too wet and because high shrinkage would prevent development of the bond that is essential to a good repair. A third method of ensuring good bond is use of an epoxy bonding resin. Epoxies bond best to dry concrete. It may be necessary to dry the hole immediately prior to dry packing using hot air, a propane torch, or other appropriate method.

The concrete temperature however should not be high enough to cause instant setting of the epoxy or to burn the epoxy when it is applied. After being mixed, the epoxy is thoroughly brushed to cover all surfaces, but any excess epoxy is removed. Dry-pack mortar is then applied immediately, before the epoxy starts to harden.

The epoxy must be either fluid or tacky when dry packing takes place. If it appears that the epoxy may become hard before dry packing is complete, fresh fluid epoxy can be brushed over epoxy that has become tacky.

If the epoxy becomes hard, it must be comes hand, it must be removed before a new coat is applied. The epoxy ensures a good bond between the dry-pack repair and the old concrete. It also reduces can loss of hydration water from the repair to the surrounding concrete, thus assisting in good curing; however, the epoxy-bonded dry pack still requires curing as discussed below. Where appearance is not important, epoxy has sometimes been used on the surface in place of a curing compound. This procedure is not recommended.