on surfaces not prominently exposed, where the defects are too wide for dry pack filling or where the defects are too shallow for concrete filling and no deeper than the far side of the reinforcement that is nearest the surface.
Repairs may be made either by use of shotcrete or by hand application methods. Replacement mortar can be used to make shallow, small size repairs to new or green concrete, provided that the repairs are performed within 24 hours of removing the concrete forms.
The use of replacement mortar to repair old or deteriorated concrete has been permitted on Reclamation projects in the past. It is now recognized, however, that accomplishing successful mortar repairs to old concrete without the use of a bonding resin is unlikely or extremely difficult. Evaporative loss of water from the surface of the repair mortar, combined with capillary water loss to the old concrete, results in unhydrated or poorly hydrated cement in the mortar. Additionally, repair mortar bond strength development proceeds at a slower rate than compressive strength development. This causes workers to mistakenly abandon curing procedures prematurely, when the mortar
Guide to Concrete Repair
"seems strong." Once the mortar dries, bond strength development stops, and bond failure of the mortar patch results.
For these reasons, using cement mortar without a resin bond coat to repair old concrete is discouraged and is not allowed under
Reclamation's Standard Specifications for the Repair of Concrete, M-47.
A portland cement mortar patch is usually darker than the surrounding concrete unless precautions are taken to match colors (figure 39). A leaner mix will usually produce a lighter color patch. Also, white cement can be used to produce a patch that will blend with the surrounding concrete. The quantity of white cement to use must be determined by trial.
(a) Preparation.—Concrete to be repaired with replacement mortar should be prepared in accordance with the provisions of section 8.
After preparation, the areas should be cleaned, roughened, if necessary (preferably by wet sandblasting), and surface dried to a saturated surface condition. The mortar should be applied immediately thereafter.
(b) Materials.—Replacement mortar contains water, portland cement, and sand. The cement should be same type as used in the concrete being repaired. The water and sand should be suitable for use in concrete, and the sand should pass a No. 16 sieve. The cement to sand ratio should be between 1:2 to 1:4, depending on application technique. Only enough water should be added to the cement-sand mixture to permit placing.
(c) Application.—Best results with replacement mortar are obtained when the material is pneumatically applied using a small gun.
Equipment commonly used for shotcreting is too large to be satisfactory for the ordinarily small size mortar repairs of new concrete. With shotcreting equipment, neat work is difficult in the usual small areas, and cleanup costs are high because cleanup is seldom done promptly.
However, small size equipment such as shown in figure 40 has been satisfactory for small scale
repair work when the mortar was premixed, including water, to a consistency of dry-pack material. No initial application of cement, cement grout, or wet mortar should be made. If repairs are more than 1 inch deep, the mortar should be applied in layers not more than 3/4 of an inch thick to avoid sagging and loss of bond.
After completion of each layer, there should be a lapse of 30 minutes or more before the next layer is placed. Scratching or otherwise
preparing the surface of a layer prior to addition of the next layer is unnecessary, but the mortar must not be allowed to dry.
In completing the repair, the hole should be filled slightly more than level full. After the material has partially hardened but can still be trimmed off with the edge of a steel trowel, excess material should be shaved off, working from the center toward the edges. Extreme care must be used to avoid impairment of bond.
Neither the trowel nor water should be used in finishing. A satisfactory finish may be obtained by lightly rubbing the surface with a soft rag.
For minor restorations, satisfactory mortar replacement may be performed by hand. The success of this method depends on complete removal of all defective and affected concrete, good bonding of the mortar to the concrete, elimination of shrinkage of the patch after placement, and thorough curing.
Replacement mortar repairs can be made using an epoxy bonding agent as described in section 26. This technique, while not required by Reclamation's M-47 specifications, is highly recommended.
(d) Curing.—Failure to cure properly is the most common cause of failure of replacement mortar. It is essential that mortar repairs receive a thorough water cure starting
immediately after initial set and continuing for 14 days. In no event should the mortar be allowed to become dry during the 14-day period following placement. Following the 14-day water cure and while the mortar is still saturated, the surface of the mortar should
Figure 39.—A portland cement mortar patch seldom matches the color of the original concrete unless special efforts are taken to blend white cement with normal portland cement.
Figure 40.—A small size pneumatic gun can be used to apply portland cement mortar. Regular shotcreting equipment would be too large for this application.
Guide to Concrete Repair
be coated with two coats of a wax-base or water-emulsified resin base curing compound meeting Reclamation specifications. If this curing procedure cannot be followed or if conditions at the job are such that this curing procedure will not be followed, money would be saved by using another repair material.
26. Dry Pack and Epoxy-Bonded Dry