Guide to Patching with Repair Mortars

See Repair Mortars for information about individual products

Proper surface preparation and removal of unsound concrete is critical. As a general rule, it is important to spend more time and money on the surface prep than on the actual patching material. The "best" patching material will not make up for or perform over a poor job of surface preparation.
All loose or unsound concrete must be removed down to sound concrete. An excellent way to check for delaminations in unsound concrete is to tap the surface with a hammer or to drag a chain across the surface and listen for a hollow sound. The hollow sound indicates a delamination in the concrete that must be removed. An important fact to keep in mind is that all unsound concrete must be removed with a minimum amount of damage to the sound concrete below.
If the concrete base for the repair material is sound and no concrete removal is necessary, then all dirt, oil, grease, curing compounds, coatings and any other surface contaminations must be removed.
Methods for surface preparation include shotblasting, sandblasting, waterblasting, scarifying and chemical cleaning. For chemical cleaning an environmentally safe and a biodegradable product like Dayton Superior J-48 is recommended. Grinding is not recommended because it may make the concrete smooth and polished, thus reducing the bond with the repair material.
All areas to be repaired should have a defined edge to which the material should be worked. This sawcut edge should be a maximum depth of 1/2" (1.3 cm). The smooth edge created by the sawcut should be roughened prior to application of the repair mortar. This will increase the bond strength at the edges and reduce perimeter cracking. A high surface profile (min. 1/16") is desired for promoting a mechanical bond.
Reinforcing steel should be thoroughly cleaned with sandblasting or very high-pressure waterblasting if signs of rust and corrosion are present. Badly deteriorated rebar may need to be totally replaced. Dayton Superior J-62 or J-58 is recommended when the rebar requires an epoxy coating.
Before placing the repair material, the surface should be primed by one of the following methods:
- Brush on a bonding agent, such as Dayton Superior bonding agents J-40 or J-58. (J-58 is the Dayton Superior premium epoxy bonding agent.) The repair material must be placed onto the bonding agent before it dries or the bonding agent could act as a bondbreaker.
- All surfaces to be repaired should be in a saturated surface dry (SSD) condition, with no standing water on the surface, then brush a scrub coat of the repair material into the substrate. The scrub coat must be thoroughly brushed into the surfaces that will come in contact with the material. Place the repair material before the scrub coat dries.
When mixing less than a full unit, always first agitate the bag/pail so that a representative sample is obtained.
If possible, the repair area should be formed and the repair material poured into the formed area. This ensures good consolidation and initiates contact with the concrete base. If this is not possible, placing the material with a trowel is acceptable. However, the material should be firmly worked into the sides and bottom of the patch area, eliminating any air pockets and assuring maximum bond. Where practical, work from one side to the other and screed to the level of the surrounding concrete.
For additional information on successful concrete repairs, consult ACI or ICRI standards and guidelines or call Dayton Superior Technical Service at (866) 329-8724.
Aggregate Extension. When placing repair materials in larger masses, typically 1½" to 2" (3.8 - 5cm) or greater in thickness, aggregate extension is recommended. Aggregate extension will operate as a "heat sink" for fast setting patches, thus absorbing the heat and reducing the possibility of cracks in the repair material. Coarse aggregates, like ³/₈" (1cm) pea gravel reduce the amount of water required for mixing the material. This reduction of water will reduce drying shrinkage of the repair material and increase the physical properties. For example, compressive strength and long-term durability of the repair material will increase.
It is good concrete practice to cure all cement based repair materials. The application of curing compounds like Dayton Superior J-11-W, J-18, J-19 is recommended.