Surface Preparation for Concrete Repair

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Surface preparation for the concrete repair works is the important step for the development of proper bond with the new concrete. Various methods are used for the preparation of surface for concrete repair works such as chemical cleaning, acid etching, mechanical preparation and abrasive method. In many repair situations, the proposed repair requires only surface roughening, exposure of coarse or fine aggregate, removal of a thin layer of damaged concrete, or cleaning of the concrete surface. Each repair material needs a particular type of surface preparation but generally the concrete surface must not be too smooth, too rough, or too irregular. In this article we discuss about the different methods used for the preparation of surface for concrete repair works.

Typical Methods of Surface Preparation 

1. Chemical Cleaning

Detergents, tri-sodium phosphate, and various other concrete cleaners are used for surface preparation for certain coatings but are not usually considered good for surface preparation in case of concrete repairs. Solvents are not recommended because they dissolve the contaminant and carry it deeper into the concrete. In most cases, ACI committee (546R) recommended not to use chemical cleaning, only under special conditions with application of certain coatings, this method can be used.

2. Acid Etching

Acid etching on concrete surfaces has been used for long time to remove dirt (normal amount) and laitance. The acid applied on concrete surface successfully remove significant amount of cement paste leaving a rough surface that can received replacement material with improved bond strength. Acids may penetrate the concrete surface through cracks, may promote corrosion of underlying reinforcing steel and may damage the paste of the remaining concrete. ACI 503R has recommended not to use acid and ACI 515.1R recommended to use acid on concrete when other alternatives of surface preparation cannot be used.

3. Mechanical Preparation

In this technique, thin layer of concrete can be removed from surface and depending on removal equipment, different surfaces are obtained. Different equipment used in this method are impact tools like breakers, scabblers, grinder and scarifier.

4. Abrasive Preparation

Abrasive blasting propels dry or moist abrasive in a stream of compressed air. Upon impact, the abrasive particles penetrate the substrate, dislodging fragments of mortar and fines, producing an overall eroding effect. Abrasive blasting removes surface contaminants, unsound concrete, coatings and adhesive films, and imparts a profiled surface. Abrasive equipment such as sandblasters, shotblasters, or high-pressure water blasters, usually followed by water or air blasting, vacuuming, or other techniques. A profile number can be determined indicating the surface roughness necessary for various overlay applications or it may be defined by sandpaper grit size.

5. Rotomilling

A rotomiller is a scarifier on steroids, so large that it must be driven, with teeth mounted to the drum instead of washers. The impact of the teeth fracture the concrete into chips and dust, creating striations and deep grooving. A rotomiller can only be used on horizontal surfaces.

6. Needle Scaling

Needle scalers pulverize concrete surfaces by the pounding action of steel rods, driven by pneumatic or hydraulic pulses. Needle scalers are commonly used to remove efflorescence and other brittle encrustations. The impacts produce a cratered surface profile.

Points to be remembered for Surface Preparation for Concrete Repair

1. All existing coatings and other surface contamination must be removed.
2. Preparation may be carried out using scarification, brushing or grinding, abrasive blasting, shot-blasting, and flame cleaning.
3. Dust and debris produced as a result of surface preparation must be removed before applying the surface treatment. Presence of surface contamination can result in poor adhesion of protective coating with the substrate.
4. The surface after preparation must be sound, dry and according to the satisfaction of the engineer.
5. Preparation must include the repairs of shallow delaminations, surface scaling, aggregate popouts, the grinding of rough surfaces, or the treatment of any other surface defects required to achieve the proper performance of the product.
6. Surfaces should be relatively smooth for applying most liquid-applied membranes and other thin coatings.
7. Trowel grade materials can be used to cover minor imperfections of surfaces but liquid coatings do not have this capability. The pH of the substrate must be compatible with the product to be installed.
8. When repair is to be made with cement concrete or mortar, the surfaces are saturated after cleaning and then are brought to the surface dryness condition before placing the new material. The surfaces must be kept moist for several hours to assure saturation.
9. Just before placing of the repair material, the surface must be coated with a slush coat of mortar having the same proportions as the matrix of the repair concrete and thickness not exceeding 3 mm.
10. Slush should not be applied in case of using dry pack material.
11. When using dry packing, the moist surfaces are dusted lightly with cement using a small dry brush. There should not be any dry cement adhering to the surfaces.

Standards codes of surface preparation for concrete repair

ACI 503R Use of Epoxy Compounds with Concrete ASTM D 4260-88- Standard Practice for Acid Etching ASTM D 4262-83- Test Method for pH for chemically cleaned or etched Concrete Surfaces ASTM D 4263-83- Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method.