When it comes to concrete repair works, surface preparation is a crucial step that must not be overlooked. Proper surface preparation ensures that the new concrete will form a strong bond with the existing concrete. There are several methods used for preparing the surface for concrete repair works. These methods include chemical cleaning, acid etching, mechanical preparation, and abrasive method.
The specific method used for surface preparation depends on the repair required. In some cases, surface roughening is all that is needed, while in other cases, the coarse or fine aggregate may need to be exposed. In addition, a thin layer of damaged concrete may need to be removed or the concrete surface may simply need to be cleaned. Each repair material requires a particular type of surface preparation, but the general rule is that the concrete surface must not be too smooth, too rough, or too irregular.
In summary, proper surface preparation is a crucial step in concrete repair works. It ensures that the new concrete forms a strong bond with the existing concrete. Different methods can be used for surface preparation, depending on the specific repair required. However, it is important to ensure that the concrete surface is not too smooth, too rough, or too irregular for the best results.
Typical Methods of Surface Preparation
1. Chemical Cleaning
Detergents, tri-sodium phosphate, and other cleaners are commonly used to prepare surfaces for coating, but they are not recommended for surface preparation in concrete repairs. The reason for this is that solvents dissolve contaminants and cause them to penetrate deeper into the concrete, making it difficult to remove them completely.
The ACI committee (546R) advises against the use of chemical cleaning for concrete repairs in most cases. However, under certain conditions and with the application of specific coatings, chemical cleaning may be used.
It is important to note that chemical cleaning should only be used in special circumstances and with caution. In general, other methods of surface preparation, such as mechanical or abrasive cleaning, are preferred for concrete repairs. These methods are more effective at removing contaminants and preparing the surface for repair.
Fig 1: Chemical Cleaning of concrete surface.
2. Acid Etching
Acid etching has been a well-established method for cleaning concrete surfaces and removing excess cement paste, also known as laitance. By applying acid onto the concrete surface, the acid effectively dissolves and removes a substantial amount of cement paste, resulting in a roughened surface that is conducive to receiving replacement materials with enhanced bond strength. This technique has been widely used for a considerable period of time as an effective means of cleaning and preparing concrete surfaces for further treatment or repairs.
Fig 2: Acid etching of concrete floor.
Acid etching has been a well-established method for cleaning concrete surfaces and removing excess cement paste, also known as laitance. By applying acid onto the concrete surface, the acid effectively dissolves and removes a substantial amount of cement paste, resulting in a roughened surface that is conducive to receiving replacement materials with enhanced bond strength. This technique has been widely used for a considerable period of time as an effective means of cleaning and preparing concrete surfaces for further treatment or repairs.
3. Mechanical Preparation
The technique involves the removal of a thin layer of concrete from a surface, which can result in different surface finishes depending on the equipment used for the removal process. There are various types of equipment utilized in this method, including impact tools like breakers, scabblers, grinders, and scarifiers. The use of these tools can yield a range of surface textures and finishes. For instance, some equipment may produce a smoother surface, while others may create a more textured surface. Ultimately, the choice of equipment depends on the desired outcome for the surface being worked on.
Fig 3: Surface preparation using scabblers.
4. Abrasive Preparation
Abrasive blasting is a process that involves propelling dry or moist abrasive using compressed air. When the abrasive particles hit the surface, they penetrate the substrate and produce an eroding effect that dislodges fragments of mortar and fines. This process is effective in removing surface contaminants, unsound concrete, coatings, and adhesive films while imparting a profiled surface.
To achieve the desired surface texture, various abrasive equipment such as sandblasters, shotblasters, or high-pressure water blasters can be used, followed by techniques like water or air blasting, vacuuming, or other methods. The surface roughness necessary for different overlay applications can be determined by a profile number, or it can be defined by sandpaper grit size. Fig 4 provides an example of the surface texture that can be achieved through grinding, shot blasting, and scarifying.
5. Rotomilling
A rotomiller is a heavy-duty scarifier used for breaking up concrete. Unlike typical scarifiers, rotomillers are large and require a driver to operate them. Instead of washers, the machine has teeth mounted on a drum. These teeth impact the concrete with a powerful force, fracturing it into chips and dust. As a result of this process, striations and deep grooving are created on the surface of the concrete.
It is worth noting that rotomillers are designed to be used exclusively on horizontal surfaces. Due to their size and power, they are not suitable for use on vertical surfaces or in tight spaces. However, for large-scale concrete removal projects, a rotomiller can be an incredibly efficient tool, saving time and effort compared to using manual methods.
Fig 5: Rotomilling of concrete pavement
6. Needle Scaling
A tool known as a needle scaler is frequently used to remove brittle encrustations and efflorescence from concrete surfaces. The tool operates by driving steel rods with pneumatic or hydraulic pulses, resulting in a pounding action on the surface. As a result of these impacts, the concrete surface is pulverized, creating a profile with a textured, cratered appearance.
Fig 6: Needle Scaling for surface preparation of concrete repair
Points to be remembered for Surface Preparation for Concrete Repair
The proper preparation of surfaces is crucial for the successful application of protective coatings. All existing coatings and surface contamination must be removed before the application of the surface treatment. The removal process may involve scarification, brushing or grinding, abrasive blasting, shot-blasting, and flame cleaning. It is important to remove any dust or debris produced during the surface preparation process as the presence of surface contamination can result in poor adhesion of the protective coating to the substrate.
After the surface has been properly prepared, it must be sound, dry, and meet the satisfaction of the engineer. Surface preparation may also involve repairing any shallow delaminations, surface scaling, aggregate popouts, grinding rough surfaces, or treating any other surface defects required to achieve the proper performance of the product. For the application of most liquid-applied membranes and thin coatings, surfaces should be relatively smooth. Trowel grade materials can be used to cover minor imperfections of surfaces, but liquid coatings do not have this capability.
It is important to ensure that the pH of the substrate is compatible with the product to be installed. When repairing surfaces with cement concrete or mortar, the surfaces are saturated after cleaning and then brought to the surface dryness condition before placing the new material. The surfaces must be kept moist for several hours to assure saturation. Just before placing 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 the thickness should not exceed 3 mm. However, slush should not be applied when using dry pack material.
When using dry packing, the moist surfaces are lightly dusted with cement using a small dry brush. It is important to note that there should not be any dry cement adhering to the surfaces.
Fig 7: Scale of surface preparation for concrete repair
Standards codes of surface preparation for concrete repair
The context provided consists of four different standards or practices related to the use of epoxy compounds with concrete, acid etching of concrete, measuring the pH of chemically cleaned or etched concrete surfaces, and indicating moisture in concrete using the plastic sheet method. These standards are identified by their respective codes: ACI 503R, ASTM D 4260-88, ASTM D 4262-83, and ASTM D 4263-83.
ACI 503R pertains to the use of epoxy compounds with concrete, providing guidelines and recommendations for the proper application and use of these compounds.
ASTM D 4260-88 is a standard practice for acid etching of concrete, outlining the procedures and requirements for using acid to prepare a concrete surface for coating or other treatments.
ASTM D 4262-83 is a test method for measuring the pH of chemically cleaned or etched concrete surfaces, providing a means of determining the level of acidity or alkalinity of the surface.
ASTM D 4263-83 is a test method for indicating moisture in concrete using the plastic sheet method, a technique that involves placing a plastic sheet on the surface of the concrete and observing any moisture that accumulates underneath it. This method can be used to determine the level of moisture in a concrete slab or to check for the effectiveness of waterproofing measures.