Sometimes cracks need to be repaired, but there are so many options, how do we design and choose the best repair option? This is not as difficult as you think.
After investigating the cracks and determining the repair goals, designing or selecting the best repair materials and procedures is quite simple. This summary of crack repair options involves the following procedures: cleaning and filling, pouring and sealing/filling, epoxy and polyurethane injection, self-healing, and “no repair”.
As described in “Part 1: How to evaluate and troubleshoot concrete cracks”, investigating the cracks and determining the root cause of the cracks is the key to choosing the best crack repair plan. In short, the key items needed to design a proper crack repair are the average crack width (including the minimum and maximum width) and the determination of whether the crack is active or dormant. Of course, the goal of crack repair is as important as measuring crack width and determining the possibility of crack movement in the future.
Active cracks are moving and growing. Examples include cracks caused by continuous ground subsidence or cracks that are shrinkage/expansion joints of concrete members or structures. The dormant cracks are stable and are not expected to change in the future. Normally, the cracking caused by the shrinkage of concrete will be very active at the beginning, but as the moisture content of the concrete stabilizes, it will eventually stabilize and enter a dormant state. In addition, if enough steel bars (rebars, steel fibers, or macroscopic synthetic fibers) pass through the cracks, future movements will be controlled and the cracks may be considered to be in a dormant state.
For dormant cracks, use rigid or flexible repair materials. Active cracks require flexible repair materials and special design considerations to allow future movement. The use of rigid repair materials for active cracks usually results in cracking of the repair material and/or adjacent concrete.
Photo 1. Using needle tip mixers (No. 14, 15 and 18), low-viscosity repair materials can be easily injected into hairline cracks without wiring Kelton Glewwe, Roadware, Inc.
Of course, it is important to determine the cause of the cracking and determine whether the cracking is structurally important. Cracks that indicate possible design, detail, or construction errors can cause people to worry about the load-bearing capacity and safety of the structure. These types of cracks can be structurally important. Cracking may be caused by the load, or it may be related to the inherent volume changes of concrete, such as dry shrinkage, thermal expansion and shrinkage, and may or may not be significant. Before choosing a repair option, determine the cause and consider the importance of cracking.
Repairing cracks caused by design, detail design, and construction errors is beyond the scope of a simple article. This situation usually requires a comprehensive structural analysis and may require special reinforcement repairs.
Restoring the structural stability or integrity of concrete components, preventing leaks or sealing water and other harmful elements (such as deicing chemicals), providing crack edge support, and improving the appearance of cracks are common repair goals. Considering these goals, maintenance can be roughly divided into three categories:
With the popularity of exposed concrete and construction concrete, the demand for cosmetic crack repair is increasing. Sometimes integrity repair and crack sealing/filling also require appearance repair. Before choosing repair technology, we must clarify the goal of crack repair.
Before designing a crack repair or choosing a repair procedure, four key questions must be answered. Once you answer these questions, you can more easily select the repair option.
Photo 2. Using scotch tape, drilling holes, and a rubber-head mixing tube connected to a handheld dual-barrel gun, the repair material can be injected into the fine-line cracks under low pressure. Kelton Glewwe, Roadware, Inc.
This simple technique has become popular, especially for building-type repairs, because repair materials with very low viscosity are now available. Since these repair materials can easily flow into very narrow cracks by gravity, there is no need for wiring (ie install a square or V-shaped sealant reservoir). Since wiring is not required, the final repair width is the same as the crack width, which is less obvious than wiring cracks. In addition, the use of wire brushes and vacuum cleaning is faster and more economical than wiring.
First, clean the cracks to remove dirt and debris, and then fill with a low-viscosity repair material. The manufacturer has developed a very small diameter mixing nozzle that is connected to a handheld dual-barrel spray gun to install repair materials (photo 1). If the nozzle tip is larger than the crack width, some crack routing may be required to create a surface funnel to accommodate the size of the nozzle tip. Check the viscosity in the manufacturer’s documentation; some manufacturers specify a minimum crack width for the material. Measured in centipoise, as the viscosity value decreases, the material becomes thinner or easier to flow into narrow cracks. A simple low-pressure injection process can also be used to install the repair material (see Figure 2).
Photo 3. Wiring and sealing involves first cutting the sealant container with a square or V-shaped blade, and then filling it with an appropriate sealant or filler. As shown in the figure, the routing crack is filled with polyurethane, and after curing, it is scratched and flush with the surface. Kim Basham
This is the most common procedure for repairing isolated, fine and large cracks (photo 3). It is a non-structural repair that involves expanding cracks (wiring) and filling them with suitable sealants or fillers. Depending on the size and shape of the sealant reservoir and the type of sealant or filler used, wiring and sealing can repair active cracks and dormant cracks. This method is very suitable for horizontal surfaces, but can also be used for vertical surfaces with non-sagging repair materials.
Suitable repair materials include epoxy, polyurethane, silicone, polyurea, and polymer mortar. For the floor slab, the designer must choose a material with appropriate flexibility and hardness or stiffness characteristics to accommodate expected floor traffic and future crack movement. As the flexibility of the sealant increases, the tolerance for crack propagation and movement increases, but the material’s load-bearing capacity and crack edge support will decrease. As the hardness increases, the load-bearing capacity and crack edge support increase, but the crack movement tolerance decreases.
Figure 1. As the Shore hardness value of a material increases, the hardness or stiffness of the material increases and the flexibility decreases. In order to prevent the crack edges of cracks exposed to hard-wheeled traffic from peeling off, a Shore hardness of at least about 80 is required. Kim Basham prefers harder repair materials (fillers) for dormant cracks in hard-wheeled traffic floors, because the crack edges are better as shown in Figure 1. For active cracks, flexible sealants are preferred, but the load-bearing capacity of the sealant and crack edge support is low. The Shore hardness value is related to the hardness (or flexibility) of the repair material. As the Shore hardness value increases, the hardness (stiffness) of the repair material increases and the flexibility decreases.
For active fractures, the size and shape factors of the sealant reservoir are as important as choosing a suitable sealant that can adapt to the expected fracture movement in the future. The form factor is the aspect ratio of the sealant reservoir. Generally speaking, for flexible sealants, the recommended form factors are 1:2 (0.5) and 1:1 (1.0) (see Figure 2). Reducing the form factor (by increasing the width relative to the depth) will reduce the sealant strain caused by the crack width growth. If the maximum sealant strain decreases, the amount of crack growth that the sealant can withstand increases. Using the form factor recommended by the manufacturer will ensure the maximum elongation of the sealant without failure. If needed, install foam support rods to limit the depth of the sealant and help form the “hourglass” elongated shape.
The allowable elongation of the sealant decreases with the increase of the shape factor. For 6 inches. Thick plate with a total depth of 0.020 inches. The shape factor of a fractured reservoir without sealant is 300 (6.0 inches/0.020 inches = 300). This explains why active cracks sealed with a flexible sealant without a sealant tank often fail. If there is no reservoir, if any crack propagation occurs, the strain will quickly exceed the tensile capacity of the sealant. For active cracks, always use a sealant reservoir with the form factor recommended by the sealant manufacturer.
Figure 2. Increasing the width to depth ratio will increase the sealant’s ability to withstand future cracking moments. Use a form factor of 1:2 (0.5) to 1:1 (1.0) or as recommended by the sealant manufacturer for active cracks to ensure that the material can stretch properly as the crack width grows in the future. Kim Basham
Epoxy resin injection bonds or welds cracks as narrow as 0.002 inches together and restores the integrity of the concrete, including strength and rigidity. This method involves applying a surface cap of non-sagging epoxy resin to limit cracks, installing injection ports into the borehole at close intervals along horizontal, vertical or overhead cracks, and pressure injecting epoxy resin (photo 4).
The tensile strength of epoxy resin exceeds 5,000 psi. For this reason, epoxy resin injection is considered a structural repair. However, epoxy resin injection will not restore the design strength, nor will it reinforce concrete that has broken due to design or construction errors. Epoxy resin is rarely used to inject cracks to solve problems related to load-bearing capacity and structural safety issues.
Photo 4. Before injecting epoxy resin, the crack surface must be covered with non-sagging epoxy resin to limit pressurized epoxy resin. After injection, the epoxy cap is removed by grinding. Usually, removing the cover will leave abrasion marks on the concrete. Kim Basham
Epoxy resin injection is a rigid, full-depth repair, and the injected cracks are stronger than the adjacent concrete. If active cracks or cracks acting as shrinkage or expansion joints are injected, other cracks are expected to form beside or away from the repaired cracks. Only inject dormant cracks or cracks with a sufficient number of steel bars passing through the cracks in order to limit future movement. The following table summarizes the important selection features of this repair option and other repair options.
Polyurethane resin can be used to seal wet and leaking cracks as narrow as 0.002 inches. This repair option is mainly used to prevent water leakage, including injecting reactive resin into the crack, which combines with water to form a swelling gel, plugging the leak and sealing the crack (photo 5). These resins will chase water and penetrate into the tight micro-cracks and pores of the concrete to form a strong bond with the wet concrete. In addition, the cured polyurethane is flexible and can withstand future crack movement. This repair option is a permanent repair, suitable for active cracks or dormant cracks.
Photo 5. Polyurethane injection includes drilling, installation of injection ports and pressure injection of resin. The resin reacts with the moisture in the concrete to form a stable and flexible foam, sealing cracks, and even leaking cracks. Kim Basham
For cracks with a maximum width between 0.004 inch and 0.008 inch, this is the natural process of crack repair in the presence of moisture. The healing process is due to the unhydrated cement particles being exposed to moisture and forming insoluble calcium hydroxide leaching from the cement slurry to the surface and reacting with the carbon dioxide in the surrounding air to produce calcium carbonate on the surface of the crack. 0.004 inches. After a few days, the wide crack can heal, 0.008 inches. The cracks may heal within a few weeks. If the crack is affected by fast-flowing water and movement, healing will not occur.
Sometimes “no repair” is the best repair option. Not all cracks need to be repaired, and monitoring cracks may be the best option. If necessary, cracks can be repaired later.
Post time: Sep-03-2021