Concrete Cracking
Photo 1.
Technician is injecting epoxy through ports into cracks. Entry port spacing depends on crack width, type of epoxy, and thickness of concrete member. The surface seal (epoxy paste) contains the liquid epoxy during injection and curing.
Photo 2.
Grinders with special blades V or square shaped called "crack chasers" routes out sealant reservoir. Small diameter blades can cut smaller radii so they can chase the crack better than larger diameter blades.
Photo 3.
Before repair - random dry shrinkage crack in polished, integral colored floor.
Photo 4.
After repair: crack sealed using a two-part polyurea, colored sand and powered coloring to match integral colored concrete. To minimize crack width repair, so sealant reservoir was used. Reservoir not required for dormant cracks.
Photo 5.
Polyurethane injection is primarily used to stop water leaks. Here the technician is pumping resin through ports to seal water leaks around steel pipe.
Photo 6.
Dry packing a low-water cement mortar into a widened crack is an economical repair option for wide, dormant cracks.
Photo 7.
Thin polymer-modified cement overlay can be used to seal multiply surface cracks in slabs and pavements. Special design consideration must be given to active cracks and joints to avoid reflective cracking through the overlay.
It happens. Here's how to fix it.
Guest Writer: Kim Basham PhD, PE

Concrete cracks and that's a fact! Because of this reality, sometimes we need to repair cracks. But with so many options, how do we choose the best repair procedure? It's not as hard as you may think. After investigating the cracks and establishing the repair objectives, selecting the best repair method can be easy.

This summary of crack repair options includes

  • epoxy injection,
  • routing and sealing,
  • stitching and doweling,
  • gravity filling,
  • polyurethane injection,
  • dry packing,
  • overlays and surface treatments,
  • autogenous healing and
  • "no repair."

Investigating Cracks
Investigators typically collect and evaluate lots of crack information. Commonly, they evaluate the number and location of cracks, crack patterns (horizontal, vertical, diagonal, eggshell, etc.), crack lengths, widths and depths, age or time of occurrence, and reinforcement details including the size and number of bars passing through cracks plus the concrete cover. All this information is important when considering repair options, but one of the keys to selecting the best repair procedure is determining whether the cracks are active or dormant.

Active cracks are moving and growing. Examples include cracks resulting from continuing foundation settlement, or cracks acting as contraction and expansion joints. Dormant cracks are stable and future movements are not anticipated. Typically, cracking caused by drying shrinkage will be active cracks at the beginning but eventually stabilize and become dormant. Also, if there is enough reinforcing crossing through the crack, future movements are controlled and the crack can be considered dormant.

Once a crack is determined to be either active or dormant, these basic repair procedures are indicated:

  • For dormant cracks, use either rigid or flexible repair materials.
  • Active cracks require flexible repair materials and special design considerations to allow for future movements. Using a rigid repair material for active cracks typically results in cracking of the repair material and/or the adjacent concrete.
Of course, it is important to establish the cause of cracking and to determine if the cracking is structurally significant or not. Cracks that indicate there may be design or construction errors and raise concerns regarding the load-carrying capacity and safety are structurally significant. Cracks can be the first signs of serious trouble or they may be the total extent of the damage and insignificant with regards to the load-carrying capacity of the member. Cracking can be load-induced or related to intrinsic volume changes such as drying shrinkage, thermal expansion and contraction and may or may not be significant. Before selecting a repair option, identify the cause and consider the significance of the cracking.

Repairing cracks caused by design and construction errors is beyond the scope of this article. Such a situation typically requires a comprehensive structural analysis and special strengthening techniques.

Repair Objectives
Repair objectives for cracks commonly include

  • Restoring the structural soundness or integrity of a concrete member,
  • Stopping water leaks or sealing out water and other harmful elements such as deicing chemicals, and
  • Improving the appearance of the crack are common repair objectives.
Considering these objectives, repairs can be loosely divided into three categories:

  1. Integrity repairs (restores the member to the original strength),
  2. Crack sealing, and
  3. Cosmetic repairs.
With the popularity of architectural concrete in recent years, the demand for cosmetic crack repairs is growing. Sometimes integrity repairs and crack sealing are required to be cosmetic repairs too. Before selecting the repair technique, be sure to clearly establish the objectives of the crack repair.

In summary, there are three key questions that must be answered before the best repair option can be selected:

  1. Are the cracks active or dormant?
  2. Is the repair an integrity repair or crack sealing?
  3. Is the repair a cosmetic repair?
Once these questions are answered, it is easy to select one of the following repair options.

Crack Repair Options
Epoxy Injection
Epoxy injection bonds or welds cracks as narrow as 0.002 inches together and restores concrete soundness and integrity. This method consists of installing injection ports into drilled holes at close intervals along horizontal, vertical or overhead cracks and pressure-injecting epoxy. Cracks injected with epoxy need to be dormant and not actively leaking. While moist cracks can be injected, water or other contaminates will reduce the effectiveness of the epoxy repair. (PHOTO 1)

Tensile strengths for epoxies exceed 5,000 psi. For this reason, epoxy injection is considered an integrity repair. Epoxy injection will not restore design strengths nor strengthen cracked members damaged because of design or construction errors. Seldom will injecting cracks with epoxy resolve issues associated with load-carrying capacity and structural safety concerns.

Epoxy injection is a rigid, full-depth repair where the injected crack will be stronger than the adjacent concrete. If active cracks or cracks acting as contraction or expansion joints are injected, expect other cracks to form next to or far away from the repaired crack. Only inject dormant cracks or cracks that have sufficient amounts of reinforcing crossing the crack so future movements are restrained. Table 1 summarizes the important selection features of this and the other repair options.

Routing and Sealing
This is the simplest and most economical means to repair isolated, fine and large cracks. It is a nonstructural repair that consists of enlarging the crack and filling it with a suitable joint sealant. Depending on the size and shape of the sealant reservoir and the type sealer used, routing and sealing can seal both active and dormant cracks. This method is ideal for horizontal surfaces but can also be used for vertical, overheads and curved surfaces.

Appropriate sealants include epoxies, urethanes, silicones, polyureas, asphaltic materials and polymer mortars. For floors, designers must choose a sealant with suitable flexibility and hardness or stiffness properties to accommodate both the anticipated floor traffic and future crack movements. As the flexibility of the sealant increases, tolerance to crack growth and movement increases but the load carry-capacity of the sealant and crack edge support decreases. As the hardness increases, load carry-capacity and crack edge support increases but the crack movement tolerance decreases. For dormant cracks, harder sealants are preferred because of the better edge support. For active cracks, flexible sealers are preferred but the load-carrying capacity of the sealant and crack edge support should not be sacrificed.

Some designers prefer to err on the side of specifying a sealant that is too hard so that any crack repair failures, if they occur, are limited to sealant cracking or tearing. Resealing a sealed crack is easier than repairing crack edge spalling.

For active cracks, the size and shape factor of the sealant reservoir is just as important as selecting a suitable sealant that can accommodate the anticipate crack movements. The shape factor is the depth to width ratio of the sealant reservoir. In general, recommended shape factors are 1:2 () and 1:1 ( See Figure 1). Reducing the shape factor (by increasing the width relative to the depth) will decrease the maximum sealant strain caused by crack growth. As the maximum strain is reduced, the amount of crack growth the sealant can tolerate increases.

As shown in Figure 1, the maximum strain in the sealant increases as the shape factor increases. For a 6-inch thick slab with a full-depth 0.020 inch crack, the shape factor without a sealant reservoir is 300 (6.0"/0.020" = 300). This explains why active cracks sealed with a flexible sealant without a reservoir commonly fail. Without the reservoir, the maximum strain quickly exceeds the tensile capacity of the sealant if any crack growth occurs. A sealant reservoir should always be used for active cracks.

ACI recommends a reservoir depth (groove depth) from inch to 1 inch and a shape factor from 1:1 to 1:2. Therefore, reservoir sizes should fall between inch x inch and 1 inch (depth) x 2 inch (width). As discussed, the larger reservoir will tolerate more crack growth. Figure 2 shows several sealant reservoir configurations. (PHOTO 2 & 3)

Stitching and Doweling
Stitching involves drilling holes on both sides of the crack and anchoring U-shaped metal staples with short legs across the crack as shown in Figure 3. Doweling consists of drilling holes and anchoring straight steel dowels at a 45 degree angle through the crack. Stitching can be used for slabs with any thickness but doweling only works for 6-inch or thicker slabs. Because stitching and doweling restores the tensile strength across the crack, they are integrity repairs. Either rigid or flexible materials can be used to seal the cracks and staples. Both repair techniques will stabilize an active crack and may cause the concrete to crack elsewhere, especially if the repaired crack was acting as a contraction or expansion joint.

Gravity Filling
Low viscosity urethanes, high-molecular-weight methacrylates (HMWA) and some epoxies can gravity fill cracks with widths from 0.001 to 0.080 inches. Lower viscosity materials are used to fill narrow cracks. This method is ideal for areas with multiple surface cracks that are dormant such as plastic shrinkage cracks. The area and cracks are cleaned with air or water blasting (and allowed to dry) before flooding the area with the monomer or resin. If cracks are full of dirt, moisture or other contaminants, penetration of the repair material into cracks is poor. The material is worked into the cracks with brooms, rollers or squeegees then the excessive material is removed to avoid shiny, slick areas. Broadcasting sand over the surface before the repair material cures can help provide a slip-resistant surface.

Polyurethane Injection
Polyurethane resins can be used to seal wet and leaking cracks as narrow as 0.002 inches. This repair option is primarily used to stop water leaks and consists of injecting a reactive resin into cracks that combines with water to form an expanding gel that chokes off the leak and seals the crack. Hydrophilic resins will chase the water and penetrate into tight micro-cracks and pores of the concrete creating a firm bond to the wet concrete. Also, cured polyurethanes are flexible and will tolerate future crack movements. This repair option is a permanent repair and works with either active or dormant cracks. (Photo 4)

Dry Packing
Using a saw and chipping hammer, the crack is first opened to a minimum depth and width of about 1 inch. After scrubbing a cement bond coat or a commercial bonding agent into the concrete, a low-water content cement mortar is placed and compacted using a blunt hardwood stick and hammer. After tamping the final lift, the repair is smoothed with a float. Tamping a low-water content mortar into place forces the mortar into the pores of the concrete creating a tight, durability repair with little shrinkage. Dry packing is an economical repair for wide, dormant cracks. (Photo 5)

Overlays and Surface Treatments
Fine surface cracks in slabs and pavements can be repaired using bonded overlays or surface treatments if the cracks are dormant and not full-depth. Thin overlays consist of polymer-modified cement, silica fume mortars and surface treatments consisting of low solids and low viscosity resin-based systems. To avoid reflective cracking at active cracks and working joints, special design considerations are required at these locations. (Photo 6)

Autogenous Healing
It is a natural process of crack repair that occurs in the presence of moisture for cracks with maximum widths between 0.004 and 0.008 inches. The healing process is due to the hydration of unhydrated cement particles that are exposed to moisture as the crack opens. Also, the healing process is helped by the formation of insoluble calcium carbonate from the calcium hydroxide in the cement paste and carbon dioxide in the surrounding air. A 0.004 inch-wide crack can heal after several days and a 0.008 inch crack can heal in several weeks. Healing will not occur if cracks are subjected to fast flowing water and movements.

No repair
Finally, sometimes "no repair" is the best repair option. Not all cracks require repair and monitoring the crack may be the best choice. If needed, the crack can be repaired later.

ACI 224.1R-93 Causes, Evaluation and Repair of Cracks in Concrete Structures (Reapproved 1998), American Concrete Institute, Farmington Hills, MI.

ACI 504R-90 Guide to Sealing Joints in Concrete Structures (Reapproved 1997), American Concrete Institute, Farmington Hills, MI.

about the author:
Kim D. Basham, PhD, PE Senior Structural Engineer
Registered Professional Engineer: WY, CO, AZ

Contact Information
Concrete Engineering Specialists, LLC
1716 Capitol Avenue
Cheyenne, WY 82001 (307) 635-7240

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© 2006 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2006.

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