Cracking is a natural, expected, and accepted behavior of concrete. However, cracks may affect the appearance, function, durability, service life, or more seriously, the structural integrity of the concrete. For these reasons, designers, concrete producers, and contractors are always trying to control or minimize the amount and severity of concrete cracking. However, crack-free concrete is seldom achievable, especially for standard unreinforced or reinforced concrete.
Basically, concrete cracks. That’s a fact.
For some projects, the specifications require cracks in concrete to be repaired and may specify a crack repair procedure or may simply require cracks to be repaired as directed by the engineer. Regardless of what is required by the specifications, cracks should be investigated before designing or performing repairs, especially for elevated structures. Otherwise, repairs may not address the root cause of cracks resulting in poorly repaired cracks that fail prematurely or repairs that may not restore the as-designed condition of the structure. More importantly, a proper investigation can determine if a crack is the first sign of serious trouble indicating the load-carrying capacity of the structure may be jeopardized by either a design, detailing, or construction error.
Here are some recommendations for troubleshooting concrete cracks prior to designing or performing repairs.
The Fundamental Cause of Cracking
There are many types of cracks related to different causes but the shared cracking mechanism for all common cracks is tension. The tensile capacity of concrete is limited for both plastic or hardened concrete. Concrete is not a ductile material; it will not stretch when subjected to tensile stresses. When the tensile stresses exceed the tensile capacity of the concrete (about 10% of the compressive strength), concrete cracks. Of course, the tensile capacity increases as freshly placed concrete hardens but while plastic and at early ages, concrete has little tensile capacity so it is very vulnerable to cracking.
A crack investigation should determine the root cause of the tension (figure 1) so the cause can be considered when designing or performing crack repairs. Once the cause of the tension has been identified, the type of crack or a crack label can be assigned such as plastic settlement, plastic shrinkage, early-thermal contraction, restrained drying shrinkage which are common types of cracking.
Typically, cracks are categized as “before hardening” while the concrete is still plastic and “after hardening” or after the concrete has hardened. Because tensile capacity increases as concrete hardens and gains strength, a third category called “early age” exists. This category refers to the time period after hardening but prior to the concrete gaining full strength. Many cracks occur when early-age concrete is subjected to tensile stresses, especially concrete volume changes associated with restrained thermal contraction (cooling) or concrete drying shrinkage.
For slabs-on-ground, restraints include the base material and reinforcing if installed, and adjacent foundations and walls if not isolated from the slab with isolation joints. For walls and elevated slabs, connecting concrete elements act as restraints that prevent volume changes of the concrete from occurring. Restraints lock the concrete in place so that volume changes of the concrete (i.e., thermal and drying shrinkage) cannot occur resulting in tensile stresses within the concrete.
Ideally, a crack investigation should always start with a site visit and a detailed visual inspection. At a minimum, do the following during the site visit:
- Create a crack map showing the locations and characteristics of the cracks. Also, show other important elements such as joints, openings, penetrations, and restraints such as foundations, walls, columns, etc.
- Measure and record crack widths and lengths. For slabs-on-ground, measure contraction joint spacings and sawcut depths.
- Take lots of photographs.
If possible, obtain and review the construction drawings and specifications including the reinforcement details and concrete cover to assist with the visual inspection and to better understand the possible causes of the cracks and the designed crack control provisions. Obtain and review the concrete mix design, batch tickets, and any placing and concrete reports that are available. For slabs-on-grade, try to obtain the sawcut timing for the contraction joints. Also, obtain concrete curing information and review the site conditions including weather during and after concrete placement. Obtain the time when cracks first appeared. Documentation should include sufficient information so others can fully understand the nature and severity of the cracking without visiting the project.
Sketch the cracks and record the measured crack widths and lengths directly on printed copies of the construction drawings. Try to sketch the cracks to scale and close as possible to their actual locations. Note any crack patterns that may exist and if possible, the depth of cracks. Be neat and organized so either the actual crack map created onsite can be easily used to quantify the cracking or used to create an electronic version. Mark locations of photographs so photographs can be used to enhance the value of the crack map.
Crack maps should contain sufficient information so a detailed crack inventory can be created for the purpose of designing and estimating the cost of repairs. For example, a crack inventory can categorize and list cracks according to widths, depths, locations, cracks with and without edge spalling, or other characteristics for the purpose of assigning different repair materials and procedures for each category of crack. Be sure the crack map inventory has sufficient information to accurately estimate repair costs or for repair contractors to bid on the work. Figure 2 shows an example of a crack map for a structural slab supported by beams/girders and piles.
Measure crack widths using a crack comparator card or an optical crack width measurer. As shown in photo 1, use the crack comparator card by aligning the proper line thickness over the crack and noting the width of the line. Be sure to measure the actual crack width and not the surface width formed by any crack edge spalling that may have occurred. If needed, measure the surface width where edge spalling has occurred and the actual crack width just below the surface as shown in figure 3. Crack widths are needed to help select the most appropriate repair material and procedure. Also, it is easier to record and manage crack widths using the unit of “mils” than using inches where a mil is equal to 0.001 inches. For example, 0.004 inches equals 4 mils.
Active or Dormant?
Determining whether cracks are active or dormant is critical for designing a crack repair, or more specifically, selecting an appropriate repair material and procedure. Active or live cracks are moving cracks or cracks that are likely to grow in width and perhaps length or open/close with changing concrete temperatures. Dormant or stable cracks are unlikely to grow in width or open/close with changing concrete temperatures. Active cracks should be repaired or sealed with flexible repair materials to accommodate future crack movements. Whereas, dormant cracks can be repaired with either flexible or rigid repair materials.
Specially designed crack gauges attached to the concrete (photo 2) can help to determine if cracks are active or dormant. Of course, other factors should be considered including the age of the concrete and the amount of concrete shrinkage yet to occur, exposure conditions or more specifically, concrete’s exposure to solar heating/cooling, and daily/seasonal temperature changes that cause thermal expansion and contraction of the concrete. Also, consider the effects of loads on cracks (photo 3). If in doubt about future crack movements, assume the crack is active.
Depending on type and amount, typical concrete reinforcing including steel rebars, steel fibers, and macro-synthetic fibers can control crack width movements so cracks can be considered as dormant with regards to designing a crack repair procedure. It is important to determine the type and amount of reinforcing crossing through cracks during the investigation. If needed, a rebar locator can be used to verify the presence and location of steel reinforcing.
Type of Cracks
There are two basic types of concrete cracks: intrinsic or non-structural and structural. Intrinsic are naturally occurring cracks that are related to concrete as a material such as plastic settlement and shrinkage, freezing of freshly placed concrete, crazing, thermal and drying shrinkage, seasonal temperature changes, corrosion of reinforcement, and alkali-aggregate reactions. Whereas, structural cracks are typically associated with overloads from either construction or service loads. These cracks typically require an engineer’s involvement to determine if the load carrying capacity of the structure has been jeopardized and to determine what type of repair or strengthening is required to ensure the structural integrity and safety of the structure.
The most common types of intrinsic cracking that are commonly repaired are plastic shrinkage, early-thermal contraction, and especially, drying shrinkage cracks. Restrained drying-shrinkage cracks in both slabs and walls are the most common type of cracks requiring repairs to improve the overall appearance, to provide crack edge support to prevent edge spalling, to seal cracks to keep water and dirt out, and to seal cracks in environmental structures. Of course, the decision to repair cracks depends on the severity of the cracking, exposure conditions, and the design function of the concrete.
After determining the root and possibly, secondary causes of cracks, an appropriate crack repair material and procedure can be selected and/or performed. Part 2 of this article, addresses crack repairs and how to design or select the proper materials and procedures. Procedures addressed include: clean and fill, rout and seal/fill, epoxy and polyurethane injections, autogenous healing, and no repair. Yes, that’s correct… no repair. Look for Part 2 soon.