How Can You Tell If Concrete Crack Needs Repair

Cracking is a natural, expected, and accepted behavior of concrete. Yet, cracks may impact the appearance, role, durability, service life, or more than seriously, the structural integrity of the physical. For these reasons, designers, concrete producers, and contractors are e'er trying to control or minimize the amount and severity of concrete smashing. However, crack-free concrete is seldom achievable, especially for standard unreinforced or reinforced concrete.

Basically, concrete cracks. That'due south a fact.

For some projects, the specifications require cracks in concrete to be repaired and may specify a crack repair procedure or may just require cracks to be repaired every bit directed by the engineer. Regardless of what is required by the specifications, cracks should be investigated before designing or performing repairs, specially for elevated structures. Otherwise, repairs may non address the root crusade of cracks resulting in poorly repaired cracks that neglect prematurely or repairs that may non restore the as-designed condition of the construction. More than chiefly, a proper investigation can determine if a crack is the starting time sign of serious trouble indicating the load-carrying capacity of the structure may be jeopardized by either a blueprint, detailing, or construction error.

Here are some recommendations for troubleshooting concrete cracks prior to designing or performing repairs.

The Fundamental Cause of Neat

There are many types of cracks related to different causes only the shared keen mechanism for all mutual cracks is tension. The tensile capacity of physical is limited for both plastic or hardened physical. Concrete is not a ductile material; information technology volition not stretch when subjected to tensile stresses. When the tensile stresses exceed the tensile capacity of the concrete (about 10% of the compressive strength), physical cracks. Of course, the tensile capacity increases as freshly placed concrete hardens simply while plastic and at early ages, concrete has little tensile capacity then it is very vulnerable to cracking.

Effigy 1. Red arrows shown perpendicular to the crack simulate the tensile stresses that created the crevice. Determining what acquired the tensile stresses is the key to determining what caused the scissure and assigning a characterization such as plastic-shrinkage, thermal, or drying-shrinkage crack, etc. Kim Basham A crack investigation should decide 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 scissure or a crack characterization can be assigned such every bit plastic settlement, plastic shrinkage, early-thermal contraction, restrained drying shrinkage which are mutual types of neat.

Typically, cracks are categized as "earlier 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 fourth dimension period after hardening but prior to the concrete gaining full strength. Many cracks occur when early-age concrete is subjected to tensile stresses, especially physical book changes associated with restrained thermal wrinkle (cooling) or concrete drying shrinkage.

For slabs-on-ground, restraints include the base of operations cloth and reinforcing if installed, and side by side 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 and so that book changes of the concrete (i.e., thermal and drying shrinkage) cannot occur resulting in tensile stresses within the physical.

Fissure Investigations

Ideally, a crack investigation should always first with a site visit and a detailed visual inspection. At a minimum, do the post-obit during the site visit:

Create a crack map showing the locations and characteristics of the cracks. Too, bear witness other important elements such as joints, openings, penetrations, and restraints such as foundations, walls, columns, etc.
Measure and tape crevice widths and lengths. For slabs-on-ground, measure contraction joint spacings and sawcut depths.
Have lots of photographs.

If possible, obtain and review the construction drawings and specifications including the reinforcement details and concrete embrace to assist with the visual inspection and to meliorate sympathise the possible causes of the cracks and the designed crack control provisions. Obtain and review the physical mix pattern, batch tickets, and any placing and concrete reports that are available. For slabs-on-form, try to obtain the sawcut timing for the contraction joints. Also, obtain concrete curing information and review the site conditions including atmospheric condition during and after physical placement. Obtain the time when cracks get-go appeared. Documentation should include sufficient information and then others can fully understand the nature and severity of the cracking without visiting the projection.

Crack Maps

Figure two. Example of a scissure map showing crack widths in mils. Highlighted cracks (yellow and orange) indicate two levels of crack border spalling. Not show is the crack inventory that summarized cleft widths, lengths and amount of crack edge spalling per gird squares for the floor. Kim Basham Sketch the cracks and tape the measured crack widths and lengths straight on printed copies of the construction drawings. Try to sketch the cracks to scale and shut as possible to their actual locations. Annotation any crevice 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 heighten the value of the scissure map.

Crack maps should comprise sufficient information and then a detailed crack inventory tin can be created for the purpose of designing and estimating the cost of repairs. For case, a crack inventory can categorize and listing 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 guess repair costs or for repair contractors to bid on the work. Effigy 2 shows an example of a crack map for a structural slab supported past beams/girders and piles.

Photo 1. To use a cleft compactor card, place on crevice and algin line that matches the crack width, and read line width. Kim Basham

Fissure Widths

Figure three. Be sure to measure out crack widths and not spall widths unless measuring both. Kim Basham Measure scissure widths using a scissure comparator card or an optical scissure width measurer. As shown in photo 1, use the crack comparator carte du jour past aligning the proper line thickness over the crack and noting the width of the line. Exist sure to measure the bodily cleft width and not the surface width formed past any crack border spalling that may have occurred. If needed, measure the surface width where edge spalling has occurred and the actual crack width only below the surface every bit shown in figure three. Crack widths are needed to help select the most advisable repair material and procedure. As well, it is easier to record and manage fissure widths using the unit of measurement 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 agile or dormant is critical for designing a crack repair, or more specifically, selecting an appropriate repair material and process. Agile or live cracks are moving cracks or cracks that are likely to grow in width and mayhap length or open up/close with changing concrete temperatures. Fallow or stable cracks are unlikely to grow in width or open/close with changing concrete temperatures. Active cracks should exist repaired or sealed with flexible repair materials to accommodate hereafter crevice movements. Whereas, dormant cracks can be repaired with either flexible or rigid repair materials.

Photo two. Using either screws or epoxy, mountain the two-piece calibrated crack monitor on the concrete over the crack. The red crosshairs on the clear piece move relative to the grid on the white piece graphically showing the direction and amount of motility. Kim Basham

Photo 3. Digital gauges tin can be used to measure crack width opening and endmost due to a heavy wheel load. Kim Basham Specially designed crack gauges attached to the concrete (photo two) can help to determine if cracks are active or fallow. Of course, other factors should be considered including the age of the physical and the amount of concrete shrinkage withal to occur, exposure weather condition or more specifically, concrete'due south exposure to solar heating/cooling, and daily/seasonal temperature changes that crusade thermal expansion and contraction of the concrete. Also, consider the furnishings of loads on cracks (photograph 3). If in doubt about time to come cleft movements, presume the fissure is active.

Reinforced?

Depending on type and amount, typical physical reinforcing including steel rebars, steel fibers, and macro-synthetic fibers can command crack width movements so cracks tin be considered as dormant with regards to designing a scissure 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 2 basic types of physical cracks: intrinsic or non-structural and structural. Intrinsic are naturally occurring cracks that are related to concrete as a textile 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'due south involvement to determine if the load carrying chapters 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.

Adobe Stock Images | Past phatthanit The almost mutual types of intrinsic cracking that are commonly repaired are plastic shrinkage, early-thermal contraction, and peculiarly, drying shrinkage cracks. Restrained drying-shrinkage cracks in both slabs and walls are the most mutual type of cracks requiring repairs to better the overall appearance, to provide crack edge support to prevent edge spalling, to seal cracks to proceed water and dirt out, and to seal cracks in environmental structures. Of course, the determination to repair cracks depends on the severity of the cracking, exposure conditions, and the design role of the physical.

Repairing Cracks

Subsequently determining the root and possibly, secondary causes of cracks, an appropriate crack repair textile and procedure can be selected and/or performed. Part 2 of this commodity, addresses crack repairs and how to blueprint or select the proper materials and procedures. Procedures addressed include: clean and fill, rout and seal/fill up, epoxy and polyurethane injections, autogenous healing, and no repair. Yes, that's right… no repair. Wait for Part 2 before long.

References
ACI 201.1R-08 Guide for Conducting a Visual Inspection of Concrete in Service, American Concrete Plant, www.concrete.org
ACI 224.ane-07 Causes, Evaluation, and Repair of Cracks in Physical Structures, American Physical Institute, www.concrete.org
Technical Study No.22, Non-structural Cracks in Concrete, fourth Edition (2010) Report of a Physical Society Working Political party, Concrete Society, world wide web.concrete.org.uk