Assessing cracks in concrete is an essential step before initiating any repairs. The primary objective of this evaluation is to determine the location and severity of the cracks and identify if they are indicative of any present or future structural problems. It is crucial to consider the existing and expected future loading conditions to evaluate the extent of damage accurately. Identifying the cause of cracking is also imperative before planning any repairs. A thorough review of drawings, specifications, construction and maintenance records, and on-site observations can help identify the underlying cause. However, if this information is insufficient, a field investigation and structural analysis may be necessary before proceeding with repairs.
Cracks in concrete structures can have various causes, and it is essential to determine the underlying reason before proceeding with repairs. Evaluating the observed cracking can help identify which of these causes apply in a particular situation. Repairs are necessary when cracks reduce the strength, stiffness, or durability of the structure to an unacceptable level or significantly impair the function of the structure. In some cases, the function of the structure itself may require repair, such as in water-retaining structures, even if strength, stiffness, or appearance are not significantly affected. Cracks in pavements and slabs-on-grade may require repair to prevent further damage and maintain structural integrity.
In addition to maintaining structural integrity, repairing cracks can also enhance the appearance of the concrete structure. Repairs that improve the surface’s appearance can be desirable for both aesthetic and functional reasons. For instance, cracks in concrete pavements and slabs-on-grade can lead to edge spalls and the migration of water to the subgrade, resulting in further damage. Therefore, repairing such cracks can prevent more significant damage and improve the overall appearance of the structure.
In conclusion, a detailed evaluation of observed cracking in concrete structures is crucial to identify the location, extent, and cause of the cracks. If necessary, a field investigation and structural analysis should be conducted before planning any repairs. The underlying cause of the cracking should be determined before proceeding with repairs to ensure that they are effective and provide long-term solutions. Repairs may be required to maintain structural integrity, prevent further damage, and enhance the appearance of the concrete structure.
Determination of location and extent of Cracks in Concrete
Observing the condition of concrete in a structure can be achieved through various means, such as direct and indirect observations, nondestructive and destructive testing, and core sampling. These methods can provide valuable information regarding the extent and location of any cracks present in the concrete. Additionally, records pertaining to the construction and maintenance of the structure, as well as drawings, can also provide useful information in assessing the overall condition of the concrete.
Direct observation involves physically examining the concrete to identify any visible signs of damage or distress, such as cracks, spalling, or discoloration. Indirect observations may include using tools such as borescopes or moisture meters to evaluate the condition of the concrete. Nondestructive testing can also be employed, using methods such as ultrasonic or radar scanning, to assess the internal condition of the concrete without causing any damage.
Destructive testing, such as taking cores from the structure, can provide valuable information on the condition of the concrete. Cores can be analyzed to determine factors such as compressive strength, density, and porosity. This information can then be used to assess the overall quality of the concrete and identify any areas of weakness.
Records pertaining to the construction and maintenance of the structure, as well as drawings, can also provide valuable insights into the condition of the concrete. These documents may provide information on the types of materials used in the construction of the structure, as well as any repairs or maintenance that have been performed. This information can be used to assess the overall condition of the concrete and identify any areas of concern that require further investigation.
Direct and indirect observation of Concrete Cracks
Accurately noting the locations and widths of cracks on a structure is important, and can be achieved through the use of a sketch. A grid can be marked on the surface of the structure to facilitate precise identification of crack locations on the sketch. To determine the width of cracks, a crack comparator can be used. This device is a hand-held microscope that includes a scale on the lens closest to the surface being viewed, and is capable of measuring crack widths to an accuracy of approximately 0.001 inches or 0.025 millimeters.
Overall, accurately documenting the location and extent of cracks on a structure is crucial for assessing its structural integrity and determining the appropriate course of action to address any issues. The use of a sketch and grid system can aid in precise identification of cracks, while a crack comparator can provide accurate measurements of crack widths. By gathering this information, engineers and other professionals can make informed decisions about how to address any structural concerns and ensure the safety and longevity of the structure.
Fig.1: Comparator for Measuring Width of Cracks in Concrete
Mechanical movement indicators are commonly used to monitor crack movement. These indicators provide valuable information about the displacement and rotation of cracks. Fig. 2.2 (a) shows an indicator that directly reads the crack displacement and rotation. This type of indicator is useful in obtaining precise measurements of the crack movement.
Another type of indicator, shown in Fig. 2.2 (b), amplifies the crack movement by a factor of 50. This indicator measures the maximum range of movement during a specific period, providing valuable information on the crack behavior. This type of indicator is known to be effective in monitoring and detecting changes in crack behavior over time.
By using these mechanical movement indicators, engineers and researchers can track crack movement and analyze the structural integrity of the material being tested. The information gathered from these measurements is crucial in predicting potential failure or damage and can help in developing preventive measures to ensure the safety of the structure being monitored.
Fig.2: Monitoring Crack Movement in Concrete
Supplementing sketches with photographs that document the state of a structure during an investigation can be useful. To conduct a condition survey of concrete that is in use, various guidelines are available. These guidelines include ACI 201.1R, ACI 201.3R, ACI 207.3R, ACI 345.1R, and ACI 546.1R.
Nondestructive testing of to Determine Concrete Cracks
Nondestructive testing methods are used to determine the presence of internal cracks and voids, as well as the depth of penetration of surface cracks. Simple techniques like tapping the surface with a hammer or using a chain drag can help identify laminar cracking near the surface. When there is a hollow sound, it indicates one or more cracks parallel to the surface.
To determine the presence of reinforcement, a tool called a pachometer can be used. There are various types of pachometers available that range in capability, from indicating the mere presence of steel to allowing an experienced user to determine the depth and size of reinforcing steel. However, in congested reinforcement areas, it may be necessary to remove the concrete cover through drilling or chipping to identify bar sizes or to obtain accurate cover measurements.
Overall, nondestructive testing methods are essential for identifying potential issues in concrete structures without causing damage. By using these methods, engineers and construction professionals can effectively evaluate the safety and integrity of structures, helping to ensure the safety of those who use them.
Fig.3: Pachometer – Reinforcement Bar Locator in Concrete
When investigating the cause of cracking in concrete, one potential culprit to consider is corrosion. To examine for corrosion, the simplest approach is to remove a portion of the concrete and directly observe the steel. Another method is to use electrical potential measurements with a reference half cell, such as the commonly used copper-copper sulfate half cell (ASTM C 876). This technique also requires access to the reinforcing steel. Ultrasonic nondestructive test equipment can also be used to detect cracks, with the most common technique being through-transmission testing using commercially available equipment (Malhotra and Carino 1991; Knab et al. 1983). This involves transmitting a mechanical pulse to one face of the concrete and receiving it at the opposite face. The time taken for the pulse to pass through the member is electronically measured and the pulse velocity can be calculated if the distance between the transmitting and receiving transducers is known. If access is not available to opposite faces, transducers may be located on the same face, but the interpretation of results is not as straightforward. A significant change in measured pulse velocity can occur if an internal discontinuity results in an increase in path length for the signal. Generally, higher pulse velocity indicates higher quality concrete. The interpretation of pulse velocity test results can be improved with the use of an oscilloscope that provides a visual representation of the received signal.
Fig.4 : Ultrasonic Testing of Concrete Cracks
Tests on Concrete Cores to Evaluate Cracks in Concrete
Cores taken from selected areas within a structure can provide valuable information, such as precise measurements of crack width and depth. Compressive strength tests can also reveal information about concrete quality, but cores containing cracks should not be used to determine concrete strength. It is essential to note that ultrasonic equipment requires a trained operator, and the results should be interpreted cautiously by an experienced person. This is because factors like moisture, reinforcing steel, and embedded items can impact the results. Fully saturated cracks can render ultrasonic testing ineffective, making it challenging to differentiate between a group of small cracks and a single larger one.
The pulse-echo technique is an alternative to through-transmission testing that employs a simple transducer to send and receive ultrasonic waves. However, developing a practical pulse-echo test for concrete has been challenging. Petrographic examinations of cracked concrete can identify the material causes of cracking, such as alkali reactivity, cyclic freezing damage, “D” cracking, expansive aggregate particles, fire-related damage, shrinkage, and corrosion. Petrography can also identify other factors related to cracking, such as the water-to-cement ratio, relative paste volume, and distribution of concrete components. In addition, petrography can determine the relative age of cracks and identify secondary deposits on fracture surfaces, which affect repair schemes.
Chemical tests for excessive chlorides can indicate the potential for corrosion of embedded reinforcement. It is essential to note that petrographic examinations and chemical tests require trained personnel to operate the equipment and interpret the results.
Review of Drawings and Construction Data
To ensure that the concrete thickness and quality, as well as the installed reinforcement, meet or exceed the strength and serviceability requirements specified in the relevant building codes, it is necessary to review the original structural design and reinforcement placing, as well as other shop drawings. This review should be thorough and detailed, taking into account any actual applied loading and comparing it to the design loads specified in the relevant building codes. By doing so, any potential issues or deficiencies can be identified and addressed, ensuring that the structure is safe, durable, and able to perform its intended function over its expected service life.
Selection of Repair Procedures of Cracks in Concrete
Different objectives can be achieved through the careful evaluation of the extent and cause of concrete cracking. These objectives may include restoring or increasing strength and stiffness, improving functional performance, enhancing appearance, providing water tightness, improving durability, and preventing the development of a corrosive environment at the reinforcement.
Depending on the nature of the damage, one or more repair methods may be selected. For example, cracks may be injected with epoxy or other high-strength bonding agents to restore tensile strength. Additional reinforcement or post-tensioning may be necessary to provide further strength. Flexural stiffness can be restored by epoxy injection, provided that further cracking is not expected.
Cracks that cause leaks in water-retaining or storage structures should be repaired, unless the leakage is deemed minor or if the crack is sealed by autogenous healing. Repairs in such structures can be complicated due to the need to perform them while the structures are still in service.
Cracks in concrete may need to be repaired for cosmetic purposes. However, the crack locations may still be visible, requiring a coating over the entire surface. To prevent future deterioration due to corrosion, cracks exposed to a moist or corrosive environment should be sealed.
Several crack repair methods are available to achieve the objectives mentioned above. These methods are described in detail in the resource “METHODS OF CONCRETE CRACK REPAIR.”