The Constructor

Evaluation of Cracks in Concrete to find Location and Extent of Cracking

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When anticipating repair of cracks in concrete, it is important to first evaluate cracks in concrete to identify the location and extent of cracking. It should be determined whether the observed cracks are indicative of current or future structural problems, taking into consideration the present and anticipated future loading conditions. The cause of the cracking should be established before repairs are specified. Drawings, specifications, and construction and maintenance records should be reviewed. If these documents, along with field observations, do not provide the needed information, a field investigation and structural analysis should be completed before proceeding with repairs. The causes of cracks are discussed here. A detailed evaluation of observed cracking can determine which of those causes applies in a particular situation. Cracks need to be repaired if they reduce the strength, stiffness, or durability of the structure to an unacceptable level, or if the function of the structure is seriously impaired. In some cases, such as cracking in water-retaining structures, the function of the structure will dictate the need for repair, even if strength, stiffness, or appearance are not significantly affected. Cracks in pavements and slabs-on-grade may require repair to prevent edge spalls, migration of water to the sub grade, or to transmit loads. In addition, repairs that improve the appearance of the surface of a concrete structure may be desired.

Determination of location and extent of Cracks in Concrete

Location and extent of cracking, as well as information on the general condition of concrete in a structure, can be determined by both direct and indirect observations, nondestructive and destructive testing, and tests of cores taken from the structure. Information may also be obtained from drawings and construction and maintenance records.

Direct and indirect observation of Concrete Cracks

The locations and widths of cracks should be noted on a sketch of the structure. A grid marked on the surface of the structure can be useful to accurately locate cracks on the sketch. Crack widths can be measured to an accuracy of about 0.001 in. (0.025 mm) using a crack comparator, which is a small, hand-held microscope with a scale on the lens closest to the surface being viewed (Fig. 1).

Fig.1: Comparator for Measuring Width of Cracks in Concrete

Crack movement can be monitored with mechanical movement indicators of the types shown in Fig. 2.2. The indicator, or crack monitor, shown in Fig. 2.2 (a) gives a direct reading of crack displacement and rotation. The indicator in Fig. 2.2 (b) (Stratton et al. 1978) amplifies the crack movement (in this case, 50 times) and indicates the maximum range of movement during the measurement period.

Fig.2: Monitoring Crack Movement in Concrete

  Sketches can be supplemented by photographs documenting the condition of the structure at the time of investigation. Guidance for making a condition survey of concrete in service is given in ACI 201.1R, ACI 201.3R, ACI 207.3R, ACI 345.1R, and ACI 546.1R.

Nondestructive testing of to Determine Concrete Cracks

Nondestructive tests can be made to determine the presence of internal cracks and voids and the depth of penetration of cracks visible at the surface. Tapping the surface with a hammer or using a chain drag are simple techniques to identify laminar cracking near the surface. A hollow sound indicates one or more cracks below and parallel to the surface. The presence of reinforcement can be determined using a pachometer (Fig. 3) (Malhotra 1976). A number of pachometers are available that range in capability from merely indicating the presence of steel to those that may be calibrated to allow the experienced user a closer determination of depth and the size of reinforcing steel. In some cases, however, it may be necessary to remove the concrete cover (often by drilling or chipping) to identify the bar sizes or to cerebrate cover measurements, especially in areas of congested reinforcement.

Fig.3: Pachometer - Reinforcement Bar Locator in Concrete

If corrosion is a suspected cause of cracking, the easiest approach to investigate for corrosion entails the removal of a portion of the concrete to directly observe the steel. Corrosion potential can be detected by electrical potential measurements using a suitable reference half cell. The most commonly used is a copper-copper sulfate half cell (ASTM C 876; Clear and Hay 1973); its use also requires access to a portion of the reinforcing steel. With properly trained personnel and careful evaluation, it is possible to detect cracks using ultrasonic nondestructive test equipment (ASTM C 597). The most common technique is through-transmission testing using commercially available equipment (Malhotra and Carino 1991; Knab et al. 1983). A mechanical pulse is transmitted to one face of the concrete member and received at the opposite face, as shown Fig. 4. The time taken for the pulse to pass through the member is measured electronically. If the distance between the transmitting and receiving transducers is known, the pulse velocity can be calculated. When access is not available to opposite faces, transducers may be located on the same face [Fig. 4(a)]. While this technique is possible, the interpretation of results is not 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, the higher the pulse velocity, the higher the quality of the concrete. The interpretation of pulse velocity test results is significantly improved with the use of an oscilloscope that provides a visual representation of the received signal [Fig. 4(b)].

Fig.4 : Ultrasonic Testing of Concrete Cracks

Tests on Concrete Cores to Evaluate Cracks in Concrete

Significant information can be obtained from cores taken from selected locations within the structure. Cores and core holes afford the opportunity to accurately measure the width and depth of cracks. In addition, an indication of concrete quality can be obtained from compressive strength tests; however, cores that contain cracks should not be used to determine concrete strength. Ultrasonic equipment should be operated by a trained person, and the results should be evaluated cautiously by an experienced person, because moisture, reinforcing steel, and embedded items may affect the results. For example, with fully saturated cracks, ultrasonic testing will generally be ineffective. In some cases, it is difficult to discern between a group of close cracks and a single large crack. An alternative to through-transmission testing is the pulse-echo technique in which a simple transducer is used to send and receive ultrasonic waves. It has been difficult to develop a practical pulse-echo test for concrete. Petrographic examinations of cracked concrete can identify material causes of cracking, such as alkali reactivates, cyclic freezing damage, "D" cracking, expansive aggregate particles, fire-related damage, shrinkage, and corrosion. Petrography can also identify other factors that may be related to cracking such as the water-to-cement ratio, relative paste volume, and distribution of concrete components. Petrography can frequently determine the relative age of cracks and can identify secondary deposits on fracture surfaces, which have an influence on repair schemes. Chemical tests for the presence of excessive chlorides indicate the potential for corrosion of embedded reinforcement.

Review of Drawings and Construction Data

The original structural design and reinforcement placing or other shop drawings should be reviewed to confirm that the concrete thickness and quality, along with installed reinforcing, meets or exceeds strength and serviceability requirements noted in the governing building code(s). A detailed review of actual applied loading compared to echo technique. design loads should get.

Selection of Repair Procedures of Cracks in Concrete

Based on the careful evaluation of the extent and cause of cracking, procedures can be selected to accomplish one or more of the following objectives: Depending on the nature of the damage, one or more repair methods may be selected For example, tensile strength may be restored across a crack by injecting it with epoxy or other high strength bonding agent. However, it may be necessary to provide additional strength by adding reinforcement or using post-tensioning. Epoxy injection alone can be used to restore flexural stiffness if further cracking is not anticipated (ACI 503R). Cracks causing leaks in water-retaining or other storage structures should be repaired unless the leakage is considered minor or there is an indication that the crack is being sealed by autogenous healing. Repairs to stop leaks may be complicated by a need to make the repairs while the structures are in service. Cosmetic considerations may require the repair of cracks in concrete. However, the crack locations may still be visible and it is likely that some form of coating over the entire surface may be required. To minimize future deterioration due to the corrosion of reinforcement, cracks exposed to a moist or corrosive environment should be sealed. The key methods of crack repair available to accomplish the objectives outlined are described in METHODS OF CONCRETE CRACK REPAIR.
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