Assessment of cracks in reinforced concrete by means of electrical resistance and image analysis
Introduction
The influence of cracks on the durability performance of concrete structures is still under research. Current design codes determine the maximum allowed crack width based on empirical studies and the influence of exposure conditions upon the structure. For marine environment or de-icing salt exposure, maximum crack values are between 0.15 to 0.30 mm [1], [2], [3]. Interestingly, crack width calculations may suggest that larger concrete covers result in larger surface crack widths, while a larger cover depth would generally be considered as improving durability. This contradiction discourages the design of concrete structures for durability performance. The condition assessment of cracked structures and their durability performance is currently based on the crack width at the concrete surface. Field engineers studying the condition of cracked structures are limited to crack width measurements at the concrete surface. So far, the relationship between crack width and durability performance remains unclear.
Current codes disregard the influence of cracks that remain undetected from the surface but that may influence the durability of a cracked concrete element. An experimental study performed by Goto [4] found that microcracks (secondary cracks) formed along the reinforcement when reinforced concrete specimens were subject to tensile stress. Since steel reinforcement is subject to tensile stresses when embedded in concrete, the presence of secondary cracks can be expected. Moreover, research studies have shown that defects in the concrete–steel interface could lead to deterioration mechanisms including reinforcement corrosion [5], [6], [7], [8]. If these secondary cracks are present, the service life of reinforced concrete structures can be reduced significantly. The influence of this type of crack in both corrosion of steel reinforcement [9], [10], [11] or chloride penetration [12], [13], [14] requires further studies.
Electrical resistance and its durability indicator counterpart, electrical resistivity, are parameters that are commonly related to transport properties of concrete. Measured values are usually between 101 to 105 Ω m, which are dependent on concrete composition, cement type, age and environmental conditions [15], [16], [17]. Since electrical current is carried by ions dissolved in the pore solution, electrical resistance/resistivity can be used as an indirect measure of changes in transport of electrical current due to moisture. Differences between moist and dry concrete can be of several orders of magnitude [18]. Recently, Reichling and Raupach [19] developed a technique that simulates the behaviour of electrical equipotential lines in a reinforced concrete element. For this, a Wenner array with different configurations was employed. This technique would allow detecting the presence of layers within the concrete with different resistance properties (e.g. different saturation levels).
In the work carried out by Boulay et al. [20], a cylindrical specimen was subject to tensile splitting while a reservoir of a conductive solution filled the crack. Results showed a linear correlation between the COD (crack opening displacement) at the surface of the specimen and electrical conductivity. While conductivity increased according to larger COD values, electrical resistance decreased. Lataste et al. [21] used concrete resistivity as a non-destructive technique for localising cracks in reinforced concrete. In their approach, tapered cracks were considered to behave similarly to several resistors in parallel to account for conductive behaviour of the crack. This approach accounts for a combination of isolating and partially conductive crack properties. On the other hand, the behaviour of electrical resistance can be modified by the presence of a conductive element such as reinforcing steel. Karhunen et al. [22] focused on conductive materials embedded in concrete, such as steel, and their influence under electrical current flow. Results show that equipotential lines were disturbed by the presence of conductive materials such as steel reinforcement, but also by the presence of non-conductive materials. Air-filled cracks represent a non-conductive medium for electrical current and thus studying the behaviour of electrical current in concrete can assess their influence.
In this paper, reinforced concrete specimens were subject to continuous monitoring of electrical current during tensile cracking. Modelling of the influence of cracks on electrical resistance is performed by a Lattice-model, which accounts for air-filled cracks that are considered to be non-conductive. Changes in the behaviour of electrical current under cracking in both experiments and the model are discussed. Subsequently, image analysis is applied on photographs obtained from cut sections containing the crack. By these means, a crack volume in the specimens is estimated. Finally, correlations between the estimated crack volume and both COD and the relative increase in electrical resistance are presented.
Section snippets
Materials and specimen fabrication
Reinforced concrete specimens were fabricated with ordinary Portland cement (CEM I 52.5R). In total, 4 specimens were made for cracking experiments. Two reinforcing steel bars of 120 mm in length and 12 mm in diameter were embedded at a cover depth of 60 mm as shown in Fig. 1a. The distance between reinforcing bars was 120 mm as shown in Fig. 1b. Before casting, a PVC profile with a cross section of 40 × 40 mm2 was mounted on the mould to obtain a recess. Two stainless steel electrodes were employed
Cracking of concrete specimens
Fig. 3 shows the results of the load–displacement curves obtained from the cracking of the four concrete specimens. In all four specimens, maximum compressive load values ranged between 2 and 2.5 kN. At this loading condition, the average displacement of the LVDTs is in the range of 20–30 μm (A). As the test continues, the maximum COD is obtained at different values for each specimen. Once the target COD is obtained, the unloading of the specimen is started and crack closing occurs. The values of
Concrete cracking and COD
Results found in this paper are similar to those obtained by Goto [4], where tensile cracks propagate along the reinforcement. The extent of the damage, described as the relative increase in electrical resistance or the estimated crack volume, has shown to be linearly correlated to the COD at the concrete surface.
Both measurements and modelling of electrical resistance have shown that cracks have a significant influence on electrical resistance between embedded electrodes. When considering
Conclusions
Reinforced concrete specimens containing Portland Cement were subject to cracking while COD and electrical resistance between embedded electrodes were monitored during loading and subsequent unloading. Later, the crack volume was estimated by image analysis of cut surfaces. Results showed that the presence and propagation of cracking influenced the electrical resistance. Image processing showed that the estimated crack volumes ranged between 43 and 86 ml, respectively. These values had a linear
Acknowledgements
Financial support by the Dutch Technology Foundation (STW) for the project IS2C 10978-Measuring, Modelling, and Monitoring Chloride ingress and Corrosion initiation in Cracked Concrete (M3C4) is gratefully acknowledged.
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