Degradation processes in reinforced concrete structures that affect durability are partially controlled by transport of aggressive ions through the concrete microstructure. Ions are charged and the ability of concrete to hold out against transfer of ions greatly relies on its electrical resistivity. Hence, a connection could be expected between electrical resistivity of concrete and the deterioration processes such as increase in permeability and corrosion of embedded steel. Through this paper, an extensive literature review has been done to address relationship between concrete electrical resistivity and its certain durability characteristics. These durability characteristics include chloride diffusivity and corrosion of reinforcement as these have major influence on concrete degradation process. Overall, there exists an inverse or direct proportional correlation between these parameters. Evaluated results, from measuring the concrete electrical resistivity, can also be used as a great indicator to identify early age characteristics of fresh concrete and for evaluation of its properties, determination of moisture content, connectivity of the micropores, and even condition assessment of in-service structures. This paper also reviews and assesses research concerning the influential parameters such as environmental conditions and presence of steel rebar and cracks on measuring electrical resistivity of concrete. Moreover, concrete resistivity concept, application, and its various measurement techniques are introduced.
The durability of concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other deterioration process to retain its original form, quality, and serviceability when exposed to harsh environment [
Over the last few decades, a great deal of attention has been paid to research and development of electrical resistivity measurement techniques as a nondestructive technique (NDT) to evaluate the durability of concrete structures. This method is becoming more popular especially for field evaluations due to its simplicity, rapidness, and cost during test conduction. However, the inclusion of these methods into the standards and guidelines is quite slow. Electrical resistivity has been standardized in 2012 by ASTM C1760 [
Electrical resistivity is a material property that can be used for various purposes, one of which is to identify early age characteristics of fresh concrete. When the fresh concrete sets and hardens, depercolation (discontinuity) of the capillary pore space leads to an increase in its electrical resistivity. Since electrical current is conveyed by dissolved charged ions flowing into the concrete pore solution, it is a good indicator of concrete pore structures [
Several researchers attempted to characterize the effects of various parameters on electrical resistivity measurements. One of the important factors affecting the measurements is environmental conditions such as temperature, rainfall, and relative humidity. During testing, good electrical connection between concrete and electrodes as well as specimen geometry plays a key role in having a reliable measurement. The electrical resistivity measurements are highly influenced by the moisture content of concrete. For instance, when the moisture content is reduced, the resistivity is increased significantly. Therefore, considering all these influencing parameters for on-site resistivity measurements and to make meaningful conclusions is not a simple task.
In this paper, the correlation between electrical resistivity and certain durability characteristics of concrete is discussed. These concrete characteristics include chloride permeability, corrosion rate, and compressive strength. Also, different approaches in the measurement of concrete resistivity including bulk and surface resistivity measurements are presented. This paper reviews the effect of several influencing parameters such as external environment (e.g., temperature) and concrete mixture on the electrical resistivity. In addition, some of bulk and surface resistivity test setups (both of laboratory and field tests) conducted by authors are also presented.
Electrical resistivity (
Electrical resistivity measurements can be performed in several ways nondestructively: using electrodes positioned on a specimen surface, or placing an electrode-disc or linear array or a four-probe square array on the concrete’s surface. Types of device techniques that can be used typically to measure resistivity physically include (1) bulk electrical resistivity test, (2) surface disc test, (3) Wenner four-point line array test, and (4) four-probe square array test.
In the bulk resistivity method (or uniaxial method), two electrodes are placed on the concrete surface (usually two parallel metal plates) with moist sponge in between (Figure
Electrical resistivity measuring techniques: (a) two-point uniaxial method and (b) four-point (Wenner probe) method (reproduced from [
The electrode-disc test method includes an electrode (disc) placed over a rebar and measuring the resistance between the disc and the rebar, as shown in Figure
Setup of one electrode-disc: measurement of concrete resistivity (reproduced from [
The Wenner probe technique was first introduced for the geologist’s field in order to determine soil strata by Wenner at the National Bureau of Standards in the 1910s and then modified through time for concrete application [
The four-probe square array consists of the four probes that are arranged in square position with spacing of 50 to 100 mm [
Electrical resistivity can be related to certain performance characteristics of concrete and can be used as a promising quality assurance tool for fresh or hardened concrete [
The primary objective of this paper is to review the existing state of practice on the electrical resistivity measurements technique. This paper also identifies the applicability and limitation of electrical resistivity method and reviews the correlation between resistivity and certain durability properties of concrete. Correlation between surface and bulk electrical resistivity and their applications is also discussed. Finally, key parameters affecting the electrical resistivity readings are identified for future research in the area.
An extensive literature search was undertaken from most relevant publications in the area. A comparison was made of the experimental setup (Section
Details of the specimen geometry (in terms of specimen size), material type, and number of specimens.
Refs | Specimen configuration | Materials type | w/b ratio | Rebar presence | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Type | Size (mm) | TNVM | Mix type | OPC | SLG | FA | MK | SF | Others | |||
[ |
Disk | Φ100 × 50 | 44 | CON/CEM | × | × | × | — | × (5% & 10%) | NP (30% and 50%) | 0.35, 0.4, 0.45 | N |
[ |
Slab | 254 × 76 × 12.7 | 18 | CON | × Type I | — | × | — | — | — | 0.26 | N |
[ |
Cylinder | Φ47 × 95 | 30 | CEM | × Type I | — | × (classes C & F) | — | — | CaCO3 (10% & 15%) | 0.35 | N |
[ |
Slab | 250 × 200 × 120 | 1 | CON | × | — | — | — | — | — | 0.5 | Y |
[ |
Slab, cube, cylinder | 300 × 300 × 135, |
6 | CON | × (blended) | — | × (20% blended) | — | — | — | 0.4 | Y |
[ |
Prism | 300 × 300 × 150 | 7 | CON | × Type I | — | × | — | — | — | 0.42 | Y |
[ |
Cylinder, prism | Φ100 × 200, |
NR | CON | × Type I | — | — | — | — | — | 0.4, 0.6 | Y |
[ |
Cylinder, prism | Φ150 × 300, |
NR | MOR | × | — | — | — | — | — | 0.5 | Y |
[ |
Prism | 400 × 400 × 100 | 6 | MOR | × | — | — | — | — | — | 0.6 | Y |
[ |
Cylinder | Φ150 × 220 | 4 | CON | × | — | — | — | — | — | 0.4, 0.6 | Y |
[ |
Slab, cube | 650 × 650 × 100, |
1 | CON | × | — | — | — | — | — | 0.5 | N |
[ |
Slab | 600 × 600 × 120 | 9 | CON | × | — | — | — | — | — | 0.36, 0.48, 0.61 | Y |
[ |
Beam | 1500 × 200 × 100 | 2 | CON | × | — | — | — | — | × | 0.7 | N |
[ |
Cylinder | Φ100 × 200 | 21 | CON | × Type I/II | × (grades 100 & 120, 50%) | × (classes C & F, 20%) | — | × (5% & 10%) | — | 0.35, 0.5, 0.65 | N |
[ |
Cylinder | Φ100 × 200 | 33 | CON | × Type I/II | — | × | — | × | — | 0.41 | N |
[ |
Cube | 150 × 150 × 150 | 47 | CON | × | × | — | — | — | — | 0.4–0.55 | N |
[ |
Prism | 100 × 100 × 170 | 12 | CON | × | × (50% and 70%) | — | — | — | WPC | 0.3, 0.42, 0.55 | N |
[ |
Cylinder | Φ150 × 300 | 3 | CON | × Type I | — | — | — | — | — | 0.45, 0.55, 0.65 | N |
[ |
Cylinder | Φ75 × 150 | NR | CON | × Type V | — | — | — | — | — | 0.45 | N |
[ |
Cube, cylinder, block | 100 × 100 × 100, |
NR | CON | × (blended) | — | × (18% blended) | — | — | — | 0.4 | N |
[ |
Cube | 150 × 150 × 150 | NR | CON | × | × | — | — | × | PFA | 0.59–0.7 | N |
[ |
Cylinder | Φ100 × 200 | 12 | CON | × | × | × | × | × | Micro-FA | 0.3–0.4 | N |
[ |
Cube | 150 × 150 × 150 | 33 | CON | × Type I & Type V | × | — | — | — | — | NR | N |
[ |
Cylinder | Φ100 × 200 | 19 | CON | × | × | × | — | × | — | 0.41 | N |
[ |
Slab, cylinder | 280 × 280 × 102, |
NR | CON | × | × | × | × | — | MS | 0.35–0.45 | N |
[ |
NR | NR | NR | MOR/CEM | × Type I | — | — | — | — | — | 0.42 | N |
[ |
Cylinder | Φ100 × 200 | 12 | CON | × | × | × | — | — | — | 0.41 | N |
[ |
Block | 300 × 300 × 200 | 3 | CON | × | × | — | — | — | PFA | 0.39, 0.4, 0.44 | Y |
[ |
Slab, cylinder | 610 × 610 × 152, |
10 | CON | × | × | × (class F) | — | — | MS | 0.43 | Y |
[ |
Prism | 1000 × 1000 × 300, |
NR | CON | × | — | × | — | — | — | 0.35–0.65 | Y |
[ |
Cylinder | Φ100 × 200 | NR | CON | × Type I/II | — | × (20%) | — | × (8%) | — | 0.4 | N |
[ |
Cylinder | Φ100 × 200 | 10 | CON | × | × (SC) | — | — | — | — | 0.45, 0.65 | Y |
[ |
Prism | 100 × 100 × 300 | 12 | CON | × | × | × | — | — | — | 0.4, 0.45, 0.5 | Y |
[ |
Cylinder, cube | Φ100 × 200, |
12 | CON | × | × | — | — | × | — | 0.25, 0.28, 0.35 | N |
[ |
Cylinder | Φ100 × 200 | 12 | CON | × | × | × | × | × | (Super fine fly ash) | 0.28–0.49 | N |
[ |
Cylinder | Φ100 × 200 | 11 | CON | × | × | × | — | — | — | 0.37–0.45 | N |
[ |
Cylinder | Φ100 × 200 | 343 | CON | × Type I/II | — | × (class C, 25%) | — | — | — | 0.42 | N |
[ |
Cylinder | Φ100 × 200 | 514 | CON | × | × | × (class F) | — | — | — | 0.41 | N |
[ |
Cylinder | Φ100 × 200 | 57 | CON | × Type I | — | — | × | × | RHA, NP | 0.4–0.6 | N |
[ |
Cube, slab | 100 × 100 × 100, |
10 | CON | × | × | — | (5%, 10%, 20%) | — | MS (5% and 10%) & PFA (30%) | 0.52 | N |
[ |
Disk | Φ100 × 50 | 6 | CON | × | × (30%) | — | — | × (10%) | — | 0.25, 0.28, 0.35 | N |
[ |
Cylinder | Φ150 × 300 | 24 | CON | × | × (50%) | — | — | 8% | NP (12%, 25%) | 0.28–0.6 | N |
[ |
Cylinder | Φ150 × 200 | 4 | CON | × | — | — | — | — | — | 0.4, 0.6 | Y |
[ |
Cylinder, prism | Φ102 × 178, |
NR | MOR | × | — | — | — | — | WPC | 0.42 | N |
[ |
Cylinder | Φ100 × 200 | 33 | CON | × Type II–V | (grades 100 & 120) | (classes C & F) | × | × | — | 0.44 | N |
[ |
Cylinder | Φ100 × 200, |
23 | CON | × Type II–V | × (grade 120) | (class F, 20%) | × | × | — | 0.44 | N |
Authors | Hollow section, cylinder | Inner dia. 152 and external dia. 304, |
18 | CON | × | × | — | × | — | CA (2%), PLC | 0.5 | Y |
Details of the reinforcements and measurement methods used to record corrosion rate.
Refs | Reinforcement | Cause of corrosion | Corrosion rate | ||||
---|---|---|---|---|---|---|---|
Φ (mm) | Length (mm) | Cover depth (mm) | Technique | Details | Correction for ohmic drop | ||
[ |
10 | 200 | 1, 10, 20 | Carbonation | — | — | — |
[ |
10 | 300 | 50 & 75 | Not studied (NS) (only effect of rebar presence on resistivity measurement was considered) | — | — | — |
[ |
16 | 300 | 50 | Cyclic ponding with sea water | — | — | — |
[ |
4 | 110, 160, 200 | Various (53.5–100) | NS | — | — | — |
[ |
8 | 40 | 80 | NS | — | — | — |
[ |
13, 19, 25 | 410 | 20, 30, 40 | NS | — | — | — |
[ |
10 | 250 | 150 | NaCl solution/marine exposure | LPR | Embedded steel rebar | N |
[ |
10 | — | 25 | NS | — | — | — |
[ |
16 | 200 | 50 | NaCl solution/marine exposure | — | — | — |
[ |
NR | NR | NR | NaCl solution | LPR | NR | NR |
[ |
NR | NR | 70 | NaCl solution | — | — | — |
[ |
16 | NR | 42 | NaCl solution | NR | Embedded steel rebar | NR |
[ |
8 | 150 | 10 or 30 | NaCl solution | LPR | Embedded CE & RE on the surface | N |
[ |
10 | 200 | 15 | NaCl solution | — | — | — |
Authors | 10 | 914 | 19–38 | NaCl solution | LPR | Embedded steel rebar | Y |
Details of the curing conditions, exposure conditions, and measurement period.
Refs | Curing conditions | Temperature (°C) | Exposure conditions | Measurement period (days) |
---|---|---|---|---|
[ |
Lime-saturated water tank | 20 (except one type mixture kept in water having 5, 20, 35 temperature) | Lime-saturated water tank/lab condition and oven dry state (only one type mixture) | 90 (except ten of the mixtures tested at age of 365 days) |
[ |
Lime-saturated water tank | 23 ± 1 | Lime-saturated water tank | 28 |
[ |
NR | 15, 26, 40 | Plastic wrapped | 1 |
[ |
NR | 20 | Laboratory (air dry) | 1000 |
[ |
Water tank | 20 | Water tank (except two slabs were kept in air after 7 days) | 30 |
[ |
NR | NR | Various | 120 |
[ |
Lime-saturated water tank | 23 | Various | 28 |
[ |
100% relative humidity in a chamber | 20 ± 2 | 100% relative humidity in a chamber | 28 |
[ |
Water tank | 20 | Water tank | 45 |
[ |
NR | NR | Seashore Exposure/immersed in saline solution | 1000 |
[ |
Plastic wrap | 20 | Room temperature | 90 |
[ |
NR | NR | NR | 28 |
[ |
Room temperature | 25 ± 2 | Room temperature | 28 |
[ |
Lime-saturated water tank | Laboratory | Lime-saturated water tank | 56 |
[ |
NR | 10–45 | Room condition with RH > 95% | 2,190 |
[ |
Water tank | 20 | Water tank | 181 |
[ |
Wet chamber with RH > 95% | 23 ± 2 | Wet chamber with RH > 95% | 91 |
[ |
Water tank | 23 ± 0.5, 105 ± 2 | Various | 28 |
[ |
Wet burlap | 20 | Oven dried and then water bath | 31 |
[ |
Water tank | 20 & 5 | Water tank (after 7 days, some cylinders subjected to air condition) | 30 |
[ |
Water tank | 20 ± 2 | Water bath | 720 |
[ |
Lime-saturated water tank | 23 ± 2 | Lime-saturated water tank | 91 |
[ |
Water tank | NR | Water tank | 365 |
[ |
Various | 21–45 | Various | 1,100–2,200 |
[ |
Lime-saturated water tank/wet burlap | NR | Lime-saturated water tank/wet burlap for 3 or 7 days | 91 |
[ |
Lime-saturated water tank | Various (10–45) | Various | 65 |
[ |
Various | Various | Various | 500 |
[ |
Wrapped in damp Hessian and stored under polythene tentage | 15–20 | Maine exposure | 140 |
[ |
Various | 18–32 | Salt ponding | 90 |
[ |
Water tank and laboratory air | 20 ± 2 | Actual tidal zone, wet and dry cycle | NR |
[ |
Five various curing regimes (tap water, NaCl solution, rog room) | Room | Various | 1500 |
[ |
Various | 20 ± 2/uncontrolled | Various | 90 |
[ |
Six days in fog room/three weeks in room condition | 20 | 26 weekly cycles of 24 h 3% NaCl solution penetration and drying for 6 days/after 30 weeks, various conditions | 52 |
[ |
Water tank | 25 | Water tank | 90 |
[ |
Moist room with a sustained 100% humidity | 23 | NaCl solution | 1,092 |
[ |
Lime-saturated water tank | 22 | Lime-saturated water tank | 56 |
[ |
Changing exposure | 23 ± 2 | Changing exposure | 90 |
[ |
Lime-saturated water tank | 21 or 36 | Various | 1000 |
[ |
Lime-saturated water tank | 23 ± 2 | Lime-saturated water tank | 28 |
[ |
Water tank for 14 days/14 days in drying cabinet at 40°C | 20 ± 1 & 40 ± 1 | Salt ponding (wet & dry cycle) | 270 |
[ |
Lime-saturated water tank | 25 | Lime-saturated water tank | 90 |
[ |
Chamber with RH > 95% | 21 | Chamber with RH > 95% | 28 |
[ |
Laboratory environment (kept in plastic bag) | 14–27, 3–13 | Seashore condition | 1,000 |
[ |
Water tank | 21 ± 3 | Water tank | 7 |
[ |
Lime-saturated water tank | NR | Lime-saturated water tank | 1 |
[ |
Lime-saturated water tank | NR | Lime-saturated water tank | 730 |
Authors | 14 days wet burlap and 14 days air dry | Uncontrolled | Natural environment and simulated seashore condition | 720 |
Details of the different measurement methods used in the literature.
Refs | Measurement technique | |||
---|---|---|---|---|
Concrete resistivity | Rapid chloride permeability test | Others | ||
Two-electrode | Four-point method | |||
[ |
× | — | — | Rapid chloride migration test (NT Build 492) & ASTM C1760 |
[ |
× | — | — | — |
[ |
× | — | — | — |
[ |
× | × | — | Multiring electrodes |
[ |
× | × | — | NT Build 492 |
[ |
— | × | — | — |
[ |
— | × | — | — |
[ |
× | × | — | — |
[ |
— | × | — | — |
[ |
— | × | — | Steel potential |
[ |
× | × | — | — |
[ |
— | × | — | Ultrasonic Pulse Velocity |
[ |
— | × | — | Electric imaging |
[ |
— | × | × | — |
[ |
— | × | — | — |
[ |
× | — | — | NT Build 492 |
[ |
— | × | — | — |
[ |
— | × | — | — |
[ |
× | — | — | — |
[ |
× | × | — | NT Build 492 |
[ |
× | — | — | — |
[ |
× | × | — | — |
[ |
× | × | — | — |
[ |
— | × | — | — |
[ |
× | × | × |
— |
[ |
× | × | — | — |
[ |
— | × | — | — |
[ |
× | — | — | — |
[ |
× | × | × |
Half-cell potential |
[ |
× | × | — | — |
[ |
— | × | — | — |
[ |
— | × | — | — |
[ |
× | — | — | Steel corrosion potential |
[ |
× | — | — | NT BUILD 492 |
[ |
— | × | — | ASTM C1556-04 |
[ |
× | × | × |
— |
[ |
— | × | × | KDOT Boil Testing |
[ |
— | × | — | Bulk diffusion test (NT Build 443), NT Build 492 |
[ |
— | × | × | Water Penetration Depth |
[ |
— | — | — | Resistivity using disc method (one external electrode) |
[ |
× | × | — | NT Build 492 |
[ |
— | × | — | Natural diffusion test (90 days) |
[ |
— | × | — | Half-cell potential method |
[ |
× | × | — | — |
[ |
× | × | — | — |
[ |
× | × | — | — |
Authors | × | × | × | UPV, half-cell potential, infrared camera |
In this section, experimental setups developed by other researchers have been summarized in Tables
Frequently, in the electrical resistivity studies, samples with dimensions between 100 and 400 mm were used (Table
According to data in Table
The specimens were cured and exposed to various and/or changing conditions over the testing period (Table
Either two-electrode or four-point electrode (Wenner probe setup) techniques were employed to record concrete electrical resistance, which is then converted into resistivity by multiplying it with an appropriate geometrical factor. The limitations of 2-electrode method resulted in using Wenner probe configurations in most studies specially for field investigations. In experimental studies that attempted to find correlation between concrete electrical resistivity and its durability parameters, other destructive and nondestructive testing techniques from standardized measuring protocol including Rapid Chloride Permeability (RCP) test, Rapid Chloride Migration (RCM) test, Bulk Diffusion (BD) test, and Ultrasonic Pulse Velocity (UPV) were employed. Authors ongoing work also employs use of both 2-electrode and 4-electrode techniques as well as UPV technique.
In summary, the experimental setup can have a significant effect on the electrical resistivity measurements. Specifically, the measurement methods and environmental conditions comprise a variety of parameters affecting the obtained data. The geometry of specimen and the general setup have a minor influence on the recorded resistivity values. To investigate electrical resistivity, the material, curing condition, and exposure condition should be carefully selected. Simulating the real-world conditions is in any case desirable since the recorded data can be used later as input in prediction models. As field survey data is rarely reported in the reviewed studies, it seems critical to identify possible deviations between laboratory investigations and field conditions. Also, authors’ focus currently is to find these deviations to fill this knowledge gap by measuring concrete resistivity on both laboratory-size and field-size specimens.
In the following sections, investigated parameters in published literature that influences the electrical resistivity readings have been discussed. For simplicity, they have been divided into two subgroups: (1) factors affecting the intrinsic electrical resistivity of concrete and (2) factors affecting the electrical resistivity measurements.
Generally, water to cement (w/c) ratio is one of the main factors contributing in permeability of concrete and its properties. Higher w/c ratio results in a high percentage of porosity (more voids) and leads to a lower electrical resistivity value indicating a more permeable concrete [
In general, aggregates depending on their location and size have a higher electrical resistivity compared to hardened cementitious paste because they have less porosity; thus electrical current can easily flow through the pore system of the paste. Hence, a number of researchers attempted to investigate aggregates’ effect on electrical resistivity measurements. The experimental study performed by Sengul [
As reported in the Sengul study [
The resistivity evolution of concrete is affected by the curing regimes [
A number of researchers have been exploring the effect of embedded rebar presence on concrete electrical resistivity through experimental and numerical investigation. Theoretically, electrical current fluxes take pathways having the least amount of resistivity and when there is embedded rebar in concrete, the current field is distorted. However, the alternation in current field is dependent on many factors such as orientation of rebar with respect to the probe, rebar diameter and spacing, and depth at which it is located [
Millard [
Practical general guidelines were developed by Polder’s work [
Resistivity using four electrodes at various spots in the same area to minimize influence of rebars [
Another similar experimental investigation, done by Sengul and Gjorv [
Five Wenner probe configurations with respect to embedded rebar tested by Sengul and Gjorv [
Presuel-Moreno et al. [
Salehi et al. [
Probe configuration with respect to rebar mesh suggested to reduce electrical resistivity measurement error [
For cylindrical concrete specimens with a single embedded steel rebar, study conducted by Chen et al. [
The effect of rebar presence on mortar electrical resistivity conducted by the four-point Wenner method was also investigated numerically and experimentally by Garzon et al. [
The last and recent study in this category belongs to Sanchez et al. [
Due to the presence of cracks, apart from embedded rebar, the electrical resistivity measurements may vary using Wenner probe technique because it is initially assumed that concrete is homogeneous and isotropic with semi-infinite geometry. In this section, some researchers’ investigations in order to characterize cracks in concrete conducted by electrical resistivity measurements are summarized.
Lataste et al. [
Four-probe square array principle [
A couple of assumptions and conclusions in Lataste et al.’s work [
Goueygou et al. [
Experimental study conducted by Wiwattanachang and Giao [
Electrical resistivity image of a concrete beam with cracks [
It was also numerically found that when cracking and delamination were present in reinforced concrete structures, electrical resistivity measurements were different from when they were not present [
A comprehensive numerical study by Salehi et al. [
Concrete is considered to be a heterogenous material and this is one of the assumptions behind the Wenner probe method. However, this assumption appears to be a likely source of error since aggregates inside concrete typically have greater resistivity and they propagated widely in different locations with various sizes. Hence, this inconsistency in the initial assumption of concrete homogeneity may affect resistivity measurements. To mitigate this issue, some researchers recommended considering enough wide space between electrodes (usually between 20 mm and 70 mm) in order to reduce the influence of nonhomogeneity due to the aggregates presence [
One recommendation to help reduce variance in resistivity measurements is to consider probe spacing 1.5 times higher than the maximum aggregate size [
Effect of contact spacing on resistivity measurement [
According to Polder [
The contact area between the electrodes and concrete surface may alter the electrical resistivity measurements using four-point Wenner probe. The experimental investigation in electrolytic tanks and finite element modeling resulted in the fact that the contact between the concrete surface and the probes has no significant influence on Wenner probe resistivity measurements [
In two-electrode method (bulk resistivity measurements), poor contact between the plate electrode and the test cylinder surface is mainly responsible for electrode resistance. One solution to minimize the contact resistance effect is to use flexible electrodes [
In the four-point Wenner method, the electrical resistivity measurements are initially presumed to be performed on the domain of semi-infinite medium which is not a practically accurate assumption. This assumption leads to deviation from the ideal condition of having infinitely large geometry which can possibly occur in different electrode orientations. For relatively small size concrete elements (e.g., cylinder or prism specimens), constriction of current to flow into a different field pattern is one of the major reasons for this deviation. Even though several researchers have realized the effect of specimens’ geometry, only very limited information is available on this topic.
To account for interference between current flow and coarse aggregates in a small size sample, a suggested correction coefficient factor (or geometry correction factor) has been established in Spragg et al.’s work [
For a standard cylinder size (
Cell constant correction to determine the concrete resistivity [
According to Millard [
Due to resistor-capacitor circuits’ behaviour in saturated cementitious system, it introduces a phase difference between electrical current imposing and the measured potential (impedance) [
Schematic representation of the AC Impedance response of concrete [
Two signal shapes used usually for electrical resistivity measurements are sine-wave and square-wave AC current. Ewins [
Temperature variation has been reported to have a significant influence on electrical resistivity of concrete, and an increase in temperature results in a decrease in resistivity. Electrical current generally flows through the ions dissolved in the pore solution and can be affected by temperature which causes changing the ion (Na+, K+, Ca2+,
Studies conducted by Millard et al. [
It was found that the above equation is only applicable over a limited interval of ±5°C to the reference temperature [
Several researchers also defined a wide agreement for this correlation using Arrhenius law [
The activation energy of conduction (
Moisture content is one of the influencing factors that can inversely affect the concrete electrical resistivity measurements. Essentially, electrical conductivity increases with an increase in moisture content due to change in the ion mobility [
Through following sections, relationship between electrical resistivity and the two main concrete durability characteristics including chloride ingress and corrosion of embedded reinforcements will be presented.
Diffusivity is the controlling parameter which determines the time it takes for chloride ions to diffuse into concrete and reach the critical chloride threshold for corrosion initiation. Typically, this can be measured through Rapid Chloride Migration (RCM) test, Rapid Chloride Permeability Test (RCPT), or Bulk Diffusion (BD) method [
In theory, the relationship between diffusivity of ion species
Several researchers have also conducted various experiments to investigate the relationship between concrete resistivity and chloride diffusivity. A strong correlation between these two parameters has been reported for various concrete mixtures at different ages [
Classification of concrete permeability to surface resistivity values.
RCP test versus surface resistivity | |||||
---|---|---|---|---|---|
Chloride ion permeability | RCP test charged passed (Coulombs) | Surface Resistivity Test | |||
|
|
Semi-infinite slab |
KDOT |
||
High | >4000 | <12 | <9.5 | <6.7 | <7.0 |
Moderate | 2000–4000 | 12–21 | 9.5–16.5 | 6.7–11.7 | 7.0–13.0 |
Low | 1000–2000 | 21–37 | 16.5–29 | 11.7–20.6 | 13.0–24.3 |
Very low | 100–1000 | 37–254 | 29–199 | 20.6–141.1 | 24.3–191 |
Negligible | <100 | >254 | >199 | >141.1 | >191 |
Similar experimental investigation conducted by the Louisiana Transportation Research Center (LTRC) also found a good correlation between resistivity and RCP test while the classified concrete permeability to surface resistivity values are equal to those proposed by FDOT as shown in Table
In addition to the experimental study on laboratory specimens, the correlation between electrical resistivity and apparent diffusivity coefficients (
A good correlation between RCM coefficients and electrical resistivity measured by two-electrode method was reported in European Union-Brite EuRam III experimental investigation [
Once rebar is depassivated and corrosion is initiated by chloride ions or carbonation, corrosion rate will be the most influential parameter that determines how fast the reinforced concrete structure is deteriorating. It is dependent on many parameters including oxygen availability, ratio of anodic/cathodic area, relative humidity (RH), and concrete electrical resistivity (Figure
Schematic descriptions of factors which may affect corrosion rate of steel in concrete: (i) O2 availability and (ii) electrical resistance of concrete (reproduced from [
Among all the studies conducted so far, there is an agreement that corrosive environment in reinforced concrete and electrical resistivity of concrete have an inverse relationship. As the electrical resistivity of concrete decreases, the rate of steel reinforcement corrosion increases. A theory by Glass et al. [
Combining both half-cell potential and electrical resistivity measurements techniques makes it possible to examine corrosion probability and corrosion rate once it is initiated [
Assessment of corrosion probability in concrete slabs through half-cell potential and resistivity measurements [
A number of researchers as well as commercial Wenner probe instrument manuals (Proceq and Giatec Scientific Inc. [
Concrete resistivity and risk of corrosion of steel reinforcement.
Corrosion risk | Resistivity values (kΩ·cm) | ||
---|---|---|---|
Polder [ |
Song and Saraswathy [ |
Commercial Wenner probe instrument manuals [ | |
High | <10 | <5 | ≤10 |
Moderate | 10–50 | 5–10 | 10–50 |
Low | 50–100 | 10–20 | 50–100 |
Negligible | >100 | >20 | ≥100 |
A linear relationship between concrete electrical conductivity and corrosion rate has been found in several articles [
Two other similar models have been proposed by DuraCrete R17 [
In most cases,
In summary, it can be concluded that still large range and scatter exist for correlation between corrosion rate and concrete resistivity. Also, effect of moisture state and temperature as well as corrections to corrosion rate measurements should be considered during an investigation on finding correlation between resistivity and corrosion rate. Knowledge is still lacking in the literature to understand which mechanism dominates the corrosion process and how resistivity measurements are impacted. To practically determine the corrosion and resistivity relationship, more field data should be collected and analyzed. In order to address some of these issues, authors of this paper initiated a project with field specimens as shown in Figure
Instrumented circular hollow columns being studied by authors to establish the relationship between electrical resistivity and durability characteristics.
One of the most important mechanical properties of concrete is compressive strength. It can be simply measured by compression testing machine, as load at the failure divided by area of specimen gives the compressive strength of concrete [
As it is discussed in previous sections, every electrical resistivity (
Theoretically, the ratio of surface and bulk resistance for standard size cylinder specimen (100 mm × 200 mm) and probe spacing of
The ratio of two different resistivity types (
Also, a number of researchers experimentally attempted to study this correlation between bulk and surface resistivity data. Studies performed by Ghosh and Tran [
Both surface and bulk resistivity measurement methods have been used to determine the presence of a heterogeneity problem [
Schematic of the (a) parallel and (b) series models of heterogeneous systems (reproduced from [
In summary, the coefficient of determination value (
Coefficient of determination (COD) value for linear trend between bulk and surface resistivity in the literatures.
Reference | Coefficient of determination value ( |
---|---|
Sengul and Gjorv [ |
0.99 |
Sengul and Gjorv [ |
0.99 |
Spratt et al. [ |
0.9986 |
Ghosh and Tran [ |
0.82–0.95 |
Gudimettla and Crawford [ |
0.98 |
Authors’ work (2016) | 0.979 |
Through an extensive literature review, this paper identifies several factors which might have potential influence on the electrical resistivity of concrete. Effect of each parameter is briefly summarized below. In agreement with most studies, when there is an embedded rebar in the concrete, the electrical current field is distorted, and thus errors can result in the electrical resistivity measurements. To minimize this effect, it is suggested to place all electrodes perpendicular to the embedded rebar on the concrete surface and take at least five measurements, each a few millimetres in distance from one another. Also, a correction factor should be applied to resistivity measurements once rebar is present in concrete. However, effect of rebar presence on the resistivity measurements is well-understood; just a few studies could be found to identify the rebar presence effect and more field investigations are still needed in this area. Presence of cracks in concrete was also identified as an influential parameter on electrical resistivity since it is initially presumed that concrete is homogenous, isotropic, and uncracked. Depth of crack, orientation of probes on crack, and type of crack (conductive or isolated) can individually affect the resistivity readings. As suggested for embedded rebar in the concrete, all electrodes should be placed in perpendicular direction to cracks. It was reported that higher resistivity readings were obtained from conductive cracks whereas lower resistivity values were gained for insulated cracks. However, no information is provided to show how much cracking induced by corrosion influences concrete resistivity. In addition when both insulated and conductive cracks are bridged together, their integrated influence on electrical resistivity is not well-understood. The moisture state and temperature of concrete during resistivity measurements were also found to be of major influence on recorded data. As temperature increases, the ions mobility becomes faster; consequently, electrical conductivity of concrete also increases. To lessen the temperature effect on resistivity results, no practical correlation is still published for real-world conditions. Essentially, electrical resistivity reduces with an increase in moisture content as a result of changing in the ion movement. It is strongly recommended to take resistivity measurements when concrete is in Saturated Surface Dry (SSD) condition. Yet, more investigations are required to understand how much time is needed for water to infiltrate the concrete to obtain constant moisture level through the bulk sample. The studies related to resistivity measurement test device confirmed that electrical signal shape and frequency, electrode contact with concrete surface, and probe spacing of surface resistivity measurement device can affect the resistivity results. To minimize the effect of signal shape and frequency, using low frequency range square-wave signal for surface resistivity and high frequency range for bulk resistivity is recommended. Proper contact should also be provided to not mislead resistivity readings. Using a saturated sponge between electrodes and concrete surface can reduce this effect. In 4-point Wenner probe method, as it is assumed that concrete is homogenous material, aggregates inside concrete also affect its homogeneity due to their higher resistivity. Therefore, providing enough wide spaces between electrodes is essential to diminish aggregates effect. As a rule of thumb, probe spacing 1.5 times higher than the maximum aggregate size should be considered. Aggregate content and type were identified to have an influence on concrete resistivity. Increase in aggregate content results in higher resistivity values due to their less porosity and lower electrical conductivity. Also, aggregates with rough surface texture were found to have higher resistivity as their tortuosity is higher. Therefore, the effect of aggregates content and type should be accounted for in resistivity measurements. Carbonation process in aged concrete forms a multilayered system that results in various resistivity values through the concrete depth. So, its effect should be mitigated during resistivity measurements. Lower resistivity is also generated when w/b ratio is high due to higher percentage of porosity.
Correlation between concrete electrical resistivity and its certain durability characteristics such as chloride diffusivity, compressive strength, and corrosion potential/rate was discussed in this paper and is summarized below. Concrete resistivity is inversely related to chloride ingress, where lower resistivity indicated the area where chloride diffusion will be faster. A retardation of chloride can be taken into account through the introduced reaction factor ( Furthermore, a strong correlation can be found between increasing electrical resistivity of concrete and the corrosion rate. The relationship can be seen when corrosion has initiated (active conditions). It will not be valid in the case of saturated concrete, where although the resistivity is low, the corrosion rate will be small because of lack of oxygen. Field data was considered in just a few investigations and, thus, it is of high interest to gather more field experience. In addition, concrete compressive strength and its electrical resistivity have a direct relationship with each other as both directly depend on the porosity of the matrix at early age. However, at a later age, the conductivity of the pore solution and the degree of concrete saturation both influence this relationship. No practical relationship was identified in the literatures between compressive strength and electrical resistivity. Four common measurement electrode geometries that have been employed in several studies to conduct electrical test on cementitious cylinders were introduced in this article. Among them, a strong direct linear correlation between two common methods of resistivity measurements (concrete surface and bulk resistivity) was presented. However, more attention should be paid to multilayered cementitious composite systems in the field as electrical current flows differently in these two techniques and for the types of layered electrical properties that can happen because of moisture gradients, chemical changes, and ionic gradients.
Concrete
Cement paste
Mortar
Metakaolin
Silica fume
Rice Husk Ash
Ordinary Portland Cement
Slag
Fly ash
Natural pozzolan
Ultrasonic Pulse Velocity
White Portland Cement
Pulverized-Fuel Ash
Microsilica
Linear Polarization Resistance technique
Total Number of Various Mixtures
Water/binder ratio
Not reported
Counter electrode
Reference electrode
Slag cement
Crystalline admixtures
Portland Limestone Cement.
The authors declare that there are no conflicts of interest regarding the publication of this paper.