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ELECTRICAL RESISTIVITY TESTING FOR AS-BUILT CONCRETE
PERFORMANCE ASSESSMENT OF CHLORIDE PENETRATION
RESISTANCE
Rob B. Polder (1,2), Willy H.A. Peelen (1)
1) TNO Structural Reliability, Delft, The Netherlands
2) Delft University of Technology, Materials & Environment, Delft, The Netherlands
Abstract
The electrical resistivity of concrete can provide information about its transport properties, which is relevant for durability performance. For example, resistivity is inversely proportional to chloride diffusion, at least within similar concrete compositions.
A methodology is proposed for on-site assessment of concrete cover resistance against chloride penetration, based on on-site resistivity testing. As such, resistivity testing can extend existing service life approaches to assessing on site performance. For example, the Dutch Guideline for Service life design of structural concrete (in chloride contaminated
environment) is based on chloride transport testing in the prequalification stage and
production control by resistivity testing of wet-cured control cubes. Adding on-site resistivity testing would extend this approach with testing for as-built quality of the concrete cover.
Applying this method requires that corrections are made for the effects of reinforcing bars, inhomogeneities due to drying out and resistivity increase due to cement hydration. A
correction for the presence of reinforcement can be obtained by numerical modelling based on combined (simultaneous) cover depth and resistivity measurements. Effects of hydration and drying out can be accounted for using long-term resistivity data for concrete with different cement types and water/cement ratios in different moisture conditions.
1. INTRODUCTION
Performance based service life design of concrete has been introduced by DuraCrete in the late 1990s (DuraCrete 2000), was further described by Gehlen (Gehlen 2000) and was widely published in the fib model code for service life design (fib 2006). These publications provided a framework and methods for probabilistic, model based design of reinforced concrete
structures for a predetermined time to initiation of corrosion due to chloride ingress or
carbonation. Limiting ourselves here to chloride ingress, such predictions require information on the aggressive load (chloride surface content), the rate of transport (diffusivity), the
concrete cover depth to the steel (the transport distance) and the chloride level initiating reinforcement corrosion (the critical chloride content). The load is dictated by the environment; it cannot significantly be influenced by concrete parameters in the design (additional preventive measures such as hydrophobic treatment are ignored here). In the 1990s the critical chloride content was too difficult a problem to be addressed as a variable that could be influenced by the design. Even today, the influence of concrete variables on the
Page 2 critical chloride content is unclear. Thus, the design method for a predetermined serviced life was focused on the chloride transport. The concrete to be used should be tested in the
laboratory for its chloride migration coefficient. A mix considered suitable in combination with a particular cover depth would then be used for the structure. Realising that concrete produced in a plant may not have the same properties as concrete tested in the laboratory, a method for quality control was proposed based on electrical resistivity testing on wet cured cubes used for strength control (at 28 days age). The method involved placing two steel plates on opposite sides of a cube, hence called the Two Electrode Method (TEM). Obviously, a correlation is required between the chloride migration coefficient and the resistivity of the concrete. The existence of such a general correlation has been reported by various studies (Andrade 1994, Polder 1997, Polder & Peelen 2002). The correlation between inverse resistivity measured on wet-cured cubes and Rapid Chloride Migration values from a wide range of concretes (differing by binder type, w/c and age) is shown in Figure 1. The resistivity method for production control was used in a number of cases, apparently with success, as reported by a.o. (Rooij et al. 2007). A recent simplified, semi-probabilistic service life design method also proposes resistivity testing of water cured cubes for quality control (Polder et al. 2011, Wegen et al. 2012).
Figure 1 Correlation of inverse resistivity measured on wet-cured cubes with Rapid Chloride Migration results for a range of concretes with different binders and w/c at different ages (indicated in the legend in days)
Page 3 An alternative methods for service life design has been based on concrete resistivity only, taking into account chloride transport and chloride binding (Andrade 2004, Andrade et al. 2014). It will be clear that such a resistivity based service life design method would also benefit from a method for on-site control of resistivity.
This paper intends to take quality control one step beyond production control on wet cured cubes: to test the quality of concrete as realised in the field. Obviously, the real concrete cover may not have the same quality as wet cured cubes for strength testing: its quality should be tested in the structure. A method for on-site quality control is proposed here. Despite the fact that no results can be given yet, it appears to the authors that the approach is feasible.
2. RESISTIVITY TESTING FOR ON-SITE PERFORMANCE ASSESMENT
In order to use on-site resistivity testing for “as built” quality control with confidence, several issues need to be solved. First and foremost, a suitable resistivity testing method must be available. In principle, using a Wenner probe allows completely non-destructive resistivity testing (Polder et al. 2000, Polder 2001). However, besides transport properties of the
concrete with regard to chloride also other phenomena affect the Wenner measurement values. These phenomena are:
- The interface contact between the Wenner probe electrodes and the concrete - The presence of reinforcing bars.
- Moisture gradients in the concrete cover - Concrete temperature
- The effect of cement hydration; as testing may be carried out several months after casting, cement hydration will increase resistivity over time, which complicates comparison to a reference value established at 28 days.
These issues are addressed here consecutively.
The Wenner method comprises placing four equally spaced point electrodes on the concrete surface, applying a current to the outer two electrodes and measuring the potential difference between the two inner electrodes. The method was derived from soil resistance measurement (Wenner 1916). It presumes a current and potential distribution based on a semi-infinite volume with homogeneous resistivity. For measuring resistivity of concrete alternating current (AC) is applied to avoid electrode polarisation effects. The interfacial electrical resistance between the Wenner electrodes and the concrete surface may have a large effect on the measured values. In the measurement procedure this is accounted for by wetting the concrete surface. However, this remains an issue, since it is difficult to wet the surface in a reproducible way. This can be solved by a more elaborate measurement procedure, or by adapting the electrodes, e.g by applying an hydrogel interfacial layer.
Even if the concrete would have a homogeneous resistivity (but see below), reinforcing bars affect the electrical field because they have a much lower resistivity. In the RILEM recommendation for resistivity measurement, it was recommended to measure with the electrodes at different positions with regard to a rebar mesh, as indicated in Figure 2 (Polder et al. 2000). However, for service life control on site the concrete cover depth to the steel must be measured, as it is an important parameter in the design. Consequently, the
distribution of reinforcing bars will be known (their general position and diameters should be known from structural design). The cover depth and position of rebars can be relatively easily
Page 4 measured using modern scanning cover meters. Once the position of rebars is known,
numerical modelling can be used to investigate the influence on the electrical field applied for the Wenner test and derive the resistivity by Finite Element Modelling, e.g. using COMSOL Multiphysics. Recent work suggests that resolving rebar effects is feasible by what is called Electrical Resistivity Tomography, using multiple electrode arrangements (Reichling et al., 2012; 2014). Other recent studies have attempted to account for rebar presence by correction factors (Garzon et al. 2012). In fact, modelling the influence of rebars is a simple case of modelling of macro cell corrosion (Redaelli et al. 2006) or of cathodic protection current flow in concrete (Polder & Peelen 2009, Polder et al. 2009). For the present purpose, taking a large number of Wenner measurements on multiple positions, while knowing where rebars are located, should provide sufficient information to resolve the influence of reinforcing bars.
Figure 2 Measuring Wenner resistivity with the electrodes at different positions (A, B, C) with regard to a rebar mesh
Another issue to be solved is the presence of moisture gradients. After casting and demoulding, concrete in the field will dry out, depending on atmospheric temperature and humidity. Drying out of concrete will increase its resistivity. However, if the test is carried out no more than one to a few months after casting, drying out should be rather superficial. There may be two types of solution here. Assuming that the concrete has dried out no more than a few millimetres, this can be accounted for by numerical simulation, see also (Reichling et al., 2012; 2014). An alternative is to wet the concrete before testing, for example for 24 hours. The additional effort may be justified by obtaining more precise data (than without wetting).
Page 5 This may not work however, on mature concrete that has dried out over several years. In such cases, more complex modelling is needed with input from a database with resistivity values for different concrete types as a function of humidity and time (see below).
Correcting for temperature differences should not be very difficult, as the overall effect is well characterised (Bürchler 1996, Polder et al. 2000, Presuel-Moreno & Liu 2012).
The final issue mentioned is the time dependency of concrete resistivity. As cement hydrates, the pore structure becomes more dense and the resistivity increases; in addition the pore solution composition may change as a result of reactions between dissolved alkalis and reactive additions. In particular concrete made with slag and fly ash (blended) cements have shown to have ongoing resistivity increase for many years (Osterminski et al. 2012).
Consequently, resistivity development with time is a function of cement type and to a lesser extent, w/c. To the authors’ knowledge presently no models exist for prediction of resistivity based only on binder composition. This may become feasible in the near future, however, with advanced mineral phase modelling. Another influencing factor is drying out, which will increase resistivity. The magnitude of this effect strongly depends on the external humidity. In temperate climates with frequent precipitation, the effect may be significant but not extreme. In hot and dry climates, the effect may be very strong. Presently no practical models exist to predict the influence of drying. The authors propose to solve the issue of time dependency (both driven by hydration and drying out) empirically, using a database containing resistivity as a function of time for a large number of concrete compositions and a range of external humidities. The information to build such a database can be derived from long-term exposure testing. Data are available from various series of experiments. One series contains Portland cement and blast furnace slag cement concrete with three w/c’s, that has been exposed since 1989, measured up to 17 years age (Osterminski et al. 2012). Other series exposed since 1998 contain Portland, slag and fly ash cements at different w/c’s, whose resistivities have been measured up to 12 years age (Pacheco et al. 2012).
3. PROPOSED METHODOLOGY FOR ON-SITE PERFORMANCE
ASSESSMENT
Here we propose to assess the as built concrete quality with regard to chloride penetration resistance as follows, schematically shown in Figure 3. The service life design involves specifying (maximum) chloride diffusion resistance of concrete and (minimum) cover depth to reinforcement. During the prequalification stages, different concrete mixes are tested for chloride migration coefficient (or any other test) and their electrical resistivity is measured, at least at 28 days age, but preferably at more points in time. For the accepted concrete mix, the dependence of resistivity (and migration coefficient) is determined by varying the w/c in small steps. Subsequently, target values for resistivity (and diffusivity) are set at 28 days, or any other point in time as desired.
Page 6 Figure 3 Schematic of proposed method for on-site control of diffusivity
Page 7 Concrete produced in the mixing plant is tested for resistivity parallel to compressive strength on wet cured cubes or cylinders (at 28 days). These data are used to confirm
‘potential’ resistivity during production. After casting and curing, the concrete on site is tested by measuring four point (Wenner) resistivity on finished sections (test areas) at an age of 28 days and/or repeatedly at a few points in time. Simultaneously cover depths and position of reinforcing bars are measured (diameters are known). Resistivity values measured on site are corrected for the presence of rebars by numerical modelling, producing an ‘apparent’
resistivity for each test area. If applicable, a temperature correction is applied. If necessary, apparent resistivities are transformed to 28 day values by correcting for effects of cement hydration (and if considered necessary for drying out) using a database of resistivities as a function of concrete composition (cement type, w/c), age and external humidity.
4. CONCLUSIONS
This paper proposes a methodology for assessment of performance of as-built concrete with regard to chloride penetration resistance. It is based on measuring on-site electrical resistivity and correcting measured values for various influences. The method can be used with any service life design approach that includes concrete resistivity as one of the parameters. It comprises the following steps.
The service life design determines the required chloride penetration resistance and cover depth to reinforcement; bar diameters are known from structural design.
The correlation between chloride penetration resistance and concrete resistivity is determined in the prequalification stage, including the effect of small variations of w/c.
This correlation is further established in the production stage.
In the production stage, resistivity is measured on wet cured cubes for strength testing. In the as-built structure, resistivity and the position of reinforcing bars are measured in test areas.
The apparent resistivity is derived by correcting for the presence of reinforcing steel by numerical modelling.
The apparent resistivity is corrected for the effects of time (cement hydration and drying out) using a database of resistivity values for various concrete compositions.
Obviously the proposed method requires further work. The authors’ institutes would welcome any collaboration to develop this method for practical use.
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