Abstract
Dielectric constant and conductivity, the so-called EM properties (electromagnetic), are widely adopted for NDT (Nondestructive Technique) in order to detect damage or evaluate performance of concrete without damage to existing RC (reinforced concrete). Among deteriorating agents, chloride ion is considered as one of the most critical threats due to rapid penetration and direct effect on steel corrosion. In the work, cement mortar samples with 3 w/c (water-to-cement) ratios and 4 levels of chloride addition are considered. Conductivity and dielectric constant are measured in the normal frequency range. They increase with strength of mortar and more chloride ions due to denser pore formation. Furthermore, the behaviors of measured EM property are investigated with carbonation velocity and strength, which shows an attempt of application to durability evaluation through EM measurement.
1. Introduction
Nondestructive Techniques (NDT) are widely used for evaluation of physical and durability performance in concrete structures. Among various techniques, the techniques using electromagnetic (EM) properties are increasingly applied as an alternative for performance evaluation in RC (reinforced concrete) structure [1, 2]. Several researchers have focused on EM properties in cement-based materials. These researches can be classified into several categories. Many studies related to the characterization of EM properties in cement-based materials have been performed [3–5]. They cover not only Ordinary Portland Cement (OPC) but also cement-based matrix with mineral admixtures such as fly ash [6–9]. Some of the studies focused on the variations of EM properties in wide frequency range [3]. Recently, EM properties are applied for durability evaluation in concrete such as permeability and the diffusion coefficient [10–12]. Usually, the models which are made up of porosity, solid paste, and saturation of water can provide dielectric constant and loss factor through complex equations [2, 13–15]. The models have been applied to the deteriorated RC structures considering permittivity in saline water. The evaluation techniques like monitoring and condition assessment are also proposed through the unique characteristics of EM measurement [15–18]. Recently, NDT techniques using EM properties are applied to the evaluation of delamination in composite member repaired with glass fiber reinforced plastic (GFRP) [19–21].
The porosity and saturation in concrete have a great effect not only on the EM properties but also on the durability characteristics in concrete. During hydration process, pores are naturally generated in the concrete and the amount of pores (porosity) has a close relationship with the structural and durability performance [22–24]. As porosity plays an important role in intrusion of deteriorating agent, it is mainly considered to explain the mechanism of diffusion and permeation of ion [25–28].
This work is focused on the characterization of EM properties in cement mortar with chloride ion. The measured EM properties are compared with typical concrete performance like carbonation velocity and strength. In the practical point of view, the characteristics of EM properties observed in the work can be utilized for not only strength evaluation but also durability evaluation.
2. Conductivity and Dielectric Constant
2.1. Conductivity and Dielectric Constant in Porous Media [3, 20]
All materials have a unique set of EM properties. Nonmetallic materials like concrete can be characterized by two independent electromagnetic properties: (1) the complex permittivity () and (2) the complex permeability () [3]. Since most of the dielectric materials such as concrete are nonmagnetic, the permeability of the dielectric materials is very close to that of free space ( henries/meter). Thus, it is enough to focus on the complex permittivity defined by [3, 20]where and are the real and imaginary parts of the complex permittivity and . Equation (1) can be rewritten as (2) when considering the permittivity in free space ( farad/m). Equation (2) can be rewritten as (3) which is dimensionless [3, 20]:where is the relative complex permittivity. and are the real and imaginary parts of the relative complex permittivity, respectively.
The real part of the relative complex permittivity, , also known as the dielectric constant, is a depiction of how much energy can be stored in a material from an external electric field. The imaginary part of the relative complex permittivity, , represents how dissipative a material is to an external electric field. This value can be simply referred to the loss factor [3, 22]. The equivalent conductivity (mhos/m) can be written in terms of the imaginary part of the complex permittivity, , as shown in where is angular frequency of EM wave (rad/sec), is loss tangent (the ratio of energy lost to energy stored in a material), and is the number of cycles per second (Hz). These EM properties are not constant and are dependent upon the frequency, temperature, moisture content, chloride content, and concrete mix constituents [2–4].
The influencing parameters can be summarized as Table 1 in the view of porous media. Exterior conditions like temperature and humidity and the measurement condition such as the range of frequency also have effects on the EM properties.
3. Conductivity and Dielectric Constant in Concrete
3.1. Experimental Program
3.1.1. Measurement of Dielectric Constant and Conductivity
The OPC mortars with 3 different w/c ratios (0.45, 0.55, and 0.65) are prepared. For each case of w/c ratios, 4 different chloride contents (0.0, 0.6, 1.2, and 3.6 kg/m3) are added at the mix stage. The chloride ion within 1.2~3.6 kg/m3 is meaningful since chloride content of 1.2 kg/m3 is regarded as a critical threshold which can cause steel corrosion in concrete. Thus, a total of 12 mix conditions are prepared for the comparative investigation. Each sample was kept in air-curing condition for 4 weeks after mixing. The mix proportions for OPC mortar are listed in Table 2.
For the test, cement mortar is used for EM properties in order to prevent a measurement error due to direct contact on the coarse aggregate. Cylindrical samples (100 mm × 200 mm) are made for the compressive strength test and the disk type specimens (100 mm diameter × 20 mm thickness) are prepared for EM property measurement. A concrete depth of over 5 mm was reported to have insignificant effect from reflected microwaves [18].
The measurement setup is shown in Figure 1. The package for EM measurement has a dielectric probe kit which includes analyzing software, an open ended coaxial probe (OECP), a network analyzer, and a laptop computer. Conductivity and dielectric constant are measured by placing the probe in contact with a flat face of a solid surface. The network analyzer sends and receives microwaves through the probe over a frequency range of 0.2 GHz to 20 GHz with an interval of 0.4 GHz.
For the calibration of the test system using the OECP, predefined property values for the reflected signal should be prepared over the measuring frequency range [29]. In the test, the calibration is performed in the air, namely, free space, and in the water with 25°C prior to the measurement.
3.1.2. Compressive Strength
Compressive strength tests are performed referring to Korea Standard [30]. Trifold cement mortar samples with 100 mm of diameter and 200 mm of height are prepared. The samples have been kept for 4 weeks in the same curing condition and compressive strength test was performed after 1-day exposure to room condition.
3.1.3. Carbonation Test
After 4 weeks of curing, the samples were kept in the condition of relative humidity at 60% and 25°C temperature for 2 weeks. And then the accelerated carbonation test is performed for 8 weeks referring to the Korea Standard [31, 32]. For 1-dimensional accelerated carbonation test, the sides of specimens are coated with epoxy except for top surface. The conditions for accelerated carbonation test are listed in Table 3. The photos for compressive strength test and carbonation test are shown in Figure 2. The carbonation test is performed on cement mortar samples (50 mm of thickness) without chloride contents.
(a) Compressive strength
(b) Samples in the carbonation chamber
3.2. Test Results
3.2.1. Measurement of Dielectric Constant and Conductivity
In the test, the measurements from OECP are repeatedly performed 10 times for each disk samples, and the values of dielectric constant and conductivity are averaged. The range of frequency is 0.2~20 GHz and several points of 0.2, 5, 10, 15, and 20 GHz are selected and measured in a room condition with 60% relative humidity and 20~22°C temperature. The measured results with different w/c ratios are plotted in Figures 3~5.
(a) Conductivity
(b) Dielectric constant
(a) Conductivity
(b) Dielectric constant
(a) Conductivity
(b) Dielectric constant
As shown in Figures 3~5, conductivity becomes larger and dielectric constant decreases with the increase of frequency. The changing behavior with frequency is consistent with the previous researches [18, 21]. With the smaller pores, mortar surface has more contact area with probe and this causes higher conductive and dielectric constant. The larger addition of NaCl causes more dense porosity [33, 34].
3.2.2. Compressive Strength Results
The compressive strength shows a close relationship with w/c ratios in cement mortar but it shows insignificant variations with chloride amount included in the test. The drop of pH due to the addition of calcium chloride (CaCl2) can accelerate the hydration activity so that the compressive strength in early-aged stage rapidly increases [33]. Concrete with sodium chloride (NaCl) is reported to show little change in hydration activity but significant effects on drying shrinkage due to the altered pore distribution [34]. The test results of compressive strength are shown in Figure 6 where one can indicate that the compressive strength has no clear relationship with the chloride contents. The similar results related to the compressive strength can be found in previous research [24]. Traditional strength reduction is found in Figure 6 with increasing w/c ratio.
3.2.3. Accelerated Carbonation Test Result
It is also found that concrete with low w/c ratio shows small carbonation depth since it has low diffusion coefficient of CO2 and large amount of hydrates such as CHS and Ca(OH)2 which have potential to keep high pH in pore water [26, 35]. Carbonation velocity is usually adopted for carbonation resistance based on the assumption that carbonation depth has linear relationship with square root of exposed time [36, 37]. In Figure 7, the results of accelerated carbonation test are plotted, which shows slower carbonation progress with lower w/c ratio. Carbonation velocity through regression analysis can be summarized in Table 4.
4. Characteristics of EM Properties for Durability Evaluation
4.1. EM Characteristics and w/c Ratios
For the purpose of comparison of the results, EM properties corresponding to 5~20 GHz range are, respectively, averaged as one value to obtain parameter. Each averaged value of conductivity and dielectric constant is plotted in Figure 8 with w/c ratios considering chloride contents.
(a) Averaged conductivity
(b) Averaged dielectric constant
Typically, porosity in cement mortar has effect on the conductivity and dielectric constant. In cement mortar, porosity becomes smaller with decrease in w/c ratios due to abundant hydrates, and the reduced porosity causes higher EM properties [22, 23, 38]. Usually, permittivity of cement gel is in the level of 5~10 and this is larger than that of air (=1.0) [2, 13]. One can also indicate that, for the samples containing the same w/c ratios, more chloride content leads to higher EM properties due to reduced porosity.
4.2. EM Characteristics and Compressive Strength
As in Section 3.2.2, the addition of NaCl cannot cause a big change in strength. The averaged EM properties can compare the results of compressive strength as in Figure 9. EM properties (dielectric constant and conductivity) without chloride content are used for the comparison with the results since they are varying significantly with chloride addition. Compressive strength is reported to have linear relationship with w/c ratio [22]. As shown in Figure 8, EM properties without chloride content have linear relationship with w/c ratio with high determination coefficient (), so that linear relationship is found between EM properties and strength as shown in Figure 9.
(a) Conductivity and strength
(b) Dielectric constant and strength
4.3. EM Characteristics and Carbonation Velocity
For porous media, the diffusion mechanism in concrete is also closely related with porosity characteristics [26–28]. In Figure 10, the averaged EM properties are compared with the carbonation velocity obtained from the accelerated carbonation test. The results are only for the case without chloride contents.
The carbonation velocity is observed in linear proportion to the w/c ratios and the trend can be found in conventional equations for the prediction of carbonation depth [26, 36]. For the cement mortar without chloride amount, it is found that that the averaged EM properties decrease with the high carbonation velocity, which is consistent with the results in high w/c and low compressive strength. Considering the results in strength and carbonation depth, the lower w/c ratio in cement mortar causes larger content of hydrates, which yields high strength and low porosity. The dense structure shows relatively high EM properties. With increasing strength due to larger hydrates and lower porosity, the resistance to carbonation also increases due to low CO2 diffusion and more carbonatable materials—Ca(OH)2. In Figure 10, higher carbonation velocity, which means coarse structure, shows lower conductivity and electric constant.
4.4. EM Characteristics and Chloride Content in Cement Mortar
In the water-submerged condition, chloride ion does not significantly affect dielectric constant; however, loss factor which causes attenuation is greatly changed due to increase in ohmic conductivity [3]. In cement mortar, both conductivity and dielectric constant increase with larger amount of chloride amount. It is because the chloride ion in initial mix causes smaller pore distribution in OPC mortar, and sodium ion dissolved in pore water can increase conductivity and dielectric constant. In Figure 11, the effect of chloride on EM properties is plotted with varying chloride contents. The result in higher w/c ratio shows more rapid increase in both conductivity and dielectric constant because the coarse pore structure in OPC mortar with higher w/c ratio is more affected by chloride ion which can cause the densification of pore structure.
(a) Conductivity variation
(b) Dielectric constant
The regression analysis is performed with increasing ratios to noncontaminated OPC mortar and is plotted in Figure 12. The linear relationships between EM properties and chloride content are summarized in Table 5. In Table 5, the gradient of slope means the effect of chloride ion on EM properties. Lower w/c ratio shows higher strength in concrete and lower dependency of chloride ion; however, mix conditions with high w/c ratio (0.65) show clear increase in chloride effect. When chloride ion is added to 3.6 kg/m3, conductivity and dielectric constant increase to 173% and 134%, respectively. The relationship shows clear linearity with high determination coefficient over 0.9, which strongly shows applicability as NDT for evaluation of chloride contamination for cement mortar.
(a) Normalized conductivity
(b) Normalized dielectric constant
5. Concluding Remark
Among various nondestructive testing (NDT) techniques, electromagnetic properties of conductivity and dielectric constant are attempted for an evaluation of chloride effect in cement-based construction material. The conclusions on effect of chlorides on conductivity and dielectric constant in hardened cement mortar are as follows.(1)Through the measurement of electromagnetic properties within the 0.2~20 GHz frequency range, the patterns of dielectric constant and conductivity are investigated in cement mortar with 0.0~3.6 kg/m3 of chloride addition. The relationships between electromagnetic properties and engineering parameters like w/c ratios, carbonation velocity, and the induced chloride content are compared as well.(2)The carbonation velocities and compressive strength are measured for OPC mortar with different w/c ratios. Then, the results are compared with averaged EM properties. Both conductivity and dielectric constant linearly decrease with increasing carbonation velocity and decreasing compressive strength with high determination coefficient. With more addition of chloride content to cement mortar, dielectric constant and conductivity are observed to linearly increase. Referred to the results without chlorides, the cement mortar containing 3.6 kg/m3 of chloride amount shows 173% increasing of dielectric constant and 134% increasing of conductivity. The linear regression with high determination coefficient is also observed with more chloride content.
Competing Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (no. 2015R1A5A1037548).