Comparison of Water and Saltwater Movement in Mortar Based on a Semi-Empirical Electromagnetic Model

The presence of chloride ions in steel-reinforced structures leads to the corrosion of the reinforcement thus compromising the integrity and strength of the structure. Thus, it is of great importance to be able to non-destructively detect and evaluate the free chloride content in concrete. To that end, an investigation was initiated where two mortar cubes were soaked in distilled water and saltwater solutions, respectively. Their temporal microwave reflection properties were measured using open-ended rectangular waveguides on a daily basis for three cycles, each lasting 35 days. A semi-empirical electromagnetic model was then developed to simulate the reflection properties of the cubes. The outcome of the model describes the water and saltwater distribution within the cubes. In addition, the distribution curves also depict the manner in which the water and saltwater contents vary within the cubes from day to day. This paper presents a comparison between the water and saltwater distributions obtained from this model. The results of such a comparison would then indicate the mechanism of mass transport within the cubes

Evaluation of Microwave Reflection Properties of Cyclically Soaked Mortar Based on a Semiempirical Electromagnetic Model

Detection of chloride ingress and evaluation of its distribution and temporal movement in reinforced concrete structures is an important practical issue. Steel reinforcing bars embedded in good quality concrete are normally protected from corrosion. However, the presence of a sufficient concentration of free chloride ions in the region of the reinforcing steel can initiate the process of corrosion. Therefore, it is important to be able to detect ingress of chloride ions and their distribution in cement-based materials. Moreover, it is important to obtain this information nondestructively. In recent years, near-field microwave nondestructive evaluation methods, using open-ended rectangular waveguide probes, have proven effective for evaluating many important properties of cement-based materials, including the detection of salt, added to the mixing water and when entering these materials through exposure to salt solution. Additionally, successful electromagnetic modeling of the interaction of microwave signals with moist cement-based materials has provided the necessary insight for evaluating the distribution and movement of moisture within these materials, leading to the current study involving ingress of sodium chloride solution. To this end, a mortar cube was subjected to cycles of wetting in a sodium chloride bath with a salinity of 2.8%, followed by episodes of drying. Subsequently, the microwave reflection properties of the cube were measured at 3 and 10 GHz using open-ended rectangular waveguides for several cycles, each lasting about 35 days. A semiempirical electromagnetic model, representing the cube as a stratified structure with a nonuniform dielectric property profile, was then developed to simulate the measured reflection properties. The simulated and the measured results at both frequencies and for all cycles were in good agreement. Subsequently, the effect of ingress of salt solution in terms of the temporal distribution of moisture along with the dissolved salt (i.e., pore solution) within the cube for every cycle was also estimated. This paper presents a brief description of the measurement approach and a detailed description of the model and its results

Electromagnetic Modeling of Saltwater Ingress in Mortar at Microwave Frequencies

Corrosion of reinforcing steel is a major cause of damage and deterioration in reinforced concrete structures. Therefore, as the presence of a sufficient concentration of chloride ions can lead to the onset of corrosion in embedded steel, it is of utmost importance to be able to determine the free salt content and its distribution in these materials. Moreover, it is important to obtain this information nondestructively. Previous investigations have shown the capability of near-field microwave nondestructive evaluation methods, using open-ended rectangular waveguide probes, to evaluate many important properties of cement-based materials. In this Investigation, the temporal microwave reflection properties of a mortar cube, subjected to cycles of wetting in a saltwater bath with a salinity of 2.8% and drying were measured at 3 GHz and 10 GHz using open-ended rectangular waveguides for several cycles, each lasting about 35 days. A semi-empirical electromagnetic model was then developed, representing the cube as a stratified structure with a dielectric constant profile to simulate the measured reflection properties. The issue of representing a continuous media as a stratified structure was also explored. The simulated and the measured results at both frequencies, and for all cycles were In good agreement. Subsequently, the volumetric free salt distribution, left In the cube, was also calculated. This paper presents a brief description of the model and its results at 3 GHz for the first cycle

An Electromagnetic Model for Evaluating Temporal Water Content Distribution and Movement in Cyclically Soaked Mortar

Evaluation of water distribution and its temporal movement in cement-based materials is important for assessing cement hydration, curing, and long-term performance. From a practical standpoint, it is also important to obtain this information nondestructively. Near-field microwave nondestructive evaluation methods have proven effective for evaluation of cement-based materials for their various mixture properties, including the detection of salt added to the mixing water and chloride ions entering these materials through exposure to salt water solutions. Electromagnetic modeling of the interaction of microwave signals with moist cement-based materials can provide the necessary insight to evaluate water content distribution and movement in these materials. To this end, the temporal microwave reflection properties of a mortar cube, subjected to cycles of wetting and drying, were measured at 3 and 10 GHz using open-ended rectangular waveguides for several cycles, each lasting about 35 days. A semiempirical electromagnetic model, based on modeling the cube as a layered structure with each layer having a different dielectric constant, was then developed to simulate the measured reflection properties. The simulated and measured results were obtained for both frequencies and, for all cycles, were in good agreement. The most important outcome of the model is the temporal behavior of water content distribution and, hence, its movement in the mortar cube. This paper presents a brief description of the measurement approach and a detailed description of the model. A detailed discussion of the results and its sensitivity to various parameters is also provided

Microwave Reflection Properties of Concrete Exposed Periodically to Chloride Solution of 3% Salinity and Compression Force

Microwave nondestructive evaluation (NDE) techniques applied to mortar have proven successful in the past for detecting mixture constitutes, salt ingress, loading and cure state monitoring. In this paper, a similar microwave NDE technique is used to evaluate reflection properties of concrete for cyclical exposure to salt ingress and loading at S-(2.6-3.95 GHz) and X-(8.2-12.4GHz) microwave frequency bands. Four identical cubic specimens were prepared: one soaked in distilled water, one soaked in a 3%-salt solution, one soaked in a 3%-salt solution and loaded and finally one that was neither soaked nor loaded. Using both magnitude and phase of microwave reflection coefficient, it is shown that the cubes can be distinguished from one another using this technique

Microwave Reflection Properties of Concrete Periodically Exposed to Chloride Solution of 3% Salinity and Compression Force

Corrosion of steel rebar in a concrete structure compromises its structural integrity and hence its performance. Chloride intrusion into concrete can lead to depassivation of the steel and initiation of corrosion. Methods exist to detect chlorides in concrete, but the practical use of many of these may be problematic because they are destructive and time consuming, and cannot be used to analyze large structures. Microwave nondestructive evaluation techniques applied to mortar have proven successful for detecting mixture constituents, chloride ingress, and cure-state monitoring. In this paper several concrete samples are cyclically soaked in distilled water and saltwater while also experiencing compression force. Compression force, simulating in-service loading, results in increased microcracking and permeability, which promotes chloride ingress. The daily microwave reflection properties of these samples were measured at 3 GHz. The results show the capability of these microwave measurements for detecting the increased level of chloride permeation as a function of increasing number of soaking cycles. In addition, comparisons between the reflection properties of mortar and concrete cubes soaked in distilled water exhibit similarity in trends, indicating that the various phenomena that occur within them are systematically similar

Microwave Reflection and Dielectric Properties of Mortar Subjected to Compression Force and Cyclically Exposed to Water and Sodium Chloride Solution

Corrosion of the reinforcing steel is a major cause of damage and deterioration in reinforced concrete structures such as concrete bridge decks and columns. Chloride intrusion into concrete can lead to depassivation of the steel and initiation of corrosion. Thus, it is very important to be able to nondestructively detect and evaluate the free chloride content in concrete. Near-field microwave nondestructive testing techniques, using open-ended rectangular waveguide probes, have shown great potential for evaluating various properties of concrete, including the successful detection of sodium chloride added to mortar mixing water. In this study, several mortar samples are cyclically soaked in distilled and salt water while also experiencing compression force. Compression force, simulating in-service loading, causes microcracking, which results in increased microcracking and permeability, promoting chloride ingress. The daily microwave reflection and dielectric properties of these samples were measured at 3 GHz. The results show the capability of these microwave measurements for detecting the increased level of chloride permeation and loading as a function of the increasing number of soaking cycles. The influence of salt ingress is shown to be more prominent in the loss factor, while the effect of loading is more evident in the permittivity of the samples

Cure-State Monitoring of Cement-Based Materials using Microwave Near-Field Techniques

The compressive strength of concrete after 28 days of curing is the vital parameter in the structural design of concrete structures. In many practical applications, it is also of interest to obtain strength information before the prescribed 28 days. The parameter that influences the compressive strength is water-to-cement ratio (w/c). The (w/c) has an effect on both the relative gain of strength over time as well as the compressive strength after 28 days. In this study, the microwave reflection properties of several cement paste cubes having different (w/c) were measured on a daily basis for the first 28 days of the curing period at two different frequency bands (S-band and X-band). The primary goal of this investigation is then to correlate microwave reflection properties to cure-state, and hence the compressive strength of these samples on a daily basis