Microwave Near-Field Reflection Property Analysis of Concrete for Material Content Determination

One of the most important parameters associated with concrete is its compressive strength. Currently, there is no reliable nondestructive testing technique that is capable of robust determination of this parameter. Concrete is a heterogeneous mixture composed of water, cement powder, sand (fine aggregate), rocks of various size or grade (coarse aggregate), and air (porosity). Water and cement powder chemically combine into a cement paste binder which, in due curing time (28 days), produces concrete with its specified compressive strength. Compressive strength of concrete is strongly influenced by its water-to-cement (w/c) ratio as well as its coarse aggregate-to-cement (ca/c) ratio. Therefore, if these two parameters are determined using a nondestructive testing technique, then they may be correlated to the compressive strength. Near-field microwave nondestructive testing techniques, employing open-ended rectangular waveguide (OERW) probes, have shown tremendous potential for evaluating concrete constituent make-up. In this paper, the results of an extensive set of measurements, using these probes, are presented. The results demonstrate that the statistical distribution of the multiple measurements of the magnitude of reflection coefficient of concrete specimens with various constituent make-ups follows two well-known distributions as a function of frequency. It is shown that for the specimens investigated this distribution is Gaussian at 10 GHz and uniform at 3 GHz. Furthermore, the standard deviation of the measured magnitude of reflection coefficient at 10 GHz is shown to correlate well with concrete (ca/c) ratio, whereas, the mean of this parameter at 3 GHz is correlated well with concrete (w/c) ratio. Subsequently, these parameters may be used in conjunction with well established formulae or a look-up table to determine the compressive strength of a given concrete specimen

Cure-State Monitoring and Water-to-Cement Ratio Determination of Fresh Portland Cement-Based Materials using Near-Field Microwave Techniques

Quick and nondestructive determination of curestate and water-to-cement (w/c) ratio in fresh Portland cementbased materials is an important issue in the construction industry since the compressive strength of these materials is significantly influenced by w/c ratio. This is especially true since current techniques are not reliable and require a priori testing of test specimens as calibration for subsequent on-site monitoring of a cast in-place structure. Recently, the sensing of Portland cementbased materials using microwave techniques has received much attention. Microwave nondestructive techniques have already shown the potential for determining w/c ratio, sand-to-cement (s/c) ratio and coarse aggregate-to-cement (ca/c) ratio in cured cement paste, mortar, and concrete. In this paper, the results of a study demonstrating the potential for early determination of cure-state and w/c ratio of Portland cement-based materials, using a near-field microwave inspection technique, are presented. This technique utilizes the reflection properties of an open-ended rectangular waveguide probe radiating into Portland cementbased materials at 5 GHz (G-band) and 10 GHz (X-band). The results demonstrate the ability of near-field microwave sensing techniques to determine the state of hydration of cement paste and concrete with 0.50 and 0.60 w/c ratios and varying aggregate contents. In fact, it is shown that cement-based materials that have been moist-cured for three days and then left to cure at ambient temperature and humidity for the remainder of the prescribed 28-day curing period, are fully cured after only 12 days. An empirical formula relating the magnitude of reflection coefficient to the curing time is presented. Using this empirical relationship, the w/c ratio of cement paste and concrete can be unambiguously determined when daily monitoring of the reflection properties of the specimens is performed. The potential for utilizing this technique for on-site monitoring of cure-state and w/c ratio (and compressive strength) determination is also discussed

A Simple, Robust, and On-Site Microwave Technique for Determining Water-to-Cement Ratio (w/c) of Fresh Portland Cement-Based Materials

Inspection and evaluation of cement-based materials such as concrete is of great interest to the construction industry. In particular, real-time and on-site evaluation of water-to-cement ratio (w/c) is an important practical issue, since the compressive strength of a concrete structure is significantly influenced by its w/c. Currently, there is no single real-time, on-site, relatively in-expensive, easy-to-implement, and operator friendly technique for evaluating this parameter. Microwave nondestructive testing and evaluation techniques have shown great promise when used for inspection and evaluation of the properties of cement-based materials. In this paper, the optimal design of a monopole antenna probe used to evaluate w/c of fresh cement-based materials in real-time and in-situ is presented. This probe, operating at 3 GHz, is used along with a reflectometer whose dc output voltage is shown to be linearly correlated to w/c of fresh cement paste and fresh concrete specimens. This paper presents the optimal probe design procedure, the experimental verification of the results, and the results of using the custom-made reflectometer for quick and robust w/c measurement of fresh cement paste and concrete

Microwave processing of cement and concrete materials - towards an industrial reality?

Each year a substantial body of literature is published on the use of microwaves to process cement and concrete materials. Yet to date, very few if any have lead the realisation of a commercial scale industrial system and is the context under which this review has been undertaken. The state-of the–art is evaluated for opportunities, and the key barriers to the development of new microwave-based processing techniques to enhance production, processing and recycling of cement and concrete materials. Applications reviewed include pyro-processing of cement clinker; accelerated curing, non-destructive testing and evaluation (NDT&E), and end-of-life processing including radionuclide decontamination

Microwave material characterization of alkali-silica reaction (ASR) gel in cementitious materials

Since alkali-silica reaction (ASR) was recognized as a durability challenge in cement-based materials over 70 years ago, numerous methods have been utilized to prevent, detect, and mitigate this issue. However, quantifying the amount of produced ASR byproducts (i.e., ASR gel) in-service is still of great interest in the infrastructure industry. The overarching objective of this dissertation is to bring a new understanding to the fundamentals of ASR formation from a microwave dielectric property characterization point-of-view, and more importantly, to investigate the potential for devising a microwave nondestructive testing approach for ASR gel detection and evaluation. To this end, a comprehensive dielectric mixing model was developed with the potential for predicting the effective dielectric constant of mortar samples with and without the presence of ASR gel. To provide pertinent inputs to the model, critical factors on the influence of ASR gel formation on dielectric and reflection properties of several mortar samples were investigated at R, S, and X-band. Effects of humidity, alkali content, and long-term curing conditions on ASR-prone mortars were also investigated. Additionally, dielectric properties of chemically different synthetic ASR gel were also determined. All of these, collectively, served as critical inputs to the mixing model. The resulting developed dielectric mixing model has the potential to be further utilized to quantify the amount of produced ASR gel in cement-based materials. This methodology, once becomes more mature, will bring new insight to the ASR reaction, allowing for advancements in design, detection and mitigation of ASR, and eventually has the potential to become a method-of-choice for in-situ infrastructure health-monitoring of existing structures --Abstract, page v

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

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

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

Measurement and Monitoring of Microwave Reflection and Transmission Properties of Cement-Based Specimens

The results of measurement and monitoring of reflection and transmission properties of cement-based specimens (blocks of mortar, concrete) obtained by using a simple and an inexpensive measurement system at microwave frequencies (X-band) are presented. Dependencies of the reflection and transmission coefficients on water-to-cement (w/c) ratio, preparing and curing conditions of the specimens are demonstrated. It is shown that the amplitudes of reflection and transmission coefficients, together with thickness of the specimens, determine the complex dielectric permittivity of the hardened cement-based specimens. The expected applications of the results for the determination of physical properties of cement-based materials are discussed. The causes and effects of measurement errors and uncertainties are also discussed

Microwave Materials Characterization and Imaging for Structural Health Monitoring

The relatively small wavelengths and large bandwidths associated with microwave signals make them great candidates for inspection of construction materials and structures, and for materials characterization and imaging. Signals at these frequencies readily penetrate inside of dielectric materials and composites and interact with their materials characteristics and inner structures. Water molecule is dipolar and possesses a relatively large complex dielectric constant, which is also highly sensitive to the presence of ions that increase its electrical conductivity. Consequently, chemical and physical changes in construction materials affect their complex dielectric constant. This can be measured, and through analytical and empirical dielectric mixing formulae, correlated to those changes. Examples of applications would be, presence of delamination in a bridge deck and pavement, permeation of moisture behind retaining walls or corrosion of reinforcing steel bars which can be imaged with microwave techniques. One of the critical trade-off issues is between the microwave signal penetration into concrete vs. frequency of operation. Dielectric of concrete, particularly when moist, has a relatively high loss factor. As such, lower microwave frequencies are suitable to achieve reasonable penetration. Image resolution degrades as a function of decrease in operating frequency, therefore, a balance must be reached when using these techniques for imaging cement-based materials. In this webinar, issues related to concrete materials property evaluation and high-resolution imaging will be discussed, and examples will be provided