(ASR) Gel

Microwave NDE Method for Health-Monitoring of Concrete Structures Containing Alkali-Silica Reaction (ASR) Gel A. Hashemia, S. Hatfielda, K.M. Donnella, R. Zoughia K.E. Kurtisb a

Applied Microwave Nondestructive Testing Laboratory, Electrical and Computer Engineering Department, Missouri University of Science and Technology, 301 W 16th St., Rolla MO 65409, USA

b

Georgia Institute of Technology, School of Civil and Environmental Engineering, 788 Atlantic Drive, Atlanta, GA 30332, USA Abstract. The presence of reactive aggregates combined with sufficient moisture and concentration of alkalis are the three basic requirements for damage due alkali-silica reaction (ASR) gel formation and expansion in concrete. For healthmonitoring of concrete structures, and in order to investigate the potential for detecting ASR gel formation in existing structures, one potential technique involves studying changes in the temporal complex dielectric constant of concrete structures. In this paper, a microwave nondestructive testing approach is proposed which involves soaking two hardened mortar samples and measuring the change in their temporal complex dielectric constant in order to distinguish between the sample containing ASR gel and the one devoid of it. Part of the free water becomes bound in the sample containing ASR gel and since a portion of the microcracks in this sample contain ASR gel, the rate of evaporation of water of the two samples is expected to be different. The complex dielectric constant of the samples is significantly dependent upon the volumetric level and movement (in and out of the samples) of free water. Therefore, studying the relative different temporal rate of change in this parameter is expected to yield information about the presence or absence of ASR gel. Keywords: Alkali-silica reaction (ASR), materials characterization, microwave nondestructive testing, dielectric properties, cement-based materials. PACS: 81.70.-q, 78.20.Ci, 84.40.-x, 78.70.Gq, 13.40.-f

INTRODUCTION As service lives of concrete structures are extended, detection and monitoring of damaging reactions, such as alkali-silica reaction (ASR), is increasingly of interest. The ASR product is a gel, which may expand and lead to cracking. ASR gel forms from reactive siliceous minerals in certain aggregates and the alkaline pore solution in concrete. Moisture (free water) is the other crucial agent needed for damage by this reaction since the ASR gel expands by imbibing water. While a concrete structure is in service and once ASR gel has been formed, factors such as aggregate type (i.e., reactive or non-reactive), alkali concentration and availability of moisture dictate further the extent of the reaction and the level of damage to the concrete. Currently, there is no effective nondestructive testing method for detecting and evaluating ASR gel in existing concrete structures, but microwave techniques appear to be promising for this application. Microwave signals interact with material media at the molecular and physical levels [1]. To date, several microwave nondestructive evaluation (NDE) techniques have been developed for characterization of cement-based materials such as: cure state monitoring and determination of water-to-cement ratio (w/c), sand-to-cement ratio (s/c), coarse aggregate-to-cement ratio (ca/c) [2], coarse aggregate content distribution [3], aggregate segregation [4], and chloride permeation in mortar along with extensive EM modeling for the same [5]-[6]. Most recently, detection of alkali-silica reaction (ASR) gel in mortar containing reactive aggregates has been demonstrated [7]. This paper presents the results of temporal microwave dielectric property characterization of two different mortar samples, one containing reactive aggregate and ASR gel and the other with non-reactive aggregates, during three cycles of soaking and subsequent drying in ambient conditions. The objective of these measurements has been to distinguish between the two samples through evaluating the relative rate of change of dielectric properties in each of the samples during the drying periods. This is based on the expectation that the ASR gel in the sample containing the reactive aggregates imbibes free water during the soaking cycle. A portion of the microcracks in this sample is also filled with ASR gel which may affect the amount of free water uptake compared to the sample without the gel.

Consequently, the interaction of the remaining free water with each of these two samples is expected to be different (i.e., due to rates of evaporation, etc.). This difference in the level and transformation of free water is then detected using microwave dielectric property characterization. Based on the successful results of this investigation and future optimization of the technique, a simple test may be devised with which to obtain information about the presence and, ultimately the volume content, of ASR gel in existing structures that have been exposed to a certain amount of water (i.e., a portion is soaked for some time). The measurement approach and the results of this investigation are outlined in this paper.

METHODOLOGY The two mortar samples used in this investigation were prepared as a part of a previous study to evaluate dielectric properties of mortar with and without ASR gel [8], and had been left in ambient conditions for more than a year. The complete specification of the samples and the manner by which they were prepared are described in [8] and for brevity will not be repeated here. These samples were used in this investigation by soaking them in distilled water for ~24 hours, and then leaving them in ambient conditions for 65 days. The temporal complex dielectric constant of the samples was measured at S-band (2.6-3.95 GHz) during the entire period and the soaking-drying cycle was repeated two more times. The well-known completely-filled waveguide technique [9] was used to measure the relative (to free-space) complex dielectric constant of the samples, as denoted by r:

er = e 'r - je "r

(1)

where the real part (relative permittivity) represents the ability of a material to store microwave energy and the imaginary part (relative loss factor) represents the ability of a material to absorb microwave energy. The mortar samples were prepared to tightly fit inside an S-band (72 x 34 mm) waveguide sample holder, as shown in Fig. 1a. The measurements were conducted using an Agilent 8510C vector network analyzer (VNA) consistent with the procedure described in [9], as depicted in Fig. 1b. In addition to the dielectric constant measurements, the temporal sample masses were also measured. Fig. 1c shows a flowchart of the potential procedure that may be used to identify the presence of ASR gel (or aggregate reactivity) based on the changes in the microwave dielectric constant of the samples as a function of drying time.

Soak the known samples for ~24 hours

Soak unknown samples for ~24 hours Model

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(a) Temporal microwave dielectric constant measurements

Behavioral model for different aggregates

Temporal microwave dielectric constant measurements

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FIGURE 1. a) Two Mortar Samples, b) Measurements Apparatus, and c) Potential Measurement Technique for Detecting ASR Gel.

MEASUREMENT RESULTS Figures 2a-d show the measured temporal permittivity and loss factor of the two samples along with their relative rates of change compared to the first day of first cycle. In this investigation, the first day of each soaking cycle was selected as the reference for calculating the respective relative rates of change. However, this reference point may be any arbitrary day (e.g., the day before soaking) as long as all measurements are compared with respect to that particular day. In these figures, day 0 represents the day prior to the soaking event, day 1 shows the measurements after samples were soaked in water for ~24 hours and in the days following, the samples were left in ambient conditions. Measured relative permittivities of the samples are shown in Fig. 2a. The results clearly show the permittivity having the lowest value at day 0 (i.e., first day of each cycle) which is followed by an abrupt change in the next day corresponding to immediately after 24 hours of soaking. This increase is the direct result of the free water uptake by the samples [10]. After day 1 and as the samples are kept in lower-relative humidity ambient conditions, they begin to gradually lose the up-taken water through evaporation (while some may also be initially drawn deeper into the samples through capillary draw, but may eventually evaporate out of the sample). This results in the relative permittivity values to correspondingly decrease. As it can be seen in the same figure, during the early days of drying cycles, the permittivity decays rapidly, indicating rapid evaporation of water from the samples. Subsequently, the water evaporates out of the samples at a much slower rate for the remaining days in each cycle. Also, during the early days of each cycle, when the samples hold most of the soaked water, the permittivities of the two samples are almost identical. However, as more water evaporates from each sample, these values begin to diverge to the point that at day 10, the values of permittivity are distinctly different. This behavior indicates that the dielectric constant of water overwhelms the overall dielectric constant of each sample during the early days of each cycle. However, as the influence of free water diminishes over the course of drying cycle, the dielectric constant of the two samples becomes less dependent on dielectric constant of water. 0

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FIGURE 2. a) Permittivity Measurements, b) Loss Factor Measurements, c) Relative Rate of Change of Permittivity, and d) Relative Rate of Change of Loss Factor.

Fig. 2b shows the measured temporal behavior of relative loss factor for the two samples. As expected, minimum values occur prior to the soaking day, and the values are maximum right after soaking as the direct result of the presence of free water [10]. Figs. 2c and 2d indicate the rate of change of dielectric constant of the reactive and non-reactive samples relative to the day 1 of the first cycle. The results show that the two samples behave differently in terms of permittivity and loss factor, and this difference is more considerable in loss factor. As reported in previous investigations [8], it is assumed that existence of more air in the microcracks/pores of non-reactive sample compared to the reactive sample (where ASR gel is present as well) is responsible for different rates of change in moisture uptake/loss and accordingly different rates of change in loss factor. This means that loss factor of the non-reactive sample changed more than the reactive sample (substantially) and once drying began, the loss factor of each sample reached the same “indistinguishable” value. Therefore, water was taken up in each sample differently. In other words, due to presence of ASR gel in the reactive sample, the evaporation of water, due to its binding within the gel, occurs more slowly than the evaporative process in the non-reactive sample. When water evaporates more readily, a higher rate of change in dielectric loss factor is observed. To quantify the different rates of change of the two samples, loss factor data were curve-fitted, resulting in equations (2)-(3) showing the difference between the two rates of change. The measured values and the results of the two equations are plotted in in Fig 3. In the equations, Y represents the loss factor rate of change as a function of time (days) during the first drying cycle. Thus, the ASR-affected sample can be distinguished by means of these equations based on which curve the relative loss factor rate of change belongs. The goodness parameters of the fits are summarized in table 1 and as reported in the table, both summed square of residuals (SSE) and root mean square error (RMSE) indicate usefulness of the fits for prediction. Likewise, R-square and adjusted R-square corroborate that a great proportion of variance is accounted for by the fit.

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FIGURE 3. Fitted Curve for Reactive and Non-reactive Data Points of the First Cycle.

YNon-reactive-sample = -2.5X -0.75 + 2.5

(2)

YRe active-sample = -2.1X -0.9 + 2.1

(3)

TABLE 1. Goodness of Fit

Goodness of Fit Properties SSE R-square Adjusted R-square RMSE

Reactive Fit 0.03377 0.9924 0.9917 0.03832

Non-reactive Fit 0.02783 0.9954 0.9950 0.03479

DISCUSSION OF RESULTS Cement-based materials are some of the most complex composite materials. The chemical activities that take place (i.e., curing, ASR gel formation, carbonation, leaching, etc.) in these materials, in addition to physical changes (i.e., water uptake and drying), makes for a complex study when investigating property changes over time. In this investigation, a preliminary attempt was made to study the changes in the temporal complex dielectric constant of two mortar samples; one containing ASR gel and the other having no ASR gel. Physical and chemical reactions of these samples resulting from free water uptake and subsequent drying were evaluated as a means for detecting preexisting ASR gel. Free and bound (by ASR gel) water behave markedly different when exposed to microwave radiation, through exhibiting significantly different complex dielectric constants. Also, evaporation of free water from the sample, if affected by the presence of ASR gel, can be readily detected through this type of measurement. The results showed this to be true, particularly when studying the temporal behavior of the loss factor of the two samples. Complete characterization of dielectric properties of ASR-affected structures involves developing a comprehensive dielectric mixing model based on all pertinent chemical and physical parameters that influence the dielectric behavior of such structures. For example, one such parameter is carbonation which takes place in either fresh or mature concrete structure and is simply chemical reaction of CO 2 and Ca(OH)2 that finally results in CaCO3 (calcium carbonate), as well as H2O. The dielectric properties of calcium carbonate and water can be measured (or are known in the case of H2O) and included in such a dielectric mixing model. In this investigation a destructive pH indicator test was conducted using phenolphthalein solution after the conclusion of the third drying cycle. The pH indicator test revealed different levels of alkalinity in both samples which can indicate carbonation or leaching. These effects should be evaluated in further research and will be necessary to consider in any dielectric mixing model development. In conclusion, a potential microwave NDE technique for structural health-monitoring of hardened (existing) cement-based materials was presented. It was demonstrated that the sample with ASR gel retains and releases ingressed moisture differently compared to the non-reactive sample. Thus, potentially water may be ingressed in a small portion of an existing structure, and while drying; different rates of change in microwave measurements can reveal the presence of ASR gel. This technique can be further extended through examining several samples having different aggregate types as well as various mix designs.

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF), as a Collaborative Grant between Missouri University of Science and Technology (S&T) and Georgia Institute of Technology, under Award No. 1234151. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the NSF.

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