Paul S. Nowak

MICROWAVE NONDESTRUCTIVE ESTIMATION OF CEMENT PASTE COMPRESSIVE STRENGTH

Dielectric and reflection properties of our cement paste samples with water-cement ratios (w/c) of 0.31, 0.40, 0.50, 0.60 were measured daily for 27 days using microwave frequencies of 4, 9, 13, and 17 GHz. The dielectric properties of these samples, and hence their reflection coefficients, decreased as a function of increasing w/c. This is opposite to what is expected for physical mixing (no chemical interaction or hydration). The presence of curing as evidenced by this result indicates that microwaves could be used to monitor the amount of curing of a concrete member. The variation in the reflection coefficient of these samples as a fucntion of w/c followed a trend similar to the variation of compressive strength as a function of w/c. Subsequently, a correlation between compressive strength and reflection coefficient of these blocks was obtained. This result can be used to directly and nondestructively estimate the compressive strength of a cement paste block, thus opening up the possibility of nondestructively testing the compressive strength of concrete.

Microwave Nondestructive Determination of Sand-to-cement Ratio in Mortar

Abstract Microwave nondestructive testing methods have shown great promise for the inspection of cement-based materials. Previously it has been shown that the magnitude of reflection coefficient from cement paste specimens can be correlated to their water-to-cement (w/c) ratios and more importantly to their compressive strengths. For mortar specimens, the sand-to-cement (s/c) ratio plays an important role in its physical, mechanical, and microwave reflection properties. To determine the w/c ratio and other properties of mortar specimens, one also needs to know its s/c ratio. To illustrate the ability of microwave reflection measurements for this purpose, two sets of four mortar specimens are produced with w/c ratios of 0.50 and 0.60 and with s/c ratios of 1.0, 1.5, 2.0, and 2.5. The microwave reflection coefficients of these specimens are measured using an open-ended rectangular waveguide sensor, in the G- (3.95–5.85 GHz), J- (5.85–8.2 GHz), and X-band (8.2–12.4 GHz) frequency ranges. It is shown that a s...

Cement paste compressive strength estimation using nondestructive microwave reflectometry

Microwave reflection properties of four cement paste samples with various water-cement (w/c) ratios were measured daily for 28 days using microwave frequencies of 5, 9, and 13 GHz. The dielectric properties of these samples, and hence their reflection coefficients, were measured daily and shown to decrease as a function of increasing w/c ratio. This is as a direct result of curing (no chemical interaction or hydration). The presence of curing as indicated by this result indicates that microwaves could be used to monitor the amount of curing in a concrete member. The variation in the reflection coefficient of these samples as a function of w/c ratio followed a trend similar to the variation of compressive strength as a function of w/c ratio. Subsequently, a correlation between the measured compressive strength and reflection coefficient of these blocks was obtained. The early results indicated that lower frequencies are more sensitive to compressive strength variations. However, further investigations showed that there may be a frequency around 5 GHz which is the optimum measurement frequency. This result can be used to directly and nondestructively estimate the compressive strength of a cement paste and mortar blocks.

Feasibility study of inflatable structures for a lunar base

The design of a structure on the moon requires addressing a host of issues not encountered on Earth. A modular inflatable structure consisting of thin membranes of composite material integrated with supporting columns and arches is proposed. An initial linear analysis of the structure is briefly reviewed. The actual response of an inflatable membrane is nonlinear and, hence, a nonlinear numerical analysis of the stresses and displacements was undertaken. Results indicate that an inflatable structure is a feasible concept for a lunar structure.

Microwave nondestructive detection of chloride in cement based materials

Preliminary results pertaining to the near-field microwave nondestructive detection and evaluation of chloride in cement paste and mortar specimens are presented. The technique used for this purpose utilizes an open-ended rectangular waveguide at the aperture of which the reflection properties of the specimens are measured. It is shown that the magnitude of reflection coefficient is a useful parameter for detecting chloride in these specimens. Furthermore, the difference in the amount of chloride present in these various specimens, at the time of mixing, can also be determined. Reflection property measurements were conducted in S-band (2.6 GHz-3.95 GHz) and X-band (8.2–12.4 GHz) for two sets of four mortar specimens with 0.50 and 0.60 water-to-cement ratio and varying salt (NaCl) contents added to the mixing water used in producing these specimens. It is shown that the reflection properties of these materials vary considerably as a function of their chloride content. Also, by monitoring the daily variatio...

Inflatable structures for a lunar base habitat

Design and construction of a structure on the moon requires addressing a host of issues not encountered on earth. A modular quilted inflatable structure consisting of thin membranes of composite material integrated with supporting columns and arches is proposed. An initial linear analysis of the proposed structure is briefly reviewed. The actual response of an inflatable membrane is nonlinear and, hence, a nonlinear numerical analysis for the stresses and displacements was undertaken. Initial results clearly indicate that an inflatable structure is a feasible concept and is ideally suited for a lunar structure.

Application of near-field microwave sensing techniques for segregation detection in concrete members

In this presentation, a simple, low-cost near-field microwave nondestructive inspection technique for segregation detection in concrete members is presented. This process employs information from the measured magnitude of reflection coefficient at the aperture of an open-ended rectangular waveguide sensor. These measurements, whose results will be presented, were conducted using a Hewlett-Packard HP8510B network analyzer. However, in practice a simple and relatively inexpensive inspection apparatus constructed from discrete microwave components can easily be employed. It is shown that the standard deviation of magnitude of reflection coefficient measurement is linearly correlated with the aggregate density in concrete. Furthermore, for concrete in which the aggregate has segregated, this measurable parameter will change as a function of vertical position of the microwave scan. Results correlating the microwave measurements to the actual aggregate density of a well consolidated concrete specimen and a spec...

Dielectric properties of concrete at S and X bands: a near-field investigation

When inspecting concrete structures with microwaves (radars, embedded microwave sensors, modulated scattering techniques, etc.) the dielectric properties of the concrete are considered as a ground truth data and must be known. During the past three years, extensive microwave near-field measurements of the reflection properties of concrete specimens with varying water-to-cement (w/c) ratios, sand-to-cement (s/c) ratios and coarse aggregate-to-cement (ca/c) ratios have been conducted. These experiments were conducted using open-ended rectangular waveguide probes radiating into a half-space of these concrete specimens. These measurements were conducted at S- (2.6 - 3.95 GHz) and X-bands (8.2 - 12.4 GHz). Moreover, an electromagnetic model, which took into account the presence of higher-order modes at the waveguide aperture, was also used to model this process. Finally, a root finding technique was applied to calculate the effective dielectric properties of the concrete specimens. This paper presents the results of these measurements and calculations as they related to determining the dielectric properties of concrete. Since concrete is a heterogeneous material, the results from many locations in a specimen are reported rendering effective dielectric properties showing the mean and standard deviation of the measurements and calculations at these frequency bands. The results of the dielectric constant can also be used to predict the reflection properties of concrete when using a standoff distance (i.e. non-contact measurements) or when using other types of microwave sensors.