Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Distributive type parameters
Reexamination Certificate
2001-07-16
2003-09-02
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Distributive type parameters
C324S632000, C324S643000, C324S647000
Reexamination Certificate
active
06614240
ABSTRACT:
The invention described herein was made in the performance of work under a National Science Foundation grant identified by #CMS-9817695.
FIELD OF THE INVENTION
This relates to the determination of the content of one or more materials in a cement containing composition, such as the amount of chloride in a cement based composition.
BACKGROUND OF THE INVENTION
The construction industry is interested in new techniques for nondestructive inspection of materials. Currently the techniques used are adequate in some cases, but may not be the case in others. One solution to this problem would be to combine one technique with others.
Chloride has been found to corrode steel members in reinforced structures. In solving this problem, early detection and close monitoring of chloride contaminated structures is essential in maintaining these structures. A nondestructive technique for determining chloride contamination would be beneficial.
Research has found that non-conducting materials, i.e. dielectric materials, can be analyzed by using microwave nondestructive testing (NDT) techniques. Microwave NDT techniques have been used to find surface and sub-surface degeneration in layered materials due to impact damage, and to measure the thickness of dielectric sheets. Such techniques also find unfilled spaces and air bubbles (locally and distributed) in dielectric materials, and are used to locate and evaluate disbond and delamination in multi-layered structures. Microwave signals can be used to measure the dielectric properties of a material. By knowing the dielectric properties of cement, aggregate, and sand, microwave signals can be used to measure the properties of the combined mixture. Also, this can be used to determine the curing rate and the presence of chemical reactions in the mixture.
Research has been conducted in this area. It has been found within recent years that this technique can be utilized for inspecting cement based construction composition. Near and far field techniques were the two main groups studied. The near and far field regions are based on the distance in which the sensor and the composition are separated from each other. Ground penetrating radars are an example of a far field technique and have been used successfully. Although it has been a success, there are still disadvantages to this technique. For example, repetitious calibration of the measurement equipment, the system spatial resolution, the inaccuracy of the data needed due to unwanted objects and the tedious signal processing needed to analyze the data are drawbacks. These are avoided when using the near field technique. The setback of operating in this region is that the electric and magnetic fields are very complicated to model.
The measured magnitude of reflection coefficient is shown to increase as a function of decreasing w/c ratio for cured cement paste. At first glance this seems inconsistent with the fact that higher water content should render a higher magnitude of reflection coefficient measured at a waveguide aperture. However, a closer look reveals that during the curing process water molecules bond with cement molecules, and some of the remaining free water evaporates. Thus, the water becomes less free and more bound over the curing time. Free water has much higher dielectric properties compared to those of cement powder, whereas bound water has similar dielectric properties to those of cement powder. In addition, higher w/c ratio specimens lose more of their free water to evaporation. Thus, the measured magnitude of reflection coefficient of these specimens decreases as a function of increasing w/c ratio.
The magnitude of reflection coefficient has been shown to be distinctly correlated to the w/c ratio of cement paste, and subsequently to its 28-day compressive strength (moist cured for 3 days in a hydration and thereafter in an air room temperature).
A simple expression predicting the microwave reflection properties of cement paste as a function of time has been obtained. Consequently, the w/c ratio of a cement paste specimen may be obtained by comparing two reflection coefficient measurements conducted several hours or a few days apart after the paste has been cast. In addition, it is possible to correlate the compressive strength of cement paste during curing to the measured microwave reflection properties (as a percentage of the 28-day strength).
A relationship between the standard deviation of the magnitude of reflection coefficient at higher frequencies and the s/c (sand/cement) ratio of a mortar specimen, has been established. Information on the w/c ratio of mortar specimens is obtained when the average value of the measurements is taken at relatively low microwave frequencies.
Mortar is a homogeneous dielectric mixture (even when measured at a frequency of 10 GHz). A simple dielectric mixing model has been obtained which predicts the constituent volume content of a mortar specimen. Consequently, the porosity (volume content of distributed air) of a mortar specimen can also be determined.
The statistical behavior of the microwave reflection properties of concrete as a function of w/c, s/c and ca/c (coarse aggregate/cement) ratios and the frequency of operation has been studied. It has been determined that the probability distribution functions of the measured magnitude of reflection coefficient of concrete, measured at high and low frequency bands, possess distinct and well-known distributions. At higher frequencies, the distribution is Gaussian whereas at low frequencies the distribution is uniform. With the use of the modifiable parameters in each of these distributions, the constituent volume distribution of a given concrete mixture can be determined from its scattering characteristics.
Similar to mortar, the results of the reflection property measurements indicate that the w/c ratio in concrete, and hence its strength, can be correlated to the average value of the magnitude of reflection coefficient measured at several independent locations on a specimen at lower frequencies (i.e., about 3 GHz). At lower frequencies the influence of aggregate size distribution is less on the measured magnitude of reflection coefficient than at higher frequencies since the aggregates electrically “look smaller” at lower frequencies.
Similarly, the standard deviation and the statistical distribution of the measured magnitude of reflection coefficient at higher frequencies is a function of the aggregate size and volume distributions. Hence, the constituent volume fraction and distribution of a concrete specimen may be determined at higher frequencies (i.e., about 10 GHz).
It has been shown that the cure state of concrete specimens, containing different w/c ratios and constitute makeup, can be unambiguously determined when making daily measurements of the magnitude of reflection coefficient.
It has also been shown that the w/c ratio of fresh concrete can be unambiguously determined independent of its s/c and ca/c ratios. This is an important finding since now an operator is capable of determining the w/c ratio of a batch plant concrete at the time of pouring.
It has been demonstrated that the extent of aggregate segregation in concrete placement can be evaluated using the statistics of the measured magnitude of reflection coefficient. This information can be easily obtained for concrete members such as walls and columns in which aggregate segregation may be an important practical issue.
Using an optimal frequency of operation, it has been effectively demonstrated that using a simple near-field and nondestructive microwave inspection technique employing an open-ended rectangular waveguide probe at 3 GHz (S-band) one can easily distinguish between empty and grout-filled masonry cells. In addition, a simple and extremely effective custom-built microwave inspection system has been designed and assembled for this purpose. This system has been successfully tested on a variety of masonry blocks.
Up to this point, the near field microwave NDT technique has been successfully applied to the inspection and cha
Benally Aaron D.
Bois Karl J.
Kurtis Kimberly
Zoughi Reza
Colorado State University Research Foundation
Le N.
Nguyen Vincent Q.
Sheridan Ross. P.C.
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