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l Short Communication

Magnetic Resonance Imaging, Vol. 14, Nos. 7/8, pp. 931-932, 1996 Copyright 0 1996 Elsevier Science Inc. Printed in the USA. All rights reserved

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DETERMINATION OF MOISTURE PROFILES IN POROUS BUILDING MATERIALS BY NMR

L. FEL, K. KOPINGA, AND H. BROCKEN

Eindhoven University of Technology, Department of Physics, P.O. Box 513, 5600 MB Eindhoven, The Netherlands

Because moisture in porous building materials can give rise to several kinds of damage, a detailed knowledge of the moisture transport is essential. For such studies it is important to measure dynamic moisture profiles quantitatively. Nuclear Magnetic Resonance (NMR) offers a powerful technique to measure such profiles in a nondestructive way. Because of the large amount of paramagnetic ions present in many of these materials (0.1~5% Fe) standard NMR imaging equipment cannot be used. A NMR apparatus will be described that was especially developed to study the moisture transport in porous building materials. At the moment, one-dimensional moisture profiles in samples with a diameter of 20 mm can be measured with an accuracy of 1% and a resolution of 1 mm. It takes about 40 s to determine the moisture content at a specific position. Copyright 0 1996 Elsevier Science Inc.

Keywords: Moisture profiles; Porous building material; NMR.

THE NUCLEAR MAGNETIC RESONANCE TECHNIQUE

Assuming a single exponential relaxation and T, + T2, the magnitude of the NMR spin-echo signal is given by:

S - p[l - exp(-TRIT,)] exp(-TEIT,). (1)

Here, p is the density of the hydrogen nuclei, T, the spin-lattice relaxation time, TR the repetition time of the spin-echo experiments, T2 the spin-echo relaxation time, and TE the so-called spin-echo time.

Almost all studies on moisture transport in porous building materials up to now were limited to materials like limestone and sandstone, for which a good signal is observed for TE = 2-20 ms and TR > 300 ms. These materials can, therefore, be imaged by standard MRI equipment. However, in many common porous building materials usually large amounts of paramag- netic ions (e.g., Fe) are present, which complicates the NMR measurements by two effects. First, T2 will be drastically decreased by paramagnetic centers at the surface of the pores. Secondly, local field gradients

will be induced, which broaden the resonance line and thereby limit the spatial resolution.

In porous materials the spin-spin relaxation process cannot be described by a single exponent, r and, hence, T, varies with TE. The results of T2 measurements on various building materials for TE < 1 ms are summa- rized in Table 1. In this table the values of 7’, are included. For fired-clay brick an effective T2 of the order of 200 hs is found. This explains that with stan- dard NMR equipment, which usually employs a TE of a few ms or more, hardly any signal will be found.

To check whether T2 can be related to the amount of Fe in these materials, magnetization measurements were performed. Table 1 also lists the experimental values of the susceptibility x = dM/dH at B = 0.78 T, corresponding to the field at which the NMR experi- ments are performed. Inspection of this table reveals that T2 tends to decrease when x, and, hence, the Fe content, increases.

EXPERIMENTAL SET-UP

The entire apparatus is home built. A description of the RF section and the data acquisition can be found

Address correspondence to L. Pel, Eindhoven University MB Eindhoven, The Netherlands. of Technology, Department of Physics, P.O. Box 5 13, 5600

931

932 Magnetic Resonance Imaging l Volume 14, Numbers 718, 1996

Table 1. The relaxation times of hydrogen nuclei in various types of porous building materials determined

from NMR as well as the differential magnetic susceptibility x per gram at 0.78 T

Type of material T b-4 T2 (ps) x (10m6 emu g-‘)

Fired-clay brick 300 180 3.43 Sand-lime brick 45 850 0.53 Mortar 35 1000 0.13 Gypsum 50 4100 -0.26

in Refs. 2 and 3. This apparatus uses a conventional electromagnet, generating a field of 0.78 T (33 MHz). This field was found to be an acceptable compromise between the signal-to-noise ratio of the spin-echo sig- nal and the required spatial resolution.

Our samples are rods with a diameter of 20 mm and a length between 20 and 180 mm. The vertical position of the sample is controlled by a step motor. Because our aim is to perform quantitative measure- ments, special attention was given to the impedance matching of the NMR probe. To reduce the effects of the variation of the dielectric permittivity by a chang- ing moisture content in the sample, a Faraday shield was placed between the LC circuit (Q = 40) of the probe head and the sample. Anderson gradient coils were employed, yielding magnetic field gradients in one spatial direction up to 0.3 T/m. No attempts were made to switch off these gradients during the individ- ual spin-echo sequences. The spin-echo signal was ex- cited by straightforward (90,-r- 180,,) Hahn pulse se- quences (T,. = 15 ps) . After Fourier analysis of the spin-echo signal the moisture distribution is obtained over 2 to 3 mm.

TYPICAL PERFORMANCE

From measurements of the moisture distribution of completely wetted samples with a flat top or bottom, the one-dimensional spatial resolution of the equip- ment was found to range between 0.8 and 1 .O mm. The sensitivity was calibrated by measurements on various series of samples with a different moisture content. To measure all water in the material (also the fraction with a small T2) the distance between the pulses of the Hahn sequences was kept as short as possible, i.e., 80 ps (TE = 190 pus). In all cases a perfect linear relation between the integrated moisture profiles and the corresponding masses of water is found. With the NMR apparatus it takes about 40 s to determine the moisture content at a specific position with an inaccu- racy of 1%.

d

0 20 40 60 60 100

position (mm)

Fig. 1. Moisture profiles measured during the absorption of water in a fired-clay brick. The curves are only meant as a guide to the eye, whereas the times are only given as an indication of the elapsed time. In the inset, a sketch of the experimental configuration is given.

To illustrate the potential capabilities of our equip- ment, we show the results of an absorption experiment on a sample of fired-clay brick. In Fig. 1 we have plotted the raw data, i.e., a number of partial profiles, each corresponding to five data points around the RF center frequency. Such a partial profile was acquired in 12 s. Because, especially just after the start of the experiment, the velocity of the wetting front may be rather high, a time stamp is added to each set of points, to allow a meaningful interpretation in terms of appro- priate theoretical models. The entire experiment took about 1 h. A detailed interpretation of these measure- ments can be found in Ref. 3.

REFERENCES

1. Brownstein, K. R.; Tarr, C. E. Importance of classical diffusion in NMR studies of water in biological cells. Phys. Rev. A 19:2446-2453; 1979.

2. Kopinga, K.; Pel, L. One-dimensional scanning of mois- ture in porous materials with NMR. Rev. Sci. Instrum. 653673-3681; 1994.

3. Pel, L. Moisture Transport in Porous Building Materials, Ph.D. thesis, Eindhoven University of Technology, the Netherlands; 1995.