Bundesanstalt für Materialforschung und -prüfung

International Symposium (NDT-CE 2003)

Non-Destructive Testing in Civil Engineering 2003
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Concrete hardening and moisture distribution in sandstones monitored by One-Sided 1H-Nuclear Magnetic Resonance

Frédéric Kohl, Bernd Wolter, Frank Knapp
Fraunhofer-Institut für zerstörungsfreie Prüfverfahren IZFP, Saarbrücken, Germany

Abstract

The nuclear magnetic resonance (NMR) phenomenon has been well known to physicists since the early fifties. Practical applications of this technique can nowadays be found in branches like medical imaging, protein analysis etc. However, conventional NMR measuring devices cannot be used for in-situ industrial application. This disadvantage is mainly due to the need for sample extraction required by these enclosing systems, but also to their large size and their fragility. Thanks to its light-weighted and rugged design, the one-sided access 1H-NMR device, which was recently developed at IZFP, offers the ability of being used in an industrial context. The aim of this paper is to show the suitability of this new concept for an almost unlimited number of applications in civil engineering. The results obtained in the last years and presented in this paper set the focus on two essential domains in common civil engineering, namely quality control during construction, where NMR was used e.g. in order to monitor the hardening degree of different fresh concrete samples and preventive diagnostic, e.g. recording of moisture depth profiles of sandstones commonly used in monuments considered as a part of our cultural heritage.

As these examples are not exhaustive, the conclusion will focus on further possibilities of implementing OSA-1H-NMR in civil engineering and similar domains.

Physical background

Most of the non-metallic liquid or solid materials encountered in everyday living contain hydrogen (e.g. hardening concrete, polymers, organic chemicals). The purpose of 1H-Nuclear Magnetic Resonance (1H-NMR) is to interact with hydrogen-containing matter in order to acquire a quantified signal, so that a proportionality relation can be established between the signal amplitude and the concentration of hydrogen atoms. The simplified principles of this technique are shown below.


Figure 1: Spin processes during a simplified NMR-Experiment; 1) No magnetic field is applied. Spins are arranged statistically; 2) Spins align according to Boltzmann's statistics when a static magnetic field B0 is applied; 3) A RF magnetic field B1 is superimposed to B0 according to the condition B0 B1. Resonance effect due to Larmor condition is observed (see text); 4) A NMR signal is caused by the phase coherence of the spins. Additionally, the longitudinal and transversal relaxation processes (T1 & T2) occur.

The initially statistically arranged magnetic dipole moments align along a static magnetic field B0. According to Boltzmann's statistics, the parallel aligned population is slightly higher than the anti-parallel one. Furthermore, according to the condition w0= g *B0 ( w0 is the so-called Larmor or resonance frequency; is the so-called specific gyromagnetic ratio) an orthogonal dynamic magnetic field B0( w0) is applied in order to subdue the dipole moments to an angular rotation q(t) (in figure 1 q(t)=90°). The dephasing of the dipole moments in this orthogonal plane can be measured as an induced voltage. This signal corresponds to the so-called FID (Free Induction Decay). More detailed information can be found in [1].

2 Monitoring of the hardening behavior of fresh concrete

2.1 NMR and concrete characterization

Nowadays a lot of characterization methods for cement based materials can be found in literature (e.g. ultrasonic or electrical resistance measurement methods). The need for sample extraction and even sample destruction often restricts these methods to laboratory use. NMR concrete characterization capabilities are known since the early fifties [2]. OSA-NMR (One-Sided Access NMR), which corresponds to the portable version of conventional NMR, creates the possibility of non-destructive in-situ examination for the first time.

2.2 Sample preparation

The OSA-NMR measurements were performed on five different concrete samples. The samples were prepared so that the whole set reflects typical real cases (see table below).

Cement [g] H2O [g] w/c [-] Gravel [g] Retarder [g]
Sample #1 200 120 0.6 - -
Sample #2 200 120 0.6 - 4
Sample #3 200 70 0.35 - -
Sample #4 200 120 0.6 200 -
Sample #5 200 120 0.6 200 4

The measurements were performed during three days with a mean measuring interval of about one hour. Due to technical considerations, measurements could not be performed during nighttime. The cement paste is based on Portland cement. The retarder is manufactured by Isola Bauchemie GmbH&Co.KG and is of the type VZ 2 (aqueous solution of tetra-potassium pyrophosphate). For both sample #2 and #5 the retarder was dosed in order to delay hardening by about 20 hours. The employed gravel corresponds to the diameter type 5 - 8 mm.

2.3 Results

In order to characterize the hardening behavior of the fresh samples described in the chapter above, a so-called CPMG experiment was performed. This experiment renders the determination of the T2-curve by recording several NMR-echoes possible. A typical T2-process can be described by the following decreasing (multi)-exponential model:

In the considered case, the model is limited to a mono-exponential case, so that only two parameters, namely A1 and T2 are generated. The results thus obtained are shown in figure 2.


Figure 2: Behavior of the fit parameter A1 (logarithmic scale). This parameter describes the NMR signal amplitude. It can be considered as a scale for the amount of free protons in the sample. In the current case this amount is directly correlated to the quantity of free water in the concrete sample. The decrease of this parameter can be observed from 10 h measuring time onwards.

Figure 3: Behavior of the fit parameter T2. Its progression describes the molecular mobility of water in the pores. The hardening process directly affects the pore diameter, which tends to become smaller with increasing setting time. The influence of the retarder can be observed on both sample #2 and #5 (~ 20 h).

The influence of the retarder can be observed on sample #2 and #5, respectively. A hardening delay of about 20 h can, as expected, be noticed for both samples (for parameter T2 in figure 3).
Furthermore the influence of gravel can be observed by comparing sample #4 with sample #1 (without gravel). By interpreting the T2-results (Figure 3) it is remarkable that gravel has a slight retarding effect. This phenomenon might be explained by the water retention effect caused by gravel. This "blocked" water is freed at a later point of time, so that the hardening reaction can be fully completed. A further influence of gravel can be observed in the upper diagram (Figure 2). Due to its size, gravel reduces the water density in the sample and consequently in the measuring volume (so-called sensitive volume). This lower water density is reflected by the signal amplitude, which is evidently smaller for both gravel samples.

3 Determination of the moisture distribution in common sandstones

3.1 Sample preparation

The whole sample set consists of three sample subsets of common sandstones (German Ruethen, Lime, Sander). Due to practical reasons each subset is divided into four identical samples (each 50x50x50 mm). Further preparation details are shown in the following table.

Subset 1 Subset 2 Subset 3
Sandstone type Ruethen Lime Sander
Subset shape (identical for all samples)
Oven-dry mass [g] 248.91 245.93 216.94 201.83 246.57 243.94
244.72 242.82 207.46 194.96 244.70 240.30
1st treatment Limited water supply (bottom of the sample, 2 min), water transport through capillary pores Limited water supply (bottom of the sample, 20 min), water transport through capillary pores Limited water supply (bottom of the sample, 60 min), water transport through capillary pores
Mass after 1st treatment [g] 257.98 254.82 221.24 208.11 249.93 247.28
254.89 251.00 212.86 201.54 248.18 243.24
2nd treatment - Limited water supply (bottom of the sample, 85 min), water transport through partially saturated capillary pores Air drying (whole sample, 124 min)
Mass after 2nd treatment [g] - 226.50 215.62 249.07 246.46
219.03 210.98 247.35 242.46

The measurements were performed by shifting the whole sample subset through the sensitive volume, beginning with the moistened bottom. The step increment, which defines the spatial resolution, is 1 mm. The maximum measurement depth is limited by the position of the sensitive volume (respectively by the measuring frequency) to 20 mm. Comparative measurements by using conventional calibrated NMR-imaging methods (device manufactured by Bruker BioSpin GmbH, Germany) and g-absorption methods were performed by Fraunhofer IBP in Holzkirchen, Germany.

3.2 Results

3.2.1 Ruethen sandstone

For this sample a complete depth profile was measured. The measurement was carried out by flipping the sample. Thus, the two extremities of the sample were covered with a measuring depth of 20 mm. The lack of data in the center of the sample is compensated by a linear interpolation. Figure 4 shows the results obtained. The red curve shows the comparative results obtained by performing -absorption measurements respectively spatially resolved NMR measurements (MRI).


Figure 4: Complete depth profile of the moistened Ruethen sandstone. The results of comparative measurements are displayed in the upper and lower diagram, respectively.

By computing the mathematical correlation coefficient the result r2=0.84 is observed ( g-absorption). Furthermore, the computation of this coefficient for the data pair OSA-NMR/MRI yields r2=0.94.

3.2.2 Lime sandstone

Lime sandstone corresponds to a highly porous sandstone (typically ~0.29 m3/m3). After only 20 min treatment a high water saturation level of the pores can be observed. This saturation is characterized by the blue curve (Figure 5). The red curve shows the progressing water saturation after the 2nd treatment.


Figure 5: Depth profile of the moistened Lime sandstone. The effect of the 2nd treatment (re-moistening) is well visible in the last third of the red curve. It can be noticed that despite re-moistening, the average signal amplitude of the red curve is slightly smaller than the amplitude of the blue one. This is mainly due to the continuing water transport through the capillary pores, which results in water evaporation at the other side of the sample.

The correlation of the measured curves (1st & 2nd treatment) to the MRI results has shown that 0.8 < r2 < 0.9. Due to an important lack of data, no correlation could be established to the g-absorption data.

3.2.3 Sander sandstone

Due to its low porosity (≤ 0.17 m3/m3) and inversely to its high density ( (≥ 2120 kg/m3) the Sander sandstone is a typical example for bad results. The low water content is responsible for an important decrease of the SNR (Signal-to-Noise ratio). Thus, the results obtained only show a moderate correlation to those acquired by performing reference measurements with the two methods mentioned above.


Figure 6: Depth profile of the moistened and subsequently air-dried Sander sandstone. The low water content of this sample results in a typical poor signal-to-noise ratio for the 1st measurement as well as for the 2nd measurement.
For both g-absorption and MRI the linear correlation factor r2 does not exceed 0.4, neither after the 1st nor after the 2nd treatment. It follows that Sander sandstone conditioned as described in the table above is a typical example for the limits which can be reached by implementing OSA-NMR for moisture measurement.

4 Conclusion and outlook

The examples presented in this paper demonstrate the versatility and the suitability, but also the limits of One-Sided Access Nuclear Magnetic Resonance in civil engineering.

In future, the focus will be set on further applications on this area. Measurements will for example be performed on steel beam reinforced concretes or low water-content concretes (w/c < 0.3). Furthermore, the influence of modern additives such as plasticizers, accelerators or sealers will be systematically examined with OSA-NMR.

Finally, occasional moisture profile measurements will be performed outdoor on real buildings (churches, castles...).

5 References

  1. E. Fukushima, S. Roeder, "Experimental Pulse NMR, a Nuts and Bolts Approach", Addison-Wesley Publishing Company, 1981
  2. K. Kawachi, M. Murakami, E. Hirahara, "Studies on the Hydratation and Hardening of Cement (Experimental Studies on Nuclear Magnetic Resonance of Water Molecules in Cement)", Bulletin of the Faculty of Engineering, Hiroshima University, 1955, p. 95
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