Aus dem Berichtsband zu Darmstadt Concrete 1998, Seite 131-141
Moisture measuring on recycled aggregates
SUMMARY
Recycled aggregates are highly porous and absorb water in a range between 5 -
10 percent of their dry weight. So it is necessary to take this amount of "suction water" into account, when recycled aggregate is used to mix concrete.
A determination of the water distributed in the various materials making up recycled aggregates is important to calculate the amount of water absorption in concrete. English literature lists many methods and studies as well as the German. In this Research British literature will be examined as well as the German. Obviously, in both countries the methods of determining the amount of water, recycled concrete will absorb have not been studied properly yet. A lot of methods to specify the water absorption of original concrete are available, but there is no precise study of the water absorption of recycled material.
This investigation deals with monitoring water absorption on single grains of recycled aggregates. Samples are tested in the laboratory by using the gravimetric method, a copper sulfate powder and especially the application of a water indicating paste from the company (Merck). Enclosed pictures illustrate the results.
1. THEORETICAL ASPECTS
The theoretical aspects of capillary water suction have been accurately studied /1-6/ and many different methods have been developed and documented. The methods differ in complexity of their apparatus, accuracy cost, precondition time and the amount of time each test requires. The results, however, still depend on the applied method. They are not yet comparable, since each method leads to different parameters that do not depend from each other.
Because differing methods lead to different results it is very important to modify the methods so that the test results are comparable. The most important methods in the English literature are the sorptivity test, the ISAT (Initial Surface Absorption Test), the CAT (Covercrete Absorption Test), TDR (Time Domain Reflectometry) using an electromagnetic signal to measure dielectric constant, neutron radiography which detects neutron scattering through water, y-rays, X-rays, MRI (Magnetic Resonance Imaging) and the gravimetric method.
The German methods are divided into two categories. The conventional method as the Resistance and the Capacity measuring method, the conventional drying of the sample (Darrmethode) dries the materials in order to get the amount of absorbed water and the final gravimetric reading. The second category consists of measuring specific radiation such as y-rays, X-rays, ultrasound, microwave, the neutron method, the tracer method and the magnetic resonance imaging (Kernmagnetische Resonanz) method. Most methods determine either the distribution or the amount of the absorbed water. Only the MRI method seems to be able to accurately define the distribution and the exact amount of water fast enough without falsifying the results by the previously described spreading of the absorbed water.
In the literature the different coefficients of variation of the various methods are not comparable or even left out completely. There have been studies to compare the results from the different methods like GAT, ISAT and sorptive tests but still it is not possible to compare all of the methods regarding their accuracy, the precondition time and the cost. In spite of the large amount of research involved in developing new tests to evaluate the amount and distribution of the water absorbed by recycled concrete, little effort has been made towards comparing these tests and their results. For example, ISAT and CAT both study the water absorption of concrete using similar techniques and yielding permeation characteristics with identical units measurements. However, there is no indication yet why CAT always produces higher values than ISAT.
After reviewing the literature the CAT was selected from those mentioned earlier as a promising and easily modified method for further investigation, and in particular for in-situ-application. A conspicuous advantage of the in-situ CAT test is that apart from the test apparatus (which is very simple construction) no specialized methods or machines (e.g. coring) are required.
The sorptive test is only applicable in the laboratory, while others are used both in the laboratory and on site, e.g. ISAT (Levitt 1969), and CAT (Dhir et al. 1987).
However, the non-damaging test methods are to be preferred, such as MRI or even the easily applicable gravimetric method. These tests allow the continued testing of samples after a period of time, so the distribution or amount of absorbed water can be determined over time. The gravimetric method is able to build the correlation between the absorbed water and the testing time. The MRI method determines the exact distribution. MRI is often mentioned and its exact results make it very mature. In connection with the gravimetric method the MRI is able to determine the amount as well as the distribution without damaging the sample. The detection time is variable. Water suction, that is, observing the H (proton) resonance into porous media such as recycled concrete, mostly dried at 105°C, has been studied by various authors (1-4). The typically broad spectrum of porous systems which limits the spatial resolution in standard imaging experiments and the short transverse relaxatian time make studies af water suction in concrete difficult and due to the even more porous nature of a recycled concrete even worse. There are different types of MRI to detect the distribution of the water. Not all are applicable on a concrete sample since some of them have a poor signal-to-noise ratio attributed to relatively large amounts of rest water. The Deuterium MRI enables to test even moist samples. To avoid problems with the rest water, distribution tests are performed with deuterated water as a suction liquid.
2. SCOPE OF WORK
The distribution as well as the precise amount of the absorbed water shall be determined. In the first tests the amount is read by using the easily applied gravimetric method (DIN 4226). The distribution is then determined with the earlier mentioned copper sulfate powder and a water indicating paste. The laboratory cast samples are dried with two different temperatures, 105°C and 37°C, in order to see the difference in the absorbed water. Then they are exposed with water. After preset times (1 min, 3 min, 5 min, 10 min and 24 h) the water supply is shut off and the weight of the sample is registrated.
Pic 1: Water absorption of single grains from the rgain fraction 16/32 mm
This allows the determination of the absorbed water. Simultaneously another group of samples is broken open after the same times. The copper sulfate powder is evenly spread over the surface to indicate the water distribution after the preset times. The following representative illustrations are taken after running several tests.
Pic.2: Cracked aggregate grain after five min of exposure to water followed by the application ofthe indicator paste: a) brick b) concrete
The waterfront is not clearly defined therefore another test is run with the same type of recycled concrete. The samples are equally dried then put into water and broken open after the water supply is shut off. Now the waterfront can be detected easily. Using the paste is an easy, fast and accurate way to determine the water distribution. And this is a low-cost application.
3. CONCLUSION
The MRI method as a non-destructive method is to be preferred. The only disadvantage is the very expensive required equipment. The laboratory tests show that the distribution as well as the amount of the absorbed water can easily be detected with a very low cost application system.
4. REFERENCES
| 1. | Krus. M. and Kiessl k. NMR-Messung kapillarer Feuchtigkeitsbewegungen in porösen Gesteinen und Ableitung neuer Kapillartransport-Kenngrößen. Jahresbericht aus dem Forschungsprogramm Steinzerfall-Steinkonservierung, Fraunhofer Institut für Bauphysik (D), 1989. |
| 2. | Gummerson R.J., Hall C. und Hoff W. Unsaturated flow within porous materials observed by NMR imaging. Nature 1979, 281, 56. |
| 3. | Carpenter T.A. et al. Capillary water migration in rock: process and material properties examined by NMR imaging. Mater.Struct., 1993, 26, 286-292. |
| 4. | Kaufmann J., Studer W. , Link J. and Schenker K. : Study of water suction of concrete with magnetic resonance imaging methods. Magazine af concrete research. 49, 157-167, 1997. |
| 5. | Baratta M., Study of the distribution and amount of absorbed water of recycled concrete dried at 37°C and 105°C. The study includes literature as well as laboratory experiments. TU-Darmstadt. June 1998. |