CAPÍTULO VII. CASO PRÁCTICO 7.1 PLANTEAMIENTO DEL PROBLEMA
7.3 INTERPRETACIÓN DE LOS DATOS
The water uptake of drill cores can be determined by immersing drill cores completely under water (so- called atmospheric water uptake—(DIN) EN 13755:2000) or by immersing the cores only with one surface up to 1 cm along the sides of the cores (so- called capillary water uptake—(DIN) EN 15148:2003).
In order to determine the water uptake, the weight of the cores has to be determined in regular intervals, e.g., 24 h, until the mass is constant. Then the cores are dried at 105°C until a constant mass is reached, and thus by the difference in mass, the water uptake can be determined.
If it is not possible to extract drill cores out of a structure, so- called Karsten’s tubes can be used to determine the water uptake of a building material. Figure 3.72 shows a Karsten’s tube, which is a glass tube with a height of 10 cm and a glass fitting. The glass fitting is con-nected to the surface by using dough. After connecting the Karsten’s tube to the surface, the tube itself is filled with water and the reduction of the water level within the tube is recorded over time. Usually the test times are between 5 and 30 min. The water uptake of the building material is then given in kg/ m²h0.5.
3.2.8.6 Sensors (MRE)
In order to measure the water content as well as the water distribution within the cross sec-tion of a structural element, sensors can be used that measure continuously the electrical resistivity of the material instead of the water content. The determination of the electri-cal resistivity can be done with more accuracy than the measurement of the water content.
By using suitable calibration curves, the measured resistivity can be transferred into a water content of the material.
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For more than 20 years the so- called multiring electrode (MRE) has been used to monitor the water content of the concrete as a function of the distance to the exposed concrete sur-face as a profile over time (see Raupach 1992). As most types of corrosion (frost, ASR, rebar corrosion, etc.) are to a considerable extent dependent on the water content of the concrete, long- term measurements (e.g., over one year) can give important information on the time dependence and maximum values of the corrosion rates.
The multiring electrode is made out of nine stainless steel rings, which are stacked together without any electrical contact in between. See Figure 3.73. Each ring—stainless Figure 3.72 Top: Karsten’s tube mounted on a brick wall. Bottom: Distribution of Karsten’s tubes on a
brick wall.
steel as well as the plastic spacer—has a width of 2.5 mm. Each transverse axis of the rings is 5 mm apart, so that the resistivity of the material can be measured in depths between 7 and 42 mm. By applying an alternating current to each abreast ring, the resistivity can be determined between rings 1 and 2, 2 and 3, and so on until 8 and 9. The specific resistivity (see Section 3.2.7.5) can be calculated by using a cell constant of k = 0.1 m.
The installation of the MRE is done either by installing the sensors before placing the concrete or by placing the sensors into drill holes by using a special anchoring mortar. Both methods are displayed in Figure 3.74. In both cases the installation has to be done carefully so that the multiring electrode does not detach from the casting during the concreting, and the anchoring mortar has to be inserted without any air entrapment. The sensors are con-nected to a measuring and recording device.
The measurement of the resistivity (see Section 3.2.7.5) has to be done with an alternat-ing current because the frequency does affect the measured values due to polarisation (see Catharin 1972; Warkus 2003).
The specific resistivity of concrete varies between roughly 50 Ωm for a water- saturated and hydrated concrete and above 10 MΩm for concrete that could dry off for a long time, e.g., interior building elements. Thus, concrete can be a fairly good electrical conductor but also an isolator. Figure 3.75 illustrates the time- as well as depth- dependent development of the resistivity of a dry concrete during the first three days of water storage. It can be noted that the water reaches the first measurement depth (7 mm) very quickly, while the inner layers need much more time until the water content rises, and thus the resistivity drops.
In order to interpret the results of the multiring measurements properly the following aspects have to be taken into account:
• The temperature has to be recorded and taken into account, e.g., by the Arrhenius correlation.
• The resistivity of concrete increases during the hydration process; thus, for young con-crete the increase of the resistivity does not necessarily correlate with a drying process.
• The properties of the anchoring mortar have to be taken into account if the electrodes are placed into drill holes.
Distance from concrete surface in mm Cables
Figure 3.73 Left: Multiring electrode (MRE) as a monitoring sensor for the distribution of the water con-tent of the concrete. Right: Schematic drawing of the resistivity distribution in over depen-dency of the depth.
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Figure 3.74 Schematic representation (top) of the installation of an MRE into the concrete by drilling a hole, inserting the MRE, and photographs of the gap filled with mortar (bottom) and MRE’s installed before placement of the concrete (right).
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Increase of the resistance
Figure 3.75 Time- and depth- dependent development of the resistivity due to water storage of concrete at 10°C after storage at 20°C/65% rh. (From Raupach, M., Schriftenreihe des deutschen Ausschusses für Stahlbeton (1992), no. 433, PhD thesis.)
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