International Symposium (NDT-CE 2003)Non-Destructive Testing in Civil Engineering 2003
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Non-destructive Determination of the Water-Content in the Concrete Cover using the Multiring-Electrode
PART I: ASPECTS OF CONCRETE TECHNOLOGY
Prof. Dr. Ing. W. Brameshuber Prof. Dr.-Ing. M Raupach
1 ELECTROLYTIC RESISTIVITY OF CONCRETE
The electrolytic resistivity of concrete essentially depends on the chemical composition of the pore solution and the pore structure (concrete composition, compaction and curing) as well as the concrete moisture and temperature. The building material concrete can act as either an insulator or a good electrical conductor with respect to its electrical properties depending on its moisture content. A dense aggregate normally acts as an insulator on account of its very high resistivity, e.g. quartz
Figure 1 shows the ionic conduction in the hardened cement paste according to [3, 4]. The hardened cement paste can be classified into cement gel, open pores (e. g. capillary pores), closed pores and dead end pores. The pore solution contains both positive and negative ions (essentially Na+, K+, Mg++, Ca++, OH-, Cl-, SO4-) that are distributed evenly within the hardened cement paste without being affected by external currents. The positive ions (cations) move to the cathode and the negative ions (anions) to the anode when an electric field is applied over via two electrodes. The charge carriers in the closed pores or dead ends are blocked and act as condensers without affecting the ohmic resistance of the concrete .
Ionic conduction generally takes place via the liquid phase of the pore solution of the cement paste in water-saturated concrete . With semi-moist concretes the ionic conduction takes place via the mono-molecular water film adsorbed on the pore walls . The concrete acts as an insulator when it becomes very dry, the conductivity drops to lower values.
2 INFLUENCING VARIABLES ON THE RESISTIVITY OF CONCRETE
Apart from the moisture content of the concrete, the resistivity also depends on the pore structure, which depends on the concrete technology influencing parameters e. g. w/c-ratio, cement type..
Type of cement and additives
At high age blast furnace slag cements have a higher content of chemically bonded water and a higher content of gel pores in the overall gel pore volume than Portland cements . Both properties lead to an increased resistivity.
Over the past few years, coal fly-ash has increasingly been used as a concrete additive in concrete construction work. Coal fly-ashes are an industrial by-product from the generation of electrical energy in coal power stations and are classified as synthetic pozzolans. The effect of fly-ash additives on the resistivity of hardened concrete underlies in the reduction of the capillary pore content through the pozzolanic reaction of fly-ash in cement and the associated restriction of the ionic conduction. The resistivity of hardened cement paste with fly-ash is 3 to 4 times higher than that of hardened cement paste with only Portland cement under the same conditions .
Water cement ratio of the concrete and storage conditions
For water-saturated concretes the capillary porosity and capillary water content increase with higher w/c-ratio (> 0.40) and thus the resistivity decrease. Conversely, the resistivity increase with a higher w/c-ratio for concretes appropriate cured after manufacture. After the curing the specimens have to store in air so they can dry out. Then the concrete contains only little or no capillary water for the current and/or ionic transport (fig. 2).
Concrete age and degree of hydration
During the first 20 days after production a rapid increase can be measured in the resisance in cement paste and hardened cement paste (Portland cement). After 30 days the resistance of the hardened cement pastes with OPC reached approx. 90% of its 128-day value with a w/c-ratio of 0.80. The hydration can be regarded as being largely complete after 128 days.
The content of free water drops steadily with an increasing hydration age until hydration has been completed. The bonded water is no longer available to carry the current, the restisivity rises. In addition, the space between the cement particles fills up with cement gel and the capillary pore space that was initially present is reduced.
The hardened cement paste pore space can decrease by up to 20 % . Hardened cement paste with a high content of blast furnace slag or pozzolan cements carbonates slightly faster and stronger than that from Portland cement under otherwise identical conditions. This is because the lower Ca(OH)2 for the reaction and the increase in density is lower on account of the lower content of CaCO3. In addition, this may also be aided by the relatively larger gel pore space in case of lower w/c-ratios. This leads to an increase in the resistivity.
Rel = Electrolytic resistance at temperature T (e. g. 20 °C)
Values of between 3,000 and 5,000 K are quoted in the pertinent literature for the temperature constant b. Under normal conditions a change in temperature of 1 °C causes a change in resistance of approx. 3 % [10, 11].
3 DETERMINATION OF THE WATER CONTENT
The moisture distribution on concrete building structures are interesting for many building materials because the building material characteristics in particular the durability depends on the moisture content. In that way the rate of corrosion of the steel in the concrete, the frost resistance, the resistance of the concrete against aggressive materials, for example of sulphates, and the rate of the alkali silicate reaction are crucial affect by the concrete moisture. It is also necessary for the estimation of concrete surface coatings in regard to their ability to prevent the penetration of water into the concrete to know the moisture content in the concrete cover zone. Not only the moisture content but also the moisture distribution must be known.
For the determination of the moisture distribution in concrete cover zone it is possible to used the measurement method based on the resistivity.
The Institute for Building Materials (ibac) at the Aachen developed the Multiring-Electrode. With this method it is possible to determine the electrical resistance from a series of electrodes, which are one above the other arranged in defined distances.
Follow mathematical correlation between the resistivity and the moisture content could be calculated :
w = moisture content [% by mass]
Under laboratory conditions concrete specimen were manufactured with and without Multiring-Electrodes to determine the necessary measured data. In regular time intervals the depth dependent electrical resistance was determined. In addition discs with a thickness of approximately 15 mm were demerged from separate specimens which were stored under the same conditions like the specimens with the Multiring-Electrodes. In that way it was possible to get a depth dependent moisture profile with the appropriate resistivity. The moisture content was determined by weighing and drying of the discs at a temperature of 105 °C.
The data of the moisture content of the concrete and the electrical resistance charted in one diagram. The functional correlation is shown in figure 3.
The experiential parameters of equation (2) have to be determined by a regression analysis for any concrete. The calculated data have a satisfactory correlation to the measured data. With this functional correlation it is possible to get the real water content of this concrete when the resistivity is well-known. Furthermore it is noted that the measured resistivity data have to be corrected by the temperature. In that the temperature of the concrete has an essential influence on the mobility of the ions (chap. 1.3).
Research work is in progress. First results are found concerning the dependence of resistivity on water content. At the moment investigation are carried out on the determination of the water content for different types of concrete with a maximum aggregate size of 16 mm respectively 32 mm. The above mentioned mathematical correlation (equation (2)) between the water content and the resistsivty of concrete was determined on concretes with a maximum aggregate size of 8 mm.
The types of concrete for the determination of the water content were concrete from site. The concrete were placed in different structures e. g. tunnels, bridges, locks. The Multiring-Electrode was directly installed in the formwork to compare the results from the field structure to the laboratory. The concrete stored for 1 year and more under laboratory condition (20 ° C / 65 % relative hunidity). This was necessary to eliminate the hydration particularly when a slowly setting cement with a high re-hardening was used e. g. blast furnace cement or fly ash.
Furthermore the influence of the carbonation on the resistivity and the water content will be investigated. The determination of the water content will be carried out on concrete discs with a thickness of 15 mm. Different water contents are regulated by drying the concretes discs. The equilibrium moisture over the complete cross-section of the disc are achieved by shrink-packing. The resistivity are measured via the Two-Electrode-Method. First results show a reasonable correlation between the water content and the resistivity. The determined resistivity determined by the Two-Electrode Method correlates satisfactory with the one determined by Muliring-Electrode.
PART II: MEASURING TECHNIQUEProf. Dr.-Ing. Michael Raupach,
Dipl.-Ing. Christoph Dauberschmidt
The target of this research project in regard to the measuring technique is to determine the humidity-balance of the concrete cover by measuring the resistivity of the concrete in steps of 5 mm to a depth of 82 mm. Therefore two staged Multiring-Electrodes (MRE) are embedded in the concrete for resistivity measurement in combination with a Multitemperature-Probe (MTP) to compensate the temperature influence on the resistivity of the concrete. These three sensors are defined as one measuring point, measured by one Digital-Measuring-Unit (DMU) permanently. Each construction consists of four measuring points located at different exposure conditions of the structure. One central Datalogger is equipped with a Multiplexer and Cellular Engine (wireless modem) for data exchange and remote control. As power supply a solar panel combined with a lead accumulator is installed in cases without external electricity supply. In total 10 of these monitoring systems are or will be installed in bridges, tunnels, lock structures, purification plants, quay walls and a trough bridge al over Germany.
The data will be recorded for a period of at least two years. This will give further information on the water content and humidity changes of the concrete cover and thus the classification of structural members into exposure conditions in accordance to the new DIN 1045.
The principle of the measuring technique is shown in Fig. 1.
The resistivity of the concrete cover is measured with a special sensor, the Multiring-Electrode. This sensor has been developed more than 10 years ago at ibac and until now many applications of this non-destructive system have been carried out , , like testing the effectiveness of surface coating systems, evaluation of the quality of the concrete or measuring the electrolyte resistivity of concrete to assess the risk of corrosion of the reinforcement. The sensor consists of several 2.5 mm thick stainless steel rings which are kept separated at a distance from one another by 2.5 mm thick insulating polymer rings. Cable connections in the sensor enable the resistivity of the concrete to be determined between each pair of neighbouring stainless steel rings. The sensor used in this research project has 9 rings of stainless steel to measure the resistivity at 8 depth steps between 7 and 42 mm from the concrete surface. A second sensor is embedded in a distance of 40 mm from the concrete surface to measure the resistivity in the range of 47 to 82 mm depth. Fig. 2 shows the set-up of a MRE schematically with a photograph of the sensor.
In new structures the sensor is embedded before concreting by fixing it to the mould (see Fig. 1) or to the reinforcement. For existing structures a coupling technique has been developed by using a coupling mortar. Therefor a hole Æ 25 mm has to be drilled into the concrete. Then the sensor is fixed centric into this hole. Finally the space of approx. 2 mm between the sensor and the concrete is filled with a non-shrinking polymer modified coupling mortar. After a certain period of time this coupling mortar is in a balanced humidity to the surrounding concrete.
As the resistivity is strongly dependent not only on the water content of the concrete but also on the temperature, the influence of the temperature has to be compensated. Therefore the temperature of the concrete has to be determined simultaneously to the resistivity of the concrete in different depths. The Multitemperature-Probe (MTP) enables to measure the temperature in 8 different depths from the concrete surface by using 8 PT 1000 probes. As the temperature is likely to change more strongly near the surface, the arrangement of the PT 1000-sensors is narrower near the concrete surface. Fig. 3 shows the set-up of this probe and the arrangement of the sensors for one measuring point.
The principle of the temperature compensation is shown in Fig. 4: using the Arrhenius-Equation according to  the influence of the temperature on the resistivity can be compensated. Investigations show that using a b-value of approx. 3000 1/K results in a satisfactory compensation.
4 DIGITAL MEASURING UNIT, MULTIPLEYER AND DATA-LOGGER
The Digital Measuring Unit (DMU) has been developed at ibac to measure 2 MRE and 1 MTP sensors by means of AC resistance measurement at a measuring frequency of 108 or 10.8 Hz respectively and a maximum voltage of 1 V , . The resistances between the rings are measured one after the other and the DMU produces a digital output in ASCI with 24 measuring values via a RS 232-interface. The measuring unit and its plugs are watertight according protection class IP 67.
Four of these DMU are connected to the multiplexer at the data-logger. The commercialised data-logger provides a remote control function in combination with a cellular engine as well as data saving and a battery. A photograph is shown in Fig. 1.
A non-destructive method to determine the water-content of the surface near concrete by measuring the resistivity of the concrete has been developed. Besides the sensors (Multiring-Electrode and Multitemperature-Probe) the measuring and data-logging technique have been advanced to use off-site. Since November 2001 these systems operate without problems exposed to natural weathering.
The research project is financed by the Federal Ministry of Transportation, Construction and Housing, the Federal Institution of Water Engineering (BAW) and the Deutscher Ausschuss für Stahlbeton (DAfStb).