Moisture content of concrete has a significant influence on its performance in various types of structures. A number of methods have been developed for determining the moisture content of concrete. Most of the methods in use today are indirect, i.e. a calibration curve, giving the relation between moisture content and some other property of concrete, such as electrical impedance, is required. Impedance measurement method has been used extensively for studying the moisture condition of various building materials. Usually a circuit with a voltmeter and ammeter is employed to determine the impedance of the gauge, which is related to the moisture content. However, the method can, under certain circumstances, give fallacious results, because the gauge does not behave as a simple resistance, but has an appreciable capacitance also. The impedance of such a gauge should, therefore, be determined with an a C bridge, which balances both resistance and capacitance. An "electrical double layer", which is formed on the electrode surfaces, gives rise to the capacitance of the gauge. The properties of the double layer are described in the report. Moreover a review of the various theories of the origin of double layer are given. Some results of impedance measurements on gauges stored in different humidity environments are also included. The test results indicate that the electrode material and the age and quality of concrete have a large influence on the capacitance and the impedance of a gauge. The capacitance decreases as the material dries out. Since the capacitance is frequency-dependent and decreases with increasing frequency impedance measurements, particularly with the voltmeter-ammeter method, should be carried out at a frequency of 1000 hz. The effect of the double layer, on measurements using moisture sensors, can be reduced appreciably by giving the electrodes a coating of platinum black.

  • Availability:
  • Corporate Authors:

    Statens Raad Foer Byggnadsforskning

    St Goeransgatan 66
    Stockholm,   Sweden 
  • Authors:
    • Bhargava, J
    • Rehnstroem, A A
  • Publication Date: 1978

Media Info

Subject/Index Terms

Filing Info

  • Accession Number: 00189552
  • Record Type: Publication
  • Source Agency: National Technical Information Service
  • Report/Paper Numbers: Document D 11:1978 Monograph
  • Files: TRIS
  • Created Date: Apr 12 1979 12:00AM