half-cell potential and resistivity testing

Excel Building Surveying Ltd have recently been commissioned to complete half-cell potential and concrete resistivity testing as part of wider structural investigation works at various sites including York, London and Bournemouth. Half-cell potential testing can provide a guide to the positions of likely reinforcement corrosion by measuring the electrochemical activity of concrete. The test equipment requires connection to embedded reinforcement in order to complete a circuit. The potential for active corrosion can be measured (in mV) on a voltmeter connected to the reinforcement and the concrete surface. Test node spacing, the presence of surface coating and the half-cell material itself can all have an effect on the readings. Both the test regime and interpretation of the results rely heavily on the experience of the surveyor/ engineer to provide useful, accurate results. Connection to reinforcement ordinarily requires a small breakout of the concrete. In order to minimise the extent of the breakout required, scanning is usually completed using a Proceq Profoscope electromagnetic cover meter. This is also used to measure the depth of concrete cover to reinforcement at each test node. Half-cell potential has been completed recently using Proceq Canin+ with both wheel and rod electrodes. The wheel electrode is used to cover large areas (slabs, carparks and the like) and as such results are based on the average reading taken over a large (programmable) unit- typically 500mm to 1m grid. The rod electrode is generally used for more focussed measurements- typically 300mm nodes or smaller dependent upon findings. Excel surveyors have the requisite experience to determine a suitable test regime. Results of half-cell testing to balconies of several blocks of flats in Bournemouth revealed that reinforcement corrosion was probably not contributory to a number of defects thus altering proposed refurbishment plans. Concrete resistivity testing has recently been completed on a multi-storey office block in London. Testing was completed using Proceq Resipod test equipment. The internal environment was generally so dry that surface resistivity readings were beyond the measurable range of the equipment. Small surface break-outs (3-5mm deep) were completed to enable contact between the 4-pin probe and material containing pore water. The surface was wetted by fine water spray. Readings showed very high resistivity and the likelihood of active corrosion was therefore relatively low.