Electrical Resistance Method To Assess Moist Curing Efficiency Of Concrete

Proper and adequate curing, especially adequate curing during early stage affect significantly the desired properties of concrete. However, in practice in China, it has not been paid an attention to that the curing of concrete structure and member in-site for a long time. Curing has always either been ignored or performed perfunctorily by specifiers or contractors. Lack of necessary care needed to insure good curing can be attributed to lack of a means by which a resulting poor end product can be identified at early age. Part of this problem may be due to at present lack of a effective method for making a quantitative assessment of how well concrete is cured and to contract documentation that makes no provision for penalties in the event of curing not being carried out. Therefore, it is beneficial for quality control of concrete through assessing its curing history accurately.The electrical resistivity of concrete is very sensitive to internal moisture content, the higher moisture content of concrete means the lower resistivity. The idea was that if a specimen was adequately cured, it would have a higher humidity within the near-surface layer concrete and smaller resistivity difference between the internal and near-surface layer during early stage, and in a case of poor curing, the opposite would be true. In this research, an improved method using potentiostat for measuring the electrical resistance of near-surface layer concrete at different depths is presented. The effects of different curing conditions on change in the electrical resistance of near-surface concrete with depths were investigated by the method. Moreover, the electrical resistivity method for assessing curing efficiency of concrete in-site was explored.To ensure the preciseness of measured values of resistivity, the stability and reproducibility of the electrical resistance test method performed on PS-6 potentiostat were investigated. Twelve standard resistances, resistance values varying from 0.33 k? to100 k?, were tested by potentiostat and digital multimeters, respectively. The test results were shown that the resistance values obtained by two test methods had a good correlation. And the high resistance values obtained by potentiostat were more stable. Thus, using potentiostat to measure the resistance of concrete is more appropriate for typically the resistance of concrete is higher?An investigation of the influence of the distance between two electrodes, ionic concentrations and electrode contact areas on the measuring results was carried out. The results indicate that the potentiostat is suitable for measuring the resistance of concrete with preciseness and stability.The small copper electrodes were embedded horizontally 50 mm within the surface 10, 20, 30, 40 mm and 50 mm of the specimens at the time of casting, the test of the electrical resistance of concrete was performed on PS-6 potentiostat. In this research, we made normal concrete and concrete containing fly ash. Fly ash concrete was prepared with the cement partially replaced by 15%, 30% and 50% of equal mass of fly ash, respectively. Three water binder ratios were 0.25?0.35 and 0.45, respectively. The curing conditions included 6?13 d or 27 d water curing, natural curing, plastic sheet or wet burlap curing and curing agent curing, etc. Simultaneously, the effect of different curing conditions on the electrical resistance change of near-surface concrete was examined. It is found that curing conditions have pronounced influence on the electrical resistivity of near-surface layer of concrete. Thus it affect mainly the resistivity of the concrete within about 0-2 cm from the surface, and the contribution of curing condition to electrical resistivity of the concrete beneath some 3 cm from the surface is negligible. Fly ash concrete was more sensitivity to curing condition. The longer the water curing duration, the smaller the resistivity changes with depths. The resistivity of the fly ash concrete specimens exposed to shorter duration of water curing was much higher than that of the plain concrete specimens. The results indicate that the resistivity of near-surface layer concrete is sensitive to relative humidity; when relative humidity decreases, the resistivity of near-surface layer concrete increases, and when relative humidity increases, the resistivity of near-surface layer concrete decreases.Based on the fact, i.e., the resistivity difference between the internal and near-surface layer is normally small under adequate curing condition. The change rule of resistivity under different curing conditions, water binder ratios and fly ash contents was investigated. The electrical resistivity method to assess curing efficiency of concrete was established based on the relationship between resistivity values at different positions. It is indicated that the difference (?d) between the average value of the resistivity values of 1 cm (?1) and 2 cm (?2) and the average value of the resistivity values of 4 cm (?4) and 5 cm (?5) is most suitable for evaluating curing efficiency of concrete. Moreover, it can eliminate the effect of mix proportion itself, only related to curing conditions.To make the method more effectively applied to field test, the test of the electrodes embedded in advance and embedded afterwards electrodes by drilling hole was performed. It is found that the testing results of the electrodes embedded in advance can be well correlated with those of the embedded afterwards electrodes by drilling hole. For large-scale specimens measuring 1000 mm×1000 mm×100 mm under natural ambient condition, the electrical resistivity of concrete within about 0-2 cm from the surface is very sensitive, and the electrical resistivity of the concrete beneath some 3 cm from the surface is not sensitive regardless of by the electrodes embedded in advance or the embedded afterwards electrodes by drilling hole. The difference between internal and surface resistivity (?d) was smaller under continuous plastic sheet curing than natural ambient condition. Test results for large-scale specimens were consistent with that for small-scale specimens. In practical field testing, the electrical resistance is measured by drilling hole, then embedding electrodes, it is not only beneficial for selecting freely test location, but also being used in-site. The electrical resistance test method that can be used to assess the curing history of concrete was validated using large-scale specimens. The resistivity difference between internal and near-surface (?d) at 28 days obtained regardless of by the electrodes embedded in advance or the embedded afterwards electrodes by drilling hole is shown to be very sensitive to curing condition. The resistivity difference between internal and near-surface (?d) at 28 days obtained by this testing method can reflect accurately the curing history of concrete.The test procedure is quick and uncomplicated, and found to be extremely sensitive to reflect changes in concrete curing condition. It will can be developed a new approach for making a quantitative assessment of how well concrete is cured in-site. By studying resistance profiles at different depths of water immerged specimens, concrete curing history and efficiency can accurately be evaluated.