Resistivity Method To Monitor And Evaluate Curing Of Concrete

Modern concrete preparing have the characteristic of large content ofcementitious materials and low water binder ratio, so that drying shrinkage andcracking of cement paste at early age is increased. The requirement of curing is higherand higher. However, there is no efficient method to monitor curing process andevaluate curing effect. This prompts that curing technology is not carried out strictly inconcrete construction process, and make the performance of concrete become poor.Usually concrete curing process is to keep the surface of concrete have higher wetdegree after casting. Generally, under dry condition, concrete can be regarded asinsulator; while under wet condition concrete has much pore solution, so itsconductivity improves. Therefore, using the characteristics of concrete resistivity tomonitor curing process and evaluate curing effect.The resistivity of concrete can be measured by a variety of devices. But due to thecomplex composition of concrete, it is difficult to test concrete resistivity accuratelyand stable. The stability and accuracy of three test methods of resistivity has beencompared. They are electrochemical Impedance Spectroscopy method, potentiostatmethod and digital multimeter method. The results show that, using the method ofdigital multimeter to read the initial resistivity can effectively avoid polarization in theconcrete resistance measurement. The test results are stable and accurate. Besides, theequipment is cheap and the testing process is fast. So it is suitable for rapidly andnondestructive test on-site. In addition, the influence of solid phase components ofconcrete (fly ash, carbon fiber) and the internal moisture content of concrete toresistivity have been studied. The results show that the internal moisture of concretehas a significant impact on its resistivity. As well as the internal moisture content isinfluenced by curing condition. So, through measuring resistivity of concrete canreflect curing process. But except the influence of curing, solid phase components ofconcrete also have a certain influence on its resistivity. Considering the difference ofconcrete types, material composition, mix proportion and other factors, usingresistivity to evaluate the curing effect must be based on comparison with thehomogeneous concrete. Therefore, in order to monitor curing effect of practicalconcrete projects, concrete specimens with the same mix proportion are necessary. In this paper, two mix proportions of concrete which design as C30and C60werecast. Three different curing conditions were applied to concrete specimens:(1) aircuring;(2) water curing;(3) dry curing. By embedding electrodes at different depthsfrom the surface of concrete, and testing the resistivity of concrete in different curingprocess in immersion state under water, the resistivity changes were analyzed anddiscussed. It is shown that there is significant relationship between curing process andthe resistivity changes between surface concrete(1cm、2cm) and internal concrete(5cm).Also, there is remarkable correlation between curing process and the resistivitychanges of surface concrete in immersion state under water, especially, high w/c ratioconcrete.The resistivity of inner region concrete or the same mix proportion concrete curedin standard conditions can be considered as the reference resistivity. Then，theresistivity of concrete after practical curing process is compared with the referenceresistivity. Curing process can be monitored according the difference between theresistivity of practical curing process and the reference resistivity. At the same depthsform the surface, resistivity of practical curing process is higher than the referenceresistivity, it can be considered that concrete is bad cured. Furthermore, the greaterdifference between the resistivity, the bad degree of curing. Similarly, when surface ofhardened concrete is immersed under water, the variation of its resistivity is greaterthan the variation of reference resistivity, the curing effect is poor, and the largerdifference between them, the worse effect of curing.