مقایسه عملکرد روش RCMT برای ارزیابی سریع مقاومت بتن در برابر نفوذ یون کلر با روش‌های RCPT و مقاومت الکتریکی

Abstract: Chloride penetration resistance of concrete is considered a key parameter affecting durability of structures exposed to marine environments. Chloride diffusivity of concrete can be determined by immersion or ponding tests. However such tests are time consuming and costly. These facts have lead researches to develop accelerated tests such as the rapid chloride penetration test RCPT (ASTM C1202). In the RCPT test, the electrical charge passing during six hours through a 50 mm concrete disc, under potential difference of 60 volts is measured. A main concern expressed for the RCPT test has been the rise in temperature of concrete specimens which results in a reduction in concrete resistivity. The other criticism towards the RCPT test is the role of ions other than chloride ion, particularly the (OH)^- ion in conductance of electrical charge. It has therefore been suggested that some complementary cementitous materials can cause a reduction in the electrical charge passed, by reducing the concentration of (OH)^- ions in pore solution. Another quick method for measuring chloride resistance of concrete is the Rapid Chloride Migration test RCMT (AASHTO TP64). This test is in general similar to the RCPT method. However for avoiding the heating of specimen, the applied voltage is adjusted in accordance with the initially passed charge. Also in the RCMT test, for avoiding the influence of other charge carrying ions, the actual chloride ion penetration into the specimen is determined. A further method proposed for appraisal of chloride resistance of concrete, is determination of electrical resistance. For avoidance of heating of concrete specimens, electrical conductivity measurements are carried out for short durations. This research was carried out with the aim of comparing the results of various quick methods for determination of chloride resistance of concretes containing various supplementary cementitous materials. In the experimental study the control mix had a water cement ratio of 0.38 and cement content of 420 Kg/m³. Mixes containing various amounts of supplementary cementitous materials including silica fume, fine fly ash, pumice, fly ash and slag were studied. For the RCPT test, the temperature effect resulted in considerable overestimation in the effect of using of supplementary cementitous materials in improving chloride resistance of concrete. Temperature rise did not occur during the RCMT test, and this test therefore does not suffer In the case of optimum fiber value, the obtained results from laboratory tests have shown a reduction in compressive strength and value of ultrasonic test around 8.7% and 2.5% respectively. Moreover it has shown an increase in tensile strength, flexural strength, electrical resistance and value of VB test about 33%, 10%,11% and 51% respectively. Consequently utilizing such fibers in sleeper concrete could be suggested for practical applications.