Abstract:
Reinforced concrete is one of the widely embraced and accepted construction materials all over the world. However, it can be threatened by various factors especially in aggressive environment. One on these factors is corrosion caused by ingression of chloride ion in concrete to reach reinforcement. Hence the need to study and evaluate available methods of determining the rate of chloride ion penetration in concrete and the resistance of such concrete to the penetration. These methods are categorized into two and they are either Diffusion tests (short term tests) or Electro migration tests (long term tests). Electro migration tests used in this research were Rapid Chloride Permeability (RCPT) and Rapid Migration Test (RMT) while the diffusion tests used are Bulk Diffusion test and 90-day Salt Ponding Test which allow a more natural penetration of the concrete by the chlorides.Thirty-six concrete samples were prepared using M20 (1:2:4) concrete typically used in construction in Nigeria. Nine samples were prepared for the RCPT of which three samples each were analysed on the 7th day, 28th day and the 56th day. Their diffusion coefficients were also calculated from the RCPT total charge passed through them. Nine samples were also prepared for the RMT and three samples each were analysed on the 7th day, 28th day and the 56th day. The diffusion coefficients of the RMT samples were calculated from the measured penetration depth of the chloride ions into the concrete sample detected by the application of Silver Nitrate solution on the surface of the axially split sample after the test. Diffusion coefficients were determined from the Bulk Diffusion test using a 35, 56 and 90 days of age concrete sample while three samples are analysed on the 90th day for the salt ponding test and their average diffusion coefficient were calculated.The diffusion coefficients obtained in all the test methods decreases with age. This is an indication that concrete’s resistance to chloride ion penetrability increases with age because as the concrete ages, the water cement ratio decreases which strengthens the pore spaces of the concrete.The results show that concrete’s diffusion coefficient decreased rapidly during the first 28 days by 54.2% and then decreased slightly by 12.7% during age 28 to 56 in the two short term tests.The Diffusion Coefficients calculated from the AASHTO T-259 (ponding) test are higher to those obtained from the AASHTO T-277 (RCP), which has been attributed to thehigher surface concentrations (CS) achieved in the natural ponding test.
