Discussion of the evolution of the chloride migration coefficient of Portland cement concrete tested by the rapid chloride migration (RCM) test at long-term curing periods up to 5 years
Chloride-induced corrosion of reinforced concrete is one of the main deterioration mechanisms leading to shortening of the service life of concrete structures. Therefore, assessment of the resistance of concrete to chloride ingress plays an important role in predicting the service life of such structures. The Rapid Chloride Migration (RCM) test is one of the accelerated test methods commonly used in Europe. It is commonly assumed that for a properly cured Portland cement concrete without degradation, the chloride migration coefficient should decrease with increasing curing age due to on-going hydration and microstructure densification. However, recent experiments show that the rapid chloride migration coefficient of Portland cement concrete decreases at early curing age, but increases at later curing ages up to 5 years. In this paper, the evolution of the rapid chloride migration coefficient of Portland cement concrete during long-term curing periods up to 5 years is discussed. The test conditions for the RCM test and the test results obtained from RCM tests are described. Finally, possible causes for the increase in DRCM that occurred during late curing of the samples are investigated in order to explain this increase.
Ageing of portland cement concrete cured under moist conditions
Deterioration of microstructure in cement concrete will cause changes in the transport properties of the concrete. Transport properties at different ages of the concrete provide information about the microstructural changes of the material. A way to measure the transport properties, i.e. the chloride diffusion coefficient, in the laboratory is rapid chloride migration (RCM) Test. With this test, the resistance of concrete to chloride ingress is evaluated. Changing losing resistance represents deterioration and ageing of concrete. In this study, RCM tests were carried out for Portland cement concrete cured in moist conditions at various curing ages up to 3 years. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analysis were performed to identify the reaction product, like delayed ettringite. The experimental results show that the resistance of Portland cement concrete to chloride ingress decreases when ettringite is formed in concrete. Based on the result it is concluded that delayed ettringite formation contributes to the deterioration of Portland cement concrete, and should thus be considered as a relevant factor in the ageing process of concrete structures at moist curing condition.
The early evolution of the surface of hydrating cement
The development of surface area during the hydration of Portland cement has been followed by small angle x-ray scattering for times ranging from 10 minutes to 1 year. Up to the time of initial set little change in surface area is observed. After final set surface area develops steadily with time to an upper value of about 600 m2/g (dry basis) after one year. The results are correlated with isothermal conduction calorimetry and compared with other investigations of surface area development.