Cement Energy and Environment

ume, t. Both the variables are calculated from best fit analysis of chloride profile, in which the levels of chloride c. at various depths are measured after different exposure periods. This is explained in the Figure 4. Figure 4. Application of Fick's law to chloride ingress in concrete The defence against corrosion of steel is integral of cover depth and chloride diffusion coefficient (D). The effective chloride diffusion coefficient (D) depends upon the progress of hydration of cement, and, therefore, reinforcement. Other concerns like sulphate resistance or alkali silica reaction (ASR) are addressed through choice of the binder system. Presence of moisture is necessary for deleterious chemical reactions in concrete. It is quite apparent that transports of fluids - both liquid and gases - are important considerations. The main worry is about three fluids which can enter into concrete by different modes. These are water - pure or carrying aggressive ions; carbon dioxide; and oxygen. Various modes of fluid transport are sorption, Permeation and ionic diffusion. In view of so many possible modes, one should really be concerned with a notion of collective 'penetrability'; nevertheless, the commonly accepted term is 'permeability', which is mostly adopted to describe transport of fluids through concrete. The following solution to Fick's second law of diffusion can express permeation of chloride ion through concrete; X Cx = Cs [1 - erf-------------1 20{D.t) Where, C 5 =surface chloride level, X =depth from surface, c. = chloride level at depth, X, t = exposure time, D = chloride diffusion coefficient, and erf = error functi on From the above equation , it is clear that the value of Cx Decreases, as X increases. Build up of chloride ions inside concrete (Cx) is controlled by both the chloride diffusion coefficient (D) and t~e surface chloride level. (C 5 ) and the passage "of the age of concrete, Type of binder, and water-binder ratio. Chloride diffusion coefficient • Decreases with age of concrete, • Is lower, lower the water-binder ratio, • Is lower for blended cement and mineral admixtures. Typical relationships are shown in Figure 5 below. 1&10~~ tE-•1 1E-t2 ~- · · 'J -~Pfm · ,. :.=:::-r:::-=- -· No - ~ ~I ~ .... 10 Age (yn11) - •...:....L-....1 ·~ :MPaPC i so 60 '·I 70 .. ' 100 • pc ~pc, l:estflt 1 sf ••• sf, best m 0 pta & ggbs - - pfa&ggbs 1E·14 '-------...i__ _ _j 0.2 0 3 0 4 05 06 0 7 08 Watar·blnder ratio Figure 5. Dependence of chloride diffusiol' coefftcient on age water-bindC~r ratio and cement type 10