INGRESS OF CHILORIDE IN CONCRETE

Abstract

This thesis reports the work and findings on the ingress of chloride ions with and without water movement in concrete subjected to local climatic and aggressive environmental conditions. A method based on experimental and analytical work is developed to predict the long¬ term performance of concrete under different exposure conditions. The governing equations for water diffusion, sorptivity and permeability coupled with chloride dispersion, advection and adsorption, as well as initial and boundary conditions, are incorporated in the modeling so that chloride ingress with and without water movement in concrete are better represented and therefore more realistic. The water diffusion and sorptivity tests, together with the accelerated water permeability test, are used to develop a model for water movement; the chloride diffusion test for the development of a model for chloride ingress without water movement; and the accelerated chloride permeability test as well as the site exposure test to establish a model for chloride ingress with water movement. Since the experimental and site data agree well with the finite element solutions from analytical work performed under similar conditions, the validity of these models is warranted. This confirms the applicability and reliability of the proposed prediction method, thus enabling an engineer who performs core sampling to predict the long-term chloride concentration profile of new or existing reinforced concrete structures located in different exposure zones, based on the semi-empirical models and current site data.

A study on OPC concretes with and without ground granulated blast furnace slag (GGBFS) and silica fume (SF) is also carried out to investigate chloride ingress in concrete containing mineral admixtures in relation to the pore size distribution and strength development, including the influence of ambient temperature. It is found that even with a denser microstructure and thus a greater early strength development due to a higher temperature, the rate of chloride ingress is faster than that at a lower temperature. This suggests a need to consider the effect of ambient temperature in the design code of reinforced concrete structures, in relation to durability requirements, to cater for local climatic and aggressive conditions in tropical and hot countries. For a lower ambient temperature, water-binder ratio, applied hydrostatic pressure gradient and exposure percentage to seawater, as well as a higher replacement percentage of OPC by GGBFS or SF, a smaller depth of chloride penetration and a lower total chloride concentration profile are observed.

It is found that improvement in the resistance to chloride ingress can be achieved by choosing a concrete mix with a lower water-binder ratio and/or using a higher replacement percentage of OPC by GGBFS (in the range of 30 to 70%) or SF (in the range of 5 to 10%). However, an increase in the fineness of GGBFS ranging from 3000 to 8000 cm2/g has less influence on this improvement. While the addition of GGBFS provides better resistance to chloride ingress due to its high chloride binding capacity, the use of SF gives better resistance due to its high fineness to achieve a much denser microstructure.