Concrete--Additives

The purpose of this research project is to compare the strength and durability
characteristics of rice husk ash-modified concrete with those of normal concrete in
the marine environment. Specimens prepared from concrete mixes with watercementitious
ratios of 0.40 and 0.55, and rice husk ash content of 0%, 10%, and 20%
were tested. The rice husk ash used was obtained from Agrilectric, power plant
located in Lake Charles, Louisiana. The grinding of the ash to particle size of 7-J..tm
to 45-J..tm was done by Process Research ORTECH Inc., Ontario, Canada. Strength
and durability tests were performed, following ASTM (American Society for Testing
and Materials) Standards. The significant fmdings are that the properties and quality
of the rice husk ash-modified concrete are as good, if not better than normal concrete.

This report describes the results obtained from reinforced concrete slabs having different fly ash and silica fume content. The specimens are submitted to periodic seawater ponding. PH measurements as well as free and total chloride analyses were achieved at 1213 days in order to study the alkalinity, resistance to chloride ingress and binding properties afforded by each mix design. Water absorption experiments were also conducted at different relative humidities and in water. Pore water pH was found to decrease with increasing admixture content and increasing relative humidity. The permeability to chloride ions was significantly reduced in the fly ash blends whereas most of the silica fume blends exhibited effective diffusion coefficients and chloride concentrations at the depth of steel marginally better than the controls. Both the fly ash and silica fume blends showed similar binding capacity at a given replacement ratio, the percentage of bound chloride increasing with increasing admixture content.

A study was performed of the effect of reinforcing bars on chloride diffusion into concrete. These bars act as obstacles to chloride movement, and this obstruction allows the chloride concentration to build-up faster along the top of the reinforcing bar than elsewhere. As a consequence, the critical chloride threshold to initiate corrosion of the steel is reached sooner than otherwise expected. This research was performed using two different methods. First, chloride analyses were performed on chloride exposed concrete blocks by taking cores in the concrete and drillings along the trace of the top bars. Second, concrete blocks models were analyzed by finite element analysis and the effect of the obstruction by the bar quantified. The role of the reinforcing bar in the chloride diffusional flow is discussed considering these two methods.