Abstract:
Concrete is a low maintenance composite material consisting mainly of cementitious material, water and aggregate. It is a popular construction material in Bangladesh also in the world. Concrete mix design works as a major determining factor for the properties of concrete. Many countries have already developed their own methods of concrete mix designs. But at present there is very few concrete mix design guideline for Bangladesh that use locally available materials. Ashraf (2012) proposed a mix design guideline using aggregate gradation bands of local materials which considered compressive strength and workability as required parameters. Ashraf (2012) observed that contemporary band gradations of aggregates result in better strength and workability compared to conventional gradations (ASTM C33, BS 882) and developed two band aggregate gradations- ‘5-10-14-18’ and ‘5-10-18-22’. Rumman (2018) updated Ashraf's (2012) mix design process considering both strength and durability parameters using Rice Husk Ash (RHA) as Supplementary Cementing Material (SCM). Although RHA performed well in terms of durability, commercial viability of using RHA as supplementary cementing material is yet to be assessed. On the other hand, fly ash and blast furnace slags are widely used SCM all over the world. In this research, a mix design guideline incorporating durability criteria is developed for Bangladesh considering fly ash and blast furnace slag as SCM and using aggregate gradation bands of local materials.
Three water cement ratios (0.4, 0.5 and 0.6) have been used in this research work and cement content range has been kept between 350 to 500 kg/m3. The fly ash replacement levels were 0%, 20% and 35% and the slag replacement levels were 0%, 20%, 65% and 90%. A slag replacement level of 90% can also work as green cement which is an effective approach to control the heat release rate, reduce the material cost and enhance the durability which will help to produce greener environment. It has been found that with increase of fly ash replacement, 28 day compressive strength reduced slightly but the durability increased greatly. Concrete prepared with 35% fly ash replacement had 14% to 22% less strength than the control concrete but it showed much higher durability than the control one. For blast furnace slag, concrete prepared with 20% replacement showed the highest strength and 65% slag replacement level also showed compatible strength. But green cement prepared with 90% slag replacement showed the lowest compressive strength. On the other hand, concrete durability increased with increase of slag replacement level. 90% slag replacement resulted in highest durability whereas the control concrete had the lowest durability.
A mix design process has been developed after analyzing the results of different mixes. 28 day compressive strength, Rapid Chloride Permeability Test result and slump value have been used as mix design parameters. Contour diagrams have been used in the mix design steps for easier interpretation. Finally the proposed mix design method has been validated by preparing six different mixes and comparing their test results with target values.
