A thesis submitted in partial fulfillment of requirements for the degree of Master of Engineering, School of Civil Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 2000

The purpose of this research is to investigate the influence of classified rice husk ash on the flexural strength of high strength concrete. This research involved the investigation of the properties of high strength concrete with partial replacement by different finenesses of rice husk ash in fresh and hardened states, and the relationship of flexural and splitting tensile strengths with compressive strength. The percentage replacements of cement by each type of rice husk ashes were 0, 15 and 30% by weight of cementitious material. The total cementitious material ratio, fine to total aggregate ratio and aggregate to cementitious material ratio were kept constant. The slumps of high strength concrete were controlled to be 18±2 cm by varying the dosage of superplasticizer. Test results showed that the dosage of superplasticizer increased with increase in fineness and percentage replacement of rice husk ash. The replacements of all types of rice husk ashes and all percentage replacements reduced the early strength for both flexural and splitting tensile strengths as compared to the conventional concrete. This reduction was more significant with the replacements of original, medium and fine rice husk ashes in that order. The replacements of medium and fine rice husk ashes for 15% replacement increased the long-term flexural and splitting tensile strengths. The replacements of all types of rice husk ashes for 30% replacement did not increase significantly the flexural and splitting tensile strengths. The flexural and splitting tensile strengths of fine rice husk ash concrete were higher than those of medium and original rice husk ashes· concrete regardless of percentage replacement for both early and long-term ages. The ratio of flexural strength to the 28-day compressive strength of high strength concrete containing rice husk ash was found to be 6.97% at early age and 12.54% at long-term age regardless of type and percentage replacement of rice husk ash. The ratio of splitting tensile strength to the 28-day compressive strength of high strength concrete containing rice husk ash was 5.19% at early age and 7.73% at long-term age regardless of type and percentage replacement of rice husk ash. It was also found that the relationship between the flexural and compressive strengths as proposed by ACI Committee 363 can be applied for high strength concrete containing rice husk ash.