Effect of Silica Fume and Fly Ash on the Properties of Self Compacting Cementitious Systems

Abstract

Self-compacting concrete does not require manual compaction and is capable to flow under its own weight. Self-compacting concrete has found diverse applications in modern engineering practices including placements in heavily reinforced sections such as bridge piers, prestressed concrete members; high rise buildings especially beam column joints etc. Self-compacting cementitious systems afford uniform durability and compaction at sufficiently low mixing water content containing suitable SRMs to improve its response in both fresh and hardened concrete. The principal aim of the study is to determine the suitability of powdered silica fume in “as packaged” condition without first sieving it through finest sieve. In addition, feasibility of inclusion of Fly ash (FA) and two grades of powdered silica fume namely Elkem SF (920) and SF (971) in 25 and 50 kg bags have been employed. The study commenced with the response of SCP systems and thereafter, SCM formulations were also studied. Comparative study of results revealed that Silica fume if used in “as packaged” condition has higher water demand and requires higher super-plasticizer content (compared with those of FA systems) to achieve target flow of 30±1 cm. Mixing water content has been taken equal to water demand of each system to impart cost effectiveness and durability. It is believed that inclusion of powdered and sieved SF in self-compacting cementitious systems generally enhances strength and improves microstructure, strength and durability. However, in this investigation FA based formulation gave much better strength results than those having “as packaged” SF. The reduction in strength can be attributed to agglomeration of SF particles prevalent in “as Packaged” condition. The agglomeration effect adds to the porosity of the system. It is recommended that powdered SF when used in HP cementitious systems must be firstly sieved or a high shear rate should be employed in the mixing regime. A part of experimental work of present study was also carried out in Technical University Bergakademie, Freiberg, Germany under DAAD sponsorship.