Use of Viscosity-Modifying Admixture to Reduce Top- Bar Effect of Anchored Bars Cast with Fluid Concrete

With the increasing use of highly fluid concrete to facilitate the casting and consolidation of congested reinforced concrete sections, it is essential to ensure adequate stability of fresh concrete to reduce micro-structural defects at the interface between the hardened cement paste and embedded reinforcement. Accumulation of bleed water under the reinforcement and minute separation of fresh paste from the reinforcement caused by segregation and settlement can significantly reduce bond with anchored or overlapped reinforcement. The reduction in bond with horizontally embedded bars located in the upper sections of structural elements as opposed to those located near the bottom is known as the top-bar effect (top-bar factor). This paper aims at investigating the effectiveness of incorporating viscosity-modifying admixture to enhance the stability of high-slump concrete and reduce the top-bar factor of anchored bars. A total of 25 specimens measuring 500 mm, 700 mm, and 1,100 mm in height were cast with various mixtures with slump values of 220 mm and 190 mm and self-consolidating mixtures with slump flow values of 600-690 mm. The specimens were prepared to evaluate the effect of viscosity-modifying admixture content, specimen height, and mode of consolidation on external bleeding, surface settlement, segregation, and relative bond strength to horizontally embedded bars. The findings indicate that regardless of the slump, specimen height, and mode of consolidation, the reduction in surface settlement (that is related to bleeding and segregation) resulting from incorporating a viscosity-modifying admixture (welan gum) can significantly reduce the top-bar factor. Highly stable, self-consolidating concrete with low settlement had low top-bar factors comparable to specimens cast with 190- and 200-mm slump concrete. The use of 0.07% welan gum in concrete made without any silica fume developed greater stability and lower top-bar factor than similar concrete containing 0.035% of welan gum and 8% silica fume replacement.