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 cas ting and consolidation of congested reinforced concrete sections, it i s essential to ensure adequate stability of fresh concrete to reduce m icro-structural defects at the interface between the hardened cement p aste and embedded reinforcement. Accumulation of bleed water under the reinforcement and minute separation of fresh paste from the reinforce ment due to segregation and settlement can significantly reduce bond w ith anchored or overlapped reinforcement. The reduction in bond with h orizontally embedded bars located in the upper sections of structural elements as opposed to those located near the bottom is known as the t op-bar effect (top-bar factor). This paper aims at investigating the e ffectiveness 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, 700, and 1100 m m in height were cast with various mixtures with slump values of 220 a nd 190 mm and self-consolidating mixtures with slump flow values of 60 0 to 690-mm. The specimens were prepared to evaluate the effect of vis cosity-modifying admixture content, specimen height, and mode of conso lidation on external bleeding, surface settlement, segregation, and re lative bond strength to horizontally embedded bars. The findings indic ate that regardless of the slump, specimen height, and mode of consoli dation, the reduction in surface settlement (that is related to bleedi ng and segregation) resulting from incorporating a viscosity-modifying admixture (welan gum) can significantly reduce the top-bar factor. Hi ghly stable, self-consolidating concrete with low settlement had low t op-bar factors comparable to specimens cast with 190- and 220-mm slump concrete. The use of 0.07 percent welan gum in concrete made without any silica fume developed greater stability and lower top-bar factor t han similar concrete containing 0.035 percent of welan gum and 8 perce nt silica fume replacement.