INFLUENCE OF BAR DIAMETER ON THE TOP-BAR EFFECT IN SELF-COMPACTING CONCRETE

Settlement and bleeding of fresh concrete cause the formation of voids under horizontal reinforcement bars and reduce steel-concrete bond. The reduction in bond of 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". It is influenced by several factors including the properties of the concrete, the position and the shape of the bars, the methods of concrete placement, and the type of formwork. This paper describes the results of a study carried out to assess the impact of the application of self-compacting concrete (SCC) on bond properties around steel reinforcement and the top-bar effect in column elements. Pull-out tests were carried out to determine the bond strength between reinforcing steel bars and concrete. Two optimized SCC mixtures with slump flow values larger than 700 mm and a control concrete with a slump of 20-30 mm were investigated. The results show that the maximum bond strength not always decreases when the diameter of the steel bar increases from 10 to 12 to 16 mm or when a lower strength class is used. The top-bar effect is significantly larger in the column cast with conventional vibrated concrete while the bond strengths in the SCC mixtures show little variations along the height.