Fundamental Mechanisms of Concrete Bleeding in Bored Piles

Abstract

Concrete bleeding in bored pile scan cause substantial defects such as channelling or air pockets in pile shaft of diaphragm walls. The repair of such damage can be costly and time consuming. The mechanism of concrete bleeding in bored piles and diaphragm walls is well known among construction professionals, but there a sons how concrete bleeding occurs remain insufficiently understood to date. This paper introduces a potential model to explain the fundamental mechanism of concrete bleeding or channelling in deep foundations (e.g. concrete bored piles or diaphragm walls). The model is based on a well-established theory from the disciplines of soil mechanics and geotechnical engineering. The transfer of knowledge from the discipline of geotechnical engineering to another (concrete technology) assumes that fresh concrete is a three-phase system consisting of aggregate (gravel and sand), fluid (cement paste and excess design water) and air. The application of external pressure on to fresh concrete inside a deep foundation due to self-weight of the fresh concrete column causes the redistribution of pore-water pressure, resulting in a reduction of void space inside the aggregate matrix. This change in aggregate density is likely to cause concrete bleeding if potential drainage paths exist inside the fresh concrete matrix. Such drainage paths will provide ‘escape routes’ to release the excess pore-water pressure (water or cement paste) to the surface of the pile by forming bleeding channels or voids inside the hardened concrete. The existence of potential drainage paths, the lack of fines in the fresh concrete matrix in combination within sufficient aggregate grading and the addition of too much design water (above the optimal water content for a given aggregate combination) have been identified as key factors contributing to concrete bleeding and channelling in deep foundations (e.g. bored piles and diaphragm walls).