ABSTRACT

This chapter discusses the behaviour of concrete during each of these stages and how they affect construction practice. The constituent materials of the concrete are of differing relative particle density and therefore while the concrete is in its semi-fluid, plastic state the aggregate and cement particles tend to settle and mix water has a tendency to migrate upwards. The most obvious manifestation of bleeding is the appearance of a layer of water on the top surface of concrete shortly after it has been placed; in extreme cases this can amount to 2% or more of the total depth of the concrete. Plastic settlement and plastic shrinkage cracks that occur soon after placing the concrete can be overcome by re-vibrating the surface region, particularly in large flat slabs. All concretes, no matter how great or small their tendency to bleed, must be protected from moisture loss from as soon after placing as possible, and for the first few days of hardening.