Segregation: Segregation is defined as the separation of the constituents of a homogeneous mixture of concrete. It is caused by the differences in sizes and weights of the constituent particles. Segregation can be controlled by properly choosing the grading of aggregates and by carefully handling wet mixes. In relatively lean and dry mixes, segregation can be caused by the coarser particles separating out because they travel farther along the slope or settle to a greater extent than finer particles. The second form of segregation occurs in very wet mixes in which the cement – water paste separates from the mix. Segregation can also be caused by poor handling, such as dropping wet concrete from a considerable height, passing long chutes along a slope, and discharging concrete carelessly against some firm obstruction. It may also be caused by the vibration of concrete. Though vibration provides a useful means of compaction, over – vibration leads to segregation. This can happen when vibration is allowed to continue for too long. It leads to the separation of coarse aggregates from the mix. These aggregates settle at the bottom, and the cement – water paste moves to the top in the form of laitance (scum). This laitance is different from bleed water. Segregation is difficult to measure. However, its occurrence is easily detected. The flow test can indicate the susceptibility of a mix that is likely to segregate. In dry mixes, heavier particles move away and occupy the edges of the flow table. In wet mixes, the cement paste trends to move away from the middle and the centre of the flow table is left only with coarser particles. Bleeding: Bleeding is also known as ‘water gain’. It is the accumulation of water at the surface, which accompanies the sedimentation of freshly mixed concrete. This happens due to the inability of the solid constituents of the mix to hold all the mixing water and they settle downwards due to gravity and the water moves upwards. Bleeding is expressed quantitatively as the total settlement per unit height of concrete or as the percentage of mixing water. In extreme cases, this can be nearly 20%. Bleeding is a function of (a) air velocity, (b) temperature, and (c) humidity. If the rate of bleeding is roughly equal to the rate of evaporation, then bleeding will not cause any problem. If the rate of bleeding is less than the rate of evaporation, then the surface becomes dry, because of which cracks appear on it. The restraint at the bottom encourages such cracks. The evaporation of water from the surface of concrete depends on (a) the relative humidity of the surrounding air, (b) the ambient temperature, and (c) the velocity of wind.