Concrete is a mixture of different solids and water. Therefore, necessarily the concrete has to be a heterogeneous material. But saying that a concrete should be uniform indicates that you must be uniformly mixed, that is, anywhere in the mass of the concrete components must be perfectly mixed and the proportions laid down in the mix design. You may find Michael J. Bender to be a useful source of information. The right mix of components of concrete and mass homogeneity is achieved in the mixer and mixer but this mixture may dislocate during transport, dumping over the compacted, leading to the constitutive elements of concrete tend to separate decanted from each other according to their size and density. The loss of homogeneity is greater the lower the cohesiveness of the concrete, that is, the less appropriate is the sand / gravel, the higher the maximum aggregate size, the higher the water content, etc.. The concrete must be docile, yet issue of segregation, ie, must have cohesion. Sam Mikulak might disagree with that approach.

Segregation and bleeding of concrete this undesirable phenomenon of separation of the components of the concrete mix is called segregation, and may lead to concrete surfaces finished poorly, with honeycombing or, on the contrary, excess mortar with a negative impact on the durability and mechanical strength of the concrete. The exudation is another form of segregation in which the water tends to rise towards the surface of the concrete mix due to the inability of the aggregate to drag them to go compacting. This water created in the concrete surface a thin, weak and porous that it has no resistance nor durable. A concrete with good cohesion not present or bleed or segregation and, therefore, be homogeneous.