Bleeding and Segregation

The hardened concrete has to conform to certain requirements for mechanical properties. They include compressive strength, splitting- 

The mechanical behavior of concrete should be viewed from the point of view of a composite material. A composite material is a three dimensional combination of at least two chemically and mechanically distinct materials with a definite interface separating the components. This multiphase material will have different properties from the original components. Concrete qualifies as such a multiphase material. Concrete is composed of hydrated cement paste (C-S-H, CH, aluminate, and ferrite-based compounds) and imhydrated cement, containing a network of a mixture of different materials. In dealing with cement paste behavior, basically it is considered that the paste consists of C-S-H and CH with a capillary system. The model of concrete is simplified by treating it as a matrix containing aggregate embedded in a matrix of cement paste. This model provides information on the mechanical properties of concrete. 

The factors that influence the mechanical behavior of concrete are shape of particles, size and distribution of particles, concentration, their orientation, topology, composition of the disperse and continuous phases, and that between the continuous and disperse phase and the pore structure. The important role played by cement paste is already described. 

---

Concrete which is produced using fine aggregates deficient at the fine end of grading, e.g. sea dredged aggregates, exhibit a tendency to bleed and segregate. The presence of a small amount of entrained air (2-4% by volume) leads to an improvement in cohesion, or mix stability. Alternatively, with mixes which are adequate in this respect, a reduction in sand content can be made when air is entrained without loss of cohesion. The amount that can be removed is approximately equal on a volume basis and leads to a reduction in water-cement ratio to minimize the effect of entrained air on compressive strength. 

Bleeding and segregation can occur in flowing concrete if the mix design and concrete production are not properly controlled. In particular the proportion of fines in the mix is important and, as a... 

Appropriate workability should ensure easy and correct placing of the fresh mix in the forms, without excessive bleeding and segregation. The workability should be maintained during a certain timeframe, for example one hour or more, which is needed for necessary transportation and operations on the site. 

Air-entrainment is the process whereby many small air bubbles are incorporated into concrete. Air-entrainment is essential for increasing the durability of concrete exposed to freeze thaw conditions. Air-entrainment improves workability of concrete and also decreases bleeding and segregation. Salts of wood resins, synthetic detergents, salts of sulfonated lignin, salts of petroleum acids, salts of proteinaceous materials, fatty and resinous... 

---