Properties of concrete - Concrete Mix Design, Quality Control and Specification - page 104

5.4 PLASTIC PROPERTIES
5.4.1 Bleeding
Factors affecting bleeding are
1. Amount of fine material (including cement, slag, fly ash, silica fume,
and natural pozzolans)
2. Air entrainment
3. Water reduction through admixtures or lower slump
4. Continuity of grading (especially including fine aggregate grading)
5. Use of VMAs (viscosity modifying admixtures)
6. Increased retardation, which delays gel formation and so extends the
period during which bleeding can occur
Essentially the fresh mortar in concrete consists of a mixture of saturated
solids surrounded by water and any entrapped or entrained air that are
lighter than the solids component and will tend to move upward. The
better the particles pack together and the more difficult it will be for water
to pass through the mass, air bubbles will also tend to block passageways.
Cement, slag, fly ash, entrained air, rice husk ash, VMAs, and silica fume
(in generally increasing order of effectiveness) are good inhibitors of
bleeding. Very fine calcium carbonate (limestone) is a recent development
and the superfine material in manufactured sand (crusher fines) is now
considered very desirable to control bleeding in some circumstances.
Silica fume is the most effective inhibitor of bleeding. It is many times
finer than cement and particles of it fill the interstices between the cement
particles. Small amounts (as little as 10 to 30 kg per cubic metre) are
sufficient to prevent bleeding almost completely. It should be noted that
the effectiveness of silica fume is greatly reduced if it is not completely
dispersed. Essentially this means that silica fume should always be either
batched as a slurry or given adequate mixing time especially together with
coarse aggregate to shear the agglomerates and, of course, used with an
appropriate superplasticiser.
It should be noted that eliminating or greatly reducing bleeding can
create problems with plastic shrinkage cracking. Such concrete may require
careful attention to preventative measures such as the use of liquid aliphatic
alcohol evaporation retardant (Confilm) or polythene sheeting, mist sprays,
and so on.
5.4.2 Rheology
Rheology describes the workability of the concrete. It is a critical fea-
ture of most concrete and there is much more to this property than
is revealed by the still widely used slump test. Workability testing is