Factors Affecting Workability
There are the following factors effecting on workability of concrete
1) Water content of the mix
2) Influence of aggregates mix proportions
3) Aggregates properties
4) Temperature and time
5) Cement characteristics
1) Water Content of the Mix is a Factors Affecting Workability
Water requirement is associated with
- Absorption by aggregate surface
- To fill the voids b/w aggregates
- Lubricating particles by separating them with a film of water.
Fine aggregates with more surface area will have more water requirement.
However a certain amount of fines are required for the plasticity of mix.
2) Influence of Aggregate Mix Proportions
Two factors are important
- Amount of aggregate
- Relative proportions of fine and coarse aggregates.For given water cement ratio:
a) Increase in aggregate/cement ratio will decrease workability
b) When final aggregate grading is used, more cement is needed i.e. to cost particles
c) A deficiency in fine aggregate results in a harsh mix
I.e. prone to segregation and difficult to finish
d) An excess of fine aggregate (over sanded) will result in a more permeable mix and less economical concrete, through it will be easily workable.
3) Aggregate Properties
Fineness of the fine aggregate is reflected in the fineness modulus. A fine sand (low fineness modulus) has a much higher paste requirement for good workability
Smoother texture aggregate will provide a more workable mix. Due both to particle inter lock and paste requirements to coat particle
Flat or elongated particles will require increase the amount of sand cement and water to provide adequate workability.
Porosity of the aggregate will affect the absorbance of water so must be accounted for in the mix design.
4) Temperature and Time
If increase ambient temperature will decrease workability. Increase evaporation and hydration rate
5) Cement Characteristics is a Factors Affecting Workability
The cement characteristics are less important factor than aggregate characteristics. If increase fineness cement then workability is decrease, this is due to increased water requirement and likely more rapid hydration. Hot cement (>60 C0 to 80 C0) can result in rapid hydration and possibly flash set.