Factor Controlling Workability of Concrete
• Aggregate size
• Aggregate shape and their surface texture
• Grading of aggregate
• Application of admixture
• Mix proportions
Besides these we can explain influencing factors on workability based on only three terms
• Water/cement ratio
• Aggregate/cement ratio
• Water content
Some can also include external factor like temperature, air flow etc. Again cement content and aggregate grinding has also influence on initial workability and change in workability of mix.
Water content:
Water content in fresh concrete has significant influence on workability of concrete. This is the main factor that defines fluidity of concrete, the higher the value for a given volume of concrete , the higher the fluidity of concrete (expressed as kg (or liters) per cubic meter of fresh concrete). As it is often misunderstood fluidity as workability, an inexperienced supervision frequently depends on inclusion of more water in concrete to improve workability.
This practice becomes very common as it is the one of the simplest way that can be adopted at site. It should be noted that addition of water is the last option to improve workability of concrete (applicable even to uncontrolled concrete). In case of controlled concrete, arbitrary increase in water content is not permitted. To keep water/cement ration of the mix unchanged with the addition of water, cement content also have to be increased to meet the design strength of hardened concrete.
Aggregate size:
A large size of aggregate will offer least amount of surface area for given volume of concrete. Thus the surface area available to be wetted by water will be less, again less amount of paste or matrix will be required to lubricate surface of aggregate. Thus less matrix and less amount of water is required to overcome internal friction between particles. For particular amount of water and paste, larger size of aggregate will offer higher workability, but within some limit.
Aggregate shape and their surface texture:
Shape of aggregates has good influence on workability. Flaky or elongated and angular aggregate make mix very harsh as compared to cubical or round shape aggregate. Rounded shaped aggregates contribute better than others, as for a given volume (or weight) such aggregate will have least surface area then flaky or angular aggregate. They also can be packed with least voids and by nature a round shape particles exert least frictional resistance. This is why sand and gravel collected from river bed offer greater workability them sand and aggregate derived from crushed stone.
Very low water/cement ratio is essential in production of high performance and high strength concrete. When water/cement ratio as low as 0.25, all the factors, may be minor, has to be taken into account and in that case shape of aggregate is a vital factor in present construction industry. It should be remembered that natural source of sand will be exhausted very soon or become very costly and there will be no choice to choose manufactured sand. The shape of sand from crushed source is not very useful in respect of workability but now some modern crushers are available that can produce well graded and well-shaped aggregates. . The shape of sand from crushed source is not very useful in respect of workability but now some modern crushers are available that can produce well graded and well-shaped aggregates.
When surface texture of aggregate is concern, it has also influence on workability by again considering surface area available to receive and get wetted by water. Having same size and shape of aggregate, a rough textured aggregate need more water and paste than a smooth textured aggregate. Thus rough textured aggregate in concrete mix will produce a less workable concrete than that have glassy or smooth textured aggregate. Not only the water demand play roles in defining workability but also to some extent low friction between aggregate particles offer higher workability.
Grading of aggregate:
Grading has significant, can be said maximum, influence on concrete workability. Well graded aggregate means all sizes of aggregates remain in the mix which will render compact packing of mix leaving least voids with in particles for a particular volume of concrete. Low amount of voids means excess paste, if other parameters remain constant, is available. Excess paste will produce a mix of fatty and cohesive nature which significantly reduces segregation of constituent particles. Inter-particle resistance is reduced and an aggregate particle can slide relative to other ensuring ease in compaction. Thus better grading will produced less void with more cement paste and yield concrete of better workability for given volume of paste.
Application of admixture:
Admixtures are introduced to concrete to achieve special properties in concrete. Application of plasticizers and superplasticizers greatly improve workability of concrete. The reference concrete mix means concrete without admixture where other factors described above can be manipulated to reach initial slump of 20~30 mm. without initial slump workability of the mix can be increased but higher dosage of superplasticizer is required.
Sometimes air entraining agents are introduced in concrete to improve durability of concrete. These agents are surfactant which decrease inter-particle friction and improve workability. The air bubbles formed in mix can be treated as fine aggregates having smooth surface. The ball-bearing behavior of air bubbles between aggregate particles renders easy mobility to those particles and they can slide past each other with less compactive effort. Finely grounded pozzolanic materials, without increasing surface area, can produce improved lubricating effects yielding more workable concrete.
Mix proportions:
A leaner concrete having more aggregate in mix contains less amount of paste to make mix lubricated. Thus there have not enough paste to provide mobility of aggregate and workability of the mix is affected. On the contrary, a rich concrete mix having more paste results a fatty and cohesive mix with better workability.
When mix proportioning is considered as factor to influence workability of concrete mix, we should take care about three factors
• Water/cement ratio
• Aggregate/cement ratio
• Water content
Among these three, only two factors are independent. Let’s take an example, when aggregate/cement ratio of a mix is decreased without changing water/cement ratio (kept constant), the water content is obviously increased which will yield more workable concrete. The explanation is reduction in aggregate/cement ratio means richer concrete having more paste and with more water content; the mix gets lubricated to yield better workability. Again with a constant water content of mix if aggregate/cement ratio is decreased, the water/cement ratio will decrease; interestingly in this case workability is not affected significantly. The explanation is reduction in aggregate/cement ratio means increase in cement content in mix with constant water content it yields reduction in water/cement ratio. At the same time less aggregate/cement ratio will result more surface area of constituent solids (comprise of total surface area of cement and aggregate particles). More surface area needs more water to provide mobility; thus workability is reduced to some extent. In such case designer can choose coarser graded aggregate to offset this problem; altering fineness of cement will also be beneficial but yet not well established.
As discussed above if water content with other parameters of mix is kept constant, workability is greatly influenced by properties of aggregate (size, shape, grading and surface texture); but water/cement ratio and grading have to be taken into account together. This is due to grading resulting highest workable mix for a particular water/cement ratio may not produce even workable mix for other values of water/cement ratio. In practical situations, a higher value of water/cement ratio of the mix need finer grading of aggregate to reach highest workability.
The actual fact is that for a particular water/cement ratio there will be a value of aggregate ratio (coarse/fine using provided materials) which will yield a peak in workability. Moreover, it can be said that for a particular workability, there will be a value of aggregate ratio that requires lowest amount of water.
It should keep in mind that when we are discussing grading in respect of satisfactory workability, concrete proportioned by mass is only applicable for aggregate having same specific gravity. Volume occupied by different sizes of particles in the mix controls workability; these requires a mix having mixed types of aggregate having variable specific gravity to be proportioned based on absolute volume for each fraction of aggregate This situation may appear in case of some type of lightweight aggregate or when lightweight aggregate is mixed with ordinary aggregate. Air entrained concrete is also example of such case as entrained air within mix act as fine particles having no weight. Interestingly a decrease influence of characteristics of aggregate is found on workability with increase in richness of concrete mix and in some cases found disappeared completely a very low aggregate/cement ratio like (2.5~2).