Factors affecting workability
Workable concrete shows very less internal friction between particles and overcomes the frictional resistance with just the amount of compacting efforts provided. Workability of the concrete depends on a number of interrelating factors. Water content, aggregate properties, use of admixtures, fineness of cement are the factors affecting workability.
- Water content: The increase in water content increases the fluidity of the concrete thus providing greater lubrication. This helps to increase the workability of the concrete. Increasing the water content should be the last resort to improve the workability in the concrete as this will seriously affect the strength of the concrete. Even if more amount of water is to be added, more cement also should be added so that the water/ cement ratio remains the same and hence the strength of the concrete remains unaffected.
- Size of aggregates: The surface area of bigger aggregates is less and hence less amount of water is required for lubricating the surface to reduce the friction. Thus the concrete having large sized aggregate is more workable (of course, within certain limits).
- Mix proportions: Aggregate/ cement ratio is the measure of how lean or rich the concrete is. If aggregate/ cement ratio is higher, the concrete becomes leaner. In lean concrete less paste is available for the lubrication of the aggregate, while in rich concrete with low a/c ratio, more paste is available which makes the mix more cohesive and hence provides better workability.
- Shape of aggregates: Rounded aggregates have considerably less surface area and less voids in comparison to angular or flaky aggregates which provide better possibility of overcoming the frictional resistance. Further, angular and flaky aggregates make concrete very harsh.
- Surface texture of aggregates: The aggregates having smooth or glossy texture have less surface area compared to rough textured aggregates. This provides better workability as less amount of water is required for lubricating effect. But, taking into account the poor interlocking action provided by the glossy textured aggregate, its use is generally discouraged in high strength concrete.
- Grading of aggregates: Well graded aggregate is the one with least amount of voids in a given volume. If the grading of aggregate is good, the voids will be less and hence higher the workability.
- Use of admixtures: Use of admixtures in concrete is the major factor that affects the workability. The use of plasticizers and super-plasticizers amply increase the workability of the concrete. Air entraining agents produce air bubbles which act as rollers between particles and provide better mobility thus improving the workability.
- Time and temperature: Fresh concrete gets stiffened as the time flows. This is because some of the water used to mix the concrete gets evaporated and some gets absorbed by the aggregates. Thus the workability of concrete reduces with time. This loss of workability with time is known asslump loss. The effect of temperature on workability of concrete is noteworthy. As the temperature increases, the workability of the mix reduces.