FACTORS AFFECTING WORKABILITY OF CONCRETE-ALL YOU NEED TO KNOW
Workability of fresh concrete mix is mainly affected by (i) proportions of different ingredients of concrete, (ii) properties of those ingredients, and (iii) environmental conditions. The effects of some important factors among these are outlined below.
Water content
Higher water content per unit volume of concrete leads to improved consistency/fluidity of concrete. However, increasing water content while keeping quantities of other ingredients same may lead to a loss of cohesiveness of mix and inhibit proper strength development of concrete. Therefore, an increase in water content should be associated with an increase in cement content as well, so that all of the increased water is not free water. Some of the increased water will be consumed in hydration. A practical approach is to increase water and cement contents while keeping the water-cement ratio constant.
Mix proportioning
Aggregate-cement ratio is an important parameter influencing workability of the concrete mix. While aggregates form the skeleton of the concrete, cement paste acts as lubricant in the mix. Therefore, higher aggregate-cement ratio means that the mix has impaired mobility and cohesiveness, and therefore low workability. A lower aggregate-cement ratio means that more cement paste per unit surface area of aggregate particles is available for lubrication. This improves both consistency and cohesiveness of the mix resulting in high workability.
Shape of aggregates
Irregular shape and rougher texture of angular aggregate demand more water than the round shaped aggregate. For fixed volume or weight, rounded or subrounded particles have less surface area and less void and they have less friction resistance too. Hence round shaped aggregates show higher workability than angular, flaky or elongated aggregates.
Size of aggregates
Larger aggregate particles have lower surface area per unit mass. Therefore, use of larger size aggregates lowers the lubrication requirement and hence increases workability of the concrete mix. However, this is true up to a certain limit. If the size of aggregates is increased beyond a limit, the mix may not remain cohesive. Since fine aggregates have much higher surface area to be lubricated as compared to coarse aggregates, gradation of fine aggregates is much more important than that of coarse aggregates. Another important factor to be looked is ratio of fine aggregates to coarse aggregates (IS 10262 provides a table of the ratio as a guideline). An optimal choice of fine to coarse aggregate ratio is needed for a suitably workable concrete mix. An unsuitable choice may lead to honeycombing or segregation in the concrete mix. In Section 1.6, we mentioned that usually a mix of 10 mm and 20 mm MSA coarse aggregates are used in construction. An expert concrete mix designer should be able to suitably adjust this proportion to get the desired workability of the concrete mix.
Texture of aggregates
While use of rough textured aggregates result in concrete mix with poor workability, use of glassy or smooth textured aggregates improve workability of the mix. This is because rough aggregate particles have a lot of surface pores/voids which increase the lubrication requirement and therefore lead to poor workability. Rough particles also increase the frictional resistance leading to poor mobility of particles in the cement paste, further reducing the workability. From above discussion, it can be observed that shape, size and texture of aggregate particles have significant effects on workability of the concrete mix. It should also be understood that all these effects are related to surface area per unit mass of aggregate particles.
The effects explained above assume that water content is kept constant across mixes with different aggregate properties. The effects of shape, size and texture of aggregates become even more prominent in case of high strength concrete mixes where the water-cement ratio is in below 0.30.
Grading of aggregates
In Section 1.6, we discussed how aggregate samples can be classified as well-graded, poorly graded, uniformly graded and gap graded based on the gradation curve. Wellgraded aggregates comprise of particles of different sizes in the right proportion. This leads to lesser voids and therefore better packing. If other factors are kept constant, concrete mix with lesser voids have an excess cement paste available for lubricating the aggregate particles. As a result, the mix becomes cohesive and shows no segregation. This implies that use of well-graded aggregates improves workability of the mix. On the other hand, use of poorly graded, uniformly graded or gap graded aggregates results into a mix with poor workability.
Water absorption and free moisture in aggregates
Different aggregates show different water absorption depending on properties of parent rock and method of crushing used. Aggregates with large number of surface pores tend to absorb more water. Higher water absorption by aggregates results into lesser water availability for hydration of cement, filling gel pores and mobility of aggregates. This leads to a mix with poor workability. Therefore, in order to obtain a mix with desired workability, it is important to determine the water absorption capacity of coarse and fine aggregates prior to obtaining concrete mix design. Further, aggregates with water absorption capacity exceeding 2% should not be used.
On the other hand, availability of free moisture in aggregates can lead to excess water available in the mix. Though it can enhance mobility of aggregates in the mix, it can also lead to loss of cohesiveness resulting in a poorly workable mix. It is therefore important to determine the free moisture content present in aggregates prior to designing a concrete mix. It is common to find dry coarse aggregates and moist fine aggregates on construction sites. Therefore, water absorption capacity and free moisture content in coarse and fine aggregates should be determined, and necessary corrections should be made while obtaining the proportions of different ingredients in the mix. These corrections are outlined in the unit on mix design.
Properties of cement
Fineness of cement particles influence workability of the concrete mix. Rapid Hardening Portland Cement (RHPC) contains higher proportions of C3S, C3A and SO3 compared. to Ordinary Portland Cement (OPC). These compounds have finer particles with higher surface area per unit mass compared to C2S, and therefore need more water for lubrication. If water content is kept same, use of RHPC leads to a reduction in workability. On the other hand, blended cements such as Portland Pozzolana Cement (PPC) and Portland Slag Cement (PSC) contain pozzolanic materials such as fly ash and slag, which result in secondary cementitious products. Water requirement for hydration of these cement types is lower compared to OPC. This makes additional free water available for lubrication and mobility of aggregates, leading to improved workability of the mix.
On a general note, the influence of cement on workability is lesser than those of properties and grading of aggregates. to Ordinary Portland Cement (OPC). These compounds have finer particles with higher surface area per unit mass compared to C2S, and therefore need more water for lubrication. If water content is kept same, use of RHPC leads to a reduction in workability. On the other hand, blended cements such as Portland Pozzolana Cement (PPC) and Portland Slag Cement (PSC) contain pozzolanic materials such as fly ash and slag, which result in secondary cementitious products. Water requirement for hydration of these cement types is lower compared to OPC. This makes additional free water available for lubrication and mobility of aggregates, leading to improved workability of the mix. On a general note, the influence of cement on workability is lesser than those of properties and grading of aggregates.
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