Each and every process and materials involved in concrete mixing affects the workability of concrete. Workability of concrete is measured in terms of ease with which it can be mixed, transported to construction site, placed in forms and compacted. It is easy to work with a highly workable concrete as it can be easily mixed, transported, placed and compacted.
Workability and strength of concrete are inversely proportional. When workability of normal concrete increases, the strength of concrete decreases which affects the durability of concrete.
Factors which affect workability of concrete are:
Cement content of concrete
Water content of concrete
Mix proportions of concrete
Size of aggregates
Shape of aggregates
Grading of aggregates
Surface texture of aggregates
Use of admixtures in concrete
Use of supplementary cementitious materials
The primary materials of concrete are cement, fine aggregates (sand), coarse aggregates and water. Many times admixtures are used in concrete to enhance its properties. Therefore, properties of these materials and their content affect the workability of concrete. Following are the general factors affecting concrete workability:
Cement Content of Concrete:
Cement content affects the workability of concrete in good measure. More the quantity of cement, the more will be the paste available to coat the surface of aggregates and fill the voids between them. This will help to reduce the friction between aggregates and smooth movement of aggregates during mixing, transporting, placing and compacting of concrete. Also, for a given water-cement ratio, the increase in the cement content will also increase the water content per unit volume of concrete increasing the workability of concrete. Thus increase in cement content of concrete also increases the workability of concrete.
Type and Composition of Cement:
There are also effect of type of cement or characteristics of cement on the workability of concrete. The cement with increase in fineness will require more water for same workability than the comparatively less fine cement. The water demand increased for cement with high Al2O3 or C2S contents.
Water / Cement Ratio or Water Content of Concrete:
Water/cement ratio is one of the most important factor which influence the concrete workability. Generally, a water cement ratio of 0.45 to 0.6 is used for good workable concrete without the use of any admixture. Higher the water/cement ratio, higher will be the water content per volume of concrete and concrete will be more workable.
Higher water/cement ratio is generally used for manual concrete mixing to make the mixing process easier. For machine mixing, the water/cement ratio can be reduced. These generalised method of using water content per volume of concrete is used only for nominal mixes. For designed mix concrete, the strength and durability of concrete is of utmost importance and hence water cement ratio is mentioned with the design. Generally designed concrete uses low water/cement ratio so that desired strength and durability of concrete can be achieved.
Mix Proportions of Concrete:
Mix proportion of concrete tells us the ratio of fine aggregates and coarse aggregates w.r.t. cement quantity. This can also be called as the aggregate cement ratio of concrete. The more cement is used, concrete becomes richer and aggregates will have proper lubrications for easy mobility or flow of aggregates. The low quantity of cement w.r.t. aggregates will make the less paste available for aggregates and mobility of aggregates is restrained.
Size of Aggregates:
Surface area of aggregates depends on the size of aggregates. For a unit volume of aggregates with large size, the surface area is less compared to same volume of aggregates with small sizes. When the surface area increases, the requirement of cement quantity also increase to cover up the entire surface of aggregates with paste. This will make more use of water to lubricate each aggregates. Hence, lower sizes of aggregates with same water content are less workable than the large size aggregates.
Shape of Aggregates:
The shape of aggregates affects the workability of concrete. It is easy to understand that rounded aggregates will be easy to mix than elongated, angular and flaky aggregates due to less frictional resistance. Other than that, the round aggregates also have less surface area compared to elongated or irregular shaped aggregates. This will make less requirement of water for same workability of concrete. This is why river sands are commonly preferred for concrete as they are rounded in shape.
Grading of Aggregates:
Grading of aggregates have the maximum effect on the workability of concrete. A well graded aggregates have all sizes in required percentages. This helps in reducing the voids in a given volume of aggregates. The less volume of voids makes the cement paste available for aggregate surfaces to provide better lubricating to the aggregates.
With less volume of voids, the aggregate particles slide past each other and less compacting effort is required for proper consolidation of aggregates. Thus low water cement ratio is sufficient for properly graded aggregates.
Surface Texture of Aggregates:
Surface texture such as rough surface and smooth surface of aggregates affects the workability of concrete in the same way as the shape of aggregates. With rough texture of aggregates, the surface area is more than the aggregates of same volume with smooth texture. Thus concrete with smooth surfaces are more workable than with rough textured aggregates.
Use of Admixtures in Concrete:
There are many types of admixtures used in concrete for enhancing its properties. There are some workability enhancer admixtures such as plasticizers and super plasticizers which increases the workability of concrete even with low water/cement ratio. They are also called as water reducing concrete admixtures. They reduce the quantity of water required for same value of slump.
Air entraining concrete admixtures are used in concrete to increase its workability. This admixture reduces the friction between aggregates by the use of small air bubbles which acts as the ball bearings between the aggregate particles.
Use of Supplementary Cementitious Materials:
Supplementary cementitious materials are those which are used with cement to modify the properties of fresh concrete. Fly ash, fibers, silica fume, slag cements are used as supplementary cementitious materials.
The use of fly ash in improves the workability of concrete by reducing the water content required for same degree of workability or slump value.
The use of steel or synthetic fibers in concrete reduces the workability of concrete as it makes the movement of aggregates harder by reducing the lubricating effect of cement paste.
The workability of concrete is reduced and increased based on the quantity of silica fume. The use of silica fume in concrete can improves workability when used at low replacement rates, but can reduce workability when added at higher replacement rates. Silica fume are used as pumping aid for concrete when used as 2 to 3% by mass of cement.
The use of slag cement also improves workability but its effect depends on the characteristics of the concrete mixture in which it is used.
References: ACI 238.1R-08 – Report on Measurements of Workability and Rheology of Fresh Concrete