Finding the Workability of Concrete with the Slump Test

Finding the right ratio of cement, water and admixtures used to reinforce a concrete mix will all affect a property called "workability." Workability of concrete is directly proportional to the strength and overall performance of a mixture once it sets. It also gives construction crews a good idea of how easy or difficult a mixture will be to work with. With just a few pieces of equipment and a simple slump test, the workability of concrete can be determined.

Definition: Workability of Concrete

Concrete workability is not a quantitative property. Instead, it is a subjective term that describes how easily fresh concrete can be mixed, placed and finished without much loss of homogeneity. The workability of concrete can affect everything from the appearance and strength of a mixture once it sets to the cost of labor for placing and finishing the concrete at a construction site.

Types of Concrete Workability

1. Unworkable/Harsh Concrete: concrete with a low water to cement ratio; very difficult to mix and work with; doesn't maintain homogeneity well.
2. Medium Workable Concrete: the "Goldilocks" zone of concrete; used in most construction works because it is easy to mix, transport and place; doesn't lose much homogeneity.
3. Highly Workable Concrete: the easiest ratio to mix, transport, place, etc.; high workability of concrete has high flowability and settles easily, but also has a high chance of lost homogeneity.

Factors Affecting the Workability of Concrete

There are several different factors that can affect the workability of concrete, but the most common and significant factors are water/cement ratio, aggregate size and shape and the use of admixtures.

• Water/Cement Ratio: in general, the more water added to a cement mixture, the more workable it becomes. However, an excessive amount of water will cause the mixture to lose homogeneity, and the strength of the resulting concrete will be compromised.
• Aggregate Size/Shape: smaller aggregates are less workable than large aggregates, even if the water content remains the same; rounded aggregates will require the same water content as angular, flaky aggregates because they have less frictional resistance, making them more workable.
• Using Admixtures: admixtures are used to enhance a concrete mixture's properties, such as increasing its plasticity or entraining air to fortify the concrete against winter conditions; they can increase flowability without sacrificing homogeneity, but can also create a sticky concrete mixture that's difficult to finish. The effect of admixtures should be observed and accounted for on a case-by-case basis.

Determining Workability of Concrete with a Slump Test

The slump test is mainly used to determine whether the water/cement ratio used for a concrete mixture is appropriate — if there's not enough water, the mixture will be stiff, but if there's too much, it will lose structural integrity and collapse. There are three kinds of slump:

• True Slump: the concrete relaxes as it settles, but maintains its shape
• Shear Slump: the top of the concrete sample slips off to the side, leaving it lopsided
• Collapse Slump: the concrete sample totally collapses

Slump tests are performed using a slump cone, which is placed on a level base and filled to the brim with three equal layers of fresh-mixed concrete. Next, the cone is lifted away and the concrete is allowed to settle. Subtracting the initial height of the sample from the final height gives you the slump, which then determines how workable a mixture will be. In general, slumps between 4-6 inches will be both easily workable and suitable for placement and finishing.

Get the Equipment Needed to Determine the Workability of Concrete at Certified MTP

From slump cones to tamping rods, Certified Material Testing Products has all the concrete slump test equipment you need to efficiently and accurately determine the workability of concrete. Need other concrete testing equipment for your civil engineering project? We have you covered there, too. View our entire inventory today.