Workability of Concrete Definition
Concrete workability is a broad and subjective term describing how easily freshly mixed concrete can be mixed, placed, consolidated and finished with minimal loss of homogeneity. Workability is a property that directly impacts strength, quality, appearance, and even the cost of labor for placement and finishing operations. But opinions don’t matter much when testing and documenting concrete properties, so how can we define these qualitative characteristics in quantitative terms? This blog post will cover everything you need to know about concrete workability, factors that can affect it, and necessary concrete slump testing equipment.
There are different expectations and agendas across the spectrum of the design/construction team when it comes to engineering concrete design. The structural engineer wants higher strength and a good bond with reinforcing steel. The architect obviously wants cosmetic appeal; Strength is attractive to the owner because it allows smaller cross sections of structural elements and therefore more usable floor space. A laborer needs a mix that can be moved, placed and consolidated properly, and a finisher is looking for something that takes a durable, high-quality finish quickly and easily. Designing a concrete mix with good workability brings all these factors together in balance to result in a quality product with long service life.
Factors Affecting Workability
- Water/Cement Ratio: a higher proportion of cement or cementitious materials usually means greater strength, and with the proper amount of water, more paste is coating the surface of aggregates for easier consolidation and a better finish. Not enough water means poor strength development and an uncooperative mix that resists easy placement and finishing. Adding excessive water could be said to increase workability because it makes it easier to place and consolidate. However, the negative impact on segregation, finishing operations and final strength can be so detrimental that it should be approached very cautiously. A water to cementitious material ratio (w/cm) of 0.45 to 0.6 is the sweet spot for production of workable concrete.
- Aggregate Size and Shape: As aggregate surface area increases, more cement paste is needed to cover the entire surface of aggregates. So mixes with smaller aggregates are less workable compared to larger size aggregates. Elongated, angular and flaky aggregates are difficult to mix and place and have greater surface area to cover, decreasing workability. Rounded aggregates have less surface area, but lack the angularity to develop sufficient bond strengths with the cement paste.
- Admixtures: Many types of admixtures alter the workability of fresh concrete, either by design or as a side-effect. Some surfactants such as super plasticizer reduce attraction between cement and aggregate particles, allowing mixes that can be quite flowable without the negative strength and segregation effects of too much water. Air entraining admixtures for freeze-thaw resistance produce air bubbles of a controlled size that can make for easier finishing, although using too much produces a sticky mix with the opposite effect.
Workability of Concrete by Slump Test
Which brings us to what is by far the most popular (some say overused) measure of workability. The slump test originated as a way for workmen to judge how easy their day was going to be using freshly mixed concrete. A Slump Cone is placed on a solid, level base and filled with fresh concrete in three equal layers rodded in a specified manner to consolidate. The concrete is struck even with the top of the cone, and the cone is carefully lifted away. The concrete then settles or slumps, and the final height is subtracted from the original height of the cone and recorded. This value is considered to be a measure of workability and a rough indicator of water/cement ratio. Slumps of 4 to 6 inches (100 to 150mm) are generally considered to be in a desirable range for typical placement and finishing operations, provided they also have suitable w/cm ratios and meet mix design requirements when placed. The form or profile of the slump is also noted and used to judge the reliability of each test:
- True slump - concrete subsides, maintaining its general form
- Shear slump - the top portion of the concrete shears off and slips sideways
- Collapse slump - the concrete collapses completely, likely from the mix being too wet
Shear or collapse slumps often indicate a problem with execution, and the test should be repeated with a fresh sample.
This concrete slump chart (click to print or download) shows different ranges of slump values and how they might be used.
To learn more about the slump test and workability of concrete, contact Gilson’s technical support. The American Concrete Institute has a page on frequently asked technical questions related to this topic as well.