Let’s Talk About Slump

Using a properly designed concrete mix with the correct slump for the particular application is a critical parameter to the success of any construction project. Concrete mixture materials and proportions are carefully determined to provide a robust, efficient, and economical high performance building material where the workability needs are a key driver in the design. Although we all have a basic understanding of "slump", there are a number of factors that need to be considered to ensure we are using the proper material design with the proper methods for the right application.

What is slump and how is it tested?

In simplest terms, slump is a measure of the CONSISTENCY of fresh concrete. The two most common methods for testing concrete consistency are the slump test (CSA Test Method A23.2-5C) for conventional concrete and the slump flow test (CSA Test Method A23.2-19C) for self-consolidating concretes. The slump cone is filled and consolidated, as per the procedure by a technician certified under an industry-recognized program, and then lifted where the vertical distance that the concrete subsides from its original height is the measured slump, or the horizontal spread of the concrete on the board is measured as the slump flow.

What does consistency of concrete mean?

Consistency of concrete is often loosely associated with words such as workability, placeability, and finishability. Those words are considered to be related to the concrete mixture’s capacity to be mixed, transported, placed, consolidated, and finished properly without harmful segregation. It also embodies concepts such as moldability, cohesiveness, and compactability. Consistency is more properly defined under ACI 211 as the relative mobility of the concrete mixture and is related to, but not synonymous with, workability.

So slump is a measure of workability?

Not entirely, but it does give us a relative idea and it is easy to use and test in practice. The workability of the concrete depends on its overall rheological properties, where the slump only gives us part of that picture. Other test methods are available to provide more information, but generally these methods are not as practical in the field. The workability and finishability of concrete depends highly on its paste and mortar viscosity, shear-strain properties, density, consistency, aggregate interlock, cohesivity or segregation tendency, and time to subside. All slumps are not created equal - some lower slump mixes can be easier to work with and finish than an improperly designed high slump mixture.

Is the slump a measure of water content or concrete strength?

No. In the early days of concreting, where mixes consisted of only cement, aggregate, and water the slump value was very highly correlated to water content and was thus indicative of concrete quality. Today however, concrete also contains supplementary cementitious materials, optimized aggregates, and admixtures which not only enhance performance, but in many cases directly improve the workability of concrete without addition of water. The slump is no longer a direct estimate of water content and concrete quality, although it can provide insight into the performance and variability of batches of the same concrete mixture.

What are the benefits of using the proper consistency?

The benefits of proper slump and good workability are associated with the ease of placing, consolidating, and finishing the concrete, and its cohesion, homogeneity, and resistance to segregation. These aspects are critical to ensuring a high density profile of the concrete element which affects its strength, shrinkage characteristics, and long term durability. Further to that, a properly designed concrete mix will reduce overhead costs, facilitate production schedules, and reduce the risk of worker strain and injury. A mix that is difficult to place and consolidate, however, will increase the cost of handling and lead to poor strength, durability, and appearance in the final structure.

What slump should we target?

There is no definitive answer to this question as the concrete should have the right workability for the tools being used to place, consolidate, and finish it, and for the particular application, concrete element design, and the requirements for plastic concrete mobility or stiffness. Construction considerations include the concrete performance specifications, type of construction work, methods of mixing, transporting, placing, consolidating, and finishing, the concrete section thickness, whether the element is vertical or horizontal, the extent of reinforcement, and environmental factors. It is better to choose the proper slump during the initial planning stages than to try to “fix” a problem after the concrete has started being produced.

The slump and workability requirements are best addressed by the DESIGNER, CONTRACTOR, and PRODUCER DISCUSSING THE PROJECT REQUIREMENTS TOGETHER with the contractor leading the discussion and advocating for their needs. The old rules of “maximum 3 inch slump for all concrete” are no longer relevant in today’s world with the advances in concrete technology and performance. In addition, limiting a slump range to other than the tolerances specified in CSA A23.1 4.3.2.3.2 hinders the concrete supplier from providing a consistent mixture and limits the contractor’s ability to perform the work, with potential cost impacts due to unnecessarily rejected loads and project delays. The selection of optimal concrete proportions involves a balance between economy, workability, strength, durability, sustainability, and appearance. Designing a concrete mix with good workability brings all these factors together in balance to result in a quality product with long service life.

What factors affect the slump and workability of concrete?

A number of factors affect the workability of concrete including: the water-to-cementitious materials ratio, entrained air content, the paste and mortar contents, the aggregate size, gradation, shape, and texture, cement and supplementary cementing materials types and proportions, mixing time and intensity, and the use of chemical admixtures, particularly water reducing and super-plasticizing admixtures. Generally, the higher the slump requirements, the higher the fines portion in the concrete (powder and sand) and the higher the water-reducing admixture dose to balance slump and cohesion while maintaining strength and durability. That dollar spent up front can very well save you ten over the course of a placement under the right application!

Temperature, and other ambient conditions such as moisture are the biggest external factors which can have a SIGNIFICANT impact on the workability and plastic lifespan of the concrete. Remember - cement chemically reacts with the water (hydration) over TIME and these factors will accelerate or slow down those reactions.

What are the effects of adding water on-site to increase the slump?

Water addition on site is permitted under CSA A23.1 5.2.5.3.2 as long as the specified water-to-cementitious materials ratio is not exceeded, no more than 60 minutes has elapsed from time of batching (may be longer with use of retarders or hydration stabilizers), not more than 16L/m3 or 10% of the mixing water is added, and as long as the mixer drum is turned for at least 30 revolutions at mixing speed and the amount of water added and by who’s authority is recorded and signed for on the delivery ticket. Water addition outside of these limitations WILL negatively impact the concrete's setting and bleeding characteristics, plastic properties (placeability and finishability due to segregation of paste and mortar from stone - this is a particular killer for concrete pumping!), and overall hardened performance (strength, permeability, durability, air void system parameters and freeze-thaw resistance, and shrinkage-cracking resistance).

The Final Word?

The slump is just one part of the whole of the concrete system and changing it usually requires modifications to the rest of the material proportions in the mix design to ensure a quality product with long service life. Concrete is the most resilient, flexible, sustainable, and widely-used building material available, but…

-We need to have a basic understanding of the fundamental mechanisms to understand what our product is doing.

-We need to respect our product and treat it accordingly.

-A little extra planning, forethought, and communication between parties can make all the difference for a successful project.

-The quality of your product depends on the quality of the WHOLE CONCRETING SYSTEM, from the concrete element design and specifications, to raw material selection and mix design, to batching, mixing, and delivery, through to proper construction practices and curing.