Understanding Concrete Slump

Submitted by Deco-Crete Supply on Fri, 07/16/2021 - 3:11pm

Have you ever taken the time to consider how important concrete is to the world? Some have even claimed that concrete is the foundation of modern society. At the very least, you can not deny the fact that concrete is one of the most widely used and versatile building materials in the world. With concrete being used for so many different applications and situations, we need a way to measure and define its flowability.

The term "slump" is commonly used in the industry to describe the consistency or flowability of concrete.

So the question is, how do we measure it?

Even if we were to limit this to just decorative concrete, We would still use a different slump for different applications.

When you place your order for the concrete, you will need to tell the ready-mix plant how stiff or flowable you want the concrete to be. This is done by associating a number to it. An example of these numbers might be 4,5,6 and so on, referring to inches of slump. 

In order to measure slump, we are going to need: a testing cone, the concrete, and a tape measure. First, fill the test cone with the concrete. This should be done in thirds, packing the concrete the same amount each time. After the cone is full, we can level off the top and pull the cone. As we do this, the concrete will start to sag, or "slump", from gravity. Now all we have to do is pull up the handle and measure the distance the concrete slump from where it started. If the measurement is 4", we should have a 4" slump. If it is 6", we have a 6" slump, and so on. You probably won't be performing this kind of test on every one of you residential jobs, but it is important to understand what a 4", 5", and 6" slump actually looks like. In decorative concrete, we would rarely pour tighter than a 4" slump. With the exception of walls or fire pits, it would not be beneficial to pour looser than 5-1/2"-6". We get a lot of comments at our training classes on how tight we pour we decorative concrete. We are not just trying to make everyone work harder! A common theory is that it's helpful to pour wetter on hot days. This is not necessarily true for stamped concrete. While pouring a high slump might buy you some time getting the concrete down, concrete poured at a 6.5" slump will be more problematic and never texture as well as that poured at 5". Keep in mind that Mini Delay Packs will increase your slump by approximately 1", so if you are planning to use them you will need to start at a lower slump than normal. In cooler weather, a 5" slump is the wettest we would ever pour. All the problems that come along with stamped concrete in cold weather get magnified when pouring a high slump.

It is important to know how to change or affect the slump of your mix. The common practice is to order the concrete at a lower slump than you intend to pour (keep in mind you can only increase slump on a job site and there is no good way of lowering it). This means you will need to add something onsite to bring the slump up to the desired consistency. The most common way of doing this is with water. A good rule of thumb is one gallon of water per cubic yard will increase the slump by 1". If you have 8 yds of concrete in the truck at a 4" slump and you add 8 gallons of water, your slump will be approximately 5". You may still need to fine-tune the mix after that, but this formula is a great way of deciding how much water to add when the truck first gets there. Keep in mind that every gallon of water you add to your concrete will affect the water to cement ratio. Too much water could lower the strength of the concrete beneath its intended PSI. 

Another way to effect concrete slump is with an admixture. Water reducers and plasticizers are very effective in increasing slump without adding excessive water. A mid-range water reducer will help you manage pours at any time of the year. High-range water reducers and superplasticizers will allow you to pour at a high slump in the case of poured walls or even fire pits. 

Just remember, most residential decorative concrete jobs will not require any certain slump, nor will there be a certified tester onsite. This means it is up to you to decide what slump will be the most beneficial to both you and your customers. Understanding the basics of what slump is and the best way to affect it will go a long way in creating the highest-end stamped concrete possible. 

Please don't hesitate to contact us if you have any questions about slump or anything else relating to decorative concrete. 

-Jeff Hersherger