So here we are, wondering what a slump of concrete is. During my early career, it didn’t mean much to me, but the further I got – the more it meant.
When we work with concrete at the construction site or at a factory, the quality of the product is one of the things we’re quite concerned about. Bad quality products can’t be sold; with most concrete products, it means the concrete itself, rebar, or the attachment for bolds or welding, etc.
So what is a slump test of concrete and what does it have to do with the quality of the product? I will try to answer these questions to the best of my ability so you can easily get the results you want. I’ve written before about concrete density and the concrete slump has everything to do with that.
That being said, we have to get to the bottom of what is the concrete slump means. The short answer is, the slump test measures the concrete density and how flowing it is. So to test that, we have to devise a way to measure those qualities.
But concrete is just concrete you might think. The truth is there are multiple levels of how hard concrete can be and multiple factors affect that like cement, amount of water, other additives, etc. How flowing the concrete is doesn’t tell us straight how hard it will be, but if we know the recipe we can take measures to predict it.
These should be basic qualities needed from the concrete slump explained. Let us move on to see how exactly it’s done and what the results will tell us.
Short concrete slump guide
The traditional way a concrete slump is measured is with a cone that resembles a small traffic cone but is made with metal and usually attached to a movable platform tightly so it won’t move while it’s being filled.
It also has the top off so it can be filled from there. And there are handles on the sides so it can be lifted after it has been filled with concrete and some of them have some sort of measuring stick on the side to make it easier to measure the slump.
What is the slump cone test?
So you fill the cone attached to a platform with concrete and then what? Sounds pretty uninteresting I can imagine.
After the cone is removed by lifting it upwards, gravity will pull the concrete downwards, right? The slump test is how much the concrete will give in at this point, and how many inches it will go down.
Now, what does a concrete slump test tell you at this point? It tells you how much flow the concrete has as stated at the beginning and from that you can also make an educated guess about the density.
The flow is also important for us when we have deep or thin forms or if there is lots of rebar reinforcing the future structure. If there is no flow, the concrete won’t fill up the whole form or there will be air pockets or uneven surfaces. So we want the concrete to be workable for difficult structures or long-distance pours.
It’s one of the go-to tests done at construction sides where measuring on the spot is needed. Why is that? Slump cone test equipment, as you can see, is dead simple to use and it will give you fast directions about the quality of concrete you’re working with.
5 steps for how to test concrete slump
So you have the basic idea now about the slump cone test for concrete. Now you’re asked to do it at the site so how do the slump cone test? Do you pour the concrete in and lift the cone or what?
Step 1: Start by putting the cone where it’s not on the way, but not far from where you’re supposed to be pouring.
Step 2: Put moisture inside the cone by spraying it with water or another method. It’s for making it easy to remove the cone. If you shake it when you lift it, it will naturally cause the concrete to slump because of the vibration so you want to be able to lift it up smoothly to get proper results.
Step 3: To have the concrete in the cone without much air, you fill it in multiple parts. You can do something like ¼ or 1/3 and in between filling, tamp the concrete with something so it’s there as settled as possible.
Step 4: Now it’s filled to the top and you’ve made the top concrete level with the cone. It’s time to lift it up. Carefully lift the cone upwards and put it aside.
Step 5: Measure how much the concrete gave in by using the possible helping tool in the cone platform. If there is none, place the cone next to the slump, use a straight piece of wood or something else on top of it, and the slump.
You can measure down from the wood towards the slump to see how much it sunk.
Slump flow test
Earlier we established that flow is also important when planning a difficult pour. You can also run into a slump flow test during your career or hobbies. It’s a different test for concrete flowing enough to be done with the cone and steel plate.
The concrete is poured into a cone without compaction. After that, the top of the cone is leveled again the cone is lifted. This time, we know it’s going to spread on the plate.
We wait until the concrete slump has spread on the plate and then we measure how wide it is in diameter in 2 perpendicular directions.
How to judge concrete slump
What is the normal slump of concrete? There must be some standards now that we know there is a way to measure.
Concrete slump chart
A concrete slump chart is something that tells you what the qualities of the concrete are by the slump measured. I’ve seen many during the years by manufacturers and some by guidebooks.
Usually, the lower the concrete slump numbers are, the less workable it is. You can imagine it by the concrete having less water in it or it has started to cure so it will be rougher and more effort has to be used to get it in the forms without leaving voids in it.
This can actually be a challenge even to the ones working with concrete daily. It has to be vibrated a lot layer by layer and with big concrete elements every hand is busy already so it’s easy to miss a spot.
So the higher the concrete slump numbers are, the easier it is to pour. The concrete slump range is usually between 1 inch and 7 inches. 1 inch could be something that is used to build roads with, 2 inches could be for foundations, 3-4 inches for reinforced walls, etc.
Don’t take these values to heart as stated before, different manufacturers can provide slightly different info. It’s good always to check local standards when you build with concrete so you won’t have trouble with authorities. Or with the building itself.
Now you should know what a slump of concrete is about. If you’re running your own project or work where concrete is poured, you should be able to do DIY slabs with these instructions.
Working with proper density concrete is important even when doing home projects. Imagine if we build something with heavy weight on top of the slab? If the concrete doesn’t have proper density it might crack or give in.
For that reason it’s important to follow the concrete manufacturer’s instructions to reach the end product’s intended strength.
These are something that is also reported and written down. Sometimes, it might even cause trouble if the slump is higher or lower than needed.
If you’re curious about concrete slump vs strength, the higher the slump value, the more water there is on the mix. Regular concrete means weaker strength for the concrete, which is why lower slumps are used for foundations.
So what is the ideal value of the slump will depend on the use. If you want foundations, it is lower and for walls, it can be a little higher. This is why we need to refer to a proper table that should be provided by manufacturers.