As the water evaporates from this concrete mix, chemical changes occur in the cement paste.
A concrete slump test is important for several reasons. The more slumped concrete you create, the easier it flows through forms (and on-site). But, it also gets weaker due to less compaction in its matrix. On the other hand, too low values mean that you will get cracks later. These arise from improper consolidation during placing and finishing. Besides that, critical dimensions on objects made out of it can be off by several inches. This makes uniformity in work almost impossible. There's a need to determine an optimum value between strength and workability. Then, all calculations are done based on this value.
When mixing materials for hydraulic cement mixtures, water is added. The percentage of water creates a difference in the consistency of a mixture and its ability to flow. This is called a slump. Carrying out a concrete slump test ensures you have the correct amount of water in your mixture. If you find a collapse slump, zero slump, or shear slump then you need to repeat the test.Concrete, when mixed, is very fluid with a minimum amount of force required for moving it. A concrete mix must be consolidated after placement. Otherwise, it would develop cracks. For this process, proper strength is needed. You will then have a high slump test value. However, if the value gets too high, the material might run off forms or destroy itself while being moved.
Slump is one of the most important measurements in concrete work.
The concrete slump test is very simple. It's done by taking a small amount of fresh, non-sealed concrete mix and placing it in a cone-shaped container (looks like a miniature volcano). When you pull the sides away from the sample and they break, that's considered good enough for most purposes. The closer to zero the value is, the better. If your value is too low, you're going to have big problems placing concrete onsite. A true slump means having material with medium to high values. This happens when there's lots of water in the mixture so it flows easily but doesn't cause cracks after removal from forms. It also doesn't get damaged during transportation because of low density.A common concrete slump test can be done with a cone.
You should never add water to your concrete mix after placing it or during transportation. This is the most important rule to remember. Doing so will cause major issues for many reasons: First, the mixture is less dense and thus doesn't become as solid as it should be. Second, the chance of developing cracks later increases. This is because there's no way for cement paste to meld together properly when you constantly add more material to it. Third, good workability probably won't happen anymore because not enough water evaporates from concrete while being placed. The result is very weak concretes which are prone to damage while being moved around.
If you have low slump values from your slump test, you can add more cement. You can also try to mix another batch with slightly less water. If the concrete slump is way too low, you will have zero slump. If this happens, you're done. You need to start all over again and make fresh concrete. It's simply not possible to achieve a true slump in any other way than through the trial and error method.
Slump is the distance that a cone of concrete will slump (fall) under its own weight. How do you measure concrete slump? A concrete slump test measures the consistency and workability of freshly mixed concrete. This is done by rolling it into a 3-inch diameter log and dropping it 6 inches into a cone-shaped container filled with water. The number of inches the sample drops before stopping is called "slump." Slump ranges for concrete are as follows:
1 - 3 inches is a coarse slump. This means that your mix is slightly wet. Thus, it will require tamping during placement. It will also have a tendency to segregate.
3 - 6 inches is medium. This means that your concrete has a high workability mix. It shows that the consistency is good and without too much or too little water. But, it may still segregate during transportation if care isn't taken to prevent this occurrence.
6 - 8 inches is fine. Your concrete mix contains just the right amount of water so it won't segregate during transportation. It doesn't need excessive vibration or tamping during placement to achieve proper compaction. It's an ideal measurement of consistency for general-purpose construction applications. Here good flow-ability, moderate surface finish, and early strength are required.
8 - 12 inches is an extra-fine slump. This means your mixture contains more water than ideal for most construction applications. This will increase segregation potential during transportation. And, it will require exceptional care to avoid problems with compaction during placement. It's not advisable to use an extra-fine mix without adequate vibration or tamping equipment. If you do this, you may get a weak finished product (particularly if it contains aggregate larger than 3/8").
The ideal slump measurement value is between 6-8 inches. A common concrete slump test can be conducted using a standard 120 mm diameter slump cone. This metal cone is also known as an Abrams cone. It is filled with fresh concrete mixed with normal consistency. Then, remove excess concrete at the top. The mix should slowly fall away from the sides of the slump cone without dropping out horizontally or vertically. This means you have achieved well-slumped concrete. If it drops vertically more than 25 mm or horizontally more than 150mm, it is too wet. Thus you will get a collapse slump. In this case, more cement should be added to the mix. If it doesn't drop out at all or drops out less than 25mm vertically and 150mm horizontally, it falls under dry mixes. More water should then be added to the mix before carrying slump testing again. A slump cone helps to ensure you do not create an improperly mixed batch of concrete.
Slump loss is the amount of concrete mixture that falls from a slump cone when lifted vertically. This follows 5 minutes of hardening after casting. If your slump test value changes drastically after this time has passed, there was too much water in your previously tested mixture. You need to add extra cement and repeat the slump test until an ideal value can be found. You want to avoid things like segregation during transportation. When your materials settle into layers before shipping, then during placement they'll be mixed together again. Hence, your slump value will change. You could have a mixture that falls more than 150mm or 25mm straight out. Then, you know it has settled into layers somehow. Its consistency is probably not ideal anymore.