ASTM Standard Test Method C143: Slump of Hydraulic Cement Concrete
Significance and Use of ASTM C143
Slump is a measure of the consistency of a sample of concrete, and tells you how fluid the concrete will be. It can help give an idea of workability, telling you how easy or hard it will be to place, and can help to predict strength.
You can also get a general idea of how much water is in the mix and if it is too wet or too dry for the project’s specifications. One gallon of added water per cubic yard equals 200 to 300 psi less strength, so knowing the slump of concrete when it's fluid is vital to knowing how strong and durable it will be once it solidifies. One gallon of water per cubic yard also equals about 1 more inch of slump (slump is measured by how far downwards the concrete sinks when the slump cone is raised and the concrete is no longer held in place).
The smaller the stone or gravel size in the batch, the more water will need to be added, as smaller stones have a greater surface area and have more areas for cement and water to bond to.
As air content in the concrete increases, so does the slump. Superplasticizer, a substance that makes concrete flow easier, may be added at times to increase the slump without adding water or air and compromising the concrete’s strength.
Equipment Needed for the Slump Test
Slump cone – Must have an average thickness of at least 0.06 inches for metal cones. Must have a base that is 8 inches in diameter and a top that is 4 inches in diameter. All diameters must be within 1/8 of these dimensions. The inside of the cone needs to be smooth and clean, and the cone must be free of dents and deformations.
- Tamping rod – Must have a smooth hemispherical tip and must have a 5/8 inch diameter ± 1/16 inches. Must be at least 4 inches longer than the depth of the slump cone. The rod itself cannot be longer than 24 inches. The rod must be smooth and clean.
- Scoop – Must be large enough to get a representative scoop of concrete and small enough to where you don’t spill any concrete on the ground as you are pouring concrete into the cone.
- Ruler or measuring tape – Must have markings at the ¼ inch mark or smaller. Must be at least 12 inches long.
- Slump plate – Must have a clean, level surface with no gouges, grooves or indentations. If it has clamps, they must be able to move freely and be fully released without moving the slump cone during the test. Must be larger than the diameter of the slump cone’s base.
- Bucket of water and rag – for cleaning your equipment before and after the test.
- Sampling receptacle – usually a wheelbarrow. Must be able to hold at least 1 cubic foot of concrete, and be able to be wheeled comfortably without spilling the sample.
ASTM C143 Procedure
- Sample the concrete, first checking with the crew to make sure all water has been added, according to ASTM C172, and mix it thoroughly. Set up your test area in a place that is free of debris and traffic, and make sure your slump plate is on the most level surface possible. Get a bucket of water and moisten the surface of the slump plate and the inside of the slump cone, to keep concrete from sticking to it.
- Put your slump cone on the plate and either clamp it down or stand on the foot pieces. If standing on it, do not step off until the cone is full and ready to be lifted.
- Fill your first layer to 2 and 5/8 inches (1/3 of the cone by volume), making sure it is even inside the cone.
- Rod the layer 25 times, making sure to cover all the surface area inside the cone, slightly angling the rod to get the edges. Here, unlike other tests, you do not tap the side of the cone, because that causes artificial subsidence and your slump will come out higher than it really is.
- Fill the second layer to 6 and 1/8 inches (2/3 of the cone by volume) making sure the concrete layer is even.
- Rod the layer 25 times, and this time make sure you penetrate the first layer by 1 inch. Again, do not tap the side of the cone.
- Fill the last layer up to the top, where the concrete is slightly overflowing.
- Rod the layer 25 times and penetrate the second layer by 1 inch. Remember to not tap the sides of the cone. This top layer needs to be kept full at all times, so add a bit of concrete if it starts to go below the rim of the cone.
- Strike off the excess concrete, and if you are standing on the cone keep it steady and do not get off yet. Clean around the rim of the cone, making sure it is full. (Optional: Place a penny in the center of the cone, so you can see where the center is displaced when you lift it.)
- Put your hands on the handles of the cone and push down while unclamping the cone or stepping off of it. Keep it steady.
- Lift the cone straight upwards with no sideways motion or twisting. The lifting process should take between 3 and 7 seconds, count if necessary.
- Flip the cone upside down next to the slumped concrete, and place your tamping rod on top of the cone and over the slumped concrete. Measure from the displaced center to the rod, and record the slump to the nearest quarter of an inch. The entire slump procedure needs to take place in 2.5 minutes.
- Clean your equipment and discard the used concrete. If the slump is out of spec, perform a second test to be certain, and then tell the site superintendent if both slumps were out of spec.
Common Errors Associated With The Slump Test
- Placement of the slump plate on an uneven surface. Make sure the plate is level before putting concrete into your mold.
- Pulling the slump cone up too fast or jerking the cone in a horizontal direction. Pull the cone straight up, smoothly and steadily.
- Failure to measure to the displaced original center. The penny trick is very helpful when finding the center.
- Equipment being out of spec or dirty. Slump cones and plates should be calibrated yearly, and cleaned after every test, especially on the inside surface of the cone and the surface of the plate.
- Slumps taken on the first 1/4 or last 1/4 of the load cannot be used to reject concrete, as they are not the main characterization of the batch.
- Using sampled material that has not been mixed up properly.
- Starting the slump test too late when the concrete is beginning to harden.
How to Perform the Slump Test
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