Slump testing determines concrete consistency, allowing for adjustments to be made to achieve the desired stiffness. Proportions of aggregate, cement, and water affect stiffness, with less water producing a stronger finished product. Contractors order wet concrete with a degree of slump suitable for the job, and slump tests are performed by filling a cone-shaped mold with wet concrete and measuring the amount of collapse. A drop of 1-3 inches is best for commercial work, while 4-5 inches is best for residential jobs. Homeowners can perform a slump test using a paper cup.
Slump testing is a method used to determine the consistency of concrete. Texture, sometimes described in terms of stiffness, generally indicates how much water has been used in the dough. The test allows builders and construction crews to get a good idea of how the mixed product will perform in a variety of settings, and also gives them the opportunity to add more powder or more water to get the balance right. Different projects have different specifications when it comes to concrete stiffness, and testing results is usually much more accurate than simply following prepared instructions or making informed assumptions.
Proportional variables
Concrete generally starts out as a powder and in almost all cases comes as a combination of aggregate, which is gravel and sometimes sand as well; cement; and water. However, the proportions of the mix can vary depending on the source and manufacturer, which can make it difficult to guess or measure accurately. A higher proportion of water produces a softer, wetter concrete mix that is easier to handle during pouring and requires less care during curing. It also results in a slightly softer and less durable finished product. Less water results in a stiffer, wetter concrete mix, which is harder to handle but makes for a stronger, more durable finished product that is capable of withstanding more weight.
Contractors typically order wet concrete with a degree of slump suitable for the job they’re trying to do, which means they tell the supplier what kind of proportions and ratings they’re looking for. Ideally, mixed concrete arrives at the job site ready to pour, but in most cases, builders and others will want to test it before they get to work. Delivery slump tests are a good way to double-check stiffness, since it’s much easier to return a load than not to pour it if it turns out to be too watery or too thick.
How the test is performed
Manufacturers typically perform slump testing by filling a specialized test mold with wet concrete. In most cases the mold is shaped more or less like a cone that is missing its pointed end. A 5 cm (8/1.5 inch) diameter metal rod is inserted into the filled cone and pumped up and down about 25 times, mainly to remove air bubbles and set the concrete. Anything sticking out of the top of the mold needs to be leveled and then the mold is usually removed; without the support of the mold, the concrete collapses under its own weight. The amount of collapse is the most important thing.
The rod is then usually placed horizontally over the slumped concrete pile and balanced on the empty mold, which most measurers will place nearby. A ruler is used to measure the distance between the rod and the concrete, which represents the height lost when the concrete collapsed. This number is the “slump” for that particular batch of concrete.
Evaluation specifications
In most cases, a drop of 1 to 3 inches (2.5 to 7.6 cm) indicates a very stiff mix and is best suited for commercial work. This is sometimes expressed as “1 to 3” on packaging and in sales information. A stiff mix like this can be really hard to work with, and usually requires a mechanical vibrator to get it to set properly. It generally produces the hardest and strongest concrete.
A 4 to 5 inch (10.1 to 12.7 cm) slump is generally thought to be most appropriate for residential jobs, and concrete pouring is easiest with this amount of slump. This type of material can usually be worked by hand and in most cases a strong concrete is still obtained. Most experts state that a slump greater than 5 is usually too runny to work with, and with few exceptions, a mix with a slump greater than 7 inches (17.8 cm) shouldn’t be used at all. In most cases it will be too runny to set properly and may lack a lot of the strength and durability people generally expect.
Home testing alternatives
A die and measuring rod are the standard means by which vacuum is measured, but these tools aren’t necessarily needed, particularly if only an estimate is really needed. Homeowners or small-project craftsmen who work with small amounts of concrete can perform a slump test without purchasing special equipment by removing the bottom from a paper cup and inverting it onto a flat surface. The form should be filled with concrete, gently compacted and leveled the top. Testers can then remove the cup and measure the height of the concrete, comparing that height to the height of the paper cup to determine the amount of collapse.
The collapse will naturally be less using this method than it would be if a commercial mold was used, but much of this is due to the fact that the overall quantities of concrete are lower. If the concrete collapses about 75% of the cup’s height, that’s usually a good consistency for pouring. This isn’t as accurate as a test using a standard die, but it can be a good general guide.
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