Concrete slump testing is by far the most common test done on new concrete in any application. The slump test has been known for a long time, but in 1918, concrete pioneer Duff Abrams created the method for determining the consistency or workability of new concrete. He and other researchers discovered that preserving the optimal slump value for a concrete mix made it easier to lay, consolidate, and finish the material.

The slump test is now an important aspect in determining a concrete batch’s conformity with the authorized mix design, as well as an eleventh-hour check on its fitness for installation. The concrete slump represents the water-to-cementitious materials ratio (w/cm) in a given batch and indicates when the mixture is out of equilibrium.

The w/cm value is an important aspect in concrete strength development, however the exact relationship between slump and w/cm is not always evident. According to ASTM C143 and AASHTO T119 test procedures, a clear and consistent strength relationship may not be observed under field settings.

Why is the Slump Test Important?

Concrete slump is one of several fresh concrete tests, including air content, unit weight, temperature, and the molding of laboratory samples for strength testing, as prescribed by ASTM C94. These tests capture the qualities of the concrete when it is delivered to the project site. Fresh concrete tests are done immediately before to concrete placement and repeated at regular intervals during the pour to record product uniformity.

In a previous article, we examined the workability of concrete and the many tests performed to assess it. For this essay, we will just discuss the slump test, how to execute it correctly, and the best testing equipment to utilize.

What is Slump in Concrete?

Different slump levels are beneficial to various concrete uses. Curbs or pavement constructions using a slip-form paver necessitate a strong mix that keeps its shape immediately after installation. Casting walls or columns with complicated geometries and dense reinforcement necessitates a mix that flows well around reinforcing steel and into tight places.

The concrete slump test begins by consolidating a new concrete sample into a cone-shaped mold. The concrete sample subsides or slumps when the mold is elevated vertically because it is no longer supported. The slump value is the vertical distance between the concrete’s original and displaced top surfaces, measured in inches or millimeters.

Concrete slump is a quick and simple test to execute correctly, but it has several stumbling blocks that can lead to mistakes. Improper testing may limit the use of suitable concrete or allow for the laying of concrete that does not meet the criteria.

The slump of concrete changes continually after the batch is mixed. Aggregate size and shape, chemical admixture kinds, pumping placement, temperatures, elapsed time after batching, and other external factors all have an impact on how a mix performs in the test.

Individual operators’ activities throughout the test also have an impact on concrete slump results. Minor changes in the specimen’s location or consolidation, as well as the speed with which the slump cone is removed, might affect the test result. The slump test is a hands-on process, and too many differences in approach might reduce its accuracy and reproducibility.

Not All Mixes Respond Well to the Slump Test:

What you Need to Know

ASTM C143 and AASHTO T119 are the standard slump test techniques for hydraulic-cement concrete with a maximum aggregate size of 1-1/2in (37.5mm). Coarser mixes are inappropriate for slump tests, although they can be changed by sifting off aggregate bigger than 1-1/2 in accordance with ASTM C172.

Self-consolidating concrete is designed to flow past crowded reinforcing steel and consolidate well in complex formwork. Standard slump tests on these mixtures provide extremely high results and are ineffective for characterisation. As shown in this blog post, the ASTM C1611 and C1621 tests employ a modified slump cone and various testing equipment to quantify slump flow and SCC passing capabilities.

Slump tests for certain specialized concrete mixes that contain high-range water-reducing admixtures (superplasticizers) or unusual combinations of aggregate sizes and volumes may not be applicable or should not be used at all.

Concrete Slump Testing Equipment: Putting Together Your Toolkit

Equipment that meets the standards for slump testing is commonly accessible and reasonably priced. The equipment is basic and cheap to maintain, but it will ultimately need to be replaced owing to dents, corrosion, and the buildup of concrete residue. Concrete slump test kits consisting the required components are available for convenience.

Traditional slump cones are made of seamless metal in a shape produced by the spinning process in the proper form of truncated cones. Slump cones permitted in ABS plastic have an advantage over such traditional units: They weigh  less and are stronger but resist denting and deformation and contain no rust. Separately, either inch and metric unit measurement models may be supplied.

Base plates are optional, making the test easier to execute, thereby yielding consistent results. The plate itself becomes a readily available non-absorbing flat surface with integral clamps which will keep foot tabs of the slump cone free.

Tamping rods conform to the standard tests, carrying on their head correct diameter, along with the hemispherical tip which will make perfect sample consolidation. The rod in length allows penetration in case rodding needs are met.

To avoid repetitive sampling of fresh concrete, sample scoops should be manageable in size, allowing for equitable distribution and sufficient capacity.

The droop cone filling funnel is a handy way to fill the cone and save waste.

Measuring devices are typically tape measures, but rulers with a minimum length of 12in (305mm) and 1/4in (5mm) or finer graduations are also acceptable. Some tamping rods include an etched scale for measuring slump.

Performing the Perfect Slump Test: Representative Samples are the Key

The possibility of an accurate test result depends upon an accurate sample. ASTM C172 outlines the acceptable methods of obtaining a representative sample of fresh concrete for tests including slump, air content, unit weight, temperature, and the molding of strength specimens.

For revolving drum truck mixers and paving mixers, two or more portions are collected from the middle portion of the batch within a 15-minute period during discharge and combined into a composite sample. Alternate procedures are noted for sampling from other types of paving mixers, agitators, and open containers. Some projects may call for point-of-placement sampling, such as sampling from the discharge end of a concrete pump or from a concrete bucket.

Once the composite sample is complete, the clock begins to tick. Testing for slump, air content, and temperature must commence within 5 minutes after sampling. The slump test itself must be completed in an elapsed time of 2 ½ minutes. This gives only a very short time period in which to have your slump testing equipment set up and ready for use.

A flat, level, non-absorbent surface free of vibration and large enough to contain the slumped concrete is necessary to place the slump cone. The cone must be damp but with no excess water and secured to the surface or base plate, either by standing on the foot tabs or by clamping to a base plate.

Pro Tip: Standing on the foot tabs throughout the test is somewhat clumsy. The test method allows clamping the cone to a non-absorbent base plate rather than standing on the foot tabs. This is preferable for practicality and ease of use.

Step-By-Step Slump Test Procedure
Follow the Test Method for Consistent Results

Scoop:
The mould is filled with three equal volume layers, not depth. The cone’s depths of equal sample volumes work out to 2 5/8in (70mm) for the first layer and 6 1/8in (160mm) for the second layer. When placing concrete in the cone move the scoop around the opening to distribute the sample evenly. The final layer is heaped over the top, and excess is maintained during rodding if required.

Pro Tip: Save time and guesswork by marking the outside of the slump cone with the required depths of the first and second layers of concrete.

Consolidate:
The use of the correctly sized and shaped tamping rod is essential to the successful operation of the test. Tamping with a piece of rebar or similar material that does not contain a hemispherical nose crushes down aggregate particles rather than uniformly compacting the concrete. Too-short rods are hard to get a good grip on, and may prevent layers to be rodded into an adequate depth.

Consolidate each layer with twenty-five evenly distributed rodding strokes throughout its depth. Penetrate the bottom layer without forcefully striking the baseplate or surface. For subsequent layers, allow the rod to penetrate about 1in (25mm) into the layer below. The vertical strokes of the tamping rod must be angled slightly at first to ensure even consolidation around the perimeter of the bottom layer. Continue spiraling toward the center with evenly distributed vertical strokes.

Pro Tip: Never tap the cone to consolidate the sample in the same manner that strength cylinders or beams are molded. If the slump cone is tapped, moved, or disturbed during the test, the sample must be discarded and the test starts over with a new sample.
After the rodding is completed, use only the tamping rod to strike off the concrete, even with the top of the cone. A horizontal screeding motion combined with a rolling action of the rod provides a clean strike-off without compacting the sample.

Lift:
While holding the cone down by one of the handles, use the other hand to clean away excess concrete from the base of the cone. Remove your foot from the opposite foot tab or release the clamping mechanism. Keeping the cone immobile while switching hands, repeat the process on the second side. With a hand on each handle, carefully lift the cone straight up without twisting. The vertical lift should be completed in 5 ±2 seconds.

When the slump cone is removed, the unsupported concrete sample slumps.

Measure:

After the slump cone is removed, the concrete settles or, in some high-slump mixes, collapses. The slump value is calculated as the vertical difference between the top of the mold and the displaced center of the top of the specimen, rounded to the closest 1/4in (5mm). If a section of the specimen rips away, the test must be discarded and a new sample tested instead.

Measure slump by inverting the slump cone and using the tamping rod as a reference.

Compare & Report:
The acceptable tolerances of the concrete slump are specified in the project specifications or as part of the concrete mix design. Slump test values that deviate from project requirements should be reported to stakeholders immediately and noted in inspection and testing reports.

The chart below shows different ranges of slump test values and how they might be used.

Concrete Slump Range
Slump, in (mm)	Degree of Workability	Application
0–1 (0–25)	Very low	Very dry mixes in paving machines with high-powered vibration
1–2 (25–50)	Low	Low-workability mixes used for foundations with light reinforcement; Pavements consolidated by hand-operated vibration
2–4 (50–100)	Medium	Medium workability mixes manually consolidated flat slabs. Normal reinforced concrete manually placed; heavily reinforced sections with mechanical vibration.
4–7 (100–175)	High	High workability concrete for sections with congested reinforcement; May not respond well to vibration.

Take a Closer Look: Using Knowledge and Experience to Add Value

Writing down the slump measurement on a testing report fulfills the responsibility to execute the test technique, but the concrete sample still has a tale to share. Collecting, testing, and inspecting fresh concrete early in the placing process might provide an excellent opportunity to act if a problem is discovered.

Understanding the behavior of concrete after the cone is removed enables predictive troubleshooting. Learning to assess the look of a slumping concrete sample may be a very useful diagnostic tool. Potential issues can be identified and addressed while they are still fixable. A basic visual assessment, supported by knowledge and experience, can reveal some important concerns.

If the concrete sample in a completed slump test exhibits an unexpected reaction or appearance, the default step should be to discard it and perform the test immediately with a fresh replacement sample. The slump test is easy and quick, therefore re-testing should be the standard anytime there is any uncertainty.

Typical profiles for completed slump test samples.

Types of Slump:

True Slump appears “normal.” After the cone is removed, the concrete sinks almost straight down and remains fairly undamaged. The sides bulge out, and the bottom may grow in diameter, but the center of the original top remains about the same position. Even as the concrete material alters position, the aggregate and paste ingredients do not separate much.

These features indicate that the concrete mix proportions are reasonably balanced and should perform reliably throughout placement, consolidation, and finishing. They also show that the combination is suitable for slump testing. Some specific concrete mixes including unusual amounts of aggregates, mortar, or admixtures may not respond well to slump tests, producing deceptive results.

Shear Slump occurs when a part near the top of a sample shears off and falls to the side. Although the difference between the original and slumped heights is quantifiable, the sample separation renders the test worthless and should be ignored. Shear slumps occur when the mix’s plasticity is reduced due to a low paste or high sand content.

Collapse Slump occurs when the majority of the sample fails to keep together after removing the slump cone. It might indicate that the mix’s sand-size content is too low and lacks cohesiveness.

Segregation Observing a slump sample can help detect segregation, a specific problem. When the cone is removed, the concrete aggregate and paste sections separate easily, with no cohesion or flexibility. Signs of segregation might suggest problems with coarse aggregate content, mixture proportions, insufficient mixing, or an inadequately obtained test sample.