Concrete Slump Test

Definition

Concrete Slump Test is a measurement of concrete's workability, or fluidity. It's an indirect measurement of concrete
consistency or stiffness. A slump test is a method used to determine the consistency of concrete. The consistency, or
stiffness, indicates how much water has been used in the mix. The stiffness of the concrete mix should be matched to
the requirements for the finished product quality

Concrete Slump Test

The concrete slump test is used for the measurement of a property of fresh
concrete. The test is an empirical test that measures the workability of fresh
concrete. More specifically, it measures concrete consistency between batches.
The test is popular due to the simplicity of apparatus used and simple procedure.

Principle of Slump Test

The slump test result is a measure of the behavior of a compacted inverted cone of
concrete under the action of gravity. It measures the consistency or the wetness of
concrete which then gives an idea about the workability condition of concrete mix

Slump Test Apparatus

 Slump cone,
 Scale for measurement,
 Temping rod (steel)

Procedure of Concrete Slump test:

1. The mold for the concrete slump test is a frustum of a cone, 300 mm (12 in) of height. The base is 200 mm (8in) in
diameter and it has a smaller opening at the top of 100 mm (4 in).
2. The base is placed on a smooth surface and the container is filled with concrete in three layers, whose workability is to be
tested.
3. Each layer is temped 25 times with a standard 16 mm (5/8 in) diameter steel rod, rounded at the end.
4. When the mold is completely filled with concrete, the top surface is struck off (leveled with mould top opening) by
means of screening and rolling motion of the temping rod.
5. The mould must be firmly held against its base during the entire operation so that it could not move due to the pouring of
concrete and this can be done by means of handles or foot - rests brazed to the mold.
6. Immediately after filling is completed and the concrete is leveled, the cone is slowly and carefully lifted vertically, an
unsupported concrete will now slump.
7. The decrease in the height of the center of the slumped concrete is called slump.
8. The slump is measured by placing the cone just besides the slump concrete and the
temping rod is placed over the cone so that it should also come over the area of slumped
concrete.
9. The decrease in height of concrete to that of mold is noted with scale. (Usually
measured to the nearest 5 mm (1/4 in).

Precautions in Slump Test

In order to reduce the influence on slump of the variation in the surface friction, the
inside of the mould and its base should be moistened at the beginning of every test, and
prior to lifting of the mould the area immediately around the base of the cone should be
cleaned from concrete which may have dropped accidentally.
Types of Concrete Slump

The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as;

1. Collapse Slump
2. Shear Slump
3. True Slump

Collapse Slump

In a collapse slump the concrete collapses completely. A collapse slump will generally mean that the mix is too wet or
that it is a high workability mix, for which slump test is not appropriate. It means the water-cement ratio is too high,
i.e. concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.

Shear Slump

In a shear slump the top portion of the concrete shears off and slips sideways. OR

If one-half of the cone slides down an inclined plane, the slump is said to be a shear slump. The shear slump indicates
that the result is incomplete, and concrete needs to be retested for valid results.

1. If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated.
2. If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix.

True Slump

In a true slump the concrete simply subsides, keeping more or less to shape

1. This is the only slump which is used in various tests.

2. Mixes of stiff consistence have a Zero slump, so that in the rather dry range no variation can be detected between mixes
of different workability.

However, in a lean mix with a tendency to harshness, a true slump can easily change to the shear slump type or even to
collapse, and widely different values of slump can be obtained in different samples from the same mix; thus, the slump
test is unreliable for lean mixes.

Applications of Slump Test

1. The slump test is used to ensure uniformity for different batches of similar concrete under field conditions and to
ascertain the effects of plasticizers on their introduction.
2. This test is very useful on site as a check on the day-to-day or hour- to-hour variation in the materials being fed into the
mixer. An increase in slump may mean, for instance, that the moisture content of aggregate has unexpectedly increases.
3. Other cause would be a change in the grading of the aggregate, such as a deficiency of sand.
4. Too high or too low a slump gives immediate warning and enables the mixer operator to remedy the situation.
5. This application of slump test as well as its simplicity, is responsible for its widespread use.
Indian Standard Slump Test
 Indian standard: IS 1199 – 1959

How to Determine Concrete Slump

slump is a measured indication of the workability, or flow, of a mix. The drier the mix, the lower or “tighter” the
slump. Conversely, the wetter or “looser” the mix, the higher the slump. The slump measurement process is
defined by ASTM C143 – Standard Test Method for Slump of Hydraulic-Cement Concrete.

The Procedure to Determine Concrete Slump

Tools Needed:

 Slump Cone Mold—The mold is 12-in. in height and known as an Abrams cone.
 Scoop—To place the concrete sample into the mouth of the cone.
 Steel Tamping Rod—This is a 5/8-in. diameter, smooth-surfaced, 24-in. long rod with hemispherical (rounded) tips.
 Baseplate—Or some similar non-absorbent surface.
 Tape Measure—Or a ruler to record the test result.

Preparing the Concrete Slump Test

 Obtain a sample using ASTM C172 – Standard Practice for Sampling Freshly Mixed Concrete methods. 
 Begin slump test within five minutes of collecting and remixing the sample.
 Ensure the test base is smooth, non-absorbent, and placed on a level surface.
 Once the slump test has started, it must be completed within 2 minutes and 30 seconds.

Test Procedure
Secure slump cone to the base plate using the cone tabs or by standing on the foot holds. The cone MUST remain still
throughout the test.

1. Using the scoop, fill 1/3 of the cone with the concrete sample.

Fill the cone in three stages. At only 1/3 full, use the steel tamping rod evenly across the entire sample .
1. Rod the material 25 times with the steel tamping rod, taking care to distribute rodding evenly over the entire cross-section
of the sample. While rodding, you may touch but do not hit or “thump” the base plate.
2. Fill the cone to 2/3 full.
3. Rod the second layer 25 times, making sure to penetrate the previous layer by about an inch. Continue rodding in a
circular fashion, working to the middle—the same pattern as discussed in step 3. 
4. For the final layer, fill the cone to slightly overflowing.
5. Rod the final layer 25 times with rod penetrating, but not through, the second layer. Distribute rodding evenly over the
entire cross-section of this layer. If the mounded material sinks below the lip of the mold, stop rodding, refill, then
continue rodding, picking up where you left off in the 25 stroke count.

Fill the Abrams cone to two-thirds full and rod again.

Now that Abrams cone is slightly overfull, use the steel tamping rod and level off the top.

6. Strike off the excess concrete from the top of the cone, using the tamping rod as a screed in a rolling or sawing motion.
7. Clean excess concrete overflow from the base of the cone. Use caution to avoid disturbing the cone during this process.
8. Holding the cone firmly, unlock or step off the cone tabs. Slowly lift the cone vertically in a steady motion, without any
rotational movement, straight up to clear the sample. The lifting action should take 5 to 10 seconds.
 9. Invert the cone mold—small side down—and set it next to, but not touching, the concrete sample on the base.

After lifting the cone, measure the difference in height of the Abrams cone to the height of the slumped concrete. Place it inverted near, but not touching, your
sample.

10. Place a straight edge, preferably the tamping rod, across the slump mold base, with one end suspended over the pile of
concrete. Using a tape or ruler, measure the distance from the bottom of the straight edge to the displaced center of the
slumped concrete. Record your results to the nearest 1/4 of an inch.

The recorded results are the slump of the concrete. 

Sampling Bagged Concrete is Different

Bagged concrete has its own set of rules as defined by ASTM C387 - Standard Specification for Packaged, Dry,
Combined Materials for Concrete and High Strength Mortar. This standard stipulates the slump range of bagged
concrete (without water-reducing admixtures) to be between 2 and 3 inches.

This is a much tighter or lower slump than you’ll see out of a ready-mix truck chute. However, looks can be deceiving.
The smaller coarse aggregate used in bagged concrete mixes makes it much easier to consolidate than most truck-
delivered concretes. Resist the urge to make your bagged mixes match ready mix consistency!

If standard concrete is mixed to a higher-than-specified slump using excess water the set time will be substantially
delayed, and—even worse–the end strength of the concrete will be adversely affected.

Make sure to follow procedures to the letter when working with concrete. Whether ready-mix or bagged, concrete may
appear to be a simple mixture, but there’s a lot of science going on under the surface.