The concrete slump test is also known as the slump cone test of fresh concrete. It is the easiest way to determine the workability & consistency of fresh concrete prior to pouring. The values obtained from the concrete slump test helps to correct the concrete mix according to the workability required. The concrete slump test can be carried out in the lab as well as on the construction site.
The slump cone test is the most simple and low-cost test to determine concrete workability and get immediate results. the test can be carried out from batch to batch or as required on the site.
Generally, the Slump test of concrete is a measure of its consistency & workability before its sets.
What is the equipment required for the concrete slump test?
The concrete slump test equipment is inexpensive but specialized and must be complied with international standards. The equipment is consists of a Slump Cone, Temping rode, measuring tape, and a nonporous base plate. The slump cone height is 30 cm, the bottom diameter is 20 cm and the top diameter is 10 cm. The temping rod is 16mm steel in the length of 60 cm. A trowel and steel wire brush is also required to assist in the slump cone test.
International standards for concrete slump test
It is important for the site engineers to study the international codes conforming to the concrete slump test, such as;
- American Society of Testing Materials (ASTM) has the following codes, ASTM C143, ASTM C11611, ASTM C192, ASTM C1712.
- American Society of state highways and transportation officials (AASHTO) ASSHTO T119 and AASHTO BS1881.
- Indian Standards (IS) IS 1199 (1959)
- British & EU Standards BS EN 12350-2
Concrete Slump Test Procedure
A concrete mix of M40 (the most common mix used in Rcc structure) or as described in the specification is tested in the Readymix lab prior to the dispatch of the concrete. Every site engineer must check the concrete dispatch slip confirming the dispatch time, concrete strength, and slump values. Once the concrete has arrived at the construction site; the procedure before the cast is as under;
- Confirm the concrete grade, dispatch time, and slump values carried out in the Readymix lab.
- Collect a sample in a wheelbarrow from each concrete ready mix truck and confirm the temperature of concrete.
- Clean the internal surface of the slump cone by applying oil.
- Place the cone on a flat nonporous base plate.
- Fill the cone with the help of a trowel, the cone should be filled in 4 equal layers.
- Each layer must be tamped 25 times uniformly by the round end of the tamping rod. For each layer, the rod must penetrate the subsequent layer of the concrete.
- Once the slump cone is filled, clean the top of the cone and the base plate to prepare for slump cone lifting.
- Raise the slump cone slowly but rotating diagonally in the vertical direction.
- Measure the slump as the difference between the height of the slump cone and the height of the concrete sample.
- Cast the cubes for concrete compression test.
The concrete slump test result value shall be observed in millimeters (mm). the graphic diagram is as under.
Concrete slump test results
Once the slump test is carried out by the technician, the following shapes of concrete are expected.
True slump is the general drop of concrete mass evenly once the slump cone is lifted. The value of the true slump ranges from 25-50 mm (1”-2”) depending on the concrete mix. The measurement is taken from the top of the slump cone to the top of the sample concrete.
Shear Slump indicates the lack of coherence in the concrete mix. Once the cone is lifted, one side of the specimen concrete will be collapsed. In the event of a shear slump, the concrete might be tested again to get the true slump, it is possible by mixing the concrete well or adding more dry mix.
Collapsed indicates that the water-cement ratio is very high, because of the high water, the concrete is collapsed and not suitable for use in any structure except for not load-bearing PCC.
If the concrete did not collapse at all, it is called a zero slump. This indicates that the concrete mix is dry. This kind of concrete is suitable for roads and highways.
Factors affecting the concrete slump test
There are some factors that greatly affect the slump cone test of the concrete.
- Concrete mix design and batching, mixing, and transportation.
- Material properties such as size, quality, moisture, texture, cleanliness of the aggregates.
- Chemical admixtures in the concrete to adjust the properties of concrete according to the requirements.
- The temperature of the concrete ( suitable temperature 10°C to 32°C)
- Air content in the concrete ( maximum 6% for ¾” aggregates)
- Condition of the slump test equipment and expertise of the testing technician.
- Water cement ratio ( acceptable 0.4-0.6)
- Concrete mixing time and dispatch from the plant. ( acceptable less than 30 mins)