Concrete Slump or Measuring Consistency of Concrete
Introduction
According to ACI, concrete slump is the measure of concrete consistency and fluidity. Generally, the term “workability” refers to the ease of pouring, compactibility, ease of concrete finishing, and concrete resistance to exfoliation. Unfortunately, there are no experiments so far fully expressing workability, but efforts have been made to measure performance easier with some physical features, but none of them have been perfect. One of these experiments is “slump”.
It should be noted that the slump test does not measure workability, but describes the consistency of the concrete. Workability, as defined in ASTM C 125-93, is the property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity. According to ASTM C 125-93 workability is defined as the property of freshly mixed concrete or mortar which determines the case and homogeneity with which it can be mixed, placed, consolidated and finished. The definition of consistency by ACI is as follows: Consistency is the relative mobility or ability of freshly mixed concrete to flow. This property of concrete is obtained by the slump test. Due to all the mentioned limitations, slump test is very useful for controlling and measuring changes hour by hour.
Two good factors for the test
- If the material and the grading of the aggregates are uniforms, the test shows changes of the water-cement ratio.
- If the amount of water in the mixture and the aggregates’ moisture is unchanged, the test indicates changes in the grading of the aggregates or inaccuracy of the weight of the cement and the sand. The consistency of the concrete coefficient (concrete slump) for the ordinary buildings is about 5-7 cm. In the case of using a vibrator, the desirable amount of this coefficient is 5 cm.
Slump Test Accessories
- Slump Test Truncated Cone with the height of 300mm, and the internal diameter of 100 mm at the top and 200mm at the bottom.
- Tamping Rod with the diameter of 16 mm and 600mm long, and one end rounded to hemispherical tip with 16mm diameter
- Scoop
- Cement
- Sand and aggregate
- Scale with gram accuracy
- Trowel
- Slump pan
- Cube mold
- Oil
Test Procedure
First, scale the sand, aggregate, cement, and water according to the determined proportions in the mixing design, then mix them in the pan until the fine and coarse grains are mixed well. After the mixture is ready, dampen the cube mold and place it in a damp, flat, and smooth surface that does not absorb moisture.
Put your feet on either side of the cone and fill it with the prepared mixture in 3 layers. Tamp each layer with 5 strokes of the tamping rod.
The height of each layer after compaction should be approximately one-third of the height of the mold (According to the standard No. 492 in Iran, the mold must be filled in three stages. The one-third volume of the cone is approximately the height of 6.5 cm, and the two-thirds volume is approximately the height of 15 cm.
For the bottom layer, it is necessary to tilt the tamping pod slightly towards the inside and tamp some strokes spirally from the circumference towards the center. In this layer, the tamping should penetrate into the underlying layer. Compact the other 2 layers in such a way that the tamping penetrates a little in the subsequent layer. Before compressing the top layer, the mold must be filled sufficiently with concrete. If the top layer of concrete is lower than the mold edges after compaction, pour some concrete on it again and level the surface with a rod in a vertical and rolling motion.
Take the slump cone handles and pull it up slowly. Lifting the mold should be done within 5 to 10 seconds without any lateral or rotational movement of the mold and concrete. Standard No. 492 of Iran stipulates in paragraph 3 that raising the mold shall take approximately 5 seconds or in a steady upward movement such that no lateral or rotational movement transfers to the concrete.
Then, place the cone next to the subsided concrete and place the tamping rod on it, to determine the slump by calculating the difference in the height of the mold and the height of the concrete after lifting the mold. (According to paragraph 4 of Standard No. 3202 of Iran) this should be calculated with approximately 5 mm accuracy.
After raising the mold, if the concrete collapses or falls on one side, the test result should be discarded and re-tested with another concrete sample. If this happens again, it means that the concrete is not pastry and adhesive enough to perform the slump test.
The standard No. 492 of Iran does not specify a place to measure. Regulation BS1881:83 Section 102 states that the amounts of concrete subside shall be measured up to its highest point in order to determine the slump of concrete.
Types of Slumps
- Natural or True slump: This means that the concrete has a good to an excellent consistency (about 5 inches)
- Shear Slump: In this slump, the height of the concrete decreases, shears off and slips sideways (almost one half of the cone slips down). In this case, the test result is rejected and concrete must be retested with another concrete sample. If the shear slump persists in the second test (as happens in harsh mixtures), then it indicates that the mixture lacks cohesion.
- Collapse Slump: The concrete collapses completely. This slump means that the mixture is too wet, harsh, or with low cement content.
Note:
- The strokes should be spread evenly on the layer surface. For the bottom layer, it is necessary to tilt the tamping pod slightly towards the inside and tamp some strokes spirally from the circumferencetowards the center.
- If a shear slump is achieved, a retest should be done until a true slump shape is achieved. If the shear slump persists, this may mean that there are some weaknesses in the mixing design.
- If the concrete collapses (without an external force to the concrete mix), all the ratios should be recalculated.
- This test is done for the concretes that their maximum size is 40 mm.
Differences between ISIRI Standards No. 492 and No. 3203
- In paragraph 3 of Standard 492 it is stated that the concrete should be divided into three volume layers (the height of the first layer is 6.5 cm, the height of the second layer is 15 cm, the height of the third layer is the whole cone), while paragraph 4 of Standard 3203 states that the concrete should be filled with three layers such that the height of each layer after compaction should be approximately one-third of the height of the cone.
- Paragraph 3 of Standard 492 states that the entire test shall be performed (from the beginning of filling to lifting the mold) in an uninterrupted period of 1.5 minutes, while this time is not being specified in standard 3203.
- Clause 4-4 of the standard 3203 states that the subsidence should be calculated immediately after the mold is lifted, by subtracting the height of the mold from the height of the while in Standard 492, no measurement is specified.
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Differences between ASTM and BS
- According to section 102 of the BS1881: 93, the concrete subsidence is measured to its highest point.
- According to ASTMC143-90a the concrete subsidence is measured to the displaced center of the concrete.
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