Concrete Slump Test - Theory and Lab 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

Concrete SlumpThe 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

Concrete Slump TestIn 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
Types of 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.
Degree of workability
Slump
Compacting
Factor
Use for which concrete is suitable
mm
in
Very low
0-25
0-1
0.78
Very dry mixes; used in road making. Roads vibrated by power operated machines.
Low
25-50
1-2
0.85
Low workability mixes; used for foundations with light reinforcement. Roads vibrated by hand operated Machines.
Medium
50-100
2-4
0.92
Medium workability mixes; manually compacted flat slabs using crushed aggregates. Normal reinforced concrete manually compacted and heavily reinforced sections with vibrations.
High
100-175
4-7
0.95
High workability concrete; for sections with congested reinforcement. Not normally suitable for vibration
Table : Workability, Slump and Compacting Factor of concrete with 19 or 38 mm (3/4 or 11/2 in) maximum size of aggregate.

Slump Test Standards

The concrete slump test is referred to in several testing and building code, with minor differences in the details of performing the test.

Indian Standard Slump Test

  • Indian standard: IS 1199 – 1959

Slump Test Video

Questions and Answers:

Q: What is the ideal value of slump?
Answer: 1
In case of a dry sample, slump will be in the range of 25-50 mm that is 1-2 inches. But in case of a wet concrete, the slump may vary from 150-175 mm or say 6-7 inches. So the value of slump is specifically mentioned along the mix design and thus it should be checked as per your location.
Factors affecting concrete workability are properties of concrete ingredients - aggregates etc. Also temperature has its effect on slump value. So all these parameters should be kept in mind when deciding the ideal slump
Q: How does a super plasticizer effect the slump of concrete?
Answer: 2
Value of Slump can be increased by the addition of chemical admixtures like mid-range or high-range water reducing agents (super-plasticizers) without changing the water/cement ratio.
Q: How much time one should take to raise the cone?
Answer: 3
Once the cone is filled and topped off [ excessive concrete from top is cleared ] raise the cone within 5-10 seconds.
Q: What are the dimensions of the cone used in Slump Test?
Answer: 4
The dimensions are:
Top Diameter - 10cm
Bottom Diameter - 20cm
Height - 30cm

Comments

Post a Comment

Popular posts from this blog

Railway sleepers

-
Image
Sleepers The members which are laid transverse to the rails on which the rails are Supported and fixed to transfer the loads from rails to the ballast and sub grade below. Functions 1)       To hold the rails to correct gauge. 2)       To hold the rails in proper level. 3)       To act an elastic medium in between ballast and rails. 4)       To distribute the load from rails to index area of ballast underlying it. 5)       To support the rails at a proper level in straight tracks and proper super elevation on curves. 6)       It also gives lateral and longitudinal stability of the permanent track. 7)       To rectify the track geometry during service life. Requirement        i.           They should be economical i.e. should be minimum initial and maintenance cost.       ii.           Fittings of sleepers should be so adjusted during maintenance operations.      iii.           The weight of sleepers should have moderate weighty, for easy handling.      iv
Read more

Different Types Of Rail Joint

-
Different Types Of Rail Joint Rail Joint Definition Encyclopedia has defined the word “rail joint” as the following: A splice connecting the adjacent ends of rails, in distinction from a chair, which is merely a seat. The two devices are sometimes united. Among several hundred varieties, the fish joint is standard. See Fish joint, under Fish. Rail joint plays a jointing function (connecting function) in the railroad. High quality railway joint can effectively reduce the influence of wheels when passing through the jointing sections of the steel rail and increase the steadiness and continuity of the passing trains. As one of the important parts in railway, railway joint bar are widely used both in light rail and heavy rail to guarantee the safety of railroad transportation. Rail joint for light rail: 8kg, 9kg, 12kg, 15kg, 18kg, 22kg, 24kg and 30kg Rail joint for heavy rail: 38kg, 43kg, 50kg and 60kg Rail joint for crane rail: QU70, QU80, QU100 and QU120 Different Typ
Read more

Factors affecting the alignment of hill road

-
Image
The main factors to be considered while deciding the alignment of hill roads, are as discussed. Length : The cost of construction of a hill road per kilometer length is comparatively very high. It should therefore be ensured that length of the road connecting two stations should be minimum possible, adopting ruling gradients along its most of the length. Altitude of the road : At lower altitudes, large number of cross drainage works are required to be constructed. Where as at higher altitudes, the road pavement may witness snowfall during winter. This is why the align-ment of hill roads should preferably by provided at an altitude between 900 m slopes exposed to sum. The hill slopes which are subjected to high winds should never be selected unless and until there is no other alternative. In northern hemisphere, southern slopes of the hills are more suitable than northern slopes which remain tin shades and are usually subjected to high winds. Saddles or Passes : While locating
Read more