How to Determine the Workability of Concrete ? | Concrete Technology

Usually the tests that can be adopted to determine the workability of concrete are: 1. Slump Test 2. K. Slump Test 3. Kelly Ball Test 4. Compacting Factor Test 5. Flow Test.

1. Slump Test:

This test is used extensively at the site of Work all over the world. Actually this test does not measure the workability of concrete but is useful for finding the variations in the uniformity of a mix of given nominal proportions and specifies procedure for determining the consistency of concrete where the nominal maximum size of the aggregate does not exceed 38.0 mm.

Apparatus:

The mould for the slump test is a frustum of a Cone having internal diameters of bottom and top as 20 cms & 10 cms respectively and 30 cms height. The mould will be made of metal of at least 1.6 mm thickness, having smooth internal surface. To facilitate in lifting the mould in the vertical direction, suitable guide attachment should be provided in the mould. (2) Tamping rod of steel 60 cms long and 1.6 cms in diameter and round at one end.

Test Procedure:

The internal surface of the mould is cleaned thoroughly and oiled or greased. Then this mould is placed on a smooth; horizontal, rigid and non-absorbent surface, such as a carefully levelled metal plate with the smaller opening at the top. The mould should be held firmly in place while filling it with concrete. It is filled in four layers, each approximately 7.5 cms in thickness and each layer is tamped with twenty five strokes of the rounded end of the tamping rod.

The strokes should be distributed uniformly over the whole area of cross section of the mould and for the second and subsequent layers should penetrate into the underlying layer. The bottom layer should be tamped throughout its depth. After tamping the top layer, the top surface is struck off level with a trowel or rolling motion of tamping rod such that the mould is exactly filled. Any mortar leaked out between the mould and its base should be wiped out immediately. Now the mould should he lifted up vertically slowly and carefully.

This will allow the unsupported concrete to subside or slump, hence the name of the test. The decrease in the height of the centre of the slumped concrete is called slump and is measured to the nearest of 5 mm. Thus slump is the difference between the original height of concrete in the mould and the highest part of concrete in subsided position. The experiment should be conducted at a place which is free from vibration or shock and within a period of 2 minutes after taking sample from the mix.

If instead of slumping evenly all-round as in the case of true slump, one half of the Cone slides down an inclined plane, then a shear slump is said to have taken place and the test should be repeated. If shear slump persists as in the case of harsh mixes, then it is an indication of non-cohesive mix.

Mixes of stiff consistency have zero slumps and in the dry range of mixes no variation can be detected between mixes of different workability. Rich mixes behave satisfactorily, their slump being sensitive to variations in workability. However in a lean mix with a tendency to harshness a true slump can change easily to the shear type slump and widely different values of slumps can be obtained in different samples for the same mix.

Thus this test is not a true guide to workability. Essentially it is a measure of consistency or wetness of the mix. This test is suitable for concrete having 25 to 125 mm of slump i.e. for medium to high workability. For example a harsh mix cannot be said to have the same workability as one with a large proportion of sand even though they have the same slump. Similarly 12.5 mm slump in concrete made with 75 mm maximum size crushed aggregate will not necessarily give the same workability as that given by 20 mm maximum size gravel concrete.

The recommended values of slump for different works are shown in Table 7.2. These values depend upon the method of compaction possible at the site in each case. Smaller slump values are required where concrete can be rammed hard or where there is no obstruction like reinforcement etc.

The patterns of slump is shown in Fig.7.3. It indicates the characteristics of concrete in addition to the slump value. If the concrete slumps evenly, then it is called true slump. If one half of the cone slides down, then it is called shear slump. In this case the slump value is measured as the difference in heights of mould and the average value of subsidence. It also shows that the concrete is non-cohesive and indicates segregation characteristics. If concrete spreads then it is called collapse.

Slump test gives fairly good results for a plastic mix. This test is not sensitive for a stiff mix or zero slump mix. In case of dry mix, no variation can be detected between mixes of different workability. In case of rich mixes, the value is satisfactory, their slump being sensitive to variations in workability. IS 456-2000 suggests that in the “very low” category of workability, where strict control is necessary as for pavement quality concrete, workability should be determined by compacting factor apparatus. A value of 0.75 to 0.80 of compacting factor should be adopted.

In case of very high category of workability, measurement of workability by flow test will be more appropriate. In case of a lean mix with a tendency of harshness a true slump can easily change to shear slump 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.

Despite the above limitations, slump test is very useful on the site to check the day to day variations in the materials being fed into the mixer. An increase in slump may mean either moisture content of aggregate has increased unexpectedly or grading of aggregate has changed (deficiency in sand). Thus too high or too low a slump gives a warning to the mixer operator to remedy the situation.

Simplicity of this test is another reason of its popularity inspite of the fact that many other workability tests are in vogue. In the past IS adopted compacting factor test values for denoting the workability of concrete. Even IS 10262-1982 recommended the use of compacting factor values for denoting workability of the mix, but IS 456-2000 reverted back to slump values to denote workability.

2. K. Slump Test:

Very recently a new apparatus known as K. slump tester has been developed. It can be used to measure the slump directly in one minute after it has been inserted into the fresh concrete to the level of the floater disc. The K-sIump tester can be used to measure the slump of the concrete in one minute in cylinders, buckets, wheel barrows, slabs or in any desired location where the fresh concrete is placed.

Thus K-slump tester is very simple, practical, handy, and economical to use both in the laboratory and field. It has been found on the basis of about 500 tests carried out with K-slump tester, that it has a good correlation with slump cone apparatus results.

Apparatus:

The apparatus is consisted of the following four parts:

i. A chrome Plated Steel Tube:

The external and internal diameters of this tube are 1.9 and 1.6 cms respectively. The length of the tube is 25 cms. The lower part of the tube is used to measure the slump of the concrete. The length of this part is 15.5 cms including the length of lower solid cone which facilitates the insertion of the tube into the concrete.

In this lower tube following types of openings are provided:

(a) 4 slots of 5.1 cms length and 0.8 cm width marked A-A.

(b) 22 round openings of 0.64 cm diameter shown as B-B. All these openings are distributed uniformly in the lower part as shown in Fig.7.4.

 

ii. A disc floater, 0.24 cm thick and 6 cm in dia­meter. This disc divides the tube into two parts as upper and lower. The upper part serves as a handle and the lower one as tester. This disc also serves as a check for the tester to sink into the concrete upto a correct pre-determined level.

iii. A Hollow Plastic Rod:

This rod is 25 cm long and 1.3 cm in diameter. This rod contains a gra­duated scale in centi­meters. This rod can move freely inside the tube and can be used to measure the height of mortar that flows into the tube and stays there. At each end this rod is plugged with a plastic cap to prevent cement paste or other material from seeping into the tube.

iv. An aluminium cap of 2.25 cm length and 3 cm diameter. It has a little hole and a screw which can be used to set and adjust the reference zero of the apparatus. In the upper part of the tube there is also a small pin which is used to support the measuring rod at the beginning of the test.

Weights:

The total weight of the apparatus is 0.226 kg or 226 gram.

Procedure:

Following procedure is adopted:

(a) Wet the tester with clean water before use and shake off the excess water.

(b) Now raise the measuring rod of the tester and tilt it slightly so that the measuring rod may rest on the pin located inside the tester for this purpose.

(c) Level the surface of concrete to be tested. Insert the tester in the levelled concrete surface verti­cally down wards until the disc floater comes in contact with the concrete surface. The tester should not be rotated while inserting or removing,

(d) After 1 minute, lower the measuring rod slowly till it rests on the surface of the concrete that has entered into the tube. Now read the K-slump directly on the scale of the measuring rod.

(e) Raise the measuring rod again and let it rest on the pin.

(f) Remove the tester from the concrete vertically up wards and again lower the measuring rod slowly till it touches the surface of the concrete retained in the tube and read workability (W) directly on the scale of measuring rod.

3. Ball Penetration Test:

This is a simple field test devised by J.W. Kelly and is known as kelly ball test after the name of its inventor Kelly, This test has not been covered by Indian standard specifications, but it has been covered by ASTM standard 360-92.

Apparatus:

The apparatus is consisted of 15.2 cm diameter spherical metal ball weighing 14 kg (30 lbs) with a measuring scale on the stem (vertical rod) of the handle.

Principle:

In this test, the penetration of the ball of the apparatus in the fresh concrete is measured. The depth of pene­tration gives an idea of the fluidity of concrete. Actually it works on the same principle as that of slump test. It is also used for routine checking of consistency for control purposes.

Procedure of Test:

The surface of the concrete is levelled, avoiding excess working. On the levelled concrete surface the ball of the apparatus is lowered gradually. The depth of penet­ration of the ball in the concrete is read from the scale of its stem to the nearest 6 mm. The test can be performed with in about 15 seconds.

Advantages:

Following advantages have been observed of the test:

1. This test is simpler and quicker than slump lest.

2. It can be applied to the surface of concrete in the actual form work or in a wheel harrow,

3. This test can be performed with greater precision.

4. This test can be used as an alternative of slump test.

Disadvantages:

1. This test requires a large sample of con­crete and it cannot be carried out in case the concrete is placed in a thin section.

2. The minimum depth of concrete must be at least 20 cm and the minimum distance from the centre of the ball to the nearest edge of concrete should be 23 cms or the least lateral dimension of the concrete should be 460 mm.

Remark:

Actually there is no simple correlation between penetration and slump test. They are used to measure specific condition. In practice the ball penetration test is used essentially to measure the variation in the mix, such as those of a variation in the moisture content of the aggregate. When a particular mix is used, correlation can be found as shown in Fig. 7.6.

4. Compacting Factor Test:

This test is designed primarily to determine the workability of concrete in the laboratory where maximum size of aggregate does’ not exceed 38 mm, however it may also be used in the field. It is more sensitive and precise than slump test and is particularly useful for concrete mixes of very low workability as are normally used for concrete to be compacted by vibration, such concrete may fail to slump.

This test known as compacting factor test was developed at Road Research Laboratory England. The workability is defined as the amount of work required to place and compact concrete fully. The degree of compaction, called compacting factor is measured by the density ratio. It is the ratio of the actually achieved density in the test to the density of the same fully compacted concrete. Thus compacting factor can be defined as the ratio of the actual density obtained during the test to the density of fully compacted concrete.

Apparatus:

The apparatus consists essentially of two hoppers, each of the shape of a frustum of a cone and one cylinder. The hoppers are hinged on a vertical frame one above the other as shown in Fig. 7.7. The hoppers have hinged doors at their bottoms. The inside surfaces of the hoppers are polished to reduce friction. The dimensions of the hoppers, mould and distances between them are shown in table 7.3 below.

Dimensions of the compacting factor apparatus for use with aggregate not exceeding 38 mm.

Procedure:

The sample of concrete to be tested is placed gently in the upper hopper A by the hand scoop in such a way that no work is done on the concrete at this stage to produce compaction. While filling concrete in the upper hopper A, hopper B and cylinder C should be covered, so that no concrete should fall into them. After filling the concrete in hopper A, the cover from hopper B is removed and the hinged bottom door of hopper A is released and the concrete is allowed to fall into the lower hopper B.

The hopper B being smaller than upper hopper A is filled to over flowing state. Thus it always contains approximately the same amount of concrete in a standard state. This reduces the influence of the personal factor in filling the top hopper. Now the cover from the cylinder is removed and the bottom door of the lower hopper B is released and concrete is allowed to fall into the cylinder. The excess concrete in the cylinder is cut off by the help of two trowels sliding across the top of the cylinder.

The outside of the cylinder is wiped clean and cylinder weighed. The net weight of the concrete in the cylinder is called as weight of partially compacted concrete and dividing it by the volume of cylinder, its density is determined. Now the concrete in the cylinder is filled in four layers & fully compacted and the density of the fully compacted concrete is obtained. The ratio of the two densities or weights of partially compacted concrete & fully compacted concrete will give its compacting factor.

i.e. Compacting factor = Weight of partially compacted concrete/ Weight or fully compacted concrete

The value of compacting factor varies from 0.78 to 1.0.

The apparatus shown above in Fig. 7.7 is 1.2 m high. For concrete with a maximum size of over 20mm & upto 40 mm a large apparatus of height 1.8 m may be employed. For the same concrete higher apparatus gives from 1.0% to 2.65% higher values of compacting factor than smaller apparatus, for very high to very low degree of workability. For this reason usually bigger apparatus is not used. The values of compacting factors for different workability are shown in Table 7.4 below.

Limitation of Compaction Factor Method:

i. This method is not very good for dry mixes.

ii. Coarse and dry mixes are unduly favoured as the initial compaction of such mixes is greater for a given quantum of work than that in latter stages when full compaction can be attained by the reorientation of the particles in the mix.

iii. In case, the maximum size of aggregate is very large than the mean size of aggregate particles, the drop into the lower hopper will produce segregation and will give unreliable comparison of weights with other mixes of small maximum aggregate size.

iv. The method of introducing concrete into cylinder does not bear any relation to any method of placing and compacting high quality concrete.

v. This method does not indicate any tendency of segregation of fine and coarse materials. It also does not indicate the ease with which the concrete can be finished and smooth sur­face obtained.

vi. This method is more accurate for medium to low workability i.e., for compacting factor 0.9 to 0.8. For concrete having compacting factor 0.7 or less, the test is not suitable. The relationship between slump and compacting factor is shown in Fig. 7.8.

5. Measurement of Consistency or Flow of Cement Concrete by the Use of Flow Table:

This method gives an indication of the consistency and cohesiveness of concrete and also proneness to segregation by measuring the spread of a pile of concrete subjected to jolting. This test is of greatest value with regard to segregation. However it also gives a good assessment of consistency of stiff, rich and cohesive concrete mixes. Normally this method is used for concrete, where nominal size of aggregate does not exceed 38 mm. The flow test gives satisfactory results for concrete of the consistencies for which slump test may be used.

Apparatus:

The test requires two apparatus as follows:

1. Mould:

A mould in the form of a frustum of cone of base diameter 25 cm, top diameter 17 cm and height 12 cm is used. The internal surface of the mould should be smooth. It should also be provided with two handles. A tamping rod 61 cm long and 1.6 cm in diameter having one end rounded is also needed for rodding the concrete.

2. Flow Table:

It is a 76.0 cm diameter brass top table. It is mounted in such a way that it can be jolted by a drop of 13 mm. This table is bolted to a concrete base having a height of 40 cm to 50 cm and weighing not less than 140 kg.

Procedure:

Just before the test, the top of table and inside surface of the mould is wetted and cleaned of all gritty material and excess water removed. The mould is placed at the centre of the table and held firmly in place and filled with concrete in two equal layers and each layer is rodded with 25 strokes of a straight 1.6 cm diameter and 61cm long rod, having one end rounded as in case of slump test.

The strokes should be uniformly distributed over the full cross- section of the mould and should pene­trate into the underlying layer. The bottom layer should be rodded throughout its depth. After rodding the top layer, the top surface should be struck off with a trowel, filling the mould exactly upto its top, i.e., the top surface of concrete should be horizontal and the excess concrete should be removed and table cleaned.

Now the mould is lifted up vertically by a gradual upward pull and the concrete stands on its own without support. Now the table is raised and dropped through a height of 13 mm, 15 times in 15 seconds. Due to this action, the concrete spreads on the table. The diameter of the spread concrete is measured with the help of a caliper in six directions nearest to 5 mm and the average diameter is determined.

The reading should be correct upto 5 mm, then-

Flow percent = (spread diameter in cms. –25/25) x 100

The value of flow percentage varies from 0 to 150. The jolting during the test encourages the seg­regation. Further it should be noted that this test does not measure the workability as concretes having the same flow may differ in their workability considerably. It gives good indication of segregation of concrete.