THE USE AND ABUSE OF THE SLUMP TEST FOR MEASURING THE

WORKABILITY OF CONCRETE

Christopher Stanley*, Unibeton Ready Mix LLC, United Arab Emirates

36th Conference on OUR WORLD IN CONCRETE & STRUCTURES: 14 - 16 August 2011, Singapore

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36th Conference on Our World in Concrete & Structures

Singapore, August 14-16, 2011

THE USE AND ABUSE OF THE SLUMP TEST FOR MEASURING THE

WORKABILITY OF CONCRETE

Christopher Stanley*

*Technical Director, Unibeton Ready Mix LLC Abu Dhabi, United Arab Emirates

Street 12 Sector 39 Old Mussafah, 110353 e-mail: <cstanley@unibetonrm.com> webpage: http://www.unibetonrm.com/

Keywords: Specifications, Workability, Slump Test, Concrete

Abstract. There are more than 60 published ways of measuring the so called ‘workability’ of concrete. Every inventor of a new method is convinced that he has the ultimate test for workability. In reality most methods are totally impractical, generally too expensive, and maybe measure some obscure property of concrete that is perhaps only indirectly related to the workability of concrete. The Slump test, which was invented by an American named Chapman in 1913, is the only test which has stood the test of time and is understood throughout the world. Mention, say, a 100mm slump to any producer or user of concrete almost anywhere in the world, and he can instantly picture the requirements of that particular concrete. In essence the Slump measures the consistency of the concrete rather than its true workability but owes its popularity to the fact that it is a simple and easily, portable test. The problem is that in this day and age the level of concrete knowledge is generally declining despite the ever growing sophistication of modern structures. The slump is a relatively crude but effective test, unfortunately it has become hijacked by people masquerading as ‘Consulting Engineers’ who have probably never carried out a slump test in their entire life. As a result slumps are being specified with values of 240mm with tolerances of 5mm, which apart from being stupid, is almost impossible to achieve. Another problem is that years ago, high slump values used to be equated with high water contents in the mix. Nowadays with the advent of the modern generation of high range water reducing admixtures it is possible to make highly workable concretes with very low water/ cement ratios. But most Consultants are unaware of this and still have a habit of specifying concretes with ridiculously low workabilities; which are hard to pump, place, and compact, and as a consequence end up taking an inordinate length of time to cast and often result in the production of concrete with poor quality finishes. And yet with a higher workability they could have achieved a much higher overall quality concrete with enhanced properties of both strength and durability, together with a high quality of finish, and with a reduction in the construction period. This paper reviews the use of the slump test on both large and small projects and how a sensible approach to the specification of workability can greatly improve the quality of the overall project.

Christopher Stanley

1 INTRODUCTION

There are over 60 published methods of measuring the workability of concrete. Most of them are highly academic or impractical and never see the light of day outside the university or company where they were developed. An exception is the slump test. In almost every country in the world people refer to the workability of concrete in terms of its slump. The reason for the popularity of the slump is that the equipment is light and portable and the test is simple and easy to carry out.

2 THE SLUMP TEST

The slump test was invented by Chapman in 1913. Originally he used a cylinder, but because the concrete became stuck in the cylinder, especially if it was stiff or cohesive, he put a taper on the cylinder and the slump cone has remained the same dimension ever since – 100mm diameter at the top, 200mm diameter at the bottom and 300mm in height. Interestingly, if the slope angle of the cone is changed the recorded slump will remain the same. This was noted by R.V.Watson in 1973 when he did an evaluation of the slump test as his thesis for the Advanced Concrete Technology Examination, a requirement for membership of the Institute of Concrete Technology. Watson did slump tests on specially made cones with a different slope angle as part of his evaluation of the test. Soon after its invention it was popularized by Duff Abrams in 1917 and in some literature it is referred to as the ‘Abram’s Cone’

Figure 1: Slump Cones with three different slope gradients prepared in 1973 by R.V. Watson as

part of his ICT Thesis. He used a standard cone and two others with a base diameter of 250mm

and 300mm. For a range of concretes he was able to show the measured slumps with each

cone was almost the same.

Although it is called a workability test the slump is really a measure of the consistence of the concrete. If batches of the same concrete are produced with nominally the same, or similar values of slump, the concrete can be said to be consistent, in that the batches are containing an almost identical amount and proportion of ingredients per cubic metre. Indirectly the slump is a measure of workability because the higher the slump the easier the concrete is to place and compact. But the slump is more related to the overall rheological properties of the concrete. For example a concrete with a slump of 100mm made from 20mm crushed rock aggregate will be more workable than a slump of 100mm in concrete made from crushed rock 10mm aggregate. This is because of particle interlock playing a part in the way the concrete slumps and flows when the cone is lifted off the moulded concrete. Similarly concrete made with a rounded aggregate will have a greater slump for the same mix proportions.

Christopher Stanley

Figure 2: Carrying out a slump test on samples of 20mm, 10mm and fine aggregate. The finer

the aggregate the smaller is the angle of repose. The slump of the concrete is a function of the

predominant size of aggregate in the mix.

The slump is also dominated by the angle of repose of the particles of aggregate in the mix. For example if stockpiles of dry aggregate are observed it will be noted that the 20mm aggregate has a steeper angle of repose than the fine aggregate. Therefore if it is required to make more workable concrete it follows that this is achieved by increasing the proportion of fine aggregate in the mix. In other words the workability of the concrete is dominated by the predominant size of the aggregate in the mix. Many mix design methods increase the amount of fine aggregate in the mix as the required workability increases. Another important aspect of this is also to reduce the tendency of the concrete to segregate as the workability increases. However the relationship between the slump and the predominant size of aggregate in the mix is not very often fully appreciated. Over the years there have been minor changes in the way the slump test is carried out. In the earlier British Standards the slump test was carried out by filling the cone in four equal layers and tamping each layer 25 times to compact it. This was later changed to three layers and 25 tamps in line with the ASTM method. Earlier the slump was measured to the mid height but now both methods measure to the highest point but the ASTM method measures to the highest point within the central area. The height of the slump is measured by standing the cone alongside the slumped concrete; placing the slump tamping rod across the top of the cone and measuring the height from the highest point of the concrete to the underside of the rod. This measurement is very critical in the ASTM where the slump is reported to the nearest 5mm. In the British & BSEN standards the slump is measured to the nearest 10mm. With either of these measurements it is difficult to bend down low enough to accurately measure the slump and the accuracy of measurement is further complicated by taking the measurements against the curved surface of the bar and somewhat prone to error. The writer believes it is better to use a straight piece of flat wood in preference to a slump cone rod for a more accurate measurement. Even though it means carrying around an additional object, it helps to minimize the testing error. Especially, where the slump test is incorrectly specified, by using small tolerances in the measured value. It is important to check that the footrests on the slump cone are manufactured so that they are

Christopher Stanley

flush with the base. This ensures that the cone can not rock when a person steps off the rests on completion of the test. Also important is that the test is carried out on a rigid non-absorbent base. Safety aspects are important, including wearing of correct safety gear, especially gloves, when carrying out the test. Cones often become deformed, especially when they are run over by vehicles on site and the attempts made to straighten them! Strict dimensional tolerances are set out in the standards and these should be checked and adhered to. Very few people use the correct type of scoop when carrying the test and this is important especially in maintaining a representative sample of concrete throughout the test. Thorough mixing of the sample before the start of the test is absolutely essential.

Figure 3: Typical measurement of the slump of a concrete which leads to inaccuracy because

of round tamping bar. The use of a flat piece of wood in place of the bar makes the

measurement easier and more accurate.

Slumps are usually specified at true, shear, or collapse slumps, however with modern highly workable concretes, shear slumps, usually the result of low workability concretes, lacking a sufficient amount of fine aggregate in the mix, nowadays are a comparatively rare occurrence. Some slump cones are supplied with a filling funnel which can be placed over the top in order to prevent concrete spilling down the sides whilst the cone is being filled with concrete. This funnel does not form part of the standard test apparatus. There is a tendency to over fill the funnel with concrete which tends to result in a lower recorded slump being obtained when the filling funnel is used.

MEASURED SLUMP (mm)

Using a Filling Funnel Standard Slump

170 175 175 170

180 185 185 185

Table 1: The variation in the measured slump when a filling funnel is placed on the top of a

slump cone for ease of filling the cone with concrete.

The slump cone should be filled on its own in order to obtain a realistic result from the test. Another

Christopher Stanley

important aspect of the test which is often overlooked is that the inside of the cone should be wiped with a damp cloth before the test commences. This prevents concrete adhering to the cone and being uplifted during the removal of the cone, resulting in a reduction in the measured slump.

NOMINAL SLUMP 160 mm

Slump Cone Interior Condition

Moist Dry

Slump mm Slump mm

160 170 170 165

150 160 155 155

Table 2: The effect on the measured slump if the inside surface of the cone is not moistened

before use.

NOMINAL SLUMP 180mm

Time Taken to Lift Cone (Sec)

Measured Slump (mm)

3

5

7.5

10

20

30

205

190

185

180

175

165

Table 3: The rate at which the cone is raised from the concrete after completing the filling also

has an impact on the recorded slump. The standard specifies 2-5 seconds. The table above

shows that lifting the cone too slowly has a tendency to reduce the recorded slump value.

The slump test is a simple rough and ready test, and this needs to be appreciated whenever the test is specified as a quality control test to monitor concrete production. When the writer began working in the construction industry almost all concrete was specified to have a 25mm slump because it was placed either by chute or by crane and skip. Such concrete tended to be cohesive and stick in the chute or skip and had to be assisted to flow with a shovel. The workability by the mid 1960’s increased to about 50mm slump and the slump tolerance then tended to be specified as +/-25mm, or one third of the specified slump value, whichever was the greater. Then with the rise in the use of concrete pumps, as a means of placing concrete, the slump specified increased to 75mm with a tolerance of one third of the slump. Even that was not a high enough slump value and there were many instances of pump blockages and excessive wear on pipelines. In the mid 1990’s with the advent of superplasticisers and PHC admixtures, it became possible to place highly workable concretes with very low water/cement ratios. Workabilities increased to slumps around 240mm. Unfortunately, university education did not keep pace with the advances in new technology, and there was still a persistent misunderstanding that workable concrete could only be achieved with the addition of excess water in the concrete mix. This has resulted in misinformed engineers writing ridiculous specifications where concretes are sometimes specified with a slump of 240mm and a tolerance of +/- 5mm in the slump. This is virtually impossible to achieve. In one project involving the casting of over 7000 cubic

Christopher Stanley

metres of concrete in a heavily reinforced raft foundation the Consultant arrogantly stated that he would rigorously enforce such a specification. At such a high slump where the concrete flows on removal of the cone, maintaining such tight tolerances is an almost impossibility. It was however accomplished by slightly increasing the 10mm aggregate fraction of the mix. As the concrete flowed out on removal of the cone, a mini mountain of coarse material remained in the middle of the spread of concrete. This remained constant even though slight changes of the workability were evident in the overall flow characteristics. The result was that only two trucks were rejected in the entire pour for being 5mm out of tolerance. Without this change in the mix, the end result could have been catastrophic, with many loads of perfectly satisfactory concrete being rejected.

Table 4: Summary of the first 30 results on a pour of over 800 m3. The remaining results were

similar. This emphasizes the tight production control that can be obtained using a highly

computerized batching plant.

Table 5: Typical Actual slump ranges obtained on a number of sites for different grades of

concrete and specified workabilities.

The concrete may be batched at a plant and have to travel, say 15km, to the site. The concrete therefore has to be batched with an increased workability to allow for some loss in workability during transit, and this is particularly difficult to calculate, especially in conditions of hot drying winds or high

Grade 40/20 Concrete OPC + MS Mix

Fully Automated Batched Concrete Measured Slump (mm)

Concrete Batched 2018 hrs – 0538 hrs Total Supply 841 m³ Specified Slump 125 ± 25 Nominal Target 140 -150mm Batch Size 8-10 m³ Concrete Temperature 24ºC - 31ºC

140 145 140 145 145 140 135 140 140 145

140 145 140 145 140 145 140 135 145 140

145 145 140 135 145 140 145 140 140 145

Mean Slump 142 mm Minimum Slump 135 mm Maximum Slump 145 mm SD Slump 3.3 mm

Concrete Grade Specified Slump (mm) Minimum Slump at

Site (mm)

Maximum Slump at

Site (mm)

C 25

C 30

C 40

C 50

C 60

C 60 [ Slump Flow ]

160 ± 40

110 ± 30

180 ± 40

180 ± 40

200 ± 40

650 – 800

190

130

200

180

220

650

200

140

220

210

240

750

Christopher Stanley

ambient temperatures or a combination of both. In addition the loss in workability is also influenced by the traffic flow. During ‘rush hours’ it may take trucks longer to reach their destination whereas at night time the journey time may be considerably reduced. Even so the concrete is expected to arrive on site with exactly the right slump.

Slump loss of concrete in the UAE

0

100

200

300

TIME AFTER MIXING - hrs

SLUMP L

OSS - m

m

Conc Soc Unibeton

Conc Soc 220 150 65 15

Unibeton 210 155 65 20

Initial slump After 1hr After 2hrs After 3 hrs

Table 6: The loss of workability with time in a hot arid environment where the concrete does

not contain a workability retention admixture. A comparison between values published by the

Concrete Society in its “Guide to Construction of Reinforced Concrete in the Arabian

Peninsula” and comparative values obtained by a local Ready Mixed concrete company.

If say ten technicians are given a sample of concrete and asked to perform a slump test, even if the test is carried out strictly in accordance with the standard procedure, it is doubtful if all the results could come within 10mm of each other. Some years ago, during the writer’s teaching career, he was able to produce any desired slump to order whilst carrying out the test strictly in accordance with the standard procedure. Whilst this paper is not the place to elaborate on the details, sufficient to say, that anyone with sufficient knowledge of the slump test, can easily produce a test to the required slump if required. If concrete arrives on site with a slightly higher slump than required, the correct procedure is to retain the concrete until the required slump is reached, rather than rejecting a perfectly good truckload of concrete. Many tests have been carried out which confirm that small differences in the measured slump value have no effect whatsoever on the properties of the hardened concrete or any effect on its performance or long term service life. If the concrete arrives on site with a slightly lower slump than specified, the workability can be corrected, not by the addition of extra water, but by careful addition of extra admixture. It is important to realize that some admixtures may slightly retard the early setting and hardening process, especially where cement replacements such as GGBFS or PFA are incorporated in the mix, and therefore their addition needs to be carefully controlled and supervised. As attitudes have changed towards the specification of the slump test, changes in the permitted tolerances have also been implemented. The table below shows a range of values in old and present specifications, together with a current suggested recommendation for Slump Class S4 in BS 8500.

Christopher Stanley

Reference

standard/year

Requirement is

180mm

Permitted slump of

“snatch” samples

Permitted slump of

incremental samples

BS5328 1981,

1990,1997 180mm 130mm to 250mm 120mm to 240mm

BS EN 206, 2000 Slump class S4 160 -

210mm 140mm to 240mm 150mm to 230mm

BS EN 206, 2000 Fixed value > 180mm 130mm to 240mm 140mm to 230mm

BS 8500, 2002 Slump class S4 160 -

210mm 140mm to 240mm 150mm to 230mm

Current suggested

recommendation

Slump class S4 of

200mm 160mm to 240mm (± 40mm)

ASTM C 94/C 94M Over 100mm ±40mm of specified slump

Table 7: Range of specified tolerances in different standard concrete specifications.

Raw material or measured

concrete property Control Option 1 Option 2 Option 3

Cement (kg/m3) 400 400 310 400

Coarse aggregate (kg/m3) 985 1060 1075 985

Fine aggregate (kg/m3) 715 800 850 715

Water (kg/m3) 200 160 155 200

Admixture Nil 3.88 2.96 3.88

Water/Cement ratio 0.50 0.40 0.50 0.50

Slump 100 100 100 200

Compression strength MPa

1 day 10.5 15.5 11.5 11.5

3 days 23 30 22.5 23

7days 35 48 36.5 36

28 days 42 56 40 42

Table 8: Table of concrete mixes showing that by the addition of 3.88 kg of an admixture, the

same properties of the concrete can be obtained relative to the control mix but with double the

slump.

Christopher Stanley

For many years people have tried to control the slump of concrete during the initial period of the mixing process. This is accomplished by measuring the power consumption of the mixer in Watts. The stiffer the concrete, the greater will be the power consumption. The problem is that it takes several seconds after the addition of materials for the mixed concrete to achieve a steady state, and in this time it is likely that the concrete may have already been slightly overdosed with water, admixture, or a combination of both. Where ready mixed concrete is being supplied the workability can be assessed by coupling a pressure transducer into the hydraulic drive of the mixer drum. The hydraulic pressure is a function of the power to turn the drum which is a function of its workability; the stiffer the concrete the greater the hydraulic pressure. Years ago when the pressure was shown on Bowden gauges the accuracy was not sufficient, especially at the higher workability ranges. However, now with the use of LCD displays, the accuracy is much greater, and the trucks can be pre-calibrated for a range of slumps in both full and part loads. In addition greater accuracy can be obtained by checking the hydraulic pressure at both slow and fast drum speeds.

Figure 4: Hydraulic Pressure Sensor. This can be connected to the hydraulic drive of the ready

mix vehicle’s truck drum and the pressure to turn the drum which is a function of the concrete

workability can be calibrated against the slump of the concrete and shown as a numerical

value on an LCD display.

Another technique is to insert a probe through the wall of a truck drum. The pressure exerted against the probe as the drum rotates can also be calibrated to give an indication of the slump of the concrete within the truck.

Figure 5: Concrete Mixer Truck showing the use of a workability probe developed by Dr. Denis

Beaupre which can be calibrated against the slump of the concrete.

Christopher Stanley

In addition the probes can be used to measure the concrete temperature and the number of truck revolutions and this information can be sent back to a monitoring station at the plant. Such devices can minimize the amount of concrete being wasted carrying out slump tests on site, which in one area typically result in the wastage of about 12,500 m3 of concrete in a typical month. With the advent of highly workable and self- compacting concretes, attention has focused on another use of the slump cone for the slump flow test. In this test the slump cone is filled in one continuous layer before being lifted and the concrete allowed to flow across a flat base. The diameter of the flow is measured and is usually required to be within about 650mm – 750mm, so the slump test has found a new application and is likely to be a standard test for many years to come.

3 CONCLUSION

It is important to understand both the advantages and the disadvantages of the slump test and correctly specify meaningful slump limits, in order to ensure a successful uninterrupted supply of concrete to any job site. Whatever the slump of the concrete, providing it can be placed and properly compacted with the available equipment on site, without re-tempering the concrete with water, the concrete can be used without any detrimental effect to the building or structure concerned. It is wasteful to dispose of any concrete that can still be used whether there is a difference in the slump or the time it has been waiting to be placed. Common sense must prevail, together with the need to conserve resources and sustain the environment.

4 REFERENCES

[1] Watson, R.V. The performance of a modified slump cone. Advanced Concrete Technology thesis. December 1973. (Copy currently held by Information Services, The Concrete Society.) London.

[2] Titford, E.M. The Golden Age of Concrete. London 1964

[3] British Standards Institution. BS 1881: Part 2: 1970. Testing Fresh Concrete.

[4] British Standards Institution. BS 1881: Part 102: 1983. Testing Concrete. Method of determination of Slump.

[5] Abrams, D.A. Experimental Studies of Concrete. Chicago 1918

[6] Neville, A.M. Properties of Concrete 4th Edition. London 2005

[7] Neville, A.M. & Brooks J.J. Concrete Technology 2nd

Edition. London 1987

[8] British Standards Institution. BS 8500-1: 2006. Method of Specifying and Guidance for the Specifier.

[9] British Standards Institution. BS EN 12350-2: 2009. Testing Fresh Concrete – Part 2: Slump Test.

[10] American Society of Testing Materials. C143/C143M – 10a. Test for the Slump of Hydraulic Cement Concrete.

[11] American Society of Testing Materials C94/C 94 M-07. Specification for Ready Mixed Concrete.

[12] THE CONCRETE SOCIETY. Guide to the Construction of Reinforced Concrete in the Arabian Peninsula, The Concrete Society. London 2008