Acknowledgement: Jacques Smith -Go Consult 082 309 1884)

Then you should be able to answerthese few questions, (which we answerfor you below):

  1. Sampling offresh concrete for the slump test should be done within 15 minutes from the time of obtaining the sample? True or false
  2. A sample for testing should not be less than 10 litres? True or false
  3. If the ordered slump was 100mm then the customer should accept the concrete at 70mm slump? True or false
  4. What is the maximum slump that should be measured with the slump test?
  5. Is it ok to measure the slump by putting the steel float on the cone and not the tamping rod? True or false
  6. If the concrete fails the tolerance, I may request and actually insist on a repeat test? True or false

It costs money and hurts if in-spec concrete is rejected:
The wetter the concrete is,the higher the slump that is measured. It is important to note that this simple test has cost Readymix companies lots of money in the past, and frequently not justified. A readymix supplier should know the details of this test to prevent trucks from being sent back to the plant, at high cost, with concrete actually meeting the specification.

Incorrect sampling:
Do not sample the first nor the last 15% of the concrete being discharged from the truck (SANS 5878 par. 12.2). It is best to sample from a moving streama nd not from concrete that has been placed. (Definitely not acceptable it if was dropped more than 500mm) (SANS 5861-2 par 6.1.3). Sample within 30minutes after truck has arrived on site. After that the responsibility of the readymix supplier has lapsed. The slump test must be done within 15minutes afters ampling (SANS 5878 12.3).

A sample must be atleast 1.5 times the volume needed for the test, within aminimum of 10 lites (SANS 5861-2 par 5)

The equipment:
The standard equipment everyone knows so well. The cone must be visibly round (within 2mm), clean from old concrete and damp. The rod is not a piece of reinforcing, but a purpose made 600mm rounded edges steel rod. The base plate on solid ground not at a level and best on a bed ofsand. Equipment should be damp when the test is performed.

Doing the test:
25 tamps perlayerforthree equal layersisstandard. Hold the cone down allthe time and then liftit off over 5 – 10 seconds. Thisfeelslike a long time forthat, butshould be adhered to. Thismeansthat pulling the slump cone upwardsin one quickmotion is not acceptable. The entire slump testshould last notmore than 150 seconds(one cannotfillthe cone and only take it off 30minuteslater(SANS 5862 par 6.4).

Putting the slump rod acrossthe inverted cone is common practice, but notspecific. It would be easierto read the slump accurately ifthe steelfloatisrested on top ofthe slump cone. (Trying to read where the bottomof a round steelrod is, is not easy).

If there is a collapse or a shearslump,repeatthe test. Ifit happens again, itis out ofspecification. (Rememberthat themaximumslumpmeasured with thistestis 175mm and minimum 5mm).

When doesitfail?
Wetter concrete tendsto bemore variable in slump and hence amore relaxedmeasurement. The following are the tolerances(SANS 5878: 2004 table 1)

Bear in mind that the maximum valid slump measurementis 175mm. But this table also implies that is a slump of 120mm is specified, that concrete should be accepted from 80mm to 160mm.

Important to note : if the concrete falls outside of these limits above, you may insist that the test be repeated. It only fails once the second test also is outside of the limits (SANS 5878 pa 12.4)

After the test:
It is good practice to tamp the baseplate and note the way in which the concrete moves. If the base widens, it is an indication of good workability. If it falls apart, it might not be cohesive enough. Fill an upside down slump cone without tamping and lift it to about 100mm above the baseplate. If it passes through without blocking, it has a good chance to pump.

If the equipmentis properly cleaned and handled with reasonable respect, it will last so much longer and still give a valid test result nexttime.