So you know The Slump Test?

 
Do not be tricked!

Acknowledgement: Jacques Smith -Go Consult 082 309 1884)

Then you should be able to answer these few questions, (which we answer for you below):

  1. Sampling of fresh 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 100 mm then the customer should accept the concrete at 70 mm 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 stream and not from concrete that has been placed. (Definitely not acceptable it if was dropped more than 500 mm) (SANS 5861-2 par 6.1.3). Sample within 30 minutes after truck has arrived on site. After that the responsibility of the readymix supplier has lapsed. The slump test must be done within 15 minutes after sampling (SANS 5878 12.3).

A sample must be at least 1.5 times the volume needed for the test, within a minimum of 10 litres (SANS 5861-2 par 5)

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

Doing the test:
25 tamps per layer for three equal layers is standard. Hold the cone down all the time and then lift it off over 5 - 10 seconds. This feels like a long time for that, but should be adhered to. This means that pulling the slump cone upwards in one quick motion is not acceptable. The entire slump test should last not more than 150 seconds (one cannot fill the cone and only take it off 30 minutes later (SANS 5862 par 6.4).

Putting the slump rod across the inverted cone is common practice, but not specific. It would be easier to read the slump accurately if the steel float is rested on top of the slump cone. (Trying to read where the bottom of a round steel rod is, is not easy).

If there is a collapse or a shear slump,repeat the test. I fit happens again, it is out of specification. (Remember that the maximum slump measured with this test is 175 mm and minimum 5 mm).

When does it fail?
Wetter concrete tends to be more variable in slump and hence a more relaxed measurement. The following are the tolerances (SANS 5878: 2004 table 1)

 

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

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 100 mm above the baseplate. If it passes through without blocking, it has a good chance to pump.

If the equipment is properly cleaned and handled with reasonable respect, it will last so much longer and still give a valid test result next time.

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