Then you should be able to answer these few questions, (which we answer for you below):
- Sampling of fresh concrete for the slump test should be done within 15 minutes from the time of obtaining the sample. True or false?
- A sample for testing should not be less than 10 litres. True or false?
- If the ordered slump was 110mm then the customer should accept the concrete at 70 mm slump. True or False?
- What is the maximum slump that should be measured with the slump test?
- It is ok to measure the slump by putting the steelfloat on the cone and not the tamping rod. True or False?
- 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.
It would not be practical to sample for the slump test after the first 10% had been offloaded, because what about that concrete that had gone into the structure now? It is widely accepted to take a spot sample in a wheelbarrow after thorough mixing in the truck to perform the slump test, even before offloading will start.
Sample within 30minutes after the 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, with a minimum of 10 lites (SANS 5861-2 par 5).
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 and near 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 specified. 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 so easy.)
If there is a collapse or a shear slump, repeat the test. If it happens again, it is out of specification. (Remember that the maximum slump measured with this test is 175mm and minimum 5mm).
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):
Specified slump (mm)
50 and less
-15 to +25
More than 50, up to 100
More than 100
Bear in mind that the maximum valid slump measurement is 175mm. But then this table also implies that if 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 par 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 equipment is properly cleaned and handled with reasonable respect, it will last so much longer and still give a valid test result next time.
Jacques Smith Pr Eng (Civil)
Cell 082 309 1884