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Heading: 1.3.1Slump of Concrete
I have a doubt about the test method for Slump of Hydraulic-Cement Concrete.

The story is this: In my country we have very coarse sands, the Fineness Modulus usually varies between 3.15 and 3.35. According the ASTM C 33, the F.M. of the Fine Aggregate must be between 2.3 and 3.1, so it's obvious our sands doesn't comply with the Specifications.

This problem causes that we have to increase the Sand Percentage and lower the Gravel Percentage. Usually, the thing goes like this: Sand - 55% vs. Gravel - 45%.

I know the story is kinda long, but my doubt dwells on this situation:

When we perform the Slump Test Method, the concrete's batch doesn't displace, instead of that, the batch collapses. All this, because our aggregates are very coarse (explained above). In conclusion, the center of the cone gets unmeasurable. Unfortunately, we have to take a "point" in the collapsed's batch to measure (with measuring-tape) the slump.


1- Are we doing the slump test right? Taking into account of the conditions.
2-Does this happen in other countries?

Jorge Moquete

Dear Sir

You need to make the concrete mix cohesive.
This can be done by either using fine sand in combination with your regular coarse sand,or you can blend crush rock fines ,a by product of stone industry.

Alternatively,you can also try PFA ,if a nearby source is there to thicken the mix and bring more cohesion to the mix.

Further,you can also try to incorporate slight amount of air entraining admixture to introduce micro pockets of air,which will enhance cohesion /pumpability.In such a case care must be taken to see that you do not use high amounts of such an admixture as it may cause drop in strength.

Please note that such situations are there in many countries and is based purely on ingradients such as poorly graded or coarse sands.


Ken and Jois are of course correct; you should start by trying to make the concrete cohesive.

My response was based on the understanding (perhaps wrongly) that the concrete mix being considered was, although harsh, fit for purpose.
Alan Kirby
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