Low Compressive Strength 2

[concretealc]

I am getting low compressive strengths for my C40 concrete.i use cubes to test the strengths.my 3day M Pa is 30 but this is using 600kg/m3 of cement. to me this is a bad result.I should be acheiving that strength using about 400kg/m3 of cement.My w/c ratio is very low;.32-.35.I cant figure out what could be wrong.

 

[BigH]

To concretealc - I would say that the amount of quarry dust would be dependent on the overall workability of the mix that you can develop - including admixtures (air entraining for example). ASTM allows up to about 7%, off the top of my head, of fines (less than #200 sieve) if the fines are the quarry dust. One thing - you might be able to go more if you can show the quarry dust to be pozzalonic. But, really you would, in my view, develop the mix based on the workability and ease in placement, consolidation and getting it out of the bucket/mixer.

 

[concretemasonry]

Fines can be either cubic (good) or flat (bad). - It just depends on the type of rock crushed.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[concretealc]

Yes I agree,Aggregate quality makes a huge difference...I got the same results with dirty material at 7days compared to clean material after 4 days.

 

[Zambo]

I'll try and look for a typical mix design later when I have my external hard drive with me. But I think 600kg/m3 is excessive and apart from the cost will cause shrinkage cracks or at least more difficulties than you need with curing and temperature control.

Quarry dust is more correctly referred to as crushed rock fines and maybe googling this will be helpful.

 

[keppel]

Is your quarry dust stored in a silo ? If so, maybe when they came to uload,the delivery person unloaded the dust into your cement silo. This has happened to me, but it was slag.

 

[concretealc]

No my quarry dust is not stored in a silo.

 

[Zambo]

Of course your quarry dust isn't stored in a silo.

I think dirty aggregates are the problem and also you need to look at the grading of the quarry dust (crushed rock fines)

But many years ago I worked in West Africa and the local cement could not produce 40MPa concrete at a reasonable cement content. We imported cement from Europe.