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.

 

[Ron]

If you're getting 75% of f'c at 3 days, I wouldn't be concerned.

 

[concretealc]

Hey
thanks for the response but this is using 600kg/m3 of cement.According to other designs ive seen, one should obtain that strength from about 400kg/m3

 

[hokie66]

So why did you use so much cement? A 3 day result could be 50% of the 28 day, so you may get 60 MPa with this mix. Try using 400 kg/m3 cement, then see what you get at 28 days.

 

[concretealc]

my results with 400Kg/m3 are really low...thats why i had to increase the amount of cement.

 

[hokie66]

Then I suggest there is something else wrong with your mix design, or perhaps the aggregate is unsound. Perhaps if you can give the details of your mix design, some advice could be given, but we can't solve your problem here. What 28 day strength did your 400 kg/m3 mix develop?

 

[concretemasonry]

What as the design mix/materials for the concrete mix that was used for the approval?

Have you looked at the sampling, sample preparation, handling, curing and testing practices?

Also is there a possible problem with excessive of fines in the aggregates? This can increase the amount of water also.

Dick

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

 

[concretealc]

cement 600km/m3
20mm 680
10mm 425
quarry dust 249
water 190L

 

[hokie66]

We still don't know why you consider 30 MPa at 3 days to be a poor result for a 40 MPa mix.

I don't know what "quarry dust" means, but I suggest you have an insufficient quantity of correctly graded fine aggregate. I would expect more than twice that amount to adequately fill the space between the coarse aggregates.

With a properly graded mix, and competent aggregates, you should be able to produce 40 MPa concrete with 400 kg/m3 cement. Is this mix based on any design calculations, or it is just trial and error?

By the way, all your components, including the water, only add to 2144 kg/m3, which is very light.

 

[BigH]

Is this an "open-graded" concrete? You don't seem to have any fine aggregate in the mix (249kg of dust) as compared to what I thought would be about 900kg of fine aggregate (sand). For North Americans, please note he is using cubes not cylinders - so his 30 MPa cube is really about 24 MPa cylinder.

 

[concretealc]

sorry my mistake

sand 500
cement 600
Quarry dust(fines)249
water 190
20mm 680
10mm 425

At 400KG cement/m3 I am not getting the 40MPa results.The 3 day results I got which was 30MPa was using 600kg cement /m3. This should not be ...ok heres a question...would 30MPa after 3 days be a good result for C60 concrete then?

 

[hokie66]

Who knows? Wait until 28 days, and you shall see.

 

[keppel]

with 600 kg. of cement, you should be getting more than 40mpa in 28 days. Try looking at your course and fine agg. To me it sounds like your aggregates are to dirty. start there and also check your testing procedures and sample handling.

 

[concretealc]

Hey I should be getting way more than 40MPa..but I will wait for my 28day test results.At 600 Cement, after 4 days I am getting 30Mpa so that should be a good sign!?

 

[keppel]

Another reason for your low strength could be your cement powder. Cast some cubes using just the cement powder, and contact your cement supplier. You may have got a couple of bad loads of powder.

 

[concretealc]

Thanks all for your comments..appreciate it.I will let you know the final outcomes.etc

 

[concretealc]

Can anyone tell me what their typical C40 Mix design would be..So i could weigh all possible causes of low strengths.my top three causes which I am investigating are:
1.Dirty Material
2.Mix Design
3.Handling/Curing Practices

Thanks

 

[Zambo]

I agree with Keppel, try to wash your aggregates. It won't take long to wash enough aggregates so that they are reasonably free of dust then try a trial mix - or full scale batch.

Quarry dust is the fines from the crusher in the quarry where the aggregates are being crushed. This a perfectly good replacement for sand, but should still have a reasonable grading. Try comparing your quarry dust gradings against a standard sand and see how thay compare (have you got a standard for quarry dust grading?)

 

[concretemasonry]

If cubes of just cement and water are made what are the proper standards and correlation to any standard? There are factors than the "powder". Perhaps it is an interground cement containing fly ash that will always slow the cement gain, but still be within the 28 day requirements. If there are questions about the early strength there is always the option of using a high early strength mix instead of waiting for test results

The cement suppliers in most countries probably have volumes of daily reports on the chemistry, fineness and performance. - Contact the concrete supplier or the cement supplier for a quick answer if it is a time-critical application.

Dick

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

 

[concretealc]

zambo mentioned that the Quarry dust is perfectly good replacement which I agree.In a typical mix design,what do you recommend the percentage of quarry dust should be in your fine aggregate.