Our execution team casted a raft foundation for an elevated tank, but the compressive results was not satisfactory with only 14 MPa after 7 days. we came up with many solutions but expensive, from your experience what could be the most cost effective solution?
Best Regards
For enhancing the strength of concrete lowering of w/c ratio with high range water reducer and viscosity modifying admixture is one of the solution which may be cost effective i.e. less expensive.
Another way of enhancing the strength of concrete is replacement of cement with pozzolanic material such as silica fume. But it will be little bit expensive.
Dear Musab
Can you specified the compressive strength of concrete expected at 7 days, 21 days and 28 days.
At the moment your are only at 7 days. The value at 28days is the relevant one. I think you can improve the concrete strength through proper curing since it cast already. I suggest a ponding curing if possible.
- Musab Salih if cubes is not achieved strength as per your mix design first you have to find out the reason. When we are doing concreting on site and collecting Samples for the test, there are four possible combinations behind Concrete cube fails in test as listed below.
1. It may be, Concrete is strong; cube is also strong.
2. It may be, Concrete is weak; cube is also weak.
3. It may be, Concrete is strong; but cube is weak.
4. It may be, Concrete is weak; but cubes strong.
you have to wait till 28 days strength because the strength basically depend on the cement(chemical properties)(SiO2). it works for 28 days and beyond that.
You must have to check IR (Insoluble Residue) which was present in cement. It also responsible for the less strength.
since the concrete has been already cast, I think the reason why the strength was not acheive was because the curing process of the concrete was not done accurately too beside the mix design. Curing process of the concrete is very important and plays an important role in getting the correct the strength too. So I think if you try curing the cast structure by covering it with blck plastic and designating someone who will be in charge in spraying water onto is days and night for the remaining part of the curing process days at respected time interval and using curing agent like some CRYSO product mix water this might help to acheive close to the required strength
The concrete has been already cast, I think the reason why the strength was not achieved was because of problem in hydration reaction of cement, one of the reason may be due to compatibility issues between cement and the admixture used. Pl. check the COMPATIBILITY OF CHEMICAL ADMIXTURE WITH CEMENT by MARSH CONE TEST.
If you think your concrete sample could be defective, try to run an ultrasonic pulse velocity test on your concrete structure at 28 days and hope for the best.
Article Combination of Compressive Strength test and Ultrasonic Puls...
Yes, ultrasonic pulse velocity test on your concrete structure will give some idea, however, the results are not conclusive.
Dear All
thank you very much for your informative insights, as you just mentioned that the reason might be lack of proper hydration and curing. however, the reason is not of highly importance right now, this is a foundation for an elevated tank and the amendments interests me more than the reason.
Dear Dan Le
yes the concrete cant reach the designed strength.
The raft is about 2m*4m*0.4m
Your raft foundation is not too big.
Depending on the load-bearing direction of the structure, you can reinforce your raft foundation. Some solutions which i think it useful for you:
1. Try to check the strength of a specimen which take from real structure (drill core) again. Sometimes, cube sample can be failed because the worker’s skill who made it is not really good, but not your raft foundation.
2. If concrete really have problem. Extend thickness of your raft foundation with concrete and rebar inside. Before casting above layer, you should make rough the surface to facilitate connection.
Thank you very much
I really appreciate your insights, it was an enormous help.
Carry out the assessment of strength by NDT methods..
and, then based on results go for injection grouting etc.
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