No lids on strength tests specimens during initial curing at the jib site...hmmm...it would probably be easier to put the lids on the cylinders when made rather than try to quantify the negative impact of not following standardized procedures. By not putting the lids on the cylinders the bleed water was lost. That lost moisture could have been used to hydrate cement.
To paraphrase Ron from an earlier thread: The purpose of taking cylinder specimens under standardized procedures, curing them under standardized procedures, and testing them under standardized procedures is to see what the MIX DESIGN is capable of doing. The cylinders represent the mix design.
When did lids become standard procedure? I've never seen lids being required but it may be in a later than 2002 spec. Advise? Certainly, I have never seen lids used - interesting that CSA and ASTM had di-opposite views on layering of top protection with respect to burlap bag.
NOTE 5—Asatisfactory moisture environment can be created during the initial curing of the specimens by one or more of the following procedures: (1) immediately immerse molded specimens with plastic lids in water saturated with calcium hydroxide, (2) store in properly constructed
wooden boxes or structures, (3) place in damp sand pits, (4) cover with removable plastic lids, (5) place inside plastic bags, or (6) cover with plastic sheets or nonabsorbent plates if provisions are made to avoid
drying and damp burlap is used inside the enclosure, but the burlap is prevented from contacting the concrete surfaces.
11. Transportation of Specimens to Laboratory
11.1 .....Prevent moisture loss during transportation by wrapping the specimens in plastic, wet burlap, by surrounding them with wet sand, or tight fitting plastic caps on plastic molds......
Since I'm hauling wet sand or wet burlap I think the lids are the way to go.
we all agree on the standard of curing with lid, no complain there, at all. I have a job that failing by 1500 psi in 28 days, the producer are claiming this was due to lids not being in place the first 24 hours. After this time the sample was in the moist curing room.
I have no question about the standard. Just trying to figure off the record, how much 24 hour non curing would matter.
If the mix design which the cylinders represent is 6000psi or less and if during the initial curing period the ambient air temp was maintained between 60F and 80F with a relative humidity around 100% I'd say the strength loss due to not having lids is near zero.
Similarly if the mix design which the cylinders represent is for 6000psi or greater and if during the initial curing period the ambient air temp was maintained between 68F and 78F with a relative humidity around 100% I'd say the strength loss due to not having lids is near zero.
If, however, the contractor failed to provide an environment for Initial Curing meeting the requirements of ASTM C-31 and as a result the strength test specimens were subject to non-standardized Initial Curing conditions and the lab manager failed to provide the tech with lids....well then your guess is as good as the next guy's.
(Notice how I gave the tech the benefit of the doubt on the lid thing?)
By not accurately following the accepted testing procedures you just open the door and invite in these kinds of criticisms. Didn't your dad teach you not to run to the hardware store and buy the rope for your own hanging?
To give you an idea, the difference in strength at 28 days between a cylinder moist cured for the first 7 days and one in air the entire time could be as high as 35%. Considering that only the lid was missing (the whole surface of the cylinder was not exposed) and it was only for 24 hrs, you should find a different reason to justify your loss of 1500 psi. Most of your losses cannot be related to not having provided the lid.
The question is of lids or no lids. The note quoted to me denotes 6 different methods of initial curing - as far as handling/care of samples go (not the temperature environment). As per the specification note, all 6 methods are acceptable to produce a test result compatible with Clause 4.2. I then agree with mitchelon's point that there is a different reason to justify loss of 1500 psi. In almost all my projects, we use steel moulds - so plastic lids don't do a thing for us. Also, working overseas as I have, it is a real chore to try to get them to follow even a bare modicum of the intitial curing requirements - yet, we had very few, if any, failures of concrete on testing. Sure, if done properly, the strengths might have been a bit higher but the fact is that the mix design was confirmed to the requirements of Ivory Snow. I saw a table once of the effects of various non-standard 'events' in casting/curing/testing a concrete specimen and there were some amazing reasons for lower strengths - I'll try to find the table and see if SlideRuleEra will post it on his site. Too, again, I have seen some take the sample out of the waterbath 24 hours before testing - mmmmm, there, too!
"it is a real chore to try to get them to follow even a bare modicum of the initial curing requirements - yet, we had very few, if any, failures of concrete on testing."
That's because contractors insist on mixes with enough cement to overcome the lower strengths associated with not performing initial curing because they DO NOT want to pay to perform correct initial curing.
A credible argument could be made that not performing initial curing increased the worldwide shortage of quality cement powder.
boffintech - always respect and learn from your posts. Your point on my quote may be correct, say for North America (and in this I am including Europe, Australia, etc). But working in remote areas of China with local subcontractors - or similarly in India or in Laos, or in Cambodia or in Indonesia - it is not relevant in my experience to the quote at hand. Curing is not in their mindset - no matter how much you rant and rave - no matter how many letters and issuances of non-conformances to specifications. You likely have not seen Chinese contractors using a cement-mortar mix, place it in 40deg sun and then go for 4 hours of rest to get out of the sun. No, they don't cure it. Which, of course, is why it crumbles on touch and why they have to redo and redo and redo it. Or having to truck water 20 or more kilometres in little tankers because there is no water at the site - or if there is, it is easier to pay someone to "bucket" the curing water onto the structure rather than use a pump and other suitable means - but being human, those who are to do the job decide to sit out of the sun with the rest of their mates because they don't want to be out in the hot sun all by themselves.
You are correct, though, that additional cement is used in most mixes. It is more of a point that since there is no "history" of concrete manufacturing in remote areas using specific aggregates and cement types, then you have a target strength at the mix design level that is in the order of 8 to 10 MPa higher than the required 28-day strength (if the standard deviation of a mix isn't known, you have to up considerably the target strength). Of course, as a "history" does develop (say after 30 or 40 test results), they could reduce the cement content. But, it is seldom done not the least bit of which is likely due to overseeing authorities not wanting to accept a reduction of the original mix design consituents - or the contractor note wanting to be bothered to save a bag of cement per m3.
in my lab we use the sure cure system to monitor and control the temps of cylinders, out 4X8 cylinder molds have no lids as they are 10" in diameter, but when we use plastic cylinder molds we use the same method, we cover the cylinder with cheapo shower caps we get from the dollar tree
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