concrete specimen initial cure
concrete specimen initial cure
(OP)
i've thumbed through a few threads that mention initial cure of concrete specimens/cylinders. i occassionally run across jobs where low breaks show up and the contractor tries to blame the testing firm for the low break since there is no storage box on site. unless the specifications or client specifically direct me to provide a storage box on site, it falls back on the contractor to provide such a box since they are responsible for their site, storage of all materials on the site, and protection of the materials on site. and as usual, the supplier and contractor try to throw us "under the bus". the issue is usually resolved because the contractor provides a storage box, they reduce placement times, and they improve onsite handling of the concrete. i have many construction jobs going on very near this site and none of those guys are experiencing the same problem. so i'm interesting in some other opinions on here since i'm sure contractors, suppliers, engineers, testing firms etc visit this site.
i've got a job that has many low breaks. the low breaks are typically within 500psi of the design strength (but too many low breaks to satisfy the aci low break criteria). the passing breaks are all very marginal and usually within 300psi of the design strength. low breaks started showing up at the beginning of the job prior to the temps being high (highs were about 80-85). the contractor said that the low breaks were due to the testing firm not storing the samples properly. i told the project team that we will gladly store the samples where directed at the jobsite and can put them in a storage box once one is provided by the contractor. (if no box is available, we attempt to cover the samples with a box or semi-bury the cylinders to get them out of the sun. we also use plastic bags or plastic tops on the cylinder molds). and on a side note, we learned the first round of low breaks was due to the wrong mix being sent to the jobsite--correct id number on the tickets but the plant had the wrong mix in their system. i have also learned that the particular plant is having difficulties with batching consistency. so, we cored the area at 70 days old and the core breaks were 100-350 psi higher than the design strength. the contractor stated that "we reviewed the break results and all the results are passing therefore the cylinders must be made incorrectly". i tried to be political and said that i reviewed the results and they looked reasonable to me given the age and strength--in other words, i didn't make personal attacks like the contractor is making toward me.
they finally provided a storage box so we've been placing the samples there. the low breaks are continuing with the passing results still marginal. now they're saying that the testing firm is responsible because we're not keeping the initial cure temps regulated (in their storage box). anyone have any thoughts on how to be political about this without calling the contractor an idiot? i have maintained that we cast and test the cylinders in accordance with astm but that we're not responsible for the initial onsite storage conditions since it's not our jobsite. i also noted that aci says that the placement times should be reduced in hot weather but the contractor continues to simply say we're leaving samples in the sun and not following astm (by leaving them in the sun, he means the storage box they provided is in the sun but implies we're leaving the cylinders laying out in the sun).
a little more background: the placement times vary from 1 hour to 2 hours. concrete temps at the time of placement vary from 80-96 degrees. slumps are generally 5-6" range with occasional 8". 7 day breaks are typically about 50-70% of design strength (3000-3500psi) and the 28 days are 85-110% of design strength. 56 day strength are typically only 10% higher than the 28 day strengths. ~70 day old core breaks are typically 101% to 120% of design strength.
a little more background: these guys are very difficult to deal with since during the masonry work, they had never heard of building code requirements and were ready to fight when they were told to cover their materials onsite and to consolidate grout. we have not been extremely picky over the masonry inspections since the architect is pretty lax about the requirements and required inspections. in other words, the contractor is doing his best to be difficult and get back at us because of his own lack of knowledge of the requirements.
any thoughts on the best way to get this settled so that i can get on with work?
i've got a job that has many low breaks. the low breaks are typically within 500psi of the design strength (but too many low breaks to satisfy the aci low break criteria). the passing breaks are all very marginal and usually within 300psi of the design strength. low breaks started showing up at the beginning of the job prior to the temps being high (highs were about 80-85). the contractor said that the low breaks were due to the testing firm not storing the samples properly. i told the project team that we will gladly store the samples where directed at the jobsite and can put them in a storage box once one is provided by the contractor. (if no box is available, we attempt to cover the samples with a box or semi-bury the cylinders to get them out of the sun. we also use plastic bags or plastic tops on the cylinder molds). and on a side note, we learned the first round of low breaks was due to the wrong mix being sent to the jobsite--correct id number on the tickets but the plant had the wrong mix in their system. i have also learned that the particular plant is having difficulties with batching consistency. so, we cored the area at 70 days old and the core breaks were 100-350 psi higher than the design strength. the contractor stated that "we reviewed the break results and all the results are passing therefore the cylinders must be made incorrectly". i tried to be political and said that i reviewed the results and they looked reasonable to me given the age and strength--in other words, i didn't make personal attacks like the contractor is making toward me.
they finally provided a storage box so we've been placing the samples there. the low breaks are continuing with the passing results still marginal. now they're saying that the testing firm is responsible because we're not keeping the initial cure temps regulated (in their storage box). anyone have any thoughts on how to be political about this without calling the contractor an idiot? i have maintained that we cast and test the cylinders in accordance with astm but that we're not responsible for the initial onsite storage conditions since it's not our jobsite. i also noted that aci says that the placement times should be reduced in hot weather but the contractor continues to simply say we're leaving samples in the sun and not following astm (by leaving them in the sun, he means the storage box they provided is in the sun but implies we're leaving the cylinders laying out in the sun).
a little more background: the placement times vary from 1 hour to 2 hours. concrete temps at the time of placement vary from 80-96 degrees. slumps are generally 5-6" range with occasional 8". 7 day breaks are typically about 50-70% of design strength (3000-3500psi) and the 28 days are 85-110% of design strength. 56 day strength are typically only 10% higher than the 28 day strengths. ~70 day old core breaks are typically 101% to 120% of design strength.
a little more background: these guys are very difficult to deal with since during the masonry work, they had never heard of building code requirements and were ready to fight when they were told to cover their materials onsite and to consolidate grout. we have not been extremely picky over the masonry inspections since the architect is pretty lax about the requirements and required inspections. in other words, the contractor is doing his best to be difficult and get back at us because of his own lack of knowledge of the requirements.
any thoughts on the best way to get this settled so that i can get on with work?





RE: concrete specimen initial cure
Response to team:
An environment created by climate controlling equipment and facilities suitable for strength test specimen Initial Curing complying with the requirements of ASTM C-31 is not available on site.
The lack of on-site Initial Curing facilities exposes strength test specimens to harsh extreme weather conditions potentially detrimental to specimen performance (strength gain) which may result in the reporting of inconsistent and/or less than accurate strength test values.
An Initial Curing facility for concrete strength test specimens is not in our scope of work. Per the project specifications, as outlined below, Initial Curing facilities are the responsibility of the contractor:
ACI-301 SPECIFICATIONS FOR STRUCTURAL CONCRETE
1.6.3 Testing responsibilities of Contractor
1.6.3.2 Duties and responsibilities—Unless otherwise specified in the Contract Documents, the Contractor shall assume the duties and responsibilities given in 1.6.3.2.a through 1.6.3.2.g:
1.6.3.2.d Provide and maintain adequate facilities on the project site for safe storage and initial curing of concrete test specimens as required by ASTM C 31/C 31M for the sole use of the testing agency.
RE: concrete specimen initial cure
this whole thing is ridiculous to begin with but the contractor is just looking for something to put the blame on someone else and their taking cues from the supplier (they even emailed a link from the ready mix website that says it's the testing lab's responsibility). i've got jobsites all over (one 300 feet from this site) and they're all going smoothly because the field procedures are much better on the part of the contractor. i actually have one site about 5 miles away the contractor doesn't want to bother with a box at all and they've been doing just fine. they did have one marginally low break and the supplier brought out a cooler with an ice bath. we actually stuck an extra one of our cylinders in his cooler just to see what happens due the different initial curing. for the 28 day breaks, 2 sets had the iced cylinders 100-150psi lower, 1 set within 50psi, and 1 set had the iced cylinder 250 psi higher. the range and scatter was identical to all the previous breaks. however, on that particular job, the contractor knows it's not our problem and have been great to work with.
thanks for the reference location. i knew it was out there somewhere but i could not recall where exactly it was. that will save me some time monday. the specs on this job say the contractor is responsible for providing safe areas to perform tests and storage of samples but i guess the contractor didn't read that part (they apparently didn't bother to read the statement of special inspections or general notes either).
i've been trying to keep from kicking the contractor but they're leaving me little choice. i was hoping they would motivate themselves to improve their placement techniques instead of dumping on me. i have little respect for contractors that cut corners but i have absolutely no respect for contractors that cut corners and then point fingers at everone else instead of acting like grown men and accepting responsibility for their own actions.
thanks again
RE: concrete specimen initial cure
Seriously? Have you been out in the hot sun with the cylinders to demonstrate the detrimental effects of being in extreme heat for too long?
Motivating themselves to do better is not in the genetic code of contractor DNA.
RE: concrete specimen initial cure
and i'm well aware of contractor's mentality as i once worked as a contractor for one of the largest contractors in the world (let's just say that i've got insight in their minds). now, as an engineer for the testing firm, i refuse to do their jobs for them--contractors are paid to at least know basic construction techniques. i attempt to educate contractors that are willing to listen when ever possible. i have the foresight to know that i will see the same contractors again and again (especially on school projects) and i would rather try to help the contractors help themselves in order to make my job easier down the road. this particular contractor has taken it personally because they have never been exposed to IBC requirements and think everyone is picking on them. they've now got a vendetta against me because i won't simply sign off on their work. they threw out the line, "i've been doing this for 30 years..." ha, they even said something to the effect of "yep, this code stuff is just way too conservative. it's all because of those crappy contractors that cut corners."--i just chuckled and told him that i would agree with that statement about crappy contractors to some extent.
and i did try to talk with the superintendent on the jobsite about some of the problems and he pretty much blew me off. as i said, i've been trying to keep from having to kick the contractor in front of the entire project team, but i think the time for trying to be helpful has past.
thanks again for the 301 reference.
RE: concrete specimen initial cure
RE: concrete specimen initial cure
"we reviewed the break results and all the results are passing therefore the cylinders must be made incorrectly"
Did you core your cylinders? oh wait those were broken.
i'm guessing that letter wasn't sealed by a PE! :)
When dealing with contractors like this, what bothers me the most is the gut feeling i have that they would not honorably report the facts if the roles were reversed. Also, when was "made" in question, hasn't it always been about curing? (oh no, that could be associated with our on-site super..., or oh no, that could mean that the cylinders could be representative of something on-site that wasn't cored. or Oh no, What the @#$% was I talking about?) What about all the "good" breaks made by your firm. logic has the contractor s$#$canning those too.
Don't take it personally, the project manager is probably playing hardball so that they don't get "negotiated" into paying for the additional testing. The super is probably just being a d!@k b/c he usually doesn't have to waste his time babysitting his masonry subs on other jobs without special inspections, or because he has had 30 years of training in this field.
here's what i would do:
Buy an air conditioner with a thermostat (also the hi-lo thermometer you should have there anyway if you don't) and connect it to the contractor's power before the report goes out.
write a report to all about the structural concrete involved. Establish that the concrete in those areas meets the project specifications and requirements of ACI-301 based on the review of laboratory and field testing of concrete both during placement and in-situ.
At this point i would discuss how the core testing does not invalidate the results of concrete testing on this project to date or in localized areas. (some people would dwell on this, i keep it simple. point being a general attack on testing methods like that one is an attack on the entire project testing)
Then describe the initial curing as Boffintech writes. Say it has been addressed temporarily by your firm. Seal it and ship it.
of course, it needs to be fact-checked, short, and written better than above, but you get the point.
crap. this thing is too long. i've been nibbling on it in my back browser all day.
adios
dsg
RE: concrete specimen initial cure
RE: concrete specimen initial cure
RE: concrete specimen initial cure
It is interesting that Neville (Properties of Concrete - Chapter 7) indicates that "The loss of strength at 28 days seems to be directly related ot the loss of water which occurred during the first 3 days; the temperature (20 or 40 deg C (68 or 104 deg F) has no effect." (Referenc P. Nischer, General report: effects of early overloading and insufficient curing on the properties of concrete after complete hardning, in Proceedings of RILEM International Conf on Concret of Early Ages, Vol.II pp 117-26, (Anciens ENPC, Paris, 1982). Neville has a relationshipo of loss of strength vs mass of water loss (percent) ranging from 100% at 0 loss to 25% at 5% loss - and the storage at 20 or 40 degC both straddle the trend line. - so the question may be of how much water loss was there in the initial curing time? Just a thought
RE: concrete specimen initial cure
we did look at the statistical data across the different mixes for the job. it looks like the mix is consistent (whether it's strong enough is another argument but i have not seen anything jump out that would suggest it wouldn't work). the tests results also look consistent. both of those referring to aci 214.
again, the box that the contractor is now using was just being used 100 yards away on an adjacent job by another contractor for the same client. the concrete total was about 4x as much with different supplier and had 3 or 4 low breaks total (only had to core two of those and they came up ok).
the one thing i'm not sure about is what admixtures are being used. maybe once i see the batch tickets i'll see something there. i'm hoping that we can just narrow it down to initial curing but my gut feeling says that's not it. i've seen cylinders initially stored under brutal conditions and not see drastic changes in the breaks, but there again, i have no other explanation for this scenario. one thing that we should be seeing if initial curing is the problem would be that the strengths would be taking off fast at first. there coming in 50-65% at 7 days. let's say the box is getting up to 120 degrees. i would expect the cement to take off and the strength sky rocket for the 7 day break and then flatten off for the 28 day. also if the other job with the same mix is having no problems with less than optimal initial curing, i'm left with two suspects as far as i can tell: the contractor's onsite efficiency and admixtures (or batch plant errors). some of the low breaks are showing placement times of right at an hour and temps in mid 80's so that doesn't seem to be the entire culprit (maybe partial). since the batch plant did send out the wrong mix and inside sources tell me that particular plant has had control problems, my gut tells me that's going to be the source of the problem but we'll see what the data shows. i would much rather it be the initial curing and storage box since we work all over with that supplier (i probably shouldn't really care since they throw us under the bus every chance they get but still i'd rather not throw gas on the fire).
as far as the moisture loss after initial curing, that should be easy enough to measure. with the new storage trough, we should be able to make a comparison between the moisture loss from the two different storage methods. bigh, any thoughts on how much moisture loss is possible with specimens in plastic molds and sealed with plastic bags (assume the box hits say 120 with no air flow)?
RE: concrete specimen initial cure
you may already know this now, but if not it's worth knowing or could help other people who use this post.
there are some references to some of the mixes having slag. Blast furnace slag and fly ash are pozzolans <sp> that have a secondary reaction with concrete hydration byproducts for increased strength (along with some other benefits). since it is a secondary reaction, the concrete strength development time can exceed and usually exceeds the 28-day standard test cycle because it doesn't kick in until those byproducts are available. (you still have to test and meet code/spec at 28-days when you use pozzolans in a mix.) But, IF there are pozzolans in the original concrete described in your post, there is an argument that the 70+ day field core tests would indicate that your 28-day breaks are more accurate than not. Especially so if they are having problems batching.
also, for all you testers out there that haven't read the ACI 301 Structural Concrete - Duties & Responsibilities of the Contractor. there is are some other good items in there. the contractor is responsible to:
1.6.3.2.b
"Furnish any necessary labor to assist Owner's testing agency in obtaining and handling samples at the project site or at the source of materials"
What that means.... We're going to have to work out some other way than having our tech going up and down stairs with 5 gallon buckets of concrete, and yes it's your problem.
RE: concrete specimen initial cure
fly ash not allowed on these jobs. we're going to make 4 extra cylinders on next pour. 2 will go in the box that's been used. 2 will go out on the open slab in the sun. the regular 4 will go in the tub. we should be able to make a reasonable assessment of the results based on 1 7 day and 1 28 day compared to the regular set. i'm willing to bet it's negligible. it's starting to point awful hard at the supplier or mix.
RE: concrete specimen initial cure
ASTM C-31 does NOT require a box. What is required is a environment between 60-80F <6Ksi and 68-78F >=6ksi without loss of moisture.
ASTM C-31 "The storage temperature shall be controlled by use of heating and cooling devices, as necessary."
Wooden boxes sitting in the sun are like ovens!
If they thought 24 hours of heat was detrimental to strength gain why not try leaving the cylinders at the job-site for the full 48 hours that ASTM C-31 allows for Initial Curing.
RE: concrete specimen initial cure
i'm 99% sure the supplier screwed up their calculations and so the results will be less than 100psi off of our results.
the only other deviation from the mix i see is that there's no #89 stone. instead, they bumped up the #57 stone. i'm not a mix design guru but i wouldn't think that would be killing the strengths. anyone else have an opinion?
RE: concrete specimen initial cure
RE: concrete specimen initial cure
You are correct; they are wrong and off base - is there not a way to turn this out so they don't lose face? Think like an Asian.
I am NOT saying to back down - but there are two paths you can take - the Nolan Ryan fast ball - down your throat - or the Gaylord Perry "goop" ball that slips and slides - gets to the catcher all the same but follows a different path. The batter can always then feel exonerated for striking out due to the "goop" (or in the case of Hoyt Wilhelm, the knuckler). Ryan gives him no such option. A thought.
RE: concrete specimen initial cure
the supplier works for the contractor, so the contractor is probably directing him to keep his mouth shut. the best route for the supplier to take is to break the 28-day cylinders and report them as they really are (which should be around 3500psi if they break like the rest have for the job). then, their able to say, "yep, the first break sure did look odd and it appears that it was weird". that way, we don't have to call them out and completely embarrass them in front of the entire team (and county).
we'll do our correlations between the storage box, open air and water bath initial cure with all of our own samples. i am fairly certain i know what the results will be. hopefully, since the supplier is a very large one and reputable, i'm hoping they will take the high road and either say to throw out their results or bring out real 28-day breaks. it's going to be hard for them since they (along with the contractor) essentially tried to discredit all of our results and our company in the meeting with the county. we'll see if they get a new attitude and work to change and correct the situation. there's a small question mark in the back of a few people's minds about my company/personnel because of these clowns and i don't intend to leave it that way. i'll give them the opportunity to do the professional thing and save face at the same time...but the clock is ticking.
(and i'm sure nolan has intentially beaned a few players in his day too--gotta be careful since they might charge and beat your ____. "the best defense is a good offense")
RE: concrete specimen initial cure
This situation would drive me absolutely up a wall. I'm impressed that you're dealing with it so well. Where did the supplier cure there specimines initially? If they came up with 5000+ @ 7 days, which is ludicrous given the difference with your results, and the core for that matter, you could just ask them how they did it, do it the exact same way. If you don't get that 5000+, you can just ask them why the heck not. Not only that, but you're the testing agency. They're not on record for this, you are. It's your responsibility to get the right results. Though I'd say when involved in this type of war, best to arm yourself with a hi-lo thermometer and record on your break sheet. Then there's nothing they can say about it.
Sounds like you're dealing with a crime ring or something. I'd say you're going way overbudget on your patience and effort spent, though it may go a long way for the next time you deal with the same contractor/supplier.
RE: concrete specimen initial cure
we had to core more footings last week and while we were on site, the supplier came out and ran a windsor probe (isn't that about pointless). they mentioned that they ran some on the previous sections back during the meeting. we and the engineer told them that was useless. we said it's plausable if you've correlated the results back to core breaks. so i suppose they're planning on using our core breaks as the correlation. here's the kicker, they ran a set of 3 shots at one location and left. we suggested that they run multiple trials at all 4 places that were being cored that day (4 seperate low breaks from different days). they declined and left. the core ended up having about 2 inches of grout on top of the footing. no wonder the guy in the field said his windsor probe results were 3500-4000psi (grout has been breaking at 4500 at 28 days).
the supplier and contractor are bracing themselves for a lawsuit in case they're told to rip out the concrete or to give a credit of some sort to the client since the client didn't get what he paid for.
as far as being over budget, the only good aspect about this whole deal is that the client does not set budgets with us for this very reason. they know that we're out there protecting their butts and trying to see that they get a good product. a few thousand dollars to us saves them many many times that. during the grading on this project, i already saved them well over 20,000+ cy of pay rock due the contractor and surveyor errors, so you can almost say they owe me one.
RE: concrete specimen initial cure
contractor/surveyor error my ass.
i don't get it. supposedly blasters work within the project specifications on every construction job they go to, but then they forget about pay limits and need to be taught it to them again and again.
Multiple choice test for these bozos:
A) They're corrupt as hell and shouldn't be working!
B) They're idiots and shouldn't be able to purchase explosives!
C) Both A & B
Pick.
RE: concrete specimen initial cure
i've said it before and i'll say it again...from my previous experience as a contractor (and surveyor), never trust contractors without first proving to yourself that they're correct.
RE: concrete specimen initial cure
RE: concrete specimen initial cure
here's an update: our 28-day breaks came in right in line with the rest of the breaks (400psi lower - 200 psi higher than the 28-day design strength). the supplier's breaks came in (drumroll please).....300 psi lower to 200 psi higher than the 28-day design strength (so the concrete lost 1000 to 2000psi between the 7 day and 28 day breaks)...WHOOPS!
so it looks like regardless of the curing temperatures (within 20-40 degrees fahrenheit from that 80 degree mark), it's not that huge of a difference. again, we use plastic molds with plastic bags to hold in moisture so it might be different if the moisture was able to readily leave the samples. either way, the contractor is on the hook for initial curing and storage. it is appearing to me that the problem is potentially within the plant since some low breaks were cast less than an hour after batching. maybe the cement is not up to par...i really don't know and to be completely honest, don't really care on the friday before holiday weekend after i received the news above about the breaks out today.
have a great holiday weekend!
RE: concrete specimen initial cure
RE: concrete specimen initial cure
p.s. i'm still trying to take the high road and not gloat too much since the supplier actually broke the real samples. they did the professional thing and let the results happen instead of pulling other specimens to keep from hurting their image.
RE: concrete specimen initial cure
concretemasonry (Structural) 3 Sep 07 14:45
Technically, when the materials are weighed and mixed, they are "batched". The gray area occurs when the concrete is a "transit mix" variety and not from a "central-mix" plant where water is initially added according to the mixing slump.
In reality, it is not that simple. Coarse aggregate usually contains only absorbed moisture (1/2% by weight). Fine aggregate (usually sand) can contain a significant amount of moisture - up to 7% by weight (mostly available surface moisture) depending on the exposure history. The first loads of the day can contain a significant amount of moisture (bottom of the bins/storage piles) that will reduce the mixing water that may be added at the mixer in a central-mix plant or by a driver/contractor in the case of a loosely controlled transit-mix operation. - This can obviously effect the timing of the beginning of cement hydration.
One other very important factor is the age and temperature of the cement. During certain times of the year (late season and/or cement shortage conditions) a concrete supplier may receive "hot cement" that is from a very fresh cement clinker ot is freshly ground from older clinkers. In either condition, the cement is much more reactive and will set quicker. Typical cement mill reports are averages (chemistry, inital set, etc.) of the cement produced and will not accurately reflect the properties of the cement when batched.
The temperature of the cement at the time of placement, while effected by the aggregate temperature, can be an indication of the amount/rate of cement hydration if you have some history of testing.
Thank concretemasonry
for this valuable post!
-------------
msucog (Civil/Environme) 3 Sep 07 15:15
good points concretemasonry...
is there any good way to identify hot cement from outside the batch plant? (i'm guessing no--this leads back to my other thread where i'm now pondering why the breaks are coming up low and not gaining a lot of strength after the 7 day breaks and virtually zero strength gain after 28 days--i do not anticipate a lot of help from the supplier)
------------
concretemasonry (Structural) 3 Sep 07 17:32
msucog -
The variation caused by hot cement is probably less than the problems you seem to encounter on your posts regarding curing and handling of cylinders. Fortunately, the long term concrete curing process provides a "cushion", but not an absolute "cure".
There are clearly defined standards for cylinder preparation, curing and handling. Variation from these standards render the results questionable. Unfortunately, hot cement coincides with hot weather and the problems maintaining proper conditions for the cylinders independent of the site conditions. Comfining cylinders in a hot box is the same as using hot cement and the results cannot be documented in the real world. All sampling and testing guidelines must be established before the actual start of construction and must be rigorously enforced early to establish a solid baseline.
Hot cement is a minor "blip" in the process that presents some unusual problems for an engineer to fully understand. When I was in college during the dark ages, our concrete lab instructor turned his back on the pranks involving mix constituants (sugar, flyash, accellerators), sample preparation (rebar, missing rodding), sample handling (impact) and testing procedures (unlevel capping, off-center testing machine alignment) that were always present. - In the end, the class had to come up with an anaylysis of the causes that turned out to be more beneficial than the "pure" testing procedure that should happen in a lab or controlled site.
As I mentioned in my previous post, you need a reliable historical base to operate off of before you try to trouble-shoot.
Dick
Thank concretemasonry
for this valuable post!
------
msucog (Civil/Environme) 3 Sep 07 19:24
as mentioned in my thread, the testing looks solid. even the supplier's test results are right there with our (probably not more than 5% off of our results based on the companion testing--even with the different initial curing). our technicians are the best around and we hound our techs about what they should/shouldn't be doing (i've even got the onsite fulltime tech watching and helping the techs doing the concrete testing). i would point fingers at the contractor more but even some of the trucks that got placed in less than an hour had low breaks. overall, the breaks seem consistent (consistently marginally passing or low). statistically, everything is consistent so i'm 90% certain it's something in the mix. i would not think hot cement would be the source since we're so far from a cement source and since this job has been seeing low breaks for many months now but i suppose it's still possible. i'm simply running out of things to look at as being the problem. i'll post this on the other thread to keep from overrunning this thread.
RE: concrete specimen initial cure
RE: concrete specimen initial cure
RE: concrete specimen initial cure
RE: concrete specimen initial cure
we had another pour where we made extra cylinders. cured the project cylinders in the new water bath provided by the contractor, cured a set in the box, and one set field cured. the field cured were the highest, box second and water bath lowest...again, probably tied to the initial temps accelerating the strength gain. i expect the 28-day breaks to possibly be slightly flip-flopped...i don't think the water bath will be much higher than the others by 28 days but maybe just slightly. the field cured were 700-900 psi higher than the water bath. the water bath came in at 60% of the design strength. we'll see what the 28-day breaks come out at.
i took a look at a low break cylinder the other day and it had aggregate that you could break apart with your hands...wonder if that has something to do with the low break...
RE: concrete specimen initial cure
RE: concrete specimen initial cure
consequently, i just realized yesterday that this is not the first project of mine that this supplier has tried this on when low breaks came up. the last client pretty much dumped us as the testing firm over it (maybe because i was not as aggressive on that project--we tried to play the "let's play nice and be political" card).
RE: concrete specimen initial cure
Clearly ASTM requires initial curing of the cylinders at the job and clearly ACI301 requires that the contract perform that initial curing.
Also, if I'm the Owner and I understand the above two facts, then I also understand that I've paid the contractor to take care of this through the contract so if it's not being done and there is all this headache because of it WHEERE IS MY MONEY GOING?
RE: concrete specimen initial cure
p.s. pay attention to the breaks i mention about the last strength breaks between water bath cured, field cured, and storage box cured (at 120 deg). the water bath had the lowest 7-day breaks...it might be the highest 28 day break...we will have to wait and see. i'm willing to bet it won't be 100-200psi higher (if even that much).
if it were my money, i'd be bucking for a cash credit...the county just wants the school open since there's so many schools being built and still so much overcrowded.
RE: concrete specimen initial cure
RE: concrete specimen initial cure
it does look like the supplier fixed something or bumped up the cement on this last pour since these were the highest breaks yet at 7 days.