concrete specimen initial cure 1

[msucog]

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?

 

[DarthSoilsGuy]

20,000+ cy! i would say they owe you one...... point five million.

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.

 

[msucog]

it's funny how when you're dealing with big areas that have deep cuts and mass rock that elevations suddenly get forgotten about. in that case i mention, the contractor "forgot" that 20+ feet of cut had already taken place to get to the top of rock. they came up with 45,000cy and i had 22,000cy. we went round and round for two weeks. i kept telling them to send me their calculations so that i could see where the difference was. we finally sat down at a meeting to look over the calulations with the head county guy and the architects (this is same job as mentioned in my thread except different contractor). i show up with pencil and paper and they plop a computer out on their mahogony table that pulls up a 3D rendering on the 50+" plasma tv of what the cut area looks like and then he says "there you go". (just how the hell are you supposed to come to a resolution when all you have is a 3D model of what the rock looks like? they said "well the program says 45,000cy"). i lucked up and finally recognized that they're depths to rock in the test holes were from a ground surface that looked something like original grade and i finally got them to say it was. so they had original elevations instead of the ground surface elevations at the time of test drilling.

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.

 

[civilperson]

How do you maintain the required cure temperature without a insulated box? Box is not specified , but the temperature limits are not to be exceeded in any weather.

 

[msucog]

water bath with ice...air conditioner i suppose would work.

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!

 

[BigH]

msucog - looks like my post by Neville was right on - so long you don't have moisture loss - which you did with the plastic bags. Interesting.

 

[msucog]

sure does. we're still waiting for the last pour which i'll still have extra cylinders made simply for data points now that the contractor's theory is debunked. i'll post the actual numbers next week. between the box (which reached 120+) and the shady area which was probably 90-95, the strength difference is negligible. we'll see what the difference will be between ice bath, ambient air temp, and insulated box sitting in the sun allowed to reach max temp.

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.

 

[msucog]

from another thread but trying to keep it on this one....

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.

 

[Rjeffery]

Usually, when I write my proposals, I state that I exclude on-site initial curing facilities, siteing the contract documents and ACI 301. But in the event the contractor is unable to provide the proper initial cure facilities that I will supply a forney LA-1302 Curing Box which has heating and cooling abilities for an additional $500 per week per box, that it is (they are) in use on site to protect the owners test specimens... (~8 weeks pays for the box...)

 

[msucog]

hey, that's good. i'll keep that in mind. have you run in to any problems with "being responsible" for the specimens while they are onsite if you provide the box (or do you clarify that too)? that would be my concern that i somehow get sucked in to being expected to stand guard to protect the specimens.

 

[Rjeffery]

I try to come at the project personnel (owners and general contractor/owner's rep) with the attitude that I am an extra pair of eyes. But in proposals and contract documents it is stated very clearly that the GC or Owners Rep is to provide us (the Independent Testing and Inspection Agency) with all assistance and access required to perform our contracted duties. A statement such as: “By ITIA supplying X, Y or Z does not absolve GC or OR of the responsibility of protection of X, Y or Z per the contract documents.” Is usually good enough on the addendum to the contract when good concrete contractors go bad

 

[msucog]

well, supplier and contractor still have not provided the results for their "not so magical" cylinders' 28-day breaks. i talked with the architect and engineer the other day and they are still waiting too...i believe their patience is wearing thin. it's just making the supplier and contractor look like idiots in their eyes as well as the county's eyes. i'm haven't been stoking the fire since the contractor/supplier seem to be doing a fine job of stoking it themselves. funny though: the bullying and finger pointing by the contractor has been all quiet since those magic cylinders broke low at 28 days...imagine that.

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...

 

[BigH]

msucog - interesting on the aggregate. Does the concrete supplier do QC on the quality of the aggregate? Do you/can you get copies? Perhaps, at the "right" time, you should suggest to the owner/architect that you do a sampling of the aggregate prior to the next pour. You can run your own LA Abrasion/Micro Deval/Aggregate Impact test for durability - also absorption, etc. as well. Mmmmm

 

[msucog]

since the supplier is a very large one (probably one of if not the largest), so i suspect they do quite a bit of QC...perhaps you can never have enough QC though. the owner doesn't want to go on a witch hunt since all the breaks are just marginally low and the cores have been coming up ok even some are just marginally ok. this was the only cylinder so far that we saw the aggregate like this on but then again, it was one of the only ones we cut in half because something didn't look right. i suspect that as long as the cores come up okay, the issue will be "resolved" in the team's eyes. considering that this is not a huge building and the concrete is only the footings and slab on grade, i'm not going to pursue the issue unless someone asks us to. however, i am going to make sure my firm's name is cleared on paper from the finger pointing that occurred earlier. i'm also attacking this on the front end of new projects now to make sure the contractor/supplier do not even have this hand to play about initial curing being my problem since i have some large, rather critical projects coming up with this same supplier. i suspect that our material expert might be making a courtesy phone call or two well above the heads of those we've been dealing with over at the supplier. apparently, there are lots of old friendships that go way back well up the line of the company and i'm sure they'd be interested to hear about how all this was handled by their people.

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).

 

[boffintech]

What's the EOR position on this issue of initial curing?
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?

 

[msucog]

they all only want the numbers to work out...per the contract documents, coring is required if the cylinders break low. if the cores break okay, then it's all okay. and the owner's money is not the owner's money...it is the county's money so there's not the personal connection. it is however MY MONEY. but they're paying my paycheck so i'll let it go as long as there's not gross negligence and since they ultimately are the ones to sign off on it.

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.

 

[boffintech]

Is this a design-build project where the engineer works for the same company as the contractor?

 

[msucog]

nope. engineer actually works for the architect. the architect and engineer are good to work with from my previous job with them. they're sometimes a little too lax on some things (i guess for the sake of being political and keeping the job moving). but all in all, they're good to deal with. since all the low breaks are marginally low, no one is afraid the building is going to fall.

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.