I think that this may have something to do with partial set and retempering although I cannot give you anymore data on this.

Unless a retarder been used and continuous agitation been maintained-

The cement in the mix will start to hydrate, meaning-
- The concrete will loose workability.
- Will be difficult to pour and vibrate.
- Will produce honeycomb.
- Will affect the in place properties of the concrete, meaning, strength, air content, serviceabilty, etc.

Out here in Arizona where the summertime humdities (May through June, before the monsoon) can be as low as 4-5%, concrete specifications have limited the concrete delivery and discharge to 90 minutes maximum as measured from batching time to delivery down the chute.  When air temp is between 85 and 90 deg F, this is reduced to 75 minutes, and when air temp is above 90, this reduced to 60 minutes.

This will prevent undue time in the truck and premature hydration and "setting" of the concrete.  We do not allow addition of water to our mixes on site, except in very limited and unusual situations.  We are not shy about rejecting trucks.

For your late placing times (approaching 3 hours) you might need to research the addition of ice to your mix to keep the concrete cool during transport.  Your 150 minutes time seems awfully long to me.

The questions i would make sure i knew were:

Was concrete testing done?
how old was the concrete when the cylinders were taken?
Are there any retarders in the mix?
God forbid are there any accellerators in the mix?
Was water added on-site and how much?
What was the temperature during placement?
What was the temperature of the concrete tested?

90 minutes is an industry/code standard cut-off time for unaccellerated mixes.  perhaps someone else will give a research paper that compares strength with placement time at different temps for different mixes, but i don't know one.

If the mix is not heavily retarded:
With 150 minutes, your concrete was probably hydrating inside the truck. Since this gets broken up by the mixer with the spinning, it is essentially replacing batch cement with dust and means strength that you will never ever get back. The truck driver will add water since its been on-site "drying up" or "heating up". In addition to the destruction of those initial crystalline structures your water/cement ratio goes up for the rest of your unhydrated cement.

on a side note, a smart concrete supplier will heavilly retard a CMU blockfill mix since pours can be very very slow. you didn't say what kind of concrete this was though.

Not only the increase in ambient temperature will reduce the set time, also the increase in cementitous material will do the same. Increased cement content in the mix will increase the heat of hydration and inturn reduce the set time.

I have seen rich mixes on a warm day losing workability with only 60 minutes of delay in the pour.

These mixes on a 70 degree day maintained about 82 to 85 degrees mix temperature.

Correction to previous post:

Delete:
"Was water added on-site and how much?"

Replace with:
"How much water was added on-site?"

Most concrete I've seen is in the 45 minutes to 60 minutes range.

I can't imagine concrete slowly turning in the mixer for 150 minutes.  i would not hestitate to reject that.  Of course I don't know the specifics of the mix, but just assuming it was typical, it would go.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

One more thing, it sounds as though you're looking for something definitive for either your sanity or the contractor who's fighting to use the load and not have his crew sitting around.

If so and you reference concrete specs to ACI, then I would suggest checking out ACI 301 which states

4.3.2.2 Time of discharge—Time for completion of
discharge shall comply with ASTM C 94/C 94M unless
otherwise permitted. When discharge is permitted after more
than 90 min have elapsed since batching or after the drum
has revolved 300 revolutions, verify that air content of airentrained
concrete, slump, and temperature of concrete are as specified.

Obviously anything not complying should be rejected.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

"How much water was added on-site?"

Look out and be ready to do it over again.

Calcium silicates and Aluminates form cement which is the bonding agent in concrete determining the strength of concrete.  Aluminates hydrate rapidly determining the initial setting of concrete ( approx from half hour) and then silicates start to hydrate and finish from 10 hours plus.
So without retarders the conc should be placed and compacted and left undisturbed before initial setting of conc time is over. When we add retarders supposedly they insulate each cement particle from water for certain time. This extra time to retard the process of hydration is determined by and sole responsibility of admixture supplier. So you should not allow any time over admixture’s supplier recommendation. Lastly adding water later at site is general mal practice which you should never allow.

As soon as the water is added and mixed the Portland cement starts hydrating.  Thus every minute the mix is tumbling it is breaking the bonds created by fast setting (finer ground) portion.  I sure there is test data out there that lead to the 90 minute rule.  You have been hustled by an incompetent and/or dishonest contractor.  Why wouldn't you just not delay adding the water until the ready mix truck was on site?  Special ready mix batch plant trucks are available in medium to large markets that can mix 1/4yrd to 7yrds of nearly any strength mix at the pour.  Even the mechanical action of the tumbling over 300 revolutions grinds the aggregate down into a clay fraction that further degrades the ultimate strength of the concrete.  And of course the mix is heated by the tumbling further speeding the set.  The upside is the mixer drums should be real shiny when they got back to the plant.

Does batching mean loading the truck, but not mixing, or mixing when the truck arrives on site.

i'd call batching from the time that water is introducted to the mix and i think it's actually defined that way.

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.

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)

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

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.

Msucog, How far is the site from the concrete plant in miles/time?  If the breaks were testing consistently low why wouldn't you just pay for/require an additional X number of lbs of Portland per yard to get your breaks to meet or exceed spec?  Is there any chance of systemic fraud/error @ the batch plant?  I'd eye ball the Portland scale for buildup.

Is it Chinese Cement?  

Mike McCann
McCann Engineering