what happens after 90 minutes

huangyhg

what happens after 90 minutes
what happens to concrete when it is placed later than 90 minutes after batching?  i can't seem to find anything definitive in aci, and i've got a project with some rather late placing times (150+ minutes).
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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.