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Posted by DanG on October 17, 2006, 12:12 am
Amen, and quite well written.
______________________________
Keep the whole world singing . . . .
DanG (remove the sevens)
dgriff237@7cox.net
>
> Daniel Vance wrote:
>> I'm a GC in central oregon. My concrete guy poured out 15
>> yards yesterday
>> for the flatwork around a house I'm building. For some darn
>> reason he
>> didn't think he'd need blankets last night. Needless to say,
>> it got down to
>> 21 degrees last night. I noticed this morning that there were
>> a few areas
>> that were starting to turn light gray. It looks like the thing
>> is ready to
>> freeze pop. I've put in a call to his cell saying that we'll
>> need blankets
>> for tonight.
>>
>> Basically, if the thing freeze pops and starts spawling, then
>> I'm going to
>> be in a fight with him on getting paid...and ripping the job
>> out.
>>
>> Does this seem like he should have to take care of it or am I
>> off base?
>>
>> -Dan
>>
>> Dan,
> The light grey color could be caused by many factors, and may or
> may
> not be an indiaction of trouble. I would be more concerned with
> the
> question of wether he cured the concrete with a curing material
> than
> the fact that it was not covered on the first night.
> Concrete needs to be cured properly to gain strength and
> durability.
> Proper curing involves keeping the concrete temperature above 40
> degrees F, and moist, until the design strength is achieved.
> Concrete
> creates heat as it hydrates, and some contractors rely on this
> to keep
> the material from freezing. This doesn't always do the job, and
> blankets should be used to keep the concrete warm enough to gain
> strength. The curing material keeps moisture in the surface
> while the
> concrete is gaining strength. Concrete that is allowed to dry
> during
> early ages will loose as much as 40% of it's ultimate strength
> potential on the surface. When concrete is allowed to dry
> during
> curing, the chemical process changes from Hydration, making
> concrete,
> to carbonation, making chalk. Chalk is weak, porous, and
> lighter
> colored than the same material that is cured properly.
> If no curing agent was used, you may have a slab with a surface
> that
> will absorb water and not be strong enough to withstand freeze
> thaw
> cycles. Throw some water on the slab, if it is absorbed
> quickly, there
> is no curing agent there. If the water doesn't soak in, there
> probably
> was a curing agent used.
> Regarding blankets, they should be used when temperatures are
> expected
> to drop into the 30's, however, expect them to leave permanent
> marks on
> the surface.
> There are admixtures available that allow placement of concrete
> at
> temperatures as low as 20 degrees F without using blankets.
> They
> basically reduce the amount of water available to freeze, treat
> the
> water to lower freezing point, and accellerate the strength gain
> so
> that the concrete rapidly gains enough strength to resist freeze
> thaw.
>> Check with your local ready mix concrete contractor or the
>> local association, to set up a training session on Cold weather
>> concrete practices. You could also require ACI Concrete
>> Finisher Certification training for your concrete subs.
>
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