Jim Brown wrote:
> On Fri, 7 May 2010 12:53:49 -0400, Mark Robinson wrote:
>
>> I poured the base for my Rohn 45 this morning. This tower is held upright
>> with a wall bracket at 18 feet. How long should I leave the concrete to cure
>> before starting work to add the next sections?
>
> Concrete cures as a "sort of" inverse exponential function over time. One
> month is the generally accepted time to wait.
>
>
There are a variety of concrete handbooks out there (go to cement.org,
for instance).. 28 days is when the concrete will have reached its "data
sheet" spec. It still keeps getting stronger for years. You're about
50% of the 28 day strength in 7 days. You MUST keep the surface wet for
the entire cure time.
If you let the surface dry out, it actually cures to 50% strength
faster, but will never get to the 28 day strength. There's an
interesting chart I found some years ago that shows if you air cure, the
max strength after months is about 50%. If you keep it under water for
3 days, then let it dry, it will get to 80%, 7 days of under water gets
it to 90%
So, if you're pouring a slab for a patio, you may not worry if you just
keep it wet for a few days.. But the footings for your house or your
tower? Here in southern California, a popular strategy is to use
plastic film over the concrete to keep it wet (along with the water
under the film, of course). There's also various chemicals (some as
simple as diesel fuel) they spray on which reduce some of the evaporation.
It sort of depends on what you want to do. Will it hold up to the full
design loads in a week? Probably not. Is it strong enough to erect the
rest of the tower, if the weather is going to be good and you're not
going to see the rated, probably.. It's a gamble.. do you feel lucky?
If you want more information on this than any ham needs to know, here's
the Federal Highway Administration's
"Guide for curing of portland cement pavements"
http://www.fhwa.dot.gov/pavement/pccp/pubs/02099/index.cfm
It has charts and graphs, and equations to model the evaporation of the
water coming out, factoring in temperature and wind, and how to maintain
the optimum hydration profile.. Hey, if you're building a freeway at
millions of dollars/mile, you want that concrete to be as good as it
gets..Memorize this puppy, and when you're at Dayton in a couple weeks,
and folks start spouting various theories of concrete use, you can just
stop them in seconds.. You can talk about water reducing admixtures,
bleed rates, etc.
By the way, the above document talks a bit about how cement chemistry
has changed over the last few decades. On big construction projects,
waiting time is big money, so there's a lot of interest in not just
saying "it's cured after 28 days". So, they want to predict (and
measure) the strength accounting for temperature, hydration, etc.
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