This Is What You Want Your Concrete Doing....

[extexthai]

Those of you building with concrete footings, piers and beams...do not let it dry out! Insist....go buy some rolls of plastic sheeting, have it on hand and DEMAND it get tied around any and all freshly poured concrete as SOON as those forms come off. It will NOT cure if it dries out...for some unknown reason that is how it is done here in LOS. They will tear off the forms as soon as it can stand up by itself. Okay, perhaps they will leave the staging under beams for a while, but still let the beam itself dryout.

Concrete wants water....it is an exothermic chemical reaction and produces heat on its own as it cures. By wrapping it in plastic and tieing it with that colorful flat poly ribbon stuff it will self-baste and keep wet for as long as the plastic is kept tied to it. You do not have to wet it if you wrap it. If you want the strength concrete is known for, let it cure correctly. Dried out, uncured concrete becomes brittle, cracks real easy and is actually prone to crumbling. Studies have shown that it still cures after a decades of being poured...if cured correctly. Some say 28 days, and I have heard at least 10 days, but the longer the better.

Oh, and if you don't do the first one to show them, more than likely it won't get done correctly. Most of the rolls of plastic are about 1 meter wide after you unfold the crease. Unroll it and cut it to length ...... something as obvious as the nose on your face, but for some other unknown reason....if left to someone else it will get cut the other way and then you have a lot of wasted, useless plastic sheeting. Good luck and when the next big earth movement hits here, you might be glad you did this....ett

[sezze]

In Thailand , many times the concrete is not mixed perfectly anyway . If brought by cpac then it should be but still there is not really a point . I'm not saying you aren't right , but this unless it is full sun in the hot season ( like starting now ) then small thin slabs will not really improve . Thick pieces , will improve but standard practice around the world is wetting the concrete a couple of times a day ( sprinkle it 2 to 3 times a day ) .

Yes , concrete keeps curing about all of it's lifetime ( yes , longer then 20 or 40 years ) but i think 80% is reached after 10 days and 90 % at the 14 day mark ( i forgot the exact nrs) .

[trogers]

In Thailand , many times the concrete is not mixed perfectly anyway . If brought by cpac then it should be but still there is not really a point . I'm not saying you aren't right , but this unless it is full sun in the hot season ( like starting now ) then small thin slabs will not really improve . Thick pieces , will improve but standard practice around the world is wetting the concrete a couple of times a day ( sprinkle it 2 to 3 times a day ) .

Yes , concrete keeps curing about all of it's lifetime ( yes , longer then 20 or 40 years ) but i think 80% is reached after 10 days and 90 % at the 14 day mark ( i forgot the exact nrs) .

And local designers will specify concrete strength to a minimum test strength (as inspected through test cylinders), rather than average test strength. Thus, strength of concrete used is probably over the design requirement by 10-12%.

But little importance is usually given to sufficient concrete cover over reinforcement steel. The thicker should be the cover when lower strengths of concrete are specified - 240 KSC should have a cover of 30mm, 300 KSC - 25mm, and 350 KSC - 25mm.

Edited March 2, 2011 by trogers

[bbradsby]

Normal structural concrete - without special admixtures - achieves its design strength in 28 days, but continues to strengthen indefinitely, interestingly enough... but the change is rapidly approaching zero after about a month.

And I whole-heartedly commend the OP on this post - concrete must be kept wet to properly cure. BEsides wrap, there are also liquid curing compounds, but be aware they can impede bonding of later-applied materials. Please hire an expert - very cheap insurance - as failures of concrete structures can be catastrophic and fatal. Proper rebar detailing, and actual fabrication cost no more, but you need proper supervision to actually get these details installed in the field.

[bbradsby]

In the US, for normal residential structural concrete, minimum coverage over rebar is 1.5" [38mm], typical, to give the rebar something to bite into without spalling the concrete; and 3" [75mm] minimum where in ground contact. This protects the rebar from premature failure via rust. Measure from face of form to face of rebar.

Edited March 6, 2011 by bbradsby

[bbradsby]

Can't edit my own post for some reason, so here is the corrected post:

Normal structural concrete - without special admixtures - achieves its design strength in 28 days, but continues to strengthen indefinitely, interestingly enough... but the change is rapidly approaching zero after about a month.

And I whole-heartedly commend the OP on this post - concrete must be kept wet to properly cure. footings & grade beams I don't worry about - but slabs, columns & elevated beams will crack more extensively (all concrete cracks) if not kept wet. BEsides wrap, there are also liquid curing compounds, but be aware they can impede bonding of later-applied materials.

As an aside, for concrete structures, please hire an expert - very cheap insurance - as failures of concrete structures can be catastrophic and fatal. Proper mix design, delivery time duration, mix verification onsite, rebar & concrete placement, rebar detailing, and fabrication cost no more, but you need proper supervision to actually get these in-place the field to get good concrete. I know this is self-help forum, but with life safety being paramount DO use some judgement as to what you can take on safely. Would you design & build your own airplane and put your family in it?

Concrete looks simple, but there are a lot of critical steps that can quickly go wrong via lack of expertise... have you ever seen concrete thrown or dropped from buckets over about a meter?

[trogers]

Do you think local engineering undergrads in Thailand have even mixed concrete during their studies?

[H2oDunc]

Back in the UK every load of concrete that arrives on site is sampled and tested to destruction. I have never seen this done anywhere in Thailand. The wet mixes arriving at some sites is also alarming.

[extexthai]

Left to most Thais....it is more or less just liquid "brick" that as soon as it can stand on its own....off comes the forms and then it dries out as fast as possible. Actually all it is, is to just cover up unsightly rebar...who wants to have to look at that stuff, anyway? It is faster and easier than laying brick...afterall, it should look sort of nice after it is laid and consequently, takes more time...so just pour it! Tear off the form and voila....nice and pretty. If there are some honeycombs, just mix a bit of cement and sand, rub it on and you're done! Beautiful....ett

[bbradsby]

laughing to keep from crying

[Khun Jean]

Concrete looks simple, but there are a lot of critical steps that can quickly go wrong via lack of expertise... have you ever seen concrete thrown or dropped from buckets over about a meter?[/size][/font]

Out of curiosity, what happens to the concrete?

[bbradsby]

Concrete looks simple, but there are a lot of critical steps that can quickly go wrong via lack of expertise... have you ever seen concrete thrown or dropped from buckets over about a meter?[/size][/font]

Out of curiosity, what happens to the concrete?

the concrete's constituent parts separate out as it flies through the air - tossed from buckets or dropped- causing it to loose strength. the mix needs to be a uniform matrix.

[powderpuff]

They are refurbing all the bridges on Onnut. They do lay wet burlap over the concrete after the slab is poured from the Redi-Mix trucks. They're making a really nice job of it.