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I am often amazed, even dumbfounded by the Filipino building methods and standards.
Being in construction, my eyes are always drawn to structures, and I do it without thinking sometimes, just an automatic response.
The commonest mistake I have seen in Phils is leaving the reo bars at the top of a column or at the end of a beam exposed.
I am guessing they do this to facilitate possible future extension of the building (which judging by the amount of corrosion on the bars,
will never happen). BIG MISTAKE. The bars must be completely sealed and protected. When the time comes to extend,
you hammer drill in at least 200 mm, 8 ", then use a purpose-made 2-pack epoxy to glue some new bars
about 600 mm, 24" long, 400mm sticking out. Tie the new steel ligatures to them, box it up then pour.
The use of a vibrator is critical to remove all air pockets, and new beams must be supported (not by bars).
One of the biggest causes of structural failure in concrete is moisture getting in to the steel reo bars.
It's like a cancer, the disaster often not apparent until it's too late. When steel rusts, it expands wildly, splitting the concrete,
thus allowing more moisture in, thus causing more rapid structural failure. The relationship between steel and concrete is symbiotic,
so to speak, one cannot exist without the other.
In my view, cracks are unacceptable, and there are a number of factors causing them.
First and foremost, the structure must have a soild foundation. If the ground is weak, piles must be drilled down to solid substructure.
But as AsiaBill pointed out, the commonest cause of cracking is drying too fast.
It's difficult to avoid heat in Phils, so at least try to avoid pouring on those hot sunny days.
Concrete dries by a chemical process, not by evaporation, so when heat and sunlight cause evaporation, cracks appear.
The strongest concete actually dries under water, and that is how concrete companies here "dry" their batch tests.
The next best thing is to cover with hessian and keep it wet for at least 2 days. It is also important to bear in mind that
concrete does not reach full structural strength for 28 days.
Chiselling and filling cracks is ok if "bondcrete" is used. It is a liquid rubber compound you mix with the grout being used to fill the holes,
and sticks like sh*t to a blanket. (By the way, it is also used in concrete to make fishponds as it completely seals the finished product
and prevents the lime from leaching out into the water and causing slow alkaline death to the fish). There is another method of fixing
cracks involving epoxy injection, but too complicated to describe here, and the chip/fill method is adequate for a house.
Another method for building walls is to use what we call "Besser blocks", I think called "cinder blocks" elsewhere. They're the type
with 2 hollow sections. As the wall rises, a length 12mm (half-inch) reo bar is placed vertically in the middle of each cavity
spanning 5 or 6 courses, and concrete is poured and vibrated in around them. The result is almost bomb-proof and the
concrete fill can be mixed on-site, thus giving you more control over what actually goes into the mix. I would be very
dubious about what comes in the agitator from the concrete companies there.
I could go on about this for hours, but basically there is no excuse for cracks appearing in concrete. It is not "normal".
Happy building,
Jess.
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