Jump to content

Recommended Posts

A_Simple_Man

Most of the Cement topics I can recall talk about the need for rebar.

 

I just watched Discovery Channel to see one of the worlds 'largest and strongest' dams being built in Turkey . . and they are using NO rebar.

 

So is rebar really all that important?  And if so is that dam in Turkey going to collapse?  When?

Link to post
Share on other sites
smokey

i too heard of special cement that dont need rebar but i also heard it was over 50 dollars a sack ... 

Link to post
Share on other sites
Headed that way

Okay, I studied this in college so here is a short course on reinforced concrete.

 

Concrete is strong in compression and weak in tensile strength. 

 

Steel (rebar) is weak in compression but strong in tensile strength.

 

Both have the same modulas of expansion, i.e., it won't rip itself apart from normal expansion and contraction.

 

Keep the rebar buried several inches under the concrete, tie it in place well with wire ties to prevent it from moving during concrete placement and use the plastic supporters to keep it off the ground.  Use plastic sheeting under the concrete to keep the ground from wicking moisture away from the concrete while it is curing.4

 

Rebar must be placed in the  proper place, where it will be in tension.   Depending on how the beam is supported, and even slabs of concrete are beams, you might need rebar at the bottom and at the top.  The center of the beam or slab is the neutral axis where there is little tension or compression so don't put rebar in the center.  

 

Try to have the rebar clean and not rusty before placing and use a concrete vibrator if possible to make sure the concrete is in intimate contact with the rebar for max strength and to prevent rusting.   After the concrete is a day or two old keep it wet constantly and covered to allow the concrete to cure properly.  After 21 days it has cured enough to support itself if you are making a roof or a beam but concrete continues to cure for many years and get harder if water is present.

 

Use as little water as possible in your concrete to avoid weakening the final product.  Just enough to get it to flow around the rebar.  Use a hammer on the side of the forms if you don't have a  concrete vibrator to avoid  voids in the concrete against the forms.  Get one of those expanded metal grid concrete tamps to tamp the gravel down and bring the sand and grout to the top to make it easier to trowel a stiff concrete mix.

  • Like 3
Link to post
Share on other sites
Buko Beach

I would never build in the Philippines without using rebar. I don't trust the Philippine cement companies and/or local builders to use super high-strength cement. They would charge you $50 a bag but use the standard mix. Noooooo way, rebar is the safer way to build.

 

Look what happened in Haiti when a big earthquake hit, death and destruction because rebar was not widely used.

Link to post
Share on other sites
thebob

Headed that way posted while I was typing, here is the simple answer.

 

It is all about design. You can squeeze concrete very hard, before it breaks, (compression) but if you try to stretch it (tension) it snaps easily.

 

Rebar is the other way round, it is very difficult to stretch, but is not strong when squeezed.

 

If you imagine a beam, the middle of the beam tends to sink because of it's weight. The top of the beam is squeezed, but the bottom of the beam is stretched. So the beam breaks by tearing from the bottom to the top. Now if you have put rebar close to the bottom of the beam, the rebar cannot stretch so it is a much stronger beam than with no rebar.

 

If you imagine a post, if you try to force the top of the post one way the concrete is stretched on the side of the post you are pushing, so rebar on that side will strengthen the post. As we don't know which side the force is coming from, we make it the same on all sides.

 

The dam was probably a gravity dam that uses it's own weight for strength.

Edited by thebob
  • Like 2
Link to post
Share on other sites
thebob

If you had complete control of the design, you could build very strong houses without much material. Unfortunately we need to stick to building codes. These are drawn up allowing for quite high margins of building error. So lots of materials are used, to account for construction mistakes.

 

Many architects here try to over specify beams and floor slabs, in the mistaken belief that thicker means stronger. It doesn't. If you double the size of a beam, it becomes 4 times as heavy so most of its strength is wasted holding its own weight. Doubling the height of a beam, but halving the depth, uses the same material but makes a stronger beam. That is why "I" section steel joists look like they do.

 

You can buy the building code at National Book Store, when your architect draws out your house make sure that he doesn't increase those dimensions, because a couple of inches here and there adds up to a huge increase in material.

 

Of course then you have the problem of getting your contractor to build your house to specification. He wants to do it like he always has. Sometimes it is easier for him to arbitrarily increase some dimensions because it makes the build easier. That can cost you a lot of material.

 

Consistency is also very important, not only using a mixer, but measuring the correct ratio of components, including water. It is much easier to work with a very wet mixture for the contractor, so you will have problems trying to convince him that he is using too much water. He is not an engineer, and probably has no idea on the chemistry involved, and will try and tell you that it needs to be wet to be strong.

 

Block work seems to fly up, and then finishing takes for ever. That is because contractors like to cover up all of the little problems with a very thick skim of rendering that is unnecessary, leads to cracking, and costs a lot in materials. Don't worry about how much cement is in each block, as long as they are strong enough to handle and lay, that is enough. It doesn't matter if the joints are staggered. The strength is in the screed which should be pretty thin, less than half an inch, but without seams and of a consistent mix. 

  • Like 3
Link to post
Share on other sites

Great synopsis from 'Headed that way.'  Just to add one point: I work to a rule of thumb not to remove the formwork for 8 days after casting.  If the rebar has become rusty before use then give it a quick wire brush to remove any loose scale and/or a coat of rust killer.  As Htw says, you want good contact between the cement and the rebar.

 

If concrete is good in compression then why use rebar in columns?  The reason is twofold: a rebar cage bound with stirrups helps to resist bursting and can almost double the max weight bearing of the column, the rebar does not have to be very large for this purpose; second, the rebar resists swaying, but for this the larger the better.  (Concrete is rigid and if the column sways, it cracks laterally.)

 

If building during the dry season keep the freshly poured/laid concrete damp for the eight day period.  Sacking covering a slab helps to retain moisture.

 

Although it is true that the less water used in the mix, the stronger the concrete, it is necessary to make concrete used for casting columns quite wet otherwise it wont flow into all the recesses and air pockets will occur.

Edited by GoHuk
Link to post
Share on other sites
Headed that way

Thebob filled in the gaps quite nicely on this topic.   

 

 

Double the height (or depth, depending how you look at it) and you quadruple the strength.  Double the width and you merely double the strength.  But you have to worry about stiffness or brace it somehow with what we called coffers (think waffles) to prevent buckling.

 

I used to make concrete forms and watch the forms (save their asses by repairing the forms if they poured too quickly) on industrial jobs like power plants.  Imagine pouring 2,000 yards using cranes and huge buckets in one pour 40 feet in the air on scaffolding for a generator pad where perfection is required for the 200 ton generator to fit.  I've worked on smoke stacks for the power plants, foundations, what we called conduit which is concrete casing over 4" pvc pipes that carried the electricity around the plant to the transformers and such, cooling towers, bridges coming in to the power plants, even worked on an aborted nuclear plant but not the reactor, never got that far.

 

Pretty cool stuff this concrete and tough if you do it right.

  • Like 4
Link to post
Share on other sites
thebob

 

Pretty cool stuff this concrete and tough if you do it right.

 

Cement is an amazing adhesive, it is cheap, strong versatile and we owe most of modern hygiene to it. It has probably done more to eradicate disease than any drug ever made.

Link to post
Share on other sites
Headed that way

Ah, we have a bumper crop of concrete experts here.

 

I worked high rises too, up too.   You poured columns, set what we called pans in place and poured the floors.  Yank the pans in seven to ten days depending on the weather, put in jacks which were 4 x 4 or 4 x 6 posts using these clamping jacks.  You had a lever/wrench tool that shoved the bottom of the top post up tight while you were driving this stirrup shaped/rectangle clamp down tight and nailing it off for safety (damn laborers would jamb the concrete vibrator or get it stuck and it could back out a sixteen penny duplex nail before you could don anything about it.).   When you got done you had a forest of these posts supporting the fresh deck/ceiling and you began pouring the next story of columns. 

 

Sometimes you had scaffolding instead of the jacks and you just left the scaffolding in place to hold the fresh concrete till the 21 day curing period.  Butt cold, dangerous work or scalding hot on summer days trying to work with steel forms that were too hot to touch much less sit on while you were bolting them in place.  Madhouse of men and machines with cranes working constantly overhead.  Wasn't uncommon for some dufus to fall off the building and spatter himself.  Worst case was a dude falling fifty stories and becoming hash after hitting a set of unpoured rebars, chopped him up in about 8 to 12" squares.   I've seen grown men step backwards off a scaffold board where they were overlapped, with his foot dropping the extra inch and a half, and the dude scream like a woman thinking he was falling off the side of the building.

 

The smoke stacks were the worst, constant pour 24/7 till done.   Not a lot of men could work at once and fewer as the stack got higher and smaller diameter.   Several collapses happened on towers around the U.S while I was doing towers, green concrete collapsing cause a superintendent pushed the project too fast or the perimeter walkways collapsing, sounds like a zipper when the would go.  One snap tie breaks, doubles the load on the next, and if you are unlucky it goes pop..... pop... pop... pop..pop. pop.popoppopop!  You hear anything close to that sound and everyone stampedes to the interior scaffolding or jumps up on the form itself.

 

Glad I got out of that line of work.

 

Yup, gravity dams are built in an arch like the Hoover dam and rely on compression jamming the ends of the dams into a pocket hewed out of bedrock.  Arches don't work under tension, just compression, unless there is side load but a dam is an arch laid on its side.  Lose one of the ends and you lose the dam though.

  • Like 2
Link to post
Share on other sites
A_Simple_Man

 

gravity dams are built in an arch like the Hoover dam

 

OK, now I get it.  The Dam in question is called the Deriner thin arch dam and is likened to the Hoover Dam.  Here is a minute and a half segment from it that is quite impressive

 

http://www.youtube.com/watch?feature=player_detailpage&v=g2WWH5uGoR0

  • Like 1
Link to post
Share on other sites
JohnD

They are probably adding fibers. These can be used as a replacement for rebar.

Link to post
Share on other sites
LastManStanding

Every house I have seen being built here including mine, the rebars sit outside for a week and all of them have a coating of rust on them.  Is this a long term problem? I've never seen them clean anything either.

Link to post
Share on other sites

Every house I have seen being built here including mine, the rebars sit outside for a week and all of them have a coating of rust on them.  Is this a long term problem? I've never seen them clean anything either.

 

Probably not a problem as has been pointed out above most designs are over engineered.  Rebar should be at least 4 cm below the surface of the concrete otherwise staining (from rusted rebar) or further degredation of the iron may take place (depends upon the additives; waterproofing helps).

 

I'd be more concerned about the quality of the mix.

  • Like 1
Link to post
Share on other sites
smokey

Every house I have seen being built here including mine, the rebars sit outside for a week and all of them have a coating of rust on them.  Is this a long term problem? I've never seen them clean anything either.

it can be a problem is your still kicking 30 years from now... is that possible 

Link to post
Share on other sites
Guest
This topic is now closed to further replies.
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use, Privacy Policy and Guidelines. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue..

Capture.JPG

I Understand...