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| Posted in Construction Techniques on
I just read the Q&A in March 2007 “Reducing the risk of cracks in concrete slabs” and have a question. It says:”If the mesh isn’t cast into the center of the slab’s thickness, it loses its ability to reinforce the concrete”
I don’t think that this is correct. I believe the rebar should be 1/4 to 1/3 of the slab thickness from the bottom. The reason is because the lower part of the slab (from the center of the thickness down) is in tension when the load is on the top of the slab. Concrete is not strong in tension and the rebar is.
Am I missing something??
Replies
McDesign | |#1
I agree for rebar, but maybe mesh has a different job?
Forrest
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brownbagg | |#2
yes,and yes.
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jwork3 | |#3
could be, but why go to rebar then?
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Geoffrey | |#4
Because the re-bar is easier to keep in a fixed location while pouring the slab, mesh is more difficult to keep from being pushed to the ground.
Geoff
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jwork3 | |#5
what is the porpose of mesh??
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Geoffrey | |#6
"Reducing the risk of cracks in concrete slabs"
Geoff
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brownbagg | |#8
"Reducing the risk of cracks in concrete slabsto reduced stress cracking due to curing and to hold the concrete together with micro cracking. Does not control load factor cracking.They both do two different jobs. I myself is a firm believer of fiber..
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JoeArchitect | |#32
>>"Reducing the risk of cracks in concrete slabs" <<
In each direction.
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ibew481 | |#7
6x6x10 mesh prevents control joints from seperating after they crack.It keeps a 1/16 inch crack from becoming a 3 inch wide abyss.Wire meshserves adifferent purpose than fiber which prevents shrinkage cracking and spalling. Fiber is commonly substituted for mesh even though they play different roles. Rebar on chairs is the most accurate way of placing reinforcement in a slab. With wire you never get real accurateand consistent depth.
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Catskinner | |#9
We do this about twice a year. I ought to keep it in a Word file. <G>
Rebar and remesh do not perform the same function.
Rebar and remesh do not perform the same function.
Rebar and remesh do not perform the same function.
There, now it's a political poem by Jesse Jackson. <G>
The only purpose of remesh is to maintain aggregate interlock, which means to prevent lateral seperation equal to more than 1/2 the diameter of the large aggregate.
Like exactly what 'Bagg said.
As long as the concrete does not crack and seperate in such a way that the large aggregate comes unlocked, if all you have is a slab laying there supporting its own weight plus furniture and people, it's no big deal.
The remesh (formerly known as 6x6/10-10) acts as a tensile member to accomplish that. If you get it in the middle of the slab more or less it works great.
If you get it on the bottom of the slab it don't work so good.
But in any case, from the perspective of a structural engineer, it imparts NO shear or tensile structural properties to the slab.
From the perspective of the demolition contractor (me), the stuff works really well. <G>
Rebar, on the other hand, as 'Bagg has also pointed out, lays there and does nothing until you load the concrete in such a way that the rebar picks up the structural load (tension or the shear).
Concrete and reinforced concrete are two completely different materials.
So yes, the placement of rebar is absolutely critical for this and several other reasons.
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hasbeen | |#10
Wouldn't a tied grid of rebar also act to hold a slab from cracking and sliding apart like a calved iceberg? If so, isn't that also what the job of remesh?Not a concrete guy! Just trying to learn.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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brownbagg | |#11
two types of cracking also. stress cracking due to cure and load factor cracking due to structure failer.
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Catskinner | |#14
Good point.Let me follow up on that for the previous question.Concrete has many ways to fail if it is batched, placed, or cured wrong, or if it is uased in the wrong application.The stress during cure that 'Bagg speaks of can be at least three types that I can think of. 1) Concrete will shrink a certain amount during cure, and if the force resulting from that shrinkage exceeds the tensile strength of the material at that time (key consideration), then you get a crack no matter how much steel you have put down.There are many ways to address this. The majority of the concerns were effectively addressed by Gabe many times here. It comes down to consistency in slab thickness and cure rate, and for this uniform slab to be able to contract unrestrained. That means a smooth, flat surface under the slab.More to that story, but it's enough for now.2) Plastic shrinkage; if the slab dries faster than it cures (remember it's a chemical process) it is going to crack.3) Curling; I think this is one of the big ones that is overlooked. If the top and the bottom of the slab cure at different rates the shrinkage will not be uniform and the slab will actually curl. When gravity takes over and the stress from the curl exceeds the strength of the material at that time (remember it has not hit much strength yet, we're only talking a few days), the you get a crack as that stress is relieved.This is the source of the vapor barrier debate. You hear always and never a lot when it comes to vapor barrier placement, but the truth is that it depends upon your local climate conditions. In a humid climate a VB right under the slab helps, in a dry climate it does not. But that is a really complex digression, and if you need a VB there are always ways to do it right.As 'Bagg points out we have not even gotten to load factors yet. <G>
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brownbagg | |#18
one more point, you have two truck pouring mud. one is a six inch slump and one is a three inch slump. it will crack where the different in slump are. so concrete much be consistent..
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Catskinner | |#20
Yep, good point to bring up.Funny stuff, this concrete. It's such an important part of the building and so difficult and expensive to repair when it goes wrong.So why do we get all these #$@!& dumb#$$5& doing the work?A good concrete sub is worth a lot of money.
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dovetail97128 | |#22
same problems with one truck having warmer (temp) concrete or one having a residue from it's last pour left in it when they fill it to bring out to your job. (especially if the last pour happened to have accelerator in it)
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Catskinner | |#12
<<Wouldn't a tied grid of rebar also act to hold a slab from cracking and sliding apart like a calved iceberg? If so, isn't that also what the job of remesh?>>To some extent, yes, a properly installed and properly sized rebar grid will prevent gross displacement, but compare the 6-inch grid of remesh to the 16" grid of a tied mat and you will see a different behavior. The remesh is 10-gauge (typically) and the rebar is 1/2-inch (typically). This is going to make the concrete behave differently, and when it comes to steel, more is not always better, and after a certain point, more is definitely worse.The best answer I can give you is I have seen slabs where remesh should have been used but rebar was substituted and the result was awful. The cracks just follwed the pattern of the rebar.<<Not a concrete guy! Just trying to learn.>>Fair enough, and good questions.<<"Doubt is not a pleasant condition, but certainty is absurd." >>BTW, great quote, and to me, a compelling arqument for a certain degree of humility. <G>
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hasbeen | |#15
Well, I guess I'll have to wait and see, since I put rebar in my shop and basem*nt 4" slabs. We did a 16" grid of #3-40. It's been about 2 1/2 years and there are no cracks at all so far. No fiber in it.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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Catskinner | |#16
See AlsoThe Science Behind Concrete and Why it Matters for ProsWhy Concrete Is So Much Stronger With Just a Little Bit of RebarWhat is the difference between over reinforced beam and under reinforced beams ?What to Do if Concrete Cylinder Test Fails at 28 Days? - The ConstructorSounds like you did a good job.
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hasbeen | |#23
One more question, if you don't mind. You said after a point more steel doesn't help the strength of the wall. My inspector said he thought I'd wasted the steel and didn't need but half what I put in. Now I'm wondering if I screwed up. (At least I bought the steel right before the price started to climb too much.)I formed and poured my own foundation wall and footing. We have two lengths of #4-60 in the footings with verticle #4-60 every 16" and horizontal #4-60 every 16". The wall is about 5' high at the highest. The wall is 8" thick. The bar in the wall is as close to center as we could keep it. It's been over three years and we have no cracks in the walls.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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Catskinner | |#24
I don't mind a bit. <G>I don't know what seismic zone you are in, but generally speaking it really does sound like you just did a good and professional job.That's exactly how I would have done it if it were my garage.
Edited 1/23/2007 5:08 pm by Catskinner
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hasbeen | |#25
Thanks for your answer.Not much seismic action here, but we do have expansive soils.It was a bit humorous dealing with my town building inspector. He told me I didn't need to have the foundation engineered since I was one who was going to live there! I love small towns.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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Catskinner | |#27
<<Thanks for your answer.>>You're welcome. Concrete and dirt are about the only topics around here that my opinion might be worth much more than a beer f#rt, so on the rare occasions I can give something back I'm happy to.I learn a lot more on Breaktime than I could ever teach, that's for sure.<<Not much seismic action here, but we do have expansive soils.>>An ounce or prevention is worth 50 or 60 pounds of cure when it comes to expansive soils. I have a love/hate relationship with expansive clay. <G><<It was a bit humorous dealing with my town building inspector. He told me I didn't need to have the foundation engineered since I was one who was going to live there! I love small towns.>>I hear you. Me too.
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brownbagg | |#26
rebar does nothing till you put a load factor on it, doing the calculation it was proven that 2 #4 is fine but you put in 7#4 then that would be a waste of 5 bars of rebar.Rebar in patio, does nothing, no load factor except if you want a hot tub. rebar in sidewalk does nothing, rebar in curb does nothing except the impact of a car tire, so its needed only once in a whileand rally rebar does nothing in footers of stick frame houses, single story but code requires it..
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hasbeen | |#28
So I probably wasted rebar and labor. Oh well.Doesn't it matter that I have expansive soils? I thought the rebar was protection from occasional high lateral pressure that our foundation may experience.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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segundo | |#29
i wouldn't say it is a waste, and i want to point out that while catskinner may be correct in his statement that at some point the rebar does more harm than good, (or was it no additional good?) i have worked on many structures that had way more rebar than that, and it was in areas with very expansive soils and seismic zone 4
i am not the engineer, and i can't make the call that the engineers are wrong in how much rebar, what size and location. while some of it does seem like overkill for the most part its there for a reason.
if i were building my walls i would try my best to get the reinforcing steel exactly 2" from the finished surface of the concrete, and i would try to do the same with a slab or a footer. i have had people tell me many times that it is more than required to have that much rebar in the crete, and it may well be a waste of rebar but....if you are building in extremely expansive soils that amount of rebar prevents the cracks.
one of the statements i have heard repeated on many jobsites over the years (although it probably doesn't pertain to a demo contractor) "it isnt a mistake until the concrete's poured". the definition means while nobody wants to re-frame a wall, or the rafters that could be done, once the concrete is poured you are not going to redo that unless the caltrans inspector makes you!
i am curious how many have demo'ed a pre-stressed or post stressed structural concrete member? let me tell you from personal experience that breaking of a pre-stressed concrete pile is hard. they don't break when the pile driver hammer hits them, and when they are down as far as they will go you have to cut them! same with a post tensioned box section reinforced structure, (overpass or bridge) i was with the shoring crew that demo'ed the elevated freeway on the embarcadero after loma prieta and we had to go to extreme lengths to get a little of it to break at a time. we didn't want it to all fall at once, so even though it didn't surviveto carry traffic, it was still tough to break apart.
so while it may be a waste to reinforce my footers as though it were a structural grade beam, i know it won't break if expansive soils move and if structure settles it can be jacked (or pressure grouted) back to original grade. i grew up in coastal california with terrible clay on a very steep hill. almost all of the foundations on the buildings in my small home town were failing when i wasyoung. it was almost impossible in that application to overdo the reinforcing steel.
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hasbeen | |#30
Thanks for your input.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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Catskinner | |#33
Good points.'Bagg is assuming a properly prepared subgrade on suitable soils in his comments about what rebar does in a slab. He works on sand a lot, I don't know if they have seen expansive clay there in a few million years. <G>In this case, my implication was "no additional good". Certainly you can put enough in to "do more harm than good" but I don't think anyone is suggesting that here. That's a LOT of rebar.<G>Your point about expansive soil is correct of course and well taken.Questionable or known-bad subgrade conditions changes everything.Now we are not talking about cracking due to shrinkage or curl, we are talking about a structural failure.In this case, yes, properly specified and placed rebar is one of several good answers.I have used #4 16"ocew in monopours on soils that put the geotechnical engineers into a nervous twitch and it worked. <G> We're talking 80% pasing a #200 sieve.On really bad soils I have worked on jobs where the floor slab ended up being 9 inches to 12 inches thick with two #5 mats 12"ocew and #6 bar in the turndowns.As you pointed out, it all depends upon what the job requires.
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brownbagg | |#35
He works on sand a lot, I don't know if they have seen expansive clay there in a few million years. <G>I have seen it one time in 23 years and it moved a slab six inches east. I had no idea what it was. It was about 200 miles north of here.It was an elementry school slab, place in two slab, we had a heavy rain and it slide the whole slabe over six inches but let the other one alone..
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Catskinner | |#36
Weird stuff, isn't it?Scary, actually. You just never know what or when or how much it's gonna make a mess.
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jrnbj | |#48
"I have used #4 16"ocew" I'm guessing OCEW means "on center each way"....is that a standard architectural abbreviation?"We're talking 80% pasing a #200 sieve."Would that be describing a fine dry silt soil?Just trying to keep on learning, so thanks for your time....
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brownbagg | |#49
We're talking 80% pasing a #200 sieve."that a very silty clay, almost useless for constuction, maybe landfill liner.
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Catskinner | |#51
And really tough on lungs and air filters too.
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Catskinner | |#50
You guessed correctly on both counts.The #4 bar 16"ocew was turned down into a fairly wide monopour footing with enough #5 in it to function as a big raft to float on this bubble of overexcavated and replaced really bad dirt. So in this case the slab really is structural, but only in a limited sense. As for the soil, that means scary bad. A #200 sieve pretty much passes cake flour. Well, not that bad, but almost. This stuff was a lot like working in pure dust.Normally you want well-graded soil, which means a mixture from coarse to fine. The client could not afford to pull 400+ cy of dirt out and replace it with import so we took the time and care to blend it right to optimal moisture content and recompact it. That was not easy. It took several differnt pieces of compaction before we could find a harmonic that would work.Don't even get me going on the rudiments of compaction . . . 'Bagg is probably the only other one here who cares anyway. <G>This is not something you want to do for most clients but these were folks I knew I could trust. You can build on just about anything if you know how and can afford it, but some sites are better than others.
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SamT | |#52
I care.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and gettheir money. JDRHI<!----><!----> 84310.51
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Catskinner | |#53
<G>
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jrnbj | |#54
Thanks....
Another question....does all fine soil like that turn into clay when it gets wet....i.e. do silts have different wetting properties?
Most peoples' mental image of clay is like pottery clay....and I would think most fine silts (at least in a building site) would be in a climate where they don't often get saturated? -
dovetail97128 | |#55
At least here in western Ore. we have enough clay to export some if you need it and yes it gets plenty wet for 7 months out of the year.
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Catskinner | |#56
Good question, long answer.The reason why clay is clay instead of something else is about the size of the particles, the shape of the particle, and the chemical composition.Clay is finer than silt, and if you looked closely the particles would be somewhat flattened (unlike silt), and the particles would have a polar electrostatic (maybe you could say ionic, I don't know) charge.The reason is that clay is made out of fractured crystals of rock. When those crystals break, they break in layers, hence the polarity. Water molecules are also polar (105 degree bond angle between those two hydrogens), so the behavior of water with clay is not like that of water with silt or sand.To complicate matters, once the clay goes below a certain moisture content (less than about 6% to 8% in my experience) it becomes hydrophobic and rejects water unless you mechanically work it in.Even worse, below about 50 degrees F ambient, the clay will not give the water back very easily -- again a chemical trait -- and you can have a mess that is difficult to correct.Any farmers here may well be familiar with this -- certain types of clay do not help with watering crops in cool weather.Anyway, clay can be a real problem. Some clays are expansive, some dramatically so. Changes in the moisture level can change the elevation of your house, and not uniformly.Some amount of clay is desirable in structural fill for certain tasks, but not much. Just enough to produce cohesion. Slightly more is good for unpaved roads for the same reason, but again not much.Silt particles are bigger than clay but smaller than sand and not shaped the same, nor are they polar. Silt can also be difficult to wet, and turns to snot when it is over-wetted, which is easy to do. Achieving optimal moisture content for compaction can be a little tricky because it goes from too dry to too wet very quickly, and sometimes it's hard to recognize this until you hit it, and then it's too late. Something about the finely divided particles and they way they accept moisture. My guess is it's about the amount of surface area relative to the volume of each particle and the relative size of voids between the particles in a poorly graded material.By poorly graded I mean that most of the particles are pretty much the same size, which is real trouble from a geotechnical perspective if we are in silt or clay. Not so much in sand, but still not ideal.Well-graded soils will typically exhibit better mechanical properties (think about how concrete is made and why).All soils except organics will have a theoretical maximum density (as measured against a reference like a Standard Proctor) at what is termed "optimal moisture content."That moisture curve is one of the keys to compaction, the other are frequency, amplitude, and experience.If you are interested, I'll continue.
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brownbagg | |#57
If you are interested, I'll continue.please dont.
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Catskinner | |#58
<LOL>You can't get away from it, amigo, it's following you everywhere.
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dovetail97128 | |#59
Please do continue.
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Catskinner | |#61
See AlsoIs Rust a Problem on Rebar?OK. Another installment of The Dirt Chronicles. <G>The are several concerns with clay, silt, and the poorly-graded soils that result from too much of either or both.Houses don't actually weigh that much, and with an appropriately sized footing, a very weak but otherwise stable soil can still be made to work if it is properly compacted and the moisture content is maintained. That is sometimes easier said than done. Any how you cut it, it's not the best of all possible worlds. Typically if soils are questionable, you'll want to amend them by adding suitable material. If they are REALLY bad you might want to take it all out to a certain depth under the bottom of footing and replace it all. I have had to take anywhere from nothing to 7 feet under the footing and replace it with structural fill depending upon the site.So when the geotech guys come out they are going to take a bucket of dirt back to the shop. Some of that dirt will be run through a set of sieves, and they will produce a report that says what percentage of dirt passes which sieve. On the basis of that report you can learn a lot about what you are actually dealing with and make some predictions about the behaviour of the dirt.They will also take some of the dirt and mix up a few different samples at different moisture contents and hit it with a specified standard force and measure the density. It's called a Proctor test. At a certain moisture content (optimal) you will get the best densities. A skilled geotech can get you the optimal moisture pretty quickly. The lab will also provide a moisture curve, which will also tell you a lot about how the dirt will act as you try to compact it.Compaction is measured by a percentage of theoretical density. That is not really the absolute max density, if you hit it harder (up to a certain point), it gets denser. There is a Modified Proctor, which basically means you are hitting it harder (again, measured to a standard). A Standard Proctor is good for homebuilding, you will see a Modified Proctor for a critical application.In general, 90% of optimal density (Standard Proctor) meets code. Architects are usually going to ask for 95% or greater. Partly CYA. partly out of a concern for settlement.By the way, someday I want to talk with an engineer who has a sound understanding of how compaction force, density, bearing competence, and shrinkage are related. My experience on site says it's probably a relatively complex problem that involves log curves.Any engineers reading this?For example, trees will grow easily in soils that are at 85%. 90% passes the criteria of the 1997 Uniform Building Code. 95% makes architects sleep well at night. 100% is not that hard to hit with some soils,very difficult with others. I have hit 104% when we get the harmonics right. The compaction effort is not proportional. But then some soils go right to 92% and it then takes tremendous effort to hit 97%. Anyway, now we know how dense the material will be at what moisture content, and we can make some sort of general or rough predictions about the strength of the soil.You will also get a note on the report about Atteburg limits.(Mr. Brownbagg just left the room spitting and cussing and mumbling unrepeatables. He hates Dr. Atteburg and his entire family, distant relatives included. <G>)You will see something called the liquid limit, the plastic limit, and the plasticity index, which is the arithmatic differnce. Sometimes they leave the PL out because you can infer that.If you take dry dirt and start adding water, at some point it will start acting like a plastic if it has the ability to do so (some dirt is non-plastic). An example is pick up some silty or clayey dirt and add water until you can roll it out like a worm. See how long and how thin of a worm you can roll. You might as well, because your geotech guy will do the same, and he is going to charge you for it. <G>It's more technical than that, and there is a procedure to follow, but at some moisture content the lab will say that this is plastic. Then they will keep adding water and again, against a standard, it will behave like a liquid.The resulting plasticity index will tell you a real lot about the soil, including its permeability to liquid, its probability of having an expansive potential, and you can make some intelligent guesses about the shear strength.The engineers I work with seem to like a PI of about 15 or slightly lower for over-ex and replace jobs. That keeps the water from passing through to lower strata which may be unpredictable, it gives you enough cohesion (yet another topic) to be easily workable, minimizes the expansive potential, and provides good mechanical or structural properties. But that is not printed in gold. As 'Bagg has pointed out many times and correctly, pure sand (non-plastic material) works great for certain applications.So now we know something about the material in question. We have a theoretical density, a good understanding of the gradation (has much to do with shear strength), a moisture curve, and the Atteburg limits.So now we can look at this report and decide if the dirt is suitable for the intended purpose, or if it needs to be modified (and if so, how) or rejected.Once we decide that, we can then make some decisions about how to actually attack the job.
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brownbagg | |#62
quit talking, you're making me sad...........
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Catskinner | |#63
Maybe I can make it up to you . . . how about you and I get good and drunk and hop in the pickup and go over to ol' Doc Atteburg's house and cut doughnuts all over his lawn and run over a buch of stuff and throw beer bottles everywhere and shoot all the windows out?
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brownbagg | |#64
well he will proberly come running out and make me classified the soil we dug up..
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Catskinner | |#65
<LMAO!>
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dovetail97128 | |#66
Thanks,
I have had two sites in the last few yrs that were dramaticlly different. One all clay, sub irragated, the other enough rock we had to blast, hidden springs , mixed with "severely weathered basalt" (severely meaning it crumbled when you stepped on it).
I spent a lot of time with Geo techs and Soils Engineers on both of them . Learned a lot. Just got me more interested in whats to learn. -
Catskinner | |#67
<<I spent a lot of time with Geo techs and Soils Engineers on both of them . Learned a lot. Just got me more interested in whats to learn.>>Well then, let's hear it. <G> I'd be most interested in your experiences on these sites.I know what you mean. About the time you think "so what , dig a hole, build a foundation" you see something new. It really is interesting.As land values increase, we are building on sites that would have been ignored just a few years ago. As personal wealth increases, we have the ability to do so. As the discipline of architecture matures, we see architects trying to give expression to ideas that would not have been taken seriously just a few years ago.It's a wonderful and exciting time to be a builder.If I could invent my dream job it would be to work as a part of a design-build team that did all of the architecture, most of the engineering, and all of the construction in-house. A team with some vision and enough fortitude to push the edges of the discipline a little bit.But now, as a sub, mostly I spend my days educating architects, negotiating with clients, and fighting with general contractors about getting these cartoons stood up in 3-D in such a way that nobody gets sued or hurt and the budget does not get blown out to Neptune or beyond.I think that the real problem is simply that these considerations are not addressed in the schematic design phase. Ideas don't cost anything to produce and very little to correct. Structural deficiencies do.And it all starts with the dirt and the concrete, which I really enjoy.
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dovetail97128 | |#68
Perhaps I will .
For now I will agree with your dream job. In a sense I have been able to do some of that. I would love to have the opportunity to do more.
Recent years have been good to me in that I was able to transfer my experience as a true custom home builder into being a site super on several fairly large custom wineries.
Baptism by fire into the world of custom commercial/industrial construction. I am afraid that it would bore readers here to read the deatils of some of the oversights and screw ups and bad planning that occured.
One minor one was having to place a set of water tanks under ground on the upslope of the building on 25% sloped site that had required blasting to set the building . Building was nearing completion when size of tanks was finally determined. Shop drawings checked by all, crane arranged to be on site day after tank delivery., 14' x 14' x 65' hole pecked into weathered basalt.
Tanks arrive late and are unloaded at 4 pm. Super for the excavator and I are looking at things and discover from paper work that the tanks weigh twice what we had been told, scramble to get the largest crane we can for delivery the next morning at 7 am. New crane will require half again more room to set up than we had alloted on the site
Re- check tanks for diameter and length and discover that one of them is 8' longer than spec'd. The hole is now 8'-10' too short with daylight fading and excavators crew gone for the day.
5 am. excavators crew arrives, starts digging out hole , we can only dig out some from each end to enlarge the hole. Place bedding , grade and compact and then have to take the track hoe off site and re-enter site from the other side because of lack of room to move it between the building and the hole.
I am on the phone meantime with soils engineers, soils techs and structural engineer about the siting of the tank in relation to the lateral loads being placed on the 30' tall concrete walls of the structure from the tanks new location. We agree that we have about 1' to spare in that regards.
Get together with excavator and check all dimension, 100 ton arrives and sets up. #1 Tanks unloaded #3 Tanks being placed
#2 Hole at end of day one #4 Tanks being placed
#5 Tanks set , ready for connections. Problem solved by noon. All in a days work .. right? -
Catskinner | |#69
Nice work. I see you guys even have the OSHA-mandated ladder.What size was that Hitachi?Yep, some days be that way, as hard as we may try to avoid them. I've had a few tank-and-crane days like that myself. <G>
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dovetail97128 | |#71
I couldn't tell you. The machine belonged to the excavators. I am a nail banger by training so when I talk about the size of machines like that it is in terms of smaller and bigger.. depends on where I need them to get into and what I need them to do . ;-)
My excavators shake their heads and laugh at me . -
Catskinner | |#73
That's how I am with anything that floats. I worked in a Naval shipyard for a while and they all looked like BGBs or LGBs to me. <G>
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dovetail97128 | |#74
Here some pics of the site during and after excavation, blasting and prep for the footings. Took out somewhere near 23,000 cu. yds.
Notice the compacted fill where the footings are to rest.
We ended up with the blast going to deep, shattering and disturbing the "native soil" so we had to dig out trenches and then replace with engineered fill.
One thing I learned is that you will never ever get the blaster to commit to just how deep the shock wave will effect.!! In the end I was the one who had to make the call for drilling depth because no one else would .
Engineer explained to me that the Proctor testing equiptment was useless on the stuff we had. He stayed on site the entire time we prepped the footing areas overseeing the digging, placement and compaction of the trenches.
You mentioned OSHA.. I had to go around the site every day and test the walls of the excavation with a Pocket Vane Shear Tester and log the results.... Useless , but if I didn't I wasn't meeting OSHA requirements and they could have nailed me. Osha did however allow us to not "bench" back the excavation as long as I did perform the test daily and ensured the bank netting was maintained .
So we were able to build 28' tall CIP walls within 4' of the cut bank.Edited 1/30/2007 8:32 pm ET by dovetail97128
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Catskinner | |#75
That's a really good looking site and a nice job from what I can see.How much delay was the blaster using?What did you use for backfill?Man, that is a boatload of material you took out of there. What did you do with it?
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dovetail97128 | |#76
I don't know about the delay. It was set off using a magneto or power pack from about 1/2 mile away. We had one of the states busiest secondary highways about 800 ft. down slope from the site so I had to co-ordinate a "rolling shut down " with the state for the blast.
Our hope was to get sheer walls good enough to "gunnite" but the rock just wasn't anywhere near good enough for that .
Back fill under the building was done with 3/4 crushed for the over excavated areas, the balance we used as much of the blasted rock as possible.
Most of the back fill on the high side was done with 3/8/in. pea gravel shot into the void with a conveyer truck. If I remember correctly it was in the amount of 800 yds. The entire hill above the site is fractured weathered basalt so we had a huge drainage problem to deal with.
One benifit of the deep blast was that we opened up the rock enough that water free flows right out under the building. All we needed for PSF bearing was 2500 PSF.
Excavated material from blast and site prep was trucked 1/4 mile away into an old quarry at no charge for the use of the pit(right across the road from my site.) Dumped there, the pits owners then brought in a portable crusher and crushed it and sold it. Lot of dirt in it though.
I priced out having it crushed on site for us but couldn't make it cost effective and had almost zero room on this site for storage etc.
( I had enforce trades people carpooling to the site to save on parking spaces.)
Part of the deal with the rock was the supplier to us for all the rock we needed was the owner of the pit. Worked out for both of us.
Bad deal was the first excavating co.
They gave us a bid for $5 per yd. My bosses assumed that meant bank yards, as that is the standard around here. They claimed after the work was done it was for truck yards. Ended up in mediation and they won. Differance between bank and truck was almost 13,000 yds. I had a lot of trouble with them . Very happy with the replacement company though who did the tanks and finished up the job for us.
Couple of shots of the area after the blast. -
Catskinner | |#77
Great photos, tough site, and some really skillfull project management/supervision/coordination.Was blasting cheaper than ripping with a dozer? Or was that not even an option?
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dovetail97128 | |#78
Thank You.
I "lived" on that site from 6 am til past dark every day for close to 14 months to meet the deadline to have it operating. Great subs for the most part. My routine is to go around the site every day first thing and ask what everybody needs that day to make their life easier.
I work hard to build a team out of the people working under me. In return for what ever I can do for them I get the co-operation I need without having to be an ####****
One of the lessons was about the soils report and ripping vs. blasting.
The originial geo tech report contained this phrase concerning the test pits
" Used a track hoe and dug to refusal"
Assumption was made by my bosses that the "Hoe" was a fair sized machine. Turned out after we learn what we are really dealing with that it was a tiny yard excavator. So we pursued blasting , all the arrangments were made, top soils stripped and stock piled. I arranged for a blasting consultant with 40 yrs. experience to come out and assess the situation, lay out the shot for us. Quarry right across the county road from us etc.. He was non commital , telling us that we may have hard rock , maybe not.
So we bring in the blasters and rock drilling rig then after the drillers get a major portion drilled they tell us that we have very uneven rock formation, maybe we could rip the rock to remove it. The original excavator didn't have equipment big enough so he demurred on the ripping and claimed that it was unrippable.(I found out about the lack of equipment later)
Once again the buck stops with me ... I decide it is not worth turning back , blast it excavtion will go faster . If we tried the ripping and hit stuff that needed blasting we would lose time we didn't have at that point so blast .
I think I made the right call because even with the blasting we ended up in some small areas with rock that ended up literally having to be scracthed out the last few inches to the grade we needed. Hard to say though.
I had to move septic tanks, holding tanks , water tanks etc to avoid the hard rock areas and fit it all togeher.
What can I say , I love doing these challanges. Just crazy I guess;-) -
Catskinner | |#81
We need more supers like you in the world.You made the right call under the circ*mstances. If I can make a respectful suggestion here (you probably already thought of this, but just in case not) it's worth talking with the excavation sub about his exact plan of attack. That really wasn't on you in this case, the geotechnical engineer should have known better.But as you now know, "refusal" is a pretty relative term.If you run into this again on an equally large scale, you might also talk with the geotechnical engineers or the local Caterpillar dealership about acoustic testing. They can measure how fast sound moves through the rock and tell you if it is rippable.The reason why I mention this is I have been on sites where the plumber's Case 580 backhoe did nothing but make screeching noises and my D-4G ate the rock up like we'd go through a pizza. It's amazing what a modern small dozer will do on a residential site.
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dovetail97128 | |#87
Catskinner,
To be fair to the originial excavator the plan of attack that was given to him for estimating purposes was one in which blasting may occur. It wasn't until after we discovered the uneven rock formations that he was asked about ripping. Things were complicated by some "Good Old Boy" network stuff as well.
The originial Geo Tech firm was fired for their lack of clarity in their initial report, unfortunatly they probably did what was asked of them by the property owner who, I would bet, was unwilling to spend a lot of money on the report. Another lesson for me as super, even if I don't make the big money contract decisions it behooves me to spend time on the phone and in person checking everything I can out.
I had never heard of the acoustical testing before this, I will remember it for sure . Thank You.As I said when I asked that you continue your discourse on site work I have a lot to learn and will never learn it all. Been bored silly lately because I have no big project going , everything is in the "planning / financing stages".
Here are some pics of the finished project from the low side.
Sorry about the size of them. -
icektruth | |#85
I did a lot of it in the Army. We typically used explosives though. Prestressed is tough stuff but if we could get the right pop at the right point, that 'prestressing' and gravity would start doing a strangedance together.
Rebaris amazing to me. I have seen reinforced concrete structures that literally blown into chuncks of concrete held together by a web of rebar.
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Pierre1 | |#31
Around here, foundations in expansive soils require an engineeredgrid such as you used. Bar size and spacing may vary depending on site conditions.
What many don't do well is the bar overlaps, and the use of hooks at footings, or bends in corners. Inspectors don't say nothin'. Go figure.
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hasbeen | |#38
Well, inspector or not, I did do the things you mentioned so at least I didn't cause that sort of potential failure.
"Doubt is not a pleasant condition, but certainty is absurd."
~ Voltaire
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JoeArchitect | |#34
The most critical part of the wire mesh is standing over the concrete guys pouring the slab and making sure they raise the mesh with the hook immediately when the concret is poured. Otherwise, it's at the bottom and is just a waste of money.
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Catskinner | |#37
That is a fact.I have annoyed the concrete crew beyond polite repartee with that one. <G>As it happens, some years I remove more concrete that I place, and as the guy on the track hoe, I can affirm that your point cannot be understated.I love seeing wire on the bottom when I'm the guy doing the demolition. <G>
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fingersandtoes | |#39
I thought the mesh was placed to give me a non-slip walking surface on the vb while I was pouring. Won't do me any good if someone keeps digging their rake in and putting it mid-slab.
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Devin | |#40
Abit on an aside but as we were pouring a basem*nt slab this past summer, the guy building next to me came over - he's an older man - and he suggested that I put down 6 mil plastic over the pour to create a greenhouse effect over the slab. As I had recieved good advice from him in the past I did it. I believe his idea was to keep the concrete wet while it cured. There were some folks on the job (mainly a mason) that said it was a waste of money and that concrete always crack (around here the joke is you know two things about concrete - it's grey and it cracks) - anyway I was wondering if some of the more enlightened folks on this thread would care to share their thoughts, in the spirit of shared enlightenment, about the 6 mil plastic over a freshly poured slab - btw I live in CT.
Devin
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brownbagg | |#41
its code down here, cover with plastic for seven days with a springler hose under the plastic. Highway dept use Burlap..
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segundo | |#43
one contractor i worked foron highway jobs used old rugs to cover the pour and then wet them down, freakin smelled terrible, no no not sweat PP i bet!
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brownbagg | |#42
..
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dovetail97128 | |#44
The object is to keep evaporation to an absolute minimum, aids the cure.
Plastic does work , As do commercially produced "cures" that are sprayed or roller applied, watering , and almost anything that will keep the top of the slab wet to damp for a minimum of 7 (but I like 21) days.
We always used to roll out 15# felt just as soon as the slab could be walked on , tape the seams and build with the felt in place. We would pull it up after drywall and painting was finished. Slabs were still damp and "green " when we did pull it up. -
Snort | |#45
Rebar is not required in most residential footings with a framed floor in NC. Thing is, everybody specs it, and they spec #5. I don't know any builders that don't use it, it's considered good building practice.Interesting...would rebar help if footings are dug too deeply in places, and then filled back to save on crete?<> "But to be honest some folks here have been pushing the envelope quite a bit with their unnecessary use if swear words. They just put a character in to replace a letter. But everyone knows what they're saying." Sancho
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Catskinner | |#46
Anything that keeps the slab moist for the first week (assuming temps above freezing) is a good thing except if you are using colored concrete or applying stain.That 6-mil plastic will leave a visible pattern forever.
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tuolumne | |#47
A wet cure will helpmitigate curling for one. It is good to prevent the surface from drying too fast. In colder weather, cover will also keep in the heat of hydration and keep the slab warm. Too warm is not good, so in the summer the cover should be removed and the slab constantly rewetted. We specify wet cure on all commercial projects. If you do a lot of slabs you may want to invest in a "synthetic burlap" type product. It comes in wide rolls and can be reused.
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semipro | |#60
Devin:
Your neighbor is right. Concrete needs to be keep wet as it sets. Concrete does not "dry" as many say, it is the chemical hydration reaction that makes concrete set. For this reaction to work adequate water must be available to the concrete. Inadequate water during setting will result in a decreased strength of the concrete, often seen later as "spalling" or "dusting" as the inadequately-hydrated concrete pulls away from the rest of the slab.
All:
However, I'm surprised that no one has mentioned(sorry if I missed it) that too much water in the concrete mix canseverely reduce its strength and result in cracking.Water-to-cement ratio is critical for strong, dimensionally-stable concrete.
I buildslabs with proper surface prep, rebar, plastic fiber, air entrainment admixture added, anda poly cover for at least a week.Expansion and contraction joints are also added. This may seem like overkill to some but I've yet to get substantial cracking other than very small surface cracks in my slabs and cracksat expansion joints(where they belong).
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cutawooda | |#70
Ok while we are discussing concrete, maybe you can answera question that has puzzled me for a while: We built a brnd new house and within 6 months we developed our first of 5 brick cracks. Slab foundation..5/8 rebar in the beams. What causes this? There is no cracks in the slab. Has the house sheered? I can understand cracking bricks if the slab dipped but this is not the case.
what is your take.
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Catskinner | |#72
Really hard to say.Are we talking about a brick veneer or a structural wall?Are the bricks themselves breaking or the mortar joints?How do the breaks relate to each other visually?Any observable pattern?How big are the cracks?Any change in the size of the cracks?
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cutawooda | |#79
stair stepped. Some right through the brick. ON the opposite side of that corner there is a horizontal crack in the mortar joint.
there is one area that really stumps me: the fireplce hearth. 2 rows of brick placed on the concrete slab. A crack from the front to the back all the way into the firebrick. Hasnt moved in 2 years. I know this is all settling issues. Red clay soil. Concrete man didn't compact the fil properly. But still,..these are things that stump me. Is it sheering or slab failure..one may never know. I find it hardto believe it is sheering, with all the osb attached. I will send you some pic today so you can see for yourself..it is snowing here so I have nothing better to do.
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Catskinner | |#82
Sure, post a photo and let's have a look.Clay is scary stuff.
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cutawooda | |#88
heres are some pics...should swept my fireplace but I am lazy. The third brick pic is horizontal and goes for 5 bricks. Hasn't movedin a while. I sloped the earth away from the house as best I could and put up gutters...seemed to have helped..or maybe the settling stopped. Someone once told me concrete slabs can bend...is this true?
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Catskinner | |#89
Interesting photos. I'm going to go back and look at them again, but my first reaction is to say that you probably don't have much of a problem there if the crack is not getting any wider of longer and the building is not moving (two seperate considerations).There are catalogs that sell inspection tools for the building trades, one of the things in the catalog is a strain gauge for just such an occasion. As I recall they are not expensive. Next time one comes across the desk I'll try to remember to post the website.You could probably just as well glue a small piece of glass over the crack with a few dabs of a hardening (non-elastic) glue and get the same result.I think "bend" might be overstating it, but every building component does have some acceptable level of deflection, and something took place to exceed that for the brick, which may not necessarily have exceeded that for the concrete. In other words, I am not yet convinced that you have a structural problem there.What do you know about the soil conditions under the foundation?
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jvhannah | |#90
Concrete can bend, flex(almost the same thing), creep, sag, shrink, and crack sometimes all at once. There are plastic cracks which happen during initial curing, there are creep and shrinkage cracks which happen later in curing and finally there are shear, bending or flexural cracks which can happen from self-weight or overloading.
But remember Concrete is a brittle material even with rebar or wwf in it.
Jim
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tuolumne | |#13
A slab will crack whether there'smesh in it or not. For a slab on grade this is a result of drying shrinkage, temperature changes and settlement. The purpose of the mesh in a slab on grade is not to reinforce a slabs tension zone as with reinforced concrete,but to control the width of the cracks. Mesh should acutally be specified in the top third of the slab (with a minium 3/4" cover). It is difficult to support it there. Usually the middle of the slab is a good compromise. At the bottom of the slab it cannot do its job and therefore becomesa good waste of money.
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BenM | |#17
Generally, you're right, it should be in the bottom third if the bottom of the slab is in tension. However, for a slab in contact with the ground it is good practice to have 3 inches of cover over the bar to prevent corrosion. And since a typical slab is usuallyno more than 6 inches thick that puts the rebar in the center.
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CStanford | |#19
Concrete great in compression, weak in tension, therefore mesh and rebar?
I'm dumb though and a wannabe.
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Catskinner | |#21
I know this was addressed to someone else, but the point bears repeating;concrete and reinforced concrete are two completely different materials.That's where a lot of the confusion comes from.Concrete is great in compression, not much in shear or tension.Reinforced concrete, with the proper selection and placement of steel is designed to address that.Yes, the steel provides the tensile strength.Think about a truss and how the top chord, bottom chord, and webs act and this will all be clear.The illustration I used (when I needed such a thing) was to take a grout sponge, draw vertical lines on the edge of it with a magic marker, and the deflect the sponge as if it were a beam at work. The lines illustrate the stress/strain pattern.
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CapnMac | |#80
I believe the rebar should be 1/4 to 1/3 of the slab thickness from the bottom.
Depends on the slab, too.
If we presume a simple slab with no turn-down, or mat-edge, then, really, with WWM, it "ought" to be dead center. Huh? Well, loads on a slab com in two directions, from above and below. Ah-ha! So, for a driveway, the bottom third will make more sense. For a "rat slab" almost at the bottom is also good. Except that most WWM is useless--but I'm biased.
Occupational hazard of my occupation not being around (sorry Bubba)
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Danno | |#83
I am studying (finally) for the builder's license test and was surprised that the answer to where to place reinforcement bar was in the middle third of the footing! Seems like in the middle is where there is no stress in terms of tension or compression, so what good would it do there? Seems to me that you'd want it in the lower third where thefooting is in tension.
Edited 1/31/2007 11:08 am ET by Danno
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SamT | |#84
Seems to me that you'd want it in the lower third where thefooting is in tension.
Well . . . It not under any stress except vertical compression at all points until there is a soil failure.
When a soil disturbance occurs it could be from settling or from expansion. In either case, there will bea shear point on each side of the disturbance, but in which direction, up or down?
You also can't predetermine whether the disturbance wiil be putting the bottom, or top, of the footing in compression or tension.
The best compromise is dead center.SamT
Guys that don't do things correctly the first time.....then argue that they did nothing wrong.....if made to agree to fix the problem, rarely put the time and effort into truely doing it properly. they'll just look for the quickest fix to appease you and gettheir money. JDRHI<!----><!----> 84310.51
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Danno | |#86
Aha! Makes sense. Thanks for the answer!
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