View Full Version : Rebar vs. Wire mesh and fiber in concrete
02-18-2001, 10:53 AM
I would like some input on what some of you like to use in your concrete. I use #3 rebar on most all my 4" slab work on 2'to 3'o.c. The inspectors here like to use wire mesh and fiber. In my experience, it is useless. I have tore out numerous slabs containing the mesh and fiber because of settlement and cracking. I know concrete is going to crack, but the rebar I have used has stopped the settltment and stopped the cracks from widening or shifting to where one side is higher than the other.
02-18-2001, 02:03 PM
Boy, if wire mesh doesn't work on a slab I think you may have a sub-base and compaction problem if it takes #3 rebar to get the job done.
02-18-2001, 02:27 PM
It works for me,if you dont pull it up when you pour the concrete,then it will be t the bottom,but if its in the midle like its supposed to be,it works well.Im sure #3 rebar is ging to help,but I still use wire mesh on every pour i do,which isnt many,I think ive poured about 15 slabs in last 5 yrs-,but all of them are doing fine.Are you using 4000 PSI mix? I find it works much better and is easier to get the disired finish with 4000 mix.
On a 4" slab, I would never use rebar, unless of course there is heavy traffic on it. Then I would think about making then slab thicker to put rebar in. I say that because, and I have no evidence on this but, I think there is not enough crete in a 4" slab when the rebar is encased. To be more clear, 4" thick, rebar about halfway inside, so less than 2" above the rebar, and the possibility of it failing it the thin spots. Thats why if I used rebar, the slab would have to be thicker. Thats just my personal experience with pouring crete, which has been too many to count, in my 9 years. If the slab that is 4" requires re-enforcing, I would only use wire mesh, squares not rolled. Also when using the wire mesh, it was taught to me that it should installed at the top of the bottom third of the crete. I've also used "chairs" to place the wire at that specified height. Altough this is not needed, a rake to pull it up is fine I think. Depends on where and what the slab is used for, I've poured many sidewalks with no re-enfocement at all. I also would like a clairifacation if my theory about the rebar is right. That's just my rule of thumb that I have aquired over the years.
02-19-2001, 01:21 AM
Walt,your right,whenever we pour a 4"floor that will carry a wall on the interior,we deepen a channel to 8" about a ft on each side of where the wall will go,then we lay the rebar in the there,that is the only place we use rebar in a 4" floor.
Yeah, Stem Walls, or preparing for a future wall are a excellent situations of making a thickend edge, but keeping most of the main area about 4". I take it you did not mean a concrete wall, so you bend rebar to a "L" shape into the new floor. But also idea when pouring a stem wall and having rebar protruding straight out of the channel for the new wall itself.
I am looking for some litature about the rebar and reinforcing concrete. It was a long time ago I read this, but I believe it was a Corp of Engineer book. Looking on internet for it too with no luck. :( It's got me curious now, and it doesn't hurt to brush up anyway. Also thinking after I read concreteman's post again, that #3 rebar is rather small, less than 0.4" Dia. Welded wire fabric also comes in large gage steel to if you feel you just had to use something bigger in that pad. Plus saves you time and backaches from tying your rebar grid.
[Edited by WALT on 02-19-2001 at 02:11 AM]
02-19-2001, 10:17 AM
Both http://www.nahb.com and http://www.jlc.com have good sources for books related to construction.
02-19-2001, 06:27 PM
Thanks for the responses. In response to keeping the mesh up in the slab, it is easy to pull it up the first time but when you walk back in it to strike it off with a straight edge you walk it back down. The #3 bar is only 3/8" dia. I hate to mess with the aggrevation and danger of rolling out the mesh. Then if you have to wheelbarrow it you run the chance of tripping or hanging the front of the wheelbarrow on it. When I said a 4" slab is approx. it usually runs between 4 & 5". It's so much easier to use the rebar and cheaper. What do you think about the fiber that you can put in the concrete now. I had a salesman tell me that it would replace rebar and mesh. I very rarley use it. The only thing I found it helps is to keep the top from popping off from shirnkage cracks or from where salt eats the top off. If someone could help me find info on the fiber it would be appreciated.
My experiances with fiber crete was on an airfield where strength is a must. The problem with that stuff is the unvoidable contact that happens, and the price is considerbly higher. As you probabley know the fiber in the crete really will irritate the skin, and damage the eyes, more than regular concrete. It's mainly used as a secondary reinforcement, and as use said, reduces cracking, even hairline. Some studies have shown when doing a large pour where joints are needed, that fiber concrete will hold the joint together so tight that allows independent movement of the slabs, (freeze/thaw, or expansion). That would be a disadvantage of the fiber of course.
I hate using rolled mesh too, thats why I said only use squares and tie these together. Pulling it up can be done, but not advisable of course if walking over it again, even chairs or stands to keep it up. By all meens screed it off from the sides, and/or work the crete from the edges, e.g bull float/skim float, use knee boards, or dangle the luckey finisher choosen from a piece of eqipment (excavator bucket) to get the hard spots HA HA :) I didn't say that did I? :) How do you pull up the rebar grid that you walk on? Or does the chairs able to hold the rebar with weight of a person on them. Also there is less give from rebar if your foot does get tangled in the grid, at least you give chance to save your @ss from tripping and falling.
I also found this question that was asked on World of Concrete.Com. This kind of supports what I was saying before about cracking above the rebar, Although it is #4 rebar, it's the same idea. I think this will get the point across, and that size rebar you use is cool as long as you have at least 2" above the steel, I wouldn't take the chance.
We do finish work for a pool contractor who insists on using #4 rebar in all his flatwork. We get hairline cracks above the rebar locations except where concrete is 6 inches thick. We have suggested that he use welded wire mesh but he refuses. What can we do to stop cracks?
Your suggestion of using wire mesh (which has a smaller diameter than #4 bars)--and your observation that cracking doesn't occur when the steel is deeper in the concrete--both agree with the recommendation of the ACI 302, Section 2.2.2, which appears in Concrete International, June 1980.
The first paragraph of that section says:
"Plastic settlement cracking over rebars is caused by inadequate concrete cover over steel, larger rebar size and/or higher slump concrete or a combination of these. To help avoid these problems, concrete cover should be at least 2 inches (50 millimeters) where possible. Other measures to avoid this are smaller bar size, low slump concrete (2 inches or 50 millimeters) and/or revibration of concrete."
This recommendation is based on data presented by Fadh. H. Dakil, Philip D. Cady and Roger E. Carrier in the article "Cracking of Fresh Concrete as Related to Reinforcement," ACI Journal, August 1975, pages 421-428.
ACI 302 also says (Section 11.1.3):
"Some cracking may occur before the concrete has hardened. This may complicate the finishing operations considerably. Some examples are: . . . (2) cracking from settlement of concrete around reinforcing bars or other embedments."
I hope this helps...
02-19-2001, 08:38 PM
Concrete man do you know DIG'EM? How do you like the computer? How do you like lawnsite ?
02-19-2001, 08:42 PM
I will snip a few wires in the mesh and bend them down and the mesh will stand on these "legs" and it seems to keep it up in the middle of the slab. Seems to work okay for sidewalks but I dont know what long term problems this may cause.
02-19-2001, 08:57 PM
Thanks Walt that was nice to know. Let me ask you a question. What have you had luck with in the summer time to cut down on the craze cracks or spider cracks on the top of a slab? The concrete company here gave me some evert surface ******er, not the same as ******er for exposed aggregat. I tried it last summer it helped a little but then I had trouble with the top 1/8" start to blister off when I start to run my trowel machine. The concrete company had to pay to fix a basement floor and garage that the top came off of it. If digman or whatever your name is reads this email or call later. You need a different name like gravedigger.
When I used to live in Georiga, that was a constant problem, even worse when the humidity was lower. The stuff would dry so quick and the urgency to finish the placement increased. We used a liquid curing compond,that was sprayed on after all the finishing was complete. I can't remember the brand, but make sure it's non-yellowing. This worked well by actually creating a thin membrane, retaining the water from hydrating so quick. It's important that when it's hot out to be sure to apply the compound when the water has dissapeared. The longer it hydrates the better. Water curing is not reccomended on hot days, and plastic too retain the water on cold days is good. Never the less, curing is very important process that shouldn't be shortened or overlooked.
Some other problems may be the slump. 3" is good for flat work, much more slump then that causes the water to cement ratio to be off. Never trust the company, take a slump test at the site and don't hessitate to send it back if it's not what you want.
Over-working or Over-finishing causes too much segregation, bullfloating too much is the most common I've seen, or screeding along with the use of a jitter-bug does this, and cause to much paste to rise, giving that weak 1/8 you talked about. And wait until the excess water is off the top before hand floating. This is personnely the problem I look out for on my concrete jobs. If they work for me, they WILL know at least that cause I watch EVERYONE to make sure that does not happen. I've seen scalling and spalling due to that, and is a peeve of mine.
I hope this helps... :)
02-19-2001, 11:36 PM
I use a curing compound whenever I get done slick finishing a slab or even brooming. We try to keep it a good 4" slump from the plant. I started tring not to close up my concrete to much with the bullfloat. I found it sets up a little faster if it is still alittle rough, and it doesn't trap the water under that top surface. I'm going to try to find the flat mesh you recommended. My dad just died in may and I'm still learning a few things.
I am always learning too and when I answer questions, or see other techniques, it helps me much also by refreshing my memory. Sorry to here about your Dad, and wish you good luck. Concrete is one of my favorite occupations and always looking for new techniqes. Oh and welcome to Lawn Site... :)
02-21-2001, 01:10 AM
I only use rebar on an Alaskan slab #4 2' oc 6" thick. However I have seen it in parking garages and structural slabs. The mesh should work fine. I was on a job with a huge basement and the owner wanted no cuts in the floor. Around all the coulmns and corners where it would crack we used two layers of mesh. Nothing more then a slight surface crack.
I give you credit for pouring a slab at a 4. someone said a three not me not on a slab. If you have an inspector on the site watching you i'd pay for the super plasticiser. Don't go nuts with the water but there is a safety factor built into the concrete 3000 psi is realy like 3400 psi you can have a littlt water. It's right in your ACI manual. I was an Inspector for a summer you can do some amazing things to concrete. It depends on the suiplier. If the suplier has good concrete you will get better rersults.
With the mesh settling to the bottom try puting pieces of broken block under it. Definitly use the sheets, it lies nice and flat and it's eassy to tie together.
Have any of you tried daratard. I saw it used on the top deck of a parking garage in August. It was about 95 degrees it worked out ok.
When the top layer comes off when you trowel it's because the top is hard and underneath it's still soft. When it's hot and you're in the sun you have to have the ******er put in right at the plant and dont let it get old 90 min max! Tell them the conditions and what has happened in your experience they know their product and what to do to fix a problem. When you're finishing dont squirt water on the surface....I know everybody and their brother blesses a slab when you're floating troweling etc but it's lowering the cement water ration right at the surface. Don't finish to soon that can ruin your finish.
This is what I've seen as an inspector as a contractor as a loborer. Comments questions think I'm way off with something I'd love to hear about it I learn something every day and it comes from you guys.
You are on the money, my friend. Especially the points on finishing, very important. A point I'd clairify, just in case somebody is confused, when you utilize the 2 layers of mesh, they are spaced out about evenly in the pad. The bottem layer increasing the strength and top layer for preventing cracks as well as strength. Always making sure to have enough concrete around the reinforcement, limiting voids.
I hope to find out tips from you, I am sure as an inspector you seen some wild stuff, both right and wrong.
Just curious, were you an inspector and was it for the gov't?
What does it take (education, etc.) to be an inspector?
Look foward to more insight..Thanks
02-21-2001, 01:39 PM
I got the inspector job through school it was like an internship I got credits for but I also got paid. All you realy need to be an inspector is ACI certification. I wasn't actualy a building inspector I was a technician for a testing lab. I monitered the slump air content temp revolutions on the barrel time from batch water added strength at 7 and 28 days and aroung 56 days if you have a lot of fly ash in the mix. To work for the gvt it may require some school I'm not sure those guys are strict. I did a couple of bridge jobs and they dont hesitate to throw away a $1500 load of concrete or charge it to an inspector or a pump company for being late to the job.
Here's a wild story I was on a job at a parking garage. Satae Building inspectors Engineers the works. #5 rebar @1' O.C. 5000psi concrete w/ water reducer and some other hi tech stuff real expensive. It was wierd stuff it almost stuck to the scoop I used to do the test. On the first day the contractor found out that the concrete wasn't user friendly. It was actualy sliping under the power screed and not coming out very flat. It was also seting up before they could realy float the high and low spots out. To remedy this situation they started power troweling it before it set. They were leaving foot prints about 3" into the concrete. The trowel was throwng around concrete like a side discharge lawn mower. They also ended up using a straight edge in stead of the power screed. Surprisingly none of the engineers or state inspectors knew that troweling it like this was bad. Only me and the contractor knew, we were friends so neithre of us said a thing. I haven't been able to go back and see this slab but I'd bet it cant be in very good condition. After the first placement they changed the mix design and the next 1300 yards went pretty smoth. Accept for a barrel that went hard while it was being pumped. 2.5 hrs old they wanted to make cylinders and let it go so i did as they asked. This all happened right infront of the state inspector and the engineer on the project. The driver got fired and if the super hadn't had around 30 years in the company he would have gotten fired too.
my name's Casey incase you were wondering 66 construction is the name of my company
02-22-2001, 10:59 PM
hey cretewalker you really eating this stuff up. you got a good post .Mine is on fire .
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