concrete forms: plywood thickness?

[mburrus]

hey everyone, i am getting ready to pour footers monday, they dont need to be formed, but i am trying to plan for the next steps.

i was able to get a pretty good amount of 1/2" plywood from work. i was wondering: would that be usable for column forms? these columns will be in between cbs block, so i will have a solid surface to anchor to. my carpenter says that half inch is too weak and will blow out under the weight of the concrete... i personally think it will be ok with a bit of extra 2x4 bracing.

anyone have any experience or opinions in this arena? i am trying to save money on wasting plywood on form work... has anyone used OSB for form work?? i can also get a. good amount of that too.

 

[Lx460]

How high are these columns? If you don't know what you are doing in forming them up (no disrespect intended) you will be very unhappy with the outcome. I've seen columns blowout by professionals that do this every day. It makes a huge mess, wastes concrete, and you still don't have your column.

Most blowouts are caused by too wet concrete pumped too fast from my experience. If you're doing it yourself start with stiffer mud and let it setup a bit instead of just trying to pump the whole thing at once.

 

[southalabama]

2x4 bracing ??

Nah. I wouldn't use a 2x4 on anything involving concrete.

 

[fury9]

are you using clamps, cross ties, and walers? If so 5/8 is what we use in commercial form building

 

[Josey Wales]

If your stuck on free, you could double that 1/2 inch up. IMHO 1/2 isn't stiff enough.

 

[pstnbly]

My forms are 1" HDO marine plywood, but sometimes I use 3/4" for custom fillers. A double layer of 1/2" should be good with adequate bracing. That means 2x4 wailers on 16" centers through tied on 16" centers. Don't expect the forms to hold without tying through the form. Lx460 is right, custom form work is probably not a good idea for a first time form carpenter, I've done tons of custom formwork and I still get a surprise now and then. You really can't appreciate how much concrete weighs until you try to stack it in a form 8' high.

 

[PT Doc]

Whatever you choose to do, please take photos.

 

[gc427]

hey everyone, i am getting ready to pour footers monday, they dont need to be formed, but i am trying to plan for the next steps.

i was able to get a pretty good amount of 1/2" plywood from work. i was wondering: would that be usable for column forms? these columns will be in between cbs block, so i will have a solid surface to anchor to. my carpenter says that half inch is too weak and will blow out under the weight of the concrete...i personally think it will be ok with a bit of extra 2x4 bracing.

anyone have any experience or opinions in this arena? i am trying to save money on wasting plywood on form work... has anyone used OSB for form work?? i can also get a. good amount of that too.

What size plywood did your carpenter tell you he needed?

 

[Iowa Mark]

The size of the plywood isn't the issue. It is the stress put on the formwork that is the problem. Most wooden concrete forming is done with a good quality solid core plywood or straight grained two by planking, supported by additional backer bracing secured to cross ties that transfer the loading to the supports. Column forming has the added problems of the weight of the concrete exerting it's hydraulic pressure to the formwork and then the increase in pressure as vibration is introduced to fill voids in the forms. Look at it this way. Take a balloon and build a box around it that will keep it's shape and not blow out when you pump it up to a few hundred psi. depending on the size of your columns that number could be in the tons rather than pounds.

 

[Firebird 1]

What kind of column are you planning. Sono tubes work very well for columns. I have formed and poured more boxes, walls, columns etc. than I care to remember. We always use 5/8 plyform. It is more expensive than free 1/2", but worth it. If you have little or no experience making forms, get some help from someone who does. I would NEVER use osb for forming. 2 x 4's are fine for form work if you know how to use them.

 

[reader2580]

I built a concrete retaining wall along the edge of my driveway more than a decade ago. I just used various bits of scrap plywood I had laying around. I think some was 3/8". It was done in two pours. The first pour I didn't have enough support behind the plywood and the plywood bowed out a bit. The second pour we added more supports behind the plywood and everything came out straight.

This was not a structural pour. I would never do concrete that way for a footing or something that requires precision and it structural.

 

[mburrus]

carpenter advised 5/8 minimum... sonotubes will not work. this is a cbs block building, and the columns are basically in the block. so the forms are on 2 sides only and affixed to the block, and the block provides form for the other 2 sides. i am out looking for more plywood in dumpsters at other construction sites, im trying to manage my costs as best as possible, especially if its going to end up as waste at the end of the job.

by 2x4 bracing, i meant across the back of the plywood to add rigidity.

what about 1x8 planks? i also found a number of them today in good shape.

 

[welder4956]

Listen to your carpenter.

 

[toomany]

I have no advice as far as the forms go.

But....if you are trying to cut costs by doing it this way, can you afford to pay for the clean up/re-pour if the form doesn't hold? Sometimes being cheap on the front end costs you double on the back.

 

[Iowa Mark]

It sounds like you are forming pilasters in a gap in a stacked block wall? The problem on pours like this is, they usually get filled up all at once so that the concrete is exerting all of it's hydraulic pressure against the formwork as you run the ******** in and out of that stack of mud. That will pillow the plywood that is between any bracing, jack the edges that are against the block wall and pull any kind of anchors you try to drill into those blocks. You will need through ties as close to the vertical edges as you can place them, and additional through ties in the field all depending on how wide and tall these pilaster are. When columns fail during a pour, they are usually pretty dramatic. Like a zipper from the bottom up.

 

[mburrus]

iowa mark, thats exactly it. i believe we are going to use some (plenty?) of steel ties through the center of the forms... columns will be 1' wide, by... 15-16' tall. plenty of concrete... i was thinking about using thicker wood at the bottom and using the half towards the top of the column...

good point on doing "double" the work for a blow out... i am pooling my resources to see if i can get a source for used wood... dont know how it will pan out, but we will see.

 

[flat tire]

wall ties, cam locks and 3/4" plyform

 

[mburrus]

pt doc, i am chronicling my build in another thread... go there for pictures.

http://www.garagejournal.com/forum/showthread.php?t=275597&showall=1

it looks like we are going to use doubled up 1/2" as much as possible, or maybe pour the columns separate from the tie beam so that we dont have the weight of the tie beam concrete on the forms...

i am hitting other construction sites to see if they are trashing any of their old form wood.

 

[JimbosGarage]

We use 3/4'' plywood with 2 x 4 bracing every 16'' to' 24'' on center. never had a problem. Don't forget kickers every 24'' on center as well.