The proverbial wise man bldng on a rock
My concrete contractor just bailed on me! Took one look at my site conditions and made a graceful exit. I can’t blame him honestly but since, quite possibly, I am going to be forming my slab on my own now, I need some info.
The main concern is that I’m on a rocky site. By rocky I mean a solid slab of limestone several feet thick under about 6″ to 8″ of loose, flat boulders and dirt. I had a backhoe operator dig my grade beams and he snapped a tooth on his bucket first pull. This is some pretty hard stuff.
I can’t see digging out rock and replacing it with softer concrete so there is no real need to go deeper than the top of the slab rock. I’ve swept some of the bottoms of my grade beam trenches with a broom and the slab is virtually seamless. My engineer has assured me that my slab design will be far more than adequate in these conditions. Even with just 6″ to 8″ grade beams.
Here is the problem. How do I secure my forms? I bought a bunch of 18″x 3/4″ steel pins to set my batter boards but I was bending them trying to drive them into the rock. I bought a long chisel for my air hammer and made some progress by pre-punching the holes but it’s slow going. I am considering pre-drilling with a rotary hammer and a long 5/8″ diamond core bit then driving the pins into the holes but I would think it would be hard to fudge them back and forth as I’m tweaking the forms into alignment. (shims maybe?) Not to mention expensive on my rent-a-tool budget since I will be doing this little by little on evenings and weekends. At any rate, my limited forming experience using 2×4 support stakes ain’t gonna help me here.
I’m really stumped on this one. I’ve never had to deal with site conditions quite this rough before. At least the view is worth it- I think. I appreciate any suggestions.
Kevin Halliburton
“The hard stuff we do right away- the impossible just takes a little longer” -Sign above the counter at a custom fab shop-
Replies
Just a top of the head amateur guess, but if the forms can be tied to the steel in the slab well enough to resist the outward pressure of the concrete, perhaps some sort of adhesive would be enough to keep them in the right place. Perhaps pads of plywood stuck down to the rock with construction adhesive could be enough to anchor the ends of braces. Adhesives depend on area for strength, so if you can get enough area, you can get enough strength. Of course, it could also turn out to be a huge mess to get rid of when you strip the forms. Maybe for grins the rock you've swept clean could be coated with the kind of bonding agent that's used for adding on to old concrete.
-- J.S.
I can't tell for sure if you are trying to pour footers for kneewalls or form a monolithic slab. You seem to have used both terms here.
Excellence is its own reward!
If the rock is limestone I would try to use 3" tapcons .
Maybe cut a bunch of 2' 2x4's and secure with 2 tapcons apiece. They work very well and very fast on concrete and I'm not sure , but think that limestone is about the same density.
Once you have a cleat attached to the rock you can brace pretty easy off of that.
You should be able to drill limestone pretty easily with a hammer drill. One thing you might consider is to drill holes same size as lengths of rebar. (#4, or 1/2", will be a drive fit in a 1/2" hole. In fact, you'll probably have to cut the rebar off with an oxy/acet torch when you're through).
Drive your rebars around the perimeter of the outside of your form boards. Put in a second row of rebar 16" or so outboard of that line to act as a set of posts. Get some landscape staples and secure the form rebar to the forms. Lay a 2 X on the inside of your rebar posts. Use this as a "heel" to prop your braces against.
I hope I'm clear enough, because I did this once on a slab of basalt bedrock and it was pretty straight-forward and worked fine. (And you don't need a diamond core drill...a masonry bit for a hammer drill will work just fine).
And I've used a mid-priced Milwaukie sds hammerdrill to successfully bore a variety of rock types, including granite, jasper, chert and a few others.
Hope this helps.
Jules Quaver for President 2004
Edited 2/11/2003 9:37:18 PM ET by Notchman
Off the top of my head as well is a method I have seen to hold forms 6" or more, but only on one or two sides; 55 gallon drums filled with H2O, holding the side of the form where nothing could be nailed to driven stakes, or no stakes could be driven. Provided that your forms are well secured on the ends, and you've got an equivalent amount of weight backing the form, which your engineer could tell you. (How many drums vs. weight of concrete pressing against forms.) This method works in several pinches, whether steep grades, saturated soil conditions, coastal work, etc. Remember to give a wide space from the barrels to the form to allow yourself to screed properly, say a 2x6 laid flat for your form stringer. When the concrete's cured, drain the barrels and go to work.
"If left is wrong, then right is the only thing left, right?
Why not use the old redneck method and just pile dirt or an occasional concrete block around the outside of the forms if only 8" thick? Or, if you don't need to insulate the slab edge, just overlap by a foot or 2 with stiff mix and don't even use a form. Have done both and the simple stuff works best.
If you're going to pour the "walls" of the foundation first and the slab later, use snap-ties and plywood forms. Drill a couple of rebars into the stone for anchors inside the walls, and fasten to forms to them with tie wire.
Do it right, or do it twice.
An approach that will work is to place a 4-6in layer of very low strength concrete (must be low slump or it'll run everywhere) straight onto the rock (I'll let you convert 5MPa to the equivalent PSI). Make the pad wider than your beams and once it has set up for a couple of days you'll be able to drive steel spikes into it with a hammer to anchor your main forms for the proper concrete.
you need to consider the relative costs of your time and tools vers the extra concrete. In my experience the extra concrete is usually cheaper than drilling every hole.
Ian
A proverbial wise man building on a rock deserves a rotary hammer and a Diamond bit to have and to hold always. This is your chance to go shopping . Go man !
Tim Mooney
go buy a nice rotary hammer with splined bits.. drill your #4 rebar anchors INSIDE your forms... drill 1/4 " holes in your form boards and run tie-wire from the rebar drifts with a loop thru the holes in the form boards... put a 16d nail in the loop and turn it as a tensioner ....
design your forms to resist uplift and spread..
set your form tops with a laserMike Smith Rhode Island : Design / Build / Repair / Restore
"This is your chance to go shopping . Go man !"
Tim,
Actually, that's a pretty good suggestion- if I do the forms myself. I'll take it out of the dough I was planning to spend on form labor. Then again, there are a lot of tools I need that would get a lot more use later on that I could buy with that same dough... maybe I've got a buddy or two with a hammer drill I can borrow. I rarely pass up a good excuse to buy tools but, since I'm building at the top end of my budget, every penny counts.
Kevin Halliburton
"In a country well governed, poverty is something to be ashamed of. In a country badly governed, wealth is something to be ashamed of." -Confucius-
This is just speculation, because I've never tried nailing limestone, but it seems to me you ought to be able to pin some chunks of 2x4 to chunks of angle iron with a ramset, then pin the angle iron to the rock, and nail forms, braces, etc. to the 2x4 just like it was a stake.
Why did the concrete guy "make a graceful exit"?
I love playing cards with children. They can't tell you're dealing off the bottom of the deck.
"Why did the concrete guy "make a graceful exit"?"
I give up. Why?
Because the plans weren't set in concrete?
More importantly, ..........the wise man is just a proverb? Say it ain't so.
View Image
Barry E
Edited 2/12/2003 9:00:59 AM ET by Barry E
Thanks Barry -
That really cleared things up.
Ever think about running for Congress ???....................(-:This morning I woke up to the unmistakable scent of pigs in a blanket. That's the price you pay for letting the relatives stay over.
I had not thought of running and if elected.....pork barrel here I come
Hey, who started this political stuff? <g>
View Image
Barry E
For those who asked for clarification- I forgot that foundations are done a whole lot different in most parts of the country than they are down here in Texas. We are about 99% monolithic slab on grade in this part of the country. Dig 12" wide grade beams about 24" deep, set your forms, tie your steel, pour slab and beams all in one shot.
As to why my contractor bailed- We are about a 20 minute drive from town, several McMansion sub divisions going up in town. Soft soil, 5 or six identical slabs within feet of each other so you can string a big crew out down a block and assembly line the whole process. All of the concrete contractors around here have a backlog of slabs waiting for them right now. The ones that don't are the guys I really don't want on my job if you know what I mean. I guess he just preferred to stick to the easy ones.
The last job this contractor did in my area he broke the chain on his trencher and he still has a bad taste in his mouth over it. We are using SIPS and staining the slab to serve as finished floor so I have some pretty restrictive slab specs that are a little uncommon in the area. At least for residential work. It's a long way off but right now my wife and I are planning on retiring in this house so we want it done right. A lot of the contractors in town are allowing concrete to be thinned to almost self leveling viscosity. Not on my house! The list goes on but you get the idea...
I am a firm believer in giving contractors the whole story on a project even if it means their bid price will go up. There is nothing more frustrating than riding a contractor to do a job right when he knows he is loosing money so before I had him sign on the dotted line I asked him to come out and thoroughly look over the site. After seeing it He felt like the job was more trouble than it was worth. No problem, I understand- that's why I had him do a site visit. Hate it- but I understand.
I do have one contractor interested but his bid is about $10,000.00 over my budget. Way higher than my other two bids but perhaps it is because he is fully aware of what it will take. Since I now have the grade beams dug and have decided to use flagstone instead of concrete on the wrap around porch I plan to call him and negotiate a little. I am also going to be calling a few other contractors today that I haven't given the opportunity to bid yet. Picked my top three guys first- oh well, move on down the line.
Whether I do the forms myself or find another contractor to pick up the job, this thread has been very helpful. I at least have several suggestions to offer that will hopefully make the job seem a little less daunting for the next guy. Thanks for your help.
Kevin Halliburton
"It is only through labor and painful effort, by grim energy and resolute courage, that we move on to better things" -Theodore Roosevelt-