Foundation site prep

[JB4310]

What I've found when demolishing concrete with fiber in it is a poor quality concrete that breaks fairly easy but stays together a little better due to the fiber.

If you plan on driving anything heavy in there then go to 5" and 3500 psi min. Do the rebar as you mentioned and I would use 6GA 6"x6" wire in the slab. you can skip the fiber, don't know if joints are needed in FL but maybe cut some 10 - 20 ft squares when it's green.

Go for a roller skate finish with industrial cure/sealer, it may be a little slippery when wet but will stay much cleaner and dust free. It's not a bad idea to install some heavy steel eye bolts or equivalent in the concrete to have a secure anchor point to lock things up to with a chain or cable.

JB.

 

[skidoo]

Even if you only scrape a little material from the top, you need to compact the site because there will be a compactible layer of disturbed dirt from the scraping.

Just curious... What if the virgin soil at the desired level is natural gravel? Does compacting do anything?

 

[sixdogs]

What I've found when demolishing concrete with fiber in it is a poor quality concrete that breaks fairly easy but stays together a little better due to the fiber.

If you plan on driving anything heavy in there then go to 5" and 3500 psi min. Do the rebar as you mentioned and I would use 6GA 6"x6" wire in the slab. you can skip the fiber, don't know if joints are needed in FL but maybe cut some 10 - 20 ft squares when it's green.

Go for a roller skate finish with industrial cure/sealer, it may be a little slippery when wet but will stay much cleaner and dust free. It's not a bad idea to install some heavy steel eye bolts or equivalent in the concrete to have a secure anchor point to lock things up to with a chain or cable.

JB.

The thread is older than dirt but the last sentence of the above post makes it worth a repeat. Why didn't I think of this?

Also, you never save any money doing site prep yourself and I've been on lots of jobs to try to fix what saved $18 many years earlier. Rarely can do because buildings, trees, underground stuff and turning radius are in the way.

Do as you wish but grading and site prep are as important as the building itself. Ask around.

 

[kenhar]

There seems to be a lot of misconception about making concrete stronger. The hardness is determined by the mix (3000#, 3500#) etc. The rebar, wire, fiber is to merely hold the concrete together when it cracks. That's why the footing in a building needs to be stronger than the rest of the floor because it supports the outer walls. Naturally speaking, heavy loads on a floor needs heavier reinforcement. Rebar if you plan to drive a vehicle or equipment on it. Fiber if it is mainly foot traffic (patio). Imagine the difference in down pressure of a 200 lb person and a 20,000 lb piece of equipment. Concrete, as hard as it may be, contracts with the seasons. That is why expansions joints are needed in a large slab. The bumps you hear on a bridge is from the expansion joints. When concrete breaks or cracks, the reinforcement is what keeps it from separating. One guarantee about concrete is it is going to crack somewhere.

 

[aircommuter]

You definitely need to compact the subgrade. The you should use 3/4” drain rock 4-6” thick as a capillary break, then cover that with 10 mil plastic and finally 2-4” of sand. Bare minimum of 4” thick concrete with #3 bar on 16” centers. If you plan on a hoist or other heavy use the go up to 6” concrete. If you are planning any electrical or other conduits put them beneath the rock layer and fill with sand.

 

[jigs_n_fixtures]

OK, most of what I have read so far in this thread scares me.

Standard plans for a residential driveway are typically 4-inches of 3500-psi concrete, over 4-inches of of “road base” gravel, compacted to 90% of the Proctor theoretical, and 1/4-inch welded wire mesh at 6-inch spacing, embedded at mid slab (it needs to be in the middle third of the slab to work.

Standard plan for a commercial driveway, is 6-inches of 4500-psi concrete, over 6-inches of “road base’ gravel at 95% of the Proctor theoretical, with #4 (12-mm) rebar 12-inches on center, both ways. Again it needs to be mid-slab to work.

This will vary based on local soils, but will generally work any where.

At the level of cost you’re headed towards with a building that size, I highly recommend that you get a Licensed Engineer at this point, before you get too far into things.

Most building departments require, sealed Engineering Specs and Drawings for buildings this size, and the building manufacturer probably has an Engineer they deal with fairly regularly, who they can recommend.

If I were doing this building for me, the slab would be at least two placements, in a checkerboard pattern 10X10-ft segments, using lost metal forms, and slip dowels between the slabs. With the metal forms at 10-foot on center, it is very easy to rest the 12-foot screed on the tops of the forms, and get a very smooth level surface. Plus 6-inch concrete wants to crack at about 12-ft on center, so the shrinkage cracking will occur at the forms and not in the slab. Locally we have issues with radon, and require gas barriers. So I’d place a bonded and reinforced barrier of two ten-mil thick polyethylene sheeting on top of the gravel. This would also help ensure there would be no cracking in the slab segments.

 

[jigs_n_fixtures]

Just curious... What if the virgin soil at the desired level is natural gravel? Does compacting do anything?

Depends on the gravel, and how much it got fluffed by the grading. Most alluvial gravels will get firmer with compaction because the rock is rounded and smooth. Highly granular talus materials with benefit less, but still benefit some form compaction.

 

[aircommuter]

Standard soils engineering plans call for minimum 90% compaction 12” deep on sub grade native soils. Personally I like #3 bar on 16” centers instead of 6x6 wire because it gets walked on so much during placement. With the larger space they can step inside. Side note about price, I just added some walkways and the price of just the concrete, 9 yards on the truck was $196.00 per yard. Of course this is California for you. I have used a little over 450 cubic yards on my property.

 

[Flogger]

I was 6" above grade at my highest point. Wish it would have been 8 or 10".

 

[aircommuter]

I usually setup for 12” above grade with the finished floor. I also put a 3/4”x 10” drop at the big door entrances. Then water hitting the door goes out instead of ponding and migrating in.