Pouring concrete over joists??

[383 240z]

I have a wooed floor over dirt in one part of my shop. I plan on moving my mill and lathe into this "room". The rest of the shop is 6" of fiber and steel reinforced concrete. The plan is to pull up the 3/4" plywood, exposing the PT joists. Frame in a section where the mill and lathe will sit, and start pouring, then strike it flush with the plywood. My one big concern is the concrete in direct contact with the PT joists. Should I cut them out, frame it up and wrap them with tar paper then pour? Your thoughts? Keith

 

[dirttracker18]

Wood should never directly touch concrete. It will wick the moisture out and prematurely rot the wood, even PT.

Cut them out and wrap the section that will be in contact with plastic, 6mil poly will work.

IMHO

 

[MoonRise]

In general, you usually don't want to embed wood in the concrete. Some possible 'issues' that can occur would be: rot (yes, even PT can rot), moisture, and strength/depth of the concrete itself.

Let's see, most of the shop floor is a 6" thick fiber-and-steel reinforced concrete slab on grade. But somehow you have a 'room' with some PT floor joists resting right on the ground (like a shed floor) and 3/4 plywood on top of those PT joists. Pouring some concrete in/on/in-between those PT joists and then having it up at the level of the plywood floor (and thus probably touching the plywood as well) I think is just asking for moisture and/or insect problems.

And how would you excavate for the gravel underneath the concrete slab without undermining the on-grade PT joists (which are probably 'sized' for being on-grade and most likely not sized to actually have any span to them)?

If you have any pics/images in your post, I can't see them right now because they are not coming from GJ but some off-site locale and hence aren't 'getting through' here.

To me a mill and a lathe would indicate a need (or at least a strong desire) for a strong and solid floor underneath them in order to both support the machines and to support them solidly without vibration.

PT joists right on grade with plywood on top may be strong enough for many things, but I think that putting big $$ machinery on top of that type of floor is just asking for moisture issues from the ground as well as vibration.

YMMV and all this is just IMHO. I have also been accused of a wee bit of 'overkill' in building things too.

 

[383 240z]

The PT is not on grade. the PO of this place had a mud floor when I bought it. He built a wood shop inside the 32x40. When I poured the slab for the rest I left the room. It is about 4" above grade. That means I have 4" of free space, 6" of joist height and 3/4" plywood.

the main pad was dug out. I put 6" of gravel down then poured. Does that make it a bit clearer? I doubt it, for some reason today I am failing to have command of the English language. Keith

 

[KELLHAMMER]

Isolate the pt joist from the concrete with a layer of felt or self adhering flashing such as Grace ice and water shield. The PT may rot due to direct contact with the concrete but it will take a long time. PT tends to loose it resistance much faster when exposed to the weather. Even untreated wood will last longer with a barrier between it and the concrete

 

[bczygan]

First thing I would want to do is to make sure the wood flooring system would support the weight of the mill and/or the lathe during the transport across the floor to the new conc. pads. Will the mill be disassembled? What is the spacing and span of the joists?
Second, I would want to pour the pads as one monolithic piece for each one. That means cutting out pieces of joist and headering off the opening with a double header at each end. Since you could not get access to double the joists you will have less strength in this area of floor. This could be critical as this is where you will be supporting the machines while moving them to the pads. I would provide at least some temporary masonry supports (Conc. block solids and wood blocking of different sizes as needed) in these areas to cut spans and increase load capacities. This could be left in place.
You could then use 30# building paper to cover the joists around the opening to protect them before pouring. This would also create a joint to allow the dissimilar materials of wood and concrete to expand and contract differently and to move independently of each other.
It might just be easier to cut out a large area of floor or the entire floor and pour a new slab. This would allow for future rearrangement of the mill and lathe. Your call.

 

[383 240z]

the joists are 18" on center, span is 10' The lathe weighs 1100 lbs. the mill comes in at 2300 lbs. I will be moving them in pieces the largest one weighs about 800 lbs. it will need to travel a max of 14' on the wooden deck. Keith

 

[MoonRise]

You have a 'room' with a wooden floor (and wooden floor joists), all surrounded by a big 'shop' with a concrete slab-on-grade floor?

I'd say just remove the wood, dig down a bit for a good gravel base, and pour a concrete slab. Done.

As to the machinery on or going across that wooden floor, nope not really. 2x6 floor joists at 18" OC (???, oddball spacing, as most joist/stud/rafter spacing in the US is 16" OC or 19.2" OC metric-derived, or 24" OC) spanning 10' is undersized for those heavy machinery pieces and their 'concentrated' floor loading.

 

[383 240z]

MoonRise. You been reading my mind. I guess the big reason for not doing the whole floor is my wallet. My plan is to do it in sections. As to the odd size joist spacing, you have NO IDEA!!! The PO of my farm was nucking futz. He has this other room in the shop before I tore it out for pouring the slab. it was built like a ******** bomb shelter, insulated 2x6" 12" oc walls CDX on both sides of the walls. This was to hold all of his daughters stuff when she moved back home after college. yet the window seats he built My wife ripped them out with her bare hands and no she is not a big amazon!!! Keith