PVC pipe in place of Plastic Chairs

[RememberTheAlamo]

Hi All,

Local practices and inventories have driven us to step "out-of-the-box" for sourcing of the plastic chairs, which our mesh should sit on.

Since I am spending $3K for mesh/materials . . . I really want this mesh in the middle of the concrete (not at the bottom).

Unfortunately, neither the W6 mesh nor a sufficient number of plastic chairs are available locally.

So we will be using 8x20 sheets of W4xW4 4x4 mesh and supporting these with 2 1/2" high 4" diameter Schedule 40 pipe sections (a lot of them).

The slab will be 96' by 48', 6" in depth (a little lower around the drains), fiber reinforced 4,000 psi concrete, pumped.

The idea is to cut 40+ 2 1/2" lengths of pipe from a 10' PVC pipe. These would be laid on the vapor barrier and we would then lay the mesh on top of the pipe sections (tying off the sheets).

During the pour, when walking on the mesh, the pipe sections won't collapse like the plastic chairs sometimes do, nor tip over like composite chairs will do. We will still use the plywood "lilly pads" for walking to reduce disruption of the mesh.

This definitely isn't "typical" . . . so I ask the audience your thoughts and recommendations.

The foreman was going to cut bricks to do the same (plastic pipe is way easier to cut and move) . . . and the inspector doesn't want to see those bricks!

So we will end up with around 1600 4" diameter, white plastic 2 1/2 " tall rings supporting the mesh . . . not the greatest solution, but probably better than the promise to "pull the wire up as we go".

Thank you,

Thoughts?

Larry

 

[cvg]

bricks work better and won't cause those annoying voids in your slab that 4" pipe will. by the way, bricks are generally allowed, what is the problem with the inspector.

 

[RememberTheAlamo]

Hi Cvg,

I kinda like the brick idea . . . it is masonary.

Don't have a physics explanation why they don't encourage bricks . . . just know it is either plastic or metal chairs.

Not in a place to really argue this one . . . sometimes the game is about picking the places to challenge . . but not to challenge all.

Hopefully the circular pipe sections would fill easily and not create any vugs or air pockets . . . imagine, when the vibrator hits the mesh, just about all air will pop out of the concrete.

Make sense?

larry

 

[Ron]

Bricks are rectangular and if they don't bond to the concrete, they create re-entrant corners that propagate cracks. Further, if your bricks don't bond, water coming from beneath has a direct path to the rebar.

If you use bricks they must be concrete bricks, not clay bricks.

 

[cvg]

i meant concrete bricks, and though I agree with Ron's comment, pipes would be even worse. There is very little chance of bonding with a smooth, round pvc pipe and very high likelyhood of having voids inside each pipe leading up to the steel.

concrete blocks are specified by UFGS for all federal projects, I suspect that they should perform well.

2.6 SUPPORTS
Bar supports for formed surfaces shall be designed and fabricated in accordance with CRSI 10MSP and shall be steel or precast concrete blocks. Precast concrete blocks shall have wire ties and shall be not less than 100 by 100 mm 4 inches square when supporting reinforcement on ground. Precast concrete block shall have compressive strength equal to that of the surrounding concrete.

For slabs on grade, supports shall be precast concrete blocks, plastic coated steel fabricated with bearing plates, or specifically designed wire-fabric supports fabricated of plastic.
Bar supports shall comply with the requirements of ACI SP-66.

 

[concretemasonry]

Concrete brick have been used for decades. A typical brick is very cheap and measures 2 1/4" x 3 5/8" x 7 5/8" and can easily split along the length to make two pieces that are more cubic. Concrete bonds well to them since they are so compatible. They can be laid flat for a 4" slab or one edge for a 6" slab since wire will always sag somewhat. They also provide a good strength and wider bearing for stability even for the "big boys" to walk on.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[RememberTheAlamo]

Hi All,

Good points!

The concrete bricks make good sense. Water should not be an issue. Structure is on top of a hill and there is a french drain that runs down the center of the slab, which should take water away before it rose up into slab.

But the inspectors feel strongly about not using brick . . . which would mean months before we get the approvals.

Would have thought the rounded nature of the pipes would have limited the crack proprogation . . . looks like I am in a situation of selecting the best of the worst alternatives.

Do appreciate the help . . . does make me pause.

Thank you,

Larry

 

[DTGT2002]

What is stated with regard to supports by the project specifications?


If ACI 301-10 is invoked, you should find Section 3.2.1.10 to your liking as it covers rebar supports and provides guidance on "Precast concrete reinforcement supports."

Not to sure how your "inspectors" could choose to deviate from items specified in the specifications - though submittals may certainly be required. That assumes 301 is used or borrowed in the reinforcing section.

One might make the case that your PVC rings are not in spec, as they are not specifically a recognized "chair," though the 301 does not preclude manufacturing your own supports, only speaks to materials.


Some folks call the precast block dobies. Bricks always brings an image of a clay block to my mind.


Daniel

 

[csd72]

Any clay product is not a good idea for a number of reasons:

1. brick/clay pipes are porous and therefore will both absorb a significant amount of the water in the mix as well as allowang contaminants suchas salt in to the reinforcement.
2. Bricks/clay pipes will swell when moisture is added and this can occur over an extended period of time.
3. most bricks/clay pipes are not as strong as concrete (though I agree some can be stronger).
4. The bond between bricks/clay pipes and concrete is not particularly good, they can therefore act as crack inducers particularly when you also consider 2.
5. The bricks/clay pipes do not offer the same resistance to steel corrosion as concrete.

I am sure I could think of a few others if I had sufficient time.