Concrete Slab Questions/ Input

[Brandon]

I have formed for a 20' X 12' patio on partial backfill.  It will be poured up aganist an exsisting patio, so I will pin (drill into existing slab) with rebar.  The field will be 3/8" rebar on 2' centers.  I was told by a concrete contractor to overcome the problem of potential settling to dig 6-10 post holes to the original grade and tie rebar into them.  This would prevent the slab from settling becasue of it being on partial backfill. 3,500psi 3 1/2" thick.

Questions:

Is 2' centers appropriate?

How far do I drill into existing slab for rebar to prevent the two patios from heaving apart?

Should I use a plate compactor to compact the sand and backfill?

I appreciate the input.

[n74tg]

I am forming up for a 12x22 basement room pour.  I am using #3 rebar on 12" centers both ways (ie a grid); slab will be same thickness as yours and probably 4,000 psi concrete (six bag mix).  

I would definately use the plate compactor.  If you can stand to drill that long I think I would go a foot into the old slab for the rebar if possible.  Depending on what type of backfill you have I would let that play into whether I'd use 12" or 24" rebar spacing.  

Good luck.  Let us know how it turns out; w pics if possible.

[peg_688]

#1:  I have formed for a 20' X 12' patio on partial backfill.  It will be poured up aganist an exsisting patio, so I will pin (drill into existing slab) with rebar.

#2 (A): The field will be 3/8" rebar on 2' centers.


#2 (B):   I was told by a concrete contractor to overcome the problem of potential settling to dig 6-10 post holes to the original grade and tie rebar into them.  

#3:  This would prevent the slab from settling becasue of it being on partial backfill. 3,500psi 3 1/2" thick.

Questions:

#4:   Is 2' centers appropriate?

#5:    How far do I drill into existing slab for rebar to prevent the two patios from heaving apart?

#6    Should I use a plate compactor to compact the sand and backfill?

#7:     I appreciate the input.

 #1: Good idea  I'd go about 2' OC. With #4 bar (1/2").

 #2 (A):  Are you in a seimic / hurricane / tornado zone ?   Rarely is a rebar grid needed / required,  at least in my 20+ years of building . Bank vaults , walls / floor / ceiling is the only place I recall a rebar grid in a  slab type situation.  And that is a commerical application. A very large slab for a store that would have various loads applied might also require a grid

  There could be some reason if they require one in your area . I live and work in Siemic zone D IIRC, PNW, and we don't use a grid of rebar for normal slabs.

 Once in a great while someone will spec wire , for garage slabs we use fiber mesh which is mixed in at the batch plant.

  #2 (B):  The post holes in theroy should work to do what he said . Again I've never done it.

 #3: 4" is normally  what is spec'ed.

 #4: For the rebar grid or the tie in pins ?

 #5: 6 to 8" is normal , be sure to check if existing plumbing lines / radaint floor heat tubs are at all possible in the locations / wall above if it's to a aslab on grade house . ya never know what some plumber might have done. If ya feel like it you could epoxy then in they sell a caulking gun tube setup mix it witha drill motor , caulking gun it into the hole .

#6:  Depends on how thick the backfill is . It should have been compacted in about 6 to 12" lifts / sections .  It can't hurt I guess but if your fill is 5' deep / thick,  water settling over a period of a week or so might do a "better " job . [highlight]So again it depends on your specific conditions IMO. [/highlight]

 
#7: We'll see if you still do

 G/L , PEG

 

[glenn-k]

The post holes should help - I do the plunger pier slabs for walking on - about 1" thickslab  2 to 3' spacing on the piers - same idea - common in places in India and England.

I have also seen 6 to 8 " as PEG says - Simpson set epoxy is relatively cheap and Home Depot carries it and a manual application gun  as well as other places - not my favorite -up to 24 hours set time -- Hilti HY 150 is better -usually sets in 15 minutes -- but more expensive and harder to get.  Brush - blow --brush --blow  - get the dust out - hole should be dry -compressor is best but a hose to the bottom blown by mouth will work.  Start filling from the bottom to keep from traping air in the hole- pump a little out first to be sure to get a good mix -- a couple pumps will do.  Epoxy should just push out the hole when the bar is all the way in.  Twist the bar as you insert it.

The reinforcing will help keep things together if settling occurs - plate compactor is a good idea.

[peg_688]

   plate compactor is a good idea.

 Even if the fill is 3 or 4' deep?  Wouldn't it just be compacting the surface material and be more show than go?

 I'd think water settling would be more effective[highlight]  IF [/highlight] the fill is deep and was not compacted as it was brought in .

[glenn-k]

I'm probably not reading close enough --- the plate compactor I have says 12 inches or so max per lift but about 6 inch lifts max is best properly dampened. -- not overly wet.

[glenn-k]

Vibration would be beneficial even a few times as the water settling s drying out over a period of a few days  -- probably still not as sure as doing it right though ---  still -- it is not a precision high tech project.  Just less chance of problems if done right.  Sometimes under certaion conditions a hole filled with too much water will not dry for a very long time - depends on soil conditions.  Doing it right is best.  Light use area the pier holes should work fine.

The plunger pier floor actually uses loose fill under it to cause settling later for an insulation space separating the floor from the ground.  The piers provide thefoundation for the suspended floor -- it does use hessian for reinforcing but steel would work.