Minimum Steel for Crack Concrol? 2

[CIVENG95]

Is there a minimum steel requirement for Temp. & Shinkage cracking in a residential slab on-grade (stemwall foundation)? In place of 6x6" 10/10 W.W.M., I would like to use #3 reinforcing bar grid but not sure of spacing?
Thanks

 

[DaveAtkins]

There is no need for any reinforcing in a slab-on-grade, if you saw control joints into the slab at a close spacing (36 times the slab thickness, maximum). #3 at 48" oc each way would give you approximately the same area of steel as WWF 6x6 - W1.4xW1.4 (the correct designation for 6x6 10/10 WWM). DaveAtkins

 

[Zulak]

If you're going to use #3 rebar, I wouldn't go any further than 18"oc with the rebar, since local cracks may form. That's why wwf works well because the grid spacing is close.

A better way for residential construction is to add Fibermesh to the mix. Good crack control and no hassle of placing reinforcement.

 

[boo1]

I have found in residental construction flat work, the wire or fiber contibutes little. Typically the wire is walked on pushing it to the bottom of the slab. I have removed dozens of slabs and driveways and found the rusting wire presses into the poly.

If you are trying to reduce Temp. & Shinkage cracking i found the most common reasons for cracking were:
1. Excess water in the mix (no water added on site)
2. Rapid Drying or improperly curing concrete slabs (subs often want to pull forms the day of the pour, use a cureing top coating)
3. Lack of control joints.
4. sub grade must be prepared according to your soil conditions (I see all the time sites dumping truck after truck putting the fill in 3 ft lifts, limit to 8" compact between lifts)

 

[Watermelon]

Hi CIVENG95:

I like #4 bars at 12" C/C each way. Workers can step in the open spaces without pushing the bars to the bottom of the slab. Put chairs at 36" max each way. Lap all joints. Stagger all joints.

 

[frogit22]

Rebar is not very effective once it is spaced more than 3x the slab thickness (i.e. 12" on center for a 4" slab). However, you could get away without reinforcing if you keep the water in the mix low and cure it properly.

I usually show a 6x6 w1.4xw1.4 reinforcing on 4" residential type basement slabs.