Residential slab/footing shrinkage reinforcement 1

[MrJB]

All, we are looking at a residential house design that is approximately 40m long and 8m wide. Located in moderately reactive soils and builder would like to pour as a waffle slab. It falls outside of the applicable residential standard and we would like to avoid having an expansion joint in the slab. Gut feel is that we need to up the slab reo rate to be about 1% in the long direction yet does anyone have a more scientific approach?

 

[SlideRuleEra]

Gut feel is that we need to up the slab reo rate to be about 1% in the long direction yet does anyone have a more scientific approach?

Absolutely, the U.S. Federal Highway Administration (FHWA) "Continuously Reinforced Concrete Pavement Manual", 0.7% minimum. Continuous reinforcement does not prevent cracks, it's just that cracks are so small and closely spaced they can't be seen or cause "trouble".

I started applying this technique to some of our industrial slabs (not highways), about 40 years ago, it works well.

 

[bones206]

I’ve had success at 0.5% in jointless slabs. My go-to reference is the control joint spacing vs reinforcement ratio table in ACI-350.

 

[dik]

There's a price to pay for not having joints. I generally use about 0.8%.

You have to take care that there are no obstructions that can restrain the slab and make allowance for them... eg. reentrant corners, footing/pile caps or piers, etc. I generally place the reinforcing with about 1-1/2" conc cover for sawcutting. That's near the top, but that's where the cracks form. If not sawcut, then maybe 1" cover. Not sawcut, I'd still use 1-1/2" cover. You have to make sure your base materials are uniform and well compacted (true with any slab, but for one without sawcuts, maybe a little more critical).

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

Excellent publication SRE... incredibly detailed. I was surprised that epoxy bars were still referenced.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Tomfh]

The default waffle slabs are around 0.25%, which is closer to minimum reinforcement.

We do a lot of waffle slabs in the 30-40m range, with around 0.4% in total. Bars at 100 cts (eg SL81 mesh).

1% would be pretty heavy. Almost twice as much as AS3600’s “strong” crack control.