What reinforcement to 150mm slab on No Fines Concrete

[sybie99]

I am designing a slab to a water treatment works reactor where the slab is to rest on a layer of no fines concrete that acts as a drain should any effluent get through the slab. The design from our water department is for 12m long slab sections. The slab is 150mm thick. I know that the no fines will bond to the slab cast over it and will resist almost all movement. This will reslut in cracking. Now to limit my crack widths to say 0.2-0.3mm, what rebar would you consider sufficient?

I am using BS8007, liquid retaining structures, and get Y10's @ 200mm centres. But would it be prudent to have even more rebar seeing that the slab is cast onto a substrate with which it will form a very strong bond and resist any shrinkage movement at the bottom?

 

[asixth]

I have always found 9mm @ 200crs mesh to be good at keeping the cracks together well for joint spacing up to 8m.

 

[hokie66]

That is only about 0.26% Ag reinforcement. I am not up to date on BS8007, but remembered the requirement for a water retaining structure to be more like 0.5 to 0.6% Ag.

With joints that far apart, you may as well leave them out. The joints won't prevent any of the cracking which will occur.

Is the effluent corrosive? If so, I would use galvanized bars.

 

[Ron]

With no fines concrete, I would consider compacting the NF concrete to achieve a relatively flat surface and then use a geotextile, even two layers, to separate the slabs and allow shrinkage in the overlying slab to be less restrained. Further, I'm a fan of less reinforcement for slabs on grade and proper jointing for crack control, with joints spaced at 3m to 4m max for a 150mm slab.

If you sawcut the control joints properly and provide proper spacing, you can then seal the joints with a semi-rigid epoxy and achieve good performance.

 

[hokie66]

Agree with Ron's suggestion about using the geotextile as a bond breaker, whether you provide joints as he suggests, or leave them out as I wouuld if it is designed as a water retaining structure.

If this is not a slab subject to traffic, and you adopt Ron's jointed approach, I would use a flexible polyurethane sealant rather than the semi-rigid epoxy.

 

[sybie99]

Thanks for the replies, I think I will go for the jointless construction with more rebar and a bond breaker.