Slab on grade-no reinforcing

[haynewp]

How many of you have used a slab on grade without any reinforcing at all? We did this on one project and I am not aware of any problems resulting from it. I have discussed doing this before with a concrete professor/engineer who thinks this is the best way to go (referring to slabs with warehouse type loads or less on decent soil). His thought is that the wwf that normally ends up on the bottom of the slab due to the highly ineffective pull-up method does more harm than good.

That is, use a better mix and tighter control joint spacing and leave out the reinforcing completely.

 

[fndn]

I agree that WWF ends at the bottom but I prefer to have minimum shrinkage, temperature etc. rebars in the middle third. Geotechnically speaking, should the subgrade soils be sandy and you have a fluctuating water table near the surface, the reinforced slab will be able to span distances that have no soil support due to erosion. Likewise expansive soils and collapsible soils benefit more if the slab is reinforced.

Let us say the warehouse is later sold to another buyer that has more forklifts or stores heavier loads, with rebar you have better quality.

As engineers, we have to be cautious of designing the same way just because failure has not occured and also explain to clients that it is cheaper to do it better now than later have to pay investigation and fixing costs.

Nearly one third of our firms forensic studies involve problems with unreinorced slabs, 20 to 30 years after they constructed.

 

[haynewp]

Rebar in the middle third should be the safest way to go but of course more expensive for the labor. I have thought about tack welded mats of reinforcing that could be easily chaired but have not proposed this before.

WWF is normally specified to be placed in the top 1/3 (for crack control at surface from shrinkage) so even if it ended up where it is supposed to be, it would not help a whole lot for the case of loss of soil support.

Have the forensic investigations been with slabs with no reinforcing or were they slabs with fibers only? I understand fibers are supposed to be good for shrinkage crack control but are not to be relied on for structural means (though the manufacturers are likely to tell you differently).

 

[fndn]

Most of the failures are with unreinforced slabs. I work in Orange county, CA and due to alkaline soils with higher soluble sulphates, Type V cement is mostly chosen. As a result, most owners are weary of paying for fibers or admixtures that increase concrete cost. My other battle with slabs are to require thicker visqueen-I insist on 15 mil. This part of the world is very litigious and jobs you did in the past 10 yrs can be questioned.

You bring good point about cost. Most developers of large warehouses insist on no reinforcement. For those clients, if the site is on sandy soils, obviously no frost and very deep Ground water, then I accept 7.5" slab without rebar vs 6" reinforced. However if they are on clayey soils then we give them thicker aggregate base plus 15 mil plus minimal reinforcemnt(#4 @ 24" o.c.) for example.

 

[dik]

I have some literature from PCA and ACI that indicates that reinforcing steel has little effect on the behaviour of a SOG. I like to add rfg but use bars only located 2" from the top of the slab. Missed by sawcutting and is near the top where you want any cracks to be 'held' together.

I don't use WWF... too many instances of it being on the bottom and spacing too close to allow workmen from easily walking between the bars <G>.

Dik

 

[JAE]

Some similar discussions:

thread507-126594

thread507-144777

 

[jike]

If using WWF, we insist that it be walked in after the concrete has been poured 1/2 to 2/3 in thickness. This works well, although it requires more labor than hooking. It is hard to push it completely down as it is to pull it up. Please note that it is recommended that alternate wires be cut at control joints and no laps within say 2 feet of a joint.

I used to work for a major US industry and we always poured warehouse slabs "unreinforced" for forktruck traffic.

I prefer "polypropylene fibers" for commercial grade structures.

 

[Ron]

...and age-old discussion that will continue!

In general, there is not a good reason to reinforce a soil-supported slab unless the loads are tremendous, which they usually are not. As an example for pavement loads, including fork lifts, they are typically analyzed as plain concrete slabs. The primary design parameter is the tensile stress at the bottom of the slab induced by loads on the top....(extreme fiber stress for the beam jockeys)

WWF vs. fiber has long been an argument, with both sides raising good points and lieing about the bad ones!

Proper control joint design, proper mix design for the application, proper placement, proper finishing, and proper curing will do more to achieve good performance from the slab than either the WWF or fiber, without regard to fiber type such as polypropylene or steel.

Fluctuating groundwater typically does not cause underslab erosion. That's typically a surface runoff issue and lack of slab edge protection.

 

[dik]

Ron: Glad to see you again... just to add that depending on the availability of water and the type of soil, it's possible to get 'pumping' at the joints with moving loads that can undermine the slab edge... one of the reasons that joints should be caulked/sealed.

Dik

 

[Ron]

Dik...thanks. I agree.

 

[dmhb]

What is a typical minimum thickness for a sog with no reinforcing for institutional use? I am pouring several large floors on grade (well compacted sandy fill), isolated from structural loads (independant of wall/column foundations) for classrooms and conference rooms. Very low loading, in other words, but I am a little concerned with cracking that would affect the ceramic tile finish. Any suggestions?

 

[dik]

I would think an absolute minimum of 4" and likely 5" min. I would be reluctant to use it with a quality brittle finish such as tile and sawcutting would have to be within 8 hours of finishing or use softcut saw. Panels would have to be small and coordinated with the tile joints using a flexible material in lieu of grout at these locations. Assuming foot or light cart traffic only.

Dik

 

[dmhb]

Reading between the lines I assume you would always use a WWF or other reinf. to conrol crack size?
I have started a new thread since I am asking more questions:
"slab on grade for tile floor"
thread592-155870 (how do you link)

Thanks.

 

[dik]

Generally use plain concrete for residential stuff and sidewalks only... else, use reinforcing steel... not WWM

Dik