Exterior loading area slab on grade - questions

[P205]

I have an industrial building with loading docks. The exterior approach slab is a concrete slab on grade that the contractor wants to be 8in (200mm) thick with macro-synthetic fibers. I have several questions. Using the modulus of subgrade reaction from the geotechnical report, I determined that the slab thickness would need to be about 9in (225mm)based on the PCA method found in the Concrete Floors on Ground book.(I have searched on the forum for similar topics)

1) Is the exterior slab on grade part of the building permit? I have not design the exterior slabs on grade for any other building I've design so far in my short career.

2) From my understanding, the steel reinforcing is simply to control shrinkage, is this correct? Despite the large truck load? Am I understanding this correctly?

3) If the reinforcement is simply a shrinkage issue, then I would think that there would be an acceptable solution using macro-synthetic fibers. But from my limited research, it seems to be discouraged.

Other details:
- I have searched on the forum for similar topics
- I'm in Canada and using CSA A23.3-04 (R2010)
- I would use american or international information if available on this topic
- This is a design-build project
- The contractor is asking why we can't use fibers. So I need to be able to hang my hat on something saying we can't, or we can and here is the documentation showing how to design it or how it meets code. Does that make sense?

 

[JAE]

1) For us usually the exterior pavement is a civil engineering issue and we don't typically specify it. That's not to say that an agreement may require the structural engineer to do pavement design (we do interior slabs-on-grade design all the time). Just not typically done in our neck of the woods.

2) Steel bars (and wires to some extent depending on position) can help in slab toughness and stiffness but typically they are used and intended simply for crack control.

3) We've used fibers in interior slabs. I've not seen them used in highways and pavement as much.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[P205]

I've since talked to the building official and he confirmed what you said JAE, that the exterior slab on grade is a pavement and is covered in the site plan by the civil guys. I guess the contractor wasn't aware of this and asked the question to me (structural) since there was no information about the slab on the site plan drawings.

 

[atrizzy]

If you have very heavy loads and soft subgrade you may get to the point where your rebar is required for flexural resistance, but in my experience this is rare. Even in such a case your slab should be thick enough to limit flexural stresses enough that the rebar would serve as temperature/shrinkage/crack control.

 

[P205]

Thank you atrizzy,your point is very clear and makes sense to me.