Concrete strength for retail sidewalks

[stevenspm]

Reading one of the recent threads about salt on concrete reminded me of a conversation/argument I had with a contractor/developer. I designed a commercial/retail building in central New York and called out 4500 psi concrete because it will be subject to freeze/thaw cycles or de-icing chemicals. He asked about this because he thought it should be 3500 psi concrete, I gave him a brief explanation why it is 4500 psi and thought that was that. He later came back with an ACI 318 reference for plain concrete stating that because it is a sidewalk slab on grade it does not need to be designed based on the code. I replied with the building code sec 1901.1 where it states that with the exceptions of 1904 and 1910 yes sidewalk slab on grade does not need to be design per the code. 1904 however deals with durability and states that concrete subject to freeze/thaw cycles shall be per the ACI 318-05 sec 4.2.2 which says to use the 4500 psi concrete. He looks it up and see the table and tries to use that because it has sidewalks using 3500 psi not reading that the table is only for Group R occupancy, which I tell him. He then calls the project manager, who is an architect, at which point the project manager and I talk about it where I show him all the code references I used, at which point the project manager use the building code's chapter 19 sec 2 to agree with the developer. After that I do not know what he told the developer nor do I know what was actually place since at this point the project should have been done. it might be possible the 4500 psi was placed and the developer was going though checking the budget/reviewing orders reciets looking for items he can pawn off on others.
What I am wondering is if I was reading the codes correctly or is the developer and project managers reading correct?
Thanks
Phil

 

[manstrom]

This has been a big point of contention lately. It's like people just noticed it.

I started in residential and all exterior flatwork was 3500 psi concrete. If you look at the chart in the ACI, it asks for 4500 psi with xx WC ratio. I always felt that this was odd.

If it's just a sidewalk, I don't normally care. This is civil territory to me. Use whatever you normally use. A garage slab on grade *should* be 4500 psi. But if you for to a house, all it requires is 3500 psi.

I normally spec 3500 unless the county makes a stink about it.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[Canuck67]

In my area, the basic parameters for sidewalks are 32 MPa (4640 psi) concrete, with air entrainment and specific controls on the w/c ratio. These are all factors for long term performance, i.e. durability and resistance to freeze thaw cycles.

 

[JAE]

I think you are correct in your strict logic in the code.
However, nobody follows this in my area - usually sidewalks for any type of building is in the 3,000 to 3,500 psi range.

I think the higher strength is prudent and wise to provide a better, durable concrete.....but not a direct life safety issue.
Our own office building replaces the sidewalks about every year.

The only "out" that I can see is that exterior sidewalks are not really, technically part of "structural concrete for buildings" and thus to me fall outside ACI 318 no matter what the IBC says.

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[bridgebuster]

NY City DOT sidewalk standard is 3,000 psi (minimum), air entrained concrete. State DOT is also 3,000 (psi) minimum. However, their mix typically produces higher strengths.