AS3600 concrete strength for slab on ground

[demayeng]

Hi,

Nice easy Friday arvo question..

AS3600 is clear in saying that for exposure classification B2 (above ground exterior within 1km of coast) you must use 40 MPa concrete. Does 'above ground' apply to slabs on ground?

At this office (my first structural workplace) this is common for suspended slabs, but I've never seen it for slabs on ground. I've just asked two old draftsmen and they have never specified 40MPa for a slab on ground.

Whadya reckon?

 

[BAretired]

If you need it for suspended slabs, what is the harm in using it on a slab on grade? We are talking here about endurance, not strength. Go for the higher strength because it will give you durability.

BA

 

[apsix]

If the only exposed edge is the vertical slab edge of an otherwise internal slab the cover can be increased to 65mm with N32 concrete (Table 4.10.3.2).

If it's an external pavement slab AS3600 doesn't strictly apply but it would be wise take extra precaution against corrosion, perhaps by increasing the cover.
Austroads' road design manual may have some advice.

 

[demayeng]

Thanks guys

It's an external concreted area. 25 MPa has been advised.
AS3600 doesn't apply because reinforcement is not included for strength?

 

[apsix]

Pavement slabs aren't structures and therefore don't have to comply with AS3600.
It can, of course, still be used for guidance; such as table 4.7 where N25 is only recommended for light traffic.

I would have used N32 and generous cover.

 

[civeng80]

apsix

exactly what I would be doing.

Actually a difference like chalk and cheese between 25 MPa and 32 (or above)MPa concrete.

 

[rowingengineer]

I am going to have a bark hear; maybe I am barking up the wrong tree.
I have a concern that because this is a slab on ground, People decided that there is very little design required for this item. Now if this is a car park for a little church, yes very unlikely anyone is going to do more than spec a 125mm slab and some mesh or similar assuming good soil conditions.

However if this slab is going to see traffic either of high frequency or heavy loading, Then it needs to be designed like any other "structural item".

For instance abrasion resistance normally determines the concrete strength. The exposure will not generally effect the concrete strength but it has a great impact on the curing you spec. I personally wouldn’t go below N32 for an external application. Even though the heavier class will cause more shrinkage and curl, I attempt to deal with the curl with good joint details

You should also spec a finish, while your architect may have an input here, I would consider it prudent to nominate, others may disagree. But to often i see a burnished finish on a N25 concrete slab it just does not work full stop.

Then you should design for strength, I use the CCA T48-1999 for my designs. For this you will need to look at soil conditions, expected loadings ect. Note a 3t forklift is about a 7.5t axial load, common mistake, and something that kills pavements everyday. I can hear one screaming on the wind right not.

Now once you have a good design, one that you can stand by in court, it is time to joint this animal, 6m crts is a good starting point. But be sure to understand your joint design, there are many products that can be used, just spec’ing the standard office detail isn’t designing.

I have spec’ed many pavements over 40mpa, with thickness’s in excess of 300mm. So please take the time to learn about the methods of design, as I hate doing investigations as to why a slab has just been nominally spec’ed without design to back it up. Then the client wonders why the pavement has cracked to all buggery, Believe you me, you can’t stand there and just say concrete cracks in one area when it doesn’t in another.

Grrrrr, I shall stop barking now and get off the soap box, as i am fairly sure that i am barking at the wrong tree, please note i bark at no-one inparticular, but the world in general.


When in doubt, just take the next small step.

 

[apsix]

rowingengineer
I won't argue with most of what you have written but will comment on two issues:
- No one implied that the slab wouldn't be properly designed, but that's OK. Better too much information than not enough.
- AS3600 makes a clear link between exposure class, concrete strength and cover. Why would you say otherwise?

 

[rowingengineer]

apsix,
-Your right no-one suggested that the slab wouldn't be designed correctly; I was off on a tangent apologies all round.

- While AS3600 dose make a clear link as you quite rightly pointed out between exposure class, concrete strength and cover. Technically the design of a slab on ground is a plain concrete member with reinforcement added only for crack control. (Would like to see if anyone disagrees with this statement?). Which will give you an A1 classification most days of the week, which means from this part you could use, N20 concrete. But a quick check of abrasion and bang you must use N25 as a minimum, Even N32.

Sure if reinforcement for shrinkage is added then adhere to Note 1, "In this context, reinforced concrete includes any concrete containing metals that rely on the concrete for protection against environmental degradation. Plain concrete members containing reinforcement or other metallic embedment should, therefore, be treated as reinforced members, when considering durability."

But even as I read this now it can be taken as ambiguous at best, does this mean you can have reinforcement for N20, and just provided appropriate cover for the OP’s situation?, or should you up the concrete to N32 as a B2 class and then do cover???? I have seen many different interpretations, and many different solutions. My personal view is B1 external minimum aka N32.

So in summing up I don't mean for anyone to not check exposure classification, this is not my intent, I just want to make sure once exposure classification is checked (and if A1 were adopted) then check abrasion and ensure the curing period is correct, thus that the concrete slab is of a minimum concrete strength. This would at least ensure the concrete pavement stays for a while.



When in doubt, just take the next small step.

 

[csd72]

While 40 MPa concrete is rare for slabs on ground it is not unheard of. The main criteria for the concrete strengths of industrial slabs is abrasive strength and often a 32MPa grade will be used to achieve this.

I would recommend you refer to the slab on ground design guide (or whatever its called)

 

[demayeng]

Thanks guys.

As I'm finding with most topics in structural engineering, there are massive grey areas. Interpretation of the code is required and most engineers do things differently. So every green engineer like myself wastes plenty of time finding this out instead of there being a more comprehensive code that avoids all the ambiguity.

Just a point - the concreted area is in a school and subject to foot traffic only.

 

[rapt]

demayeng,

A Code is supposed to list design limits.

What you are asking for is a "cookbook"! That is not a Code. A Green Engineer should ask his supervisor! If he does not have one, he should not be practicing until he has changed colours!!

 

[civeng80]

rapt,

demayeng has a right to ask any question he wishes to do with structural engineering.
We all learn from these questions young and old, experienced or green.
His question was a perfectly legitimate one.

Your comment does nothing for his question or the group.

 

[demayeng]

thanks for the backup

I meant that the code could easily be less ambiguous about this particular issue. Then you wouldn't see the range of answers that you see here.

 

[csd72]

Demayeng,

In this case 25MPa should be sufficient unless the school is particularly close to the ocean. The bracketed covers in the code for one sided exposure can be used.

That said, most of these paths are done on cookbook specifications based on previous experience. Be careful with the joint layout as this is probably the most important thing.

 

[rowingengineer]

One extra little bit of information to think about here in schools which is often ignored and not of particular importance to you, is that schools should be designed for a longer return period. but worth a consideration for a second; thinking out load the durability for the code is about 50 years, say you want 100yrs, you may wish to use N32, such that it is more durable.

We all know that there will never be any maintenance on the slab.


When in doubt, just take the next small step.

 

[apsix]

In the defence of rapt:

I almost responded to this statement myself;
'So every green engineer like myself wastes plenty of time finding this out instead of there being a more comprehensive code that avoids all the ambiguity.'

And I generally agree with rapt's comments. I don't believe that he meant that demayeng's question here was out of order.

Thinking through and obtaining other opinions on seemingly simple concepts should not be seen as wasting time.

 

[csd72]

I agree, I do it all the time when I am at a new company just to understand the way they think.