Concrete Grade for External Slabs on Ground

[rscassar]

Hi

What is the industry practice for specifying concrete grade for slabs on ground. Builders (mainly residential and smaller commercial projects) will naturally order N25 concrete because that is what they have always provided. I myself have been on-site before and instinctively instructed N25 concrete for an external slabs on ground.

I was reading through AS3600-2009 today to find whether N25 concrete is applicable for external slabs on ground. As defined in the code, when the surface of the member is in an above-ground exterior environment within 50km of the coastline, the exposure classification is B1 and requires a minimum 32MPa concrete.

Would 32MPa concrete be appropriate for even the most minor structure (i.e. Importance level 1 defined in the BCA).

I am looking for arguments to suggest that N25 concrete is acceptable and the only resource I have found is AS2870 which says that the minimum concrete grade to be used is N20.

Am I too conservative in using classification B1.

 

[rowingengineer]

you may wish to read this thread

thread507-251461


ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION.”

 

[rscassar]

Thanks RE

I don't know how I missed that post.

Regards

 

[csd72]

There used to be a clause in AS3600 that allowed a lower strength if only one face was exposed and you used the higher (bracketed) covers.

This is how we used to justify it.

 

[apsix]

That's an optimistic interpretation csd72.

The AS3600 commentary shows an example of a suspended edge beam with the 1 vertical face exposed. I think that is the intent of the clause.

However, for slabs on grade it's probably justifiable. I wouldn't do it for a suspended slab.

 

[csd72]

Yes and the other interpretation is that the failure of the slab on grade will not lead to collapse and therefore the slab could potentially be a plain concrete member.

 

[rscassar]

What can the structural engineer do if the concrete test results consistently do not come up to grade. I.e. cyclinder tests for 32MPa are constantly below 28MPa at 28 days.

Thanks and regards.

 

[csd72]

kikflip,

a good question with a complex answer.

The answer depends on what the strength is needed for.

If it is a post tensioned beam or a column then the strenght is most likely critical for strength, for most other cases the strength of the concrete is dictated by durability criteria.

If necessary for strength then it may be the case that you could wait for another couple of weeks before testing the remaining cubes in the hope that the mix continues to gain strength at a reasonable rate. If you find that it is still not of sufficient strength then demolition and rebuilding of the effected parts may be necessary.

If it is required for durability only then there are a number of surface coatings that will increase the durability of a weaker concrete.

I would suggest that you check their curing regime for future pours.

 

[apsix]

Pray that you get a better result at 56 days.