Concrete Grade for External Slabs on Ground
Concrete Grade for External Slabs on Ground
(OP)
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.
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.





RE: Concrete Grade for External Slabs on Ground
thread507-251461: AS3600 concrete strength for slab on ground
ANY FOOL CAN DESIGN A STRUCTURE. IT TAKES AN ENGINEER TO DESIGN A CONNECTION."
RE: Concrete Grade for External Slabs on Ground
I don't know how I missed that post.
Regards
RE: Concrete Grade for External Slabs on Ground
This is how we used to justify it.
RE: Concrete Grade for External Slabs on Ground
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.
RE: Concrete Grade for External Slabs on Ground
RE: Concrete Grade for External Slabs on Ground
Thanks and regards.
RE: Concrete Grade for External Slabs on Ground
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.
RE: Concrete Grade for External Slabs on Ground