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Minimum grade for structural concrete

Minimum grade for structural concrete

Minimum grade for structural concrete

I 'know' that the minimum grade is 20 MPa (although I only use 20 MPa for houses) but is this minimum stated in an AS standard anywhere?

We've been given the task of assessing the effect of 13 MPa concrete as supplied instead of N32 as specified.

I'd like to cut to the chase and say it can't be classed as structural concrete and therefore is not acceptable.

RE: Minimum grade for structural concrete

Breaking news;
I've re-read the report and 13 MPa is the 7 day break result, so the 28 day result is likely to be around 20 MPa.

RE: Minimum grade for structural concrete

I only know of the minimum grade for abrasion clause. Whenever i get problem I reject based on the lap lengths and detailing being wrong.

An expert is a man who has made all the mistakes which can be made in a very narrow field

RE: Minimum grade for structural concrete

Thanks RE

I'm looking at a retaining wall and at pedestals so there are 3 criteria (discounting deflection);
- strength
- lap length
- durability (cover to rebar)

RE: Minimum grade for structural concrete

AS 3600-2001, in 1.1.2(a), gives a range for which the Standard is applicable, 20 to 65 MPa.  This range has been extended in the latest code, to I think 100 MPa.  If below this, you would have to rely on the "Use of Alternative Materials or Methods" provisions.

RE: Minimum grade for structural concrete

Thanks Hokie, that's probably the nearest that I can expect, although it doesn't rule out <20MPa completely.

RE: Minimum grade for structural concrete

I would be concerned with the loss of strength and durability. If it is detrimental to the design I would be rejecting the concrete? Do you know when 20MPa came to site instead of 32MPa?

I have read through AS3600-2009. Clause 1.1 says the code is applicable for 20MPa to 100MPa (as hokie mentioned).

Otherwise the minimum requirements for strength based on exposure classification, durability, abrasion etc is all in section 4.

Reject it unless other rectification solutions are present.

RE: Minimum grade for structural concrete

Table 4.4 in AS 3600 requires at least Grade 32 for exposure classification B1.  That would be a perfectly valid reason for rejecting concrete that would reach Grade 20 at best.

Doug Jenkins
Interactive Design Services

RE: Minimum grade for structural concrete

The problem with lower grade concrete is often durability - it is usually not as dense or well compacted and therefore does not protect the stteel as well.

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