Are concrete mix details required in drawing set? 12

[StructureMan44]

Sometimes I have seen concrete note drawings specifically list the required concrete mix details. Is this necessary or is specifying ACI 318 (mainly chapter 3-Materials) sufficient?

 

[XR250]

A structural engineer MUST know the materials upon which his/her design is based. To specify a 3000 psi concrete and give no further direction or limitation is irresponsible, does not meet the standard of care and leaves the engineer open to a variety of claims.

I think it depends on the type of job. The only time I specify a concrete mix is when I do exterior suspended slabs and I get that mix from a concrete consultant friend of mine.. Most of my other work is only light commercial and residential footings and interior slabs. I typically just specify the compressive strength and am done with it. Same as my competitors - so I meet the local standard of care. If I was doing R/C buildings, it would be a different story. I don't pretend to be a concrete expert and I can actually see getting in more trouble by over or incorrectly specifying it.

 

[MacGruber22]

If I was doing R/C buildings, it would be a different story.

I don't pretend to be a concrete expert and I can actually see getting in more trouble by over or incorrectly specifying it.

Those comments seem to conflict - Or are you saying that you would learn more concrete mix design if you were designing framed concrete buildings?

Not to minimize the importance of framed concrete, but I believe that concrete repairs on prestressed concrete garage structures is one of the most important types of building structural work that warrants the EOR's careful attention to mix design. It seems that most engineers around my area leave way too much up to the repair contractor. Sometimes I wonder why some clients even bother hiring a consultant who essentially puts together documents which points out defects with an estimated quantity to be repaired. Often times, the contractor performs the repairs without the repair engineer performing inspections. *I am going to gloat a bit* I believe that my firm is very busy and successful with concrete repair-work, because the level of attention paid to understanding the properties of concrete and related materials. I know it is a lot of information - concrete is such a complex material, but I think it is so worth it.

What gripes me, though, is when a spec says to use a w/c ratio of such and such and a requirement on strength but also gives a minimum cement content.

Yeah, that is a problem related to the "total cement content" being a hindrance on durability.



"It is imperative Cunth doesn't get his hands on those codes."

 

[Ron]

BigH...specifying a minimum cement content and w/c ratio (as I'm sure you know from experience) is done by an engineer who doesn't know what he's asking for and doesn't know the difference between compressive strength and durability. As Neville has told us (and we should listen), if we design for durability, compressive strength will follow! We can get compressive strength easily.....durability, well, not necessarily so!

 

[XR250]

Those comments seem to conflict - Or are you saying that you would learn more concrete mix design if you were designing framed concrete buildings?

Correct. I don't do these types of buildings due to my lack of experience and my preference for smaller jobs.

 

[BUGGAR]

Yeah, we used to specify less than 2000 psi concrete to avoid special inspection then decrease the w/c ratio for "durability" (and to achieve our design strength). That was common. And of course only for certain work.

 

[BeFEA]

In Europe, when I use the Eurocodes I usually specify concrete strength class, aggregate size and environmental exposure class. Regulations (EN 206) actually require some more specifications, but I am not the only one giving so few specifications.

Aggregate size is especially important when you have designed rebars with a small spacing somewhere in the structure and when you have used (or presumed) the aggregate size somewhere in your design. Maybe for this reason the aggregate size is not specified in some projects.