to them at all. "Why should I give you concrete test cylinder results?
I didn't allow fot that in my price!" Reply: The Building Code requires
3 cylinders per 50 cubic metres. "Never heard of it and it doesn't
apply to us".
Gary
Benjamin Maxwell wrote:
> This is a classic example of why concrete should be packaged by
> engineers as a performance-based product. The concrete industry has
> changed so much in the past few decades, and so much has changed in
> terms of material options, aggregate availability, the myriad of
> admixture products, and so on, that it really requires a _exacting
> command_ of concrete science to proportion a mix. Increasingly so,
> this is _NOT_ the engineer's realm.
>
> Does specifying a prescriptive mix really matter in terms of
> structure? In general, we want concrete of a certain strength and
> weight. More specifically, we may want concrete with low
> shrinkage and/or concrete with a certain air content. Very
> specifically, for exacting applications, we could need concrete with
> low porosity, high corrosion resistance, or controlled heat of
> hydration (mass concrete).
>
> Who is more qualified to determine the materials, proportions,
> delivery methods, placement methods, and curing of concrete that meet
> a set of qualifications, the engineer, or the concrete producer?
> I argue the latter.
>
> We care about the qualities of the end product. How it gets there
> should not be the responsibility of the engineer.
>
> We're not on the hook to determine the exact amount of carbon in a
> heat of steel. We simply specify the appropriate ASTM standard for
> the material we want. Wood is similar and we don't need to tell CMU
> manufacturer's how to make their masonry - just that it meets the
> appropriate ASTM standard. Why is concrete so out of bounds with
> respect to the other major building materials? Why should it be?
>
> It should be up to the concrete producer to provide, deliver, and
> place satisfactory concrete material in conformance with a set of
> engineering performance requirements. We just need to establish and
> standardize the performance requirements.
>
> Performance specification of concrete gets tricky in terms
> of shrinkage control, permeability, and mass concrete, but there are
> tests and practices established, in-place, tested, and in use that
> have demonstrated that it is possible to use a performance-based
> methodology.
>
> Not to get too long-winded about this, but we should not be hung out
> to dry because of issues like Andrew's where the contractor can easily
> pass blame and the responsibility for bad concrete rests on our
> shoulders. I'm sure every engineer on this list can recount scores of
> stories about being coerced and jostled into making decisions about
> concrete mixes beyond their true ability to fully realize the
> consequences. We shouldn't have to roll dice.
>
> -Ben Maxwell
>
>
> */"Andrew Kester, P.E." <akester@cfl.rr.com>/* wrote:
>
> Listers,
> We normally have a fibermesh mix of 3.5lbs/CY of 1.5" polyprop.
> fiberson our standard SOG in lieu of WWF. We have never had a
> comment before, but now a GC wants to use less "because his
> concrete company cannot provide that much and can I use less...?."
>
> We are getting this info through the client so we don't know what
> exactly that means. All the fibermesh specs I have looked at
> online say 1.5lb/cy MINIMUM.
>
> What do you guys use if you don't mind me asking?
>
>
> Andrew Kester, P.E.
> Principal/Project Manager
> ADK Structural Engineering, PLLC
> 1510 E. Colonial Ave., Suite 301
> Orlando, FL 32803
> M 407-921-1617
> F 321-249-0349
>
>
>
>
> /Benjamin H. Maxwell, S.E./
>
> ------------------------------------------------------------------------
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