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Re: fibermesh rates - Performance-Based Concrete...
- To: seaint(--nospam--at)seaint.org
- Subject: Re: fibermesh rates - Performance-Based Concrete...
- From: Benjamin Maxwell <enginerd666(--nospam--at)yahoo.com>
- Date: Wed, 13 Feb 2008 10:07:19 -0800 (PST)
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).
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.
"Andrew Kester, P.E." <akester(--nospam--at)cfl.rr.com> wrote:
Benjamin H. Maxwell, S.E.
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.
ADK Structural Engineering, PLLC
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803
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