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George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding

Post Number: 316
Registered: 11-2004
Posted on Wednesday, June 06, 2007 - 06:49 pm:   Edit PostDelete PostPrint Post

We traditionally use lightweight concrete over metal deck for floor construction. We are being asked by a CM to use normal weight concrete because lightweight takes longer to cure to a point where flooring can be installed. We are also told that normal weight concrete is less expensive than lightweight.

Our concern is with the additional mass to our structure, and what it will add to our steel frame and our seismic design.

Has anyone run into this issue before, and do you have insights to share? We are particularly interested in hard background data on cure rates related to floor finish. This is an elevated slab, not slab on grade. Thanks.
Dave Metzger
Senior Member
Username: davemetzger

Post Number: 197
Registered: 07-2001
Posted on Wednesday, June 06, 2007 - 07:02 pm:   Edit PostDelete PostPrint Post

I have run into this issue not for interior slabs, but for roof decks where we have adhered roofing membranes. Because of the higher water content in lightweight concrete, there can be a substantially longer time period (compared to normal weight concrete) until the moisture level in the concrete is reduced to the point where the membrane will adhere.

Per American Hydrotech's specifications: cure times for normal weight concrete, 28 days recommended, 14 days minimum; for lightweight concrete, 60 days recommended, 28 days minimum.

George, I realize this is for a roofing membrane and not finish flooring, but it gives you a sense of the magnitude involved.
Posted on Wednesday, June 06, 2007 - 06:55 pm:   Edit PostDelete PostPrint Post

Colin Gilboy
Senior Member
Username: colin

Post Number: 98
Registered: 09-2005
Posted on Wednesday, June 06, 2007 - 07:18 pm:   Edit PostDelete PostPrint Post

You also have to watch the fire ratings on the floor. Structural lightweight (110 pcf I believe) will give 1- and 2-hour ratings at about 60% of the thickness of the 150 pcf versions. If you changed to the equivalent thickness the contractor may have to fireproof the deck - which will be much more expensive. This does not include any seismic-required changes.

Roof lightweight insulating concrete is an entirely different animal with a 40 pcf density for products like Zonolight.
Ronald L. Geren, RA, CSI, CCS, CCCA, SCIP
Senior Member
Username: specman

Post Number: 477
Registered: 03-2003
Posted on Wednesday, June 06, 2007 - 09:47 pm:   Edit PostDelete PostPrint Post

The structural engineers that I have worked with in the past have always specified normal weight concrete.
George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding

Post Number: 317
Registered: 11-2004
Posted on Thursday, June 07, 2007 - 10:44 am:   Edit PostDelete PostPrint Post

To Colin's point, the CM is aware of the need to increase the thickness of the normal weight concrete for the fire rating. That just makes the additional mass more of a problem. The issue seems to be the increase in seismic resisting steel more than just bearing the extra deadload.

Dave, your suggestion of twice the cure time for lightweight is consistent with other information we've found.
Mark Gilligan SE, CSI
Senior Member
Username: markgilligan

Post Number: 183
Registered: 05-2005
Posted on Friday, June 08, 2007 - 05:44 am:   Edit PostDelete PostPrint Post

In California with our obsession with earthquakes lightweight concrete on metal deck is very common.

I would suggest we differentiate between curing and drying. To facilitate curing we try to prevent moisture loss while the emphasis on floors is focused on encouraging the evaporation of free water in the concrete pores.

Lightweight concrete takes no longer to cure than does other concrete. It may take longer to dry out than other concrete that I do not know. Because of the pressure on drying floors it seems that the lenght of curing specified has been reduced.

I have found in a number of instances that the equilibrium density of lightweight concrete listed in the submitted mix design exceeds the 110 pcf density listed in the rated assembly which could possibly mean that we do not have the fire rating specified. I do not believe that most structural engineers are checking for this when reviewing mix design submittals
John Regener, AIA, CCS, CCCA, CSI, SCIP
Senior Member
Username: john_regener

Post Number: 305
Registered: 04-2002
Posted on Sunday, June 10, 2007 - 08:35 pm:   Edit PostDelete PostPrint Post

What does this thread have to do with construction contract administration?
George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding

Post Number: 321
Registered: 11-2004
Posted on Tuesday, June 12, 2007 - 01:44 pm:   Edit PostDelete PostPrint Post

[See Mortar Net]

John - I am not sure where this discussion belongs, but I posted it here, even though we are still obviously in the design phase, because it seemed to directly involve construction related issues - specifically, a suggested substitution driven by a tight schedule. And while there is no construction contract yet so therefore nothing to administer, the question did seem to warrant comment from those with CA backgrounds and experience.

Having CM's involved early in the design phase introduces a whole chain of construction related discussions way before construction actually commences. So I'd pose this question, where would you post this thread if not here?

I'm not sure that Specifications is totally correct - either you'd spec lightweight concrete or normal weight concrete. Once the decision is made, no real discussion, it is one specification or the other. And as far as floor finishes, you'd be concerned with testing the dryness of the concrete, regardless of what kind of concrete was used. So, it didn't seem there were really any specification issues.

Maybe Products is a better place, but that always seemed to me to be about manufactured "things" rather than materials. Maybe that is too narrow a view?

Although I read all three fairly regularly -- Specs, Products and CA -- I get the feeling that others prefer one area of the board based on their backgrounds or their current activities, and I guess my posting here was an effort to get responses from those people who have a construction background.
John Regener, AIA, CCS, CCCA, CSI, SCIP
Senior Member
Username: john_regener

Post Number: 309
Registered: 04-2002
Posted on Sunday, June 24, 2007 - 11:45 am:   Edit PostDelete PostPrint Post

Everything on this forum is "construction" but not everything ought to be under the heading of construction contract administration.

The technical subject (curing of lightweight aggregate concrete) is a valid one to discuss and it would be helpful for future retrieval and even renewed construction to have it in the Products heading.

I do understand the underlying issue: How to deal with changes initiated or directed by owners, construction managers and general contractors. It would be appropriate to discuss this under the construction contract administration heading and get suggestions about how to respond to proposed changes which the architect/engineer disagrees with (usually "value engineering" proposals that reduce initial costs and ignore long term performance).

At the just-concluded CSI SHOW (& convention), the subject of CM-dictated Division 1 specifications was discussed in one of the sessions I attended. One response was to demand CM's license stamp and signature since the CM was assuming the role of design professional of record.

I'd like to see someone initiate discussion of these sorts of issues, under the Construction Contract Administration heading.
John Bunzick, CCS, CCCA
Senior Member
Username: bunzick

Post Number: 742
Registered: 03-2002
Posted on Monday, June 25, 2007 - 08:36 am:   Edit PostDelete PostPrint Post

John, which session discussed CM's Division 01. I'd like to download it.
George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding

Post Number: 323
Registered: 11-2004
Posted on Monday, June 25, 2007 - 10:16 am:   Edit PostDelete PostPrint Post

I think he is referring to one of Phil Kabza's sessions (Engineering Coordination?). Phil's handouts weren't on the CD we got, but may be available online: I haven't checked this morning.
Mark Gilligan SE, CSI
Senior Member
Username: markgilligan

Post Number: 187
Registered: 05-2005
Posted on Monday, June 25, 2007 - 11:16 am:   Edit PostDelete PostPrint Post

As long as the Division 1 provisions are related to procedures for administering the contract and relate to financial risk as opposed to physical performance of the completed structure, then they do not require the stamp of the design professional. In such a case we may believe our Clients decision to be ill advised but we are in no position to prevent him from making these changes.
Posted on Monday, June 25, 2007 - 12:46 pm:   Edit PostDelete PostPrint Post

Of course, in CSI's infinite wisdom, the Education Handouts have been removed from the website. You get the dates to click on and a blank screen after you click. Hopefully it is an oversight that can be corrected when they read my e-mail asking the handouts to be reinstated.
Robert W. Johnson
Senior Member
Username: bob_johnson

Post Number: 149
Registered: 08-2004
Posted on Monday, June 25, 2007 - 02:57 pm:   Edit PostDelete PostPrint Post

The education session handouts are back up on the CSI website (http://www.csinet.org/s_csi/sec_csishow.asp?CID=1659&DID=12520).

You can find them under:
The CSI Show
Categories, CSI Show Handouts

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