Slabs-on-grade

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Author Message Gerard Sanchis (Unregistered Guest) Unregistered guest Posted on Thursday, March 19, 2009 - 07:33 am:    We have for years specified a minimum of 2 inches of damp sand on the vapor retarder (VP) before concrete is cast to minimize slab curling of slabs-on-grade. This was recommended by ACI in the past. To my knowledge, we haven’t had problems with this approach in the past 30 plus years. ACI now recommends casting the slab directly on the VR (no sand bed). It may work in more humid climate but would probably result in slab curling in dry climates such as the South West where we live and work. Is there a consensus on sand bed over vapor retarder (VR) for slabs-on-grade, or is there a regional approach to its use? Jerry Tims Senior Member Username: jtims Post Number: 61 Registered: 04-2005 Posted on Thursday, March 19, 2009 - 09:08 am:    We don't do the sand thing on our projects. (We're in Dallas.) Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 137 Registered: 10-2007 Posted on Thursday, March 19, 2009 - 09:21 am:    The change in practice proposed by ACI was in response to the concern with slab moisture impacting floor finishes. Read ACI 302 if you have not done so already. Curling of the slab should not be a problem if the slab is cured as it should. In addition the use of reinforcing steel helps to minimize curling. We also have some hot dry areas in California and I am not hearing of problems with curling. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1027 Registered: 03-2002 Posted on Thursday, March 19, 2009 - 11:45 am:    We don't use the sand bed either. One of the problems is that if it rains prior to placing the concrete, the sand bed absorbs a lot of moisture which it gives back into the slab over a long period of time. Nathan Woods, CCCA, LEED AP Senior Member Username: nwoods Post Number: 288 Registered: 08-2005 Posted on Thursday, March 19, 2009 - 11:52 am:    In Southern California, it is not uncomon to cast directly against the vapor barrier, but its not widespread. Some contractors look at me as if I am speaking in tongues. Others are all over it and prefer it. Its a bit like the Global Climate Change debate. Science and comon practice/belief's ar at diametric odds at this point. I prefer casting against the slab. It does require more care/thought in execution, but I get a better slab. Phil Kabza Senior Member Username: phil_kabza Post Number: 376 Registered: 12-2002 Posted on Friday, March 20, 2009 - 08:09 am:    The ASTM flooring committee prevailed upon ACI to change this practice after they determined that the sand course was absorbing and holding excess moisture that later was resulting in flooring adhesive failures. This all played out about 7 years ago; it's interesting how long it takes to change habitual practices in the industry. The sand course did help sometimes with curling issues. When moving to a direct-to-vapor barrier concrete slab, look carefully at your concrete mix. Avoid using more cement than necessary, as it is the volume of water in the mix, not the water/cement ratio, that affects the amount of shrinkage cracking and curling issues you'll experience pouring on top of an impermeable membrane. Look at using WRDA or similar admixtures to offset some water. Take curing seriously. Threaten concrete masons who poke holes in the membrane with nasty consequences. Don Harris CSI, CCS, CCCA, AIA Senior Member Username: don_harris Post Number: 220 Registered: 03-2003 Posted on Friday, March 20, 2009 - 12:40 pm:    We don't use the sand bed and haven't for years. Most of the time I need to remove the requirement from the S.E.'s specs and have them change their detail. I'd much rather deal with some grinding and filling, if some curling should occur, than deal with the repercussions of a failed floor system. Howard Yancey (Unregistered Guest) Unregistered guest Posted on Sunday, March 22, 2009 - 11:57 am:    Here is a link to an article from 1998. http://www.4specs.com/articles/vapor_retarder.pdf ------------------------- Howard Yancey Butterfield Color Randy Cox Senior Member Username: randy_cox Post Number: 53 Registered: 04-2004 Posted on Monday, March 23, 2009 - 09:07 am:    Energy Star’s Indoor Air Package Specifications still lists a sand bed as an option to crushed stone (their item 1.3) http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/IAP_Specification_041907.pdf Richard L Matteo, AIA, CSI, CCS Senior Member Username: rlmat Post Number: 322 Registered: 10-2003 Posted on Monday, March 23, 2009 - 11:08 am:    I don't specify the sand bed either. It's been more of a battle with the geotech's than the structural engineers, but I think I'm winning. I normally reference the ACI 302. I also specify a maximum W/C ratio of 0.45 The only other problem I have is that I'm being deluged with slab treatment products, most of which I consider to be "snake oil". I'm being forced by some in my firm (prodded by owners or contractors) and mostly by owners (also prodded by contractors) to specify one of these. The Phoenix Chapter of CSI produced a "Green Sheet" on this subject back in 2003 which is available from the Phoenix Chapter. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1029 Registered: 03-2002 Posted on Monday, March 23, 2009 - 01:24 pm:    When you say "slab treatment products" are you referring to the moisture mitigation membranes? Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 144 Registered: 10-2007 Posted on Monday, March 23, 2009 - 02:43 pm:    I have found that structural engineers, geotechnical engineers, and architects all believe that slabs on grade and what goes below them is part of their scope of work. This is complicated by manufactures of flooring products that make recommendations regarding what I see as a structural issue. At times this can call for a bit of diplomacy to reconcile the differing world views. Complicating this is the practice of having the structural engineer specify the slab subgrade and the membrane and show them on structural drawings. I see the slab subgrade as the geotechnical engineer’s scope and the membrane as being in the architect’s scope. Thus I would be happy not to show these items. When others want to digress from the ACI 302 recommendations I will inform them of the issues and as long as I do not have a problem from a purely structural point of view I will defer to what the Architect wants to do. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 432 Registered: 04-2002 Posted on Monday, March 23, 2009 - 06:52 pm:    The parochial approach to construction specifications ("this is the geotechnical engineer's scope" or "this is the architect's scope") is not at all helpful. Not only is Masterformat not organized according to trade and subcontract jurisdictions, it isn't organized (unlike the drawings) according to design discipline. The geotechnical engineer provides only RECOMMENDATIONS. Frequently, these recommendations include things such as "10 mil visqueen" for the vapor barrier, a product that has not be been manufactured in the US for at least 10-15 years. And the infamous 2" sand layer is also commonly included "to protect the membrane," damn any other consequence. And damn any spec writer who dares to challenge or deviate from the recommendations in the geotechnical report. Excesssive water vapor emission and the resulting flooring failure is obviously not a STRUCTURAL problem, but it is a genuine problem for the Architect and the Owner. What is the solution? Two cast-in-place concrete Sections, one for structural engineer to write and another for the architect to write? Then, somehow the construction manager or the general contractor will figure out how to combine the two for bidding and construction? Let's hear it for the team approach to design. ... but BIM or Whole Building Design or Smart Building will take care of the problem so why worry, huh? Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 145 Registered: 10-2007 Posted on Monday, March 23, 2009 - 08:05 pm:    The team approach is for all parties to coordinate and discuss their differences. To do this they need to understand the reasons why they recommend certain practices. It is also necessary for each party to understand what their primary concerns are and what are secondary issues. There are some things that, while I may think I know more about than the geotechnical engineer or the architect, I will defer to the other because it is outside my contractual scope. Richard L Matteo, AIA, CSI, CCS Senior Member Username: rlmat Post Number: 324 Registered: 10-2003 Posted on Tuesday, March 24, 2009 - 10:32 am:    When I prepare the Earthwork and Concrete specs I send a copy to both the Structural Engineer and the Geotechnical Engineer for their review and comment. I also include the references to ACI 302 I generally get their concurrence. Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 146 Registered: 10-2007 Posted on Tuesday, March 24, 2009 - 11:12 am:    I would suggest that the team approach would also recognize that at times it is better for consultants to author specification sections as opposed to editing ones authored by the architect. Editing a specification section authored by others definitely is less satisfactory, from a technical perspective, than starting from a specification section you have used over time. I propose that the originator of a specification section should be the subject matter expert who is most knowledgeable regards the majority of the work in the section. Nathan Woods, CCCA, LEED AP Senior Member Username: nwoods Post Number: 289 Registered: 08-2005 Posted on Tuesday, March 24, 2009 - 11:22 am:    Mark, on every single job I have ever built (and all I do is full time CA), the Geotech Report, the structural section detail, the div 03300 concrete slab on grade section, and the vapor barrier section are all different. Drives me nuts. In most cases, these sections are written independantly by the consultants. I think the architect should be the point of origion for the slab design parameters, or perhaps "functional goals" is a better term. It is my hope that at some point in my life time, I will not have to fight this battle getting out of the ground. Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 147 Registered: 10-2007 Posted on Tuesday, March 24, 2009 - 12:46 pm:    No disagreement with the need for coordination being orcrestrated by the architect. Also no disagreement with the Architect defining the approach to control moisture flow. I just do not want the concrete specification sections distorted because I am forced to edit a section that is weak in many areas. These differences also drive me nuts. From a CA perspective the Geotechnical Report should not be an issue since it is not a construction document Propose that the structural details just refer to the stuff below the slab-on-grade as "slab subgrade". The grading specification would address the composition of the subgrade as well as any membranes. Then the architect can provide a detail showing the membrane and its relationship to the subgrade. This approach would facilitate coordination, eliminate redundancies, and eliminate conflicts. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 433 Registered: 04-2002 Posted on Tuesday, March 24, 2009 - 02:03 pm:    Concrete is a subject that most of the design professionals on a project have a stake in. Civil: Cast in place concrete drainage structures. Landscape: Cast in place concrete planters; footings for fencing and monument signs. Architect: "Non-structural" (non-load bearing), regular weight aggregate concrete fill at stairs and landings; concrete pads under rooftop HVAC units for acoustical purposes; concrete curing and finishing for flatness, levelness and visual qualities (color additives/color staining); moisture vapor emission control and mitigation; surface hardness and abrasion resistance. Mechanical/Plumbing/Fire Protection: Thrust blocks on underground pressure lines; equipment pads (boilers, HVAC equipment) Electrical: Equipment pads; foundations for lighting standards; underground electrical duct encasement. And there are retaining walls that no one wants responsibility for. How do these get covered when the cast-in-place concrete section is only for utilitarian, structural aspects? Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 148 Registered: 10-2007 Posted on Tuesday, March 24, 2009 - 03:01 pm:    Cast-in-place concrete is not necessarily for only structural aspects. What I emphasize is coordination and a sense of perspective. Regarding Civil and Landscaping, their issues can usually be incorporated into one CIP section with the structural work. In my experience there has not been an interest in coordination and thus we often find seperate CIP sections for the different work scopes. To some extent the problem with Civil is that they are focused on public works specifications such as Caltrans which makes a single section difficult. On one project I did in Texas where I had no response to my offer to help coordinate the Civil and Strucutral concrete work, the Civil concrete section referenced standards 20 years out of date. MEP equipment pads are typically detailed, but not located, on the structural drawings and are covered by the structural concrete. Concrete finishes need to be defined by the Architect in discussion with the engineer. The Architect should schedule concrete finishes. The specifications should include an aticle of two defining the finishes. On projects with sophisticated concrete finish requirements it often makes sense for the Architect to createa Concrete Finsihes section thus allowing most of the finishes issues to be deleted from the CIP specification. Construction joints and reveals also need to be coordinated. All too often requests to help coordinate finish issues have been ignored. Once design responsibility for site retaining walls is agreed how they are addressed in the specifications is not a major problem. If the site walls are small Landscaping takes the lead. If they are good size the structural is given an add service and he deals with these walls. If people are willing to cordinate their work these issues are usually not a problem. The problem is that I do not see a lot of coordination. Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 750 Registered: 03-2003 Posted on Tuesday, March 24, 2009 - 03:48 pm:    Mark: Just a minor correction to your post before last. The Geotechnical Report is a construction document; however, it is not (or it shouldn't be) a contract document. Ronald L. Geren, AIA, CSI, CCS, CCCA, SCIP RLGA Technical Services www.specsandcodes.com Robert W. Johnson Junior Member Username: robert_w_johnson Post Number: 2 Registered: 03-2009 Posted on Wednesday, March 25, 2009 - 11:02 am:    This discussion has diverted to a good specifier/consultant specifier coordination discussion that probably should be relocated to the Specifications area from Products. There was a similar discussion about a year ago that I have taken the time to research. I find that in my practice the number of consultants involved in the preparation of a project manual seems to be growing. We have MEP, structural (seem to be more wanting to write their own specs), landscape, civil (on the rare occasions that they don't use public work secs), LEED consultants, roofing/waterproofing/exterior envelope consultants, hardware, elevators, specialty equipment, etc., etc. There are always coordination problems. Consultant specs that do not recognize a Division 01 whether they are given a early copy or not. The coordination of structural and architectural aspects of concrete as discussed above. The coordination of who covers what - the SOG vapor barrier is a good example. The coordination of fonts and formats so that they at least appear to have been coordinated rather than a collection of different documnts. I had a devil of a time just getting the architect to establish the date of the documents. Specialty consultants who know little about specification standards. LEED consultants who know little about the principles of contract documents. The list could go on and on. I am sure you could add many others. That is not the point at the moment; we all know and have experienced the various problems. This would appear to me to be a significant wide-spread problem that is worth of some collective action. Discussion of the problem here by a few individuals is fine, but where does it go? Is there enough representation of all the parties - Mark seems to be the only (or one of very few) consultants that participates in this forum. What significant long-term benefit results? Probably not much. Why not consider forumlating a task team that includes representation of all the interested parties to more thoroughly investigate the common problem and try to come up with some recommendations of how to improve it? Could be a good task for a CSI chapter or region. Could produce a document that could that we could all use to educate people when we have coordination problems. Maybe the results might somehow be included or referred to in the next edition of the PRM - work on the next edition is just starting. Any takers?