How critical is steel trowel finish?

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Author Message ken hercenberg Senior Member Username: khercenberg Post Number: 115 Registered: 12-2006 Posted on Wednesday, December 14, 2011 - 11:23 am:    Considering some of the previous discussions about moisture vapor mitigation in concrete slabs and decks, it seems one culprit that has been identified as trapping water is steel troweling of the concrete. If this is truly the case, how far off-base would we be to just float the concrete and come back later with a thin coat of underlayment? Despite the added cost it should still be cheaper than a vapor mitigation system presuming our floor is sufficiently flat and level. Marc C Chavez Senior Member Username: mchavez Post Number: 428 Registered: 07-2002 Posted on Wednesday, December 14, 2011 - 11:38 am:    BINGO a guy from Dayton Superior talked about this several years ago. and supercap (now owned by laticrete) recommends bull floating the floor and then just walking away - sending 4/5 of the finishing crew home. THEN come back later and float a FLAT floor with the cementitious underlayment $$ for the underlayment is off set by the extra finishing crew and time. I'll let supercap finish their own sales pitch ...but if you need FLAT floors for large tiles or other lab/medical needs this is the way to go - I also agree with some of the water thing as I've been told that "burning" the floor makes it so dense at the top that glues will sometimes not work (no place for the water in the glue to go) Lynn Javoroski FCSI CCS LEED® AP SCIP Affiliate Senior Member Username: lynn_javoroski Post Number: 1348 Registered: 07-2002 Posted on Wednesday, December 14, 2011 - 12:02 pm:    And DITTO! I've heard the same thing, not only for concrete floors that will receive an impervious finish floor, but also for concrete floors that will be stained/polished. (the cap should be thick enough to tolerate this - 2-3 inches) You then get a relatively clean concrete surface, rather than one that's had beer, soda, coffee, oil, gas, and who knows what all else spilled on it - which is seriously detrimental to a good-looking stained concrete. I spec a wet-cure for concrete that will have an adhesively applied finish floor and actually had a GC agree with me! Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 433 Registered: 10-2007 Posted on Wednesday, December 14, 2011 - 12:54 pm:    This sounds bogus to me. I do not believe that steel troweling will have a significant impact on moisture mitigation. Let us see the test data. Thin topping slabs have a real possibility of delaminating. Gerard Sanchis Senior Member Username: gerard_sanchis Post Number: 61 Registered: 10-2009 Posted on Wednesday, December 14, 2011 - 01:05 pm:    I agree with Mark. Although steel troweling densifies the top 1/8 to 1/4 inch of the concrete, the concrete is still porous; we have a staining problem on one of our project and are trying to determine what the absorption rate is (which would also translate into diffusion). To my knowledge the best we can get with cast stone (a form of precast concrete), which is fabricated under controlled conditions, is 6 percent. If anyone has the absorption numbers for steel troweled concrete with a low W/C ratio, I’d appreciate getting them. Gerard Sanchis Senior Member Username: gerard_sanchis Post Number: 62 Registered: 10-2009 Posted on Wednesday, December 14, 2011 - 01:19 pm:    Reading the thread above brings more thoughts and comments. For a topping slab to work, it needs to be wet cured. Anything else will result in curling (and I don't mean the game played on ice). I think Lynn is very lucky to have had a GC agree to a wet cure; in over 40 years of practice, I'm still waiting to meet that rare bird. I spend quite a bit of time in Switzerland and, out of professional curiosity, I can't help but spend time on job sites. The Swiss always pour a rough slab. Then when the wet work is completed they pour a topping slab about 2 inches thick which is wet cured. As for setting large format tiles, several mortar manufacturers make medium setting bed which can be installed up to 3/4 inch thick and can provide a super flat floor without the trouble and expense of getting a substrate in monolithic concrete that will achieve the same results. Marc C Chavez Senior Member Username: mchavez Post Number: 430 Registered: 07-2002 Posted on Wednesday, December 14, 2011 - 01:26 pm:    I have a lot of luck with getting wet curing. however you are right there is some push back but I tell them they get to shot blast the floor to pull the cure seal off to make it compatible with flooring and underlayments - that usually helps - but not always... also I'm talking about underlayments NOT topping slabs so curing is a different matter I would like to see some test results as well. the info I have is from several reps - I will dig deeper Lisa Goodwin Robbins, RA, CCS, LEED ap Senior Member Username: lgoodrob Post Number: 158 Registered: 08-2004 Posted on Wednesday, December 14, 2011 - 02:10 pm:    I'm skeptical, but I'd like to hear more about this too. I went to the excellent SGH presentation about concrete floor finishes at CSI Chicago this past September. Broom vs. smooth finishing was not mentioned. Contractors and sales reps are recommending all sorts of weird things now, but without much hard data to recommend them. - Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 1222 Registered: 07-2002 Posted on Wednesday, December 14, 2011 - 06:51 pm:    Gerard: the wet cure is much less common in LA than in the northwest -- where you sometimes get a wet cure whether you wanted one or not. However, on medical work in the LA area, I typically did get a wet cure -- the contractors were just different and more willing to take the time, I guess. I still think the "medium setting bed" is not the right way to go with really large tiles - I don't think it adequately supports the floor. John Bunzick, CCS, CCCA, LEED AP Senior Member Username: bunzick Post Number: 1347 Registered: 03-2002 Posted on Thursday, December 15, 2011 - 11:30 am:    Even if the steel trowel finish does not reduce moisture vapor transmission (which I have heard, too, possibly from the ACI's publication on moisture in slabs), deleting it still could solve other problems. With the overlay, you will get a flatter floor; and, no big worries about having to do a lot of hole patching and flash-patching since the self-leveler takes care of all of it. Finally, if you DO find moisture problems when your are nearing the point to install finishes, you can use a moisture mitigation membrane with the self-leveling over it to give a surface that is better for adhering finish flooring. I never had much luck convincing contractors of the merits of this approach, though. Don Harris CSI, CCS, CCCA, AIA Senior Member Username: don_harris Post Number: 250 Registered: 03-2003 Posted on Thursday, December 15, 2011 - 02:10 pm:    I have been told by a number of reps that in NYC this has been the standard practice for years; bull float and then a self leveling topping. Saves money and they don't have to lug the trowling machines around. With the labor rates in NYC, it's probably a big deal. If it actually helps with the levelness, that's an additional plus. Full disclosure, I never actually participated in a project in NYC, but this is what I have been told. ken hercenberg Senior Member Username: khercenberg Post Number: 116 Registered: 12-2006 Posted on Thursday, December 15, 2011 - 03:36 pm:    A major concrete sub in the Washington, DC area has told me that they probably wouldn't change their price much in terms of whether or not we included steel troweling but he does think that not troweling will make some difference in the drying out of the decks and slabs. I can't find any studies where this has been documented but have heard a number of people claim it as true (and we all know what that's worth). My guess is that unless it becomes a popular practice GCs will freak out and include a large line item to cover self-leveling underlayment everwhere, even if just at 1/8 inch or less. I was just curious if anyone had actually stumbled across this in practice anywhere. Thanks for the feedback everyone. Jim Sliff Senior Member Username: jim_sliff Post Number: 107 Registered: 08-2010 Posted on Monday, December 19, 2011 - 07:54 pm:    When I was on the other side of the fence (specs & project mgmt for a coatings/waterproofing firm) we saw no difference in MVT between hard (steel) troweled concrete or bull float/topping or any other method *except* when so-called "curing compounds" were applied (and shotblasting or a scarification method was nearly always required). To apply any floor coating or stain you need surface penetration (ad usually mechanical adhesion), and the concrete curing/finish process rarely made much o a difference. In 80%+/- of the new concrete slabs we worked on we had to bring in a shotblasting crew to get the proper surface profile - in the other 20% the specification called for "broom finish" - and then we still ran moisture tests (usually calcium chloride, sometimes via electronic meter and a few by the ancient "mat" test). All this work had to be done by the finish work contractor regardless whether the concrete was hard troweled, broom finish or float/overlay. All the methods require MVT testing at a minimum, and surface profile examined one way or another(hardly ever measured per particular "standards"; a visual and "fingertip feel test" is just as good IMO). Usually not addressed, though, but very common is the problem known as "vapor drive". This isn't MVT (although the two sometimes go together), but is a phenomenon where air pressure escapes through the slab - wet OR dry. I've not seen a huge amount of related research but it is a significant problem, usually tracked to a poorly installed, failed, or nonexistent vapor barrier under the slab. When a floor coating job failed and I was called out to look at it, I'd often find what I call a "blister farm" - that is bone dry underneath, with the surface often having the appearance of flawless preparation. It occurs primarily in heated structures during cold weather (and on the west coast we don't have a huge temperature difference but still have the problem - I'd imagine it'd be worse in less temperate areas. The dry surface when tested by any of the established methods baffles many inspectors. When encountered in new construction it's usually far too late in the job to fix the vapor barrier - so a between-slab membrane of an epoxy primer followed by 80-100 mils of plural-component polyurea with a +/- 2" cementitious overlay was what we found to be the most cost-effective solution. It still required a bucketload of change orders, though, as the floors were generally raised 1-2". However, once that system was down (since it's totally seamless) it solved both vapor drive and MVT. We did so much post-construction "fixit" work of that type that about 2 years ago I noted a marked increase in the number of slab specs calling for a vapor barrier, slab with a broom finish, a fluid-applied polyurea or urethane between-slab membrane, overlay and THEN a coating...or stain...system.