Polished Concrete Topping Slab Thickness

Log Out | Topics | Search Moderators | Register | Edit Profile

Author Message John Hunter Senior Member Username: johnhunter Post Number: 67 Registered: 12-2005 Posted on Thursday, April 02, 2009 - 08:33 pm:    Does anyone have recommendations for the minimum thickness for a concrete topping slab to receive a polished finish (Retroplate, Vexicon, etc.)? We have both slab on grade and elevated slab conditions. The elevated deck assembly is planned to have a 6 1/4 inch composite deck with the topping slab installed over an anti-fracture membrane. This project is in seismic zone 4, so elevated mass in the structure is a concern. It was originally planned to be 2 inches thick, but the wisdom of a 2 inch concrete slab has subsequently been called into question. Any assistance will be appreciated. Nathan Woods, CCCA, LEED AP Senior Member Username: nwoods Post Number: 292 Registered: 08-2005 Posted on Thursday, April 02, 2009 - 10:46 pm:    We commonly do a 2.5" - 3" normal weight concrete topping slab in multifamily wood framed construction, over 1-1/8" plywood, from 2 to 5 stories in height. Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 154 Registered: 10-2007 Posted on Friday, April 03, 2009 - 12:42 am:    The problem is that if the topping slab is too thin you will get a lot of cracking. The 2.5 to 3 inch topping sounds reasonable. Recommend that the structural slab be reinforced to control cracking that can be reflected to the topping slab. The wire mesh is typically not adequate to do this. This slab cracking is typically not a problem if there is a carpet or linoleum floor. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 438 Registered: 04-2002 Posted on Friday, April 03, 2009 - 11:41 am:    As I understand "polished concrete" floors, the most important factor for the finish is flatness. I checked one manufacturer's spec and it noted Face Number of FF50. The most convenient resource I had was a Masterspec Evaluation for concrete. It noted that FF50 is a flatness used for tv studio floors. It's much flatter than typical, high quality, exposed concrete floors that get applied, reflective floor coverings or finishes. In a tv studio, I assume the flatness is to minimize rocking of the image when a camera is moved across the floor. For polished concrete, my understanding is that there are two considerations. One is to get even reflection across the floor (mitigated by lower gloss polishing and finishing). The other is to cut down the top surface to an even depth, so that exposed aggregate is consistently sized. The high points get cut off and the low points get scipped. This very flat (not superflat) floor is costly, a fact not mentioned in the promotional material I've seen for polished concrete. Also, the hassle factor of trying to finish portions of the concrete slab to different standards is significant. Given the contemporary environment for public construction where concrete finishers come from the Home Depot parking lot, quality control would take exceptional effort. Regarding additional reinforcement, does the fuzzy stubble from cut-off fibers from the concrete create visual problems? Mark Gilligan SE, CSI Senior Member Username: mark_gilligan Post Number: 155 Registered: 10-2007 Posted on Friday, April 03, 2009 - 12:09 pm:    The non-metalic fibers are most effective in controlling plastic shrinkage cracking and will not do a lot to help with other types of cracking. Note that there are limitations on measuring flatness on suspended floors. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 439 Registered: 04-2002 Posted on Saturday, April 04, 2009 - 02:52 pm:    Mark: You are correct about suspended floors. To get the necessary flatness at the time of finishing (polishing), shoring of the suspended concrete floor might be necessary. That's another hidden cost for a finish that's supposed to save maintenace cost for the Owner. I wonder about deflection during construction (finishing). Does that not mean the concrete thickness and reinforcement should increase at suspended concrete floors to make them stiffer (less deflection)? ken hercenberg Senior Member Username: khercenberg Post Number: 17 Registered: 12-2006 Posted on Thursday, April 09, 2009 - 04:45 pm:    I've sent this thread to RetroPlate to get their 'take' on this issue. We should be seeing a reply from Vernon Talbot soon. Vernon Talbot (Unregistered Guest) Unregistered guest Posted on Thursday, April 09, 2009 - 12:10 pm:    These are some great points made regarding polished concrete. When it comes to polished concrete, the flatter the better and you are correct that some "concrete finishers come from the Home Depot parking lot" - it is very important to use a concrete finisher that can provide a flat, nicely finished floor. The concrete is your floor finish and if your canvas is bad, it is difficult to create a masterpiece. A FF rating of 50 would be great, but a 30 - 40 FF rating, along with a good finish is sufficient for a polished floor and will not increase the cost of the floor as much as a 50 FF rating. The original question about slab topping thickness - I generally recommend 2" or more. Consult the structural engineer to make sure there are no structural issues requiring a certain thickness, but generally I see 2" or more. The topping slab is done extensively throughout Europe and quite a bit in Canada, but I rarely see it here in the U.S. Whether it is a topping slab, or a full-depth pour, if the floor is flat and finished well (without a lot of trowel marks) the floor will be a great candidate for RetroPlate polished concrete. Lisa Goodwin Robbins, RA, CCS, LEED ap Senior Member Username: lgoodrob Post Number: 16 Registered: 08-2004 Posted on Monday, April 13, 2009 - 09:03 am:    Our local Ardex rep, Barry Cullen, also recommends a concrete topping at least 2 inches thick.