Polished concrete

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Author Message David Axt, AIA, CCS, CSI Senior Member Username: david_axt Post Number: 1109 Registered: 03-2002 Posted on Friday, August 27, 2010 - 02:15 pm:    Does anyone have any tips on polished concrete floors? Several of our projects have polished concrete floors and we have had mixed results. Probably the major headache is the poor quality joints. Lisa Goodwin Robbins, RA, CCS, LEED ap Senior Member Username: lgoodrob Post Number: 84 Registered: 08-2004 Posted on Friday, August 27, 2010 - 02:55 pm:    Good timing David. I have an Architect who wants to know how wet curing affects the polished concrete system. Their client wants a floor with NO cracks whatsoever. The structural engineer (another consultant) is ignoring her questions. The contractor sent photos of a recently completed project that I thought looked great, but it also had hairline cracks. I wrote the spec for that project too. They chose a dark color that looked really deep, similar to black epoxy lab tops. Splotchiness (if that is a word) wasn't an issue, possibly because of the dark color. Is it possible to get a new concrete floor without tiny little cracks? How does the curing method affect cracking and color consistency? (Unregistered Guest) Unregistered guest Posted on Friday, August 27, 2010 - 04:27 pm:    The first "tip" is to remind the owner that a polished concrete floor is an artisan finish and not just something some guy off the street can do. Whenever I've had a polished concrete floor that we really wanted to look good, it always involved a pretty good sized (10'x 10') mockup and prior approval of the workman/artisan who would be doing the floor. If you're adding a stain on the floor, its even more critical to get a mockup done. As for curing: I have had good results with wet curing if the contractor knows how to do it properly. As for joints: you have to have joints in well thought out places and place them a little more close together than you think you need them. As for color consistency: I've gotten the best results with a two-phase method: adding a stain to the wet concrete AND using a colored wax (up to five coats) on the cured surface. Starbucks and a number of other restuarants used to use this two-part method to achieve their stained and polished floors. this is primarily a workmanship issue. if you want a ""quick and cheap"" floor, polished concrete isn't the way to go. J. Peter Jordan Senior Member Username: jpjordan Post Number: 449 Registered: 05-2004 Posted on Friday, August 27, 2010 - 04:37 pm:    This is one of my ongoing lines of investigation, and we have had two different "applicators" speak to us in the last week and a half with slightly different suggestions. It is my impression that "polished concrete" finish s the result of grinding operations with one set of tools and polishing operations with another set of tools. The grinding operations step is really critical in determining what the stuff will finally look like since this is the phase in which more or less of the existing surface is ground off and more or less of the aggregate is exposed. A light grinding is likely to expose only the "fines" in the mix, you will need to grind more to expose some of the larger aggregate which may result in a terrazzo-lke appearance. Aside from this consideration, the concrete polishing guys are working with "structural concrete" with mix parameters set by the structural engineer and an actual mix design determined by the readimix guys. There may be variations caused by slight variations in mix design (one used early in the day and one used later), temperature at the time of pour, curing methods, and finishing (the polishing guys say not to trowel it too smooth). Of course, the type and size of aggregate in the mix can affect it as well. One area may have great aggregate for polishing with marble or granite in the mix while other aggregate will be less attractive. Larger aggregate in the mix may be forced down in the slab, preventing it from ever being exposed. Floor flatness is an issue as well, with both guys I spoke with recommending an FF 40 slab. One guy suggested the concrete ought ot have a compressive strength of 3000 psi while the other insisted it be 4000 psi. I would suggest that the lower strength mix will be easier to grind, and that particular person also stated that they would prefer to get on the slab before the concrete has fully cured and reached its design strength. It should not be too difficult to get a few field samples of "typical" concrete from a local project, 12 by 12 by 2 to 3 inches for a concrete polishing outfit to demonstrate several different effects. Work with the structural consultant to get a little more control if the final appearance is critical, and you may want to add some color to the concrete mix. Some polishers are able to use dyes (not stains) to obtain certain colors and effects, but application of the dyes will be a part of the polishing process not part of the concrete pour. Do a larger onsite mockup, perhaps as large as 10 by 10 feet, poured at the same time as the slab (using the same mix), but separate from the slab so you can see what you can expect to get. One can "seed" the freshly poured slab with a different aggregate or other material (broken glass, bits of non ferrous metal) which is forced into the top of the slab. Cracking is another issue altogether. Concrete is almost always going to crack unless the amount of water in the concrete and the curing process are very carefully controlled. Moisture curing may yield better results than curing compounds, but good mix design will help as well. True hairline cracks (those less than a thickness of a playing card) may actually go away during the grinding process since they may get filled with the dust. The polisher can mix a grout paste of the dust and water or latex to produce a crack filler that may be acceptable. However, if there is too much water in the concrete and evaporation is not controlled, the type and amount of cracking may well be unacceptable; pour a topping and polish that (can you say terrazzo?). The stuff that goes on on a slab during construction may cause aesthetic issues for the final appearance of the polished concrete. These guys will want to do it early (right after the slab is poured) and then protect it during construction. Although this may be doable, this is certainly not the way the slab gets used during construction. Materials used to protect the surface may also affect the finished surface. I would suggest that dealing with a hairline cracks is the least of your worries. Recommendations: (1) Get samples for concrete in your area to see what types of results are possible. (2) Work with your structural engineer and possibly even the ready mix supplier to get a concrete mix that will be more consistent. (3) Increase the flatness requirements. (4) Decide how much you want to grind off and how much aggregate exposure you want. (5) Do a field mockup and get "buy-in" from owners and designers. Typical floor slabs are sows' ears that many people think can be dressed up for a black tie affair. Polished concrete can be very attractive and has a certain aesthetic, but you will have to work for it. Mark Gilligan SE, Senior Member Username: mark_gilligan Post Number: 300 Registered: 10-2007 Posted on Friday, August 27, 2010 - 05:12 pm:    The Architect who insists on concrete with no cracks is likely to be disappointed. You can do a lot to limit the size and number of cracks but expecting none is unrealistic. A good designer designs with an understanding of the limitations of materials he is using. John Regener, AIA, CCS, CCCA, CSI, SCIP Senior Member Username: john_regener Post Number: 466 Registered: 04-2002 Posted on Sunday, August 29, 2010 - 02:35 am:    Check the requirement for "flatness" of the concrete floor slab at polished concrete. From what I've found, a Face number of FF50 is required. Checking on what this means in real life, my understanding is that it is the flatness of a floor used in a tv studio. There, the exceptional flatness requirement is so the video image does not shift as the camera moves across the floor. FF50 should be contrasted against FF35 which is for a very good floor under resilient floor coverings with their reflective surfaces. The consequence of waviness in the concrete surface is that the grinding becomes uneven. More is taken off the high spots and less is taken off at the low spots. This means that the size of the exposed aggregate varies. So, in addition to the potential for cracks to foul up the appearance, there's the irregularity of the exposed aggregate. I'll leave it up to those with more hands-on construction expertise to explain how to control the flatness during concrete finishing. Certainly, an entire floor slab does not need FF50; only the areas to be polished need it. I'll also leave it to others to explain how fibers added to the concrete mix to help prevent cracking will not affect the appearance of the polished concrete. And the effect of the money- and time-saving curing and sealing compound (remember the discussion about moisture vapor emissions?) is also something I'll leave for the polished concrete and concrete sealing compound peddlars to explain. I guess the moisture coming up from the concrete with a more porous surface will have no negative effect on the polished concrete sealer. I wonder how the cost compares between polished concrete done right, including FF50 floor slab finish, staining, grinding/polishing and sealing, and an epoxy terrazzo finish. J. Peter Jordan Senior Member Username: jpjordan Post Number: 450 Registered: 05-2004 Posted on Sunday, August 29, 2010 - 11:41 am:    I was recently told that the cost of polished concrete was $3 to $5 per sq. ft. I am assuming that this is for the process itself, not the premium for flatness and special mix requirements. David R. Combs, CSI, CCS, CCCA Senior Member Username: davidcombs Post Number: 319 Registered: 08-2004 Posted on Monday, August 30, 2010 - 08:13 am:    Many years ago, I had a very good concrete foreman tell me "There are three undisputable things you need to know - and can't change - about concrete: It's hard, it cracks, and you can't steal it." I would submit that any design professional that insists on ZERO cracks in a concrete floor is asking for perfection - the cost of which the client probably can't afford, and the implementation of which the contractor probably can't provide. Lisa Goodwin Robbins, RA, CCS, LEED ap Senior Member Username: lgoodrob Post Number: 85 Registered: 08-2004 Posted on Monday, August 30, 2010 - 09:27 am:    David, I like that quote. Architects and other design professionals need to understand and accept the limitations of materials. Perhaps good design results from working with limitations, not trying to break the laws of physics. Harry Peck, CSI, CCS Advanced Member Username: harryspec Post Number: 5 Registered: 05-2010 Posted on Tuesday, August 31, 2010 - 07:55 am:    Have an awareness of the spacing between joints. The nature of concrete is to crack so we must apply our knowledge to control cracking. Avoid use of fiber reinforcing unless some type of shake is placed in the concrete surface to push down the fiber. This especially important if the polishing process is "wet". Require levelness be achieved to the greatest extent by screeding rather than troweling. There is a risk of burnishing the concrete and causing the "cap" to tighten and make polishing more difficult. Consider minimizing power troweling as the final finish is polishing. If using ASTM E 1155 to measure flatness and levelness, consider extending measurements to the edge of pour and accross joints. This highlights some of my "lessons learned" from specifying a few automotive manufacturing facilities where the plant floor was polished. Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 998 Registered: 07-2002 Posted on Tuesday, August 31, 2010 - 12:37 pm:    I was the "unregistered guest" -- I forgot to sign in. I haven't been as exacting with the flatness requirements for a polished concrete floor unless the usage of the space requires a superflat floor. But I have often required that the concrete polisher work with the concrete installer and "accept" the installation as being suitable for their work. Never used fiber resinforcing, but have had fiber mesh (sort of like nets) used sort of mid-way through the slab for additional reinforcement. I think screening the applicator and mockups are key. And, on the west coast, while the prices are all over the map, I think $5 to $8 per square foot is more typical. ken hercenberg Senior Member Username: khercenberg Post Number: 59 Registered: 12-2006 Posted on Saturday, September 04, 2010 - 03:27 pm:    Excellent thread. It's amazing how much we've learned over the past few years about concrete as a finished product. As an aside, if you're thinking about depressing the structural slab and pouring a topping slab, take serious issue with minimum thickness of the topping slab, reinforcing, design mix, curing, etc. Concrete still wants to be concrete. Cementitious overlays act differently and may not give you the look you think you're going to get. Another thought is to add terrazzo strips to dress up your polished concrete. Again this adds cost. I've only had minimal experience with projects in Europe but I understand that the tendency there when using polished concrete is to reduce the size and spacing of the reinforcing bars, thereby minimizing cracking. Apparently there are even some technologies that permit the elimination of control joints, but my understanding is that there is an awful lot of steel in the concrete, small bars spaced pretty closely together, as well as low cement mixes and low water-cement ratios to really control the water content. Obviously the cost of using the European method adds a lot more to the overall cost of the concrete before even considering Ff, grinding, polishing, and finishing. As to fiber reinforcing, hairy concrete does not look good when polished. Give me cracks and let the finisher fill it with the grout paste they can make during the finishing process. With enough controls (reinforcing, design mix, etc) you can minimize cracking but don't expect to eliminate it. After all, concrete will be concrete. Oh, and you can steal it but it takes some effort and you really have to want it pretty badly. Unregistered Guest (Unregistered Guest) Unregistered guest Posted on Wednesday, September 08, 2010 - 05:30 pm:    For zero cracks, try Type K shrinkage compensating concrete, also known as KSC. Manufacturer is very responsive on consulting about specing and placement, leading to good outcomes. And polishers tell me it is an excellent polishing surface. Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 1005 Registered: 07-2002 Posted on Thursday, September 09, 2010 - 02:00 pm:    Ken- we seem to always look to Europe as the example for seamless plaster and seamless concrete, but their seismic conditions are considerably different than ours are and they don't always look as great after the photos have been published. many years ago, I helped a chain store develop their polished concrete specs and we always called for a minimum of a 2-1/2" topping slab with well situated control joints. I have seen and used some thinner toppings but don't have enough years behind them to know the eventual outcome. I have had a couple of jobs where the whole topping had to be scooped up and disposed of -- because this is not an area where "saving money" is necessarily a good idea.