Successful paintings need to start with a good quality canvas. When referring to Polished Concrete, our canvas is a good, quality concrete floor. Proper mix design, professional finishing techniques and optimal curing are some of the obvious elements of an exceptional Polished Concrete floor.
Let's start with the mix design of the concrete. Keep in mind we still have economics, LEED® and practicality to consider when designing the concrete mix. Some specifications insist on straight cement mix designs with no admixtures allowed. This is slightly “old school,” not exactly state-of-the-art. We usually allow up to 15% cement replacement using fly ash, a recycled material. A 15% fly ash mix can still be properly finished. Remember: Fly ash overuse may cause blistering and delamination as it delays the slab's set time.
Conventional water reducers and accelerators work well in polished concrete. However, there have been several unexplainable instances where specialty admixtures, such as shrinkage compensating products, yielded some strange results. Fiber reinforcement is definitely not desirable as they detract from the purity of the surface.
The bottom line: If you do not have any examples of a product used successfully on a polished concrete project, don't use it, or do a mockup whenever possible.
Placing & Finishing:
Placing and finishing is key in the outcome of a good polishing job. First off, power screeding is essential in driving out the entrapped air and bringing enough paste to the top to meet the specification. Striking-off concrete with a 2” x 4” by hand simply does not produce a quality job. Most specifications call for a FF of 40 – 50 for polished concrete. This means using advanced finishing methods. Don't use a 42” conventional bull float. Instead, start with a 60” channel float for initial straightening and then follow that with a bump cutter and check rod. These tools range from 8' to 24' in width. Cutting and filling the surface with these tools, working at 90-degree angles to each pass, ensures the flattest floor possible.
The desired finish on an ACI Class 5 and Class 6 floor specification is for a minimum of 3 passes without “burning” the surface. Occasionally, concrete finishers think that when the floor is going to be ground, a smooth finish is not required. This is not true. Polishers want it flat and well finished.
Curling is the next big obstacle. Once we've produced a perfect floor, the challenge is keeping it that way. Reducing curling is a function of maintaining temperature and keeping moisture content the same on the top and the bottom of the slab. This is difficult to do when the designer insists on a vapor barrier directly below the concrete, as opposed to placement over 2” – 4” of compacted granular fill as ACI suggests. Immediate curing after final finishing is one of the best ways to approach the issue.
Our recommended method is to cure the concrete with L&M CURE RTM. This is a membrane-forming cure that meets ASTM C-309 and dissipates as construction traffic wears the product. The remaining residue will easily come off with the first metal bond grinding step. The reason I prefer a tightly applied cure is that it ensures a mottle-free, final appearance. Differential cure is then virtually eliminated. Protection methods may then be employed without the fear of discoloration, striping, etc.
Wet cure blankets are still a popular method of curing. Although expensive, they do a good job of curing in a short period of time without leaving membranes that need to be removed. Overlapping sheets or taping-off the seams will cause differential cure and leave permanent marking in the floor. Also, remember to immediately rinse and scrub the floor with an auto-scrubber after removing the sheets. Failure to do so will cause a build-up of salt that is very difficult to remove from the concrete surface.
Saw cutting the slab properly is critical to avoid curling and cracking. Plan your saw cutting on 10' – 12' grids and no more than 12 '- 6”. Always cut off re-entrant corners and embedment items.
See chart above and the Concrete News article, Pinwheel Joint Techniques, from September 2008 (Vol. 8, No. 2.) at www.lmcc.com/concretenews
Protection is a tricky endeavor as there are so many variables that play into the success or failure of a floor. A recent job by one of my installers required them to grind, polish and dye the concrete when it was only 8 days old. The job was then masterfully covered with a breathable material.
The cover was removed 58 days later when he noticed that the dye had adhered to the covering. In addition, the slab was covered with all of the salts from the bleed water still trying to escape. The entire job had to be re-done—starting from scratch—this time with all of the woodwork in place. Don't let this happen to you.
Lesson: Be sure to wait for the slab to de-water before you grind. This subject just requires some common sense before attaching anything to the floor. A dissipating cure helps remove some of the unknown variables.
It is next to impossible to put a time frame on how long it takes concrete to de-water to an acceptable level as pouring conditions vary greatly. This is not a problem in hot, dry climates, but many of us live in the cold climates where a “day is not a day.” Waiting the recommended 28 days means the environment needs to be at 70 degrees for 28 days. A winter pour in North Dakota may not see 70 degrees in the building for several months. Therefore, the timetable changes proportionately.
In pouring environments with variable conditions, the best motto is “Don't guess—test.”
About the Author:
Bill Butler is a sales and tech rep for LATICRETE. He has worked in the concrete industry since 1976 and has been involved with ready mix trucks, concrete admixtures and construction products for the concrete industry. His approach to helping contractors and installers “do things right the first time” or when necessary, “doing things right the second time” is to ask good questions, be thorough, and learn from mistakes instead of repeating them.
Phone: 920-450-2932 • e-mail: email@example.com
© 2014 L&M Construction Chemicals, Inc. | ConcreteNews Winter 2014.