|Posted on October 16, 2012 at 10:40 AM|
A professionally ground and polished concrete floor can be equated to a work of art. However no matter how great the artist is, if the artist's canvas and tools are substandard then producing beautiful work of art is rather difficult.
Before pouring a new concrete floor slab, you can get the best floor finish by getting involved early in the process to help effect the final outcome of polished concrete floor finish.
To produce a uniformly polished concrete finish, the slab needs to be poured and placed perfectly flat and as level as possible. If pouring slab-on-grade concrete, the specification should meet all minimum standard requirements for strength, and should not contain any curing additives or accelerators.
Once the concrete slab is installed it well known, that concrete often will not dry uniformly. Non uniform curing of concrete may result in concrete curling somewhat similar to what happens to wet mud when moisture is released and it dries up. The top of the concrete surface dries usually much faster than the bottom and the corners of the slab panels tend to slightly rise as uneven curing is occurring.
When grinding the concrete slab prior to polishing, more cement paste must be removed from the surface where curling took place, resulting in uneven aggregate exposure of the surface. Although this is what makes polished concrete flooring so unique as no 2 polished concrete floors usually look exactly alike. Many professional new concrete finishers recommend a well-graded aggregate system which reduces the concrete shrinkage potential. Concrete mixtures can be analyzed using a Concrete Coarseness Factor Chart to help in the process.
Cutting expansion joints reduces curling. It may be worth the cost and the sacrifice of visual appeal by cutting and filling in the expansion joints with clear or coloured epoxy to minimize the curling effect versus the non-uniformity created by grinding and polishing curled areas of concrete. Generally it is recommended that the maximum expansion joint spacing should be as follows;
Expansion joint cut should be for every 10 feet in length, and for a 4-inch-thick slab, 12.5 feet for a 5-inch slab, and 15 feet for a 6-inch (or greater) slab.
Engineers and contractors often mistakenly believe that curing eliminates concrete floor curling. Not so, curing only delays concrete curling process for awhile, but the top concrete will dry up faster then the bottom of the slab, and curling begins to occur. More often then not, the benefits of curing concrete is outweighed by issues resulting directly from the actual curing methods used. Concrete curing compounds are generally detrimental to the process of concrete polishing, and it can be expensive to fix. Water curing blankets are also not recommended as they can dry non-uniformly, and create permanent discolouration marks in the surface of concrete that will be exposed when the concrete floor is ground and polished.
The effects of a new concrete pour on polished concrete is very significant from finishing procedures of the newly poured slab and could have a dramatic impact on the final esthetically appearance of a polished concrete slab.
Hard troweled concrete surfaces have an extremely dense and hard surface layer which is not conducive for polishing concrete. If the owner desires any sort of aggregate exposure, we then must cut through this layer and it's very time consuming to do.
Currently the concrete polishing industry is working on guidelines and standards to determine what are the ideal finishing techniques when pouring new concrete that will minimize the thickness and hardness of the top surface paste layer while still providing the required flatness level floor needed. Many experience polished concrete companies believe that the use of synthetic trowel blades assist in the effort of preventing the surface from hardening too much. As well many endorse plastic troweling blades help by minimizing any surface colour variation created in new concrete pour and cure.
There is no detail that is overlooked when it comes to polishing a concrete floor slab. Good new concrete pour contractors must know that when a concrete slab is diamond ground and polished, it will uncover all faults and any embeded objects that the contractor may have dropped under the surface. As an example; (and we have seen this plenty of times before), a previously unseen bootprints, or small debris such a cigarette butt, that was covered over with the finishing cement paste will all of the sudden magically re-appear after a concrete floor is diamond ground and fully polished, and there is nothing we as concrete polishing company can do remove these kind of faults in the surface.
So having a detail oriented new concrete pour contractor, that has the experience needed and understanding of the basic fundamentals of polishing concrete so as not to create faults or problems for us and for their client.
As a polished concrete contractor we want our diamond tools to be in alignment with the strength of the concrete that will be grinding and polishing. We often perform a hardness test with picks to scratch the concrete’s surface. The number that determines the hardness level is used to select the proper diamond tools to be used in grinding the surface. Generally, the harder the concrete, the softer the matrix that bonds the diamonds to the metallic grinding pad.
frequently, when testing for strength, we will drill and break cores. Cores provide the overall strength of the concrete, but offer little value in determining the strength or hardness of the concrete’s floor surface.
When we grind and polish an area where there is more than one pouring of concrete floors, visual variability can often come into play. Different concrete mixtures will likely be visible in the finished project. A mixture containing 20% fly ash and coarse aggregate will create a different look than a straight cement-limestone mixture.
Moisture content can and does play a large role, particularly if colour dye is specified and is applied to the polished concrete floor. The colour dye enters the micro pores of the concrete surface to produce a desired colouring. Concrete floors that has low moisture content absorb colour dyes differently then concrete slabs with naturally higher moisture content. A simple 24 hour moisture test can be performed on a slab to determine whether a moisture control treatment may be required prior to colour treating the concrete floor.
The information gathered before the start, we provide to the customer prior to commencing the project so our customer is aware of potential pit falls.
A beautiful polished concrete floor is like most things in life: The better the communication between all involved, the better the final outcome.
We have ground and polished hundreds of projects and many hundreds of thousands of square feet of concrete, and we seldom see a perfect polished concrete floor. The inclusions and natural faults is what gives polished concrete character and it's beautiful unique appearance.