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Cement and concrete technology continue to undergo significant changes each year, making the job of selecting the right concrete, for the right application, more challenging. The specification and use of the correct concrete mix is arguably the most important factor in obtaining good results in floor construction.
Cement manufacturing uses significant amounts of energy and produces high amounts of carbon dioxide. There are numerous initiatives underway by the cement and concrete industries to reduce these emissions. Cement contents have been changing rapidly as manufacturers try to find new ways to minimize the use of cement and improve its efficiency. The cement content is "the" critical component in floors which produces the necessary paste to obtain a well finished surface. Without an adequate amount of cement in a concrete mix, it is almost impossible to produce a good finished result.
It was not long ago that we needed to use cement contents of 300 kgs/m3 to obtain a 30 MPa strength. These days we are seeing 30 MPa strengths with cement contents as low as 260 kgs/m3. This reduction in cement content creates difficult challenges for the concrete finisher and other trades as well. As the cement content decreases there are impacts: 1) obtaining a smooth finish becomes impossible, 2) the concrete becomes more porous to vapour transmission, 3) longer drying periods are required before applied finishes can be adhered, and 4) the control shrinks more.
The CSA A23.1 Standard also defines classifications for exterior concrete pavements and agricultural purposes, but it has been traditionally silent on what to do for interior slab surfaces (Type N Concrete). CSA A23.1 states:
Surfaces which are to be covered with an applied finish are usually not of as great concern as exposed floors. Defects can be repaired by grinding and patching which become invisible after the application of the applied finish. Floors with a low cement contents take longer to dry, thus creating excessive delays in the installation of applied finishes as well. Bumpy surfaces can also result from using too lean a concrete mix.
The CFCA's Standard Concrete specification for exposed concrete floors subject to traffic stipulates the following for concrete:
Cement replacing materials have been used for decades to efficiently economize the use of cement and utilize the waste by-products of other industries (this is good business). Slag cement is a by-product of steel manufacturing and flyash is a by-product of coal burning. The use of these products can dramatically affect the concrete set and finishing characteristics of the concrete. They can also improve sulfate resistance, heat of hydration and other useful benefits as well. In order to promote "GREEN" concrete thinking, the CFCA recommends a maximum 25% slag or 15% flyash replacement in hot weather conditions. At these replacement levels there is little effect on the characteristics of the conrete as compared to conventional mixes produced without these cement replacing materials. Percentage replacemet in excess of these amounts do create significant plastic shrinkage concerns that can be problematic. We want to be as GREEN as possible, but there are practical limits that should not be exceeded if you wish to maintain a good quality finished floor product.
Slag cement has cementitious properties similar to portland cement, but flyash is a very different material. The quality of flaysh can vary significantly. Increasing percentages of both flyash and slag cement can cause significant reductions in concrete set and the bleed rate of the concrete. A reduced bleed rate is significant to cement finishers becuase this also reduces the plasticity of the cement paste at the surface of the concrete. This can inhibit the use of surface applied sry shake hardeners and also produce crusted surfaces that are bumpy.
Flyash comes in two forms: Type C flyash (which is hydraulic like cement) and Type F (which reacts with the hydrated cement to gain strength). Liquid hardeners also react with the hydrated cement, therefore the increased use of flaysh can also reduce the effectiveness of a liquid hardener to densify and chemically seal the surface of the concrete.
As always, we strongly suggest that you pre-qualify members of The Concrete Floor Contractors Association of Ontario in order to ensure that all work is carried out correctly - we are the voice of our industry and we are here to help you !
If you have any questions, please feel free to e-mail.
"To allow proper finishing and wear resistance, Type N concrete intended for use in an industrial concrete floor with a toweled surface exposed to wear shall have a minimum cementing materials content of 265 kg/m3."
It is our recommendation that this minimum 265 kgs cement content apply to ALL interior exposed concrete floor surfaces.
"Concrete shall conform to CSA A23.1 having a minimum strength of 25 MPa [or higher as required per the structural design], minimum 265 kg cement content, a maximum 0.55 water:cement ratio, a 60 mm ± 20 mm initial slump, normal setting plasticizing admixture to a final 120 mm ± 20 mm slump, 40 mm coarse aggregate for slabs screeded with a machine and 20 mm coarse aggregate for slabs screeded by hand. All admixtures shall be normal setting and non-retarding. A maximum 10% fly ash or slag cement replacement shall be acceptable in cold weather conditions and up to 25% slag cement or 15% flyash replacement in hot weather conditions."
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