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Cement and concrete technology continue to evolve significantly each year, making the job of selecting the right concrete, for the right application, more and more challenging - purchasing concrete is not just about issuing a purchase order ! There is no such thing as "normal" concrete anymore. The specification, selection and management of the concrete mix is a key factor in obtaining good concrete floor performance.
The cement and ready mixed concrete industries have been making great efforts to reduce the carbon footprint of this product through the optimizing of cement contents and the replacement of normal portland cement with other cement replacing materials (eg: slag, flyash and limestone). New to CSA A23.1 is the inclusion of up to 15% cement replacement with interground limestone dust ("GUL Cement"). Unfortunately daily changes seem to be occurring without much consultation of the constituents. What is known is that any change to the cementitious materials will have an effect on the workability and finishability of the concrete as well.
All things being equal, the cement content (also relating to the water:cement ratio) is "the" critical component to produce a concrete mix that can be finished well. Without an adequate amount of cement in a concrete mix, it is almost impossible to produce a good finished result. As the cement content decreases there are impacts: 1) obtaining a smooth finish becomes almost 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 concrete shrinks more.
The new CSA A23.1 Standard (2009) defines a standardized mandatory concrete mix for all interior concrete floors with a trowel finish as follows:
8.12.1 General
Interior concrete floors with a steel trowelled finish other than those covered in Clause 8.13.3 shall be
designed to a maximum 0.55 w/cm and a minimum compressive strength of 25 MPa at 28 d, as well as
designed for placing methods finishability, set, and serviceability, as required for intended service.
Notes:
CSA A23.1-09 also includes guidance for specifiers to reduce the drying time of concrete for the application of applied finishes:
Concrete slabs shall be allowed to dry sufficiently before the application of moisture-sensitive floor
coverings. A vapour retarder shall be placed below slabs on grade in accordance with ACI 302.3R to
reduce future moisture migration.
Note: To reduce the drying time of concrete for applied finishes, the following should be considered:
"Supplementary cementing materials", which are truly cement replacing materials, have been used for decades to efficiently economize the use of cement and to develop improved exposure properties of the cement paste for different exposure classifications. Slag cement is a by-product of steel manufacturing and flyash is a by-product of coal burning. The use of these products can improve some durability aspects of concrete mixes, but they can also dramatically affect the concrete set and finishing characteristics of the concrete 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 finishability of the concrete as compared to conventional mixes produced without these cement replacing materials. Percentage replacemet in excess of these amounts creates plastic shrinkage concerns that can be problematic if not managed carefully in the field by the concrete purchaser (See Position Statement on Concrete Purchasing).
Slag cement has cementitious properties similar to portland cement, but flyash is a very different material. There are two types of flyash: 1) type C which is cementitious in nature and 2) Type F which reacts with the hydrated cement by-products. The quality of flyash can also vary significantly. Increasing percentages of both flyash and slag cement can cause significant increases in concrete set times and reductions in the plasticity (bleed rate) of the concrete. A reduced bleed rate is significant to cement finishing because this also reduces the workability of the surface of the concrete. This can result in surfaces that are bumpy and contain plastic shrinkage cracks. Liquid silicate/lithium hardeners react with the hydrated cement like Type F flyash, therefore the increased use of flyash can also reduce the effectiveness of liquid hardeners to densify and chemically seal the surface of the concrete.
The CFCA's standard specification for concrete floors stipulates the following:
As always, we strongly suggest that you pre-qualify members of The Concrete Floor Contractors Association in order to ensure that all work is carried out professionally !
If you have any questions, please feel free to e-mail.
"8.12 Concrete mixes for interior concrete floors
(1) See ACI 302 for further information on concrete slabs and concrete mixes.
(2) The water content of the concrete mix should be minimized to reduce the effects of shrinkage and the slump increased using a normal setting plasticizing admixture.
(3) Mixes most suited for floors should have a slump of 100 mm at the point of placement. Higher workability or flow
should be achieved with the addition of plasticizing admixture only.
(4) SCM use and chemical admixtures in concrete mixes can reduce the amount of bleed water available at the concrete
surface unless other changes are made to the mix to address bleed rate. A reduction in available bleed water at the
surface can create difficulties in finishability and in the application of dry shake surface hardeners and may increase the need to protect the slab from rapid evaporation of surface moisture. See Clause 7.4.1.2.
(5) The use of air entrained concrete is not recommended for interior ice rink slabs and freezer slabs with a steel trowelled finish. They have been found to perform satisfactorily without entrained air if an adequate period of drying is provided before the initial freezing."
"8.12.3 Drying time for applied finishes
(a) decreasing the water content of the concrete mix;
(b) decreasing the water-to-cementitious ratio;
(c) using 3 d of continuous wet curing (not use curing membrane);
(d) protecting the slab surface from environmental re-wetting after the curing period; and
(e) minimizing slab thickness consistent with structural requirements and desired joint spacing."
"Concrete shall conform to CSA A23.1 having a minimum strength of 25 MPa [or higher as required per the structural design], a maximum 0.55 water:cement ratio [or lower for increased durability or lower moisture permeability], a 60 mm ± 20 mm initial "water" slump, the addition of a normal setting plasticizing admixture to a final 120 mm ± 20 mm slump at the point of placement, 40 mm coarse aggregate for slabs screeded by machine OR 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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