M.A. Concrete Ltd.
Concrete toppings since 1977
Authorized Applicator
Resources
Final Floor Requirements
a.k.a. "THE GAP"
First published in the Canadian Flooring Resources Journal
Discussion
It may be obvious that different final floor finishes require different tolerances from the concrete underlayment. It is, however, not always understood that a large percentage of the most common type of flooring requires a tolerance that is outside the concrete specifications indicated on the approved drawings. In fact, some common flooring requirements like hardwood or resilient flooring are well outside normal concrete specifications.
The standard for concrete specified in virturally all drawings in Canada is CSA A23.1-94 “Concrete Materials and Methods for Concrete Construction”. This standard applies unless deviations are specifically stated by the Owner.
CSA A23.1-94 Table 16 “Slab and Floor finish classifications” describes the “Straightedge Tolerance, Flatness/Levelness (F-numbers), and Waviness (SWI Index) requirements for a Conventional (smooth) concrete slab. The floor flatness (FF) and floor levelness (FL) do not apply to “… unshored surfaces or surfaces after the removal of shores …”. The F-number therefore never applies to slabs poured over an unshored wood floor assembly. For reinforced concrete floors, measurement of the “F-numbers” is not permitted once the shores have been removed. The SWI index is derrived from a series of equations and relates to the standard deviation of a number of survey points from a calculated baseline. The methodology for the data collection and the level of mathematical analysis required is onerous and renders this measurement unusable without professional assistance. This leaves the “Straightedge Tolerance” as the only usefull measurement according to the standard. For Conventional (smooth) classification the tolerance is 68mm.
The final condition that tends to surprise even the most experienced of project managers is that no measurement referred to in this standard can be used unless the following condition is met; “Slab or floor tolerance measurements shall be made no more than 72 hours after completion of the concrete floor finishing” (CSA A23.1-94-22.1.1). This section of the code reflects the changes likely to occur in the concrete slab due to changes in the substructure and floor assembly. These changes normally occur after 72 hours, however, an aggressive construction schedule may cause these changes to occur even sooner.
The technique for measuring the “Straightedge Tolerance” is described in CSA A.23.1-94-22.1.2. Although I have seen this measurement taken in many creative ways, the standard is specific about the type of equipment and the method that must be used. In a large percentage of the cases, the unapproved, but commonly used test methods would produce a fail where the actual test method would produce a pass. In fact, according to the standard, only 80% of the measurements taken must meet the standard (A.23.1-94-22.1.2.4) in order to be in compliance with the tolerance requirement.
THE ACTUAL STANDARD, 68mm using the 3m straightedge method, has been loosely and liberally translated into “1/4” in 10 feet”. Since most flooring contractors (especially in the case of hardwood) would require better than this standard in virtually all areas, we have what I refer to as … “THE GAP”.
Widening of "THE GAP"
If “THE GAP” is the difference between the insitue condition of the concrete floor and the tolerance required for the final flooring, then factors that degrade the performance of the floor would widen “THE GAP”. Choosing a lessor quality concrete or choosing flooring with tighter tolerances would also produce the same effect.
“THE GAP” = $$$ … The wider the gap, the greater the cost to close the gap.
The conditions that effect the final performance of the concrete slab can be listed under three main categories;
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Post-Construction Deformations (settlement)
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Mix design and water content
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Curing Conditions
Although would frame structures generally, will undergo greater movement during the construction process than its steel or reinforced concrete counterpart, all structures will deform to some extent. Some of these deformations are predictable and can be accurately calculated, and some are not so obvious and vary greatly depending on environmental factors.
POST-CONSTRUCTION DEFORMATION: Most floor assemblies are designed for the best economy. From a design point of view this means that you design as close as possible to the allowable critical values for shear, bending and deflection. The allowable design criteria in deflection for most floors is L/360 for live loading and L/240 for total loading. This applies whether it is a suspended reinforced concrete slab or a wood frame floor assembly. If you use an average 12 foot span, this would give you an allowable deflection of 0.4 inch for live loading and 0.6 inch for total loading. The wood design manuals all note that a wet service condition can increase these values by at least 15%. This means that some floor assemblies, although legal, would not be considered serviceable for concrete topping. Engineered products can increase the problem as the same dimension profile can support a longer span (greater ‘L’ value) and therefore each member has a greater allowable deflection.
A soils report for any substantial structure will likely predict between 0.5 inch and 1.5 inch of post-construction settlement. Settlement alone is not necessarily a problem, however, the extent of differential settlement can be a significant factor. Differential settlements at or above the predicted settlements are regularly observed across bearing walls constructed of different materials., (ie. Wood frame vrs concrete block) and between bearing and non-bearing walls.
Especially relevant to wood construction on the West Coast is the post-construction deformations caused by shrinkage. The Canadian Design Manual states that wood held in a wet service condition for any period of time can be considered to have a moisture content of 24% or greater. Wood will achieve an equilibrium with respect to moisture content after a certain time in an interior heated space at a level of approximately 6%. The shrinkage associated with drying will vary however, the manual predicts a shrinkage factor of 2.5% to 4.0% depending on species of wood and other factors. This can obviously create significant movement within the structure.
MIX DESIGN AND WATER CONTENT: All concrete mixes, including most “non-shrink” mixes will shrink to some extent. In general the more cementitious materials and sanded aggregates are used as a percentage of the mix the more shrinkage will occur. Mixes with a high water content generally will shrink more than the same mix with less water. The actual amount of shrinkage is a function of the design, final water content and the curing conditions. Remember that some mixes are specifically designed for use as toppings. Due to the higher cement content, these mixes tend to be more expensive than a regular mix. Regular concrete, although less expensive, is not a suitable substitute for a good quality topping mix. There is a significant variation in concrete performance that will still meet the requirements of CSA A23.1. Unsuitable or lower quality concrete may meet the minimum requirement but will likely widen “THE GAP”. In most cases, the cost of bridging a widened “GAP” is greater than the savings enjoyed with a lessor mix.
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CURING CONDITIONS: All Concrete mixes are designed to have an optimum rate of hydration that will produce a desired result. A faster or slower rate of hydration than designed will likely reduce the overall performance and can lead to cracking of the slab. A large difference in temperature and sunlight exposure between a sunny south side and a much cooler north side will create differential rates of hydration and additional internal tension within the material. In some cases, differential hydration can be more damaging that substandard hydration rates. As noted, water is the key. You have to have just the right amount, at all stages of the chemical reaction, in order to achieve maximum performance. This can be very difficult to achieve in adverse and/or changing conditions for the duration of the cast-in-place process.
There are numerous environmental factors that will alter the rate of hydration. With respect to the placing of concrete, there is not much that can be done to alter the effects of structural deformation (settlement). There is however, opportunity to mitigate the effects environmental factors will have on the concrete. Most measures taken to address the individual environmental factors will impove the performance to some extent and will have a positive effect on the reduction in cracking due to shrinkage and thereby narrow “THE GAP”.
The Solutions
1. Use as many of the following techniques as possible to reduce the negative effects of the environment on the critical curing period;
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Pre-wet floors to a SSD* condition.
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Ensure doors and windows are installed to block wind.
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Darken windows to prevent sunlight.
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Avoid placing during high ambient temperatures.
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Avoid placing during periods of low humidity.
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Use fibre reinforcing in the mix.
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Use anti shrinkage admixtures.
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Use post placement cure & seal products or mist surface**.
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Reinforce concrete at corners and columns.
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Separate perimeter, corners and columns with a flexible material like insulation or sill gasket.
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Install crack control ie. cant strips to ½ depth of the concrete at critical or flooring transition locations.
Consult your supplier and/or placing contractor to obtain more information on these techniques and other techniques that may be unique to your site.
2. Suppliers should ensure that placing contractors are properly trained and have sufficient experience to place concrete toppings. Ongoing training and a program that creates “licensed or approved contractors” similar to other industries is highly recommended. Concrete topping products have a small window in which they work well and requires experience specific to this type of application. Regular concrete placing techniques will often be ineffective or detrimental to the process of placing a thin topping.
3. The final, and in my opinion the most important solution, is that Builders must have a line item in a budget for “floor preparation”. The size of the budget depends on the size of “THE GAP”. The tighter the tollerance for a particular selected flooring, the larger the required budget. Whether it is the concrete contractor, the flooring contractor, or an independent floor preparation/repair contractor, this work should be tendered similar to any other work on the site. Failure to deal in advance with this necessary and normal requirement has led to stoppages in work as flooring contractors who do not have “floor prep” in their budget may refuse to proceed with installing flooring. Since the floor preparation can take a significant amount of time, there is a risk to incurring a delay in the construction. Furthermore, any such tender should specify all work to bring the floor from “CSA A23.1” to a condition that will satisfy the reqirements of the flooring to be used. In otherwords, to bridge “THE GAP”.
Steven Schwartz
President
* SSD – saturated surface dry condition. In this case, it means a saturated floor with no surface water (puddles)
** mist surface – caution: only mist if the surface has not gone “dry”. Misting a dry surface can create elastic tension as the concrete alternates between wet and dry. This may actually create additional cracking.