Manual Test

4. Concrete

.01 Concrete substrates for floor finishes shall conform to the following general requirements

.02 Granular Fill Requirements:

The thickness, compaction, and level of granular fill for concrete slabs-on-grade is critical for flooring finishes on concrete. In accordance with CSA A23.1 the maximum elevation variation of a compacted granular base is + 10 mm (3/8") and the average thickness of concrete may be 10 mm (3/8") less that that specified with a maximum local variation of 20 mm (3/4"). In the worst case this may result in concrete slab thickness below allowable limits that may lead to future problems. Fill levels must be set using a laser level and surfaces must be inspected and any deficiencies in thickness, levels, and compaction rate corrected before concrete is placed.

Granular bases must be of thickness, compaction rate, and levels within specified tolerances before any concrete is placed.

.03 Concrete Formwork Requirements: (suspended concrete slabs)

.1 All suspended concrete formwork shall be designed to accommodate slab deflection under dead load and long-term sag between supports in accordance with Building Code load and deflection requirements and to provide a smooth, level, and structurally sound system free of vertical movement detrimental to flooring materials installed. In some instances formwork may be cambered to accommodate deflection and sag after formwork is removed.

.04 Concrete Quality:

Unless concrete quality control measures are observed, including those dealing with workability of materials, ambient conditions, and method and quality of screeding operations to an accurate bench mark and level, undesirable properties in the hardened concrete may occur - particularly at the wearing surface - which may lead to a soft or dusting surface, permeable concrete, cracking or poor durability.

All materials used in concrete shall conform to current / applicable CSA and/or ASTM requirements established for cement type, aggregate size, mix design, and end-use of concrete.

Unless otherwise required, concrete should be of standard density, with a low water / cement ratio consistent with placing and finishing requirements in accordance with applicable CSA or ASTM requirements for interior slab finishes and levels.

Even if the above noted requirements are adhered to the quality of concrete is still dependent on the use of skilled concrete installers / finishers using appropriate equipment.

.05 Lightweight and Cellular Concrete:

Lightweight and cellular concrete substrates with a density of less than 1600 kg / m3 (100 lbs/ft3.), have generally such low surface strength that they are unable to support common on-site used static or rolling loads and are therefore, considered unsuitable for the direct installation of resilient flooring, unless 25 mm (1") or more of Standard Concrete is used as a topping.

Lightweight concrete substrates must have a minimum density of 1600 kg/m3 (100 lbs/ft3) and a minimum compressive strength of 3500 psi or greater to be acceptable for regular traffic use.

.06 Concrete Classifications: Both new and existing concrete substrates (or subfloors) can be grouped into three general classifications:

Below Grade Concrete Substrates: concrete substrates in contact with earth or fill and below ground level.

On Grade Concrete Substrates: concrete substrates in contact with earth or fill, but not below the ground level at any point. In all other cases, substrates in contact with the earth at/or above ground level are to be considered as on grade type substrates.

Suspended Concrete Substrates: concrete substrates having no contact with earth or fill.

.07 Concrete Handling, Placing, and Finishing:

All concrete handling and placing operations shall be such as to eliminate the possibility of segregation of aggregate and cement fines.

Concrete finishing operations should be performed with care. The following placing and finishing methods are used:

Consolidation: this operation is performed immediately after the concrete is placed to compact and mould concrete within the forms and around embedded items and reinforcement and to eliminate stone pockets, honeycombing, and entrapped air. Vibration, either internal or external, is the most widely used method for consolidating concrete. When concrete is vibrated, the internal friction between the aggregate particles is temporarily eliminated resulting in the concrete behaving like a liquid settling it into forms by gravity and large entrapped air voids rising more easily to the surface.

Screeding or strike off: this operation is performed immediately after the concrete is placed to remove excess concrete and to roughly level the surface. Concrete must be screeded to an established benchmark and levels checked using laser levels.

Darbying: this operation using a stiff wood or metal straight edge is used to embed surface aggregates and eliminate ridges and voids left by screeding.

Floating: this operation using a bull or hand float is performed after the water sheen disappears off the placed concrete and the concrete is set enough to support the weight of a person to further embed coarse aggregate, to remove slight surface imperfections left by edging and jointing, and to consolidate / compact cement mortar at the surface.

Trowelling: this operation using hand or power operated equipment is used to provide a smooth finish to concrete surfaces within level and smoothness tolerances noted in the applicable standards or particularly specified for the type of floor finish to be applied. Surfaces should be hand or power floated before any trowelling operation is performed. If necessary, tooled joints and edges should be rerun before and after trowelling to maintain true lines and depths. Time should be allowed between successive trowelling operations to permit the concrete to become harder. As the surface stiffens, each trowelling operation should be made with successively smaller trowels tipped at progressively higher angles so that sufficient pressure can be applied for proper finishing.

Excess water should not be left on the surface, as it will cause dusting, crazing, and scaling.

While slight imperfections may be hidden by a carpet's texture or backing, by carpet cushion or a flooring underlayment, or by hard flooring (hardwood, laminate, bamboo, etc.), any slab imperfection, including minor waviness, hairline cracks, and even sand particles inadvertently left on the surface, will telegraph or show through most resilient floor coverings particularly those with high gloss finishes. It is therefore imperative, that concrete surfaces be carefully reviewed for imperfections and that such surfaces are correctly prepared and thoroughly cleaned and vacuumed before the application of any floor covering materials.

Unless otherwise required by the floor covering material manufacturer, all concrete substrates (including acceptable lightweight or cellular concrete surfaces) shall be steel towelled to a smooth, dense, level, and even finish.

It must be noted however that the installation of finish flooring materials may transpire anywhere from 4 to 18 months after concrete placement and floor flatness may change with time due to curling of slabs-on-grade or dead load sag of suspended concrete slabs. This makes it very difficult if not impossible to predict concrete floor flatness at the time of flooring installation. This being said however it is critical that every effort be made to achieve a flat and level substrate surface when concrete is installed and finished.

Due to conditions noted above and where the quality of finishing work is suspect or will make it difficult if not impossible to provide level tolerances required for flooring, consideration should be given to provide a self-levelling cementitious underlayment as noted under "rectifying concrete levels" below.

.08 Concrete Levels:

There are a number of methods used to specify concrete substrate "flatness" or level tolerances required for floor finishes which are endorsed by the Cement Association of Canada (CAC) (formerly the Canadian Portland Cement Association) and by the American Concrete Institute in publications ACI 117 and ACI 302.1R.

Two main Consensus Standard organizations define these methods:

The Canadian Standards Association (CSA) provides three options in CAN/CSA A23.1, Concrete Materials and Methods of Concrete Construction, namely: the Straightedge Method, the Waviness Method based on ASTM E1486, and the F-Number Method based on ASTM E1155.

The American Society for Testing and Materials (ASTM) provides two different test methods, namely: ASTM E1486, Standard Test Method for Determining Floor Tolerances Using Waviness, Wheel Path and Levelness Criteria, and ASTM E1155, Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers.

There are three methods used to define / measure level tolerances:

Straightedge Method: Random measurements of one per 10 m² (300 ft²) are taken using a 3000 mm (10 ft) metal straightedge placed parallel and then at right angles to the long direction of the substrate surface. Measurements shall be to the closest 1 mm and compliance is considered satisfactory if 90% of the measurements are less than or equal to the specified method's designated Class tolerance, i.e., "Straightedge Values" of CSA A23.1 Table 19 – Slab and Floor Finish Classifications.

Waviness Method: Measurements for each area are taken in accordance with ASTM E1486 requirements along surveyed transverse and longitudinal lines of the substrate surface and waviness deviations are calculated and analyzed. Compliance is considered satisfactory if the Surface Waviness Index (SWI) of the entire floor area is less than or equal to the specified method's designated Class value (e.g. "Surface Waviness Index" of CSA A23.1 Table 19 - Slab and Floor Finish Classifications).

F-Number Method: This measurement method measures deviations of floor flatness (FF) and floor levelness (FL). Floor flatness relates to the bumpiness of the floor, while floor levelness relates to the tilt or pitch of the floor. The higher the F-Number, the better that characteristic of the floor. F-Numbers are linear, so an FF 20 is twice as flat as an FF 10, but only half as flat as an FF 40. Slabs-on-grade are usually specified with an FF number and an FL number (the FF is always listed first), such as: FF 25 / FL20. Because of deflection, elevated slabs are usually specified using FF only. When a floor is described as an "F 25", it usually means "FF 25".

Note to concrete finishers

Note

As previously noted concrete floor slab surface classifications and level tolerances have been defined by both the Cement Association of Canada (CAC) and the American Concrete Institute with such measurements conducted in accordance with CSA and ASTM consensus standards. The CSA A23.1 standard has defined the requirements for concrete floors and finishes as follows:

Prior to 2004, slab and floor finish classifications and level tolerances defined in accordance with CSA A23.1-00 were as follows:

Current (2006) slab and floor finish classifications and level tolerances defined in accordance with CSA A23.1-04, Table 19 are as follows:

Notwithstanding the slab and floor finish classifications and level tolerances noted in current CSA A23.1, ATSM E1155, or ASTM E1486 consensus standards noted above, the slab and floor finish classifications and level tolerances required by NFCA at the time of floor covering installation shall be as follows:

Note

In addition, where a variety of floor finishes (i.e., resilient carpet, hardwood, laminate and ceramic flooring) are to be installed over a concrete surface (such as a floor plate in high-rise construction), such surfaces (i.e. the whole floor plate in this case) shall be finished at time of placement to suit the most critical flatness and levelness requirements unless such areas are locally separated by concrete walls. As this may not be easily or realistically attainable due to quality of finishing work, slab deflection, etc. then the following or other means must be considered by the Owner, Design Authority / Specifier to provide such levels.

.09 Adjusting Concrete Levels:

Minor discrepancies in new or existing surfaces levels can be adjusted by using patching and filling compounds. This is considered, within reason, part of the flooring contractor's work.

Where level discrepancies are too large, i.e. where the thickness of patching and filling compounds required exceeds NFCA tolerances noted herein, floor levels must be corrected by using a self-levelling cementitious underlayment capable of bonding to prepared substrate surfaces and being installed from 3mm (1/8") to 13 mm (1/2") thick in one pour and up to 25 mm (1") thick in small areas. The material must be capable of being feather edged or tapered without fracturing in order to not leave ridges and to match existing elevations. Such material should achieve a compressive strength of 5300 psi after 28 days by air cure only, be capable of being walked on after 3 hours without damage, and be capable of being coated after 24 hours at 21°C (70°F). Substrate preparation is critical for bonding of this material.

The application of cementitious underlayments shall be done by others (the General Contractor or Owner) in a timely manner before the installation of flooring materials or may be undertaken by the floor covering contractor as an additional cost to flooring work.

.10 Concrete Curing:

Sufficient curing time should always be allowed for concrete substrates to develop adequate strength prior to subjecting them to loading or finish work. Curing should be started as soon as the curing method or medium (of types pre-approved by the applicable flooring material manufacturers) can be applied without damaging the surface.

During initial curing, concrete surfaces should be kept uniformly wet or moist. Partial drying of any part of the surface should not be allowed as it may result in crazing or cracking of the surface.

Curing methods using water (such as sprinkling and the use of wet coverings) are the most effective and should be used whenever practical. Although not usually as effective as curing with water and wet coverings, moisture retention membranes (curing compounds) are widely used because of their convenience.

Curing compounds should not be used for slabs intended for further surface treatment, or for slabs intended for adhered flooring materials unless they are known to be compatible with adhesive, since bonding properties of adhesive or other materials may be affected.

Concrete substrates, even with adequate curing time, can present an unacceptable moisture condition by allowing excessive amounts of moisture vapour to pass through to the surface. This can be a problem even on suspended concrete floors.

Concrete substrates containing lightweight aggregate or concrete substrates cast in steel or plastic pans retain moisture for long periods of time and may need a much longer drying time. Such substrates should not be covered with resilient floor covering unless proven to be dry.

It is not enough to assume that a concrete substrate has cured after a designated time period. There are, in fact, a number of variables that affect concrete curing:

The amount of water in mix versus cement and aggregate content.

Inclusion of fly ash and admixtures that will affect the curing time. Note that the use of fly ash will noticeably extend concrete curing times.

Time of year when placed as the relative humidity (above 80%) and temperature (below 20°C) will affect curing time.

Use of sealers after installation that will retard the release of moisture.

The actual slab thickness - the thicker the concrete the longer it will take to "dry" all factors remaining the same.

The type of slab (i.e. slab-on-grade versus a suspended slab). Suspended slab can dry on both sides speeding up the process.

If a vapour retarder/ waterproof membrane is used under the slab.

The type of formwork material used for suspend slabs – slabs formed using metal or plastic pan systems may take longer to cure than those formed using wood framing.

The amount of heat within the area from the sun or from heating sources to aid in the drying process

The amount of natural or forced ventilation available to exhaust moisture.

The amount of surface porosity of the finished slab – a steel towelled and polished (densified) surface will take longer to dry.

There are also a number of methods that may be used to speed up the drying process. Subject to the pre-approval of the Structural Consultant, these are:

Increasing the temperature within the area and providing additional ventilation to permit the escape of moisture-laden air.

Placing dehumidifiers in strategic locations to extract moisture from the area and from the slab.

Subject to the finish requirements for the type of flooring to be installed, the use of curing and sealing compounds should be restricted as they slow concrete hydration resulting in slow curing slabs thus retaining moisture within the concrete long after the curing period. The use of such compounds may also be incompatible with the type of flooring adhesives to be used.

Subject to the finish requirements for the type of flooring to be installed, the use of curing and sealing and hardening compounds or the presence of a very dense / polished surface finish may retain moisture within the concrete long after the curing period. These treatment and finishes could be removed by grinding, sanding, or shot blasting to allow for the faster emission of static and dynamic moisture from the slab.

New concrete substrates must be properly cured and thoroughly dry before commencement of any flooring installation. As curing depends on such things as the environmental conditions, the location of the slab and the time of year, substrate surfaces may require more drying time than normal before they may be considered ready for moisture testing.