What lurks beneath: Preventing failures due to insufficient floor preparation

by sadia_badhon | June 25, 2021 4:24 pm

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by Howard Jancy, CSI, CDT

It is lurking there, hidden, out of sight. Waiting. It is lurking there, unmoving, seemingly harmless. Waiting. It is lurking there, waiting for some obscure moment to finally reveal itself, and when it does…It is too late. You cannot stop it. Damage will be done. What is it? It is the poorly prepared concrete substrate underneath your failed tiled floor.

Once the damage starts, thoughts about what should have been done prior to the tiles’ installation are quickly dimmed by the impending tear out and replacement project on the horizon.

The weak link

Inadequate substrate preparation is the Achilles’ heel of any tile floor installation. There is often a thought that surface preparation can be casually omitted without consequence, since no one will ever know once the concrete is covered by a membrane, mortar, and tile. No one will know until tiles start to delaminate or crack, and grout starts to crumble. Regardless of the quality of the setting materials and tiles, it is really the quality of the methods and materials used to address the substrate’s deficiencies that dictate the functional and aesthetic longevity of a tile floor. The omission is often driven by several factors.

Substrate evaluation

An inaccurate or incomplete assessment of a substrate’s deficiencies is made, particularly when an existing floorcovering is still in place when the specification is written, or the project is bid. As a result, the flooring subcontractor only includes minor surface preparation in the bid, which normally does not include extensive flattening of the substrate.

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Construction delays

As delays compress the construction schedule, the general contractor (GC) or project owner is looking (demanding) for ways of expediting the floor’s installation by eliminating substrate preparation.

Value engineering

As the construction schedule is compressed, so too is the project’s budget. Consequently, substrate preparation is value-engineered out of the floor installation.

Time and money

The flooring installer, aware of these potential speed bumps to a timely and profitable installation, chooses to flatten the floor by applying additional tile bonding mortar during tile installation for the purpose of smoothing a substrate, rather than including self-leveling underlayment in the bid. This creates additional problems, addressed later in the article.

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What are the consequences?

Surface preparation is not a ‘new’ requirement for tile projects. However, the tile size has evolved over the last few years. Gauged porcelain tile panels now reach 1.5 x 3 m (5 x 10 ft) in dimension.

Large-format ceramic and stone tiles, greater than 380 mm (15 in.) on a side, require a keener eye when evaluating remedial treatment of a concrete substrate. Most importantly, are high/low points within the required tolerances, as defined by the American National Standards Institute (ANSI) A108.02, General Requirements: Materials, Environmental and Workmanship: For sub-floor surfaces, the maximum allowable variation for large-format tiles is no more than 3 mm (¹/₈ in.) in 3 m, and no more than 1.6 mm (¹/₁₆ in.) in 0.6 m (2 ft).

If substrate is not flattened prior to a tile installation, it would lead to several deficiencies in the finished floor, all of which are generally the result of inadequate mortar coverage. ANSI A108.5, Installation of Ceramic Tile with Portland Cement Mortar, defines proper mortar coverage as 80 percent for dry area installations and 95 percent for wet area installations. Coverage should be evenly distributed on the substrate as well as the underside of the tile to fully support it, particularly at the edges and corners. Though not required by ANSI, 95 percent mortar coverage should also be considered for stone tiled floors, since it is not as strong as ceramic ones. Further, both ceramic and stone installations are subjected to thermal cycling (Figure 1).

Mortar must be applied in a uniform thickness to ensure proper coverage and performance after a tile is embedded. Typically, as the installer combs the mortar with the correctly configured notched trowel, the trowel’s edge will follow the substrate’s contour between high and low spots. If the low spots, more than 3 mm in 3 m, are not filled or flattened, there is a greater likelihood of uncollapsed mortar ridges under the tiles. Uncollapsed ridges create voids, often located by the distinct hollow sound when the tile is tapped. They will cause the tiles to crack from either impact or rolling loads since the underside of the tile is unsupported by mortar. Attempts by the installer to correct for potential voids during tile installation often compound the problem.

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During installation, the tile installer will periodically lift random tiles after embedment to assess mortar coverage and distribution. If there are uncollapsed mortar ridges, or mortar voids on the substrate or on the back of tile, the installer could, for example, change to a larger-notched trowel to distribute more mortar. Unfortunately, the more typical corrective action is to randomly apply spots of mortar. Spot bonding adds more mortar to the substrate, but when the tile is once again embedded, the additional mortar does not evenly distribute across the substrate or on the tile’s underside.

If the tiles remain intact despite the poor surface preparation and marginal mortar coverage, there are other issues to address: excessive tile lippage, crooked grout joints, irregular grout joint widths, and inadequate slope for drainage. None of these issues are a simple fix and may require tear out to remediate.

Constructing a flat concrete floor

For larger concrete installations, a floor flatness (Ff) number is specified to indicate the required slab flatness. Ff numbers can range from 20 to 150, with higher values indicating a greater degree of flatness. A concrete slab for a commercial office space may only require an Ff of 30, whereas a concrete slab for a TV studio or narrow aisle warehouse requires a Ff number approaching 100. Unfortunately, a concrete slab constructed to a specified Ff number may not meet the sub-floor tolerance outlined in ANSI A108.02, at the time of the tile or stone flooring installation.

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ASTM E1155, Standard Test Method for Degerming FF Floor Flatness and FL Floor Levelness Numbers, prescribes the measurement of Ff numbers within
72 hours of concrete placement. However, a floor that is measured or verified as flat at 72 hours does not remain flat. The residual moisture in concrete after placement will bleed to the concrete’s surface and evaporate if uninhibited by a curing membrane. As evaporation and slab drying occurs, the concrete slab shrinks and curls. The deformation is most pronounced at slab edges and construction joints. Consequently, the measured flatness within 72 hours will not be accurate after weeks and months of drying, particularly when the tile contractor is ready to start floor installation. Upon understanding concrete shrinks and curls after installation and does not remain flat, a design professional could attempt to solve the problem by simply specifying a higher Ff number than normally required for the project.

Ff number conundrum

As mentioned, ANSI A108.02 prescribes sub-floor surface variation for large-format tiles no greater than 3 mm in 3 m and no more than 1.6 mm in 0.6 m (¹/₁₆ in. in 2 ft), which is determined by measuring the gap between the substrate and a 3-m straight edge. There is no exact correlation between Ff numbers and straight edge measurements, but accepted industry correlations indicate a floor constructed to an Ff number of 50 approximately equates to a measurement of 3 mm in 3 m, and an Ff of 100 is around 1.6 mm in 0.6 m.

So, based on the approximate relationship between Ff numbers 50 and 100, and straight-edge measurements, one could assume there should not be any sub-floor flatness issues at the time of installation. While this is true theoretically, concrete drying shrinkage as previously described, will likely curl the slab out of spec for flatness even if Ff is high.

Another concern with specifying high Ff numbers is the concrete slab’s installation cost. The degree of floor flatness is a function of the finishing methods, tools, and precision employed by the concrete contractor. A hand-troweled concrete garage floor will not be as flat as one finished with a mechanical trowel. The former is cheaper to install. Consequently, achieving high Ff-numbered floors will be costly (Figure 2).

A better approach to substrate flatness

Achieving a substrate that remains flat until the time of tile installation is a challenge. Concrete drying shrinkage and the accompanying slab curl is almost a given. The money spent on initially achieving a high Ff number, high precision floor would be better spent on addressing floor flatness requirements through the installation of a self-leveling cementitious underlayment (SLU), at a lower cost and with a higher degree of flatness.

As a guide to SLU selection and specification, as well as product installation, two new ANSI standards are pending, with likely publication later this year:

• ANSI A118.16, Flowable Hydraulic Cement Underlayment/Self-leveling Underlayment; and
• ANSI A108.21, Interior Installation of Flowable Hydraulic Cement Underlayment/Self-leveling Underlayment, (Figure 3).

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Some important elements of the proposed product standards are:

• only applicable to tile and stone installations;
• covers standard and rapid-hardening formulas;
• ≥ 20.7 MPa (3000 psi) compressive strength required for all formulations; and
• must be rated for Extra Heavy Service in accordance with ASTM 627, Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson-type Floor Tester.

Also, the currently proposed installation standard provides specific instructions when floor leveling, not just floor flattening; is required.

When substrate leveling to designated elevations is required on a project, this intent needs to be clearly specified along with tolerances. Substrate leveling requires extensive surveying and additional costs in labor and materials. Leveling may not be plausible for the project due to adjacent finishes and/or passageways, especially in remedial work. SLU mortars may not achieve a level surface without additional pinning, preparation, and labor. It is advisable to consult the SLU manufacturer for specific recommendations not addressed by this standard.

More mortar leads to problems

Instead of using a self-leveling cement underlayment for the purposes of truing a floor prior to the tile installation, a contractor may choose to utilize the tile setting mortar for flattening and tile setting. The intent is to save time, money, and labor by eliminating SLU. This tack is acceptable for a substrate that is within the tolerance of ≤ 3 mm in 3 m. However, using mortar to flatten substrates 3 mm in 3 m should be avoided.

ANSI A118.4, Modified Dry-set Cement Mortar, and A118.15, Improved Modified Dry-set Cement Mortar, establish maximum mortar thickness for large and heavy tiles (LHT) at 13 mm (½ in.) after tile embedment. Applications exceeding this value because the mortar was used to flatten the floor, and adhere and support the tile, causes mortar shrinkage. As the adhered mortar shrinks, it can pull the tile with enough force to crack or deform it. Spot bonding of cementitious mortars, an installation method not endorsed for floors by the Tile Council of North America (TCNA) or ANSI A108, A118, and A136, will contribute to excessive mortar thickness and shrinkage (Figure 4).

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Dabbing mortar randomly, instead of unidirectionally combing or troweling utilizes less time, material, and labor. So, spot bonding is done to cheapen an installation. More often though, it is used to correct uncollapsed mortar ridges and insufficient mortar coverage because the substrate was not flattened first with SLU before the mortar was applied (Figure 5).

SLU selection

Setting material manufactures provide several self-leveling underlayment formulations optimized for specific floor leveling situations. Whether by specification or a contractor’s choice, a project’s site conditions and flooring requirements can be better addressed with the right product, potentially saving time and labor, as well as achieving a more suitable substrate for tile installation. Figure 6 illustrates the range of products available to manage the project’s budget, construction schedule, degree of flattening (or leveling), and product working time.

Surface preparation

Cementitious self-leveling underlayments should only be applied to clean and structurally sound concrete substrates. It is recommended to remove bond-breakers, such as grease and oils or curing compounds and damaged and loose concrete. Surface should be roughened or profiled to promote a better mechanical bond of the primer and SLU to the concrete.

SLUs can also be applied over gypsum-based underlayments, oriented strand board (OSB) underlayments, cutback adhesive residues, epoxy and cement terrazzo, metal, ceramic and stone tile, and well-adhered resilient flooring. Careful evaluation of these conditions must be made before installing a self-leveling product. Manufacturers will have specific preparation requirements for these conditions and could in some instances, require a complete tear out before attempting to install a SLU.

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Additional items to evaluate

Moisture vapor: A moisture vapor emission rate (MVER) in excess of 1.4 to 2.5 kg/93 m² (3 to 5 lb/1000 sf)/24 hours or 75 to 80 percent relative humidity (RH) could exceed the flooring manufacturer’s guidelines or restrictions for their product. Ceramic and stone tiles as well as cement-based SLUs are more compatible with excess moisture vapor. However, the effects of the vapor, such as production of alkaline deposits on the substrate, efflorescence formation on the flooring surface, staining, mold growth, and warping, may plague the finished floor, particularly luxury vinyl tile (LVT), carpet, and wood. MVER or RH readings that are not compatible with the flooring product must be remediated with 100 percent solids, two-part epoxy, moisture mitigation membrane.

Construction joints and cracks: Movement joints and cracks that may have developed after the concrete’s installation require specific methods when installing underlayments and ceramic tile and stone. The TCNA Handbook provides several useful details such as:

• EJ171, Movement Joint Guidelines for Ceramic, Glass, and Stone – Expansion joints in the concrete must brought up through the tile assembly, including SLU, in the same plane;
• F205 (on-ground concrete) and F205A (above-ground concrete) – Both flooring details include a cementitious self-leveling underlayment, and are recommended for concrete slab construction where no bending stresses occur, and Ff is critical (F205 and F205A are similar, but the latter requires a bonding mortar that is approved by the manufacturer for above-ground use; and
• F125 – Partial – Allows for the relocation of a static joint or crack through the tile assembly by incorporating a partial crack isolation membrane in the assembly, applicable to F205 and F205A flooring installations.

Additionally, full slab depth joints and cracks require filling to prevent the self-leveling underlayment from flowing through the slab into the room below or soil subgrade. If SLU seeps through the concrete slab after the products’ heal time, ≥ 10 minutes in accordance with ANSI A118.16, a permanent depression can form in the SLU’s surface directly over the crack. This depression can reflect through resilient flooring.

Substrate priming: Concrete is a porous material. If not correctly primed so as to not absorb SLU’s mix water, the SLU’s workability, flow and heal rates, adhesion, strength, and surface flatness are compromised. A properly applied primer penetrates the surface, forming a uniform film, free of voids and pinholes. Non-porous substrates, such as hard-troweled concrete may also be a problem and could require mechanical preparation before a primer is applied.

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ASTM F3191, Standard Practice for Field Determination of Substrate Water Absorption, is a simple test for assessing the substrate’s porosity. Apply drops of water onto the concrete, particularly on suspected problem areas. If the water is readily absorbed, creating a darkened spot, the concrete is porous. If the water drops bead up on the surface, and no dark damp spot is visible, the concrete is non-absorbent. Extremes in porosity are equally problematic and require special attention. The primer will need multiple applications on porous concrete, whereas repellent surfaces may require mechanical preparation or a special primer. Also, primer applications over gypsum-based or OSB underlayments, cutback adhesive residues, epoxy and cement terrazzo, metal, ceramic and stone tile, and well adhered resilient flooring may require special consideration before priming.

Going with the flow

Self-leveling cement underlayments can be mixed and applied using automated mixing and pumping equipment or manually mixed onsite with drill-powered ‘eggbeater’ paddle and poured from the mixing barrels. Both methods are suitable for commercial and residential installations.

A wet edge must be maintained between successive pours of SLU. Fresh material poured adjacent to product that has already started to harden can form a ridge or bump along the unintended cold joint, which could create a visible ridge in resilient flooring. The window of time for mixing to final finishing is approximately 20 minutes for standard set SLUs. Temperature, humidity, and air movement must be factored into the installation so that the SLU’s working time and flow and heal rates are not compromised.

Cold temperatures are also a potential problem. The ideal ambient, substrate, and product temperature is 10 C (50 F) to 32 C (90 F). Manufactures provide rapid and delayed setting formulations when installing at temperature extremes. Strategic material and equipment staging, and adequate labor must be managed for a successful installation.

As material placement proceeds, another contractor will be pushing and pulling the flowable product to the required thickness with a gauge rake. The gauge rake has adjustable feet so that the blade is set and maintained at the correct elevation above the substrate. Gauge raking is followed by lightly pulling a smoother across the SLU (Figure 7).

The smoother breaks the surface tension of the SLU. This tooling contributes to product flow and distribution across the concrete, allows entrapped air to escape and refines surface smoothness. Gauge raking and smoothing must be completed before the product’s working, flow, and healing times are exceeded. Any attempts to work the SLU once it loses workability will leave permanent depressions on the surface.

Generally, non-moisture sensitive ceramic and stone tiles can be installed after the SLU has cured for four hours, and other floorcoverings after 16 hours. SLU drying time and subsequent flooring installation can be impacted by temperature, humidity, and material thickness. It is advisable to confirm moisture limitations of flooring and adhesives before installing over a self-leveler (Figure 8).

Conclusion

What is under the tiled floor should not be a mystery or reveal itself through failure. If specified and correctly executed, there are many methods, materials, standards, and guidelines that will minimize the possibilities of failure due to insufficient floor preparation or any other stage of an installation.

[10]Howard Jancy, CSI, CDT, is a commercial architectural services representative for Custom Building Products. He has 30 years of experience with tile, stone, and concrete flooring design, installation, and remediation. His responsibilities include specification writing and review, technical services, and continuing education. He can be reached at howard.jancy@cbpmail.net[11].

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