Section 3: Wall Constructs

3.1 Structural Considerations (Walls and Tunnels)

As in Section 2 with floor applications, the same criteria for surface and structural considerations applies to wall applications. Basically, the wall constructs must be structurally sound, dimensionally stable, meet the maximum allowable standard for deflection of L/360 for ceramic tile, L/480 for stone (the MIA requires L/720 for stone applications) under total anticipated loads and be free from any bond breaking or bond inhibiting substances (please refer to Section 2.2 for more information on structural considerations and live and dead loads). For more information on direct adhered ceramic tile, stone and thin brick exterior façade applications, please reference the “Direct Adhered Ceramic Tile, Stone and Thin Brick Façade” Technical Design Manual, by Richard P. Goldberg AIA, CSI available at www.laticrete.com.

3.2 Wall Types

Concrete Wall Types

One of the most common substrate types that will be found in mass transit applications is concrete. This section will examine the various concrete construction types that can be encountered and their common characteristics.

Tilt-Up Concrete

Tilt-up and tilt-wall are two terms used to describe the same process. For a tilt-up concrete building, the walls are created by assembling forms and pouring large slabs of concrete, called panels, directly at the job site. The concrete panels are then tilted up into position around the buildings slab to form the walls. Because the concrete tilt-wall forms are assembled and poured directly at the job site, no transportation of panels is required. A major benefit of this technique is that the size of the panels is only limited by the needs of the building and the strength of the concrete panels themselves.

Tilt-up construction panels can sometimes be extremely wide and/or tall. Tilt-up concrete panels have been as large as 69′ (21 m) across and almost 93′ (28.3 m) high. Thus, architects and tilt-up concrete contractors have a great deal of flexibility in planning and creating their buildings.

A tilt-up construction project begins with job site preparation and pouring the slab(s). During this phase of the project, workers install footings around the slab in preparation for the panels. The crew then assembles the panel forms on the slab. Normally, the form is created with wooden pieces that are joined together. The forms act like a mold for the cement panels. They provide the panel’s exact shape and size, doorways and window openings, and ensure the panels meet design specifications and fit together properly. Next, workers tie in the steel grid of reinforcing bars into the form. Inserts and lift hooks are embedded for lifting the panels and then attaching them to the footings, the roof, and to each other.

Once the concrete panels have hardened and the forms have been removed, the crew connects the first panel to a crane with cables that hook into the inserts. Workers help to guide the concrete panel into position and the crane sets it into place. An experienced crew can erect as many as 30 panels in a single day.

Because concrete tilt-up walls are poured outdoors, contractors are at the mercy of climatic conditions. When temperatures drop below freezing, curing the concrete panels becomes more difficult and expensive. This makes this process attractive in warm climates. While tilt-up concrete buildings are built in northern areas, the window of time for temperate weather is smaller and less predictable. This can make construction schedules more difficult to meet.

Pre-Cast Concrete

The pre-cast concrete building process is similar to tilt-up construction, but it addresses the challenges presented by weather. For pre-cast concrete buildings, work crews do not set up forms at the job site to create the panels. Instead, workers pre cast concrete panels at a large manufacturing facility. Because the pre-cast concrete forms are poured indoors, this activity can take place regardless of the weather conditions. After curing, the pre-cast concrete panels are trucked to the job site. From this point, pre-cast concrete buildings are assembled in much the same manner as tilt-wall buildings.

The fact that pre-cast concrete walls are formed at a manufacturing facility resolves the weather issue, but presents a different limitation not found in tilt-up construction. Because the panels must be transported, sometimes over long distances, this places a substantial limitation on how wide or tall each panel can be. It would be impossible to load pre-cast panels that were 60′ (20 m) wide or 90′ (30 m) long onto trucks and transport them any distance. For a pre-cast construction project, the panels must be smaller and more manageable to allow trucks to haul them over the road to their final destination. This places certain design restrictions on architects and limits the applications where pre-cast construction can be used.

Cast-in-Place Concrete

Cast-in-place concrete is a common substrate for the direct adhesion of tile. Cast-in-place concrete is poured into forms (sprayed with form release agents) where steel reinforcing has previously been placed. The condition of vertically formed concrete is extremely variable, due to the numerous potential defects that can occur with mix design, additives, forming, placement, and curing. There may be concerns with poured-in-place concrete in relation to the long term performance of a tile installation.

Laitance

As noted in Section 2.3, laitance is a thin layer of weakened portland cement fines that have migrated to the surface of the concrete. This condition is especially prevalent in vertically formed concrete, where excess water migrates by gravity, aided by the vibration of concrete and pressure to the surface against the wall form. The excess water gets trapped by the form where is stays until the form is removed. Once the forms are removed and the water has had a chance to evaporate, it leaves behind a thin layer of what appears to be a hard concrete surface, but in reality is weakened due to the high water to cement ratio at the surface. Laitance has very low tensile strength, and therefore the adhesion of tile will be limited by the low strength of the laitance. Laitance should be removed from the concrete surface prior to the installation of tile.

Honeycombing

Honeycombing is a condition where concrete is not properly packed or consolidated by vibration during the pour, where steel reinforcement is too close to the form, where there is internal interference with the flow of concrete during the consolidation procedure, or where there is poor mix design. These conditions can result in voids in the surface or core of the concrete. Surface honeycombing defects must be properly prepared and patched using a bonding agent to ensure proper adhesion to the concrete prior to installation of the tile or other finish material.

Unintended Cold Joints

In vertical walls, cold joints are usually unintended, and can result in a weakened plane. This weakened plane is subject to random shrinkage cracking which could transfer to the surface of the tile installation. These conditions usually result from delays or equipment breakdowns and can be prevented by proper coordination of concrete delivery and proper maintenance and use of installation equipment.

Concrete Forms

Smooth formwork for concrete walls can result in a surface that is too smooth for direct adhesion of tile or stone with a portland cement based tile adhesive. A smooth surface provides little or no mechanical key for the initial grab required when applying wet cement based mortars. These surfaces do not typically facilitate absorption of cement paste and subsequent mechanical locking provided by the growth of cement crystals into the pores of the substrate. High-pressure water blast, mechanically grinding or vertical scarification can be used to achieve a better concrete surface to accept a direct adhered tile installation. Epoxy based tile installation materials (e.g. LATAPOXY® 300 Adhesive or LATAPOXY 210 Adhesive) do not rely on open pore structure to achieve exceptional bond and may be a better choice for a smooth concrete finish.

Form Release Agents

There are a wide variety of form release agents on the market today. These products range from used motor oil and diesel fuel to sophisticated water based products. Any type of oil based or other potential bond breaking contaminant must be physically removed prior to the direct adhesion of tile.

Curing Compounds

The variety of materials and the unique characteristics of proprietary formulations require that you follow the same recommendations above for form release agents.

Concrete Additives

There are numerous concrete additives, which, depending on the properties they impart to the concrete, could be detrimental to the adhesion of the tile to the concrete wall. For example, super plasticizers are a type of concrete additive that allows extremely low water to cement ratios and resultant high strength, without sacrificing workability of the concrete. This type of additive can induce bleed water, and facilitate the formation of laitance. Similarly, additives that react with free minerals in the concrete produce an extremely dense and water-resistant pore structure (e.g. crystalline type additives) and may be detrimental to good adhesive bond. It is therefore imperative to communicate to the concrete subcontractor, and to write into the concrete specification, which areas of the concrete are scheduled to receive ceramic tile/stone finishes. This communication can help ensure that the concrete is fully compatible with the direct bond method of ceramic tile/stone installation using a latex fortified portland cement based or epoxy adhesive.

3.3 Concrete Curing

The installation of ceramic tile/stone over concrete can only begin once the concrete reaches satisfactory cure. As concrete cures, it loses moisture and shrinks. A common misconception is that concrete cures completely and all concrete shrinkage takes places within 28 days of placement. This is simply not true. Thick sections of concrete could take over 2 years to reach the point of ultimate cure. 28 days at 70°F (21°C) is the period of time it takes for concrete to reach its full design strength. At that point, concrete should reach its designed tensile strength, and can better resist the effects of shrinkage and stress concentration.

Depending on the humidity and exposure to moisture in the first 28 days, there may be very little shrinkage that occurs within that period. So while more flexible adhesives, like latex portland cement adhesive mortars can accommodate the shrinkage and stress that may occur in concrete less than 28 days old, it is recommended to wait a minimum of 30–45 days to reduce the probability of concentrated stress on the adhesive interface. Some building regulations may require longer waiting periods (up to 6 months). After this period, resistance to concentrated stress is provided by the tensile strength gain of the concrete, and its ability to shrink as a composite assembly. The effect of remaining shrinkage is significantly reduced by its distribution over time and accommodated by the use of low modulus or flexible adhesives.

3.4 Concrete Masonry Unit (CMU)

Concrete masonry unit (CMU) construction is a suitable substrate for many ceramic tile/stone applications. When standard aggregate and density CMU is built to plumb and levelness tolerances (including the mortar joints), no further preparation is needed except for final water cleaning, unless there is a specific need or specification for an anti-fracture (e.g. LATICRETE® Blue 92 Anti-Fracture Membrane) or waterproofing membrane (e.g. LATICRETE Hydro Ban or LATICRETE 9235 Waterproofing Membrane) which typically are installed directly to the CMU (following the manufacturer’s installation instructions).

Both standard and lightweight aggregate concrete masonry units present several other material specific concerns. Typically, CMU walls are fairly porous. Therefore, care must be taken to prevent possible pre-mature absorption of moisture, required for proper hydration of latex portland cement adhesive mortars, into the CMU. The CMU walls should be wiped down with a damp sponge prior to the application of any membrane or adhesive mortar. This will increase the working time of the membrane or adhesive mortar and also provide a final cleaning of the wall.

In some cases, where test panels may indicate poor adhesion at the CMU/adhesive interface, it is recommended to skim coat the CMU (1/8" {3 mm} maximum thickness) with a latex portland cement mortar (e.g. LATICRETE 254 Platinum or LATICRETE 211 Powder gauged with LATICRETE 4237 Latex Additive) to seal the rough surface texture of the CMU. With the proper latex portland cement mortar, the thin skim coat will harden quickly without risk of moisture suction. Another concern is the cohesion or tensile strength of the CMU material which may be less than the tensile bond strength of the adhesives; this is more of a concern with lightweight aggregate or cellular CMU.

Cellular or gas beton CMU (also commonly known as ytong or Aerated Autoclaved Concrete [AAC]), is manufactured with gases to entrain air spaces and reduce weight and density. Typically, AAC block does not have good tensile and shear strength (<7 kg/cm2). Due to the low shear strength, slight shrinkage of conventional cement mortars may tear the surface and result in delamination. Similarly, the low density (40–50 lbs/ft2 [500–600 kg/m2]) of this material results in a coefficient of thermal expansion which is significantly different enough from typical cladding materials to cause concern about differential movement. The porous structure of this material also requires careful consideration to compensate for suction of hydration moisture from cement based adhesives. Most of the cellular or gas beton CMU block manufacturer’s require the use of a latex portland cement based skim coat (e.g. LATICRETE 254 Platinum or LATICRETE 211 Powder mixed with LATICRETE 4237 Latex Additive) prior to the installation of the tile adhesive mortar, cement based render or membrane.

3.5 Framed Wall Substrates

Light gauge metal (galvanized steel) studs are commonly used as a back-up wall structure for directed adhered cladding. The metal stud frame can employ a variety of sheathings, the type of sheathing dependent on whether the wall is a barrier wall requiring direct adhesion of the cladding material, or a cavity wall where the sheathing type does not affect adhesion. Metal stud walls can also be used for both pre-fabrication of panels, or construction in-place. Metal stud size and gauge are selected based on known structural properties required to resist live and dead loads. The predominant live load in external applications is wind, therefore stiffness usually controls size of metal studs. Empirical experience has shown that 6" (150 mm) wide, 16 gauge studs spaced 16" (400 mm) on center are appropriate for most applications. However, engineering calculations may show that other widths, and gauge are required. Deflection (measure of stiffness) of metal stud back-up wall construction for exterior facades should be limited to 1/600 of the unsupported span of the wall under live (wind) loads. Interior applications of ceramic tile should be limited to L/360 and L/480 for stone applications. While these are the current allowable deflection standards for metal stud back-up walls, some studies on conventional masonry veneer cavity walls have shown cracking can occur on walls that have significantly less deflection. There have been no definitive studies conducted on metal stud barrier walls used for direct adhered cladding, but empirical evidence indicates that the composite action of rigid cladding materials, high strength adhesives, and proper specification of sheathing material and attachment method to metal studs does create a more rigid diaphragm compared to a metal stud back-up wall separated by a cavity. Metal stud framing typically requires lateral bracing to, or integration within the structural steel frame of a building. Bracing is dependent on the configuration and unsupported length of the stud frame.

Empirical experience also provens that integration within the structural steel system not only provides a stiffer metal stud wall by reducing the unbraced lengths of studs, but also improves accuracy and reduces errors by providing an established framework where studs are used as infill rather than the entire framework. There are a wide variety of sheathing materials to choose from for metal stud walls, ranging from low cost exterior gypsum sheathing or plywood for cavity wall sheathing, to cement backer board, or lath and cement plaster for barrier walls requiring direct adhesion of the cladding material. Gypsum sheathings historically have not been a very durable material for cavity walls, although new gypsum based sheathings with fiberglass facings and silicone impregnated cores have improved performance. Cement plaster is an ideal sheathing for exterior metal stud back-up walls. This sheathing provides a seamless substrate with no exposed fasteners, resulting in good water and corrosion resistance. The integral reinforcement also provides necessary stiffness, resistance to shrinkage cracking, and positive imbedded attachment points for anchorage to the metal stud frame. The attachment of the reinforcing in a cement plaster sheathing and resulting shear and pull-out resistance of the fasteners within the sheathing material is superior to that of pre-fabricated board sheathings such as gypsum or cement backer unit boards (CBU). This factor is important in more extreme climates where there is more significant thermal and moisture movement which can affect sheathings that are poorly fastened or have low shear or pull-out resistance to fasteners.

Cement backer unit boards (CBU), fiber cement and calcium silicate boards are other choices for metal stud back-up walls requiring direct adhesion of the cladding material. CBU board is pre-fabricated, and provides an efficient, cost effective cementitious substrate for adhesion of cladding materials. While CBU is technically water resistant, it requires waterproofing for exterior and interior wet area applications, as the minimal thickness and corrosion potential of fastener attachments increase the possibility for minor cracking, leaks, deterioration, and defects such as efflorescence.

Fiber cement underlayments can be sensitive to moisture, and requires waterproofing to resist dimensional instability that may be caused by both infiltrated rain water and condensation on the back side of the board. Check with the fiber cement underlayment manufacturer for use in exterior environments.

There are proprietary direct adhered wall systems which employ corrugated steel decking as sheathing and as a substrate for exterior cladding adhered with special structural silicone adhesives. Because these systems employ spot bonding rather than a continuous layer of adhesive, the combination of open space behind the cladding and the corrugation of the steel decking provide a cavity for drainage and ventilation. This cavity anticipates water penetration, and re-directs water back to the exterior wall surface. However, the underlying metal decking and framing are subject to corrosion facilitated by abrasion of galvanized coatings during construction. Leakage may also occur due the difficulty in waterproofing the steel and multiple connections/penetrations. Corrugated steel sheathing cavity walls have a limited service life similar to that of barrier walls.

Generally, the light weight and minimal thickness of most sheathing materials for metal stud barrier back-up walls make them more susceptible to differential structural movement and dimensional instability from thermal and moisture exposure. Therefore, careful engineering analysis of cladding-adhesive-sheathing material compatibility, and analysis of the anticipated behavior of the sheathing and its attachment are critically important.

Cementitious Backer Units (CBU)

There are a wide variety of product formulations in this category of substrates, such as pure portland cement, cement-fiber, and calcium silicate boards. Some of these boards are manufactured with an adhered layer of rigid insulation attached to the framed side of the board for use in vertical applications. This board type is designed for use on floors, walls, ceilings in wet or dry areas and is applied directly to wood or metal framing. Ceramic tile/stone can be bonded to it with latex/polymer modified portland cement mortar, or epoxy adhesive by following the backer board manufacturer’s instructions.

The ceramic tile industry supplies the following installation instructions for CBU applications. It is important to note that many of the other board types, including coated glass mat water resistant gypsum backer board, fiber cement underlayments, fiber-reinforced water-resistant gypsum backer board and cementitious coated foam boards follow many of the same industry recognized installation instructions. However, the specific board manufacturer’s installation instructions will take precedence over the general installation instructions.

1. Systems, including the framing system and panels, over which tile will be installed shall be in conformance with the International Building Code (IBC) for commercial and industrial applications, or applicable building codes. The project design should include the intended use and necessary allowances for the expected live load, concentrated load, impact load, and dead load including the weight of the finish and installation materials.

2. All CBU must comply with American National Standards Institute Inc. (ANSI) “Standards for Test Methods and Specifications for Cementitious Backer Units (ANSI A118.9)” – and ASTM C1325 (Standard specification for non-asbestos fiber-mat reinforced dementitious backer units). CBU installation must comply with ANSI “Interior Installation of Cementitious Backer Units (ANSI A108.11)”.

3. Provide expansion movement/expansion joints for ceramic tile, stone and thin brick installations as per the current TCA Handbook for Ceramic Tile Installation – EJ171.

4. Fasten the CBU with 7/8" (22 mm) minimum length, non-rusting, self-imbedded screws for wood studs (or appropriate fasteners for steel framing). Fasten the boards every 6" (150 mm) at the edges and every 8" (200 mm) in the field. Tape all the board joints with the alkali resistant 2" (50 mm) or 4" (100 mm) wide reinforcing mesh (provided by the CBU manufacturer) imbedded in the same thin-set mortar used to install the ceramic tile, stone or thin brick.

5. To prevent water leakage through the walls, especially in high water exposure areas apply a waterproofing membrane (e.g. LATICRETE® 9235 Waterproofing Membrane or LATICRETE Hydro Ban) directly on the CBU. Please refer to membrane manufacturer’s written installation instructions. Some applications may require an additional vapor barrier installed behind the CBU.

6. Before applying the ceramic tile/stone it is essential that the CBU be wiped down with a damp sponge to remove dust and to increase working/adjustability time over hot, dry surfaces. This will ensure that the thin-set mortar (e.g. LATICRETE 254 Platinum) has an opportunity to hydrate properly without the CBU absorbing the water. Apply the mortar or adhesive, using the flat side of the trowel to work the material into good contact with the CBU. Then comb on additional material with the notched side of the trowel. Spread only as much material as can be tiled in 15–20 minutes. Use the correct size notched trowel and “back butter” the tiles, if necessary, to achieve the correct coverage. It is recommended to pull tiles occasionally to ensure proper coverage is being achieved. Once the thin-set mortar or epoxy adhesive has cured for the appropriate amount of time, grouting can take place.

Coated Glass Mat Water-Resistant Gypsum Backer Board

Coated glass mat water resistant gypsum backer board should conform to ASTM C1178 (Standard Specification for Coated Glass Mat Water-Resistant Gypsum Backing Panel) and be suitable for use as a ceramic tile backer board. This type of board should only be recommended for use on walls and ceilings over wood or metal framing in mass transit applications. Ceramic tile/stone can be bonded to a coated glass mat water resistance gypsum backer board with latex/polymer modified portland cement mortar or an epoxy adhesive by following the backer board manufacturer’s instructions.

Fiber Cement Underlayment

A dispersed fiber-reinforced cement backer and underlayment designed for use on walls and ceilings in mass transit applications. This board is typically applied directly to wood or metal framing. Ceramic tile/stone can be bonded to it with latex/polymer modified portland cement mortar or an epoxy adhesive by following the backer board manufacturer’s installation instructions. General interior installation and material specifications are contained in ANSI A108.11 and ASTM C1288 (Standard Specification for Discrete Non-Asbestos Fiber-Cement Interior Substrate Sheets).

Fiber-Reinforced Water-Resistant Gypsum Backer Board

Fiber-Reinforced Water-Resistant Gypsum Backer Board should conform to ASTM C1278 (Standard Specification for Fiber-Reinforced Gypsum Panel). This board is typically used on walls and ceilings, and is applied directly to wood or metal framing in mass transit applications. Ceramic tile/stone is adhered to this board with latex/polymer modified portland cement mortar or an epoxy adhesive by following the backer board manufacturer’s recommendations.

Cementitious Coated Foam Board

Cementitious coated foam board is a waterproof backer board constructed from extruded polystyrene and coated with a cementitious coating which is designed as a substrate for ceramic tile walls in wet and dry areas and is applied directly to wood or metal framing. Ceramic tile/stone can be adhered with a latex/polymer modified portland cement mortar or an epoxy adhesive. Follow the manufacturer’s recommendations for installation instructions.

3.6 Substrate Condition and Preparation

Evaluation of Substrate Condition

As previously mentioned in Section 2.3, the first step in any installation is the evaluation of job site conditions. The extent of substrate preparation will not be known until the surface is examined for compliance with industry standards for substrate tolerances, plumb, surface defects and substrate contaminates.

In relation to the overall cost of the installation, preparation of the substrate is neither costly nor time consuming. However, proper preparation is the one of the most important steps that leads to a successful, long term tile/stone installation and helps prevent “call backs”.

Adhesive Compatibility

As mentioned in Section 2.3, adhesive compatibility plays an important role in determining adhesion between the substrate and the tile/stone being installed. Both the substrate and the finish type must be compatible with the type of adhesive being used and recommended for use in the environment in which it will be installed. The ability of a substrate to be ‘wetted out’ by an adhesive is essential to good adhesion and important in determining the performance of the adhesive in bonding to the substrate. The highest strength adhesives and the most careful application to the best concrete wall will not overcome a dirty or contaminated substrate.

Site Visit and Conference

Prior to commencing ceramic tile/stone work, the tile contractor shall inspect surfaces to receive tile and accessories, and shall notify the architect, general contractor, or other designated authority in writing of any visually obvious defects or conditions that will prevent a satisfactory tile/stone installation. Installation work shall not proceed until satisfactory conditions are provided. Commencing installation of tile work deems acceptance of substrate conditions.

Substrate Preparation

Wall substrates scheduled to receive tile/stone will always be exposed to varying degrees of airborne contamination, exposure to other trades and site applied products. This can include, but is not limited to, form release agents, curing compounds, sealers, efflorescence, laitance, or any other potential bond inhibiting or bond breaking materials.

Therefore, any type of oily or other potential bond breaking contaminant must be removed prior to the installation of tile/stone on concrete walls. These types of contaminants may require mechanical scarification, grinding, shot blasting or other methods of mechanical removal.

It is important to note that if concrete walls are poured in place (cast in place), they had to be formed. Therefore, if they are formed, then it must be assumed that form release oils were used to coat the forms. The form release oils allow the forms to be stripped from the concrete after it has hardened. It must also be assumed that form release oils or other potential bond breakers exist in all of these conditions and must therefore proceed accordingly. This information is also appropriate for pre-cast tilt up concrete walls that may be treated with curing compounds during the casting phase.

The following methods can be utilized to prepare vertical substrates scheduled to receive ceramic tile/stone finishes:

Waterblasting

High-pressure water blasting using pressures over 3,000–10,000 psi (21–69 MPa) will remove the surface layer of concrete and expose aggregate to provide a clean, rough surface. Thorough rinsing of the surface with water after water blasting is necessary to remove any weakened cement paste (laitance) residue. Water blasting is only recommended on concrete because the high pressure will damage surfaces of thin, less dense materials such as cement boards or brick masonry.

Mechanical Chipping, Scarifying and Grinding

For preparation of walls, this method is recommended only when substrate defects and/or contamination exist in isolated areas and require bulk surface removal greater than 1/4"(6 mm) in depth. Chipping with a pneumatic square tip chisel, or grinding with an angle grinder are common scarifying techniques.

Shotblasting

This is a term for a surface preparation method which uses proprietary equipment to bombard the surface of concrete with pressurized steel pellets. The pellets, of varying diameters, are circulated in a closed, self-contained chamber which also removes the residue in one step. This is the preferred method of substrate preparation when removal of a thin layer of concrete surface is required, especially removal of surface films or existing painted concrete. However, only hand held equipment is currently available for vertical concrete, so preparing large areas with this method is inefficient.

Sandblasting/Gritblasting

The coatings industry now employs a new generation of cleaner, safer, and less intrusive grit-blasting which employs water soluble low-silica grit materials (sodium bicarbonate). Sandblasting is acceptable if other safer and less intrusive methods of bulk removal are not available.

In addition to the above mentioned methods, other methods to mechanically abrade contaminants from concrete also exist.

Cracks

Plastic and Shrinkage Cracks

Freshly placed concrete undergoes a temperature rise from the heat generated by cement hydration, resulting in an increase in volume. As the concrete cools to the surrounding temperature, it contracts and is susceptible to what is termed “plastic shrinkage” cracking due to the low tensile strength within the first several hours or days after the concrete is placed. Plastic shrinkage can be controlled by reduction of aggregate temperature, cement content, size of pours/members, deferring concreting to cooler temperatures, damp curing, and the early removal of forms.

Concrete also undergoes shrinkage as it dries out, and can crack from build-up of tensile stresses. Rapid evaporation of moisture results in shrinkage at an early stage where the concrete does not have adequate tensile strength to resist contraction. Concrete is most susceptible to drying shrinkage cracks within the first 28 days of placement. After 28 days concrete typically develops adequate tensile strength to resist a more evenly distributed and less rapid rate of shrinkage. It is for this reason that it is recommended to wait 30–45 days before application of adhesive mortars. Just like floors, treat any shrinkage cracks with an anti-fracture membrane (e.g. LATICRETE® Blue 92 Anti-Fracture Membrane) to prevent the transmission of cracks through the finish surface.

Structural Cracks

Cracks that are greater than 1/8" (3 mm) in width, are displaced or not in plane, and occur throughout the cross section of a concrete wall or structural member, are an indication of a structural defect and must be corrected before the tile is adhered to the wall. Structural cracking on vertical applications can be repaired using low viscosity epoxy or methacrylate pressure injection methods. Once the cracks are stabilized and properly repaired, the tile/stone installation process can commence. Methods for corrective action vary depending on the severity of the structural cracks. Methods can range from simply routing out and patching the cracked areas to more sophisticated pinning and epoxy injection systems.

In-plane cracks that are 1/8" (3 mm) or less in width are typically non-structural shrinkage cracks. While these types of cracks do not require structural correction, they require suppression by means of a crack isolation membrane (e.g. LATICRETE Blue 92 Anti-Fracture Membrane for crack isolation; or, LATICRETE Hydro Ban or 9235 Waterproofing Membrane if crack isolation and waterproofing are required). The crack isolation membrane is applied to the crack with a 6" (150 mm) wide treatment (3" {75 mm} applied on either side of the crack). Next, another layer of the crack isolation membrane treatment, that is at least three times the width of the tile/stone, is applied over the previous layer (For more information on this method, please refer to LATICRETE ES-F125 at www.laticrete.com/ag). This treatment ensures that the tile/stone will sit directly on the membrane and will provide the full capabilities of the crack isolation membrane. An alternative method treats the entire vertical substrate with the crack isolation membrane to help prevent existing cracks and any future non-structural cracks from telegraphing through to the tile surface (for more information on this method, please refer to LATICRETE ES-F125A at www.laticrete.com/ag).

Plumb and Level

It is imperative to evaluate how plumb a wall is before applying tile/stone. The TCNA stipulates maximum variation in the substrate shall not exceed 1/4" in 10′ (6 mm in 3 m) or 1/16" in 1′ (1.5 mm in 300 mm) from the required plane for most tile/stone installations. At times, the design professional may specify a more stringent tolerance of 1/8" in 10′ (3 mm in 3 m). If this variation is not achieved then a leveling coat or mortar bed may be necessary. For mass transit applications, concrete, concrete masonry units and cement backer units over steel framing are generally the vertical substrates most frequently used. At times, the substrate may require a skim coating of latex/polymer fortified thin-set mortar (e.g. LATICRETE 254 Platinum) to fix any minor irregularities (<1/4" [6 mm] thickness), all the way up to a full render application that includes a scratch and brown coat (e.g. LATICRETE 3701 Fortified Mortar Bed) in order to make the walls plumb and true.

Although some walls may be plumb, they may not necessarily be level. Tile /stone installations can overcome a wall that is not perfectly level, however, there could be consequences to setting tile/stone on a wall that is not flat, the most serious being inadequate bond.

Tile and stone also have certain tolerances when it comes to their manufacturing process (ANSI A137.1). For example, the greater the tolerance for tile thickness, the greater the chances are that the tile wall will appear wavy and irregular in profile. The quality of the tile can also play an important role in the final appearance of the finish.

After the walls have been brought into compliance with industry substrate tolerance standards, the installation of tile/stone can commence. Prior to installing tile/stone on walls, it is important to clean the wall surface just prior to installing tile/stone so that dust and contaminants will not affect the bond of the installation.

Specialty and medium bed mortars (e.g. LATICRETE® 255 MultiMax™^ or LATICRETE 220 Marble & Granite Mortar mixed with LATICRETE 3701 Mortar Admix) can alleviate small variations in the wall and tile tolerances without the need of a leveling coat or thick mortar bed. Follow the manufacturer’s recommendation of thickness with these special setting materials.

Mass transit walls that employ the epoxy spot bonding method (e.g. LATAPOXY® 310 Stone Adhesive (see Details ES W260 and ES W215 in section 10 for more information) generally tolerate greater deviations from a flat plane. Maximum deviation is a function of the recommended thickness and working properties of the adhesives such as sag resistance. Generally, this adhesive type can accommodate +/-1/2" (12 mm) from the finish plane for a maximum build up of 1" (25 mm). Follow the manufacturer’s installation instructions when utilizing the epoxy spot bond method.

Surface and Ambient Temperature

During the placement of concrete and installation of other substrate types, cold or hot temperatures may cause numerous surface or internal defects, including shrinkage cracking, a weak surface layer of hardened concrete caused by premature evaporation, or frost damage. Prior to curing, extreme temperatures of both the ambient air and surface of the substrate will also affect the normal properties of adhesive mortars.

Warmer ambient air and surface temperature will accelerate the setting of cement and epoxy adhesives. Cooler ambient air will cause the adhesives to cure for a longer period of time.

The two general rules are;

1. For every 18° F (10°C) below 70°F (21°C) cement based and epoxy based materials will take twice as long to cure, and

2. For every 18° F (10°C) above 70°F (21°C) cement based and epoxy based materials will take half as long to cure.

Washing and dampening walls as described previously will not only help to remove any loose contaminants off the wall, but it also serves to lower surface temperatures in warmer climates, and lower the absorption rate of the substrate. It is important to follow the manufacturer’s recommendations for temperature ranges for all tile installation materials (see section 8.1 for more information on installing tile and stone in hot or cold weather conditions).