Section 2 – Floor Constructs
2.1 Structural Considerations
Types of Structural Movement
It is essential that all floor applications be designed to accommodate all types of structural movement. Structural movement can transmit through the adhesive connection and tile, accumulate, and then exert stress on the floor, resulting in cracking, buckling, or loss of bond between the tile and adhesive.
The different types of structural movement are individually quantifiable through mathematical calculations which, for mass transit floors, will mainly be restricted to concrete substrates. Fortunately, the structural theory used in most building codes dictates the use of “worst case” conditions; the calculated movements are of the highest possible magnitude in order to provide a safety factor when exposed to the most extreme, actual conditions.
Types of Structural Movement Include:
Thermal Movement
Creep
Differential Settlement
Seismic
Thermal Movement
Thermal movement is a term that refers to the expansion or contraction of a substance in response to changes in temperature. All materials react to changes in temperature. While all materials move in response to temperature, all materials can exhibit differences in both the speed of the reaction and the degree of movement when subjected to similar temperature changes. When two dissimilar materials react dramatically different in the same environment, the ability of a tile adhesive to maintain strong bond through such challenges can be tested. In situations where dissimilar materials meet and tile spans both materials, cracking or complete loss of bond may be the likely consequence. Allowing for movement within the substrate layer and the tile installation is critical to assure long term, problem-free installations.
There are two factors to consider in analyzing thermal movement:
1. The rates of expansion of different materials (i.e. linear coefficient of thermal expansion)
2. The anticipated temperature range exposure
Some building materials respond rapidly when exposed to temperature changes while concrete can respond more slowly. Some tile products have a higher tensile strength than concrete and may also respond to temperature changes at a different rate. Stresses applied to the tile installation and concrete, as a result of the rapid or continuous movement of dissimilar materials, can be that the concrete cracks horizontally just below the bond line and the system can fail at that point.
Thermal movement can be rapid and reoccurring. Rapid changes can be explained when normal conditions are introduced to extreme high temperatures (i.e. direct sunlight) or extreme cold (e.g. freezing conditions). Temperature changes do not have to be dramatic for movement to occur. Slower more repetitive temperature changes can be equally destructive. In these situations, there can be continuous stress at the bond line caused by such things as daily recurring temperature changes. These temperature changes, in conjunction with time, can fatigue the weaker material at the bond line. Over time the weaker material (i.e. concrete), may cause the same failure as if it were exposed to rapid temperature changes. The conditions that might cause loss of bond are not always obvious. Some conditions to be aware of are:
Exterior Applications
Interior Applications Exposed to Sunlight Through Windows, Curtain Walls, Doors and Skylights
Rapid or Wide Changes in Ambient Temperature
Application of Cold Water to Hot Surfaces (e.g. When Rain Showers Occur and Cool Down Exterior Applications That Have Been Exposed to Hot Conditions)
When selecting materials for a tile floor, be aware of the above conditions. Additionally, cleaning and disinfecting protocols requiring hot water or steam need to be considered, especially if the area being cleaned is normally kept cool, as in the case of controlled public environments.
In applications of extreme temperature change, it may be necessary to use a coarser aggregate than what is used in typical concrete. Thinner ceramic tile and stone bonding systems (e.g. LATICRETE 254 Platinum) react well to thermal stress because they are often too thin to exhibit destructive energy at the bond line. In a conventional tug of war, tile installation materials are more at the mercy of the concrete properties than the other way around. However, if the application receives heavy vehicular traffic or extremely heavy loads, make sure the tile installation materials fit the service requirement of TCNA “Extra Heavy” when tested in accord with ASTM C627 (Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson-Type Floor Tester).
The primary goal in analyzing thermal movement is to determine both the cumulative and individual differential movement that occurs within and between components of the floor assembly.
For example, a porcelain tile has an average coefficient of linear expansion of between (4–8 x 10-6 mm/°C/mm) of length. Concrete has an average expansion rate of 9–10 x 10-6mm/°C/mm. The surface temperature of a porcelain tile in an application exposed to direct sunlight may reach as high as 160°F (71°C); an ambient temperature in a moderately cold climate may be 0°F (-18°C) or colder. The temperature variation within this tile installation can vary by as much as 160°F (89°C). The temperature range of the concrete, insulated from the temperature extremes by the tile and tile installation mortars, as well as length of exposure, may only be 85°F (30°C). For a building that is 50 m wide, the differential movement can be calculated as follows:
Concrete 0.000010 mm x 50 m x 1000 mm x 30°C = 15 mm. Tile 0.000006 mm x 50 m x 1000 mm x 70°C = 21 mm.
Because the thermal expansion of the tile is greater, this figure is used. The general rule for determining the width of a movement joint is 2 – 3 times the anticipated movement, or 3x21 mm (.82") = 63 mm (2.5"). The minimum recommended width of any individual joint is 10 mm (3/8"), therefore, a minimum of 6 joints across a 50 m (154′) floor, each 10 mm (3/8") in width is required just to control thermal movement under the most extreme conditions.
Creep
Deformation movement in concrete structures, also known as creep, occurs more slowly and can increase initial deflection by 2–3 times. Creep is the time dependent increase in strain of a solid body under constant or controlled stresses. The placement of movement joints is critical in the success of the structure. Also the realistic prediction of both the magnitude and rate of creep strain is an important requirement of the design process. While there are laboratory tests that can determine the deformation properties of concrete, it is often skipped because of the time consuming nature and high cost of the test. In cases where only a rough estimate of the creep is required, an estimate can be made on the basis of only a few parameters such as relative humidity, age of concrete and member dimensions. Ideally a compromise has to be sought between an estimate of the prediction procedure and the laboratory testing and mathematical and computer analyses.
Differential Settlement
Buildings structures and concrete placement pours used for access ways are typically designed to allow for a certain tolerance of movement in the foundation, known as differential settlement. In most cases, the effect of normal differential settlement movement on the flooring system is considered insignificant because much of the allowable settlement has occurred before the flooring system has been installed. Differential settlement of a building foundation and concrete that occurs beyond the allowable tolerances is considered a structural defect which can cause significant problems to any flooring system, including the tile or stone finish. At that point, one would need to address the root cause of the problem and come to a solution before the flooring system can be repaired. Patching the visible problem areas in the flooring system will not provide an adequate solution, and one can expect repetition of the same issues in the floor.
Controlling Stresses With Movement (Expansion) Joints
One of the primary means of controlling the stresses induced by building movement, concrete shrinkage and typical concrete curing is with movement joints (also known as expansion, dilatation, or control joints). All buildings and building materials move to varying degrees, and therefore the importance of movement joints cannot be understated. At some point in the life cycle of an interior floor, there will be a confluence of events or conditions that will rely on movement joints to maintain the integrity of the floor system. Maintaining integrity of the floor can be made as simple as preventing cracks in grout joints, to preventing complete adhesive bond failure of the tile. Proper design and construction of movement joints requires consideration of the following criteria:
Location
Frequency
Size (Width:Depth Ratio)
Type and Detailing of Sealant and Accessory Materials
MOVEMENT JOINTS
Location of Movement Joints
The primary function of movement joints is to isolate the tile or stone from other fixed components of the structure, and to subdivide the substrate and finish materials into smaller areas thereby compensating for the cumulative effects of building movement (see section 10 for specifications and details). While each floor is unique, there are some universal rules for location of movement joints that apply to any floor installation. Many of the universal rules for movement joints can be found in the current edition of the Tile Council of North America’s (TCNA) TCA Handbook for Ceramic Tile Installation, EJ-171.
Existing Structural Movement Joints
Movement joints may already be incorporated in the underlying structure to accommodate thermal, seismic or other load types. These movement joints must extend through to the surface of the tile or stone, and equally important, the width of the underlying joint must be maintained to the surface of the tile or stone.
Changes of Plane
Movement joints should be placed at all locations where there is a change in plane, such as outside and inside corners and changes in elevations (e.g. ramps).
Location – Dissimilar Materials
As stated earlier in this section, different materials have different rates and characteristics of movement. Movement joints must be located wherever the floor tile and underlying adhesive and leveling mortars meet a dissimilar material, such as metal, penetrations, and many different types of tile.
Frequency of Movement Joints
Guidelines for movement joints are every 20′ to 25′ (6 m – 7.5 m) in every direction for interior applications, and 8′ to 12′ (2.4 m – 3.6 m) in every direction for exterior applications and any interior tile work exposed to direct sunlight or moisture, or, as stated in the current TCA Handbook for Ceramic Tile Installation EJ-171. The placement of movement joints need to be incorporated where tile work abuts restraining surfaces such as perimeter walls, dissimilar floors, curbs, columns, pipes, ceilings, and where changes occur in backing materials, but not at drain strainers. All expansion, control, construction, cold, and seismic joints in the structure should continue through the tile work, including such joints at vertical surfaces. Joints through tile work directly over structural joints must never be narrower than the structural joint.
Size of Movement Joints
The proper width of a movement joint is based on several criteria. Regardless of the width, as determined by mathematical calculations, the minimum functional width of a movement joint should be no less than 1/4" (6 mm); any joint narrower than this makes the proper placement of backer rods and sealant materials impractical, and does not provide adequate movement allowance.
The width of a movement joint filled with sealant material must be 3 to 4 times wider than the anticipated movement in order to allow proper elongation and compression of the sealant. Similarly, the depth of the sealant material must not be greater than half the width of the joint to allow for proper functioning of the movement joint (width:depth ratio). For example, if 1/4" (6 mm) of cumulative movement is anticipated in the floor, the movement joint should be 3/4 – 1" (19 – 25 mm) wide and 3/8 – 1/2" (6 – 9 mm) deep (a rounded backup rod is inserted in the joint to control depth, and to keep the sealant from bonding to the substrate). Sealants are products that are designed to bond to two parallel surfaces (the sides/flanks of two tiles). Sealant bonding to 3 surfaces (the sides/flanks of two tiles and the substrate) means that the sealant can lose 75% of its effectiveness. So the backer rod, which the sealant does not bond to, is very important to the success of the sealant.
Sealants
Sealants should be a neutral cure, high performance (also known as Class A, or have a Shore-A hardness of 25 or greater), viscous liquid type capable of +/-25% movement. Silicone sealants can have the ability to compress to 50% of its original width and expand up to 100%. Floors exposed to heavy vehicular traffic may require a sealant with a higher A-Shore hardness as specified. Check with the sealant manufacturer for acceptability on each application.
Pre-fabricated movement joints, which typically consist of two L-shaped metal angles connected by a cured flexible material often may not meet the above movement capability required for a demanding horizontal application where extreme temperature changes occur. Similarly, the selection of non-corroding metal, such as stainless steel, is required to prevent corrosion by alkaline content of cement adhesive or galvanic reactions with other metals.
Pre-fabricated movement joints are commonly installed in advance of the tile, so it is critical to prevent excessive mortar from protruding through the punched openings in the metal joint. The hardened mortar may subsequently prevent proper bedding of the tile onto the floor in these areas.
Mechanical Properties
Sealants should have good elongation and compression characteristics, as well as tear resistance to respond to dynamic loads, thermal shock, and other rapid movement variations which are not unusual for floors subjected to heavy loads and use. Many floors in mass transit projects are exposed to extreme vibrations resulting from railway cars, heavy foot traffic, carts, vehicles and machinery and are therefore constantly under stress from these vibrations.
Compatibility
Some sealants may stain tile or stone. In addition, curing by-products may be corrosive to concrete, metals, or waterproofing membranes. There are many types and formulations of sealant products, so it is important to verify compatibility and acceptability for the intended use. Compatibility varies by manufacturer’s formulations, and not by sealant or polymer type. For example, acetoxy silicones cure by releasing acetic acid and can be corrosive; neutral cure silicones do not exhibit this characteristic.
Fluid migration and resultant staining is another compatibility issue to consider with sealants. There is no correlation with polymer type (i.e. silicone vs. polyurethane) and fluid migration. Fluid migration is dependent solely on manufacturer’s formulation and type of tile or stone. Dirt contamination is another common problem and can be associated with type of exposure, surface hardness, type of and length of cure, and formulation, but not the sealant polymer type. Performing a test area to determine compatibility is recommended to make sure that problems are not encountered in the field during installation.
Adhesion
Sealants must have good tensile adhesion to non-porous or porous tile surfaces, ideally without special priming or surface preparation.
Subjective Criteria
Color selection, ease of application, toxicity, odor, maintenance, life expectancy, and cost are some of the additional subjective criteria that do not affect performance, but do require consideration.
Types of Sealant
High performance sealants are synthetic, viscous liquid polymer compounds known as polymercaptan, polythioether, polysulfide, polyurethane, and silicone. Each type has advantages and disadvantages. As a general rule, polyurethane and silicone sealants are a good choice for ceramic tile, pavers and stone.
Polyurethanes and silicones are available in either one component cartridges, sausage packs, or pails; some polyurethanes come in two-component bulk packages, which require mixing and loading into a sealant applicator gun. Both types of sealants are typically available in a wide range of colors.
Installation of sealants and accessories into movement joints requires skilled labor familiar with sealant industry practices. The installation must start with a clean, dry and dust free surface. Some products or materials require use of a primer to improve adhesion or prevent fluid migration. If a primer is necessary, it should be installed before installation of backer rods and it may be necessary to protect underlying flashing or waterproofing to avoid deterioration by primer solvents. Any excess mortar, spacers or other restraining materials must be removed to preserve freedom of movement. If necessary, protect the tile or stone surface with masking tape to facilitate the cleaning process. The use of a suitable backer rod or bond breaking tape is typically used to prevent three-sided adhesion and to help regulate depth of the sealant. Once the sealant has been applied, it is necessary to tool or press the sealant to ensure contact with the tile edges; the backer rod also aids this process by transmitting the tooling force to the tile edges. Proper tooling of the sealant joint also gives the sealant a slightly concave surface profile consistent to the interior surface against the rounded backer rod. This allows even compression/elongation, and prevents a visually significant bulge of the sealant under maximum compression.
2.2 Structural Considerations
Loads
Forces that act on structures are called loads, and there are two types of loads which are taken into consideration when designing structures; dead loads and live loads. Typically, dead loads are static in nature, which means they either do not change or change infrequently. Dead load is essentially the weight of the structure itself; anything permanently attached to the structure would be considered part of the dead load. This would include walls, floors (and flooring), roofs, columns, and so on.
Live loads are the weight of items in the building. Live loads are not static as they can change. Examples of live loads would be people, furniture, railway cars, trolleys, carts, vehicular traffic, etc… Live loads can have a profound effect on the success of a tile installation and on the long term performance of the entire structure. Suitable allowance must be made for all anticipated live loads with enough allowance to meet any additional loads placed on the system in the future. Anticipated live loads must be accounted for in mass transit applications. Design assumptions should be calculated on ‘worst case scenarios’. In other words, structures must be designed to anticipate the confluence of all loads, including dead and live loads, and the potential affect on the structure.
Requirements of Building Design
Buildings, platforms, structural concrete pours, and tunnels must be designed for the specific use for which they will be utilized. The architect or engineer has to know what the structure is going to be used for in order to properly calculate the different live and dead loads involved. If a railway car platform were designed to be constructed for an elevated condition, the design professional would have to calculate the total anticipated live load. Suitable allowance must be made for all anticipated live loads with enough allowance to meet any additional loads placed on the system in the future. Adequate structural reinforcement is required for demanding mass transit applications. The American Concrete Institute (ACI) is a good source of information pertaining to the design of concrete pours and structures for mass transit projects.
In addition, The Tile Council of North America’s (TCNA) – TCA Handbook for Ceramic Tile Installations provides a floor tiling installation guide that depicts which installation methods are appropriate for the performance level requirements of an application. The performance levels of specific installation methods are determined by ASTM Test Method C627 “Standard Test Method for Evaluating Ceramic Floor Tile Installation Systems Using the Robinson Type Floor Tester”. The test method exposes a tiled floor construct to a 300 lb. (135kg) concentrated load which turns on three wheels. As the cycles increase, the wheel type is changed from hard rubber wheels to steel wheels. An installation method that passes all 14 cycles is considered “Extra Heavy”. An installation method that passes cycles 1 through 12 is considered “Heavy” and an installation method that passes cycles 1 through 10 is considered “Moderate”. Typically, mass transit floor applications require installation methods that comply with “Extra Heavy” or “Heavy” service ratings.
In some applications, “Moderate” service ratings may also be applicable. It is important to note that the Marble Institute of America, Inc. (MIA) references the TCNA Handbook for the installation of certain dimension stone. The MIA’s Dimension Stone Design Manual also provides information on this matter.
Vibration and Noise
Mass transit applications are subjected to varying levels of vibration and noise. Railway cars, buses, automobiles, and other mechanized equipment moving in and around mass transit structures place stresses on building components, including the tile and stone constructs and their installation systems. In many cases, shock absorbing fasteners, sound barriers, shock absorbing mounts, ballast mats, resiliently supported ties, floating concrete slabs and other anti-vibration ballast materials and equipment is specified and utilized in an effort to help reduce the transfer of vibration and noise through a building’s structure. Multi-story mass transit facilities are designed to integrate shopping, housing and various transportation modes within the same facility. The vibration that can occur from the elements within a structure can affect the long-term performance of building components that are not designed to accommodate these stresses. In addition, excessive vibration and noise can have an adverse impact on humans and their ability to carry out their daily functions.
The United States Federal Transit Administration (FTA) has issued its second report entitled “Transit Noise and Vibration Impact Assessment” on this matter. These guidelines specify how to measure, predict and evaluate noise and vibration levels from mass transit sources.
Noise and Vibration Basics
The A-weighted noise level measured in decibels (dBA) is the basic measure of sounds.
Lmax = Maximum noise level during a single event (without taking into account the duration, time of day or number of events that occur).
Ldn = The equivalent day-night noise levels. This measurement assesses the annoyance level at residences and hotels. This is a 24 hour measurement which includes penalties for events that occur in the evenings.
Leq = The hourly-equivalent noise levels. This measurement is used to assess annoyance from noise at locations with daytime use such as schools or libraries.
Vibrations are the movements of building surfaces produced by the forces of mass transit vehicles which can transmit through rails, roadways, structures, soil and other building elements. The basic measure of vibration is vibration velocity level in decibels (VdB). Humans can feel vibrations in structures above 65 VdB. Typically, vibration levels over 100 VdB can start to fatigue structures and begin to cause minor damage.
It is a science onto itself to research, assess, predict and construct mass transit facilities to reduce vibration and noise impact on humans and building structures. A qualified noise engineering service or an engineering firm specializing in this discipline should be consulted for aviation, rail and other mass transit projects for a comprehensive noise and vibration assessment.
Ceramic tile and stone installations can resist the traditional vibration stresses when installed with high performance shock resistant installation materials. The LATICRETE Systems Materials referenced throughout this technical design manual are designed to hold up under the demanding stresses typically found in mass transit applications.
Deflection
Floor systems, over which the tile will be installed, shall be in conformance with the International Building Code (IBC) or applicable building codes for mass transit applications. Historically, for ceramic tile and paver applications, the maximum allowable deflection should not exceed L/360 under total anticipated load and L/480 for stone installations. The Marble Institute of America sets the maximum allowable deflection standard at L/720.
The ceramic tile and stone industry abides by the following note on deflection: the owner should communicate in writing to the project design professional and general contractor the intended use of the tile (or stone) installation, in order to enable the project design professional and general contractor to make necessary allowances for the expected live load, concentrated loads, impact loads, and dead loads including the weight of the tile (or stone) and setting bed. The tile installer shall not be responsible for any floor framing or sub-floor installation not compliant with applicable building codes, unless the tile installer or tile contractor designs and installs the floor framing or sub-floor.” (see section 10 Building Codes and Industry Standards for more information).
2.3 Substrate Condition and Preparation
Evaluation of Substrate Condition
The first step in substrate preparation is the evaluation of the type of substrate and its surface condition. This includes the levelness (plane or flatness deviation), identification of general defects (e.g. structural cracks, shrinkage cracks, laitance, etc…), and the presence of curing compounds, form release agents, surface hardeners, and contamination. Concrete should have a wood float or light steel trowel finish for proper adhesion of thin-set mortars, renders, screeds or membranes. Over finishing a concrete surface can close the pores and may inhibit proper adhesion of thin-set mortars, renders, screeds and membranes.
The ability of a substrate to be wetted by an adhesive is essential to good adhesion and important in determining the performance of the adhesive in bonding to the substrate. This means that not only should the substrate possess a balance between porosity and texture, but also that the surface must be clean of any contamination such as dust or dirt that would prevent wetting and contact of an adhesive. The levelness tolerance or smoothness of a substrate surface also play an important role in allowing proper contact and wetting with an adhesive. Typically, the greater the surface area to which the adhesive is in contact, the better the adhesion.
Adhesive Compatibility
Compatibility plays an important role in determining adhesion between the substrate and the finish material being installed. The substrate material must be compatible not only with adhesive attachment, but also with the type of adhesive under consideration. This means that the substrate material must have good cohesive qualities to resist tensile and sheer stresses and not have adverse reactions with the proposed adhesive. Similarly, the tile or stone being installed must also be compatible with the adhesive.
A general consideration in determining compatibility with adhesives is as follows; the installation of any finish material with an adhesive will only be as good as the setting materials and the substrate to which the finish material will be bonded. The highest strength adhesives and most careful application with the best quality tile or stone will not overcome a weak or dirty substrate.
This section provides information on the identification of common substrate characteristics and defects, and the preventative and corrective actions necessary for proper surface preparation.
Site Visit and Conference
Prior to commencing ceramic tile or stone work, the contractor shall inspect surfaces to receive the tile or stone and any installation accessories (e.g. waterproofing membranes, crack-isolation membranes, vapor reduction membranes, etc…), 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 or stone installation. Installation work shall not proceed until satisfactory conditions are provided. Commencing installation of tile or stone work typically means acceptance of substrate conditions.
Job Site Conditions
The following items are examples of potential issues that may need to be addressed prior to commencing the installation:
Moisture Content of Concrete
Finishes and installation materials used in mass transit applications can be affected by moisture during the installation and curing phase. For example, the strength of cementitious adhesives can be reduced from constant exposure to wet or damp substrates. Some materials, such as waterproofing membranes, may not cure properly or may delaminate from a continually wet substrate. A damp substrate may also contribute to the formation of efflorescence.
There are generally three tests that are used to determine moisture content in concrete. The three tests are Calcium Chloride (ASTM F1869 – Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloors Using Anhydrous Calcium Chloride), Relative Humidity (ASTM F2170 – Standard Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes) and Plastic Sheet Method (ASTM D4263 – Standard Test Method for Indicating Moisture in Concrete by the Plastic Sheet Method). The Calcium Chloride test involves placing a petri dish of calcium chloride (covered by a plastic dome adhered to the concrete) on the concrete and allowing the petri dish to remain in place between 60 – 72 hours. The calcium chloride absorbs any moisture vapor that transmits through the concrete within the plastic dome. The results of a calcium chloride test measures the amount of moisture absorbed and results are stated in pounds per 1,000 ft2 (92.9 m2) in a 24 hour period. The Relative Humidity test involves placing probes in the concrete and taking readings with a hygrometer. A relative humidity reading of 75% or below is acceptable for most tile/stone applications. The Plastic Sheet Method involves taping a 24" x 24" (600 mm x 600 mm) piece of plastic on the concrete and allowing the plastic to remain in place for 18 – 24 hours to determine if any moisture has accumulated under the plastic when it is removed. Both ASTM F 1869 and ASTM F2170 are quantitative tests (stating approximately how much moisture is present) while ASTM D 4263 is a qualitative test (stating that moisture is present but not how much), and all are a “snapshot” of moisture vapor emission during the testing period. Please refer to Section 2.5 for more information on moisture content in concrete.
For substrates scheduled to receive a waterproofing membrane, maximum amount of moisture in the concrete/mortar bed substrate should not exceed 5 lbs/1,000 ft2/24 hours (283 µg/s•m2) per ASTM F1869 or 75% relative humidity as measured with moisture probes. Consult with finish materials manufacturer to determine the maximum allowable moisture content for substrates under their finished material.
Surface and Ambient Temperatures
During the placement of concrete and installation of other types of substrates, extreme 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. Once the concrete is cured, extreme temperatures of both the ambient air and surface of the substrate can also affect the normal properties of tile/stone adhesives.
Elevated ambient air and surface temperatures (>90°F [32°C]) will accelerate the setting of cement, latex cement and epoxy adhesives. Washing and dampening floors will serve to lower surface temperatures for latex cement mortars and epoxy adhesives. Shading the substrate, if exposed to sunlight, is also effective in lowering surface temperatures, but if ambient temperatures exceed 100°F (38°C), it is advisable to defer work with adhesives to a more suitable time. Humidity may also have an effect on the curing of membranes and portland cement based adhesives and grouts. Higher humidity will work to slow down cure rates while low humidity will accelerate the curing process.
Flatness and levelness
A flat substrate is an important concern for any tile/stone installation requiring a direct bond adhesive application. Acceptable tolerance is 1/4" in 10′ (6 mm in 3 m) and 1/16" (1.5 mm in 300 mm) from the required plane to conform with the ANSI specifications for ceramic tile/stone installations (see ANSI A108.01 – General Requirements: Subsurfaces and Preparations by Other Trades – Current Version). Greater deviations prevent the proper installation of tile/stone into the adhesive, which may result in numerous problems, including loss of bond or lippage.
If levelness tolerance is exceeded, then it may be necessary to employ remedial work, such as re-construction, patching, grinding, or installation of a self-leveling underlayment (e.g. LATICRETE® 86 LatiLevel™) or a mortar bed (e.g. LATICRETE 3701 Fortified Mortar Bed).
If the tolerance is within specifications, then the use of a medium bed mortar and a larger size notch trowel can alleviate any minor defects in the substrate. Please note that while a medium mortar may be used to correct minor substrate defects, it is important to stay within the product manufacturers guidelines for thickness of the setting material.
Concrete Curing – Age of Concrete
The age of a concrete substrate is important due to the fact that as concrete cures and loses moisture, it shrinks. A common misconception is that concrete completes shrinking in 28 days. This is not true. Thick sections of concrete may take over 2 years to reach the point of ultimate shrinkage. Under normal conditions, 28 days is the time that it typically takes for concrete to reach its full design strength. At that point, concrete will have maximum tensile strength and can better resist the effects of shrinkage and stress concentration.
Depending upon the curing techniques, exposure to humidity or moisture, there may be very little shrinkage in the first 28 days. Flexible adhesives, certain latex or polymer fortified thin-set mortars or (e.g. LATICRETE® 254 Platinum or LATICRETE 211 Powder gauged with LATICRETE 4237 Latex Additive), can accommodate the shrinkage movement and stress that may occur in concrete less than 28 days old. In some cases it may be recommended to wait a minimum of 30–45 days to reduce the probability of concentrated stress on the adhesive interface. Some building regulations or codes may require longer waiting periods of 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 the remaining shrinkage is significantly reduced by its distribution over time and accommodated by the use of flexible adhesives.
Cracking
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 after the pour.
Concrete also undergoes shrinkage as it dries out, and can crack from build-up of tensile stress. Rapid evaporation of moisture results in shrinkage at an early stage where the concrete does not have adequate tensile strength to resist even contraction. Concrete is most susceptible to drying shrinkage cracking within the first 28 days of placement during which it 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 direct application of adhesive mortars.
Plastic shrinkage occurs before concrete reaches its’ initial set, while drying shrinkage occurs after the concrete sets. These types of shrinkage cracks generally do not produce cracks larger than 1/8" (3 mm) in width.
Treating Shrinage Cracks
There are two different ways to treat shrinkage cracks. The first way is detailed in the LATICRETE Architectural Guidebook – ES-F125 (available at www.laticrete.com/ag) or the Tile Council of North America’s (TCNA) TCA Handbook for Ceramic Tile Installation – F125. This method only treats the individual crack and not the entire floor. Be sure to follow the LATICRETE Execution Statement and detail ES-F125 or the TCA Handbook for Ceramic Tile Installation – F125 for proper installation recommendations.
The second method of treating the shrinkage crack would be detailed in the LATICRETE Architectural Guidebook – ES-F125A (available at www.laticrete.com/ag) or the TCA Handbook for Ceramic Tile Installation – F125A. This method uses the anti-fracture membrane over the entire floor. Following this method will help to protect the finished installation from cracks currently in the concrete substrate and any cracks which may develop over time. Mass transit applications are generally subjected to greater incidents of vibration which can affect the finish materials. Therefore, the use of crack isolation/anti-fracture membranes is recommended for these applications.
Structural Cracks
There is no tile/stone installation practice or method for treating any crack over 1/8” (3 mm) wide or structural cracks that experience differential vertical movement. These cracks are considered structural in nature and would require determination of the cause of the crack. Once the cause of the structural movement is determined, it must be remedied prior to repairing the tile installation. Repair techniques can vary (i.e. epoxy injection and pinning systems) and a structural engineer should be consulted prior to any remediation or installation of a tile/stone system.
Excessive foundation settlement and movement can be caused by building on expansive clay, compressible or improperly compacted fill soils, or improper maintenance around foundations. Whatever the cause, settlement can destroy the value of a structure and even render it unsafe. In any case, water is the basic culprit in the vast majority of expansive soil problems. Specific components of certain soils tend to swell or shrink with variations in moisture. The extent of this movement varies from soil type to soil type.
When unstable soils are used as a base for a foundation, the tendency for movement is transmitted to the foundation. Since soil movement is rarely uniform, the foundation is subject to a vertical differential movement or upheaval. If all the soil beneath a foundation swells uniformly, there usually is no problem. Problems occur, however, when only part of the slab settles. Then, differential movement causes cracks or other damages. Once again this condition must be corrected before any tile/stone installation can occur.
Potential Bond Breaking Materials
A tile/stone installation is only as good as its adhesion to the substrate and the tile/stone. An adhesive, in any form, will bond to the first thing it comes in contact with. If that material is dirt, dust, paint, or any other impediment that is lying on a surface, then the adhesion to that substrate can be compromised. The importance of a good, clean surface cannot be over emphasized, regardless of the substrate or tile/stone adhesive.
Laitance
Laitance is a surface defect in concrete where a thin layer of weakened portland cement fines have migrated to the surface with excess “bleed” water or air from unconsolidated air pockets. Once the excess water evaporates, 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 a very low tensile strength, and therefore the adhesion of tile/stone will be limited by the low strength of the laitance.
Mechanical methods, including the use of chipping hammers, scarifying machines or high-pressure water blast, are recommended. Concrete should be removed until sound, clean concrete is encountered. Measurement of surface tensile strength and the absence of loose material are good indicators of sound concrete.
Abrasive blasting by means of dry, wet or bead/shot blast methods are preferred for the removal of laitance on new and fully cured concrete. Compressed air used in these methods must be oil free. Since wet abrasive blasting reintroduces moisture into the concrete, sufficient drying time must be allowed.
Curing Compounds and Sealers
Liquid curing compounds and sealers are topically applied spray-on materials, which are designed to keep moisture in the slab. The constant amount of water kept in the concrete by the curing compounds helps accelerate the curing time and improve the performance of the concrete. Curing compounds and concrete sealers are used more frequently in all types of construction, especially in fast track jobs. Unfortunately, all types of curing compounds, concrete sealers and surface hardeners (including form release agents) must be completely removed from the slab prior to the installation of tile/stone. The best method to remove these curing compounds from the surface would be to bead-blast or shot-blast the concrete surface.
There is a very simple and effective test to identify the presence of curing compounds, sealers or other bond breaking conditions. Simply sprinkle a few drops of water onto the substrate and see what happens. If water absorbs into the slab then it is usually suitable for the direct adhesion of tile. On the other hand, if the water beads up on the concrete surface (like water on a freshly waxed car) then there is something present on the concrete surface that can inhibit proper adhesion of the tile adhesive.
Substrate Preparation Equipment and Procedures
To determine if bond inhibiting contamination such as oil or curing compounds are present on concrete, conduct the following test: taking proper safety precautions, mix a 1:1 solution of aqueous hydrochloric (muriatic) acid and water, and place a few drops in various locations. If the solution causes foaming action, then the acid is allowed to react freely with the alkaline concrete, indicating that there is no likely contamination. If there is little or no reaction, chances are the surface is contaminated with oil or curing compounds. Acids do not affect or remove oily or waxy residue, so mechanical removal may be necessary.
Contamination Removal
Any surface to receive tile or stone finishes will always be exposed to varying degrees of contamination, especially normal construction dust and debris. Tile and stone is often included in the last phase of building construction. Multiple trades have been in the area and finished their certain part of construction (i.e. sheet rock, plumbing, painting, etc…). There is often paint, drywall compound, oil and other materials on the concrete from prior trades that need to be removed. One of the most difficult jobs for any installer is the preparation of the surface before the installation of the tile or stone commences. But, it is one of the most important steps, if not the most important step, in providing for a successful, long lasting tile installation. Cleaning the surface is mandatory before tile is placed, and sometimes multiple washings will have to take place before tiling. Just sweeping the floor is not good enough!
With most adhesives or cement leveling mortars/renders, such as latex cement mortars or moisture insensitive adhesives, the substrate can be damp during installation; however, it cannot be saturated. The objective is not to saturate the floor, but to make sure all the dust and debris is removed before tiling.
If contamination removal is required, or if surface damage or defects exist, bulk surface removal may be necessary to prepare the substrate. There are several methods of removal, but it is important to select a method that is appropriate to the substrate material and will not cause damage to the sound material below the surface. The following methods are recommended:
METHODS OF REMOVAL
Mechanical Chipping, Scarifying and Grinding
Mechanical chipping, scarifying or grinding methods are 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 and grinding with an angle grinder are common mechanical removal techniques.
Shot-blasting
This is a surface preparation method, which uses proprietary equipment to pummel the surface of concrete with steel pellets at high velocity. The pellets are of varying size, and are circulated in a closed, self contained chamber, where the pellets and debris are separated. The debris is collected in one container and the pellets are re-circulated for continued use. This is the preferred method of substrate preparation when removal of a thin layer of concrete surface is required, especially the removal of surface films (e.g. curing compounds or sealers) or paint.
Water-Blasting
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 laitance. Water-blasting is only recommended on concrete because of the high-pressure. Proper allowance must be made to allow for the excess water in the slab to dry. This method is commonly used on vertical surfaces.
Acid Etching
Acid etching or cleaning is never recommended to clean a surface prior to receiving tile/stone. If an acid is not neutralized or cleaned properly after the cleaning takes place, it can continue to weaken the portland cement in the concrete and tile installation materials when in the presence of moisture. Acid must be neutralized with Tri-Sodium Phosphate or baking soda mixed with water and then completely rinsed to ensure all the acid is removed from the surface. Again, acid is not recommended for cleaning concrete, since it has an adverse affect on portland cement. A chemical reaction occurs when portland cement and acid are introduced to each other that can destroy the cement matrix. The interaction between the acid and the portland cement exposes the concrete aggregates and weakens the concrete.
Acid can also leave a white powdery substance on the surface which can act as a bond breaker for any tile/stone installation material. To avoid any potential problems it is best to avoid the use of acids as a substrate preparation method.
Final Surface (Residue) Cleaning
The final and most important step of substrate preparation is the final cleaning, not only of the residue from contamination and bulk removal processes described above, but also cleaning of loose particles and dust from airborne contamination.
The final cleaning is considered minimum preparation for all substrates. Final cleaning can be accomplished by pressurized water as mentioned above, but can also be accomplished with standard pressure water and some agitation to eliminate the bond breaking effect of dust films. In some cases, airborne contamination is constant, requiring frequent washing just prior to installation of cement leveling plaster/renders, adhesive mortars, or membranes.
There is no exception from this general rule; and the only variation is the drying time of the substrate prior to the application of the adhesive. Drying time is dependent on the type of adhesive being used. With most adhesives, the substrate can be damp, with no standing water. A surface film of water will inhibit grab and bond of even water insensitive cement and epoxy based adhesives. The use of a damp sponge just prior to installation of tile/stone is an industry accepted method to ensure that the substrate is cleaned of any dirt and construction dust on the properly prepared substrate.
Typical Concrete Surface Profiles to Accept Tile/Stone Finishes
Ideally concrete slabs should be finished to a wood float or light steel trowel finish when scheduled to receive tile, stone or associated installation materials (e.g. waterproofing membranes, anti-fracture membranes, sound control membranes, self-leveling underlayments, vapor reduction membranes, bonded mortar beds, etc…). However, at times a wood float or light steel trowel finish has not been achieved and additional mechanical abrasion is required. Mechanically abrading over troweled slabs (shiny or burnished slabs) opens up the concrete surface pores which improves the bond of the topping materials.
To what extent / amount of mechanical scarification or bead blasting is required in order to achieve the satisfactory results? Is there a point where there can be too much mechanical abrasion?
As a reference guide, the International Concrete Repair Institute (ICRI) has created a concrete surface profile (CSP) chart:
CSP 1 – Acid Etching
CSP 2 – Grinding
CSP 3 – Light Shot Blasting
CSP 4 – Light Scarification
CSP 5 – Medium Shot Blast
CSP 6 – Medium Scarification
CSP 7 – Heavy Abrasive Blast
CSP 8 – Scabbling
CSP 9 – Heavy Scarification
These models are replicates of concrete surfaces that represent the degrees of roughness ranging from CSP 1 (nearly smooth) to CSP 9 (very rough). Ordered in ascending roughness, surface profiles are meant to correspond to acid etching, grinding, light shot blasting, light scarification, medium shot blast, medium scarification, heavy abrasive blast, scabbing and heavy scarification.
If a floor is scarified to a point that it is too rough (e.g. CSP 6 or rougher), a LATICRETE® fortified underlayment can be used to correct the imperfections. Waterproofing or crack isolation membranes should be applied over smooth concrete slabs or slabs that has been smoothed or leveled with a LATICRETE fortified underlayment.
It is important to note that simply achieving the desired CSP rating does not necessarily indicate that all of the potential bond breaking or bond inhibiting materials have been thoroughly removed. All bond breaking and bond inhibiting materials must be removed regardless of the CSP rating of a concrete floor.
Contaminated Slab Alternative
On contaminated concrete slabs where it is not feasible to remove the top surface by a suitable method, an unbonded (wire-reinforced) mortar bed would be the best alternative. Please refer to the LATICRETE ES-F111available at www.laticrete.com/ag or in Section 10 for more information.
ANSI A108.01 provides typical service ratings and mortar bed thickness for unbonded mortar bed applications. It is important to note that an unbonded mortar bed may not be appropriate for all application types in mass transit applications. Bonded mortar beds may be better suited for many of the applications that will be encountered in these applications.
Service rating / thickness of wire reinforced mortar beds installed over a cleavage membrane, specify reinforcing and thickness. Mortar beds in excess of 3-1/2" (87 mm) thick may require heavier reinforcing, larger aggregate, richer mix, greater compaction and must be detailed by appropriate authority.
2.4 Uncommon Substrates
Asphaltic Waterproofing Membranes
Asphaltic (petroleum based) waterproofing placed over substrate surfaces are generally not compatible with tile installation adhesives. The presence of this type of waterproofing would dictate the method of installation that would have to be used. An unbonded wire reinforced mortar bed (ES-F111), available at www.laticrete.com/ag, would be the best option for installing over this type of waterproofing product. (See Section 10 for executions statement on this method).
Steel and Metal
Steel and metal substrates require an epoxy adhesive or the mechanical fastening of diamond metal lath to the steel and the installation of a mortar bed due to the high density and very low porosity of this type of material. Portland cement or latex portland cement adhesives, by themselves, do not develop adequate bond to metals without expensive preparation or special adhesive formulations.
There are two methods for the installation of tile over steel or metal substrates;
The preferred method would be to tack weld or mechanically fasten 3.4# diamond metal lath complying with the current revision of ANSI A108.1 (3.3 Requirements for lathing and Portland cement plastering), ANSI A108.02 (3.6) Metal lath, and A108.1A (1.0 – 1.2, 1.4 and 5.1).
Next, apply LATICRETE 3701 Fortified Mortar Bed; or, LATICRETE 226 Thick Bed Mortar gauged with LATICRETE 3701 Mortar Admix to float and fill in the wire lath. Float surface of scratch/leveling coat plumb, true and allow mortar to set until firm. Once the mortar bed is firm and dry the installation of the membrane (e.g. LATICRETE Hydro Ban™, LATICRETE 9235 Waterproofing Membrane or LATICRETE® Blue 92 Anti-Fracture Membrane), if specified can commence. Tile/stone can be installed directly to the membrane using LATICRETE 254 Platinum. Grout using an appropriate LATICRETE Grout (e.g. LATICRETE SpectraLOCK® PRO Grout†) and use LATICRETE Latasil™ for any movement or isolation joints.
The alternative (and most frequently used) method to set tile over a steel or metal substrate is as follows;
Make sure the steel or metal substrate is cleaned thoroughly, meets deflection ratings and can support the weight of the installation. Wash steel or metal with a strong detergent to ensure that all manufacturing oils are removed. Rinse completely and allow the steel or metal to air dry. If possible scuff up the surface to receive tile with sand paper or emery cloth and then re-wash the surface, rinse completely and allow to air dry. Once the surface is dry you may set the tile/stone using an epoxy adhesive (LATAPOXY® 300 Adhesive). Grout using a suitable LATICRETE Grout (e.g. LATICRETE SpectraLOCK PRO Grout). Use LATICRETE Latasil for movement and isolation joints.
The metal or steel substrate must be rigid enough to withstand the weight of the mortar bed, any membranes, setting materials, tile and grout.
(See Section 10 for an execution statement on these methods). Please refer to LATICRETE ES-S313 and ES-S314 at www.laticrete.com/ag or in Section 10 for more information.
Plywood
Plywood and other wood-based products generally have high water absorption rates, and undergo rates of volumetric swelling and subsequent shrinkage that make these materials unsuitable as a substrate types in mass transit applications. In addition, The Tile Council of North America (TCNA) classifies most plywood floor substrates as residential and light commercial use. Therefore, this classification would negate the use of plywood in many types of mass transit applications.
2.5 Concrete – Slab-on-Grade
Placement of Concrete Slab
The vast majority of all mass transit tile installations are adhered directly to concrete. The most important factor for good, hard concrete is the water to cement ratio. Concrete needs water to hydrate and harden, but too much water can have a detrimental effect on concrete. Too little water will also affect the final performance of the concrete product. Understanding water and its effect on concrete is critical to achieving the desired results from a concrete slab. Water escapes from concrete via evaporation and also transpires through concrete from other sources and passes through as moisture vapor.
The water used to mix concrete must be clean (potable) and free of acids, alkalis, oils, or sulfates. This is necessary for proper hydration and curing of the concrete. There is a direct relationship between the strength characteristic of portland cement based concrete and the amount of water used per weight of cement. This is known as Abram’s Law (Duff Abrams, 1918). Essentially, the lower the water to cement ratio the higher the resultant physical properties of the concrete will be. Rule of thumb – LESS WATER = BETTER CONCRETE.
A properly designed concrete mixture will possess the desired workability for the fresh concrete and the required durability and strength for the hardened concrete. Typically, a mix is about 10 – 15% cement, 60 to 75% aggregate (fine and coarse combined), 15 to 20% water and 5 to 8% entrained air. The project engineer or design professional is responsible for specifying the actual concrete properties as required for each individual project.
Concrete will very often have an excess amount of water added to make the concrete easily workable. However, because portland cement only requires a certain percentage of its weight to hydrate, the excess water (water of convenience) will eventually escape. Much of the excess water will escape through capillary action (bleeding) while the concrete is in its plastic state during consolidation and finishing operations. Proper cure of concrete to attain the desired physical properties requires that moisture in concrete be maintained for a minimum of 3 to 7 days depending on temperature, humidity, type of cement, and type of admixtures used.
Typically, the first thing that a concrete contractor will do on a job site is perform a slump test to make sure that the concrete meets the slump criteria for that particular concrete. Unfortunately, many concrete contractors do not like the workability of concrete that passes the slump test. If this is the case, then the next words heard on the job site is “ADD MORE WATER”. The concrete contractor may, without their knowledge, be affecting the final performance of the concrete. The fact is; if one extra gallon (3.8 ) of water is added to a cubic yard (1 m3) of 3,000 psi (21 MPa) concrete then one or more of the following problems may occur;
1. Finished concrete can develop 5% less than its intended design strength,
2. Slump may increase by 1" (25 mm),
3. Compressive strength can be lowered by 150psi (1 MPa) or more,
4. The effect of 1/4 sack of concrete can be wasted,
5. Shrinkage potential increases,
6. Resistance to attack by de-icing salts is decreased, and
7. Freeze/thaw resistance can be decreased by 20%.
Importance of Vapor Retarders
Vapor retarders are necessary because concrete is a moisture and vapor permeable material. In fact, concrete can be thought of as being a very hard, dense sponge. Moisture vapor easily passes through concrete and can lead to problems with certain types of impervious tile, membranes, setting materials, and other types of flooring materials. In many cases, the vapor retarder is typically a 10 mil (.25 mm) thick polyethylene sheet placed directly under the concrete slab. Choosing the proper vapor retarder can be important since many polyethylene sheet materials are made with some recycled organic content. This organic content can decay over time leaving voids or holes through the sheeting; rendering it as an ineffective barrier. For better long term performance, architects and engineers are recommending 100% virgin polyethylene or 15 mil reinforced polyolefin as the vapor retarder. Proper placement and installation of the vapor retarder should also be specified by a qualified architect or engineer and shown in project details. No matter what material is used as the vapor retarder, it should conform to ASTM E 1745 (Standard Specification for Water Vapor Retarders Used in Contact with Soil or Granular Fill Under Concrete Slabs).
A vapor retarder must have a maximum perm rating of 0.3 perms (0.2 metric perms) when tested by ASTM E 96 (Standard Test Method for Water Vapor Transmission of Materials).6 To give you an example of what a perm is; 7003 perms translates to 1 lb/1,000 ft2/24 hours (57 mg/s•m2) of moisture vapor as determined using the calcium chloride test (ASTM F 1869 – Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride). This means that moisture vapor can transpire through the vapor retarder but at an extremely low rate. A properly specified and placed vapor retarder will not allow any passage of moisture vapor through penetrations in the slab or at the perimeter. Good detailing, seaming and sealing of the vapor retarder are necessary to ensure that the required performance is attained. A good, properly placed and installed vapor retarder can also help to limit radon infiltration through a slab and into the structure.
Placement of Vapor Retarder
ACI Committee 302, “Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials” (ACI 302.2R-06)” states in section 7.2 that some specifiers require concrete to be placed on the vapor retarder, and others require placement of a granular blotter layer between the concrete and the vapor retarder. As with many engineering decisions, the location of a vapor retarder is often a compromise between minimizing water vapor movement through the slab and providing the desired short- and long-term concrete properties.
There are benefits and drawbacks to each method. Therefore, proper detailing is very important to not only the performance of a flooring system but to the potential health and safety of building occupants.
The original method places the vapor retarder directly onto the compacted soil. Next a 4" (100 mm) granular base blotter layer is placed on the vapor retarder with concrete poured on top. Based on the review of problem installations incorporating this method, it became clear that the fill course above the vapor retarder can take on water from rain, wet-curing, wet grinding or cutting, and cleaning. Unable to drain, the wet or saturated fill provides an additional source of water that contributes to moisture vapor emission from the slab. These moisture vapor emission rates can be well in excess of the 3 to 5 lb/1,000 ft2/24hr (170 – 283 mg/s•m2) recommendation by many of the floor covering manufacturers.
As a result of these experiences, and the difficulty in adequately protecting the fill course from water during the construction process, caution is advised as to the use of the granular fill layer when moisture sensitive finishes are to be applied to the slab surface. The committees believe that when the use of a vapor retarder is required, the decision whether to locate the material in direct contact with the slab or beneath a layer of granular fill should be made on a case-by-case basis. Each proposed installation should be independently evaluated to consider the moisture sensitivity of subsequent floor finishes, anticipated project conditions and the potential effects of slab curling and cracking. It is also very important to lap up the vapor retarder onto the vertical plane and to seal off any penetrations through the sheeting to ensure maximum protection against vapor and moisture intrusion.
Drivers of Moisture Vapor
There are some very common reasons for having high moisture vapor emission problems in slabs. The most obvious would be a concrete slab without the placement of a vapor retarder. Without a vapor retarder there is nothing to prevent or limit any moisture underneath the slab from passing through the concrete. Soil capillarity can contribute as much as 12 gallons (45 L) per 1,000 ft2 (92.9 m2) per day to unprotected slabs from saturated shallow water tables. Broken pipes or leaking sewer lines can saturate the slab without obvious loss of water pressure. Some industrial applications have sump pumps underneath the slab to remove heavy chemicals and water used to clean machinery and floors. The pipes for these pumps can become corroded and eventually compromised by these chemicals and the soil underneath the slab can become saturated. Over-watered plant beds are another obvious contributor of water to building slabs as well.
When there is a vapor pressure differential, the higher pressure system will force moisture into the lower pressure system. Moisture vapor will consistently move from areas of high pressure to areas of low pressure. If sufficient moisture volume exists at the source and the concrete slab has low resistance to moisture, then the potential for floor covering or coating failure increases. Under the right conditions, there may also be sufficient moisture available to encourage the colonization of fungi. Indoor air quality and human health issues can be a far costlier outcome of excessive concrete moisture vapor emission than simply the loss of a floor.
Negative Hydrostatic Pressure
A common misconception points to negative hydrostatic pressure as the culprit floor covering failures.
Negative hydrostatic pressure can only occur when there is a physical water source higher than the slab. Therefore, it is very rare that a negative hydrostatic pressure condition exists on a project.
The chart (Figure 2.15) above helps to explain how temperature and humidity work to draw moisture into a structure through walls and concrete slabs. If the temperature of the soil under a structure is 55°F (13°C) and the relative humidity is 100% then the static pressure equals 0.214; if the building interior is at 70°F (21°C) and the humidity is 30% then the static pressure equals 0.108. This means that the moisture is driven into the building through the slab moving from the area of high pressure to the area of low pressure. Proper placement of a suitable vapor retarder can help to minimize moisture vapor transmission.
Testing for Moisture in Concrete
Many variables affect the results of moisture and pH tests commonly used to determine the moisture related acceptability of concrete floors. Failure to run the test correctly can produce erroneous and misleading results. Owners and contractors must understand that accurate floor tests must be conducted after the HVAC system is operating and the building has been at service conditions for 48 hours or longer. Most floors will not even begin to dry until the building has been enclosed and the HVAC system is running.
The building owner or general contractor should hire an independent testing agent to conduct floor moisture testing. Testers should be trained and certified. The test results should be reviewed by the design professional or a knowledgeable consultant to determine whether the floor is ready to receive an applied finish.
Most moisture tests, whether for moisture vapor emissions, relative humidity, or moisture content, measure a property that changes after the tile or other floor covering is installed. Concrete at the bottom of the slab in contact with the vapor retarder contains more moisture than the concrete at the surface. The moisture condition at the interface between the concrete and finish flooring changes because evaporation at the surface is hindered after the flooring is installed. This is true even if the vapor retarder is properly installed, the water to cement ratio is less than 0.50, and the floor is protected to prevent re-wetting. Water moves from the bottom of the slab toward the top driven by differences in vapor pressure between the high relative humidity at the bottom and the lower relative humidity at the top (as noted in Figure 2.15). Changes in temperature and relative humidity above and below the slab affect the static pressure and, in turn, the subsequent drive of the moisture vapor.
Current practice (if required) is for the tile installer to measure the moisture and pH of the floor and submit the results to the general contractor or construction manager. Too often these results are not transmitted to the design team, nor are the tests performed that the design team might have preferred. Moisture vapor emission rates are critical to the long term performance of a tile installation that incorporates a waterproofing or crack isolation membrane. Typical liquid applied waterproofing/anti-fracture membranes (e.g. LATICRETE® Hydro Ban™) require that the maximum amount of moisture in the concrete substrate not exceed 5 lbs/1,000 ft2/24 hours (283 mg/s•m2) per ASTM F1869 or 75% relative humidity as measured with moisture probes as per ASTM F2170.
High alkalinity in conjunction with a high moisture vapor emission rate may affect the long term performance of certain types of adhesives and “peel n’ stick” asphaltic type membranes. These adhesives and membranes may soften and deteriorate when subjected to high alkalinity. Alkalinity can be measured by performing a standard concrete surface pH test in compliance with ASTM F710 (Standard Practice for Preparing Concrete Floors to Receive Resilient Flooring).
The design team should not leave the testing to the tile installer. Specifications should require the owner’s testing agency conduct these tests and report the test results to the tile installer, general contractor or construction manager, and the design team. The specifications also should require that each test be conducted in accordance with ASTM standard test methods, or that any deviations from these methods be approved by the design team.
Commonly Used Moisture Test Procedures
Calcium Chloride Test (ASTM F1869 – Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride)
A calcium chloride test measures Moisture Vapor Emission Rates (MVER) passing through or from concrete and gives results measured in pounds of moisture per 1,000 ft2 (98.3 m2) in a 24 hour period. Three calcium chloride tests should be conducted for the first 1,000 ft2 (98.3 m2) and one additional test per 1,000 ft2 (98.3 m2) within a 60 – 72 hour time frame or as required by design team. These tests are a “snapshot” for the specific time/date when the testing takes place and results can vary when calcium chloride tests are performed on different dates. Calcium chloride tests should only be performed after a building has been completely enclosed and the HVAC system has been operating for a prescribed length of time. Check with the manufacturer of the moisture test kit for complete instructions and recommendations.
Relative Humidity Testing (ASTM F 2170 – Standard Test for Determining Relative Humidity in Concrete Floor Slabs Using In-Situ Probes)
Typically, relative humidity testing (also referred to as in situ testing) involves drilling a hole into the concrete and inserting a plastic sleeve. The sleeve is sealed and pressure is allowed to equalize for a prescribed length of time. A hygrometer probe is inserted into the sleeve and the reading is taken. Some relative humidity testers do not require drilling a hole. Testing equipment methodology and procedures may vary by manufacturer. Instructions for frequency and location of testing should be followed as recommended by design professional or engineer. Relative humidity testing can measure at selected depths of the concrete depending on the depth of the hole that is measured by the probe.
The results of relative humidity testing are measured in percentages. Although there is no direct correlation between results based on ASTM F1869 testing and relative humidity testing, a reading of 75% roughly translates into 3 lbs/1,000 ft2/24 hours (1.4kg/98.3 m2/24 hours) as measured by a ASTM F1869 calcium chloride test (see 2.4.4.1). A reading of 80-85% roughly translates into 5 lbs/1,000 square feet/24 hours (283 mg/s•m2).
Plastic Sheet Test (ASTM D 4263 – Standard Test for Determining Moisture in Concrete by the Plastic Sheet Method)
This test method is qualitative and only provides static results at the moment that the test is completed. This test method will not provide quantitative moisture level results and is strictly used to determine if moisture is present. This is generally considered an outdated method to measure moisture transmissions.
Efflorescence
Efflorescence is a white crystalline deposit that forms on or near the surface of concrete, masonry, grout and other cement based materials. It is the most common post-installation condition in tile, stone and brick masonry installations.
Efflorescence can range from a cosmetic annoyance that is easily removed, to a serious problem that could cause adhesive bond failure or require extensive corrective construction and aggressive removal procedures.
Efflorescence starts as salt, present in portland cement products, which is put into solution by the addition of water. The salt is then transported by capillary action (or gravity on walls) to a surface exposed to the air. The solution evaporates, the salts react with carbon dioxide and a white crystalline deposit remains. Efflorescence can also occur beneath the surface or within ceramic tile or brick.
Efflorescence occurs when the three conditions listed below occur. While theoretically, efflorescence cannot occur if one condition does not exist, it is impractical to completely eliminate the confluence of these conditions.
Causes of Efflorescence
Presence of Soluble Salts
Presence of Water (for Extended Period)
Transporting Force (Gravity, Capillary Action, Hydrostatic Pressure, Evaporation, etc…)
Presence of Soluble Salts
There are numerous sources of soluble salts listed in the Table below. There is always the potential for efflorescence when concrete and cement mortars, adhesives and grouts are exposed to the weather. Other sources of soluble salts can be monitored, controlled or completely eliminated. For more information on efflorescence please see Section 9.6.
Cement Hydration – The most common source of efflorescence is from portland cement based materials (e.g. concrete, cement plasters/renders, concrete masonry units, cement backer board units, and cement-based mortars, including latex cement adhesive mortars). One of the natural by-products from cement hydration (the chemical process of hardening) is calcium hydroxide, which is soluble in water. If portland cement based products are exposed to water for prolonged periods and evaporate slowly, the calcium hydroxide solution evaporates on the surface, combines with carbon dioxide and forms calcium carbonate, one of the many forms of efflorescence. Once the calcium hydroxide is transformed to calcium carbonate efflorescence, then it becomes insoluble in water, making stain removal difficult.
Calcium Carbonate Contamination – A common source of soluble salts is either direct or airborne salt-water contamination of mixing sand and the surface of the substrate. Mixing water can also be contaminated with high levels of soluble salts. Typically, water with less than 2,000 ppm of total dissolved solids will not have any significant effect on the hydration of portland cement, although lower concentrations can still cause some efflorescence.
Presence of Water
While it is difficult to control naturally occurring soluble salts in cementitious materials, proper design, construction and maintenance of a concrete floor and its finish materials can minimize water penetration. Without sufficient quantities of water, salts do not have adequate time to dissolve and precipitate to the surface of a concrete slab or tile installation, and efflorescence simply cannot occur. Using less “water of convenience” can also help to minimize the occurrence of efflorescence.
For exterior installations, rain and snow are the primary sources of water. For interior installations, the primary source is cleaning water. Broken pipes, poor soil drainage and inadequate rainwater evacuation can also contribute to high moisture levels within a building.
Sealers and Coatings
Water repellent coatings are commonly specified as a temporary and somewhat ineffective solution to fundamentally poor slab design and construction. In some cases, water repellents may actually contribute to, rather than prevent the formation of efflorescence. Water repellents cannot stop water from penetrating cracks or movement joints in the slab. As any infiltrated water travels to the surface by capillary action to evaporate, it is stopped by the repellent, where it evaporates through the coating (most sealers have some vapor permeability) and leaves behind the soluble salts to crystallize just below the surface of the water repellent. The collection of efflorescence under the repellent coating may cause spalling of the concrete.
Effects of Efflorescence
The initial occurrence of efflorescence is primarily considered an aesthetic nuisance. However, if the fundamental cause (typically water infiltration) is left uncorrected, continued efflorescence can become a functional defect and affect the integrity and safety of a tile/stone installation.
The primary concern is the potential for bond failure resulting from continued depletion of calcium and subsequent loss of strength of cementitious adhesives and underlying cement based components. The crystallization of soluble salts can exert more pressure on a tile/stone system than the volume expansion forces of ice formation.
Efflorescence Removal Methods and Materials
Prior to removal of efflorescence, it is highly recommended to analyze the cause of efflorescence and take corrective action to prevent recurrence. Analysis of the cause will also provide clues as to the type of efflorescence and recommended cleaning method without resorting to expensive chemical analysis.
Determine the age of the installation at the time the efflorescence appeared. In buildings less than one year old, the source of salts is usually from cementitious mortars and grouts, and the water source is commonly residual construction moisture. The appearance of efflorescence in an older building indicates a new water leak or new source of salts, such as from acid cleaning residue. Do not overlook condensation or leaking pipes as a water source. Location of the efflorescence will offer clues as to the entry source of water
Chemical analysis of efflorescence can be conducted by a commercial testing laboratory using several techniques to accurately identify the types of minerals present. This procedure is recommended for buildings with an extensive problem, or where previous attempts to clean with minimally intrusive methods have failed.
Removal methods vary according to the type of efflorescence. Therefore, it is of critical importance to evaluate the cause and chemical composition of efflorescence prior to selecting a removal method.
Many efflorescence salts are water soluble and will disappear with normal weathering or dry brushing. Washing is only recommended when temperatures are warm so that wash water can evaporate quickly and not have the opportunity to dissolve more salts.
Efflorescence that cannot be removed with water and scrubbing requires chemical removal. The use of muriatic acid is a conventional cleaning method for stubborn efflorescence, however, even with careful preparation, acid etching can occur. There are less aggressive alternatives to muriatic acid, including a less aggressive sulfamic acid, available in powdered form. Sulfamic acid, dissolved in water to a concentration between 5–10%, should be strong enough to remove stubborn efflorescence without damage to the cementitious material.
Regardless of the cleaning method selected, the cleaning agent should not contribute additional soluble salts. For example, acid cleaning can deposit potassium chloride residue (a soluble salt) if not applied, neutralized and rinsed properly.
Acids should not be used on polished stone or glazed tile, because the acid solution can etch and dull the glaze or polished surface. Acids can react with compounds in the tile glaze and deposit brown stains on the tile surface which are insoluble and impossible to remove without ruining the tile.
Before applying any acid or cleaning solution, always test a small, inconspicuous area to determine if any adverse effects may occur. Just prior to application, saturate the surfaces with water to prevent acid residue from absorbing below the surface. While most acids quickly lose strength upon contact with a cementitious material and do not dissolve cement below the surface, saturating the surface is more important to prevent absorption of soluble salts residue (potassium chloride) which then cannot be surface neutralized and rinsed with water. This condition in itself can be a source of soluble salts and allow recurrence of the efflorescence problem intended to be corrected by the acid cleaning.
Application of acid solutions should be made to small areas less than 10 ft2 (1m2) and left to dwell for no more than 5 minutes before brushing with a stiff acid-resistant brush and immediately rinsing with water. Always follow the acid manufacturer’s directions for diluting, mixing, application, initial rinsing, neutralization, and final rinsing techniques.
2.6 Suspended Concrete Slabs
With advancements in concrete and concrete placement technology, the number of suspended (elevated) concrete slabs being placed is increasing around the world. There are numerous types of cast-in-place and pre-cast concrete floor systems available that can satisfy any structural, span or loading condition. Since the cost of a floor system is a major part of the structure, and the building cost, then selecting the most effective floor system is important to achieving overall performance of the building.
The ability to customize load capacity to suit the usage requirements, deflection, inherent fire-resistance, ease of installation, and the ability to create long spans makes concrete the material of choice for mass transit applications. The ability to finish the floor with a wide variety of finish materials (including tile and stone), permanently mount heavy machinery and the capability to stand up to extreme conditions are added benefits of concrete.
Defining the proper suspended slab type, reinforcement method, thickness, span, load bearing capacity, and all other performance requirements is the responsibility of a qualified design professional and/or structural engineer and is based on expected loads, usage, environment and much more.
We will take a look at 3 different types of suspended concrete slabs;
Cast-in-Place Concrete Slabs
The main components and expenses of cast-in-place concrete slabs are the concrete, reinforcement (either mild or post-tensioned) and formwork. A major emphasis of the need for reinforcement in suspended concrete slabs is the fact that concrete, while strong in compression, is weak in tensile and flexural strengths. Steel is strong under forces of tension, so combining concrete and steel together makes for an extremely strong and versatile building material. By combining the properties of reinforcing steel with concrete, you achieve a building material that can easily resist both compressive and tensile forces.
Benefits can also be achieved by using the reinforcing materials to place additional forces on the concrete to place it in compression. By compressing the concrete, additional tensile strength can be realized. This additional tensile strength (stiffness) can provide an architect with the ability to achieve longer spans with a thinner concrete slab. Another benefit of tensioning concrete raises the capability of the slab to resist the development of shrinkage cracks. In theory, the more the concrete is squeezed together, the less likely it is that concrete slab shrinkage cracks will develop.
Mild Reinforcement Concrete Slabs
Mild reinforced concrete slabs are poured in place, over framework, and around a steel reinforcement (rebar) grid. These rebar grids are most often assembled on site as defined by installation drawings with concrete poured around the reinforcing. This type of reinforcement is most often used in steel frame (deck) concrete slabs.
Post Tensioned Concrete Slabs
Post-tensioning is a method of stressing concrete in which tendons have tension applied after the concrete has hardened and the pre-stressing force is primarily applied through the end plates or anchorages. Unlike pre-tensioning, which can only be done at a pre-cast manufacturing facility, post-tensioning is performed in-situ on the job site.
Concrete slabs usually utilize ultra high-strength steel strands to provide post-tension forces to the slab. Typically, these steel strands have a tensile strength of 270,000 psi (1,860 MPa), are about 1/2" (12 mm) in diameter and are stressed to approximately 33,000 lbs (15,000 kg).
Reinforcing wire tendons are usually pre-manufactured at a plant, based on specific requirements, and delivered to the jobsite, ready to install. These tendons are laid out in forms in accordance with installation drawings that indicate how they are spaced, what their profile height should be, and where they will be stressed. After the concrete is poured and has reached required strength (up to 5,000 psi [34.5 MPa]) the tendons are stressed and anchored. These tendons, like rubber bands, want to return to their original length but are prevented from doing so by the anchorages. The fact that the tendons are kept in a permanently stressed state causes a force in compression to act on the concrete. The compression that results from the post-tensioning counteracts the tensile forces created by subsequent loading (machinery, people, equipment, flooring, etc…).
Pre-Tensioned (Pre-Cast) Concrete Slabs
While pre-tensioning is similar to post-tensioning in the fact that steel tendons are exerting stresses onto concrete to increase tensile strength, the method of placement is different. In post-tensioning, the steel tendons are stressed after the concrete hardens; in pre-tensioning, the steel tendons are stressed to 70 – 80% of their ultimate strength prior to the concrete being placed or (poured into the molds – in the case of pre-cast concrete) around the tendons. Once the concrete reaches the required strength, the stretching forces are released. As the steel reacts to return to its original length, the tensile stresses are translated into a compressive stress in the concrete.
Pre-tension concrete members must be poured at a production facility and shipped to the job site individually. Each member is then installed as required and supported by columns, beams or other structural member.
Pre-cast concrete planking is another form of pre-tensioned concrete. The pre-cast concrete planks are tensioned prior to the concrete being poured. The concrete planks are then slipped together and mortared in place. Typically, a 2" (50 mm) thick concrete topping slab is required to create a monolithic concrete slab that is suitable to receive a ceramic tile or paver finish.
Advantages of pre-stressed concrete slabs is shallower depth for the same deflection rating as a thicker slab and greater shear strengths than plain reinforced slabs of the same depth.
Steel Frame (Deck) Concrete Slabs
To achieve desired tensile strength in pre-tension and post-tension slabs, tendons are required that have stresses applied to them. In steel frame (deck) construction, the steel deck and additional mild steel reinforcing will provide the tensile strength required for the concrete slab. Post-tensioning is typically not necessary.
Modern profiled steel pan sheeting, specifically designed for the purpose, acts as both permanent formwork during concreting and tension reinforcement after the concrete has hardened. Shear connections are mechanical fasteners used to develop composite action between the steel beams and the concrete and maintains solid structural integrity. At this final stage the composite slab consists of a profiled steel sheet and an upper concrete topping which are interconnected in such a manner that horizontal shear forces can be resisted at the steel-concrete interface.
Composite floor construction has certain advantages over typical concrete construction;
1. It is used in very tall buildings,
2. It is lightweight and strong,
3. It is prefabricated, so it assembles quickly.
Tile Installation Over Suspended Concrete Slabs
The TCNA recommends method F-111 for installation over a suspended concrete slab, or, for installations where an unbonded mortar bed is impractical, follow TCNA method F122 which requires an anti-fracture or waterproofing/anti-fracture membrane. Please reference www.laticrete.com/ag for further information on the LATICRETE recommended installation methods (ES-F111 and ES-F122) to comply with the above mentioned TCNA guidelines.
Concrete or Mortar Bed Substrates
Once the mortar bed (e.g. LATICRETE 226 Thick Bed Mortar mixed with LATICRETE 3701 Mortar Admix or LATICRETE 3701 Fortified Mortar Bed) hardens and is cured properly, most waterproofing membranes can be installed directly over the mortar bed. Follow the membrane (e.g. LATICRETE 9235 Waterproofing Membrane or LATICRETE Hydro Ban) installation instructions for proper cure time of the mortar bed prior to application of the membrane. When using an epoxy setting material or other epoxy membrane, full cure of the mortar bed is required.
A typical 2" (50 mm) thick mortar bed weighs roughly 24 lbs per square foot (95 kg per m2).