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Single wythe concrete masonry units (CMU) are a durable, cost-effective, aesthetically versatile form of masonry construction that has been widely used worldwide for decades. A sustainable, effective solution for owners and architects looking for permanence and low maintenance, single wythe construction is defined as a stone, brick, or concrete wall that is one masonry unit thick. A single wythe wall offers the economic advantage of serving as the structural system with multiple finish options on the exterior, and the interior as well. But as single wythe walls do not require the backup of a traditional cavity wall construction, in order to provide full protection from the elements they must be carefully detailed and constructed. This article will identify the factors that ensure proper performance and discuss the details and specifications that should be considered during the design process in order to achieve superior masonry buildings.
Concrete masonry units are manufactured blocks used in construction. They come in a variety of sizes, colors, and finishes, the most common size being 8 inches deep by 8 inches tall by 16 inches long. They are generally comprised of sand, cement, stone or aggregate, and can have a color pigment such as iron oxide. Admixtures, such as integral water repellants, are also included to improve performance. CMUs are typically produced with hollow centers to reduce weight, with the added benefit of allowing insulation to be placed in the cores if specified.
Wet and dry casting are the two most widely used methods today, with each requiring a unique mix of natural sands and aggregates. Wet cast concrete typically has 33 percent moisture content whereas in dry casting, there is 6 percent moisture content. The dry cast mix is consolidated by intense tamping with an air hammer until it is densely compacted and ready for removal from the mold. Dry-cast concrete has zero slump, and the forms can be stripped as soon as the concrete has been consolidated. After that, the concrete block is cured in an environment in which high humidity levels are maintained for at least 24 hours. A key benefit of this method is that only one set of forms is needed to mass produce a specific product, and the form can be stripped immediately. The production of CMUs has progressed over the years from a one-at-a-time manufacturing process to a highly automated, state-of-the art procedure. Inventors started developing concrete block right after the Revolutionary War. In 1832 the first concrete block was patented, but commercial success was found in England. Up until 1906, concrete block producers were experimenting with refining the manufacturing process. In this early hand tamp one-at-a-time machine, a workman would fill the machine with cement and aggregate, hand mixed on the ground. He would tamp the mix in and discharge the finished block. In 1915, block was produced from ash or cinders from coal, hence being known as “cinderblock.” By 1920, there were several thousand small plants producing 50 million CMU. Due to mechanization in 1941, 500 million units were being produced by fewer plants, and as of 2004, some 4.5 billion units were being produced by 600 companies in the U.S. The original tamping method has been replaced with vibration and compaction, and today’s block machine produces 1,200 8-inch block equivalents per hour.
From the beginning, CMU has been a sustainable material produced with local materials by local manufacturers using recycled content. In terms of length of service life, one has only to look at the pyramids and the great cathedrals of Europe to see the durability of masonry construction. By its very nature it conveys permanence and quality. Masonry endures, with little or no maintenance, and CMUs continue to be identified with longevity and low life-cycle cost. They are relatively inexpensive compared to competing products, and the variety of shapes, colors, textures, and sizes available gives architects a full palette. CMUs also get high marks in energy efficiency. Materials such as insulation that have a high R-factor are usually associated with greater energy efficiency. However, this is not the entire picture as it neglects the benefit of thermal mass, which is a significant measure of a material’s capacity to store heat for future distribution. Because they are high in mass, masonry walls offer excellent thermal insulation. Their slow rate of heat discharge keeps interiors warm in winters, and their high rate of heat absorption makes for cool interiors in summers. When used with complementary products or systems, CMUs are particularly energy efficient. The mass of a masonry building also pays off in preventing easy sound transmission, reducing noise pollution, and helping to achieve a quiet environment, a feature much sought after in public buildings that accommodate large numbers of people. Using masonry can also lead to savings on insurance and maintenance costs as the material won’t burn, dent, rot, rust, or suffer insect infestation.
Today, manufacturers offer a wide range of CMU products that include gray block, architectural block, landscape block, half-high concrete brick, and several types of specialty block. Split face block, for example, is a concrete masonry unit composed of specialty colored aggregates and color additives put into the mix design. The split face texture offers a more rugged stone appearance and can add interest and dimension to an otherwise plain masonry wall. Manufactured in a variety of colors and sizes, half-high concrete offers the advantage of a veneer-like appearance in economical single wythe construction. Half highs offer the same quality exterior finish as a cavity wall but with a shorter construction time. Halfhigh brick-like units are integrally colored and produced to the same standards as conventional masonry and share the latter’s strength and resistance to fire and wind and ability to create a maintenance-free façade that is appropriate for new construction and particularly desirable in historic renovations. In ground face block, also known as burnished block, diamond grinding heads are applied to the face to expose the aggregates. Grinding takes place after the block is cured; it is then sealed with a heat-treated factory acrylic. After the block is cleaned, manufacturers strongly recommended another post-applied coat of sealer for moisture protection and to bring out the color.
In filled and polished block, after the initial grinding process, the pores are filled by hand and rubbed with a cementitious slurry that is color specific to the block aggregates. The block is oven baked and the slurry then becomes an actual part of the block. The block faces are polished again in a multistage process, then lightly ground. A factory-applied clear satin gloss acrylic finish accentuates the natural beauty of the aggregates and provides moisture and graffiti resistance. Architects should make sure the filled and polished block they are specifying meets ASTM C-744 Standard Specification for Prefaced Concrete and Calcium Silicate Masonry Units. Another specialty option is the glazed block, in which a thermoset glazing compound is permanently molded to one or more faces. Heat-treated and cured, the compound becomes an integral part of the unit. The result is a tight impervious surface that is easy to clean. This type of block is exceptionally resistant to staining, abrasion, impact, and chemicals. It is graffiti resistant and virtually impenetrable to spray paint, permanent markers, grease, or crayon. Glazed block is ideal for clean rooms as well, as it does not allow collection of dust, germs, or bacteria. The designer’s palette and opportunities for creativity are not limited to exterior applications for masonry. There are many fine examples of what can be accomplished with masonry interiors. Their inherent color variations and texture make CMUs an attractive design element, and with staggered placement, lightly contrasting mortar, or other treatment, they can add life to a space, creating an artistic look that reflects a smart modern design aesthetic.
CMUs must meet several specifications. In addition to ASTM E-119 Fire Safety and ASTM E-514 Water Penetration, ASTM C-90 is a standard specification that covers hollow and solid concrete masonry units made from hydraulic cement, water, and mineral aggregates with or without the inclusion of other materials. Parameters that must be met according to the standard include:
There are three classes of CMUs—normal weight, medium weight, and lightweight, with all three suitable for both loadbearing and nonloadbearing applications. Less than 105 pounds per cubic foot is considered light weight; less than 125 pounds per cubic foot, medium weight; and more than 125 pounds or greater per cubic foot, normal or heavy weight. Lightweight units are made from expanded shale or clay, and are best for fire rating and thermal performance, because of air voids in the units. The ASTM C-90 Standard for compressive strength stipulates a minimum 1,900 psi (average of three), and no unit must fall below 1,700 psi. Materials used in the unit affect the fire ratings, as does the size of the unit. As an example, an 8-inch x 8-inch x 16-inch hollow block would have a 2-hour rating at medium weight, a 2.75-hour rating at light weight and a 3-hour rating at an ultra light weight, which is a custom product. Architects can obtain a full list of hourly ratings from NCMA TEK 7.1 with the Equivalent Thickness Methodology, which nearly all specifications use or, alternatively, from laboratory test UL 618 or ASTM E119 which, however, is an expensive procedure. While ASTM C-90 does set standards in several areas, it does not address thermal performance, sound transmission rating, or color and texture issues.
One of the most significant concerns in designing a wall is moisture. This section will explore the best way to design walls with moisture penetration in mind and with multiple lines of defense. The main objective is to keep water from penetrating or entering the wall in the first place. In addition to precipitation, moisture can enter masonry walls from several different sources, including capillary action, water vapor, and ground water.
According to the National Concrete Masonry Association, successful moisture mitigation in concrete masonry walls involves a variety of techniques including flashing and counter flashing, weeps, vents, water repellents, sealants, post-applied surface treatments, vapor retarders, and crack control measures, with all components considered for redundant use yet with the appreciation that not all techniques will be suitable for all wall systems. The preferred approach to controlling moisture is to provide redundancy in a four-level line of defense including surface protection, internal protection, and drainage and drying, the idea being that water tightness of the wall will still be maintained even if one of these systems fails. This is referred to as the belt and suspenders approach.
In a cavity wall, designers rely on gravity and an unobstructed 2-inch airspace to get water down to the flashing and the weeps. This is the basic “rainscreen principle” wall. In a cavity wall are veneer, cavity, flashing and weeps, and backup. In Chicago, the cost to construct such a system is in the $30-per-square-foot range, based on the Masonry Advisory Council Cost Guide. Applying the rainscreen principle lines of defense to a single wythe wall, the components are an integral water repellent, drainable cores, flashing and weeps, an interior face shell, and post-applied water repellent. Again, according to the Masonry Advisory Council Cost Guide, the cost to construct such a system in Chicago is in the $12.50-per-square-foot range, some 58 percent less than the cavity wall.
Integral water repellents are an important part of the moisture control strategy in a single wythe wall. Repellency characteristics manufactured and locked in the design mix reduce the concrete’s absorption properties and ensure permanent performance. Consequently, the entire unit is treated so as to provide a back-up layer of protection that lasts the service life of the unit and protects it from moisture during construction. Architects will want to note whether the manufacturer includes an integral water repellent as a standard feature of the CMU. The figure below shows CMU blocks with and without a water repellent additive. On the block at the left, water poured on top of the block penetrates into the block. The block on the right wicks the water. For best moisture prevention, the integrated water repellent additive should be specified in the block and in the mortar. In fact, the NCMA recommends that the same manufacturer’s water repellent for mortar and concrete block be incorporated for compatibility and the same reduced capillary action characteristics. These water repellent admixtures both serve as a vapor barrier and reduce the ability of moisture to travel via capillary action within the CMU. However, these admixtures do not stop moisture that enters through cracks in the wall, and they prevent any water that does penetrate from exiting easily. Consequently, other moisture reduction methods such as flashing and control joints are critical to achieve full mitigation.
Applying a clear treatment, paint, or opaque elastomeric coating can enhance the water tightness of a wall. It is important to verify with the manufacturer that post-applied surface coatings are compatible with the block’s integral water repellents. However, post-applied water repellents are less successful in moisture prevention than integral repellents. They have a limited surface life of approximately two to seven years depending on the manufacturer. While post-applied coatings can be a good surface repellent, they do nothing to prevent an untreated unit from getting wet prior to sealing. If using surface protection, most manufacturers recommend a two-coat system.
While integrated and surface-applied water repellents can enhance the watertightness of a wall, they are not a substitute for proper design that incorporates flashing. When flashing is used, the importance of proper detailing cannot be overemphasized. Water mostly moves downward—a principle that informs the locations for flashing on single wythe walls. Flashing is best located at the top of the wall, the window head or bond beams, the window sill, and the wall base. In the accompanying figure (see the online version of this course), note the proper detailing of flashing at the window sill.
Also important in producing dry walls are the type of mortar and mortar joint. The lowest strength mortar required for durability and watertightness should be selected as this mortar is typically more workable and able to produce a weather-resistant seal at the unit interface. For best results, mortar joints should be tooled to a concave profile. This improves resistance to water penetration as it steers water away from the surface of the wall. The shape of the tool also compacts the mortar against the CMU to more effectively seal the joint. Because they don’t compact the mortar, other types of joints including raked, flush, struck, beaded, or extruded, will create ledges that interrupt water that streams down the face of the wall and are not recommended in exposed exterior walls. It is also important for water resistance that head and bed joints extend the full width of the face shells. In summary, single wythe walls offer an attractive cost savings compared to cavity construction. Moisture protection, collection, drainage, and back-up are important in achieving “rainscreen” performance in single wythe masonry, and a combination of integral water repellents and sealers offer a “belt and suspenders” line of defense. For best results, flashing and weeps are required at horizontal breaks.
To accommodate movement, expansion joints are used with clay brick, and control joints are used in concrete masonry construction. Control joints are one way of relieving horizontal tensile stresses due to shrinkage of CMUs, mortar, or grout. In addition to relieving horizontal tensile stresses, control joints reduce restraint, permit longitudinal movement, and separate dissimilar materials. Control joints, which are essentially vertical planes of weakness built into the wall to reduce restraint and allow longitudinal movement due to anticipated shrinkage, should be located where stress concentrations may occur. While these locations can be difficult to pinpoint in practice, following are some rules of thumb in locating the control joints. According to the National Concrete Masonry Association, these are:
Types of control joints are shown in the illustration above.
By their very nature, CMUs have good thermal properties. Yet technology is ever expanding, and insulated units may well be the next generation of masonry. This type of unit contains an insulated thermal barrier, which effectively creates highinsulated thermal mass, high heat capacity, and a long thermal lag time—attributes that combine to create walls that require just a fraction of the energy normally needed to keep the interior cool in the summer and warm in the winter. Because they consume less energy, a smaller HVAC can be utilized, which promotes cost savings and reduction of building energy use. The block design creates an energy barrier between the exterior wall veneer and the interior core, with the installation creating a thermal mass design and reducing air temperature fluctuations. Some insulated units have attained an equivalent performance value of an R-22 wall and an STC rating of 53. In addition to delivering these benefits, insulated CMUs may contribute to LEED points in several categories including recycled materials. In any event, the thermal performance of building materials is governed by building codes which benefit public safety and support the industry’s need for one set of standards without regional limitations. Prime among them is the International Energy Conservation Code (IECC), which is published by the International Code Council (ICC). The ICC develops model codes and standards used in the design, building, and compliance process to construct safe, sustainable, affordable, and resilient structures, and its I-Codes are a complete set of comprehensive, coordinated building safety and fire prevention codes.
Now, consider Table 1, the Maximum Overall Assembly U-Factor Path Table. The Zone 3 U-factor of the whole wall assembly must be 0.11 to meet the IECC 2012 code. In the Maximum Overall Assembly U-Factor Path Table, the Zone 3 U-Factor of the whole wall assembly must be 0.11 to meet 2012 IECC. Architects have but to simply download the free NCMA Thermal Catalog and choose a system that meets this requirement. Option B is to use a wall that has a total U factor of .078 for the entire wall assembly in 2012 code, it’s 0.90 in the 2009 code which is 11.11. This translates to an R-value of 12.82.
Software allows an analysis of the components of an entire building envelope’s performance, and enables the use of trade-offs to achieve energy efficiency and compliance, conferring the advantage of more design flexibility. Increased glazing, for example, can be offset by increases in insulation. Usually the analysis is implemented using free easy-to-use software. The software adds efficiency to the process. Basic building data is entered and saved and can be easily modified, with the designer immediately determining whether those changes have achieved compliance. Most, but not all states allow use of prescribed software to show compliance, and some states allow it only for certain counties or other jurisdictions.
This approach requires a total building performance analysis that simulates one full year of building operation. The building is treated as a discrete system, and evaluates the “big picture” of the entire building system, not just the components. It takes into account many more variables, such as window orientation and shading coefficients, and offers credit for renewable energy sources (solar, fuel cells, thermal energy storage). Fee-based software is generally utilized to model the equation and though this method enables greatest design freedom, it also requires the most rigorous analysis.
Curtailing air leakage through the building envelope is one way to boost the energy efficiency of a building. In the IECC, air leakage requirements are mandatory, regardless of which compliance path is used. The 2009 edition of the code requires the same building envelope air leakage requirements as previous editions. Essentially, openings and penetrations must be sealed and gasketed consistent with construction material and location specifics. Sealing materials spanning joints between construction materials must be able to accommodate expansion and contraction of those materials. The 2012 IECC has adopted requirements for air leakage in commercial buildings. The code lists the following as compliant assemblies: fully grouted cells; paints and coatings; one coat high quality latex, or two coats commercial grade latex paint; heavy duty polyisocyanurate applied to interior; gypsum wallboard with joints sealed; and exterior insulation and finish systems. In terms of air leakage, typical masonry construction has integral advantages over frame wall construction. First, masonry construction does not have the same leakage sites as their frame wall counterparts. There are no sills in masonry construction as the wall extends from the footing in a continuous assembly. There is usually a tie-beam or bond beam atop a masonry wall, with trusses or rafters set to the top plate. As a matter of course, quality sealing and caulking are critical at the ceiling edge as well as at access to the attic and around all wall penetrations. According to the National Concrete Masonry Association, air leakage rates compiled for a variety of buildings in various climates, though all constructed after 1980, showed that 84 percent of masonry buildings had whole building air leakage rates of less than 2 cfm/square foot at a pressure differential of 1.57 lb/square foot. By contrast, only 30 percent of frame buildings had whole building air leakage rates less than the above mentioned parameters. While the association states that the data was drawn from different references over several climate zones, making conclusions somewhat difficult to draw, the results do indicate that in terms of existing buildings, masonry buildings tend to have lower air leakage rates than framed buildings.
As with any building material proper cleaning and maintenance of masonry go a long way to promoting performance. In-progress cleaning is very important, though often overlooked. All excess mortar must be removed and any exposed masonry must be dry brushed prior to the end of each workday. The wall must also be protected from mud spatter and mortar droppings. In terms of a final cleaning, the correct proprietary cleaning must be used. High pressure cleaning methods should never be utilized, and nozzle pressure should not exceed 500 pounds per square inch. The water flow should be at least four gallons per minute and fan nozzle should be at least 40 degrees. The nozzle should be kept at least 18 inches from the face of the units.
A durable, cost-effective form of masonry construction, the single wythe wall is a viable solution to many of today’s pressing concerns. When well designed, this type of construction offers sound solutions in controlling moisture and air leakage as well as meeting low maintenance requirements with a minimum of effort. Properly designed, CMUs can give architects a proven solution for a smart sustainable modern aesthetic.Back