Tag Archives: 04 22 00−Concrete Unit Masonry

Durable Waterproofing for Concrete Masonry Walls: Field Testing Methods of Water Repellency

by Robert M. Chamra, EIT and Beth Anne Feero, EIT

There are two main field testing methods used for water repellency of concrete masonry units (CMUs), for quality assurance before being placed in a wall: droplet and RILEM tube testing. Completed assemblies can also be tested with RILEM tubes or other standard water spray tests such as ASTM E514, Standard Test Method for Water Penetration and Leakage Through Masonry.

Droplet testing
The droplet test is a quick and simple test to observe the water mitigation capabilities of a CMU. This test requires the unit to be placed horizontally on a level surface with the face shell oriented upward. Droplets are placed at different locations around the unit from a height of 50 mm (2 in.) or less.

The specimens are to be placed in ambient temperature (22.9 ± 5.6 C [75 ± 10

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F]) and moderate relative humidity (50 ± 15 percent) and are monitored for evaporation facilitated by sunlight or wind; they are recorded at one-, five-, and 10-minute intervals. At the conclusion of the test, the droplets are classified as standing, partially absorbed, totally absorbed, or dry. Additional testing methods should be implemented to further evaluate failed droplet tests.*

Commencement of a droplet test on a concrete masonry unit (CMU) containing integral water repellent.

Commencement of a droplet test on a concrete masonry unit (CMU) containing integral water repellent.

After fi ve minutes, the originally beaded droplet has been partially absorbed into the CMU containing integral water repellent.

After five minutes, the originally beaded droplet has been partially absorbed into the CMU containing integral water repellent.

RILEM tube testing
The standard RILEM tube can hold 5 ml (0.17 oz.) of water, which correlates with the static pressure of a 158-kph (98-mph) wind-driven rain. The short RILEM tube was developed for porous materials that are unable to pass a standard RILEM test. A short RILEM tube (approximately 2 ml [0.06 oz.] of water) correlates with a 97-kph (60-mph) wind-driven rain.

Both RILEM tubes are plastic cylinders that are securely placed against the unit for testing using an impermeable putty. Once the RILEM tube is attached to the CMU, water is placed into the tube up to the 0 ml (0 oz) mark (top of tube). The RILEM tube is monitored at five-, 10-, 20-, 30-, and 60-minute intervals for any noticeable changes in the water column. Previous testing has shown specimens that hold water for 20 minutes will also typically hold for 60; this allows for shorter experiments. If 20 percent of the water is lost within a 20-minute interval, the CMU is considered to have failed the test—if such losses are not observed, then the CMU has passed.**

A standard RILEM tube is shown at the left CMU cell, while a short RILEM tube is shown at the right CMU cell.

A standard RILEM tube is shown at the left CMU cell, while a short RILEM tube is shown at the right CMU cell.

A standard RILEM tube test has failed on this CMU with integral water repellent.

A standard RILEM tube test has failed on this CMU with integral water repellent.

 

 

* See NCMA’s, Standard Test Methods for Water Stream and Water Droplet Tests of Concrete Masonry Units from 2009.
** See the article, “Testing the Test: Water Absorption with RILEM Tubes,” by Adrian Gerard Saldanha and Doris E. Eichburg in the August 2013 issue of The Construction Specifier. Visit www.constructionspecifier.com and select “Archives.”

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Durable Waterproofing for Concrete Masonry Walls: Redundancy Required

All images courtesy Building Diagnostics Inc.

All images courtesy Building Diagnostics Inc.

by Robert M. Chamra, EIT and Beth Anne Feero, EIT

Single-wythe concrete masonry walls are popular because they are inexpensive to construct, and combine structural support and cladding in one system. However, they can be associated with leakage when the waterproofing design is simplistic. A single-wythe wall can, and should, have multiple waterproofing components.1

Concrete masonry units (CMUs) are characteristically porous building materials. When manufactured in accordance with the industry standard, ASTM C90, Standard Specification for Load-bearing Concrete Masonry Units, commonly used lightweight CMUs absorb up to 17 percent of their weight in water.

CS_July_2014.inddThis porosity is due in part to their composition. The mix for the units contains the usual concrete components of water, cement, and aggregates, but that third component will be a smaller coarse aggregate (i.e. gravel) than cast-in-place concrete. The smaller aggregate decreases the workability of the mix if all other variables are held constant. In some cases, this decrease in workability is compensated by the addition of water to the mix. Similar to cast-in-place concrete, the higher the water-to-cement (w/c) ratio in the CMU mix, the higher the permeability of the units. However, even a good-quality mix will remain permeable (Figure 1).

Furthermore, the geographical location where the CMUs are manufactured affects permeability. The types of aggregate available in different regions varies, which results in mixes with identical proportions of components, but with much different absorption. For this reason, a prescriptive approach for waterproofing CMUs cannot be applied globally. The guidelines for methods of waterproofing remain the same, but the proportions of water repellents must be tailored for the available materials.

An additional factor affecting the porosity of CMUs is the unit-forming process. After the components have been combined, the mix is compacted and vibrated in molds. If properly compacted, a large volume of the interconnected pores within the unit is eliminated. If poorly compacted, the resulting interconnected pores can provide a path for water to migrate through the unit. Even if the overall unit is compacted, extremely porous localized pockets can remain, as demonstrated in the testing described in this article.

Similarly, a CMU containing cracks will be prone to moisture migration. The curing process CMUs undergo after forming will limit shrinkage cracking within the units, but it does not prevent all subsequent shrinkage—especially when CMUs are installed immediately after manufacturing (21 days of curing is recommended). In addition to drying shrinkage, creep (i.e. time-dependent deformation) can occur in concrete masonry walls after sustained loading.2 The resulting hairline cracks from these phenomena will provide routes for water through the unit.

CS_July_2014.inddIn addition to the units themselves, the mortar joints can provide water sources into a concrete masonry wall assembly. If the mortar loses the water it needs to complete curing—due to wind, sun, or suction from the CMUs—shrinkage cracks and separations between units and mortar will develop. Similar to the CMUs, the mortar will also undergo creep after sustained loading—up to five times as much as the CMUs—since the mortar is less stiff than the concrete.3

For waterproofing, cracks within the mortar are worse than cracks within the units, since it is common to have mortar only at the inside and outside faces of the masonry (i.e. face shell bedding). Then, water only has to travel the thickness of the unit wall, approximately 32 mm (1 1/4 in.) to penetrate the assembly (Figure 2).

Recommendations
National Concrete Masonry Association (NCMA) publishes technical articles to provide recommendations for the design and construction of concrete masonry. TEK 19-2B, Design for Dry Single-wythe Concrete Masonry Walls, outlines waterproofing strategies for single-wythe concrete masonry walls at the surface, within the CMU, and at the drainage path. NCMA recommends redundancy to protect concrete masonry from water penetration, including surface repellents or coatings, integral repellents (admixtures), and adequate drainage systems.4

Surface repellents for concrete masonry—typically silicones, silanes, and siloxanes—provide waterproofing at the exterior of the wall assembly. They are applied by a roller or spray equipment after the mortar has had an opportunity to cure. The product is absorbed into the units and mortar and coats the pores. While some products can penetrate deeper, most surface repellents remain within 12.7 mm (1/2 in.) of the CMU surface. In addition to their ability to repel water, surface repellents provide other benefits, such as reducing dirt and staining on the wall’s surface.

Split-face units, shown here being tested with a RILEM tube, are even more challenging to waterproof than smooth CMUs because of the fractured surface.

Split-face units, shown here being tested with a RILEM tube, are even more challenging to waterproof than smooth CMUs because of the fractured surface.

Surface repellents typically allow water vapor to be transferred in and out of the wall, and drying when water does penetrate the assembly through cracks or other penetrations.5 These products have varying ultraviolet (UV) resistance, but most need to be reapplied at intervals recommended by their manufacturers.6

Integral water repellents are available to be incorporated into CMUs as admixtures during manufacturing and into mortar during site mixing to limit water migration through the wall assembly. Since the mortar is mixed onsite and not in the unit plant, it is crucial masons also provide proper admixture quantity and mixing practices for the mortar to avoid a waterproofing weakness within the wall assembly. Integral water repellents also improve efflorescence control. Despite concerns with changes to the concrete’s properties, research has shown integral water repellents do not interfere with the assembly’s bond strength.7

Although it may seem counterintuitive, it is better to use mortar of lower strength to limit cracking.8 High-strength mortars are stiffer; they crack at a lower strain compared to low-strength mortars. Movement related to thermal and moisture changes, as well as foundation shifting, can cause cracking in strong and stiff wall assemblies. These cracks may not impair the wall’s structural performance, but all cracks add opportunities for water’s entry into the assembly.

The mortar’s installation can be as important to the mortar joints’ performance as the materials used. Proper tooling practices help protect concrete masonry walls from unwanted moisture penetration. Choosing a concave or V-joint mortar joint profile will push the mortar against the CMUs to improve bond and provide drainage when the assembly is wet. Raked joints decrease the bond between the CMU and mortar, and provide an area to trap water.9

CS_July_2014.inddIn addition to surface repellents or coatings and integral repellents, NCMA’s other primary recommendation is to provide adequate drainage systems for moisture penetrating the wall assembly. For ungrouted assemblies, through-wall flashing can be installed at bond-beams and floor slabs. Flashing is often eliminated in fully grouted walls to avoid severing the grout which makes it important to consider supplemental waterproofing measures.

These suggestions, along with other considerations found in TEK 19-2B, are given to help ensure moisture will not penetrate the masonry. Although CMUs are characteristically permeable, they can be used successfully in single-wythe walls by following NCMA’s recommendations. Since water penetration can come from various sources, the need for a careful and comprehensive waterproofing approach is essential to providing dry and durable concrete masonry construction.

Laboratory testing
Absorption testing of 24 lightweight CMUs was performed by the authors. Half the units contained an integral water repellent. An informal droplet test was performed initially on selected CMUs from each group; then, all the CMUs underwent a RILEM tube test.10 For additional information about these test methods, see “Field Testing Methods of Water Repellency.”

CS_July_2014.inddThe units tested were smooth-faced CMUs. Split-face blocks, with their more aesthetically appealing surfaces, would likely be even more porous because of the fracturing that creates the appearance (Figure 3).

Absorption testing
To comply with ASTM C90, CMUs must meet maximum absorption requirements dependent on the units—the denser the unit, the less absorption the standard allows. ASTM C140, Standard Test Methods for Sampling and Testing Concrete Masonry Units and Related Units, outlines the absorption testing procedures to comply with ASTM C90. Each CMU in this study underwent ASTM C140 absorption testing (Figure 4).

The addition of integral water repellent to the CMUs resulted in a 34 percent reduction in absorption (and nearly 50 percent less than allowed by ASTM C90). However, these low absorption values do not correlate with water penetration through the units; the low-absorption CMUs still allowed water to penetrate during water-spray testing. The authors believe this disconnect is a leading reason for leakage in single-wythe concrete masonry walls—the industry standards for the components address absorption, rather than water penetration.

Droplet testing
The CMUs without integral water repellent had droplet test results classified as ‘totally absorbed’—immediately after placing the droplet on the unit, the water was absorbed, but the surface remained slightly damp. For the units with the integral water repellent, the classification was ‘partially absorbed.’ Once the water was placed on the unit, some of the water was absorbed, but there was still partial beading and standing water remaining on the unit. After a five-minute period, most of the beaded water had absorbed into the units with integral water repellent and appeared the same as units without integral water repellent.

CS_July_2014.inddThese observations show an integral water repellent can aid in preventing water from penetrating into the unit. However, the integral water repellent was not impenetrable—some water made its way into the units during the droplet tests. More importantly, there was an extreme range of absorptions on the surface of individual CMUs, which indicates porous pockets of less consolidated concrete were present as described earlier (Figure 5).

RILEM tube testing
The second procedure conducted on the concrete masonry units was RILEM tube testing. When tested using a standard 5-ml (0.16-oz) tube, all 24 specimens failed. However, units containing an integral water repellent were able to hold the water column of a short RILEM tube test for more than 20 minutes with little to no reduction in the water level, thus passing the less-severe testing method.

The units without integral water repellent quickly failed even when tested with a short RILEM tube. In a matter of one to two seconds, the entire water column had been depleted, and significant water penetration could be seen in the unit surrounding the RILEM tube and putty. These results clearly indicate the necessity for CMUs to have deliberate waterproofing components to avoid catastrophic leakage.

Medium- or normal-weight CMUs would be expected to perform better than their lightweight counterparts because research indicates water repellents’ effectiveness correlates with concrete density. This is another reason for water ingress in single-wythe concrete masonry walls—the repellents most commonly employed are least effective on lightweight CMUs. In some regions, lightweight units dominate the market despite their poor water penetration performance. This point alone indicates the benefit of using redundant waterproofing components.

CS_July_2014.inddConclusion
Concrete masonry units are porous structural elements that need to be properly installed with appropriate components to prevent water infiltration in single-wythe exterior walls. High-quality CMUs and mortar (complying with ASTM standards), integral water repellents, and good design and construction practices (following NCMA recommendations) are important steps. However, these measures may not suffice.

Redundant waterproofing components are required because of the likelihood of cracks, mortar joint separations, and variable absorption characteristics in a single-wythe concrete masonry wall (Figure 6). The variability of available materials in a given region supports the need for tailoring the design to achieve the desired performance. Field testing during the construction phase is recommended to confirm performance. Even adding a surface-applied repellent will not stop water from migrating through cracks. An elastomeric wall coating should be considered for crack-bridging ability.11

Notes
1 The authors gratefully acknowledge the continuing support and leadership of David W. Fowler, PhD, PE—the faculty advisor for the research being performed at The Durability Lab, a testing center at The University of Texas at Austin. Also, the authors thank Featherlite Building Products for donating concrete masonry units for lab testing. (back to top)
2 For more, see Failure Mechanisms in Building Construction, edited by David H. Nicastro, PE (ASCE Press, 1994). (back to top)
3 See Note 2. (back to top)
4 See NCMA’s TEK 19-2B, Design for Dry Single-wythe Concrete Masonry Walls. (back to top)
5 See NCMA’s TEK 19-1, Water Repellents for Concrete Masonry Walls. (back to top)
6 See the article, “Testing the Test: Water Absorption with RILEM Tubes,” by Adrian Gerard Saldanha and Doris E. Eichburg in the August 2013 issue of The Construction Specifier. (back to top)
7 See NCMA TEK 19-7, Characteristics of Concrete Masonry Units with Integral Water Repellent. (back to top)
8 See Note 4. (back to top)
9 See Note 4. (back to top)

Robert M. Chamra, EIT, is a project engineer with Building Diagnostics Inc., specializing in the investigation of problems with existing buildings, designing remedies for those problems, and monitoring the construction of the remedies. He participates in the research being performed at The Durability Lab—a testing center established by Building Diagnostics at The University of Texas at Austin (UT). He can be reached by e-mail at rchamra@buildingdx.com.

Beth Anne Feero, EIT, is completing her master’s degree in architectural engineering at UT. She serves as the graduate research assistant for The Durability Lab, which researches and tests the durability of building components, identifying factors causing premature failure. She can be reached via e-mail at bfeero@buildingdx.com.

Designing Masonry Buildings to the 2012 Energy Code: Thermal Mass Basics

A material’s thermal mass denotes its ability to store heat within a cycle of time. K-values, generally calculated on a 24-hour cycle, are important because they give general references to a material’s capabilities for storing heat. All materials may be considered for use in a thermal mass calculation, but steel, aluminum, and other metal claddings tend to cycle too quickly, while wood tends to cycle too slowly to offer desirable design values.

Masonry—such as concrete masonry unit (CMU), stone, and brick—offers a good blend of characteristics for the thermal mass design based on several values. Storing heat well, the dense material can be designed with wall thicknesses that allow for normal window and door jamb details with reasonable per-area costs to construct.

In most cases, thermal mass should be measured on a cycle representative of both a typical heating and cooling cycle or a variable daily winter cold temperature cycle. While this is done for either season with the same principals, external factors contribute to the winter wall calculations in a more direct way. Building orientation, ceiling heights, lighting, solar heating, soffits, wall finishes, number of occupants, and usage round out a general list for design.

In colder climates, thermal mass is based on the function of interior heating cycling through the core of the wall. As the evening temperatures fall and the interior begins to feel cooler, warmth that was gained and stored during the daylight hours can then reverse the heat path and move back to the interior space of the building.

Summer cycles seem a bit clearer when explained, as the heat of the day penetrates toward the core of the wall. The term ‘decrement property’ takes into account the wall’s material density (e.g. concrete mix), final façade finishes, and exposure. The decrement factor dictates the speed at which the heat can be absorbed into the building. The design should stop the absorption of the heat before it alters the interior of the building’s cooler temperature and cycles the heat to the exterior of the structure as the afternoon temperatures begin to fall.

‘R-value’ has become a term familiar to even consumers, as it is listed on every insulation package in the home improvement stores. The general thought often reduces this metric’s significance to ‘the higher the R-value, the better the product when placed in the wall.’ However, as a unit of thermal resistance, R-value is the conduction rate of heat flow through a combination of materials comprising a wall. Mass-enhanced R-value walls are a combination of thermal mass walls and use of materials that offer high resistance to heat flow. They are extremely useful in climates where the external building temperatures rise well above and fall well below the interior space daily temperatures.

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Designing Masonry Buildings to the 2012 Energy Code

All images courtesy Mortar Net Solutions

All images courtesy Mortar Net Solutions

by Steven Fechino

The 2012 International Energy Conservation Code (IECC) will bring tremendous change to the way buildings are designed, constructed, and renovated. Several of the code’s changes have already been implemented throughout the industry, with many of the currently specified systems and products meeting these new codes. However, there are also materials and assemblies that will need to evolve to remain compliant.

For instance, HVAC systems will require improvements to the mechanical systems and ductwork. Window glazing is becoming more energy-efficient to meet ever-tightening performance criteria. Further, there is this author’s focus—the insulation requirements for masonry construction have been written to higher performance levels. There are many rumors about how the changes will limit the available products with which masonry structures can be built and designed. This article will address some of those rumors by providing simple explanations of the code and some helpful insight into how the industry is dealing with the changes on a positive level.

This diagram shows the elements of a cavity wall This particular assembly includes an insect barrier and mortar-dropping-collection device.

This diagram shows the elements of a cavity wall This particular assembly includes an insect barrier and mortar-dropping-collection device.

The 2012 energy code has been adopted state by state, and jurisdiction by jurisdiction, so the changes have not been applied uniformly across the country.1 Nevertheless, the updated IECC is important to all design/construction professionals, because it is a positive step toward reducing the country’s energy consumption through the design, construction, and operation of more efficient structures. It is important to adapt to these changes as soon as possible, since all regions will likely be affected by the code sooner or later. Preparing now will make the eventual transition to the new energy standards much easier.

CMUs and continuous insulation
The prescriptive energy code for the masonry industry is based primarily on the requirement for continuous insulation (ci) within the wall envelope. This becomes an issue when one looks at the standard concrete masonry unit (CMU)—the cross-webs prevent continuous insulation within the block because they allow thermal bridging. By reducing the cross-web dimension, thermal bridging is reduced and the thermal efficiency of the unit is increased. However, this, in itself, is not the solution to code compliance. It is important to look at all the compliance criteria.

In some cases, a CMU assembly’s mass and resistance to heat transfer (i.e. R-value) are all that is necessary to meet the code, but only in warmer climates. Differing temperature conditions means various types of insulation are used in designing the many single-wythe and cavity wall systems specified across the country. Rigid insulation, foam inserts, dry loose fill, injected foam, spray-on foam, and proprietary block design round out the field of techniques for increasing R-value, with typical gains of 5 to 25.

An important factor for determining energy efficiency of a CMU wall is the envelope’s design, specifications, and the materials making up the assembly—various concrete masonry manufacturers will have similar, but ultimately different, mixes. This is one factor that can change a CMU wall’s R-value and the thermal mass performance of otherwise similar envelopes. (For more, see “Thermal Mass Basics.”)

Other factors include geographical climate history, insulation specifications (within either the CMU or the cavity), and the actual cross-section of the masonry units comprising the wall design. For assistance with this, the National Concrete Masonry Association’s (NCMA’s) TEK 6-2B, R-Values and U-Factors of Single-wythe Concrete Masonry Walls, discusses thermal performance of a CMU wall and its thermal properties based on material properties.2

This church was built with masonry cavity walls and brick veneer. An important factor for determining such a wall’s energy effi ciency is the envelope’s design, specifi cations, and the materials making up the assembly.

This church was built with masonry cavity
walls and brick veneer. An important factor
for determining such a wall’s energy efficiency
is the envelope’s design, specifications, and the materials making up the assembly.

The three paths to compliance
It is important to clear up the rumor that all masonry walls will require continuous insulation in order to meet the new standards. There are many ways a designer can achieve compliance using complete building systems to meet the new IECC requirements rather than relying solely on continuous insulation.

Right now, there are three methods available for determining code compliance, and many of the current masonry designs will show acceptable numbers in at least one of them. These methods are:

  • prescriptive compliance;
  • compliant software (also known as ‘performance method’); and
  • whole building analysis.

Prescriptive method
The prescriptive method uses a series of material or assembly requirements to meet compliance. For example, designers can employ tabulated values for mass walls that specify requirements for continuous insulation to determine compliance. This is the method most manufacturers and designers use today. Many of the products and systems on the market gain compliance through this path.

However, this prescriptive method may not be part of the next energy code in 2015, so it is important to keep an open mind to developing newer technologies and improvements to existing systems for future compliance to the code. Using the prescriptive tables is easy and straightforward, but this method also limits design flexibility and makes some masonry wall types difficult or impractical to build.

Performance method
The performance or compliant software method uses computer programs developed specifically to determine whether an assembly meets the code. There are two popular programs: the American Society of Heating, Refrigerating, and Air-conditioning Engineers’ (ASHRAE’s) EnvStd and the U.S. Department of Energy’s (DOE’s) COMcheck.3 Though the programs differ in their capabilities, they can both offer the designer thermal property constants for various masonry wall configurations. Depending on which part of the energy code the designer needs to meet, these programs can offer wall configurations that meet prevailing codes and which also comply with IECC in many cases.

COMcheck is a bit more complex to use than EnvStd, but it offers options to modify many components within the structure that can then be compiled to achieve compliance, offering the design community the ability to use the products they know how to bid, construct, and sell in energy-efficient buildings. If the designer compiles all the information about the project and compliance is not achieved, he or she can adjust various individual properties of the building envelope to meet the code requirements. This method allows more design flexibility because the designer can test how multiple building components interact to achieve compliance.

Whole building analysis
Whole building analysis is not yet widely used. However, it will likely be the prevailing method in the future because it takes into account everything about the building, and can produce accurate guidelines for the most energy-efficient sources.

This method can analyze annual total energy use rather than individual component compliance. It demonstrates when new design methods can reduce energy costs as compared to standard building methods. The whole building method not only takes into account the various wall types, but also includes entire building envelope information, plus mechanical and lighting specifications to determine compliance.

At left, a mason ‘butters’ a brick with mortar for installation in a masonry cavity wall. In the photo on the right, one can see the detail of a masonry cavity wall comprising a concrete-unit structural wall and brick veneer.

At left, a mason ‘butters’ a brick with mortar for installation in a masonry cavity wall. In the photo on the right, one can see the detail of a masonry cavity wall comprising a concrete-unit structural wall and brick veneer.

iStock_000013773064Medium

Other changes
Beyond IECC, there are a couple of other standards that have recently undergone changes of which those working with masonry design should be aware. ASTM C90-11b, Hollow Load-bearing Concrete Masonry Units, for example, allows cross-web configurations to regulate by cross-sectional area, not by web thickness. The reasons for paying close attention to this change include:

  • R-values may be increased,4 while structural characteristics and performance will not change;
  • a reduction in cost may be achieved (i.e. using less material in each block); and
  • less demand for materials to produce the units, which reduces energy costs for manufacture and transportation.

Another important change is associated with National Fire Protection Association (NFPA) 285-12, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components. Language in the next energy code will exempt this NFPA test when:

  • material flame indexes are met to published standards (cited below); and
  • air spaces that contain insulation are kept to 25 mm (1 in.) or less.

The new language approved for inclusion in the code that permits exclusion refers to:

Envelopes where rigid or spray-applied insulation is encased by at least one inch of masonry, and there is no gap between the insulation and the masonry; or the insulation and the CMU are not separated by an air space greater than one inch, and the insulation has an index for flame rate meeting requirements of ASTM E84 [Standard Test Method for Surface Burning Characteristics of Building Materials] or [Underwriters Laboratories] UL 723 [Test for Surface Burning Characteristics of Building Materials].

Conclusion
Change is coming to the building industry, driven by a need for far more efficient energy use in the built environment. Masonry has many qualities that make it an ideal building material for energy-efficient construction, including its thermal mass, sustainability, high level of availability, and design flexibility. A combination of new building materials, a better understanding of building dynamics, and improved design software is making it possible for designers to create masonry buildings that meet the new energy codes; skilled masons will be key to making these energy-efficient buildings a reality.

Notes
1 To determine how the new IECC will affect a particular project, visit the U.S. Department of Energy (DOE) website at www.energycodes.gov. (back to top)
2 Visit www.ncma.org/etek/Pages/Manualviewer.aspx?filename=TEK%2006-02B.pdf. (back to top)
3 Visit www.ashrae.org/resources–publications/publication-updates/standard-90-1-users-manual-software-envstd-4-0 and www.energycodes.gov/comcheck, respectively. (back to top)
4 R-values may be increased because of a reduction in thermal bridging via the cross-webs and additional space for insulation. (back to top)

Steven Fechino is the engineering and construction manager for Mortar Net Solutions. He provides engineering support services and product training. Fechino has a bachelor’s of science degree in civil engineering technology and two associate degrees in civil engineering and drafting and drafting and design specializing in building construction. He can be contacted at sfechino@mortarnet.com.

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Selecting Joint Reinforcement

Photo courtesy Neumann/Smith Architecture

Photo courtesy Neumann/Smith Architecture

by Dan Zechmeister, PE, FASTM, and Jeff Snyder, MBA

In a time of increasingly complex building enclosure systems, the masonry industry is striving to rediscover the simplistic principles that have made it a frequent material choice throughout history. One of these is the ‘less-is-more’ principle, which holds true when it comes to selecting wire reinforcement for reinforced masonry wall systems.

Standard 9-gauge (MW11), ladder-shaped wire fabricated with butt-welded cross-rods spaced 406-mm (16-in.) on center (oc) better facilitates structurally required rebar placement, grout flow and consolidation, and shrinkage control for concrete masonry unit (CMU) walls. To understand why, it is important to know the history and rational behind horizontal joint reinforcement.

According to the National Concrete Masonry Association (NCMA) TEK 12-2B (2005), Joint Reinforcement for Concrete Masonry, CMU joint reinforcement was “initially conceived primarily to control wall cracking associated with horizontal thermal or moisture shrinkage or expansion and as an alternative to masonry headers when tying masonry wythes together.” The TEK note goes on to state it “also increases a wall’s resistance to horizontal bending, but is not widely recognized by the model building codes for structural purposes.”

The most dramatic design change in single and multi-wythe masonry walls since wire reinforcement became the norm in the 1960s was the shift to vertical and horizontal steel reinforcement (rebar) in CMU in the 1990s. This encompassed all of North America’s unreinforced markets, not just seismic zones.

According to Table 2 in NCMA TEK 10-3 (2003), Control Joints for Concrete Masonry Walls−Alternative Engineered Method (“Maximum Spacing of Horizontal Reinforcement to Meet the Criteria As > 0.0007 An”), for ungrouted or partially grouted walls, vertical spacing of wire is 406-mm (16-in.) oc for 203- and 305-mm (8- and 12-in.) CMU block. Additionally, Table 2 states the 406-mm (16-in.) spacing applies to 9-gauge (MW11) wire with two wires (one wire per face shell of the block). A CMU wall without frequently spaced vertical rebar(s) and corresponding bond beam(s) with rebar encapsulated in grout is rare.

Ladder-shaped wire promotes code required rebar centering.  [CREDIT] Images courtesy John Maniatis

Ladder-shaped wire promotes code required rebar centering. Images courtesy John Maniatis

Truss-shaped wire interferes with code-required rebar centering.

Truss-shaped wire interferes with code-required rebar centering.

 

Truss versus ladder
Horizontal joint reinforcement has evolved quite a bit over the decades. In the beginning, the truss shape was the norm for unreinforced masonry walls. As NCMA TEK 12-2B implies, the truss shape offered some resistance to wall spanning in the horizontal direction because of three wires—two longitudinal and one diagonal. However, since most masonry walls are now, typically, designed to span in the vertical direction, steel rebar and grout are placed vertically.

Rebar placement
When structural engineers design reinforced masonry, they typically call for the vertical bar to be placed in the center of block cells. In Articles 3.4 B.11.a & b, the 2013 Masonry Standard Joint Committee (MSJC) Building Code Requirements and Specification for Masonry Structures,requires placement tolerance for vertical rebar to be ± 12.7 mm (½ in.) across the width of the block, and ± 50.8 mm (2 in.) along the length of the block, measured from the center of the block cell.

Shape matters
Ladder-shaped wire has perpendicular cross-rods butt-welded at 406-mm (16-in.) oc to the longitudinal wires. It is placed with cross-rods centered directly over the webs of the block (Figure 1). Placement of ladder wire in this manner eliminates obstructions caused by diagonal cross-rods common with the truss shape, especially where block cells are designed to contain vertical bars (Figure 2).

Grout flow
Another advantage of ladder-shaped wire is evident when grout is placed and consolidated. The absence of diagonal (truss) cross-wires improves the flow and consolidation of grout. Under Articles 3.43 B.4.d, the MSJC Code typically requires CMU block (i.e. hollow units) to be placed so vertical cells to be grouted are aligned. This provides an unobstructed path for grout flow. According to NCMA TEK 12-2B, “Because the diagonal cross wires may interfere with the placement of vertical reinforcing steel and grout, truss-type joint reinforcement should not be used in reinforced or grouted walls.”

Shrinkage control
Ladder-shaped wire placed with cross-rods centered directly over block webs has yet another distinctive advantage. It positions butt-welded T-intersections of each longitudinal wire with cross-rods directly over T-intersections where block face shells meet each web. When laid in running bond pattern, two-cell block are placed with face shell mortar bedding only. Block webs are only mortar-bedded adjacent to vertically reinforced cells.

Face-shell mortar bedding will extrude at the webs when compressed during block placement, completely encapsulating the wire T-intersections, bonding the wire to the concrete masonry (Figure 3). Hence, the net result should be improved shrinkage crack control.

Ladder-shaped wire improves shrinkage control.

Ladder-shaped wire improves shrinkage control.

Heavy-duty 4.8-mm (3/16-in.) diameter wire leaves inadequate room for mortar coverage.

Heavy-duty 4.8-mm (3/16-in.) diameter wire leaves inadequate room for mortar coverage.

Standard 9-gauge versus heavy-duty 3/16
Besides its shape (i.e. truss or ladder), wire thickness is important in the placement process. The most common mortar joint thickness specified is 9.5 mm (3/8 in.). The largest diameter of wire allowed by Section 6.1.2.3 of MSJC Code would be half the mortar joint thickness—4.8 mm (3/16 in.). There are compelling reasons why the use of 9-gauge wire (i.e. 3.8 mm [0.148 in.) is more appropriate than a larger wire size that is heavy-duty (i.e. 4.8 mm [3/16 in.]).

Placement tolerances
The MSJC Code tolerance for the placement of the mortar bed joint thickness is ± 3.2 mm (1/8 in.), as clarified in Article 3.3 F. 1. b. Therefore, a specified mortar joint of 9.5 mm (3/8 in.) would be allowed to vary from 12.7 to 6.4 mm (½ to ¼ in.) in thickness. With an as-built mortar joint thickness of ¼ to 3/8 inch, using heavy-duty 3/16-in. wire with hot-dip galvanized coating (per MSJC Code Section 6.1.4.2), would leave inadequate room for mortar cover to encapsulate the wire (Figure 4). Quite literally, block could be placed directly on the wire (i.e. block on wire on block).

In an article in the January 1995 issue of Masonry Construction magazine, “Selecting the Right Joint Reinforcement for the Job,” author Mario J. Catani states:

One compelling reason to use 9-gauge reinforcement is for fit and constructability. While the code allows joint reinforcement to have a diameter one half the mortar joint width, the tolerances allowed for units, joints and the wire itself can hinder the placement of large diameter reinforcement. Use it only when there is no other choice.

Forming corners
There is some debate regarding the merits of ordering factory prefabricated inside and outside corners versus field-forming them onsite. Since the MSJC Code does not distinguish the merits of either method (and, indeed, barely recognizes them), some interpretation is necessary.

The standard for lapping wire reinforcement at any location is always the same—it requires a 152 mm (6 in.) minimum whether lapping straight 3.1-m (10-ft) sections one to another or where a straight section meets a corner (per Article 3.4 B.10.b). This requirement can also be applied to field-formed corners. The inside longitudinal wire can be cut and bent to form a 90-degree angle with a minimum of 152 mm (6 in.) of lap paralleling the newly formed inside longitudinal wire (Figure 5).

Factory-prefabricated corners may seem like the natural choice, but this can require additional lead time and cost for any size or configuration other than standard (8- or 12-in.) two-wire reinforcement. This is especially the case for custom-made adjustable hook and eye configurations.

Field-formed corners have many advantages. They meet all MSJC Code requirements and are easily formed to fit any corner condition. Each leg can be formed to fit to length, plus lapped in each direction off a corner, minimizing wasteful leftover from 3.1-m lengths that would otherwise head to a landfill. Field-formed corners eliminate lead time, cost less per lineal foot than factory-fabricated pieces, and only take a minute to cut and form to fit at the work station.

This shows a simple three-step sequence to field form corners.

This shows a simple three-step sequence to field form corners.

Code approved mesh ties are safe, economical, and readily available.  [CREDIT] Image courtesy Matt Fowler

Code approved mesh ties are safe, economical, and readily available. Image courtesy Matt Fowler

 

 

 

 

 

 

 

 

 

Intersecting walls
MSJC Code allows prefabricated T-horizontal wire reinforcement sections where an interior non-loadbearing masonry wall intersects another for lateral support. However, this may not be the best selection. Such T-sections are typically embedded 406-mm (16-in.) on center during construction in the longitudinal wall, leaving the projecting leg of the T-section extending out approximately 609-mm (24 in.) until the intersecting wall is constructed.

Many masons will agree the exposed wire sections can be dangerous onsite, especially at eye height. Fortunately, MSJC Code also allows 6.3-mm (1/4-in.) mesh galvanized hardware cloth for interior non-loadbearing interesting walls (Figure 6). Additionally, MSJC Code allows Z-strap anchors for walls that intersect where shear transfer is desired. Projecting Z-straps share similar safety concerns with exposed T-sections. They only need to be used where the structural engineer indicates shear transfer. When applicable, mesh ties are typically the best choice. They are readily available, simple, and economical to install, and can be safely bent out of the way until the intersecting wall reaches their height.

Finish options
The two most common finishes for wire reinforcement are mill galvanized and hot-dip galvanized. The first category is allowed by the MSJC Code for most interior applications not in contact with moisture or high humidity. These standard mill galvanized finishes are produced through electro-galvanization—a process where a layer of zinc is bonded to steel when a current of electricity is run through a saline/zinc solution with a zinc anode and steel conductor. This process is undertaken when wire is in its raw state, before fabrication (i.e. cut and welded to shape) into wire reinforcement.

This guide outlines joint reinforcement selection. [CREDIT] Image courtesy Masonry Institute of Michigan

This guide outlines joint reinforcement selection. Image courtesy Masonry Institute of Michigan

Hot-dip galvanization is required for all exterior applications, as well as any interior walls exposed to moisture or high humidity. It is a process of coating steel with a heavy layer by immersing it in a bath of molten zinc. This process is undertaken after wire is fabricated to form reinforcement.

Myriad advantages
Unfortunately, not all who design or specify wire reinforcement have kept pace with the shift to reinforced CMU. There are many pockets of the country where antiquated truss shape and/ or heavy-duty wire are still in use. Figure 7 reviews the advantages and disadvantages of ladder and truss shapes, along with standard 9-gauge versus heavy-duty wire reinforcement.

Additionally, ladder-shaped wire with 9-gauge side- and cross-rods has other advantages including lower production, packaging, and shipping costs. Lighter bundle weight reduces risk of back injury when they are handled on the jobsite. Ladder configuration also streamlines wire, rebar, and grout installation—this, in turn, enhances bricklayer productivity.

Specification
The following, and Figure 8, provides an example of the recommended wording for horizontal joint reinforcement in single and multi-wythe masonry walls:

PART 2 PRODUCTS
2.1 Masonry Reinforcing
A. Joint Reinforcement, General: ASTM A 961
1. Interior Walls: Mill galvanized, ASTM A 641 (0.10 ounces per square foot), carbon steel.
2. Exterior Walls: Hot-dip galvanized, ASTM A 153 Class B-2 (1.50 ounces per square foot) carbon steel.
3. Interior Walls Exposed to High Humidity: Hot dip galvanized, ASTM A 153 Class B-2 (1.50 ounces per square foot) carbon steel).
4. Wire Size and Side Rods: W1.7 or 0.148 inch diameter (9 gage).
5. Wire Size and Cross Rods: W1.7 or 0.148 inch diameter (9 gage).
6. Wire Size for Veneer Ties: W2.8 or 0.1875 inch diameter (3/16 inch).
7. Spacing for Cross Rods: 16 inches on center
8. Provide in lengths of 10 feet.

  • B. Masonry Joint Reinforcement for Single-Wythe Masonry: Ladder type with single pair of side rods.
  • C. Masonry Joint Reinforcement for Multi-Wythe Masonry: Ladder type with adjustable (two-piece) design, with separate double eye butt welded to side rod 16 inches on center. Double hook ties that engage eyes welded to reinforcement and resist movement perpendicular to wall. Hook tie length shall be sufficient to extend 1/2 inch minimum into outer face shell for hollow units and 1-1/2 inch minimum into solid units, but with a minimum 5/8 inch cover at outside face.

 

Ladder-shaped wire, code required minimum lap, and butt-welded adjustable eye options are shown here. Images courtesy John Maniatis

Ladder-shaped wire, code required minimum lap, and butt-welded adjustable eye options are shown here. Image courtesy John Maniatis

Conclusion
To control potential shrinkage cracking in a concrete masonry wall, it requires proper placement of control joints (CJs), along with placement of horizontal joint reinforcement. Horizontal joint reinforcement in a CMU wall does not prevent cracking, but controls it. Without it in a concrete masonry wall, shrinkage cracks may be visible and of a size penetrable by Mother Nature.

With 9-gauge ladder-shaped joint reinforcement in a concrete masonry wall, the longitudinal wire will go into tension as the concrete masonry is shrinking. Hence, an occasional microscopic crack should not be noticeable and would be less vulnerable to the elements. Use of truss-shaped wire does not meet code compliance and may negatively impact the integrity of a reinforced concrete masonry wall.

When it comes to masonry wire reinforcement, the old adage ‘less is more’ could not be any more true. Ladder-shaped wire, fabricated in 3.1-m (10-ft) long pieces with 9-gauge continuous side-rods and butt-welded 9-gauge cross-rods spaced 406-mm (16-in.) oc is the ideal choice for high-performance, economically designed CMU wall systems.

Dan Zechmeister, PE, FASTM, has been the executive director and structural services director of the Masonry Institute of Michigan (MIM) since 1986. He is an active member of ASTM, and a 2012 recipient of its International Award of Merit. Zechmeister also serves as a board member of the American Institute of Architects (AIA) Building Enclosure Council of Greater Detroit. He can be contacted at dan@masonryinfo.org.

Jeff Snyder, MBA, is the president of Masonpro Inc., a provider of specialty accessories to unit mason contractors. He has extensive field experience, including project managing for mason contractors in Texas and New Mexico. Snyder is a trustee with MIM, serving on its Generic Wall Design Committee. He can be contacted at jeff@masonpro.com.