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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.”

To read the full article, click here.

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.

Troubleshooting Exterior Masonry Walls

Photo © BigStockPhoto/Ronald Hudson

Photo © BigStockPhoto/Ronald Hudson

by Michael Gurevich

In a sense, all buildings are alive, and they mainly breathe and move through their exterior walls. If a design/construction professional tries to restrain breathing or movements of the exterior walls, then side effects should be expected. Therefore, when a new building is designed and constructed, the brick veneer expansion joints should be provided to accommodate the movement in the brick veneer.

Volume Changes: Analysis and Effects of Movement, and 18A, Accommodating Expansion of Brickwork, with recommendations for the brick veneer wall system. It advises vertical expansion joints in the brick veneer be located approximately 7.6 m (25 ft) apart in addition to the joints at each side of the exterior corner within 1.5 m (5 ft) from the corner.1

Clay brick veneer of the masonry walls normally would have a moisture expansion and a thermal expansion/contraction, which should be absorbed by the brick veneer expansion joints. For example, the moisture expansion of a 12-m (40-ft) long or high brick veneer panel could be calculated with the BIA formula:

0.0005 x L
= 0.0005 x 40 ft x 12 in.
= approximately 6.4 mm (1/4 in.).

The moisture expansion behavior of the clay brick primarily depends on the raw materials and secondarily on the firing temperatures. Additionally, the clay brick’s moisture expansion is an irreversible process with most expansion taking place during the first months of manufacture, but expansion will continue at a much lower rate for several years.

A vertical crack developed in the brick veneer in the middle of this large, first-floor window. It began at the window head shelf angle as a wide, open crack, and disappears into a hairline above.

A vertical crack developed in the brick veneer in the middle of this large, first-floor window. It began at the window head shelf angle as a wide, open crack, and disappears into a hairline above. Photos courtesy New York City Brickwork Design Center

Dealing with cracks
In this hypothetical example, crack development in the brick veneer was the evidence the expansion joints were not adequately provided to absorb the brick veneer movements. For example, with restoration projects of exterior masonry walls, some design professionals are trying to cut vertical expansion joints in 30-year-old brick veneer at 7.6 m (25 ft) apart with additional joints at each side of the corner. However, the problem is the moisture expansion in this brick veneer happened some three decades ago, and Mother Nature has provided the brick veneer with cracks (i.e. natural expansion joints). In other words, the cracks can be replaced with expansion joints, but one should not cut them at 7.6 m apart. (The aforementioned BIA recommendations are mainly for new construction.)

American Concrete Institute (ACI) 530/American Society of Civil Engineers (ASCE) 5/The Masonry Society (TMS) 402, Building Code Requirements for Masonry Structures, has been adopted into the International Building Code (IBC). Chapter 6, “Anchored Veneer,” of the standard has the following requirements for brick veneer:

The horizontally spanning element supporting the masonry veneer shall be designed so that deflection due to dead plus live loads does not exceed L/600.

In Figure 1, a vertical crack had developed in the brick veneer in the middle of the large first-floor window. It started at the window head shelf angle as a wide, open crack, and had disappeared as a hairline crack above. This serves as a perfect example of the spandrel beam excessive deflection, which was translated into the brick veneer as the vertical crack.

The problem is this building was designed in the early 1980s, when spandrel beam deflection had limits of L/360. To fix this problem, one must reinforce the spandrel beam, which would be an expensive proposition. Another option would be to treat the crack as a movement joint, which means to brace the brick veneer on each side with masonry restoration anchors before sealing the fissure.

Another option would be to reinforce the brick veneer with helical stainless steel rods—a technique popular in the United Kingdom with landmark building renovations. Horizontal mortar joints should be raked for a minimum depth of 25 mm (1 in.), and be extended for approximately 406 mm (16 in.) on each side of the crack. The rods should be embedded into the raked joint with a soft grout; they should be located every three to six brick courses apart to reinforce the veneer.

Other examples
In Figure 2, one can see the horizontal crack in the parapet brick wall mortar joints at the roof level with approximately 6.4 to 9.5-mm (¼ to 3/8-in.) vertical displacement in the crack. In this case, the author observed a vertical crack, which started as a hairline at the horizontal crack location, and was wide open at the top of the parapet wall. It looks as if some forces had pushed brick up from the horizontal crack location to the top of the wall. Rust had most likely developed at the top of the steel spandrel beam.

This horizontal crack in the parapet brick wall mortar joints at the roof level resulted in approximately 6.4 to 9.5-mm (¼ to 3/8-in.) vertical displacement.

This horizontal crack in the parapet brick wall mortar joints at the roof level resulted in approximately 6.4 to 9.5-mm (¼ to 3/8-in.) vertical displacement.

This author’s team opened the wall to expose the top of the steel beam, and observed the rust—which had delaminated and expanded up to 9.5 mm—built up atop the beam. Steel beam rust could expand up to 10 times, creating tremendous forces, which pushed the masonry wall up—this created horizontal and vertical cracks in the masonry walls.

This is not, it must be made clear, a ‘masonry wall problem.’ When this building was constructed in the 1920s, no waterproofing was installed at the spandrel beam. Today, one must remove the masonry wall to expose the steel beam for the structural engineer’s evaluation and reinforcement (when necessary). Then, the masonry wall needs to be rebuilt with the steel beam waterproofing and a drainage system installation.

A similar problem can be seen in Figure 3. Multiple cracks had developed at the brick parapet wall located at the exterior corner. Cracked masonry was removed from the wall, and this author observed badly rusted steel beams. About 3.2 mm (1/8 in.) was lost from the steel beam web, given rust expanded eight times. It means rust had expanded for 25 mm (1 in.) and then forced masonry up or out for another 25 mm, developing multiple cracks in the masonry walls.

Multiple cracks had developed at the brick parapet wall located at the exterior corner. Cracked masonry was removed from the wall; badly rusted steel beams were observed.

Multiple cracks had developed at the brick parapet wall located at the exterior corner. Cracked masonry was removed from the wall; badly rusted steel beams were observed.

In Figure 4, one can see a building partially one and two stories. The photo is looking at the ‘low roof,’ toward the brick veneer exterior corner. A vertical crack had developed in the brick veneer at the corner’s roof side, which was located approximately 102 mm (4 in.) from the corner. This author observed vertical displacement of approximately 4.8 mm (3/16 in.) at this crack.

The problem is the brick veneer located at the roof side of the corner was supported by the roof spandrel beam and did not move. The brick veneer facing the street was supported by foundation 5.5 m (18 ft) below. When the brick veneer facing the street had reached the low roof level, it had irreversible moisture expansion and the thermal expansion, which shifted this part of the brick veneer from foundation up to the low roof level for approximately 4.8 mm.

Differential movement of the brick veneer supported by various elements at the different levels had caused this crack. What is the remedy? This vertical crack could be treated as an expansion joint. Brick veneer on each side of the crack became unbraced, and should be anchored to the wall backup system with masonry restoration ties located within 203 mm (8

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in.) from the crack on each side of the crack. The crack should be cut straight, and filled with backer rod and sealant.

A vertical crack had developed in the brick veneer at this corner’s roof side. Vertical displacement of approximately 4.8 mm (3/16 in.) was noted at this crack.

A vertical crack had developed in the brick veneer at this corner’s roof side. Vertical displacement of approximately 4.8 mm (3/16 in.) was noted at this crack.

The low roof parapet had a vertical expansion joint located between the end of the parapet wall and the brick veneer facing the roof. It was the proper location of the vertical expansion joint, but the author observed the end of the parapet wall had leaned toward the roof with the horizontal displacement of approximately 19 mm (¾ in.) from the brick veneer facing the street.

The problem was the end of the parapet wall became unbraced at the vertical expansion joint, and the brick veneer facing the street had expanded up from the foundation 5.5 m (18 ft) below toward the top of the parapet wall. This caused the unbraced end of the parapet wall shifting towards the roof.

The original design/construction should be providing the vertical rebar in the parapet wall backup system to reinforce the unbraced end of the parapet wall. The structural engineer should evaluate this problem; he or she will call for the parapet wall reinforcement if necessary.

Conclusion
For new projects with brick veneer cavity wall systems, architects and engineers should provide the vertical and horizontal expansion joints following the BIA recommendations. Before beginning the exterior wall assembly, design/construction professionals should call for a preconstruction meeting to verify locations and size of brick veneer expansion joints.

Existing building cracks in brick masonry walls should go through the process of design/construction evaluation and diagnostics. Proper rehabilitation techniques must be employed to replace cracks with the expansion joints. After all, it is important to remember expansion joints are essentially just ‘pre-cracking’ the veneer in the proper locations.

Notes
1 Visit www.gobrick.com/Technical-Notes. (back to top)

Michael Gurevich is a masonry consultant at the New York City Brickwork Design Center (NYCBDC), which conducts free seminars on a variety of topics—such as brick veneer metal stud backup exterior walls—for CSI and American Institute of Architects (AIA) chapters. He has 25 years of experience working with exterior masonry walls. Gurevich holds a master’s degree in structural engineering from Belarussian State Polytech University in Minsk. He can be contacted via e-mail at nycbdc@aol.com.

Preserving 10 Light Street’s Exterior Façade with Restoration

Photos courtesy RMF Engineering

Photos courtesy RMF Engineering

by John Hovermale, PE

One of Baltimore’s most visible and recognizable buildings is located at 10 Light Street, close to the waterfront. The 34-story structure is considered to be the first skyscraper in the city; it has remained its tallest building for nearly 50 years.

The construction of the tower started in July 1928 and was completed in 15 months. This is a remarkable feat given its magnitude—approximately 46,450 m2 (500,000 sf) and standing taller than 152 m (500 ft). The building also showcases a high level of architectural detail on its interior and exterior design. It is one of the historic gems of the Maryland Historic Trust, the City of Baltimore’s Commission for Historic Architectural Preservation (CHAP), and Preservation Maryland.

While the designer, Taylor & Fisher-Smith & May, and the builder, J. Henry Miller, created the iconic tower to last forever, age and natural elements necessitated efforts to reinforce and renew the exterior features so it could remain a prominent fixture in the commercial real estate market.

Baltimore’s skyscraper
The 10 Light Street structure has a steel skeleton frame with masonry wall construction comprising terracotta backup faced with brick and limestone. The floor slabs consist of a terracotta flat arch system common to the period. The structure type is known as a ‘transitional façade,’ which was often seen in the United States from the 1890s to the mid-1950s.

One of the several limestone medallions at the entrance – each medallion has a unique carving.

One of the several limestone medallions at the entrance – each medallion has a unique carving.

When structural steel became a common building material in the early 1900s, it allowed building structures to reach new heights. Transitional structures bridged the gap between those with load-bearing masonry barrier walls that preceded them, and the curtain walls common today. In transitional façades, the floor slabs and masonry walls encase the structural steel at the building’s perimeter. Generally, structural steel buildings, particularly buildings of several stories, tend to be flexible and move when subjected to lateral loading. The steel frame and masonry were not detailed to accommodate the differential movement. Additionally, traditional façades were not always properly designed to resist moisture infiltration.

The owner of 10 Light Street noticed cracks and dislocated masonry in the façade and knew if those issues were not addressed, they could progress, presenting a higher risk to public safety and resulting in expensive future repairs.

Assessing the damage
Baltimore-based RMF Engineering completed a detailed study of the façade to assess the project’s magnitude and set budgets. The condition assessment employed 29 swing stage drops providing close access to the façade for inspection. (Given the vertical nature of the work, ‘drop’ is the term used to describe the wall area accessible from a mast climber or swing stage.)

Not every square foot of the façade was inspected during the study; however, enough drops were completed to provide an assessment at each unique structural and architectural building detail, enabling reliable extrapolation of the data for those areas not inspected during the study.

Detail from the original architectural drawings. Image courtesy Taylor & Fisher-Smith & May

Detail from the original architectural drawings. Image courtesy Taylor & Fisher-Smith & May

RMF provided construction documents for the restoration work and followed through project completion as the construction inspector. More than 325 m2 (3500 sf) of the original Italian marble was replaced. Approximately 122 m (400 ft) of helical joint reinforcement was installed at vertical cracks in the brick, and 6800 helical pins were used to restore the limestone. Further, more than 4645 m2 (50,000 sf) of spot tuck pointing was completed, in addition to brick replacement, relief angle replacement, and structural steel reinforcement. The restoration was successfully completed with more than 35,000 hours of labor and no injuries.

The construction budget was set by the owner based on the condition assessment report. The delivery method was guaranteed maximum price (GMP). Although there was a detailed study, the inherent risks and unknowns related to façade restoration work remained. The GMP approach provided for better cost management where funds can be focused on the most critical repairs, as well as shifting funds from low-to high-priority repairs to address unforeseen conditions. Funds can also be shifted from repairs that were conservatively estimated in the design to other areas. Moreover, the GMP process allows the owner, engineer, and contractor to evaluate the approach to each repair and determine a solution addressing design and constructability.

Throughout the construction of 10 Light Street, RMF inspected each drop with the contractor and reviewed the remedial work required, providing an opportunity to verify what was illustrated on the construction documents and make adjustments where necessary. If conditions varied from the construction documents, the owner was immediately notified and a discussion about costs and procedure followed. After the contractor completed the work on the drop, RMF would punch out the drop and document the findings. Although a final walk-through was performed for the entire project, most of the project was punched out on a drop-per-drop basis as the façade access migrated along the building.

Logistics
Access to perform the repairs was one of the project’s most challenging aspects. The building has a straight vertical face for the bottom 22 floors, accommodating the use of mast-climbers. However, the floor plate diminishes and steps back several times between Floor 22 and the top of the building with a number of small roof levels in the upper section making mast-climbers impossible to use. Individual swing stages of various lengths, including small, single-cable stages called ‘buckets,’ were used. In a few isolated areas, bosun chairs were the only solution. More than 60 drops in all were required.

The building is located in a busy area of the city with a bus stop on one side of the structure and a service entrance on the other—both had to be maintained throughout the construction. Scaffolding with overhead protection was erected over all the sidewalk areas from the face of the building to the curb line of the streets; this also provided a landing level for the mast climbers and swing stages above the sidewalk level.

The design was completed in a phased approach, and the construction documents were separated into three packages:

  • Package 1–Marble Replacement;
  • Package 2–Phase 1, General Repairs; and
  • Package 3–Phase 2, General Repairs.

The Marble Replacement Package was completed first given the replacement stone had a long lead-time. The team determined the stone thickness and the overall quantity the contractor needed to order. While the stone was being quarried, cut into 30-mm (1.18-in.) thick slabs and shipped, the initial mast-climber was being erected at the first drop. This marble removal provided valuable information on the conditions of the existing backup, and also provided insight on the appropriate stone anchorage details subsequently added to the Marble Replacement Package.

A before and after photo of the repair performed at a steel spandrel beam. The moment splice plate behind the screw jacks is needed for the new steel member installation. [CREDIT] Photo courtesy RMF Engineering

A before and after photo of the repair performed at a steel spandrel beam. The moment splice plate behind the screw jacks is needed for the new steel member installation. Photos courtesy RMF Engineering

10light_spandrelbeam_after

Repair plans
The original stone was an Italian marble containing intermixed serpentine, and classified as Soundness Group D by the Marble Institute of America (MIA). Soundness Group D stone is often the most attractive but, it is the least durable of the Groups A through D and contains large portions of natural faults and variations.

Although prized for its rich burgundy color and decorative veining, it is not suitable for exterior exposure in this climate. As a result, all the exterior marble was replaced. This same marble, used extensively inside the building and a few well-protected exterior areas, was in good condition. It maintains its polish and gives credence to the deleterious effects on the marble when exposed to the elements over time. The replacement stone used on the project is not a marble product, but rather a granite quartzite stone from a Brazilian quarry. The granite quartzite is highly durable and an excellent visual match to the original marble.

The stone was mounted in two distinct conditions:

  • recessed into the wall with a limestone surround; and
  • mounted in a steel frame integrated into the window system as a spandrel panel.

The primary methods of attachment for the recessed stone were stainless steel drop pins in the surrounding limestone at the upper section, epoxy-set stainless steel pins at the bottom section, and blind stainless steel Type 31 anchors in the brick backup.

The pin locations had to be carefully placed to logistically allow the stones to be set onto the epoxy pins at the bottom and tilted into place with tight tolerance all around. The recessed stones have unique shapes; this required the stone supplier to field measure and make templates of each individual piece given the tight tolerances. The stone fabrication rate was largely driven by the accessibility to the façade to allow for field measurements and template-making.

This is the original marble in the framed opening. Not only is there a crack in the panel to the right along one of the natural soft veins in the stone, but the marble had also lost its luster and burgundy color.

This is the original marble in the framed opening. Not only is there a crack in the panel to the right along one of the natural soft veins in the stone, but the marble had also lost its luster and burgundy color.

New granite quartzite in framed spandrel panel.

New granite quartzite in framed spandrel panel.

This existing marble at the main entrance was replaced in the ornamental brass framing.

This existing marble at the main entrance was replaced in the ornamental brass framing.

 

 

 

 

 

 

 

 

Additionally, the stone fabrication had to be closely coordinated with the mast-climber and swing-stage sequence. At the spandrel panel installation, the external steel components that held the stone in place were re-installed or replaced with new steel components matching the existing profile to maintain architectural integrity. All steel components were cleaned and painted with a high-performance direct-to-metal acrylic coating. Missing from the original design, a weep system was added directly above the sill frame to prevent water buildup behind the stone. A fluid-applied air barrier membrane was installed on the brick backup in all cases.

The Phase 1 and 2, General Repair Packages included repairs to the limestone and brick with the Phase 1 Package including higher priority repairs. Funds left over after completion of Phase 1 were attributed to the Phase 2 scope of work.

The limestone was on the bottom five floors and the building’s upper portion between Floors 19 and the top of the building. Less than one percent of the ornamental limestone pieces (e.g. medallions, rosettes, and carved lion heads) required patching. The limestone pieces not mechanically anchored to the backup or keyed into the backup or other limestone proved to be an issue—a few of the stones had shifted out of place. After removing the stone for inspection during the report phase, the joint mortar and mortar buttered on the stone’s back were found to be the only mechanisms holding it in place.

Although this performed well for numerous years, the mortar joints were deteriorated and allowed moisture to migrate behind the stone, slowly loosening them. While consideration was given to removing and re-anchoring without impacting the face of the stone, the risk of removing stones of this size on a swing stage, particularly with heights upward of 122 m (400 ft), was too great.

This is one of many limestone lion heads on the 22nd floor.

This is one of many limestone lion heads on the 22nd floor.

The decision was made to keep the stones in place, re-align if necessary, and anchor with helical pins through the stone’s face. The small circular recess left in the face of the stone from the drill at each pin was patched with a mortar specifically designed for a natural stone substrate. The patching product was also tinted to match the limestone color. In addition to the pinning, all of the limestone joints were raked out and repointed. The new mortar was made compatible based on petrographic examinations and chemical analysis on the existing mortar—a critical step in any façade restoration work. The results yielded ASTM C270, Standard Specification for Mortar for Unit Masonry, Type M or S to be suitable. The existing brick mortar was also tested resulting in ASTM C270 Type S.

Given its age, the original brick’s condition and mortar between Floors Five and 22 was good—there were only a few areas where the brick was repaired and re-pointed. The steel lintels at the window openings were also in good condition. In the upper elevations of the building, where the building geometry becomes more complex with the extensive use of limestone, the brick and mortar was more distressed. The abrupt change in the façade condition would suggest the several building setbacks, roof levels, and parapets allowed the building envelope to take on more moisture in the upper elevations which also experienced rust-jacking in some of the steel columns and lintels.1

The façade damage caused by rust-jacking areas was addressed, including remediation of the structural steel. The brick was removed in front of the steel where the rust-jacking occurred to expose the steel for inspection. Although the column steel experienced some section loss, it remained structurally adequate. However, some lintels and spandrel beams needed to be replaced. Switching out the short lintels was straightforward, but the spandrel beam replacement presented challenges in providing temporary support of the wall and roof structure during the beam remediation.

Ultimately, the existing beam stayed in place and a portion of the outboard flanges of the existing beam was removed. A new steel member was installed with a similar depth and narrow profile allowing one wythe of brick to bypass the new steel. Given the existing beam’s condition and selective removal of the flanges, structural calculations were performed at each phase of the construction to verify adequate support of the wall and roof structure throughout the process. The vertical cracks in the brick caused by rust-jacking of the steel columns were repaired using L-shaped helical joint reinforcement recessed into the horizontal mortar joints. The reinforcement was spaced vertically at 0.6 to 1.2 m (2 to 4 ft) on center (oc), and wrapped around the corner to stabilize the brick. The cracked brick was replaced and the area repointed. The limestone integrated into the corner masonry was pinned to the backup.

A falcon head on the 29th floor.

A falcon head on the 29th floor.

Conclusion
Façade issues are frequently left unaddressed until a major disaster occurs, or until the damage is so extensive it becomes cost-prohibitive to repair. The 10 Light Street façade restoration project is an excellent example of proactive façade inspection followed up with repairs in order to successfully preserve an historic structure.

For its first 84 years, the building served primarily as a banking center, and provided retail and office space for thousands of employees, customers, and visitors every day. It is currently under renovation to be repurposed as a residential complex with tenant fit-out and building support functions on the lower three floors. Approximately 460 living spaces will be integrated into 31 floors of the building, ranging in size of approximately 42 m2 (450 sf) for the studio units to 186 m2 (2000 sf) for the multiple bedroom and loft units. These apartments are primarily designed for the student population attending nearby teaching hospitals and universities, and are expected to be ready for occupancy in 2015.

Notes
1 Rust-jacking is the process where steel, in contact with masonry, corrodes then expands pushing the masonry outward to the point where the masonry cracks and can eventually become dislocated. (back to top)

John Hovermale, PE, is a partner in the structural engineering department at the Baltimore branch of RMF Engineering Inc. He has been with RMF for 20 years, and has been involved in façade restoration projects for a decade. Hovermale has a bachelor’s degree in civil /structural engineering from the University of Maryland, College Park. He can be reached via e-mail at john.hovermale@rmf.com.

EIFS: Then and now

Our September 2013 article on entrapped moisture touched on issues related to early iterations of exterior insulation and finish systems (EIFS). It led to this letter to the editor from Scott Robinson, of the EIFS Industry Members Association (EIMA).

While going through the September 2013 issue of The Construction Specifier, I read, “Claddings and Entrapped Moisture: Lessons Learned from Early EIFS,” by John Koester. As manager of public affairs with EIMA—a national non-profit technical trade association comprising EIFS manufacturers, suppliers, distributors, and applicators—the article gave me pause. Continue reading