March 28, 2019
by Jonathan Dickson, P.Eng., BSS
Brick masonry has, and will continue to be, a prevalent building material as it has several desirable attributes. However, it is these same material attributes that can also potentially make brick masonry undesirable for use in the exterior walls of buildings. For example, durability and esthetics can be considered as both positive and negative attributes of masonry depending on the quality of design and installation. The long-term performance of brick masonry enclosures requires technically sound design and quality construction practices supported by diligent maintenance procedures throughout the life of the structure.
Brick masonry assemblies are a combination of brick units and mortar. The brick can be arranged to either form the entirety of the enclosure (mass wall) or as a separate cladding over supplemental control layers of the exterior wall assembly (veneer). Within historical structures, the masonry assembly usually completed most of the required functions of the exterior wall enclosure. Multiple layers of masonry, also called wythes, were employed in these buildings and tied together to create an assembly capable of supporting the structure while providing the required control functions (air and moisture) of the uninsulated exterior wall assembly (Figure 1).
As buildings became insulated (thermal control), a vapour control function was a good practice in cold climates. Buildings also became taller and were required to be more economical to construct. This resulted in the use of other materials for the structure of the exterior walls (e.g. wood, steel, and concrete). Thus, brick masonry went from a structural component used in a mass wall configuration to a cladding material hung off the primary structure in a veneer configuration retained for its durability and esthetics. Regardless of the type of brick masonry configuration, the performance of the assembly requires both the brick masonry unit and the mortar to function as intended.
While the mortar joints may occupy less than 20 per cent of the total wall area, the impact to the wall’s long-term performance is disproportionally significant. Modern mortar is a combination of lime, Portland cement, and sand proportioned to achieve desired compressive strengths (Figure 2). Historic mortars are composed of pure lime or are lime-centric and have lower compressive strengths than the modern Portland cement-based varieties.
The mortar in the masonry assembly is designed to fail in the sense that should differential movement occur the mortar fails before the brick units in the building. When rehabilitating older brick masonry structures, it is important to maintain this relationship between the bricks and mortar. It is pertinent to note that there could be undesirable consequences when replacing historic, soft mortars with modern, hard ones (Figure 3). If a historic brick masonry wall assembly repointed with strong, modern mortars undergoes differential movement, the path of least resistance will become the brick units instead of the mortar. The subsequent widespread cracking through the units will be difficult to control and repair. Such an occurrence would be significant for a historic building where masonry is often of key esthetic importance.
Best practice #1: Before specifying mortar for restoration of an older masonry building, determine the composition of the existing mortar to decide on the appropriate mortar for the application.
The joint profile of the mortar can also have a significant impact on the long-term performance of the brick masonry wall. Further, it is not recommended to base the selection of the joint solely on esthetics especially for buildings prone to wind-driven rain events. Figure 4 provides a snapshot of typical joint profiles. A slightly concave joint is ideal as it allows for the mortar to be installed with sufficient force to ensure compaction.
Best practice #2: Avoid raked mortar joint profiles in exposed exterior applications.
A raked joint may help create the desired look of a masonry wall. However, it is this author’s opinion the profile should be avoided in exterior locations unprotected by a canopy or similar rain-shedding features because it allows for rain and/or snow to collect on the top surfaces of the masonry units, thereby degrading the mortar bond line and increasing potential for moisture ingress as shown in Figure 5.
Brick masonry units comprise the vast majority of the masonry assembly and have the largest impact on an exterior wall’s durability and esthetics.
Brick masonry is generally viewed in the design community as a durable material suitable for use within exterior, impact-prone environments. Masonry has a reasonable resistance to the elements, is ultraviolet (UV) radiation stable, and has desirable moisture retention and dispersion and fire-resistance properties. However, based on the quality of the materials used and installation methods, this brick masonry could also be susceptible to widespread cracking and spalling, thereby creating a public safety liability and financial burden for building owners.
Cracking of brick masonry usually occurs as a means for relieving stress within brick enclosures due to differential movement. In this author’s experience, differential movement is often a result of settlement of the foundation or deflection of floor slabs. As the structure of the building moves, the cladding also needs to shift accordingly. If this movement is restricted, stress develops and will be relieved through cracks. In the case of brick veneer, this cracking follows the path of least resistance, typically along the mortar joints. However, as noted previously, cracking could also occur through the brick units if incorrect mortar is used. In the event of vertical cracking (i.e. cracking through a brick masonry unit) when an appropriate mortar was used, the cause was more likely localized and significant such as a point load placed on the brick veneer. Significant cracking may impact lateral connection of the veneer to the backup wall and should be investigated as soon as possible.
To reduce potential for cracking, it is recommended to follow the guidelines of the Canadian Standards Association (CSA) A371, Masonry Construction for Buildings. It outlines locations and sizes for movement joints in brick veneer. Of particular importance are the movement joints below shelf angles of brick veneer. As floor slabs deflect due to dead and live loading between columns, the shelf angle attached to the floor slabs also deflects. This may result in the brick veneer below the shelf angle taking on unintended loading, leading to associated crack development (Figure 6). Rehabilitation at these conditions would be required to allow for the occurrence of differential movement without loading the brick veneer. It is advisable to supplement existing tie-backs of the brick veneer to the backup wall as required if the new control joints leave a portion of the brick veneer unsupported.
Best practice #3: Install movement joints within brick veneer at intervals and locations at a minimum as recommended by CSA A371.
If the cracking is determined to exist from differential settlement of the foundation, remediation of the foundation wall should be completed prior to repairing the cracking within the brick wall. Failure to stabilize the foundation first may result in further cracking of the brick masonry.
Spalling of brick masonry is a very common deterioration mechanism in cold climates and most concerning to building owners. Spalling brick can potentially result in falling hazards that increase liabilities for owners. Prevention of spalling brick starts with sound design and quality installation. However, this must be supplemented through the life of the building with diligent review and rehabilitation practices.
Spalling occurs as a result of water intrusion into the brick combined with freezing temperatures that causes the water to expand and pop off the face of the brick unit (Figure 7).
It also exacerbates the rate of failure of adjacent bricks as they are now more prone to water ingress and future spalling. Additionally, spalling carries an increased potential for esthetic consequences ranging from minor localized brick units to significant large areas of painted brick.
Best practice #4: Liabilities associated with spalling brick are significant. Have all noted locations of brick spalling reviewed to determine causes and consequences.
Regardless of the duration of time an owner intends to carry a property, whether a couple of months awaiting redevelopment or an intent for long-term building ownership, the liabilities from deteriorated brick masonry far outweigh the costs of an assessment by an engineer. Ownership is therefore strongly recommended to investigate and also protect the area of spalling brick immediately.
Spalling that occurs as a result of freeze-thaw deterioration requires the presence of the following two conditions:
The outer wythe of historical masonry buildings reach critical saturation several times over their lifespan during periods of freezing temperatures, yet remain in sound condition. The primary reason for this is the masonry has not reached freezing temperatures as a result of significant heat loss from the uninsulated mass wall building. As energy becomes more costly, an increasing number of owners are choosing to insulate their historic mass masonry buildings. Due to heritage designations and esthetic concerns, these owners are constrained to only apply insulation to the interior of the exterior wall. Further, these insulations tend to be vapour closed (e.g. closed-cell sprayed polyurethane foam [SPF]) to reduce wall thickness. This inhibits the drying of a mass wall previously exposed on both sides. This type of insulation reduces heat flow from radiation and drying from convection (Figure 8). This may lead to deterioration of the exterior masonry and embedded wood elements, a consequence often overlooked during analysis of insulation options (Figure 9).
Best practice #5: Prior to insulating the interior of a previously uninsulated building, complete site-specific hygrothermal analysis of the exterior wall assemblies in order to confirm their capability of accommodating reduced temperatures.
Hygrothermal modelling software can be employed to understand the temporal impacts to temperature and moisture flows as a result of the proposed insulation system. The software is capable of using site-specific data based on material testing to develop a model that is a reasonable simulation of reality. For these models to be applicable, it is imperative the existing masonry be tested and used as the basis for the modelled masonry performance parameters because critical saturation values for masonry can vary widely.
Brick masonry is seen as a desirable esthetic in both new and old buildings. The esthetics of the masonry can be both directly and indirectly impacted by the quality of installation. Directly, the quality of the installation can create negative esthetics through offset or misaligned masonry units, and varying joint sizes (this can also negatively reduce structural performance of the masonry assembly). Indirectly, the esthetic consequences can be more significant.
Spalled bricks can be an eyesore on a building façade especially for painted brick. If spalling occurs, the painted face of the brick is dislodged, thereby exposing the base colour of the brick unit. This makes the spalling more noticeable and the esthetic consequences more pronounced (Figure 10).
Efflorescence is an indirect consequence of the performance of the brick masonry wall. Efflorescence occurs when the salts deposited on the surface of the masonry leave a white residue (Figure 11). This salt can originate from one of the following two sources:
Efflorescence is a symptom of the more significant problem of bulk water ingress into the wall assembly. If this staining appears below windows or roofs it would typically correspond with a location of water ingress into the system (Source #1). The occurrence of a horizontal line of efflorescence about a metre above grade would suggest capillary suction of salts into the system (Source #2).
A third source could also be air leakage through the exterior wall assembly in cold climates where warm moist air from the interior condenses on the back of the brick and dries to the exterior (although this pathway is less common than the first two sources). The first step to fix efflorescence involves the identification and correction of the source. This may include installing or rehabilitating water-shedding features on the façade, setting up capillary breaks, employing materials less prone to capillary suction, or sealing air leakage pathways. Once rectified, the efflorescence can be removed from the surface of the masonry with a stiff fibre brush. A mildly acidic cleaning solution can be employed if the brush is not 100 per cent effective.
Periodic rehabilitation of brick-clad structures is essential to maintain the integrity, performance, and longevity of the building envelope. The best practices presented here are intended to assist with the maintenance but require the diligence of building operators, owners, managers, and consultants to identify, investigate, and diagnose the sources and not just the symptoms. The future of brick and mortar retail may be unclear but the buildings are here to stay.
Jonathan Dickson, M.Eng., P.Eng., BSS, LEED GA, is senior project manager at Pretium Engineering. He has been actively involved in the industry since 2010 with extensive experience in restoration of existing buildings. His experience ranges from localized leak assessments to prime consultant on multimillion dollar rehabilitation projects. Dickson can be reached at firstname.lastname@example.org.
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