by Katie Daniel | March 6, 2017 10:32 am
By Paul Potts
Often enduring more than a hundred years, brick masonry is testimony to the traditional collaboration between architect, engineer, and mason. Beauty and historical significance are often the most compelling features of brickwork, but focusing only on this rich appearance and permanence oversimplifies the engineering knowledge, design requirements, and workmanship needed to arrive at a result that is not only beautiful and enduring, but also structurally sound and a defense against the elements. (This article is intended as a guide for inspection and not a handbook for designing or constructing masonry. Its contents are the result of the author’s experience and research on the issues of masonry inspection. Architects’ and engineers’ specifications and building codes take precedence over anything stated in this article. Further, this author has not included discussions of engineering issues that are the basis of masonry structural design or concrete masonry unit (CMU) construction.)
This article discusses developing a working relationship with the mason, construction practices, design details, material properties, and workmanship issues basic to successful brick masonry construction. With the information presented in this article, the architect’s construction administrator (CA) or independent inspector will be better able to examine and report on the brickwork, enforce the requirements of the plans and specifications, and prepare representative reports while communicating confidently with the mason.
Getting acquainted at the progress meeting
Before and after the weekly progress meeting (which takes place away from the distractions of construction activities with a full set of construction documents available) is the ideal opportunity for developing rapport with the masonry superintendent. The superintendent can easily bring the CA or inspector up to speed on the current progress of the work, but it is important to keep in mind this requires a willingness to be helpful—the result of a harmonious, and not contentious, relationship.
During the progress meeting, the construction administrator should be taking notes and listening for the ‘narrative’ (or gossip, if you will) that has developed around the jobsite. The owner is influenced by this gossip, and the CA will show better client management skills if they are on top of it.
Masonry construction practices
Some issues can be observed while walking the site. These include ensuring all bagged products and brick, mortar, and concrete masonry units (CMUs) are protected from the weather to prevent wetting of the materials, and that only non-chloride admixtures are being used in mortar or grout.
Masons should not drop mortar into the void behind the brickwork, as it piles up on the masonry ties and plugs up the drainage material. They should not attempt to cut off mortar fins protruding into the cavity; instead, they should minimize the excess mortar squeezed out of the bed joint by bevelling the mortar away from the space behind the brick.
Brick movement joints should be kept free of mortar, and horizontal reinforcement should be stopped on either side of the joint. Brick Industry Association (BIA) Technical Bulletin 18A, Accommodating Expansion of Brickwork, provides a good discussion of expansion joint construction. The tops of masonry must be covered when not being worked on to prevent rain and snow from getting into the cores. Additionally, the sharp corners of the extended metal drip flashing should be flattened to protect the public.
Some mortar additives put a shading on the mortar. For this reason, masons must be careful of where they stop and start adding it. Mortar should be used consistently in each elevation so the shading does not start or stop in the middle of an elevation.
Weep joints and rainscreen wall design
Head-joint weeps and through-wall flashing are a part of the traditional masonry drainage wall system. Weep openings provide a pathway for water to drain out of the cavity and should be in the first course of brick on the through-wall flashing. Head-joint weeps must be raked clear of mortar down to the flashing, and be kept free of mortar droppings to provide an unobstructed path for water to drain. They are essential for brick veneer or cavity wall masonry, and should be discussed with the architect-engineer if not shown on the drawings.
In addition to this traditional drainage wall system, a pressure-equalized rainscreen wall design incorporates air vents at the top and bottom of the exterior wall to circulate water vapor out of the cavity and minimize water infiltration. Without these air vents, suction in the cavity (created by high-velocity wind hitting the exterior of the brick) draws moisture from the outside face into the cavity, through breaches in the mortar joint. Venting, provided by the pressure-equalized rainscreen wall design, reduces cavity suction by equalizing air pressure inside and outside the brick wythe. Vents should be at the top and bottom of the cavity, below any parapet or brick ledge that compartmentalizes any portion of the cavity and three courses above the flashing. For an in-depth discussion of the rainscreen wall principle, refer to Canadian Building Digest 40 (CBD-40), Rain Penetration and Its Control, and BIA Technical Note 27, Brick Masonry Rainscreen Walls.
The most common defects allowing moisture into the cavity are separations (breaches) between the mortar and brick, which develop from poor workmanship or moisture incompatibility between the brick and mortar, and contraction cracks in the mortar—very little moisture gets through the actual mortar or brick unit. ‘Slushing’ mortar, the practice of throwing mortar onto the brick with the edge of the trowel, should not be permitted, as it results in leaky joints.
Tooling mortar joints improves the water resistance of the brick masonry. Striking the mortar joint with a jointer tool presses the mortar into the brick and produces a glaze on the surface of the mortar that is more water-repellent than mortar that has not been struck. Mortar is ready for tooling when it is fingerprint-hard.
Type N mortar and workability
Aside from cushioning and locking the masonry units together, the most important function of mortar in exterior face brick is to make the joint weather-resistant. The mortar type, moisture compatibility between the mortar and the brick, and workability (i.e. ease with which the mason can get the mortar to spread and stick to the brick), along with the craftsmanship of the mason, are essential to reducing water penetration of the masonry joint.
Workability, perhaps the most critical factor for moisture resistance, is improved by using the lowest-strength mortar consistent with structural requirements. Thus, Type N mortar (i.e. the lowest-strength commonly used mortar at 17 MPa [750 psi]) is recommended for face brickwork except where prohibited by building codes.
Other mortars include Types M, S, and O. Type O is the weakest mortar, and is rarely used in brick masonry. Type M (16.5 MPa [2400 psi]), on the other hand, is the strongest in compressive strength and is used mostly for below-grade applications. Type S (12.4 MPa [1800 psi]) is recommended for below-grade applications and interior and exterior structural-bearing masonry.
Although it is the weakest of the three common mortar types, Type N mortar can easily support brick masonry more than a few storeys high. It is weaker in compression than Type S due to its manufactured properties and mixing proportions (higher sand and water content), but these proportions and other manufactured properties improve workability, making Type N mortar stickier and thus easier for the mason to spread across the entire joint. How much water penetrates the joint into the cavity behind the brick has a direct relationship to how completely the mason butters the head joints.
The bond between brick and mortar
The two characteristics of the bond between brick and mortar—extent of bond and strength of bond—are, for somewhat different reasons, both related to the compatibility of the moisture content of the brick and the water retention quality of the mortar.
The extent of bond, the critical factor in making weather-resistant joints, refers to how completely the mortar is spread in intimate contact with the brick, across the entire surface of the joint—a result of the moisture properties of the brick and mortar, skill of the mason, and mortar workability. There is no field test for determining mortar has the right workability, but an experienced mason feels when it is right and can make field adjustments to the mortar to improve workability.
The strength of bond is not explored in this article, but it involves the force required to break the mortar and brick apart. This is related to structural engineering considerations, such as compressive strength and modulus of elasticity.
Initial rate of absorption and extent of bond
When brick and mortar are brought together, a tug of war begins between the suction of the brick (trying to draw water out of the mortar) and the water retention qualities of the mortar (trying to keep its water to itself). The objective is to keep the water and cement together so they can be absorbed as paste, and to let the brick absorb as much of that paste as possible. That makes a good extent of bond. Water retention of mortar is a manufactured quality, and can vary between manufacturers and types of mortar—whether it is Portland cement/lime mortar, masonry cement, or mortar cement.
Suction properties of brick, measured as the initial rate of absorption (IRA), produce the bond between brick and mortar when a certain amount of cement/lime paste is absorbed into the pores of the brick. However, if the brick is too dry (i.e. an IRA above 30 g of water per minute per 0.02 m2 [30 si] of surface area), the brick suction prematurely draws too much water from the mortar, leaving a dry paste behind. If it is too wet (i.e. an IRA below 30 g), the brick has too little suction—reducing the extent of bond and weakening compressive strength. An experienced mason can feel if the mortar is not sticking properly to the brick because of mismatched moisture properties and make adjustments by rewetting brick or retempering mortar that has gotten too dry on the board.
|TEN COLD-WEATHER REQUIREMENTS|
|Certain requirements apply when using masonry during cold weather.
Workability is directly related to the amount of moisture remaining in the mortar. Retempering is replacing water lost to evaporation by adding a small amount of water to mortar on the board. Many specifications do not allow retempering mortar, likely due to prohibitions on rewetting concrete.
However, recommendations by ASTM and the Portland Cement Association (PCA) state mortar can be retempered as necessary, but should be discarded after two hours from the time it leaves the mixer. This time limit will be shortened in hot or dry weather. Coloured mortar should not be retempered, as it might dilute the colouring agent.
The mason must get permission from the architect-engineer before retempering mortar if it is prohibited by
Types and grades of brick
Face brick is classified by type (i.e. standards for dimension and shape), and grade (i.e. durability when exposed to extremes of weather). Types of brick are divided into FBX, FBS, and FBA. The first category has the highest standard of dimensional stability and uniform shape, and is preferred by most architects and designers. The irregular shape and dimensions of FBS brick are sometimes desired for their ‘colonial look.’
The grade is divided into SW–Severe Weather Exposure and MW–Moderate Exposure. Virtually all exterior brick in Canada and the United States is ordered as Severe Exposure (SW) grade. Bricks in Canada should be manufactured to CAN/CSA A82.1, Burned Clay Brick (Solid Masonry Units Made from Clay or Shale).
The safest way to clean brick is with a paddle and brush, using a proprietary cleaner. (Diluted muriatic acid is not allowed.) The mason must protect windows, doors, and other hardware and appliances from damaging effects of the cleaner.
Power-washing can also be effective, but precautions must be taken to prevent damage to the brick. The use of power-washing and the choice of the proprietary cleaner must be approved by the brick manufacturer as well as the architect-engineer. Additionally, some specifications do not allow power-washing, so permission from the architect or engineer must be obtained.
For high-pressure washing, the nozzle must not be too narrow and must not come closer than 152 mm (6 in.) from the face of the brick. Cleaning with power-washing is a four-step process:
This article has explored storage of materials onsite and workmanship practices that can be inspected during a walk-through, along with a discussion on weep holes and the rainscreen wall design. It examined the selection of mortar and workmanship practices leading to weathertight masonry joints, and suggested ways to select appropriate type and grade of brick for the desired esthetic effect and durability qualities. Further, it touched on proper mortar cleaning procedures and warnings about the hazards of pressure-washing.
While this is a good overview of what one should look for while surveying brickwork construction, it is not possible to explain each issue in depth within the confines of a single article. It is the responsibility of readers to do further research if they are interested.
|If the architect/engineer’s representative is concerned about the uniformity of the head joints and bed joints—usually indicated by fatter and thinner joints or poor alignment—the inspector can settle the question with a story pole. To do this, the mason first lays up a portion of the wall where the joints are acceptable to both. The inspector then makes a measuring pole using a 3-m (10-ft) long oak or birch board, marking off the joint spacing on the pole. From there, the inspector uses the pole to occasionally check the alignment and dimensions of the joints.|
Paul Potts is a technical writer and construction administrator. He has worked in the construction industry as an independent contractor and construction administrator for architects, engineers, and owners in Michigan. Potts can be contacted via e-mail at email@example.com.
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