Bonding with Masonry 2024 Q4

Words: David Biggs

This issue’s questions come from an Engineer, an Architect, and a Mason Contractor. What questions do you have? Send them to info@masonrymagazine.com, attention Technical Talk.

Q. An Engineer asked about the Brick Industry Association (BIA) TECHNICAL NOTE 18A, Accommodating Expansion of Brickwork, specifically Figure 4. Figure 4 shows a wall section bounded between movement joints( expansion joints). The Engineer states that this layout of windows is commonly used by many of their architectural clients to avoid placing movement joints at the windows.


Figure 4 from BIA Technical Note 18A

The technical note explains there could be cracking at the heads of the piers and sills of the windows due to the differential expansion of the brick veneer over and under the windows in comparison to the piers themselves. However, the note does not provide a solution to mitigate the cracking. So, the Engineer asks what could be done in the design.

A. Thank you for the question and this is a common design issue. While there is the potential for these designs to have the cracking shown in Figure 4, not every project experiences cracking. The BIA solution is to add vertical movement joints at the window jambs. However, this affects the aesthetics of the masonry by adding many joints that are accompanied by more sealants to maintain. In addition, the detailing of the lintels and flashings is more complicated for the Architect.

There are alternative techniques that have been used on various projects. These are not discussed in any BIA Technical Notes so it is up to the designer whether you choose to try them.

One technique is to attempt to restrict the brick expansion above and below the windows to reduce the differential movement that could cause horizontal and step cracks.

BIA Technical Note 18A states “Vertical expansion joint spacing should not exceed 25 ft (7.6 m) in brickwork without openings and 20 ft (6.1 m) for brickwork with multiple openings.” So, let us assume the spacing of the movement joints is 25 feet for this example. Also let us use the total unrestrained movement of the brickwork (inches) is mu = 0.0009L where L is in inches (see Equation 1, BIA Technical Note 18, Volume Changes – Analysis and Effects of Movement). This gives mu = 0.27 inches of brick expansion over 25 feet.

Assuming the expansion is approximately zero at mid-panel (Figure 1), the expected differential movement at each end of the panel is half the total or approximately 0.14 inches. By adding horizontal joint reinforcement, we can restrain some of this expansion.


Figure 1 – expansion extends from the centerline of the wall panel


If M1.7 (9 ga.) ladder-type, horizontal joint reinforcement is placed at 8 inches on center vertically above and below the windows, we could calculate that the anticipated expansion might be reduced to 0.11 inches. This is not a significant change, but it has been effective on several projects.

A second technique is to provide a horizontal relief joint in the mortar joints where we anticipate a crack may form at the head of the windows (red lines in Figure 2). The relief joint is a mortared joint where the surface mortar is raked to a depth of approximately ½ inch to allow the placement of a backer rod or tape and sealant. The goal is to provide a weakened joint that avoids the step cracks at the ends. The sealant is to hide the actual crack and limit water infiltration.


Figure 2 with relief joints at head of windows

The same logic could be used to add horizontal relief joints at the bottom of the windows. However, those possible cracks are less likely due to the restraint provided by the bottom support of the panel.

If relief joints are used, the designer should specify that veneer anchors should be within 8 inches above and below the joint.

Summary:

Horizontal reinforcement could be added to restrain some of the brick expansion above and below the windows.
Relief joints help control the location of horizontal cracks should they develop. The sealant will mask the cracks.
These suggestions are just that; they are not BIA recommendations.


Q. An Architect states that the CMU manufacturer has recommended she include horizontal joint reinforcement, integral water repellant in the CMU veneer units as well as the mortar, and a post-applied sealer I her design and specifications. She asks if these are all necessary?

A. The short answer is yes. Each is an industry recommendation for both CMU veneer and exterior single-wythe CMU.

The horizontal joint reinforcement is recommended for crack control (not crack elimination) due to shrinkage of the CMU and to extend the distance between movement (control) joints. This is explained in CMU-TEC-009-23, Crack Control Strategies for Concrete Masonry Construction (https://www.masonryandhardscapes.org/resource/cmu-tec-009-23/)

Integral water-repelling admixtures in both the CMU and mortar have proven to be effective at reducing water absorption (https://masoncontractors.org/2005/04/05/integral-water-repellent-mortar/#newsletter).

CMHA (Concrete Masonry and Hardscape Association), formerly NCMA, notes that its crack control strategy is to attempt to limit cracks to less than 0.02 inches. While these cracks may be visually acceptable, they still offer the opportunity for water infiltration. Many post-applied coatings that include specific paints, stains, and sealers are formulated to bridge cracks of 0.02 inches and improve the water-resistance of the wall. Check the coating manufacturer’s literature to see that their product is intended for this use or speak to the CMU supplier. See NCMA TEK 19-1, Water Repellents For Concrete Masonry Walls, and NCMA TEK 19-2B, Design For Dry Single-Wythe Concrete Masonry Walls. The masonry wall must be cleaned before applying any coating.

Summary:

Crack control and water resistance of CMU walls require a system design strategy.
Horizontal reinforcement can be used for crack control
Integral water repellents and post-applied coatings improve water infiltration resistance.


A Mason Contractor was traveling in Europe and photographed a building from the 1600s. It is now a restaurant. He asks about the stone lintels because they have a vertical joint over the opening (Figure 3). It is a load-bearing wall. How is this possible?


Figure 3 – Stone lintel with joint


What a great question! We love that you are noticing such magnificent masonry.

This is an example of masonry arching. Let us look at the elements of the theoretical arch. First there are the abutments (Figure 4). The thick stone walls and the ends of the lintel stones interlock and form a solid abutment at each.


Figure 4- Stone abutments

The theoretical arch itself is within the lintel stones (Figure 5). Because the lintel stones are deep, a theoretical arch forms within the stones. The vertical joint between the lintel stones is in compression at the top. The bottom of the vertical joint is uncracked although it could be in tension. The continuous stone over the vertical joint provides some shear strength to the joint by distributing the weight above to the lintel stones.


Figure 5 – Theoretical arch

For the Engineer readers, the analysis could be modeled as a three-hinged arch (Figure 6) or a fixed base arch (Figure 7). The stone elements are in compression.


Figure 6 – Three-hinged arch                                 Figure 7 – Fixed-base arch

Constructing these lintel stones would have required temporary support below the vertical joint until the walls and stone lintel cured.

Summary:
There are arches and there is arching action. Many historic structures are supported by arching action but do not look like what we think of as an arch.
In this case, the depth of the lintels was useful for creating the arching action.

Thank you again for following this column. Remember, by bonding, we get stronger! Keep the questions coming. Send them and your comments to info@masonrymagazine.com, with attention to Technical Talk. If you have missed any of the previous articles Technical Talk, Bonding with Masonry at Masonry Design magazine, you can find them and other useful articles online at https://masonrydesignmagazine.com/PageList?typeID=328.

David is a PE and SE with Biggs Consulting Engineering, Saratoga Springs, NY, USA (www.biggsconsulting.net), and an Honorary Associate Professor with the University of Auckland, NZ. He specializes in masonry design, historic preservation, forensic evaluations, and masonry product development.



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