Bonding with Masonry 2024 Q2

Words: David Biggs

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

Q. An Architect writes that they like the aesthetic of curved walls on buildings. However, when using CMU veneers over CMU cavity walls or cold-formed metal framing (CFMF) cavity walls, how is the horizontal joint reinforcement to be installed? Can it be cut into short lengths or eliminated?

A. Thank you for the question. Masonry construction offers many aesthetic options, and radial or curved walls are fine examples of the flexibility of masonry.

One issue to be addressed immediately is that CMU construction undergoes shrinkage. Therefore, horizontal reinforcement is preferred for crack control for both straight and curved walls. The reinforcement can be installed in short lengths but always lap spliced between movement joints. For CMU constructed in a running bond, an alternative is to eliminate the horizontal reinforcement and closely place movement joints (approximately one-half the spacing normally used with horizontal reinforcement). For CMU constructed other than running bond (stack bond), there is no option, TMS 402-16, Section 4.5 mandates the use of horizontal reinforcement.

The second issue is the construction of masonry in a radial pattern. CMHA (formerly NCMA) TEK Note 5-10A, CONCRETE MASONRY RADIAL WALL DETAILS is an excellent reference for the layout of curved CMU. The allowable radius is geometrically determined by:
       a. Unit length
       b. Unit thickness
       c. Mortar joint thickness at the exterior of the curve and interior of the curve.
       d. Bond type (either running bond or stack bond)

The TEK Note provides formulas and tables to determine the minimum possible radii. The reader should obtain a free download of this and other TEK notes at

The Brick Industry Association does not specifically address curved walls. It does have BIA Technical Note 29A - Brick in Landscape Architecture (Garden Walls) that discusses serpentine walls which are a specific variation on curved walls. Because it is only geometry, the formulas provided in CMHA TEK 5-10A can also be applied to clay brick by using the specific brick parameters, mortar thickness, and bond type desired.

Now that we can geometrically design curved walls, let’s discuss the two CMU examples you provided regarding placing horizontal joint reinforcement. The response will also apply to clay brick and stone veneer.

1.) CMU veneer over CMU cavity wall.
For the CMU backup, horizontal joint reinforcement will not bend to the radius unless the cross wires are cut and bent individually. (Alternatively, bent bars can be used in bond beams).

When used with a veneer, the cross wires of the horizontal joint reinforcement should be cut and then bent to the horizontal curvature of the face shell of the wall.

Figure 1 – Modified 270-2X LADDER EYE-WIRE REINFORCEMENT by H&B

The veneer can use two single wires bent to its radius. To avoid interference, do not place horizontal reinforcement in the courses with the veneer ties.

2.) CMU veneer over CFMF cavity wall
The veneer anchors should be attached to the CFMF studs as with a straight wall and the veneer reinforcement can be two single wires as previously discussed.


1.) Radial walls can be constructed with CMU or CFMF backup and a veneer of CMU, clay brick, or stone.
2.) Horizontal reinforcement for CMU is preferred for selecting movement joints and is required for stack-bonded walls of any masonry.
3.) Horizontal joint reinforcement can be cut to the wall radius. Lap splice between movement joints.

Q. An Engineer writes that some designers are mixing in autoclaved aerated concrete (AAC) units with wall sections that have CMU and/or clay brick units. These designers are using the thermal properties of the AAC as justification for their use. Is this common and acceptable? One such use is shown in Figure 2; the AAC is used below a clay brick veneer over the foundation.

Figure 2 – Detail of veneer support on the foundation wall

A. This is an interesting question that has two parts. The first is whether it is appropriate to intermix AAC with CMU and/or clay brick. The second part is whether Figure 2 is a good detail.

Regarding intermixing AAC and CMU or clay brick, TMS 402 does not specifically address this issue. While AAC is usually set in thin-set mortar, it can also be set in conventional mortar. Thus, while it seems reasonable that they could be intermixed, it is advisable to get approval from the building official as an alternative design.

Regarding Figure 2, there are significant problems. First, AAC is not an approved veneer material by TMS 402/602. TMS 402 specifically states that AAC veneer must be approved by the building official as a special system.

The second problem is that the AAC in Figure 2 is being used below grade and will be exposed to wet conditions. In the 2008 article, Using Autoclaved Aerated Concrete Correctly, by Richard Klingner in Masonry magazine( states “Unprotected exterior AAC deteriorates when exposed to cycles of freezing and thawing while saturated.” In 2004, Keith Itzler, PE wrote Autoclaved Aerated Concrete, A New Tool in the Structural Engineer’s Toolbox in STRUCTURE magazine not to use AAC below grade unless waterproofed.

Finally, we notice the cavity is open below the flashing and extends below grade. This is contrary to standard masonry construction recommendations. NCMA TEK Notes for CMU veneer and BIA Technical Notes for clay brick veneer both recommend grouting the cavity solid to minimize water from entering the cavity and freezing. The lack of grout in Figure 2 below the flashing is not advisable. The net result is that Figure 2 is not a recommended detail.

Another detail that has been showing up on projects is Figure 3 at a parapet.

Figure 3 – Detail of parapet with AAC intermixed

In this detail, the AAC is intermixed structurally with the CMU. As previously stated, this intermixing is not addressed in TMS 402/602 and it’s advisable to obtain building official approval as a special system.

The AAC is protected by the roofing and air/vapor barrier. However, roof leaks could result in deterioration of the AAC. This should be considered by designers.


1.) Mixing AAC with CMU and/or clay brick is not specifically addressed by the building code and should be approved by the building official.
2.) AAC units are not approved veneer material.
3.) AAC should not be used below grade without moisture protection.

Q. A Mason Contractor doing commercial work asks why mortar can’t be used in place of grout. Aren’t they the same materials?

A. Mason contractors have been asking this question for years. The short answer is that mortar as grout is not recognized in ASTM standards governing commercial work and therefore it does not appear in TMS 402 or TMS 602.
Mason contractors doing residential work have used mortar as grout for decades because it is allowed by the International Residential Buildings Code, Section 6060.2.12 Grout.

“R606.2.12 Grout
Grout shall consist of cementitious material and aggregate in accordance with ASTM C476 or the proportion specifications of Table R606.2.12. Type M or Type S mortar to which sufficient water has been added to produce pouring consistency shall be permitted to be used as grout.” Emphasis added by author.

With that, there are commercial CMU projects where mortar as grout has been accepted on projects by the building official under the alternative provisions of TMS 402-17, Section 1.3 and Section 104.3 Alternative Materials, Equipment, Appliances, Designs, and Methods of Construction of the International Building Code (IBC).

“1.3 — Alternative design or method of construction
The provisions of this Code and the TMS 602 Specification referenced within this Code are not intended to prohibit a design or exclude a method of construction within the scope of this Code and the Specification but not specifically prescribed therein, provided such design or method of construction has been approved by the authority having jurisdiction.” Emphasis added by author.

The basis for accepting mortar as the grout in commercial construction has been research financed by the International Masonry Institute (IMI), the National Lime Association, the Bricklayers and Allied Craftworkers, Local No. 1, New York, and the National Concrete Masonry Association Research and Education Foundation. The research was published in conference proceedings as well as a paper in the Journal of ASTM International titled Type S Portland Cement-Lime Mortar as a Low-Lift Grout and co-authored by D.T. Biggs and M.L.Thomson Vol 4, No 2, February 2007.

The research compared compressive strength and reinforcement pull-out strength (Figure 4) of mortar fill, pourable mortar, and conventional grout.

Figure 4 – Reinforcement pull-out test

The results were that:
The mortar as grout mixes should be limited to Type S or M mortar using only Portland cement and hydrated lime consistent with the research. Type N mortar did not provide adequate strength. Mortar as grout should have a slump between 4 and 6 inches.
Consistent with TMS 402, the grout mix (mortar fill or pourable mortar) should achieve a compressive strength of at least 2000 psi or f’m, whichever is greater, when tested per ASTM C 1019.
Construction should be limited to concrete masonry units since clay masonry and AAC were not tested.
Construction should be limited to a modified low-lift procedure with four-foot pour heights and one-foot lifts. All lifts should be rodded or tamped in place; mechanical vibration is not required.
Grouting is subject to continuous special inspection.

1.) Type S and M mortar have been allowed for use in residential construction in place of grout for decades.
2.) There is evidence that Type S or M mortar could be used as grout for commercial construction under certain conditions noted. Only Portland cement and hydrated lime mixes have been tested.
3.) Previous projects have received approval under the alternative method of construction in TMS 402 and the IBC.

Thank you again for following this column. Remember, by bonding, we get stronger! Keep the questions coming. Send them and your comments to, with attention to Technical Talk. If you’ve missed any of the previous articles, you can find them online for Technical Talk, Bonding with Masonry at Masonry Design magazine

David is a PE and SE with Biggs Consulting Engineering, Saratoga Springs, NY, USA (, 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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