Bonding with Masonry 2024: Q3

Words: David Biggs

This issue’s questions come from a Mason Contractor, an Architect, and an Engineer.  What questions do you have? Send them to info@masonrymagazine.com, attention Technical Talk.
 
Q.  A Mason Contractor writes that they have a project to install a concrete masonry veneer over existing interior concrete walls.  The design uses:

·       2-inch (nominal) thick (actual 1-5/8 inches), 8-inch high ASTM C90 concrete masonry units with a 1-inch cavity,
·       Type S polymer-modified mortar in the cavity (backing mortar) meeting ASTM C 1714 and ANSI A118.4,
·       ASTM C270 Type S bedding mortar, and
·       veneer anchors are required to be spaced at a maximum of 16” vertically and 24” horizontally.
 
The required veneer anchors were not specified. The Mason Contractor issued an RFI asking for the anchors to be specified and received a response from the Architect to have the anchor supplier make a recommendation.
 
The Mason Contractor has not installed a veneer like this before and asks for comments.
 
A.  Thank you for the question.  This is a unique application.
The information indicates the design uses aspects of both anchored veneers and adhered veneers. Both are described in TMS 402/602 Building Code Requirements and Specifications for Masonry Structures.
 
Some conditions to qualify as an anchored veneer:
1.     The veneer can be ASTM C90. but must be at least 2-5/8 inches thick.
2.     Anchors embedded in mortar must be at least 1-1/2 into the mortar and have at least 5/8 inches of cover.
3.     No backing mortar is required.
4.     Horizontal joint reinforcement can be used to provide crack control. NCMA recommends reinforcement at 16 inches on center.
 
Some conditions to qualify as an adhered veneer:
1.     The veneer must meet ASTM C1670, Standard Specifications for Adhered Manufactured Stone Masonry Veneer Units, or ASTM C1877, Standard Specification for Adhered Concrete Masonry Units.
2.     The veneer must be less than 2-5/8 inches thick and not weigh more than 15 pounds per square foot (psf) per TMS 402. (Note: There is a conflict in that TMS 402 does not distinguish between interior and exterior adhered veneer while IBC Section 1404.10.3,  Interior Adhered Masonry Veneers restricts interior masonry veneer to 20 psf.)
3.     Type S polymer-modified mortar that meets either ASTM C1714 or ANSI 118.4 is acceptable as backing mortar.
4.     The veneer shall develop a shear bond strength with the mortar substrate and concrete of not less than 50 psi.
 
Based on this information, the specified veneer system does not qualify as either an anchored or an adhered system.

·       As an anchored system, the veneer units are too thin, and embedded anchors would have insufficient mortar cover. 
·       As an adhered system, the veneer units do not meet ASTM C1670 or ASTM C1877 criteria and the units would be too heavy using normal-weight concrete (TMS criteria).
 
Summary:
           
1.     The veneer system specified is neither an anchored nor adhered system.
2.     Preferably, the design should be modified to meet the complete criteria of either an anchored or an adhered system.
3.     Another option would be to have the design tested and reviewed by the building official as an Alternative Construction Method per TMS 402. However, the building official is under no obligation to approve the system even with testing.
 
Q.        An Architect states that they specify masonry veneer ties and anchors that reduce thermal bridging. Figure 1 shows an example of an anchor (steel plate) they use when the support structure is galvanized cold-formed metal framing. For most projects, they use hot-dip galvanized ties and anchor plates, but on specific projects, they prefer to specify stainless steel for the tie and anchor plate for greater corrosion protection.
 
When they specify stainless steel anchors, a consultant has advised them to have the anchor plate painted where it will be in contact with the metal stud.  This recommendation only applies to stainless steel anchor plates and not to hot-dip galvanized anchor plates, so they ask, “Why is that?.”
 

Figure 1 - Thermal Tie™ (courtesy of FERO Corporation)
 
A.        Another interesting question! The short answer is that painting the stainless steel reduces the chance of galvanic action between the zinc of the hot-dip galvanized steel and the stainless steel.
 
For this answer, we turn to an article titled “Using Paint as an Electrical Insulator Between Dissimilar Metals” by the American Galvanizers Association (https://galvanizeit.org/knowledgebase/article/using-paint-as-an-electrical-insulator-between-dissimilar-metals). While zinc galvanizing and stainless steel are considered corrosion resistant, electrically they are still dissimilar materials.
 
Figure 2 shows a chart of the galvanic series of metals. You notice that zinc is on the anode side relative to stainless steel on the cathodic side. So, if a galvanic cell occurs, electrons will flow from the zinc to the stainless steel and the zinc will corrode. There is no certainty a reaction will occur because there needs to be moisture in all of this, either through a leak or condensation. Therefore, the probability of occurrence may be low if the wall is properly built.
 

Figure 2 – Galvanic Chart (courtesy of American Galvanizers Association)
 
So, back to the painting of the stainless steel, the American Galvanizers Association notes the priority of preference for protecting the materials as:
1.     Have both the cold-formed metal stud and the anchor plate of the same material, either hot-dip galvanized or stainless steel. This eliminates galvanic action between the two parts.
2.     If that is not possible, isolate the two materials with a rubber pad or other insulating material.
3.     Finally, using a coating of paint separating the two materials is an option that can weaken the possible galvanic action. However, they have additional recommendations regarding the paint application.
 
They state that paint may be an effective method of weakening one element of the galvanic cell by reducing the material’s exposure to an electrolytic solution. This will only be an effective method if
·       both the anodic material [hot-dip galvanized steel] and the cathodic material [stainless steel] are painted, or
·       if only the cathodic material [stainless steel] is painted. However, the anodic material should never be painted while the cathodic material is left bare.
 
The highlighted text (by the author) states what has been recommended by your consultant (paint just the stainless steel) when using stainless steel anchor plates in contact with hot-dip galvanized metal. Painting has the lowest priority of preference, and it is also an inexpensive method to further reduce the already low probability of galvanic action.
 
 Summary:
           
1.     If stainless steel anchor plates are to be in contact with hot-dip galvanized cold-formed metal framing, painting the stainless steel plate at the interface is one means to help reduce galvanic action. This is a judgment call by the designer as to whether the added protection of the paint is necessary.
2.     If the anchor plates and the metal studs are both hot-dip galvanized steel, there is no need for paint or insulation material between them.
 
 
Q.   An Engineer asks a question about a 20-foot tall concrete retaining wall that is being designed to be faced with an anchored veneer (cladding) over a 1-inch cavity/air space. She states that the Architect wants to use 3-inch thick granite in an ashlar pattern. Her question is whether it is appropriate to extend the cavity to the top of the footing (see the conceptual section, Figure 3).

Cavity extends to footing

Figure 3 – Conceptual structural section of retaining wall
   
A.    The cavity provides for drainage for any water that penetrates the stone veneer (and it will). If the cavity extends to the footing, the collected water will puddle in the cavity and freeze in the winter; both conditions are detrimental to the veneer.  Therefore, the cavity should not be extended below grade. 
 
The International Building Code (IBC) Section 1404.4.2 is intended for buildings, but the advice is appropriate for your retaining wall. It requires “Flashing and weep holes in anchored veneer designed in accordance with Section 1404.6 shall be located not more than 10 inches (245 mm) above finished ground level above the foundation wall or slab.”
 
The recommendation is to install veneer flashing with cavity weeps 6” to 10” above the finished grade.  Below the flashing, the cavity/air space should be filled solid to limit water into the cavity below grade.  This is the same detailing we would expect for a building cavity.
 
Looking at your section, there are two additional observations:
1.     There is a through-wall weep hole through the concrete. This should be extended to provide an extension so the water exiting the pipes does not run down the veneer.
2.     The batter of the wall combined with the thin stone veneer will present a construction challenge for the mason. Before the bedding mortar cures, the stone will want to roll into the cavity. Consider including 1-inch rigid insulation in the cavity or allow the mason to spot mortar behind the stone in the cavity to buttress the stone during construction.
 
 
 
Summary:
1.     Site retaining walls with an anchored cavity should be flashed and weeped like a building wall.
2.     Battered veneers need to be stabilized during construction. Discuss options with your local masons during the design phase.
 
 
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.
 
Keywords for this issue:  adhered veneer, anchored veneer, galvanic action, veneer anchors, cavity wall, battered retaining wall. 


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