Adhering thin-set veneers on multi-storey buildings

by Katie Daniel | May 25, 2017 11:20 am

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By Mark D. Hagel, PhD, P.Eng., and Nicholle Miller
The use of adhered thin-set veneers over a structural backup gained popularity over the last two decades as the Canadian construction industry moved toward lighter and quicker methods of construction. Lighter and often quicker to install than full-bed stone veneers, these veneers can replicate their look and feel.

It is interesting to note the use of thin-set veneers (albeit mechanically anchored rather than adhered) is more traditional than one would suspect. At the end of the 19^th century, natural thin-set veneers (less than 50 mm [2 in.] thick) began to be commonly used, and one of their earliest applications was marble cladding on the Colosseum in Rome. (For more, read Michael J. Scheffler’s 2001 article, “Thin-stone Veneer Building Façades: Evolution and Preservation,” originally published in APT Bulletin Vol. 32, No. 1.)

Adhered masonry veneers first gained popularity with single-family homes, but have more recently entered the multi-storey commercial and residential building markets. Unlike tied masonry veneer systems, adhered masonry veneers rely solely on the setting bed mortar to attach the thin masonry cladding to the support structure. Multi-storey installations can create greater stresses on adhered veneers from differential movements and larger structural loads, and this results in a greater risk of injury or damage to property if the bond fails and units fall from a greater height.

With the release of the National Energy Code for Buildings (NECB) 2011, many veneer products are also being installed over rigid or semi-rigid insulation. These factors increase the importance of the setting bed mortar’s ability to provide sufficient structural capacity to anchor the adhered masonry veneer to the structure. This article explores shear bond strength test results of adhered manufactured stone veneer units when traditional Type N and Type S mortars are employed as the setting bed. The results of using polymer-modified stone veneer mortars and improved modified dry-set cement 
(i.e. thin-set) mortars were also investigated. Design and installation recommendations that can help mitigate bond failures on multi-storey buildings were extrapolated from this testing.

Examples of adhered stone veneer failures observed in the field
In the past, Type N and Type S mortars used in traditional full-bed masonry veneers have been employed to construct the scratch coat that embeds the metal lath. They have also been used for the setting bed that bonds the adhered masonry unit to the scratch coat in adhered thin masonry veneer.

In full-bed masonry veneers, these mortars are predominantly relied on for strength in compression in the bed joint and to aid in mechanical anchorage of the masonry veneer to the structural wall using metal brick ties. Bond failures in the field with Type N and Type S mortars have been observed to occur at the setting bed/masonry unit interface (referred to as Failure Mode 1), as illustrated in Figure 1, or at the setting bed/scratch coat interface (Failure Mode 2), as illustrated in Figure 2. The former has been most frequently observed in the field by this article’s authors.

The pre-blended mortar and grout industry has responded by introducing polymer-modified stone veneer mortars and adapting modified dry-set cement mortars from directly adhered ceramic-tile applications. A testing program was commissioned by Alberta Masonry Council to investigate the performance of several of these mortars.

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Adhered stone veneer bond testing
The Alberta Masonry Council testing investigated the shear bond strength of four popular brands of adhered manufactured stone, with four types of mortar used for the setting bed and with veneers adhered to six different substrates. Adhered manufactured stone veneer units are typically made from lightweight concrete and must weigh less than 0.72 kPa (15 psf) according to ASTM C1670-16, Standard Specification for Adhered Manufactured Stone Masonry Veneer (AMSMV) Units. (For more, read Michael J. Scheffler’s 2001 article, “Thin-stone Veneer Building Façades: Evolution and Preservation,” originally published in APT Bulletin Vol. 32, No. 1.) The substrate is typically a 13-mm (½-in.) mortar embedding a self-furring metal lath. The mortar (i.e. scratch coat) is scored to provide better bond, and is required to be a minimum 13 mm and a maximum 20 mm (¾ in.) according to ASTM C1780-2016a, Standard Practice for Installation Methods for Adhered Manufactured Stone Masonry Veneer. The mortar used to embed the metal lath can be Type N, Type S, or modified.

Exterior-grade cement board is also becoming more common, and is now permitted by ASTM C1780. This replaces the metal lath and mortar scratch coat, but requires the use of a modified dry-set cement mortar compliant with ANSI A118.4, American National Standard Specification for Modified Dry-set Cement Mortar, or ANSI A118.15, American National Standard Specification for Improved Modified Dry-set Cement Mortar. Regardless of the substrate (scratch coat and lath or cement board), Clause 7.2.1 of ASTM C1670 and Clause X1.2 of ASTM C1780 require a shear bond strength of 0.35 MPa 
(50 psi) using a shear bond test modified from ASTM C482-02, Standard Test Method for Bond Strength of Ceramic Tile to Portland Cement Paste.

Figure 3 outlines the combinations of stone brand, substrate, and setting bed investigated in the testing program. Shear bond tests of three samples for each combination were completed. The samples consisted of full-size manufactured stone units adhered to a mortar block or cement board substrate and tested according to a modified ASTM C482-02 test. As well, stone veneer units were provided from suppliers, rather than directly from the manufacturers, but the thickness of the manufactured stone samples varied depending on the manufacturer.

Stone samples of approximately 100 x 100 mm (4 x 4 in.) in dimension were cut as needed. The scratched surface was created with a 6-mm (¼-in.) square notched trowel. After the mortar blocks had air-cured for 24 hours (with a typical laboratory temperature of 21 C [70 F] and 25 per cent relative humidity [RH]), a mortar for the setting bed was mixed and applied to the manufactured stone unit sample using a pointed (brick) trowel, or a fluid-applied structural weather-resistant barrier (WRB) was applied to the cured simulated scratch coat and allowed to cure. Cement board substrate samples were constructed using a 13-mm (½-in.) exterior-grade cement board fastened with the manufacturer’s fasteners to a 2×6 wood stud cut to 150 mm (6 in.) in length. A fluid-applied structural WRB was then applied to the cement board and allowed to cure.

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A fully buttered manufactured stone unit sample was then adhered to the simulated scratch coat mortar block or cement board according to the mortar or stone manufacturer’s installation instructions. The sample was air-cured at room conditions for seven days prior to testing. The samples best represent a dry-stack adhered stone installation, or a jointed application where the joints are installed with a grout bag after the stones are laid. The testing program deviated from the required test method required by ASTM C1780 and ASTM C1670, which modify ASTM C482-02. However, it was felt the revised method better represented actual construction practices in Alberta. Some of the modifications to ASTM C482-02 included:

• reduced curing time;
• scratched mortar block surface; and
• use of coupons cut from full-size 
stone units from suppliers rather than from manufacturers.

A fixture used to load the specimen in shear was fabricated using the specifications of the “Fixture” prescribed in ASTM C482-02 (Figure 4). One or more metal strips were set on a flat face to help localize the force applied by a manual hydraulic press.

The two distinct failure modes observed in the field were also observed with the shear bond tests when pre-bagged Type N, Type S, and polymer-modified stone veneer mortars were used for the simulated scratch coat and setting bed (Figure 5).

In contrast, ANSI A118.15-compliant, improved modified dry-set cement mortar only had one mode of failure, regardless of whether the substrate was a Type N simulated scratch coat with or without the structural WRB, or cement board with the structural WRB (Figure 6).

Figures 7 and 8 provide the average values from the shear bond strength testing, and are organized according to failure mode. Figure 9 offers the maximum shear bond values obtained from the testing of the three samples.

Figure 9 illustrates the use of Type N and Type S mortars for the setting bed did not achieve the 0.345 MPa (50 psi) shear bond requirement of ASTM C1670 and ASTM C1780 when tested according to the modified ASTM C482-02 procedure used in this testing program. It is important to note the modifications to ASTM C482-02 were implemented to better represent the installation methods for dry-stack adhered manufactured stone veneers typically employed in Alberta, and may be solely relevant to this province.

Figure 9 demonstrates the use of an improved modified dry-set cement mortar for the setting bed guarantees Failure Mode 2 regardless of the substrate. The shear bond strength values exceed the 0.345 MPa (50 psi) minimum in each case, but the information from Failure Mode 2 provides the shear bond capacity of the substrate, rather than the bond between the stone unit and substrate. 
For example, an average strength of 0.71 MPa 
(103 psi) for the cement board structural WRB and thin-set mortar combination in Figure 9 only indicates the shear capacity of the cement board is 0.71 MPa. This value does not represent the ultimate bond strength of the setting bed to the substrate or the setting bed to the stone unit.

Recommendations for multi-storey adhered stone veneer installations
A typical 100 x 200-mm (4 x 8-in.) stone unit weighs approximately 1 kg (3 lb) and could, hypothetically, support 2.71 kN (610 lb) in shear. In theory, the shear capacity should never be exceeded. However, building movements from thermal loads, moisture, or loading can exceed 0.345 MPa (50 psi). The direct-adhered ceramic tile and thin-brick design guide  requires use of ANSI A118.4-compliant mortars; it also recommends horizontal movement joints be installed at floor levels on multi-storey buildings, and vertical movement joints at 5 m (20 ft) on centre (o.c.), to help mitigate these forces.

The design guide also recommends a deflection limit of h/600 be used for the structural backup, rather than the typical h/360. A similar philosophy should be adopted for multi-storey adhered masonry veneer applications, as their installation is very similar and relies solely on the bond of the setting bed to the unit and substrate. These recommendations are also supported by the test results. Figure 10 illustrates foundation- and floor-level details.

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Recommendations for adhered stone veneer installations over exterior insulation
In order to comply with NECB 2011’s high R-value requirements in colder climates, adhered masonry veneer products are being installed over exterior insulation. Similar recommendations 
are utilized to ensure sufficient bond for installations of adhered masonry veneers over exterior insulation.

Most installations over exterior insulations use Z-bars to fur out the wall. Although long fasteners can be employed, it is often more difficult to engage a stud backup wall, and the fasteners tend to be more robust and expensive than Z-bars. Focusing on the Z-bar installation, the temptation is to design the Z-bars to be installed horizontally (i.e. perpendicular to the studs) to reduce thermal bridging and, in theory, eliminate cutting the insulation boards (typically 610 mm [24 in.] wide) down to 406 mm (16 in.), as is necessary to fit between Z-bars when they are vertically oriented and fastened to studs typically spaced at 406 mm o.c. However, installing Z-bars horizontally loads them along their weak axes, and if this method is used, an increase in the Z-bar gauge is required to reduce the deflection. In addition to excess deflection, horizontally oriented Z-bars can more readily trap moisture, and gaps must be intentionally left to allow water to flow past them.

The required spacing for lath or cement board is 406 mm (16 in.) o.c. This eliminates the possibility of spacing horizontal Z-bars to be consistent with the insulation board width of 610 mm (25 in.). Therefore, it is recommended to install Z-bars vertically (parallel to the studs). The other recommendation is to use cement board to replace both the lath and scratch coat, as doing so means the designer is guaranteed the thickness specified. Ideally, this size is 16 mm (⁵/8 in.) to reduce deflection between the Z-bar supports. The use of cement board requires ANSI A118.4-compliant mortars. A 13-mm (½-in.) oversizing of the Z-bar can provide drainage behind the adhered stone veneer without the necessity for a drainage mat. Figure 11 provides a foundation and floor detail of adhered stone over exterior insulation.

Conclusion
Adhered masonry veneers replicate the look and feel of full-bed masonry, but their reliance on mortar bond to anchor them to the structural backup makes it extremely important to use the proper products and pay additional attention to installation practices when exceeding 3 m (10 ft) above-grade. This becomes even more important when a multi-storey building uses different materials, such as a combination of steel stud backup walls and concrete floors, or when cantilevering over rigid insulation.

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[5]Mark D. Hagel, PhD, P.Eng., is the executive director of the Alberta Masonry Council. He holds bachelor’s degrees in actuarial science/applied mathematics and civil engineering, and a doctorate in civil engineering. Hagel was previously employed as a technical services engineer for the Canadian Concrete Masonry Producers Association (CCMPA) and as a structural and building envelope engineer with a Calgary design firm. He can be reached via e-mail by contacting markhagel@albertamasonrycouncil.ca[6].

 

[7]Nicholle Miller is the marketing co-ordinator of the Alberta Masonry Council. She holds a degree in business administration with a double major in marketing and human resources. Prior to her employment with the Alberta Masonry Council, Miller worked as a proposal co-ordinator with Aecom, formerly Flint Energy Services. Additionally, she has served as a Calgary Corporate Challenge co-ordinator and a CANstruction co-ordinator for the Calgary Food Bank. Miller can be reached via e-mail at nicholle@albertamasonrycouncil.ca[8].

Source URL: https://www.constructioncanada.net/adhering-thin-set-veneers-on-multi-storey-buildings/

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