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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