Resistance to Tear Propagation

In addition to a sealant’s tensile adhesion strength and elongation capacity, its tear propagation resistance can play an important role under strain and repeat loading. Some structural silicone formulations propagate tears more readily than others and a product weak in this aspect would lead to a lower system drift capacity, as shear failure could occur earlier (assuming a tear site emerged in the structural seal during earthquake racking and was thus susceptible to

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Rochester Institute Of[43]Technology pursuant to License Agreement. No further reproductions authorized.

TABLE 1—High strain rate data in tension and shear.

High strain rate data for two-part high modulus structural sealant

Strain rate, m/s (average values from ten specimens, each configuration ASTM Cl 135 or C961 [15] test and strain rate дц specimens failed cohesively)

specimen with dimensions noted

(see Figs. 16 or 17) 0.00085a 0.5 1.1

о Max load—Newton (lb), % change0

1192 (268)

1842 (414), 55 %|

1975 (444), 66 %T

“Max stress—MPa (psi), % change0

1.23 (179)

1.90 (276), 55 %T

2.04 (296), 66 %T

Elongation at max load, % change0

79

128, 62 %|

145, 84 %T

Elongation at break, % change0

99

184, 86 %T

209, 111 %T

Tension 0135, Newton (lb) 0.5 wide x 0.5 deep

x 2 in. long (12.7

x 12.7 x 50.8 mm), see Fig. 16

Max load—Newton (lb), % change0

743 (167)

1254 (282), 69 %

1383 (311), 86 %T

Max stress—MPa (psi), % change0

1.15(167)

1.94 (282), 69 %T

2.14(311), 69 %T

% Elongation at max load, % change0

126

210, 67 %|

230, 83 %T

% Elongation at break, % change0

141

254, 80 %T

290, 106 %T

Shear C961, Newton (lb) 0.25 thick x 0.75 long x

1 in. wide (6.4 x

19 x 25 mm), see Fig. 17

Max load—Newton (lb), % change0

529(119)

1058 (238), 100 %T

1019(229), 92 %|

Max stress—MPa (psi), % change0

1.10(159)

1.6(317), 99 %T

1.6 (305), 92 %|

% Elongation at max load, % change0

112

153, 37 %|

149, 33 %T

% Elongation at break, % change0

131

207, 58 %T

Tension 0135, Newton (lb) 0.25 wide x 0.75 deep x 2 in. long (6.4 x 19 x 25 mm), see Fig. 16

propagate) than with a product more resistant to this action. This type of behav­ior may have possibly been exposed in a field inspection of a four-story four­sided SSG facade following the 1994 Northridge earthquake near Los Angeles, U. S. [8]. The building inspected was glazed and constructed in 1979-1980 utiliz­ing an acetoxy cure structural silicone sealant, a recognized trait of which is lower resistance to tear propagation. Had a structural silicone with increased resistance to tear propagation been used in that design, it is possible that less damage to that curtain wall system may have occurred from the movements estimated and realized during that event.

The Guideline for European Technical Approval for Structural Sealant Glazing Kits (ETAG 002) [16] provides one measurement of this property. Sec­tion 5.1.4.6.4 of this guideline presents a method to assess “resistance to tear­ing," which encompasses purposefully introduced cuts or “nicks" of defined dimension into the rubber of standardized specimens, see Fig. 6. These nicked specimens are then pulled to destruction on tensile-elongation equipment with the result of the test categorizing the product into one of two categories. Use category 1 represents a rubber that retained at least 75 % of un-cut control specimens, and use category 2 represents a rubber that retained at least 50 % but less than 75 % of un-cut control specimens. A rubber not retaining at least 50 % of controls does not meet the requirements of the document and may not be used in adhesively bonded SSG construction. Considering the above discus­sion regarding resistance to tearing, logic would suggest that a product that tests to the “use category 1" classification should bear consideration for seismic SSG designs.

Table 2 below shows the ETAG 002 tested category of several commercially available structural sealants as outlined in their respective ETA document.

From the information contained in Fig. 5 and Table 2, it is evident that there is sufficient variability in the tested mechanical properties of current com­mercially available structural silicone sealants and that some may be more appropriate for use in seismic regions than others. These highlighted properties of a structural sealant can play a more important role in the performance of SSG systems in seismic regions and product selection should be a consideration for use in such designs. In addition to the mechanical properties discussed, it

FIG. 6—Tear resistance specimen configuration and location of nicks.

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TABLE 2—Tear propagation use category per ETAG 002.

Sealant type

Claimed movement

classa

Availability

Use

category

2-Component structural silicone

12

Americas, Asia, Europe

2

2-Component structural silicone

25

Americas, Asia

1

1-Component structural silicone

25

Euro, Asia

1

2-Component structural silicone

25

Americas

2

2-Component structural silicone

Europe, Asia

1

1-Component structural silicone

Europe

2

aWhen tested to ASTM C719 [17].

goes without saying that adhesion quality and adhesive bond durability are equally important factors in the successful use of these systems.