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Structural silicon sealants were originally applied to bond glass panes to aluminum subframes in curtain wall facades of high rise buildings (structural sealant glazing systems, SSGS). However, structural silicones are increasingly being used to achieve soft structural connections between glass and aluminum or stainless steel or between glass and glass. These connection are employed to create ‘transparent’ glass structures where mechanical fixings are replaced by structural silicone joints (e. g. glass corners, glass fins bonded to fully glazed vertical glass). Two different types of structural silicone sealants are available:

u One-component siliconesstart curing as soon as they come into contact with moisture in the air. Optimum conditions for application are 24◦_{C at minimum 50% relative}

humidity. The diffusion-controlled curing process imposes practical limits on the geometry of the seal: recommended thickness> 6 mm, maximum width < 15 − 20 mm. The ratio of joint thickness to joint width must be at least 1 : 1 but no more than 1 : 3. A ratio of 1 : 2 is ideal. Depending on the thickness curing times up to 3 weeks have to be considered. If the seal is too thick, the interior parts may never cure completely.

u Two-component siliconesare cured by the polymerization reaction that is triggered by the mixing of the two components that consist of a base compound (about 90% by volume) and a catalyst (about 10% by volume). The curing does not require outside chemical components. Diffusion lengths among the two components are very small and curing will progress relatively quickly (curing time less than 3 days), homogeneous and independent of the joint size. The recommended minimum thickness is 6 mm, the maximum width is 50 mm. Depending on manufacturer recommendations or design codes a maximum ratio of of joint thickness to joint width of 1 : 4 is allowed. Proper mixing is very important and must be checked frequently during application. Therefore application of two-component silicones on the building site is generally problematic and should be avoided.

Material properties may differ from one manufacturer to another common values are given in Table 7.18. In small-scale short-term laboratory testing structural silicone sealants typically achieve tensile strengths of around 0.8 MPa to 1.8 MPa for dynamic loading, depending on the temperature. However, allowable stresses for wind loads are usually much lower. Creep is initiated under long-term loading stresses equivalent to roughly

10% of the short term strength. Long-term strains in excess of creep levels will lead to relaxation. This will reduce the stresses but no failure will occur, as long as permissible strains are not exceeded. Structural silicone joints are normally designed in terms of allowable stresses (Table 7.18) which are in turn based on the ultimate strength and a safety factor of 6. This means that the allowable design strain range is±12.5% of the ultimate strains. At these relatively low level of strains used for design purposes it is sensible to assume an elastic behaviour.

The very low modulus of elasticity constitutes both an advantage and a disadvantage. On one hand it reduces stress concentrations, but on the other hand structural silicone sealants are not suitable to transfer high shear forces required for built-up sections of glass (e. g. T-section, H-section).

When used in combination with with laminated safety glass, structural silicone sealants show good behaviour in case of protective glazing (i. e. where facades are subjected to impact or blast loads). This is due to the soft material behaviour which has a capacity to absorb high amounts of energy.

The design of a structural silicone joint must allow for sufficient load-carrying strength in order to transfer the applied loads. At the same time the allowable strain of the silicone sealant must not be exceeded. The maximum strain is particularly critical if two materials with different coefficients of thermal expansion are bonded together. For this reason joint geometries with adhesion to three surfaces such as L-shaped joints have small displacement capacities (Figure 7.19) and should be avoided for example when the glass panel is glued with all edges onto an aluminium frame in a curtain wall facade element. The influence of SSG joints on the load carrying behaviour of glass panels is studied in [327]. In case of a high glass edge rotation due to glass deformations in combination with large structural sealant joints the resulting additional tensile stress in the joint has to be taken into account. Due to the Poisson’s ratioµ of nearly 0.5 the stiffness of a structural sealant joint depends strongly on the geometry of the joint - this explains the recommended

Table 7.18: Typical material properties of structural silicone sealants (manufacturers data). Allowable tensile stress, short term loads σall,short MPa 0.14

Allowable tensile stress, long term loads σall,long MPa 0.014

Allowable shear stress, short term loads τall,short MPa 0.070-0.128

Allowable shear stress, long term loads τall,long MPa 0.007-0.011

Young’s modulus of elasticity, short term loads Eshort MPa 1.0-2.5

Maximum allowable strain[213] "all – ±12.5%

Poisson’s ratio ν – 0.49

Figure 7.19:

Example of a good (two sided) and a poor (L-shaped) structural silicone sealant joint.

t allowable deformation t/2 12.5% t glass structural silicone sealant glass structural silicone sealant a allowable deformation 12.5% a t Good structural

width to thickness ratios given by manufacturers. For structural applications, i. e. the fixing of glass fins or any in plane load introduction, different joint configurations are shown in Figure 7.20. The load carrying behaviour depends on the ratio between face side bond length and lateral bond length. The load is mainly transferred over the face side joint which is subjected to tension. Due to the lateral elongation restraint in the U-channel the lateral bond length has only a small influence on the joint stiffness. Approved design methods for these types of connections do not yet exist and research is still ongoing [54, 186].

a) U-shape joint b) T-shape joint c) L-shape joint Structural

silicone

F F F Figure 7.20:

Different structural silicon joints.

In some countries such as Germany or France structural silicone sealant cannot be used to carry permanent loads (i. e. dead load) and SSGS facades require additional mechanical fixings to prevent the glass from falling once the silicone fails.

Furthermore, the chemical compatibility of all materials in contact with the silicone must be ensured in order to ensure long-term performance and prevent damage. The compatibility of any material that the structural silicone adhesive comes in contact with (e. g. gaskets, spacers, backer materials, setting blocks) has to be approved by the manufacturer or should be tested in the laboratory. EPDM, neoprene, bitumen, asphalt and other organic-based membranes, coatings and gaskets often cause discoloration of light coloured silicone sealants. These materials are often approved for incidental contact with the structural silicone but are not approved for full contact as a structural spacer material[7].

The design values provided by manufacturers are based on the assumption that the sealant fails due to loss of cohesion (i. e. failure within the silicone) rather than adhesion (i. e. failure at the silicone-adherend interface). Adhesion quality is mainly affected by the surface quality of the connected materials (adherends). Flat surfaces such as glass provide the best conditions while surfaces with pores are unfavourable as they only allow adhesion to the local peaks in the material. Aluminum, anodized aluminum, stainless steel (not brushed or with satin finish) and some powder coatings offer good conditions for adhesion. Some glass coatings (e. g. most self-cleaning coatings) are not suitable with structural silicone. All surfaces have to be primed prior to the silicone application[86].

In Europe the application of SSGS are regulated in[52, 74, 161]. A European Technical Approval (ETA) for any silicone used in structural applications is needed. EOTA 1998 [161] defines permissible stresses for loading in dynamic (short-term) tension, dynamic shear and in permanent shear (but not for permanent tension). For dynamic tension it requires that the 5%-fractile value of the strengths measured on small scale tests must exceed the permissible stress by a factor of 6. For permanent shear a minimum creep

factor of 10 is defined. Therefore permissible stresses for permanent shear loads are usually 10 to 15 times lower than for dynamic shear loads. The design method given in EOTA 1998[161] is limited to four side supported glass (SSGS glued or mechanical fixed) panels with a linear SSGS joint over the entire glass edge. This quite rough design approach does not take into account the stiffness of the supporting frame or the non linear stress distribution along the glass edge in the SSGS joint due the deformation of the glass panel. An application of_{[161] for any structural silicone sealant connection is therefore} not useful._{[161] requires all SSGS connections to be made in the factory rather than on} site. This is because proper execution of an SSGS joint requires a controlled climate and clean surroundings. However, in all-glass structures some SSGS joints may have to be applied on-site. This will require special measures to ensure a proper environment and even more stringent quality assurance procedures. Even so, it should be noted that the structural quality of an SSGS joint cannot be tested non-destructively.

In the Unites States, SSGS applications are regulated by[1, 14]. The design principle is similar to the European approach. Dow Corning provides a detailed design guide and examples on detailing[85].