CWCT TN20 Design of Sealant Joints (PDF)

Technical Note No. 20 DESIGN OF SEALANT JOINTS Introduction assumptions and if necessary repeating the design with modified values until all A joint may be defined as a discontinuity in...

Technical Note No. 20 DESIGN OF SEALANT JOINTS Introduction assumptions and if necessary repeating the design with modified values until all A joint may be defined as a discontinuity in the requirements are satisfied. fabric located in a predetermined position between either similar or dissimilar materials. Joint location Joints take many forms and are required to fulfil different functions. General requirements for The location of joints is often dictated by: joints are described in Technical Note 16 Joints in the building envelope.
The aesthetic requirements of the façade;
Materials (e.g. dimensional changes that This Technical Note gives guidance on the must be accommodated); design of joints that are to be sealed with a wet applied sealant.
The size of individual panels/components. Design requirements A design may rely on a small number of widely- spaced joints designed to accommodate large The requirements for a sealant joint are movements, or a large number of more closely- generally as follows: spaced joints which demand less of each individual sealant joint. The decision will be
Provide a weathertight seal; influenced by the factors above, for example, closer spacing generally permits narrower and
Accommodate variations in joint size arising hence less conspicuous (but more numerous), from induced deviations (tolerances); joints.
Accommodate inherent deviations (movement); Choice of sealant
Durable; There are four generic types of high
Aesthetically acceptable; performance sealant currently in use as follows:
Buildable.
Acrylic, The key steps in the joint design process are as follows:
Polysulfide,
Selecting the joint locations;
Polyurethane,
Choice of sealant;
Silicone.
Determining the geometry of the joint. Technical Note 19 Selection and use of sealants The design process may require assumptions to describes their properties. be made to allow an initial joint design to be carried out followed by checking against the © CWCT 1999 January 2000 This document has been printed from the CWCT ‘Cladding Forum’, access to which is restricted to subscribing Members of the Centre for Window & Cladding Technology. Information about the availability of CWCT publications and membership is available at our website – www.cwct.co.uk – or from the address at the end of this note. Design of sealant joints TN20 Detailing the joint Joint types Gaps between components may be detailed as Joint components butt, lap or fillet joints, each of which vary in A typical sealant joint comprises a number of terms of: components as shown in Figure 1.
Appearance; Backing strip
The forces induced in the sealant bead (and hence its ability to accommodate movement);
Protection against degradation (from weathering, vandalism);
Access (i.e. ease or sealing/resealing). Primer
Butt joint Butt (tension/compression) joints between Sealant ideally parallel and flush joint sides are readily accessible for installation, inspection, Figure 1 Components of a typical sealant joint maintenance and replacement. The ability of all butt joints to accommodate in-plane deviations
Primer - improves the inherent adhesive is limited; for example, if constructed too strength and/or bond durability between the narrow, the joint will be hard to prepare and the sealant and the prepared substrate. It may movement capability of the sealant will be also help limit plasticizer migration. reduced. Out-of-plane deviations in the joined
Backing strip - forms the correct joint depth. components can lead to variations in joint depth, These should preferably be based on closed- which may result in sealant failure. Due to their cell cellular polymers (e.g. polyethylene), exposure sealed butt joints are vulnerable to the and are commonly supplied in the form of weather and attack from vandals, birds etc., cut sections, sheets or rods. although they can be recessed to provide a degree of protection.
Bond-breaker tape - ensures the sealant is bonded only to the two faces of a movement
Lap joint joint so the sealant mass is free to deform; it may be a self adhesive strip of polyethylene For a given movement, a lap (or shear) joint or PTFE. The bond-breaker tape may not be places much less stress on the sealant than the required with all types of backing strip. equivalent butt joint. Therefore, under appropriate conditions a sealant will be able to
Joint filler - can be used to help form the accommodate greater movement in this joint prior to sealant installation; they are configuration. However, lap joints are usually often retained in the joint in the final works more difficult to design into a structure and are to provide support to the sealant or to less easily sealed and maintained, although they exclude debris that could block the joint are less exposed to the weather. faces. Joint fillers may be made of wood fibre/bitumen or cork and resin/bitumen in sheet or strip form.
Sealant - provides the weathertight flexible seal and the finished outer surface. 2/6 Design of sealant joints TN20 stress to the sealant under movement, and in Sealant the latter two cases to ensure adequate depth for optimum service life. Elastic sealants: 2:1 Width Depth Elasto-plastic sealants: 2:1 to 1:1 Plasto-elastic sealants: 1:1 to 1:2 Backing strip Plastic sealants: 1:1 to 1:3 Figure 2 Lap joint
Lap
Fillet joint To maximise movement accommodation the optimum configuration is width  depth Fillet joints work in combined (Figure 2). For fixed lap joints, the sealant tension/compression/shear and can be depth may be increased relative to joint particularly useful where the close proximity of width in order to ensure a weathertight seal. adjacent components precludes the use of a butt Sealants in lap joints are subject to the same joint. Fillet joints are only suitable for certain minimum depth requirements for adhesion as low movement applications (e.g. to seal some butt joints. window perimeters) and are vulnerable to the weather and attack.
Fillet Joints need to be the correct shape and size. The bulk of the sealant acts as a considerable barrier to movement. To maximise movement, diagonal geometry back-up rods, which omit the sealant at the root of the Slightly concave fillet, may be used. The sealant should ideally be applied such that it is at least 6mm thick with at least 6mm ‘bite’ onto the adjacent surfaces. Concave fillet seals are easier to tool whereas convex seals Slightly convex accommodate greater movement and perform better when properly made (Figure 3). Figure 3 Fillet joints Joint functions Sealant bead aspect ratios Joints may be static (i.e. fixed) or dynamic (i.e. moving). Fixed joints may be provided simply All types of sealant joint need the correct to: geometry and size of sealant in combination with bond breakers to perform successfully.
Enable manageable-sized components to be assembled into a complete element on site,
Butt joints
Accommodate positional and dimensional The following sealant width/depth ratios are variations in the structure and cladding. normally recommended, in the first two cases to ensure adequate bond area to minimise 3/6 Design of sealant joints TN20 Most joints in window, cladding and curtain components. Site construction/erection walling systems are dynamic and will have to processes (e.g. of the building structure and accommodate the following additional cladding openings) are more variable and requirements: therefore of greater importance in the joint design process. Sources of information on
Single uni-directional movements (e.g. variability are listed in Table 1 of Technical concrete drying shrinkage and creep, ground Note 21 Tolerance, fit and appearance of settlement); cladding.
Repeated reversible movements (e.g. Joints (particularly sealed butt joints) should not dimensional changes due to thermal and be expected to overcome situations where the moisture movement). agreed tolerances in the structure or cladding have been exceeded because to do so can A major step in the design of sealant joints is compromise the durability of the sealant if the selection of the joint width. The width must be joint width is too narrow or wide. sufficiently great to allow joint movements to occur without causing excessive stress in the Calculation of joint width sealant. This must be achieved over the range of possible joint widths resulting from the Most sealant manufacturers assign a movement tolerances in the cladding components and accommodation factor (MAF) to each of their ambient conditions at the time the joint is products to provide a basis for the calculation of sealed. However, if the joint is excessively joint width. The MAF is defined as the wide, it may be uneconomic due to the large maximum movement that the sealant can quantity of sealant required, visually accommodate expressed as a percentage of the unacceptable and the sealant may be difficult to minimum joint width. The minimum joint width apply and prone to slumping. is therefore given by: To calculate the joint width it is, therefore, M  100 w (1) necessary to know the range of movement to be MAF accommodated, the total tolerance on the joint width and the amount of movement that the where: selected sealant can accommodate.
M is the total expected movement; Movements and dimensional changes
w is the minimum joint width The total expected movement of a dynamic cladding sealant joint can be estimated by In practice, while the joint may be designed to combining: be, say, 16mm wide, and sealed with a sealant having a MAF of 25 per cent, the joint may
Thermal and moisture movements of the actually be sealed when thermal expansion has components (BRE Digest 228); given rise to, say, 2mm of movement. This results in a reduction of the actual joint width
Movements due to externally applied loads from 16 to 14mm. This joint would then cool (e.g. ground settlement/heave, creep, dead and subsequently subject the sealant to the full and live load deflections) anticipated by the movement of, 4mm. Thus, the joint could open project structural engineer. from 14 to 18mm - an extension of 28.6 per cent. This problem is overcome by adding the Tolerances total expected movement to the design joint British Standards stipulate the permissible width. Thus equation 1 becomes: deviations in the manufacture of most cladding 4/6 Design of sealant joints TN20 M  100 Depth of sealant w M (2) MAF The optimum sealant shape will provide adequate bond area to the substrates, yet impart Equation ‘2’ has also to be modified to take minimal stress to the sealant under joint account of variability in the production and movement. positioning of components (induced deviations), which again results in an increase in design joint
Too deep a joint will create resistance to width giving the final design formula as follows: movement and may increase the risk of failure M  100 w MV (3)
Too shallow may risk concentrating MAF movement over too small a sealant depth, where V is the overall induced deviation or increasing the risk of splitting, or may variability. provide inadequate bond area or inadequate resistance to weathering. Irrespective of movement accommodation, an actual width of at least 6mm is essential to The depth of the sealant bead can be calculated ensuring adequate access for joint preparation from the width and the permissible aspect ratio and sealing. for the type of sealant. This value can then be compared with the acceptable limits for the type Having established a value for the design joint of sealant. In most cases this will be between 6 width the range of possible joint widths which and 20mm. may be obtained can be calculated and compared with other requirements of the joint Installation such as appearance and practical limitations on the width at which the sealant can be applied. Satisfactory performance requires good site practice in addition to good design. The For example, if in the joint described above the following should be considered. overall tolerance is 5mm, the design joint width would be 25mm. The actual joint width Uniformity would range from 20mm to 30mm depending on Joint movement will often occur during sealant the actual deviations present and would be cure - when modulus and bond strength are subject to a total movement of 4mm. If the joint under development - and form unsightly and gap is set when the components are near the mid uneven bulges or hollows in the seal. Slow point of their movement range, the joint width curing sealants (e.g. one-part polysulfides and would range from 18mm to 32mm. However, if polyurethanes) and lightweight, dark coloured the joint width is set when the components are cladding (e.g. aluminium sheeting) are most at at an extreme condition the joint width could risk. range from 16mm to 34mm. Out-of-plane deviations in the joined Values at the upper end of this range may be components can lead to variations in joint depth, aesthetically unacceptable or too wide to which may result in sealant failure. prevent slumping of the chosen sealant. In this case it would be necessary to redesign the joint Tooling varying one or more parameters such as the type The tooling of butt joints against generally of sealant, spacing of joints or tolerance on circular backing rods tends to produce concave components. top and bottom surfaces. This maximises the area of sealant in contact with the substrates and 5/6 Design of sealant joints TN20 minimises the ‘bulk’ of sealant that resists design, specification and construction, movement, so providing the ideal geometry for Construction Industry Research and Information sealed butt movement joints. Tooling also Association. expels trapped air within the installed sealant and aids adhesion to the sides of the joint. Component temperatures The joint will be able to accommodate the greatest range of movement if it is sealed when the joint gap is close to the mid point of its range which will normally mean avoiding extremes of temperature. Application temperatures should also be limited due to the effect on properties of the sealant. At high temperatures the viscosity of the sealant may be reduced increasing the risk of slumping. The working life of the sealant may also be reduced. At low temperatures the viscosity may be increased and impair toling of the sealant. Summary The joint design process comprises the following stages: 1. Calculate joint movement; 2. Estimate joint size variability; 3. Select joint seal; 4. Size joint; 5. Detail joint; 6. Specify joint configuration and seal materials. References and Bibliography BRE, 1988, Estimation of thermal and moisture movements and stresses: Part 2, Building © CWCT 2000 Research Establishment Digest 228. CWCT Technical Notes 1 – 30 have been part-funded BS 6093, 1981, Code of practice for Design of by the DETR under research contract 39/3/338 (CI 1354) joints and jointing in building construction, University of Bath, Claverton Down, Bath, BA2 British Standards Institution. 7AY Tel: 01225 826541; Fax: 01225 826556; email: CIRIA Report R 178, 1998, Sealant joints in the [email protected]; website: www.cwct.co.uk external envelope of buildings: a guide to 6/6