01.07 Sealant joints

Categories: Envelope Sealing

Introduction
General requirements for joints are described in Section 01.01 and Section 01.02.  This Section gives guidance on the design of joints that are to be sealed with a wet applied sealant.
 

Design requirements
The requirements for a sealant joint are generally as follows:

• Provide a weathertight seal;
• Accommodate variations in joint size arising from induced deviations (tolerances);
• Accommodate inherent deviations (movement);
• Durable;
• Aesthetically acceptable;
• Buildable.

The key steps in the joint design process are as follows:

• Selecting the joint locations;
• Choice of sealant;
• Determining the geometry of the joint.

The design process may require assumptions to be made to allow an initial joint design to be carried out followed by checking against the assumptions and if necessary repeating the design with modified values until all requirements are satisfied.
 

Joint location
The location of joints is often dictated by:

• The aesthetic requirements of the façade;
• Materials (e.g. dimensional changes that must be accommodated);
• The size of individual panels/components.

A design may rely on a small number of widely-spaced joints designed to accommodate large movements, or a large number of more closely-spaced joints which demand less of each individual sealant joint.  The decision will be influenced by the factors above, for example, closer spacing generally permits narrower and hence less conspicuous (but more numerous), joints.
 

Choice of sealant
There are four generic types of high performance sealant currently in use as follows:

• Acrylic,
• Polysulfide,
• Polyurethane,
• Silicone.

Their properties are described in detail in Section 01.08.
 

Joint components
A typical sealant joint comprises a number of components:

• Primer - improves the inherent adhesive strength and/or bond durability between the sealant and the prepared substrate. It may also help limit plasticizer migration.
• Backing strip - forms the correct joint depth, image.  These should preferably be based on closed-cell cellular polymers (e.g. polyethylene), and are commonly supplied in the form of cut sections, sheets or rods.
• Bond-breaker tape - ensures the sealant is bonded only to the two faces of a movement joint so the sealant mass is free to deform; may be self adhesive strips of polyethylene or PTFE, image. The bond-breaker tape may not be required with all types of backing strip.
• Joint filler - can be used to help form the joint prior to sealant installation; they are often retained in the joint in the final works to provide support to the sealant or to exclude debris that could block the joint 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.

Joint types
Gaps between components may be detailed as butt, lap or fillet joints, each of which vary in terms of:

• Appearance;
• 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).

Butt joint
Butt (tension/compression) joints between ideally parallel and flush joint sides are readily accessible for installation, inspection, maintenance and replacement, image.  The ability of all butt joints to accommodate in-plane deviations is limited; for example, if constructed too narrow, the joint will be hard to prepare and the movement capability of the sealant will be reduced.  Out-of-plane deviations in the joined components can lead to variations in joint depth, which may result in sealant failure.  Due to their exposure sealed butt joints are vulnerable to the weather and attack from vandals, birds etc., although they can be recessed to provide a degree of protection.

Lap joint
For a given movement, a lap (or shear) joint, image, places much less stress on the sealant than the equivalent butt joint.  Therefore, under appropriate conditions a sealant will be able to accommodate greater movement in this configuration.  However, lap joints are usually more difficult to design into a structure and are less easily sealed and maintained, although they are less exposed to the weather.

Fillet joint
Fillet joints, image, work in combined tension/compression/shear and can be particularly useful where the close proximity of adjacent components precludes the use of a butt joint.  Fillet joints are only suitable for certain low movement applications (e.g. to seal window perimeters) and are vulnerable to the weather and attack.
 

Sealant bead aspect ratios
All types of sealant joint need the correct geometry and size of sealant in combination with bond breakers to perform successfully.

Butt joints
The following sealant width/depth ratios are normally recommended, in the first two cases to ensure adequate bond area to minimise stress to the sealant under movement, and in the latter two cases to ensure adequate depth for optimum service life.

Elastic sealants: 2:1
Elasto-plastic sealants: 2:1 to 1:1
Plasto-elastic sealants: 1:1 to 1:2
Plastic sealants: 1:1 to 1:3

Lap
To maximise movement accommodation the optimum configuration is for the width to be greater than or equal to the depth.  For fixed lap joints, the sealant depth may be increased relative to joint width in order to ensure a weathertight seal.  Sealants in lap joints are subject to the same minimum depth requirements for adhesion as butt joints.

Fillet
Joints need to be the correct shape and size, image; 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 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 accommodate greater movement and perform better when properly made.
 

Joint functions
Joints may be static (i.e. fixed) or dynamic (i.e. moving).  Fixed joints may be provided simply to:

• Enable manageable-sized components to be assembled into a complete element on site,
• Accommodate positional and dimensional variations in the structure and cladding.

Most joints in window, cladding and curtain walling systems are dynamic and will have to accommodate the following additional requirements:

• Single uni-directional movements (e.g. concrete drying shrinkage and creep, ground settlement);
• Repeated reversible movements (e.g. dimensional changes due to thermal and moisture movement).

A major step in the design of sealant joints is selection of the joint width. The width must be sufficiently great to allow joint movements to occur without causing excessive stress in the sealant. This must be achieved over the range of possible joint widths resulting from the tolerances in the cladding components and ambient conditions at the time the joint is sealed. However, if the joint is excessively wide, it may be uneconomic due to the large quantity of sealant required, visually unacceptable and the sealant may be difficult to apply and prone to slumping.

To calculate the joint width it is, therefore, necessary to know the range of movement to be accommodated, the total tolerance on the joint width and the amount of movement that the selected sealant can accommodate.
 

Movements and dimensional changes
The total expected movement of a dynamic cladding sealant joint can be estimated by combining:

• Thermal and moisture movements of the components, Section 01.10,
• Movements due to externally applied loads (e.g. ground settlement/heave, creep, dead and live load deflections), Section 01.09.

Joint design to accomodate movement is covered in Section 01.11.
 

Tolerances
Joints (particularly sealed butt joints) should not be expected to overcome situations where the agreed tolerances in the structure or cladding have been exceeded because to do so can compromise the durability of the sealant if the joint width is too narrow or wide. Joint design to accomodate tolerance is covered in Section 01.11.
 

Depth of sealant
The optimum sealant shape will provide adequate bond area to the substrates, yet impart minimal stress to the sealant under joint movement.

• Too deep a joint will create resistance to movement and may increase the risk of failure
• Too shallow may risk concentrating movement over too small a sealant depth, increasing the risk of splitting, or may provide inadequate bond area or inadequate resistance to weathering.

The depth of the sealant bead can be calculated from the width and the permissible aspect ratio for the type of sealant. This value can then be compared with the acceptable limits for the type of sealant. In most cases this will be between 6 and 20mm.
 

Installation
Satisfactory performance requires good site practice in addition to good design. The following should be considered.

Uniformity
Joint movement will often occur during sealant cure - when modulus and bond strength are under development - and form unsightly and uneven bulges or hollows in the seal.  Slow curing sealants (e.g. one-part polysulfides and polyurethanes) and lightweight, dark coloured cladding (e.g. aluminium sheeting) are most at risk.
Out-of-plane deviations in the joined components can lead to variations in joint depth, which may result in sealant failure.

Cleaning
The faces of the joint should be properly cleaned to achieve a full bond between the sealant and the substrate and avoid an adhesive failure.  Suitable cleaning methods for different substrate materials are shown in this image,

Masking
Substrates may be masked on each side of the joint using a low tack tape.  The tape should be removed immediately after the sealant has been applied.

Gunning
When placing sealant in the joint it is important that the sealant gun is moved at an even and correct speed.  Moving the gun to fast leads to under filling of the joint.  Moving the gun too slowly over fills the joint. image.

Built-up-joint
Wider joints are made by gunning in a series of small beads of sealant to build up the required body of sealant, image. The complete joint is then tooled as if a single bead of sealant had been placed.

Tooling
The tooling of butt joints against generally circular backing rods tends to produce concave top and bottom surfaces, image.  This maximises the area of sealant in contact with the substrates and minimises the ‘bulk’ of sealant present to resist movement, so providing the ideal geometry for sealed butt movement joints.  Tooling also 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.  Sealant joints should be made at temperatures in the range 5^oC to 40^oC.

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 opposite effects may occur.