01.11 Sealant joint design

Categories: Envelope Sealing

Introduction
The design of sealant joints can be confusing as different guidance documents use terms differently. A major confusion arises from the definition of the movement a sealant can accommodate. Some specifications give movement capacity as the amount of compression or extension the sealant can accommodate from its unstressed state whereas others give movement capacity as the total range from fully compressed to fully extended.

The procedure adopted here is that given in BS ISO 11600 which has also been adopted in the 2000 revision of BS6213. This differs from the previous edition of BS6213 and BS 6093. BS ISO 11600 categorizes sealants into movement classes of 7.5, 12.5, 20 and 25 where the number indicates the permissible extension or compression that the sealant can accommodate as a percentage of the unstressed width.
 

Joint design process
Key stages in the design of sealed joints are:

• Calculation of M, expected movements
• Selection of sealant of given flexibility (movement class)
• Calculation (including allowance for deviations) of joint width
• Check - is joint width acceptable?

If, after calculating the joint width by these means, the result is unacceptable, it will be necessary to consider:

• Reassessment of movements/deviations to ensure they are realistic
• Use of sealant of different movement class
• More frequent joints
• Different, more accommodating joint system (e.g., use of lap joints if they can be
• installed correctly)

This ‘loop’ will be visited many times in most joint designs until the correct balance of material and joint type is obtained.  Once the correct sealant width has been established, an appropriate width:depth ratio must be selected prior to specification, Section 01.07.
 

Assessment of joint width
The amount of movement a joint can accommodate depends on the properties of the sealant and  the width of the joint at the time the sealant is applied according to the following relationship

Movement accommodation ³ (Movement x 100) / joint width

However the movement the joint can accommodate is also dependent on the position of the joint in its movement cycle at the time the sealant is applied. If the joint is sealed when at the mid point of its movement range it will subsequently expand and contract and the sealant will operate in both compression and extension. For example if a gap 10mm wide is filled with a sealant with a movement class of 20, the sealant could subsequently be extended to a width of 12mm or compressed to a width of 8mm.

However, if the joint is sealed when at its minimum width it will subsequently expand but the ability of the sealant to accommodate compression will not be utilized. Similarly if it is sealed at its maximum width the sealant will only experience compression. The amount of movement the joint can accommodate will also differ slightly as the width of the unstressed sealant will vary. Taking the example above if the gap is sealed at its minimum width of 8mm it would only be able to expand to 9.6mm, a movement of 1.6mm.

As the designer does not know the conditions under which the joint will be sealed, BS 6213 recommends that the design of the joint is carried out assuming it is at its minimum width at the time of sealing. The above equation then becomes

Movement accommodation ³ (M x 100) / W_min

Where M is the total movement of the joint and M_min is the minimum joint width.

This relationship may be used either to determine the required movement class of sealant for a proposed joint design or to determine the required joint width for a given sealant movement class and joint movement.

The minimum joint width calculated above will occur under extreme conditions, for example when the cladding is at its maximum temperature. The nominal width of the joint used in calculation of sizes of adjacent components should relate to typical conditions, say a temperature of 10 to 15^oC. In most cases it will be sufficiently accurate to take the nominal joint width as the mid point of the movement range, but where the extension and contraction of the joint from typical conditions are likely to differ significantly, it may be necessary to carry out more refined calculations.

The nominal joint width should also make allowance for induced deviations (tolerances). The cladding components adjacent to the joint may not be manufactured or erected to precisely to the specified dimensions. For the joint to perform satisfactorily the minimum joint width must still be obtained when the joint width is reduced by these deviations. A conservative approach would be to increase the joint width by the sum of all the deviations which may affect the joint width.  However, it is unlikely that all deviations would be at their most adverse at the same time and it is common to combine deviations as the square root of the sum of the squares of the individual deviations. Hence if the individual deviations are d₁, d₂……d_n, the total deviation to be used in the joint width calculation Y is given by:

Y  = (d₁² + d₂² + d₃² + … d_n² )^0.5

The nominal joint width is therefore given by

W = W_min + (M/2) + Y

If the joint is too wide the sealant may be difficult to apply due to slumping and the appearance of the joint may not be acceptable. The maximum joint width should therefore be calculated to check that it is within acceptable limits. The maximum joint width will occur if the joint is sealed when movements have caused the maximum opening and the adjacent components are undersize by the permitted deviations. It may be calculated as follows

W_max = W_min + M + 2Y

BS ISO 11600 gives movement classes rather than allowing movement capability to be determined on a continuous spectrum. Where calculations show that a sealant is required with movement capability intermediate between the class boundaries the higher class sealant should be used. For example if a sealant with a movement capability of 15 is required a sealant of class 20 should be used. Where a movement capability greater than 25 is required the joint may be redesigned to permit the use of a sealant of lower movement class. If this is not possible sealants with greater movement capability are available but they are not covered by BS ISO 11600 and should be used with greater caution.

The standard tests to assess movement capability are carried out on standard substrates (glass, mortar or anodised aluminium). When selecting a sealant it is important to ensure that it will give the required performance with the proposed joint surfaces. This may require additional tests to be carried out.
 

Visco-elastic properties
Comparing the predicted rate and type of building movements with the capabilities of anyproposed sealant is important, Section 01.08.

As outlined in Section 01.08, sealant materials can be classified into elastic, elasto-plastic, plasto-elastic and plastic characteristics.  While elastic sealants, for example, exhibit excellent recovery after movement, their ability to show stress relaxation(the gradual decay of an applied stress at given strain) is limited.  Elasto-plastic sealants,
however, show reasonable elasticity within a constant fluctuating joint, yet have the abilityto stress-relieve under permanent deformation, then showing good elasticity in the new geometry.

Hence, in a structure where deformations lead to a permanent opening or closing of ajoint, an elastic sealant may be more prone to failure than, say, an elasto-plastic
sealant.  Conversely, in a structure of joints subject to rapidly fluctuating movements, anelastic sealant may be more appropriate.

Plastic sealants are capable of considerable stress relaxation accompanied by change ofgeometry.  This may, however, result in adhesive failure of the sealant if permanent opening joint movements become excessive.