consideration must be made of the function of the joint, the substrates that make up the joint, the conditions the sealant is exposed to, and the mechanical properties required of the sealant. The two main functions of joint sealants are to provide weather seals and structural seals. The mechanical properties the sealant will be required to provide include joint movement capability, tensile strength, and hardness. Also, consider the following criteria in selecting a sealant, temperature range, chemical exposure, UV exposure, and the projected life span. The substrate issues include the ability of the sealant to adhere to the substrate and the compatibility of the sealant to the substrate, this includes any ancillary materials such as backing rods and glazing tapes.

Desian of the Sealant Joint
The best performing joints are ones where the sealant forms a continuous bead, and is adhered to both substrates. Sealant failure most commonly comes from poor adhesion of the sealant, poor joint design causing the sealant to split and weathering causing the sealant to harden and crack. Silicone sealants offer far greater longevity because of their inorganic chemical make up, giving them far superior UV resistance to other sealants.
Several issues discussed later in this document, and the other related document, Surface Preparation and Sealant Application may affect the joint design. Issues such as the ability of a sealant to gain adhesion to a substrate, the ability to apply the sealant and the mechanical properties of the optimum sealant may cause a re-think of the design.

Weather Seals
Weatherseals occur where the sealant is filling a gap between two or more panels. The sealant has to adhere to the panels, accommodate the movement of the joint, as the panels move, and control the movement of heat, dust, water and insects from passing through the joint. The sealant is not holding the panel in place.

Joint Movement
The causes of joint movement are:
Changes in temperature (thermal movement),
Changes in moisture content,
Building loading (compression as new floors are added to high rise construction and changing fumiture etc. in existing buildings) .
Building settlement, and
Environmental forces (wind, earthquakes etc.)

The sealant needs to be able to move in the joint and accommodate these changes. The movements usually happen over several cycles, for example thermal movement occurs on a daily basis, as the day warms up and on an annual basis as the seasons change from warm to cool.

Width of the Sealant Bead
The minimum width is determined by using the following calculation. As a rule elastomeric sealant beads should not be less than 6mm wide, and not over 30 mm wide as it is hard to provide a good surface finish on a sealant. The depth should not be less than 6mm, and as a general rule is half the width (Le. 12mm wide and 6mm deep), however a depth greater than 10mm is usually unnecessary (Le. 25mm wide by 10mm deep is usually satisfactory).
The width of the sealant bead governs the amount of movement that the joint will accommodate without sealant failure. The wider the joint, the greater its movement capability. Therefore, to calculate the optimum width of the sealant bead you need to calculate the expected total joint movement. Thermal movement is relatively easy to calculate; but a suitably qualified person is required to calculate other forms of movement. Two calculations are required to calculate the minimum joint size. Firstly, calculate the expected movement, and then convert this movement into a joint width.

Expected Joint Movement
To calculate the expected movements of the substrate use the following formula:
M =(MT-T)xSxL
Where: M = the movement in millimetres of the substrate.
MT = the maximum temperature that substrate is likely to reach in degrees centigrade. This temperature is to include the temperature increase that occurs because of the absorption of radiant heat, as well as ambient heat. For example, if the ambient heat is 40°C a dark aluminium section in direct sunlight can reach temperatures up to 80°C.
T = the minimum temperature that the substrate is likely to reach in degrees centigrade. This temperature must include wind chill factors etc.
S = the Substrate coefficient of thermal expansion, a list of typical coefficients are included below.
L= the length of the material in metres.
There are two cycles of joint movement. There is the daily cycle of temperature fluctuation between day and night, and there is the annual cycle between summer and winter. The sealant needs to have the properties to handle these movement cycles over an extended number of years without losing its elastomeric properties. It also needs to be able to handle the movement, i.e. from the largest the joint will ever be, to the smallest the joint will ever be, calculated from the actual joint size at the date of sealant installation.

Figure 2 Panel Movement - direction of movement

In fig 2 the panels are supported from below, therefore all the movement associated with dimension Y will effect the horizontal joint, above the panel. However, the movement associated with dimension X is shared approximately 50% on either side. The joint between panels A and B would experience 100 percent of the movement, as each panel is moving the same amount. The joint between panel B and the wall would only experience 50% of the horizontal movement, as the framing is fixed to the wall and therefore moving horizontally as the wall moves. However, if thepanels are of differing sizes calculate each panel size and consider the effect of each panel on the joint. In addition, if the joint is between materials with different coefficients of thermal expansion, i.e. glass and aluminium panel curtain walls then calculate the substrate movement for each element separately. Their different effects on the total joint movement have to be included, proportionate to their effect.

Movement can also occur for other reasons. For example, deflection of suspended slabs as a building is loaded with fixtures and fittings, or with the addition of other floors during construction. Also porous materials such as concrete or bricks will expand and contract as their moisture content changes. A qualified structural engineer should calculate these changes of dimensions. Always incorporate a tolerance for time of application. During a hot summer day joints tend to be at the narrower, because of the thermal expansion of substrates, and the sealant will be extended for the bulk of its working life. Whereas during a cold winter day the joints tend to be wider and will be compressed all of its life.

The formula only calculates the minimum joint width. The characteristics of the sealant and the use of the joint also effect the joint width actually used. We do not recommend a joint width less than 6mm.
There is also a practical maximum width for joints. The limiting factors are the slump of the sealant, the tooling time and curing characteristics of the sealant. For silicone, the maximum is 40-45mm.

DEPTH OF THE SEALANT BEAD
The expected use of the joint and the properties of the sealant will dictate the optimum depth of the sealant. The following are generalisations that are relevant to most sealants.

WEATHER SEALS
Generally, the depth of the sealant is half the width. If the depth is excessive there is more volume to change shape of, and the amount the sealant becomes concave or convex during movement will increase. This could lead to sealant failure due to high stress at the bond line. If the sealant bead is too thin then it will be subject to failure due to a lack of body or stress concentrations caused by air bubbles etc. Plasticised sealants, such as butyl's, can also suffer from the loss of plasticiser. Therefore a weather seal should not be deeper than 12mrn nor shallower than 6mm.

Sealant Selection
Joint Functions
Weather seals
Some non-silicone type sealants harden with age and continuous exposure to the elements especially damaging ultra violet rays. Therefore when selecting a sealant the expected service life of the sealant needs to be a consideration. Weather seals fill a gap between materials. They keep the elements, wind, rain and dust etc. from passing through the gap. Therefore, the sealant must be able to bond adequately to the substrate and stretch or compress sufficiently to cope with the changes in joint dimensions, created by movement in the substrates. Silicone sealants have excellent UV resistance, maintaining near constant modulus, neither do their elastomeric properties change greatly through a temperature range of -50 to + 150 degrees Celsius.

Structural Seal
Structural seals occur when the sealant is used to bond one substrate to another while at the same time coping with the tensile and shear stress. Therefore, these joints or seals have to provide a degree of structural strength that is quantified by evaluation to standard engineering requirements. The structural strength of a sealant is indicated by the modulus and tensile strength, the higher the value the stronger the product. Another important consideration with structural seals is that the bond between the sealant and the substrate will not break down with age. The reliable long life performance of silicones means that they are suitable for use as structural sealants.
An obvious example of a structural seal is structural glazing. These many other common uses of silicone as a structural adhesive, including aquariums, mirrors, fin glazing.

Mechanical Properties
Movement capability
The movement capability of the sealant is listed as Dynamic Movement Capability in the cured properties section of the Technical Data Sheet. Normally use a high tolerance, as the actual joint width is not always the same as is detailed on the architectural drawings. Also, allow for other variables that may effect the joint performance, such as surface preparation etc.

Tensile Strength
The amount of force required to break a sample and indicates which product is most likely to hold two parts securely together, if the sealant gains satisfactory adhesion to the substrates.

Hardness
We measure hardness as units of Shore A. A sealant with a higher Shore A hardness is better for a trafficked pathway, as damage is less likely to occur from impact and scuffing than from a softer product. Softer sealants are generally more flexible and can be used where high joint movement is likely to occur.

Conditions
Different sealant types and different formulations within a type of sealant will behave differently when subjected to various conditions.
The various conditions that affect sealants include:
. UV Exposure
. Chemical Exposure
. Temperature range
Which includes both a range for application of sealant (the ability of it to cure satisfactorily), and their service range (the temperature where the cured sealant characteristics start to change, or where the cured product starts to deteriorate).
. Projected Life Span of the Joint.

Refer to the individual products Technical Data Sheet for specific information on the strengths and weaknesses of various products.

Substrates
Adhesion
Adhesion tests are relevant for all projects not just the structural projects. It is important to test the adhesion of the silicone to all batches of coating, intended for use on the project. This is to protect against batch variations in substrates or coatings providing different surface characteristics, which could affect the adhesion between the sealant and the substrate.
Termicide offers to carry out as many tests as are deemed necessary in order to minimise the chance of batch variation causing a problem. Once complete we submit a typed and signed adhesion report, along with a copy of an application specification specific for the project.

Staining
To minimise the chance of the sealant staining stone substrates we strongly recommend the completion of stain testing before the commencement of the project. The stain tests are to be between the preferred sealant and a sample of the actual stone batch nominated for the project. For major projects where the stone is likely to be quarried over a lengthy period we usually repeat the tests throughout the project. This protects against staining of the stone caused by variations in the properties of the stone quarried from different locations in the quarry.

The test procedure we use is ASTM D 2203-84. This test is purpose designed for testing for staining between natural stone and elastomeric sealants. An independent authority develops it, therefore it is unlikely to be accused of the bias that results from the use of sealant suppliers Qorporate Iest Method, and it provides quantifiable results. As well, we often use our own C.T.M., however we
only use this internally as a back up method.

Compatibility
Incompatibility can cause poor sealant cure, discolouration of the sealant, or deterioration of either material. Typical products that cause incompatibility are plasticised sealants, such as butyl sealants and rubber setting blocks. Compatibility testing takes three weeks to complete.