Sealant joints may be the weakest link in a façade.
Peter Mayer of Building LifePlans runs through
key whole life performance and costs issues
for sealant options.
Sealants are commonly used externally to weather-proof junctions between moving components. They are short-life components in a building element that, in the case of brickwork, may perform for centuries. It is important to maximise their whole-life performance.
Guidance is given in BS 6093, the British Standard code of practice for design of joints in building construction. Further guidance is published in the Construction Industry Research and Information Association’s Manual of good practice in sealant application and Sealant joints in the external envelope of buildings. Some design issues with whole life cost implications include:
• Design joint width-to-depth ratio and sealant movement capability. It is important to minimise tensile stresses between the joint faces to reduce the risk of adhesive failure.
• Consider protecting sealants from direct weathering or ultraviolet radiation, but not at the cost of being unable access the joint for maintenance or inspection.
• Ensure the sealants are non–staining and compatible with adjoining materials. Tests may be needed.
BS 6213 provides guidance on the selection of sealants. This standard refers to ISO 11600, which classifies sealants by application and performance characteristics but does not refer to sealants by material. ISO 11600 defines 11 sealant classes which give performance assurances with regard to elastic recovery, tensile properties, adhesion and cohesion properties at variable and constant temperatures, and after water immersion and loss of volume. Sealants are defined by:
• The end use.
• Movement capability usually up to 25%. Before specifying sealants with movement capacity greater than 25%, long-term performance should be confirmed with the manufacturer.
• Elasticity. Plastic sealants do not recover their original position as well as elastic sealants after movement.
• Modulus — applies to elastic sealants. Low modulus sealants require only small forces to stretch or compress them, meaning these are most suitable where constant movement is expected or when the substrate is weak or friable.
ISO 11600 does not predict sealant durability. Long-term performance should be confirmed by manufacturers.
Acetoxy cure silicone sealants are slightly cheaper, have a quicker cure time but poorer adhesion. They release acetic acid making them unsuitable for use with substrates affected by acids such as concrete or some metals. Neutral cure silicone sealants are more expensive and have a longer cure period, but have better adhesion and low modulus. External life expectancy is five to 20 years.
These tend to be used less as they have high modulus and are not completely elastic. External life expectancy is 20 years.
These are slow curing sealants, with moderate resistance to ultraviolet radiation and chemicals. They show excellent resistance to abrasion and indentation. External life expectancy is 20 years.
MS polyethers sealants
These show excellent adhesion on metals, plastics, wood and ceramics used without primers. A conservative life expectancy estimate is 15 to 20 years, although some think it could be as much as 30 years.
Refer to BS 8000–16. Good preparation is essential:
• Surfaces should be clean and dry
• If required, apply the correct primer, filler, back–up or bond break.
• Sealants should be given the correct profile: the exposed face of triangular fillet joints should ideally be convex or flat, preferably not concave. Rectangular joints should be finished flat or slightly concave.
BS 6093 recommends inspection of movement joints after the first year and all joints at intervals of one–fifth of their expected service life. Maintenance may be necessary where sealants attract dust, mould or mildew. Cleaning can be minimised by appropriate selection of sealants.
Net present value for 60 years£/m
1 part low modulus silicone sealant to ISO 11600, Type F with life expectancy of 25 years
1 part low modulus silicone sealant to ISO 11600, Type F with life expectancy of 20 years
1 part low modulus silicone sealant to ISO 11600, Type F with life expectancy of 15 years
1 part polyurethane sealant to ISO 11600, Type F
MS polymer sealant
1 part MS polymer sealant to ISO 11600, Type F
1 part polysulfide sealant to ISO 11600, Type F
2 part polysulfide sealant to ISO 11600, Type F
• Sealant joint size 12mm by 6mm, includes back-up for a nominal masonry movement joint.
• Replacement costs include for removal of existing sealant, making good, new back-up and application of new sealant. Access costs are excluded.
• The costs represent averages based on a range of sealants with similar performance qualities for each sealant type. Best value should be determined from a whole life assessment of sealant options which satisfy project specific criteria.
• A discount rate of 3.5% is used to calculate net present values.