Tag Archives: fixings

Tag Archives: fixings

Joints in Precast Concrete Cladding

Joints can generally be split into two types, butt joints and lap joints.  Lap joints have the advantage of being able to accommodate larger movement and being less obtrusive.

However, they are difficult to install and maintain and are hence rarely used.  There is also a triangular fillet joint but this should not be specified as it offers very limited movement capacity.

Joints in Precast Concrete Cladding 1Joints in Precast Concrete Cladding 2It is good practice to apply a primer before using sealants even if the sealant manufacturer claims that it is not needed.  Experience has shown that the application of a primer is extremely beneficial in reducing loss of adhesion to the concrete.  It also reduces the possibility of staining/migration.

If the outer surface of the panel is ‘rough’ e.g. exposed aggregate/grit blasted, then care must be taken that the actual joint surfaces are detailed and formed with a ‘smooth’ surface.

The ‘design joint width’ (the width of the sealant) may not always be the same as the ‘gap width’ (the clear space between units) as shown.  Depending on the joint profile, the sealant may then come into contact with surfaces which are closer together than the design width of the joint.  This reduces the distance over which the sealant can ‘stretch’ and can cause failure.  In these cases a bond breaker is required in the form of a tape.  This ensures that the sealant only adheres where intended.

On concrete, silicone seals should be nominally10 mm deep, regardless of the width, and not the often quoted 2:1 width: depth which applies to polysulphides.  For joints over 30 mm wide, the depth may be increased to a maximum of 15 mm.  Although not indicated in the diagrams above, it is good practice for the outer edge of sealant to be stopped some 2 – 3 mm from the front face of the joint.  This minimises the risk of primer and sealant being accidentally applied to the ‘seen’ face.  In the case of a recessed joint, this figure can be considerably greater. Continue reading

Tag Archives: fixings

Joints for elements of construction

Where concrete units butt up against each, or other elements of construction, it is necessary to provide a gap.

This serves several purposes:

  • it allows for manufacturing tolerances,
  • it provides room for positional adjustment,
  • it allows room for thermal movement,
  • it allows sealant to be introduced to seal against moisture.

Each of these is important, but when considering details, it is often easier to group joints into two types, those with no movement, and those where movement may occur.

By definition, no vertical movement should occur at the support since this is a fixed point.  Thus the joint at the level of the supports should not need to take movement.  The main factors determining size will be visual impact and practicalities of introducing sealants.

From a visual point of view it is nearly always preferable for these joints to be the same width as those taking movement. Continue reading