Opendrained joints

The most popular gasket joint is the open-drained baffle joint, first introduced from Scandinavia in the 1950s. With proper detailing, correct specification and control of installation on site, it has proved to be fairly successful. Correct detailing, however; is essential. Open-drained joints are explained in BRE Digest No 85 Joints between concrete wall panels: open drained joints (BRE, I 967). This covers the principles of and recommendations for details, especially flashings at crossover joints (Fig. 1.24).

The principle of the open-drained joint is to use the geometry of the joint to trap most of the rain in

Grout

Sealant depth?

Grout

Sealant depth?

Circular or rectangular dosed cell polyethylene bacl-.ing strip

Two-part polysulphide sealant

1.23 Polyethylene backing strip to ensure correct depth of sealant.

an outer zone and to provide a barrier (air seal) at the back of the joint to prevent air and water (that gets past the baffle) from penetrating into the building. Although the baffle strip acts as a primary barrier to wind-driven rain, the real effectiveness of the joint depends upon maintaining the performance of the air seal.The theory is that, because the seal is positioned at the back of the joint, it is sheltered by the baffle from the ultraviolet light and direct rain, and is subject to less thermal and moisture movement than if it were positioned nearer the face of the panel.

The main elements of the open-drained joint are therefore:

- the baffle strip;

- the baffle groove;

- an effective upstand at the base of the panel;

- the horizontal flashing at crossover joints.

Baffle strips

Neoprene is the most common material for baffle strips, and is normally used in 50, 63 and 75 mm widths.There are various ways of supporting the baffle strip, including fixing it into plugs in the top surface of the panels.There can be some difficulty in preventing spalling of the concrete using these plugs, and another method devised to facilitate renewal of the baffle is to turn it over a stainless steel pin restrained by bolting through the return ends (Fig. 1.25).The baffle should not be fixed over the pin using staples alone (Fig. 1.26). The steel pin is located in grooves on either side of the panel joint, which should be wide enough to contain the diameter of the pin (not less than 6 mm) plus two thicknesses of baffle. This will probably be wider than the baffle groove in the side of the panel.

1.24 Crossover open-drained joint.
1.25 Baffle fixed over stainless steel pin for easy renewal.

When using baffle strips these should not be stretched before cutting at the base of the panel, otherwise they spring back up the joints, and are then short.

Baffle grooves

Grooves are normally formed by fixing a timber insert to the edge of the mould, and a width of 15-25 mm is normally sufficient to allow a screw fixing and demould-ing from the mould.The depth of the groove will vary according to the joint range expected, but 35 mm is normally adequate to accommodate a 75 mm baffle.

The recommended distance of the baffle groove from the front panel is 50 mm, which should be increased to 75 mm if an additional drainage groove is provided (Fig. 1.27).

Air seals

Air seals can be in the form of flexible membranes, gaskets, compressible foam strips and sealants. In practice, this seal has proved difficult to apply. The main difficulty is that of providing a satisfactory air seal at the back of the joint, especially in positions where panels pass in front of floor slabs or where panel joints occur on the centre line of concrete columns. In this case it is not normally possible to apply the seal directly to the back face of the joint.

Upstands

The matching upstand and downstand profile at the horizontal joint is one of the primary features of the open-drained system.The heights of the upstand and

Width of groove to locate pin

Stainless steel pin

Stainless steel pin

Bolted not stapled Stainless steel bolt

Width of baffle groove

1.26 Method of restraining the baffle using a stainless steel pin.

downstand are related to the site exposure conditions. The effective lap should be 50 mm, and for severe conditions this should be increased to I 00 mm

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