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THOMAS ARMSTRONG

(CONCRETE BLOCKS)

LTD

We are Part of the Thomas Armstrong Group

Vertical & Horizontal Movement Joints | Concrete Block Wall Ties

Concrete blocks regardless of manufacturer will dry out and shrink slightly once the building is complete. This is unavoidable and perfectly natural for most construction products based on natural materials. This drying out can sometimes cause very slight differential movement and stresses between building elements and material types, which may result in hairline cracks in blocks or mortar joints.

These cracks are rarely anything more serious than a visual nuisance and are very easy to avoid completely if this movement is accommodated for in the design stage and during the build. The guidelines in this section are industry-standard and have been proven to work for many decades.

  • Vertical Movement Joints

    Internal walls of low-rise dwellings and houses do not normally require movement joints and the use of bed-joint reinforcement is usually sufficient and movement joints are never usually required below dpc. Movement joints are particularly suitable at the interfaces between dissimilar materials; at intersecting walls for example. For acoustic party walls, vertical movement joints should never be used.

     

    Wherever possible, blockwork should be designed as a series of ‘panels’ separated by movement joints. Generally, length should not exceed 3 x height of the panel, except where bed joint reinforcement is incorporated.

     

    Care must be taken when introducing wide window openings as the blockwork immediately above and below may exceed the 3 to 1 length/height ratio. Where possible storey height openings should be designed, forming the masonry into discrete panels without interfering with the overall design aspect.

    General Guidelines for Vertical Movement Control:

    • Ultralite, Insulite and Dense Concrete block walls should be divided into approximately 8 - 12m long panels for internal walls and 6 - 9m long panels for external walls. For Airtec block walls, the panels should be no longer than 6m in length whether they be internal or external walls
    • Each panel should be separated by a suitable vertical movement joint placed in an unobtrusive location (note: vertical movement joints are not to be used in acoustic separating party walls).
    • The first movement joint should not be more than 3m distance from the end of the wall or an internal / external corner or angle.
    • The adverse effects of movement can be minimised by protecting the blockwork from the extremes of moisture during and after construction. Recommendations can be found in BS 5628-3.
    • Overly strong mortars should be avoided to reduce the effect of movement and to prevent the blocks from cracking. This is especially important for Airtec block walls.

    • Additional wall ties should be placed either side of the movement joint at every block course up the length of the joint and within 150mm of the joint.

    • Movement joints can be formed by butting the block up against a 10mm strip of rigid filler which is left in place until the wall is completed. This can remain in place or be removed. However, additional flexible sealing will be required to maintain airtightness placed at least 10mm deep to ensure a good bond. Stop beads should be used at the ends of the joint.

    • Movement joints should be carried through external the render layer if used and formed using stainless steel render stop beads.

  • Horizontal Movement Control

    Bed joint reinforcement is particularly suitable for houses and low-rise dwellings wherever a vertical movement joint is impractical and for acoustic separating party walls where vertical movement joints are prohibited.

     

    Horizontal movement control can be in the form of steel ladder-type mesh or flexible movement control mesh (particularly designed for use with Airtec thin joint mortar construction).

     

    Horizontal movement control is particularly effective at relieving stresses around areas of point loading such as joists and lintels as well as above and below openings such as doors and windows.

    General Guidelines for Horizontal Movement Control

    • Horizontal movement control should be placed in the 2 courses above and below any opening and below any area of higher stress such as lintels and floor joists.
    • The reinforcement should extend at least 600mm beyond the width of the opening into adjacent blockwork.

     

    • Ideally, the reinforcement should be of a suitable length so that any stresses are transmitted to vertical movement joints or adjacent areas of blockwork.
    • Reinforcement should normally have a mortar cover of at least 13mm on the face of the internal blockwork and 25mm on the external blockwork faces.
    • Where a wall is supported by a floor, the first two courses above the floor junction should be bed-joint reinforced.
    • Masonry grade reinforcement should be used. Plastering grades are not suitable.
    • Reinforcement must never bridge a vertical movement joint.
    • A piece of dpc placed immediately below lintel bearings and joists will provide a slip plane and additional protection from cracking in these areas of higher stress.

  • Wall Ties

    Wall ties should be flexible enough to accommodate the relative movement between both leaves of a cavity wall by stiff enough to transmit axial loads. Stainless steel wall ties should be specified conforming to BS EN 845-2 or BS DD 140-2.

    General Guidelines for Wall Ties

    • For conventional 10mm mortar construction, butterfly or double triangle type wall ties should be embedded at least 50mm into the mortar joint on each leaf.
    • When using partial-fill internal cavity insulation, suitable retaining devices should be used to support the cavity insulation batts in position.
    • The leaves should be coursed so that the ties slope slightly down towards the outer leaf and the drip is facing downwards in order to prevent water penetration across the cavity to the inner leaf.

    Wall Tie Spacing and Positioning:

    • Wall tie spacing and positioning should be in accordance with BS 5628 with a tie density of 2.5 ties per m² of wall.
    • Ties should be evenly distributed over the entire wall area using a staggered pattern when both leaves are greater than 90mm thick.
    • The distance from a vertical movement joint or the vertical edge of any opening (e.g. door, window) should be no greater than 150mm.

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