Movement Due to Expansion and Contraction in Construction Using Aircrete Blocks

Celcon manufactures a wide range of quality aircrete products, available in a variety of grades and dimensions, and all conforming to BS 6073. This is the excellence of Celcon aircrete.

Accommodation of Movement

Movement can be due to expansion and contraction (shrinkage) related to the physical characteristics of the materials and the environment, and to differential settlement or other movement of parts of the structure. Such movements are termed either reversible or irreversible.

General Considerations for Wall Construction

In the design of walls, movement should be accommodated by following the recommendations of BS 5628-3:2001, Clause 5.4.2, which can be summarised as follows:

1.      Internal walls of housing and other low-rise dwellings do not normally require movement joints and the use of bed-joint reinforcement at openings is usually sufficient to avoid cracking (See BS 5628-3:2001, Clause 5.4.5).

2.      With the exception of party walls, a rendered outer leaf and solid rendered external walls, neither movement joints nor bed-joint reinforcement are normally required in dry-lined constructions.

3.      Concrete block walls in excess of 6m should be designed as a series of panels separated by movement joints at maximum 6m centres unless the wall is to be reinforced (see “Location of movement joints”).

4.      Where the masonry is continuous at an internal or external corner, the first joint from the corner should be positioned to be within 3m of the corner.

5.      At openings, or at changes of wall height, thickness or direction, bed-joint reinforcement can be used to reduce the risk of cracking caused by a concentration of stress.

6.      Particular care is needed when designing shallow walls, wherever the length of a panel exceeds twice its height.

7.      Normally dissimilar materials should not be bonded together but should be separated by forming a vertical movement joint or by incorporating a horizontal slip plane such as a dpc type material if needed.

8.      Over-strong mortars should not be used; lower-strength mortars improve the ability of the wall to accommodate movement.

Location of Movement Joints

Where movement joints are required they are best positioned:

  • At intersecting walls and columns
  • At changes of wall height or thickness, or where
  • Chases occur
  • To coincide with movement joints in adjacent elements of structure (floor or roof)
  • At junctions of dissimilar materials
  • Where architectural or structural features create a ‘weak’ section

Where a movement joint is required, but aesthetic or practical reasons do not allow (e.g. in separating walls where a movement joint may impair sound insulation) the wall panel may be reinforced; see Bed-joint reinforcement.

Movement joints are not normally required below DPC level.

Construction of Movement Joints

Straight, unbonded vertical joints are the most common type of movement joint, and can be formed by butting the Celcon blocks against both sides of a strip of rigid filler. When the wall is complete, the filler can be left in place or removed and the joint left unfilled, or sealed, as required.

Flat strip ties across a movement joint are required only if structural continuity/restraint across the joint is required. See 'wall ties' for extra cavity ties needed at vertical movement joints at 300mm maximum spacing.

Movement joints should normally be continuous through all surface finishes, with stop beads used to end the finish, either at one or both edges of the joint. Alternatively, a proprietary type of cover strip can be used, or an architrave can be pinned to one edge of the joint.

Where stability of the design requires continuity across the joint, slip-ties should be set in every second or third bedding joint. These can be in the form of metal strip, 200 x 25 x 3mm, set parallel to the plane of the wall. One end of the slip-tie should be free to move within the bedding joint, for example by wrapping the appropriate end in polythene or building paper. Proprietary types of slip-ties with one end sleeved are also available.

Wall ties, or ties with anchored ends, for example fish-tail ties, should not be used since this would result in bonding of the joint which would prevent the wall from moving.

Bed Joint Reinforcement

Reinforcement laid in horizontal bed joints is particularly appropriate for areas of high stress, for example at openings and under concentrated imposed loads. Any such reinforcement should be of adequate length to distribute stresses to nearby movement joints or into adjacent areas of blockwork, extending for example at least 600mm into the adjacent blockwork each side of an opening. Reinforcement should normally have a mortar cover of 13mm (minimum thickness) on the face of internal blockwork, and 25mm to external faces.

Entire wall panels may be reinforced in such a way that movement joints, which would otherwise be advisable, can be omitted. This also applies to separating walls which should not contain a movement joint.

Where a wall is supported by a floor, which itself may be subject to deflection, the first two courses of the wall should be reinforced.

In all cases, masonry-grade reinforcement should be used; plastering grade types are not suitable.

Dissimilar Materials

Where Celcon blocks abut other materials (e.g. brickwork, steel etc.) differential movement can occur. If dissimilar materials bear onto Celcon blocks (e.g. cast in-situ concrete slabs, precast concrete beams or floor units, and lintels bearing onto Celcon blocks) a separating layer should be created at the bearing using two sheets of a smooth, non-compressible material, such as DPC.

This information has been sourced, reviewed and adapted from materials provided by H + H Celcon.

For more information on this source, please visit H + H Celcon.

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