Positioning of movement joints
Movement joints are used to eliminate or control cracking within the fabric. The joints are located in strategic positions and allow slight, predetermined movement without subsequent cracking to the masonry.
Spacing, design and location of movement joints depends principally on the dimensions and shape of the wall in question, detailing techniques and the complexity of the structure. The joints should be located at junctions which may be vulnerable to cracking.
The position of movement joints will be strongly influenced by structural and architectural considerations but the following may prove useful as an indication of where control joints may be necessary.
Straight walls Cracking is most likely to occur in long, straight walls. Blockwork walls of excessive length should be divided into a series of rectangular panels with movement joints between each panel.
The effects of end restraint need to be assessed when deciding on the panel length. This can add either 25% or 50% to the actual length, if one or two ends are fully restrained. It is upon this total length that the empirical rules are then applied.
In the case of lightweight aggregate blocks, the maximum length between movement joints is 6m and 9m for dense aggregate blocks. The length of each panel should not normally be in excess of twice the height of the panel and areas of blockwork above and below openings should be treated as individual panels.
The following gives general guidance for methods of controlling movement in blockwork panels.
| |
Spacing of control joint |
|
Length to height
ratio of panel (L:H)
More than |
Lightweight aggregate
reinforcement blocks |
Dense aggregate
blocks |
Spacing of bed
joint reinforcement |
| 2:1 |
4.5 to 6m |
6 to 9m |
- |
| 2.5:1 |
4.5 to 6m |
6 to 9m |
600/675mm |
| 3:1 |
4.5 to 6m |
6 to 9m |
400/450mm |
| 4:1 |
4.5 to 6m |
6 to 9m |
200/225mm |
Notes:
1 For general design purposes only
2 Consult Hanson’s Product Services department, or a qualified structural engineer regarding specific applications where greater spacing of control joints is envisaged
Changes of height, thickness and profile
Movement joints should be provided where changes of height, thickness and profile(either on plan or section) occur in the wall since these junctions are particularly vulnerable to cracking.
Movement control joints Control joints will be required at the junctions of intersecting walls and junctions with columns, and also where walls are penetrated by major service pipes and ducts, or where large chases are incorporated to accommodate services.
Types of movement joints
Continuous vertical joints Various forms of movement joints are possible. Normally, these should be 10mm wide as for normal perpends, at not greater than 6.0m centres for lightweight aggregate
blocks and 9.0m centres for dense aggregate blocks.
The simplest version is to construct a continuous vertical joint as work proceeds by placing suitable compressible boards (e.g. bitumen impregnated fireboard) between the joints. Joints can be filled with a compatible mastic for external applications and plastic or metal cover strips are available to conceal internal joints.
Bed joint reinforcement Internal and external finishes should not be taken across the movement joint unless suitable provisions are made. Bed-joint reinforcement (parallel wire type) can be used to increase joint spacing and help resist high stresses around openings.
In general, bed joint reinforcement should be located in all concrete masonry panels where openings occur and where the distance between movement joints has been extended. The reinforcement should be placed in two courses above and below the opening, if possible, and extended at least 600mm beyond the opening limits. The reinforcement must be curtailed or made discontinuous at movement joints.
Flat bar joints Where increased stability of panels on either side of a movement joint is important, non-corroding flat bars, debonded at one end, can be placed across the joint in alternate courses. The bars should be located at least 25mm from the face of the wall.
Similar ties can be used at the intersection of two block walls where separation is required. The vertical spacing of the ties would depend on the load bearing requirements of the wall. As a minimum, Hanson recommends ties at 450mm vertical centres.
In cavity walls, wall ties should be placed across the cavity, within 225mm either side of the movement joint, in every course.