All masonry walls are subject to movement, which can be caused when the walls dry out. This is as a result of cycles of heating and cooling or from changes in moisture content. If provisions are not made to allow movement without restraint, then there can be a build-up of compressive or tensile forces that may cause the masonry to bow or crack.
It is important that at the design stage the designer recognises the factors that may affect a building’s performance and makes provisions to accommodate any likely movement.
To aid the designer, along with the information in this section of the manual, there is detailed information in BS 5628: Part 3 on movement in masonry and its accommodation.
In concrete masonry walls, including Thermalite® walls, the movement that occurs is primarily a consequence of moisture movement whilst drying out (shrinkage) and therefore, it is normally only necessary to accommodate this movement in simple construction joints.
Positioning movement joints When positioning movement joints it is essential that the designer ensures that the locations of the joints do not impair the stability of the walls in which they occur.
It is important to provide movement joints in the long runs of walls that occur in commercial and industrial buildings as well as in large residential buildings. It is also important to provide movement joints in solid external walls and the outer leaves of external walls that are subject not only to drying out movement, but also to changes in temperature and moisture content.
The inner leaves and internal walls of traditional dwellings do not normally require movement joints. In addition, the use of bed joint reinforcement at openings is usually sufficient to avoid the risk of movement cracks.
Following the guidance in BS 5628: Part 3, movement joints should be provided at 6.0m centres to divide up the walls into a series of rectangular panels. Where the wall is continuous at an internal or external corner, the first movement joint should be approximately 3.0m from the corner.
The risk of movement cracking increases where the length of the wall panel exceeds twice the height. In such cases it is advisable to provide movement joints at closer centres than the nominal 6 metres.
When positioning movement joints, the following features, which may influence the movement joint locations, should be taken into account:
- intersections of walls, piers and columns
- positions of doors and window openings
- changes in height and thickness of walls
- locations of chases in walls
- locations of structural movement joints in the building
- positions where dissimilar materials meet
Lateral restraint
Where the design criteria require continuity across the movement joint, whilst still allowing horizontal movement, flat metal strips measuring 200 x 25 x 3mm or similar should be built into alternate bed joints, spanning the movement joint. One end of each metal strip should be debonded by wrapping in polythene or building paper to ensure free movement. Proprietary movement joints with one end sleeved are also available.
Wall finishes All wall finishes should be discontinued at movement joints. In the case of plaster and render finishes, proprietary metal stop beads should be used on each side of movement joints.
Where ceramic tiles are to be fixed to the finished wall, the movement joints should be continued through the tiled finish. BS 5385: Part 1 recommends that large tiled areas be divided up into panels with movement joints at 3.0 to 4.5m centres.
Services Any services or other fittings that are required to span the movement joints should be designed to allow for movement.
Dissimilar movement Where Thermalite® block walls join with other materials, such as brickwork, other concrete blockwork, structural steel or concrete etc, differential movement will occur.
To allow such movement to take place, a vertical movement joint should be provided to separate the different materials.
Where dissimilar materials bear onto Thermalite® walls, such as in-situ concrete floors, pre-cast beams or floor units etc., a separating layer should be introduced using a non-compressible material such as a bituminous damp proof course.
Construction of movement joints Movement joints should be built in as work proceeds. The simplest form of movement joint is a vertical butt joint; this can be dry, or filled with a compressible joint filler. In the case of an external wall the joint will need to be sealed to prevent the ingress of moisture.
The blockwork should be rendered to parallel square-nosed stop beads and the movement joint sealed with a proprietary mastic sealant. The selection and use of all sealants should be in accordance with BS 6213 and BS 5628: Part 3.
Movement joints in fire walls should be designed so that the ability of the wall to function as a fire barrier is not impaired. This is generally achieved by sealing the joint filler with intumescent sealant or strip.
Junctions of internal walls Where movement has to be allowed for at junctions of internal walls, the movement joints may be incorporated into either or both of the walls.
Junctions with structural members Where blockwork is tied to a structural steel or reinforced concrete column, the design of the joint should allow for any movement that may occur in the structural member.
The Thermalite® blockwork should be tied to the columns with 100mm x 100mm angles of 25mm x 3mm flat steel strip, fixed to the columns and built into alternate horizontal movement joints. Alternatively, proprietary ties may be used. It is advisable to debond the flat strip in the mortar joints.
Movement should be accommodated with debonding ties at both ends of a masonry panel (e.g. between columns).
Fire protection of columns
Where blockwork is used to provide fire protection to columns, movement joints can be incorporated into the blockwork casing.
Bed joint reinforcement
In walls containing openings, movement joints may need to be provided at more frequent centres than 6 metres. Alternatively, the masonry above and below the opening should be reinforced with masonry grade bed joint reinforcement.
The reinforcement should be contained in the horizontal mortar joints so that the mortar cover is 13mm on the face of the internal blockwork and 25mm on the external face. The reinforcement should extend at least 600mm beyond each side of the opening to ensure that any stresses are distributed into adjacent blockwork.
In some instances, it may be appropriate to include bed joint reinforcement in the mortar joints over the full length of the wall in such a way that movement joints, which would normally be recommended, may be omitted. An example would be separating walls, which should not contain movement joints.
Mortar strength
The strength of the mortar should be compatible with the strength of the blocks. In accordance with the definitions in Table 13 of BS 5628: Part 3, mortar designation (iii) should be used.