Movement control

Fyfestone Movement Control Picture of a wall close upFyfestone Movement Control picture of a wall

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.

 

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 Fyfestone blocks, the maximum length between movement joints is 9m. 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 data below is a general rule of thumb for methods of controlling movement in blockwork panels.

LENGTH TO HEIGHT RATIO OF PANEL (L:H) SPACING OF CONTROL JOINT (architectural masonry) SPACING OF BED JOINT (reinforcement (mm))
> 2:1 6-9m -
> 2.5:1 6-9m 600/675
> 3:1 6-9m 400/450
> 4:1 6-9m 200/225

 

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.

 

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.

 

A number of factors should be considered by the designer when using Fyfestone:

  • providing control joints at suitable centres
  • using bed-joint reinforcement
  • protecting the blocks before and during construction
  • specifying the correct mortar

 

Detailed information is covered within BS5628: Part 3:

Control joints are vertical separations built into a wall and located where cracking may occur due to excessive stresses caused mainly by drying shrinkage.

As a general guide, the joints should be at regular spacing up to 9m in external and internal walls.

 

It should be noted that on south and east elevations, the effect of the early morning sun on these faces can raise the temperature very rapidly and cause greater movement to take place than elsewhere in the structure. It is recommended that the above rules be strictly adhered to unless further advice is taken.

 

Control joints may also be required at:

  • changes in wall height and thickness.
  • junctions with other forms of construction e.g. steel stanchions and reinforced concrete columns
  • intersections with other walls and partitions – particular attention should be given where return walls occur.
  • return angles in L-, T- and U shaped buildings.
  • chases or recesses for piping, pilasters, fixtures, etc.

 

movement joints in roof and floor slabs. These joints in the main structure must be continued through the wall construction. The width of the wall joint and the compressible filler should be similar to that used in the roof and floor slabs. one or both sides of some large wall openings, e.g. windows, louvres or doors. However, the addition of localised bed joint reinforcement above and below openings can often eliminate the need for control joints.

FORMATION OF CONTROL JOINTSFyfestone Movement Control Junctions of Columns diagramFyfestone Movement Control Around Window diagramFyfestone Movement Control Change of Wall Height diagramFyfestone Movement Control Change of Wall Thickness diagramFyfestone Movement Control Expansion Joint In Floor Slab dgrmFyfestone Movement Control Joint Hidden By Downwater Pipe dgrmFyfestone Movement Control Junction of Steel Frame dgrmFyfestone Movement Control At Large Chases or Recesses digramFyfestone Movement Control Large Duct digramFyfestone Movement Control Staggered Joints in Cavity Wall digram

The wall is built in the ‘normal’ half bond manner with the exception that on alternate courses, half-length blocks are used to form a straight vertical joint.

 

The sealant should be one of the following or similar:

  • an acrylic based sealant
  • two-part polysulphide
  • a silicone-based sealant

 

Internally, the joint should be left open as long as possible to enable the wall to dry out thoroughly. Control joints should be carried through all finishes. In cavity walls, the control joints in each leaf should be offset. The flexibility of the cavity ties is normally sufficient to compensate for the very small differential movement between two leaves. Generally, the joint spacing is greater on the inner leaf so the staggering of joints is relatively simple.

HORIZONTAL CONTROL JOINTS

Limitation on uninterrupted height when the method of limiting the uninterrupted height is adopted in accordance with BS 5628 : Part 1, the outer leaf should be supported at intervals of not more than every third storey or every 9m, whichever is less. This method employs shelf angles and vertical joints, which subsequently provides a means of vertical movement control. However, for buildings not exceeding four storeys or 12m in height, whichever is less; the outer leaf may be uninterrupted for its full height.

MOVEMENTS DUE TO THERMAL EFFECTS

In general, movements due to normal changes in temperature of internal walls are likely to be small and not an important cause of cracking. However, in long walls or partitions where variations in temperature are likely to be large, care should be taken to accommodate movement.

MOISTURE MOVEMENT

All concrete units shrink slightly after manufacture but part of this shrinkage is reversible due to dry units expanding slightly when wetted. However, care should be taken to ensure that all units comply with the relevant European Standard for drying shrinkage requirements. To minimise moisture movement, all masonry units should be kept as dry as possible prior to laying and when laying, they should not be soaked in order to adjust the suction for the mortar.

DIFFERENTIAL MOVEMENT

If practical, products with different characteristics should not be rigidly bonded together. Thus clay and concrete units should not be mixed unless consideration has been made into allowing independent movement.

 

All concrete products shrink slightly over a period of time, the initial shrinkage taking place during the first six months to a year. Likewise some clay bricks built on top of concrete brick plinth walls may cause severe cracking in the plinth wall. Where dissimilar materials are used, for example in cavity walls, it is good practice to avoid the use of rigid ties and permanent bonding together of brick or block work with the opposing component.

PRECAUTIONS

Procedures to minimise cracking:

  1. Use the correct grade of mortar: an unnecessarily strong grade of mortar will concentrate the effects of any differential movement on few and wider cracks. A weaker mortar will accommodate small movements so that any cracking will be distributed as hair cracks in the joints where they are less noticeable
  2. Bonding: cracking of masonry is more likely to occur if bond patterns are not maintained and alignment of vertical joints results. This can be exacerbated by poor workmanship
  3. Use masonry of low potential shrinkage. Ensure that the units are dry before use and during laying
  4. Reinforce the masonry with joint reinforcement at locations of high stress
  5. Allow for thermal and shrinkage stress by the correct incorporation of control joints at appropriate intervals

 

Joint design should comprise a sealant cover either side of the joint with compressible joint backing. The choice of sealant is important. Fyfestone recommend the use of sophisticated sealant such as two-part polysulphides or acrylic resin polymers. The former is the most proven performer.

The need for periodic maintenance of joints should be recognised and considered when positioning joints.

LOCATION OF MOVEMENT JOINTS

Movement joints in masonry may either be expansive or contraction joints; the latter being free to accommodate any tensile stresses. The incorporation of movement joints within a wall structure should be in accordance with
BS 5628 : Part 3: 2005.

 

Movement joints should be installed at not greater than 9m centres and may be formed as a straight butt joint. Or in a toothed fashion in the coursework using a separate medium, such as dpc material, and subsequently pointed in a suitably coloured mastic. Flexible damp proof courses may also be used to act as a slip planes between materials of different properties.

Care should be taken to ensure that all movement joints also comply with the requirements of the other functions of the wall, ie. lateral restraint, fire resistances, etc.

 

In estimating the correct spacing of movement joints, it is best to design walls of concrete units into a series of panels. Areas above and below windows and doors should be treated as individual panels except where it is permissible to control cracking by the use of bed reinforcement.

 

Generally the shape of the panels is critical in deciding the susceptibility to cracking. It is more likely to occur if the length exceeds about 1.5 to 2 times its height.

Care should be taken that there is no excessive end restraint and if lateral stability is likely to be affected then dowels, one end of which is not fixed, should be used to provide stability.

WALL TIES

Wall ties should conform to BS EN 845-1. For guidance on selection and use of wall ties see BS 5628 : Part 3 : 2005.

TYPE OF MOVEMENT JOINTS

Examples of where movement control joints could be located

  • Expansion Joint
  • Contraction Joint - Internal
  • Contraction Joint - Internal and External
  • At large chases or recesses
  • At change of wall height
  • Where wall is penetrated by large duct or services
  • At change of wall thickness
  • Expansion joint in floor slab
  • At junction of columns, etc.
  • Junction of steel frame
  • Joint hidden by downwater pipe
  • At staggered joints in a cavity wall

CONTROL OF CRACKING USING REINFORCEMENT

If it is impractical to divide walls into a large number of panels because of, for example, a large number of openings, then reinforcement can be introduced to distribute the areas of high stress. It is still important to maintain panel length/height ratio at 2-2.5. The reinforcement should be long enough, or have enough overlap, to distribute stresses adequately and be provided with the correct cover. All bed joint reinforcement must be used in accordance with the manufacturers’ recommendations.

At window opening