Bricks and brickwork like other materials, exhibit dimensional instability in service. The amount of movement that takes place in brickwork depends on varying circumstances. The consequences of movement, however, must be anticipated and the necessary design allowances must be made. If the effects of movement are ignored:
- Damage may be caused to adjacent materials;
- The material which is attempting to move, if restrained and unable to contain the forces set up may crack or deform;
- Rain penetration may result in buildings.
In order to make provisions for movement it is necessary to know the nature and extent of movements in brickwork.
All bricks are subject to reversible thermal movements which persist for the life of the building. The amount of movement will be greater for dark coloured materials and for those exposed to the sun for long periods. The magnitude of this movement is approximately 0,15 mm/m of brickwork.
Calcium silicate bricks are subject to some initial and mostly irreversible shrinkage soon after completion of the building. In addition, these units exhibit reversible expansion and contraction on wetting and drying. SABS 285 allows calcium silicate bricks to exhibit a reversible movement of 0,04% when tested in accordance with the specification. The actual amount of shrinkage which will take place in a completed wall is generally less than this and is of the order to 2,5 – 3,5 mm per 10 m of wall.
All fired clay products exhibit irreversible moisture expansion over their life. This expansion is fairly rapid during the early life of the brick, being 50% complete in approximately 6 months and continuing at an ever decreasing rate throughout the life of the product.
Reinforced concrete frame buildings exhibit a shrinkage of between 4 and 6 mm per story height of building. This movement is attributable to initial shrinkage of cement in aggregate, elastic deformation of elements under load and plastic creep or flow during the life of the building. Concrete frames also undergo reversible expansion and contraction due to temperatures changes.
The accommodation of the relative movements of building materials in design
If movement problems are anticipated in building design, careful attention to certain fundamental principals will ensure that building damage will not result. The most common means of protecting a building from damage due to movement is through the provision of adequate movement joints. The actual positioning of movement joints needs careful consideration and although each case should be treated on its merits, there are certain fundamental principals which can be followed.
(a) initial shrinkage
up to 28 days.
(b) elastic deformation.
(c) creep of follow.
Parapets and free standing walls.
These types of walls need to be given special consideration because they are exposed to more severe temperature change and driving rain than any other part of the building and will, in general, be liable to more movement.
The waterproofing of parapets and free standing walls therefore needs special consideration. Additionally, because of their relatively unsupported nature, special attention also needs to be given to their structural stability.
In designing walls of these types special attention must be given to the requirements of the local building by-laws (or the SBR’s, whichever is applicable).
The following general rules are offered as a guide:
- Always use class I mortars that is 1:1/4:3 – cement, lime, sand.
- In the case of parapets, movement joints of 10 mm thickness should be provided for every 7,5 m or less. The table for joint thickness should be referred to for different brick types. It is essential to ensure that the movement joint runs right through the coping.
- In the case of garden walls, use the recommendations contained in table 2, again ensuring that movement joints run right through copings.
- Provide a correctly detailed and constructed coping on the top of the wall. This will provide protection against rain penetration into the structure in the case of parapets, and prevent unsightly efflorescence developments due to water penetration, on both types of wall.
- Parapets should always be detailed in cavity construction.
Construction changes-effect on Brickwork
Concrete frame/brick infill panel construction
The major design consideration in relation to movement is the potentially large differential movement between the concrete frame and brick panel.
In high-rise construction it is not unusual to expect the concrete frame to shrink between 4 and 6 mm per story due to:
- Shrinkage of the concrete.
- Elastic deformation under load.
- Plastic creep due to loading.
In the same situation, the brickwork panel can be expected to expand up to 3 or 4 mm per story height.
A combined movement of 10 mm. on average, therefore needs to be allowed for in the design of the building. A typical movement detail is shown below.
Brick infill panel in R.C. frame, high-rise
- DPC protrudes through joint. This avoids unwanted load transfer across the joint with resultant danger of pushing off brick slips.
DPC is sandwiched between wet mortar.
Movement joint sealed on outside with polysulphide filler. Drip provided at cavity, foamed polystyrene sheet fills joint. Internal rendering does not spill into joint.
Vertical movement joints
In designing long runs of walling sufficient movement joints should be placed in the structure to prevent damage due to brickwork expansion alone. Care should be taken particularly to protect short returns as these details are especially vulnerable.
- Do not fill joints with non-compressible materials.
- Do allow sufficient joint width so that the expected movement will not operate beyond the compressibility of the joint filler.
- Two part polysulphides, although expensive, are durable and have a high compressibility of approx 50%.
(This means that a 10 mm joint can accommodate 5 mm of movement before the filler begins to transmit load).
- The polysulphide filler should penetrate to between 10 and 20 mm into the joint. Fill the balance of the joint with a highly compressible foam polystyrene sheet as means of controlling polysulphide thickness.
Movement of calcium silicate brickwork.
Calcium silicate brickwork, has the common shrinkage characteristic of all cement based products. It is for this reason, that calcium silicate bricks are particularly suited to concrete frame brick in-fill panel construction methods. It is still necessary, however, to provide for differential movement between the frame and the brick panel to accommodate under load deflection and creep of the concrete frame due to loading. It is therefore good design practice always to accommodate differential movement between frame and brick panel through the provision of a horizontal movement joint.
Calcium silicate walling should be provided with vertical movement joints at intervals not exceeding 9,0 m. The reason for this is to avoid random cracking appearing in the walling.
Shrinkage movement is limited to the places where it has been provided for.