Brick Growth
Clay bricks grow. Therefore clay brickwork increases in size with increasing time.
The locations and widths of the horizontal and vertical joints required to control this movement should be considered in the architectural design of all brick buildings.
The locations and widths of the horizontal and vertical joints required to control this movement should be considered in the architectural design of all brick buildings.
The expansion of clay bricks commences as soon as the bricks leave the kiln, and continues at a decreasing rate for some years.
It is therefore necessary to consider the requirement for vertical and horizontal control joints in brick walls.
Each brick type has a different rate of expansion. Therefore the spacing and width of control joints depends on the brick type selected for the project.
This paper describes the nature of expansion, gives typical values for the expansion of several brick types, and presents tables for the selection of both horizontal and vertical control joint spacings and widths.
Causes of Expansion
There is a three-dimensional time-dependent increase in the size of each brick when it is exposed to the atmosphere after leaving the kiln. This causes increasing length, height and width of brick walls with time. Measurements on test bricks showed that there was a high early rate of expansion during the first month, followed by an almost constant rate, and that they were still expanding at an average rate of 0.006 percent per annum after standing in air for nearly five years (Ref. 1).
The magnitude of the total expansion of a brick wall depends on:
- the characteristic coefficient of expansion of the bricks,
- the length of time that the bricks were exposed to atmospheric moisture before laying,
- the degree of exposure after laying (exposure to the sun increases expansion, and sealing with render and paint reduces expansion),
- the elapsed time after laying,
- the length and height of the brick wall between control joints.
Anderson (Ref. 2) reported that Australian clays are characteristically rich in mica, that alkaline oxides are contained mostly in the mica, that the micas are changed by firing at high temperatures into amorphous material which is highly reactive, and when exposed to atmospheric moisture, is the cause of the expansion. Exposure to high (sun) temperature accelerates this process, which can be reversed only by re-firing the bricks.
The expansion process commences as soon as the bricks are taken from the kiln and exposed to atmospheric moisture: typical expansion-age graphs are shown in Fig. 1 for samples of bricks measured in the laboratory. The magnitude of expansion depends on the nature of the raw materials (mica content of the clay), the method of manufacture (by pressing or by extrusion), and the temperature and duration of firing.
Damage Caused by Expansion
Schubert (Ref. 3) reported the investigation of brickwork in 63 damaged buildings. Movement was noticeable at the return ends of walls because:
a) “The expanded wall causes a local length of the return wall to slide sideways.
b) The expanded wall causes a vertical crack in the return wall . . . “
Table 1 indicates the frequency of occurrence of significant movement and/or cracking in walls with end returns and no control joints. Damage occurred frequently in walls more than 9m long and in almost all walls more than 45m long. Cracking at offsets (jogs) occurred in seven out of nine walls. In 35 cases of parapet walls 230 thick and 10 courses high, most of the walls that were more than 15m long exhibited expansion damage. Many building walls without control joints that exhibited cracking, appeared to have expanded only 2mm to 4mm; the maximum expansion measured was 16mm in a wall 40m long. The parapet wall damage was similar in that relatively small measured expansions induced cracking in return walls: the maximum total expansions were 35mm in a wall 44m long, and 50mm in a wall 78m long.
Internal Walls
Joints are not usually necessary in internal walls because of the shorter wall lengths between restraints, and the lower potential expansion. Internal walls are usually sealed by cement render and/or paint finishes, they usually do not experience prolonged exposure to heat from the sun, and so their expansion is relatively low. Because the external skin jointing is more important, and the external skin relies on the internal skin and its fixing to the structure for its stability, the provision of effective internal joints would be both aesthetically undesirable and expensive. Therefore, expansion joints are usually omitted from internal walls. Anderson (Ref. 4) recommends that internal wall bricks should have fifteen-year unrestrained expansions of less than 1.1mm/m.
In unusual structures where long runs of unrestrained internal brickwork occur, joints should be considered.
External Walls
It is usual to consider the use of both vertical and horizontal control joints in external walls and parapets. The design of the joint width and spacings depends on the coefficient of expansion, which is an estimate of the brick expansion in the first fifteen years after manufacture, determined by a standard test procedure.
Values of the coefficient of expansion (15 year em) are available from the manufacturers for each brick type they produce (see Table 5).
Vertical Control Joints
It is assumed that the brickwork between vertical control joints will expand equally in
both directions from its centre. To minimise interference with associated elements,
the maximum movement of brickwork into the joint gap should be restricted to 6mm to 8mm, giving a total maximum movement of, say, 15mm. Allowing 5mm for joint filler results in a maximum joint width of 20mm (Ref. 4).
both directions from its centre. To minimise interference with associated elements,
the maximum movement of brickwork into the joint gap should be restricted to 6mm to 8mm, giving a total maximum movement of, say, 15mm. Allowing 5mm for joint filler results in a maximum joint width of 20mm (Ref. 4).
Recommended maximum spacings between vertical control joints are given in Table 2, based on the fifteen-year unrestrained expansion value obtained from the brick supplier. These spacings take into account the restraint afforded to normal walls by vertical load and friction, and the effects of mortar shrinkage and creep. No such restraint has been considered for parapet walls.
Vertical joints should be as close to corners as possible, but at no greater distance than half the joint spacing.
Horizontal Control Joints
The vertical expansion of brickwork requires horizontal control joints to avoid damage to lintels, corbels, windows and doors. Anderson (Ref. 4) referred to the desirability of maintaining an outward slope on wall ties and recommended a 10mm closure for a 15mm wide joint. Table 3 gives the maximum spacings between horizontal joints for closures of 7mm and 10mm, taking into account the probable vertical shortening of the building frame.
The vertical expansion of brickwork requires horizontal control joints to avoid damage to lintels, corbels, windows and doors. Anderson (Ref. 4) referred to the desirability of maintaining an outward slope on wall ties and recommended a 10mm closure for a 15mm wide joint. Table 3 gives the maximum spacings between horizontal joints for closures of 7mm and 10mm, taking into account the probable vertical shortening of the building frame.
Design Assumptions (Ref. 7)
- ‘In most walls there are conditions of restraint which reduce the actual long-term expansion of the brickwork to half that indicated by the long-term (5-year) unrestrained expansion of the bricks themselves. In parapets, however, such restraint appears not to exist and the full amount of long-term unrestrained brick expansion is assumed to occur. On the other hand, in base brickwork which is not more than about 600mm high between ground level and a sheet damp-proof course, experience shows that the risk of damage is slight if expansion gaps are omitted.
- Vertical expansion gaps are spaced so that the maximum movement at each gap is limited to 7 to 8mm from the wall section on each side of the gap, that is, a total movement at the gap of 15mm.
- Since corners and offsets are the places most vulnerable to damage, the first gap shall be located at a distance from each such corner or offset of not more than half the calculated gap spacing for walls in general. Hence, any wall or section of wall whose length between corners or offsets is more than half the calculated gap spacing shall be protected by the inclusion of a vertical control gap at approximately the centre of its length.
- The gap width shall be not less than 15mm plus the compressed thickness of the joint filler plus a small margin for safety – say 5mm – which gives an overall gap width of at least 20mm.
- Gaps shall be cleaned to ensure that no hard materials such as mortar droppings remain in the gaps to prevent their proper functioning.
- Any joint filler used shall be of a highly compressible type. Rigid foam polystyrene and impregnated softboards are both too rigid to be suitable fillers for a closing gap.’
Design Procedure
A design procedure and an alternative series of recommendations for joint spacings was recommended by McNeilly (Ref. 7) and is:
- Determine the characteristic expansion – the em value – of the clay bricks to be used. Information is available from the brick manufacturer or BDRI or Table 5.
- Select appropriate vertical gap spacings for walls and parapets from Table 4 noting the maximum distances allowance between salient corners and the first gap in each direction. Provide slip-joints at re-entrant corners.
- Check that the vertical spacings of horizontal gaps do not exceed the maximum given in Table 4.
- Check to ensure that the gaps do not make the walls unstable.
- Select appropriate construction details and use flashings, wall-ties and gap sealants with the correct properties.
Joint Sealants and Fillers
The movement of clay bricks at horizontal or vertical brickwork expansion control joints tends to close the joint, and the joint filler must therefore be compressible.
Alternative construction methods are to either build the brickwork leaving a gap which is subsequently cleaned out and filled, or to build the brickwork against the filler. Common fillers are either polystyrene, polyurethane or polyethylene foam, which are all very compressible. For aesthetic reasons, the filler is often covered with a sealant, which must also be compressible.
Alternative construction methods are to either build the brickwork leaving a gap which is subsequently cleaned out and filled, or to build the brickwork against the filler. Common fillers are either polystyrene, polyurethane or polyethylene foam, which are all very compressible. For aesthetic reasons, the filler is often covered with a sealant, which must also be compressible.
Elastomeric fillers may offer considerable resistance to compression, and so oil or butyl based mastic materials are considered to be more effective (Ref. 2). If a gap is built into the brickwork, a filler such as ‘Compreband’ may be used.
The sealing material should generally be a low modulus polysulphide: this should only be applied over a polyethylene foam and not over a bitumen-impregnated foam.
“A simple means of sealing gaps is first to clean them out and then insert either a bitumen impregnated plastic-foam strip as the complete seal or a closed-cell polyethylene foam circular rod as a backing for a gun-applied butyl caulking compound. In either case the finished seal should be kept well back (approximately 25mm) from the face of the wall to avoid an unsightly squeezing out of the compressed seal brought about by the closing of the gap.
For horizontal gaps under shelf angles or haunches, the seal will usually have to be located close to the face to reduce the risk of water penetration into the brickwork below”. (Ref. 5).
Conclusion
The need for vertical and horizontal expansion control joints should be considered by the Architect for most brick buildings. The 15 year expansion value for the selected brick should be used in conjunction with Tables 2, 3 and 4 to determine appropriate joint locations and widths. If these joint locations are found to be unacceptable, it may be necessary to select a brick with a lower coefficient of expansion.
- Hosking, J.S., Hueber, H.V., Waters, E.H. and Lewis, R.E., “The Permanent Moisture Expansion of Clay Products”, CSIRO Division of Building Research, Tech, Paper No. 6, 1959.
- Anderson, G.W., “Permanent Moisture Expansion of Clay Bricks”, ACSE Seminar on Brick Growth and Brickwork/Concrete Interaction, August 1978.
- Schubert, T.J., “Long-Term Expansion of Brickwork”, Dept of Works, CEBS, Technical Records 52/75/383.
- “The Design of Brickwork for Differential Movement”, Brick Development Research Institute, Techniques 6, Jan 1979.
- “Detailing of Clay Masonry Walls”, Clay Brick Paving Institute, 2000.
- Taylor, P.J., “Expansion in Clay Brickwork”, Building Materials and Equipment, No. 135, Feb-Mar, 1981.
- McNeilly, T., “Moisture Expansion of Clay Bricks: An Appraisal of Past Experience and Current Knowledge”, CBPI Research Paper 9, Feb. 1985, and May 2002.
