Clay Brick and Paver Institute
 
  Introduction
  Brickwork Design
  Walls
      Loadbearing
      Non-Loadbearing
      Lintels & Openings
      Reinforced Brickwork
      Brick Rod
 

    Mortar & Joints

      Control Joints
      Weather Resistance
      Thermal Performance
      Acoustic & Fire
  Case Studies
  Bibliography
  Appendices

Mortar and Joints

Regulations on joint thickness

Most brickwork is laid with "normal" mortar joints about 10mm thick. The AS3700 Masonry Code also allows for "thin bed" joints, which are used in some systems with very accurately made bricks or blocks. It also allows for hollow block masonry units, which are not a common form but are occasionally manufactured in clay masonry; and for cored units, which are normal bricks with holes through them. The Code refers to joints:

4.9.1 Thickness
For other than thin-bed mortar, the design thickness or mortar joints, including bed joints and perpends, shall not exceed 10 mm unless the effects of greater thickness on compression and flexural strength are taken into account in the design.
The design thickness of thin-bed mortar joints shall be not less than 2 mm and not greater than 4 mm.
4.9.2 Finishing
With the exception of thin-bed mortar, the surfaces of joints in exposure environments 4 (c), 5, 6, 7 and 8, as given in Table 5.1, shall be tooled to give a dense water-shedding finish. [these environments are more severe than normal].

Joints in walls constructed with hollow unit, ungrouted masonry shall not be raked. In other masonry, the depth of raking, if any, shall be not closer than 5mm to any perforation in cored unit masonry or 20mm in hollow unit masonry.

Table 8.1 specifies the following tolerances:

"(g) Deviation from specified thickness of bed joint: 3mm average in any 3 m length.
(h) Deviation from specified thickness of perpend: - 5, + 10mm."

However, C8.8 of the Commentary on the Code suggests that:

"to achieve a satisfactory appearance, designers may need to reduce the tolerances specified in Table 8.1 when products of regular dimensions are used. Similarly, when using products with uneven surfaces or varying dimensions in non-structural applications, the specified tolerances may need to be relaxed."

 

Mortar-mix design

Apart from engineered loadbearing brickwork and walls below the dampcourse, much brickwork is laid in 1:1:6 cement, lime and sand mortar. A large colour range is possible within a 1:1:6 mix. To whiten mortar, the use of white cement (rare, due to cost) and off-white cement is common, together with a light sand. Ready-mix mortars tend to use whiter sands than most bricklayers' site-mixed mortars, which are between 50% and 75% bush sand. The introduction of yellow ochre colour is possible by raising the bush sand content. After a point, the only way to match brick colour in the mortar is to add oxide colouring, a process which may not be successful unless the whole job batch is dry mixed in one operation.

Insistence upon fine texture sand only (e.g. all Sydney sand) results in too sloppy a mix, fickle in its hardening patterns and prone to cracking. (See Appendix C.)

 

Type of joint

Flush, or very lightly tooled joints are popular with smooth-faced and polychromatic bricks. This tends to 'hold' brick colour areas together in a band or panel, and simplifies planes and crisp detail. Raked joints are more common with heavily textured bricks.

Royal Agricultural SocietyCommonly, joints separating brick colours are also raked to sharpen the colour contrast. Flush joints have their own bricklaying challenges. They bear upon the brick selection process, as the arris quality and trueness of shape greatly affects the evenness of joint appearance. Different effects can be achieved by raking the bed joints and flushing up the perpends (which emphasises the horizontal lines of the brickwork), or by raking an occasional bed joint.

This subtle banding at the Royal Agricultural Society building, Homebush Bay, is achieved by raking every seventh bed joint and lightly tooling the others. The bricks are all the same colour.

 

Slip joints

In cases where bands of different coloured bricks are being used, and the 'e' values of the different brick types vary by more than 0.3 mm/metre, the recommended practice is to use two layers of coated aluminium DPC material, 80 mm wide, as a slip joint to permit differential movement. Where the 'e' values are of borderline concern an alternative is to use deep raked joints in a lime rich mortar mix (e.g. 1:2:9).

 

Efflorescence

The likelihood of efflorescence (lime and other salts formation on the face of brickwork, common in the first 12 months), should be taken into account at the design stage. EfflorescenceOverly subtle brick colour combinations will be considerably compromised by efflorescence in the short term, and by pollution build-up in the long term. Omission of water-shedding details such as sill overhangs and copings increase the chances of such problems, as the efflorescence is a result of dissolved salts being deposited when water from within the brickwork evaporates on the surface.

 

Cleaning Brickwork

There is a consensus held by many in the building industry that brick cleaning is a problem. That is, bad brick cleaning too often damages the results of good brickwork design, brick selection and laying. In the past, bricklayers cleaned their work, but for some time brick cleaning has operated as a separate trade. It is a mark of a good bricklayer that they will tend to keep brickwork clean while laying, leaving little mortar on the face. However, only a minority show that care, but even with skilled bricklayers, some cleaning down will be required to get rid of mortar droppings or other stains that can accumulate during the building processes.

An attitude can develop that mortar stains on face brickwork are not of great consequence, as the brick cleaner will fix it. This sloppy attitude, combined with the open surface of most extruded bricks, results in much ingrained mortar left on face brickwork. As brick cleaning has developed as a separate trade, the head contractor will, in many cases, be more anxious about mortar staining than the bricklayer. Every supervising architect will be familiar with the damage caused to brick faces, fretting of joints and damage to other elements by excessive cleaning at high pressure. The problem will only be resolved when the brick cleaning trade becomes trained and licensed or the trade eliminated and cleaning revert to bricklayers.

Techniques such as bagging mortar flush joints (by carpet or polystyrene), while very successful in the short term, can create difficulties by masking the whole face of brickwork with mortar smears. It becomes very difficult for the architect or bricklayer to see the brick quality and check for brick batch colour problems until cleaning is done and when rectification may be too late.

Common problems include: excessive mortar smears, water jet pressures that are too high, too much acid in the cleaning solution and inadequate washing down after acid cleaning. The presence of acid in the brickwork will hasten corrosion of any metalwork built in or attached to the surface. Acid left on internal face brickwork will never be removed by rain, and can result in corrosion of metal furniture placed against the walls.

The Code of Practice "Cleaning of Clay Masonry" prepared by the Clay Brick Association of NSW and attached as Appendix B (link) has detailed guidelines on good cleaning practices.

img011
Erosion of joints and bricks by excessive water jet pressure

 

Return To Top Next section:  Control Joints