Successful brick walls rely on good materials and good building practice. A well built brick wall requires the use of bricks, mortar, accessories and a frame that act together to form a structurally sound, weather tight and decorative wall that will last for many decades without the need for maintenance or repairs. A brick wall correctly specified and built will outlast most other building envelope.

Frame
Timber frames must comply with NZS 3604 (Timber Framed Buildings) particularly the dimensional characteristics detailed in Section 2.3. Variations from these tolerances can result in a reduced cavity between brick and frame resulting in poor water tightness particularly if insulation is installed in the cavity. The cavity must be between 40 and 75 mm but a minimum of 50 mm is recommended to allow for timber warpage.

Bricks
All bricks produced by Austral Bricks are manufactured to comply with the requirements of the Australian and New Zealand Standard, unless otherwise specified in the technical data sheet made available for each product. They are rigorously tested to AS/NZS 4455 (Masonry units segmental pavers) using the test methods of AS/NZS 4456 (Masonry units and segmental pavers - methods of test). This testing is performed in accredited and registered laboratories to ensure every batch of bricks meets both the minimum statutory requirements and the advertised specification for Austral Bricks product.

Compressive Strength

Characteristic Unconfined Compressive Strength
Austral Bricks Pty Ltd makes available the characteristic unconfined compressive strength (f'uc) of their products as required by the joint Australian New Zealand Standard AS/NZS 4455. The f'uc values are determined using the test method detailed in AS/NZS 4456.4. The test method involves subjecting the masonry unit to increasing load by compressing it between two metal platens. The friction between the platens and the masonry unit acts to restrain the sideways spread of the unit, as shown in the diagram below.

The effect of this restraint is more pronounced in shorter specimens than in taller specimens of the same width. The incorporation of an aspect ratio (height to thickness ratio) factor, allows the results to be converted to an equivalent characteristics unconfined compressive strength value (f'uc). That is, a correction factor, directly related to the dimensions of the unit, is used to compensate for the restraining effects of the platens during testing. The f'uc allows direct comparison of all masonry units, regardless of the material or the size.

The characteristic unconfined compressive strength (f'uc) values now supplied for a traditional sized brick are approximately 60% of the previously used characteristic confined compressive strength values. It is important to note that this difference is due to the altered approach used when determining the values.

Durability

Definition

All products manufactured by Austral Bricks Pty Ltd are classified by their durability. The durability of a brick is a measure of its resistance to attack by soluble salts.

The test method used to determine the durability of bricks is given in AS/NZS 4456.10. The suitability of the units for use in a given environment determines their salt attack resistance category.

Salt Attack Resistance Categories

1) Exposure Grade: Suitable for use in external walls exposed to aggressive environments, such as:

• in areas where walls are subjected to salts in the soil, adjacent material or ground water,

• in coastal areas where walls are exposed to attack from windborne salt spray, or

• retaining walls that may be subjected to fertilisers and ground salts.

2) General Purpose: Suitable for use in an external wall under ordinary exposure conditions.

3) Protected: Suitable for use in internal and external walls only when above a sheet or membrane damp-proof course and protected by an adequate coping, eaves, roof or similar top covering. These units should not be directly exposed to saline environments.

Durability Required

The severity of the environmental conditions, such as the amount of moisture and the availability of soluble salts, determines the durability grade required. New Zealand Standard NZS 4210 (Masonry Construction: Materials and Workmanship) states that masonry units must have appropriate salt attack resistance to ensure their durability. The durability requirements set out in NZS 4210 are summarised in the following table:

Exposure Categories (Note 1)

Exposure Categories (Note 1)	Masonry Unit Durability Requirements	Mortar Durability Requirements (Note 2)	Classification of Built- in Components  (Note 3)
Seaspray	Exposure	M4	R4
1 & 4	General Purpose	M4	R3
2 & 3	Protected	M3	R3
Closed Interior	Protected	M2	R1
Geothermal Hotspot	Exposure	M4	R5

Note 1: The exposure categories are defined in NZS 3604 (Timber Framed Buildings) using a corrosion zone map

Note 2: The requirements of mortar to meet these classifications is given in the section on mortar and in NZS 4210

Note 3: The classifications are defined in AS/NZS 2699: Part 1 Wall Ties, Part 2 Connectors and Accessories, Part 3 Lintels and Shelf Angles.

Note: When weatherproofed to the requirements of Section 2.21.2.2-2.21.2.5 of NZS 4210 then Exposure categories 1, 2, 3 & 4 (NZS 3604) can be reduced to Closed Interior, thereby reducing the durability requirements of the masonry unit. This is normally achieved by rendering or coating with a heavy duty acrylic coating.

NZS 3604 (Timber Framed Buildings) classifies the exposure categories primarily using geographical location. Please refer to the corrosion zone map provided in NZS 3604 or in NZS 4210 for full details.  In general, a geothermal hotspot is found within 50m of a bore, mudpool, steam vent or other fume source. A sea spray zone is defined as being within 500m of the sea, 100m from tidal estuaries and sheltered inlets or in the additional sea spray zones specified in the corrosion zone map. It is important to take into consideration the given environment during the design and construction of brickwork buildings, to minimise the potential for salt attack.

The minimum durability classification of built-in components is particularly relevant to the use of wall ties in masonry constructions. AS /NZS 2699.1 classifies the durability of masonry wall ties as:

• R3 - galvanised to a coating weight of at least 470g/m2 on both sides, in accordance with AS/NZS 4680

• R4 - stainless steel grade AS 1449/316 or AS 1449/316L

Durability class R5 has no deemed-to-satisfy specifications as it is intended for critical applications, the wall tie manufacturer should be contacted for further information.

Fretting of Brickwork (Salt Attack)

Fretting of brickwork is linked to the durability of bricks and directly related to the exposure environment in which they are placed. The fretting (flaking or crumbling) of bricks can be prevented by the adequate maintenance of your wall structure. An extreme case of fretting is shown below.

Fretting is caused by the action of salt migration in the walling system. Water which has salt dissolved in it migrates through the brick to the brick surface. As the brick dries, the salt is left behind and forms a salt crystal. The salt crystals grow in the voids within the brick. As more salt is left behind by the evaporation of water, the salt crystal grows larger and larger. The strength of the growing salt crystal can be stronger than the elements that hold the brick together. If this occurs, the brick face begins to crumble and fall away. This is also true for mortar joints.

For salt attack to occur the following three conditions are required:
- There must be salts present
- There must be water entering the wall
- The water must evaporate from the wall

The absence of any of these conditions will prevent salt attack. The fretting of bricks can be exacerbated at specific locations around a house, which undergo increased wetting and drying cycles. The brickwork will continue to deteriorate unless moisture movement through the masonry is prevented.

When treating fretting, "prevention is the best cure". The source of the salt may be airborne salt from sea spray or salts that are naturally present in the soil, or introduced by fertilizers and salt-water swimming pools. The use of bore water may also provide the source of the salt.

The first step in treating fretting is to identify the source of the salt and where the salt might be coming from. The best method of preventing fretting is to prevent the salt from being absorbed into the wall. This may be a physical barrier such as plastic sheeting, digging the soil away from the wall or rearranging the sprinklers so that they do not spray directly onto the wall.

Once fretting has begun it will not stop until the salt source is identified and removed. It could then take another 12-24 months before the fretting process comes to a complete halt. Once the fretting process has stopped the brickwork can then be repaired. Any repairs made to the brickwork while the fretting action is still occurring will generally fail.

Fretting brickwork may be treated using breathable sealers that penetrate the brickwork and consolidate loose particles. However, depending on the degree of fretting it may be necessary to render or replace the affected brickwork. For further information, contact Austral's Technical Department.

Initial Rate of Absorption

The initial rate of absorption (IRA) is a function of the size and extent of the porosity of the bricks. The IRA is a measure of how quickly a brick wall will absorb water. The test method for initial rate of absorption is given in AS/NZS 4456.17.

The ability of bricks to absorb water affects the bond formed between brick and mortar. A tug-of-war occurs between the bricks ability to absorb water and the capacity of the mortar to retain water. If either the brick or the mortar wins, a poor bond will result. Therefore, the water retentively of the mortar needs to be matched to the IRA of the bricks to ensure that a strong bond forms.

Lime Pitting

Lime pitting is a defect occurring in the surface of a brick due to the expansion of large lime particles just below the surface. The lime originates from the raw materials used in the manufacture of the bricks. Lime pitting is observed when the lime particles are present just below or on the surface of the brick. The volume expansion of the lime particle, resulting from the presence of moisture, can cause it to pop out of the brick or break the brick surface, generating a defect. An example of a large lime pit is shown on the right: Lime pitting is defined in AS/NZS 4456 and the degree of lime pitting can be determined in accordance with AS/NZS 4456.13.  Lime pitting is classified by the number and size of lime pits on the face or ends of the brick, as described below:

Bricklaying Procedures

Bricks must be laid in stretcher or running bond so that each brick is surrounded by 6 other bricks. This prevents long vertical cracks forming in masonry walls leading to weakness in the structure. Stack bond is strictly forbidden for this reason. Mortar joints must be 10 mm ± 3 mm except for the first course where the bed may be up to 20 mm to accommodate slab deviation.

Control and Articulation Joints

Bricks naturally expand very slightly after manufacture. This expansion happens slowly and the rate of expansion decreases with time.  Austral Bricks’ test their products to AS/NZS 4456.11 to ensure that the expansion is kept as low as possible. To accommodate this expansion control joints are inserted into walls. These control joints may also act as articulation joints to accommodate ground movements resulting in slabs bowing. Control joints can normally be inserted along the edges of windows or doors or located behind down pipes to disguise their presence. Control joints are recommended for runs of brickwork greater than 10 metres long and should be installed at 8 metre centres. It is also recommended that control joints be used to separate disimilar materials. Control joints should be laid with a clear 20 mm gap with no mortar dags in the joint. The joint should be filled with a compressible foam or rubber insert and detailed with a durable flexible material such as mastic.

Mortar
Mortar must be made from Portland Cement. It is strongly recommended that lime be used as it makes the mortar workable and the finished mortar self heals if any micro-cracking occurs. The use of clay to make mortar workable is discouraged as is reduces bond strength and results in mortar smearing that is difficult to remove. Admixtures such as set retarders or water thickeners are only allowed if the manufacturers’ instructions are strictly followed. Colour oxides are allowed but only up to 3% of the total cement amount.

The use of a gauge box or bucket for consistent quantities is recommended as sand in particular can be easily misjudged when shoveling if damp.

NZS 4210 specifies the ratios required to make mortar for particular durability classes.

Durability	Cement	Lime	Sand
M4	1	0-0.25	3
M3	1	0.5	4.5
M2	1	1	6

It is recommended that the mason as a competent tradesman should have the compressive strength of the mortar checked at regular intervals, particularly if the supply of sand has changed. It should achieve a minimum compressive strength of 12.5 MPa at 28 days. It should also achieve a minimum bond strength of 200 kPa for one storey veneers and between 500 and 1000 kPa for two storey unreinforced veneers or reinforced multi-storey veneers.

For this reason Austral does not recommend the use of second hand bricks as the pores of the brick are already filled with cement paste crystals and this results in poor bond strength. Mortar joints should be finished as either concave tooled and burnished, or raked and pointed Mortar must not be allowed to accumulate at the bottom of cavities or sit on wall ties as this will prevent the cavity performing its job of maintaining weathertightness.

Wall Ties and Accessories

All wall ties, lintels and other metal accessories must be of the correct durability grade for the environmental conditions. Wall ties in particular are hidden from view after construction and therefore cannot be checked for corrosion at a later date.

Wall ties must be spaced according to the provisions of NZS 4210. This spacing depends on the mass of the veneer, the earthquake zone and the type of tie.

Wall ties must be screw fixed to the frame and must be embedded at least half the depth of the veneer, that is at least 35 mm in a 70 mm brick and 45 mm in a 90 mm brick. There must also be at least 15 mm of mortar cover from the outside of the wall. Laying the tie on the dry brick before mortar coverage is permitted. This situation has been tested by BRANZ. Wall ties must be installed so that they slope to the outside of the building.

Lintels must be embedded at least 100 mm each side for spans up to 2 metres and 200 mm each side for spans over 2 metres unless covered by specific engineering design.

Further information on brick veneer construction along with detailing design can be found in the Standards New Zealand Handbook SNZ HB 4236 Masonry Veneer Wall Claddings as well as the relevant New Zealand Standards:

• NZS 4210 Masonry Construction: Materials and Workmanship
• NZS 4230 The Design of Masonry Structures
• NZS 3604 Timber Framed Buildings

If you have any further questions please contact Austral Bricks New Zealand on 0800 287 8725