It is important that the type of bedding (full or face-shell) specified in the documents should be followed in the construction. This is because the strength of masonry depends on the type of bedding. Joint thickness in uences strength as well as appearance and must therefore be constructed as specified. Tolerances are prescribed for any variations in joint thickness.
The vertical joints (perpends) must be filled with mortar unless the documents indicate otherwise; filling of these joints has an effect on horizontal bending strength, water penetration resistance, and other properties such as sound transmission. The base course should always be laid on a freshly prepared, horizontal bed of mortar, applied to a clean surface.
Various general construction details also require attention. The bonding
of the masonry units, in particular the unit overlap and the use of header units, must be as specified by the designer. Toothing must not be used where one wall intersects another and they are built at different times. Examples of bad practice in toothing walls before connecting it to return walls abound. It is preferable for the wall built first to be racked back or metal ties to be embedded during construction of the rst wall and incorporated into the second wall when it is built. In general, the use of cut units and the cutting of holes and chases should be minimised as far as possible. Chases can have serious adverse effects on strength, re resistance and sound transmission and should only be used within the limits permitted by the design.
The rate of construction of masonry should match the conditions. Construction that is too rapid can lead to slumping of the work, and excessively hot and dry or freezing conditions should be avoided. Once constructed, the masonry should be allowed to set without disturbance. Initial bond between a masonry unit and mortar is formed quite rapidly by transport of water from the mortar into the unit, and it is most important that the unit should not be moved after being tapped into position.
During wet weather, the tops of walls should be covered to prevent rainwater entering the units, which could lead to damage, excessive shrinkage and ef orescence. Weep-holes and cavities between masonry leaves should be kept free of mortar and other materials. Cavities can be kept clean by using a cavity batten that is pulled up as the work proceeds. It is also good practice to hose out cavities at the end of each day’s work. Weep holes can be formed by inserting a duct or other insert, or by using a rod that is removed once the mortar has stiffened sufficiently.
Mortar joints (both bed and perpend) are usually specified as nominally
10 mm in thickness. Any raking, if specified, should not exceed 10 mm depth and should not penetrate closer than 5 mm to any core or perforation in cored units or to within 20 mm of the cores in hollow units. Tooling of joints is particularly benefcial in improving durability and must always be carried out as specified.
Control Joints - Control joints are used in clay masonry to accommodate movements within the masonry or between the masonry and other parts of the building.
Such movements can arise from various sources including foundation movements, expansion of the clay masonry units, thermal movements and shrinkage of concrete elements. When foundation movement is the cause, the joints are referred to as articulation joints.
Control joints must be kept clear of mortar droppings and other materials and properly back- lled with sealant as speci ed. A temporary gap- lling material can be inserted in a control joint to ensure that it is kept free of mortar. However, it is essential that this temporary filler is removed and the joint properly filled when construction of the wall is complete. To ensure proper functioning, it is particularly important that expansion joints in clay masonry should be filled only with specified compressible material.
Flexible sealant should be left well back from the surface to avoid unsightly squeezing out as the joint closes. If the flexible sealant has squeezed out, it may be trimmed with a sharp knife.
Control joints should have appropriate flexible ties (not cavity ties) installed across the joint.
Flashings and damp proof courses
Correct installation of damp-proof courses and flashings is one of
the most important construction considerations for masonry. The standards require damp-proof courses and flashings to be provided for the following purposes:
- To prevent moisture from moving upward or downward through the masonry.
- To prevent moisture passing from the exterior to the interior of a building, including passing across a cavity.
- To shed moisture from a cavity to the outer face of a masonry wall.
While chemical parging of damp-proof courses has proven successful in some areas (for example Western Australia) the use of membrane damp-proof course materials is by far the most common. Recommended locations for flashings and damp-proof courses are given in Think Brick Australia Manual 9 – Detailing of Clay Masonry.
Traditionally, membrane damp- proof courses (DPC) and flashings have been embedded in joints with mortar above and below, not directly laid on the units. However, there is evidence to suggest that slip joints, which are often implemented with two layers of DPC material, are more effective if they are laid directly on the units. The specification should be carefully followed in this regard. The bed joint on which the DPC is laid should be flushed up with mortar, but this does not mean that core holes should be filled or that the DPC should necessarily be sandwiched within the mortar joint.
Joints at the ends of DPC and flashing material should be lapped to a length at least as great as the thickness of the masonry leaf, to guard against moisture migrating along the lap and penetrating the wall. Care should be taken in storage and handling to avoid puncturing DPC and flashing materials, thereby allowing moisture to pass through.
The materials for damp-proof courses, copings, flashings and weatherings must comply with AS/NZS 290410 and must be corrosion resistant and compatible with all materials they will contact in service. The following are common materials used for this purpose:
Copper and copper alloys
Bituminous materials without metal centres
It is essential that the membrane DPC should be visible at the front surface of the wall after construction. This is best achieved by allowing the material to project while the masonry is under construction, followed by cutting it off flush or turning the edge down when construction is complete. The most common cause of dampness in masonry buildings is bridging of the DPC, either because of insufficient projection from the surface of the joint or by the application of a render coating after construction of the wall. Any external landscaping or rendering of the wall must not be allowed to bridge the DPC and form a path for moisture to pass above the DPC level.
Where the wall is to be rendered to below the DPC, the DPC should not be cut off until after the rendering is complete. The most common reason for render failing is saline ground water wicking up through the render, drying and depositing salt, which builds up between the render and the brick, eventually popping the render off.
Wall ties and connectors
Wall ties that interconnect the leaves of a cavity wall or connect a masonry wall to a backup frame of timber or steel stud are essential structural components of the building. If their integrity fails, there is a signi cant risk of the masonry skin falling off the building in high wind or earthquake. This was one of the most signi cant causes of failure in the Newcastle earthquake of 1989. The wall tie standard AS/NZS 2699.111 specifies the required characteristics of ties, while AS 3700, AS 4773.1 and AS 4773.2 specify how they should be designed and installed. It is essential that these standards should be complied with.
Acoustic brickwork ties are designed to provide structural stability whilst attenuating noise and vibration between cavity walls.
An important feature of AS/NZS 2699.1 is the labelling requirements. These require each package of ties to show the strength rating (light, medium or heavy duty), the rated cavity width, the durability category (R1 to R5) and the fastening requirements (for veneer ties). The individual ties must be colour coded or stamped to indicate the durability rating. If colour coding is used it should be as follows (in order of increasing durability):
Green R0 or R1
- Blue R4
Connectors used in masonry, for example across control joints and
to tie the tops of walls must comply with the appropriate standard AS/ NZS 2699.2. These connectors must also have a durability rating (R1 to R5) and must be selected and installed in accordance with the speci cation. The same colour coding scheme used for wall ties (see above) or another form of stamping or labelling must be used to indicate the durability rating of connectors. Manufacturers test these products and provide strength values, which are used for the design of critical connections such as the tops of walls. It is therefore essential that the appropriate types, at the speci ed spacings, should be used in the construction.
All ties and connectors must be built into the masonry as the work proceeds, to ensure that they are properly embedded in fresh mortar.
AS/NZS 2699.1 requires the tie manufacturer to supply the fastener for use with veneer ties. This is an important requirement, necessary to guard against the risk of electrolytic corrosion caused by dissimilar metals being in contact. Any such corrosion, leading to disintegration of the fastening, would quickly render the ties useless. For this reason, veneer ties must always be installed using the fastener supplied with them. While veneer ties have traditionally been attached to timber framing with nails, recent research has shown that this can lead to a loss of strength under cycles of reversing load, particularly for face fixed ties. Consequently, there is a trend in Australia to require screw fixings and this is already the common practice in New Zealand. For this reason, Clause 4.10 of AS3700-2011 requires face fixed ties for masonry veneer more than 3.0 m above the ground to be positively attached by screw fixing. For side fixed ties, either screw or nail fixing can still be used.
All ties must be installed to prevent water transfer across the cavity
from the outer masonry leaf to the inside of the building. Ties are tested to ensure that they will not permit water transfer if installed level, but an additional safeguard is provided by giving them a slight slope towards the outside of the wall. However, this slope should not be too great, or the strength of the tie connection will be affected. Careful planning and setting out should be used to avoid excessive coursing differences between the leaves of a cavity wall.
The strength of embedment of the ties in the masonry affects their ability to transfer forces. The standards require all ties to be embedded at least 50 mm into the mortar joint and to have at least 15 mm remaining cover to the outer surface of the mortar joint. In the case of hollow masonry units, laid in face shell bedding, the cores should be filled with grout or mortar (at least where the ties are located) to provide Sufficient embedment for the ties.
Ties must be installed at the correct spacing, as specified on the documents for the job. It is important to remember that this spacing can vary from one job to the next and even for different areas within the same building. The spacing should never exceed 600 mm in either the vertical or the horizontal direction. There
will generally be an overall spacing of ties for the wall, for example 450 mm by 450 mm and a requirement for additional ties to be installed in some locations.
Alongside openings, control joints and edges of a wall, the first row of ties should be within 300 mm of the edge of the masonry. This is to ensure that all parts of the masonry are adequately supported. This requirement also applies opposite intersecting walls and at other points of support for the wall. Where a two-storey masonry veneer is continuous past a floor level, there should be a row of ties within 300 mm below the floor and a separate row within 300 mm above the level of the floor. This is to ensure that the masonry in both the upper and lower storeys is adequately supported, considering that the floor membrane acts as a very stiff point of support. These rows are also each required to contain additional ties (see below).
Double the usual number of ties must be installed in rows in the following locations:
At the top of a single storey veneer.
Opposite vertical lateral supports (for example intersecting walls) in both veneer and cavity walls.
For a continuous multistorey veneer, in the row immediately above and the row immediately below the intermediate oor level.
As an alternative to using double the number of ties, ties of a higher strength rating could be used but this will not usually be practicable. The standards no longer require additional ties around the edges of door and window openings or at control joints. In general, the requirements should be speci ed on the drawings.
There are some common mistakes or abuses in the use of wall ties, many
of which came to light in the 1989 Newcastle earthquake. These all detract from the performance of the masonry in terms of either its integrity or its durability. The more common mistakes are:
- Not engaging bending down ties. This can be avoided by using two-part ties or polymer ties.
- Not embedding the ties for a sufficient distance (often due to cavity width, greater than specified for the tie used. Not embedding the ties properly in mortar, resulting in low pullout strength.
- Use of ties with strength or corrosion resistance specifications below what is required
- Misalignments of ties across the cavity, resulting in water transfer into the building and reduced strength.
- Mortar dags in the cavity, leading to water penetration.
- The cavity cluttered with cables, debris and so on, leading to water penetration.
- Galvanised or stainless steel
- Reinforced or prestressed concrete (precast or in situ)
- Reinforced or prestressed masonry stone
All lintels and shelf angles must also comply with AS/NZS 2699.3:2002 Built In Components for Masonry Construction.
This standard provides for durability ratings using the same classi cation system (R0 to R5) as that used for ties and connectors.
The standard requires lintels tom have identifying markings and, in particular, to be colour coded to indicate the durability class, using the same scheme as for wall ties and connectors. This colour coding and the identifying marks should be visible when the lintel is embedded in the wall.
Lintels, especially those of steel with galvanized or duplex coatings, must be handled carefully to prevent damage before they are installed. Any damage to the coating arising from dropping the lintel, or other impact, will compromise the corrosion protection and shorten the life of the lintel.
Correct installation is also vital if lintels are to perform satisfactorily. The speci ed bearing distances at each end of the lintel must be complied with and any speci ed propping procedure must be followed. To ensure proper composite action with the supported masonry, some lintels should be propped during construction. Where they are used, the props should not be removed until the masonry has hardened suf ciently, usually seven days after the masonry is built above the lintel. Steel angle lintels should be installed with the longer leg vertical and should always have any space between the vertical leg and the masonry packed with mortar to prevent twisting.
When masonry is grouted, it is most important that the grout should flow into all cavities and fully surround all reinforcement. Vibration or rodding should always
be used to ensure complete filling. Before commencement of grouting, the cores and cavities should be free of any debris and excessive mortar dags. During the grouting process, care must be taken to keep the reinforcement in its specified position; AS 3700 provides tolerances on the reinforcement position that must be maintained at all times.
The height of individual grout lifts depends on the type of units and the strength of the mortar joints. If too high a lift is attempted or the joints have not hardened suf ciently, there is a risk of joints blowing out under pressure of the uid grout. This will be less of a risk if the units are of high suction so that the grout stiffens quickly as a result of water being drawn into the units.
After the cores are lled with grout they must be topped up to compensate for shrinkage. This is usually best done approximately 30 minutes after initial lling. This top- up should be rodded to ensure that it merges with the previous grout lling.
The most important requirement for the grout is that it should have at least 300 kg of cement per cubic metre, to ensure adequate corrosion protection for the reinforcement. It is only necessary to sample and test the strength of grout if this is called for in the specification.
The final stage of construction of clay masonry is cleaning and removal
of mortar residue from the units. Guidance on appropriate techniques for this cleaning and for the removal of stains is given in Think Brick Australia Manual 13 – Clay Masonry Cleaning Manual. It is essential that cleaning be carried out with care, especially if it involves the use of high pressure water sprays. Disfiguring and damage to the mortar can happen if high pressure cleaning is not done correctly.