General
The most common problems associated with masonry construction derive from a failure to comply with AS 3700, particularly in respect of durability, mortar mix, ties and flashings.
Brick Type
AS 3700 Table 5.1 specifies that, under some circumstances such as a severe marine environment (i.e. within 1 kilometre of a surf coast) or within 1 kilometre of polluting industry, exposure grade bricks or blocks are required. In such circumstances, the builder should seek written assurance from the masonry unit supplier that the units are exposure grade.
Mortar Type
It has become common practice for bricklayers and blocklayers to ignore the requirements of AS 3700 Table 10.1, which requires that all mortars contain either lime (in the specified proportions) or methyl cellulose water thickener (not detergent type air entraining agent). It is becoming increasingly common for such practices to result in litigation, and in some cases, demolition of the building. The situation is further complicated by the different requirements for general-purpose portland cements, blended cements and masonry cements. The Builder should clearly specify which mixes are permissible, purchase the materials, supply them to the tradesmen and monitor the construction.
Corrosion Resistance
AS 3700 and the ancillary standards referred to therein provide clear specification for the corrosion resistance of ties, connectors and lintels. However, the higher specifications are often difficult to obtain. The Builder should clearly specify the required durability requirements, purchase the materials and supply them to the tradesmen.
Reinforcement Cover
If reinforcement is to be incorporated into the masonry as vertically reinforced cores or in horizontal bond beams, the cover requirements of AS 3700 Table 5.1 must be achieved and a grout with 300 kg/m3 of portland cement must be used.
Brick Gauge
In order to achieve the correct brick and block coursing at the underside of suspended slabs and the tops of doors or windows, the brick gauge should be measured down from these points to near the finished level of the footings. If necessary, bring the footing up to the underside of the bottom course using a concrete screed.
Cavity Ties
Cavity ties are often incorrectly inserted, sloping towards the inner leaf or with excessive slope towards the outer leaf. AS 3700 requires that cavity ties slope towards the outer leaf, but not more than 10 mm.
Veneer Ties
Veneer ties often slope towards the frame and are sometimes not nailed to the frame. The Builder should ensure that veneer ties have the correct slope and are properly nailed to the frame. When plywood bracing is fixed to studs, face-fixed ties will be required rather than side-fixed ties.
Flashings
Flashings and damp-proof courses should pass through the brickwork, preventing salt damp problems, which result from ground water rising up the wall by capillary action.
Cleaning Cavities
All cavities should be cleaned during construction to prevent the passage of moisture from the outside brickwork to the interior of the building. In brick veneer construction, this can be done before the wall sheeting is fixed. However, in cavity walls it is best achieved by hosing out the cavities before any brick cleaning is done.
Termite Barrier
Any termite barrier must be positioned to comply with AS 3660.1 whilst not interfering with the flashing of the cavity. In some cases, this may be achieved by combined termite barriers and flashings.
Efflorescence
White calcium carbonate staining, formed by the carbonation of calcium hydroxide from the mortar, often appears on the face of the brickwork. To minimise this staining:
- Flash the top of all cavities
- Vent all cavities
- Install flashings through the brickwork to protrude 25 mm beyond the face.
Water Penetration
Water penetration of masonry can represent a significant failure, particularly in cavity walls where it is hard to remedy. Water may enter the wall from any of the following sources.
Whilst clay masonry units are relatively impermeable, some concrete masonry units can be quite permeable. The permeability of concrete blocks varies greatly, depending on the mix, vibration, surface texture and nature of any cores. Many commercially available concrete blocks have a permeability of approximately 2 mm per minute head loss, when tested to AS/NZS 4456.16. Blocks with a head loss in excess of 6 mm per minute would be excessively permeable and would have a significant contribution in permitting water to enter the cavity.
Mortar is also permeable and can contribute to the overall permeability of a wall. AS 3700 Tables 5.1 and 12.2 require M3 or M4 mortars in particular applications for durability considerations. These mortars are relatively impermeable and will assist is excluding water from the building. AS 3700 Table 10.1 and 12.1 specify the particular mix. There are no definitive tests for permeability of mortar. However, subjective assessments can be applied.
- If mortar “dusts” easily when scratched by nail or similar, this usually indicates a deficiency of cement and/or lime.
- If mortar absorbs water rapidly when partly immersed in water and when a small quantity of water is placed on a horizontal piece of mortar, it usually indicates significant permeability.
- Ties
It is a well-known phenomenon that the external leaves of masonry cavity walls permit water to pass through into the cavity. This is reflected in the AS 2699.1 and AS 3700 provisions for tie design and installation. If ties have dags on them or if they are sloping towards the inner leaf, they could be a source of moisture crossing the cavity.
As noted above, water commonly enters the cavity, and on occasion crosses via the ties. It then runs down the cavity until it reaches the flashing. A properly designed flashing and weep-hole system will consist of a flashing that is tucked into the inner leaf (or an over-flashing could be used). It then slopes down to the weep holes and passes through the mortar joint to the outside of the outer leaf. The following details are of critical importance.
- The flashing must not form a channel lower than the weep holes. It this occurs, water will collect in the bottom of the flashing, and run along until it reaches the end or a join, where it will pass into the building.
- The flashing should be turned up at the end to prevent water from running out into the building.
- Weep holes should be positioned such that they do not channel water into the cavity.
- Weep holes are often omitted or filled with mortar droppings. They should be formed at 1200mm centres immediately above the flashings, by omitting perpendicular joints. The bottom of all weep holes must be at or below the lowest part of the flashing to enable them to drain (as described above). They must not be in positions where rain water can run back into them during heavy rain.
- Parapet Permeability and Flashing
The tops of parapets must be capped and sealed to prevent the ingress of rain water. Any moisture that does enter the top of a parapet must be re-directed to the face by a flashing.
External paved patios and the roof areas surrounded by solid parapet walls with only a few small outlets into the drainage system ay cause rain water to pond. If these drains become blocked, the water will back up, flow into the weep holes, along the flashings and into the interior of the building as described above. This problem will be exacerbated if the following conditions exist.
- The weep holes are very close to floor level, and thus tolerate very little water build -up before allowing water to enter.
- The membrane is turned up only a small distance or is not well fixed adhered to the wall.
- There are gaps in sealants or membranes. Any of these gaps could be permit ponded water to pass into the masonry.
- Depending on the effectiveness of the membrane, any small gaps or cracks in floor tiles could be permit ponded water to pass under the tiles
Differential Movement
If there is differential foundation movement along or across a building, the transverse walls could pull away from longitudinal walls. There are two different ways to deal with this problem, and you must choose the most appropriate for your situation.
- Tie the top and bottom of the wall into the existing structures via the bond beam(s) and ensure that there is enough strength and anchorage (to a substantial support) to resist rupture if movement occurs. This solution may be difficult to achieve; or
- Break the construction at the ends of the new wall with an articulation joints. Any bond beams would have to incorporate a sliding dowelled joint. In this case there could be large differential movement and opening or closing of articulation joints and this must be accommodated in the finishes.
Horizontal cracking may occur at the horizontal junction of any clay brickwork and concrete blockwork. It is advisable to incorporate a slip material and to form a proper joint in any coating or treatment to be applied at this point.
Bond Beams at Suspended Floors
The inclusion of a horizontal bond beam at each suspended floor level provides additional integrity to partially reinforced blockwork, and also provides a sound substrate for the fixing of floor anchors.
Galvanised Reinforcement
AS 3700 does not require reinforcement to be galvanised, although some local authorities do. The principal means of avoiding corrosion is via sufficient cover of correctly specified grout.
