Our most frequently asked questions and answers.
Well-made clay bricks should only require wetting before laying under extremely hot and windy conditions. Poor quality clay bricks from non-accredited sources could be too porous. The water absorption rate is therefore high and so these bricks might need to be soaked with water before being used.
The amount of water added to a mix must be enough to make the mix workable and plastic. Use cement that has the SABS mark (SABS ENV 197-1). Masonry cement that complies with SABS ENV 413-1; strength class 12.5 or higher may be used for mortar and plaster.
A builder’s wheelbarrow usually has a capacity of 65 litres
For laying bricks and blocks in normal applications (SABS Class II)
To lay 1000 bricks = 3 bags cement + 0.6 cu. m. sand
1 Bag of cement to 3 wheelbarrows of building sand
For exterior and interior work
To lay 100sq.m. (15mm thick) = 10 bags cement + 2 cu. m. sand
1 Bag of cement to 3 wheelbarrows of plaster sand
(Measure with a container such as a bucket, drum or tin)
1 unit of mortar to 5 units of mortar sand.
1 unit of mortar to 5 units of plaster sand and stone.
Clay Bricks are fired at high temperatures to stabilise their structure, and have a high thermal mass which helps them insulate for heat. In a domestic fireplace or braai they are really a total “over specification”. Refractory Bricks are normally rated from around 1600’C upwards. They are expensive and not easily available.
A braai will never reach this kind of heat and all facebricks are fired to around 1000 – 1250’C. The heat in the fireplace must get higher than this to have any effect on the brick. Generally your mortar joint will fail before the brick. What I always suggest to people is the following.
Build the outer skin of the sides of the braai/fireplace with a standard mortar mix (1 bag cement : 3 Barrows Sand) with brickforce and cavity ties. Build the inner skin with a well burnt clay face brick using a weaker mix (1 bag : 6 Barrows Sand) – this allows more joint movement and reduces cracking from heating.
Concrete bricks or any regular concrete product made with Portland cement cannot stand up to high temperatures and will disintegrate at the temperatures required by a pizza oven or braai.
With excessive heat, cement and mortar between the bricks can crumble. I recommend you use an air-set refractory cement (as oppose to a fire-set refractory cement), and minimize the joint space between the bricks – use “buttered joints” i.e. have the bricks very close together.
The heat to the walls of a grill is not that high (compared with the floor where the fire is built or the ceiling of a pizza oven) so ordinary tile cement can be used to adhere tiles.
Efflorescence is a powdery deposit of salts which forms on the surface of bricks and paving. It is usually white but efflorescence can be yellow, green or brown. Efflorescence does not in any way influence the structural strength of clay bricks or pavers. Roman buildings that have been standing for 2 000 years can show efflorescence.
A temporary efflorescence is particularly common on new brickwork and paving as soluble salts are dissolved and transported to the surface of the brickwork and paving by water. The efflorescence on new well fired brickwork and paving may be unsightly, but it will not cause damage unless the brickwork and paving are under-fired or soft-fired (in which case serious efflorescence can cause flaking of the surface of the brick or paver.)
Efflorescence can occur from a variety of sources. Besides the brickwork, soluble salts can originate from the sand, mortar and the water used. Ground waters that are naturally salt-bearing can be drawn into the base of the brickwork and paving.
The best removal method is simply to brush off the deposit with a stiff dry bristle brush after the wall has dried out. Collect the removed salts with a dust pan or a vacuum cleaner to prevent the salts re-entering the brickwork. Wetting the wall by methods such as hosing usually dissolves efflorescence back into the brickwork, allowing it to reappear again when the wall dries out.
Acid or alkaline treatments are not recommended as they do more harm than good because they add to the total salt content.
The efflorescence can be removed by washing with a high pressure washer, some builders pre wet the surface with a dilute solution of pool acid prior to using the high pressure washer.
Moss, lichens and algae should not grow on clay bricks unless the area is heavily shaded, is under trees, or is not laid to an adequate fall. If such growth does occur and is considered undesirable then the area should be treated with a proprietary moss killer used in accordance with the manufacturer’s instructions.
Such products take some days to be effective and work best when applied during a spell of dry weather. Any thick growths should be scraped off first and the chemical treatment well brushed in.
Some treatments leave a residue to discourage the re-growth of the moss and algae, but this will only be of limited value if the paving remains damp and in shade.
Properties that require consideration when buying or specifying clay brick are:
- Compressive strength (varies from 7MPa to 50MPa)
- Water absorption (face brick up to 10%, NFP up to 16%)
- Modulus of rupture – strength in bending
- Moisture expansion – important when detailing longer structures
Irreversible moisture expansion takes place with all clay products predominently during the first 24 months after manufacture. This expansion is dependent on the clay minerals present in the product and the firing temperature of the process. The movement takes place in both the vertical and horizontal directions.
Initial rate of absorption.
This affects the bond between the mortar and the brick. Bricks with high rates of absorption may require pre-wetting well before laying to prevent absorbing excessive water from the mortar mix. Trial and error experiments when bricks arrive on site will determine the need to pre-wet bricks.