Compressive strength AS/NZS 4456.4
Fairly obviously, this is the ability of the masonry unit (brick or block) to resist crushing loads, eg the weight of the roof that the wall is supporting, plus
the weight of the wall itself. The designer of the structure needs to be sure that the masonry unit will be able to carry the load being placed upon it, including any live loads.
Salt attack resistance AS/NZS 4456.10
In some situations, bricks can be attacked by salts from ground water, swimming pools , spas, sea spray etc. How resistant a given brick is to the effects
of salt crystallisation will depend on things like the porosity of the paver and the strength of the material the paver is made from, as well as the concentration and type of salt, and moisture and
Bricks and blocks may be classed as Exposure Grade if they either have a history of coping with a salty environment, or have passed a laboratory test which
simulates such conditions. Units are required to be exposure grade, according to AS 3700 Masonry Structures, wherever they are
- in contact with aggressive soils (eg with high concentrations of salts in the ground-water), or
- in a “severe marine environment”, ie within 1 km of a surf coast or 100 m of a bayside coast.
The lab test puts small segments of the masonry units through a series of 40 cycles of alternate soaking in a salt solution, then drying in an oven. If the
specimens survive the 40 cycles with less than a specified loss in mass, they’ve passed the test.
Moisture expansion AS/NZS 4456.11
All fired clay bricks expand slowly after their manufacture, by taking up moisture out of the atmosphere. The expansion continues for many years, and needs
to be taken into account in building design. To predict the long-term expansion, a brick length can be accurately measured before and after a laboratory steam treatment and the change in length
used to estimate how much the brick will expand in 15 years.
Information on how far a used brick has already expanded (“past expansion”) can be gained from re-firing the brick in a laboratory kiln, and measuring its
shrinkage. How far the used bricks still have to expand (“residual expansion”) can be assessed by the difference between these two measurements.
These expansion characteristics depend on the clays used in making the bricks, and on the manufacturing process itself.
AS/NZS 4455 Masonry units and segmental pavers calls for bricks and blocks to be classified into dimensional categories based on their deviation from
their work size, or the size specified in manufacture. This is usually a standard size; it is important that deviations from this be controlled to a low level so bricklayers are able to
build the structure to the designer’s specifications and with minimum joint thickness variation.
Under this standard test, 20 units can be either measured individually for length, width and thickness, or they can be placed side by side, end to end, etc and
their cumulative dimensions measured.
Potential to effloresce
Efflorescence is a deposit of salts, usually white, on the surface of bricks and blocks after being laid. The salts usually come from ground water or out
of the mortar, but may come from within the masonry units themselves.
This test predicts the likelihood that the units will display such unsightly deposits from salts that they already contain.
A standard soaking-in-water test can determine the porosity of bricks and blocks, which can then be used as an indication of the potential for the development of
problems related to the penetration of salts and other materials into the units, such as salt attack and efflorescence.
Initial rate of absorption AS/NZS 4456.17
As soon as the bricklayer puts the mortar on a brick, the brick starts to absorb water out of the mortar. The microscopic pores in the brick soak up the
water, which carries with it some of the partly-dissolved cement and lime. It’s the setting of this cementitious material within the brick pores that provides most of the bond between the brick
and the mortar, and thus gives the wall its strength.
To get the best bond, it’s important to match the “suction” of the brick to the water-retaining properties of the mortar. If the initial rate of absorption
of the brick is too high for the mortar that’s being used, the mortar may dry out too quickly and stiffen before the next course can be laid. If it’s too low, not enough cementitious material
is drawn up into the brick pores. In either case, the bond strength will suffer.
The Initial Rate of Absorption test measures the amount of water a dry brick can soak up during the first minute of contact with water.