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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
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
temperature conditions.
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.
Dimensions
AS/NZS
4456.3
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
AS/NZS 4456.6
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.
Water
absorption
AS/NZS
4456.14
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. | 