Advanced Structural
Designs ACN 097 789 87
92 Vasey Cres Telephone
61612171
Facsimile 61612170 Email mal@structuraldesigns.com.au
What causes Cracking in Brickwork
or other masonry (esp brick house cracking) and who’s responsible?
The cause of cracks in brickwork and
other masonry can generally be broken into 5 categories
Foundation movement.
Support structure deflection.
Thermal movement and moisture uptake or loss
Failure under external loads.
Arch failure.
The
majority of cracking in houses in Canberra ACT is from foundation movement,
whereas residential units and commercial premises tend to poor expansion joint
design. There is however no general
answer to the problem and at times a number of the above causes act in unison
making diagnosis more difficult.
Foundation
movement
In
In a
building constructed on a flat block, founded on uniform soil and with no
gardens close by, this swelling would lift the outside foundations uniformly
and only have a tendency to crack inner walls.
This is because the moisture regime under the house would tend to stay
the same while the perimeter would be affected by the usual seasonal
variations.* In normal circumstances
however the ground moisture and soil parameters vary around the house and
cracking can occur in any number of locations.
It is the
responsibility of the building designers to control the amount of cracking by
judicious use of control joints and by appropriate footing design. In this design consideration is given to the reactivity
of the soil, the variability of the soil, the moisture history of the soil and
any other constraints such as block boundaries, service trenches etc.
The
building owner also has a responsibility to maintain plumbing services and to
not unduly vary the moisture conditions around the property. Some guidance on
what these responsibilities are for residential home owners is outlined in
AS2870 Appendix B. If you ring CSIRO
publishing on 1800 – 645051 for a few dollars they can send you a Information sheet
called “Guide to home owners on foundation maintenance and footing performance”
which is also referenced by AS2870. To summarise if you are planting trees near
the walls of the house or watering shrubs up against it you are likely to be
contributing to the problem.
Foundation
movement normally results in stepped cracking often emanating from the corners
of windows and doors. Such cracks are
normally widest at the top.
For
residential home owners considering negligence action against their engineer,
classifier or builder they should be aware of the performance criteria in the
AS2870 which acknowledges the risk of significant cracking to a small number of
houses. The point is, that if the
engineer, classifier and builder have all acted in good faith, they are well
protected by the code.
If on the
other hand, any of the parties can be shown to have increased the probability
of cracking beyond that deemed acceptable by the code there may be grounds to
take action against them even if the cracking is within code limits. This might be true if for example an engineer
has allowed strip footings and infill slabs on a class M site and the infill
slabs move causing cracking to internal masonry or Gyprock or causing windows
to jam.
If you are
looking to arrest cracking resulting from soil swelling we normally recommend
trying to control the moisture regime in the soil around the building
first. This can take a number of forms
from watering in dry periods to introducing paths close to the building or
installing root barrier to block tree roots.
If you are considering underpinning do not let anyone talk you into
underpinning only the cracked portion of the building on a reactive clay site
as this will normally only move the problem, and more often than not worsen
it. The exceptions to this rule in the
local region is if the house has strip footings and is mostly on rock with a
smaller portion on clay or poorly compacted soil, or if subsidence is occurring
due to adjacent trenching.
It has
recently been brought to our attention that one engineer practicing in the ACT
is recommending partial underpinning of problem foundations on deep reactive
clays. This same engineer is failing to
carry out even a rudimentary soil investigation and is pricing to carry out the
work himself. If you have such a
recommendation we strongly suggest getting a second opinion before you waste a
very large amount of money and potentially worsen the problem. If you are currently suffering the results of
this type of “repair” (which normally involves cracking immediately adjacent to
the underpinned area) you will be well within your rights to seek retribution
from the engineer involved.
If the
crack only partially closes with judicious watering it can often be given an
extra nudge using resin grouting techniques such as those used by Uretek. In this case the grout injected under the
footing chemically expands and effectively jacks the footing back into
place. The procedure generally involves
fewer risks than partial underpinning as the soil structure under the footing
remains essentially the same. It should
be emphasised that this is not a “fix all” solution and it is one that works
best when poor fill compaction was a contributing factor. Controlling the soil moisture regime afterwards
is still a very important consideration.
Another benefit of this system is that it is less disruptive and does
not damage adjacent paths and gardens.
Another
area that exhibits significant cracking is where house extensions are carried
out and the new brickwork is toothed into the old. Whilst this without a doubt provides the most
attractive outcome in the short term (assuming the bricks match or it’s
rendered) it never is in the long term if the house is founded on reactive
clays. This is because it is difficult
to ensure adequate continuity of reinforcement in the footing. Most builders opt to tie the footings
together with a couple of dowels at the footing base which results in the
footing being effectively hinged at the join.
We generally recommend dowelling top and bottom of footings but also
specify a control joint over so that any rotation at the joint does not crack
the brickwork.
The crack
below is in a set of units in Aranda. Judging by the once fashionable streaks
in the mortar I’d say built 1968-69. An
interesting point is that in almost any part of Aranda rock is within 1 m of
the surface. We can’t help but think
this builder could have dug a little deeper.
As is often the case, the additional cost of the correct footing would
be less than 1% of the loss in resale value of the property.
Moisture
change in reactive clays are not the only cause of foundation movement as
founding on poorly compacted fill or adjacent to deep trenches may also result
in footing settlements. Assessing the
lay of the land and identifying services can often yield useful information
when diagnosing a problem.
It is worth
noting here that the old ACT building act prescribed shallow singly reinforced
footings until 1986 when AS2870 (the Residential Slabs and Footings Code) was
released. Although we designed to this
code from the day of it’s release many engineers did not, and the ACT building
authorities did not move to correct this situation until 1988. If your house was built after 1988 it should therefore
have had soil tests carried out to determine the reactivity of the clay and
should have had footings designed to suit the conditions.
Support
Structure Deflection
Masonry may
be supported in defined areas to create openings using lintel beams or may be
completely supported by a series of steel or concrete beams carrying the load
to the ground via concrete or steel columns.
The movement of the support structure can sometimes lead to cracking of
the masonry.
Contributing
causes to this type of cracking can be:
- Poorly designed support
structure (most common).
- Inadequate jointing (if
specified).
- Overly rich mortar mix
increases wall stiffness (especially true where lime is specified in the
mix but not used).
- Rapid construction loading
(incorrect design assumptions made).
- Support propping arrangements
(props removed too late when brickwork is brittle).
It is
always worth remembering that the concrete structure supporting masonry walls
is not only deflecting but also shrinking and creeping. Masonry can sometimes be overloaded because
it is bounded by the columns connected to creeping and shrinking prestressed
floors. This is especially true if the masonry
itself is trying to expand on moisture uptake, and can lead to some quite
spectacular failures.
Thermal
Movement and moisture uptake or loss
Clay bricks
take up moisture and grow whereas silica and concrete bricks lose moisture and
shrink over time. To complicate matters
the mortar between the joints is also shrinking which results in different
vertical and horizontal movements and the amount of movement is affected by the
degree of restraint or confinement.
These movements are generally of no concern if the expansion joints have
been correctly designed (for more info see How do I design Expansion
Joints.htm).
Where
cracking from thermal movement and moisture variation does occur it is normally
vertical and often worst at the building corners. This type of cracking is more common in
residential apartments and commercial buildings but as the plan area of
residential houses increases the problem is becoming more widespread there as
well.
The
location and detailing of expansion joints is generally on the architectural
drawings and when the joints fail to accommodate movement the architect is
often pursued for negligence. Many
architects ask that their structural engineer sign off on the design, which is
certainly a service we provide free of charges on projects we are documenting.
The
responsibility is however, not always with the architect or structural designer
especially when not engaged to oversee construction. Joints are sometimes
unable to function because of mortar dags in the joints, the use of non sliding
wall ties or even non compressible fillers. Swapping brick sources are also
potential reasons for expansion joint problems as the potential expansion of a
different brick may drastically affect the required spacing of the joints.
Failure
under External Loads.
Gravity
loads, wind, earthquake or other external load such as soil pressures or tree
roots can act to stress masonry beyond its acceptable limits. Cracking from gravity, wind or soil pressure
is often the result of a design or construction error. The most common failure
of this type are retaining walls where the failures are generally due to poor
design, incorrect bonding pattern or poor drainage. The other fairly common failure type is
freestanding walls where the failures are generally due to poor (or no) design.
Damage from
earthquake or other seismic loads such as blasting is often seen as diagonal
cracking in both directions. This type
of cracking is extremely rare in Canberra. If there is talk of blasting in the
vicinity it is wise to have the façade of the building carefully photographed
before any blasting commences.
Arch
failure
Many
archways detailed for houses around Canberra were not designed by structural
engineers and were destined to failure from the outset. Fortunately they are now out of fashion and
rarely causing problems. It is important
to note that even correctly designed arches should not be detailed where
footings rest on expansive soils as foundation movement is likely to compromise
the integrity of the arch. This recommendation
can be found in the Cement and Concrete Associations’ TN 61 which is referenced
by AS2780.
The above
photograph shows a typical arch which has failed partly because the left abutment
was not strong enough to carry the thrust from the arch to the ground. Some relative movement of the left and right
abutments have also contributed to the problem.
It is worth remembering that the shallow arches have high lateral
forces.
We have had
cause to look at masonry arch in Church building in Ainslie where it was
overstressed almost to the point of causing a total roof collapse. If you see a crack in an arch structure or
its abutting walls, call a structural engineer to check it out.
Further
Information
In most
cases, causes of cracking in masonry can be determined and remedied or
practices put in place to minimise their extent. When you are looking to spend
money repairing cracks in masonry it is worth having an expert look at your particular
problem. If you have a particular
problem in mind and need a structural engineers perspective in Canberra ACT or
surrounding districts contact Mal Wilson from Advanced Structural Designs on 02
61612171.
* This
point is not strictly true as the house has a tendency to shade some areas more
than others from the drying effects of the sun.