cracked gauged brick arch (Photo: Charterbuild)
with the conservation of historic buildings, or more mundane commercial
or institutional buildings, it is imperative that the cause of
cracking in masonry structures is well understood. Perhaps one
of the most significant differences between working on relatively
modern structures and historic buildings is that often, in the
latter case, it will not be possible to deal with the root cause.
However, this does not in any way reduce the need for understanding,
because a suitable repair strategy can only be determined once
the cause of the problem has been identified.
because the real world is not static: whether at a macro or a
micro level, materials respond to changes in their environment
by trying to move. There are several generic causes for movement
(a more specific listing would run to several pages):
Ground movement (beneath foundations) Examples include
shrinkage in clay sub-soils (often tree-related), loss of
fine particles from granular material (caused by drain failure
or ground water movement for example), land slip and even
the activities of burrowing animals.
Foundation failure Consolidation of rubble foundations,
decay of soft clay brick, and attack of concrete by aggressive
chemicals all fall into this category.
Decay of superstructure Most structures are composite,
being made up of different materials each with its own life
expectancy under any given environmental exposure. When one
material fails prior to another, movement and cracking arise.
A classic example would be the decay of timber wall plates
in masonry, or the corrosion of iron cramps in stone walls.
Moisture movement Materials either expand or contract
as they gain or lose moisture. In some cases (the expansion
of new clay bricks for instance) the movement is irreversible
but, if anticipated originally, the construction may have
been designed to accommodate it. In others, movement caused
by variations in moisture content are reversible and because
of their cyclical nature, often harmless, but they can lead
to progressive movement, often out of plane. Seasonal changes
in timber are a good example, and of course as timber is a
natural material, movement will not be uniform.
Thermal movement As temperature increases or decreases,
materials either expand or contract. If such movements are
prevented, very great stresses can occur. This form of movement
is also cyclical and can lead to the progressive deterioration
Inherent defects Structures, particularly historic
structures, often lack (by today's standards) sufficient lateral
restraint of vertical elements, resulting in lateral movement.
Often progressive alteration of structures can concentrate
load in areas ill-suited to carry it. Movements associated
with such defects are often progressive and unless arrested
can eventually end in structural failure.
Inappropriate specification Modern repair techniques
are often too strong or too rigid for the repair of structures
built with pliable lime-based mortars. Pointing with Portland
cement-rich mortar is a good example, as strong mixes do not
behave elastically and, if overstressed, the new mortar can
fracture or, more usually, the surrounding masonry fractures.
Deflection under load Suspended structures such as
floors tend to deform under load, and even vertical elements
subject to load will compress by a small amount. Any infill
(which is by definition non-structural) must be detailed to
accommodate such movements or cracking will occur. A classic
defect of 1960s buildings is delamination of brick slips caused
by the deflection of reinforced concrete supporting structures.
exhaustive, this list serves to illustrate the complex interactions
that can occur, and the difficulties that may arise in gaining
an understanding of cause.
an inevitable response to the inability of a structure to accommodate
the movement to which it is subjected. There are two issues to
be considered when assessing the reasons for cracking: the first
is the nature and significance of the cause of movement (as described
above); the second is the ability of the structure to accommodate
movement. The latter will depend on the nature of the material
or, in the case of composite materials like masonry, the nature
of the combination of materials used in the structure.
For new buildings,
designers can try to eliminate many of the causes of cracking
and design tolerance for those factors that remain either by introducing
joints, or by choosing movement-tolerant materials. The latter
is the traditional approach to design, however materials such
as fat lime (lime putty) and hydraulic lime can now be specified
to give structures the ability to absorb considerable movement.
are what they are, and when considering performance an assessment
must be made as to the likely cause of movement and the degree
of the movement-tolerance inherent in any given structure. The
cracks may be predominantly associated with an external cause
(as in the case of subsidence) or with the material itself (an
over-specified cement render for example).
London Road, Chippenham: (top ) the front elevation showing
new and old stone; (middle) detail over a door showing a lintel
cut to shape to accommodate settlement; and (bottom lateral
movement in the front elevation due to the decay of embedded
timbers both to front and side elevation (note the buttressing).
wall showing settlement where it was built over an old well
buildings are concerned it is essential to employ someone with
the experience and expertise to recognise the patterns of movement
taking place, and who will be able to plan a programme of investigation
to verify their assessments. Research is essential and can save
hours of expensive time, and minimise the need for the disruption
that investigations can cause.
structural engineer will be able to form an opinion on the basis
of a visual inspection, but even when good archival information
is available (from a previous structural survey, for example),
some site investigation will still be required to tease out the
various interdependencies, and to establish the most important
factors at work.
This can be an iterative process as investigations
can raise more questions than they answer.
It is important
to establish a brief before embarking on any programme of repair.
On the assumption that both the cause of movement and the degree
of movement-tolerance have been established, a range of options
will be available. These may encompass major intervention on the
one hand (and a good prospect of avoiding major future problems)
to limited cosmetic action in the knowledge that further maintenance
work will be required on a regular basis. The most important consideration
is whether movement is likely to be progressive. If the structural
condition is going to deteriorate, then for a structure to be
preserved, intervention becomes essential.
an approach to underlying problems, the result of the intervention
must be assessed. For instance, new foundations can be expected
to settle following underpinning, as the ground beneath consolidates.
This is particularly an issue where clay forms the sub-strata.
Premature or inappropriate repair of crack damage can be an expensive
In effect, the likelihood and the nature of future movements
need to be assessed for the new condition created by intervention.
Detailing will be dependent on the kind of movement likely to
occur in the future. In most cases structures will still be subject
to moisture content changes and variations in temperature, as
short and long term changes in environment occur.
an inherent weakness in a structure. This is why a superficial
approach to repair, limited to filling and decorative work, will
almost always fail. Even the smallest of movements will lead to
In engineering terms, this is explained by the
size of cracks in relation to the rest of the structure. Cracks
are usually narrow, and if movement is concentrated at these positions
of weakness, the strain will be high. The stress created by movement
is a function of strain, and will vary depending on a parameter
called the Young's modulus which, in effect, describes the stiffness
of a material.
For this reason,
materials with high values of Young's modulus, such as a rich
Portland cement (OPC) mix, should not be used to repair a crack
in a masonry wall, because the stresses that arise will cause
the repair to fail. Materials with a low modulus, or with the
ability to deform in a plastic manner, will offer better prospects
for success. A high calcium hydraulic lime, for example, would
be much better than OPC in an area of high strain.
the width of a repair will also reduce strain and, therefore,
stress. In practical terms this can be achieved by cutting out
cracks and substituting a formed joint, but this is major work
and may not be acceptable visually or in terms of cost. Rebonding
masonry using hydraulic or fat limes is usually preferable, as
this will give the necessary plasticity, providing a mechanism
to accommodate strain.
is to insert corrosion-resistant metal reinforcement (usually stainless
steel) into bed joints to redistribute strain and therefore stress
over a wider area, thus reducing the risk of failure. Care must
be taken to vary the width of reinforcement across the crack as
abruptly terminating a bar in a wall or mesh in a render in parallel
lines on either side of the crack can also cause stress concentrations
leading to further cracks.
require the basic form of construction to be robust and in relatively
good condition. This is not always the case, and sometimes it
is necessary to carry out remedial work to stabilise a structure
before it can be repaired. To assess this need, the basic form
of construction must be understood, and the degree of distortion
estimated. Generating cross sections to show the deflection and
deformation can be helpful, allowing the extent of voids and other
defects to be estimated and taken into account.
are suspected, the use of instruments such as boroscopes can be
invaluable to see inside structures to verify the extent of the
problems. There are other more sophisticated methods of non-destructive
investigation that can be employed, including radar and thermography,
but these can be expensive and complex to interpret. These methods
are discussed further in 'Non-Destructive Investigations', a Building
Conservation Directory article by Robert Demaus, also available
on this website.
response to voids is usually either reconstruction (often unavoidable)
or grouting to fill them with cementitious material. Grouting
requires the voids to be accessed by drilling holes in the structure
through which the grout can be poured. There is no single 'best
way' to approach grouting, but some methods are better than others.
- use trials to find the best grout mix
- use rounded fine aggregates rather than sharp sands
- make several holes to access voids
- start below and work up; grout appearing in the next hole
is a sign of success
- pre-wetting of the wall will prevent moisture-loss from
the grout and reduce clogging of the grout points with dust
- grout can exert high levels of hydrostatic pressure, so
limit the pour height and the pressure head used
- where walls are in poor condition it may be preferable to
slowly feed the grout into the wall using clay applied to
the wall and shaped into cups
- grout can take time to penetrate walls
- use a grout mix that is compatible with the fabric of the
- consider the future performance of the wall when specifying
are from an engineering perspective. There will be other factors
influencing the specification for repair, and it may be appropriate
to depart from the 'ideal' engineering solution.
repair of cracks in masonry structures requires a fundamental
understanding of the reasons for the cracking. Intervention must
respect the structure, and it is important to assess the likelihood
of progressive movement. An appropriate repair will accommodate
future movements in the wall, but may require stabilisation works
if it is to be successful.
article is reproduced from
Building Conservation Directory, 2003
ALLEN PhD BEng MICE MIOSH is a consulting engineer and a partner
in Ellis and Moore. His research experience in cementitious
building materials has been applied to both repair and refurbishment
projects, particularly listed buildings, and he is collaborating
with Bristol University and major industrial partners in the
development of hydraulic lime for mainstream construction
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