THE MECHANICAL CUT AGAINST RISING DAMP
Mlouise Musumeci
Dissertation submitted to the Institute for Masonry Construction and Research in
partial fulfillment of the requirements for the Degree of Master of Science in
Conservation Technology for Masonry Buildings
ABSTRACT
If any city in the world has a problem of rising capillary moisture, that city is Venice,
where the buildings literally stand in the water. Behind the attractive elevations of the
canal-side buildings are damp decaying houses, unfit for habitation which have long been
abandoned by their inhabitants. In Venice, as a result of efforts over the past decade to
improve the condition of the city’s buildings, all known methods of damp proofing can be
seen. In fact it was the Venetians who invented a system called cuci e scuci to bar the rise
of water inside their walls.
Since then technology has taken a twist and diverse methods of cutting, various water
barriers and state of the art techniques are being proposed. A few of the most popular
types of damp proofing interventions adopted are discussed in chapter 2. Although the
latter interventions have been used in various countries, the author feels that the most
successful intervention against damp proofing a building against rising damp is the
mechanical cut and insertion of a physical barrier, be it PVC or resin. This, together with
the fact that locally cutting and insertion of a damp proof barrier is becoming a popular
practice, instigated the author to research into this type of intervention by way of this
study.
The fact that certain waterproofing products have proved successful when applied directly
to new construction, or on a particular type of construction, does not automatically imply
that they are an all round solution against rising damp. Situations may arise where, after
one has completely resolved the problem of damp, a new problem, such as structural
cracks or wall instability is introduced as side effects. Hence a product should be closely
scrutinised from two different aspects. Firstly, the physical and chemical properties of the
product and how such properties interact and affect the existing masonry should be
studied. Should such a study indicate negative and deleterious effects to the existing
building fabric, then the use of this product should be discarded at this point.
If on the other hand the aforementioned properties of the product do not exhibit negative
effects on the existing construction, nor does their use appear to induce undesirable
consequences, then the next level of study should be embarked upon. This would
typically involve studying the product once it is incorporated all along into the building
structure. It is at this point that one should remember that a relatively thin layer of a
completely different material of that of the building fabric, will be incorporated all along
the lowermost part of the masonry wall. It is hardly surprising, therefore that the effects
on the shear strength and stiffness of the wall caused by the intervention will be altered.
Hence determining properties such as shear strength across the joint and the elasticity of
the material becomes of paramount importance.
Masonry wall panels are frequently subjected to horizontal racking shear in addition to
compression, such as a wall under seismic loading. Malta has been placed under Zone 2
in the draft Buildings Regulations for the Maltese islands. The latter establishes an
acceleration of 0.05g to 0.1g. To date a seismic risk hazard analysis has not been drawn
up for the Maltese Islands, however one can compare the latter accelerations to studies
carried out on Chilean low rise masonry buildings. Chilean engineers studied various
heights of low rise buildings subject ot seiesmic action and found that masonry buildings
sustains shear under accelerations ranging from 0.3g to 0.4g, till a maximum height of
five stories. At this point shear failure occurred in the wall elements.
Other common situations were one finds walls subjected to shear are, retaining walls and
walls serving as buttresses to arches. The latter is very common locally especially in
historic buildings were roofing with msonry arches was a common practice.
Published results of research work illustrated that the shear strength of a wall is directly
related to the shear strength of the masonry joint. Mortar and brick/block properties are of
paramount importance when considering shear failure of a masonry wall and likewise
damp proofing materials inserted in the mortar joint.
The importance of the Elastic Modulus of masonry arises from the fact that the modulus
of elasticity of masonry affects the distribution of loads between elements in a structure
and thus an accurate prediction of the value of elastic modulus of masonry is necessary to
correctly estimate stress distribution in a structure in the elastic range. The introduction
of a very stiff material or very flexible at the base of a wall will inadvertently affect the
stress paths in the wall.
It is therefore the intention of the author, by way of this study, to analyze primarily, the
structural consequence of the introduction of a waterproofing material, mainly polyester
resin and PVC strips, into a horizontal cut along the base of a wall. By carrying out such
laboratory tests on small specimens of globigerina limestone walls which were
respectively cut and consequently injected with resin or with PVC strips, it was possible
to obtain test results for shear strength and Elastic Modulus for each different construction
type. By comparing the behaviour of the resin specimens with those for a normal wall
and similarly the PVC specimens with that of a normally constructed wall, one would
then be in a position to comment on possible structural effects, such an intervention
would have on the existing building, employing local masonry construction techniques.