Stone 219
2.4 Limestone Weathering in an Urban Environment: Illustration of
the Interaction of the Three Factors
Degradation cannot be pinned solely on one factor – it is the spatially- and
temporally-variable interaction of the above factors that produce degradation.
An example of how these factors act in combination can be gauged from the
example of limestone weathering in an urban or other polluted environment.
The material itself, although it may be a natural rock type, is not exposed as
a natural rock. Limestone is likely to have been worked before being placed
in the environment, at the very least to form it into a shape and size that will
fit within a building. This could set up stresses within the stone surface that
agents of degradation could exploit. Likewise, the location of a piece of stone
in a building, a major determinant of its immediate environment, is not acci-
dental. The location of each stone is designed beforehand, in some cases to
take advantage of particular perceived physical properties such as durability.
Stones used for copings, for example, tend to be viewed as durable stones as
they are expected to have a harder time upon exposure, i.e.have greater runoff
over their surfaces and greater exposure to wind and driving rain. It cannot,
therefore be assumed that all stones are expected to behave in an equivalent
manner in a building. In fact, it could be argued that differential degradation
is designed into the building. Sometimes, design requirements conflict with
the stone properties being exploited. Delicate carving, for example, is gener-
ally easiest and most spectacular in stone that is regarded as ‘soft’ or less
durable. The carvings are exposed as much as more durable stone and so tend
to degrade at a faster rate. Their propensity for degradation under a given set
of conditions is greater.
An initial and important distinction needs to be made between two micro-
environments – exposed and sheltered. Exposed surfaces have water flow
across their surfaces either from direct rainfall or from runoff or both.
However the magnitude and frequency of this flow may vary between sur-
faces. Sheltered surfaces do not experience any water flow; products of
degradation cannot, therefore, be removed from the surface by water flow.
This can result in the build-up of a crust of degradation products that could
protect the limestone surface from further alteration. On an exposed surface,
the build-up of any degradation products is likely to be temporary. Micro-
environmental variations in exposure begin to define the type of degradation
forms that can be expected to develop on different parts of the building.
For degradation forms to develop, however, it is necessary for reactions to
occur that can alter or cause stress in the stone. Delivery of potential reactants
to a limestone surface can be by two pathways, wet or dry. Wet deposition
occurs when gaseous and particulate pollutants are incorporated into water
droplets, falling as rain. This solution is usually acidic; natural rainfall hav-
ing a about pH of about 5.6 due to weak carbonic acid; urban rainfall falls as