Prospective Study 1: Aqueduct
Carthage-Zaghouan, Tunisia
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Prepared by: Department of Geology |
Comments and conclusions
The
materials
a) The stones
The results concerning the natural material show the use of different
materials in the aqueduct, mainly Oligocene sandstone forming the majority of
the blocks in the studied area. The other materials have been used in very
specific parts of the aqueduct.
The petrographic analyses comparing the materials deriving from the
quarry and from the monument suggest minimal differences.
In the case of sandstone the differences are limited to the presence/absence of accessory minerals (chert in the quarry and volcanic quartz and zircon in the monument). Anyway, the above quoted differences are due to changes in the depositional environment for instance: concentration of some minerals depending on their hydrodynamic behavior. The differences are never associated to alterations produced once the materials are placed in the monument
The sampling at the Roman quarry and further development of the
mineralogical and geological study will contribute to clarify many mainly
questions on the Roman building phases and the later restorations.
Regarding to the Eocene limestone there is a very good correspondence
when comparing analysis from the quarry and the monument.
b) The mortars
and concrete
In the materials used in the concrete, particularly in the contact
stone-mortar, it is observed that the porous are filled of mortar.
The artificial materials, mortar and concrete, are widely used in the aqueduct building. There are not important weathering actions because the climate are usually very dry and the material are in good condition. All the problems could be focused on structural aspects (Mas-Guindal 1992)
Normally, mortars used in the Roman conduits present the following
disposition:
a)
a
internal coarser-grained mortar placed upper the concrete that filled the
pillars and the arches,
b)
a
second mortar with small fragments of red ceramics suggesting the use of fine
debris from the mosaic industry as load in the concrete,
c)
finally,
a very fine-grained mortar impervious to water
used as plaster. In the internal conduits surface, a crystalline
calcareous crust was settled over.
In the successive interventions a high quality mortars were used, and
their conservation state is also very good.
At least three types of bonding materials can be distinguished: Roman
and Haphside mortars (fine-grained aggregates) and Roman concrete with stones
and large particles embedded as aggregates in the mortar filling the pillars
and the arches. Mineralogical analysis showed that all mortars are essentially
composed of calcite and quartz with minor vaterite. Roman mortars display a
higher content in the fine-grained aggregate (sand) comparatively to the
Haphside bulk mortar in reconstructed structural elements, pillars and arches
Mortars used in constructing Roman conduits present a great diversity. A
layer of very fine-grained mortar impervious to water (stucco) was
coarser-grained mortar. Quite frequently this mortar carries crystals of rare
minerals (like feldspars, amazonite and adularia) along with small fragments of
red ceramics.
There are in the aqueduct many portions that need an urgent intervention
in order to save their integrity. Also it is necessary to organize a global
planning for determine the interventions more important at present. Finally it
is very interesting to make a complete documentation of the restoration works,
that has been realized at the present time (Hassine 1998).
The study of stones and mortars and the characterization of their actual decay state has been considered (Chiari 1992).
The study of stones (limestone and
sandstone) from the monument and from surrounding quarries-in some cases
recognized as having been used in roman times-was considered a major task with
the aim of identifying possible sources of adequate materials for restoration
purposes. (Casatany 1955, Burrollet 1975, Ghaumi 1980)