Spanish fort made of cut coral blocks

Anna Shepherd <a_k_shepherd [at] hotmail__com> writes

>I am currently assisting a local heritage team on a small island in
>the Philippines with the conservation of their fort. It was built by
>the Spanish in the mid 1600's, and is made of cut coral with lime
>mortar. At the moment the fort has extensive structural damage due
>to weathering and vegetation growth on and around the fort. Many of
>the blocks are falling away due to mortar loss, the stresses have
>caused turrets to break off from the fort walls, and one wall is
>completely bowed from modern infilling material placed behind the
>wall.
>
>Having not had much experience in architectural conservation, I am
>wondering if anyone has any information on how best to repair such a
>site, in particular the leaning towers and major structural cracks
>which are threatening to collapse. Also, I am wondering if anyone
>has any information on an appropriate mortar that can be used in
>conjunction with cut coral bricks. Being a small provincial island,
>with most of the work being volunteer, it is extremely difficult and
>exorbitantly expensive to obtain chemicals here, therefore any
>information on easily obtainable 'recipes' that would not be
>confused with the original mortar would be greatly appreciated.

It would appear from the description that the fort in question has
been neglected for a substantial period of time during which no
maintenance has been undertaken. Being located on a small coral
island it would be reasonable to assume a high level of salt
contamination and that the loss of mortar is due to a loss of its
structural integrity as a result crystallisation cycling of the
contaminating salts as a result of fluctuations in the ambient
relative humidity. This problem would be greatly exacerbated by the
colonisation of plants in the mortar joints leading to further
fragmentation, dislocation and loss of bond strength as a result of
root penetration.

One must assume that the original mortar was adequate for the
application as the fort has survived to the present. Therefore the
clue to a successful conservation mortar can be found in the
original composition. It is unlikely that the constituents of the
original mortar travelled far and were most probably prepared at the
site. Analysis of the mortar composition is a matter of very simple
quantitative analysis and can be undertaken with no more resources
than are found in a school laboratory, percentages of lime to
aggregate should be deduced and also the size range and fractional
percentage of aggregates. From this information and conservation
mortar closely resembling the original can be produced from locally
available materials.

The composition of a mortar is unfortunately not the end of the
story as its method of application is equally, if not more critical
to a successful application. Lime mortars require time to cure and
the original water content and rate of vapour loss must be
controlled over a period of days if the mortar is to cure
successfully. Shrinkage will occur and must be addressed by the
reworking of surfaces and possibly the surface removed on final
drying to increase porosity.

Before any application can be undertaken, perished mortar, botanical
colonisation and soil must be removed and on bowed walls and areas
of dislocated blocks this operation can lead to collapse. This
problem can be overcome by planned treatments where only small
section of wall are cleaned and therefore temporally destabilised,
therefore reducing the chance of collapse. But before advancing with
any treatment on dislocated masonry a structural engineer with
conservation experience should be consulted.

A safer alternative for extremely unstable structures would be
dismantle and rebuild, but this is an extremely contentious
approach, as to preserve authenticity ever block must be replaced in
its original position. Is the bowing wall really due to the modern
infill or could it be subsidence that occurred at the time of
building? Is this the Leaning Tower of the Philippines?

Mortar recipes are I am afraid of little use to you; the author
could give you a recipe for a mortar that works well in Crimea,
another that works well up a mountain in Turkey and others that work
well for standing buildings across Europe, but none are what you
need. Luckily the answers are in the original construction materials
and methods of application and should only require locally available
materials and a little practice and guidance in preparation and
application.

Chris Cleere
Object and Site Conservation Consultant
Flat A
131 Malden Road
London NW5 4HS
United Kingdom


                                  ***
                  Conservation DistList Instance 21:18
                 Distributed: Wednesday, August 1, 2007
                       Message Id: cdl-21-18-004
                                  ***