Sonic tests for the restoration of San Francisco church in Cuba | Team Arch-indagini

Thanks to a renewed interest in Cuba's cultural architectural heritage, recent years have seen restoration projects involving some of the colonial churches in the city of Santiago de Cuba that have also involved Arch-indagini professionals.

Operating in distant territories, working with construction materials and techniques that are often little known and not widespread in Europe, or even no longer used and not documented in the scientific literature, has been a great challenge for the professionals involved.

In the case of the church of Santiago, the reconnaissance of the historical data of the buildings was essential: a complex search due to the lack of historical documents on site, although fortunately all the monumental construction of the Spanish colonies in South America was designed in the motherland, so copies of many plans are now preserved in Seville, at the Archivo de Indias.

Preliminary study: historical floor plans and structural diagnostics

The San Francisco complex in Santiago de Cuba dates from the mid-18th century and consists of two basic elements: the second most important church in the city and a convent, designed to house the friars and allow them to carry out charitable activities.
In order to delve deeper into the construction peculiarities of colonial churches, especially of the Iglesia de San Francisco, the Arch-survey team began with a preliminary study of the documentation available in the archives of the East Indies of Seville, from which some points emerged that needed to be clarified regarding the load-bearing structure of the church.

In fact, the plan MP-Santodomingo 588, which shows the progress of construction in 1796, shows some small squares inside the pillars, with an unspecified function. What prompted the planners to investigate these unclear construction aspects was the fact that when comparing the actual structures of other colonial churches in Santiago, some common features emerged.
For example, the fact that each church has an exterior wall built of brick and/or stone masonry, but the interior pillar structure and roof are always made of wood. Why? This construction technique was developed to enable religious and government buildings to withstand strong winds, as hurricanes and tropical storms are common in the area, as well as earthquakes. In the San Francisco church, the second largest after the cathedral, the pillars dividing the 3 aisles are externally made of regular brick masonry. The cognitive investigations performed wanted to verify precisely whether there were indeed wooden elements inside them.

This objective was achieved by deepening the knowledge of the buildings through diagnostic techniques aimed at determining their overall geometry, the modifications made over time, the characteristics of the morphology and masonry texture, the chemical-physical and mechanical behaviors of the components and materials, along with their state of degradation.

The diagnostic project, therefore, was planned first and foremost to provide an overview of the construction quality of the masonry structures, especially to characterize any hidden structures in their thicknesses: the large pillars were studied through sonic tests in various configurations, following which direct masonry assays through video-endoscopy were planned.

Performing in situ sonic tests and interpreting the data

For an initial interpretation of the wall morphology, sonic tests were conducted on the opposite faces of the pillars inside the church. The tests were developed by following a mesh of orthogonal acquisition trajectories: this specific arrangement was aimed at identifying discontinuities in the masonry structure, in order to verify whether indeed inside the pillars were contained the so-called horcones, wooden trunks with the function of absorbing the oscillations of the roof, which is also wooden, resulting from a possible seismic event or strong wind.

Sonic tests were performed on 3 types of masonry structures in the church: isolated cruciform and composite piers, recessed piers, and bearing walls.

Data processing consisted of defining the travel time taken by the sonic wave from its origin to the point of arrival: knowledge of the distance between the point of emission and reception, deducible from the geometric survey, allows calculation of the pulse velocity, understood as the ratio between the distance between the test points and the relative crossing themes. We adopted a test mode by transparency, i.e., with the corresponding grid points placed on the opposite faces of the structure, to achieve higher resolution of the results and less influence of the acquisition geometry during the passage of sonic waves inside the wall solid.

The visual representation of the results consists of a velocity distribution map that is assigned a color scale, from orange-red, which identifies areas where the masonry is more compact, to blue, for areas where more discontinuities are present.

The results obtained required further investigation to better locate the position of the wooden logs within the pillars. The acquired velocities were then reprocessed on the basis of a mesh of points inside the section of the pillars, given by the intersection of the orthogonal trajectories: each point was given the average velocity of the 2 perpendicular trajectories intersecting it. This also served to identify the most significant areas for subsequent visual assays conducted by video-endoscope.

Why performing non-destructive sonic and in situ testing on historic buildings

Preservation interventions on a historic building are, in general, all the more effective the deeper the knowledge of the building in all its aspects. To meet this need, it is essential to appropriately apply in situ and laboratory diagnostic investigations so that the results obtained can be used to understand all conservation aspects affecting the materials and structures.

In the case study of the church of San Francisco in Cuba, a knowledge path characterized by increasing levels of depth was defined: the first level of knowledge made it possible to collect essential information, such as history, general geometry (through plans, elevations and plan sections), as well as masonry characteristics, the scheme of the structural system and all documentation related to the damage processes that occurred over time. Later, sonic tests and in situ surveys were added to this theoretical research phase.

"The information and data collected determined the knowledge of construction techniques that have been abandoned today, but which are indispensable for tackling a restoration project that is aware of the problems of preservation and of the historical value of such an important monumental building. The background of construction notions from the past, even if no longer handed down, constitutes a valuable historical resource that can and must be rediscovered and developed for local civil buildings as well, especially since it involves the use of natural materials, such as wood and stone, which are readily available and workable in their territory of origin", commented Architects L. Bolondi, R.D. De Ponti, of Arch-indagini.