Old Akko (Aka; Acre)
Technological and Structural Aspects in the Conservation of Old Akko
Ofer Cohen and Yael F. Na’aman
This essay deals with technological and structural aspects of the conservation of building remains in Old Akko. The basic concept is one of authentic conservation by means of preserving the original materials and facades without disturbing the ongoing everyday live in the city. The source for the information presented below was derived from physical-engineering surveys that were conducted in the city over the past decade.
|Damage to Buildings and Material Decay
Damage is a condition in which the building has partly or completely lost its load bearing capability and is liable to collapse partially or entirely. The damage is usually marked by such causes as cracks, collapse, crushing, crumbling and the breaking away of and deformation of elements. Decay is the deterioration and weathering of the material, a condition that usually leads to a reduction in its resistance and increased brittleness and porosity. The process whereby material is lost stems from physical and chemical action and usually begins on the outside and works inward.
The mechanisms of damage and decay are actuated by a number of factors: the absence of proper maintenance, the lack of scientific knowledge, the use of a construction beyond its life expectancy, imperfections in the original design and the introduction of new factors that were not foreseen. All of these factors lead to a reduction in the structural strength, in other words, the reduction in the load bearing capacity together with an increase in the effects of the actions involved. Three factors are at work here: the kind of action, the quality of the materials and the type of structure. The action involved can be some sort of dynamic or static mechanical action, and a physio-chemical action linked to the atmosphere and environment. The materials’ resistance is effected by the climate and weathering as a result of physio-chemical processes. The decay is connected with the natural environment such as humidity, rain, temperature fluctuation; and factors such as traffic, pollution and the lack of maintenance also accelerate natural processes. Decay can be chemical, physical or biological and is related to environmental factors, the characteristics of the building materials and the specifications that protect the building (e.g.: roofs, drain pipes). The structural behavior depends mainly on the kind of materials used, the shape and size of the structure, connecting specifications between the elements and the environmental conditions that border on the building. Damage is caused due to an increase in the mechanical action and reduction in the structural efficiency, whether from natural phenomena or as a result of an action caused by man. When these occur without careful control they can negatively impact the building (Croci 1998: 41-46).
Old Akko was built of kurkar masonry stones (walls and vaults) held fast by lime based bonding materials and wood that was used in the roofing, window and door specifications. Other elements were also used such as hard limestone (for the cantilever steps, pavement in public spaces, window openings and decorated elements) and marble. In later periods materials such as terrazzo pavements and painted concrete, iron beams, Marseilles roof tiles and of course concrete were also utilized. In recent years we have witnessed the use of a variety of modern materials, among them surfacings, plasterboard walls, aluminum and ceramic.
The most common structural problems in the Old City are concentrated in the buildings’ walls, vaults and ceilings. Each one of these is characterized by problems that stem from the construction technique, quality of the building materials, the destructive factors of the building and the decay factors of the material.
The Processes Characteristic of Building Disintegrating
Two factors are destroying the buildings in Akko: (1) the decay of the material – weathering as a result of natural processes; (2) damage as a result of human intervention. The natural process of material decay is influenced by an especially high level of humidity and moisture, as well as salt crystallization, the properties of the air, ground characteristics, water (precipitation and proximity to the sea), temperature and man’s intervention with improper maintenance.
Damage caused by the direct intervention of man is commonplace, for example: the renovation actions where unsuitable materials were used; when part of the building is dismantled in order to adapt the built space to the user’s needs or for the purpose of clearing an area needed for new construction, in order to create new access routes or improve existing ones; dismantling part of a structure so it can be put to secondary use elsewhere, or dismantling for the purpose of removing an immediate danger.
A schema of the disintegration process of buildings includes prolonged decay of ceiling joists on the upper story until they collapse, an accelerated process of the walls falling apart on the upper story and of the vault on the ground floor and the continued disintegration of the building’s outer walls.
1. In the wake of the collapse of a wooden ceiling on the upper story the walls remain, standing tall and thin relative to the surroundings.
The danger in this situation is that the stability of the walls will be undermined
that will result in several stones falling or the collapse of wall sections.
The solution in this case is to stabilize the remains by creating a flat element on
top of the walls and/or a support anchored to the vault. (Fig.1).
2. Collapse of a wooden ceiling on the upper story and of the vault on the ground floor and the partial disintegration of the walls.
In this instance remains are created that are tall relative to the surroundings and
the remains of the vaults are unstable.
The solution to this is to restore the spatial function to the ground floor and/or
support the remains to the ground or an adjacent structure. (Fig.2).
3. Collapse of a wooden ceiling on the upper floor and the vault on the ground floor and the disintegration of the building’s outer walls.
In this situation a protruding stump remains that is not stabilized by the vault. The
solution requires stabilization of the vault stump or the controlled removal of
dangerous parts. (Fig.3).
The described disintegration schema is also valid in three story buildings in which there are two wooden ceilings borne atop a vaulted story.
The most significant and common factor in the disintegration processes in the city is the inclination of the walls, in other words their becoming out-of-plumb. It can be said that all of the instances of building disintegration in Akko stem from a lack of proper maintenance or as a result of physical damage. These factors cause a chain reaction of prolonged damage and decay and constant deterioration in the physical condition of the building.
Most of the walls on the ground floor in Akko are built of two rows of stones with a debesh fill in between. The construction was carried out in horizontal rows: first one course of stone of the outer faces of the wall was laid and the space between them was filled with debesh and afterwards the second row of stones was laid in the same manner and so one, one course of stone atop another. The walls’ outer faces were built of dressed kurkar masonry stones but the repairs that were made were done with stones that were not dressed. The core of the wall consists of small stones and bonding material. In the construction of the city extensive use was made of two kinds of bonding material, one based on lime and the on soil.
Most of the walls are 80-120 centimeters wide. Their primary function is to bear the load from the vaulted story and to direct loads from the tall walls (one stone wide) of the upper stories. The initial impression one gets from looking at the walls is that they are homogenous. Nevertheless, when we observe the crumbling bonding material in areas where the wholeness of the wall was damaged or in instances of decay, a different picture is revealed: the walls’ building material and the construction mass are undergoing a process of disintegration.
We can classify the walls in Akko into four types:
Type 1: Regular construction utilizing stones that have five dressed surfaces. The height of the stone and the course is c. 45 centimeters; the stone is 50-100 centimeters long and the width of the joint between the stones ranges from 5-10 millimeters. In general we can say that the quality of this type wall is quite good.
Type 2: Regular construction utilizing stones that have five dressed surfaces. The height of the stone and the course is 18-37 centimeters; the stone is 18-45 centimeters long and the width of the joint between the stones is 5-10 millimeters. The quality of the wall is usually good.
Type 3: Regular construction using roughly hewn stones. The height of the stone and the course is 23-40 centimeters; the stone is 18-60 centimeters long and the width of the joint between the stones is 5-10 millimeters. In general we can say that this type wall is of average quality.
Type 4: Irregular construction utilizing quarried stone. The size and shape of the stone varies and the courses are not of uniform length. The width of the joints is not uniform and the vertical joints frequently extend through more than one course. The quality of the wall is poor. Walls such as these mainly occur as repairs or retaining walls.
Most of the walls’ structural problems involve: a lack of stones in the wall’s outer surface, structural faults in the outer surface, cracks, faults in the plane of the wall that are mainly characterized by horizontal shifts, voids in the core of the wall or bonding material that is missing from the core of the wall. These problems are caused due to the use of inferior quality materials, the tops of the walls that have not been sealed, percolating water, the crumbling of bonding materials, mechanical damage, as well as wear and erosion of the core of the walls leading to the formation of voids inside them. Frequently wear occurs when the bonding materials gradually crumble and are washed away through cracks and joints that were emptied by this process. In addition to these one must add the absence of ongoing maintenance which accelerates the natural weathering processes.
Lack of Stones
A number factors lead to an absence of stones from the outer surface of a wall:
(1) Direct mechanical damage to one or more stones leading to the localized loss of stone. Such damage usually results in other stones falling from the wall. One or several stones suffering from intense wear will cause one or more stones to fall from the course above it. This process will stop when the damaged area is re-stabilized.
(2) Poor construction quality of the wall. In this instance the inside surface that comes in contact between the different courses is rather small, the depth of the stones is small and they are not sufficiently anchored to the wall. Because of this a minimal amount of pressure will lead to the detachment of the stones and their falling.
(3) Collapse of a section of the outer face of the wall as a result of deformation caused by distention.
(4) Removal of a wall or partition perpendicular to the wall will result in damage to the wholeness of the wall, to loss of stones and accelerated decay.
The objective in treating missing stones is to restore the original loading bearing capacity of the wall, to integrate the outer surface as an inseparable part of the wall and renew the original constructive system.
Possible solutions in this case are: (1) restoring limited and relative stability to the region (Fig. 4); (2) a more extensive restoration of stability through the use of supports (Fig. 5). Stabilizing by completing the stonework is a preferred solution from all aspects of the building’s conservation. The installation of supports is suitable as an intermediate phase in the stabilization process or in cases in which the deformation in the wall calls for it.
Fig. 4. Restoring localized stability by completing the stonework.
Fig. 5. Restoring extensive stability through the use of supports.
This condition is characterized by a section of the wall protruding from the original line of the building. This phenomenon is a result of a number of destructive mechanisms:
1. Cracks and/or voids in the core of the wall creating an excess load on its outer surface, which leads to distention followed by the collapse of the same section of wall.
2. Detachment of the outer surface of the wall from the core due to the absence of sufficient adhesion properties.
3. A process that accelerates the distention is the crumbling of the bonding materials and its falling into the space between the core of the wall and the outer stone surface. In this situation the bonding material acts as an accelerator in the deformation process.
The objective in this situation, like the previous one, is to restore the wall to its original loading bearing capacity, to reattach the outer surface of the wall and renew the original static system.
Possible solutions: fill the voids in the core of the wall (grouting) and pointing up the joints, install anchors or dismantle the affected wall section in order to rebuild it.
The property that differentiates building remains from wall remains is the possibility to restore their spatial-structural function. A number or combination of factors brings the building to a state where it will be defined a remain, for example the collapse of a vaulted ceiling due to sundry reasons such as excess load; wall stability that is low relative to the lateral pressure of the vaults; failed implementation; collapse of one of the load bearing walls; collapse of a wooden ceiling due to various reasons such as natural wear; moisture problems that lead to enrooting in the wood beams; excess load; wall deformation or an act by man such as the opening of new access routes or the removal of a space for the sake of a new structure. In all of these cases accelerated wear of the ceiling and walls is apparent prior to the collapse. The objective in this situation is to stabilize the different structural elements and, to the extent possible, restore its spatial function. In buildings that are only slightly damaged, it is preferable to restore the spatial function by means of constructing a ceiling using traditional technology while at the same time replacing the missing stones. In reality, building remains in an advanced state of destruction are currently not undergoing conservation in Old Akko and they are either being demolished intentionally or neglected.
This element includes walls of various heights and widths, suffering from different degrees of damage, without any possibility of restoring spatial function. A number of factors or a combination of them leads to this situation:
A wall that was originally part of a building that was destroyed and today nothing remains of the original spatial context. The quality of the construction is a significant factor in the condition of the walls; when this is extremely poor we notice the accelerated decline in the constructional properties of the structural element or of the entire building. A wall that was originally built as a single element and part of it was destroyed by a natural process, such as stone or core erosion as a result of weather damage leading to the collapse of a section of the wall; the defective sealing of the top of the wall or the absence of sealing in this case intensifies the rate of erosion and its toll. In addition to these we should add the human factor which manifests itself through the creation of new approaches or new construction.
The objective in these instances is to stabilize the wall, remove any immediate danger and ensure that the wall can carry the anticipated load.
In this category we include walls that stand no higher than 1.5 m above their surroundings and which do not represent an immediate danger. For example, a 1 meter high wall located on the roof of a building may be considered dangerous because its height above the ground exceeds 1.5 m. However, wall remains less than 2.5 m are considered stabile if the wall’s height does not exceed five times its width when it is structurally complete.
Tall Walls or Walls that Constitute an Immediate Danger
Remains of these walls are tall (thin walls that are more than 1.5 m higher than their surroundings) and constitute an imminent danger and require temporary or permanent spatial support, during the course of conservation and renovation work and/or after it. Conservation measures in these instances will include preparation and stabilization of the wall in accordance with its characteristics, filling cracks and voids in the stone, repair one or more worn stones, pointing up joints and the installation of supports.
In tall walls the preparation measures will include the removal of loose materials from the top of the wall, removal of loose and weakened materials in the areas where plaster is missing on the inside of the wall and the removal of loose bonding material from the joints on the outside part of the wall. Conservation measures will include stabilizing and sealing the top of the wall, replacing missing bonding material in the joints and replacing missing plaster while at the same time creating a straightened surface.
In double-faced walls the preparation will include the removal of loose stones, loose core materials and crumbling bonding materials from the joints. The stabilizing measure for these walls will consist of replacing missing stones and back filling them with bonding material, completing missing stone courses to the extent required, pointing up joints and sealing the top of the wall.
In both instances conservation measures are required for weathered stones, filling voids and cracks in stone, and pointing the joints as part of the measures for stabilizing the wall.
|Supporting Tall Walls
Supporting tall walls is considered a short-term measure until a permanent solution is provided for stabilizing the wall. In any case the preferred objective is a spatial solution, in other words completing the building. As a rule, the design of the supports will include a specification of a wooden layer that separates the new materials from the stone.
Fig. 6. Support on the ground floor – from one side
(detail – separation layer)
Fig. 7. Support from an adjacent structure
Fig. 8. Support on an upper story
Sections of vaults and arches in different states of preservation are integrated in the city’s building remains. The remains of the vault are limited in size and the most common phenomenon is the survival of the vault’s springing connected to the upright walls in the stable part of the building. In most instances the vault remains constitute an immediate danger owing to their height and location above a passage. The immediate danger is one of stones falling into the space below them. The destructive mechanisms that lead to this condition are the collapse of the vault due to different reasons and/or intervention by man in order to open passages or remove a room for the sake of a new building.
The objective in this case is to stabilize the remains and remove any imminent danger of stones and other parts of the vault from falling. Sometimes it is sufficient to stabilize the core, but when the angle between the top of the vault’s upper stone and the vertical is less than 30°, the upper stone needs to be anchored to a stabilized core, or conversely it should be removed out of safety considerations.
Fig. 9. A section of vault remains prior to stabilization
Fig. 10. After the treatment to stabilize the core and anchoring the top stone
The structural problems in Old Akko were surveyed many times as part of the ongoing measures conducted by the Antiquities Authority in city. Analyzing and understanding them has led us to conclude that in many instances the root of the problem lies in inferior construction and the use of poor quality materials. The most significant factor in the state of the physical preservation is the long-term absence of proper maintenance and the lack of awareness. This fact has accelerated the action of the destructive mechanisms and the natural decay that is occurring in the city.
There are different ways to conserve the building elements in the city. Choosing a treatment is a stage in a methodical and structured process that includes identifying the problem, understanding the historic and active factors at the site, formulating a theoretical concept for treatment based on a broad perspective of the aspects bound up in the conservation of a historic city, and planning and implementing conservation measures using the resources available to the property owner and the conservator working on his behalf. On more than one occasion a conservator has found himself with his hands tied owing to budgetary constraints. The actions taken in the city’s residential buildings over the past decade were mostly determined because of demolition orders issued for the removal of a danger and not because of the resident’s pro-conservation attitude or an overall broad conservation concept adopted by the city. We are hopeful that things will change in wake of the city having been declared a world heritage site and the approval of a new local master plan.