Retrofitting Mechanisms of Valuable Heritage Buildings: Al-Kifl Shrine as a Case Study

The research concerns the concepts related to restoration and conservation, as well as its theories and methods. It focuses on preserving historical buildings, knowing the associated concepts, and determining the methods and degrees of intervention to reach an ideal conservation work that achieves the continuity of the architectural and structural value of the historic building.

Some of the historic buildings are exposed to ineffective restoration work in previous restorations, such as the lack of accurate documentation of the structural and decorative condition of buildings. This has made assessing the quality of these works more difficult and may contradict the recommendations of the Venice Charter, paragraph 16 [1]. Also, ineffective restoration sometimes included the removal or covering of many original inscriptions and decorations in a manner that contravenes international conservation principles [2].

There is a need to adopt modern technologies within sustainable retrofit mechanisms. These mechanisms include advanced technologies, documentation, cleaning, and reinforcement techniques, without compromising the building's historical appearance, while preserving the authenticity of the historic building [3, 4].

3.1 The concept of retrofitting

Retrofitting is the installation of new or modified parts or equipment in an existing object or its adaptation for a new purpose [5, 6]. In heritage building conservation, retrofitting is a conscious intervention that balances preserving original values with meeting modern needs using modern technologies [7]. It aims to improve the structural performance of the building using modern technologies. It may include updating the function of the building to adapt it to a modern function without conflicting with its historical and architectural values. The retrofitting process relies on the integration of modern technologies in various stages: surveying and documentation (photography, 3D scanning), inspection and diagnosis), reinforcement and consolidation works, cleaning and treatment (physical, chemical, biological), and insulation works [8]. Retrofitting contributes to preserving the authenticity of the building using compatible materials and techniques [7].

3.2 Retrofitting and conservation

Retrofitting represents an advanced level of conservation, going beyond maintenance and restoration to include targeted intervention to improve the condition and function of the building, and requires a comprehensive strategy that takes into account the building’s location, value, condition, and proposed function, to preserve and enhance the building’s value [9].

Conservation interventions are limited and reversible, meaning that any future restoration or repair can be removed or replaced. International conventions refer to the concept of reversibility. The Venice Charter of 1964 states in paragraph (12): "All additions intended to replace missing parts must be recognizable from the original and must be designed and implemented in such a way that they can be removed without damaging the original structure of the building" [10].

The Burra Charter of 1979 also emphasized the principle of reversibility in paragraph (15.2): "New changes or additions must be capable of being removed or retracted without affecting the original materials or causing damage to the original elements" [11].

The ICOMOS Charter guidelines stated: "Interventions in historic structures must be as reversible as possible and must be carried out in a manner that does not damage the original historic fabric" [12].

The aim is to maintain the continuity of the building over time through appropriate use and modifications [13].

Several levels of conservation are determined according to the nature of the level of intervention required to preserve the historic building, noting that any conservation work can include a wide range of these levels and concepts and according to the problems that the building is exposed to. Each conservation method has its own limits and proportions of intervention, and interventions can be carried out using modern or traditional materials and techniques. They can be:

Retrofitting, regeneration, maintenance, restoration, reshaping, rehabilitation, transfer of the monument, reconstruction, reproduction, adaptation and reuse [14]. Table 1 shows the levels of conservation.

Table 1. The levels of conservation

The study focused on retrofitting mechanisms through modern technologies, starting with virtual and non-virtual scanning and documentation techniques and moving on to reinforcement and cleaning techniques, as follows:

5.1 Technologies for virtual scanning and documentation processes

The use of the digital revolution can contribute to documenting historical buildings better than traditional methods in the architectural documentation processes of historical buildings, whether separate buildings or in groups, and studying their potential in the process of exchanging experiences between the various parties interested in the conservation process, which enables them to make sound decisions in this field [18].

For example, laser scanning can be used to obtain the shape and texture of existing landmarks, while three-dimensional modelling can be used to restore damaged or incomplete cultural heritage assets. The process includes 3D digitization, digital data processing and storage, archiving and management, representation, and reproduction [19]. Each device has its own use depending on the available space, required accuracy, and output method.

The benefits of these techniques include the accurate identification of the locations and causes of damage, which helps in choosing the most appropriate methods of treatment and intervention and reducing unnecessary interventions by:

• Providing accurate information about the condition of the building.
• Avoiding interventions that may harm it.
• Providing basic information for restoration and reinforcement operations, which helps in identifying the appropriate materials and techniques for restoration and reinforcement.
• Using inspection techniques periodically to monitor the condition of the building after restoration and detect any new signs of damage [20].

They can be classified into:

• Visual Documentation, Measurement and Exploration.
• Invisible Documentation, Measurement and Exploration are as follows.

5.1.1 Visual documentation, measurement and exploration

With the development of science and technology, many exploratory and documentation programs and devices have emerged that document the visual parts of these devices such as [20-23]:

• Photo Metric. Camera.
• Photogrammetry technique.
• Total Station.
• Close-range photogrammetry.
• 3D Laser Scanning.
• Building Information Modeling (BIM).
• Virtual Reality (VR).
• Geographic Information Systems (GIS).
• Use of Drones.
• Augmented Reality (AR).
• Smartphone applications for heritage documentation.
• Portable LIDAR technology.

Figure 2 demonstrates Laser Scanning and photogrammetry as most techniques used in visual documentation of heritage buildings.

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Laser Scanning [24]

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Photogrammetry [25]

Figure 2. Most techniques used in visual documentation of heritage buildings

5.1.2 Invisible documentation, measurement and exploration

Many exploratory and documentary programs and devices have emerged that document and explore the invisible parts of the historical building’s parts and elements such as [26-29]:

• Fibre optics microscopy
• Colorimetry
• Ultrasonic
• Endoscopy
•  Micro-Drilling
• Infrared Thermography
• Ground-Penetrating
• Micro Gravimetric
• Acoustic Emissions (AE) Technology

Figure 3 shows two techniques mostly used in invisible documentation of heritage buildings.

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Inside don church (Valencia, Spain) [29]

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Detecting water leakage by IR [30]

Figure 3. Two techniques mostly used in invisible documentation of heritage buildings

5.2 Cleaning techniques in retrofitting

Historic building cleaning is an essential step in the retrofitting process, following the survey and documentation processes, and aims to remove dirt and harmful materials accumulated on the building surfaces while preserving the integrity of the original materials. Choosing the appropriate cleaning method requires an understanding of the nature of the dirt and materials that make up the building [31]. Cleaning techniques used in historic buildings are classified into three main categories:

(1) Physical techniques: They depend on mechanical force to remove dirt and include:

• Water cleaning: Pressurized water (water jet) or spray is used, and the water can be hot or cold. This method is effective in cleaning limestone and marble, low cost, and easy to control. However, it may not be suitable for silicon stones or deteriorated buildings [32].
• Steam cleaning: Steam is used to remove dirt, sulfate scale, and some types of paint, and is useful for stone surfaces [33]. Figure 4 shows a portable steam cleaner used to clean decorative marble pieces.

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Figure 4. A portable steam cleaner used to clean decorative marble pieces [29]

• Mechanical cleaning: involves scraping and chiseling using hand tools or pneumatic machines. This method is used to remove fragile contamination and paint, but may not be suitable for polished surfaces [33].

• Micro-blasting: Uses a fine abrasive material (such as marble powder) with water and compressed air to remove dirt [34].

(2) Chemical techniques: These rely on chemical reactions to remove dirt, and include the use of acids, alkalis, solvents, detergents, and biocides. These techniques require extreme caution in use, and neutralization or rinsing with water after application. Figure 5 introduces cleaning technique using ammonium carbonate in a fibre and paper poultice. Examples of materials used include acidic detergents (such as hydrofluoric acid), alkalis (such as caustic soda), alkaline gels, and enzymes [35, 36].

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Figure 5. Cleaning of cement plaster façade details using ammonium carbonate in a fibre and paper poultice [35]

(1) Special techniques including poultices technique, ultrasonic technology, laser cleaning technology, and biological cleaning technology [37, 38]. Figure 6 demonstrates laser cleaning of Greek marble.

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Figure 6. Greek marble head after laser cleaning without damage [36]

From the above, there are important considerations for choosing the appropriate technique:

• The cleaning method must be chosen based on the nature of the dirt and the type of materials that make up the building.
• The cleaning methods must be safe for the original materials of the building, and not cause any damage to them.
• It is preferable to start with the least harmful methods and move to more powerful methods when necessary.
• Tests must be conducted on small areas before applying any cleaning method on a large scale.

5.3 Modern techniques for treating the infrastructure

Foundation Reinforcement: Micropiles are used to fix foundations without leaving any trace of their use. Micropiles are driven into the soil to support the foundation to prevent soil degradation and provide a new, permanent foundation for the building and maintain its integrity without disturbing the surrounding soil. Problems with this technique include narrow spaces, service conflicts near exterior walls, exposure of some archaeological features during excavation, and changes in the structural behavior of the monument when exposed to earthquakes [39].

5.4 Modern techniques for structural treatment

They are used for structural reinforcement such as wall reinforcement, arches, bars, minarets, cracks and internal fillings. They include:

5.4.1 Reinforcing the structural frame using an anchors system

It works to improve the structural performance of the building by resisting horizontal loads and resisting cracks that have been treated. It is also used to reconnect walls separated from each other and the ceiling [40].

5.4.2 Pressure wall injections strengthen walls

They add a liquid binding material to the building to fill gaps in the thickness of the walls, especially in the walls (Double Skin Construction with Core), where moisture dissolves the internal mortar of the wall, which leads to cracks and separation between the two walls. They are characterized by their speed of implementation, and their disadvantages include the possibility of swelling in the wall when injected in a large, uncalculated quantity [41].

5.4.3 Structural elements reinforcement techniques using carbon fibers (FRP)

It strengthens structural elements that need to increase their load-bearing capacity without compromising the original material. It is implemented using a group of solid fibers assembled from carbon or glass, which are fixed to the surface to be strengthened by coating them with epoxy materials. It is characterized by ease and speed of implementation, and the fibers are made of a material that does not rust, corrode, or rot and is invisible to the eye [42]. Its disadvantages include its vulnerability to heat and combustion and its sensitivity to ultraviolet rays. This technique cannot be used if there are engravings or decorations on the element to be strengthened [43, 44].

Therefore, using glass fiber-reinforced plastic in heritage buildings must be done according to precise standards that balance achieving structural safety and preserving architectural and historical values. This is confirmed by international conservation charters, such as the Charter of the International Council on Monuments and Sites (ICOMOS, 2003) [12].

5.5 Modern techniques in horizontal insulation works

The rise in the groundwater level in historical areas due to leaks from nearby sewage networks has led to an increase in the percentage of salts and the deterioration of the walls. Therefore, it is necessary to isolate the roofs to stop the sources of salts. There are two methods:

• Chemical insulation: This is done by injecting the walls with silicone materials that prevent water absorption.
• As for facade insulation, the insulating material is sprayed on the facade stones to be insulated after strengthening them.
• Physical insulation: This is summarized in cutting the entire wall at the level required to be insulated using a hydraulic saw and inserting PVC strips specifically for insulation in place of the cuts [40].