Renovation of architectural monuments
The term redevelopment is no longer used today due to the changed and unclear meaning of the word in connection with monument protection and monument preservation. Nevertheless, irreversible interventions in the historical substance, for example the foundation or the masonry, may be necessary to save architectural monuments from decay. These interventions exceed the principles of monument protection and preservation according to the Charter of Athens, namely the preservation or repair of any historical substance, as well as the reversibility of the measures taken to preserve it. These remedial measures are therefore considered special cases. In practice, these interventions can only be approved after a detailed assessment of the substance by the monument authorities. The special methods of rehabilitation of the masonry and the foundation of architectural monuments are described here. Note the requirements of monument protection that the monument offices create the country. The structural engineering measures can be limited to the pure preservation of the current condition. This often already includes extensive construction measures such as foundation reinforcement, installation of anchors, etc. But you can also aim to restore the original condition as much as possible. Two typical examples of these different objectives are the preservation of the ruins of the Kaiser Wilhelm Memorial Church in Berlin and the reconstruction of the Frauenkirche in Dresden.
Preservation of architectural monuments
According to the definition of the German monument protection regulations, architectural monuments are selected structures that are exemplary for the respective epoch in terms of their execution, their architectural design and the use of materials. In order to protect and preserve these architectural monuments, the governments of most countries have issued ordinances on the protection of monuments. All construction work on the protected objects, from renovation through renovation to demolition, require special approval from the responsible monument protection authority .
The financial expenditures necessary for the preservation of architectural monuments are often only to be represented if the building is put to a meaningful use ( change of use ). In many cases, this use will not be the same for the purpose of which a structure was originally built. This means that the structure must meet today's comfort requirements and with regard to its stability and fire safety, today's building regulations. In many cases, more extensive measures are required, such as adapting the supporting structure , breakthroughs for escape routes or installing sanitary facilities.
Since the 2009 amendment to the Energy Saving Ordinance, more and more monuments have been renovated with the aim of reducing CO 2 emissions and heating costs. Since a large part of the heat is lost through the building envelope, the energetic upgrading of the existing building is often reduced to the insulation of the outer walls. The usual facade insulation for energy-efficient renovation cannot be used on listed facades. The use of thermal insulation composite systems on house facades in particular has negative consequences for both the appearance and the substance of monuments. Instead of standardized renovation procedures, a differentiated set of instruments should be used for energetic renovation of historical buildings: installation of efficient heating systems, insulation of roof and basement ceilings, installation of solar systems in hard-to-see places, consideration of existing energy storage options, zoning of the building according to solar and geothermal energy generation, etc. Um To enable monument owners to carry out an energetic renovation that is eligible for funding and that does not damage the house through external insulation, the KfW Bank expanded its support program for energy renovation on April 1, 2012 to include the monument efficiency house class.
Causes of damage
As historical buildings are mostly made of natural stone masonry , the typical causes of damage are primarily chemical-physical processes that attack and destroy the natural stone or the grout .
Moisture penetration
In most cases, water is the essential prerequisite for the chemical and physical processes that cause the damage. As long as it is possible to keep the masonry completely dry, all the processes of destruction that are summarized under the collective terms weathering and corrosion can hardly occur. The water that penetrates the masonry is usually rainwater that penetrates through destroyed joints or moisture that rises from the groundwater due to the capillarity of the stone material.
However, moisture damage can also occur due to humidity condensing on the inner surfaces of the masonry . In heated rooms with large public traffic, such as assembly rooms, churches, etc., there are also different relative humidity levels, especially in winter, with very different indoor and outdoor temperatures, so that water-saturated indoor air can condense on the colder wall surfaces and soak them.
Weathering and chemical erosion
Monuments made of natural stone are exposed to both natural erosion and erosion caused by air pollution from human activities. Both causes of erosion are mutually dependent and partly superimposed, so that it is difficult to name this or that cause as the sole or main responsible erosion mechanism.
weathering
The destruction of stone by media penetrating the pores of the rock from the atmosphere is not an invention of modern times. Even the old master builders had to deal with the complex physical-chemical processes that begin at the latest when the natural stone is extracted, and which we summarize under the term weathering . The increasing pollution of the atmosphere with gaseous aggressive chemical compounds - especially from the combustion of sulfur-containing petroleum products - has dramatically accelerated the natural weathering processes today. At Cologne Cathedral was z. B. determined that the Schlantdorfer sandstone, which was partly used there, weathered eight times as fast today as it did in 1880.
The rate of erosion is determined by the composition of the rock (especially the pore content and type of binding agent in sedimentary rock ), the regional climatic conditions including air pollution , and finally the location of the material in the structure (inside or outside). The most important climatic influences are rain, ice, wind and temperature differences. Weathering occurs mainly through mechanical erosion. The water that penetrates pores or cracks caused by temperature differences freezes at temperatures below zero degrees, the increase in volume resulting from ice formation loosens the structure or splits off parts, and the loose parts are then removed by wind and rain.
Another natural cause of stone destruction is so-called biological erosion caused by growth of algae, moss or lichens. The growth prevents soaked components from drying out and the washing out of harmful substances from the atmosphere by rainwater. Larger plants can destroy the rock where they have once taken root in crevices due to the pressure of the expanding roots.
Chemical erosion
Chemical erosion requires the presence of water and aggressive gases in the atmosphere. In our latitudes, water is abundant as rainwater all year round. The aggressive gases that destroy the rock by dissolving the binding agent cementing the grains of the rock are primarily sulfur dioxide and chlorides .
As a rule, the pollutants dissolved in the rainwater or in the pore water of the rock surface work by either dissolving the binding agents - mostly lime mortar - ( acids ), or by forming new crystals with components of the binding agent, greatly increasing the space, and thus the rock from the inside out to burst ( Ettringit driving ). The mechanism of the formation of sulfuric acid from the atmosphere is typical for this type of stone destruction : When sulfur-containing coal or heating oil is burned, sulfur dioxide (SO 2 ) is released into the air with the exhaust gases . Oxidation with atmospheric oxygen turns it into SO 3 , and when this gas is dissolved in rainwater, H 2 O and SO 3 become sulfuric acid (H 2 SO 4 ).
In the strong dilution in which the sulfuric acid penetrates the pores of the rock, it can hardly attack the binding agents. However, since the water evaporates when the material dries out, after many passes of moisture penetration and drying, a sulfuric acid concentration results that can very well dissolve and destroy the structure of the rock from within.
Cracks
Cracks in supporting structures are usually both the damage and the cause. The cause of all cracks are deformation hindrances or the deformation ability of the building material is exceeded. Since the masonry usually also has to fulfill a sealing function at the same time, cracks are wide open entrance gates for water and aggressive media. There are a variety of possible causes of cracks. The most important features that indicate the cause of a crack are:
- Crack movement. Plaster marks or crack monitors that are placed across the crack show whether the crack has already come to rest (dead crack) or is still working (living crack).
- Depth of the crack. The determination of whether a crack only runs on the surface, or to a certain depth, or across the entire component, allows conclusions to be drawn about the type of crack cause. ( Shrinkage , settlement ).
- The course of the crack and, above all, the displacement of the crack edges against each other, provide information on the direction of the acting force.
The most common causes of cracks in the masonry of historical buildings are primarily shear ( e.g. arching ) and tensile forces, changes in the load-bearing capacity of the subsoil (e.g. due to lowering of the groundwater ), or dynamic loads from heavy goods traffic flowing past the building. Cracks can also be caused by short-term disaster effects such as fire or earthquake .
Ettringen drive
Damage from ettringite drifting often occurs where masonry containing gypsum has been repaired with normal cements in the past, or where such cements were injected to strengthen the masonry. In historical buildings, plaster of paris or lime with sulphate components was often used as a binding agent. If such mortars come into contact with normal cement, the reaction of the tricalcium aluminate (C 3 A) contained in these cements can lead to the formation of ettringite and drift phenomena, which burst the mortar or the rock from the inside out. In the case of renovations carried out in previous years, this problem was not sufficiently known and therefore not taken into account.
Rehabilitation process
In order to preserve the old building fabric, it would of course be ideal to carry out renovation work largely with the same building materials that were used when the building was erected at the time. However, this will very often not be possible for building law reasons, since - especially in buildings with public traffic - today's requirements for stability and fire protection must be met. So you will often have to resort to modern materials such as concrete , prestressing steel and plastics. Of course, these materials are to be installed in such a way that the character of the structure is largely retained.
Shotcrete
Shotcrete is primarily used to install reinforcements, supporting structures such as arches and abutments for anchors. The high impact pressure creates a non-positive connection with old masonry parts.
Cement injections
The injection of a water-cement mixture serves to consolidate and seal masonry. Furthermore, in the case of anchors, to fill the borehole and to produce a force-fit connection between the built-in steel anchor and the masonry, and to protect the anchor from rust. For better flow behavior, the water-cement mixtures are often modified by adding plastic emulsions (mixture of plastic and water). For the reasons already mentioned, only C3A-free cements should be used.
Steel anchor
Steel anchors are used to clamp together cracked structural parts and to anchor parts at risk of cracking to stable parts of the structure or newly installed support structures. The anchors can be installed slack and then act through their jacket adhesion to the grout of the borehole. Or they can also be prestressed and bring the prestressing force into the masonry to be consolidated via the anchor plates.
Plastics
Plastics should only be used to a limited extent for the renovation of historical buildings due to their deformation behavior, which differs greatly from the old materials. They are mainly used for surface preservation, as thin liquid solutions for sealing capillary pores, into which cement particles can no longer penetrate because of their size, and as additives for sealing plasters or paints. Plastic-bound mortars are also used to replace broken parts on sculptures or frescoes , whereby the choice of different colored additives, color and structure of the replacement can largely be adapted to the existing material.
Horizontal barriers
To prevent capillary rising moisture from the subsoil, a few layers above the foundation base and mostly horizontal sealing layers are to be installed below the first ceiling. In the old masonry of historical buildings, this horizontal barrier is often missing , or a tarred cardboard that was put in at the time has rotted away. The water that penetrates the foundation base or the basement walls in contact with the ground is transported upwards through the capillaries of the masonry, escapes in the rooms used on the walls and destroys paintwork and plaster. To prevent this damage, there is little point in sealing off the damp areas from the inside with a special sealing plaster. The water would rise further and emerge above the sealing plaster. Another ineffective method is masonry dehumidification by inserting evaporation tubes into the masonry. The more moisture evaporates, the more salts and substances that attack stone or mortar are transported and the destruction processes are accelerated. An effective horizontal barrier against capillary rising damp is possible by three methods.
Subsequent installation of a sealing film
With this safest but also the most complex method, the masonry is sawn or pried open in sections and then mortared again after a sealing film has been installed. Metal or reinforced plastic foils are used here, which are laid in at least two layers. By creating a smooth support surface (leveling with cement mortar) it must be ensured that the foil is not perforated by the pressure exerted by the masonry.
Installation of an injection barrier
It is carried out using individual bores up to about 2/3 of the masonry thickness offset in a grid and injecting a cement emulsion (mixture of cement and water) or hydrophobic chemicals. There may be a two-stage injection, in which cement is first pre-pressed and the larger holes and crevices are closed, and then further pressurized with chemicals to block the capillaries. Pressing is generally used. Under favorable conditions, it is sometimes sufficient to drill into the masonry and, if necessary, to fill the drilled holes at an angle downwards with a pore-clogging solution based on silica or water glass . Due to the force of gravity and the capillarity of the masonry, the liquid water glass penetrates into very fine cavities and thus blocks the passage of water after it has hardened.
Electro-osmosis dehumidification
In the case of dehumidification by electroosmosis , the physical effect is used that water in an electric field always moves to the negative pole (cathode). By installing electrodes, a negative voltage is generated near the base of the foundation, which pushes the water downwards and does not transport it upwards through the capillarity of the masonry. A distinction is made here between the passive method (both poles are short-circuited, which results in a certain current flow even without external voltage) and the active method, in which a constant current source with very low voltage is applied. While active electroosmosis achieves verifiable results and drains walls, this effect is very small with passive methods, the moisture penetration limit only moves a few centimeters down after commissioning. A practical application for dehumidifying or drying walls can therefore only be achieved with an active process.
Ground consolidation
In the course of construction work (such as underground railway construction) there are often changes in the load-bearing capacity of the building ground. In order to secure the foundations and to avoid subsidence, soil consolidation is then required. Depending on the type and composition of the subsoil, this is done using an injection seal .
With cement injection, a thin water-cement paste is pressed into absorbent soil layers. Colloidally prepared cement mixtures are also used . These are water-cement mixtures that are whipped into the cement in high-speed propeller mixers with the addition of foam-forming agents in such a way that a colloidal, very stable and flowable suspension is formed. The lowest limit of the injectability of cement suspensions is sand with a grain size of about 1.0 mm. For smaller grain sizes down to 0.04 mm, chemical soil consolidation is used by injecting two-component chemical mixtures. The mixtures harden when the two components come into contact in the soil and cement the grains of the subsoil together.
Foundation reinforcement with injection piles
With this method, boreholes with a diameter of around 20 cm are drilled under the existing foundation from the deepest accessible base of the structure. After the borehole has been drilled, a reinforcement cage or a single steel tendon (e.g. GEWI system ) is inserted and the pile is concreted under pressure. Usually the concrete is compacted with compressed air after it has been poured into the borehole. The concrete is thereby pressed against the walls of the borehole. After hardening, the result is a pile with a coarse surface, which can transfer the structural loads through skin friction.
literature
- Günther Ruffert: Restoration of architectural monuments , Beton-Verlag, Düsseldorf 1981, ISBN 3764001445 .
- Andrei Walther : Building diagnosis on modern monuments: User-oriented building renovation , Bauhaus University Weimar , no year, ISBN 978-3-86068-421-4 .
- Kurt Schönburg: Natural substances on buildings, properties, application ,: Editor: German Institute for Standardization eV -DIN-, Beuth Verlag, 2010, 280 S. ISBN 978-3-410-17355-7
Web links
- H. Künzel: Causes of damage to old buildings: rising damp, hygroscopic damp or condensation? In: IPB announcement 25th Fraunhofer Society for Building Physics, 1998, accessed on February 20, 2015 .
Individual evidence
- ^ Bavarian State Office for the Preservation of Monuments: Construction work on monuments. In: Monument preservation information . Munich 2008, p. 7f
- ↑ On the energetic renovation of monuments: Monument Debates ( Memento of the original from June 15, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ ÖNORM B 3355-2 PDF
- ↑ Investigation report Scherpke / Schneider TU Wien ( Memento of the original of February 2, 2006 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
