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Material properties of historical masonry of the Frauenkirche and the masonry guideline for reconstruction
Construction and Building Materials 17 (2003) 651–667
Material properties of historical masonry of the Frauenkirche and the masonry guideline for reconstruction ¨ 1,* Frank Pohle, Wolfram Jager Faculty of Architecture, Chair for Planning of Load Bearing Structures, University of Technology Dresden, D-01062 Dresden, Germany Received 16 January 2001; received in revised form 18 August 2003; accepted 19 August 2003
Abstract This contribution provides insight into the characteristics of the masonry of the historical Frauenkirche and the requirements for its reconstruction. Comprehensive material studies were performed in the course of preparation for and implementation of the project, and guidelines for evaluation, design and construction were established on the basis of those studies. The material used for the reconstruction is sandstone, coming from quarries in the immediate vicinity, as well as mortar composed on a lime basis with hydraulic additives. The execution is monitored within the scope of a quality management system specially created for the reconstruction project. This system simultaneously provides a detailed documentation of the building materials used in the project. 䊚 2003 Elsevier Ltd. All rights reserved. Keywords: Masonry; Sandstone; Mortar; Compressive; Strength; Fire tests
1. Introduction The reconstruction of the Frauenkirche in Dresden, aside from the archeological removal of the rubble, has been preceded by a comprehensive analysis of the historical structure w1–3x. The focal point of this analysis was the issue of weaknesses of the building prior to its destruction (Fig. 1). If a historically accurate reconstruction was to be attempted, it was necessary to bring the level of safety up to today’s accepted standards and to guarantee the performance capability. One path followed in this respect represented the gentle improvement of material properties and the use of defined material quality grades. At the same time, the engineers responsible for the reconstruction project wanted to have clarity about the failure mechanisms of masonry made from Elbe sandstone and to create a design process on the basis of the latest engineering knowledge. For this purpose, they *Corresponding author. E-mail address: [email protected]¨ dresden.de (W. Jager). 1 ¨ Ingenieurgemeinschaft Frauenkirche Dresden Prof. Jager & Prof. Wenzel, RadebeulyKarlsruhe, Wichernstr. 12, D-01445 Radebeul, Germany.
selected the verification procedure according to the ultimate limit state method, which permits a differentiation of safety factors for the effects and the resistance of load bearing structures. When reviewing the German building code DIN 1053 for masonry structures in force at the time the rebuilding of the church was prepared, the reconstruction in accordance with this code was not feasible in view of the forecast level of stresses and loads. The permissible stresses were in part lower than the maximum working stresses, which were determined through calculations of the undamaged structure as being between 5 and 7.5 Ny mm2. The use of masonry after testing it for qualification became possible only after the publication of the revised DIN 1053-1 and -2 (refer to w5,6x) and was not available in the early 1990s. Therefore, it was necessary to perform corresponding studies of load bearing capacities of sandstone masonry, which at the same time could serve as a scientific basis for the theory of failure and its description of masonry in general. Of further interest was the issue of load bearing capacity of the existing masonry of the ruin and the cellar, which were intended to be integrated in the reconstruction.
0950-0618/03/$ - see front matter 䊚 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0950-0618Ž03.00062-X
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2. Stone material
Fig. 1. Photo of a destroyed pillar capital from the restoration.
Experimental and analytical studies were carried out on stones, mortar, masonry and the metal components present in the original structure. It was necessary to test the stone material of the rubble, the parts of the ruin that were left standing, the cellar as well as new stone materials, in order to gain adequate knowledge of their characteristic strength values. The analyses of metals, particularly the wrought iron used in the construction of the church, had a predominantly architectural and historical background w7x. Aside from strength issues, it was also necessary to clarify building material related problems having a bearing on the varieties of sandstone and the mortar to be used. In order to be able to realize the planning specification in practice, technological considerations had to be made and the verification of the quality of the reconstruction had be taken into account as well.
Fig. 2. Stone specimen in compression test with the typical crack pattern.
The result of testing stone material samples with regard to their compressive strength was that the average values of the foundation masonry are lower than those of the ruined parts higher up in the structure. It should be noted, however, that the samples from the cellar could only be taken by drilling parallel to the sedimentation plane. As a rule for Saxon sandstone, the compressive strength perpendicular to the sedimentation plane is approximately 10–15% higher compared to the one parallel to the sedimentation plane. In comparing the various strength test results, account had to be taken of the specimen dimensions (Fig. 2). Random sample taken from the heap of rubble (Fig. 3) yielded a mean value approximately corresponding to the overall values of historical sandstone from quarries in the Elbe valley. The stone material recovered from the rubble that was deemed reusable will be used in the reconstruction, in accordance with the aim of archaeological accuracy, after undergoing a reconditioning process. The data collected from the rubble were consolidated after the reconditioning of the lining stones and thus confirmed in their statistical value. In the evaluation of test samples of new stone material of the Posta variety, which were required within the scope of masonry tests (refer to w8,9x), a distinction was made between normal Posta sandstone and the particularly strong sandstone from the so-called ‘white bed’. The last is a very homogenous and solid layer of a nearly white color, which is found in the quarries (Fig. 4). In order to assure the desired progress in construction, another variety has been used in the meantime in accordance with the specifications of the engineers. This variety is the so-called Reihnhardtsdorf sandstone, which has a higher content of clay.
Fig. 3. View on the remains of the Frauenkirche in 1989 prior to the removal.
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Fig. 4. Sandstone quarry in Wehlen nearby Dresden.
3. Mortar Analyses of the mortar used in the original construction of the church were also performed at the time of the removal of the rubble (Fig. 5). It was particularly interesting to find out whether any distinction was made in relation to different parts of the structure and what the composition of the mortar had been (Fig. 6). With a view to differentiation, the only differences found were those between normal masonry and that of the
Fig. 5. Look at the remains (north-west staircase tower) during the archeological removal of the rubble in 1993.
sandstone pillars. While, as a rule, normal masonry is well carbonated, evidence of poor carbonation was found in the contiguous remains of the pillars. The sands used in the lining masonry exhibited good grain gradation and grain sizes of approximately 6 mm, partially with sharp-edged surfaces. In some places the addition of brick dust could be proven. The proportion of binding agent was, in relation to weight, approximately 1:3.45. Lime was found to be used as a cementing agent. For the purpose of reconstruction, clearness was required with regard to the processing technologies to be used in the construction of the new masonry (Fig. 7). According to the experience and practice of stone masons over many decades, ashlars have been placed on small pre-balanced lead pads, lining bricks were laid with a mortar suitable to be applied with a trowel, the joints in the facade were stuffed with hemp and the coursing joints were filled with grouting compound. The hemp is removed after the grouting compound has set and the joints are filled with a special pointing mortar. The stones of the masonry of the pillars (Fig. 8) with thin joints subjected to high stresses and loads are placed with a thin layer mortar suitable for trowel
Fig. 6. Historic masonry of the remains (ashlar stone masonry).
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erate and stabilize the setting process. The cement content has been reduced to the absolute minimum. Alkali-poor materials are used as additives in order to control the potential for blooming. Furthermore, the mortar for the units was supplemented by a mortar for masonry with thin joints as well as by the pointing and grouting compound. 4. Masonry Tests on masonry test samples (Fig. 9) were also necessary for the assessment of the load bearing capacity of ashlar stone work of Saxon sandstone. These tests concentrated on the following focal points:
Fig. 7. Re-erected masonry of the exterior walls (in a height of the first floor).
– compressive load bearing capability of sandstone work with various joint widths; – shearing load bearing capacity of sandstone masonry at various compressive loads (Fig. 10); – creep characteristics of dimensioned stonework with various joint widths; – behavior in fire of sandstone masonry. 4.1. Compression tests of sandstone masonry
application, whose main characteristics are a high degree of water retention capacity and a long time period of correctability. The above mentioned lead plates may cause peak stresses in the masonry, which is why they were omitted in the masonry of the pillars with higher strength requirements. Very high demands are made on the prefabrication of the stones, since a 6-mm joint does not allow the correction of any tolerances. Measures for the compensation of tolerances were not necessary in the execution of the pillars, since the surveying work done during the ongoing construction permitted a very high level of accuracy. On the basis of the analyses performed, as well as the experience of skilled craftsmen, proposals were made for the mortar to be used in the reconstruction of the Frauenkirche. These proposals were reviewed and evaluated by experts of the department of building materials ¨ Massivbau und Baustofftechnologie of of the Institut fur the University of Karlsruhe and finally modified in adjustment to the technology used by the manufacturer, TUBAG of Kruft. During the initial phase, the recipe had been based on the use of the mortar as a site mortar. However, the necessary rationalization of technical processes by the construction company required the use of a dry mortar that is supplied in silo trucks. An adaptation to these requirements followed. Special attention was paid to the compatibility of the mortar and the sandstone material. The sandstone itself contains small amounts of soluble alkali, as does the mortar, in order to bring about a hydraulic reaction. A small cement content is used, which is intended, as a hydraulic component, to accel-
The series of tests, already under way in Dresden in 1992, was expanded by tests of the load bearing capacity of sandstone masonry with narrow joints, which were performed on behalf of the Stiftung Frauenkirche Dresden e.V. w8x. Later, test samples with simulated original masonry were added to the series of tests as well. The tests included four-stone pillars as well as RILEM test samples. Parallel to the test samples, the strength
Fig. 8. Stone masonry with thin layer mortar of an interior pillar.
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fb t mm dB dyd9 Ez F|xq0.7 q0.3 C1y x hD d G~ y h9 1ymm
fs w
10 cm T t d9stq B h9sminU p FE T tanC q F Vh D4 2G xsfbkyfstk S
Fig. 9. RILEM test specimen for the compression test of sandstone masonry.
characteristics of the stone material and the mortar were tested as well. The test series have shown that fractures occur due to transverse tension failure in sandstone masonry as well. It was determined that the strength of the stone material, the thickness of the joints and the type of mortar have a decisive influence on the strength of the masonry. The minimum joint thickness achievable with normal mortar was approximately 5 mm; however, at this thickness difficulties began to arise in the application of the mortar. However, a significant increase of the masonry strength was found in masonry with joints of 5 mm or less. At this joint thickness, a masonry resistance to fracture of 80–85% of the stone strength was achieved. This was a value that prompted the engineers responsible for the reconstruction of the Frauenkirche in Dresden to voice their recommendation that it should be possible to reconstruct the church with sandstone masonry. The end result of the test series was the creation of a formula for the determination of the compressive strength of masonry and a proposal for the assessment of sandstone masonry on the basis of the limit state method with partial safety factors w8x. The advantage of this method lies in the fact that individual influences can be grasped and considered more realistically and concretely. Consequently, after intensive discussions between the joint engineering task force and the check proof engineer,2 the proposal for assessment was adopted, and in the process the partial safety factors (actions, materials) for the actual application at hand were determined. Thus, a verification method according to the current state-of-the-art forms the basis for the reconstruction of the Frauenkirche in Dresden. 2 Check proof engineer for the reconstruction of the Frauenkirche ¨ Peter from Stuttgart. in Dresden is Prof. Dr.-Ing. Jorg
t h h9 d f fb f bk f stk mm F
Joint thickness Height of the stone Effective height of the stone Width of the stone Mean compressive strength of masonry Mean compressive strength of stones Characteristic compressive strength of stone Characteristic split tensile strength of stone Poission’s ratio (M5 s0.4; M10 s0.3) Inner angle of friction
4.2. Shear-compression tests Since shear-compression loads occur in the masonry at a non-orthogonal direction of forces in relation to the coursing joint due to the anisotropy, this type of influence had to be investigated as well. It is exactly this type of load that is quite significant in the Frauenkirche, due to the occurrence of ring tensile forces, descending forces, shearing forces of the flat inner cupola and forces due to the additional horizontal load application. The tests were conceived in co-operation with Berndt w10x, performed in the Otto-Mohr-Laboratory of the Faculty for Civil Engineering at the Dresden University of Technology. A special test rig was developed, approximately simulating the shear-compression loads to be
Fig. 10. Test specimen of a shear-compression test with the test data recording equipment.
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Fig. 12. Curve of ultimate load bearing capacity due to shear-compression loads.
Fig. 11. Shear-compression test in the Otto-Mohr-Laboratory of the Dresden University of Technology.
expected in the actual structure as realistically as possible. Various shear-compression ratios were investigated in the tests (Fig. 11). The final test results yielded an assessment proposal, which in turn was developed on the basis of the method of partial safety factors and led to the shear-compression failure curves. Fig. 12 shows an example of a curve for the determination of the shear strength in dependence of the design compressive stress. 4.3. Requirement categories In accordance with the load flux and loads determined for the entire structure according to the planned load bearing concept w3x, requirement categories were defined by the joint engineering task force and underscored with corresponding assessment values (Fig. 13) w12x. It was also necessary to assess the existing masonry. This was based on a statistical evaluation of joint thicknesses, which then, combined with the above-mentioned taking of stone specimens, permitted the establishment of assessment values. With a masonry quality matched to the actual load flow, the reconstruction of the Frauenkirche in Dresden is possible and feasible in sandstone masonry (0). The decisive factor was, and continues to be, that the calculated values are actually achieved in the execution of the reconstruction.
which has been implemented in ANSYS (refer to w11x). The cohesion c and the angle of internal friction g of the mortar are required as input parameters. It was possible to recreate the experimental results quite well with the analyses. Even for thin joints, it was possible to show evidence of the typical stresses patterns in the area of the contact surface between stone and mortar. However, narrow joints with hard mortar no longer exhibit the typical joint cracks near the edges. Further parameter studies could be performed with the calibration of the mechanical model in accordance with the available test results, making it possible to forego elaborate subsequent tests. At the same time, experience with the material masonry and its illustration in FEM analyses were gained, which came in very handy for the component studies of the cellar and the cupola. 4.5. On-site tests For the validation of the mechanical modeling of the remaining masonry it was necessary to forego on-site
4.4. Evaluation of results obtained with FEM analyses The theoretical assessment was supported by parallel FEM analyses by Bergander et al. w11x. For the purpose of these analyses, the two-component masonry as such has been modeled and calculated close to reality. The stone behaves in linear fashion, while the mortar exhibits plasticity. The obvious choice for modeling the plastic behavior of the mortar joint was the isotropic, idealplastic material model according to Drucker-Prager,
Fig. 13. Application of the various requirement categories in the structure.
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Fig. 14. Sample of test cores of the historic masonry nearby the choir.
tests. Beside the check of the assumed compressive strength of sandstone units were carried out tests on cut out mortar samples of size 20=20=12 mm3, which indicated mortar strengths of approximately 1.5 up to 5 Nymm2. In order to be able to control the load bearing capacity of the highly stressed apse pillars, the masonry strength in this area was determined according to the tensile splitting test method of cylinders drilled over a joint (refer to w13x). To this end, joints were drilled perpendicularly with a core drilling machine, the core (sandstone–mortar–sandstone) was extracted and subsequently tested for split tensile strength (Fig. 14). By extracting cores next to the location of the joint bores it was possible to determine the strength of the stone and to assess the strength of the masonry. Load bearing design resistance of 4.3 Nymm2 were found w3x. Finally, the general condition of the two highly stressed apse pillars was tested for hollow spaces by means of geo radar. The masonry mass of the cellar was inspected visually and by means of endoscopy.
strength masonry. It was intended that the stress level corresponded with the actual service conditions of use. In order to be able to separate the creep deformations of the stone from those of the mortar, the compression set of the tri-stone test sample were determined with the aid of inductive distance sensors and those of the center stone with a settling tensiometer. The creep deformations of the entire test sample were determined as a mean value of the four measured individual values. The deformations of the center stone were measured with four measuring points along two segments (refer to Fig. 16). The test samples were subjected to load for approximately 5 months in a standard reference atmosphere. In accordance with commonly used procedures, the final amount of creep was measured during a certain time segment towards the end of the test period and the final creep value of the test sample was determined via the relation to the elastic compression set at the beginning of the test period:
wk`s´k`y´el
(1)
With the aid of the final creep value, the modulus of elasticity can then be determined, taking creep into consideration, as follows: EwsEelyŽ1qwk, `..
(2)
Fig. 16 shows the course of the creep curves of both test samples of the highly stressed masonry. The tests have shown that the most significant creep influence comes from the joint and depends on its thickness as well as on the type of mortar that was used. Creep is determined exclusively by the mortar. The
4.6. Creep tests In the course of reconstruction work on the Frauenkirche in Dresden, creep tests on tri-stone test samples with mortars of various strengths were performed in the test laboratory of the Chair for the Planning of Load Bearing Structures at the Dresden University of Technology, under the supervision of Berndt w14x. The corresponding test program also comprised two tests on the issue of creep behavior of masonry with a particularly high load bearing capacity and thin joints. The tests were performed in creep frames of the same dimensions, making it necessary to select test samples with different sizes (Fig. 15). The compressive strength in the normal masonry test samples was 2–3 Nymm2 and 8 Nymm2 in the high-
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Fig. 15. Test laboratory with prepared creep frames.
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stone exhibited no compressive setting over the test period. This fact appears to be logical, since the sandstone was subjected to natural prestressing in its geological location of deposit. The share of creep in masonry made with mortar of Group I (low strength) w6x and normal joints is approximately double that of masonry made with mortar of Group III (high-strength) and narrow joints. These differences had to be observed accordingly in the calculations and verifications, since the structure is being erected with various different qualities of masonry. 4.7. Fire tests Experimental studies on the behavior of sandstone were required in order to verify constructional fire protection. To keep the expenditures within reasonable limits, it was finally decided to implement a two-stage program. As a first step, small sandstone cubes were subjected to thermal loads and then tested mechanically in co¨ operation with the Materialprufamt Sachsen as well as ¨ Geotechnik of the Dresden University of the Institut fur Technology. The temperature stages were selected in intervals of 100 8C. For each stage, five cubes with the dimensions 50=50=50 mm3 were heated in a muffle furnace and maintained at that temperature over a certain period of time. After cooling down, the remaining compressive strength was determined. In the case of sandstone from the ‘white bed’ a continuous drop of strength was determined at increasing temperatures. It begins to be particularly pronounced at temperatures above 300 8C. The test samples already fell short of the required 5% fractile value for stone strength of 44.4 Nymm2 at a temperature of 400 8C. A residual strength of approximately 65% was found at temperatures in excess of 1000 8C. The materials tested included sandstone from the ‘white bed’ for highly stressed masonry of requirement category 4 as well as the normal, Posta variety, which exhibited a slightly different behavior. Its most significant drop of strength at approximately 30% was found to occur between 500 and 600 8C. A significant connection between red
Fig. 17. Sandstone masonry wall in the cellar damaged by fire.
discoloration and clay content could not be proven (Fig. 17). In order to be able to evaluate the behavior of sandstone under load during a fire, the second step of the test program included testing of masonry slices under vertical loads in a fire test on the basis of the socalled standard temperature time curve (Figs. 18 and 19). The duration of fire resistance required for a 24cm-thick wall was proven. In this process, the assumptions with regard to the size of the chipped off shells were confirmed as well. 4.8. Investigation of durability Owing to the alternating freezing–thawing loads in the climate of Germany, the sandstone facade of the Frauenkirche is subject to heavy stresses. These stresses are concentrated in the joints and the adjacent areas of the stones. Many historical structures exhibit chipping of the stone in the area of the edges adjacent to the joints.
Fig. 16. Side views of test samples with measuring segments to investigate creep behavior and experimentally determined creep curves for highly stressed masonry.
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Fig. 18. Masonry specimen under vertical loading in the fire test.
In order to be able to employ the latest technology in this area as well, comprehensive experimental and theoretical investigations have been carried out with the aim of achieving a sufficient measure of durability. This includes special studies on the selection of stones for the cupola as well as on the mortar to be used in vertical sections of the facade and in areas subjected to particularly heavy stresses, such as the cupola. The studies are being performed at the University of Karlsruhe w16x and at the Dresden University w14,15,22x (Figs. 20 and 21). 5. Technological aspects After the clarification of the strength of existing masonry as well as masonry to be newly erected and after planning and specifying of the requirement categories, thought had to be given to their technological feasibility. For this purpose, a joint workgroup with
Fig. 19. Spelled stone material in the fire chamber after the test.
Fig. 20. Full scale test specimen of the outer wall of the stone cupola in the test laboratory of the Chair for the Planning of Load Bearing Structures of the Dresden University of Technology w22x.
members from the architectural and engineering offices was appointed to address this topic (Fig. 22). In this process, the following focal points needed to be clarified: – prefabrication of stones with different requirements on accuracy, – verification of accuracy, – mortar application, – brick laying technology, – use of technical aids. Special provisions had be to made to maintain a basic minimum temperature of above 5 8C, which is required
Fig. 21. Drawing of test sample as section of the cupola in full scale with installation of the climatic hood w22x.
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Fig. 23. Screeding the applied mortar by a straight edge.
Fig. 22. Trial of the stone laying technology of the pillar masonry with thin bed joints.
for the setting process of lime mortar with hydraulic additives for the construction work during the cold weather period. Initially, handling of water posed some problems. In order to achieve good adhesion of the mortar to the stone, the lining units had been immersed in water prior to being laid. That resulted in the mobilization of the free alkali and in heavy blooming. Thereupon the use of water was reduced in order to curb the migration of alkali. Particular attention was paid to the highly stressed masonry. This required the preparation of special aids and devices (templates, drawing trowel). The work steps for the placement of the pillar stones in their proper sequence may be used as an example here: – preparing the bed surface by moderate moistening and closing of the pores with mortar; – attaching and aligning the templates for drawing off the joint; – slinging and adjusting the stone on the suspension gear of the portal crane; – applying the mortar on the prepared bed surface; – screeding the applied mortar (Fig. 23); – placing the stone (Fig. 24); – verifying and readjusting the stone; – filling the cross joints with hemp; – filling the perpendicular joints with grouting mortar; – pointing the joints after removing the hemp.
Frauenkirche in Dresden w12x defines the established requirement categories with corresponding characteristic values and basic conditions for the assessment of the load bearing capability of sandstone masonry and its manufacture. The guideline forms the basis for the static verification of the existing structure as well as the newly erected masonry. Furthermore, the guideline contains all specifications and characteristic values required for the implementation of the quality control system as well as notes for their execution. All essential findings from the various test series have been incorporated. In order to avoid any inconsistency in the calculation of the strength of the respective masonry within the scope of verification, the strength and the modulus of elasticity have been specified for the established requirement categories. This guarantees uniform handling and interpretation of findings, formulae and definitions for the entire reconstruction project. The requirements for the basic materials (stone and mortar) as well as for the constructed masonry are formulated in detail in the full length version of the masonry guideline. In order to guarantee security and effectiveness in daily use, the most important specifications for planning and execution have been additionally published in form of individual, clearly designed
6. Masonry guideline for the reconstruction of the Frauenkirche The masonry guideline for the reconstruction of the
Fig. 24. Laying of stones of requirement category 4 for the pillars.
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and handy leaflets providing so-called ‘directives’ in abbreviated fashion. These loose leaf instructions list the parameters to be monitored including their limiting values to be adhered to, separately for stones, mortar and masonry on clearly arranged pages. The validity of the masonry guideline is limited to the project of the ‘Reconstruction of the Frauenkirche Dresden’. This concerns, on the one hand, the applicability as well as the material used in the project. It is certainly possible to draw conclusions from this guideline in order to apply them to other projects, but the applicability must be verified in each individual case by comparing the basic conditions of the projects. 7. Quality management and assurance system 7.1. Basic ideas One important cause of damages to the Frauenkirche in the past was the fact that stresses occurred in areas subject to high loads that could no longer be dissipated locally by the materials of the masonry (stone, mortar). Damaged stones in these areas had to be replaced in the years 1904–1905 and 1924–1932. These replacements were approached in a very thorough manner according to the technological standards at the time, particularly those in the 1920s. Only approved stone material, meeting the specified requirements with regard to strength, was used for the replacements. Therefore, the engineers responsible for the work considered the quality control of raw materials for the highly stressed masonry and their processing properties one of the essential prerequisites to be met in order to be able to meet the demands established in project planning for the structure. The quality control system is broken down, according to generally accepted principles, into a receiving inspection and a quality assurance test of the raw materials prior to being used for the construction of masonry. In this process, the receiving inspection represents the basic test, in which it is determined whether the material to be used for construction or offered by the contractor is principally suitable for its intended use. The quality assurance test is employed to verify the uniform adherence to characteristic values over the entire process from delivery to manufacture. A quality assurance system has been developed by the engineers responsible for the required quality assurance activities w17x, which was eventually integrated by the contractor in its own system after some modifications. It is well known that reasonable efforts extended towards early recognition of nonconforming items to help to avoid consequential costs incurred for the elimination of damages and deviations (Fig. 25). The applicable German standards (see Refs. w5,6x and, respectively w4x) do not contain sufficiently binding
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specifications for the quality assurance of natural stone masonry. For that reason, a quality assurance guideline w12x has been developed specifically for the reconstruction of the Frauenkirche in Dresden. For the purpose of this guideline, the current state-of-the-art of science and technology has been compiled and edited for practical usability for this particular concrete application case. The guideline mainly covers the control of statically relevant properties of masonry and its raw materials. 7.2. Check lists Check lists are being used to control the manufacture and delivery of raw materials as well as their processing within the scope of the quality assurance system. In order to guarantee a satisfactory and manageable quality control, requirements, quantities, processes and responsibilities have been uniformly defined following today’s customary standard quality assurance systems. A number of appropriate forms are intended to facilitate quality control activities for all those involved in manufacture and construction. At the same time, a uniform documentation of the materials used in construction and the achieved quality levels in the structure is guaranteed. Fig. 26 shows an example of a quality check list. 7.3. Natural stone 7.3.1. Strength tests of natural stone Prior to use of stones from a newly developed quarry, respectively, a newly cut bed, a qualification test must be performed with regard to the determination of the characteristic values, in order to assure the principal qualification and comparability with the specifications of the intended materials. For the construction of the load bearing masonry structure in the reconstruction of the Frauenkirche in Dresden, the regular inspections of stone parameters of an entire quarry were deemed to be insufficient, since the characteristic values within a quarry and between different points in time may fluctuate too much. Therefore, the following points must be determined within the scope of qualification tests for new stones: – – – –
raw density, compressive strength, bending tensile strength, standard characteristic values of a particular bed, such as – water absorption under atmospheric pressure, – frost resistance, – wear test and – resistance to break-outs at the anchor mandrel bore.
In case of use of sandstone varieties other than those from Posta, the following additional characteristic values must be determined:
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Fig. 25. General flow diagram of the quality assurance system.
– split tensile strength, – static modulus of elasticity, – swelling and shrinking behavior with water storage and drying, – thermal expansion, – thermal capacity and, – thermal conductivity. A decision is then made about the qualification of the respective material on the basis of these values, a comparison with the characteristic values of the new stones used up to this point as well as the specified values. Up to now, the raw material for highly stressed masonry has been supplied by three separate quarries from the Saxon Elbe sandstone district. In the course of tests of the stone materials, differences became apparent
between the three quarries with regard to strength levels and dispersion of test results. After the initial qualification tests, regular quality inspections must be carried out parallel with progress of stone work of the raw sandstone blocks in the quarry. These quality inspections are used to control whether or not the characteristic values determined in the qualification test are being adhered during stone work process. The scope and frequency of the tests have been defined differently in relation to the various requirement categories. In case of utilization of stones for the highest requirement category 4 with the highest stresses, representative test samples for the verification of the stone strength within the quality inspection must be taken from each raw block while it is still at the quarry. These test samples are used to determine the following character-
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Fig. 26. Example of a quality check list.
istic values, which are essential for the load bearing capacity: – raw density, – compressive strength and, – split tensile strength. It is assumed that, due to well-known correlations, the other characteristic values will be similar to those determined in the qualification test. A sensible and meaningful quality assurance system begins at the quarry. Particularly high-strength masonry
requires stones of a homogenous structure. An initial visual inspection for homogeneity is carried out in the quarry prior to stone work and testing. 7.3.2. Statistical evaluation of strength tests The calculation of the characteristic strength of masonry of the various requirement categories requires the determination of the 5% fractile values of the stone, since these are part of the calculation formulae valid for the reconstruction.
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Stone strengths of randomly selected stones or raw blocks were determined exemplary within the scope of the qualification tests. A quality inspection has been established for each raw block to be used for highly stressed masonry, while fewer samples are taken in the course of stone work for the other requirement categories. Since it is not possible, with the extraction of five test samples each for compressive strength and split tensile strength tests, to perform a complete and secured determination of the characteristic stone strengths of the raw block itself or the general entirety, it is necessary to include the results of a broken lot of stones in the statistical evaluation. The strengths are determined within the quality inspections by means of the same prism patterns used in the qualification tests, in order to obtain directly comparable strength values. For the tests of stones of the highest requirement category, cubes (lywyhs50 mm) were used for testing compressive strength and prisms (lywyhs160y40y40 mm) were used for testing split tensile strength. The definition of the geometry of test samples resulted from the qualification tests of the respective stone material. The influences of shape and structure, for which there are no confirmed values available at this time, must be taken into consideration in comparisons of the test results of test samples with different geometries. In order to keep the number of test samples in manageable limits, the customary approach used in concrete construction was adopted for the statistical evaluation of the tests performed in this project. However, the mean value and the standard deviation was determined from all test results that could be grouped together for the purpose of this evaluation due to their location in the quarry. It may be assumed that, with a certainty of 90%, no more than 5% of the strength values of the basic entirety, with respect to the lot in question, will fall short of the specified or comparative value. The calculation of the lower fractile value was performed from the mean value of the respective test results and the standard deviation, which must be multiplied by a factor depending on the number of test samples. fksfyks
(3)
where f k is characteristic strength (5% fractile), f is mean value strength, k is factor in accordance to the number of test specimens and s is standard deviation. The factor must be selected according to the number of test results that can be grouped together, adhering to the basic rules of statistics w18x. A symmetrical normal distribution was assumed for the statistical distribution of the strength values on the basis of literature of known test results of Saxon sand-
Fig. 27. Control of the masonry.
stone, which eventually were confirmed during the course of the ongoing evaluations. 7.3.3. Dimensional inspections of the cut stones Aside from the strength tests, the geometry of the stones being prepared for use in construction must be inspected as well. This becomes all the more important, the smaller the maximum permissible and minimum possible joints are. The limiting value for the greatest permissible joint thickness resulted from the necessity to assure the required strength, while the smallest joint thickness depended on the placement technology employed. In the case of highly stressed masonry, it is no longer sufficient to inspect the individual dimensions. Possible deviations of flatness and form errors must also be verified (Fig. 27). 7.4. Mortar Aside from the natural stone, the mortar also has an influence on the load bearing capacity of the constructed masonry and thus must be supplied and processed in accordance with quality specifications. For that reason, the quality of the mortar must also be controlled in order to achieve a uniform strength of the masonry (Fig. 28, Table 1). Standard production mortar could not be used in the reconstruction of the Frauenkirche due to the special requirements. Mix specifications were developed within the scope of planning work for load bearing structures, which, however, required final adjustment under consideration of the actual raw materials and the equipment that was eventually used in construction. The designed mix ratio, after being put in more concrete terms, were first tested in the lab with a view towards their main characteristic values and subsequently in practical testing of experimental applications. Particular attention was paid to the compatibility of the mortar and the natural
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Fig. 28. Small dry mortar silo with an anger type forced mixer.
stone. While observing the specified test period, suppliers are obliged to perform complete qualification tests of the mortar in good time prior to the start of construction. In order to guarantee the production of mortar of uniform quality, quality inspections must be performed in regularly scheduled intervals during processing of the mortar. The scope of the quality inspection is limited to the major characteristic values. The comparative values
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should be based on the results of the preceding qualification test of the same designed mix ratio. With regard to the test procedures, measures should be taken to adjust the test conditions and the uniformity of the geometry of the test samples between the supplier and the construction management. The mortar tests are performed on standard prisms (160=40=40 mm3). They are prepared in the customary metal mould and then stored, until the seventh day, in a climate of 20 8C and 95% relative humidity and, until the time of the test, at 20 8C and 65% relative humidity. The test of the compressive strength is performed in accordance with DIN 18555, Part 3 w19x. Experience in handling of various different test sample geometries was gained in the course of the reconstruction project of the Frauenkirche w20x. The corresponding form factors depend on geometry, structure, age and strength. An adoption of form factors from the sector of concrete and cement is not permissible. When applying the guideline of the Deutsche Gesells¨ Mauerwerksbau w21x, it should always be chaft fur noted that this guideline is of a supplemental nature and the test results may fluctuate considerably depending on the materials used. In this case, special preliminary tests are advisable. The initial attempt of using test samples with deviating geometries in connection with lime mortars did not prove successful. In case of mortar for thin joints, the essential increase in strength takes place during the first 28 days. Thus, the 28-day strength of these mortars is meaningful, as far as quality inspections are concerned. In case of longer test periods, the reaction to deviations becomes
Table 1 Characteristics for qualification tests and quality inspections of the mortar to be used in reconstruction Value
Qualification test
Quality test
Dry mortar Mixing ration Grading curve of aggregates Content of soluble alkali in relation to proportion of binding agent
= = =
= = =
Fresh mortar Consistency (slump value, flow value) Raw density of fresh mortar Air content Water content with waterybinding agent ratio Pot life Correctibility time (thin layer mortar)
= = = = = =
= = = =
Solid mortar Raw density of solid mortar Bending tensile strength Compressive strength Share of soluble alkali and sulfates Adhesive shear strength Water absorption with saturation value Amount of shrinkage
= = = = = = =
= = =
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rather more difficult. In case of predominantly lime mortars, the 70-day strength must be determined, at least within the scope of the qualification test, even though the corresponding 28 day is used for the purpose of quality inspections. In this case, a correlation to the 70-day strength must be established at the time of the qualification test. Aside from quality control measures performed parallel to manufacture and construction, follow-up checks of newly constructed masonry as well as old masonry may be necessary, if applicable. For this purpose, test samples may be cut from the masonry and tested in accordance with the DGfM guideline w21x. However, in case of thin joints of 6-mm thickness, extraction and testing of samples becomes questionable. The procedures followed in the reconstruction of the Frauenkirche have been proven for masonry of the various requirement categories with thicker joints, on the basis of cuboides 20=20=12 mm3. 8. Summary Valuable experience in the use of natural masonry with appropriate static stresses was gained in the reconstruction of the Frauenkirche. These experiences not only include load bearing behavior but also issues of fire protection, durability, manufacturing technology and quality control. Guidelines have been established for planning, execution and quality control, guaranteeing the achievement and realization of design specifications for the masonry, while at the same time providing a comprehensive documentation of the material used in construction. This documentation can be very useful and necessary for future assessments, particularly in case of difficult engineering projects involving historical structures. Acknowledgments The represented results were acquired within the scope of project planning for the reconstruction and financed by the Foundation Frauenkirche Dresden. The authors wish to thank the Foundation Frauenkirche and Building Director Dipl.-Ing. E. Burger for the opportunity to participate in this project. References w1x Jager ¨ W. Bestandserkundung, materialtechnische Untersuchun¨ gen, Bemerkungen zur ingenieurtechnischen Ausfuhrung und ¨ den Wiederaufbau der Frauenkirzu Tragwerksalternativen fur ceh Dresden. Wissenschaftliche Zeitschirft der Technischen ¨ Dresden, vol. 45. Sonderheft zum Wiederaufbau Universitat der Frauenkirche Dresden; 1996. p. 54–8. w2x Jager ¨ W, Pohle F. Einsatz von hochfestem Natursteinmauerwerk beim Wiederaufbau der Frauenkirche in Dresden. MauerwerkKalender. Berlin: Ernst & Sohn, 1999. p. 729 –55.
w3x Jager ¨ ¨ W, Wenzel F. Der archaologische Wiederaufbau der Frauenkirche zu Dresden als Ingenieuraufgabe—Zur Entwicklung des statisch-konstruktiven Konzepts und zum Stand der Umsetzung. Beton-und Stahlbetonbau 1996;91(11):257 –76. w4x DIN 1053, Teil 1: Mauerwerk; Rezeptmauerwerk; Berechnung ¨ und Ausfuhrung. Berlin: Beuth Verlag; February 1990. w5x DIN 1053-1, 11-96: Mauerwerk. Teil 1: Berechnung und ¨ Ausfuhrung. Berlin: Beuth Verlag; November 1996. w6x DIN 1053-2, 11-96: Mauerwerk. Teil 2: Mauerwerksfestigkeit¨ sklassen aufgrund von Eignungsprufungen. Berlin: Beuth Verlag; November 1996. w7x Lambrecht J, Weiß Ch, Hennig GK, Jager ¨ W. Die Armierungseisen der Dresdner Frauenkirche. In: Die Dresdner Frauenkirche. Jahrbuch zu ihrere Geschichte und zu ihrem Wiederaufbau, vol. 4. Society for the Support of the Reconstruction of the Frauenkirche Dresden e.V. & Foundation Frauenkirche Dres¨ den. Verlag Hermann Bohlaus Nachfolger Weimar; 1998. p. 101–22. w8x Berndt E, Schone ¨ I. Ein Bemessungsvoirschlag fur ¨ Mauerwerk aus Elbsandstein auf der Grundlage experimentell ermittelter ¨ Tragfahigkeiten. Erhalten historisch bedeutsamer Bauwerke. ¨ Baugefuge, Konstruktion, Werkstoffe. Sonderforschungsbereich 315, Yearbook 1992. Berlin: Ernst & Sohn; 1994. S. 41– 60. w9x Berndt E, Popp Th, Bergander H. Untersuchungen zur Auswahl ¨ eines besonders tragfahigen und geeigneten Natursteinmauerwerkes aus Elbsandstein. Compiled on behalf of the Foundation Frauenkirche Dresden by the Engineering Partnership Frauen¨ kirche Dresden Prof. JageryProf. Wenzel. Dresden; 1993, unpublished manuscript. w10x Berndt E. Gutachterliche Aussagen zur Schub-Druck-Tragfa¨ ¨ Quadermauerwerk aus Elbsandstein. Bemessungshigkeit fur gleichungen, Versuchsbericht mit Auswertung. Compiled on behalf of the Foundation Frauenkirche Dresden in collaboration with the Engineering Partnership Frauenkirche Dresden Prof. ¨ JageryProf. Wenzel. Dresden September; 1995, unpublished manuscript. w11x Bergander H, Jager ¨ W, Fleischer Th. FEM-Modellierungen beim Wiederaufbau der Frauenkirche in Dresden. Tagungsband 3. FEMyCAD-Tagung Darmstadt, Erfahrungsaustausch und Technologietransfer der Baupraxis am 10.y11.03.1994 at the TH Darmstadt (Ed.) Darmstadt: TH Darmstadt; 1994. p. 169– 76. w12x Mauerwerksrichtlinie Frauenkirche. Richtlinie zur Beurteilung ¨ der Mauerwerkstragfahigkeit und Anforderungen an die Aus¨ fuhrung von Sandsteinmauerwerk. Worked out by the Engineering Partnership Frauenkirche Dresden under co-operation with the Working Group ‘Masonry’. Dresden; 1996, unpublished manuscript. w13x Berger F. Zur nachtraglichen ¨ ¨ Bestimmung der Tragfahigkeit ¨ von zentrisch gedrucktem Ziegelmauerwerk. Erhalten historisch ¨ bedeutsamer Bauwerke. Baugefuge, Konstruktion, Werkstoffe. Sonderforschungsbereich 315, yearbook 1986. Berlin: Ernst & Sohn; 1994. S. 41–60. w14x Burkert T, Weller B, Jager ¨ W. Investigation into structural design of the sandstone surface layer for the dome reconstruc¨ tion of the Dresden Frauenkirche. In: Brebbia CA, Jager W, editors. Structural studies, repairs and maintenance of historical buildings VI. Southampton, Boston: WIT Press, 1999. p. 875 – 85. w15x Grunert S, Szilagyi J. Untersuchungen zur Auswahl von Stein¨ die Kuppel der Frauenkirche Dresden. Final report. material fur Compiled by the Engineering Partnership Frauenkirche Dres¨ den Prof. JageryProf. Wenzel on behalf of the Foundation
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w16x
w17x
w18x
w19x
Frauenkirche Dresden. Institute for Geotechnique, University of Technology Dresden, Dresden; 1998. ¨ ¨ ¨ Muller HS, Horenbaum W, Jager W. Investigations of sandstone ¨ for the reconstruction of the Frauenkirche in Dresden. In: Jager W, Brebbia CA, editors. The revival of Dresden. Southampton, Boston: WIT Press, 2000. ¨ Jager W, Gigla B, Preusche Th, Pohle F, et al. Wiederaufbau der Frauenkirche zu dresden. Mauerwerksrichtlinie Frauenkir¨ che. Instrumente des Qualitatssicherungssystems (QSS)— Checklisten. Unpublished manuscript. Compiled by the ¨ Engineering Partnership Frauenkirche Dresden Prof. Jagery Prof. Wenzel on behalf of the Foundation Frauenkirche Dresden. Dresden; 1997. Storm R. Wahrscheinlichkeitsrechnung, mathematische Statis¨ tik und statistische Qualitatskontrolle. Leipzig: Fachbuchverlag, 1986. ¨ ¨ DIN 18555, Teil 3. 09.82. Prufung von Morteln mit mineral¨ ischen Bindemitteln; Festmortel; Bestimmung der Biegezugfes-
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tigkeit, Druckfestigkeit und Rohdichte. Berlin: Beuth Verlag; 1982. w20x Jager ¨ ¨ W, Pohle F. Ermittlung von Umrechnungsfaktoren fur ¨ ¨ ¨ Mortelprufkorper unterschiedlicher Geometrien—Zwischenbericht. Worked out by Ingenieurgemeinschaft Frauenkirche Dres¨ den Prof. JageryProf. Wenzel by order of the Foundation Frauenkirche Dresden. Dresden Mai; 1998, unpublished manuscript. w21x Vorlaufige ¨ ¨ ¨ Richtlinie zur Erganzung der Eignungsprufung von ¨ Mauermortel; Druckfestigkeit in der Lagerfuge; Anforderun¨ ¨ Mauerwerksbau e.V., gen, Prufung. Deutsche Gesellschaft fur Bonn August 1992. w22x Burkert T. Untersuchungen zur baukonstruktiven Ausbildung und zum Verwitterungsverhalten der Kuppeldeckschicht beim Wiederaufbau der Frauenkirche Dresden. Dissertation, Dresden University of Technology, Institute for Building Technology in co-operation with the Chair for Load-Bearing Structures. Dresden; 2001.
Material properties of historical masonry of the Frauenkirche and the masonry guideline for reconstruction ¨ 1,* Frank Pohle, Wolfram Jager Faculty of Architecture, Chair for Planning of Load Bearing Structures, University of Technology Dresden, D-01062 Dresden, Germany Received 16 January 2001; received in revised form 18 August 2003; accepted 19 August 2003
Abstract This contribution provides insight into the characteristics of the masonry of the historical Frauenkirche and the requirements for its reconstruction. Comprehensive material studies were performed in the course of preparation for and implementation of the project, and guidelines for evaluation, design and construction were established on the basis of those studies. The material used for the reconstruction is sandstone, coming from quarries in the immediate vicinity, as well as mortar composed on a lime basis with hydraulic additives. The execution is monitored within the scope of a quality management system specially created for the reconstruction project. This system simultaneously provides a detailed documentation of the building materials used in the project. 䊚 2003 Elsevier Ltd. All rights reserved. Keywords: Masonry; Sandstone; Mortar; Compressive; Strength; Fire tests
1. Introduction The reconstruction of the Frauenkirche in Dresden, aside from the archeological removal of the rubble, has been preceded by a comprehensive analysis of the historical structure w1–3x. The focal point of this analysis was the issue of weaknesses of the building prior to its destruction (Fig. 1). If a historically accurate reconstruction was to be attempted, it was necessary to bring the level of safety up to today’s accepted standards and to guarantee the performance capability. One path followed in this respect represented the gentle improvement of material properties and the use of defined material quality grades. At the same time, the engineers responsible for the reconstruction project wanted to have clarity about the failure mechanisms of masonry made from Elbe sandstone and to create a design process on the basis of the latest engineering knowledge. For this purpose, they *Corresponding author. E-mail address: [email protected]¨ dresden.de (W. Jager). 1 ¨ Ingenieurgemeinschaft Frauenkirche Dresden Prof. Jager & Prof. Wenzel, RadebeulyKarlsruhe, Wichernstr. 12, D-01445 Radebeul, Germany.
selected the verification procedure according to the ultimate limit state method, which permits a differentiation of safety factors for the effects and the resistance of load bearing structures. When reviewing the German building code DIN 1053 for masonry structures in force at the time the rebuilding of the church was prepared, the reconstruction in accordance with this code was not feasible in view of the forecast level of stresses and loads. The permissible stresses were in part lower than the maximum working stresses, which were determined through calculations of the undamaged structure as being between 5 and 7.5 Ny mm2. The use of masonry after testing it for qualification became possible only after the publication of the revised DIN 1053-1 and -2 (refer to w5,6x) and was not available in the early 1990s. Therefore, it was necessary to perform corresponding studies of load bearing capacities of sandstone masonry, which at the same time could serve as a scientific basis for the theory of failure and its description of masonry in general. Of further interest was the issue of load bearing capacity of the existing masonry of the ruin and the cellar, which were intended to be integrated in the reconstruction.
0950-0618/03/$ - see front matter 䊚 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0950-0618Ž03.00062-X
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2. Stone material
Fig. 1. Photo of a destroyed pillar capital from the restoration.
Experimental and analytical studies were carried out on stones, mortar, masonry and the metal components present in the original structure. It was necessary to test the stone material of the rubble, the parts of the ruin that were left standing, the cellar as well as new stone materials, in order to gain adequate knowledge of their characteristic strength values. The analyses of metals, particularly the wrought iron used in the construction of the church, had a predominantly architectural and historical background w7x. Aside from strength issues, it was also necessary to clarify building material related problems having a bearing on the varieties of sandstone and the mortar to be used. In order to be able to realize the planning specification in practice, technological considerations had to be made and the verification of the quality of the reconstruction had be taken into account as well.
Fig. 2. Stone specimen in compression test with the typical crack pattern.
The result of testing stone material samples with regard to their compressive strength was that the average values of the foundation masonry are lower than those of the ruined parts higher up in the structure. It should be noted, however, that the samples from the cellar could only be taken by drilling parallel to the sedimentation plane. As a rule for Saxon sandstone, the compressive strength perpendicular to the sedimentation plane is approximately 10–15% higher compared to the one parallel to the sedimentation plane. In comparing the various strength test results, account had to be taken of the specimen dimensions (Fig. 2). Random sample taken from the heap of rubble (Fig. 3) yielded a mean value approximately corresponding to the overall values of historical sandstone from quarries in the Elbe valley. The stone material recovered from the rubble that was deemed reusable will be used in the reconstruction, in accordance with the aim of archaeological accuracy, after undergoing a reconditioning process. The data collected from the rubble were consolidated after the reconditioning of the lining stones and thus confirmed in their statistical value. In the evaluation of test samples of new stone material of the Posta variety, which were required within the scope of masonry tests (refer to w8,9x), a distinction was made between normal Posta sandstone and the particularly strong sandstone from the so-called ‘white bed’. The last is a very homogenous and solid layer of a nearly white color, which is found in the quarries (Fig. 4). In order to assure the desired progress in construction, another variety has been used in the meantime in accordance with the specifications of the engineers. This variety is the so-called Reihnhardtsdorf sandstone, which has a higher content of clay.
Fig. 3. View on the remains of the Frauenkirche in 1989 prior to the removal.
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Fig. 4. Sandstone quarry in Wehlen nearby Dresden.
3. Mortar Analyses of the mortar used in the original construction of the church were also performed at the time of the removal of the rubble (Fig. 5). It was particularly interesting to find out whether any distinction was made in relation to different parts of the structure and what the composition of the mortar had been (Fig. 6). With a view to differentiation, the only differences found were those between normal masonry and that of the
Fig. 5. Look at the remains (north-west staircase tower) during the archeological removal of the rubble in 1993.
sandstone pillars. While, as a rule, normal masonry is well carbonated, evidence of poor carbonation was found in the contiguous remains of the pillars. The sands used in the lining masonry exhibited good grain gradation and grain sizes of approximately 6 mm, partially with sharp-edged surfaces. In some places the addition of brick dust could be proven. The proportion of binding agent was, in relation to weight, approximately 1:3.45. Lime was found to be used as a cementing agent. For the purpose of reconstruction, clearness was required with regard to the processing technologies to be used in the construction of the new masonry (Fig. 7). According to the experience and practice of stone masons over many decades, ashlars have been placed on small pre-balanced lead pads, lining bricks were laid with a mortar suitable to be applied with a trowel, the joints in the facade were stuffed with hemp and the coursing joints were filled with grouting compound. The hemp is removed after the grouting compound has set and the joints are filled with a special pointing mortar. The stones of the masonry of the pillars (Fig. 8) with thin joints subjected to high stresses and loads are placed with a thin layer mortar suitable for trowel
Fig. 6. Historic masonry of the remains (ashlar stone masonry).
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erate and stabilize the setting process. The cement content has been reduced to the absolute minimum. Alkali-poor materials are used as additives in order to control the potential for blooming. Furthermore, the mortar for the units was supplemented by a mortar for masonry with thin joints as well as by the pointing and grouting compound. 4. Masonry Tests on masonry test samples (Fig. 9) were also necessary for the assessment of the load bearing capacity of ashlar stone work of Saxon sandstone. These tests concentrated on the following focal points:
Fig. 7. Re-erected masonry of the exterior walls (in a height of the first floor).
– compressive load bearing capability of sandstone work with various joint widths; – shearing load bearing capacity of sandstone masonry at various compressive loads (Fig. 10); – creep characteristics of dimensioned stonework with various joint widths; – behavior in fire of sandstone masonry. 4.1. Compression tests of sandstone masonry
application, whose main characteristics are a high degree of water retention capacity and a long time period of correctability. The above mentioned lead plates may cause peak stresses in the masonry, which is why they were omitted in the masonry of the pillars with higher strength requirements. Very high demands are made on the prefabrication of the stones, since a 6-mm joint does not allow the correction of any tolerances. Measures for the compensation of tolerances were not necessary in the execution of the pillars, since the surveying work done during the ongoing construction permitted a very high level of accuracy. On the basis of the analyses performed, as well as the experience of skilled craftsmen, proposals were made for the mortar to be used in the reconstruction of the Frauenkirche. These proposals were reviewed and evaluated by experts of the department of building materials ¨ Massivbau und Baustofftechnologie of of the Institut fur the University of Karlsruhe and finally modified in adjustment to the technology used by the manufacturer, TUBAG of Kruft. During the initial phase, the recipe had been based on the use of the mortar as a site mortar. However, the necessary rationalization of technical processes by the construction company required the use of a dry mortar that is supplied in silo trucks. An adaptation to these requirements followed. Special attention was paid to the compatibility of the mortar and the sandstone material. The sandstone itself contains small amounts of soluble alkali, as does the mortar, in order to bring about a hydraulic reaction. A small cement content is used, which is intended, as a hydraulic component, to accel-
The series of tests, already under way in Dresden in 1992, was expanded by tests of the load bearing capacity of sandstone masonry with narrow joints, which were performed on behalf of the Stiftung Frauenkirche Dresden e.V. w8x. Later, test samples with simulated original masonry were added to the series of tests as well. The tests included four-stone pillars as well as RILEM test samples. Parallel to the test samples, the strength
Fig. 8. Stone masonry with thin layer mortar of an interior pillar.
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655
fb t mm dB dyd9 Ez F|xq0.7 q0.3 C1y x hD d G~ y h9 1ymm
fs w
10 cm T t d9stq B h9sminU p FE T tanC q F Vh D4 2G xsfbkyfstk S
Fig. 9. RILEM test specimen for the compression test of sandstone masonry.
characteristics of the stone material and the mortar were tested as well. The test series have shown that fractures occur due to transverse tension failure in sandstone masonry as well. It was determined that the strength of the stone material, the thickness of the joints and the type of mortar have a decisive influence on the strength of the masonry. The minimum joint thickness achievable with normal mortar was approximately 5 mm; however, at this thickness difficulties began to arise in the application of the mortar. However, a significant increase of the masonry strength was found in masonry with joints of 5 mm or less. At this joint thickness, a masonry resistance to fracture of 80–85% of the stone strength was achieved. This was a value that prompted the engineers responsible for the reconstruction of the Frauenkirche in Dresden to voice their recommendation that it should be possible to reconstruct the church with sandstone masonry. The end result of the test series was the creation of a formula for the determination of the compressive strength of masonry and a proposal for the assessment of sandstone masonry on the basis of the limit state method with partial safety factors w8x. The advantage of this method lies in the fact that individual influences can be grasped and considered more realistically and concretely. Consequently, after intensive discussions between the joint engineering task force and the check proof engineer,2 the proposal for assessment was adopted, and in the process the partial safety factors (actions, materials) for the actual application at hand were determined. Thus, a verification method according to the current state-of-the-art forms the basis for the reconstruction of the Frauenkirche in Dresden. 2 Check proof engineer for the reconstruction of the Frauenkirche ¨ Peter from Stuttgart. in Dresden is Prof. Dr.-Ing. Jorg
t h h9 d f fb f bk f stk mm F
Joint thickness Height of the stone Effective height of the stone Width of the stone Mean compressive strength of masonry Mean compressive strength of stones Characteristic compressive strength of stone Characteristic split tensile strength of stone Poission’s ratio (M5 s0.4; M10 s0.3) Inner angle of friction
4.2. Shear-compression tests Since shear-compression loads occur in the masonry at a non-orthogonal direction of forces in relation to the coursing joint due to the anisotropy, this type of influence had to be investigated as well. It is exactly this type of load that is quite significant in the Frauenkirche, due to the occurrence of ring tensile forces, descending forces, shearing forces of the flat inner cupola and forces due to the additional horizontal load application. The tests were conceived in co-operation with Berndt w10x, performed in the Otto-Mohr-Laboratory of the Faculty for Civil Engineering at the Dresden University of Technology. A special test rig was developed, approximately simulating the shear-compression loads to be
Fig. 10. Test specimen of a shear-compression test with the test data recording equipment.
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Fig. 12. Curve of ultimate load bearing capacity due to shear-compression loads.
Fig. 11. Shear-compression test in the Otto-Mohr-Laboratory of the Dresden University of Technology.
expected in the actual structure as realistically as possible. Various shear-compression ratios were investigated in the tests (Fig. 11). The final test results yielded an assessment proposal, which in turn was developed on the basis of the method of partial safety factors and led to the shear-compression failure curves. Fig. 12 shows an example of a curve for the determination of the shear strength in dependence of the design compressive stress. 4.3. Requirement categories In accordance with the load flux and loads determined for the entire structure according to the planned load bearing concept w3x, requirement categories were defined by the joint engineering task force and underscored with corresponding assessment values (Fig. 13) w12x. It was also necessary to assess the existing masonry. This was based on a statistical evaluation of joint thicknesses, which then, combined with the above-mentioned taking of stone specimens, permitted the establishment of assessment values. With a masonry quality matched to the actual load flow, the reconstruction of the Frauenkirche in Dresden is possible and feasible in sandstone masonry (0). The decisive factor was, and continues to be, that the calculated values are actually achieved in the execution of the reconstruction.
which has been implemented in ANSYS (refer to w11x). The cohesion c and the angle of internal friction g of the mortar are required as input parameters. It was possible to recreate the experimental results quite well with the analyses. Even for thin joints, it was possible to show evidence of the typical stresses patterns in the area of the contact surface between stone and mortar. However, narrow joints with hard mortar no longer exhibit the typical joint cracks near the edges. Further parameter studies could be performed with the calibration of the mechanical model in accordance with the available test results, making it possible to forego elaborate subsequent tests. At the same time, experience with the material masonry and its illustration in FEM analyses were gained, which came in very handy for the component studies of the cellar and the cupola. 4.5. On-site tests For the validation of the mechanical modeling of the remaining masonry it was necessary to forego on-site
4.4. Evaluation of results obtained with FEM analyses The theoretical assessment was supported by parallel FEM analyses by Bergander et al. w11x. For the purpose of these analyses, the two-component masonry as such has been modeled and calculated close to reality. The stone behaves in linear fashion, while the mortar exhibits plasticity. The obvious choice for modeling the plastic behavior of the mortar joint was the isotropic, idealplastic material model according to Drucker-Prager,
Fig. 13. Application of the various requirement categories in the structure.
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Fig. 14. Sample of test cores of the historic masonry nearby the choir.
tests. Beside the check of the assumed compressive strength of sandstone units were carried out tests on cut out mortar samples of size 20=20=12 mm3, which indicated mortar strengths of approximately 1.5 up to 5 Nymm2. In order to be able to control the load bearing capacity of the highly stressed apse pillars, the masonry strength in this area was determined according to the tensile splitting test method of cylinders drilled over a joint (refer to w13x). To this end, joints were drilled perpendicularly with a core drilling machine, the core (sandstone–mortar–sandstone) was extracted and subsequently tested for split tensile strength (Fig. 14). By extracting cores next to the location of the joint bores it was possible to determine the strength of the stone and to assess the strength of the masonry. Load bearing design resistance of 4.3 Nymm2 were found w3x. Finally, the general condition of the two highly stressed apse pillars was tested for hollow spaces by means of geo radar. The masonry mass of the cellar was inspected visually and by means of endoscopy.
strength masonry. It was intended that the stress level corresponded with the actual service conditions of use. In order to be able to separate the creep deformations of the stone from those of the mortar, the compression set of the tri-stone test sample were determined with the aid of inductive distance sensors and those of the center stone with a settling tensiometer. The creep deformations of the entire test sample were determined as a mean value of the four measured individual values. The deformations of the center stone were measured with four measuring points along two segments (refer to Fig. 16). The test samples were subjected to load for approximately 5 months in a standard reference atmosphere. In accordance with commonly used procedures, the final amount of creep was measured during a certain time segment towards the end of the test period and the final creep value of the test sample was determined via the relation to the elastic compression set at the beginning of the test period:
wk`s´k`y´el
(1)
With the aid of the final creep value, the modulus of elasticity can then be determined, taking creep into consideration, as follows: EwsEelyŽ1qwk, `..
(2)
Fig. 16 shows the course of the creep curves of both test samples of the highly stressed masonry. The tests have shown that the most significant creep influence comes from the joint and depends on its thickness as well as on the type of mortar that was used. Creep is determined exclusively by the mortar. The
4.6. Creep tests In the course of reconstruction work on the Frauenkirche in Dresden, creep tests on tri-stone test samples with mortars of various strengths were performed in the test laboratory of the Chair for the Planning of Load Bearing Structures at the Dresden University of Technology, under the supervision of Berndt w14x. The corresponding test program also comprised two tests on the issue of creep behavior of masonry with a particularly high load bearing capacity and thin joints. The tests were performed in creep frames of the same dimensions, making it necessary to select test samples with different sizes (Fig. 15). The compressive strength in the normal masonry test samples was 2–3 Nymm2 and 8 Nymm2 in the high-
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Fig. 15. Test laboratory with prepared creep frames.
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stone exhibited no compressive setting over the test period. This fact appears to be logical, since the sandstone was subjected to natural prestressing in its geological location of deposit. The share of creep in masonry made with mortar of Group I (low strength) w6x and normal joints is approximately double that of masonry made with mortar of Group III (high-strength) and narrow joints. These differences had to be observed accordingly in the calculations and verifications, since the structure is being erected with various different qualities of masonry. 4.7. Fire tests Experimental studies on the behavior of sandstone were required in order to verify constructional fire protection. To keep the expenditures within reasonable limits, it was finally decided to implement a two-stage program. As a first step, small sandstone cubes were subjected to thermal loads and then tested mechanically in co¨ operation with the Materialprufamt Sachsen as well as ¨ Geotechnik of the Dresden University of the Institut fur Technology. The temperature stages were selected in intervals of 100 8C. For each stage, five cubes with the dimensions 50=50=50 mm3 were heated in a muffle furnace and maintained at that temperature over a certain period of time. After cooling down, the remaining compressive strength was determined. In the case of sandstone from the ‘white bed’ a continuous drop of strength was determined at increasing temperatures. It begins to be particularly pronounced at temperatures above 300 8C. The test samples already fell short of the required 5% fractile value for stone strength of 44.4 Nymm2 at a temperature of 400 8C. A residual strength of approximately 65% was found at temperatures in excess of 1000 8C. The materials tested included sandstone from the ‘white bed’ for highly stressed masonry of requirement category 4 as well as the normal, Posta variety, which exhibited a slightly different behavior. Its most significant drop of strength at approximately 30% was found to occur between 500 and 600 8C. A significant connection between red
Fig. 17. Sandstone masonry wall in the cellar damaged by fire.
discoloration and clay content could not be proven (Fig. 17). In order to be able to evaluate the behavior of sandstone under load during a fire, the second step of the test program included testing of masonry slices under vertical loads in a fire test on the basis of the socalled standard temperature time curve (Figs. 18 and 19). The duration of fire resistance required for a 24cm-thick wall was proven. In this process, the assumptions with regard to the size of the chipped off shells were confirmed as well. 4.8. Investigation of durability Owing to the alternating freezing–thawing loads in the climate of Germany, the sandstone facade of the Frauenkirche is subject to heavy stresses. These stresses are concentrated in the joints and the adjacent areas of the stones. Many historical structures exhibit chipping of the stone in the area of the edges adjacent to the joints.
Fig. 16. Side views of test samples with measuring segments to investigate creep behavior and experimentally determined creep curves for highly stressed masonry.
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Fig. 18. Masonry specimen under vertical loading in the fire test.
In order to be able to employ the latest technology in this area as well, comprehensive experimental and theoretical investigations have been carried out with the aim of achieving a sufficient measure of durability. This includes special studies on the selection of stones for the cupola as well as on the mortar to be used in vertical sections of the facade and in areas subjected to particularly heavy stresses, such as the cupola. The studies are being performed at the University of Karlsruhe w16x and at the Dresden University w14,15,22x (Figs. 20 and 21). 5. Technological aspects After the clarification of the strength of existing masonry as well as masonry to be newly erected and after planning and specifying of the requirement categories, thought had to be given to their technological feasibility. For this purpose, a joint workgroup with
Fig. 19. Spelled stone material in the fire chamber after the test.
Fig. 20. Full scale test specimen of the outer wall of the stone cupola in the test laboratory of the Chair for the Planning of Load Bearing Structures of the Dresden University of Technology w22x.
members from the architectural and engineering offices was appointed to address this topic (Fig. 22). In this process, the following focal points needed to be clarified: – prefabrication of stones with different requirements on accuracy, – verification of accuracy, – mortar application, – brick laying technology, – use of technical aids. Special provisions had be to made to maintain a basic minimum temperature of above 5 8C, which is required
Fig. 21. Drawing of test sample as section of the cupola in full scale with installation of the climatic hood w22x.
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Fig. 23. Screeding the applied mortar by a straight edge.
Fig. 22. Trial of the stone laying technology of the pillar masonry with thin bed joints.
for the setting process of lime mortar with hydraulic additives for the construction work during the cold weather period. Initially, handling of water posed some problems. In order to achieve good adhesion of the mortar to the stone, the lining units had been immersed in water prior to being laid. That resulted in the mobilization of the free alkali and in heavy blooming. Thereupon the use of water was reduced in order to curb the migration of alkali. Particular attention was paid to the highly stressed masonry. This required the preparation of special aids and devices (templates, drawing trowel). The work steps for the placement of the pillar stones in their proper sequence may be used as an example here: – preparing the bed surface by moderate moistening and closing of the pores with mortar; – attaching and aligning the templates for drawing off the joint; – slinging and adjusting the stone on the suspension gear of the portal crane; – applying the mortar on the prepared bed surface; – screeding the applied mortar (Fig. 23); – placing the stone (Fig. 24); – verifying and readjusting the stone; – filling the cross joints with hemp; – filling the perpendicular joints with grouting mortar; – pointing the joints after removing the hemp.
Frauenkirche in Dresden w12x defines the established requirement categories with corresponding characteristic values and basic conditions for the assessment of the load bearing capability of sandstone masonry and its manufacture. The guideline forms the basis for the static verification of the existing structure as well as the newly erected masonry. Furthermore, the guideline contains all specifications and characteristic values required for the implementation of the quality control system as well as notes for their execution. All essential findings from the various test series have been incorporated. In order to avoid any inconsistency in the calculation of the strength of the respective masonry within the scope of verification, the strength and the modulus of elasticity have been specified for the established requirement categories. This guarantees uniform handling and interpretation of findings, formulae and definitions for the entire reconstruction project. The requirements for the basic materials (stone and mortar) as well as for the constructed masonry are formulated in detail in the full length version of the masonry guideline. In order to guarantee security and effectiveness in daily use, the most important specifications for planning and execution have been additionally published in form of individual, clearly designed
6. Masonry guideline for the reconstruction of the Frauenkirche The masonry guideline for the reconstruction of the
Fig. 24. Laying of stones of requirement category 4 for the pillars.
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and handy leaflets providing so-called ‘directives’ in abbreviated fashion. These loose leaf instructions list the parameters to be monitored including their limiting values to be adhered to, separately for stones, mortar and masonry on clearly arranged pages. The validity of the masonry guideline is limited to the project of the ‘Reconstruction of the Frauenkirche Dresden’. This concerns, on the one hand, the applicability as well as the material used in the project. It is certainly possible to draw conclusions from this guideline in order to apply them to other projects, but the applicability must be verified in each individual case by comparing the basic conditions of the projects. 7. Quality management and assurance system 7.1. Basic ideas One important cause of damages to the Frauenkirche in the past was the fact that stresses occurred in areas subject to high loads that could no longer be dissipated locally by the materials of the masonry (stone, mortar). Damaged stones in these areas had to be replaced in the years 1904–1905 and 1924–1932. These replacements were approached in a very thorough manner according to the technological standards at the time, particularly those in the 1920s. Only approved stone material, meeting the specified requirements with regard to strength, was used for the replacements. Therefore, the engineers responsible for the work considered the quality control of raw materials for the highly stressed masonry and their processing properties one of the essential prerequisites to be met in order to be able to meet the demands established in project planning for the structure. The quality control system is broken down, according to generally accepted principles, into a receiving inspection and a quality assurance test of the raw materials prior to being used for the construction of masonry. In this process, the receiving inspection represents the basic test, in which it is determined whether the material to be used for construction or offered by the contractor is principally suitable for its intended use. The quality assurance test is employed to verify the uniform adherence to characteristic values over the entire process from delivery to manufacture. A quality assurance system has been developed by the engineers responsible for the required quality assurance activities w17x, which was eventually integrated by the contractor in its own system after some modifications. It is well known that reasonable efforts extended towards early recognition of nonconforming items to help to avoid consequential costs incurred for the elimination of damages and deviations (Fig. 25). The applicable German standards (see Refs. w5,6x and, respectively w4x) do not contain sufficiently binding
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specifications for the quality assurance of natural stone masonry. For that reason, a quality assurance guideline w12x has been developed specifically for the reconstruction of the Frauenkirche in Dresden. For the purpose of this guideline, the current state-of-the-art of science and technology has been compiled and edited for practical usability for this particular concrete application case. The guideline mainly covers the control of statically relevant properties of masonry and its raw materials. 7.2. Check lists Check lists are being used to control the manufacture and delivery of raw materials as well as their processing within the scope of the quality assurance system. In order to guarantee a satisfactory and manageable quality control, requirements, quantities, processes and responsibilities have been uniformly defined following today’s customary standard quality assurance systems. A number of appropriate forms are intended to facilitate quality control activities for all those involved in manufacture and construction. At the same time, a uniform documentation of the materials used in construction and the achieved quality levels in the structure is guaranteed. Fig. 26 shows an example of a quality check list. 7.3. Natural stone 7.3.1. Strength tests of natural stone Prior to use of stones from a newly developed quarry, respectively, a newly cut bed, a qualification test must be performed with regard to the determination of the characteristic values, in order to assure the principal qualification and comparability with the specifications of the intended materials. For the construction of the load bearing masonry structure in the reconstruction of the Frauenkirche in Dresden, the regular inspections of stone parameters of an entire quarry were deemed to be insufficient, since the characteristic values within a quarry and between different points in time may fluctuate too much. Therefore, the following points must be determined within the scope of qualification tests for new stones: – – – –
raw density, compressive strength, bending tensile strength, standard characteristic values of a particular bed, such as – water absorption under atmospheric pressure, – frost resistance, – wear test and – resistance to break-outs at the anchor mandrel bore.
In case of use of sandstone varieties other than those from Posta, the following additional characteristic values must be determined:
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Fig. 25. General flow diagram of the quality assurance system.
– split tensile strength, – static modulus of elasticity, – swelling and shrinking behavior with water storage and drying, – thermal expansion, – thermal capacity and, – thermal conductivity. A decision is then made about the qualification of the respective material on the basis of these values, a comparison with the characteristic values of the new stones used up to this point as well as the specified values. Up to now, the raw material for highly stressed masonry has been supplied by three separate quarries from the Saxon Elbe sandstone district. In the course of tests of the stone materials, differences became apparent
between the three quarries with regard to strength levels and dispersion of test results. After the initial qualification tests, regular quality inspections must be carried out parallel with progress of stone work of the raw sandstone blocks in the quarry. These quality inspections are used to control whether or not the characteristic values determined in the qualification test are being adhered during stone work process. The scope and frequency of the tests have been defined differently in relation to the various requirement categories. In case of utilization of stones for the highest requirement category 4 with the highest stresses, representative test samples for the verification of the stone strength within the quality inspection must be taken from each raw block while it is still at the quarry. These test samples are used to determine the following character-
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Fig. 26. Example of a quality check list.
istic values, which are essential for the load bearing capacity: – raw density, – compressive strength and, – split tensile strength. It is assumed that, due to well-known correlations, the other characteristic values will be similar to those determined in the qualification test. A sensible and meaningful quality assurance system begins at the quarry. Particularly high-strength masonry
requires stones of a homogenous structure. An initial visual inspection for homogeneity is carried out in the quarry prior to stone work and testing. 7.3.2. Statistical evaluation of strength tests The calculation of the characteristic strength of masonry of the various requirement categories requires the determination of the 5% fractile values of the stone, since these are part of the calculation formulae valid for the reconstruction.
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Stone strengths of randomly selected stones or raw blocks were determined exemplary within the scope of the qualification tests. A quality inspection has been established for each raw block to be used for highly stressed masonry, while fewer samples are taken in the course of stone work for the other requirement categories. Since it is not possible, with the extraction of five test samples each for compressive strength and split tensile strength tests, to perform a complete and secured determination of the characteristic stone strengths of the raw block itself or the general entirety, it is necessary to include the results of a broken lot of stones in the statistical evaluation. The strengths are determined within the quality inspections by means of the same prism patterns used in the qualification tests, in order to obtain directly comparable strength values. For the tests of stones of the highest requirement category, cubes (lywyhs50 mm) were used for testing compressive strength and prisms (lywyhs160y40y40 mm) were used for testing split tensile strength. The definition of the geometry of test samples resulted from the qualification tests of the respective stone material. The influences of shape and structure, for which there are no confirmed values available at this time, must be taken into consideration in comparisons of the test results of test samples with different geometries. In order to keep the number of test samples in manageable limits, the customary approach used in concrete construction was adopted for the statistical evaluation of the tests performed in this project. However, the mean value and the standard deviation was determined from all test results that could be grouped together for the purpose of this evaluation due to their location in the quarry. It may be assumed that, with a certainty of 90%, no more than 5% of the strength values of the basic entirety, with respect to the lot in question, will fall short of the specified or comparative value. The calculation of the lower fractile value was performed from the mean value of the respective test results and the standard deviation, which must be multiplied by a factor depending on the number of test samples. fksfyks
(3)
where f k is characteristic strength (5% fractile), f is mean value strength, k is factor in accordance to the number of test specimens and s is standard deviation. The factor must be selected according to the number of test results that can be grouped together, adhering to the basic rules of statistics w18x. A symmetrical normal distribution was assumed for the statistical distribution of the strength values on the basis of literature of known test results of Saxon sand-
Fig. 27. Control of the masonry.
stone, which eventually were confirmed during the course of the ongoing evaluations. 7.3.3. Dimensional inspections of the cut stones Aside from the strength tests, the geometry of the stones being prepared for use in construction must be inspected as well. This becomes all the more important, the smaller the maximum permissible and minimum possible joints are. The limiting value for the greatest permissible joint thickness resulted from the necessity to assure the required strength, while the smallest joint thickness depended on the placement technology employed. In the case of highly stressed masonry, it is no longer sufficient to inspect the individual dimensions. Possible deviations of flatness and form errors must also be verified (Fig. 27). 7.4. Mortar Aside from the natural stone, the mortar also has an influence on the load bearing capacity of the constructed masonry and thus must be supplied and processed in accordance with quality specifications. For that reason, the quality of the mortar must also be controlled in order to achieve a uniform strength of the masonry (Fig. 28, Table 1). Standard production mortar could not be used in the reconstruction of the Frauenkirche due to the special requirements. Mix specifications were developed within the scope of planning work for load bearing structures, which, however, required final adjustment under consideration of the actual raw materials and the equipment that was eventually used in construction. The designed mix ratio, after being put in more concrete terms, were first tested in the lab with a view towards their main characteristic values and subsequently in practical testing of experimental applications. Particular attention was paid to the compatibility of the mortar and the natural
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Fig. 28. Small dry mortar silo with an anger type forced mixer.
stone. While observing the specified test period, suppliers are obliged to perform complete qualification tests of the mortar in good time prior to the start of construction. In order to guarantee the production of mortar of uniform quality, quality inspections must be performed in regularly scheduled intervals during processing of the mortar. The scope of the quality inspection is limited to the major characteristic values. The comparative values
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should be based on the results of the preceding qualification test of the same designed mix ratio. With regard to the test procedures, measures should be taken to adjust the test conditions and the uniformity of the geometry of the test samples between the supplier and the construction management. The mortar tests are performed on standard prisms (160=40=40 mm3). They are prepared in the customary metal mould and then stored, until the seventh day, in a climate of 20 8C and 95% relative humidity and, until the time of the test, at 20 8C and 65% relative humidity. The test of the compressive strength is performed in accordance with DIN 18555, Part 3 w19x. Experience in handling of various different test sample geometries was gained in the course of the reconstruction project of the Frauenkirche w20x. The corresponding form factors depend on geometry, structure, age and strength. An adoption of form factors from the sector of concrete and cement is not permissible. When applying the guideline of the Deutsche Gesells¨ Mauerwerksbau w21x, it should always be chaft fur noted that this guideline is of a supplemental nature and the test results may fluctuate considerably depending on the materials used. In this case, special preliminary tests are advisable. The initial attempt of using test samples with deviating geometries in connection with lime mortars did not prove successful. In case of mortar for thin joints, the essential increase in strength takes place during the first 28 days. Thus, the 28-day strength of these mortars is meaningful, as far as quality inspections are concerned. In case of longer test periods, the reaction to deviations becomes
Table 1 Characteristics for qualification tests and quality inspections of the mortar to be used in reconstruction Value
Qualification test
Quality test
Dry mortar Mixing ration Grading curve of aggregates Content of soluble alkali in relation to proportion of binding agent
= = =
= = =
Fresh mortar Consistency (slump value, flow value) Raw density of fresh mortar Air content Water content with waterybinding agent ratio Pot life Correctibility time (thin layer mortar)
= = = = = =
= = = =
Solid mortar Raw density of solid mortar Bending tensile strength Compressive strength Share of soluble alkali and sulfates Adhesive shear strength Water absorption with saturation value Amount of shrinkage
= = = = = = =
= = =
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rather more difficult. In case of predominantly lime mortars, the 70-day strength must be determined, at least within the scope of the qualification test, even though the corresponding 28 day is used for the purpose of quality inspections. In this case, a correlation to the 70-day strength must be established at the time of the qualification test. Aside from quality control measures performed parallel to manufacture and construction, follow-up checks of newly constructed masonry as well as old masonry may be necessary, if applicable. For this purpose, test samples may be cut from the masonry and tested in accordance with the DGfM guideline w21x. However, in case of thin joints of 6-mm thickness, extraction and testing of samples becomes questionable. The procedures followed in the reconstruction of the Frauenkirche have been proven for masonry of the various requirement categories with thicker joints, on the basis of cuboides 20=20=12 mm3. 8. Summary Valuable experience in the use of natural masonry with appropriate static stresses was gained in the reconstruction of the Frauenkirche. These experiences not only include load bearing behavior but also issues of fire protection, durability, manufacturing technology and quality control. Guidelines have been established for planning, execution and quality control, guaranteeing the achievement and realization of design specifications for the masonry, while at the same time providing a comprehensive documentation of the material used in construction. This documentation can be very useful and necessary for future assessments, particularly in case of difficult engineering projects involving historical structures. Acknowledgments The represented results were acquired within the scope of project planning for the reconstruction and financed by the Foundation Frauenkirche Dresden. The authors wish to thank the Foundation Frauenkirche and Building Director Dipl.-Ing. E. Burger for the opportunity to participate in this project. References w1x Jager ¨ W. Bestandserkundung, materialtechnische Untersuchun¨ gen, Bemerkungen zur ingenieurtechnischen Ausfuhrung und ¨ den Wiederaufbau der Frauenkirzu Tragwerksalternativen fur ceh Dresden. Wissenschaftliche Zeitschirft der Technischen ¨ Dresden, vol. 45. Sonderheft zum Wiederaufbau Universitat der Frauenkirche Dresden; 1996. p. 54–8. w2x Jager ¨ W, Pohle F. Einsatz von hochfestem Natursteinmauerwerk beim Wiederaufbau der Frauenkirche in Dresden. MauerwerkKalender. Berlin: Ernst & Sohn, 1999. p. 729 –55.
w3x Jager ¨ ¨ W, Wenzel F. Der archaologische Wiederaufbau der Frauenkirche zu Dresden als Ingenieuraufgabe—Zur Entwicklung des statisch-konstruktiven Konzepts und zum Stand der Umsetzung. Beton-und Stahlbetonbau 1996;91(11):257 –76. w4x DIN 1053, Teil 1: Mauerwerk; Rezeptmauerwerk; Berechnung ¨ und Ausfuhrung. Berlin: Beuth Verlag; February 1990. w5x DIN 1053-1, 11-96: Mauerwerk. Teil 1: Berechnung und ¨ Ausfuhrung. Berlin: Beuth Verlag; November 1996. w6x DIN 1053-2, 11-96: Mauerwerk. Teil 2: Mauerwerksfestigkeit¨ sklassen aufgrund von Eignungsprufungen. Berlin: Beuth Verlag; November 1996. w7x Lambrecht J, Weiß Ch, Hennig GK, Jager ¨ W. Die Armierungseisen der Dresdner Frauenkirche. In: Die Dresdner Frauenkirche. Jahrbuch zu ihrere Geschichte und zu ihrem Wiederaufbau, vol. 4. Society for the Support of the Reconstruction of the Frauenkirche Dresden e.V. & Foundation Frauenkirche Dres¨ den. Verlag Hermann Bohlaus Nachfolger Weimar; 1998. p. 101–22. w8x Berndt E, Schone ¨ I. Ein Bemessungsvoirschlag fur ¨ Mauerwerk aus Elbsandstein auf der Grundlage experimentell ermittelter ¨ Tragfahigkeiten. Erhalten historisch bedeutsamer Bauwerke. ¨ Baugefuge, Konstruktion, Werkstoffe. Sonderforschungsbereich 315, Yearbook 1992. Berlin: Ernst & Sohn; 1994. S. 41– 60. w9x Berndt E, Popp Th, Bergander H. Untersuchungen zur Auswahl ¨ eines besonders tragfahigen und geeigneten Natursteinmauerwerkes aus Elbsandstein. Compiled on behalf of the Foundation Frauenkirche Dresden by the Engineering Partnership Frauen¨ kirche Dresden Prof. JageryProf. Wenzel. Dresden; 1993, unpublished manuscript. w10x Berndt E. Gutachterliche Aussagen zur Schub-Druck-Tragfa¨ ¨ Quadermauerwerk aus Elbsandstein. Bemessungshigkeit fur gleichungen, Versuchsbericht mit Auswertung. Compiled on behalf of the Foundation Frauenkirche Dresden in collaboration with the Engineering Partnership Frauenkirche Dresden Prof. ¨ JageryProf. Wenzel. Dresden September; 1995, unpublished manuscript. w11x Bergander H, Jager ¨ W, Fleischer Th. FEM-Modellierungen beim Wiederaufbau der Frauenkirche in Dresden. Tagungsband 3. FEMyCAD-Tagung Darmstadt, Erfahrungsaustausch und Technologietransfer der Baupraxis am 10.y11.03.1994 at the TH Darmstadt (Ed.) Darmstadt: TH Darmstadt; 1994. p. 169– 76. w12x Mauerwerksrichtlinie Frauenkirche. Richtlinie zur Beurteilung ¨ der Mauerwerkstragfahigkeit und Anforderungen an die Aus¨ fuhrung von Sandsteinmauerwerk. Worked out by the Engineering Partnership Frauenkirche Dresden under co-operation with the Working Group ‘Masonry’. Dresden; 1996, unpublished manuscript. w13x Berger F. Zur nachtraglichen ¨ ¨ Bestimmung der Tragfahigkeit ¨ von zentrisch gedrucktem Ziegelmauerwerk. Erhalten historisch ¨ bedeutsamer Bauwerke. Baugefuge, Konstruktion, Werkstoffe. Sonderforschungsbereich 315, yearbook 1986. Berlin: Ernst & Sohn; 1994. S. 41–60. w14x Burkert T, Weller B, Jager ¨ W. Investigation into structural design of the sandstone surface layer for the dome reconstruc¨ tion of the Dresden Frauenkirche. In: Brebbia CA, Jager W, editors. Structural studies, repairs and maintenance of historical buildings VI. Southampton, Boston: WIT Press, 1999. p. 875 – 85. w15x Grunert S, Szilagyi J. Untersuchungen zur Auswahl von Stein¨ die Kuppel der Frauenkirche Dresden. Final report. material fur Compiled by the Engineering Partnership Frauenkirche Dres¨ den Prof. JageryProf. Wenzel on behalf of the Foundation
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w16x
w17x
w18x
w19x
Frauenkirche Dresden. Institute for Geotechnique, University of Technology Dresden, Dresden; 1998. ¨ ¨ ¨ Muller HS, Horenbaum W, Jager W. Investigations of sandstone ¨ for the reconstruction of the Frauenkirche in Dresden. In: Jager W, Brebbia CA, editors. The revival of Dresden. Southampton, Boston: WIT Press, 2000. ¨ Jager W, Gigla B, Preusche Th, Pohle F, et al. Wiederaufbau der Frauenkirche zu dresden. Mauerwerksrichtlinie Frauenkir¨ che. Instrumente des Qualitatssicherungssystems (QSS)— Checklisten. Unpublished manuscript. Compiled by the ¨ Engineering Partnership Frauenkirche Dresden Prof. Jagery Prof. Wenzel on behalf of the Foundation Frauenkirche Dresden. Dresden; 1997. Storm R. Wahrscheinlichkeitsrechnung, mathematische Statis¨ tik und statistische Qualitatskontrolle. Leipzig: Fachbuchverlag, 1986. ¨ ¨ DIN 18555, Teil 3. 09.82. Prufung von Morteln mit mineral¨ ischen Bindemitteln; Festmortel; Bestimmung der Biegezugfes-
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tigkeit, Druckfestigkeit und Rohdichte. Berlin: Beuth Verlag; 1982. w20x Jager ¨ ¨ W, Pohle F. Ermittlung von Umrechnungsfaktoren fur ¨ ¨ ¨ Mortelprufkorper unterschiedlicher Geometrien—Zwischenbericht. Worked out by Ingenieurgemeinschaft Frauenkirche Dres¨ den Prof. JageryProf. Wenzel by order of the Foundation Frauenkirche Dresden. Dresden Mai; 1998, unpublished manuscript. w21x Vorlaufige ¨ ¨ ¨ Richtlinie zur Erganzung der Eignungsprufung von ¨ Mauermortel; Druckfestigkeit in der Lagerfuge; Anforderun¨ ¨ Mauerwerksbau e.V., gen, Prufung. Deutsche Gesellschaft fur Bonn August 1992. w22x Burkert T. Untersuchungen zur baukonstruktiven Ausbildung und zum Verwitterungsverhalten der Kuppeldeckschicht beim Wiederaufbau der Frauenkirche Dresden. Dissertation, Dresden University of Technology, Institute for Building Technology in co-operation with the Chair for Load-Bearing Structures. Dresden; 2001.