The paper describes a structural survey of the brick arched bridge from the 17th century located in Portz Insel near the city Mikulov—in the historic cultural landscape on the Moravian-Austrian border. The bridge is consummate engineering work, originally equipped with a wooden lifting deck, reaching a respectable length of 91.5 m and with 3.7 m. It consists of 15 semi-circular vaulted arches divided by pilasters with double sizes pyramidal edges. It fascinates by its age and it is unparalleled as to the selection of the building material and design. The basic material analysis of the physical and mechanical properties of bridge bricks (moisture, water absorption, resonant frequencies, tensile and compressive strength) were tested including the assessment of the freeze resistance of the original bricks confirming their high quality and durability. The construction was not far from sinking into oblivion until recently when the reconstruction was carried out from 2019 to 2020 and the bridge masonry has been stabilised. Thanks to the inimitable quality of the bricks the construction has survived. The reconstruction of the bridge was awarded the title Monument of the year 2020 by National Heritage Institute.
- historic bridge
- structural survey
- experimental testing
- historic technologies
The large cultural landscape along the border of South Moravia and Lower Austria used to be one continuous area. A varied array of unique monuments related to the evolution of the landscape from ancient times to the present has been preserved. This area became rich thanks to the extraordinary financial potential of the then landowners (the Dietrichsteins, the Liechtensteins, and many other noble families). Their wealth, broad cultural vision, purposefulness, ambition and other contemporary values, along with the effect of positive rivalry, are permanently imprinted on the distinctive character of the landscape.
The project “
The completed project aimed to restore the historically valuable area situated in the locality of the peninsula named Portz Insel. It used to be part of the designed peri-urban landscape, whose development was commissioned by Franz Cardinal Dietrichstein in the 17th century. It was one of the earliest generously urbanised landscapes in Moravia and likely also in the larger territory north of the Alps. The island complex, accessible over the long brick bridge supported by arched abutments and connected to the City of Mikulov through a two and a half kilometre long allée, originally an access road for carriages, was an extraordinary work of architecture of its time. It was still described by historians as a landscape jewel of unique beauty in the late nineteenth century .
Extraordinary changes spread through the Mikulov region’s cultural landscape in the extremely fruitful era of the late Renaissance and early Baroque, associated with the well-known members of the Dietrichstein aristocratic family. Pond cascades, stream and millrace beds, water mills and fisheries, avenues of trees and orchards, bridges and summer houses all created an almost magical landscape of which, regrettably, only fragments have survived. Outstanding among these fragments, the complex near the pond formerly named Portz, harbours the summer residence and ruins amidst the forest. In the 17th century, the complex was surrounded by gardens and cultivated vegetation, offering enjoyment and respite, and showing the status of the Mikulov Princes of Dietrichstein. Franz Cardinal Dietrichstein is a widely known figure of central European history in the early modern times. He would purposefully bring to Moravia the period concepts of Italian Recatholization culture that he was intimately familiar with .
2. Historic brick bridge
The bridge was located at the narrowest point of the pond (Figure 2), still reaching a respectable length of 91.5 m, its width being 3.70 m. It consists of semi-circular vaulted arches, divided by pilasters with double-sided pyramidal edges (Figure 3). The bridge deck was lined with a solid, approx. 75 cm high parapet wall. The outer fully bricked sections were ended by short open wings with decoratively shaped fenders at the ends (Figure 4).
The tenth section from the south had originally been equipped with a wooden lifting deck; it was still present on the maps from as late as the 1830s (Figure 5), but shortly afterward it was removed and the empty section was bricked up similarly as the other parts of the bridge. After being vaulted, the part of the deck in the originally unfilled segment was also paved with limestone blocks (Figure 6) .
The existing intact paving in the entire area of the bridge deck made of limestone blocks was found during the restoration, accompanied by an archaeological survey. The paving is well-preserved on about 90% of the surface and during the fieldwork was carefully covered. The stone pavement had an approx. 5–10 cm thick layer of fine limestone gravel on it.
The engineering design of the bridge was very well thought-out. The part of the columns below and slightly above the water level was made of large blocks of hewed limestone, the remainder was built of brick (Figure 7). The bridge arches themselves widen at two points towards the base and the bricks on the front face are hewed in the shape of a decorative arch keystone. Also, the pyramidal shape of the edges was obtained through the final hewing of the brick masonry to shape (Figure 8). According to the documentation, the bridge edges were plastered. On the surface of several bricks, there are still noticeable plaster-coat remains.
Uniquely preserved is the drainage solution for the bridge deck, using drainage openings lined with curved roof tiles, known as tegula. Based on samples of the original historical material of the bridge deck and their laboratory evaluation, it was found that the original bridge deck was threshed on an aluminous-stone backfill with clay waterproofing at the level of the drainage pipes. The thickness of the clay layer was about 60–100 mm.
The Mikulov estate had sufficient quarries supplying stone for construction elements of the bridge as well as for its pavement and penning of the deck. The 17th century land registers record 13 of them. They were in the different localities of the estate, limestone was predominantly quarried between Mikulov and the Portz Island (geological locality Mušlov is a former large sandpit, the main rock type is sands with algal limestone bodies, containing mollusc fauna). Bricks, too, were of local origin, as shown by their marking with the letter N—Nicolsburg (currently Mikulov) (Figure 9), a mark or production stamp of identification. The bricks are slightly larger than the current solid fired bricks (approx. 150 mm/300–310 mm).
Good quality of original bricks was given by the technology of the production process, in the 17th century, building materials were produced more slowly. The bricklayers of that time stuffed the clay into the moulds and let them dry for a month before firing. This production process guarantees high-strength bricks. In the first half of the 17th century there was only one brickyard in Mikulov, but with its three kilns surprisingly large for its time. Given the same technology, it was also possible to burn lime at the brickyards, used in turn to make mortar for the bridge construction. In the mid 17th century, perhaps owing to the consequences of the Thirty Years´ War, only two brick kilns were left in operation.
The golden era of the summer residence complex suddenly came to an end in 1872 when, in consequence of building a railway line to Mikulov (Figure 10), the pond was drained and the bridge lost its main function. It continued to serve as the crossing over the Rybniční brook. The pond was later restored in the northern surroundings of the island, though due to the railway line it could not be refilled in its original scope, its southern part changed into a wetland forest and meadow. The island became a peninsula and the essentially disused bridge was no longer maintained. Thanks only to the inimitable quality of the bricks made by Mikulov’s skilful brick makers of the 17th century, the construction has survived until today. As the whole area had been tightly enclosed over the period of 40 years under the totalitarian regime, the bridge was in a state of disrepair (Figure 11), when the restoration was carried out from 2019 to 2020.
The bridge is, in all respects, a unique structure also on a broader European scale. It fascinates by its age, and it is unparalleled as to the selection of the building material and design. It differs from the common pond and river bridges also in its function. Being built as part of a well-considered architectural plan including the other buildings on the island and landscaping, its form elevated the aesthetic effect of the whole majestic complex. The builder had not only in the mind the visual impact of the bridge, but also, through the incorporation of the lifting deck as an element of defensive architecture, he sought to create an impression of a fortress-like character of the place. It is a consummate engineering work that leaves us with the question of who was its originator—perhaps one of the northern Italian fortress master builders, who at that time worked for influential, politically active aristocrats in Bohemia and Moravia.
3. Structural survey of the historic brick bridge
One of the fundamental preconditions of the structural survey is detailed analyses of the consequences of the interaction between the external environment and degradation processes in time including and taking into account namely the specificities of repeated cyclic temperature and moisture effects. In prominent cultural and historical monuments, these requirements are of utmost significance and necessity. Protection of structures against these effects is a fundamental part of preservative measures avoiding degradation. Mechanical states of stress due to these effects very often exceeded the stresses and deformation caused by force effects. In the case of moisture, attention must also be paid to chemical and microbiological degradation processes strongly dependent on cyclically changing climatic conditions.
Main objectives of the survey:
Monitoring and evaluation of non-force effects and influences affect historic structures
Setting reliability and time dependencies in relation to defined properties of the exterior environment
Monitoring of effects of the intensity of degradation and durability on applied bricks
Classification of degradation processes in individual parts of the brick bridge
Complementation of basic material analysis of the bridge: physical and mechanical properties of bricks (moisture, water absorption, capacity, resistance, tensile and compressive strength, modulus of elasticity)
Design of efficient direct and indirect rehabilitation measures to be applied in historical bridge structures.
3.1 Laboratory testing of bricks
Experimental testing remains the most practised approach towards remedial techniques and information obtained in real buildings can be very useful, especially if the systematic survey was performed.
A total of 14 pieces of original bricks from the 17th century were delivered to the laboratory. The set was marked “N” according to the brand, which most likely means the designation of origin (Nicolsburg—Mikulov). Depending on their appearance and dimensions, the bricks actually correspond to early Baroque bricks—they are lower and wider than usual for bricks from a later period as probably brick N 12 (marked “GN” and other shapes). The bricks, with the exception of brick N 12, had a similar appearance, shard colour and dimensions. Two samples were stored in the archive, 12 pieces of bricks were selected for testing (Figure 12).
The samples were divided according to non-destructive tests—ultrasonic and resonant. If the samples were divided into sets only on the basis of a visual impression or even at random, the results could be significantly affected. Based on the long-term experience, the first individual frequency of transverse oscillation was used as crucial for the brick classification. The frequency of oscillation responds very sensitively to both the quality of the material and, in particular, the internal faults in the material, outside invisible.
3.1.1 Brick bulk density test
The bricks were first dried at 105°C to a constant weight, then they were measured, weighed and the bulk density was calculated in the dried state ρd, u.
The bulk density of individual brick samples was relatively uniform, ranging from 1546 to 1641 kg/m3.
3.1.2 Brick water absorption test
Furthermore, they were subjected to a water absorption test, and the water absorption of the bricks was calculated from the difference in weights in both boundary moisture states. The weight absorption was also very balanced—from 15.4 to 18.1% (for a different sample N 12).
3.1.3 Brick frequency test
The bricks were selected for the following sets according to the resonant frequencies as well as the anomalies in the frequency curves:
Frozen set—bricks N 1, N 3, N 5, N 7, N 10 and N 12,
Comparative (unfrozen) set—bricks N 2, N 4, N 6, N 8, N 9 and N 11.
In addition to the natural oscillation frequencies, the magnitude of the oscillation amplitude and the sharpness of the frequency curve are also monitored. High oscillation frequency, clear and sharp oscillation amplitude (basically high, clear and clean tone, which the brick sounds when knocking) is typical for quality material without defects, lower frequency together with indistinct or double amplitude, on the contrary, indicates defects or discontinuities in the internal structure of the brick. All samples showed very sharp vibration curves, except for samples N 3, where there were signs of internal structure defects .
3.1.4 Brick freeze resistance tests for 25 cycles
According to ČSN 722609 technology of brickmaking, the samples selected for frost resistance tests were first saturated with boiling. Then the resonant oscillation frequencies were measured again. This was followed by alternating freezing and thawing for 25 cycles in an automatic freezing device including the calculation of the so-called relative dynamic modulus of elasticity.
From the results, it is evident that practically all samples passed on frost resistance after 25 cycles, only in sample N 3 found anomalies in the longitudinal resonance frequency. It was a sample that resulted in a greater decrease in resonance frequencies in the saturated state, which can predict a certain disorder in the internal structure. In addition, the samples were dried and subjected to a tensile strength test for bending and strength in pressure.
3.1.5 Brick tensile and compressive strength
The average value of the flexural tensile strength was 3.64 MPa for the comparative samples, and 4.26 MPa for the frozen ones. This is given by the selection criteria into individual sets—the better bricks have been selected for freezing, but it is clear that virtually all samples have safely passed on 25 freezing cycles. The exception is sample N 3 (fb,f = 0.94 MPa), for which resonant test indicated a possible failure in the internal structure. This was indeed reflected in the bending strength test, which was significantly lower than that of the other samples.
According to ČSN 722609, the compressive strength after freezing shall not decrease by more than 15% against the declared compressive strength, in this case against the strength in the pressure of the comparative (non-frozen) bodies. The average value of the bend tensile strength was 22.6 MPa, for frozen bricks up to 24.8 MPa. This is because it is basically unrealistic to create two completely comparable sets of bricks, on the other hand, it is indicative of the excellent frost resistance of the original bricks from the bridge. According to the comparative bodies, the original bricks can be attributed to the strength of the P20 (the mean compressive strength of 22.60 MPa, minimum 19.2 MPa). From the point of view of frost resistance, when the main criterion is the drop of compressive strength, all samples have been passed for 25 cycles .
The material analysis confirmed the good quality of the bridge bricks and together with evaluation of the quality of the environment, it provided grounds for establishing the principal degradation agents. During the visual inspection the following failures of the bridge were identified:
dampness, non-functional waterproofing system (clay sealing and ceramic drains),
The historic brick bridge may serve for demonstrating the seriousness of external effects, their growing aggressiveness and the result of mutual structure environment interaction including the surrounding climatic and biological ecosystems and anthropogenous factors .
3.2 Mechanical failures
Mechanical failures were mainly manifested by cracking ranging from hairlines to prominent tension and shear cracks, going through the joints of brickwork masonry and stone blocks. The extent and intensity of masonry degradation of individual vaults arches varied. Some vaults’ arches were damaged by prominent longitudinal cracks up to several millimetres wide which extend through several masonry layers. The respective cracks were mostly situated too close to vault edges. Other cracks mostly local non-continuous ones were situated at various points and did not follow any traceable patterns. Parapet walls were damaged by loose bricks and cracks in the footing bed joint between the vault and the wall.
3.3 Moisture analysis of the masonry
In terms of dampness the foundation masonry and pillars, which were currently covered with soil, were the most stressed. The moisture penetrated the structure through direct contact with the soil of adjacent terrain. The high moisture of the masonry was detected at the side of the performed probes. There was direct flooding of the probes. Building materials were disturbed and the binder was being washed off.
Rainwater was the main reason causing the high dampness of the bridge’s brickwork. The water from the bridge deck seeped into layers of the backfill up to the vault (the content of moisture reached 15% by weight on top of the vaults). On several places there is no backfill, the rainwater affected directly the vault and further seeped to its lower face. Diversion of water was supposed to be done by the brick drainpipes. Considering the state of the pipes, it was possible that water levels were locally higher within particular sections of the bridge.
One of the possible causes of collapsing of the parapet wall was saturation with water from the entire backfill due to broken and non-functional waterproofing and thus increase of active pressure on the back of the parapet wall and increase of tension on the face side, which masonry was no longer able to transmit.
A very exposed part of the bridge is face surfaces—parapet walls, cornices and masonry railings. The wind-driven rain penetrated the masonry. On the windward side, there is a synergy of negative phenomena, which caused significant degradation of the masonry due to weathering manifested by leaching of the binder and scaling of surface layers. The arches were covered with soil and airborne vegetation.
Based on the analysis the summary of reasons could be stated:
4. Reconstruction of the historic bridge
Starting summer 2019, a vast reconstruction of the bridge structure had carried out. During the fieldwork after lowering the terrain around the bridge, a problem of much a worse state of the pillars in their lower part, as well as a higher level of groundwater, arose. Some of the pillars had to be statically supported.
The reconstructing the historic bridge is described briefly in the following processes:
Digging up the terrain around the bridge.
Vegetation surrounding the bridge was removed. Subsequently, all historical bricks from the area around the bridge were collected, cleared up and stored. The bricks were cleaned from moss and other vegetation.
Diversion of the stream and remediation of pillars in the original stream bed, damaged bricks replacement.
In order to enable repairs and remediation of the middle pillars of the stream bed, it was crucial to temporarily divert the stream bed. The terrain around the pillars was manually dug away while the vaults were gradually supported (Figure 14). Within deeper layers, it had to be needed to drain the leaking groundwater.
Damaged and irregularly shaped bricks had to be taken away carefully and replaced by new replicas produced of at least the same quality compared to the original ones, meaning the quality of “klinker brick these bricks were inserted into cleaned holes after missing or removed ones in order to be statically integrated (activated) at the place of contact with the original brickwork by a special and appropriate expandable mortar mixture—a filler substance intended for caulking of load-bearing joints, the substance is freeze and weather conditions resistant as well.” After returning the stream into its original position, gradual remediation of other bridge piers followed.
When dismantling the masonry it was necessary to preserve or avoid breaking the original backfill especially in the area below the level of the drain pipes, meaning under the clay waterproofing. Historical backfill above this level had to be cleared of roots and contaminating soil and preserved for reuse, i.e. to replenish the lower part of the backfill. Replicas of hand-fired bricks were made after laboratory analysis. A lime mortar was used for masonry. Replicas of the drainpipes were installed in the original places just above the clay waterproofing on the same slope. These replicas were made of fired ceramic precisely according to the preserved remains of the original historical pipes, the exact length was determined on-site so that the face of the parapet walls is exceeded at least 40 mm (Figure 15). The pipes were hydrophobized from inside before mounting.
Based on the probes for ascertaining the structure condition below the terrain, it was decided that the soil was removed only to the level of limestone blocks, not to the base of stone packing, in order to avoid possible movements of the stone foundations.
Discovered original bricks were used to complete the lower faces of the vaults. Leaning-out retaining walls at the beginning and the end of the bridge were dismantled and straightened up. The described situation was repeated in other sections—it was essential to perform manual demounting of existing bricks from damaged parts including their cleaning and reusing in particular sections (Figure 15). Based on the experimental analysis the original bricks were compared with brick production in a nearby brickyard. Laboratory analysis revealed that the newly produced bricks were not suitable for use due to poor frost resistance and it was necessary to find an adequate replacement of another supplier.
Completion of the original fillings and construction of the road threshing surface.
The bridge construction that connected the Portz Island to land was not far from sinking into oblivion until recently. Thanks only to the inimitable quality of the bricks made by Mikulov’s skilful brick makers of the 17th century, the bridge construction has survived until today (Figure 16).
Based on the presented extensive review on reconstruction and assessment of the historic brick bridge, the following conclusions are drawn:
Samples of original bricks from the 17th century taken from the “Portz Insel” bridge, have been tested for the assessment of their freeze-resistance. On the basis of the test results, the original bricks from the bridge comply with the requirements for the strength mark P 20. These bricks from the bridge construction have been exposed to external influences over a period of approximately 280 years. At the same time, they are visually very similar, do not differ either of their dimensions nor the volume mass or the absorbency. Practically there are no cracks in them, the brick brittle fracture under tensile test in bending was mostly straight, the ceramic shard is also good quality and well-burned. It is obvious that these are extremely well-produced and durable bricks.
Currently, the bridge masonry has been stabilised. The accuracy of the results critically depends on the availability of all input parameters (materials features, meteorological data and soil conditions), hence, the results could be used to predict the behaviour of existing structures. Nevertheless, experimental testing remains the most practised approach, especially if the systematic survey was performed.
The main benefit of the project is the suggestion in using historical technologies and traditional material solutions for the purpose of reconstruction (bricks, clay sealing in combination with ceramic drainage). The proposed historical materials are inert, environmentally friendly and renewable with long service life. The project shows that historical materials together with modern technologies can be profitable for optimal realisation the consummate bridge was saved for the future.
The bridge restoration was awarded for the best reconstruction of the year 2020 by the National Heritage Institute.
The results presented in this article were obtained in the framework of institutional research of the Faculty of Civil Engineering CTU in Prague. Stated facts were analysed in relation to the design and realisation of the reconstruction of the historic bridge located in Portz Insel. Acknowledgements belong to everyone who participates in the reconstruction with the financial support of the Project Interreg Austria—Czech Republic.
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