Protection of Historical Buildings, PROHITECH 09 – Mazzolani (ed)© 2009 Taylor & Francis Group, London, ISBN 978-0-415-55803-7

The effectiveness of strengthening historic brick vaults by

contemporary methods

J. Jasienko, Ł. Bednarz & T. Nowak

Wrocław University of Technology

ABSTRACT: This paper discusses problems relating to the technical condition of historic brick arches andvaults. Different methods of strengthening such brickwork structures, with reinforcement placed on the extradosconsidering that there are usually frescos and other decorations on the intrados, are described. The results of testson a technical scale, carried out in the Laboratory of the Institute of Building Engineering at Wrocław Universityof Technology are presented. Moreover, the effectiveness of the different reinforcements used (FRCM nets andFRP strips) is assessed.

1 INTRODUCTION

Historic brick domes, vaults and arches under theinfluence of various factors (changes in the load-ing configuration, material degradation, temperature,dampness, subsoil settlement, conversion to otherfunctions, a change in water-soil relations, delayedphenomena, etc.) often fail to perform their origi-nal structural functions. It happens that the originaldesign solutions and assumptions no longer hold goodwhereby the structures undergo new static loadingschemes. In order to prevent damage to historic struc-tures, measures improving their static performance andstrengthening them should be taken.

Damaged and cracked vaults and arches ofchurches, castles, palaces, gate buildings, bridges andresidential houses can be saved in many different waysdescribed in, e.g., [1], [2], [3], [4], [5]. The commonlyused repairs and reinforcements of curved brickworkstructures are:

– stays;– reinforced concrete coatings;– stuck on and glued in steel plates and bars;– timber-frame and steel-frame constructions for sus-

pending ribs and squinches;– mats, strips and nets made of FRP materials;– other (injection, impregnation, re-erection).

The static behaviour of historic structures after rein-forcement depends on the multiplane process of thephenomena occurring in the reinforced structure. Forthis reason, as opposed to newly erected structures, theanalysis of the static performance of reinforced struc-tures involves the checking of the following three limitstates:

– the ultimate limit state of the (composite) structureafter reinforcement,

– the serviceability limit state of the (composite)structure after reinforcement,

– the ultimate limit state of the bond between thereinforcing element and the reinforced one.

Historic buildings typically include brick arches anddomes representing the cultural heritage and so theirconservation is of major importance.

2 STRENGTHENING METHODS

Innovative materials and technologies are used forstrengthening historic buildings. Particularly effec-tive are fibre composites since considering their lightweight they show the best strength properties. Com-posites with polymer matrices (thermoplastic andthermohardening resins) and with mineral matrices(FRCM), reinforced with fibres and (carbon, graphite,glass, boron and aramid) fabrics are mainly used.Recently a technology of reinforcing historic brick-work structures with SRP/SRG (Steel-ReinforcedPolymer/Steel Reinforced Grout) laminates has beenintroduced. Such materials have very good parameters,can be quite easily manufactured and are relativelyinexpensive.

FRP (Fibre Reinforced Polymer) composites areincreasingly often used to reinforce and repair wholemasonry structures and their individual components(e.g. vaults, arches and domes), especially historicstructures which need to be maintained in servicecondition for cultural and safety reasons.

One of the first reinforcements of this kind (com-posite strips laid latitudinally in order to stop deforma-tions increasing as a result of tension) was applied to adome inThessaloniki in Greece.Aramid fibre compos-ites were used to strengthen the vaults in the Basilica ofSt. Francis of Assisi in Assisi and to restore their load-bearing capacity [6] after they had been damaged byan earthquake and required immediate intervention toprevent damage to the surviving domes. In the years

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that followed composites began to be increasinglyused to reinforce arches, vaults and domes, mainly inItaly [7], but also in other countries. In Poland thebarrel vault in the Church of St. Peter and St. Paulin Cracow, the dome of the Cathedral in Lublin andthe arches and vaults in the Archaeological Museumin Cracow were reinforced in this way. Studies on thestatics of reinforced arches were presented in, e.g., [8].Brick arches reinforced with FRP strips, subjected tostatic loading were tested. The test results quite clearlyindicate the suitability of this kind of reinforcement forrepairing curved brickworks structures. Similar stud-ies on the behaviour of brickwork structures reinforcedwith FRP are presented in [9]. The effect of differentkinds of reinforcing fibres (carbon, glass) and that ofthe location (on the intrados or extrados) of FRP mate-rials were experimentally examined. On the basis of theexperimental results the mechanism of failure of thereinforced arches was determined and analytical mod-els were defined. Despite the increasing use of FRPmaterials in reinforcing brickwork structures there areno fully reliable models for designing, and furtherresearch in this area is needed. In order to explain thebehaviour of brick vaults reinforced with FRP, researchwas undertaken in the University of Padua [1]. Sixmodels were tested under a static load. The verticalload was applied in a location most unfavourable forthe arch so that it could also be used in seismic loadsimulations.

The aim of the tests was to:

– compare the effect of various composite materi-als (CFRP strips and GFRP mats) placed on thedifferent sides of the vault,

– develop an analytical model for the estimation of theultimate force, depending on the failure mechanism.

It was found that reinforcement in the form of CFRPstrips stuck on the intrados and extrados significantlychanges the pattern of pressures in the brick arch.The proposed theoretical model was experimentallyverified and compared with similar (but rather few)models available in the literature. The load-bearingcapacity of the reinforced cross section (subjected tocompression and bending) depended on the magni-tude of the compressive force and the strength of thestrip. Assuming linear elastic behaviour for the rein-forcing material and a rectangular stress diagram forthe masonry, a mathematical calculation model wasproposed. Laboratory tests yielded promising resultsconcerning the model. Investigations of eight brickarch models are presented in [11]. The reinforcementwith GFRP materials was made in different versions(reinforcement on the intrados, reinforcement on theextrados, continuous reinforcement, segmental rein-forcement). It was shown that there was a dependencebetween the different reinforcement versions and thearch collapse models differing from Heyman’s clas-sic four-hinge mechanism [12]. The collapse modelswith the hinges formed and the test results show thatreinforcements of this type are suitable for brick archstructures. FRP materials used to reinforce curved

brickwork structures on the intrados must be anchored.When there is no anchorage the reinforcing materialtends to get unstuck from the structure. The ItalianCNR manual [13] seems to be interesting and usefulin the design of reinforcements based on compositematerials.

The above brief survey of the world-wide researchin this field indicates that the use of innovative tech-nologies of reinforcing historic brick arches and vaults(especially by means of innovative materials) has notbeen sufficiently explored yet.

2.1 New reinforcement technologies

A few years ago FRCM (Fibre Reinforced Cementi-tious Matrix) systems consisting of carbon or BPO(polybenzobisoxazole) fibre fabrics became the nov-elty. FRP (carbon fibre, Kevlar, glass, etc.) materialswhen used to reinforce structural members are bondedto the base using (epoxy or polyester) resins. Asopposed to FRP, in FRCM systems inorganic mortar,consisting of a hydraulic binding agent and additives(chemically, physically and mechanically compatiblewith the substratum, especially with masonry), is used.

The use of mineral mortar as the matrix bondingthe fibre reinforcement with the structural member hasmany advantages:

– high-temperature resistance similar to that of thebrickwork substratum;

– the reinforcement can be applied onto a damp sub-stratum (FRP systems usually can be applied onlyonto a dry substratum since polyester and epoxyresins do not bind in the presence of water);

– the reinforcement can be easily applied, even ontorough and irregular surfaces (the layer of mineralmortar evens out roughness whereby, as opposed toFRP systems, it is not necessary to presmooth thesubstratum);

– the task is easily executed: the material is mixed withwater and the mortar obtained in this way is appliedas ordinary mortar and subsequently the reinforcingfabric is embedded (sunk) in it;

– workability: there is not much difference in the useof the system in a temperature range from +5 ◦Cto +40 ◦C (because of the synthetic resins, FRPsystems’ range of temperature and dampness islimited);

– the FRCM system is safer than the FRP system(based on resins): when applying the mortar it isenough to follow the directions for the use of mineralmortars;

– the tools used in applying the mortar can be cleanedwith water (FRP systems require solvents for thispurpose and often they cannot be reused).

By reinforcing masonry structures with FRCM sys-tems the load-bearing capacity of the former can beincreased as a result of tensile stress distribution overa larger surface area. Moreover, some of the load istaken over by the reinforcement thanks to the strongbond between the mineral mortar with the embedded

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one or two layers of (e.g. carbon fibre) fabric andthe surface of the masonry structure being reinforced.Structural reinforcements made using FRCM compos-ites were tested in the Institute of Building Engineeringat Wrocław University of Technology [14].

3 DESCRIPTION OF TEST MODELS

The tests were carried out on several brick archeshaving the following dimensions: thickness – 0.12 m,width – 077 m, span – 4 m and radius – 2 m (fig. 1).

The arches, having the shape of a barrel vault sec-tion, were made from materials resembling the onesfound in historic buildings (grade 100 solid ceramicbricks and lime mortar), using a free hand techniqueto generate imperfections characteristic of historicvaults. The same static support and load configurationwas used for all the arches (fig. 1). All the arch mod-els were subjected to monotonically increasing staticloading applied to the whole width of the arch at onethird of the span until failure by a servomotor.

Each of the arches was differently reinforced.Model A1 – a reference (unreinforced) arch (fig. 2),Model A2 – an arch reinforced with an FRCM sys-tem (carbon fibre net embedded in mineral mortar onthe extrados) (fig. 3), Model A3 – an arch reinforcedwith two 150/2000 CFRP strips (100/1.4 in cross sec-tion) stuck at an axial spacing of 40 cm on the extradosand with an FRCM system (carbon fibre net embed-ded in mineral mortar on the extrados) (fig. 4), Model

Figure 1. Model of tested arch.

Figure 2. Model A1.

A6 – an arch reinforced with two 150/2000 CFRP strips(100/1.4 in cross section) stuck at an axial spacing of400 cm on the extrados (fig. 6).

The way in which the arches were reinforced isschematically shown in figs 2–5.

Care was taken to place the reinforcing materials onthe extrados (figs 6 and 7). This way of reinforcing isespecially suitable for historic arches richly decoratedon the intrados.

Only a few similar, comprehensive studies coveringso many methods (acceptable from the conservationdoctrine point of view) of reinforcing (particularlywith FRCM systems) historic brickwork structures,can be found in the literature on the subject.

Figure 3. Model A2.

Figure 4. Model A3.

Figure 5. Model A6.

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Figure 6. Actual view of one of tested arches (arch 2) with

mounted displacement gauges.

Figure 7. FRCM net being glued to CFRP strips on extrados

of arch A3.

4 TEST RESULTS

The test results show that the reinforcement has a verybeneficial effect on the load-bearing capacity and stiff-ness of the arches. The static equilibrium paths of thetested arches are compared in fig. 8.The boundary ulti-mate force (Fmax) values in arch models A1, A2, A3and A6 are shown in fig. 9. A considerable increase inthe ultimate force (in comparison with the referencearch) is observed for all the reinforced arches. Thereinforced models achieved much larger deflectionsthan the reference arch (fig. 10) at a simultaneous,considerable increase in the failure load.

The above particularly applies to the models rein-forced with carbon fibre fabric, in which deformationand collapse proceed very smoothly and slowly. Thereis no jump connected with the violent separation of thereinforcing material, as in the models reinforced withCFRP strips.

The change in the collapse mechanism (from theclassic three-hinge collapse to shear failure only on thesupports) and the increase in the load-bearing capac-ity of the arch models prove that the reinforcementsare effective and that they were correctly made. Incomparison with unreinforced arch A1, the collapsemechanism has changed in all the reinforced archmodels.

Figure 8. Paths of static equilibrium.

Figure 9. Limit values of failure force Fmax in tested models.

Figure 10. Deformations u in measuring point no. 3 (load

application point) in particular tested models.

This indicates a change in the static behaviour ofthe reinforced arches and confirms the effectivenessof the reinforcements.

One should also note the mechanisms of failure ofthe bonds between the reinforcing material and theparticular arches:

– in arches A2 and A3 the carbon fibre fabric togetherwith pieces of brick separated from the masonry,which shows that the failure (tearing) took place inthe brick;

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Figure 11. Utilization of load-bearing capacity of CFRP

strips in arch A3.

Figure 12. Utilization of load-bearing capacity of CFRP

strips in arch A6.

– in arch A6 the FRP strip separated together withpieces of brick from the brick substratum, whichindicates that the failure (tearing) also took place inthe brick.

The load-bearing capacity of all the reinforced archmodels increased considerably. The increase is verylarge: more than threefold (3.77) in arch A2, nearlysixfold (5.76) in archA6 and as large as ninefold (9.25)in arch A3.

Summing up the tests on the arch models on atechnical scale, one can state that:

– all the models were prepared having in mind thepractical use of the innovative techniques in theconservation of brickwork structures,

– the test results have confirmed that FRCM rein-forcement (carbon fibre net embedded in mineralmortar) can be used to effectively reinforce historiccurved brickwork structures,

– the FRCM systems have been found to be easilyapplicable to arch and vault structures,

– the tests have confirmed that the combination ofCFRP strips and FRCM composites is a viable solu-tion thanks to which the beneficial properties of thetwo kinds of systems can be exploited (the utiliza-tion of the load-bearing capacity of the CFRP stripsin the case of arches A3 and A6 is compared infigs 11 and 12).

5 CONCLUSION

The merits of reinforcement with FRP and FRCMcomposites become apparent in especially the con-servation of monuments. The main advantage is theratio of the reinforcing material weight and dimen-sions to the obtained increase in load-bearing capacity.Moreover, the reinforcement can be easily hidden inthe structure so as to preserve the latter’s originalappearance.

Besides FRP and FRCM composites, quite recentlySRP/SRG laminates have been introduced and begunto be tested with a view of using them in structuralconservation.

The presented innovative technologies and meth-ods of reinforcing brick arches and vaults are usuallyused in combination. It is often only their combina-tion which brings the desired results. One should bearin mind that each of the methods requires an individ-ual analysis of its applicability to the given structure,as regards both the materials and the available spaceabove the structure, as well as an analysis of the struc-ture’s historic and aesthetic values. The selection ofa suitable method should take into account the tar-get static behaviour of the reinforced structure and itsdefects before the reinforcement.

In the case of conservation and reinforcement (alsowith innovative materials) of historic structures rep-resenting global cultural achievements and being indanger of failure, which as a rule are characterized bya complicated static system and heavy dead load, itbecomes necessary to employ FEM.

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