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Contribution of clayey–calcareous silicite to the mechanical properties of structuralmortared rubble masonry of the medieval Charles Bridge in Prague (Czech Republic)

R. Přikryl ⁎, A. ŠťastnáInstitute of Geochemistry, Mineralogy and Mineral Resources, Faculty of Science, Charles University, Albertov 6, 128 43 Prague 2, Czech Republic

a b s t r a c ta r t i c l e i n f o

Article history:Received 14 January 2010Received in revised form 20 May 2010Accepted 19 June 2010Available online 25 June 2010

Keywords:Mortared rubble masonryHydraulic lime mortarSiliciteMechanical properties

The Gothic fill masonry of the arches and pillars of the Charles Bridge in Prague (Czech Republic) can becharacterised as structural mortared rubble masonry (MRM). The mortar, a mixture of hydraulic lime binderand fine-grained filler (river sand), fastens together larger pieces of natural stone, which are a porous fine-grained clayey–calcareous silicite. A set of specimens were subjected to the uniaxial compressive load. Thespecimen set was composed of some of the mortars alone, the natural stone alone, or variable mixtures ofboth (i.e. mortared rubble masonry materials). Along with the ultimate strength, the sample's stress–strainbehaviour was recorded. The compressive strength, as well as the modulus of elasticity of the mortaredrubble masonry, increased with the proportion of natural stone in the specimens (having a compressivestrength of 12.55 MPa for a specimen having almost 4 vol.% of coarse aggregate, and 61.49 MPa for aspecimen having almost 82 vol.% of coarse aggregate; with a modulus of elasticity of 6 GPa for a specimenhaving almost 4 vol.% of coarse aggregate, and over 14 GPa for a specimen having 23 vol.% of coarseaggregate). The compressive strength of the mortar (6–11 MPa) is lower than that of MRM; however, themodulus of elasticity (7–16 GPa) can reach the same values as that for MRM. The natural stone, used as thecoarse aggregate, showed the highest compressive strength (80–140 MPa) and the highest modulus ofelasticity (25–28 GPa) from among the materials studied. This observation suggests that it is not only thehydraulic lime-based mortar, but also the coarse aggregate (clayey–calcareous silicite), as well as favourablebonding on the natural stone–mortar interface contribute to the superlative mechanical properties of thestudied MRM.

© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Mortars in antique and medieval monuments comprise animportant part of these structures, and significantly contribute totheir stability (Adams et al., 1993; Giavarini et al., 2006). Prior to thewidespread use of modern concrete employing Portland cement,inorganic binders were used for mortars. They were mainly appliedfor: (1) the joining of two courses of natural stone or bricks; and/or(2) for wall plastering. Less frequently larger volumes of concrete-likematerials were applied for the construction of antique structures;those of the highest regarded importance are characterised by(among others) the piers of Mediterranean harbours (Oleson et al.,2004), aqueducts (Malinowski, 1979), and symbolic structures (e.g.temples, such as the Pantheon in Rome) (Marusin, 1996). However,the term ‘concrete’ should be avoided for these ancient materials. Theterm ‘mortared rubble masonry’ (Delaine, 2001) represents a moreappropriate coinage, due to the heterogeneous granulometry and

significantly coarser particles in these coarse aggregates, than is foundin modern concrete.

Most of the recent studies on historic mortars have focused upontheir phase (mineralogical) composition, chemistry (Paama et al.,1998; Ingo et al., 2004; Silva et al., 2005), and/or evaluations of theconditions for their preparation (Velosa et al., 2007). In contrast, veryfew papers have dealt either with the physical and/or mechanicalproperties of these materials (Oleson et al., 2004; Giavarini et al.,2006). Such data are crucially important for a precise evaluation of thefuture stability of such ancient structures (Šejnoha et al., 2008) and/orfor the correct formulation of mortars to be used for preservation andrestoration (Beck and Al-Mukhtar, 2008; Hanley and Pavía, 2008). Inorder to determine the mechanical properties of these ancientmortars, most studies have relied on a computation of theirmechanical properties from their dynamic elastic properties (Mor-opoulou et al., 2005), or on the testing of small specimens havingirregular shapes (Drdácký, 2007; Drdácký et al., 2005, 2008). Thereason for the current paucity of knowledge should be obvious, assampling on historic (and often protected) monuments is generallyrestricted.

In order to avoid the problems that can arise from data based uponcalculations or measurements of other properties, as well as from the

Engineering Geology 115 (2010) 257–267

⁎ Corresponding author. Tel.: +420 221951500; fax: +420 221951496.E-mail address: [email protected] (R. Přikryl).

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testing of irregularly shaped specimens, our study focused onexperimental testing of materials obtained in sufficient quantities toallow for the preparation of regularly shaped specimens; from whichthe mechanical data could then easily be determined. The experi-mental materials were sampled from the fill masonry of the GothicCharles Bridge in Prague (Czech Republic), one of Prague's iconicmonuments (Fig. 1). The stress–strain behaviour and the ultimatestrength testingwas designed to explore not only the properties of themortar and/or larger pieces of natural stone, but precisely thoseproperties of the mortar/natural stone mixture that comprise theunique materials of the fill masonry. Thus, our experimental datacontributes to the technical historic knowledge base about theBridge's construction, and to the usage of specific types of buildingmaterials during the medieval period.

2. Site description

The Charles Bridge, the oldest stone arch bridge in Prague, wasconstructed under the auspices of Charles IV, Czech King and HolyRoman Emperor during the period from 1357 to 1406 (Vítovský,1994). The Bridge, a listed national historic monument, is locatedwithin the UNESCO-protected historic city centre; it measures 516 mlong and 9.5 m wide. Its facing masonry, built of regularly shapedsandstone ashlars (“Opus quadratum”) provide the external protectionto the fill masonry, the nature of which had not been seriously studiedprior.

Some parts of the Bridge, including the fill masonry, have beenadversely affected by localised failures due to river floods dating fromduring its construction (in 1359, 1367, 1370, 1373, and 1374), as wellas after its completion (in 1432, 1655, 1784, and 1890). Despitenumerous repairs, major portions of the bridge's masonry fill remain

in their original state and composed of the original materials, thuscharacterising a Gothic original. According tomodern information, theoriginal medieval fill masonry is preserved in 13 out of a total of 15pillars (i.e. 2 were rebuilt after collapse due to the flood of 1890) andin 13 arches (out of 16 total, 3 having been re-erected after the 1890flood).

Previous research, conducted in 1990s, reported on the criticalstate of the fill masonry, arguing that it was of low technological valueas well as being in a state of extreme deterioration due to thecrystallization of water-soluble salts and high moisture content(Witzany et al., 2005). The mechanical properties at that time werestated to be of very poor quality.

The masonry fill materials are commonly hidden by the naturalstone facingmasonry (Opus quadratum) and are not accessible. Duringongoing research, prior to reparations on the Bridge, there was thechance to perform sampling at several sites on the Bridge, granted tous by the National Institute of Cultural Heritage Care. Along with thebasic characteristics of the materials (mineralogy, chemistry, indexphysical properties, moisture contents, and presence of water-solublesalts) that are thoroughly discussed elsewhere (Přikryl et al., 2009),the current materials research focused on an examination of themechanical properties.

3. Materials studied

3.1. Sampling

Materials from the fill masonrywere obtained from three sites: thebase of the pillar 8, and arches XI and XIV (Fig. 2). The fill masonry ofthe pillar no. 8 was explored by means of a diamond core drill. Twohorizontal drills penetrated the fill masonry at the base of pillar 8 in

Fig. 1. View of the Charles Bridge in Prague (Czech Republic) from the south.

Fig. 2. Position of the sampling sites on the Charles Bridge.

258 R. Přikryl, A. Šťastná / Engineering Geology 115 (2010) 257–267


2005, when the basement of this pillar was being stabilised as a part ofa flood-protection project. The drills (10 cm in diameter) went inapproximately 1 m deep; about half of this length containing the fillmasonry, with the remaining portion belonged to the stone ashlars ofthe facing masonry. There was 100% recovery of these compactmaterials.

The fill masonry of arch XIV was reached by means of a manuallydug exploration pit in late 2007 (Fig. 3). The main reason for this pitwas to explore the actual state of the fill masonry within this arch,which exhibited the most serious damage to the surface of the facingmasonry, manifested by extensive weathering of the stone ashlars.The pit penetrated through modern concrete and a waterproofinglayer inserted in 1960s–70s, and then passed through the entireprofile of the original fill masonry, reaching the internal side of thefacing masonry of the arch (for a cross-section see Fig. 4). Additionalsamples were obtained from exploration pit K5 that was dug in thearch XI in 2004.

3.2. Macroscopic description of the materials

In situ observations of the fill masonry in the bridge archesprovided another very significant, although unexpected finding. Itwas related to the presence of the second arch on the transition zonebetween the base of the fill masonry and internal side of the facingmasonry (Figs. 4, 6). This arch was constructed from slabs (8–12 cmthick and 45–50 cm long) out of the same stone that occurs within thefill masonry.

The materials obtained by sampling can be subdivided into thefollowing categories: (1) mortar, (2) mortared rubble masonry(MRM) (Fig. 5), (3) natural stone used as coarse aggregate withinthe MRM, and (4) natural stone from the facing masonry ashlars. Theterm mortar, as used here, is understood to be a mixture of fineaggregate (i.e. river sand with grain sizes of less than 4 mm) and abinder having the character of hydraulic lime.

Based on the in situ observations (from the exploratory pit), aswell as on the interpretations of the drill cores, the MRM is composedof two materials mortar and natural stone which are intimatelyinterconnected. The natural stone, which was used as a coarseaggregate, is present in the form of irregular blocks in variable sizecategories. The largest blocks have sizes from 20 to 50 cm (infre-quently up to 100 cm), with the smallest fragments being in the rangeof 5–15 cm.

The volume of natural stone in the fill masonry has been calculatedfrom data obtained by computer-assisted image analysis of thephotographs taken of the walls of the exploratory pit. In total, 12photos of the Gothic MRM were taken. The area of each photo coversapproximately 0.5 m2 (i.e. the total area analysed was 6 m2). Theprocedure for the image analysis (Přikryl, 2001) consisted of manually

outlining the individual pieces of coarse aggregate in the digitalimage, and then the conversion of these outlines into the vectors withthe subsequent analysis done by using image analysis softwareSigmaScan Pro © (v. 5.0.0., SPSS Inc.). The 2-D data was thenconverted into the 3-D by standard stereological procedures (Under-wood, 1970). The measured volumes of the coarse aggregate fromindividual photos ranged from 62 vol.% (the minimum) to 78 vol.%(maximum); with the average being 69 vol.% (Fig. 6).

3.3. Petrographic description of the natural stone used as coarse aggregate

The natural stone, used as the coarse aggregate within the fillmasonry, is macroscopically very fine-grained (aleuropelitic) sedi-mentary rock of light beige to ochre colour. The methods used during

Fig. 3. General view of the exploration pit on arch XIV of the Charles Bridge in Prague.

Fig. 4. Cross-section through thefillmasonry of arch XIV (adopted fromPřikryl et al., 2009).Explanation: [1–5]=the concrete layers with waterproofing inserted in 1960s–1970s, [6]original (Gothic) fill masonry having the character of mortared rubble masonry (pieces ofcoarse aggregate are shown by light colour) with hydraulic lime mortar; [6a] internal archbuilt of natural stone slabs (having the same petrographic character as the coarse aggregatein the fill masonry); and [7] external arch built of rectangular sandstone ashlars (bothUpper Cretaceous quartz sandstone and Carboniferous arkoses). Note: all dimensions areexpressed in mm.

259R. Přikryl, A. Šťastná / Engineering Geology 115 (2010) 257–267


the petrographic investigation and source locality determinationswere governed by the extreme fine grain, which requires acombination of thin section study (rock fabric description anddetermination of minerals having silt-size granulometry) and X-raydiffraction measurements (detection of the minerals present in thematrix). The amounts of carbonate (calcite) were calculated from theweight differences of the materials before and after leaching in HCl.

Based on the mineralogical and petrographical examination, therocks studied can be subdivided in 4 subtypes, which also differ intheir index physical properties (Table 1). The rocks studied werecomposed of dominant silica (both amorphous/cryptocrystallineforms such as opal, CT opal, and tridymite making up 45–55 vol.%;with subordinate silt-size clastic quartz grains comprising from 5 to10 vol.%). These were followed by calcite (micritic mud grains andsome fragments of microfossils, specifically foraminifers and spongespicules; with the total volume of calcite ranging from 15 to 45 vol.%),and clay minerals (kaolinite, illite, and some glauconite; in totalmaking up 6–15 vol.%). The crystallinity of the silica was evaluated byusing X-ray diffraction data, following themethodology of Murata andNorman (1976), Williams et al. (1985), and Lynne et al. (2007). K-feldspar, muscovite, Fe-oxyhydroxides, and Ti-oxides are commonaccessories within the studied rocks. Regarding their grain sizes, mostof the grains were within (or very close to) a clay-size fraction (90–95 vol.%), the remainder was made up of a silt-size clastic component(Fig. 7a,b). The silicite shows parallel to subparallel, mostly discon-tinuous, planar lamination; and additionally showing a locally chaoticto nodular microfabric.

Based on the mutual proportions of these major rock-formingminerals, the rock can be classified as clayey–calcareous silicite, withsome overlap into the sphere of calcareous silicite (Fig. 9). In theliterature, alternative classifications of these rocks as marlstones or ascalcareous mudstones can be found (Laurin and Uličný, 2004).

The mineralogical and petrographic data obtained were furtheremployed for a determination of the source locality. Fingerprinting ofthe source locality was facilitated with the aid of the Lithotheque ofCzech historical building and sculptural stones (Přikryl et al., 2001)and the analytical data assembled during its compilation (Přikryl,2007). Based on this approach, the source locality of the studiedsilicite was found to be within the beds of the Upper Cretaceoussedimentary rocks that outcrop on Bílá Hora Hill in Prague. Theseancient quarries were located 1–2 km from the Bridge and from thePrague Castle. Certain varieties of these rocks have been favoured forsome the finest sculptural endeavours during the Romanesque andGothic artistic periods (see e.g. Schütznerová-Havelková, 1979);however, their use for exterior work was later abandoned, due totheir limited stability when exposed to the direct effects ofatmospheric conditions (Přikryl et al., 2003a). However, these rocksremained one of the most popular sources of both building stones andinterior sculptural stone in the Czech Republic up until the 20thcentury. Furthermore, certain written records (Balbín, 1986), as well

Fig. 5. Macroscopic appearance of the mortared rubble masonry of the Charles Bridge(wall of the pit in arch XIV).

Fig. 6. Interior arch built from the slabs of clayey–calcareous silicite. This arch makes upan intimate part of the fill masonry, but can be also interpreted, together with theexterior arch built from rectangular sandstone ashlars, as a double protection for thebridge's stability.

Table 1Compositional and petrographical details of the studied silicite used as a coarseaggregate within the mortared rubble masonry of the Charles Bridge in Prague.

Number ofspecimens

Subtype I Subtype II Subtype III Subtype IV

3 2 4 2

Form andcontentof carbonates

Micritic calcite15–23 wt.%

Micritic calcite44–45 wt.%

Micriticcalcite (16–25 wt.%)

Calcite(22–35 wt.%),mostlymicritic,smallamountsof sparite

Amount ofall SiO2

forms

60–65% 45–50% 55–60% 55–60%

SiO2

crystallinityindex

2.7–3.4 2.3–2.7 2.2–3.0 2.5

Clay minerals Kaolinite (10%)and illite(1–2%)

Kaolinite(4–6%) andillite (2–3%)

Kaolinite(10%) andillite (5%)

Kaolinite(7-9%) andillite (2-3%)

Accessories Feldspars2–3%, rutile,goethite, andglauconite

Feldspars,rutile, goethite,and glauconite

Feldspars1–2%, rutile,goethite,andglauconite

Feldspars,rutile,goethite, andglauconite

Bioclasts Very lowamount;spicules ofsponge, andfewforaminifers

High proportionof silicifiedbioclasts(spicules ofsponge) (30–40 vol.%)

Silicifiedbioclasts(spicules ofsponge)(10–15 vol.%)

Very rare,foraminifers

Clastic(silt-size)component

Commonsubangularquartz grains(10 vol.%)

Subangularquartz grains(3–5 vol.%),feldspars(b1 vol.%)

Subangularquartz grains(5–8 vol.%)

Subangularquartz grains(3–7 vol.%)

Rockmicrofabric

Parallellamination,locally chaoticto nodular; andaleuropelitic

Weak parallellamination; andaleuropelitic

Parallellamination,locallynodular; andaleuropelitic

Massive andaleuropelitic

Bulk density(g/cm3)

1.823–1.913 2.148–2.178 1.836–1.934 1.907–1.982

Real density(g/cm3)

2.521–2.587 2.600–2.603 2.530–2.587 2.572–2.577

Porosity (vol.%) 24.26–30.15 16.22–17.50 24.26–27.43 23.07–25.86



as personal observations of the authors suggest that during the Gothicand Baroque periods, along with its use as a common building stone, italso served as an admixture for the burning of hydraulic lime binders.

The silicite shows highly favourable physical properties, specifi-cally those concerning bulk density (1823–2178 kg/m3, mean value1930 kg/m3) and porosity (16.22–30.15 vol.%). The porosity corre-lates inversely with the micritic carbonate content (i.e. the lowestporosity of 16.22–17.50 vol.% was recorded for the silicite containingthe highest amounts of micritic calcite) (Table 1). The same rule canbe applied for the real density values (Table 1).

3.4. Description of the mortars

Themortars that were studied presented a mixture of a binder andfiller (Fig. 8a). The fine aggregate filler is common river sand,homogenously distributed in the mixture. Microscopic examinationconfirmed the prevalence of clastic quartz grains (mostly rounded,monocrystalline and polycrystalline clasts of magmatic and meta-morphic origin, with grain sizes ranging from 0.08 to 1.5 mm). Clastsof feldspar, micas, hornblende, and/or pyroxene only make up minoradmixtures. Along with the mostly monomineral grains, clastic rockfragments are common, among which granodiorites and clayey–ferruginous siltstone prevail.

A very fine-grained binder fastens together the filler. Based uponthe quantitative microscopy, by use of image analysis (Přikryl, 2001),the proportions of filler and binder range from 2.6:1 to 3.2:1. Due tothe very fine-grained character of the filler, its phase composition hasbeen studied by X-ray diffraction; confirming the presence ofhydrated calcium silicates (CSH phases), aluminates (CAH phases)and aluminosilicates (CASH phases) (see Přikryl et al., 2009 fordetails). Calcite and vaterite also make up the common phases ofthese binders.

Larger pieces (2–5 mm in diameter) of burnt limestones (so-called ‘lumps’) make up a characteristic feature of these mortars(volume proportion from 2 to 4%). Their actual composition ischaracterised by variable proportions of CSH, CAH, and CSAH phases(detected by X-ray diffraction). In addition to these limestones,fragments of clayey–calcareous silicite (sized from 0.3 to 0.8 mm)make up a common admixture (of about 2–5 vol.%). Based onmicroscopic observation, a reaction rim on the binder–siliciteinterface can be observed (Fig. 8b), indicating a probable chemicalreaction between the binder and the stone.

Pore space is another characteristic feature of these studiedmortars. It is present in the form of roughly equidimensional, mostlyisolated, pores. Microcracks, formed due to the contraction of the

Fig. 7. Microphotographs of the studied silicite: (A) clayey–calcareous silicite showingthe highest amounts of calcite (44–45 wt.%) and high proportions of silicified bioclasts;and (B) clayey–calcareous silicite showing the lowest amounts of calcite and highestamounts of silica.

Fig. 8. Microphotographs of the studied mortars: (A) general view of the mortarmicrostructure documenting the character of the filler and proportions of filler andbinder; (B) oval fragment in the centre is unburnt clayey–calcareous silicite on whichthe reaction rim on the binder-stone interface is clearly visible (the reaction is due tothe low crystalline or amorphous forms of SiO2 with the hydraulic lime binder).



hardening mixture, can also be found. The porosity of the studiedmortars, as determined by mercury intrusion porosimetry, rangesfrom 27.67 to 32.68 vol.% with average 30.26 vol.% (Přikryl et al.,2009). The total volumes of these pores comprise from 153 to193 mm3/g. Most of the pore space belongs to mesopores, the contentof which makes up 50–71% of the mercury intrusion porosity. Thecharacteristic radii of the pores, as determined by the mercuryintrusion porosimetry, range from 11 to 23 nm.

The bulk density of the studied mortars is lower than that of thenatural stone used as the coarse aggregate; ranging from 1570 kg/m3

to 1676 kg/m3, with an average value of 1635 kg/m3. Their totalporosity is higher than that of the natural stone used as the coarseaggregate; ranging from 31.72 vol.% to 38.91 vol.%, with an averagevalue of 36.07 vol.%. The real density of the mortars studied rangedfrom 2449 kg/m3 to 2610 kg/m3, with an average value of 2552 kg/m3.

4. Experimental

Themechanical properties of the studiedmaterials (for a summarysee Table 2) were evaluated by applying a uniaxially-orientedcompressive load on cylindrical specimens. These were obtained bydiamond core drilling from the original drill cores (i.e. materialssampled from pillar 8; the diameter of this drill core being 100 mm)and from larger pieces of fill masonry sampled in the exploratory pitsin arches XI and XIV. The limited quantity of the materials and thesizes of the samples proscribed the relatively small sizes of thespecimens to be used for the experimental tests (60 mm in height and30 mm in diameter). With the intent to study the stress–strainbehaviour on the specimens, unaffected by a low height/diameterratio, our test specimens were prepared with the h/d=2:1.

The axial/lateral deformation during uniaxial loading (using a“soft” press, and constant stress rate of 0.05 MPa/s) on selectedspecimens was controlled by glued foil-type electrical resistivitygauges. Based upon these measurements, the stress–strain curveswere recorded. From the stress–strain data, the secant and tangentYoung's modulus at 30% and 50% of the ultimate strength werecalculated. For these specimens, the uniaxial compressive strength(UCS) and the tangent Young's modulus at 50% of the UCS wereplotted onto a Deere and Miller stress–strain diagram (Deere andMiller, 1966), in order to evaluate the type of stress–strain behaviour.

5. Results

5.1. Strength behaviour

According to our experimental results, the uniaxial compressivestrength of the clayey–calcareous silicite, which had been used as acoarse aggregate in the Gothic MRM, ranged from 80 to 140 MPa, withan average value of 103.8 MPa (see Table 3). On the other hand, puremortar specimens (i.e. without any admixture of the coarseaggregate) showed uniaxial compressive strengths from 6 to11 MPa (Table 3).

The uniaxial compressive strength (UCS) of the MRM effectivelyincreases with the increasing content of coarse aggregate in thespecimens tested. This was evident, even with a very slight presenceof the coarse aggregate in the specimens (less than 4 vol.% of siliciteresulted in a UCS of 12.55 MPa, 10 vol.% increased the UCS to17.67 MPa, 23 vol.% of coarse aggregate shifted the UCS value over24 MPa, and 82 vol.% of silicite produced a UCS of 61 MPa (Table 3)).Accordingly, the effect of the coarse aggregate on the strength of MRMis quite evident.

Fig. 9. Classification of studied rocks in the system of clay-rich rocks (mudrocks), carbonate-rich rocks (limestones), and biogenic/chemogenic silica-rich rocks (silicites). Thediagram clearly shows that the natural stone used as a coarse aggregate within the fill masonry of the Charles Bridge mostly belongs in the range of clayey–calcareous silicite withsome overlaps with the calcareous silicite. In spite of the 4 distinct determined subtypes, based on the mineralogical and fabric variations, all studied samples come from one sourcelocality which can be found very close to the Charles Bridge. The limits of the compositional variations of the rocks from the source locality are outlined by a dotted line. The field ofmarlstone is also indicated (left side of the diagram) documenting the distinct mineralogical difference between marlstone and the studied rocks.



5.2. Stress–strain behaviour

The studied materials show distinct stress–strain behaviours(Fig. 10). The mortars used within the arches mostly showed elasto-plastic behaviours; with the elastic proportion up to 80% of the UCS(specimens KMP8/E1, K7/ZD4 and K7/ZD5) (see Table 3 and Fig. 8A).Only one sample (KM5/VI/3) was characterised by plastic behaviourin the initial stages of loading (Fig. 10A).

The MRM stress–strain curves differ depending on the proportionof coarse aggregate in the mixture (Fig. 10B). Specimen KM5/VI/2,with the lowest volume of coarse aggregate (below 4 vol.%), mostlyshowed plastic behaviour over the entire stress range, which is verysimilar to the stress–strain behaviour of mortars. Specimen KMP8/D1,containing about 10 vol.% of the coarse aggregate, showed a similarbehaviour to the previously mentioned one, up to about 60% of theUCS but then exhibited a much steeper stress–strain relationship athigher stresses. This could be due to the concentration of the stress inthe coarse aggregate, which shows greater stiffness than the mortar.Specimen KMP8/D2, composed of 23 vol.% of coarse aggregate, wascharacterised by a non-linear proportion in the 0–25% range of theUCS (probably due to the closure of oriented microcracks), butthereafter turned into a quasilinear and relatively steep slope of thestress–strain curve (Fig. 10B).

Natural stone from the MRM (specimens KMP8/B1 and KMP8/B2)mostly showed elasto-plastic behaviours (Fig. 10C). The elastic(quasilinear) proportion of the curve occurred up to 60 to 70% ofthe UCS. Clearly, this thus shows evidence of plastic behaviour.

6. Discussion

6.1. Interpretation of the geomechanical data

Based on the experimental results, the clayey–calcareous silicite(coarse aggregate within the MRM) demonstrated mechanicalbehaviour in the range of materials possessing a high (to veryhigh) strength, with medium stiffness (Fig. 11). In contrast, onespecimen, composed exclusively of mortar (no fragments of coarseaggregate were present), showed very low strength and yielding,according the classification of Deere and Miller (1966). Wheninterpreting the real mechanical properties of the fill masonry, onemust be very careful when considering the mechanical parametersof those materials composed exclusively of either natural stone or ofmortar. The fill masonry presents a hardened mixture of binder(hydraulic lime), fine-grained aggregate (river sand), and coarseaggregate (larger pieces of siliceous–calcareous mudstone); and itsmechanical properties must be compared against a material havingequivalent proportions of the above mentioned components.

Due to the limited amount of authentic materials available fortesting from this historic monument, only a small number ofspecimens with unpredictable proportions of these componentswere available. In our case, we were able to prepare 3 specimens,from which the volume of natural stone fragments varied from 4 to23 vol.%, with one specimen having over 82 vol.% of stone fragments(the volumetric proportion of natural stone fragments withinindividual specimens was calculated after preparation of thesespecimens). None of these correspond precisely to the proportions ofnatural stone in the real masonry which makes up approximately62–78 vol.% (this is obtained by computer image analysis of thephotographs documenting the sections through the fill masonry in

Table 2Overview of the test specimens, their material characteristics and place of extraction.

Specimen Type of material Place of extraction

KMP8/B1 Coarse aggregate(clayey–calcareous silicite)

Pillar No. 8 (specimen preparedfrom drill core)















KMP8/D1 Mortared rubble masonry(mixture of fine and hydrauliclime binder, coarse aggregatemakes 10.3 vol. %)






KMP8/E1 Mortar (mixture of fine aggregateand hydraulic lime binder)


K7/ZD4 Mortar (mixture of fine aggregateand hydraulic lime binder)

Exploration pit in the arch No. XIV(specimen prepared from largerirregularly shaped sample)

K7/ZD5 Mortar (mixture of fine aggregateand hydraulic lime binder)

Exploration pit in the arch No. XIV(specimen prepared from largerirregularly shaped sample)

KM5/VI/2 Mortared rubble masonry(mixture of fine and hydrauliclime binder and coarse aggregatemakes 3.8 vol.%)

Exploration pit in the arch No. XI(specimen prepared from largerirregularly shaped sample)

KM5/VI/3 Mortar (mixture of fine aggregateand hydraulic lime binder)

Exploration pit in the arch No. XI(specimen prepared from largerirregularly shaped sample)

Table 3Results of geomechanical tests of the fill masonry materials from the Gothic CharlesBridge in Prague. The % values provided in the brackets for the specimens marked Dshow the proportions of coarse aggregate (clayey–calcareous silicite) in the respectivespecimen. Explanation of symbols: σUCS=uniaxial compressive strength, E-T50=tangent Young's modulus at 50% of uniaxial compressive strength, ES50=secantYoung's modulus at 50% of uniaxial compressive strength, ET30=tangent Young'smodulus at 30% of uniaxial compressive strength, ES30=secant Young's modulus at 30%of uniaxial compressive strength, and ν=Poission ratio.

Specimen σUCS

[MPa]ET50[GPa]

ES50[GPa]

ET30[GPa]

ES30[GPa]

ν

Coarse aggregate (clayey–calcareous silicite)KMP8/B1 80.71 24.99 28.07 27.86 29.53 0.24KMP8/B2 107.47 27.98 29.43 28.71 30.00 0.22KMP8/B3 119.19 – – – – –

KMP8/B4 107.81 – – – – –

KMP8/B5 92.19 – – – – –

KMP8/B6 81.00 – – – – –

KMP8/B7 101.45 – – – – –

KMP8/B8 140.55 – – – – –

Mortared rubble masonry (mixture of fine and coarse aggregate and hydraulic limebinder)

KM5/VI/2 (3.8%) 12.55 6.15 7.34 6.86 8.27 0.30KMP8/D1 (10.3%) 17.67 12.44 11.47 11.65 11.71 0.24KMP8/D2 (23.4%) 24.45 15.87 14.58 15.87 14.08 0.34KMP8/D3 (82.6%) 61.49 – – – – –

Mortar (mixture of fine aggregate and binder)KMP8/E1 9.69 7.00 6.93 6.90 6.91 0.30K7/ZD4 6.44 16.04 16.23 18.05 15.94 0.36K7/ZD5 11.34 13.98 14.44 14.69 14.49 0.48KM5/VI/3 10.69 7.01 9.40 7.89 10.00 0.42



the exploratory pit and in the drill core). Despite this disadvantage,the UCS and Young's modulus systematically increased withincreasing volume fraction of coarse aggregate (natural stone) inthe specimens (Table 3). Assuming that the proportions between thecoarse aggregate and the mortar in the MRM vary between 3:1 and2:1, we can interpolate the mechanical properties of the MRM in therange of 46–58 MPa of the UCS (high strength field according toDeere and Miller, 1966) and in the range of 18–22 GPa for Young'smodulus (low to medium stiffness field according to Deere andMiller, 1966).

The higher stiffness of themortars from the fill masonry of the archXIV (empty squares on Fig. 11), when compared to the mortars frompillar 8 (black squares on Fig. 11) and arch XI is due to their lowerporosity (from 31 to about 35 vol.%). The porosity of the mortars frompillar 8 and arch XI reached 36–39 vol.%. Similar effects of porosity onthe mechanical properties of both modern and ancient mortars havebeen recorded in numerous previous studies (Lanas and Alvarez,2003; Arandigoyen and Alvarez, 2007).

6.2. Comparison of geomechanical data measured on regularly andirregularly shaped specimens

Our UCS data, measured on regularly shaped specimens preparedfrom the fill masonry of the Charles Bridge, showed higher values thanthose determined by other researchers on irregular specimens fromthe same structure (Drdácký and Slížková, 2008). The reasons forthese differences can principally be explained by the effects of sizeand shape. The use of regularly shaped specimens has majoradvantages, among the obvious of these is the possibility to measuredeformation during uniaxial loading.

The possibility to observe the failure modes of the specimens afteruniaxial compression tests presents another advantage of testingregularly shaped specimens (Přikryl et al., 2003b). As is shown by ouranalysis of macroscopically visible fractures, produced by the ultimateload, the failure mode gradually developed from: the arrays of axiallyoriented sets of fractures, combined with the final shear planes (forpure mortar and specimens with a low content of coarse aggregate);to axially oriented splitting of specimens (those with a higher contentof coarse aggregate) (Fig. 12).

6.3. Comparison with mechanical properties from other MRM structures

When comparing the mechanical properties of MRM obtainedfrom the Charles Bridgewith experimental results from other ancientmortars and ‘concretes’, one can generally find similar results (seeTable 4); however, also some differences. The major differencebetween the mechanical behaviours of MRM from the fill masonry ofthe Charles Bridge and thematerials from ancient harbours and otherstructures (see Oleson et al., 2004; Giavarini et al., 2006) seems to bein the relatively greater strength and stiffness of the fill masonrymaterial utilized in the Charles Bridge. Thismight be explained by thedesire to obtain a fill masonrymaterial that would sustain the greaterhorizontal loads that would be expected within individual arches(Audenaert et al., 2008). The mechanical properties of the fillmasonry of ancient bridges, although extremely rarely reported,show values similar to our records (e.g. 39±10 MPa for Romanbridges in Campania, Italy (Baratta and Coletta, 1998)).

The other difference is due to the deformational and strengthcharacteristics of the natural stone used as a coarse aggregate invarious types of the mortared rubble masonry. Despite the fact that

Fig. 10. Stress–strain behaviour of mortars (A) mortared rubble masonry (B), andnatural stone used as a coarse aggregate (C); (only the axial strain is shown). For a moredetailed explanation see text.



only limited data is currently available, it was possible to compare theCharles Bridge data with the antique mortared rubble masonryreported by Giavarini et al. (2006). The clayey–calcareous silicite ofthe Charles Bridge's fill masonry shows much higher strength andstiffness than do the tuffs of volcanic origin used in antique mortars(Table 3).

7. Conclusions

Based on the study of its composition and properties, the studiedmaterials of fill masonry from the Gothic Charles Bridge in Prague canbe declared as typical mortared rubble masonry material also knownas “Opus caementicium”. The combination of hydraulic lime mortar

Fig. 11. Classification of the studied materials in the Deere–Miller diagram (Deere and Miller 1966, note that the position of the boundaries have been changed compared to theoriginal). Sequence of strength/deformational properties of mortar (black squares)/mortared rubble masonry (black dots)/and natural stone used as a coarse aggregate (blacktriangles) are shown for the materials obtained from the fill masonry of pillar 8. The mortars from the fill masonry of the arch XIV (empty squares) show a substantially highermodulus of elasticity than similar materials from the pillar due to their lower porosity than for mortars from pillar 8. Based on the measured data, the deformation/strengthproperties of MRM showing an average 2:1 proportion of natural stone tomortar is interpolated to the high strength field and to the low tomedium stiffness field (shown by the greysquare on the diagram).

Fig. 12. Fracture pattern in the test specimens produced by uniaxial loading. A=specimen composed of mortar (i.e. mixture of hydraulic lime binder with fine aggregate),B=specimen containing 10.3 vol.% of coarse aggregate, C=specimen containing 23.4 vol.% of coarse aggregate, D=specimen containing 82.6 vol.% of coarse aggregate. The fracturepattern is indicated by solid red lines (axially oriented cracks) and dotted lines (secondary shear fracture planes). The part of the specimen composed of coarse aggregate (silicite) isfilled in with grey colour.



and pieces of natural stone (coarse aggregate), in the ration from 1:2to 1:3, resulted in favourable physical and mechanical properties. Thenatural stone used as a coarse aggregate has been classified as clayey–calcareous silicite, from which a source locality that has beenidentified close to the Bridge.

The experimentally obtained mechanical data (uniaxial compres-sive strengths ranging from 6.44 MPa to 11.34 MPa for specimenscomposed of mortar from 12.55 MPa to 61.49 MPa for specimensdenominated as mortared rubble masonry, and 80–140 MPa fornatural stone) allowed us to interpolate the strength of the bulk fillmasonry material consisting of from 68 to 72% of coarse aggregate tobe 46–58 MPa. Corresponding deformational properties (tangentYoung's modulus at 50% of UCS), from 18 to 22 GPa, suggests arelatively plastic material that might be favourable for the interior fillof bridge arches, affected by high horizontal stresses. Based upon theexperimentally determined mechanical properties, the studied mate-rials of 6 1/2 century-old fill masonry is of excellent quality, which canbe compared to modern lightweight structural concrete.

Acknowledgements

The experimental materials were prepared with the financialassistance from the Prague City Council. Part of the experiments werekindly supported by the Czech Ministry of Education, Youth andSports (project MSM 002160855 “Material flow mechanisms in theupper spheres of the Earth”), GACR project no. 205/08/0676 and no.205/09/P138, as well as project FR— TI1/381 “Information system as atool for the proposal of the repair of stone bridges”. Help from ZdeněkErdinger and Julie Erdingerová during the rock mechanical tests washighly appreciated. The English has been edited and improved byPeter R. Lemkin.

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Table 4Comparison of the mechanical properties (uniaxial compressive strength and modulus of elasticity) of mortared rubble masonry (Opus caementicium) from ancient structures.

Type of specimens Uniaxial compressive strength (MPa) Modulus ofelasticity (GPa)

Reference

Opus caementicium samples Bulk samples 0.98–6.7 0.8–9 Giavarini et al.(2006)Tuffs (natural stone used as coarse

aggregate)2–3.7 (soft variety), 10 (mediumvariety), and 36–50 (Peperino variety)

n.d.

Specimens of rubble mortared masonry from antique peers (Mediterranean harbours)4.9–9.4

4.9–18.8 Oleson et al.(2004)

Mortars from mortared rubble masonry of Charles bridge in Prague (14th c.) 2.9–10.5 (mean value 7.5) n.d. Drdácký andSlížková (2008)

Mortared rubble masonry (regular shape ofspecimens) of Charles bridge in Prague (14th c.)

Mortars without coarse aggregate 6.44–11.34 7.0 This studyBulk mortared rubble masonry material(mixture of mortar and natural stone)

12.55–61.49 11.30–14.41

Clayey–calcareous silicite (natural stoneused as coarse aggregate)

80.71–140.55 24.99–27.98



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Author's personal copy Contribution of clayey–calcareous silicite to the mechanical properties of structural mortared rubble masonry of the medieval Charles Bridge in Prague (Czech