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Learning from Failure

Long-term Behaviour of HeavyMasonry Structures

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Learning from Failure

Long-term Behaviour of HeavyMasonry Structures

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Politecnico di Milano, Italy

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Chapter 4

Effects of creep on new masonry structures................................................. 83

N.G. Shrive & M.M. Reda Taha

4.1 Introduction............................................................................................. 83

4.2 The step-by-step in time approach to modeling

time-dependent effects ............................................................................ 84

4.3 Case 1: An axially loaded column .......................................................... 85

4.3.1 Creep model................................................................................ 85

4.3.2 Effect of coupling creep and damage in concentrically

loaded columns........................................................................... 89

4.3.3 Examining the effect of rehabilitation ........................................ 914.4 Case 2: Composite structural element subject to bending ...................... 92

4.4.1 Development of model ............................................................... 92

4.4.2 Application to a beam................................................................. 97

4.4.3 Masonry walls subject to axial load and bending....................... 103

4.5 New mathematical approaches to modeling creep.................................. 103

4.6 Discussion ............................................................................................... 104

4.7 Conclusions ............................................................................................. 105

Chapter 5

Experimental study on the damaged pillars

of the Noto Cathedral ..................................................................................... 109

A. Saisi, L. Binda, L. Cantini & C. Tedeschi

5.1 Introduction ............................................................................................. 109

5.2 The collapse and the decision for reconstruction.................................... 109

5.3 On-site investigation on the remaining parts of the collapsed

pillars....................................................................................................... 110

5.3.1 Layout of the section and of the masonry morphology.............. 1115.3.2 General characterisation of the materials ................................... 111

5.3.3 Damage description .................................................................... 114

5.3.4 Laboratory testing....................................................................... 114

5.3.4.1 Mortars........................................................................ 115

5.3.4.2 Stones.......................................................................... 115

5.3.4.3 Injectability tests ......................................................... 117

5.3.5 On-site tests ................................................................................ 117

5.3.5.1 Flat-Jack tests.............................................................. 1175.3.5.2 Application of sonic pulse velocity test to

pillars........................................................................... 118

5.3.6 Design decisions ......................................................................... 119

5.3.7 The dismantling of the remaining pillars.................................... 120

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Chapter 6

Monitoring of long-term damage in long-span

masonry constructions.................................................................................... 125P. Roca, G. Martnez, F. Casarin, C. Modena, P.P. Rossi,

I. Rodrguez & A. Garay

6.1 Introduction ............................................................................................. 125

6.2 Monitoring and long-term damage.......................................................... 125

6.3 Role of monitoring in the study of ancient constructions ....................... 127

6.4 Monitoring: methodology and requirements........................................... 128

6.4.1 Technology ................................................................................. 128

6.4.2 Distinction between dynamic and static monitoring .................. 129

6.4.3 Requirements .............................................................................. 131

6.5 Measuring damage and deformation related to historical

or long-term processes ............................................................................ 133

6.5.1 Monitoring and long-term damage............................................. 133

6.5.2 Structural deformation................................................................ 133

6.5.3 Tensile damage in arches and vaults .......................................... 135

6.5.4 Damage of compressed members ............................................... 135

6.5.5 Fragmentation............................................................................. 139

6.6 Structural modelling and monitoring ...................................................... 1406.7 Case studies ............................................................................................. 141

6.7.1 Dynamic monitoring of Mallorca Cathedral .............................. 141

6.7.2 S. Maria Assunta Cathedral, Reggio Emilia, Italy ..................... 145

6.7.3 Vitoria Cathedral ........................................................................ 148

6.8 Conclusions ............................................................................................. 151

Chapter 7

Modelling of the long-term behaviour of historicalmasonry towers ............................................................................................... 153

A. Taliercio & E. Papa

7.1 Introduction ............................................................................................. 153

7.2 A continuum damage model for masonry creep ..................................... 154

7.2.1 Unidimensional viscoelastic model with damage ...................... 154

7.2.2 Three-dimensional viscoelastic model with damage.................. 157

7.2.3 Identification of the model parameters and comparisons

with experimental results............................................................ 1607.3 Structural analyses of two masonry towers............................................. 166

7.3.1 The Civic Tower of Pavia........................................................... 166

7.3.2 The bell-tower of Monza Cathedral............................................ 167

7.4 Remarks and future perspectives ............................................................ 171

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Preface

On March 17 1989, the Civic Tower of Pavia collapsed without any apparent warning

signs killing four people. Subsequently, L. Binda, together with four colleagues

from DIS, Politecnico of Milan, was nominated a member of a Committee that had the

aim of helping the Prosecutor of the Procura della Repubblica in Milan find the

causes of the collapse. After an experimental and analytical investigation lasting

nine months, the collapse cause was found. Progressive damage dating back many

years, due mainly to the heavy dead load put on top of the existing medieval tower

with the addition of a massive bell-tower in granite, was to blame.

This type of long-term behaviour of masonry structures was not as well researchedas it was for concrete and steel structures and for rocks. Experimental research

aimed at showing the reliability of this interpretation was carried out, and is still

continuing, that is more than sixteen years of research since 1989. After careful

interpretation of the experimental results, also based on experiences from rock

mechanics and concrete, the modelling of the phenomenon for massive structures,

such as creep behaviour of masonry, was implemented by collaboration with E.

Papa and A. Taliercio from the same department.

Other case histories were collected such as the collapse of the Sancta Maria

Magdalena bell-tower in 1992 in Dusseldorf, the damage to the bell-tower of theMonza Cathedral, Italy, and to the Torrazzo in Cremona, Italy. Later on, in 1996 the

collapse of the Noto Cathedral, Italy, showed that similar progressive damage can

take place in pillars of churches and cathedrals.

Collaborations on the topic first started with the University of Padua (C. Modena)

and later on with the University of Minho, Portugal (P. Lourenco). Then the University

of Calgary, Canada (N. Shrive) and the University of Barcelona (P. Roca) were

involved.

The editor would like to thank the technicians Mr Antico, Mr Cucchi and

Mr Iscandri for their collaboration in the experimental research and Mrs C. Arcadi

for her help in the editing of the chapters.

The Editor

2007

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CHAPTER 1

Failures due to long-term behaviour ofheavy structures

L. Binda, A. Anzani & A. SaisiDepartment of Structural Engineering, Politecnico di Milano,Milan, Italy.

1.1 Introduction

The authors interest towards the long-term behaviour of heavy masonry struc-tures started after the collapse of the Civic Tower of Pavia in 1989, when L. Bindawas involved in the Committee of experts supporting the Prosecutor in the trial,which involved the Municipality and the Cultural Heritage Superintendent afterfour people died under the debris of the tower.

The response required by the Committee concerned the cause of the failure;

therefore an extensive experimental investigation on site, in the laboratory and inthe archives was carried out and the answer was given within the time of ninemonths. Several hypotheses were formulated and studied before finalizing themost probable one, from the effect of a bomb to the settlement of the soil caused bya sudden rise of the water-table, to the effect of air pollution, to the traffic vibrationand so on.

Several documents were collected concerning the sudden collapse of other tow-ers even before the San Marco tower failure and the results of the investigationwere interesting. In fact, the failure of some towers apparently happened a fewyears after a relatively low intensity shock took place. In other cases, the collapsetook place after the development of signs of damage, such as some crack patterns,for a long time. This suggests that some phenomena developing over time had prob-ably to be involved in the causes of the failure, combined in a complex synergeticway with other factors.

As for the experimental investigation carried out on some prisms cut out fromthe large blocks of the collapsed walls of the Pavia tower found on the site, the

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2 Learning from Failure

attention was more and more concentrated on the dilatancy of the masonry undercompressive monotonic and creep tests and on the fatigue behaviour of masonryunder cycling loads.

This chapter discusses the investigation carried out on the materials of the CivicTower of Pavia and the conclusion reached by the previously mentioned Commit-tee. Furthermore, the phenomena of early and retarded deformations of historicmasonry structures will be described together with the results of an investigationcarried out on other damaged structures.

Finally the research campaign carried out on site and in laboratory on the bell-tower of the Cathedral of Monza and the bell-tower of the Cathedral of Cremona.The investigation shows that the damaged state of the structures or of structuralelements can be precociously detected by the recognition of the typical crack pat-terns, based on simple visual investigation.

Collapses may be prevented by detecting the symptoms of structural decay, par-ticularly the crack patterns, through on-site survey, monitoring the structure move-ments for long enough periods of time, choosing appropriate analytical modelsand appropriate techniques for repair and strengthening the structures at recog-nized risk of failure.

1.2 The collapse of the Civic Tower of Pavia:

search for the cause

The Civic Tower of Pavia, an eleventh-century tower apparently made of brickworkmasonry, suddenly collapsed on 17 March 1989 (Fig. 1.1). Several hypotheses were

Figure 1.1: The ruins after the collapse, seen from the arcade opposite to theCathedral.

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According to the few historical documents found, the first order and half of thesecond order can be dated between 1060 and 1100 AD [1, 2], the part from themiddle of the second order and the fifth perhaps were built between the twelfth and

thirteenth centuries; the tower was surmounted by a brick belfry and a timber roof.Between 1583 and 1598 the granite belfry weighing 3,000 tons, designed by thefamous architect Pellegrino Tibaldi was set on top of the tower.

A staircase built into the wall ran along all four walls from the south-west cornerup to the belfry. The staircase was covered by a small barrel vault apparently madeof conglomerate.

1.2.2 First experimental results and interpretation of the failure causes

The few documents available at the time of the collapse [3] were insufficient togive an accurate geometric configuration of the tower.

Consequently, in order to draw prospects and sections of the tower the followingoperations, described in detail in [4, 5], were carried out:

topographic survey of the remains of the tower (Fig. 1.3), and partial rectifica-tion of existing photographs to define the precise plan and the thickness andmorphological features of the cross-section of the masonry;

reconstruction of the geometry of the belfry from a survey of the granite parts,practically all recovered from the internal portion of the remaining part of thetower;

assessment of the overall height of the tower from an existing aerial photogram-metric survey;

perspective plotting from existing photographs to reconstruct the geometry ofthe staircase and the arrangement of the architectural elements.

1.2.2.1 Structure and morphology of the wallsThe medieval walls, built according to the techniques normal at that time for tow-ers, were characterized by two external brick cladding ranging from 120 to 400 mm

Figure 1.3: Photogrammetric survey of the remains of the tower.

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Failures due to Long-Term Behaviour of Heavy Structures 5

with an average of 150 mm, with the intermediate portion of the walls consistingof irregular courses of large pebbles of brick and stones alternated with mortar,constituting a sort of conglomerate (Fig. 1.4). The walls of the second building

phase were characterized by a much more irregular filling and by thinner externalfacings.

Figure 1.5 shows one of the large blocks among the remains revealing part ofthe section of the wall with the external cladding. Figure 1.6 shows a completecross-section of the wall of the present remains of the tower (south side), the ratiobetween the thickness of the external leaf of the wall and the internal one wasapproximately 1:16.

The section of the wall near the staircase was composed by an external wall

similar to the one described above, but 1400 mm thick, a stairwell 800 mm wide,and an internal wall 600 mm thick. The latter wall was of the rubble type and wasparticularly heterogeneous.

Figure 1.4: Cross-section of the wall of the Civic Tower of Pavia.

Figure 1.6: View of the completesection of the bearing wall. Notehow thin the external facing is incomparison with the total thicknessof the wall.

Figure 1.5: Part of the section of thebearing wall (2.8 m thick), showing theexternal brick cladding.

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and the second case 4583 kN/m2was calculated. The unit load on the soil was1161 kN/m2and the safety factor was therefore 2.4 and 3.95, respectively. Theeffective safety factor will lie between these and even the lower value can be

considered sufficient to guarantee the stability of the foundation.In the period from January 1987 to February 1989, the maximum measured

variation in level was 400 mm.Even though there are no reasons to believe that the variations around the tower

were greater, the effect of an abnormal drop in level of 3 m was examined. The soilwas considered deformable down to the depth at which svthe variation in porepressure stale is about 0.2 of the svgeostatic pressure. The average settlementcalculated was 8 mm. Since the ground around the tower is relatively uniform, it

must be assumed that the differential settlements are negligible. In order to evalu-ate the maximum theoretical distortion possible, penetrometric profiles were cal-culated at opposite sides using all the minimum and maximum values recordedduring the various tests at various depths. Maximum settlements of 11 mm and aminimum of 6 mm were obtained. The ultimate differential settlement would, there-fore, be 5 mm and consequently of negligible effect on the stressstrain conditionwithin the structure.

1.2.2.3 Physical, chemical and mechanical tests on the componentsTo determine the effect of any possible chemical or physical degradation ofthe masonry, numerous samples of mortar were taken from the large blocks ofmasonry. The bricks and stones showed no signs of degradation except in the out-ermost area; in fact, even in the most deteriorated areas of the examined blocks,the degradation did not penetrate any deeper than 80100 mm.

Chemical and mineralogical/petrographic analyses were performed on 22 sam-ples of mortars. The chemical analyses revealed that the binder used for the mor-tars during the first building phase consisted chiefly of lime putty (soluble silica0.280.40%) and that the aggregate was mainly siliceous (unsoluble residuebetween 69.94 and 82.04%). The binder/aggregate ratio varied from 1:3 to 1:5.Similar values were obtained for the mortars of the second and third buildingphases. The porosity was around 1213% and the bulk density about 18.5 kN/m3.In most cases, the sulphur trioxide content was negligible (around 0.06). Opticalinspection of thin sections of the mortar revealed numerous porous areas whichwere sometimes covered by a layer of carbonates of relatively recent formation,thus making the surface of the mortar far more resistant. This could be the result

of calcareous matter being deposited by flows of water. Similar deposits have beenfound in different areas of the masonry [6] and in each case the covering layerstrengthened the surface of the mortar.

Thin section mineralogical/petrographic analysis also confirmed the total car-bonation of the mortars and the siliceous nature of the aggregate and revealedcorrosion along the surface of contact between certain aggregates (pebbles ofstained quartz and flintstone, etc.) and the binder. As it is quite common [7], how-ever, the reaction products cause no fissures inside the mortars. The adhesion

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between mortar, bricks and stone was also fairly good (except in cases where thebuilding techniques had left large voids).

The mortars were consistent, as the mechanical tests confirmed, had a low con-

tent of sulphates and did not show heavy deterioration except for the outermostones.

The possibility of any significant reduction in structural strength of the masonrydue to the chemical or physical degradation of the mortars or other materials was,therefore, excluded.

Since the collapse was not caused by the degradation of the building materialsor sudden or differential settlement, attention was turned to how dead and liveloads might have affected the mechanical behaviour of the materials over time.

Compression tests were performed on small cubes of mortar [5] (with sidesranging from 2.7 to 3.5 mm) taken from the mortar joints of the inner conglomer-ate. The strength was 2.9213.37 N/mm2, with a mean value of 6.45 N/mm2andSD 49% (n= 11). Since the specimens are very small these results are merelyindicative. Nevertheless it can be said that the results confirm the chemical andphysical analyses; in general, the mortar was consistent despite its heterogeneityand very hard and strong when sampled.

The compressive strength of the bricks, on the other hand, as tested on cubeswith sides of 4050 mm was rather low: the mean value was 13.37 N/mm2, with

SD 26% (over 50 specimens). The elastic modulus between 20 and 60% of thepeak stress was 1973 N/mm2for the bricks and 905 N/mm2for the mortars [5].

Tests reported later show that the strength of the masonry was less than that ofthe mortar, suggesting that the low carrying capacity of the masonry was mainlydue to the construction technique.

1.2.2.4 Compression tests on masonry prisms

Compression tests were performed on prisms of masonry, cut from large blocksthat had remained intact, in order to obtain the stressstrain curve up to and beyondfailure [5].

Fatigue tests were then performed using a load value reproducing the stressinduced by the dead load and applying a cyclic load, the amplitude of which repro-duced the stress variations due to the effects of the wind.

Lastly, a survey of the effects of the dead load of the tower on the behaviour ofthe materials over time was carried out by means of constant load tests.

Prisms measuring 4000 600 700 mm approximately were obtained from the

recovered blocks. These dimensions were chosen so as to simulate the behaviourof the masonry, which was very thick (2.8 m) compared to height (60 m) and planform (12.3 12.3 m) of the tower. The load-control or displacement-control com-pression tests were carried out with a 2250 kN, servo-controlled MTS hydraulicpress, with programmed cycles.

Monotonic compression tests to failure were conducted on seven masonryprisms from the first two building phases. The tests were carried out underdisplacement-control, at rates of 3.85 103mm/s and 9.62 104mm/s.

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Figure 1.7 shows the curves obtained for the seven prisms, two of which (102Aand 102B) were tested by applying the load in the direction of the horizontal joints.The peak strengths and ultimate strain values vary quite considerably: low ulti-mate strains appear to correspond to higher strengths. Strength varies from 2.0 to4.1 N/mm2, ultimate strains from 3.0 to 5.5 103and the modulus of elasticity,defined between 20 and 40% of the peak stress, varies from 719 to 1802 N/mm2.

Five prisms were tested to failure by means of loading and unloading cyclesapplied every 0.5 N/mm2under displacement-control conditions up to and beyondthe peak stress. Strength varied from 1.8 to 3.3 N/mm2, the elastic modulus from

544 to 1455 N/mm2and the ultimate strains from 3.6 to 8.5 103.A typical curve is shown in Fig. 1.8. Although on average the strength is lower

than that of the seven prisms subjected to the monotonic tests, the cycles appearnot to have any great influence on the securve, the peaks of which at each load-ing approximate well to points of the monotonic curve.

1.2.3 Long-term tests

The behaviour detected from cycling tests and particularly the evident increasein deformation while the stress was kept constant (Fig. 1.8) led to a study of theeffects of fatigue and long-term tests at constant load. The experimental researchis described in the following sections.

1.2.3.1 Fatigue tests

It is well known that repeated load cycles cause damage to the material. The dam-age originates from imperfections in the material itself, such as small cracks which

Figure 1.7: securves ofprisms subjected to monotoniccompression tests.

Figure 1.8: securve obtained for a cycliccompression test.

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hydrated lime [9]. This can be the case, for instance, of very thick ancient wallsor of masonry characterized by very thick mortar joints like those of St. Vitale inRavenna shown in Fig. 1.13 [10].

Increasing deformation due to heavy dead or cyclic loads can vary the geometryof masonry walls in a visible way already during the construction. These modifica-tions can occur locally, or involve a whole structural element. Large displacementsand deformations frequently involve piers and columns like those of gothic cathe-drals (Fig. 1.14) [11]due to the horizontal thrust exerted by vaults and arches ordue to soil and structure settlements.

Generally speaking, old or ancient structures are continuously subjected tomodifications concerning their geometry and their state of stress and strain. J.L.

Taupin [12] says that time moulds the structure of towers, cathedrals, bridges etc.which we would like to consider immutable. Time plays a role both in the shortand in the long run dispersing and returning energy in three ways: through defor-mations and settlements, through vibration, and through material modification ordeterioration.

Figure 1.15 shows a detail of the well-known Hagia Sophia at Istanbul, wherethe rotation of a column and the deformation of an arch in the north gallery can beclearly observed [13]. Figure 1.16 shows a much less famous small Romanesquechurch, St. Maria la Rossa in Milan, dating from the tenth to thirteenth century.

This single aisled brickwork masonry building is covered by a timber gable roof,with a chancel comprised by two small chapels besides the choir, terminating witha semicircular apse. The church was subjected to different transformations duringcenturies, and its present aspect is due to the restoration works done in the 1960s.In the picture the tilt of the lateral walls and the deformation of the central arch canbe seen [14].

Figure 1.13: View of the Basilica of San Vitale in Ravenna (Sixth century AD).

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Figure 1.14: Pillar of the Cathedral of Salisbury [11].

Figure 1.15: Hagia Sophia, northgallery looking west.

Figure 1.16: Church of St. Maria laRossa, view of the central navelooking east.

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at a height of 30 m, together with a damaged zone at a height of 1125 m with amultitude of very thin and diffused vertical cracks.

A similar crack pattern is visible on the Torrazzo, a medieval brickwork tower adja-cent to the Cathedral of Cremona (Fig. 1.23a and b) [21]. The precise date of construc-tion is not known but is assumed to be around the thirteenth century. It belongs to agroup of monuments, including the Cathedral, the Baptistery, the Town Hall Palace,the Militia Loggia, which forms one of the most beautiful Italian squares.

The external load-bearing walls of the tower, which is about 112 m tall, havebeen showing several cracks for many years [21]; since the crack pattern has expe-rienced an evolution, a time-dependent behaviour of the material may possibly beassumed to cause the phenomenon.

1.5 The role of investigation on the interpretation of the

damage causes

On the basis of the previous experience the authors have developed investigationprocedures for the safety of these structures; the idea came first when studying thecollapse of the Civic Tower in Pavia.

Figure 1.19: The bell-towerof the St. Marco Basilica inVenice.

Figure 1.20: Detail of the collapse with thereinforced pillar [18].

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The procedure is based on the following steps: (i) historic research to know theevolution of the structure over time, (ii) geometrical and crack pattern surveys,which allow one to understand the evolution of the structure, to calculate weightsand give a first interpretation of the crack pattern, (iii) geognostic investigation andmonitoring, to understand the soilstructure interaction, (iv) on-site mechanical andnon-destructive testing (radar, sonic, etc.) to define local states of stress and stressstrain behaviour of the material, (v) chemical, physical and mechanical tests onmortars, brick and stones to find their composition and their characteristics, (vi) ifnecessary, passive and active dynamic tests on site to survey the overall structuralbehaviour and (vii) monitoring system applied to the structure when necessary.

1.5.1 The bell-tower of the Cathedral of Monza

The bell-tower of the Cathedral of Monza, is a masonry structure 70 m high, witha square plan (a side is 9.7 m long) with solid brick walls 140 cm thick. The towerconstruction started in 1592, probably following the design of Pellegrino Tibaldi,the architect of the Pavia tower belfry, and ended in 1605 [19, 22]. The only dam-age to the tower reported by the documents occurred in 1740 and was due to a firewhich started in the bell-tower and caused the collapse of the belfry dome androof and the fall of the bells with their supporting frame down to the vault of the

Figure 1.21: The church of St. Magdalena in Goch (Germany) after the collapse ofthe bell-tower, 1993.

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first floor at 11 m. No damages were reported in other known calamities, such aslightning or thunderstorms through the centuries. Nevertheless cracks are presentsince 1927 or even before, as mentioned above. From 1978 the cracks have beensurveyed with removable extensometers: they show a slow increase of their open-ing through time. From 1988 the rate of opening seems to be increasing faster.The trend of widening of the three main cracks was calculated as 30.6, 31.3 and39.7 m/year from 1978 to 1995. Actually if this trend is considered from 1988 to1997 the values change, respectively, into 41.2, 35.2 and 56.2.

Figure 1.22: Survey of the crackpattern for the bell tower of theCathedral of Monza: (a) west and(b) east sides.

Figure 1.23: Survey of the crack patternfor the Torrazzo tower: (a) west and(b) east sides.

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The first step of the investigation procedure [19] was the geometrical survey[20]. A geodetic network set up in the square of the Cathedral in 1993, was usedas support. No relevant leaning was measured due to the small subsidence which

is taking place in the square. Two distinct products were obtained: (i) a detailedthree-dimensional model from which the external and internal prospects and thevertical sections were obtained and (ii) a simplified model for which only theessential aspects of the geometry were preserved for the structural analysis.

The survey of the crack pattern showed that the tower walls have a dangerous distri-bution of passing-through cracks on the western and eastern load-bearing walls formore than 50 years, and of a net of thin vertical cracks from a level of 11 m up to 30 m(Fig. 1.22). Other cracks can be seen on the internal walls of the tower; they are very

thin, vertical and diffused along the four sides of the tower and deeper at the sidesof the entrance where the stresses are more concentrated. The thin diffused cracks run450 mm deep inside the section, reducing its total working thickness from 1400 mmto no more than 900 mm. From laboratory tests it was found that the mortar is veryweak and made with putty lime and siliceous aggregates; also the bricks were of poorstrength (between 4 and 12 N/mm2measured on 40 mm-side cubes).

On-site single flat-jack tests were carried out at different heights of the tower(5.4, 5.6, 13.0, 14.0, 31.5 and 38.0 m) and the stress values against the height areplotted in Fig. 1.24.The maximum compressive stress acting in the tower, mea-

sured on site by the flat-jack test, is about 2.2 N/mm2. The most interesting infor-mation came from the double flat-jack test results, where it was possible to see thereal risky situation if compared with the local state of stress measured by the singleflat-jack (Fig. 1.25). Passive dynamic tests using the bell ringing were also carriedout monitoring the dynamic excitation of the extensometers applied across the

Figure 1.24: Single flat-jack tests of Monza.

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Archive research did not clarify the date of construction; nevertheless the high-est number of reference data collected locates the date of construction between theeighth and the thirteenth centuries. In 1491, the porch of the Bertazzola was added

connecting the Torrazzo with the Cathedral and in 1519 the Loggia was builtresting on the arches of the porch. Maintenance works were carried out startingfrom the fifteenth century. These works mainly concerned the highest part of thetower damaged by storms and lightening, especially the stone and brick columnswhich were sometimes substituted. The last intervention at the Ghirlandina wascarried out in 1977. The works performed were the following: connection of struc-tural and decorative elements, construction of a concrete frame sustaining the twincolumns of the Stanza delle Ore (at 85 m height) and surface treatments of stone

and brick elements with an epoxy resin.The first step of the investigation carried out in 1998was the geometrical sur-vey. A principal network defining fixed points in the horizontal and vertical planwas set up having 21 nodes inside and around the tower made with fixed nails. Theco-ordinates of the nodes were determined with a T2000 WILD equipment. Thevertical and horizontal profiles were determined by rays starting from the networknodes, using a TC1600 DIOR system and an auto scanning Laser System MDL. Aphotogrammetric survey of the external prospects was also carried out usingTC1600-DIOR and T460* DISTO equipment. The prospects were obtained by a

Rollei special software, MSR. The survey enabled the finding of some irregulari-ties of the structure: (i) a 21 cm horizontal displacement of the centre of the towerin direction north-east, calculated from the ground level to the top at 112 m, (ii) anon-symmetrical reduction of the plan dimensions from the ground level to the topat 31 cm for the north-east corner and 66 cm for the south-west corner, (iii) theGhirlandina not being perfectly centred on the square part of the tower, but with aslight counter-clockwise rotation toward west.

The presence of a diffused crack pattern particularly on the western and theeastern sides of the tower and on the Ghirlandina can indicate high states of stressdue to the dead loads, the temperature variations and/or to a slight leaning. Thesurvey was carried out on the outer surfaces by reaching the height of 60 m thanksto a special crane. The crack pattern is certainly also influenced by differentialmovements due to temperature variation between one side and the other of thetower. The highest variations certainly occur between the north and the south side.The west side has a diffused fissuration with passing-through cracks; the cracksare mostly vertical and start from approximately 20 m. Important cracks appearalso between 48 and 60 m from the ground level (Fig. 1.23a). The north side is

cracked in the centre between 27 and 40 m and at the north-east corner. The eastside is cracked between 6 m and 20 m from the ground level and between 35 and60 m (Fig. 1.23b). The south side has few cracks located between 14 m and 27 m.The Ghirlandina shows the most important cracks, on the buttress and on the brickcolumns particularly on the south-west corner. Also the internal part of the tower,along the staircase and inside the rooms shows a diffused crack pattern with somepassing-through cracks. Three thresholds were established concerning the measureof the opening of the cracks: 10 mm. The crack

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where the leaf seems to be detached from the rest of the wall; following theseresults the application of NDT technique was required in order to map the detachedareas which represent structurally a reduction of the wall section to be taken into

account when modelling. All the areas from where samples were taken were thenrepaired with similar bricks and mortars.

Flat-jack tests: Single and double flat-jack tests were carried out on the Torrazzo.The single flat-jack test was also used to study the behaviour of the external leafof the wall. Different types and dimensions of flat-jacks were used: (i) 240 mm 12 mm rectangular jacks where the detachment of the external leaf was suspected,(ii) 400 mm 200 mm rectangular jacks and (iii) 350 mm 240 mm semicircularjack where no detachment was suspected and for the double jack-test.

Twenty-one tests were carried out, of which 19 were with single flat-jack and 2 withdouble flat-jack: 3 single flat-jack tests at between 1 and 5 m from the ground, 7 singleflat-jack tests at 7 m, 10 single flat-jack between 15 and 18 m and 1 single flat-jack at22 m. The double flat-jack tests were carried out at 7.2 and 19 m from the ground.

The results of the single jack tests are reported in Fig. 1.26 and show clearly twosituations: a state of stress varying between 0.4 and 0.9 N/mm2 where the testfound a detached leaf and a state of stress varying from 1.01 and 1.81 N/mm 2where no detachment was found.

Also double flat-jack tests were performed and Fig. 1.27shows the stressstrain

plots. It was impossible to carry out tests at higher levels due to the lack of scaf-folding and of appropriate means for carrying the jack equipment. In future othertests will be carried out.

1.7 m

7.2-7.4 m

16.6-17.8 m

15.2-16.5 m

7.2-7.7 m

7-7.7 m

5 m

15.4-19.1 m

16-17.8 m

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

Stress[N/mm2]

masonry section 3.3 m

masonry section 1 m

presence of veneer

inner walls (rooms)

Figure 1.26: Single flat-jack tests of Cremona.

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1.6 Comparison between the two towers

Since the bell-tower of Monza is considered a building with high risks of collapse,a comparison between the data collected on both towers seems to be useful tounderstand better the real situation of the Torrazzo.

As mentioned above, the mortar composition of the two bell-towers does not dif-fer much from one another,though the Torrazzo mortar seems to be more consistent.The bricks of the Monza tower are generally weaker than those of the Torrazzo(Fig. 1.28a and b)except for the brown type, which is mainly used on the outsidesurface of the bearing walls and very seldom used in the interior. On the contrarythe brown and the red bricks are evenly distributed in the Torrazzo walls.

It is also interesting to compare the results of single and double flat-jack testscarried out on the two towers.

The results of four tests, two for each tower are discussed. In Fig. 1.27a and bmaximum stress reached with the double flat-jack test on the Torrazzo togetherwith the values obtained with the single one, respectively, at 7 and 19 m height areconsidered, showing an elastic linear behaviour up to, respectively, 2.45 and 2.7 N/

mm2

. The maximum stress level when cracks clearly appear is, respectively, 3.77and 3.77 N/mm2and the state of stress measured is 1.5 and 1.5 N/mm2. So in thesetwo cases the safety coefficient at collapse is certainly more than 3.

In Fig. 1.25a and b the results of two tests at the height of 5 and 13 m, out of thefour carried out on the walls of the Monza tower, are considered. Here the linearelastic behaviour stops at, respectively, 1.65 and 1.1 N/mm2and the maximum stressreached before cracks propagated was 2.62 and 1.87 N/mm2. The measured localstate of stress was, respectively, 1.67 and 0.98 N/mm2. In these two cases the safety

-3.00 -2.00 -1.00 1.000.00 2.00 3.00 -3.00 -2.00 -1.00 1.000.00 2.00 3.00

Strain [m/mm] Strain [m/mm]

h vh v

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00(a) (b)

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

Stress[N/mm2]

Stress[N/mm2]

Localstate

ofstress

Localstate

ofstress

Figure 1.27: Stressstrain plot at (a) 7.2 m and (b) 19 m height.

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coefficient at failure is much lower than in the first one and certainly less than 2.Furthermore in the case of the Torrazzo the modulus of elasticity is much higher andthe Poisson ratio much lower than in the case of the Monza tower.

1.7 Conclusions

The investigation carried out on the specimens cut from the walls of the Pavia towerafter its collapse allowed formulating for the first time on an ancient masonry thehypothesis of a collapse due to the long-term behaviour of the material. Probablysince the construction of the bell-tower in the sixteenth century the structure wasunder a high state of stress and the damage very slowly but continuously increasinguntil the collapse. The creep behaviour of the material was shown clearly duringthe experimental research which started in 1989 and is still developing, as will beshown in Chapter 2. Examples of other similar situations were found in the historyof collapses of towers and damages or collapses of churches (Noto cathedral).

The two experiences of investigation on tall towers allow some concluding

remarks:

the on-site and laboratory tests carried out following the methodology describedin the first section allowed one to detect situations of danger and to characterizethe materials and calculate input parameters for the structural analysis;

the laboratory tests were able to show the difference of properties of the ma-sonry in the two buildings and that where the materials used are weaker, thedamage is more;

-20 -16 -12 -8 -4 0 4 8 12 16 20Strain [m/mm]

0246

810121416182022242628

303234(a) (b)

Stress[N/mm2]

red brickbrown brick

h

h

v

v

-20 -16 -12 -8 -4 0 4 8 12 16 20Strain [m/mm]

0

2

4

6

8

10

12

14

16

1820

22

24

26

Stress[N/mm2]

red brickbrown brick

Figure 1.28: Stressstrain plot for (a) Monza tower bricks and (b) Torrazzobricks.

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[12] Taupin, J.L., Rflexions sur la cathedrale Saint-Pierre de Beauvais.ANAGKH, 12, pp. 86100, 1995.

[13] Mainstone, R.J.Haghia Sophia:Architecture, Structure and Liturgy of theJustinians Great Church, Thames and Hudson, 1985.

[14] Binda,L., Mirabella Roberti,G. & Guzzetti,F., St. Vitale in Ravenna: aSurvey on materials and structures,International Symp. on Bridging LargeSpans (BLS) from Antiquity to the Present, Istanbul, Turkey, pp. 8999,2000, ISBN 975-93903-02.

[15] Jaeger, J.C. & Cook N.G., Fundamentals of Rock Mechanics, 2nd edn,Chapman & Hall: London, 1976.

[16] Lenczner, D. & Warren, D.J.N., In situ measurement of long-term move-

ments in a brick masonry tower block. Proceedings of the 6th IBMaC,Rome, pp. 14671477, 1982.[17] Anzani, A., Binda, L. & Mirabella Roberti, G., The behaviour of ancient

masonry towers under long-term and cyclic actions, in Proc. ComputerMethods in Structural Masonry 4, Computer & Geotechnics: Swansea,pp. 236243, 1998.

[18] Fradeletto, A., et al., Il campanile di S. Marco riedificato. Studi, ricerche,relazioni, ed. Comune di Venezia, Carlo Ferrari: Venezia, 1912.

[19] Binda, L., Tiraboschi, C. & Tongini Folli, R., On site and laboratory inves-

tigation on materials and structure of a bell-tower in Monza.Int. Zeitschriftfr Bauinstandsetzen und baudenkmalpflege, 6, Jahrgang, AedificationPublishers, Heft 1, pp. 4162, 2000.

[20] Binda, L., Tongini Folli, R. & Mirabella Roberti, G., Survey and investigationfor the diagnosis of damaged masonry structures: the Torrazzo of Cremona.12th Int. Brick/Block Masonry Conf., Madrid, Spain, pp. 237257, 2000.

[21] Binda, L. & Poggi, C., Ricerca volta a stabilire le condizioni statiche edil comportamento meccanico della muratura del campanile del Duomo diCremona.Relazione Finale, Contratto Consiglio della Chiesa Cattedrale diCremona, 1999.

[22] Scotti, A.,Let dei Borromei in Monza.Il Duomo nella storia e nellarte,Electa: Milano, 1989.

[23] Binda, L., Falco, M., Poggi, C., Zasso, A., Mirabella Roberti, G.,Corradi, R. & Tongini Folli, R., Static and dynamic studies on the Torrazzoin Cremona (Italy): the highest masonry bell tower in Europe. Int. Symp.on Bridging Large Spans from Antiquity to the Present, Istanbul, Turkey,pp. 100110, 2000.

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CHAPTER 2

Experimental researches into long-termbehaviour of historical masonry

A. Anzani1, L. Binda1& G. Mirabella Roberti21Department of Structural Engineering, Politecnico di Milano,Milan, Italy.2Department of History of Architecture, University Iuav of Venice,Venice, Italy.

2.1 Introduction

The time-dependent behaviour of ancient masonry structures, often characterizedby non-homogeneous load-bearing sections, is considered among the factorsaffecting the structural safety of monumental buildings. Together with othersynergetic aspects, this has proved to be involved in collapses, which occurredduring the last thirty years.

Exploiting the ancient (from the Middle Ages to the sixteenth century) masonrycoming from the ruins of the collapsed tower of Pavia, several experimentalprocedures have been adopted to understand the phenomenon, from creep topseudo-creep tests at different time intervals, and various rheological models havebeen applied to describe the creep evolution and creep-induced damage, asexplained later in this book in Chapter 7.

Purpose of the testing activity has been initially the identification of the creepbehaviour as a possible cause of the collapse of buildings, then the study of factorsaffecting creep (rate of loading, stress level, etc.) and the set-up of the most suit-able testing procedures to understand the phenomenon, and finally the individua-tion of significant parameters (e.g. strain rate of secondary creep phase) that maybe referred to as risk indicators in real structures.

After the first tests carried out on prisms of dimension 400 600 700 mmdescribed in Chapter 1, long-term tests on six prisms coming from the ruins ofthe tower of Pavia and one from the crypt of Monza were performed, someof which lasted 1000 days. Considering that long-term tests require constant

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thermo-hygrometric conditions and especially designed testing apparatus, a morerapid and therefore more convenient testing procedure was subsequently preferred.The so-called pseudo-creep tests were carried out applying the load by subsequent

steps corresponding to a constant value (generally 0.25 or 0.3 MPa) kept constantfor a specific time interval. Different durations of the time interval have beenexperimented (from 300 to about 30,000 seconds) that allowed one to indirectlyobserve the influence of the rate of loading. In fact, these tests characterized by aregular load history tend to simulate, by discrete load steps, monotonic tests wherethe load increases continuously at an equivalent rate that can be calculated. Theygive the opportunity to satisfactorily catch the limit between primary and second-ary creep phase.

Considering tertiary creep, pseudo-creep tests imply a disadvantage. In fact, theload value that in a monotonic test at the equivalent rate of loading would causefailure may not correspond exactly to one of the applied load steps, but to an inter-mediate value (Fig. 2.1). Therefore, if the applied load is higher than that whichwould have caused tertiary creep and failure, the specimen collapses instanta-neously, without showing the tertiary creep phase. The latter is not particularlyimportant in itself; what is interesting is the secondary creep strain rate just beforefailure. The problem could be solved by simply prolonging the time interval of thelast load step so as to reach the failure limit curve (see Fig. 2.1) in constant load

conditions instead of at increasing load. Actually, before reaching it, the stress atfailure is not known; however, the failure conditions may be roughly previewed byestimating the ultimate stress through sonic tests and by controlling accurately thestrain rate in real time during the pseudo-creep test. In some of the cases describedbelow tertiary creep was therefore recorded.

v

vfailure limit curve

viscosity limit curve

Figure 2.1: Pseudo-creep testing: simulation of monotonic test at an equivalentrate of loading.

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LONG-TERMBEHAVIOUROFHISTORICALMASONRY 31

In the case of the masonry from the Pavia tower, different prism dimensions wereadopted in order to use as much as possible the material coming from the irregularblocks taken from the ruins, compatibly with the capacity of the testing machine. On

the contrary, from the crypt of Monza two big blocks were purposely extracted fromwhich prisms of the same dimensions were obtained. When relevant, comments onthe influence of the specimen dimensions on the test results will be given.

The mechanical test series were carried out on the prisms previously cappedwith 1:3 cement mortar; PTFE sheets were interposed between the sample basesand the machine platens (Fig. 2.2a); a hydraulic compressive machine MTS (2500KN) was used, connected with a control unit for data-acquisition, a plotter produc-ing load-displacements diagrams, a PC for storing data. If not differently specified,vertical and horizontal displacements were measured directly on the prisms usingfour LVDT with a base of 150 mm and four LVDT with a base of 180 mm, respec-tively; two overall vertical readings were also taken from plate to plate of themachine in case the other LVDTs had fallen during the tests (Fig. 2.2b and c).

2.2 Tests on the masonry of the Civic Tower of Pavia

After the sudden collapse of the Civic Tower of Pavia (built from eleventh tosixteenth century), during the investigation into the causes, many prisms of differ-

ent dimensions were obtained out of the large blocks coming from the ruins of thetower and constituting the medieval trunk of the structure (Fig. 2.3). The prisms,subjected to mechanical tests, had mainly been obtained from the conglomerateforming the very thick inner core of the 2800 mm three-leaf walls (Fig. 2.4); afew of them were coming from the fairly regular external layers made of romanbrick masonry of thickness varying between 150 and 490 mm; no specimens wereinitially sampled from the plain masonry belonging to the sixteenth-century belfry,as it was not involved in the initiation of the collapse [1] although this addition

may be suspected as a remote trigger responsible for the collapse [2].

(a) (b) (c)

Figure 2.2: Preparation and instrumentation of the prisms for mechanical testing.

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Only recently, it was decided to study also the sixteenth-century plain masonrybelonging to the upper part (Figs 2.5, 2.6). In fact, its behaviour may provide auseful comparison for other structures of the same age and constructive technique,being a historical masonry not usually available for mechanical testing.

Prisms of larger dimensions were cut first and progressively smaller specimenswere also obtained subsequently in order to exploit the historical material as muchas possible. The 400 600 700 mm masonry prisms were identified by a number

Figure 2.3: Civic Tower of Pavia before failure.

Eleventh

twelfth

centuries

Sixteenth

century

Figure 2.4: Detail of the cross-sectionof the 2800 mm thick medievalmasonry.

Inner leaf

Outerleaf

Figure 2.5: Cutting the sixteenth-century plain masonry.

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Two prisms of the sixteenth-century plain masonry of dimension 200 200 350 mm were tested, the results of which are reported in Table 2.2. A higher peak

stress and a lower value of maximum vertical strain than in the case of the innerleaf have been registered, indicating an important influence of the constructiontechnique on the masonry mechanical behaviour.

The stressstrain diagrams of the larger size prisms are shown in Figs 2.9 and2.10. Considering the vertical strains, the marked initial locking branch appearingin Fig. 2.9 is due to the fact that, in this case, the plotted strain has not been readdirectly on the specimens, but calculated from the displacement between themachine platens. In all cases, a linear part can be seen during which no visible

Figure 2.7: Apparatus forsonic tests.

1000 1200 1400 1600 1800 2000sonic velocity [m/s]

1.0

1.5

2.0

2.5

3.0

3.5

4.0

m

[N/mm2]

Figure 2.8: Compressive strength vs. sonicvelocity: tests on 400 600 700 mm masonryprisms.

Table 2.1: Results of monotonic tests on the masonry of the inner leaf.

Dimensions sfv ef

v sf

v, ave efv, ave

Sample (mm) (MPa) (103) (MPa) (103)

67C 400 600 700 2.0 4.094B 3.1 3.094D 2.5 3.0100C 3.0 3.9 2.8 4.14100D 2.5 5.5102A 2.4 5.2102B 4.1 4.4Y 200 200 350 2.26 2.5

18-10M 100 100 180 3.86 2.20 3.83 2.6319-10A 3.60 3.9019-10H 3.00 2.90102-10B 3.90 2.14102-10A 4.80 2.04

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values followed an increasing trend up to the peak stress, and in general were higher

than the values of the tangent modulus calculated for monotonic tests [6].More specific tests were then carried out on 100 100 200 mm prisms tounderstand the effect of cyclic actions, e.g. that of wind or that of thermal cycles.

Figure 2.12 shows the results obtained on a sample which was loaded monotoni-cally and submitted to cycles of 0.5 Hz frequency and 0.05 N/mm2amplitude, atdifferent stress levels. Cycles acting within a relatively small load range provedcapable of provoking noticeable material strain. When applied at a high stress level,relative to the material strength, load cycles appeared particularly effective, havinginduced failure at a lower stress level than the estimated peak stress [3].

2.2.4 Creep tests on prisms of 300 300 510 mm

Six prisms of dimensions 300 300 510 mm were tested in compression in con-trolled conditions of 20C and 50% RH at ENEL-CRIS Laboratory (Milan), usinghydraulic machines capable of keeping constant a maximum load of 1000 KN. Thedimensions adopted for the prisms were the maximum compatible with the testingmachine. The load was applied in subsequent steps, kept constant until either the creep

strain reached a constant value or a steady state was attained. The first stress level waschosen between 40 and 50% of the static peak stress of the prisms, estimated by sonictests. The test results are reported in Table 2.3 and in Fig. 2.13a and b.

From the experimental data, the development of all the creep phases was evi-dent, with secondary creep showing even at 41% of the estimated material peakstress and tertiary creep showing at about 70%; material dilation took place undersevere compressive stress corresponding to high values of the horizontal strain dueto slow crack propagation until failure [3].

00.0 1.0 2.0 3.0

1

2

3

4

5

v[N/mm2]

v(103)

Figure 2.12: Results of a cyclic teston a 200 200 400 mm masonryprism.

420 6 8 10

0

1

2

3

4

5

v[N/mm2]

v(103)

Figure 2.11: Results of a cyclic teston a 400 600 700 mm masonryprism.

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The strain vs. time values of one of the prisms tested are reported in Fig. 2.14aand b. Due to technical problems, after 630 days from the beginning of the test theload was unintentionally lowered to zero for 90 days. Undesired unloading causedonly partial strain recovery without affecting very much the test results.

Despite the apparent scatter due to the sensitivity of the calculated value to ran-dom reading errors, the volumetric strain seems nearly constant during the first loadsteps of the test. Subsequently it starts to decrease markedly: the slope of the plot

Table 2.3: Results of long-term tests on inner leaf prisms of 300 300 510 mm.

Sample Test duration (days) sfv(MPa) ef

v(103)

19-30A 1170 2.0 4.7519-30B 1082 2.0 2.9267-30B* 163* 1.3* 0.70*47-30A 524 2.0 3.5041-30B 894 2.3 2.70102-30A 894 2.9 2.90Average 2.24 3.35

*Collapsed by premature failure; not included in average calculation.

0 200 400 600 800 1000 1200

time [days]

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4(a)

(b)

h(103)

v(103) Primary creep

Tertiary creep

Secondary creep

dilation

0 200 400 600 800 1000 1200

time [days]

0.0

0.5

1.0

1.5

2.0

2.5

3.0

v

[N/mm2]

Figure 2.13: Results of creep tests on prisms of 300 300 510 mm from theinner leaf.

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2.2.5 Pseudo-creep tests on prisms of 100 100 180 mm

Twelve prisms of dimensions 100 100 180 mm were tested in compression by

subsequent load steps corresponding to 0.3 MPa kept constant for different timeintervals, respectively, of 300, 900, 3600 and 10800 s, as indicated in Table 2.4. Aninitial step of 0.6 MPa was first applied. Before the application of any new load step,the sample was completely unloaded so the unloading Young modulus could be eval-uated [4]. Looking at Table 2.4, it is interesting to notice that the average peak stresstends to decrease and the corresponding strain tends to increase at extending the timeinterval, indicating that the stressstrain behaviour is strongly time-dependent.

The results of a test carried out at 10800 s are reported, as an example, in

Fig. 2.16: at each load step primary creep occurred and, during the last load step,secondary and tertiary creep.

2.2.6 Pseudo-creep tests on prisms of 200 200 350 mm

A first series of four prisms, three coming from the external layers of the masonryand one coming from the inner part constituting the trunk of the tower of Pavia,was tested applying constant load steps of 0.25 MPa and keeping them constant for10800 s. The test results are reported in Table 2.5 and in Fig. 2.17.

Strictly speaking, a direct comparison between these results and those previouslypresented could not be done, due to various factors: different testing procedures,different dimensions of the specimens, non-homogeneity of the material, differenttexture of these prisms with respect to the previous ones. Anyway, this last aspectseems to have played a major role, since higher values of peak stress and lowervalues of the corresponding strain have on average been obtained in this case.

Table 2.4: Results of pseudo-creep tests on inner leaf prisms of 100 100 180 mm

at different timeintervals. Time sf

v ef

v sf

v, ave efv, ave

Sample interval (s) (MPa) (103) (MPa) (103)

18-10A 300 3.24 3.19 3.68 3.1718-10B 4.18 3.6018-10C 3.63 2.7318-10D 900 2.69 4.71 2.73 4.2018-10E 2.99 4.15

18-10F 2.52 3.7518-10G 3600 2.89 5.48 2.47 4.1218-10H 2.39 3.9418-10I 2.15 2.9618-10J 10800 2.59 5.78 2.68 5.8718-10K 2.88 10.2118-10L 2.58 1.65

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The horizontal strain takes higher absolute values than the vertical strain, indi-cating that at failure dilation takes place. The results of a test on a single specimenof the first series are shown in Fig. 2.18a and b. Considering the trend of the stressstrain plot (Fig. 2.18a), a linearly elastic behaviour can be observed below a stress

value of 3.25 MPa. Correspondingly, the straintime plot shows that within thisinterval only primary creep develops. After that level, the stressstrain diagramindicates non-linear behaviour; and the straintime plots exhibit the steady-state(or secondary creep) and, eventually, the tertiary creep phases. The volumetricstrain (Fig. 2.18b) keeps almost naught values approximately during the firsttwelve load steps and subsequently gets decreasing negative values until collapse.As appears also from the data previously presented, decreasing volumetric straincan be certainly interpreted as a sign of increasing material damage.

60 3 9 12v(103)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

v[N/mm2]

60

120

180

t[min]

Figure 2.16: Pseudo-creep tests on a 100 100 180 mm prism of the inner leaf.

Table 2.5: Results of compression tests on medieval outer leaf prisms of 200 200 350 mm, at constant load step.

Sample Test interval (s) sfv(MPa) ef

v(103)

40-20B 10800 5.25 1.1740-20C 7.00 1.2757-20A 4.50 1.80Average 5.583 1.413

102-20A* 3.50 0.90*Inner leaf, not considered for average calculation.

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Figure 2.19 shows the crack pattern of a prism after the test. A more regulartexture than that appearing in Fig. 2.15 characterizes this sample, with the pres-ence of whole bricks lying horizontally; nevertheless a great difference betweenthe four faces of the same prism has to be pointed out. The vertical cracks tend tofollow the directions corresponding to the interfaces between mortar and bricks

and split the prism faces from bottom to top.A second series of pseudo-creep test was then carried out on additional prismsrecently obtained from the ruins of the tower. A total of four prisms coming fromthe inner medieval masonry (labelled Q, B, M and S) and four coming from thesixteenth-century solid masonry (labelled Ec, W, K and In) were tested applyingsubsequent load steps of 0.3 MPa kept constant for intervals of 28800 s (Table 2.6).On the average, higher peak stresses (s

f) and lower strains at failure (e

vmax, e

hmax)

were registered on the plain masonry.

0 1 2 3 4 5 6 87

v

(103)2

4

6

v(Mpa)

120008000

4000

time[sec]

0 40000 80000 120000 160000 200000 240000

time[sec]

-30

-20

-10

0

10

20(b)(a)

(10

3)

'v

'h

evol

Figure 2.18: (a) Vertical stress vs. vertical strain and vertical strain vs. time on prism40-20B. (b) Deviatoric and volumetric strains vs. time on prism 40-20B.

0 100000 200000 300000 400000

time [sec]

-20

-16

-12

-8

-40

4

8

vol(103)

v(103) 40-20b

40-20b

40-20c

40-20c

57-20a

57-20a

102-20a

102-20a

Figure 2.17: Pseudo-creep tests of the first series.

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In Fig. 2.20 the results of the pseudo-creep tests of the second series on the innerleaf and on the plain masonry are compared. As expected, the prisms of the innerleaf reached failure well before the prisms of the plain masonry.

In Figs 2.21 and 2.22 the results of a test carried out on the masonry of the innerleaf and those of a test carried out on the plain masonry are, respectively, shown.In both cases, all the creep phases are visible.

2.3 Tests on the masonry of the crypt of the Cathedral

of Monza

2.3.1 Preparation of prisms of 200 200 350 mm

After the plane rearrangement of the Museum of the Cathedral of Monza in 1994,the removed material, resulting from a door opening on the northern wall of the

Face A Face B Face C Face D

Figure 2.19: Prism of the outer leaf: crack pattern at the end of the test.

Table 2.6: Results of pseudo-creep tests of the second series.

sf e

vmax e

hmax

Specimen Masonry (MPa) (m/mm) (m/mm)

Prism Q Sixteenth-century 6.07 9.52 20.39Prism B solid masonry 4.36 5.98 26.46

Prism M 5.27 4.67 15.56Prism S 4.13 7.19 17.23Average 4.96 6.84 19.91

Prism Ec Medieval 3.59 8.43 11.84Prism W inner leaf 2.75 4.00 7.89Prism K 1.56 3.99 45.35Prism In 2.47 4.78 18.30Average 2.59 5.30 20.85

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crypt, was collected for experimental testing. According to historical informa-tion, the construction of the crypt (concluded in 1577) is nearly contemporaryto the construction of the bell tower (15921605), a building made of solid brickmasonry that was badly damaged by compression and is now undergoing a repair

intervention [8]. As it has become clear after the collapses of the last fifteen years,towers as well as pillars of the cathedrals turn out to be particularly vulnerable tothe effects of persistent loading; therefore, achieving a better experimental knowl-edge on their creep behaviour became crucial. Since considerable amounts of his-torical masonry are not normally available, it was a good opportunity of gainingoriginal sixteenth-century masonry (Fig. 2.23).

Two large blocks were extracted by coring their perimeter in order to obtain asmuch as possible undisturbed material. Subsequently, they were cut by a diamond

0 200000 400000 600000 800000time [sec]

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

h(103)

v(103) S

S

Q

Q

M

M

B

B

Ec

Ec

In

In

K

K

W

W

Figure 2.20: Pseudo-creep curves: (---) inner leaf, () solid masonry.

0

1

23

45

40000

30000

20000

10000

0

t[sec]

-15-20-30-40 -35-45 -25 -10 -5 0 5 10[m/mm]

h v

v[N/mm

2]

Figure 2.21: Results obtained on prismK of the inner leaf.

-15-20-30-40 -35-45 -25 -10 -5 0 5 10[m/mm]

h v

0

1

23

4

5

v[N/mm2]

40000

30000

20000

10000

0

t[sec]

Figure 2.22: Results obtained onprism B of the solid masonry.

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saw into prisms of dimensions 200 200 350 mm to be subjected to differentmechanical testing (Fig. 2.24).

As shown in Fig. 2.24c, the crypt is mainly made of solid brick masonry appar-ently regularly laid which nevertheless includes stones, voids and cracks, and has

therefore to be considered as a non-homogeneous material. In fact, the differentprisms appeared damaged to rather different extents, those subjected to monotonictests showing the most evident signs of damage.

2.3.2 Characterization by sonic tests

Before mechanical tests, the prisms were characterized by sonic tests as describedin Section 2.2.1. The results are reported in Fig. 2.25.

Figure 2.23: Sampling of the masonry from the crypt of the Cathedral of Monza.

(a) (b) (c)

Figure 2.24: Cutting scheme of the masonry sampled from the crypt of theCathedral of Monza.

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failure and the estimated static maximum stress (sm*) is also indicated the latter hav-ing been calculated on the basis of the initial elastic modulus. In fact, the averageratio between the initial elastic modulus and the maximum vertical stress obtainedwith monotonic tests on samples IIp6 and IIp9 was used for the calculation; theresults obtained on prism IIp13 were not included in calculating the average, as thestressvolumetric strain diagram of this sample showed a very dilatant behaviour,probably due to some local effects of the material. Of course, the estimated values ofs

m* cannot be considered statistically relevant, but still they can give an indication on

the severity of the testing procedure with respect to the strength of the material [9].In Fig. 2.27 vertical stress vs. vertical and volumetric strain diagrams obtainedon prism Ip1 are shown as an example. It can be observed that during the applica-tion of the cyclic load a deformation takes place. Moreover, considering the volu-metric strain, it appears that dilation occurs to the material after very low stressvalues are reached.

The strain rate per cycle was calculated for the prisms tested cyclically, as pre-sented by Taliercio and Gobbi [10] relatively to cyclic tests on concrete specimens.

-15 -10 -5 0 5 10 15

vol(103) v(103)

0

1

2

3

4

v[MPa]

IIp13IIp13

IIp6IIp6

IIp9IIp9

Figure 2.26: Monotonic tests.

Table 2.8: Results of fatigue tests. s

m e

v(103) Test duration

Sample (MPa) at failure Ei(MPa) s

m/s

m* (no. of cycles)

Ip1 5.00 9.21 3118 0.97 110286Ip6 5.00 6.13 3774 0.81 51507Ip7 4.25 4.14 2805 0.92 24243

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It was interesting to notice that plotting the strain rate vs. the number of cycles allowsthe primary, secondary and tertiary creep phases to be distinguished quite clearly.Figure 2.28 shows the results relative to the last series of cycles obtained on eachprism: though the number of test results is not particularly significant, a relationshipbetween the strain rate of the secondary creep phase, corresponding to the portion ofthe diagram with horizontal tangent, and the fatigue life of the material, correspond-ing to the total number of cycles at failure, can be found. In particular, the higher thestrain rate of the secondary creep phase, the shorter the fatigue life.

-24 -20 -16 -12 -8 -4 0 4 8 12 16vol(103) v(103)

0

1

2

3

4

5

6

v[MPa]

Figure 2.27: Results of a fatigue test carried out on prism Ip1.

0 2000 4000 6000 8000 10000cycles

0.0

0.5

1.0

1.5

2.0

2.5

v/n(106)

Ip1

Ip6Ip7

Figure 2.28: Strain rate vs. number of cycles in the last series of cycles.

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2.3.5 Creep test on one prism of 300 300 510 mm

The creep test was carried out at the ENEL-CRIS laboratory (Milan) in an espe-

cially designed apparatus, in controlled conditions of 20C temperature and 50%RH. During the 630-day test, three load increments of 1.4, 2 and 2.25 MPa and anunloading phase were applied. In Fig. 2.29 the vertical and volumetric strain areplotted vs. time.

It appears that after load removal, it took more than 100 days for the material tocompletely recover the accumulated creep strain.

2.3.6 Pseudo-creep tests, first series

Compression tests in displacement control were carried out loading the prismsmonotonically until a stress value of 2.25 MPa equal to 65% of the average shortterm strength obtained by monotonic tests and then applying the load in subse-quent steps kept constant for periods of about 5400 s, during which creep straintook place. In this case also unloading reloading cycles had been necessary over-night. Table 2.9 shows the results obtained with this test series: the maximumvertical stress, the vertical strain at failure, the initial elastic modulus calculatedduring the monotonic phase, the test duration and the ratio between the maximum

vertical stress and the static maximum stress are reported.Figure 2.30 shows the diagrams obtained from all the tested prisms and

Fig. 2.31 shows the results obtained on a single prism. Results similar to thoseobtained by the pseudo-creep tests previously presented can be seen. A clear dila-tancy phenomenon is evident, with considerable volumetric strain developingwhen approaching failure, as a typical feature of brittle materials.

2MPa

0 90 180 270 360 450 540 630time[days]

-4

-3

-2

-1

0

1

2

3

4

vol(10

3)

v(103)

0MPa

2.25MPa

1.4MPa

Figure 2.29: Creep test.

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0 20000 40000 60000 80000 100000 120000 140000 160000 180000 200000

tempo [sec]

-20

-15

-10

-5

0

5

10

vol(103)

v(103)

II 4p

II 4p

I 5p

I 5p

I 4p

I 4p

II 11p

II 11p

Figure 2.30: Pseudo-creep tests: first series.

Table 2.9: Results of pseudo-creep tests, first series.

sm e

v(103) e

h(103) E

i

Sample (MPa) at failure at failure (MPa) sm/sm*IIp4 2.75 7.57 7.57 2122 0.79Ip5 2.80 5.07 5.07 2159 0.79Ip4 4.25 8.87 8.87 3024 0.85IIp11 5.30 3.98 3.98 4298 0.75

Comparing the strength values obtained on the masonry of Monza with the dif-ferent test series, it appears unexpectedly that the prisms tested monotonicallyshowed the lowest strength values, which apparently is not coherent with viscous

behaviour. Actually, some aspects which may have influenced the results haveto be considered: first of all, masonry does not fulfil the conditions of continu-ity, homogeneity and isotropy which have normally to be assumed in continuousmechanics. In addition, the masonry studied here is an ancient one; at present it isdamaged. Moreover, the amount of cracks already present before testing was notthe same for all the samples, those tested monotonically showing more signs ofdamage. However, if the ratio between the actual maximum vertical stress and thestatic strength (s

m/s

m*) is evaluated through the initial elastic modulus as shown

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in Tables 2.8 and 2.9, it appears that cyclic and constant load step tests damagedthe material lowering its strength on average by 85%.

2.3.7 Pseudo-creep tests, second series

The second series followed a more regular procedure: no monotonic phase wascarried out; the load steps were applied since the start of the test and kept constantfor a time interval of 10800 s (Table 2.10). As shown in Figs 2.32 and 2.33 moreregular data were obtained.

Considering the trend of the stressstrain plot, it can be noted that in this casealso the behaviour can be considered linearly elastic below a stress value of 2.5MPa. Correspondingly, the straintime plot shows that within this interval onlyprimary creep develops. After that level, the stressstrain diagram indicates

v(103)

0

2

4

6

v[M

Pa]

1 2 3 4 5 6 7 8 9

2000

4000

6000

8000

time[sec]

Figure 2.31: Results of a compression test with constant load steps on prism Ip4.

Table 2.10: Results of pseudo-creep tests, second series.Sample s

m(MPa) e

v(103) at failure e

h(103) at failure

II8pN 2.00 13 16II2pN 3.25 4 9I8pN 4.00 6 9II16pS 4.00 4 12II12pS 4.25 8 9II5pS 4.75 6 9

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non-linear behaviour, and correspondingly the straintime plots exhibit the steady-state (or secondary) and, eventually, the tertiary creep phases. In Fig. 2.34 thecrack pattern across the specimen at the end of the test is represented.

It is apparent from the drawings that the prism was characterized by the pres-ence of a large portion of stone, occupying most of face D and large parts of facesA and C. The crack pattern has basically developed in sub-vertical direction, with

0 40000 80000 120000 160000 200000

Time [sec]

-20

-15

-10

-5

0

5

10

vol(1

03)

v(103)

II 12

II 8 I 8 II 5

II 2

II 16II 2

II 8

I 8

II 16 II12II 5

Figure 2.32: Pseudo-creep tests: second series.

-10 -8

vol(103) v(103)

21600

18000

14400

10800

7200

3600

0

Time[sec]

420-2-4-6 6 8 10

0

1

2

3

4

5

v[MPa]

Figure 2.33: Results obtained on prism II 12.

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fissures opening preferably along discontinuities already present at start of the test,whereas bricks were not cracked.

2.4 Comments

The effect of persistent loads on the damage of ancient masonry has been experi-

mentally studied and their effects on the mechanical properties of the materialhave been shown. Constant load step tests turned out to be a suitable procedure foranalysing creep behaviour, having the advantage of being carried out more easilythan long-term tests.

Primary, secondary and tertiary creep phases have been clearly observed,together with their relationship with the stress level, a damage development beingassociated to an increase of the stress level.

The action of cyclic and persistent loads has proved to cause a severe damageon the mechanical properties of ancient masonry. The fatigue life of masonryunder uniaxial cyclic compression is related to the secondary creep strain rate,which is the strain rate during the phase of stable cyclic damage growth.

Having tested the masonry coming from two historical buildings and built bydifferent constructive techniques, a comparison can be made by using differentmechanical parameters. In Fig. 2.35 the compressive peak stress obtained throughpseudo-creep tests have been plotted vs. the sonic velocity.

A strong linear correlation can be clearly observed; the prisms obtained fromthe outer leaf of the medieval part of the tower of Pavia achieved the highest

strength values, whereas those from the inner leaf present the lowest ones, beingthe sixteenth-century plain masonry in between. In fact, this is coherent with thetexture characteristics of the three materials (Figs 2.19 and 2.24). The outer leaf isthe highest quality one, built to be the visible part of the structure; the inner leaf isconstituted by rubble masonry. The values of the crypt of Monza, the texture ofwhich shows an intermediate pattern between the previous two, with the presenceof bricks and stones, are overlapped to those of the inner leaf and those of the plainmasonry.

II 8I 8

II 2

II 16II 2

II8

I 8

II 16

II 8 I 8

II 2

II16II2

II 8

I 8

II 16 I

II 8I 8

II 2

II 16II2

II 8

I 8

II 16II

II 8I 8

II 2

II 16II 2

II8

I 8

II 16

Figure 2.34: Crack pattern of prism II 12 at the and of the test, faces A, B, C and D.

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accordingly, can be used as a reliable parameter to predict the residual life of amaterial subjected to a given sustained stress. In view of preserving the historicalheritage, it would be useful to define similar relationships to evaluate, for instance,

the results of a monitoring campaign on a massive historic building subjected topersistent load, to judge whether the creep rate indicates a critical condition interms of safety assessment. Of course, the precocious recognition of a critical statewill allow one to design a strengthening intervention to prevent total or partialfailure of the construction.

Acknowledgements

Architects C. Curallo, M. Garau, P. Garau, L. Giannecchini, M.G. Paccapelo,R. Tassi and S. Sironi are gratefully acknowledged for their assistance in theexperimental work and data elaboration; Mr Marco Antico for his technicalsupport in the laboratory. The research has been partially supported by COFIN2002 MIUR funds.

References

[1] Binda, L., Gatti, G., Mangano, G., Poggi, C. & Sacchi Landriani, G., Thecollapse of the Civic Tower of Pavia: a survey of the materials and structure.Masonry International, 6(1), pp. 1120, 1992.

[2] Anzani, A., Binda, L. & Taliercio, A., Application of a damage model tothe study of the long term behavior of ancient towers. Proc. 1st CanadianConference on Effectiveness Design of Structures, Hamilton, Ontario1013/07/2005, CD ROM.

[3] Anzani, A., Binda, L. & Melchiorri, G., Time dependent damage of rubble ma-sonry walls. Proceedings of the British Masonry Society, 2(7), pp. 341351,1995.

[4] Anzani. A., Mirabella Roberti. G. & Binda, L., Time dependent behav-iour of masonry: experimental results and numerical analysis. StructuralRepair and Maintenance of Historical Buildings, Vol. III, Bath: STREMA,pp. 415422, 1993.

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