SEISMIC STRENGTHENING OF MASONRY IN BUILDINGS AND CULTURAL HERITAGE

Ramiro A. SOFRONIE Professor University of Bucharest Bucharest Romania ABSTRACT The paper presents a summary of research results regarding seismic protection of masonry buildings. The issue of masonry and starting data of research are first discussed. The summary of laboratory tests gives a general view of the concepts and methods used in the experimental work carried out during a decade and followed by numerical validation. Then, the paper successively presents the sandwich effect, the polymer grids as reinforcement, the method of reinforcing masonry in bed layers, confining masonry, homogenisation and shaping of masonry buildings, the snap of whip effect, the resistance of masonry to jerks, shocks and environmental actions, and finally the design approach of masonry composed with polymer grids. 1. INTRODUCTION 1.1. Use of masonry In the first century of the third millennium masonry is the construction material mostly used for buildings and structures. It has been constantly employed during centuries in spite of the developing new and advanced technologies. Nowadays, more than 60% of social buildings and 90% of cultural heritage buildings are made of masonry. This widely spread use of masonry is motivated by two reasons: 1) The composing materials of masonry are everywhere easy available; 2) the manpower and labour productivity are supported by gravity. The masonry became so familiar that it is used even in seismic prone areas.

82 SÍSMICA 2004 - 6º Congresso Nacional de Sismologia e Engenharia Sísmica 1.2. History of Science It is assumed that masonry consisting of burned clay bricks and lime mortars has been invented about 9,000 years ago somewhere in a Palestinian region poor in stone but rich in clay. There are proofs that masonry was widely used in the Plain of Shinar where famous ziggurats as hexahedral towers were erected. They were pyramidal, stepped temple towers that is an architectural and religious structure characteristic of the major cities of Mesopotamia. The legendary Tower of Babel built about 1100 B.C.E. has been associated with the ziggurat of the great temple of Marduk in the City of Babylon. 1.3. The Bible Three technical facts of scientific interest are clearly mentioned in the holy book: 1) Clay mixed with straw was burnt to produce lightweight porous bricks (Exodus 5, 7-9). 2) Lime was currently used for mortars (Genesis 11, 3). 3) The plumb wire was known and used as a device for masonry. The influence of gravity on masonry during its erection and service was consciously recognised (Amos 7, 7-8). 1.4. Cultural heritage buildings Masonry is the main construction material used for city walls. Historians consider that the walled cities have marked the beginning of civilization. Castles, churches and mosques with their vaults, steeples and towers were also easily shaped with the aid of masonry. Masonry was used by the Romans also to bridges, aqueducts and viaducts. The three lobed plan of the Eastern Churches built in seismic areas is also due to masonry. 1.5. Does masonry hide a secret? This is a natural question. Indeed, how it was possible that such simple construction material like masonry, put into work anytime, everywhere and by anyone with the only aid of a plumb wire could last so long without even being a composite material. Due to such questions masonry was the most searched construction material from all the available ones. During the last century it was many times micro and macro modelled by both mathematical and physical tools but no special disclosure has been reported yet. 1.6. History of technology Modern masonry essentially differs from the original one. After the Industrial Revolution of the 18th century, in order to increase the bearing capacity of masonry, two changes occurred: the weak porous bricks were replaced by strong ceramic bricks while in mortars the cement took the place of lime. Later, in 19th and 20th centuries, from ergonomic reasons, the ceramic solid bricks were replaced with cored bricks which are even more ceramic than the formers. Thus, the specific weight of masonry decreased to 16kN/m3 or even less while labour productivity increased almost five times, and the profit accordingly. Nowadays, this type of masonry is world wide spread although it is too brittle for seismic prone areas. The question is whether the factories producing cement and ceramic bricks to be closed and people in seismic

Ramiro A. SOFRONIE 83 prone areas return to the porous bricks with lime mortars without disclosing the secret of millennial masonry? 2. PRELIMINARY REMARKS 2.1. Original masonry It could be defined as an association of elastic porous bricks and ductile lime mortars wherefrom a non-homogeneous and anisotropic artificial stone results. Since the bricks are produced by burned clay and the mortars based on lime are dry the two materials are compatible, and their mechanical proprieties are complementary. In addition, the strength of lime mortars is lower than that of porous bricks what is in accordance with the basic rule of binding. The coefficient of linear thermal expansion of original masonry, α = 0.4x10-1 ºC-1, is 2.5 times smaller than that of concrete what means the two materials cannot be associated each others in any way. 2.2. Modern masonry It could be defined as an association of brittle ceramic bricks and brittle cement mortars wherefrom a non-homogeneous and anisotropic artificial stone results. Since the bricks are produced by burned clay while the mortars based on cement are wet the two materials are incompatible or antagonist and their mechanical proprieties are no longer complementary but only comparable as brittleness. In addition, the strength of cement mortars is higher than that of ceramic bricks and therefore against the basic rule of binding. The coefficient of linear thermal expansion of modern masonry, α= 10-1 ºC-1, is almost identical with that of concrete what means the two materials can be always associated each other. 2.3. Composite material Neither original masonry nor modern masonry is a composite material because none of them does obey St. Venant’s Principle of geometric continuity of strains. Bricks and mortars always deform differently. This is why the two types of masonry cannot be reinforced with fibbers, for instance. All the attempts failed a priori. 2.4. Cored bricks The ceramic cored bricks used in modern masonry are thin walled units. Their surface in contact with mortars in bed layers is sometimes less than 50% of that of solid bricks what involves high concentrations of normal and tangential stresses. In addition the thermal isolation qualities, often claimed by builders of modern masonry, are not true as long as the cement mortars, containing water of constitution, are used. The only reason for using cored bricks is of ergonomic nature and consists in their high labour productivity. This is why they are called efficient bricks.

84 SÍSMICA 2004 - 6º Congresso Nacional de Sismologia e Engenharia Sísmica 2.5. Terminology Modern masonry is in fact a lightweight concrete preserving but the colour of masonry. Lowering the specific weight of concrete from 28kN/m3 to 16kN/m3 an artificial stone very brittle but with lower inertia is obtained. It should be called more accurately MASCRETE while the term MASONRY is further preserved for the original material. Mascrete can be reinforced or prestressed like any concrete with steel bars but never with polymer grids according to Romanian Patent [1]. On the other hand, synthetic reinforcement is proper for brick masonry with lime mortars there where steel and RC does not apply. 3. SUMMARY OF LABORATORY TESTS 3.1. Static tests INCERC Iasi, Romania: 1D-12 short columns and 2D-18 panel walls, 1995/96 [2-4]. 3.2. Pseudo-dynamic tests 3.2.1. LNEC Lisbon, Portugal: 2D – 4 reduced scale infill panels 1997 [5]. 3.2.2. JRC Ispra, Italy: 2D – 4 full scale infill panels, 1998/99 [6-9]. 3.3. Seismic tests 3.3.1. Enel.Hydro Seriate, Italy: 3D - 10 models as follows:

• Two models of masonry without RC members and two storey, 1996/97 [10-14]. • Two models of RC frame with masonry infill and two storey, 1998/99 [15-23]. • Three models of masonry with cored bricks, RC slab as roof and one storey, 2001/03 • [24-29]. • Three models of masonry with solid bricks, wooden roof and one storey, 2001/03

[30-35]. 3.3.2. LNEC Lisbon, Portugal: 3D – 2 models ready for testing on June 2004 [36].

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Figure 1: Reinforcing masonry in columns at each or second bed layer

Figure 2: Reinforcing masonry in wall panels at second bed layer Figure 3:Masonry in columns and wall panels confined by wrapping around with polymer grids

86 SÍSMICA 2004 - 6º Congresso Nacional de Sismologia e Engenharia Sísmica 3.4. Anti-terror and environmental tests Enel Hydro Seriate, Italy: 3D – 1 model tested to shock. January 2003 [37]. INCERC Iasi, Romania: 2D – 3 models tested to shock and thermal transfer, March 2003 [38-39]. ASTRA Ploiesti, Romania: 3D – 1 real masonry building submitted to military explosions, May 2003 [40]. 3.5. Numerical validation. The philosophy of above testing programs was based on a relativity approach. It consisted in planning as much as possible comparable models and then, after testing, comparing the results obtained in the same conditions of loading or exciting. All testing programs were kept under analytical and numerical control. That means the tests were not at random. The aims of tests were precise, and the results always checked up. SAP 2000 was used as computing program. The cracks resulted by tests were mapped on the fields of principal stresses. It was made a clear distinction between the normal phenomena caused by seismic actions and the influence of boundary conditions imposed by modelling on the shaking table. 4. THE SANDWICH EFFECT When a structural member of masonry is loaded concentrations of stresses occur around some structural faults, like vertical joints between bricks. As a consequence, under those stresses reaching their limit values in the lime mortar of bed layers, plastic strains are resulting. Spontaneously, and according to the Principle of minimum compulsion of Gauss-Hamilton, the stresses located around geometrical imperfections are gradually redistributed to neighbouring less heavily loaded areas. By this phenomenon of self-adaptation under the permanent action of gravity the original masonry protects itself against overloading and consequently becomes long lasting. Since 2000 this phenomenon was called sandwich effect [15]. With the aid of Prandtl’s Mathematical Theory of Plasticity published in 1923 was calculated the expulsion force developing in bed layers under vertical and lateral actions. The phenomenon is a consequence of the complementarities between elastic bricks and plastic mortars. It is the mark of a genuine masonry. Any masonry without such outstanding quality is only an ordinary artificial stone and not longer a true masonry. Once the sandwich effect discovered it should be put under control. There is a rather large field where such complementarity between elastic bricks and plastic mortars works. However, only in a narrow domain that effect is optimum. For practical purposes it can be controlled with the aid of polymer grids. 5. POLYMER GRIDS There are strong reasons to associate the polymer grids, specially produced by Netlon Ltd. in Blackburn, UK, for RichterGard System, with masonry as follows:

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Figure 4: Details of anchoring the grids around the opening, inside of the model with cored brick masonry, and the strips of grids confining the vaults in the other model with solid brick masonry

Figure 5: Model confined by wrapping around on external surfaces with special care for vaults and openings. The role of fixing devices is to ensure good coverage on both sides of grids during plastering. Their function ceases after hardening of mortar. Equidistance of fixing devices is important. 5.1. Mechanical reasons: 1) Both materials have elastic and plastic proprieties in the same proportions; 2) The grids and masonry obey similar laws of deformation what means they have almost the same Young modules of elasticity, and the Neumann ratio of equivalence n=Egrid/Emasonry is 1(one) or near to it; 3) The available grids have convenient levels of strength and high capacity of dissipation of the induced energy.

88 SÍSMICA 2004 - 6º Congresso Nacional de Sismologia e Engenharia Sísmica 5.2. Geometric reasons: 1). The grids are shaped as bars of equal strength to tension and developed in their both principal directions according to the topologic law of isomorphism. 2). The ribs are stiff enough to transmit beside tension stresses also shear stresses while the joints are solid and together with the ribs are integrated in grids. 3). The apertures of grids are shaped by four continuously connected hyperbola, and their small dimensions of only 39x39 mm ensure a uniformly distribution of stresses as Bernoulli’s Principle requires. 5.3. Physical and environmental reasons: 1).The grids are inert to all chemicals found in mortars and has no solvents at ambient temperatures. 2). The grids were certificated by INCERC Iasi, Romania for thermal actions and mechanical shocks. 3) The durability of grids is guaranteed by producer for a normal service of 120 years. 5.4. Eurocode 8, prEN 1998-3:200X, by clauses C.5.1.1. (3), C.5.1.1. (4), C.5.1.1. (5) and C.5.1.8, at pages 70-72, formally recognises polymer grids as reinforcement for masonry. 6. REINFORCING MASONRY IN BED LAYERS The aim of reinforcement in bed layer is to prevent “the sandwich effect” in the masonry of new buildings. That effect occurs only in lime and lime-cement mortars. Cement mortars without lime or another similar ductile component do not develop such effect. Polymer grids with integrated solid joints, as it was described above, are the most appropriate as reinforcements for that purpose. They also compensate the lack of strength of lime mortars and bring much more strength than cement mortars. The mechanism of stress transfer from mortars to grids takes place by anchoring. The phenomenon of anchoring is not influenced by the thermal expansion proprieties of grids. The stresses due to loads are concentrating around joints wherefrom are distributed to ribs. Since grid joints are not far from each other the distribution of stresses is rather uniform. Only shear and tension stresses are transmitted by grids due to a spontaneous phenomenon of self-control. Such mechanism of stress transfer essentially differs from that known in reinforced concrete where “the clamping effect” occurs. It is based on the shear stresses developed around the lateral surfaces of the cylindrical bars of steel as a consequence of concrete shrinkage. The adherence of concrete to steel is possible only because the two materials have the same coefficients of thermal expansion. Masonry reinforced with polymer grids in bed layers does not become a composite material but its bearing capacity to compression and shear forces increases. It is worth mentioning that as a consequence of reinforcing masonry in bed layers the 45º inclined cracks no longer occur. Often the same thing happens even when not all horizontal joints of masonry are reinforced but only some, for instance, at three or five bed joints. However, the polymer grids act only in horizontal planes in spite of the fact that some of their effects are also felt on vertical directions. The cost of one cubic meter of masonry reinforced in bed layers increases by only 0.6% in comparison with the cost of one cubic meter of plain masonry. This method of reinforcing masonry was patented in Romania since 1995 (Figures 1-3).

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Figure 6: Wall panel of cored brick masonry confined by wrapping around with grids

Figure 7: Wall panel of cored bricks masonry confined with polymer grids after tests

Figure 8: Hysteresis diagrams for plain (in blue) and reinforced (in red) masonry

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Figure 9: Envelope curves for the two panels of plain and reinforced masonry 7. CONFINING MASONRY The aim of confining is to develop three directional stresses of compression in masonry of both new and existing buildings. The bearing capacity of masonry increases several times due to existing internal reserves of strength and also to the strength of grids. The confinement of masonry is induced by wrapping around with polymer grids and then by rendering either building bodies as such or only some of their structural members. Confined masonry becomes a composite material fulfilling the requirements of St.Venant Principle. The outstanding advantage of confining is the uniform distribution of stresses. Any attempts of stress concentrations around some structural faults are annihilated by the plastic deformations of lime in render mortars. In order to make these advantages true, the adherence of render to masonry surface should be perfect. The joints between bricks should be deepened about to 15 mm, and the grids should be fixed at the same distance to masonry surface to be equally and uniformly covered on their both sides. The grids are joined by overlapping and no special devices except the fixing ones are necessary. The function of fixing devices will cease after four weeks when the render has hardened, and they would never be put in the position to develop concentrations of stresses around them. The thickness of plaster must be 18 to 24 mm and no more. Since all the strength comes from grids only lime or lime-cement plaster is recommended. The polymer grids can also be easily bent around outer corners or when the openings for doors and windows are bordered. Some special care is necessary when inner corners are met. If it is not possible to avoid such cases the grids should be fixed with bolts crossing the whole thickness of walls on both directions of corners. Obviously, in new buildings masonry can be both reinforced and confined while in existing buildings only confined. The cost of one cubic meter of confined masonry increases by about 6% in comparison with the cost of one cubic meter of plain masonry. This method has been also patented in Romania since 1995 (Figures 4-9).

Ramiro A. SOFRONIE 91 8. HOMOGENISATION OF MASONRY BUILDINGS In the case of new buildings when the masonry is reinforced and confined with polymer grids the structural members of reinforced concrete are no longer necessary and can be eliminated. This means both technical improvements and cost lowering. Indeed, during earthquakes the columns and beams of reinforced concrete act as concentrators of stresses and their cooperation with masonry structural members like walls or infills cannot be assessed. Such mixed buildings are strongly non-homogeneous and with great non-uniformities in the distribution of masses what is against the “Basic principles of conceptual design”, clause A3, provided by Eurocode 8. This structural solution of compromise, when masonry was associated with reinforced concrete, was accepted so many years because no alternative was available. From now on, by composing masonry with polymer grids the progress in seismic protection of masonry buildings as well as of masonry infills in buildings and structures has become possible. 9. SHAPING OF MASONRY BUILDINGS Structural irregularities in plan and elevation are according to Eurocode 8 for both new and existing buildings of special interest because they generate strong concentrations of stresses.

Figure 10: Horizontal cracks developed in the confined masonry of model as a consequence of “the snap of whip” effect during the tests on shaking table

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Figure 11: Induced and peak accelerations on vertical direction of the model with cored brick masonry reinforced only in bed layers. The acceleration recorded at the top of model by A13z at the 11th test is 17 times higher than the acceleration of shaking table recorded by ATz.

Figure 12: Induced and peak accelerations on vertical direction of the model with cored brick masonry reinforced in bed layers and confided by wrapping around with polymer grids. The acceleration recorded at the top of model by A13z at the 11th test is 5 times higher than the acceleration of shaking table recorded by ATz. Theory of dislocation explains how and why severe failures occur to such buildings. By confining the masonry of the existing buildings the effects of irregularities can be mitigated and good safety factors at convenient costs could be obtained. However, the most important achievement consists in shaping the new masonry buildings with the requested irregularities by considering the opportunity of reinforcing and/or confining masonry with polymer grids.

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Ramiro A. SOFRONIE 93 10. SNAP OF WHIP EFFECT Eurocode 8 and all national codes are deeply concerned about the torsion effects of seismic actions on irregular buildings and lots of preventing provisions are available. It is caused by yaw moment in vertical direction, and some of the Eastern Churches in Romania have been protected since 1512 against its destructive effects. However, there is another phenomenon also dangerous consisting in the amplification of seismic response on the vertical direction of buildings. Paradoxically, it is not mentioned in any code. Because of its extreme violence it is known as “the snap of whip” and was identified during the tests carried out on the shaking table on full scale 3D models of masonry buildings. The effects of the amplification phenomenon were materialized by horizontal cracks in masonry as well as by violent upheavals of the buildings from their foundations. This amplification of seismic response is caused by pitch and roll moments against their horizontal axes and is typical for masonry buildings without reinforced concrete columns. The confinement of the external surfaces of the building walls with polymer grids proved to be an effective measure of protection against the snap of whip effect. Not only local concentrations of stresses are prevented by confining but all structural members concur to increase the stiffness of buildings (Figures 10-12). 11. RESISTANCE OF REINFORCED MASONRY TO JERKS AND SHOCKS Earthquakes, mainly the strong ones, are dangerous for masonry buildings because the occurrence of jerks may crush the ceramic bricks or even cause dislocations of some structural members. Jerks are due to sudden variations in time of accelerations and they were first used by Jacobi in his doctoral thesis in Mathematics presented on August 13, 1825. For human beings the lower threshold is 30 m/s3, and for an earthquake of IX degree on Mercalli scale the jerk reaches 496 m/s3. If the masonry is reinforced or confined with polymer grids, under jerk actions they behave elastically. Between two consecutive changes of acceleration there is no time for grids to develop plastic deformations and therefore their ductile qualities are not used. The induced energy by earthquakes is dissipated only by the friction forces. The comparative tests carried out on the shaking table have shown that during the ultimate limit state the model of masonry with solid bricks resisted better than the model of masonry with cored bricks. Indeed, in the first case the solid bricks remained undamaged and the grids were torn off, what is easy to repair by replacing the synthetic reinforcement. In the second case the bricks were crushed and the grids remained undamaged what can not be repaired without evacuating all the rubble and replacing it with new masonry. The other tests carried out on several different models and submitted to shocks, according to ISO 7892/1998, have shown a rapid amortisation of the induced oscillations by their equal distribution in all directions. The tests also correspond to the requirement of the clause 2.9.6., in Eurocode 8. This is why the masonry reinforced and confined with polymer grids seems appropriate as an anti-terror protection (Figures 13-18).

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Figure 13: Position of TNT explosive over the ground in front of the wall reinforced with grids

Figure 14: Plan of the masonry building and the positions of accelerometers

Figure 15: The broken window and the damage plaster reinforced with polymer grids

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Figure 16: The reinforcement of plaster not affected by the explosion of 1kg TNT 12. RESISTANCE OF REINFORCED MASONRY TO ENVIRONMENTAL ACTIONS Comparative tests have been carried out on three panels of plain masonry, masonry reinforced with steel grids and masonry reinforced with polymer grids. It was successively determined: thermal conductivity, resistance to thermal transfer, thermal transmittance, thermal permeability and the equivalent thermal conductivity, all according to the European codes. The results have shown that masonry with polymer grids has normal physical characteristics which are stable for large fields of variation of the environmental factors. 13. DESIGN APPROACH OF MASONRY COMPOSED WITH POLYMER GRIDS RichterGard System is a holistic concept consisting in the examination, analysis, design and technical assistance for the installation, completion, monitoring and maintenance of masonry in buildings and structures located in seismic prone areas. Obviously, for such an advanced concept there is a corresponding program of analysis and design. It is based on the relative positions of the two intrinsic centres, gravity centre CG and stiffness or rotation centre CR, both allowing global analysis of the building bodies and detailed analysis of each structural member. For the new buildings the concept also allows a seismic shaping while for the existing buildings only the putting into value of all real reserves of strength and stiffness. A special computing program supports all the above mentioned analyses in accordance with the existing European and Romanian national codes. A recent geological map printed by the European Seismological Commission shows that seismic movements could also occur in some plains of Belgium, Germany and Netherlands that formerly were considered quiet. Those regions are called zones of low seismic intensity (Figures 19-20).

96 SÍSMICA 2004 - 6º Congresso Nacional de Sismologia e Engenharia Sísmica 14. CONCLUSION The answer to the former question is negative. The factories producing cement and ceramic bricks have been saved by the polymer grids and they should not be closed. Once the millennial secret of masonry disclosed and recognised as true in seismic prone areas, the ceramic solid or cored bricks associated with lime or lime-cement mortars can be further used provided that they be reinforced and/or confined with polymer grids according to RichterGard System. This is a revolutionary system addressed to clever, honest and courageous people so as it should be properly understood, trustingly recognised and accordingly promoted. The system easily applies to new and existing buildings, including those of cultural heritage, being more expeditious and cost effective than any of the known techniques. In the case of new buildings it allows to eliminate heavy and expensive structural members of reinforced concrete while in the case of existing buildings often the lodgers should not be evacuated. The system is based on an advanced concept of design what allows to reach the highest safety for the investment paid.

Figure 17: Crushed cored bricks behind integer reinforcement after tests on shaking table

Figure 18: Torn reinforcement in front of integer solid brick masonry after test on shaking table

Ramiro A. SOFRONIE 97 Figure 19: Case study two storey old masonry building in Bucharest, July 2001 Figure 20: Case study two storey new masonry building in Bucharest, July 2003 15. REFERENCES [1] Sofronie, R.; Feodorov, V. – “Method of seismic reinforcement of masonry works”

Romanian Patent Office RO 112373 B1, Bucharest 1995. [2] Sofronie, R. - “Antiseismic reinforcement of masonry works” in Proceedings of the

International Conference “New Technologies in Structural Engineering.” July 1997, Lisbon, Portugal, p.373-380.

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[14] Sofronie et al. – “Restoring techniques based on polymer grids” in Proceedings of the 5th International Congress on Restoration of Architectural Heritage FIRENZE 2000. September 17-24, 2000, Florence, Italy, p.152-163.

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Americas: Hemispheric Workshop Directions. April 23 & 24, 2001, Mayaguez, Puerto Rico, paper #24.

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[21] Sofronie, R. – “Vulnerability of Romanian Cultural Heritage to Hazards and Prevention Measures” in Proceedings of the ARCCHIP Workshop ARIADNE 4, August 18-24, 2001, Trest, Czech Republic p. 1-16.

[22] Sofronie, R. – “Strengthening and restoration of Eastern Churches” in Proceedings of th International Congress of ICOMOS and UNESCO “Two thousands years, and more, in the history of structures and architecture”, September 10-12, 2001, Paris, France

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[24] Sofronie, R. – “Repair and retrofitting masonry buildings” in Proceedings of the 12th European Conference on Earthquake Engineering, September 9-13, 2002, Barbican Centre, London UK, Paper Reference #183.

[25] Sofronie, R. – “Use of polymer grids in masonry retrofitting”. Special Session on Advances in earthquake experimental studies in Proceedings of the 12th European Conference on Earthquake Engineering, September 9-13, 2002, Barbican Centre, London UK, Paper Reference #5.

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