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KeywordsMasonry, seismic strengthening, polymeric grids.

1. IntroductionMany buildings in Romania are simple buildings of brick,

designed and executed after rules which are not in accordance with the requirements of the current provisions, especially in terms of earthquake safety (The Romanian Seismic Code for the Assessment of Existing Buildings, P100 -3: 2008; performance – based assessment with three performance levels; limit states; distinction between ductile and fragile structural elements etc. and from SR EN 1998-3:2005/NA:2010 - Design of structures for earthquake resistance). It is well known that Vrancea seismogenic zone is the most important seismic zone, taking into account the energy, the extent of the macro seismic effects and the persistent and confined character of the earthquakes that occur in this narrow area. The behavior of masonry structures during 1977 Vrancea earthquake and others (1986, 1990) was characterized by cracks and partial failure due to bad mortar quality or poor workmanship and the masonry structures have presented damage by wall corners and crossings cracking, diagonal cracking, local failure of untied walls. Hence the importance of increased seismic reinforcement to improve the behavior of masonry buildings.

2. The basis of reinforcing masonry with polymer grids

From the point of view of dislocation theory (Lev Landau Theory), the vertical joints of masonry are presented as structural geometric imperfections (Figure 1). The unitary efforts from figure are inversely proportional to the area A and directly proportionally with concentrated force P, which in the Bernoulli hyperbole explain that for a large area result small unified efforts and for a small area the efforts remain uniformly distributed but very large [1, 2, 3]. When there is a structural imperfection, the efforts are not uniformly distributed and cause large efforts around the focus joints/points [1, 2, 3]. A simple and practical solution to remove the concentration of efforts that occur due to geometric imperfections is to consider their coverage by a polymeric grid with high tensile strength, long-term controlled deformations etc. This grid with its regular geometry and tensile strength will

take the efforts of masonry and redistribute them evenly cross sections neighboring, removing the danger from dislocations or fractures, both meaning damages (Fig. 1).

Figure 1. Unitary efforts σ are uniformly distributed under a force P, for a homogeneous material. In case of geometric imperfections

(the vertical joints between the bricks) and earthquake action, the local concentration of efforts (σmax) appears. Effect of

superposition of polymer grid. Bernoulli hyperbole [1, 2, 3].

The following theoretical reasons are most important: both materials have elastic and plastic proprieties in the same proportions and they have almost the same Young modules of elasticity; 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, and, also they are inert to all chemicals found in mortars and has no solvents at ambient temperatures etc. [1, 2, 3].

Seismic strengthening of masonry using some advanced composite materials

C.S. Dragomir 1,3,a, D. Dobre 2,3,b, E.S. Georgescu 3,c

1University of Agronomical Science and Veterinary Medicine, Faculty of Land Reclamation and Environmental Engineering, Department of Environment and Land Improvement, Bucharest, Romania.

2Technical University of Civil Engineering Bucharest, Bucharest, Romania.3National Institute for Research and Development in Construction, Urban Planning and Sustainable Spatial

Development “URBAN-INCERC” & European Center for Building Rehabilitation- ECBR, Bucharest, Romania.

E-mail: adragomirclaudiusorin@yahoo.com, bdobred@hotmail.com, cemilsevergeorgescu@gmail.com

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3. Strengthening of the masonry walls by reinforcing, with carbon fiber blades and with carbon fiber fabric over the carbon fiber blades

The present study puts into evidence some results obtained by technique reinforcement with carbon fiber blades for strengthening of masonry walls [4, 5, 6, 7]. Masonry specimens were those of standard diagonal tests, ca. 1.20 x 1.20 m and 30 cm thickness, made of clay blocks, out of which some were with a special profiling for earthquake zones, Figure 2. The INCERC Bucharest universal testing machine of 400 tf was used, loading the specimen on its vertical diagonal until nominal failure. During the initial test, the failure pattern followed the joints, in steps, as the adherence between blocks and mortar failed. In this state, cracks were developed along of the mortar joint and/or through some hollow blocks, but specimens were still keeping integrity, thus such a situation was similar to a moderate post-earthquake cladding damage that may deserve a strengthening.

Figure 2. (left) Three panels specimens (1.20 m x 1.20 m, with 30 cm wide), after the application of carbon fiber blades

on both sides, without further repair of joints. From left to right, specimens: code sample no. 2, sample

code no. 3, sample code no. 1. Strengthening masonry walls reinforced with carbon fiber fabric over the carbon fiber blades

(right) [4, 5, 6].

The basic concept of the experiments at a large scale, on modular elements of masonry building, was to test the same specimens masonry by a standardized test - the test diagonal - in two situations: in the initial state, according to the usual procedures, and after consolidations through applying carbon fiber blades bonded with specific adhesives. As a reference the capable force of initial testing was chosen and the percentage change in value of force was shown. These values

are actually of interest for interventions after post-seismic current situations.

The relationships between the shear stress τ and relative deformations were represented, on phase of charge, within available pairs of values, corresponding to each separately test. Images from the measurement and disposal of blocks Porotherm time statements are presented for each sample in Figures 3÷6. At the failure only capable final loading value was registered, respectively of the unified effort, which was calculated the ratio related to the initial state.

Figure 3. Diagonal compression test. Sample no. 1. Dimensions and the type of load [4].

By comparison, it is found that the two samples made of blocks for seismic zones, the intake percentage of consolidation with blades of carbon fiber is apparently low, but this reflects in fact the initial absolute values higher of supported load, while consolidation type was similar.

If the tests present a significant parameter is the failure stress τ equal to the tensile strength of main effort, which takes into consideration and the average area of work samples, slightly different from case to case. At the current attempts, the values of this parameter τ are higher related to the values of the original test which prove the effect of consolidation. Through initial tests, the mechanism of failure was mainly through the joint, in scale, by breaking the adhesion between the blocks and mortar.

Results obtained after the application of carbon fiber fabric on both sides of panel [4]:

• Sample no.1: Maximum taken load was over 242% compared to the initial supported load by the masonry panel

• Sample no. 2: Maximum taken load was over 282% compared to the initial supported load by the masonry panel

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• Sample no.3: Maximum taken load was over 386% compared to the initial supported load by the masonry panel

In the case of strengthening masonry walls reinforced with carbon fiber fabric over the carbon fiber blades, it is noted that the application of fabric determines reduced deformations on both sides of the model. In these circumstances, the conclusion would be that the element of masonry that has suffered two successive earthquakes can be brought through consolidation with double blades and carbon fiber fabric at the bearing capacity representing 230% from the initial state.

4. Model instrumentation of a natural stone masonry

In order to emphasize the influence of geometry on seismic response, a natural stone masonry model was designed, with geometrical irregularities in plan and without a symmetry axis, and was instrumented for the seismic response [2, 3, 5]. As excitation has been accepted Herzegovina type with two degrees of freedom after the directions north-south and east-west (Figure 7). This type of input was induced successively with increasing intensity, from 0.05 g. The first series of tests stopped at an input of 0.37 g when it was noticed that the wall with the largest surface of gaps varies strongly. Although the oscillation amplitude reached large levels, the natural stone masonry did not give up from dislocations.

Figure 4. Front and side view of the consolidated model – sample no.1. The relationship τ effort - relative compression / relative

elongation corresponding to sample no. 1 ( relative compression on side I, relative elongation on side I, relative compression on side II, relative

elongation on side II) [4].

Figure 5. Front and side view of the consolidated model – sample no.2. The relationship τ effort - relative compression /

relative elongation corresponding to sample no. 2. ( relative compression on side I, relative elongation on side I, relative compression on side II, relative

elongation on side II) [4].

Figure 6. Front and side view of the consolidated model – sample no.3. The relationship τ effort - relative compression /

relative elongation corresponding to sample no. 3 ( relative compression on side I, relative elongation on side I, relative compression on side II, relative

elongation on side II) [4].

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5. Numerical analysis of the main stresses, for simple and reinforced masonry

A seismic calculation according to code P100-1: 2013 for ground acceleration ag = 0.24g and corner period Tc = 1.6 s was done. The main normal and tangential efforts values were compared and the space deformation of structure in the case of simple masonry and of the reinforced masonry with polymer grids are represented. Of comparative representation of the two types of masonry it is showed that the state of efforts changes with increasing material resistance and the masonry which is reinforced with polymer grids performs well in earthquakes compared to the simple, unreinforced masonry (Figure 8). It notes that appears concentrate of efforts in cracked areas of the model.

6. ConclusionsThere are several methods to improve the behavior of

masonry walls at dynamic actions, selected according to type of masonry, physical characteristics and properties of walls,

seismic requirements, the structural system, the existing dislocations etc. (reinforcing steel in masonry, masonry coating, embedded columns and coupling bars, welded mesh etc.).

Strengthening of the masonry walls, by reinforcing with carbon fiber blades and with carbon fiber fabric over the carbon fiber blades, increases the maximum taken load and the shear strength capacity (in-plane or out- of-plane), removes the concentration of efforts, involves lower values for relative deformations, the masonry becomes a ductile material etc. Compared to other methods for masonry consolidation, this strengthening method presents a number of advantages, but a cost-benefit analysis should be done.

On the other hand, the basic concept and methodology of testing masonry specimens influences greatly the significance of the conclusions. When as reference are taken specimens without consolidation and are tested and then other new specimens consolidated from beginning are considered and are tested, the situation is of a preventive rehabilitations of buildings that have subjected to a seismic motion. When a sample without consolidation

Figure 8. Maps of normal stresses σxx, σyy , τ [kPa], for plain masonry and reinforced masonry. Spatial deformation of masonry model.

Figure 7. General scheme of the instrumentation of the model. Inputs acceleration and displacement on the NS and EW directions. Extreme values of seismic response recorded at the top of the natural stone masonry [2, 3, 5]

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is taken as a reference and the same sample is consolidated, the situation is that of a frequent post-earthquake rehabilitation.

From the presented results, the tested systems are efficient, but in addition it is advisable the application of blades on the diagonals of panels on two directions close to those of the main efforts so that the blades to work on tension.

AcknowledgementsThis article is based on some results obtained in Contract

31031/2007- RISKLESSCONS, in Core Program PN 16-10 01 01 and in BIGSEES Project, financed by the Romanian National Authority for Research and Innovation (ANCS).

References[1]. N. D. Stoica, M. Barnaure, Despre utilizarea grilelor polimerice la structurile de zidărie/About using polymeric grids for masonry structures, Revista Romana de Materiale/Romanian Journal of Materials 2015, 45 (4), 411-418[2]. C. S. Dragomir, Influenţa fenomenului de amplificare dinamică asupra răspunsului seismic al construcţiilor din zidărie (in Romanian)/ Influence of the dynamic amplification phenomena on the seismic response of masonry structures, Thesis U.T.C.B., Bucharest 25 January 2008.

[3]. R. A. Sofronie, Performances in seismic strengthening of masonry, 13th World Conference on Earthquake Engineering Vancouver, B.C., Canada August 1-6 2004, Paper No. 182.[4]. Contract no. 31031/2007, Structural and architectural integrated technical solutions, technologies and advanced materials to reduce risk, increase safety of constructions and security of life in extreme actions (earthquake, impact - terrorism, explosions, tornadoes etc.) - RISKLESSCONS, Project funded by the Romanian National Authority for Scientific Research, INCD URBAN- INCERC, 2007 (in Romanian).[5]. Information on http://www.revistaconstructiilor.eu/index.php/2009/12/08/protectia-constructiilor-din-zidarie-la-actiuni-seismice, Accessed 1.03.2016.[6]. Information on http://sd.utcb.ro/_upload/content/docs/262_matei_claudiu-lucian_-_rezumat_ro.pdf, Accessed 1.03.2016.[7]. E.S. Georgescu, M.S. Georgescu, E. Albota, Towards a Multi-Hazard Engineering and Architectural-Urbanistic Design in Seismic Zones of Romania, 14ECEE, 2014.

Lecture presented in the 8th International conference “Innovative technologies for joining advanced materials - tima16”, June 2-3, 2016, Timişoara, Romania.