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1 Structural Analysis of Historical Constructions Jerzy Jasieńko (ed) 212 DWE, Wrocław, Poland, ISSN , ISBN NlN-abpTorCTfsb TbpT lc BofCK ClirMNp rpfnd CeANdb fn cobnrbncy ANa fnbotancy obpmlnpb Arkadiusz Kwiecień 1, Jarosław Chełmecki 2, Piotr Matysek 3 ABpToACT In the paper testing method based on the value and the frequency shift of an inertance function, obtained during impact excitation with modal hammer was presented. The acceleration response of masonry columns, investigated under various compression load levels, was analyzed to assess the change of stiffness (after cracks appearance) and the inertance function distributions. Tests were carried out on brick columns executed in laboratory and on columns cut from historical structure while conducting dismantle works. Keywords: Masonry, Synamic testing, Non-destructive testing NK fntolarctfln Non-Destructive Testing is the widely developed part of science in the last decades. It is applied to recognize material properties and structural conditions, especially connected with detection of structural damages, which can influence safety of the object [1-3]. One of the application areas of the NDT techniques is assessment of the safety of historical masonry structures and efficiency of masonry repair. These topics are a subject of investigation of various international teams (e.g. RILEM Committees: TC 216-SAM, TC MCM) and were analyzed in many publications. Typically used NDT methods are: Sonic Tests, Tomography and methods using frequency shift [3-6]. From previously conducted researches it appeared that interesting results may be obtained by applying NDT in connection with other testing methods. In the paper there are presented results of masonry walls tests in compression with assumed constant value of displacement velocity for which there was measured the dynamic response for different load level. OK abpcofmtfln lc TbpTba ClirMNp Four masonry columns were tested dynamically and next statically up to failure (Fig. 1). Columns Z1 and P1, were of dimension mm, consisted of 7 brick layers of thickness 65 mm made of the Polish Bonarka bricks (dimension of mm 3 ). Brick layers were joined with cement mortar layers of thickness 7.5 mm made of Izolbet (specimen Z1) and with polymer layers of thickness 7.5 mm made of polymer PS (specimen P1). Columns F5-1 and F9A-2 with dimensions mm 3 and mm 3 respectively, were cut from buildings walls while carrying out repair and dismantle works. Column F5-1 was taken from wall of building constructed on lime mortar in the 7-ties of 19 th century, while F9A-2 speciment from structure realized on cement and lime mortar in the middle of 2 th century. Thickness of joints in obtained columns was at average equal to 2mm. Masonry elements from 19 th century had dimensions equal to mm 3, while those from 2 th century mm 3. Some selected characteristics of tested colums are listed in Table 1 and the general views of elements before testing are presented in Fig Ph.D., Cracow University of Technology, akwiecie@pk.edu.pl 2 M.Sc., Cracow University of Technology, jchelm@o2.pl 3 Ph.D., Cracow University of Technology, pmatysek@tlen.pl 2437

2 (1) Pillars notation Type of material in joint Table N Characteristics of tested pillars Compressive strength of bricks f B [MPa] Compressive strength of masonry f [MPa] Elasticity modulus for masonry E [MPa] Strain at maximum compressive force P-1 polymer (1) 26,5 2, ,6 F5-1 lime mortar 24,6 3, ,5 F9A-2 cement-lime mortar [ ] 25,4 8, ,5 Z-1 cement mortar 26,5 17, polymer PS Young modulus of 8 MPa, tensile strength of 2.2 MPa, shear strength of.8 MPa, adhesion of 2.5 MPa and ultimate strain of 45% Z1 F9A-2 F5-1 P-1 cigk N Tested brick columns Elasticity moduli for masonry were determined at the level of,33f max (F max maximum compressive load registered in testing). Detailed description of tests for brick columns is given in chapter 3.2. Compressive strength for masonry elements was evaluated on halves of bricks with thin layer of cement mortar. PK aynamfc TbpT lc MAplNoY ClirMNp PKNK Test description The tested dynamically masonry columns were compressed up to failure under static load. The research was carried out at Cracow University of Technology, using the universal testing machine Zwick Z6, which is able to generate the compression load of 6 kn (Fig. 2a). The compression tests of all columns were realized with constant displacement ratio of.5 mm/min. All specimens were initially loaded before the test with the stress level of.1 MPa. The tested columns were excited dynamically and analyzed at various load levels normalized to the maximum force value, namely: %, 33%, 66%, 9%, 1%, 9% (*) of F max (9% (*) of F max measured in the post critical zone of the test, after the ultimate force occurrence). Four masonry specimens (fixed between the compression plates of the universal testing machine) were excited using a modal hammer. Impulse excitations were generated in the middle of tested columns in a horizontal direction (Fig. 2b) and dynamic responses of specimens were measured using one accelerometer fixed to the columns also in the middle of them but on the opposite surface (Fig. 2c). There were used: the accelerometer 356B18 PCB, the modal hammer PCB 86D5 and the recording system LMS SCADAS MOBILE (Fig. 2d). The data of excitation impulse forces and of acceleration 2438

3 responses were collected in time domain. The sampling frequency was 496 samples/sec. Each specimen was excited by the modal hammer 3 times at the considered load level and the most representative signal had been chosen for analysis. a) b) c) d) cigk O Masonry specimen during compression test (a), impulse excitation by a modal hammer (b), accelerometer (c), recording system (d) PKOK aynamic analysis Recorded signals in the time-domain of the force impulse (Fig. 3a) and the acceleration response (Fig. 3c) were transformed to the frequency-domain by the Fast Fourier Transform (FFT). The force frequency characteristics F ( and the acceleration frequency characteristics X & ( are presented in Fig. 3b and 3d respectively. The obtained characteristics allowed determining [7] the modulus of the inertance function I (, which converse defines the apparent mass M ( given by equation (1). cigk P Dynamic characteristics of measured signals: impulse force in time domain (a) and its FFT (b), acceleration response of a column in time domain (c) and its FFT (d) I( = X&& ( F( = 1 M ( (1) I ( = I( - I( (2) column+ device device 2439

4 F ' ( = X&& ( I( = X& ( M ( (3) where: X & ( = acceleration frequency characteristic (FFT) of the tested column F ( = excitation force frequency characteristic (FFT) of the modal hammer I ( = real inertance frequency characteristic of the tested column F '( = inertial force frequency characteristic w = frequency in [Hz] To avoid influence of the stand vibration of the universal testing machine on the results of the tested columns, the specimen s responses were corrected in frequency domain according to equation (2). The modulus of the inertance function I ( allows describing the dynamic response characteristics of a structure under changing load levels and allows recognizing damage process developing in a structure. The load increase causes higher stress in masonry, thus a shift of characteristic frequencies spectrum to the higher frequencies is observed. On the other hand, the structural damage causes changes in masonry stiffness, visible in a shift of characteristic frequencies spectrum to the lower frequencies (Fig. 5). Moreover, the apparent mass M ( indicates the value of the structure mass, which is activated during vibrations with particular response frequency. This indicator allows determining consistence of the masonry substance. Higher values of M ( (lower values of I ( ) indicates high consistence (Fig. 5), activating high level of the elastic dynamic response of structure. On the other hand, lower values of M ( (higher values of I ( ) indicates low consistence (because of cracks appearance), activating inelastic dynamic response. Knowing values of apparent mass M ( and assuming constant the constant value of the excitation acceleration X & ( in the whole frequency range, it is possible to calculate the inertial force frequency characteristic F '(, according to equation (3). This function allows assessing how changes the masonry structure ability to absorbing seismic energy by means of an action of inelastic deformation, caused by cracked structure. The proposed functions could be especially useful in the non-destructive testing (NDT) of masonry civil and heritage structures, because allow monitoring changes of the structure state to detect the appearance of invisible structural damages (cracks and micro-cracks) and their influence on structural behavior. PKPK aiscussion of obtained dynamic results Analysis of the calculated inertancy functions I ( were carried out for four tested brick columns, at various load levels normalized to the maximum force value F max. The inertancy functions I ( of all tested columns are presented in Fig. 4 for the load levels of 33% F max and 9% F max. It was observed that lower frequency characteristics were connected with the columns with the flexible mortar layers made of polymer PS (specimen P1) and the lime mortar layers (specimen F5-1). Stiffer ones (higher frequency characteristics) were connected with specimens with the lime-cement mortar layers (specimen F9-2) and with the cement mortar layer (specimen Z-1). It was also visible (Fig. 4) that increase of the load levels shifted the dynamic characteristics to the higher frequencies. Analyzing the inertancy functions of the specimens F5-1, F9A-2, Z-1 two more or less dominant resonance frequency bands were observed (lower and upper). The lower frequency band was located in the range of 7-85Hz (the lower mortar strength is the higher frequency). From relationships presented in Fig. 5, Fig. 6, Fig.7, one may conclude that after exceeding F max there appears a sudden change in inertancy, manifested by lowering the frequency and significant increase in its value (pointing the decrease in column consistency). That is due to the meaningful cracking of masonry column and its division into smaller elements (decrease in masonry consistency). The process of cracking the masonry columns influences also the mode of inertancy function change in the upper frequency band, located within the range of 95-11Hz. These changes are determined from one side by progressive process of loosening the masonry structure (lowering the frequency) and from the other by the effect of unloading and stress redistribution in bricks and mortar resulting locally in returning to higher consistency of materials constructing the column (columns were tested with constant displacement velocity after exceeding the value of F max force dropped). 244

5 By comparing relationships in Fig.4 and Fig.8 it may be noticed that for column P-1, characterized by very low value of elasticity modulus and significant strains registered for force F max (values many times greater than measured for other columns), the inertancy function course is different. Significant deformability of polymer joints resulted in classical vertical splitting of bricks at very low load levels (33-66% F max ). Gradually, with the increase of load, the number of vertical cracks in bricks was growing, but at the same time significant material compression in joints took place. N2 N Z-N m-n c5-n c9^ N N2 N2 N Z-N m-n c5-n c9^-2 9% cmax N N2 cigk Q Inertancy functions calculated for the tested masonry columns, presented for two load levels normalized to the ultimate load:.33% F max (top) and.9% F max (bottom) N8 N6 N4 N2 N % cmax 66% cmax 9% cmax N% cmax 9%(*F cmax Z-N N N2 cigk R Change of the inertancy function with the load level for specimen Z

6 N8 N6 N4 N2 N % cmax 66% cmax 9% cmax N% cmax 9%(*F cmax c9^ N N2 cigk S Change of the inertancy function with the load level for specimen F9A-2 N8 N6 N4 N2 N % cmax 66% cmax 9% cmax N% cmax 9%(*F cmax c5-n N N2 cigk T Change of the inertancy function with the load level for specimen F5-1 N8 N6 N4 N2 N % cmax 66% cmax 9% cmax N% cmax 9%(*F cmax m-n N N2 cigk U Change of the inertancy function with the load level for specimen P-1 These superimposed processes determined the character of relationship presented in Fig. 8, where with the increase of load the stiffness of the whole column decreases, but the material composite brickpolymer consistency increases. Defects in the form of cracks were more uniformly distributed within the structure of P-1 column than in other tested columns (lack of visible peaks for inertancy function). 2442

7 Z1 F9A-2 F5-1 P-1 cigk V Failure modes of the tested columns Failure modes for masonry columns are presented in Fig.9. For specimens Z-1, F9A-2 and F5-1 the characteristic feature constitute vertical cracks containing few layers of bricks and mortars, as well as surface crumbling of ceramic and joints materials. Within the structure of masonry there are formed isolated columns with much higher slenderness and lower stiffness, which results in change of inertancy function course (relationships in Fig. 5, Fig. 6 and Fig. 7). Modes of columns cracking for elements Z-1, F9A-2 and F5-1 correspond to classical destruction form for walls subjected to compression. For column P-1 there were not registered the cracks in vertical polymer joints even for high deformation of columns structure. Bricks in individual layers were subjected to multiple cracking. But significant suspectibility of joint material made it impossible to form continuous vertical cracks in column. QK ClNCirpflNp In the paper there were presented the results of tests made on brick columns constructed from different materials. Tested columns were cut from existing structures with lime and cement-lime mortars (buildings from 19 th and 2 th century, respectively) or prepared in laboratory on polymer and cement mortars. Materials used made it possible to test masonry columns within the wide spectrum of strength and deformability characteristics. Columns were tested under compression with registration of dynamic response for different load levels. Presented results indicate that the most important role in the process of masonry columns destruction under compression plays the type of material applied for the joint. Material type in joints influences also the character of dynamic response of masonry, which in the paper was identified in the form of inertancy function. Results showed that dynamic responses of the tested specimens change significantly with the load level and the state of masonry work. But formulation of specific relationships requires further researches made on bigger population of samples. In authors opinion, presented method of dynamic testing may be useful in determining the cracking load level for masonry as well as for technical state estimation of existing structures (in co-operation with for ex. flat-jack method) and is planned to be developed in these directions. obcbobncbp [1] Binda L.: Learning from failure Long-term behaviour of heavy masonry structures. WIT Press, Southampton, Boston 28. [2] Atkinnson R.H.; Noland J.L.: Kingsley G.R: Application of NDE to Masonry Structures, Conservation of Historic Brick Structures, Donhead Publishing Ltd

8 [3] Maierhofer Ch.: Zerstörungfreie prüfung zur Beurteilung von Mauerwerk, Instandsetzung und Ertüchtigung von Mauerwerk, Teil 3, Mauerwerk-Kalender 27. [4] Binda L.; Saisi A.; Tiraboschi C.: Application of sonic tests of the diagnosis of the damaged and repaired structures, NDT&E International 34, 21. [5] Da Porto F., Valluzzi M.R., Modena C.: Use of sonic tomography for the diagnosis and the control of intervention in historic masonry buildings. International Symposium NDT-CE 23, BAM, Berlin 23. [6] Ramos L.F., Lourenço P.B., De Roeck G., Campos-Costa A.: Damage identification in masonry structures with vibration measurements. Structural Analysis of Historic Construction D Ayala & Fodde (eds) Taylor & Francis Group, London 28. [7] Kwiecień A.: Flexible polymer adhesives versus stiff mineral and epoxy adhesives tested dynamically on masonry columns strengthened using of bonded GFRP mesh. Proceedings of the 8th International Conference on Structural Dynamics, EURODYN 211, Leuven