Experimental results on masonry wall anchored ties

Transcription

1 Experimental results on masonry wall anchored ties E. Giuriani, G.A. Plizzari, C. Bassini Department of Civil Engineering, University of Brescia ing. unibs. it Abstract The strengthening technique of historical buildings is often based on steel ties. In precious facades, external anchor plates cannot be adopted so that ties can be restrained to the masonry wall by using grouting material. The tie efficiency depends on the properties of the injection mortar as well as on the masonry mechanical properties. In the present work preliminary results of pull-out tests on threaded steel bars anchored in masonry walls with different types of grouting materials are presented. 1 Introduction The restoration of historical buildings often requires structural strengthening. Frequently the strengthening technique is based on the insertion of steel ties which link the masonry walls or part of them together.' This kind of intervention is well accepted because the ties can be eventually removed without significant damage to the original structure. As an example, arches and vaults often decay because of an insufficient confinement of the horizontal forces transmitted to the walls. The adoption of intrados or extrados ties is one of the most appropriate ways to restore the correct static behaviour^ (Fig. la). Another significant example concerns the ancient buildings in seismic areas, where strengthening of the floors aims to obtain stiff horizontal diaphragms that restrain the masonry walls subjected to seismic actions (Fig. Ib). Also in this case, the floor-to-wall

2 56 Structural Studies, Repairs and Maintenance of Historical Buildings linking is usually obtained by means of steel ties. When steel ties are adopted, particular attention has to be paid to the connection to masonry walls. Furthermore, in precious facades external anchor plates cannot be adopted so that ties should be restrained to the surrounding masonry by using grouting material.' ^ Tie efficiency depends on the properties of the injection mortar as well as on the masonry mechanical properties/ The lack of specific test results cannot provide information for the design of injected anchors and complementary standards for concrete cannot be used because of the remarkably different behaviour/ Preliminary studies on anchors with cement mortar have already given some indications for the design/ This kind of mortar allows for a high bond strength but it is not usually well accepted because of the well-known incompatibility with ancient lime mortar. (a) Figure 1: Extrados tie in a vault" (a); confining action of steel ties in a masonry wall under seismic actions (b). Aim of the present work is to shed some new light on the behaviour of steel ties anchored in masonry walls with grouting materials that are chemically compatible and that have mechanical characteristics similar to the original mortar. To this purpose, pull-out tests on steel bars anchored in masonry walls were performed. As grouting materials, mortars that are usually adopted for the injection in ancient masonry walls were taken into consideration. The results concern the anchorage stiffness and strength. Furthermore, tests show the different collapse modes which can occur at the steel-mortar-masonry interfaces or within the masonry itself.

3 Structural Studies, Repairs and Maintenance of Historical Buildings 57 The experiments are performed on steel threaded-bars anchored in walls made in a laboratory with new bricks and a hydraulic lime mortal having mechanical properties quite similar to the one of an ancient masonry. The use of new masonry walls with homogeneous properties allows to better evidence the influence of the injected mortars. 2 Materials The grouting materials adopted are mortars that the producers declare compatible with ancient masonry walls; they are identified by their commercial names: Mape-Antique is a hydraulic bonding material (suggested w/c ratio=0.40); Microlime 900 is a mixture of bonding materials without cement (suggested w/c ratio = 0.30); Grout Flow R is a natural hydraulic lime without soluble salts (suggested w/c ratio = 0.60). In addition, in order to make a comparison with the preliminary tests performed on smaller diameter bars'*, the following cement based mortar was adopted: Macflow is an expansive cement with superplasticizer (suggested w/c=0.32). The adopted water/cement ratios (by weight), indicated above, are the values suggested by the producers for injection in masonry walls. The flexular strength was measured at the time of the pull-out tests from three-point-bending tests on small beams having dimensions of 40x40x160 mm according to UNI-EN 196 code. The compressive strength was measured from the two pieces obtained after bending failure, on a base of 40x40 mm; the results are shown in Figs. 2a,b. E, i rm n Materials Materials Compressive strength (a) Flexural strength (b) MP'-Mape-Antique; ML=Microlune 900, MC=Maflow\ GF=Grout Flow R Figure 2: Mechanical properties of the adopted grouting materials. The anchored bars, made of C40 steel, were subjected to stresses (Os.max~240 MPa) remarkably lower than the yield strength (f^=630 MPa). They were 1000 mm long and were threaded for a length of 550 mm at the anchored side, to improve bond strength, and for a length of 150 mm at the other side to allow the connection of the load cell (Fig. 3).

4 58 Structural Studies, Repairs and Maintenance of Historical Buildings pull out (a) Figure 3: Instrumentation (a) and particular of the circular steel plate to support the LVDTs at the loaded end of the bar (b). The steel bars were anchored in three masonry walls made with new bricks and a low-strength hydraulic lime mortar; the adopted cement: water aggregate ratios were equal to 1:0.57:3.33. The maximum size of the aggregate was 3 mm and the thickness of the masonry mortar bed joints was about 15 mm. During the wall construction, several beam specimens (dimensions of 40x40x160mm) were made with the bed joint mortar. Its average flexural strength (determined as for the injection mortars) was 0.7 MPa, while the average compressive strength was 1.90MPa. 3 Experiments The anchors were prepared by first making a horizontal hole with a diameter of 42 mm in the wall, by using a circular saw. After cleaning and pre-wetting the drill hole, the steel bars were inserted, centred with spacings and grouted with low pressure (about 2 bar). In order to completely fill the cylindrical hole, its ends were sealed with mortar and two small plastic tubes were placed to inject the mortar from one side, and to allow the air to exit from the other side. The anchored bars were suitably instrumented to measure the bar-to-masonry displacements at the free and at the loaded ends of the bar. The latter was measured both at the bar-to-masonry interface and between the bar and a point of the masonry wall which was sufficiently far from the bar to be considered as

5 Structural Studies, Repairs and Maintenance of Historical Buildings 59 undisturbed. A cone-shaped disturbed volume with vertex at the bar free-end and a vertex angle of 90" was assumed (Fig. 3). The two loaded end displacements were measured by two series of three inductive transducers (LVDT, Linear Variable Differential Transformer), placed radially at 120, to avoid the disturbance of bar rotation (Fig. 3). The average displacement measured by the three inner transducers as well as the displacement at the unloaded end of the bar can be assumed as the slip between the bar and the surrounding masonry. A circular aluminium plate, placed at the loaded end of the bar, supported the three inner LVDTs and the three aluminium braces to hold the three outer transducers (Fig. 3b). The pull-out load was measured by means of a load cell, made by applying strain gauges to a steel bar having the same diameter as the anchored bar. The relationship between the load and the strain was checked by means of a calibration test. A 300-kN tension jack was used for loading the bar (Fig. 4). The reaction points of the pull-out apparatus were outside the circular base of the ideal "disturbed" cone by using a steel tripod (Fig. 4). A small pre-load was applied to the bar for keeping the tripod in the horizontal position. Figure 4: Test arrangement. All anchored bars were pulled-out after days from the injection. The load was progressively increased up to failure with an initial average loading rate of 30 N/sec; when the load was approaching the maximum value, an average displacement rate of 2u.m/sec was applied (outer displacement at the loaded end). Some specimens were unloaded when the load was about half of the maximum load, determined in a previous test.^

6 60 Structural Studies, Repairs and Maintenance of Historical Buildings The measurements from the strain gauges and from the LVDTs were converted from analogue to digital by a HBM UPM 100 multipoint measuring-unit, and then stored in a PC. Two specimens from each material were tested by drilling four holes in two masonry walls having a dimension of 1600x1600x500 mm (Fig. 5). The third wall was made without vertical bed joints of mortar, as may occur in ancient buildings. In this wall, two anchors with cement mortar (Macflow) and two anchors with lime mortar (Grout Flow R) were tested. The bars were placed at a distance that allowed to avoid superimposition of the assumed failure cones (Fig.5). Figure 5: View of the masonry walls with anchored steel ties. In order to simulate the typical case of a wall in an ancient building with two or three floors, all the walls were prestressed before drilling the holes by means of a stiff steel beam and two vertical steel bars with bolts at the lower end, under the floor of the lab. The average compressive stress in the walls was 0.2 MPa. A load cell placed between the steel bars allowed to measure the compressive load on the masonry wall during the test (Fig. 5). Further details on the experimental set-up can be found in Giuriani et al.^ 4 Results Typical bond-stress versus relative displacement, as obtained from bars anchored with Grout Flow R (GF) and Mape-Antique (MP), are shown in Figs. 6 and 7 respectively. Here, the bond stress is conventionally assumed as uniformly distributed along the bar and is evaluated along the external perimeter of the threaded bar (< )=30mm); furthermore, the plotted curves concern the displacements measured at the free-end and at the loaded end of the bar. One can

7 Structural Studies, Repairs and Maintenance of Historical Buildings 61 noticed that all curves from specimen GF3 keep increasing during the test and that the two loaded-end curves are very close, since bond failure occurred at the mortar-to-masonry interface, as shown in Fig. 8a. On the contrary, a crack developed within the masonry in specimen MP1, where the inner loaded-end displacement remained constant during the pull-out of the masonry block around the anchor bar (Fig. 8c). A similar behaviour occurred in the other Mape-Antique specimens and in bars anchored with Micro Lime (ML) and Mac Flow (MC) mortars (Figs. 8b,d). In some specimens, a failure cone with a vertex at about mm from the loaded end of the bar formed in the masonry wall (Figs. 8c,d), while in specimens where the hole was drilled in a masonry brick on the external wall-surface (and not in a joint), the masonry brick itself was pulled-out with the steel bar since shear failure occurred along the masonry mortar joints (Figs. 8b). In all anchors, after failure the first mm of grouting material was attached to the bar at its loaded end, while the remaining injected mortar remained attached to the masonry wall (Figs. 8a,d). Specimen GF3 free-end loaded-end (inner) loaded-end (outer) Displacement [mm) Figure 6: Load versus free and loaded-end slips from specimen GF3 with Grout Flow R grouting mortar. The maximum bond stress obtained from the anchor specimens is shown in Table 1. One can notice the remarkable bond strength obtained from specimens MC, MP and ML, where the anchor involved the masonry failure. A lower bond strength was obtained from GF specimens, based on natural lime mortar; however, a better bond strength should be obtained by using a lower water/cement ratio. In fact, with the w/c ratio suggested by the producer, the grouting mortar was very fluid, as required for common masonry injections. For anchorages, it would be however injectable with a lower w/c ratio. Furthermore, the lower values of the bond strength measured from push-off tests with respect to the conventional bond strength obtained from pull-out tests can be attributed to the different failure mechanisms involved.

8 62 Structural Studies, Repairs and Maintenance of Historical Buildings Specimen MPl free-end loaded-end (outer) loaded-end (inner) Displacement mm Figure 7: Load versus free and loaded-end slips from specimen MPl Antique grouting mortar. with Mape- Mape-Antique (MP1) (c) Microlime (ML 1) (d) Figure 8: Failure types of the bars injected with the different mortars adopted.

9 Structural Studies, Repairs and Maintenance of Historical Buildings 63 Table 1 shows the maximum pull-out load (F,^x), the loads correspondent to free-end slips of 0.01 mm (Fo.oi) and 0.1 mm (Fo.i), and the maximum conventional bond stress (%ma\), as obtained from all the specimens tested. Tests on specimens MC3, MC4, GF2 and GF3 were performed on the wall without vertical bed joints of mortar. The little lower bond strength of specimens MC3 and MC4, with respect to the other specimens of the same material, can be observed (Tab. 1). On the contrary, in the GF specimens, where the masonry failure did not occur, bond strength of specimens GF2 and GF3 was similar to the one of specimen GF1. Table 1: Values of the maximum load (F^*), load at free-end slips of 0.01 and O.lmm (Fo.i), and maximum conventional bond stress (TmaxX as obtained from the anchor tests. SPECIMEN MCI MC2 MCJ(*) MC4(*) MP1 MP2 ML1 ML2 GF1 GF1(*) GFJ(*) 5 Concluding remarks Fmax [kn] FO,OI [kn] F,,,, [kn] (*) Specimen in wall without vertical bed joints of mortar ^mav [MPa] B L06 Pull-out tests on threaded steel-bars anchored in laboratory-made masonry walls by using different types of grouting materials are performed. Some kinds of mortars, usually adopted for the injection in ancient masonry walls, were adopted as grouting materials. The results concern the bond stiffness and strength (Figs. 6 and 7). The bond strength values obtained are helpful for the design of the tie anchorage-length. In spite of the low compressive strength of the grouting materials, relatively strong connections have been achieved (Tab. 1). A lower bond strength was obtained from anchors with natural lime mortar. However, because of the high compatibility of this mortar with ancient masonry walls, it is particularly suitable to restore historical monuments. To increase its strength, multi-branch anchorages could be considered. Moreover, since a very high w/c ratio was adopted for this mortar, a better bond strength should be obtained by using a lower w/c ratio. Tests showed different collapse modes which can occur at the steel-mortarmasonry interfaces or within the masonry itself (Fig. 8).

10 64 Structural Studies, Repairs and Maintenance of Historical Buildings Finally, the adopted experimental technique seemed suitable to perform pull-out tests. In-situ tests on the same threaded bars in actual masonry walls, by using the same experimental technique, are in progress. References 1. Cecchinato, P. & Modena, C., Research on the interaction mechanism between steel bars and hollow clay unit masonry, Proc. of the Fourth North American Masonry Conference, eds. G.C. Hart & J. Kariotis, University of California Los Angeles, pp , August 1987, The masonry society. 2. Giuriani, E. & Gubana, A., Extrados ties for structural restoration vaults, Proc. of the Fourth Int. Conf on Structural Studies of Historical Buildings, eds. C.A. Brebbia & B. Leftheris, Crete, Greece, Computational Mechanics Publications, Southampton, pp , Giuriani E., Recupero e consolidamento delle strutture, Percorsi del restauro in San Faustina a Brescia, ed. G. Mezzanotte, University of Brescia, pp , in Italian. 4. Gigla, B. & Wenzel, F., The repair of historic masonry structures by injection anchors, Proc. of the 11th International Brick/Block Masonry Conference, Tongji University, Shanghai, China, pp , October Psilla, N. & Vintzileou, E., Pullout of horizontal reinforcement embedded in masonry, ACI SP , Bond and development of reinforcement, ed. R. Leon, pp , Giuriani, E., Plizzari, G.A. & Bassini, C., Gli ancoraggi di bar re in acciaio iniettate nelle murature, Technical Report, Dept. Of Civil Engineering, University of Brescia, in Italian. Acknowledgements The assistance and skill of Mr Augusto Botturi, Mr Domenico Caravaggi and Mr Alessandro Coffetti of the Laboratory for Testing Materials "P. Pisa" of the University of Brescia in preparing the tests is gratefully acknowledged. Also, the authors would like to thank MAC (Treviso, Italy) for providing Macflow, Mapei (Milan, Italy) for providing Mape-Antique, Phoenix (Vicenza, Italy) for providing Grout Flow R and Volteco (Treviso, Italy) for providing Microlime POO.