Repair of Cracked Historical Masonry Structures by Use of the Flexible Joint Method ( FJM ) – Laboratory Tests

Cracked historical buildings function in a new stress state of static equilibrium that is a consequence of post damage redistribution of the internal forces. This concerns especially unreinforced masonry (URM) buildings damaged by irregular settlements or acting of inertial forces during dynamic excitation. The Flexible Joint Method (FJM) for repairing of cracked historical masonry structures is proposed in this paper. It is based on “softening” of a structure and energy dissipation absorbed by the new joint constructed in place of cracks. In the FJM special polymer fills the cracks of the damaged buildings and thus introduces a new strength. There is given description of the FJM and also results of laboratory tests of polymer and results of tests of bricks joined with investigated polymer are presented. Practical examination of the proposed method was carried out on an unreinforced masonry wall damaged by shear stress, which cracks were filled in with investigated polymer. 2 RETROFITING METHODS OF CRACKED HISTORICAL MASONRY BUILDINGS 2.1 Short review of strengthening methods of cracks in monument masonry structures Generally, in case of cracked masonry heritage structures the repair and strengthening techniques must be chosen with exercise of special care. First of all, during whole repair process at any moment is not permitted to generate any deterioration of the technical state of whole or part of repaired structure. Secondly, the strengthening method should be suitable for the majority of cracks and should be born in mind, dependent upon aesthetic, environmental and real loading conditions. Usually, the diagonal or similar to vertical cracking are observed. The repair method dependent upon internal stresses and suitable recognition of all factors, which produced them (Freeman et al. 1994). In case of cracks connected with increasing of compressive stresses the most typically solution is to fill the crack with appropriate (Lime or Cement-Lime) mortar mix, well rammed in, and repoint the surface. Sometimes is also permitted removing and replacing shaded cracked units with new. Quite different situation is observed when cracks are produced by appearance tensile and high concentrated stresses in plane of masonry (Parkinson et al. 1996). In such cases is necessary to introduce into existing structure some elements being able to safely transfer the tensile stresses. The most popular is usage of reinforcing stitch bars (made of stainless steel and usually have diameter not exceeds 6 mm) grouted in at appropriate bed joints. Much more effective is putting into bed joints the special stainless steel spiral bars, more flexible, characterised by higher tensile strength and have very good adherence to mortar (Drobiec, Ł. and Jasiński, R.and Kubica, J. 2000). Sometimes, especially in case of stone masonry structures are also in use stiff steel bolts fixed by resin concrete injection across the crack into earlier prepared boreholes. Alternatively, where load the transfer of enhanced structural continuity is required, a stitch stiff lintel or rigid beam could be incorporated into the wall across the crack. This solution is acceptable only in structures with plastering of the masonry surface. 2.2 Applying of new materials in retrofitting works It should be noted that applying of too strong strengthening material to cracked structures (in comparison to the material properties of the wall) could cause additional damages. It is highly preferred to select properly innovative materials to make them work together with the existing materials with better “compatibility” from the mechanical point of view. This is to reduce as much as possible high stress concentration, which could occur using high strength/high stiffness modern bonding materials like resin or high strength mortars in cracked URM masonry (Modena 2004). The main criterion in selection should concern much more deformability than strength, especially in seismic areas. It is necessary to investigate material properties of masonry before applying a retrofit method based on various materials. The compressive strength and modulus of elasticity define the loadbearing capacity of masonry walls at gravity, whereas the tensile strength and shear modulus define the load-bearing capacity at seismic loads. Additional information regarding the ductility, damping and energy dissipation capacity, as well as strength and stiffness degradation and deterioration is also of relevant importance (Tomaževič 2004). 3 THE FLEXIBLE JOINT METHOD (FJM) IN REPAIR OF CRACKED MASONRY 3.1 Description of the Flexible Joint Method The most of retrofit methods applied up till now tend to restore the primary stiffness of the object and its properties from the pre-damage state. Cracks are usually filled in with rigid inject to assure co-operation of members separated by a crack and receive a new external strengthening. Generally strengthening is typically accompanied by stiffening but in certain cases “softening” can be better than strengthening (Bachmann 2002), particularly in case of cracked historical masonry structures. The new proposed Flexible Joint Method (FJM) allow to join the disrupted structure elements by means of flexible joints made of deformable elastic-plastic polymers and reinforced poly1448 Structural Analysis of Historical Constructions Arkadiusz Kwiecień, Boguslaw Zają mers filling in the cracks, see Fig. 1. This method is registered in the Polish Patent Department with No. P-368173 and was described in details in papers (Ciesielski and Kwiecień 2004), (Kwiecień and Zając 2004), (Kwiecień et al. 2005). The new method of protection and retrofit may be regarded as a passive one. Application of this sort of joining structural elements does not involve any additional stress in the damaged object, which could cause new damages in the weakened structure of the building (e.g. in historical buildings). The FJM permits safe work of the retrofitted structure in the new stabilized the state of balance. This is the method particularly conductive to objects, in which a redistribution of stress occurred in consequence of damage (cracks). It permits further safe exploitation of the object under additional static and dynamic loads. The FJM should be treated as a complement to the existing retrofit methods, which could be applied at determined work conditions of the structure under the protection and retrofit process. It is obvious that reason of damage should be removed before applying of a retrofitted method. 3.2 Cooperation of a flexible joint with a structure element Details of work of a flexible joint, constructed in a cracked masonry structure, are presented on the basis of an example given in diagrams, see Fig. 2. The strength of the joining material (polymer) is assumed to be lower than the strength of the structure materials (fj = 0.67 fs) and the joining material is much more flexible than the structure material (εs = 0.35%, εj = 100%). Under an exploitation load, a tension principal stress is much lower than tension strength of the structure and an input energy (area under σ-ε curve) is absorbed by the elastic deformational energy of the structure (Step 1). Under an ultimate load, the tension principal stress is reaching the value of tension strength and the input energy is absorbed by the total deformational energy of the structure. If the input energy grows up, the crack is appearing and the tension principal stress is starting to be equal zero in this place (Step 2). After filling in of crack with polymer the tension principal stress is still equal zero (Step 3). Under the new exploitation load, the input energy is absorbed simultaneously by both elastic deformational energy of the structure and of the new flexible joint (Step 4). Under the additional load, both absorb the input energy simultaneously: the elastic deformational energy of the structure and also by the elastic and non-linear deformational energy of polymer (Step 5). Under the new ultimate load, the tension principal stress is reaching the value of the polymer tension strength, and polymer undergoes destruction in the joint (Step 6), preserving this way the structure from destruction. Beginning of polymer destruction (visible in form of shear bends) is the signal for a new structural intervention in the cracked structure. Figure 1 : Application of the Flexible Joint Method: (a) cracked masonry wall, (b) flexible joint made of polymer, filling in crack and joining disrupted structural elements. (a) (b)