Experimental research on strengthening of masonry vaults using FRP

Transcription

1 Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland Experimental research on strengthening of masonry vaults using FRP J. Witzany, T. Čejka & R. Zigler Czech Technical University in Prague, Faculty of Civil Engineering, Prague, Czech Republic ABSTRACT: The article presents partial results of experimental research of segment masonry barrel vaults focusing on gaining new knowledge on the vaults failure mechanism in relation to their vertical rise, initial shape and geometric imperfections and the problems of the efficiency of methods used for the strengthening and rehabilitation of impaired masonry barrel vaults or masonry barrel vaults with an insufficient load-bearing capacity. 1 INTRODUCTION Floor structures of masonry buildings built until the start of the 20th century over underground storeys, over gateways, in corridors and staircase spaces are often made with masonry vaults usually applying simple segment barrel vaults of solid burnt bricks or are shaped into masonry barrel arches. For this reason, i.e. considerable frequency of the occurrence of masonry barrel vaults in historical masonry buildings, the research (Witzany at al., 2004) is, among other problems, oriented towards the problems of strengthening and rehabilitation of barrel vaults. 2 EXPERIMENTAL RESEARCH OF STRENGTHENING BARREL VAULT MASONRY STRUCTURES Experimental research was performed as part of an extensive experimental research programme of strengthening barrel vault masonry structures (vaults and pillars) focused on the verification of the reliability and efficiency of selected rehabilitation methods (Witzany at al., 2004). The research was aimed at the verification of the efficiency of strengthening barrel vault masonry structures reinforced with carbon fabric and additionally inserted helical reinforcement of highstrength steel. The research is a follow-up to previous research of segment masonry barrel vaults whose objective was the verification of the strain and failure mechanism of barrel vaults (Witzany et al. 2005). Parallel to experimental research, the non-linear modelling of strengthened vaults is performed using the finite-element method (FEM) in the Atena programme. Experimental research was performed on segment barrel vault masonry structures with a span of 3000 mm, a width of 750 mm and three different vertical rises of vaults (375 mm, 750 mm and 1000 mm) so as to obtain knowledge on the effect of the vertical rise and the vault reinforcement method on the vault failure mechanism. The vaults were built of bricks P15 (compressive strength 15 MPa) laid in lime mortar MV2 (compressive strength 2 MPa). The vault imposts were made using steel bases on which loading areas with a slope corresponding to the vertical rise of the vault were concreted. The bases were secured against horizontal displacement by means of bolted joints of the steel vault bases and steel sections embedded in the floor and further by means of a pair of steel tie rods anchored into the steel bases. Load was imposed onto the vaults by a pair of isolated forces exerted by hydraulic presses with a maximum force of 400 kn

2 The vaults were fitted with opto-electric incremental deformation gauges LARM (6 pcs of vertical deformation gauges and 6 pcs of horizontal deformation gauges). The resistance of supports against shift was checked by a pair of mechanical deformation gauges. In the case of strengthening masonry barrel vaults by carbon fabric the carbon composite was fitted with 6 pcs of strain gauges for the determination of stress. Table 1 presents a summary of experimental tests of segment barrel vaults performed Table 1. Overview of experiments performed Tested vault labelling Span Width/Thickness Vertical rise Strengthening K2a 3000 mm 900/150 mm 750 mm Without strengthening K mm 900/150 mm 375 mm Without strengthening K mm 900/150 mm 1000 mm Without strengthening K08a 3000 mm 750/150 mm 1000 mm CFRP extrados/intrados K09a 3000 mm 750/150 mm 750 mm CFRP extrados/intrados K mm 750/150 mm 375 mm CFRP extrados/intrados K mm 750/150 mm 1000 mm HELI intrados K mm 750/150 mm 750 mm HELI intrados K mm 750/150 mm 375 mm HELI intrados K mm 750/150 mm 750 mm HELI extrados/intrados K mm 750/150 mm 375 mm HELI extrados/intrados 3 REHABILITATION AND STRENGTHENING OF MASONRY BARREL VAULTS WITH CARBON FABRIC (CFRP) The vault strengthening with carbon fabric was made on both the vault extrados and intrados at the point of expected tension areas (specified by the FEM numerical calculation), at a width of 750 mm identical to the vault width and with an anchorage length of approximately 150 mm in the compressed area of vault cross-sections (Fig. 1). The strengthening was performed using Tyfo SCH-41 carbon fabric glued on with Tyfo S epoxy resin. The application procedure was carried out under the supervision of the supplier strictly observing the designed technological procedure. Figure 1. Vault K08a (K09a, K10) vertical rise 1000 mm (750mm, 375 mm), layout of measuring apparatus and fitting of strengthening carbon fabric (carbon fabric position is shown in bold) - 2 -

3 4 REHABILITATION AND STRENGTHENING OF SEGMENTS OF BARREL VAULT MASONRY WITH HELICAL REINFORCEMENT (HELI) The vault strengthening with helical reinforcement of high-strength steel additionally inserted into the grooves was performed for the case of the reinforcement installation only on the intrados, along the whole length of the vault arch (Fig. 2) and for the case of the reinforcement installation along the whole length of the intrados and on the extrados in the extent of 1/3 of the arch length in the vault base area at the point of so-called hazardous cross-sections (Fig. 3). The strengthening was performed using helical reinforcement Kompakt VAH with a diameter of 8 mm inserted into three grooves with a width of min. 12 mm and depth of min. 35 mm. The reinforcement was anchored into the grooves by high-strength polymer-cement mortar Kompakt MPC 50. The application procedure strictly observed the designed technological procedure. Figure 2. Vault K11 (K12, K13) vertical rise 1000 mm (750 mm, 375 mm), layout of measuring apparatus and installation of strengthening helical reinforcement (position of inserted rods is shown in bold) Figure 3. Vault K14 (K15) vertical rise 750 mm (375 mm), layout of measuring apparatus and installation of strengthening helical reinforcement (position of inserted rods is shown in bold) - 3 -

4 Table 2. and Fig. 4 to Fig. 7 display partial results of experimental research. Table 2. Experimentally established limit load of limit deformation of segment barrel vaults K08a K09a K10 K11 K12 K13 K14 K15 Load (kn) Vertical def. (crown, gauges K,H) Vertical def. (1/4, gauges L,G) Vertical def. (3/4, gauges I,J) Horizontal def. (1/3, gauges FC) Horizontal def. (2/3, gauges AB, ED) SUMMARY OF RESULTS OF EXPERIMENTAL RESEARCH The experimental research performed to-date has proved a relatively significant effect of the strengthening of vaults with carbon fabric (CFRP) or additionally inserted helical reinforcement of high-strength steel (HELI) on their ultimate bearing capacity, deformation and inelastic strain. With a view to the above-mentioned, the strengthening of vaults with carbon fabric may be assessed as more significant: The increase in the bearing capacity of segment masonry barrel vaults strengthened with carbon fabric (CFRP) or additionally inserted helical reinforcement of high-strength steel (HELI) loaded symmetrically by two vertical loads at thirds of the vault span (Fig. 4) as compared to the bearing capacity of non-strengthened masonry vaults (Witzany et al. 2005) ranges within values by approximately 1.5-times to 2-times higher (Fig. 5a), Segment masonry barrel vaults strengthened with carbon fabric (CFRP) or additionally inserted helical reinforcement of high-strength steel (HELI) as compared to nonstrengthened vaults showed - under comparable load (approaching the ultimate bearing capacity of non-reinforced vaults) - lower deformation values (80 50% of limit deformation of non-reinforced vaults (Witzany et al. 2005)) (Fig. 6), Segment masonry barrel vaults strengthened with carbon fabric (CFRP) or additionally inserted helical reinforcement of high-strength steel (HELI) as compared to nonstrengthened vaults show a considerably wider area of elasto-plastic to plastic deformations. The limit deformations of strengthened vaults at reaching the limit load reached values by 1.5-times to 2.2-times higher in vaults strengthened with carbon fabric (CFRP) than the values of limit deformations of non-reinforced vaults, In vaults strengthened with additionally inserted helical reinforcement of high-strength steel (HELI), in vaults with a vertical rise of 750 mm and 1000 mm the limit deformation reached values comparable to limit deformations of non-reinforced vaults, while in vaults with a vertical rise of 375 mm the limit deformation ranged from 2- to 3- times the values of the limit deformations of non-reinforced masonry barrel vaults (Fig. 6), The application of carbon fabric (CFRP) in the vault area exposed to tensile stresses (the lower vault intrados at the crown in the extent of approximately 1/3 of the vault arch length, on the extrados in the area of vault bases in the extent of approximately 1/3 of the vault length at both bases at the point of so-called hazardous cross-sections) limited not only the appearance and development of characteristic tensile cracks at these vault cross-sections, but also significantly increased the vault stability against buckling (Fig. 5b), The position of strengthening elements, mainly the carbon fabric (CFRP) affected the failure mechanism. In the case of the vault strengthening with carbon fabric (CFRP) in the extent of approximately 1/3 of the vault arch length at the crown (intrados) and at the bases (extrados) a prominent inclined shear crack arose and vault cross-sections were displaced at the point where the anchoring areas of carbon fabrics located on the vault extrados and intrados overlapped (Fig. 7), - 4 -

5 In the case of the vault strengthening with additionally inserted helical reinforcement of high-strength steel (HELI) the characteristic vault failure by the opening of bed joints in the area of so-called hazardous cross-sections followed by the vault buckling and collapse (vaults failure accompanied by a stability loss) occurred only on the lower vault intrados in the segment vault with a vertical rise of 1000 and 750 mm, In the case of both-sided strengthening of the barrel vault with carbon fabric or helical reinforcement (on the intrados at the crown and on the extrados at the bases) the vault limit load was reached in which the compression stress-state of the vault cross-section approached its ultimate bearing capacity, i.e. the strain due to the exhaustion of the vault cross-section bearing capacity prevailed prior to the vault failure by the loss of its stability, In the case of the vault reinforcement only on the intrados (by additionally inserted helical reinforcement) failure occurred by the buckling of the vault with a vertical rise of 1000 mm and 750 mm, which was preceded by the opening of the bed joint on the intrados in the area of so-called hazardous cross-sections. This vault buckling (loss of stability) in the area of hazardous cross-sections was assisted by the one-sided installation of the helical reinforcement on the vault intrados, mainly in the case of the vault vertical rise of 100 mm. This method of one-sided reinforcement of segment barrel vaults with a vault vertical rise (height) greater than 1/4 of the span cannot be recommended. Research of the rehabilitation and strengthening of segment barrel vaults has contributed to the extension of current knowledge of the behaviour of vaults strengthened with carbon-based fabrics and additionally inserted helical reinforcement. Figure 4. Tested vault strengthened with carbon fabric (a) and with additionally inserted reinforcement vault extrados and intrados (b) Figure 5. Comparison of load bearing capacities of segment barrel vault structures in relation to their vertical rise and strengthening performed (a), deformation patterns of segment barrel vault structures strengthened with carbon fabric (CFRP) under various loads (b) - 5 -

6 Figure 6. Comparison of vertical deformations of segment barrel vault structures in relation to the vertical rise and strengthening performed (a), comparison of horizontal deformations of segment barrel vault structures in relation to their vertical rise and strengthening performed (b) Figure 7. Failure of the tested vault strengthened with carbon fabric (a, b), tensile cracks on the vault extrados (in the area of hazardous cross-sections) during loading in a vault strengthened with additionally inserted reinforcement on the vault intrados (c) and on the vault extrados and intrados (d) The article was written with support from Research Plan MSM "Reliability, optimization and durability of building materials and structures. REFERENCES Witzany J., Čejka T. and Zigler R. (2004). Experimental Research of Masonry Vault Structrures. Proceedings of VI. konfrencia Staticko-konštkrukčné a stavebno fyzikálné problémy stavebných konštrukcií, Tatranská Lomnica (SK), , pp , ISBN Witzany J., Čejka T. and Zigler R. (2005). Barrel Vault Masonry Structures, Stavební ročenka 2006 (Structural yearbook 2006), JAGA GROUP, Bratislava (SK) 2005, pp , ISBN