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Intrados strengthening of brick masonry arches with composite materials
Composites: Part B 42 (2011) 1164–1172
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Intrados strengthening of brick masonry arches with composite materials Antonio Borri, Giulio Castori ⇑, Marco Corradi Department of Civil and Environmental Engineering, University of Perugia, via Duranti 93, 06125 Perugia, Italy
a r t i c l e
i n f o
Article history: Received 1 August 2010 Received in revised form 14 January 2011 Accepted 18 March 2011 Available online 25 March 2011 Keywords: A. Glass fibers A. Plates B. Debonding Masonry arches
a b s t r a c t The objective of this study is to investigate the effectiveness of an innovative technique for strengthening masonry arches at their intrados, based on the use of carbon plates. Although FRP sheets or strips are successfully used as strengthening elements for this kind of application, they present several critical issues that can compromise these upgrading works, leading to the local collapse mechanism, which corresponds to the debonding of reinforcement from the masonry substrate. As an alternative to FRP sheets, the use of FRP plates presents instead several interesting aspects which make them very attractive for intrados strengthening. More precisely, FRP plates have an inherent bending and axial stiffness that may overcome problems concerning premature peeling. Fifteen prototypes of brickwork arches strengthened at their intrados with GFRP sheets or CFRP plates were tested under a monotonic vertical load applied at the keystone. The influence of the types of reinforcement (glass fibers and carbon plates), properties of the bonding system and masonry substrate and the presence of anchor spikes has been investigated. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to movements in the abutments, structural alterations induced by architectural changes, or increased service loads, masonry arches and vaults often need repair and/or strengthening. Because structural remedial measures might be needed after a structural evaluation, a significant concern in actual research is the need for efficient strengthening techniques to re-establish the performance of these structures, ensuring that under severe seismic conditions they will not collapse upon their occupants or passers-by. In the development of these methodologies, on the one hand, structural safety, including seismic hazard, must be accounted for, while on the other, the original construction must be preserved from both an aesthetic and a structural perspective. With regard to the many issues associated with the maintenance and restoration of historic buildings [1], the interest of workers in this field has become increasingly devoted to the development of innovative materials and advanced technologies. Specifically, there is an increasing interest in fiber-reinforced polymer (FRP) composites, whose use in the structural reinforcement of RC (reinforced concrete) structures has been widespread now for more than a decade [2–4]. FRP reinforcement provides designers with outstanding combination of properties, including low self-weight, and high strength. Moreover, the use of these materials does not alter the natural behavior of the structure since they do not add mass. In addition, they are removable, and they can be made either ⇑ Corresponding author. Tel.: +39 0755853906; fax: +39 0755853897. E-mail address: [email protected] (G. Castori). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.03.005
invisible or visible, in order to comply with modern restoration requirements. For these reasons, FRP reinforcement in the form of externally bonded at either the intrados or the extrados of the arch or vault with the wet lay-up technique is an effective solution, as demonstrated by the available experimental and theoretical studies [5–8]. As found in literature [8–10], the use of composite materials enables in fact masonry structures to carry substantial tensile stresses, eliminating their greatest mechanical shortcoming at an acceptable cost. More specifically, reinforcement is incapable of preventing masonry from cracking (to do so, it would either have to have a stiffness several times greater than the masonry, or it would have to be pre-stressed), but it does transmit the tension force between the two faces of the crack, i.e., it stitches the crack. Hence cracks may form at a reinforced boundary, but cannot open, since the tension force is transmitted by the reinforcement in lieu of the cracked masonry, i.e., the tension force bypasses the cracks and passes into the reinforcement. This means that reinforcing the extrados or the intrados allows to prevent all mechanisms (hinged mode failures) from occurring, forcing such structures to fail by other failure modes (i.e., crushing, sliding, debonding or FRP rupture). Parametric analyses [9,10] have been developed for predicting the ultimate load associated with each failure modes, the lowest of which constitutes the strength of the reinforced masonry arch. According to such analysis and to the abovementioned experimental studies, when FRP strips are bonded at the intrados, the effectiveness of the strengthening scheme was found to be highly dependent on the bond between composite strip and existing
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structure. In the presence of concavely-curved soffits, the FRP laminates tends to get straightened under tension, leading to a multiaxial stress state, which combines the shear stresses (s), parallel to the bonding masonry boundary, with transverse tensile stresses (r), which accelerate debonding failure [11,12]. Under such conditions, peeling and debonding became critical considerations in the design of masonry arches strengthening since the load-carrying capacity for this failure mode is commonly much lower than that of the other failure modes [6]. Nevertheless, quite surprisingly, few studies are available on masonry arches strengthened at their intrados [13], despite the fact that such strengthening arrangements are often found in practice. The main objective of the present research is finding out an effective way of applying FRP reinforcement at the intrados of masonry arches to improve their performance, overcoming problems concerning premature peeling. Fifteen prototypes of brickwork arches were built, reinforced and tested to compare their behavior up to collapse by testing several types reinforcement (glass fibers and carbon plates), looking after the properties of both the adhesive system and the masonry substrate, possibly influenced by the presence of anchor spikes. 2. Behavior of masonry arches strengthened at their intrados Traditional design and analysis of masonry arches and vaults [14] is based on a number of simplifying assumptions: (i) masonry has no tensile strength and infinite compressive strength, and (ii) sliding failure of an arch with rigid abutments is unrealistic. The consequence of these assumptions is that failure of a masonry arch theoretically occurs by formation of a sufficient number of hinges transforming the arch into a mechanism, and stability under given loads depends essentially on the geometry of the structure. As abovementioned, the presence of a bonded FRP strengthening modifies completely the structural behavior of a masonry arch. Since reinforcement enables masonry structures to carry substantial tensile stresses, it may stitches the crack, preventing the boundary opposite an FRP strip from hinging. Depending on the position and amount of the reinforcement and on the loading pattern, the formation of hinges may be either altered (hinges form at different locations than in the unstrengthened arch) or completely prevented. Therefore, the capacity of the arch may be controlled by other failure mechanisms, which depend on the strength limits of the constituent materials (masonry and reinforcement) and on their structural interactions at the local level (bond and localized shear). Specifically, if the FRP strip is bonded at the intrados, the effectiveness of the strengthening scheme is highly dependent on the bond between composite strip and existing structure. In this case, the interfacial shear stresses (s) between FRP and masonry should be in equilibrium with the normal ones (r) resulting from the shape of the concave substrate. Basic theoretical evaluation proves that the relationship that links the tension force in the reinforcement (T) with the normal stresses (r) is given by [13]:
r¼
T ; breinf R
ðnormal stressÞ
resultant state of stress (rd = f(r, s)), acting in a generic cracked section, by considering only the contribution of normal stresses (rd = r). The crucial point is to establish the limit (rRd) of the rd. rRd can be measured by in situ tests (pull-off tests) or obtained from the technical literature. Failure occurs once rd reaches rRd in a generic cross section. 3. Experimental program 3.1. Characterization of masonry mortar The arches were constructed using a hydraulic lime mortar (ratio sand/binder = 5/2 in volume). To obtain the mechanical properties of the, flexural and compressive strength tests were performed in accordance with the following specifications: ASTM C 348 [15] and ASTM C 349 [16]. As for the flexural strength tests, six prismatic specimens (160 40 40 mm) were used. Accordingly, the mean value of the flexural strength was 0.36 MPa, while the coefficient of variation was equal to 0.09. Compression tests were performed on 12 specimens, obtained from the flexural test specimens by breaking each prism into two halves. The compression strength is equal to 6.95 MPa, while the coefficient of variation was equal to 0.02 MPa. 3.2. Characterization of the bricks Solid clay bricks (250 125 55 mm3) were used for the construction of the masonry arches. The mechanical characteristics of the bricks were obtained by means of compression and bending tests carried out on six samples each. Uniaxial compression tests gave a mean strength and a coefficient of variation of 20.99 MPa and 0.11 MPa, respectively, whereas the mean value of the bending tensile strength and its coefficient of variation were 0.81 MPa and 0.27 MPa, respectively. 3.3. Characterization of the reinforcement The composite system used to strengthen the arches included glass fibers, carbon plates and FRP anchor spikes. Table 1 summarizes the geometrical and mechanical properties of the glass fibers and carbon plates. In both cases, coupon specimens were tested in tension to determine the mechanical properties and to draw their stress–strain curves. Test results indicated that both materials behave linearly to failure and there is practically no yielding. The FRP anchor spikes were constructed in-house and consisted of a precured fiber portion and a dry fiber portion. Because of their outstanding toughness and excellent resistance [17,18], aramid fibers were used to manufacture the FRP anchor spikes. Unidirectional aramid fibers were first bundled together and half of the fiber length was covered with plastic and duct tape. The uncovered bundled fibers were then impregnated and saturated thoroughly with epoxy resin. Lastly, the saturated fibers were passed through a circular hole in a steel plate (a die) to obtain the desired diameter of the anchor spikes. In this experimental program the diameter of the anchor spikes used was 8 mm. The saturated fibers were cured
ð1Þ
where breinf = width of the reinforcement; and R = radius of curvature. According to Eq. (1), r stresses are proportional to the tension in the fibers (T); as a consequence the use of reinforcements with high strength induces very high r stresses that are far above s stresses, which are consequently negligible. Consequently, in order to obtain an analysis model, able to describe the debonding failure mode, it is assumed to assess the
Table 1 Mechanical properties of the reinforcement. Property
Thickness (mm) Tensile strength (N/mm2) Elastic modulus (N/mm2) Ultimate strain (%)
Reinforcement type Glass fibers
Carbon plate
0.231 1940 70,804 2.8
1.400 3252 205,381 1.6
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in ambient temperature for 24–48 h and the plastic sheet was removed from the anchor spike to free the unsaturated dry fibers. These dry fibers were to be used for bonding purposes, suitably trimmed to different lengths according to specific requirements. Standard pull-out tests were conducted for these anchor spikes. In this particular situation, the aramid fibers were fully impregnated with saturant along their length to make AFRP bars for these tests. These anchors were embedded in 150 mm hydraulic lime mortar cubes over different lengths of 25 mm, 51 mm, 76 mm and 102 mm to perform the pull-out tests. A 530 kN Tinus-Olesen machine was used for these pull-out tests. The recorded average pull-out loads at failure were 22 kN, 29 kN, 36 kN and 31 kN, respectively. Based on these results, it was decided that in the main experimental program on masonry arches tests, the anchor spikes would be embedded 80 mm. 3.4. Interaction between masonry and reinforcement To investigate the mechanisms of local interaction among the constituent materials, a series of strengthened specimens of small dimensions have been tested. Adhesion tests on samples strengthened by glass fibers and carbon plates have been carried out for loads perpendicular (pull-off tests) to the bond surface (Fig. 1). Experimental results revealed that both glass fibers and carbon plates specimens failed in the substrate (Fig. 2a and b). The mean
(a)
Fig. 2. Pull-off tests: glass fibers (a) and carbon plate (b) specimens failure.
(b)
values of the tensile bond strength were 1.27 and 1.13 MPa for specimens strengthened by glass fibers and carbon plates, respectively. This indicated, according to fib provisions [19], that, since the failure of all specimens is a cohesion failure, the obtained values correspond to the masonry tensile strength under perpendicular tension rather than to the bond strength between reinforcement and masonry. 3.5. Test matrix
Fig. 1. Pull-off test setup.
To study the behavior of the strengthened arches, 15 specimens built with solid clay bricks arranged in a single layer (125 mm of thickness) have been tested under monotonic vertical loads applied at the keystone. A semi-circular shape, typical of many historic bridges and roman constructions, was chosen as directrix of the arches. A span of 2000 mm therefore means that the height of the arch is 1000 mm above the springing level. As already mentioned, the arches were tested altogether, varying type of reinforcement (glass fibers and carbon plate), number and position of anchor spikes and the boundary conditions (i.e., properties of the bonding system and masonry substrate). All the specimens, except for the control one (UN.01), where strengthened using a different combination of the above test variables (Table 2). The arches are identified by a two index code, in which the first
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A. Borri et al. / Composites: Part B 42 (2011) 1164–1172 Table 2 Test matrix. Specimen
Reinforcement type
Number and position of anchor spikes
Boundary conditions
UN.01 GF.01 GF.02 GF.03 GF.04 GF.05 GF.06 GF.07 GF.08 CP.01 CP.02 CP.03 CP.04 CP.05 CP.06
–
– – – 22 (from 15° to 160°, every 7°) – – – – – 2 (15°–165°) 2 (15°–165°) 3 (15°–90°–165°) 4 (15°–60°–120°–165°) 4 (15°–60°–120°–165°) 5 (15°–60°–90°–120°–165°)
– – – – Bedding band
Glass fibers
Carbon plate
Defects on the masonry substrate
– – – – – –
indicates the type of reinforcement (UN = unreinforced; GF = glass fibers; CP = carbon plate), while the second indicates the identification number of the specimen. A single ply of laminate or plate (150 mm and 100 mm wide, respectively) was applied to each arch, following the recommendations of ACI 440.2R-02 (ACI 440) provisions for FRP materials. Moreover, to avoid expected premature peeling, in CP specimens and arch GF.03, AFRP anchor spikes were adopted, in addition to the reinforcement. Installation of the spikes was carried out as follows. After the surface preparation had been completed, holes of
10 mm diameter and 85 mm depth were drilled into the masonry soffit prior to the strengthening. The holes were cleaned with pressurized air. The bond surface, including the holes, was primed, then the first layer of putty (or saturant) was applied on the masonry surface and the same saturant was used to partially fill the holes. After applying the reinforcement, the precured portions of the anchor spikes were inserted into the holes (Fig. 3a). The fiber bundle was then spread in circular fashion, folded and pressed over the carbon plate (or glass fibers), and impregnated with a layer of saturant (Fig. 3b). Table 2 shows the location of the anchor spikes.
Fig. 3. Application of AFRP anchor spikes: (a) anchor before gluing; (b) anchor after gluing.
Fig. 4. (a) Bedding band; (b) defects on the masonry substrate.
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Fig. 5. Test setup.
Finally, it is known that with masonry arches is very usual to have a soffit characterized by the presence of a layer of cementitious grout (bedding band), used to eliminate the irregularities of masonry substrate, or a different kind of substrate, with lower mechanical properties, that become in this way the elements of weakness of the whole strengthening system. Therefore, in two specimens (GF.04 and GF.05 tests), in order to investigate the influence of bonding system properties, the reinforcement was applied interposing a layer of cementitious grout (bedding band, Fig. 4a), whereas three specimens (GF.06, GF.07 and GF.08 tests), with a surface of application characterized by the presence of a substrate with lower mechanical properties (Fig. 4b), were used to simulate the presence of defects on the masonry substrate.
3.6. Test setup As shown in Fig. 5, all tests were conducted using a close-loop load configuration, where no external reaction is required. The load always applied at keystone was generated by means of a 100 kN manual hydraulic jack reacting against a steel frame and a 100 kN load cell, placed on bottom of the jack, was used to record load levels. Horizontal reaction was supplied by a reaction frame, built with equal leg angles and flat bars. In addition, a total of three Linear Variable Displacement Transducers (LVDTs) were used to
Table 3 Comparison among the experimental results. Specimen
a
Ultimate load capacity (kN)
Load point deflection (mm)
Reinforcement strain (le)
Mode of failure
Mechanism Laminate debonding Laminate debonding Masonry crushing Laminate debonding Laminate debonding Shear sliding + laminate debonding Shear sliding + laminate debonding Shear sliding + laminate debonding Shear sliding Shear sliding Masonry crushing Masonry crushing Masonry crushing Masonry crushing
UN.01 GF.01
0.70 7.07
1.50 16.04
– 5184
GF.02
6.41
11.68
3394
GF.03
6.90
18.61
4783
GF.04
4.71
14.01
2958
GF.05
4.48
5.74
2930
GF.06
4.85
8.44
4075
GF.07
5.93
10.95
4909
GF.08
4.88
14.28
4122
CP.01 CP.02 CP.03
13.44 14.20 15.81
1.69 2.33 5.83
779a 925a 2066
CP.04
18.55
7.78
2377
CP.05
11.50
4.17
1113a
CP.06
11.18
2.44
2046
Gauge produced unreliable data from this point.
Fig. 6. Laminate debonding: (a) arch GF.01; (b) arch GF.05.
A. Borri et al. / Composites: Part B 42 (2011) 1164–1172
register deflections, while four strain gauges were placed near the predicted hinge position to ensure the maximum strain was recorded. 4. Test results In the following, the results of the experimental tests carried out on the arches are grouped for type of the reinforcement (Table 3). 4.1. Unstrengthened arch The unreinforced arch showed a brittle failure, due to the formation of four hinges (arch displacement mechanism). Failure occurred suddenly (without any warning), for small displacements and just after the maximum load (equal to 0.7 kN) has been reached. 4.2. Arches strengthened with glass fibers The arches strengthened with glass fibers exhibited three different patterns of collapse: laminate debonding, masonry crushing and shear sliding. More precisely, in specimens GF.01 and GF.02 failure was dictated by the detachment of the reinforcement under the loaded section (the mean value of the failure load was 6.73 kN), with part of the masonry substrate attached to the laminate (Fig. 6a), denoting a good interface bond between masonry and reinforcement. Arches GF.04 and GF.05, reinforced by interposing a layer of cementitious grout (bedding band), exhibited the same
Fig. 7. (a) Masonry crushing: arch GF.03; (b) shear sliding and laminate debonding: arch GF.07.
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failure mode (laminate debonding), the difference being failure occurred at the interface between masonry and bedding band (Fig. 6b), but a lower ultimate load capacity (4.60 kN). Conversely, in arch GF.03, employing AFRP anchor spikes able to delay expected premature peeling, collapse occurred for an ultimate load of 6.90 kN (Fig. 7a) due to a crushing of the keystone masonry blocks, while the GFRP strip remained fixed. In this latter case, as well as in the previous ones, the structure did not reach a state of collapse, since the reinforcement contributed in holding the bricks together during the last phase. Finally, arches GF.06, GF.07 and GF.08, characterized by the presence of a substrate with lower mechanical properties, failed due to the sliding along the mortar joint close to the point of application of the load, resulting in the consequent detachment of the reinforcement from the masonry (Fig. 7b). Under such conditions, debonding progressed as long as the reinforcement contributed to holding the bricks together, then the arch load-carrying capacity (5.22 kN) decreased suddenly and the specimens collapsed under their own weight. 4.3. Arches strengthened with carbon plates As for the arches strengthened with carbon plates, the detected failure mechanisms involved the crisis of mortar joints (shear sliding) or of bricks in compression. More precisely, in arches CP.01 and CP.02, employing only two AFRP anchor spikes, failure (the mean value of the failure load was 13.82 kN) was dictated by the sliding between brick and mortar at the arch haunch (Fig. 8a). Conversely, arch CP.03, reinforced with three AFRP anchor spikes, failed due to a crushing of the keystone masonry blocks
Fig. 8. (a) Shear sliding: arch CP.02; (b) masonry crushing: arch CP.03.
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(Fig. 8b) after a significant increment of the load-carrying capacity (15.81 kN). It is worth noticing that such a failure mode is caused and governed by the pin localization, which is due to the bricks’ rotation and governed by the stitching action of the plate. More specifically, as expected (see Section 2), the reinforcement is incapable of preventing masonry from cracking, but it stitches the crack, preventing the boundary opposite an FRP strip from hinging. In such a context, the blocks close to the point of application of the load are cracked, but stitched by the reinforcement. The tension force in the CFRP plate is thus the result of a substantial relative rotation of the bricks around the boundary opposite the plate. The opposite boundary thus behaves like a pin. The pin localizes the contact between the units that rotate around it. Consequently, the stress profile lies at a limited depth and when the compressive strength was overcome, said stresses lead the keystone masonry blocks to crack along a horizontal plane, following the top side of the arch. Finally, arches CP.04 and CP.05, reinforced with four AFRP anchor spikes, and arch CP.06, reinforced with five AFRP anchor
spikes, presented a different ultimate load capacity despite having shown the same failure mode (masonry crushing). Specifically, while arch CP.04 exhibited a substantial increase of the failure load (18.55 kN), the ultimate loads of arches CP.05 and CP.06 were considerably lower (11.50 and 11.18 kN, respectively). Due to the notable variations in results typical of tests on masonry and considering the presence of undesirable variables (such as handwork) that may have arisen from the construction of the specimens, this fact may be attributed to a lower value of the masonry compressive strength. It is worth noticing that all specimens did not reach a state of collapse because the reinforcement contributed in holding the brick together during the last phase.
5. Analysis of the results The experimental investigation carried out on the strengthened arches is aimed at assessing the potential of carbon plates to
Fig. 9. Load–deflection (a) and load–strain (b) curves (measured at the location where load was applied).
A. Borri et al. / Composites: Part B 42 (2011) 1164–1172
provide a reinforcing system alternative to FRP laminates for intrados strengthening of masonry arches. According to this goal, analysis of the test results is conducted first with respect to arches strengthened with glass fibers, investigating the influence of the properties of the bonding system and masonry substrate; and then, arches using the same strengthening arrangement with different materials (glass fibers and carbon plates) are compared (Fig. 9). Remarks on the influence of the presence and number of anchor spikes are also presented. According to such an analysis the following can be highlighted: 1. As for the arches strengthened with glass fibers, it was observed that much of the success of masonry arches reinforced at their intrados lies with the integrity of the bond between GFRP strip and masonry substrate. Laboratory outcomes highlighted that the load-carrying capacity of the GF arches is significantly affected by the surface preparation (e.g., use of bedding bands) and masonry substrate properties. More specifically, compared with arches GF.01 and GF.02, the arches reinforced by interposing a layer of cementitious grout (arches GF.04 and GF.05) provided an ultimate strength approximately 30% smaller, whereas the presence of a substrate with lower mechanical properties (arches GF.06, GF.07 and GF.08) evidenced an average decrease in strength equal to 23%. Furthermore, it is worth noticing that even if AFRP anchor spikes allowed to eradicate the problem of premature peeling, they were incapable of increasing the arch load-carrying capacity. In spite of a different failure mechanism (masonry crushing), the ultimate load capacity of arch GF.03 was in fact similar (+2%) than that of the arches without anchor spikes (arches GF.01 and GF.02). 2. The use of carbon plates and AFRP anchor spikes eradicates the problem of premature peeling by resisting the transverse tensile stress that would otherwise limit the capacity of the structure leading to premature collapse. Strength increases provided by carbon plates were greater than those obtained using glass fibers. Regardless of the number of anchor spikes, carbon plates permitted the attainment of strength increases ranging between 16 and 26 times the original, while the maximum increase obtained with glass fibers was no more than 10 times the original. 3. As for the CP arches, it was observed that the number and the position of the anchor spikes have a limited influence on the reinforcing action of the carbon plates. Compared with arches reinforced with two anchor spikes (arches CP.01 and CP.02), arches CP.03 and CP.04, reinforced with three and four anchor spikes respectively, permitted the attainment of strength increases ranging between 11% and 38%, while the ultimate strength of arches CP.05 and CP.06, reinforced with four and five anchor spikes respectively, was approximately 20% smaller. 4. Important information is also provided by the analysis of LVDT and strain gauges readings at the keystone of the arch. The load–deflection and stress–strain curves (Fig. 9a and b) for series GF and series CP reveal a different behavior pattern. While in CP arches the failure was brittle, GF arches tends to attain a bilinear shape, characterized by a long pre-peak branch, which provides the structure with important ductility behavior. In fact, the peak load was achieved for a displacement even 11 times greater than the ones corresponding to the CP specimens. Conclusions The following conclusions are deduced from the experimental results: The use of AFRP anchor spikes to secure the GFRP strip to the arch soffit, albeit able to eradicate the problem of premature
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peeling, was shown to be labor intensive (the spacing of the anchors is small) and, above all, incapable of increasing the load-carrying capacity of GF arches. In spite of a different failure mechanism (masonry crushing), the ultimate load capacity of arch GF.03 was in fact close to of the arches without anchor spikes. A key factor that may explain such a discrepancy is the damage on the masonry region adjacent the pre-drilled holes used for the insertion of AFRP anchor spikes. The presence of too many anchors, inserted to a depth of 2/3 of the arch thickness, are likely, in fact, to justify a serious weakening of masonry blocks. Carbon plates and AFRP anchor spikes were effective in helping to prevent debonding and premature peeling and provided an overall better performance, in terms of ultimate load capacity, than glass fibers. This is because the FRP sheet is generally so thin and its bending stiffness so small that when it detached from the masonry it acts like a membrane, which cannot adhere more to the masonry substrate. As a consequence, since it cannot carry the stresses occurring at the tensed edges, it becomes incapable of preventing masonry from hinging. As an alternative to FRP sheets, the use of FRP plates presents instead several interesting aspects which make them very attractive for intrados strengthening. These can be summarized as follows: FRP plates have an inherent bending and axial stiffness that may overcome problems concerning premature peeling. More precisely, if the ends of the FRP system are sufficiently anchored to the soffit of the arch (experimental analysis pointed out that, since the number AFRP anchor spikes have a limited influence on the reinforcing action of the carbon plates, even two anchors are enough) and no slip is allowed at the interface between masonry and reinforcement, even when the transverse stresses reach the tensile resistance of the adhesive or masonry support, the FRP plate, due to its greater bending and axial stiffness, tends to maintain its shape and hence it can continue to adhere to the masonry substrate, carrying the tensile stresses and preventing masonry from hinging. Reinforced arches exhibited a different postcapacity behavior with respect the unreinforced ones. While the unreinforced arch failed due to the complete loss of capacity, both the GF and CP arches (with the exception of arches GF.06, GF.07 and GF.08, characterized by the presence of a substrate with lower mechanical properties) did not reach a state of collapse because the reinforcement contributed in holding the brick together during the last phase. This means that either classical GFRP sheets and CFRP plates are effective in preventing the collapse of masonry arches when subjected to ultimate stress limits. The difference being that GF specimens, exhibiting a larger deformation capacity prior to failure, also provide the arches with important ductility behavior.
Acknowledgements The authors would like to acknowledge the support of FIDIA for providing the strengthening materials, and TECINN Laboratory technicians for their assistance during the construction of the AFRP anchor spikes. References [1] ICOMOS. International scientific committee for analysis and restoration of structures of architectural heritage. Recommendations for the analysis, conservation and structural restoration of architectural heritage. In: Guidelines of ICOMOS 14th General Assembly. Victoria Falls; October, 2003. [2] Fanning P, Kelly O. Shear strengthening of reinforced concrete beams: an experimental study using CFRP plates. In: Proceedings of structural faults + repair 99 conference. London; July 1999.
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Intrados strengthening of brick masonry arches with composite materials Antonio Borri, Giulio Castori ⇑, Marco Corradi Department of Civil and Environmental Engineering, University of Perugia, via Duranti 93, 06125 Perugia, Italy
a r t i c l e
i n f o
Article history: Received 1 August 2010 Received in revised form 14 January 2011 Accepted 18 March 2011 Available online 25 March 2011 Keywords: A. Glass fibers A. Plates B. Debonding Masonry arches
a b s t r a c t The objective of this study is to investigate the effectiveness of an innovative technique for strengthening masonry arches at their intrados, based on the use of carbon plates. Although FRP sheets or strips are successfully used as strengthening elements for this kind of application, they present several critical issues that can compromise these upgrading works, leading to the local collapse mechanism, which corresponds to the debonding of reinforcement from the masonry substrate. As an alternative to FRP sheets, the use of FRP plates presents instead several interesting aspects which make them very attractive for intrados strengthening. More precisely, FRP plates have an inherent bending and axial stiffness that may overcome problems concerning premature peeling. Fifteen prototypes of brickwork arches strengthened at their intrados with GFRP sheets or CFRP plates were tested under a monotonic vertical load applied at the keystone. The influence of the types of reinforcement (glass fibers and carbon plates), properties of the bonding system and masonry substrate and the presence of anchor spikes has been investigated. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Due to movements in the abutments, structural alterations induced by architectural changes, or increased service loads, masonry arches and vaults often need repair and/or strengthening. Because structural remedial measures might be needed after a structural evaluation, a significant concern in actual research is the need for efficient strengthening techniques to re-establish the performance of these structures, ensuring that under severe seismic conditions they will not collapse upon their occupants or passers-by. In the development of these methodologies, on the one hand, structural safety, including seismic hazard, must be accounted for, while on the other, the original construction must be preserved from both an aesthetic and a structural perspective. With regard to the many issues associated with the maintenance and restoration of historic buildings [1], the interest of workers in this field has become increasingly devoted to the development of innovative materials and advanced technologies. Specifically, there is an increasing interest in fiber-reinforced polymer (FRP) composites, whose use in the structural reinforcement of RC (reinforced concrete) structures has been widespread now for more than a decade [2–4]. FRP reinforcement provides designers with outstanding combination of properties, including low self-weight, and high strength. Moreover, the use of these materials does not alter the natural behavior of the structure since they do not add mass. In addition, they are removable, and they can be made either ⇑ Corresponding author. Tel.: +39 0755853906; fax: +39 0755853897. E-mail address: [email protected] (G. Castori). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.03.005
invisible or visible, in order to comply with modern restoration requirements. For these reasons, FRP reinforcement in the form of externally bonded at either the intrados or the extrados of the arch or vault with the wet lay-up technique is an effective solution, as demonstrated by the available experimental and theoretical studies [5–8]. As found in literature [8–10], the use of composite materials enables in fact masonry structures to carry substantial tensile stresses, eliminating their greatest mechanical shortcoming at an acceptable cost. More specifically, reinforcement is incapable of preventing masonry from cracking (to do so, it would either have to have a stiffness several times greater than the masonry, or it would have to be pre-stressed), but it does transmit the tension force between the two faces of the crack, i.e., it stitches the crack. Hence cracks may form at a reinforced boundary, but cannot open, since the tension force is transmitted by the reinforcement in lieu of the cracked masonry, i.e., the tension force bypasses the cracks and passes into the reinforcement. This means that reinforcing the extrados or the intrados allows to prevent all mechanisms (hinged mode failures) from occurring, forcing such structures to fail by other failure modes (i.e., crushing, sliding, debonding or FRP rupture). Parametric analyses [9,10] have been developed for predicting the ultimate load associated with each failure modes, the lowest of which constitutes the strength of the reinforced masonry arch. According to such analysis and to the abovementioned experimental studies, when FRP strips are bonded at the intrados, the effectiveness of the strengthening scheme was found to be highly dependent on the bond between composite strip and existing
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structure. In the presence of concavely-curved soffits, the FRP laminates tends to get straightened under tension, leading to a multiaxial stress state, which combines the shear stresses (s), parallel to the bonding masonry boundary, with transverse tensile stresses (r), which accelerate debonding failure [11,12]. Under such conditions, peeling and debonding became critical considerations in the design of masonry arches strengthening since the load-carrying capacity for this failure mode is commonly much lower than that of the other failure modes [6]. Nevertheless, quite surprisingly, few studies are available on masonry arches strengthened at their intrados [13], despite the fact that such strengthening arrangements are often found in practice. The main objective of the present research is finding out an effective way of applying FRP reinforcement at the intrados of masonry arches to improve their performance, overcoming problems concerning premature peeling. Fifteen prototypes of brickwork arches were built, reinforced and tested to compare their behavior up to collapse by testing several types reinforcement (glass fibers and carbon plates), looking after the properties of both the adhesive system and the masonry substrate, possibly influenced by the presence of anchor spikes. 2. Behavior of masonry arches strengthened at their intrados Traditional design and analysis of masonry arches and vaults [14] is based on a number of simplifying assumptions: (i) masonry has no tensile strength and infinite compressive strength, and (ii) sliding failure of an arch with rigid abutments is unrealistic. The consequence of these assumptions is that failure of a masonry arch theoretically occurs by formation of a sufficient number of hinges transforming the arch into a mechanism, and stability under given loads depends essentially on the geometry of the structure. As abovementioned, the presence of a bonded FRP strengthening modifies completely the structural behavior of a masonry arch. Since reinforcement enables masonry structures to carry substantial tensile stresses, it may stitches the crack, preventing the boundary opposite an FRP strip from hinging. Depending on the position and amount of the reinforcement and on the loading pattern, the formation of hinges may be either altered (hinges form at different locations than in the unstrengthened arch) or completely prevented. Therefore, the capacity of the arch may be controlled by other failure mechanisms, which depend on the strength limits of the constituent materials (masonry and reinforcement) and on their structural interactions at the local level (bond and localized shear). Specifically, if the FRP strip is bonded at the intrados, the effectiveness of the strengthening scheme is highly dependent on the bond between composite strip and existing structure. In this case, the interfacial shear stresses (s) between FRP and masonry should be in equilibrium with the normal ones (r) resulting from the shape of the concave substrate. Basic theoretical evaluation proves that the relationship that links the tension force in the reinforcement (T) with the normal stresses (r) is given by [13]:
r¼
T ; breinf R
ðnormal stressÞ
resultant state of stress (rd = f(r, s)), acting in a generic cracked section, by considering only the contribution of normal stresses (rd = r). The crucial point is to establish the limit (rRd) of the rd. rRd can be measured by in situ tests (pull-off tests) or obtained from the technical literature. Failure occurs once rd reaches rRd in a generic cross section. 3. Experimental program 3.1. Characterization of masonry mortar The arches were constructed using a hydraulic lime mortar (ratio sand/binder = 5/2 in volume). To obtain the mechanical properties of the, flexural and compressive strength tests were performed in accordance with the following specifications: ASTM C 348 [15] and ASTM C 349 [16]. As for the flexural strength tests, six prismatic specimens (160 40 40 mm) were used. Accordingly, the mean value of the flexural strength was 0.36 MPa, while the coefficient of variation was equal to 0.09. Compression tests were performed on 12 specimens, obtained from the flexural test specimens by breaking each prism into two halves. The compression strength is equal to 6.95 MPa, while the coefficient of variation was equal to 0.02 MPa. 3.2. Characterization of the bricks Solid clay bricks (250 125 55 mm3) were used for the construction of the masonry arches. The mechanical characteristics of the bricks were obtained by means of compression and bending tests carried out on six samples each. Uniaxial compression tests gave a mean strength and a coefficient of variation of 20.99 MPa and 0.11 MPa, respectively, whereas the mean value of the bending tensile strength and its coefficient of variation were 0.81 MPa and 0.27 MPa, respectively. 3.3. Characterization of the reinforcement The composite system used to strengthen the arches included glass fibers, carbon plates and FRP anchor spikes. Table 1 summarizes the geometrical and mechanical properties of the glass fibers and carbon plates. In both cases, coupon specimens were tested in tension to determine the mechanical properties and to draw their stress–strain curves. Test results indicated that both materials behave linearly to failure and there is practically no yielding. The FRP anchor spikes were constructed in-house and consisted of a precured fiber portion and a dry fiber portion. Because of their outstanding toughness and excellent resistance [17,18], aramid fibers were used to manufacture the FRP anchor spikes. Unidirectional aramid fibers were first bundled together and half of the fiber length was covered with plastic and duct tape. The uncovered bundled fibers were then impregnated and saturated thoroughly with epoxy resin. Lastly, the saturated fibers were passed through a circular hole in a steel plate (a die) to obtain the desired diameter of the anchor spikes. In this experimental program the diameter of the anchor spikes used was 8 mm. The saturated fibers were cured
ð1Þ
where breinf = width of the reinforcement; and R = radius of curvature. According to Eq. (1), r stresses are proportional to the tension in the fibers (T); as a consequence the use of reinforcements with high strength induces very high r stresses that are far above s stresses, which are consequently negligible. Consequently, in order to obtain an analysis model, able to describe the debonding failure mode, it is assumed to assess the
Table 1 Mechanical properties of the reinforcement. Property
Thickness (mm) Tensile strength (N/mm2) Elastic modulus (N/mm2) Ultimate strain (%)
Reinforcement type Glass fibers
Carbon plate
0.231 1940 70,804 2.8
1.400 3252 205,381 1.6
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in ambient temperature for 24–48 h and the plastic sheet was removed from the anchor spike to free the unsaturated dry fibers. These dry fibers were to be used for bonding purposes, suitably trimmed to different lengths according to specific requirements. Standard pull-out tests were conducted for these anchor spikes. In this particular situation, the aramid fibers were fully impregnated with saturant along their length to make AFRP bars for these tests. These anchors were embedded in 150 mm hydraulic lime mortar cubes over different lengths of 25 mm, 51 mm, 76 mm and 102 mm to perform the pull-out tests. A 530 kN Tinus-Olesen machine was used for these pull-out tests. The recorded average pull-out loads at failure were 22 kN, 29 kN, 36 kN and 31 kN, respectively. Based on these results, it was decided that in the main experimental program on masonry arches tests, the anchor spikes would be embedded 80 mm. 3.4. Interaction between masonry and reinforcement To investigate the mechanisms of local interaction among the constituent materials, a series of strengthened specimens of small dimensions have been tested. Adhesion tests on samples strengthened by glass fibers and carbon plates have been carried out for loads perpendicular (pull-off tests) to the bond surface (Fig. 1). Experimental results revealed that both glass fibers and carbon plates specimens failed in the substrate (Fig. 2a and b). The mean
(a)
Fig. 2. Pull-off tests: glass fibers (a) and carbon plate (b) specimens failure.
(b)
values of the tensile bond strength were 1.27 and 1.13 MPa for specimens strengthened by glass fibers and carbon plates, respectively. This indicated, according to fib provisions [19], that, since the failure of all specimens is a cohesion failure, the obtained values correspond to the masonry tensile strength under perpendicular tension rather than to the bond strength between reinforcement and masonry. 3.5. Test matrix
Fig. 1. Pull-off test setup.
To study the behavior of the strengthened arches, 15 specimens built with solid clay bricks arranged in a single layer (125 mm of thickness) have been tested under monotonic vertical loads applied at the keystone. A semi-circular shape, typical of many historic bridges and roman constructions, was chosen as directrix of the arches. A span of 2000 mm therefore means that the height of the arch is 1000 mm above the springing level. As already mentioned, the arches were tested altogether, varying type of reinforcement (glass fibers and carbon plate), number and position of anchor spikes and the boundary conditions (i.e., properties of the bonding system and masonry substrate). All the specimens, except for the control one (UN.01), where strengthened using a different combination of the above test variables (Table 2). The arches are identified by a two index code, in which the first
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A. Borri et al. / Composites: Part B 42 (2011) 1164–1172 Table 2 Test matrix. Specimen
Reinforcement type
Number and position of anchor spikes
Boundary conditions
UN.01 GF.01 GF.02 GF.03 GF.04 GF.05 GF.06 GF.07 GF.08 CP.01 CP.02 CP.03 CP.04 CP.05 CP.06
–
– – – 22 (from 15° to 160°, every 7°) – – – – – 2 (15°–165°) 2 (15°–165°) 3 (15°–90°–165°) 4 (15°–60°–120°–165°) 4 (15°–60°–120°–165°) 5 (15°–60°–90°–120°–165°)
– – – – Bedding band
Glass fibers
Carbon plate
Defects on the masonry substrate
– – – – – –
indicates the type of reinforcement (UN = unreinforced; GF = glass fibers; CP = carbon plate), while the second indicates the identification number of the specimen. A single ply of laminate or plate (150 mm and 100 mm wide, respectively) was applied to each arch, following the recommendations of ACI 440.2R-02 (ACI 440) provisions for FRP materials. Moreover, to avoid expected premature peeling, in CP specimens and arch GF.03, AFRP anchor spikes were adopted, in addition to the reinforcement. Installation of the spikes was carried out as follows. After the surface preparation had been completed, holes of
10 mm diameter and 85 mm depth were drilled into the masonry soffit prior to the strengthening. The holes were cleaned with pressurized air. The bond surface, including the holes, was primed, then the first layer of putty (or saturant) was applied on the masonry surface and the same saturant was used to partially fill the holes. After applying the reinforcement, the precured portions of the anchor spikes were inserted into the holes (Fig. 3a). The fiber bundle was then spread in circular fashion, folded and pressed over the carbon plate (or glass fibers), and impregnated with a layer of saturant (Fig. 3b). Table 2 shows the location of the anchor spikes.
Fig. 3. Application of AFRP anchor spikes: (a) anchor before gluing; (b) anchor after gluing.
Fig. 4. (a) Bedding band; (b) defects on the masonry substrate.
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Fig. 5. Test setup.
Finally, it is known that with masonry arches is very usual to have a soffit characterized by the presence of a layer of cementitious grout (bedding band), used to eliminate the irregularities of masonry substrate, or a different kind of substrate, with lower mechanical properties, that become in this way the elements of weakness of the whole strengthening system. Therefore, in two specimens (GF.04 and GF.05 tests), in order to investigate the influence of bonding system properties, the reinforcement was applied interposing a layer of cementitious grout (bedding band, Fig. 4a), whereas three specimens (GF.06, GF.07 and GF.08 tests), with a surface of application characterized by the presence of a substrate with lower mechanical properties (Fig. 4b), were used to simulate the presence of defects on the masonry substrate.
3.6. Test setup As shown in Fig. 5, all tests were conducted using a close-loop load configuration, where no external reaction is required. The load always applied at keystone was generated by means of a 100 kN manual hydraulic jack reacting against a steel frame and a 100 kN load cell, placed on bottom of the jack, was used to record load levels. Horizontal reaction was supplied by a reaction frame, built with equal leg angles and flat bars. In addition, a total of three Linear Variable Displacement Transducers (LVDTs) were used to
Table 3 Comparison among the experimental results. Specimen
a
Ultimate load capacity (kN)
Load point deflection (mm)
Reinforcement strain (le)
Mode of failure
Mechanism Laminate debonding Laminate debonding Masonry crushing Laminate debonding Laminate debonding Shear sliding + laminate debonding Shear sliding + laminate debonding Shear sliding + laminate debonding Shear sliding Shear sliding Masonry crushing Masonry crushing Masonry crushing Masonry crushing
UN.01 GF.01
0.70 7.07
1.50 16.04
– 5184
GF.02
6.41
11.68
3394
GF.03
6.90
18.61
4783
GF.04
4.71
14.01
2958
GF.05
4.48
5.74
2930
GF.06
4.85
8.44
4075
GF.07
5.93
10.95
4909
GF.08
4.88
14.28
4122
CP.01 CP.02 CP.03
13.44 14.20 15.81
1.69 2.33 5.83
779a 925a 2066
CP.04
18.55
7.78
2377
CP.05
11.50
4.17
1113a
CP.06
11.18
2.44
2046
Gauge produced unreliable data from this point.
Fig. 6. Laminate debonding: (a) arch GF.01; (b) arch GF.05.
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register deflections, while four strain gauges were placed near the predicted hinge position to ensure the maximum strain was recorded. 4. Test results In the following, the results of the experimental tests carried out on the arches are grouped for type of the reinforcement (Table 3). 4.1. Unstrengthened arch The unreinforced arch showed a brittle failure, due to the formation of four hinges (arch displacement mechanism). Failure occurred suddenly (without any warning), for small displacements and just after the maximum load (equal to 0.7 kN) has been reached. 4.2. Arches strengthened with glass fibers The arches strengthened with glass fibers exhibited three different patterns of collapse: laminate debonding, masonry crushing and shear sliding. More precisely, in specimens GF.01 and GF.02 failure was dictated by the detachment of the reinforcement under the loaded section (the mean value of the failure load was 6.73 kN), with part of the masonry substrate attached to the laminate (Fig. 6a), denoting a good interface bond between masonry and reinforcement. Arches GF.04 and GF.05, reinforced by interposing a layer of cementitious grout (bedding band), exhibited the same
Fig. 7. (a) Masonry crushing: arch GF.03; (b) shear sliding and laminate debonding: arch GF.07.
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failure mode (laminate debonding), the difference being failure occurred at the interface between masonry and bedding band (Fig. 6b), but a lower ultimate load capacity (4.60 kN). Conversely, in arch GF.03, employing AFRP anchor spikes able to delay expected premature peeling, collapse occurred for an ultimate load of 6.90 kN (Fig. 7a) due to a crushing of the keystone masonry blocks, while the GFRP strip remained fixed. In this latter case, as well as in the previous ones, the structure did not reach a state of collapse, since the reinforcement contributed in holding the bricks together during the last phase. Finally, arches GF.06, GF.07 and GF.08, characterized by the presence of a substrate with lower mechanical properties, failed due to the sliding along the mortar joint close to the point of application of the load, resulting in the consequent detachment of the reinforcement from the masonry (Fig. 7b). Under such conditions, debonding progressed as long as the reinforcement contributed to holding the bricks together, then the arch load-carrying capacity (5.22 kN) decreased suddenly and the specimens collapsed under their own weight. 4.3. Arches strengthened with carbon plates As for the arches strengthened with carbon plates, the detected failure mechanisms involved the crisis of mortar joints (shear sliding) or of bricks in compression. More precisely, in arches CP.01 and CP.02, employing only two AFRP anchor spikes, failure (the mean value of the failure load was 13.82 kN) was dictated by the sliding between brick and mortar at the arch haunch (Fig. 8a). Conversely, arch CP.03, reinforced with three AFRP anchor spikes, failed due to a crushing of the keystone masonry blocks
Fig. 8. (a) Shear sliding: arch CP.02; (b) masonry crushing: arch CP.03.
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(Fig. 8b) after a significant increment of the load-carrying capacity (15.81 kN). It is worth noticing that such a failure mode is caused and governed by the pin localization, which is due to the bricks’ rotation and governed by the stitching action of the plate. More specifically, as expected (see Section 2), the reinforcement is incapable of preventing masonry from cracking, but it stitches the crack, preventing the boundary opposite an FRP strip from hinging. In such a context, the blocks close to the point of application of the load are cracked, but stitched by the reinforcement. The tension force in the CFRP plate is thus the result of a substantial relative rotation of the bricks around the boundary opposite the plate. The opposite boundary thus behaves like a pin. The pin localizes the contact between the units that rotate around it. Consequently, the stress profile lies at a limited depth and when the compressive strength was overcome, said stresses lead the keystone masonry blocks to crack along a horizontal plane, following the top side of the arch. Finally, arches CP.04 and CP.05, reinforced with four AFRP anchor spikes, and arch CP.06, reinforced with five AFRP anchor
spikes, presented a different ultimate load capacity despite having shown the same failure mode (masonry crushing). Specifically, while arch CP.04 exhibited a substantial increase of the failure load (18.55 kN), the ultimate loads of arches CP.05 and CP.06 were considerably lower (11.50 and 11.18 kN, respectively). Due to the notable variations in results typical of tests on masonry and considering the presence of undesirable variables (such as handwork) that may have arisen from the construction of the specimens, this fact may be attributed to a lower value of the masonry compressive strength. It is worth noticing that all specimens did not reach a state of collapse because the reinforcement contributed in holding the brick together during the last phase.
5. Analysis of the results The experimental investigation carried out on the strengthened arches is aimed at assessing the potential of carbon plates to
Fig. 9. Load–deflection (a) and load–strain (b) curves (measured at the location where load was applied).
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provide a reinforcing system alternative to FRP laminates for intrados strengthening of masonry arches. According to this goal, analysis of the test results is conducted first with respect to arches strengthened with glass fibers, investigating the influence of the properties of the bonding system and masonry substrate; and then, arches using the same strengthening arrangement with different materials (glass fibers and carbon plates) are compared (Fig. 9). Remarks on the influence of the presence and number of anchor spikes are also presented. According to such an analysis the following can be highlighted: 1. As for the arches strengthened with glass fibers, it was observed that much of the success of masonry arches reinforced at their intrados lies with the integrity of the bond between GFRP strip and masonry substrate. Laboratory outcomes highlighted that the load-carrying capacity of the GF arches is significantly affected by the surface preparation (e.g., use of bedding bands) and masonry substrate properties. More specifically, compared with arches GF.01 and GF.02, the arches reinforced by interposing a layer of cementitious grout (arches GF.04 and GF.05) provided an ultimate strength approximately 30% smaller, whereas the presence of a substrate with lower mechanical properties (arches GF.06, GF.07 and GF.08) evidenced an average decrease in strength equal to 23%. Furthermore, it is worth noticing that even if AFRP anchor spikes allowed to eradicate the problem of premature peeling, they were incapable of increasing the arch load-carrying capacity. In spite of a different failure mechanism (masonry crushing), the ultimate load capacity of arch GF.03 was in fact similar (+2%) than that of the arches without anchor spikes (arches GF.01 and GF.02). 2. The use of carbon plates and AFRP anchor spikes eradicates the problem of premature peeling by resisting the transverse tensile stress that would otherwise limit the capacity of the structure leading to premature collapse. Strength increases provided by carbon plates were greater than those obtained using glass fibers. Regardless of the number of anchor spikes, carbon plates permitted the attainment of strength increases ranging between 16 and 26 times the original, while the maximum increase obtained with glass fibers was no more than 10 times the original. 3. As for the CP arches, it was observed that the number and the position of the anchor spikes have a limited influence on the reinforcing action of the carbon plates. Compared with arches reinforced with two anchor spikes (arches CP.01 and CP.02), arches CP.03 and CP.04, reinforced with three and four anchor spikes respectively, permitted the attainment of strength increases ranging between 11% and 38%, while the ultimate strength of arches CP.05 and CP.06, reinforced with four and five anchor spikes respectively, was approximately 20% smaller. 4. Important information is also provided by the analysis of LVDT and strain gauges readings at the keystone of the arch. The load–deflection and stress–strain curves (Fig. 9a and b) for series GF and series CP reveal a different behavior pattern. While in CP arches the failure was brittle, GF arches tends to attain a bilinear shape, characterized by a long pre-peak branch, which provides the structure with important ductility behavior. In fact, the peak load was achieved for a displacement even 11 times greater than the ones corresponding to the CP specimens. Conclusions The following conclusions are deduced from the experimental results: The use of AFRP anchor spikes to secure the GFRP strip to the arch soffit, albeit able to eradicate the problem of premature
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peeling, was shown to be labor intensive (the spacing of the anchors is small) and, above all, incapable of increasing the load-carrying capacity of GF arches. In spite of a different failure mechanism (masonry crushing), the ultimate load capacity of arch GF.03 was in fact close to of the arches without anchor spikes. A key factor that may explain such a discrepancy is the damage on the masonry region adjacent the pre-drilled holes used for the insertion of AFRP anchor spikes. The presence of too many anchors, inserted to a depth of 2/3 of the arch thickness, are likely, in fact, to justify a serious weakening of masonry blocks. Carbon plates and AFRP anchor spikes were effective in helping to prevent debonding and premature peeling and provided an overall better performance, in terms of ultimate load capacity, than glass fibers. This is because the FRP sheet is generally so thin and its bending stiffness so small that when it detached from the masonry it acts like a membrane, which cannot adhere more to the masonry substrate. As a consequence, since it cannot carry the stresses occurring at the tensed edges, it becomes incapable of preventing masonry from hinging. As an alternative to FRP sheets, the use of FRP plates presents instead several interesting aspects which make them very attractive for intrados strengthening. These can be summarized as follows: FRP plates have an inherent bending and axial stiffness that may overcome problems concerning premature peeling. More precisely, if the ends of the FRP system are sufficiently anchored to the soffit of the arch (experimental analysis pointed out that, since the number AFRP anchor spikes have a limited influence on the reinforcing action of the carbon plates, even two anchors are enough) and no slip is allowed at the interface between masonry and reinforcement, even when the transverse stresses reach the tensile resistance of the adhesive or masonry support, the FRP plate, due to its greater bending and axial stiffness, tends to maintain its shape and hence it can continue to adhere to the masonry substrate, carrying the tensile stresses and preventing masonry from hinging. Reinforced arches exhibited a different postcapacity behavior with respect the unreinforced ones. While the unreinforced arch failed due to the complete loss of capacity, both the GF and CP arches (with the exception of arches GF.06, GF.07 and GF.08, characterized by the presence of a substrate with lower mechanical properties) did not reach a state of collapse because the reinforcement contributed in holding the brick together during the last phase. This means that either classical GFRP sheets and CFRP plates are effective in preventing the collapse of masonry arches when subjected to ultimate stress limits. The difference being that GF specimens, exhibiting a larger deformation capacity prior to failure, also provide the arches with important ductility behavior.
Acknowledgements The authors would like to acknowledge the support of FIDIA for providing the strengthening materials, and TECINN Laboratory technicians for their assistance during the construction of the AFRP anchor spikes. References [1] ICOMOS. International scientific committee for analysis and restoration of structures of architectural heritage. Recommendations for the analysis, conservation and structural restoration of architectural heritage. In: Guidelines of ICOMOS 14th General Assembly. Victoria Falls; October, 2003. [2] Fanning P, Kelly O. Shear strengthening of reinforced concrete beams: an experimental study using CFRP plates. In: Proceedings of structural faults + repair 99 conference. London; July 1999.
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