Bond Behavior of Historical Clay Bricks Strengthened with Steel Reinforced Polymers (SRP)

Transcription

1 Materials 2011, 4, ; doi: /ma rticle OPEN CCESS materials ISSN Bond Behavior o Historical Clay Bricks Strengthened with Steel Reinorced Polymers (SRP) Ernesto Grande, Maura Imbimbo and Elio Sacco * Department o Mechanics, Structures and Environments, University o Cassino Italy/via G. Di Biasio 43, 03043, Cassino (FR), Italy; s: e.grande@unicas.it (E.G.); mimbimbo@unicas.it (M.I.) * uthor to whom correspondence should be addressed; sacco@unicas.it; Tel.: ; Fax: Received: 18 February 2011 / ccepted: 18 March 2011 / Published: 21 March 2011 bstract: In the strengthening interventions o past and historical masonry constructions, the non-standardized manuacture processes, the ageing and the damage o masonry units, could signiicantly aect the properties o the suraces where strengthening materials are applied. This aspect requires particular care in evaluating the perormance o externally bonded strengthening layers, especially with reerence to the detachment mechanism. The bond response o old masonries could be very dierent rom that occurring in new masonry units which are the ones generally considered in most o the bond tests available in technical literature. The aim o the present paper is the study o the bond behavior o historical clay bricks strengthened with steel reinorced polymers (SRP) materials. In particular, the results o an experimental study concerning new manuactured clay bricks and old bricks extracted rom dierent historical masonry buildings are presented. The obtained results, particularly in terms o bond resistance, detachment mechanism and strain distributions, are discussed or the purpose o analyzing the peculiarities o the historical bricks in comparison with new manuactured ones. Some considerations on the eicacy o the theoretical ormulations o the recent Italian code are also carried out. Keywords: bond; masonry; SRP; historical bricks; racture energy 1. Introduction Masonry structures constitute an important part o existing buildings in several countries and, in many cases, they also represent the cultural heritage o these countries, which has to be preserved and

2 Materials 2011, protected. The degradation o materials due to the ageing o structures and the damage caused by past events, primarily earthquakes, has increased the vulnerability o these structures. lthough several standards codes and documents contain speciic indications about the strengthening interventions on existing masonry structures, one o the key aspects, in the case o cultural heritage, is to select strengthening solutions characterized by high eectiveness, low invasiveness and reversibility [1]. The strengthening interventions based on the use o iber composite materials (FRP, Fiber Reinorced Plastic) certainly represents an attractive solution able to guarantee all the above remarked properties. In recent years, new composite materials have been developed or the purpose o improving their compatibility with the structural materials in civil constructions and reducing costs. mong the new generation o composite materials, there is a great relevance o unidirectional ultra high tensile steel sheets made o steel ibers, characterized by high tensile strength and reduced diameters and impregnated with epoxy resin, in the case o Steel Reinorced Polymer (SRP), or impregnated with mortars (cement or hydraulic) in the case o Steel Reinorced Grout (SRG). The steel ilaments are braided into cords o various geometries and characterized by dierent values o strength and glued on a polypropylene scrim inorder to simpliy installation procedures. The application o SRP/SRG materials seems to be very suitable to masonry structures, thanks to the possibility that these materials can be impregnated in situ, can be easily removed, and are extremely compatible with the substrate maintaining excellent mechanical properties. However, most o the applications have been done or concrete structures. Only quite recently, the use o SRP/SRG materials has been extended also to the strengthening o masonry structures; some examples are the interventions made in important Italian constructions, such as the Church o San Gaetano in Padua, the Museum Diocesano in Faenza, and the building o Contucci in Montepulciano [2,3]. crucial point in the design and use o externally bonded reinorcement or the strengthening o masonry constructions is the bond mechanism between the reinorcement and masonry. Indeed, in recent years there has been an increasing interest in theoretical and experimental studies on bond behavior. However, most o these studies reer to the strengthening o new manuactured masonry units, which, in contrast to the old ones, are generally produced using a standardized manuacturing process, and are characterized by high compactness o clay and having very tough suraces. Moreover, investigations have been developed or FRP strengthening materials; while very ew studies reer to old bricks reinorced by SRP. For this purpose, the present paper shows and discusses the results o experimental bond tests involving new and historical clay bricks strengthened by SRP. The obtained results have particularly evidenced the role that some parameters such as the compressive strength, the porosity and the damage o the surace o bricks play in the local and global bond behavior o historical clay bricks reinorced by SRP. On the basis o the obtained results, the eicacy o the theoretical ormulations provided in the recent Italian code CNR-DT 200 [4] is also discussed. The paper is composed o three main parts. The irst part concerns the description o the specimens, the experimental setup and the preliminary tests or characterizing the bricks; the second part concerns the discussion o the bond tests results; the third part reers to the comparison between the experimental results and the theoretical ormulations. conclusive section is provided at the end o the paper.

3 Materials 2011, Experimental Study The experimental study presented in this paper consists o double shear push-pull tests perormed on new and historical clay bricks strengthened with SRP materials. dditional tests are also provided in order to characterize the bricks. In particular, three series o ive specimens, or a total o iteen bricks, were tested or the bond experiments, and compressive tests were perormed on cubic specimens extracted rom the bricks. The results obtained rom the tests are presented in terms o bond resistance, detachment mechanism and strain distribution along the strengthening component. The compressive strength and the characteristics o the external surace o the bricks are also examined in order to analyze the obtained results Materials Clay Bricks Three dierent types o clay bricks have been considered; they are denoted in the ollowing as new, old-1 and old-2 : the new bricks, illustrated in Figure 1a, are produced in the south o Italy; the old-1 bricks, illustrated in Figure 1b, were extracted rom a warehouse built between and located in the south o Italy (Teano, Caserta); the old-2 bricks, illustrated in Figure 1c, were extracted rom a construction adjacent to a convent, which was built between in the center o Italy (Ceccano, Frosinone). In particular, the warehouse is a one story building with rectangular dimensions about m 2 (see Figure 2). Both the external and the internal suraces o the walls, composing the structure, are not covered by plaster and show a regular texture composed o a double layer o clay bricks. timber roo was present on the structure until the years , when it completely collapsed due to the degradation o materials. The bricks used or the tests were extracted rom the lateral wall near the door. Figure 1. Examined bricks. (a) (b) (c)

4 Materials 2011, Figure 2. Warehouse (rom which the old-1 bricks were extracted). The masonry construction, rom which the old-2 bricks were extracted, is a simple two-story house (see Figure 3) next to an important convent ( convent o Padri Passionisti in Ceccano, Frosinone). In this case, the walls o the construction are composed o tu blocks and clay bricks. In addition, according to a traditional rule o construction, clay bricks are also inserted along the corners o the building (see Figure 3) and around the doors and the windows. ll the açades are covered in plaster and the examined bricks were extracted rom the irst story during a restoration intervention. Figure 3. House next to the convent (rom which the old-2 bricks were extracted). ll the examined bricks have similar dimensions, about mm, although there are several dierences, particularly in terms o characteristics o exterior suraces. Whilst new bricks show compact suraces with regular and uniorm porosities or all the examined specimens, the old-1 and old-2 bricks present dierent properties also in comparison to the new bricks. The old-1 bricks present compact and tough suraces with porosity lower than the new bricks and, in some cases, with macro-irregularities due to the presence o several proturbances aecting the surace. The old-2 bricks present uniorm characteristics in terms o surace coniguration with no macro-irregularities. However, in comparison to new bricks, they are characterized by greater and non-uniorm porosity, lower compactness and, also, less tough suraces. The measurement o these properties is quite complex, in particular or the porosity and the compactness which generally involve only a limited depth o the brick surace. It is worth noting that although the dierences in terms o porosities were evident, old and new bricks are characterized by similar weights. ll the above aspects play an important role in the bond behavior since they aect the adherence between the support and the strengthening system and, consequently, the interaction process in terms o the stress transer mechanism. nother important parameter analyzed in the investigation is the compressive strength o the bricks which has been evaluated by perorming compressive tests, by means o the universal testing machine

5 Materials 2011, (Galdabini SUN60) on cubic specimens o dimensions equal to 50 mm. In particular, in the case o new bricks, the compressive test was perormed on six cubic specimens all extracted rom a brick which has not been used or the bond tests and the results are reported in Table 1. In the case o old bricks, the compressive tests were carried out on cubic specimens extracted rom each brick used or bond tests. In particular, ater the bond tests, the strengthening portion has been removed rom the bricks, the surace o the bricks has been leveled by removing the adhesive residues and six or eight cubic specimens have been obtained by cutting the bricks (Figure 4). Beore carrying out the compressive tests, each specimen has been dried in an oven and weighed by using an electronic balance. The results obtained or old bricks are summarized in Table 2. Table 1. Compressive test results (new bricks). compressive cubic specimen strength [MPa] average value 38.5 Table 2. Compressive test results (old bricks) and bond tests results (new and old bricks). Type new old-1 old-2 Legend: Specimen verage compressive strength [MPa] Bond resistance [N] S See Table 1a S S S S verage S S S B S C S C verage S B S B S B S B S B verage Debonding mechanism : removal o a deep portion o the support material at the end o the reinorcement together with a thin layer o the support material along the other part o the strip B: detachment o a uniorm and thin layer o the support material along the strip C: detachment o the adhesive -B: combined mechanism

6 Materials 2011, Figure 4. (a) Specimens extracted by cutting the bricks used or bond tests; (b) compressive tests Strengthening System a. b. For all the specimens, steel reinorced polymer strips, 25 mm wide, have been used. In particular, 3X2-B hardwire (medium density) steel reinorced polymer has been applied on the brick surace by using a bi-component epoxy resin (Betontex, RC02-LC202). The main properties o the strengthening system, supplied by the producer (TEC.INN-srl, Italy) are reported in Table Specimens and Setup Table 3. Properties o the strengthening system. FIDSTEEL 3X2-B HRDWIRE Tensile strength o strip 3070 MPa Ultimate deormation o strip 1.60% Elastic modulus 190 GPa Equivalent thickness mm Betontex, RC02-LC202 Tensile strength 35 MPa Ultimate deormation >2.8% Elastic modulus >2.5 GPa Five specimens or each type o brick have been tested in the bond experiments (Table 1). In particular, all specimens have been prepared according to a standardized procedure proposed by the RILEM committee (RILEM technical committee 223-MSC) and considering the coniguration shown in Figure 5. single strip, 25 mm wide, has been bonded on two opposite suraces o the brick leaving an un-bonded zone o 40 mm and a bonded zone o 160 mm. For all types o the examined bricks, two specimens have been equipped with our strain gauges applied on the strengthening portion according to the coniguration shown in Figure 5. For the other specimens, only the bond resistance and the detachment mechanism have been evaluated rom the bond tests. Bond tests have been perormed by using the special device illustrated in Figure 6; the proposed test setup allows carrying out bond tests by simply using a universal testing machine present in several laboratories. In particular, the same machine used or perorming the compressive tests has been adopted or the bond tests. The device consists o two main components: a steel rame made o two steel plates connected by our bars, and an additional system which consists o a steel roller or pulling the reinorcement. Other authors have also used a similar device or perorming bond tests [5-9].

7 Materials 2011, Figure 5. (a) Specimens coniguration; (b) Strain gauges positions. a. b. Figure 6. Test setup. 3. Results The irst results analyzed rom the bond tests were the bond resistance and the detachment mechanisms. These data are reported in Table 1 where the bond resistance reers to one hal o the bond orce in order to take into account the resistance o a single strip. The same data are also plotted in Figure 7. The detachment mechanisms are shown in Figure 8. Figure 7 shows that the new bricks are characterized by similar values o bond resistance, varying between 10.1 kn to 14.6 kn, and the same type o detachment mechanism; in act, in this case, the detachment is characterized by the removal o a deep portion o the support material at the end o the reinorcement together with a thin layer o the support material along the other part o the strip (mechanism, Figure 8).

8 Bond Force [N] Materials 2011, Figure 7. Bond resistance and detachment mechanism B -B -B B C -B C B "new" "old1" "old2" S1 S2 S3 S4 S5 specimens In the case o old-1 bricks, Figure 7 shows that the examined specimens are characterized by dierent values o bond resistance and dierent types o detachment mechanisms. In act, specimens S1 and S2 showed similar values o bond resistance, about 10 kn, and the same bond mechanism (mechanism ) which also characterizes the new bricks. However, specimen S3 collapsed at a lower value o bond resistance, equal to 8.3 kn, with a mixed detachment mechanism involving mechanism (but with a reduced depth o the removed support material) and, also, mechanism B which consists o a detachment o a uniorm and thin layer o the support material along the strengthening portion (see Figure 8). Figure 8. Detachment mechanisms: a. mechanism ; b. mechanism B; c. mechanism C. a. b. c. Specimens S4 and S5 showed the lowest values o bond resistance (less than the 50% o the maximum value o the bond resistance o the others specimens) and a dierent debonding mechanism, characterized by the detachment o the adhesive (mechanism C, Figure 8). s observed in the previous section, these two specimens are characterized by irregular suraces with signiicant irregularities and proturbances. In the case o the old-2 bricks, specimens S1, S3, S5 are characterized by low values o bond resistance varying rom 4.8 kn to 5.9 kn and by the same type o debonding mechanism, consisting o the detachment o a uniorm and thin layer o support material along the whole length o the strengthening portion (mechanism B). Specimens S2 and S4 showed greater values o bond resistance and a mixed debonding mechanism, which involves both mechanism and mechanism B.

9 strain strain strain Materials 2011, The bond tests also provided, or some specimens ( new -55, old-1 -S5, old-2 -S3), the measurements o the strain distribution. In the case o new bricks, whose ailure occurs with mechanism, the measured strain distribution is reported or dierent load levels, in Figure 9. The igure shows a concentration o strains in the initial portion o the strengthening portion until the load reaches the 80% o bond resistance when the strain concentration becomes lower and moves toward the central part o the strengthening portion. In the same igure, the strain distributions measured in old-1 and old-2 bricks, which collapse in mechanisms B and C respectively, are also reported. The igure shows that or the brick with mechanism C, the strain distribution is uniorm along the strip. However, or the brick with mechanism B, the strain distribution is somehow intermediate between those occurring in mechanism B and C; in act, the strain concentration is less evident than that which occurred in mechanism and is placed in the centre o the strengthening portion. From Figure 9 it is also evident that the maximum values o the strain occurred using mechanism, whilst the minor ones occurred using mechanism C. Figure 9. Strain distributions. 3,00E-03 F/Fmax [%] 2,50E-03 2,00E-03 NEW-S5 - mechanism 1,50E-03 1,00E-03 5,00E-04 0,00E distance [mm] 3.00E-03 F/Fmax [%] 2.50E-03 OLD2-S3 - mechanism B 2.00E E E E E+00 3,00E-03 2,50E-03 2,00E distance [mm] OLD1-S5-mechanism C F/Fmax [%] ,50E-03 1,00E-03 5,00E-04 0,00E distance [mm]

10 F max / FRP [MPa] F max / FRP [MPa] Materials 2011, Discussion and Remarks The experimental tests have so ar evidenced a dierent bond behavior o historical clay bricks in comparison to new bricks. In particular, old bricks have shown a major variability o bond resistance and o the detachment mechanism. These dierences can be correlated to some characteristics o old bricks and, primarily, the compressive strength and the properties o the exterior suraces where the strengthening is applied Compressive Strength In order to analyze the inluence o the compressive strength, the values o the bond resistance, divided by the cross section area o the FRP versus the compressive strength, are reported in Figure 10, or each specimen. The data, represented by cross marks, are collected on the basis o the collapse mechanism occurring in the specimens. The tendency line (dotted line) is also shown in the same igure. The irst plot reers to the specimens using mechanism, which involves the removal o a deep portion o the support material; while the second plot relates to mechanism /B, which involves the removal o a deep portion o the brick material at the end o the strip and the detachment o a thin layer o the brick material along the remaining zone o the strip. From the plots, the inluence o the support s compressive strength on the experimental values o the bond strength o specimens is evident. This result is in agreement with the type o observed ailure modes o specimens in which the core material o the brick is responsible or the compressive strength o the support. In the case o mechanism B, which involves the removal o a thin and uniorm layer o the brick material along the whole length o the strip, the compressive strength o the support does not inluence the bond strength o the specimens. This result is also in agreement with the observed ailure mode; in this case, the bond strength depends only on the shear strength o the material composing the external layer surace o the brick, without involving the inner material o the support. Finally, by examining the two specimens collapsing with mechanism C, which involves the detachment o the adhesive layer, it is evident that, also in this case, the compressive strength o the support does not inluence the bond strength. Figure 10. Bond resistance vs. compressive strength mechanism mechanism /B compressive strength [MPa] compressive strength [MPa]

11 F max / FRP [MPa] F max / FRP [MPa] Materials 2011, Figure 10. Cont mechanism B mechanism C compressive strength [MPa] compressive strength [MPa] 4.2. External Surace Regarding the properties o the external surace o the bricks, it has been observed that, in the case o new bricks, the external surace is characterized by uniorm porosities and a good level o compactness (absence o external layers with mechanical properties lower than those characterizing the core material o the brick), actors which guarantee a good adherence o the reinorcement (due to the penetration o the adhesive) and a high resistance o the support surace with respect to the detachment. On the other hand, old bricks are characterized, in several cases, by macro-irregularities (specimens S4, S5 o old-1 bricks) which do not guarantee an adequate level o adherence o the strengthening to the support, or by weak suraces with high porosity, which guarantee a good adherence but lead to a low bond strength o the exterior support material (specimens S1, S3, S5 o the old-2 bricks). These dierences are also conirmed by the strain distributions which showed a variability o the stiness o the interace layer between the FRP and the masonry support; this variability is due to the mechanical properties o the support and the adhesive material but, also, depends on the characteristics o the external surace o the bricks (porosity and regularity). In act, similar detachment mechanisms have been observed or bricks characterized by dierent values o the compressive strength and similar characteristics o the external suraces (see or instance old-2 -s1 and old-2 -s3) and, conversely, dierent detachment mechanisms have occurred or bricks characterized by similar values o compressive strength and dierent eatures o external suraces (see or instance old-2 -s1 and old-2 -s3). It is clear that these aspects must be accounted or in theoretical models, in order to also capture the bonding behavior in the case o historical bricks. 5. Theoretical Formulation The results obtained rom the experimental tests on the dierent types o examined bricks have evidenced so ar the role o dierent actors on the bond behavior o clay bricks strengthened by SRP materials. On the basis o these experimental results, it would be worthy to veriy the eicacy o the theoretical ormulation provided by the Italian code CNR-DT 200 [4] and the adopted assumptions.

12 Materials 2011, In particular, the CNR-DT200, according to other codes which reer to concrete supports [10,11] suggests evaluating the bond strength dd o masonry elements strengthened by FRP materials through the ollowing equation: 1 2 E dd ( i Lb Le ) (1.a) t d M L L b b 2 ( ) dd, rid dd i L L b e L L e e where d is a saety actor that takes into account the modality o the application o the reinorcement system; M is a saety actor concerning the masonry material; E and t are respectively the Young s modulus and the thickness o the strengthening; is the racture energy; dd, rid is the reduced value o the design bond strength; L b is the bond length o FRP element; L reers to the optimal bond length o FRP corresponding to the minimal bond length able to carry e the maximum anchorage orce, which can be deduced through the ollowing equation also reported in the CNR-DT200: where mtm the masonry average tensile strength. L E t /2 e mtm urther indication contained in the CNR-DT200 concerns the evaluation o the racture energy. With regard to this last parameter, the CNR-DT200 gives the ollowing relation based on the compressive strength mk and the tensile strength mtm o the support: 1 mk mtm 0.5 (1.b) c (3) where c 1 is an experimental coeicient which, in the case o masonry supports, is suggested to be assumed equal to i experimental results on the racture energy are not available. (2) 5.1. Remarks on the Coeicient c 1 The evaluation o the racture energy is generally very diicult to obtain because o the rapid development o the detachment phenomenon during the tests. In act, also in the tests discussed in this paper, it was not possible to evaluate the racture energy despite the act that the tests were conducted by a displacement control procedure with low velocities equal to 0.3 mm/min. In addition, this datum is generally not available when a preliminary design o the strengthening system is carried out. This implies that also the experimental coeicient c 1 is generally assumed by adopting the code indications. On the basis o the above considerations, it is worth analyzing i the value suggested by the code can be considered valid also or historical bricks strengthened by SRP. To this purpose, considering Equations (1.a) and (3), assuming an unitary value or the saety coeicients and setting the tensile strength equal to 0.10 mk as suggested by the code, the coeicient c 1 can be evaluated through the ollowing ormulation:

13 Materials 2011, c 1 2,exp t 2E mk FRP FRP mtm (4) where is the experimental value o the bond strength. The bond strength is deined as the bond,exp orce resulting rom the tests divided by the cross section area o the FRP strip. The values o the coeicients c 1 deduced rom Equation (4) are reported in Table 4. Table 4. Experimental values o the coeicient c1 (Equation 4). Specimen / debonding mechanism Coeicient c 1 new / mechanism old-1 / mechanism old-2 / mechanism B old-1 / mechanism C old-1, old-2 / mechanism -B The comparison between the values obtained or new bricks with those obtained or old bricks, suggests the ollowing observations: - old bricks ( old-1 /mechanism and old-1, old-2 /mechanism -B), characterized by values o the compressive strength similar to new bricks (dierences less than 20%) and by regular suraces with a uniorm distribution o porosities, show a reduction o the average value o the coeicient c 1 approximately equal to 15%; - old bricks ( old-2 /mechanism B and old-1 mechanism C), characterized by irregular suraces or by values o compressive strength less than the values characterizing new bricks (dierences greater than 40%), show a reduction o the average value o the coeicient c 1 greater than 65%. In all the above cases, the obtained values o c 1 are signiicantly greater than the value proposed by the code Inluence o the Compressive Strength The theoretical values o the bond strength evaluated by Equation 1.a, assuming the saety actors equal to 1, are charted as a continuous line in Figure 10, as previously discussed in Section 4. The irst two plots, reerring to mechanism and /B, show that the theoretical ormulation is able to represent the dependency o the support s compressive strength on the experimental values o the bond strength in accordance with the experimental results. However, in the case o mechanism, the experimental values and the dependency o the bond strength on the compressive strength are much higher than the results provided by the theoretical model. Moreover, the results carried out or new bricks show a great variability which could probably be due to the act that the compressive strength was not directly evaluated on the specimens used or bond tests. On the other hand, in the case o mechanism /B, the theoretical ormulation leads to a better estimation o the dependency o the bond strength on the compressive strength o the support.

14 Le [mm] Materials 2011, In the case o mechanism B, and also mechanism C, the theoretical values are in good agreement with the experimental values, although, in this case, the dependency on the compressive strength given by the theoretical ormulation is not conirmed by the experimental results Optimal Bond Length urther parameter examined in light o the theoretical ormulation provided by the code, concerns the optimal bond length L e which is particularly inluenced by the characteristics o the support. In Figure 11 the comparison between the theoretical values o the optimal bond length L e (vertical bars) and the experimental values (horizontal segments) is reported. In particular, the experimental values o L e, deduced on the basis o strain-gauges measurements, has been assumed equal to the distance between the section where the maximum strain value is attained and the section where a strain value not greater than 5% o the maximum strain is attained. On the contrary, the arrow symbols indicate the cases where the minimum measured value o strain is greater than 5% o the maximum strain: this means that the experimental optimal bond length is greater than the length o SRP. From the plot, it is evident that all the new specimens (characterized by mechanism ) show a theoretical value o L e greater than the experimental ones, whilst the contrary occurs or the old-1 bricks and the specimen S3 o the old-2 bricks. This result is probably due to the act that the porosity and the regularity o the surace o the new bricks guarantees an excellent anchorage level o the reinorcement to the support; whereas the irregular surace and the minor porosity o the old-1 bricks, together with the dierent ailure mechanisms, lead to a reduction o the anchorage eect o the SRP. The above observations, again, underline the role o the support s surace on the bond behavior and, also, emphasize the dierent behavior o the historical bricks in comparison to new manuactured elements as already discussed in the previous section. Figure 11. Optimal bond length / / B C B B C S3 S4 S5 new old1 old2 B specimens 6. Conclusions In this paper the results o bond tests conducted on new manuactured and old bricks have been discussed.

15 Materials 2011, The dierent bond behavior that emerged rom the tests has been examined on the basis o the dierent characteristics o the bricks, particularly in terms o the compressive strength and the properties o the exterior suraces where the strengthening is applied. The obtained results have clearly shown the important role o both characteristics. In particular, the tests showed that regular suraces with uniorm porosities distribution, which are commonly ound in new bricks and some old bricks, lead to a good level o adherence between the strengthening system and the support, and are characterized by a debonding mechanism which involves the detachment o the support material. In this case the compressive strength o the support material plays an important role in the bond resistance and, thus, sti suraces and bricks materials characterized by high values o compressive strength contribute to increase the bond resistance. On the other hand, bricks characterized by macro-irregularities, such as some types o old bricks, do not guarantee an adequate level o adherence o the strengthening to the support and are characterized by a debonding mechanism which involves the detachment o the adhesive with the lowest values o bond resistance. Finally, bricks with weak suraces but high porosity, such as others types o old bricks, can guarantee a good adherence, but show a low bond strength o the exterior support material which leads to a debonding mechanism involving the detachment o a thin layer o the support material. In both these last cases the compressive strength lightly aects the debonding resistance. It is evident that these aspects have to be particularly considered when a preliminary estimation o the bond resistance is carried out by using the design ormulations contained in the codes provided or when reerring to results deduced with reerence to the same type o masonry units but with dierent characteristics. Finally, it is important to note that, although this study has shown important peculiarities concerning the strengthening o historical constructions with innovative composites materials, urther aspects and additional inormation can be developed by considering other masonry materials and a greater number o experimental tests. Nevertheless, as emphasized in this paper, it is very important to improve the actual studies on the discussed problem and the relevant code provisions. cknowledgements The tests were conducted at the GEOLB SUD s.r.l (San Vittore (FR), ITLY) who are grateully acknowledged or their technical support, particularly Francesco Di Paolo or his technical assistance. The authors wish to thank TEC.INN.srl and in particular dott. ing. Paolo Casadei or supplying strengthening materials. Reerences 1. Carta Italiana del Restauro. Ministero della Pubblica Istruzione (in Italian) Borri,.; Casadei, P.; Castori, G.; Hammond, J. Strengthening o brick masonry arches with externally bonded steel reinorced composites. J. Compos. Constr. 2009, 13, Borri,.; Castori, G.; Corradi, M. Masonry columns conined by steel iber composite wraps. Materials 2011, 4,

16 Materials 2011, CNR CNR-DT200. Guide or the Design and Construction o externally bonded FRP Systems or Strengthening Existing Structures, Materials, RC and PC structures, Masonry Structures. National Research Council: Rome, Italy, RILEM technical committee 223-MSC: Masonry Strengthening with Composite Materials. vailable online: (accessed on 21 March 2007). 6. Briccoli Bati, S.; Rotunno, T.; Rovero, L.; Tonietti, U. Experimental study on CFRP-brick bonded joints. In Proceedings o Fourteenth International Conerence on Mechanics o Composite Materials, Riga, Latvia, 5 9 ugust iello, M..; Sciolti, M.S. Bond analysis o masonry structures strengthened with CFRP sheets. Constr. Build. Mater. 2006, 20, Capozucca, R. Experimental FRP/SRP historic masonry delamination. Compos. Struct. 2010, 92, Grande, E.; Imbimbo, M.; Sacco, E. simple model or the bond behavior o masonry elements strengthened with FRP. J. Compos. Constr vailable online: (SCE)CC (accessed on 2 December 2010). 10. Fib-TG9.3. Externally Bonded Fiber Polymer Reinorcement (FRP EBR) or Reinorced Concrete Structures; The International Federation or Structural Concrete: Lausanne, Switzerland, CI R-02. Guide or the Design and Construction o Externally Bonded FRP Systems or Strengthening Concrete Structures; CI merican Concrete Institute: Farmington Hills, MI, US, by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions o the Creative Commons ttribution license (