- Email: [email protected]
Shear strength of brick masonry walls assembled with different types of mortar
Construction and Building Materials 40 (2013) 1038–1045
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Shear strength of brick masonry walls assembled with different types of mortar Valerio Alecci, Mario Fagone, Tommaso Rotunno ⇑, Mario De Stefano Department of Construction and Restoration, University of Florence, 50121 Florence, Italy
h i g h l i g h t s " Brick masonry walls assembled with different kinds of mortar are tested. " Shear tests on triplets and diagonal compression tests on panels are performed. " The shear strength under zero normal stress is the parameter obtained. " A comparison between the values of the shear strength is presented.
a r t i c l e
i n f o
Article history: Received 8 May 2012 Received in revised form 16 November 2012 Accepted 22 November 2012 Available online 28 December 2012 Keywords: Masonry Shear strength Experimental test
a b s t r a c t The prediction of masonry shear strength, by direct way, requires appropriate experimental tests on triplets, in line with standard EN 1052-3, or diagonal compression tests on panels according to ASTM 5092010 and RILEM LUMB6. In the present paper the results of an experimental investigation, carried out by these two types of tests on brick masonry walls assembled with different kinds of mortar are reported. A comparison between the values of the masonry shear strength, calculated applying the three formulas available in literature for the diagonal compression test data, and those obtained by laboratory tests on shear triplets, is presented. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Masonry constructions still constitute today a large part of the building stock throughout the world. The historical and artistic heritage as well as the ‘‘common’’ residential buildings in the old urban and rural city centres are usually made of masonry. Masonry material can scarcely bear tensile stress and it is, therefore, known as no-tension material. Furthermore, as it is a heterogeneous material, its mechanical behaviour depends on the geometric texture and the properties of the constituent materials. In the last decades, seismic events which hit and badly damaged large areas of high density masonry buildings (such as the Umbria and Abruzzo regions in Italy) have increased the interest of the scientific community towards more appropriate modelling strategies to assess the seismic vulnerability of such buildings. Although structural engineers are developing even more sophisticated numerical procedures, the ‘‘accuracy’’ of the modeling results always depends on the correct identification of a few mechanical parameters required to characterize masonry material.
⇑ Corresponding author. E-mail address: rotunno@unifi.it (T. Rotunno). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.11.107
The shear strength under zero normal stress is one of these parameters; its exact definition plays a crucial role in the prediction of masonry behaviour under seismic actions. Italian Seismic Code [1] in line with Eurocode 6 [2], allows to determine the shear characteristic strength fvk0 either through an estimate using a pre-calculated table of values or by a direct way which requires appropriate experimental tests on triplets, in line with standard EN 1052-3 [3]. For existing masonry walls, Eurocode 8 [4] suggests the direct determination of this parameter by diagonal compression tests according to ASTM 509-2010 [5] and RILEM LUMB6 [6] specifications. Diagonal compression tests are performed on new masonry walls also, as available in literature [7–11] and suggested by Italian Guidelines [12]. Although the diagonal compression test is largely used, the interpretation of the test outcomes and the formula to calculate the masonry shear strength according to ASTM 509-2010 and RILEM LUMB6 specifications have been questioned by several researchers; currently, various interpretations of the test results and different formulas are available in literature [13,14]. In the context of the seismic design of new masonry constructions, the shear strength can be determined by two different kinds of tests, shear triplets and diagonal compression. The former is
1039
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
preferred due to the simplicity of test setting and even for providing test data without any doubt of interpretation. On the other hand, on existing masonry buildings, the shear triplet test is hardly performable. On the contrary, the diagonal compression test is currently executed, even though it is invasive [4,15,16], but, as explained above, such test data is subject to various interpretations which involve different formulas [13,17]. All the facts described above show that the determination of masonry shear strength is a not straightforward operation. Two types of tests can be used to determinate this parameter: shear tests on triplets and diagonal compression tests on panels. The former is performable only on new masonry structures, the latter both on new and existing ones. Concerning the new masonry constructions it is worth pointing out that neither the seismic international Codes nor the scientific community provide univocal instructions about the most appropriate type of test to be carried out. In fact, even in the specific literature there still does not exist a comparative study of the experimental outcomes, obtained by the two types of tests, in order to provide precise operative instructions. Therefore, in this context, it becomes necessary to perform an experimental investigation by using the two different procedures, aimed at comparing the data results and their mechanical interpretation also in reference to the complexity of the tests setting. In the present paper the results of an extensive experimental investigation performed by shear tests on triplets and diagonal compression tests on square panels, are reported. The experimental results obtained using both the types of tests on the same kind of brick masonry assemblages are compared. For the diagonal compression test data analysis, the three formulas available in literature were applied in order to evaluate the degree of accuracy of such formulas. Experimental tests, carried out at the Constructions and Restoration Department Laboratory of the University of Florence, were performed on brick masonry specimens of various sizes assembled with mortar of different composition.
2. Determination of masonry shear strength by direct way 2.1. Diagonal compression test The diagonal compression test on masonry panels is defined by ASTM E519-2010 and RILEM LUMB6 specifications. According to these standards, the masonry shear strength must be determined by loading a square panel in compression along one diagonal, until failure.
(a)
The nominal size of this panel cannot be less than 1.2 1.2 m2 (ASTM E519-2010) or less than four units wide (standard RILEM LUMB6); the thickness depends on the wall type being tested. The diagonal compression test can be carried out in a laboratory, on made-to-measure masonry panels, and in situ, on a portion of masonry opportunely cut and isolated from the rest of the existing masonry wall. Only in the in situ tests, the panel remains anchored to the rest of the masonry wall by a 0.7 m part of the lower horizontal edge. The failure usually occurs with the specimen splitting apart parallel to the direction of the load. Cracks, starting from its centre, develop along the mortar joints and, in some cases, through the blocks. In the standard interpretation of the test, as provided by both the specifications, it is assumed that the stress state at the centre of the panel is of pure shear and the principal directions coincide with the two diagonals of the panels. This stress state is properly represented by Mohr’s circle shown in Fig. 1a. The principal tensile stress is equal to shear stress and can be calculated as:
rI ¼ smax ¼ 0:707
P An
ð1Þ
where P is the applied load and An is the net area of the specimen, equal to:
An ¼
wþh t 2
ð2Þ
being t the wall thickness and w and h the face dimensions. Finally, the masonry shear strength value is determined by the formula (1), by assuming term P equal to the load value at failure Pult:
s0 ¼
0:707Pult An
ð3Þ
A different interpretation of the test results is obtained modeling the masonry panel as if it is an isotropic and homogeneous material and running a linear elastic analysis: the results highlight that the stress state at the centre of the specimen is not a pure shear state, although the principal directions still coincide with the two diagonals of the panels. This interpretation gives the values of the principal stress state localized at the centre of the panel as:
rI ¼ 0:5
P ; An
rII ¼ 1:62
P An
ð4Þ
The relative Mohr’s circle is plotted in Fig. 1b. As available in literature, the stress state (4) has two different interpretations for evaluating the masonry shear strength. The first
(b)
Fig. 1. Mohr’s circles: (a) pure shear stress state; and (b) stress state by Eq. (4).
1040
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
Fig. 2. Failure modes of specimens subjected to triplet tests.
interpretation assumes the value of the masonry shear strength be equal to the ultimate principal tensile stress:
s0 ¼ rI ¼ 0:5
Pult An
ð5Þ
The second interpretation of the elastic solution, starting from the value of the ultimate principal tensile stress rI and adopting the Turnsek–Cacovic criterion [18], provides the masonry shear strength by the following formula:
s0 ¼
1 P ult Pult ¼ 0:33 0:5 1:5 An An
ð6Þ
2.2. Triplet test The triplet test, defined by EN 1052-3 [3], covers the determination of shear strength by testing no less than six specimens constituted by blocks and mortar joints. The test can be performed following two different procedures (A and B) with or without lateral pre-compression. Expected failure modes for this kind of test are illustrated in Fig. 2. In particular, referring to the first two modes (A/1 and A/2), the failure occurs with the separation of the mortar from the brick, due to the weakness of the brick–mortar interface. The use of a low strength mortar to constitute bed joints provides a failure in the mortar layer (B mode). Finally, when high strength mortar with good adhesive property is used, the specimen fails with cracks passing through one or both bricks (‘‘C’’ and ‘‘D’’ modes). The shear strength fvoi of each specimen is calculated as follows:
fv oi ¼
F i;max 2Ai
ð7Þ
where Fi,max is the ultimate load and Ai is the specimen area parallel to the mortar joint. The characteristic shear strength fvok is determined by the following formula:
fv ok ¼ 0:7 fv o
ð8Þ
being fvo the average strength value. 3. Experimental tests The experimental investigation was organized in three phases: the first was devoted to the characterization of the component materials, the second to the diagonal compression tests and the third to the triplet tests. In particular, regarding the component materials, tests on brick specimens and mortar specimens of different types (lime, cement– lime and cement) were carried out. Concerning both the diagonal compression tests and the triplet tests, 18 panels and 18 triplets respectively were produced and tested.
3.1. Mechanical properties of the component materials In order to define the mechanical properties of the bricks, uniaxial compression tests on six specimens of 20 20 40 mm3 size, obtained by cutting common bricks produced by San Marco Laterizi Terreal Italy Srl, were performed. With regard to the mortar, the composition of the mixtures used to assemble both the panels and the triplets is reported in Table 1. The mortar components were mixed by means of a kneading machine with the gradual addition of water until the achievement of the optimum consistency. As required by EN 1015-11 [19] three prismatic specimens of 160 40 40 mm3 size for each type of mortar were produced. After 28 days of curing, these were subjected to bending tests on three points. Subsequently, the two stumps, produced by the rupture of each prism, were subjected to uniaxial compression tests. In Table 2 the average values of the brick compressive strength and of the mortars compressive and flexural strength are reported. 3.2. Panel geometry and loading conditions for the diagonal compression test In order to carry out a wide experimental campaign on masonry walls assembled with different types of mortar, small-scale panels (1:2.5) were realized and tested after 28 days of curing. On the other hand, only three full-scale panels assembled with cement– lime mortar were realized. Of the latter, the first panel was tested after 28 days of curing, in order to compare the data results with those obtained on the small-scale panels, while the remaining two were tested after 90 days of curing in order to evaluate possible age-related effects. Overall, the tests were conducted on 18 single leaf brick masonry panels, divided into four groups as described below:
Table 1 The composition of the mixtures used for mortars. Mortar
Water/cement/lime/sand (weight ratio)
Lime mortar (CA) Cement–lime mortar (MB) Cement mortar (CE)
2/–/2/8 2/1/1/8 2/2/–/8
Table 2 Average values of the brick compressive strength and of the mortars compressive and flexural strength.
rc (MPa) rf (Mpa)
Brick
Lime mortar
Cement–lime mortar
Cement mortar
17 –
0.96 0.17
2.75 0.89
8.33 2.63
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
1041
Fig. 3. Metallic shoes.
Six panels in scale 1:2.5 of 400 400 50 mm3 size (code ‘‘PaCA1–PaCA6’’), assembled with lime mortar joints and realized according to RILEM LUMB6 specifications. Six panels in scale 1:2.5 of 400 400 50 mm3 size (code ‘‘PaCE1–PaCE6’’), assembled with cement mortar joints and realized according to RILEM LUMB6 specifications. Three panels in scale 1:2.5, of 400 400 50 mm3 size (code ‘‘PaMB1–PaMB3’’), assembled with cement–lime mortar joints and realized according to RILEM LUMB6 specifications. Three panels in scale 1:1, of 1200 1200 120 mm3 size (code ‘‘PaGMB1–PaGMB3’’), assembled with cement–lime mortar joints and realized according to ASTM E519-2010 specifications. Two apparatuses with the same test mechanism but different in size, depending on the dimensions of the masonry element to be tested, were used. Both the apparatuses were constituted by a rigid frame in which the specimen was positioned. The centered and plumb placement of the panel was guaranteed by two metallic shoes (Fig. 3) fixed at the two corners of the diagonal coinciding with the load direction. The metallic shoes, carefully designed, were anchored to the lower beam of the frame, one, and interposed between the upper corner of the panel and the load cell, the other. The metallic shoes had the function of distributing the load on a larger surface avoiding concentration of compression stresses and, consequently, local failures at the corners. The spaces between the specimen and the side-confining metallic shoe were filled with gypsum capping material. The load was applied using a displacement control device in order to describe the whole load path. Displacements were imposed at a uniform rate until the failure. Two displacement transducers were placed on the top of the panel; 2 X transducers were positioned on the lateral faces along the directions of the two diagonals (Fig. 4).
Fig. 4. The apparatus of diagonal compression test.
Six specimens assembled with cement mortar joints (code ‘‘TpCE1–TpCE6’’). Six specimens assembled with cement–lime mortar joints (code ‘‘TpMB1–TpMB6’’). Triplet tests were carried out using a displacement control device made up of a rigid frame with a screw jack, which can be controlled through a flywheel. Displacements were imposed at a uniform rate until the failure. The use of a displacement control device allowed to observe the whole loading history. This test apparatus is schematized in Fig. 5. A metallic plate and cylinder of 15 mm, respectively in thickness and in diameter, were positioned under the lateral bricks of the specimen. The load was applied on the top of the central brick by a spherical hinge placed on a double metallic plate with interposed two metallic cylinders of 120 mm in length (equal to the brick width). The 18 triplets were tested without lateral compression loads, as indicated in the procedure B of the EN 1052-3 specifications.
3.3. Geometry of the triplets and relative loading conditions The triplet tests were conducted on 18 specimens, each of which was realized with three bricks and two mortar joints, as indicated in EN 1052-3 specifications. The 18 triplets were divided into three groups: Six specimens assembled with lime mortar joints (code ‘‘TpCA1–TpCA6’’).
Fig. 5. The apparatus of the triplet test (UNI EN 1052-3).
1042
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
3.4. Results of the diagonal compression tests
along a non-diagonal direction (indicated with ‘‘ND’’) (Fig. 7). In this last case, the failure occurs because the load exceeds the
Masonry panels subjected to diagonal compression tests showed two different failure modes: cracks developed along the load direction (indicated with ‘‘D’’) (Fig. 6), as could be expected in the case of a diagonal compression test, and cracks developed
Fig. 9. Panel PaCE1: load–displacement diagram.
Fig. 6. Diagonal compression test: failure mode D.
Fig. 7. Diagonal compression test: failure mode ND.
Fig. 10. Panel PaMB1: load–displacement diagram.
Fig. 8. Panel PaCA3: load–displacement diagram.
Fig. 11. Panel PaGMB2: load–displacement diagram.
1043
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045 Table 3 Results of the diagonal compression tests. Group code
Specimen code
Maximum load (N)
Shear strength Eq. (3) (MPa)
Shear strength Eq. (5) (MPa)
Shear strength Eq. (6) (MPa)
Failure mode
PaCA
PaCA1 PaCA2 PaCA3 PaCA4 PaCA5 PaCA6 Average
4136 2558 3499 3401 3597 3195 3397
0.157 0.096 0.133 0.129 0.142 0.121 0.130
0.103 0.064 0.087 0.085 0.090 0.080 0.085
0.068 0.042 0.058 0.056 0.059 0.053 0.056
ND ND ND ND ND ND –
PaCE
PaCE1 PaCE2 PaCE3 PaCE4 PaCE5 PaCE6 Average
41,846 58,359 31,497 48,451 44,110 42,683 44,491
1.572 2.183 1.172 1.804 1.660 1.678 1.678
1.046 1.459 0.787 1.211 1.102 1.067 1.112
0.690 0.963 0.520 0.800 0.728 0.704 0.734
D D D D D D –
PaMB
PaMB1 PaMB2 PaMB3 Average
8281 9359 6713 8117
0.371 0.427 0.303 0.367
0.207 0.234 0.168 0.203
0.137 0.154 0.111 0.134
ND ND ND –
PaGMB
PaGMB1 PaGMB2 PaGMB3 Average
86,828 140,042 135,632 120,834
0.420 0.670 0.654 0.581
0.301 0.486 0.471 0.419
0.199 0.321 0.311 0.277
ND ND ND –
Table 4 Results of the triplet tests. Group code
Specimen code
Maximum load (N)
Shear strength Eq. (7) (MPa)
Failure mode
TpCA
TpCA1 TpCA2 TpCA3 TpCA4 TpCA5 TpCA6 Average
2328 4661 1585 2489 1983 2807 2642
0.039 0.077 0.026 0.041 0.033 0.047 0.044
A/1 A/1 A/1 A/1 A/1 A/1 –
TpCE
TpCE1 TpCE2 TpCE3 TpCE4 TpCE5 TpCE6 Average
28,071 33,588 35,080 35,679 32,136 26,645 31,866
0.468 0.559 0.585 0.595 0.536 0.444 0.531
A/2 A/2 A/2 A/2 A/2 A/2 –
TpMB
TpMB1 TpMB2 TpMB3 TpMB4 TpMB5 TpMB6 Average
15082.2 16405.2 8281.1 8124.2 7242.2 19610.8 11932.7
0.256 0.279 0.141 0.138 0.123 0.334 0.212
A/2 A/2 A/1 A/1 A/1 A/2 –
tensile strength of the mortar used to realize the joints. However, it cannot be excluded that the stress distribution before failure corresponds to that of the model described in Section 2.1: this reason made it possible, although with a higher degree of approximation, to still apply the formulas (3)–(6). It can be observed that the ‘‘D’’ failure mode has occurred exclusively on the panels with cement mortar (PaCE) joints (Fig. 6). Moreover, the maximum load values recorded for this group of specimens are significantly greater than those recorded in both the other cases (PaCA and PaMB). The specimens assembled with lime mortar and those assembled with cement–lime mortar have maximum load values that closely resemble each other. The load–displacement diagrams (Figs. 8–11) show the differences described above. In particular, the diagram recorded for the specimen with cement mortar shows
Fig. 12. Triplet test: failure mode A/1.
Fig. 13. Triplet test: failure mode A/2.
a brittle failure, while the other diagrams, concerning the specimens with lime mortar and cement–lime mortar, show a descending path after the maximum load.
1044
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
It should be specified that in the case of the tests performed on the three panels in scale 1:1 (PaGMB), the execution of the tests was interrupted after reaching the maximum load, therefore the diagram (Fig. 11) does not describe the post-peak behaviour. 3.5. Results of the triplet tests The results of the triplet tests are summarized in Table 4. For each specimen, the maximum load, the shear strength calculated by the formula (7) and the failure mode (A/1 or A/2 as illustrated in Figs. 2–13) are reported. It can be observed that the maximum load values reflect the mechanical properties of the mortars used (Table 2). 3.6. Remarks The experimental investigation was carried out on different kinds of brick masonry walls by using the two types of shear tests in order to compare the data results. Furthermore, the outcomes obtained by the diagonal tests were analyzed in the light of the different interpretations provided in the literature, as reported in Section 2.1. The two types of tests are different from each other with regard to the specimen assemblage, the test setting and the interpretation of the data results. The triplet specimen can be made very easily and the relative test can be performed using minimal apparatus. This test procedure is particularly suitable for new masonry walls. On the contrary, the diagonal compression test is more onerous on new masonry walls, due to the panel and apparatus setting, while it is suitable for existing masonry walls, although its execution requires the partial demolition of the wall. Concerning the interpretation of the test outcomes, it is possible to highlight that the triplet shear test allows the evaluation of the adhesion between mortar and brick and the measuring of the shear strength by the only available formula (7). On the other hand, the shear strength, by diagonal test, can be determined by different formulas as described in Section 2.1. The experimental results obtained by using both types of tests evidence that the masonry shear strength values depend on the mortar: in fact the shear strength value for masonry assembled with lime mortar is lower than that made with cement–lime mortar; the highest shear strength value is obtained on the cement mortar brick masonry. In particular, as regards the diagonal compression tests, the experimental results reported in Table 3 show substantial differences between the shear strength values determined by the three interpretations described in Section 2.1. It is worth noting that, for all the types of masonry tested, the shear strength value determined by the diagonal compression test using formula (6) is very close to the one calculated by formula (7) on the data resulting from the triplet test. Furthermore, these values, though not coincident, are the closest to those tabulated in Eurocode 6, where the fvko term values are estimated relating to different types of mortar and masonry units. In particular, in the case of the tested cement mortar, fallen within the M9 class, the masonry strength value calculated by formula (6) (0.73 MPa) is significantly higher than the value estimated by Eurocode 6 table (0.2 MPa). On the contrary, in the case of lime and cement–lime mortar, fallen within the M1 and M2.5 mortar classes, the mean values of the masonry shear strengths calculated by formula (6), respectively equal to 0.06 MPa and 0.13 MPa, fall approximately within the range estimated by Eurocode 6 (from 0.1 MPa to 0.2 MPa). Therefore, referring to the diagonal compression test, it can be considered that the value of the shear strength of masonry walls calculated by formula (6) is the more reliable one.
4. Conclusion In the present paper, the results of an experimental investigation aimed at evaluating masonry shear strength, under zero normal stress, are reported. For this goal, Eurocode 6 requires shear tests on masonry triplets, while Eurocode 8 requires diagonal compression tests on masonry walls. The triplet test can be performed on new brick masonry structures, while the diagonal compression test can also be used on existing masonry walls. The former is very straightforward and the resulting data can be easily interpreted, the latter, on the other hand, is more onerous and the data can be subject to various interpretations: in Section 2.1 three different formulations, as available in literature, are reported. This study was carried out in order to assess significant differences between the two types of tests pointing out the problem of choosing the test more suitable to determine the masonry shear strength. For this aim, the experimental campaign was carried out using both types of tests, as provided by current codes (Eurocode 6, Eurocode 8), on three different kinds of masonry made of bricks alternated with lime mortar, cement–lime mortar and cement mortar. When masonry specimens were subjected to triplet tests, the shear strength value was determined by applying the only available formula while, in case of masonry specimens tested for diagonal compression, the shear strength value was calculated applying the three formulas available in literature. The results pointed out a clear correlation between the masonry strength and the mechanical properties of the components. The comparison between the shear strength values determined using both types of tests on the three types of masonry provides a significant result: the strength values obtained by triplet tests are in good measure along the line of the results determined by diagonal tests and calculated by formula (6). This formula is obtained by adopting the Turnsek and Cacovic criterion referring to the stress state at the centre of a panel modeled as an isotropic and homogeneous material. Furthermore, these values are the closest to those tabulated in Eurocode 6. Accordingly, in order to predict the shear strength of brick masonry panels, one can consider formula (6) more suitable than formulas (3) and (5). Concerning the choice of the more appropriate type of test, the facts that emerged from the present experimental study permit to assert that the triplet test is very straightforward and provides reliable data results and, accordingly, it can be considered the more convenient as well as more suitable one. Of course, other experimental tests, now in the planning phase, will be useful for further validation of the present results.
References [1] EC6, Eurocode EN 1996. European Union norm on construction. Design of masonry structures. [2] D.M. del 14 gennaio 2008. Norme Tecniche per le Costruzioni; 2008 [in Italian]. [3] EN 1052-3. Methods of test for masonry. Part 3: Determination of initial shear strength. European standard; 2007. [4] EC8, Eurocode EN 1998. European Union norm on construction. Design of structures for earthquake resistance. [5] ASTM E519-2010. Standard test method for diagonal tension (shear) in masonry assemblages. American Society for Testing Material; 2010. [6] RILEM LUMB6. Diagonal tensile strength tests of small wall specimens (1991)0. Rilem recommendations for the testing and use of constructions materials. RILEM; 1994: p. 488–9. [7] Calderini C, Cattari S, Lagomarsino S. The use of the diagonal compression test to identify the shear mechanical parameters of masonry. Constr Build Mater 2010;24:677–85. [8] Corradi M, Tedeschi C, Binda L, Borri A. Experimental evaluation of shear and compression strength of masonry wall before and after reinforcement: deep repointing. Constr Build Mater 2008;22:463–72. [9] Prota A, Marcari G, Fabbrocino G, Manfredi G, Aldea C. Experimental in-plane behavior of tuff masonry strengthened with cementitious matrix–grid composites. J Compos Constr 2006;10:223–33.
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045 [10] Valluzzi MR, Tinazzi D, Modena C. Shear behavior of masonry panels strengthened by FRP laminates. Constr Build Mater 2002;16:409–16. [11] Mahmood H, Ingham JM. Diagonal compression testing of FRP-retrofitted unreinforced clay brick masonry wallettes. J Compos Constr 2011;14:810–20. [12] Circolare n.617 del 2 febbraio 2009. Istruzioni per l’Applicazione Nuove Norme Tecniche Costruzioni; 2009 [in italian]. [13] Binda L, Cardani G, Castori G, Corradi M, Saisi A, Tedeschi C. Procedure sperimentali per la determinazione delle caratteristiche della muratura. In: Borri A, editor. Manuale delle murature storiche – vol. I. Roma: Tipografia del Genio Civile; 2011. p. 316–8. [14] Chiostrini S, Galano L, Vignoli A. On the determination of strength of ancient masonry walls via experimental tests. In: Proc. of the 12th world conference on earthquake engineering (CD-ROM). New Zealand: Auckland; 2000 [paper no. 2564].
1045
[15] Borri A, Castori G, Corradi M, Speranzini E. Shear behavior of unreinforced and reinforced masonry panels subjected to in situ diagonal compression tests. Constr Build Mater 2011;25:4403–14. [16] Corradi M, Borri A, Vignoli A. Experimental study on the determination of strength of masonry walls. Construct Build Mater 2003;17:325–37. [17] Brignola A, Frumento S, Lagomarsino S, Podestà S. Identification of shear parameters of masonry panels through the in situ diagonal compression test. Int J Archit Heritage 2009;3(1):52–73. [18] Turnašek V, Cacovic F. Some experimental results on the strength of brick masonry walls. In: Proc. of the 2nd international brick masonry conference. United Kingdom: Stoke-on-Trent; 1971 [p. 149–56]. [19] EN 1015-11. Methods of test for mortar for masonry. Determination of flexural and compressive strength of hardened mortar. European Standard; 2007.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Shear strength of brick masonry walls assembled with different types of mortar Valerio Alecci, Mario Fagone, Tommaso Rotunno ⇑, Mario De Stefano Department of Construction and Restoration, University of Florence, 50121 Florence, Italy
h i g h l i g h t s " Brick masonry walls assembled with different kinds of mortar are tested. " Shear tests on triplets and diagonal compression tests on panels are performed. " The shear strength under zero normal stress is the parameter obtained. " A comparison between the values of the shear strength is presented.
a r t i c l e
i n f o
Article history: Received 8 May 2012 Received in revised form 16 November 2012 Accepted 22 November 2012 Available online 28 December 2012 Keywords: Masonry Shear strength Experimental test
a b s t r a c t The prediction of masonry shear strength, by direct way, requires appropriate experimental tests on triplets, in line with standard EN 1052-3, or diagonal compression tests on panels according to ASTM 5092010 and RILEM LUMB6. In the present paper the results of an experimental investigation, carried out by these two types of tests on brick masonry walls assembled with different kinds of mortar are reported. A comparison between the values of the masonry shear strength, calculated applying the three formulas available in literature for the diagonal compression test data, and those obtained by laboratory tests on shear triplets, is presented. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Masonry constructions still constitute today a large part of the building stock throughout the world. The historical and artistic heritage as well as the ‘‘common’’ residential buildings in the old urban and rural city centres are usually made of masonry. Masonry material can scarcely bear tensile stress and it is, therefore, known as no-tension material. Furthermore, as it is a heterogeneous material, its mechanical behaviour depends on the geometric texture and the properties of the constituent materials. In the last decades, seismic events which hit and badly damaged large areas of high density masonry buildings (such as the Umbria and Abruzzo regions in Italy) have increased the interest of the scientific community towards more appropriate modelling strategies to assess the seismic vulnerability of such buildings. Although structural engineers are developing even more sophisticated numerical procedures, the ‘‘accuracy’’ of the modeling results always depends on the correct identification of a few mechanical parameters required to characterize masonry material.
⇑ Corresponding author. E-mail address: rotunno@unifi.it (T. Rotunno). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.11.107
The shear strength under zero normal stress is one of these parameters; its exact definition plays a crucial role in the prediction of masonry behaviour under seismic actions. Italian Seismic Code [1] in line with Eurocode 6 [2], allows to determine the shear characteristic strength fvk0 either through an estimate using a pre-calculated table of values or by a direct way which requires appropriate experimental tests on triplets, in line with standard EN 1052-3 [3]. For existing masonry walls, Eurocode 8 [4] suggests the direct determination of this parameter by diagonal compression tests according to ASTM 509-2010 [5] and RILEM LUMB6 [6] specifications. Diagonal compression tests are performed on new masonry walls also, as available in literature [7–11] and suggested by Italian Guidelines [12]. Although the diagonal compression test is largely used, the interpretation of the test outcomes and the formula to calculate the masonry shear strength according to ASTM 509-2010 and RILEM LUMB6 specifications have been questioned by several researchers; currently, various interpretations of the test results and different formulas are available in literature [13,14]. In the context of the seismic design of new masonry constructions, the shear strength can be determined by two different kinds of tests, shear triplets and diagonal compression. The former is
1039
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
preferred due to the simplicity of test setting and even for providing test data without any doubt of interpretation. On the other hand, on existing masonry buildings, the shear triplet test is hardly performable. On the contrary, the diagonal compression test is currently executed, even though it is invasive [4,15,16], but, as explained above, such test data is subject to various interpretations which involve different formulas [13,17]. All the facts described above show that the determination of masonry shear strength is a not straightforward operation. Two types of tests can be used to determinate this parameter: shear tests on triplets and diagonal compression tests on panels. The former is performable only on new masonry structures, the latter both on new and existing ones. Concerning the new masonry constructions it is worth pointing out that neither the seismic international Codes nor the scientific community provide univocal instructions about the most appropriate type of test to be carried out. In fact, even in the specific literature there still does not exist a comparative study of the experimental outcomes, obtained by the two types of tests, in order to provide precise operative instructions. Therefore, in this context, it becomes necessary to perform an experimental investigation by using the two different procedures, aimed at comparing the data results and their mechanical interpretation also in reference to the complexity of the tests setting. In the present paper the results of an extensive experimental investigation performed by shear tests on triplets and diagonal compression tests on square panels, are reported. The experimental results obtained using both the types of tests on the same kind of brick masonry assemblages are compared. For the diagonal compression test data analysis, the three formulas available in literature were applied in order to evaluate the degree of accuracy of such formulas. Experimental tests, carried out at the Constructions and Restoration Department Laboratory of the University of Florence, were performed on brick masonry specimens of various sizes assembled with mortar of different composition.
2. Determination of masonry shear strength by direct way 2.1. Diagonal compression test The diagonal compression test on masonry panels is defined by ASTM E519-2010 and RILEM LUMB6 specifications. According to these standards, the masonry shear strength must be determined by loading a square panel in compression along one diagonal, until failure.
(a)
The nominal size of this panel cannot be less than 1.2 1.2 m2 (ASTM E519-2010) or less than four units wide (standard RILEM LUMB6); the thickness depends on the wall type being tested. The diagonal compression test can be carried out in a laboratory, on made-to-measure masonry panels, and in situ, on a portion of masonry opportunely cut and isolated from the rest of the existing masonry wall. Only in the in situ tests, the panel remains anchored to the rest of the masonry wall by a 0.7 m part of the lower horizontal edge. The failure usually occurs with the specimen splitting apart parallel to the direction of the load. Cracks, starting from its centre, develop along the mortar joints and, in some cases, through the blocks. In the standard interpretation of the test, as provided by both the specifications, it is assumed that the stress state at the centre of the panel is of pure shear and the principal directions coincide with the two diagonals of the panels. This stress state is properly represented by Mohr’s circle shown in Fig. 1a. The principal tensile stress is equal to shear stress and can be calculated as:
rI ¼ smax ¼ 0:707
P An
ð1Þ
where P is the applied load and An is the net area of the specimen, equal to:
An ¼
wþh t 2
ð2Þ
being t the wall thickness and w and h the face dimensions. Finally, the masonry shear strength value is determined by the formula (1), by assuming term P equal to the load value at failure Pult:
s0 ¼
0:707Pult An
ð3Þ
A different interpretation of the test results is obtained modeling the masonry panel as if it is an isotropic and homogeneous material and running a linear elastic analysis: the results highlight that the stress state at the centre of the specimen is not a pure shear state, although the principal directions still coincide with the two diagonals of the panels. This interpretation gives the values of the principal stress state localized at the centre of the panel as:
rI ¼ 0:5
P ; An
rII ¼ 1:62
P An
ð4Þ
The relative Mohr’s circle is plotted in Fig. 1b. As available in literature, the stress state (4) has two different interpretations for evaluating the masonry shear strength. The first
(b)
Fig. 1. Mohr’s circles: (a) pure shear stress state; and (b) stress state by Eq. (4).
1040
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
Fig. 2. Failure modes of specimens subjected to triplet tests.
interpretation assumes the value of the masonry shear strength be equal to the ultimate principal tensile stress:
s0 ¼ rI ¼ 0:5
Pult An
ð5Þ
The second interpretation of the elastic solution, starting from the value of the ultimate principal tensile stress rI and adopting the Turnsek–Cacovic criterion [18], provides the masonry shear strength by the following formula:
s0 ¼
1 P ult Pult ¼ 0:33 0:5 1:5 An An
ð6Þ
2.2. Triplet test The triplet test, defined by EN 1052-3 [3], covers the determination of shear strength by testing no less than six specimens constituted by blocks and mortar joints. The test can be performed following two different procedures (A and B) with or without lateral pre-compression. Expected failure modes for this kind of test are illustrated in Fig. 2. In particular, referring to the first two modes (A/1 and A/2), the failure occurs with the separation of the mortar from the brick, due to the weakness of the brick–mortar interface. The use of a low strength mortar to constitute bed joints provides a failure in the mortar layer (B mode). Finally, when high strength mortar with good adhesive property is used, the specimen fails with cracks passing through one or both bricks (‘‘C’’ and ‘‘D’’ modes). The shear strength fvoi of each specimen is calculated as follows:
fv oi ¼
F i;max 2Ai
ð7Þ
where Fi,max is the ultimate load and Ai is the specimen area parallel to the mortar joint. The characteristic shear strength fvok is determined by the following formula:
fv ok ¼ 0:7 fv o
ð8Þ
being fvo the average strength value. 3. Experimental tests The experimental investigation was organized in three phases: the first was devoted to the characterization of the component materials, the second to the diagonal compression tests and the third to the triplet tests. In particular, regarding the component materials, tests on brick specimens and mortar specimens of different types (lime, cement– lime and cement) were carried out. Concerning both the diagonal compression tests and the triplet tests, 18 panels and 18 triplets respectively were produced and tested.
3.1. Mechanical properties of the component materials In order to define the mechanical properties of the bricks, uniaxial compression tests on six specimens of 20 20 40 mm3 size, obtained by cutting common bricks produced by San Marco Laterizi Terreal Italy Srl, were performed. With regard to the mortar, the composition of the mixtures used to assemble both the panels and the triplets is reported in Table 1. The mortar components were mixed by means of a kneading machine with the gradual addition of water until the achievement of the optimum consistency. As required by EN 1015-11 [19] three prismatic specimens of 160 40 40 mm3 size for each type of mortar were produced. After 28 days of curing, these were subjected to bending tests on three points. Subsequently, the two stumps, produced by the rupture of each prism, were subjected to uniaxial compression tests. In Table 2 the average values of the brick compressive strength and of the mortars compressive and flexural strength are reported. 3.2. Panel geometry and loading conditions for the diagonal compression test In order to carry out a wide experimental campaign on masonry walls assembled with different types of mortar, small-scale panels (1:2.5) were realized and tested after 28 days of curing. On the other hand, only three full-scale panels assembled with cement– lime mortar were realized. Of the latter, the first panel was tested after 28 days of curing, in order to compare the data results with those obtained on the small-scale panels, while the remaining two were tested after 90 days of curing in order to evaluate possible age-related effects. Overall, the tests were conducted on 18 single leaf brick masonry panels, divided into four groups as described below:
Table 1 The composition of the mixtures used for mortars. Mortar
Water/cement/lime/sand (weight ratio)
Lime mortar (CA) Cement–lime mortar (MB) Cement mortar (CE)
2/–/2/8 2/1/1/8 2/2/–/8
Table 2 Average values of the brick compressive strength and of the mortars compressive and flexural strength.
rc (MPa) rf (Mpa)
Brick
Lime mortar
Cement–lime mortar
Cement mortar
17 –
0.96 0.17
2.75 0.89
8.33 2.63
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
1041
Fig. 3. Metallic shoes.
Six panels in scale 1:2.5 of 400 400 50 mm3 size (code ‘‘PaCA1–PaCA6’’), assembled with lime mortar joints and realized according to RILEM LUMB6 specifications. Six panels in scale 1:2.5 of 400 400 50 mm3 size (code ‘‘PaCE1–PaCE6’’), assembled with cement mortar joints and realized according to RILEM LUMB6 specifications. Three panels in scale 1:2.5, of 400 400 50 mm3 size (code ‘‘PaMB1–PaMB3’’), assembled with cement–lime mortar joints and realized according to RILEM LUMB6 specifications. Three panels in scale 1:1, of 1200 1200 120 mm3 size (code ‘‘PaGMB1–PaGMB3’’), assembled with cement–lime mortar joints and realized according to ASTM E519-2010 specifications. Two apparatuses with the same test mechanism but different in size, depending on the dimensions of the masonry element to be tested, were used. Both the apparatuses were constituted by a rigid frame in which the specimen was positioned. The centered and plumb placement of the panel was guaranteed by two metallic shoes (Fig. 3) fixed at the two corners of the diagonal coinciding with the load direction. The metallic shoes, carefully designed, were anchored to the lower beam of the frame, one, and interposed between the upper corner of the panel and the load cell, the other. The metallic shoes had the function of distributing the load on a larger surface avoiding concentration of compression stresses and, consequently, local failures at the corners. The spaces between the specimen and the side-confining metallic shoe were filled with gypsum capping material. The load was applied using a displacement control device in order to describe the whole load path. Displacements were imposed at a uniform rate until the failure. Two displacement transducers were placed on the top of the panel; 2 X transducers were positioned on the lateral faces along the directions of the two diagonals (Fig. 4).
Fig. 4. The apparatus of diagonal compression test.
Six specimens assembled with cement mortar joints (code ‘‘TpCE1–TpCE6’’). Six specimens assembled with cement–lime mortar joints (code ‘‘TpMB1–TpMB6’’). Triplet tests were carried out using a displacement control device made up of a rigid frame with a screw jack, which can be controlled through a flywheel. Displacements were imposed at a uniform rate until the failure. The use of a displacement control device allowed to observe the whole loading history. This test apparatus is schematized in Fig. 5. A metallic plate and cylinder of 15 mm, respectively in thickness and in diameter, were positioned under the lateral bricks of the specimen. The load was applied on the top of the central brick by a spherical hinge placed on a double metallic plate with interposed two metallic cylinders of 120 mm in length (equal to the brick width). The 18 triplets were tested without lateral compression loads, as indicated in the procedure B of the EN 1052-3 specifications.
3.3. Geometry of the triplets and relative loading conditions The triplet tests were conducted on 18 specimens, each of which was realized with three bricks and two mortar joints, as indicated in EN 1052-3 specifications. The 18 triplets were divided into three groups: Six specimens assembled with lime mortar joints (code ‘‘TpCA1–TpCA6’’).
Fig. 5. The apparatus of the triplet test (UNI EN 1052-3).
1042
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
3.4. Results of the diagonal compression tests
along a non-diagonal direction (indicated with ‘‘ND’’) (Fig. 7). In this last case, the failure occurs because the load exceeds the
Masonry panels subjected to diagonal compression tests showed two different failure modes: cracks developed along the load direction (indicated with ‘‘D’’) (Fig. 6), as could be expected in the case of a diagonal compression test, and cracks developed
Fig. 9. Panel PaCE1: load–displacement diagram.
Fig. 6. Diagonal compression test: failure mode D.
Fig. 7. Diagonal compression test: failure mode ND.
Fig. 10. Panel PaMB1: load–displacement diagram.
Fig. 8. Panel PaCA3: load–displacement diagram.
Fig. 11. Panel PaGMB2: load–displacement diagram.
1043
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045 Table 3 Results of the diagonal compression tests. Group code
Specimen code
Maximum load (N)
Shear strength Eq. (3) (MPa)
Shear strength Eq. (5) (MPa)
Shear strength Eq. (6) (MPa)
Failure mode
PaCA
PaCA1 PaCA2 PaCA3 PaCA4 PaCA5 PaCA6 Average
4136 2558 3499 3401 3597 3195 3397
0.157 0.096 0.133 0.129 0.142 0.121 0.130
0.103 0.064 0.087 0.085 0.090 0.080 0.085
0.068 0.042 0.058 0.056 0.059 0.053 0.056
ND ND ND ND ND ND –
PaCE
PaCE1 PaCE2 PaCE3 PaCE4 PaCE5 PaCE6 Average
41,846 58,359 31,497 48,451 44,110 42,683 44,491
1.572 2.183 1.172 1.804 1.660 1.678 1.678
1.046 1.459 0.787 1.211 1.102 1.067 1.112
0.690 0.963 0.520 0.800 0.728 0.704 0.734
D D D D D D –
PaMB
PaMB1 PaMB2 PaMB3 Average
8281 9359 6713 8117
0.371 0.427 0.303 0.367
0.207 0.234 0.168 0.203
0.137 0.154 0.111 0.134
ND ND ND –
PaGMB
PaGMB1 PaGMB2 PaGMB3 Average
86,828 140,042 135,632 120,834
0.420 0.670 0.654 0.581
0.301 0.486 0.471 0.419
0.199 0.321 0.311 0.277
ND ND ND –
Table 4 Results of the triplet tests. Group code
Specimen code
Maximum load (N)
Shear strength Eq. (7) (MPa)
Failure mode
TpCA
TpCA1 TpCA2 TpCA3 TpCA4 TpCA5 TpCA6 Average
2328 4661 1585 2489 1983 2807 2642
0.039 0.077 0.026 0.041 0.033 0.047 0.044
A/1 A/1 A/1 A/1 A/1 A/1 –
TpCE
TpCE1 TpCE2 TpCE3 TpCE4 TpCE5 TpCE6 Average
28,071 33,588 35,080 35,679 32,136 26,645 31,866
0.468 0.559 0.585 0.595 0.536 0.444 0.531
A/2 A/2 A/2 A/2 A/2 A/2 –
TpMB
TpMB1 TpMB2 TpMB3 TpMB4 TpMB5 TpMB6 Average
15082.2 16405.2 8281.1 8124.2 7242.2 19610.8 11932.7
0.256 0.279 0.141 0.138 0.123 0.334 0.212
A/2 A/2 A/1 A/1 A/1 A/2 –
tensile strength of the mortar used to realize the joints. However, it cannot be excluded that the stress distribution before failure corresponds to that of the model described in Section 2.1: this reason made it possible, although with a higher degree of approximation, to still apply the formulas (3)–(6). It can be observed that the ‘‘D’’ failure mode has occurred exclusively on the panels with cement mortar (PaCE) joints (Fig. 6). Moreover, the maximum load values recorded for this group of specimens are significantly greater than those recorded in both the other cases (PaCA and PaMB). The specimens assembled with lime mortar and those assembled with cement–lime mortar have maximum load values that closely resemble each other. The load–displacement diagrams (Figs. 8–11) show the differences described above. In particular, the diagram recorded for the specimen with cement mortar shows
Fig. 12. Triplet test: failure mode A/1.
Fig. 13. Triplet test: failure mode A/2.
a brittle failure, while the other diagrams, concerning the specimens with lime mortar and cement–lime mortar, show a descending path after the maximum load.
1044
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045
It should be specified that in the case of the tests performed on the three panels in scale 1:1 (PaGMB), the execution of the tests was interrupted after reaching the maximum load, therefore the diagram (Fig. 11) does not describe the post-peak behaviour. 3.5. Results of the triplet tests The results of the triplet tests are summarized in Table 4. For each specimen, the maximum load, the shear strength calculated by the formula (7) and the failure mode (A/1 or A/2 as illustrated in Figs. 2–13) are reported. It can be observed that the maximum load values reflect the mechanical properties of the mortars used (Table 2). 3.6. Remarks The experimental investigation was carried out on different kinds of brick masonry walls by using the two types of shear tests in order to compare the data results. Furthermore, the outcomes obtained by the diagonal tests were analyzed in the light of the different interpretations provided in the literature, as reported in Section 2.1. The two types of tests are different from each other with regard to the specimen assemblage, the test setting and the interpretation of the data results. The triplet specimen can be made very easily and the relative test can be performed using minimal apparatus. This test procedure is particularly suitable for new masonry walls. On the contrary, the diagonal compression test is more onerous on new masonry walls, due to the panel and apparatus setting, while it is suitable for existing masonry walls, although its execution requires the partial demolition of the wall. Concerning the interpretation of the test outcomes, it is possible to highlight that the triplet shear test allows the evaluation of the adhesion between mortar and brick and the measuring of the shear strength by the only available formula (7). On the other hand, the shear strength, by diagonal test, can be determined by different formulas as described in Section 2.1. The experimental results obtained by using both types of tests evidence that the masonry shear strength values depend on the mortar: in fact the shear strength value for masonry assembled with lime mortar is lower than that made with cement–lime mortar; the highest shear strength value is obtained on the cement mortar brick masonry. In particular, as regards the diagonal compression tests, the experimental results reported in Table 3 show substantial differences between the shear strength values determined by the three interpretations described in Section 2.1. It is worth noting that, for all the types of masonry tested, the shear strength value determined by the diagonal compression test using formula (6) is very close to the one calculated by formula (7) on the data resulting from the triplet test. Furthermore, these values, though not coincident, are the closest to those tabulated in Eurocode 6, where the fvko term values are estimated relating to different types of mortar and masonry units. In particular, in the case of the tested cement mortar, fallen within the M9 class, the masonry strength value calculated by formula (6) (0.73 MPa) is significantly higher than the value estimated by Eurocode 6 table (0.2 MPa). On the contrary, in the case of lime and cement–lime mortar, fallen within the M1 and M2.5 mortar classes, the mean values of the masonry shear strengths calculated by formula (6), respectively equal to 0.06 MPa and 0.13 MPa, fall approximately within the range estimated by Eurocode 6 (from 0.1 MPa to 0.2 MPa). Therefore, referring to the diagonal compression test, it can be considered that the value of the shear strength of masonry walls calculated by formula (6) is the more reliable one.
4. Conclusion In the present paper, the results of an experimental investigation aimed at evaluating masonry shear strength, under zero normal stress, are reported. For this goal, Eurocode 6 requires shear tests on masonry triplets, while Eurocode 8 requires diagonal compression tests on masonry walls. The triplet test can be performed on new brick masonry structures, while the diagonal compression test can also be used on existing masonry walls. The former is very straightforward and the resulting data can be easily interpreted, the latter, on the other hand, is more onerous and the data can be subject to various interpretations: in Section 2.1 three different formulations, as available in literature, are reported. This study was carried out in order to assess significant differences between the two types of tests pointing out the problem of choosing the test more suitable to determine the masonry shear strength. For this aim, the experimental campaign was carried out using both types of tests, as provided by current codes (Eurocode 6, Eurocode 8), on three different kinds of masonry made of bricks alternated with lime mortar, cement–lime mortar and cement mortar. When masonry specimens were subjected to triplet tests, the shear strength value was determined by applying the only available formula while, in case of masonry specimens tested for diagonal compression, the shear strength value was calculated applying the three formulas available in literature. The results pointed out a clear correlation between the masonry strength and the mechanical properties of the components. The comparison between the shear strength values determined using both types of tests on the three types of masonry provides a significant result: the strength values obtained by triplet tests are in good measure along the line of the results determined by diagonal tests and calculated by formula (6). This formula is obtained by adopting the Turnsek and Cacovic criterion referring to the stress state at the centre of a panel modeled as an isotropic and homogeneous material. Furthermore, these values are the closest to those tabulated in Eurocode 6. Accordingly, in order to predict the shear strength of brick masonry panels, one can consider formula (6) more suitable than formulas (3) and (5). Concerning the choice of the more appropriate type of test, the facts that emerged from the present experimental study permit to assert that the triplet test is very straightforward and provides reliable data results and, accordingly, it can be considered the more convenient as well as more suitable one. Of course, other experimental tests, now in the planning phase, will be useful for further validation of the present results.
References [1] EC6, Eurocode EN 1996. European Union norm on construction. Design of masonry structures. [2] D.M. del 14 gennaio 2008. Norme Tecniche per le Costruzioni; 2008 [in Italian]. [3] EN 1052-3. Methods of test for masonry. Part 3: Determination of initial shear strength. European standard; 2007. [4] EC8, Eurocode EN 1998. European Union norm on construction. Design of structures for earthquake resistance. [5] ASTM E519-2010. Standard test method for diagonal tension (shear) in masonry assemblages. American Society for Testing Material; 2010. [6] RILEM LUMB6. Diagonal tensile strength tests of small wall specimens (1991)0. Rilem recommendations for the testing and use of constructions materials. RILEM; 1994: p. 488–9. [7] Calderini C, Cattari S, Lagomarsino S. The use of the diagonal compression test to identify the shear mechanical parameters of masonry. Constr Build Mater 2010;24:677–85. [8] Corradi M, Tedeschi C, Binda L, Borri A. Experimental evaluation of shear and compression strength of masonry wall before and after reinforcement: deep repointing. Constr Build Mater 2008;22:463–72. [9] Prota A, Marcari G, Fabbrocino G, Manfredi G, Aldea C. Experimental in-plane behavior of tuff masonry strengthened with cementitious matrix–grid composites. J Compos Constr 2006;10:223–33.
V. Alecci et al. / Construction and Building Materials 40 (2013) 1038–1045 [10] Valluzzi MR, Tinazzi D, Modena C. Shear behavior of masonry panels strengthened by FRP laminates. Constr Build Mater 2002;16:409–16. [11] Mahmood H, Ingham JM. Diagonal compression testing of FRP-retrofitted unreinforced clay brick masonry wallettes. J Compos Constr 2011;14:810–20. [12] Circolare n.617 del 2 febbraio 2009. Istruzioni per l’Applicazione Nuove Norme Tecniche Costruzioni; 2009 [in italian]. [13] Binda L, Cardani G, Castori G, Corradi M, Saisi A, Tedeschi C. Procedure sperimentali per la determinazione delle caratteristiche della muratura. In: Borri A, editor. Manuale delle murature storiche – vol. I. Roma: Tipografia del Genio Civile; 2011. p. 316–8. [14] Chiostrini S, Galano L, Vignoli A. On the determination of strength of ancient masonry walls via experimental tests. In: Proc. of the 12th world conference on earthquake engineering (CD-ROM). New Zealand: Auckland; 2000 [paper no. 2564].
1045
[15] Borri A, Castori G, Corradi M, Speranzini E. Shear behavior of unreinforced and reinforced masonry panels subjected to in situ diagonal compression tests. Constr Build Mater 2011;25:4403–14. [16] Corradi M, Borri A, Vignoli A. Experimental study on the determination of strength of masonry walls. Construct Build Mater 2003;17:325–37. [17] Brignola A, Frumento S, Lagomarsino S, Podestà S. Identification of shear parameters of masonry panels through the in situ diagonal compression test. Int J Archit Heritage 2009;3(1):52–73. [18] Turnašek V, Cacovic F. Some experimental results on the strength of brick masonry walls. In: Proc. of the 2nd international brick masonry conference. United Kingdom: Stoke-on-Trent; 1971 [p. 149–56]. [19] EN 1015-11. Methods of test for mortar for masonry. Determination of flexural and compressive strength of hardened mortar. European Standard; 2007.