Methodological issues for the mechanical characterization of unfired earth bricks

Methodological issues for the mechanical characterization of unfired earth bricks J.D. Rodr´ ıguez-Mariscal, M. Sol´ ıs∗, H. Cifuentes Escuela T´ ecnica Superior de Ingenier´ ıa, Universidad de Sevilla, Camino de los Descubrimientos, 41092 Sevilla, Spain Abstract Earth is a traditional building material used through History in many areas around the world. Nowadays, there is also a significant revival of its use because of its ecological value and architectural performance. However, there is still a lack of knowledge about its actual mechanical behavior. This paper is aimed at providing experimental data for the development of consistent methodologies for the characterization of this building material. Compression tests of prismatic, cubic and cylindrical specimens were carried out. The compressive strength, Young modulus and the stressstrain constitutive law are obtained and analyzed. The paper shows that the Unconfined Compression Strength of the material should not be obtained by applying existing correction factors for other materials. The paper also analyzes the meaning and usefulness of different estimates of the Young modulus. Finally, the paper proposes a simplified method for estimating stress-strain relationships from the compressive strength and its corresponding strain of tested samples by using reference normalized curves. Keywords: earth bricks, compressive behavior, aspect ratio, Young modulus, constitutive law 1. Introduction1 Earth is a construction material widely used throughout the world since ancient times. It is estimated that approx-2 imately 30% of the world population lives in earth buildings and that in developing countries this percentage rises3 to approximately 50% [1]. Earth exhibits several attractive features as a building material. It provides good thermal4 and acoustic isolation properties, it is cheap, easy to handle, and it is a zero polluting and ecological material. The5 main disadvantage of earth is its poor mechanical properties, which is specially critical in seismic areas [2, 3, 4, 5, 6].6 This also limits the use of earth to low-rise buildings, although some exceptions can be found around the world as the7 well-known city of Shibam (Yemen), where buildings of more than 30m high can be found.8 Despite the advantages of the earth as a building material and its extensive use around the world, there is a9 lack of scientific knowledge about its structural behavior. Hence, there are no rigorous standards for its use, as10 they exist for other building materials. Only some qualitative and empirical standards currently exist for earthen11 ∗Corresponding author Email address: [email protected] (M. Sol´ ıs ) Preprint submitted to Construction and Building Materials November 29, 2022 construction [7, 8, 9]. The heterogeneity of the constituents of earth and the different building techniques around the12 world make even more difficult the development of building standards and recommendations. However, there is a13 need for investigating earth from a mechanical point of view in order to eventually address the needs for building safe14 and sustainable houses for the large population for which the earth is the only affordable or available building material15 and also the needs of the rapid developing of earthen modern architecture. Besides, this investigation is required for16 preserving the valuable existing earthen heritage sites and constructions. Fortunately, there is a increasing interest17 from the academic research community to address this issue and the numbers of related scientific papers has increased18 in the last years.19 The methodology for the determination of the fundamental mechanical properties such as the compressive strength20 or the Young modulus of earthen materials is still an open issue. This paper is aimed at providing a new contribution21 for the development of knowledge about the mechanical behavior of earthen building materials and for the definition22 of standards for the characterization of its compressive mechanical properties.23 There are different traditional techniques for using raw earth as a building material, and different compositions or24 additives may be used in different areas around the world. Adobe (unfired and air-dried earth blocks use for building25 adobe masonry constructions) is one of the most widespread technique. This paper is focused on the investigation of26 the properties of adobe, although it can be useful for future research on other traditional earthen building techniques27 such as cob or rammed earth, or modern type earthen masonry made of Compressed Earth Blocks (CEB). Although28 each building technique exhibit its own singularities, there are obviously strong similarities between all of them from29 a structural point of view.30 Values of fundamental mechanical properties of adobe (namely compressive and tensile strength and Young mod-31 ulus) obtained by different authors have been summarized in [10, 11] and an interesting state of the art review about32 the tests involved can be found in [12]. The scattering of the experimental results obtained up to now is significant.33 This is partly due to the different properties of the material (proportions and properties of each constituent, addition34 of fibers, moisture content, stabilizers, etc) [13, 14, 15, 16], but also due to the different types of tests performed. For35 instance, the size and shape of the specimens for a standard compressive test is not established. Different sizes of36 cubic, cylindrical and prismatic specimens can be found in the literature, but the effect of specimen size and shape on37 the obtained results are not clear yet. Some research works [17, 18, 19, 16] have highlighted the importance of con-38 sidering the confinement effect on the specimens. They have even illustrated how a low aspect ratio (the ratio between39 the dimension parallel to the compressive loading direction and the minimum dimension of the cross-section of the40 specimen) can lead to unreasonable estimations of the compressive strength. In [18, 19] the authors also investigated41 the application of correction factors (used for other materials) to obtain a consistent value from different types of42 specimens. This issue have been also addressed for adobe samples of existing constructions [20] and for compressed43 earth blocks [21].44 The Young modulus is probably one of the most uncertain properties of earthen materials [10, 14, 20, 22, 23, 24].45 The stress-strain relationship is non-linear and the criteria for the determination of a reference value is not established46 2 yet. Moreover, the experimental procedure to measure strains definitely affects the results [20]. However, some47 empirical relationships between the compressive strength and the Young modulus have been proposed [7, 8, 24, 20,48 10]. The complete stress-strain constitutive law has been also studied by several authors for adobe [20, 25, 12, 23, 26]49 and also for rammed earth and cob [15, 22]. The complete stress-strain relationship, including not only the rising50 branch but also the post-peak or softening behavior after the compressive strength is reached, is needed for a complete51 understanding of the mechanical behavior of the material. This experimental information can be used to asses the52 compressive fracture energy and develop simplified elastic-plastic models for modelling masonry assemblages [23].53 In this research work, simple compression tests are performed on brick, cubic, prismatic and cylindrical speci-54 mens. The specimens are made from soils from the riverbank of Guadalquivir river in Sevilla (Spain). To the authors’55 knowledge, the paper is the first research work about the analysis of mechanical properties from local materials and56 building techniques in this area. The compressive strength, Young modulus and complete constitutive law are ob-57 tained for each type of specimen. Normalized curves are provided and compared to others existing in the literature.58 The obtained average and normalized stress-strain curves can be useful for future analysis, building numerical models59 or comparison purposes. In addition, the paper analyzes the influence of the shape of the specimen, the confinement60 effect, the manufacturing process and the direction of loading (anisotropy) on the experimental results. It also illus-61 trates the need of using a reliable method for the determination of the actual strains and it analyzes several approaches62 for characterizing the Young modulus.63 2. Materials and methods64 2.1. Experimental set-up65 Specifications from standards for masonry characterization [27] were considered for conducting the tests. Two66 different loading directions were considered: the vertical (hereafter longitudinal) and horizontal (hereafter transversal)67 directions during the molding process (Fig. 1). The longitudinal would be equivalent to the usual vertical compressive68 direction of the bricks as part of an adobe masonry wall, whereas the transversal direction would be equivalent to a69 horizontal compressive one. Thus, considering both directions, information about the anisotropic behavior of the70 adobe bricks can be obtained.71 (a) (b) (c) Figure 1: Scheme of the specimens obtained from cutting the bricks and loading direction: cubes loaded in (a) longitudinal and (b) transversal direction, and (c) prisms 3 A universal mono-axial servo-hydraulic testing machine was used (50kN load capacity and 200mm stroke) and72 all tests were displacement controlled.The applied load and the displacement of the actuator was measured. Two73 LVDT type displacement sensors (12mm range model DC-EC250 from Schaevitz company) measured the relative74 axial displacement between two points of the prismatic and cylindrical specimens. They were installed at opposite75 sides of the specimen in order to cancel out any bending effect by computing the average reading of the two sensors.76 All the output analog signals from the displacement sensors and the testing machine were recorded through a SCXI77 data acquisition system from National Instruments company.78 From the displacement readings, the average strain can be computed. A global strain value for the whole specimen79 is estimated from the displacement of the actuator, whereas the displacement sensors provide a more local strain value80 which is not contaminated by the local effects near the contact areas between the specimens and platens of the testing81 machine. Because of their size, the LVDT displacement sensors could not be used for bricks and cubic specimens.82 The use of traditional strain gages for obtaining a more precise and local measurement of strains was not feasible83 because of the dificulty to obtain a proper and effective bond between the gage and the surface of adobe specimen.84 The tilt movement of the upper platen of the testing machine was released in order to diminish the effects of85 irregularities and lack of flatness of the specimen sides, so the load is distributed as uniformly as possible on the86 specimen. The effect of irregularities is more significant at the first stages of loading [12], as it will be illustrated in87 next sections.88 Table 1 summarizes the description of the different types of specimens and tests performed. The loading direction89 is indicated as Tfor the transversal direction and Lfor the longitudinal direction. The type of measuring technique90 for strain readings is indicated as ’Mach.’ for the displacement of the actuator of the testing machine and Sen. for the91 displacement sensors. The tests were performed complying with the European Standard for the determination of the92 compressive strength of masonry units [27].93 Table 1: Description of the different types of specimens and tests. Number Size(mm) Loading Loading Strain Type of tests width/depth/height Direction rate readings Bricks 8 320/160/80 L 50kN/min Mach. Bricks 6 320/80/160 L 50kN/min Mach. Cubes 7 80/80/80 L 3mm/min Mach. Cubes 7 80/80/80 T 3mm/min Mach. Prisms 22 80/80/160 T 2mm/min Mach./Sen. Cylinders 5 150/300 L 4mm/min Mach. /Sen. Bricks.A total of 14 adobe bricks were tested: 8 in horizontal position and 6 in vertical position (Fig. 2). The force94 rate was controlled at 50kN/min, complying with European masonry standards [27].95 4 Cubes.Fourteen cubic specimens were tested (Fig. 2), seven of them loaded in the longitudinal direction and the96 other seven were loaded in the transversal direction. According to [27], a controlled displacement rate of 3mm/min97 was applied during the tests.98 Prisms.Twenty two prismatic specimens were tested (Fig. 2). All of them were tested applying the load parallel99 to the highest dimension (transversal direction). A controlled displacement rate of 2mm/min was applied during the100 tests.101 Cylinders.Five cylindrical specimens were tested (Fig. 2). All of them were tested in the vertical direction during102 the drying process (longitudinal direction). A controlled displacement rate of 4mm/min was applied during the tests.103 (a) (b) (c) (d) (e) Figure 2: Tests of bricks in (a) horizontal and (b) vertical position, (c) cubic, (d) prism and (e) cylindrical specimens. For each type of specimen, the stress-strain relationships are obtained from the readings of the applied load and104 the relative displacements (from both the actuator of the testing machine and the displacement sensors). The stress is105 obtained by simply dividing the load by the area of the cross-section of the specimen. It is assumed that this cross-106 section remains undeformed, as it is usually considered for other materials like concrete. The strains are obtained by107 dividing the measured displacement by the original height of the specimen (for the readings from the testing machine)108 5 or the distance between the fixing points of the displacement sensors. The obtained results for the bricks, cubes,109 prisms and cylinders are presented in next sections. The experimental stress-strain curves for each test are shown and110 the mean curve of each type of specimen is obtained. This mean curve is approached by a third order polynomial111 fitting. The polynomial fitting is defined for two different intervals: the first one for strains smaller than the strain112 corresponding to the peak strength (εc(σcmax )) and the second one for higher strains.113 Three stages can be observed during the tests. Initially, there is an adjustment and settlement of the platens on114 the specimen. This initial stage is influenced by the irregularities and lack of flatness of the sides of the specimen.115 Strain measurements are not reliable during this stage and the resulting apparent stiffness is very small. This initial116 stage and its undesirable effect on the stress-strain relationship are more significant when computing strains from the117 displacement readings of the testing machine. It is also more clearly observed for prismatic and cubic specimens118 due to their smaller cross-section. During the second stage, a more uniform compression on the whole specimen119 is achieved, the stiffness increases and the peak strength is reached. Finally, a softening branch on the stress-strain120 relationship can be observed during the third stage, where large deformations are measured. An asymptotic perfect121 plastic behavior takes place at about 40 −50% of the peak strength.122 The next sections include a specific analysis of the stress-strain curves obtained for each type of specimen.123 2.2. Experimental results124 2.2.1. Experimental stress-strain relationship of bricks.125 Adobe masonry is made of bricks of adobe. Therefore, the strength and constitutive law of the adobe material126 should ideally be obtained from adobe bricks. However, this can not be done in a straightforward way because127 of the shape of bricks. According to the usual vertical compressive load of any adobe masonry structure, when128 performing a compressive test on an adobe brick it should be laid horizontally and the load direction should be129 applied vertically. However, because of the very low slenderness (aspect ratio) of the brick (smaller height than any130 of the cross-section dimensions), the confinement of the brick between the platens of the testing machine artificially131 increases the compressive strength. Even when the brick is actually cracked and useless from a structural point of132 view (Fig. 3), the applied compressive load is still growing. Fig. 4(a) shows the obtained stress-strain relationship133 for this test. It can be observed that a stress higher than 2.5MPa is reached, and no peak strength is observed. The134 unrealistic behavior obtained from this type of tests will be illustrated by comparing this result with those shown135 in next sections for other types of specimens. The confinement phenomenon when testing adobe bricks or rammed136 earth plates has been also previously reported in the literature [14, 28, 18, 12]. Although the change in the slope137 of the obtained stress-strain law have been used as an indicator of the compressive strength of the adobe brick [28],138 this method is unreliable and may provide unrealistic estimations of the actual values. Moreover, because of the139 handmade manufacturing process of adobe bricks and shrink effects during the drying process, the shape of adobe140 bricks are usually quite irregular, the external faces may not be parallel to each other and they may be uneven. As a141 consequence, the scattering of the obtained results may be significantly high. From all these reasons, this type of test142 6 should not be considered for the determination of the compressive behavior of adobe.143 Figure 3: Failure mode of a brick tested in horizontal position. (a) (b) Figure 4: Stress-strain relationship of bricks tested in (a) horizontal and (b) vertical position. As a first attempt to circumvent the undesirable confinement effects on adobe bricks, this paper presents the results144 of compressive tests on adobe bricks laid vertically (the compressive load applied on the largest lateral sides of the145 brick). The obtained results are shown in Fig. 4(b). Although results are different to those corresponding to the146 horizontal direction, no reliable nor realistic stress-strain relationship is obtained. Thus, other types of specimens147 different to adobe bricks should be used. The influence of the specimen size and shape will be discussed in the148 following sections.149 2.2.2. Experimental stress-strain relationship of cubes.150 Fig. 5 shows the experimental stress-strain relationships obtained for the cubic specimens when they are loaded151 in the longitudinal (Fig. 5(a)) and transversal direction (Fig.5(b)). By comparing both figures, it is clear that there is152 an anisotropic behavior of the material. The compressive strength and strains are smaller when the load is applied in153 the transversal direction, and the resulting stiffness is higher.154 Eqs. (1) and (2) show the third order polynomial curve fitting coefficients for the longitudinal and transversal155 directions respectively.156 7 (a) (b) Figure 5: Stress-strain relationship for cubes loaded in (a) longitudinal and (b) transversal direction. σc(εc) =    −2.263e3ε3 c+32.86ε2 c+24.41εcf or 0≤εc≤εc(σmax c) 4.657e3ε3 c−249.50ε2 c−1.65εc+1.10 f or εc(σcmax )<εc (1) σc(εc) =    −4.137e4ε3 c+1.236e3ε2 c+32.20εcf or 0≤εc≤εc(σcmax ) 1.763e3ε3 c−116.25ε2 c−7.18εc+0.96 f or εc(σcmax )<εc (2) 2.2.3. Experimental stress-strain relationship of prisms.157 Fig. 6 shows the experimental stress-strain relationships for the prismatic specimens. Fig. 6(a) considers the strain158 measurements obtained from the displacement of the platens of the testing machine, whereas Fig. 6(b) considers159 the average strain obtained from the relative displacement measured by the two displacement sensors fixed to each160 specimen. It can be seen from Fig. 6 that the strain measurements from the displacement sensors are smaller than the161 measurement from the displacement of the platens.162 (a) (b) Figure 6: Stress-strain relationships for prisms obtained from relative displacements measured by (a) the testing machine and (b) the displacement sensors. Eqs. (3) and (4) show the third order polynomial curve fitting coefficients for the mean curve obtained from the163 strain measured from the testing machine and from the displacement sensors, respectively.164 8 σc(εc) =    −6.733e5ε3 c+1.182e4ε2 c+33.86εcf or 0≤εc≤εc(σmax c) −1.059e4ε3 c+1.146e3ε2 c−45.04εc+0.959 f or εc(σcmax )<εc (3) σc(εc) =    4.352e6ε3 c−7.834e4ε2 c+4.700e2εcf or 0≤εc≤εc(σmax c) 1.136e3ε3 c+51.83ε2 c−16.92εc+0.94 f or εc(σcmax )<εc (4) 2.2.4. Experimental stress-strain relationship of cylinders.165 Stress-strain curves for cylinders are obtained considering the strains measured from the displacement of the166 testing machine (Fig. 7(a)) and the displacement sensors (Fig. 7(b)). As for the prisms, the apparent global strain of167 the specimen computed from the displacement of the platens is overestimated because of the settlement process of the168 platens on the specimen.169 (a) (b) Figure 7: Stress-strain relationships for cylinders obtained from relative displacements measured by (a) the testing machine and (b) the displacement sensors. The equations of the polynomial approaches of the mean values of the experimental curves for cylinders are170 presented for the strains measured from the testing machine (Eq. 5) and from the displacement sensors (Eq.6)171 σc(εc) =    −4.026e5ε3 c+8.502e3ε2 c+48.22εcf or 0≤εc≤εc(σmax c) 4.941e4ε3 c−1.508e3ε2 c−25.25εc+1.30 f or εc(σcmax )<εc (5) σc(εc) =    6.030e7ε3 c−4.940e5ε2 c+1.385e3εcf or 0≤εc≤εc(σmax c) −2.214e7ε3 c+4.3032e4ε2 c−34.26εc+1.33 f or εc(σcmax )<εc (6) 3. Analysis of experimental results172 This section analyzes the experimental stress-strain curves obtained from the compression tests shown before.173 Characteristic values and testing parameters (compressive strength, peak strains, Young modulus, aspect ratio, anisotropy,174 strain measurement techniques) are determined and analyzed. Moreover, normalized stress-strain relationships are175 presented and compared with previous results from other authors.176 9 3.3. Normalized stress-strain relationship315 In order to compare the stress-strain curves for all types of specimens in a straightforward way, the polynomial316 curve fitting curves shown in section 2.2 for each type of specimen are normalized so the stresses are divided by317 their corresponding compressive strength ( ¯ σc=σc/fck) and the strains are divided by their corresponding peak strains318 (¯ εc=εc/εk). Thus, all the normalized stress-strain curves go from (0,0) to (1,1). The normalized equations are written319 in Eqs. (7) and (8) for cubic specimens loaded in the transversal and longitudinal direction, respectively, Eqs. (9) and320 (10) for prisms considering strains measured from the displacement of the testing machine and displacement sensors,321 respectively, and Eqs. (11) and (12) for cylinders considering strains measured from the displacement of the testing322 machine and displacement sensors, respectively.323 ¯ σc(¯ εc) =    −1.046 ¯ εc3+1.078 ¯ εc2+0.968 ¯ εcf or 0≤¯ εc≤1 0.0446¯ ε3 c−0.1014¯ ε2 c−0.2161¯ εc+1.0008 f or 1<¯ εc (7) ¯ σc(¯ εc) =    −0.562 ¯ εc3+0.125 ¯ εc2+1.436 ¯ εcf or 0≤¯ εc≤1 1.1582¯ ε3 c−0.9547¯ ε2 c−0.0976¯ εc+1.0000 f or 1<¯ εc (8) ¯ σc(¯ εc) =    −1.541 ¯ εc3+2.082 ¯ εc2+0.458 ¯ εcf or 0≤¯ εc≤1 −0.0244¯ ε3 c−0.2029¯ ε2 c−0.6119¯ εc+1.0000 f or 1<¯ εc (9) ¯ σc(¯ εc) =    1.000 ¯ εc3−2.999 ¯ εc2+2.999 ¯ εcf or 0≤¯ εc≤1 2.6110e−4¯ ε3 c+0.0020¯ ε2 c−0.1080¯ εc+1.0000 f or 1<¯ εc (10) ¯ σc(¯ εc) =    −1.388 ¯ εc3+1.777 ¯ εc2+0.611 ¯ εcf or 0≤¯ εc≤1 0.1705¯ ε3 c−0.3153¯ ε2 c−0.3201¯ εc+1.0000 f or 1<¯ εc (11) ¯ σc(¯ εc) =    1.218 ¯ εc3−3.327 ¯ εc2+3.109 ¯ εcf or 0≤¯ εc≤1 −0.4475¯ ε3 c+0.2899¯ ε2 c−0.0769¯ εc+1.0000 f or 1<¯ εc (12) The normalized curves are shown in Fig. 11, along with the previously normalized curves proposed by Illampas et324 al. [12] and by Adorni et al. [25]. In [12], the curves were obtained from compression tests on cylindrical specimens325 with aspect ratio value of 1. The curves obtained in this paper for the cubic specimens in the longitudinal direction,326 which also have a unity aspect ratio, are in very good agreement with the curves proposed in [12] for strains smaller327 than the peak strain. In addition, the strains in [12] were also measured by the displacement of the testing machine.328 However, there is a poor agreement for the post-peak part of the curve.329 The results for prisms and cylinders clearly illustrate the influence of the method for measuring strains. However,330 cylinders and prisms provide very similar curves for both types of measuring techniques, even though the loading331 direction is different and there are other factors that can affect the different behavior of the cylinders (remarked in332 16 Figure 11: Normalized stress-strain relationships for each type of specimen loaded in the longitudinal (L) or transversal (T) directions and from global strain measurements from the testing machine (mach) or displacement sensors (sens). section 3.1). As for the prisms and cylinders, very similar curves are obtained for the cubes loaded in the longitudinal333 and transversal directions. Thus, the anisotropy of the material has little effect on the normalized curves.334 The curve from [25] is in between the curves for the cubes and for the cylinders and prisms obtained from dis-335 placement readings of the testing machine. The samples tested in [25] were prismatic with an aspect ratio between336 1.5 and 1.8 and they were obtained by cutting samples from an archaeological site. The scattering of the specimens337 geometry and the results presented in [25] is very high, so they should be considered cautiously. In addition, no results338 are provided in [25] for strains higher than the peak strain.. Despite of this fact, they are consistent with the presented339 results.340 Results from Fig. 11 are very promising since very similar normalized curves are obtained for different types341 of specimens, with different size and shape, different loading directions, and even different materials. However, the342 agreement between the different curves is more significant for the ascending branch of the curve than for the post-343 peak part. The scattering of the results increases after the compressive strength is reached. The only factors that are344 affecting the curves in Fig. 11 are the aspect ratio and the method for measuring strains. Therefore, by measuring just345 the compressive strength and the peak strains (which are easier to measure than the whole stress-strain curve), one346 could obtain an accurate approach of the whole stress-strain curve from the normalized curves.347 17 4. Conclusions348 This paper has analyzed the experimental compressive behavior of unfired earthen bricks made from local mate-349 rials from the Guadalquivir river bank in Sevilla (Spain). Tests on whole bricks and cubic, prismatic and cylindrical350 specimens were carried out in the laboratories of School of Engineering of the University of Seville (Spain). Full351 stress-strain curves have been obtained and analyzed for all specimens. Third order polynomial curve fitting equa-352 tions have been obtained for each type of test. Values of compressive strength and Young modulus have been obtained353 and analyzed.354 Strains have been measured from the displacement of the actuator of the testing machine and from displacement355 sensors. The paper shows how readings from the testing machine overestimates the computed strains and lead to356 erroneous estimations of stress-strain curves and Young modulus.357 The anisotropic behavior of the material has been identified from the different behavior observed when the load is358 applied in the vertical (longitudinal) or horizontal (transversal) direction during the molding process. The anisotropy359 affects the rigidity of the material and the compressive strength. Higher strength and lower stiffness has been obtained360 for specimens loaded in the longitudinal direction. This anisotropic behavior can be induced by the orientation of the361 clay platelets and the straw fibers.362 The effect of the aspect ratio of the specimens on the compressive strength have been also analyzed. The obtained363 results show that existing correction factors for other materials may lead to erroneous estimations of the Unconfined364 Compressive Strength. The confinement effect might have a different influence on earthen materials because of its365 lower stiffness and ratio between tensile and compressive strength. Thus, existing correction factors for other materials366 should not be considered valid for earthen materials until a more extensive research on this issue is carried out.367 The Young modulus of the material has been analyzed by obtaining values of the secant modulus at 1/3 and 2/3368 of the compressive strength and the chord modulus between those two points. The paper has shown that previously369 proposed empirical correlations between Young modulus and compressive strength should not be considered as reli-370 able since they are based on invalid strain measurements obtained from the displacement of the actuator of the testing371 machine. Moreover, it is highlighted the importance of specifying in any research work which estimate of Young372 modulus is used since very different values can be obtained for secant or tangent modulus at different points. This373 paper proposes the use of the tangent modulus between 1/3 and 2/3 of the compressive strength since it gives a good374 approach of the stress-strain relationship in this range.375 Finally, the paper has obtained normalized stress-strain curves for each specimen. Good agreement has been376 obtained between different types of specimens and with results from other authors. This is a promising result that377 can help the standardization of methodologies for the characterization of earthen construction materials and also to378 develop standard constitutive models. Normalized stress-strain curves could be used to estimate the whole stress-strain379 curve by just determining the compressive strength and peak strain of the material.380 The paper has obtained relevant conclusions for the development of methodologies for the characterization of the381 18 compressive behavior of earthen bricks. However, it also definitely shows that further research is needed on this topic.382 The authors are now developing a new experimental campaign for addressing some of the open issues highlighted383 in this paper. This campaign also includes the experimental characterization of the mechanical behavior of adobe384 masonry walls in order to analyze the relationship between the compressive behavior of the bricks and the masonry.385 Acknowledgments386 This work was supported by the Consejer´ ıa de Econom´ ıa, Innovaci´ on, Ciencia y Empleo of Andaluc´ ıa (Spain)387 under project P12-TEP-2546 and the Spanish Ministry of Economy and Competitiveness (Ministerio de Econom´ ıa y388 Competitividad, Secretar´ ıa de Estado de Investigaci´ on, Desarrollo e Innovaci´ on) through research projects BIA2016-389 7531-R and BIA2016-75042-C2-1-R. The financial support is gratefully acknowledged.390 References391 References392 [1] H. Houben, H. Guillaud, CRAterre., Intermediate Technology Publications., Earth construction : a comprehensive guide, Intermediate Tech-393 nology Publications, 1994.394 [2] M. Blondet, J. Vargas, N. Tarque, C. 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