https://doi.org/10.14311/APP.2022.33.0370 Acta Polytechnica CTU Proceedings 33:370–376, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licence Published by the Czech Technical University in Prague EXPERIMENTAL INVESTIGATION ON THE MECHANICAL BEHAVIOUR OF AAC BLOCKS FOR SUSTAINABLE CONCRETE MASONRY Elena Michelini∗, Daniele Ferretti, Alice Sirico, Roberto Cerioni University of Parma, Department of Engineering and Architecture, Parco Area delle Scienze, 181/A, 43124 Parma, Italy ∗ corresponding author: elena.michelini@unipr.it Abstract. To satisfy the increasing demand of energy efficient buildings, AAC manufacturers are nowadays encouraged to produce blocks with ever lower densities. However, a compromise between energy-saving requirements and mechanical performances is needed to ensure structural safety, as well as an adequate structural durability. This paper reports a comprehensive experimental study on AAC mechanical properties (compressive and tensile strengths, as well as fracture energy), and on their dependency from material density and moisture content. The collected data are compared with some well-known analytical relations taken from the literature, which are often used for the calibration of mechanical parameters required for mathematical and/or finite element modelling of AAC load-bearing masonry, as well as of AAC masonry-infilled framed structures. These comparisons highlight some critical issues in the formulation of analytical relations having a general applicability; however, it was found that RILEM suggestions are appropriate for the considered AAC productions, at least for densities greater than 400 kg/m3. Keywords: Autoclaved aerated concrete, mechanical properties, sustainable concrete masonry. 1. Introduction The increasing interest in energy efficient buildings has promoted a growing use of innovative masonry products in the construction market. This tendency to search for new and non-standard masonry tech- niques is related to the widespread awareness that a large proportion of thermal losses in buildings is due to masonry walls. Therefore, one of the most efficient strategies for the reduction of greenhouse gases pro- duced by space heating is the adoption of products with increased thermal insulation performances [1–3]. Among them, Autoclaved Aerated Concrete (AAC) is becoming a quite common solution for the realization of sustainable concrete masonry units (CMUs), to be used in the construction of new buildings, as well as in the retrofitting of existing ones [4–10]. AAC is a porous lightweight concrete whose cellular structure is generally obtained through a gas-producing chemi- cal reaction of sand, lime, gypsum, and cement slurry, with the addition of an expanding agent (usually alu- minium powder). Other waste materials, such as fly ash, bottom ash, air-cooled slag, slate waste, glass or perlite waste, agriculture and industrial waste, etc. can be added to increase the sustainability of the pro- duction process [11–14]. Thanks to the large amount of entrapped air - ranging from 60% to 85% by vol- ume - the volume of the final product is up to five times that of the raw materials used in the production process. Adequate strength and dimensional stability are obtained through autoclaving at high tempera- ture and pressure, with the formation of a crystalline binder, called tobermorite [15]. Thanks to the presence of small air voids (in the range of 0.1 − 1 mm) uniformly distributed in the cement paste, AAC masonry blocks have many ad- vantages in comparison with conventional concrete: lighter weight (typically from one-sixth to one-third of conventional concrete), lower transportation and building costs, excellent workability and easiness of laying, reduced construction times, remarkable acoustic and thermal insulation. Besides its low ther- mal conductivity (which can reach 0.08 W/(mK), or even less for lower density values, see [16]), AAC is characterized by an outstanding air-tightness, which further enhances building thermal performances, cre- ating a comfortable living environment. Further- more, the use of AAC, with its simple construction details, reduces energy losses due to thermal bridges at junctions [6]. The flip side of the coin is that the reduction of AAC density, which improves thermal properties and reduces structural masses (and consequently the self- weight and the seismic forces acting on the build- ing), exerts an unfavourable effect on material me- chanical strengths. This could represent a limita- tion when AAC blocks are used for the realization of load-bearing masonry walls, since in this case it is mandatory to find a compromise between satisfactory insulation requirements and minimum compressive strength values, according to Standard Codes [17]. For example, regardless of the type of adopted ma- 370 https://doi.org/10.14311/APP.2022.33.0370 https://creativecommons.org/licenses/by/4.0/ https://www.cvut.cz/en vol. 33/2022 Mechanical Behaviour of AAC Blocks Figure 1. Sampling scheme for compression and flexural tests. sonry units, EN 1998-1 [18] requires a minimum nor- malised compressive strength of 5 MPa, so to ensure adequate stability and robustness of masonry walls in seismic areas. Even if not explicitly required by design Stan- dards, minimum tensile strength values (and conse- quently minimum compressive strengths, since these two properties are strictly related to each other) should be also provided for cladding and infill panels in framed structures, to limit crack formation under static and seismic loads. It is indeed well known that one of the most common drawbacks in using AAC for the realization of cladding and infills is related to its limited fracture toughness and to its brittleness, which can reduce in some cases the durability of ma- sonry panels, causing the appearance of premature and excessive damages [19, 20]. Aim of this work is to provide a further insight on the relation between density and mechanical proper- ties of AAC. Despite the large number of research works on this topic, a well-established relation be- tween compressive strength and density has not yet been found, since available experimental data are re- ferred to different raw materials in the admixture and to different autoclaving conditions (e.g., [3, 21–24]). On this point, another critical aspect is that temper- ature, pressure and curing time values are often not specified, despite their marked influence on the for- mation process of a stable form of tobermorite. A further complication relies on the fact that compres- sive strength is also dependent from the moisture con- tent at time of testing, as well as from the possible treatments undergone by the specimen to smooth its surfaces (i.e. use of sandpaper, or of a water grind- ing machine). To better clarify these aspects, an ex- perimental campaign is presented on AAC specimens characterized by four different densities and four dif- ferent moisture contents. The main mechanical prop- erties, i.e. compressive and flexural tensile strengths, and the fracture energy, were determined according to relevant Standards [25–27]. The best-known rela- tionships between these properties and the material density, as taken from the literature [23, 24, 28], were then checked against the collected data. 2. Experimental program 2.1. Preparation of AAC specimens All the specimens were cut from commercially avail- able AAC masonry blocks with nominal dimensions equal to 600 × 250 × 300 mm. The blocks were man- ufactured at the production plant starting from the following raw materials: sand with high silica content, cement, lime, water, gypsum, and aluminium powder as expanding agent. The blocks were autoclaved for 11 hours, at a temperature of 180 ◦ C and a pressure of 12 bars. Four different types of commercial blocks were investigated, with nominal density equal to 300, 350, 480 and 580 kg/m3 (respectively indicated with symbols A, EN, E, and S in Table 1). The sampling scheme followed for compression and flexural tests is depicted in figure 1. As can be seen, 6 cubes with 100 mm side and three 50 × 50 × 200 mm prisms were cut from the bottom, middle and top part of each block (with respect to the direction of rising), by using a diamond blade cutter and a hacksaw for AAC. Three of these six cubes were used 371 E. Michelini, D. Ferretti, A. Sirico, R. Cerioni Acta Polytechnica CTU Proceedings Figure 2. Test setup for the determination of: (a) compressive strength; (b) flexural tensile strength; (c) fracture energy. for the determination of the oven-dry density (DB, DM and DT), while the remaining three were tested in compression (CB, CM, CT), according to [25, 29]. Prismatic samples were instead tested in flexure (FB, FM, FT) according to [26]. For each examined den- sity, the sampling was repeated on six blocks (so hav- ing a total of 144 cubes and 72 prisms). According to the adopted Standards, before the tests, specimens were cured at laboratory conditions until the attain- ment of a moisture content equal to (6 ± 2) %. In order to investigate the effect of moisture con- tent on mechanical strengths, additional samples (108 cubes and 54 prisms) were realized from 18 AAC blocks with nominal density of 350 kg/m3. These specimens were subjected to different curing condi- tions so to achieve a moisture content respectively equal to (0 ± 2) %, (15 ± 2) % and an "upper limit" equal to the moisture content of the samples at their delivery to the laboratory (approximately ranging from 20 to 30%). Before conducting the tests, flatness and paral- lelism requirements were checked; in some cases, it was necessary to smooth sample surfaces with sand- paper. Regarding fracture energy determination, the only available Standard for AAC is represented by RILEM Recommendations [28], which suggests to carry out wedge-splitting tests. These tests are quite complex to be performed and require a completely different setup with respect to flexural tests. On the contrary, for standard concrete, fracture energy is usually de- termined by means of three-point bending tests on notched specimens. Previous works (i.e. [22]) proved that this test method can be extended also to AAC, providing results in accordance with those of wedge- splitting tests. For this reason, in this work fracture energy tests were carried out on notched prismatic samples having the same geometry as those tested in flexure, following the Japanese Code [27]. It was decided to extract only one prismatic specimen from the middle of each block, together with two cubes for the determination of the corresponding oven-dry density and compressive strength. Three blocks were tested for each considered density and moisture con- tent (also in this case, the latter was varied only for those blocks with a nominal density of 350 kg/m3). 2.2. Determination of oven-dry density As already stated, three twin cubes extracted from each block (DB, DM and DT according to Figure 1) were used for the determination of the oven-dry den- sity. To this aim, the specimens were dried in a ven- tilated oven at the temperature of (105 ± 5) ◦ C un- til a constant mass was reached. The oven-dry bulk density was then calculated as the oven-dried mass divided by the volume. The moisture content was determined as the ratio between the loss of mass dur- ing drying and the corresponding oven-dry mass. 2.3. Compression tests According to [25], compression strength was calcu- lated as the average of the results obtained on three standard cubes with an edge length of 100 mm, cut from the top, middle and bottom part of each block. Compression tests were performed by using an In- stron 5882 press working under loading control (Fig- ure 2a), with a loading rate of 0.05 MPa/s, as sug- gested for masonry elements with an expected com- pressive strength lower than 10 MPa. Loading was applied perpendicular to the direction of rise. 2.4. Flexural tests AAC modulus of rupture (tensile strength in bending, so-called MOR) was obtained from prismatic samples subjected to three-point bending over a net span of 150 mm. An Instron 8862 press was used to the scope, with a loading rate of 10 N/s (Figure 2b). Flexural tensile strength was determined as [26]: fct = 1.5 F l bf r h 2 f r (1) where F is the failure load, l is the net span, bf r and hf r are the specimen dimensions in correspondence of the cracked section. 372 vol. 33/2022 Mechanical Behaviour of AAC Blocks Figure 3. Evaluation of the average fracture energy for specimens with nominal density of 350 kg/m3 and moisture content ws of (0 ± 2) % by considering: a) the load P -deflection δ curve; b) the load P-CMOD curve, according to [27]. Density class Thermal conductivity Nominal density Oven-dry density Compressive strength Flexural tensile strength (W/(mK)) (kg/m3) (kg/m3) (MPa) (MPa) A 0.07 300 ± 50 295 (0.007) 1.93 (0.04) 0.65 (0.13) EN 0.08 350 ± 50 346 (0.008) 2.56 (0.03) 0.77 (0.08) E 0.11 480 ± 50 506 (0.002) 3.94 (0.08) 1.13 (0.05) S 0.13 580 ± 50 588 (0.005) 5.49 (0.08) 1.44 (0.05) Table 1. Properties of AAC specimens for different density classes, with (6 ± 2) % moisture content. 2.5. Fracture energy tests The test setup for the determination of fracture en- ergy was similar to that adopted for flexural tests (Figure 2c). However, in this case, the specimens were notched at their mid-length (with a notch depth equal to 0.3 times the beam depth) and the tests were performed under Crack Mouth Opening Dis- placement (CMOD) control, with a loading rate of 0.01 mm/min. The specimens were also instru- mented with a Linear Variable Displacement Trans- ducer (LVDT) for the measurement of midspan de- flection. Fracture energy Gf was calculated as [27]: Gf = 0.75 W0 + W1 Alig (2) W0 being the area below load-CMOD curve up to specimen failure, W1 the work done by the speci- men deadweight and by that of the loading jig, and Alig the area of the broken ligament. The appli- cability of this relation (which was originally devel- oped for standard concrete) to AAC was preliminary checked by comparing the so obtained fracture en- ergy with that calculated as the total work of fracture given by the area under the complete load-midspan deflection curve, divided by the ligament area [30]. The deadweight of the specimen and of the load- ing arrangement was taken into account also in this case. As an example, fracture energy deduced from the load-displacement curve (where the midspan de- flection was that measured by the LVDT) and that calculated from the load-CMOD curve according to the Japanese Standard [27] is reported in figure 3 for three prismatic samples with nominal density of 350 kg/m3 (class EN), and a moisture content of (0 ± 2) %. 3. Results and discussion 3.1. Oven-dry density For the four density classes considered in this study, the average oven-dry density was found to be equal to 295, 346, 506 and 588 kg/m3, with a Coefficient of Variation (COV) - reported in brackets in the table - below 0.01. These densities were almost coincident with the nominal values provided by the manufac- turer, as can be seen in table 1. The declared nom- inal values of thermal conductivity are also reported in the table for each density class. 4. Compressive strength The average cube compressive strength is summa- rized in table 1 for all the considered density classes, for a predefined moisture content equal to (6 ± 2) %. As can be seen from Figure 4a, compressive strength increased with density due to the corresponding re- duction in material porosity, but the relation between the two variables was found to be non-linear. The experimental relation between compressive strength and density was checked against some well- known analytical expressions available in the litera- ture. As can be seen, experimental data laid within the range suggested by RILEM Recommended Prac- tice [28], except for the lower density values. In this case, experimental compressive strengths were higher 373 E. Michelini, D. Ferretti, A. Sirico, R. Cerioni Acta Polytechnica CTU Proceedings Figure 4. Comparison between experimental data and analytical relations available in the literature: a) cube compressive strength fcc vs. density ρ; b) dimensionless cube compressive strength fcc/fcc (6 %) vs. moisture content ws. Figure 5. (a) Comparison between experimental data and analytical relations available in the literature in terms of flexural tensile strength fct vs. cube compressive strength fcc; (b) experimental relation between fracture energy Gf and density ρ, for a moisture content of (6 ± 2) %.. than the predicted ones. In the same graph, the re- lations proposed by Argudo [24] and by Chen et al. [23] were also plotted. In these relations, which were obtained from the best-fitting of experimental data collected by the Authors, the compressive strength is expressed as a function of the bulk density corre- sponding to a moisture content of 10% (ρ10 %), by as- suming that the ratio ρ10 %/ρdry is equal to 1.1. The comparison between the experimental data collected in this study and the two considered analytical rela- tions shows that a clear relation between compressive strength and density has not yet been found for AAC. Figure 4b highlights the influence of moisture con- tent at time of testing on the dimensionless com- pressive strength. It can be seen that the maximum strength was found at the end of the drying process. Even if standard Codes allow to perform compressive tests on dried specimens (by introducing a correction factor on the so obtained strength), the actual mois- ture content normally reached by AAC blocks in one or two years in external constructions is about 4−6 % [30]; for this reason, it seems more reasonable to pro- vide the compressive strength value corresponding to that condition. In addition, the compressive strength of specimens tested in dry conditions may be influ- enced by the required thermal treatment. As can be seen in Figure 4b, experimental normalized strength at dried condition was found to be significantly lower than that suggested by RILEM Recommended Prac- tice [28], while experimental values for other moisture contents fell inside the proposed range. 4.1. Flexural tensile strength and fracture energy Experimental values of flexural tensile strength com- ing from three-point bending tests are summarized in Table 1. As for ordinary concrete, flexural ten- sile strength is generally expressed as a function of concrete compressive strength, instead of density. As can be seen in Figure 5a, the relation suggested by 374 vol. 33/2022 Mechanical Behaviour of AAC Blocks RILEM [28] was found to well fit the experimental data collected in the performed experimental cam- paign, while the semi-empirical equations suggested more recently by Argudo [24] and by Chen et al. [23] had a significantly lower precision, especially for higher compressive strengths. This is a further evi- dence of the limits of semi-empirical relations avail- able in the literature, which suffer the dependency of AAC mechanical properties from the raw materials adopted in the production process, the autoclaving conditions and the moisture content of the specimens at time of testing, and consequently they could hardly be generalized. A possible strategy to overcome these limitations could be the formulation of different rela- tionships associated to distinct production processes, and to fix a predefined value of moisture to be reached before testing. Even if tensile strength is the main parameter gov- erning crack formation, it can be useful also to know the fracture energy, since it affects crack propaga- tion within the material. When masonry behaviour is analysed through finite element analyses, the knowl- edge of fracture energy is necessary for the calibra- tion of a proper block cohesive law. However, the dependency of fracture energy with material density has been little studied so far. A possible relation based on the data collected in this experimental pro- gram is shown in Figure 5b. This relation well fit also the average of the data collected by Ferretti et al. [22] in a previous experimental work, on a dif- ferent AAC production with a density of about 550 kg/m3. However, a further validation with other ex- perimental programs is required. 5. Conclusions To reduce thermal losses in buildings, an increasing attention is being paid to the reduction of CMUs ther- mal conductivity and, consequently, of their density. However, minimum mechanical properties should be guaranteed also for lower density blocks, so to ensure adequate structural performances and the compliance with durability requirements (i.e. limiting crack for- mation in infills and claddings). This work focuses on the mechanical characteriza- tion of AAC blocks belonging to four density classes. Based on the obtained experimental results and their comparison with some semi-empirical relations avail- able in the literature (calibrated on other AAC pro- ductions), the following conclusions can be drawn: • an almost linear relation can be inferred between thermal conductivity and density, as well as be- tween fracture properties (fracture energy) and density, while the relation between compressive strength and density seems to have an exponential form; • the relation between MOR and compressive strength is almost linear; • significant variations may be obtained in compres- sive strength for a given density value, mainly due to different production processes and material cur- ing; • eco-mechanical indexes, which are often used as synthetic indicators of environmental and mechan- ical performances for concrete (see, e.g. [4]), are hardly applicable in the considered case, unless modifying the definition of the reference environ- mental data (which are typically related to the pro- duction process, i.e., global warming potential and embodied energy, and are consequently quite sim- ilar for the four density classes). 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