construction of reinforced AAC buildings

Autoclaved Aerated Concrete (AAC) is commonly used in Europe for infill walls in reinforced concrete/steel structures and constructing masonry buildings due to its advantages such as lightweight, good insulation and high quality. These advantages of the material can be combined with reinforcing elements to create an added value product of structural panel with load carrying capability. Current use of engineered reinforced AAC panels are limited to partition walls with little emphasis on using them as structural elements. This study presents the results of the recent experimental and numerical work conducted in Turkey toward developing design guidelines for reinforced AAC panel wall buildings for energy efficient and rapid construction of low-rise buildings. Research findings, the published building code, examples and the future developments are discussed.

Masonry is one of the most popular choice for low-rise building systems worldwide. Clay bricks, AAC blocks and pumice blocks are the commonly used engineered construction material for masonry. AAC has some unique material properties such as being light weight, good insulation, airtightness and fire resistance. In Europe, approximately 60% of the new building constructions uses different types of AAC elements [1]. Among these materials, AAC, when produced with reinforcement as panel elements, can offer a comprehensive precast construction alternative for low-rise buildings. Reinforced AAC panels, produced with a length of 60 cm up to 6 m height and desired thickness are usually used in the housing industry as partition walls. On the other hand, they can be used to construct load bearing walls by placing desired number of them side by side. Additional longitudinal reinforcing bars are placed at the ends of each panel and the gaps around the reinforcement are filled with grout. Hence, AAC panel construction can increase the speed of erection compared to classical brick masonry while allowing to design the reinforcement for a target failure mode. In addition, reinforced AAC vertical panel building systems have additional advantages in seismic zones such as less building weight resulting in smaller lateral (inertial) forces, better ductility due to the presence of reinforcement.

The seismic performance of reinforced AAC wall panels was investigated in the past [2-8] by physical testing and simulations. Except the four vertical panel wall tests of Varela et al. [7], there were no experimental data on the lateral cyclic behaviour of reinforced vertical panel AAC walls with different number of panels. A comprehensive experimental campaign was initiated at Middle East technical University with the objectives of: i) understanding the seismic performance of reinforced AAC wall panels, ii) observing the seismic performance of a full-scale structure, iii) proposing design guidelines for low- rise AAC buildings. In this study, the results of the research conducted in Turkey toward developing design guidelines for reinforced AAC panel wall buildings for energy efficient and rapid construction of low-rise buildings are briefly summarized. Research findings, the published building code, examples and the future developments are discussed. Firstly, the experimental results on the in-plane seismic behavior of AAC vertical load-bearing wall panels is discussed. Then, an in-situ lateral loading test on a sample AAC building is shown.

Cyclic behaviour of reinforced AAC wall panels

An experimental program was initiated to determine the lateral load behavior of reinforced AAC wall panels. Six specimens with different arrangements were tested under the effect of two-way cyclic deformations. The load-deformation curves of each specimen were determined and the crack propagations of all specimens were detected. The reinforcement ratio, axial load, and geometry were selected as the test parameters compatible with the objectives of the tests. To study the effect of aspect ratios on the capacity of AAC wall panels, the number of panels was changed during the test program. Tests were conducted with and without axial load to determine the influence of axial force on the capacity of AAC walls. Specimens with two and four vertical wall panels were tested with and without axial load. A test without axial load and another test with a window opening were carried out for specimens with six AAC wall panels. More details on the experimental programme could be found in Taghipour et al. [9].

The height, length and thickness of each panel are 2400 mm, 600 mm and 200 mm, respectively. The AAC panels has a compressive strength of 4 MPa, a modulus of elasticity of 2,250 MPa and a density of 600 kg/m3. The reinforcements had a yield and ultimate strength of 413 MPa and 545 MPa, respectively. Each AAC panel was reinforced vertically equally spaced six bars with a dimeter of 4 mm (6ø4/240mm). ın addition, the transverse reinforcement was selected as ø4/125mm. The sketches of the reinforcement used in AAC panels are presented in Fig. 1.

The cyclic tests were conducted by using a test setup enabling simultaneous application of lateral and vertical loads (Fig. 2). Lateral load was applied to the AAC panel wall system by using hydraulic cylinders. The hydraulic cylinders were pin connected both to the reaction wall and loading beam in order to prevent any accidental moment application to the specimens. The axial load on Specimens S2 and S4 were applied by an axial loading system composed of two hydraulic cylinders, two steel rods and a cross beam. As shown in Fig. 2, the rods were passing through the cross beam. The rods were con- nected to the base by using hinges. At the beginning of each test, the axial load was applied was tried to be kept constant during the tests. The behaviour of each specimen was monitored by using Linear Variable Displacement Transducers (LVDTs) and load cells. The LVDTs were utilized to record i) horizontal displacement at top, middle and bottom of the wall height, ii) horizontal displacement at the mid height of the loading beam, iii) slip between the AAC panels and foundation and slip between the foundation and reaction floor, iv) vertical displacement at the bot- tom of the specimen at different levels, v) lifting-off of the foundation and vi) displacement of the AAC panels relative to each other.

Test results of all specimens will be critically discussed in this section. Displacement ductility (μ∆) curvature ductility (μφ) and behavior factor (Rμ) of the AAC walls were determined by assuming equal energy principal. In these calculations, Vu was taken as 80% of the maximum applied lateral load, Vmax. In addition, the value of ∆y was obtained by crossing the horizontal line passing through the maximum load and the line drawn from origin through 70% of lateral load capacity. These values are presented in Tab. 1. The Turkish Earthquake Code (TEC 2019 [10]) proposes the structural behavior factor (R) for reinforced AAC panel wall structures as 3. The value of Rμ, which is reduction factor due to ductility, should be multiplied by over-strength factor which can be taken as approximately 1.2-1.5 to get structural system behavior factor (R). According to all R values are higher than 3 as proposed in TEC 2019. Therefore, it can be concluded that the proposed R value of 3 in TEC 2019 is on the safe side for reinforced AAC panel wall structures (Tab. 2). In addition, it could be concluded that the ultimate drift ratios were greater than 1% with the exception of Specimen S1. Therefore, ultimate drift ratio of walls composed of reinforced AAC walls was 1%.

In-situ experiment on a sample AAC building

A full-scale building test was used to investigate the performance of the numerical method and it was constructed with reinforced AAC vertical panels. This test building was tested in the scope of this study i) to observe the seismic performance of a sample building composed of AAC vertical panels and ii) to unveil the seismic safety of the slab wall connections of this kind of structures. The full-scale building was constructed with vertical reinforced AAC panels and tested under increasing two-way cyclic deformation demands. The drawings of the test building and its loading system are presented in Fig. 3. The building was loaded in a reversed cyclic manner according to predefined drift ratios. Approximately 70 LVDTs were used to measure the deformations of the structure and their locations are presented in Fig. 3. There were longitudinal reinforcements between each panel at the walls and floors. There were also rein- forced concrete beams on top of the panels and they were used to connect the floors to the AAC vertical panels.

The test was conducted in displacement-controlled manner. After each predefined drift ratio was attained, the test was paused and all the walls were photographed in order to track the crack formations and possible failures at each loading step. More details on this experimental study could be found in Gokmen et al. [11].

First story shear versus first story displacement, second story shear versus second story displacement and base shear versus second story displacement curves are presented in Fig. 4. From this figure, it could easily be inferred that the lateral load capacity was reached its maximum value (580 kN) at a drift ratio of 0.19%. This load was about 1.6 times the weight of the test building. After the maximum capacity was attained, the lateral load capacity decreased by 20% at a drift ratio of 0.35%. This corre- sponded to an average ductility value of about 3.5. The structural behavior factor (R) considering an overstrength of about 1.5 could be found as about 3.5 by assuming equal energy principle.

The building capacity was estimated by using comprehensive simulations in Gokmen et al. [11]. That study showed that standard section analysis can be used to compute the moment capacity of AAC vertical panel walls in groups of at most three adjacent walls. The reason of this limitation was the vertical cracking between wall panels occurring around 1% interstory drift ratio.

Conclusions

This study presents the results of the recent experimental and numerical work conducted in Turkey toward developing design guidelines for reinforced AAC panel wall buildings for energy efficient and rapid construction of low-rise buildings. Research findings, the published building code, examples and the future developments are discussed. Tests conducted on six specimens with different properties and geometries are presented in this study. Test samples were made of various numbers of wall panel units, laid vertically. From these limited test results following conclusions can be drawn:

• The suggested value of 4 for the structural behavior factor (R) in the new proposed Turkish Earthquake Code (TEC 2019 [10]) seemed to be acceptable but perhaps slightly high according to the test results.

• The specimens with no axial load reached to their maximum capacities at lower drift ratios (0.35%), whereas the ultimate load capacities of specimens with axial load occurred at higher drift ratios (1%). Additionally, specimens with axial load rep- resented more ductile behavior compared to their companion specimens.

• The ultimate drift ratio of reinforced AAC walls should be limited to 1% in the light of experimen- tal observations.

• The moment capacity of reinforced AAC vertical panels should be calculated considering at most 3 adjacent wall panels due to vertical cracking at panel interfaces.

• Flexural and sliding were the most dominated failure modes in the experiments. Shear cracks [8] are not observed during the experiments and web shear failure mode was never dominated. It should be noted that, base leveling mortar was [9] not used in the construction of all of the specimens.

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