Failure Mechanism of Confined Masonry Wall under Pushover Load

. The confined masonry wall has various performances and failure mechanisms under earthquake excitation. This excitation, which is applied to the specimen in laboratory tests, can be simplified by conducting pushover loading. This research aims to exhibit the capacity and failure mechanism of confined masonry walls due to pushover loading that was applied on the top of the specimen. The pushover load was applied to the full-scale specimen with the size of 3 m x 3 m. The specimen failed due to bed-joint sliding failure with the maximum pushover load of 34.716 kN. The crack was observed on the contact between the bottom reinforced concrete tie-beam and the brick masonry wall. Thus, the vertical bars that connect the beam and the wall are essential to apply to the tie-beam to hold the shear force on the area.


Introduction
The majority construction system of dwelling buildings in many cities around the globe [1], including Aceh Province-Indonesia, is confined masonry walls.A confined masonry wall is constructed usually by laying down bricks bonded with cement paste over the reinforced concrete bottom tie-beam and later the reinforced concrete tie columns followed by the top tie-beam are cast to confine the masonry wall.This wall system performs diversely from well to poor under earthquake excitation due to unstandardized materials, and informal construction methods [2]- [4].Up until now, local red clay bricks in Aceh are still traditionally produced by groups of people in different villages across the province.
Earthquake loading in a laboratory is simplified as a pushover load that works in the plane of a confined masonry wall specimen [5], [6].The failure modes or mechanisms of this type of wall under in-plane pushover loading are a shear diagonal failure, bed-joint sliding failure, rocking failure, and toe-crushing failure which complied with the last experience earthquakes where the damages triggered by shear and flexure [1], [7].This research objective is to study the capacity and failure mechanism of a confined masonry wall made of red clay brick that was produced locally in Aceh Besar District.The data from this research can expectantly be used in the computational modeling of this type of structure.

Research method
Two parts of the work were conducted in this research.The first work was to construct and test the specimen of the confined masonry wall, and the second work was to prepare and test all small specimens to check the quality of red clay bricks, mortar to lay down the masonry wall, concrete mixture of tie-beams and tie-columns, and steel rebars.All specimens were made based on regular workmanship to obtain real work of confined masonry wall construction in Aceh.The works are explained in detail below.

Properties of material
The confined masonry wall specimen was made from red clay bricks, mortar, concrete, and steel rebar.Mortar was used for bonding bricks and covering the wall surfaces.The composition of mortar was one part of Portland Cement (PC) and two parts of fine aggregate.The concrete mixture was made from one part of Portland Cement, two parts of fine aggregate, and three parts of coarse aggregate.The steel bar had a diameter of 6 mm for the stirrups, and 12 mm for the longitudinal rebars.Table 1 shows the characteristics of the red clay brick that was used in this research, and Table 2 describes the material mechanical properties.All data in Table 2 was obtained from standard testing that was conducted based on the Indonesia National Standard (SNI) and the American Society for Testing and Materials (ASTM) for each type of test.Shear Modulus (MPa), [11] Bond Strength (MPa), [12] Elastic Modulus (MPa), [13] Tensile Strength (MPa), [13] Concrete bottom tie-beam 15.905 ---------------Concrete of tiecolumn and top tiebeam  [11] Shear Modulus (MPa), [11] Bond Strength (MPa), [12] Elastic Modulus (MPa), [13] Tensile Strength (MPa), [13] Plain Steel Rebar Diameter 12mm ------------123,613.9 376.224 Plain Steel Rebar Diameter 6mm ------------163,776.2 448.001

The confined masonry wall specimen
The confined masonry wall specimen was made with full-scale (3 m x 3 m) as shown in Fig. 1.The size of reinforced concrete tie-beams and tie-columns was 10 cm x 10 cm, with 4 longitudinal rebars, and the stirrup distance of 15 cm.The construction of the specimen in this research was initiated by casting down the reinforcement of the tie-beam on top of the base reinforced concrete plate which was the plate attached to the strong floor with bolts.The reinforcement of the bottom tie-beam was connected to the reinforced concrete (RC) plate with Dynabolts that were placed every 50 cm.Then the construction work was continued by assembling the vertical tie-column reinforcement.The tie-column reinforcement is also linked to the reinforced concrete plate by connecting the tie-column steel rebar to the plate rebar.The wooden molds were put around the bottom tie-beam reinforcement to cast in the concrete mixture.This tie beam concrete mixture composition was made of one part Portland cement, two parts aggregate, and three parts coarse aggregate.The concrete mixture was poured into the bottom tie-beam wooden molds, and after 24 hours the first half height of the brick masonry wall was laid down over the tie-beam.The red clay bricks of the wall were put together by mortar with the mixing of one part of Portland cement and two parts of fine aggregate.Later, the wooden molds for tie-columns were put on, and the concrete mixture was poured into the molds of the tie columns at the same height as the masonry wall.Then after 24 hours, the second half height of the masonry wall was laid down, followed by pouring the concrete into the last half of the tie-column molds.The next step was to cast the steel rebars of the top tie-beam over the brick masonry wall, and to put the wooden molds in place to hold the concrete mixture that was poured into the tie-beam mold.
The final step was covering both sides of confined masonry walls with around 1.85 cm thickness of mortar after 24 hours of the last work.The mortar composition was similar to the mortar to bond the red clay bricks, which were one part of Portland cement and two parts of fine aggregate.

Pushover testing of the confined masonry wall specimen
The pushover loading of the confined masonry wall was conducted with the setup shown in Fig. 1.The load actuator was placed in line with the center of the top tie beam.The load was applied gradually, and the crack propagation was also observed in the process of loading.Some linear variable differential transformers (LVDTs) were put in some places to observe the movement of the specimen.On the opposite side of the load, the LVDTs were put on the top and the middle height of the wall.The out-of-plane movement of the wall is also detected by attaching an LVDT perpendicular to the wall.Horizontal and vertical movement of the RC base plate were monitored by LVDTs.

Results and discussion
The relationship between pushover load and drift of the confined masonry wall specimen is shown in Fig. 2. Point A shows the sample starting to crack on the contact layer of the reinforced concrete bottom tie-beam and brick masonry wall.It shows that there is a difference in the stiffness of the wall due to the crack.The crack started and wider progressively in this area following the increment of pushover load.Point B shows the maximum pushover load of 34.716 kN that can be held by the specimen.Then the load decreases abruptly because of the loss of contact between the connection of the RC base plate and the tie column.The progression of a bigger uplift happened to the wall specimen, and the pushover load was stopped when the displacement of the specimen reached 1.484% drift (44.505 mm of displacement) for safety reasons.
The propagation of cracks on the contact of the bottom tie-beam and wall happens due to no vertical bar that supports the shear force that working on the CMW.It is important to note that the vertical reinforcement that connects the reinforced concrete bottom tie-beam and the masonry wall is important to hold the shear force from the pushover load.The confined masonry wall specimen under pushover loading started to crack on the bottom of the specimen at the layer where the top part of the reinforced concrete tie-beam meets the bottom part of the masonry wall.The progression of the crack continued to develop on the same spot where the gap between the cracks increased at the pushover load around 34.716 kN.After this load, no more increasing pushover load can be held by the specimen because the bottom tie-beam was separated from the reinforced concrete base plate due to the separation of the connector of the tie-beam and the RC base plate.
Both sides of the confined masonry wall behave almost identically, and the failure pattern is presented in Fig. 3 and Fig. 4 below.No crack was observed on both tie columns of the confined masonry wall due to flexural failure, where the plastic hinge usually occurred at this place.

Summaries
The failure mechanism of confined masonry wall specimens is dominated by bed-joint sliding failure in the contact layer of the top part of the reinforced concrete tie-beam with the bottom part of the masonry wall.The tie-column at the far end of the confined masonry wall specimen, with the same level of wall layer that has cracks, suffers slight bending which means started to behave as a plastic hinge.The shear-sliding failure mechanism can be reduced by applying some vertical reinforcement that connects the tie beam and the masonry wall.

Fig. 1 .
Fig. 1.Testing setup of the confined masonry wall specimen and equipment.

Fig. 2 .
Fig. 2. Relationship of the pushover load to the displacement of the confined masonry wall.

Fig. 3 .
Fig. 3. Failure on the bottom of the south side wall at the last stage of pushover loading.

Fig. 4 .
Fig. 4. Failure on the bottom of the North side wall at the last stage of pushover loading.

Table 1 .
Characteristics of red clay brick per unit

Table 2 .
Materials mechanical properties