Study on Seismic Performance of Concrete Columns and Beams Containing Recycled Brick Aggregate

In order to study the reinforced concrete beam and column components composed of natural concrete (NC), recycled aggregate concrete (RAC), recycled brick grain concrete (RBGC) and recycled brick aggregate concrete (RBC), the reinforced columns under the axial tension and compression cyclic load and the horizontal cyclic load and the simply supported beam under concentrated mid-span cyclic load are simulated respectively. The transient time history analysis is conducted to analyze their failure characteristics, hysteretic performance and energy dissipation performance. The results show that the damage of the column under horizontal cyclic loading is more obvious than that under vertical cyclic loading, and the energy dissipation is better, while the energy dissipation capacity of the beam is not as good as that of the column due to local concentration. The seismic performance of reinforced concrete columns and beams composed of RBGC and RBC materials is lower than that of NC and RAC materials. The materials of RBGC and RBC are more suitable for beam members.


Introduction
The construction of new buildings and the demolition of old buildings have produced a large amount of construction waste, including waste concrete and waste clay bricks.One of the effective ways to reuse waste concrete and waste clay bricks is to use them again as aggregates in buildings.There are several types of recycled aggregate materials, such as recycled aggregate concrete (RAC), recycled brick grain concrete (RBGC), recycled brick aggregate concrete (RBC), etc.Among them, RAC is the most widely used recycled aggregate and it is made from crushed waste concrete and used as coarse aggregates in newly cast concrete.When brick grain is added into the coarse aggregates, it becomes RBGC.RBC means the coarse aggregates is made from the wasted brick and used as coarse aggregates in newly cast concrete.In terms of cube compressive strength, axial compressive strength, and elastic modulus, NC is the largest, RAC is in the middle, and RBGC and RBC are smaller [1][2].During the preparation of recycled aggregate into new concrete, due to its rough surface, it effectively increases the mechanical bonding force with the mortar.And the brick aggregate is lightweight and has good fire resistance [3].Many studies on the seismic performance of recycled aggregate concrete columns have been carried out.SAI S et al. [4], Choi et al. [5], Lu X et al. [6] concluded that the failure modes of RAC and RBC columns is comparable to that of NC.Wang S et al. [7] carried out horizontal low cyclic loading experiments on NC, RAC and RBGC columns.It was found that the ductility, bearing capacity and energy dissipation capacity of recycled brick frame columns were lower than those of NC and RAC frame columns.Deresa S et al. [8] found that the crack morphology and crack development of RAC beams and columns are similar to those of NC beams and columns.Under horizontal cyclic loading, the rotation rate of RAC columns is slightly higher than that of NC columns.Kang T H K [9] found that the cracking load of RAC beams is lower than that of NC beams.Arezoumandi M et al. [10], Lee M et al. [11] found that the shear performance of RAC beams is similar to that of NC beams.González J S et al. [12] found that the highest acceptable substitution rate forprecast prestressed beams with RBC is 35%.It shows that a series of current research on the seismic performance of brick aggregate components mainly focused on columns under horizontal cyclic loading, while few of them discussed those under vertical cyclic loading.Moreover, few of the research discussed the cyclic properties of beams.Therefore, this paper focuses on the seismic performance of NC, RAC, RBGC and RBC reinforced concrete columns under horizontal and vertical cyclic loading and the seismic performance of beams.

Basic Material Property
The recycled replacement rate of RAC, RBGC and RBC is set to be 30% for comparison.The mechanical performance parameters of NC, RAC, RBGC and RBC are as follows:

Selection of Constitutive Model
The stress-strain relationship of NC and RAC [1] is expressed as equation (1) and equation (2), while for RBGC and RBC, equation (3) gives this relationship [2].The stress-strain curves of each material are shown in figure 1.

Modeling
Three models are considered respectively, which are detailed as follows.
(1) A reinforced concrete column with a length of 1800 mm and a width of 600 mm, reinforced with GB HRB335 and with a stirrup spacing of 200 mm and a concrete cover thickness of 30 mm is considered (figure 2).The finite element model of the column is established by using the general finite element ANSYS, and the separated modeling method is adopted, that is, the concrete is established by using the 8-node solid element SOLID65, and the steel bar is simulated by using the LINK8 element.The bond slip between the rebar and the concrete is overlooked for all the three models.The bottom of the column is fixed to ground and the top is subjected to vertical displacement cyclic load.The maximum compressive displacement is d, and the maximum tensile displacement is 0.1d so that the effective displacement range is 1.1d, and the displacement d is set to be 0.4mm, 0.8mm, 1.7mm, 2.5mm, 4.5mm and 5.5mm, respectively.
(2) A reinforced concrete column is the same as Model 1.The bottom of it is fixed.The vertical distributed load of 7MPa is applied at the top to simulate the axial force and the horizontal cyclic displacement load is applied.The maximum displacement in both two directions is d and thus the effective displacement range is 2d.The displacement d is 1mm, 3mm, 5mm, 7mm and 9mm respectively.
(3) A reinforced concrete simply supported beam, as shown in figure 3, with a length of 2000 mm, a width of 150 mm and a height of 300 mm.The beam is reinforced similarly with the above columns.Both ends of the beam are fixed on the ground, and a concentrated cyclic load is applied at the top of the center of the beam.Since the model is completely symmetrical, so a 1 / 4 model is taken for modeling.The maximum compressive and tensile displacements are both set to d and the effective displacement range is 2d.The displacement d is set to 0.1mm, 0.2mm, 0.3mm, 0.4mm, 0.5mm and 0.6mm, respectively.

Failure Characteristics
From the literature review and simulation, it is found that the failure modes of NC, RBC, RBGC and RBC reinforced concrete components are similar and thus only that of RBC are discussed.

The Column Under Vertical Cyclic
Loading.The RBC column under d = 4.5mm is taken as an example.It is found from figure 4 that the compression distribution within the column is uneven.At the bottom where the component is fixed, the stress is more concentrated and the steel bar undergoes a relatively large deformation.Moreover, the cracks and crushing contour of the concrete are drawn as figure 5.It is found that the crushing phenomenon is obvious within concrete at the bottom, which leads to failure.

The Column under Horizontal Cyclic
Loading.The RBC column under d = 9mm is taken as an example.it is found from figure 6 that the compression distribution of the column is uneven, but its difference from the column under vertical cyclic load is that the stress concentration position is more dispersed.The deformation of the steel bar at the bottom is still obvious.The crack and crushing contour is shown as figure 7. It is found that, compared with the column under vertical cyclic loading, the concrete crushing area of the column under horizontal cyclic loading is larger and the damage is more serious, which may stem from more sufficient energy consumption.

Hysteretic Performance
Hysteretic performance reflects the mechanical properties of components under cyclic loading, which is the comprehensive reflection of seismic performance.

The Column under Vertical Cyclic
Loading.With the increase of displacement loading amplitude, the hysteresis curve area of the concrete column increases gradually, and the elastic-plastic energy dissipation of the component also increases gradually as shown in figure 10.Based on the comparison between the hysteresis curves of the four types of material columns, it is found that the area of the hysteresis curve of NC is the largest, which means that the component can consume more energy in the earthquake, and the seismic performance is relatively better.Secondly, the hysteresis curves of RAC, RBGC and RBC are small, which indicates that the strength of the added brick aggregate is lower than that of ordinary stone aggregate or ordinary recycled aggregate, so the strength of these two kinds of recycled concrete with brick is slightly lower and the energy dissipation capacity is also decreased.

The Column under Horizontal Cyclic
Loading.The results are listed in figure 11.It is found that the area of the hysteresis curve of the NC column is the largest and the RAC is second largest.The hysteresis curve of the RBGC and the RBC is smaller.Compared with the column under vertical cyclic loading, the area enclosed by the hysteresis curve is larger, which shows that the column under horizontal loading has sufficient energy dissipation capacity. -1000 Figure 11.Hysteretic curve of column under horizontal loading.

Simply Supported Beam under Mid-span Cyclic
Loading.The hysteretic curve results are illustrated in figure 12.It is found that the area of the hysteresis curve of the concrete beam is generally small, which indicates that the energy dissipation capacity of the simply supported beam under cyclic load is smaller than that of the column.This may stem from the local stress concentration near the loading position.The hysteresis curves of the four types of material beams are similar to that of the above concrete columns, where NC beam has the largest area, the RAC follows while the areas of the RBGC and the RBC beams are smaller, but the phenomenon is not obvious.This shows that the reduction of the seismic performance of reinforced concrete beams containing recycled brick concrete and brick aggregate recycled concrete is not as obvious as that of columns.

Conclusions
In this paper, the hysteretic characteristics of a reinforced concrete column under axial cyclic load and under horizontal cyclic load and a simply supported beam under mid-span cyclic load made from NC, RAC, RBGC and RBC are respectively discussed.The results show that: (1) The failure modes of the three components are different.For the column under vertical cyclic loading, the stress concentration distribution position is more concentrated.For the column under horizontal cyclic loading, the stress concentration position is more dispersed.For the simply supported beam under mid-span cyclic load, the stress is more concentrated near the supported and the loading position where the concrete compression occurs.
(2) The hysteresis curve area of the three components is different.The hysteresis curve area is moderate, large and small respectively for the column under vertical cyclic loading, the column under horizontal cyclic loading and the simply supported beam under mid-span concentrated cyclic loading.Therefore, the column has the best energy dissipation characteristics under horizontal cyclic loading.
(3) The hysteresis curves of the components made from the four types of material are different.The differences of the four hysteresis curves indicating four types of material are large, moderate and small respectively for the column under vertical cyclic loading, the column under horizontal cyclic loading and the simply supported beam under mid-span cyclic loading.Therefore, the reduction of seismic performance is relatively small for simply supported beams made from RBGC and RBC, while it is especially large for the column under vertical cyclic loading.

Figure 4 .
Figure 4.The principal stress cloud diagram.Figure 5. Crack and crushing diagram.

Figure 5 .
Figure 4.The principal stress cloud diagram.Figure 5. Crack and crushing diagram.

3. 1 . 3 .
Failure Characteristics of Beam Members.The RBC beam under d = 0.6mm is taken as an example and the principal stress contour is shown in figure8, the form of damage is shown in figure9.It is found from the figure that the compression distribution is uneven, and the stress is more concentrated near the fixed position and the loading position.Moreover, the concrete in the above two positions shows local crushing phenomenon and leads to failure, which may stem from local stress concentration near the loading position.

Figure 10 .
Figure 10.Hysteretic curve of column under vertical cyclic loading.

Figure 12 .
Figure 12.Hysteretic curves of beam members under different displacement loading.

Table 1 .
Basic parameters of different materials.