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Seismic performance of steel-reinforced recycled concrete columns under low cyclic loads
Construction and Building Materials 48 (2013) 229–237
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Seismic performance of steel-reinforced recycled concrete columns under low cyclic loads Hui Ma a,⇑, Jianyang Xue b, Xicheng Zhang b, Daming Luo b a b
School of Civil Engineering and Architecture, Xi’an University of Technology, Xi’an, Shaanxi, China School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi, China
h i g h l i g h t s The seismic performance of composite columns was investigated under low cyclic loads. The composite columns behave similarly in the aspects of the failure mode. The influence of parameters on the seismic performance of columns is analyzed. It is feasible to apply the composite columns in the practice of civil engineering.
a r t i c l e
i n f o
Article history: Received 30 August 2012 Received in revised form 3 June 2013 Accepted 10 June 2013 Available online 31 July 2013 Keywords: Steel-reinforced concrete Recycled coarse aggregate Column Cyclic loading Seismic performance
a b s t r a c t This paper describes an experimental study of the seismic performance of steel-reinforced recycled concrete (SRRC) columns. Based on low cyclic loading tests of seven 1:2.5-scaled column specimens, the failure modes, hysteresis loops, skeleton curves, ductility, energy dissipation capacity, and stiffness degradation of SRRC columns were analyzed. The influence of recycled coarse aggregate (RCA) replacement percentages, axial compression ratios, and stirrup ratios on the seismic performance of SRRC columns was investigated in detail. The test results show that the seismic performance of SRRC columns decreases slightly as the RCA replacement percentage increases. The results also indicate that appropriate design of the axial compression ratio and stirrup ratio can improve the seismic performance of SRRC columns. The average values of the ductility factor and the equivalent viscous damping coefficient with respect to the loop of ultimate load of the columns were 3.47 and 0.217, respectively, which reflect the SRRC columns’ good performance in terms of earthquake resistance. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Rapid urbanization has brought about many negative problems, such as exploitation of nonrenewable natural resources and the production of large amounts of construction waste. Deciding what to do with huge quantities of construction waste is a problem for governments all around the world. To minimize the exploitation of natural resources, we must try to reuse construction waste. Some countries have adopted the use of waste concrete, i.e., recycled concrete, as a replacement for concrete aggregate. A considerable amount of experimental work on the material properties and structural behavior of recycled concrete aggregate has been carried out worldwide. Most previous investigations have focused on the mix design and the physical and mechanical properties, including the durability, of recycled concrete. The research on recycled concrete has been extensively reviewed and summarized by Nixon [1], ⇑ Corresponding author. Tel.: +86 15029923059. E-mail address: [email protected] (H. Ma). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.06.019
Hansen [2], Dhir [3], the ACI Committee 555 [4], Khatib [5], Casuccio [6], and Tabsh [7]. Although the research results show that some mechanical properties of recycled aggregate concrete (RAC) may be inferior to those of normal concrete (NC), they are still suitable for use in civil engineering applications through reasonable design. The structural behavior of recycled concrete has also been studied. Some investigations involving the behavior of reinforced concrete beams [8–10], columns [11–13], beam–column joints [14,15], and frames [16] made from recycled aggregate concrete have been reported. The major findings of most investigators have been positive. The cracking patterns and failure modes of reinforced recycled concrete (RRC) are similar to those of ordinary reinforced concrete (RC), but the bearing capacity of RRC is somewhat reduced, to an allowable extent, compared to that of RC. Some research has examined the performance of composite structures using recycled aggregate concrete, such as tubular steel columns filled with recycled aggregate concrete [17,18]. The results showed that tubular steel columns filled with recycled
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
Nomenclature r n
qsv k fcu fy fu Es Ec V P, D
RCA replacement percentage axial compression ratio stirrup ratio shear span ratio cube compressive strength yield strength of I-steel and rebar ultimate strength of I-steel and rebar modulus of elasticity of I-steel or rebar modulus of elasticity of concrete vertical load lateral load and corresponding displacement, respectively
Py, Dy Pm, Dm Pu, Du
l he
crack load and corresponding displacement, respectively yield load and corresponding displacement, respectively maximum load and corresponding displacement, respectively ultimate load and corresponding displacement, respectively ductility factor equivalent viscous damping coefficient
ratio of the RCA mass to the mass of all the coarse aggregates in the concrete), which were 0, 70%, and 100%. The measured mechanical properties of recycled concrete are listed in Table 3.
Table 1 Properties of I-steel and rebar. I-steel and rebar
fy (MPa)
fu (MPa)
Es (MPa)
446.5 474.9 560.9
1.99 105 1.98 105 2.03 105
607.0
2.02 105
No. 14 I-steel
Flange Web
Longitudinal reinforcement
/14
311.5 325.6 358.0
/8
479.9
Stirrup
Pcr, Dcr
aggregate concrete have good mechanical properties and that RCA can be used in composite structures. In view of the advantages of steel-reinforced concrete (SRC) structures in terms of their bearing capacity, stiffness and seismic performance, steel-reinforced concrete columns (SSRCs) containing recycled aggregate concrete were investigated by Cui [19]. The test results showed that SRRC columns exhibit better behavior than normal reinforced concrete and reinforced recycled concrete under static loads. However, there has been no research on the seismic performance of steelreinforced concrete structures or structural members with recycled aggregate concrete either at home or abroad. In this study, seven SRRC columns with shear span ratios of k = 3.25 were made, and low cyclic loading tests were conducted. The failure modes, hysteresis loops, skeleton curves, ductility, energy dissipation capacity, and stiffness degradation of the SRRC columns were analyzed. The influence of design parameters (i.e., RCA replacement percentages, axial compression ratios, and stirrup ratios) on the seismic performance of the SRRC columns were analyzed in detail. 2. Experimental program 2.1. Materials Typical ordinary Portland cement (C) with a 28 day nominal compressive strength of 42.5 MPa was used in this research. Crushed aggregate and river sand (S) were chosen as the natural coarse aggregate (NCA) and fine aggregate, respectively. Commercially available recycled coarse aggregate (RCA) produced from demolished concrete structures was also used in this investigation. The physical properties of the recycled coarse aggregate meet the requirements of Chinese code GB/T25177-2010[20]. The No. 14 I-steel of Q235 was adopted in the specimens, and HRB 335 rebar (crescent-ribbed bars with diameters of 8 mm and 14 mm) were used for stirrups and longitudinal reinforcement. The mechanical properties of the I-steel and rebar used in this research are listed in Table 1.
2.3. Design and fabrication of specimens Seven 1:2.5-scaled rectangular SRRC columns were fabricated for this experiment with 240 mm 180 mm cross-sections and heights of 780 mm (from the bottom to the loading point of each column). The design parameters include the RCA replacement percentage, the axial compression ratio and the stirrup ratio, the values of which are given in Table 4. All the rectangular columns were reinforced with I-steel and four longitudinal reinforcement bars and were also reinforced transversely by stirrups. The percentages of I-steel and longitudinal reinforcement for all the columns were 4.98% and 1.42%, respectively. The thicknesses of concrete cover of the stirrups and steel flanges were 20 mm and 50 mm, respectively. Typical reinforcement details for the specimens are shown in Fig. 1. 2.4. Test devices and methods The tests were carried out in the State Key Laboratory of Architecture, Science and Technology in West China at Xi’an University of Architecture & Technology, PR China. The test set-up is shown in Fig. 2. All specimens were tested under low cyclic lateral loads with vertical force. The force diagram of the specimens is shown in Fig. 3. The vertical loads were applied by a hydraulic jack before testing. After the vertical loads reached a stable value, lateral force was applied using an electrohydraulic servo test machine. The lateral loading procedure included two main steps, namely, a load-controlled step and a displacement-controlled step, which are illustrated in Fig. 4. To monitor the lateral displacement of the top of column, one linear variable differential transducer (LVDT) was installed along the centerline of the loading cross-section of the column. The lateral movement of the column basement was also measured. The strains in the steel flanges, steel webs, longitudinal reinforcement bars, and transverse stirrups were measured using strain foil and strain rosettes attached to the steel flanges, steel webs, longitudinal bars, and transverse stirrups and embedded in concrete in advance. The measurement points of the specimens are shown in Fig. 5.
3. Results and discussion 3.1. Cracking and failure modes After the vertical loads were applied to the tops of the columns, lateral cyclic loads were applied according to the loading procedure shown in Fig. 4. In the early stage of loading, no cracks were found on the specimens’ surfaces because the deformation of the
Table 2 The mix ratios of concrete with RCA.
2.2. Concrete mixture proportions
Concrete strength
r (%)
Because of the water absorption capacity of RCA, the RCA was presoaked in water before mixing. The design 28 day strength for all the concrete mixtures used was 40 MPa. The mix ratios of the concrete are given in Table 2. The main parameters of these three groups are the ratio of water to cement (W/C), the cement content, the sand content, and the RCA replacement percentages considered (i.e., the
C40 C40 C40
0 70 100
Unit mass (kg/m3) W/C
C
S
NCA
RCA
W
0.44 0.43 0.42
466 478 488
571 549 527
1158 347.4 0
0 810.6 1158
205 205 205
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237 Table 3 Properties of recycled coarse aggregate concrete. Concrete strength
r (%)
fcu (MPa)
Ec (GPa)
C40 C40 C40
0 70 100
47.70 51.82 48.89
34.26 29.56 27.32
Table 4 Design parameters of specimens. Specimen no.
r (%)
k
n
qsv (%)
Stirrup spacing (mm)
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
0 70 100 100 100 100 100
3.25 3.25 3.25 3.25 3.25 3.25 3.25
0.6 0.6 0.6 0.3 0.9 0.6 0.6
1.36 1.36 1.36 1.36 1.36 1.02 2.04
90 90 90 90 90 120 60
Fig. 2. Test set-up.
V P
P L
columns was in the elastic range. When the lateral loads reached 30–40% of the maximum lateral load, tiny transverse cracks were observed at the bottom of the columns. As the magnitudes of the lateral loads increased, the numbers of transverse cracks in the columns increased, and some transverse cracks gradually extended. As some transverse cracks extended to the surfaces of the steel flanges, the transverse cracks began to develop relatively slowly into diagonal cracks because of the constraints of the steel flanges. At the same time, some tiny vertical cracks appeared at the bottom of the columns. As the loads continued to increase, the original diagonal cracks developed slowly, but the transverse cracks developed rapidly and linked together gradually at the bottom of columns. When the lateral loads reached 75–85% of the maximum lateral load, most of the specimens were already in the yield stage. Displacement-controlled cyclic loading was then applied. Because
Specimen
PL Bending moment
Shear force
V PL Fig. 3. Force diagram of specimens.
of the core concrete of the columns, which maintains its strength because it is constrained by the surrounding steel, the bearing
Fig. 1. Geometry and reinforcing bars of specimens.
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Loads or displacements
232
Loading cycles
Fig. 4. Loading procedure for tests.
capacities of the specimens increased to some extent. As the displacement magnitude increased, no new cracks appeared, but the previously formed cracks became longer and wider. Some transverse cracks at the bottom of the column specimens penetrated through completely. At the same time, the concrete cover in the vicinity of column bottom began to crush and fall off. After the specimens reached the maximum lateral load, the resisting force of the columns began to drop. With more and more concrete cover at the bottom of the columns falling off, the longitudinal reinforcement yielded absolutely due to the pressure, and local buckling occurred in the steel. At that time, the lateral loads on the specimens dropped rapidly, indicating the specimens’ failure. The failure modes of the specimens are shown in Fig. 6. All can be summarized as flexural failures. All the investigated SRRC columns behaved similarly in terms of the failure pattern under low lateral loading, regardless of differences in the values of their design parameters. 3.2. Characteristic loads Table 5 lists the measured crack loads, yield loads, maximum loads, and ultimate loads of the columns. Fig. 7 shows the influences of the design parameters on the specimens’ maximum lateral loads. Table 5 and Fig. 7 together show the following:
3.3. Characteristic displacement and ductility factor Table 6 summarizes the deformation features of the columns, including the crack displacements, the yield displacements, the peak displacements and the ultimate displacements corresponding to the crack loads, yield loads, maximum loads, and ultimate loads, respectively. In addition, each specimen’s ductility factor l, which is defined as the ratio of the ultimate displacement to the yield displacement (i.e., l = Du/Dy), is also presented in Table 6. Fig. 8 illustrates the influences of design parameters on the specimens’ ductility factors. As Fig. 8 shows, a specimen’s ductility factor decreases as the RCA replacement percentage and axial compression ratio increase, while a specimen’s ductility factor increases as the stirrup ratio increases. The average ductility factor
150
(1) The crack loads were almost the same for SRRC11, SRRC12, and SRRC13. Compared to SRRC11, which contained natural coarse aggregate, the yield load of SRRC12 was 1.17% lower, while that of SRRC13 was 6.28% higher. The maximum load
and ultimate load of SRRC12 increased by 5.24%, while those of SRRC13 decreased by 0.41%. The maximum load is almost the same for SRRC11, with natural coarse aggregate, and SRRC13, with recycled coarse aggregate (i.e., the RCA replacement percentage is 100%). This indicates that the recycled coarse aggregate did not influence the characteristic loads of the columns. It should also be noted that the mechanical properties of RCA were also not influenced by the replacement of natural aggregate with recycled aggregate, as shown in Table 3. This may be due to the contributions of the steel or steel bars and the geometry of the columns. The reason why the characteristic loads were not influenced by the quantity of recycled aggregate still needs to be investigated. Fortunately, compared to the steel-reinforced concrete column, the characteristic loads of the SRRC column were only slightly different. The test results indicate that these SRRC column designs and materials can satisfy the bearing capacity requirements for the SRRC columns. (2) The crack loads, yield loads, maximum loads, and ultimate loads of the specimens increased as the magnitude of the axial compression ratio increased, which suggests that increasing the axial compression ratio improves the bearing capacity of SRRC columns. (3) The maximum loads and ultimate loads of the specimens increased as the magnitude of the stirrup ratio increased, which suggests that increasing the stirrup ratio improves the bearing capacity of SRRC columns.
Steel flange
Steel web
Strain rosette Strain foil
50
LVDT
500
Specimen
Steel flange 730
930
1430
780
150
LVDT
450
240
450
Stirrup Longitudinal reinforcement
1140
(a) external measure points
(b) internal measure points Fig. 5. Measurement points of specimens.
H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
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Fig. 6. Failure patterns of specimens.
Table 5 Characteristic loads of columns. Specimen no.
Pcr (kN)
Py (kN)
Pm (kN)
Pu (kN)
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
60.8 60.4 60.1 40.7 60.4 41.0 50.8
119.3 117.9 126.8 105.6 144.6 110.1 122.4
146.9 154.6 147.5 135.7 168.9 142.4 166.5
124.9 131.4 125.4 115.3 143.5 121.1 134.5
value was 3.47, which indicates the good seismic performance of the SRRC columns. 3.4. Hysteresis loops Fig. 9 shows the hysteresis loops observed in the tests, which illustrate the relationship between the lateral loads and displacements at the tops of the columns under cyclic loading. Based on Fig. 9, the following observations can be made:
(1) The spindle-shaped loops indicate the good energy dissipation capacity of all the columns. In the early stage of loading (before cracking), the loads and displacements have linear relationships, which indicate that the specimens are in an elastic state. In the elasto-plastic state, as cracks appeared at the bottom of the specimens, the slopes of the hysteresis loops began to decrease, and a larger residual deformation was observed when the lateral load was removed. In the displacement-controlled stage, the hysteresis curves of the columns became larger and wider as the displacements became larger. With respect to the loading process at each displacement level, both the strengths and stiffnesses of the specimens decreased as the number of displacement cycles increased. This deterioration mainly reflects the damage accumulation in the specimens. (2) The different design parameters have different influences on the shapes of the hysteresis loops. First, as the RCA replacement percentage increases, the areas of the hysteresis loops seem to decrease somewhat. Second, the larger the axial compression ratio is, the narrower the hysteresis loops become. Third, as the stirrup ratio increases, the hysteresis loops of the specimens become wider.
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
(a) The influence of RCA replacement percentages on the maximum load
(a) The influence of RCA replacement percentage on the ductility factor
(b) The influence of axial compression ratio on the maximum load
(b) The influence of axial compression ratio on the ductility factor
(c) The influence of stirrup ratios on the maximum load (b) The influence of stirrups ratio on the ductility factor
Fig. 7. The influences of design parameters on the maximum load.
Fig. 8. The influences of design parameters on the ductility factor. Table 6 Characteristic displacement and ductility factor. Specimen no.
Dcr (mm)
Dy (mm)
Dm (mm)
Du (mm)
l
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
1.43 1.49 1.37 1.32 1.61 0.97 1.70
4.67 4.28 5.24 5.97 5.18 4.82 5.90
8.90 9.68 8.99 15.18 8.47 10.92 12.92
16.14 14.32 17.25 31.08 11.99 15.05 20.88
3.47 3.35 3.30 5.22 2.19 3.13 3.64
3.5. Skeleton curves A skeleton curve reflects the relationship between the peak loads and corresponding displacements from the hysteresis loops of the specimens. The skeleton curves of all seven specimens are plotted in Fig. 10. The follow observations can be made based on Fig. 10: (1) The crack loading points, the yield loading points, the maximum loading points, and the ultimate loading points can be easily recognized, and the loading process can thereby be divided into the elastic, elasto-plastic, and failure stages. The initial stiffness is relatively large because of the existence of steel in the columns. Most of the skeleton curves show the resistance of the specimens to loading decreasing slowly, which indicates that the SRRC columns have good ductility.
(2) The influence of the RCA replacement percentage on the skeleton curves of the specimens is shown in Fig. 10a. The skeleton curves of all three specimens overlap before the yield load point of the column, which indicates that the initial stiffnesses of the columns were similar, regardless of the RCA replacement percentages. After the yield load point of the column, the three skeleton curves exhibit some differences in the elasto-plastic and failure phases. These plots show that the RCA replacement percentage had some influence on the bearing capacity and ductility of the specimens, but the influence is not very obvious. (3) As Fig. 10b shows, the initial stiffnesses of the columns increased as the axial compression ratios increased. After the yield load points of the columns, the three skeleton curves show obvious differences in the elasto-plastic and failure phases. The strength attenuation of the specimens with high axial compression ratios decreased rapidly, which suggests that selection of reasonable axial compression ratios is very significant in the seismic design of SRRC columns. (4) Fig. 10c shows that the skeleton curves of the three specimens exhibited obvious difference in the initial stiffnesses of the columns with increasing stirrup ratio. After the yield load points of the columns, the three skeleton curves exhibit obvious differences in the elasto-plastic and failure phases.
H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
235
Fig. 9. Hysteresis loops of specimens.
The bearing capacity of the specimen with a high stirrup ratio decreased slowly, which suggests that using a high stirrup ratio can improve the seismic performance of SRRC columns. 3.6. Stiffness degradation Fig. 11 shows the degradation of the secant stiffnesses of the seven columns versus the lateral displacements at the tops of the columns under cyclic loading. All the columns had a larger initial stiffness because of the existence of steel in the specimens. Fig. 11 shows that the stiffnesses of most of the columns decreased slowly during the initial stage of loading. When cracks appeared in the specimens, the stiffnesses decreased dramatically, and when the columns were in the yield stage, their stiffnesses decreased significantly. Beyond that point, the stiffness degeneration tended to be slow and show no obvious abrupt changes. Fig. 11a illustrates the influence of the RCA replacement percentage on the stiffness degradation of the specimens. The initial stiffnesses were almost the same for all three investigated
columns. After cracks appeared in the specimens, no obvious differences were observed in the investigated columns. Fig. 11b shows that the initial stiffnesses of the columns with high axial compression ratios were greater than those of the columns with low axial compression ratio. Unfortunately, the rate of stiffness degradation after cracking in the columns with high axial compression ratios was greater than that in columns with low axial compression ratios. Fig. 11c shows that the initial stiffnesses of columns with high stirrup ratios were greater than those with low stirrup ratios. After cracks appeared in the specimens appeared, the rate of stiffness degradation in the columns with high stirrup ratios was slower than in the columns with low stirrup ratios.
3.7. Energy dissipation capacity The energy dissipation capacity of a structure or structural component reflects its seismic energy absorption ability. The equivalent viscous damping coefficient he is used to quantify the
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
(a) The influence of RCA replacement percentages on the stiffness degradation
(a) The influence of RCA replacement percentage on the skeleton curves
(b) The influence of axial compression ratio on the skeleton curves
(b) The influence of axial compression ratio on the stiffness degradation
(c) The influence of stirrup ratio on the skeleton curves
(c) The influence of stirrup ratio on the stiffness degradation Fig. 11. Stiffness degradation of specimens.
Fig. 10. Skeleton curves of specimens.
seismic energy absorption ability of structures and structural components. Table 7 lists the equivalent viscous damping coefficients versus the different characteristic loading points of the specimens. The equivalent viscous damping coefficients hey, hem and heu correspond to the yield loading, the maximum loading and the ultimate loading of the columns, respectively. Fig. 12 shows the influence of the design parameters on the energy dissipation capacities of the specimens. Table 7 shows that the energy dissipation capacity of an SRRC column increases with increasing loading steps (especially in the displacement-controlled loading stage). The average values of the equivalent viscous damping coefficients hey, hem and heu of the tested specimens were 0.081, 0.155 and 0.217, respectively, which indicate that the SRRC columns had relatively good energy dissipaTable 7 The equivalent viscous damping coefficients of the specimens. Specimen no.
hey
hem
heu
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
0.090 0.080 0.079 0.096 0.068 0.069 0.085
0.161 0.159 0.158 0.183 0.124 0.144 0.159
0.225 0.217 0.212 0.260 0.177 0.193 0.232
tion capacity. Fig. 12 illustrates the influence of some of the design parameters on the equivalent viscous damping coefficients of the columns. Table 7 and Fig. 12 together show the following: (1) Compared to SRRC11, with natural coarse aggregate, the equivalent viscous damping coefficients heu of SRRC12 and SRRC13 were 3.56% and 5.78% lower, respectively. In addition, the equivalent viscous damping coefficients hey and hem decreased slightly as the RCA replacement percentage increased. These results suggest that the RCA replacement percentage had no obvious influence on the energy dissipation capacity of the columns. (2) Compared to SRRC14, with a low axial compression ratio, the equivalent viscous damping coefficients heu of SRRC13 and SRRC15 were 18.46% and 31.92% lower, respectively. In addition, the equivalent viscous damping coefficients hey and hem decrease considerably as the axial compression ratio increases. These results suggest that the axial compression ratio had an obvious influence on the energy dissipation capacity of the columns. Therefore, the axial compression ratio must be controlled strictly in seismic design. (3) Compared to SRRC17, with a high stirrup ratio, the equivalent viscous damping coefficients heu of SRRC13 and SRRC16 were 8.62% and 23.71% lower, respectively. Moreover, the
H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
(a)The influence of RCA replacement percentages on the equivalent viscous damping coefficient
237
(3) The hysteresis loops, skeleton curves, stiffness degradation, and energy dissipation capacities of the columns indicate that the seismic performance of SRRC columns is comparable to that of conventional concrete. Appropriate design of the axial compression ratio and stirrup ratio can improve the seismic performance of SRRC columns. (4) The average values of the ductility factor and equivalent viscous damping coefficient of the SRRC columns were 3.47 and 0.217, respectively. These values indicate that SRRC columns are capable of resisting an earthquake and that it is feasible to employ SRRC columns in the practice of civil engineering.
Acknowledgements The work presented herein was carried out at the Xi’an Univ. of Architecture and Technology in PR China and was funded by the Chinese National Natural Science Foundation, under Grant No. 51178384. The project was also funded by Shaanxi Province as a scientific research project under Grant No. 12JK0902. The financial support provided by these agencies is gratefully acknowledged.
(b) The influence of axial compression ratio on the equivalent viscous damping coefficient
(c) The influence of stirrups ratio on the equivalent viscous damping coefficient Fig. 12. The influence of design parameters on the equivalent viscous damping coefficient.
equivalent viscous damping coefficients hey and hem increase as the stirrup ratio increases. These results suggest that increasing the stirrup ratio improves the energy dissipation capacity of SRRC columns. 4. Conclusions This paper discusses an experimental study on the seismic performance of steel-reinforced recycled concrete (SRRC) columns under low cyclic lateral loads. The results show that the following: (1) All the investigated SRRC columns behaved similarly in terms of their failure pattern under low lateral loading, regardless of differences in the values of their design parameters. The failure patterns of the columns can be classified as flexural failure. (2) The RCA replacement percentage had no obvious influence on the bearing capacity of the columns, but both the ductility and the energy dissipation capacity decreased as the replacement percentage increased. The bearing capacity of the columns increased as the axial compression ratio increased, while the ductility decreased considerably. In addition, the bearing capacity and the ductility of the columns increased as the stirrup ratio increased.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Seismic performance of steel-reinforced recycled concrete columns under low cyclic loads Hui Ma a,⇑, Jianyang Xue b, Xicheng Zhang b, Daming Luo b a b
School of Civil Engineering and Architecture, Xi’an University of Technology, Xi’an, Shaanxi, China School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi, China
h i g h l i g h t s The seismic performance of composite columns was investigated under low cyclic loads. The composite columns behave similarly in the aspects of the failure mode. The influence of parameters on the seismic performance of columns is analyzed. It is feasible to apply the composite columns in the practice of civil engineering.
a r t i c l e
i n f o
Article history: Received 30 August 2012 Received in revised form 3 June 2013 Accepted 10 June 2013 Available online 31 July 2013 Keywords: Steel-reinforced concrete Recycled coarse aggregate Column Cyclic loading Seismic performance
a b s t r a c t This paper describes an experimental study of the seismic performance of steel-reinforced recycled concrete (SRRC) columns. Based on low cyclic loading tests of seven 1:2.5-scaled column specimens, the failure modes, hysteresis loops, skeleton curves, ductility, energy dissipation capacity, and stiffness degradation of SRRC columns were analyzed. The influence of recycled coarse aggregate (RCA) replacement percentages, axial compression ratios, and stirrup ratios on the seismic performance of SRRC columns was investigated in detail. The test results show that the seismic performance of SRRC columns decreases slightly as the RCA replacement percentage increases. The results also indicate that appropriate design of the axial compression ratio and stirrup ratio can improve the seismic performance of SRRC columns. The average values of the ductility factor and the equivalent viscous damping coefficient with respect to the loop of ultimate load of the columns were 3.47 and 0.217, respectively, which reflect the SRRC columns’ good performance in terms of earthquake resistance. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Rapid urbanization has brought about many negative problems, such as exploitation of nonrenewable natural resources and the production of large amounts of construction waste. Deciding what to do with huge quantities of construction waste is a problem for governments all around the world. To minimize the exploitation of natural resources, we must try to reuse construction waste. Some countries have adopted the use of waste concrete, i.e., recycled concrete, as a replacement for concrete aggregate. A considerable amount of experimental work on the material properties and structural behavior of recycled concrete aggregate has been carried out worldwide. Most previous investigations have focused on the mix design and the physical and mechanical properties, including the durability, of recycled concrete. The research on recycled concrete has been extensively reviewed and summarized by Nixon [1], ⇑ Corresponding author. Tel.: +86 15029923059. E-mail address: [email protected] (H. Ma). 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.06.019
Hansen [2], Dhir [3], the ACI Committee 555 [4], Khatib [5], Casuccio [6], and Tabsh [7]. Although the research results show that some mechanical properties of recycled aggregate concrete (RAC) may be inferior to those of normal concrete (NC), they are still suitable for use in civil engineering applications through reasonable design. The structural behavior of recycled concrete has also been studied. Some investigations involving the behavior of reinforced concrete beams [8–10], columns [11–13], beam–column joints [14,15], and frames [16] made from recycled aggregate concrete have been reported. The major findings of most investigators have been positive. The cracking patterns and failure modes of reinforced recycled concrete (RRC) are similar to those of ordinary reinforced concrete (RC), but the bearing capacity of RRC is somewhat reduced, to an allowable extent, compared to that of RC. Some research has examined the performance of composite structures using recycled aggregate concrete, such as tubular steel columns filled with recycled aggregate concrete [17,18]. The results showed that tubular steel columns filled with recycled
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Nomenclature r n
qsv k fcu fy fu Es Ec V P, D
RCA replacement percentage axial compression ratio stirrup ratio shear span ratio cube compressive strength yield strength of I-steel and rebar ultimate strength of I-steel and rebar modulus of elasticity of I-steel or rebar modulus of elasticity of concrete vertical load lateral load and corresponding displacement, respectively
Py, Dy Pm, Dm Pu, Du
l he
crack load and corresponding displacement, respectively yield load and corresponding displacement, respectively maximum load and corresponding displacement, respectively ultimate load and corresponding displacement, respectively ductility factor equivalent viscous damping coefficient
ratio of the RCA mass to the mass of all the coarse aggregates in the concrete), which were 0, 70%, and 100%. The measured mechanical properties of recycled concrete are listed in Table 3.
Table 1 Properties of I-steel and rebar. I-steel and rebar
fy (MPa)
fu (MPa)
Es (MPa)
446.5 474.9 560.9
1.99 105 1.98 105 2.03 105
607.0
2.02 105
No. 14 I-steel
Flange Web
Longitudinal reinforcement
/14
311.5 325.6 358.0
/8
479.9
Stirrup
Pcr, Dcr
aggregate concrete have good mechanical properties and that RCA can be used in composite structures. In view of the advantages of steel-reinforced concrete (SRC) structures in terms of their bearing capacity, stiffness and seismic performance, steel-reinforced concrete columns (SSRCs) containing recycled aggregate concrete were investigated by Cui [19]. The test results showed that SRRC columns exhibit better behavior than normal reinforced concrete and reinforced recycled concrete under static loads. However, there has been no research on the seismic performance of steelreinforced concrete structures or structural members with recycled aggregate concrete either at home or abroad. In this study, seven SRRC columns with shear span ratios of k = 3.25 were made, and low cyclic loading tests were conducted. The failure modes, hysteresis loops, skeleton curves, ductility, energy dissipation capacity, and stiffness degradation of the SRRC columns were analyzed. The influence of design parameters (i.e., RCA replacement percentages, axial compression ratios, and stirrup ratios) on the seismic performance of the SRRC columns were analyzed in detail. 2. Experimental program 2.1. Materials Typical ordinary Portland cement (C) with a 28 day nominal compressive strength of 42.5 MPa was used in this research. Crushed aggregate and river sand (S) were chosen as the natural coarse aggregate (NCA) and fine aggregate, respectively. Commercially available recycled coarse aggregate (RCA) produced from demolished concrete structures was also used in this investigation. The physical properties of the recycled coarse aggregate meet the requirements of Chinese code GB/T25177-2010[20]. The No. 14 I-steel of Q235 was adopted in the specimens, and HRB 335 rebar (crescent-ribbed bars with diameters of 8 mm and 14 mm) were used for stirrups and longitudinal reinforcement. The mechanical properties of the I-steel and rebar used in this research are listed in Table 1.
2.3. Design and fabrication of specimens Seven 1:2.5-scaled rectangular SRRC columns were fabricated for this experiment with 240 mm 180 mm cross-sections and heights of 780 mm (from the bottom to the loading point of each column). The design parameters include the RCA replacement percentage, the axial compression ratio and the stirrup ratio, the values of which are given in Table 4. All the rectangular columns were reinforced with I-steel and four longitudinal reinforcement bars and were also reinforced transversely by stirrups. The percentages of I-steel and longitudinal reinforcement for all the columns were 4.98% and 1.42%, respectively. The thicknesses of concrete cover of the stirrups and steel flanges were 20 mm and 50 mm, respectively. Typical reinforcement details for the specimens are shown in Fig. 1. 2.4. Test devices and methods The tests were carried out in the State Key Laboratory of Architecture, Science and Technology in West China at Xi’an University of Architecture & Technology, PR China. The test set-up is shown in Fig. 2. All specimens were tested under low cyclic lateral loads with vertical force. The force diagram of the specimens is shown in Fig. 3. The vertical loads were applied by a hydraulic jack before testing. After the vertical loads reached a stable value, lateral force was applied using an electrohydraulic servo test machine. The lateral loading procedure included two main steps, namely, a load-controlled step and a displacement-controlled step, which are illustrated in Fig. 4. To monitor the lateral displacement of the top of column, one linear variable differential transducer (LVDT) was installed along the centerline of the loading cross-section of the column. The lateral movement of the column basement was also measured. The strains in the steel flanges, steel webs, longitudinal reinforcement bars, and transverse stirrups were measured using strain foil and strain rosettes attached to the steel flanges, steel webs, longitudinal bars, and transverse stirrups and embedded in concrete in advance. The measurement points of the specimens are shown in Fig. 5.
3. Results and discussion 3.1. Cracking and failure modes After the vertical loads were applied to the tops of the columns, lateral cyclic loads were applied according to the loading procedure shown in Fig. 4. In the early stage of loading, no cracks were found on the specimens’ surfaces because the deformation of the
Table 2 The mix ratios of concrete with RCA.
2.2. Concrete mixture proportions
Concrete strength
r (%)
Because of the water absorption capacity of RCA, the RCA was presoaked in water before mixing. The design 28 day strength for all the concrete mixtures used was 40 MPa. The mix ratios of the concrete are given in Table 2. The main parameters of these three groups are the ratio of water to cement (W/C), the cement content, the sand content, and the RCA replacement percentages considered (i.e., the
C40 C40 C40
0 70 100
Unit mass (kg/m3) W/C
C
S
NCA
RCA
W
0.44 0.43 0.42
466 478 488
571 549 527
1158 347.4 0
0 810.6 1158
205 205 205
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237 Table 3 Properties of recycled coarse aggregate concrete. Concrete strength
r (%)
fcu (MPa)
Ec (GPa)
C40 C40 C40
0 70 100
47.70 51.82 48.89
34.26 29.56 27.32
Table 4 Design parameters of specimens. Specimen no.
r (%)
k
n
qsv (%)
Stirrup spacing (mm)
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
0 70 100 100 100 100 100
3.25 3.25 3.25 3.25 3.25 3.25 3.25
0.6 0.6 0.6 0.3 0.9 0.6 0.6
1.36 1.36 1.36 1.36 1.36 1.02 2.04
90 90 90 90 90 120 60
Fig. 2. Test set-up.
V P
P L
columns was in the elastic range. When the lateral loads reached 30–40% of the maximum lateral load, tiny transverse cracks were observed at the bottom of the columns. As the magnitudes of the lateral loads increased, the numbers of transverse cracks in the columns increased, and some transverse cracks gradually extended. As some transverse cracks extended to the surfaces of the steel flanges, the transverse cracks began to develop relatively slowly into diagonal cracks because of the constraints of the steel flanges. At the same time, some tiny vertical cracks appeared at the bottom of the columns. As the loads continued to increase, the original diagonal cracks developed slowly, but the transverse cracks developed rapidly and linked together gradually at the bottom of columns. When the lateral loads reached 75–85% of the maximum lateral load, most of the specimens were already in the yield stage. Displacement-controlled cyclic loading was then applied. Because
Specimen
PL Bending moment
Shear force
V PL Fig. 3. Force diagram of specimens.
of the core concrete of the columns, which maintains its strength because it is constrained by the surrounding steel, the bearing
Fig. 1. Geometry and reinforcing bars of specimens.
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Loads or displacements
232
Loading cycles
Fig. 4. Loading procedure for tests.
capacities of the specimens increased to some extent. As the displacement magnitude increased, no new cracks appeared, but the previously formed cracks became longer and wider. Some transverse cracks at the bottom of the column specimens penetrated through completely. At the same time, the concrete cover in the vicinity of column bottom began to crush and fall off. After the specimens reached the maximum lateral load, the resisting force of the columns began to drop. With more and more concrete cover at the bottom of the columns falling off, the longitudinal reinforcement yielded absolutely due to the pressure, and local buckling occurred in the steel. At that time, the lateral loads on the specimens dropped rapidly, indicating the specimens’ failure. The failure modes of the specimens are shown in Fig. 6. All can be summarized as flexural failures. All the investigated SRRC columns behaved similarly in terms of the failure pattern under low lateral loading, regardless of differences in the values of their design parameters. 3.2. Characteristic loads Table 5 lists the measured crack loads, yield loads, maximum loads, and ultimate loads of the columns. Fig. 7 shows the influences of the design parameters on the specimens’ maximum lateral loads. Table 5 and Fig. 7 together show the following:
3.3. Characteristic displacement and ductility factor Table 6 summarizes the deformation features of the columns, including the crack displacements, the yield displacements, the peak displacements and the ultimate displacements corresponding to the crack loads, yield loads, maximum loads, and ultimate loads, respectively. In addition, each specimen’s ductility factor l, which is defined as the ratio of the ultimate displacement to the yield displacement (i.e., l = Du/Dy), is also presented in Table 6. Fig. 8 illustrates the influences of design parameters on the specimens’ ductility factors. As Fig. 8 shows, a specimen’s ductility factor decreases as the RCA replacement percentage and axial compression ratio increase, while a specimen’s ductility factor increases as the stirrup ratio increases. The average ductility factor
150
(1) The crack loads were almost the same for SRRC11, SRRC12, and SRRC13. Compared to SRRC11, which contained natural coarse aggregate, the yield load of SRRC12 was 1.17% lower, while that of SRRC13 was 6.28% higher. The maximum load
and ultimate load of SRRC12 increased by 5.24%, while those of SRRC13 decreased by 0.41%. The maximum load is almost the same for SRRC11, with natural coarse aggregate, and SRRC13, with recycled coarse aggregate (i.e., the RCA replacement percentage is 100%). This indicates that the recycled coarse aggregate did not influence the characteristic loads of the columns. It should also be noted that the mechanical properties of RCA were also not influenced by the replacement of natural aggregate with recycled aggregate, as shown in Table 3. This may be due to the contributions of the steel or steel bars and the geometry of the columns. The reason why the characteristic loads were not influenced by the quantity of recycled aggregate still needs to be investigated. Fortunately, compared to the steel-reinforced concrete column, the characteristic loads of the SRRC column were only slightly different. The test results indicate that these SRRC column designs and materials can satisfy the bearing capacity requirements for the SRRC columns. (2) The crack loads, yield loads, maximum loads, and ultimate loads of the specimens increased as the magnitude of the axial compression ratio increased, which suggests that increasing the axial compression ratio improves the bearing capacity of SRRC columns. (3) The maximum loads and ultimate loads of the specimens increased as the magnitude of the stirrup ratio increased, which suggests that increasing the stirrup ratio improves the bearing capacity of SRRC columns.
Steel flange
Steel web
Strain rosette Strain foil
50
LVDT
500
Specimen
Steel flange 730
930
1430
780
150
LVDT
450
240
450
Stirrup Longitudinal reinforcement
1140
(a) external measure points
(b) internal measure points Fig. 5. Measurement points of specimens.
H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
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Fig. 6. Failure patterns of specimens.
Table 5 Characteristic loads of columns. Specimen no.
Pcr (kN)
Py (kN)
Pm (kN)
Pu (kN)
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
60.8 60.4 60.1 40.7 60.4 41.0 50.8
119.3 117.9 126.8 105.6 144.6 110.1 122.4
146.9 154.6 147.5 135.7 168.9 142.4 166.5
124.9 131.4 125.4 115.3 143.5 121.1 134.5
value was 3.47, which indicates the good seismic performance of the SRRC columns. 3.4. Hysteresis loops Fig. 9 shows the hysteresis loops observed in the tests, which illustrate the relationship between the lateral loads and displacements at the tops of the columns under cyclic loading. Based on Fig. 9, the following observations can be made:
(1) The spindle-shaped loops indicate the good energy dissipation capacity of all the columns. In the early stage of loading (before cracking), the loads and displacements have linear relationships, which indicate that the specimens are in an elastic state. In the elasto-plastic state, as cracks appeared at the bottom of the specimens, the slopes of the hysteresis loops began to decrease, and a larger residual deformation was observed when the lateral load was removed. In the displacement-controlled stage, the hysteresis curves of the columns became larger and wider as the displacements became larger. With respect to the loading process at each displacement level, both the strengths and stiffnesses of the specimens decreased as the number of displacement cycles increased. This deterioration mainly reflects the damage accumulation in the specimens. (2) The different design parameters have different influences on the shapes of the hysteresis loops. First, as the RCA replacement percentage increases, the areas of the hysteresis loops seem to decrease somewhat. Second, the larger the axial compression ratio is, the narrower the hysteresis loops become. Third, as the stirrup ratio increases, the hysteresis loops of the specimens become wider.
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
(a) The influence of RCA replacement percentages on the maximum load
(a) The influence of RCA replacement percentage on the ductility factor
(b) The influence of axial compression ratio on the maximum load
(b) The influence of axial compression ratio on the ductility factor
(c) The influence of stirrup ratios on the maximum load (b) The influence of stirrups ratio on the ductility factor
Fig. 7. The influences of design parameters on the maximum load.
Fig. 8. The influences of design parameters on the ductility factor. Table 6 Characteristic displacement and ductility factor. Specimen no.
Dcr (mm)
Dy (mm)
Dm (mm)
Du (mm)
l
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
1.43 1.49 1.37 1.32 1.61 0.97 1.70
4.67 4.28 5.24 5.97 5.18 4.82 5.90
8.90 9.68 8.99 15.18 8.47 10.92 12.92
16.14 14.32 17.25 31.08 11.99 15.05 20.88
3.47 3.35 3.30 5.22 2.19 3.13 3.64
3.5. Skeleton curves A skeleton curve reflects the relationship between the peak loads and corresponding displacements from the hysteresis loops of the specimens. The skeleton curves of all seven specimens are plotted in Fig. 10. The follow observations can be made based on Fig. 10: (1) The crack loading points, the yield loading points, the maximum loading points, and the ultimate loading points can be easily recognized, and the loading process can thereby be divided into the elastic, elasto-plastic, and failure stages. The initial stiffness is relatively large because of the existence of steel in the columns. Most of the skeleton curves show the resistance of the specimens to loading decreasing slowly, which indicates that the SRRC columns have good ductility.
(2) The influence of the RCA replacement percentage on the skeleton curves of the specimens is shown in Fig. 10a. The skeleton curves of all three specimens overlap before the yield load point of the column, which indicates that the initial stiffnesses of the columns were similar, regardless of the RCA replacement percentages. After the yield load point of the column, the three skeleton curves exhibit some differences in the elasto-plastic and failure phases. These plots show that the RCA replacement percentage had some influence on the bearing capacity and ductility of the specimens, but the influence is not very obvious. (3) As Fig. 10b shows, the initial stiffnesses of the columns increased as the axial compression ratios increased. After the yield load points of the columns, the three skeleton curves show obvious differences in the elasto-plastic and failure phases. The strength attenuation of the specimens with high axial compression ratios decreased rapidly, which suggests that selection of reasonable axial compression ratios is very significant in the seismic design of SRRC columns. (4) Fig. 10c shows that the skeleton curves of the three specimens exhibited obvious difference in the initial stiffnesses of the columns with increasing stirrup ratio. After the yield load points of the columns, the three skeleton curves exhibit obvious differences in the elasto-plastic and failure phases.
H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
235
Fig. 9. Hysteresis loops of specimens.
The bearing capacity of the specimen with a high stirrup ratio decreased slowly, which suggests that using a high stirrup ratio can improve the seismic performance of SRRC columns. 3.6. Stiffness degradation Fig. 11 shows the degradation of the secant stiffnesses of the seven columns versus the lateral displacements at the tops of the columns under cyclic loading. All the columns had a larger initial stiffness because of the existence of steel in the specimens. Fig. 11 shows that the stiffnesses of most of the columns decreased slowly during the initial stage of loading. When cracks appeared in the specimens, the stiffnesses decreased dramatically, and when the columns were in the yield stage, their stiffnesses decreased significantly. Beyond that point, the stiffness degeneration tended to be slow and show no obvious abrupt changes. Fig. 11a illustrates the influence of the RCA replacement percentage on the stiffness degradation of the specimens. The initial stiffnesses were almost the same for all three investigated
columns. After cracks appeared in the specimens, no obvious differences were observed in the investigated columns. Fig. 11b shows that the initial stiffnesses of the columns with high axial compression ratios were greater than those of the columns with low axial compression ratio. Unfortunately, the rate of stiffness degradation after cracking in the columns with high axial compression ratios was greater than that in columns with low axial compression ratios. Fig. 11c shows that the initial stiffnesses of columns with high stirrup ratios were greater than those with low stirrup ratios. After cracks appeared in the specimens appeared, the rate of stiffness degradation in the columns with high stirrup ratios was slower than in the columns with low stirrup ratios.
3.7. Energy dissipation capacity The energy dissipation capacity of a structure or structural component reflects its seismic energy absorption ability. The equivalent viscous damping coefficient he is used to quantify the
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H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
(a) The influence of RCA replacement percentages on the stiffness degradation
(a) The influence of RCA replacement percentage on the skeleton curves
(b) The influence of axial compression ratio on the skeleton curves
(b) The influence of axial compression ratio on the stiffness degradation
(c) The influence of stirrup ratio on the skeleton curves
(c) The influence of stirrup ratio on the stiffness degradation Fig. 11. Stiffness degradation of specimens.
Fig. 10. Skeleton curves of specimens.
seismic energy absorption ability of structures and structural components. Table 7 lists the equivalent viscous damping coefficients versus the different characteristic loading points of the specimens. The equivalent viscous damping coefficients hey, hem and heu correspond to the yield loading, the maximum loading and the ultimate loading of the columns, respectively. Fig. 12 shows the influence of the design parameters on the energy dissipation capacities of the specimens. Table 7 shows that the energy dissipation capacity of an SRRC column increases with increasing loading steps (especially in the displacement-controlled loading stage). The average values of the equivalent viscous damping coefficients hey, hem and heu of the tested specimens were 0.081, 0.155 and 0.217, respectively, which indicate that the SRRC columns had relatively good energy dissipaTable 7 The equivalent viscous damping coefficients of the specimens. Specimen no.
hey
hem
heu
SRRC11 SRRC12 SRRC13 SRRC14 SRRC15 SRRC16 SRRC17
0.090 0.080 0.079 0.096 0.068 0.069 0.085
0.161 0.159 0.158 0.183 0.124 0.144 0.159
0.225 0.217 0.212 0.260 0.177 0.193 0.232
tion capacity. Fig. 12 illustrates the influence of some of the design parameters on the equivalent viscous damping coefficients of the columns. Table 7 and Fig. 12 together show the following: (1) Compared to SRRC11, with natural coarse aggregate, the equivalent viscous damping coefficients heu of SRRC12 and SRRC13 were 3.56% and 5.78% lower, respectively. In addition, the equivalent viscous damping coefficients hey and hem decreased slightly as the RCA replacement percentage increased. These results suggest that the RCA replacement percentage had no obvious influence on the energy dissipation capacity of the columns. (2) Compared to SRRC14, with a low axial compression ratio, the equivalent viscous damping coefficients heu of SRRC13 and SRRC15 were 18.46% and 31.92% lower, respectively. In addition, the equivalent viscous damping coefficients hey and hem decrease considerably as the axial compression ratio increases. These results suggest that the axial compression ratio had an obvious influence on the energy dissipation capacity of the columns. Therefore, the axial compression ratio must be controlled strictly in seismic design. (3) Compared to SRRC17, with a high stirrup ratio, the equivalent viscous damping coefficients heu of SRRC13 and SRRC16 were 8.62% and 23.71% lower, respectively. Moreover, the
H. Ma et al. / Construction and Building Materials 48 (2013) 229–237
(a)The influence of RCA replacement percentages on the equivalent viscous damping coefficient
237
(3) The hysteresis loops, skeleton curves, stiffness degradation, and energy dissipation capacities of the columns indicate that the seismic performance of SRRC columns is comparable to that of conventional concrete. Appropriate design of the axial compression ratio and stirrup ratio can improve the seismic performance of SRRC columns. (4) The average values of the ductility factor and equivalent viscous damping coefficient of the SRRC columns were 3.47 and 0.217, respectively. These values indicate that SRRC columns are capable of resisting an earthquake and that it is feasible to employ SRRC columns in the practice of civil engineering.
Acknowledgements The work presented herein was carried out at the Xi’an Univ. of Architecture and Technology in PR China and was funded by the Chinese National Natural Science Foundation, under Grant No. 51178384. The project was also funded by Shaanxi Province as a scientific research project under Grant No. 12JK0902. The financial support provided by these agencies is gratefully acknowledged.
(b) The influence of axial compression ratio on the equivalent viscous damping coefficient
(c) The influence of stirrups ratio on the equivalent viscous damping coefficient Fig. 12. The influence of design parameters on the equivalent viscous damping coefficient.
equivalent viscous damping coefficients hey and hem increase as the stirrup ratio increases. These results suggest that increasing the stirrup ratio improves the energy dissipation capacity of SRRC columns. 4. Conclusions This paper discusses an experimental study on the seismic performance of steel-reinforced recycled concrete (SRRC) columns under low cyclic lateral loads. The results show that the following: (1) All the investigated SRRC columns behaved similarly in terms of their failure pattern under low lateral loading, regardless of differences in the values of their design parameters. The failure patterns of the columns can be classified as flexural failure. (2) The RCA replacement percentage had no obvious influence on the bearing capacity of the columns, but both the ductility and the energy dissipation capacity decreased as the replacement percentage increased. The bearing capacity of the columns increased as the axial compression ratio increased, while the ductility decreased considerably. In addition, the bearing capacity and the ductility of the columns increased as the stirrup ratio increased.
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