- Email: [email protected]
Seismic behavior of recycled aggregate concrete beams under cyclic torsion
Construction and Building Materials 129 (2016) 193–203
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Seismic behavior of recycled aggregate concrete beams under cyclic torsion Xiaohan Wang a,b,⇑, Bingkang Liu b, Cong Zhang a a b
School of Environment and Civil Engineering, Jiangnan University, Wuxi 214000, China School of Civil Engineering, Hefei University of Technology, Hefei 230000, China
h i g h l i g h t s Seismic behavior of recycled aggregate concrete (RAC) beam under cyclic torsion was presented. Cyclic torsion performance of RAC beam and natural aggregate concrete (NAC) beam were compared. Research on seismic performance of RAC members under cyclic torsion is still very limited now.
a r t i c l e
i n f o
Article history: Received 22 July 2016 Received in revised form 19 October 2016 Accepted 24 October 2016 Available online 31 October 2016 Keywords: Recycled aggregate concrete Seismic behavior Cyclic torsion Hysteresis loop Ductility
a b s t r a c t Although the material property and structural behavior of recycled aggregate concrete (RAC) have been widely investigated, the research on seismic performance of RAC members under cyclic torsion is still very limited. In this paper, the failure mode, hysteresis loop, strain of steel reinforcement, principal strain of concrete, energy dissipation capacity, skeleton curve, deterioration of stiffness and ductility factor of RAC beams and natural aggregate concrete (NAC) beams under cyclic torsion were compared and investigated. The results indicate that although both the RAC and NAC torsional beams present a torsional failure mode with spiral cracks, the cracks of RAC beams are larger than that of NAC beams. The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, but the energy dissipation capacity and ductility factor of RAC torsional beams are about 20% and 7% higher than those of NAC beams, respectively. The use of RAC in cyclic torsional beams does not bring apparent influence on the deterioration of stiffness, but the bearing capacity of RAC torsional beams is lower than that of NAC beams. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid development of economy and urbanization all over the world, many existing concrete structures need to be removed and rebuilt, thus generating a huge amount of waste concrete. In order to relieve the pressure of environmental protection, it is urgent to process these waste concrete in a reasonable way. After recycling, crushing, screening and blending, recycled aggregate can be obtained, thus producing the so-called Recycled Aggregate Concrete (RAC) [1–4]. It can not only solve the problem of environmental protection, but also meet the demand of sustainable society. Existing research of RAC mainly focuses on two points: properties of RAC materials and performances of RAC members. Compressive ⇑ Corresponding author at: School of Environment and Civil Engineering, Jiangnan University, Wuxi 214000, China. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2016.10.101 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
strength [5–7], tensile strength [8], elastic modulus [9], constitutive relationship [10–13], durability [14–17] of RAC materials and modified RAC materials [18–21] have been widely investigated. Flexural and shear performance of RAC beams [22–26], compressive performance of RAC column [27–30], seismic performance of RAC beam-column joint [31,32], RAC shear wall [33] and RAC frame structure [34] have also been reported in previous papers. Within many theoretical and experimental studies available in literature on the use of RAC for the structural members, the case of torsional loads does not seem to have been exhaustively investigated, although it frequently occurs in civil engineering. Not too much attention has previously been given to the behaviors of such members subjected to torsion moment because of the fact that, except for exceptional cases, the structural strength is not compromised by the torsional capacity. A torsional moment in building structures always arises when the resultant force acts eccentrically relative to the longitudinal axis of an element. Examples of reinforced concrete elements often loaded with a torsional moment are: lateral beams of
194
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
Fig. 1. Morphology of recycled aggregate.
control the overall structural behavior [35]. For this reason, the torsional behavior of RAC members should be studied and comprehended as well as their flexural or shear behavior. In this paper, experiments were carried out to better understand the seismic performance of RAC members under torsion loading. 2. Material and methods 2.1. Materials
Fig. 2. Grading curve for the recycled aggregate used in this study.
stairways, edge floor joists, spatial frames, spiral stairs, bridge decks subject to highly eccentric loads and reinforced concrete arches loaded perpendicularly to their plane. It is very important that the cracked torsional stiffness of a reinforced concrete beam may be much smaller than its uncracked stiffness. In some cases torsion becomes a primary effect and torsional response of beams may also
Two types of concrete mixtures have been prepared, one of which with natural aggregates (crushed limestone) only and the other one by replacing 100% coarse natural aggregate with coarse recycled concrete aggregate. Morphology and grading curve of recycled aggregate are shown in Figs. 1 and 2, respectively. Basic properties of recycled aggregate and natural aggregate are given in Table 1. Cement (PS 32.5), river sand (0–5 mm) and water are also used in this study. Mix design of concrete with or without recycled aggregate is illustrated in Table 2. The 28d compressive strength of the natural aggregate concrete (NAC) and RAC is 22.2 MPa and 28.2 MPa, respectively. Mechanical properties of steel reinforcement bars are shown in Table 3. 2.2. Design and construction of specimen Two types of concrete beams have been manufactured, two NAC beams (NAC-1 and NAC-2) and two RAC beams (RAC-1 and RAC-2),
Table 1 Basic properties of coarse aggregate used in this study. Coarse aggregate type
Size/mm
Crush index/%
Water absorption/%
Apparent density /(kg/m3)
Fine powder content/%
Natural Recycled
10–25 10–25
3.5 16
0.6 4.21
2810 2700
0.9 3.2
Table 2 Mix proportion of concrete/(kg/m3). Concrete type
Cement
Water
Sand
Crushed aggregate
W/C ratio
Natural Recycled
484 433
218 195
519 538
1103 1143
0.45 0.45
Table 3 Mechanical properties of steel reinforcement bars. Diameter/mm
Yield strength/MPa
Ultimate strength/MPa
Elongation/%
Elastic modulus/GPa
8 (stirrup) 10 (longitudinal)
298.1 455.3
385.2 541.8
30 8.5
210.9 200
195
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) Profile in plan
(b) Profile in longitudinal plan
(c) Section 1-1
(d) Section 2-2
(e) Section 3-3
Fig. 3. Dimension and steel reinforcement details of specimens.
Fig. 4. Fabrication of concrete specimens.
according to specifications in Eurocode 8. These four beams have the same dimension and reinforcement ratio, as illustrated in
Fig. 3. In order to fix the beam, an enlarged head has been made in the fixed end of the tested beam, as shown in Fig. 3a.
196
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
Fig. 5. Outline of the concrete beam under test.
Fig. 6. Diagram of imposed load control and displacement control cyclic loading test.
Fig. 4 demonstrates the construction procedure of the tested beams. Similar to the construction in real projects, the construction steps of the NAC and RAC beams include the steel binding, the casting of concrete and the curing of the specimens before loading test. 2.3. Loading program and measurement As illustrated in Fig. 5, four hydraulic jacks (500 kN each one) were employed to fix the enlarged head and four hydraulic jacks were used to provide a cyclic antisymmetrical load, which can cause a cyclic torque we need in this study. The cyclic torsion test was performed through two steps, namely a load controlled step and displacement controlled step,
as illustrated in Fig. 6. During the load controlled step, the load increment is 0.1 Py and the initial load is 0.3 Py, where Py is the calculated yield load of the tested beam. After every cyclic loading step, the strain of steel rebar and the width of concrete crack were recorded. When the bottom longitudinal reinforcement bars were yielded, the corresponding yield load and yield torsional curvature uy were obtained. Then the loading process was changed to displacement controlled stage. The displacement increment was Duy and each displacement controlled step was cycled for three times until the beam failed. As shown in Fig. 5b, two linear variable differential transducers (LVDT) were employed to monitor the angular displacement of the tested beam. Three strain gauges were used to measure the strain of longitudinal reinforcement bars (Z1, Z2 and Z3) and eight strain gauges were employed to monitor the stirrup strain (G1 and G2), as illustrated in Fig. 7a–c. A strain rosette and three clock gauges were used to measure the maximum principal strain of concrete, as shown in Fig. 7d.
3. Results and discussion 3.1. Failure modes Table 4 shows the general experimental observations during the full cyclic torsion test. It can be seen that the failure process of RAC torsional beams is very similar to that of NAC torsional beams. However, the crack width of RAC beams is larger than that of NAC beams (Figs. 8 and 9). During cyclic torsion test, the NAC beams (NAC-1 and NAC-2) present a torsional failure mode with spiral cracks, as shown in Fig. 8.
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
197
(a) Strain gauges for steel reinforcement bars
(b) Strain gauges for stirrup G1
(c) Strain gauges for stirrup G2
(d) Strain rosette and clock gauge on the side surface of concrete beam Fig. 7. Schematic diagram for the arrangement of measuring device.
Table 4 Testing process and associate phenomena. Stage of loading
Load level
Crack width/mm
Other associate phenomena
Initial crack stage
NAC beam RAC beam
0.6 Pu 0.7 Pu
0.1 0.2
45° oblique cracks, can close after unloading 45° oblique cracks, can close after unloading
Yield stage
NAC beam RAC beam
0.8 Pu 0.8 Pu
0.2–0.3 0.3–0.5
More and larger cracks, can’t close after unloading More and larger cracks, can’t close after unloading
Ultimate stage
NAC beam RAC beam
Pu Pu
2–3 3–5
Oblique through cracks Oblique through cracks, larger and faster than NAC beams
Failure stage
NAC beam RAC beam
After Pu After Pu
– –
Concrete block caving, brittle failure mode Concrete block caving, more severe than NAC beams, brittle failure mode
198
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) NAC-1
(b) NAC-2
Fig. 8. Crack distribution of NAC beams under cyclic torsion.
(a) RAC-1
(b) RAC-2
Fig. 9. Crack distribution of RAC beams under cyclic torsion.
Similar to NAC beams, the RAC beams also perform a torsion failure mode, but with larger spiral cracks, as illustrated in Fig. 9.
3.2. Hysteresis loops Hysteresis loop shows the relationship between the moment of torsion and torsional curvature in the low cyclic loading, which is an important basis for the seismic design of structures and structural members. Fig. 10 illustrates the hysteresis loops of NAC and RAC beams under cyclic torsion. It can be seen that in the early stage of loading, the moment of torsion and corresponding torsional curvature presented a linear relationship prior to cracking. The area of hysteresis loops was very small and narrow, which indicated that the torsional beams were in the elastic state. As the torsional loads increased, the slopes of the hysteresis loops of the torsional beams decreased gradually due to the cracking behavior and slip between the reinforcement and concrete of torsional beams. Residual deformation can be found after removing the torsional load, which indicated the beams were in an elasticplastic state. And after the torsional beams yielded, the hysteresis loops gradually rotated towards the horizontal axis and increased with the increasing magnitude of torsional loads. Under the same displacement load level, the peak load of the last cycle was lower than that of the early cycles, indicating that the bearing capacity and stiffness of the torsional beams decreased gradually as the number of loading cycles increased, which shows the damage accumulation in the torsional beams. With further torsional loading, the shape of hysteresis loops in yield stage changed from spindle-shape to anti-S shape. Slip of hysteresis loops can be found due to the shear caused torque, thus leading to an obvious pinch
effect. The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, which indicates that the NAC and RAC torsional beams have a nearly equivalent hysteretic performance. 3.3. Strain of steel reinforcement bars Strain of longitudinal reinforcement bars (Z3) in NAC and RAC beams under cyclic torsion are shown in Fig. 11. It can be obtained that the strain of longitudinal reinforcement bars was very low before cracking of concrete. Before the torsional beams yielded, the longitudinal reinforcement strain of NAC and RAC beams were at the same level. However, this strain of RAC beams became larger obviously than that of NAC beams after the torsional beams yielded. Fig. 12 shows the strain of stirrup reinforcement bars in NAC and RAC beams under cyclic torsion. It can be seen that the stirrup strain-torsional moment curves performed a butterfly-shape due to the cyclic torsion loading. Compared to NAC beams, the stirrup strain of RAC beams is larger, indicating a more serious shear cracking. 3.4. Principal strain of concrete Principal strain direction angle of NAC and RAC beams under cyclic torsion is shown in Fig. 13. It can be found that the principal strain (stress) direction angle of concrete is nearly 45° (compared to the longitudinal axis of torsional beam), indicating the concrete is under a pure shear state and the crack direction angle is also nearly 45°. After cracking, the principal strain (stress) direction
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) NAC-1
(c) RAC-1
(b) NAC-2
(d) RAC-2
Fig. 10. Hysteresis loops of NAC and RAC beams under cyclic torsion.
(a) NAC beam
(b) RAC beam
Fig. 11. Strain of longitudinal reinforcement bars (Z3) in NAC and RAC beams under cyclic torsion.
199
200
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) Strain of G2-D in NAC beam
(c) Strain of G2-D in RAC beam
(b) Strain of G2-L in NAC beam
(d) Strain of G2-L in RAC beam
Fig. 12. Strain of stirrup reinforcement bars in NAC and RAC beams under cyclic torsion.
(a) NAC beam
(b) RAC beam
Fig. 13. Principal strain direction angle of NAC and RAC beams under cyclic torsion.
201
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) NAC beam
(b) RAC beam
Fig. 14. Principal strain of NAC and RAC beams under cyclic torsion.
Table 5 Equivalent viscous damping coefficient in each cyclic loading. Cycle time
6 7 8 9 10 11 12 13 14
R
Fig. 15. Definition of energy consumption.
3.5. Energy dissipation capacity The energy dissipation capacity of torsional beam can be measured based on the area surrounded by the hysteresis loop, which also can indicate its seismic performance. Larger surrounded area means better energy dissipation capacity and seismic behavior. Usually, the equivalent viscous damping coefficient he is employed to quantify the energy dissipation capacity of structural members
RAC beam NAC-2
RAC-1
RAC-2
0.038 0.093 0.110 0.073 0.058 0.171 0.086 0.084 0.080 0.793
0.052 0.129 0.091 0.087 0.045 0.024 0.109 0.058 0.064 0.659
0.051 0.126 0.145 0.135 0.084 0.058 0.172 0.093 0.088 0.952
0.037 0.090 0.159 0.130 0.079 0.053 0.170 0.091 0.094 0.902
and structures, as illustrated in Eq. (1) and Fig. 15. The higher the he value is, the better the beam’s energy dissipation capacity is.
he ¼ angle of concrete shows an irregular change due to the cracking behavior of concrete. Fig. 14 shows the principal tensile strain and principal compressive strain of NAC and RAC beams under cyclic torsion. It can be seen that the principal strain is symmetrical before the cracking of concrete, which indicates that the principal stress direction angle is nearly 45° before cracking. After cracking of concrete, although the increase of principal strain (tensile and compressive) is irregular, the principal compressive strain is larger than principal tensile strain and the principal strain direction angle is always smaller than 45°. The development of principal compressive strain in RAC beam is very similar to that of NAC beam. However, the principal tensile strain of RAC beam is larger than that of NAC beam.
NAC beam NAC-1
1 SðABCþCDAÞ 2p SðOBEþODFÞ
ð1Þ
where S(ABC+CDA) is the areas of regions ABC and CDA, indicating the energy dissipation of torsional beam during one cyclic loading; S(OBE+ODF) is the areas of regions OBE and ODF, indicating the energy dissipation of torsional beam at elastic stage. The equivalent viscous damping coefficient of NAC and RAC beams in each cyclic loading are shown in Table 5. It can be observed that the value of he increased with cycle time before peak load. After the beam yielded, the value of he increased much more quickly. The energy dissipation capacity of RAC torsional beam is about 20% higher than that of NAC beam. 3.6. Skeleton curves The skeleton curves of NAC and RAC beams under cyclic torsion can be obtained by connecting the peak points of the hysteresis loops, which can reflect the mutual relationship between the peak torsional moment and the corresponding torsional curvature in each loading stage. The skeleton curves of NAC and RAC beams under cyclic torsion are shown in Fig. 16. It can be observed that the skeleton curves of all beams are very similar to each other. The skeleton curves are almost linear before cracking, suggesting that the beams are in the elastic stage. After cracking, the skeleton curves became nonlinear, indicating that the stiffness of the beams
202
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203 Table 7 Ductility factor of NAC and RAC beams under cyclic torsion. Beams
Loading direction
hy /(deg/m)
hf/(deg/m)
Ductility factor l
NAC-1
Positive Reverse Positive Reverse Positive Reverse Positive Reverse
0.009 0.006 0.007 0.006 0.005 0.007 0.005 0.007
0.030 0.022 0.025 0.024 0.017 0.027 0.019 0.030
3.333 3.667 3.571 4.000 3.400 3.857 3.800 4.286
NAC-2 RAC-1 RAC-2
3.7. Deterioration of stiffness After yielding, the stiffness deteriorates gradually due to the damage accumulation in torsional beams. In this study, the secant stiffness Gj was employed to evaluate the deterioration of stiffness, as illustrated in Eq. (2).
Fig. 16. Skeleton curves of NAC and RAC beams under cyclic torsion.
Pn jT ij j Gj ¼ Pi¼1 n i¼1 jhij j
ð2Þ
where Gj is the stiffness of torsional beam at the j level of cyclic loading; Tij is the torsional moment of the i times cycle at the j level of cyclic loading; hij is the torsional curvature of the i times cycle at the j level of cyclic loading. Herein, the ratio of Gj/G1 was used to illustrate the deterioration of stiffness, as shown in Fig. 17. It can be observed that the stiffness of torsional beams decreases along with the increase of torsional curvature. The stiffness of torsional beams deteriorates seriously when the concrete is cracking and when the beams are yielded. Compared to NAC beams, the stiffness deterioration of RAC beams is very similar. It seems that the use of recycle aggregate concrete in cyclic torsional beams does not bring apparent influence on the deterioration of stiffness. 3.8. Ductility factors Fig. 17. Deterioration of stiffness for NAC and RAC beams under cyclic torsion.
declines and the beams are in the elastic-plastic stage. The skeleton curves present a softening behavior after the peak load, which shows the deterioration of the bearing capacity and stiffness of NAC and RAC beams. From Fig. 16, it can be found by comparing the skeleton curves of NAC and RAC beams that there is no obvious difference in skeleton curves and initial stiffness before yielding. After the beams yielded, the skeleton curve slop of RAC beams is a little higher than that of NAC beams. However, after the peak point, the load of RAC beams decreases much faster than that of NAC beams, suggesting that the bearing capacity and ductility of RAC beams are worse than that NAC beams.
Table 6 lists the characteristic torsional moment T and corresponding torsional curvature h of NAC and RAC torsional beams. It can be seen that the cracking torsional moment and cracking torsional curvature of RAC beam are about 17% and 20% lower than that of NAC beam, respectively. However, the yield torsion moment, ultimate torsional moment, failure torsional moment and their corresponding torsional curvature of RAC and NAC beams are approximately the same. The ductility factor l was employed to evaluate the ductility of NAC and RAC beams under cyclic torsion. The larger the ductility factor is, the better plastic deformation capacity and seismic behavior of torsional beams are. The definition of ductility factor is illustrated in Eq. (3).
Table 6 Characteristic torsional moment and corresponding torsional curvature of NAC and RAC beams. Beams
Loading direction
Initial cracking stage
Yield stage
Ultimate stage
Failure stage
Tcr/kNm
hcr/(deg/m)
Ty/kNm
hy/(deg/m)
Tu/kNm
hu/(deg/m)
Tf/kNm
hf/(deg/m)
NAC-1
Positive Reverse
6.440 6.370
0.001 0.001
8.270 8.128
0.009 0.006
10.860 10.210
0.021 0.021
9.231 8.679
0.030 0.022
NAC-2
Positive Reverse
7.000 6.930
0.001 0.002
8.684 8.253
0.007 0.006
11.060 10.010
0.018 0.020
9.401 8.509
0.025 0.024
RAC-1
Positive Reverse
5.600 5.460
0.001 0.001
8.854 8.101
0.005 0.007
10.360 10.610
0.016 0.024
8.806 9.019
0.017 0.027
RAC-2
Positive Reverse
5.530 5.530
0.001 0.002
8.043 8.360
0.005 0.007
10.500 10.710
0.015 0.024
8.925 9.104
0.019 0.030
Note: Tf is the failure torsional moment, which equals 0.85 Tu.
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
l ¼ hf =hy
ð3Þ
where hf is the failure torsional curvature and hy is the yield torsional curvature. Ductility factor of NAC and RAC beams under cyclic torsion is summarized in Table 7. It can be observed that the ductility factor of RAC and NAC torsional beam is 3.400–4.286 and 3.333–4.000, respectively. The ductility factor of RAC torsional beam is nearly 5.3% higher than that of NAC torsional beam. 4. Conclusions In this paper, the failure mode, hysteresis loop, strain of steel reinforcement, principal strain of concrete, energy dissipation capacity, skeleton curve, deterioration of stiffness and ductility factor of RAC beams and natural aggregate concrete (NAC) beams under cyclic torsion were compared and investigated. The following conclusions can be drawn. (1) The failure process of RAC torsional beams is very similar to that of NAC torsional beams with spiral cracks. However, the crack width of RAC beams is larger than that of NAC beams. (2) The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, which indicates that the NAC and RAC torsional beams have a nearly equivalent hysteretic performance. (3) Before the torsional beams yielded, the longitudinal reinforcement strain of NAC and RAC beams were at the same level. However, this strain of RAC beams became larger obviously than that of NAC beams after the torsional beams yielded. Compared to NAC beams, the stirrup strain of RAC beams is larger, indicating a more serious shear cracking. (4) The development of principal compressive strain in RAC beam is very similar to that of NAC beam. However, the principal tensile strain of RAC beam is larger than that of NAC beam. (5) The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, but the ductility factor of RAC torsional beams are about 5.3% higher than those of NAC beams, respectively. (6) The use RAC in cyclic torsional beams does not bring apparent influence on the deterioration of stiffness, but the bearing capacity of RAC torsional beams is lower than that of NAC beams.
Acknowledgements The authors acknowledge the National Natural Science Foundation of China (Grant: 50908102), the National Natural Science Foundation of China (Grant: 51378240), Science and Technology Project of Housing and Urban-rural Development of China (Grant: 2013-K4-46). References [1] D. Soares, J. De Brito, J. Ferreira, Use of coarse recycled aggregates from precast concrete rejects: mechanical and durability performance, Constr. Build. Mater. 71 (2014) 263–272. [2] J. Xiao, W. Li, Y. Fan, An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (6) (2012) 364–383. [3] D. Matias, J. De Brito, A. Rosa, et al., Mechanical properties of concrete produced with recycled coarse aggregates – influence of the use of superplasticizers, Constr. Build. Mater. 44 (2013) 101–109.
203
[4] Q. Liu, J.Z. Xiao, Z.H. Sun, Experimental study on the failure mechanism of recycled concrete, Cem. Concr. Res. 41 (10) (2011) 1050–1057. [5] T.C. Hansen, H. Narud, Strength of recycled concrete made from crushed concrete coarse aggregate, Concr. Int. 5 (1) (1983) 79–83. [6] M. Tavakoli, P. Soroushian, Strengths of recycled aggregate concrete made using field-demolished concrete as aggregate, ACI Mater. J. 93 (2) (1996) 182– 190. [7] R.S. Ravindrarajah, Y.J. Loo, C.T. Tam, Strength evaluation of recycled aggregate concrete by in-situ tests, Mater. Struct. 21 (1998) 289–295. [8] J.Z. Xiao, Y. Lan, Investigation on the tensile behavior of recycled aggregate concrete, J. Build. Mater. 8 (2) (2006) 154–162. [9] Q. Hu, C. Song, C.Y. Zou, Experimental research on the mechanical properties of recycled concrete, J. Harbin Inst. Technol. 41 (4) (2009) 33–39. [10] G.F. Belén, M.A. Fernando, C.L. Diego, Stress-strain relationship in axial compression for concrete using recycled saturated coarse aggregate, Constr. Build. Mater. 25 (51) (2011) 2335–2342. [11] J.Z. Xiao, Experimental investigation on complete stress-strain curve of recycled concrete under uniaxial loading, J. Tongji Univ. 35 (11) (2007) 1445–1454. [12] T. Du, W.H. Wang, Z.X. Liu, The complete stress-strain curve of recycled aggregate concrete under uniaxial compression loading, J. Wuhan Univ. Technol. 25 (5) (2010) 862–867. [13] T. Hao, Z.H. Du, L.X. Liu, Study on complete stress-strain curves of recycled concrete, in: 2nd International Conference on Waste Engineering and Management, ICWEM 2010, RILEM Proceedings 2010, vol. 73, pp. 506–518. [14] F.T. Olorunsogo, N. Padayachee, Performance of recycled aggregate concrete monitored by durability indexes, Cem. Concr. Res. 32 (2002) 179–185. [15] V.W.Y. Tam, X.F. Gao, C.M. Tam, Microstructural analysis of recycled concrete produced from two-stage mixing approach, Cem. Concr. Res. 35 (6) (2005) 1195–1203. [16] A. Gokce, S. Nagataki, T. Saeki, Freezing and thawing resistance of airentrained concrete incorporating recycled coarse aggregate: the role of air content in demolished concrete, Cem. Concr. Res. 34 (5) (2004) 799–806. [17] J.S. Ryu, An experimental study on the effect of recycled aggregate on concrete properties, Mag. Concr. Res. 34 (11) (2003) 1975–1980. [18] M. Surya, P. Lakshmy, V.V.L. Kanta Rao, Study on some engineering properties of recycled aggregate concrete with fly ash, Adv. Struct. Eng. (2015) 1773– 1783. [19] M.A. Ali, T. Murat, R. Kambiz, Mechanical and durability performance of concrete incorporating fine recycled concrete and glass aggregates, Mater. Struct. 48 (2015) 2629–2640. [20] R.A. Kutalmıs, C. Özgür, I. Metin, Properties of polypropylene fiber reinforced concrete using recycled aggregates, Constr. Build. Mater. 98 (2015) 620–630. [21] S.C. Kou, C. Spoon, Compressive strength, pore size distribution and chlorideion penetration of recycled aggregate concrete incorporating class-F fly ash, J. Wuhan Univ. Technol. 21 (4) (2006) 130–136. [22] K. Ishill, Flexible characteristic of RC beam with recycled coarse aggregate, in: Proceeding of the 25th JSCE Annual Meeting, Kanto Branch, 1998, pp. 886–887. [23] G.F. Belen, M.A. Fernando, Shear strength of recycled concrete beams, Constr. Build. Mater. 1 (2006) 1–7. [24] M. Arezoumandi, A. Smith, An experimental study on shear strength of reinforced concrete beams with 100% recycled concrete aggregate, Constr. Build. Mater. 53 (2014) 612–620. [25] W.C. Choi, H.D. Yun, Long-term deflection and flexural behavior of reinforced concrete beams with recycled aggregate, Mater. Des. 51 (2013) 742–750. [26] G.F. Belén, M.A. Fernando, M.L. Isabel, Structural shear behaviour of recycled concrete with silica fume, Constr. Build. Mater. 23 (2009) 3406–3410. [27] W.C. Choia, H.D. Yunb, Compressive behavior of reinforced concrete columns with recycled aggregate under uniaxial loading, Eng. Struct. 41 (2012) 285– 293. [28] Y.F. Yang, L.H. Han, Experimental behavior of recycled aggregate concrete filled steel tubular columns, J. Constr. Steel Res. 62 (2006) 1310–1324. [29] Y.F. Yang, G.L. Ma, Experimental behaviour of recycled aggregate concrete filled stainless steel tube stub columns and beams, Thin Walled Struct. 66 (2013) 62–75. [30] J.Z. Xiao, Y.J. Huang, Zh. Sun, Seismic behavior of recycled aggregate concrete filled steel and glass fiber reinforced plastic tube columns, Adv. Struct. Eng. 17 (5) (2014) 693–707. [31] C. Valeria, L. Viviana, M. Giacomo, Behaviour of beam–column joints made of recycled-aggregate concrete under cyclic loading, Constr. Build. Mater. 25 (2011) 1877–1882. [32] C. Viviana, G. Letelier, M. Giacomo, The influence of recycled concrete aggregates on the behavior of beam–column joints under cyclic loading, Eng. Struct. 60 (2014) 148–154. [33] J.W. Zhang, W.L. Cao, S.B. Meng, Shaking table experimental study of recycled concrete frame-shear wall structures, Earthq. Eng. Eng. Vib. 13 (2) (2014) 257– 267. [34] J.Z. Xiao, Y.D. Sun, H. Falkner, Seismic performance of frame structures with recycled aggregate concrete, Eng. Struct. 28 (1) (2006) 1–8. [35] F. Okay, S. Engin, Torsional behavior of steel fiber reinforced concrete beams, Constr. Build. Mater. 28 (2012) 269–275.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Seismic behavior of recycled aggregate concrete beams under cyclic torsion Xiaohan Wang a,b,⇑, Bingkang Liu b, Cong Zhang a a b
School of Environment and Civil Engineering, Jiangnan University, Wuxi 214000, China School of Civil Engineering, Hefei University of Technology, Hefei 230000, China
h i g h l i g h t s Seismic behavior of recycled aggregate concrete (RAC) beam under cyclic torsion was presented. Cyclic torsion performance of RAC beam and natural aggregate concrete (NAC) beam were compared. Research on seismic performance of RAC members under cyclic torsion is still very limited now.
a r t i c l e
i n f o
Article history: Received 22 July 2016 Received in revised form 19 October 2016 Accepted 24 October 2016 Available online 31 October 2016 Keywords: Recycled aggregate concrete Seismic behavior Cyclic torsion Hysteresis loop Ductility
a b s t r a c t Although the material property and structural behavior of recycled aggregate concrete (RAC) have been widely investigated, the research on seismic performance of RAC members under cyclic torsion is still very limited. In this paper, the failure mode, hysteresis loop, strain of steel reinforcement, principal strain of concrete, energy dissipation capacity, skeleton curve, deterioration of stiffness and ductility factor of RAC beams and natural aggregate concrete (NAC) beams under cyclic torsion were compared and investigated. The results indicate that although both the RAC and NAC torsional beams present a torsional failure mode with spiral cracks, the cracks of RAC beams are larger than that of NAC beams. The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, but the energy dissipation capacity and ductility factor of RAC torsional beams are about 20% and 7% higher than those of NAC beams, respectively. The use of RAC in cyclic torsional beams does not bring apparent influence on the deterioration of stiffness, but the bearing capacity of RAC torsional beams is lower than that of NAC beams. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid development of economy and urbanization all over the world, many existing concrete structures need to be removed and rebuilt, thus generating a huge amount of waste concrete. In order to relieve the pressure of environmental protection, it is urgent to process these waste concrete in a reasonable way. After recycling, crushing, screening and blending, recycled aggregate can be obtained, thus producing the so-called Recycled Aggregate Concrete (RAC) [1–4]. It can not only solve the problem of environmental protection, but also meet the demand of sustainable society. Existing research of RAC mainly focuses on two points: properties of RAC materials and performances of RAC members. Compressive ⇑ Corresponding author at: School of Environment and Civil Engineering, Jiangnan University, Wuxi 214000, China. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2016.10.101 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
strength [5–7], tensile strength [8], elastic modulus [9], constitutive relationship [10–13], durability [14–17] of RAC materials and modified RAC materials [18–21] have been widely investigated. Flexural and shear performance of RAC beams [22–26], compressive performance of RAC column [27–30], seismic performance of RAC beam-column joint [31,32], RAC shear wall [33] and RAC frame structure [34] have also been reported in previous papers. Within many theoretical and experimental studies available in literature on the use of RAC for the structural members, the case of torsional loads does not seem to have been exhaustively investigated, although it frequently occurs in civil engineering. Not too much attention has previously been given to the behaviors of such members subjected to torsion moment because of the fact that, except for exceptional cases, the structural strength is not compromised by the torsional capacity. A torsional moment in building structures always arises when the resultant force acts eccentrically relative to the longitudinal axis of an element. Examples of reinforced concrete elements often loaded with a torsional moment are: lateral beams of
194
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
Fig. 1. Morphology of recycled aggregate.
control the overall structural behavior [35]. For this reason, the torsional behavior of RAC members should be studied and comprehended as well as their flexural or shear behavior. In this paper, experiments were carried out to better understand the seismic performance of RAC members under torsion loading. 2. Material and methods 2.1. Materials
Fig. 2. Grading curve for the recycled aggregate used in this study.
stairways, edge floor joists, spatial frames, spiral stairs, bridge decks subject to highly eccentric loads and reinforced concrete arches loaded perpendicularly to their plane. It is very important that the cracked torsional stiffness of a reinforced concrete beam may be much smaller than its uncracked stiffness. In some cases torsion becomes a primary effect and torsional response of beams may also
Two types of concrete mixtures have been prepared, one of which with natural aggregates (crushed limestone) only and the other one by replacing 100% coarse natural aggregate with coarse recycled concrete aggregate. Morphology and grading curve of recycled aggregate are shown in Figs. 1 and 2, respectively. Basic properties of recycled aggregate and natural aggregate are given in Table 1. Cement (PS 32.5), river sand (0–5 mm) and water are also used in this study. Mix design of concrete with or without recycled aggregate is illustrated in Table 2. The 28d compressive strength of the natural aggregate concrete (NAC) and RAC is 22.2 MPa and 28.2 MPa, respectively. Mechanical properties of steel reinforcement bars are shown in Table 3. 2.2. Design and construction of specimen Two types of concrete beams have been manufactured, two NAC beams (NAC-1 and NAC-2) and two RAC beams (RAC-1 and RAC-2),
Table 1 Basic properties of coarse aggregate used in this study. Coarse aggregate type
Size/mm
Crush index/%
Water absorption/%
Apparent density /(kg/m3)
Fine powder content/%
Natural Recycled
10–25 10–25
3.5 16
0.6 4.21
2810 2700
0.9 3.2
Table 2 Mix proportion of concrete/(kg/m3). Concrete type
Cement
Water
Sand
Crushed aggregate
W/C ratio
Natural Recycled
484 433
218 195
519 538
1103 1143
0.45 0.45
Table 3 Mechanical properties of steel reinforcement bars. Diameter/mm
Yield strength/MPa
Ultimate strength/MPa
Elongation/%
Elastic modulus/GPa
8 (stirrup) 10 (longitudinal)
298.1 455.3
385.2 541.8
30 8.5
210.9 200
195
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) Profile in plan
(b) Profile in longitudinal plan
(c) Section 1-1
(d) Section 2-2
(e) Section 3-3
Fig. 3. Dimension and steel reinforcement details of specimens.
Fig. 4. Fabrication of concrete specimens.
according to specifications in Eurocode 8. These four beams have the same dimension and reinforcement ratio, as illustrated in
Fig. 3. In order to fix the beam, an enlarged head has been made in the fixed end of the tested beam, as shown in Fig. 3a.
196
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
Fig. 5. Outline of the concrete beam under test.
Fig. 6. Diagram of imposed load control and displacement control cyclic loading test.
Fig. 4 demonstrates the construction procedure of the tested beams. Similar to the construction in real projects, the construction steps of the NAC and RAC beams include the steel binding, the casting of concrete and the curing of the specimens before loading test. 2.3. Loading program and measurement As illustrated in Fig. 5, four hydraulic jacks (500 kN each one) were employed to fix the enlarged head and four hydraulic jacks were used to provide a cyclic antisymmetrical load, which can cause a cyclic torque we need in this study. The cyclic torsion test was performed through two steps, namely a load controlled step and displacement controlled step,
as illustrated in Fig. 6. During the load controlled step, the load increment is 0.1 Py and the initial load is 0.3 Py, where Py is the calculated yield load of the tested beam. After every cyclic loading step, the strain of steel rebar and the width of concrete crack were recorded. When the bottom longitudinal reinforcement bars were yielded, the corresponding yield load and yield torsional curvature uy were obtained. Then the loading process was changed to displacement controlled stage. The displacement increment was Duy and each displacement controlled step was cycled for three times until the beam failed. As shown in Fig. 5b, two linear variable differential transducers (LVDT) were employed to monitor the angular displacement of the tested beam. Three strain gauges were used to measure the strain of longitudinal reinforcement bars (Z1, Z2 and Z3) and eight strain gauges were employed to monitor the stirrup strain (G1 and G2), as illustrated in Fig. 7a–c. A strain rosette and three clock gauges were used to measure the maximum principal strain of concrete, as shown in Fig. 7d.
3. Results and discussion 3.1. Failure modes Table 4 shows the general experimental observations during the full cyclic torsion test. It can be seen that the failure process of RAC torsional beams is very similar to that of NAC torsional beams. However, the crack width of RAC beams is larger than that of NAC beams (Figs. 8 and 9). During cyclic torsion test, the NAC beams (NAC-1 and NAC-2) present a torsional failure mode with spiral cracks, as shown in Fig. 8.
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
197
(a) Strain gauges for steel reinforcement bars
(b) Strain gauges for stirrup G1
(c) Strain gauges for stirrup G2
(d) Strain rosette and clock gauge on the side surface of concrete beam Fig. 7. Schematic diagram for the arrangement of measuring device.
Table 4 Testing process and associate phenomena. Stage of loading
Load level
Crack width/mm
Other associate phenomena
Initial crack stage
NAC beam RAC beam
0.6 Pu 0.7 Pu
0.1 0.2
45° oblique cracks, can close after unloading 45° oblique cracks, can close after unloading
Yield stage
NAC beam RAC beam
0.8 Pu 0.8 Pu
0.2–0.3 0.3–0.5
More and larger cracks, can’t close after unloading More and larger cracks, can’t close after unloading
Ultimate stage
NAC beam RAC beam
Pu Pu
2–3 3–5
Oblique through cracks Oblique through cracks, larger and faster than NAC beams
Failure stage
NAC beam RAC beam
After Pu After Pu
– –
Concrete block caving, brittle failure mode Concrete block caving, more severe than NAC beams, brittle failure mode
198
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) NAC-1
(b) NAC-2
Fig. 8. Crack distribution of NAC beams under cyclic torsion.
(a) RAC-1
(b) RAC-2
Fig. 9. Crack distribution of RAC beams under cyclic torsion.
Similar to NAC beams, the RAC beams also perform a torsion failure mode, but with larger spiral cracks, as illustrated in Fig. 9.
3.2. Hysteresis loops Hysteresis loop shows the relationship between the moment of torsion and torsional curvature in the low cyclic loading, which is an important basis for the seismic design of structures and structural members. Fig. 10 illustrates the hysteresis loops of NAC and RAC beams under cyclic torsion. It can be seen that in the early stage of loading, the moment of torsion and corresponding torsional curvature presented a linear relationship prior to cracking. The area of hysteresis loops was very small and narrow, which indicated that the torsional beams were in the elastic state. As the torsional loads increased, the slopes of the hysteresis loops of the torsional beams decreased gradually due to the cracking behavior and slip between the reinforcement and concrete of torsional beams. Residual deformation can be found after removing the torsional load, which indicated the beams were in an elasticplastic state. And after the torsional beams yielded, the hysteresis loops gradually rotated towards the horizontal axis and increased with the increasing magnitude of torsional loads. Under the same displacement load level, the peak load of the last cycle was lower than that of the early cycles, indicating that the bearing capacity and stiffness of the torsional beams decreased gradually as the number of loading cycles increased, which shows the damage accumulation in the torsional beams. With further torsional loading, the shape of hysteresis loops in yield stage changed from spindle-shape to anti-S shape. Slip of hysteresis loops can be found due to the shear caused torque, thus leading to an obvious pinch
effect. The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, which indicates that the NAC and RAC torsional beams have a nearly equivalent hysteretic performance. 3.3. Strain of steel reinforcement bars Strain of longitudinal reinforcement bars (Z3) in NAC and RAC beams under cyclic torsion are shown in Fig. 11. It can be obtained that the strain of longitudinal reinforcement bars was very low before cracking of concrete. Before the torsional beams yielded, the longitudinal reinforcement strain of NAC and RAC beams were at the same level. However, this strain of RAC beams became larger obviously than that of NAC beams after the torsional beams yielded. Fig. 12 shows the strain of stirrup reinforcement bars in NAC and RAC beams under cyclic torsion. It can be seen that the stirrup strain-torsional moment curves performed a butterfly-shape due to the cyclic torsion loading. Compared to NAC beams, the stirrup strain of RAC beams is larger, indicating a more serious shear cracking. 3.4. Principal strain of concrete Principal strain direction angle of NAC and RAC beams under cyclic torsion is shown in Fig. 13. It can be found that the principal strain (stress) direction angle of concrete is nearly 45° (compared to the longitudinal axis of torsional beam), indicating the concrete is under a pure shear state and the crack direction angle is also nearly 45°. After cracking, the principal strain (stress) direction
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) NAC-1
(c) RAC-1
(b) NAC-2
(d) RAC-2
Fig. 10. Hysteresis loops of NAC and RAC beams under cyclic torsion.
(a) NAC beam
(b) RAC beam
Fig. 11. Strain of longitudinal reinforcement bars (Z3) in NAC and RAC beams under cyclic torsion.
199
200
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) Strain of G2-D in NAC beam
(c) Strain of G2-D in RAC beam
(b) Strain of G2-L in NAC beam
(d) Strain of G2-L in RAC beam
Fig. 12. Strain of stirrup reinforcement bars in NAC and RAC beams under cyclic torsion.
(a) NAC beam
(b) RAC beam
Fig. 13. Principal strain direction angle of NAC and RAC beams under cyclic torsion.
201
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
(a) NAC beam
(b) RAC beam
Fig. 14. Principal strain of NAC and RAC beams under cyclic torsion.
Table 5 Equivalent viscous damping coefficient in each cyclic loading. Cycle time
6 7 8 9 10 11 12 13 14
R
Fig. 15. Definition of energy consumption.
3.5. Energy dissipation capacity The energy dissipation capacity of torsional beam can be measured based on the area surrounded by the hysteresis loop, which also can indicate its seismic performance. Larger surrounded area means better energy dissipation capacity and seismic behavior. Usually, the equivalent viscous damping coefficient he is employed to quantify the energy dissipation capacity of structural members
RAC beam NAC-2
RAC-1
RAC-2
0.038 0.093 0.110 0.073 0.058 0.171 0.086 0.084 0.080 0.793
0.052 0.129 0.091 0.087 0.045 0.024 0.109 0.058 0.064 0.659
0.051 0.126 0.145 0.135 0.084 0.058 0.172 0.093 0.088 0.952
0.037 0.090 0.159 0.130 0.079 0.053 0.170 0.091 0.094 0.902
and structures, as illustrated in Eq. (1) and Fig. 15. The higher the he value is, the better the beam’s energy dissipation capacity is.
he ¼ angle of concrete shows an irregular change due to the cracking behavior of concrete. Fig. 14 shows the principal tensile strain and principal compressive strain of NAC and RAC beams under cyclic torsion. It can be seen that the principal strain is symmetrical before the cracking of concrete, which indicates that the principal stress direction angle is nearly 45° before cracking. After cracking of concrete, although the increase of principal strain (tensile and compressive) is irregular, the principal compressive strain is larger than principal tensile strain and the principal strain direction angle is always smaller than 45°. The development of principal compressive strain in RAC beam is very similar to that of NAC beam. However, the principal tensile strain of RAC beam is larger than that of NAC beam.
NAC beam NAC-1
1 SðABCþCDAÞ 2p SðOBEþODFÞ
ð1Þ
where S(ABC+CDA) is the areas of regions ABC and CDA, indicating the energy dissipation of torsional beam during one cyclic loading; S(OBE+ODF) is the areas of regions OBE and ODF, indicating the energy dissipation of torsional beam at elastic stage. The equivalent viscous damping coefficient of NAC and RAC beams in each cyclic loading are shown in Table 5. It can be observed that the value of he increased with cycle time before peak load. After the beam yielded, the value of he increased much more quickly. The energy dissipation capacity of RAC torsional beam is about 20% higher than that of NAC beam. 3.6. Skeleton curves The skeleton curves of NAC and RAC beams under cyclic torsion can be obtained by connecting the peak points of the hysteresis loops, which can reflect the mutual relationship between the peak torsional moment and the corresponding torsional curvature in each loading stage. The skeleton curves of NAC and RAC beams under cyclic torsion are shown in Fig. 16. It can be observed that the skeleton curves of all beams are very similar to each other. The skeleton curves are almost linear before cracking, suggesting that the beams are in the elastic stage. After cracking, the skeleton curves became nonlinear, indicating that the stiffness of the beams
202
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203 Table 7 Ductility factor of NAC and RAC beams under cyclic torsion. Beams
Loading direction
hy /(deg/m)
hf/(deg/m)
Ductility factor l
NAC-1
Positive Reverse Positive Reverse Positive Reverse Positive Reverse
0.009 0.006 0.007 0.006 0.005 0.007 0.005 0.007
0.030 0.022 0.025 0.024 0.017 0.027 0.019 0.030
3.333 3.667 3.571 4.000 3.400 3.857 3.800 4.286
NAC-2 RAC-1 RAC-2
3.7. Deterioration of stiffness After yielding, the stiffness deteriorates gradually due to the damage accumulation in torsional beams. In this study, the secant stiffness Gj was employed to evaluate the deterioration of stiffness, as illustrated in Eq. (2).
Fig. 16. Skeleton curves of NAC and RAC beams under cyclic torsion.
Pn jT ij j Gj ¼ Pi¼1 n i¼1 jhij j
ð2Þ
where Gj is the stiffness of torsional beam at the j level of cyclic loading; Tij is the torsional moment of the i times cycle at the j level of cyclic loading; hij is the torsional curvature of the i times cycle at the j level of cyclic loading. Herein, the ratio of Gj/G1 was used to illustrate the deterioration of stiffness, as shown in Fig. 17. It can be observed that the stiffness of torsional beams decreases along with the increase of torsional curvature. The stiffness of torsional beams deteriorates seriously when the concrete is cracking and when the beams are yielded. Compared to NAC beams, the stiffness deterioration of RAC beams is very similar. It seems that the use of recycle aggregate concrete in cyclic torsional beams does not bring apparent influence on the deterioration of stiffness. 3.8. Ductility factors Fig. 17. Deterioration of stiffness for NAC and RAC beams under cyclic torsion.
declines and the beams are in the elastic-plastic stage. The skeleton curves present a softening behavior after the peak load, which shows the deterioration of the bearing capacity and stiffness of NAC and RAC beams. From Fig. 16, it can be found by comparing the skeleton curves of NAC and RAC beams that there is no obvious difference in skeleton curves and initial stiffness before yielding. After the beams yielded, the skeleton curve slop of RAC beams is a little higher than that of NAC beams. However, after the peak point, the load of RAC beams decreases much faster than that of NAC beams, suggesting that the bearing capacity and ductility of RAC beams are worse than that NAC beams.
Table 6 lists the characteristic torsional moment T and corresponding torsional curvature h of NAC and RAC torsional beams. It can be seen that the cracking torsional moment and cracking torsional curvature of RAC beam are about 17% and 20% lower than that of NAC beam, respectively. However, the yield torsion moment, ultimate torsional moment, failure torsional moment and their corresponding torsional curvature of RAC and NAC beams are approximately the same. The ductility factor l was employed to evaluate the ductility of NAC and RAC beams under cyclic torsion. The larger the ductility factor is, the better plastic deformation capacity and seismic behavior of torsional beams are. The definition of ductility factor is illustrated in Eq. (3).
Table 6 Characteristic torsional moment and corresponding torsional curvature of NAC and RAC beams. Beams
Loading direction
Initial cracking stage
Yield stage
Ultimate stage
Failure stage
Tcr/kNm
hcr/(deg/m)
Ty/kNm
hy/(deg/m)
Tu/kNm
hu/(deg/m)
Tf/kNm
hf/(deg/m)
NAC-1
Positive Reverse
6.440 6.370
0.001 0.001
8.270 8.128
0.009 0.006
10.860 10.210
0.021 0.021
9.231 8.679
0.030 0.022
NAC-2
Positive Reverse
7.000 6.930
0.001 0.002
8.684 8.253
0.007 0.006
11.060 10.010
0.018 0.020
9.401 8.509
0.025 0.024
RAC-1
Positive Reverse
5.600 5.460
0.001 0.001
8.854 8.101
0.005 0.007
10.360 10.610
0.016 0.024
8.806 9.019
0.017 0.027
RAC-2
Positive Reverse
5.530 5.530
0.001 0.002
8.043 8.360
0.005 0.007
10.500 10.710
0.015 0.024
8.925 9.104
0.019 0.030
Note: Tf is the failure torsional moment, which equals 0.85 Tu.
X. Wang et al. / Construction and Building Materials 129 (2016) 193–203
l ¼ hf =hy
ð3Þ
where hf is the failure torsional curvature and hy is the yield torsional curvature. Ductility factor of NAC and RAC beams under cyclic torsion is summarized in Table 7. It can be observed that the ductility factor of RAC and NAC torsional beam is 3.400–4.286 and 3.333–4.000, respectively. The ductility factor of RAC torsional beam is nearly 5.3% higher than that of NAC torsional beam. 4. Conclusions In this paper, the failure mode, hysteresis loop, strain of steel reinforcement, principal strain of concrete, energy dissipation capacity, skeleton curve, deterioration of stiffness and ductility factor of RAC beams and natural aggregate concrete (NAC) beams under cyclic torsion were compared and investigated. The following conclusions can be drawn. (1) The failure process of RAC torsional beams is very similar to that of NAC torsional beams with spiral cracks. However, the crack width of RAC beams is larger than that of NAC beams. (2) The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, which indicates that the NAC and RAC torsional beams have a nearly equivalent hysteretic performance. (3) Before the torsional beams yielded, the longitudinal reinforcement strain of NAC and RAC beams were at the same level. However, this strain of RAC beams became larger obviously than that of NAC beams after the torsional beams yielded. Compared to NAC beams, the stirrup strain of RAC beams is larger, indicating a more serious shear cracking. (4) The development of principal compressive strain in RAC beam is very similar to that of NAC beam. However, the principal tensile strain of RAC beam is larger than that of NAC beam. (5) The hysteresis loop shape and corresponding inflection points of NAC and RAC torsional beams are very similar, but the ductility factor of RAC torsional beams are about 5.3% higher than those of NAC beams, respectively. (6) The use RAC in cyclic torsional beams does not bring apparent influence on the deterioration of stiffness, but the bearing capacity of RAC torsional beams is lower than that of NAC beams.
Acknowledgements The authors acknowledge the National Natural Science Foundation of China (Grant: 50908102), the National Natural Science Foundation of China (Grant: 51378240), Science and Technology Project of Housing and Urban-rural Development of China (Grant: 2013-K4-46). References [1] D. Soares, J. De Brito, J. Ferreira, Use of coarse recycled aggregates from precast concrete rejects: mechanical and durability performance, Constr. Build. Mater. 71 (2014) 263–272. [2] J. Xiao, W. Li, Y. Fan, An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (6) (2012) 364–383. [3] D. Matias, J. De Brito, A. Rosa, et al., Mechanical properties of concrete produced with recycled coarse aggregates – influence of the use of superplasticizers, Constr. Build. Mater. 44 (2013) 101–109.
203
[4] Q. Liu, J.Z. Xiao, Z.H. Sun, Experimental study on the failure mechanism of recycled concrete, Cem. Concr. Res. 41 (10) (2011) 1050–1057. [5] T.C. Hansen, H. Narud, Strength of recycled concrete made from crushed concrete coarse aggregate, Concr. Int. 5 (1) (1983) 79–83. [6] M. Tavakoli, P. Soroushian, Strengths of recycled aggregate concrete made using field-demolished concrete as aggregate, ACI Mater. J. 93 (2) (1996) 182– 190. [7] R.S. Ravindrarajah, Y.J. Loo, C.T. Tam, Strength evaluation of recycled aggregate concrete by in-situ tests, Mater. Struct. 21 (1998) 289–295. [8] J.Z. Xiao, Y. Lan, Investigation on the tensile behavior of recycled aggregate concrete, J. Build. Mater. 8 (2) (2006) 154–162. [9] Q. Hu, C. Song, C.Y. Zou, Experimental research on the mechanical properties of recycled concrete, J. Harbin Inst. Technol. 41 (4) (2009) 33–39. [10] G.F. Belén, M.A. Fernando, C.L. Diego, Stress-strain relationship in axial compression for concrete using recycled saturated coarse aggregate, Constr. Build. Mater. 25 (51) (2011) 2335–2342. [11] J.Z. Xiao, Experimental investigation on complete stress-strain curve of recycled concrete under uniaxial loading, J. Tongji Univ. 35 (11) (2007) 1445–1454. [12] T. Du, W.H. Wang, Z.X. Liu, The complete stress-strain curve of recycled aggregate concrete under uniaxial compression loading, J. Wuhan Univ. Technol. 25 (5) (2010) 862–867. [13] T. Hao, Z.H. Du, L.X. Liu, Study on complete stress-strain curves of recycled concrete, in: 2nd International Conference on Waste Engineering and Management, ICWEM 2010, RILEM Proceedings 2010, vol. 73, pp. 506–518. [14] F.T. Olorunsogo, N. Padayachee, Performance of recycled aggregate concrete monitored by durability indexes, Cem. Concr. Res. 32 (2002) 179–185. [15] V.W.Y. Tam, X.F. Gao, C.M. Tam, Microstructural analysis of recycled concrete produced from two-stage mixing approach, Cem. Concr. Res. 35 (6) (2005) 1195–1203. [16] A. Gokce, S. Nagataki, T. Saeki, Freezing and thawing resistance of airentrained concrete incorporating recycled coarse aggregate: the role of air content in demolished concrete, Cem. Concr. Res. 34 (5) (2004) 799–806. [17] J.S. Ryu, An experimental study on the effect of recycled aggregate on concrete properties, Mag. Concr. Res. 34 (11) (2003) 1975–1980. [18] M. Surya, P. Lakshmy, V.V.L. Kanta Rao, Study on some engineering properties of recycled aggregate concrete with fly ash, Adv. Struct. Eng. (2015) 1773– 1783. [19] M.A. Ali, T. Murat, R. Kambiz, Mechanical and durability performance of concrete incorporating fine recycled concrete and glass aggregates, Mater. Struct. 48 (2015) 2629–2640. [20] R.A. Kutalmıs, C. Özgür, I. Metin, Properties of polypropylene fiber reinforced concrete using recycled aggregates, Constr. Build. Mater. 98 (2015) 620–630. [21] S.C. Kou, C. Spoon, Compressive strength, pore size distribution and chlorideion penetration of recycled aggregate concrete incorporating class-F fly ash, J. Wuhan Univ. Technol. 21 (4) (2006) 130–136. [22] K. Ishill, Flexible characteristic of RC beam with recycled coarse aggregate, in: Proceeding of the 25th JSCE Annual Meeting, Kanto Branch, 1998, pp. 886–887. [23] G.F. Belen, M.A. Fernando, Shear strength of recycled concrete beams, Constr. Build. Mater. 1 (2006) 1–7. [24] M. Arezoumandi, A. Smith, An experimental study on shear strength of reinforced concrete beams with 100% recycled concrete aggregate, Constr. Build. Mater. 53 (2014) 612–620. [25] W.C. Choi, H.D. Yun, Long-term deflection and flexural behavior of reinforced concrete beams with recycled aggregate, Mater. Des. 51 (2013) 742–750. [26] G.F. Belén, M.A. Fernando, M.L. Isabel, Structural shear behaviour of recycled concrete with silica fume, Constr. Build. Mater. 23 (2009) 3406–3410. [27] W.C. Choia, H.D. Yunb, Compressive behavior of reinforced concrete columns with recycled aggregate under uniaxial loading, Eng. Struct. 41 (2012) 285– 293. [28] Y.F. Yang, L.H. Han, Experimental behavior of recycled aggregate concrete filled steel tubular columns, J. Constr. Steel Res. 62 (2006) 1310–1324. [29] Y.F. Yang, G.L. Ma, Experimental behaviour of recycled aggregate concrete filled stainless steel tube stub columns and beams, Thin Walled Struct. 66 (2013) 62–75. [30] J.Z. Xiao, Y.J. Huang, Zh. Sun, Seismic behavior of recycled aggregate concrete filled steel and glass fiber reinforced plastic tube columns, Adv. Struct. Eng. 17 (5) (2014) 693–707. [31] C. Valeria, L. Viviana, M. Giacomo, Behaviour of beam–column joints made of recycled-aggregate concrete under cyclic loading, Constr. Build. Mater. 25 (2011) 1877–1882. [32] C. Viviana, G. Letelier, M. Giacomo, The influence of recycled concrete aggregates on the behavior of beam–column joints under cyclic loading, Eng. Struct. 60 (2014) 148–154. [33] J.W. Zhang, W.L. Cao, S.B. Meng, Shaking table experimental study of recycled concrete frame-shear wall structures, Earthq. Eng. Eng. Vib. 13 (2) (2014) 257– 267. [34] J.Z. Xiao, Y.D. Sun, H. Falkner, Seismic performance of frame structures with recycled aggregate concrete, Eng. Struct. 28 (1) (2006) 1–8. [35] F. Okay, S. Engin, Torsional behavior of steel fiber reinforced concrete beams, Constr. Build. Mater. 28 (2012) 269–275.