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Strength and deformation of recycled aggregate concrete under triaxial compression
Construction and Building Materials 156 (2017) 1043–1052
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Strength and deformation of recycled aggregate concrete under triaxial compression Zhiheng Deng, Yumei Wang ⇑, Jun Sheng, Xu Hu College of Civil Engineering and Architecture, Guangxi University, 100 University Road, Nanning, Guangxi 530004, China
h i g h l i g h t s Mechanical properties of recycled concrete under triaxial compression were studied. The effect of RCA content and stress ratio on mechanical properties were analyzed. Triaxial strength of recycled concrete were depicted in octahedral stress space.
a r t i c l e
i n f o
Article history: Received 11 April 2017 Received in revised form 13 August 2017 Accepted 31 August 2017
Keywords: Recycled aggregate concrete Triaxial compression Replacement ratio of recycled coarse aggregate Stress ratio Strength Deformation
a b s t r a c t Experiment was carried out to better understand the behaviors of recycled aggregate concrete under triaxial compressive stresses. The test was performed on 100 mm 100 mm 100 mm cubic specimens with five replacement ratios of recycled coarse aggregate (RCA) at four kinds of stress ratios, using a servo-controlled static-dynamic triaxial machine (TAWZ-5000/3000). The failure modes, ultimate strength and deformation under triaxial compression were observed, and the effect of RCA replacement ratio and stress ratio on these characteristics was analyzed. The results show that the triaxial compressive strength of the specimens is much higher than the corresponding uniaxial strength, and the strength and deformation depend on both RCA replacement ratio and stress ratio. Moreover, the factors affecting triaxial strength were quantitatively analyzed by variance calculation. Based on the triaxial strength, the relationship between normalized octahedral normal stress roct =f c and octahedral shear stress soct =f c with RCA replacement ratio and stress ratio was discussed. Ó 2017 Published by Elsevier Ltd.
1. Introduction Recycled coarse aggregate (RCA) obtained from construction and demolition (C&D) wastes has become a valuable resource as an alternative material to natural aggregate [1]. Using cracked concrete as coarse aggregate can not only decrease the usage of natural aggregate, but also reduce pollution. The main difference between recycled aggregate and natural aggregate is the presence of residual mortar in RCA. Some work has been carried out to explore the important properties of recycled concrete [2–5]. Compared with natural aggregate concrete, recycled aggregate concrete has higher porosity, lower density, and weaker interfacial transition zones (ITZs), the property of RCA has a great influence on the mechanical properties of recycled concrete. Tam et al. [6–8] found that the microstructure of recycled concrete is much more complicated than that of the conventional concrete, recycled con-
⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2017.08.189 0950-0618/Ó 2017 Published by Elsevier Ltd.
crete has two ITZs: one is between the original aggregate and the residual mortar; the other is between the recycled aggregate and the fresh paste, a two-stage mixing approach (TSMA) has been proposed to strengthen the weakened ITZs. Poon et al. [9–11] found that the microstructure of recycled aggregate has an adverse effect on the mechanical properties of concrete, recycled aggregate from high strength concrete is recommended to reduce the adverse effects. Xiao et al. [12–15] analyzed the failure mechanism of recycled aggregate concrete under uniaxial compression and simulated the process. Casuccio et al. [16] and Kwan et al. [17] found that the failure of recycled concrete was characterized by low strength, high peak strain and low fracture energy. Poon et al. [18,19] pointed out the influence of RCA on the long-term mechanical properties of concrete, and studied the performance of low-grade recycled concrete. Hansen [20] found that the compressive strength of recycled aggregate concrete can be reduced by 25%, depending mainly on the quality of recycled concrete aggregate. However, some researchers observed that the compressive strength of concrete was not affected, and some slightly increased
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when RCA replacement ratio was less than 25% [3,21–23]. Most of the structures are actually in multiaxial stress state, such as beamcolumn connection, steel tube confined concrete structure, shell structure, etc. Therefore, the structural design based on uniaxial strength theory is neither economic nor reasonable [21,24–26]. It is necessary to study the mechanical behavior of recycled aggregate concrete under multiaxial stress state. The mechanical properties of concrete under multiaxial stress states are different from those of uniaxial mechanics, the strength of concrete greatly improved, and many researchers have revealed these properties [27–31]. Although the mechanical behavior of ordinary concrete under multiaxial stress has been extensively studied, the study of recycled concrete under multiaxial stress is rare.He et al. [32] revealed the strength and failure criterion of recycled aggregate concrete under biaxial and triaxial compression with two kinds RCA replacement ratio, which is the only published article on multiaxial compression of recycled concrete. In this study, the mechanical properties of recycled concrete under triaxial compression were tested, and the effects of RCA replacement ratio and stress ratio on failure modes, strength and deformation were analyzed. The purpose of this research is to provide experimental data for the application of recycled aggregate concrete in structural engineering.
2. Materials and Testing procedure 2.1. Materials and mix proportions Waste concrete was obtained from urban road under demolition in the Guangxi area in China, no detailed information was available on the age of the road, but most constructions in this area took place for no more than thirty years. Large parts of the waste concrete were crushed on site, then the crushed concrete were transported to the laboratory and broken by workers into pieces with size smaller than 20 mm and sieved with a proper gradation of 5–20 mm. The natural aggregate used were common crushed stone. Table 1 shows some of the measured physical properties of the natural and recycled aggregate. As expected, due to the presence of the attached mortar, recycled aggregate has higher water absorption and smaller density than natural aggregate. Using heat-resistant separation method can obtain the attached mortar content, the measured content of 30.55% is much higher than that of natural aggregate, and this feature directly affects the physical and mechanical properties of recycled concrete. Based on Regulation of Common Concrete Mix Design, the water-cement ratio and the unit water consumption were deter-
mined according to the design requirement, and then the amount of cement was determined. Finally, the aggregate content was calculated according to the reasonable sand ratio. For objective comparison on triaxial strength of concrete with different RCA replacement ratio, the unit water consumption was fixed and the water-cement ratio was adjusted to get the same uniaxial strength among different groups. The fine aggregate was natural river sand with a fineness modulus of 2.8. The cementitius material was 42.5R Portland cement, water was tap-water. Table 2 shows the dryweight proportions of mix designs. 2.2. Samples and testing method Samples were casted in plastic cube molds and cured for 24 h before demolded, and then placed in a moist room with a relative humidity of 95% at 20 3 C. RCA replacement ratio and stress ratio were the main factors that affect the mechanical properties of recycled concrete. Five levels of RCA replacement ratio (0%, 30%, 50%, 70%, 100%) and four kinds of stress ratios (a ¼ 0:1 : 0:25 : 1, 0:1 : 0:5 : 1, 0:1 : 0:75 : 1, 0:1 : 1 : 1) were considered in this study. The principal stresses were expressed as r1 P r2 P r3 , compression denoted as negative and tension denoted as positive. 100 mm 100 mm 100 mm cubes were used to measure the strength of specimens under triaxial compression using a servo-controlled test setup (TAWZ-5000/3000) at State Key Laboratory of Structural Engineering in Guangxi University. The test setup consisted of four independent hydraulic actuators, the vertical actuator with loading capacity of 5000 KN, and the others with capacity of 3000 KN. Two linear variable differential transformers(LVDT) were attached along each side of the specimen in loading direction. The tests were carried out at a loading rate of 0:5 MPa=s. A data acquisition system was used to record the applied load and LVDT readings. The deformation of the specimens was obtained by averaging the reading of the two LVDTs. The specimen and test setup are shown in Fig. 1. 3. Test results and discussion 3.1. Failure modes The crack patterns and failure modes were observed,in general, there was no fundamental difference between the failure modes due to different RCA replacement ratios. The typical crack patterns are shown in Fig. 2. The failure mode was mainly affected by stress ratio. At stress ratio a ¼ 0:1 : 1 : 1, due to the constraints of r2 and r3 , tensile strain occurred parallel to r1 direction. As the load
Table 1 Physical properties of coarse aggregate. Classification
Gradation (mm)
Apparent density (kg/m3)
Crush Index (%)
Water absorption (%)
Attached mortar content
NCA RCA
5–20 5–20
2708 2485
4.6 13.4
0.3 3.0
– 30.55%
Table 2 Mix proportions. Unit weight (kg/m3) Specimens
RCA%
Water/cement
Cement
Sand
Water
NCA
RCA
RAC0 RAC30 RAC50 RAC70 RAC100
0% 30% 50% 70% 100%
0.47 0.46 0.45 0.44 0.43
415 424 433 443 453
631 643 656 669 681
195 195 195 195 195
1121 767 536 314 0
0 329 536 733 1021
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Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
(a) specimen wrapped with loading plates
(b) triaxial loading setup Fig. 1. Specimen and test setup.
¦ 3Ò
¦Ò 2
¦Ò 1
¦Ò 1
¦ 3Ò (a) plate-splitting cracks
¦Ò 3
¦Ò 1
¦Ò 1
¦Ò 3 (b) slant-shear cracks Fig. 2. Failure mode of RAC under triaxial compression.
¦Ò 2
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Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
Fig. 3. ITZ of RAC.
increasing, the cracks gradually appeared on surface of r2 and r3 . When exceeding ultimate strain, plate-splitting cracks were formed on surface of r2 and r3 . The failure mode is shown in Fig. 2(a). At stress ratio a ¼ 0:1 : 0:25 : 1; 0:1 : 0:5 : 1 and 0:1 : 0:75 : 1, due to large shear stress ðr1 r3 Þ=2, slant-shear cracks appeared parallel to the direction of r2 . It can be seen that the single-shear and double-shear cracks were formed on the surface of r2 , and formed an angle about 20–30° with direction r3 . The failure mode is shown in Fig. 2(b). From a microscopic point of view, with the use of RCA, the structure of recycled concrete is much more complicated than that of natural concrete. As shown in Fig. 3, Recycled concrete has two weaker ITZs, one is between original aggregate and old mortar in RCA, and the other is between new and old cement paste. A large amount of pores and microcracks existed in the attached mortar, the pores and microcracks critically decreased the bonding strength of the concrete, therefore, the recycled aggregate concrete specimens presented more minute cracks with higher RCA replacement ratio.
3.2. Stress-strain relationship The stress-strain curves of recycled aggregate concrete under triaxial compression are shown in Fig. 4. The stress and strain increases proportionally during the initial stage of loading, the curve is approximately a straight line. With the increasing of stress, plastic deformation occurred in concrete specimens, the slope of the curve slowed down. The stress ratio has obvious influence on triaxial strength of recycled concrete. The maximum principal stress r3 is obtained at the stress ratio a ¼ 0:1 : 0:5 : 1, and the minimum value is obtained at a ¼ 0:1 : 1 : 1. The principal strain e3 under triaxial compression is obviously higher than that under uniaxial compression, this result indicates that the lateral restraint could effectively improve the stiffness and ductility of test specimens. The maximum strain e3 is obtained at the stress ratio a ¼ 0:1 : 0:25 : 1, and the minimum value is obtained at a ¼ 0:1 : 1 : 1. Due to the effects of stress r2 and r3 , tensile stress occurrs in the direction of r1 , therefore, the strain e1 is positive at all stress ratios. At the stress ratio a ¼ 0:1 : 0:25 : 1, strain e2 is positive, at other stress ratios, the value e2 turns to negative. It illustrates that stress r2 is relatively sensitive to the change of intermediate stress ratio. When the intermediate stress ratio is small, r2 is tensile stress, and then the stress value gradually becomes compressive as the intermediate stress ratio increased. The RCA replacement ratio has great effect on stress-strain relationship of recycled concrete. With the increase of the replacement ratio, the
principal stress r3 decreases gradually except for RAC30 which stress value is slightly higher than RAC0 due to the lateral restriction. For all test specimens, the principal strain e3 increases with the raise of RCA replacement ratio. In general, the linear section of stress-strain curve decreases and the slope of the curves is gradually gentle, it indicates that the initial modulus of elasticity decreases with the increase of RCA replacement ratio.
3.3. Triaxial strength Table 3 shows the ultimate strength of each specimen under triaxial compression. It can be seen that the strength of most specimens is higher than 200 MPa, the value is much greater than that of under uniaxial compression. At least three specimens were tested in each group, deleted the discrete values, taken the average value of the three samples that meet the requirements. Fig. 5(a) shows the influence of RCA replacement ratio on the average value of stress r3 . The triaxial strength of RAC30 is slightly higher than that of RAC0, when replacement ratio of RCA is higher than 30%, the triaxial strength gradually decreases with the increase ratio of RCA. The main reason for the higher strength of RAC30 can be explained as: for one thing, the uniaxial strength of RAC30 and RAC0 are close to each other, and the strength value of RAC0 is only 0.5% higher than that of RAC30; for another, lateral forces restrict the development of micro cracks, the adverse effects of internal micro cracks on compressive strength under triaxial stress states has been weakened due to lateral constraint. Fig. 5(b) demonstrates the influence of stress ratio on average stress r3 , it can be seen that the maximum triaxial compressive strength is obtained at stress ratio a ¼ 0:1 : 0:5 : 1, and the minimum strength is obtained at a ¼ 0:1 : 1 : 1. The average triaxial strength is normalized by uniaxial strength and expressed as r3 =f c . Fig. 6(a) shows the influence of RCA on normalized value of r3 =f c . Similar to the trend of stress r3 variation, the normalized value r3 =f c slightly increases as RCA replacement ratio changing from 0% to 30%, and decreases with the increase usage of RCA when the replacement ratio is larger than 30%. Fig. 6(b) demonstrates the influence of stress ratio on r3 =f c , it can be seen that the highest value occurred at stress ratio a ¼ 0:1 : 0:5 : 1, and the triaxial strength is about 5–6 times of the uniaxial strength. The minimum value of r3 =f c is obtained at a ¼ 0:1 : 1 : 1, and the triaxial strength is about 4–5 times of uniaxial strength. Overall, the strength of concrete specimens under triaxial compression is much higher than that of under uniaxial compression, the triaxial strength decreases with the increase of RCA replacement ratio except RAC30 which strength is slightly higher than RAC0, the
Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
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Fig. 4. Stress-strain curves for RAC under triaxial compression.
trend of triaxial strength with stress ratio is a-0.1:-0.5:-1 > a-0.1:-0.75:-1 > a-0.1:-0.25:-1 > a-0.1:-1:-1. A variance calculation of test data is made and the quantitative analysis of two factors: RCA replacement ratio and stress ratio on triaxial strength is given. Taken r3 =f c as the dependent variable, stress
ratio(A) and RCA replacement ratio(B) as independent variables. There are four levels of factor A(r = 4): 0.1:0.25:1, 01:0.5:1, 0.1:0.75:1 and 0.1:1:1, and five levels of factor B(s = 5): 0%, 30%, 50%, 70%, 100%. Variation of dependent variable caused by the two factors A and B can be calculated by equations as follows:
1048 Table 3 Principal stress
Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
r3 under different stress ratios (MPa).
No
r1 : r2 : r3
r3 01(MPa)
r3 02(MPa)
r3 03(MPa)
RAC0
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
210.69 287.07 260.65 239.30
250.26 274.95 241.15 217.72
271.49 259.38 258.80 210.11
RAC30
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
236.44 279.92 248.01 225.92
238.66 274.07 257.71 211.66
272.09 286.69 287.63 242.52
RAC50
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
227.30 210.90 235.80 211.30
213.10 251.00 192.90 197.90
212.70 244.80 257.60 187.00
RAC70
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
218.08 223.67 229.27 194.32
206.62 209.70 195.25 173.25
192.27 204.10 203.73 177.36
RAC100
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
182.95 193.72 193.09 164.91
209.78 185.10 209.06 175.59
169.58 223.68 174.16 169.49
360
-0.1:-0.25:-1 -0.1:-0.75:-1
360
-0.1:-0.5:-1 -0.1:-1:-1
300
MPa 3
MPa
300 240
3
180
180 120
60
60 0 0%
30%
50% RA%
70%
RAC30 RAC70
240
120
0
RAC0 RAC50 RAC100
100%
0.25
0.50
0.75 2
(a) with different RCA replacement ratio
/
1.00
3
(b) with different stress ratio
Fig. 5. Triaxial strength with RCA replacement ratio and stress ratio.
8
6
6
3
3
c
c
8
4 2 0
0.1:0.25:1 0.1:0.75:1
0.1:0.5:1 0.1:1:1
4 2 0
0%
30%
50% 70% 100% RA% (a) with different RCA replacement ratio
0.25
0.50 / 2
RAC0 RAC50 RAC100
RAC30 RAC70
0.75
1.00
3
(b) with different stress ratio
Fig. 6. Normalized triaxial strength with RCA replacement ratio and stress ratio.
1049
Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052 r X 2 SSA ¼ s ðX i X i Þ
ð1Þ
Table 4 Analysis of influencing factors.
i¼1 s X 2 SSB ¼ r ðY j Y j Þ
ð2Þ
SSA
SSB
SST
RA
RB
2.171
2.890
5.196
41.79%
55.61%
j¼1
SST ¼
r X s X 2 ðX ij X i Þ
ð3Þ
i¼1 j¼1
RA ¼ ðSSA=SSTÞ 100%
ð4Þ
RB ¼ ðSSB=SSTÞ 100%
ð5Þ
where X i ; Y j are the average value of factor A and B under different levels, the calculated results is shown in Fig. 7. X i and Y j are the general average values, X i ¼ Y j ¼ 4:95. SSA is the variation of r3 =f c caused by factor A, and SSB is the variation of r3 =f c caused by factor B, SST is the total sum of variations. RA is the influence rate of factor A, RB is the influence rate of factor B. Fig. 7(a) shows the average value of r3 =f c under different levels of factor A. It can be seen that the maximum value is obtained at intermediate stress ratio r2 =r3 ¼ 0:5, the minimum valued is obtained at r2 =r3 ¼ 1. Fig. 7(b) shows the average value of r3 =f c under different levels of factor B. It can be seen that the maximum value occurred when RCA replacement ratio is 30%, the general trend of r3 =f c with RCA replacement ratio is RAC30 > RAC0 > RAC50 > RAC70 > RAC100. Table 4 gives the results calculated by Eqs. (1)–(5). It can be seen that the value of RA is 41.79%, the value of RB is 55.61%, and it indicates that the influence of factor A on r3 =f c is slightly smaller than the influence of factor B. The results demonstrate that the strength of RAC under triaxial compression is influenced both by RCA replacement ratio and stress ratio. 3.4. Octahedral stress The strength of RAC under triaxial compression can be depicted in octahedral stress space[33], the normal stress roct and the shear stress soct can be calculated from:
1 3
roct ¼ ðr1 þ r2 þ r3 Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðr1 r2 Þ2 þ ðr2 r3 Þ2 þ ðr3 r1 Þ2
ð7Þ
3.5. Deformation characteristic Table 5 shows the peak strain of RAC specimens under triaxial compression, positive means tensile and negative means compression. Fig. 9(a) shows the influence of stress ratio on principal strain e3 . The value of e3 decreases with the increase of intermediate stress ratio r2 =r3 , the maximum strain is obtained at stress ratio a ¼ 0:1 : 0:25 : 1, and the minimum strain is obtained at a ¼ 0:1 : 1 : 1, the general trend of e3 with intermediate stress ratio is a0:25 > a0:5 > a0:75 > a1 . Fig. 9(b) illustrates the influence of RCA replacement ratio on strain e3 . It shows clearly that the strain value increases as RCA replacement ratio increased. The pores and microcracks existed in attached mortar critically increase the deformation of RAC specimens. Therefore, strain e3 of RAC is slightly higher than that of normal concrete. Fig. 10(a) shows the influence of stress ratio on strain e2 . It can be seen that strain e2 is positive as the intermediate stress ratio r2 =r3 ¼ 0:25, and the value changes to negative at other stress ratios. The main reason for this change is that the stress in direction r2 is less than the extrusion pressure from direction r3 , tensile stress is eventually occurred in direction r2 due to Poisson’s effect, so tensile strain is appeared in direction r2 when intermediate stress ratio is small. Fig. 10(b) demonstrates the influence of RCA replacement ratio on strain e2 . It can be seen that the strain e2 increases as the RCA replacement ratio increased. It illustrates that whether tensile or compressive stress occurs in direction r2 , micro cracks in recycled concrete increase the deformation in this direction. Fig. 11(a) shows the influence of stress ratio on strain e1 . It can
6
6 5.34
5.31
5.46
5.10
4.90
4.85
4.64
4.44
4
c
c
4.47
4
3
1 3
3
soct ¼
ð6Þ
Fig. 8 shows the relationship between normalized octahedral normal stress roct =f c and octahedral shear stress soct =f c with RCA replacement ratio and stress ratio. Fig. 8(a) indicates that the variation of octahedral shear stress soct =f c with stress ratio is not obvious. Stress ratio has a great effect on octahedral normal stress roct =f c , especially from the intermediate stress ratio a ¼ r2 =r3 ¼ 0:25 to a ¼ 0:5, and from a ¼ 0:5 to a ¼ 0:75. Fig. 8(b) shows the influence of RCA replacement ratio on roct =f c and soct =f c , it can be seen that except RAC 30, the roct =f c decreases with the RCA replacement ratio increased, the same trend is performed in soct =f c .
2
2
0
0 0.25
0.50
/ 2
0.75
1.00
0%
30%
3
(a) with different stress ratio
50% 70% RA%
100%
(b) with different RCA replacement ratio
Fig. 7. Influence of stress ratio and RCA replacement ratio on
r3 =f c .
Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
2.4
2.0
2.0
oct
c
2.4
oct
c
1050
1.6
1.6 0.1:0.25:1 0.1:0.75:1
1.2
2.0
2.4
2.8
3.2 oct
RAC0 RAC50 RAC100
0.1:0.5:1 0.1:1:1
3.6
1.2
4.0
2.0
2.4
2.8
c
oct
(a) with different stress ratio
RAC30 RAC70
3.2
3.6
4.0
c
(b) with different RCA replacement ratio
Fig. 8. The relationship between octahedral normal stress and octahedral shear stress.
Table 5 Triaxial strain under different stress ratios (103 ). Specimens
r1 : r2 : r3
e1 (103)
e2 (103)
e3 (103)
RAC0
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
13.97 16.05 18.49 24.04
6.27 12.42 23.36 30.04
39.76 37.51 34.89 30.04
RAC30
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
15.15 17.78 20.33 26.62
7.86 14.15 24.01 32.82
41.69 39.28 36.51 32.82
RAC50
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
17.56 19.04 20.17 27.26
9.29 17.22 26.14 35.76
44.28 42.49 39.78 35.76
RAC70
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
18.45 19.79 21.27 28.29
9.83 18.39 27.87 37.48
46.39 43.96 41.81 37.48
RAC100
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
18.71 20.35 22.18 28.31
10.5 20.17 28.49 39.69
47.78 45.67 42.79 39.69
-60
-60
-45
-45
-30
-30
-15 0
(a)
0.25
0.50
RAC0 RAC50 RAC100
RAC30 RAC70
0.75
1.00
with different stress ratio
-15 0
(b)
0.1:0.25:1 0.1:0.75:1
0%
30%
0.1:0.5:1 0.1:1:1
50% 70% 100% RA%
with different RCA replacement ratio
Fig. 9. Influence of stress ratio and RCA replacement ratio on
e3 .
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Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
-60
-60
-45
-45
-30
-30
-15
-15
0 15
(a)
0.25
0.50
RAC0 RAC50 RAC100
RAC30 RAC70
0.75
1.00
with different stress ratio
0.1:0.25:1 0.1:0.75:1
0 15
(b)
0%
30%
50% 70% 100% RA%
with different RCA replacement ratio
Fig. 10. Influence of stress ratio and RCA replacement ratio on
30
30
25
25
20
20
15
15
10
(a)
e2 .
10 RAC0 RAC50 RAC100
5 0
0.1:0.5:1 0.1:1:1
0.25
0.50
0.75
RAC30 RAC70
1.00
with different stress ratio
0.1:0.25:1 0.1:0.75:1
5 0
(b)
0%
30%
50% 70% 100% RA%
with different RCA replacement ratio
Fig. 11. Influence of stress ratio and RCA replacement ratio on
be seen that the strain e1 is positive under all kinds of stress ratios due to the expansion force produced by extrusion pressure from r2 and r3 , and the value increases as the intermediate stress ratio increased. Fig. 11(b) shows the influence of RCA replacement ratio on strain e1 , it reveals that the value increases slightly with the increase of RCA replacement ratio, micro cracks in recycled concrete also increase the strain e1 . 4. Conclusion
0.1:0.5:1 0.1:1:1
e1 .
(4) The stress ratio mainly affects the octahedral normal stress roct =f c while RCA replacement ratio affects both roct =f c and soct =f c . (5) The adverse effects of internal pores and micro cracks on triaxial strength of recycled concrete have been weakened due to lateral constraint when RCA replacement ratio is less than 30%, but the triaxial strain continues to increase with the increase of RCA content, for widely used in construction, the quality and dosage of RCA should be taken into consideration.
Based on the experimental work and the analysis of the test results, the following conclusions can be drawn: (1) Under triaxial compression, there are two typical crack patterns of test specimens: one is plate-splitting crack at stress ratio a ¼ 0:1 : 1 : 1, the other is slant-shear crack when at other stress ratios, and the crack patterns do not depend on RCA content. (2) The difference in stress-strain curves of recycled concrete is attributed to the effect of RCA replacement ratio, the initial slope of the curves is gradually more gentle with the increase of RCA replacement ratio (3) The strength of recycled concrete under triaxial compression is much higher than that of under uniaxial compression, the trend of triaxial strength with stress ratio is a0:1:0:5:1 > a0:1:0:75:1 > a0:1:0:25:1 > a0:1:1:1 , and the trend with RCA content is RAC30 > RAC0 > RAC50 > RAC70 > RAC100.
Acknowledgements This work is supported by National Natural Science Foundation of China (Grant No. 51478126). References [1] J. Xiao et al., An overview of study on recycled aggregate concrete in China (1996–2011), Constr. Build. Mater. 31 (2012) 364–383. [2] T. Li et al., Experimental study on mechanical behaviors of concrete with large-size recycled coarse aggregate, Constr. Build. Mater. 120 (2016) 321–328. [3] A. Abd Elhakam, A.E. Mohamed, E. Awad, Influence of self-healing, mixing method and adding silica fume on mechanical properties of recycled aggregates concrete, Constr. Build. Mater. 35 (2012) 421–427. [4] R.V. Silva, J. de Brito, R.K. Dhir, Properties and composition of recycled aggregates from construction and demolition waste suitable for concrete production, Constr. Build. Mater. 65 (2014) 201–217.
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[5] C.S. Poon, Z.H. Shui, L. Lam, Strength of Concretes Prepared with Natural and Recycled Aggregates at Different Moisture Conditions, in Advances in Building Technology, Elsevier, Oxford, 2002. pp. 1407–1414. [6] V.W.Y. Tam, X.F. Gao, C.M. Tam, Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach, Cem. Concr. Res. 35 (6) (2005) 1195–1203. [7] V.W.Y. Tam, C.M. Tam, Diversifying two-stage mixing approach (TSMA) for recycled aggregate concrete: TSMAs and TSMAsc, Constr. Build. Mater. 22 (10) (2008) 2068–2077. [8] V.W.Y. Tam, C.M. Tam, Y. Wang, Optimization on proportion for recycled aggregate in concrete using two-stage mixing approach, Constr. Build. Mater. 21 (10) (2007) 1928–1939. [9] Z.H. Duan, C.S. Poon, Properties of recycled aggregate concrete made with recycled aggregates with different amounts of old adhered mortars, Mater. Des. 58 (2014) 19–29. [10] C.S. Poon, Z.H. Shui, L. Lam, Effect of microstructure of ITZ on compressive strength of concrete prepared with recycled aggregates, Constr. Build. Mater. 18 (6) (2004) 461–468. [11] C.S. Poon et al., Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete, Cem. Concr. Res. 34 (1) (2004) 31–36. [12] Y. Huang, J. Xiao, C. Zhang, Theoretical study on mechanical behavior of steel confined recycled aggregate concrete, J. Constr. Steel Res. 76 (2012) 100–111. [13] W. Li et al., Failure processes of modeled recycled aggregate concrete under uniaxial compression, Cem. Concr. Compos. 34 (10) (2012) 1149–1158. [14] Q. Liu, J. Xiao, Z. Sun, Experimental study on the failure mechanism of recycled concrete, Cem. Concr. Res. 41 (10) (2011) 1050–1057. [15] J. Xiao, H. Falkner, Bond behaviour between recycled aggregate concrete and steel rebars, Constr. Build. Mater. 21 (2) (2007) 395–401. [16] M. Casuccio et al., Failure mechanism of recycled aggregate concrete, Constr. Build. Mater. 22 (7) (2008) 1500–1506. [17] W.H. Kwan et al., Influence of the amount of recycled coarse aggregate in concrete design and durability properties, Constr. Build. Mater. 26 (1) (2012) 565–573. [18] S.-C. Kou, C.-S. Poon, M. Etxeberria, Influence of recycled aggregates on long term mechanical properties and pore size distribution of concrete, Cem. Concr. Compos. 33 (2) (2011) 286–291. [19] S.-C. Kou, C.-S. Poon, H.-W. Wan, Properties of concrete prepared with lowgrade recycled aggregates, Constr. Build. Mater. 36 (2012) 881–889.
[20] T.C. Hansen, Recycled concrete aggregate and fly ash produce concrete without portland cement, Cem. Concr. Res. 20 (3) (1990) 355–356. [21] N. Fonseca, J. de Brito, L. Evangelista, The influence of curing conditions on the mechanical performance of concrete made with recycled concrete waste, Cem. Concr. Compos. 33 (6) (2011) 637–643. [22] M. Etxeberria et al., Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete, Cem. Concr. Res. 37 (5) (2007) 735–742. [23] S.B. Huda, M.S. Alam, Mechanical behavior of three generations of 100% repeated recycled coarse aggregate concrete, Constr. Build. Mater. 65 (2014) 574–582. [24] L. Butler, J.S. West, S.L. Tighe, Effect of recycled concrete coarse aggregate from multiple sources on the hardened properties of concrete with equivalent compressive strength, Constr. Build. Mater. 47 (2013) 1292–1301. [25] S.-C. Kou, C.-S. Poon, Effect of the quality of parent concrete on the properties of high performance recycled aggregate concrete, Constr. Build. Mater. 77 (2015) 501–508. [26] S.-C. Kou, C.-S. Poon, F. Agrela, Comparisons of natural and recycled aggregate concretes prepared with the addition of different mineral admixtures, Cement Concr. Compos. 33 (8) (2011) 788–795. [27] Z. He, Y. Song, Failure mode and constitutive model of plain high-strength high-performance concrete under biaxial compression after exposure to high temperatures, Acta Mech. Solida Sin. 21 (2) (2008) 149–159. [28] H.S. Shang, Y.P. Song, Experimental study of strength and deformation of plain concrete under biaxial compression after freezing and thawing cycles, Cem. Concr. Res. 36 (10) (2006) 1857–1864. [29] E.K. Tschegg, M. Elser, S.E. Stanzl-Tschegg, Biaxial fracture tests on concrete — Development and experience, Cement Concr. Compos. 17 (1) (1995) 57–75. [30] P. Folino, H. Xargay, Recycled aggregate concrete – mechanical behavior under uniaxial and triaxial compression, Constr. Build. Mater. 56 (2014) 21–31. [31] Z.-J. He, Y.-P. Song, Triaxial strength and failure criterion of plain high-strength and high-performance concrete before and after high temperatures, Cem. Concr. Res. 40 (1) (2010) 171–178. [32] Z.-J. He, J.-X. Zhang, Strength characteristics and failure criterion of plain recycled aggregate concrete under triaxial stress states, Constr. Build. Mater. 54 (2014) 354–362. [33] C.-Z. Wang, Z.-H. Guo, X.-Q. Zhang. Experimental investigation of biaxial and triaxial compressive concrete strength, ACI Mater. J. (1987) March-April: 92–100.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Strength and deformation of recycled aggregate concrete under triaxial compression Zhiheng Deng, Yumei Wang ⇑, Jun Sheng, Xu Hu College of Civil Engineering and Architecture, Guangxi University, 100 University Road, Nanning, Guangxi 530004, China
h i g h l i g h t s Mechanical properties of recycled concrete under triaxial compression were studied. The effect of RCA content and stress ratio on mechanical properties were analyzed. Triaxial strength of recycled concrete were depicted in octahedral stress space.
a r t i c l e
i n f o
Article history: Received 11 April 2017 Received in revised form 13 August 2017 Accepted 31 August 2017
Keywords: Recycled aggregate concrete Triaxial compression Replacement ratio of recycled coarse aggregate Stress ratio Strength Deformation
a b s t r a c t Experiment was carried out to better understand the behaviors of recycled aggregate concrete under triaxial compressive stresses. The test was performed on 100 mm 100 mm 100 mm cubic specimens with five replacement ratios of recycled coarse aggregate (RCA) at four kinds of stress ratios, using a servo-controlled static-dynamic triaxial machine (TAWZ-5000/3000). The failure modes, ultimate strength and deformation under triaxial compression were observed, and the effect of RCA replacement ratio and stress ratio on these characteristics was analyzed. The results show that the triaxial compressive strength of the specimens is much higher than the corresponding uniaxial strength, and the strength and deformation depend on both RCA replacement ratio and stress ratio. Moreover, the factors affecting triaxial strength were quantitatively analyzed by variance calculation. Based on the triaxial strength, the relationship between normalized octahedral normal stress roct =f c and octahedral shear stress soct =f c with RCA replacement ratio and stress ratio was discussed. Ó 2017 Published by Elsevier Ltd.
1. Introduction Recycled coarse aggregate (RCA) obtained from construction and demolition (C&D) wastes has become a valuable resource as an alternative material to natural aggregate [1]. Using cracked concrete as coarse aggregate can not only decrease the usage of natural aggregate, but also reduce pollution. The main difference between recycled aggregate and natural aggregate is the presence of residual mortar in RCA. Some work has been carried out to explore the important properties of recycled concrete [2–5]. Compared with natural aggregate concrete, recycled aggregate concrete has higher porosity, lower density, and weaker interfacial transition zones (ITZs), the property of RCA has a great influence on the mechanical properties of recycled concrete. Tam et al. [6–8] found that the microstructure of recycled concrete is much more complicated than that of the conventional concrete, recycled con-
⇑ Corresponding author. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.conbuildmat.2017.08.189 0950-0618/Ó 2017 Published by Elsevier Ltd.
crete has two ITZs: one is between the original aggregate and the residual mortar; the other is between the recycled aggregate and the fresh paste, a two-stage mixing approach (TSMA) has been proposed to strengthen the weakened ITZs. Poon et al. [9–11] found that the microstructure of recycled aggregate has an adverse effect on the mechanical properties of concrete, recycled aggregate from high strength concrete is recommended to reduce the adverse effects. Xiao et al. [12–15] analyzed the failure mechanism of recycled aggregate concrete under uniaxial compression and simulated the process. Casuccio et al. [16] and Kwan et al. [17] found that the failure of recycled concrete was characterized by low strength, high peak strain and low fracture energy. Poon et al. [18,19] pointed out the influence of RCA on the long-term mechanical properties of concrete, and studied the performance of low-grade recycled concrete. Hansen [20] found that the compressive strength of recycled aggregate concrete can be reduced by 25%, depending mainly on the quality of recycled concrete aggregate. However, some researchers observed that the compressive strength of concrete was not affected, and some slightly increased
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when RCA replacement ratio was less than 25% [3,21–23]. Most of the structures are actually in multiaxial stress state, such as beamcolumn connection, steel tube confined concrete structure, shell structure, etc. Therefore, the structural design based on uniaxial strength theory is neither economic nor reasonable [21,24–26]. It is necessary to study the mechanical behavior of recycled aggregate concrete under multiaxial stress state. The mechanical properties of concrete under multiaxial stress states are different from those of uniaxial mechanics, the strength of concrete greatly improved, and many researchers have revealed these properties [27–31]. Although the mechanical behavior of ordinary concrete under multiaxial stress has been extensively studied, the study of recycled concrete under multiaxial stress is rare.He et al. [32] revealed the strength and failure criterion of recycled aggregate concrete under biaxial and triaxial compression with two kinds RCA replacement ratio, which is the only published article on multiaxial compression of recycled concrete. In this study, the mechanical properties of recycled concrete under triaxial compression were tested, and the effects of RCA replacement ratio and stress ratio on failure modes, strength and deformation were analyzed. The purpose of this research is to provide experimental data for the application of recycled aggregate concrete in structural engineering.
2. Materials and Testing procedure 2.1. Materials and mix proportions Waste concrete was obtained from urban road under demolition in the Guangxi area in China, no detailed information was available on the age of the road, but most constructions in this area took place for no more than thirty years. Large parts of the waste concrete were crushed on site, then the crushed concrete were transported to the laboratory and broken by workers into pieces with size smaller than 20 mm and sieved with a proper gradation of 5–20 mm. The natural aggregate used were common crushed stone. Table 1 shows some of the measured physical properties of the natural and recycled aggregate. As expected, due to the presence of the attached mortar, recycled aggregate has higher water absorption and smaller density than natural aggregate. Using heat-resistant separation method can obtain the attached mortar content, the measured content of 30.55% is much higher than that of natural aggregate, and this feature directly affects the physical and mechanical properties of recycled concrete. Based on Regulation of Common Concrete Mix Design, the water-cement ratio and the unit water consumption were deter-
mined according to the design requirement, and then the amount of cement was determined. Finally, the aggregate content was calculated according to the reasonable sand ratio. For objective comparison on triaxial strength of concrete with different RCA replacement ratio, the unit water consumption was fixed and the water-cement ratio was adjusted to get the same uniaxial strength among different groups. The fine aggregate was natural river sand with a fineness modulus of 2.8. The cementitius material was 42.5R Portland cement, water was tap-water. Table 2 shows the dryweight proportions of mix designs. 2.2. Samples and testing method Samples were casted in plastic cube molds and cured for 24 h before demolded, and then placed in a moist room with a relative humidity of 95% at 20 3 C. RCA replacement ratio and stress ratio were the main factors that affect the mechanical properties of recycled concrete. Five levels of RCA replacement ratio (0%, 30%, 50%, 70%, 100%) and four kinds of stress ratios (a ¼ 0:1 : 0:25 : 1, 0:1 : 0:5 : 1, 0:1 : 0:75 : 1, 0:1 : 1 : 1) were considered in this study. The principal stresses were expressed as r1 P r2 P r3 , compression denoted as negative and tension denoted as positive. 100 mm 100 mm 100 mm cubes were used to measure the strength of specimens under triaxial compression using a servo-controlled test setup (TAWZ-5000/3000) at State Key Laboratory of Structural Engineering in Guangxi University. The test setup consisted of four independent hydraulic actuators, the vertical actuator with loading capacity of 5000 KN, and the others with capacity of 3000 KN. Two linear variable differential transformers(LVDT) were attached along each side of the specimen in loading direction. The tests were carried out at a loading rate of 0:5 MPa=s. A data acquisition system was used to record the applied load and LVDT readings. The deformation of the specimens was obtained by averaging the reading of the two LVDTs. The specimen and test setup are shown in Fig. 1. 3. Test results and discussion 3.1. Failure modes The crack patterns and failure modes were observed,in general, there was no fundamental difference between the failure modes due to different RCA replacement ratios. The typical crack patterns are shown in Fig. 2. The failure mode was mainly affected by stress ratio. At stress ratio a ¼ 0:1 : 1 : 1, due to the constraints of r2 and r3 , tensile strain occurred parallel to r1 direction. As the load
Table 1 Physical properties of coarse aggregate. Classification
Gradation (mm)
Apparent density (kg/m3)
Crush Index (%)
Water absorption (%)
Attached mortar content
NCA RCA
5–20 5–20
2708 2485
4.6 13.4
0.3 3.0
– 30.55%
Table 2 Mix proportions. Unit weight (kg/m3) Specimens
RCA%
Water/cement
Cement
Sand
Water
NCA
RCA
RAC0 RAC30 RAC50 RAC70 RAC100
0% 30% 50% 70% 100%
0.47 0.46 0.45 0.44 0.43
415 424 433 443 453
631 643 656 669 681
195 195 195 195 195
1121 767 536 314 0
0 329 536 733 1021
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(a) specimen wrapped with loading plates
(b) triaxial loading setup Fig. 1. Specimen and test setup.
¦ 3Ò
¦Ò 2
¦Ò 1
¦Ò 1
¦ 3Ò (a) plate-splitting cracks
¦Ò 3
¦Ò 1
¦Ò 1
¦Ò 3 (b) slant-shear cracks Fig. 2. Failure mode of RAC under triaxial compression.
¦Ò 2
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Fig. 3. ITZ of RAC.
increasing, the cracks gradually appeared on surface of r2 and r3 . When exceeding ultimate strain, plate-splitting cracks were formed on surface of r2 and r3 . The failure mode is shown in Fig. 2(a). At stress ratio a ¼ 0:1 : 0:25 : 1; 0:1 : 0:5 : 1 and 0:1 : 0:75 : 1, due to large shear stress ðr1 r3 Þ=2, slant-shear cracks appeared parallel to the direction of r2 . It can be seen that the single-shear and double-shear cracks were formed on the surface of r2 , and formed an angle about 20–30° with direction r3 . The failure mode is shown in Fig. 2(b). From a microscopic point of view, with the use of RCA, the structure of recycled concrete is much more complicated than that of natural concrete. As shown in Fig. 3, Recycled concrete has two weaker ITZs, one is between original aggregate and old mortar in RCA, and the other is between new and old cement paste. A large amount of pores and microcracks existed in the attached mortar, the pores and microcracks critically decreased the bonding strength of the concrete, therefore, the recycled aggregate concrete specimens presented more minute cracks with higher RCA replacement ratio.
3.2. Stress-strain relationship The stress-strain curves of recycled aggregate concrete under triaxial compression are shown in Fig. 4. The stress and strain increases proportionally during the initial stage of loading, the curve is approximately a straight line. With the increasing of stress, plastic deformation occurred in concrete specimens, the slope of the curve slowed down. The stress ratio has obvious influence on triaxial strength of recycled concrete. The maximum principal stress r3 is obtained at the stress ratio a ¼ 0:1 : 0:5 : 1, and the minimum value is obtained at a ¼ 0:1 : 1 : 1. The principal strain e3 under triaxial compression is obviously higher than that under uniaxial compression, this result indicates that the lateral restraint could effectively improve the stiffness and ductility of test specimens. The maximum strain e3 is obtained at the stress ratio a ¼ 0:1 : 0:25 : 1, and the minimum value is obtained at a ¼ 0:1 : 1 : 1. Due to the effects of stress r2 and r3 , tensile stress occurrs in the direction of r1 , therefore, the strain e1 is positive at all stress ratios. At the stress ratio a ¼ 0:1 : 0:25 : 1, strain e2 is positive, at other stress ratios, the value e2 turns to negative. It illustrates that stress r2 is relatively sensitive to the change of intermediate stress ratio. When the intermediate stress ratio is small, r2 is tensile stress, and then the stress value gradually becomes compressive as the intermediate stress ratio increased. The RCA replacement ratio has great effect on stress-strain relationship of recycled concrete. With the increase of the replacement ratio, the
principal stress r3 decreases gradually except for RAC30 which stress value is slightly higher than RAC0 due to the lateral restriction. For all test specimens, the principal strain e3 increases with the raise of RCA replacement ratio. In general, the linear section of stress-strain curve decreases and the slope of the curves is gradually gentle, it indicates that the initial modulus of elasticity decreases with the increase of RCA replacement ratio.
3.3. Triaxial strength Table 3 shows the ultimate strength of each specimen under triaxial compression. It can be seen that the strength of most specimens is higher than 200 MPa, the value is much greater than that of under uniaxial compression. At least three specimens were tested in each group, deleted the discrete values, taken the average value of the three samples that meet the requirements. Fig. 5(a) shows the influence of RCA replacement ratio on the average value of stress r3 . The triaxial strength of RAC30 is slightly higher than that of RAC0, when replacement ratio of RCA is higher than 30%, the triaxial strength gradually decreases with the increase ratio of RCA. The main reason for the higher strength of RAC30 can be explained as: for one thing, the uniaxial strength of RAC30 and RAC0 are close to each other, and the strength value of RAC0 is only 0.5% higher than that of RAC30; for another, lateral forces restrict the development of micro cracks, the adverse effects of internal micro cracks on compressive strength under triaxial stress states has been weakened due to lateral constraint. Fig. 5(b) demonstrates the influence of stress ratio on average stress r3 , it can be seen that the maximum triaxial compressive strength is obtained at stress ratio a ¼ 0:1 : 0:5 : 1, and the minimum strength is obtained at a ¼ 0:1 : 1 : 1. The average triaxial strength is normalized by uniaxial strength and expressed as r3 =f c . Fig. 6(a) shows the influence of RCA on normalized value of r3 =f c . Similar to the trend of stress r3 variation, the normalized value r3 =f c slightly increases as RCA replacement ratio changing from 0% to 30%, and decreases with the increase usage of RCA when the replacement ratio is larger than 30%. Fig. 6(b) demonstrates the influence of stress ratio on r3 =f c , it can be seen that the highest value occurred at stress ratio a ¼ 0:1 : 0:5 : 1, and the triaxial strength is about 5–6 times of the uniaxial strength. The minimum value of r3 =f c is obtained at a ¼ 0:1 : 1 : 1, and the triaxial strength is about 4–5 times of uniaxial strength. Overall, the strength of concrete specimens under triaxial compression is much higher than that of under uniaxial compression, the triaxial strength decreases with the increase of RCA replacement ratio except RAC30 which strength is slightly higher than RAC0, the
Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
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Fig. 4. Stress-strain curves for RAC under triaxial compression.
trend of triaxial strength with stress ratio is a-0.1:-0.5:-1 > a-0.1:-0.75:-1 > a-0.1:-0.25:-1 > a-0.1:-1:-1. A variance calculation of test data is made and the quantitative analysis of two factors: RCA replacement ratio and stress ratio on triaxial strength is given. Taken r3 =f c as the dependent variable, stress
ratio(A) and RCA replacement ratio(B) as independent variables. There are four levels of factor A(r = 4): 0.1:0.25:1, 01:0.5:1, 0.1:0.75:1 and 0.1:1:1, and five levels of factor B(s = 5): 0%, 30%, 50%, 70%, 100%. Variation of dependent variable caused by the two factors A and B can be calculated by equations as follows:
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Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
r3 under different stress ratios (MPa).
No
r1 : r2 : r3
r3 01(MPa)
r3 02(MPa)
r3 03(MPa)
RAC0
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
210.69 287.07 260.65 239.30
250.26 274.95 241.15 217.72
271.49 259.38 258.80 210.11
RAC30
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
236.44 279.92 248.01 225.92
238.66 274.07 257.71 211.66
272.09 286.69 287.63 242.52
RAC50
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
227.30 210.90 235.80 211.30
213.10 251.00 192.90 197.90
212.70 244.80 257.60 187.00
RAC70
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
218.08 223.67 229.27 194.32
206.62 209.70 195.25 173.25
192.27 204.10 203.73 177.36
RAC100
0.1:0.25:1 0.1:0.5:1 0.1:0.75:1 0.1:1:1
182.95 193.72 193.09 164.91
209.78 185.10 209.06 175.59
169.58 223.68 174.16 169.49
360
-0.1:-0.25:-1 -0.1:-0.75:-1
360
-0.1:-0.5:-1 -0.1:-1:-1
300
MPa 3
MPa
300 240
3
180
180 120
60
60 0 0%
30%
50% RA%
70%
RAC30 RAC70
240
120
0
RAC0 RAC50 RAC100
100%
0.25
0.50
0.75 2
(a) with different RCA replacement ratio
/
1.00
3
(b) with different stress ratio
Fig. 5. Triaxial strength with RCA replacement ratio and stress ratio.
8
6
6
3
3
c
c
8
4 2 0
0.1:0.25:1 0.1:0.75:1
0.1:0.5:1 0.1:1:1
4 2 0
0%
30%
50% 70% 100% RA% (a) with different RCA replacement ratio
0.25
0.50 / 2
RAC0 RAC50 RAC100
RAC30 RAC70
0.75
1.00
3
(b) with different stress ratio
Fig. 6. Normalized triaxial strength with RCA replacement ratio and stress ratio.
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Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052 r X 2 SSA ¼ s ðX i X i Þ
ð1Þ
Table 4 Analysis of influencing factors.
i¼1 s X 2 SSB ¼ r ðY j Y j Þ
ð2Þ
SSA
SSB
SST
RA
RB
2.171
2.890
5.196
41.79%
55.61%
j¼1
SST ¼
r X s X 2 ðX ij X i Þ
ð3Þ
i¼1 j¼1
RA ¼ ðSSA=SSTÞ 100%
ð4Þ
RB ¼ ðSSB=SSTÞ 100%
ð5Þ
where X i ; Y j are the average value of factor A and B under different levels, the calculated results is shown in Fig. 7. X i and Y j are the general average values, X i ¼ Y j ¼ 4:95. SSA is the variation of r3 =f c caused by factor A, and SSB is the variation of r3 =f c caused by factor B, SST is the total sum of variations. RA is the influence rate of factor A, RB is the influence rate of factor B. Fig. 7(a) shows the average value of r3 =f c under different levels of factor A. It can be seen that the maximum value is obtained at intermediate stress ratio r2 =r3 ¼ 0:5, the minimum valued is obtained at r2 =r3 ¼ 1. Fig. 7(b) shows the average value of r3 =f c under different levels of factor B. It can be seen that the maximum value occurred when RCA replacement ratio is 30%, the general trend of r3 =f c with RCA replacement ratio is RAC30 > RAC0 > RAC50 > RAC70 > RAC100. Table 4 gives the results calculated by Eqs. (1)–(5). It can be seen that the value of RA is 41.79%, the value of RB is 55.61%, and it indicates that the influence of factor A on r3 =f c is slightly smaller than the influence of factor B. The results demonstrate that the strength of RAC under triaxial compression is influenced both by RCA replacement ratio and stress ratio. 3.4. Octahedral stress The strength of RAC under triaxial compression can be depicted in octahedral stress space[33], the normal stress roct and the shear stress soct can be calculated from:
1 3
roct ¼ ðr1 þ r2 þ r3 Þ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðr1 r2 Þ2 þ ðr2 r3 Þ2 þ ðr3 r1 Þ2
ð7Þ
3.5. Deformation characteristic Table 5 shows the peak strain of RAC specimens under triaxial compression, positive means tensile and negative means compression. Fig. 9(a) shows the influence of stress ratio on principal strain e3 . The value of e3 decreases with the increase of intermediate stress ratio r2 =r3 , the maximum strain is obtained at stress ratio a ¼ 0:1 : 0:25 : 1, and the minimum strain is obtained at a ¼ 0:1 : 1 : 1, the general trend of e3 with intermediate stress ratio is a0:25 > a0:5 > a0:75 > a1 . Fig. 9(b) illustrates the influence of RCA replacement ratio on strain e3 . It shows clearly that the strain value increases as RCA replacement ratio increased. The pores and microcracks existed in attached mortar critically increase the deformation of RAC specimens. Therefore, strain e3 of RAC is slightly higher than that of normal concrete. Fig. 10(a) shows the influence of stress ratio on strain e2 . It can be seen that strain e2 is positive as the intermediate stress ratio r2 =r3 ¼ 0:25, and the value changes to negative at other stress ratios. The main reason for this change is that the stress in direction r2 is less than the extrusion pressure from direction r3 , tensile stress is eventually occurred in direction r2 due to Poisson’s effect, so tensile strain is appeared in direction r2 when intermediate stress ratio is small. Fig. 10(b) demonstrates the influence of RCA replacement ratio on strain e2 . It can be seen that the strain e2 increases as the RCA replacement ratio increased. It illustrates that whether tensile or compressive stress occurs in direction r2 , micro cracks in recycled concrete increase the deformation in this direction. Fig. 11(a) shows the influence of stress ratio on strain e1 . It can
6
6 5.34
5.31
5.46
5.10
4.90
4.85
4.64
4.44
4
c
c
4.47
4
3
1 3
3
soct ¼
ð6Þ
Fig. 8 shows the relationship between normalized octahedral normal stress roct =f c and octahedral shear stress soct =f c with RCA replacement ratio and stress ratio. Fig. 8(a) indicates that the variation of octahedral shear stress soct =f c with stress ratio is not obvious. Stress ratio has a great effect on octahedral normal stress roct =f c , especially from the intermediate stress ratio a ¼ r2 =r3 ¼ 0:25 to a ¼ 0:5, and from a ¼ 0:5 to a ¼ 0:75. Fig. 8(b) shows the influence of RCA replacement ratio on roct =f c and soct =f c , it can be seen that except RAC 30, the roct =f c decreases with the RCA replacement ratio increased, the same trend is performed in soct =f c .
2
2
0
0 0.25
0.50
/ 2
0.75
1.00
0%
30%
3
(a) with different stress ratio
50% 70% RA%
100%
(b) with different RCA replacement ratio
Fig. 7. Influence of stress ratio and RCA replacement ratio on
r3 =f c .
Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
2.4
2.0
2.0
oct
c
2.4
oct
c
1050
1.6
1.6 0.1:0.25:1 0.1:0.75:1
1.2
2.0
2.4
2.8
3.2 oct
RAC0 RAC50 RAC100
0.1:0.5:1 0.1:1:1
3.6
1.2
4.0
2.0
2.4
2.8
c
oct
(a) with different stress ratio
RAC30 RAC70
3.2
3.6
4.0
c
(b) with different RCA replacement ratio
Fig. 8. The relationship between octahedral normal stress and octahedral shear stress.
Table 5 Triaxial strain under different stress ratios (103 ). Specimens
r1 : r2 : r3
e1 (103)
e2 (103)
e3 (103)
RAC0
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
13.97 16.05 18.49 24.04
6.27 12.42 23.36 30.04
39.76 37.51 34.89 30.04
RAC30
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
15.15 17.78 20.33 26.62
7.86 14.15 24.01 32.82
41.69 39.28 36.51 32.82
RAC50
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
17.56 19.04 20.17 27.26
9.29 17.22 26.14 35.76
44.28 42.49 39.78 35.76
RAC70
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
18.45 19.79 21.27 28.29
9.83 18.39 27.87 37.48
46.39 43.96 41.81 37.48
RAC100
0.1:0.25:1 0.1:0.50:1 0.1:0.75:1 0.1:1.00:1
18.71 20.35 22.18 28.31
10.5 20.17 28.49 39.69
47.78 45.67 42.79 39.69
-60
-60
-45
-45
-30
-30
-15 0
(a)
0.25
0.50
RAC0 RAC50 RAC100
RAC30 RAC70
0.75
1.00
with different stress ratio
-15 0
(b)
0.1:0.25:1 0.1:0.75:1
0%
30%
0.1:0.5:1 0.1:1:1
50% 70% 100% RA%
with different RCA replacement ratio
Fig. 9. Influence of stress ratio and RCA replacement ratio on
e3 .
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Z. Deng et al. / Construction and Building Materials 156 (2017) 1043–1052
-60
-60
-45
-45
-30
-30
-15
-15
0 15
(a)
0.25
0.50
RAC0 RAC50 RAC100
RAC30 RAC70
0.75
1.00
with different stress ratio
0.1:0.25:1 0.1:0.75:1
0 15
(b)
0%
30%
50% 70% 100% RA%
with different RCA replacement ratio
Fig. 10. Influence of stress ratio and RCA replacement ratio on
30
30
25
25
20
20
15
15
10
(a)
e2 .
10 RAC0 RAC50 RAC100
5 0
0.1:0.5:1 0.1:1:1
0.25
0.50
0.75
RAC30 RAC70
1.00
with different stress ratio
0.1:0.25:1 0.1:0.75:1
5 0
(b)
0%
30%
50% 70% 100% RA%
with different RCA replacement ratio
Fig. 11. Influence of stress ratio and RCA replacement ratio on
be seen that the strain e1 is positive under all kinds of stress ratios due to the expansion force produced by extrusion pressure from r2 and r3 , and the value increases as the intermediate stress ratio increased. Fig. 11(b) shows the influence of RCA replacement ratio on strain e1 , it reveals that the value increases slightly with the increase of RCA replacement ratio, micro cracks in recycled concrete also increase the strain e1 . 4. Conclusion
0.1:0.5:1 0.1:1:1
e1 .
(4) The stress ratio mainly affects the octahedral normal stress roct =f c while RCA replacement ratio affects both roct =f c and soct =f c . (5) The adverse effects of internal pores and micro cracks on triaxial strength of recycled concrete have been weakened due to lateral constraint when RCA replacement ratio is less than 30%, but the triaxial strain continues to increase with the increase of RCA content, for widely used in construction, the quality and dosage of RCA should be taken into consideration.
Based on the experimental work and the analysis of the test results, the following conclusions can be drawn: (1) Under triaxial compression, there are two typical crack patterns of test specimens: one is plate-splitting crack at stress ratio a ¼ 0:1 : 1 : 1, the other is slant-shear crack when at other stress ratios, and the crack patterns do not depend on RCA content. (2) The difference in stress-strain curves of recycled concrete is attributed to the effect of RCA replacement ratio, the initial slope of the curves is gradually more gentle with the increase of RCA replacement ratio (3) The strength of recycled concrete under triaxial compression is much higher than that of under uniaxial compression, the trend of triaxial strength with stress ratio is a0:1:0:5:1 > a0:1:0:75:1 > a0:1:0:25:1 > a0:1:1:1 , and the trend with RCA content is RAC30 > RAC0 > RAC50 > RAC70 > RAC100.
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