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Mechanical properties of recycled concrete made with different types of coarse aggregate
Construction and Building Materials 134 (2017) 497–506
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Mechanical properties of recycled concrete made with different types of coarse aggregate Chunheng Zhou a, Zongping Chen a,b,⇑ a b
College of Civil Engineering and Architecture, Guangxi University, 530004 Nanning, China Key Laboratory of Disaster Prevention and Structure Safety of Chinese Ministry of Education, 530004 Nanning, China
h i g h l i g h t s Mechanical properties test for two types of recycled coarse aggregate concrete were performed and the the results were discussed. Equations of relationships between the compressive strength and flexural strength of recycled aggregate concrete were presented. Expressions of stress-strain curves for recycled coarse aggregate concrete were proposed.
a r t i c l e
i n f o
Article history: Received 17 July 2016 Received in revised form 27 December 2016 Accepted 28 December 2016
Keywords: Recycled aggregate concrete Recycled crushed rock aggregate Recycled pebbles aggregate Mechanical properties Strength relationship equation Stress-strain equation
a b s t r a c t One of the most important issues that determine many properties are the types of coarse aggregate in recycled concrete. This paper is arming to experimental study the influence of two different types of coarse aggregate (recycled crushed rock aggregate and recycled pebbles aggregate) on the mechanical properties of recycled concrete. The properties of these two types of recycled coarse aggregate (RCA) derived from waste concrete were investigated. Using these RCA and corresponding types of natural coarse aggregates, recycled concrete specimens according to different replacement percentage were produced and tested. Finally, Analysis and comparisons of the mechanical properties of RAC, including compressive strength, flexural strength, elastic modulus and Poisson’s ratios etc., were made between these recycled concretes. The results show that different types of RCA have significant variance in mechanical properties. The recycled concrete containing crushed rock aggregate presents a lower relative strength and elastic modulus than that containing pebble aggregate while the toughness of them is contrary. The theoretical expression for stress-strain relationship and equations for the relationships between various strength of recycled concrete with each type of coarse aggregate are also presented. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Mix proportions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Specimens preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Test results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Failure pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Cube compressive failure mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Prism compressive failure mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Stress-strain curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Flexural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author at: College of Civil Engineering and Architecture, Guangxi University, 530004 Nanning, China. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.163 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
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4. 5. 6.
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3.5. Static modulus of elasticity and Poisson’s ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Energy absorption capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationships between strengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approximation of the stress-strain relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction With economy development, rapid growth in urbanization has led to huge scale new construction, especially in some emerging economies countries. These construction works require large quantities of consumption and production of natural aggregate, which result in the intensification of natural aggregate resources shortage and the difficulty of sustainable development. Furthermore, a large number of old structures, which are nearing the end of their life span, need to dismantle and replace in many countries, result in the production of large amounts of construction and demolition waste. In China, the amount of construction and demolition waste has amounted to 30%–40% of the total city solid wastes [1], which will significantly increase the load of landfill. Therefore, recycling of concrete demolition waste as recycled aggregate to partially or fully substitute natural aggregate for recycled aggregate concrete (RAC) has been recognized as an effective way to offset the shortage of natural aggregate, disposal of waste concrete and related environment problem [1,2]. In recent years, even though reuse of RCA to make RAC has received increasing research interest in academic and has been extensively studied, the practical engineering application of RAC is still low [3] or mainly in non-structural concrete [4] explained by the disadvantage and discreteness of their properties, including strength, elastic modulus, toughness, stress-strain relationship, and so on, compare with natural aggregate concrete (NAC) [5– 12]. There are many factors related to the deterioration of properties for RAC, which limits its widespread used in structural concrete. Firstly, the replacement percentage (RP), which is defined as the ratio between the weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix [8], is deemed as a vital influence on the properties of RAC [13]. Xiao et al. [6] and Topçu et al. [14] found that the strength of RAC decreases as RP increases while Ho et al. [7] and Etxeberria et al. [9] observed the opposite results. The contradiction of the results was found due to the quality loss of RCA which to be mainly depended on the specific gravity of RCA [18], the moisture state of RCA [5,19], the amount of old adhered mortars [20]. Furthermore, the strength [15,16] and the age of original concrete [17] have a significant impact on the properties of RAC. Researches show that the concrete made with RCA from original concrete with weak strength resulted in lower strength. But the quality of RCA had little influence on the strength of RAC when comparing it to those of high performance conventional concrete. The properties of RAC made with RCA crushed at age 3 days were worse than those made with aggregate crushed at age 1 or 28 days. Most studies have indicated that, no matter what factors, the fluctuation and adverseness of the mechanical and physical properties of RAC is ultimately due to bad quality and weak link of aggregate-cement matrix interfacial transition zone, which the failure of RAC is often occurring [11,21–24]. However, previous research has shown that different coarse aggregate types strongly influence the mechanical properties of the interfacial transition zone (ITZ) [25] due to the significant difference of their surface and shape. Wu et al. [26] and Rocco et al. [27] considered the impact of coarse aggregate type on mechanical properties of con-
502 503 504 504 505 505 505
crete, and they found that different types of coarse aggregate has significant effect on the fracture energy further result in the variation of concrete strength. Concrete made with crushed aggregates provides higher values of the fracture energy than for concrete made with spherical ones. Ribeiro et al. [28] also found that, due to their smoother surface and therefore weaker anchorage, pebble aggregates present areas dislodged from the matrix, leading to concretes with them have lower fracture energy than those with crushed rock. Summarizing the existing research found that the influence of different types of coarse aggregate on mechanical performance of RAC is not a well-known area while that has been extensively studied on conventional concrete. Nevertheless, due to the adhered old mortar, the RCA originated from crushed concrete with different types of natural coarse aggregate have further variation compared with accordingly natural coarse aggregate (NCA). The mechanical performance of recycled concrete made with them is a meaningful area requiring further research. Consequently, this paper presents an experimental study on the mechanical properties, including compressive strength, flexural strength, stress-strain curves, elastic modulus, Poisson’s ratios and energy absorption capacity, of recycled concrete made with different types of coarse aggregate. 2. Experimental program 2.1. Materials The constituent materials describe as follows: Ordinary Portland cement with a 28-day compressive strength of 42.5 MPa. Fine natural aggregate (medium-coarse river sand). Two types of NCA: crushed rock aggregate (CRA) and pebbles aggregate (PA). They all had a maximum size of 20 mm and a minimum size of 5 mm. Two types of RCA originated from crushed waste concrete with crushed rock and natural pebbles as coarse aggregate respectively, which are denominated RCRA and RPA. They are in the same maximum and minimum size of NCA. The crushed waste concrete collected from concrete block produced in laboratory with target strength of 30 MPa . All RCA were produced by a jaw crusher, and were then sieved to obtain aggregates with required size. After that, they were washed with water to remove surface fine particles such as dust and clay. The RCRA is composed of crushed natural rock and mortar. And likewise, the RPA is composed of natural pebbles and mortar. However, the content of mortar for RCRA and RP is different due to the various shapes and surfaces of CRA and PA. CRA is irregular and has a rough surface while PA is approximately rounded and has a smooth surface. In Fig. 1, two types of natural coarse aggregate and two types of recycled coarse aggregate are presented. The physical properties determined on NCA and RCA were tested according to Chinese code GB/T 14685-2011 [29]. The amount of adhered mortar in RCA was measured using the
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(a) CRA
(b) PA
(c) RCRA
(d) RPA
Fig. 1. Characteristics of NCA and RCA.
Table 1 Physical properties of NCA and RCA. Coarse aggregate
Grading (mm)
Moisture content (%)
Water absorption (%)
Apparent density (kg/m3)
Bulk density (kg/m3)
Adhered mortar (%)
CRA RCRA PA RPA
5 20 5 20 5 20 5 20
0.01 1.82 0.01 1.69
0.05 3.16 1.51 3.54
2722 2655 2685 2640
1435 1270 1540 1203
0 5.5 0 3.8
methodology given in [20]. And the percentage of adhered mortar was calculated using Eq. (1)
M c ð%Þ ¼
m1 m2 100 m1
ð1Þ
where Mc (%) is the amount of adhered mortar in RCA, m1 is the total mass of the RCA sample, m2 is the mass of the same NCA sample. Specific physical properties, including moisture content, water absorption, apparent density, bulk density and adhered mortar content are shown in Table 1. From the Table 1, it is clear that the recycled aggregates have higher water absorption, lower apparent density and bulk density than corresponding natural aggregate. This is because the attached mortar, which is an important composition of recycled aggregate, has very low specific gravity and high water absorption due to porousness. RPA has much lower relative density and water absorption in reference to PA than that of RCRA in reference to CRA, and is maybe because that the mortar adhered to RPA is less than RCRA due to the difference between the shapes and surfaces of CRA and PA. 2.2. Mix proportions To investigate the effect of coarse aggregate type, four different kinds of coarse aggregates were used to produce concrete mix with 28-days target strength of 30 Mpa. A total of 22 concrete mixes were designed by means of the various coarse aggregate types and replacement percentages. One category of RAC mixes (hereafter called RCRC) which were prepared by replacing the RCRA to CRA with different replacement of 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% by weight of total coarse aggregate
content were labeled as RCRC-x, with ‘‘x” representing the replacement percentage of RCRA. The other category of RAC mixes (hereafter called RPC) which were prepared by replacing the RPA to PA with the same replacement percentage as the former were labeled as RPC-x, with ‘‘x” representing the replacement percentage of RP. In the case of RCA replacement percentage equal 0%, the concrete are normal concrete, which are regarded as control mix and named CRC and PC respectively. Because RCA has higher water absorption than NCA as previously stated, RAC often produced with the presaturation of RCA method [19] and mixing water compensation method [30] to meet the workability of freshly mixed. In this text, the RAC were produced with coarse aggregate at air-dried condition, and the amount of water was adjusted by the actual moisture contents and the water absorption capacity of aggregates according to [5,31]. Due to the water absorption capacity of RCA is much higher than that of natural aggregate, the amount of water increased as the RP increased even considering that RCA had higher moisture contents. All the 22 mixtures of fresh concrete bring out good workability. The workability of concrete was measured following concrete slump test according to GB/T 50080-2002 and the slump of all kinds of concrete mixes were 100–120 mm. All proportions of coarse aggregates, fine aggregates, cement and water were calculated based on constant water-cement ratios of 0.49 and 0.47 for RCRC and RPC respectively. There are no superplasticizers were used in all mixes. The detail mixture proportions of different concrete are shown in Tables 2 and 3. 2.3. Specimens preparation The procedure of preparing the concrete mixtures was conducted in laboratory conditions. The coarse aggregates, sand and
Table 2 Mix proportions (kg/m3) of RCRC. Mix
RP (%)
Water-cement ratio
Cement
Water
Sand
CRA
RCRA
RCRC-0 RCRC-10 RCRC-20 RCRC-30 RCRC-40 RCRC-50 RCRC-60 RCRC-70 RCRC-80 RCRC-90 RCRC-100
0 10 20 30 40 50 60 70 80 90 100
0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49
398 398 398 398 398 398 398 398 398 398 398
195.0 196.6 198.2 199.8 201.4 203.0 204.6 206.2 207.8 209.4 211.0
614 614 614 614 614 614 614 614 614 614 614
1193 1074 954 835 716 597 477 358 239 119 0
0 119 239 358 477 597 716 835 954 1074 1193
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Table 3 Mix proportions of recycled RPC. Mix
RP (%)
Water-cement ratio
Cement
Water
Sand
PA
RPA
RPC-0 RPC-10 RPC-20 RPC-30 RPC-40 RPC-50 RPC-60 RPC-70 RPC-80 RPC-90 RPC-100
0 10 20 30 40 50 60 70 80 90 100
0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47
404 404 404 404 404 404 404 404 404 404 404
208.4 208.9 209.3 209.7 210.1 210.6 211.0 211.4 211.8 212.3 212.7
614 614 614 614 614 614 614 614 614 614 614
1228 1105 982 860 737 614 491 368 245 123 0
0 123 245 368 491 614 737 860 982 1105 1228
minutes by a concrete vibrating table and demolded 24 h after casting, and then were cured in a curing chamber (20 ± 2 °C temperature,90% relative humidity) for 28 days. They were then kept in laboratory at temperature of about 26 °C until testing. 2.4. Tests The loading tests were performed on a RMT-201 rock and concrete material test setup as shown in Fig. 2. During the loading procedure, the axial compression and displacement of specimens were recorded by data acquisition system automatically. The cube compressive strength, axial compressive strength, flexural strength and elastic modulus were determined in accordance with Chinese code GB/T 50081-2002. In order to obtain the complete stress-strain curves of concrete, the load process on 150 150 300 mm prisms was controlled by displacement under a constant rate of 0.005 mm/s. The Poisson’s ratio was measured by two strain gauges. One was placed vertically in the middle a third part of specimen to measured the vertical strain, the other was placed at 180° for vertical loading direction in the middle part of specimen to measured the hoop strain. In order to obtain the accurate deformation of concrete, each specimen was pre-loaded to about 30% of the estimated peak load to eliminate the effect of the gap between specimen and test setup. 3. Test results and discussion The test results of stress-strain curves, cube strength, axial strength, flexural strength, elastic modulus and Poisson’s ratios of RAC were determined on types and replacement percentage of RCA. Table 4 lists the test results which derived from the average of the three specimens at same mix. 3.1. Failure pattern
Fig. 2. Test setup.
cement were mixed for two mins before water was added, and than were mixed for three mins when water was added. For each of concrete mixture, three 150 mm cubes, three 150 150 300 mm prisms and three 150 150 450 mm prisms were cast in steel moulds. The cubes were used to determine the cube compressive strength. The 150 150 300 mm prisms were used to obtain the Poisson’s ratio, static elasticity modulus, axial compressive strength and stress-strain curve. As for the 150 150 450 mm prisms were used to test the flexural strength. The specimens were vibrated and compacted about three
3.1.1. Cube compressive failure mode With regard to same type of coarse aggregate, the failure processes and modes for concrete specimens were similar. In the initial loading stage, i.e., the stage for loading on the specimen before 30% ultimate load, no visible cracks can be seen on the surface of sample. With the increase in compressive loading, a few thin and vertical micro cracks were observed in the middle of specimen, and they then propagated to some inclined cracks whose angle are about 45° with respect to the load-direction. When the loading on the specimen was close to the peak load, it was a crack zone for a shape of an X to be formed into the surface of the specimen. Whereafter, the concrete was spalling gradually. After the loading procedure, it can be observed that scarcely any fracture of coarse aggregates could be seen from the fracture plane except a few flaky particles in CRA and RCRA. It indicated that the cracks mainly exist
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C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506 Table 4 Mechanical index of RAC. RP (%)
fcu (MPa)
fc (MPa)
fr (MPa)
Elastic modulus (GPa)
Poisson’s ratio
RCRC
RPC
RCRC
RPC
RCRC
RPC
RCRC
RPC
RCRC
RPC
0 10 20 30 40 50 60 70 80 90 100
41.71 37.50 43.48 42.03 44.01 42.40 42.33 43.94 50.50 43.73 44.26
29.88 27.95 29.72 34.86 32.60 29.84 32.39 37.02 40.11 31.85 41.16
37.01 29.66 39.54 38.33 33.82 42.20 42.33 40.70 44.31 42.32 43.57
26.82 25.80 28.86 28.59 27.08 29.09 30.68 32.67 35.22 26.00 35.27
4.30 4.31 4.20 3.96 4.80 5.11 4.31 5.19 6.28 4.27 5.07
5.20 4.87 5.52 5.80 5.85 5.48 5.54 6.29 6.39 5.11 6.30
46.40 44.66 39.74 38.53 38.29 39.50 40.87 40.43 40.78 41.27 40.80
44.39 39.33 39.36 39.59 36.50 41.00 35.77 36.46 38.24 34.18 36.33
0.21 0.21 0.21 0.22 0.21 0.21 0.21 0.21 0.21 0.22 0.21
0.22 0.22 0.22 0.23 0.22 0.22 0.22 0.22 0.22 0.23 0.22
Note: fcu is cube compressive strength; fc is axial compressive strength; fr is flexural compressive strength.
for different replacement percentages were similar. The typical failure patterns of cube specimens are shown in Fig. 4. 3.2. Stress-strain curves Base on the measured data of axial load and deformation, the stress and strain could be calculated using Eq. (2)
r ¼ N=A; ¼ Dl=l (a) RCRC-20
(b) RPC-20
Fig. 3. Typical failure patterns of cube specimens.
ð2Þ
where N is the axial load, A is the cross-sectional area of the specimen, Dl is the axial deformation, l is the vertical length of the specimen. As the stress-strain curves of the three specimens with the same mix are similar to each other, one of them was chosen as representative curve. The stress-strain curves for all mixes of concrete are shown in Fig. 5. It can be seen clearly from Fig. 5 that the shapes of the stressstrain curve for concrete with different types of coarse aggregate are considerably different. The curvatures of the ascending branch of RCRC are higher than those of RPC while the descending branches of RPC are more flat than these of RCRC. Most of the strains at the peak stresses of RCRC are higher than those of RPC. It is also worth mentioning that the strain at the peak stress is increased with the increase of RP, either RCRC or RPC, due to the decrease of elasticity modulus. 3.3. Compressive strength
(a) RCRC-10
(b) RPC-70
Fig. 4. Typical failure patterns of prism specimens.
in the interface of mortar and aggregate. The typical failure patterns of cube specimens are shown in Fig. 3. 3.1.2. Prism compressive failure mode In the early loading stage, the prism specimens also did not show any cracks. When the load reaches the peak load of specimen, the sound caused by cracking could be heard while some discontinuous vertical micro cracks appeared on the surface of the specimen. After the strain exceeds the peak strain, the cracks propagated quickly to penetrate the specimen. Through the comparison for concretes with different types of coarse aggregate, it is of interest to note that the failure process of RCRC was faster than that of RPC. The failure modes of prisms of recycled concrete
Fig. 6 shows the relative cube compressive strength and axial compressive strength of different RAC mixes with respect to the strength of accordingly control mix. It can be observed that the variation on relative compressive strengths with the change of replacement is significantly, which is consistent with the results of previous study [7,9]. The strength of RAC is comparable and even exceeded the control concrete, either cube compressive strength or axial compressive strength. In addition, the compressive strength fluctuates, but has a tendency to increase with the increase of RP due to the much higher absorption of RCA. It would give rise to a decrease of the effective water-cement ratio of concrete mixture in the interfacial transition zone consequently improving bonding strength between aggregate and cement, when the RCA which absorbed more water during mix in air-dried state were used in the mixes, with the increase of RP. It can also be noted that most of the RPC have a higher relative strength than RCRC. It may be attributed to that the RPA was crushed along with its own original texture result in its more angular than the PA, which would reinforced the weakest interface between new cement and aggregate.
C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506
40
CRC PC
30
Stress (Mpa)
20 10
40 RCRC-10
30
RPC-10
20 10
0
2
4 6 8 Strain (10-3)
10
2
4
6
8
10
0
(b) RP=10%
20 10 0
RCRC-40
30
Stress (Mpa)
Stress (Mpa)
RPC-30
RPC-40
20
4 6 8 Strain (10-3)
10
10
RPC-60
Stress (Mpa)
RCRC-60
20 10 0
2
4 6 8 Strain (10-3)
10
40
RCRC-70
30
RPC-70
0
10
10
4 6 8 Strain (10-3)
10
40
RCRC-80
30
RPC-80
20 10 0
0
2
4
6
8
10
Strain (10 )
0
4 6 8 Strain (10-3)
(h) RP=70%
(i) RP=80%
-3
(g) RP=60% Stress (Mpa)
2
(f) RP=50%
20
40
RCRC-90
30
RPC-90
Stress (Mpa)
4 6 8 Strain (10-3)
RPC-50
10
0 2
RCRC-50
30
(e) RP=40%
30
0
40
0 0
(d) RP=30% 40
10
20
0 2
4 6 8 Strain (10-3)
(c) RP=20%
40 RCRC-30
0
2
Strain (10-3)
40
Stress (Mpa)
10 0
0
(a) RP=0% 30
RPC-20
20
0
0
Stress (Mpa)
RCRC-20
30
Stress (Mpa)
Stress (Mpa)
40
Stress (Mpa)
502
20 10
40
RCRC-100
30
RPC-100
2
10
20 10 0
0 0
2
8 4 6 Strain (10-3)
10
0
2
4
6
8
10
Strain (10-3)
(k) RP=100%
(j) RP=90%
Fig. 5. Representative stress-strain curves of specimens.
1.6
1.4 1.2 1.0 0.8
RCRC RPC
0.6 0.4 0
10 20 30 40 50 60 70 80 90 100
Relative axial compressive strength
Relative cube compressive strength
1.6
1.4 1.2 1.0 0.8
RCRC RPC
0.6 0.4 0
10 20 30 40 50 60 70 80 90 100
RP (%)
(a) cube compressive strength
RP (%)
(b) axial compressive strength
Fig. 6. Relative compressive strength of RAC.
3.4. Flexural strength The relative flexural strength of all concrete mixes is presented in Fig. 7. It is of interest that the flexural strength also has a rising tendency with the increase of RP due to the decrease of actual water-cement ratio. However, RPC did not have a higher relative strength than RCRC absolutely. It may be that the interface
between new cement and aggregate is no longer the weakest interface under the condition of bending. 3.5. Static modulus of elasticity and Poisson’s ratio The relative static elastic modulus of all concrete mixes is shown in Fig. 8. It can be noticed from Fig. 8 that all elastic modu-
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50 RCRC RPC
1.4 1.2
40
f c (MPa)
Relative flexural strength
1.6
1.0 0.8
RCRC RPC
0.6
30 GB50010-2010
0.4
20 0
10
20
30
40
50
60
70
80
90
100
20
30
40
50
60
f cu (MPa)
RP (%) Fig. 7. Relative flexural strength of RAC.
Fig. 10. Relationship between cube compressive strength and axial compressive strength.
1.4
8
RCRC RPC
1.2
7
1.0
6
f r (MPa)
Relative elastic modulus
1.6
0.8 0.6
RCRC RPC
5 4
0.4 0
10
20
30
40
50
60
70
80
90
JTG D40-2011
100 3
RP (%)
20
30
40
60
Fig. 11. Relationship between cube compressive strength and flexural compressive strength.
0.15
0.10 RCRC RPC
0.05
0.00 0
10
20
30
40
50
60
70
80
90 100
RP (%) Fig. 9. Toughness of RAC.
lus of RAC were lower than that of the control concrete due to the decreasing of stiffness and bulk density of RCA [2]. For RCRC, the reduction of elastic modulus was range from 4% to 18% of the control concrete while the reduction of those was range from 10% to 22% for RPC. Furthermore, the increase of RP reduces the elastic modulus overall for both RCRC and RPC, which can also attribute to the decreasing of stiffness and bulk density of RCA, but in the case of the elastic modulus of RCRC with RP greater than 50% become constant. From the Table 4 it can be seen that not only the Poisson’s ratio of RCRC but also those of RPC was comparable and even little higher (about 5%) than those of control concrete.
calculated up to different specified stress or strain value in previous study [32,33]. In this study, the area under stress-strain curve, which is competed up to 60% peak stress value at the descending branch, was used to evaluate the toughness of RAC. The calculated results of the toughness for all mixes of concrete are shown in Fig. 9. From the Fig. 9, it is clear that most of the toughness of RCRC were higher than those of RPC which indicated that the RAC with RPA has higher energy absorption capacity. The fact can be attributed to concrete with crushed rock aggregates have higher fracture energy than those with pebble aggregates due to the irregular and rougher surface [28]. With an increase of RP, the toughness has a
50
Predicted values of f c (MPa)
0.20
Thoughness (Mpa)
50
f cu (MPa)
Fig. 8. Relative static elastic modulus of RAC.
40
30 RCRC RPC
20
3.6. Energy absorption capacity The energy absorption capacity (i.e. toughness) of concrete in compression had been defined as the area under stress-strain curve
20
30
40
50
Experimental values of f c (MPa) Fig. 12. Comparison of experimental and predicted values by Eq. (5).
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tionship between cube compressive strength and axial compressive strength, and the relationship between cube compressive strength and flexural strength for RAC.
Predicted values of f r (MPa)
7
6
f c ¼ ðar2 þ br þ cÞ 0:89f cu 2
ð5Þ
0:75
f r ¼ ðlr þ mr þ nÞ 0:4f cu
5
4
RCRC RPC
3 3
4
5
6
7
Experimental values of f r (MPa) Fig. 13. Comparison of experimental and predicted values by Eq. (6).
tendency in general to decrease first than increase both for RCRC and RPC. The first decrease is due to the bad quality of RCA which result from the attached old mortar. However, the later increase is mainly because of that the decrease of actual water-cement ratio in the interfacial transition zone. 4. Relationships between strengths The relationship between cube compressive strength and axial compressive strength, and the relationship between cube compressive strength and flexural compressive strength for conventional concrete stipulated by Chinese code GB50010-2010 [34] and JTG D40-2011[35] are expressed as follow respectively:
f c ¼ 0:76f cu
ð3Þ
2=3
f r ¼ 0:438f cu
ð4Þ
where: fc is axial compressive strength, fcu is cube compressive strength, fr is flexural compressive strength. Comparisons of experimental values and calculated values of fc and fr by Eqs. (3) and (4) are shown in Figs. 10 and 11 respectively. From them it can be concluded that the relationship equations adopted by codes for conventional concrete are not applicable to those for RAC. It may be attributed to the different aggregatemotor bond of RCA under various forces. As a result, we suggest the following regression equations based on the experimental results in this study for expressing the rela-
where r is replacement percentage of RAC, a, b, c, l, m and n are constant parameters to be determined for RAC with different types of coarse aggregate. Base on the experimental data in this study, the parameters were obtained by analysis of regression as follows: For RCRC, a = 0.0227, b = 0.1030, c = 0.9620, l = 0.0788, m = 0.1796, n = 0.6429, For RPC, a = -0.1037, b = 0.0473, c = 1.0101, l = -0.2729, m = 0.2164, n = 1.0206. It can be noted from Figs. 12 and 13, which show the comparisons of the experimental and predicted values by Eqs. (5) and (6), that the regression equations fit the experimental results satisfactorily.
5. Approximation of the stress-strain relations If e/e0 (e0 is the strain at peak stress) is regarded as x, r/fc is regard as y, the stress-strain curve can be inversed into a nondimensional form. Fig. 14 shows the non-dimensional stressstrain curves for all mixes of concrete under uniaxial compression. For structural analysis and design in practical, the expression of stress-strain curve for normal concrete adopted by GB50010 [34], which was proposed by Guo and Zhang [36], is given as follows:
( y¼
þ x; x P 1
0.6
0.8 0.6 RPAC-10 RPAC-30 RPAC-100
0.4
0.4 RGAC-100 RGAC-60 RGAC-80
RGAC-20
0.4
0.8
1.2
1.6
/
(a) RCRC
2.0
0.2 2.4
NPAC RPAC-60 RPAC-50 RPAC-90 RPAC-20 RPAC-40 RPAC-70 RPAC-80
1.0
/f
0.8
/f
x bðx1Þ2
1.2 RGAC-40 RGAC-10 RGAC-30 NGAC RGAC-70 RGAC-50 RGAC-90
1.0
0
ax þ ð3 2aÞx2 þ ða 2Þ; 0 6 x < 1
2.8
ð7Þ
where a and b are fitting constants, represented as the initial elastic modulus and related to shape of descending branch of the stressstrain curve, respectively. By the expressions which are extended to apply for RAC in this study, the parameters determined base on the experimental stressstrain curve to be a = 1.15, b = 12.43 for RCRC, and to be a = 1.17, b = 4.58 for RPC respectively. The comparison between the experimental average curves and the theoretical curves provided by Eq. (7) are shown in Fig. 15 for RCRC and RPC. The theoretical curves are well coincident with the accordingly experimental curves.
1.2
0.2
ð6Þ
0
0.4
0.8
1.2
1.6
2.0
/
(b) RPC
Fig. 14. Non-dimensional stress-strain curves of RAC under uniaxial compression.
2.4
2.8
C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506
1.2 1.0
1.2 1.0
Theoretical
0.8
0.8
0.6
0.6
0.4
505
Experimental
Theoretical
0.4 Experimental
0.2 0
0.2
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(a) RCRC
(b) RPC
Fig. 15. The comparison of the theoretical and experimental stress-strain curves.
6. Conclusions Acknowledgment From the experimental results for mechanical properties of recycled concrete made with different types of coarse aggregate, the following conclusions can be drawn: (1) In contrast with the accordingly NCA, The RCA (both RCRC and RPC) has higher water absorption, lower apparent density and bulk density. RPA has much lower relative density and water absorption, which are the respective ratios between density and water absorption of RCA to those of accordingly NCA, than that of RCRA due to the various attached mortar result from the difference between their shape and surface. (2) The compressive failure processes and modes for RAC with different RP are similar. In the final failure state, the cracks mainly exist on the interface of mortar and aggregate. The failure process of RCRC is faster than that of RPC. (3) The type of coarse aggregate has significant influence on the shape of stress-strain curves for RAC. The curvature of ascending branches of RCRC is higher than that of RPC, and the descending branch of the former are more flat than that of the latter. Meanwhile, the peak strain of RCRC is higher than that of RPC. (4) The compressive strength and flexural strength of RAC are comparable and even exceeded the conventional concrete. They all have a tendency to increase with the increase of RP due to the much higher absorption of RCA which is improving bonding strength between aggregate and cement in the same mix and compaction process. The RPC has a higher relative compressive strength than the RCRC’s. (5) The elastic modulus of RAC decreases as the RP increase generally due to the lower stiffness and bulk density of RCA. The maximum reductions of elastic modulus are 18% and 22% for RCRC and RPC respectively. Most of the toughness of RCRC is higher than those of RPC due to the difference between the shape and surface of RPA and RCRA. The Poisson’s ratio of RAC is comparable and even little higher than those of conventional concrete. (6) Based on the experimental data, equations for the relationship between cube compressive strength and axial compressive strength, and the relationship between cube compressive strength and flexural strength of RAC were proposed for future research and application. (7) The theoretical expression of stress-strain curves initially proposed for NAC was extended to RCRC and RPC in this study, and the theoretical stress-strain curve of RAC is coincident with its experimental curve.
The authors gratefully acknowledge the financial support of this work provided by National Natural Science Foundation of China (No. 51578163 and 51268004), the key project of Natural Science Foundation of Guangxi Province (Project No. 2016GXNSFDA380032) and by Innovation Project of Guangxi Graduate Education (YCBZ2015009). References [1] J.Z. Xiao, W.G. Li, C.S. Poon, Recent studies on mechanical properties of recycled aggregate concrete in China – A review, Sci. China Technol. Sci. 55 (2012) 1463–1480. [2] 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. [3] I.A. Rahman, H. Hamdom, A.M.A. Zaidi, Assessment of recycled aggregate concrete, Modern Appl. Sci. 3 (2009) 47–54. [4] C.S. Poon, S.C. Kou, L. Lam, Use of recycled aggregates in molded concrete bricks and blocks, Constr. Build. Mater. 16 (2002) 281–289. [5] C.S. Poon, Z.H. Shui, L. Lam, H. Fok, S.C. Kou, Influence of moisture states of natural and recycled aggregates on the slump and compressive strength of concrete, Cem. Concr. Res. 34 (2004) 31–36. [6] J.Z. Xiao, J.b. Li, C. Zhang, Mechanical properties of recycled aggregate concrete under uniaxial loading, Cem. Concr. Res. 35 (2005) 1187–1194. [7] N.Y. Ho, P.K.L. Yang, W.F. Lim, T. Zayed, K.C. Chew, G.L. Low, S.K. Ting, Efficient utilization of recycled concrete aggregate in structural concrete, J. Mater. Civ. Eng. 25 (2013) 318–327. [8] M. Chakradhara Rao, S.K. Bhattacharyya, S.V. Barai, Influence of field recycled coarse aggregate on properties of concrete, Mater. Struct. 44 (2011) 205–220. [9] M. Etxeberria, E. Vázquez, A. Marí, M. Barra, Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete, Cem. Concr. Res. 37 (2007) 735–742. [10] A. Ajdukiewicz, A. Kliszczewicz, Influence of recycled aggregates on mechanical properties of HS / HPC, Composites 24 (2002) 269–279. [11] T. Du, W. Wang, Z. Liu, H. Lin, T. Guo, The complete stress-strain curve of recycled aggregate concrete under uniaxial compression loading, J. Wuhan Univ. Technol. Mater. Sci. Ed. 25 (2010) 862–865. [12] G.F. Belén, M.A. Fernando, C.L. Diego, S.P. Sindy, Stress-strain relationship in axial compression for concrete using recycled saturated coarse aggregate, Constr. Build. Mater. 25 (2011) 2335–2342. [13] R.V. Silva, J.D. Brito, R.K. Dhir, The influence of the use of recycled aggregates on the compressive strength of concrete: a review, Eur. J. Environ. Civ. Eng. 19 (2014) 825–849. [14] I.B. Topçu, N.F. Günçan, Using waste concrete as aggregate, Cem. Concr. Res. 25 (1995) 1385–1390. [15] S.W. Tabsh, A.S. Abdelfatah, Influence of recycled concrete aggregates on strength properties of concrete, Constr. Build. Mater. 23 (2009) 1163–1167. [16] A. Gonzalez, M. Etxeberria, Experimental analysis of properties of high performance recycled aggregate concrete, Constr. Build. Mater. 52 (2014) 227–235. [17] A. Katz, Properties of concrete made with recycled aggregate from partially hydrated old concrete, Cem. Concr. Res. 33 (2003) 703–711. [18] K.H. Yang, H.S. Chung, A.F. Ashour, Influence of type and replacement level of recycled aggregates on concrete properties, ACI Mater. J. 105 (2008) 289–296. [19] J. de Brito, M. Barra, L. Ferreira, Influence of the pre-saturation of recycled coarse concrete aggregates on concrete properties, Mag. Concr. Res. 63 (2011) 617–627.
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Contents lists available at ScienceDirect
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Review
Mechanical properties of recycled concrete made with different types of coarse aggregate Chunheng Zhou a, Zongping Chen a,b,⇑ a b
College of Civil Engineering and Architecture, Guangxi University, 530004 Nanning, China Key Laboratory of Disaster Prevention and Structure Safety of Chinese Ministry of Education, 530004 Nanning, China
h i g h l i g h t s Mechanical properties test for two types of recycled coarse aggregate concrete were performed and the the results were discussed. Equations of relationships between the compressive strength and flexural strength of recycled aggregate concrete were presented. Expressions of stress-strain curves for recycled coarse aggregate concrete were proposed.
a r t i c l e
i n f o
Article history: Received 17 July 2016 Received in revised form 27 December 2016 Accepted 28 December 2016
Keywords: Recycled aggregate concrete Recycled crushed rock aggregate Recycled pebbles aggregate Mechanical properties Strength relationship equation Stress-strain equation
a b s t r a c t One of the most important issues that determine many properties are the types of coarse aggregate in recycled concrete. This paper is arming to experimental study the influence of two different types of coarse aggregate (recycled crushed rock aggregate and recycled pebbles aggregate) on the mechanical properties of recycled concrete. The properties of these two types of recycled coarse aggregate (RCA) derived from waste concrete were investigated. Using these RCA and corresponding types of natural coarse aggregates, recycled concrete specimens according to different replacement percentage were produced and tested. Finally, Analysis and comparisons of the mechanical properties of RAC, including compressive strength, flexural strength, elastic modulus and Poisson’s ratios etc., were made between these recycled concretes. The results show that different types of RCA have significant variance in mechanical properties. The recycled concrete containing crushed rock aggregate presents a lower relative strength and elastic modulus than that containing pebble aggregate while the toughness of them is contrary. The theoretical expression for stress-strain relationship and equations for the relationships between various strength of recycled concrete with each type of coarse aggregate are also presented. Ó 2017 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experimental program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Mix proportions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Specimens preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Test results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Failure pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Cube compressive failure mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Prism compressive failure mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Stress-strain curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Flexural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author at: College of Civil Engineering and Architecture, Guangxi University, 530004 Nanning, China. E-mail address: [email protected] (Z. Chen). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.163 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
498 498 498 499 499 500 500 500 500 501 501 501 502
498
4. 5. 6.
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3.5. Static modulus of elasticity and Poisson’s ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6. Energy absorption capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationships between strengths. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Approximation of the stress-strain relations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction With economy development, rapid growth in urbanization has led to huge scale new construction, especially in some emerging economies countries. These construction works require large quantities of consumption and production of natural aggregate, which result in the intensification of natural aggregate resources shortage and the difficulty of sustainable development. Furthermore, a large number of old structures, which are nearing the end of their life span, need to dismantle and replace in many countries, result in the production of large amounts of construction and demolition waste. In China, the amount of construction and demolition waste has amounted to 30%–40% of the total city solid wastes [1], which will significantly increase the load of landfill. Therefore, recycling of concrete demolition waste as recycled aggregate to partially or fully substitute natural aggregate for recycled aggregate concrete (RAC) has been recognized as an effective way to offset the shortage of natural aggregate, disposal of waste concrete and related environment problem [1,2]. In recent years, even though reuse of RCA to make RAC has received increasing research interest in academic and has been extensively studied, the practical engineering application of RAC is still low [3] or mainly in non-structural concrete [4] explained by the disadvantage and discreteness of their properties, including strength, elastic modulus, toughness, stress-strain relationship, and so on, compare with natural aggregate concrete (NAC) [5– 12]. There are many factors related to the deterioration of properties for RAC, which limits its widespread used in structural concrete. Firstly, the replacement percentage (RP), which is defined as the ratio between the weight of recycled coarse aggregate to the total weight of coarse aggregate in a concrete mix [8], is deemed as a vital influence on the properties of RAC [13]. Xiao et al. [6] and Topçu et al. [14] found that the strength of RAC decreases as RP increases while Ho et al. [7] and Etxeberria et al. [9] observed the opposite results. The contradiction of the results was found due to the quality loss of RCA which to be mainly depended on the specific gravity of RCA [18], the moisture state of RCA [5,19], the amount of old adhered mortars [20]. Furthermore, the strength [15,16] and the age of original concrete [17] have a significant impact on the properties of RAC. Researches show that the concrete made with RCA from original concrete with weak strength resulted in lower strength. But the quality of RCA had little influence on the strength of RAC when comparing it to those of high performance conventional concrete. The properties of RAC made with RCA crushed at age 3 days were worse than those made with aggregate crushed at age 1 or 28 days. Most studies have indicated that, no matter what factors, the fluctuation and adverseness of the mechanical and physical properties of RAC is ultimately due to bad quality and weak link of aggregate-cement matrix interfacial transition zone, which the failure of RAC is often occurring [11,21–24]. However, previous research has shown that different coarse aggregate types strongly influence the mechanical properties of the interfacial transition zone (ITZ) [25] due to the significant difference of their surface and shape. Wu et al. [26] and Rocco et al. [27] considered the impact of coarse aggregate type on mechanical properties of con-
502 503 504 504 505 505 505
crete, and they found that different types of coarse aggregate has significant effect on the fracture energy further result in the variation of concrete strength. Concrete made with crushed aggregates provides higher values of the fracture energy than for concrete made with spherical ones. Ribeiro et al. [28] also found that, due to their smoother surface and therefore weaker anchorage, pebble aggregates present areas dislodged from the matrix, leading to concretes with them have lower fracture energy than those with crushed rock. Summarizing the existing research found that the influence of different types of coarse aggregate on mechanical performance of RAC is not a well-known area while that has been extensively studied on conventional concrete. Nevertheless, due to the adhered old mortar, the RCA originated from crushed concrete with different types of natural coarse aggregate have further variation compared with accordingly natural coarse aggregate (NCA). The mechanical performance of recycled concrete made with them is a meaningful area requiring further research. Consequently, this paper presents an experimental study on the mechanical properties, including compressive strength, flexural strength, stress-strain curves, elastic modulus, Poisson’s ratios and energy absorption capacity, of recycled concrete made with different types of coarse aggregate. 2. Experimental program 2.1. Materials The constituent materials describe as follows: Ordinary Portland cement with a 28-day compressive strength of 42.5 MPa. Fine natural aggregate (medium-coarse river sand). Two types of NCA: crushed rock aggregate (CRA) and pebbles aggregate (PA). They all had a maximum size of 20 mm and a minimum size of 5 mm. Two types of RCA originated from crushed waste concrete with crushed rock and natural pebbles as coarse aggregate respectively, which are denominated RCRA and RPA. They are in the same maximum and minimum size of NCA. The crushed waste concrete collected from concrete block produced in laboratory with target strength of 30 MPa . All RCA were produced by a jaw crusher, and were then sieved to obtain aggregates with required size. After that, they were washed with water to remove surface fine particles such as dust and clay. The RCRA is composed of crushed natural rock and mortar. And likewise, the RPA is composed of natural pebbles and mortar. However, the content of mortar for RCRA and RP is different due to the various shapes and surfaces of CRA and PA. CRA is irregular and has a rough surface while PA is approximately rounded and has a smooth surface. In Fig. 1, two types of natural coarse aggregate and two types of recycled coarse aggregate are presented. The physical properties determined on NCA and RCA were tested according to Chinese code GB/T 14685-2011 [29]. The amount of adhered mortar in RCA was measured using the
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(a) CRA
(b) PA
(c) RCRA
(d) RPA
Fig. 1. Characteristics of NCA and RCA.
Table 1 Physical properties of NCA and RCA. Coarse aggregate
Grading (mm)
Moisture content (%)
Water absorption (%)
Apparent density (kg/m3)
Bulk density (kg/m3)
Adhered mortar (%)
CRA RCRA PA RPA
5 20 5 20 5 20 5 20
0.01 1.82 0.01 1.69
0.05 3.16 1.51 3.54
2722 2655 2685 2640
1435 1270 1540 1203
0 5.5 0 3.8
methodology given in [20]. And the percentage of adhered mortar was calculated using Eq. (1)
M c ð%Þ ¼
m1 m2 100 m1
ð1Þ
where Mc (%) is the amount of adhered mortar in RCA, m1 is the total mass of the RCA sample, m2 is the mass of the same NCA sample. Specific physical properties, including moisture content, water absorption, apparent density, bulk density and adhered mortar content are shown in Table 1. From the Table 1, it is clear that the recycled aggregates have higher water absorption, lower apparent density and bulk density than corresponding natural aggregate. This is because the attached mortar, which is an important composition of recycled aggregate, has very low specific gravity and high water absorption due to porousness. RPA has much lower relative density and water absorption in reference to PA than that of RCRA in reference to CRA, and is maybe because that the mortar adhered to RPA is less than RCRA due to the difference between the shapes and surfaces of CRA and PA. 2.2. Mix proportions To investigate the effect of coarse aggregate type, four different kinds of coarse aggregates were used to produce concrete mix with 28-days target strength of 30 Mpa. A total of 22 concrete mixes were designed by means of the various coarse aggregate types and replacement percentages. One category of RAC mixes (hereafter called RCRC) which were prepared by replacing the RCRA to CRA with different replacement of 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90% and 100% by weight of total coarse aggregate
content were labeled as RCRC-x, with ‘‘x” representing the replacement percentage of RCRA. The other category of RAC mixes (hereafter called RPC) which were prepared by replacing the RPA to PA with the same replacement percentage as the former were labeled as RPC-x, with ‘‘x” representing the replacement percentage of RP. In the case of RCA replacement percentage equal 0%, the concrete are normal concrete, which are regarded as control mix and named CRC and PC respectively. Because RCA has higher water absorption than NCA as previously stated, RAC often produced with the presaturation of RCA method [19] and mixing water compensation method [30] to meet the workability of freshly mixed. In this text, the RAC were produced with coarse aggregate at air-dried condition, and the amount of water was adjusted by the actual moisture contents and the water absorption capacity of aggregates according to [5,31]. Due to the water absorption capacity of RCA is much higher than that of natural aggregate, the amount of water increased as the RP increased even considering that RCA had higher moisture contents. All the 22 mixtures of fresh concrete bring out good workability. The workability of concrete was measured following concrete slump test according to GB/T 50080-2002 and the slump of all kinds of concrete mixes were 100–120 mm. All proportions of coarse aggregates, fine aggregates, cement and water were calculated based on constant water-cement ratios of 0.49 and 0.47 for RCRC and RPC respectively. There are no superplasticizers were used in all mixes. The detail mixture proportions of different concrete are shown in Tables 2 and 3. 2.3. Specimens preparation The procedure of preparing the concrete mixtures was conducted in laboratory conditions. The coarse aggregates, sand and
Table 2 Mix proportions (kg/m3) of RCRC. Mix
RP (%)
Water-cement ratio
Cement
Water
Sand
CRA
RCRA
RCRC-0 RCRC-10 RCRC-20 RCRC-30 RCRC-40 RCRC-50 RCRC-60 RCRC-70 RCRC-80 RCRC-90 RCRC-100
0 10 20 30 40 50 60 70 80 90 100
0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49
398 398 398 398 398 398 398 398 398 398 398
195.0 196.6 198.2 199.8 201.4 203.0 204.6 206.2 207.8 209.4 211.0
614 614 614 614 614 614 614 614 614 614 614
1193 1074 954 835 716 597 477 358 239 119 0
0 119 239 358 477 597 716 835 954 1074 1193
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Table 3 Mix proportions of recycled RPC. Mix
RP (%)
Water-cement ratio
Cement
Water
Sand
PA
RPA
RPC-0 RPC-10 RPC-20 RPC-30 RPC-40 RPC-50 RPC-60 RPC-70 RPC-80 RPC-90 RPC-100
0 10 20 30 40 50 60 70 80 90 100
0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47 0.47
404 404 404 404 404 404 404 404 404 404 404
208.4 208.9 209.3 209.7 210.1 210.6 211.0 211.4 211.8 212.3 212.7
614 614 614 614 614 614 614 614 614 614 614
1228 1105 982 860 737 614 491 368 245 123 0
0 123 245 368 491 614 737 860 982 1105 1228
minutes by a concrete vibrating table and demolded 24 h after casting, and then were cured in a curing chamber (20 ± 2 °C temperature,90% relative humidity) for 28 days. They were then kept in laboratory at temperature of about 26 °C until testing. 2.4. Tests The loading tests were performed on a RMT-201 rock and concrete material test setup as shown in Fig. 2. During the loading procedure, the axial compression and displacement of specimens were recorded by data acquisition system automatically. The cube compressive strength, axial compressive strength, flexural strength and elastic modulus were determined in accordance with Chinese code GB/T 50081-2002. In order to obtain the complete stress-strain curves of concrete, the load process on 150 150 300 mm prisms was controlled by displacement under a constant rate of 0.005 mm/s. The Poisson’s ratio was measured by two strain gauges. One was placed vertically in the middle a third part of specimen to measured the vertical strain, the other was placed at 180° for vertical loading direction in the middle part of specimen to measured the hoop strain. In order to obtain the accurate deformation of concrete, each specimen was pre-loaded to about 30% of the estimated peak load to eliminate the effect of the gap between specimen and test setup. 3. Test results and discussion The test results of stress-strain curves, cube strength, axial strength, flexural strength, elastic modulus and Poisson’s ratios of RAC were determined on types and replacement percentage of RCA. Table 4 lists the test results which derived from the average of the three specimens at same mix. 3.1. Failure pattern
Fig. 2. Test setup.
cement were mixed for two mins before water was added, and than were mixed for three mins when water was added. For each of concrete mixture, three 150 mm cubes, three 150 150 300 mm prisms and three 150 150 450 mm prisms were cast in steel moulds. The cubes were used to determine the cube compressive strength. The 150 150 300 mm prisms were used to obtain the Poisson’s ratio, static elasticity modulus, axial compressive strength and stress-strain curve. As for the 150 150 450 mm prisms were used to test the flexural strength. The specimens were vibrated and compacted about three
3.1.1. Cube compressive failure mode With regard to same type of coarse aggregate, the failure processes and modes for concrete specimens were similar. In the initial loading stage, i.e., the stage for loading on the specimen before 30% ultimate load, no visible cracks can be seen on the surface of sample. With the increase in compressive loading, a few thin and vertical micro cracks were observed in the middle of specimen, and they then propagated to some inclined cracks whose angle are about 45° with respect to the load-direction. When the loading on the specimen was close to the peak load, it was a crack zone for a shape of an X to be formed into the surface of the specimen. Whereafter, the concrete was spalling gradually. After the loading procedure, it can be observed that scarcely any fracture of coarse aggregates could be seen from the fracture plane except a few flaky particles in CRA and RCRA. It indicated that the cracks mainly exist
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C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506 Table 4 Mechanical index of RAC. RP (%)
fcu (MPa)
fc (MPa)
fr (MPa)
Elastic modulus (GPa)
Poisson’s ratio
RCRC
RPC
RCRC
RPC
RCRC
RPC
RCRC
RPC
RCRC
RPC
0 10 20 30 40 50 60 70 80 90 100
41.71 37.50 43.48 42.03 44.01 42.40 42.33 43.94 50.50 43.73 44.26
29.88 27.95 29.72 34.86 32.60 29.84 32.39 37.02 40.11 31.85 41.16
37.01 29.66 39.54 38.33 33.82 42.20 42.33 40.70 44.31 42.32 43.57
26.82 25.80 28.86 28.59 27.08 29.09 30.68 32.67 35.22 26.00 35.27
4.30 4.31 4.20 3.96 4.80 5.11 4.31 5.19 6.28 4.27 5.07
5.20 4.87 5.52 5.80 5.85 5.48 5.54 6.29 6.39 5.11 6.30
46.40 44.66 39.74 38.53 38.29 39.50 40.87 40.43 40.78 41.27 40.80
44.39 39.33 39.36 39.59 36.50 41.00 35.77 36.46 38.24 34.18 36.33
0.21 0.21 0.21 0.22 0.21 0.21 0.21 0.21 0.21 0.22 0.21
0.22 0.22 0.22 0.23 0.22 0.22 0.22 0.22 0.22 0.23 0.22
Note: fcu is cube compressive strength; fc is axial compressive strength; fr is flexural compressive strength.
for different replacement percentages were similar. The typical failure patterns of cube specimens are shown in Fig. 4. 3.2. Stress-strain curves Base on the measured data of axial load and deformation, the stress and strain could be calculated using Eq. (2)
r ¼ N=A; ¼ Dl=l (a) RCRC-20
(b) RPC-20
Fig. 3. Typical failure patterns of cube specimens.
ð2Þ
where N is the axial load, A is the cross-sectional area of the specimen, Dl is the axial deformation, l is the vertical length of the specimen. As the stress-strain curves of the three specimens with the same mix are similar to each other, one of them was chosen as representative curve. The stress-strain curves for all mixes of concrete are shown in Fig. 5. It can be seen clearly from Fig. 5 that the shapes of the stressstrain curve for concrete with different types of coarse aggregate are considerably different. The curvatures of the ascending branch of RCRC are higher than those of RPC while the descending branches of RPC are more flat than these of RCRC. Most of the strains at the peak stresses of RCRC are higher than those of RPC. It is also worth mentioning that the strain at the peak stress is increased with the increase of RP, either RCRC or RPC, due to the decrease of elasticity modulus. 3.3. Compressive strength
(a) RCRC-10
(b) RPC-70
Fig. 4. Typical failure patterns of prism specimens.
in the interface of mortar and aggregate. The typical failure patterns of cube specimens are shown in Fig. 3. 3.1.2. Prism compressive failure mode In the early loading stage, the prism specimens also did not show any cracks. When the load reaches the peak load of specimen, the sound caused by cracking could be heard while some discontinuous vertical micro cracks appeared on the surface of the specimen. After the strain exceeds the peak strain, the cracks propagated quickly to penetrate the specimen. Through the comparison for concretes with different types of coarse aggregate, it is of interest to note that the failure process of RCRC was faster than that of RPC. The failure modes of prisms of recycled concrete
Fig. 6 shows the relative cube compressive strength and axial compressive strength of different RAC mixes with respect to the strength of accordingly control mix. It can be observed that the variation on relative compressive strengths with the change of replacement is significantly, which is consistent with the results of previous study [7,9]. The strength of RAC is comparable and even exceeded the control concrete, either cube compressive strength or axial compressive strength. In addition, the compressive strength fluctuates, but has a tendency to increase with the increase of RP due to the much higher absorption of RCA. It would give rise to a decrease of the effective water-cement ratio of concrete mixture in the interfacial transition zone consequently improving bonding strength between aggregate and cement, when the RCA which absorbed more water during mix in air-dried state were used in the mixes, with the increase of RP. It can also be noted that most of the RPC have a higher relative strength than RCRC. It may be attributed to that the RPA was crushed along with its own original texture result in its more angular than the PA, which would reinforced the weakest interface between new cement and aggregate.
C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506
40
CRC PC
30
Stress (Mpa)
20 10
40 RCRC-10
30
RPC-10
20 10
0
2
4 6 8 Strain (10-3)
10
2
4
6
8
10
0
(b) RP=10%
20 10 0
RCRC-40
30
Stress (Mpa)
Stress (Mpa)
RPC-30
RPC-40
20
4 6 8 Strain (10-3)
10
10
RPC-60
Stress (Mpa)
RCRC-60
20 10 0
2
4 6 8 Strain (10-3)
10
40
RCRC-70
30
RPC-70
0
10
10
4 6 8 Strain (10-3)
10
40
RCRC-80
30
RPC-80
20 10 0
0
2
4
6
8
10
Strain (10 )
0
4 6 8 Strain (10-3)
(h) RP=70%
(i) RP=80%
-3
(g) RP=60% Stress (Mpa)
2
(f) RP=50%
20
40
RCRC-90
30
RPC-90
Stress (Mpa)
4 6 8 Strain (10-3)
RPC-50
10
0 2
RCRC-50
30
(e) RP=40%
30
0
40
0 0
(d) RP=30% 40
10
20
0 2
4 6 8 Strain (10-3)
(c) RP=20%
40 RCRC-30
0
2
Strain (10-3)
40
Stress (Mpa)
10 0
0
(a) RP=0% 30
RPC-20
20
0
0
Stress (Mpa)
RCRC-20
30
Stress (Mpa)
Stress (Mpa)
40
Stress (Mpa)
502
20 10
40
RCRC-100
30
RPC-100
2
10
20 10 0
0 0
2
8 4 6 Strain (10-3)
10
0
2
4
6
8
10
Strain (10-3)
(k) RP=100%
(j) RP=90%
Fig. 5. Representative stress-strain curves of specimens.
1.6
1.4 1.2 1.0 0.8
RCRC RPC
0.6 0.4 0
10 20 30 40 50 60 70 80 90 100
Relative axial compressive strength
Relative cube compressive strength
1.6
1.4 1.2 1.0 0.8
RCRC RPC
0.6 0.4 0
10 20 30 40 50 60 70 80 90 100
RP (%)
(a) cube compressive strength
RP (%)
(b) axial compressive strength
Fig. 6. Relative compressive strength of RAC.
3.4. Flexural strength The relative flexural strength of all concrete mixes is presented in Fig. 7. It is of interest that the flexural strength also has a rising tendency with the increase of RP due to the decrease of actual water-cement ratio. However, RPC did not have a higher relative strength than RCRC absolutely. It may be that the interface
between new cement and aggregate is no longer the weakest interface under the condition of bending. 3.5. Static modulus of elasticity and Poisson’s ratio The relative static elastic modulus of all concrete mixes is shown in Fig. 8. It can be noticed from Fig. 8 that all elastic modu-
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50 RCRC RPC
1.4 1.2
40
f c (MPa)
Relative flexural strength
1.6
1.0 0.8
RCRC RPC
0.6
30 GB50010-2010
0.4
20 0
10
20
30
40
50
60
70
80
90
100
20
30
40
50
60
f cu (MPa)
RP (%) Fig. 7. Relative flexural strength of RAC.
Fig. 10. Relationship between cube compressive strength and axial compressive strength.
1.4
8
RCRC RPC
1.2
7
1.0
6
f r (MPa)
Relative elastic modulus
1.6
0.8 0.6
RCRC RPC
5 4
0.4 0
10
20
30
40
50
60
70
80
90
JTG D40-2011
100 3
RP (%)
20
30
40
60
Fig. 11. Relationship between cube compressive strength and flexural compressive strength.
0.15
0.10 RCRC RPC
0.05
0.00 0
10
20
30
40
50
60
70
80
90 100
RP (%) Fig. 9. Toughness of RAC.
lus of RAC were lower than that of the control concrete due to the decreasing of stiffness and bulk density of RCA [2]. For RCRC, the reduction of elastic modulus was range from 4% to 18% of the control concrete while the reduction of those was range from 10% to 22% for RPC. Furthermore, the increase of RP reduces the elastic modulus overall for both RCRC and RPC, which can also attribute to the decreasing of stiffness and bulk density of RCA, but in the case of the elastic modulus of RCRC with RP greater than 50% become constant. From the Table 4 it can be seen that not only the Poisson’s ratio of RCRC but also those of RPC was comparable and even little higher (about 5%) than those of control concrete.
calculated up to different specified stress or strain value in previous study [32,33]. In this study, the area under stress-strain curve, which is competed up to 60% peak stress value at the descending branch, was used to evaluate the toughness of RAC. The calculated results of the toughness for all mixes of concrete are shown in Fig. 9. From the Fig. 9, it is clear that most of the toughness of RCRC were higher than those of RPC which indicated that the RAC with RPA has higher energy absorption capacity. The fact can be attributed to concrete with crushed rock aggregates have higher fracture energy than those with pebble aggregates due to the irregular and rougher surface [28]. With an increase of RP, the toughness has a
50
Predicted values of f c (MPa)
0.20
Thoughness (Mpa)
50
f cu (MPa)
Fig. 8. Relative static elastic modulus of RAC.
40
30 RCRC RPC
20
3.6. Energy absorption capacity The energy absorption capacity (i.e. toughness) of concrete in compression had been defined as the area under stress-strain curve
20
30
40
50
Experimental values of f c (MPa) Fig. 12. Comparison of experimental and predicted values by Eq. (5).
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C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506
tionship between cube compressive strength and axial compressive strength, and the relationship between cube compressive strength and flexural strength for RAC.
Predicted values of f r (MPa)
7
6
f c ¼ ðar2 þ br þ cÞ 0:89f cu 2
ð5Þ
0:75
f r ¼ ðlr þ mr þ nÞ 0:4f cu
5
4
RCRC RPC
3 3
4
5
6
7
Experimental values of f r (MPa) Fig. 13. Comparison of experimental and predicted values by Eq. (6).
tendency in general to decrease first than increase both for RCRC and RPC. The first decrease is due to the bad quality of RCA which result from the attached old mortar. However, the later increase is mainly because of that the decrease of actual water-cement ratio in the interfacial transition zone. 4. Relationships between strengths The relationship between cube compressive strength and axial compressive strength, and the relationship between cube compressive strength and flexural compressive strength for conventional concrete stipulated by Chinese code GB50010-2010 [34] and JTG D40-2011[35] are expressed as follow respectively:
f c ¼ 0:76f cu
ð3Þ
2=3
f r ¼ 0:438f cu
ð4Þ
where: fc is axial compressive strength, fcu is cube compressive strength, fr is flexural compressive strength. Comparisons of experimental values and calculated values of fc and fr by Eqs. (3) and (4) are shown in Figs. 10 and 11 respectively. From them it can be concluded that the relationship equations adopted by codes for conventional concrete are not applicable to those for RAC. It may be attributed to the different aggregatemotor bond of RCA under various forces. As a result, we suggest the following regression equations based on the experimental results in this study for expressing the rela-
where r is replacement percentage of RAC, a, b, c, l, m and n are constant parameters to be determined for RAC with different types of coarse aggregate. Base on the experimental data in this study, the parameters were obtained by analysis of regression as follows: For RCRC, a = 0.0227, b = 0.1030, c = 0.9620, l = 0.0788, m = 0.1796, n = 0.6429, For RPC, a = -0.1037, b = 0.0473, c = 1.0101, l = -0.2729, m = 0.2164, n = 1.0206. It can be noted from Figs. 12 and 13, which show the comparisons of the experimental and predicted values by Eqs. (5) and (6), that the regression equations fit the experimental results satisfactorily.
5. Approximation of the stress-strain relations If e/e0 (e0 is the strain at peak stress) is regarded as x, r/fc is regard as y, the stress-strain curve can be inversed into a nondimensional form. Fig. 14 shows the non-dimensional stressstrain curves for all mixes of concrete under uniaxial compression. For structural analysis and design in practical, the expression of stress-strain curve for normal concrete adopted by GB50010 [34], which was proposed by Guo and Zhang [36], is given as follows:
( y¼
þ x; x P 1
0.6
0.8 0.6 RPAC-10 RPAC-30 RPAC-100
0.4
0.4 RGAC-100 RGAC-60 RGAC-80
RGAC-20
0.4
0.8
1.2
1.6
/
(a) RCRC
2.0
0.2 2.4
NPAC RPAC-60 RPAC-50 RPAC-90 RPAC-20 RPAC-40 RPAC-70 RPAC-80
1.0
/f
0.8
/f
x bðx1Þ2
1.2 RGAC-40 RGAC-10 RGAC-30 NGAC RGAC-70 RGAC-50 RGAC-90
1.0
0
ax þ ð3 2aÞx2 þ ða 2Þ; 0 6 x < 1
2.8
ð7Þ
where a and b are fitting constants, represented as the initial elastic modulus and related to shape of descending branch of the stressstrain curve, respectively. By the expressions which are extended to apply for RAC in this study, the parameters determined base on the experimental stressstrain curve to be a = 1.15, b = 12.43 for RCRC, and to be a = 1.17, b = 4.58 for RPC respectively. The comparison between the experimental average curves and the theoretical curves provided by Eq. (7) are shown in Fig. 15 for RCRC and RPC. The theoretical curves are well coincident with the accordingly experimental curves.
1.2
0.2
ð6Þ
0
0.4
0.8
1.2
1.6
2.0
/
(b) RPC
Fig. 14. Non-dimensional stress-strain curves of RAC under uniaxial compression.
2.4
2.8
C. Zhou, Z. Chen / Construction and Building Materials 134 (2017) 497–506
1.2 1.0
1.2 1.0
Theoretical
0.8
0.8
0.6
0.6
0.4
505
Experimental
Theoretical
0.4 Experimental
0.2 0
0.2
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
(a) RCRC
(b) RPC
Fig. 15. The comparison of the theoretical and experimental stress-strain curves.
6. Conclusions Acknowledgment From the experimental results for mechanical properties of recycled concrete made with different types of coarse aggregate, the following conclusions can be drawn: (1) In contrast with the accordingly NCA, The RCA (both RCRC and RPC) has higher water absorption, lower apparent density and bulk density. RPA has much lower relative density and water absorption, which are the respective ratios between density and water absorption of RCA to those of accordingly NCA, than that of RCRA due to the various attached mortar result from the difference between their shape and surface. (2) The compressive failure processes and modes for RAC with different RP are similar. In the final failure state, the cracks mainly exist on the interface of mortar and aggregate. The failure process of RCRC is faster than that of RPC. (3) The type of coarse aggregate has significant influence on the shape of stress-strain curves for RAC. The curvature of ascending branches of RCRC is higher than that of RPC, and the descending branch of the former are more flat than that of the latter. Meanwhile, the peak strain of RCRC is higher than that of RPC. (4) The compressive strength and flexural strength of RAC are comparable and even exceeded the conventional concrete. They all have a tendency to increase with the increase of RP due to the much higher absorption of RCA which is improving bonding strength between aggregate and cement in the same mix and compaction process. The RPC has a higher relative compressive strength than the RCRC’s. (5) The elastic modulus of RAC decreases as the RP increase generally due to the lower stiffness and bulk density of RCA. The maximum reductions of elastic modulus are 18% and 22% for RCRC and RPC respectively. Most of the toughness of RCRC is higher than those of RPC due to the difference between the shape and surface of RPA and RCRA. The Poisson’s ratio of RAC is comparable and even little higher than those of conventional concrete. (6) Based on the experimental data, equations for the relationship between cube compressive strength and axial compressive strength, and the relationship between cube compressive strength and flexural strength of RAC were proposed for future research and application. (7) The theoretical expression of stress-strain curves initially proposed for NAC was extended to RCRC and RPC in this study, and the theoretical stress-strain curve of RAC is coincident with its experimental curve.
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