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Influence of crushing index on properties of recycled aggregates pervious concrete
Construction and Building Materials 135 (2017) 112–118
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of crushing index on properties of recycled aggregates pervious concrete Zhiquan Zhang a, Yufen Zhang b,⇑, Changgen Yan c, Yunxiao Liu a a
School of Civil Engineering, Chang’an University, Xi’an 700061, PR China School of Civil Engineering, North China University of Technology, Beijing 100144, PR China c Highway College of Chang’an University, Xi’an, Shaanxi Province, PR China b
h i g h l i g h t s Natural aggregate was replaced with discarded concrete and clay brick aggregates. Addition of recycled clay brick aggregates increases the crushing index. Increase of crushing index exerts a negative impact on mechanical properties. The effects of crushing index on the porosity and permeability can be ignored.
a r t i c l e
i n f o
Article history: Received 11 July 2016 Received in revised form 5 December 2016 Accepted 29 December 2016
Keywords: Recycled aggregates Pervious concrete Crushing index Mechanical properties Water permeability
a b s t r a c t In this study, recycled aggregates (RA) produced by mechanical shredding from discarded concrete and clay bricks were used to make pervious concrete. Six groups of recycled aggregates pervious concrete (RAPC) were designed with different crushing index of aggregates under the condition of same concrete mixture ratio. The crushing index of aggregates increases with the replacement of RA increases. Experiments about compressive strength, elasticity modulus, flexural strength, permeability coefficient, total void ratio were conducted, including freezing and thawing cycle test. Test data of RAPC indicated that compressive strength, flexural strength and elasticity modulus of 28 days decreased by 36%, 28% and 21% respectively when crushing index changed from 9% to 37%, simultaneously strength loss rate increased from 6.6% to 18.7% and mass loss rate increased from 2.3% to 8.5%. Especially when crushing index is greater than 24%, except permeability coefficient and total void ratio, other properties of RAPC decreased significantly due to the lower quality of RA. Therefore, experimental results show that increase of crushing index has significant effects on compressive strength, elasticity modulus, flexural strength and freeze-thaw durability of RAPC. However, the effects of increasing crushing index on the permeability coefficient and total void ratio of RAPC can be ignored. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Concrete mainly consists of cement, sand, pebble (or crushed stone), and each cubic meter of concrete consumes about 1700– 2000 Kg aggregates. With economic development, concrete aggregate consumption grows very rapidly. Depletion and difficulty of mining sand and gravel resources are increasing. Meanwhile, billions of ton of construction waste were produced per year, including waste concrete, brick and others [1,2]. Such a large number of construction waste, if not well handled and effective utilized, the urban environment will suffer enormous damage. Using the waste ⇑ Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.203 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
concrete and brick can protect resources and reduce environmental pollution, so it has significant economic and social benefits [3]. Pervious concrete (PC) uses a small amount or no fine aggregate during preparation, and the right amount of cements are used to paste and wrap the bonding aggregate particles, then connected pore can quickly drain [4]. Its previous behavior depends on the porosity and the particle size of the aggregate. PC can reach a certain intensity and water permeability through rational design of mixture ratio [5,6]. PC as a unique and effective eco-friendly materials have been widely used at low-grade roads, squares, parking lots and other areas in North America, Europe and Japan [7]. It can regulate urban microclimate and sound absorption, maintain ecological balance, reduce the adverse effects of heavy rain or flood [8,9]. There were also many achievements published about PC
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design [4], strength and water permeability [10], frost resistance [11], microstructure [12], workability [13,14]. Recycled aggregate pervious concrete (RAPC), as one new type of ecologically concrete, can reduce environmental pollution, and help to protect the ecological balance [15]. So in recent years, some researchers have studied the different behaviors of RAPC including strength, total void ratio, and permeability coefficient. Research results showed that pervious concrete containing recycled aggregate (RA) gave lower mechanical properties than those containing natural aggregate (NA) [3]. Although, there is not enough research about the effect of RA on total void ratio and water permeability of RAPC, it is reported that the gradation and shape of RA can affect the void texture and void ratio of porous concrete [5]. Test research on the shrinkage and frost-resistance of RAPC showed that after 25 times and 50 times of freeze-thaw cycles, the strength loss rate of the pervious concrete were 5.6% and 18.9% [16]. The main factors that result in the strength loss are large pore structure of PC and weak bond between aggregates. The characteristic and mechanism of RA were studied in the Refs. [17–19], and it was concluded that durability of concrete changed greatly with the RA behaviors because RA differed from NA in pore volume and size on a micro level, water absorption and apparent density on a macro level. Kwan et al. have conducted the influencing test of aggregate size, grading and content on the behaviors of RAPC, showing the remarkable influence of water cement ratio and the RA content [20–22]. However, few researches of RAPC were relative to crushing index of aggregate. NA are hard and compact with low porosity, while RA produced by crushing waste concrete and brick is rough with many edges and corners, so RA has porosity property and high water absorption [23]. Crushing index is related to the strength, shape, water absorption, density, micropore and microfracture of aggregate, which can represent the quality of aggregate [24]. Consequentially, different crushing index can affect the performance on RAPC greatly. This paper focused on the sorts of recycled aggregates grouped by different crushing value, and some tests on compressive strength, flexural strength, elasticity modulus, porosity, permeability, frost resistance were conducted for 6 groups of RAPC specimens.
RA particles produced by mechanical shredding from discarded concrete and clay bricks, ranging from 5 mm to10 mm in size. The particle size distribution curve of the RA is shown in Fig. 1. Crushing index, water absorption and distribution curve were tested according to Pebble and crushed stone for construction (GB/ T14685-2011) [29]. In this study, NA is crushed stone, while RA is discarded concrete and clay brick particle. Six groups of aggregate, the recycled aggregate took different content of the total aggregate, were prepared for RAPC. Crushing index of each group was measured as 37%, 34%, 30%, 24%, 19% and 9% respectively. Water absorption of each group was measured as 12.8%, 9.54%, 7.12%, 4.67%, 2.85% and 0.9% respectively. The mixing water was tap water. Water-reducing admixture (WRA) was polycarboxylate superplasticizer, and the recommended participate amount is 1%. 2.2. Mix proportions The mix proportion of RAPC was set at 1.00:0.20:0.34:4. 34:0.012 for Cement:FA:Water:RA:WRA as shown in Table 3. Six mix proportions were set for crushing index 37%, 34%, 30%, 24%, 19% and 9% respectively. Some tests were conducted about compressive strength, flexural strength, elasticity modulus, permeability coefficient, total void ratio and freeze-thaw durability. All mixtures were mixed in a standard mixer. Firstly, all solid ingredients were mixed together for about 1 min before adding water, then water and WRA were added in and mixed for 2 min. The mixture was then placed into molds and demoulded after 24 h. After demoulded, all specimens were cured for 3, 7 and 28 days in a standard condition where the relative humidity is more than 95% and the temperature is 20 ± 2 °C [30]. 2.3. Testing methods
2. Experiment program
The crushing index was determined in accordance with Pebble and crushed stone for construction (GB/T14685-2011) [29]. Firstly, 3000 g of sample (M1) was put into the determinator under pressure testing. Then test machine was started by the velocity of 1 kN per second until the load of 200 kN. Kept the load for 5 s, and uninstalled it. Finally, the crushed fine particles were screened out using sieves with holes in 2.36 mm. The leaves of the sample was measured as M2. The crushing index (Qe) was calculated by
2.1. Materials
Qe ¼
All the parameter indexes of cement are shown in Table 1. The value of strength, setting time, soundness and fineness of cement were measured according to Method of testing cementsdetermination of strength (GB/17671-1999) [25], Test methods for water requirement of normal consistency setting time and soundness of the Portland cement (GB/1346-2001) [26] and Test method for fineness of cement (GB/1345-2005) [27]. The Fineness was measured on 45 lm sieve. The value of water demand ratio, loss on ignition (LOI), fineness and main chemical composition of fly ash (FA) are shown in Table 2. Fineness was measured on 45 lm sieve. It can be classified as ClassⅡ according to Technical code for application of fly ash concrete (GB/T50146-2014) [28].
where, Qe is the crushing index (%); M1 is the initial mass of sample (g); M2 is the mass of leaves of sample (g). The results of compressive strength, flexural strength and elasticity modulus were reported by the average readings. The compressive strength, flexural strength and elasticity modulus were measured according to Standard for test methods of mechanics performance of common concrete (GB50081-2011) [31]. The compressive strength and elasticity modulus were both measured on three prism specimens 150 150 300 mm3 in size. The flexural strength was measured on prism specimens 150 150 600 mm3 in size. The compressive load was applied by a servo-controlled hydraulic testing machine. In compressive strength test, specimens were tested at a constant loading rate of 9.5 kN/s. For the flexural
M1 M2 100% M1
ð1Þ
Table 1 Cement parameter index. Fineness (%)
3
Soundness
up to standard
Setting time (min)
Strength (MPa)
Initial setting
Final setting
7d
28d
160
285
25.5
48.8
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Table 2 Fly ash parameter index. Water demand ratio (%)
LOI (%)
Fineness (%)
102
5.3
14.6
Main chemical composition (%) SiO2
AI2O3
Fe2O3
CaO
61.2
22.5
5.0
0.7
100 standardized upper bound standardized lower bound RA
90
Percentage passing, %
80 70 60 50 40 30 20
Fig. 2. Water permeability set-up for RAPC.
10 0 2
4
6
8
10
12
14
16
particle size, mm Fig. 1. Particle size distribution curve of the RA.
strength, specimens were tested at a constant loading rate of 0.5 kN/s. The elasticity modulus was measured as a secant modulus in the elastic state. Each of these specimens was equipped with two dial indicators with the capable of measuring deformation 0.002 mm and then loaded by 40% of the ultimate load three times. The first set of readings of each cylinder was discarded and the modulus was reported as the average of the rest readings [30]. The total void ratio of RAPC was determined in accordance with construction and recent applications of porous concrete [32], curing period of specimens was 28d. The total void ratio (VR) was calculated by
VR ¼ 1
ðM 4 M3 Þ=qw 100% V
ð2Þ
where, VR is the total void ratio (%); M3 is the initial mass of specimen in water (g); M4 is specimen dried 24 h in air measured final mass (g); V is the volume of specimens (cm3); qw is the density of water (g/cm3). Permeability coefficient of RAPC was also determined in accordance with Construction and recent applications of porous concrete [32], curing period of specimens was 28 days. Fig. 2 shows the test methods for pervious concrete permeability coefficient. Permeability coefficient was calculated by
Kr ¼
H Q h A ðt 2 t1 Þ
ð3Þ
where, Kr is the permeability coefficient (cm/s); H is the length of the specimen (cm); Q is the amount of water t1 time to t2 time specimens discharged (cm3); h is the head difference (cm); t2, t1 are the time (s); A is the cross-sectional area of the cylindrical specimens (cm2). Strength loss rate and mass loss rate of pervious concrete under freezing and thawing were both measured according to Standard for test methods of long-term performance and durability of ordinary concrete (GB/T50082-2009) [33]. Freezing and thawing specimens were chosen as 100 100 100 mm3 cube, and the curing period was 28 days. Freeze-thaw test chamber was kept with air temperature from 20 °C to 18 °C in freezing period, the temperature of water in freeze-thaw test chamber was kept from 18 °C to 20 °C in thawing period. 3. Results and discussions Test results of compressive strength, flexural strength and static elasticity modulus are listed in Table 4. Test results of permeability coefficient, total void ratio, mass loss rate and strength loss rate are listed in Table 5. 3.1. Compressive strength, flexural strength and static modulus of elasticity The compressive strength of RAPC is shown in Fig. 3. Compressive strength of RAPC decreases with the increases of crushing index, and the downward trend of 3-day (3d) strength, 7-day (7d) strength and 28-day (28d) strength is same. When crushing index increases from 9% to 37%, 28d strength decreases from 24.2 MPa to 15.5 MPa, about 36 percent, while the decrease of 7d
Table 3 Mix proportions and crushing index. Specimens No.
RAPC RAPC RAPC RAPC RAPC RAPC
1 2 3 4 5 6
Weight per cubic meter (kg/m3) C
FA
Water
RA
350 350 350 350 350 350
70 70 70 70 70 70
118 118 118 118 118 118
1520 1520 1520 1520 1520 1520
W/C + FA
Water-reducing admixture (kg/m3)
Crushing index (%)
0.28 0.28 0.28 0.28 0.28 0.28
4.2 4.2 4.2 4.2 4.2 4.2
37 34 30 24 19 9
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Z. Zhang et al. / Construction and Building Materials 135 (2017) 112–118 Table 4 Testing results of compressive strength, flexural strength and static modulus of elasticity. Specimens No.
RAPC RAPC RAPC RAPC RAPC RAPC
Compressive strength (MPa)
1 2 3 4 5 6
Flexural strength (MPa)
Static modulus of elasticity (GPa)
3d
7d
28d
3d
7d
28d
3d
7d
28d
5.0 5.3 6.3 6.9 8.0 8.9
10.5 11.9 12.6 13.5 16.6 17.5
15.5 16.5 17.8 20.0 23.6 24.2
0.66 0.71 0.75 0.78 0.90 1.02
1.81 1.82 1.96 2.01 2.35 2.45
2.55 2.59 2.77 3.02 3.41 3.56
2.6 2.6 2.7 2.8 3.1 3.4
11.2 11.1 11.5 12.2 12.8 13.1
14.1 14.5 15.1 16.4 17.4 17.9
Table 5 Testing results of permeability coefficient, total void ratio, mass loss rate and strength loss rate. Permeability coefficient (mm/s)
Total void ratio (%)
RAPC RAPC RAPC RAPC RAPC RAPC
3.5 3.8 3.1 3.2 3.5 3.7
15.3 16.5 15.9 16.4 15.1 16.3
1 2 3 4 5 6
Compressive strength, MPa
30 3d 7d 28d
25 20 15 10 5 0 5
10
15
20
25
30
35
40
Crushing index, % Fig. 3. Compressive strength of RAPC versus crushing index.
strength and 3d strength are 40% and 44%respectively, so the decrease amplitude gradually increases from 28d strength to 7d strength to 3d strength. Take 28d strength for example, when crushing index changed from 9% to 19%, the strength was 24.4 MPa and 23.6 MPa respectively, keeping little changes. But when crushing index was greater than 24%, 28d strength was 20.0 MPa, 17.8 MPa, 16.5 MPa and 15.5 MPa for crushing index 24%, 30%, 34%, and 37% respectively, so the downward trend more obvious. In the reference, it was reported that the compressive strength of RAPC decrease about 30% with RA content changed from 0% to 100%, i.e. values range from 20 to 13.9 MPa [34]. Another study claimed the replacement level of NA with RA will reduce the compressive strength of the concrete. However, when the replacement was smaller than 80%, the target strength was still achieved by employing the DoE mix design method [20]. In this study, the strength decreased about 36% with crushing index changed from 9% to 37%, so crushing index has evident influence on the compressive strength of RAPC. It also was observed that the strength keeping little change when crushing index changed from 9% to 19%. It means that it is feasible to use right amount recycled aggregates for making pervious concrete with acceptable compressive strength. According to Technical specifications for pervious con-
The mass loss rate after freezethaw cycles (%)
The strength loss rate after freeze-thaw cycles (%)
25 times
50 times
25 times
50 times
4.9 3.8 3.0 1.8 1.1 0.9
8.5 7.2 5.7 4.5 2.9 2.3
7.7 7.5 6.6 5.7 3.0 2.5
18.7 17.1 14.5 12.3 7.5 6.6
crete pavement (CJJ134-2009) [36], when crushing index is smaller than 24% the compressive strength of RAPC are acceptable. In general, compressive strength of pervious concrete is related to aggregate strength and total porosity. For this research, strength decrease with increased crushing index is due to the increased brick particles. Firstly, the strength of the brick particles is relatively low, about 10–20 MPa, so it affects the function of concrete strength. Secondly, water-cement ratio of concrete mixing ratio design is 0.28, and the lowest water-cement ratio is 0.23, while higher water absorption of brick particles results in incomplete hydration of cement [35], so it indirectly leads to lower concrete strength. Test data of flexural strength are shown in Table 3. 3d strength, 7d strength and 28d strength are all decrease with increasing crushing index. The flexural strength of RAPC is presented in Fig. 4. When crushing index changes from 9% to 37%, 28d strength, 7d strength and 3d strength decreased respectively from 3.56 MPa, 2.45 MPa and 1.02 MPa to 2.55 MPa, 1.81 MPa and 0.66 MPa, and decrease amplitudes are 28%, 26% and 35% respectively. Meanwhile, when crushing index is greater than 24%, downward trend of strength is more obvious. The reason is that drawbacks of RA result in the decrease of flexural strength and toughness. As crushing index increases, brick
5
Flexural strength, MPa
Specimens No.
3d 7d 28d
4 3 2 1 0 5
10
15
20
25
30
35
Crushing index, % Fig. 4. Flexural strength of RAPC versus crushing index.
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10
25 3d 7d 28d
20
Permeability coefficient, mm/s
Static moidulus of elasticity, GPa
116
15 10 5 0 5
10
15
20
25
30
35
8 6 4 2 0
40
5
10
15
Crushing index, % Fig. 5. Static modulus of elasticity of RAPC versus crushing index.
3.2. Permeability coefficient and total void ratio Test results of permeability coefficient and total void ratio of RAPC are shown in Figs. 6 and 7. When crushing index changes from 9% to 37%, permeability coefficient of RAPC is 3.7 mm/s, 3.5 mm/s, 3.2 mm/s, 3.1 mm/s, 3.8 mm/s, 3.5 mm/s for six groups respectively. The minimum is 3.1 mm/s, and the maximum is 3.8 mm/s. All the data change slightly, basically a horizontal line as shown in Fig. 6. The porosity data of RAPC have no principle with the value between 15.1% and 16.5%, and the average is 15.8%, so increasing crushing index only has faint influence on total
25
30
35
40
Fig. 6. Permeability coefficient of RAPC versus crushing index.
30 25
Total void ratio, %
particles increases hardened cement mortar attached to the surface of aggregates, then the connection between old and new mortar becomes weak. At the same time, RA in the preparation and crushing process inevitably produces cracks, causing part of the damage [23], so these factors make recycled aggregate performance degradation, thus influence the flexural strength of RAPC. It is reported that the flexural strength of RAPC decreased with RA content increased in the Ref. [34]. In this study, compressive strength and bending strength decreased 36%, 28% respectively when the crushing index changed from 9% to 37%. It can be concluded that the influence of crushing index on compressive strength is stronger than bending strength. According to Technical specifications for pervious concrete pavement (CJJ134-2009) [36], all the bending strength of the RAPC is acceptable. Elasticity modulus is an important mechanical property of concrete material, and it is one necessary parameter to calculate deformation, cracks and temperature stresses of mass concrete. The static modulus of elasticity of RAPC is presented in Fig. 5. When crushing index value changed from 9% to 37%, 28d, 7d and 3d elastic modulus decreased respectively from 17.9 GPa, 13.1 GPa and 3.4 GPa to 14.1 GPa, 11.2 GPa and 2.6 GPa, and decrease amplitudes are 21%, 15% and 24% respectively. Using four different mixtures, the effects of aggregate gradation, amount, and size on elasticity modulus of pervious portland cement concrete were compared [13]. Results show that less paste lowers the elasticity modulus and compressive strength, which is available for aggregate bonding. There was no obvious influence for elasticity modulus using different sizes aggregate. The effect of aggregate size on the static elasticity modulus is needed to analyze further. So the performance of aggregate is the major contributor to influence the strength and elastic modulus of concrete. In this study, considering the gradation and size of aggregate are immobile and the crushing index of aggregates increases with the replacement of RA increases, so the elastic modulus of RAPC decreases with the increasing crushing index because of bad quality of waste brick aggregate.
20
Crushing index, %
20 15 10 5 0 5
10
15
20
25
30
35
40
Crushing index, % Fig. 7. Total void ratio of RAPC versus crushing index.
void ratio. In this study the results showed that the performance of RA has a great influence on the strength of RAPC, but the effects of increasing crushing index on the permeability coefficient and total void ratio of RAPC can be ignored. The reason is that permeability coefficient depends on the porosity of RAPC, while the porosity is determined mainly by aggregate size, shape and mix proportion [8]. Therefore crushing index of RAPC had no direct relationship with permeability coefficient and porosity. Some research showed that PC with macadam aggregate type has larger porosity than PC with pebble aggregate type [19]. In the Ref. [3], it was reported that the permeability coefficient and total void ratio of RAPC were similar to those of conventional pervious concrete. On the contrary, the porosity of RAPC was described to increase 10% than PC with NA [34]. The main reason may be that the NA was chosen as pebble with good compactness, while angular shape of RA resulted in under compaction of RAPC, so porosity increased. In this study, the NA is crushed stone, and RA is also a multi angular shape, so the void ratio was basically not changing with the crushing index. Therefore, angular shape of aggregate played an important role in total void ratio of PC when the concrete composition and aggregate size are the same. And all the test data about permeability coefficient and total void ratio of RAPC in this study are acceptable according to Technical specifications for pervious concrete pavement (CJJ134-2009) [36].
3.3. The strength loss rate and mass loss rate In the freezing and thawing cycles, recycled aggregate produced residual deformation. When the residual deformation is greater
Z. Zhang et al. / Construction and Building Materials 135 (2017) 112–118
The strength loss rate %
20 25 times 15
50 times
10
5
0 5
10
15
20
25
30
35
40
Crushing index % Fig. 8. The strength loss rate of RAPC versus crushing index.
The mass loss rate %
10
117
to improve freeze-thaw durability of pervious concrete. The reason is that fine aggregate increases particle contact area, which increases strength and durability. On the other hand, 8 groups of coarse aggregate with different quality were used to conduct freezing and thawing cycle tests to study the behavior of low quality concrete aggregate, showing that coarse aggregate quality has much influence on the freezing resistance because low quality coarse aggregate has high water absorption, low strength, and lots of micropore and microfracture [18]. In this study, strength loss rate and mass loss rate of RAPC are 18.7% and 8.5% respectively, so it is feasible to use recycled aggregates for making pervious concrete. Test research on the shrinkage and frost-resistance of RAPC conducted by WANG Jun-qiang also showed that after 25 times and 50 times of freeze-thaw cycles, strength loss rate were 5.6% and 18.9%, while mass loss rate were 2.3% and 7.5% [11,16]. Therefore, it can be concluded that the quality of aggregate decreases with the improvement of RA replacement, meanwhile freezing resistance of RAPC decreases with increased crushing index since the quality of aggregate deteriorates. 4. Conclusions In this study, recycled aggregates (RA) from crushed clay bricks were used to make recycled aggregates pervious concrete (RAPC), and the influence of crushing index on its properties were presented. Based on the findings of this summary, the following items may be drawn as following.
8 25 times 50 times
6
4
2
0 5
10
15
20
25
30
35
40
Crushing index % Fig. 9. The mass loss rate of RAPC versus crushing index.
than the cumulative residual strain of mortar, there was a greater tensile stress inside mortar, even reach the ultimate mortar strength, so the concrete cracked, splitted and dropped pieces, etc. These phenomena caused concrete strength reduction and quality loss [18]. The strength loss rate and mass loss rate of the test results are shown in Figs. 8 and 9 respectively. It can be concluded that both strength loss rate and mass loss rate increased by substantially with crushing index of recycled aggregate. For example, at 50 times freeze-thaw cycles, strength loss rate increased from 6.6% to 18.7%, and the mass loss rate increased from 2.3% to 8.5%. When crushing index is greater than 24%, the increasing trend accelerated. That is because the aggregate quality is relatively good when crushing index is less than 24%. There is less accumulated residual strain generated by the freeze-thaw cycles, even lower than mortar cumulative residual deformation, so RAPC at this state has sufficient durability of anti-freezing and thawing, with a relatively small strength loss rate and mass loss rate. However, when crushing index value is greater than 24%, aggregate quality is relatively poor. There are much difference from the good aggregate in strength, internal microstructure, porosity and gap size, thereby the frost resistance of RAPC is greatly influenced, showing relatively larger strength loss rate and mass loss rate. Generally, after 25 freeze-thaw cycles, the strength loss rate and the mass loss rate less than 20% and 5% respectively, the freezethaw durability can be acceptable for pervious concrete [36]. It is reported in the Ref. [22], that fine aggregate is major determinant
(1) The value of compressive strength, flexural strength and static modulus of elasticity reduce significantly with increased crushing index of recycled aggregates. The most reduction of strength occurs when crushing index is more than 24%. (2) With increased crushing index, the permeability coefficient and total void ratio change slightly, so the effects of increasing crushing index on the permeability coefficient and total void ratio of RAPC can be ignored. (3) Strength loss rate and mass loss rate of RAPC both increases as crushing index increases, and increasing trend accelerates when crushing index is greater than 24%. The overall results indicate that it is feasible to use recycled aggregates for making pervious concrete with acceptable properties including permeability coefficient and total void ratio, and the properties of RAPC are greatly related to crushing index of recycled aggregates. When crushing index is greater than 24%, pervious concrete using recycled aggregates behaves lower mechanical properties and durability. Acknowledgements The research work was jointly funded by the National Natural Science Foundation of China (Nos. 51379015 and 51579013) and Key International Cooperative Program of Shaanxi Province of China (2013KW13-01). References [1] M. Etxeberria, A.R. Marí, E. Vázquez, Recycled aggregate concrete as structural material, Mater. Struct. 40 (5) (2007) 529–541. [2] M. Limbachiya, M.S. Meddah, Y. Ouchagour, Use of recycled concrete aggregate in fly-ash concrete, Constr. Build. Mater. 27 (1) (2012) 439–449. [3] V. Sata, A. Wongsa, P. Chindaprasirt, Properties of pervious geopolymer concrete using recycled aggregates, Constr. Build. Mater. 42 (5) (2013) 33–39. [4] Z.H. Zhang, Q.F. Wang, J.J. Yang, J.Q. Liao, J. Yang, Study and design on pervious concrete mix proportion, Concrete 06 (2008) 120–126 (in Chinese). [5] H.K. Kim, H.K. Lee, Acoustic absorption modeling of porous concrete considering the gradation and shape of aggregate and void ratio, J. Sound Vib. 329 (7) (2010) 866–879.
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[22] J.T. Kevern, V.R. Schaefer, K. Wang, M.T. Suleiman, Pervious concrete mixture proportions for improved freeze-thaw durability, J. ASTM Int. 5 (2) (2008) 1– 12. [23] Q.S. Li, D.P. He, Comparative study of porous concretes using natural and recycled aggregates, J. Beijing Univ. Technol. 41 (1) (2015) 89–94 (in Chinese). [24] B. Zhang, S.L. Wang, Y.F. Du, L.P. Jing, Influence of crushing value index on compressive strength of recycled aggregate concrete, Ind. Build. 43 (11) (2013) 1–6 (in Chinese). [25] The State Bureau of Quality and Technical Supervision of the People’s Republic of China GB/17671-1999. Method of testing cements-determination of strength. [26] The State Bureau of Quality and Technical Supervision of the People’s Republic of China GB/1346-2001. Test methods for water requirement of normal consistency, setting time and soundness of the Portland cement. [27] The State Bureau of Quality and Technical Supervision of the People’s Republic of China GB/1345-2005. The test method for fineness of cement. [28] Ministry of Housing and Urban-Ural Construction of the People’s Republic of China GB/T50146-2014. Technical coed for application of fly ash concrete. [29] Ministry of Housing and Urban-Ural Construction of the People’s Republic of China GB/T14685-2011. Pebble and crushed stone for construction. [30] J. Lv, T.H. Zhou, Q. Du, H.H. Wu, Effects of rubber particles on mechanical properties of lightweight aggregate concrete, Constr. Build. Mater. 91 (5) (2015) 145–149. [31] Ministry of Housing and Urban-Ural Construction of the People’s Republic of China GB50081-2011 Standard for test methods of mechanics performance of common concrete. [32] Japan Concrete Institute, Construction and recent applications of porous concrete, Proceedings of the JCI Symposium on Design, Japan Concrete Institute, Tokyo, 2004, pp. 1–10. [33] Ministry of Housing and Urban-Ural Construction of the People’s Republic of China GB/T50082-2009 Standard for test methods of long-term performance and durability of ordinary concrete. _ [34] Erhan Güneyisi, Mehmet Gesog˘lu, Qays Kareem, Süleyman Ipek, Effect of different substitution of natural aggregate by recycled aggregate on performance characteristics of pervious concrete, Mater. Struct. 49 (1) (2016) 521–536. [35] P. Kumar Mehta, Paulo J.M. Monteiro, Concrete: Microstructure, Properties and Materials, third ed., McGraw Hill, New York, 2005. [36] Ministry of Housing and Urban-Ural Construction of the People’s Republic of China CJJ134-2009 Technical specifications for pervious concrete pavement.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Influence of crushing index on properties of recycled aggregates pervious concrete Zhiquan Zhang a, Yufen Zhang b,⇑, Changgen Yan c, Yunxiao Liu a a
School of Civil Engineering, Chang’an University, Xi’an 700061, PR China School of Civil Engineering, North China University of Technology, Beijing 100144, PR China c Highway College of Chang’an University, Xi’an, Shaanxi Province, PR China b
h i g h l i g h t s Natural aggregate was replaced with discarded concrete and clay brick aggregates. Addition of recycled clay brick aggregates increases the crushing index. Increase of crushing index exerts a negative impact on mechanical properties. The effects of crushing index on the porosity and permeability can be ignored.
a r t i c l e
i n f o
Article history: Received 11 July 2016 Received in revised form 5 December 2016 Accepted 29 December 2016
Keywords: Recycled aggregates Pervious concrete Crushing index Mechanical properties Water permeability
a b s t r a c t In this study, recycled aggregates (RA) produced by mechanical shredding from discarded concrete and clay bricks were used to make pervious concrete. Six groups of recycled aggregates pervious concrete (RAPC) were designed with different crushing index of aggregates under the condition of same concrete mixture ratio. The crushing index of aggregates increases with the replacement of RA increases. Experiments about compressive strength, elasticity modulus, flexural strength, permeability coefficient, total void ratio were conducted, including freezing and thawing cycle test. Test data of RAPC indicated that compressive strength, flexural strength and elasticity modulus of 28 days decreased by 36%, 28% and 21% respectively when crushing index changed from 9% to 37%, simultaneously strength loss rate increased from 6.6% to 18.7% and mass loss rate increased from 2.3% to 8.5%. Especially when crushing index is greater than 24%, except permeability coefficient and total void ratio, other properties of RAPC decreased significantly due to the lower quality of RA. Therefore, experimental results show that increase of crushing index has significant effects on compressive strength, elasticity modulus, flexural strength and freeze-thaw durability of RAPC. However, the effects of increasing crushing index on the permeability coefficient and total void ratio of RAPC can be ignored. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction Concrete mainly consists of cement, sand, pebble (or crushed stone), and each cubic meter of concrete consumes about 1700– 2000 Kg aggregates. With economic development, concrete aggregate consumption grows very rapidly. Depletion and difficulty of mining sand and gravel resources are increasing. Meanwhile, billions of ton of construction waste were produced per year, including waste concrete, brick and others [1,2]. Such a large number of construction waste, if not well handled and effective utilized, the urban environment will suffer enormous damage. Using the waste ⇑ Corresponding author. E-mail address: [email protected] (Y. Zhang). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.203 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
concrete and brick can protect resources and reduce environmental pollution, so it has significant economic and social benefits [3]. Pervious concrete (PC) uses a small amount or no fine aggregate during preparation, and the right amount of cements are used to paste and wrap the bonding aggregate particles, then connected pore can quickly drain [4]. Its previous behavior depends on the porosity and the particle size of the aggregate. PC can reach a certain intensity and water permeability through rational design of mixture ratio [5,6]. PC as a unique and effective eco-friendly materials have been widely used at low-grade roads, squares, parking lots and other areas in North America, Europe and Japan [7]. It can regulate urban microclimate and sound absorption, maintain ecological balance, reduce the adverse effects of heavy rain or flood [8,9]. There were also many achievements published about PC
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design [4], strength and water permeability [10], frost resistance [11], microstructure [12], workability [13,14]. Recycled aggregate pervious concrete (RAPC), as one new type of ecologically concrete, can reduce environmental pollution, and help to protect the ecological balance [15]. So in recent years, some researchers have studied the different behaviors of RAPC including strength, total void ratio, and permeability coefficient. Research results showed that pervious concrete containing recycled aggregate (RA) gave lower mechanical properties than those containing natural aggregate (NA) [3]. Although, there is not enough research about the effect of RA on total void ratio and water permeability of RAPC, it is reported that the gradation and shape of RA can affect the void texture and void ratio of porous concrete [5]. Test research on the shrinkage and frost-resistance of RAPC showed that after 25 times and 50 times of freeze-thaw cycles, the strength loss rate of the pervious concrete were 5.6% and 18.9% [16]. The main factors that result in the strength loss are large pore structure of PC and weak bond between aggregates. The characteristic and mechanism of RA were studied in the Refs. [17–19], and it was concluded that durability of concrete changed greatly with the RA behaviors because RA differed from NA in pore volume and size on a micro level, water absorption and apparent density on a macro level. Kwan et al. have conducted the influencing test of aggregate size, grading and content on the behaviors of RAPC, showing the remarkable influence of water cement ratio and the RA content [20–22]. However, few researches of RAPC were relative to crushing index of aggregate. NA are hard and compact with low porosity, while RA produced by crushing waste concrete and brick is rough with many edges and corners, so RA has porosity property and high water absorption [23]. Crushing index is related to the strength, shape, water absorption, density, micropore and microfracture of aggregate, which can represent the quality of aggregate [24]. Consequentially, different crushing index can affect the performance on RAPC greatly. This paper focused on the sorts of recycled aggregates grouped by different crushing value, and some tests on compressive strength, flexural strength, elasticity modulus, porosity, permeability, frost resistance were conducted for 6 groups of RAPC specimens.
RA particles produced by mechanical shredding from discarded concrete and clay bricks, ranging from 5 mm to10 mm in size. The particle size distribution curve of the RA is shown in Fig. 1. Crushing index, water absorption and distribution curve were tested according to Pebble and crushed stone for construction (GB/ T14685-2011) [29]. In this study, NA is crushed stone, while RA is discarded concrete and clay brick particle. Six groups of aggregate, the recycled aggregate took different content of the total aggregate, were prepared for RAPC. Crushing index of each group was measured as 37%, 34%, 30%, 24%, 19% and 9% respectively. Water absorption of each group was measured as 12.8%, 9.54%, 7.12%, 4.67%, 2.85% and 0.9% respectively. The mixing water was tap water. Water-reducing admixture (WRA) was polycarboxylate superplasticizer, and the recommended participate amount is 1%. 2.2. Mix proportions The mix proportion of RAPC was set at 1.00:0.20:0.34:4. 34:0.012 for Cement:FA:Water:RA:WRA as shown in Table 3. Six mix proportions were set for crushing index 37%, 34%, 30%, 24%, 19% and 9% respectively. Some tests were conducted about compressive strength, flexural strength, elasticity modulus, permeability coefficient, total void ratio and freeze-thaw durability. All mixtures were mixed in a standard mixer. Firstly, all solid ingredients were mixed together for about 1 min before adding water, then water and WRA were added in and mixed for 2 min. The mixture was then placed into molds and demoulded after 24 h. After demoulded, all specimens were cured for 3, 7 and 28 days in a standard condition where the relative humidity is more than 95% and the temperature is 20 ± 2 °C [30]. 2.3. Testing methods
2. Experiment program
The crushing index was determined in accordance with Pebble and crushed stone for construction (GB/T14685-2011) [29]. Firstly, 3000 g of sample (M1) was put into the determinator under pressure testing. Then test machine was started by the velocity of 1 kN per second until the load of 200 kN. Kept the load for 5 s, and uninstalled it. Finally, the crushed fine particles were screened out using sieves with holes in 2.36 mm. The leaves of the sample was measured as M2. The crushing index (Qe) was calculated by
2.1. Materials
Qe ¼
All the parameter indexes of cement are shown in Table 1. The value of strength, setting time, soundness and fineness of cement were measured according to Method of testing cementsdetermination of strength (GB/17671-1999) [25], Test methods for water requirement of normal consistency setting time and soundness of the Portland cement (GB/1346-2001) [26] and Test method for fineness of cement (GB/1345-2005) [27]. The Fineness was measured on 45 lm sieve. The value of water demand ratio, loss on ignition (LOI), fineness and main chemical composition of fly ash (FA) are shown in Table 2. Fineness was measured on 45 lm sieve. It can be classified as ClassⅡ according to Technical code for application of fly ash concrete (GB/T50146-2014) [28].
where, Qe is the crushing index (%); M1 is the initial mass of sample (g); M2 is the mass of leaves of sample (g). The results of compressive strength, flexural strength and elasticity modulus were reported by the average readings. The compressive strength, flexural strength and elasticity modulus were measured according to Standard for test methods of mechanics performance of common concrete (GB50081-2011) [31]. The compressive strength and elasticity modulus were both measured on three prism specimens 150 150 300 mm3 in size. The flexural strength was measured on prism specimens 150 150 600 mm3 in size. The compressive load was applied by a servo-controlled hydraulic testing machine. In compressive strength test, specimens were tested at a constant loading rate of 9.5 kN/s. For the flexural
M1 M2 100% M1
ð1Þ
Table 1 Cement parameter index. Fineness (%)
3
Soundness
up to standard
Setting time (min)
Strength (MPa)
Initial setting
Final setting
7d
28d
160
285
25.5
48.8
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Table 2 Fly ash parameter index. Water demand ratio (%)
LOI (%)
Fineness (%)
102
5.3
14.6
Main chemical composition (%) SiO2
AI2O3
Fe2O3
CaO
61.2
22.5
5.0
0.7
100 standardized upper bound standardized lower bound RA
90
Percentage passing, %
80 70 60 50 40 30 20
Fig. 2. Water permeability set-up for RAPC.
10 0 2
4
6
8
10
12
14
16
particle size, mm Fig. 1. Particle size distribution curve of the RA.
strength, specimens were tested at a constant loading rate of 0.5 kN/s. The elasticity modulus was measured as a secant modulus in the elastic state. Each of these specimens was equipped with two dial indicators with the capable of measuring deformation 0.002 mm and then loaded by 40% of the ultimate load three times. The first set of readings of each cylinder was discarded and the modulus was reported as the average of the rest readings [30]. The total void ratio of RAPC was determined in accordance with construction and recent applications of porous concrete [32], curing period of specimens was 28d. The total void ratio (VR) was calculated by
VR ¼ 1
ðM 4 M3 Þ=qw 100% V
ð2Þ
where, VR is the total void ratio (%); M3 is the initial mass of specimen in water (g); M4 is specimen dried 24 h in air measured final mass (g); V is the volume of specimens (cm3); qw is the density of water (g/cm3). Permeability coefficient of RAPC was also determined in accordance with Construction and recent applications of porous concrete [32], curing period of specimens was 28 days. Fig. 2 shows the test methods for pervious concrete permeability coefficient. Permeability coefficient was calculated by
Kr ¼
H Q h A ðt 2 t1 Þ
ð3Þ
where, Kr is the permeability coefficient (cm/s); H is the length of the specimen (cm); Q is the amount of water t1 time to t2 time specimens discharged (cm3); h is the head difference (cm); t2, t1 are the time (s); A is the cross-sectional area of the cylindrical specimens (cm2). Strength loss rate and mass loss rate of pervious concrete under freezing and thawing were both measured according to Standard for test methods of long-term performance and durability of ordinary concrete (GB/T50082-2009) [33]. Freezing and thawing specimens were chosen as 100 100 100 mm3 cube, and the curing period was 28 days. Freeze-thaw test chamber was kept with air temperature from 20 °C to 18 °C in freezing period, the temperature of water in freeze-thaw test chamber was kept from 18 °C to 20 °C in thawing period. 3. Results and discussions Test results of compressive strength, flexural strength and static elasticity modulus are listed in Table 4. Test results of permeability coefficient, total void ratio, mass loss rate and strength loss rate are listed in Table 5. 3.1. Compressive strength, flexural strength and static modulus of elasticity The compressive strength of RAPC is shown in Fig. 3. Compressive strength of RAPC decreases with the increases of crushing index, and the downward trend of 3-day (3d) strength, 7-day (7d) strength and 28-day (28d) strength is same. When crushing index increases from 9% to 37%, 28d strength decreases from 24.2 MPa to 15.5 MPa, about 36 percent, while the decrease of 7d
Table 3 Mix proportions and crushing index. Specimens No.
RAPC RAPC RAPC RAPC RAPC RAPC
1 2 3 4 5 6
Weight per cubic meter (kg/m3) C
FA
Water
RA
350 350 350 350 350 350
70 70 70 70 70 70
118 118 118 118 118 118
1520 1520 1520 1520 1520 1520
W/C + FA
Water-reducing admixture (kg/m3)
Crushing index (%)
0.28 0.28 0.28 0.28 0.28 0.28
4.2 4.2 4.2 4.2 4.2 4.2
37 34 30 24 19 9
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Z. Zhang et al. / Construction and Building Materials 135 (2017) 112–118 Table 4 Testing results of compressive strength, flexural strength and static modulus of elasticity. Specimens No.
RAPC RAPC RAPC RAPC RAPC RAPC
Compressive strength (MPa)
1 2 3 4 5 6
Flexural strength (MPa)
Static modulus of elasticity (GPa)
3d
7d
28d
3d
7d
28d
3d
7d
28d
5.0 5.3 6.3 6.9 8.0 8.9
10.5 11.9 12.6 13.5 16.6 17.5
15.5 16.5 17.8 20.0 23.6 24.2
0.66 0.71 0.75 0.78 0.90 1.02
1.81 1.82 1.96 2.01 2.35 2.45
2.55 2.59 2.77 3.02 3.41 3.56
2.6 2.6 2.7 2.8 3.1 3.4
11.2 11.1 11.5 12.2 12.8 13.1
14.1 14.5 15.1 16.4 17.4 17.9
Table 5 Testing results of permeability coefficient, total void ratio, mass loss rate and strength loss rate. Permeability coefficient (mm/s)
Total void ratio (%)
RAPC RAPC RAPC RAPC RAPC RAPC
3.5 3.8 3.1 3.2 3.5 3.7
15.3 16.5 15.9 16.4 15.1 16.3
1 2 3 4 5 6
Compressive strength, MPa
30 3d 7d 28d
25 20 15 10 5 0 5
10
15
20
25
30
35
40
Crushing index, % Fig. 3. Compressive strength of RAPC versus crushing index.
strength and 3d strength are 40% and 44%respectively, so the decrease amplitude gradually increases from 28d strength to 7d strength to 3d strength. Take 28d strength for example, when crushing index changed from 9% to 19%, the strength was 24.4 MPa and 23.6 MPa respectively, keeping little changes. But when crushing index was greater than 24%, 28d strength was 20.0 MPa, 17.8 MPa, 16.5 MPa and 15.5 MPa for crushing index 24%, 30%, 34%, and 37% respectively, so the downward trend more obvious. In the reference, it was reported that the compressive strength of RAPC decrease about 30% with RA content changed from 0% to 100%, i.e. values range from 20 to 13.9 MPa [34]. Another study claimed the replacement level of NA with RA will reduce the compressive strength of the concrete. However, when the replacement was smaller than 80%, the target strength was still achieved by employing the DoE mix design method [20]. In this study, the strength decreased about 36% with crushing index changed from 9% to 37%, so crushing index has evident influence on the compressive strength of RAPC. It also was observed that the strength keeping little change when crushing index changed from 9% to 19%. It means that it is feasible to use right amount recycled aggregates for making pervious concrete with acceptable compressive strength. According to Technical specifications for pervious con-
The mass loss rate after freezethaw cycles (%)
The strength loss rate after freeze-thaw cycles (%)
25 times
50 times
25 times
50 times
4.9 3.8 3.0 1.8 1.1 0.9
8.5 7.2 5.7 4.5 2.9 2.3
7.7 7.5 6.6 5.7 3.0 2.5
18.7 17.1 14.5 12.3 7.5 6.6
crete pavement (CJJ134-2009) [36], when crushing index is smaller than 24% the compressive strength of RAPC are acceptable. In general, compressive strength of pervious concrete is related to aggregate strength and total porosity. For this research, strength decrease with increased crushing index is due to the increased brick particles. Firstly, the strength of the brick particles is relatively low, about 10–20 MPa, so it affects the function of concrete strength. Secondly, water-cement ratio of concrete mixing ratio design is 0.28, and the lowest water-cement ratio is 0.23, while higher water absorption of brick particles results in incomplete hydration of cement [35], so it indirectly leads to lower concrete strength. Test data of flexural strength are shown in Table 3. 3d strength, 7d strength and 28d strength are all decrease with increasing crushing index. The flexural strength of RAPC is presented in Fig. 4. When crushing index changes from 9% to 37%, 28d strength, 7d strength and 3d strength decreased respectively from 3.56 MPa, 2.45 MPa and 1.02 MPa to 2.55 MPa, 1.81 MPa and 0.66 MPa, and decrease amplitudes are 28%, 26% and 35% respectively. Meanwhile, when crushing index is greater than 24%, downward trend of strength is more obvious. The reason is that drawbacks of RA result in the decrease of flexural strength and toughness. As crushing index increases, brick
5
Flexural strength, MPa
Specimens No.
3d 7d 28d
4 3 2 1 0 5
10
15
20
25
30
35
Crushing index, % Fig. 4. Flexural strength of RAPC versus crushing index.
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Z. Zhang et al. / Construction and Building Materials 135 (2017) 112–118
10
25 3d 7d 28d
20
Permeability coefficient, mm/s
Static moidulus of elasticity, GPa
116
15 10 5 0 5
10
15
20
25
30
35
8 6 4 2 0
40
5
10
15
Crushing index, % Fig. 5. Static modulus of elasticity of RAPC versus crushing index.
3.2. Permeability coefficient and total void ratio Test results of permeability coefficient and total void ratio of RAPC are shown in Figs. 6 and 7. When crushing index changes from 9% to 37%, permeability coefficient of RAPC is 3.7 mm/s, 3.5 mm/s, 3.2 mm/s, 3.1 mm/s, 3.8 mm/s, 3.5 mm/s for six groups respectively. The minimum is 3.1 mm/s, and the maximum is 3.8 mm/s. All the data change slightly, basically a horizontal line as shown in Fig. 6. The porosity data of RAPC have no principle with the value between 15.1% and 16.5%, and the average is 15.8%, so increasing crushing index only has faint influence on total
25
30
35
40
Fig. 6. Permeability coefficient of RAPC versus crushing index.
30 25
Total void ratio, %
particles increases hardened cement mortar attached to the surface of aggregates, then the connection between old and new mortar becomes weak. At the same time, RA in the preparation and crushing process inevitably produces cracks, causing part of the damage [23], so these factors make recycled aggregate performance degradation, thus influence the flexural strength of RAPC. It is reported that the flexural strength of RAPC decreased with RA content increased in the Ref. [34]. In this study, compressive strength and bending strength decreased 36%, 28% respectively when the crushing index changed from 9% to 37%. It can be concluded that the influence of crushing index on compressive strength is stronger than bending strength. According to Technical specifications for pervious concrete pavement (CJJ134-2009) [36], all the bending strength of the RAPC is acceptable. Elasticity modulus is an important mechanical property of concrete material, and it is one necessary parameter to calculate deformation, cracks and temperature stresses of mass concrete. The static modulus of elasticity of RAPC is presented in Fig. 5. When crushing index value changed from 9% to 37%, 28d, 7d and 3d elastic modulus decreased respectively from 17.9 GPa, 13.1 GPa and 3.4 GPa to 14.1 GPa, 11.2 GPa and 2.6 GPa, and decrease amplitudes are 21%, 15% and 24% respectively. Using four different mixtures, the effects of aggregate gradation, amount, and size on elasticity modulus of pervious portland cement concrete were compared [13]. Results show that less paste lowers the elasticity modulus and compressive strength, which is available for aggregate bonding. There was no obvious influence for elasticity modulus using different sizes aggregate. The effect of aggregate size on the static elasticity modulus is needed to analyze further. So the performance of aggregate is the major contributor to influence the strength and elastic modulus of concrete. In this study, considering the gradation and size of aggregate are immobile and the crushing index of aggregates increases with the replacement of RA increases, so the elastic modulus of RAPC decreases with the increasing crushing index because of bad quality of waste brick aggregate.
20
Crushing index, %
20 15 10 5 0 5
10
15
20
25
30
35
40
Crushing index, % Fig. 7. Total void ratio of RAPC versus crushing index.
void ratio. In this study the results showed that the performance of RA has a great influence on the strength of RAPC, but the effects of increasing crushing index on the permeability coefficient and total void ratio of RAPC can be ignored. The reason is that permeability coefficient depends on the porosity of RAPC, while the porosity is determined mainly by aggregate size, shape and mix proportion [8]. Therefore crushing index of RAPC had no direct relationship with permeability coefficient and porosity. Some research showed that PC with macadam aggregate type has larger porosity than PC with pebble aggregate type [19]. In the Ref. [3], it was reported that the permeability coefficient and total void ratio of RAPC were similar to those of conventional pervious concrete. On the contrary, the porosity of RAPC was described to increase 10% than PC with NA [34]. The main reason may be that the NA was chosen as pebble with good compactness, while angular shape of RA resulted in under compaction of RAPC, so porosity increased. In this study, the NA is crushed stone, and RA is also a multi angular shape, so the void ratio was basically not changing with the crushing index. Therefore, angular shape of aggregate played an important role in total void ratio of PC when the concrete composition and aggregate size are the same. And all the test data about permeability coefficient and total void ratio of RAPC in this study are acceptable according to Technical specifications for pervious concrete pavement (CJJ134-2009) [36].
3.3. The strength loss rate and mass loss rate In the freezing and thawing cycles, recycled aggregate produced residual deformation. When the residual deformation is greater
Z. Zhang et al. / Construction and Building Materials 135 (2017) 112–118
The strength loss rate %
20 25 times 15
50 times
10
5
0 5
10
15
20
25
30
35
40
Crushing index % Fig. 8. The strength loss rate of RAPC versus crushing index.
The mass loss rate %
10
117
to improve freeze-thaw durability of pervious concrete. The reason is that fine aggregate increases particle contact area, which increases strength and durability. On the other hand, 8 groups of coarse aggregate with different quality were used to conduct freezing and thawing cycle tests to study the behavior of low quality concrete aggregate, showing that coarse aggregate quality has much influence on the freezing resistance because low quality coarse aggregate has high water absorption, low strength, and lots of micropore and microfracture [18]. In this study, strength loss rate and mass loss rate of RAPC are 18.7% and 8.5% respectively, so it is feasible to use recycled aggregates for making pervious concrete. Test research on the shrinkage and frost-resistance of RAPC conducted by WANG Jun-qiang also showed that after 25 times and 50 times of freeze-thaw cycles, strength loss rate were 5.6% and 18.9%, while mass loss rate were 2.3% and 7.5% [11,16]. Therefore, it can be concluded that the quality of aggregate decreases with the improvement of RA replacement, meanwhile freezing resistance of RAPC decreases with increased crushing index since the quality of aggregate deteriorates. 4. Conclusions In this study, recycled aggregates (RA) from crushed clay bricks were used to make recycled aggregates pervious concrete (RAPC), and the influence of crushing index on its properties were presented. Based on the findings of this summary, the following items may be drawn as following.
8 25 times 50 times
6
4
2
0 5
10
15
20
25
30
35
40
Crushing index % Fig. 9. The mass loss rate of RAPC versus crushing index.
than the cumulative residual strain of mortar, there was a greater tensile stress inside mortar, even reach the ultimate mortar strength, so the concrete cracked, splitted and dropped pieces, etc. These phenomena caused concrete strength reduction and quality loss [18]. The strength loss rate and mass loss rate of the test results are shown in Figs. 8 and 9 respectively. It can be concluded that both strength loss rate and mass loss rate increased by substantially with crushing index of recycled aggregate. For example, at 50 times freeze-thaw cycles, strength loss rate increased from 6.6% to 18.7%, and the mass loss rate increased from 2.3% to 8.5%. When crushing index is greater than 24%, the increasing trend accelerated. That is because the aggregate quality is relatively good when crushing index is less than 24%. There is less accumulated residual strain generated by the freeze-thaw cycles, even lower than mortar cumulative residual deformation, so RAPC at this state has sufficient durability of anti-freezing and thawing, with a relatively small strength loss rate and mass loss rate. However, when crushing index value is greater than 24%, aggregate quality is relatively poor. There are much difference from the good aggregate in strength, internal microstructure, porosity and gap size, thereby the frost resistance of RAPC is greatly influenced, showing relatively larger strength loss rate and mass loss rate. Generally, after 25 freeze-thaw cycles, the strength loss rate and the mass loss rate less than 20% and 5% respectively, the freezethaw durability can be acceptable for pervious concrete [36]. It is reported in the Ref. [22], that fine aggregate is major determinant
(1) The value of compressive strength, flexural strength and static modulus of elasticity reduce significantly with increased crushing index of recycled aggregates. The most reduction of strength occurs when crushing index is more than 24%. (2) With increased crushing index, the permeability coefficient and total void ratio change slightly, so the effects of increasing crushing index on the permeability coefficient and total void ratio of RAPC can be ignored. (3) Strength loss rate and mass loss rate of RAPC both increases as crushing index increases, and increasing trend accelerates when crushing index is greater than 24%. The overall results indicate that it is feasible to use recycled aggregates for making pervious concrete with acceptable properties including permeability coefficient and total void ratio, and the properties of RAPC are greatly related to crushing index of recycled aggregates. When crushing index is greater than 24%, pervious concrete using recycled aggregates behaves lower mechanical properties and durability. Acknowledgements The research work was jointly funded by the National Natural Science Foundation of China (Nos. 51379015 and 51579013) and Key International Cooperative Program of Shaanxi Province of China (2013KW13-01). References [1] M. Etxeberria, A.R. Marí, E. Vázquez, Recycled aggregate concrete as structural material, Mater. Struct. 40 (5) (2007) 529–541. [2] M. Limbachiya, M.S. Meddah, Y. Ouchagour, Use of recycled concrete aggregate in fly-ash concrete, Constr. Build. Mater. 27 (1) (2012) 439–449. [3] V. Sata, A. Wongsa, P. Chindaprasirt, Properties of pervious geopolymer concrete using recycled aggregates, Constr. Build. Mater. 42 (5) (2013) 33–39. [4] Z.H. Zhang, Q.F. Wang, J.J. Yang, J.Q. Liao, J. Yang, Study and design on pervious concrete mix proportion, Concrete 06 (2008) 120–126 (in Chinese). [5] H.K. Kim, H.K. Lee, Acoustic absorption modeling of porous concrete considering the gradation and shape of aggregate and void ratio, J. Sound Vib. 329 (7) (2010) 866–879.
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