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Permanent deformation behaviour of unbound recycled mixtures
Construction and Building Materials 37 (2012) 573–580
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Permanent deformation behaviour of unbound recycled mixtures G. Cerni a, F. Cardone b,⇑, M. Bocci b a b
Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, via G. Duranti 93, 06125 Perugia, Italy Dipartimento di Ingegneria Civile Edile e Architettura, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy
h i g h l i g h t s " Intrinsic properties of recycled fine fraction assure a good permanent deformation resistance for the mixture. " Innovative approach for ranking the unbound recycled mixture as regards rutting potential. " C&D wastes as sustainable and cost-effective alternative to natural aggregate.
a r t i c l e
i n f o
Article history: Received 5 March 2012 Received in revised form 14 June 2012 Accepted 22 July 2012 Available online 7 September 2012 Keywords: Recycling C&D aggregate Repeated triaxial loading Permanent deformation
a b s t r a c t Nowadays, the use of recycled waste materials in road construction and rehabilitation processes aimed to preserve non-renewable resources and to solve management problems related to the expansion of landfills become a promising engineering solution. The current paper deals with the feasibility of using material from construction and demolition wastes (C&D) as aggregate for unbound layers (base and/or subbase) of road pavements. In particular, the permanent deformation behaviour of a recycled C&D mixture under repeated triaxial loading was investigated and compared with that of two natural granular mixtures selected as reference materials. An analytical model, proposed by the authors, was used to describe the long-term permanent strain accumulation of materials. The results, in addition to showing the ability of the model to predict change in mechanical behaviour depending on the different nature of materials, highlight how the C&D mixture performs better under specific stress and moisture conditions compared with traditional materials. Finally, the results obtained in this study not only provide a practical approach for ranking granular materials for pavement with regard to their rutting potential but also promote the use of C&D materials as a sustainable and effective alternative to traditional aggregates. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, all developed countries have been dealing with social and economic problems related to the environment. First issue according to sustainable development is the protection of the limited natural resources due to the ever-increasing demand for high quantities of adequate materials for the construction and maintenance of civil infrastructures with a consequent increase in construction costs. The second aspect is connected with the management of the waste materials produced by means of industrial processes or by the construction and demolition of civil infrastructures (C&D waste) because of the dwindling number of landfills and continuous increasing costs of disposal. These problems have led to greater social and political sensitivity to the environment with significant consequences on road engineering. The need to reduce construction costs and time have led to consider recycling of waste materials as a suitable alternative to virgin ⇑ Corresponding author. Tel.: +39 071 220 4507; fax: +39 071 220 4510. E-mail address: [email protected] (F. Cardone). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.062
materials in road construction and rehabilitation works. As such, the use of C&D materials as a source of aggregate in the construction of unbound base and/or subbase layers seems to be a valid possibility to solve the aforementioned problems as also confirmed by several researches [1–7]. As well known, unbound granular material layers provide the most important structural contribution in road pavements because in most cases, as the soil subgrade assures a sufficient bearing capacity, the rutting phenomenon takes place mainly in the granular base and subbase layers causing pavement depression and progressive fatigue cracking of bituminous layers [8–10]. Basically, granular materials show resilient and permanent deformations when subjected to repeated loading. Resilient deformations are related to the stiffness properties of the material that affect the fatigue cracking of overlying asphalt layers, whereas the gradual accumulation of permanent deformations, although very small during each loading cycle, could lead to the collapse of the structure due to excessive rutting. Therefore, the conventional road pavement design approach is based on providing adequate thickness of layers in such a way that the pavement structure does
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not experience shear failure and that unacceptable permanent deformations occur in each layer. On the basis of this evidence, an appropriate understanding and characterisation of permanent deformation behaviour of unbound granular materials (UGMs) is needed in order to perform a successful pavement design [8–14]. The main objective of the present research was to better understand the mechanical behaviour of recycled mixtures containing C&D materials in order to evaluate whether they are feasibly useful as unbound material in the base or subbase layer of road pavement. In particular, the permanent deformation behaviour of C&D mixture through repeated load triaxial tests was investigated. An analytical model, developed by the authors [12], was used to predict the permanent strain accumulation phenomenon of the C&D materials. Moreover, the comparison between the selected recycled material and two virgin granular blends allowed the reliability of the model to characterise the mechanical response under cyclic loading of alternative materials, as the C&D wastes, to be evaluated. 2. Experimental program This study presents a laboratory investigation aimed to characterise the permanent deformation behaviour of unbound recycled granular mixtures. The first part of research was dedicated to a physical characterisation of C&D materials by traditional tests. The second part was entirely dedicated to the study of the permanent deformation behaviour of recycled mixture by means of repeated load triaxial tests. In this phase, two natural granular blends were also considered as reference materials for performance comparison. 2.1. Material The recycled material, referred to as C&D mixture, consisted mainly of construction and demolition waste coming from a recycling plant located in the Umbria region (Central Italy) near the town of Foligno. The planned production procedures of the plant, based on preliminary sorting and separation processes, create a homogenous material without undesirable component as wood, plastic, metal, paper, glass and asbestos. Table 1 shows the results of the physical characterisation performed on C&D material according to European Specifications [15]. The recycled material had a maximum diameter of 16 mm and its composition consisted mainly of lithic material as cementitious elements, bricks, tiles and crushed rocks. The undesiderable materials were in negligible amounts. Moreover, the determination of consistency limits showed that the investigated recycled material under study was non-plastic. The Proctor compaction analysis revealed that the recycled mixture needed a high optimum moisture content to guarantee maximum dry density. This is mainly attributed to the high water absorption of the C&D particles due to the presence of highly porous ceramic elements. As absorption can change because of heterogeneity of these materials, the moisture content had to be rigorously controlled during the in situ compaction process in order to evaluate the need to be changed to improve the compaction of unbound layer. The values of shape and flakiness index mean that the recycled blend was characterised by a predominance of cubic particles. The high Los Angeles coefficient denotes the marked tendency to breakage of particles, that results in a high production of fine fraction during the compaction process. Both above mentioned aspects can contribute to a better densification of the unbound recycled mixture. As far as the mechanical properties are concerned, the CBR index, approximately 90%, showed a good bearing capacity of recycled mixture undoubtedly comparable to that of a natural well-graded crushed aggregate mixture and widely adequate to meet the requirements of technical specifications.
2.2. Blend and specimen preparation The gradation of the C&D granular mixture was designed according to Italian Specification for the construction of unbound base or subbase pavement layers [16]. In particular, the mean curve of the reference band was used as design grain-size distribution as shown in Fig. 1. After blending, the recycled mixture was compacted at its respective maximum Proctor Dry density and Optimum Moisture content (Table 1). Compaction was carried out by means of a shear gyratory compactor (SGC) in order to produce cylindrical samples of 100 mm diameter and 200 mm height, dimensions suitable for materials with a particle size distribution of 0/16 mm according to European Standards [17]. Moreover, in order to guarantee uniform density conditions, each specimen was compacted in four layers of equal size (h = 50 mm).
2.3. Test procedure The cyclic tests were performed by means of a repeated load triaxial apparatus, consisting essentially of a main pneumatic load device and a removable pressure chamber, which was suitable for applying a constant confining pressure and a cyclic axial load through the test on the specimen [17]. In addition, two LVDTs measured axial deformations at the top of the specimen. Load and deformation data were then stored by a proper data acquisition system. All tests were performed in drained conditions. The details of the loading program, in terms of confining pressure r3 and cyclic axial stress r1, are shown in Table 2. In particular, additional axial stress with a sinusoidal wave frequency of 2 Hz was applied for each test after the set confining pressure was reached. The tests were carried out up to 10,000 load cycles. Such an analysis period allowed on the one hand a testing time compatible with laboratory work constraints and on the other a practical and rapid method to characterise the mechanical behaviour of these materials to be obtained. Triaxial tests were performed on the investigated mixtures in both optimum water content and saturated conditions, in order to evaluate the effect of water on the permanent deformation behaviour of the materials. To this end, a proper laboratory procedure to soak unbound specimens was adopted (Fig. 2), which is described in detail elsewhere [12]. Each specimen was soaked for 96 h and immediately afterwards subjected to repeated load triaxial test according to the schedule listed in Table 2. It is right to note that results shown in the following paragraphs refer to a single replicate carried out in each condition deriving from the combination of the considered test variables (moisture and stress condition). Even though it is widely recognised that a permanent deformation response can lead to a scatter of test results, in this specific phase of investigation the authors wanted primarily to investigate the effects of a wide range of stress levels at the expense of test repeatability.
3. Results and discussion As mentioned above, the permanent deformation behaviour of recycled mixture was analysed and compared with the mechanical response of two unbound natural granular materials. Hence, in the following the results of a previous research performed on these natural aggregates were reported also [12]. The two considered reference blends, referred to as mixtures A and B respectively, were characterised by a different nature of the fine fraction. In particular, mixture A was a crushed limestone aggregate, whereas mixture B was obtained by replacing only the calcareous fine fraction (passing to 0.063 mm sieve) of mixture A with a silty clay soil fraction, characterised by high water susceptibility (plasticity index = 21.7%), without any further change in
Table 1 Physical properties of C&D material.
a
Property
Result
Technical requirement
Property
Result
Technical requirement
Bituminous material (Ra) Bricks, ceramics, tiles (Rb) Concrete, concrete products, mortar (Rc) Unbound aggregates, natural stone (Ru) Glass (Rg) Metals, plastic, rubber, gypsum (X1) Wood, paper, cellulose (X2) Maximum aggregate size Apparent particle density Water absorption
2.0% 29.5% 25.6% 42.0% 0.6% 0.2% 0.1% 16 mm 2.57 t/m3 7.72%
<5%
Sand equivalent Plasticity index Crushed particles Shape index Flakiness index Los angles coefficient Maximum dry density Optimum moisture content California bearing ratio Total sulphur content
27.4% NP 72% 28% 26.5% 37% 1.92 t/m3 12% 90% 0.2%
>40% NP >60% <40% <35% <30% – – >30% –
Requirement on (Rb + Rc + Ru).
>90%a <5% <0.4% <0.1% <63 mm – –
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In the figure it can be seen that the magnitude of accumulated permanent deformations increase rapidly in the first phase of the test, while subsequently the strain increase stabilizes to a constant value. These typical results were described by a linear-exponential law, which was suggested by the authors and based on the following equation:
100 90
Design curve
Passing (%)
80
Reference band
70 60 50
ep ¼ A þ B N C eDN
40 30 20 10 0 0.01
0.1
1
10
100
Sieve Size (mm) Fig. 1. Design particle-size distribution for tested materials.
grading. This mixture was used in order to investigate the influence of a small amount of plastic material as silty clay soil on the strain behaviour of an unbound granular material layer. Before testing, both blends were compacted at their maximum Proctor dry density and optimum moisture content (2.29 t/m3 and 2.27 t/m3 and 4% and 5% for mixture A and mixture B, respectively) and under the same compaction method. Table 3 summarises the details of testing program adopted for mixtures A and B. 3.1. Permanent deformation behaviour analysis In order to characterise the long-term deformation behaviour of the investigated materials, the results from RLT tests were analysed according to a new constitutive model proposed by the authors [12]. The analytical model showed to be reliable to describe the gradual accumulation of permanent strain with the number of load applications taking into account the applied stress effects. Fig. 3 shows the typical permanent deformation results through a repeated triaxial test in terms of the relationship between permanent strain and number of load cycles at a prefixed stress level (r1, r3) for all three materials.
ð1Þ
where A, B, C and D are regression parameters depending on the characteristics of the material and applied load level. The equation consists of two terms that well describe the two main phases of permanent deformation accumulation. The initial phase of the trend, characterised by a rapid increase in strains and related mainly to post-compaction phase, is well depicted by the exponential term of Eq. (1). Afterwards, the increase in the level of permanent strain rate tends to stabilize with the number of load cycles up to a constant value which can be described by the linear term of Eq. (1). The analysis of the long-term strain behaviour allows the exponential term of Eq. (1) related to the transitory post-compaction period to be neglected and thus Eq. (1) can be simplified as follows:
ep ðNÞ ¼ A þ B N
ð2Þ
where parameter A is a constant expressing the variability of permanent strain accumulated during the first load cycles, while coefficient B represents the strain rate per load cycle which can be considered as a parameter characterising the long-term permanent deformation behaviour of the material. Therefore, this simplification will allow the strain behaviour of the material to be exclusively investigated by analysing the percentage of the strain accumulated every N load cycles, denoted with e_ p . In particular, the following relationship gives the percentage of the strain accumulated every 1000 load cycles:
e_ p ð%Þ ¼
B ð%=103 cyclesÞ 10
20 70 150
ð3Þ
In order to evaluate the stress level influence on permanent deformation accumulation, the strain rate e_ p is depicted as function of axial stress r1 and confining pressure r3 (Fig. 4). Fig. 4 shows the results for mixtures A, B and C&D, respectively, which were all tested in the two moisture conditions.
Table 2 Stress level program.
r3 (kPa)
575
Permanent deformation stress levels Optimum moisture condition r1 (kPa)
r1 (kPa)
30, 40, 60, 120, 160, 200, 240, 280, 380 105, 140, 210, 320, 350, 370, 400, 420, 470, 500 225, 300, 375, 450, 525, 600, 700, 710, 720
30, 40, 80, 90, 120, 160, 200, 240 105, 140, 210, 260, 320, 330, 350, 400 225, 300, 375, 450, 570, 600, 630
Fig. 2. Equipment for soaking the specimen.
Saturated condition
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Table 3 Testing program for mixtures A and B.
r3 (kPa)
Mixture A
Mixture B
Optimum moisture condition
Saturated condition
Optimum moisture condition
r1 (kPa) 20 70 150
Saturated condition
r1 (kPa)
30, 40, 60, 120(2), 160, 200, 240(2) 105(2), 140, 210(2), 280, 350 225, 300, 375, 450(2), 600
30, 40, 60, 80, 90, 120, 160 105, 140, 175, 210, 220, 280, 290, 350 225, 280, 300, 375, 400, 450
30, 40(2), 60, 160, 240 95, 105, 140, 210, 420 165, 180, 225, 300,450
30, 40, 60, 90, 120 85, 95, 105, 130, 140, 175 180, 225, 250, 280, 300
‘‘2’’, Two replicates.
Mixture A (70_210)
Mixture B (70_210)
Mixture C&D (70_210)
Permanent strain (με)
7000 6000 5000 4000 3000 2000 1000 0 0
2000
4000
6000
8000
10000
Number of load cycles N Fig. 3. Permanent deformation results in optimum condition (r1 = 210 kPa and r3 = 70 kPa).
levels. On the other hand, the recycled mixture shows a similar trend also in optimum moisture condition. This is probably a direct consequence of the stronger tendency of recycled aggregate to break under loads because of its high brittle nature compared with a virgin aggregate [1,2]. The greater grain breakages result in large plastic strain rate, hence causing a loss in the stiffness response of the recycled material. This result confirms the importance of the particle breakage aspect in the deformation behaviour of granular systems [18]. As far as the permanent deformation modelling is concerned, only the linear trend describing a stable response of the materials was considered. In particular, the analysis of Fig. 4 shows that confining pressure also affects the strain rate trend. Thus the dependency of strain rate e_ p on the principal stresses (r1, r3) can be described by the following general law:
e_ p ¼ f ðr3 Þr f1ðr3 Þ As shown in the figure, the permanent strain rate is widely influenced by the stress levels; in particular it increases with the increase in axial stress and decreases with the increase in confining pressure for all testing conditions. It is important to highlight that two different data trend are evident in the figure. The first trend, clear in all graphs, is related to all experimental data that in a logarithmic scale can be well fitted by a linear trend with the increase in axial stress r1 for each confining pressure. The second trend (clearly marked by rectangles), obtained only for mixtures A and B in saturated conditions and for mixture C&D in both moisture conditions and apparently outtrend, shows a sudden change in the slope of the before-mentioned linear trend, denoting a rapid deformation accumulation of the materials. This trend reveals at high stress level and thus supposes the existence of a load threshold that separates a stable from an unstable permanent strain accumulation condition. Thus, it can be assumed that these stress states, which cause the rapid increase in the strain rate, are close or exceed the limiting value at which the material starts showing an incremental collapse response according to the shakedown theory [9–11]. Moreover, it is right to note that the high strain rate values come from the analysis of those curves showing an anomalous shape or curves that are somehow different from the typical trend of the permanent deformation rise (Fig. 3) and for which the proposed model did not turn out to be optimal to fit the experimental data. This different deformation behaviour can be explained considering that at low stress levels the initial plastic strains are mainly due to grain re-orientation and limited interparticle slips that result in a hardening response of the material under loading. On the other hand, at higher stress levels, significant and unrecoverable intergranular slip occurs together with particle crushing that produces an unstable aggregate skeleton characterised by low stiffness so that large plastic strains become possible. From Fig. 4, it is possible to note that for each investigated material the rapid increase in strain rate always occurs in saturated condition. This result clearly shows the lubricating action of excess water that promotes the interparticle slips under lower stress
ð4Þ
This equation describes a linear trend with the increase in axial stress on the bi-logarithmic plane, where the intercept and the slope of the line depend on the confining pressure. The specialisation of Eq. (4) for mixtures A and B was performed in a previous research [12] based on the following functional relationships:
e_ p ¼ arb3 rc1
ð4:1Þ
e_ p ¼ aebr3 rc1þdr3
ð4:2Þ
in which a, b, c, d are regression parameters depending on the material. Table 4 lists all regression parameters of the above illustrated equations and the corresponding R2 values for each material tested in both moisture conditions. Focusing on the results, the high values of R2 yield in all cases denote the reliability of the adopted models to describe the permanent strain behaviour of the mixtures. In particular, as regards the virgin materials Eq. (4.1), representing a group of parallel lines on the ðe_ p ; r1 Þ plane, was employed to predict the permanent strain behaviour of mixture A, seeing that in both moisture conditions, R2 reached the highest values. Whereas Eq. (4.2), a model of four parameters that generates a group of lines whose slopes depend on the confining pressure value, better describes the permanent strain behaviour of mixture B. The reliability of these analytical laws is also based on their capability to well interpret the physical behaviour of the virgin mixtures. In fact, Eq. (4.1) shows that e_ p increases fast (b < 0) as r3 decreases and tends up towards infinity as r3 approaches zero. In practice, this means that a granular material always needs suitable confinement in order to avoid excessive plastic strains confirming the typical frictional behaviour of mixture A, which is lacking in cohesive properties. On the contrary, by considering Eq. (4.2), it can be noted that even though e_ p increases (b < 0) as r3 decreases, it always assumes a finite value also as r3 approaches zero. Thus, a granular mixture does not reach the collapse even in
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1.E+00 20kPa 70kPa 150kPa
1.E-01
(%/103 load cycles)
(%/103 load cycles)
1.E+00
1.E-02
1.E-03
20kPa 70kPa 150kPa
1.E-01
1.E-02
1.E-03
Mix A - Wopt
Mix A - Wsat 1.E-04
1.E-04 10
100 1
10
1000
1
1.E+00
1000
(kPa)
1.E+00 20kPa 70kPa 150kPa
1.E-01
(%/103 load cycles)
(%/103 load cycles)
100
(kPa)
1.E-02
1.E-03
20kPa 70kPa 150kPa
1.E-01
1.E-02
1.E-03
Mix B - Wsat
Mix B - Wopt 1.E-04 10
100 1
1.E-04 10
1000
1
1.E+00
1000
(kPa)
1.E+00 20kPa 70kPa 150kPa
1.E-01
(%/103 load cycles)
(%/103 load cycles)
100
(kPa)
1.E-02
1.E-03
20kPa 70kPa 150kPa
1.E-01
1.E-02
1.E-03
Mix C&D - Wsat
Mix C&D - Wopt 1.E-04 10
100 1
1000
1.E-04 10
100
(kPa)
1
1000
(kPa)
Fig. 4. Permanent strain rate versus axial stress at any confining pressure.
Table 4 Regression parameters of models. Parameter
Mixture A
Mixture B
Mixture C&D
Optimum moisture condition
Saturated condition
Optimum moisture condition
Saturated condition
Optimum moisture condition
Saturated condition
1.734E05 1.857 2.565 – 0.941
1.143E04 1.435 1.882 – 0.858
3.371E04 0.504 0.890 – 0.899
2.407E04 1.148 1.788 – 0.718
1.259E04 0.610 1.104 – 0.939
1.332E03 0.666 0.810 – 0.896
2.112E06 0.097 1.945 1.210E02 0.934
5.635E05 0.060 1.077 8.293E03 0.656
1.564E04 0.022 0.748 2.767E03 0.924
2.077E04 0.101 1.170 1.564E02 0.836
9.904E05 0.032 0.794 4.194E03 0.957
5.495E04 0.027 0.600 3.137E03 0.891
Eq. (4.1)
a b
c d R2 Eq. (4.2)
a b
c d R2
the absence of confinement as it can rely on the contribution of the cohesive shear strength component. From the analysis of Table 4, it is possible to note that for the recycled mixture in both moisture conditions the best data fitting
is obtained by Eq. (4.2) similar to mixture B. Therefore, on the basis of foregoing considerations it can be stated that the recycled material shows a typical permanent deformation behaviour of a cohesive granular mixtures. This apparent cohesive component can be
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directly attributed to the light effects of the self-cementing phenomenon that occurs due to the unhydrated cementitious materials present in the recycled mixture [19]. Moreover, the high amount of recycled fine fraction produced during the compaction phase, because of a high susceptibility to fragmentation (high Los Angeles coefficient), results in a decrease in the volume of macropores (higher densification grade), that can lead to benefits in terms of mechanical response including intergranular tensile and bond strength. Fig. 5 shows the results of the proposed models in terms of strain rate as function of applied stress state (r1, r3) for the investigated materials in both moisture conditions. In optimum moisture condition (Fig. 5a), it is seen that for the cohesive materials, mixtures B and C&D, the model gives lines closer, meaning that such as mixtures show a permanent strain behaviour depending less on confining pressure due to the contribution of the cohesive properties of the fine fraction. Otherwise, the same decrease in lateral confinement causes a higher loss in permanent deformation resistance of the frictional behaviour material (mixture A), as shown by the corresponding lines that shifted towards a higher strain rate. In addition, mixtures B and C&D show a similar and lower slope of lines than that of mixture A, which highlights their lower sensitivity to the axial stresses. In particular, Fig. 5 shows that at the same stress state (r1, r3), except for the low axial stresses, the recycled mixture experiences a lower strain rate denoting its capability to accumulate lower permanent deformation under loading. As far as saturated condition is concerned (Fig. 5b), first of all it can be noted that all mixtures show higher strain rate at the same stress level compared with those at optimum moisture condition, and so highlighting the negative effects of water. In particular, the curves characterising the strain behaviour of mixture B are more spaced and inclined than before, whereas mixtures A and C&D show a similar trend with a slight change in their response. This result denotes a higher loss in the permanent strain resistance
(%/103 load cycles)
(a) 1.E-01
Mix A Mix B
1.E-02
Mix C&D
1.E-03 Optimum Moisture Condition
1.E-04 10
100
1000
1 (kPa)
(%/103 load cycles)
(b) 1.E-01
Mix A Mix B
1.E-02
Mix C&D
1.E-03 Saturated Condition
1.E-04 10
100 1
σ3:
20kPa
1000
of the cohesive mixture B under loading due to the strong water susceptibility of the fine fraction. Contrarily, the recycled mixture experiences the water action less, and so it is able to assure an adequate permanent deformation resistance also in presence of water. Moreover, from Fig. 5 it is interesting to see that the recycled mixture compared with the mixture A is able to suffer a higher stress level before reaching the incremental collapse state, although at low stress levels it shows a lower permanent strain resistance (higher strain rate values). Therefore, on the one hand the cohesive component of the recycled mixture immediately leads to the development of plastic strain, but on the other it assures enhanced performance as loads increase. Previous finding confirms the importance of cohesive component on the strain behaviour of granular mixture and at the same time highlights good moisture resistance guaranteed by the recycled mixtures contrary to the mixture containing plastic fine fraction that shows an anomalous mechanical behaviour in presence of water. Results from Eqs. (4.1) and (4.2) can be depicted on a Mohr– Coulomb diagram. In particular, Fig. 6 shows an example of the possible Mohr circles obtained for an assigned strain rate as the axial stress and the confining pressure vary. In the graph, the envelope curve of the Mohr circles is also represented. This curve, defining all the possible stress conditions which cause a specific strain rate condition (e_ p ), can be considered to express a permanent deformation resistance of the material. Fig. 7 shows the envelope curves of the investigated mixtures in both moisture conditions for the strain rates of 0.003%, 0.004%, 0.005% and 0.006%, respectively. As can be seen, such a representation of results allows the physical–mechanical behaviour of the different materials to be distinctly distinguished. In fact, the envelope curves that intercept the s axis reveal the existence of a cohesive contribution on the permanent deformation resistance of mixtures B and C&D, coherently with the previous results. While, the envelope curves of mixture A (blue lines1) converge at s = 0 as the confining pressure is zero, thereby showing the typical frictional behaviour of the mixture. Similar results can be observed in both moisture conditions. From Fig. 7a, it can be seen that the recycled mixture is able to undergo a higher stress state before reaching the same strain level compared with the other mixtures. Thus, this allows the recycled mixture to be characterised by a higher permanent deformation resistance. In particular, the higher slope of the envelope curves, representing the internal friction angle of mixture, at a specific strain rate, would highlight that recycled mixture can rely on a higher frictional contribution due to larger interlocking particles. Moreover, results show that recycled mixture is characterised by a higher internal cohesion compared with the silty clay material (mixture B). The higher cohesion of the recycled mixture can be related to the particular nature and higher amount of fine fraction produced under loading. On the other hand, in a saturated condition (Fig. 7b), the two extreme responses of silty clay (mixture B) and frictional (mixture A) material are evident. In fact, after soaking, mixture A showed an unchanged mechanical behaviour, whereas mixture B showed a drastic decay in strain response due to a significant decrease in internal cohesion (low intercepts) and friction angle (lower slopes), denoting its marked moisture susceptibility. However, it is interesting to note that the recycled mixture also experiences a loss in cohesive properties, but the evident increase in slope of curves at high stress states supposes the existence of significant interlocking grains also in presence of water, that results in an increase in permanent deformation resistance.
(kPa) 70kPa
150kPa
Fig. 5. Strain rate prediction according to the models.
1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
G. Cerni et al. / Construction and Building Materials 37 (2012) 573–580
Fig. 6. Mohr–Coulomb representation of strain rate envelope.
Fig. 7. Permanent strain rate envelopes for investigated mixtures.
These results highlight the potential of this approach to well describe also the mechanical behaviour of recycled granular material on the basis of their permanent deformation response. Therefore, the use of this approach as a performance comparison tool with the traditional virgin materials allows to evaluate realistically whether C&D materials are adequate for unbound granular course of road pavement, overcoming the limits of the traditional design procedure, still based on static and simple index parameters (Table 1). In particular, this method clearly highlights the stress conditions over which the mixture shows an undesirable strain response. Hence, this is very significant information mainly for the design of low-traffic pavements that suffer a rutting phenomenon due to the excessive stresses in unbound granular layers. 4. Conclusion This research presents the results of a laboratory investigation aimed to analyze the permanent deformation behaviour of a recycled C&D mixture under repeated triaxial loading for its use in
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unbound layers of road pavements. The selected mixture was tested in optimum moisture and saturated conditions in order to evaluate the influence of moisture on their mechanical response too. Two further natural granular mixtures, were selected as reference materials for a performance comparison. A preliminary physical characterisation showed the investigated C&D material as an unbound mixture of satisfying quality according to a classical analysis based on the acceptance prescriptions by Italian technical specifications for road construction, except for sand equivalent and Los Angeles coefficient. Really, although the high Los Angeles abrasion can be considered as a limiting property for recycled aggregate mixture as its high value highlights the marked breakage potential of C&D aggregate, it is important to state that this apparent restrictive characteristic can contribute to a better densification of the recycled mixtures with benefits for mechanical properties as also confirmed by the found high CBR value. The long-term deformation behaviour of the recycled mixture was analysed according to the constitutive model proposed by the authors and based on the functional dependency of strain accumulation rate on the number of load applications and applied stress state. Firstly, test results showed that the marked tendency of the recycled aggregates to break under loads, due to its prevailing brittle nature, has a significant influence on the change from a stable to an unstable permanent strain accumulation condition. The model analysis showed that in an optimum moisture condition the C&D mixture was less sensitive to confining pressure and axial stress, denoting a typical permanent deformation behaviour of a cohesive granular mixture. This apparent cohesive component can be directly attributed to the intrinsic properties of fine recycled fraction combined to the high amount of fine fraction produced during the compaction phase. Moreover, the recycled mixture showed a higher capability to accumulate lower permanent deformation under specific stress levels as compared to the reference natural granular mixtures. As far as the saturated condition is concerned, the recycled mixture experienced the moisture effect less contrary to the cohesive natural mixture that suffered the high water susceptibility of the fine fraction. However, compared to the frictional natural mixture, it was able to undergo a higher stress level before showing any failure. In addition, a Mohr–Coulomb diagram representation of the results allowed permanent deformation resistance curves of the recycled material to be identified and compared with those of the traditional granular mixtures. Such an approach clearly highlights the stress and moisture conditions over which the recycled mixture seems to guarantee no critical strain response, hence providing useful information for pavement design. In conclusion, the results obtained in this study on the one hand provide a practical and innovative method for ranking granular material for pavement design on the basis of a performance-related approach such as permanent deformation analysis, on the other they support the use of C&D materials as a sustainable and cost-effective alternative to traditional aggregates. References [1] Leite FdC, Motta RS, Vasconcelos KL, Bernucci L. Laboratory evaluation of recycled construction and demolition waste for pavements. Constr Build Mater 2011;25:2972–9. [2] Santagata FA, Cardone F, Pannunzio V, Graziani A. An experimental investigation on unbound mixture containing recycled materials. In: Proceedings of 6th international conference on maintenance and rehabilitation of pavements and technological control (MAIREPAV6), vol. 2; 2009. p. 904–13. [3] Poon CS, Chan D. Feasible use of recycled concrete aggregates and crushed clay brick as unbound road sub-base. Constr Build Mater 2006;20:578–85. [4] Bennert T, Papp Jr WJ, Maher A, Gucunski N. Utilization of construction and demolition debris under traffic-type loading in base and subbase applications. Trans Res Rec 2000;1714:33–9.
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[5] Park T. Application of construction and building debris as base and subbase materials in rigid pavement. J Transp Eng ASCE 2003;129(5):558–63. [6] Molenaar AAA, van Niekerk AA. Effects of gradation, composition, and degree of compaction on the mechanical characteristics of recycled unbound materials. Trans Res Rec 2002;1787:73–82. ~ ez JT, San Jose´ JT, Urzelai A. Construction demolition wastes, [7] Vegas I, Iban Waelz slag and MSWI bottom ash: a comparative technical analysis as material for road construction. Waste Manage 2008;28:565–74. [8] National Cooperative Highway Research Program. Performance-related tests of aggregates for use in unbound pavement layers. NCHRP report 453, Washington (DC, USA); 2001. [9] Lekarp F, Dawson A. Modelling permanent deformation behaviour of unbound granular materials. Constr Build Mater 1998;12:9–18. [10] Werkmeister S, Dawson A, Wellner F. Permanent deformation behaviour of granular materials. Road Mater Pave 2005;6:31–51. [11] Boulbibane M, Collins IF, Ponter ARS, Weichert D. Shakedown of unbound pavements. Road Mater Pave 2005;6(1):81–96. [12] Cerni G, Cardone F, Virgili A, Camilli S. Characterisation of permanent deformation behaviour of unbound granular materials under repeated triaxial loading. Constr Build Mater 2012;28:79–87.
[13] Perez I, Gallego J. Rutting prediction of a granular material for base layers of low-traffic roads. Constr Build Mater 2010;24:340–5. [14] Hornych P, Chazallon C, Allou F, El Abd A. Prediction of permanent deformations of unbound granular materials in low traffic pavements. Road Mater Pave 2007;8(4):643–66. [15] EN 13285. Unbound mixtures – specifications. European Committee for Standardization; 2010. [16] Ministero delle Infrastrutture e dei Trasporti. Norme tecniche di tipo prestazionale per Capitolati Speciali d’Appalto – Formazione di strati in misto granulare. Centro Interuniversitario di Ricerca Stradale e Aeroportuale (CIRS); 2001. [17] EN 13286-7. Unbound and hydraulically bound mixtures – part 7: cyclic load triaxial test for unbound mixtures. European Committee for Standardization; 2006. [18] McDowell GR, Bolton MD. On the micro mechanics of crushable aggregates. Geotechnique 1998;48(5):667–79. [19] Poon CS, Qiao XC, Chan D. The cause and influence of self-cementing properties of fine recycled concrete aggregates on the properties of unbound subbase. Waste Manage 2006;26:1166–72.
Contents lists available at SciVerse ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Permanent deformation behaviour of unbound recycled mixtures G. Cerni a, F. Cardone b,⇑, M. Bocci b a b
Dipartimento di Ingegneria Civile ed Ambientale, Università di Perugia, via G. Duranti 93, 06125 Perugia, Italy Dipartimento di Ingegneria Civile Edile e Architettura, Università Politecnica delle Marche, via Brecce Bianche, 60131 Ancona, Italy
h i g h l i g h t s " Intrinsic properties of recycled fine fraction assure a good permanent deformation resistance for the mixture. " Innovative approach for ranking the unbound recycled mixture as regards rutting potential. " C&D wastes as sustainable and cost-effective alternative to natural aggregate.
a r t i c l e
i n f o
Article history: Received 5 March 2012 Received in revised form 14 June 2012 Accepted 22 July 2012 Available online 7 September 2012 Keywords: Recycling C&D aggregate Repeated triaxial loading Permanent deformation
a b s t r a c t Nowadays, the use of recycled waste materials in road construction and rehabilitation processes aimed to preserve non-renewable resources and to solve management problems related to the expansion of landfills become a promising engineering solution. The current paper deals with the feasibility of using material from construction and demolition wastes (C&D) as aggregate for unbound layers (base and/or subbase) of road pavements. In particular, the permanent deformation behaviour of a recycled C&D mixture under repeated triaxial loading was investigated and compared with that of two natural granular mixtures selected as reference materials. An analytical model, proposed by the authors, was used to describe the long-term permanent strain accumulation of materials. The results, in addition to showing the ability of the model to predict change in mechanical behaviour depending on the different nature of materials, highlight how the C&D mixture performs better under specific stress and moisture conditions compared with traditional materials. Finally, the results obtained in this study not only provide a practical approach for ranking granular materials for pavement with regard to their rutting potential but also promote the use of C&D materials as a sustainable and effective alternative to traditional aggregates. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, all developed countries have been dealing with social and economic problems related to the environment. First issue according to sustainable development is the protection of the limited natural resources due to the ever-increasing demand for high quantities of adequate materials for the construction and maintenance of civil infrastructures with a consequent increase in construction costs. The second aspect is connected with the management of the waste materials produced by means of industrial processes or by the construction and demolition of civil infrastructures (C&D waste) because of the dwindling number of landfills and continuous increasing costs of disposal. These problems have led to greater social and political sensitivity to the environment with significant consequences on road engineering. The need to reduce construction costs and time have led to consider recycling of waste materials as a suitable alternative to virgin ⇑ Corresponding author. Tel.: +39 071 220 4507; fax: +39 071 220 4510. E-mail address: [email protected] (F. Cardone). 0950-0618/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2012.07.062
materials in road construction and rehabilitation works. As such, the use of C&D materials as a source of aggregate in the construction of unbound base and/or subbase layers seems to be a valid possibility to solve the aforementioned problems as also confirmed by several researches [1–7]. As well known, unbound granular material layers provide the most important structural contribution in road pavements because in most cases, as the soil subgrade assures a sufficient bearing capacity, the rutting phenomenon takes place mainly in the granular base and subbase layers causing pavement depression and progressive fatigue cracking of bituminous layers [8–10]. Basically, granular materials show resilient and permanent deformations when subjected to repeated loading. Resilient deformations are related to the stiffness properties of the material that affect the fatigue cracking of overlying asphalt layers, whereas the gradual accumulation of permanent deformations, although very small during each loading cycle, could lead to the collapse of the structure due to excessive rutting. Therefore, the conventional road pavement design approach is based on providing adequate thickness of layers in such a way that the pavement structure does
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not experience shear failure and that unacceptable permanent deformations occur in each layer. On the basis of this evidence, an appropriate understanding and characterisation of permanent deformation behaviour of unbound granular materials (UGMs) is needed in order to perform a successful pavement design [8–14]. The main objective of the present research was to better understand the mechanical behaviour of recycled mixtures containing C&D materials in order to evaluate whether they are feasibly useful as unbound material in the base or subbase layer of road pavement. In particular, the permanent deformation behaviour of C&D mixture through repeated load triaxial tests was investigated. An analytical model, developed by the authors [12], was used to predict the permanent strain accumulation phenomenon of the C&D materials. Moreover, the comparison between the selected recycled material and two virgin granular blends allowed the reliability of the model to characterise the mechanical response under cyclic loading of alternative materials, as the C&D wastes, to be evaluated. 2. Experimental program This study presents a laboratory investigation aimed to characterise the permanent deformation behaviour of unbound recycled granular mixtures. The first part of research was dedicated to a physical characterisation of C&D materials by traditional tests. The second part was entirely dedicated to the study of the permanent deformation behaviour of recycled mixture by means of repeated load triaxial tests. In this phase, two natural granular blends were also considered as reference materials for performance comparison. 2.1. Material The recycled material, referred to as C&D mixture, consisted mainly of construction and demolition waste coming from a recycling plant located in the Umbria region (Central Italy) near the town of Foligno. The planned production procedures of the plant, based on preliminary sorting and separation processes, create a homogenous material without undesirable component as wood, plastic, metal, paper, glass and asbestos. Table 1 shows the results of the physical characterisation performed on C&D material according to European Specifications [15]. The recycled material had a maximum diameter of 16 mm and its composition consisted mainly of lithic material as cementitious elements, bricks, tiles and crushed rocks. The undesiderable materials were in negligible amounts. Moreover, the determination of consistency limits showed that the investigated recycled material under study was non-plastic. The Proctor compaction analysis revealed that the recycled mixture needed a high optimum moisture content to guarantee maximum dry density. This is mainly attributed to the high water absorption of the C&D particles due to the presence of highly porous ceramic elements. As absorption can change because of heterogeneity of these materials, the moisture content had to be rigorously controlled during the in situ compaction process in order to evaluate the need to be changed to improve the compaction of unbound layer. The values of shape and flakiness index mean that the recycled blend was characterised by a predominance of cubic particles. The high Los Angeles coefficient denotes the marked tendency to breakage of particles, that results in a high production of fine fraction during the compaction process. Both above mentioned aspects can contribute to a better densification of the unbound recycled mixture. As far as the mechanical properties are concerned, the CBR index, approximately 90%, showed a good bearing capacity of recycled mixture undoubtedly comparable to that of a natural well-graded crushed aggregate mixture and widely adequate to meet the requirements of technical specifications.
2.2. Blend and specimen preparation The gradation of the C&D granular mixture was designed according to Italian Specification for the construction of unbound base or subbase pavement layers [16]. In particular, the mean curve of the reference band was used as design grain-size distribution as shown in Fig. 1. After blending, the recycled mixture was compacted at its respective maximum Proctor Dry density and Optimum Moisture content (Table 1). Compaction was carried out by means of a shear gyratory compactor (SGC) in order to produce cylindrical samples of 100 mm diameter and 200 mm height, dimensions suitable for materials with a particle size distribution of 0/16 mm according to European Standards [17]. Moreover, in order to guarantee uniform density conditions, each specimen was compacted in four layers of equal size (h = 50 mm).
2.3. Test procedure The cyclic tests were performed by means of a repeated load triaxial apparatus, consisting essentially of a main pneumatic load device and a removable pressure chamber, which was suitable for applying a constant confining pressure and a cyclic axial load through the test on the specimen [17]. In addition, two LVDTs measured axial deformations at the top of the specimen. Load and deformation data were then stored by a proper data acquisition system. All tests were performed in drained conditions. The details of the loading program, in terms of confining pressure r3 and cyclic axial stress r1, are shown in Table 2. In particular, additional axial stress with a sinusoidal wave frequency of 2 Hz was applied for each test after the set confining pressure was reached. The tests were carried out up to 10,000 load cycles. Such an analysis period allowed on the one hand a testing time compatible with laboratory work constraints and on the other a practical and rapid method to characterise the mechanical behaviour of these materials to be obtained. Triaxial tests were performed on the investigated mixtures in both optimum water content and saturated conditions, in order to evaluate the effect of water on the permanent deformation behaviour of the materials. To this end, a proper laboratory procedure to soak unbound specimens was adopted (Fig. 2), which is described in detail elsewhere [12]. Each specimen was soaked for 96 h and immediately afterwards subjected to repeated load triaxial test according to the schedule listed in Table 2. It is right to note that results shown in the following paragraphs refer to a single replicate carried out in each condition deriving from the combination of the considered test variables (moisture and stress condition). Even though it is widely recognised that a permanent deformation response can lead to a scatter of test results, in this specific phase of investigation the authors wanted primarily to investigate the effects of a wide range of stress levels at the expense of test repeatability.
3. Results and discussion As mentioned above, the permanent deformation behaviour of recycled mixture was analysed and compared with the mechanical response of two unbound natural granular materials. Hence, in the following the results of a previous research performed on these natural aggregates were reported also [12]. The two considered reference blends, referred to as mixtures A and B respectively, were characterised by a different nature of the fine fraction. In particular, mixture A was a crushed limestone aggregate, whereas mixture B was obtained by replacing only the calcareous fine fraction (passing to 0.063 mm sieve) of mixture A with a silty clay soil fraction, characterised by high water susceptibility (plasticity index = 21.7%), without any further change in
Table 1 Physical properties of C&D material.
a
Property
Result
Technical requirement
Property
Result
Technical requirement
Bituminous material (Ra) Bricks, ceramics, tiles (Rb) Concrete, concrete products, mortar (Rc) Unbound aggregates, natural stone (Ru) Glass (Rg) Metals, plastic, rubber, gypsum (X1) Wood, paper, cellulose (X2) Maximum aggregate size Apparent particle density Water absorption
2.0% 29.5% 25.6% 42.0% 0.6% 0.2% 0.1% 16 mm 2.57 t/m3 7.72%
<5%
Sand equivalent Plasticity index Crushed particles Shape index Flakiness index Los angles coefficient Maximum dry density Optimum moisture content California bearing ratio Total sulphur content
27.4% NP 72% 28% 26.5% 37% 1.92 t/m3 12% 90% 0.2%
>40% NP >60% <40% <35% <30% – – >30% –
Requirement on (Rb + Rc + Ru).
>90%a <5% <0.4% <0.1% <63 mm – –
G. Cerni et al. / Construction and Building Materials 37 (2012) 573–580
In the figure it can be seen that the magnitude of accumulated permanent deformations increase rapidly in the first phase of the test, while subsequently the strain increase stabilizes to a constant value. These typical results were described by a linear-exponential law, which was suggested by the authors and based on the following equation:
100 90
Design curve
Passing (%)
80
Reference band
70 60 50
ep ¼ A þ B N C eDN
40 30 20 10 0 0.01
0.1
1
10
100
Sieve Size (mm) Fig. 1. Design particle-size distribution for tested materials.
grading. This mixture was used in order to investigate the influence of a small amount of plastic material as silty clay soil on the strain behaviour of an unbound granular material layer. Before testing, both blends were compacted at their maximum Proctor dry density and optimum moisture content (2.29 t/m3 and 2.27 t/m3 and 4% and 5% for mixture A and mixture B, respectively) and under the same compaction method. Table 3 summarises the details of testing program adopted for mixtures A and B. 3.1. Permanent deformation behaviour analysis In order to characterise the long-term deformation behaviour of the investigated materials, the results from RLT tests were analysed according to a new constitutive model proposed by the authors [12]. The analytical model showed to be reliable to describe the gradual accumulation of permanent strain with the number of load applications taking into account the applied stress effects. Fig. 3 shows the typical permanent deformation results through a repeated triaxial test in terms of the relationship between permanent strain and number of load cycles at a prefixed stress level (r1, r3) for all three materials.
ð1Þ
where A, B, C and D are regression parameters depending on the characteristics of the material and applied load level. The equation consists of two terms that well describe the two main phases of permanent deformation accumulation. The initial phase of the trend, characterised by a rapid increase in strains and related mainly to post-compaction phase, is well depicted by the exponential term of Eq. (1). Afterwards, the increase in the level of permanent strain rate tends to stabilize with the number of load cycles up to a constant value which can be described by the linear term of Eq. (1). The analysis of the long-term strain behaviour allows the exponential term of Eq. (1) related to the transitory post-compaction period to be neglected and thus Eq. (1) can be simplified as follows:
ep ðNÞ ¼ A þ B N
ð2Þ
where parameter A is a constant expressing the variability of permanent strain accumulated during the first load cycles, while coefficient B represents the strain rate per load cycle which can be considered as a parameter characterising the long-term permanent deformation behaviour of the material. Therefore, this simplification will allow the strain behaviour of the material to be exclusively investigated by analysing the percentage of the strain accumulated every N load cycles, denoted with e_ p . In particular, the following relationship gives the percentage of the strain accumulated every 1000 load cycles:
e_ p ð%Þ ¼
B ð%=103 cyclesÞ 10
20 70 150
ð3Þ
In order to evaluate the stress level influence on permanent deformation accumulation, the strain rate e_ p is depicted as function of axial stress r1 and confining pressure r3 (Fig. 4). Fig. 4 shows the results for mixtures A, B and C&D, respectively, which were all tested in the two moisture conditions.
Table 2 Stress level program.
r3 (kPa)
575
Permanent deformation stress levels Optimum moisture condition r1 (kPa)
r1 (kPa)
30, 40, 60, 120, 160, 200, 240, 280, 380 105, 140, 210, 320, 350, 370, 400, 420, 470, 500 225, 300, 375, 450, 525, 600, 700, 710, 720
30, 40, 80, 90, 120, 160, 200, 240 105, 140, 210, 260, 320, 330, 350, 400 225, 300, 375, 450, 570, 600, 630
Fig. 2. Equipment for soaking the specimen.
Saturated condition
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Table 3 Testing program for mixtures A and B.
r3 (kPa)
Mixture A
Mixture B
Optimum moisture condition
Saturated condition
Optimum moisture condition
r1 (kPa) 20 70 150
Saturated condition
r1 (kPa)
30, 40, 60, 120(2), 160, 200, 240(2) 105(2), 140, 210(2), 280, 350 225, 300, 375, 450(2), 600
30, 40, 60, 80, 90, 120, 160 105, 140, 175, 210, 220, 280, 290, 350 225, 280, 300, 375, 400, 450
30, 40(2), 60, 160, 240 95, 105, 140, 210, 420 165, 180, 225, 300,450
30, 40, 60, 90, 120 85, 95, 105, 130, 140, 175 180, 225, 250, 280, 300
‘‘2’’, Two replicates.
Mixture A (70_210)
Mixture B (70_210)
Mixture C&D (70_210)
Permanent strain (με)
7000 6000 5000 4000 3000 2000 1000 0 0
2000
4000
6000
8000
10000
Number of load cycles N Fig. 3. Permanent deformation results in optimum condition (r1 = 210 kPa and r3 = 70 kPa).
levels. On the other hand, the recycled mixture shows a similar trend also in optimum moisture condition. This is probably a direct consequence of the stronger tendency of recycled aggregate to break under loads because of its high brittle nature compared with a virgin aggregate [1,2]. The greater grain breakages result in large plastic strain rate, hence causing a loss in the stiffness response of the recycled material. This result confirms the importance of the particle breakage aspect in the deformation behaviour of granular systems [18]. As far as the permanent deformation modelling is concerned, only the linear trend describing a stable response of the materials was considered. In particular, the analysis of Fig. 4 shows that confining pressure also affects the strain rate trend. Thus the dependency of strain rate e_ p on the principal stresses (r1, r3) can be described by the following general law:
e_ p ¼ f ðr3 Þr f1ðr3 Þ As shown in the figure, the permanent strain rate is widely influenced by the stress levels; in particular it increases with the increase in axial stress and decreases with the increase in confining pressure for all testing conditions. It is important to highlight that two different data trend are evident in the figure. The first trend, clear in all graphs, is related to all experimental data that in a logarithmic scale can be well fitted by a linear trend with the increase in axial stress r1 for each confining pressure. The second trend (clearly marked by rectangles), obtained only for mixtures A and B in saturated conditions and for mixture C&D in both moisture conditions and apparently outtrend, shows a sudden change in the slope of the before-mentioned linear trend, denoting a rapid deformation accumulation of the materials. This trend reveals at high stress level and thus supposes the existence of a load threshold that separates a stable from an unstable permanent strain accumulation condition. Thus, it can be assumed that these stress states, which cause the rapid increase in the strain rate, are close or exceed the limiting value at which the material starts showing an incremental collapse response according to the shakedown theory [9–11]. Moreover, it is right to note that the high strain rate values come from the analysis of those curves showing an anomalous shape or curves that are somehow different from the typical trend of the permanent deformation rise (Fig. 3) and for which the proposed model did not turn out to be optimal to fit the experimental data. This different deformation behaviour can be explained considering that at low stress levels the initial plastic strains are mainly due to grain re-orientation and limited interparticle slips that result in a hardening response of the material under loading. On the other hand, at higher stress levels, significant and unrecoverable intergranular slip occurs together with particle crushing that produces an unstable aggregate skeleton characterised by low stiffness so that large plastic strains become possible. From Fig. 4, it is possible to note that for each investigated material the rapid increase in strain rate always occurs in saturated condition. This result clearly shows the lubricating action of excess water that promotes the interparticle slips under lower stress
ð4Þ
This equation describes a linear trend with the increase in axial stress on the bi-logarithmic plane, where the intercept and the slope of the line depend on the confining pressure. The specialisation of Eq. (4) for mixtures A and B was performed in a previous research [12] based on the following functional relationships:
e_ p ¼ arb3 rc1
ð4:1Þ
e_ p ¼ aebr3 rc1þdr3
ð4:2Þ
in which a, b, c, d are regression parameters depending on the material. Table 4 lists all regression parameters of the above illustrated equations and the corresponding R2 values for each material tested in both moisture conditions. Focusing on the results, the high values of R2 yield in all cases denote the reliability of the adopted models to describe the permanent strain behaviour of the mixtures. In particular, as regards the virgin materials Eq. (4.1), representing a group of parallel lines on the ðe_ p ; r1 Þ plane, was employed to predict the permanent strain behaviour of mixture A, seeing that in both moisture conditions, R2 reached the highest values. Whereas Eq. (4.2), a model of four parameters that generates a group of lines whose slopes depend on the confining pressure value, better describes the permanent strain behaviour of mixture B. The reliability of these analytical laws is also based on their capability to well interpret the physical behaviour of the virgin mixtures. In fact, Eq. (4.1) shows that e_ p increases fast (b < 0) as r3 decreases and tends up towards infinity as r3 approaches zero. In practice, this means that a granular material always needs suitable confinement in order to avoid excessive plastic strains confirming the typical frictional behaviour of mixture A, which is lacking in cohesive properties. On the contrary, by considering Eq. (4.2), it can be noted that even though e_ p increases (b < 0) as r3 decreases, it always assumes a finite value also as r3 approaches zero. Thus, a granular mixture does not reach the collapse even in
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1.E+00 20kPa 70kPa 150kPa
1.E-01
(%/103 load cycles)
(%/103 load cycles)
1.E+00
1.E-02
1.E-03
20kPa 70kPa 150kPa
1.E-01
1.E-02
1.E-03
Mix A - Wopt
Mix A - Wsat 1.E-04
1.E-04 10
100 1
10
1000
1
1.E+00
1000
(kPa)
1.E+00 20kPa 70kPa 150kPa
1.E-01
(%/103 load cycles)
(%/103 load cycles)
100
(kPa)
1.E-02
1.E-03
20kPa 70kPa 150kPa
1.E-01
1.E-02
1.E-03
Mix B - Wsat
Mix B - Wopt 1.E-04 10
100 1
1.E-04 10
1000
1
1.E+00
1000
(kPa)
1.E+00 20kPa 70kPa 150kPa
1.E-01
(%/103 load cycles)
(%/103 load cycles)
100
(kPa)
1.E-02
1.E-03
20kPa 70kPa 150kPa
1.E-01
1.E-02
1.E-03
Mix C&D - Wsat
Mix C&D - Wopt 1.E-04 10
100 1
1000
1.E-04 10
100
(kPa)
1
1000
(kPa)
Fig. 4. Permanent strain rate versus axial stress at any confining pressure.
Table 4 Regression parameters of models. Parameter
Mixture A
Mixture B
Mixture C&D
Optimum moisture condition
Saturated condition
Optimum moisture condition
Saturated condition
Optimum moisture condition
Saturated condition
1.734E05 1.857 2.565 – 0.941
1.143E04 1.435 1.882 – 0.858
3.371E04 0.504 0.890 – 0.899
2.407E04 1.148 1.788 – 0.718
1.259E04 0.610 1.104 – 0.939
1.332E03 0.666 0.810 – 0.896
2.112E06 0.097 1.945 1.210E02 0.934
5.635E05 0.060 1.077 8.293E03 0.656
1.564E04 0.022 0.748 2.767E03 0.924
2.077E04 0.101 1.170 1.564E02 0.836
9.904E05 0.032 0.794 4.194E03 0.957
5.495E04 0.027 0.600 3.137E03 0.891
Eq. (4.1)
a b
c d R2 Eq. (4.2)
a b
c d R2
the absence of confinement as it can rely on the contribution of the cohesive shear strength component. From the analysis of Table 4, it is possible to note that for the recycled mixture in both moisture conditions the best data fitting
is obtained by Eq. (4.2) similar to mixture B. Therefore, on the basis of foregoing considerations it can be stated that the recycled material shows a typical permanent deformation behaviour of a cohesive granular mixtures. This apparent cohesive component can be
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directly attributed to the light effects of the self-cementing phenomenon that occurs due to the unhydrated cementitious materials present in the recycled mixture [19]. Moreover, the high amount of recycled fine fraction produced during the compaction phase, because of a high susceptibility to fragmentation (high Los Angeles coefficient), results in a decrease in the volume of macropores (higher densification grade), that can lead to benefits in terms of mechanical response including intergranular tensile and bond strength. Fig. 5 shows the results of the proposed models in terms of strain rate as function of applied stress state (r1, r3) for the investigated materials in both moisture conditions. In optimum moisture condition (Fig. 5a), it is seen that for the cohesive materials, mixtures B and C&D, the model gives lines closer, meaning that such as mixtures show a permanent strain behaviour depending less on confining pressure due to the contribution of the cohesive properties of the fine fraction. Otherwise, the same decrease in lateral confinement causes a higher loss in permanent deformation resistance of the frictional behaviour material (mixture A), as shown by the corresponding lines that shifted towards a higher strain rate. In addition, mixtures B and C&D show a similar and lower slope of lines than that of mixture A, which highlights their lower sensitivity to the axial stresses. In particular, Fig. 5 shows that at the same stress state (r1, r3), except for the low axial stresses, the recycled mixture experiences a lower strain rate denoting its capability to accumulate lower permanent deformation under loading. As far as saturated condition is concerned (Fig. 5b), first of all it can be noted that all mixtures show higher strain rate at the same stress level compared with those at optimum moisture condition, and so highlighting the negative effects of water. In particular, the curves characterising the strain behaviour of mixture B are more spaced and inclined than before, whereas mixtures A and C&D show a similar trend with a slight change in their response. This result denotes a higher loss in the permanent strain resistance
(%/103 load cycles)
(a) 1.E-01
Mix A Mix B
1.E-02
Mix C&D
1.E-03 Optimum Moisture Condition
1.E-04 10
100
1000
1 (kPa)
(%/103 load cycles)
(b) 1.E-01
Mix A Mix B
1.E-02
Mix C&D
1.E-03 Saturated Condition
1.E-04 10
100 1
σ3:
20kPa
1000
of the cohesive mixture B under loading due to the strong water susceptibility of the fine fraction. Contrarily, the recycled mixture experiences the water action less, and so it is able to assure an adequate permanent deformation resistance also in presence of water. Moreover, from Fig. 5 it is interesting to see that the recycled mixture compared with the mixture A is able to suffer a higher stress level before reaching the incremental collapse state, although at low stress levels it shows a lower permanent strain resistance (higher strain rate values). Therefore, on the one hand the cohesive component of the recycled mixture immediately leads to the development of plastic strain, but on the other it assures enhanced performance as loads increase. Previous finding confirms the importance of cohesive component on the strain behaviour of granular mixture and at the same time highlights good moisture resistance guaranteed by the recycled mixtures contrary to the mixture containing plastic fine fraction that shows an anomalous mechanical behaviour in presence of water. Results from Eqs. (4.1) and (4.2) can be depicted on a Mohr– Coulomb diagram. In particular, Fig. 6 shows an example of the possible Mohr circles obtained for an assigned strain rate as the axial stress and the confining pressure vary. In the graph, the envelope curve of the Mohr circles is also represented. This curve, defining all the possible stress conditions which cause a specific strain rate condition (e_ p ), can be considered to express a permanent deformation resistance of the material. Fig. 7 shows the envelope curves of the investigated mixtures in both moisture conditions for the strain rates of 0.003%, 0.004%, 0.005% and 0.006%, respectively. As can be seen, such a representation of results allows the physical–mechanical behaviour of the different materials to be distinctly distinguished. In fact, the envelope curves that intercept the s axis reveal the existence of a cohesive contribution on the permanent deformation resistance of mixtures B and C&D, coherently with the previous results. While, the envelope curves of mixture A (blue lines1) converge at s = 0 as the confining pressure is zero, thereby showing the typical frictional behaviour of the mixture. Similar results can be observed in both moisture conditions. From Fig. 7a, it can be seen that the recycled mixture is able to undergo a higher stress state before reaching the same strain level compared with the other mixtures. Thus, this allows the recycled mixture to be characterised by a higher permanent deformation resistance. In particular, the higher slope of the envelope curves, representing the internal friction angle of mixture, at a specific strain rate, would highlight that recycled mixture can rely on a higher frictional contribution due to larger interlocking particles. Moreover, results show that recycled mixture is characterised by a higher internal cohesion compared with the silty clay material (mixture B). The higher cohesion of the recycled mixture can be related to the particular nature and higher amount of fine fraction produced under loading. On the other hand, in a saturated condition (Fig. 7b), the two extreme responses of silty clay (mixture B) and frictional (mixture A) material are evident. In fact, after soaking, mixture A showed an unchanged mechanical behaviour, whereas mixture B showed a drastic decay in strain response due to a significant decrease in internal cohesion (low intercepts) and friction angle (lower slopes), denoting its marked moisture susceptibility. However, it is interesting to note that the recycled mixture also experiences a loss in cohesive properties, but the evident increase in slope of curves at high stress states supposes the existence of significant interlocking grains also in presence of water, that results in an increase in permanent deformation resistance.
(kPa) 70kPa
150kPa
Fig. 5. Strain rate prediction according to the models.
1 For interpretation of color in Fig. 7, the reader is referred to the web version of this article.
G. Cerni et al. / Construction and Building Materials 37 (2012) 573–580
Fig. 6. Mohr–Coulomb representation of strain rate envelope.
Fig. 7. Permanent strain rate envelopes for investigated mixtures.
These results highlight the potential of this approach to well describe also the mechanical behaviour of recycled granular material on the basis of their permanent deformation response. Therefore, the use of this approach as a performance comparison tool with the traditional virgin materials allows to evaluate realistically whether C&D materials are adequate for unbound granular course of road pavement, overcoming the limits of the traditional design procedure, still based on static and simple index parameters (Table 1). In particular, this method clearly highlights the stress conditions over which the mixture shows an undesirable strain response. Hence, this is very significant information mainly for the design of low-traffic pavements that suffer a rutting phenomenon due to the excessive stresses in unbound granular layers. 4. Conclusion This research presents the results of a laboratory investigation aimed to analyze the permanent deformation behaviour of a recycled C&D mixture under repeated triaxial loading for its use in
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unbound layers of road pavements. The selected mixture was tested in optimum moisture and saturated conditions in order to evaluate the influence of moisture on their mechanical response too. Two further natural granular mixtures, were selected as reference materials for a performance comparison. A preliminary physical characterisation showed the investigated C&D material as an unbound mixture of satisfying quality according to a classical analysis based on the acceptance prescriptions by Italian technical specifications for road construction, except for sand equivalent and Los Angeles coefficient. Really, although the high Los Angeles abrasion can be considered as a limiting property for recycled aggregate mixture as its high value highlights the marked breakage potential of C&D aggregate, it is important to state that this apparent restrictive characteristic can contribute to a better densification of the recycled mixtures with benefits for mechanical properties as also confirmed by the found high CBR value. The long-term deformation behaviour of the recycled mixture was analysed according to the constitutive model proposed by the authors and based on the functional dependency of strain accumulation rate on the number of load applications and applied stress state. Firstly, test results showed that the marked tendency of the recycled aggregates to break under loads, due to its prevailing brittle nature, has a significant influence on the change from a stable to an unstable permanent strain accumulation condition. The model analysis showed that in an optimum moisture condition the C&D mixture was less sensitive to confining pressure and axial stress, denoting a typical permanent deformation behaviour of a cohesive granular mixture. This apparent cohesive component can be directly attributed to the intrinsic properties of fine recycled fraction combined to the high amount of fine fraction produced during the compaction phase. Moreover, the recycled mixture showed a higher capability to accumulate lower permanent deformation under specific stress levels as compared to the reference natural granular mixtures. As far as the saturated condition is concerned, the recycled mixture experienced the moisture effect less contrary to the cohesive natural mixture that suffered the high water susceptibility of the fine fraction. However, compared to the frictional natural mixture, it was able to undergo a higher stress level before showing any failure. In addition, a Mohr–Coulomb diagram representation of the results allowed permanent deformation resistance curves of the recycled material to be identified and compared with those of the traditional granular mixtures. Such an approach clearly highlights the stress and moisture conditions over which the recycled mixture seems to guarantee no critical strain response, hence providing useful information for pavement design. In conclusion, the results obtained in this study on the one hand provide a practical and innovative method for ranking granular material for pavement design on the basis of a performance-related approach such as permanent deformation analysis, on the other they support the use of C&D materials as a sustainable and cost-effective alternative to traditional aggregates. References [1] Leite FdC, Motta RS, Vasconcelos KL, Bernucci L. Laboratory evaluation of recycled construction and demolition waste for pavements. Constr Build Mater 2011;25:2972–9. [2] Santagata FA, Cardone F, Pannunzio V, Graziani A. An experimental investigation on unbound mixture containing recycled materials. In: Proceedings of 6th international conference on maintenance and rehabilitation of pavements and technological control (MAIREPAV6), vol. 2; 2009. p. 904–13. [3] Poon CS, Chan D. Feasible use of recycled concrete aggregates and crushed clay brick as unbound road sub-base. Constr Build Mater 2006;20:578–85. [4] Bennert T, Papp Jr WJ, Maher A, Gucunski N. Utilization of construction and demolition debris under traffic-type loading in base and subbase applications. Trans Res Rec 2000;1714:33–9.
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