CONSTRUCTION COSTS OF ROAD PAVEMENTS BUILT USING RECYCLED AGGREGATE
A. Marradi, S. Mannucci
University of Pisa – Civil Engineering Department
ABSTRACT
The findings from numerous studies show that appropriately selected and properly utilized recycled construction and demolition waste can be used to constitute bound and unbound road pavement layers, yielding a performance equivalent to layers built of good quality virgin aggregate.
This paper reports on the experiences of the Road and Transport Section of the Civil Engineering Department of the University of Pisa, focusing on construction costs of some types of pavement structures utilizing recycled aggregate. The results showed that such materials allowed an appreciable savings on costs, with equal pavement service life.
1. INTRODUCTION
Numerous tests on experimental rural road sections have shown that the performance and service life of unbound pavement layers constructed with recycled C&D waste is not inferior to that of traditional materials [1]. Experiences by other Authors [2] regarding pavement structures of similar roads have demonstrated good bearing capacity and durability, partly attributable to the self-cementing properties of different types of recycled aggregate [3]. It has also been suggested that the road design phase could take these findings into account by suitably increasing the values of parameters that characterize the bearing capacity of pavement layers constructed with recycled aggregate.
Specific tests conducted to evaluate fatigue resistance and deformability of cement stabilized recycled crushed concrete have shown that these mixtures achieve a performance equal to that of traditional cement stabilized aggregate [4].
2. REFERENCE PAVEMENTS
We therefore aimed to conduct a comparison, under equal performance and environmental conditions, between the costs of traditional pavement structures and pavements constructed with recycled C&D waste. Experimental sections of three different pavement types with a subgrade bearing capacity of E
o= 60 MPa, were set up (Figure 1). The project required the pavements to guarantee, over a twenty-year service life in environmental conditions characterized by absence of frost actions, mean annual air temperature of 14°C and normal rainfall for the Region of Tuscany, the passage of five million commercial vehicles subdivided according to the following traffic spectrum of main and secondary rural road traffic [5].
Axle 1.5t 4t 5t 6t 8t 9t 3tG 8tG 9tG 10tG 11tG 12tG 13tG
% 5.6 19.0 12.5 4.4 10.1 3.3 5.6 21.3 3.2 8.6 5.6 0.2 0.6 Subscript "G" indicates twin wheel axles
Table 1 – Design traffic
Pavement Type 1 is a traditional flexible pavement. Type 2 differs from the latter only in the presence of recycled aggregate constituting the subbase. In Type 3, which is semirigid, the base course is composed of cement stabilized crushed concrete.
Figure 1 – The three types of reference pavements
The main characteristics of the materials constituting the various pavement layers are shown in Table 2 below.
Pavement Type 1-2-3 1-2-3 1-2 3
Mixture type Wearing course Binder Base Cement stabilized aggregate
Void % 4.5 5.0 6.5 6.5
Bitumen % 5.5 5.0 4.5 4.5
Bitumen penetration (dmm) 70-100 50-70 50-70 50-70
Cement % - 3
Table 2 –Physical-qualitative characteristics of bound pavement layers
3. PAVEMENT STRESS-STRAIN PATH
Strain and stress-induced pavement deformation due to traffic loads was determined by modeling the three pavements as elastic, homogeneous and isotropic multilayers, with each layer characterized by dynamic modulus values (E), Poisson’s coefficient (ν) and thickness value. Values of these parameters for pavements Type 1 and 2 are shown in Table 3, while Table 4 shows the corresponding values for pavement Type 3.
It is known that the mechanical resistance of cement stabilized aggregate decays over time, resulting in behavior, after cracking, that becomes similar to the behavior of unbound aggregate [6] [8]. Therefore this study was conducted under the hypothesis that during pavement service life, cement stabilized aggregate behaves according to two distinct models: pre-cracked or subject only to shrinkage cracking (phase 1 - pre-cracked) and cracked (phase 2 - cracked).
To estimate the dynamic modulus of the asphalt concrete layer, the University of Nottingham’s correlation [7] was used, with a cautionary expectation of mean heavy vehicle speed of 20 km/h and asphalt concrete temperature of 20°C. The phase°1 (pre-cracked) cement stabilized aggregate modulus was determined by laboratory tests [3], while the phase°2 (cracked) modulus was derived from the technical literature [8] [9] on cracked cement stabilized aggregate.
Unbound layer mechanical characteristics were determined using the Heukelom
formula adopted by the CNR N. 178/95 Bulletin [5] for the subgrade and the
Edwards and Walkering relation for the subbase. The Edwards and Walkering relation, which expresses the subbase modulus as a function of the subgrade modulus and of layer thickness, was also used here for recycled aggregate, as specific experimental tests [10] have demonstrated its applicability to such material.
Asphalt concrete
(22 cm) Subbase
(35 cm) Subgrade
E [MPa] 3800 170 60
ν 0.35 0.45 0.45
Table 3 – Pavement Types 1 and 2
Asphalt Concrete
(12 cm)
Cement stabilized aggregate - Phase 1 (pre-cracked) - (18 cm)
Cement stabilized aggregate - Phase 2 (cracked) - (18 cm)
Subbase
(35 cm) Subgrade
E [MPa] 3800 3500 700 170 60
ν 0.35 0.20 0.35 0.45 0.45
Table 4 – Pavement Type 3
4. PAVEMENT LIFE ESTIMATION
Pavement life was estimated for each of the three different reference pavements, on the basis of the design traffic loads. Thus for each axle type indicated in the design traffic spectrum, the stress-strain path affecting the base of each layer and the top of the subgrade was determined by means of the CIRCLY program. After obtaining the values of the resulting deformation, fatigue laws (1), (2) and (3) shown below were adopted to determine the damage attributable to each type of axle by the end of 20 years of service life (Tables 5 and 6).
Asphalt concrete
6 5.6180 10 209−
⎟⎠
⎜ ⎞
⎝
⋅⎛
=
ε
N
(Nottingham) (1)
Cem. stab. aggr.
SR= =−0.093⋅Log( )
N +1.032tr t
σ
σ for
SR≤0.5 N=∞[4] (2)
Subgrade
6 4
10 569
−
⎟⎠
⎜ ⎞
⎝
⋅⎛
= v
N
ε
(Shell) (3)
4t 5t 6t 8t 9t 3tG 8tG 9tG 10tG 11tG 12tG 13tG
A.C. 0.019 0.035 0.026 0.148 0.086 0.000 0.093 0.025 0.107 0.109 0.006 0.025 C.S.A. 0.005 0.009 0.006 0.042 0.022 0.000 0.055 0.013 0.054 0.051 0.003 0.011
Table 5 – Damage caused by each axle (Pavement Types 1 and 2)
4t 5t 6t 8t 9t 3tG 8tG 9tG 10tG 11tG 12tG 13tG
A.C. 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 C.S.A. 0.000 0.000 0.000 0.000 0.584 0.000 0.000 0.000 0.000 0.000 0.043 0.374 S. 0.000 0.001 0.000 0.003 0.002 0.000 0.004 0.001 0.004 0.004 0.000 0.001
Damage caused at the end of Phase 1 (pre-cracked) by 36% of design traffic
4t 5t 6t 8t 9t 3tG 8tG 9tG 10tG 11tG 12tG 13tG
A.C. 0.047 0.082 0.048 0.113 0.065 0.001 0.087 0.023 0.095 0.086 0.004 0.015 S. 0.003 0.005 0.004 0.027 0.014 0.000 0.035 0.009 0.035 0.033 0.002 0.007
Damage caused at the end of Phase 2 (cracked) by 64% of design traffic
Table 6 – Damage caused by each axle (Pavement Type 3)
With regard to the subbase layer, we checked that the maximum vertical stresses resulting from the passage of each axle was always lower than 0.2 MPa and that tensile stresses were 40% lower than vertical stresses at the point considered.
Finally, the amount of damage caused by each axle was summed by the law of linear damage accumulation (Miner), checking that the Cumulated Damage Coefficient CDF = Σ(N
i/N
max,i) was lower than 1 and that the pavement would therefore be capable of bearing the design loads. Miner’s Law was likewise applied to sum the damage caused in Phase 1 (pre-cracked) and Phase 2 (cracked) with regard to Type 3 pavement. Thus for Type 3 pavement, the transition between the two phases was determined by investigating what percentage of total traffic load would be necessary in order to cause fatigue rupture of the cement stabilized aggregate layer. It was found that the number of load repetitions required for the cement stabilized aggregate to reach the unit value corresponded to passage of 1.8·10
6vehicles (equivalent to 36% of overall expected volume).
The cumulated damage value of the three pavements is shown in Table 7
Pav. Type 1 Pav. Type 2 Pav. Type 3 (pre-cracked)
Pav. Type 3 (cracked)
Pav. Type 3 (total) A.C. CDF = 0.68 CDF = 0.68 CDF = 0.00 CDF = 0.67 CDF = 0.67
C.S.A. Absent Absent CDF = 1.00 Cracked Cracked
S. CDF = 0.27 CDF = 0.27 CDF = 0.02 CDF = 0.17 CDF = 0.19
Table 7 – Cumulated damage coefficients
Comparison among the cumulated damage coefficients revealed that the three pavements analyzed reached failure after the same number of load repetitions, and presented a 1.5 safety factor with regard to fatigue damage affecting the surface bituminous layer. In addition, Pavement Type 3 presented a higher safety margin concerning the risk of accumulation of permanent subgrade deformation.
5. COST ANALYSIS
Cost analysis was conducted by evaluating costs required for construction of the three pavement types, leaving out additional costs needed to give a functionally complete road and maintenance costs, as these do not influence the comparative analysis carried out. Unit costs, shown in Table 8, represent the minimum and maximum values established by the Region of Tuscany, as specified in the Price List enclosed with N. 11/2006 of the official Engineering Bulletin.
The quantities of material and the road works computed were based on the hypothesis of constructing a Type C1 rural highway (platform with width 10.5 m).
Price (€) Art. Entry U.
Min. Max.
1 Road subbase no less than 30 cm thick; virgin quarry aggregate of 0/50 sieve size, including rolling and compaction to 95% of the Modified AASHTO test.
m3 22,10 29,50 2 Road subbase no less than 30 cm thick; recycled aggregate of 0/50 sieve
size, including rolling and compaction to 95% of the Modified AASHTO test. m3 19,80 24,50 3
Road subbase 10-20 cm thick; recycled aggregate of 0/50 sieve size supplemented with 3% R 32.5 cement per cubic meter of aggregate, spread with grader, including rolling including acid 55% bituminous emulsion layer for complete surface cover and subsequent sand dusting.
m3 37,10 42,40
4
Hot-mix asphalt concrete base layer with aggregate and filler with 0/32 mm continuous granulometric curve and type 50-70 or 70-100 distilled bitumen, with 4% to 7% residual voids on Marshall samples, for a 10 cm compacted thickness laid with paver finisher, including tack coat with 0.80 kg/mq of 55% bituminous emulsion and rolling with vibrating roller.
m2 12,34 15.14
4b Increase/decrease for every additional cm or every cm less at 10 cm of the
asphalt concrete base layer. m2 1,17 1,45
5
Hot-mix asphalt concrete binding layer (Binder) with aggregate and filler with 0/20 mm continuous granulometric curve and type 50-70 or 70-100 distilled bitumen, with 3% to 7% residual voids on Marshall samples, for a 6 cm compacted thickness laid with paver finisher, including tack coat with vibrating roller.
m2 8,16 9,87
5b Increase/decrease for every additional cm or every cm less at 6 cm of the
asphalt concrete binding layer. m2 1,25 1,55
6
Hot-mix asphalt concrete wearing course composed of aggregate and filler with 0/10 mm continuous granulometric curve, and 50-70 or 70-100 distilled bitumen, with 3% to 6% residual voids on Marshall samples, for a 3 cm finished compacted thickness laid with paver finisher, including tack coat with 0.80 kg/mq of 55% bituminous emulsion and rolling.
m2 4,58 5,99
6b Increase/decrease for every additional cm beyond the first 3 cm of the
asphalt concrete wearing course. m2 1,36 1,82
Table 8 – Unit price list
The costs in Euro/km, calculated for each pavement structure and indicated in Table 9, were computed by considering the mean value of the price for each of the entries given above. The results show that under equal service life conditions, Pavement Type 3, constructed with recycled waste material, allows a savings of roughly 19% on construction costs as compared to pavements built with traditional stabilized virgin aggregate (Reference Pavement Type 1). And even if the use of recycled aggregate is limited to the subbase (Pavement Type 2), a savings not lower than 3% can be estimated
Pav. Type 1 (€/km) Pav. Type 2 (€/km) Pav. Type 3 (€/km)
Wear layer 72.240,00 72.240,00 72.240,00
Binder 94.710,00 94.710,00 124.110,00
Base 171.780,00 171.780,00 -
Cement stab. aggreg. - - 75.222,00
Subbase 94.815,00 81.401,25 81.401,25
Total 433.545,00 420.131,25 352.973,25
