The Authors present the results obtained in a experimental investigation carried out to provide technical support to the construction of a cold-recycled bituminous sub-base layer of a major Italian motorway. Compliance of the employed materials and of the resulting recycled mixture to requirements set in Technical Specifications and to the adopted job-mix formula were verified by means of laboratory tests. Field observations focused on the evaluation of the void content of the compacted sub-base layer and on its stiffness evolution in the short term curing phase. Finally, mix design was carried out by following a procedure based on the use of the gyratory shear compactor and on the optimization of the composition of the fluid phase. The procedure was validated by performing volumetric and mechanical tests both on the optimized mixture and on a mixture prepared according to the job-mix formul

Chiappinelli, Giuseppe

Riviera, Pier Paolo

Santagata, Ezio

PORTO Publications Open Repository TOrino

Politecnico di Torino
Porto Institutional Repository
[Proceeding] Experimental Investigation for the Analysis of Cold-Recycled
Bituminous Mixtures
Original Citation:
Santagata E; Chiappinelli G; Riviera P.P. (2007). Experimental Investigation for the Analysis of
Cold-Recycled Bituminous Mixtures. In: MAIREPAV5, Park City, Utah, USA, 8-10 August 2007. pp.
463-468
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EXPERIMENTAL INVESTIGATION FOR THE ANALYSIS OF COLD-RECYCLED 
BITUMINOUS MIXTURES 
 
Ezio Santagata, Giuseppe Chiappinelli, Pier Paolo Riviera  
Department of Hydraulics, Transportation and Civil Infrastructures 
Politecnico di Torino 
Turin, Italy 
ezio.santagata@polito.it 
 
 
Abstract: The Authors present the results obtained in a experimental 
investigation carried out to provide technical support to the construction of a cold-
recycled bituminous sub-base layer of a major Italian motorway. 
Compliance of the employed materials and of the resulting recycled mixture to 
requirements set in Technical Specifications and to the adopted job-mix formula 
were verified by means of laboratory tests. Field observations focused on the 
evaluation of the void content of the compacted sub-base layer and on its stiffness 
evolution in the short term curing phase. Finally, mix design was carried out by 
following a procedure based on the use of the gyratory shear compactor and on 
the optimization of the composition of the fluid phase. The procedure was 
validated by performing volumetric and mechanical tests both on the optimized 
mixture and on a mixture prepared according to the job-mix formula. 
 
INTRODUCTION 
 
The use of cold-recycled bituminous mixtures in flexible road pavements is currently viewed as 
one of the most attractive options available to designers, Administrators and contractors. This is 
certainly due to the environmental benefits which are associated to the adoption of cold-mix 
recycling construction techniques, but also to the satisfactory performance which has been 
observed in service (Asphalt Institute, 1983; AASHTO-AGC-ARTBA Joint Committee, 1998). 
Nevertheless, a number of critical issues still need to be addressed and improvements are 
absolutely necessary in many aspects of material selection, mix design and quality control. 
 
Coherently with this scenario, the Pavement Research Group of the Politecnico di Torino has 
worked in the area of cold-recycling for a number of years (Santagata, 1999), focusing both on 
laboratory characterization and on field evaluation of cold-recycled mixtures (Santagata & 
Chiappinelli, 2002). In this paper a further contribution is given to both topics, by showing the 
results obtained in a experimental investigation which was carried out to provide support to the 
construction of a cold-recycled bituminous sub-base layer of a major Italian motorway.  
 
Mixtures were produced in a plant by employing reclaimed asphalt pavement (RAP) material,  
preliminary processed through a crusher and thereafter sieved on a 25 mm screening deck, and a 
60/40 modified emulsion. No virgin aggregates were added to the mixture. The job-mix formula, 
declared by the contractor and based on the assumption that RAP had approximately 3% water 
content, included 3.0% bitumen emulsion, 1.0% added water and 1.5% Portland cement.  
 
  
EXPERIMENTAL INVESTIGATION 
 
The goals of the project were to evaluate the compliance of the materials to requirements set in 
Technical Specifications (SATAP, 2004) and to provide suggestions to improve field 
performance. Based on the Authors’ previous experience in other research projects (Santagata & 
Chiappinelli, 2003 and 2004), this latter aspect was addressed by performing additional tests 
both in the laboratory and in the field, and by carrying out a full mix design of the recycled 
mixture according to a specifically devised procedure. 
 
Laboratory compliance testing 
 
Requirements included in the Technical Specifications referred both to the plant-produced 
mixture and to its components (RAP, aggregates extracted from the RAP, emulsion, water, 
cement), which were therefore sampled at the production plant and from the paver.  
 
Due to the fact that it was stockpiled with no protection, RAP material had a high water content 
(average value equal to 4.27%). Such a result is considered acceptable by Specifications 
provided that no extra water is added to the recycled mixture. Average binder content was equal 
to 5.75%, lower than the maximum acceptance limit (6.5%).  
 
Average particle size distributions and Specification limits, both referred to RAP and to extracted 
aggregates, are given in Table 1, which also contains the results obtained for the recycled 
mixture. Violations to the upper and lower acceptance limits were observed for both materials.  
 
Table 1. Particle size distribution test results 
 RAP Extracted aggregates Mixture 
D (mm) P (%) Pmin-Pmax (%) P (%) Pmin-Pmax (%) P (%) 
25 100 80-100 100 95-100 100 
15 91.2 60-90 95.7 75-95 94.1 
10 78 40-80 86.6 60-85 85.4 
5 52.3 25-55 65.6 45-60 63.3 
2 32.8 15-40 48.8 30-45 46.0 
0.4 6.0 7-20 19.5 13-22 20.2 
0.18 1.4 4-15 8.6 7-16 10.3 
0.075 0.3 2-8 3.4 4-9 4.6 
 
Control of the emulsion, cement and water dosage was carried out by considering the results 
obtained from the analysis of the recycled mixture (Santagata & Chiappinelli, 2003). The 
average values of both total binder content (7.40%) and total water content (5.80%) were 
acceptable when compared to Specification upper limits, respectively equal to 8% and 7%. 
However, both contents were lower than those calculated from the job-mix formula (respectively 
equal to 7.54 and 6.26%) considering the average water and binder contents of the RAP. Thus, it 
was concluded that the production plant provided an underdosage of emulsion (of the order of 
2.7%), while variations of the water content of the RAP were presumably responsible for the 
deficiency of total water content. Cement dosage on the contrary was found to be very precise 
since calculations lead to identical values of the target and measured values of the percent 
aggregate passing the 0.075 mm sieve. 
 
  
Field testing 
 
Technical Specifications required static plate loading tests to be performed on the sub-base layer 
24 hours after compaction with a minimum acceptable value of the deformation modulus set at 
60 MPa. However, in order to monitor the curing process in the short term, the Authors resorted 
to Light-Weight Drop Tester (LWDT) tests, which yield the so-called dynamic modulus (Ed), 
calculated by means of the following equation: 
 Ed = 22.5 / s (1)  
where the dynamic modulus (Ed) is expressed in MPa and the vertical displacement of the 
loading plate (s) is expressed in mm. 
 
Tests were carried out on a test section which had a length of approximately 400 m and was laid 
in two different days. Dynamic modulus was measured in 68 test points, homogeneously 
distributed on the test section. Pavement and air temperature, respectively equal to 5 and 5.5°C, 
were constant throughout the investigation. The average dynamic modulus measured on the 
underlying unbound foundation was equal to 106 MPa. Experimental results, derived from 
statistical analysis, are shown in Table 2.  
 
Dynamic modulus was reported to increase in the first 24 hours due to the progressive curing of 
the bituminous mastic, with a corresponding reduction of the dispersion of measured values, 
revealed by the decrease of standard deviation and coefficient of variation (COV). The layer was 
not overlaid immediately, and as a consequence of the cold weather and of intense rain, after 1 
week of curing the modulus exhibited a slight decrease and a greater variability. 
 
Table 2. Dynamic modulus results 
 Dynamic modulus Ed 
Curing time Average (MPa) Std. Dev. (MPa) COV (%) 
 20 min 118 17 14.5 
 2 hours 144 15 10.4 
 24 hours 154 15 9.4 
 1 week 123 21 17.0 
 
In order to have information on the level of compaction obtained in the field, cores were taken 
from the test section and thereafter subjected to laboratory tests for the determination of dry 
density at 25°C  and Theoretical Maximum Density of the loose dry mixture. Unfortunately, 
even though coring operations were carried out after 2 months of curing, recovery of undamaged 
cores proved to be quite difficult. The corresponding test results yielded an extremely high value 
of the average void content, equal to 19.6%. 
 
Mix design  
 
The two-stage mix design procedure followed during the investigation is based on the use of the 
gyratory shear compactor (GSC) and of specially designed moulds in which a uni-directional 
flow of the fluid phase (water and emulsion) is induced (Santagata & Chiappinelli, 2004). In the 
first phase of design the optimal fluid content (%FFopt) of the mixture is evaluated by compacting 
mixtures made of RAP and cement, with an added water content variable within a conveniently 
wide range: the resulting cylindrical samples are subjected to density evaluation and the optimal 
  
fluid content is defined as the value at which density reaches its maximum. The conversion of 
such a value in water and emulsion dosage (%W and %E) takes place in the second stage, in 
which the composition of the mixture is defined according to the following relationship: 
 %FFopt = %W + (a +Kb)%E (2)  
where: a and b correspond to the weight ratios of the water and bitumen phase, respectively, to 
the total emulsion; K is a fluidity index which quantifies the type of contribution which the 
bitumen phase gives to compaction when compared to water (variable between 0 and 1).  
By considering a number of combinations of %W and %E which satisfy the above given 
relationship, different mixtures are prepared, compacted in the GSC and thereafter subjected to 
the evaluation of volumetric and (optionally) mechanical properties. Based on the comparative 
analysis of the corresponding results, the design mixture is finally defined. 
 
During the investigation, mix design was carried out by employing materials sampled from the 
production plant. In order to simulate the winter conditions in which construction was being 
carried out (December), before mixing and compaction all materials, tools and moulds were 
conditioned at 5°C. Each GSC sample was prepared by following an internal test protocol which 
requires a total of approximately 3200 g of material, a 150 mm diameter mould and 100 
gyrations. For all mixtures the cement dosage was kept constant, equal to 1.5%, which was also 
the maximum value accepted by Technical Specifications.  
 
The average density values obtained in the first and second stage of design are given in Table 3. 
Tests were carried out after a preliminary conditioning phase during which specimens were dried 
in a forced-draft oven at 40°C for 4 days. Subsequently, dry density (Gdry) at 25°C was measured 
by following the procedure which requires paraffin specimen coating.  
 
Table 3. Results of density tests carried out in mix design 
 First stage Second stage 
%W (%) 4.0 5.0 6.0 2.88 2.45 2.03 1.60 
%E (%) - - - 2.5 3.0 3.5 4.0 
Gdry (g/cm
3) 2.107 2.147 2.107 2.132 2.139 2.154 2.145 
 
It can be observed that the optimal fluid phase content calculated at the end of the first design 
stage, equal to 5%, was converted in the second stage, by adopting a value of K equal to 0.75, to 
an emulsion dosage (%E) of 3.5% and a corresponding water content (%W) of 2.03%. Thus, it is 
shown that according to the results of the mix design procedure, the job-mix formula used at the 
production plant is characterized by a slight emulsion underdosage (equal to 3%) but also by 
excess total water (4.0%, approximately twice the optimal dosage). This latter observation is 
coherent with previous findings of the Authors, who have frequently noted that contractors tend 
to overdose water, either as a result of the need of keeping the plant and dump trucks sufficiently 
clean or as a consequence of unprotected stockpiling of the RAP. 
 
The project included an experimental validation of the results of the mix design procedure. This 
was carried out by comparing the volumetric and mechanical properties of the newly designed 
mixture with those of a reference mixture proportioned according to the job-mix formula. To 
eliminate the effects caused by production variability, the second mixture was replicated in the 
laboratory by following the same mixing and compaction procedures adopted for the optimized 
one. 
  
 
Evaluation of the void content of the mixtures was carried out by considering specimens 
compacted by means of the above described GSC protocol and also by employing a static 
compactor, which applies a load of 5 tons for 5 minutes on a 3000 g sample contained in a 150 
mm diameter mould with a specially designed perforated base plate. The use of the alternative 
compaction method was deemed necessary since the Authors have frequently used it on site for 
quality control purposes (Santagata & Chiappinelli, 2002). Moreover, Technical Specifications 
contained a maximum void content value (%v), equal to 10%, derived from a similar compaction 
procedure (in which, however, no controlled water drainage from the sample is allowed). A 
synthesis of the obtained results is given in Table 4 which also includes Theoretical Maximum 
Density (TMD) values measured on dried mixture samples. 
 
Table 4. Volumetric characteristics of the recycled mixtures 
  GSC Static compactor 
 TMD 
(g/cm3) 
Gdry 
(g/cm3) 
%v 
(%) 
Gdry 
(g/cm3) 
%v 
(%) 
Job-mix formula 2.500 2.137 14.52 2.145 14.20 
Design 2.431 2.169 10.78 2.157 11.27 
 
It should be noted that the mixture prepared according to the mix design results can be 
compacted to a void content which is significantly lower than that of the mixture selected by the 
contractor for production. Moreover, a good agreement between the results obtained by means of 
the two compaction techniques is observed. Finally, by means of the design improvements, the 
obtained void contents is drawn very close to the maximum value given in Specifications (10%). 
 
Mechanical testing was carried out on GSC compacted specimens and included repeated load 
indirect tensile tests (RLIT) (at 20°C after 1, 2 and 45 days of curing), indirect tensile strength 
tests (ITS) (at 25°C after 24 hours of curing) and static uni-axial creep tests (SUC) (at 20°C, after 
24 hours of curing). In all cases curing of the specimens took place in a climatic cell set at 20°C. 
Elastic stiffness values (E) were obtained from RLIT tests carried out with 124 ms rise time and 
7 m target horizontal deformation. As required by Specifications, ITS tests were carried out at 
25°C with a displacement speed of 0.847 mm/s. SUC tests were carried out at a relatively low 
temperature, 20°C, in order to consider worst case conditions in which no curing enhancement 
occurs as a result of temperature increase; 100 kPa compressive stress was applied for 1000 s and 
strain recovery was monitored for 3000 s. Results were expressed by referring to compliance 
calculated after 1 s loading (J1) and to permanent strain recorded after recovery (p).  
 
Results obtained in this phase of the investigation are shown in Table 5, where the percent 
differences between the two mixtures are also given. As expected, the denser mixture derived 
from design exhibited , when compared to the one which replicates plant production, a 
significant reduction in the short term of compressive compliance and of the corresponding 
permanent deformation. The more closely packed aggregate skeleton also caused a slight 
increase of short term indirect tensile strength, which however was in both cases quite low if 
compared to other reference cold-recycled mixtures (Santagata & Chiappinelli, 2003). Elastic 
stiffness data provided information on the progression of curing of the bituminous matrix: while 
in the long term the two mixtures tend to very similar modulus values as a result of complete 
water elimination, significant differences are observed in the short term. In particular, after 2 
  
days of curing the design mixture, as a result of its lower water dosage, proved to be significantly 
stiffer than the reference plant mixture. Data obtained after 1 day of curing only seem to 
contradict this observation, but it should be taken into account that at such low stiffness levels 
RLIT test results tend to be less reliable and are affected by a greater dispersion. 
 
Table 5. Mechanical properties of the recycled mixtures 
 E1 day 
(MPa) 
E2 days 
(MPa) 
E45 days 
(MPa) 
ITS 
(N/mm2) 
J1 
(mm2/N) 
p 
Job-mix formula 594 775 2284 0.059 7.55E-02 1.56E-02 
Design 461 1153 2476 0.065 4.98E-02 1.15E-02 
 (%) -22.4 +48.8 +8.4 +10.2 -33.4 -26.1 
 
CONCLUSIONS 
 
The results collected during the investigation described in this paper allowed the Authors to 
provide the highway Administration with a number of suggestions to improve field performance. 
As a result of the specific particle size distribution of the employed RAP, in order to increase dry 
density values obtained in the field after compaction it was recommended either to allow the use 
of additional virgin aggregates or to perform a preliminary separation of the RAP in different 
size fractions to be separately fed in the mixing plant. RAP protection from environmental 
precipitations was also highlighted as necessary in order to ensure a closer control of the water 
content which can adversely affect compaction. Moreover, it was pointed out that the contractor 
should perform an optimization of the compaction scheme followed by the rollers by means of 
appropriately monitored trial sections.  
Further conclusions were drawn from the results obtained in the mix design phase of the study. 
With the RAP available at the plant a more easily compactable mixture was obtained by 
appropriately adjusting the emulsion and water content. Such a mixture was characterized, when 
compared to the one prepared according to the job-mix formula adopted by the contractor, by a 
greater stiffness and indirect tensile strength. Moreover, its tendency to accumulate permanent 
strains was also significantly reduced. As a result of these observations the Authors strongly 
recommended to included the proposed mix design method in the acceptance procedures 
indicated in Technical Specifications. 
 
REFERENCES  
 
AASHTO-AGC-ARTBA Joint Committee. (1998). Report on Cold Recycling of Asphalt 
Pavements (Task Force 38 Report, March 1998). 
Asphalt Institute. (1983). Asphalt Cold-Mix Recycling, Manual Series n. 21, 1983. 
Santagata, E. (1999). Caratteristiche meccaniche dei riciclati a freddo. Le Strade, 1345(3), 47-50. 
Santagata, E. & Chiappinelli, G. (2002). La ricerca nel settore del riciclaggio a freddo. Le Strade, 
1379(7/8), 54-56. 
Santagata, E. & Chiappinelli, G. (2003). Experimental evaluation of cold-recycled bituminous 
mixtures used for major rehabilitation works. 3
rd
 MAIREPAV Conference, Guimarães, Portugal. 
Santagata, E. & Chiappinelli, G. (2004). Improvements in the mix design of cold-recycled 
bituminous mixtures”, 3rd Eurasphalt&Eurobitume Congress, Vienna, Austria. 
SATAP. (2004). Norme Tecniche d’Appalto, Torino, Italy. 


Experimental Investigation for the Analysis of Cold-Recycled Bituminous Mixtures

The Authors present the results obtained in a experimental investigation carried out to provide technical support to the construction of a cold-recycled bituminous sub-base layer of a major Italian motorway.
Compliance of the employed materials and of the resulting recycled mixture to requirements set in Technical Specifications and to the adopted job-mix formula were verified by means of laboratory tests. Field observations focused on the evaluation of the void content of the compacted sub-base layer and on its stiffness evolution in the short term curing phase. Finally, mix design was carried out by following a procedure based on the use of the gyratory shear compactor and on the optimization of the composition of the fluid phase. The procedure was validated by performing volumetric and mechanical tests both on the optimized mixture and on a mixture prepared according to the job-mix formula

SANTAGATA, Ezio

CHIAPPINELLI, GIUSEPPE

RIVIERA, PIER PAOLO

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

30 November 2022POLITECNICO DI TORINORepository ISTITUZIONALEExperimental Investigation for the Analysis of Cold-Recycled Bituminous Mixtures / Santagata, Ezio; Chiappinelli,Giuseppe; Riviera, PIER PAOLO. - (2007), pp. 463-468. ((Intervento presentato al convegno MAIREPAV5 tenutosi aPark City, Utah, USA nel 8-10 August 2007.OriginalExperimental Investigation for the Analysis of Cold-Recycled Bituminous MixturesPublisher:PublishedDOI:Terms of use:openAccessPublisher copyright(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/1849132 since:  EXPERIMENTAL INVESTIGATION FOR THE ANALYSIS OF COLD-RECYCLED BITUMINOUS MIXTURES  Ezio Santagata, Giuseppe Chiappinelli, Pier Paolo Riviera  Department of Hydraulics, Transportation and Civil Infrastructures Politecnico di Torino Turin, Italy ezio.santagata@polito.it   Abstract: The Authors present the results obtained in a experimental investigation carried out to provide technical support to the construction of a cold-recycled bituminous sub-base layer of a major Italian motorway. Compliance of the employed materials and of the resulting recycled mixture to requirements set in Technical Specifications and to the adopted job-mix formula were verified by means of laboratory tests. Field observations focused on the evaluation of the void content of the compacted sub-base layer and on its stiffness evolution in the short term curing phase. Finally, mix design was carried out by following a procedure based on the use of the gyratory shear compactor and on the optimization of the composition of the fluid phase. The procedure was validated by performing volumetric and mechanical tests both on the optimized mixture and on a mixture prepared according to the job-mix formula.  INTRODUCTION  The use of cold-recycled bituminous mixtures in flexible road pavements is currently viewed as one of the most attractive options available to designers, Administrators and contractors. This is certainly due to the environmental benefits which are associated to the adoption of cold-mix recycling construction techniques, but also to the satisfactory performance which has been observed in service (Asphalt Institute, 1983; AASHTO-AGC-ARTBA Joint Committee, 1998). Nevertheless, a number of critical issues still need to be addressed and improvements are absolutely necessary in many aspects of material selection, mix design and quality control.  Coherently with this scenario, the Pavement Research Group of the Politecnico di Torino has worked in the area of cold-recycling for a number of years (Santagata, 1999), focusing both on laboratory characterization and on field evaluation of cold-recycled mixtures (Santagata & Chiappinelli, 2002). In this paper a further contribution is given to both topics, by showing the results obtained in a experimental investigation which was carried out to provide support to the construction of a cold-recycled bituminous sub-base layer of a major Italian motorway.   Mixtures were produced in a plant by employing reclaimed asphalt pavement (RAP) material,  preliminary processed through a crusher and thereafter sieved on a 25 mm screening deck, and a 60/40 modified emulsion. No virgin aggregates were added to the mixture. The job-mix formula, declared by the contractor and based on the assumption that RAP had approximately 3% water content, included 3.0% bitumen emulsion, 1.0% added water and 1.5% Portland cement.     EXPERIMENTAL INVESTIGATION  The goals of the project were to evaluate the compliance of the materials to requirements set in Technical Specifications (SATAP, 2004) and to provide suggestions to improve field performance. Based on the Authors’ previous experience in other research projects (Santagata & Chiappinelli, 2003 and 2004), this latter aspect was addressed by performing additional tests both in the laboratory and in the field, and by carrying out a full mix design of the recycled mixture according to a specifically devised procedure.  Laboratory compliance testing  Requirements included in the Technical Specifications referred both to the plant-produced mixture and to its components (RAP, aggregates extracted from the RAP, emulsion, water, cement), which were therefore sampled at the production plant and from the paver.   Due to the fact that it was stockpiled with no protection, RAP material had a high water content (average value equal to 4.27%). Such a result is considered acceptable by Specifications provided that no extra water is added to the recycled mixture. Average binder content was equal to 5.75%, lower than the maximum acceptance limit (6.5%).   Average particle size distributions and Specification limits, both referred to RAP and to extracted aggregates, are given in Table 1, which also contains the results obtained for the recycled mixture. Violations to the upper and lower acceptance limits were observed for both materials.   Table 1. Particle size distribution test results  RAP Extracted aggregates Mixture D (mm) P (%) Pmin-Pmax (%) P (%) Pmin-Pmax (%) P (%) 25 100 80-100 100 95-100 100 15 91.2 60-90 95.7 75-95 94.1 10 78 40-80 86.6 60-85 85.4 5 52.3 25-55 65.6 45-60 63.3 2 32.8 15-40 48.8 30-45 46.0 0.4 6.0 7-20 19.5 13-22 20.2 0.18 1.4 4-15 8.6 7-16 10.3 0.075 0.3 2-8 3.4 4-9 4.6  Control of the emulsion, cement and water dosage was carried out by considering the results obtained from the analysis of the recycled mixture (Santagata & Chiappinelli, 2003). The average values of both total binder content (7.40%) and total water content (5.80%) were acceptable when compared to Specification upper limits, respectively equal to 8% and 7%. However, both contents were lower than those calculated from the job-mix formula (respectively equal to 7.54 and 6.26%) considering the average water and binder contents of the RAP. Thus, it was concluded that the production plant provided an underdosage of emulsion (of the order of 2.7%), while variations of the water content of the RAP were presumably responsible for the deficiency of total water content. Cement dosage on the contrary was found to be very precise since calculations lead to identical values of the target and measured values of the percent aggregate passing the 0.075 mm sieve.    Field testing  Technical Specifications required static plate loading tests to be performed on the sub-base layer 24 hours after compaction with a minimum acceptable value of the deformation modulus set at 60 MPa. However, in order to monitor the curing process in the short term, the Authors resorted to Light-Weight Drop Tester (LWDT) tests, which yield the so-called dynamic modulus (Ed), calculated by means of the following equation:  Ed = 22.5 / s (1)  where the dynamic modulus (Ed) is expressed in MPa and the vertical displacement of the loading plate (s) is expressed in mm.  Tests were carried out on a test section which had a length of approximately 400 m and was laid in two different days. Dynamic modulus was measured in 68 test points, homogeneously distributed on the test section. Pavement and air temperature, respectively equal to 5 and 5.5°C, were constant throughout the investigation. The average dynamic modulus measured on the underlying unbound foundation was equal to 106 MPa. Experimental results, derived from statistical analysis, are shown in Table 2.   Dynamic modulus was reported to increase in the first 24 hours due to the progressive curing of the bituminous mastic, with a corresponding reduction of the dispersion of measured values, revealed by the decrease of standard deviation and coefficient of variation (COV). The layer was not overlaid immediately, and as a consequence of the cold weather and of intense rain, after 1 week of curing the modulus exhibited a slight decrease and a greater variability.  Table 2. Dynamic modulus results  Dynamic modulus Ed Curing time Average (MPa) Std. Dev. (MPa) COV (%)  20 min 118 17 14.5  2 hours 144 15 10.4  24 hours 154 15 9.4  1 week 123 21 17.0  In order to have information on the level of compaction obtained in the field, cores were taken from the test section and thereafter subjected to laboratory tests for the determination of dry density at 25°C  and Theoretical Maximum Density of the loose dry mixture. Unfortunately, even though coring operations were carried out after 2 months of curing, recovery of undamaged cores proved to be quite difficult. The corresponding test results yielded an extremely high value of the average void content, equal to 19.6%.  Mix design   The two-stage mix design procedure followed during the investigation is based on the use of the gyratory shear compactor (GSC) and of specially designed moulds in which a uni-directional flow of the fluid phase (water and emulsion) is induced (Santagata & Chiappinelli, 2004). In the first phase of design the optimal fluid content (%FFopt) of the mixture is evaluated by compacting mixtures made of RAP and cement, with an added water content variable within a conveniently wide range: the resulting cylindrical samples are subjected to density evaluation and the optimal   fluid content is defined as the value at which density reaches its maximum. The conversion of such a value in water and emulsion dosage (%W and %E) takes place in the second stage, in which the composition of the mixture is defined according to the following relationship:  %FFopt = %W + (a +Kb)%E (2)  where: a and b correspond to the weight ratios of the water and bitumen phase, respectively, to the total emulsion; K is a fluidity index which quantifies the type of contribution which the bitumen phase gives to compaction when compared to water (variable between 0 and 1).  By considering a number of combinations of %W and %E which satisfy the above given relationship, different mixtures are prepared, compacted in the GSC and thereafter subjected to the evaluation of volumetric and (optionally) mechanical properties. Based on the comparative analysis of the corresponding results, the design mixture is finally defined.  During the investigation, mix design was carried out by employing materials sampled from the production plant. In order to simulate the winter conditions in which construction was being carried out (December), before mixing and compaction all materials, tools and moulds were conditioned at 5°C. Each GSC sample was prepared by following an internal test protocol which requires a total of approximately 3200 g of material, a 150 mm diameter mould and 100 gyrations. For all mixtures the cement dosage was kept constant, equal to 1.5%, which was also the maximum value accepted by Technical Specifications.   The average density values obtained in the first and second stage of design are given in Table 3. Tests were carried out after a preliminary conditioning phase during which specimens were dried in a forced-draft oven at 40°C for 4 days. Subsequently, dry density (Gdry) at 25°C was measured by following the procedure which requires paraffin specimen coating.   Table 3. Results of density tests carried out in mix design  First stage Second stage %W (%) 4.0 5.0 6.0 2.88 2.45 2.03 1.60 %E (%) - - - 2.5 3.0 3.5 4.0 Gdry (g/cm3) 2.107 2.147 2.107 2.132 2.139 2.154 2.145  It can be observed that the optimal fluid phase content calculated at the end of the first design stage, equal to 5%, was converted in the second stage, by adopting a value of K equal to 0.75, to an emulsion dosage (%E) of 3.5% and a corresponding water content (%W) of 2.03%. Thus, it is shown that according to the results of the mix design procedure, the job-mix formula used at the production plant is characterized by a slight emulsion underdosage (equal to 3%) but also by excess total water (4.0%, approximately twice the optimal dosage). This latter observation is coherent with previous findings of the Authors, who have frequently noted that contractors tend to overdose water, either as a result of the need of keeping the plant and dump trucks sufficiently clean or as a consequence of unprotected stockpiling of the RAP.  The project included an experimental validation of the results of the mix design procedure. This was carried out by comparing the volumetric and mechanical properties of the newly designed mixture with those of a reference mixture proportioned according to the job-mix formula. To eliminate the effects caused by production variability, the second mixture was replicated in the laboratory by following the same mixing and compaction procedures adopted for the optimized one.    Evaluation of the void content of the mixtures was carried out by considering specimens compacted by means of the above described GSC protocol and also by employing a static compactor, which applies a load of 5 tons for 5 minutes on a 3000 g sample contained in a 150 mm diameter mould with a specially designed perforated base plate. The use of the alternative compaction method was deemed necessary since the Authors have frequently used it on site for quality control purposes (Santagata & Chiappinelli, 2002). Moreover, Technical Specifications contained a maximum void content value (%v), equal to 10%, derived from a similar compaction procedure (in which, however, no controlled water drainage from the sample is allowed). A synthesis of the obtained results is given in Table 4 which also includes Theoretical Maximum Density (TMD) values measured on dried mixture samples.  Table 4. Volumetric characteristics of the recycled mixtures   GSC Static compactor  TMD (g/cm3) Gdry (g/cm3) %v (%) Gdry (g/cm3) %v (%) Job-mix formula 2.500 2.137 14.52 2.145 14.20 Design 2.431 2.169 10.78 2.157 11.27  It should be noted that the mixture prepared according to the mix design results can be compacted to a void content which is significantly lower than that of the mixture selected by the contractor for production. Moreover, a good agreement between the results obtained by means of the two compaction techniques is observed. Finally, by means of the design improvements, the obtained void contents is drawn very close to the maximum value given in Specifications (10%).  Mechanical testing was carried out on GSC compacted specimens and included repeated load indirect tensile tests (RLIT) (at 20°C after 1, 2 and 45 days of curing), indirect tensile strength tests (ITS) (at 25°C after 24 hours of curing) and static uni-axial creep tests (SUC) (at 20°C, after 24 hours of curing). In all cases curing of the specimens took place in a climatic cell set at 20°C. Elastic stiffness values (E) were obtained from RLIT tests carried out with 124 ms rise time and 7 m target horizontal deformation. As required by Specifications, ITS tests were carried out at 25°C with a displacement speed of 0.847 mm/s. SUC tests were carried out at a relatively low temperature, 20°C, in order to consider worst case conditions in which no curing enhancement occurs as a result of temperature increase; 100 kPa compressive stress was applied for 1000 s and strain recovery was monitored for 3000 s. Results were expressed by referring to compliance calculated after 1 s loading (J1) and to permanent strain recorded after recovery (p).   Results obtained in this phase of the investigation are shown in Table 5, where the percent differences between the two mixtures are also given. As expected, the denser mixture derived from design exhibited , when compared to the one which replicates plant production, a significant reduction in the short term of compressive compliance and of the corresponding permanent deformation. The more closely packed aggregate skeleton also caused a slight increase of short term indirect tensile strength, which however was in both cases quite low if compared to other reference cold-recycled mixtures (Santagata & Chiappinelli, 2003). Elastic stiffness data provided information on the progression of curing of the bituminous matrix: while in the long term the two mixtures tend to very similar modulus values as a result of complete water elimination, significant differences are observed in the short term. In particular, after 2   days of curing the design mixture, as a result of its lower water dosage, proved to be significantly stiffer than the reference plant mixture. Data obtained after 1 day of curing only seem to contradict this observation, but it should be taken into account that at such low stiffness levels RLIT test results tend to be less reliable and are affected by a greater dispersion.  Table 5. Mechanical properties of the recycled mixtures  E1 day (MPa) E2 days (MPa) E45 days (MPa) ITS (N/mm2) J1 (mm2/N) p Job-mix formula 594 775 2284 0.059 7.55E-02 1.56E-02 Design 461 1153 2476 0.065 4.98E-02 1.15E-02  (%) -22.4 +48.8 +8.4 +10.2 -33.4 -26.1  CONCLUSIONS  The results collected during the investigation described in this paper allowed the Authors to provide the highway Administration with a number of suggestions to improve field performance. As a result of the specific particle size distribution of the employed RAP, in order to increase dry density values obtained in the field after compaction it was recommended either to allow the use of additional virgin aggregates or to perform a preliminary separation of the RAP in different size fractions to be separately fed in the mixing plant. RAP protection from environmental precipitations was also highlighted as necessary in order to ensure a closer control of the water content which can adversely affect compaction. Moreover, it was pointed out that the contractor should perform an optimization of the compaction scheme followed by the rollers by means of appropriately monitored trial sections.  Further conclusions were drawn from the results obtained in the mix design phase of the study. With the RAP available at the plant a more easily compactable mixture was obtained by appropriately adjusting the emulsion and water content. Such a mixture was characterized, when compared to the one prepared according to the job-mix formula adopted by the contractor, by a greater stiffness and indirect tensile strength. Moreover, its tendency to accumulate permanent strains was also significantly reduced. As a result of these observations the Authors strongly recommended to included the proposed mix design method in the acceptance procedures indicated in Technical Specifications.  REFERENCES   AASHTO-AGC-ARTBA Joint Committee. (1998). Report on Cold Recycling of Asphalt Pavements (Task Force 38 Report, March 1998). Asphalt Institute. (1983). Asphalt Cold-Mix Recycling, Manual Series n. 21, 1983. Santagata, E. (1999). Caratteristiche meccaniche dei riciclati a freddo. Le Strade, 1345(3), 47-50. Santagata, E. & Chiappinelli, G. (2002). La ricerca nel settore del riciclaggio a freddo. Le Strade, 1379(7/8), 54-56. Santagata, E. & Chiappinelli, G. (2003). Experimental evaluation of cold-recycled bituminous mixtures used for major rehabilitation works. 3rd MAIREPAV Conference, Guimarães, Portugal. Santagata, E. & Chiappinelli, G. (2004). Improvements in the mix design of cold-recycled bituminous mixtures”, 3rd Eurasphalt&Eurobitume Congress, Vienna, Austria. SATAP. (2004). Norme Tecniche d’Appalto, Torino, Italy. 

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Experimental Investigation for the Analysis of Cold-Recycled Bituminous Mixtures

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