Evaluation of High Recycled Asphalt Pavement Contents in Warm Mix Asphalt Technologies

Transcription

1 Evaluation of High Recycled Asphalt Pavement Contents in Warm Mix Asphalt Technologies Juan Antonio González-León* (corresponding author) Centre de Recherche Rhône-Alpes ARKEMA, Rue Henri Moissan, B.P. 63 Pierre-Bénite, France phone : , fax : , juan-a.gonzalez@arkema.com Patrick Caujolle Centre de Recherche Rhône-Alpes ARKEMA, Rue Henri Moissan, B.P. 63 Pierre-Bénite, France Vincent Luca Centre de Recherche Rhône-Alpes ARKEMA, Rue Henri Moissan, B.P. 63 Pierre-Bénite, France Eric Jorda Arkema Inc. 900 First Av. King of Prussia, PA phone : eric.jorda@arkema.com Submission date: May 20 th, 2011 Presentation ID: WM30 Word count:

2 González-León et al Abstract It has been demonstrated that the ensemble of warm mix asphalt technologies are compatible with most of the asphalt mixtures normally produced at standard hot temperatures. The advantages of using warm mix asphalts (WMA), in particular on the reduction of polluting emissions and energy consumption, have been reported on several occasions in literature described by laboratory and field tests. However, the biggest challenge remains to be efficiently combining WMA with higher amounts of reclaimed asphalt pavement (RAP). The incorporation of higher amounts of RAP into asphalt mixtures produced at reduced temperatures is not an easy task. The final asphalt mix properties are highly dependent on the proper blending between the aged bitumen, present on the RAP, and the fresh added binder. The blending of aged and fresh binder is a process that depends on the viscosity of the binders, their mixing time, and among other factors the operating conditions. It is easy to imagine that this blending process is more difficult at lower production temperatures, as found in WMA, than in regular hot mix asphalt conditions. In addition, since the WMA technologies are based on different physical phenomena, such as bitumen viscosity reduction, use of surface-active chemicals, or bitumen foaming, a different result from the incorporation of higher amounts of RAP may be expected in each case. In this work, the challenges of using increased amounts of RAP with warm asphalt mix technologies are discussed and demonstrated through laboratory experiments based on rheology. Simple asphalt mixing experiments are used to show the effects of different WMA technologies on their ability to reincorporate the aged bitumen from RAP. Finally, mechanical tests on asphalt mixes containing high amounts of RAP, such as ITS, were carried out to evaluate the impact of those technologies on the final performance of the mix

3 González-León et al INTRODUCTION There is an increasing interest in recycling all of the different materials we currently use in our daily lives. Road materials are not the exception and significant work has been done in order to decrease raw material consumption and reduce the amount of waste generated. In addition, recycling asphalt pavement can induce significant economic savings, especially at times when the binder price is relatively high. There are many challenges in order to reuse reclaimed asphalt pavement (RAP). Other than RAP s managing and pre-treatment (separation, storage, transport etc ), there are concerns about the aging of the binder present in the reclaimed pavement. As it is well known, bitumen suffers a series of physical and chemical changes during its service life that result in an aged, hard binder (1). The aged binder is oxidized, and having lost some of its lighter components, becomes a harder and more viscous bitumen than when first used. Higher temperatures are required to mobilize this bitumen and use it as binder for hot mix asphalts (HMA). It has to be taken into account that this aged binder is already well adsorbed (and potentially partially absorbed in the porosity of the aggregate) making it harder to reuse. In practice, RAP is usually incorporated into HMA at only a few percent of the formulation (10% to 20%) (2). When RAP is added cold, the fresh mineral aggregates are overheated to compensate for the RAP introduced. This actually sets up a limit on the amount of RAP that may be used; the fresh aggregates can not be heated to excessively high temperatures since they may degrade the binder when put in contact with the RAP. When the RAP is heated or just warmed up before the mixing to the fresh aggregates, a higher amount of recycled material may be used, in some cases up to 100% (3). This however, requires dedicated equipment to the heating of RAP. In any case, attention should be given into the properties of the final asphalt obtained when a significant amount of RAP is used. The hardened binder in RAP could transfer properties to the asphalt similar to those obtained with higher grade, stiffer binders. If the amount of RAP is important, a softer fresh binder should be used to compensate the higher viscosity of the binder present in the RAP. This is usually performed by extracting and characterizing the bitumen present in the RAP to be incorporated, and then adding an optimal amount of soft binder to obtain the desired binder properties (4). These design methods assume that the mixing between the RAP and fresh binder is complete, which may not be the case, particularly when cold RAP is used. It is currently believed that there is only a partial amount of available binder in RAP (3). This amount corresponds to the fraction of RAP binder that is actually mixed with the fresh bitumen during the asphalt production process. The determination of this availability is not an easy task. It is quite hard to determine the properties of its constitutive binder in the asphalt mix, especially if it is not homogeneous throughout. Recently there have been some published methods that allow the estimation of this available binder in RAP; however they are still very labor intensive and depend on theoretical models to interpret the mechanical properties of the mix (5, 6). The amount of actual RAP binder available or effectively incorporated into the HMA is most likely a function of the process temperature, this concern would be relevant in warm mix asphalts. In warm mix asphalts (WMA) the production temperature is reduced by 20 C up to 50 C (36 F to 90 F). This is possible through the many different WMA technologies present on the market today, all based on diverse physical processes. Nevertheless, there have been many

4 González-León et al successful WMA trials and actual jobs with different amounts of RAP incorporated, where no particular problems were observed. WMA technologies offer the opportunity to potentially increase the amount of RAP used by allowing the compaction of mixes at lower temperatures. As an example, Figure 1 shows the theoretical aggregate temperature that would be required for a given amount of RAP. The values shown correspond to the addition of cold rap (15 C, 59 F) with 4 w% humidity. The red line shows the temperatures that would be required to obtain a final mix with a temperature of 160 C (320 F) as in a standard HMA. The blue line shows the temperatures that would be required to obtain a final mix with a temperature of 120 C (248 F), as possible with some WMA technologies. Aggregate T ( C) % RAP Tmix 120 C Tmix 160 C FIGURE 1 Calculated aggregates temperature required as a function of RAP at the indicated mix production temperatures. One of the first things to observe is that the relationship between aggregate temperatures and RAP percent is not linear. This is explained by the fact that with increasing RAP content, a smaller amount of fresh aggregates must be used to compensate for a higher amount of heat. From this calculation, it can be seen why incorporating large quantities of RAP (added cold) into HMA can be difficult to achieve. It can be observed that when compaction is carried out at lower temperatures, similar to WMA, the amount of RAP content could be increased by about 10% to 15% for the same aggregate temperature. Although these values are theoretical, meaning that the heat is assumed to be at equilibrium, (thus no thermal gradients inside the aggregates, which most likely is not the case given the short mixing times normally used in practice), the potential advantages of WMA with higher RAP contents are clearly demonstrated. This advantage does not take into account the previous discussed matter of the amount of actual RAP binder mixed with the added fresh binder and the impact of the final properties of the

5 González-León et al pavement. The effect of the different WMA technologies on the incorporation of RAP, in particular their effect on the actual mixing of aged and fresh binder, is still not fully understood. In this work, the results from experiments carried out to better understand the actual mixing process between the aged binder from RAP and a soft binder at WMA conditions are showed and discussed. Rheology experiments were set up to better understand the basic mixing process between a hard and a softer binder under shear. Simple tests with RAP aggregates were also carried out with different WMA technologies to identify potential differences. The objective of this paper is discuss some of the issues involving RAP use with WMA and share some observations encountered during the experimentation. MATERIALS AND METHODS Two Layer Rheology In order to better understand the basic phenomena occurring during the mixing of the hard bitumen present in the RAP and a soft binder, an experimental setup to measure the viscosity of two layers of different binders simultaneously as a function of time was designed. The harder binder was first placed on the bottom plate of a dynamic shear rheometer at a sufficiently high temperature for it to flow. Then, it was covered by a piece of silicon treated paper and pressed with the upper plate to a defined gap setting (0.7mm). In order to remove the silicon paper, the temperature was decreased to room temperature (see Figure 2-a). Once the paper was removed, the temperature was increased so that the softer bitumen may flow, but the harder bitumen would remain solid. At this temperature the soft binder was placed on top the hard binder and pressed with the upper plate (see Figure 2-b) to obtain a total gap of 1mm. The two layers were then trimmed simultaneously with a hot spatula and then the temperature was raised to 120 C to carry out the viscosity measurement. A Physica MCR301 dynamical shear rheometer with 25mm parallel plate geometry was used for these experiments. Measurements were carried out at 120 C at 30 s -1 and 50 s -1 shear rate. The lower and top bitumen layers consisted on a 10/20 and a 70/100 1/10mm penetration binders, respectively (close to a PG82-16 and PG58-28 binders). Viscosity measurements were also carried out on each binder independently (120 C, 30s -1, 25mm parallel plate geometry, 1mm gap) and on mixtures between them. These mixtures were prepared by stirring different amounts of each binder at 160 C (320 F) for 10 min in a metal beaker to achieve complete mixing

6 González-León et al a) b) soft binder hard binder 238 c) soft binder hard binder FIGURE 2 Pictures showing the assembly steps of the two layers of bitumen. Manual RAP mixing Simple mixtures between RAP aggregates and fresh aggregates were carried out to observe the transfer and repartition of binder between the two. The RAP was fractioned so that only large aggregates were used (passed through a 6/10mm sieve). The fresh aggregates, chosen for their color (calcareous origin), were also sieved. The experiment consisted of adding 2g of soft binder (70/100 1/10mm penetration bitumen) to 100g of heated RAP aggregates (at 120 C, 248 F); placing the samples in an oven for 5 minutes at 120 C, adding 100g of heated fresh aggregates, and then mixed manually for 1 minute. The final mix was then observed for coverage quality. This was performed for a reference mix and four other mixes, where different WMA additives were used (also at 120 C). WMA techniques where the additive is introduced into the bitumen were used. The chemical surfactant additive (0.8w%) and the wax (4w%), were prepared by stirring hot bitumen at 160 C (320 F) for ten minutes after the additive was introduced. In the case of the zeolite additive, it was introduced at the same time the binder was poured into the RAP (0.3% to the total mix). An additional binder was also prepared by adding aromatic oil (20 w%) to the binder to make it softer. 100% RAP Samples Compacted samples with 100% RAP were prepared using the same WMA technologies described above. RAP aggregates (0/10 fraction) were heated to 120 C (248 F) for about 3 hours before adding 2% of 50/70 1/10mm penetration bitumen. The mix was then homogenized

7 González-León et al with a laboratory mixer and poured into a gyratory compactor mold. It was then compacted to a fixed height (to have comparable void percentage in all samples). After cooling down, the samples were taken out of the mold and kept at 18 C (64.4 F) overnight before testing. Their indirect tensile strength (ITS) was determined using an Instron universal testing machine at 50mm/min. Additional samples, without any warm mix asphalt additive, were also prepared following the same procedure described above as references. Bitumens of different penetration grades (35/50, 50/70 and 70/100 1/10mm) were used for the reference samples. Three specimens for each additive or bitumen tested were prepared and analyzed. RESULTS AND DISCUSSION Two Layer Rheology As discussed above, a main concern is the understanding of the actual mixing between the aged binder present in the RAP and the fresh added binder. The measurement of the diffusion process between a hard and soft bitumen by rheology have been previously described in literature (7). As described, two layers of bitumen, one harder than the other were measured under an oscillatory shear for a period of several days. A model was used to fit the obtained data and calculate the diffusion coefficients out of their measurements. The continuous shear that takes place during the mixing of the RAP with the asphalt mix should play an important role on the diffusion between the RAP binder and the fresh binder. In the present work, rheology measurements on two layer bitumen assemblies are carried out at temperatures relevant to the WMA processes (higher then that in the previous cited publication) and under a continual rotational shear rate. The values of shear rate used in these experiments were chosen so that they were compatible with the experimental setup attempted (two layer assembly, instrument characteristics, and quality of data). To better understand the two layer viscosity measurements (and to be able to compare to actual mixtures between the soft and the hard binder) a correlation between the individual viscosities of the soft, η soft, and hard bitumen, η hard, to the viscosity of the complete mix, η Total, was found. The viscosity of the mix may be calculated by using a logarithmic mixing rule of the mass fractions of both bitumens (x soft + x hard =1). It was found that with an empirical modification of this mixing rule by an exponential factor,α, with a value of 1.4 on the volume fraction of the hard bitumen, the calculated complete mix viscosity more accurately represented the measured viscosity. The equation used was: ln η = x lnη + x lnη eq. 1 Total soft soft Figure 3 shows the measured values and the values obtained with equation 1. The relationship as expected is far from linear. It can be seen that the viscosity actually changes very little with lower amounts of hard bitumen (for example, in a case where a relative small quantities of RAP is used). However, large changes in viscosity are observed within just a few percent difference of soft binder at higher hard bitumen contents (case of high amounts of RAP with just a few percent of soft binder). This importance on the hard bitumen viscosity seems to be captured on the parameter α, were a value higher than 1 was required to obtain a better fit. α hard hard

8 González-León et al The mix viscosity is actually the viscosity that will be obtained when complete mixing of the two binders occurs. However, the value of apparent viscosity, η apparent, meaning the viscosity of the two unmixed layers of binder, as measured together, can be calculated. The calculation is based on modeling each layer of bitumen, knowing their independent viscosities, and then taking into consideration that the shear stress is constant through all of the different bitumen layers and the thickness of each layer (h hard and h soft ). The resulting equation is shown here below: η apparent η soft η hard = eq. 2 η h + η h Figure 3 shows the values of apparent viscosity of the 2 layer assembly, calculated using equation 2, assuming h hard +h soft =1. It can be seen that at lower hard bitumen fractions there is little difference between the completely mixed and the separated two layer values. However, as it may be expected, larger differences are observed at higher hard bitumen contents. This difference is captured in the viscosity measurement of the two layer sample as a function of time. soft hard hard soft Measured complete mix viscosity Calculated complete mix viscosity Calculated 2 layers apparent viscosity Viscosity (Pa s) Fraction of hard bitumen FIGURE 3 Calculated and measured viscosities of complete mixtures and two layer assembly between a hard and a soft bitumen. Figure 4 shows the measured viscosity (or apparent viscosity) of the two layers, hard and soft during 1 hour. The values of viscosities shown are the apparent viscosities as measured by the rheometer as if the 2 layers were a homogeneous material (calculated from the measured torque and the chosen geometry factors). The assembly was set to have a hard bitumen layer of

9 González-León et al mm and a soft bitumen layer of 0.3 mm. However, after a careful measurement of the hard layer it was found that is was actually 0.55 mm (modified probably by the weight of the paper). The model value for the two layer apparent viscosity is 1.1 Pa s and for the complete mixture it is 1.8 Pa s. Both are close to what was observed at the initial and final values of the measured curve. There is not a linear evolution of the two layer viscosity toward the fully mixed value. It is fast at the beginning, while approaching asymptotically to the fully mixed value at the end, as expected for a diffusion phenomenon. Another experiment, not shown, was carried out at a higher shear rate of 50 s -1, resulting in a similar curve, but the fully mixed value was achieved in a shorter time Viscosity (Pa s) Time (min) FIGURE 4 Viscosity of two layers of bitumen (soft at the top, hard and the bottom) as a function of time. Although this test remains a model situation of the mixing between the hard binder from RAP and a softer binder, it demonstrates that the full mixing can take place under shear on a time scale that is on the same order of magnitude as in pavement production (minutes, instead of hours or days) at temperatures that are lower than the usual production temperatures for the hard bitumen used (120 C, 248 F). Many parameters may be adjusted for this test to approach conditions more consistent to industry standard RAP use conditions, such as type of bitumen, shear rates, temperatures, or layer thickness. The result of the modification of those parameters will be the subject of a future publication. RAP Mixing for Visual Inspection

10 González-León et al Mixing experiments were carried out to test if by simple inspection, differences on the amount of bitumen mixed and transfered could be identified. The mixes were done at 120 C (248 F) using different WMA techniques. The chosen techniques differ greatly on the mechanism by which they allow a reduction of the asphalt temperature. A binder with an aromatic oil was also tested (to simulate a binder containing a large amount of rejuvenator). The final mixes are shown on Figure 5. Sufficient transfer of bitumen to the fresh aggregates was not found in any of the mixtures. Figures 5-a and 5-b show the mixes where a chemical additive and a zeolite were used. As can be seen, the fresh aggregates are barely covered with bitumen and look very much like the reference mix (Figure 5-c). A slight improvement was observed with the bitumen containing the oil, Figure 5-d, where a better coverage seemed to take place. The addition of a wax, Figure 5-e, also seemed to be a bit better than the reference. In any case, the bitumen transferred seemed to be the added soft bitumen with perhaps only a fraction of the aged binder from the RAP. Figure 5-f shows the case where no additional bitumen was added to the aggregates, but it was heated to 180 C (356 F) before mixing. It can be seen that the binder from the RAP is barely softened by the heat treatment and a very small amount is transferred to the fresh aggregates. a) b) c) d) e) f) FIGURE 5 Simple mixtures between RAP and fresh aggregates at 120 C using different WMA techniques. See text for description

11 González-León et al This method turns out to be an insufficient test to distinguish any difference on the possible effects between the different WMA techniques on RAP usage. In any case, it shows once again the difficulties encountered when trying to incorporate RAP and effectively use its binder. 100% RAP Samples As simple inspection did not show any significant differences between the WMA techniques with RAP, mechanical tests were carried out to try to quantify the possible differences. The compacted samples, prepared as described above, were compressed and their indirect tensile strengths were determined. The obtained values are shown in Figure ITS (MPa) /100 reference 50/70 reference 35/50 reference Wax 50/70 Surfactant additive 50/70 Zeolite 50/70 Surfactant additive 70/ FIGURE 6 Indirect Tensile Strength values of samples containing 100% RAP aggregates prepared with the WMA additives and bitumens showed. As can be seen, the bitumen penetration has an important impact on the cohesion of the sample. The reference with the softer binder, 70/100 1/10mm penetration, has the lowest ITS while the harder binder, with 35/50 1/10mm penetration (close to a PG70-22) has the highest. The fresh binder added seems to control the final mechanical properties of the sample. Additional reference samples (data not shown) were carried out at 160 C (320 F) instead of 120 C (248 F) with the 50/70 and 35/50 binders. Their ITS values were higher and actually quite close to each other (3.28 and 3.31 respectively) suggesting that at temperatures where a more complete mix between the binder occurs, the aged binder controls the properties. When the different WMA additives were used to prepare the samples, similar results to the reference were obtained, except for when the chemical additive was used. In this case, a higher ITS value was obtained, even higher than observed with lower penetration bitumen (harder binder was used). A similar effect was observed with the 70/100 binder, although the improvement is less significant. Since the chemical additive is known not to change the binder viscosity (8) its increase on the ITS might be due to a partial mixing with the RAP binder. A deeper study would be needed to further understand the impact of WMA additives on the use of higher amounts of RAP that would include other techniques, such as asphalt modulus and fatigue.

12 González-León et al CONCLUSIONS A novel test based on rheological measurements of binders was developed to understand the mixing of different bitumens under shear. The effect the hard binder had on the viscosity of the mix and particularly the non-linearity with respect to the composition was well-observed. Simple inspection of the bitumen transfer from a RAP aggregate to a virgin aggregate, under the tested conditions, is not sufficient to observe significant differences between the WMA techniques used. The ITS of compacted samples with 100% RAP at 120 C showed that it is dependent on the penetration of bitumen added. Harder bitumens resulted in higher ITS values. Most of the WMA techniques and additives showed a similar ITS with respect to the reference sample, except in the case of the surfactant chemical additive, where a higher value was obtained. The observed value was in fact higher than when a harder bitumen was used, and since the additive is claimed to not change the viscosity of the binder, it suggests that it may help make available a larger fraction of the binder in the RAP. REFERENCES (1) Sa da Costa, A. et al. Chemical and Thermal Characterization of Road Bitumen Ageing. Materials Science Forum Vols pg (2) Sullivan, J. Pavement Recycling executive summary and report. Publication FHWA-SA FHWA. U.S. Department of Transportation, (3) Doyle, J.D. and Howard, I.L. Compactability and bitumen utilization of 100% Warm mixed RAP. CD-ROM. Transportation Research Board of the National Academies, Washington, D.C., (4) Asphalt Institute. Asphalt Handbook 7 th edition MS pg (5) Ma, T., Mahmoud, E. Bahia, H. Development of a Testing Procedure for the Estimation of RAP Binder Low-Temperature Properties without Extraction. CD-ROM. Transportation Research Board of the National Academies, Washington, D.C., 2010 (6) Bennert, T., Dongré, R. A Backcalculation Method to Determine Effective Asphalt Binder Properties of RAP Mixtures.. CD-ROM. Transportation Research Board of the National Academies, Washington, D.C., 2010 (7) Karlsson, R., Isacsson, U. and Ekblad, J. Rheological characterisation of bitumen diffusion. J; Mater Sci Vol pg (8) Grampré, L, Gonzalez-Leon, J,A. and Barreto, G. Enrobés tièdes par additivation chimique. Revue Générale des Routes, No 866,2008, pp