# Assessing RAP Multi-Recycling Capacity by the Characterization of Recovered Bitumen Using DSR

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## Abstract

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## 1. Introduction

## 2. Background

## 3. Materials and Methods

#### 3.1. Materials

#### 3.2. Experimental Procedures

^{TM}software. The latter converts dynamic mechanical data from the frequency domain to the time domain and vice versa, hence giving the possibility of determining the master curves of stiffness from the dynamic data. The time-temperature superposition was performed by shifting the isothermal curves using a modified non-linear Marquadt–Levenburg least-squares optimization [39]. The software numerically optimizes the number of relaxation/retardation modes used in the analysis [57]. The shifting procedures use the methodology defined by Gordon and Shaw [48]. Both the CAM model and the Generalized Sigmoidal model (Richards model) were used to calculate the master curves of all the bitumen.

## 4. Results and Discussion

#### 4.1. Softening Point and Penetration Value

#### 4.2. Rheological Analysis

#### 4.2.1. Complex Modulus and Phase Angle

#### 4.2.2. Master Curve Development from Dynamic Shear Rheometer Data

#### 4.2.3. Ageing Indices

#### 4.2.4. Rheological Index

#### 4.2.5. Fatigue Criteria

_{R&B}increases and the penetration value decreases. Similar tendencies are observed for the increase in R-value.

## 5. Conclusions

- The bitumen recovered from the second recycling cycle RAP mixtures revealed a lower softening point (4.4% and 4.1%) and a higher penetration value (60.0% and 50.7%) than the bitumen recovered from the first recycling cycle RAP mixtures (non-aged and aged, respectively).
- The penetration index increased from −1.00, in the bitumen from the non-aged virgin mixture, to 0.16, in the second recycling cycle aged RAP mixture, which seems to indicate a decreasing ageing susceptibility throughout the recycling cycles and with ageing.
- The bitumen rheology plotted on the Black diagram showed that the successive recycling cycles did not affect the rheological performance of the bitumen. The use of a rejuvenator containing a percentage of virgin bitumen led to the obtention of a behavior that does not compromise the performance of the bituminous mixture.
- The bitumen recovered from the RAP mixtures and virgin mixtures presented similar ageing indices, which shows that the bitumen blends (aged bitumen from RAP + rejuvenator + virgin bitumen) were not prone to ageing when compared with the virgin bitumen.
- The R-value-${\omega}_{c{T}_{d}}$ space diagrams showed that the successive recycling cycles were not translated into an increased tendency toward crack development. Furthermore, the bitumen from the second recycling cycle RAP mixtures presented lower R-values (2.8% and 5.4%) and higher ${\omega}_{c{T}_{d}}$ (141.2% and 88.0%) compared with the bitumen from the first recycling cycle RAP mixtures, for non-aged and aged conditions, respectively. This demonstrates the multi-recycling potential of the RAP.
- The results plotted on the Black diagram of the complex modulus and phase angle, determined for a frequency of 0.005 rad/s at 15 °C (G-R parameter), confirmed that the bitumen recovered from the RAP mixtures did not present a tendency to develop cracking, as the values obtained were less than G-R = 180 kPa. Moreover, it was verified that the bitumen from the second recycling cycle RAP mixtures presented lower G-R values (77.8% and 55.3%) than the bitumen from the first recycling cycle RAP mixtures, for non-aged and aged conditions, respectively. This indicates that the rheological properties of the bitumen can be recovered using a rejuvenator. The bitumen from the second recycling cycle RAP mixture and from the virgin mixture were similar.
- All bitumen presented values far from the SHRP fatigue criteria limit (G″ ≤ 5000 kPa), hence confirming the results obtained for the G-R parameter.
- It was found that both the softening point and penetration tests presented a good correlation with the G-R parameter and that the softening point presented a strong correlation with the R-value. Despite the tests failing to give further information on the rheological performance of the bitumen and, consequently, on the cracking and ageing susceptibility, these tests could be used as a first indicator in practices of multi-recycling feasibility; however, rheological characterization is fundamental to an accurate evaluation.

## Author Contributions

## Funding

## Acknowledgments

^{TM}software and for all the provided support.

## Conflicts of Interest

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**Figure 7.**Complex modulus and phase angle for an angular frequency of 0.005 rad/s tested at a temperature of 15 °C.

Recycling Cycle | Softening Point (°C) | Penetration 25 °C (10 ^{−1} mm) |
---|---|---|

1st recycling cycle | 67.5 | 19.5 |

2nd recycling cycle | 68 | 22 |

Samples | Bitumen | Mixture | |||
---|---|---|---|---|---|

Neat Bitumen (by Mass of Total Bitumen) | RAP Bitumen (by Mass of Total Bitumen) | Rejuvenator (by Mass of RAP Bitumen) | Ageing | Composition | |

0RAP | 100% | 0% | 0% | No | Virgin aggregates |

0RAPA | 100% | 0% | 0% | Yes | |

75RAP0 | 30% | 70% | 4.5% | No | 25% virgin aggregates 75% RAP |

75RAP0A | 30% | 70% | 4.5% | Yes | |

75RAP1 | 28% | 72% | 5.0% | No | 25% virgin aggregates 75% RAP |

75RAP1A | 28% | 72% | 5.0% | Yes |

Parameter | Configuration 1 | Configuration 2 |
---|---|---|

Temperature | 10–40 °C (5 °C intervals) | 25–80 °C (5 °C intervals) |

Frequency | 0.1–20 Hz | 0.1–20 Hz |

Geometry | Parallel plates ø 8 mm | Parallel plates ø 25 mm |

Gap | 2 mm | 1 mm |

Bitumen | Softening Point (°C) | P (10^{−1} mm) | Penetration Index |
---|---|---|---|

0RAP | 55.2 | 31 | −1.00 |

0RAPA | 57.9 | 27 | −0.75 |

75RAP0 | 62.7 | 20 | −0.40 |

75RAP0A | 67.5 | 15 | −0.12 |

75RAP1 | 60.0 | 32 | 0.00 |

75RAP1A | 64.7 | 23 | 0.16 |

Bitumen | 0RAP | 0RAPA | 75RAP0 | 75RAP0A | 75RAP1 | 75RAP1A |
---|---|---|---|---|---|---|

G*_{min} (Pa) | $3.4\times {10}^{1}$ | $5.2\times {10}^{1}$ | $1.3\times {10}^{2}$ | $2.4\times {10}^{2}$ | $7.7\times {10}^{1}$ | $1.4\times {10}^{2}$ |

G*_{max} (Pa) | $3.6\times {10}^{7}$ | $4.9\times {10}^{7}$ | $4.9\times {10}^{7}$ | $5.4\times {10}^{7}$ | $2.2\times {10}^{7}$ | $3.9\times {10}^{7}$ |

δ_{min} (°) | $34.7$ | $34.3$ | $31.7$ | $29.7$ | $36.1$ | $32.7$ |

δ_{max} (°) | $88.3$ | $89.3$ | $86.6$ | $86.1$ | $88.0$ | $87.6$ |

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**MDPI and ACS Style**

Antunes, V.; Moreno, F.; Rubio-Gámez, M.; Freire, A.C.; Neves, J.
Assessing RAP Multi-Recycling Capacity by the Characterization of Recovered Bitumen Using DSR. *Sustainability* **2022**, *14*, 10171.
https://doi.org/10.3390/su141610171

**AMA Style**

Antunes V, Moreno F, Rubio-Gámez M, Freire AC, Neves J.
Assessing RAP Multi-Recycling Capacity by the Characterization of Recovered Bitumen Using DSR. *Sustainability*. 2022; 14(16):10171.
https://doi.org/10.3390/su141610171

**Chicago/Turabian Style**

Antunes, Vítor, Fernando Moreno, Maria Rubio-Gámez, Ana Cristina Freire, and José Neves.
2022. "Assessing RAP Multi-Recycling Capacity by the Characterization of Recovered Bitumen Using DSR" *Sustainability* 14, no. 16: 10171.
https://doi.org/10.3390/su141610171