Figure 1:
Research framework illustrating the identification of the research gap, formulation of objectives, experimental methodology, performance-based evaluation framework, and engineering recommendations for RAP–soil blends
Figure 2:
Natural soil sample collected for laboratory characterization prior to index property and compaction testing
Table 1:
Soil properties
| Property | Value |
|---|---|
| Soil classification (USCS) | CL (Lean Clay) |
| Specific gravity (Sg) | 2.67 |
| Liquid limit [%] | 44 |
| Plastic limit [%] | 23 |
| Plasticity index [%] | 21 |
| % Passing 75 µm sieve | 61 |
| Sand fraction [%] | 28 |
| Gravel fraction [%] | 11 |
| Natural moisture content [%] | 12.3 |
Figure 3:
Reclaimed Asphalt Pavement (RAP) particles showing aged asphalt coating and granular morphology used for preparing RAP–soil mixtures.
Table 2:
RAP properties
| Property | Test standard (IS Code) | Value |
|---|---|---|
| RAP source | Field milling | Flexible pavement rehabilitation |
| Maximum particle size [mm] | IS 2386 Part 1 | 19 |
| Specific gravity | IS 2720 Part 3 | 2.48 |
| Asphalt binder content [%] | Ignition method | 4.8 |
| % Passing 75 µm sieve | IS 2386 Part 1 | 7 |
| Gradation type | Sieve analysis | Well-graded gravelly sand |
| Particle angularity | Visual assessment | Sub-angular to angular |
| Surface condition | Visual assessment | Asphalt-coated particles |
| Water interaction behaviour | Qualitative | Hydrophobic tendency |
Figure 4:
Atterberg limits testing procedure
Figure 5:
Particle size distribution curves of natural soil and Reclaimed Asphalt Pavement (RAP), illustrating the complementary gradation characteristics of the constituent materials
Table 3:
Mix design matrix
| Mix ID | RAP [%] | Soil [%] | Preparation details |
|---|---|---|---|
| M0 | 0 | 100 | Control soil; oven-dried and moisture-conditioned |
| M10 | 10 | 90 | Dry blending followed by moisture conditioning |
| M20 | 20 | 80 | Uniform mixing with gradual water addition |
| M30 | 30 | 70 | Mechanical blending to ensure homogeneity |
| M40 | 40 | 60 | Increased RAP proportion with consistent mixing protocol |
| M50 | 50 | 50 | Equal soil–RAP blend; controlled moisture distribution |
| M60 | 60 | 40 | RAP-dominant mix; extended blending time |
| M70 | 70 | 30 | High RAP content: uniform particle dispersion ensured |
| M80 | 80 | 20 | RAP-dominated mixture; moisture adjusted carefully |
Figure 6:
Standard proctor compaction apparatus used for evaluating moisture–density behaviour of RAP–soil mixtures
Figure 7:
Experimental methodology workflow for performance-based evaluation of RAP–soil mixtures
Figure 8:
Standard Proctor compaction curves showing the variation of dry density with moisture content for RAP–soil mixtures containing different RAP replacement percentages
Figure 9:
Variation of maximum dry density with increasing RAP percentage
Figure 10:.
Relationship between optimum moisture content and RAP replacement level
Table 4:
Influence of constituent material properties on the compaction behaviour of RAP–soil mixtures
| Property | Natural soil | RAP | Influence on compaction behaviour |
|---|---|---|---|
| Specific gravity | 2.67 | 2.48 | Lower RAP specific gravity contributes to reduced MDD at high RAP contents |
| Particle shape | Fine cohesive particles | Angular/sub-angular | RAP increases interparticle friction and mechanical interlocking |
| Surface characteristics | Hydrophilic | Asphalt-coated(hydrophobic) | RAP reduces water demand and lowers OMC |
| Dominant interaction | Cohesion | Friction | Governs transition from cohesion-controlled to friction-controlled behaviour |
| Packing mechanism | Fine matrix | Granular skeleton | Optimum packing occurs at intermediate RAP replacement levels |
Table 5:
Compaction parameter summary
| Mix ID | Maximum dry density (kN/m3) | Optimum moisture content (%) | Trend observation |
|---|---|---|---|
| M0 | 17.6 | 17.8 | Control cohesive behaviour |
| M10 | 17.9 | 17.1 | Improved packing begins |
| M20 | 18.2 | 16.3 | Density increase continues |
| M30 | 18.5 | 15.6 | Optimal particle interaction |
| M40 | 18.7 | 14.9 | Peak packing efficiency |
| M50 | 18.6 | 14.2 | Transition toward granular response |
| M60 | 18.3 | 13.6 | Reduced cohesion observed |
| M70 | 17.9 | 13.0 | Granular behaviour dominates |
| M80 | 17.4 | 12.5 | Density reduction due to RAP dominance |
Table 6:
Mean values and standard deviations of Maximum Dry Density (MDD) and Optimum Moisture Content (OMC) for RAP–soil mixtures
| Mix ID | MDD [kN/m3] | SD [kN/m3] | OMC [%] | SD [%] |
|---|---|---|---|---|
| M0 | 17.6 | 0.06 | 17.8 | 0.15 |
| M10 | 17.9 | 0.05 | 17.1 | 0.13 |
| M20 | 18.2 | 0.07 | 16.3 | 0.17 |
| M30 | 18.5 | 0.06 | 15.6 | 0.14 |
| M40 | 18.7 | 0.05 | 14.9 | 0.16 |
| M50 | 18.6 | 0.07 | 14.2 | 0.18 |
| M60 | 18.3 | 0.08 | 13.6 | 0.15 |
| M70 | 17.9 | 0.09 | 13.0 | 0.17 |
| M80 | 17.4 | 0.08 | 12.5 | 0.16 |
Table 7:
Mechanistic interpretation of the transition from cohesion-controlled to friction-controlled behaviour in RAP–soil mixtures and its influence on compaction characteristics
| Rap content range (%) | Dominant behaviour | Particle interaction mechanism | MDD trend | OMC trend | Engineering interpretation |
|---|---|---|---|---|---|
| 0 | Cohesion-controlled | Clay cohesion and capillary forces dominate | Baseline | Highest | Moisture-dependent compaction of cohesive soil |
| 10–20 | Cohesion–friction transition | RAP particles fill voids while soil matrix remains continuous | Increasing | Slightly decreasing | Improved packing and particle rearrangement |
| 30–40 | Balanced cohesion and friction | Optimum void filling and granular interlocking | Maximum | Moderate | Highest compaction efficiency and optimum packing |
| 50–60 | Friction-controlled transition | Reduced fine matrix and increased granular contacts | Slightly decreasing | Lower | Granular skeleton begins to dominate behaviour |
| 70–80 | Friction-controlled | Angular RAP particles dominate with limited void filling | Lowest | Lowest | Reduced packing efficiency due to excessive granular structure |
Figure 11:
Variation of performance index with RAP content showing optimal performance range
Table 8:
International RAP Utilization Practices in Pavement Applications
| Country | Typical RAP content range | Primary application | Implementation context |
|---|---|---|---|
| United States | 20–40(%) | Base layers, recycled asphalt mixtures | Widely adopted for cost and sustainability benefits |
| Germany | 30–50(%) | Structural pavement layers | Strong recycling policies and material recovery systems |
| Netherlands | 25–50(%) | Base and surface recycling | Emphasis on circular economy principles |
| Japan | 30–60(%) | Asphalt pavement recycling | High material recovery rates and controlled processing |
| India | 20–40(%) | Pavement rehabilitation and base layers | Increasing adoption under IRC guidelines and sustainable infrastructure initiatives |
Figure 12:
Cost ratio (CR) variation with RAP content and corresponding cost-saving behaviour
Table 9:
Comparison to the literature
| Reference | RAP Range [%] | Peak density range [%] | OMC reduction [%] | Reported optimum RAP [%] |
|---|---|---|---|---|
| Gui et al. [24] | 0–60 | Increase up to ~40 | 10–15 | 30–40 |
| Wang et al. [25] | 0–70 | Progressive increase to ~35 | 12–18 | ~35 |
| Hamedi et al. [26] | 0–50 | Minor change | 15–20 | ~30 |
| Santos et al. [27] | 0–60 | Peak at moderate RAP | 10–20 | 25–45 |
| Uniyal et al. [29] | 0–80 (stabilized) | Increase up to ~60 | <10 | 50–60 |
| Present Study | 0–80 | Peak at ~40 | 25–30 | 30–50 |