Many asphalt plant manufacturers incorporate RAP directly in the pugmill, without any drying process. This method is the simplest, but no more than 5% RAP can be added. This limitation exists because the RAP is heated only through contact with the hot virgin aggregate exiting the dryer drum. The aggregates must be overheated in an attempt to “dry” the RAP, but this simply does not work. Overheating the aggregate consumes excessive burner energy and, consequently, excessive fuel.

When superheated virgin aggregate abo 180 °C comes into contact with cold RAP at 20–25 °C, a severe thermal shock occurs (Bonaquist, 2011; Shah, 2007). This extreme temperature gradient accelerates oxidation of the asphalt binder, damaging both the RAP binder and the virgin binder. Because RAP does not warm gradually, its binder does not properly soften or blend, instead undergoing localized overheating and additional oxidative hardening (Shah, 2007; Bennert et al., 2011). As a result, the final blended binder becomes harder and more brittle, reducing pavement durability and increasing susceptibility to cracking (Jamshidi et al., 2013; Zhao et al., 2015).

Some brands claim that their systems can incorporate up to 30% RAP by mixing it directly in the pugmill, but this is not realistic. High-RAP systems are thermally and mechanically complex and cannot rely only on superheated virgin aggregate. As RAP percentages rise, the risk of overheating the material increases sharply, leading to excessive aging of the binder and visible blue-smoke emissions if no proper gas-filtration or incineration system is used.

Laboratory studies using pugmill-style mixing have shown that even when virgin aggregates are preheated to typical plant temperatures and room-temperature RAP is added and mixed for approximately one minute, only a limited amount of binder is actually transferred from the RAP to the virgin aggregate. Shah (2007) demonstrated that when mixing time or temperature is insufficient, RAP particles behave essentially as pre-coated “black rock,” meaning the aged binder remains on the RAP and does not adequately blend with the virgin binder.

Mixing RAP directly in the pugmill also requires longer mixing time to transfer enough heat to the RAP. However, the pugmill residence time is very short—typically only a few seconds. Extending the mixing time to properly heat the RAP significantly reduces the plant’s production rate. And even when production is slowed down, the limited heat-transfer capability and short residence time inside the pugmill make it impossible for the RAP to reach the required temperature for proper binder activation.

According to FHWA guidance and several laboratory and field studies, simply dropping cold, moist RAP into the pugmill and relying on superheated virgin aggregate results in insufficient and non-uniform heating, limited RAP binder activation, higher fuel consumption, and an increased risk of blue-smoke emissions.

In real-world operation, plants that claim very high RAP contents using direct pugmill incorporation are frequently forced to reduce the actual RAP percentage to meet emission regulations, avoid binder burning, but excessive oxidation, mix quality aren’t good.

The reduction in production temperature raises concerns about several factors that influence both construction and long-term pavement performance. Lower temperatures can negatively affect mix workability and compactability, and they also limit binder blending efficiency. In high-RAP WMA mixtures, these effects contribute to increased moisture susceptibility and reduced rutting resistance (Bonaquist, 2011; Bennert et al., 2011; Jamshidi et al., 2013; Zhao et al., 2015).

Adequate workability is essential for any asphalt mixture to ensure proper pavement construction. Because asphalt binder is highly temperature-dependent, the workability of an asphalt concrete mix is directly influenced by the temperatures at which it is produced and handled. In conventional HMA, mixing and compaction temperatures are selected to achieve a binder viscosity that provides sufficient coating of aggregate particles and adequate lubrication during compaction.

When RAP is incorporated, however, the effective binder grade increases due to the presence of aged, stiff RAP binder blending with the virgin binder. As a result, higher temperatures may be required to attain the same level of workability achieved in mixtures containing only virgin materials.

This occurs because the reduced production temperatures used in WMA may not supply enough thermal energy to adequately activate and blend the RAP binder with the virgin binder. When this happens, RAP behaves essentially as “black rock,” contributing little to the effective binder content of the mixture. The reduction in effective binder impairs constructability and weakens the mixture’s overall mechanical stability and durability.

Black rock: Describes RAP particles whose aged binder remains stiff and inactive during mixing, causing the RAP to behave like inert aggregate rather than contributing usable asphalt binder.

When temperatures are too low, the RAP binder may not mobilize or blend with the virgin binder. In these cases, RAP behaves like an inert “black rock,” reducing the effective binder, increasing air voids, and weakening mixture durability (Bonaquist, 2011).

Increasing production temperatures directly affects the air voids in the asphalt mixture. A study by Oliveira et al. (2012):

Figure 3 - Air void content at different mixing temperatures for (a) virgin WMA and HMA & (b) 50% WMA and 50% HMA mixes; (adpoted from Oliveira et al. 2012).

The study shows that lowering the mixing temperature has little effect on virgin mixes, but it significantly affects RAP mixes. At 50% RAP, lower temperatures lead to higher air voids, meaning poorer compaction. This occurs because the aged binder in the RAP requires more heat energy to blend and mobilize. Without that heat, the RAP behaves like a stiff “black rock,” reducing the effective binder and making the mixture harder to compact (Oliveira et al., 2012).