Rheological properties of asphalt binders with chemical tensoactive additives used in Warm Mix Asphalts (WMAs)

Abstract

Warm Mix Asphalts (WMAs) have been developed with the objective of minimizing the CO₂ emissions in the production and placement process of Hot Mix Asphalts (HMAs) by reducing temperatures at which these are mixed and compacted. However, this reduction must not affect the manufacturability and final performance of the mixture. WMA additives allow reducing the production temperature while maintaining mixture workability during the mix process and without compromising the final performance of concrete asphalt. There are different additives, some of which modify the rheological behavior of asphalts (wax or paraffin) while others, in theory, allow for unaffected rheological behavior (chemical additives).

In this work the differences between the rheological properties of conventional and polymer modified asphalts, with and without chemical tensoactive additives, obtained from HMA and WMA, were studied by a Dynamic Shear Rheometer (DSR). Additionally, rutting resistance and moisture susceptibility were evaluated in these mixtures.

The rheological properties of polymer modified asphalt were affected by the WMA additives, while in the conventional asphalt the rheological properties were not significantly affected. The WMA additives improved the workability of the mixtures in the production process. Performance improvements in the submerged wheel-tracking test were observed for WMA when additives were used, yielding results similar to HMA.

Introduction

Due to the importance of environmental concerns in the manufacture of Hot Mix Asphalts (HMAs), new technologies have been developed. Warm Mix Asphalt (WMA) has appeared with the objective of minimizing CO₂ emissions in the production and placement process of HMA by reducing the temperatures at which these are mixed and compacted. This not only allows minimizing the amount of energy required, it also reduces emissions and odors and improves the welfare of workers.

Other important aspects of WMA are its paving benefits, including the ability to pave in cooler temperatures, to haul the mix longer distances, to compact the mixture with less effort and the possibility to pave and open to traffic in a short time period.

Traditionally, conventional temperatures used to produce HMA attempt to achieve a good coating of the aggregate and help in the laying process of mixture. The reduction of mixing and compaction temperatures can bring problems in the final mixture properties; therefore, it is important to consider this when trying to maintain an acceptable pavement performance for a mixture. Research studies have reported a reduction in rutting performance and moisture damage with some of the WMA technologies used. Su and co-workers [1] found lower rutting performances and increased moisture damage of WMA produced with the inclusion of a chemical synthetic wax to the mixture. Hurley and Prowell [2] found that the rutting potential did increase with decreasing mixing and compaction temperatures and conclude that the lower compaction temperature used when producing WMA may increase the potential of moisture damage.

The different WMA technologies seek to improve the workability and compactability of mixtures and can be classified into two major types: those that use water and those that use some form of additive incorporated into the asphalt to obtain the temperature reduction [3].

Processes that introduce small amounts of water into hot asphalt, either via a foaming nozzle or a hydrophilic material such as zeolite, or damp aggregate, rely on the fact that when a given volume of water is dispersed in hot asphalt, it results in an expansion of the binder phase and a corresponding reduction in the mix viscosity; making it possible to reduce the temperature in this way [3].

Organic additives (Fischer–Tropsch, montan waxes and fatty amides [3]) or chemical tensoactive additives [4] are also incorporated into asphalts. The former ones produce a decrease in asphalt viscosity when the mixing and placement temperatures are above the melting point of the wax, while the other additives reduce the surface tension of the asphalt binder without modifying, in theory, the rheological properties.

Several studies refer to the use of zeolites and waxes and their related mixture performances [5], [6], [7], [8]. Little information can be found regarding the use of tensoactive additives in WMA and its related asphalt properties and mixture performances [9].

In this work the rheological properties of conventional and polymer modified asphalts, with two types of chemical tensoactive additives were extracted from the WMAs and their resistances to rutting and moisture susceptibilities were evaluated. Additionally, the same properties for traditional HMA and WMA (made without additive) were studied to compare with the former ones.