Cementitious binders and reclaimed asphalt aggregates for sustainable pavement base layers: Potential, challenges and research needs

Highlights

• High replacement levels natural aggregates with reclaimed asphalt lead to lower mechanical properties.

• Low mechanical properties can be compensated by increasing the amount of cementitious materials.

• Alkali-activated binders can be a suitable binder if adequately designed.

Abstract

Reclaimed asphalt pavement (RAP) has been increasingly used in the past few decades as a replacement of natural aggregates in pavement layers. However, low replacement level is employed, and consequently, large quantities of RAP are still left unused. Researchers often neglect foundation pavement layers, but it has higher prospects to consume recycled materials due to their increased thickness. RAP has inferior properties compared to natural aggregates; therefore, cementitious binders are used to produce a (sub)base layer using high volumes of RAP. This paper reviews the use of RAP aggregates with cementitious materials for pavement foundation layers. Special attention is given to the use of alkali-activated materials (AAM) as a binder in substitution for Portland cement (PC). This review discussed the properties of fresh and hardened RAP in cementitious matrices, as well as changes in the microstructure. The biggest challenge on the use of RAP on both systems, RAP-PC and RAP-AAM, is the bond issues caused by the presence of asphalt on the surface of the aggregates. Some researches addressed how physical or chemical pre-treatments to the RAP could improve the adherence to the paste, but few studies focused on the optimisation of the binder. A literature survey indicated that an optimised mix design, durability studies and life cycle assessment (LCA) are important research needs towards the development of RAP-AAM. Despite the lack of research evidence, RAP-AAM is a promising solution for foundation pavement layers.

Introduction

The road infrastructure comprises 21 million kilometers worldwide [1], and the related industrial sectors are said to be responsible for 24% of the global Greenhouse Gas (GHG) emissions worldwide [2], [3]. Natural aggregates are a non-renewable source and a major component of pavements, any small recyclability actions may significantly reduce the environmental impact of the infrastructure sector. Currently, pavement sustainability actions comprise the employment of several types of recycled aggregates. The most used recycled aggregates are recycled concrete [4], steel slag [5], waste foundry sand [6], waste glass [7], crushed brick [8] and reclaimed asphalt pavement (RAP) [9].

RAP is a waste produced during the asphalt road rehabilitation; Europe and USA combined produced well over 100 million tons of RAP in 2017 [10]. In Europe, 68% of the reclaimed asphalt is reused in asphalt mixes, 19% is used as granular materials in unbound layers, 1.25% finds application in other civil engineering projects and 11% ends on landfills [10]. Most studies recommend the use between 20 and 50% of RAP as an aggregate replacement; in the US, many of the State Department of Transportation (DOT) agencies limit the RAP content to 15% to avoid variabilities in the hot mix asphalt [11]. Zhang et al. [12] presented a detailed review of the production process and performance of RAP-containing asphalt hot mixes. In Belgium, RAP also finds applications in cement-stabilized and loose bases / sub-bases [13].

Worldwide a relatively high amount of RAP is yet not recycled and therefore disposed of in stockpiles. Other recycling actions are in place, such as the utilisation of RAP as a replacement for natural aggregates in Portland cement (PC)-based materials (mainly concrete). However, the poor adhesion of RAP granules to the cement paste compromise the mechanical and durability properties of the concrete [14].

Alkali-activated materials (AAM) emerged in the last two decades as sustainable building materials to replace PC for some applications. These new binders rely on the alkaline activation of natural materials (mainly clay and metakaolin) and industrial residues or wastes (such as pulverised fly ash, ground granulated blast furnace slag, mining residues, among others). AAM has been studied as stabilisers in base-subbase layers and more recently employed in combination with RAP aggregates [15], [16]. Although the results appear to be promising, the literature on this subject is still scarce. To the author’s knowledge, there is no publication addressing the advantages and shortcomings of alkali-activated materials containing RAP aggregates (RAP-AAM).

Most of the researches on sustainable materials for pavement sections focus on the asphalt layers and undermine the potential of the foundation layers (base and subbase). The foundation layers are thicker than asphalt layers and therefore, can consume higher volumes of recycled materials. However, the use of larger quantities of recycled aggregates represents, in general, a loss of mechanical properties that need to be compensated with the use of cementitious binders or stabilisers. The most used stabiliser for base layers is PC, a material with a high carbon footprint and therefore not sustainable. One of the possible substitutes for PC is AAM. An adequately designed alkali-activated binder could contribute to a reduction on carbon footprint, high early strength, low water permeability and higher resistance to chemical attacks [17], [18]. However, designing the alkaline-activated binder is not an easy task, especially when aspiring for sustainability: the type and amount of raw materials must be carefully selected. Miranda et al. [19] performed a test track using AAM for the soil stabilisation of base layers and observed an equivalent mechanical performance to traditional binders (PC and lime). The authors suggested that the mix design for this kind of application must be further optimised and recommended the use of solid activators.

Recently there has been a significant rise in publications investigating construction materials with high RAP content and the use of alternative binders (AAM). The use of RAP-AAM could reduce the environmental impact of the construction industry as well as the costs. The main question to be answered is whether RAP-AAM is a sustainable and suitable material for pavement layers, particularly base layers.

This review focus on the use of RAP as an aggregate replacement in cement bound pavement sections and the use of alkaline binders as alternative cementitious systems. We provide an overview of what has been published in the literature, the existing knowledge gaps and available opportunities. Some of the topics discussed herein are:

• How the research of RAP in cement bound pavement layers has evolved over time.

• Which AAM is more suitable for road application.

• Which methodologies are the most used to study RAP-AAM.

• How the performance of RAP-AAM compares with the currently used practice.

• How RAP-AAM is related to sustainability

After this introduction, Section 2 presents the state of the art of the subject. It will start with the main properties of RAP, how it is being investigated as an aggregate replacement for pavement layers, and the properties of RAP in PC matrices (fresh, mechanical and durability). The use of alkali-activated binders as PC replacement will also be introduced in Section 2, followed by the main differences between systems with high calcium content and low calcium content, and the extent of the research so far in RAP-AAM. Section 3 debates the literature findings and proposes future research needs, and Section 4 presents the conclusion.