1. Introduction
The durability of an aggregate used in a road structure can be defined as the capacity of the material to be stored over time and to preserve its initial characteristics, in particular, its particle size distribution vs. various stresses (since its treatment until the destruction of the road) [
1,
2]. This definition involves three key concepts that should be specified, namely the conservation of the particle size distribution, the life cycle of material and the defects by solicitations present within the material [
3,
4,
5].
An unbound granular mixture is an assembly of grains of different sizes and nature that touch each other at different points of the surface and are separated by intergranular spaces filled, with either free water or capillaries or air [
6]. Each phase is characterized by distinct properties that influence the overall behaviour of the material [
7].
In the presence of stresses within the material, a part of the stresses is concentrated in the liquid phase (in the form of interstitial pressure), whereas the other part is concentrated in the solid phase and, in particular, in certain zones such as the contact points between grains, areas of weakness of the rock (cracks, pores, etc.), or at the water–solid interface [
8,
9,
10]. If these stresses grow, they can cause fractures within the grains as well as their polishing, which results in their decomposition and the formation of fine particles [
11]. Therefore, this causes a change in the granular structure of the material that often results in the loss of geotechnical characteristics [
9,
12], not only of the layer concerned but also of the entire structure [
13].
The conservation of the particle size distribution is therefore an important factor in the durability of an aggregate [
14,
15]. This is why the majority of sustainability tests provide results in the form of fines produced during these tests [
16]. The presence of fines within a granular material can have several positive or negative effects depending on the content on the following three parameters [
17,
18,
19,
20,
21]:
(a) The compactness, the bearing capacity of the material as well as the resistance to the different types of deformations: the fines tend to fill the intergranular spaces during the compacting operation [
22,
23]. This improves the density of the material and therefore its compactness. Therefore, from a certain content (between 20–25% depending on the initial porosity of the coarse material), the addition of fines generates a reverse phenomenon, namely a decrease in density [
24]. The fines no longer occupy only the interstices but they separate the large elements and the initial porosity of the material becomes more and more important [
25]. It should be noted that the specifications allow a fines content of up to 9% for load bearing layers such as the foundation.
(b) Permeability: the fines present in the interstices reduce the permeability of the material by filling the pores [
24]. However, as long as their mass percentage remains moderate, the water continues to circulate in the interstices without much problem. On the other hand, from a certain value (related to the sieving curve of the material), the fines favour the accumulation of the water in the material rather than the circulation of the latter. Thus, as already mentioned by Casagrande [
26], the criterion concerning the content of elements with a diameter of less than 20 μm should be less than 2%. Currently, on the basis of the different experiments carried out since then and for practical reasons (the mechanical sieving stops at 63 μm), the majority of the specifications limit the content of fines (Ø < 63 or 80 μm) to a lower or equal value to 7% for layers with draining role, such as the sub-foundation.
(c) Frost sensitivity: in case of frost, stationary water freezes more easily than moving water. The transformation from liquid to solid state causes an increase in the volume (swelling). This swelling is characterized either by an uplift of the upper structure or by an increase of stresses on the different materials [
27]. This therefore facilitates the breakage and the production of fines and accelerates the phenomena of mechanical degradation of the material. A non-draining and therefore rich in fines structure accelerates the propagation of frost and thaw fronts.
These considerations are of particular concern when using recycled concrete aggregates (RCA) which are more and more used in road foundations [
27,
28]. RCA are composed of a mix of natural aggregates and adherent hardened cement paste [
29,
30,
31,
32]. The latter is usually much more porous than natural aggregates [
33,
34] and leads to a large water demand which makes RCA less easy to recycle into concrete [
1,
35,
36,
37]. Properties of RCA such as water absorption and porosity can deeply influence the fresh properties of concrete as well as mechanical properties and durability of concrete made with RCA [
16,
38,
39,
40,
41]. Over the past decades, many researchers have investigated the possibility of using C&DW materials in the road base and sub-base [
27,
42,
43,
44,
45,
46,
47]. They confirm that the properties of recycled aggregates could influence the performances of unbound road layers [
27,
43,
48,
49,
50].
Molenaar and van Niekerk [
45] investigated the effects of gradation, composition and degree of compaction on the mechanical characteristics. The results showed that crushed recycled concrete and masonry rubble can be used to produce good quality road bases. The results indicated that the composition, particle size and degree of compaction have a strong influence on the mechanical properties of recycled materials, but the degree of compaction is the most important influencing factor. This was an important conclusion for construction in practice, since the degree of compaction is much easier to realize and to control than the other parameters.
Park [
46] investigated the characteristics of RCA as base and sub-base materials. The results showed that RCA can be used as alternative materials to crushed natural aggregates for roadways. The stability and shear resistance of RCA in wet conditions were lower than in dry conditions, and the reduction rate was comparable with that of natural aggregates. In addition, the deflection of the RCA section was similar to that of natural aggregates section in the field.
Poon and Chan [
47] presented the feasible use of RCA and crushed clay brick as unbound road sub-base materials. The results demonstrated that the use of 100% of RCA decreased the maximum dry density and increased the optimum moisture content of the sub-base materials compared to those of natural sub-base materials. The California bearing ratio (CBR) value of sub-base using crushed clay brick was lower than that with RCA. Nevertheless, the soaked CBR value for all recycled sub-bases were 30% greater than that of the minimum strength requirement in Hong Kong.
Leite et al. [
44] evaluated the composition and compaction influences on the mechanical behaviour of road base and sub-base layers based on the C&DW materials. The CBR value and the resilient moduli obtained with recycled C&DW materials were similar to those obtained with natural aggregate commonly used in road construction.
Barbudo et al. [
42] studied the relationship between different constituents of recycled aggregates and their mechanical behaviour for the road application through a statistical analysis. The results showed that the correct selection of materials and the removal of impurities in a plant with pre-screening and double crushing are important to improve the quality of recycled aggregates. They concluded that recycled aggregate can be used as sub-base materials in roadways.
Jiménez et al. [
51] evaluated the behaviour and environmental impact of recycled aggregates from selected C&DW in field conditions. The results demonstrated that both recycled aggregates (4/40 mm, recycled concrete aggregate and recycled mix aggregate) were of good quality and met all limits. The static plate load test showed an excellent bearing capacity in both structural layers. In addition, the use of recycled aggregates in unpaved rural roads did not present a risk of environmental contamination to ground and underground water.
Arulrajah et al. [
11] investigated the possibility of using RCA, crushed brick (CB), reclaimed asphalt pavement (RAP), waste excavation rock (WR) and recycled glass in unbound pavement base/sub-base applications. RCA, CB and WR were found to meet the physical and shear strength requirements for aggregates in pavement base/sub-base applications. RAP and recycled glass have to be blended with higher quality aggregates to further enhance their physical and strength properties, particularly the Los Angeles abrasion and California bearing ratio in order to meet road authority requirements.
Sangiorgi et al. [
52] focused on the development of the stiffness of recycled materials during construction, as well as how it modified over time. An experimental embankment with four sections of different recycled materials was tested and fields were made from two structural layers, forming a homogenous thickness of about 80 cm. The structural performance of the embankment was determined using different types of lightweight deflectometers. The results showed that recycled aggregates performed well when properly compacted and may showed some positive self-cementing properties.
Silva et al. [
53] presented the representative case studies of several applications undertaken in several countries worldwide, namely recycled aggregates in unbound in road and building constructions. For some time now, there have been several real-scale road applications comprised of recycled aggregates, the experience of which has indicated the material’s technical and economic viability.
Finally, some authors investigated the permeability of mixtures prepared with varying RCA contents [
13]. The results showed that the use of 75% RCA allowed permeability values closer to those obtained in natural aggregates. However, the influence of fine particles into recycled aggregates on the performance of road foundation is still limited [
27,
43,
54], in particular the shape and grading during the construction procedure and weathering ageing have not been previously reported [
34,
43].
This study focuses on the recycled fine C&DW specific characterization for exploring the effect of recycled fine materials in road construction. The main objectives of this study are twofold: firstly, the influence of treatment in recycling plants on the properties of recycled aggregates has been explored. Afterwards, an experimental road foundation has been built and material has been excavated for analysing the evolution of particles and materials induced by the construction process and environmental ageing. In addition, special attention has been provided for the shape analysis of fines.
2. Materials
Recycled aggregates have been collected from FEREDECO, Fernelmont, Belgian recycling plants for analysis and characterization [
55]. Four types of samples were investigated, specifically fine particles.
(1) Raw materials (M84). Four raw materials (noted as M84A, M84B, M84C and M84D, respectively) were directly collected from four different recycling sites. The quantity of each raw material was about 20 m
3 and the origin of the raw material was unknown. A pre-treatment operation was necessary before sampling. This operation consisted of eliminating the fraction greater than 100 mm by means of a comb screen mounted on a mobile unit of treatment. In order to facilitate manual sorting on the different raw samples, a sieving separation was first performed on each of them. The sample was fed on a conveyor belt equipped with a magnetic pulley at its end which removed the ferrous metal elements from the flow of material [
56]. Then, a double pass was provided on a double-stage vibrating screen in order to obtain four particle size fractions in order to carry out the visual characterization: fraction (+40 mm); fraction (−40 + 16 mm); fraction (−16 + 4 mm); and fraction (−4 mm).
Four samples referenced M84Af to Df (these represent the fraction under 4 mm) were collected from materials M84A to D, respectively.
(2) Recycled materials after treatment (M37). Four recycled materials (noted as M37A, M37B, M37C and M37D, respectively) were produced by means of a crushing treatment of M84 samples (M84A, M84B, M84C and M84D, respectively) after decontamination [
56]. The main steps of a C&DW recycling facility are as follows [
57]:
- (a)
Scalping: pre-screening in order to separate fine particles and soil before crushing;
- (b)
Crushing (primary and eventually secondary);
- (c)
Iron extraction by electromagnetic strip;
- (d)
Manual extraction of impurities;
- (e)
Screening to the desired fraction.
Four samples referenced M37Af to Df (these represent the fraction under 4 mm) were collected from materials M37A to D, respectively.
(3) Recycled materials after compaction (CRR11). Experimental road foundation (10 × 3 × 0.3 m) was built with M37A (0/31.5 mm) and material was excavated after one week for analysing the evolution of particles and materials induced by the construction process. The treated recycled material M37A was spread by means of a blade over the entire surface of the board (over a thickness of around 30 cm) with the incorporation of temperature sensors at different depths. The compaction of the layer using a BOMAG BW 213D compactor (Bomag, Hamme, Belgium) according to the method commonly used on site by the contractor, namely four passes with a low amplitude vibrating roller and two passes with a non-vibrating roller in order to close the layer. CRR11f refers to the fraction lower than 4 mm from CRR11.
(4) Recycled materials after freeze–thaw cycles (CRR22). These materials were the same as CRR11 but were exposed outside without cover after 6 months’ natural environmental ageing and 17 recorded freeze–thaw cycles (December 2014–May 2015, Belgian climate conditions). The lowest recorded freeze and highest thaw temperature were −4 °C and 12 °C, respectively, according to the sensors on the layer’s surface. CRR22f refers to the CRR22 fraction under 4 mm.