2.2.1. Testing Protocol for Raw Materials Characterization
A series of laboratory tests were performed on CMWR, FA and HRB samples. As unconventional materials with unknown applications in road techniques for Moroccan pavement design, the geotechnical characterization of the CMWR samples were codified based on the [
17] standard. The particle size distribution was obtained by dry sieving method for an element with a diameter greater than 80 μm while a granulometric test by using sedimentation approach was performed for size fraction less than 80 μm following NF EN ISO 17892-4 standard [
18]. The optimal moisture content (
WOPN) and maximum dry unit weight (
ρd OPN) of CMWR were determined using the Proctor test according to NF P94-093 standard [
19]. The specific gravity (Gs) was measured using a helium gas pycnometer. The calcium carbonate (CaCO
3) content measurements within CMWR sample were obtained based on NF P94-048 standard [
20]. The plasticity index (
PI) of CMWR samples was achieved using the Atterberg limits test. The liquid limit (W
L) was measured using the Casagrande cup method while the plastic limit (
WP) was performed by a rolled thread method of NF EN ISO 17892-12 standard [
21]. Regarding the methylene blue absorption test (
MBV), the studied samples were sieved on 5 mm mesh under NF P94-068 standard [
22]. The specific surface area (Ss) of the fine fraction was determined by using the Brunauer–Emmett–Teller (BET) method.
The compressive/swelling behaviour of the CMWR sample was determined using the oedometer test. This later was carried out on 0–20 mm size fraction at the optimal moisture content and the maximum dry density conditions of the standard Proctor test. The saturated specimen was subjected to axial stress levels ranging between 5 and 800 kPa, according to the XP P94-090-1 standard [
23]. The swelling-shrinkage behaviour of clay fraction was assessed using many fundamental parameters such as the total specific surface area (
SST). The
SST value can be calculated based on the methylene blue absorbed value (Vb) by the clay fraction as expressed in Equation (1).
Furthermore, the swelling potential (Sp %) related to the presence of expansive clayey materials was estimated using Lautrin classification. Lautrin [
24] proposed a classification of soil activity (A
CB) based on methylene blue adsorption value. The A
CB is defined as the ratio of 100
MBV to the clay fraction. However, the
MBV value is obtained for the 0–5 mm fraction of the CMWR, including from the granulometric point of view clays, silts and sands. It is then interesting to be able to obtain the
MBV on the clay fraction. To estimate the colloidal behaviour of CMWR, the methylene blue value is calculated for 100 g of clay fraction as follows (Equation (2)):
To assess the sensitivity of CMWR material to fragmentation under mechanical stresses and immersion-drying cycles, the fragmentability (FR) and degradability (DG) coefficients were determined on the 10–20 mm aggregates fraction according to NF P94-066 [
25] and NF P94-067 [
26] standards, respectively. The resistance of CMWR aggregates to degradation by abrasion and wear was evaluated on 10–14 mm fraction using Los Angeles (LA) and Micro-Deval (MD) tests based on NF EN 1097-2 [
27] and NF EN 1097-1 [
28] standards, respectively. The Californian bearing ratio (CBR) which measures the resistance to punching and heavy machines traffic is a fundamental parameter to characterize the strength in the empirical pavement design. The CBR tests namely, the CBR at 4 days of immersion in water CBR(4i) and immediate bearing index (IBI) were determined conforming to NF P94-078 standard [
29]. The IBI was determined for compacted specimens in the CBR mould at both modified and normal Proctor efforts without any soaking in water or overloading. The two parameters CBR(4i) and IBI reflect on the sensitivity to water and the immediate stability of the tested material, respectively.
A direct shear test was performed on a 0–5 mm size fraction of the CMWR material. The test sample was consolidated under drained shear conditions at various confining pressures (50, 100, and 200 kPa) according to NF-P94-071-1 standard [
30]. The unconfined compressive strength (
UCS) and the direct tensile strength (Rt) tests were conducted according to NF EN 13286-41 [
31] and NF EN 13286-40 [
32] standards, respectively. The cylindrical specimens were prepared at the optimum of the standard Proctor references and stored under standardized conditions at room temperature or subjected to water immersion at 20 °C before running tests at 28, 90 and 360 days.
On the other hand, the content of major and trace elements presented in the studied samples were measured using X-ray fluorescence (Bruker, Tiger Model, Bruker, Billerica, MA, USA) and inductively coupled plasma with atomic emission spectroscopy (ICP-AES) (Perkin Elmer Optima 3100 RL, Waltham, MA, USA). The crystalline phases were identified by the X-ray diffraction measurements (Bruker, AXS Advance D8, Bruker, Billerica, MA, USA). The Diffrac Plus EVA and TOPAS software programs (
https://www.bruker.com/products/x-ray-diffraction-and-elemental-analysis/x-ray-diffraction/xrd-software/eva.html) were used to identify and quantify mineral species and abundances, respectively. The total sulfur (S) and total inorganic carbon (C) were determined by induction furnace analysis (ELTRA CS-2000, ELTRA, Haan, Germany). Moreover, the toxicity characteristic leaching procedure test (TCLP) EPA-1311 [
33] was applied to determine the concentration of leached pollutants from the CMWR sample. The samples were prepared by crushed CMWR to pass through a 9.5 mm sieve, while the solutions were separated from the solid phase by filtration through a 0.45 µm. The obtained metal concentrations are then compared with the United States Environmental Protection Agency (US-EPA) limits.
2.2.4. Evaluation of the Designed Mixes for Pavement Applications
The specimens of solidified materials were prepared by blending 0–20 mm size fraction of CMWR with FA, HRB and eventually water. The specific technical criteria should be checked for designing unconventional material in road paving use, among them, immediate stability, water sensitivity and long-term mechanical performance. To evaluate the ability of the designed mixes for capping layer use, the following requirements, as specified in the French technical guide [
35], were adopted:
where, UCS (28 + 32i) is the unconfined compressive strength (MPa) measured after 28 days in standard conditions followed by soaking in water for 32 days at 20 °C, and UCS (60) is the unconfined compressive strength measured after 60 days in standard curing conditions. Moreover, according to the French guide, the pavement application requires the fulfilment of the following immediate stability conditions: IBI > 50 for the base layer and IBI > 35 for the sub-base layer.
For the average daily traffic of heavy vehicles from T5 to T1 (traffic classes) for the case of the foundation layers and from T5 to T2 for the base layers, the use of such material in pavement structure is mainly conditioned by the elastic modulus and direct tensile strength measurements at 360 curing days. It should be mentioned that the elastic modulus (
E) test was not performed in this study, but it was estimated at desired age using the following equation (Equation (3)) which was recommended by ACI 318-95 [
36] for normal weight concert:
where
UCS is unconfined compressive strength (MPa) and
E is the elastic modulus (GPa). This equation is the most suitable to this particle case as it tends to evaluate the elastic modulus taking into account the effect of coarse aggregate type on mechanical properties of CMWR (degradability). The estimated results of E and the measured Rt (direct tensile strength) values at 90 days curing time were reported in a specific abacus to predict the structural class of the designed mixes which must lead at least to the mechanical performance of zone “5” (zone 5 is the minimum structural class required) SETRA-LCPC [
37]. Regarding the pavement layers application, the estimated results of E and the measured Rt values at 360 days ageing were reported in a specific graph to predict the structural class of the tested specimen which must lead at least to S2 class based on the NF P98-113 standard [
38].
Table 3 gives the experimental plan for monitoring the proposed parameters.