1. Introduction
Economy, sustainability, and safety carry high profiles nowadays in transportation engineering, and pavements, the inevitable component of most transportation infrastructures, are immediately engaged with all these factors. The majority of pavements around the globe are surfaced with asphalt, and among the different types of which, hot mix asphalt (HMA) prevails in volume [
1,
2]. However, the use of HMA currently raises a lot of questions due to its environmental costs, as does the matter of sustainability, which applies to almost all types of construction materials [
3,
4].
The production of HMA consumes a huge amount of energy to heat the two main asphalt ingredients, aggregate and bitumen. Additionally, the process emits pollutant gases, resulting in the categorisation of HMA within environmentally costly products [
5]. Therefore, researchers try to alleviate the issues by developing new technologies to reduce production temperature and emittance. Recent works have shown merits in cold bitumen emulsion mixtures (CBEMs), a type of cold asphalt mixture family. The advantage of a CBEM is that it is produced, mixed, and compacted at normal ambient temperature [
6]. However, the mechanical properties of CBEM are inferior to those of conventional HMA. CBEMs have a low early life strength and high porosity [
7,
8,
9]. Such disadvantages result in there being little interest in substituting HMA with a CBEM. Several methods have so far been examined by researchers to develop and advance this technology, including the use of filler types [
10,
11,
12,
13,
14,
15], advanced polymers [
16,
17,
18,
19], compaction energy [
10,
20,
21,
22], heating techniques via microwave [
23,
24,
25], polymers [
18,
26], and crumb rubber [
27,
28]. While these methods show a slight modification in volumetric properties, other properties have shown better improvement.
Researchers have been trying to apply affordable techniques, such as heating, to control the porosity of CBEMs in comparison to that of conventional HMA without leaving negative effects on the other improved characteristics of CBEM. Al-Busaltan et al. [
23] found that subjecting a CBEM to microwave heating up to 100 °C had a significant impact on the engineering properties of the mixture. The process improved the final product’s resistance to permanent deformation while it decreased porosity to an acceptable level. The CBEM’s water damage and ageing characteristics were comparable to those of conventional HMA and better fatigue characteristics were achieved. Additionally, Dulaimi et al. [
24] concluded that pre-compaction heating led to significant effects such as a reduction in the porosity and mixture sensitivity to water damage and an improvement in the mixture’s early life properties.
The term half-warm bituminous emulsion mixture (HWBEM) refers to the technique of applying post-heating, whether conventional or microwave radiation (the process occurs within a temperature not >100 °C), to a loosened CBEM before compaction. Hence, HWBEM is a method for producing asphalt mixtures at temperatures between 65 and 100 °C [
24,
29,
30,
31]. Mixtures such as emulsified bitumen, foamed bitumen, and modified bitumen with fluxing oil can be made using several kinds of bituminous binders [
32,
33]. According to Van de Ven et al. [
34], a HWBEM can provide comparable monotonic qualities at high temperatures in addition to similar fatigue properties in comparison to HMA. The HWBEM offers a variety of advantages due to its lower production, laying, and compacting temperatures, including but not limited to improved working conditions, lower GHG emissions, less energy usage, a longer paving window, and longer hauling lengths [
35].
Accordingly, this study is aimed at examining the cracking characteristics of the HWBEM. The improvement in volumetrics post-heating is examined through mechanical properties. The investigation is an attempt to cover the phenomena of expected cracking failure in pavements, which will facilitate more understanding of the HWBEM. To date, this subject is rarely discussed in the literature for the CBEM and HWBEM.
3. Test Results and Discussion
Figure 5 shows the density of the studied mixtures. Increasing the AR content in HWBEM samples resulted in some reduction in the density of the HWBEM samples containing AR. These samples contained excess water because this water formed a continuous phase with the AR. This volume of water was released from the mixture during the curing process, leading to a noticeable decrease in the density of these samples if heat was not applied. Likewise, the presence and loss of water had an impact on the air voids in the mixtures, as demonstrated in
Figure 6.
Adding AR to the HWBEM in an amount of up to 2.5% increased the ITS considerably. As shown in
Figure 7, even a small AR content of 1.25% led to a higher ITS when compared to that of HMA, and doubling the AR content caused the HWBEM to perform superiorly compared to HMA. However, any further increase in AR caused a cliff fall in the tensile strength of the HWBEM. It is understood that the addition of AR initially led to the development of some interlocking in the aggregate phase of the mixtures. However, a further increase after the optimum point reduced binder–aggregate adhesion at the binder–aggregate interface.
Observing the trend of changes in failure energy (
Figure 8) confirms the results. Incorporating AR up to the optimal level enhanced the HWBEM’s ability to absorb energy before failing. However, any increase beyond this point resulted in mixture failure at much lower energy levels. A higher failure energy level is directly associated with improved material performance under repeated loading conditions.
Cracking, though itself a type of distress, is the onset of future distress usually related to water ingress. Therefore, retarding crack initiation is vital for pavement performance, durability, and preservation. The energy required for the crack initiation (Gf-CI) of the mixtures in the study is presented in
Figure 9. Heat treatment certainly improved the GF-CI value, but the inclusion of AR up to the optimum value of 2.5% tremendously enhanced the capacity of the material to absorb energy before crack initiation. As another confirmation of the effect of AR within the HWBEM mixtures, any further addition of AR considerably dropped the energy required before any crack initiation.
Figure 10 shows loading–displacement curves for all the study mixtures. It can be noticed that heat treatment was effective at enhancing the maximum indirect cracking load of the HWBEM when compared with that of the CBEM. Adding AR in an amount up to 2.5% certainly improved the HWBEM’s response to loading although it caused the HWBEM-1.25%AR and HWBEM-2.5%AR to be more brittle than HMA and CBEM (associated with the displacement position of
Pmax compared with that of HMA). The viscous phases of HMA were more extensive than those of HWBEMs, possessed an enhanced capacity to dissipate energy and respond to loading, and therefore possessed a higher capacity to retain tensile strength, than that of CBEM.
Figure 11 reports the CT indices found for the studied mixtures. As expected, the
CTindex of HMA shows superior characteristics to that of the CBEM. It is known that the
CTindex for asphalt mixture ranges from 31 to 255 [
55]. Moreover, the HWBEM, after being extensively heat-treated, demonstrates a slight improvement in the
CTindex when compared with that of the CBEM. On the other hand, adding AR in an amount up to led to a continued improvement in the
CTindex. Now, the criterion for an optimum value for the AR content will need to be discussed. As expected enough now, adding AR in an amount greater than 2.5% caused the mixtures to show a little tolerance to cracking. What is understood from the literature is that the
CTindex depends on various factors, namely failure energy, the slope of the post-peak inflection point, and the strain value at 75% of the peak load value. These indices are highly affected by the ingredients of the mixture and volumetric properties. Noticeably, in terms of volumetric properties,
Figure 4 and
Figure 5 demonstrate the lower density and higher air void for the HWBEM containing more than 2.5% of AR. In terms of ingredients, AR extends a reinforced network within the binder (emulsion residue), leading to improved adhesion and cohesion, and ultimately enhancing crack resistance. Inversely, the extra AR predominated the binder materials and affected the role of the binder material, whereas the created networks dispersed the binder, affecting adhesion and cohesion inferiorly.
As mentioned previously, the slope of the curve in the mixture’s post-peak behaviour (m
75) is one of the factors that affect the magnitude of
CTindex; theoretically, the sharper the slope, the smaller the
CTindex. In other words, a sharper slope indicates that the mixture has a weaker ability to tolerate the onset of cracking and less resistance to cracking propagation after the maximum tensile stress is borne. The slope of the mixtures’ behaviour in their post-peak stage was studied and is presented in
Figure 12. However, the previous claim is correct only when comparing the CBEM with HWBEM-0%AR. Inversely, the remaining m
75 values results show higher magnitudes. Moreover, some mixtures with a higher
CTindex are associate with a higher m
75, as for example, in the case of HWBEM-1.25%AR compared with HWBEM-5%AR. Therefore, it would lead to fruitless results to use the m
75 slope index result alone to evaluate the cracking resistance. Its effect must be accommodated inclusively within the
CTindex.
The strain at 75% of the post-peak load can reflect the brittleness of the asphalt mixture and/or crack initiation, whereas the higher value is associated with ductile material. Results in
Figure 13 demonstrate such an indicator, where the hydric filler material in the CBEM produced the secondary binder (hydridic products have brittle characteristics) that controlled the ability of the asphalt binder (primary binder) to be ductile. However, introducing the heat treatment increased the brittleness as a result of extra hydraulic product creation due to the microwave heating process. This is also confirmed by the value of
Pmax and/or ITS, as shown in
Figure 6, when comparing the CBEM with the HWBEM with up to 2.5% of AR. Then, the inferior effect of extra AR and volumetric properties works to weaken the mixture with higher amounts of AR. Nevertheless, a controlled dosage of AR of up to 2.5% facilitates there being greater ductility for the mix due to the polymer-created network that reinforces the binders (both primary and secondary binders). Additional conformation can be confirmed through observing
Figure 8, where the crack initiation energy is negatively affected by heating but AR inclusion rises to 2.5%.
The CRI-index can reveal the normalization of different mixtures against crack resistance, as it is determined by dividing the failure energy (
Gf) over the peak load (
Pmax). Comparing
Figure 7 with
Figure 14, the same trend can be observed, but there is a noticeable variation in the ratio. An explanation for this would be the difference between these mixtures in their responses to loading. The load–displacement curve provides insights into the characteristics of a mixture. While a mixture might exhibit high peak load resistance, it may simultaneously possess a limited capacity to absorb energy. This relationship becomes apparent when considering the area under the load–displacement curve. However,
Figure 14 reveals the significance of AR in extending the energy of failure noticeably with an increase in the peak load with less range, as can be seen with the HWBEM with up to 2.5% of AR.
The toughness index (TI) demonstrates the post-cracking characteristics, or it explains to what extent post-cracking is associated with the total toughness of the material, where the brittle material has a significantly lower value compared with that of the ductile material. However, the highest toughness index obtained for HWBEM-2.5%AR is further proof of the effect of AR (of up to 2.5% in content) in improving the HWBEM’s performance, as can be seen in
Figure 15. Both 1.25% and 2.5% AR contents caused the HWBEM to demonstrate much higher toughness than that of HMA itself. Furthermore, the heat treatment resulted in a minimization of the toughness index, as can be seen when comparing the CBEM with HWBEM-0%AR.