Concerns about virgin aggregate sources and increasing demands for construction materials of transport infrastructures as the key parameters in development are the most important reasons, which convinced pavement engineers to develop new methods in order to use higher amount of recycled asphalt pavement (RAP). One of the common methodologies to produce mixtures containing RAP is foamed bitumen mix (FBM). In addition, according to previous research studies, incorporating various types of fibers and hydraulic binders such as cement could significantly improve the mechanical performance of mixtures. The present research study evaluated FBM containing 100% RAP and two types of fiber and Portland cement. Dynamic modulus, unconfined dynamic creep compression, and indirect tensile strength were evaluated in the laboratory at optimum moisture content, which was investigated in this research. Both types of fiber and cement proved to enhance specific properties of mixtures.
Cold recycling method is a widely accepted strategy among pavement industries with regard to the lower environmental effects due to reduced emissions, decreasing amount of virgin aggregates, and economic benefits. In addition, the possibility of using high amount of recycled asphalt pavement (RAP) made this method even more interesting for pavement communities; however, some production and mechanical performance considerations still need to be cleared.
As the production temperature of cold-recycled mixes is lower than conventional methods, an agent is required to improve the workability of bitumen during mixing and compaction process. On account of that, foamed bitumen (FB) or bitumen emulsions are currently used, often in combination with Portland cement. The application of foamed bitumen was first modified and presented by Mobil Australia in 1968 and used in cold in place pavement strategy since then [
Design and curing process of foamed bitumen mixtures can highly affect the final performance of the mixture. For that reason, optimum foamed content (OFC) and optimum moisture content (OMC) must be carefully considered through mechanical laboratory tests [
Previous studies on inclusion of fiber in hot asphalt mixtures presented several improvements in performance of mixtures such as dynamic modulus, moisture susceptibility, and permanent deformations [
According to the abovementioned background, this research study aimed to further investigate the mechanical performance of foamed bitumen mixtures containing 100% RAP and application of two types of polypropylene fiber and Portland cement as active filler. Laboratory tests included two parts; in the first part, optimum content of foamed bitumen (FB) on the optimum moisture content (OMC) was determined while the second part, based on the obtained results, studied the effect of two types of fibers and Portland cement on mechanical characterization of FBM.
The aggregate used in this study was RAP, which was sourced from a local asphalt plant in northern Italy. The classification of materials was based on 20 mm maximum aggregate size (MAS) and followed the foamed bitumen mixtures recommendations provided by South Africa Asphalt Academy (TG2) [
Basic characteristics of RAP materials.
Parameter | RAP | RAP + filler |
---|---|---|
Optimum moisture content (%)1 | 6 | 6.5 |
Maximum dry density (Kg/cm2)1 | 1960 | 2155 |
| 2.57 | — |
Residual bitumen content (%) | 5% | — |
RAP gradation.
The virgin bitumen used to produce the foamed bitumen was classified 70–100 dmm penetration grade [
Characteristics of bitumen.
Pen | R&B (°C) | | Viscosity @ 160°C (Pa s) | Viscosity @ 135°C (Pa s) | Foaming temperature (°C) | | | H- |
---|---|---|---|---|---|---|---|---|
75 | 46 | −1.4 | 0.175 | 0.500 | 160 | 3 | 14 | 2 |
Two types of polypropylene fiber were used in this study, namely, F1 and F2 (Figure
Properties of fibers.
Code | Type | Form | Specific gravity | Length (mm) | Decomposition temperature (°C) | Tensile strength (MPa) |
---|---|---|---|---|---|---|
F1 | Polypropylene | Mesh | 0.91 | 20 | 160 | 350 |
F2 | Monofilament | 0.91 | 15 | 160 | 450 |
Polypropylene fibers.
The experimentation was divided into two main parts as mentioned before.
The first part is dedicated to find the optimum FB content of mixtures on 75% of OMC and also to reassess the OMC for identified FB. On one hand, OMC (FB) was assessed while the 3.0% FB was kept constant to verify the moisture content at 50%, 75%, and 100% of the OMC which was previously obtained from the Proctor test on RAP. Then, the OFB content was evaluated considering 1.5%, 3.0%, and 4.5% of foamed bitumen (by the weight of RAP) on constant value of OMC.
The second part of the experimental campaign focused on the evaluation of the effect of two types of fibers (fiber content ranged from 0.30% to 0.075% by weight of RAP) and 2.0% of Portland cement. Improvements were first analyzed separately while the combined effects of fiber (0.075% F2) and cement were studied at a later stage.
A laboratory asphalt foaming unit and a twin-shaft pug mill mixer was used to produce the cold foamed bitumen (foaming bitumen temperature of 160°C) with 3% of foaming water by mass. RAP material was put in a climatic chamber at 25°C prior to mixing. In addition, Proctor test has been used to determine the OMC for RAP optimal compaction. Based on the Proctor hammer results, dry density and OMC of mixtures with filler were, respectively, 2150 kg/cm3 and 6.5%; however, the present research adopted 75% of OMC value as the final moisture content of mixtures based on outcomes from previous studies [
Prior to compaction, RAP and filler (and in second part of study Portland cement) were mixed together for one minute using the abovementioned mixer. When included, fibers were then added and the mixing process continues for another minute to produce a homogeneous mixture. Prior to foaming bitumen, water was added to the RAP and mixed for one minute. Finally, foamed bitumen was sprayed in the mixer directly onto the aggregates and mixing process was continued for one minute. To prepare the Marshall samples, 75 compaction blows at
The obtained samples were identified by an ID which described the presence of filler (indicated by letter F), percentage of foamed bitumen (FB), moisture content based on OMC, type and amount of fiber (F1 or F2), and cement content. For example, the mixture ID containing filler, 3.0% of FB, 75% of OMC, 2.0% cement, and 0.075% (by the weight of the RAP) of fiber type 2 will be F3075 + 2% CEM + 0.075F2.
The entire laboratory investigation was based on volumetric and mechanical tests. Volumetric characteristics were assessed by calculating the void content [
In order to evaluate the effects of bitumen content, moisture, types of fiber, and cement content on the mechanical performance of mixtures, laboratory tests were conducted on three replicates for each test. Dynamic modulus at 25°C [
Moisture and foamed bitumen content affect mixtures as depicted in Figures
Effect of OMC content on mixtures: (a) voids, (b) stiffness, (c) UAS, (d) CSS, and (e) ITS. E: Stiffness Modulus.
Effect of FB content on mixtures containing 75% OMC: (a) voids, (b) stiffness, (c) UAS, (d) CSS, and (e) ITS. E: Stiffness Modulus.
It is important to note that foamed bitumen mixes exhibited very high voids and normally encompasses void values between 9 and 16%. In fact, the higher void contents were observed to FBM without any quantity of filler and generally the higher the FBC the higher the void content [
On the other hand, it should be noticed that differences in void content values on mixes are lesser than one percentage point. For example, varied results may be influenced by the high void content of specimens and the standard method for determining the bulk density which was the conventional one for HMA [
The results of void content and dynamic modulus for mixtures containing three different contents (0.075%, 0.15%, and 0.30% by weight of RAP) of two types of fiber (F1 and F2) are shown in Figure
Effect of fiber types on (a) voids, (b) stiffness, (c) UAS, (d) CSS, and (e) ITS. E: Stiffness Modulus.
Figures
Based on ITS results of mixtures (Figure
Generally, based on the overall results of previous tests, mixtures containing 3.0% of foamed bitumen, 75% of OMC, and 0.075% content of F2 (F3075 + 0.075F2) have been selected in order to evaluate the effect of Portland cement as active filler on the performance of the mixtures.
As illustrated in Figure
Effect of cement on (a) voids, (b) stiffness, (c) UAS, (d) CSS, and (e) ITS. E: Stiffness Modulus.
The effect of cement on creep performance of mixtures can be seen in Figures
Figure
The research was conducted to evaluate the effect of foamed bitumen content, moisture, two types of fiber, and cement as active filler on the performance of cold-recycled mixtures containing 100% of RAP. The laboratory tests included dynamic modulus, unconfined creep compression, and indirect tensile strength tests; based on the results, the following conclusions can be drawn: The preliminary laboratory tests on optimum content of foamed bitumen in mixtures with 100% RAP showed that samples containing 3.0% FB had greater stiffness with 75% OMC. It should be noticed that the aforementioned moisture content was also selected according to previous research. Both types of polypropylene fibers increased the mechanical performance of recycled mixtures. Mixtures containing 0.075% F1 and 0.15% F2 exhibited greater stiffness (12% and 16% greater than samples without fibers), while samples containing 0.15% of both fibers displayed the greatest resistance to rutting (i.e., lowest creep strain slope). 0.30% F1 and 0.075% F2 provided greater values of ITS although F2-mixtures performed better (up to 15% greater than control mixture). Based on the results, addition of 2.0% of cement could significantly increase the mechanical properties of mixtures such as dynamic modulus (up to 150%), tensile strength (up to 53%), and improved permanent deformation (7% lower creep strain slope and 56% lower ultimate axial strain). In case of adding polypropylene fiber to samples containing cement, the values were greater where tensile strength increased up to 68% and stiffness increased up to 180%.
Finally, the laboratory investigation showed that inclusion of polypropylene fibers and cement in foamed bitumen recycled mixtures containing 100% RAP can significantly increase specific properties of mixtures although further investigation should be conducted to evaluate the interaction of fibers and active fillers in mixtures.
The authors declare no competing interests regarding the publication of this article.