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Article

Influence of Anti-Stripping Green Additives on Binder Performance

by
Andrei Forton
,
Adelin Stirb
* and
Paul Marc
Department of Overland Communication Ways, Foundation and Cadastral Survey, University Politehnica Timisoara, 1A Ioan Curea, 300224 Timisoara, Romania
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(5), 4560; https://doi.org/10.3390/su15054560
Submission received: 28 January 2023 / Revised: 23 February 2023 / Accepted: 1 March 2023 / Published: 3 March 2023
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Abstract

:
The purpose of this study was to evaluate the influence of anti-stripping green additives on the behaviour of a fresh binder. Firstly, the adhesiveness (spectrophotometric method) and affinity (rolling bottles method) of a binder with a penetration grade of 50/70 on two types of aggregates (natural quarry aggregates from two different sources) were investigated. The results show a weak bond and stripping between the 50/70 binder and the aggregates. Therefore, two different anti-stripping green additives (named A and B) were used in three percentages. A total of six blends were tested to establish the optimal content of the additive by performing a series of conventional tests combined with the adhesiveness test and the affinity test. Finally, the rheological behaviour of the optimal blends and of the fresh binder was investigated by performing DSR tests. The 0.4% dosage of green additive B was considered the most effective due to the fact that this dosage did not have a significant influence on the conventional properties and rheological behaviour of the 50/70 binder but had a major impact on the adhesiveness (15% increase), affinity after 6 h (13% increase), and affinity after 24 h (25% increase).

1. Introduction

Many research efforts have been expended in recent decades in order to find better methods to extend the life cycle of the bituminous mixtures that are included in the different layers of road pavements. Different strategies have been developed and applied to reduce the use of natural resources and to reduce the environmental impact. Most of them are related to the reuse of reclaimed asphalt pavement (RAP) or other construction materials with different additives or rejuvenators in order to produce new bituminous mixtures, such as HMA (hot mix asphalt (HMA) or warm mix asphalt (WMA) [1,2,3,4,5,6]. However, each strategy or method should be adapted to the region where it will be applied.
Various sustainability measures are increasingly being promoted in the field of transport infrastructure, focusing on the reuse and recycling of existing materials, the use of local materials, and the use of renewable products (bio-binders and green additives). In addition to materials, road pavement construction technologies play an important role in the green roads rating system [7].
Several studies have highlighted the potential of different green products, such as natural fibres, oil palm fibres [8], pulp [9], bio-based polyurethane binder [10], biocrude oil from microalgae [11], and many other by-products derived from renewable and biodegradable sources, which could be used in the production of bituminous mixtures. The use of such by-products could lead to several advantages related to cost, environment, and technical performance. However, comprehensive studies are still needed on this topic in order to evaluate the impact of green additives on the long-term performance of HMAs and WMAs.
For all types of bituminous mixtures, including HMA or WMA with or without RAP, the adhesiveness or affinity between the binder and aggregates is an important characteristic that can have a crucial impact on the service life of road pavements.
Different aspects can affect the binding between the binder and the aggregates, including the physical and chemical characteristics of each material, the nature of the aggregates, the moisture and the form of the aggregates, the porosity, the presence of impurities, and the binder viscosity [12,13,14,15].
As the stripping phenomenon in bituminous mixtures is one of the most important types of road pavement damage, many researchers have investigated this aspect [16]. For example, Aguilar-Moya et al. [17] explained the four individual theories and other combined theories that describe the affinity between aggregate and binder, which are related to a chemical reaction, molecular arrangement, surface energy, and mechanical adhesion. Zhang et al. [18] highlighted that some repulsion interactions can appear between a binder and acid aggregates, which can lead to a partial bond between them.
Many studies have concluded that most of the time, the presence of residual water is the principal factor that leads to a partial bond and, finally, to a failure of the affinity between the binder and the aggregates [19,20,21,22].
However, when WMA is produced, due to the lower fabrication temperature in some cases, residual water is not entirely evaporated during the mixing process, a fact that could lead to stripping phenomena [23].
In order to increase the bond/adhesiveness/affinity between binders and aggregates, many products have been used and investigated in different studies: Amines, silanes, rubbery polymers [24], chemical anti-stripping products [25], fillers [26], and nanomaterials [27].
Therefore, the use of anti-stripping agents or additives of different origins in the production of bituminous mixtures has become a common approach, with similar goals to those of other eco-friendly methods related to the reduction of production temperatures, better workability, and energy saving.
An additive can be considered as being an anti-stripping agent only if it has an ‘activation’ effect (active adhesion) on the binder, by inducing the ability to eliminate the water from the surface of the aggregates [28].
Some comprehensive studies were conducted by several authors, who investigated the beneficial effect of anti-stripping additives on the behaviour and mechanical performance of bituminous mixtures. Junior et al. [29] found that the use of anti-stripping additives in the production of bituminous mixtures leads to an improvement in fatigue life and water resistance. Additionally, Yousefi et al. [30] concluded that the use of the proper dosage of anti-stripping additive led to an increase in the rutting, cracking, and moisture damage of warm mix asphalts produced with RAP material and recycling agents.
The interface between the binder and the aggregates and the wetting behaviour of the binder–aggregate interface were investigated in terms of mechanical properties by Liu et al. [31], Zhang et al. [15], and Boulange et al. [32]. The conclusions of these studies highlighted that the interface depends on several physical and chemical factors, but the greater impact on the shipping damage corresponds to the characteristics/properties of the aggregates.
Furthermore, Maia et al. [16] showed that the use of adhesion promoters contributed to an improvement in the permanent deformation resistance and better resistance to the moisture damage of bituminous mixtures.
On the other hand, several studies evaluated the effects of different anti-stripping additives on binder performance. Hossain et al. [33] highlighted that 0.5% of an amine-based liquid anti-stripping additive can have a positive effect on the low temperature and fatigue resistance of a modified binder, without reducing its viscosity, but with a significant impact on the high-performance temperature grade of the aged binder.
Another study by Maciejewski et al. [34] investigated the effect of a liquid anti-stripping additive (adhesion product for hot and warm mixes) on two binders, with penetration grades of 35/50 and 50/70. The conclusions of this study highlight that this additive caused a significant increase in the penetration values of the binders, without an effect on the softening point, and with a small decrease in their dynamic viscosity. In particular, the additive had a major impact on the 35/50 binder, causing a significant increase in the non-recoverable compliance and some important changes in the high-temperature stiffness.
Ameli et al. [35] studied the influence of three anti-stripping additives, ground tire rubber, and waste polyethylene on the behaviour of an 85/100 pen. grade binder and the corresponding bituminous mixtures. For all binders, the addition of the anti-stripping additives led to a decrease in the penetration values with an increase in the softening point values, viscosity, and rutting parameters. An improvement in the fatigue life of the binders was also observed.
However, it is not very clear whether the anti-stripping additives improve only the binding between the binder and aggregates and the moisture resistance of the bituminous mixtures, or whether they have a positive or negative effect on the binder performance (mechanical characteristics and rheological behaviour).
Therefore, it is important to investigate how an anti-stripping additive impacts the behaviour of the binders.
Therefore, the purpose of this study was to evaluate the influence of anti-stripping green additives on the mechanical and rheological behaviour of a fresh 50/70 binder. Two different anti-stripping green additives (named A and B) were used in three percentages, 0.2%, 0.4%, and 0.6%, by the weight of the binder. A total of six blends were tested in order to establish the optimum content of the additive by performing a series of conventional tests (penetration, softening point, Fraass breaking point, and elongation tests), combined with the adhesiveness test (spectrophotometric method) and the affinity test (rolling bottles method). Finally, the rheological behaviour of the two blends and of the fresh binder was investigated by performing the DSR test. More details are provided in the following sections.

2. Materials

2.1. Binder

A straight-run binder with a penetration grade of 50/70 was used as a fresh binder. A series of conventional tests were performed on the fresh binder in order to verify whether this material meets the requirements specified in the Romanian Norms. Therefore, penetration tests at 25 °C (SR EN 1426), ring and ball tests (SR EN 1427), elongation tests (SR 61), and Fraass tests (SR EN 12593) were performed. The RTFOT ageing by hardening under the influence of heat and air (RTFOT method) was then performed, and the mass variation, the residual penetration, the increase in the softening point, and the elongation after the RTFOT ageing method were investigated. All results are reported in Table 1, together with the calculated penetration index and the requirements of the norm SR EN 12591.
As can be observed from the results listed in Table 1, the fresh binder is a straight-run 50/70 penetration grade binder that meets the requirements imposed by the norms; therefore, it can be used in the production of bituminous mixtures.

2.2. Aggregates

In Romania, 0–4, 4–8, and 8–16 fractions of quarry-crushed aggregate, 0–4 fragments of natural sand, and limestone filler are commonly used as aggregates to produce bituminous mixtures with a maximum aggregate size of 16 mm.
One of the main objectives of this investigation was to evaluate the adhesion and affinity between the tested binder and two types of natural aggregates. In the study, the authors used 4–8 fractions of crushed aggregates from two different natural stone quarries (named X and Y) in Romania.
Since the rock component of a bituminous mixture makes up approximately 95% of its total mass and 80–85% of its volume, the properties of the aggregates are crucial during the design stage [36].
The Romanian Norm AND 605 clearly outlines the specifications that each aggregate fraction must meet in order to be used in the production of bituminous mixtures. These specifications are related to the size, shape, and granularity of the granules, as well as the type of rock that is used.
The main characteristics of the 4–8 fractions from the two natural stone quarries were investigated in the laboratory and the results are reported in Table 2.
The results listed in Table 2 show that the 4–8 fractions of crushed quarry aggregates meet the Romanian Norms specifications and can be used in the production process of bituminous mixtures.

2.3. Anti-Stripping Green Additives

To increase the bond between the binder and the aggregates, in this study, two different anti-stripping green additives (named A and B) were used. These two green additives were used to ensure a better cover of the aggregates with the binder and to increase the adhesion and the affinity between them, which will finally lead to an improvement in the workability of the bituminous mixtures during the production stage.
Both additives are products made by mixtures of different oils and different amino derivatives, reasons for which they were considered anti-stripping green additives. The density and viscosity at 20 °C were determined in the lab for each additive. The results are reported in Table 3.

3. Methodology

As previously described, first, the adhesiveness (spectrophotometric method, nondestructive method) and affinity (rolling bottle method) of the 50/70 penetration grade binder to the two types of aggregates were investigated.
To increase the adhesion between aggregates and the binder, two different anti-stripping green additives (named A and B) were used in three percentages (0.2%, 0.4%, and 0.6% by the weight of the binder).
A total of six blends were tested to establish the optimal content of the two additives by performing a series of conventional tests: Penetration, softening point, and elongation tests. These tests were combined with the adhesiveness and affinity tests performed on the samples produced between the 6 blends and the two types of aggregates (quarry X and Y). A total of 12 spectrophotometric and affinity tests were performed.
For ease of use, the blends were named based on the fresh base binder penetration grade, as well as the percentage and name of the anti-stripping green additive (for example, 50/70 + 0.2% A).
Finally, the rheological behaviour of the optimal blends and of the fresh binder was investigated by performing DSR tests. The experimental campaign is presented in Figure 1.
The procedure used to produce the above-mentioned binders was selected in order to reproduce in the lab the actual production process of an asphalt mixture with a 16 mm maximum aggregate size containing an anti-stripping green additive.
Therefore, the fresh binder was heated to 145 °C (this temperature was chosen with respect to the Romanian technical specifications for the production of bituminous mixtures) for 45 min. Before blending, the additives were not heated. To simulate the actual industrial process of producing bituminous mixtures, the additive and fresh binder were manually mixed for two minutes. In this process, the anti-stripping additive is added to the fresh binder tank when all of the components (aggregates and fresh binder) are heated before mixing. The blend was then used to produce the specimens for all tests.
Regarding the methods used to investigate the properties of the six blends, it should be mentioned that the conventional tests were performed according to the European Norms, as follows: Penetration test at 25 °C (SR EN 1426), ring and ball test (SR EN 1427), elongation test at 25 °C (SR 61), Fraass test (SR EN 12593).
Concerning the procedure used to produce materials for the adhesiveness test (the spectrophotometric method, the non-destructive method, SR 10969), first, from the 4–8 fractions of quarry-crushed aggregates a sample of approximately 500 g of 6–8 fractions was washed on the sieve until the washing water became clean, and then washed with distilled water and dried in a thermostatic oven at 110 °C to constant mass [37]. Secondly, 100 g of the aggregate sample, prepared as previously mentioned, was heated in the thermostatic oven at a mixing temperature (145 °C) for 2 h. From the binder sample prepared according to the procedure described above, 5 g of the binder was weighed in a mixing container and kept in the thermostatic oven for 1 h at the mixing temperature.
This procedure was followed according to the specification of the Romanian Norm SR 10969.
The aggregate sample was added to the binder sample and manually mixed with a spatula. The mixing process was performed on a sand bath heated at the mixing temperature.
After homogenization, the container was removed from the sand bath. The mixing process continued until the binder film no longer flowed from the surface of the natural aggregate. The mix was then transferred to another recipient and cooled at room temperature for 1 h. The mix was kept for 22 h in distilled water at a temperature of 25 °C. Before starting the test, the mix was introduced in a water bath at 25 °C for 1 h. Two mixes were prepared for each spectrophotometric test. Additionally, for each test, an aggregate sample prepared as previously described (uncoated with a binder) was tested.
To perform the spectrophotometric adhesiveness test, some preliminary operations should be performed in order to plot the calibration curve of the dye solution. A dye solution was prepared using distilled water with a concentration of 0.02% by weight of 0.2 g of 4G red pigment, introduced into a recipient of 1000 cm3 volume. This solution was homogenized until the 4G red pigment dissolved.
Subsequently, 1 cm3 of the above solution was inserted into a 50 cm3 volume. A similar operation was conducted with 2 cm3, 3 cm3, 4 cm3, and 5 cm3 of the solution. The recipients were filled with distilled water and homogenized. Therefore, five solutions with concentrations of 0.4 × 10−3%, 0.8 × 10−3%, 1.2 × 10−3%, 1.6 × 10−3%, and 2.0 × 10−3% were obtained. Each solution was then successively introduced into the perfectly clean and dry analysis cuvettes of the spectrophotometer.
The absorbances were determined one by one in comparison with those of distilled water at the wavelength for which the absorbance of the dye solution was maximum. The determination of the working wavelength is achieved by measuring with a spectrophotometer the wavelength at which the absorbance value of the 0.1·10−3% concentration solution is at its maximum, using distilled water as a reference. The standard curve was plotted graphically, marking the known concentrations on the abscissa and the extinctions on the ordinate. Finally, a reference line (calibration curve) was plotted, as can be observed in Figure 2.
After these preliminary operations, the spectrophotometric adhesiveness tests were started. A solution with a concentration of 1·10−3% was prepared by weighing 0.01 g of 4G red pigment for each litre of solution. The cuvette of the spectrophotometer was filled with the solution, and the absorbance A0 was determined using distilled water as a reference. This result was introduced into the calibration curve to obtain the corresponding concentration, C.
From the aggregate sample prepared as previously presented, the mineral granules were separated from each other, underwater, and mixed in a circular direction 100 times in 3 min. Aggregate separation and mixing were completed in a maximum of 10 min. The coated aggregate was then left for a few minutes on wax paper to drain the water and then placed on the sieve in the glass vessel of the equipment. A total of 1300 mL of the C concentration solution was added to the test sample in the recipient of the spectrophotometer. The pump was started to recirculate the dye solution and the recirculation was maintained for 90 min. Figure 3 shows the tools used to perform the spectrophotometric adhesiveness test.
After recirculation, with the help of a pipette, the cuvette of the spectrophotometer was filled with dye solution from the glass vessel of the equipment and the absorbance A2 was determined spectrophotometrically, using distilled water as a reference. This result was then introduced into the calibration curve and the corresponding concentration, C2, was determined. A second test sample was tested following the same procedure.
The initial aggregate sample, 100 g without binder coating, was placed on the sieve in the glass vessel of the recirculation equipment, 1300 mL of the C concentration solution was added, and the recirculation was started for 90 min. Finally, the absorbance A1 and the corresponding concentration C1 were determined.
The adhesiveness obtained from the spectrophotometric test was determined using Equation (1):
A = 100 − C − C 2 C − C 1 × 100
where:
  • A—the adhesiveness obtained from the spectrophotometric test, %.
  • C—the concentration of the initial dye solution, %.
  • C1—the concentration of the solution after its recirculation over the natural aggregates, %.
  • C2—the concentration of the solution after its recirculation over the natural aggregates coated with binder, %.
The adhesiveness result was considered as the mean value of two determinations that must not differ between them by more than 3% in absolute value, with an accuracy of 0.01%.
The affinity test was performed according to the rolling bottles method described in the European Norm EN 12697-11. It must be mentioned that this test was performed on aggregate samples with granules from 8 mm to 11.2 mm. For each material, three samples were tested simultaneously. All tests were performed at the same rotation speed of 60 rotations/min.
The test was stopped after 6 h, and the degree of binder coating of the aggregates was estimated by visual observation to the nearest 5%. The test was again started for a total time of 24 h (including the previous 6 h) using the same procedure.
For each rotation time, the degree of the average value of the binder coating of the aggregates obtained on the three samples was calculated and rounded to the nearest 5%.
In order to investigate whether each anti-stripping green additive has an effect or an impact on the rheological behaviour of the final binder, complex shear modulus tests were performed by using a DSR Anton Paar rheometer. The tests were carried out at 6 temperatures ranging from 35 °C to 85 °C (10 °C increment) and at 10 frequencies ranging from 0.1 Hz to 10 Hz. A 25 mm diameter plate–plate configuration and a 1 mm gap were used. All tests were performed at 5% target shear strain. From the test results, the norm of the complex shear modulus |G*| and the phase angle φ were calculated. All results are presented in Section 4.3.1.
The complex shear modulus tests were performed on 3 binders: the fresh 50/70 binder, and two blends between the fresh binder and 0.40% of each anti-stripping green additive A and B by mass of the binder.
To further investigate the rheological behaviour, the steady shear viscosity of the three tested binders was determined from the complex shear modulus test results, which can provide information about the viscosity of the binders.
Therefore, the steady shear viscosity of the tested binders was determined using the same procedure presented by Forton et al. [38]. The results are presented in Section 4.3.2.
Finally, the DSR high critical temperatures were calculated according to AASHTO T315-10 for unaged binders at an angular frequency of 10 rad/s when |G*|⁄sin φ = 1.0 kPa.
The same measurements were then used to obtain the DSR high critical temperatures by using the procedure proposed by Forton et al. [39] in which some linear interpolations were performed between the data at 1.29 Hz and 2.15 Hz, in order to determine the values at 10 rad/s.
Following that, plots of |G*|⁄sin φ values in relation to temperature were created and DSR high critical temperatures were obtained for each tested binder. More details regarding the obtained results are shown in Section 4.3.3.

4. Experimental Results and Discussion

4.1. Preliminary Study

The results of the adhesiveness and affinity tests of the 50/70 penetration grade binder to two types of aggregates are reported in Table 4.
The results show a weak bond and stripping between the 50/70 binder and the aggregates that could cause the premature ageing of the bituminous mixture and could lead to some instabilities that will affect the pavement performance. As an example, in Figure 4, the aggregates freshly coated with the binder (a) and the sample analysed in the case of affinity after 6 h of rotation (b) are presented.
It should be mentioned that in the Romanian Norm AND 605, there is only one specification regarding the adhesiveness obtained by performing the spectrophotometric test. Therefore, according to this norm, the adhesiveness should be a minimum of 80%.
As observed, the experimental results for the adhesiveness are lower than the minimum required by the norms. Therefore, in order to increase the adhesion between the aggregates and the binder, two different anti-stripping green additives were used.

4.2. Optimal Anti-Stripping Green Additives Content

As mentioned previously, two different anti-stripping green additives, named A and B, were used in three percentages, 0.2%, 0.4%, and 0.6%, by the weight of the binder, in order to establish their optimal content (most effective).
First, six blends were produced, as mentioned in Section 3, and tested in order to highlight the influence of the two anti-stripping green additives on the conventional properties of the binders. The results in terms of the conventional properties of the six binders are reported in Table 5.
As the amount of the additive increases, the softening point values of the tested binders had a reducing trend (a maximum decrease of 0.30 °C per 0.20% additive increase amount for green additive B) and the penetration values had a reverse trend (a maximum increase of 6 1/10 mm per 0.20% additive increase in the case of green additive B). In the case of the breaking point results, it was observed that each additive has a different effect. However, the increase/decrease in these values is limited to a maximum of 0.20 °C compared to the one obtained for the fresh binder. These results lead to the idea that the addition of green additives can make the binder stiffer or softer depending on the amount and type of the additive, which can finally impact the rutting performance of the corresponding bituminous mixtures.
On the other hand, for all binders, the elongation at 25 °C was higher than the length of the test machine, which is limited to 150 cm. Therefore, for these results, it was not possible to highlight the effect of the additives.
However, the two anti-stripping green additives do not have a significant impact on the conventional properties of the tested binders. For this reason, the adhesiveness and affinity of the six blends combined with the two crushed aggregate sources, X and Y, were investigated using the spectrophotometric method and the rolling bottles method, respectively, after 6 h and 24 h. All of the results obtained for adhesiveness by the spectrophotometric method are reported in Figure 5.
From Figure 5, it can be observed that the adhesiveness results obtained by applying the spectrophotometric method increase with increasing content of both additives. Furthermore, some linear relationships were observed between these results and the content of additives, depending on the source of crushed aggregates (X and Y), confirmed by the high R2 values (always higher than 0.984).
It can be observed that even for 0.20% of each anti-stripping green additive A and B by mass of the binder, a higher adhesiveness was obtained than the minimum value of 80% specified in the Romanian Norm. However, from these results, the optimal content of the green additives could not be determined. As already mentioned, even if from this method, according to the Romanian Norm SR 10969, the ‘adhesiveness’ is determined by spectrophotometric measurements, this method in fact evaluates the counting degree of the binder on the crushed aggregates. Therefore, the increase in the above-mentioned results with the increase in the additive content was expected.
For this reason, the affinity of the binder to the crushed aggregates was investigated using the rolling bottles method after 6 h and 24 h. All of the results are reported in Figure 6.
From Figure 6a,b, it can be observed that the affinity results obtained after 6 h of rotation increase with the increase in the content of both additives, excluding the values obtained for the 0.6% content of each additive by mass of the binder. In the case of each additive and each aggregate source, the maximum affinity value was obtained for the 0.4% content of each anti-stripping green additive.
Moreover, in the case of each additive and each aggregate source for the 0.6% content of each anti-stripping green additive, a decrease in affinity was observed. These results decrease by a maximum of 4% from the affinity results obtained in the case of the materials prepared with 0.4% content of each anti-stripping green additive. This decrease could be explained by the fact that the corresponding blend between the fresh binder and the additive becomes a bit softer than the fresh binder. This observation is supported by the results obtained for the conventional properties of the blends (Table 5), for which an increase in the penetration results of a maximum of 10% was observed in the case of the corresponding blends (50/70 + 0.40% B and 50/70 + 0.60% B).
Similar tendencies were observed in the case of the affinity results obtained after 24 h of rotation (Figure 6c,d).
From these experimental results, it can be concluded that the 0.40% content of each anti-stripping green additive for both aggregate sources represents the most advantageous additive content, for which an increase in the adhesiveness of approximately 15%, an increase in the affinity after 6 h of approximatively 13%, and an increase in the affinity after 24 h of approximatively 25% for the materials produced without additives were obtained. This 0.40% content of both anti-stripping green additives is the most effective content from the affinity point of view and is the reason this content is considered optimal in this article.

4.3. Investigation of the Rheological Behaviour

4.3.1. Complex Shear Modulus Test Results

The obtained results for the three tested binders are reported in Black diagrams in Figure 7. As can be observed, all tested binders present a simple thermo-rheological behaviour. Moreover, at high temperatures, the two additive binders present a similar/closer behaviour to that obtained in the case of the straight-run 50/70 binder. Therefore, the two green additives do not have an important impact on the DSR test results.
The master curves of the norm of the complex shear modulus |G*| and phase angle φ at a reference temperature of 45 °C were built for all binders (Figure 8).
The time–temperature superposition principle was applied to obtain temperature shift factors, aT. Values of aT shift factors were approximated using the Williams–Landel–Ferry (WLF) equation and the C1 and C2 constants were obtained. In Figure 9, the shift factors aT and WLF fitting with a close-up of the results obtained at 55 °C and 65 °C for the tested binders are plotted. The values of the WLF constants for the three tested binders are listed in Table 6.
The master curves of the norm of the complex shear modulus and phase angle, as well as the temperature shift factors and the WLF constants obtained in the case of the three tested binders, present a closer or similar behaviour over the same temperature and frequency ranges.
Regarding the values of C1 and C2, it was observed that when each green additive was used, some higher values of the WLF constants were obtained. However, these values are closer to those obtained in the case of the fresh binder.
A small reduction in the phase angle values of the binder 50/70 + 0.40% B compared to those obtained in the case of the fresh binder was observed, which could lead to the improvement of the elastic response of the binder.
From a thermo-mechanical point of view, these results show that the use of each additive does not significantly affect the rheological behaviour of the fresh binder. However, in the case of green additive B, rheological behaviour closer to that obtained for the fresh binder was observed.

4.3.2. Steady Shear Viscosity

In Figure 10, the results for all tested binders of the norm of complex shear modulus are plotted as a function of the angular frequency (log–log scale), at a reference temperature of 45 °C. In order to determine η0 at 85 °C, the part of the plotted curve corresponding to the high temperature/low frequency was analysed by highlighting the Newtonian behaviour by performing a linear regression (45° slope line imposed). The results are plotted in Figure 10.
As can be observed in Figure 10, the addition of additives leads to an increase in the η0 values at 85 °C compared to those obtained for the fresh binder. When green additive A was used, an increase of approximately 32% was observed for the steady shear viscosity value obtained in the case of the fresh binder. A similar observation was found in the case of additive B, for which an increase of approximately 4% from that in the case of the 50/70 binder was obtained.
The viscosity at high temperatures of binders is usually considered an important characteristic that can influence the workability and construction temperatures of bituminous mixtures. High production temperatures of bituminous mixtures are generally used in the case of binders with a low viscosity. On the other hand, the durability of bituminous mixtures can be affected by binders with high viscosity. Further investigations related to the viscosity of these blends produced with green additives are needed. However, this study of the steady shear viscosity of the three binders was performed in order to investigate whether the addition of green additives had a significant impact on this characteristic of the fresh binder.
By analysing the results from Figure 10, it can be concluded that the steady shear viscosity of the blend produced with green additive B remains more stable compared to the results obtained in the case of green additive A.
Therefore, a similar conclusion to that made in the previous subsection can be drawn in the case of binder 50/70 + 0.40% B, for which a closer η0 value to the one obtained for the fresh binder was observed.

4.3.3. DSR High Critical Temperatures

As an example, in Figure 11, the procedure of determining the DSR high critical temperature for the binder 50/70 + 0.40% A is presented. The same procedure was applied to the other two tested binders. The values of the DSR high critical temperatures of all binders are reported in Table 7.
Similar observations as those made in the case of steady shear viscosity can be made in the case of the results obtained for the TDSR high critical values. When green additive A was used, an increase of approximately 4% to the TDSR high critical value obtained for the fresh binder was observed.
In the case of the use of additive B, the TDSR high critical value of the corresponding binder was nearly the same as that obtained in the case of the 50/70 binder (difference of 0.06 °C).
These results highlight that the use of green additive A can lead to an improvement in the temperature at which the binder should resist rutting. This aspect should also be investigated in the case of aged binders. Additionally, the low critical temperatures obtained by performing BBR tests should be determined in order to investigate if the green additives affect the cracking resistance of binders. However, the presented study was performed only to highlight that green additive B has an insignificant effect on the DSR high critical temperature of the fresh binder.

5. Conclusions

The main objective of the presented work was to evaluate the influence of two anti-stripping green additives on the performance of a fresh binder by performing a series of tests (conventional tests and DSR tests) and evaluating the adhesiveness (the spectrophotometric method and non-destructive method) and the affinity (the rolling bottles method) of the binders with two types of aggregate.
The following conclusions can be drawn:
  • The experimental results obtained for the adhesiveness of the fresh 50/70 binder to the two aggregate sources are lower than the minimum required by the norms (80%). Additionally, the affinity results show a weak bond between the binder and the aggregates. Therefore, two different anti-stripping green additives were used in order to increase the bond between the aggregates and the binder.
  • Conventional tests on binders combined with adhesiveness and affinity tests were performed in order to establish the optimum dosage of additives.
  • The softening point values of the tested binders decreased with the increase in the dosage of additives; a reverse trend was observed in the case of penetration values. Each additive had a different effect on the breaking point results.
  • However, the use of anti-stripping green additives did not have a significant impact on the conventional properties of the tested binders; all additive binders still met the specifications of the norms for a 50/70 pen. grade binder.
  • The 0.40% amount of both anti-stripping additives by mass of the fresh binder was considered the most effective additive dosage based on the experimental results obtained for the affinity, presented in Section 4.2.
  • The DSR test was performed on three binders (fresh binder and the two blends produced with the optimum additive content); all tested binders present a simple thermo-rheological behaviour.
  • It was observed that at high temperatures, the two additive binders present a similar/closer behaviour to that obtained for the straight-run 50/70 binder. Therefore, the two green additives do not have an important impact on the DSR test results.
  • When the anti-stripping green additive A was used, an increase of nearly 32% in the steady shear viscosity and an increase of approximately 4% in the TDSR high critical value, compared to the values obtained for the fresh binder, were observed.
  • When the anti-stripping green additive B was used, an increase of nearly 4% in the steady shear viscosity and an increase of 0.06 °C in the TDSR high critical value, compared to the values obtained for the fresh binder, were observed.
  • These results led to the idea that the addition of green additives can make the binder stiffer or softer depending on the amount and type of the additive, which can finally impact the rutting performance of the corresponding bituminous mixtures.
  • The 0.4% dosage of green additive B can be considered the most advantageous/effective due to the fact that it does not have a significant influence on the conventional properties and rheological behaviour of the 50/70 binder but has a major impact on the adhesiveness (15% increase), affinity after 6 h (13% increase), and affinity after 24 h (25% increase).
Future comprehensive studies on this topic are needed in order to better understand the influence of green additives on the behaviour of different binders, mastics, and bituminous mixtures. It must be mentioned that this study forms the basis of new research that the authors are conducting in which a combined method is studied between the two standard methods (adhesiveness and affinity). In this new method, the affinity test will be continued with the spectrophotometric method in order to eliminate the visual evaluation of the binder coating of each granule.

Author Contributions

Conceptualization, A.F. and A.S.; methodology, A.F.; validation, P.M.; formal analysis, P.M.; investigation, A.F. and A.S.; resources, A.F.; data curation, A.F.; writing—original draft preparation, A.F.; writing—review and editing, A.S.; visualization, P.M.; supervision, P.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are not publicly available. The data were gathered by the first author (Andrei Forton) through laboratory-based experimental investigation.

Acknowledgments

This paper was financially supported by the project “Network of excellence in applied research and innovation for doctoral and postdoctoral programs”/InoHubDoc, co-funded by the European Social Fund financing agreement no. POCU/993/6/13/153437.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Experimental campaign.
Figure 1. Experimental campaign.
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Figure 2. Calibration curve of the 4G red pigment dye solution.
Figure 2. Calibration curve of the 4G red pigment dye solution.
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Figure 3. Spectrophotometer test equipment.
Figure 3. Spectrophotometer test equipment.
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Figure 4. Samples before and after the affinity test: (a) Aggregates freshly coated with binder; (b) Sample analysed after 6 h of rotation.
Figure 4. Samples before and after the affinity test: (a) Aggregates freshly coated with binder; (b) Sample analysed after 6 h of rotation.
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Figure 5. Adhesiveness results by spectrophotometric method. (a) Anti-stripping green additive A. (b) Anti-stripping green additive B.
Figure 5. Adhesiveness results by spectrophotometric method. (a) Anti-stripping green additive A. (b) Anti-stripping green additive B.
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Figure 6. Affinity results. (a) After 6 h, anti-stripping green additive A. (b) After 6 h, anti-stripping green additive B. (c) After 24 h, anti-stripping green additive A. (d) After 24 h, anti-stripping green additive B.
Figure 6. Affinity results. (a) After 6 h, anti-stripping green additive A. (b) After 6 h, anti-stripping green additive B. (c) After 24 h, anti-stripping green additive A. (d) After 24 h, anti-stripping green additive B.
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Figure 7. DSR complex shear modulus test results: Black diagram.
Figure 7. DSR complex shear modulus test results: Black diagram.
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Figure 8. DSR complex shear modulus test results: Master curves of the norm of the complex shear modulus |G*| and phase angle φ at a reference temperature of 45 °C.
Figure 8. DSR complex shear modulus test results: Master curves of the norm of the complex shear modulus |G*| and phase angle φ at a reference temperature of 45 °C.
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Figure 9. DSR complex shear modulus test results: Temperature shift factors and WLF fitting.
Figure 9. DSR complex shear modulus test results: Temperature shift factors and WLF fitting.
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Figure 10. Steady shear viscosity at 85 °C of three tested binders.
Figure 10. Steady shear viscosity at 85 °C of three tested binders.
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Figure 11. Determination of TDSR high critical from complex shear modulus data for the binder 50/70 + 0.40% A.
Figure 11. Determination of TDSR high critical from complex shear modulus data for the binder 50/70 + 0.40% A.
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Table
Table 1. Conventional characteristics of the fresh 50/70 binder.
Table 1. Conventional characteristics of the fresh 50/70 binder.
ParametersExperimental ResultsRequirements SR EN 12591
Penetration at 25 °C, 1/10 mm (pen.)5450 … 70
Softening point, °C (TR&B)48.4046 … 54
Elongation at 25°C, cm (elong.)>150>100
Fraass breaking point, °C (TFraass)−14.00≤8.00
Penetration index−1.40−1.50 … +0.70
Resistance to hardening under the influence of heat and air (RTFOT method)
    -
mass variation, %
+0.04<0.50
    -
residual penetration, %
67.31≥50
    -
increase in softening point, °C
5.50≤9.00
    -
elongation after RTFOT, cm
100≥50
Table
Table 2. Characteristics of 4–8 fractions of quarry-crushed aggregates.
Table 2. Characteristics of 4–8 fractions of quarry-crushed aggregates.
CharacteristicsQuarry XQuarry Y
Type of rockdacitediorite
Granularity sort, d/D4–84–8
GranularityGC 90/10GC 85/15
Purity-fine particles (63 μm), %f 1.5f 2
Shape index, %SI 25SI 20
Flakiness index, %FI 20FI 20
Dry real density, kg/m32.652.60
Water absorption, %WA24 0.8WA24 2.0
Resistance to fragmentation—Los Angeles coefficientLA 15LA 20
Resistance to wear—micro-Deval coefficientMDE 10MDE 15
Table
Table 3. Characteristics of anti-stripping green additives.
Table 3. Characteristics of anti-stripping green additives.
ParametersAnti-Stripping Green Additives
AB
Density at 20 °C, g/cm30.981.05
Viscosity at 20 °C, cP185404
Table
Table 4. Preliminary results—adhesiveness and affinity test results.
Table 4. Preliminary results—adhesiveness and affinity test results.
SampleAdhesiveness Spectrophotometric Test Results (%)Affinity Test Results after 6 h (%)Affinity Test Results after 24 h (%)
50/70 + aggreg. quarry X78.5975.8856.33
50/70 + aggreg. quarry Y78.7374.5054.87
Table
Table 5. Conventional properties of the tested binders.
Table 5. Conventional properties of the tested binders.
BindersPenetration at 25 °C,
1/10 mm (pen.)		Softening Point,
°C (TR&B)		Fraass Breaking Point, °C (TFraass)Elongation at 25 °C,
cm (elong.)
50/70 + 0.20% A5648.20−14.20>150
50/70 + 0.40% A5748.00−14.20>150
50/70 + 0.60% A6047.90−14.00>150
50/70 + 0.20% B5548.30−14.00>150
50/70 + 0.40% B5648.00−14.20>150
50/70 + 0.60% B6247.80−14.20>150
Table
Table 6. WLF constants at a reference temperature of 45 °C.
Table 6. WLF constants at a reference temperature of 45 °C.
BindersC1C2
50/707.3688.52
50/70 + 0.40% A7.5088.99
50/70 + 0.40% B7.5888.58
Table
Table 7. TDSR high critical values of the three tested binders.
Table 7. TDSR high critical values of the three tested binders.
BindersTDSR high critical
50/7067.28
50/70 + 0.40% A70.23
50/70 + 0.40% B67.34
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Forton, A.; Stirb, A.; Marc, P. Influence of Anti-Stripping Green Additives on Binder Performance. Sustainability 2023, 15, 4560. https://doi.org/10.3390/su15054560

AMA Style

Forton A, Stirb A, Marc P. Influence of Anti-Stripping Green Additives on Binder Performance. Sustainability. 2023; 15(5):4560. https://doi.org/10.3390/su15054560

Chicago/Turabian Style

Forton, Andrei, Adelin Stirb, and Paul Marc. 2023. "Influence of Anti-Stripping Green Additives on Binder Performance" Sustainability 15, no. 5: 4560. https://doi.org/10.3390/su15054560

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