Patent Application
20250019545
This application claims the benefit of priority under 35 U.S.C. § 119(e) from Portugal Patent Application No. 118815, filed on Jul. 12, 2023, which is hereby incorporated by reference as if set forth in its entirety herein.
The present disclosure relates to asphalt concrete mixtures for surface, binder and regulating courses (AC 14 surf/bin/reg) and binder/base courses (AC 20 bin/base), incorporating high amounts of steel slag aggregates (SSA) and reclaimed asphalt (RA).
The steel industry, particularly the steel producer, generates significant amounts of waste, including slag, commonly known as “steel slag”. It is a granular material with a high micro-porosity, very high density, around 3.5 Mg/m3, an angular shape, and excellent wear resistance that can be used as aggregates in asphalt concrete mixtures [1].
Using steel slag to substitute aggregates in asphalt concrete mixtures to produce a high-performance mixture has gained significant interest recently as a value-added option to recycle steel slag [2]. This slag has excellent properties for asphalt concrete mixtures, being especially suitable for surface courses where skid resistance and durability are critical functional requirements and for producing asphalt mixtures subjected to heavy traffic [3-5]. Some previous research has shown that using this material improves the mechanical properties of asphalt mixtures, like durability, workability, stiffness, permeability, stability, resistance to fatigue, and permanent deformation [6-11].
However, the geometric characteristic of steel slag causes an increase in the air voids of the asphalt mixture or implies the use of higher amounts of added bitumen. In traditional asphalt mixture design, the air voids content is the most important volumetric property because it can affect the performance of the mixture [12]. It has been widely recognised that the bearing capacity and durability of asphalt pavement strongly depends on the density or air void content of the asphalt mixture. Too high or too low densities may cause early distresses in the asphalt pavement [13]. Thus, the use of steel slag in the asphalt mix should be limited to partial replacement of natural aggregates because the asphalt mix with 100% steel slag is highly susceptible to bulking and air voids problems because of its angular shape [14].
Existing asphalt pavement materials are milled during resurfacing, rehabilitation, or reconstruction operations. Once removed and processed, the pavement material becomes reclaimed asphalt (RA), consisting of valuable non-renewable resources, i.e., approximately 95% wt. of aggregates and 5% wt. of an aged asphalt binder. It can be reused in new asphalt mixtures, reducing the demand for virgin aggregates and bitumen [14].
Previous research has shown that using RA can improve basic properties such as resilient modulus, susceptibility to moisture, permanent deformation, and fatigue [15]. According to Abdel-Jaber et a. [16], the permanent deformation, considered one of the critical properties of asphalt mixtures, has been reduced as the percentage of RA content in the mixtures increased, which in turn means higher rutting resistance.
In addition to the advantages related to the mechanical performance of the asphalt mixtures, incorporating SSA and RA in the pavements reduces the volume of these materials deposited in landfills and the demand for virgin aggregates and bitumen, promoting a circular economy.
Document CN104926233 discloses a high RA content asphalt mixture comprising (percentage by weight) 40-65% of milled waste material (RA), 30-50% of aggregates, 1.5% of mineral powder, 1.5% of ordinary Portland cement, 0.3% of polyester fibre or basalt fibre, 2.8-3.9% of substrate asphalt. Steel slag is also part of the asphalt mixture, but in a low concentration.
Document CN113511842 describes a thermal regeneration steel slag asphalt mixture comprising: 6-24 parts of steel slag, 27-29 parts of old asphalt mixture, 3.9-4.5 parts of asphalt, 42-60 parts of basalt and 5.4-5.6 parts of mineral powder, wherein the optimal asphalt-aggregates ratio is 3.9-4.5%. The resulting thermal regeneration steel slag asphalt mixture test piece has characteristics of good high temperature stability, good low temperature crack resistance and good water stability. However, the amount of steel slag is below the amount of reclaimed asphalt, thus not being the main component of the mixture.
These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.
The present disclosure relates to an asphalt concrete mixture for surface, binder and regulating courses (AC 14 surf/bin/reg) and binder/base courses (AC 20 bin/base), incorporating high amounts of steel slag aggregates (SSA) and reclaimed asphalt (RA). These wastes replace most of the natural aggregates and part of the virgin asphalt binder to produce asphalt mixtures with better mechanical performance, lower dependence on petroleum-based resources, and a significant reduction in CO2 emission (48% for AC 14 surf/bin/reg and 49% for AC 20 bin/base). Using waste materials reduces the amount of material sent to overcrowded and overused landfill areas.
The practice of recycling reclaimed asphalt (RA) has been limited primarily due to a lack of confidence in the performance of recycled pavement, increased operational complexity, variability of RA material, and uncertainty about the mobilization degree of the aged binder [17,18]. This lack of confidence is linked to high RA content and concerns about the unknown durability of such pavements, which may require more frequent maintenance operations. However, increasing RA incorporation involves additional costs with processing, testing, pollution control, and rejuvenator use [19]. The aged asphalt binder in RA may compromise the performance of the final mixture when high recycling rates are used without rejuvenators. Therefore, in this disclosure, the incorporation of RA is intentionally limited to levels below 30% by weight and the mixtures do not need a rejuvenating agent.
The asphalt concrete mixtures of the present disclosure surprisingly show an improved mechanical performance, by combining two types of waste: steel slag aggregates (SSA, mainly used as coarse aggregates) and reclaimed asphalt (RA, mainly used as fine aggregates and partial substitute of bitumen). Steel slag is a by-product generated as waste during steelmaking and can be a cost-effective and environmentally acceptable alternative to replace natural aggregates. Reclaimed asphalt results from milling/demolition of distressed pavements and contains valuable asphalt binder and high-quality aggregates, thus being a cost-effective pavement construction material. This disclosure defines the adequate combination of steel slag aggregates and RA to produce asphalt concrete mixtures with improved mechanical performance using high amounts of waste (more than 75% of steel slag aggregates and reclaimed asphalt aggregates) as a raw material, in order to achieve the sustainability and circular economy principles that are now one of the main goals of the industry and global economy.
The present disclosure relates to an asphalt concrete mixture, or bituminous concrete mixture, comprising: 50% to 75% (w/w) of steel slag aggregates, wherein the size of the steel slag aggregates is up to 20 mm, measured by sieving; up to 30% (w/w) of reclaimed asphalt, wherein the size of the aggregates of reclaimed asphalt is up to 14 mm, measured by sieving; 3% to 25% (w/w) of natural stone aggregates and/or a filler, preferably 5% to 25% (w/w); up to 4% (w/w) of virgin bitumen.
The disclosed asphalt concrete mixtures of the present disclosure uniquely combining over 75% steel slag aggregates (SSA) and reclaimed asphalt (RA) to significantly enhance mechanical performance, demonstrating an unexpected synergistic effect, thereby addressing the technical challenge of creating high-performance, cost-effective, and environmentally sustainable asphalt concrete that aligns with the principles of circular economy.
An aspect of the present disclosure relates to an asphalt concrete mixture comprising
The asphalt concrete mixture of the present disclosure surprisingly demonstrate better mechanical performance using a lower percentage of virgin bitumen than conventional mixtures with natural aggregates. The percentage of virgin bitumen was reduced by 30%, significantly saving non-renewable resources. The incorporation of reclaimed asphalt in this disclosure is intentionally limited to levels below 30% by weight, ensuring that the mixtures do not require rejuvenator agents and maintaining the asphalt concrete mixture performance. The asphalt concrete mixture of the present disclosure surprisingly allows a significant reduction in CO2 emissions and promotes sustainability and circular economy principles by utilizing high amounts of waste materials.
In an embodiment for better results, the mass ratio between steel slag aggregates and reclaimed asphalt aggregates ranges from 1.6:1 to 4.3:1; preferably 2.5:1 to 4:1.
In an embodiment for better results, the asphalt concrete mixture comprises, 50% to 65% (w/w) of steel slag aggregates; 15% to 30% (w/w) of reclaimed asphalt aggregates; 10% to 20% (w/w) of natural stone aggregates and/or filler; and 2% to 4% (w/w) of virgin bitumen.
In an embodiment for better results, the asphalt concrete mixture comprises 55% to 65% (w/w) of steel slag aggregates.
In an embodiment for better results, the asphalt concrete mixture of the present disclosure is absent of a rejuvenating agent.
In an embodiment for better results, the asphalt concrete mixture is an asphalt concrete (AC) mixture for surface, binder or regulating courses (surf/bin/reg), or an asphalt concrete mixture for binder or base (bin/base) courses.
For the scope and interpretation of the present disclosure, the term “AC 20” and “AC 14” relates to asphalt concrete (AC) with nominal maximum aggregate size of 20 or 14 mm, respectively.
In an embodiment, an asphalt concrete mixture, or bituminous concrete mixture, is applied in layers of different thicknesses ranging from 4 to 6 cm, preferably 4 to 5 cm, for AC 14 surf/bin/reg mixture. In another embodiment, the asphalt concrete mixture, or bituminous concrete mixture, is applied in layers of different thicknesses ranging from 5 to 12 cm, preferably 6 to 8 cm for AC 20 bin/base mixture.
In an embodiment, the size of the steel slag aggregates ranges from 4-16 mm; preferably 10-14 mm, preferably for AC 14 surf/bin/reg mixtures.
In an embodiment, the asphalt concrete mixture comprises 15-20% (w/w) of reclaimed asphalt aggregates.
In an embodiment, the natural stone aggregates and/or filler for the asphalt concrete mixtures are selected from: limestone, cement, lime, granite, basalt, or mixtures thereof; preferably, the filler can be selected from limestone, cement, lime, or mixtures thereof; preferably, the natural stone aggregates can be selected from limestone, granite, basalt, or mixtures thereof.
In an embodiment, the air void content of the disclosed asphalt concrete mixture ranges from 3% to 5% (v/v); preferably, the air void content ranges from 3.5% to 4.5% (v/v).
In an embodiment, the size of the aggregates of reclaimed asphalt ranges from 4-10 mm.
In an embodiment, the disclosed asphalt concrete mixture comprises 3.0% (w/w) to 3.5% (w/w) of virgin bitumen, preferably a 50/70 bitumen.
In an embodiment, the asphalt concrete mixture comprises 3% (w/w) to 5% (w/w) of filler.
In an embodiment, the asphalt concrete mixture comprises 11% (w/w) to 13% (w/w) of natural stone aggregates.
In an embodiment, the asphalt concrete mixture further comprises at least one additional additive selected from the group consisting of acids, polymers, anti-stripping agents, lime, gilsonite and fibers.
In an embodiment, the sizes of the steel slag aggregates are divided into fractions:14 to 20 mm, 10 to 14 mm, 4 to 10 mm, and up to 4 mm; preferably, the sizes of the steel slag aggregates are divided into three fractions: 14 to 20 mm, 4 to 14 mm, and up to 4 mm.
In an embodiment for better results, up to 20% (w/w) of the steel slag aggregates have a size ranging from 14 mm to 20 mm.
In an embodiment for better results, 31% to 50% (w/w) of the steel slag aggregates have a size ranging from 4 to 14 mm.
In an embodiment for better results and synergistic effect, 18% to 30% (w/w) of the steel slag aggregates have a size ranging from 10 to 14 mm and 13% to 20% (w/w) of the steel slag aggregates have a size ranging from 4 mm to 10 mm.
In an embodiment for better results, 11% to 15% (w/w) of the steel slag aggregates have a size of up to 4 mm.
The present disclosure also relates to an infrastructure comprising the disclosed asphalt concrete mixture.
In an embodiment, the infrastructure is a building, a road, a driveway, or a pavement, such as: road construction, airport runways, driveways, walking paths, cycling paths, bridge decks, industrial or race tracks, among others. These applications benefit from the mechanical performance, durability, and cost-effectiveness of asphalt concrete mixtures of the present disclosure, namely those incorporating reclaimed asphalt (RA) and steel slag aggregates (SSA).
The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of the invention.
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1D: Embodiment of a comparison between the cut sections of conventional asphalt mixtures ((a) AC 14 surf/bin/reg with natural aggregates and (c) AC 20 bin/base with natural aggregates) and the disclosed asphalt mixtures ((b) AC 14 surf/bin/reg with SSA and RA and (d) AC 20 bin/base with SSA and RA).
The present disclosure relates to an asphalt concrete mixture comprising: 50% to 75% (w/w) of steel slag aggregates, wherein the size of the steel slag aggregates is up to 20 mm, measured by sieving; up to 30% (w/w) of reclaimed asphalt, wherein the size of the of reclaimed asphalt is up to 14 mm, measured by sieving; 3% to 25% (w/w) of natural stone aggregates and/or filler; and up to 4% (w/w) of virgin bitumen. Infrastructures comprising the described asphalt concrete mixture are also disclosed.
In an embodiment, the present disclosure relates to asphalt concrete mixtures for surface, binder, or regulating courses (AC 14 surf/bin/reg) and binder or base courses (AC 20 bin/base), wherein the mixture comprises 50% to 75% (w/w) of SSA, preferably 50% to 65% (w/w) of SSA, up to 30% (w/w) of RA, preferably 15% to 30% (w/w) of RA, 3% to 25% (w/w) of natural aggregates and/or filler, preferably 10% to 20% (w/w) of natural aggregates and/or filler, and up to 4% (w/w) of virgin bitumen, preferably 2% to 4% (w/w) of virgin bitumen. In an embodiment, the resulting mixtures show better mechanical performance using a lower percentage of virgin bitumen incorporation than conventional mixtures with natural aggregates. The percentage of virgin bitumen was reduced by 30%, significantly saving non-renewable resources.
The collective term “aggregates” is used for the mineral materials such as sand, gravel, and crushed stone, used with a binding medium (i.e., bitumen) to form compound materials (such as asphalt concrete). The term aggregates is also used for base and subbase courses for flexible and rigid pavements. Natural aggregates are generally extracted from large rock formations through an open excavation. Extracted rock is typically reduced to usable sizes by mechanical crushing.
In an embodiment, the mechanical performance of the disclosed asphalt concrete mixtures was compared to that of identical conventional mixtures produced with 94% to 96% (w/w) natural aggregates, and 4% to 6% (w/w) virgin bitumen. The results showed that asphalt concrete mixtures of the present disclosure present excellent water sensitivity and rutting resistance, superior to the performance of the conventional asphalt concrete mixtures. The result is even more impressive considering that it was achieved with 80% waste materials and a reduction of 30% in the bitumen added.
Most works with SSA incorporation in asphalt mixtures showed difficulties obtaining adequate values of air void content (Va), typically higher than expected. Due to the physical characteristics of steel slag, namely its angular shape and rough surface, it is not easy to compact the mixtures with this type of material, maintaining Va values within the specified limits. This problem is solved by the asphalt concrete mixture of the present disclosure, which shows Va values according to the regulatory specifications (3.0% to 5.0% (v/v) for AC 14 surf/bin/reg and 3.0% to 6.0% (v/v) for AC 20 bin/base).
In an embodiment, the asphalt concrete mixture comprises SSA, RA, natural aggregates (NA) (e.g., limestone, granite, basalt), filler (e.g., limestone, cement, lime, granite, basalt), and bitumen with penetration grade 50/70. The amount of each material for the mix design of AC 14 and AC 20 mixtures is presented in Table 1 in kg/ton. The size of aggregates was obtained by particle size distribution—sieving method. In an embodiment, the values given in Table 1 have a tolerance of 10% depending on specific designs to consider the variability of the waste materials.
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1D show the morphological differences between conventional asphalt mixtures and the disclosed asphalt mixtures. This figure was obtained with a HD digital camera after sawing (with a circular cut-off saw) slabs of asphalt mixtures produced according to the processes presented in this disclosure.
In an embodiment, the volumetric characteristics of the asphalt concrete mixtures were obtained using the maximum theoretical density (MTD) and the bulk density. The bulk density of the samples was obtained by method B of the standard EN 12697-6:2020. The maximum theoretical density (MTD) was determined according to EN 12697-5:2018, using a pycnometer and a disaggregated mixture sample. After determining the bulk density of the specimens and the MTD of each mixture, it is possible to calculate the air voids content (Va) according to Equation 1:
The volumetric characteristics of the asphalt concrete mixtures developed can be seen in Table 2, which shows the average values of each asphalt concrete mixture. The values obtained for Va (%) are considerably lower as compared to the 6% to 7% values presented in the state-of-the-art [4,9], showing that the problem of high air void contents observed in asphalt mixtures with high amounts of SSA is solved by the disclosed asphalt concrete mixtures.
In an embodiment, the asphalt concrete mixtures' mechanical performance was evaluated through water sensitivity and permanent deformation resistance tests. The water sensitivity test was carried out following EN 12697-12:2018 as the preliminary performance control test. It is necessary to store three specimens at room temperature, without conditioning in water, while a second group is conditioned in water for three days at 40° C. Finally, all the samples are tested by indirect tension at 15° C. (EN 12697-23:2018). The test results are the average indirect tensile strength of the conditioned in water (ITSw) or dry (ITSd) specimens. The indirect tensile strength ratio (ITSR) is the ratio between ITSw and ITSd and is the main parameter used to evaluate water sensitivity. According to current practice, the asphalt mixtures should have ITSR values higher than 70% or, ideally, 80% to ensure adequate performance [20,21].
A comparison between the results obtained for conventional (AC 14 50/70 surf/bin/reg and AC 20 50/70 bin/base) and the disclosed mixtures is present in Table 3. The conventional AC 14 50/70 surf/bin/reg mixture was produced with 92% (w/w) of natural stone aggregates, 3.2% (w/w) of recovered filler and 4.8% (w/w) of bitumen 50/70. The conventional AC 20 50/70 bin/base was produced with 93% (w/w) of natural stone aggregates, 2.6% (w/w) of recovered filler and 4.4% (w/w) of bitumen 50/70. There was an increase of 7.5% in the ITSR for the AC 14 mixture and 1% for the AC 20 mixture, demonstrating the improved durability of the new mixtures in the presence of water. Surprisingly, even with a reduction of 30% in the amount of binder added, the disclosed asphalt concrete mixtures showed excellent ITSR results
In an embodiment, the wheel-tracking test (WTT) was used to evaluate the resistance to permanent deformation according to EN 12697-22:2020. Briefly, the method consists of repeatedly passing a wheel over the asphalt mixture at a high operating temperature (60° C.) while measuring the evolution of the wheel rut depth with the number of cycles (up to 10 000 cycles). As for the results obtained in this test, the main parameter is the wheel tracking slope (WTSAIR), which indicates the increase in deformation per thousand cycles between the 5000th and the 10000th cycles. The other parameters obtained in this test are the mean proportional rut depth (PRDAIR) and the final rut depth (RDAIR). The RDAIR is the total deformation after 10000th cycles, while the PRDAIR is the ratio between RDAIR and sample thickness.
A comparison between the conventional and the new mixtures is presented in Tables 4 and 5. The disclosed asphalt concrete mixtures showed an improvement (decrease) of 75% to 77% in the permanent deformation rate (WTSAIR) compared to the conventional asphalt mixtures.
Based on the abovementioned results, the disclosed asphalt concrete mixture differs from previously developed solutions since it shows improved mechanical and environmental performance and volumetric properties that were not previously achieved when using natural, SSA and RA materials. Surprisingly, the asphalt concrete mixtures now disclosed comprise more than 75% (w/w) of waste materials and 30% (w/w) less of added bitumen, which resulted in improved mechanical and environmental performance and volumetric properties. The disclosed mixtures are then the optimum solution after a series of combinations that were not able to meet the expected properties, namely:
Asphalt concrete mixtures with different compositions did not meet the expected results. Table 6 lists the composition of comparative examples which were not able to meet the expected properties. Several examples presented in Table 6 were not able to meet the maximum air voids content limit of 5% (v/v) for AC14 (7.8% (v/v) for Example 2 and 6.4% (v/v) for Example 4) or 6% (v/v) for AC 20 (7.3% (v/v) for Example 1 and 11.6% (v/v) for Example 5). The water sensitivity of some of these compositions, measured as a percentage of strain retain after conditioning the samples in water, was also below the minimum limit value of 80% (77% for Example 1 and 74% for Example 3).
As used in the specification and claims, the singular forms “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a sample” includes a plurality of samples, including mixtures thereof.
Whenever the term “at least,” “greater than,” or “greater than or equal to” precedes the first numerical value in a series of two or more numerical values, the term “at least,” “greater than” or “greater than or equal to” applies to each of the numerical values in that series of numerical values. For example, greater than or equal to 1, 2, or 3 is equivalent to greater than or equal to 1, greater than or equal to 2, or greater than or equal to 3.
Furthermore, it is to be understood that the invention encompasses all variations, combinations, and permutations in which one or more limitations, elements, clauses, descriptive terms, etc., from one or more of the claims or from relevant portions of the description is introduced into another claim. For example, any claim that is dependent on another claim can be modified to include one or more limitations found in any other claim that is dependent on the same base claim.
Furthermore, where the claims recite a composition, it is to be understood that methods of using the composition for any of the purposes disclosed herein are included, and methods of making the composition according to any of the methods of making disclosed herein or other methods known in the art are included, unless otherwise indicated or unless it would be evident to one of ordinary skill in the art that a contradiction or inconsistency would arise.
Where ranges are given, endpoints are included. Furthermore, it is to be understood that unless otherwise indicated or otherwise evident from the context and/or the understanding of one of ordinary skill in the art, values that are expressed as ranges can assume any specific value within the stated ranges in different embodiments of the invention, to the tenth of the unit of the lower limit of the range, unless the context clearly dictates otherwise. It is also to be understood that unless otherwise indicated or otherwise evident from the context and/or the understanding of one of ordinary skill in the art, values expressed as ranges can assume any subrange within the given range, wherein the endpoints of the subrange are expressed to the same degree of accuracy as the tenth of the unit of the lower limit of the range.
The term “comprising”, whenever used in this document is intended to indicate the presence of stated features, integers, steps, or components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.
The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof. The above described embodiments are combinable.
The following claims further set out particular embodiments of the disclosure.
The following citations are incorporated herein by reference:
| Number | Date | Country | Kind |
|---|---|---|---|
| 118815 | Jul 2023 | PT | national |
