A Discussion on the Mix Design of SBS-Modified Asphalt

　　Abstract: Under favorable design mix proportions and construction conditions, SBS-modified asphalt can significantly enhance the durability and high-temperature stability of asphalt pavements. Based on the construction test results from South Second Road, this paper briefly outlines the construction technical requirements for SBS-modified asphalt.

　　Keywords: SBS asphalt mixture, mix design, technical requirements

　　Polymer-modified asphalt is a new type of high-quality road construction material characterized by its high technological content and added value. By incorporating polymers into road asphalt, its performance is significantly enhanced, leading to a substantial extension of pavement service life, reduced noise levels, and improved driving comfort and safety. SBS asphalt, as a modified asphalt binder, first emerged in the 1990s. Since SBS is a thermoplastic rubber copolymer, its usage is extensive and relatively costly. Due to economic constraints, it has not been widely adopted domestically until recently. In 2002, the Research Institute of East China University of Petroleum successfully developed an SBS asphalt modifier. In collaboration with the Materials Division of the Dongying Municipal Highway Bureau, they began producing SBS-modified asphalt mixtures and conducted a pilot construction project on South Second Road in Dongying City. Below, based on the construction and supervision experience from the first phase of the South Second Road project in Dongying City, we will discuss the design considerations for SBS asphalt mix proportions as well as key points to note during the construction process.

　　1. Overview of SBS-modified asphalt

　　SBS-modified asphalt is produced by blending 2.5%, 3.0%, or 4.0% of SBS modifier into the original matrix asphalt (AH-70). Compared with the original asphalt, the modified asphalt exhibits increased high-temperature viscosity and a higher softening point. Under favorable mix design and construction conditions, the durability and high-temperature stability of asphalt pavements are significantly improved. Extensive laboratory tests were conducted during the production process of SBS-modified asphalt, and after production, field tests were carried out to evaluate its technical performance. The test results show that the modified asphalt containing 3.0% SBS meets all the specification requirements for modified asphalt in terms of softening point, penetration, and other key indicators, making it suitable for use in the mix design of asphalt mixtures.

　　2 Mix Design of SBS Asphalt Mixture

　　To ensure that the designed mixture achieves the desired implementation results, it is necessary to set forth the following requirements from various aspects, including material specifications, construction techniques, quality control standards, and quality control methods. We hope these requirements will receive due attention.

　　(1) Raw Material Requirements

　　1-1 Coarse Aggregate:

　　Coarse aggregates used for the surface layer of modified asphalt mixtures should preferably be crushed stone or crushed gravel, and their particle size specifications and quality requirements shall both comply with the provisions of the "Technical Specifications for Construction of Highway Asphalt Pavements" (JTJ032-94).

　　1) Coarse aggregate shall be clean, dry, free from weathering and harmful impurities, and possess a certain degree of hardness and strength.

　　2) Coarse aggregates should have good particle shape. When crushed gravel is used for expressways and first-class highways, large-gravel crushing should be employed, and the crushed gravel should have at least two or more fracture surfaces.

　　3) For the coarse aggregate used in anti-skid surface layers, hard rocks (neutral or basic igneous rocks) should be selected. Given the significant differences in adhesion between hard rocks and asphalt, the adhesion between coarse aggregate and asphalt should be no less than Grade 4. As for the 3–5 mm stone chips, due to their relatively low content and their role in forming an interlocking structure within the asphalt mixture, it is recommended to use hard rock chips (basalt).

　　1-2 fine aggregate

　　Fine aggregates include manufactured sand and natural sand. For the surface layer of asphalt pavements, manufactured sand is preferred as the fine aggregate. The fine aggregate should be clean, dry, free from weathering and harmful impurities, have an appropriate particle size distribution, and exhibit good adhesion to modified asphalt. Natural sand, due to its significant variability in quality (mostly medium-to-coarse sand) and relatively rounded particle shape, has poorer adhesion to asphalt and can significantly affect the performance of asphalt mixtures. For asphalt mixtures used on expressways and first-class highways, the content of natural sand should not exceed 20%; stone dust with a particle size of 0–3 mm can be used as a substitute for natural sand.

　　1-3 filler

　　The filler used for the surface layer of modified asphalt mixtures shall be clean and dry, and its quality shall meet the technical requirements specified in the "Technical Specifications for Highway Asphalt Pavements."

　　1. The filler for modified asphalt mixtures should preferably be mineral powder obtained by finely grinding water-absorbent aggregates such as strongly basic rocks (limestone and igneous rocks). The mineral powder must be dry and clean; recycled powder recovered from dry dust removal during mixture production is not recommended.

　　2. When using cement and hydrated lime powder as fillers, their dosage should not exceed 2% of the total aggregate volume.

　　3. It is not recommended to use recycled powder in the production of asphalt surface layer mixtures. However, when the plasticity index is less than 4 and the hydrophilicity coefficient is less than 0.8, partial use of recycled powder is permissible after testing; the amount of recycled powder used per batch must not exceed one-quarter of the total mineral powder content.

　　1-4. Technical Requirements for SBS-Modified Asphalt

　　Technical Specifications: SBS-modified Asphalt

　　Penetration at 25℃, 100g, 5s (0.1mm): minimum 60

　　Penetration Index PI minimum: 1 -0.2

　　Elongation at 5℃, 5 cm/min (cm): minimum 30

　　Softening point, TR&B (℃) minimum 55

　　Wax content (distillation method) (%) maximum 3

　　Kinematic viscosity at 135℃ (Pa·s): Maximum 2.3

　　Flash point (℃) minimum 230

　　Solubility (%) minimum 99

　　Separation, softening point difference (℃) maximum: 3 2.5

　　Elastic recovery at 25℃ (%) minimum 65

　　Residue after the Rotating Thin-Film Oven Test (RTFOT) 『4』

　　Mass loss (%) maximum 1.0

　　Penetration at 25℃ (%) minimum 60

　　Elongation at 5℃ (cm): Minimum 20

　　Note: [1] The penetration index PI is determined by performing linear regression on the penetration values measured at three or more different temperatures—such as 15°C, 25°C, and 30°C—using Equation 1: Gp = AT + K. After obtaining the parameter A through this regression, the PI is calculated using the following formula. The correlation coefficient R of the linear regression must be no less than 0.997.

　　PI = (20 - 500A) / (1 + 50A)

　　[2] The kinematic viscosity at 135℃ in the table can be determined using the “Asphalt Viscosity Test Method (Brookfield Viscometer Method)” specified in the “Test Procedures for Asphalt and Asphalt Mixtures in Highway Engineering” (JTJ052-93).

　　Perform the determination. If the modified asphalt can be easily pumped and mixed at a temperature that does not alter its physical and mechanical properties and meets safety requirements, or if tests demonstrate that appropriately raising the pumping and mixing temperature can ensure the quality of the modified asphalt and facilitate construction, then the determination may not be required. When conditions permit, the dynamic viscosity of the modified asphalt at 60℃ should be determined using the capillary method.

　　[3] If the modified asphalt is used immediately after being prepared on-site or if it undergoes continuous stirring or pumping circulation during storage, the requirements for segregation test indicators may be waived.

　　[4] (China Asphalt Network) The aging test shall be conducted according to the Rotating Thin-Film Oven Test (RTFOT) method; the Thin-Film Heating Test (TFOT) may be used as an alternative, provided that this is explicitly stated in the report and such results shall not be used as arbitration outcomes.

　　1-5. Points to Note in SBS-Modified Asphalt Testing

　　1. Sampling of test specimens

　　Each truckload of modified asphalt used during construction must be inspected. Sampling must be uniform and representative. Each sample should be heated and immediately poured into the required test mold in one go; repeated heating and reuse are not recommended. The temperature for pouring the test molds must reach above 160°C, and before pouring the molds and preparing the mixture, the materials must be thoroughly mixed until completely homogeneous.

　　2. When conducting the softening point test, the sample must be heated according to the test procedure until it flows freely before being poured into the sample ring. Other methods of filling the sample ring are not permitted; otherwise, the test results will be significantly inaccurate.

　　(2) Asphalt Mixture Proportion Design:

　　The mix design for modified asphalt mixtures shall follow the three stages—target mix design, production mix design, and trial mixing and paving—as specified in the "Technical Specifications for Construction of Highway Asphalt Pavements," to determine the aggregate gradation and the optimal dosage of modified asphalt.

　　2-1 gradation

　　The gradation for asphalt mixture mix design shall be determined using the Böhler method. The principle for selecting the gradation is as follows: For AC-13I mixtures, the percentage passing through the 2.36 mm sieve should be set at the lower limit of the gradation range to achieve compaction and interlocking.

　　2-2 powder-to-binder ratio

　　The proportion of particles smaller than 0.075 mm significantly affects the volumetric properties and pavement performance of asphalt mixtures. In the mixture gradation, the content of particles smaller than 0.075 mm must take into account the dust portion that should naturally be present in the coarse aggregate. The mineral powder content is required not to exceed the asphalt content; the ratio between the fraction smaller than 0.075 mm and the asphalt content—the so-called fines-to-asphalt ratio—should fall within the range of 1 to 1.2. For asphalt surface layer mixtures, the mineral powder content is recommended to be between 4.5% and 5%.

　　Technical Specifications for 2-3 Mixture

　　To effectively enhance the performance of asphalt pavements, the surface-layer asphalt mixture must, on the one hand, meet the requirements for water drainage and, on the other hand, avoid excessive compaction. Therefore, further restrictions need to be imposed on the volumetric properties of the asphalt mixture, as shown in the table below.

　　Test Project Technical Requirements Remarks

　　Number of compaction blows (times): 75 per side

　　The theoretical maximum relative density of asphalt mixtures shall be determined by the measured method.

　　Stability (kN) > 7.5

　　Flow value (0.1mm): 20-40

　　Porosity AV (%) 3-6

　　Saturation VFA (%) 70-85

　　The voids in mineral aggregate (VMA, %) shall be determined according to the specification based on the maximum nominal size.

　　Compaction temperature (℃): 160

　　Residual stability (%) >80

　　Dynamic Stability of Rutting Test (cycles/mm) > 2000

　　2-4 Precautions

　　The following issues should be given attention during the design process of modified asphalt mixtures.

　　A. The mixing and compaction temperatures for the mixture shall be determined in accordance with the Construction Technical Specifications for Modified Asphalt Pavements and based on the viscosity-temperature relationship curve of the asphalt binder. During laboratory mix design, the mixing and compaction temperatures should be consistent with the mixing temperature at the batching plant and the compaction temperature at the construction site. Recommended temperatures are shown in the table below:

　　Testing and Construction Temperatures for Modified Asphalt

　　Temperature-graded asphalt mixture

　　Mixing temperature not less than 160℃

　　Initial compaction temperature not less than 150℃

　　Repressurization temperature not less than 140℃

　　Final compaction temperature not less than 120℃

　　B. During sampling and mixing for the test, it is essential to ensure the uniformity of the asphalt binder. The prepared binder should be thoroughly mixed before sampling and preparing the mixture.

　　C. The determination of volumetric indices for mixtures shall be standardized. For the determination of density of compacted asphalt mixture specimens, the surface-dry method should be uniformly adopted.

　　D. The water stability of modified asphalt mixtures shall meet the following two index requirements; if these requirements are not met, anti-stripping measures shall be adopted:

　　(1) The residual Marshall stability obtained from the 48-hour water-immersed Marshall stability test, as determined by the “Marshall Stability Test for Asphalt Mixtures” method, shall not be less than 80%.

　　(2) The splitting strength ratio determined by the “bituminous mixture freeze-thaw splitting test” method shall not be less than 80%.

　　3 Construction of SBS Asphalt Mixture

　　SBS asphalt is an elastomeric-modified asphalt. When used properly, it can significantly enhance the rutting resistance of asphalt pavements, improve durability, increase resistance to aging, and extend the service life of roads. Compared with AH-70 matrix asphalt, SBS asphalt exhibits markedly higher viscosity and softening point. The transportation, storage, and pavement construction of SBS asphalt have certain requirements that differ from those for matrix asphalt. Only by using it correctly can the desired performance be achieved.

　　1) Technical requirements for transportation

　　The loading temperature of SBS asphalt at the production plant must be maintained above 160℃. The temperature of the asphalt upon arrival at the mixing plant should not fall below 140℃. The transport vehicles must deliver the asphalt to the designated location within 24 hours and promptly pump it into the asphalt storage tanks.

　　2) Technical requirements for storage at asphalt mixing plants

　　The storage temperature of SBS asphalt should be maintained at around 150℃. If the temperature falls below the required storage range, the viscosity of the SBS asphalt will become excessively high, leading to blockages in the oil pipelines of the asphalt tank and ultimately forcing a shutdown for repairs.

　　Asphalt hot-mix plants should store SBS asphalt for as short a period as possible, using it immediately upon arrival and keeping larger quantities when it’s in use while minimizing storage when it’s not needed. Storage duration should not exceed 24 hours.

　　After completing the day’s construction tasks, you should try your best to use up all the asphalt in the tank, or fill the asphalt tank completely, or transfer any remaining small amount of asphalt into other storage tanks. This will minimize the surface area of asphalt exposed to air, thereby helping to prevent the asphalt from aging.

　　Most storage tanks at asphalt mixing plants are horizontal. To ensure the uniformity of SBS-modified asphalt, a mixer should be installed at the top of the storage tank, or the tank itself should be equipped with a built-in mixer. The mixer should be operated every 3 hours, with each mixing cycle lasting 20 minutes.

　　3) Technical requirements for pumping

　　SBS asphalt has relatively high requirements for transportation and storage temperatures. When producing asphalt mixtures, the asphalt needs to be pumped into the mixing plant using an asphalt pump. Since asphalt pumps are equipped with filters that can easily become clogged by certain substances, this may impair the pump's ability to deliver asphalt effectively. Therefore, it is recommended to use filters with larger mesh sizes (9.5 mm or greater). At the same time, it is important to strengthen insulation measures along the asphalt pipelines to prevent the SBS asphalt temperature from dropping and causing blockages in the pipelines.

　　4) Technical requirements for mixing and transportation

　　To ensure greater stability in the quality of asphalt mixtures and more accurate asphalt dosage, it is advisable to use a batch-type mixer for mixing. Mixing must be uniform; only when the SBS asphalt modifier is completely dispersed throughout the asphalt can its full effectiveness be realized. For dense-graded (AC-13I) mixtures, the mixed material must be uniformly consistent after mixing, with no separation of fine and coarse aggregates, nor any signs of mottling or clumping. Since SBS-modified asphalt mixtures require relatively high construction temperatures, it is recommended that the mixing temperature be controlled at 160°C. Transport vehicles must be covered with tarpaulins or other insulating materials to prevent the binder from hardening on the surface. To ensure continuous paving and compliance with technical specifications regarding smoothness and evenness, at least two trucks must be waiting ahead of the paver to unload material; under no circumstances should the paver have to wait for trucks. Other requirements shall comply with the technical specifications for modified asphalt pavement construction.

　　5) Technical requirements for paving

　　When paving SBS asphalt mixtures, construction should be carried out as continuously as possible to minimize interruptions to the paver and roller. Transverse joints should be minimized to improve the smoothness of the pavement surface. To enhance pavement smoothness, the surface layer should adopt a sled-type paving thickness control method that maintains a consistent height difference before and after paving. Since SBS asphalt has high viscosity and strong adhesion, some pavers are equipped with rear sleds that feature rubber wheels. These rubber wheels tend to accumulate fine particles from the mixture, thereby compromising the smoothness of the pavement. Therefore, pavers equipped with rubber-wheel rear sleds must have their rear sleds replaced with steel slipper-type structures. The paving speed should be controlled at 2 meters per minute, ensuring slow, uniform, and continuous paving without interruption. Any arbitrary changes in speed or mid-paving stops are strictly prohibited.

　　Increase the pre-compaction density during paving. At high temperatures, SBS-modified asphalt mixtures rely primarily on the interlocking action of coarse aggregates. Therefore, the vibration frequency of the tamping hammer can be appropriately increased to reduce the residual compaction coefficient and minimize any visible traces from the initial compaction, thereby ensuring the ultimate smoothness of the pavement surface.

　　6) Technical requirements for compaction

　　For dense-graded mixtures, the appropriate compaction temperature range is 130℃ to 150℃, and the final compaction temperature should not be lower than 110℃.

　　The compaction process for SBS asphalt mixtures is carried out according to the following principles: Compaction shall be performed in accordance with the “follow closely, compact slowly, use high frequency, and maintain low amplitude” eight-character guideline. The roller must follow closely behind the paver. Better compaction results can be achieved only when compaction is conducted under high-temperature conditions. The compaction speed should be controlled within 4–5 km/h. The compaction speed must be uniform; when reversing, the vibration function should be turned off, and the direction should be changed gradually—twisting or turning sharply while rolling is strictly prohibited. At the start or end of each pass, slight twisting of the roller may be applied to eliminate wheel tracks at the joints. It is absolutely forbidden to turn, make U-turns, or shift positions left or right on freshly laid asphalt mixtures. Sudden braking or stopping for rest is also not allowed. Based on the SBS asphalt trial section along the first and second contract sections of South Second Road, the recommended compaction procedure involves using either a DD110 or DD130 roller, operating in gears 2–3, and performing three passes per gear—namely, one initial pass, two passes of high-frequency, low-amplitude vibration compaction, and two final passes. Special attention: During construction, if the roller becomes sticky, it is best to use a solution of laundry detergent and water.

　　7) Quality Control of SBS Asphalt Mixtures

　　For asphalt pavement mixtures, the compaction effect at the site should be controlled simultaneously by both void ratio and compaction degree. The maximum theoretical density required for calculating the void ratio shall be based on daily field measurements, and the test shall be conducted according to “Test Method for Maximum Relative Density of Asphalt Pavement Mixtures (Vacuum Method) (T0711-93).” The void ratio of in-situ asphalt mixtures typically ranges from 3% to 6%. For the compaction degree of surface-layer asphalt mixtures, inspection shall be based on core samples actually obtained from the site.

　　4 Conclusion

　　As SBS-modified asphalt continues to be promoted in China, it will undoubtedly play an important role in the country’s road construction. We hope that the construction technical requirements outlined above—regarding the production and construction of SBS-modified asphalt—will prove helpful to all of you. We also welcome valuable feedback from our colleagues regarding any shortcomings or areas for improvement.