Stone Matrix Asphalt: A Durable Solution for Modern Road Networks

The global construction industry plays a pivotal role in shaping our urban environments; it also has a significant environmental footprint. From the extraction of raw materials to energy-intensive construction processes and construction waste, traditional construction methods have been associated with resource depletion and environmental degradation. However, there is a positive shift happening in the industry due to the growing emphasis on sustainable construction materials.

Selecting the right sustainable construction materials for road projects depends on factors like design requirements of the project, and environmental and climatic factors. Sustainable road construction practices consider the entire life cycle of the materials and the roads themselves to minimize environmental and social impacts while ensuring long-term durability and cost-effectiveness.

As traffic volumes surge on our road networks, the quest for paving materials with enhanced durability and reliability becomes ever more critical. Traditional options like Bituminous Concrete (BC) frequently fall short in addressing the challenges like heavier vehicle loads, increased truck traffic, and elevated temperatures. Stone Matrix Asphalt (SMA) emerges as a promising solution capable of meeting the demands of today's road infrastructure.

Stone Matrix Asphalt (SMA)

Developed in Germany during the 1960s, SMA is composed of high-quality aggregates, asphalt-rich mortar, mineral filler, and cellulose fibres, offering enhanced performance and longevity compared to traditional mixes. It quickly gained recognition for its superior resistance to rutting and exceptional durability.

In 2008, SMA specifications were set out in IRC: SP:79-2008, giving clear guidelines for using Stone Matrix Asphalt in modern road construction. These specifications detailed the range of binder content, mix composition, and construction methods to ensure the best performance and durability. Recently, IRC SP 79-2023 was published as an update to the earlier IRC specifications.

Advantages of SMA

Stone Matrix Asphalt (SMA) offers a significant advantage with its elevated binder content, typically falling within the range of 5.8% to 7%. This higher binder content provides an improved coating on the aggregate particles thereby enhancing its resistance against water infiltration leading to increasing the durability of the mixture. Mitigate bitumen, drain down during both transportation and paving, a small amount of cellulose fibre is introduced into the mix. Furthermore, with over 75% coarse aggregates, the SMA mix is less prone to deformation under traffic loading, enhancing the overall stability and longevity of the pavement.

Key Factors in Stone Matrix Asphalt Production

The successful implementation of SMA in any project necessitates a thorough integration of essential components and precise methodologies throughout each stage of the process. CubeTech has the experience of implementing SMA with polymer modified bitumen for a 20 km long National Highway section back in 2018 wherein the section was experiencing heavy loads and high temperatures.

Performance data of the same section collected for the past 5 years gives a promising picture. Currently, CubeTech is also monitoring the implementation of SMA in 3 National Highway projects as Project Management Consultants (PMC). The implementation experience along with the detailed technical knowledge, has allowed CubeTech to identify the critical factors influencing SMA production, wherein emphasis is provided on aggregate selection, filler usage, bitumen application, fibre incorporation, hot mix plant operations, transportation logistics, and execution strategies, which include trial patch protocols, paving procedures, and compaction techniques.

Aggregate: The Foundations

The preference for VSI (vertical shaft impactor) aggregates in SMA production is due to their exceptional quality, strength, and favourable shape characteristics, all of which contribute to the overall quality and performance of the final mix. To ensure optimal results, it's advisable to assign personnel at the crusher site to oversee tasks such as monitoring sieve conditions and addressing any operational challenges that may arise. Utilizing VSI dust instead of dust from cone and jaw crushers is recommended for enhanced outcomes in SMA production. Additionally, proper stockpiling of aggregates is crucial to maintain mix quality standards.

Filler: Consistency and Quality Control

On-site filler quality control is paramount. Verification of lime hydration and purity through iodine or sugar tests is essential, with a target purity of no less than 90%. Bag house filler assumes a pivotal role in SMA execution, underscoring the importance of checking its gradation in lower sieves, particularly at 45 microns, to ensure consistency. This consistency significantly impacts the optimal binder content for SMA.

Bitumen: Selecting the Best

Selecting the appropriate bitumen is crucial for SMA implementation. For projects exceeding 50 km in length, it is advisable to engage a minimum of three vendors to ensure timely supply, quality, and consistency. PG testing on every batch of binder and third-party quality checks on the binder at least once every two months is key to ensure that the material meets the relevant standards.

Fiber: A Critical Component

Choosing the appropriate fibers is a critical aspect of Stone Matrix Asphalt projects. Before settling on a particular type, it is essential to undergo comprehensive testing to assess factors such as the ash content, moisture content, bitumen content, and PH value of the fibers.

SMA Implementation Methodology

The methodology for designing Stone Matrix Asphalt involves a series of steps outlined in the flowchart (figure 2). The goal is to find the best combination of aggregate and asphalt for the mix.


To achieve this, the SMA mix design is carried out in the following locations:

• Third-Party Laboratory: This is a fully equipped lab where the mix design can be carefully tested and analysed.
• Field-Based Laboratory: This is a lab set up at the construction site, allowing for real-time testing and adjustments.

After verification of mix design at site lab, the SMA mix design can be fine-tuned to ensure optimal performance for the specific conditions of each project.

Hot Mix Plant (HMP): Production Standards

Utilizing batch mix plants equipped with auto-feeding mechanisms of fibre is recommended for SMA production. Drum mix plants are discouraged for SMA due to potential quality concerns. A silo for filler storage having a capacity of at least ten metric tons and adequate bitumen storage with circulation mechanism is essential. During HMP production, adequate dry and wet mixing durations should be ensured to achieve a homogeneous, well-coated mix.

Transportation of Mix: Safe Practices

It is advisable to use non-petroleum solvents for cleaning tipper trucks. Petroleum-based solvents have the potential to adversely affect the cohesion of the bituminous mix, which poses risks to the quality of the SMA. Alternatively, using a solution of detergent mixed with water can yield satisfactory results for cleaning.

Paver: Precision and Consistency

It is recommended to use multiplex technology paver for SMA. Tamping and screed vibration significantly impact compaction, with targets close to 90%. It is to be ensured that the paver is heated for at least 30 minutes before the start of paving and the paver speed should be within 2-3 meters per minute. Proper co-ordination and planning during material supply is key to avoid frequent stops during paving due to a lack of trucks or mix, as this creates multiple transverse joints and increases unevenness and roughness.

Roller: Optimal Compaction

Only static rollers with vibratory attachments are suitable for SMA. The breakdown pass should be vibratory, followed by a plain pass for intermediate and final rolling. Pneumatic type rollers (PTR) should be avoided. Rollers should not stop on the mat after rolling; instead, they should be kept on the shoulder. Use a non-nuclear density gauge (NNDG) to check SMA density and compaction, calibrated with core samples. A core diameter of 150 mm is recommended per MoRTH/IRC guidelines.

Trial Patch: Testing Before Implementation

Before laying SMA, trial patch/sections should be undertaken to determine rolling patterns, paver functioning, and proper mixing times. It is advised to perform the trial patch on the main carriageway to achieve these objectives.

Execution: Attention to Detail

The final execution should happen two days after the trial patch. This allows time to fix any issues found during the trial patch. Once the final mat is laid, let it set for at least 24 hours before allowing traffic on it. The water permeability test is an effective way to check how the mix reacts to moisture. It helps us see how the mix will perform in different weather conditions and how long it will last.

Performance Testing

Performance testing, such as rut resistance, indirect tensile strength (ITS), and resilient modulus, is vital for assessing the quality of SMA mixes and ensuring long-lasting and reliable roads. Proper mix design, high-quality production, and adherence to application standards are necessary to achieve the best results.

Conclusion

Stone Matrix Asphalt has proven to be a valuable paving material over the past six years of our experience, but its adoption did come with initial challenges. These challenges can be addressed by establishing comprehensive Standard Operating Procedures (SOPs) for the HMP plant, site lab, and execution processes. These SOPs ensure that all team members are aligned and working effectively to optimize the performance of SMA.

Despite the higher upfront costs associated with SMA, its longer service life offers a superior return on investment. SMA extends the lifespan of road networks by 5-10 years, significantly reducing the frequency and cost of maintenance over time. Proper application and adherence to established guidelines allow SMA to reach its full potential, providing more durable, long-lasting roads.