In asphalt pavement construction and maintenance projects, the precision of asphalt mixture design directly determines the quality and service life of the pavement. As the core indicator of the asphalt mixture design system, the stability of the bitumen / aggregate ratio serves as a critical defense line for controlling construction quality. Pavement defects such as loose surfaces, oil bleeding, rutting, and cracking caused by unstable bitumen / aggregate ratios not only increase maintenance costs but also compromise traffic safety, becoming a major bottleneck hindering project quality improvement. This paper systematically analyzes the signs, core causes, and impacts of bitumen / aggregate ratio instability on mixing plant performance, starting from its fundamental concepts and significance. It then proposes targeted solutions and long-term stability management strategies, providing comprehensive guidance for precise control of the bitumen / aggregate ratio during asphalt mixing plant production.
The bitumen / aggregate ratio is a core mix design parameter in asphalt mixtures. It specifically denotes the percentage value of asphalt weight relative to the total weight of mineral materials (including aggregates, fines, and powdered admixtures), calculated as: bitumen / aggregate ratio = (Bitumen Weight / Total Mineral Material Weight) × 100%. Typically, the bitumen / aggregate ratio for SMA (Stone Matrix Asphalt) binder-treated chip seal ranges from 5.5% to 5.6%, while for AC (Asphalt Concrete) fine-graded mixes, it falls between 4.5% and 6.01%. Simply put, it directly reflects the relative asphalt content in the mixture and serves as a key indicator for assessing the blending ratio between bitumen and mineral aggregates.
The bitumen / aggregate ratio is a core indicator for controlling the construction quality and service performance of asphalt pavements. Its rationality directly determines the pavement’s strength, stability, durability, and traffic safety, specifically manifested in the following aspects:
An unstable bitumen / aggregate ratio manifests through multiple observable and detectable indicators. These signs not only serve as early warnings of quality anomalies but also provide crucial directions for troubleshooting:
Fluctuating asphalt consumption data is the most direct quantitative indicator of an unstable bitumen / aggregate ratio. It manifests as deviations in asphalt consumption per unit of work volume from the design value, with fluctuations exceeding specifications. Core causes include: First, metering system failures such as zero-point drift in asphalt scales or mineral powder scales, or blockages/leaks in mineral powder silo doors. Second, deviations in raw materials and production parameters—such as mix design deviations, elevated fine aggregate content (which adsorbs more asphalt), or fluctuations in sand/stone moisture content and cold aggregate silo shortages—all trigger abnormal oil consumption.
In production control, daily verification of the bitumen / aggregate ratio can validate the bitumen / aggregate ratio. If verification results fluctuate frequently, promptly calibrate the measurement system or adjust the mix design.
Uneven asphalt color and texture serve as visual indicators of unstable bitumen / aggregate ratio, manifesting as “speckled mix” (insufficient aggregate coating), localized stickiness/dryness, or inconsistent density. Core causes include imbalanced bitumen / aggregate ratio and inadequate mixing: a low ratio often produces speckled mix, while a high ratio leads to localized stickiness and excessive gloss.
Three specific triggers exist: First, raw material issues where surface impurities or moisture on aggregates hinder bonding. Second, mix design deviations causing imbalanced bitumen / aggregate ratios. Third, mixing process defects where insufficient mixing time prevents proper fusion. Additionally, when aggregate moisture exceeds 7% after rainfall, fine materials adhere to undried aggregates, forming extensive patchy areas—particularly noticeable on rough surface aggregates.
A sudden surge in smoke and intensified pungent odors during mixing serve as critical early warnings of unstable bitumen / aggregate ratios, closely linked to excessively high ratios or abnormal heating temperatures. Asphalt releases smoke at construction temperatures between 160-250°C, with controllable emission levels under normal conditions. However, when the bitumen / aggregate ratio is too high, excess asphalt undergoes excessive high-temperature volatilization. Combined with abnormal temperatures caused by burner or damper malfunctions, this significantly increases smoke volume and intensifies odors (such as oily or burnt smells).
Additionally, excessively high clay content in fine aggregates adsorbs asphalt, passively increasing asphalt consumption and elevating the bitumen / aggregate ratio. Simultaneously, high-temperature reactions between clay particles and asphalt intensify odors. Excessive smoke contains harmful gases like carbon monoxide and sulfur dioxide, posing environmental pollution risks alongside safety and health hazards.
Frequent failure in mixture quality testing serves as core quantitative evidence of unstable bitumen / aggregate ratio. This ratio directly impacts critical indicators like Marshall stability, flow value, void ratio, and compaction degree. When these metrics frequently exceed specifications during standardized testing, it is highly probable due to an imbalanced bitumen / aggregate ratio.
Specific manifestations correlate with deviation direction: – When the bitumen / aggregate ratio is too low, mixture strength is insufficient, leading to Marshall stability below specifications (e.g., AC-13 type ≥8kN), void content exceeding the 3%-5% design range, low compaction, and loose core samples. When the bitumen / aggregate ratio is too high, internal friction within the aggregate decreases, leading to insufficient high-temperature stability. The flow value exceeds the upper limit of 4.5mm, increasing susceptibility to rutting. Additionally, compaction degree becomes excessively high, raising the risk of bleeding.
Furthermore, testing errors in the bitumen / aggregate ratio (e.g., mineral powder leakage in centrifugal extraction methods, non-representative sampling) can also cause test failures. These issues require resolution through standardized procedures and personnel training.
The impact of unstable bitumen / aggregate ratios ultimately manifests in construction outcomes, triggering field problems and customer complaints—the most direct end-user manifestations. Problem characteristics vary depending on the direction of deviation from the optimal bitumen / aggregate ratio.
When the bitumen / aggregate ratio is too low, insufficient asphalt bonding leads to pavement defects such as loose surfaces, pitting, cracking, and water penetration. When the bitumen / aggregate ratio is too high, excess asphalt causes oil bleeding, glossy surfaces, rutting, and oil pockets, compromising smoothness and skid resistance while endangering traffic safety.
Common complaints center on poor smoothness, premature cracking/oil bleeding/looseness, and water penetration during rain. These require tracing back to production stages to investigate metering systems, raw material quality, mixing processes, and other factors to address the root causes of unstable bitumen / aggregate ratio.
The root causes of unstable bitumen / aggregate ratio can be traced to multiple critical stages throughout the production process, encompassing equipment systems, raw material characteristics, control procedures, and human management. Key contributing factors include:
The measurement precision of bitumen weighing systems directly determines the foundational accuracy of the bitumen / aggregate ratio. Inaccuracies primarily stem from three issues: First, aging load cells. Prolonged exposure to high temperatures and vibration causes internal component wear or performance degradation, leading to distorted signal transmission and inaccurate reflection of actual asphalt weight. Second, infrequent calibration. Failure to calibrate asphalt scales regularly per specifications allows measurement deviations to accumulate beyond permissible limits as equipment wears. Third, asphalt temperature impacts weighing accuracy. Within the construction temperature range of 160-250°C, temperature fluctuations cause density variations. Without dynamic temperature compensation in the weighing system, weight measurements become inaccurate.
Unstable aggregate moisture content is a critical raw material factor disrupting the bitumen / aggregate ratio balance, manifested as follows: First, seasonal and weather-related moisture variations cause significant increases during rainy seasons and plum rains, while winter dry conditions result in lower moisture levels. Cyclical environmental changes make maintaining constant aggregate moisture challenging; Second, inadequate aggregate drying performance—due to insufficient drying equipment capacity or insufficient drying time—fails to reduce aggregate moisture content to the design control range, resulting in residual moisture. Third, residual moisture directly impacts aggregate effective weight by increasing apparent weight, causing metering systems to misjudge aggregate usage. This leads to deviations in the actual bitumen / aggregate ratio from the designed mix proportion. Each 1% fluctuation in moisture content may also reduce the mixing plant’s production capacity by approximately 10%, further exacerbating mix imbalance.
The stability of aggregate feeding systems directly impacts measurement accuracy. Common problems include: First, belt scale wear—prolonged conveying of hard aggregates causes abrasion of the weighing surface and tension changes, resulting in inaccurate weight signals from load cells; Second, uneven gate openings in feeders due to malfunctioning or worn gate adjustment mechanisms cause fluctuating aggregate discharge rates at different times, preventing uniform and quantitative feeding. Third, blockages or segregation in cold aggregate bins—where aggregate may separate due to particle size differences during storage or clog bin openings due to moisture adhesion—result in discharge interruptions or sudden flow changes, disrupting feeding continuity.
The bitumen pump and pipeline system is responsible for asphalt transportation and metered supply. Its malfunction directly causes abnormal asphalt supply:
First, pump wear or unstable pressure. After prolonged operation, internal seals and impellers in the asphalt pump wear out, leading to pump leakage or fluctuating output pressure, causing asphalt flow rates to fluctuate erratically. Second, trapped air in pipelines. After switching asphalt delivery lines or system maintenance, residual air not fully purged from pipelines distorts flow measurement, disrupting accuracy like “adding sand to the mix.” Third, inadequate temperature control causes asphalt viscosity changes. Malfunctions in pipeline heating/insulation systems allow asphalt temperature to deviate from setpoints, altering viscosity and affecting flow velocity within pipelines, which in turn causes supply fluctuations per unit time.
As the “brain” of the production process, failures in the control system and software can cause the bitumen / aggregate ratio control logic to malfunction:
First, outdated PLC programs. As production processes upgrade or raw material properties change, existing PLC (Programmable Logic Controller) programs may not be updated synchronously, failing to adapt to new production requirements and causing control algorithms to lag or deviate. Second, incorrect parameter settings occur when operators input critical parameters like mix proportions or temperature compensation coefficients erroneously, or fail to confirm adjustments promptly, causing the system to operate based on incorrect data. Third, communication failures between weighing modules arise when signal transmission between the control system and weighing modules (e.g., asphalt scales, aggregate scales) is disrupted by electromagnetic interference or aging wiring, leading to communication interruptions or data delays that destabilize coordinated module control.
The standardization of human operations and management is crucial for ensuring stable bitumen / aggregate ratio. Key issues include: First, inadequate operator training, where personnel lack sufficient understanding of equipment principles, parameter settings, and fault diagnosis methods, hindering timely identification and handling of abnormal signals during production; Second, erroneous manual interventions occur during automated production. Due to inexperience or misjudgment, operators arbitrarily alter equipment parameters or measurement results, disrupting stable bitumen / aggregate ratio control. Third, the absence of standardized operating procedures (SOPs) and well-defined production workflows, equipment calibration standards, and quality control protocols leads to operational variations across shifts and personnel, increasing the risk of bitumen / aggregate ratio fluctuations.
The stability of the asphalt mixing ratio directly affects the operational efficiency, production costs, equipment wear, and operational reliability of asphalt mixing plants. An unstable ratio negatively impacts plant performance in multiple dimensions, specifically:
An unstable bitumen/aggregate ratio directly disrupts the consistency balance of the asphalt mixture, leading to decreased workability. When the bitumen/aggregate ratio is too high, excess asphalt forms excessive “free asphalt” between aggregate particles, acting as excessive lubrication. This significantly reduces internal friction between particles, lowering mixture consistency and causing it to become soft and loose. This often leads to segregation and oil bleeding issues, making it difficult to control surface smoothness during subsequent paving. When the bitumen/aggregate ratio is too low, insufficient asphalt fails to fully coat aggregate particles, resulting in uneven mixture consistency and localized loose areas. This can create “white spots” that compromise the mixture’s cohesive integrity. Such abnormal consistency fluctuations cause the mixing plant’s output to fail construction specification requirements, necessitating frequent production parameter adjustments and indirectly reducing the plant’s effective production efficiency.
Unstable bitumen/aggregate ratios significantly elevate fuel and asphalt consumption costs at mixing plants. On one hand, when aggregate moisture fluctuations disrupt the bitumen/aggregate ratio, drying equipment must consume more fuel to extend drying time or increase drying temperatures to reduce aggregate moisture and ensure mix quality. Statistics show that a 1% fluctuation in aggregate moisture reduces plant production capacity by approximately 10%, with corresponding increases in fuel consumption. On the other hand, an excessively high bitumen/aggregate ratio directly increases asphalt consumption. Conversely, an insufficient ratio resulting in substandard mix quality necessitates reprocessing of scrap material, consuming additional asphalt and fuel. Furthermore, abnormal mix consistency caused by unstable ratios complicates mixing, requiring extended mixing times and further fuel consumption. Furthermore, deviations in the bitumen/aggregate ratio caused by metering system errors may lead to continuous, excessive asphalt consumption at the mixing plant without awareness, resulting in material waste.
Unstable bitumen/aggregate ratios accelerate wear on core mixing plant components, leading to higher maintenance frequency and increased maintenance costs. When the bitumen/aggregate ratio is too high, the soft mixture adheres to surfaces like the mixing drum, mixing arms, and blades, forming stubborn asphalt deposits that are difficult to clean. This not only reduces mixing efficiency but also causes uneven stress distribution, accelerating wear and deformation of mixing arms and blades. Simultaneously, excess asphalt seeps into component joints, corroding seals and triggering oil leaks or material spills. When the bitumen/aggregate ratio is too low, inadequately coated hard mineral particles generate stronger impact and friction during mixing, intensifying wear on mixing components—particularly noticeable on mixing blades. Furthermore, unstable bitumen/aggregate ratios necessitate frequent shutdowns for equipment parameter adjustments and component cleaning, further increasing maintenance frequency and downtime while reducing the mixing plant’s continuous operation capability.
Quality issues in the mix caused by unstable bitumen/aggregate ratios directly undermine the cooperative trust between the mixing plant, contractors, and project owners. Abnormal mix consistency and failed quality inspections resulting from imbalanced ratios lead to pavement defects such as loose surfaces, cracking, oil bleeding, and rutting. These defects compromise project quality and service life, triggering complaints and accountability claims from project owners. In modern highway construction, project management often employs intelligent monitoring platforms that collect real-time production data such as bitumen/aggregate ratios and mix proportions from mixing plants. Frequent fluctuations in the bitumen/aggregate ratio trigger platform alerts, indicating inadequate production process control and damaging the mixing plant’s industry reputation. For long-term cooperative projects, diminished trust erodes the mixing plant’s competitive edge and may even hinder future market expansion.
To tackle the core causes of bitumen/aggregate ratio instability, a comprehensive closed-loop control system must be implemented across the entire process. This includes targeted measures in measurement accuracy, raw material control, equipment maintenance, system upgrades, and personnel management to ensure the bitumen/aggregate ratio remains stable within design parameters. Specific key measures are as follows:
Asphalt weighing precision forms the foundation for controlling the bitumen/aggregate ratio. Reliability must be reinforced through three safeguards: First, implement regular calibration by performing zero-point and full-scale calibrations on asphalt scales at least monthly per specifications. Metrological certification is required before commissioning new equipment and after major overhauls to ensure weighing errors remain within ±0.5%. Second, upgrade to high-precision load cells by replacing aged or degraded sensors with high-accuracy units resistant to high temperatures and vibrations, enhancing signal transmission stability and accuracy. Third, install an asphalt temperature compensation system to collect real-time asphalt temperature data. System algorithms dynamically correct density fluctuations caused by temperature changes, ensuring consistent weighing results across varying temperatures.
Fluctuations in aggregate moisture are a key factor causing bitumen/aggregate ratio imbalance. Comprehensive control is required across detection, storage, and drying processes:
First, deploy online moisture sensors by installing real-time moisture detection equipment at aggregate conveyor belts. Synchronize data with the control system to enable dynamic moisture monitoring and automatic compensation, triggering alerts when moisture fluctuations exceed 0.5%. Second, enhance aggregate yard management by implementing zoned storage and rain-proof covering to shield aggregates from precipitation. Store aggregates from different batches and moisture levels separately; mixing is strictly prohibited. Third, optimize dryer performance and flame control. Conduct regular maintenance on drying equipment, clean accumulated material from drums to ensure uniform heating. Precisely adjust combustion intensity via an intelligent flame control system and dynamically modify drying time based on online moisture data to consistently maintain aggregate discharge moisture content below 0.5%.
Ensuring uniform and quantitative aggregate supply is critical for stable bitumen/aggregate ratios. Key optimizations for feeding and weighing include:
First, replace worn belt scales. Regularly inspect weighing surface wear and tension status, replacing scales promptly when wear exceeds limits to ensure measurement accuracy meets specifications. Second, balance the flow rate of the cold feeder. Repair and adjust the gate adjustment mechanism to ensure uniform gate opening. Implement closed-loop flow control using variable frequency speed regulation to prevent discharge fluctuations. Third, ensure uniform aggregate gradation by strictly controlling particle size distribution during crushing and screening. Install segregation prevention devices in cold aggregate bins to prevent pile-ups and blockages caused by gradation variations, guaranteeing continuous and uniform discharge.
The stable operation of the bitumen supply system directly impacts bitumen delivery volume. Establish a routine maintenance mechanism:
First, conduct regular inspections of pumps and valves. Weekly checks should cover the seals and impeller wear of asphalt pumps, with timely replacement of damaged components. Monthly maintenance of valves in asphalt delivery pipelines ensures smooth operation and leak-free performance, maintaining stable asphalt delivery pressure. Second, maintain proper pipe insulation and heating. Fully insulate asphalt delivery pipelines and regularly inspect heating devices to prevent asphalt cooling and solidification within pipes or localized overheating. Third, maintain asphalt temperature within optimal ranges. Stabilize delivery and mixing temperatures at 160-180°C based on asphalt grade specifications. Utilize intelligent temperature control systems to dynamically adjust heating power, preventing viscosity fluctuations caused by excessive or insufficient temperatures and ensuring consistent asphalt flow rates.
Enhancing control system intelligence is central to achieving precise bitumen/aggregate ratio control:
First, adopt intelligent asphalt mixing plant control systems to replace outdated PLC systems. Select intelligent systems with multi-module collaborative control capabilities to improve control algorithm accuracy and response speed; Second, implement real-time data monitoring and alerts by establishing a production data monitoring platform. Continuously collect critical metrics such as bitumen/aggregate ratio, mix proportions, temperature, and flow rate. Set data thresholds to automatically trigger audible and visual alarms when the bitumen/aggregate ratio fluctuates beyond ±0.3%, with alerts pushed to management terminals. Third, integrate automatic mix ratio calibration. The system dynamically adjusts aggregate-to-asphalt ratios based on real-time moisture detection and weighing data, minimizing human intervention errors.
Operator compliance is critical to implementing all measures. Enhance training and process oversight through:
First, establish Standard Operating Procedures (SOPs) defining operational workflows and quality standards for equipment operation, parameter settings, calibration maintenance, and quality inspection. This ensures consistent practices across shifts and personnel. Second, conduct regular technical training, organizing at least one monthly session covering equipment principles, parameter adjustment standards, fault diagnosis and resolution, and safety protocols. Personnel must pass assessments before being authorized to work. Third, implement data-driven performance evaluations by incorporating metrics like bitumen/aggregate ratio stability and production yield rates into operator performance assessments. Quantifying operational quality through data incentivizes standardized practices.
With the application of intelligent technologies in asphalt mixing plants, advanced facilities integrate three core intelligent systems to enhance bitumen/aggregate ratio control precision at the source, enabling proactive prevention and precise resolution of instability issues. Specific solutions include:
The intelligent weighing and batching system is the core of bitumen/aggregate ratio control in advanced plants. Compared to traditional systems, it achieves measurement precision and batching coordination through multi-dimensional upgrades. The system employs high-precision digital load cells with automatic zero tracking and nonlinear compensation capabilities, capable of withstanding harsh conditions like high temperatures and vibrations. Weighing accuracy is maintained within ±0.2% (exceeding the traditional ±0.5% standard). Integrated with intelligent batching algorithms, it optimizes mixing logic based on aggregate properties and asphalt grade, synchronizes weighing rhythms across multiple scales, and prevents mix deviation. It also features self-diagnostic weighing data capabilities, automatically triggering calibration alerts and locking production upon sensor anomalies or measurement overruns to prevent substandard material output.
Addressing aggregate moisture fluctuations, the advanced mixing plant’s fully automatic moisture compensation system achieves closed-loop control through “real-time detection – dynamic calculation – precise compensation.” High-frequency microwave moisture sensors installed before and after aggregate drying collect real-time data transmitted to the central control system. Algorithms automatically calculate asphalt compensation volumes and drying parameter adjustments—within 0.5 seconds per 1% increase in aggregate moisture content. The system automatically increases asphalt supply and modifies dryer parameters to ensure post-drying moisture content remains below 0.3%. Compared to traditional manual adjustments, this system achieves over 10 times faster response speeds, eliminates human error, and prevents moisture content from affecting the asphalt-to-aggregate ratio at its source.
Advanced mixing plants integrate a quality control module to enable real-time monitoring and intelligent regulation of the bitumen/aggregate ratio throughout the entire process, implementing proactive quality management. The module combines three key functions: First, it employs online infrared spectroscopy to analyze asphalt content in the mix in real time, comparing it to design values and automatically triggering mix adjustments when deviations exceed ±0.3%. Second, leveraging big data algorithms, it predicts critical indicators like Marshall stability and void ratio based on production data such as bitumen/aggregate ratio and temperature, proactively mitigating quality risks. Third, it automatically records production data for each batch, creating traceable quality archives for efficient troubleshooting. Additionally, the module interfaces with the project’s intelligent management platform to enable remote monitoring and sharing of production data, enhancing quality control transparency.
Achieving long-term stability requires establishing a routine preventive closed-loop system. This involves daily inspections, regular maintenance, data monitoring, and continuous improvement to mitigate fluctuation risks at the source. Specific measures include:
Establish a tiered inspection mechanism to promptly identify potential hazards. Daily inspection priorities:
First, calibrate the measurement system’s zero point to ensure accurate benchmarks.
Second, verify critical equipment status—confirm asphalt pump pressure, pipeline temperature, etc., and check for leaks or jams.
Third, confirm raw material conditions—inspect aggregate rain protection, spot-check moisture content, and verify asphalt grade inventory. Fourth, production data verification to calculate asphalt-aggregate ratio deviations and document causes of anomalies.
Weekly inspection priorities: First, spot checks on weighing system accuracy to ensure error compliance; second, sensor and detection equipment calibration; third, equipment cleaning and lubrication, clearing accumulated materials and maintaining moving parts; fourth, safety and environmental facility inspections to ensure proper operation of exhaust treatment equipment, etc.
Develop a full-cycle maintenance plan specifying intervals based on equipment lifespan and operating conditions to prevent sudden failures affecting the bitumen/aggregate ratio. Short-term (monthly): Maintain metering systems, clean drying drums, calibrate control systems; Medium-term (quarterly): Overhaul asphalt supply systems, clean aggregate bins, upgrade control software; Long-term (annual): Conduct in-depth inspections of core equipment, obtain third-party certification for metering systems, evaluate and upgrade smart modules.
Establish historical asphalt-to-aggregate ratio archives using intelligent systems to predict risks through trend monitoring. Periodically aggregate data to identify fluctuation patterns, common causes, and develop countermeasures. Utilize big data modeling to forecast volatility trends, trigger alerts, and provide adjustment recommendations (e.g., preemptively optimizing moisture content thresholds and asphalt compensation strategies during rainy seasons).
Establish an integrated internal-external audit mechanism to drive control system optimization. Conduct monthly internal self-inspections to review check records, maintenance execution, and identify weaknesses; perform semi-annual external audits to evaluate measurement accuracy and process compliance. Post-audit, compile issue lists and improvement reports, incorporate them into performance evaluations, and continuously refine control processes and technical measures.
In summary, maintaining stable bitumen/aggregate ratio at asphalt mixing plants is a core quality control point throughout the production process, directly impacting mix quality, pavement durability, and operational efficiency. Instability stems from multiple factors including equipment precision, raw material characteristics, system performance, and human operation, often leading to quality failures, equipment wear, and diminished trust. Resolving this issue requires establishing a closed-loop control system across the entire process. Targeted measures such as enhancing measurement accuracy and stabilizing raw material conditions should be implemented to control risks at the source. Advanced intelligent systems should be leveraged for precise prevention and control, supported by routine maintenance, data monitoring, and continuous audits to ensure long-term stability. In the future, the deep application of intelligent and digital technologies will drive more precise and efficient bitumen/aggregate ratio control, providing a foundation for improving asphalt pavement engineering quality and supporting the high-quality development of highway construction.
