In asphalt pavement construction, asphalt content is the core parameter to determine the pavement performance, which directly affects the smoothness, compactness, waterproof, rutting resistance, cracking resistance and service life of pavement.Too much asphalt will cause road surface diseases such as oil flooding, encapsulation, rutting and displacement; too little asphalt will cause loose mixture, excessive void ratio, easy water seepage, threshing and cracking, which will greatly reduce road durability.There is no fixed and uniform asphalt addition value in the project, which needs to be accurately determined in combination with mixture type, raw material characteristics, construction technology and specifications.Combined with current highway engineering specifications and practical operation experience, this paper comprehensively explains the value range, determination method and control points of asphalt addition amount of asphalt mixture.

Common asphalt mixture conventional asphalt content interval

Asphalt content is marked by asphalt aggregate ratio (percentage of asphalt mass to total mass of mineral aggregate) or asphalt content (percentage of asphalt mass to total mass of mixture) in the project, and asphalt aggregate ratio is generally used in domestic road construction.The conventional optimum dosage interval of hot mix asphalt mixture of different types and particle sizes is relatively fixed, which is the basic basis of preliminary construction proportion.
1. Fine-grained asphalt concrete (AC-10, AC-13): mostly used in pavement upper surface layer, wearing layer, aggregate particle size is small, specific surface area is large, more asphalt is needed to wrap aggregate, the best asphalt aggregate ratio is generally 4.8%~5.8%, modified asphalt mixture value deviation interval upper limit.
2. Medium-grained asphalt concrete (AC-16, AC-20): mostly used in pavement surface layer, trunk road structure layer, comprehensive performance balance, the best asphalt aggregate ratio is 4.2%~5.0%, is the most commonly used proportion interval for urban roads and ordinary highways.
3. Coarse-grained asphalt concrete (AC-25, AC-30): mostly used in the lower layer of pavement, base load-bearing structure, large aggregate particle size, small specific surface area, low asphalt content, the best asphalt ratio is 3.8%~4.5%.
4. Special function mixture: SMA asphalt mastic macadam mixture skeleton dense structure, high asphalt content, the best asphalt aggregate ratio of about 5.5%~6.5%;OGFC open-graded permeable asphalt mixture porosity, asphalt aggregate ratio generally controlled in 4.5%~5.2%, need to give consideration to water permeability and aggregate bond strength.
The above interval is the engineering experience reference value, which shall not be directly applied in formal construction. The accurate optimal dosage must be determined through laboratory mixture ratio test, and the test result shall not exceed the dosage range given in the specification.

Standard method for determining optimum asphalt dosage

The asphalt mixture ratio design of highway engineering in my country takes the Marshall test method as the core standard, combined with the Superpave design method to assist verification, and accurately determines the optimal asphalt content (OAC) in three stages. The whole process strictly follows the "Specifications for Asphalt Pavement Construction and Acceptance""requirements.

Experimental grouping design

In the preliminary estimated dosage interval, 5 groups of different asphalt dosage are set at intervals of 0.5% to prepare standard Marshall test pieces, so as to ensure that the mineral aggregate gradation, mixing temperature and compaction technology are completely consistent, and other variable interference is eliminated.

Core indicators detection

Test key technical indexes of each group of test pieces, including gross volume relative density, Marshall stability, flow value, void fraction, mineral aggregate void fraction (VMA) and asphalt saturation (VFA), and all indexes shall meet the specification limits.Among them, porosity is the core control index, and the target porosity of conventional asphalt mixture is mostly controlled at about 4%.

Optimum dosage

The first step of calculating the value is to determine the asphalt dosage corresponding to the optimal index: take the four asphalt dosages corresponding to the maximum bulk density, the maximum Marshall stability, the standard void ratio median and the standard saturation median respectively, and calculate the average value to obtain the initial optimal dosage OAC1.
The second step is to determine the upper and lower limits of asphalt content allowed by the specification, and obtain OAC2 by taking the median of the interval.
The third step, the final optimal asphalt content OAC is the average of OAC1 and OAC2 to ensure that mechanical properties and volume indicators are considered.

Validation adjustment of production phase

The optimal dosage determined by the laboratory is the target mixture ratio parameter. Trial mixing and trial paving shall be carried out before formal production, and the standard asphalt dosage for production shall be finally determined by fine adjustment according to the actual conditions such as construction site temperature, compaction equipment and aggregate moisture content. Meanwhile, the pavement performance of the mixture shall be verified by rutting test and water permeability coefficient test.

Key factors affecting asphalt dosage

For the same type of mixture, the asphalt dosage is not fixed, and the characteristics of raw materials and construction conditions will directly change the optimal dosage. The core influencing factors are as follows:

Grading of mineral aggregate and aggregate characteristics

The finer the aggregate particle size, the higher the powder content, the larger the total specific surface area, more asphalt is needed to form a complete cohesive oil film, and the asphalt content needs to be appropriately increased; the mixture with high coarse aggregate proportion and coarse gradation has a small specific surface area, and the asphalt content can be appropriately reduced.At the same time, when the aggregate surface is rough and the water absorption rate is high, more asphalt will be adsorbed, and the dosage needs to be adjusted to compensate.

Bitumen types and modification methods

Ordinary base asphalt has low viscosity and low conventional dosage;SBS modified asphalt has high viscosity and strong cohesion. In order to ensure the workability and compactness of the mixture, the dosage needs to be increased by 0.2% to 0.5% compared with ordinary asphalt.Warm mix asphalt mixture can optimize asphalt content slightly and adapt to low temperature construction scene under the premise of ensuring performance.

Construction environment and technology

In summer construction at high temperature, the fluidity of asphalt is enhanced, and the dosage can be appropriately reduced by 0.1%~0.3% to avoid road flooding deformation; in autumn and winter construction at low temperature, the asphalt dosage needs to be slightly increased to ensure the adhesive strength and compaction effect of the mixture.The greater the compaction work, the stronger the compaction ability of equipment, the easier the mixture compaction, the optimal asphalt content can be slightly reduced.

Road usage scenarios

Expressway, heavy traffic sections, focus on rutting resistance, deformation resistance, need to appropriately reduce asphalt content within the specification range, reduce plastic deformation of mixture; municipal branch roads, scenic roads and other light load sections, can appropriately increase the content, improve pavement compactness, waterproof and wear resistance.

Common mistakes in construction and principles of dosage control

Common construction errors

In some construction sites, asphalt dosage is fixed only by experience, ignoring the influence of moisture content and gradation fluctuation of aggregate, which is easy to cause quality problems; excessive pursuit of compactness blindly increases asphalt dosage, resulting in insufficient pavement high temperature stability, package and rutting in summer; low asphalt dosage causes loose and seepage of mixture, freeze-thaw cracking and early damage in winter.

Core principles of dynamic regulation

One is to take the test data as the only basis, all dosage adjustments must be based on Marshall test and field trial paving results, and empirical values must be eliminated; the other is to make small adjustments gradually, and the adjustment range of single dosage shall not exceed 0.2%, so as to avoid sudden changes in performance; the third is to give consideration to multiple performance balance, which not only ensures compact bonding, but also considers comprehensive road performance such as high temperature rutting resistance, low temperature cracking resistance and water damage resistance.
There is no universal fixed value for the asphalt addition amount of asphalt mixture, and there is a clear empirical reference interval for all kinds of conventional mixtures, but the precise amount must be determined by Marshall test mix design, combined with the characteristics of raw materials, construction environment and road load level dynamic optimization.The core logic of the project is as follows: fine aggregate, modified asphalt, low temperature construction and light load scenario shall be appropriately increased; coarse aggregate, matrix asphalt, high temperature construction and heavy load scenario shall be appropriately reduced. Finally, the optimal asphalt dosage shall be determined through test verification and on-site trial paving to ensure long-term stable and durable operation of asphalt pavement.