It is mainly used for waterproof and impervious pavement, repairing micro-cracks, improving pavement smoothness, delaying pavement aging, adapting to high-speed, first-class highway and other high-load, high-traffic standard pavement. Different from ordinary highway slurry seal layer, high-grade pavement has higher requirements on wear resistance, interlayer adhesion, slip resistance and durability of seal layer. Scientific and accurate mixture ratio design is the core key to prevent peeling, grain loss, water seepage and displacement. According to Technical Specification for Preventive Maintenance of Highway Asphalt Pavement (JTG/T 5142-01-2021) and construction conditions of high-grade pavement, this paper comprehensively expounds mixture ratio selection, material parameters, design process, performance control and site fine adjustment standards of slurry seal. 

Selection of slurry seal adaptation gradation for high-grade pavement

Slurry seal is divided into ES-1, ES-2 and ES-3 standard gradations. Fine ES-1 gradation is strictly prohibited for high-grade asphalt pavement, ES-2 and ES-3 gradation suitable for heavy load, wear resistance and waterproof requirements are preferred. The applicable scenarios and mesh gradation control ranges of different gradations are as follows. The fillers shall be fully included in the mineral aggregate gradation statistics:
1. ES-2 type is medium and fine gradation, excellent compactness and waterproof property, suitable for overall overlay, anti-seepage maintenance and mild disease repair of high-grade pavement;ES-3 type is medium and coarse gradation, with stronger aggregate embeddedness and wear resistance and skid resistance, suitable for expressway lane, heavy load section and pavement skid resistance recovery construction, and is the main gradation type of high-grade pavement.
2. Standard mesh gradation control range The production gradation of high-grade pavement construction shall be strictly controlled. The allowable fluctuation range of each mesh passing rate is: ±4% for 4.75~1.18mm mesh, ±3% for 0.6~0.15mm mesh and ±2% for 0.075mm mesh. It is strictly prohibited to exceed the specification extreme value range.

The core material mix ratio parameter standard

Slurry seal mixture ratio consists of mineral aggregate, modified emulsified asphalt, mineral powder filler, mixing water and admixture. Modified emulsified asphalt system shall be adopted for high-grade pavement, and the content of each component shall strictly match performance index to prevent common emulsified asphalt from mixing.

Modified emulsified asphalt

SBR modified emulsified asphalt must be used for high-grade pavement, and ordinary cationic emulsified asphalt is strictly prohibited to ensure high temperature stability, low temperature crack resistance and interlayer bonding strength.The optimum asphalt dosage is 7.0%~9.0% for ES-2 type and 6.5%~8.5% for ES-3 type.Too low asphalt dosage will lead to loose aggregate, particle loss, insufficient wear resistance; too high dosage is easy to appear oil flooding, bag, road slip disease.

Coarse and fine aggregate

Aggregate shall be hard, wear-resistant and clean basalt and limestone sand, sand equivalent ≥65%, impurity-free and weathered particles.Through screening and blending, synthetic gradation is formed, which completely fits ES-2 and ES-3 gradation interval. Coarse aggregate provides skeleton extrusion strength, fine aggregate fills voids, and ensures dense and wear-resistant sealing layer.

The mineral powder filler

Aopts dry and clean limestone mineral powder without caking and impurities, and the doping amount is 5%~12% of the total mass of dry mineral material.Mineral powder can fill the gap of mixture, improve the compactness, optimize the demulsification effect of emulsified asphalt, greatly improve the waterproof performance and overall strength of seal layer, and is the key component of high grade pavement seal layer.

Mixing water

Drinking water is preferred, and sewage containing salt and pollutants is strictly prohibited.The total water consumption (including natural moisture content of aggregate + additional mixing water) shall be accurately controlled to ensure that the slurry mixture is uniformly flowing paste, free of segregation and agglomeration, and suitable for the construction requirements of paving equipment.

Special additive

According to construction temperature and wind speed, add retarding and accelerating additives flexibly, and the amount shall be determined by test, so as not to negatively affect road performance.Add retarder in high temperature weather above 25℃ to prolong mixing time; add coagulant in low temperature weather between 10 ℃ and 20℃ to accelerate demulsification molding and ensure construction continuity and molding quality.

Standardized mix ratio design process

It is strictly prohibited to directly apply empirical mix ratio to slurry seal layer of high-grade pavement. Systematic mix ratio design must be completed through indoor test, which is divided into five stages of test flow to finally determine the optimal construction mix ratio:

Raw material performance testing

The modified emulsified asphalt, aggregate, mineral powder and admixture shall be tested one by one, and each index shall conform to the construction standard of high-grade pavement. The unqualified materials shall be directly eliminated, and the basic quality of mixture ratio shall be controlled from the source.

Composite grading design

According to the selected ES-2 or ES-3 grading interval, through multi-grade aggregate screening and blending, the synthetic grading with the median value of the grading is debugged to ensure the continuous grading and dense skeleton, and avoid the problems of coarse loose grading and fine oil flooding.

Preliminary test selection

Select 1~3 groups of preliminary test proportion within the standard asphalt dosage range, carry out mixing test and cohesion test, and screen out reasonable preliminary test scheme with uniform mixing, no segregation and stable molding.At 25℃, the mixing time of fast open traffic type is ≥120s, and that of slow open traffic type is ≥180s.

Determination of optimum oil-stone ratio

5 groups of oil-stone ratios were set up according to ±0.3% gradient, taking the best oil-stone ratio of initial test as median value, and wet wheel wear and load wheel adhesion sand test were carried out.The upper and lower limits of the oilstone ratio were determined by taking the wet wheel wear value ≤800g/m² and the adhered sand amount ≤450g/m² as the control standards, and the optimal performance value was selected as the optimal oilstone ratio for design.

Calibration of production mix ratio

After the indoor mix ratio is determined, the test section of no less than 200m shall be paved. In combination with the fluctuation of field equipment, air temperature and aggregate moisture content, the mix ratio parameters shall be finely adjusted. The deviation of production mix ratio shall not exceed ±0.2%. The fluctuation of grading shall strictly comply with the allowable range of specifications. It shall be regarded as the formal construction mix ratio after acceptance by the supervisor.

Core performance control index of high-grade pavement mixture

The mix ratio design shall finally meet the road performance requirements, and all indexes of high-grade slurry sealing mixture shall meet the standards. The specific control standards are as follows:
- Cohesion index: 30min initial cohesion ≥1.2N·m, 60min forming cohesion ≥2.0N·m, ensure early forming fast, strong anti-rolling ability, avoid early traffic damage.
- Wear resistance: wet wheel wear loss ≤800g/m² after immersion at 25℃ for 1h, ensuring no particle loss and wear disease under repeated rolling of heavy-duty vehicles.
- Anti-sticking performance: the amount of sand adhered to the loaded wheel is ≤450g/m², effectively eliminating the problems of road surface flooding, sticking wheels and road pollution.
- Forming stability: Demulsification uniform, compact molding, no white, segregation, peeling phenomenon, waterproof and impervious performance to meet the long-term use requirements of high-grade pavement.

Dynamic fine adjustment and quality control points of field mixture ratio

Indoor proportion is the benchmark, and the site needs to be dynamically adjusted according to the working conditions to adapt to the complex construction environment and ensure the molding quality:

Temperature and humidity adjustment

In high temperature and dry weather, properly increase mixing water and fine adjust retarder content to avoid rapid demulsification and segregation of mixture; in low temperature and humid weather, reduce water consumption, properly increase modified asphalt content and coagulant content, accelerate curing and molding, and eliminate long-term loose and non-curing problems.

Aggregate moisture correction

Check the natural moisture content of aggregate before construction every day, draw the moisture content-dry mineral mass relationship curve, adjust the added water amount in real time, ensure the consistency of mixture is stable, and avoid excessive thin flow of slurry due to too high moisture content, and dry and uneven mixing due to too low moisture content.

It is strictly forbidden to change the proportion at will

During construction, it is forbidden to adjust the ratio of oil aggregate, gradation and additive content without permission.If the batch of raw materials is replaced and the working conditions change greatly, indoor test and test section paving shall be carried out again, and the production mix ratio shall be updated.At the same time, calibrate the metering system of paving vehicle every day to ensure accurate proportioning and metering.

VII. SUMMARY

The service life and pavement performance of slurry seal of high-grade asphalt pavement depend entirely on the accuracy of mixture ratio design and field control.During construction, it is necessary to strictly follow the JTG/T 5142-01-2021 specification, give priority to ES-2 and ES-3 standard gradations, adopt a modified emulsified asphalt system, determine the optimal oil-stone ratio and component dosage through indoor tests, and combine the on-site Dynamic fine-tuning of temperature, humidity and aggregate status.Strictly control gradation fluctuation, oil-stone ratio deviation and various performance indexes of mixture, avoid various proportioning diseases, effectively guarantee the core performance of slurry seal layer waterproof, wear resistance, anti-sliding and strong bonding, and meet the long-term, heavy-load and high-frequency traffic requirements of high-grade highway.