The paving thickness of asphalt pavement is the core index to determine the road bearing capacity, driving comfort, waterproof and impermeability performance and service life, and is also the key control item of road construction quality acceptance.Insufficient thickness will cause road subsidence, cracking, rutting, seepage damage and other diseases, too thick thickness will cause material waste, increase engineering costs, but also may affect the overall structural stability of the road.There are clear specifications and standards for highway and urban road engineering in China, and fine thickness requirements are formulated for different road grades, structural layers and asphalt mixture types. Combined with current standards such as Highway Asphalt Pavement Design Code (JTG D50-2017) and Urban Road Pavement Design Code (CJJ169-2012), this paper systematically combs the core requirements and construction control points of asphalt pavement paving thickness.

Core principles of asphalt pavement thickness design

Asphalt pavement adopts layered paving structure, which is divided into upper layer, middle layer and lower layer as a whole. Some pavements are equipped with functional structures such as sealing layer and penetration layer. The thickness of each layer shall follow three core principles.One is particle size matching principle, the minimum compaction thickness of asphalt mixture paving shall not be less than 2.5~3 times of the nominal maximum particle size of aggregate, so as to ensure the compaction and stability of mixture, and avoid the problems of excessive porosity and loose segregation; the other is load matching principle, the overall thickness of high-speed and heavy-load traffic pavement is larger, and the thickness of ordinary municipal roads and light traffic roads shall be appropriately reduced; Third, the principle of structural adaptation, combining the strength of base layer, subgrade stability and regional climate conditions to optimize the thickness of each layer, so as to ensure the overall stress balance of pavement.

Standard laying thickness of asphalt surface layer at different levels

Asphalt pavement structure layer function is different, thickness standard difference is significant, expressway, trunk highway, city road layer thickness has clear specification limit, is the core basis of engineering construction.
(1) The upper layer (wearing layer) is the surface layer of the road surface, which directly bears vehicle wear, rain erosion and temperature changes. It mainly has wear-resistant, anti-skid and waterproof functions, and its thickness is generally small.The core of expressway and urban expressway adopts fine-grained modified asphalt concrete (AC-13) and modified asphalt mastic macadam (SMA-13). The standard paving compaction thickness is 40mm, and the minimum paving thickness is not less than 30 mm.The upper layer thickness of urban trunk roads and ordinary trunk roads shall also be 40 mm. Light traffic branch roads and community roads shall be relaxed to 30~35mm, and it is strictly prohibited to be lower than 25 mm.
(II) The middle layer (load-bearing layer) is the main load-bearing structure of the pavement, bearing the vertical load of vehicles, resisting the deformation of the pavement and rutting diseases, and the thickness shall be adapted to the traffic load grade.Medium-grained modified asphalt concrete (AC-20) is generally used for high-grade highways and heavy-duty traffic roads, and the standard compaction thickness is 60mm; the thickness of the middle layer of ordinary trunk highways and urban trunk roads is controlled at 50~60mm; light traffic roads can be simplified, and the thickness is not less than 40 mm.The special specification for heavy traffic is clear. The thickness of asphalt middle course of heavy traffic trunk highway shall not be less than 50mm to ensure the fatigue resistance and deformation resistance of pavement.

Requirements for total thickness of asphalt pavement of different grades of roads

- Expressway and Class I highway: the total thickness of conventional three-layer asphalt surface layer is 180mm (40mm upper layer +60mm middle layer +80mm lower layer), the heavy load expansion section can be optimized to 190~ 200mm, and the total thickness of the overall pavement structure including base and subbase shall not be less than 760mm.
- Urban expressway and trunk road: the total thickness of asphalt pavement shall be controlled at 140~ 180mm, and the double-layer structure of 40mm +50mm shall be adopted for light traffic trunk road, with the total thickness of more than 90mm.

Construction thickness deviation control and acceptance criteria

The allowable deviation value of surface layer thickness of high-grade highway and urban expressway shall be ±5mm, and the deviation of single point extreme value shall not exceed-8 mm; the allowable deviation value of ordinary highway and urban main and secondary trunk road shall be ±10mm, and there shall be no large area of insufficient thickness.The acceptance shall adopt drilling core and radar scanning detection mode. The qualified rate of single point thickness shall reach more than 95%. The unqualified points shall be reworked and rectified to eliminate the hidden danger of thin layer construction.
In construction, the substandard thickness mostly results from improper paving control, compaction deviation, insufficient flatness of base course and so on. Targeted control can effectively ensure the construction quality.Firstly, strictly control the paving thickness, calibrate the paver sensor and balance beam in advance, and pave at a constant speed to avoid uneven thickness caused by sudden and slow speed; secondly, accurately control the compaction process, strictly follow the process of initial pressing, re-pressing and final pressing, adjust the compaction speed and times according to the mixture temperature, and eliminate thickness deviation caused by underpressure and overpressure; thirdly, deal with base diseases in advance, and rectify the flatness and elevation deviation of base course in advance to avoid uneven thickness of asphalt surface caused by uneven base course; Fourthly, the mixture shall be selected by classification, and the mixture of structural layer shall be matched strictly according to the principle of particle size matching, so as to eliminate the problem of non-compaction caused by the laying of thin layer of large-particle mixture.
Asphalt pavement laying thickness is the foundation guarantee for the safe and durable operation of pavement structure. The whole construction process shall strictly comply with the current national specifications and standards, and accurately control the layered thickness and total thickness according to road grade, traffic load and structure level.From mixture selection, virtual paving thickness calculation to paving, compaction and acceptance, the fine control of the whole process can not only avoid pavement diseases caused by insufficient thickness, but also avoid material waste and cost exceeding standard, comprehensively guarantee the traffic quality and long-term service life of asphalt pavement, and lay a solid core foundation for road engineering quality standard.