Asphalt pavement thickness is the core design and construction index to control pavement structural strength, rutting resistance, fatigue resistance and waterproof durability, which directly determines road traffic capacity, driving comfort and long-term service life.Insufficient thickness is easy to cause early diseases such as pavement cracking, settlement, seepage, displacement, rutting, etc. If the thickness exceeds the standard, it will cause asphalt material waste, increase project cost, and destroy the stress balance of the overall pavement structure.According to national highway standards such as Design Specification for Highway Asphalt Pavement (JTG D50-2017) and Technical Specification for Construction of Highway Asphalt Pavement (JTG F40-2004), combined with general construction design scheme of expressway pavement in China, this paper specially expounds the standard of layered paving thickness, design principle, thickness requirement under special working conditions and construction control points of acceptance of expressway asphalt pavement, providing accurate technical basis for expressway pavement construction, quality inspection and acceptance.

Core Design Principles of Asphalt Pavement Thickness of Expressway

Expressway traffic flow is large, heavy vehicles are many, driving speed is fast, pavement thickness design is different from ordinary highway and municipal road, follow three exclusive core principles, all layer thickness must meet the requirements of the specification bottom line.
First, the particle size matching principle.The minimum thickness of high-speed asphalt mixture after compaction shall not be lower than 2.5~3 times of the nominal maximum particle size of aggregate, so as to eliminate the problems such as non-compaction, excessive porosity and loose pavement caused by thin-layer paving of large-particle mixture, so as to ensure that the compaction degree of each structural layer reaches the standard.
Second, overload adaptation principle.According to the characteristics of high-speed, high-frequency and heavy-load and high-speed driving load, the layer thickness increases layer by layer, focusing on strengthening the bearing thickness of the middle and lower layers, improving the overall deformation resistance and fatigue resistance of the pavement, and resisting long-term vehicle rolling loss.
Third, the principle of structural synergy.Combined with the strength of high-speed subgrade, base stiffness, regional temperature difference and rainfall conditions, the thickness of each layer shall be matched to realize the stress balance of surface layer, base layer and subbase layer, and avoid structural damage caused by local stress concentration.

Requirements for Layered Thickness of Expressway Standard

The three-layer asphalt pavement structure (upper layer + middle layer + lower layer) is generally adopted for the main lines of newly-built and reconstructed expressways in China, which is a general standard structure in the industry. The type, function and compaction thickness of mixture at each level have clear standard fixed values, which is also the core design parameters of expressway construction drawings.

Upper layer (anti-skid layer)

The upper layer is the pavement surface layer, mainly responsible for wear resistance, anti-skid, waterproof and noise reduction functions, directly exposed to the natural environment and vehicle wear, using fine-grained modified asphalt mixture.SMA-13 modified asphalt mastic macadam or AC-13 fine-grained modified asphalt concrete shall be uniformly adopted for the main line of expressway. The standard compaction thickness shall be 40mm, and the minimum compaction thickness shall not be less than 30mm in the specification.The thickness can effectively guarantee the flatness and friction coefficient of the road surface, meet the safety requirements of high-speed driving, and resist the aging damage of the surface layer caused by ultraviolet rays and rainwater erosion.

Middle layer (main load-bearing layer)

The middle surface layer is the core load-bearing structure of expressway pavement. It can withstand vertical vehicle loads and horizontal shear forces. It is a key layer to resist rutting and fatigue cracking, and has the highest requirements for thickness.AC-20C medium-grain SBS modified asphalt concrete is adopted for high-speed special middle course, and the standard compaction thickness is 60mm.For special sections such as heavy traffic density, long downhill, front and rear toll stations, it is strictly prohibited to lower than this thickness to ensure the high temperature stability and shear resistance of the road surface, and eliminate permanent deformation caused by long-term heavy load.

Lower layer (basic force transmission layer)

The lower layer connects the asphalt surface layer and cement stabilized macadam base, and its main function is to disperse the upper load, transfer the buffer stress, seal the base interface, and ensure the overall structural stability of the pavement.AC-25C coarse grained asphalt concrete is adopted for the surface layer under the main line of expressway. The standard compaction thickness is 80mm, which is the layer with the largest thickness in the three-layer structure. It can effectively reduce the tensile stress at the bottom of the layer, improve the overall bearing capacity of the pavement, and avoid the pavement settlement caused by uneven stress of the base layer.

Standard total thickness requirements

The total compaction thickness of three-layer asphalt pavement on the main line of conventional newly-built expressway is 180mm (40mm +60mm +80mm), which is the general standard thickness of domestic expressway pavement.For the reconstruction and expansion of high-speed sections with heavy load expansion and large traffic volume, the thickness can be optimized to 190~200mm to further improve the durability of pavement.In the supporting pavement structure, the total thickness of high-speed base and subbase shall not be less than 560mm, and the total thickness of the overall pavement structure shall be more than 760mm, meeting the demand of high-speed long-term heavy-load traffic.

Special requirements for asphalt thickness of special sections of expressways

In addition to the standard sections of the main line, for special working conditions such as highway ramps, bridge decks, tunnels, and old road resurfacing, the thickness standards shall be subject to the special requirements of the specifications separately, and the main line standards shall not be applied.

Interchange ramp section

Expressway hub interchange, service interchange ramp speed is low, vehicles start and stop frequently, shear load is large, generally adopt double-layer asphalt structure, the standard total thickness is 100mm, namely 40mm fine particle modified asphalt upper layer +60mm medium particle asphalt lower layer, adapt to ramp driving force characteristics.

Bridge deck pavement section

The asphalt pavement layer of expressway bridge shall give consideration to waterproof, bonding and shock absorption functions, avoid damage of asphalt layer caused by deformation of bridge structure, standardize the total thickness of pavement as 100mm, adopt special asphalt mixture suitable for bridge stress, and prohibit super-thick or thin layer paving.

Road surface inside tunnel

The pavement of high-speed tunnel is mostly composed of composite asphalt structure. The thickness of the overall asphalt pavement shall refer to the main line standard. Meanwhile, flame retardant asphalt materials shall be adopted. The thickness deviation shall strictly comply with the acceptance standard of high-speed first-class pavement to ensure the tunnel traffic safety and pavement durability.

Asphalt overlay on old cement pavement

According to JTG D50-2017 specification, the minimum total thickness of asphalt overlay shall not be less than 100mm when the asphalt structure is paved on the old cement concrete pavement of expressway, which can effectively slow down the reflection cracks of the old pavement and ensure the stability of the overlay structure.

Description of functional thin layer structure

Permeable layer, adhesive layer, slurry sealing layer and waterproof adhesive layer in high-speed pavement construction are all functional thin layers, which only play the role of bonding, waterproof and sealing, and are not included in the total thickness of asphalt surface structure. In construction, only coating uniformity needs to be controlled, and there is no requirement for structural thickness.In addition, the high-speed asphalt penetration macadam layer is only used for special auxiliary sections, and the standard thickness is controlled at 40~80 mm.

Deviation standard for acceptance of asphalt thickness construction of expressway

Expressway belongs to high-grade highway, acceptance standard is stricter than ordinary highway, compaction molding thickness is the only acceptance basis, virtual paving thickness needs to be accurately calculated in advance according to mixture compaction coefficient (1.15~1.25).
According to the highway engineering quality acceptance specification, the allowable deviation of highway asphalt pavement thickness is ± 5mm, the maximum negative deviation of single point shall not exceed-8mm, and large area of insufficient thickness is strictly prohibited.The acceptance adopts the dual inspection methods of drilling core and road surface radar nondestructive scanning. The pass rate of single point of road surface thickness must reach more than 95%. The unqualified points must be thoroughly reworked and rectified. Thin layer covering and repair are strictly prohibited.

Key construction points of expressway thickness quality control

Thickness deviation of highway pavement is a high-frequency quality problem in construction, which is mainly caused by insufficient paving accuracy, improper compaction technology, poor flatness of base course and other factors. The whole construction process needs to be controlled emphatically.
One is to accurately calibrate paving equipment.Before construction, strictly calibrate the balance beam and ultrasonic sensor of the paver, set accurate virtual paving thickness, keep the paver uniform and continuous paving, and eliminate uneven thickness caused by midway shutdown and speed fluctuation.
The second is to strictly control the compaction process matching.Strictly implement the standardized process of initial compaction, recompaction and final compaction, dynamically adjust the compaction speed and compaction times according to the delivery temperature and paving temperature of asphalt mixture, and eliminate the problems of too large thickness and insufficient compaction caused by under-pressure or ultra-thin thickness and aggregate crushing caused by overpressure.
Third, rectify grass-roots diseases in advance.Before paving high-speed pavement, the elevation, flatness and disease defects of cement stabilized macadam base must be comprehensively rectified. The unevenness and elevation deviation of base will directly lead to insufficient local thickness of asphalt surface layer, which is an important cause of pavement damage in the later period.
Fourth, strictly match the particle size of the mixture.Resolutely put an end to the paving of thin layers of large-particle mixtures, strictly follow the matching specifications of particle size and thickness, ensure the compaction and stability of each structural layer, and avoid potential quality hazards from the source.

Conclusion

Asphalt pavement thickness of expressway is the basic main control index of structural safety and durable operation. All construction and design must strictly comply with national standards such as JTG D50-2017.The 180mm standard three-layer structure of the main line, the special thickness of ramp and bridge deck, and the accurate acceptance deviation of plus or minus 5mm constitute the core system of expressway pavement thickness control.Fine control shall be implemented during construction, from mixture selection, false paving thickness calculation to paving, rolling and acceptance, so as to avoid all kinds of pavement diseases caused by insufficient thickness, avoid material waste and cost out of control, and comprehensively guarantee the traffic quality, bearing capacity and long-term service life of expressway pavement.