Abstract: The application of old cement concrete pavement gravel technology is one of the best technologies for the maintenance and reconstruction of old cement concrete pavement at present.

Key words: gravel technology; Construction quality standards; Structural combination; Conditions of use

1 overview

1. 1 Definition of Fragments

Cement concrete pavement crushing is a crushing treatment technology of old cement concrete pavement and an important means of overhaul or reconstruction of old cement concrete pavement. This technology is to crush the old cement concrete pavement slab into a flexible structure embedded with debris by special equipment at one time, which can make full use of the residual strength of the old road, protect the environment and save resources. This structure not only has a certain bearing capacity, but also has the function of preventing or limiting the occurrence and development of reflection cracks. After crushing, the particle size is 2~40cm, and the mechanical mode tends to be graded gravel.

1.2 main features of crushing technology

Through rolling, the structural strength of the old cement concrete pavement is reduced to some extent, which can prevent the occurrence of reflection cracks and achieve a better balance between structural strength and reflection cracks. After the old cement concrete pavement is crushed, it has the following characteristics: crushing can make the original cement concrete slab have uniform strength distribution on the plane; Rolling can retain the strength of the original cement concrete pavement; Crushing can eliminate the diseases of the original cement concrete pavement; Gravel particle size is reasonable, and stress concentration will not occur.

1.3 main advantages of crushing technology

The old cement concrete pavement can be directly used as the base or subbase of the new pavement structure after rolling. If the old cement concrete pavement has high strength and can meet the requirements of road bearing, it can be used as the pavement base for direct covering. The new pavement layer can be asphalt concrete pavement or cement concrete pavement.

1.4 special equipment and characteristics of crushing technology

The main equipment for crushing is MHB (multi-hammer crusher) multi-hammer crusher and Z-roll.

Multi-hammer crusher (MHB) consists of two parts, the first half is the diesel engine power system, and the second half is the crushing system. There are 3 pairs of 650kg hammers in the middle two rows and 865kg wing hammers on both sides/kloc-0. The lifting height of each pair of hammers can be adjusted as required, and the maximum lifting height is 1 10cm.

The crushing mechanism of MHB is to produce instantaneous and point-like impact on cement concrete slab through the fall of heavy hammer, which has the following characteristics: (1) multi-point crushing the width of the whole lane at one time; Hammer power can be easily adjusted; High crushing efficiency; The composition characteristics of the crushed particles are good; The surface flatness after crushing is high; Convenient adjustment and flexible operation.

Z-shaped roller is a vibrating roller with Z-shaped texture on the surface of steel wheel, and its weight is not less than 10 ton. Its function is to further crush the crushed road surface and provide a flat surface for paving.

1.5 strength formation mechanism of petrochemical technology

After rolling, cement concrete pavement is divided into three levels: loose surface layer, upper rolling layer and lower rolling layer.

The surface layer of (1) is about 2~5cm after being crushed. In the process of compaction, the particles are compacted to form an embedded thin layer, which has high adhesion, certain strength and stability through oil injection in the layer.

(2) The thickness of the upper part of the gravel layer is about 10cm, and the strength mainly includes: first, it comes from the internal friction angle, and the larger the particle size, the greater the internal friction angle; Second, it comes from prestress. When the cement concrete slab breaks, the concrete will expand laterally. The smaller the particle size of concrete, the greater the expansion trend and the greater the prestress.

(3) The thickness of the lower part of the gravel layer is about 10cm, which is a block structure of "cracking without breaking, good bonding and interlocking". This kind of structure is statically determined and self-stable, which is embodied in various forms of occlusal beam and dental arch structure. Under the action of external force, occlusal impaction occurs, which is larger than ordinary interlocking, provides higher strength and has better structural stability.

2 MHB macadam construction quality standard

2. 1 requirements for particle size range of broken pavement

Cement concrete slabs are generally between 20 and 26 cm, and the particle size of the top surface is smaller and the particle size of the lower part is larger after crushing. The particle size of broken pavement is the key parameter to control the strength of base and prevent early reflection cracks of newly paved pavement. As a key index to control the crushing process, referring to foreign data and domestic research results, the particle size of crushed pavement should meet the requirements in the table.

2.2 Requirements for equivalent resilient modulus and resilient deflection of the top surface of broken pavement.

The equivalent elastic modulus of the top surface of broken cement concrete pavement is one of the basic parameters in the design of new pavement structure. In general, for the pavement structure directly paved with asphalt concrete, the average resilience modulus should be controlled between 150~500MPa. There is a certain relationship between rebound deflection value and rebound modulus after gravel. When the broken plate and its substructure are regarded as the same material, it can be obtained by referring to the pavement reinforcement formula:

Ez=( 1000pD/l0)m 1m2

In which: p- deflection is used to measure the tire pressure of the vehicle;

D- bearing diameter equal to the track area of double circle of deflection measuring vehicle;

L0- original pavement deflection calculation value;

M 1—— The ratio of the deflection value measured on the original road surface with a standard axle load vehicle to the rebound deformation value measured with a bearing plate under the same pressure condition, that is, the wheel-plate ratio, which is generally taken as1.1;

M2 expansion coefficient of equivalent resilience modulus of original pavement.

2.3 MHB macadam construction quality standards and testing frequency

In order to meet the technical requirements of direct overlay and ensure the construction quality of overlay, according to the research of the subject and the test of the test road, combined with the specification requirements of pavement design, the quality standard and detection frequency of MHB macadam construction are put forward.

At present, there is no corresponding regulation in China's technical specifications for paving asphalt pavement directly with gravel layer as the base. This technical index requirement is put forward on the basis of referring to China's current technical standard "Technical Specification for Highway Pavement Base Construction" (JTJ034-2000) and the original technical standard (JTJ034-93), combined with the actual situation of the test road, and can be flexibly grasped in concrete implementation. If the surface smoothness of the gravel layer is quite different from the above requirements, it must be treated before paving the asphalt road. Treatment measures mainly include:

(1) Select the model of asphalt mixture according to the flatness;

(2) After leveling and rolling, fill graded macadam, spread hot asphalt or emulsified asphalt, and then compact;

(3) Use other appropriate technical measures for leveling. If leveling is not carried out, it may affect the smoothness of asphalt pavement and the use effect of pavement.

3 asphalt overlay structure combination after crushed stone

3. 1 structural combination principle

The research shows that the following structural combination principles can be adopted in the case of different particle sizes or elastic modulus that may occur in engineering:

(1) Crushing should refer to the recommended particle size range and resilience modulus as far as possible. Within this range, asphalt overlay needs dense graded asphalt concrete, and waterproof seal coat can be considered;

(2) When the particle size of crushed stone is slightly larger and the modulus of resilience is higher, the structural combination of open graded large-particle permeable asphalt crushed stone (LSPM for short) and waterproof seal coat can be considered, and the asphalt concrete on it still needs dense gradation;

(3) When the particle size is slightly smaller and the modulus of resilience after rolling is lower, in order to ensure the total thickness of the overlay, FDAC anti-fatigue layer can be considered to prevent fatigue cracking, and other asphalt layers still need dense gradation;

(4) When the modulus of resilience is less than 120MPa, it is necessary to consider adding reinforced layers and design according to the new pavement structure. 3.2 asphalt overlay four mechanism combinations

(1) After prime coat and seal coat, directly pave the upper, middle and lower layers of dense graded asphalt concrete;

(2) Laying LSPM, and then adopting the form of two layers and two sides;

(3) After the anti-fatigue layer is added, asphalt concrete is added;

(4) Pave the inorganic binder to stabilize the base, and then pave the asphalt surface.

According to the research results, the resilience modulus of crushed stone can be roughly divided into five grades, and the corresponding combination form of cover structure can be selected according to the standards in Table 3.

4. Applicable conditions and precautions of gravel technology

4. 1 Application conditions of crushing technology

4. 1. 1 crushing technical conditions

Crushing technology is one of the main schemes of old cement concrete pavement reconstruction technology. Research and engineering practice at home and abroad prove that as long as the old cement concrete pavement meets the conditions listed in Table 4, it can be reconstructed by crushing technology. Other factors, such as plate fracture degree, potholes, joint damage, surface cracks, delamination and peeling, are not necessary conditions to determine the application of crushing technology.

4. 1.2 Economic conditions of crushing

There is an economic balance between the application of gravel technology and the repair of the original pavement, which can be reflected by the repair rate. In foreign examples, the restoration ratio is about 13%, and the economic balance point of Shandong is that when the restoration area is 20%~25%, it is more economical to carry out crushing transformation.

4.2 Technical requirements for direct overlay

When paving asphalt pavement directly after cement concrete pavement is broken, the following principles should be followed:

The average resilience modulus of (1) is generally around 150~500MPa, and the resilience modulus of some original pavement cement concrete materials will be larger when they are good. In the field test, the individual values are 600MPa and 700MPa, so when designing the superstructure, the bending and tension index must be taken as the main design index.

(2) On the highway with higher grade, the asphalt concrete structure on the gravel layer should not be less than12 cm; ;

(3) The test section has been used in cement concrete pavement with 80% (the whole pavement) broken slabs, and there will be no problem if it is used below 80%;

(4) The upper layer must be dense graded waterproof asphalt mixture;

(5) Drainage facilities must be improved;

(6) The degree of gravelling is high, and when the measured data of resilience modulus is small, attention should be paid to the fatigue resistance of the lower layer.

4.3 Matters needing attention in the application of gravel technology

On the premise of meeting the requirements of technical and economic conditions, the following factors need to be comprehensively considered before using MHB for crushing:

(1) The damage degree of cement concrete pavement base determines the particle control and technological requirements of its gravel construction. For severely damaged cement concrete pavement, it is necessary to judge its base state. Generally speaking, the higher the degree of damage at the grass-roots level. The smaller the particle size after crushing.

(2) The damage degree of cement concrete pavement base is an important criterion to judge whether the severely diseased pavement can be rolled by gravel technology; When the base is seriously damaged, the broken board is easy to lose the embedding function between particles, which leads to the decrease of modulus and the fatigue failure of asphalt pavement layer. At this time, the application of gravel should pay attention to improving the design safety of the upper pavement structure.

(3) Drainage facilities are necessary auxiliary works for sand and gravel. Perfecting drainage facilities is an important measure to prevent water damage of asphalt overlay after gravel.

All the requirements here, * * * together constitute the application conditions and decision-making basis of gravel technology, which is the necessary condition to determine whether the old cement concrete pavement can implement gravel technology and whether the asphalt concrete pavement can be paved directly.

Conclusion: The key technologies such as application conditions, strength mechanism, overlay combination, construction quality standards and detection frequency of crushed stone are emphatically introduced, which can provide reference for the reconstruction of old cement concrete pavement.

refer to

[1] Application and discussion on the technology of cement concrete pavement crushed stone reconstruction [J]. Beijing: Expressway, May 2004.

[2] Study on the application of flexible base with large crushed stone asphalt mixture in old road reinforcement [J].Xi 'an: China Journal of Highway and Transportation, March 2004.

[3] Application of MHB Equipment in Cement Concrete Pavement Crushing [J]. Beijing: Highway Traffic Technology, March 2005.

[4] Asphalt overlay design after the old cement concrete pavement is broken [J]. Beijing: Highway Traffic Technology, February 2006.

[5] Brief introduction of foreign cement concrete pavement crushing technology [J]. Beijing: Expressway, September 2003.

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