1 Overview
Bridge deck pavement layer directly withstand the role of traffic load, beam deformation and environmental factors, its deformation and stress characteristics and the main beam and bridge deck structure type is closely related, on the one hand, can disperse the load and participate in the stress of the deck, on the other hand, the role of linking the main beam common stress; both the bridge deck protection layer and the bridge deck structure of the common stress layer, so it has sufficient strength and good integrity, and has sufficient Crack resistance, impact resistance, wear resistance.
In recent years, the rapid development of China’s highway bridge construction, bridge structure innovation, large span bridges have been very common, but the design and construction of the bridge deck pavement still follow the traditional customary practice, in the design of the bridge structure, the bridge deck pavement layer is generally not a special calculation analysis. With the increase of traffic and heavy vehicles, bridge deck pavement problems are common. This not only hinders the normal traffic, affects the beauty of the bridge deck, more likely to cause traffic accidents, but also brings great difficulties to the maintenance work. In recent years, people have paid enough attention to the direct and indirect economic losses caused by bridge deck pavement problems. The early damage of the bridge deck pavement has become a major disease affecting the function of the highway and inducing traffic accidents.
Bridge deck flexible pavement can greatly ease the impact of traffic on the bridge deck plate, easier to achieve the smooth and comfortable operation requirements, with the improvement of asphalt material performance, the application will be more widely. But the current specifications for the design of asphalt pavement structure mainly from the materials used, practices and thickness, etc. for guidance, on the specific design theory and methodology is still blank, the design of the pavement layer without a chapter to follow. This caused in the actual design, bridge deck pavement layer only as a subsidiary structure of the bridge project, the designer spent little effort on it, thus laying a hidden danger for the early damage to the bridge deck pavement. Therefore, we should accelerate the research on bridge deck pavement, especially the structural damage mechanism and design theory.
2 damage form
Asphalt concrete bridge deck pavement and normal pavement and cement concrete bridge deck pavement, compared with the damage form is different. There are mainly :
① pavement layer inside a large shear stress, causing uncertainty about the shear deformation of the damage surface, or due to poor bonding between the pavement and bridge deck plate layer, the horizontal shear resistance is weak, the relative displacement in the horizontal direction of shear damage, resulting in pushing, embracing the package and other diseases;
② due to temperature changes and accompanied by the bridge deck plate or beam structure of the large deflection and cracks, in the vehicle load and infiltration of water under the action of the surface layer loose and pit damage.
Europe and the United States since the 1970s in the bridge deck pavement widely used in waterproofing, with the increase in traffic, there are some new problems, such as early damage to the surface layer, cracking, potholes, waterproofing layer and surface layer and bridge deck bonding strength is not enough and produce nudging and other diseases. In recent years, in some areas of China, such as Beijing, Tianjin and other places of the bridge deck waterproofing layer also appeared in the corresponding disease.
With a waterproof layer of cement concrete bridge deck asphalt concrete pavement in the form of damage under the action of traffic load is generally shear damage, often manifested as a crowded package and pushing phenomenon. Shear damage in two cases: one is the bridge deck reinforced concrete modulus is much larger than the modulus of asphalt concrete and waterproofing layer, coupled with the asphalt concrete layer thickness is thin, asphalt layer within the larger shear stress caused by the shear deformation without determining the damage surface; two is the waterproofing layer and asphalt concrete surface layer and bridge deck layer between the bonding force is insufficient and shear damage occurs. Therefore, shear damage is a waterproof layer of cement concrete bridge deck asphalt concrete pavement damage is the main reason, so in the actual design should be based on two forms of shear damage are calculated and analyzed.
3 Disease analysis
3.1 Structural theory and design
(1) The structural theory of the bridge deck pavement is almost zero, the current specification only gives the recommended value of the thickness, the engineering community has been used in all levels of highway for decades. With the increase in traffic, the current pavement and heavy, super-heavy vehicles and the increase in speed has been incompatible. Bridge deck pavement layer directly bear the impact of wheel load, bridge deck pavement part or all involved in the deformation of the main beam structure, so the bridge deck pavement is a complex dynamic system, various forms of the main beam and pavement structure itself are affected by the distribution of its stress.
(2) The current bridge regulation 3.2.2 stipulates: …… If there is no accurate calculation method, the box girder can also refer to the T-shaped beam provisions [9]. From the design of many box girders, most of the designers treat the box girder members as T-shaped girders. The actual force of box girder is similar to that of T-beam and different from that of T-beam, and the difference is even greater for continuous box girder. Especially in recent years, the bridge deck of the box girder is getting wider and wider, the ratio of bridge span to bridge width is getting smaller and smaller, the box girder is still according to the T-beam kind of long thin bar design reinforcement, it is more and more unsuitable, resulting in the design of the box girder skeleton reinforcement by beam in the actual state of stress is difficult to play the role of the main reinforcement like T-beam. Therefore, the hypothetical state of design is not consistent with the actual stress state of the box girder.
(3) With the development of material industry and the improvement of bridge load-bearing structure, the main girders of bridges can achieve the force requirements with a more flexible structure, and the lateral width of large-span bridges of high-grade highways is getting wider and wider. In particular, the design calculation focuses on the longitudinal analysis of the main girder, but not enough attention is paid to the transverse stiffness of the bridge, and the transverse construction measures are unfavorable so that the bridge deck pavement shares too much of the secondary internal forces.
(4) for continuous beam bridge, arch bridge and cantilever beam bridge and other bridge structures, due to the role of load and negative moment or tension, so that the deck pavement layer by the action of tension and negative moment zone cracks, thus causing damage to the bridge deck pavement.
(5) in the traffic organization and management of the highway, due to the different functions of the lanes, artificially forced to make the bridge structure operation is always in the biased load state, so that the main lane of the pavement bear much higher than the super lane (the amount of value can be three to four times) the level of operational stress, so accelerate the fatigue of the main lane pavement layer. Especially with the development of private transport industry, freight owners in pursuit of short-term economic interests, by changing the carriage structure such as lengthening the carriage and raising the axle spring, etc. to make the car load, axle weight and wheel load multiplied. These vehicles have a serious destructive effect on the pavement, and make the bridge structure local overload, accelerating the development of the main lane pavement disease. Therefore, in the design should be based on the actual distribution of vehicle loads in operation, in a clear bridge structure on the basis of the force, the bridge deck pavement force calculation.
3.2 Construction process
(1) The thickness of the pavement layer is small.
Because the bridge superstructure in the construction of the bracket settlement and prestressing anti-arch can not be very accurate prediction, or due to poor control of the construction process, construction of the top elevation of the main beam with the design value is more difficult, generally in the measurement of the top elevation of the main beam to adjust it to ensure the thickness of the bridge deck. If the adjustment is not good, it will cause the thickness of the pavement layer is not uniform, so that the thickness of some places is small.
(2) the top of the beam is not cleaned up, resulting in a poor combination of pavement and the main beam.
3.3 The impact of bridge deck waterproofing layer
As the strength of the flexible waterproofing layer and the main board and pavement layer strength difference, its presence makes the upper structure according to the modulus of the formation of rigid-flexible-rigid plate body stress system, the middle of the flexible sandwich will increase the bridge deck plate plate in the middle of the plate bottom tensile stress. In the waterproofing layer on the pavement layer once cracked, in the dynamic load of the wheel, the gap between each other bigger and bigger, until the loose off. In addition, the use of waterproofing layer so that the pavement shear damage chance greatly increased.
3.4 Bridge deck pavement constraints
Bridge deck pavement by the constraints of the bridge structure, after loading its boundary conditions and the general roadway is very different, coupled with the deflection of the beam, twisting and other deformation of the coupling effect, to the pavement layer work performance caused by adverse effects.
4 Bridge deck pavement design method discussion
At present, the research on bridge deck pavement is still very immature, and the existing research mainly focuses on material design and pavement technology, while the research on theoretical analysis and structural calculation is very little.
In addition, the calculation method of the thickness of the asphalt pavement layer of the bridge deck was proposed by combining the requirements of the waterproof layer, levelness, construction process and rutting index. Zhang Zhanjun [11] et al. used the finite element method to analyze the calculation of interlayer shear stress in the asphalt pavement structure of cement concrete bridges with waterproofing layer, and discussed the influence of parameters such as thickness of waterproofing layer, modulus, Poisson’s ratio, thickness and modulus of asphalt concrete pavement layer on the interlayer shear stress of the structural layer. It is believed that the maximum shear stress between layers depends mainly on the thickness of the surface layer and the modulus of the waterproofing layer; in the case of the same modulus of the waterproofing layer, increasing the thickness of the surface layer is the most effective means of reducing the interlayer shear stress. Reasonable control index is an important basis for structural design, is also the subject of future research to focus on an aspect.
From the existing structural analysis methods, mainly using three-dimensional isoparametric model for analysis, the more commonly used is three-dimensional eight-node and twenty-node units.
A reasonable finite element model is the premise of the calculation and analysis, from the current state of research, there are several aspects that need to be discussed.
In short, how to simulate the contact between the layers, especially how to consider the influence of the waterproof layer, is a key issue in the establishment of a reasonable finite element model, is a key point in the study of structural design theory of the pavement. To take a combination of theoretical calculations and experimental analysis, the calculation results are compared with the test and measurement results to find a model that matches the actual force of the structure.
As the bridge deck is a special pavement structure, how to simplify the load model and how to distribute the load in the transverse and longitudinal directions is also directly related to the accuracy of the calculation results.
Another important issue that needs to be studied is the deflection and other deformation of the bridge under load, and how these factors affect the mechanical properties of the pavement, and how to consider these effects, which is also an aspect of the bridge deck pavement different from the general composite pavement. At present, there is no discussion specifically for this aspect in China.
To solve the problem of bridge deck pavement reasonably, we need to start from both theoretical analysis and structural calculation, the correct theoretical basis is fundamental, and the reasonable mechanical model is the key. Through calculation and analysis and actual measurement comparison, better solve the above contact model, load simplification and other problems, to clarify the impact of other factors; but also to strengthen the model size and convergence conditions of the study; in the conditions allowed, to strengthen the study of its dynamic properties. On the basis of the analysis of the damage form of the pavement, determine the key factors, control indicators and establish the corresponding damage criteria to provide a basis for the design, to achieve this goal requires a lot of basic research work.
5 Summary
This paper analyzed and researched the early damage of flexible deck pavement of reinforced concrete bridge and its causes, summarized the main methods of structural analysis of bridge deck pavement in China, proposed some problems that need attention by finite element analysis, and pointed out the main research direction in the future.
The most urgent task is to accelerate the further research on asphalt concrete bridge deck pavement, in order to clarify the calculation model of each structural layer of the deck pavement, mechanical properties and related parameters, so as to provide guidance for the design of the bridge deck pavement.
At the same time, strengthen the pavement layer material performance indicators and testing technology research, the development of new materials to adapt to the damage mechanism of the bridge deck; in addition, we must improve the paving technology and improve the quality of construction, to ensure the accuracy of the design model, to fundamentally solve the problem of early damage to the bridge deck pavement
