Lead: Asphalt concrete pavement refers to the pavement with asphalt concrete as the surface layer. A mixture made by manually selecting mineral aggregates (crushed stone or gravel, stone chips or sand, mineral powder, etc.). ) mixed with a certain proportion of road asphalt materials under strictly controlled conditions.
First, the current outstanding problems of asphalt concrete pavement
According to relevant statistics, the cost of asphalt concrete pavement accounts for 1/4 ~ 1/3 of the total highway cost, and most of the maintenance funds are used for pavement. There are two common technical and quality problems in asphalt concrete pavement.
First, the durability is poor. At present, the service life of asphalt concrete pavement in China is long, ranging from 8 years to 12 years, which is generally shorter than the design service life (15-20 years). Moreover, due to the poor natural environment and operating conditions, some pavements even need 3-5 years to be completely renovated.
Second, the pavement was seriously damaged in the early stage. Some new roads need maintenance after 2 ~ 3 years, or 6 ~ 8 years, such as potholes, cracks, rutting, insufficient skid resistance and so on.
Since 1980s, although the level of highway design, construction and raw materials in China has been greatly improved, there are still many early damages to highway pavement. In addition to the design and construction reasons, the main external factors are the rapid growth of traffic volume, heavy load and serious overload, which makes the road overloaded for a long time; The main internal cause is the poor performance of materials (asphalt, aggregate, etc.). ), resulting in the pavement resistance to tensile stress (such as cold and hot, dry and wet expansion and contraction stress, load bending fatigue stress, reflection crack expansion stress, etc.) is relatively insufficient. ), shear stress, water seepage, bond aging.
The adverse consequences caused by the above reasons are shocking. Even if the heavy traffic road asphalt and the mixed aggregate and gradation specified in the specification are used, the pavement can hardly bear the present situation of road transportation.
Second, the shortcomings of modified asphalt and SMA technology
In order to solve these serious and urgent problems, China's highway departments have adopted some measures and methods and explored some successful experiences on the basis of learning advanced foreign experiences, among which modified asphalt and SMA pavement are the two new technologies that are most widely used. For example, Beijing Highway Bureau is used in? The first road to the country? (Capital Airport Road)? Air China line one? (Capital Airport Runway)? China first street? In the construction and transformation of (Chang 'an Street), good use effect and economic benefit have been achieved. If two technologies are adopted at the same time, the effect will be better.
However, there are still some defects in the application of modified asphalt and SMA pavement in highway engineering:
1) The special equipment and materials (high-quality orthopedics, modifiers and fillers) it needs are hard to be found in general areas, and the one-time investment is large, and the required construction technology is also more difficult than the current routine practice.
2) With the recent rise in oil prices at home and abroad, the already high cost of the two new technologies will increase again. According to the price of 1998, the cost per kilometer of SMA pavement needs to increase by 97,000 yuan, and if modified asphalt is used at the same time, it will increase by about 388,000 yuan. The increase in cost is related to whether foreign raw materials, processing equipment and different modifiers are used. For a long time, imported road asphalt has been imported to China at a price close to twice that of the international market, making huge profits. For example, in the United States, Singapore and other countries, the price of asphalt is about $90/ton, while the sales price in China is1$35/ton. In recent years, the annual output of road asphalt in China is more than 2.2 million tons, which is far from meeting the actual demand in quantity and quality, leading to an increase in imported asphalt. Therefore, these two new technologies are only used in highways, important sections of high-grade highways, intersections, parking lots, overpasses, airports and so on. In China, they can't be widely used as in developed countries.
3) From the mechanism of increasing pavement crack resistance, seepage prevention, isolation and diffusion, it is not optimal to rely too much on increasing the elasticity and cohesive force of asphalt binder, increasing the plastic strength and reducing the void ratio of mixture. Because the modification of asphalt binder improves its tensile and shear mechanical properties, but the amplitude is not too high after all; Not all areas have high-strength aggregates; The decrease of porosity of the mixture is close to the limit. Because of the difference of physical and mechanical properties of materials, there will always be water seepage channels at the interface between orthopedics and adhesive. In addition, it can do nothing to improve the bearing capacity of foundation, but requires more.
4) The problems of sun aging, high temperature softening, low temperature embrittlement and impact fatigue of asphalt materials have yet to be solved.
Thirdly, geosynthetics are introduced into asphalt concrete pavement.
Under the action of general vehicle load, the dynamic stability of highway three-layer structure basically depends on the surface layer, and has little to do with the middle and lower layers. At present, under the condition of widely using asphalt for heavy traffic roads and aggregate gradation of mixture specified in specifications, if appropriate geosynthetics are used on the lower surface or bottom surface of pavement, it can not only improve the crack resistance and impermeability of pavement, but also improve the bearing capacity of base or subgrade, which can not only save costs relatively, but also effectively extend the maintenance period and reduce the degree of damage.
For example, when pure cement concrete bears a high load, it will not be widely used so far if only the mechanical properties of cement or the mixture ratio of aggregate are considered, and proper steel bars (nonmetals such as polymers) are not added to bear excessive tensile, compressive and shear stresses in its work. Why don't highway and municipal departments pay attention to the experimental research and design application of reinforced asphalt concrete? According to foreign statistics, geosynthetics used in highway engineering have accounted for more than 1/3 of its total consumption.
1993 the application research of geogrids in road engineering is listed as? Eight-five? The joint research project of "Study on geogrids to prevent asphalt pavement cracking in cold areas" undertaken by Heilongjiang Provincial Communications Department. Highway departments in Henan, Hunan, Hubei, Shaanxi, Heilongjiang, Beijing and other places have built some geogrid test roads. Held in Lianyungang city, Jiangsu province on April 26th, 2000? National highway geosynthetics application technology exchange meeting? Li, deputy director of the Highway Department of the Ministry of Communications, made a speech entitled? Should we pay attention to the application of geosynthetics in highway engineering? The report introduces three measures: formulating specifications, establishing key geosynthetics enterprises and building demonstration projects.
Since 1980s, field experiments have been carried out in various places. For example, 1986, Shaanxi Highway Bureau carried out experimental research on cracking of asphalt pavement to Baotou Railway; Study on crack treatment of Fuxia road surface of Tongji University 65438-0985: Study on the disease treatment of cement concrete runway of Shanghai Airport of Hebei Polytechnic University: Study on the application of geotextile in asphalt pavement of Yichang Highway Bureau, Hubei Province: Study on the application of Dalian Harbor Engineering Company in the reinforcement of external highway pavement: Study on the longitudinal joint treatment of multifunctional geogrid of Dalian Huanghai Avenue: Study on the treatment of road frost boiling with geotextile of Jiamusi Highway Bureau: The recent experiment of Liaoning Highway Bureau using glass fiber mesh on Shen Tie Expressway pavement, etc. In addition, we can also see many published articles about numerical calculation of highway pavement or base reinforcement effect.
For these valuable practices, we have the following experiences: the treatment of highway diseases should combine long-term and short-term, treat both the symptoms and root causes, consider the treatment of pavement and grass-roots (including subgrade) in a unified way, combine technology with economy, and adopt new materials and new technologies.
So far, the geosynthetics used in the experiment mainly include nonwoven geotextile (nonwoven needle-punched geotextile), woven geotextile (woven geotextile), geotextile, geogrid, geomembrane, drainage belt and so on. Recently, there are glass fiber mesh (glass fiber grid) and multifunctional geogrid.
According to the previous practical experience, in order to reduce or delay the new cracks in the newly-built road base (due to drying shrinkage) or the old pavement (due to various tensile stresses), the national standards "Technical Specification for the Application of Geosynthetics" (GB50290-98) and "Technical Specification for the Application of Highway Geosynthetics" (JTJ/T0 19-98) are in section 7.6. Glass fiber mesh can also be used, and its aperture should be 0.5 ~ 1.0 times of the maximum particle size of asphalt concrete pavement, with ultimate tensile strength greater than 50kN/m and maximum load elongation less than 3%. However, their actual mechanisms and functions under the surface are varied (not limited to the prevention and treatment of reflection cracks), and they are the same but different.
1) Thin nonwoven needle-punched geotextile (nonwoven geotextile) has low tensile strength and modulus and a tiny three-dimensional pore structure. After soaking in oil in the bonding layer, the original permeability coefficient will be reduced by about two orders of magnitude, and the asphalt cloth formed has better impermeability and heat transfer performance than asphalt concrete. When it is bonded with the bottom of asphalt concrete pavement, the tensile strength and modulus of the pavement will be improved and the cracks will be greatly reduced. If cracks appear in the old pavement or subgrade, they will be absorbed and relaxed when reflected to the new pavement layer because of its toughness and certain creep, thus playing its role as a stress barrier.
2) Recently, the glass fiber mesh has high tensile strength and modulus, no long-term creep, good thermal stability, good surface compatibility with asphalt, chemical stability of the material and the function of the grid clavicle material after treatment. Therefore, when it is laid on the lower part or bottom of asphalt concrete pavement, the deformation resistance of pavement can be enhanced. At the same time, because its paving area is generally large, and sometimes it is paved in the whole road section, under the condition of compatible interface deformation, the pressure, secondary bending and tensile stress, shear force and temperature tensile stress brought by axial load on the surface layer will be transferred to it and spread through it within its coverage, which can improve the fatigue strength of the surface layer, enhance the anti-rutting and anti-bulging ability, and also reduce the deflection value.
It can be seen that the above two types of geosynthetics play an irreplaceable role in improving pavement durability and preventing early damage, and cannot be regarded as short-term, temporary and emergency passive engineering measures.
Note: (a) If the subgrade, base and actual bearing capacity can't meet the requirements of axle load and traffic volume and need to be reinforced, they should be considered separately or solved by using suitable geosynthetics, and high-strength lightweight plastic one-way and two-way geogrids can be preferred.
(b) Whether there are diseases such as water seepage, boiling and subsidence. At the same time, drainage and seepage control measures should be considered separately, and corresponding geosynthetics can be used to solve the problem, such as reinforced composite geomembrane to meet the requirements of seepage control, drainage, reinforcement and isolation.
Fourth, the economic benefit analysis
The economic benefit of geosynthetics application has always been a key issue affecting its popularization. Many application examples at home and abroad have carried out economic benefit analysis, all of which have obvious advantages, but there are still many obstacles to popularization at present. We believe that the following methods can be used for economic benefit analysis.
According to the current practice, it takes about 150 yuan per square meter to build a new expressway (high-speed, first-class) to 200 yuan, in which the pavement needs about 50 yuan/m2 to 70/m2, and the pavement maintenance needs about 50 yuan/m2. The normal design service life is 15 years, 5 years for minor repairs and 8 years for major repairs. Analyze the economic benefits according to three situations: new construction, maintenance and unified consideration of the two. Determine the best solution through comparison.
1) New situation
If the newly-established fund is decomposed according to different service life, then:
When the service life is 15 years (design), the annual average is 10.0 ~ 13.3 yuan;
When the service life is 12 years (4/5), the average annual per square meter 12.5 ~ 16.7 yuan;
When the service life is 10 year (2/3), the annual average per square meter is 15.0 ~ 20.0 yuan;
When the service life is 7.5 years (1/2), the average annual price is 20.0 ~ 26.6 yuan per square meter.
If the 75 yuan per square meter is increased in order to reach the design life of 15 years (225 yuan /m2 ~ 275 yuan /m2 in total), in this case, although the annual average per square meter is increased to 15.0 ~ 18.3 yuan, it is still higher than the current annual average of 10 years. Judging from the current market situation, the increase in cost per square meter is more than enough for 75 yuan to choose filament nonwoven geotextile or grid to cover filament nonwoven geotextile.
2) Maintenance situation
According to the experience provided by existing related engineering examples, when laying suitable geosynthetics under the surface (corresponding construction technology should be kept up), the maintenance period can generally be delayed by more than 1 times. In this way, it only needs to be repaired for about 3 times within the design life (15 years), and the maintenance cost can be saved by 3? Every square meter. 50 yuan = 150 yuan.
3) If the cost of new construction and maintenance is considered in the whole service life, and the project quality responsibility is contracted by a company, then even if the estimated cost of new roads is not increased, if half of the maintenance cost saved by using geosynthetics is 150 yuan/㎡, that is, 75 yuan/㎡, the actual service life of roads will not only meet the design requirements, but also be extended by using geosynthetics in advance.
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