In recent years, with the increase in investment in transportation infrastructure, China's highway mileage has increased rapidly year by year. By the end of 2006, the total mileage of highways opened to traffic nationwide had reached 3.45 million kilometers, of which highways reached 453,000 kilometers. It is foreseeable that, corresponding to the rapid development of highway construction in China, a large number of original pavements will need to be repaired and reformed in the future to maintain good road performance. Pavement performance testing is a key and basic technology in highway construction and management. It is not only important for testing and controlling project quality, but also determines the scientific nature of road network maintenance decisions and directly affects the rationality of maintenance capital allocation.
According to the relevant regulations in China, the main indicators of the old pavement inspection include deflection, flatness, friction coefficient, damage status, etc. In addition, indicators such as rut, thickness, and grass-roots integrity can also be added based on actual project requirements. Traditional detection methods mainly include: (1) Beckman beam deflection instrument, dial indicator, and standard axle-loaded Yellow River vehicle, using the principle of lever to test the road surface rebound deflection; (2) using a 3 meter ruler to test Pavement longitudinal smoothness and transverse section rutting; (3) Pendulum friction coefficient meter is used to manually test the friction coefficient of the pavement point by point; (4) A core coring machine is used to drill the core sample to test the thickness of the pavement and judge the integrity of the core sample; (5) Adopt manual damage investigation to understand the condition of pavement damage. These early test methods not only took time and effort, affected traffic, but also destroyed the pavement structure, and the data accuracy was difficult to be reliably guaranteed. Therefore, they have been gradually replaced by new testing equipment. The following highlights some of the new detection technologies that are currently in use.
Pavement deflection detection Deflection is an important indicator of pavement detection, and its detection and analysis technology has developed rapidly. Since Benke Iman invented the beam deflectometer in 1953, road surface deflection testing equipment has developed from static deflectometer, steady-state dynamic deflectometer to pulsed dynamic deflectometer, from single-point maximum deflection test Developed into the detection of pavement deflection basin, and will be limited to the concept of deflection in the sense of flexible pavement, into the structural evaluation and design analysis of rigid pavement, the evaluation of pavement structural performance has also developed from the evaluation of the overall strength of pavement to the right Back analysis of the stiffness of each layer of the pavement.
The use of the Beckman beam method to determine the rebound deflection value of the pavement is simple and widely used, but the test is manual. The test results are greatly affected by human factors and the speed is slow. The basic working principle of the automatic deflectometer is the same as that of the Beckman beam. They all use a simple lever principle. The measuring vehicle travels at a certain speed in the detection section. It will be installed in front of the test vehicle and the deflection measuring beam under the chassis between the rear axles. Placed at the front of the vehicle chassis and supported on the ground to remain stationary. When the rear axle double wheel clearance passes through the probe, the deflection is automatically recorded by a displacement sensor and other devices. At this time, the measuring beam is dragged to double The car's speed is dragged to the next test point, and it is continuously measured forward and back. The general test speed is maintained between 1.5km / h-4 0km / h.
The Falling Weight Deflectometer (FWD) is a typical representative of the pulse type dynamic deflectometer. Its technical characteristics are mainly manifested in: fast speed measurement, high accuracy, and a good simulation of the actual driving load on the road's dynamic effect. Many countries are widely used in pavement inspection and evaluation. The main principle is as follows: the hydraulic system under computer control lifts and releases a weight, which imposes a pulse load on the road surface. The load size is adjusted by changing the weight of the hammer and lifting height, and acts on the road surface through the rigid disc. The deflection of the road surface is measured by 5 to 9 sensors, which can more accurately reflect the shape of the deflection basin, thus providing a basis for the back calculation of the road surface modulus. With the modulus, the stress and strain of the pavement structure can be further analyzed, and the bearing capacity can be evaluated. Since the 1980s, FWD has been widely used internationally. So far, more than 50 countries and regions have introduced FWD. After a comparative analysis, the US Federal Highway Administration confirmed that FWD is an ideal device for evaluating the pavement carrying capacity, and selected it as an important device for implementing the pavement carrying capacity assessment part of the SHRP plan.
At present, the main research carried out around FWD at home and abroad is the stable and reliable modulus inversion technology. Through the analysis of the FWD deflection basin data, the dynamic modulus of each structural layer of the pavement is retrieved, and then the bearing silicon force is discriminated. The focus of this technology at home and abroad is on the simulation of pavement mechanical characteristics, the reliability of the back analysis of the modulus, and the verification of the inversion results. In addition, FWD can also be used for judging voiding of old cement concrete pavement bodies, judging the load-carrying capacity of joints, dynamic monitoring during roadbed construction, and evaluation of the impact compaction effect of roadbeds.
In addition to FWD, a new generation of deflectometer RWD (roller-type deflectometer) expected to be developed in Denmark and the United States is in the research stage. It uses high-frequency laser scanning to continuously record the bending of the test car on the road surface during driving Shen has the advantage of recording the real stress state of the road surface, and its speed measurement is much greater than FWD, so it has less impact on traffic and is an ideal deflection detection equipment.
Pavement flatness detection Pavement flatness is an important indicator in pavement evaluation and pavement construction quality acceptance, mainly reflecting the flatness of the pavement longitudinal profile curve. When the profile curve of the road surface is relatively smooth, it means that the road surface is relatively smooth and the driving comfort is good; otherwise, the smoothness is relatively poor. The detection of pavement smoothness can provide important information for decision makers, so that decision makers can make optimal decisions for the maintenance and repair of pavement. On the other hand, the detection of pavement smoothness can accurately provide information on pavement construction quality, and provide an objective indicator of quality assessment for pavement construction.
In the 1970s, the flatness measurement was mainly level, three-meter ruler, etc., with low accuracy and slow speed. After the 1990s, the flatness detection methods have been gradually improved, and a series of new detection equipment such as continuous flatness meters, bump accumulators, and laser profilers have appeared.
At present, pavement roughness testing equipment is mainly divided into two categories: cross-section and reaction. The cross-section type is actually to measure the unevenness of the pavement surface, such as continuous flatness meter and laser cross-section meter. The reaction class is the flatness index directly felt by the driver and passengers, so it is actually a comfortable performance index, such as a bump accumulator, etc. The principle is that the test car runs on the road at a certain speed, which is caused by uneven roads. Excited vibration, the one-way displacement cumulative value VBI between the rear axle and the car is measured by a mechanical sensor. The larger the VBI value, the less comfortable the driving. Because VBl is not a standard international flatness index, it is necessary to establish a conversion relationship between the international flatness index IRI value of the cross-section equipment through calibration test and calibration conversion.
In general, profile equipment is the mainstream product for the development of flatness testing at home and abroad. The early product was a continuous flatness meter, and its detection principle was very simple, that is, the front and rear wheels with a spacing of three meters were used as the fulcrum to build the balance beam, and a displacement sensor detected the change in the vertical distance from the center point of the balance beam to the road The amount is then converted to the standard deviation of flatness. Due to the slow test speed of the continuous flatness meter, the normal speed is about 5km / h, which is mainly used for detection during the construction process.
Laser profiler is a cross-section test equipment that is widely used at present. The normal speed is about 80 km / h. It has the characteristics of fast test speed and high accuracy. It can be used for the test of flatness and other indicators. The basic principle is to use a laser sensor to measure The distance between the car body and the road surface, while using the accelerometer to measure the vertical displacement of the car body itself, thereby obtaining a profile of the longitudinal section of the road surface, and then using this profile to calculate the international flatness index in real time. The research carried out around the laser profiler is mainly the repeatability and reproducibility of the test. Both Europe and the United States have conducted large-scale reproducibility and reproducibility studies, and established a correlation between the mainstream devices used. At present, there are many brands of laser cross-sectional analyzers used in China. These devices have begun to be used in large quantities. However, since there has not been a system reproducibility study, the comparability of data between different devices needs to be investigated.
Pavement rutting detection Rutting refers to the band-shaped grooves generated along the longitudinal direction of the road at the concentration of vehicles. Due to the combined influence of traffic growth, vehicle canalized traffic, sustained high temperature and other factors, rutting has become an early damage to asphalt pavement in China A common pavement disease. Rutting has a significant impact on driving safety, especially after rain, it is easy to cause the vehicle to slide laterally and cause traffic accidents. Therefore, the detection of this indicator has received widespread attention.
The early rut test mainly used the 3m ruler method, which has the advantages of low cost, convenient and intuitive, and the disadvantages of slow speed, low efficiency, and affecting traffic. With the rapid development of computer technology, ultrasonic technology and laser technology, new types of rut testing equipment such as ultrasonic rut tester and laser profiler have appeared. Among them, the ultrasonic rut tester is generally composed of about 30 ultrasonic sensors, the interval between the sensors is about 100mm, and the test width is about 3m. Describe the cross-section of the road surface by measuring the distance from the road surface, and determine the maximum rut depth of the road surface by ruler analysis. Its advantages are low price, dense distribution along the lateral direction, and good cross-section continuity. The disadvantage is that the accuracy of a single sensor is lower than the laser sensor, which is affected by the outside world. The impact is large and can only go vertically downward. In addition to measuring the flatness, the laser profiler can also test rutting, that is, measuring the height from the road surface through 5 to 9 laser sensors distributed laterally, and simulating the cross section of the road surface through the elevation of several measuring points to quickly calculate the rut.
In recent years, a new laser rut scan test system has been developed and prototypes have been developed. The system contains two laser scanners with cross-section, which can collect data from 1 280 points in the range of m, with a sampling rate of 25 cross-sections / In seconds, it can more realistically reflect the actual situation of road rutting in engineering applications. The system is not affected by temperature, humidity, pavement color and flatness, and can be tested in rainy days. In addition, the laser rut scan test system has high repeatability and accuracy, and the accuracy of the test height is ± 1mm. It is expected that such products will become the future development trend.
Pavement friction coefficient detection The anti-skid performance of pavement is an important part of pavement performance, which directly affects the safety of road traffic. Pavement anti-skid performance includes both longitudinal and lateral aspects. The longitudinal anti-skid performance determines the vehicle's sliding distance when braking, and has a direct role in avoiding rear-end traffic accidents. The lateral anti-skid performance determines the vehicle's direction control ability. The safety of driving on curved roads is more important. In recent years, with the improvement of people's safety awareness, the anti-skid performance of the road has begun to get people's attention. However, at this stage, the pendulum friction coefficient instrument commonly used in China's specifications has shortcomings in the application of friction coefficient testing, mainly manifested in affecting road traffic, slow test speed, low efficiency, and potential safety hazards for operators.
In response to this situation, automated friction coefficient testing equipment has gradually been introduced into China from the United Kingdom, Sweden and other countries in recent years. According to different test methods, such equipment can be divided into three categories. Lateral force coefficient tester, brake type friction coefficient tester, incomplete brake type friction coefficient tester, etc.
The lateral force coefficient tester is the most widely used in China. Due to the relatively high price of imports from abroad, localization was achieved in the mid-1990s. The basic principle of the equipment is to set the test wheel at a certain angle with the driving direction. The ratio of the lateral force to the load of the test wheel on the pavement is the lateral force coefficient, which reflects the risk of the vehicle slipping on the road. In the process of driving, the brake type friction coefficient tester automatically brakes the test wheel every specified distance, and the test wheel slides on the road during braking. Based on the force recorded by the sensor, the braking force coefficient can be calculated. This equipment is one of the standard equipments for anti-skid ability test in the United States, and the test speed can reach up to 110km / h. The test wheel and the running wheel of the incomplete braking friction coefficient tester are connected with a coaxial gear and chain of unequal diameter, so that the rolling linear speed of the test wheel is lower than that of the running wheel. During the normal test, it shows the motion state of continuous rolling and sliding, and the road friction coefficient can be calculated according to the data recorded by the force sensor. The test speed of this device on the road surface is about 50km / h, which is widely used in Europe. Since it is not a collection device specified in the current specification, a comparison with a pendulum or lateral force coefficient tester is required when performing the friction coefficient test Experiment to establish the relationship between the two.
At present, the pendulum instrument is still mainly used in the test of pavement anti-skid ability. The lateral force coefficient instrument has gradually had a considerable number of users. At present, there are only a few users of brake and incomplete brake friction coefficient testers. It is foreseeable that due to advantages in safety and accuracy, automated friction coefficient meters will become mainstream in China.
Road surface damage survey The road surface damage status is often the road users' intuitive perception of the quality of pavement construction and maintenance. Therefore, highway departments at all levels in China have always paid more attention to the pavement damage. At present, this index mainly relies on manual collection. In addition to its high subjectivity and low efficiency, there are still great security risks. In response to this situation, some domestic units have introduced road surface breakage testing systems in recent years. The basic principle is to continuously collect road surface images through a camera system, and then use post-processing software to automatically process and combine manual interpretation to identify, classify and statistical road tables damaged. The road surface damage test system greatly improves the efficiency of the second crop and avoids the danger of manual damage investigation. With the rapid development of China's road construction, it will surely become a widely used equipment.
At present, the road surface breakage test system mainly includes products from several countries such as the United States and Canada. Due to the relatively high price of imported equipment, a few domestic units have conducted independent research and development and early products have been put into use. According to the investigation of such products, the main problems include: (1) the current equipment can mainly identify crack-type diseases, and it cannot accurately identify three-dimensional diseases such as congestion and subsidence; (2) the post-processing workload is large due to This type of product has not yet achieved automatic identification of damage, and the rate of misjudgment and missed judgment is high. For example, it is easy to identify the pollution of the surface as a pit, etc., so it needs to be interpreted by the manual after the picture, resulting in a long processing time: Human and weather factors have a certain influence on the accuracy of the test results. For example, the recognition effect is different under different weather conditions. In response to this problem, various equipment vendors are improving, focusing on the automatic identification and classification of surface damage, reducing the rate of misjudgment and omission of judgment, and automatically output indicators such as road surface damage rate.
Pavement Thickness and Integrity Testing At present, China's highway pavement thickness testing mainly uses the coring method to determine, and at the same time determine the integrity of the base layer through manual observation. With the development of electromagnetic wave technology, road radar has begun to be used at home and abroad. This technology combines transient electromagnetic field theory, time domain measurement technology, nanosecond pulse source technology, ultra-wideband antenna technology and signal processing technology. The main principle is to use the propagation and reflection of electromagnetic waves in the pavement structural layer, determine the thickness according to the echo time, amplitude and waveform, and at the same time determine the base layer looseness rate through the change of the dielectric constant after the base layer is loose, so as to understand the base layer integrity status. In this process, the focus is on the analysis and study of the dielectric properties of the road surface. Because the reflected waves received by the radar are a function of the dielectric properties of the medium, the interpretation, interpretation, and inversion of road radar image data depend on the medium Analysis of electrical performance, therefore, in-depth analysis of dielectric characteristics is the key technical point of current radar technology applications.
The application of pavement radar in engineering has just started, there are about 20 domestic devices at present. The brands of these devices are different, mainly produced in the United States and Europe, but the test principle is basically the same. The higher the test frequency, the higher the accuracy and the deeper the detection depth. shallow. Pavement radar has become an important part of pavement non-destructive testing technology, and represents the development direction of pavement structural layer thickness, compactness, base condition, water content, asphalt content and other testing technologies.
At present, the accuracy of pavement radar in the detection of the thickness of asphalt concrete surface is about 3%. The research on the integrity of structural layers such as the determination of the emptying of cement concrete slabs and the determination of the looseness of the base layer still needs to be further deepened. The research on other important performance indicators of pavement such as compactness, porosity, water content, asphalt content, etc. is also in the exploratory stage and has not been widely used in engineering. In addition, because the actual situation is often difficult to judge objectively, different detection methods can be used to verify each other, such as the use of a drop hammer deflectometer and road radar to jointly detect the plate vacancy and the bearing capacity of the base layer, so as to find the pavement structural layer Hidden dangers exist in the road, master the inherent quality and service life of the road, and guide the maintenance and repair of the road.
In addition to the accuracy of the radar antenna, the post-processing software is also very important for the application of road radar. It can be said that the equipment provides a means of detection, and the software determines the breadth and depth of the application, which should attract enough attention from domestic users. Each radar manufacturer has its own post-processing software. In addition, there are some more professional post-processing software developed by professional research institutes, especially in the United States and Finland.
It can be considered that the speed of future road surface radar technology promotion and application mainly depends on the development speed and depth of practical software.
Overall, new testing equipment has continuously emerged in recent years, providing us with richer information. Therefore, how to better use automated testing technology to evaluate pavement performance and propose a reasonable maintenance program will be the next stage of testing equipment The focus of user attention.
The general trend of road surface detection technology is from manual detection to automatic detection technology, from damage detection to non-destructive detection technology, from low speed and low precision to high speed and high precision. In recent years, there have been more and more automated pavement non-destructive testing equipment. Correspondingly, the research carried out on automated testing equipment will also be improved in depth. In general, the development direction of pavement testing technology in China is as follows; (1) more and more users of advanced non-destructive testing equipment, and gradually realize domestic assembly and localization; (2) the research on testing technology will gradually deepen, Especially the evaluation technology, which is promoted through the marketization of relevant practical software; (3) Comprehensive evaluation of the pavement condition using various non-destructive testing equipment test results, and decision-making of maintenance technology routes; (4) Direct import of various testing data Pavement management system to realize information management
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