The Application of Road Surface Inspection Technology and the Development Trend of Highway Construction Technology in China

In recent years, with the increasing investment in transportation infrastructure, the mileage of highways in China has been growing rapidly year by year. As of the end of 2006, the total length of highways in China had reached 3.45 million kilometers, including 4.53 million kilometers of expressways. It can be foreseen that in response to the rapid development of highway construction in China, a large number of existing road surfaces will need to be repaired and renovated in the coming period to maintain good road performance. Pavement performance testing is a critical and fundamental technology in highway construction and management. It is not only crucial for detecting and controlling engineering quality, but also determines the scientificity of road network maintenance decisions, directly affecting the rationality of maintenance fund allocation.
According to relevant regulations in China, the main indicators for detecting old road surfaces include deflection, smoothness, friction coefficient, and damage condition. In addition, indicators such as ruts, thickness, and base integrity can be added according to actual project requirements. The traditional detection methods mainly include: (1) using a Beckman beam deflection meter, a dial gauge, and a standard axle load Yellow River vehicle to test the rebound deflection of the road gauge using the lever principle; (2) Using a 3-meter ruler, test the longitudinal smoothness of the road surface and the rutting condition of the transverse section; (3) Using a pendulum friction coefficient meter, manually test the road friction coefficient point by point; (4) Using a coring machine, drill core samples to test the thickness of the road surface and determine the integrity of the core samples; (5) Using manual damage investigation to understand the condition of road surface damage. These early testing methods were not only time-consuming and labor-intensive, affecting traffic, but some also had to damage road structures, making it difficult to reliably guarantee data accuracy. Therefore, they have gradually been replaced by new detection equipment. The following focuses on introducing some new detection technologies that are currently widely used.

Road surface deflection detection
As an important indicator for road surface detection, the detection and analysis technology of deflection has developed rapidly. Since the invention of the beam deflectometer by Benke Iman in 1953, road surface deflection detection equipment has evolved from static deflectometers and steady-state dynamic deflectometers to pulse type dynamic deflectometers, from single point large deflection detection to detection of road surface deflection basins. The concept of deflection, which is limited to flexible road surfaces, has been developed into the structural evaluation and design analysis of rigid road surfaces. The evaluation of road surface structural performance has also evolved from the overall strength evaluation of the road surface to the back analysis of the stiffness of each layer of the road surface.
The use of Beckman beam method to determine the rebound deflection value of road surface is easy to operate and widely used, but the test is manually operated, and the test results are greatly affected by human factors, resulting in slow speed measurement. The basic working principle of an automatic deflectometer is the same as the Beckman beam principle, both using a simple lever principle to measure the deflection of a vehicle traveling at a certain speed on a test section. The deflection measuring beam installed under the chassis between the front and rear axles of the test vehicle is placed at the front end of the vehicle chassis and supported on the ground to remain stationary. When the double wheel clearance of the rear axle passes through the measuring head, the deflection is automatically recorded by displacement sensors and other devices. At this time, the measuring beam is dragged to the next measuring point at twice the speed of the vehicle, and continuously measured forward repeatedly. Generally, the testing speed is maintained between 1.5km/h-40km/h.
The falling hammer deflectometer (FWD) is a typical representative of pulse type dynamic deflectometers, characterized by fast speed measurement, high accuracy, and good simulation of the dynamic effects of actual driving loads on the road surface. It has been widely used in road detection and evaluation in many countries. The main principle is as follows: a hydraulic system controlled by a computer lifts and releases a heavy hammer, thereby applying a pulse load to the road surface. The load size is adjusted by changing the weight of the hammer and the lifting height, and is applied to the road surface through a rigid disc. The deflection of the road surface is measured by 5 to 9 sensors, which can accurately reflect the shape of the deflection basin and provide a basis for calculating the modulus of the road surface. With the modulus, the stress and strain conditions of the pavement structure can be further analyzed, and the bearing capacity can be evaluated. Since the 1980s, FWD has been widely used and has been introduced in over 50 countries and regions. After comparative analysis, the Federal Highway Administration of the United States has confirmed that FWD is an ideal equipment for evaluating road bearing capacity and has been selected as an important equipment for implementing the road bearing capacity evaluation part of the SHRP plan.
At present, the main research on FWD at home and abroad is the stable and reliable modulus inversion technology. By analyzing the FWD deflection basin data, the dynamic modulus of each structural layer of the road surface can be inverted to determine the bearing silicon capacity. The focus of attention on this technology both domestically and internationally is on simulating the mechanical properties of road surfaces, ensuring the reliability of modulus back analysis, and verifying the results of the back analysis. In addition, FWD can also be used for determining the detachment of old cement concrete pavement panels, assessing the load-bearing capacity of joints, dynamically monitoring the construction process of roadbeds, and evaluating the impact compaction effect of roadbeds. Its application is becoming increasingly widespread.
In addition to FWD, the expected development of a new generation of roll deflectors (RWD) in Denmark, the United States, and other countries is currently in the research stage. It uses high-frequency laser scanning to continuously record the deflection generated by the test vehicle on the road surface during driving. Its advantage is that it records the true stress state of the road surface, and the speed is much higher than FWD, so it has less impact on traffic and is an ideal deflection detection device.

Road surface smoothness detection
Road surface smoothness is an important indicator in road surface evaluation and construction quality acceptance, mainly reflecting the smoothness of the longitudinal profile curve of the road surface. When the longitudinal profile curve of the road surface is relatively smooth, it indicates that the road surface is relatively flat and the driving comfort is good. Conversely, when the curve is relatively smooth, it indicates that the smoothness is relatively poor. The detection of road surface smoothness can provide important information for decision-makers, enabling them to make optimized decisions for road maintenance and upkeep. On the other hand, the detection of road surface smoothness can accurately provide information on the quality of road construction, providing an objective indicator for quality assessment of road construction.
In the 1970s, flatness measurement was mainly carried out using leveling instruments, three meter rulers, etc., which had low accuracy and slow speed. After the 1990s, the methods of flatness detection gradually improved, and a number of new detection equipment such as continuous flatness meters, bump accumulation meters, and laser section meters emerged.
At present, road surface smoothness testing equipment is mainly divided into two categories: cross-sectional and reactive. The section type is actually used to measure the unevenness of the road surface, such as continuous flatness meters, laser section meters, etc. Reaction type refers to the smoothness index directly perceived by drivers and passengers, therefore, it is actually a comfort performance index, such as a bump accumulator. Its principle is to test the vehicle driving at a certain speed on the road surface. Due to the uneven road surface, the vehicle is excited, and the accumulated value VBI of one-way displacement between the rear axle and the carriage is measured by mechanical sensors. The larger the VBI value, the more uncomfortable the driving is. Since VBl is not a standard flatness index, it is necessary to establish a conversion relationship between the flatness index IRI value of cross-sectional equipment through calibration experiments for calibration conversion.
Overall, cross-sectional equipment is currently the main product for flatness testing development both domestically and internationally. The early product was a continuous flatness meter, which had a simple detection principle. It used front and rear wheels with a spacing of three meters as support points to build a balance beam. A displacement sensor detected the change in the vertical distance from the midpoint of the balance beam to the road surface, and then converted it into the standard deviation of flatness. The continuous flatness meter is mainly used for testing during construction due to its slow testing speed, with a normal speed measurement of around 5km/h.
Laser section meter is currently a widely used section testing equipment, with a normal speed measurement of around 80 km/h. It has the characteristics of fast testing speed and high accuracy, and can be used for testing indicators such as flatness. Its basic principle is to use a laser sensor to measure the distance between the vehicle body and the road surface, and use an accelerometer to measure the vertical displacement of the vehicle body itself, thereby obtaining the profile of the longitudinal section of the road surface, and then use this profile to calculate the flatness index in real time. The research conducted around the laser section analyzer mainly focuses on the repeatability and reproducibility of testing. Both Europe and the United States have conducted large-scale studies on repeatability and reproducibility, establishing correlations between the mainstream devices they use. At present, there are various brands of laser section meters used in China, and these devices have begun to be widely used. However, due to the lack of systematic reproducibility research, the comparability of data between different devices needs to be investigated.

Road rutting detection
Rutting refers to the strip-shaped grooves formed on the road surface along the longitudinal direction of the road at locations where vehicles are concentrated. Due to the combined effects of increasing traffic volume, vehicle channeling, and sustained high temperatures, rutting has become a common pavement disease in the early stages of asphalt pavement damage in China. Car ruts have a significant impact on driving safety, especially after rain, which can easily cause lateral skidding of vehicles and cause traffic accidents
Therefore, the detection of this indicator has received widespread attention from people.
The early rutting test mainly used the 3m straightedge method, which has the advantages of low cost, convenience and intuitiveness, but the disadvantages are slow speed, low efficiency, and affecting traffic. With the rapid development of computer technology, ultrasonic technology, and laser technology, new types of rutting testing equipment such as ultrasonic rutting testers and laser section testers have emerged. Among them, the ultrasonic rutting tester generally consists of about 30 ultrasonic sensors, with a spacing of about 100mm between the sensors and a testing width of about 3m. By measuring the distance from the road surface, the cross-section of the road surface is depicted, and the depth of the large vehicle rutting on the road surface is determined through straightedge analysis. Its advantages are low price, can be densely distributed along the transverse direction, and good continuity of the cross-section; The disadvantage is that the accuracy of a single sensor is lower than that of a laser sensor, and it is greatly affected by external factors, so it can only be vertically downward. In addition to testing smoothness, the laser section meter can also test ruts, that is, by using 5-9 laser sensors distributed horizontally to test the height from the road surface, and simulating the road cross-section through several measuring points, ruts can be quickly calculated.
In recent years, a new laser rutting scanning testing system has been developed and a prototype has been released. The system includes two cross-sectional laser scanners that can collect data from 1280 points within a range of m, with a sampling rate of 25 sections per second. In engineering applications, it can more accurately reflect the actual situation of road rutting. The system is not affected by temperature, humidity, road color, and flatness, and can also be tested on rainy days. In addition, the laser rutting scanning testing system has high repeatability and accuracy, with an accuracy of ± 1mm for testing height. It is expected that such products will become a future development trend.

Road friction coefficient detection
The anti-skid performance of road surface is an important component of road performance, which directly affects the safety of road driving. The anti-skid performance of the road surface includes both longitudinal and transverse aspects. The longitudinal anti-skid performance determines the sliding distance of the vehicle when braking and has a direct determining effect on avoiding rear end traffic accidents; The lateral anti-skid performance determines the directional control ability of the vehicle and is important for the safety of vehicle cornering. In recent years, with the improvement of people's safety awareness, the anti-skid performance of road surfaces has begun to receive widespread attention. However, at present, the commonly used pendulum friction coefficient tester in China has shortcomings when applied to friction coefficient testing, mainly manifested in affecting road traffic, slow testing speed, low efficiency, and potential safety hazards for operators.
In response to this situation, automated friction coefficient detection equipment has gradually been introduced to China from countries such as the UK and Sweden in recent years. According to different testing methods, this type of equipment can be divided into three categories: lateral force coefficient tester, brake type friction coefficient tester, non brake type friction coefficient tester, etc.
The lateral force coefficient tester is widely used in China, and due to the high cost of importing it from abroad, it was localized in the mid-1990s. The basic principle of this device is to set the test wheel at a certain angle to the driving direction, and the ratio of the lateral force to the road load of the test wheel is the lateral force coefficient, which reflects the danger of the vehicle sliding on the road surface. The normal speed measurement is about 50km/h. The brake type friction coefficient tester automatically brakes the test wheel at intervals during driving, and during braking, the test wheel slides on the road surface. According to the sensor recording
Force can be used to calculate the braking force coefficient. This device is one of the standard equipment for anti-skid capability testing in the United States, with a high testing speed of up to 110km/h. The test wheel and the driving wheel of the non braking friction coefficient tester are connected by coaxial gears and chains of unequal diameters, so that the rolling linear velocity of the test wheel is lower than that of the driving wheel. During normal testing, it presents a rolling and sliding motion state, and the road friction coefficient can be calculated based on the data recorded by the force sensor. The testing speed of this device on the road is about 50km/h, and it is widely used in Europe. As it is not a collection device specified by current regulations, a comparative test with a pendulum or lateral force coefficient tester is required to establish the relationship between the two when conducting friction coefficient testing.
At present, the pendulum instrument is still mainly used for testing the anti-skid ability of road surfaces, and the lateral force coefficient instrument has gradually gained a considerable number of users. Currently, there are only a few users for brake and non brake friction coefficient testers. It can be foreseen that automated friction coefficient meters will become mainstream in China due to their advantages in safety and accuracy.

Investigation of road surface damage condition
The condition of road surface damage is often the intuitive feeling of road users towards the quality of road construction and maintenance. Therefore, various levels of highway departments in China have always attached great importance to the condition of road surface damage. At present, this indicator mainly relies on manual collection, which not only has high subjectivity and low efficiency, but also poses significant safety hazards. In response to this situation, some domestic units have introduced road surface damage testing systems in recent years. The basic principle is to continuously collect road surface images through a camera system, and then automatically process them through post-processing software combined with manual interpretation to identify, classify, and statistically analyze road surface damage. The road surface damage testing system greatly improves the efficiency of secondary work and avoids the danger of manual damage investigation. With the rapid development of highway construction in China, it will become a widely used equipment.
At present, the road surface damage testing system mainly includes products from several countries such as the United States and Canada. Due to the high cost of imported equipment, a few domestic units have independently developed and put early products into use. According to the investigation of this type of product, the main problems include: (1) Currently, the equipment can mainly identify crack type diseases, and cannot accurately identify three-dimensional diseases such as wrapping and subsidence; (2) The post-processing workload is relatively large. Due to the inability of such products to achieve automatic identification of damage, the misjudgment and missed judgment rates are high. For example, it is easy to distinguish surface pollution as pits and grooves. Therefore, manual interpretation of each image in the later stage is required, resulting in a long processing time. (3) Human and weather factors have a certain impact on the accuracy of test results, such as different recognition effects under different weather conditions. In response to this issue, various equipment manufacturers are making improvements, with a focus on automatic recognition and classification of surface damage, reducing misjudgments and missed judgments, and automatically outputting indicators such as road damage rate.

Pavement thickness and integrity testing
At present, the thickness testing of highway pavement in China mainly adopts the core sampling method, and the integrity of the base layer is determined through manual observation. With the development of electromagnetic wave technology, road radar has begun to be used both domestically and internationally. This technology combines transient electromagnetic field theory, time-domain measurement technology, nanosecond pulse source technology, ultra wideband antenna technology, and signal processing technology, among other disciplines. The main principle is to use the propagation and reflection of electromagnetic waves in the road structure layer to determine the thickness based on the echo time, amplitude, and waveform. At the same time, the looseness rate of the base layer can be determined by the change in dielectric constant after the base layer is loosened, thereby understanding the integrity of the base layer. In this process, the focus is on analyzing and studying the dielectric properties of road media. As the reflected waves received by radar are a function of the dielectric properties of the media, the interpretation, interpretation, and inversion of road radar image data rely on the analysis of the dielectric properties of the media. Therefore, in-depth analysis of dielectric properties is currently a key technical point in the application of radar technology.
The application of road radar in engineering has just begun, with about 20 devices in China. The brands of these devices are different, mainly produced in the United States and Europe, but the testing principles are basically the same. The higher the testing frequency, the higher the accuracy, and the shallower the detection depth. Road radar has become an important component of non-destructive testing technology for road surfaces, and represents the development direction of detection technologies for road structure layer thickness, compaction degree, base condition, moisture content, asphalt content, and other aspects.
At present, the accuracy of pavement radar in detecting the thickness of asphalt concrete surface layer is about 3%. Further research is needed in the integrity of structural layers such as the determination of cement concrete slab detachment and the determination of loose base layer. The research on other important performance indicators of road surfaces, such as compaction degree, porosity, moisture content, asphalt content, etc., is still in the exploratory stage and has not been widely applied in engineering. In addition, due to the difficulty of objectively judging the actual situation, different detection methods can be used to mutually verify each other, such as using a hammer deflectometer and road radar to jointly detect the detachment of the plate and the bearing capacity of the base layer, so as to timely discover hidden dangers in the pavement structure layer, grasp 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 itself, the post-processing software is also crucial 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 be given sufficient attention by domestic users. Each radar manufacturer has supporting post-processing software, and there are also some more professional post-processing software developed by specialized research institutes, especially in the United States and Finland where research is more in-depth.
It can be considered that the speed of promoting and applying road radar technology in the future mainly depends on the development speed and depth of practical software.

Overall, new types of detection equipment have emerged in recent years, providing us with richer information. Therefore, how to better utilize automated detection technology to evaluate road performance and propose reasonable maintenance plans will be the focus of attention for users of detection equipment in the next stage.
The overall trend of road surface inspection technology is to develop from manual inspection to automated inspection technology, from damage detection to non-destructive testing technology, and from low-speed and low precision to high-speed and high-precision. In recent years, there has been an increasing number of automated non-destructive testing equipment for road surfaces. Correspondingly, research on automated testing equipment will also be deepened. Overall, the development direction of road detection technology in China is as follows; (1) More and more users of advanced non-destructive testing equipment are gradually achieving domestic assembly and localization; (2) The research on testing techniques, especially evaluation techniques, will gradually deepen and be promoted through the commercialization of relevant practical software; (3) Utilize various non-destructive testing equipment to comprehensively evaluate the road condition and make maintenance technology route decisions based on the test results; (4) Various detection data are directly imported into the road management system to achieve information management.