At present, China has built about 6,543.8+0.9 million kilometers of cement concrete pavement in expressway and 7,436 kilometers of cement concrete pavement in expressway. With the early construction of some cement concrete pavement and the recent construction of some cement concrete pavement damaged to varying degrees, it has reached the point where it needs to be renovated. How to reform has increasingly become a technical problem that highway departments must face and solve. Although some provinces in our country have made many beneficial attempts and arduous technical explorations in reforming cement concrete pavement by using asphalt overlay technology, it is undeniable that the reflection crack problem of cement concrete pavement has always been the most fundamental reason that affects the service life and performance of overlay.
1. structural design and construction technology of old cement pavement reconstruction recycled pavement
(1) Structural Combination Design of Cement Pavement Slab Recycled Pavement
According to the pavement structure combination of crack-resistant and pressure-stabilized asphalt mixture overlay, the structure and construction technology of "black on white background" and "white on white background" overlay can be adopted, including interlayer bonding design and material application, in which the prevention and control of reflection cracks is the key problem.
The preventive measures of reflection cracks include: increasing the thickness of asphalt overlay, setting asphalt mixture with good road performance such as stress absorption film interlayer and reinforced asphalt layer, setting isolation layer, and treating old pavement slab (sealing joint, rolling and stabilizing old pavement). Our experimental research on fiber asphalt mixture shows that fiber asphalt mixture has good low-temperature crack resistance and high-temperature rutting resistance, and it is planned to carry out on-site experimental research in Chengkou and Yubei, Chongqing. By improving the gradation of asphalt mixture, the rutting resistance of asphalt mixture at high temperature is improved, and the rutting resistance asphalt mixture is obtained. The excellent asphalt overlay materials such as modified asphalt concrete and SMA are also studied.
The thickness of asphalt overlay after macadam is designed by AASHTO method and program. The minimum covering thickness is required to be 15cm, and it is better to have a dense graded structure. The overlay thickness of asphalt mixture in China is generally 12 ~ 15 cm (only in Beijing-Shanghai expressway 18cm- Taihua section). According to the deflection test results of broken pavement, it is determined whether it is necessary to overlay the base.
The thickness of cement concrete overlay can be designed according to the traffic conditions and the design method of new pavement.
(2) Technology of injecting emulsified asphalt into recycled materials
After cement concrete pavement is broken, emulsified asphalt can be used to pour recycled materials between paving asphalt surface layers to strengthen the bond between broken base and surface layers and improve the overall strength of pavement. This method is relatively simple, can be constructed with emulsified asphalt spreading equipment, and is suitable for crushing technology using MHB, impact stabilizer and other equipment. The emphasis is on the control and evaluation of emulsified asphalt dosage, pouring depth and distribution uniformity. The general practice is:
Crushing the original cement concrete with MHB crusher again;
Adopt a Z-shaped roller (20-25 tons) to vibrate and compact the broken pavement for 2-3 times;
Vibrate and compact for 3 times with a tire roller;
Spraying and pouring modified emulsified asphalt with the dosage of 2.5-3.5 kg/m2;
Spray hot emulsified asphalt as waterproof adhesive layer, and its dosage is1.0 kg/m2;
Spread a layer of stone chips with a particle size of 3-5 mm, and roll them twice with a 16 ton steel wheel roller;
After the traffic is closed for 8- 12h, pave modified asphalt mixture or cement concrete.
2. Rapid detection and evaluation technology of cement concrete pavement
The rapid detection technology of cement concrete pavement includes: cement concrete pavement slab stability detection, pavement damage detection, structural bearing capacity detection, driving quality detection, skid resistance detection, traffic condition detection and so on. Study and establish a rapid detection and evaluation system and related evaluation model for cement concrete pavement conditions, and establish a criterion for judging cement concrete pavement conditions. The technical difficulty lies in the stability detection of cement concrete pavement slab, mainly how to judge the void of cement concrete pavement slab.
It is always a worldwide problem to judge the void of cement concrete pavement slab. Many research achievements have been made at home and abroad, such as using Beckman beam or FWD and pavement radar to detect the void degree at the bottom of cement pavement slab.
(1) pavement ground penetrating radar void diagnosis technology.
Ground penetrating radar (GPR) can measure the speed of more than 80km/h, which can be used to detect the thickness of pavement and airport pavement structure layer. Invalid identification; Identification of underground foreign bodies, such as steel bars, gas pipelines and drainage pipelines. Water content, compaction degree evaluation, etc. As shown in figure 1, the principle is that the suspicious position, degree and range of pavement void can be found by receiving, processing and analyzing the radar reflection signal. Because the reflected signal is greatly influenced by various noises, the accuracy of discrimination should be emphatically studied. Therefore, it is necessary to use FWD verification and manual excavation verification, which is helpful to establish the corresponding relationship between the reflected waveform and the actual cavity situation and enrich and accumulate the experience of ground penetrating radar.
(2)2)FWD fault diagnosis technology.
For the suspicious void points found by ground penetrating radar, FWD void diagnosis technology is adopted. Using AASHTO method, the deflection of three load levels at the same point on the plate edge is detected and linear regression is carried out. If the tropic of cancer passes through the coordinate origin, it is considered that there is no hole, otherwise it is considered that there is a hole.
3. Crushing technology of cement concrete pavement reconstruction
Chongqing Jiaotong University, Highway Research Institute of the Ministry of Communications and other units believe that when the broken slab rate of concrete pavement is lower than 10%, crack stabilization technology can be used to directly pave asphalt concrete overlay. For the cement pavement with broken slab rate of 65,438+00 ~ 65,438+05%, the anti-reflective crack material can be paved after the crack is stable, and then the asphalt concrete overlay can be paved. For the cement pavement with broken slab rate exceeding 15% and obvious structural damage (or the displacement (settlement difference) of adjacent slabs is greater than 4mm "the standard of American AI is 3mm", which requires broken slab), it is required to treat the problematic parts of subgrade and base locally, and then use the concrete slab as the base, and then pave the asphalt concrete overlay.
The existing crushing technologies at home and abroad mainly include roadheader series crushing technology, MHB multi-head heavy hammer crushing technology, RMI*** crushing technology, impact stabilization technology, door plate vibration crushing stabilization technology, impact pick crushing stabilization technology and so on. In the research process, MHB multi-head heavy hammer crushing technology, door plate vibration crushing and pressure stabilizing technology, impact pick crushing and pressure stabilizing technology are adopted. At the same time, the use effect of typical road sections is evaluated. The main evaluation contents include: the changing law of pavement performance, the change of reflection crack rate with time, and the effectiveness of measures to prevent reflection cracks. The project research has achieved good results in Chongqing, Hubei and Guangdong.
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