Compaction is the most important process for the success or failure of asphalt concrete pavement construction. Early failures occur in many highway asphalt concrete pavements, most related to compaction quality control.
1. Factors affecting the compactness of asphalt pavement surface
1.1 Effect of material properties on compaction degree
1.1.1 The quality of aggregate gradation directly affects the degree of compaction of the pavement
The ratio of coarse aggregate to fine aggregate and the proportion of raw materials such as mineral powder have a direct impact on the compaction of the asphalt mixture. It is found through experiments that: under the same conditions of other indicators, the mixture that is uniformly graded from coarse to fine is easier to compact, and the asphalt mixture with a large proportion of coarse aggregate is easier to obtain the required porosity. The graded mixture is easier to compact, and the obtained porosity is satisfactory.
1.1.2 The viscosity of asphalt determines the effect of particle movement and thus affects the degree of compaction
Asphalt viscosity has a direct impact on the strength of the asphalt mixture and affects the compaction of the mixture. When the viscosity of the asphalt is high, the aggregate will be locked by the high-viscosity asphalt, resulting in poor mobility. During compaction, the movement of the aggregate is not smooth, resulting in insufficient compactness; when the viscosity of the asphalt is low, the mobility of the aggregate will not be locked by the asphalt Better, the dry aggregate moves smoothly during compaction, and the compactness is relatively high. At a given temperature, low-viscosity asphalt achieves a higher degree of compaction than high-viscosity asphalt, and by increasing the compaction temperature, high-viscosity asphalt can achieve as high a degree of compaction as low-viscosity asphalt.
1.2 The temperature of the mixture
The viscosity of asphalt is increased or decreased by the influence of temperature, and the temperature should be avoided when the initial pressure is too high or too low. When the rolling temperature is too high, the viscosity of the asphalt is low, and the mixture is easy to dislocate and move, and the phenomenon of shifting is serious. Also prone to striae. When the rolling temperature is too low, the viscosity of the asphalt is low and it is difficult to compact, and if it is over-rolled, it will crack.
1.3 Compaction process
The compaction procedure is divided into initial pressure, compression, and final pressure. The purpose of initial pressure is to level and stabilize the mixture, and at the same time create conditions for recompression, which is the basis of compaction. The purpose of repressing is to make the mixture dense, stable, and shaped, and the degree of compactness of the mixture depends on this process. The purpose of final pressure is to eliminate wheel marks and finally form a flat compacted surface.
1.4 Model of roller
The commonly used asphalt pavement rollers in my country mainly include static slick wheel rollers, tire rollers, and vibratory rollers. Different rollers have their own characteristics. Choosing a suitable roller can ensure the compactness of the asphalt pavement surface. Due to the impact force generated by the vibration of the vibratory roller, the unit line pressure is greatly increased and when the vibratory roller continuously and rapidly impacts the surface, the pressure wave of the same frequency penetrates into the material layer, and the particles of the material will also be moved and rearranged. Make it dense, so vibratory rollers are currently widely used.
2. Measures to improve the compactness of asphalt pavement
2.1 Temperature control
The level of rolling temperature directly affects the compaction quality of the asphalt mixture. The so-called optimal temperature for rolling refers to the temperature at which the asphalt mixture can support the road roller without horizontal movement and the compaction resistance is small within the allowable range of the material. For general asphalt mixture, the best rolling temperature is 120~150°C, and the highest temperature should not exceed 160°C.
Increase the temperature at which rolling begins. The paving temperature of pure asphalt mixture should be between 140°C and 165°C. The temperature at the beginning of rolling every day should not be lower than 120°C~150°C. Don’t wait for 30m~50m behind the paver to start rolling. When rolling, the road roller should follow closely behind the paver, that is, the initial pressure has been pressed to the end of the paver. In short, compact the mixture while it is at a high temperature. Because it is easiest to achieve high density at this time. In order to ensure the paving temperature of the mixture, it is necessary to strictly control the factory temperature of the mixture. From the time when the mixture leaves the factory until the truck is unloaded, during this period, strict insulation measures should be taken, such as covering the mixture on the truck with tarpaulins and quilts. This measure should be taken regardless of the distance of transportation and how high the temperature is.
2.2 Selection and Combination of compaction machinery
Combined with the actual project, the selection of the type, size, and quantity of the road roller should consider factors such as the productivity of the paver, the characteristics of the mixture, the thickness of the paving, and the specific conditions of the construction site.
The productivity of the paver determines the capacity required for compaction, thereby affecting the selection of the size and number of rollers, while the characteristics of the mixture provide the basis for the selection of the size, optimal frequency, and amplitude of the rollers. The frequency and amplitude of the roller should be selected according to the thickness of the paving layer. If the thickness of the paving is less than 6cm, it is best to use a small and medium-sized vibratory roller with an amplitude of 0.35~0.6mm (2~6T). The selection of the road roller must consider the specific conditions of the construction site. If there are steep slopes and turning road sections, the flexibility of the mobile operator of the road roller should be considered.
The required quantity of the road roller can only be obtained through careful observation, measurement, and testing at the initial stage of paving. Generally, the road roller is required to follow the paver as much as possible. Under the conditions of mixture temperature thickness and underlayer temperature change, the research on the mixture cooling rate shows that the effective compaction time can be estimated quite accurately by using the temperature parameters, and this effective compaction estimation can help us determine how many road rollers are needed.
2.3 Strict compaction procedures and operating requirements
The three processes of initial pressure, re-pressure, and final pressure must be strictly operated and operated.
(1) The initial pressure should pay attention to the smoothness of the compaction. Use a vibratory roller (turn off the vibrating device) to press twice during the initial pressure. The speed is controlled at 1.5-2.0km/h, and the temperature is controlled at 110°C-130°C. After the initial pressure, check the flatness and road crown at any time, and repair it if necessary.
(2) The compactness of the mixture depends on the recompression. The re-pressing is first pressed twice with a rubber-tired road roller. Because the rubber-tired road roller is compacting, the asphalt pavement and the tires deform at the same time, the contact area is large, and there is a kneading effect, so the compaction effect is better. At the same time, the rubber-tired road roller does not damage the edges and corners of the gravel, making the gravel mutually tooth-shaped, and the road surface has better compactness. Then use a vibratory roller to press twice to improve the compactness of the road surface. Finally, press it twice with a rubber wheel roller. The rolling speed is controlled at 4-5km/h, and the re-pressing temperature should always be controlled at 90°C-110°C.
(3) The rolling speed of the final pressing is controlled at 2.5-3.5km/h, and the temperature is controlled at 70°C-90°C. The final pressure adopts a vibratory roller (turn off the vibration device).
2.4 Adopt an effective compaction process
(1) Pavement thickness
The greater the thickness of the pavement layer, the easier it is to keep warm. This is because compared with the thin layer, the effective time of compaction is relatively longer. From this point alone, it is beneficial to compaction.
(2) Pre-compaction in paving operations
In modern mechanical paving operations, the screed and vibrating device has vibrated and tamped the asphalt concrete pavement layer, and the general compaction degree can reach 83%-90%. Pre-compaction not only greatly reduces the workload of the roller, but also prolongs the effective compaction time, and because the pavement layer has an initial compaction degree, it avoids the movement, ruts, pear lines, waves, etc. during the rolling process. Whole illness.
(3) Rolling speed
Reasonable selection of rolling speed is very important for compaction. Theoretically speaking, the degree of compaction formed by low rolling speed is higher than that formed by high rolling speed. However, due to the influence of rolling temperature, compacted layer thickness, equipment factors, mixture composition, and base conditions, the actual compaction degree is not completely consistent with the theory, so a reasonable rolling speed should be determined in field experiments.
(4) Rolling times
In the range of maximum compactness, the degree of compaction increases with the number of rolling passes. In order to achieve the compaction index, it is necessary to determine the necessary number of rolling passes through experimental compaction. It should be noted that too many rolling passes may not necessarily receive the best compaction effect. On the contrary, it may cause excessive rolling, damage the surface roughness, reduce the surface strength and overall strength, and even produce defects such as longitudinal cracks and transverse cracks in the compacted layer.
To sum up, the degree of compaction has a decisive impact on the quality of asphalt pavement. Improving the standard of road compaction, increasing the compaction function of the road roller, and strictly controlling the speed of the road roller during the rolling process will have a negative impact on the asphalt pavement. The compaction quality has a noticeable effect.