Asphalt pavement has become the mainstream pavement form of highway and urban road in China by virtue of its advantages of high flatness, driving comfort, convenient maintenance, noise reduction and wear resistance.As a typical temperature-sensitive viscoelastic material, asphalt performance fluctuates easily with temperature change. Temperature environment runs through the whole construction process of asphalt mixture mixing, transportation, paving and rolling, which is the core factor to determine pavement compactness, flatness and durability.Improper control of construction temperature will directly cause early diseases such as rutting, cracking, potholes and seepage, which will greatly shorten the service life of roads.Combined with asphalt material characteristics and engineering construction practice, this paper systematically analyzes the specific impact of high temperature and low temperature environment on asphalt pavement construction, and puts forward targeted temperature control and quality optimization strategies, which provides technical reference for road engineering construction.

Temperature-sensitive core characteristics of bituminous materials

Asphalt mixture is composed of asphalt binder, aggregate and mineral powder. It has strong temperature dependence as a whole. Its characteristics are often summarized as "softening at high temperature and brittle at low temperature" in the industry.In high temperature environment, the viscosity of asphalt binder decreases greatly, the fluidity increases, the overall mixture is soft, and the deformation resistance decreases sharply; in low temperature environment, the viscosity of asphalt increases rapidly, the material hardens and becomes brittle, the flexibility decreases greatly, and shrinkage stress is easy to occur.
There is a strict "temperature window period" in asphalt pavement construction. The temperature decay rate and stable temperature range of each link directly determine the construction quality from mixture delivery, transportation to site, paving and leveling to rolling molding.The optimum construction temperature range of conventional asphalt mixture is 145℃-170℃, the suitable paving temperature is 110℃-130℃, and the initial pressure temperature shall not be lower than 110℃. Once it deviates from the standard temperature range, the pavement construction quality will be difficult to meet the standard.It can be said that the core technical key of asphalt pavement construction is the precise control of temperature chain of the whole process.

the impact of high temperature environment on asphalt pavement construction

High temperature in summer is a common working condition for asphalt pavement construction. High temperature environment can slow down the temperature loss of mixture, prolong the effective rolling window period, facilitate the full compaction of mixture, close fit of aggregate, and improve the overall compactness and integrity of pavement. Therefore, summer is also the golden construction period for asphalt pavement maintenance and renovation.However, if the ambient temperature is too high, it will cause a series of construction quality problems and safety hazards, especially the negative effects.

mixture performance imbalance, easy to breed high temperature disease

When the ambient temperature is higher than 35℃ continuously, the heat dissipation of newly paved asphalt mixture is very slow, and the internal temperature is at a high level for a long time.At high temperature, asphalt binder softens, adhesion decreases, mixture displacement, over-pressure compaction and other problems easily occur in the process of roller compaction, resulting in excessive pavement flatness.After the pavement is formed and opened to traffic, under the continuous high temperature and repeated vehicle load in summer, the softened asphalt mixture is easy to produce plastic deformation, forming permanent ruts, waves, pits and other diseases, which seriously affect the driving safety.At the same time, high temperature will accelerate the aging of asphalt, make the light components of asphalt volatilize quickly, reduce the adhesion between asphalt and aggregate, and easily cause problems such as loose pavement, spalling and water seepage in the later stage.

Increased difficulty in construction process control

Under high temperature environment, asphalt mixture temperature attenuation is uneven, surface heat dissipation is a little fast, internal heat storage is serious, it is easy to appear the hidden danger of surface compaction up to standard, internal compaction is insufficient, resulting in uneven pavement compactness, void ratio is not up to standard.In addition, high temperature operation environment will increase the operation load of construction equipment, pavers and rollers are prone to performance fluctuations, affecting the uniformity of paving and rolling.At the same time, high temperature weather is easy to cause mixture segregation, local asphalt enrichment, uneven color difference after molding, and significant difference in durability.

Outstanding hidden dangers in construction safety

Asphalt mixture factory temperature can reach 170℃, summer surface high temperature superimposed mixture radiation heat, construction site temperature is extremely high.High temperature operation not only easily leads to heatstroke and fatigue operation of construction personnel, reduces construction accuracy, but also accelerates aging of construction equipment, oil volatilization, and increases equipment failure and fire hazards.

the impact of low temperature environment on asphalt pavement construction 

Low temperature is the main unfavorable working condition of asphalt pavement construction, especially in autumn and winter, low temperature and strong wind weather, the temperature decay speed of mixture will be doubled, and the effective construction window period will be greatly compressed, which is the main cause of pavement cracking, insufficient compaction, seepage damage and other diseases. Improper control of low temperature construction will directly lead to project rework. 

Temperature decay is too fast, compaction quality is not up to standard 

When the ambient temperature is lower than 5℃ and the wind speed is higher than 3m/s, the heat dissipation speed of asphalt mixture is accelerated rapidly, and the temperature of mixture falls below the lower limit of specification rapidly during the process from transportation to paving and rolling. Low temperature hardening of asphalt, rapid increase of viscosity, poor fluidity of mixture, increased difficulty of aggregate embedding, difficulty of compaction of mixture by road roller, resulting in insufficient compaction degree and large void ratio of pavement as a whole. The pavement with excessive void ratio is easy to penetrate rain water and snow melt water, which will cause frost heaving damage under the alternating change of temperature, resulting in potholes, subsidence and other diseases. The results show that SMA-13 and other special asphalt mixtures in low temperature and strong wind environment, the construction qualification rate dropped significantly, it is not suitable to carry out paving operations. 

Low temperature shrinkage of pavement and frequent structural cracks 

At low temperature, asphalt mixture hardens and becomes brittle, and the tensile resistance and deformation resistance of the material decrease greatly. After pavement molding, continuous low temperature will cause uniform shrinkage of pavement structure. When the shrinkage stress exceeds the tensile strength of asphalt mixture, transverse and longitudinal low temperature cracks will appear. At the same time, there are differences in temperature shrinkage coefficient between base and surface materials, and the shrinkage deformation at low temperature is inconsistent, which is easy to produce interlayer void and bond failure, which further intensifies crack propagation and destroys the overall structural stability of pavement. 

Interlayer bonding failure, deterioration of pavement integrity 

Under low temperature condition, the temperature of base course and tack coat oil is low, the temperature of asphalt mixture is lost rapidly, the adhesion performance with base course and new and old pavement is greatly reduced, and problems such as interlayer separation, peeling and displacement are easy to occur. Especially in winter low temperature construction, if the base preheating and mixture insulation measures are not well done, the interlayer adhesion is insufficient, and the pavement delamination damage is easy to occur under the later vehicle load, which greatly reduces the service life of the road.

Construction quality control strategies under different temperature environments 

In view of the differential influence of temperature on asphalt pavement construction, the principle of "high temperature temperature control to prevent deformation, low temperature insulation to prevent failure, and full chain temperature control to ensure uniformity" shall be followed during construction, and the construction process shall be optimized in combination with ambient temperature and mixture type to realize accurate control of the whole process. 

Construction control measures for high temperature environment 

When the ambient temperature is higher than 35℃, focus on controlling the mixture delivery, paving and rolling temperature to avoid high temperature softening diseases. Firstly, accurately control the mixing temperature to stabilize the delivery temperature of asphalt mixture at 140℃-165℃, so as to avoid excessive temperature to accelerate asphalt aging; secondly, optimize the construction sequence, give priority to paving operation in the early morning and evening, avoid the high temperature peak value at noon, and reduce the heat storage time of mixture; thirdly, optimize the rolling technology, follow the principle of "close follow, fast press and uniform speed", shorten the interval between paving and rolling, reasonably control the rolling times, and avoid over-pressure and under-pressure loosening; Fourthly, strengthen ventilation and heat dissipation, adopt layered paving and thin layer construction mode to speed up heat dissipation of pavement surface layer and reduce internal heat storage; fifthly, do a good job in finished product maintenance, spray water to cool down and seal protection timely for high-temperature molded pavement, so as to avoid early load rolling deformation.

Control measures for construction in low temperature environment 

The ambient temperature is below 5℃ and the construction conditions shall be strictly controlled in windy weather. Construction shall not be carried out unless necessary. If construction is really necessary, heat preservation and temperature control shall be strengthened. Firstly, strictly control the construction threshold. When the temperature is lower than-5℃ and the wind speed is greater than 6m/s, asphalt pavement construction is prohibited. Warm asphalt mixing technology is preferred under the breeze environment of about 5℃ to reduce the temperature loss of mixture construction; Second, strengthen the whole process insulation. The mixture transportation vehicles shall be covered with insulation quilt in the whole process to reduce the temperature attenuation during transportation, preheat the pavement base in advance, and avoid the rapid cooling of mixture caused by heat absorption of the base layer; Third, improve construction efficiency, streamline construction procedures, shorten paving and rolling intervals, ensure that the initial pressure temperature is not lower than 110℃, and the final pressure temperature meets the specification standard; fourth, optimize maintenance measures, timely cover thermal insulation and moisture retention materials on low-temperature molded pavement, avoid shrinkage cracks caused by sudden temperature difference, and prolong pavement thermal insulation maintenance cycle. 

Establish a full-process temperature monitoring system 

Relying on infrared thermal imaging and intelligent sensing equipment, a real-time temperature monitoring system for asphalt pavement construction is set up to monitor the mixture ex-factory temperature, arrival temperature, paving temperature and rolling temperature in the whole process, record the temperature data in real time, and dynamically adjust the construction parameters. For different types of asphalt mixtures, matching the exclusive temperature control standard, the optimal rolling temperature of AC-20 asphalt mixture is controlled at about 152℃, and the optimal rolling temperature of SMA-13 mixture can reach 169℃. The compaction quality of various mixtures is guaranteed by precise temperature control. At the same time, strictly control the wind speed and temperature parameters of the construction environment, avoid adverse meteorological conditions, and eliminate the hidden dangers of construction quality caused by temperature from the source.

Conclusion 

Temperature is the core variable that determines the construction quality and service life of asphalt pavement. High temperature is easy to cause pavement deformation, rutting and asphalt aging. Low temperature is easy to cause insufficient compaction, cracks and interlayer voids. Temperature control runs through the whole life cycle of asphalt pavement construction. In the construction of the project, it is necessary to fully grasp the temperature sensitivity characteristics of asphalt materials, optimize the mixing, transportation, paving, rolling and maintenance processes according to different working conditions of high and low temperature, establish a full-chain temperature control mechanism, accurately control the construction temperature window period, and avoid various quality diseases caused by temperature. Through scientific temperature control and fine construction, the compactness, flatness and structural stability of asphalt pavement can be effectively improved, the service life of road can be prolonged, and the construction quality and driving safety of road engineering can be guaranteed.