SMA, namely asphalt mastic macadam, is an asphalt mastic binder composed of asphalt, mineral powder, fiber stabilizer and a small amount of fine aggregate, which is filled in the gaps between coarse aggregate macadam skeletons with discontinuous gradation to form asphalt mixture. Simply put, SMA consists of coarse aggregate skeleton and asphalt mastic.
SMA is a new pavement material, which has good pavement performance: SMA not only has good surface function, skid resistance, high temperature resistance and rutting resistance, low temperature crack reduction, high flatness, low noise and good visibility, but also has the advantages of strong pavement deformation resistance, good water tightness, long service life and low maintenance cost. At the same time, SMA can also reduce the thickness of surface layer, which is convenient for construction and maintenance. Due to the above advantages, asphalt mastic is widely used as high-grade pavement material in high-grade highway construction.
2. Composition materials of 2.SMA mixture
SMA is a new type of asphalt mixture that has attracted worldwide attention in recent years. It is famous for its excellent rutting resistance and skid resistance. There are many factors that affect the quality of SMA, among which the quality of raw materials is the decisive factor. Therefore, the quality of raw materials should be strictly controlled in the construction process, and its quality should be inspected and tested in strict accordance with the requirements of relevant specifications.
2. 1 asphalt binder
The quality of asphalt binder in SMA mixture must meet the needs of asphalt mortar, with high viscosity and certain requirements to ensure sufficient high-temperature stability and low-temperature toughness. In China, asphalt that meets the technical requirements of heavy traffic road asphalt must be adopted. Taiwan Province AH-70# heavy traffic asphalt was used in this test road, and 5%SBS was added to modify the asphalt.
2.2 Coarse aggregate
According to the formation mechanism of SMA, the high-temperature stability of SMA is based on the collision between high-content coarse aggregates. The quality of aggregate compaction depends largely on the toughness, particle shape and angularity of aggregate stone. These characteristics of coarse aggregate are the key to the success of SMA. Therefore, the coarse aggregate used in SMA must meet the technical requirements of anti-sliding surface mixture, SMA requires high crushing resistance of coarse aggregate, and coarse aggregate must use tough, rough and angular high-quality stone. The particle size range of the stone used in the SMA upper layer of this test road is 13.2 mm ~ 19 mm and 5 mm ~ 13.2 mm, and its physical and mechanical properties meet the requirements of the specification.
2.3 Fine aggregate
Although fine aggregate only accounts for a small proportion in SMA, it also has a great influence on the performance of SMA. It is generally believed that machine-made sand has better angularity and embeddedness than natural sand, which is beneficial to improve the high temperature stability of mixture. Therefore, this test road is made of machine-made sand, which is crushed from basalt in Guanshi, Hengyang, Hunan Province.
2.4 filling
The amount of filler in SMA is large and plays an important role in the mixture, so filler with good adhesion must be used. This test road uses dolomite to grind fine mineral powder, and the cement is Chen Wangpai 325# cement.
2.5 fiber stabilizer
Fiber stabilizer must be used when manufacturing SMA, which can effectively improve the high temperature stability of the mixture and resist the wear of the nail-embedded tire. This project adopts granular wood fiber additive produced by JRS company in Germany.
3 SMA mixture stirring
3. 1 mixing equipment
This project adopts intermittent asphalt mixer to mix SMA mixture. Before putting into production, the mixing equipment should be overhauled to make all components in good condition. The weighing device and temperature control system should be overhauled and calibrated to ensure that the quality and temperature of the mixture meet the requirements, with the metering accuracy reaching 1% and the temperature control system accuracy reaching 2%.
3.2 mixing process
(1) Aggregates, fillers, fibers and asphalt materials shall be measured in strict accordance with the production mixture ratio, and sent to the mixer for mixing. The discharging temperature is generally 175℃ ~ 185℃, and the maximum mixing temperature of the mixture shall not be higher than 195℃. If it exceeds 195℃, it shall be scrapped. The aggregate heating temperature is controlled at 190℃ ~ 200℃, because of the large amount of cold mineral powder and fiber, the temperature is not high, so it can not be fully dispersed and stirred evenly; Fillers and lignin fibers are not heated. [Next page]
(2) High-quality fibers directly enter the mixer through a special inlet. This time, granular fiber was used, which was quantitatively input in a special container, and the fiber was scattered by the impact of coarse aggregate mixing. The feeder must pay close attention to the signal to open the coarse aggregate bin to prevent missing time. Adding lignin fiber generally needs to increase the dry mixing time by 5s ~ 10s.
(3) After mixing, SMA mixture should not be stored for too long, because it will form a hard shell on the surface of the mixture if it is stored for too long; Moreover, the asphalt content of SMA is higher than that of ordinary asphalt mixture. After a long time, asphalt leakage will occur, resulting in uneven asphalt content. Therefore, it is generally stipulated that SMA mixture is not allowed to be stored overnight, that is, it must be used up the same day.
4 transportation of SMA mixture
(1) Due to the high viscosity of modified asphalt SMA asphalt mastic, the carriage of the material carrier should be coated with oil-water mixture, and the carriage should be cleaned first to ensure that there are no sundries in the carriage. Every car should be covered tightly with canvas to keep warm from rain.
(2) According to the mixing capacity and transportation distance, allocate enough vehicles to transport SMA mixture to the paving site in time.
(3) The mixture delivered to the site shall be visually inspected first. Check whether the mixture is uniform and the color is consistent; And measure the temperature of the mixture car by car, and make records. If the temperature exceeds the specified value, it should be discarded.
(4) In order to ensure the balance and continuity of paving mixture, a certain number of transport vehicles must be maintained at the site, and each paver shall have no less than 2 ~ 3 vehicles to be unloaded.
5 SMA mixture paving
5. 1 Preparation before paving
(1) Strictly check the intermediate surface, and repair or treat surface damage, oil stain, dirt, etc. Thoroughly clean the topsoil as appropriate, and rinse with water if necessary.
(Before the construction of SMA upper layer, relevant projects and cross projects need to be completed. After the construction of SMA upper layer is completed, it is no longer allowed to carry out polluting or destructive engineering operations on the upper layer.
(3) Spray modified asphalt binder oil 1d ~ 2d in advance, with the dosage less than 0.4kg/m2.
5.2 Paver machinery
Two sets of ABG423 pavers are selected for this test road. Before use, the paver should be thoroughly inspected and debugged to ensure that it is in good working condition.
5.3 Paving operation
This project adopts full-width pavement, and hot joints are used as far as possible in acceleration and deceleration lanes and harbor parking areas. The longitudinal joints of the upper construction corresponding to the middle construction are staggered by 50cm, and the transverse joints of the middle construction are staggered by 100cm. Paver operation should be carried out continuously and evenly, and no pause is allowed in principle. The paving speed is generally controlled at 1.5m/min ~ 2m/min, and the maximum speed is controlled at 4m/min；. . The paving temperature shall not be lower than 165℃.
Rolling of 6 SMA mixture
(1) This project is equipped with four double steel roller. Compaction of the mixture shall be carried out in accordance with the eight-character policy of "high temperature, close follow, high frequency and low amplitude". Practice has proved that SMA mixture can achieve the expected compaction effect at high temperature only according to the compaction process. In the process of compaction, pay close attention to the defects of coarse aggregate, such as broken, angular, embedded and soaked. Excessive rolling is the taboo of SMA structure, which will make the upper layer of pavement unstable, and the degree of compaction can only be controlled by controlling the rolling times.
(2) When rolling SMA mixture, we must pay attention to the change of compaction degree. It is best to use a nuclear densitometer to measure the change of compactness as a reference. This test road is rolled five times, that is, static pressure once, vibrating pressure three times and static pressure again. The minimum compaction degree is 98.4% and the maximum compaction degree is 99.3%. It meets the requirements of the specification well.
7 conclusion
In recent years, SMA has been widely used in the construction of high-grade highways in Jiangsu Province. Practice has proved that SMA has obvious advantages compared with traditional asphalt concrete pavement. However, SMA is a new type of pavement material that has emerged in recent years, and it is not mature enough in theory and practice. Therefore, it is of great significance to study the construction of SMA test section first and accumulate experience for large-scale construction in the future. After the construction of SMA test section was completed, many technical indexes were tested. The results show that all the test indexes are excellent, which proves that the construction of the test section is successful, but it still needs further optimization in some aspects. For example, in order to prevent the phenomenon of "oil ding" on the upper surface of SMA after rolling, the dry mixing time can be appropriately increased and the amount of mineral powder can be reduced; In order to improve the degree of compaction, we should strictly control the rolling speed and temperature, and strengthen the rolling on both sides of the road and the overlapping part of two pavers. The application of several expressways in Jiangsu Province proves that SMA has broad prospects as a pavement material for high-grade highways.