Asphalt mixture is the main material of hot-mix pavement mixed by asphalt, aggregate and mineral powder in proportion. Its temperature stability, bonding performance and gradation uniformity directly determine the pavement quality and molding durability. During the whole process of mixture discharge from mixing plant, transfer in site, temporary storage in bin and paving on site, there are risks such as temperature loss, moisture intrusion, segregation and agglomeration, asphalt aging, etc. If the storage control is improper, it is easy to cause quality diseases such as loose pavement, water seepage, poor flatness, insufficient compaction, early cracking, etc. In order to standardize the storage management of construction site, ensure the stability of mixture performance and meet the acceptance standard of pavement construction, this paper systematically combs the precautions for standardized storage of asphalt mixture according to the Technical Specification for Highway Asphalt Pavement Construction (JTG F40) and field operation experience.
Basic requirements for storage sites and silos
The storage site of asphalt mixture shall be standardized in advance to avoid pollution, ponding, cooling, segregation and other problems from the source. The finished product bin of the mixing plant is the main storage facility for mixture. The inside of the bin shall be kept clean and dry without impurities and residual accumulated materials. The bin wall shall be smooth and smooth to avoid caking and grading damage caused by adhesion of mixture on rough inner wall. The discharge gate at the bottom of the bin shall be tightly sealed and smoothly opened and closed without leakage, air leakage and water seepage, so as to prevent cold air and rainwater from invading the bin.
The temporary storage site on the construction site shall be hardened, and cement concrete or asphalt shall be used to harden the ground to ensure that the site is flat, solid, without potholes and ponding, and soil and sundries shall be prevented from polluting the mixture. Reasonable drainage slope and drainage ditch shall be set on the site. In rainy weather, accumulated water can be quickly drained to avoid moisture and water at the bottom of the mixture. At the same time, the storage area should be far away from dust sources, pollution sources, open flames and high temperature heat sources to prevent the mixture from being contaminated or the performance of the mixture from changing.
Core points of temperature storage management and control
Temperature is the core index of asphalt mixture storage control, and temperature imbalance is the main reason for mixture failure and pavement quality failure. Hot mix asphalt mixture shall be kept warm continuously after discharge. Rapid cooling and low temperature storage are strictly prohibited. The finished product bin must be equipped with complete insulation interlayer, insulation cover plate and closed system, and the whole process of closed storage can reduce heat loss and air convection cooling.
The storage temperature range of different types of mixtures shall be strictly controlled: the storage temperature of ordinary matrix asphalt mixture shall be maintained at 110℃~130℃, the storage temperature of SBS modified asphalt mixture shall not be lower than 120℃, and it shall not be lower than the critical temperature of construction paving during the whole process. During the storage process, the internal temperature and discharge temperature of the silo shall be monitored in real time. It is strictly prohibited to spread the low-temperature mixture directly. At the same time, it is prohibited to store the mixture at high temperature for a long time. If the mixture stays in the bin for too long and the temperature is too high, it will accelerate the thermal aging of asphalt, resulting in the decrease of asphalt viscosity and adhesion force, and the loose and peeling diseases of the later pavement are easy to occur.
Under normal working conditions, the storage time of asphalt mixture silo should not exceed 4 hours, and the storage time of modified asphalt mixture should not exceed 3 hours, so as to retain the original performance of mixture to the maximum extent.
stacking and unloading anti-segregation control
The gradation uniformity of asphalt mixture is the basis for the compactness and flatness of the road surface. Separation and segregation of coarse and fine aggregates are prone to occur during storage and unloading, so strict operation is required. The hopper shall be fed in a stratified and uniform manner to avoid centralized stacking at a single point, and to prevent large particle size aggregates from rolling down and accumulating, and fine materials from concentrating and accumulating, resulting in partial gradation imbalance.
During temporary stacking on site, the stacking height shall not be too high, and shall be controlled within 1.5 meters to reduce segregation and agglomeration caused by self-weight extrusion of mixture. Open the gate slowly and uniformly during unloading, keep continuous and uniform unloading, and prohibit sudden opening and closing and rapid flushing. When receiving materials, the transport vehicle shall move forward and backward to receive materials in batches, so as to avoid separation of coarse and fine materials caused by fixed point receiving materials.
During storage and stacking, if partial agglomeration, segregation and uneven color of mixture are found, direct paving and use are prohibited. It shall be inspected by on-site technicians, mixed evenly again, and the mixture can be put into construction only after its performance meets the standard. The mixture with serious segregation and agglomeration must be discarded.
Waterproof, moisture-proof and rain-proof sun protection control
Asphalt mixture is hydrophobic, but high temperature hot mixture will cool down sharply when it meets water. Meanwhile, moisture will destroy the bonding interface between asphalt and aggregate, greatly reduce the adhesion and stability of mixture, and the molded pavement is easy to appear water seepage, pit groove, spalling and other diseases. Therefore, the whole storage process must be strictly waterproof and moisture-proof.
The bin shall be kept completely airtight, and the cover plate shall be closed in the whole process to prevent rain, dew and moisture from invading. The mixture temporarily stored in the open air must be fully covered and sealed with high temperature resistant and rainproof tarpaulin, which shall be tightly attached without damage or gap, so as to prevent rainwater infiltration and dew wetting of the mixture. It is strictly prohibited to stack the mixture in the open air in rainy weather. The mixture that has been wetted and wet shall not be used for formal paving regardless of whether the temperature and appearance are normal.
During sunny and high temperature periods, there is no need to deliberately expose and ventilate, and it is necessary to keep closed and insulated for storage. Long-term exposure to the outside will aggravate the aging of asphalt, destroy the workability and bonding performance of the mixture, and affect the construction quality.
Storage duration and aging control
Asphalt mixture is an instant construction material, and its performance continues to decay with the extension of storage time. It has no long-term storage value. It must follow the principle of "mixing, storing, using, first-in, first-out". Overstock and overnight storage are strictly prohibited.
Under normal normal temperature and dry conditions, the effective storage time of ordinary hot mix asphalt mixture is ≤4 hours, and the effective storage time of modified asphalt mixture is ≤3 hours; under low temperature, strong wind and humid environment, the heat loss is accelerated, and the performance attenuation of mixture is intensified. Therefore, it is necessary to further shorten the storage time and give priority to rapid discharge and paving. It is strictly prohibited to use overnight mixture, temperature failure mixture and repeated heating mixture. Secondary heating will lead to serious aging of asphalt, hardening, dryness and poor workability of mixture. After construction, the durability of pavement will be greatly reduced, and early damage will easily occur.
Pollution prevention and control and sundries control
During the storage period of the mixture, pollution prevention and control shall be carried out throughout the whole process to prevent any impurities from mixing. The storage bin and stacking site shall be cleaned regularly without dust, soil, gravel, garbage and other sundries. The site and the inner wall of the storage bin shall be cleaned thoroughly before storage. It is forbidden to mix waste materials, ground materials and waste materials into fresh storage mixture during construction.
At the same time, oil, sewage and dust pollution mixture shall be eliminated. The asphalt adhesion performance of contaminated mixture will be completely damaged, and the adhesion strength and overall stability of pavement cannot be guaranteed. It must be cleaned up in time and prohibited from use. Arrange special personnel to inspect the storage state regularly, clean up the floating dust and sundries on the surface in time, and ensure the cleanliness of the mixture.
Precautions for storage in winter and special weather
Low temperature and strong wind weather in winter are the key difficulties in storage control of mixture. The environmental temperature is low, heat loss is fast, and low temperature failure is easy to occur. Winter storage needs to be sealed and insulated throughout the whole process, thickening the bin insulation layer, densifying the cover plate gap, reducing the cold air penetration, and appropriately raising the discharge temperature to offset the temperature loss during storage. At the same time, the storage time is greatly shortened, so that it can be used immediately, and the standing and cooling time of the mixture is reduced.
Wind, frost weather should be done well, anti-condensation measures to avoid mixture surface condensation damp, rapid cooling. It is forbidden to store in the open air in foggy and humid weather. All mixtures must be stored in a sealed manner to prevent moisture from invading and affecting the performance of materials. In summer, it is necessary to avoid long-term high temperature storage of mixture to prevent asphalt aging and deterioration.
Quality control after storage
After the mixture is stored and before the material is discharged and spread, on-site quality reinspection must be carried out. Construction can be carried out only after it is qualified. Focus on checking three indicators: first, temperature, check whether the discharge temperature meets the construction standard of corresponding asphalt type; second, appearance state, observe whether the mixture color is uniform, loose without agglomeration, no segregation, no moisture whitening phenomenon; third, workability, fine feel, good fluidity is qualified, dry and rigid, caking serious is unqualified.
For the mixture with long storage time, low temperature and abnormal state, it must be sampled for re-inspection, and the pavement can be paved only after the technician confirms that the performance meets the standard. The unqualified mixture shall be resolutely cleared, and the construction with defects shall be eliminated to ensure the pavement construction quality from the source.
Conclusion
The storage management of asphalt mixture is the key hidden procedure of pavement construction quality control, which directly affects paving effect and road service life. The core points of storage control are temperature control, time control, waterproof, anti-segregation and anti-pollution. The standardized storage requirements shall be strictly implemented on the construction site, the site layout, temperature control, stacking operation, aging management and quality re-inspection shall be standardized, the extensive storage management shall be eliminated, the original performance of asphalt mixture shall be retained to the maximum extent, the early diseases of pavement shall be effectively avoided, the smoothness, compactness and durability of asphalt pavement shall be guaranteed, and the overall construction quality of road engineering shall be improved.
