This paper mainly analyzes the formation mechanism of asphalt concrete strength, and discusses the measures to improve the strength of mixture, in order to improve the quality and durability of asphalt pavement.
At present, asphalt concrete pavement is widely used in expressways all over the world because of its many excellent properties. But at the same time, compared with cement concrete pavement, asphalt pavement has the disadvantages of high maintenance cost, short service life and poor durability. Due to improper material selection and construction technology control during construction, the damage of asphalt pavement will be aggravated. Therefore, understanding the formation mechanism of asphalt mixture (including asphalt concrete) pavement strength is particularly important for improving the service quality and durability of asphalt pavement.
(1) Asphalt mixture (including asphalt concrete) is a heterogeneous dispersion system with spatial network structure, which consists of mineral skeleton and asphalt binder. The strength of asphalt mixture (including asphalt concrete) consists of two parts: embedding force and internal friction between mineral aggregate and cohesion between asphalt and mineral aggregate. This paper discusses the strength formation mechanism of asphalt concrete and the strength measures of mixture, so as to improve the service quality and durability of asphalt pavement.
The embedding force and internal friction between mineral aggregate mainly depend on mineral aggregate gradation, size uniformity, particle shape, surface roughness and asphalt content.
According to the principle of gradation composition, asphalt mixture can be divided into the following three ways: suspension dense structure: dense mixture composed of mineral mixture with continuous gradation, in which various gradations exist continuously, and the larger particles at the same stage are squeezed out by smaller particles, and the larger particles are in the suspended state. This kind of structure is usually designed according to the principle of optimal gradation, with high density and strength and good water stability, but it is greatly influenced by the properties and physical state of asphalt and has poor temperature stability. Traditional type I and type II asphalt concrete (AC) belong to this type of structure. B skeleton void structure: coarse-grained minerals are closely connected, and the amount of fine aggregate is small, which is not enough to completely fill the void, and its void ratio is large. In this structure, the internal friction and embedding force between aggregates play an important role, which is less affected by the properties and physical state of asphalt, with good temperature stability and poor water stability. Anti-sliding surface (AK) and asphalt macadam (am) belong to this type of structure. C skeleton dense structure: it is a structure composed of the above two methods. A certain amount of coarse aggregate forms a skeleton in the mixture, and a certain amount of fine aggregate is added according to the gap of coarse aggregate to form a higher density. Intermittent gradation and composition According to this principle, crushed stone asphalt mortar concrete (SMA), multi-crushed stone asphalt concrete (SAC), thin asphalt concrete (BBM) and Superpawe(SPP) all belong to this kind of structure. This structure has strict requirements for material grading and process control of each construction link. (2) The cohesive force mainly depends on the interaction between asphalt and mineral aggregate and the cohesive force of asphalt material itself.
The interaction between asphalt and mineral aggregate is the decisive factor for the formation of asphalt mixture structure, which is directly related to a series of important properties of the mixture, such as strength, temperature stability, water stability, aging speed and so on.
After asphalt contacts with mineral aggregate, the chemical components of asphalt are rearranged on the surface of mineral aggregate, and a diffusion structure film is formed on the surface of mineral aggregate. The asphalt within the film thickness is called structural asphalt, and the asphalt outside the film is called free asphalt. There is interaction between structural asphalt and mineral aggregate, and the properties of asphalt have changed. However, free asphalt is far away from mineral aggregate and does not interact with mineral aggregate, thus maintaining its original properties.
The chemical composition of petroleum asphalt mainly includes asphaltene acid, asphaltene anhydride, oil, colloid, asphaltene, asphaltene carbon and carbon-like substances. There are many effects when asphalt interacts with mineral aggregate, including physical adsorption of asphalt layer on mineral aggregate surface, chemical adsorption of asphalt and mineral aggregate, and selective adsorption of asphalt components by some kinds of mineral aggregate.
Physical adsorption occurs when there is only molecular force between mineral aggregate and asphalt, and the structural asphalt film formed by physical adsorption is easy to peel off when it meets water. Acidic substances in asphalt (such as asphalt acid and asphalt acid bile) will change chemically with alkaline mineral substances at the contact surface, and form water-insoluble asphalt acid salt at the contact surface, and chemical adsorption will occur at this time. The structural asphalt film formed by chemical adsorption has high water resistance. Only through chemical adsorption can asphalt mixture have good water stability. Whether or not chemical adsorption occurs and the degree of adsorption depends on the chemical composition of asphalt and mineral aggregate. Asphaltic acid is the most active component in asphalt, belonging to high molecular carboxylic acid, and its polar part is carboxyl group (COOH). Hydrogen in carboxyl group can be exchanged with high valence metal cations in alkaline stone to form high valence asphaltene, which is insoluble in water and soluble in polymer hydrocarbons and oils. Asphalt molecules interact with anions (such as CO3-2) in rocks through molecular force. Hydrogen in the carboxyl group of asphalt acid exchanges with low-valent metal cations in stone, and the formed asphalt acid salt is easily soluble in water.
The function of adding mineral powder to asphalt concrete is to fill the gaps between mineral materials with structural asphalt wrapped on the surface of mineral powder, which is beneficial to increase the adhesion and water stability of asphalt mixture. The mineral powder is generally ground limestone powder, or partially ground hydrated lime powder or waste cement can be used instead of mineral powder. Sometimes the mixing station will mix some recycled dust into concrete as a substitute.
The selective adsorption of asphalt components by mineral aggregate mainly occurs in mineral aggregate with micropores (1L void diameter less than 0.02mm) on the surface, such as limestone and marl, which has strong adsorption potential. The surface of mineral aggregate adsorbs asphaltene with high activity in asphalt, resin is adsorbed in pores on the surface of mineral aggregate, and oil with small molecular weight is adsorbed into mineral aggregate along capillaries. The result of selective adsorption is that the properties of asphalt change, the contents of gum and oil are relatively reduced, asphaltene and asphaltene acid are relatively increased, asphalt thickens, cohesion is increased, thermal stability and water stability are improved, and the resistance to low temperature and fatigue cracking is reduced. (3) Improving the strength of asphalt mixture includes two aspects: one is to improve the embedding force and friction between mineral aggregate, and the other is to improve the bonding force between asphalt and mineral aggregate.
1, choose the stone with solid material, rough surface, square shape and sharp edges to improve the embedding force and friction between mineral aggregates. Priority is given to the use of alkaline stone. In the absence of alkaline stone, neutral basalt, andesite and diabase can be used.
2. Choose the mineral aggregate gradation with the lowest porosity to reduce the content of free asphalt. Perfect mixing and compaction technology can greatly improve the embedding force and water stability of the mixture.
3. Through experiments and comparison of multiple schemes, the appropriate mineral powder is selected, and the reasonable dosage is determined to improve the structural asphalt content in the mixture. If the dosage of mineral powder is too much, it is necessary to increase the stirring time accordingly, otherwise it is difficult to stir evenly and easy to agglomerate, and generally the median value within the range specified in the specification is taken.
4. By adding polymer modifiers (such as thermoplastic elastomer SBS, rubber 3BR) or modifiers such as carbon black and silica fume to asphalt, the performance of asphalt is improved, and the working performance of asphalt concrete is further improved.
5. Anti-stripping agents can be used, but there are many kinds of anti-stripping agents with different properties, so we should choose them carefully. Adding anti-stripping agent can improve the water damage resistance of asphalt mixture, but anti-stripping agent is selective to aggregate and asphalt. Therefore, it is not easy to draw a conclusion that some anti-stripping agent is not good or not, and it should be screened through careful experimental design.
To sum up, combined with the strength formation mechanism of asphalt concrete pavement and years of working experience, we should pay attention to the following points: (1) The type of asphalt pavement structure mixture should be reasonable, and graded mixture with small particle size and small gap should be selected; The design should conform to the actual thickness of the road section and should not be too large to avoid deceleration; Affect the construction period and quality; The pavement thickness of the reinforcement section should be realistic, and the situation of the old road, especially the old road, should not be imagined hastily. It should be avoided that the deflection of the reinforced section after strength reinforcement is greater than the design value, resulting in insufficient strength of the new road and serious early damage; Strictly control raw materials, if the mixture ratio is not controlled enough, especially the dosage of ingredients and leaching are not allowed, the pavement will be pushed, oily, loose and pitted in the early stage; Old and unsuitable construction machinery and equipment will affect the mixture ratio, measurement, mixing uniformity, compactness and smoothness of the mixture; The asphalt mixture is overheated, the mineral materials scorch the asphalt at high temperature during the mixing process, the asphalt is aging, the pavement strength is insufficient, and looseness and pits appear; The rolling temperature should not be too high, so we should pay attention to the rolling temperature, otherwise it will cause false compaction and produce passes and micro-cracks; Subbase, base and pavement should be cleaned up. If floating soil, floating ash and floating sand are not cleaned up, they will soften and spread to the surface under the action of rain. The loose laying coefficient of reinforcement should be well controlled, and it is not suitable to make secondary reinforcement layer. Because the secondary reinforcement layer can not be closely connected with the lower base, it is easy to be loose, pitted and cracked. It is suggested to use oil sands (i.e. oil) instead of additional layers; Under the action of driving, initial maintenance is more important for small areas with loose pits and grooves, especially for layered construction, pavement penetration and surface treatment; Strictly follow the basic construction procedures and do a good job in construction management and quality management; Put an end to the construction team without pavement construction experience, pavement equipment and technical force to undertake pavement construction and supervision, unable to strictly supervise, not in accordance with the requirements of construction technical specifications, rushing to engage in gift-giving projects. This will certainly strengthen the strength of asphalt concrete pavement, thus achieving greater benefits.