What Is Asphalt Concrete?

Asphalt concrete (bituminous concrete) is commonly known as asphalt concrete. It is artificially matched with a certain graded composition of minerals, crushed or rolled gravel, stone chips or sand, ore powder, etc., and a certain proportion of road asphalt materials. A mixture made under the same conditions.

Asphalt concrete

Asphalt concrete (bituminous concrete) commonly known

The strength of the asphalt mixture is mainly manifested in two aspects. One is the cement formed by asphalt and mineral powder.

The hot-mixed asphalt mixture should be mechanically mixed at a centralized location. Generally, a fixed hot mix plant is used, and a mobile hot mix machine should be used when the line is long. The cold-mixed asphalt mixture can be mixed intensively or on-site. The main equipment of the asphalt mixing plant includes: asphalt heating boilers, gravel storage, slag powder silos, heating rollers, mixers and weighing equipment, steam boilers, asphalt pumps and pipes, dust removal facilities, etc., and some also have hot aggregates. Re-screening and storage equipment (see Asphalt Mixing Base). Mixers can be divided into two types: continuous and batch. In the preparation process, in the past, the sand and gravel were first dried and heated, and then mixed with hot asphalt and cold mineral powder. Another method is to mix the wet aggregate with hot asphalt first, and then heat and mix to eliminate fly ash caused by the aggregate during heating and drying. When using the latter process, to prevent the moisture remaining in the mixture from affecting the service life of the asphalt concrete, it is best to use an asphalt anti-stripping agent at the same time to enhance the water resistance.

Asphalt concrete structure form

1 Traditional asphalt concrete surface (AC) ps: ordinary dense graded asphalt concrete

"Highway Asphalt Pavement Design Code" JTGD50-2006, according to the "Seventh Five-Year Plan" national scientific and technological research and the special study of revising the specification, uniformly changing the aggregate particle size standard in asphalt mixtures from round hole sieve standard to square hole sieve .

The main reasons are: the measurement standards are close to ISO international standards; grading standards that are easy to refer to similar structures in foreign countries; the number of World Bank projects is increasing, which facilitates international bidding, supervision and quality inspection; many foreign mixing equipment are square holes Sieve is standard. The symbol of asphalt concrete has been changed from LH to AC.

1.1 Classification by the particle size of the asphalt mixture aggregate

1.1.1 Fine-grained asphalt concrete: AC9.5mm or AC13.2mm.

In the literature, the test paper often appears in the form of AC-9 or AC-13

1.1.2 Medium-granular asphalt concrete: AC-16mm or AC-19mm.

In the literature, the test paper often appears in the form of AC-16 or AC-19

1.1.3 Coarse-grained asphalt concrete: AC-26.5mm or AC-31.5mm.

In literature, test papers often appear in the form of AC-26 or AC-31

The combination principle is that the maximum particle size of the asphalt surface layer aggregate should gradually increase from the upper layer to the lower layer. The upper layer should use medium-grain and fine-grain type, and the maximum particle size of the asphalt mixture aggregate in the upper layer should not exceed 1/2 of the layer thickness, and the maximum particle diameter of aggregate in the middle and lower layers should not exceed 2/3 of the layer thickness.

1.2 Classification by the porosity of the asphalt mixture after compaction

1.2.1 Type I dense-graded asphalt concrete: porosity (3% 6%)

1.2.2 Type II dense-graded asphalt concrete: porosity (4% 10%)

c. AM type open-graded hot mix asphalt macadam: porosity (greater than 10%)

The combination principle is that at least one layer of the asphalt surface layer is type I dense-graded asphalt concrete to prevent water seepage. If type II asphalt concrete is used for the upper layer and type I asphalt concrete is used for the middle layer, AM type open-graded asphalt crushed stone should not be used as the surface layer, and it can only be used as the connection layer.

2 Crushed Asphalt Concrete Surface (SAC)

2.1 Generate background

Large-volume vehicles pass safely, comfortably, and at high speed on the highway. The asphalt surface layer must have good anti-sliding performance. This requires that the asphalt surface layer not only has a large friction coefficient, but also a deep surface structural depth (structural depth is the main factor for high-speed driving to reduce noise and reduce water [LM] drift and splashing water, which affects the driver's sight). The research results show that "the anti-sliding performance of the asphalt surface layer is formed by the microstructure and macrostructure of the surface structure. The macrostructure is derived from the mix ratio of the asphalt mixture, which is mainly determined by the thickness and gradation of the aggregate. ".

In the mid-1980s, China began to build high-grade highways. From the perspective of the structure of the asphalt surface layer: Type I asphalt concrete, with a porosity of 3% to 6%, low water permeability, good durability, and the friction coefficient of the surface layer can meet the requirements. However, the depth of the surface structure is small and far from meeting the requirements. Type II asphalt concrete has a porosity of 6% to 10%, deep surface structure and strong resistance to deformation, but its water permeability and durability are poor. In order to solve the anti-sliding performance of the asphalt surface layer (especially the surface layer has a good waterproof structure under the condition of large structural depth), the multi-gravel asphalt concrete surface layer is studied and used.

2.2 Characteristics of multi-gravel asphalt concrete surface

The multi-crushed asphalt mixture is a form of asphalt mixture formed by using more coarse crushed stones to form the framework, and the asphalt sand rubber fills the pores in the framework and glues the framework together. The specific composition is: coarse aggregate content of 69% to 78%, mineral powder of 6% to 10%, oil-stone ratio of about 5%. The practice of several highways has proved that the multi-gravel asphalt concrete surface layer not only provides a deeper surface structure, but also has the smaller voids and smaller water permeability of traditional type asphalt concrete, and at the same time has better resistance. Deformation ability (high dynamic stability). In other words, "Multi-gravel asphalt concrete has both the advantages of traditional type I asphalt concrete and the advantages of type II asphalt concrete, while avoiding the shortcomings of the two traditional asphalt concrete structural forms."

3 Asphalt horseshoe gravel mixture surfacing (SMA)

3.1 Forming the background

In the 1960s, German traffic was very developed. According to the climatic characteristics of the country (about 20 ° C in summer and not too cold in winter), it is customary to build "cast asphalt concrete" pavement. The bitumen content in this structure is about 12%, and the content of mineral powder is high. It is found that the rut on the road is very serious during use. In addition, the car in that country often used tires with nails (including in some European countries) for the purpose of anti-skid. ). In order to overcome the increasingly serious rutting and reduce road wear, road workers adjusted the mix ratio of the asphalt mixture, increased the proportion of coarse aggregates, and added fiber stabilizers to form the initial shape of the SMA structure. In 1984, the German transportation department formally formulated a design and construction code for SMA pavement, and the structural form of SMA pavement was basically improved. This new pavement structure has been gradually promoted and used in Germany and some European countries. In the early 1990s, the American highway industry believed that the quality of its road surface was not as good as that of European countries. The investigation found that there were two gaps: in the use of modified asphalt; in the structural form of the pavement (ie SMA). 1991 and 1992 began to study and promote the structure of SMA, the most typical is: in 1995, Atlanta city reconstruction and construction of the highway network for the Olympic Games, all using SMA this structural form for road surface.

3.2 Principles and Characteristics of Asphalt Horseshoe Crushed Stone Mixture Pavement (SMA)

Asphalt horseshoe fat crushed stone mixture (SMA) is an asphalt horseshoe fat binder composed of asphalt, mineral powder and fiber stabilizer. It is filled in the intermittently graded mineral material skeleton to form a dense skeleton mixture. . Its composition characteristics mainly include two aspects: coarse aggregates with a large amount of content are interlocked to form a structural skeleton with high stability (strong resistance to deformation); asphalt aggregates composed of fine aggregate mineral powder, asphalt and fiber stabilizer Hoof fat glues the skeleton together and fills the voids of the skeleton, so that the mixture has better flexibility and durability.

The structural composition of SMA can be summarized as "three more and one less, that is: more coarse aggregate, more mineral powder, more asphalt, and less fine aggregate". Specifically: SMA is an intermittently graded asphalt mixture. The proportion of coarse aggregate above 5mm is as high as 70% to 80%, and the amount of mineral powder is 7% to 13%. limits). The resulting intermittent gradation rarely uses fine aggregates; In order to add more asphalt, on the one hand, increase the amount of mineral powder, while using fiber as a stabilizer; Asphalt is used more, up to 6.5% to 7%, High cohesiveness requirements, and hope to choose asphalt with small penetration, high softening point, and good temperature stability (preferably modified asphalt)

Features of SMA: Asphalt horseshoe fat gravel mixture is currently recognized internationally (used more often) as an asphalt surface mixture with strong anti-deformation ability and good durability. Due to the good coextrusion of the coarse aggregate, the mixture has very good high temperature resistance to rutting. At the same time, the low temperature deformation performance and water stability are also improved due to the bonding effect of asphalt mastic. The fiber stabilizer is added to keep the asphalt binder with high viscosity, and its paving and compaction effect is better. Discontinuous gradation forms large pores on the surface, large structural depth, and good sliding resistance. At the same time, the gap of the mixture is very small, and the aging resistance and durability are very good, thereby improving the pavement performance of the asphalt mixture.

Asphalt concrete control points

(1) Strictly controlling the mix ratio: The composition design of the mix is strictly in accordance with the design drawings and the requirements of the Technical Specification for Highway Pavement Base Construction (JTJ034-2000). Two lime soil stabilized gravel basement inspection items, table items, sub-item inspection values, or allowable deviation inspection methods: 1. Compaction (%) 98 Inspections every 200m at 4 locations. 2. Flatness (cm) Standard value 2 Maximum gap h 8. 3, Elevation (mm) +5, -10 Check with 4 levels every 200m. 4, the width (mm) is not less than the design value of the ruler, 4 for every 200m. 5. Thickness (mm) 8 Check 1 point every 200m per lane. 6. Transverse slope (%) ± 0.2 Measure 4 sections with a spirit level per 200m. 7. Compressive strength (MPa) 1.2. Press JTJ071-94 Appendix F

(2) When the construction temperature is lower than 5 and there is dense fog or rainfall, no grassroots construction will be carried out.

(3) In the construction of the grassroots, ensure that the drainage is unobstructed, and water is not allowed on the surface.

(4) After the grass roots are compacted, they will be maintained for a period of no less than 7 days. During the health period, the driving speed is limited to 30km / h, but heavy vehicles and machinery are prohibited. ^[1]

(5) Be sure to do a good job of controlling dust at the construction site.