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Asphalt Mixtures-Applications, Theory and
dinner什么意思Principles
1 . Applications
Asphalt materials find wide usage in the construction industry. The u of asphalt as a cementing agent in pavements is the most common of its applications, however, and the one that will be considered here.
Asphalt products are ud to produce flexible pavements for highways and airports. The term “flexible” is ud to distinguish the pavements from tho made with Portland cement, which are classified as rigid pavements, that is, having beam strength. This distinction is important becau it provides they key to the design approach which must be ud for successful flexible pavement structures.
The flexible pavement classification may be further broken down into high and low types, the type usually depending on whether a solid or liquid asphalt product is ud. The low types of pavement are made with the cutback, or emulsion, liquid products and are very widely ud throughout this country.
Descriptive terminology has been developed in various ctions of the country to the extent that one pavement type may have veral names. However, the general process followed in construction is similar for most low-type pavements and can be described as one in which the aggregate and the asphalt product are usually applied to the roadbed parately and there mixed or allowed to mix, forming the pavement.
cultureThe high type of asphalt pavements is made with asphalt cements of some lected penetration grade.
Fig. ·1 A modern asphalt concrete highway. Shoulder striping is ud as a safely feature.
Fig. ·2 Asphalt concrete at the San Francisco International Airport.
They are ud when high wheel loads and high volumes of traffic occur and are, therefore, often des
igned for a particular installation.
2 . Theory of asphalt concrete mix design
High types of flexible pavement are constructed by combining an asphalt cement, often in the penetration grade of 85 to 100, with aggregates that are usually divided into three groups, bad on size. The three groups are coar aggregates, fine aggregates, and mineral filler. The will be discusd in detail in later chapter.
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Each of the constituent parts mentioned has a particular function in the asphalt mixture, and mix proportioning or design is the process of ensuring that no function is neglected. Before the individual functions are examined, however, the criteria for pavement success and failure should be considered so that design objectives can be established.
A successful flexible pavement must have veral particular properties. First, it must be stable, that is to resistant to permanent displacement under load. Deformation of an asphalt pavement can occur in three ways, two unsatisfactory and one desirable.
Plastic deformation of a pavement failure and which is to be avoided if possible. Compressive defor
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eventhorizonmation of the pavement results in a dimensional change in the pavement, and with this change come a loss of resiliency and usually a degree of roughness. This deformation is less rious than the one just described, but it, too, leads to pavement failure. The desirable type of deformation is an elastic one, which actually is beneficial to flexible pavements and is necessary to their long life.anzhelika
The pavement should be durable and should offer protection to the subgrade. Asphalt cement is not impervious to the effects of weathering, and so the design must minimize weather susceptibility. A durable pavement that does not crack or ravel will probably also protect the roadbed. It must be remembered that flexible pavements transmit loads to the subgrade without significant bridging action, and so a dry firm ba is absolutely esntial.
Rapidly moving vehicles depend on the tire-pavement friction factor for control and safety. The texture of the pavement surfaces must be such that an adequate skid resistance is developed or unsafe conditions result. The design procedure should be ud to lect the asphalt material and aggregates combination which provides a skid resistant roadway.
Design procedures which yield paving mixtures embodying all the properties are not available. Sound pavements are constructed where materials and methods are lected by using time-tested tests and specifications and engineering judgments along with a so-called design method.
The final requirement for any pavement is one of economy. Economy, again, cannot be measured directly, since true economy only begins with construction cost and is not fully determinable until the full uful life of the pavement has been recorded. If, however, the requirements for a stable, durable, and safe pavement are met with a reasonable safety factor, then the best interests of economy have probably been rved as well.
With the requirements in mind, the functions of the constituent parts can be examined with consideration give to how each part contributes to now-established objectives or requirements. The functions of the aggregates is to carry the load impod on the pavement, and this is accomplished by frictional resistance and interlocking between the individual pieces of aggregates. The carrying capacity of the asphalt pavement is, then, related to the surface texture (particularly that of the fine aggregate) and the density, or “compactness,”, of the aggregates. Surface texture
varies with different aggregates, and while a rough surface texture is desired, this may not be available in some localities. Den mixtures are obtained by using aggregates that are either naturally or artificially “well graded”. This means that the fine aggregate rves to fill the voids in the coarr aggregates. In addition to affecting density and therefore strength characteristics, the grading also influences workability. When an excess of coar aggregate is ud, the mix becomes
harsh and hard to work. When an excess of mineral filler is ud, the mixes become gummy and difficult to manage.
卢卡斯莫拉The asphalt cement in the flexible pavement is ud to bind the aggregate particles together and to waterproof the pavements. Obtaining the proper asphalt content is extremely important and bears a significant influence on all the items marking a successful pavement. A chief objective of all the design methods which have been developed is to arrive at the best asphalt content for a particular combination of aggregates.
3 . Mix design principles
juice是什么意思Certain fundamental principles underlie the design procedures that have been developed. Before the procedures can be properly studied or applied, some consideration of the principles is necessary.
Asphalt pavements are compod of aggregates, asphalt cement, and voids. Considering the aggregate alone, all the space between particles is void space. The volume of aggregate voids depends on grading and can vary widely. When the asphalt cement is added, a portion of the aggregate voids is filled and a final air-void volume is retained. The retention of this air-void volume i
confident用法s very important to the characteristics of the mixture. The term air-void volume is ud, since the voids are weightless and are usually expresd as a percentage of the total volume of the compacted mixture.
An asphalt pavement carries the applied load by particle friction and interlock. If the particles are pushed apart for any reason , then the pavement stability is destroyed. This factor indicates that certainly no more asphalt should be added than the aggregate voids can readily hold. However ,asphalt cement is susceptible to volume change and the pavement is subject to further compaction under u. If the pavement has no air voids when placed, or if it los them under traffic, then the expanding asphalt will overflow in a condition known as bleeding. The loss of asphalt cement through bleeding weakens the pavement and also reduces surface friction, making the roadway
hazardous.
Fig. ·3 Cross ction of an asphalt concrete pavement showing the aggregate framework bound together by asphalt cement.
The need for a minimum air-void volume (usually 2 or 3 per cent ) has been established. In addition, a maximum air-void volume of 5 to 7 per cent should not be exceed. An excess of air voids promotes raveling of the pavement and also permits water to enter and speed up the deteriorating process. Also, in the prence of excess air the asphalt cement hardens and ages with an accompanying loss of durability and resiliency.
The air-void volume of the mix is determined by the degree of compaction as well as by the asphalt content. For a given asphalt content, a lightly compacted mix will have a large voids volume and a lower density and a greater strength will result. In the laboratory, the compaction is controlled by using a specified hammer and regulating the number of blows and the energy per blow. In the field, the compaction and the air voids are more difficult to control and tests must be made no specimens taken from the compacted pavement to cheek on the degree of compaction being obtained. Traffic further compact the pavement, and allowance must be made for this in the design. A systematic checking of the pavement over an extended period is needed to given factual information for a particular mix. A change in density of veral per cent is not unusual, however.
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Asphalt content has been discusd in connection with various facets of the ix design problem. It is a very important factor in the mix design and has a bearing an all the characteristics ld a successful pavement: stability, skid resistance, durability, and economy. As has been mentioned, the various design procedures are intended to provide a means for lecting the asphalt content . The tests will be considered in
