Pavement
A highway pavement is a structure consisting of superimposed layers of selected and processded materials placed on a subgrade,whose primary function is to support the applied traffic loads and distribute them to the basement soil.The ultimate aim is to ensure that the transmitted stresses are sufficiently reduced that they will not exceed the supporting capacity of the subgrade.Two types of pavement are arbitrarily recognized as serving this purpose-flexble pavements and rigid pavements.
A flexible pavement is a pavement structure that maintains intimate contact with ,and distribute load to ,the subgrade ;it depends upon aggregate interlock,particle friction ,and cohesion for its stability .The distinguishing feature of flexible pavement lies in its structrual mechaincs the pressure is ususlly assumed to be transmitted to the subgrade through the lateral distribution of the applied load with depth ,rather than by beam and slab action as with a concretes slab.Thus a flexible pavement can be most easily defined by contrasting it with a rigid portland cement concrete pavement.
When the subgrade deflects beneath a rigid pavement ,the concrete slab is able to bridge over localized failures and areas of inadequate support because of its rigidity and high modulus of elasticity.The major factor influencing the design of rigid pavement is the structural strenth of the concrete ,and its thickness is relatively little affected by the quality of the subgrade as long as it meets certain minimum criteria.
In direct contrast to this ,the strength of the subgrade is a major factor contrlling the design of a flexible pavement. When the subgrade deflects,the overlying flexible pavement is assumed to deform to a similar shape and extent. In fact,this does not necessarily happen,e.g. pavements with thickness and help to support the imposed loads. Nonetheless ,these pavements are still generally classed as flexible pavements,and the assumed basic design criterion is that a depth of pavement is required that will distribute the applied surface load through the various pavement layers to the subgrade so that the subgrade is not over-stressed.
In its simplest form,a flexible pavement is generally considered to be any pavement other than a concrete one. It is this definition that is accepted by great majority of practising engineers. It should be clearly understood,however ,that the term is simply one of convenience and does not truly reflect the characterisitics of the many different and composite types of construction masquerading as “flexible” pavements.
Whethet it is a pavement for an expensive motorway or a simple country road ,the basic structural cross-section of a flexible road is essentially that illustrated in Fig.10-1;it is composed of several distinct layers that make up the pavement superimposed on the subgrade in the manner indicated. The intersection of the subgrade and the pavenment is known as the formation.
The subgrade is normally considered to be the situ soil over which the highway is being constructed. It should be quite clear ,however ,that the term subgrade is also appied to all native soil materials exposed by excavation and to excavated soil that may be artically deposited to form a compacted embankment. In the later case ,the added material is not considered to be part or the road structure itself but part of the foundation of the road.
The oppermost layer of a flexible pavement is called surface course. The highway materials used in a surface course can vary from loose mixtures of soil and gravel to the very highest-quality bituminous mixtures. The choice of materials used in any particular situation depends inmost countries upon the quality of service required of the highway.
If surface course is composed of bituminous materials-as is the normal practice for flexible pavements in Britain-it may consist of a single homogeneous layer or,in the highrquality roads,of two distinct sub-layers known as a wearing course and a basecourse. The wearing course provides the actual surfacing on which the vehicles run,while the basecourse acts as a rugulating layer to provide the wearing course with a better riding quality. The basecourse is normally composed of a more pervious material than the wearing course.
The primary funtion of the surface course,and especially of its wearing component,is to provide a safe and comfortable riding surface for traffic. It must also withstand the most concentrated stresses due to traffic ,and protect the pavement layers beneath from the effects of the natural elements.
Bitumonous surfacings are generally expected : (a) to contribute to the structural strength of the pavement , (b) to provide a high resistance to plastic deformation and resistance to cracking under traffic,(c) to maintain desirable surface charateristics as good skid-resistance,good drainage ,and low tyre noise.
Roadbase must not be confused with the basecourse ,which is an integral part of thesurface.One is a sub-layer within the bituminous surfacing ,while the other is normally the thickest element of the flexible pavement on which the surfacing rests.
From a structual aspect ,the roadbase is the most important layer of a flexible pavement. It is expected to hear the burden of distributing the appied surface loads so that the bearing capacity of the subgrade is not exceeded. Sincen it provides the pavement with added stiffness and resistence to fatigue,as well as contributing to the overrall thickness ,the material used in a roadbase must always be of a reasonably high quality. Roadbase materials rang from unbound soils and/or aggregates,to chemically stabilized soils,to cement/bitumenbound materials.
In its simplest sense,subbase can be considered merely as an extension of the roadbase;in fact,it may or may not be present in the pavement as a separate layer. Whether or not it is utilized from a number of aspects,as follows.
(1)As a structural member of the pavement the subbase helps to distribute the applied loads to 1the subgrade. The subbase material must always be significantly stronger than the subgrade matetial and capable of resisting within itself the stresses transmitted to it via the roadbase.
(2)A coarse-grained material may be used in the subbase to act as a drainage layer,i.e.,to pass to the highway drainage system any moisture which falls during construction ro which enters the pavement after construction. The quality of the material used must be such that the free-draining criterion of the subbase is always met;in certain instances,this may require a dual-layer subbase, i.e.,an open-graded layer with a protective filter.
(3)On fine-grained subgrade soils a granular subbase may be provided: (a) to carry constructional traffic and act as a working platform on which subsequent layers can be constructed, (b) to act as cutoff blanket and prevent moisture from migrating upward from the subgrade,or (c) to act as a cutoff blanket to prevent the infiltration of subgrade material into the pavement.
The type of material used in any of these designs depends upon the purpose for which it is being used and the grading of the subgrade soil.
Highway subgrade (or basement soil) may be defined as the supporting structure on which pavement and its special undercourses rest. In cut sections,the subgrade is the original soil lying below the special layers designated as base and subbase material. In fill sections,the subgrades is constructed over the native ground and consists of imported material from nearby roadway cuts or from borrow pits.
The cross-sectional shape of the subgrade depends on the type of surfacing,if any ,which is to be used. On earth roads,the subgrade ,which is also the surface course,is shaped to the standard road cross section. If the road is to be surfaced ,the subgrade is graded to the same slope as the finished surface. If the trench method is used,the earth excavated to form a trench is pushed to the sides of the road to form retaining shoulders.
Before 1920 attention was focused largely on the pavement or other wearing course,and little notice was given to the materials that date increased vehicle speeds brought demands for higher standards of alignment and grade,which in turn meant deeper cuts and higher fills. About the same time ,the weight and number of vehicles began increasing,which imposed larger and more numerous wheel loads on the roadway surface. In many instances,subsidence or even total failure or the roadway resulted. Study of such failures indicated that the fault lay in the subgrade and not in the pavement. This in turn led to the investigation of the properties of subgrade soils and of their performance under service conditions.
Soils engineers agree that the properties of a soil mixture are influenced more by moisture than by any other cause. Soils that have ample strenth and supporting power under one set of moisture conditions may be entirely unsatisfactory if the percentage or moisture changes. One difficulty with soils in highway subgrades is that they are subject to such moisture changes.
Soils grains are surrounded by thin films of water. This water is attracted by the molecular charge of the soil grains and has a higher boiling point,lower freezing point,and greater cohesion than ordinary water. The first few molcular layers are almost solid and are more nearly like ice.
Then as the distance from the particle increases through the double layer water its properties change and finally become those of free or gravitational water. The force required to poll the adsorbed water from the mineral surface is prodigious ,generally ranging from 100 atm for the outer layers to 10000 atm for the closest molecular. Small forces are required to remove the double layer water ,and the force required to move it parallel to the particle surface is much less than that to remove the water entirely. However ,none of this water can be removed from the soil by pressures normally encounted in highway work.
By increasing the moisture content of a soil,its consistency can by varied from semisolid to plastic to liquid. Experience has shown that place can be directly correlated with the behavior of the material in service.
The density of a soil is its weight per cubic centimetre. It is sometimes expressed as “wet weight”,or the total weight including water. It is more commonly the “dry weight”,which is the weight of the soil particles alon,excluding the weight of the contained water. As a particular sorl becomes more dense,it will contain a great number or particles,and the (pore) volume remaining for air and water will be decreased.
The supporting power of soils increases with density. A road constructed on a given soil may be entirely satisfactory if the soil is properly compacted. On the other hand,the road may fail if the soil is insufficiently compacted,particularly if the voids become filled with water.
The soil density varies with peculiarities of the soil itself,the moisture conten ,and the compactive device and method that are used. Thus a standard weight percubic centimitre cannot be set,but must be determined in each instance.
The influence of moisture on the density of a soil is illustrated by comaction curve or morsture-density curve.
Roller or other compaction devices increase soil density by expelling air from the voids in the soil and by rearranging or forcing the soil grains into more intimate contact. Water aids as a lubricant up to the optimum moisture content. In porous soils air is easily forced out,but in heavy or tight,cohesive soils much effort is required. Because of this,heavy cohesive materials must be placed in thin layers if the air is to be expelled readily.
Compactors include tamping or sheepfoot rollers;pneumatic-tired rollers;smooth steel-wheeled rollers;segmented plate compactors;grid rollers;vibrary compactors in the form of vibrating pad,steel whedls or pneumatic itself. In some instance two types of compactors in the form of vibrating pad,steel wheels and pneumatic tires are combined in a single unit. Within most compactor types,units are available in several weights or provision is made to change the weight readily. There is small wonder then ,that compactor selection is a difficult task.
A highway pavement is a structure consisting of superimposed layers of selected and processded materials placed on a subgrade,whose primary function is to support the applied traffic loads and distribute them to the basement soil.The ultimate aim is to ensure that the transmitted stresses are sufficiently reduced that they will not exceed the supporting capacity of the subgrade.Two types of pavement are arbitrarily recognized as serving this purpose-flexble pavements and rigid pavements.
A flexible pavement is a pavement structure that maintains intimate contact with ,and distribute load to ,the subgrade ;it depends upon aggregate interlock,particle friction ,and cohesion for its stability .The distinguishing feature of flexible pavement lies in its structrual mechaincs the pressure is ususlly assumed to be transmitted to the subgrade through the lateral distribution of the applied load with depth ,rather than by beam and slab action as with a concretes slab.Thus a flexible pavement can be most easily defined by contrasting it with a rigid portland cement concrete pavement.
When the subgrade deflects beneath a rigid pavement ,the concrete slab is able to bridge over localized failures and areas of inadequate support because of its rigidity and high modulus of elasticity.The major factor influencing the design of rigid pavement is the structural strenth of the concrete ,and its thickness is relatively little affected by the quality of the subgrade as long as it meets certain minimum criteria.
In direct contrast to this ,the strength of the subgrade is a major factor contrlling the design of a flexible pavement. When the subgrade deflects,the overlying flexible pavement is assumed to deform to a similar shape and extent. In fact,this does not necessarily happen,e.g. pavements with thickness and help to support the imposed loads. Nonetheless ,these pavements are still generally classed as flexible pavements,and the assumed basic design criterion is that a depth of pavement is required that will distribute the applied surface load through the various pavement layers to the subgrade so that the subgrade is not over-stressed.
In its simplest form,a flexible pavement is generally considered to be any pavement other than a concrete one. It is this definition that is accepted by great majority of practising engineers. It should be clearly understood,however ,that the term is simply one of convenience and does not truly reflect the characterisitics of the many different and composite types of construction masquerading as “flexible” pavements.
Whethet it is a pavement for an expensive motorway or a simple country road ,the basic structural cross-section of a flexible road is essentially that illustrated in Fig.10-1;it is composed of several distinct layers that make up the pavement superimposed on the subgrade in the manner indicated. The intersection of the subgrade and the pavenment is known as the formation.
The subgrade is normally considered to be the situ soil over which the highway is being constructed. It should be quite clear ,however ,that the term subgrade is also appied to all native soil materials exposed by excavation and to excavated soil that may be artically deposited to form a compacted embankment. In the later case ,the added material is not considered to be part or the road structure itself but part of the foundation of the road.
The oppermost layer of a flexible pavement is called surface course. The highway materials used in a surface course can vary from loose mixtures of soil and gravel to the very highest-quality bituminous mixtures. The choice of materials used in any particular situation depends inmost countries upon the quality of service required of the highway.
If surface course is composed of bituminous materials-as is the normal practice for flexible pavements in Britain-it may consist of a single homogeneous layer or,in the highrquality roads,of two distinct sub-layers known as a wearing course and a basecourse. The wearing course provides the actual surfacing on which the vehicles run,while the basecourse acts as a rugulating layer to provide the wearing course with a better riding quality. The basecourse is normally composed of a more pervious material than the wearing course.
The primary funtion of the surface course,and especially of its wearing component,is to provide a safe and comfortable riding surface for traffic. It must also withstand the most concentrated stresses due to traffic ,and protect the pavement layers beneath from the effects of the natural elements.
Bitumonous surfacings are generally expected : (a) to contribute to the structural strength of the pavement , (b) to provide a high resistance to plastic deformation and resistance to cracking under traffic,(c) to maintain desirable surface charateristics as good skid-resistance,good drainage ,and low tyre noise.
Roadbase must not be confused with the basecourse ,which is an integral part of thesurface.One is a sub-layer within the bituminous surfacing ,while the other is normally the thickest element of the flexible pavement on which the surfacing rests.
From a structual aspect ,the roadbase is the most important layer of a flexible pavement. It is expected to hear the burden of distributing the appied surface loads so that the bearing capacity of the subgrade is not exceeded. Sincen it provides the pavement with added stiffness and resistence to fatigue,as well as contributing to the overrall thickness ,the material used in a roadbase must always be of a reasonably high quality. Roadbase materials rang from unbound soils and/or aggregates,to chemically stabilized soils,to cement/bitumenbound materials.
In its simplest sense,subbase can be considered merely as an extension of the roadbase;in fact,it may or may not be present in the pavement as a separate layer. Whether or not it is utilized from a number of aspects,as follows.
(1)As a structural member of the pavement the subbase helps to distribute the applied loads to 1the subgrade. The subbase material must always be significantly stronger than the subgrade matetial and capable of resisting within itself the stresses transmitted to it via the roadbase.
(2)A coarse-grained material may be used in the subbase to act as a drainage layer,i.e.,to pass to the highway drainage system any moisture which falls during construction ro which enters the pavement after construction. The quality of the material used must be such that the free-draining criterion of the subbase is always met;in certain instances,this may require a dual-layer subbase, i.e.,an open-graded layer with a protective filter.
(3)On fine-grained subgrade soils a granular subbase may be provided: (a) to carry constructional traffic and act as a working platform on which subsequent layers can be constructed, (b) to act as cutoff blanket and prevent moisture from migrating upward from the subgrade,or (c) to act as a cutoff blanket to prevent the infiltration of subgrade material into the pavement.
The type of material used in any of these designs depends upon the purpose for which it is being used and the grading of the subgrade soil.
Highway subgrade (or basement soil) may be defined as the supporting structure on which pavement and its special undercourses rest. In cut sections,the subgrade is the original soil lying below the special layers designated as base and subbase material. In fill sections,the subgrades is constructed over the native ground and consists of imported material from nearby roadway cuts or from borrow pits.
The cross-sectional shape of the subgrade depends on the type of surfacing,if any ,which is to be used. On earth roads,the subgrade ,which is also the surface course,is shaped to the standard road cross section. If the road is to be surfaced ,the subgrade is graded to the same slope as the finished surface. If the trench method is used,the earth excavated to form a trench is pushed to the sides of the road to form retaining shoulders.
Before 1920 attention was focused largely on the pavement or other wearing course,and little notice was given to the materials that date increased vehicle speeds brought demands for higher standards of alignment and grade,which in turn meant deeper cuts and higher fills. About the same time ,the weight and number of vehicles began increasing,which imposed larger and more numerous wheel loads on the roadway surface. In many instances,subsidence or even total failure or the roadway resulted. Study of such failures indicated that the fault lay in the subgrade and not in the pavement. This in turn led to the investigation of the properties of subgrade soils and of their performance under service conditions.
Soils engineers agree that the properties of a soil mixture are influenced more by moisture than by any other cause. Soils that have ample strenth and supporting power under one set of moisture conditions may be entirely unsatisfactory if the percentage or moisture changes. One difficulty with soils in highway subgrades is that they are subject to such moisture changes.
Soils grains are surrounded by thin films of water. This water is attracted by the molecular charge of the soil grains and has a higher boiling point,lower freezing point,and greater cohesion than ordinary water. The first few molcular layers are almost solid and are more nearly like ice.
Then as the distance from the particle increases through the double layer water its properties change and finally become those of free or gravitational water. The force required to poll the adsorbed water from the mineral surface is prodigious ,generally ranging from 100 atm for the outer layers to 10000 atm for the closest molecular. Small forces are required to remove the double layer water ,and the force required to move it parallel to the particle surface is much less than that to remove the water entirely. However ,none of this water can be removed from the soil by pressures normally encounted in highway work.
By increasing the moisture content of a soil,its consistency can by varied from semisolid to plastic to liquid. Experience has shown that place can be directly correlated with the behavior of the material in service.
The density of a soil is its weight per cubic centimetre. It is sometimes expressed as “wet weight”,or the total weight including water. It is more commonly the “dry weight”,which is the weight of the soil particles alon,excluding the weight of the contained water. As a particular sorl becomes more dense,it will contain a great number or particles,and the (pore) volume remaining for air and water will be decreased.
The supporting power of soils increases with density. A road constructed on a given soil may be entirely satisfactory if the soil is properly compacted. On the other hand,the road may fail if the soil is insufficiently compacted,particularly if the voids become filled with water.
The soil density varies with peculiarities of the soil itself,the moisture conten ,and the compactive device and method that are used. Thus a standard weight percubic centimitre cannot be set,but must be determined in each instance.
The influence of moisture on the density of a soil is illustrated by comaction curve or morsture-density curve.
Roller or other compaction devices increase soil density by expelling air from the voids in the soil and by rearranging or forcing the soil grains into more intimate contact. Water aids as a lubricant up to the optimum moisture content. In porous soils air is easily forced out,but in heavy or tight,cohesive soils much effort is required. Because of this,heavy cohesive materials must be placed in thin layers if the air is to be expelled readily.
Compactors include tamping or sheepfoot rollers;pneumatic-tired rollers;smooth steel-wheeled rollers;segmented plate compactors;grid rollers;vibrary compactors in the form of vibrating pad,steel whedls or pneumatic itself. In some instance two types of compactors in the form of vibrating pad,steel wheels and pneumatic tires are combined in a single unit. Within most compactor types,units are available in several weights or provision is made to change the weight readily. There is small wonder then ,that compactor selection is a difficult task.
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