2.0 Road Pavement lecture version.ppt
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TRANSPORTATION ENGINEERING II (BCE IV/I)
2.0 HIGHWAY PAVEMENT
Syllabus: Overall 1.Traffic Engineering 2. Highway Pavement 3. Road Construction Technology 4. Highway Maintenance, Repair and Rehabilitation 5. Introduction to Bridge and Tunnel Engineering
Syllabus: Going to Deal 2.0 Highway Pavement
2.1 Definition and Types of Pavements 2.2 Differences between Flexible and Rigid Pavement Structure 2.3 Loads and Other Factors Controlling Pavement Design 2.4 Design Methods for Flexible Pavements- Road Note 29, 31, CBR, AASHTO 2.5 Details of Asphalt Institute Method of Design of Flexible Pavements 2.6 Design Methods for Rigid Pavements and Westergaard’s Theory 2.7 Stress due to Load, Temperature Differential and Subgrade Friction 2.8 Details of the IRC Method of Design of Rigid Pavements for Highways
5.0 Introduction to Bridge and Tunnel Engineering (8) 5.1 Choice of Bridge Location Site 5.2 Classification of Bridges and Component Parts of a Bridge 5.3 Introduction to River Bank and Protection Structure 5.4 Types of Road and Railway Tunnels 5.5 Component Parts of Tunnel and Tunnel Cross-section 5.6 Survey for Tunnel Alignment 5.7 Drainage, Lightening and Ventilation Requirements for Tunnel 5.8 Introduction of Tunneling in Firm Soil, Soft Soil and Rock 5.9 Tunnel Lining
Definition and Types of Pavements
2.1.1 What is Pavement? 2.1.2 Objectives of Pavement 2.1.3 Functions of a Pavement 2.1.4 Types of Pavement 2.1.5 Structural Elements of a Pavement 2.1.6 Functions and Characteristics of Pavement Layers
What is Pavement?
• A pavement is a Relatively Stable Layer Constructed over a Natural Soil (i) for the purpose of Supporting and Distributing the Wheel Loads so that the bearing capacity of the underlying soil is not exceeded, and (ii) for providing an Adequate Wearing Surface. • A pavement is a multilayered structure. • The layers are placed horizontal one over other.
2.1.2 Objectives of Pavement • To provide a stable and unyielding surface due to loads caused by traffic. • To support and distribute wheel load of vehicles over a wide area of the underlying sub grade soil. • To provide an even surface so that passengers in fast moving vehicles will not be put to discomfort and unsafe. • To prevent water from entering the soil which may lead to yielding(soft/elastic) of the road. • To provide an adequate surface for the movements of vehicles with certain speed safely, comfortably and economically. • To keep temporary deformation of the pavement within the permissible limits so that the pavement can sustain a large number of repeated load applications during its design life.
2.1.3 Functions of a Pavement • Load Support • Smoothness/ Good Riding Characteristics • Drainage • Safety and Comfortable
Types of Pavements
• Based upon the Structural Behaviour of the Materials Used in the Construction, Pavements are Classified as: 1. Flexible Pavement 2. Rigid Pavement 3. Semi-Rigid Pavement 4.
1. Flexible Pavement • So named because the pavement structure Deflects or Flexes under Loading. • Pavements having Very Little Resistance to Deformation under the Wheel Loads. • Which have Very Low Flexural Strength and is Flexible in the Structural Behaviour under the Wheel Loads.
• Its layers reflect the deformation of the lower layers on to the surface of the layer. • Thus if lower layer somehow gets deformed, the surface layer of the pavement also gets deformed
Stress Distribution in Flexible Pavement
• Each layer receives the loads from the above layer, spreads them out, then passes. • Top layer experiences more stress than the descending layers. • So materials of higher quality is used on the top layer than the lower layers i.e. gradually decrease in material quality with depth Design Principle of Flexible Pavement
Materials for Flexible Pavement
• Materials which fall in the category of flexible pavement are Soilaggregate Mix, Crushed Aggregate, WBM, Granular Materials with Bituminous Binder, Bituminous Concrete, etc. • Pavement thickness is so designed that the stresses on the sub grade soil are kept within its bearing power and the sub grade is prevented from excessive settlement.
Types of Flexible Pavement • Dense-graded HMA • Stone matrix asphalt (SMA) • Open-graded HMA • Bituminous surface treatments (BSTs) • Earthen Roads-Stabilized and Unstabilized • WBM Roads, etc.
Types of Flexible Pavements
Rigid Pavement • So named because the pavement structure deflects very little under loading due to the high modulus of elasticity of the surface course. • Possesses considerable flexural strength. • Has slab action and is capable of transmitting the wheel loads through a wider area below.
Rigid Pavement • Doesn’t get deformed to the shape of the lower surface as it can bridge the minor variations of lower layers. • Can take appreciable tensile stresses . • Made of cement concrete which may be either plain, reinforced or pre-stressed. Stress Distribution in Rigid Pavement
Types Of Rigid Pavements 1. Jointed Plain Concrete Pavement (JPCP)
2.Continuously Reinforced Concrete Pavement (CRCP)
3.Jointed Reinforced Concrete Pavement (JRCP)
4. Pre-stressed Concrete Pavement (PCP)
5. Post-tensioned Concrete Pavement (PTCP)
Semi –Rigid Pavement • Has the intermediate properties between the flexible and the rigid pavement. • Has much lower flexural strength compared to concrete slabs but derives the support by the lateral distribution of loads through the pavement depth as in a flexible pavement. • Has low resistance to impact and abrasion and therefore is usually provided with the flexible pavement surface course. • Bonded materials like pozzolanic concrete(lime-fly ashaggregate mix), lean cement concrete or soil cement are used in the base or sub-base courses which has higher flexural strength than the common flexible pavement layers, But have low flexural strength than rigid pavements. • Eg. Lean-concrete Base, Soil-cement, Lime–pozzolana Concrete Constructions, etc.
Composite Pavement • Comprises of multiple, structurally significant layers of different composition. • Consists of PCC as a bottom layer and bituminous layer as a top layer. • Bottom layer (PCC) provides a strong base and bituminous layer(top) provides a smooth and nonreflective surface. • Very expensive type of pavement so rarely used. Eg. Brick -sandwitched Concrete Pavement (under research in India), Asphalt concrete overlay over a PCC slab Cement Concrete Brick ,
2.1.5 Structural Elements of a Pavement • A typical pavement structure consists of – 1. Subgrade 2. Sub-base Course 3. Base Course 4. Wearing/Surface Course
SURFACE/WEARING COURSE BASE COURSE SUB-BASE COURSE SUBGRADE
WEARING COURSE BASE COURSE ROAD BASE COURSE SUB-BASE COURSE SUBGRADE
SURFACE COURSE BINDER COURSE BASE COURSE SUB-BASE COURSE
For Flexible Pavements 1. Subgrade 2. Sub-base Course 3. Base Course 4. Wearing/Surface Course
For Rigid Pavements
1. Subgrade 2. Sub-base Course 3. Base Course 4. Wearing/Surface Course
2.1.6 Functions and Characteristics of Pavement Layers
Subgrade • Soil beneath the pavement is called the subgrade. • Subgrade is a layer of natural soil prepared to receive the other layers of the pavement. • Prformance of the pavement is affected by the characteristics of the subgrade. • Desirable properties which the subgrade should possess are: strength, drainage, ease of compaction, permanency of compaction, and permanency of strength. • The strength of subgrade is increased by compaction or in some cases by stabilization.
Function: •Subgrade is the foundation layer, the structure which must ultimately support all the loads which come on to the pavement and then dispersed to the earth mass below.
Sub-base Course • A sub-base is a layer of material between the base and subgrade. • Base course and sub-bases are used under the pavement primarily to increase the load supporting capacity of the pavement distributing the load through a finite thickness of pavement. • A sub-base material can be of a lower quality materials such as burnt clinkers, natural aggregates or slag than the base course. • The sub-base should be laid as soon as possible after final stripping to formation level, to prevent damage from rain or sun baking which could cause surface cracks.
• Primary function to give structural support but it can also: • Minimize the intrusion of fines from the subgrade into the pavement structure. • Improve drainage. • Minimize frost action damage. • Provide a working platform for construction.
Base Course • Layer of granular material which lies immediately below the wearing surface of the pavement which is the main spreading layer of the pavement. • Base course is the layer of material immediately beneath the surface or binder course. It may be composed of crushed stone, crushed slag, or other untreated or stabilized materials. • The base course lies close to the pavement surface and hence it must possess high resistance to deformation in order to withstand the high pressures imposed upon it. So, it is of superior quality materials.
Functions: •This course receives the impact of the traffic through the wearing course. •The loads are transferred to the sub-base and subgrade through it.
Surface/Wearing Course • The wearing or surface course is the top course of a pavement with which the wheels of vehicles are in actual contact. • This is the layer in direct contact with traffic loads.
Functions: • To distribute the load to the base. • To water-proof the base against the surface water. • To provide smooth riding surface, dust preventive surface.
2.2 Differences Between Flexible and Rigid Pavements
2.2 Differences Between FLEXIBLE PAVEMENT 1. Under heavy loads, yield to excessive stresses resulting in the local depression of the surface. 2. A flexible pavement with subgrade of varying thickness will adjust itself to the irregularities due to different settlements. 3. Under load worsened condition in subgrade will get depression in the pavement.
1. Under heavy loads, ruptures occur thereby producing a crack to the surface. 2. A rigid pavement with the subgrade of varying strength will not adjust the irregularities due to different settlement but acts as a beam or cantilever. 3. Capable of bridging the small weakness and depressions in the subgrade.
2.2 Differences Between FLEXIBLE PAVEMENT
4. Has healing properties (recomig to shape). 5. Temperature variations due to atmospheric conditions do not produce stresses. 6. Strength of flexible layer is a result of building up thick layers and thereby distributing the load over subgrade.
RIGID PAVEMENT 4. Doesn’t have healing properties. 5. Temperature variations produce heavy temperature stresses. 6. Strength of rigid layer is rather by bending action.
Comparison Between Flexible and Rigid Pavement
Comparison Between CRITERIA
LOWflexible pavements designs are mainly Empirical.
MORErigid pavements are designed using precise structural analysis.
10 ~ 20 years
About 40 years
3.Mainteneance Needs frequent
Needs very little
POSSIBLEintial minimum thickness can be provided and additional overlays are provided in keeping with the traffic growth
DO NOT FIT for such SCHEME
Comparison Between CRITERIA
6. Availability of Material
Bitumen scarce resource and imported
Cement can be produce in the country
7. Surface Characteristics
GOOD produces pavement surfaces free from rutting, potholes and corrugations with good riding quality
8. Penetration of Water
NOT IMPERVIOUS SURFACE water enters through pores and cracks
PRACTICALLY IMPERVIOUS except at joints where mud pumping can takes place
POSSIBLE public IMPOSSIBLE utilities such as water supply pipes, telephone cables sewer lines, etc can be buried by
Comparison Between CRITERIA
11. Traffic Dislocation
Traffic can be opened SHORTLY after it is rolled
LONGER Needs 28 days to cure and set the concrete to gain its strength
12. Environmental Considerations During Construction
MORE HAZARDOUS by burning bitumen
13. Overall Economy on a Life Cycle Basis
FAR MORE ECONOMICAL
Difference between Flexible and Rigid Plates
Flexible Pavement – Construction
Rigid Pavements ADVANTAGES 1.HIGH STRENGTH compressive, abrasion, compression-tension 2. GOOD STABILITY water, heat stability, strength increases with time 3. DURABILITY20~40 years 4. LOW MAINTENENACE COSTlarge economic gain, big investment but long design period therefore maintenance cost per year is LOW 5. SUITABILE FOR NIGHT DRIVING
DISADVANTAGES 1.NECESSITY OF CEMENT AND WATER ID LARGEfor 20 cm depth, 7 m wide cement concrete pavement for every 1 km road needs 400~500 ton cement and 25 ton water 2.HAVE JOINTS difficult to construct and maintain, cause vibration on vehicle 3. DIFFICULT TO REPAIR repairing work influences the traffic movement largely 4.PAVEMENT IS QUITE LATELY AVAILABLE FOR TRAFFIC OPERATIONS needs 15~20 days
• Concrete (Rigid) Pavement acts more like a Bridge over the Subgrade. • Inch-for-inch much Less Pressure is placed on Materials below Concrete than Asphalt (Flexible) Pavements.
2.3 Loads and Other Factors Controlling Pavement Design/Pavement Design Parameters/Pavement Design Factors
2.3 Loads and Other Factors Controlling Pavement Design 1. Traffic Factors and Loads Characterization 2. Material Characterization - Soil Factors 3. Environmental Factors 4. Failure Criteria
1. Traffic Factors and Loads Characterization • Traffic is the most important factor in the pavement design. • The key factors include: a) Tire/Wheel/Axle Loads b)Axle and Tire Configurations c) No. of Load Repetitions d)Moving Loads e) Traffic Distribution f) Contact Area g)Vehicle Speed
A) Tire/Wheel/Axle Loads • Wheel load which determines the depth of the pavement required to ensure that the sub grade soil is not failed. • Wheel configurations affect the stress distribution and deflection within a pavement. • Many commercial vehicles have dual rear wheels which ensure that the contact pressure is within the limits.
Axle -The common axis of rotation of one or more wheels whether power-driven or freely rotating. •An axle is a central shaft for a rotating wheel or gear.
Axle Load •The total weight felt by the roadway for all wheels connected to a given axle. • In another way, it is the fraction of total vehicle weight resting on a given axle.
• Single Axle Load - The total load transmitted by all wheels of a single axle extending the full width of the vehicle.
Axle Loads Practice in Nepal According to DOTM, Nepal •6 wheel : 16.2 T •10 wheel : 25 T •12 wheel (lift axle) : 31 •14 wheel : 35.2 T
No. of Axle Load Repetitions • The influence of traffic on pavement not only depends on the magnitude of the wheel load, but also on the frequency of the load applications. • The number of repetition of loads cause elastic and plastic deformations. • Each load application causes some deformation and the total deformation is the summation of all these. • Although the pavement deformation due to single axle load is very small, the cumulative effect of number of load repetition is significant. • When a load from a heavy vehicle is applied repetitively to the pavement it causes certain damage (cumulative strains in various layers of the pavement)
• The damage caused by each axle depends on its load, configuration and repetitions. • It is possible to evaluate the damage caused by the repetitions of each axle load group. • Instead of analyzing each axle load group separately, they can be converted into equivalent repetitions of a standard axle using equivalent axle load factors. • Standard Axle: Single axle with dual wheels carrying a load of 80 kN (8.16 tonnes or 18,000 lb) is defined as standard axle.
Moving Loads • The damage to the pavement is much higher if the vehicle is moving at creep speed. • Many studies show that when the speed is increased from 2 km/hr to 24 km/hr, the stresses and deflection reduced by 40 per cent.
Contact Area • The tire pressure is an important factor, as it determines the contact area and the contact pressure between the wheel and the pavement surface. • Even though the shape of the contact area is elliptical, for sake of simplicity in analysis, a circular area is often considered. • If the effect of the tire wall is ignored, the contact pressure between the tire and pavement must be equal to the tire pressure. • For low-pressure tires, contact pressures under the tire wall may be greater than at the center of the tire. • For high-pressure tires the reverse is true. • For most problem however, the assumption is made that contact pressures are uniform over the imprint area.
Traffic Distribution • Along with load type and repetitions, the load distributions across a particular pavement must be estimated. • For instance, on a 6-lane highway (3 lanes in each direction) the total number of loads is probably not distributed exactly equally in both directions. • Often one direction carries more loads than the other. • Furthermore, within that one direction, not all lanes carry the same loading. • Typically, the outer most lane carries the most trucks and therefore is subjected to the heaviest loading. • Therefore, pavement structural design should account for these types of unequal load distribution.
Traffic Distribution Contd… • Typically, this is accounted for by selecting a "design lane" for a particular pavement. • The loads expected in the design lane are either (1)directly counted or (2)calculated from the cumulative two-direction loads by applying factors for directional distribution and lane distribution.
Vehicle Speed • Speed is directly related to duration of loading. • The greater the speed, larger the modulus of elasticity and smaller the strains on the pavement. • So higher speed of the vehicles is DESIREABLE. • In general, slower speeds and stop conditions allow a particular load to be applied to a given pavement area for a longer period of time resulting in greater damage. • At bus stops (where heavy buses stop and sit while loading/unloading passengers) and intersection approaches (where traffic stops and waits to pass through the intersection)
2. Soil Factors • Soil under the pavement foundation is seldom homogeneous. • Large variations may occur in its properties. • To predict the behavior of the soil under different conditions, it is essential to carry out certain tests. • The soil strength varies with type of the soil, bulk density, moisture content, permeability, internal structure of the soil etc. • It also depends upon the method of application of load on the soil. • We know that the soil strength increase with increase density and decreasing moisture content. • As the elastic properties of soil are very low, in the design of flexible pavement, the supporting capacity of sub grade is very important factor.
2. Soil Factors Contd. • To determine the supporting capacity of sub grade generally following properties of all the soil are determined. • Shear Strength • Bearing Capacity • Penetration Resistance of the soil
3. Environmental Factors • Environmental factors affect the performance of the pavement materials and cause various damages. • Environmental factors that affect pavement are : 1. Temperature Variations 2. Frost Action and 3. Moisture/Precipitation/Rainfall
Temperature Variations • Temperature affects the resilient modulus of asphalt layers, while it induces curling of concrete slab.
• In rigid pavements, due to difference in temperatures of top and bottom of slab, temperature stresses or frictional stresses are developed.
Frost Action • Frost heave causes differential settlements and pavement roughness.
• Most detrimental effect of frost penetration occurs during the spring break up period when the ice melts and sub grade is a saturated condition.
Moisture/Precipitation/Rainfall • The precipitation from rain and snow affects the quantity of surface water infiltrating into the sub grade and the depth of ground water table.
• Poor drainage may bring lack of shear strength, pumping, loss of support, etc.
4. Failure Criteria 1. Fatigue Cracking • •
Is due to the build up of tensile strain at the bottom of Asphaltic Concrete Layer Pavement is considered failed if 20% of the surface has cracked
2. Rutting Failure • •
Is due to the build up of excessive compressive strain at the top of subgrade layer Pavement is considered failed if it exhibits a rut depth of 20 mm
Equivalent single wheel load • To carry maximum load with in the specified limit and to carry greater load, dual wheel, or dual tandem assembly is often used. • Equivalent single wheel load (ESWL) is the single wheel load having the same contact pressure, which produces same value of maximum stress, deflection, tensile stress or contact pressure at the desired depth. • The procedure of finding the ESWL for equal stress criteria is provided below.
• This is a semi-rational method, known as Boyd and Foster method, based on the following assumptions: • equalancy concept is based on equal stress; • contact area is circular; • influence angle is 450; and • soil medium is elastic, homogeneous, and isotropic half space. • The ESWL is given by:
Where, P = wheel load, S = center to center distance between the two wheels, d = clear distance between two wheels, and z = desired depth
Numerical Example 1 • Find ESWL at depths of 5 cm, 20 cm and 40 cm for a dual wheel carrying 2044 kg each. The center to center tyre spacing is 20 cm and distance between the walls of the two tyres is 10 cm.
Solution: 1) For desired depth z = 40 cm, which is twice the tyre spacing, ESWL = 2P = 22044 = 4088 kN 2) For z = 5cm, which is half the distance between the walls of the tyre, ESWL = P = 2044kN 2) For z = 20 cm,
Equivalent Single Axle Load • Vehicles can have many axles which will distribute the load into different axles, and in turn to the pavement through the wheels. • A Standard Truck has two axles, front axle with two wheels and rear axle with four wheels. • But to carry large loads multiple axles are provided. Since the design of flexible pavements is by layered theory, only the wheels on one side needed to be considered. • On the other hand, the design of rigid pavement is by plate theory and hence the wheel load on both sides of axle need to be considered.
• Legal Axle Load: • The maximum allowed axle load on the roads is called legal axle load. • For highways the maximum legal axle load in India, specified by IRC, is 10 tonnes.
• Standard Axle Load: • It is a single axle load with dual wheel carrying 80 KN load and the design of pavement is based on the standard axle load.
• Tare weight sometimes called Unladen Weight, is the weight of an empty vehicle or container.
• By subtracting it from the Gross Weight(Laden Weight), the weight of the goods carried (the net weight) may be determined. • Laden = fully loaded
Unladen = empty
Repetition of Axle Loads • The deformation of pavement due to a single application of axle load may be small but due to repeated application of load there would be accumulation of unrecovered or permanent deformation which results in failure of pavement. • If the pavement structure fails with N1 number of repetition of load W1 and for the same failure criteria if it requires N2 number of repetition of load W2, then W1N1 and W2N2 are considered equivalent. • Note that, W1N1 and W2N2 equivalency depends on the failure criterion employed.
Equivalent Axle Load Factor • An equivalent axle load factor (EALF) defines the damage per pass to a pavement by the ith type of axle relative to the damage per pass of a standard axle load. While finding the EALF, the failure criterion is important. Two types of failure criterion are commonly adopted: fatigue cracking and rutting. • The fatigue cracking model has the following form:
where, Nf = number of load repetition for a certain percentage of cracking, t = tensile strain at the bottom of the binder course, E = modulus of elasticity, and f1; f2; f3 = constants
• If we consider fatigue cracking as failure criteria, and a typical value of 4 for f2, then:
where, i indicate ith vehicle, and std indicate the standard axle. • Now if we assume that the strain is proportional to the wheel load,
• Similar results can be obtained if rutting model is used, which is:
Where, Nd = permissible design rut depth (say 20mm), c = compressive strain at the top of the subgrade, and f4; f5 = constants. • Once we have the EALF, then we can get the ESAL as given below.
where,, m = number of axle load groups, Fi = EALF for ith axle load group, and ni = number of passes of ith axle load group during the design period.
Numerical Example 1 • Find the equivalent axle load. If the number of load repetition expected by 80 KN standard axle is 1000, 160 KN is 100 and 40 KN is 10000. Solution: Refer the Table. The ESAL is given as ∑Fini = 3225 kN
Numerical Example 2 • Find the equivalent standard axle load if the equivalence criteria is rutting if the number of load repetition expected by 120 kN axle is 1000, 160 kN is 100, and 40 kN is 10,000. Assume 80 kN as standard axle load and he rutting model is where f4 = 4.2 and f5 = 4.5.
Solution: Refer the Table. The ESAL is given as ∑Fini = 8904.94 kN.
Numerical Example 3 Find the equivalent axle load using fatigue cracking as failure criteria according to IRC if the number of load repetition expected by 60kN standard axle is 1000, 120kN is 200 and 40 kN is 10000.
Solution: Refer the Table. The ESAL is given as ∑Fini = 6030.81 kN
Fig. ESWL-Equal tress Concept
2.4 Design Methods for Flexible Pavements- CBR, Road Note 29, 31, AASHTO
Design Methods for Flexible Pavements
Design Methods of Flexible Pavements
Mechanistic(Theoretical/Analytical) Design Approach Based on Boussinesq’s Theory Based on Burmister’s Theory 2. Empirical Design Approach Group Index Method CBR (California Bearing Ratio) Method Road Note 29 Method Road Note 31/ Catalogue Method IRC Method IRC Recommended CBR Method (IRC 37: 1970) IRC 37: 1984 IRC 37: 2001 AASHTO Method 3. Mechanistic-Empirical Design Approach – Semi-empirical Design Approach Triaxial Method Asphalt Institute Method