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63 Cards in this Set
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Pavement Types (5) |
1. Flexible Asphalt Concrete (AC) 2. Rigid Portland Cement Concrete (PCC) 3. Composite Asphalt + Portland Cement Concrete Stabilized base/subbase AC overlay on PCC 4. Interlocking Brick Pavements 5. Unpaved Roads |
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Flexible Layered Pavement System |
1. Surface Very strong, durable, impermeable manufactured, expensive [75-200mm] 2. Base Strong, free-draining, manufactured, less expensive USE GRANULAR A, [150-500mm] 3. Subbase Moderate Strength, free-draining, natural material, inexpensive USE GRANULAR B[150-500mm] 4. Subgrade Relatively weak, moisture sensitive, in-situ soil |
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Purpose of base/subbase |
1. Control drainage 2. Protect against frost action/volume change 3. Acts as working surface during construction 4. Increased structural capacity
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Purpose of AC/PCC Surface |
1. To provide a tough, smooth, skid-resistant surface |
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Pros of Layered Pavement System |
-Stronger surface layers sheild weaker base layers. -Stress at point of loading is very high, as depth increases the stresses decrease (think 2:1 triangle rule) |
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Perpetual Pavement |
Pros: Durable, longer lasting Less fatigue cracking Less rutting Smoother pavement No major reconstruction required Minimizing service disruption Cons: Higher initial cost - 250-350mm thick HMA vs. traditional <150mm |
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Multi- (AC) Layer Design |
Layer 1: renewable surface layer Layer 2: strong, rut-resistant, durable intermediate layer Layer 3: flexible, fatigue-resistant bottom layer |
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Flexible Highway Pavements |
Surface Layer: thin, smooth, skid resistant Binder Layer: Large aggregates, strong, rutting resistant
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Rigid Highway Pavements |
150-300mm Concrete slab thickness 100-300mm Subbase |
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Types of Rigid Pavement |
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Load Transfer of Rigid Pavements |
Due to the rigidity of the concrete, the stress is spread over the surface area of the slab. Whereas in flexible pavement the stress is concentrated at the application point |
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Differences between Flexible and Rigid Pavements |
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Interlocking Pavements |
Layers: soil subgrade, aggregate base, sand bedding layer, concree paver wearing surface, edge restriants, and drainage structures
Performs as flexible pavement when load is applied
Pros Existing 'crack' structures, no new cracks
Cons Slippery when wet |
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Factors influencing pavement performance (5) |
1. Traffic -Volume (AADT) -Distribution (% trucks, aircraft, etc) -Vehicle load and load distribution 2. Environment -Precipitation, moisture -Temperatures/gradients -Freeze/thaw, wet/dry cycles 3. Material properties of each layer 4. Pavement structure details 5. Maintenance schedule |
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Soil properties for pavement design |
1. Resilient Modulus 2. Poisson's Ratio Deformation properties under high number of repeated loads |
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Required soil tests (2) |
1. Sieve Analysis - for coarse grained material, classified by GSD 2. Hydrometer Analysis - fine grained soils, classified by Atterberg Limits
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Soil symbols used for unified soil classification system (USCS) [G, S, M, C, O, Pt] |
G - gravel S - sand M - silt C - Clay O - Organic Pt - Peat |
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Liquid limit symbols (2) |
H - high liquid limit (LL>50) L - low liquid limit (LL<50) |
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Fine Symbols (2) |
M - non-plastic fines C - plastic fines |
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Gradation symbols (2) |
W - well graded P - poor graded |
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Mechanical properties of soil |
Stiffness Coefficient of subgrade reaction, k=p/delta Poisson's Ratio, v Resilient Modulus, Mr CBR Permanent Deformation Resistance |
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Coefficient of subgrade reaction |
Not a material property, as the coefficient is dependant on the size of the plate
k=p/a=E/118R
**measured at p=6.9kPa OR 10 psi **30" (762mm) diameter plate used in test |
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Resilient Modulus |
Measures the elastic modulus of subgrade OR granular base/subbase material
Mr is sensitive to w/c when % of fines increase
Mr=Cyclic Stess/Resilient Strain |
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Factors affecting resilient modulus (7) |
An increase in ________ would result in: Bulk Stress - Major Increase Shear Stress - Major Decrease Broader Grading - Minor Increase Fines - Decrease Larger Size - Increase Density - Increase Moisture Content - Major Decrease |
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CBR |
Provides a measure of a soil's strength/ductility and moisture susceptibility
CBR = Unit load carried by specimen @ 2.5mm / Load carried by standard crushed rock CBR = Unit Load (kPa)/6900kPa |
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General Ratings of Soil Quality |
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Relationship between CBR and Mr |
CBR > 10/20 17.6CBR^0.64
CBR < 10/20 10.3CBR
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Requirements of subgrade (5) |
1. Sufficient Bearing Capacity 2. Low Deformability 3. Time-independent Properties 4. Non-susceptibility to Environmental Change 5. Good Drainage |
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Effects of Moisture Content on Subgrade |
Expansive soils can cause severe distress on the surface of the pavement
Increased w/c 1) reduction of strength & stiffness 2) volume expansion |
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Methods of moisture control (8) |
Compaction Control Methods Compact wet of optimum Maintain material uniformity Maintain uniform moisture content and compaction
Grading Control Methods Excavate deep cuts first Place expansive soils at the bottom of fill Cross haul less expansive soils for top of subgrade Cross haul to get uniform subgrade Stabilize expansive soil with cement/lime |
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Frost heace concequences |
1. heaving of pavement surface caused by formation of Ice Lense 2. Loss of strength with thawing of excess pore water 3. loss of performance due to heaving or weakening of the structure |
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Depth of Frost Penetration in Ontario |
Hamilton: 2' OR 0.6m
Ottawa Valley: 4' OR 1.2m
Northern Ontario: 6/7' oR 2m |
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Design considerations for frost heave (6) |
1. removal of frost susceptible soils (high % silt) 2. Install subdrains to lower water table 3. excavation of soil to frost line & place impermeable seal 4. Install insulating layer to prevent frost penetration 5. Increase void volume/size, via chemical action, to inhibit soil suction 6. Provision of insulating layer |
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Mud Pumping Required Conditions |
-Soils have >45% passing 0.075mm sieve, PL > 6 -Free water between slab and subgrade -Frequent heavy wheel load |
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Mud Pumping Design Consideration |
Introduce a subbase layer between the rigid slab and the subgrade |
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Role of Granular Base Course (3) |
1.Provide support to surface layer 2.Drainage Layer 3. Frost Protection
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Properties of Granular Base Course (3) |
1. High internal friction angle 2. High permeability 3. Low fines content |
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Unbound Granular Aggregate |
New Aggregates Crushed rock, gravel, etc
Recycled material RAP [Recycled Asphalt Pavement] RBM [Recycled Bituminous Material] RAC [Recycled Aggregate Concrete]
Other Materials Blast furnace slag
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RAP |
Can compose mix of up to 50% if pulverized, usually 20-30% of granular material
Reduces CBR value significantly, however Mr values remain similar
Increasing RAP content will induce large accumulative deformation and rutting |
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Objectives of Soil Stabilization (8) |
1. Increase soil strength 2. Reduce compressibility 3. Reduce moisture susceptibility of fine grained soils 4. Improve properties of borderline materials
Temporary Construction Measures 5. Dust Control 6. Moisture Control 7. Salvage Old Road 8. Construction of superior bases |
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Mechanical Methods of Stabalization |
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Admixture Stabilization Methods |
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Cement Stabilization |
- Improve strength, compressibility, expansive characteristics, and reduce frost susceptibility -Cannot be used in presence of sulphate or organic matter
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Bituminous Stabilization |
- Used for granular material, not for fines - Reduces water absorption of soil - Provides strength and cementation - Can be used as membrane to control moisure in subgrade or subbase
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Lime Stabilization |
- Used for fine grained material - avoid use in claw with low Pl, causes loss of cohesion - Slow strength gain with time - Decreases soil density, increases soil strength, and change Pl |
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Effects of water on flexible pavement (4) |
1. Fatigue Cracking 2. Subgrde Rutting 3. Continuous contact with water causes stripping of AC mix and durability cracking of concrete 4. Failure and collapsing
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Effects of water on rigid pavement (3) |
1. Mud pumping of PCC pavements leading to faulting, cracking and general shoulder deterioration 2. Fatigue Cracking 3. Failure and collapsing |
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Methods of controlling water in pavements (3) |
1. Prevention sealing the pavement surface surface drainage 2. Removal drainage layer of blanket longitudinal drain transverse drain 3. Strong pavement section Use thicker HMA or PCC slab appropriate improvement of subgrade soil, granular base/subbase material |
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Surface Prevention (Prevention) |
Pavement & Shoulders for full width, impervious surface with sealed shoulder the pavement should be maintained without cracks or holes Special Mixes Open graded hot mix can adequately drain water to the side. Then a transverse pipe can drain the water away. Curbs & Dykes Storm sewers in areas where open channels are not appropriate
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Subsurface Drainage Methods (3) (Removal) |
Interception Drain Keep water away
Subgrade Drainage Hold ground water low
Base Drainage prevent flooding of base
Through: i)Drainage Blanket ii)Longitudinal Drain iii)Transverse Drain |
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Pavement Distress Categories (5) |
1. Cracking 2. Potholes 3. Surface deformation 4. Surface Defects 5. Miscellaneous Distresses |
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Flexible Pavement Distresses ( |
1. Alligator/Fatigue Cracking 2. Transverse Cracking 3. Thermal Cracking 4. Joint Reflection Cracking 5. Longitudinal Cracking 6. Top Down Cracking (longitudinal) |
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Alligator/Fatigue Cracking/Wheel Track Cracking |
- caused by fatigue failure of asphalt under repeated traffic loading - typically occurs in wheel path, but may present itself elsewhere due to vehicle wandering - begins at bottom of asphalt concrete and propagates to road surface |
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Transverse Cracking |
- perpendicular to pavement centerline, may extend part or fully across travel lane
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Block Cracking (Thermal Cracking) |
- divides pavement into large rectangular pieces (blocks) - caused by thermal shrinkage of asphalt binder - binder age hardening is also related to these cracking types - not caused by fatigue
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Joint Reflection Cracking |
- occurs on pavement that have asphalt surfaces over concrete slabs - cracks occur over transverse and longitudinal joints where pavement was widened - caused by slab movement beneath asphalt due to thermal and moisture changes - knowing the dimensions beneath the asphalt concrete surface help identify cracks at joints. |
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Longitudinal Cracking |
-cracking occurs parallel to pavement centerline - causes: i) Load related (within wheel path) ii) Poorly constructed pavement (non-wheel path) |
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Top Down Cracking Causation (longitudinal) |
-non-uniform vertical loading (contact pressures) resulting in higher surface tensile/shear stresses
-poor/inconsistent hot-mix asphalt quality, production, placement, and compaction
-interlayer slippage or delamination
-thermal stresses
-stiffness gradients within the surface course and between asphalt concrete courses
-premature AC age hardening (binder stiffening) |
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Rutting |
- characterized by depressions that form in the wheel paths -stems from deformations occuring in any layer of the pavement/subbgrade - caused by: i) compression or lateral movement of materials due to traffic loading ii) plastic movement of asphalt in hot weather, or poor compaction during construction
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Instability Rutting |
- failure attributed to mix properties and occurs withing 50mm of the top AC layer |
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Bleeding |
-excess asphalt binder on the surface of the pavement - caused by high asphalt content, or low air void content - results in low skid resistance
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Slippage |
-characterized by half-moon shaped cracks, pointed into the direction of traffic - causes: i) lack of bond between surface layer and underlying layers ii) excessive deflection of pavement structure -found in area's of acceleration and decceleration |
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Shoving |
-longitudinal displacement of a localized area of pavement surface. -caused by braking or accelerating of vehicles -seen on hills, curves, at intersections, may result in vertical displacement |