CHAPTER 5SURFACES AND GUIDEWAYS
Surfaces provide supports for vehicles traveling on
transportation facilities. Guideways, in addition to providing
support, also provide lateral guidance. Roadway surface types
include treated soil, gravel orother aggregate surfaces, and
asphalt concrete and Portland cement concrete pavements. In
addition, many low-volume roads are completely unsurfaced. Airport
surfaces include asphalt concrete and Portland cement concrete
pavements, as well as turf or aggregate-turf surfaces for low
volume airports used by light aircraft. Guideways include monorail,
concrete guideways for rubber-tired vehicles, and conventional
duorail steel-on-steel railroad track.5.1 HIGHWAY PAVEMENTS5.1.1
Components
Figure 6.1 shows cross sections for typical asphalt concrete and
Portland cement concrete pavements. Pavements usually consist of a
surface course (of asphalt concrete or Portland cement concrete), a
base course, and a subbase constructed on top of prepared subgrade
material.Surface courses usually consist of Asphalt or Portland
cement concrete. Concrete refers to any material consisting of a
mixture of aggregates, such as sand, gravel or crushed stone,
fastened together by cement.Asphalt concrete consists of asphalt
cement and aggregate. A variety of asphalt-bound materials are used
to surface roadways. Asphalt concrete usually refers to
high-quality premixed material using in paving high-type
roadways.Portland cement concrete consists of Portland cement and
aggregate, usually sand and gravel or stone. Portland cement
concrete pavements may be either unreinforced or continuously
reinforced with conventional rebar.Base courses normally consist of
aggregates such as gravel or crushed rock. These may be simply
compacted (in which case they are known as aggregate bases) or
stabilized by means of lime, Portland cement, or asphalt.Subbases
are usually local aggregate materials. They may consist of either
unstabilized compacted aggregate.In addition, geotextiles are
sometimes used in constructing pavements. These are to add
strength, control moisture, and prevent the movement of fine
materials in drainage layers.5.1.2 Pavement Maintenance and
RehabilitationPavements are subject to varying degrees of distress
short of outright failure that reduces their serviceability.
Pavement maintenance and rehabilitation programs are intended to
restore ride quality and maintain the structural integrity of the
pavement over its full design life.Asphalt concrete pavements are
subject to a variety of types of pavements distress or failure.
These include: Alligator cracking. A series of interconnected or
interlaced cracks caused by fatigue failure of the asphalt concrete
surface under repeated traffic loading. Block cracking. Cracks
forming large interconnected polygons, usually with sharp corners
or angles. These cracks are generally caused by hardening and
shrinkage of the asphalt and/or reflection cracking from underlying
layers such as cement-treated base. Transverse cracking. Cracks
approximately at right angles to the pavement centerline. These may
be caused by shrinkage or differential thermal stress of the
asphalt concrete, or may be reflective cracks. Longitudinal
cracking. Cracks approximately parallel to the pavement centerline.
These are caused by poorly constructed construction joints and
shrinkage of the asphalt concrete surface; they may also be
reflective cracks. Ravelling. Wearing away of the pavement surface
caused by dislodging of aggregate particles and binder. This is
usually a result of insufficient asphalt binder in the mix or
stripping of asphalt from particles of aggregate. Drip track
ravelling. Progressive disintegration of the surface between the
wheel paths caused by dipping of gasoline or oil from vehicles.
Bleeding or flushing. The exuding of bitumen onto the pavement
surface, causing a reduction in skid resistance. Bleeding is
generally caused by excessive amounts of asphalt in the mix and/or
low air voids content. It occurs when asphalt fills the voids in
the mix during hot weather and then exudes out onto the surface of
the pavement.Maintenance and rehabilitation actions for asphalt
concrete pavements include bituminous seal coats of various types
which are intended to seal cracks and restore skid resistance,
asphalt concrete overlays, and recycling of asphalt concrete
pavements. In addition, failures confined to small areas, such as
isolated potholes, may be patchedTYPE OF DISTRESSPOSSIBLE
CAUSEMaintenance Suggestions
Fatigue (Alligator) Cracking
1. Excessive loading2. Weak surface, base, or subgrade3. Thin
surface or base4. Poor drainage5. Any combination of 1-4Full-depth
patch
Block Cracking
1. Old and dried out mix2. Mix was placed too dry3. Fine
aggregate mix with low penetration asphalt & absorptive
aggregates4. Aggravated by low traffic volumeAny surface treatment
or thin overlay
Edge Cracks
1. Lack of lateral support2. Settlement of underlying material3.
Shrinkage of drying out soil4. Weak base or subgrade layer5. Poor
drainage6. Frost heave7. Heavy traffic or vegetation along
edgeImprove drainage. Remove vegetation close to edge. Fill cracks
with asphalt emulsion slurry or emulsified asphaltCrack
seal/fill
Longitudinal (Linear) & Transverse Cracking1. Poorly
constructed paving joint crack2. Shrinkage of the asphalt layer3.
Daily temperature cycling4. Cracks in an underlying layer that
reflect up through the pavement5. Longitudinal segregation caused
by the improper operation of the paverImprove drainage by removing
the source that traps the waterSeal crack or fill with asphalt
emulsion slurry or light grade of asphalt mixed with fine
sand.Provide side drainage ditchesCrack seal/fill
Reflection Cracking
1. Differential movement between the asphalt and concrete
layers2. Can deteriorate further under heavy trafficCrack
seal/fill
Slippage Cracks
1. Lack of a good bond between surface layer and the course
beneath due to dust, oil, dirt, rubber, water and other
non-adhesive material2. Tack coat has not been used3. Mixture has a
high sand content4. Vehicular turning or stopping movements in
pavements with a low-strength surface mix
Partial or full-depth patch
Corrugations & Shoving
1. Mixtures too high in asphalt2. Low air voids3. Fine aggregate
content too high4. Excessive moisture or contamination in the
granular base5. Smooth or rounded aggregate6. Incorrect asphalt
grade
Deep or full-depth patch
Rutting
1. Consolidation or lateral movement of any of the pavement
layers or the subgrade under traffic2. Insufficient design
thickness3. Lack of compaction4. Weaknesses in the pavement layers
due to moisture infiltration5. Weak asphalt mixturesCold mill and
overlay or thin surface patch
Settlement/Grade Depressions1. Settlement or failure in the
lower pavement layers2. Improper construction techniquesCold mill
and overlayThin surface patchInfrared patch
Upheaval/Swell
1. Expansive soils (which swell in the presence of moisture)2.
Frost heave (in which ice lenses grow beneath the pavement, causing
the pavement to crack)Full-depth patch
Utility Cuts/Patch Failure
1. A portion of a pavement has been removed and replaced2. A
portion of a pavement where additional material has been added3.
Poor installation techniques such as inadequate compaction,
inferior or improper materials4. Failure of the surrounding or
underlying pavementReplace patch with deep or full-depth patch
Pot Hole
1. Continued deterioration of another type of distress, such as
thawing of a frozen subgrade, cracking, raveling, or a failed patch
after pieces of the original pavement surface have been dislodged2.
Poor surface mixtures3. Weak spots in the base or subgrade4.
Severity of the surrounding distress and traffic action accelerate
potholes
Partial, full-depth or injection patching
Raveling/Weathering
1. Asphalt binder has hardened excessively2. Poor-quality
mixture3. Usually requires the presence of both traffic and water
to occurAny surface treatment or thin overlay
Bleeding
1. Improperly constructed seal coat2. Too much asphalt in a
mix3. Too heavy a prime or bond/tack coat4. Excessive sealant in
the cracks or joints under an overlay5. Traffic can contribute to
bleeding if the asphalt layers become overcompacted and excess
asphalt is forced to the surface
Chip Seals, Sandwich Seals, thin overlay
Polished Aggregate
1. Soft aggregates that polish quickly under traffic
Any surface treatment except fog seal
Loss of Aggregate on Surface Treatments
1. Aggregate n ot spread immediately2. Asphalt may have cooled
to much3. Aggregate too dusty or too wet when spread4. Not rolled
immediately after placing it may not become seated5. Steel-wheeled
roller alone was used for compaction6. Weather too cool when
treatment applied7. Fast traffic too soon after application
Hot coarse sand spread over affected areasRolled immediately
after spreading with a pneumatic-tired roller
Longitudinal/Transverse Streaking
1. Aggregate not spread immediately2. Improper spray bar height
causing incorrect overlap of the spray fans3. Changing spray bar
height as the distributor load decreases4. Nozzle problems
(incorrect angle, incorrect size, different sizes, plugged or
restricted nozzles, or ones with imperfections)5. Nozzle control
linkage problem6. Inconsistent pump speed or pressure to the
nozzles, and varying distributor travel speed7. Improper
application temperature (allowing the asphalt material to cool);
and Improper binder choice (viscosity too high for existing
conditions and equipment)Re-seal surface using proper procedure and
adjustment of equipment
Types of pavement distress affecting Portland cement concrete
pavements include the following: Faulting. Elevation differences
between adjacent slabs at transverse joints. Faulting is usually
the result of pumping, and is a major source of Portland concrete
pavement failure.
Settlement. Local sagging in the pavement caused by differential
settlement, consolidation, or movement of the underlying earth
mass.
Blow ups. Localized upward buckling and shattering of the slabs
at transverse joints or cracks. Blow ups can occur when traverse
joints are filled with incompressible solid materials. They are
especially common where pavement is sanded in winter to counteract
icy conditions and in areas subject to large temperature
changes.
Joint or crack spalling. The breakdown or disintegration of slab
edges at joints or cracks, usually resulting in the loss of sound
concrete and the progressive widening of the joint or crack.
Surface attrition or surface abrasion. Abnormal surface wear,
usually resulting from poor-quality surface mortar or coarse
aggregate.
Surface polish. Loss of the original surface texture due to
traffic action.Maintenance and rehabilitation actions for Portland
cement concrete pavements include installation of edge drains to
reduce pumping, grinding of slabs to correct faulting, replacement
of badly cracked or deteriorated slabs, asphalt concrete overlays,
grooving to reduce hydroplaning, and use of special thin concrete
surface treatments.5.2 RAILROAD TRACKRailroad track serves to
provide guidance for rail vehicles and to spread wheel loads so as
to keep bearing pressure on the subgrade within acceptable limits.
This latter function is similar to that of highways pavements, and
the design of railroad tracks is similar in many ways to that of
flexible highway pavements.
5.2.1 Overall Track StructureThe overall track structure
typically consists of subgrade, ballast, ties, rails, and rail
fastenings. These are arranged as follows: The ballast rests on the
subgrade, The ties are embedded in the ballast The rails usually
rest on tie plates and are fastened to the ties by spikes Other
track fastening include joint bars, which are used to fasten rails
together longitudinally; Gage bars, which are steel bars used to
maintain gage at critical locations such as curve and switches Rail
anchors, which are used to resist the longitudinal motion of rails
relative to ties.Railways must also include special provisions to
handle intersecting, merging, and diverging track. Traffic can be
interchanged between lines only by means of diverging and merging,
since trains cannot turn sharp corners. Since the vehicles cannot
be steered, the track itself must be used to guide the train to the
proper branch at a merge or divert point, and this is accomplished
by means of a switch.5.2.2 Major Track Component: BallastMajor
track components include ballast, ties and rail. Ballast consists
of a layer of crushed stone, crushed slag, gravel, or similar
material which is placed on top of the subgrade and in which the
ties are embedded. Ballast is not universally used; it is rarely
used on bridges or in tunnels, for instance, and in these cases the
track is usually anchored directly to the bridge structure or the
floor of the tunnel. Ballast has a number of functions. These
include: Distributing tie loads to the subgrade Anchoring track
against lateral, vertical and longitudinal movement Providing for
drainage of water away from rails and ties In cold climates,
reducing frost heave by preventing capillary action and providing
insulation for the subgrade Facilitating maintenance, especially
maintenance of track grade and replacement of ties Retarding growth
of vegetation in the immediate vicinity of the track structure
Providing some resilience to help absorb dynamic loadsIn order for
it to perform these functions effectively, it is important that
ballast consist of some type of granular material. Ideally, it
would be strong, angular, open graded aggregate, which will provide
high strength and stability through interlocking of the particles
as well as high permeability to facilitate drainage.
5.2.3 Major Track Components: TiesTies are used to maintain gage
and to transmit wheel loads from the rails to the ballast. In North
America, wood ties are the most common variety, with some use of
pre-stressed concrete ties. Elsewhere in the world, non-wood ties
are fairly common.Tie dimensions are usually as follows: depth, 7
inches (180mm); width, 8 to 9 inches (200 to 230 mm); length, 8 to
9 ft (2.4 to 2.7 m) for ordinary track, but up to 22 ft (6.7 m) for
ties used in turnouts and crossovers.Ties must be spaced a minimum
of 10 inches (255 mm) face-to-face to allow tamping of ballast
under them; this means that for 8-in (200 mm) ties a
center-to-center spacing of 18 in (455 mm) is the minimum
possible.
5.2.4 Major Track Components: RailsRails support and provide
guidance for the flanged wheels on rail vehicles and transmit wheel
load to the ties. As previously stated, rails are designated in
terms of the shape of their cross sections (with certain standard
designs available) and by their weight per unit length, usually
quoted in pounds/yard. Rail weights in common use range from
approximately 85 lb/yd (42kg/m) to 150 lb/yd (75 kg/m), with
weights of 110 lb/yd (55 kg/m) or greater recommended for lines
used by 100-ton cars.Rails like ties, rarely fail because of
excessive bending stress, but are subject to a variety of types of
wear and to failures from metallurgical defects. Most cases in
which rails actually break are attributable to defects, with
transverse cracking being perhaps the most dangerous type of
defect.
5.2.5 Track MaintenanceCommon maintenance tasks include
restoration of horizontal and vertical alignment, replacement of
worn or defective ties or rails, and cleaning or restoration of
ballast. When possible, these are combined into a single operation,
often referred to as track resurfacing. This operation consists of
jacking rails up to the desired elevation, removing and cleaning
dirty ballast, and replacing defective ties and rails.