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.