Glossary
Terms are defined in the context of their use in the paving industry.
A B C D E F G H J L M O P R S T W
AASHTO
American Association of State Highway and Transportation Officials.
A.D.A.
The Americans with Disabilities Act is a comprehensive federal act that ensures compliance with standards for the benefit of people with disabilities. Most relevant to pavement are standards for disabled parking, access aisles, curb ramps, signage and markings. State and local regulations may set more stringent requirements.
Aggregate
Particulates (sand, gravel, crushed stone, slag, and recycled concrete) often used in pavement construction as base material or as components of composite materials (i.e., asphalt concrete or Portland cement concrete) that serve as reinforcement to add strength. Aggregates typically compose 92 to 96% of an asphalt mixture.
Aggregate Base Material
Free-draining materials, typically a mix of coarse and fine crushed stone, used as a base layer under asphalt pavement to prevent premature water-related failure. These materials should be highly permeable, as well as high in strength and stiffness to sustain traffic loads.
Alligator Cracking
A major structural distress of asphalt pavement consisting of interconnecting “fatigue” cracks caused by traffic loading, typically where the base or sub-grade support is inadequate.� Cracking begins at the bottom of the asphalt surface where tensile stress and strain are highest under a wheel load. Cracks propagate to the surface as longitudinal cracks, gradually connecting to form many-sided, sharp-angled pieces ranging in size from 1” to 6”. Alligator cracking occurs only in areas subjected to repeated traffic loading, such as wheel paths, and is usually accompanied by rutting.
Annual Average Daily Traffic
The average 24-hour volume, being the total number during a stated period divided by the number of days in that period. Unless otherwise stated, the period is a year. The term is commonly abbreviated as ADT or AADT.
Asphalt
A sticky, black to brown, highly viscous liquid or semi-solid that is present in most crude petroleum and in some natural deposits. This raw material is refined in several steps to produce asphalt cement (bitumen). “Asphalt” is also used as a general term for asphalt concrete pavement.
Asphalt Cement
A refined residue from the distillation process of selected crude oils which is used as the binder for aggregate particles in asphalt concrete pavement. By volume, this material makes up about 4 – 8% of the pavement mixture. Outside of North America, this product is called “bitumen.”
Asphalt cement is separated from other components in crude oil by the process of fractional distillation, usually under vacuum conditions. Further separation is achieved by processing in a de-asphalting unit and “blowing” the product (reacting it with oxygen), making the product harder and more viscous. Asphalt may also contain additives such as emulsifiers, cut back agents, polymers, etc. to modify specific properties.
Asphalt Concrete
See Hot Mix Asphalt (HMA).
Asphalt Concrete Pavement
A relatively inexpensive flexible pavement composed of aggregates bound together by asphalt cement and composed of several layers: 1) an asphalt concrete surface; 2) a granular or asphalt concrete base; and, 3) a sub-base. The entire pavement structure, which is constructed over the sub-grade, is designed to support traffic loads and distribute these loads over the roadbed. Sometimes referred to as “blacktop,” “tarmac,” or simply as “asphalt.”
Asphalt Rubber
A blend of asphalt binder, reclaimed tire rubber, and certain additives in which the rubber component is at least 15 percent by weight of the total blend and has reacted in the hot asphalt binder sufficiently to cause swelling of the rubber particles.
A blend of asphalt binder modified with crumb rubber modifier (CRM) that may include less than 15 percent CRM by mass.
Asphalt Treated Permeable Base (ATPB)
A highly permeable open-graded mixture of crushed coarse aggregate and asphalt binder placed as the base layer to assure adequate drainage of the structural section, as well as structural support.
American Society for Testing and Materials (ASTM)
ASTM International is one of the largest voluntary standards development organizations in the world – a trusted source for technical standards for materials, products, systems, and services.
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Base
A crushed stone or asphalt product (full-depth asphalt pavements) installed prior to asphalt paving. The base material provides the load bearing characteristics of the finished pavement. Its depth may vary from 3” to 4” for a residential driveway to 18” or more for parking areas or roads. Lack of adequate base material is a primary cause of pavement failures.
A layer of selected, processed, and/or treated aggregate material that is placed immediately below the surface course. It provides additional load distribution and contributes to drainage and frost resistance.
Base Course
An underlying component of asphalt pavement consisting of an asphalt mix in which the largest stone used is no larger than 3/4” (typically AASHTO #57 grade). Base courses are usually laid at a minimum depth of 2” (compacted) over a stone base.
Base Failure
Base failures occur when the layer beneath the binder layer and driving surface can no longer adequately support the weight of the structure or the traffic. Base failures can occur for a number of reasons, including: ground water, excessive load counts (too much weight), and inadequate design. The failure can be corrected by excavating the failed material and replacing it with bridging stone material.
Basement Soil/Material
See Subgrade.
Binder
The component of an asphalt pavement which coats and adheres the aggregate particles, typically about 5 – 6% of the total asphalt mixture. Asphalt cement is used in hot mix asphalt. Liquid asphalt, which is asphalt cement dispersed in water with the aid of an emulsifying agent or solvent, is used as the binder in surface treatments and cold mix asphalt pavements. The properties of binders are often improved or enhanced by using additives or modifiers.
Properties of asphalt cement that are used to evaluate their quality include:
1) penetration – relative softness or hardness of an asphalt cement (or emulsion) at a given temperature;
2) viscosity – the resistance of an asphalt cement to flow at a given temperature;
3) ductility – ability of an asphalt cement to undergo elongation under tensile stress at a given temperature.
Binder Course
The asphalt layer between the base layer of aggregate and driving surface. The binder course is usually thicker than the surface layer and is composed of coarser materials. The binder layer can be used as a first layer or a driving surface. In most cases, a stone base layer, an asphalt base course, and then a surface layer are used.
Blacktop
A common “slang” term for asphalt. This term should not be used in specifications as the term has different regional connotations. For example, sometimes “blacktop” is used to refer to a penetration pavement or hot oil treatment (see fog seal).
Bleeding
Upward movement of asphalt binder in an asphalt surfacing resulting in a film of binder on the surface which creates a shiny, glass-like, reflecting surface that can be sticky. The most common cause is too much binder in one or more of the pavement courses. Excess application of bituminous sealant or low air-void content can also be at fault. Bleeding occurs when asphalt fills the voids of the mix during hot weather and then expands onto the pavement surface. Since the process is not reversible during cold weather, binder will accumulate. See “Flushing.”
Block Cracking
A major structural distress of asphalt pavement consisting of interconnecting cracks forming rectangular pieces of various sizes caused by shrinking and hardening of aging asphalt. It is not traffic load-associated. Spacing of cracks, which is inversely proportional to age, ranges from approximately 1’ to 10’ square.
Borrow
Natural soil obtained from sources outside the roadway prism to make up a deficiency in excavation quantities.
Bumps
Small, localized upward displacements of the pavement surface caused by:
1) buckling or bulging of underlying PCC slabs;
2) frost heave;
3) Infiltration and buildup of material in a crack in combination with traffic loading, sometimes referred to as “tenting.”
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Cement Treated Permeable Base (CTPB)
A highly permeable open-graded mixture of coarse aggregate, Portland cement, and water placed as the base layer to provide adequate drainage of the structural section, as well as structural support.
Climatic Conditions
Conditions induced by the average pattern of weather for a particular region that affect pavement performance, primarily temperature and precipitation.
Coal Tar
A by-product of coke ovens in the steel production industry. Refined coal-tar has been used as a base for asphalt pavement sealers since 1938. It has become more expensive in recent years due to the shift in steel production to foreign countries.
Cold In-Place
Recycling
A process (CIPR) using grinding machines to prepare existing pavement into base material for new paving. Emulsions or foamed asphalt are often added stabilizers.
Cold Milling
A process which uses specialized equipment (a rotating drum with helically-placed teeth) to grind asphaltic pavement into pieces to the desired depth. Typically, a new overlay of asphalt concrete of a specified depth follows.
Cold Mix Asphalt
A mixture of emulsified asphalt and aggregate, that can be produced, placed, and compacted at ambient air temperature. This type of asphalt is limited to low-volume traffic paving or Patching small areas. Usually, cold mix asphalt pavement requires an overlay of hot mix asphalt or surface treatment. The components of cold mix asphalt can be mixed at a central plant or in-situ with a traveling mixer.
Compaction
Exertion of force to consolidate aggregates and minimize voids, thus increasing density, stability and strength of a composite material, such as asphalt concrete, aggregate subbase or subgrade. A well-compacted base and subgrade is essential for optimum performance of asphalt or Portland cement concrete pavements. Machinery used for this purpose, typically heavy steel-wheeled rollers, apply weight and/or vibration to achieve consolidation.
Composite Pavement
These are pavements comprised of both rigid and flexible layers.
Concrete
Any composite material composed of mineral aggregate held together by a binder, whether that binder is Portland cement, asphalt cement or epoxy.
Construction Joint
Cracks forming along the limits of previous Patching or match point areas.
Corrugation
A type of pavement distortion typified by closely-spaced ripples and valleys, typically perpendicular to the traffic direction, at fairly regular intervals (less than 10’) across the asphalt pavement surface. This type of distress is caused by traffic action combined with an unstable pavement surface or base. See “washboarding.”
Crack
Separation of the pavement due to thermal and moisture variations, consolidation, traffic action, or reflections from an underlying pavement.
Crack and Seat Overlay (CSO)
A rehabilitation strategy for rigid pavements. CSO practice requires the contractor to crack and seat the rigid pavement slabs, and place a flexible overlay with a pavement reinforcing fabric (PRF) interlayer.
Crack Filler
Material that is placed in a pavement crack or joint to fill, but not necessarily seal, the void created by the crack or joint.
Crack Sealant
A material that has adhesive and cohesive properties to seal cracks, joints or other narrow openings (less than 1-1/2 inches wide) in pavements. Typical sealants are flexible rubberized asphalts (AC20 or asphalt emulsions). The best performing sealant is a polymer-modified sealant that “relaxes” during full extension, placing less tension or stress on the bond of the sealant to the side walls of the crack and resulting in longer lasting sealant bonds.
Crack Sealing
A critical, cost-effective maintenance activity of a pavement management program in which narrow openings in asphalt concrete pavement are:
1) cleaned of dust, dirt and debris by vacuuming;
2) dried with compressed air (often heated);
3) filled with an appropriate sealant.
Sealing slows pavement deterioration by preventing the passage of water or entrance of debris into the pavement layers, subbase or subgrade.
This technique can extend pavement service life by 3 to 5 years. Sealing prior to surface treatments enhances the treatment and further extends the pavement life. Crack sealing provides the most cost-effective use of dollars over time compared to other pavement maintenance techniques. Crack-sealed pavements have better rideability 5 years later than other surface treatments such as thin overlays, slurry seals, chip seals, and micropaving.
The best time to seal is during early to mid-fall, when cooler temperatures cause pavement cracks to open up between their maximum and minimum apertures and pavement moisture content is low. By sealing cracks when the pavement temperature is average, less stress is put on the sealant bond and bonding failures are much less likely.
Cracking
Separation of the asphalt concrete pavement layer due to excessive loads, heat, or age.
Crumb Rubber Modifier (CRM)
Scrap rubber produced from scrap tire rubber and other components, if required, and processes for use in wet or dry process modification of asphalt paving.
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Deflection
Deviation of a pavement from its constructed profile due to loading.
The downward vertical movement of a pavement surface due to the application of a load to the surface.
De-lamination
Separation of the asphalt wearing surface from the asphalt binder course caused by several possible factors:
1) insufficient or poor quality tack coat material between asphalt layers;
2) construction of pavement during cold weather;
3) inadequate sub-grade support.
Dense Graded Asphalt
An asphaltic mix (DGA) that has a continuous distribution of aggregate particle size and filler (i.e. evenly distributed from coarse to fine) and a low design air void content, generally in the range of 3 to 7%. Dense graded mixes are also often referred to as asphaltic concrete (AC) and represent the most widely used form of asphalt.
This type of mix provides the greatest load carrying capacity for structural layers, as well as a range of other properties appropriate to a wide variety of wearing course applications. The durability and resistance to environmental degradation of DGA is largely determined by insitu air voids and binder content, which must be optimized for service conditions.
See Hot Mix Asphalt (HMA).
Density
The weight of a material at a specific volume (unit weight). A specific density of asphalt is achieved by mechanically compacting (rolling) the hot material after it has been placed.
Depression
Localized pavement surface areas with elevations slightly lower than those of the surrounding pavement, detectable by puddling or stains caused by ponding water. Depressions are caused by settlement of the foundation soil or are a result of improper construction. Depressions filled with water of sufficient depth can cause hydroplaning or present hazardous conditions for pedestrians.
Localized low areas of limited size that may or may not be accompanied by cracking.
Distress
Visible signs of pavement deterioration, including:
1) surface defects;
2) surface deformation;
3) cracks;
4) potholes, patches and utility cuts.
These signs are used in measuring and rating asphalt pavements in order to determine an asset management strategy.
Distress is caused by six primary factors:
1) weathering;
2) excessive loading;
3) poor drainage;
4) poor construction;
5) inadequate design;
6) fuel spills.
See the American Public Works Association AWPA PAVER: Pavement Condition Index Field Manual – Asphalt for definitions of the 19 types of asphalt distress.
DOT
U.S. Department of Transportation.
Dowel Bar
A load transfer device in a rigid slab usually consisting of a plain round steel bar.
Drainage Structures
Any built structures such as drain inlets, catch basins, manholes, piping, culverts, outfalls, etc. used to convey stormwater from the pavement surface to a collection system, much of which is typically located underground.
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Edge Cracking
Cracks parallel to and within 1’ to 2’ of the outer edge of the pavement area, caused by frost weakened base or subgrade and accelerated by traffic loading. Edge cracking is often accompanied by raveling.
Edge Drain System
A drainage system, consisting of a slotted plastic collector pipe encapsulated in treated permeable material and a filter fabric barrier, with un-slotted plastic pipe vents, outlets, and cleanouts, designed to drain both rigid and flexible pavement structures.
Embankment
A prism of earth that is constructed form excavated or borrowed natural soil and/or rock, extending from original ground to the grading plane, and designed to provide a stable support for the pavement structure.
Equivalent Single Axle Loads (ESAL’s)
Summation of equivalent 18-kip single axle loads used to convert mixed traffic volume to total accumulated traffic loading during the design life of the pavement.
Excessive Loading
Traffic conditions exceed the structural capacity of the pavement as constructed, resulting in a more rapid rate of deterioration than typically anticipated.
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FHWA
Federal Highway Administration.
Flexible Pavement
Pavements engineered to transmit and distribute traffic loads to the underlying layers. The highest quality layer is the surface course (generally asphalt binder mixes) which may or may not incorporate underlying layers of a base and a subbase. These types of pavements are called “flexible” because the total pavement structure bends or flexes to accommodate deflection bending under traffic loads.
Flushing
Excess asphalt binder present on the pavement surface caused by incorrect asphalt mix (binder content too high). See “Bleeding.”
Full-Depth Asphalt Pavement
An asphalt pavement structure using asphalt products for all components. The base material and surface courses are all made up of appropriately specified grades of hot-mix asphalt, as opposed to conventional paving using crushed stone or other aggregate materials.
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Geotextiles
Fabric-like materials manufactured for specific performance characteristics and construction uses, such as paving. Geotextiles may be used to stabilize base material to prevent migration into sub-grades, retard reflective cracking in asphalt overlays, and/or serve as moisture barriers between pavement layers.
Grade
Inclination or slope of a surface, base layer or sub-grade to aid in the drainage of water. Grading is the act of intentionally constructing such a slope prior to paving.
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Heat Lance
A device used to remove debris and vegetation, as well as to warm and dry pavement cracks prior to sealing. The lance uses a combination of propane and compressed air, ignited in a specially designed chamber, to produce an extremely hot, high-velocity stream of air.� Federal research has determined this to be the most effective preparation method. Although more expensive initially, a combination of routing and heat lance preparation can provide 10 times the life of conventional crack sealing methods.
Heaving
Expansion of sub-grade or aggregate base material caused by intrusion of water into expansive soils, or freezing of such water, beneath the pavement.
Hot Mix Asphalt (HMA)
HMA is a graded asphalt concrete mixture (aggregate and asphalt binder) containing a small percentage of voids which is used primarily as a surface course to provide the structural strength needed to distribute loads to underlying layers of the pavement structure.
Hot Mix Asphalt Concrete
HMAC is a mixture of fine and coarse aggregate with asphalt cement binder that is mixed, placed, and compacted in a heated condition. The components are heated and mixed at a central plant: the aggregate is dried to remove moisture prior to mixing; mixing is performed with the aggregate at about 300�F (roughly 150�C), and the asphalt cement at 200 �F (95�C).� Paving and compaction must be performed while the asphalt is sufficiently hot, typically in summer months. In cooler seasons, the asphalt will cool too much before it is compacted to the optimal air content.
HMAC is the form of asphalt concrete most commonly used on highly trafficked pavements such as major highways and airfields. HMAC is produced in different grades from coarse base mixes to specialized mixes for surfacing and repair.
Hot Mix Asphalt Overlay
A high quality, controlled hot mixture of asphalt cement and well-graded, high quality aggregate that is spread in a thin layer, typically 1” to 1 �” thick, over a prepared pavement surface by an asphalt paving machine and compacted with a vibrating steel drum roller.
Hot In-Place Recycling
A pavement rehabilitation treatment used to correct asphalt concrete surface distress which includes:
1) heating and removal of old asphalt concrete;
2) processing and mixing with new aggregates, new asphalt binder and/or recycling agents;
3) relaying and compacting to meet specifications for conventional asphalt concrete.
Hot Recycled Asphalt (HRA)
The use of reclaimed flexible pavement which is combined with virgin aggregates, asphalt, and sometimes rejuvenating agents at a central hot-mix plant and placed in the pavement structure in lieu of using all new materials.
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Joint
The seam where two different “pulls” of asphalt abut each other, usually highly visible after the paving operation.
Joint Reflection Cracking
Cracks in an asphalt pavement surface that are aligned with underlying cracks in a PCC slab caused by thermal or moisture induced movement of the slab beneath. This distress is not load-related, however, traffic may cause further deterioration of the surface surrounding the initial crack. Fragmentation of the asphalt along the crack is referred to as “spalling.”
Joint Seals
Pourable, extrudable or premolded materials that are placed primarily in transverse and longitudinal joints in concrete pavement to deter the entry of water and incompressible materials (such as sand that is broadcast in freeze-thaw areas to improve skid resistance).
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Laydown
The portion of the asphalt paving process where the hot asphalt is actually placed or “laid down” by the paving machine.
Lean Concrete Base
Mixture of aggregate, Portland cement, water, and optional admixtures, primarily used as a base for Portland cement concrete pavement.
Limestone
A sedimentary rock often used as the base layer in an asphalt or PCCP paving system and the major stone component for asphalt materials produced in much of the U.S.
Longitudinal Cracking
Cracks running parallel to the direction of traffic or laydown, typically in roadway pavement, caused by:
1) poorly constructed paving joints between laydown passes;
2) shrinkage of the pavement surface due to low temperatures, hardening of the asphalt or daily temperature cycling;
3) cracks in an underlying PCC slab at locations other than joints. These types of cracks are not usually load-associated.
Longitudinal Joint
A joint normally placed between traffic lanes in rigid pavements to control longitudinal cracking; and the joint between the traveled way and the shoulder.
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Macadam
A type of road construction, pioneered by John Loudon McAdam (ca. 1820), consisting of three layers of stone laid on a crowned sub-grade with side ditches for drainage. The first two layers consisted of angular hand-broken 3” (75 mm) aggregate laid to a total depth of 8” (200 mm). The top layer was composed of 1” (25 mm) aggregate about 2” (50 mm) thick. Each layer was compacted with a heavy roller, causing the angular stones to lock together.
Mat
A fresh asphalt surface behind the paving machine during the laydown and compaction phase of construction.
Median
The portion of a divided highway separating the traveled ways for traffic in opposite directions.
Microsurfacing
An enhanced slurry seal with a polymer modified binder, very high quality aggregates, and placed using specialized paving equipment.
Mineral Filler
Mineral filler consists of very fine, inert mineral matter (“fly ash”) that is added to hot mix asphalt to improve the density and strength of the mixture. Mineral fillers compose 3 to 6% of the hot mix asphalt concrete by mass.
Fillers modify properties of the mix:
1) reinforce mechanical strength to improved crack resistance;
2) retard photo-oxidation by opaque shielding from sun rays;
3) prevent evaporation and oxidation of oils and resins through absorption;
4) increase fire resistance.
Their disadvantages can be:
1) increased water absorption,
2) reduced pliability.
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Open Graded Asphalt
A hot asphalt mix (OGA) that provides improved skid resistance and drainage, as well as reduced road noise. Open graded asphalt has no sand and rock dust in the mix and, therefore, is more porous.
Open Graded Asphalt Concrete (OGAC)
See Open Graded Friction Course (OGFC).
Open Graded Friction Course (OGFC)
OGFC is a wearing course mix consisting of asphalt binder and aggregate with relatively uniform grading and little or no fine aggregate and mineral filler. OGFC is designed to have a large number of void spaces in the compacted mix as compared to hot mix asphalt.
Overlay
A thin covering of hot asphalt mix, typically 1” to 3” thick, applied and compacted over an existing paved surface (asphalt or Portland cement concrete). The existing paved surface is cleaned and sprayed with a tack coat, a crack-inhibiting geotextile fabric may be placed, and then the asphalt overlay is placed and compacted.
Asphalt overlays are good solutions for pavements which are aging, but which have not experienced severe or generalized structural failure. Overlays may be placed over areas with block cracking, thermal or shrinkage cracking. However, cracks should be sealed before placing the tack coat, if a geotextile is not used. Pavements with general alligator or block cracking, depressions, rutting, shoving or corrugations are not good candidates for overlays.
An overlay is a layer, usually hot mix asphalt, placed on existing flexible or rigid pavement to restore ride quality, to increase structural strength (load carrying capacity), and to extend the service life.
Oxidation
The process by which organic molecules in asphalt binder react with oxygen in the atmosphere. This reaction causes the structure and composition of the asphalt molecules to change, resulting in the asphalt binder becoming hardened and more brittle. One of the factors of “natural aging” of an asphalt concrete pavement.
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Partial-Depth Asphalt Pavement
Resurfacing of an existing asphalt pavement by removal, typically by milling, of only 4-3/4” or less of the surface material and replacement with an overlay of similar depth. size, shape, and depth of patch depends on the extent of pavement deterioration and must be determined by core sampling or during the removal operation.
Patching
Replacing an area of pavement with new material to repair a defective surface. Also, a patch may repair an area of previously non-distressed pavement that was removed for subsurface utility work.
In the case of asphalt concrete Patching, this entails filling of an area with hot or cold mix asphalt after removal of the deteriorated pavement, typically to a 2” depth, and thorough cleaning. Cold mix patches are considered temporary and cannot withstand excessive traffic loading.
A patch is considered a defect no matter how well constructed, as it usually does not perform as well as an original pavement section. Generally, some roughness is associated with this distress.
Pavement
The planned, engineered system of layers of specified materials (typically consisting of surface course, base, and subbase) placed over the subgrade soil to support the cumulative traffic loading anticipated during the design life of the pavement. The pavement is also referred to as the pavement structure and has been referred to as pavement structural section.
Pavement Design Life
Also referred to as performance period, pavement design life is the period of time that a newly constructed or rehabilitated pavement is engineered to perform before reaching its terminal serviceability or a condition that requires major rehabilitation or reconstruction. The selected pavement design life varies depending on the characteristics of the highway facility, the objective of the project, and projected traffic volume and loading.
Pavement Drainage System
A drainage system used for both asphalt and rigid pavements consisting of a treated permeable base layer and a collector system which includes a slotted plastic pipe encapsulated in treated permeable material and a filter fabric barrier with unslotted plastic pipe as vents, outlets and cleanouts to rapidly drain the pavement structure.
Pavement (Asphalt) Maintenance
Maintenance is the treatment of an asphalt concrete surface before significant deterioration (PCI 60 – 100, though 75+ is preferable). It typically includes an application of seal coats, crack sealant, and sometimes patches, such that the service life of the pavement is extended by prevention of distress.
Maintenance is a pro-active approach to managing pavements and is an investment in the longevity of the paved asset.
Pavement Preservation
Work done, either by contract or by State forces to preserve the ride quality, safety characteristics, functional serviceability and structural integrity of roadway facilities on the State highway system
Pavement (Asphalt) Reconstruction
Total replacement of the asphalt pavement structural section because it has performed to its maximum service life and no longer offers any utility as a paved surface. It is the permanent fix for a pavement which has completely failed or which has failed (PCI 0 – 40) to the extent that continued full depth asphalt Patching is not cost-effective.
Reconstruction is an extreme form of reactive pavement rehabilitation and is very expensive. It is more expensive than a simple pavement installation since the remnants of the existing pavement section have to be removed and discarded off-site or rejuvenated onsite (in-situ recycling) before a new paved surface can be installed. Complicating factors include governmental regulations regarding handling and disposal of old asphalt pavement.
The services of a professional civil engineer are essential in the design and installation of a reconstructed pavement.
Pavement (Asphalt) Rehabilitation
Action taken redressing significant distresses of a pavement section (PCI 40 – 60), though not repairing the base, sub-base or sub-grade. Common practices include simple overlays, mill and overlay, and large full depth patches.
Rehabilitation is inherently reactive in nature and, thus, is more expensive than maintenance treatments. Specific treatment must be tailored to the specific distress conditions. Conditions that warrant rehabilitation are severe enough to critically impact pavement performance, and costs are high enough, that the services of an engineer are necessary and cost effective.
Pavement Service Life
Is the actual period of time that a newly constructed or rehabilitated pavement structure performs satisfactorily before reaching its terminal serviceability or a condition that requires major rehabilitation or reconstruction. Because of the many independent variables involved, pavement service life may be considerably longer or shorter than the design life of the pavement.
Pavement Structure
See Pavement.
Pavement Types
AAC – Airfield Asphalt Concrete
AC – Asphaltic Concrete
APC – Asphalt over Concrete
PCC – Portland Cement Concrete
Paver Joint Cracking
Cracks forming along the edges of an original paver pass during construction.
Polishing
A smooth, slippery pavement surface caused by traffic wearing off the sharp edges of the aggregate.
Portland Cement
A product made from Portland cement clinker (limestone or some other source of lime), gypsum and other materials, which are ground up, mixed, burned in a kiln to a sintering temperature, and subsequently ground to a fine powder which will harden when mixed with water. Used as a binder with crushed stone aggregates and sand to produce “concrete.”
Portland Cement Concrete
The product of mixing Portland cement, aggregate, water, and, in some cases, additives (such as an air entraining agent or a water reducing agent) to result in a hardened structural material after hydration occurs. Commonly used as a paving material.
Pothole
Bowl-shaped depressions, usually less than 3’ in diameter, with defined edges where the asphalt surface is absent, typically caused by small sections of pavement with severe alligator cracking breaking free and exposing the base or subgrade. Expansion of potholes is accelerated by free moisture collecting inside the hole. Potholes are generally structurally related distresses and should not be confused with raveling and weathering.
Prime Coat
A low viscosity emulsion (i.e., asphalt oil) applied to the sub-base and/or base material to seal and enhance bonding, prior to laydown of hot mix asphalt concrete.
Pumping
The ejection of foundation material, either wet or dry, through joints or cracks, or along edges of rigid slabs resulting from vertical movements of the slab under traffic. This phenomena is especially pronounced with saturated structural sections.
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Random Cracking
Cracks forming due to thermal differentials in the pavement materials induced by weather conditions. Random or “thermal” cracking is directly proportional to pavement age (brittleness increases with age resulting in greater susceptibility to thermal variations).
Raveling
Progressive loss of pavement material from the surface downward caused by a lack of bonding between the asphalt binder and aggregate for several possible reasons:
1) poor compaction during initial construction;
2) insufficient asphalt cement content in the mix;
3) construction of a thin lift during cold weather;
4) defective or dirty aggregate;
5) overheating of the asphalt mix,
6) environmental aging.
Softening of the surface and dislodging of the aggregates due to oil spillage is also included under raveling.
Progressive disintegration of the surface course on asphalt concrete pavement by the dislodgement of aggregate particles and binder.
Recycling
Asphalt concrete that is removed from a pavement is usually stockpiled for later use as a base course material. This reclaimed material, commonly known by the acronym ‘RAP’ for recycled or reclaimed asphalt pavement, is crushed to a consistent gradation and added to the HMA mixing process.
See also “Hot In-Place Recycling.”
Reclaimed Asphalt Pavement
An existing asphalt concrete pavement that has been pulverized, usually by milling, and then is used as an aggregate in new asphalt concrete or blended with virgin aggregates and used in base and sub-base materials. Also referred to as “RAP.”
Reflective Cracking
Cracks presenting in overlays directly resulting (reflecting) from cracks in the underlying pavement.
Rehabilitation
Work undertaken to extend the service life of an existing facility. This includes placement of additional surfacing and/or other work necessary to return an existing roadway, including shoulders, to a condition of structural or functional adequacy, for the specified service life. This might include the partial or complete removal and replacement of portions of the pavement structure. Rehabilitation is divided into pavement rehabilitation activities and roadway rehabilitation activities.
Resurfacing
A supplemental surface layer or replacement layer placed on an existing pavement to restore its riding qualities and/or to increase its structural (load carrying) strength.
Ride Quality
An attribute of a pavement describing how well the pavement performs as perceived by a user traveling in a vehicle on its surface. Where pavement exhibits one or more signs of distress, ride quality may be compromised.
Ride Quality must be evaluated in order to establish a severity level for the following distress types:
1) bumps;
2) corrugation;
3) shoving;
4) swells.
Ride Quality is determined by riding in a standard-sized automobile over the pavement section at the posted speed limit.
Rigid Pavement
These are pavements with a rigid surface course (typically Portland cement concrete or a variety of specialty cement mixes for rapid strength concretes) which may incorporate underlying layers of stabilized or unstabilized base or subbase materials. These types of pavements rely on the substantially higher stiffness rigid slab to distribute the traffic loads over a relatively wide area of underlying layers and the subgrade. Some rigid slabs have reinforcing steel to help resist cracking due to temperature changes and repeated loading.
Roadbed
The roadbed is that area between the intersection of the upper surface of the roadway and the side slopes or curb Hues. The roadbed rises in elevation as each increment or layer of subbase, base or surface course is placed. Where the medians are so wide as to include areas of undisturbed land, a divided highway is considered as including two separate roadbeds.
Roadway
That portion of the highway included between the outside lines of the sidewalks, or curbs and gutters, or side ditches including also the appertaining structures, and all slopes, ditches, channels, waterways, and other features necessary for proper drainage and protection.
Roller Marks
Visual evidence of construction equipment used to compact the surface application caused by:
1) poor quality compaction;
2) inadequate base construction;
3) yielding sub-grade.
Routing
Enlarging of pavement cracks using a specialized machine that provides a uniform width reservoir for the sealant, greatly increasing the effectiveness and durability of crack sealing. If cracks are routed prior to sealing, there is almost a 40% greater chance of sealant success. Older-aged asphalt pavements and thin asphalt pavements may not be suitable for routing.
Rubberized Hot Mix Asphalt (RHMA)
Formerly known as rubberized asphalt concrete (RAC). RHMA is a material produced for hot mix applications by mixing either asphalt rubber or asphalt rubber binder with graded aggregate. RHMA may be gap-(RHMA-G) or open- (RHMA-O) graded.
Rutting
A distress in which surface depressions are evident on the pavement surface caused by vehicle wheel tracks. Pavement uplift may occur along the sides of a rut, usually noticeable after a rain event when the rut fills with water. Rutting stems from a permanent deformation in any of the pavement layers or subgrades caused by settling or lateral movement of pavement materials due to traffic loads. Significant rutting can lead to major structural failure of the pavement.
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Sags
Small abrupt downward displacements of the pavement surface that often accompany bumps.
Seal Coat
A surface treatment that acts as a barrier to protect asphalt surfaces. There are two primary types of seal coating materials commonly in use: 1) those made from refined coal tar, and 2) those made from asphalt. Refined coal tar — a byproduct of the coking process — is a very complex mixture and quite different in molecular structure from asphalt. This often is referred to as C.T.P.E. (Coal Tar Pitch Emulsion), denoting that it is water based, obtained by dispersing refined coal tar in a matrix of clay and water.
Asphalt emulsions deliver most of the same properties as refined coal tar-based coatings — except for the resistance to color fading due to ultraviolet degradation, salts, and petrochemicals like oils, fats, grease and solvents. These deficiencies are inherent in the asphalt binder itself. Being a petroleum derivative, asphalt has a natural affinity for petrochemicals, so it is easily dissolved by them. Asphalt emulsion seal coats are more “user friendly,” practically odorless, and do not irritate and burn the skin.
Seal, Cape
A surface treatment where a chip seal is followed by the application of either a slurry seal or microsurfacing.
Seal, Chip
A surface treatment using one or more layers of aggregate chips and asphalt binding agent.
Seal, Fog
A highly diluted asphalt emulsion in a fine spray (fog) to a roadway surface in order to seal hairline cracks, slow oxidation and restores blackness. This treatment is not generally used for parking facilities due to tracking.
Seal, Sand
A surface treatment constructed by spraying emulsified asphalt and immediately spreading and rolling a thin fine aggregate cover. Similar to a chip seal, except that finer aggregate is used in the cover.
Seal, Sandwich
Similar to a double chip seal, except the first layer of asphalt binder is omitted.
Seal, Slurry
A surface treatment where an emulsified asphalt is sprayed on an existing asphalt pavement surface followed by a ‘brooming’ action to force the emulsion into voids left after the initial application. Sand is applied over the surface, followed by compaction.
Seal, Scrub
A surface treatment of emulsified asphalt, sand, additives, and water, applied as an aqueous mixture classified as Type I, II, or III depending on the size of aggregate used.
Sealer Cracking
Cracks forming in a previously applied surface treatment that develops on the surface and penetrates downward into the pavement structure. (Also termed “sealer checking.”)
Secondary Cracking
Cracks forming along a previous crack caused by the erosion of materials from the initial site of distress. Secondary cracking is avoided through proper maintenance.
Service Life
An asphalt pavement structure (surface and base courses) is designed to support a given number of repetitions of a given load every day. A typical asphalt pavement is expected to last between 15 and 20 years without maintenance, with proper drainage and without being subjected to heavier than design loadings.
The life of asphalt pavement can be reduced by any or all of the following five factors:
1) poor drainage;
2) excessive loading;
3) weathering;
4) poor design;
5) poor construction;
6) fuel spills.
Each of these factors, however, may be prevented or managed.
Serviceability
The ability at time of observation of a pavement to serve traffic (automobiles and trucks) which use the facility. The primary measure of serviceability is the Present Serviceability Index (PSI), which ranges from 0 (impossible road) to 5 (perfect road).
Settling
Consolidation of aggregate base material or sub-grade caused by improper compaction.
Settlement
Localized vertical displacement of the pavement structure due to slippage or consolidation of the underlying foundation, often resulting in pavement deterioration, cracking and poor ride quality.
Shoulder
The portion of the roadway contiguous with the traveled way for accommodations of stopped vehicles, for emergency use, and for lateral support of base and surface courses.
Shoving
Permanent, longitudinal displacement of surface material in a localized area of pavement caused by traffic loading. When traffic pushes against the pavement, it produces a short, abrupt wave in the pavement surface. This distress normally occurs in unstable liquid asphalt mix (cutback or emulsion) pavements. If the pavement is unstable due to improper asphalt mix or poor quality aggregate, washboarding may develop.
Shoves also occur where asphalt pavements abut PCC pavements; the PCC pavement increases in length and pushes the asphalt pavement.
Simple Overlay
A thin covering of hot asphalt mix, typically less than 2” thick, applied over an existing paved surface (asphalt or Portland cement concrete) that has not been previously milled. See “Overlay.”
Slippage Cracking
Cracks presenting in crescent or half-moon patterns having two ends pointing away from the direction of traffic. This distress is produced by breaking or turning wheels that cause differential movement between an overlay surface treatment and the underlying pavement. Usually this indicates a low-strength surface mix or a poor bond between the surface and the underlying layer of pavement structure
Stone Mastic Asphalt
Stone Mastic Asphalt (SMA) provides a deformation resistant, durable, surfacing material, suitable for heavily trafficked roads. SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resists permanent deformation.
The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film and, lower air voids content. This high bitumen content also improves of flexibility.
Addition of a small quantity of cellulose or mineral fiber prevents drainage of bitumen during transport and placement. The preferred method of compaction is to use heavy, nonvibrating, steel-wheeled rollers to minimize bleeding. SMA surfacings may provide reduced reflection cracking from underlying cracked pavements due to the flexibility of the mastic.
Structural Overlay
A thin covering of hot asphalt mix, greater than 2” thick, applied over an existing paved surface (asphalt or Portland cement concrete), typically after milling. A licensed civil engineer with experience in asphalt should assist in preparing technical bid documents, including specifications to insure quality and provide a standard for construction testing.
Structural Section
See Pavement Structure.
Subbase
The layer in a flexible pavement structure immediately below the base course. Also used to describe the single aggregate layer above the subgrade in a rigid pavement structure.
Unbound aggregate or granular material that is placed on the subgrade as a foundation or working platform for the base. It functions primarily as structural support, but it can also minimize the intrusion of fines from the subgrade into the pavement structure, improve drainage, and minimize frost action damage.
Subgrade
The natural soil or rock layer, or placed earth or rock fill layer, prepared to support a pavement structure. It is the foundation of the pavement structure.
Also referred to as basement soil, is that portion of the roadbed consisting of native or treated soil on which pavement surface course, base, subbase, or a layer of any other material is placed.
Subsurface Moisture
Trapped water beneath a pavement structure caused by several possible factors:
1) depressions in the pavement surface;
2) depressions in the pavement base and/or sub grade;
3) poorly drained soils.
Surface Course
The top layer of a flexible or composite pavement, that comes into direct contact with vehicles, sometimes called the asphalt wearing course.
One or more uppermost layers of the pavement structure engineered to carry and distribute traffic loads. The surface course typically consists of a weather-resistant flexible or rigid layer, which provides characteristics such as friction, smoothness, resistance to traffic loads, and drainage. In addition, the surface course minimizes infiltration of surface water into the underlying base, subbase and subgrade. Surface course may be composed of a single layer with one or multiple lifts, or multiple layers of differing materials.
Surface Drainage Slope
Inclination of the pavement surface to allow for water movement in the direction of collection structures. Typically, the pavement surface is constructed with a 1% minimum slope and a preferred slope of 2% where conditions allow. Slopes less than the minimum cause slow movement of water resulting in potentially damaging infiltration into the pavement structure.
Surface Ponding
Areas of standing water in excess of 1/2” depth on the pavement surface caused by poor surface drainage.
Surface Treatment
Surface treatments consist of an application (or sometimes multiple applications) of emulsified or liquid asphalt and select aggregate, placed over a prepared granular base or existing surface. Following placement of the aggregate, the mixture is rolled and compacted to provide a drivable, dust-free surface. This type of pavement is common on light- to medium- volume roads that may or may not already have an existing bituminous surface.
Swelling
Distortion and displacement which occurs over large areas of the pavement surface, characterized by upward bulging in long, gradual waves more than 10’ in length. This type of distress is usually caused by frost action in the subgrade and can be accompanied by surface cracking.
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Tack Coat
An application of liquid asphalt or emulsified or cutback asphalt to an existing asphalt concrete surface prior to the placement of an asphalt concrete lift or overlay to create a bond between the old and new asphalt layers.
Tarmac
Roadways constructed using the macadam process and then sprayed with tar to create “tarbound macadam” (tarmac) as a method of ameliorating the tendency of macadam roads to produce dust and gradually ravel under motor vehicle traffic. Asphalt concrete pavements at airfields are sometimes referred to as “tarmac” for historical reasons, although they no longer contain tar and are not constructed using the macadam process.
Tenting
A specific type of bump in which a small, localized area of pavement is displaced due to infiltration and buildup of material in a crack in combination with traffic loading.
Thermal Cracking
Cracks forming due to thermal differentials in the pavement materials induced by weather conditions, specifically seasonal temperature changes and/or daily temperature cycling. See “Random Cracking.”
Tie Bars
Deformed reinforcing bars placed at intervals that hold rigid pavement slabs in adjoining lanes and exterior lane-to-shoulder joints together and prevent differential vertical and lateral movement.
Traffic Loading
The dynamic forces imposed on a pavement structure as a result of normal moving traffic operations.
Transverse Cracking
Cracks forming perpendicularly to the direction of traffic caused by: 1) shrinkage of the asphalt concrete surface due to low temperatures, hardening of the asphalt, and/or daily temperature cycling; or 2) cracking in the underlying PCC slab in locations other than joints. This distress is typically not load-associated.
Traveled Way
The portion of the roadway for the movement of vehicles, exclusive of shoulders.
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Warm Mix Asphalt Concrete
A mixture of fine and coarse aggregate with asphalt cement binder that is
produced by adding either zeolites, waxes, or asphalt emulsions in order to allow significantly lower mixing and laying temperatures. Advantages of this WMA asphalt are: 1) more rapid availability of the surface for use; 2) improved working conditions; 3) lower consumption of fossil fuels; and, thus 4) reduced levels of released carbon dioxide, aerosols and vapors.
Washboarding
A series of transverse undulations or corrugations that form in a transverse direction in the surface of an unbound road surface.
Weathering
The natural process of deterioration of a road surface over time due to exposure to the elements (e.g. sun, rain, and ice). Weathering of asphalt is specifically due to the following factors:
1) oxygen – produces oxidized derivatives, some of which are water soluble;
2) light – through photo-oxidation, light also produces oxidized derivatives;
3) heat – hastens the chemical reaction of oxidation;
4) water – washes away oxidized products, exposing layers of asphalt.
The weathering process causes an increase in the asphaltene content of asphalt along with a decrease in resins and oils. This means that increased stiffness will occur in the asphalt and cracks will begin to appear, as the lack of flexibility makes the surface less resistant to stress.
Wearing Surface
The top layer of a flexible or composite pavement, either a bituminous spray seal or a hot mix asphalt overlay, that comes into direct contact with vehicles. The bituminous spray forms a thin waterproof membrane to prevent distresses and provide a skid-resistant surface.
See also “Seal, Fog” and “Surface Course.”
Wheel Depressions
Depressed areas of pavement presenting in parking stalls in the locations where vehicle tires are typically located caused by the static load of the vehicle weight consolidating or laterally moving the asphalt surface and/or the underlying courses.
Several factors may contribute to this type of distress:
1) insufficient structural strength of the pavement structure;
2) inadequate compaction of the pavement courses or base material;
3) yielding sub-grade.