If you don’t know a cape seal from an overlay or a pothole from an alligator crack, check out Pavement Engineering’s glossary.  Although not all inclusive, it does provide a primer of terms common to pavement.

AASHTO – American Association of State Highway Transportation Officials.  This organization sets standards and publishes specifications, test protocols and guidelines which are used in highway design and construction throughout the United States.

ADA – Americans with Disabilities Act.  With regard to pavement, the Act provides regulations governing accessibility related to parking, ramps, etc. and accessible paths of travel for the disabled community.

Aggregate - materials such as sand, gravel, rock or crushed stone.  Aggregate of varying size and composition is used with a binding agent to produce asphalt concrete or Portland cement concrete.

Aggregate base - a layer of material, usually quarried rock or recycled asphalt concrete that is laid on top of native soil or an aggregate subbase.  It provides a foundation to support the surface layer of asphalt concrete or Portland cement concrete.

Aggregate subbase – a layer of evenly spread and compacted crushed stone laid on a base of native soil that provides the main load-bearing layer of a pavement, aids drainage and prevents settlement that leads to surface defects such as rutting. 

Alligator cracking – a series of interconnecting cracks in the surface of asphalt pavement caused by heavy wheel loads (fatigue) that looks like an alligator hide.  It is a clear sign of structural failure.

Asphalt – a material obtained through petroleum processing that is a generic term for asphalt concrete.

Asphalt concrete – a composite mix of aggregate and asphalt binder that is the most frequently used surface material for road and parking lot construction.

ASTM – American Society of Testing and Materials.  This organization develops uniform methods and standards for product quality.

Base – a layer of materials, usually aggregate, placed just beneath the asphalt concrete surface layer and above the subbase that provides additional load distribution and helps with drainage. 

Blacktop - a generic term for asphalt concrete.

Block cracking – a combination of longitudinal and transverse cracking in asphalt concrete that generally results as the binding agent evaporates and the asphalt hardens and shrinks.  It has a distinctive checkerboard pattern.

Cape seal – a combination of a chip (aggregate) layer overcoated with a slurry seal. The chip seal is placed first followed within a few days by the slurry seal that binds the chips and prevents loose aggregate. A cape seal provides a new wear surface, prevents water damage to the roadbed and addresses some pavement defects. 

Chip seal - a two-step process that combines a layer of aggregate followed by a high viscosity emulsion seal coat.  Used primarily on low-volume roads, it covers surface imperfections, improves surface friction and adds a new wear surface.

 Core test – measures existing pavement thickness to determine the structural capacity for road reconstruction or the thickness of the subgrade layer (foundation) for new roadway construction.  It involves coring through multiple layers of pavement to determine composition, characteristics of the native soil and moisture content.  Core test data is referred to as the R-value (resistance value), which determines how deep the subgrade needs to be to support the aggregate base and asphalt concrete surface layer that will carry the expected amount of traffic and wheel loads.

Crack seal – an inexpensive emulsion of hot rubberized asphalt used to seal longitudinal, transverse and block cracking to prevent water from seeping beneath the asphalt to the subgrade where structural damage occurs.  It also seals against abrasive dirt and sand.

Deflection test - a non-destructive method to determine the overall structural capacity and properties of existing pavement.  The test measures the resilience of the subgrade, base and surface layers, determines how flexible the pavement structure is and how it has been affected by the type and volume of traffic and other factors such as temperature and moisture. 

Digout – a localized repair or patch that involves digging out or excavating an area of damaged pavement, such as a pothole, and replacing it with new asphalt concrete. It is designed to prevent damage to the subgrade.

Dynaflect® - a dynamic-force generator equipped with a motion-sensing device that applies a vertical oscillating force on a pavement section to measure the magnitude of deflection.

ESAL - Equivalent Single Axle Load. One ESAL is equal to one 18,000-pound axle. An ESAL determines the impact of heavy wheel loads that are quantified as a Traffic Index, which helps determine how much loading a pavement will support before it begins to fail.

Fatigue – pavement deterioration caused by repetitive heavy wheel loads that results in cracking from the bottom of the pavement structure upward.  It usually appears as alligator cracking.

Flexible pavement – a structural section of road made up of asphalt concrete and one or more layers of aggregate that is designed to distribute loading to the underlying subgrade.  If properly designed, flexible pavement can expand or stretch to absorb heavy loads.

Fog seal – an inexpensive, short-lived treatment of diluted asphalt emulsion applied to an oxidized (weathered) asphalt concrete pavement to seal and restore flexibility to the pavement surface.

Full-depth asphalt – a pavement structure using hot mix asphalt (HMA) for both the base and surface materials.

HMA – Hot Mix Asphalt - a high quality, temperature-controlled hot mixture of asphalt binder and aggregate, ranging from coarse to very fine particles, that can be compacted into a uniform dense mass. It can be made from new or recycled material. 

HMAC - Hot Mix Asphalt Concrete.

Laydown – the portion of a paving process where HMA is placed or “laid down” by a paving machine.

Lime treatment – a modifier for or additive to soil subgrade that increases its R-value, improves performance and adds years to pavement life. 

Longitudinal cracking - cracks in asphalt concrete pavement that run parallel to the pavement’s centerline or laydown direction. It results primarily from environmental aging.

Maintenance – regularly timed pavement preservation treatments necessary for safety and to extend service life, typically for up to five years.  Maintenance treatments can include crack sealing, seal coats, slurry seals, chip seals and overlays or any combination of these and other treatments.

Mill and fill – a pavement rehabilitation process that involves milling (removing) the surface layer of pavement to a predetermined depth and filling it with new or recycled HMA.  It creates a smooth ride by eliminating tire ruts and other defects.

Milling - the controlled removal an existing asphalt pavement layer to correct and restore the surface to a specified profile.

MTC – Metropolitan Transportation Commission.  MTC is the regional planning organization for roads and transit in the San Francisco Bay Area.

Native subgrade - the base or soil upon which the pavement structure is built. It can be augmented by structural fill material.

Nuclear gauge – a non-destructive device that measures the density of in-place HMA to determine the level of compaction.

OGFC – Open Graded Friction Course.  OGFC is a non-structural wear course used primarily on HMA to improve wet weather skid resistance, reduce the potential for hydroplaning, reduce water splash and spray and reduce night time wet pavement glare. 

Overlay – a pavement rehabilitation process for severely deteriorated pavement that overlays bituminous asphalt concrete on top of existing pavement to strengthen its overall structure, improve ride and extend service life.

Pavement – the surface layer of a structural section of road that carries traffic.  It is composed either of asphalt concrete or Portland cement concrete. 

Pavement Management System (PMS) – a system developed by Caltrans to assess the condition of pavement biennially and prioritize pavement maintenance and rehabilitation consistent with available funding.

PCC – Portland Cement Concrete.  PCC is rigid and more durable than flexible asphalt pavement, and as such, is a pavement alternative for areas affected by heavy wheel loads from buses, garbage trucks and service vehicles.

Potholes – bowl-shaped holes caused by water damage that may extend into the base.

Profilograph - a device used to measure pavement surface roughness.  The data collected by a profilograph is used to calculate the International Roughness Index (IRI), which is expressed in units of inches/mile or millimeters/meter. IRI values range from 0 (equivalent to driving on a plate of glass) upwards to several hundred (a very rough road). The IRI value is used for road management to monitor road safety and quality issues.

Pulverization – a mechanized process that transforms the existing flexible pavement surface layer and a portion of the underlying granular layer into a uniform granular material suitable for use as a base layer. 

R-value - Resistance Value.  R-value is a measure of resistance to deformation under loading of saturated soil. 

Raveling – also called weathering, raveling is the progressive and gradual deterioration of the HMA layer. It results as the asphalt concrete binder oxidizes, separating it from the coarser aggregates and making the surface rough and uneven

Reconstruction - replacing an existing pavement structure that has reached the end of its service life or is badly deteriorated with a new, equivalent pavement structure that may use new or recycled paving materials or a combination of both.

Recycling – A process of milling (removing) the top asphalt concrete surface, which is pulverized, sized, and mixed with an additive, reshaped and compacted, and reapplied as a new surface.

Reflection cracking - cracks that generally occur on pavements where an asphalt concrete surface is placed over jointed Portland cement concrete.  Reflective cracking occurs directly over underlying cracks or joints but also can occur over existing HMA pavement cracks, cement or lime stabilized base, etc. 

Rehabilitation – a process that extends the service life of existing pavement by placing additional surfacing (overlay) or other treatment to restore an existing roadway to structural or functional adequacy for a minimum of 10 years.  Rehabilitation may include partial or complete removal and replacement of portions of the structural section.

Rejuvenation – an emulsion similar to a fog seal applied to an oxidized (weathered) pavement to restore flexibility to an existing HMA pavement surface.  The emulsion may include asphalt, polymer latex and other additives. 

RHMA – Rubberized Hot Mix Asphalt.  RHMA is a type of asphalt that combines granulated (crumb) rubber with hot asphalt to form an elastic binder with less susceptibility to temperature changes.  RHMA is generally specified to retard reflection cracking, resist thermal stresses created by wide temperature variations and add flexibility to a structural overlay.

Rigid pavement - a structural section of road constructed of rigid Portland cement concrete that is designed to distribute axle loads over a relatively wide area.

Rutting - longitudinal surface depressions in the wheel path of a pavement often caused by an inadequate structural foundation.

Seal coat –   a mix of approximately 85% emulsion and 15% aggregate used to seal rough or raveled pavement in areas with traffic speeds less than 15 mph. A seal coat fills in minor cracks and provides a smooth finish that protects against the environment.

Service life – the approximate lifespan of newly constructed pavement before major rehabilitation or reconstruction is required.  Because of traffic, climate and other variables, service life may be considerably shorter or longer than that for which is was designed. 

Slurry coat – a mix of approximately 15% emulsion and 85% aggregate used to seal rough or raveled pavement in areas with traffic speeds greater than 15 mph. A slurry seal fills in cracks and provides a smooth finish that protects against the environment. It has roughly twice the lifespan of a seal coat. 

Structural section – the designed layers of materials placed over native subgrade to support estimated traffic loads over a specified period of time.  Generally, the structural section normally consists of a subbase, base and pavement surface. 

Subbase – a layer of aggregate designed in thickness and quality placed on top of the native soil or subgrade that serves as a foundation for the base layer.

Subgrade – the native soil or portion of the roadbed on which a pavement surface, subbase, base or layer of other material is placed.

Traffic Index (TI) — determines how much loading a pavement will support before it begins to fail.  It is a measure of the number of Equivalent Single Axle Loads (ESALs) expected in the traffic lane over the pavement design life. One ESAL is equivalent to one 18,000-pound axle.

Transverse cracking – cracks in asphalt concrete pavement that run at right angles to the pavement’s centerline or laydown direction. It results primarily from environmental aging.

Washboard – also called corrugations, the regularly spaced rough transverse “bumps” caused by an unstable surface course. 

Weathering - also called raveling, weathering is the progressive and gradual deterioration of the HMA layer. It results as the asphalt concrete binder oxidizes, separating it from the coarser aggregates and making the surface rough and uneven.