Bridges & Structures

Geotechnical Aspects of Pavements Reference Manual

Appendix A. Terminology

The following provides a definition of the pavement components, along with other terms common to the geotechnical aspects of pavements as contained in this manual. (Definitions were taken from NCHRP 1-37A, where available). The terms and definitions are organized under five general headings:

• Primary Pavement Components
• Geotechnical Pavement Components
• Non-Geotechnical Components
• Design Terminology
• Pavement Distress and Failure Terminology

Primary Pavement Components

subgrade - The top surface of a roadbed upon which the pavement structure and shoulders are constructed.

subbase - The layer or layers of specified or selected materials of designed thickness placed on a subgrade to support a base course. Note that the layer directly below the PCC slab is now called a base layer, not a subbase layer.

base - The layer or layers of specified or select material of designed thickness placed on a subbase or subgrade to support a surface course. The layer directly beneath the PCC slab is called the base layer.

surface course - One or more layers of a pavement structure designed to accommodate the traffic load, the top layer of which resists skidding, traffic abrasion, and the disintegrating effects of climate. The top layer of flexible pavements is sometimes called the "wearing" course.

Geotechnical Pavement Components

The geotechnical components of a pavement system as covered in this manual include unbound granular base, unbound granular subbase, the subgrade or roadbed, aggregate and geosynthetics used in drainage systems, graded granular aggregate and geosynthetic used as separation and filtration layers, and the roadway embankment foundation. Terms related to these components are defined as follows.

aggregate base (AB) - A base course consisting of compacted mineral aggregates. Also, granular base (GB), unbound granular base.

aggregate subbase (ASB) - A subbase course consisting of compacted mineral aggregates. Also, granular subbase, unbound granular subbase.

asphalt-treated permeable base (ATPB) - A base containing a small percentage of asphalt cement to enhance stability.

asphalt-treated permeable base (ATPB) - A permeable base containing a small percentage of asphalt cement to enhance stability. Also, asphalt-treated open-graded base (ATOGB), asphalt-treated base-permeable (ATB-Perm).

cement-treated base (CTB) - A base course consisting of mineral aggregates blended in place or through a pugmill with a small percentage of Portland cement to provide cementitious properties and strengthening. Also, aggregate cement, cement-stabilized graded aggregate (CSGA), cement-stabilized base (CSB).

cement-treated permeable base (CTPB) - An open-graded aggregate base treated with Portland cement to provide enhanced base strength and reduce erosion potential.

crushed stone base - A base course of designed thickness and constructed of graded and mechanically crushed mineral aggregate compacted above a subbase course or subgrade. Also, aggregate base (AB), graded aggregate base (GAB), and crushed aggregate (CA).

crushed stone subbase - A subbase course of designed thickness and constructed of graded and mechanically crushed mineral aggregate compacted above a subgrade.

dense-graded aggregate (DGA) - A mechanically crushed, well graded aggregate having a particle size distribution such that when it is compacted, the resulting voids between the aggregate particles, expressed as a percentage of the total space occupied by the material, are relatively small.

drainable granular subbase - A subbase constructed of compacted and crushed open-graded aggregate.

geogrid (GG) - a geosynthetic formed by a regular network of tensile elements with apertures of sufficient size to interlock with surrounding fill material, used primarily as reinforcement of base and subbase layers and in stabilization of soft subgrade layers. Also used in overlays for asphalt reinforcement.

geosynthetic - a planar product manufactured from a polymeric material used with soil, rock, earth, or other geotechnical-related material as an integral part of a civil engineering project, structure, or system.

geotextile (GT) - a permeable geosynthetic made of textile materials, used as a separator between base, subbase and subgrade layers, used as filters in drainage features, and used in stabilization of soft subgrade layers. Also used in asphalt overlays as a membrane absorption and/or waterproofing layer.

gravel - Coarse aggregate resulting from natural disintegration and abrasion of rock or processing of weakly bound conglomerate. In geotechnical engineering, the particles of rock that range in size from 76.2 mm (3-in. U.S. sieve) to 4.75 mm (No. 4 U.S. sieve). To be classified as a gravel in the Unified Classification System (UCS), at least 50% of the material must be in this range. (Identification and classification of soils is covered in Chapter 5.)

gravel base - An unbound base course constructed of compacted gravel. May or may not be graded and/or crushed.

gravel subbase - An unbound subbase course constructed of compacted gravel. May or may not be graded and/or crushed.

gravel subgrade - A subgrade where a natural gravel has been used as the roadbed surface or where the native soil has been blended with a gravel additive (a.k.a. gravel-treated subgrade for the second case).

lime-treated subgrade - A prepared and mechanically compacted mixture of hydrated lime, water, and soil supporting the pavement system.

lime-flyash base (LFB or LFA) - A blend of mineral aggregate, lime, flyash, and water, combined in proper proportions and producing a dense mass when compacted.

modified or treated base - The addition of cement or asphalt (typically less than 5%) to unbound base with the primary purpose of improving the stability for construction (i.e., no improvement anticipated for stiffness or structural support).

open-graded aggregate base (OGAB) - A crushed mineral aggregate base having a particle size distribution such that when compacted the interstices will provide enhanced drainage properties. Also, granular drainable layer, untreated permeable base (UPB).

permeable base (PB) - A base course constructed of treated or untreated open-graded aggregate. Also, free-draining base.

prefabricated geocomposite edge drain (PGED) - An edgedrain consisting of an extruded plastic drainage core covered with a geotextile filter (also known as panel drains or fin drains).

roadbed - The graded portion of a highway between top and side slopes, prepared as a foundation for the pavement structure and shoulder.

roadbed material - The material below the subgrade in cuts and in embankment foundations, extending to such depth as affects the support of the pavement structure.

soil aggregate - Natural or prepared mixtures consisting predominantly of stone, gravel, or sand that contain a significant amount of minus 75-µm (No. 200) silt-clay material.

soil cement - A mechanically compacted mixture of soil, Portland cement, and water, used as a layer in a pavement system to reinforce and protect the subgrade or subbase. Also, cement-treated subgrade (CTS).

stabilized granular base - A base course with an unspecified stabilizing material, usually asphalt cement or Portland cement.

stabilized permeable base - A permeable base with an unspecified stabilizing material, usually asphalt cement or Portland cement. Also, bound drainable base.

subgrade - the top surface of a roadbed upon which the pavement structure and shoulders are constructed with the purpose of providing a platform for construction of the pavement and to support the pavement without undue deflection that would impact the pavements performance (NCHRP 1-37A). In this manual, the natural and/or prepared soil materials beneath the pavement structure that deform under pavement loading or otherwise have an influence on the support of the pavement (a.k.a. roadbed, pavement foundation).

Unbound base/subbase - compacted mineral aggregate layer that may be either untreated or treated, but has not been modified sufficiently to provide an increase in stiffness or strength for design.

Non-Geotechnical Components

As indicated in Section 1.1, the non-geotechnical components are the surficial pavement layers, including asphaltic concrete, Portland cement concrete, and bound aggregate layers. Terms related to these components are defined as follows:

asphalt concrete (AC) - A controlled mixture of asphalt cement and graded aggregate compacted to a dense mass. Also, hot-mixed asphalt (HMA), hot-mixed asphalt concrete (HMAC), bituminous concrete (BC), plant mix (PM).

asphalt concrete base (ACB) - Asphalt concrete used as a base course. Also, asphalt base course (ABC), asphalt-stabilized base, hot-mixed (ASB-HM), asphalt-treated base (ATB), bituminous aggregate base, bituminous concrete base (BCB), bituminous base (BB), hot-mixed asphalt base (HMAB).

asphalt concrete pavement (ACP) - A pavement structure placed above a subgrade or improved subgrade and consisting of one or more courses of asphalt concrete or a combination of asphalt concrete and stabilized or unstabilized aggregate courses.

asphalt concrete surface (ACS) - Asphalt concrete used as a surface course. Also, dense-graded asphalt concrete (DGAC).

continuously reinforced concrete pavement (CRCP) - Portland cement concrete pavement with no transverse joints and containing longitudinal steel in an amount designed to ensure holding shrinkage cracks tightly closed. Joints exist only at construction joints and on-grade structures.

flexible pavement - A pavement structure that maintains intimate contact with and distributes loads to the subgrade and depends on aggregate interlock, particle friction, and cohesion for stability.

jointed plain concrete pavement (JPCP) - Jointed Portland cement concrete pavement containing no distributed steel to control random cracking; may or may not contain joint load transfer devices.

jointed reinforced concrete pavement (JRCP) - Jointed Portland cement concrete paving containing distributed steel reinforcement to control random cracking and usually containing joint load transfer devices.

lean concrete base (LCB) - A base course constructed of mineral aggregates plant mixed with a sufficient quantity of Portland cement to provide a strong platform for additional pavement layers and placed with a paver.

plain concrete - PCC without reinforcing steel.

Portland cement concrete (PCC) - A composite material consisting of a Portland or hydraulic cement binding medium and embedded particles or fragments of aggregate.

rigid pavement - A pavement structure that distributes loads to the subgrade, having as one course a Portland cement concrete slab of relatively high-bending resistance.

Design Terminology

In the context of current design practice, pavement designers and geotechnical specialists must communicate using design terms with consistent definitions. Terms related to design as used in this manual include:

analysis period - (a.k.a. performance period) The time period used for comparing design alternatives. An analysis period may contain several maintenance and rehabilitation activities during the life cycle of the pavement being evaluated.

average annual daily traffic (AADT) - The estimate of typical traffic on a road segment for all days of the week over the period of a year.

average annual daily truck traffic (AADTT) - The estimate of typical truck traffic on a road segment for all days of the week over the period of a year.

axle load - The sum of all tire loads on an axle.

axle load spectrum - The full spectrum (distribution) of single, dual, triple, and other axle loads applied to a pavement structure by a given traffic stream.

bound base - The addition of a sufficient amount of cement or asphalt to change the long term stiffness and structural characteristics of unbound base to that of lean concrete.

design life - The length of time for which a pavement structure is being designed, including the time from construction until major programmed rehabilitation.

elastic layer theory - A mathematical process wherein the layers of a pavement structure are all assumed to behave elastically.

equivalent single axle load (ESAL) - A numerical factor that expresses the relationship of a given axle load to another axle load in terms of the relative effects of the two loads on the serviceability of a pavement structure. Often expressed in terms of 18,000-pound (80 kN) single axle loads.

finite element analysis - The finite element method is one wherein rigorous mathematical solution, often employing complex differential equations, of an engineering problem is approximated algebraically. The geometry of the problem is described by discrete elements of finite dimensions that are analyzed through the application of engineering mechanics principles. Results of the finite element analyses are aggregated to approximate the exact mathematical solution.

international roughness index (IRI) - A pavement roughness index computed from a longitudinal profile measurement using a quarter-car simulation at a simulation speed of 50 mph (80 km/h).

life-cycle cost analysis (LCCA) - An economic assessment of an item, area, system, or facility and competing design alternatives considering all significant costs of ownership over the economic life (which encompasses several analysis periods), expressed in equivalent dollars.

longitudinal profile - The perpendicular deviations of the pavement surface from an established reference parallel to the lane direction, usually measured in the wheel tracks.

mechanistic-empirical (M-E) - A design philosophy or approach wherein classical mechanics of solids is used in conjunction with empirically derived relationships to accomplish the design objectives.

pavement performance - Measure of accumulated service provided by a pavement (e.g., the adequacy with which it fulfills it purpose). Often referred to as the record of pavement condition or serviceability over time or with accumulated traffic.

performance period - The period of time that an initially constructed or rehabilitated pavement structure will last (perform) before reaching its terminal condition when rehabilitation is performed. This is also referred to as the design period.

present serviceability index (PSI) - An index derived by formula for estimating the serviceability rating from measurements of physical features of the pavement.

present serviceability rating (PSR) - A mean rating of the serviceability of a pavement (traveled surface) established by a panel rating under controlled conditions. The usual scale for highways is 0 to 5, with 5 being excellent.

reliability - The probability that a given pavement design will last for the anticipated design performance period.

rideability - A subjective judgment of the comparative discomfort induced by traveling over a specific section of highway pavement in a vehicle.

serviceability - The ability at time of observation of a pavement to serve traffic (autos and trucks) that uses the facility.

single axle load - The total load transmitted by all wheels whose centers may be included between two parallel transverse vertical planes 1 m (40 in.) apart, extending across the full width of the vehicle.

tandem axle load - The total load transmitted to the pavement by two consecutive axles whose centers may be included between parallel vertical planes.

traffic growth factor - A factor used to describe the annual growth rate of traffic volume on a roadway.

transverse profile - The vertical deviations of the pavement surface from a horizontal reference perpendicular to the lane direction.

user costs - Those costs realized by the users of a facility. In a life cycle cost analysis, user costs could take the form of delay costs or of changes in vehicle operating costs associated with various alternatives.

weigh-in-motion (WIM) - The process of estimating a moving vehicle's gross weight and the portion of that weight that is carried by each wheel, axle, or axle group, or combination thereof, by measurement and analysis of dynamic forces.

wheel load - The sum of the tire loads on all tires included in the wheel assembly comprising a half axle.

zero-stress temperature - temperature (after placement and during curing) at which the concrete layer exhibits zero thermal stress (at temperatures less than this, concrete exhibits tensile stress).

Pavement Distress and Failure Terminology

Distress refers to conditions that reduce serviceability or indicate structural deterioration. Failure is a relative term. In the context of this manual, failure denotes a pavement section that experiences excessive rutting or cracking that is greater than anticipated during the performance period or that a portion of the pavement is structurally impaired at any time during the performance period with incipient failure anticipated from the local distress. There are a number of ways that a pavement section can fail.

alligator cracking - Interconnected or interlaced cracks forming a pattern that resembles an alligator's hide. Also, map cracking.

blowup - An upward eruption of a PCC pavement slab near a crack or joint.

crack - A fissure or discontinuity in the pavement surface not necessarily extending through the entire thickness of the pavement.

fatigue cracking - Cracking of the pavement surface as a result of repetitive loading; may be manifested as longitudinal or alligator cracking in the wheel paths for flexible pavement and transverse cracking (and sometimes longitudinal cracking) for jointed concrete pavement.

faulting - Elevation or depression of a PCC slab in relation to an adjoining slab, usually at transverse joints and cracks.

liquefaction - the process of transforming any soil from a solid state to a liquid state, usually as a result of increased pore pressure and reduced shearing resistance (ASTM, 2001). Spontaneous liquefaction may be caused by a collapse of the structure by shock or other type of strain and is associated with a sudden but temporary increase of the prefluid pressure.

longitudinal cracking - Pavement cracking predominantly parallel to the direction of traffic.

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, or from cracks in semi-rigid pavements.

punchouts - A broken area of a CRCP bounded by closely spaced cracks usually spaced less than 1 m (3 ft).

random cracking - Uncontrolled and irregular fracturing of a pavement layer.

raveling - A pavement distress characterized by the loss of surface material involving the dislodgment of aggregate particles and degradation of the binder material.

reflective cracking - Cracks in asphalt or concrete surfaces of pavements, occurring over joints or cracks in the underlying layers.

rutting - Longitudinal depression or wearing away of the pavement in wheel paths under load.

spalling - The cracking, breaking, or chipping of pavement edges in the vicinity of a joint or crack.

thermal cracking - Cracks in an asphalt pavement surface, usually full width transverse, as a result of seasonal or diurnal volume changes of the pavement restrained by friction with an underlying layer.

transverse cracking - Pavement cracking predominantly perpendicular to the direction of traffic.

warping - Deformation of a PCC slab caused by a moisture or temperature differential between the upper and lower surfaces.