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Asphalt Paving of Treated Timber Bridge Decks


This glossary includes terms related to asphalt pavement as well as terms normally associated with bridges. Some of the terms shown may not be included in the report. Many of the definitions are from the Asphalt Institute Handbook, the Asphalt Emulsion Manufacturer's Association, the Federal Highway Administration's Bridge Inspector's Training Manual, and Timber Bridges-Design, Construction, Inspection, and Maintenance (Ritter 1990).

AASHTO: American Association of State Highway and Transportation Officials.

AWPA: The American Wood Protection Association (formerly The American Wood Preservers' Association).

Abutment: A substructure supporting the end of a single span or the extreme end of a multispan superstructure. The abutment may retain or support the approach embankment.

Aggregate: Any hard, inert material of mineral composition, such as gravel, crushed rock, slag, or sand used in pavement applications, either by itself or for mixing with asphalt.

Aggregate, dense graded or well graded: An aggregate mixture that has a particle-size distribution graded from the maximum size to smaller sizes so that a compacted layer has high stability and a relatively low ratio of void spaces.

Aggregate, open graded: An aggregate having little or no small-size gravel or rock as filler. The void spaces in a compacted layer of open-graded aggregate are relatively large and interconnected.

Asphalt (also referred to as asphalt cement and asphalt binder): A dark brown to black cementitious material in which the predominant constituent is bitumen, occurring in nature or obtained during petroleum processing. Asphalt is a constituent of most crude petroleum.

Asphalt leveling course: A mixture of asphalt and aggregate of variable thickness used to eliminate irregularities in the contour of an existing surface before placing a pavement layer.

Asphalt overlay: One or more courses (layers) of asphalt pavement placed on an existing pavement or bridge deck.

Asphalt pavement: Pavements consisting of a surface course (layer) of mineral aggregate coated and cemented together with asphalt cement on supporting courses, such as asphalt bases; crushed stone, slag, or gravel; or on Portland cement concrete, brick, or block pavement.

Asphalt pavement system: Pavements consisting of mineral aggregate coated and cemented together with asphalt cement and possible paving fabric or membranes on a supporting surface (road base course or bridge deck).

Asphalt prime coat: An application of a low-viscosity cutback asphalt product to an absorbent surface. It is used to prepare an untreated base for an asphalt surface. The prime coat penetrates into the base and plugs the voids, hardens the top, and helps bind the base to the overlying asphalt course.

Asphalt seal coat: A thin asphalt surface treatment used to waterproof and improve the texture of an asphalt wearing surface. Depending on the purpose, seal coats may or may not be covered with aggregate. The main types of seal coats are aggregate seals, fog seals, emulsion slurry seals, and sand seals.

Asphalt tack coat: A very light application of asphalt, usually asphalt emulsion diluted with water. It is used to help bond the surface being paved and the overlying course.

Assay: Determination of the amount of preservative in a sample of treated wood by appropriate physical and chemical means.

Backwall: The topmost portion of an abutment above the elevation of the bridge seat, functioning primarily as a retaining wall. It also may serve as a support for the extreme end of the bridge deck and the approach slab.

Base: Road layers below the primary structural layer.

Beam: A structural member supporting a load applied transverse to it. Beams used in bridge construction include stringers, girders, and floor beams.

Bleeding: The secretion of liquid preservative from treated wood. The secreted preservative may evaporate, remain liquid, or harden into a semisolid or solid state. Bleeding also describes the secretion of liquid asphalt to pavement surfaces and is usually caused by too much asphalt or other hydrocarbons in the pavement mix.

BMP: Best management practices.

Charge: All the wood treated at one time in one cylinder of a treating tank.

Check: A lengthwise separation of the wood that extends across the rings of annual growth, commonly caused from stress set up in wood during seasoning.

CITW: Canadian Institute of Treated Wood.

Continuous spans: Spans without joints designed to extend over one or more intermediate supports.

Creosote: A wood preservative that is a distillate of coal tar produced by high-temperature carbonization of bituminous coal.

Crown of the roadway: The vertical dimension describing the total amount of the surface that is raised from the gutter to the centerline, sometimes termed the cross-fall of the roadway.

Cutback asphalt: Asphalt cement that has been liquefied by blending it with petroleum solvents (also called diluents). On exposure to atmospheric conditions, the diluents evaporate, leaving the asphalt cement to perform its function of cementing and waterproofing.

Decay: Disintegration of wood by wood-destroying fungi.

Deck: The portion of a bridge that provides direct support for vehicle and pedestrian traffic.

Deck bridge: A bridge in which all supporting members are beneath the roadway.

Delamination: Separation of layers of pavement or of the paving from the bridge deck.

Differential deflection: Movement of a structure or displacement between two adjoining members of a structure.

Dowels: Short steel rods used to join deck panels by transferring shear forces to prevent differential deflection.

Dry: Wood with a relatively low moisture content: 19 percent for sawed lumber and 16 percent for glued-laminated lumber.

Durability: A general term for permanence or resistance to deterioration. As applied to wood, durability refers to lasting qualities or permanence in service, particularly with reference to its resistance to decay and other forms of deterioration.

Empty-cell process: Any process for impregnating wood with preservatives or chemicals in which air, trapped in the wood under pressure, is released to drive out part of the injected preservative or chemical. The intent is to obtain good distribution of preservative in the wood, leaving the cell cavities only partially filled to minimize preservative bleeding.

Emulsified asphalt: A mixture of asphalt cement and water containing a small amount of an emulsifying agent that creates a heterogeneous system with two normally immiscible phases (asphalt and water) in which the water forms the continuous phase of the emulsion and minute globules of asphalt form the discontinuous phase. Emulsified asphalts may be either positively (cationic) or negatively (anionic) charged, depending on the emulsifying agent.

Expansion joint: A joint designed to allow expansion and contraction caused by temperature changes, load, or other forces.

Floor beam: A beam located transverse to the centerline or direction of travel on a bridge that supports the deck or other components of the floor system. A deck comprised of glued-laminated panels placed transversely across longitudinal girders or longitudinally across floor beams.

Geotextiles: A fabric (woven or nonwoven) used to reinforce soils or asphalt pavement. Geotextiles are also used as filters.

Girder: A flexural member designed to resist bending that is the main or primary support for the structure. In general, a girder is any large beam.

Glued-laminated timber: An engineered, stress-rated product of a timber-laminating plant comprising assemblies of specially selected and prepared wood laminations securely bonded with adhesives.

Heartwood: The interior wood in a tree extending from the pith to the sapwood.

Hot mix asphalt (HMA): A mixture of asphalt and aggregate produced in a batch or drum-mixing facility. To dry the aggregate and obtain sufficient fluidity of the asphalt cement, both must be heated before mixing—giving rise to the term “hot mix.”

Laminated wood: An assembly made by bonding layers of veneer or lumber with an adhesive so that the grain of all laminations is essentially parallel. When the laminations are dimensional lumber (2 by 4, and so forth), they are commonly referred to as glued-laminated lumber.

Horizontally laminated: Laminated wood in which the laminations are arranged with their wider dimension about perpendicular to the direction of load.

—Vertically laminated: Laminated wood in which the laminations are arranged with their wider dimension about parallel to the direction of load.

Longitudinal: The direction of travel on a bridge parallel to the roadway or bridge centerline.

Lumber: Sawed or planed wood.

Medium-curing (MC) asphalt: Cutback asphalt composed of asphalt cement and kerosene-type diluent of medium volatility.

Moisture content: The amount of water contained in wood, usually expressed as a percentage of the weight of the wood after it has been oven dried.

Monolithic: One single piece of material. In the case of stress-laminated timber decks, a single timber slab.

Nail-laminated deck: A timber bridge deck, usually installed across longitudinal beams, comprised of a series of dimensional lumber laminations placed on edge and nailed together on their wide faces.

Open-graded asphalt friction course: A pavement surface course that consists of a high-void asphalt plant mix that allows rapid drainage of rainwater through the course and out the shoulders. The mixture is characterized by a large percentage of one-sized coarse aggregate. This surface prevents tires from hydroplaning and increases skid resistance.

Paving fabrics: A geotextile used with asphalt pavement to stabilize and reinforce the pavement.

Paving membranes: An asphalt-impregnated paving fabric (usually rubberized or polymer-modified asphalt) intended to make a waterproof layer.

Penetration: The depth to which preservative enters the wood.

Pentachlorophenol (penta): A chlorinated phenol used as a wood preservative, usually carried in a base of petroleum oil.

Performance grade (PG): Asphalt grade designation used in Superpave based on the binder's mechanical performance at critical temperatures and aging conditions. This system uses engineering principles to directly correlate laboratory testing to field performance.

Polymer-modified asphalt: Conventional asphalt cement to which a styrene block copolymer or styrene butadiene rubber (SBR) latex or neoprene latex has been added to improve performance.

Plank deck: Timber planks, usually aligned transversely and nailed to the load-carrying member.

Plant mix: A mixture produced in an asphalt mixing facility that consists of mineral aggregate uniformly coated with asphalt cement, emulsified asphalt, or cutback asphalt.

Preservatives: Insecticides injected into wood to inhibit deterioration caused by insects and fungi.

Pressure process: Any process of treating wood in a closed container where the preservative is forced into the wood under pressure. The AWPA usually specifies pressure greater than 50 pounds per square inch.

Prime coat: An application of a low-viscosity cutback asphalt product to an absorbent surface. It prepares an untreated base for an asphalt surface finish. The prime coat penetrates into the base and plugs the voids, hardens the top, and helps bind the base to the overlaying asphalt course.

Rapid-curing (RC) asphalt: Cutback asphalt composed of asphalt cement and naphtha or gasoline-type diluent of high volatility.

Reflective cracking: Cracks that migrate up from lower layers of the subgrade or a timber deck.

Retention: The amount of preservatives remaining in the wood after treating, usually expressed as pounds per cubic foot.

Retort: A steel tank, commonly horizontal, in which wood is placed for pressure treatment.

Rigid pavement system: A concrete slab serves as the structural component. Because of the slab's structural capacity, it tends to distribute loads over a wide area. Underlying bases are provided as a surface only and do not contribute to the strength of the system. Minor variations in subgrade strength have little influence on the structural capacity of the rigid pavement.

Roadway: The portion of the bridge deck intended for use by vehicles and pedestrians.

Rutting: Depressions in an asphalt pavement from traffic use over a “soft” pavement.

Sapwood: The wood of pale color near the outside of the log. Sapwood is more porous than heartwood and less resistant to decay.

Shrinkage: A change in the dimension of structural timber caused primarily by changes in moisture content.

Slow-curing (SC) asphalt: Cutback asphalt composed of asphalt cement and oils of low volatility.

Solid lumber: Sawed lumber that has not been modified, or built up by gluing.

Solvents: Carriers for chemical preservatives.

Span: The distance between the end supports center-to-center when applied to the design of beams, stringers, or girders.

Stiffener beam: A load distributor beam attached to the underside of the deck. A longitudinal stiffener beam is placed midway between the longitudinal load-carrying beams. The stiffener beam is the length of the bridge, helping reduce differential deflections between deck panels.

Stress-laminated deck: A longitudinal deck without beams or stringers, stressed into a monolithic slab by high-strength reinforcing rods.

Stringer: A longitudinal beam supporting the bridge deck.

Structural capacity: The measure of carrying capacity of a structure or member.

Subgrade: The layer in the asphalt pavement structure immediately below the base course is called the subgrade course. The subgrade soil is sometimes called foundation soil.

Substructure: The structural members that carry the loads from a bridge's superstructure to its foundation.

Superelevation: The difference in elevation between the inside and outside edges of a roadway in a horizontal curve. This elevation counteracts the effects of centripetal force.

Superpave: Short for superior performing asphalt pavement, a performance-based system for selecting and specifying asphalt binders and for designing an asphalt mixture.

Superpave mix design: A mixture design system that integrates the selection of materials (asphalt, aggregate) and their volumetric proportions with the project's climate and the designed traffic.

Tack coat: A very light application on asphalt, usually asphalt emulsion diluted with water. It ensures a bond between the surface being paved and the overlying course.

Timber: Wood members at least 5 inches in the shortest dimension that are suitable for building purposes.

Transverse: Perpendicular to the direction of travel, the roadway, or the bridge centerline.

Wearing surface: The topmost layer of material applied to a roadway or bridge that receives the traffic loads and resists the resulting tire abrasion—also known as the wearing course.

Wheel load: The total load transferred by one wheel of a vehicle.

Wingwall: The retaining wall extension of an abutment that is intended to hold the sideslope material in place.

WWPI: Western Wood Preservers Institute.

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