Category HIGHWAY ENGINEERING HANDBOOK

New Construction. Some agencies use asphalt concrete overlays on new decks and protect the deck with a waterproofing membrane below the asphalt overlay. Currently, however, concrete-wearing surfaces are more popular on new bridges than asphalt concrete. Concrete surfaces may be placed as an integral part of the structural bridge deck (monolithic), or placed as bonded overlays of various types including dense concrete, latex-modified concrete, and silica fume concrete (see Art. 4.5.1).

Repair of Existing Bridge Deck Surface. The concrete overlays discussed above can be used, in combination with patching of spalled areas, as a means of repairing deteriorated existing bridge decks. In addition to these overlays, which are at least 1/4 in (32 mm) thick and usually thicker, thin overlays 1/2 in (13 mm) or less thick are available. Binder materials include epoxy, epoxy-urethane blends, and polyester resin. Because of their thinness and light weight, they are advantageous for bridges where weight reduction is desirable, or where thicker overlays would present problems with expansion joint or scupper modification, or where railing height would be reduced more than an acceptable amount by a thicker overlay. In recent years use of methyl methacrylate (MMA)-type material has gained popularity due to its waterproofing properties coupled with extremely high strength and chemical resistance with an abrasion-resistant surface.

The repair of a spalled bridge deck involves removal of the fractured or disintegrated concrete by some means. Mechanical methods include scabblers, scarifiers, and jackhammers. Because these methods all tend to create microfractures in the sound concrete, a better method is hydroblasting, or use of very high pressure water jets. This method is selective in that it automatically removes unsound concrete while leaving the sound concrete undamaged. The operation consumes large quantities of water and is noisy, and passing motorists must be protected from stray jets and flying debris. The muddy effluent must be disposed of properly, and not allowed to flow into catch basins.

After removal of unsound concrete, or concurrently with it, the surface of the remaining good concrete is removed to a depth of about 1/4 in (6.4 mm). The entire surface to be over­laid is dried or wetted to the required moisture condition, and the overlay placed. Where deep removal areas are present, it is generally preferable to patch these areas in a separate operation from the general overlay. After texture is applied, the fresh overlay concrete is then given an appropriate cure of the required duration. In cold weather the overlay must be prevented from freezing. For this reason specifications require placement at temperatures well above freezing.

Concrete bridge decks designed by the current AASHTO method described above have large amounts of reinforcing steel. (Some say it is enough to drive on, and that the concrete is provided just to make the ride smoother!) In the past, in areas where deicing salts are used, corrosion of the top steel caused extensive spalling that led to premature repair or replacement of many decks. In some coastal areas, saltwater spray on the bottom of deck slabs has caused similar corrosion of the bot­tom reinforcing steel.

For the foreseeable future, concrete bridge decks will continue to be reinforced with steel bars, even though revised design procedures may be adopted that permit lesser amounts. Therefore, it will continue to be necessary to protect those bars against corrosion. Reinforcing bar corrosion can be prevented or forestalled by a number of means, including:

• Making concrete more resistant to penetration of chlorides (less permeable; see HPC in Art. 4.5.1)

• Preventing chlorides from penetrating the concrete by applying concrete sealers or waterproofing membranes

• Applying a physical coating to the bars to prevent contact between the chlorides and the bars

• Adding a corrosion-inhibiting admixture to the concrete mix

• Installing cathodic protection

Concrete Permeability. As discussed above, improvements have been made in the quality of concrete, and in the development of special concretes, in an effort to reduce the amount of chlorides reaching the reinforcing steel. These improvements themselves may be adequate to prevent premature corrosion, especially in areas where the applica­tion of deicing salt is moderate. In areas of greater rates of salt application, it may be necessary to provide supplementary protection of the types listed above.

Concrete Sealers. Sealers are available that can reduce the permeability of hardened concrete. Forms of silanes and siloxanes are among the best sealers. In some cases, however, the field performance of concrete sealers has not lived up to expectations based on laboratory testing. When selecting a sealer, one should avail oneself of the most current field evaluations of effectiveness over a reasonable period of time, and not rely solely on the claims of the manufacturer’s representative.

Waterproofing Membranes. Where an asphalt concrete overlay is placed on a bridge deck in an area where deicing salt is used, the salt will penetrate through the overlay unless an impermeable membrane is installed on the concrete deck. Both hot and cold rubberized materials are available, as well as more labor-intensive built-up systems. Built-up systems, like roof systems, combine layers of fabric alternated with applications of a bituminous coating. Built-up systems may cause the asphalt overlay to slide on steep grades or superelevation. All kinds of membranes are subject to development of blisters due to entrapped water vapor if the membrane cures before the vapor escapes. This can generally be prevented by placing the membrane when the temperature in the deck is decreasing, that is, during the late afternoon or evening, rather than in the morning or midday.

Epoxy-Coated Reinforcing Steel. The coating that has received the most widespread acceptance for physical encapsulation of the reinforcing bars is fusion-bonded epoxy coating. Some agencies require epoxy-coated bars for the top mat only; others require them top and bottom.

The epoxy coating is applied electrostatically in powder form to cleaned and heated bars in a continuous operation, and rapidly quenched immediately after being applied. The coating is quite hard, but must be handled carefully to avoid damage. Nylon slings are used to lift the bundles, and padding is used within the bundles. Specifications that allowed a small but liberal percentage of openings in the coating have recently been reexamined and tightened. Following the widespread adoption of epoxy-coated reinforcing steel, some unfavorable experience in marine structures has put somewhat of a damper on its enthusiastic acceptance.

A disadvantage of epoxy coating is that the coating reduces the bond between the bars and the concrete, requiring longer lap splices.

Galvanized Reinforcing Steel. About the time when many states began to install epoxy-coated bars for experimental evaluation, some states experimented with galva­nized bars. This was based partly on the contention of some corrosion experts that flaws in epoxy-coated bars would result in aggravated corrosion at those flaws. Pennsylvania adopted galvanized reinforcing steel for a time as its primary means of protecting the bars. Now at least one state has changed to that policy.

Galvanizing does not provide a permanent barrier, but creates a sacrificial coating, and consequently would be expected to have limited life expectancy when exposed to sufficient quantities of chlorides over a period of time. Because of the electrochemical nature of the way galvanizing prevents corrosion, it should not be used on only one mat of reinforcement.

Corrosion-Inhibiting Admixtures. Another means of protecting against corrosion of reinforcing steel, without application of physical coating to the bars, is the incorporation of a corrosion-inhibiting admixture in concrete. The amount of chemical added to the concrete mix is proportioned to the amount of chlorides expected to penetrate to the reinforcing steel. Therefore, the degree of effectiveness of the inhibitor is related to the accuracy of that prediction. Higher dose rates will provide greater protection, but at greater cost. A lower dose rate may not provide the necessary protection.

Some inhibitors have undesirable effects on other properties of the concrete, but one admixture that is effective without side effects is calcium nitrite.

Available publications do not provide specifications or guidelines for the evalua­tion and comparison of corrosion-inhibiting admixtures, requiring users to rely on information provided by product manufacturers. However, a National Cooperative Highway Research Program project is planned to develop test procedures to evaluate and compare the effectiveness of corrosion inhibitors, and to recommend performance criteria for their acceptance.

Cathodic Protection. Since rebar corrosion is an electrochemical reaction, an effective means of preventing or arresting corrosion is cathodic protection. The two main types of cathodic protection are sacrificial anode and impressed current. In the sacrificial anode system, disks of metal are installed at intervals in the deck before placement of the deck or overlay concrete. Corrosion activity involves the consumption of this metal rather than rusting of the bars. The impressed-current method requires the input of electricity, and therefore requires an electric source and is dependent on the continued monitoring and maintenance of the system. Power consumption is low. One reason cathodic protection was late in being implemented is that it involves the expertise of electrical or corrosion engineers rather than the structural engineers who are normally responsible for bridge design and rehabilitation.

During construction, bridge deck concrete can be supported by reusable wood forms, permanent stay-in-place steel forms, or precast prestressed-concrete planks. Where per­mitted, contractors will generally use stay-in-place steel forms rather than removable wood forms. Allowance must be made in the design of the bridge for the extra weight of the steel forms, and for extra concrete where required. The forms are corrugated. Where the bottom transverse bar spacing can be made the same as the pitch of the corrugated form, the extra concrete in the valley below the nominal bottom of the slab line is com­pensated for by the concrete displaced by the peak of the corrugation above the bottom of the slab line, and the allowance can be for the weight of the forms only. If the spacing is different from the pitch, which is usually the case for curved bridges, a greater allowance will be required because extra concrete must provide the necessary cover. For long slab spans, the stay-in place forms are corrugated, but with a flat top plate. In this case, no extra concrete is required, and the extra weight allowance is for the forms only.

Prestressed-concrete planks can also be used as support forms. In this case, the planks also serve as a component of the structural slab. Some agencies have used prestressed planks for years with success, but others have experienced problems—particularly, longitudinal cracking through the cast-in-place top slab over the ends of the planks at the supporting beams—and have discontinued their use.

When stay-in-place steel forms or prestressed-concrete planks are used, the slab overhang beyond the outside beam is generally formed separately using conventional removable wood forms.

AASHTO Standard Specification requirements for design of concrete bridge deck slabs on longitudinal beams are based on distribution of loads in the slab according to Westergaard theory and assume flexural action of the slab. On the basis of these specifica­tions, many states have developed design tables and charts for quick determination of slab thickness and both primary (transverse) and secondary (longitudinal) reinforcement. The main variables in the design of the deck slab are

• Beam spacing

• Concrete strength

• Weight allowance for future paving

• Live load (generally HS 25 or, LRFD, HL-93)

• Continuity factor for dead load

Applying the specifications, the simple dead and live load moments per unit width of slab are calculated. Dead load includes the client-specified future paving allowance, weight of any separate wearing surface, and weight of the deck slab including any monolithic wearing surface. Live load is the wheel load(s) of the client-specified HS or HL truck loading. The simple span moments are calculated for the design slab span length, and are then modified for continuity over the beams. For this factor, most states use 0.8 for both dead and live load, but some states use 1.0 for dead load. The moments are factored, and the slab is designed by the strength method using the slab thickness minus any monolithic wearing surface considered subject to loss due to traffic wear. Effective depth dimension d from the compressive face is usually different for the top and bottom steel, because a minimum cover of 1 in (25 mm) is permissible and generally adequate for the bottom of the slab but much greater cover, up to 3 in (76 mm), is specified for the top steel to provide pro­tection against intrusion of chlorides. The rebar diameter is usually different as well, since most agencies maintain the same spacing of top and bottom steel and vary the bar size. However, practices vary among agencies. For example, the New Jersey DOT keeps the bar size the same for top and bottom reinforcement. A uniform spacing makes bar placement and inspection easier and facilitates concrete placement. Secondary steel is provided in accordance with the specifications, with a lesser amount in the outer quarters compared with that in the middle of the distance between beams. But again, practices vary and some states prefer uniform spacing of secondary steel.

Slab overhang beyond outside beams is limited so that the reinforcement furnished for interior panels is adequate for the overhang, or extra reinforcement is provided if required. Slab overhang is sometimes also limited for construction reasons; the weight of fresh concrete on an excessive overhang, acting through a diagonal brace, can cause local buckling of unstiffened steel girder webs or can damage the web of a prestressed-concrete girder.

Some states such as Ohio DOT require that the top distribution reinforcement is placed above rather than below the primary steel. This practice was adopted in recog­nition of the fact that most deck slab cracking is transverse, and the distribution steel is more effective in resisting that cracking if placed closest to the surface.

Some states continue to use deck slab design tables developed using the allowable – stress design method, while other states have updated using the LRFD method. In addition, some states assign an allowable concrete stress that is less than AASHTO Standard Specifications would allow on the basis of the required 28-day strength of the specified concrete. These conservative practices reflect the prevalent attitude gained from a common experience of premature and extensive bridge deck deterioration, mostly in the form of spalling due to reinforcing bar corrosion. If the preventive measures now being taken prove to be effective in eliminating or greatly reducing this premature deterioration, those states will be more inclined to adopt less conservative design methods.

The design procedures described above have resulted in safe designs. However, research has determined that significant membrane action is present in interior panels, and actual stresses are considerably lower than design stresses calculated on the basis of flexural action. Following laboratory testing, the province of Ontario and several states have constructed and tested full-scale bridges with so-called orthotropic deck slab rein­forcement. In these designs, the reinforcement is the same size and spacing in both directions, and of a reduced total amount compared with designs by AASHTO Standard Specifications. These experimental decks have performed well, in most cases.

Bridge decks can be constructed of timber, concrete, or steel.

Timber Decks. For bridges on unpaved roads or low-volume roads in rural environ­ments, timber decks of modern construction, such as Glulam (glued laminated) decks, can be serviceable and durable. For high-traffic-volume highways, timber is at a great disadvantage because of high cost, difficulty of fitting to a variable support profile, lack of skid resistance if a separate wearing surface is not provided, and difficulty in maintaining adhesion of an asphalt wearing surface. The choice then is usually between concrete and steel, or a combination of both.

Cast-in-Place Concrete Decks. Where light weight or speed of construction is not prerequisite, cast-in-place concrete decks prevail because they easily conform to the top of the supporting superstructure and required surface profile. Cast-in-place con­crete decks also easily accommodate concrete sidewalks, median barriers, and outside safety barriers.

The durability of concrete decks became a matter of great concern after the use of roadway deicing salts became prevalent and decks began to develop spalls at an early age. Extensive investigation and development have resulted in adoption of improved design and construction requirements that show promise of extending deck life by preventing premature corrosion of reinforcing steel. Life-extending measures include greater design cover over the top reinforcing bars, tighter control of actual construction tolerances, use of lower water-cement ratio concrete, use of admixtures and special concretes such as silica fume concrete to reduce permeability, use of HPC, imposition of stricter curing requirements, use of epoxy-coated or galvanized reinforcing bars, application of various types of waterproofing surface sealants, installation of membranes or protec­tive overlays, and installation of cathodic protection.

Cast-in-Place Concrete Decks Composite with Precast Formwork. This type of concrete deck is constructed using half-depth, precast, prestressed panels as forms on which the remaining half-depth is cast-in-place. This type of construction, which elim­inates traditional formwork, is used by some states for both bridge decks and box cul­verts. The prestressed concrete panels are a minimum of 3V2 in (89 mm) thick, with greater thicknesses required for some beam spacings. The transverse prestressing strands are usually 3/8, 7/16, or 1/2 in (9.5, 11, or 13 mm) in diameter. Longitudinal reinforcement is usually No. 3 bars or equivalent welded wire fabric. The panels are designed to support the dead load of the panel, the weight of the subsequently cast portion of the deck, and any specified construction loads. In addition, the resulting full-depth composite section is designed to support the design highway live loads and any other dead loads, such as overlays.

Precast Concrete Deck Units. For rehabilitation of existing bridges requiring deck replace­ment, the use of precast prestressed-concrete deck units placed transversely across existing beams permits deck replacement at night or during other hours of reduced traffic volume. Only as much of the existing deck slab as can be replaced in a work shift is removed, and the gap between the remaining slab and the new deck units is minimized so that it can be bridged by a steel plate to maintain traffic. The deck units can be fastened by welding studs to the beams through formed holes in the deck unit, and by filling the holes with a fast-setting con­crete. Adjusting devices can be built into the deck units to control the deck profile. Longitudinal posttensioning can be used to ensure a tight deck at the grouted joints. The deck units can cantilever beyond the outside beams and have provision for barrier placement.

Composite Precast Concrete Deck Systems. A significant improvement over conventional composite construction is achieved using an upside down casting technique to create a com­posite superstructure composed of steel beams and a concrete deck. Also known by its com­mercial name, Inverset™, it results in a reduced superstructure depth because steel beams and concrete deck act as a composite unit to resist all dead loads. Units can be cast in lengths from 20 to 160 ft (6 to 49 m), depending on the capacity of the manufacturer, and utilized as span units longitudinally between abutments and piers or transversely between girders as decking. Similar to precast concrete deck, these units allow faster installation, which makes overnight deck replacements or over-the-weekend bridge replacements possible. Other benefits include minimal cracking and greater durability due to built-in prestressing, easy handling, and year-around installation. Closure pours in joints in between units, and longitudinal or transverse posttensioning would provide an integral deck system.

Steel Decks. Sometimes the weight of the deck needs to be minimized. This is true when replacing or widening the deck on an existing superstructure or substructure of limited strength. It is also important on movable bridge spans where every pound reduced in the movable span is accompanied by a similar reduction of counterweight. In these cases, steel decks can effectively reduce weight. The lightest-weight decks have been open steel-grid decks, but these decks often have an unpleasant riding quality when new, and can become slippery and unsafe with wear. Skid resistance can be restored by grinding grooves on the riding surface, which weakens the grid, or by welding studs to the surface, but the unpleas­antness of the sound and overall sensation perceived by the traveling public remains. Another type of damage to which open-grid decks are vulnerable is breakage of bars when chains, dragged from passing vehicles such as car haulers, become lodged in the grid openings. Open-grid decks are also prone to fatigue failure at the welds. The location and nature of the welds create a severe condition for fatigue susceptibility. For all the reasons above, open steel-grid decks are falling from favor.

Concrete-filled, partially filled, and overfilled steel-grid decks (now referred to as grid reinforced-concrete bridge decks) are also available and have often provided many years of service under heavy traffic with minimum maintenance. Where the concrete is filled only to the surface of the steel grid, wear of the concrete between the grid members, called cupping, can result in an unpleasant and unsafe riding condition. Therefore, overfilling is recommended.

In a few cases, concrete-filled steel-grid decks have been known to “grow,” breaking welds to the supporting members. After extensive testing and analysis, the cause was determined to be corrosion on the vertical interfaces between the steel and the con­crete fill. Although a very small expansion occurred at each interface, the accumulated expansion measured several inches at the ends of units. This phenomenon emphasizes the importance of preventing corrosion by any suitable means. In this case, the use of a corrosion-inhibiting concrete admixture seems appropriate

An attribute of the steel-grid deck, whether filled or open, is that it is or can be fabri­cated off-site, complete with concrete fill and wearing surface if necessary. This can be advantageous for speedy redecking where downtime must be held to a minimum, and may be a reason for selecting this type of deck even if weight reduction is not necessary.

A more recent (1980) variation of the concrete-filled steel-grid deck is the patented “exodermic” deck, where a thin reinforced-concrete slab is constructed on top of and made composite with the steel grid.

Another type of steel deck is the orthotropic deck, where the steel plate that supports traffic, and its stiffeners, are a part of the longitudinal load-carrying member of the bridge. Some of these decks have experienced problems with wearing surface adhesion, but the main reason they are not used more extensively is their high cost of fabrication.

Corrugated-Steel Bridge Flooring. Corrugated-steel bridge flooring, like stay-in-place steel forms but thicker (up to 3/8 in or 9.5 mm thick), can be used on bridges such as existing truss bridges where the tops of the stringers are at the same level transversely. The planks are usually galvanized. They extend the full width of the roadway but are narrow, and so can be erected without cranes. The planks are fastened to the stringer flanges by bolted clips or by plug welding in holes over the stringers, thus permitting installation by the owner’s forces. The deck is then paved with asphalt concrete. The valleys are filled first, and then the entire deck is overlaid, building in crown if necessary. To promote longevity of the plank and wearing surface, drainage holes are placed in the valleys of the plank. However, leakage of salt-laden water can corrode supporting stringers. Measures can be taken to prevent leakage, including seal-welding the seams, and eliminating the drain holes and waterproofing the entire plank surface before paving, but these measures can make this floor system costly.

Aluminum. A few bridges, including highway plate girder bridges and arch-type pedestrian bridges, have been constructed of aluminum. These bridges have generally performed well and have not required much maintenance. The plate girder bridges do not seem to have experienced problems that one might anticipate due to the difference in thermal coefficient between the alu­minum girders and the concrete deck. The main reason aluminum bridges have not captured a larger share of the market is high cost. Design specifications for aluminum bridges may be found in Guide Specifications for Aluminum Highway Bridges, AASHTO, 1991.

Aluminum railings, while not having the strength or ductility of steel, do not require maintenance painting. Aluminum posts are cast, and aluminum rail elements are extruded in shapes that are convenient for bolted assembly of the railing. Bolts to anchor aluminum railings in concrete parapets are generally stainless steel.

Rubber. Rubber, sometimes natural but more often synthetic, is used in bridge bearings and expansion joint sealing devices. Reinforced rubber sheets are used to fabricate troughs to conduct storm water that is permitted to flow through open expansion joints.

Stone. Stone is used in some states to face barriers and to provide waterline protection of piers. It is sometimes also used for aesthetic reasons.

Steel for bridges is available in several different strength levels, each of which may be specified under ASTM A709, Standard Specification for Structural Steel for Bridges.

The grade designations are indicated in Table 4.1, as well as some alternative specifications that may be more familiar. The grade designation indicates the specified minimum yield stress in kips per square inch, and a “W” indicates that it is a weathering steel composition. ASTM A709 contains supplementary requirements for notch toughness and other items that are available but apply only when specified by the purchaser. When such supplemen­tary requirements are specified, they exceed the requirements of the basic specifications such as A36 or A572.

The HPS designations indicate that the materials are high-performance steels. They are so designated because they possess superior weldability and toughness compared to conventional steels of similar strength. Grades 36, 50, and 50W are available either as structural shapes or as plates. The other grades are available only as plates. Grades 36, 50, and 50W are the most frequently used materials. In general, compared with A36 steel, where other limitations such as deflection or stiffness do not override, the extra unit cost of the higher-strength grades (50 or 50W) is more than offset by the higher – yield strength. Grades 70W and 100/100W have proven economical in longer-span structures, or the higher-stressed portions of medium-span structures. The AASHTO publication Guide Specification for Highway Bridge Fabrication with HPS-70W Steel suggests that economies can be achieved by combining the use of HPS-70W and Grade 50W steels in a structure. In a 181-ft (55-m) span bridge for the New York State Thruway Authority, the use of HPS-70W steel reduced the number of girders in the cross section from five to four, enabling a savings of 28 percent in weight and 18 percent in cost.

Weathering grades (50W, 70W, and 100W) have chemical compositions that pro­vide enhanced resistance to atmospheric corrosion. They can be used in the bare (unpainted) condition for bridges in many cases (see Art. 4.13). The savings on cost of painting and repainting frequently makes them an economical choice.

Although prices vary widely due to demand and availability, in reference to the unit price of grade 36 steel, the relative material price of the other steels in plate grades is approximately as follows:

As indicated, these are only price factors and do not consider the reduced quantity of steel that may be required as the yield strength increases. For structural shapes, grade 50 steel can usually be obtained for about the same price as grade 36 steel, but there would usually be some additional cost for grade 50W.

The cost of fabrication and erection for members of grade 36 and grade 50 or 50W steel is approximately the same. Thus, in preliminary cost studies, only the cost of the mill material for the members selected need be compared. Fabrication costs for grade 70W, grade 100, and grade 100W tend to be higher than those of the as-rolled products, and thus, the cost comparisons must include those costs.

Steels with greater strength than grade 36 tend to be economical for beams and girders in many cases, and are particularly attractive under the following conditions:

• When dead load is a major part of total load

• When deflection limits do not control

• When deflections can be reduced (composite design, continuous structure, etc.)

• When weight reduction cuts cost of foundations, shipping, etc.

• When selection avoids use of built-up members (cover plates, fabricated girder versus rolled beam, etc.)

The higher-strength steels often show advantage for tension members of trusses because the higher strength is used more effectively (for the entire depth of the member, because there is no stress gradient). The same is true for compression members of trusses where the member slenderness ratio is small to moderate (ratio of length to radius of gyration of about 80 or less, depending on grade).

The basic materials most often used to construct bridges are concrete and steel. Timber is occasionally used for deck construction and sometimes for short-span bridges.

4.5.1 Concrete

High strength is desirable for bridge concrete to reduce member size and weight, but durability is equally or more important. Component materials must be compatible with each other, and the concrete must have low permeability.

A long-term destroyer of concrete from within is alkali-silica reaction. While material specifications for concrete have been developed to preclude use of cement and aggre­gates that will produce alkali-silica reaction, the best prevention of this problem is the use of cements and aggregates from sources that have a known history of absence of this problem.

Given that required strength can be obtained by mix design with relative ease, low permeability becomes one of the most desirable properties, because bridge concrete is reinforced or prestressed and prevention of corrosion of the embedded steel is essential for long-term durability. Reduced permeability will also reduce carbonation, alkali-silica reaction, and freeze-thaw damage. Permeability can be reduced by proper mix design, by maintenance of a low water/cement ratio during concrete placement, by use of admix­tures, by compactive effort, by use of specialty concretes, or by application of concrete sealers or coatings. Often a combination of these procedures is employed.

Another desirable quality for the prevention of reinforcing steel corrosion, along with low permeability, is resistance to cracking. The cracking of bridge decks and of bridge deck overlays has been a persistent problem. The use of shrinkage-compensating con­crete using type K cement has been found to be effective in reduction of cracking, and some agencies mandate its use in construction of bridge decks. One property of shrinkage – compensating concrete that is different from regular concrete is the need for adequate amounts of mix water to cause the chemical reaction necessary for the development of the expansion. The normal rules of low water-cement ratio do not apply and must not be enforced. Another difference is that bleed water cannot be expected to appear. Waiting for bleed water to appear will result in the start of concrete hardening, making finishing very difficult. Agencies that have adopted shrinkage-compensating concrete have also adopted strict specifications for the production, placement, and curing of the concrete.

These specifications address the requirements peculiar to shrinkage-compensating concrete, but also include many requirements that are applicable to normal concrete as well. Perhaps the most important requirement is that prior to placement of the deck concrete, a preconstruction meeting be held, and that all participants in the cement manufacturing and concrete mixing, delivery, placement, finishing, curing, and inspection be required to take part. This meeting gives the type K cement manufacturer the opportunity to instruct the other participants in special requirements, and to correct any misconceptions that exist. Such meetings in themselves go a long way toward improving the quality of the concrete.

Admixtures. Various admixtures are available to enhance the properties of concrete made with the basic ingredients: coarse aggregate, fine aggregate, portland cement, and water. Admixtures may be classified as chemical admixtures such as air-entraining, water-reducing, set-retarding, accelerating, or superplasticizer; and mineral admixtures such as fly ash, silica fume, or slags. Mineral admixtures are usually added to concrete to improve workability, and resistance to thermal cracking and sulfate attack; and to reduce cement content whereas the chemical admixtures are added for entrainment of air, reduction of water or cement content, plasticization of the mixture, or control of setting time.

Air-entraining admixtures produce a distribution of bubbles that become permanent tiny voids in the concrete. This system of voids makes the concrete resistant to scaling, a surface failure that became frequent when deicing salts came into use. Air entrain­ment has virtually eliminated scaling. The use of air entrainment is recommended even with high-strength concrete.

Water-reducing admixtures make concrete mixtures workable at a lower water/cement ratio than is possible with use of “water of convenience” (water in excess of that required for hydration of the cement) alone. High-range water reducers provide great workability at very low water/cement ratios, and have been developed to provide reasonable control of duration of extra fluidity.

Dense concrete is a concrete developed by the Iowa Department of Transportation for overlayment of new and existing concrete bridge decks. It has a low water/cement ratio and relies on special compactive effort imparted by vibrating screeds to produce a dense concrete with reduced permeability.

Latex-modified concrete uses an admixture of latex, generally liquid styrene butadiene with a minimum solids content of 40 percent. It achieves a reduced permeability equivalent to dense concrete at a lesser thickness. This quality is important to the via­bility of latex-modified concrete as an alternative option to dense concrete because the cost of latex-modified concrete is higher on a volume unit measure basis.

Silica fume concrete, or microsilica concrete, incorporates extremely fine particles of microsilica. Added to concrete in powder or liquid form, it densifies the concrete, increases strength, and reduces permeability. Where silica fume concrete has been allowed as a contractor’s alternative option to latex-modified or dense concrete, it has rapidly replaced these other specialty concretes.

Calcium nitrite concrete contains calcium nitrite, a widely used inorganic corrosion inhibitor that acts at the surface of the steel reinforcement to limit the electrochemical reaction involved in the corrosion process. The calcium nitrite is added in liquid form to the concrete at a rate of 3 to 5 gal/yd3 (15 to 25 L/m3), depending on the quality of the concrete, the level of chlorides expected, and the life required for the concrete. (See Manual for Corrosion Protection of Concrete Components in Bridges, Task Force 32 Report, February 19, 1992, AASHTO, Washington, D. C.)

High-Performance Concrete. High-performance concrete (HPC) is defined as con­crete that meets special combinations of performance and uniformity requirements that cannot be achieved routinely using conventional ingredients, normal mixing and placing procedures, and typical curing practices. HPC offers many strength-related improve­ments such as higher compressive strength and modulus of elasticity, and lower creep and shrinkage. It also offers ductility-related improvements such as increased resistance to freeze-thaw, abrasion, and scaling, as well as reduced permeability. Potential benefits to owners include lower initial and life cycle costs as a result of lower construction costs, less required maintenance, longer structure life, and elimination of additional pro­tective systems. Also, there should be less disruption to the public due to the decreased maintenance requirements and longer periods between major rehabilitations.

HPC qualifies for the special federal funding allocated by the U. S. Congress (fiscal 1998 to 2003) for repair, rehabilitation, replacement, and new construction of bridges or structures that demonstrate the application of innovative materials. Missouri, Nebraska, New Hampshire, Texas, Virginia, and Washington are included in the AASHTO Strategic Highway Research Program (SHRP) lead state team for HPC implementation. HPC program bridges have already been constructed in several states including Texas, Nebraska, Virginia, Washington, New Hampshire, and Colorado. HPC bridge decks have been designed and either built or scheduled to be constructed in Ohio, New Jersey, and Puerto Rico.

HPC mixes must use pozzolan materials such as silica fines and fly ash. Pozzolans make concrete denser, thereby increasing durability. Silica fines eliminate the detri­mental effects of fly ash on concrete mechanical properties. Fly ash reduces the heat of hydration and thereby reduces plastic shrinkage. Mix design requirements vary, depending upon the application. For bridge decks, states have specified a 28-day com­pressive strength from 4000 to 8000 lb/in2 (27.6 to 55.2 MPa), and from less than 750 up to 2000°C for chloride permeability. For prestressed concrete beams, states have specified a 28-day compressive strength from 6000 to 10,000 lb/in2 (41.4 to 69 MPa), and from less than 1000 up to 2000°C for chloride permeability. The minimum specified for silica fines has ranged from 5 to 10 percent. Curing requirements have included a wet burlap covering within 10 min of finishing, and a 7-day wet cure followed by application of a curing compound.

HPC specifications typically also include production, placement, and curing trials. Testing programs include evaluations of strength, permeability, scaling, freeze-thaw, abrasion, elasticity, creep, and shrinkage. Not all tests are required for each application, but because many of the tests are of long duration, mix design may take 6 months or more.

Lightweight Concrete. Although the concrete most often used in bridge construction is normal-weight (hardrock) concrete having a unit weight, reinforced, of 150 lb/ft3 (24 kN/m3), lightweight concrete can be produced from manufactured aggregate that is available from several sources around the United States. The coarse aggregate is produced by heating shale in a kiln, which expands it. Lightweight fine aggregate can also be pro­duced but is not recommended for bridge concrete. Use of lightweight coarse aggregate can reduce the weight of reinforced concrete to 115 lb/ft3 (18 kN/m3). While the history of lightweight concrete for bridges includes premature failures, it also includes successful applications in both deck slabs and beams. It is important to recognize the different (greater) creep characteristics of lightweight concrete in structures where long-term deflec­tions are a significant design factor.

Bridge Width. Roadway width on bridges is the inside measurement to the bottom of the sidewalk curb or the bottom of the safety barrier. For bridges on roads where sidewalks are not provided, the bridge width is made equal to the approach roadway width including shoulders, so that the bottom of the barrier curb or the near face of the railing is aligned with the face of the barrier rail at the outside edge of the shoulder.

In the past, policy did not always permit full shoulders to be accommodated on bridges. Often the roadway was made narrower, particularly on longer bridges. This was done strictly to reduce bridge cost. From the traffic operations standpoint, however, it was an unwise practice. Disabled vehicles could not find refuge on the shoulder, and a full shoulder was not available for temporary maintenance of traffic during road rehabilitation or repaving. It is now recognized that a bridge is an integral part of a highway system when it comes to roadway width. The FHWA requires a minimum shoulder width of 8 ft (2.4 m) on each side of the roadway on federally funded projects.

Bridge Horizontal Clearance. For bridges over streams, the location of substructure units, and therefore the length of spans, is controlled by hydraulic requirements and by nav­igation clearance requirements established by agencies such as the U. S. Coast Guard and the U. S. Army Corps of Engineers. For bridges over navigable waters, the bridge designer should also consider the possibility of collisions from vessels. Refer to AASHTO’s Guide Specifications and Commentary for Vessel Collision Design of Highway Bridges.

For crossings of highways, the bridge columns or pier walls should clear the traveled way, shoulders, ditches where required, barrier rail, and any additional width required to provide a safe clear zone from edge of pavement. A minimum clearance of 30 ft (9.1 m) from edge of pavement is required except where this clearance is impractical, in which case the pier or wall may be placed closer to the edge of pavement, with barrier rail 2′-0" (610 mm) minimum from edge of shoulder, and pier or wall 2′-0" (610 mm) minimum from face of barrier rail. The barrier rail offset from face of pier or wall will be further controlled by the dynamic deflection of the particular system used. (See Chap. 6 for additional information.)

For crossings over railroads, the horizontal clearance requirements are usually set by the railroad company or by the state public utilities commission. In addition to clearance for safe operation of trains, including allowance for accidentally overhanging cargo, rail­road companies are cognizant of the importance of trackside drainage and require that drainage ditches be accommodated where present. In addition, a maintenance roadway for off-track equipment is often required. A horizontal clearance of 25 ft (7.6 m) from the centerline of the track is desirable and will obviate the need for pier crash walls.

If a pier adjacent to a railroad track is located closer than what is considered to be an adequate distance to prevent derailed cars from striking the pier (generally 25 ft (7.6 mm) from centerline of railroad track), the pier is required to be of heavy construction, or a sub­stantial crash wall is required to be constructed to protect the pier and prevent catastrophic collapse of the bridge. This wall should be aligned with the pier. For additional details, refer to the American Railway Engineering and Maintenance-of-Way Association (AREMA) Manual for Railroad Engineering.

AASHTO LRFD Specifications require abutments and piers located closer than 30 ft (9.1 m) to the edge of the roadway or closer than 50 ft (15 m) to the centerline of a railway track to be designed for a vehicular collision load defined in the specifications, unless protected by an embankment or a structurally independent, crashworthy, ground – mounted barrier.

Bridge Vertical Clearance. Generally, a clearance of 16 ft (4.9 m) plus an allowance for resurfacing should be provided over major state, U. S., and interstate highways, over the entire width of roadway. Over less important highways, a clear­ance of 14 ft (4.3 m) should be provided. These are AASHTO requirements. Published state standards, if different from AASHTO, should be followed.

The above vertical clearances apply to vehicular bridges. Because pedestrian bridges are narrower and lighter in weight, and therefore more vulnerable to major damage or collapse in the event of collision from overheight vehicles passing under the bridge, states are beginning to require an additional clearance of 1 ft (300 mm) for pedestrian bridges. This additional clearance is also recommended for overhead sign structures.

Vertical clearance requirements over railroads, like horizontal clearances, are set by the railroad company or state public utilities commission. A minimum clearance of 23 ft (7.0 m) above high rail is common for new bridges over nonelectrified racks. If the tracks are electrified, an additional 1-ft (300-mm) minimum clearance is required for catenary wires. Widened or rehabilitated bridges will generally be allowed to maintain the existing clearance, but no less.

Vertical clearance requirements over navigable waterways are subject to bridge permits by U. S. Coast Guard (USCG). USCG has established guide clearances for particular waterways. They are not regulatory in nature and greater or lesser clearances meeting reasonable needs of navigation for a particular location may be required or approved by USCG.

AASHTO Specifications. For many years, the basic manual for design of highway bridges has been the Standard Specifications for Highway Bridges adopted by the American Association of State Highway and Transportation Officials (AASHTO). These specifications permit use of either allowable stress design or load factor design. In 1994, however, AASHTO published a completely new alternative volume, LRFD Bridge Design Specifications. It was subsequently updated with a second edition in 1998; third edition in 2004; fourth edition in 2007; and annual Interim Specifications thereafter. Based on the load and resistance factor design method, the LRFD Specifications represent a major step in improved bridge design and analysis methods. It is anticipated that usage of the new specifications will lead to bridges with improved serviceability, enhanced long-term maintainability, and more uniform levels of safety. The initial volume resulted from a 5-year research effort conducted under AASHTO’s National Cooperative Highway Research Program. Independent consultants, technical representatives from various industries, AASHTO members, and other engineers par­ticipated in the effort to develop a draft document. Then the provisions were tested in trial designs at 14 AASHTO member departments before final specifications were adopted. One of the most useful feature included is a detailed commentary that explains the specification provisions and gives references for further study.

The use of the new specifications has been increasing throughout the country. The Federal Highway Administration (FHWA) and AASHTO have established a goal that the LRFD Specifications be used for all new bridges designed after 2007 and for all culverts, retaining walls, and other standard structures after 2010. Most state DOTs have adopted LRFD specifications for the design of their bridges so as not to lose funding for federally funded bridge projects. States unable to meet these dates were required to provide justifi­cation and a schedule for completing the transition to LRFD. The Standard Specifications would be applicable only to structures designed prior to 2007, and could be used for the maintenance and rehabilitation of existing bridges. No technical revisions will likely be made to the Standard Specifications in the future as its usage is phased out.

AASHTO specifications are developed under the direction of the AASHTO Highway Subcommittee on Bridges and Structures. This subcommittee consists of all bridge engineers of states of the United States and of Canadian provinces and officials of selected turnpike and bridge authorities. The specification development process is a deliberate one. Nevertheless, changes are made on a regular basis (some would say too frequently for the average bridge designer to stay abreast of them). Between new editions, revisions are published under the title of Interim Specifications. When identifying the AASHTO specifi­cations used for design on plans, some states refer to “AASHTO Standard Specifications for Highway Bridges, Current Edition.” A better practice is to refer to the specific edition, by number and year, along with any interims that were in effect at the time of design.

Unless there is a cogent reason for not meeting the minimum requirements of the AASHTO specifications, engineers designing bridges where they are in effect should apply and conform to them. Any exceptions should be noted on the plans. In case of litigation, one would have to explain why these recognized standards were not met.

Other AASHTO Publications. AASHTO offers numerous publications related to bridges and structures. A bridge designer should be aware of the availability of these publications and should use them where applicable.

In this chapter, references to the “AASHTO Specifications” or “AASHTO” will be to the AASHTO Standard Specifications for Highway Bridges unless otherwise noted.

Bridge Design Manuals. Many state departments of transportation publish bridge design manuals, which they develop for guidance of their own staff and consultants. States that do not have manuals often publish design memoranda. Before starting a bridge design project, a consultant should determine which of these aids are available, acquire and become familiar with them, and apply them in designing and preparing plans. Some state bridge design manuals are quite explicit, and are almost textbooks on bridge design.