ACI 345R:2011 download free.Guide for Concrete Highway Bridge Deck Construction.
3.2—Concrete and reinforcement materials
Although the specific topics of material selection for concrete mixture proportioning and bridge deck reinforcement are covered in greater detail in Chapters 4 and 5. respectively, ii is important to emphasize the influence of material selection during the design process on the long-term durability of a bridge deck. Most modem bridge deck designs generally employ some strategy for deterring corrosion and enhancing exposure-related durability. These may include the use of epoxy-coated, galvanized, or metallic-clad reinforcenient: alternative reinforcement materials such as various grades of stainless steel, specialized steel alloy formulations: or fiber-reinforced polymer (FRP) reinforcement.
The use of better-quality concrete mixtures has gained favor, either separately from, or in conjunction with. alternative reinforcement strategies. Such strategies may include minimizing the water-cementi tious material ratio (w/crn) of a concrete mixture or the use of mineral admixtures, such as fly ash, silica fume, slag cement, or metakaolin. to reduce permeability characteristics of the concrete. Many other admixtures are commercially available to address workability and placement characteristics, resistance to freezing and thawing, and increased corrosion resistance. Other products are available to reduce susceptibility to plastic and drying shrinkage.
Careful consideration should be given to the selection of deck materials. One common myth is that compressive strength is the single most important factor in specifying quality deck concrete. In fact, concrete bridge decks composed of concrete with excessively high compressive strength tend to he less flexible, have greater shrinkage potential. and have less ability to redistribute load and thermal- or shrinkage-induced strains. The result is a greater tendency toward cracking, which leads to premature deterioration from the ingress of moisture and aggressive chemicals, such as deicing salts. Recently, many agencies have considered performance-based specifications that rely more on measures of permeability than strength as criteria for acceptance.
Alternatively, reinforcing materials such as FRP bars, which are not affected by chlorides, can be considered viable alternatives to ferrous reinforcing bars. The use of FRP bars is governed by the American Association of State Highway Transportation Officials (AASHTO) LRF1) design guidelines (AASHTO 1998) and by the Canadian Highway Bridge Design Code (CAN/CSA-S6-06) (Canadian Standards Association 2006).
3.3—Positive protective systems
3.3.1 O’er1ws—1’he common forms of bridge deck deterioration. such as scaling, some types of cracking. and surface spalling. generally occur within the top 2 in. (50 mm) of a deck. Improper concrete placing and finishing practices often result in a lower-quality concrete in this area.
to, and directly over, the top primary reinforcing bars, exposing them to attack from chlorides, moiswre, and air (Fig. 3.4.3). Furthermore, the tensile stresses caused by drying shrinkage are not uniform through the depth of a concrete slab, but are largest near the drying faces. Because flexural strength is not generally the dominant factor in reinforced concrete deck design. a more effective way to control or reduce the widths of drying shrinkage cracking is to place the shrinkage and temperature reinforcement in a more strategic location, which is above the primary slab reinforcement while providing minimum 2 in. (50 mm) clear cover.
3.4.4 Prestressed box beam bridges generally experience reduced tendencies toward transverse cracking because of their stiffness. Adjacent box beam superstructures with no space between the beams, however, often have thin, non- reinforced decks that frequently exhibit undesirable longitudinal reflection cracks over the joints between adjacent beams. One solution is to post-tension the beams together transversely and use a reinforced concrete deck on top.
3.4.5 A most important consideration for durability in bridge deck design is the thickness of protective concrete cover over the top reinforcement. It is recommended that 2 in. (50 mm) of concrete, measured from top of bar, be the minimum specified protective cover over the uppermost reinforcement in bridge decks, with provisions for variability during placement (Pfeifer et al. 1987). AASHTO (2010) requires minimum 2.5 in. (65 mm) clear cover for decks exposed to deicing or subject to tire stud or chain wear and 3.0 in. (75 mm) in coastal exposures. ACI 117-10 and the discussion on reinforcement in this guide provide recommended construction tolerances. Spalling generally occurs readily on decks having inadequate cover over the bars. Similar requirements for top, bottom, and side faces for reinforcing bar cover should be considered for highly corrosive environments. It should be recognized, however, thai specified cover depths are to be a l’uncion of in-place concrete properties, intended service life, and loading and environmental conditions.
Deviations from the specified cover should be expected to occur in construction. The designer should try to anticipate conditions that could make accurate reinforcing bar placement difficult, or where the desired concrete surface might be undercut by the action of the strikeoff, as at non-uniform sections of complicated geometrical transitions, and compensate with an increased cover requirement. Furthermore. field investigations have documented that clear cover depths in cast-in-place bridge decks vary consistently, even under favorable conditions, with a standard deviation of approximately 3/8 in. (10 mm)(Weyerset al. 2003).
When FRP bars are used, issues of concrete cover and crack widths are less critical. For GFRP reinforcing bars, minimum concrete cover is dictated by issues of potential reflective cracking due to differences in transverse thermal expansion with the surrounding concrete. A concrete clear cover of only two bar diameters is sufficient to avoid this phenomenon. Further consideration needs to be made to future rehabilitation, and milling of the concrete wearing surface.
through (c). The severity of cracking is conventionally expressed qualitatively as fine, medium, and wide, based on crack width.
ACt 201.1 R-08 defines cracking severity as:
I. Fine: Generally less than 0.04 in. (I mm) wide:
2. Medium: Between 0.04 in. (I mm) and 0.08 in. (2 mm) wide: and
3. Wide: Over 0.08 in. (2 mm) wide.
A survey by the Portland Cement Association (PCA) (1970) of randomly selected bridge decks in eight states provides some insight to frequency and causes of various categories of cracking. recognizing that most cracks are caused by a number of interacting factors. The survey found comparatively little longitudinal and diagonal cracking.
3.8.1 Diagonal cracking occurred most often in the acute angle corner near abutments of skewed bridges, or over single-column piers of concrete box girder. deck girder, or hollow slab bridges.
3.8.2 Transverse cracking was observed on about one-half of the 2300 spans inspected. No one factor can be singled out as the cause of transverse cracking. Among the more important factors were:
• External and internal restraint on the early and tong- term shrinkage of the slab: and
• A combination of dead-load and live-load stresses in negative moment regions.
In general. the observed crack pattern suggests that live- load stresses alone play a relatively minor role in transverse cracking.
A study of 72 North Carolina highway bridges was completed in 1985, shortly after their construction. The study sought to determine the frequency, extent, and cause(s) of transverse cracking in decks on steel and prestressed concrete girder superstructures of both simple and continuous design. in the first of two reports, the impact of construction and materials was investigated (Cheng and Johnston 1985) and in the second, the influence of superstructure type, deck casting sequence. and superstructure vibrations under load (Perfetti ci at. 1985) was discussed. The study found the most frequent transverse cracking occurred on continuous structures, most particularly those comprising concrete decks on steel girders. The casting sequence was found to have some intluence and seemed to relate to the development of residual stresses after placement. but could not be fully correlated with observed cracking. As one might expect. weather conditions conducive to high evaporation rates and thermal contraction contributed to higher incidence of cracking. Individual contractor practices were also a tictor, as certain contractors’ work appeared to be more prone to cracking, though causality was not clearly established.
A comprehensive investigation (Krauss and Rogalla 1996) of major factors that influence transverse cracking concluded that multi-span continuous composite steel girder bridges exhibited the highest severity of transverse cracking. Also, post-tensioned bridges had the lea.st susceptibility to deck cracking. The deck and girder shrink together. as well as the post-tensioning, inducing compressive stresses in the deck. It was concluded that simply supported spaits can
surface prior to application. Sand-blasting, shot-blasting. or hydro-blasting are generally preferred, although hydroblasting is not recommended before applying most polymer materials. Manufacturer’s recommendations should be checked. Shot-blasting involves less risk or human error than sand-blasting, and is often preferred. Surface preparation for Type Ill overlays is also dependent on the kind of membrane selected. Resinous membranes for Type Ill overlays may require the same degree of surface preparation as Type I and Type 11 overlays. Bitumen membranes may require only careful sweeping.
Type I overlays should be placed on a thy surface. The degree of surface dryness required for Type Ill overlays is dependent on the type of membrane material. Most polymers will not bond well to a moist surface. Asphalt will not bond well to a wet surface. In contrast, the emulsions often used with reinforced membrane systems may bond better to a moist surface than to a dry one. Manufacturer’s Instructions should be consulted.
Type I! overlays generally bond best to surfaces that are saturated surface-dry. For low-slump dense concrete and latex-modified concrete, a bonding slurry is typically broomed on just ahead of the concrete placement. The effectiveness of bonding slurries has been questioned. however (Silfwerbrand and Paulsson 1998).
The ambient temperature is significant for nearly all overlays. Virtually all common materials require temperatures above freezing. and most above 50°F (10°C), to affect proper cure. One exception is the prefabricated sheets.
In the absence of specific information, a good rule of thumb is that all Type I overlays bond best to a clean, dry. (except emulsions), and warm deck.
7.6.2.1 Type I overlays may be applied by spraying or pouring the liquid binder. Aggregates are then broadcast over the surface. Another method is to premix the aggregates and binder, and screed the overlay, sometimes in narrow longitudinal strips. Sometimes the premix system is preceded by a primer coat. Aggregates are typically broadcast over the screeded surface.
7.6.2.2 Type 11 overlays are usually applied by screeding in place. Low-slump overlays require mobile concrete mixers and special screeds. Other overlays placed at 2 to 4 in. (50 to 100 mm) slump involve conventional screeds. High- amplitude air screeds or the use of air screeds with mixture slumps higher than 4 in. (100 mm) are not recommended due to their effect on the concrete air-void system and resulting freezing-and-thawing resistance of the overlay. Concrete overlays with HRWRAs should not he over-vibrated or over-finished to avoid durability problems.