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One third of the North American bridges are structurally deficient or functionally obsolete and $80 billion are needed for the repairing of these bridges (Cusson et al.-1999). In Canada the problem of effective rehabilitation and its funding represents a challenge to the entire infrastructure under the jurisdictions of the federal, provincial, regional and municipal governments and the private sector because only 17% of the bridges are acceptable and rehabilitation needs for the rest 83% are about $0.7 billion annually (Mirza and Murtaza -2003). A clears picture can be imagined by knowing that According to Transportation Association of Canada (TAC) the rough estimate of number bridges in Canada is 80000 with the replacement value of $ 35 billion and between 2005 and 2015 a percentage of annual increase in the replacement cost will take place.
Historically since the opening of Montréal's Metropolitan Boulevard in 1960 which was the first urban expressway in Canada, several significant highways and bridges were designed and constructed starting with Burlington Bay Bridge, the Angus McDonald Bridge and he Murray McKay suspension bridge in the 50's (Straka-2002).
The importance and urgency of the problem pushed the engineers and the decision makers towards an intensive study to define the technical meaning of maintenance and accompanying terms in order to specify the tasks of those who are involved in the problem. A primary evaluation of the bridge condition is needed to determine if it is structurally deficient i.e. having one or more deteriorated components (deck, girder, piers, etc.) resulting in restrictions on their use or functionally obsolete with an insufficient load-carrying capacity or unwanted geometrical characteristics .The professional definition of the remedial action is also needed to decide the appropriate response.
Various expressions can be employed here like the repair which means putting the bridge into action again or replacement which refers to the substitution of the damaged component .There is also the rehabilitation which has wide applications but mainly it can be defined as "modernization, alteration, or improvement to the existing condition of the structure or bridge system that is designed to correct deficiencies for a particular design life live load level" as per the definition of the Canadian Standardization Association (CSA). Other related expressions include the modernization i.e. updating by adding new features and the strengthening or retrofitting which is related to the load-carrying capacity or structural fortification of the bridge elements.
Ramcharitar (2002) stated that, many states and provinces in United States and Canada devote an amount of money for rehabilitate the deteriorated bridges. Theses bridges deteriorated due to the heavy traffic loads, old age, and weather condition, freezing, thawing and de-icing salt. These bridges were built using the ordinary reinforced concrete in the late 1950's and throughout 1960's. According to Hassanain (2003), approximately 30% to 40% of the bridges in North America have some form of deterioration on them. Canada has 80,000 bridges, over 50% of them are over 35 years old and the designed service life was 50 years (Lounis 2003). The United States have 42% of their bridges need major rehabilitation (Ahlskog 1990). Estimated the cost for these rehabilitation The FHWA estimated the cost for rehabilitate the deteriorated bridges is $92 Billion between the periods of 1987 to 2005.
The bridge deck is the physical extension of the roadway across the obstruction to be bridged. It is an important part of a bridge that is directly subjected to cyclic loading and harsh environmental conditions. Liu et al (1997) stated that the bridge deck is an important part of the bridge that is directly subjected to cyclic loading and harsh environmental conditions.
2-Delamination is the separation of concrete layers at or near the top of the outmost layer of the reinforcing steel parallel to the concrete member surface. The corrosion of reinforcing steel is the major cause of delamination .Tensile stresses, impact forces and freeze-thaw can also cause delamination of the concrete of decks (spartlin-2001). Delamination requires urgent repair in order to avoid the growing of the problem into Spalling.
3-Spalling is a defect usually occurs at the top of reinforcement .It is related to delamination and is accelerated by the increase in the use of chlorides caused concrete to pop out .The rate and severity of Spalling depends on the concrete cover, permeability of concrete, tendency of bridge deck to cracking and load intensity on the deck. Spalling can be detected in the form of horizontal cracks above the corroding bars.
4-Cracking is an incomplete separation of concrete because of tensile stresses. It can be horizontal, vertical, or random. There are many causes for over-tensile stresses and hence cracks such as the plastic shrinkage, the settlement, structural causes, reactive aggregate and corrosion. The corrosion and cracking enhance each other in the cause and effect.
5- Overloading: Ramcharitar (2002) said that today's trucks are substantially heavier, travel much faster, and induce greater impact forces on bridge decks and they account for a much larger percentage of traffic flow. Theses load increases have not only damaged the decks but also have contributed to reducing the fatigue life of deck girders. The most detrimental effect heavy load on bridges is in increase in stresses they apply on bridges.
6-Chloride contamination: Ramcharitar (2002) said that, because of the weather condition of North America, they use the de-icing salts as a mean of melting snow and improving road condition during harsh winters.
7- Honeycombing and air pockets: Xanthakos (1996) explained that these defects occur at the time of construction. Air pockets results when fresh concrete is not properly consolidated. Honeycombing occurs when there is spacing between the coarse aggregate particles, also when the cement mortar has not filled these spaces, these cause improper consolidation or leakage of mortar between sections of the formwork.
8-Other forms of deck deterioration including the wear and polishing which is related to the skid resistance of concrete , the honeycombing and air pockets which appear at the time of construction and affect the tightness of the concrete against the external causes of deterioration.
2.4 Different maintenance and repair activities
Generally, the structures do not perform satisfactorily for a long period of time. The period of accepted performance is called the service life of the structures. Bridges deteriorate with time. The decision between rehabilitation and replacement depends mainly on the economic considerations. The amount of work to be done, the economical life time of the bridge after rehabilitation or replacement and the compliance of the rehabilitated bridge with the old design/ detailing concepts are other aspects to be considered also.
In selecting the rehabilitation system, strength and serviceability, performance and durability, ease for rapid construction, minimum interference with traffic, minimum maintenance requirements, and life cycle cost are some effecting aspects.
Currently, many different repairs are used to maintain bridges based on materials, construction techniques, and types of damage.
Liu et al (1997) conclude that the conditions of bridge decks are assessed to be one of the following five deterioration levels,
Level I: deterioration is serious and is affecting the serviceability and traffic safety;
Level II: deterioration is obvious, and a detailed inspection may be necessary;
Level III: bridge deck satisfies the functional and structural requirements, but deterioration is aggravating, and further investigation is necessary;
Level IV: minor deterioration exists and needs to be put on record, but further investigation is unnecessary; and
Level V: the bridge deck is like new.
The deterioration degree is an index to quantify the deterioration condition of a bridge deck and has been adopted in much previous research.
Researchers and agencies have given different classifications and definitions to maintenance methods such as routine maintenance, repair, and rehabilitation. For simplicity, rehabilitation is assumed to be the only possible maintenance method in this research. Rehabilitation implies fairly major reconstruction of the deck and a large maintenance effort and sometimes closure of the bridge to traffic. Rehabilitation can be a measure to allow time to develop plans for deck replacement, and its effect is quantified in the deterioration model in this research by extending the life of the rehabilitated bridge deck.
Different researchers suggest different classifications and definitions of maintenance activities. There are four levels of maintenance activities that are defined by Harper and Liu et al.
1-Routine maintenance: activities that do not affect bridge structure and its function (e.g., timely cleaning and removing snow and ice).
2-Repair: activities that restore a good surface condition of bridge decks and prolong the life of them (e.g., patching and sealing).
3-Rehabilitation: activities that restore damaged structures and rehabilitate them to as-new condition (e.g., attaching additional girder or plates).
4-Major rehabilitation (Replacement): activities that completely replace components of the bridge decks with new ones when it is impossible to restore the function of the structure using repair and rehabilitation methods.
2.5 Different techniques of major rehabilitation (Deck Replacement)
When the volume of traffic on the deteriorated decks are very large with no good detouring routes, the replacement of these decks must be performed on a staged construction schedule with minimal traffic impact, i.e., the work must be accomplished in short time frames such as at night or on weekends.
Alampalli , Kunin (2002) stated that replacement is often a cost-effective option to improve the load carrying capacity. In certain cases, the load capacity of these bridges can be improved by replacing the heavy deteriorated concrete bridge decks with lighter decks.
Yehia et al 2007 conclude that deck replacement is a treatment option with the highest initial cost and this should always be treated as a last alternative. Both protective and non-protective treatments should be used to delay the deck replacement. Deck replacement may be the better option when the deterioration of the concrete is deep and greater than half the depth of the slab.
Also, the non-protective repairs were identified by the litreature that it does not provide any water proofing mechanism. Therefore, bridge decks have the tendency to deteriorate even after the repair. Protective repairs provide a water proofing mechanism that halt or delay the total bridge deck replacement. This kind of repair lasts longer than non-protective repairs and is always preferred if the funds are available.
Generally when replacing a bridge deck, three different techniques can be considered. These techniques are Precast, Exodermic and FRP. In the following sections different techniques are discussed.
There are different techniques of major rehabilitation such as the conventional (traditional) cast in situ concrete panels, precast prestressed concrete panels and use the new prefabricated polymers composites (FRP).
Jacobs (1984) stated that the replacement and rehabilitation costs were estimated based on the total surface area of each bridge deck and a nominal unit cost for completing the work.
2.5.1. Cast in-situ concrete panels
Using traditional approach, an old deck would be removed and a new framework of reinforcement put in place, with the concrete being cast onsite. The waiting time for curing is normally 4 weeks. In the traditional technique, after the existing decks are taken off, a new framework of reinforcement is tied into place and the concrete is cast on-site. Place reinforcing steel, and then pour the concrete in. After that, you have to wait for the concrete to cure and gain strength before you can put traffic on. At a minimum, it takes about 28 days.
22.214.171.124. Cast in-situ versus prefabricated bridge elements
Ralls et al (2002) acknowledged that the prefabricated bridge elements and systems can be constructed off-site and brought to the project location ready to erect. They can also be constructed adjacent to the project site but out of the way of traffic and then moved into position when they are needed. Their use provides bridge owners, designers, and contractors with options and advantages in terms of construction time, work-zone safety, environmental impact, constructability, quality and cost. The traveling public has lost patience with the extensive highway construction that is necessary today as the interstate highway system approaches its service life at the same time as urban congestion is increasing. Delays for delivery trucks and other commercial vehicles as well as for drivers traveling to and from their workplaces negatively impact commerce. Bridge construction can be a bottleneck because of its sequential nature, with foundations required before columns, and columns required before beams and deck. Innovative bridge design and construction that focuses on minimizing traffic disruption is needed.
Designers who choose prefabricated bridge elements and systems can effectively move a significant portion of the construction, i.e., the fabrication of components, away from the bridge site and the traffic that must maneuver through the construction zone. With the use of prefabricated elements and systems, time-consuming tasks such as formwork erection and removal, steel reinforcement and concrete placement, and concrete curing no longer need to be accomplished in the work zone. Conventional sequential processes can now occur simultaneously as components are fabricated off-site concurrently with on-site construction. Prefabricated components can be transported to the bridge site and quickly erected in place. The construction project timeline is thereby compressed and the traveling public experiences the disruption for a significantly reduced period of time.
Construction sites often require workers to operate close to moving traffic, over water, at high elevations, near power lines, or in other dangerous situations. Prefabrication allows bridge construction, whether concrete placement and curing or steel fabrication, to occur in safer surroundings, greatly reducing the amount of time that workers must operate in potentially dangerous settings.
Conventional bridge construction requires significant access underneath the bridge for construction personnel and equipment to perform the activities necessary to construct the bridge. Erection of formwork and placement of steel reinforcement and concrete necessitate access to specific locations. Using prefabricated bridge elements and systems gives the contractor more options and can reduce the access requirements underneath the bridge, thereby reducing the impact to the adjacent landscape. On-site construction time is also reduced. This flexibility facilitates bridge construction and can be especially beneficial in environmentally sensitive areas.
Many job sites impose difficult constraints on the constructability of bridge designs. Examples include heavy traffic on an interstate highway that runs under a neighborhood bridge, high elevations, and long stretches over water, and restricted work zones due to adjacent development. Using prefabricated bridge elements and systems allows much of the work to be done off-site, relieving constructability pressures.
Carter et al, (2007) stated that highway agencies commonly use cast-in-place concrete for building new bridge decks or replacing deteriorated ones. Cast-in- place decks have a few drawbacks, however, primarily as a result of the construction time and cost associated with forming, placing reinforcement, and casting the new deck. All of these on-site construction activities can translate into long road closure times and traffic disruption. An alternative to cast-in-place decks is prefabricated, full-depth precast concrete deck panels. These high-quality precast concrete panels are constructed off-site under controlled conditions and are brought to the site ready to be placed and connected. Using precast concrete deck panels requires significantly less on-site construction activity and ensures minimum traffic interference. In many situations, a bridge can only be repaired using night or weekend closures or staged construction (some lanes remain open to traffic while others are under construction) to maintain traffic flow in both directions.
2.5.2. Pre-cast pre-stressed concrete panels
The innovative precast deck panels consisted of a fabricated steel grid (the bottom portion) and a reinforced concrete slab (the upper portion). The top portion of the main bearing bars of the steel grid extend upward into the reinforced concrete component and act as shear connectors, assuring internal composite behavior. Advantages of this design include light weight, rapid construction, and efficiency of overall structural design.Precast concrete bridges have numerous advantages in terms of lower initial cost, lower maintenance cost, durability, fire resistance, and excellent riding characteristics. With the use of precast elements that same deck may take only a few days or hours for placement, and be ready for vehicle use immediately. Using prefabricated substructure elements reduces the heavy equipment required and the time that the equipment is onsite. The result is less potential damage to sensitive environments compared with conventional construction. Prefabricated elements and systems make bridge building less disruptive to the traveling public. (Minimized traffic disruption by facilitating replacement of the bridge superstructure without ever closing the highway to rush hour traffic), safer to build, less disruptive to the environment and make bridge designs more constructible.
Hearn et al (2006) stated that to rehabilitate and replace the decks of heavily traveled bridges, precast-prestressed concrete panels often are placed transversely on the supporting girders and posttensioned longitudinally. Portions of a deteriorated deck can be removed during night operations and the panels installed in time to open the structure to morning traffic. Other deck systems offer similarly rapid construction with the advantages of reduced dead load and enhanced durability.
Used of lightweight precast deck panels. Minimized traffic disruption by reducing construction time, and minimized equipment needed and dead load on the existing structure. This technique enables the bridge deck to be cast off-site in sections or panels. The panels then are transported to the site as soon as they are ready to be inserted. The fact that the casting is done off-site inside an enclosed building allows for better quality control. Use of the precasting technique allowed the project team the flexibility to carry out the work during lean traffic hours and not affect traffic during peak hours. The initial construction costs associated with precast, full-depth concrete deck panels are almost certainly greater than those associated with traditional cast-in-place decks. The added costs stem from the fabrication process as well as from shipping and placing the precast elements. On average, recent sources (Balakrishna 2006; Hayes, Seay, Mattern, and Mattern, 2007; Wenzlick. 2005) have reported additional premium costs for the precast deck panels on the order of $26/ft2 ($280/m2). The full-depth, precast lightweight concrete panels appeared to have performed well, with few maintenance issues observed. Reports of similar, more recent, projects have noted additional direct costs associated with precast deck systems on the order of $26 to $30 per square foot. However, sketchy information from those projects, as well as an analysis of the construction alternatives presented herein, demonstrates that use of precast deck systems for deck replacement of existing bridges can shorten construction time by several weeks or months and induce far less disruption to travel than the conventional cast-in-place alternative, resulting in a dramatic reduction in user costs. When total life-cycle costs, including those associated with road user costs, construction time, construction safety, and maintenance, are taken into account full-depth precast concrete deck panels are the more economical alternative.
The difference in road user costs can be compared between two construction scenarios: the factual case (precast scenario), in which precast concrete panels were used to replace the deck of the Woodrow Wilson Bridge; and a counterfactual case (cast-in-place scenario), in which the new deck would have been conventionally formed and cast in place. The road user costs accrue because of reductions in the average speed of the vehicles traversing the work zone and/or the queue that forms when the traffic flow exceeds the throughput capacity of the work zone. In the precast scenario, two lanes of the Woodrow Wilson Bridge were closed every night for about 8 months (Lutz and Scalia, 1984). In the cast-in-place scenario, Lutz and Scalia estimated that one-half of the Woodrow Wilson Bridge would have been closed more or less continuously for 12 months. For the purposes of this analysis, it is assumed that two 10-ft-wide (0.93 m) lanes in each direction would have been maintained through the work zone. As I-95 is a beltway rather than a radial route, the directional split is close to 50:50 and a three-lane traffic maintenance plan would have been inappropriate.