Service Life Design

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

TOPICS HIGLIGHTED

WHAT IS SERVICE LIFE?

SERVICE LIFE CONCEPTS IN REINFORCED CONCRETE STRUCTURES

SERVICE LIFE, IS IT POSSIBLE?

DURACRETE – EFFORT TO PROMOTE SERVICE LIFE DESIGN

BUSAN-GEOJE FIXED LINK – IMPLEMENTING THE CONCEPT

HANGZHOU BAY BRIDGE – CHALENGGING THE FORCE OF NATURE

STURGEON RIVER BRIDGE: KEEP IT SIMPLE SOLUTION (KISS)

ALEXANDRA BRIDGE – SUSTAINED LONGER WITH PROPER CARE

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

WHAT IS SERVICE LIFE? A product's service life is its expected lifetime. It is the time for which MTBF (mean time before failure) applies and is usually 2 to 5 years for most consumer products (for example computer peripherals and components). Most items to which this applies follow a bathtub curve of reliability and the service life is the width of the well at the bottom of the curve. A product's end of life is determined by wear out mechanisms in certain components, other than random failures due to variations in component stress withstand as a result of small manufacturing defects and other than variations in the actual stress on the components as a result of unintended overloads. For mechanical parts like fans or automobile motors the end of life is determined by loss of material from these parts to such an extent that the intended operation is not possible anymore or with greatly reduced performance. For small fans in electronic equipment have a typical life of 10,000 to 60,000 operating hours.

Bathtub Curve The bathtub curve is widely used in reliability engineering. It describes a particular form of the hazard function which comprises three parts: o o o

The first part is a decreasing failure rate, known as early failures or infant mortality. The second part is a constant failure rate, known as random failures. The third part is an increasing failure rate, known as wear-out failures.

The bathtub curve is generated by mapping the rate of early "infant mortality" failures when first introduced, the rate of random failures with constant failure rate during its “useful life", and finally the rate of "wear out" failures as the product exceeds its design lifetime. The bathtub curve is often modeled by a piecewise set of three hazard functions, while the bathtub curve is useful, not every product or system follows a bathtub curve hazard function

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

SERVICE LIFE CONCEPTS IN REINFORCED CONCRETE STRUCTURES The definition of service life of concrete involves more considerations than thirty years ago. Among several reasons, this is because only mechanical parameters were taken into account by then and, nowadays, there are other like durability that are being commonly used to define service life. The coming of new materials, mixture designs and, in general, a new generation of concrete structures has leaded to solve specific problems with specialized work. Therefore, a difficulty to predict and validate future behavior of structures has appeared because the models used take into account a mixture of variables, methods, material, and stages of service life whose future behavior is hardly predictable. It was in the 80´s when Tutti (1982) thought about a concrete damage model and has the vision, at that time, of what we know as the initiation and propagation periods of structures damage. As a result of his work, several contributions to improve his model have been developed that incorporate the advances from then to now. This is why several codes about cement and concrete have adopted durability terms and consider, among other things that the service life of a structure finishes when the environmental aggressive agents reach the reinforcement. The truth is that there is neither a universal definition of service life nor the stages of the complete life of a concrete structure because several parameters are involved and there exists a lack of work on this regard yet. During the last 30 years the evolution of concepts has allowed interesting ways to define and calculate service life in function of the cracking or not of a concrete structure, to take into account the limitations of the initiation and propagation period, to validate probabilistic and deterministic prediction models, to introduce residual life models and definitions and, recently, to introduce durability indexes. The philosophy of the concept of service life has influenced the way the structures are designed. Nowadays, it is very common the design of High Performance Concrete Structures which use simple combinations of traditional and economical materials to obtain longer service lives than those traditionally expected. There are also authors with papers where the environment is taken into account as a variable that affect the service life of the structures. Some research also shows many papers dealing with predictions of service life based on the diffusion of aggressive agents as well as others that have no complete confidence with these predictions because of several limitations from the environment. The modern, reliability-based service life design for structures is implemented in most new designs and in re-design of existing structures and has been adopted by national authorities and individual clients in countries all over the world. Tunnels and marine structures are now usually designed for a service life of 100, 120 or even 200 years. This by far surpasses the assumed design life according to most codes and standards.

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

SERVICE LIFE, IS IT POSSIBLE? Service life prediction The AASHTO LRFD Bridge Design Specifications defines service life as the period of time that the bridge is expected to be in operation. The design life is defined as the period of time on which the statistical derivation of transient loads is based. Though the subject specifications prescribe transient loads based on a design life of 75 years, they are silent on the extent of the expected service life. A bridge's ability to fulfill its intended function can be compromised due to degradation. Major causes of degradation are high transient loads and severe environmental conditions. Proper structural design addresses the effects of transient loads through adequate member proportioning and design details. Environmental conditions that cause degradation include carbonation, sulfate attack, alkali-silica reaction, freeze-thaw cycles, and ingress of chlorides and other harmful chemicals. Adverse environmental conditions, if not properly addressed, typically cause chemicals to invade the concrete's pore structure and initiate physical and/or chemical reactions causing expansive byproducts. The most damaging consequence of these reactions is depassivation and eventual corrosion of reinforcing steel causing cracking and spalling of concrete. The end of the service life of the structure occurs when the accumulated damage in the bridge materials exceeds the tolerance limit. However, the service life is typically extended by performing periodic repairs to restore the serviceability of the structure. An initiative of applying this concept has been taken by Oregon Department of Transportation (ORDOT) where they consider designing bridges with an expected service life of 100 to 120 years as it looks towards the future. Engineers who plan, design, and build bridges and highways usually think of 40, 50, or maybe 75 years for the expected service life of structures. But why not consider longer time frames? Investing in the Future ORDOT is undertaking a bold investment in their future highway infrastructure. Using a time horizon of 100 plus years, a design must pass two tests – it must be economically justifiable and technically feasible. Building a bridge for 40 years and then coming back and building a second bridge for another 40 years, and a third bridge for 40 years, involves the cost of building that bridge three times, interruption in service, public safety, and the maintenance costs. Therefore, it makes good engineering and financial sense to build the bridge one time for a life of 120 years, by investing the money up front.

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

A bridge when there’s a road beside it? Not investing for future.

Durability The key technical challenges to extending the life of bridges are corrosion resistance, freeze-thaw durability, and surface abrasion. The climate requires extensive use of ice melting chemicals. In addition, ORDOT highways receive significant studded tire traffic during the snow season. Silica fume concrete was found to have the best resistance to abrasion. It also offers reduced concrete permeability and improved corrosion protection for reinforcement. ORDOT believes, based on the number of bridges that they have built and the comfort level that the contractors have working with silica fume concrete, that there is no real additional cost in adding silica fume to the concrete. In fact, all ORDOT bridge deck concretes have silica fume at 3 to 4 percent of the total cementitious materials. Corrosion Protection ORDOT decided to look at an engineered approach for corrosion protection and to face the corrosion problem head-on. Two alternative approaches are used: o o

Completely mitigate the corrosion issue by using non-corrosive steel reinforcement and a very low permeability concrete Use regular uncoated reinforcing steel and introduce a cathodic protection system when corrosion starts

For complete mitigation of corrosion, ORDOT uses silica fume concrete to reduce chloride penetration and solid stainless steel reinforcement to prevent corrosion. With one mat of stainless steel at the top and one at the bottom of the deck, the stainless steel reinforcement will not corrode, even if the concrete becomes contaminated with chlorides. For bridges near the ocean,

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

stainless steel is used in both the deck and the concrete girders. For bridges further inland, stainless steel is used only in the decks.

Typical reinforcement bar (right) compared with the stainless reinforcement bar (top)

The cost of using stainless steel reinforcement in bridges ranges from 10 to 15 percent of the total cost of the bridge depending on whether the steel is used in the girders as well as the deck. But that cost yields a major jump from a 40-year service life of the bridge all the way to 120 years and beyond because stainless steel is never going to corrode, regardless of how much chloride is in the concrete. It realizes the need to invest more money up front to get a longer service life. The combination of silica fume and stainless steel reinforcing bars gives tremendous long-term economic advantage.

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

DURACRETE – EFFORT TO PROMOTE SERVICE LIFE DESIGN The objective of DuraCrete, an EU-funded research project is to use available knowledge of concrete deterioration through scientifically based modeling of the transport and deterioration mechanisms governing the service life performance of concrete structures. This includes a quantified classification of the aggressivity of different types of environment. Chloride induced and carbonation induced reinforcement corrosion have been the most advanced mechanisms in the modeling. The project resulted in the publication of a design guide for service life. It adopted the probabilistic approach by introducing the uncertainties of the input parameters and their distribution functions (figure below was one of the examples of input parameter affecting service life). The unique development is that the service life design approach is identical to the well-known method of designing structures for load, which then has been simplified to the so-called load-and-resistance-factor-design (LRFD) methodology. In the same way the DuraCrete approach can be presented in the framework of an LFRD procedure, just for design service life.

DuraCrete has been adopted as service life design basis for: o o o o o o o o

The large Dutch Tunnels including the world record Green Heart Tunnel The Malmø City Tunnel in Sweden The world record Sutong cable stayed bridge in China The Sitra Causeway, Bahrain The Busan-Geoje Fixed Link with a world record deep immersed tunnel and the bridges The Subaya Causeway, Kuwait The tender design of the Qatar-Bahrain Causeway The tender proposal for the Dubai Metro

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

BUSAN-GEOJE FIXED LINK – IMPLEMENTING THE CONCEPT The Busan-Geoje fixed link project comprises an 8.2km motorway link from Busan to the island of Geoje. The connection includes a 4 km immersed tunnel, the deepest in the world with 50 m water depth and two cable-stayed bridges, each 2 km in length. And the bridges and the tunnels of the Busan-Geoje Fixed Link project have to be designed to satisfy a service life of 100 years. According to the traditional design methods, concrete durability is ‘‘ensured’’ simply by adopting deem-to-satisfy requirements, which have no rational relationship to the service life and the service life as such is not defined in an operational manner. COWI (a leading consulting engineer firm) has been spearheading the international developments for rational service life designs of concrete structures, cumulating in a probabilistic based durability design approach called DuraCrete which is one of the first scientifically based recognized for service life designs. The DuraCrete approach has been adopted as service life design methodology for the reinforced concrete bridge and tunnel structures of the Busan-Geoje fixed link considering the governing deterioration mechanism, such as chloride induced reinforcement corrosion. Key durability factors governing the DuraCrete design methodology is the chloride diffusion coefficient and the age factor. Both parameters are functional requirements of the concrete specification of the project and have been determined and verified during a comprehensive pre-testing program of concrete mixes performed by DAEWOO Institute of Construction Technology (DICT). Completion of the link is expected in 2009, and total project costs have been estimated at $1.2 billion. Sunken Tube Tunnel The total length of tunnel will be approximately 4km and the immersed tunnel will be constructed at a maximum water depth of 50m - the deepest road tunnel ever constructed. 18 pre-cast tunnel elements will make up the tunnel, and will be floated into position and sunk into a pre-dredged trench. Challenges faced include ensuring water-tightness of the joints, preparing the tunnel supports and the actual placing of the sections.

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

Cable –Stayed Bridges Two cable-stayed bridges feature in this project: are two-pylon cable-stayed bridge with a 475m main span, 220m side span and 940m connecting span, and a three-pylon cable-stayed bridge with a 230m main span, 106m side span and 968.5m connecting span. The main cable-stayed bridges will be comprised of composite steel and concrete superstructures, and concrete towers linked by stay cables. Heavy caisson (large watertight chamber used for construction under water) foundations have been pre-cast and will be floated into place. At selected locations, piled foundations will also be used.

HANGZHOU BAY BRIDGE – CHALENGGING THE FORCE OF NATURE When it’s completed, the 36km long Hangzhou Bay Bridge will be the longest ocean-crossing bridge in

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

the world, spanning across the Hangzhou Bay on the East China Sea and crossing the Qiantang River at the Yangtze River Delta. The S-shaped Hangzhou Bay Bridge will be an important connection in China’s East Coast Superhighway. Starting in Jiaxing to the north, the bridge will end at Ningbo to the south. It will shorten the ground transportation distance from Ningbo to Shanghai by 120km and travel time from four hours to two hours. It will be a six-lane, two-direction highway with a 100km/h speed limit, and a 100-year, service guaranteed, and stayed-cable design. Construction work began in June 2003 and completion is scheduled for 2008. When opened to the public in 2009, it is estimated that the bridge will carry 45,000 to 50,000 vehicles per day in its first year of operation. Architecture, Design and Structure The Hangzhou Bay is a gulf in the East China Sea where one of China's natural wonders, the Qiantang River Tide, creates fast water and large waves. The area is also a typhoon prone zone. These factors made construction feasibility a major concern for the project and the plan was only finalized after nine years of consultation and over 120 technical studies.

The cable-stayed bridge design was selected for the project as it can withstand the adverse conditions, multi-directional currents, high waves, and geologic conditions at the site. The bridge structure has also been designed to seismic criteria and will retain integrity in earthquake conditions up to seven on the Richter scale. The 36km length will be of highway class road with six, 3.75m lanes, three in each direction. The overall width of the bridge will be 33m. Designed for 100 years of service life, the bridge has speed limits of 100km/h for the main spans and 120km/h for land approaches. The bridge has a height of 62m, enabling fourth and fifth generation container ships to pass through in all conditions. The total length of cable used in the project will be 32.2km. Construction Challenges

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

One major challenge faced by the project was the eruption of natural gas in a shallow layer along the bridge line. A special study was conducted and exploration was performed to investigate the distribution of the gas and the property of the soil during and after releasing the gas. The gas was released before pile driving to avoid any disturbance to the soil, collapsing of ground or eruption and flaming of gas.

Preparation of piles and process of transferring girders using giant floating crane

Mostly, construction activity will be performed on land and then the prefabricated components will be transported to the site for erection and final installation. For shipping and erecting the girders in sea, giant floating cranes with accurate anchoring devices and launching gantries are being used. Also, severe marine conditions caused difficulties in anchoring barges and construction vessels. Under turbulent tidal flow and typhoon influences, water flow currents are in the range of 2 m/s to 3.32 m/s at the Hangzhou Bay Bridge sites. The floating cranes can safely transport the 2,000t girder from the shore to the site and then anchor stably to erect and install the pre-cast concrete box girder.

Bridge Layout and Structures The Hangzhou Bay Bridge consists of nine sections. The first is the bank lead road to the north approach. The north approach rests on low piers with posttension concrete box girder spans spanning pre-stressed continuous concrete box-girders and drill-shaft pile. The north approach leads to the north navigable bridge; a cable-stayed bridge with twin diamond-shaped towers, double cable, and steel box-girders. The main span of the north approach is 448m. Including side spans the total length is 908m. The north high piers have continuous, 70m, post-tensioned, concrete box-girder spans with a total length of 1,470m. The middle bridge approach is laid on low

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

piers with 70m, post-tension, concrete box-girder spans with a total length of 9,380m. The south navigable bridge is a cable-stayed bridge with an A-shaped single tower, double-cable and steel box-girders. The main span is 318m, and the total length including side spans is 578m. The south high piers have continuous 70m, post-tension, concrete box-girder spans with a total length of 1,400m. The eighth section measures to a total of 19,373m, and is composed of three sections: • • •

6,020m in-water section with 70m girders and steel piles 10,100m mud-flat section with 50m girders and drill-shafts 3,253m land section with 30m to 80m girders and drill shaft foundations

The ninth section is Bank Lead Road at the south approach. Auxiliary Facilities and Structures At the middle of the bridge, there will be a 10,000m² service island for drivers to rest and enjoy a full range of services, including hotels, restaurants, petrol stations and a viewing tower. It is also expected the service island will become a tourist destination for watching the Qiantang River Tide. The service island will be built entirely on piers to avoid disrupting the tide. The bridge requires the installation of traffic safety devices, monitoring systems, communications equipment, toll plazas, power supply, lighting, and maintenance and office buildings. Two public parks are planned on each side of the bridge.

STURGEON RIVER BRIDGE: KEEP IT SIMPLE SOLUTION (KISS)

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

Located near Edmonton, Alberta, the Sturgeon River Bridge incorporates innovative features aimed at achieving a 100 year service life and meeting difficult geometric constraints. Integral abutment design, stainless steel-clad reinforcement, and high performance concrete were employed to achieve these objectives. The selected design consists of a single span 40m long low profile pre-cast, pre-stressed concrete girder bridge with high performance concrete in the deck and the girders. Girder Design Using high performance concrete in the girders allowed 15.2mm diameter strands to be substituted for the standard 12.7mm diameter strands, thereby, increasing the girder's capacity. Thirty-eight strands were required for each girder. A concrete strength at transfer of 45MPa and design strength of 65MPa were specified to meet the requirements. The girders were fabricated in Calgary and transported more than 300 km to the site by truck. The fabricator was able to achieve a one-day construction cycle for each girder and complete the fabrication ahead of schedule. There were initial concerns over transportation of the slender girders but no problems were encountered during fabrication or erection. Bridge Deck Design The most common service failure for bridge decks is corrosion of the top layer of steel reinforcement. Traditional methods of prolonging the onset of corrosion include reducing the permeability of the concrete, coating the steel with epoxy, or coating the top of the deck with a water proof layer. All these methods have been effective but have not yet provided the desired increase in service life. To assure a 100 year service life, a more innovative deck design was required. Rather than focusing on methods to protect the top layer of steel reinforcement, use of stainless steel-clad reinforcement was specified.

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

Stainless steel differs from regular steel in that it is essentially neutral in an alkaline environment. The chromium content of the steel allows the formation of a self-healing chromium oxide film on the steel surface. Stainless steel-clad reinforcement combines a stainless steel outer layer with a carbon steel core to provide the benefits of stainless steel at a reasonable cost. This type of reinforcement is produced at a cost about double that of epoxy-coated reinforcement. The concrete deck consisted of 50 MPa compressive strength high performance concrete with a water-cementitious materials ratio of 0.30 and incorporating both silica fume and fly ash. This provided a high quality deck with greater resistance to carbonation and a minimum 100 year period for chloride ions to diffuse to the unprotected bottom layer of deck reinforcement. The deck finishing proved to be more challenging than expected. Alberta Transportation's specifications require the use of magnesium floats to finish the concrete. For this sticky concrete mix, stainless steel floats seemed to reduce surface tearing and provided a superior finish. The new Sturgeon River Bridge was completed in 16 weeks at a unit cost lower than the average unit cost for bridges of this category in Alberta. High performance concrete allowed the difficult constraints of the project to be met and will contribute to the long-term durability of the bridge. However, it is not the use of high-tech materials or the innovativeness of any single feature alone that provides an outstanding solution but the careful selection of technologies that complement each other to achieve the challenging goals of this project.

AUTHOR: AU YONG THEAN SENG www.madisonvelocity.blogspot.com

ALEXANDRA BRIDGE – SUSTAINED LONGER WITH PROPER CARE Background Alexandra Bridge, also known as the Interprovincial Bridge, links Sussex Drive in Ottawa to Boulevard Saint-Laurent in Gatineau. It provides a link between the tourist attractions of the Byward Market, Rideau street area and the Museum of Civilization area. The structure has been designated by the Canadian Society of Civil Engineers as a "National Historic Civil Engineering Site". The bridge is owned by the Government of Canada and operated and maintained by Public Works and Government Services Canada (PWGSC). Traffic Volume The Alexandra Bridge presently carries 10% of the inter-provincial bridge traffic within the National Capital Region with approximately 18,000 vehicles per day crossing the bridge. The bridge also carries 1,300 cyclists and 2,000 pedestrians per day. This represents the highest use of all inter-provincial bridges for cyclists and pedestrians. Planned Rehabilitation Though the structure is more than 100 years old, comprehensive detailed inspections, done every two years, along with ongoing maintenance have ensured the safety of the bridge for its given use. Built in 1900, it underwent a major rehabilitation in 1975, when all deck areas were replaced to accommodate the present lane arrangements and configuration. The bridge was repainted in 1995. While it has received ongoing maintenance to extend its service life, it is now in need of major rehabilitation and upgrades. The planned rehabilitation will: • • • • •

strengthen and retrofit the steel structure and piers and increase their seismic capacity; replace the 575 meters long center lane concrete deck and guardrails; replace the wooden boardwalk and railings; install inspection walkways Install damping mechanisms to address the vibration of the structure.

Alexandra Bridge has proven to us that there is a possibility of success in service life design concept provided that proper maintenances and retrofitting works are done on the structure itself on timely basis.