Effects of fly ash on chloride migration in concrete and calculation of cover depth required against the corrosion of embedded steel reinforcement
N. Shafiq, Department of Civil Engineering, University Technology Petronas, Tronoh, Perak, Malaysia
This study investigated the effects of fly ash on chloride migration in concrete and calculation of cover depth in order to protect the embedded steel reinforcement against the occurrence of severe corrosion. A rapid chloride migration test was conducted for determination of the coefficient of chloride diffusion. The resulting coefficient of chloride diffusion was used for calculation of minimum cover depth required for a specified service life of concrete structure. Concrete made of 40 and 50% fly ash as partial replacement of cement resulted in a lower coefficient of chloride diffusion and hence a cover depth of 75 mm or less was calculated for a specified service life of 120 years. On the other hand a higher value of the coefficient of chloride diffusion was determined for 100% ordinary Portland cement (OPC) concrete, therefore a thick concrete cover such as 150 mm was calculated for a similar life span.
Chloride profiles and diffusion coefficients in structures located in marine environments
A. A. Di Maio, CONICET-LEMIT, Argentina L. J. Lima, University of La Plata, Argentina L. P. Traversa, CIC-LEMIT, Argentina
Steel corrosion is one of the main causes of damage in reinforced concrete structures. Recent studies carried out in Argentina indicate that the percentage of damage in concrete structures induced by corrosion of reinforcement is 16%. In structures located in marine environments the corrosion is due fundamentally to the action of chlorides. Chlorides penetrate concrete by different processes; in structures exposed to atmosphere, the ingress process is by diffusion. Ingress depends on the characteristics of concrete and of its distance to the sea (marine environment). In this paper, assessment of durability of concrete structures exposed to marine (Atlantic Ocean coast) environments is reported. The ages of the evaluated structures, bridges and buildings, vary between five and 67 years. Surface chlorides concentration (environmental loads) and the effective diffusion coefficient, calculated using Fick's second law, are included. Furthermore, the quality of the cover concrete is characterised by means of the specific gravity and porosity accessible to water methods.
A landmark cable-stayed bridge over the Charles River, Boston, Massachusetts
Vijay Chandra, Parsons Brinckerhoff Quade and Douglas, Inc. Anthony L. Ricci, Massachusetts Turnpike Authority Paul J. Towell, Parsons Brinckerhoff Quade and Douglas, Inc. Keith Donington, Parsons Brinckerhoff Quade and Douglas, Inc.
Boston, in the forefront of the American Revolution over two centuries ago, is now in the forefront of another revolution - in the field of cable-stayed bridge technology. A highly complicated structure, unique in the world, is complete. New technologies and innovations have become hallmarks of the Leonard P. Zakim Bunker Hill Bridge. This crossing of the Charles River at the north end of Boston was a challenging assignment from the start. It brought the community, engineers, and architects together to create a 'signature bridge' and a 'gateway' to the city. Located at a pre-eminent point where Paul Revere crossed in 1775, warning colonists that the British were coming, the bridge took on special meaning from a historical perspective. Numerous alternatives were studied for the crossing and the interchange configurations on both sides of the river. Eventually, the option known as the 'non-river tunnel' alternative was chosen, which required a ten-lane crossing of the Charles River. The ten lanes include four lanes each for Interstate 93 (I-93) northbound and southbound, as well as a two-lane ramp on the east side. Some impediments the bridge had to contend with included the Orange Line subway adjacent to and below the bridge; the close proximity of the Charles River lock and dam, and the need to maintain navigation; a major water main in the area of the south tower footing; a cantilevered two-lane ramp on only one side of the structure; the existing Storrow Drive ramps at the south end, dictating the arrangement of the stay cables in the back spans; and a new tunnel at the south end of the structure.
Boston's Central Artery/Tunnel project and its interchanges
Vijay Chandra, Parsons Brinckerhoff Quade and Douglas, Inc. Anthony L. Ricci, Massachusetts Turnpike Authority Hany Riad, Parsons Brinckerhoff Quade and Douglas, Inc. Jonathan Hren, Parsons Brinckerhoff Quade and Douglas, Inc.
Boston's Central Artery/Tunnel project is the largest infrastructure undertaking ever attempted at a single US location. It involves reconstructing Interstate 93 (I-93) by depressing it into a tunnel and subsequently removing the original elevated highway (known as the 'Central Artery'). The new I-93 will emerge from the tunnel near the south bank of the Charles River and rise to cross the river on a cable-stayed bridge. This structure, unique in the world, will be both longitudinally and transversely asymmetrical. The second phase of the project extends the Massachusetts Turnpike (I-90) through South Boston and under Boston Harbour to Logan International Airport. This paper reports on the current status of the project and its many interchanges.
Central Artery/Tunnel project: innovative use of precast segmental technology
Paul J. Towell, Parsons Brinckerhoff Quade and Douglas, Inc. Vijay Chandra, Parsons Brinckerhoff Quade and Douglas, Inc. Peter A. Mainville, Parsons Brinckerhoff Quade and Douglas, Inc. Elie Homsi, Rizzani de Eccher
The first part of this series of articles on Boston's Central Artery/Tunnel project presented an overview of the scope of the work and the many interchanges that form the hubs of the project. This second paper describes the innovative use of precast segments, as well as other construction techniques, on two major interchanges.
Advances in precast concrete in mixed construction
K. S. Elliott, University of Nottingham
Mixed construction is now being used in the majority of new multi-storey buildings, once the traditional domain of cast-in-situ concrete and structural steelwork. Mixed precast construction means the combined use of precast concrete with steelwork, timber, cast-in-situ concrete and masonry. The combination is made for the benefit of the building process at large and does not necessarily have to be designed and constructed compositely. Mixed construction maximises the structural and architectural advantages in combining components made of different materials, but it requires the cooperation of the architect, structural designer, services engineer, manufacturer, supplier and contractor. It is possible that some client and architectural demands can only be satisfied using mixed construction. In 2002 thefibCommission 6 on Prefabrication will publish a state-of-the-art report on the use of precast concrete in mixed construction. This paper summarises the report.
