Written on 19 août 2011.
Vijay Chandra, Parsons Brinckerhoff Quade and Douglas, Inc.Anthony L. Ricci, Massachusetts Turnpike AuthorityHany 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.
Written on 19 août 2011.
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 EccherThe 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.
Written on 19 août 2011.
K. S. Elliott, University of NottinghamMixed 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 the fib Commission 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.
Written on 19 août 2011.
Z. Smerda Technical University Brno, VIAPONT Ltd, Brno, Czech RepublicJ. Smerda VIAPONT Ltd, Brno, Czech RepublicThe rising volume of traffic on concrete road bridges has caused designers to consider the problem of permanent deflections caused by repeated traffic loading. However, it should be stated that the correct method for calculating such deflections is currently unclear and consequently gives rise to unrealistic estimations. Moreover, a lack of knowledge regarding the full spectrum of traffic loads and the effects of repeated loads upon concrete deformation exacerbate the problem. This paper attempts to determine the permanent deflection of prestressed concrete bridges without cracks.
Written on 19 août 2011.
Catherine G. Papanicolaou, University of Patras, GreeceThanasis C. Triantafillou, University of Patras, GreeceThis paper presents a minimum cost design procedure for precast structural sandwich panels made of HPC (high-performance concrete - i.e. high-strength and fibre-reinforced) faces and a pumice aggregate concrete (PAC) core, which might also include a layer of thermal insulating material. Attention has been focused on the selection of materials' strength and members' geometrical parameters, as well as on the material cost functions for both concrete types. The strength-based design case study resides with the format of Eurocode 2 and takes into consideration failure modes associated with in-plane loading, such as flexure, shear and local buckling of the compressed faces, with respect to the presence of the insulating layer. The solution of the optimisation problem is attained through the development of a computer program using available algorithm codes provided by Matlab®. The optimisation procedure results in the derivation of design recommendations, which encompass the objective of minimum cost for the elements under investigation.
Written on 19 août 2011.
Neil R. Short, Aston University, BirminghamJohn A. Purkiss, Aston University, BirminghamSarah E. Guise, Aston University, BirminghamPetrographic techniques have been developed to determine the nature and density of cracks that have developed after subjecting concrete to steady-state and transient heating regimes. The steady-state investigations showed that there was a good correlation between measurements of crack density and measurements of residual compressive strength. Crack density measurements taken on samples after transient heating have shown that the depth to which the compressive strength of the concrete is likely to have been significantly affected can be identified. Thus, this technique should provide very useful information for the evaluation of fire-damaged concrete.
Written on 19 août 2011.
Sérgio M. R. Lopes, University of Coimbra Ricardo N. F. do Carmo, Polytechnic of Coimbra This paper presents an experimental study on the bond of strands prestressed by pretensioning to concrete. Ten beams with different cover values were tested and the results are presented and analysed. In eight of the beams, transfer of the prestressing force was gradual, and sudden in the remaining two beams. A relationship between the transmission length and the bond draw-in at the end of the beam is proposed and compared with predictions found in the literature. The variation of the tendon force along the transmission length is also plotted on graphs and the results are compared with those proposed by Rôs. The comparison with Rôs' predictions shows that experimental values obtained in the current investigation deviate from the expected values by up to 10%.
Written on 19 août 2011.
G. De Schutter, Ghent University, Belgium K. Audenaert, Ghent University, Belgium J. De Rouck, Ghent University, Belgium LD-slag is an industrial by-product resulting from the transformation of hot metal into steel by oxygen refining. In order to avoid waste disposal, the incorporation of LD-slag in concrete armour units for breakwaters has been studied. Due to the higher density of LD-slag in comparison with traditional aggregates, a higher hydraulic stability of the armour units is obtained for a given unit size. Static loading tests showed that the mechanical behaviour of breakwater armour units made with the LD-concrete is comparable to the mechanical behaviour of units made with traditional concrete. However, durability problems might occur due to an insufficient volume stability of the LD-slag. To overcome this problem, treated slag has to be used.
Written on 19 août 2011.
Sérgio M. R. Lopes, University of Coimbra Ricardo N. F. do Carmo, Polytechnic of Coimbra This paper presents an experimental study on the bond of strands prestressed by pretensioning to concrete. Ten beams with different cover values were tested and the results are presented and analysed. In eight of the beams, transfer of the prestressing force was gradual, and sudden in the remaining two beams. A relationship between the transmission length and the bond draw-in at the end of the beam is proposed and compared with predictions found in the literature. The variation of the tendon force along the transmission length is also plotted on graphs and the results are compared with those proposed by Rôs. The comparison with Rôs' predictions shows that experimental values obtained in the current investigation deviate from the expected values by up to 10%.
Written on 19 août 2011.
M. Virlogeux, Consulting Engineer and Designer This paper aims to give an overview of the recent evolution in the design and construction of prestressed concrete bridges worldwide. Several major trends are evidenced. Certainly those which have the larger influence for the industry due to their wide applications are the development of external prestressing, now systematically used in some countries for medium span bridges; the emergence of high performance concrete which extends the possibilities at the same time as improving the durability of concrete structures; and the more and more frequent association of steel and concrete to constitute composite bridges of different types and composite elements in bridges, allowing for many innovative structures. Considering more specific applications, a section is devoted to cable-stayed bridges which received very interesting development during the last ten years; and another section evokes the more and more extensive use of heavy prefabrication in large projects, with elements up to several thousand metric tons. The paper concludes with a word on bridge architecture, showing that good structural designs can produce elegant prestressed concrete bridges.
Written on 19 août 2011.
Karin Lundgren, Chalmers University of Technology, Goteborg, Sweden Until recently, splicing of the reinforcement in frame corners had not been allowed by the Swedish Road Administration. Since this has led to reinforcement layouts that were hard to realise on site, the effect of splices placed in corner regions were examined. Four frame corners with differing detailing were tested. Furthermore, detailed three-dimensional non-linear finite element analyses of corners were conducted. The splitting stresses resulting from the anchorage were taken into account by the use of a new model of the bond mechanism. A parameter study of the importance of the loading conditions was made with two-dimensional models. The tests and analyses show that splicing the reinforcement in the middle of the corner has advantages over placing splices outside the bend of the reinforcement. They also indicate, in agreement with the previous analyses and tests, that provided the splice length is as long as required by the codes, there are no disadvantages in splicing the reinforcement within the corner of a frame.
Written on 19 août 2011.
Kjell T. Fosså, Norwegian University of Science and Technology, Trondheim, Norway Magne Maage, Selmer Skanska AS, Oslo, Norway Tests are carried out in a vertical slipform rig in order to identify the main parameters affecting the lifting stress (friction) during lifting of the slipform panel. The results show that there is an almost linear correlation between the net lifting stress and the effective pressure. This means that the net lifting stress can be calculated based on the effective pressure by using the friction law. The lifting stress is affected by both the particle shape of the aggregates, roughness of the slipform panel and the workability in the early phase. The results also show that lower air content and a finer pore system (finer particle size distribution and higher particle concentration) in the concrete will result in a higher maximum lifting stress (friction). Also a lower lifting height or lower frequency will increase the lifting stress. It is assumed that higher lifting stress will increase the risk for surface damages during lifting of the slipform.
