Cast in Situ vs Segmental Construction
Comparison of Cast in Situ vs Segmental Construction
The cast in situ construction of bridges is done on ground supported staging at project site while in segmental construction, precast segments of superstructure are brought at site and then launched and placed together with the help of launcher. The main advantage of segmental construction is the time saving of construction as segments are precast. However cast-in-situ segmental construction is also popular in long span bridges such as cantilever type construction. However the precast segmental construction is economical only when there is large number of span to be constructed in a single stretch because the launcher is very large. The cast in situ construction is useful in small projects where number of span is less. In cast-in-situ construction precast I-Beam with insitu deck slab is very popular. The cast in situ technique is very at interchanges where spans are curved. Segmental Construction is not feasible for curved spans having sharp radius. Time consuming is large in cast-in-situ construction as it involves following steps.
a) Erection on ground supported staging.
b) Casting of Concrete.
c) Dismantling of staging.
The cast-in-situ construction is preferable if there is no problem in diversion of traffic from project site and also in small projects.
Cantilever Construction is a method of progressive construction of a cantilever in segments and stitching them to the segments already completed by prestressing. It is an example of segmental construction. The cantilevering segments are constructed/erected from pier outwards one on either side and stitched back simultaneously. The segments normally 2.5 to 3.0 m long can be either cast-in-situ on traveling gantries or can be precast in yard and erected by launching truss or floating cranes. In Situ construction is economical only in case of bridges having fewer spans. Usually it takes about 4 months to complete a 120m long superstructure by cast in situ segmental method. Hence for bridges when many long spans are involved, prestressing can really speed up the work.
Though precasting involves additional investment as plant machinery and organization for a longer bridge this investment ultimately proves economical over the cost of time saved. The use of precast concrete segmental offers the following advantages.
(1) Control of high quality concrete.
(2) Manufacture of segments at casting yard instead of project site where the quality control may not be there.
(3) Great accuracy of section and profile can be obtained and problem of deflection during construction can be better overcome.
(4) Shrinkage can be practically eliminated and creep reduced by high quality of concrete.
Deep indentation shear key shall be employed at match cast joints at segments face in precast segmental construction. These shear keys shall cover as much area of the cross-section as possible. Shear keys in the webs shall be smaller in size and more in number whereas those in top and bottom flange may have larger sizes with lesser number. Shear keys shall be dimensioned in the form of trapezium.
Type and Shape Of Substructure
Plate type of piers is considered appropriate for bridges on river / stream / canal. Alternatively single circular piers with cantilever pier cap may also be used. Multi-column piers are not desirable in bridges over canals / streams as floating debris, trees, timber logs on the stream may get entangled in the pier frame resulting in obstruction to flow and hydraulic loss in case of canal structures. Piers shall be placed parallel to the flow direction under HFL condition. Suitable cut waters shall be provided at the ends to ease the water current force.
For urban interchange structures and road over bridges, the pier shape is extremely important from aesthetic considerations. The piers should look lighter and less massive. It may be useful to adopt multi-column circular piers in this cases. Elimination of pier caps by providing bearings directly over columns is also useful in achieving lightness and smooth transition between substructure and superstructure. In order to enhance the aesthetics, the columns may be provided with vertical grooves.
The abutments in case of bridges on canal / streams shall be solid cantilever type having a wall face in front. Spill through abutments may be provided in cases where the front of the abutment is well protected by means of a suitably designed stone pitching and launching aprons.
In case of urban interchange structures, the wall face of the abutment needs architectural treatment to break the feeling of massiveness. The wall face can be used to form sculptural treatment (say embossing logo of the concessionaire & client) or with vertical grooves.
Figures below shows the appropriate type of substructure for different boundary conditions.
Choice of Appropriate type of Pier
Safety Kerbs
The Kerb will be constructed with a height of 300 mm to ensure that trucks do not ride on the kerb and hit the railing. The kerb dimension will be 750mm x 300mm.
The safety kerb shall be designed as per cl-209 of IRC: 6-2000.
Wearing Course
Asphaltic concrete wearing course, 65 mm thick, as per the latest circular issued by MORTH/NHAI for National Highways, will be provided. It will comprise of 50 mm thick Asphaltic concrete laid in two layers of 25 mm each with an overlay of 15 mm thick mastic asphalt.The wearing course shall be as per MORTH Standard Book.
Note :- For the purpose of loading, the thickness of wearing coat shall be taken as 90mm
Unit Weight is taken as 22KN/m3
So Load = 22x(90/1000)= 1.98 KN/m2 = 200 kg/m2.