Several alternatives are available for the organization of the casting yard of an incrementally launched bridge. The basic decision between segmental precasting and in-place casting typically leads to in-place casting, unless the bridge to be launched is part of a large-scale modular project designed for precast segmental construction. Incremental launching construction of precast segmental bridges is indeed very rare nowadays.
In-place casting offers a great number of alternatives. Three solutions are available for casting of short segments (short-line casting and launching, long-line casting and launching, and short-line casting and launching after span completion), and three solutions are also available for longer segments (one-phase casting, two-phase casting in a single casting cell, and two-phase casting in a double casting cells). A different number of short segments may be used in the span, and long segments may be one-third of the span, one-half of the span, and even as long as the span.
The steel cage may be prefabricated fully or partially, or may be fabricated within the casting cell. The launch tendons may cross one, two or three segments and may be spliced with couplers or by crossing and overlapping. Launch prestressing may be conventional (internal launch tendons within the slabs), antagonist (combination of permanent and temporary tendons with antagonist layout and centroidal resultant), or a mix of the two. Integrative tendons may also be external or internal.
A great number of factors influence the cost-effectiveness of bridge design and project organization, and the break-even points are different for the same construction method in different countries, and often even in different regions of the same country. Some factors such as length and width of the bridge, length and uniformity of the spans, span modularity, plan and vertical curvature, time-schedule, and availability and access to the area behind the abutment, are related to the project. Other factors such as logistics and availability and cost of skilled labor are related to the project location. Ingenuity of designer and contractor, availability of resources, financial exposure and risks of the project, and contractor's propensity to risk, are other determining factors.
The investment for special equipment generates direct costs and financial exposure; however, the service life of special equipment is longer than the project duration, and different strategies are available to diminish the impact of the investment on the project. Leasing the equipment for the project duration, selling the equipment when no longer necessary, or modifying the equipment and reusing it on new projects are typical strategies. Local fiscal rules also play an important role by defining the number of years for full fiscal depreciation of the investment.
Computer simulations may be used to translate the uncertainties of the project into their potential impact on project objectives. Process analysis by computer simulation allows evaluation of construction processes, prediction of process performance under different conditions, analysis of functional relations, sensitivity analysis of process performance, process optimization, and bottleneck analysis. Computer simulation is a powerful tool for modelling construction processes and comparing alternatives, although its application within the bridge industry is still limited.
Process analysis is based on formulating a simulation model for the process, running the simulation, and analyzing and comparing the results. The process may be progressively refined and optimized during the simulation. General purpose simulation engines such as AweSim, GPSS/H and MicroCYCLONE have been designed for modeling and analysis of fabrication processes which are cyclic in nature. These programs are used to model construction operations that involve task interaction, and the resource flow routes through the work tasks are the basic rationale for sequencing of construction operations.
The simulation network for incremental launching construction of a prestressed concrete bridge involves three main operations:
- Preparation of the casting yard. This may include design and fabrication of inner and outer forms, extraction rails, launch nose and thrust systems; design and fabrication of rebar jigs, suspension frame and portal crane for cage delivery; design and construction of foundations and production support facilities; and shipping, assembly and site commissioning of equipment. In the most complex cases, the casting yard may also include batching plant, concrete delivery lines, and concrete distribution systems.
- Deck fabrication. This includes repetitive casting cycles of deck segments. Each casting cycle includes fabrication of steel cage and prestressing ducts, handling of external and internal forms, pouring of concrete, finishing of the top slab, segment curing, strand insertion and application of prestressing, and deck launching. Different deck casting processes include different number of segments, different duration of the casting cycle, and different resources.
- Deck completion. This includes, at every pier, deck lifting, removal of the launch bearings, installation and grouting of the permanent bearings, curing of bearing grout, and deck lowering on the permanent bearings. During bearing replacement, deck completion also includes application of integrative post-tensioning, tendon grouting and deck finishing.
The simulation network is identified through the tasks involved in every operation. Resources are assigned to the tasks, and the assignments are progressively calibrated during the optimization process. New tasks may be inserted into the process according to need. For example, two-phase casting in a single casting cell with cage fabrication within the casting cell, internal launch and integrative tendons, splicing of launch tendons by crossing and overlapping, no transverse post-tensioning in the top slab, and launching with rear thrust beam and draw-cables, includes 4 main operations and 62 tasks:
- Casting of bottom slab and webs. This operation includes 25 tasks: (1) cleaning, alignment and oiling of the outer form, (2) cleaning and lifting of the bottom form table, (3) cleaning, positioning and oiling of the rear bulkhead, (4) greasing of the extraction rails and placement of the launch plates, (5) oiling of the bottom form table, (6) placement of spacers for outer concrete cover, (7) placement of the outer rebar grid for bottom slab and webs, (8) placement of the support rebar for the top rebar grid of the bottom slab and for the ducts of the launch tendons, (9) fixation of the anchorages of the launch tendons to the rear bulkhead, (10) placement of pre-tested ducts for the launch tendons in the bottom slab and splicing to the ducts emerging from the front segment, (11) placement of the top rebar grid of the bottom slab, (12) placement of pre-tested ducts for the parabolic integrative tendons in the webs and splicing to the ducts emerging from the front segment, (13) temporary positioning of anchorages of the parabolic integrative tendons, (14) placement of the inner rebar grid of the webs, (15) completion of the rebar cage for pier diaphragm and tendon blisters, (16) application of the inner forms for webs, anchor blisters and pier diaphragm, (17) fixation of the anchorages of the parabolic integrative tendons to the anchor blister forms, (18) geometry control of the completed cage, (19) insertion of rubber pipes into the tendon ducts, (20) pouring and vibration of concrete, (21) finishing of the top surface of the bottom slab and of the construction joints at the top of the webs, (21) concrete curing, (22) removal of inner forms and rubber pipes from tendon ducts, (23) cleaning, inspection and repair of the interior surfaces, and (24) application of the support frames for the rolling form table for the top slab.
- Casting of the top slab. This operation includes 16 tasks: (1) rolling the form table from the front deck segment backward into position, (2) cleaning, oiling and lifting of the form table, (3) placement of spacers for concrete cover, (4) placement of the bottom rebar grid, (5) placement of the support rebar for the top rebar grid and the ducts of the launch tendons, (6) placement of scuppers and other embedded items that interrupt the top rebar grid, (7) fixation of the anchorages of the launch tendons to the rear bulkhead, (8) placement of pre-tested ducts for the launch tendons and splicing to the ducts emerging from the front segment, (9) placement of the top rebar grid, (10) placement of minor embedded items, (11) geometry control of the completed cage, (12) insertion of rubber pipes into the tendon ducts, (13) pouring and vibration of concrete, (14) screed finishing of the top slab surface, (15) concrete curing, and (16) removal of rubber pipes from the tendon ducts.
- Application of launch prestressing. This operation includes 10 tasks: (1) removal of the rear bulkhead, (2) stripping of the form table for the top slab and lowering onto the support rollers, (3) stripping and opening of the outer form, (4) stripping and lowering of the bottom form table, (5) bush-hammering and pressure washing of the joint surface, (6) cleaning of tendon anchorages, (7) cleaning of air vents and washing of the ducts, (8) strand insertion into the ducts, (9) application of anchor plates and wedges, and (10) stressing of launch tendons.
- This operation includes 11 tasks: (1) positioning and anchoring of the thrust beam to the rear end of the segment, (2) insertion of the draw-cables into the anchorages, (3) positioning of the thrust jacks, (4) distribution of personnel on the pier caps, (5) testing of the launch systems, (6) iso-tensioning of the draw-cables, (6) release of deck anchor systems, (7) staged launching, (8) final application of deck anchor systems, (9) release of the draw-cables, (10) removal of thrust jacks and thrust beam, and (11) inspection and repair of the outer surface of the segment.
Some of these 62 tasks are performed sequentially, while others may be carried out in parallel. Resources are assigned to each task in terms of labor, equipment and supervision, and process logic is assigned to each task in terms of sequence of execution and interference with other tasks. Instructions are formulated to model the direction of the resource flow between predecessor and successor tasks. Flow control statements are used to control task initiation, draw and release of resources, and conditional logical aspects that rule the process.
Simulation of deck construction shows the bottlenecks of the construction process. Cage fabrication and personnel available for launch operations are two typical bottlenecks. Sensitivity analyses can be performed to evaluate the process performance with different combinations of resources. For example, if the rebar cage is assembled by a subcontractor, the ironworkers will be available during launching, they may be included into the resources assigned to the launch tasks, and the simulation is re-run. Process optimization may also include doubling the prestressing crews, or adding a grouting crew.
The alternative casting processes for the deck segments are also analyzed and optimized. The results are compared in terms of labor demand, investment in special equipment, financial exposure and project duration to identify the optimum casting procedure for the specific bridge project.