A bridge girder doesn’t start life on a bridge. It begins in a factory, typically far from water, traffic, and anything like the structure that it will come to support. The idea of precasting is that you will make the hard part in a controlled environment and then transport the finished product to site and drop it in place. The manufacturing of precast concrete bridge girders provides the explanation for its use for highway and railway bridges throughout the world.
From the tensioning of the strands to final shipment, it walks you through the process step-by-step, using industry terminology and industry sequence. No imaginary numbers, no assumptions. The way it really works on the casting bed.
Because Manufacturing Occurs Off-Site in the First Place Precast Concrete Bridge Girders
But it is not the same as your typical concrete pouring process that takes place at the bridge site, as precast prestressed girders are made off-site and then delivered to and installed at the job site, Structure Magazine reports. The whole approach is worth detailing because that one switch, of building the girder elsewhere first, is what makes the whole thing worth doing.
Consistent temperature, controlled humidity, and quality checks not possible at a job site can only be matched in the factory. Crews don’t have to battle weather conditions or work above ground on a bridge deck. They are not only creating a part that will always be the same strength but on a predictable timetable.
This Phase Involves Setting Up the Casting Bed and Formwork.The First Phase Is the Preparation of the Casting Bed and Formwork
The bed is made of a long reinforced steel or concrete shape that conforms to the cross-section of the girder. AASHTO I-girders and bulb-tee sections are common shapes, as is a widely-used design example published by PCI/Prestressed Concrete Institute that is a 72-inch-deep bulb-tee.
It is common practice in casting to cast a number of girders in a single long bed because, according to discussion from a PCI West Prestressed Concrete Bridge Workshop, this helps to minimise wasted prestressing strand across the production run. Before anything else, the formwork is cleaned, coated with a release agent and inspected for dimensional accuracy. Even a slight error will result in an improper girded form in later stages of bearing sitting.
The Next Step Is to Tension the Prestressing Strands
Here is where the “prestressed” component comes from the name of precast girders. The same PCI design example calls for 270-ksi (3,400 psi) 7-wire, low-relaxation steel strands, usually at 0.5 inch diameter, that are threaded through the form, and anchored at firm abutments on either end of the bed.
The strands are then stretched slightly using hydraulic jacks, to a designed tension, before any concrete comes near to them, as described by Structure Magazine. This is important because the whole performance of the girder is dependent on achieving this tension. If too small, the girder will not support its intended load. But more than this and the strand may be damaged or may even destroy the anchorage system.
Now for Step #3, Placing Reinforcement and Pouring Concrete.For Step #3, Placing Reinforcement and Pouring Concrete, It’s Time
After the strands are locked at the appropriate tension, reinforcing steel which is not to be prestressed is added if required, as well as embedded components such as lifting loops and connection hardware. The concrete is then poured around the strands and compacted to fill the form and to remove air pockets which would compromise the strength of the completed section.
The choice of the mix is very significant here. The PCI design example suggests that the 28-day concrete strength of girder concrete be in the range of 6.0 ksi or greater, significantly higher than the 4.5 ksi normally used for building concrete, since these concrete members are required to support the heavy loads of the highway or rail system over long spans.
Avoid the Use of Concrete.Do Not Use Concrete – Step 4: Curing the Concrete
Fresh concrete requires time and favorable conditions to gain strength, with precasters not having the time required for the 28-day curing period that concrete normally needs to gain its design strength. Steam curing is the answer. According to PCI Gulf South, low-pressure steam is injected into a controlled environment that promotes the hydration process allowing the concrete to achieve a usable strength much quicker than air curing.
According to PCI Gulf South’s overview of the curing systems, the producers typically need to achieve a minimum of 75% of the ultimate 28-day design strength within about 12 to 15 hours after casting before they can strip the form and proceed to the next stage. That is the speed which makes a precast plant capable of turning the beds, so that the casting bed can be used for the next casting job without being tied up for weeks.
Step 5 Detensioning (Strand Release)
Detensioning is possibly one of the most important steps in the entire process. For this pre-tensioning engineering guide, once the concrete has the desired release strength (usually in the range of 3,000 to 4,000 psi), the tensioned strands are released from the end abutments, typically by sequentially cutting the strands.
When the strands are free, they will want to return to their original lengths. The bond, however, has hardened in the concrete and instead of shrinking freely, they contract towards it, transferring compressive stresses through the bond. It is at this point that a plain concrete beam turns into a prestressed concrete beam. Strand end slip is measured at release and subsequently at various times to ensure that the force was transferred as intended; such measurements have been used in research on prestress losses in precast girders.
Step 6: Quality Control and Camber Verification
The bottom of the girder being compressed, the girder will naturally develop camber after being detensioned, which is a slight upward curve. Quality control teams test this camber and compare it to design tolerances as it must be on the roadway when the girder is installed.
Research published in PCI Journal indicates that some projects require precambered girders in which the formwork and strand layout are carefully designed to create a desired vertical curve in the girder itself. This technique allows for better conformance to roadway grade requirements and minimizes additional concrete requirements in the deck slab over the grade.
Step 7: Stripping, Storage, and Final Curing
After stripping, the girder may be left to continue to cure for a period under controlled conditions before deemed ready for shipping. For some plants, girders are stored under continued moist or steam to ensure quality, while others store girders under air-dry storage for the project schedule and site scheduling.
This storage time is more significant than it sounds. On larger projects, it is important to keep a close watch on the time of casting and curing conditions, because in published curing research there was a change in age, and slight difference in strength development per girder, by the time they were installed.
Step 8: Transportation and Erection
The last stage is transporting the completed girder from the plant to the bridge location; this is frequently done on a special trailer that has been engineered to accommodate the girder’s length and weight without damage to the member during transport. After transfer to site, the crane lifts the girder to its final position, it joins with other girders and, at a later stage, is tied to the cast in place deck above.
It’s here that the benefits of precasting can be most clearly seen. The girder was delivered cured and load tested at the plant, eliminating the need for erection crews to wait for concrete to become strong on site. They have installed a complete structural component and are working on the next span.
Common Shapes of Girder Produced This Way Precast Concrete Bridge Girders
The manufacturing process is similar for each project, but the final shape depends on span length and load requirements. AASHTO-type I-girders are still a popular standard for shorter to mid-length spans because they have a very simple, well understood cross section, and many casting yards can efficiently cast them to this standard. Bulb-tee sections, such as the deep profile BT-72 discussed above, take that ability further, and facilitate longer spans by focusing more material at the top and bottom flanges. On the other hand, box girders have a hollow cross-section, which decreases the weight with torsional stiffness, especially when the bridge is curved or skewed. The correct selection of the shape occurs much before the casting bed is even set up, as it dictates the formwork, strand layout, and amount of concrete for the entire casting run.
Final Thoughts on Precast Concrete Bridge Girders
The manufacturing processes of precast concrete bridge girders are ultimately a series of processes that follow the principle: carry out the difficult, precision work in the factory, and provide the finished, load-bearing product in the field. Each stage of tensioning the girder, pouring the concrete, steaming, detensioning and quality checks are important in the production of a girder which functions exactly as it was designed to do when it is loaded with traffic.
It’s not just an academic process for engineers and project owners. It’s also why precast girders are still the preferred choice for the construction of fast-track bridges, and why so much of the quality that ensures a bridge is safe is built before the girder ever arrives on site.
