How ABC can be applied to concrete construction to save time, money and improve safety.
Aging and deteriorating infrastructure is an ongoing safety issue in our communities. It has been a challenge for many owners to address the problems and meet the traffic demands of the public.
Existing structures are required to be inspected every two years, and conditions are rated per the National Bridge Inspection Standards (NBIS) by comparing the existing condition to its new condition. The rating system is as follows: 7-9 is good, 5-6 is fair, and 0-4 is poor. Structures with severe deterioration are either posted with a lower load capacity or are permanently closed and are categorized as structurally deficient (SD).
Bridges are also checked for issues related to the functionality of the existing structure. When the geometry doesn’t meet the current traffic demand or the current design standards, the structure is called Functional Obsolete (FO).
Based on the statistical data shown in Figure 1, in 2020, there were 45,031 bridges, or about 7.3% of the structures included, classified in poor condition. This substantial quantity requiring replacement or rehabilitation presents many challenges, specifically for structures needing replacement in areas with high Average Daily Traffic (ADT). In this case, Accelerated Bridge Construction (ABC) methods can help with a reduction in onsite construction time of about 90% and improves safety.
This article will compare the traditional concrete construction method of cast-in-place to the alternative precast ABC method. Methods are compared based on schedule, safety, traffic challenges, environmental impact, cost, and quality. An essential consideration for the design and construction of ABC products is also included.
Traditional Bridge Construction Challenges
Traditional bridge construction methods include a cast-in-place concrete substructure and a partial or complete superstructure. This method requires all construction activities to be done on-site, including form installations, material storage (i.e., reinforcement), concrete mix delivery, and pouring concrete. These activities present several challenges when replacing existing structures related to schedule, safety, traffic challenges, environmental impact, cost, and quality.
Challenges & Solutions
With all those traditional bridge construction challenges, there is a need to utilize an improved process and innovative system to address the need for aging infrastructure and improve safety. The ABC method is an innovative construction technique that consists of prefabricating bridge elements offsite and transporting them to the site for installation. Due to its benefits, ABC has gained immense popularity in the bridge construction industry in recent years.
Schedule
Traditional Construction – The construction duration for a typical traditionally constructed bridge can range from a few months to a few years on larger projects. The schedule is subject to various variables, including material availability, unexpected weather, and contractor staffing, all of which can cause substantial delays. Longer duration projects leave themselves open to an increased risk of safety and labor concerns.
ABC Construction – Bridge elements are fabricated in a controlled environment. This mitigates the weather impact and helps create a realistic and predictable schedule. Pieces are then shipped to the site erected in place and connected. This process reduces the construction sequence by eliminating the need for forms on-site and the curing duration. This also offers a reduction in construction equipment quantity and mobility. The most significant reduction can be seen in closure duration, which can be as minimal as a weekend for ABC versus two to three months for traditional construction.
Safety
Highway construction zones are highly hazardous to the traffic and construction personnel on-site. According to the federal highway (Figure 2), fatalities are increasing annually. The duration of construction directly impacts safety; longer construction duration leads to higher exposure to potential accidents. Several factors are attributed to this issue are the construction staging process, traffic pattern changes, narrow roads, limited mobility space for construction equipment, and traffic congestion.
ABC Construction – The project schedule is shorter, which increases safety by limiting the exposure of the construction personnel in the construction zone and the possibility of motorist accidents.
Traffic Challenges
Traditional Construction – Traditionally, phased construction is used for high ADT structures. Traffic flow is maintained during construction on a portion of the bridge or by installing a temporary bridge adjacent to the existing bridge. Even the best of these solutions negatively impacts daily traffic with increases in delays, congestion, and accidents, which increase with the construction duration. The additional traffic also affects local businesses, activities, and schools.
ABC Construction – The project schedule is shorter, which leads to a substantial reduction in traffic impact.
Environmental impact
Traditional Construction – Traditional construction methods face several environmental challenges, including material waste, wastewater generation, water contamination, noise, and air pollution. Construction equipment utilization is on-site longer, and a larger workspace is needed, resulting in an increase in land disturbance. If over a waterway, disturbances to navigation or biological species, including the possibility of endangered species, and construction time restrictions also impact the job.
ABC Construction – The project schedule is shorter, which reduces pollution and possible land and water passage disturbance.
Cost
Traditional Construction – While the construction cost of the bridge may be reasonably predicted, there are several less obvious costs that can be impacted by the construction method. Traffic can have economic impact on motorists taking detours and waiting for traffic delays to clear. Traffic delays impact commuters, neighboring businesses, and deliveries. Safety incidents cause delays and lead to short- to long-term staff losses, thus impairing product quality.
ABC Construction – Several factors discussed earlier can lead to substantial cost savings during the construction and service life of the structure. Another potential cost benefit is that future deteriorated or damaged pieces can be replaced with less effort. However, there are costs associated with the delivery and connection of ABC elements that are not seen in traditional construction. The cost should be finalized during the design process by ensuring the proposed structure is a good candidate for ABC bridge construction.
Quality
Traditional Construction – Cast-in-place construction relies on timely and quality delivery of the concrete mix. Weather delays can significantly impact the overall schedule by delaying concrete pouring, affecting concrete mix quality, and impairing the curing process. All of this can lead to poor concrete quality with the possibility of repair needed and deterioration during its service life.
ABC Construction – The quality is higher because all elements are constructed in a controlled environment without the risk associated with concrete mix delivery. This improves the piece’s finishing and the structure’s service life, requiring less maintenance. The quality of the structure depends on the design and handling processes, which require careful consideration.
ABC Bridge Design and Construction Concerns
Depending on the structure size, the bridge likely consists of multiple pieces shipped to the site, erected, and connected together. Ensuring that all benefits are achieved will require an effective design methodology, a high-quality control process, and careful consideration of means and methods. These design and construction concerns are divided into technical and practical categories.
Technical
- Camber is an essential design factor in prestressed superstructure elements in Girders and deck panels. During the design process, it should be carefully calculated and designed to avoid potential large differential that will lead to issues during construction and service life. Those issues include excessive grouting and alignment, which may cause potential deficiencies such as cracking.
- Post-tensioning is utilized to connect bridge decks and lock them together. The design drawings should include detailed post-tensioning, properties, sequences, and procedures.
- Deck joints located at the maximum negative moment area are subject to cracking; therefore, they should be carefully considered and designed.
- Repetition is critical to gaining manufacturing efficiencies that offer cost savings in materials, time, and labor. The design of the piece’s geometry, reinforcement, shipping hardware, and connections should be considered.
- Concrete strength should be carefully selected by considering the process of handling pieces for stripping, storage, and erection. A possible effective solution for closure pours is utilizing an alternative concrete product known as Ultra-High Performance Concrete (UHPC), which can achieve a strength of 18,000 psi and higher.
- Several connection types could be utilized, including.
- Grouted splice couplers that connect precast elements are used primarily in substructure connections. Selecting the proper coupler with adequate tolerance is crucial because it will allow for field adjustments without needing modification. Coordinating with the coupler supplier is an effective way to avoid misinterpretations of utilization and design challenges.
- Mechanical threaded couplers are mainly utilized for deck panel connections.
- Grout material specification and strength should be carefully considered and specified for each application.
- Shear pockets in the deck connect the bridge deck panels with the girders; this provides the composite action between the two members. Ensuring adequate reinforcing length, concrete covers, and grout strength are crucial to mitigating potential cracks and deterioration.
- Drawings must be thoroughly reviewed during design to avoid misalignments requiring field modifications. The drawings and specifications should include allowable tolerances for all design elements, hardware, and connection details.
Practical
- Finding local precast manufacturers/contractors with the capability and efficiency to produce and construct the proposed structure. Those include skills, equipment, and transportation.
- Careful consideration of the site conditions to ensure crane mobility and operational space. This includes overhead utility lines, nearby railroads, and site closures/detours.
- Prefabricated elements for roadway transition, including approach and sleeper slabs, should be completed during the same time frame as the bridge elements.
- Local restrictions and permitting requirements for shipping and handling include pieces’ weight and geometry to avoid possible oversized loading. A possible solution for the heavy-weight pieces, specifically the substructures, is to fabricate them with voids (i.e., utilize corrugated metal pipe).
- The void can be filled with concrete on-site and utilized as a connection.
- Another solution is to use lightweight concrete, which offers a weight reduction of nearly 20% or more.
Quality control
- Ensuring all connections are properly installed and connected.
- Ensuring elements are fabricated and constructed within allowable construction tolerance.
Conclusion
Several challenges exist in providing our communities and industries with an up-to-date infrastructure system that prioritizes improving safety. Projects that are ideal candidates for ABC technology can offer several benefits over traditional construction methods. Those benefits include reduced construction duration, lessened environmental impact, lower cost minimized traffic impact, and improved quality.■
References
“U.S. Bridges Rated in Poor Condition.” U.S. Bridges Rated in Poor Condition | Bureau of Transportation Statistics, www.bts.gov/browse-statistical-products-and-data/freight-facts-and-figures/us-bridges-rated-poor-condition. Accessed 16 June 2023.
“FHWA Work Zone Facts and Statistics.” FHWA Work Zone Facts and Statistics – FHWA Office of Operations, ops.fhwa.dot.gov/wz/resources/facts_stats.htm#ftn8. Accessed 16 June 2023.
FHWA, Accelerated Bridge Construction: Experience in Design, Fabrication, and Erection of Prefabricated Bridge Elements and Systems U.S. Department of Transportation, Federal Highway Administration, Publication Number HIF-12-013, November 2011.
Guidelines for Accelerated Bridge Construction Using Precast … – PCI, www.pci.org/PCI_Docs/PCI_Northeast/Technical_Resources/Bridge/PCINE_Accelerated_Bridge_Construction_Guidlines.pdf. Accessed 16 June 2023.
Prefabricated Bridge Elements and Systems Cost Study: Accelerated Bridge Construction Success Stories.” www.Fhwa.Dot.Gov/Bridge/Prefab/Successstories/091104/Index.Cfm. Accesses 25 June 2023.