ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/768999
CONCRETE BRIDGE TECHNOLOGY Durability of Post-Tensioning Systems: The Details Matter by Timothy Barry, RS&H Today’s post-tensioning (PT) industry is facing scrutiny due to performance issue on projects that have been deemed to be in violation of project specifications. In some locations, owners are questioning the viability of post-tensioning rathr than investigating root causes and implementing improved procedures. With proper planning and execution, however, the industry has repeatedly shown that any PT project can be successful by implementing quality specifications, using quality materials, and performing the work the right way. Attention to detail matters during these operations, and it has been proven time and time again that shortcuts can lead to major problems. Preconstruction Commitment Like all construction projects, success depends on the level of engagement of all stakeholders. This starts with the owner and designer instituting the right specifications for the project. The PT industry has developed effective specifications over a number of years to account for many lessons learned. The Post-Tensioning Institute's (PTI's) and American Segmental Bridge Institute's (ASBI's) PTI/ASBI M50.3-12 Guide Specification for Post-Tensioning and the PTI M55.1-12: Specification for Grouting of Post-Tensioned Structures are examples of valuable resources providing owners a comprehensive specification that can lead to a quality final product. These specifications provide a starting point for owners and have proven to be effective through years of use. In these specifications are various personnel requirements for years of experience and specific training. These requirements, already implemented on numerous projects, have proven to be an effective way to achieve the desired construction quality. Owners should enforce similar requirements for inspection staff as well. The owner should have their own expertise, which is equally qualified, and that is looking out for the owner's long-term interests. Understanding the specifications and their specific intent goes a long way toward institutionalizing quality through construction. Grouting Operations A diligent commitment to proper grouting procedures has been shown to be one of the most effective ways to alleviate durability issues. The current PT industry specifications, when used properly, have proven to be very effective in achieving fully grouted tendons. All parties need to understand that there can be no shortcuts or missed steps when it comes to grouting procedures. This includes buy-in from the contractor’s personnel and the inspection staff at all levels of the project. This buy-in starts with the respective project managers from the contractor and the owner creating this culture of high-quality construction. It should be understood from the top that commitment to quality and adherence to proper procedures are expected. Inspection oversight of grout ing operations has become one of the most important jobs in the PT industry. There have been too many examples where flawed inspection, or simply lack of inspection, was noted on projects that experienced problems with tendon durability. No part of the grouting operation can be overlooked and strict adherence to procedures is the key. Like many things in life, preparation ahead of time improves the potential for success. For PT grouting, preparations include knowing the storage condition of the grout, tracking the expiration dates of the grout, and establishing the proper water-cement ratio, exact batch parameters, theoretical volume for the day’s operation, efflux testing of the mixed grout, unit weight of the mixed grout, inlet and outlet locations, and the sequence of operations. These are all items of information that can and should be determined long before the first batch of grout is mixed. Everyone involved in the grouting process must be aware of the details in the project specifications. Without exception, these details must be understood and followed explicitly. This translates directly to the project specific grouting plan that will be developed and approved to achieve the owner’s expectations. Development of the plan should address batching parameters, equipment, grouting sequence, methods of maintaining proper grouting pressures, material testing, and observation of the operations. This plan is also a way for the contractor to consider ahead of time how these important details will be handled.