UndergroundQ We are a local agency considering the repair of a number of older culverts with shotcrete. Like most agencies, we are trying to be creative about maximizing our funds. We work with a federal agency when it is determined that a “new” culvert is needed. In other words, the agency will not pay for maintenance repairs but will pay for “new” culverts. We are wondering if the shotcrete method has ever been viewed as a means of creating a “new” culvert. Could the existing culvert be considered as merely a form for the new culvert? Could you also speak to the life expectancy of shotcrete (life cycle) versus a new concrete culvert?
Shotcrete has been used extensively for the purpose of relining existing culverts. You are correct to visualize the existing culvert as a form for building a new structure. Because it is a stay-in-place form, it may actually act as a composite structure. Shotcrete is a method of placing concrete and will have similar, if not better, durability and life span if installed professionally with good mixtures. You can locate numerous past articles on durability of shotcrete that have appeared in Shotcrete magazine in the magazine’s archive on the ASA website, shotcrete.org. Similar work has been done in California, Colorado, and other states. This approach is currently being used as permanent tunnel lining in many places, including many of the current New York Transit projects.
A knowledgeable contractor will develop a mixture and procedures to ensure that the timing of the subsequent blast is compatible with the set time of the shotcrete. Preconstruction testing should be required to establish the set time (both early and final set) to assist in developing the sequence of operations. The set time will also be impacted by the site conditions, such as temperature.