Phased Construction Bridges: Monitoring and Analysis for Traffic-Induced Vibration

Due to the current state of deteriorating infrastructure in the region and country, the number of bridges in the state and in the country in need of replacement is expected to increase. However, the complete closure of a traffic route to allow for the construction of a new bridge is often not feasible - particularly in rural Nebraska, in which truck traffic is limited to few routes and is critical to the economic vitality of the state. To address this need and reduce detours, phased (staged) construction has become a very prevalent practice for bridge replacement, which allows the bridge to remain partially open to traffic throughout construction. While phased construction can be interpreted as a very broad term, herein it is defined as the situation where one segment of the bridge is constructed adjacent to an existing segment. Typically, the number of traffic lanes is reduced to allow for partial demolition of the bridge. Then, a new segment of the bridge is constructed - termed the first phase. Once traffic is re-routed to the new segment, the remaining bridge is demolished and replaced - the new construction termed the second phase. In most situations, rebar extends from the first phase deck and is spliced to the second phase deck reinforcement prior to pouring of the deck. Despite the practical advantages of phased construction, bridges constructed in this manner have been observed to have several constructability, serviceability, and durability issues. The most common issue associated with phased construction is differential elevation, which occurs when the second phase deck does not vertically align with the first phase deck. If this differential elevation is greater than 2 inches (NDOT 2016), a closure pour is often required which extends the duration of construction and increases costs. A number of studies have focused on the problem of differential elevation determining that many factors contribute to the differential and that no one clear mitigation measure can completely eliminate it (e.g., Azizinami ni et al. 2003, Norton and Shane 2014, Mohammadi et al. 2014). A second issue widely associated with phased construction is premature deterioration of the second-phase deck and/or closure pour region. This premature deterioration is often evidenced by transverse cracking of the second-phase deck along most of the span. This extensive early-age cracking can substantially increase the costs associated with maintenance, repair, and rehabilitation over the lifetime of the bridge; and, therefore, there is a critical need to identify the causes of this early-age cracking and determine appropriate methods to mitigate premature deterioration of the deck. There are many reasons why a concrete deck cracks, including plastic settlement, thermal shrinkage, and heavy traffic loads. However, the widespread occurrence of early-age cracking in second-phase decks where similar cracking is not observed in the first-phase decks indicates that the cause of this cracking must be directly related to the phased construction techniques. One such potential cause and the major underlying premise of this research is traffic-induced vibration, which is defined as the transfer of vibration and relative motion from the first-phase deck (which is open to traffic) to the curing concrete of the second-phase deck. This vibration can be transferred directly by the supporting formwork, transverse diaphragms (if present), the embedded reinforcing bars, and edge of the first-phase concrete. This will have two primary effects: (1) vibration of the curing concrete, and (2) deflection of the spliced reinforcing bars. While it is widely recognized that vibration of concrete is necessary for proper consolidation and to achieve sufficient strength, revibration or the process of vibrating concrete that was previously vibrated does not necessarily improve the performance of reinforced concrete components. Specifically, revibration is expected to improve bond strength in high-slump concrete, but it may significantly reduce bond strength in low-slump concretes (American Concrete Institute 2005), similar to mix designs used for Nebraska bridge decks. More targeted studies for curing bridge decks have found that traffic-induced vibration causes closure pour regions to flex substantially which primarily results in excessive cracking with relatively minor reductions in bond strength (Kwan and Ng 2007a, Kwan and Ng 2007b). However, a more general study of bond strength due to differential deflection in 6-inch cube specimens observed reductions in bond strength for deflections as low as 0.05 inch (Federal Highway Administration 2012). In addition to impacts on the bond strength, a very recent experimental study found that prolonged revibration (e.g., 6 hours) of concrete cylinders reduced the concrete compressive strength as much as 25% and is a likely source of excessive cracking in bridge decks exposed to traffic-induced vibration (Hong and Park 2015). While it is apparent from the literature that traffic-induced vibration is a definite source of premature deterioration in phased construction, there is no clear method to mitigate this damage. This project will directly address this gap in knowledge by measuring existing levels of traffic-induced vibration in the field and directly implementing varying levels of this vibration in a laboratory experiment. Results of these experiments will provide clear guidance on how to mitigate the harmful impacts of traffic-induced vibration and enhance the durability of phased construction bridge decks. The primary objective of this research is to determine the amplitude, frequency, and duration of traffic-induced vibration that results in premature deterioration of concrete bridge decks in phased construction and identify methods for mitigating its effects.