Assessment of influencing parameters on transmission length of prestressed concrete

Over the last few decades pretensioned, prestressed concrete has become very common in structural applications, particularly for bridge superstructures. Its extensive use, however, requires some important considerations on adequate bond development between concrete and prestressing strand. Structural design should be based on correct determination of the transmission length, identifying the end regions of the member needed to transfer the fully effective prestressing force into the concrete, by means of bond strength. Several investigations have been conducted in the past to measure the transmission length of prestressing strands for a variety of design variables, including strand diameter, strand spacing, concrete strength and cover thickness. A database of 535 transmission length specimens has been collected from many studies in literature. This paper presents a comparative study between recent proposals and current code provisions for calculating transmission length of prestressing strands. For this purpose, the current fib Model Code 2010 and American Concrete Institute (ACI) Code 318-14 are taken into account. It has been observed that such formulations, provided by the above regulations, do not always fit well the experimental results predicted by single authors, being in some cases not conservative. The effects of various important parameters on the transmission length of pretensioned, prestressed concrete components are therefore evaluated and discussed, to find out if they are reasonably included in the current design codes.