Bond stress-slip of strand in prestressed concrete sleepers

Continuous vertical force on sleepers gradually degrades the bonding between the concrete and the prestressing strands. In particular, the vertical wheel loads can be significantly amplified due to irregularities on the wheel or on the track. This can eventually result in cracking or splitting of concrete along the strands leading to the structural failure of concrete sleepers. The bond between strand and concrete is an important factor which can directly or indirectly influence the cracking and splitting failure of the sleeper. This paper reports a series of pullout tests undertaken on specimens with 85 MPa concrete (nominal strength) and 7-wire prestressing strands replicating typical prestressed concrete sleepers produced in New Zealand. The results are discussed and compared in terms of the loads causing the first strand slip and also the bond stress-slip relationship. The results show that longer water curing time does not necessarily increase the bond between strand and concrete in prestressed concrete sleepers. This behaviour can be attributed to the effect of shrinkage and the trade-off between strength and shrinkage influenced by water curing. The obtained results can contribute to determining optimum sleeper curing time to achieve required strength and bonding and is valuable for concrete sleeper manufacturers where curing process is associated with considerable cost and logistical issues.