Abstract
Because of its high binder content and severe construction environment, early-age cracking is one of the most important threats to concrete used in continuous box-girder bridge structures. In this study, controlled permeable formwork (CPF) liner was used to mitigate the early-age shrinkage and reduce the early-age cracking risk of box-girder concrete. Early-age shrinkage was measured by a noncontact method and started at 30 min after adding mix water until 7 d. Internal relative moisture content and pore distribution tests were also carried out to reveal the working mechanism of CPF liner. The results show that covering the concrete surface with CPF liner decreased early-age shrinkage significantly. Under the temperature of 20°C and the relative humidity of 60%, two-surface-covering and three-surface-covering CPF liner on concrete decreased the shrinkage by 44% and 48%, respectively, at 7 d compared with concrete without CPF liner covered on it. The main reason is that CPF liner enhanced the internal relative moisture content and resulted in better performance of the surface concrete.
Highlights
Continuous box-girder bridge structural systems have been widely used and developed rapidly all over the world
Plastic shrinkage increased drastically followed by a short steady period, and it climbed gently until the end of the testing duration. e development of plastic shrinkage could be divided into three phases, the sharp-grow phase (Phase I), the slight fluctuation phase (Phase II), and the gentle-increase phase (Phase III). e Phase I duration of samples covered by controlled permeable formwork (CPF) liner was quite different from that of concrete without CPF liner
It could be noted that the shrinkage rate in Phase I increased with the increase of temperature and decreased with the increase of relative humidity because higher temperature and lower relative humidity resulted in a higher water evaporation rate
Summary
Continuous box-girder bridge structural systems have been widely used and developed rapidly all over the world. Because of the high binder content of box-girder concrete and severe service environment, early-age cracking of concrete is an important risk which threats the safety and durability of continuous box-girder bridge structures. Numerous techniques have been proposed to mitigate early-age shrinkage and cracking, such as optimizing concrete mix proportion [1], blending shrinkage-reducing agent or expansive agent [2, 3], applying internal curing with super absorbent polymer (SAP) or light-weight aggregate [4, 5], and moist curing [6]. Moist curing is one of the most effective methods to inhibit early-age shrinkage and cracking; it is not always suitable to cure the box-girder concrete. Moist curing is normally applied in the form of water spraying, ponding, or damp covering. It is difficult to cover the outer surface of the bottom plate damply, and the frequent wind in the service environment makes water spraying not feasible
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