Usually, structures are subjected to a combination of permanent and imposed loads throughout its service life. The time-dependent deformation of any part of the structure due to loading is regarded as creep. All concrete structures undergo creep as soon as they are subjected to sustained loadings (change in strain with time under sustained stress). If not subjected to sustained loadings, they still undergo some form of deformation which is known as shrinkage . Many studies have been undertaken in the past to understand the creep of plain concrete, the mechanisms
A.J. Babafemi • W.P. Boshoff (H)
© RTT.EM 2017 179
P. Serna et al. (eds.), Creep Behaviour in Cracked Sections of Fibre Reinforced Concrete, RTLEM Bookseries 14,
responsible for its time-dependent deformation and the factors influencing creep behaviour of concrete. Results of such studies have attributed the time-dependent deformation of concrete to the volumetric change in the cement paste content under applied load [2-4]. The rate of the volumetric change in the cement paste is a function of a host of factors which include the water-cement ratio, environmental conditions (temperature and humidity), applied stress level and volume and properties of aggregates used . Based on the type of loading condition (compression and tension), some authors have shown that for mass concrete, uniaxial tensile creep is somewhat 20-30 % higher than specimens tested in uniaxial compression under the same stress level . The creep of unreinforced concrete can lead to cracking, excessive deflection and other serviceability and durability issues.
On the other hand, when reinforcement such as fibres, which are now being increasingly used for reinforcing structures such as ground slabs, industrial floors, bridge deck, pipes and tunnel lining [5, 6] are introduced into plain concrete, the study of its creep behaviour takes a whole new dimension. Fibres are generally known to bridge cracks when they are formed in concrete, thereby leading to significant residual tensile strength of concrete in the post-crack state. Due to the action of the fibres bridging the cracked planes, energy absorption capacity is also significantly enhanced. With fibres having diameter less than 0.3 mm, improved durability properties of concrete have been reported due to reduction in crack width and permeability of harmful substances .
To be able to understand the time-dependent deformation of concrete reinforced with fibres otherwise called fibre reinforced concrete (FRC), tests would have to be performed in the cracked state as the fibres are only functional as crack-bridging material in the cracked state. Though a number of guidelines exist on the reasonable use of fibres in concrete, there is yet to be a guide on the creep behaviour of FRC in the cracked state. This has necessitated the recent rise in research activity on the creep of cracked FRC. Some of the leading works in this respect are those by . Whereas most of these works were carried out using steel FRC, only a handful have considered the creep of cracked macro-synthetic FRC. Also, studies on the uniaxial tensile creep of cracked macro-synthetic FRC are still very limited. One of the first published work on the creep of cracked macro-synthetic FRC is that reported by Babafemi and Boshoff .
Macro-synthetic fibres (diameter > 0.3 mm) are becoming increasingly preferred to steel fibres due to several reasons: easy to work in concrete, lightweight, resistance to corrosion and cheaper in cost. However, macro-synthetic fibre has an elastic modulus (3-7 GPa) far less than that of steel fibre (210 GPa) and responds in a viscoelastic manner (being a plastic fibre) under load. If this fibre is to be safely used for structural concrete, its long-term behaviour in the cracked form under sustained loadings must be understood. For cracked specimens of macro-synthetic FRC tested under sustained loadings, possible failure modes would be fibre pull-out and fibre rupture. An understanding of the mechanism responsible for the failure is also germane to a study of the time-dependent behaviour of cracked macro-synthetic FRC. Therefore, this study examines the creep response of cracked macro-synthetic FRC and the mechanisms responsible for the creep behaviour.