Factors Influencing Creep of Cracked Fibre Reinforced Concrete: What We Think We Know & What We Do Not Know
Ravindra Gettu, Raul Zerbino and Sujatha Jose
Abstract The significance of the creep behaviour of fibre reinforced concrete (FRC) in the cracked state is a matter of debate. As FRC in service could be in the cracked state, the serviceability and failure will depend on the stability of the cracks, and the alteration of the capacity to transfer stresses. Based on the present knowledge, this paper discusses factors influencing the creep of cracked FRC to facilitate further discussions. Effects of load, temperature and humidity, and the importance of progressive debonding and pull-out, creep of the fibres and crack propagation under sustained loading have been discussed for steel and polymer fibres.
Keywords Fibre reinforced concrete Creep behaviour Pull-out Steel fibres Polymeric fibres
Over the past few decades, fibres have been incorporated in concrete in many applications, such as pavements, slabs on grade, tunnel linings and structural elements along with conventional reinforcement, to improve the service life and failure mode of structures. Since the serviceability depends on the stability of cracks and capacity to transfer sustained stresses, the creep behaviour of cracked fibre reinforced concrete (FRC) is significant. The fib Model Code 2010 raises concerns about the long-term performance of concrete reinforced with fibres that could be affected by creep.
R. Gettu (H) • S. Jose
CONICET Researcher, LEMIT-CIC, Civil Engineering Department, UNLP, Ensenada, Argentina
© RILEM 2017 3
P. Serna et al. (eds.), Creep Behaviour in Cracked Sections of Fibre Reinforced Concrete, RILEM Bookseries 14,
As is well-known, creep is defined as the deformation in a solid due to sustained stresses. Creep of plain concrete or the matrix of FRC under compression load occurs at all stress levels and is due to movement of pore water in the paste, compaction of C-S-H sheets and microcracking, especially at relatively higher stresses. Consequently, the main factors that impact the rate of creep in compression are the type of curing, age of loading, temperature and relative humidity of the environment . In plain concrete, the fracture toughness and fracture process zone size (i.e., the region over which energy is dissipated in front of the propagating crack) can decrease as the loading rate decreases. Consequently, creep could decrease the load carrying capacity of a cracked element significantly .
Researchers who have studied the long term behaviour of FRC agree on its ability to better sustain the load levels over time, reducing deflection and crack opening compared to concrete without fibres. In the case of cracked FRC, the fibres bridging the cracks are subjected to sustained loading , which could lead to progressive creep of the fibre-matrix interface, and debonding and pull-out of the fibre. These phenomena, along with matrix creep and drying shrinkage in the compressive zone ahead of the crack [4, 5], creep in the individual fibre or filament itself and, more importantly, crack propagation  can result in crack widening that can affect serviceability, long-term load-carrying capacity and durability [4, 7-11]. The behaviour varies with the fibre type and dosage, as expected [12, 13].
More than 15 research groups in the world are working now on this subject and efforts have been made to arrive at standard creep testing procedures for cracked FRC. At this point of time, when there is considerable information compared to few years ago, it appears relevant to discuss what we think we know and what we do not know about the assessment of tensile creep in cracked FRC.