# Effect of Fibre Reinforcement on Creep in Early Age Concrete
## 1 Introduction  
Fibre-reinforced concrete (FRC) is being used more and more in structural applications due to the benefits it provides [1]. The addition of fibres to concrete improves the endurance limit, flexural strength, Young's modulus, and the compressive strength. These improvements depend on the characteristics of the fibres, their material, and their dimensions. In addition, fibres or microfibres provide concrete with better behaviour after cracking [2; 3; 4; 5; 6; 7].  
The study of creep undergone by structural concretes has a great influence on the design of large structures, as well as on the definition of their construction process. Creep undergone by concrete at early ages influences the design of evolutionary construction processes, such as the construction of bridge decks using the successive cantilever technique [8], while the study of creep at infinite time influences the analysis of the countershafts used in the construction of boards with large spans [9; 10; 11].  
Some authors have studied the influence of fibres on creep in concrete, finding that fibres have a positive influence as they improve creep behaviour compared with plain concrete [10; 11; 12; 13]. Steel fibres are especially efficient for creep deformation compared to other types of fibres [1]. [PERSON] [12] compared the influence of polypropylene fibres, time of load, and applied load in creep measurement, reporting that the use of fibres improves creep behaviour of concrete loaded at early-ages and finding that Young's Modulus increases at the age of 24 h and that its influence in later stages is insignificant. He also found differences between the creep calculated based on the Code and the measured creep. [PERSON] et al. [14] concluded that it is possible to predict the creep behaviour of FRC by comparing its elasticity modulus at 28 days with that of plain concrete. That is, the modulus of FRC at 28 days is inversely proportional to its long-term specific creep. They also concluded that fibres with elastic modulus lower than concrete can increase creep and the addition of steel fibres at less than 2% can reduce creep. In addition, it was shown that temperature influences creep in concrete; the higher the temperature, the greater the creep displacement [15].  
Several authors have formulated different models for prediction of creep in non-reinforced concrete [16; 17; 18; 19]. In addition, different models can be found in the literature, including the AASHTO (American Association of State Highway and Transportation Officials, Washington, DC, US) model [20], the ACI (American Concrete Institute) model [21] or the EHE-08 model [22]. However, despite great progress in studying the short-term properties of fibre-reinforced concrete, research on the creep behaviour of FRC has not kept up [1].  
In this paper, the influence of loading age of the specimens on creep evolution at early age and long-term was analysed using a reinforced concrete with hooked-end steel fibres. In addition, after performing creep analysis, the instantaneous and delayed deformation was measured. With the results obtained, the authors proposed a formulation for the calculation of the creep evolution and for the delayed elastic strain, depending on loading age. The characterization of this reinforced concrete is a continuation of another study performed by some of the authors [7].