Creep Testing Methodologies and Results Interpretation
Nicola Buratti and Claudio Mazzotti
Abstract This paper presents a literature review on the main testing methodologies used to investigate the long-term behaviour of cracked FRC elements. Various tests methods such as pull-out tests, uniaxial tension tests, beam bending tests, and plate bending tests are illustrated and discussed. The paper originates from a round table held during the FRC-CREEP 2016 workshop.
Keywords FRC Creep Testing Pull-out Uniaxial tension Bending Plate
Many studies have contributed to a better characterization of short-term mechanical performances of FRCs and have led to the definition minimum performance requirements and design guidelines. On the other hand, a proper knowledge of the long-term behaviour of SFRCs (Steel Fibre Reinforced Concretes) and MSFRCs (Macro-synthetic Fibre Reinforced Concretes) has not yet been achieved.
Kurtz and Balaguru  tested the long-term performance of cracked beams made of concrete reinforced with polypropylene and nylon short-fibres. Creep failure occurred when the stress level was higher than a certain percentage of the failure load under short-term monotonic testing, measured using the Average Residual Strength (ARS) as defined by ASTM C1399 . Bernard  investigated the time dependent behaviour of cracked FRC round panels reinforced with either steel or MS fibres. The load applied during long-term tests was defined according to the actual residual tensile-strength measured in the pre-cracking tests. Post-crack creep coefficients were relatively insensitive to load ratio for the SFRC and for one of the two MSFRCs while for the second MSFRC the creep coefficient was sensitive to the load ratio. MacKay and Trottier  described the results of experimental tests comparing the behaviour of one SFRC and one MSFRC under long terms loads.
N. Buratti (H) • C. Mazzotti
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P. Serna et al. (eds.), Creep Behaviour in Cracked Sections of Fibre Reinforced Concrete, RILEM Bookseries 14,
They concluded that, at similar loading levels, cracked MSFRCs could experience creep coefficients larger than SFRCs by a factor two. Kusterle  tested one SFRC and three different MSFRCs. For each mix six beams were cast and tested in four-point bending. The beams were pre-cracked up to a mid-span deflection of 1.75 mm. A sustained load ranging from 50 to 60 % of the strength at 1.75 mm was applied. Kusterle concluded that MSFRCs had large long-term deformations and that a maximum creep load ratio of 50 % seemed to be the maximum for obtaining good long-term performance. SFRCS were able to sustain larger loads (60 %). Failures were observed when the load level was increased to 75-80 %. Zerbino and Barragan  studied the creep behaviour of SFRC cracked beams subjected to long-term loading. The beams were pre-cracked up to crack openings spanning from 0.2 and 3.5 mm. For small crack-openings at the beginning of long-term tests stable responses were obtained during 18 months, even when applying stress levels equal to the final stress level reached at the end of the initial cracking tests. A stable response could be observed for a pre-crack of 0.5 mm. However, for load ratios of 0.96 relatively high crack-opening rates were found, indicating the possibility of the initiation of creep failure. When the loads were further increased, a quick failure was observed in these cases. When creep rupture took place, a three-stage creep response was observed. Garcia-Tengua et al.  tested 31 SFRC specimens in four-point bending  in order to investigate the effects of various parameters on creep in cracked conditions by means of multiple linear regression. They concluded that the load-ratio had an effect on flexural creep response and that the extent of this effect depends on fibre slenderness and fibre dosage. Zhao et al. [9, 10] carried out an experimental program to investigate the long-term behaviour of SFRCS under uniaxial tensile loads by testing cylindrical specimens, pre-cracked up to either 0.05 or 0.2 mm crack openings. The time-dependent crack opening observed was almost at the same level of instantaneous crack opening after 3 months loading at around 30 % of cracking strength. They also concluded that the damage due to debonding at the fibre/matrix interface was not increasing with creep deformation at the loading level of 30 %, even though the irreversible part almost doubled during the creep loading. Babafemi and Boshoff  investigated the time-dependent behaviour of a MSFRC under long-term uniaxial tensile loading. Prismatic specimens were pre-cracked up to 0.5 mm using a displacement control machine. Babafemi and Boshoff observed that the MSFRC showed significant crack widening over time under sustained uniaxial tensile loads. Even at loads as low as 30 % of the post-peak resistance, the time-dependent crack widening did not stabilize after 8 months. Tensile creep failure occurred within 10 days for specimens loaded at 60 % of the post-peak resistance and within less than a day for a 70 % loaded specimen. Average fibre counts on the cracked face of MSFRC were found to influence the time-dependent behaviour. Higher fibre counts resulted in lower time-dependent CMOD and vice versa. Babafemi and Boshoff also performed single fibre long-term pull-out tests observing that specimens loaded at 50 % of the quasi-static capacity pulled out over time. Time-dependent crack widening under sustained loading was identified to be caused by two mechanisms: time-dependent fibre pull-out and time-dependent fibre creep. Buratti and Mazzotti found that temperature influences the creep deformation rates in particular on MSFRCs .
This paper presents the most widely experimental techniques used to evaluate creep deformations in cracked FRC sections, as discussed during a round table held during the 1st International RILEM Workshop on creep behaviour in cracked section of Fibre Reinforced Concrete, Valencia, Spain.