Flexural Creep Test Procedure

Beams measuring 225 x 75 x 700 mm (dimensions selected according to the creep apparatus configuration) were successively submitted to a pre-cracking test to reach the specified crack mouth opening displacement (CMOD), then submitted to a creep test. The pre-cracking tests were conducted on a displacement controlled closed-loop testing system using the average beam mid-span deflection as the control signal; while the flexural creep tests were conducted in a modified compression creep rig with flat Freyssinet actuator loaded by a hydraulic accumulator (Fig. 1). The pre-cracking and the flexural creep devices had the same boundary conditions, 4-point bending configuration with 200 and 600 mm spans between the loading points and the lower supports respectively. Three roller supports and one pinned-roller support placed at one of the two loading points were used. In both tests, the deflection and the CMODs were measured respectively by an LVDT and a miniature spring return potentiometer on each side of the beam.

A typical loading procedure for a beam was selected to simulate service conditions and ultimate conditions found in a reinforced SFRC structure and is illustrated in Fig. 2 with the load-CMOD curve. The curve includes the pre-cracking phase (continuous line) to a specified CMOD before and after the maximum load Pmax for part of the test in service conditions and in ultimate conditions respectively. The load measured at the specified CMOD in service, ws = 0.1 mm, was called Ps, whereas the load measured at the specified CMOD at ultimate, wu = 0.5 mm, was called Pu. The initial loading into the flexural creep frame (A), the sustained loading plateaus (B), the unloading-reloading cycles to estimate the compliance evolution during of sustained loading (C), and the final failure (?) are also presented on this figure. All creep tests were conducted until failure. The choice of ws = 0.1 mm in

Flexural creep test apparatus

Fig. 1 Flexural creep test apparatus

Typical load-CMOD curve for high sustained loads

Fig. 2 Typical load-CMOD curve for high sustained loads

service conditions is related to the durability criterion in the Canadian Bridge Design Code [8], which limits crack opening to 0.25 mm. The choice of wu = 0.5 mm in ultimate conditions is related to the fact that yielding of rebar occurs when crack width of 0.5-1 mm is achieved in reinforced SFRC structures [9].

Table 2 Loading characteristics

Service conditions

CMOD (ws)

0.1 mm

Loading level (P/Ps)

60 %

Loading period

4 weeks

Ultimate conditions

CMOD (wu)

0.5 mm

Loading level (P/Pu) and period

60 % (1 week), 75 % (2 weeks) and 90 % (1 week)

Table 2 summarizes the flexural creep test conditions such as the initial pre-cracked CMOD, the loading history and the loading period. Typically, the sustained load level (P/Ps) representing the service conditions was set to 60 % of Ps for 4 weeks, whereas the ultimate sustained load level (P/Pu) was increased from 60 to 90 % of Pu. Unloading-reloading cycle were performed every 7 days to evaluate the elastic compliance, which corresponds to the slope of the linear segment of the reloading part of the cycle. The compliance is an indicator of the damage state of the beam. The compliance measure should be understood as a measure of the length of a fictitious crack that would be mechanically equivalent to the real crack and its fracture process zone [10-12].

A total of ten beams were tested in the research project. Results of only 2 beams are presented in this paper (one in drying condition and one in sealed condition). Trends described for the 2 beams in the papers were also observed on all tested beams.

 
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