Experimental Results and Discussion
Creep Crack Width Results
For the case of the S1 beam, after 150 days of loading the values of Aw‘c (increment of crack width) registered for the specimens precracked at wp of 0.25 mm vary between 0.30 and 0.45 mm for PF (see Fig. 3a) and between 0.08 and 0.13 for SF (see Fig. 3b). The specimens with a wp of 1.50 mm, reached values of AwC between 1.20 and 1.40 mm for PF (see Fig. 3c), whereas SF reached only 0.15 mm (see Fig. 3d).
The creep curve of figure comprises two stages: primary and secondary creep. The increase of AwC is relatively high the first days just right after the sustained load is applied (primary creep), subsequently decreases progressively (secondary creep). On average, about half (50 %) of AwC recorded after 150 days of the test occurs in the first 5 days, after 30 days 70 % and after 90 days 90 % of the final AwC is reached.
In the case of the S2 beams (under ambient environmental conditions) after 90 days of testing the values AwC obtained vary between 0.30 and 0.45 mm for the specimens precracked with wp of 0.25 mm (see Fig. 4a). The S2/PF_1.50P5 precracked with a wp of 1.50 mm reach approximately 0.90 mm (see Fig. 4b).
Fig. 3 Diagram AwC _ time curves for the specimens: a S1/PF_0.25Pi; b S1/SF_0.25Pi; c S1/PF_1.50Pi; d S1/SF_1.50Pi
Fig. 4 Diagram Awtc — time curves for the specimens: a S2/PF_0.25Pi; b S2/SF_0.25Pi; c S2/PF_1.50Pi; d S2/SF_1.50Pi
Notice that the nature of the S2 creep curve is considerably different from that of S1. In the S2 specimens, creep increases monotonously in time, while in the S1 specimens creep stabilizes within several days. However, the initial rate of the S1-creep curve is larger. Unexpectedly during some initial period (about 7-8 days in our tests) the S2 creep width is less than the S1 creep for equivalent specimens. Only then the S2 creep begins to exceed the S1 creep.
During the execution of the test, none of the beams previously analysed (with wf of 0.25 and 1.5 mm) entered into the tertiary creep stage (a period of increasing creep rate, prior to failure). However, for all those beams with a wp of 2.50 mm a three-stage creep response: primary, secondary, and tertiary creep with a sudden failure was observed (see Fig. 5). In this experimental campaign, failure took place after 41, 112 and 108 days of loading for S1/PF_2.50P8 (with load ratio of %FP = 54 %), S1/PF_2.50P9 (%FP = 48 %) and S2/PF_2.50P6 (%FP = 49 %) respectively. On the contrary, none of the SF beams with a wf of 2.50 mm entered in tertiary creep. The values of AWc recorded range between 0.1 and 0.2 mm, lower values than those obtained for PF with a wf of 0.25 mm.
Fig. 5 Diagram DwC — time curves for the specimens: a S1/PF_2.50Pi; b S2/PF_2.50Pi