Issue 46

S.Y. Jiang et alii, Frattura ed Integrità Strutturale, 46 (2018) 275-284; DOI: 10.3221/IGF-ESIS.46.25 281 Fig. 6 displays the variation in midspan deflection of the beams during the pre-loading, unloading, final loading and sustained loading. It is clear that the total deflection of each beam is controlled by the instantaneous deflection. With a high preload, B-4 boasted a large residual deflection, a significant total instantaneous deflection, and the greatest final deflection; By contrast, B-2 exhibited a small total instantaneous deflection and the lowest final deflection, thanks to its small pre-load, residual deflection and final midspan deflection. Fig. 7 presents the trend of additional midspan deflection in the test period. From this figure, it can be seen that, whatever the degree of pre-cracking, the long-term deflection always developed rapidly right after final loading and then slowed down to a stable state. In terms of magnitude, the additional deflection in the first 100 days was 90%~92% of the total additional deflection in the test period, that is, most of the deflections occurred in the early phase. Under long-term load, the rate of deflection development varied with the degrees of pre-cracking. Overall, the degree of pre-cracking is positively correlated with the gentleness of the additional deflection curve and negatively with the increment of deflection. Fig. 9 shows the relationship between the additional deflection and instantaneous midspan deflection in the test period. It is observed that the ratio increased with the degree of pre-cracking. During the 300 days, the ratios of B-1, B-2, B-3 and B- 4 were 57.9%, 67%, 49.3% and 42.8%, respectively. The proportion of additional deflection decreased with the growth in the pre-cracking degree. This means the time variation of deflection is negatively correlated with the magnitude of deflection at final loading. Considering the fluctuations of the deflection and the law of temperature and humidity (Figs. 5), the varied “anti-arch” degrees among the beams can be ascribed to the uneven shrinkage of concrete under the varying temperature and humidity. Nevertheless, the two factors no longer influenced the deflection trend in the later phase of the test. Long-term cracks Fig. 9 exhibits the crack development of each beam during load holding (excluding the pre-cracks and the instantaneous cracks at final loading). Figure 9 : Cracks under sustained loading. As shown in Fig. 9, most of the cracks developed on B-3 and B-4 during the load holding period were extensions of previous cracks. By contrast, there was a considerable number of new cracks on B-1 and B-2 besides the extended ones. In terms of crack quantity and height, B-1 and B-2 had much more new cracks than B-3 and B-4, indicating that the two beams were continuous cracked as the concrete reached the cracking strain under sustained load. B-3 and B-4 had fewer new cracks because the preload-induced cracks are relatively tall and the load during crack development was limited by compressive bars and concrete compression. The concrete creeped at a fast rate in the initial phase, but the rate gradually slowed down to a stable state over the time. In the 300 days, the time-deflection curve became increasingly gentle, and the creep tendency of each beam had an obvious relationship with the additional deflection (Figs. 7 and 8). There was virtually no visible crack development from 100 to 300 days, meaning that the cracks had fully developed The above results reveal the positive correlation between pre-cracking degree and the additional deflection after reinforcement. The relationship agrees well with that obtained by M. Muller [11] through relaxation tests on pre-cracked reinforced beams.