Issue 46

S.Y. Jiang et alii, Frattura ed Integrità Strutturale, 46 (2018) 275-284; DOI: 10.3221/IGF-ESIS.46.25 277 plan. After curing, the ultimate load of 75% Fu was applied onto all the four beams (final loading) and maintained at that level (sustained loading). The midspan deflection and crack development of each beam were recorded periodically, together with the lab temperature and humidity. Figure 1 : Details of the test beams and reinforcement. E XPERIMENTAL R ESULTS Pre-loading o reflect the actual conditions of reinforcement, the beams were cracked under pre-loading prior to reinforcement. The pre-loading process is illustrated in Fig. 2, Fig. 3 and Tab. 1. As shown in Fig. 2, the loading device for pre-loading and long-term load holding consists of a reaction beam connected to the trench, a hand-type mechanical jack, a force sensor and a distribution beam. The magnitude of the applied load can be displayed in real time on the force sensor between the mechanical jack and the reaction beam. The mechanical jack was selected to meet the load requirements of long-term load holding. Overall, the loading device is easy to isntall and operate, flexible in loading and unloading, and stable in the long term. Figure 2 : Loading device. It can be seen from Tab. 1 that, with the increase of preload, the instantaneous midspan deflections of B-2, B-3 and B-4 were on the rise and the residual midspan deflections grew after unloading. Considering the crack development in Fig. 3, it is clear that the B-4 had the most severe degree of cracking under the preload, followed by B-3 and B-2 in decending order. No cracks were observed in B-2 because of its low preload level (below the cracking load). The small preload-induced deflection, coupled with the lack of residual deflection, is insufficient to form a visible fracture. The cracking situation of B- T