Water absorption rate of the recycled coarse aggregate is higher than the natural ones. Until now, there is no test method to record the instantaneous value of the water absorption although this could help us to understand the recycled coarse aggregate better. This paper developed a new device that can record the water absorption continuously and calculate the water absorption rate automatically. The water absorption curve from 0 min to 60 min can be plotted smoothly. The performances of modified recycled coarse aggregate and concretes with recycled coarse aggregate have also been studied. The result shows that the water absorption rate increases fast and about 90% water has been absorbed during the first 10 min. The slurry of silicon nitride with 500 nm particle size can reduce the 10 min water absorption rate of the recycled coarse aggregate from above 4.5% to below 2.5%. A recycled coarse aggregate wrapped with wet slurry is better than the ones wrapped with dry slurry shell for the slump of concrete. Compared to the concrete without any recycled coarse aggregate, the compressive strength and the splitting tensile strength of the concrete with recycled coarse aggregate modified by fresh cement slurry have been increased by more than 20%.
In order to obtain the law of the fatigue damage development of reinforced concrete hollow beams that has been in service for 24 years, its solid hollow beams were removed and transported to the laboratory for loading test. Two beams were selected for static loading to obtain the ultimate flexural bearing capacity, and three beams were, respectively, subjected to constant-amplitude fatigue loading with different load amplitudes. The static and dynamic behaviors of the beams were monitored in the fatigue test. The fatigue failure of the beams showed that the outermost rebar at the butt weld fractured at first, and the crack width at the fracture position of the steel bar was about 0.3 mm, which was largest in all cracks. After a rebar was broken, midspan deflection and flexibility increased by approximately 20% and 10%, respectively, relative to the initial state. The damage developed rapidly in the following range: (1) the first 10,000 fatigue cycles; (2) after fatigue fracture of the rebar; and in the intermediate stage of fatigue test, the damage development was relatively stable. As the loading amplitude increased, the stiffness degradation and the cumulative damage that occured under the same loading cycle were more significant.
The residual bearing capacity of existing bridges has been a controversial topic for engineers and technicians. In order to accurately evaluate the actual bearing capacity of a 24-year-old RC hollow beam bridge, its components with different thickness concrete leveling layer were removed and transported back to the laboratory. The representative static and dynamic responses of the two beams were monitored during the whole procedure. A quick assessment of loading capacity of bridge using crack height and a parameter correction method for the crack width prediction formula in the code were proposed. In addition, comparison of measured and current design codes GB 50010 and ACI 318 predicted behaviour of existing beams was also presented. The results showed that the bending process of the RC hollow beam went through the elastic phase to the elastic-plastic phase and to the final failure. The actual flexural capacity of two beams was 10% larger than the calculated values. The natural vibration frequencies of the beam changed slightly before plastic stage, but the modal amplitude increased with the increase of degree of damage, once the beam entered plastic stage. The predicted deflections according to GB50010 were consistent with the experimental values at about 200 kN; for the code ACI, as the loading force increased, the difference between the two gradually decreased.
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