Study On Flexural And Shear Behaviors Of Precast Concrete Beams Filled With Demolished Concrete Lumps And Fresh Concrete

To minimize the impact of casting demolished concrete lumps(DCLs)on field construction efficiency,incorporating DCLs and fresh concrete(FC)into precast members in the factory is an effective strategy.To coordinate with this strategy,a precast concrete beam connected by bonding longitudinal reinforcement with structural adhesive has been proposed.This paper carried out the following work in order to provide technical support for the engineering application of this type of precast beam.1.Seven precast concrete beams and one cast-in-situ concrete beam were tested under twopoint loading.The influences of the construction method,the replacement ratio of DCLs,the bonding anchorage depth,the bending level of longitudinal reinforcement and the stirrups spacing on the flexural behaviors of specimens have been investigated.It can be found that:(1)Good integrity of precast specimens can be found as cracks are not centralized near the interface between concrete and grouting material.Precast specimens have a flexural capacity comparable to that of cast-in-situ specimen.(2)The use of DCLs does not deteriorate the failure mode and the flexural capacity of precast specimens.(3)With the increase of the vertical bending level of longitudinal reinforcement,the flexural capacity of precast specimens decreases.(4)Under the premise that the bonding anchorage depth is not less than 20 d and the stirrups quantity is not less than C8@100,these two parameters have limited influence on the flexural behavior of precast specimens.(5)The formula given in design code GB 50010-2010 is still applicable for predicting the flexural capacity of precast specimens and the prediction results are generally conservative.2.Six precast concrete beams and one cast-in-situ concrete beam were tested under threepoint bending.The influences of the construction method,the replacement ratio of DCLs,the shear span length and the diameter of dapped-end reinforcement on the shear behaviors of the specimens have been investigated.It can be found that:(1)The failure mode and the main diagonal crack distribution of precast specimens are similar to that of cast-in-situ specimen.The shear capacity of precast specimens is not lower than that of cast-in-situ specimen,and precast specimens have a better deformation capacity than cast-in-situ specimen.(2)The use of DCLs has no significant effect on the failure process and the shear capacity of precast specimens.(3)With the increase of the diameter of dapped-end reinforcement,the shear capacity of precast specimens is improved to a certain extent,but the former one has a limited influence on the deformation capacity of precast specimens before reaching the peak load.(4)The relevant calculation method used to predict the shear capacity of precast specimens shows a safety margin similar to or even higher than that of cast-in-situ specimens.3.Further investigation on the flexural behaviors of precast concrete beam at room temperature and elevated temperature were carried out through calculation analysis.It can be found that:(1)With the decrease of the stirrups spacing,the growth rate of the flexural capacity of precast beam decreases gradually.When the bending length of longitudinal reinforcement is not less than 100 mm,its influence on the flexural capacity of precast beam is small.(2)With the increase of the horizontal/ vertical bending level of longitudinal reinforcement,the ultimate load of precast beam decreases slowly.Compared to the horizontal bending level of longitudinal reinforcement,the vertical bending level of longitudinal reinforcement shows a greater influence on the ultimate load of precast beam.(3)Bending of longitudinal reinforcement can effectively slow down the deterioration of structural adhesive at elevated temperature.When the bonding area is located at the lower part of precast beam,bending vertically of longitudinal reinforcement is more effective for slowing down the reduction of the flexural capacity of precast beam at elevated temperature.When the bonding area is located at the upper part of precast beam,bending horizontally of longitudinal reinforcement can slow down the reduction.(4)Compared with increasing the bending level of longitudinal reinforcement,increasing the bonding anchorage depth has a relatively limited effect on slowing down the reduction of the flexural capacity of precast beam at elevated temperature.