Long-term Behaviour Of Recycled Concrete Composite Slabs With Steel Tube Trusses

The production and use of recycled coarse aggregate concrete can reduce resource consumption and environmental burden,and provide ideas for the treatment and reuse of construction waste.The prestressed precast concrete bottom slab with grouting steel tube trusses is a new prefabricated floor members in recent years.Because it is factory and standardized prefabricated,it has better quality assurance.At the same time,it has light dead weight,easy to transport,and has better construction and bearing performance.The recycled coarse aggregate concrete composite slab was formed by combining the recycled coarse aggregate concrete as cast-in-place layer with prestressed precast grouting steel tube trusses bottom slab,and its long-term performance was studied.The research contents and results are as follows.A kind of physical and chemical composite strengthened recycled concrete with recycled coarse aggregate wrapped by silica fume slurry after mechanical grinding was developed and put forward.The 28-day compressive strength of silica fume slurry wrapped composite strengthened recycled concrete was compared with that of silica fume mixed composite strengthened recycled concrete,only physically strengthened recycled concrete and normal recycled concrete.The results showed that the proposed silica fume slurry wrapped composite strengthening method can improve the compressive strength of recycled concrete and inhibit its performance degradation under the influence of recycled coarse aggregate replacement rate.The long-term performance test of recycled coarse aggregate concrete composite slabs with different recycled coarse aggregate replacement rate was carried out for 360 days.The long-term performance of composite slab with 30 % replacement rate of recycled coarse aggregate in composite layer was closer to that of normal concrete composite slab,while the specimen with 50 % and 70 % replacement rate showed similar rules and changed more significantly than the former two.The deflection of specimens with replacement rate of 30 %and normal concrete composite slab developed rapidly in the early stage of the experiment,and entered the stable stage earlier.While the deflection of specimens with replacement rate of 50 %and 70 % developed slowly,and the numerical stability was also delayed.The long-term strain curves of the concrete along the height of the section of the composite slabs gradually developed in a straight line with the increase of the holding time,showing a form that conforms to the plane section assumption.The finite element software ABAQUS was used to simulate the long-term performance of the composite slabs.The deflection variation trend and the strain variation trend of the concrete at the bottom of the composite slabs under the long-term load were simulated well.The fracture distribution was studied by strain nephogram.Cracks began at the mid-span edge of the bottom of the slabs and extended toward the center.At the position of 1/2 span,transverse cracks through the whole bottom of the slabs were formed at first,and then a strip area with larger stress value appeared on both sides.The integrating method of the mid-span section was used to calculate the flexural bearing capacity of the composite slabs.The results were compared with the test values in the previous experimental study of our research group.The errors were about 10 %.Then the flexural bearing capacity of recycled concrete composite slabs studied in this paper was calculated using this method.According to the long-term deflection formulas suggested by GB50010-2010,AS3600-2009 and ACI318-19,the long-term deflections of the composite slabs were calculated.The results were conservative.It was concluded that the recycled concrete composite slabs with steel tube trusses had a relatively safe resistance to deformation in the normal use stage.Through fitting,taking into account the effect of recycled coarse aggregate replacement rate on long-term deflection and modifying the time impact coefficient,the calculation formula given in ACI318-19 had been adjusted.The correlation coefficients between the calculated values and the test values were about 10 %.It was considered that the proposed formula can predict the long-term deflection trend of such components to a certain extent.