Experimental Study On Flexural Behavior Of Recycled Aggregate Concrete Composite Beams In Precast Part With Brick Aggregates

The construction area of newly buildings reaches billions of square meters every year in China,but at the same time,hundreads of million tons of construction wastes generates.The utilization of construction waste becomes a major problem.The use of breaking con-struction waste into recycled concrete aggregates(RCA)and producing recycled aggregate concrete(RAC)can effectively alleviate the problems of large amounts of construction waste and scarce of natural resources.But RCA contains some fired bricks,and the strength of RCA is lower than that of natural aggregate(NA),it will reduce the mechanical properties of concrete.Nowadays,the practical application of RAC components is so few,and the research of RAC components is mainly focused on columns,so that the research on beams is still relatively few.With the vigorous promotion of prefabricated buildings in China,this paper appiles construction waste into prefabricated concrete composite beams.At this stage,there are some research results on the application of RCA in composite beams.As the concrete in the tensile zone cracks when the beam is bending,it will redraw from work,which pro-vides a feasible idea for the use of some inferoir materials(such as RCA)for this part.But it is worth to point out that research results maily focus on the natural aggregate concrete(NAC)in the precast part and the RAC in the cast-in-place part at this stage.The strength of RAC is relatively low and the dispersion is huge.This design method does not fully consider the advantages and disadvantages of the RAC and NAC.This paper uses the RAC in the precast part and the NAC in the cast-in-place part to form the concrete composite beams.This not only considers the advantages and disadvantages of the two types of con-crete,but also conforms to the actural engineering situation.But there is very little related research.This paper proposes a RAC in the precast part and the NAC in the cast-in-place part and explores the effect of composite beams with steel fiber in the precast part.In this paper,10 specimen beams of 1:2 scale(125mm×250mm×2500mm)are designed,including 2ordinary NAC composite beams,6 precast RAC composite beams,and 2 precast RAC composite beams with steel fiber.The test variables are:1)the concrete strength grade:C35 and C45;2)the replacement rate of recycled concrete aggregates with brick aggregate(RCA-BA)in precast part:0%,33.3%,66.7%and 100%;3)the amount of steel fiber in precast part:0 kg/m~3and 70 kg/m~3.The result shows that the increase of RCA-BA in precast part will not significantly change the flexural bearing capacity of the concrete composite beams,but will reduce the cracking moment.The increase of RCA-BA replacement rate in precast part will accelerate the crack development,the crack width will expand more and the spacing will be closer,and the ductility is slightly reduced,maximum down to 7%,but it will not significantly weaken the stiffness of the beam after cracking,the ultimate failure also does not change significantly.The mix of steel fibers can effectively increase the cracking moment of the beam,and at the same time it will effectively slow the crack development speed,but it does not significantly contribute to the flexural bearing capacity.After the steel fiber is added,the stiffness of the beam after cracking to pre-yield stage is unstable,which indicates that the joint anchorage of the concrete and steel fiber in precast part can still provide a certain rigidity after cracking.The flexural bearing capacity of recycled aggregate concrete with brick aggregates(RAC-BA)in precast part composite beams can adopt the current specifications.But we must pay more attention to the requirements of applicability and the crack width check.It is possible to use RAC with a higher replacement rate in the precast part of composite beams in engineering.In terms of applicability,it can be achieved by adding steel fiber in precast part.This paper also uses Abaqus finite element software to build and analyze the model of composite beams in precast part with RAC-BA.This paper analyzes the stress distribution of the beam from the finite element analysis,and compares the bending moment-midspan displacement(M-Δ)curves obtained from the analysis and the experiments.The accuracy and reliability of the finite element simulation are verified,and the correctness of the experimental conclusions is further confirmed.Besides,this paper also carried out the simulation of other variables(replacement rate of RCA-BA in precast part,reinforced steel bar rate and the height of cast-in-place part)through Abaqus.The simulation result found that the increase of RCA-BA in precast part does not change the mechanical properties of the beams.The increase of steel bar rate leads to dramatic decrease in ductility,and the stiffness and bearing capacity values begin to increase and tend to be gentle after overbar.When the height of cast-in-place part increases,the stiffness and ductility appear to decrease and then increase.