Research On Influencing Factors Of Cracking Of Truss Reinforced Concrete Composite Slab Bottom Plate

Thanks to the implementation of relevant policies,prefabricated buildings have developed rapidly in recent years.As an important component in prefabricated buildings,reinforced concrete laminated slabs with with steel bar truss have the characteristics of cast-insitu and precast slabs.It consists of precast and cast-in-situ layers.The precast layer is standardized in the factory,transported to the construction site,and performed on site,in the construction of cast-in-situ layers,the precast layer can be used as the bottom mold and support of the cast-in-place layer to cast the cast-in-place layer.It has the advantages of convenient construction and good mechanical properties,and has been widely used.Concrete structures are allowed to work with cracks,but the acceptance code stipulates that the appearance of precast components should not have serious defects.In actual engineering,cracks often occur in the precast floor.After investigation and statistics,the cracking rate of precast slab of the reinforced concrete laminated slab with steel bar truss used in some precast construction projects in Jinan can reach up to 30%,which seriously affects the acceptance of the project and requires the inspection agency testing and appraisal results in a waste of time and resources and restricts the healthy development of the fabricated construction industry.Aiming at the cracking problem of precast slab of reinforced concrete laminated slabs with steel bar truss,the theoretical calculation analysis and experimental research of different sizes of prefabricated floor slabs during demoulding and lifting,stacking transportation and hoisting and transportation are carried out to study the influence of different factors on the cracking of precast slabs.The specific research content is as follows:(1)The theoretical analysis of the cracking causes of the truss reinforced concrete composite slab floor during demoulding,stacking,transportation and hoisting installation.Starting from the calculation method of cracking stress of existing bending members,the cracking stress calculation formula suitable for the prefabricated bottom plate of truss reinforced concrete laminated plate is derived.Based on this formula,the analysis and calculation of the dimensions,replacement of prestressed tendons,and additional transverse reinforcement are based on the control variable method The influence of factors such as brackets on the tensile stress of concrete under the prefabricated floor.The calculation results show that cracks will occur for large-size floor plates and non-standard lifting methods.During the stacking and transportation phase,large-scale floor plates will have transverse cracks,and no longitudinal cracks will occur at this stage.It is obvious to replace the prestressed tendons and increase the transverse reinforcement bracket Increased crack load on precast floor.(2)The static loading test of three truss reinforced concrete laminated slabs was carried out to study the mechanical performance of the truss reinforced concrete slab.The various data of the slabs were obtained and compared.The longitudinal cracks on the bottom plate of the laminated slab are obviously inhibited from appearing and developing;the additional transverse steel brackets have greatly improved the rigidity of the prefabricated bottom plate,and the thickness of the plate has a significant effect on the increase in stiffness.(3)Based on ABAQUS finite element analysis software,the finite element simulation of three different forms of truss reinforced concrete composite slab prefabricated slabs was carried out during the lifting stage,the load-displacement curves of the precast slabs were extracted,and the simulation data was compared with the experimental data.It can be seen that The growth trends of the two are basically consistent,which verifies the accuracy of the finite element model.(4)Based on the established finite element model,the crack performance parameters of the truss reinforced concrete laminated slab floor are analyzed.The parameters of the number of steel bars and the upper chord diameter of the steel bar are selected.The simulation results show that the number of steel bars supports the cracking load of the precast floor The influence is small,and the diameter of the chord rebar on the reinforced support has a greater influence on the cracking load of the precast floor.