| Precast assembled monolithic concrete structure(also known as composite structure)has been widely used in practical engineering due to its advantages of both cast-in-place monolithic structure and precast fabricated structure,composite thick slabs are also common in precast assembled monolithic concrete structures,such as the assembly of monolithic concrete composite pipe rack,underground passageway and other underground box structure of the plate often appear composite thick plate.For this type of thick plate structure,there is the possibility of shear failure control design of inclined section,but there is more research on the shear performance of composite beams,and the research on the shear performance of composite thick slabs is still lacking,so it is necessary to study the shear performance of inclined section of composite thicks labs.The composite surface(bonding interface)of concrete is ubiquitous in engineering structures,in addition to the inevitable existence of the composite structure,there are problems in the construction of cold joint treatment,back pouring area and reinforcement by increasing the cross-section method,the reliable bonding of the superimposed surfaces is the key to ensure that the superimposed structure can be stressed together.At present,the indicators to measure the shear strength of new-toold concrete bonding interface are usually determined by using small-scale specimens,whether the obtained result can be reliably applied to the large-scale bonding interface in actual engineering is worth re-examining.In fact,there are more defects and more obvious uneven stress distribution in the large-scale bonding interface of engineering structure,which leads to the size effect of the shear performance of the bonding interface,but the research on the size effect of the shear performance of the composite surface(bonding interface)is rarely reported in the literature.Therefore,it is necessary to study the size effect of the new-to-old concrete bonding interface in order to determine the real shear performance of the new-to-old concrete bonding interface.Based on this,this paper has carried out experimental research and theoretical analysis on the size effect of the shear performance of the new and old concrete joints and the shear performance of the oblique section of the reinforced concrete composite slab,in order to provide a certain basis for the shear performance and size effect of composite thick slabs and bonding interface.1.Experimental Study on the Size Effect of Shear Performance for the Bonding Interface between New and Old Concrete(1)The shear test was conducted on 16 Z-shape specimens in 2 groups with different treatments(i.e.,chiseling method,the method of chiseling and post-installed rebar)and heights of the bonding interface(i.e.,heights of specimens are 100 mm,200m,330 mm and 400 mm,respectively,when width of specimen is always 200mm).The experimental results demonstrate that: under the direct shear load,the failure characteristics of specimens which chiseling-grossness are brittle,but the failure characteristics of specimens which post-installed rebar are ductile;Under the ultimate load,the slip amount of chiseling-grossness specimen was no more than 0.2mm,and the slip amount of the post-installed rebar specimen was about twice that of the chiseling-grossness specimen;The shear performance of the bonding interface for chiseling-grossness and post-installed rebar specimens show obvious size effect,the main manifestation is that as the size of the bonding interface increases,the shear strength and shear stiffness of the test specimen decrease,and the slip of the bonding interface increases.(2)The shear stress-slip curve of chiseling-grossness specimen is approximately linear,and the shear stress-slip curve of post-installed rebar specimen is divided into four stages.Based on the shear stress-slip curve of the specimen,a four-fold line is adopted to characterize the bond-slip constitutive model of the bonding surface of chiseling-grossness and post-installed rebar treatment.(3)Based on test results and theoretical analysis,the equations for characterizing size effect law of the characteristic parameters of the shear performance for the bonding interface were developed,The aforementioned models and equations can be used as reference for design and analysis of engineering structure.Based on the derivation of the size effect ratio calculation formula,Compared to the specimen with bonding interface height of 100 mm,the critical ultimate strength and elastic shear stiffness of the chiseling specimen reduced by 49% and 68%,respectively;the critical cracking strength,critical ultimate strength and elastic shear stiffness of the post-installed rebar specimen reduced by 35%,24% and 59%,respectively.When height of bonding interface is larger than 1000 mm,the size effect of characteristic parameters for describing the shear performance of bonding interface in both chiseling and postinstalled rebar specimens tends to be stable.2.Experimental study on shear behavior of reinforced concrete composite thick slabs(1)Based on 4 pieces of reinforced single layer composite thick plate and 3 pieces of double layer composite thick plate load test,the shear span ratio and the number of composite layer parameters on the composite surface and the effect of oblique section shear performance,comparison and analysis under the different shear span ratio of composite slab crack load and crack development rule,load-deflection relation,composite surface slip and shear bearing capacity and so on.Experimental results show that the shear-compression failure occurs when the critical oblique crack rises to the loading point of the plate top,the shear capacity and the critical inclination Angle of inclined crack decrease with the increase of the shear span ratio.The larger the shear span ratio is,the faster the critical inclined crack width increases,the main compressive strain inclination Angle obtained from the Rosette test of concrete can well reflect the change law of inclination Angle of inclined crack.(2)When the specimens were destroyed,the single layer composite thick plate had obvious horizontal sliding cracks on the combined interface,and the upper and lower concretes were significantly dislocated,and the double layer composite thick plate did not have combined interface sliding cracks;(3)When the shear span ratio is 1.13,the shear bearing capacity of the double layer composite thick plate is 1.22 times that of the single layer composite thick plate.When the shear span ratio is 1.29,the shear capacity of the double layer composite thick plate is the same as that of the single layer composite thick plate.1.24 times,the shear load capacity of the single layer composite thick plate test piece is about 10%lower than that of the double layer composite thick plate test piece.Comparing the test results with the calculation values of domestic and foreign codes,The experimental results demonstrate that: The calculation results of European and American codes are more conservative than those of Chinese codes.When the shear bearing capacity of composite thick plates is calculated according to the formula of super-thick members,the results are closer to the test values than those calculated according to the formula of general bending members. |
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