Study On Anchoring And Noncontact Lap Splice Performance Of Rebars In Concrete Composite Slab

The use of the bottom longitudinal rebars without protruding structures at the support of the reinforced concrete composite slab has obvious advantages in the production,transportation and installation of precast slabs.The construction of replacing the longitudinal rebars in the precast slab with additional connecting rebars mainly involves two aspects of performance.The first is the anchoring performance of the additional rebars at the support.The second is the noncontact lap splice performance of the additional steel bars in the cast-in-place layer of the composite slab and rebars at the bottom of the precast slab.Noncontact lap splice of rebars are widely used in assembled building with concrete structure.However,the design methods and construction requirements of it have not been given in China’s specifications,and sufficient relevant research is lacking.In this paper,the anchoring and noncontact lap splice properties of the rebars in the composite slab are studied.The main contents and conclusions,suggestions are as follows:(1)The bonding and anchoring properties of the rebars in the plate(the diameter is not more than 12mm generally)at short anchors and long anchors were studied by 6pull-out specimens.The results indicate that the anchor length of 15d can ensure that the rebars achored in the slab do not have anchor failure before yielding in the case of one-way tension.To obtain an accurate reinforced-concrete bond-slipτ-S curve,it is recommended to take the anchor length of the measured rebar as 5d when using pull-out test to calibrate the bond-slip relationship.Theτ-S curve proposed by GB 50010 is relatively accurate,but the corresponding feature points are significantly smaller.(2)Tests on 9 uniaxial tension specimens with noncontact lap splice indicate that it can meet the requirements of reliable lap splice when the lap length is 0.8l_a.If the critical lap length of the rebar is estimated by the effective lap length of the rebar,the lap length can be taken as 0.8l_a.The effective lap length of rebar increases as the total lap length increases.The variation of the diameter and the clear spacing of the rebars in a small range has no obvious effect on the effective lap length.(3)Bending behavior of 6 composite slab connected with post-pouring zone in which the longitudinal rebars with 90ohooks is connected by binding and noncontact lap splice are studied experimentally.The results indicate that all specimens are subject to bending failure and can reach the limit of the flexural load capacity specified in Chinese code.The longitudinal rebars in post-pouring zone connected by binding or noncontact lap splice have no significant effect on the crack development at the joint and the bearing capacity,rigidity and deformation capacity of the composite slab.Rebars can reach the tensile yield strength whether they are binded or noncontact lapped.0.4times the lap length required by Chinese code allows the rebars to be anchored and lapped reliably for rebars with a 90°hook at the end.(4)The design methods and detailing requirments for noncontact lap splice of rebars in Chinese and foreign codes were.And then effects of different factors on lap length were identified.Through the summarization and comparison of the design methods and construction requirements about rebar lap splices in Chinese and foreign codes as well as the analysis of the factors affecting lap length,it is known that sufficient configuration of transverse rebars can greatly improve the performance of the lap splices.The recommended lap length with standard hooks in tension is about0.50～0.90 times that of straight rebars in all codes.The calculated compression lap length by all codes is smaller than the tension lap length.The correction coefficientζ_lused in Chinese code to calculate the tension lap length is generally larger than the foreign codes.(5)The design method and related construction suggestions of noncontact lap splice are put forward.On this basis,the design and construction suggestions of the additional rebars at the support of the composite slab with the bottom longitudinal rebars without protruding structures are proposed.