Experimental Study on Bond and Force Transmission Properties of Steel Reinforcement in Non-Contact Lap Splice Encased in Calcium Sulphoaluminate Cement-Based Micro Steel Fiber Concrete

CSMSFC (Calcium Sulphoaluminate Cement-based Micro Steel Fiber Concrete) possesses the advantages of early strength, high strength, exceptional toughness, minimal shrinkage, and excellent bond performance with bars. When applied to NLSB (Non-contact Lap Splice of Bars) in prefabricated structures, CSMSFC enhances mechanical performance while preventing shrinkage cracking and reducing seismic damage. Additionally, it shortens construction periods for prefabricated structures and achieves a comprehensive improvement in seismic performance and construction efficiency. However, there is a lack of systematic testing of factors influencing the bond strength between CSMSFC and NLSB and the effect of CSMSFC on the force transfer performance between NLSB. Therefore, the axial tensile tests of NLSB were conducted on 51 non-contact lapping specimens to investigate the bond properties and force transfer mechanism between lapping bars and CSMSFC. The effects of lapping length, volume fraction of steel fibers, spacing of bars, and concrete cover thickness on the lapping characteristics were examined, and the comparison with OPMSFC (Ordinary Portland Cement-based Micro Steel Fiber Concrete) was also considered. The experimental results demonstrate that the bond strength between bars and CSMSFC increased by 36.8%, 42.3%, and 43.3% respectively, with volume fractions of steel fiber at 1.5%, 3%, and 4.5% compared to the absence of steel fiber. The bonding effect between CSMSFC and bars is similar to that of OPMSFC and bars. The bond strength between CSMSFC and the bars improved by 4.3% and 6.6% with the increases of the spacing of bars from 0 to 20 mm and the concrete cover thickness from 10 to 30 mm. Conversely, with the increases of the lapping length from 50 mm to 100 mm, 200 mm, and 300 mm, the bond strength decreased by 46.8%, 72.2%, and 82.0%, respectively. Finally, based on the force transmission mechanism of the non-contact lapping bars, a calculation model is proposed for determining the lapping length while considering the reinforcing effect of steel fiber “stirrups.” A formula is derived from the model to calculate the minimum lapping length of HRB400 bars in CSMSFC, considering the volume fraction of steel fibers, which can assist in designing the minimum lap length of NLSB in practical applications.