IntroductionCompared with the conventional moulded concrete process, the layer-by-layer stacking process produces a large number of interlayer interfaces in 3D printed concrete (i.e., 3DPC), leading to a low flexural and tensile strength of 3DPC and a poor structural integrity of 3D printed concrete structure, which restricts the development and large-scale application of concrete 3D printing technology. Reinforcement is considered as one of the most effective measures to improve the flexural and tensile properties of 3D printed structures and enhance the structural integrity. Various reinforcement techniques for 3DPC, such as fiber reinforcement and transverse reinforcement, are developed, but all of them fail to effectively reinforce the interlayers. A longitudinal reinforcing approach involving short-cut straight and n-shaped bars was firstly proposed in 2018, and some work showed that longitudinal reinforcing method of n-shaped bar could enhance the mechanical properties of 3DPC.In this paper, the influences of the penetration degree of the n-shaped bars and the overlapping conditions on the flexural properties of 3DPC were investigated, and the damage process of 3DPC was analyzed via crack displacement and section morphology. The different reinforcing methods were compared.MethodsPortland cement P·I 42.5 according to Chinese standard and natural river sand with a fineness modulus of 3.4 as a fine aggregate were used. Hydroxypropyl methylcellulose (HPMC) with an apparent viscosity of 198 Pa·s was selected as a viscosity enhancing agent and a bauxite-based powdered accelerator with a fineness (80 μm square-hole sieve residue) of 9% was used to adjust the thixotropy and buildability of 3DPC. A polycarboxylic acid-based high-performance superplasticizer with a solid content of 39% was used to adjust the flowability of the printed materials. In this work, a ratio of the layer number penetrated by n-shaped bar (L_P) to the total layer number (L_P) was defined as a penetration degree (P) including overlapped penetration degree P_ovr and nonoverlapped penetration degree P_non. Eight types of n-shaped bar layouts that meet the constraint requirements of the formula were investigated.To ensure the accuracy of the n-shaped bar overlapping and positioning, a manual reinforcing method was used to accurately locate and reinforce the n-shaped bars. This process was repeated for ten layers of concrete, resulting in the completion of 3DPC specimens reinforced by n-shaped bars. For the unreinforced specimens, the preparation was completed via directly printing ten layers in a row.After printing, the specimens with the sizes of 40 mm×200 mm×180 mm were placed in an indoor environment for 24 h and then transferred to a standard curing room for 28 d. In the curing period, the specimens underwent a three-point bending test using a CMT-300 universal pressure testing machine. The span between the specimen supports was 160 mm, and the loading was controlled using mid-span displacement mode at a loading speed of 0.01 mm/min. The flexural strength of each group with three specimens was obtained via calculating the average value.Results and discussionWhen P_non=0.1, the addition of n-shaped bars fails to strengthen the 3DPC, and reduces the flexural strength of 3DPC. When P_non=1.0, the maximum flexural strength reaches 5.88 MPa, which is only slightly greater than that of the unreinforced 3DPC. Under the condition of nonoverlapped, the n-shaped bar is not effectively connected at the interface, and the n-bar has little enhanced effect.P_ovr=0, Compared to the unreinforced 3DPC as P_ovr=0, the flexural strength of reinforced 3DPC increases when P_ovr=0.2, with an increase of 100%. As the penetration degree increases, the flexural strength increases by 128% when P_ovr=0.3 and by 152% when P_ovr=0.9. The flexural strength is highly improved by an overlapped n-shaped bar. The damage mode of overlapped n-shaped bar is mainly a ductile failure with obvious strain hardening characteristics.The influence weights of f_t0, P and ρ of flexural strength of reinforced 3DPC are 0.896 2, 0.836 0 and 0.766 3, respectively. Based on the analysis of these influence weights, the advantages and disadvantages of the two reinforcement methods, i.e., the rivet method and chopped bar method, are evaluated. To ensure a fair comparison, each parameter is normalized to the same order of magnitude and unit, and transformed into equivalent flexural strength. The chopped bar and n-shaped bar (overlapped) methods both have the similar reinforcing effects, with equivalent flexural strengths of 27.74 MPa and 26.25 MPa, respectively, based on the proposed equivalent flexural strength formula.ConclusionsThe interface defects at the shoulder and bottom of the n-shaped bar were increased when the n-shaped bar was nonoverlapped, having little effect on the flexural strength. All the reinforced 3DPC exhibited a brittle damage. When the n-bar was overlapped, the flexural strength of reinforced 3DPC increased with the penetration degree, but there was a saturation value for the reinforcement of flexural strength by penetration degree, and there was a most economical penetration degree in the actual project. The damage mode of 3DPC reinforced by n-shaped bar under overlapped condition was a ductile failure with strain hardening characteristics. Based on the gray entropy system theory, the flexural strength of reinforced 3DPC (f_t) of the influence weight was obtained in a decreasing order, i.e., matrix flexural strength (f_t0)>penetration degree (P)>reinforcing ratio (ρ). To improve the flexural strength of current longitudinal reinforcement methods, the chopped bar and overlapped n-shaped bar were proven to be more effective rather than the rivet and nonoverlapped n-shaped bar techniques. It was essential to carefully choose an appropriate reinforcing method corresponding to the specific requirements of the printing project.