As a novel civil engineering material, Engineered Cementitious Composite (ECC) has attracted more and more attention due to its strain-hardening characteristics, good post-cracking resistance and its unique properties. Bonding between Engineered Cementitious Composite (ECC) and rebar has a great effect on the mechanical behavior of structural members. In this paper, direct pull-out tests were conducted to understand the bond behavior between the ECC and rebar. The test parameters included rebar diameter and type, cover layer thickness, embedment length and fiber volume content. Bond-slip curves, failure and cracking pattern and bond strength were compared and discussed. The test results indicated that the bond strength decreased with the increase of embedded length. Through regression analysis with the test data, the functional relationships between bond strength and cover layer thickness and rebar diameter were fitted well. According to the positive and negative signs of the fitting parameters m and n, the relationship between the bond strength and the cover layer thickness and the rebar diameter could be determined. The bond strength increased obviously with the increase of fiber content. When the fiber volume content was 1, 1.5 and 2%, the bond strength of these specimens were 1.5, 2.5 and 3.1 times that of specimens without polyvinyl alcohol (PVA) fiber.

The technology of 3D printing concrete has undergone rapid development in the last few years due to its lower environmental impact than that of conventional concrete. To investigate the fresh and the hardened behavior as well as the microstructure of 3D printing cementitious materials (3DPC) containing nano-CaCO₃ (NC), four replacement ratios of NC to binder from 1 to 4% are investigated. 3DPC without NC was used as a control specimen. The workability, such as fluidity, extrudability, printability limit, and deformation under self-weight, and the flexural and compressive strength of 3DPC are tested. The strength development of 3DPC is compared with that of the cast specimens. The hydration products and the microstructure of specimens were also investigated by derivative thermogravimetry (DTG), thermogravimetry analysis (TGA), scanning electron microscopy (SEM), and backscattered electron (BSE) imaging. The results indicate that the fluidity, extrudability, printability limit, and deformation under self-weight demonstrate a decreasing tendency to increase the NC content. This is attributed to the large specific surface area of NC, which increases the mixtures’ consistency. Adding 2% of NC exhibited about 7.2, 39.1, and 22.5% higher compressive strength than that of the control mixture at 7, 28, and 90 days. The enhancement of strength of 3DPC with NC can be ascribed to the filler effects and the seeding effects of NC. Furthermore, NC refines the pore structure and improves the microstructure of 3DPC due to its filler effects and accelerating effects.