Failure of the structure due to poor buildability is a major concern in 3D printing of cementitious materials. Evaluation of buildability based on fresh material properties and print parameters is of significance. In this paper, the buildability of printable engineered cementitious composites was investigated and quantified at the material and the structural scale. Fresh ECC material showed excellent load capacity and deformation resistance at the material scale, therefore preventing material failure of the bottom layers, as confirmed by constant shear rate tests and incremental loading tests. To predict vertical deformation of a 3DP structure, a time-dependent strain-stress model of printable ECC was proposed and validated based on the green strength evolution of the material and the buildup rate of the designed structure. At the structural scale, the approach of predicting critical height at self-buckling failure based on stiffness evolution was validated by printing a straight wall and a cylinder structure.@en

Strain-hardening geopolymer composite (SHGC) lately emerged as a promising alternative to traditional strain-hardening cementitious composite with added advantages of industrial by-product utilization and enhanced sustainability. However, as the design of SHGC requires multi-factor optimization, the application of traditional trial-and-error method is inefficient and hinders the development of this material. This paper aims at the development of a slag/fly ash-based SHGC with low slag content using a micromechanical model to guide the composite mixture design. To this end, experimentally characterized physical properties of fiber, matrix and interface are used as input for the micromechanical model, which serves as a predictive tool for the tensile performance of SHGC. Following the guidance, a slag/fly ash-based SHGC with tensile strain capacity of 4.8% and ultimate tensile strength above 3.8 MPa was systematically developed. The feasibility and effectiveness of using micromechanics as the design basis of SHGC are demonstrated and experimentally verified.@en

3D printing of engineered cementitious composites (3DP-ECC) serves as an effective means to enable digital and automated construction while removing the dependence of steel reinforcement typical in normal concrete construction. As generally recognized, process control in 3DP is as crucial as material constituents to the performance of the printed structure. Printing parameters pose direct influence on the micro-structure of material, which further impacts the macro-scale properties of printed ECC. In this paper, the effects of different nozzle standoff distances and nozzle travelling speeds are investigated. It is found that lowering the nozzle standoff distance within a certain range increases the in-plane tensile strength and strain capacity of 3DP-ECC by 39% and 30%, respectively. The tensile performance is also enhanced by a moderate printing speed. In addition, a 141% increase in interfacial fiber bridging force is found with elevated nozzle standoff distance. Furthermore, the micro-structure observed via μ-CT correlates well with the printing parameters, micro-structure and macro-scale properties of 3DP-ECC.@en

Widely reported anisotropy in 3D printed cementitious structures has been a primary concern to structural integrity, especially for fiber-reinforced cementitious material, e.g., engineered cementitious composites (ECC). To alleviate the anisotropy present in 3D printed ECC (3DP-ECC), two innovative printing patterns, “knitting” and “tilting” filaments, were proposed, mimicking the natural crossed-lamellar structure of conch shells. 3D spatial paths were designed to allocate tensile/flexural resistance to multiple directions and to create an interwoven interface system to strengthen the structure. Four-point bending tests loading from three different directions were conducted. It was found that knitted and tilted filaments revealed superior or comparable bending performance to cast ECC in two favorable orientations. Furthermore, flexural performance in the weakest orientation was notably improved by knitting and tilting, with up-to-179% increases in flexural strength compared with that of parallel filaments. This novel approach holds great promise in alleviating anisotropy of 3DP-ECC without introducing additional reinforcement.@en