Toward 3D-Printable Engineered Cementitious Composites: Mix Design Proportioning, Flowability, and Mechanical Performance

3D concrete printing is a cutting-edge construction technique that has the potential to revolutionize the construction industry due to cost saving in terms of labor and formwork costs, efficiency in construction, lower safety-related risks, and a higher degree of automation. However, several issues still make its adoption relatively slower on a large scale. Engineered cementitious composites (ECC), a class of ultra-high-strength concrete, can be a potential solution to some of the problems currently faced by 3D concrete printing, such as reinforcement and durability. This study is a preliminary phase of a Tran-SET project focusing on the design of 3D printable ECC, considering the concrete mix design proportions and their potential effects on fresh and hardened properties to achieve an optimized, printable ECC mix. The type and content of various concrete ingredients such as cement, admixtures, aggregates, and fibers have considerable influence on the several properties in the fresh and hardened state. The investigation was done to evaluate the effect of some critical parameters of mix design proportioning, including the contents of mineral admixtures, such as fly ash, slag, metakaolin, nano-clay, silica fume, chemical admixtures, (superplasticizer), fine aggregate, and fibers. The replacement levels were 0%, 50%, and 75% for mineral admixture; 10% for silica fume; and 0.4% for nano-clay by cement mass. Locally available fine aggregates were also used at 25% and 40% by mass of binder. In addition, the influence of fiber types and contents was also investigated. Two types of fibers, polyethylene (PE) and polyvinyl alcohol (PVA) fibers, were used at different levels, such as 0%, 1%, 1.5%, and 2% of the total volume of the mix. The flowability of the ECC mixes was reduced with the incorporation of slag, metakaolin, higher aggregate content, and PE fibers. The high contents of both PVA and PE fibers (2%) (50.46 and 56.39 MPa) depicted a reduction in compressive strength as compared to the low fiber content. Moreover, the PE-ECC exhibited superior tensile ductility as compared to the PVA-ECC due to the more fiber bridging by the PE fibers at the crack interface.