Role of material selection in mix optimization of 3D printable concrete

The printing process is a novel digitally-controlled additive manufacturing method which can build architectural and structural components without formwork, unlike conventional concrete construction methods. The most important fresh properties are extrudability and buildability, which have mutual relationships with workability and open time. These properties are greatly influenced by the mix proportions and the presence of superplasticiser, retarder, accelerator and polypropylene fibres. The present study is about optimisation of material quantity to obtain a 3D printable mix with 3D printer. Cement, fly ash, silica fume, fine aggregate, water and chemical admixture are used for developing a 3D printable mix and optimum dose of chemical admixture such as PC base chemical admixture and Viscosity Modifying Admixture (VMA) is used to optimise the 3D printable mix. The study indicated that the dosage of superplasticizer needed to achieve the similar flow value increases as a/b increases. The plastic viscosity increased by about 35 % when a/b increased from 0.75 to 0.9. The flow was ranging from 160 to 225 mm which indicates dependency on the material type and packing density. Study also highlighted that yield stress of mix is important to achieve buildability and low yield stress value can lead to collapse of layers and will also prevent layer wise buildability. The optimum dose of polypropylene fibre was found to be 0.1 % to achieve 3D printable mix without clogging or shrinkage crack. The open time is found to be about 12—15 minutes for the materials. Studies reported in this paper highlights that both mix optimisation with different combination of cementitious binders and selection of optimum dosage of superplasticizer and VMA are very critical in achieving a 3D printable concrete.

Author: A. Trivedi, M.K. Mandre, B. Singh, P.N. Ojha

Section: Concrete

Keywords: 3D printing, concrete mix, flow, buildability, yield stress, open time