Three-dimensional (3D) concrete printing necessitates a balance between various ingredients of the mix composition. This study investigated the effect of hydroxypropyl methylcellulose (HPMC) dosage at various aggregate volumes on fresh, rheological, and mechanical properties of 3D printable concrete (3DPC). Accordingly, 3DPC mixtures having three aggregate volumes, namely 44, 41, and 38 %, were produced at a constant water-to-binder ratio. For each aggregate volume, three HPMC dosages, namely 0, 0.14, and 0.28 % by weight of cement, were studied. A mini-slump flow table and a manual printing gun were used to assess the flow diameter and printability. Rheological properties were determined using a rotational rheometer and extrusion device. The buildability was assessed through green strength testing. Results showed a directly proportional relationship between HPMC dosage and fresh and rheological properties of these mixtures. At a constant w/c ratio, increasing the aggregate volume led to higher green strength and extrusion pressure at all piston-moving velocities. Overall, at an early age, the effect of HPMC dosage was more significant on the static yield stress of mixtures with lower paste volume while being more accentuated on the green strength for mixes with higher paste volume. The positive impact of increasing HPMC dosage on the green strength becomes insignificant at later ages. The effect of increasing HPMC dosage, however, was more pronounced on the extrudability of mixtures with higher paste volume by preserving their extrudability at later ages. Finally, HPMC addition led to strength losses of up to 28.63 and 32.7 % for flexural and compressive strength, respectively.

Rheology and shrinkage are the key features that should be controlled to perform a successful casting process and obtain a durable ultra-high performance concrete (UHPC). It is anticipated that cast-in-situ applications of UHPC will increase in the near future, so the studies on early-age properties of UHPC have come into prominence. However, there is still no standardized method for mix design of UHPC that can ensure the desired performance. High amount of cement and low water/binder ratio increase the shrinkage values, which also increase the crack formation risk especially at early-age. The basic variable of mix proportions that determines the rheological and shrinkage properties is aggregate volume and accordingly paste volume. In the scope of this experimental study, influence of volume fraction of quartz aggregate on rheological, shrinkage, and mechanical properties of UHPC was investigated. The results showed increasing the aggregate volume led to sharpen pseudoplastic behaviour of UHPC. To be able to produce a self-compacting UHPC, upper limit of aggregate volume was found 45%. Both autogenous and drying shrinkage can be limited by increasing the aggregate volume. Early-age autogenous and drying shrinkage are far more than the shrinkage measured after demoulding during 90 days. Necessary precautions, thus, should be taken as of casting stage.