Rheological Characterization of Nano-Modified Ultra-High-Performance Concrete for 3D Printing (2024-09)¶

Additive Manufacturing of Ultra-High-Performance Concrete (UHPC) can revolutionize the optimization of structures, while leveraging the exceptional strength and durability inherent in UHPC. However, controlling concrete rheology during printing and characterizing its rheological properties remains challenging. Among these challenges is the critical requirement for a 3d printable concrete to withstand the weight of subsequent layers over time. This study addresses this by investigating how the yield stress of a 3d printable UHPC evolves over resting time, while also identifying the factors that affect the rheological behavior. The 3d printable UHPC mix consists of class-H cement, silica fume, silica sand, silica flour, and superplasticizer with a water-cement ratio of 0.2. Nano-clay is used as a rheology controller. The rheological properties were measured using building material cell (BMC) in Anton Paar’s MCR 302 rheometer. BMC has a cup and a slotted vane, the diameters of which are 70 mm and 59 mm respectively. It also has an inner cage that prevents the slippage of samples. The sample volume used is 300 ml. The tests have been repeated thrice to check reproducibility and the variance was less than 10%. The protocol of the test is illustrated in Fig. 1a, where time = 0s is defined as the time when cement was added to water. The tests started 1350s after cement was added to water. First, a preshear of 10 1/s is applied for 200s, so that the material reaches steady state, followed by a 30s rest. Then, the shear rate is increased linearly from 0 to 10 1/s in 60s and decreased linearly from 10 to 0 1/s in another 60s; this procedure is referred to as the flow curve and Fig. 1b shows an enlarged view of this step. This flow curve step is then repeated with different resting times between them (790, 780, 1680, 1680 and 2280s). The resting time is defined here as the difference between the ending time of one flow curve and the starting time of the next flow curve. The temperature during testing is set to 25°C and the same sample is used during the entire testing period.