Describing the rheology of vibrated, no-slump concrete for application in 3D-printing construction
Using an automated process like 3D printing in concrete construction could improve safety and performance while decreasing environmental impact and cost. But 3D-printed concrete construction still needs considerable development before it will be reliable. This research addresses the challenge of extruding concrete, which must be fluid enough to flow through an extruding nozzle but solid enough to retain its shape when placed in layers. It focuses on controlling the fluid/solid state of the concrete with vibration. In rheological literature, vibration has been shown to overcome the yield stress of granular suspensions, permitting them to behave as solids at low strain rates. Most models describe two regimes, one describing Newtonian behavior at low strain rates and the second describing a return to the original, non-vibrated behavior at higher strain rates. Concrete can be considered a volume of aggregates suspended in cement paste and therefore a granular suspension. Several constitutive models have been proposed to describe the rheological behavior of vibrated granular suspensions but only on idealized fluids and at small scales. This study investigated how well these models apply to granular suspensions using concrete constituents. The first series of tests were conducted on standard concrete mixtures without vibration. Stress growth tests and flow curve tests using an ICAR (International Center for Aggregate Research) rheometer and the slump test was conducted on each mixture. The ICAR rheometer was not capable of characterizing no- and low-slump concrete mixtures. It was found that the yield stress of concrete relies on the granular phase while the viscosity relies on the cement paste phase. Because of this, the aggregate stock must be controlled more carefully than is currently done in practice. The second series of tests were conducted on no-slump concrete mixtures made of a bleed-resistance cement paste and coarse limestone aggregates. The gradation was controlled so that the sizes and volume fractions of the aggregates were known. Stress growth tests and flow curve tests were conducted using the ICAR rheometer while the concrete was being vibrated. Accelerometers were used to measure the acceleration profile of the concrete in the ICAR bucket. The validity of the raw data had to be checked because the rheometer struggled to achieve steady-state conditions but enough data points from the flow curve tests were acceptable. The data fit well to a power-law model. The shear moduli and frequency parameter of the mixtures were characterized because they are required in the Hanotin constitutive model but the remaining two parameters, the critical strain and -parameter, could not be obtained from the experimental data. The shear moduli were derived from the stress growth tests. The frequency parameter was assumed to be the vibrational strain rate experienced by the concrete and calculated from the acceleration profile measured by the accelerometers. The agreement between the data and a power-law model and the fact that this vibrational strain rate was at least two orders of magnitude less than the strain rates applied during shear testing with the ICAR rheometer supported the presence of a third, intermediary regime between the expected Newtonian and Bingham regimes with a significant range of strain rates. Further experiments are required in order to validate the assumptions used in analysis and investigate the extent of this intermediary regime. In the end, fully characterizing this vibration-dependent behavior and determining a reasonable constitutive model will permit the constraints on 3D-printing to be better understood and the new construction method to be more reliable.
concrete rheology, 3D printing, vibrated concrete
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign