Comparison of Rotational Rheometers for Ready-Mix Concrete (2024-09)¶

3D printing with ordinary ready-mix concrete, pioneered by the ConPrint3D project at TU Dresden, is gaining considerable attention in both academia and industry. This highlights the need for developing a suitable test method to precisely determine the rheology of concrete, which is key for the success of large-scale 3D printing operations. In this comprehensive study, the dynamic and static rheological properties of various ready-mix concrete formulations were determined. Over ten rheological assessment devices were used to evaluate these properties. The majority of the devices were commercially available wide gap rotational rheometers designed for concrete mixes with a maximum aggregate size of at least 8 mm. Other novel devices that were developed within DFG funded Priority Program (SPP) 2005 “Opus Fluidum Futurum – Rheology of reactive, multiscale, multiphase construction materials” were also investigated. These include an open channel flow unit, where the rheological properties were assessed by means of the optical analysis of the concrete flow through the chute of a ready-mix concrete truck. The benefits of optical scanning were further utilized for 3D reconstruction of a standard slump flow and spread diameter. The reconstructed 3D image was then used to obtain rheological properties using machine learning techniques. These are quite beneficial for stiff 3D printable concrete where the precise measurement of parameters (yielded, un-yielded height, etc.) for the rheological assessment is necessary. However, this extended abstract primarily focuses on providing a concise overview of the experimental methodology and the strategy utilised for evaluating results from the wide gap rotational rheometers. Establishing an adequate shearing protocol is crucial for determining dynamic rheological properties using rotational rheometers. Due to the presence of aggregates up to 8 mm in size, a significant shear-induced particle migration can occur during the assessment. Consequently, excessive pre-shearing may lead to the characterisation of rheological properties of constitutive mortar or paste, or concrete with a lubrication layer instead those of homogenous concrete. Therefore, an optimal pre-shearing duration was determined for every concrete mix at various hydration times before the dynamic rheological properties were measured. This was achieved by operating the rheometer at maximum shearing rates until the measured torque visually reached a steady state. The maximum shear rate does not necessarily have to be at the maximum capacity of a rheometer. The shear rates in the different devices were set to be identical, theoretically, by adjusting the rotational velocities according to the geometry of each device. Once the shearing protocol was identified, the rotational rheometers were operated simultaneously to analyze over 15 concrete mixtures with varying rheological properties. Each mixture underwent testing at least three time intervals after water addition. The plastic viscosity and yield stress were determined using the Reiner-Riwlin equation with plug flow correction. Subsequently, the shearing zone, which is defined as the radial distance from the vane’s outer edge to the plug’s boundary was determined for every shear rate applied during the test. Based on these values, the mixtures were classified into three distinct categories for each rotational rheometer.