A Strain-Based Constitutive Model Ensuring Aesthetic 3D Printed Concrete Structures (2022-06)¶

Additive technologies in construction, such as 3D printing of concrete, have brought about renewed enthusiasm in the construction sector, mainly due to the principal advantages it presents including reduced cost, time, waste, formwork free manufacturing and free-form parametric design possibilities. Although holistically impressive, the aesthetic appeal brought by distinct fissility of printed structures remains highly contended among the public. A particular aspect contributing thereto, is the inconsistent filament layer thicknesses observed over the height of a printed object. This is not atypical since each filament layer must support the weight of successive depositions while still in the plastic concrete state. Consequently, differential settlement of filament layers is observed, increasing in magnitude toward the bottom critical filament layer. This paper derives a simple strain-based constitutive analytical model that determines the number of printable layers whereby the user-defined critical strain value for an individual filament layer is not exceeded. A novel material stiffness characterisation test, taking form as a modified version of the conventional unconfined uniaxial compression test, is performed on 3D printed concrete samples extracted directly from a filament layer, whereafter the model is compared against experimentally measured displacements. Digital image correlation technology is adopted for the accurate determination of displacements during printing. Experimental results indicate that the model initially underestimates displacement, followed by overestimation closer to the material yield point. Recommendations are provided toward further model improvements. This model is particularly apt in situations where the aesthetic appeal of a printed element places stricter demands than mechanical or durability requirements.