Environmental stress cracking of 3D-printed polymers exposed to concrete

Abstract

Increasing demand for complex concrete shapes in architecture has triggered the use of digital fabrication methods for producing non-standard formworks. To this end, polymer extrusion 3D printing has already shown promising results in a range of research projects. However, 3D-printed polymer formworks can suffer from sudden failures during casting, which are generally associated with the pressure exerted by the wet concrete on the thin plastic formworks. Such failures can however be substantially accelerated by the corrosive effects of the wet concrete paste on some 3D-printed thermoplastics, a phenomenon that has not been studied in depth so far. This paper first reviews the state of the art of 3D-printed polymer formworks and their applications in architecture. It focuses on the failures caused by formwork pressure and synthesizes the various solutions to this problem reported in the literature. Beyond this, additional results are introduced to provide a guideline for thermoplastic selection in 3D-printed formworks. Common thermoplastics are presented, and their behaviour during concrete casting is documented. This behaviour can be analysed through ultimate strength and creep tests performed in three-point bending on 4 × 10 × 100 mm 3D-printed polymer samples, both in dry state and exposed to an alkaline solution of pH 13, typical for cementitious materials. The tests show that contact with the alkaline solution leads to a rapid strength loss in some thermoplastics. This effect is particularly pronounced with PLA and its blends, for which the strength loss over 10 min is more than 80 % compared to dry samples. Other thermoplastics, such as ABS, ASA, HIPS, Nylon, PVA and ABS/PC have shown more limited or in some cases no deterioration under the same conditions. The material-specific mechanism of deterioration is environmental stress cracking in contact with the alkaline solutions and it occurs in all 3D printing orientations and loading directions where tensile stresses occur.