Venkatesh Naidu Nerella

Developing and Testing of Strain-Hardening Cement-Based Composites (SHCC) in the Context of 3D-Printing

Incorporating reinforcement into the practice of digital concrete construction, often called 3D-concrete-printing, is a prerequisite for wide-ranging, structural applications of this new technology. Strain-Hardening Cement-based Composites (SHCC) offer one possible solution to this challenge. In this work, printable SHCC were developed and tested. The composites could be extruded through a nozzle of a 3D-printer so that continuous filaments could be deposited, one upon the other, to build lab-scaled wall specimens without noticeable deformation of the bottom layers. The specimens extracted from the printed walls exhibited multiple fine cracks and pronounced strain-hardening characteristics under uniaxial tensile loading, even for fiber volume fractions as low as 1.0%. In fact, the strain-hardening characteristics of printed specimens were superior to those of mold-cast SHCC specimens.

Capillary Water Intake by 3D-Printed Concrete Visualised and Quantified by Neutron Radiography

Water uptake into two formulations of 3D-printed concrete via capillary suction was assessed by neutron radiography. The samples varied in their layer-to-layer deposition time intervals (TI) and the use of different binders. TI of two and 13 min were short enough to avoid preferential capillary suction at interlayer bonding areas in the fine-grained printable concretes containing supplementary cementitious materials. An increase in the time interval to 24 h gave rise to quick capillary suction through the layer-to-layer interfaces. However, moisture did not redistribute into the matrix regions from the interfaces. For mixture with Portland cement as sole binder and addition of a superabsorbent polymer (SAP), the short layer-to-layer deposition interval of two minutes resulted in tight interlayer bonds with quasi-null capillary suction. Intervals of 13 and 36 min, however, resulted in partially quick and intense absorption of water and immediate absorption by adjacent SAP particles.

Alternative Reinforcements for Digital Concrete Construction

Applications of structural concrete require use of reinforcement in one form or another. The known reinforcement concepts in additive concrete construction typically rely on conventional reinforcement approaches, which provide a solid basis for structural design, since existing guidelines and codes can be used. However, the use of conventional steel reinforcement poses serious limitations to the digitalization and automation of fabrication techniques. The article at hand presents two alternative approaches of reinforcing 3D-printed concrete structures: (1) additive manufacturing of steel reinforcement elements, (2) use of high-performance microfiber to achieve strain-hardening behavior of printed concrete. For both approaches materials and manufacturing techniques are briefly described followed by the results of mechanical testing and complimentary microscopic investigations. The printed steel bars showed similar mechanical performance in comparison to ordinary steel bars of the same cross-section area and comparable bond behavior to concrete too, as observed in pull-out experiments. The addition of 1% to 1.5% high-density polyethylene microfiber to fine-grained matrix enabled for printable strain-hardening cement-based composites (SHCC) with a tensile strain capacity of up to 3.2%.