Thixotropy and interfacial bond strengths of polymer-modified printed mortars

Abstract

3D concrete printing is an emerging construction technique for building structures layer-by-layer based on digital computer models without the need of formwork. The monitoring of interlayer bonding is crucial to ensure durability and structural integrity. This investigation aims at proposing a new methodology to assess the concurrent effects of material structural build-up (thixotropy), time gap between successive filaments, and incorporation of styrene-butadiene rubber (SBR) polymers on bond strengths of deposited layers. Four mortar series prepared with 450–750 kg/m3 binder and effective water-to-binder ratio of 0.35–0.55 are tested; the resulting thixotropy rate ranged from 0.16 to 1.1 Pa/s. Results showed that mixtures exhibiting moderate thixotropy levels (i.e., about 0.48–0.64 Pa/s) yielded the best performance regarding interfacial bonding. The incorporation of SBR proved efficient to enhance the bond strength as well as attenuate its rate of drop over the time gap between successive layers. This was related to the polymer films that coalesce in the cementitious system, thus binding the cement hydrates together and causing increased tensile strength properties. The bond strengths recorded on specimens exposed to hot temperature of 45 °C were consistently lower than those determined in standard curing conditions. The methodology proposed was successfully validated using 3D laboratory printing machine.