This paper investigates the rheological and printability characteristics of PVA fiber-reinforced cementitious composites. To fabricate 3D printable strain hardening cementitious mixtures, ordinary Portland cement, fly ash, silica fume, fine sand, water, and a polycarboxylate-based superplasticizer are used. The effects of a modified starch-based viscosity modifying agent and nano clay on the rheological properties of these mixtures are explored. A shear rheometer with a building materials cell and vane motor is used for rheological tests. First, stress-growth tests are conducted to determine the static yield stress evolution curves for the PVA fiber-reinforced cement composites. A constant low shear rate is applied to minimize the viscous contributions to yield stress. Then, structural recovery tests are conducted by applying three different shear rates that mimic initial rest, extrusion, and after deposition conditions of printable mixtures and the change in apparent viscosity is observed. Next, structural build-up of PVA fiber-reinforced cementitious composites is assessed through constant shear rate rheology tests at different rest intervals. Finally, the buildability of the PVA fiber-reinforced cementitious composites is evaluated using a 3D concrete printer equipped with a 15 mm diameter nozzle and screw pump.
The cement-based additive manufacturing, commonly known as 3D concrete printing, facilitates the use of advanced cementitious materials in construction as this construction technique minimizes waste and enables the optimal placement of the material. 3D printable cementitious mixtures should have specific consistency for successful manufacturing. In particular, they should be extruded smoothly during the printing process while maintain their shape after deposition, both of which are closely related to the rheological properties of cementitious mixture. The use of graphene in cementitious composites has been widely explored in recent years and it was shown that graphene can improve the mechanical properties and durability of cementitious composites. However, the rheological properties and printability characteristics of graphene-reinforced cementitious materials still remain underexplored. This study investigates the effects of graphene nanoplatelets (GNPs) on rheological and printability characteristics of GNP-reinforced cementitious composites. GNPs are added into cementitious mixtures, designed for 3D concrete printing applications, at concentration of 0%, 0.05%, 0.10%, 0.15%, 0.20%, and 0.25% by weight of cement. GNPs are first dispersed into water through the help of ultrasonic treatment and a polycarboxylate-based superplasticizer. The dispersion quality of GNPs is assessed through UV-vis absorption spectroscopy, optical microscopy, and Raman spectroscopy. Then, the rheological properties of GNP-reinforced mortar composites are studied using a shear rheometer via stress-growth tests, shear rate ramp up-down tests, and structural recovery tests.
Cyclic and time dependence of superelastic properties such as critical stress for stress-induced martensite (σSIM),
irrecoverable strain levels, and stress hysteresis are crucial parameters to ensure stable operation in applications of shape
memory alloys. In our studies on titanium-niobium shape memory alloys that have undergone various thermo-mechanical
processes, declines in both σSIM and stress hysteresis with increasing cycle number were observed. More
surprisingly, aging treatment at room temperature following cycling produced stress-strain behavior very similar to
behavior of samples prior to cyclic deformation, and irrecoverable strain levels did not increase monotonically with
increasing cycle number. Lowest irrecoverable strain levels and smallest evolution in the superelastic behavior were
found in precipitated cold rolled or processed specimen by equal channel angular extrusion.