PSI - Issue 10

M. Papachristoforou et al. / Procedia Structural Integrity 10 (2018) 155–162 M. Papachristoforou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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coarser than limestone filler and this is the reason why a mixture of both was also used. Maximum size of 1 mm was selected as the diameter of the printing nozzle was relatively small (~2 cm). Regarding the binder, cement type II 52.5 was used at a quantity of 500 and 830 kg/m ³ . Fly Ash (FA) with relatively high CaO free content (≈5%) , a by-product from lignite fired power plants, and Ladle Furnace steel Slag (LFS), a by-product of steel industry, were also used as alternative cementitious materials along with Silica Fume (SF) in the mixtures. CFA or LFS replaced 20 wt.% and SF 10 wt.% of cement. CFA and LFS replaced an amount of cement not only for cost reduction, but also to reduce con crete shrinkage due to high cement paste quantity. Water/binder ratio ranged from 0.34 to 0.56, depending on type of aggregates and binders used. A superplasticizer (Sika Viscocrete 300) at various addition rates (0-2.5 wt.% of binder) was used in order to obtain different levels of workability for the same mixture. The purpose of the relatively large number of mixture parameters was the production of a large number of concrete mixtures (over 20) with different workability in order to establish the limits for accepting them as printable or not.

Limestone filler

River Sand

0 25 50 75 100

Passing (%)

0.01

0.1

1

Sieve (mm)

Fig. 1. Granulometry of various aggregate mixtures used.

A prototype printing system was introduced to print and check all the latter parameters in small scale experiments. The system includes the 2 cm diameter nozzle for linear extrusion as well as the base where the nozzle prints the layers of the mixtures, which in each turn is also able to move in z axis. Regarding the extrusion mechanism, a screw-kind extrusion system was selected for the extrusion process due to some of the advantages and easiness it offers in com parison to other extrusion methods (syringe extrusion).

2.2. Concrete tests

Workability of fresh concrete was estimated using various tests that include a rotational rheometer (Koehler and Fowler (2004)), flow table test according to EN 1015-3 (1999), and Vicat apparatus according to EN 196-3 (2005). All these tests were conducted 0, 15 and 30 min after mixing in order to determine the rate of which the workability is lost for the given 3D printing system. However, for performing these tests, an adequate quantity of the material must be extracted from the printing system and results are obtained after the required testing time. Additionally, in real scale applications, workability of 3D printing concrete is prone to even small variations of environmental conditions (temperature, humidity, moisture of raw materials, etc). For these reasons, a novel method was implemented on selected mixtures in order to estimate real-time workability of fresh concrete during printing. The electric power consumption of the electric motor that rotates the screw extruder was measured and correlated with the corresponding values of workability obtained from the flow table test. This way, the properties of fresh concrete can manually or automatically be corrected by adding chemical additives in the print-head (superplasticizer, viscosity modifier, retarder etc.), using appropriate equipment as proposed by Gosselin et al. (2016). The final workability parameters that were measured are presented in Table 1. The threshold values of all the above parameters in order to characterize the concrete mixture as printable were obtained during the printing procedure, where the material must be able to be extruded from the nozzle. On the other hand, buildability can be estimated by the number of layers of the printing specimen that can be achieved without