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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collapse. Malaeb et al. (2015) proposed that 5 layers were considered to be the target and this was the method that was adopted. Additionally, dimensions of the first and fifth layer were also a criterion for buildability. The height ratio of the first layer versus fifth layer should be kept around 1 in order to accept the mixture as printable. Final criteria for printability and buildability are given in Table 2.

Table 1. Parameters tested for measuring workability of 3D printing concrete. Standard Test apparatus Parameter tested ASTM C 1749 Rotational Rheometer Static Yield Stress (Pa) EN 1015-3:1999 Flow table Expansion (mm) EN 196-3:2005 Vicat Vicatscale reading (mm) - Real-time monitoring of workability

Electric power consumption of electric motor rotating screw extruder (W)

Table 2. Criteria for accepting concrete mixture as printable or not. Characteristics of 3D printing concrete Accepted

Not accepted

Printability

1. The mixture is extruded through the nozzle 2.Good printing quality meaning no voids, no dimensional variations of extruded material 3. Five layers of printing material can be achieved without collapse 4. Height of 1 st layer versus height of 5 th layer ~ 1

If 1 or 2 do not apply

Buildability

If 3 or 4 do not apply

Regarding hardened concrete, compressive strength was measured on 40x40x40 mm cubes after 28 days of curing in the humidity chamber. Additionally, density and Ultrasonic Pulse Velocity (UPV) of concretes were also recorded. Volume stability of selected mixtures, which is very important since shrinkage problems can be severe in 3D printing concrete due to high cement paste quantity, was also tested according to Le Chatelier apparatus (EN 196-3:2005).

3. Results

Concrete mixtures produced with different aggregates, binders and different amount of water and superplasticizer were produced, tested for workability according to four different tests and printed in order to have available a wide data range of measured workability parameters and finally define their threshold values that characterize a concrete mixture as printable. In Fig.2, the mixture with river sand as aggregate, binder of cement, LFS and SF, and 2.5 % (left speci-

Fig. 2. River sand, cement, LFS, SF mixture with higher (left) and lower (right) amount of superplasticizer.