Issue 50

M. Papachristoforou et alii, Frattura ed Integrità Strutturale, 50 (2019) 526-536; DOI: 10.3221/IGF-ESIS.50.44 529 A prototype printhead was introduced to extrude the mixtures. The diameter of the nozzle was 2 cm and 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) [21]. Concrete tests Fresh concrete properties were estimated using a rotational rheometer [22,23] and the flow table test according to EN 1015-3 [24]. These tests were conducted 0, 15 and 30 min after mixing in order to determine the rate of which the work- ability is lost for the given 3D printing system and materials. However, performing of these tests requires extracting an ad- equate quantity of the material from the printing system and results are obtained after the test is completed. For this reason, a new method was implemented in order to estimate real-time workability of fresh concrete during printing by recording real time energy consumption of screw extruder motor. The method was calibrated with the rotational rheometer ASTM C1749 test [23] for measuring rheology characteristics of SCC mixtures and the flow table test. The rotational rheometer test uses a rotating four-bladed vane. A strain gage measures deformation of the shaft rotating the vane and transforms it to torque and finally, yield stress. With the proposed method, the vane is substituted by the screw extruder and the complex strain gage recording apparatus fitted on the rotating shaft by a much simpler wattometer that records the Watts of the motor that rotates the extruder. The same principle is used in ready-mix concrete plants where the power consumption of the mixer motor is recorded during mixing, making possible for the operator to adjust water quantity in order to obtain desired workability. By measuring real-time workability of mixture, it is possible to alter it manually or automatically by adding chemical additives in the print-head (superplasticizer, viscosity modifier, retarder etc.), using appropriate equipment as pro- posed by Gosselin et al. [25]. The final workability parameters that were measured are presented in Table 3. Standard Test apparatus Parameter tested ASTM C 1749 Rotational Rheometer Static Yield Stress (Pa) EN 1015-3:1999 Flow table Expansion (mm) - Real time monitoring of workability Electric power consumption of electric motor rotating screw extruder (W) Table 3 : Testes for measuring workability of 3D printing concrete. The threshold values of the above parameters were obtained in order to characterize the concrete mixture as printable, meaning that it can be extruded from the nozzle, and buildable. Buildability was estimated by the number of layers of the printing specimen that can be achieved without collapse. Malaeb et al. [26] proposed that 5 layers were considered to be the target and this was the method that was adopted. Additionally, shape stability was another criterion for buildability and was estimated by measuring the dimensions of the first and fifth layer. 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 4. Characteristics of 3D printing concrete Accepted Not accepted Printability 1. The mixture is extruded through the nozzle If 1 or 2 do not apply 2. Good printing quality meaning no voids, no dimensional variations of extruded material Buildability 3. Five layers of printing material can be achieved without collapse If 3 or 4 do not apply 4. Height of 1 st layer versus height of 5 th layer ~ 1 Table 4 : Criteria for accepting concrete mixture as printable or not. Regarding hardened concrete, compressive strength was measured on 40x40x40 mm cubes and 3-point flexural strength on 40x40x160 mm prisms after 28 days of curing in the humidity chamber. Additionally, density and Ultrasonic Pulse Velocity (UPV) of concretes were also recorded. Volume stability of mixtures, which is very important since shrinkage