Issue 50

M. Papachristoforou et alii, Frattura ed Integrità Strutturale, 50 (2019) 526-536; DOI: 10.3221/IGF-ESIS.50.44 530 problems can be severe in 3D printing concrete due to high cement paste quantity, was also tested according to ASTM C1581-04 [27]. The procedure can be used to determine the effects of variations in the proportions and material properties of mortar or concrete on cracking due to both drying shrinkage and deformations caused by autogenous shrinkage and heat of hydration. The advantage of this test is that is more realistic since it measures concrete shrinkage under restrained conditions which occur in real applications. The apparatus consists of a steel ring with an outside diameter of 330 mm and height 152 mm and two strain gages at midheight locations on the interior surface of the steel ring along a diameter. The base, the outside steel ring surface and another outer ring with inner diameter 406 mm are used as a mould and concrete is poured. As concrete shrinks, it applies stress on the inner steel ring which is recorded in the form of deformation measured from the strain gages attached, until cracking occurs. Results of the test are the age of concrete at cracking (days) and the stress rate at cracking ((MPa/day). In Fig.2, a crack formed on concrete can be seen while Fig.3 shows a typical diagram of age versus ring strain obtained from the measurements. Figure 2 : Formation of crack on concrete due to restrained shrinkage. -60 -45 -30 -15 0 0 50 100 150 200 Steel ring strain x 10 -6 Specimen age (Hours) Figure 3 : Age versus steel ring stress obtained from the restrained shrinkage test. Results Results of workability 0, 15 and 30 minutes after mixing are given in Tables 5 and 6. For the given printing system, it was soon realized that flow table expansion values, in order the mixture to be printable, should be between 18 and 24 cm. From the other tests, accurate threshold values could not be obtained. In Figs.(4,5), flow table expansion versus the other parameters is given along with least squares regression lines and printable window (blue area with expansion 18-24 cm). For expansion less than 15 cm measured in some mixtures, no further testing was conducted since the mixtures were very stiff and could not be extruded. For the series of mixtures with low binder content (Table 5), three out of five were considered as printable only for 0 minutes after mixing. After 15 minutes, all of the mixtures were considered as not printable. On the other hand, mixtures with high binder content (Table 6) could be extruded but collapse 0 minutes after