PSI - Issue 61

Zili Huang et al. / Procedia Structural Integrity 61 (2024) 252–259 Huang et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Three-dimensional (3D) concrete printing provides potential opportunities for the construction industry with the benefits of design flexibility and sustainability compared to conventional construction techniques (Hamidi & Aslani 2019; Tay et al. 2017; Wu et al. 2016). The 3D concrete structures are manufactured by stacked 2D planes, inducing weak inter-layer bonds and anisotropic properties (Panda et al. 2017). Interface weakness can cause cold joints or cracking, therefore the improvement of interlayer strength is one of the research topics to ensure technology reliability. The weak interfaces between layers induced challenges in testing, especially in fracture tests for fracture properties. Instability can happen due to abrupt failure of the interface, leading to instant load drop and hence rendering the tests out of control. This is particularly the case for indirect tensile tests using circular shape disc specimens (Brazilian tests). This type of test is appealing thanks to the ease in specimen preparation. It can give us tensile strength and also fracture properties if the tests can be carried out successfully without abrupt failure. Nevertheless, the configuration of the test and the orientation of the interfaces of 3D-printed specimens make it much more challenging than tests on “homogeneous” materials (Gell et al. 2019). Loading direction parallel to the interfaces facilitates fracture of the interface in pure mode I fracture, given maximum tensile stress in such cases is perpendicular to the interfaces. On the other hand, inclined interfaces are subjected to mixed mode conditions and when the angle between the loading direction and interfaces reaches 90 degrees, the effect of weak interfaces can be negligible, given it is subjected to compressive stress perpendicular to the interface. In this paper, the challenges in indirect tensile testing are addressed using our innovative technique in controlling the loading process. The focus is the application of this new technique for controlling fracture in Brazilian disc testing, and the associated benefits for advanced instrumentation. This technique, named AUSBIT (Adelaide University Snap Back Indirect Tensile Test; Verma et al. 2019, 2021a, 2021b; Verma, 2020) uses indirect displacement control, taking the feedback from the lateral expansion of the disc to stabilize the fracturing processes. Given the lateral expansion is a monotonically increasing quantity, its feedback to the loading machine helps control the test and allows capturing snap-back behaviour that is impossible using direct displacement control. In this sense, lateral displacement control in AUSBIT can effectively stabilize the sudden cracking of disc specimens to allow the effective use of Digital Image Correlation (DIC) to capture the full-field strain during testing. Both promising features and challenges in applying this technique to 3D-printed cement materials with weak interfaces are addressed. 2. Experiment program 2.1. Material The cementitious mortar was used in the 3D printed specimen fabrication, including five main ingredients: ordinary Portland cement (OPC), natural sand, superplasticizer and water. The OPC is used as the mixture binder produced by Adelaide Brighton Cement LTD. The sand component is procured from Marion Sand & Metal Pty. LTD, Adelaide and sieved to maintain a maximum grain size of 600 μm. The additive of ViscoCrete 10 as a superplasticizer complied with AS1478.1-2000 helps enhance the mixture fluidity without requiring an increasing amount of water content. Details on the cementitious mortar are summarized in Table 1.

Table 1. Printable mix design composition. Component

Weight ( g )

Concentration ( % )

Sand

7044.50 2555.60 1390.50

63.122 22.903 13.891

Water

Cement

Superplasticizer

9.30

0.085