PSI - Issue 64

Saim Raza et al. / Procedia Structural Integrity 64 (2024) 1200–1207 Raza / Structural Integrity Procedia 00 (2024) 000 – 000

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1. Introduction The traditional method of onsite manufacturing of concrete slabs involves using temporary timber formworks along with temporary shoring/scaffolding to support the slab until the concrete has hardened and gained enough strength to withstand the design loads. According to an estimate in Jha (2012), the temporary formwork can account for up to 35 50% of the total construction cost and 50-75% of the total construction time in conventional construction, as this formwork has to be removed once the concrete has hardened. In addition, temporary timber formwork is a major source of wastage in traditional construction as it is often discarded after a few times usage. Digitally fabricated stay in-place formwork for concrete slabs offer a promising solution to address the drawbacks of the temporary formwork used in traditional construction. Recent research studies and field practice have shown successful application of stay-in-place 3DPC formworks for structural members. However, this application has remained mainly limited to compression-dominant structures. For instance, stay-in-place 3DPC formworks have been used for building columns and segmental bridge columns as presented in Zhu et al. (2021) and Raza et al. (2024). Other applications of 3DPC structures include fabrication of unreinforced masonry arch bridge as shown in Dell'Endice et al. (2022). In addition, there have been recent proposals in Mata-Falcón et al. (2022) for using 3DPC to enable sustainable, structurally efficient (and otherwise complex to manufacture) floor systems. However, the application of 3DPC as a permanent formwork for horizontal structural members such as slabs/beams is currently limited because the deadweight of slabs is typically much higher than other structural members, and therefore a very stiff and heavily reinforced 3DPC formwork section would be required to carry the loads of the slabs with moderate to long spans, even those that have optimized geometries. To address the issue of high flexural demands, in horizontal members, stay-in-place formwork made up of ultrahigh performance concrete has been proposed in Zhang et al. (2022). Post-tensioning of the 3DPC formwork appears to offer a viable solution to address this issue. In particular, post-tensioning can help to reduce the thickness of the formwork while improving the ability to carry loads and resist cracking. This study presents a novel prefabrication concept wherein post-tensioned 3D-printed concrete is used as a stay in-place formwork for slabs. The slabs with ribbed topology are used to assess the prefabrication concept. The next section presents details of the fabrication concept, which is followed by the design details and evaluation of the printing feasibility of the 30 mm thick stay-in-place formwork with ribbed topology. The paper ends up with a discussion on the design implications of the corrugated surface of 3DPC compared to conventional flat concrete ones. 2. Fabrication concept The proposed fabrication concept consists of 3 stages. In the first stage, a thin 3DPC with a ribbed topology is fabricated using an extrusion-based robotic printer, as shown in Fig. 1 (a). In the second stage, unbonded end anchorage post-tensioning is performed to induce a precompression in the 3DPC formwork to enhance its load carrying capacity, as illustrated in Fig. 1 (b). In the final stage, the minimum reinforcement required for temperature and shrinkage is installed and the cast concrete is poured, as shown in Fig. 1 (c). 3DPC Formwork 3DPC Formwork

3DPC Formwork

Post-Tensioning Tendon

3DPC Formwork

3DPC Formwork

Post-Tensioning Tendon

(a)

(b)

Cast Concrete

Cast Concrete

3DPC Formwork

Post-Tensioning Tendon

(c) 3DPC Formwork

Post-Tensioning Tendon

3DPC Formwork

Post-Tensioning Tendon

Fig. 1. Fabrication Stages for Post-Tensioned 3D Printed Concrete Slab: a) Stage 1: Fabrication of 3DPC formwork; b) Stage 2: Post-tensioning of the 3DPC formwork; c) Stage 3: Pouring of cast concrete

Cast Concrete

3DPC Formwork

Post-Tensioning Tendon