Reinforcement Strategies for Digital Fabrication with Concrete
Author: Lukas Gebhard
Language: English
external pageDOI: 10.3929/ethz-b-000614836call_made
Abstract
The construction industry faces the challenge of reducing its large environmental impact while at the same time providing housing and infrastructure to a growing global population. One of the main causes of the negative impact is the vast amount of reinforced concrete used. Digital fabrication with concrete (DFC) might offer a path towards more sustainable construction by producing structurally optimised shapes without extra cost or effort. However, DFC technologies have been mainly applied to date to produce elements with low structural demand. One of the main reasons for this limited applicability is the challenge of integrating reinforcement to comply with existing building codes. This thesis thus aims at advancing the application of DFC for load-bearing applications by developing and assessing reinforcement strategies based on sound structural principles. To this end, five experimental campaigns were designed, examining various fabrication approaches and reinforcement strategies and assessing their performance based on established models for reinforced concrete.
In its first part, the thesis is contextualised based on the two identified current challenges, i.e., (i) of the construction industry to reduce its environmental impact and (ii) of DFC to reach structural integrity. The need for consistent structural testing of large-scale components with simple geometries is identified based on the state-of-the-art of various DFC technologies and corresponding reinforcement strategies.
The second part of the thesis presents five experimental campaigns exploring and assessing various reinforcement strategies. In the first campaign, the feasibility of aligned interlayer fibre reinforcement as shear reinforcement is explored. These fibres are combined with unbonded post-tensioning and bonded passive deformed steel reinforcing bars as longitudinal reinforcement of 3D concrete printed beams, and the resulting behaviour is compared to that of beams without and with steel cables as shear reinforcement. In the second campaign, the versatility of aligned interlayer fibres as shear reinforcement is examined for the Eggshell technology on rectangular beams and compared to fibres in the mix and conventionally cast specimens. The insights from these beams are then applied for material optimisation resulting in a T-beam with almost 50% volume reduction. The bond between 3D printed concrete and conventional reinforcement or cables is assessed on pull-out tests produced with high yield stress or set on-demand concrete in the third campaign. The fourth campaign investigates conventional reinforcement cages designed according to established building codes to fabricate an optimised T-beam with 3D concrete printing. The last campaign explores the use of reinforcing bar meshes fixed by shotcreting to an unreinforced printed shell as a reinforcement solution for water tank walls, with the structural performance being evaluated with a series of direct tension tests.
The results of these five experimental campaigns allow (i) gaining insights into the efficiency of the reinforcement strategies, (ii) assessing the influence of fabrication parameters, (iii) comparing the results to models for conventional reinforced concrete, and where needed, (iv) developing new models based on established structural principles. The lessons learnt are discussed in the third part of the thesis and put into context with other reinforcement approaches and DFC technologies, comparing the various reinforcement strategies and identifying their advantages and drawbacks, highlighting the complexity of choosing the right fabrication parameters and finally proposing future research areas for structural applications of DFC.