Correlation between destructive compression tests and non-destructive ultrasonic measurements on early age 3D printed concrete
Introduction
Presently, a rapidly growing number of innovative case-study structures is being presented that have been realized through various forms of additive manufacturing methods of concrete and cementitious materials. It is increasingly recognized by industry and clients that these technologies present a serious potential in terms of optimized material use, reduced labour, and form freedom. The focus, by and large, is on delivering proofs of concept which show the aesthetical and economical potential. In most cases, extrusion-type methods with a nozzle attached to various types of moving robots are applied [1], [2], [3], [4], [5]. These layer-wise extrusion processes are commonly referred to as (3D) concrete printing. Some associated technologies are simultaneously under development, such as D-Shape (based on binder jetting) [6], Mesh Mould [7], and Smart Dynamic Casting [8], [9]. Digitally Fabricated Concrete (DFC) is used as a generic term to refer to these innovations. Their common denominator is the perspective to move towards largely automated production. The study presented in this paper relates to 3D Concrete Printing (3DCP) technology under development at the Eindhoven University of Technology [10].
Owing to the novelty of concrete printing, the structural properties of the fabricated object, both during and after printing, are often only globally understood. Significant knowledge gaps still exists concerning the specific relations between the design, material, system, and product. It is known from other 3D printing industries, these parameters interact heavily and significantly influence the quality of the fabricated product [11]. Structural collapse during printing and layer delamination afterwards are common failures associated with insufficiently attuned processes. To avoid such failures to occur, and to prove that the manufactured object is equal to the design, quality control procedures should be introduced to DFC. As such, the authors suggest a development towards a system in which:
- (1)
the manufacturing process is monitored continuously,
- (2)
the acquired data is used for real-time quality control, and
- (3)
a closed feedback loop reacts upon the quality control when required.
Primarily, this concerns a measuring method in line with the digital nature of the manufacturing process. In conventional concrete construction, type testing (e.g. cube compression tests, slump-flow tests) is a commonly applied instrument to prove certification compliance and thus to show sufficient quality has been obtained. However, the continuous nature of the printing process and the presence of interacting parameters, make such isolated and destructive tests unsuitable. Instead, an online non-destructive monitoring system is required, which can measure properties on every position and moment in the process.
To assess the quality, or structural integrity, of the printed object based on the online measurements, initial steps towards the structural analytical and numerical modelling of the 3DCP process have been presented [12], [13]. These modelling methods aim to predict the structural performance and possible failure modes of objects during printing, based on mechanical properties as defined by the printing process. Further development of modelling methods will be required to predict other crucial properties, such as the interface adhesion, which depends on interval time and application pressure among others.
Likewise, early examples of online monitoring and feedback systems were introduced. Lloret et al. adopted a method of simultaneous penetrometer tests during extrusion to record the concrete strength evolution [8], and formwork pressure and friction measurements [9], to adjust the robot speed accordingly. Neudecker et al. [14] proposed a feedback loop for robotic spraying of concrete, where 3D scanners measure the surface finish of the sprayed concrete parts. Wolfs et al. [15] presented a continuous measurement and real-time adjustment of print height nozzle relative to the print surface (or previous layer) for the 3DCP system.
This study takes a first step in connecting the structural modelling on 3DCP objects during printing as introduced by the authors [12] with continuous monitoring of the mechanical properties that are used in such modelling. In particular, the correlation of parameters from a non-destructive test method with potential for online monitoring, to mechanical properties determined from an appropriate destructive test for concrete in the dormant state is established.
Section snippets
Concrete hydration and the 3DCP process
Concrete goes through several stages during 3D printing that can be organized by its hydration processes and relative to the printing process. The four stages of hydration are: (1) the initial hydration directly after mixing, when the cement first comes in contact with water, (2) the dormant stage in which the cementing reactions are delayed, and the mechanical properties are mainly determined by thixotropic build-up attributed to both interparticle forces and low-rate hydration reactions [16],
Material and 3D printing system
In this research, a custom designed printable mortar Weber 3D 145-1 was applied, containing Portland cement (CEM I 52.5 R), siliceous aggregate with a maximum particle size of 3 mm, limestone filler, additives, rheology modifiers and a small amount of polypropylene (PP) fibres. This is an improved version of the mortar introduced by Bos et al. [10] that has since been branded Weber 3D 115-1. The fresh concrete is extracted after mixing and pumping using the 3DCP setup [10], which consists of an
Uniaxial unconfined compression
The cylindrical samples were loaded in compression up to 30% strain. Typical load-displacement curves are shown in Fig. 3 for concrete age t = 30 min, were the five grey lines represent each individual test, and the black line indicates their average. Each load-displacement curve was then translated into a stress-strain relation, taking into account large deformations of the samples. The updated cross section, derived by image analysis, was used to calculate the actual state of stress during
UUCT
As expected from previous research, Fig. 4 shows the mechanical properties of the print mortar develop significantly within the time frame of a typical printing process. Globally, the mortar behaviour tested in this study (Weber 3D 145-1) is very similar to that of the mortar (Weber 3D 115-1) investigated in the previous study [12]. Both the strength and Young’s modulus increase with concrete age, as can be recognized from the increment of the maximum stress and of the initial slope in the
Conclusions
The early age mechanical properties which are required for structural analysis of a 3D concrete printing process can be determined directly through destructive tests or indirectly through non-destructive methods that measure associated variables. In this study, the correlation between the compressive strength and the Young’s modulus obtained from unconfined uniaxial compressions tests, and the pulse velocity taken from ultrasonic wave transmission tests (all of which are time dependent) in the
Conflict of interest
There are no conflicts of interest to disclose.
Acknowledgements
The support of the staff of the Structures Laboratory Eindhoven is greatly acknowledged. The assistance in the 3DCP research of Master track students Structural Design at the TU/e Department of the Built Environment is highly valued. For this paper, the authors appreciate the work of R. Crombez, C. Simpelaar and M. van de Ven in particular, on whose MSc project the material presented is partially based.
The TU/e research program on 3D Concrete Printing is co-funded by a partner group of
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