Letters Characterizing cement mixtures for concrete 3D printing Karthick Manikandan a , Kwangwoo Wi b , Xiao Zhang a , Kejin Wang b , Hantang Qin a,⇑ a Department of Industrial and Manufacturing Systems Engineering, Iowa State University, Ames, IA 50011, USA b Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA 50011, USA article info Article history: Received 8 July 2019 Received in revised form 27 November 2019 Accepted 4 March 2020 Available online 13 March 2020 Keywords: Concrete 3D printing Extrudability Rheological properties Cement additives abstract The need to automate the construction process for civil infrastructures has been ceaselessly propelled by the reported number of detrimental site accidents, enormous time, and material wastages in current labor-intensive approaches. Additive 3D printing could revolutionize the construction-site. Notable advantages of concrete 3D printing include wider build customizability, safer work ambiance, reduced construction time and cost. The major challenge in concrete printing is to identify and maintain the mix- ture characteristics suitable for printing and stacking up in layers. In this study, ready-to-print fresh cementitious mixtures with silica fume and superplasticizer were characterized for printability based on their rheological properties. Ó 2020 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved. 1. Introduction In the construction field, concrete is one of the globally used materials as they are highly beneficial for both in-situ construc- tions as well as setting the prefabricated constructs. Concrete pro- vides a better moldability, thermal resistance, and durability under a relatively lower production cost. Although concrete has the inherent material advantages, it requires higher labor activities starting from mixing with appropriate admixtures, casting to the required shape (formwork), until temperature-controlled curing for desired mechanical properties. According to a recent study [1], a formwork can account for about 50% of the total construction cost and takes about 70% of the construction time. The cost would be increased if the construction design is intricate. The other chal- lenge is to handle the substantial amount of waste generated from the construction sites since frames are challenging to recycle sev- eral times after casting the concrete [2]. According to the Environ- mental Protection Agency (EPA), about 578 million tons of construction and demolition debris was generated in the year 2015 in the United States [3]. The third major challenge is the reported number of accidents and fatalities in the construction sites. Occupational Safety and Health Administration (OSHA) reported that 20.7% of the fatalities in the private industries hap- pened at construction sites in the United States in 2017 [4]. The construction industry has gone through numerous automa- tion to reduce the accidents and construction time, specifically, when the single robotic arms deployed in the early 1980s reduced the labor activities on construction sites [5]. Although semi- automated and automated machines were introduced to handle the cementitious and other construction materials, the need for labors on-site has never diminished. 3D printing a constructive approach has attracted attention from various fields due to its advantages such as easy customization, efficient material con- sumption, less human–machine interaction, and relatively less production time [6]. For the past few years, researchers across the countries [7–10] studied 3D printing process as a technological revolution in the construction field, as it could reduce the material wastage and the labor activities, which thereby reduce the produc- tion cost and fatalities on the sites. The purpose of this study is to develop a schema to identify the best concrete mixture that can be 3D printed instantly. 2. Materials and methods 2.1. Printing platform and concrete additives An extrusion-based Velleman K8200 3D printer was con- structed in the lab (Fig. 1) to instantly test the printability of the concrete mixtures. The resolution in x and y axis was 0.015 mm, resolution in z axis was 0.781 mm. Maximum printing speed is 300 mm/s. The printable area was: 20 cm by 20 cm by 20 cm. The printer can print up to 60 mm of concrete layers. A 3 mm diameter nozzle was fixed constantly. The travel speed and the feed rate in the printer can be adjusted and were maintained at 65 and 25 mm/s respectively throughout the experiments. Before printing the concrete, rheological assessments were made on all the mixtures in Table 1 using the HR-2 Discovery Hybrid Rheome- https://doi.org/10.1016/j.mfglet.2020.03.002 2213-8463/Ó 2020 Society of Manufacturing Engineers (SME). Published by Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: qin@iastate.edu (H. Qin). Manufacturing Letters 24 (2020) 33–37 Contents lists available at ScienceDirect Manufacturing Letters journal homepage: www.elsevier.com/locate/mfglet