PSI - Issue 10

M. Papachristoforou et al. / Procedia Structural Integrity 10 (2018) 155–162 M. Papachristoforou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

3-D printing is an additive manufacturing technique for constructing numerous types of products and structures using various raw materials, directly from three dimensional (3D) model data. The process consists of printing success ive layers of materials on top of each other. There has been a growing interest upon 3D printing for civil engineering applications over the past decades due to the advantages this method possesses compared to traditional construction methods, such as the ability to construct complex geometry without the need of formwork, construction speed and minimize waste material and labor cost. There are many examples of such applications both from private companies and researchers. Khoshnevis (2004) has developed Contour Crafting (CC) which is based on the extrusion of cementitious materials in order to construct automatically a single house or a colony of houses, each with possibly a different design, or even habitats on other planets. The extruded surface roughness of every layer is smoothed out using a trowel while perform ing the extrusion. The 3D printhead is mounted on an overhead crane as the system is designed for on-site construction operations. Lim et al. (2011) from the department of Civil Engineering at Loughborough University were the first to study and develop a high-performance 3D printable concrete. They used a 3D printer that had a small printhead to 3D print in many layers a bench-looking structure. They were among the first to study the fresh and hardened properties of printable concrete and they maintained a lower limit of about 10 MPa of flexural strength in order the concrete to be characterized as high-performance. Also, using admixtures they were able to create a relatively flowable mixture. Cesaretti et al. (2014) created D-Shape, a technique using sand that can be hardened in pre-arranged places to create structures of different shape and size. The main advantage of this method is that the sand bed is used as the support, making it possible to built complex 3D structures. The main drawbacks of the D-Shape printing method are that building dimensions are limited by the dimensions of the printing equipment, the need for a constant supply of sand and the fact that needs a significant amount of time for the creation of a new design. Institute for Advanced Architecture of Catalonia, IAAC (2016) from Barcelona has developed a technique that uses robots to create any shape and form of buildings. It was one of the first to 3D-print a bridge in Madrid in full scale and now is planning on continuing this trend with other materials such as metal and plastic. They use high-tech drawing technology at state of the art laboratories to design and build complicated structures and they even have partnerships with MIT in some projects. Many other companies around the world have developed different techniques and equipment for concrete 3D printing (ApisCor TM , WASP, CyBe Constructions, WINSUN). Limited research has been carried out on properties of printing concrete and especially on its fresh state. Kazemian et al. (2017) proposed a framework for performance-based laboratory testing of cementitious mixtures for construction scale 3D printing, where workability of a fresh printing mixture was studied in terms of print quality, shape stability, and printability window. Print quality described using measures of surface quality and dimensions of printed layers. Experimental study of four different mixtures revealed that inclusion of silica fume and Nano-clay significantly en hances shape stability. The results of five conventional test methods, as well as four proposed tests were used to discuss the performance of mixtures as printable or not. In 2016, Perrot et al. (2016) compared the vertical stress acting on the first printed layer with the critical stress related to the plastic deformation and defined a critical failure time as a function of concrete specific weight, concrete yield stress with no time at rest, structuration rate, construction rate and a geometric factor. In this paper, criteria based on printing quality and dimensions of printed layers were established in order to accept a concrete mixture as printable or not. Concrete mixtures with different raw materials were produced and printed. Measurements of workability of fresh concrete by four different methods were conducted and printability windows were obtained. Finally, properties of hardened concrete such as compressive strength, ultrasonic pulse velocity and density were also measured.

2. Experimental program

2.1. Materials and printing system

Crushed limestone, siliceous river sand and a combination of both (50% limestone + 50% river) were used as aggregates in the concrete mixtures, and their granulometry is given in Fig.1. It can be seen that river sand is much