Issue 50

M. Papachristoforou et alii, Frattura ed Integrità Strutturale, 50 (2019) 526-536; DOI: 10.3221/IGF-ESIS.50.44 527 ApisCor [4], WASP [5], CyBe Constructions [6], WINSUN [7]), however daily practice still seems far away. This comes from the fact that construction industry is risk adverse and conservative in its practice but also because there are technical some challenges that need to be overcome to unlock and trigger all opportunities from 3D printing in building sector [8]. Perhaps the greatest challenge of 3D printed concrete is to have a successful printing, meaning that the printed structure has the desired shape and material properties. Printing system (type of printer, pump, printhead) and concrete fresh properties are the two factors that affect significantly printing quality. There are not any commercially available printers for concrete so researchers and industries have developed different types of prototype printers and printheads and properly configure mix design. Focusing on the material, it must be flowable enough so that it can be extruded through the nozzle, meaning that the pump exerts enough stress to exceed the yield stress of concrete, causing it to flow and extrude through the nozzle. But once the layer is extruded, the concrete yield stress should be higher than the stress due to self weight in order to resist deformation. Furthermore, the yield stress of the extruded concrete should increase even more in order to support the up- coming layers through thixotropy and cement hydration. Therefore, concrete yield stress that can be measured using Self Compacting Concrete (SCC) tests becomes the most important parameter for mixture design and has an optimum range in which the material is both extrudable and buildable [9]. There are various research works studying fresh properties of 3D printed concrete using different tests. Kazemian et al. [10] proposed a framework for accepting on not a mixture as printable. Workability of a fresh printing mixture was tested using the flow table test and studied in terms of print quality and shape stability using measures of surface quality and dimensions of printed layers. Perrot et al. [11] used a rotational rheometer to measure yield stress and compare the critical stress related to the plastic deformation with the vertical stress acting on the first printed layer. However, regardless the test adopted, testing procedure requires a sample of the material in order to be performed, meaning that printing procedure must be stopped for testing. For a given printing system, real time monitoring and active control of rheology during production by adding automatically chemical additives seems to be the most robust approach in order to address this challenge [8]. Mixture proportions of concrete for 3D printing differ from ordinary one. Regarding aggregates, in most cases only fine natural or crushed aggregate is used in order to allow the material to pass through the small pipes and nozzles at the printing head. While in ordinary concrete cement quantity is 270-350 kg/m³, concrete mixtures for 3D printing use higher amounts of cement (600-900 kg/m³) in order to improve consistency and strength [12]. However, increased cement content in combination with the absence of formwork that protects the freshly placed concrete against desiccation make the concrete volume more prone to shrinkage cracking compared to conventionally placed concrete. The use of supplementary cement- itious materials (SCM) such as fly ash, silica fume or slag as partially replacement of cement can improve crack resistance of concrete at early age [13]. Chemical additives are also used to control workability and open time of fresh concrete for 3D printing which are related to printability and buildability [14]. Printability can be defined as the capacity of concrete to pass through the small pipes and nozzles at the printing head and extrude continuously without blockage or fracture occurs, and buildability as the capacity to maintain its shape once deposited and not collapse under the load of subsequent layers [10]. The use of by-products in the production of 3D printed concrete has not been studied in the literature. The scope for using such alternative materials in concrete production is twofold; it can provide some technical improvements to the final product, but it is also expected to be beneficial from an environmental point of view [15]. This paper aims at developing effective and eco-friendly concrete mixtures for 3D printing by using by-products as binders and aggregates, measure fresh properties of these mixtures, correlate them with printability and buildability and finally, test mechanical properties and durability of hardened concretes in order to obtain the optimum materials and proportions. E XPERIMENTAL PROGRAM Materials and printing system he industrial by-products that were used were high calcium (CaO free content~10% wt.) Fly Ash (FA), Ladle Furnace Slag (LFS) and Limestone Filler (LF). FA is produced in lignite fire power plants, LFS during steel production process and LF comes as a by-product from crushing of limestone. FA and LFS can be used as SCMs and increase durability of concrete [16,17,18], while LF is successfully used in self compacted concrete as filler material [19,20]. Two sets of mixtures were produced in the laboratory, one with 500 kg/m³ and one with 830 kg/m³ total binder quantity. 10% of the binder was silica fume and 90% was cement type II 52.5N. In some mixtures, 30% wt. of cement was replaced by FA or LFS passing from the 100μm sieve, while in others, LF replaced 50% wt. of siliceous river sand aggregates. The granulometry of LF, river sand and combination of both is given in Fig.1. The proportions of all the mixtures produced are presented in Tables 1 and 2, along with the type of by-products used in each mixture. From preliminary tests conducted, optimum superplasticizer and water quantities were obtained in order to have adequate and similar workability for all T