Materials 2022, 15, 2750. https://doi.org/10.3390/ma15082750 www.mdpi.com/journal/materials Article Preliminary Study of the Fresh and Hard Properties of UHPC That Is Used to Produce 3D Printed Mortar Ester Gimenez-Carbo 1, *, Raquel Torres 1 , Hugo Coll 1 , Marta Roig-Flores 2 , Pedro Serna 1 and Lourdes Soriano 1 1 Institute of Concrete Science and Technology (ICITECH), Universitat Politècnica de València, 46022 Valencia, Spain; ratorrem@hotmail.com (R.T.); hucolcar@cst.upv.es (H.C.); pserna@cst.upv.es (P.S.); lousomar@upvnet.upv.es (L.S.) 2 Department of Mechanical and Engineering Construction, Universitat Jaume I, 12071 Castellon de la Plana, Spain; roigma@uji.es * Correspondence: esgimen@cst.upv.es Abstract: Three-dimensional printed concrete (3DPC) is a relatively recent technology that may be very important in changing the traditional construction industry. The principal advantages of its use are more rapid construction, lower production costs, and less residues, among others. The choice of raw materials to obtain adequate behavior is more critical than for traditional concrete. In the present paper a mixture of cement, silica fume, superplasticizer, setting accelerator, filler mate- rials, and aggregates was studied to obtain a 3DPC with high resistance at short curing times. When the optimal mixture was found, metallic fibers were introduced to enhance the mechanical proper- ties. The fresh and hard properties of the concrete were analyzed, measuring the setting time, work- ability, and flexural and compressive strength. The results obtained demonstrated that the incorpo- ration of fibers (2% in volume) enhanced the flexural and compressive strength by around 163 and 142%, respectively, compared with the mixture without fibers, at 9 h of curing. At 28 days of curing, the improvement was 79.2 and 34.7% for flexural and compressive strength, respectively. Keywords: 3D printed concrete; silica fume; setting time; workability; metallic fibers; mechanical properties 1. Introduction The expanded selection of additives in concrete technology has led to the develop- ment of new materials and the possibility of achieving ultra-high-performance fiber-rein- forced concrete (UHPFRC). This material is the product of three technologies, self-com- pacting concrete, fiber-reinforced concrete, and high-strength concrete [1], and was devel- oped with the aim of improving three important aspects, mechanical properties, durabil- ity, and workability. UHPFRC was first developed in France in the 1990s, and, according to the Associa- tion Française de Génie Civil (AFGC) [2], this cementitious matrix material has a charac- teristic 28-day compressive strength of more than 150 MPa, with high flexural strength and ductile behavior. In recent years, there have been small variations in the placement of concrete, with the development of self-compacting concrete and improvements in the techniques for the use of shotcrete, which at the time represented a great advance [3]. Shotcrete can be considered as the ancestor of additive manufacturing. These techniques are the only ones that do not use formwork for the placement of concrete. In the present work, UHPRC mixtures were developed that could be used in shot- crete as a first step until their dosages could be used to develop additive manufacturing techniques. One of these techniques was three-dimensional concrete printing (3DPC). The challenge presented by the dosages used was that ultra-high-strength concrete is manu- Citation: Gimenez-Carbo, E.; Torres, R.; Coll, H.; Roig-Flores, M.; Serna, P.; Soriano, L. Preliminary Study of the Fresh and Hard Properties of UHPC That Is Used to Produce 3D Printed Mortar. Materials 2022, 15, 2750. https://doi.org/10.3390/ma15082750 Academic Editor: Jorge Otero Received: 10 March 2022 Accepted: 7 April 2022 Published: 8 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses /by/4.0/).