Ricerc@Sapienza

Introduction
Additive manufacturing of cement-based materials is a new challenge for the construction business. Compared to traditional manufacturing, this technology offers important advantages: design freedom, reduction of production costs and times, low pollution and safety.
Printable mixture must meet certain requirements in order to guarantee good extrudability, durability and suitable mechanical properties of the final product.
University of Marche patented a material that combines remarkable printing characteristics and excellent mechanical performances. The purpose of this work is to study the possibility of modifying the printing mix by adding recycled rubber tires to replace the sand and coarse aggregate of the mixture. This strategy aims to improve some physical-mechanical properties of the material (lightness, toughness, acoustic and thermal insulation) and promote the additive manufacturing to reach its maximum cost-effective and environmental-friendly potentials.

Material and Methods
Materials used for the experimental work are:
a) cement-based printable mixture
b) rubber grain/powder from tire recycling processes: powder (0 – 0,8 mm) and granulated (2 – 3 mm). Several rubber-concrete formulations were tested: reference, sand-powder mix and powder-granulated mix. Deposition tests were performed for each mixture in order to monitor the printing properties. From printed artifacts, normed size specimens were extracted for mechanical characterization. Microscopic analysis was performed in order to investigate the fillers distribution in the cement matrix.

Results
The replacement of the aggregates with rubber grain/powder provided a material with good printability. The type of filler affect the mix composition: in the richest formulations of granulated, it was necessary to increase the mix fluidity. Increased hydration promoted a lower shape stability of the filaments during deposition. Experimental characterizations, performed on rubber-concrete mixtures, show interesting results.

Discussion
The use of tire rubber as aggregate in cement mix shows promising results in producing an innovative building material with improved performance in terms of lightness, energy absorption and ductility. The choice of the optimal replacement ratio of rubber fillers is related to the achievement of the best compromise between mechanical strength and toughness based on field of application. Future research will concern the effect of rubber on the thermal and acoustic insulation properties of cementitious material.