Resistance to impact of porcelain stoneware tiles

The impact resistance of ceramic tiles is an important technical requirement, especially for large size and/or low thickness slabs. The standard test for ceramic tiles (ISO 10545-5) is non-destructive: it determines the coefficient of restitution of a small sample (75×75 mm2) assembled on a concrete substrate under a weak impact energy (0.27 J). This method does not provide neither an impact strength nor information on the way a ceramic tile is broken by impact. In order to fill this gap, an investigation was undertaken to describe, by a phenomenological approach, how ceramic tiles break under different conditions of impact. For this purpose, unglazed porcelain stoneware tiles with different size (12×12, 25×25 and 60×60 cm2) and thickness (3.5, 5 and 8 mm) were assembled on a concrete basement and tested for the coefficient of restitution (ISO 10545-5) and Roesler index. Their physical and mechanical properties were determined by measuring: water absorption, porosity, bulk density (ISO 10545-3), flexural strength (EN 843-1), elastic moduli (EN 843-2), fracture toughness (SENB). The impact strength was measured by falling steel balls (50, 80, 200 and 500 g) with increasing energy (from 0.2 to 6 J) and with different speed at the moment of impact (from 1.9 to 5.5 m/s). The effects caused by impact were visually inspected, revealing for increasing energy the formation of an impact ring, radial cracks, one or two concentric Hertzian cone fractures, a highly fractured inner zone. Such damages were quantified by measuring the impact ring diameter as well as number, length and estimated surface of radial and conical cracks. A nearly linear dependence on the impact energy was found for crack length and surface, while the ring diameter and crack number seem to follow a quadratic law. The impact strength depends on the tile thickness, but also the sample size may somehow affect the mechanical behavior under impact. A map of seven levels of damage was drawn by contrasting increasing impact energy with tile thickness: it can help to predict the tile behavior in operation and to address the correct tile type in different applications.