Effect of freeze-thaw cycles on the bending strength of roofing slate tiles

Slate is one of the most popular natural roofing materials, because of its ability to be split into large, thin and waterproof tiles. However, while some slate roofs last up to two centuries, others disintegrate after just a few years, a fact which is closely linked to the intrinsic properties of each particular slate. The effect of these properties on durability of slates is closely related to their performance during freeze-thaw cycles, as expressed in the European Standard for Roofing Slate (EN 12326:2005), which details the tests required to market roofing slate within the European Union. This paper examines variations in the bending strength of three varieties of Spanish roofing slate tiles, upon exposure to freeze-thaw cycles. Each variety of slate was divided up into 10 groups of 3 tiles, and then subjected to freezing and thawing (from -20 degrees C to 25 degrees C) in a climatic chamber. Every ten cycles, a group was removed from the chamber, and its bending strength was tested according to EN 12326:2005. Petrographical characterization, porosity, water absorption and P-wave velocity tests were also conducted for each group of slate. In additon, in order to determine the environmental thermal changes that could affect the slate's integrity, the temperatures reached by a slate tile on a roof exposed to direct sunlight were measured over a period of 24 months. The results showed variations in the bending strength of the slate as well as other properties measured, including water absorption, P-wave velocity and pore size distribution. These variations are due to the formation of a new microcrack system, a process that is closely linked to the petrographical fabric of the slate. Understanding the behavior and durability of roofing slate under freeze-thaw conditions will help in determining which varieties of slate are best for roofs built in cold regions. (C) 2012 Elsevier B.V. All rights reserved.