Failure Analysis of a Total Damage by Hail Impact of an External Thermal Insulation Composite Systems

Abstract

A hailstorm has heavily damaged a glass fibre mesh reinforced mineral render of an externally insulated composite system (ETICS) of a family house. While a neighbouring house of identical design and of the same age showed only minor defects, for the house under investigation it was a total failure of the render. Material probes were taken from the damaged façade and analysed physically and chemically. Before renovation, the hail resistance of the façade was determined with a unique artificial hail impact test method: Clear ice balls were shot against the façade with the help of a launcher, where the kinetic energy of the projectile can precisely be controlled. The type of damage due to natural hail could be reproduced. The impact energy needed to cause a damage similar to the one caused by the natural hail was determined. The hail impact damage was also identical to results from hail testing performed in the laboratory on similar systems. Since the amount of rendering material on the façade was not according to standards of the Swiss Society of Engineers and Architects - it was by far too thin - specific laboratory tests were performed with specimens not only made of similar materials but also with a comparable thin render layer. By systematic variation of the projectile size and its velocity, the impact parameters were studied and correlated to the type and extent of the hail impact damage. A threshold, associated with the first appearance of full circle cracks in the render, was found to be a kinetic energy of 2.5 J. The assessment of a supposed thickness effect and an effect of doubling the reinforcement on the hail impact resistance and the effect of the age of the façade however was a challenge. The methodological limitations given by the circumstances and the approach used are discussed in this paper.

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