A quantitative analysis of volcanic ash damage to New Zealand roof structures and materials

Abstract

The impacts of recent volcanic ashfall on roofs and gutters is very significant and it is important to know how different types of roofing materials will react, and what effects different amounts of ash will have on gutters.

Because of the difficulty of obtaining large quantities of fresh ash, which still contains the soluble gases from the volcanic eruption, a pseudo‐ash has been prepared, using crushed rock dosed with acidic waters so that it has the same composition as fresh volcanic ash. This was then used to test various types of roofing materials. The test samples were placed in a fog chamber and subjected to alternating wetting and drying for periods varying from 1 day to 1 month, but during the relatively short time frame for the project no corrosion occurred. In future tests it will be necessary both to vary the test conditions in the fog chamber and length of time roof samples are subjected to wetting and drying.

Experiments with plastic (Marley) gutters attached to a simulated corrugated iron roof showed that at low roof angles (15o) most ash stayed on the roof and the effect on the gutter was minimal; at medium angles (25o) ash began to move downslope on the roof and collect in the gutters, particularly below the troughs of the corrugated iron roof. At high angles (45o) most ash went into the gutter, rapidly blocking it. Additional tests are needed with wet ash and different types of gutter to see what changes occur.

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Technical Abstract

There have been three aspects to this research. The first was to create a pseudo‐ash that can be used as a proxy for fresh volcanic ash (+ aerosols) from explosive eruptions, which can be used in tests on the effects of such ash on infrastructure. One use of this pseudo‐ash has been to tests effects on fresh ash falling on roofs. The chosen material was crushed basalt from Banks Peninsula, Canterbury, ‘dosed’ with acidic water from either Crater Lake, Ruapehu or from the crater on White Island. Comparisons were made between this dosed ash and fresh ash to check their properties were similar. Once this was established, the pseudo‐ash was used to evaluate the effects of ash on different types of roofing materials (galvanized iron, old corrugated iron, Coloursteel©, and zinc), using the fog chamber at the Building Research Association of New Zealand (BRANZ) facility at Porirua. Samples were subjected to 6 hours of wetting and 18 hours of air drying every 24 hours to simulate types of conditions roofs might experience for periods of 1 day, 1 week and 1 month. The samples were then examined using SEM and XPS analysis at the University of Auckland to check on any corrosion. None was found, indicating that for future tests, conditions in the fog chamber will need to be varied with for example, potentially longer periods of wetting per day or longer periods in the chamber. Checks on gutters involved dropping crushed, non‐dosed, dry basalt onto a test frame incorporating a corrugated iron roof, capable of being set at variable angles, with a plastic (Marley) gutter attached. This frame is housed in the Volcanic Ash Testing (VAT) Facility in the Department of Geological Sciences at the University of Canterbury. At low roof angles (15o) most ash remained on the roof; at intermediate angles (25o) ash started to move downslope and form small piles in the gutter. At high angles (45o) almost all the ash went into the gutter, rapidly blocking it.

This preliminary study has provided some important parameters, but has equally shown that a more complete, longer term study is needed to provide the type of information needed by planners to mitigate this type of hazard.