RMIT Durability and Fire Resistance Study

This paper presents an investigation into the durability of cereal straw-building materials after fifty years of use in the Australian climate, the effect of half a century of real-world service was assessed. The investigation indicated no change to composition or degradation to microstructure, which correlated to full retainment of the panels’ functional properties.

This study evaluated a compressed straw insulation panel (Ortech Industries Pty Ltd., Australia) comprised of a wheat straw (triticum aestivum) core and kraft paper facings. The wheat straw core did not contain any added adhesives, flame retardant additives or fillers. Two sets of panels were evaluated. Panel set no.1 was recovered from a building whose construction utilised the compressed wheat straw insulation panels manufactured in 1970, referred to as the “aged” samples.

Panel set no. 2 was manufactured after 2020 and were supplied in the as-manufactured condition. Samples from these panels are referred to as the “pristine” samples.

The building that the panels were recovered from was situated in a region characterised by a temperature range between 􀀀 6.3 ◦C and 45 ◦C. There was no observable evidence of mould or fungal growth, or decomposition within the panel.

To assess the influence of aging on compositional content of the straw a compositional analysis was conducted using straw samples from the aged and pristine panels. The analysis revealed no significant differences in the protein, fibre, carbohydrate, and mineral content of the straw.

The consistent nutritional content supports the observed absence of microbial growth noted the straw in these panels approximates a closed environment and therefore the colonisation and growth of micro-organisms is largely controlled by the moisture and nutrient contents available within the panel.

It is assumed that if some microbial growth does exist initially within the panel than one or more of the carbon, nitrogen or moisture contents will drop below the necessary level to support further growth due to the closed nature of the panel. Furthermore, during the manufacturing of the panels, the wheat straw is exposed to high pressure and -temperature steam. This corresponds to conditions that can provide either pasteurisation (≥ 70 ◦C) or sterilisation (~100 ◦C) benefits to the wheat straw inhibiting microbial growth, the colonisation and growth of microbial.

Examination of the microstructure via optical microscopy was conducted to study the possible biodegradation of the wheat straw during service. Optical microscope images confirmed no significant difference to the microstructure of the wheat straw samples attained from the aged and pristine panels indicating there is no compromise to the microstructure caused by the aging of the straw.

The similar cellulose, hemicellulose, and lignin contents combined with the unchanged microstructure of the wheat straw suggest that little to no degradation of the straw has occurred during the panels working life.

Mechanical testing of the straw fibres indicated that negligible deterioration of the wheat straw occurred over the service life of the panels and corroborates the findings from the compositional and microstructural evaluations.

The fire resistance of the pristine and aged panels was assessed by exposing the panels to a constant radiant heat flux of 40 kW/m2. The temperature of the heat exposed, and unexposed surfaces of the panels were recorded.

The kraft paper facing on the heat exposed surface decomposed and either oxidised or fell from the panel. The temperature time histories presented revealed the thermal front penetrated through the panel to the unexposed surface; the decomposition front, however, did not. The decomposition front penetrated evenly through most of the panels’ thickness, converting nearly all the straw into carbonous char. In all instances the kraft paper facing on the unexposed surface remained attached and well bonded to the remaining thin layer of uncharred straw.

Despite the extensive thermally induced damage the straw maintained its’ structure in both the charred and oxidised phases. The stability of these phases greatly enhanced the fire resistance and is likely responsible for the panels ability to withstand the fire conditions for approximately two hours. The decomposition behaviour of the pristine and aged panels was quantitatively assessed. Regardless of the panels age condition, the char front and oxidation front both penetrate roughly equally through the panel thickness.

The fire resistance of the compressed straw panel demonstrated when straw is compressed it can have self-extinguishing properties, low tendency flame spread, good through-the-thickness fire insulation, and stable char structure. When compared to polymer foam insulations (without flame retardant additives) the fire performance could generally be considered to remain superior to polymer foams which are typical characterised by high flame spread rates, high smoke toxicity, melting, and low char yields.

During their in-service life all building and construction materials must be able to survive the conditions of their environment without degradation.

This study has shown that cereal straws (wheat) can maintain their composition, microstructure, and stem mechanical properties after half a century of in-service use. Furthermore, when incorporated into a building material insulation system, the functional properties (compression, flexural, and insulation) are maintained. This study also explored the potential fire resistance behaviour of cereal straws through the examination of a compressed wheat straw insulation panel.

This study has highlighted materials incorporating cereal straw can be manufactured to have excellent fire resistance behaviour. For the compressed wheat straw insulation panel under examination this study revealed the panel to be able to withstand a high-intensity fire condition for up to two hours through the formation of a highly stable and low thermal conductivity char structure. Furthermore, the panel maintained its’ fire resistance over its’ 50-year working life.

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