Mechanical and thermal performance of cob materials

Abstract

Soil is the first construction material used by man, widely available and low energy consuming [Quagliarini 2010]. Indeed, about 30% of the current world population lives in earthen structures and, in developing countries, this rate rise to 50%, mostly rural [Houben 2006]. Moreover, earthbased materials allow an improved balance and control of thermal and acoustic indoor climate compared to industrial construction materials. However, most of earthen structures do not reach current requirements in terms of mechanical, thermal or architectural [Aymerich 2012]. To respond to these requirements, a work at scientific and craftsman levels is necessary. Cob is an earth construction type wide-spread in Normandy. In this study, the mechanical behaviour cob is studied. Two different mixes currently used for cob constructions are firstly characterized. In traditional cob, straw is usually used. Straw can act as a thermal insulation material which allows creating pleasant indoor temperatures during unpleasant weather conditions [Bouhicha 2005]. Moreover, adding vegetable fibres can reduce materials thermal conductivity [Khedariet 2005]. An important characteristic for soil-based materials is water content. Usually, in building field, the required water content is the Proctor optimum. However, cob making is done traditionally with higher water content in order to have a plastic mix. In this study, several earth-fibres formulations are developed by varying fibres’ content as well as soil mixes. Mechanical and thermal behaviour of these materials is then determined and assessed according to fibres characteristics and content used and soil mixes characteristics. Results show that a higher fibre content lead to a weaker thermal conductivity of cob. This is due to a lower thermal conductivity of straw than soil and, also, to a smaller density of earth-fibre compared to earth only.