COMPRESSIVE STRENGTH TESTING OF EARTHEN PLASTERS. FOR STRAW BALE WALL APPLICATION
Brendon Taylor, Stephen Vardy and Colin MacDougall
Department of Civil Engineering, Queen’s University, Kingston, Ontario, Canada
Straw-bale construction is an emerging building method and many builders choose to plaster the straw bales with earthen plaster to reduce the embodied energy of the structure. A better understanding of the parameters affecting earthen plaster strength is essential for safe and effective use of this building technique. This study investigated the importance of initial plaster moisture content, drying time, clay content and, moisture content at the time of testing. Clayey silt soil, bagged ball clay and lime-cement are compared as plaster binders for straw-bale applications. Compressive testing was conducted on 50-mm plaster cubes and 100-mm by 200-mm plaster cylinders. It was found that as initial moisture content increased, strength and modulus of elasticity was unaffected for the earthen plaster. As the drying time increased between 10 days and 18 days, strength was unaffected but modulus of elasticity increased proportionally. As clay content increased, strength increased proportionally and stiffness was unaffected. As moisture content at the time of testing increased, both the strength and the stiffness decreased proportionally. Plaster made with soil was found to have greater strength than the plaster made with bagged clay or lime-cement plaster.
In recent years the traditional practice of building plastered straw bale structures has seen a revival due to its economic and environmental benefits. As of 2004, over 50 permitted straw bale homes had been built in Ontario alone (OSBBC, 2004). This method of building originated in Nebraska in the late 19th century and a number of earthen rendered homes from the turn of the century are still lived in use. Load-bearing straw bale walls typically consist of a sandwich panel of stacked straw bales with plaster skins of Portland cement, lime, gypsum, clay, or a combination of these binders. Portland cement or lime-cement plasters are currently the most widely accepted, especially by building officials. However, the harmful environmental effects of cement and lime production have encouraged many environmentally conscious builders to consider earthen plasters.
Earthen plasters are typically mixed on-site and consist of local clay-rich soil, sand, water and chopped straw. They have been successfully used for centuries but are viewed with skepticism by many building officials. This is due, in part, to the lack of published research pertaining to the parameters that affect the strength of earthen plasters. Past research on earthen plasters has investigated parameters such as chopped straw content and sand content (Lerner et al. 2003; Ash et al. 2003). These tests have provided promising strength values as high as 2.00 MPa (Ash et al. 2003). These strength values are comparable to published values for Portland-cement plaster, ranging from 0.75 MPa to 1.98 MPa (Lerner et al. 2003; Vardy et al. 2005). However, some results are irreproducible due to a lack of proper soil analysis and there are many parameters yet to be investigated. A better understanding of how soil components and moisture content affect the strength of earthen plasters is essential to consistently building safe earthen rendered straw bale structures and allow for more widespread use of this environmentally friendly building material.
M. Pandey et al. (eds), Advances in Engineering Structures, Mechanics & Construction, 175-183.
© 2006 Springer. Printed in the Netherlands.
Table 1. Test matrix describing batch parameters.