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

Abstract

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.

Introduction

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.

175

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.

Moistun

Batch

Content: Drying Time

Initial M. C.

Drying Conditions

Sand/Soil

Binder Type/Source

M1

14 days

0.126

lab

1.5

Clay / Soil

М2

14 days

0.132

lab

1.5

Clay / Soil

М3

14 days

0.134

lab

1.5

Clay / Soil

M4

14 days

0.144

lab

1.5

Clay / Soil

M5

14 days

0.146

lab

1.5

Clay / Soil

Drying Time:

Batch

Drying Time

Initial M. C.

Drying Conditions

Sand/Soil

Binder Type/Source

T1

10 days

0.14

lab

1.5

Clay / Soil

T2 (М3)

14 days

0.14

lab

1.5

Clay / Soil

T3

18 days

0.14

lab

1.5

Clay / Soil

Drying Conditions:

Batch

Drying Time

Initial M. C.

Drying Conditions

Sand/Soil

Binder T ype/Source

C1

14 days

0.14

moist room (24h/48h)

1.5

Clay / Soil

C2 (М3)

14 days

0.14

lab

1.5

Clay / Soil

C3

14 days

0.14

drying oven (24h/48h)

1.5

Clay / Soil

Sand:Soil Ratio

Batch

Drying Time

Initial M. C.

Drying Conditions

Sand/Soil

Binder T ype/Source

R1

14 days

0.14

lab

1.0

Clay / Soil

R2 (М3)

14 days

0.14

lab

1.5

Clay / Soil

R3

14 days

0.14

lab

3.0

Clay / Soil

Clay Source

Batch

Drying Time

Initial M. C.

Drying Conditions

Sand/Soil

Binder Type/Source

S1

14 days

0.14

lab

3.0

Clay / Bagged

S2 (М3)

14 days

0.14

lab

1.5

Clay / Soil

Lime-cement plaster

Batch

Drying Time

W/C. M.

Drying Conditions

Binder Type/Source

P1

28 days

1.08

first 7 days moist, lab

Cement-lime / Bagged

P2

28 days

1.18

first 7 days moist, lab

Cement-lime / Bagged

P3

28 days

1.28

first 7 days moist, lab

Cement-lime / Bagged