Results

The ideal soil for mixing plaster for straw-bale application is predominantly clay-sized particles because they act as a binder for other particles. The hydrometer test on the earthen soil indicates that it consists of 69% silt, 27% clay and 4% sand and can be classified as a clayey silt. The hydrometer test on the bagged ball clay revealed that it contained approximately 80% clay-sized particles and 20% silt-sized particles. Although only 80% of the mass is clay-sized, a larger proportion is likely clay minerals, specifically hydrous aluminum silicate. It is unknown whether or not the clay-sized particles in the soils are clay minerals.

Figure 1 is a typical stress-strain curve for an earthen plaster. The response is similar to that for concrete and cement-lime plaster. At stresses up to 40% of the ultimate stress (in this case about 35

Fig. 1. Stress-strain response of cylinder A of batch C3 of earthen plaster.

MPa), the response is fairly linear. In accordance with ASTM C 469 (2002), the modulus of elasticity was taken as the slope of the stress-strain curve at 40% of the ultimate stress of the cylinder. Beyond 35 MPa, the response becomes non-linear. The ultimate stress, defined as the strength of the plaster, occurs at 0.89 MPa. The ultimate strain is 0.005.

Table 3 summarizes the average strength results for the cube and cylinder tests and modulus results obtained from the cylinder tests. The strength and modulus for each batch is the average value obtained from the three cubes or cylinders tested. Generally, there is little difference in the strength values obtained from cubes or cylinders for the earthen plasters. However, for the cement – lime plasters (P1, P2, P3), the cube strengths are significantly greater than the cylinder strengths.

Figure 2 shows the variation in strength with moisture content for the earthen plaster. There is little relationship between the initial moisture content and the strength of the plaster. As indicated by the data in Table 3, the modulus of the plaster also did not vary significantly with initial mois­ture content. This contrasts with cement-lime plasters, for which the ratio of water to cementitious materials was a critical parameter for the strength of the plaster (Vardy et al. 2005). The reason for the lack of sensitivity of earthen plasters to initial moisture content may result from the loss of water from the plasters, which was observed in these tests as visible shrinkage of the cubes and cylinders.

Figure 3 shows the plaster compressive strength obtained at various drying times. Clearly, there is no significant change in strength in the plaster between 10 and 18 days of drying. In contrast, Figure 4 shows the plaster modulus obtained at various drying times. The results indicate a significant increase in modulus between 14 and 18 days of drying. The results for batch T1, T2, and T3 in Table 3 indicate that the average modulus increases 2.4 times between 14 and 18 days of drying. Although the relationship is linear on this timescale, further investigation is necessary to determine plaster behaviour before 10 days and after 18 days of drying time.

Figure 5 shows the effect of increasing clay content on the plaster compressive strength. Batches R1, R2, and R3 as well as the bagged clay batch S1 are shown. For the earthen plasters, strength increases linearly with clay content. The bagged clay, however, does not follow the same trend,

Table 3. Summary of average compressive strengths and modulii of elasticity.

Batch

Initial Moisture Content

Cube Strength (MPa)

Cylinder Strength (MPa)

Modulus (MPa)

M1

0.126

1.5

1.4

1672

М2

0.132

1.2

1.3

1431

М3

0.134

1.1

1.2

2086

M4

0.144

1.1

1.3

1827

M5

0.146

1.0

1.2

1811

Batch

Drying Time (d)

Cube Strength (MPa)

Cylinder Strength (MPa)

Modulus (MPa)

T1

10

0.9

0.8

890

T2

14

1.0

1.1

758

T3

18

1.0

1.1

1848

Batch

Drying Environment

Cube Strength (MPa)

Cylinder Strength (MPa)

Modulus (MPa)

C1

drying oven (110 C)

1.8

2.1

2285

C2

laboratory

1.0

1.1

758

C3

moisture room (100%RH)

0.7

0.9

562

Batch

Sand/Soil by volume

Cube Strength (MPa)

Cylinder Strength (MPa)

Modulus (MPa)

R1

1.0

1.5

1.5

2500

R2

1.5

1.0

1.1

758

R3

3.0

0.7

0.8

1787

Batch

Clay Source

Cube Strength (MPa)

Cylinder Strength (MPa)

Modulus (MPa)

S1

commercial bagged clay

0.8

0.9

1731

S2

clayey silt soil

1.0

1.1

758

Batch

Water/Cementitious Mat.

Cube Strength (MPa)

Cylinder Strength (MPa)

Modulus (MPa)

P1

1.08

1.1

0.8

443

P2

1.18

1.1

0.7

839

P3

1.28

0.9

0.7

395

Fig. 2. Relationship between initial moisture content and compressive strength of earthen plaster.

Fig. 3. Relationship between drying time and the compressive strength of earthen plaster.

Fig. 4. Time dependency of the modulus of elasticity of earthen plaster.

and has a much lower strength than earthen plasters with similar clay content. The reason for these differences may be related to the vastly different gradations measured for the two types of soil. The bagged soil is 80% clay and 20% silt, while the earthen soil has 69% silt, 27% clay, and 4% sand. Further research is needed to identify the optimum soil gradation for earthen plasters. This has some important practical implications for builders using clay plasters, since the ideal is to use soil from the building site to reduce the energy needed to transport the building materials. Locally available soil may not have the optimum soil gradation to produce structural plasters.

Fig. 5. Clay content and strength, including the bagged clay plaster batch (S1).

Fig. 6. Relationship between the moisture content at time of testing and strength.

Batches C1, C2, and C3 were dried in different environments prior to testing. This varied the moisture content of the plaster at the time of testing. The moisture content of each cube or cylinder was measured immediately after the compression test was complete. Figure 6 shows the drastic changes in moisture content and strength resulting from the different environments. Placing the plaster in a 110°C oven substantially reduced the moisture content and increased the strength.

Leaving the plaster in a humid environment increased the moisture content and resulted in lower strength as opposed to a lab environment. This can be compared with the initial moisture content results of Figure 2, which indicated no relationship between initial moisture content and strength. This suggests that the range of initial moisture contents tested for batches M1,…, M5 did not result in significantly different final moisture contents. This is an area that needs further investigation. Furthermore, these results point to the critical importance of ensuring adequate moisture protection for the clay in building applications. In addition, the exposure of the plaster to hot temperatures during, for example, summer days, is likely to have a beneficial effect on the strength of the plaster.

Three batches of cement-lime plaster, P1, P2, and P3 were tested for comparison with the earthen and bagged clay plasters. The water to cementitious materials ratio (w/cm) ranged from 1.08 to 1.28. As the water to cementitious materials ratio increased, the compressive strength decreased. The results in Table 3 indicate that the average strength and elastic modulus increase as w/cm decrease, a trend noted by Vardy et al. (2005). Figure 7 compares the compressive cube strength of typical soil clay plaster (M3), bagged clay plaster (S1), and three batches of lime-cement plaster (P1, P2, P3). The average strength of the earthen plaster is slightly higher than the cement-lime plasters, and significantly greater than the bagged clay plaster. The average elastic modulus, given in Table 3, of the earthen plaster is 2086 MPa, which is significantly greater than the bagged clay plaster (1731 MPa) and the cement-lime plasters (395 MPa-839 MPa). It is encouraging that earthen plaster can equal and even surpass lime-cement plaster in strength since earthen plaster has only a small fraction of the embodied energy of lime-cement products.