Building with Earth: Design and Technology of a Sustainable Architecture

Shrinkage limit

The shrinkage limit (SL) is defined as the

boundary between the semi-solid and solid

states. It is the limit where shrinkage ceases

to occur. With clayey soil, it can be identified

optically when the dark colour of the humid

mixture turns a lighter shade due to evapo-

ration of water in the pores. Still, this is not

an exact method of measurement.

Capillary action

Water movement

All materials with open porous structures

like loam are able to store and transport

water within their capillaries. The water,

therefore, always travels from regions of

higher humidity to regions of lower humidi-

ty. The capacity of water to respond to suc-

tion in this way is termed “capillarity” and

the process of water transportation “capil-

lary action.”

The quantity of water (W) that can be

absorbed over a given period of time is

defined by the formula:

W = w √t [kg/m^2 ]

where w is the water absorption coefficient

measured in kg/m^2 h0.5and t, the time in

hours.

Determination of the water absorption

coefficient

According to the German standard DIN

52617, the water absorption coefficient (w)

is obtained in the following way: a sample

cube of loam is placed on a plane surface

and immersed in water to a depth of about

3 mm, and its weight increase measured

periodically. The coefficient (w) is then calcu-

lated by the formula:

w = [kg/m^2 h0.5]

where W is the increase in weight per unit

surface area and t the time in hours elapsed.

With this test, all four sides of the cube

should be sealed so that no water enters

from these surfaces, and only the bottom

surface is operative.

With loam samples, problems are caused by

areas that swell and erode underwater over

time. The BRL developed a special method

to avoid this: to prevent the penetration of

water from the sides as well as the swelling

and deformation of the cube, samples are

covered on all four sides by a glass-fibre

reinforced polyester resin. To avoid the ero-

sion of particles from the submerged sur-

face, a filter paper is attached beneath and

glued to the polyester resin sides. To pre-

empt deformation of the weakened loam at

the bottom during weighing, a 4-mm-thick

sponge over an acrylic glass plate is placed

underneath (see 2 .18). A test with a baked

brick sample comparing both methods

showed that the BRL method reduced

results by only 2%.

The coefficient w of different loams tested

along with the w-values of common build-

ing materials is listed in 2 .19. Interestingly,

the silty soil samples gave higher w-values

than those of clayey soil. Surprisingly, com-

parison with baked bricks shows that loam

has w-values that are smaller by a factor

of 10.

Water absorption in relation to time is also

very interesting as shown in 2.20. Visible

here is the amazing effect of a tremendous

increase in absorption caused by adding

small quantities of cement.

Capillary water capacity

The maximum amount of water that can be

absorbed in comparison to the volume or

mass of the sample is called “capillary water

capacity” ([kg/m^3 ] or [m^3 /m^3 ]). This is an

important value when considering the con-

densation phenomena in building compo-

nents. Illustration 2 .19shows these values

with the w-values.

Water penetration test after Karsten

In Karsten’s water penetration test, a

spherical glass container with a diameter of

30 mm and an attached measuring cylinder

is fixed with silicon glue to the test sample

so that the test surface in contact with the

water is 3 cm^2 (Karsten, 1983, see 2.21). The

27 Properties of earth

2 .19Water absorption

coefficient ‘w’ of loams in

comparison with com-

mon building materials

2.20Water absorption

curves of loams

W

√t

1 Clayey loam + sand

2 Clayey loam + 2% cement

3 Clayey loam + 4% cement

4 Clayey loam + 8% cement

5 Lightweight mineral loam 650

6 Lightweight mineral loam 800

7 Lightweight straw loam 450

8 Lightweight straw loam 850

9 Lightweight straw loam 1150

10 Clayey loam

11 Silty loam

12 Sandy loam

Silty loam (1900 kg/m^3 ) (3)
Clayey loam (1940 kg/m^3 ) (3)
Lightweight mineral loam (470 kg/m^3 ) (3)
Lightweight mineral loam (700 kg/m^3 ) (3)
Lightweight straw loam (450 kg/m^3 ) (3)
Lightweight straw loam (850 kg/m^3 ) (3)
Lightweight straw loam (1150 kg/m^3 ) (3)
Spruce axial (2)
Spruce tangential (2)
Cement concrete (2290 kg/m^3 ) (1)
Hollow brick (1165 kg/m^3 ) (1)
Solid brick (1750 kg/m^3 ) (1)

2 .19

2.20

3.7

1.6

1.3

1.2

0.2

2.8

1.8

8.9

25.1

0.32

0.27

0.13

0.15

0.20

0.26

0.29

3.6

3.1

2.4

0 0.2 0.4

(m^3 /m^3 )

0 10 20 30

w (kg/m^2 h0.5)

Water absorption w (kg/m^2 )

Time t (min)