Durability of Cement Stabilised Earth Walls

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Reference:Heathcote, K.A., and Moor,G.J., “Durability of Cement Stabilised Earth Walls”, FifthCANMET/ACI International Conference on Durability of Concrete, Barcelona, Spain2000.



Durability of cement stabilised earth walls

Heathcote, K.A., and Moor,G.J., “Durability of Cement Stabilised Earth Walls”, Fifth

CANMET/ACI International Conference on Durability of Concrete, Barcelona, Spain2000.

FIFTH CANMET/ACI INTERNATIONAL CONFERENCE ON DURABILITY

OF CONCRETEBARCELONA, SPAIN 2000

Synopsis: This paper examines the effect of driving rain on the erosion of

earth walls. In particular it outlines research being carried out by the

authors on the erosion of cement stabilised pressed earth blocks, although

the results will be applicable to other forms of earth walls such as Adobe and

Rammed earth. Pressed earth blocks are made of earth with around 5-8%cement pressed in steel moulds. Current methods of performance evaluation

of test specimens are in most cases applicable only to the region in which

testing was developed. The long term aim of the authors’ research is to

develop performance criteria that relate to the specific climatology of the

area in which the walls are to be built. This paper outlines a test program

which involved making specimens of 3—8% cement content, testing them in

a simulated rain test in the laboratory and then comparing these results with

the erosion resulting from four months exposure in the field. Wind—driven

rain records are also presented for the exposure duration. The conclusion of

the test program was that the exponential increase in resistance withincreasing cement content was consistent in both the field and laboratory

tests and that a minimum cement content of around 5% was necessary to

achieve an acceptable level of durability.

Keywords: cement, driving rain, durability; earth walls, stabilised earth.

INTRODUCTION

The use of earth as a building material dates back to biblical times.

Unfortunately all of these ancient buildings have succumbed to the ravagesof time and we have to move forward to more recent history to find

evidence of earth buildings. Quite clearly earth buildings are not as weather

resistant as most of the "modern" buildings constructed out of stone or fired

clay brick. Recently however there has been a trend away from the

traditional manufactured "modern" materials to more energy efficient

materials such as earth. The inherent naturalness of the material, its low

embodied energy and its thermal effectiveness have all contributed to the

resurgence of earth building over the world over the last 25 years. There are



basically three types of earth building techniques:

Mud Brick or Adobe. Adobe bricks are typically 250mm by 350mm by

100mm and are made by pouring a puddled mixture of clay and sand

into forms. Once laid the blocks are left in the sun to dry. Rammed Earth or Pise. Although the technique is centuries old

principally the French in the latter half of the 19th century developed

this technique. In this method a dryish mixture of sandy soil is

rammed into wall forms. The thickness of rammed earth walls is

typically around 600mm but more recently walls built from earth

stabilised with cement are being built with thicknesses around 300mm

Pressed Earth Bricks. This is a development of adobe that surfaced in

the second half of this century. In this method a dryish soil is placed in

a steel mould and compacted under high pressure. Typically densities

of around 2000 kg/m^2 are achieved compared to around 1700

kg/m^2 for traditional Adobe bricks.

The traditional adobe structure is coated with a protective weatherproof

coating such as stucco and is therefore protected from erosion. Many

examples of buildings built in France and Italy using this form of protection

remain today after centuries of exposure to driving rain. Recently however it

has been thought to be architecturally desirable to leave earth walls without

any exterior coating, and this has given rise to questions as to long-termresistance to driving rain. In earth buildings with large protective eaves this

is not much of a problem but where the walls are more exposed the majority

of natural soils will in the long term suffer deterioration unless regular

maintenance is carried out.

To overcome the inherent weakness of earth when exposed to driving rain

modern practitioners either resort to protective eaves or to the use of

stabilisers such as cement or bitumen. In some cases particular soils may be

more resistant to the erosive action of rain and can be used unprotected or

not stabilised but this is not common.

AIM OF PAPER

This paper presents the results of work carried out by the authors on

correlating the field performance of cement stabilised pressed earth

specimens with laboratory test results. It outlines the development of a new

spray test developed by the authors at the University of Technology Sydney



and relates testing of samples with the performance of specimens placed

near the runway at Sydney International Airport.

REVIEW OF EXISTING SPECIFICATIONS AND TEST METHODS FOR

ASSESSING THE DURABILITY OF EARTH WALLS

In areas such as New Mexico in the USA where protective coatings are

commonly applied to the surface of earth walls the question of resistance to

driving rain is not appropriate. In these cases the question of durability

relates more to the permeability of the wall and the effect moisture has on

the strength of the wall. In New Mexico the New Mexico State Building Code

(1) requires that cubic samples of the soil used to make the bricks be stood

in a saucer of water for seven days and that upon their removal they must

not have gained more than 2.5% in weight. There is also a requirement that

the minimum compressive strength of the bricks be not less than 300 psi (2

kPa).

Craterre (2) also has a similar strength requirement for dry bricks but

additionally requires that the ratio of wet to dry strength be not less than

0.5, which effectively means a minimum wet strength of 1.2 MPa.

In Israel Cytryn (3) recognised that a test that simulated the action of rain

was needed to test for resistance to the forces of driving rain. He developed

a test that involved a shower rose spraying water vertically onto specimens

from a height of 250 mm. The water pressure was 50 kPa and the exposure

time was 33 minutes. Cytryn calculated that the volume of water falling onthe block surface was equivalent to 7,500 mm of rain that is about equal to

10 years of rainfall in Israel. A block was considered to have passed this test

if not more than two of its corners deteriorated during the test and if at the

same time the surface erosion did not exceed 10%.

A spray test developed by Wolfskill (4) was adapted by Jagadish and Reddy

(5) to test pressed soil blocks in India. In their case a shower rose

approximately 100mm in diameter was held a distance of 175mm over

specimens. Water was sprayed vertically onto the specimens at a pressure

of 100 kPa and at a rate of 0.94 l/sec. Specimens were sprayed for between5 and 20 minutes. The depth of erosion following spraying was divided by

the total precipitation to produce an Erosion Ratio (ER). Jagadish and Reddy

carried out this test as well as field testing on a particular soil to compare

the severity of the test. Their results show that after three years of exposure

the field sample had an ER of 0.012 compared to a laboratory value of



0.039.

In 1960 Fitzmaurice (6) carried out a comprehensive study on the condition

of existing earth wall buildings and concluded that only stabilised walls

should be considered as permanent. In his detailed study of the properties

of stabilised earth he used the ASTM standard D559 (ASTM D559-44) fortesting stabilised earth. The test involves 12 cycles of wetting and drying

with the sample being wire brushed with a "standard" wire brush in between

cycles.

Fitzmaurice set out guidelines for the maximum weight loss that should be

considered acceptable if this test is used (Table 1).

In South Africa Webb et al (7) carried out tests on stabilised pressed earth

bricks and fired bricks using a modification of ASTM D559 and concluded

that the earth bricks made from suitable soils were equivalent to mediumquality fired stock bricks.

The Australian "Spray test" (8) involves water being sprayed horizontally out

of a special nozzle at a pressure of 50kPa. The sample is placed 470mm

from the nozzle and after an hour the sample is examined. The depth of

erosion is determined using a 10mm diameter rod. The maximum allowable

erosion is 60mm per hour. The impact area is a circle of 150mm diameter.

The nozzle has 35 holes, each of which are 1.3mm in diameter, and the flow

pressure is 50kPa. Private communication with Morris (9) indicated a

measured discharge of 0.31l/s for this test which yields a total volume of water in the one hour test of 1116 mm or approximately one years rainfall

in Sydney. The corresponding jet velocity of 6.7m/s for the above cross-

sectional area is consistent with some head loss through the nozzle.

Modifications made by Heathcote which were included in the N.Z. standard

(10) involved making the limiting erosion depth dependent on local

environmental factors such as wind speed, annual rainfall and orientation of

the wall with respect to the prevailing wind driven rain direction.

RELATIONSHIP BETWEEN EROSION LOSSES AND CLIMATICFACTORS

In 1941 Laws (11) published a relationship between raindrop size and

terminal velocity which has become the standard for work in this field (Fig.

1).

In 1943 he produced a paper (12) outlining a detailed drop size distribution



for data on rainfall which he collected in Washington. In his results there

were no reported drop sizes greater than 7mm and very few greater than

6mm. The latter diameter appears to be a limiting size above which drops

become unstable and break up into smaller droplets (13). The drop size

distribution produced by Laws was in tabular form. Assouline and Mualem

(14) developed a normalised two parameter (Intensity and drop diameter)

Weibull distribution function that yields the results shown in Fig. 2 for the

Washington data of Laws.

Laws (15) found from his experiments that raindrop size had a marked

effect on erosion with erosion losses increasing by as much as 1200% in

some cases. Laws was not the first to associate the kinetic energy of rainfall

(= _ ´ m´ v2) with soil erosion. Cook (16) identified raindrop velocity as the

principle determinant of soil erosion. However other researchers at the time

were coming to slightly different conclusions and uncertainty over the exact

relationship between single raindrops and soil detachment continues. Gilley

and Finkner (17) tested a wide range of relationships involving kinetic

energy and momentum and concluded that kinetic energy per unit of drop

circumference was the best fit to the available data.

Present soil erosion calculations are based largely on the Universal Soil Loss

Equation (USLE) developed by Wischmeier and Smith (18). Various

researchers have attempted to relate the erosion index R in the USLE to

more readily obtainable parameters of rainfall. Formulas range from a

simple linear relationship between R and Average Annual Precipitation

developed by Roose (19) for West Africa through to the more generalrelationships developed by The United States Soil Conservation Service

which relate R to the 2 year recurrence interval, 6 hour duration rainfall.

Recently Morgan et al (20) developed a method for predicting soil splash

detachment rates based on kinetic energy where the total kinetic energy

impacting the surface in one year is related to annual rainfall by the formula

E (Joules/m2) = Annual Rainfall (mm) ´ (11.9 + 8.7 log10 I (mm/hr)) where

I = 11mm/hr for temperate climates

= 25mm/hr for tropical climates and

= 30mm/hr for strongly seasonal climates

Springer (21) looked at the problem of erosion of materials due to raindrop

impacting on projectiles (such as aircraft) travelling at high speed. His

hypothesis is that erosion is caused by the cumulative action of raindrops



and that there is an "incubation" period during which time negligible erosion

takes place. The length of this incubation period depends on the fatigue

strength of the material, the raindrop size and on the pressure exerted by

the raindrops, the latter being dependant on material properties. He uses

the same raindrop distribution as Laws to determine the number of droplets

impacting per square metre of surface.

DEVELOPMENT OF NEW SPRAY TEST

A spray testing rig was built at UTS using the Bulletin 5 specifications. The

nozzle was purchased from the National Building Technology Centre who

publishes Bulletin 5 (8).

Following extensive testing with this machine it was felt that the standard

nozzle did not adequately model the impact process of driving rain. Thenozzle has a number of individual jets that work independently and produce

a series of bored holes in weak samples.

A commercial nozzle that would produce a true spray pattern was then

sought and eventually a suitable nozzle was found. The nozzle used in all

tests is a "Fulljet" narrow angle 15 degrees spray nozzle (Model 1550)

manufactured by Spraying Systems company of Illinios in the USA. A full

cone spray pattern is produced in these sprays with the aid of internal

vanes. The nozzle diameter is 4.4mm. The nozzle efficiency is 94% with

drop sizes varying between 1 and 3mm. A pressure of 70kPa was found toyield a satisfactory spray pattern and was therefore adopted in all tests. This

pressure produced a spray velocity of 10.4m/sec and a discharge of 5l/sec.

TESTING PROCEDURE AND RESULTS

A sandy clay material was chosen for the tests presented in this paper.

Further tests using other materials is presently underway but from past

experience the type of material does not significantly effect the general

trend of laboratory spray test results.

The test program involved mixing samples of this soil with various

percentages of off white cement. From past experience six cement contents

were chosen — 3%, 4%, 5%, 6%, 7% and 8%. Soils containing these

cement contents were compressed into 120 mm sections of PVC (Polyvinyl

Chloride) drainage pipes. Following curing for 28 days the sections of pipe

were cut in half, each section being 60mm thick, one of the cut faces was

then tested in the spray test apparatus whilst the other face was installed in



a test rack which was located adjacent to the main runway at Sydney’s

International Airport. In all there was a total of 12 specimens, 6 of which

were tested in the laboratory and 6 in the field

The laboratory specimens were tested in the spray test apparatus using the

4.4mm nozzle at a pressure of 100kPa. They were left in the test apparatusfor varying lengths of time (See Table 2). This was necessary in order to

achieve erosion volumes that were measurable. For instance in the case of

the 3% specimen it was necessary to stop the test after 2 minutes otherwise

the whole specimen would have broken up. On the other hand in the case of

the 7 and 8% specimens the test was stopped after a considerable length of

time because of the low erosion rates. Following testing the samples were

oven dried and re-weighed to obtain the weight loss during the test (See

Table. 2).

The field specimens were inserted in the test rack. This rack was orienteddue south and was placed adjacent to the runway at Sydney’s International

Airport for the period 15th October 1998 to the 3rd March 1999. This site was

chosen because of the availability of hourly wind and rain records. The back

of the rack was covered with a rigid plastic sheet to ensure that only the

south face was exposed to the weather. The wind and rain records were

collected every month and analysed to produce a wind-driven rain rose (Fig.

3). The wind driven rain index used in this rose is the sum of the hourly

rainfalls in mm times the corresponding wind speed in knots. It is clear from

Fig. 3 that the majority of the wind-driven rain during the period of sample

exposure came from the south and south east quadrants. The samples weretherefore directly exposed to most of the 516mm of rain that fell during the

exposure period. An average wind speed of around 10 knots was

experienced during periods of rain.

The field samples were arranged in order of cement content and following

the above period of exposure they were returned to the laboratory where

they were oven dried and re-weighed. The resulting material loss is given in

Table 3. Figure 4 shows the 4% cement specimen being weighed and

illustrate the degree of erosion that was achieved during the field exposure.

In general the erosion profile was reasonably uniform across the surface of specimens.

CONCLUSIONS

The variation in performance between the field and laboratory specimens

was consistent across the range of cement contents. Figures 5 and 6 show



the variation between mass loss and cement content. The curves generally

show a power relationship between mass loss and cement content. In the

case of the laboratory tests the decrease was roughly proportional to the

inverse of the cement content raised to the power 0.5 whilst for the field

tests the power was roughly 0.33. Both curves show a general sharp

increase in mass loss below a cement content of around 4 —5%. It should

be noted however that the mass loss for both curves is expressed in a

different way to facilitate comparison of the general trend of results. In the

case of the laboratory tests the mass loss is expressed in terms of grams

per minute of exposure to the water spray whilst in the field tests the mass

loss is measured directly in grams.

Whilst the results presented above were consistent with our aims and

expectations there was a unexplainable difference between the laboratory

and field test results in relation to the erosion per unit volume of water

falling on the specimens. This difference increases exponentially with

increasing cement content. It appears that the field specimens required

much less volumes of water to achieve similar mass losses and that this

difference could not be explained in terms of either the drop sizes or the

impingement velocities. Further testing is underway to try to explain these

differences, which may be due to atmospheric pollutants or wetting-drying

cycles.

REFERENCES

New Mexico State Building Code Section 2405 — "Unburned Clay Masonry",Construction Industries Division, Santé Fe, N.M. 1979.

CraTerre, "General Specifications for Compressed Earth Blocks", CraTerre,

Villefontaine, 1989.

Cytryn, S., "Soil Construction", State of Israel, Ministry of Labour, Housing

Division. The Weizman Science Press of Israel, Jerusalem, 1957.

Wolfskill, L.S., Dunlop, W.A., & Callaway, B.M., Handbook for Building

Homes of Earth, Department of Housing and Urban Affairs, Office of International Affairs, Washington, D.C., 1970.

Jagadish,K.S. and Venkatarama Reddy,B.V. ,"Spray Erosion Studies on

Pressed Earth Blocks", Building & Environment , Vol 22, No 2 , 1987, pp135-

140.

Fitzmaurice, R., " Manual on Stabilised Soil Construction for Housing",



Technical Assistance Program, United Nations, 1958.

Webb, T.L., Cilliers, T.F. and Stutterheim, N., " The Properties of Compacted

Soil and Soil-Cement Mixtures for use in Building", National Building

Research Institute, Pretoria, 1950.

Bulletin 5 — Earth Wall Construction, 4th Ed., National Building Technology

Centre, Sydney, Australia, 1987.

Morris, H. Unpublished Comments for consideration on Draft Standard

"Engineering Design of Earth Walled Buildings". Joint Technical Committee

Meeting BD/83 Earth Building, 1994.

NZS 4297:1998 "Engineering Design of Earth Buildings", Standards

NewZealand.

Laws, O.J. 1941. "Measurements of the fall velocities of water drops and

raindrops." Transactions of the American Geophysical Union. pp 709-712.

Laws, J.O. and Parsons, D.A. "The Relation of Raindrop- Size to Intensity".

Transactions of the American Geophysical Union.1943 pp 452-460.

Hudson, N.W. "An Introduction to the mechanics of soil erosion under

conditions of subtropical rainfall", Rhodesia Science Association Proceedings

49,1961 14-25.

Assouline, S. and Mualem, Y. "The Similarity of Regional Rainfall: ADimensionless Model of Drop Size Distribution". Transactions of the ASAE.

Vol 32(4): July- August. 1989 pp 1216 — 1222.

Laws, O.J. "Recent Studies in raindrops and erosion". Agricultural

Engineering, V21, November. 1940.pp 431 -433.

Cook H.L. "The nature and controlling variables of the water erosion

process." Soil. Sci. Soc. Amer. Proc. 1 .1936. pp 487 -494.

.N.Y.Gilley, J.E. and Finkner, S.C. "Estimating Soil Detachment Caused by

Raindrop Impact". Transactions of the ASAE. 1985. pp 140 - 146.

Wischmeier, W.H. and Smith, D.D. . "Rainfall Energy and its Relationship to

Soil Loss". Transactions of the American Geophysical Union . Vol 39, No 2,

April. 1958 pp 285 - 291.

Roose, E. . "Use of the universal soil loss equation to predict erosion in West



Africa." In Soil Erosion: Prediction and Control, Proceedings of the National

Conference on Soil Erosion, Soil Conservation Society of America, Ankeney,

Iowa, 1977 pp 60-74.

Morgan,R.P.C.,.Morgan,D.D.V., and Finney,H.J., "A Predictive Model for the

Assessment of Soil Erosion Risk, Journal of Agricultural EngineeringResearch, No 30, 1984 , pp245-253.

Springer,G.S., "Erosion by Liquid Impact", John Wiley & Sons,1976.

Table 1 - Weight Loss Limits Suggested by Fitzmaurice

Type of

Development

Weight Loss Not To

Exceed

In Any

Climate

In Dry

Climate

(<500mm

rain p.a.)

Permanent

Buildings

5 % 10 %

Rural

Buildings

10 % 10 %

Table 2 — Laboratory Spray Test Results

CEMENT

CONTENT

( % )

EXPOSURE

TIME(MINUTES)

WEIGHT

LOSS(GRAMS)

3 2 336



4 7 405

5 90 170

6 120 61

7 180 15

8 141 4

Table 3 - Field results

CEMENT CONTENT (

% )

WEIGHT LOSS

(GRAMS.)

3 662

4 343

5 148

6 66

7 43

8 31



Fig. 1 Relationship between Raindrop Size and Terminal Velocity

Fig.2 Drop Size Distribution for Washington Data



Fig. 3 — Wind Driven Rain Rose for Period of Sample Exposure

Fig.4 — Photo of 4 % Cement Test Specimen being weighed after Exposure

in Test Rig



Fig. 5 Variation of Mass Loss with Cement Content for Laboratory Tests

Fig. 6 — Variation of Mass Loss with Cement Content for Field Tests



Fig.7 — Inserting Samples into Test Rack

Fig 8 — Revised "Spray" Test Apparatus

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Durability of Cement Stabilised Earth Walls