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U.S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

Preservation o

Historic Adobe Structures—

A Status Report

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards 1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measurement system, (2) scientific and

technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to pro- mote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Institute for Computer Sciences and Technology, the Office for Information Programs, and the Office of Experimental Technology Incentives Program.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the United States of a complete and consist- ent system of physical measurement; coordinates that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of the Office of Measurement Services, and the following center and divisions: Applied Mathematics — Electricity — Mechanics — Heat — Optical Physics — Center for Radiation Research — Laboratory Astrophysics 8 — Cryogenics 2 — Electromagnetics 2 — Time and Frequency*.

THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to improved methods of measure- ment, standards, and data on the properties of well-characterized materials needed by industry, commerce, educational insti- tutions, and Government; provides advisory and research services to other Government agencies; and develops, produces, and

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the innovation process. THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS; promotes the development of the National Standard Reference Data System and a system of in- formation analysis centers dealing with the broader aspects of the National Measurement System; provides appropriate services to ensure that the NBS staff has optimum accessibility to the scientific information of the world. The Office consists of the

following organizational units:

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Office of International Standards — Office of International Relations.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Located at Boulder, Colorado 80302.
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Preservation of Historic Adobe Structures—

A Status Report

James R. Clifton

Institute for Applied Technology

National Bureau of Standards

Washington, DC 20234

Prepared for U.S. Department of the Interior National Park Service Washington, DC 20240

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U.S. DEPARTMENT OF COMMERCE

Dr. Besty Ancker-Johnson, Assistant Secretary for Science and Technology

V^i NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued February 1977
National Bureau of Standards Technical Note 934

Nat. Bur. Stand. (U.S.), Tech. Note 934, 35 pages (Feb. 1977) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.46:934). Stock No. 003-003-01740-0 Price 85 cents (Add 25 percent additional for other than U.S. mailing).
Table of Contents

Page

1. Introduction . 1

2. Composition and Properties of Adobe Soils 2

2 .

Composition and Moisture Contents of Adobe Soils 2

2 .

Physical Properties of Adobe Brick 6 2.3 Mechanical Properties of Adobe Brick 7

2 .

Concluding Remarks 8

3. Stabilized Adobe 9 3.1 Portland Cement 9 3.2 Lime 9

3 .

Bituminous and Asphaltic Emulsions 10

3.4 Sand 10

3 .

Other Stabilizing Agents 10

3 .

Concluding Remarks 12

4

Preservation of Adobe Structures 12

4 .

Moisture in Adobe Structures 13

4.1.1 Deleterious Effects of Moisture 13

4.1.2 Mitigating the Effects of Moisture 15

4.1.3 Methods for Moisture Determinations 16

4.1.4 Removal of Moisture from Adobe Structures 17 4.2 Resistance of Adobe Structures to Natural Hazards 18

4 .

Preservation Materials 19 4.3.1 Stuccoes and Plasters 19

4.3.2 Surface Coatings 20

4.3.3 Surface Impregnation Materials 21

4.3.4 Consolidation Materials 22 4.3.5 Concluding Remarks 23

5

Summary and Conclusions 23

6

References 24

iii

Preservation of Historic Adobe Structures - A Status Report

James R. Clifton

The physicochemical and mechanical properties of adobe soils and

building materials, and the technology of preserving historic adobe structures have been critically reviewed. In most cases, the deterioration of

adobe structures can be directly or indirectly correlated with the presence

of excess moisture. Therefore, the successful preservation of most historic

adobe structures depends largely on effectively protecting these structures

from water. This review indicates that the technology of preserving adobe

structures needs further development to ensure the longevity of the structures.

Areas in which research is needed have been identified and include:

(1) the development of standard methods to characterize the composition and

physical properties of adobe soils, and the mechanical properties of adobe

brick; (2) nondestructive methods to measure the water contents of and water

movement in adobe structures; and (3) the evaluation of the effectiveness

of different types of preservation materials and methods.

Key words: Adobe building materials; adobe soil; mechanical properties;

moisture determination; preservation technology.

1. Introduction

This report is based on a critical literature review of the composition, physical

properties and durability of adobe— building materials, and of methods used to preserve

adobe structures. Adobe building materials, which include sun-dried brick, adobe mortar

and cast adobe, are constituents of many of the historic structures in the arid southwestern

region of the United States which federal, state, and private organizations are endeavoring

to preserve. This review was carried out under the auspices of the National Park Service

to assess the current status of adobe preservation technology and to form the basis for

future preservation programs.

Numerous reports and books have been published on adobe and related building materials

and several large bibliographies are available [2-4], The majority of these publications,

however, are directed towards new construction e.g., references 6-7, with only a small

number of them addressing the preservation of old structures. Recently, the preservation

problems associated with adobe structures have attracted increasing attention, both in the

United States [7] and abroad [8]. Because of this increased interest a symposium has been

held recently in Iran devoted exclusively to the preservation of mud-brick structures [9].

1/ Adobe is defined as any type of clay soil which, when mixed with water to a plastic

consistency, can be made into a part of a structure [1].
Adobe and related materials are among the oldest building materials on earth, with

2/ rammed earth— construction dating back to Neolithic times (3,000 to 10,000 B.C.) [10].

The "Tower of Babel" was apparently constructed with adobe brick [11]. Under favorable

conditions, earth structures of this type can be extremely durable. For example, portions

of the Zigguart of Agar Quf, which are over 2,000 years old, still exist in Iran [12],

Houses are still being built with adobe brick and rammed earth building materials

(often stabilized with bituminous materials). However, despite mankinds' long use of earth

as a building material, this review indicates that the technology of preserving adobe

structures still needs further development to ensure their longevity.

The reader must be cautioned that it is difficult to quantitatively compare the results

of various studies because of the lack of uniformity in preparing test specimens and because

of variations in test methods and test conditions. It is strongly recommended by this

author that standard test methods, including specimen preparation, type of testing equipment,

testing conditions, and methods of reporting the data, be established so that the results

from different laboratories can be compared directly.

3/

2. Composition and Properties of Adobe Soils—

The composition and properties of adobe soils and building materials, including some remarks on rammed earth walls, are discussed in this section. Only brief reference to stabilized adobe is given because this material is the subject of the following chapter.

Primarily, attention is given in this section to the factors which appear to have the

greatest effect on the performance of the adobe building materials and, therefore, on the

durability of historic adobe structures.

2.1 Compostion and Moisture Contents of Adobe Soils

Certain types of soils appear to produce more durable adobe building materials than do

others. Several investigators [13-16] have suggested that the performance of adobe soils are largely dependent on their particle size distributions. The following, therefore, is a brief discussion of particle size classifications of soils and of the textural classification

of soils. Soil particles are usually classified as sand, silt, or clay based on their

size. Good adobe soils contain only a small amount of gravel particles. Based on the

particle size classification established by the International Society of Soil Science [17],

particles in the range of 2 to 0.02 mm are sand particles; particles in the range of 0.02

4/ to 0.002 mm are silt particles; while those smaller than 0.002 mm are clay particles—.

2/ Rammed earth is a mixture of sandy clay soil and water of a slightly moist consistency

enabling it to be compacted between shuttering for monolithic walls or in molds for making

individual blocks [1].

3/ Adobe soils denote soils which have been used to make adobe structures or soils which

appear to have the necessary properties for making durable adobe building materials.

4/ Several other principal particle size scales exist, which differ slightly; for example,

according to the particle size scale of the Federal Highway Administration, the size ranges

are 2 to 0.05 mm for sand, 0.05 to 0.005 mm for silt, and particles less than 0.005 mm are classified as clay particles [17].
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Figure 1. Chart of Soil Textural Classes
Soils are often separated into textural classes corresponding to different proportions

of sand, silt, and clay as shown in figure 1. The clay portion, and to a lesser extent,

the silt portion of a soil acts as "he binder. A good adobe soil should contain sufficient

quantities of clay and silt to form a matrix in which the sand particles are firmly embedded.

Furthermore, the sun-dried material should be cohesive [18] . It is the clay portion of a soil which is primarily responsible for both its expansion, through absorption of moisture,

and its shrinkage due to dehydration. A dimensionally stable adobe soil (defined as an adobe soil in which little dimensional change takes place between the wet and dry state)

therefore, often has a high sand content [13], Fenton has suggested [19] that a stable

adobe soil should contain about 70 percent sand by weight. Schwalen [14] prepared satisfactory adobe brick from soils containing 9 to 28 percent clay. He also associated shrinkage

and cracking of adobe bricks with high clay contents. Other investigators have confirmed

the negative effects of high clay contents on the stability of adobe bricks and have suggested

that soils for adobe bricks should contain between 70 to 80 percent sand and 20 to 30

percent silt and clay [16, 18-19]. In general, a soil should contain equal amounts of silt

and clay [14]. Similar results were obtained by Patty [20] in a study of soils for rammed

earth construction. He determined that soils should contain over 75 percent sand for

satisfactory weather resistance. In addition, he observed that the size distribution of

the sand particles had little effect on the resistance of the rammed earth walls to water

erosion. Soils which contain high amounts of clay and silt can often be mixed with additional amounts of sand to produce a satisfactory mixture for making adobe brick [13, 19].

Based on the fracture characteristics of soils, Chang [21] developed a method to predict if a soil possesses typical adobe behavior. He observed that many soils will crack

into large, irregular blocks upon drying. These primary blocks may fracture further to produce secondary macroaggregates having diameters in the range of 2 to 10 cm. If these

macroaggregates have smooth cleavage planes and angular edges, the soil should be suitable

as an adobe building material. He also postulated that soils containing either montmorillonitic

clays or properly dispersed kaolinitic clays tend to have the properties typical of adobe

soils.

The proper amount of water needed for making adobe bricks and rammed earth walls

appear to vary with the soil. Fenton [19] suggested that for each adobe soil there exists

a certain water content that will give the maximum density and strength. Long [22] recom- mended that an optimum amount of mixing water is 25 to 30 percent, on an air-dry soil

basis, for adobe brick and 12 to 15 percent for rammed earth work. The proper amount of

mixing water, however, depends on the clay and silt contents of a soil. For example, the

optimum mix water for an adobe soil with 9 percent clay was found to be 14.3 percent and was increased to 30.8 percent for an adobe soil with 28 percent clay [14]. The moisture

contents of sun-dried adobe brick also vary with their clay contents but usually are in the

range of 1 to 3 percent [11, 14-15],
The magnitude of shrinkage accompanying the sun drying of adobe brick largely depends

on the clay content, the type of clay, and the amount of mixing water. Schwalen measured

volume shrinkages ranging from 0.4 to 7.7 percent for adobe bricks prepared from 10 different

soils whose clay contents ranged from 9 to 28 percent [14]. Webb et al. [15] observed that

drying shrinkage is not a linear function of the amount of mixing water; the amount of

shrinkage increases considerably for higher water contents. They measured shrinkages

ranging from 0.17 percent to 1.1 percent when the optimum amounts of mixing water were

used. The addition of excessive mixing water in the production of adobe brick can cause significant shrinkages; for example, shrinkages as high as 37 percent have been reported

[23].

Fenton [19] studied the effect of moisture contents on the shrinkage of sun-dried

compacted cylinders of rammed earth prepared from a soil containing 46.7 percent sand, 32.6

percent silt, and 20.7 percent clay. Shrinkages ranged from near percent, if the moisture

content was less than 10 percent, to about 2.5 percent at a moisture content of 19 percent.

Eyre [13] has suggested that the strength and durability of adobe brick is affected by

its pH and that each adobe soil has an optimum pH. He further suggested that if necessary,

the pH of the adobe soil can be adjusted to its optimum value by additions of appropriate

chemicals. However, he did not investigate the relationships between pH and the properties

of adobe brick. Little, if any, other work has been carried out on the effect of pH on the

properties of adobe soils except to measure their pH's. Read et al. [18] reported that the

pH of eight adobe soils from Ellis County of Kansas State ranged from 7.15 to 7.67. Kriegh

and Sultan [24] found the pH of four adobe soils from the southwestern region of the U.S.

to range from 7.3 to 8.0.

All of the adobe soil analyses given in the reports covered by this review are incomplete in some respect. Usually the particle size distribution was given, but only a few

qualitative determinations by x-ray diffraction of the major and minor components of adobe

soils have been reported [24]. An adequate analysis of adobe soil should include at least

the chemical and phase composition, particle size distribution, pH, moisture contents, and

porosity. The porosity is important because of the strong relationships which exist between

it and both the mechanical properties and the capillary potential of soils [17, 25-26]. A

mercury porosimetry method of measuring soil porosity has been described by Sridharen et

5/ 6/

al. [27]. Other fundamental properties such as the plastic limit— and liquid limit—,

should also be determined. Methods for measuring the plastic and liquid limits are given

in ASTM D423 [28] and D424 [29], respectively.

5/ Plastic limit is that moisture content in percent of dry soil weight at which the soil

changes from a solid to plastic state [11]

6/ Liquid limit is that moisture content in percent of dry soil weight at which the soil

changes from a plastic to a liquid state [1].
2.2 Physical Properties of Adobe Brick

A wide range of values for many of the physical properties of adobe brick are given in

the literature. This variability dn the data is understandable when taking into consideration the differences in composition of adobe soils, methods of fabricating adobe brick (the

consolidation process is especially critical), the quality of workmanship, etc.

The dimensions of adobe brick appear to depend largely on the convenience of the user. Some reported dimensions of adobe in historic structures have ranged from 9x5x5 inches

(23 x 13 x 13 cm) to 16 x 5 x 5 inches (41 x 13 x 13 cm) [10]. The size of adobe brick

used for research purposes depends on the specific property being measured. Reported

3 values of the density [11, 13-15] of dried adobe brick range from 98 to 125 lb/ft (1570 to

3 2000 kg/m ) and increase with higher sand contents.

Eyre [13] reported that the coefficient of thermal expansion for adobe brick made from

—ft —ft —ft 5 different soils ranged from 0.90 x 10 to 3.0 x 10 in/in per °F (0.50 x 10 to 1.7 x

— ft 10 cm/ cm per °C) . Webb et al. [15] found that the coefficients of thermal expansion of

—ft —ft adobe bricks made from 40 different soils ranged from 11.1 x 10 to 14.2 x 10 per °C. The reason for the large differences between the two sets of data is not clear.

The expansion of oven-dried (105 °C) adobe bricks exposed to a combination of 30 and 60

percent relative humidities were measured by Webb et al. [15]. Brick specimens made from a "clay soil" expanded by 0.24 percent when exposed for 25 days to 30 percent relative humidity

followed by exposure to 80 percent relative humidity for an additional 25 days. In contrast,

specimens made from a "sandy soil" expanded by only 0.012 percent after exposure to the same conditions.

Whittemore et al. [30] reported that adobe brick and rammed earth have thermal conductivities (K) of 10.7 and 11.3, respectively. These values are higher than for many other common building materials. For example, the K value for building gypsum is approximately 3 and between 6 to 9 for normal weight concrete [31]. The apparent insulating effect of

adobe brick and rammed earth walls, therefore, can be attributed to the thickness of the

walls rather than to low thermal conductivities.

Various types of water permeability tests have been carried out on adobe brick and

rammed earth walls. Most of these tests are based on measuring the time required for

moisture to penetrate a specific thickness of material. A form of the water permeability

test developed for masonry walls [32] is probably the most reasonable method for measuring

the resistance of adobe brick walls to rain. In this test, wall specimens are exposed to water spray at controlled flow rates and the time for moisture to penetrate the wall is

measured. Furthermore, the resistance of the adobe brick wall to rain erosion can be

visually evaluated. Whittemore et al. [30] measured water permeability using a water flow

3 rate of 40 gal/hr (.15 m /hr) . They observed that in about 0.03 hr. water was visible on

the back surface of a 12 inch (30 cm) thick adobe brick wall. If the adobe was previously

stabilized, however, with a 5.6 percent (by dry soil weight) bituminous admixture, the time

required for penetration was increased to 18 hours.
Tests have been developed to simulate the penetration of adobe brick by ground water

through capillary action. In general, test specimens are placed just in contact with water

and the rate at which the water rises in the adobe is measured [13, 18]. Eyre [13] found

that the height at which water rose in different adobe specimens by capillary action after

10 minutes ranged from 0.55 to 1.60 inches (1.40 to 4.06 cm). The effects of various

admixtures were also investigated. The lowest absorption rate of 0.18 inches (0.46 cm) in

10 minutes was obtained with a mixture of 40 grams of soap, 80 grams of lime, and 650 ounces (1.84 kg) of adobe. The compressive strength of bricks made from the mixture, however, was only about 50 percent of the strength of the unadulterated adobe. The capillary

rise of water in adobe mixed with 10 percent by weight of portland cement was 1.25 inches

(3.18 cm) in ten minutes, compared to 0.55 inches (1.4 cm) in the unadulterated adobe.

However, it is well-documented that addition of pcrtland cement to soil has a positive

effect on the resistance of adobe brick to water damage [19, 30, 33-34]. Webb et al.

observed that the initial rate of water adsorption of soil cements was of the same order as that of burnt brick but decreased with time of exposure [15].

2.3 Mechanical Properties of Adobe Brick

A wide range of compressive strength values for cured adobe brick is given in the

literature. Schwalen [14] reported compressive strengths to range from 332 to 544 psi

2 (2.29 to 3.75 MN/m ) for adobe brick made from 10 different soils. Eyre [13] obtained

2 values ranging from 325 to 736 psi (2.24 to 5.07 MN/m ) for four different adobe bricks.

Long [22] measured the compressive strength of adobe brick prepared from six soils that

ranged from a sandy loam to a silty clay loam. Their compressive strengths ranged from 60

2 to 785 psi (0.41 to 5.41 MN/m ). Other investigators have reported [11, 15, 30] strength

2 values in the range of 300 to 700 psi (2.07 to 4.75 MN/m ).

Webb et al. [15] observed that the compressive strength of adobe brick increases with

age. For example, the strength of one series of adobe brick increased from 250 psi (1.72

2 2 MN/m ) after 7 days to 599 psi 14.13 MN/m ) after 28 days. This 28 day strength decreased

2 to 284 psi (1.96 MN/m ) if the adobe brick was in a "wet state" (the term "wet state" was

not defined nor is it certain that the environmental conditions were well controlled)

Patty [20] observed that the compressive strengths of three series of rammed earth blocks

increased from 437, 372, and 353 psi C3.01, 2.56, and 2.43 MN/m

2

) after 6 months to 570,

581, and 497 psi C3.93, 4.01, and 3.43 MN/m2 ), respectively, after 2 years.
Other factors which were found to affect the compressive strength of the adobe brick

include the composition of the soil, the amount of mix water used in making the adobe brick,

density of the dried brick, and the size of the test specimens. The effects of these

factors are briefly discussed in the following. The 28 day compressive strength has been

found to increase with increasing clay contents of the adobe soil until the optimum clay

content is reached, thereafter, shrinkage cracking causes reduction in the strength [11].

As the adobe brick ages over a period of years, the effect of the clay content on the strength

appears to diminish [20] . The compressive strength increases slightly with the fineness of

the soil [13] . It has been firmly established that for each adobe soil the optimum amount

of mixing water which gives the highest strength and density can be determined [15, 19].

Reasonably direct correlations between densities of dried adobe bricks and their compressive strengths have been obtained [14]. Schwalen [14] found the specimen size to affect

the measured compressive strength. For example, reducing the size of test specimens from

6x6x3 inch prisms (15 x 15 x 7.5 cm) to 4 x 4 x 3 inch prisms (10 x 10 x 7.5 cm)

resulted in an average reduction of 11 percent in the measured strength. Whittemore et al.

[30] observed a more dramatic effect, the measured compressive strength was reduced by

approximately 50 percent when the test specimen size was reduced from 11 11/16 x 15 1/4 x

4 15/16 inch (29.69 x 38.74 x 12.54 cm) to 11 9/16 x 7 7/16 x 4 15/16 inch (29.37 x 18.89 x

12.54 cm). These findings clearly indicate the need to establish a standard specimen size

for measuring the compressive strength of adobe bricks.

The tensile strengths of adobe are comparatively low and difficult to measure accurately

because of the effects of shrinkage cracks. Reported values range from 54 to 121 psi

2 (0.37 to 0.834 MN/m ) with the ratio between the compressive strength and tensile strength

being about 6 [13]. Schwalen [14] found that the modulus of rupture for 10 different series

2 of adobe bricks ranged from 29 to 88 psi (0.20 to .61 MN/m ). In general, soils with high

clay contents produce adobe brick with low flexural strengths [11].

This review indicates that the mechanical properties of adobe are not well- characterized.

For example, little information on the creep and stress-strain relationships of either dry

or wet adobe have been reported. Furthermore, the effects of moisture on the compressive

and tensile strengths of aged adobe need to be determined before the structural soundness of

historic adobe structures can be ascertained. Apparently nondestructive evaluation (NDE)

techniques such as ultrasonic pulse velocity measurements and probe techniques [35-36], have

not been employed to estimate the mechanical properties of adobe.

2.4 Concluding Remarks

The development of standard methods and tests to characterize the composition, microstructure, and the physical and mechanical properties of adobe materials is fundamental to the advancement of adobe preservation technology. The implementation of standard methods and

tests will be of assistance to the National Park Service in the assimilation and interpretation of data from different sources. Furthermore, standard tests should form the basis for

evaluating the effectiveness of preservation materials and methods.

8
Many of the important properties and features of adobe soils and building materials

that should be characterized have been identified in this section. Methods which can be

used for these and other characterizations should be identified and, if necessary, new methods developed.

3. Stabilized Adobe

Admixtures (stabilizing agents) can be added to the adobe soil and water mixture

during the production of adobe building materials to increase the weathering resistance of

the product (compressive strength may or may not be affected) [1] . Because stabilized

adobe brick are sometimes used as a replacement for deteriorated adobe in the preservation

of historic adobe structures, this class of materials is covered by this review.

The most commonly used stabilizers appear to be portland cement, lime, bituminous

emulsions, and sand. A variety of other materials have been investigated or proposed as stabilizing agents and are briefly discussed in this section.

3.1 Portland Cement

Portland cement is- an effective admixture for many adobe soils which improves both

their strength and durability [1, 5, 6, 15, 19, 20, 30, 33, 37-41]. For example, a 12 percent addition, by weight, of portland cement to a "sandy soil" increased the compressive

strength by a factor of 5 compared to the neat (unadulterated) soil [15]. Soil-cement

adobes are more resistant to weathering, rain damage, and freeze-thaw damage than are the

neat adobes [15, 19]. They shrink less during curing than the neat adobe. The thermal

expansions of adobes are only slightly affected by low additions of portland cement [15].

Almost all soils can be stabilized with portland cement, however, soils with higher

clay contents require higher percentages of cement to achieve the desired strength and

durability [40]. Recommended proportions of cement to soil vary from 1 part cement to 8 to 20 parts of soil [33, 40, 41]. The optimum amount of mix water depends on the compositions

of the soil and cement and the mix design [15]. Methods of preparing adobe soil-cement

bricks are described in detail in references 1, 5, and 6.

A disadvantage of using adobe soil-cement materials for replacing deteriorated materials

in historic structures is the difficulty in duplicating the color and texture of the

original adobe. No doubt this problem arises with the use of most stabilizing agents.

3 . 2 Lime

Slaked lime (CaO) is used alone or in combination with portland cement to stabilize

adobe. At least a 15 percent lime addition, by weight, is necessary to effectively stabilize

adobe. However, this amount can be reduced to 10 percent by adding 5 percent (based on the

weight of dry soil) portland cement [41].
The hydrated form of lime, calcium hydroxide (Ca(OH) ) has been reported [13, 41] to substantially reduce the compressive and tensile strengths of adobe (table 1) . Addition of

calcium hydroxide has little effect on the weatherability of adobe [13]

3.3 Bituminous and Asphalt ic Emulsion

Bituminous and asphaltic emulsions and similar types of materials have been used

successfully for years to waterproof adobe [1, 19, 42]. The optimum amount of bituminous

and asphaltic emulsions depends on the soil and four to eight percent, by weight, are commonly added [1]. Asphaltic emulsions are of two types, anionic and cationic. In anionic

emulsions, the asphalt droplets are negatively charged by an alkaline water phase; whereas

in the cationic emulsions, the asphalt droplets are positively charged by an acid water

phase. The anionic emulsion should be used to stabilize positively charged soils, and the

cationic emulsions should be used with negatively charged soils [42] . The method of preparing stabilized adobe with emulsions is described in reference [1]

Some bituminous and asphalt materials could, in some cases, impart an objectionable

color to adobe brick if they are used for the preservation of some historic adobe structures.

3 . 4 Sand

Sand can often be added to soils with high clay contents to yield soils of adequate

properties for adobe production [1, 6, 13, 19]. The addition of sand will reduce the early

age strengths of the adobe but the long-term effect should be small [13], The shrinkage

cracking, water absorption, and weatherability of high clay soils will usually be improved

by the additions of the proper amount of sand [19]. Worn and round sand, sometimes located

in desert regions, has been found to be an unsatisfactory material for the dilution of high

clay soils [23]

3.5 Other Stabilizing Agents

A variety of other stabilizing agents have been proposed which have generally received

little or localized attention. It is difficult to judge the effectiveness of many of these

agents on the basis of limited investigations and, therefore, they are only briefly covered.

Among the suggested stabilizing agents are blood and protein [43], vinyl acetate [44],

sawdust [45], casein glue [42], vinsol resin [46], and aniline [4Z] . A series of materials were studied by Eyre [13] and others [14, 18, 19, 22, 42, 47] in the United States during

the period of 1920 to 1950, and a qualitative assessment of the effectiveness of these

materials is given in table 1. Eyre [13] did observe that the addition of 0.1 percent, by

weight, of sodium carbonate increased with compressive strength of one adobe by 64 percent.

It has been suggested that chemicals should be used to modify the pH of adobe soils [13]

and to convert soluble salts to unsoluble salts [34]. However, a systematic investigation

of the chemical treatment of adobe soils has not been performed.

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11
A type of material which is being considered for making durable adobe brick is made

combining a soil with a polymer. Soil-polymer materials have been prepared by two processes:

(1) monomer is added during the mixing of the soil with water and after the soil specimen

is cured the monomer is polymerized by either gamma radiation or by the application of

heat; (2) cured soil specimens are impregnated with monomer which is subsequently polymerized

by gamma radiation or by the application of heat [48 J . A wide range of compressive strengths

2 (9 to 1300 psi (0.6 to 8.96 MN/m )) have been obtained depending on the method of preparation,

moisture content of the soil, and the monomer-polymer system [48, 49]. It appears that

further developmental work is necessary before these types of material can be used for

preparing adobe brick.

3.6 Concluding Remarks

No doubt other materials not covered by this review have been used and some of them

may even be effective in stabilizing adobe. It does appear that adequate stabilizing

agents such as portland cement, lime, and emulsions are available and that other promising

materials are being developed. Three important factors must be considered, however, in

selecting stabilized adobe for replacing deteriorated materials: (1) extent of color and

texture duplication between the stabilized and original adobe; (2) compatibility of the

physical properties of the stabilized and original adobe; and (3) the potential damage to the original adobe caused by using a substitute which has higher mechanical properties and

which is more durable. Usually the weakest component of a structure deteriorates the most

rapidly and replacing it with a more durable material can accelerate the deterioration of

the remainder of the structure. For example, replacing the adobe mortar joining together

adobe block with an adobe-cement mortar has been found to often accelerate the deterioration

of the adobe brick.

4. Preservation of Adobe Structures

The durability of adobe structures is largely dependent on factors encountered during

their original construction including proper design, good construction practices, good soil

selection, and the climate [42]. Unfortunately, these are factors over which the preservation scientist has no control. McHenery [50] has stated the problems well, "Restoration

can be accomplished but it requires considerably more skill to 'restore' an old adobe than

to build one from scratch." Preservation can be even more difficult than restoration,

especially if the structure has no roof [10], Systematic and effective preservation

programs must be developed to overcome these obstacles to the preservation of historic

adobe structures. The first step in developing an effective restoration program for historic

adobe structures is to achieve an understanding of the major processes causing the deterioration of the adobe building materials. Effective preservation methods can then be selected.

12
4.1 Moisture in Adobe Structures

4.1.1 Deleterious Effects of Moisture

The deterioration of adobe and similar constructions have been largely attributed to shrinkage cracks, erosion, undercutting at the base, and loss of mechanical properties

[42]. In most cases, these deterioration processes are directly or indirectly correlated

with the presence of excess moisture. For example, most adobe walls will swell upon becoming

damp and shrink upon drying, resulting in their cracking. If the wet and dry cycles continue

over a period of years, the size of cracks and extent of cracking can become sufficient to endanger the structural integrity of the wall [42]. Rain water is the main cause of erosion

and contributes, along with ground water, to undercutting of walls. The eroding effect of

wind-blown sand on adobe has been often mentioned [12, 42], but documented evidence of such

damage has not been uncovered in this review. Probably, water erosion is a more severe problem in most locations than wind-sand erosion. Excessive moisture in adobe walls,

undoubtedly, can reduce the strength of the building material sufficiently to cause the

collapse of the structure. Furthermore, excessive moisture combined with cycles of freezing temperatures can result in freeze-thaw damage [19].

In places where little or no rainfall occurs, adobe and similar structures have satisfactory durabilities [42] . This durability is evidenced by the longevity of the Zigguart

of Agar Quf in Iran [12, 22], structures at Chan-Chan, Peru, and at Sian Fu, China [1, 22,

42]; all built of earth and believed to be over 2,000 years old. The long-term preservation

of historic adobe structures, therefore, appears to largely depend on the success of keeping

the structures dry. It is difficult, however, to keep adobe structures dry, even in the

arid southwestern region of the United States where the rainfall may be 6 inches (15 cm) or less (the total annual rainfall may take place in one or two severe rainstorms, which is more harmful to adobe than a moderate rainfall [12]).

The traditional approach of protecting adobe walls, either free-standing or walls of

structures, from water has been to coat them with some type of "waterproofing" material [1,

10, 12, 42]. These materials vary in type from a thin paint coating to a thick portland

cement stucco [51]. This approach, however, addresses only one of three main processes by

which water erodes, undercuts and, in general, weathers adobe structures. These processes

[8], shown in figure 2, are:

(a) action of rain water on the top of walls resulting in the formation of deep

fissures and cracks in the top and vertical surfaces of the walls,

(b) slow erosion of the vertical surfaces of walls,

(c) undercutting at the base of walls due to the action of salt-containing

ground water or of accumulated rain water.

13
RAIN

DIRECT ACTION

CRACKS AND FISSURES

RAIN

EROSION BY GROUND WATER AND

CRYSTALLIZATION OF SOLUBLE SALTS

Figure 2. Deleterious Action of Water on Adobe Walls.

14
Torraca 18] and Steen [10] have concluded that processes (a) and (c) cause much more damage

than process (b) . Apparently, the vertical surfaces of adobe walls normally suffer little

damage when exposed to the full force of driving rains [10]. Ground water (process c) can be especially harmful because most historic adobe structures do not have adequate waterproof

foundations and water usually rises by capillary action up the wall to just above the

ground surface and then evaporates. If the ground water has a high salt content, a salt

efflorescence will be deposited just below the surface of the adobe wall as the water

evaporates. This can result in a rapid undercutting of the wall [8, 10, 12, 52].

4.1.2 Mitigating the Effects of Moisture

Battle [53] has stated that "a good tight roof with ample and positive drainage will

do more than anything to preserve an adobe structuie." This approach will certainly assist

in the mitigation of one of the most severe deleterious effects of water i.e., process (a).

Unfortunately, many historic sites have either inadequate roofs or no roofs at all.

Furthermore, often the decision is made to preserve ruins and roofless structures in their

present state rather than restoring them [10]. However, if the structure has an existing

roof or originally had a roof, the merits of repairing or reconstructing the original roof

should be considered. Another approach is that taken to protect the Casa Grande ruins

[10]; i.e., the construction of a separate high roof over the structure. The possibility

that a high roof could cause the creation of damaging wind turbulences should be considered.

If all of these approaches are unacceptable, then a routinely-scheduled maintenance program

should be implemented [53]. The eroded adobe can be replaced with new adobe or with stabilized

adobe. However, stabilized adobe must be used with caution, because as mentioned in Section

3.1.6, replacing the eroded adobe with a more durable material can accelerate the erosion

of the remainder of the structure.

The deleterious effects of ground water and accumulated rain water on the base of

adobe walls can be mitigated by providing drainage around the wall. Steen has suggested

[10] digging trenches around adobe walls and filling them with gravel of sufficient size to prevent the rise of capillary water. Alternatively, the immediate soil can be "waterproofed"

with a soil stabilizer [24] accompanied by the sloping of the adjacent ground away from the

wall.

Either of two methods is usually selected to protect the vertical surfaces of adobe

walls from slow erosion due to rain water or sand-laden wind; coating the vertical surfaces

with surface coatings or surface impregnation materials [51, 52], or replacing eroded adobe

with new adobe [10, 53]. Often, the surface coatings and surface impregnation materials

have been ineffective and at times have accelerated the deterioration of the adobe [8, 10,

53] for reasons discussed in Section 4.3. Because of the slow erosion rate of vertical

wall surfaces [13], the replacement of eroded adobe with new adobe is often sufficient to preserve the adobe structure [53].

15
4.1.3 Methods for Moisture Determinations

The determination of the source of moisture, amount of moisture, and water movement in

adohe structures is an important aspect of their preservation. For example, coating the

vertical surfaces of an adobe wall with a "waterproofing" material is a wasted effort when

the source of the moisture is a leaky roof or ground water. Moisture determinations are also important when evaluating the effectiveness of a preservation process.

The only thorough measurements of source of moisture, moisture contents, and moisture

flow in adobe structures uncovered during this literature review was that reported by

Gullini and co-workers [12, 52]. The migration of water in structures located in Iran were

monitored by taking electrical resistivity measurements. The types of and amounts of

soluble salts in adobe building materials were determined from x-ray powder diffraction

patterns of cored adobe specimens. (Briefly, the destructive effects of processes (a) and

(c) (Section 4.1.1) were confirmed.)

A major problem associated with the use of electrical resistance meters (commonly

called "moisture meters") is the lack of guidelines covering their use and the lack of

standard calibration materials. Furthermore, the electrical resistance will be affected by

the salt content of adobe. Kreigh and Sultan [24] have suggested the use of the neutron

probe technique for moisture determinations in adobe structures and the surrounding soil.

The use of the neutron probe is covered by ASTM Designation: D30.7 [54], If properly

calibrated, the neutron probe is a reliable method for measuring moisture of soils [17, 55-

58]. To measure the moisture contents of adobe walls, relatively large-sized cores (1 to 2 inches (2.5 to 5.0 cm)) must be removed to accommodate the head of the neutron probe. An

unacceptably large number of access holes would probably be required to obtain an adequate

moisture profile of an adobe structure. Backscattering gamma devices are useful for

measuring densities and thereby indirectly moisture contents of soils for depths up to 6 inches (15 cm) [17, 55, 56, 59]. The microwave technique has been used to estimate the

moisture contents of soils [60, 61], but its applicability to adobe has not been ascertained,

Other nondestructive methods to measure moisture contents of soils include: radar [62],

thermal conductivity measurements [63], ultrasonic techniques [55, 58], and visible and

infrared reflectance methods [64]

Drying soil specimens at 110°C to constant weight is the standard laboratory method of

measuring moisture contents and is covered by ASTM Designation: D2216 [65] . This method is

normally used to calibrate nondestructive methods for measuring the moisture contents of

soils [58], However, such a calibration is meaningful only if the chemical and physical

properties of the soils selected for calibration purposes are well characterized, which

suggests that standard reference materials for moisture calibration purposes should be

established.

16
4.1.4 Removal of Moisture from Adobe Structures

Removal of excess moisture from adobe structures may be necessary if the strength of

the adobe has been severely reduced to the level that the stability of the structure is

impaired. Little work, apparently, has been carried out on methods for removing moisture

from adobe structures except to protect a structure from rain and ground water and then

allow it to dry gradually [52]. The methods covered in this section have been used for

removing excess moisture from soils or for changing the direction of water flow in soils.

Electro-osmosis is an electrochemical method for stabilizing soils in which direct

electrical current flows between electrodes buried in the soil in response to an applied

potential difference of up to 100 volts [66-69]. The current is transmitted primarily by

the movement of ions through the pore water of the soil; as the ions move, they carry with

them some of the pore water. The water is generally transported in the direction of the

cathode (negative electrode) . The cathodes are usually perforated pipes to allow the water

flowing toward the cathode to escape under its own flow or, more effectively, through a pumping system. Electro-osmosis treatment stabilizes soil through several processes [70]

including: removal of water resulting in consolidation of the soil and improvement in

strength; change in pH; alteration of clay materials; and modifications in water flow

patterns. Electro-osmosis has been successfully used on a number of occasions to provide

temporary stabilization of soils during excavation [66, 71].

The electro-osmosis method is most effective for soils having low clay contents and

soils having high water contents 166, 69, 72, 73]. It is doubtful if this method can be

used to reduce the water contents of soil to its optimum value because the efficiency of

the process decreases rapidly after the removal of a small portion of the water [72]. The

removal of just a small amount of water, however, is often sufficient to produce large

strength gains [67]. Although only a small amount of water is removed, a large amount of

water can accumulate at the cathode [72] and if not rapidly removed, can cause slumping and

collapse of the soil in the vicinity of the cathode [73, 74],

Electrochemical hardening is a modification of the electro-osmosis process in which

ions are electrolytically introduced into clay soils by using an aluminum anode immersed in an aqueous solution of aluminum chloride [74], Magnesium ions [75] and calcium chloride

[73] have also been electrolytically introduced into soils and other salts, acids and bases

can be used [76]. Winterborn et al. [76] have noted that while electrochemical hardening

is a promising method for soil stabilization, a good practical field approach has yet to be

developed.

17
The moisture contents of soils can be reduced by exposure to elevated temperatures,

which are often accompanied by improvements in the mechanical properties of the soils [76].

Thermal treatments in the range of approximately 450 to 900°C can have several beneficial

effects on soils such as improving their water resistances, reducing their swelling capacities,

and reducing their plasticity [77-78]. The temperature required to stabilize soils depend

on their composition and the type of chemical bonding binding together the soil particles

[78]. According to Winterborn {76], "an effective and economical field system of thermal

stabilization has yet to be developed." The application of thermal treatment to an existing

adobe structure must be carefully considered because induced thermal stresses could affect

its structural integrity. Interestingly, merely prolonged exposure to sunlight can improve

the compressive strength and water resistance of most adobe soils [79].

Before any of the methods reviewed in this section or any other proposed methods are used to remove moisture from adobe structures, further exploratory studies are necessary to determine their effectiveness. The effect of any de-watering process on the structural

integrity of adobe structures must also be considered. For example, rapid water removal

can possibly cause extensive cracking to take place in the adobe building material because

of shrinkage or differential settling.

4.2 Resistance of Adobe Structures to Natural Hazards

Because of the low unit structural strength of adobe wall construction, adobe structures are often severely damaged during earthquakes and severe windstorms [13, 80], Much

of the poor resistance of adobe structures to earthquakes has been attributed to poor

workmanship and poor design which resulted in structures having little resistance to lateral

forces as imposed by earthquake shock [80]. To resist these forces an adobe structure

should have a monolithic foundation combined with vertical and horizontal reinforcement

with the walls well anchored together at the corners [1, 80, 81]. If possible, heavy

roofing materials should be replaced with lightweight materials and the structural roof

system should be stiff in its own plane and should be anchored to both side and end walls

to serve as a diaphragm to more evenly distribute forces and distortions to the supporting

walls [1].

Concrete footing walls have been placed along each side of old adobe walls to upgrade

the foundation. These concrete footing walls are tied together with closely spaced steel

tension bars that pass through the old adobe wall base. The new concrete is kept below

grade to preserve the authentic appearance of the adobe structure [81]. It has been

suggested [81] that, if possible, reinforced concrete beams should be installed at floor

levels and at tops of walls to provide positive anchorage with the beams being concealed

within the wall structure.

18
Kreigh and Sultan [24] have investigated the feasibility of reinforcing adobe structures

by injecting liquid epoxy into vertical and horizontal holes drilled into the adobe walls.

They formulated a slow curing, low viscosity epoxy which has been shown to penetrate the

adobe and to fill pores, cracks, and fissures. The effectiveness of this approach in

strengthening an actual structure has not been determined.

The use of steel reinforcing bars and steel wire mesh has been recommended [1, 80] for

reinforcing adobe structures located in earthquake or high wind areas. Reinforcement

should be provided around all openings in the adobe structure as well as at the corners

[80]. Bamboo has also been used to reinforce adobe structures [80].

Before any method is selected to reinforce an adobe structure, the potential dangers

of natural hazards should be evaluated. A methodology is presented in reference 82 to evaluate the amount of structural damage which could take place if a structure is exposed

to the extreme natural environments encountered in earthquakes, hurricanes, and tornadoes.

4.3 Preservation Materials

A wide range of materials have been employed to protect adobe and earth structures.

These can be roughly divided into four major categories: (1) stuccoes and plasters; (2)

surface coatings; (3) surface impregnation materials; and (4) consolidation materials. The

distinctions between these categories are made more apparent during the following discussion of the respective materials.

4.3.1 Stuccoes and Plasters

Cement stuccoes"^ have been used extensively in attempting to protect vertical surfaces

of adobe walls from rainfall [42]. This extensive use can probably be attributed to the

availability and low cost of the constituents of stucco because, in many cases, stuccoes

have not been effective in protecting adobe from water. This poor performance has been

attributed to the lack of bond development between stuccoes and adobe as well as the tendency

of stuccoes to crack [10, 52]. It has been frequently observed that moisture accumulates

at the stucco-adobe interface, which accelerates the disintegration of the adobe [10].

These processes are similar to those through which unprotected adobe walls are eroded by

water (figure 2) except that the stucco forms a barrier to the passage and evaporation of

moisture and, therefore, the deleterious effects of water at the stucco-adobe interface are often not visually apparent until a portion of the wall collapses [10].

Various methods have been developed to upgrade the performance of stuccoes including:

(1) applying stucco to wire mesh nailed to adobe walls [19]; (2) formulating stuccoes which

are lean in cement e.g., stuccoes with mix proportions of cement : lime : sand in the range of

1:2:9 to 1:3:12 have been used [42]; (3) applying the stucco to dampened adobe surfaces

[42]; and (4) the use of a primer coat prior to application of the stucco [1], However, no evidence has been uncovered during the course of this review which documents the satisfactory

performance on stucco over a period of years. Soil-cement mortars appear to be no more

effective than stuccoes [10].

_7/ An exterior finish for walls consisting of cement, sand, hydrated lime, and water [83]

19
Lime plasters have been applied successfully to adobe walls for many years [1] . These

plasters are durable if they do not crack and if moisture does not accumulate at the plasteradobe interface [1, 10].

Gypsum plasters have been used in a few arid regions, such as Cyprus, to protect earth

walls [42], However, gypsum is sufficiently soluble in water that gypsum plaster surfaces

are susceptible to water erosion [84],

8 / Dagga-mud plaster— has been used over earth walls for many centuries in many parts of

the world [1, 6]. In its unstabilized form, this plaster is probably no more durable than

the adobe substrate but the plaster is easily prepared and readily applied to adobe and,

therefore, can be incorporated in an effective maintenance program. Dagga-mud plasters

have been stabilized with asphalt emulsions with good results [1, 42] . Because of the

possiblity of a dark color, this type of stabilized plaster would not normally be useful in

preserving historic adobe structures.

4.3.2 Surface Coatings

Surface coatings form thin films on the surfaces of adobe and earth walls providing

temporary protection and also improving the appearance of the walls. Various types of

surface coatings have been used, including: oil base, resin-base and emulsion paints [42,

51]; portland cement washes and white washes [1, 6, 42]; coatings of plant extracts [1,

85]; and coatings of fresh blood [43].

In a thorough study of the effectiveness of paints and plasters for rammed-earth

walls, Patty [51] found that exterior lead-oil paints of good quality were the most satisfactory paints tested. This conclusion appears to be a controlling factor regardless of

the type of paint or wash. Probably, the most significant finding of this study was that

"contrary to expectations, the penetration of paints was undesirable. Paints and priming

coats that penetrated the wall material caused deep failures on the surface and, in no

case, did deep penetration of the material have any advantage." This negative effect of

deep penetration of paints into adobe surfaces was later verified by Legavit [86]. The

consequences of this finding are amplified in section 4.3.3 of this review which covers surface impregnating materials.

Typical primer coatings which have been applied to adobe walls to form a base for the

topcoat paints are thinned aluminum paint [51], asphalt-based aluminum paint [18], linseed

oil and glue sizing [1]. Legavit [86] reported that some of the difficulties associated

with penetration of the paint could be mitigated by applying a primer coating consisting of

the adobe soil mixed with a flour and water mixture.

Whitewash has often been applied to adobe walls to protect them from rainfall [1].

Whitewash normally consists of hydrated lime mixed with sufficient water so that it has a consistency similar to paint. Such a coating is inexpensive and easily applied but it is

neither durable nor waterproof [5]. A more durable whitewash can be prepared by adding

8/ Dagga-mud plaster is a mixture of clay and sand used as a plaster to protect adobe

walls [1].

20
caesin, trisodium phosphate, and formaldehyde to the lime-water mixture [5]. A similar

wash can be prepared by replacing the hydrated lime with portland cement [1]; although this

substitution will be accompanied by a color change unless a white type of portland cement

is selected. Washes should be applied to the substrate after it has been treated with a primer coat of the type used with paints.

Protective coatings for adobe walls have been obtained in Africa and South America by

crushing the plant "Euphorbia Lacter," a common variety of the rubber plant family. A

sticky liquid is obtained which is directly applied or mixed with slaked lime prior to application. Similar coatings are made in South Africa from the leaves of a cactus of the

Optuntin family and from agave leaves [1, 85]. The extracts from these types of plants are usually toxic and, therefore, these liquids must be used with caution [1, 85]

Coatings of fresh blood have been found to increase the water resistance of adobe

surfaces [43] . Apparently, monolayers of blood proteins are adsorbed on the clay particles

thereby temporarily improving the water repellance of the adobe. Proteins from gelatin

[87] and egg albumen [88] have also been found to be strongly adsorbed on clay particles.

Surface coatings do not form durable waterproof films on adobe and earth walls [1,

42] . For example, Patty [51] has convincingly shown that moisture can pass in and out

through paints and similar coatings applied to earth walls. These materials form thin

films which are vulnerable to scratches and abrasion. Furthermore, flaking, peeling,

blistering, and crazing also reduce the protective capabilities of these coatings. Often,

the occurrence of flaking and peeling is evidence of the presence of moisture at the coatingsubstrate interface [42]. Even with these deficiencies, surface coatings can be effective

in a preservation program stressing continued maintenance. These coatings are usually

inexpensive, easily applied, and probably will not cause any irreversible harm to the adobe

surface.

4.3.3 Surface Impregnation Materials

Surface impregnation materials are materials that penetrate the surface layers of

adobe or earth walls to a finite depth and both "waterproofs" and consolidates these layers.

Often they are organic-silicates or organic monomers which are polymerized in-situ or are prepolymerized and dissolved in a solvent prior to application. Examples of polymer systems

that have been applied to adobe walls are polyurethane [10], members of the acrylate family

[8, 81], ethyl silicate [8, 12, 52], and silicones [52].

Only limited success has been achieved with surface impregnation materials. Similar

to the case of paints, impregnation of the adobe surface has had a negative effect on its

durability [8, 10, 53]. Discontinuity of properties exist between the impregnated and

unimpregnated regions and stresses at these interfaces, caused by differential thermal

expansion or by structural loads, can result in cracking. Moisture can then enter and

accumulate at the boundary causing the weakening of the interfacial bond. Freeze-thaw

damage can also occur culminating in the complete disbonding of the treated region.

Concerning surface impregnation of adobe walls, Torraca [8] has stated, "as far as penetration

is concerned, unless one can penetrate the whole wall and produce a homogeneous material

21
material, the less penetration the better." Battle 153] and Steen [10] have reported that

while the application of a polyurethane to adobe walls gave excellent results for the first

few years, that its use ultimately resulted in the destruction of significantly larger

amounts of adobe than if the walls were not treated.

Clearly, surface impregnation materials should be used with caution and their effective- ness evaluated by appropriate research prior to their application to important adobe structures. The effects caused by these materials are essentially irreversible i.e., their

removal would be extremely difficult without severely damaging the treated regions.

4.3.4 Consolidation Materials

Consolidation materials are those materials which can be intruded into the mass of

adobe and earth structures to fill pores, voids, and cracks in the soil matrix, thereby,

enhancing their structural integrities and also "waterproofing" them. No reports were un- covered in this review of the actual intrusion of consolidation materials into the fabric

of adobe or earth structures. Some materials which may serve as consolidation materials

are: surface impregnation materials Csection 4.4.3); low viscosity epoxies [24]; and a variety of soil stabilizers such as organic resins [76, 89-91], silicates [91-92], and

petrochemical liquids [93]. Concerning soil stabilizers, Kinton [91] reported, based on a very comprehensive study lasting over 20 years in which nearly all types of materials were

considered, that "no single chemical or combination of chemicals have been found acceptably

effective or economical as a major soil stabilizer." However, he noted that current and

planned research may possibly lead to the development of chemical treatments which are effective in controlling volume changes in soils, improving moisture-density relationships,

and reducing frost action, etc. The review by Mura and Thornburn [94] on stabilization of

soils with inorganic salts and bases, also stressed the need for further research. They

concluded that the type and exact percentage of chemicals most beneficial to strength are unique for each soil and depend upon the physical, chemical, and mineralogical composition

of each soil. This conclusion certainly should be applicable to adobe soils and to salts

used in electrochemical hardening methods Csection 4.1.4).

No doubt, that with the rapid advancement in the synthetic polymer field, the use of

polymeric materials to consolidate adobe structures will be contemplated. Warner [95] has

cautioned engineers in the indiscriminate use of polymers to solidify soils. For example,

polymeric materials often deform and creep under stress so that the fundamental strength

(the load at which no load relaxation takes place) of solidified soils may be as low as 20

percent of the ultimate strength (obtained from rapid loading) [95],

22
4.3.5 Concluding Remarks

Obviously, no unique material has yet been identified which will protect adobe walls

from all the deleterious effects of water and other natural hazards. Until the effectiveness

of a preservation material has been demonstrated in well-designed experiments simulating

field conditions, it should be used with extreme caution. The damage caused by the indiscriminate application of preservation materials has often exceeded the effects of natural

weathering [10, 53].

5. Summary and Conclusions

The successful preservation of most historic adobe structures depends largely on effectively protecting the structures from natural hazards, especially water. The initial

phase of preserving adobe structures should consist of determining the processes leading to the deterioration, the extent of deterioration, and if water is involved, its source. Only

after this phase is completed should preservation methods and materials be selected. In most cases, keeping the adobe building material dry will greatly contribute to the longevity

of the structure. The selection of preservation materials and methods for an adobe structure

should be based on well-designed laboratory and field investigations in which the experimental conditions closely simulate the actual exposure conditions. Existence of an universal

preservation material or process is doubtful because many factors such as the properties of

the adobe soil, deterioration processes and extent of deterioration, sources of moisture,

etc., will vary to some extent from structure to structure. Therefore, the preservation of

each adobe structure should be considered as an unique problem. After a preservation

process is implemented or a material is applied, its effectiveness should be monitored over

a period of years and the results thoroughly documented. This review disclosed only a few

reports in which the long-term results of preservation practices were given.

An obvious need exists for the development and establishment of standard test methods

to characterize the important physicochemical and mechanical properties of adobe soils and

adobe building materials. Many of these important properties have been identified in this

review. Standard test methods can be of great assistance in the assimilation and interpretation of test results obtained at different laboratories.

23
6. References

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International Housing Activities Staff, U.S. Housing and Home Finance Agency (1955).

2. George, E., Adobe Bibliography, APT Bulletin, Vol. V (No. 2), pp. 97-103 (1973).

3. Barnes, M. R. , Adobe Bibliography, APT Bulletin, Vol. VII (No. 1), pp. 89-101 (1975).

4. Davis, L., Pise, Adobe and Cob - A List of Selected References, Research Service

Bibliographies No. 1, State Library of Victoria (Melbourne, Australia, 1966).

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Agriculture (1972)

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of Housing and Urban Development.

7. Steen, C. R. , Some Recent Experiments in Stabilizing Adobe and Stone, in Conservation

of Stone, New York Conference on Conservation of Stone and Wooden Objects, June 1970,

pp. 59-64 (International Institute for Conservation of Historic and Artistic Work,

London, 1971).

8. Torraca, G., An International Project for the Study of Mud-Brick Preservation, ref.

7, pp. 47-57.

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(1971).

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University of New Mexico Publications in Engineering No. 4 (The University of New

Mexico press, Albuquerque, 1950).

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Experiment Station, Bulletin No. 90 (1945).

24
17. Yong, R. N. and Warkentin, B. P., Soil Properties and Behaviour (Elsevier Scientific

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Studies, Science Series No. 4 (1950).

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and Water-Permeability Properties of Five Earth-Wall Constructions, Building Materials

and Structures Report BMS 78, National Bureau of Standards (U.S., 1941).

25
31. Handbook of Chemistry and Physics, 47th edition, P E-4 (The Chemical Rubber Co.,

Cleveland, Ohio, 1966).

32. Fishburn, C. C, Watstein, D., and Parsons, D. E., Water Permeability of Masonry Walls,

Building Materials and Structures Report BMS 7, National Bureau of Standards (U.S.,

1938).

33. Smith, R. G., Building with Soil-Cement Bricks, Building Research and Practice, pp. 98-

102 (March/April 1974)

34. Kirkham, J. E., Development of Earth Building Blocks, Oklahoma A & M College, Vol. 11

(No. 3), pp. 1-16 (Stillwater, Oklahoma, 1940).

35. Clifton, J. R. , Nondestructive Tests to Determine Concrete Strength, National Bureau

of Standards Report No. NBSIR 75-729 (1975); available from National Technical

Information Service, No. PB-246-858.

36. Malhotra, V. M. , Nondestructive Methods for Testing Concrete, American Concrete

Institute Monograph (Detroit, Michigan, 1976).

37. Felt, E. J., Status of PCA Soil-Cement Development, Journal of the Portland Cement

Association, Vol. 3 (No. 1), pp. 7-18 (1961).

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1942).

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Cement Association (1946)

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Institute (Kumasi, Ghana).

41. Results of Experiments in Stabilizing Soil that is to be Used as a Building Material in

Iran, Ideals and Methods Exchange No. 41, Division of International Affairs, U. S. Department of Housing and Urban Development (1967)

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and Practice, pp. 154-163 (May/June 1973).

43. Winkler, E. M. , The Effect of Blood on Clays, Soil Science, Vol. 82 (No. 2), pp. 157-

164 (1956).

44. Martlus, F. T., Method of Preserving Adobe and Other Porous Material, U.S. Patent No. 2,137,247 (1938).

45. Agueros, G., Adobe Construction and Method, U.S. Patent No. 2,560,272 (1951).

46. Sparks, M. M., Adobe Brick Construction, Office of Technical Services Report No. TAS

98, U.S. Department of Commerce (1951).

26
47. Botkin, C. W. , The Influence of Fluctuation on the Compression Strength of Gila Clay

Loam, Agricutral Engineering, Vol. 8, p. 252 0.927)

48. Autmuth, R. E., The Soil-Polymer System, Construction Engineering Research Laboratory

Report No. CERL-TM-M-72, U.S. Army Construction Engineering Research Laboratory

(Champaign, Illinois, 1974); available from National Technical Information Service,

No. AD-775-812.

49. Jones, C. W. , Effect of a Polymer on the Properties of Soil-Cement, Bureau of Reclamation

Report No. REC-OCE-20-18 (Denver, Colorado, 1970); available from National Technical

Information Service, No. PB-173-543.

50. McHenery, P. G., Jr., Adobe, Build it Yourself, pp. 146-147, The University of Arizona

Press (Tuscon, 1973)

51. Patty, R. L. , Paints and Plasters for Rammed Earth Walls, South Dakota Department of

Agricultural Engineering Bulletin No. 336 (South Dakota State College, Brookings, South

Dakota, 1940)

52. Torraca, G., Chiari, G., and Gullini, G., Report on Mud Brick Preservation, Mesopotamia,

Vol. VII, pp. 259-287 (1972).

53. Battle, D. G., Adobe Preservation, U.S. National Park Service Workshop, January 14-18,

1975 (Denver, Colorado).

54. Standard Method of Test for Moisture Content of Soil and Soil-Aggregate in Place by

Nuclear Methods (Shallow Depth), American Society for Testing and Materials Designation:

D3012-72.

55. Wilson, R. G., Methods of Measuring Soil Moisture, International Field Year for the

Great Lakes Technical Manual Series No. 1 (1971); available from National Technical

Information Service, No. C0M-73-11266.

56. Webster, S. L., Determination of In-Place Moisture and Density by Nuclear Methods,

Instruction Report S-74-1, U.S. Army Engineer Waterways Experiment Station (Vicksburg,

Mississippi, 1974); available from National Technical Information Service, No. AD-779-422.

57. Wyant, D. C, Anday, M. C, and Hughes, C. S., Refinement of Moisture Calibration

Curves for Nuclear Gage, Virginia Highway Research Council (Charlottesville, Virginia,

1973); available from National Technical Information Service, No. PB-231-388.

58. Pande, A., Handbook of Moisture Determination and Control, Vol. 2 (Marcel Dekker, Inc.,

New York, 1975).

59. Hesse, W. , Eine Neue Messmethode Zur Erfassung der Bodenfeuchtigbeit Mit Co Und Cs , Angewandtc Meteorologie, Vol. 5 (No. 3/4), pp. 197-212; translation, A New Method of

Measurement for Determining Soil Moisture with Co and Cs , National Aeronautics

and Space Administration Technical Translation No. TT F-14-219 (Washington, DC, 1972).

60. Cihlar, J. and Ulaby, F. T., Dielectric Properties of Soils as a Function of Moisture

Content, The University of Kansas Space Technology Center Report No. CRES 177-47

(Lawrence, Kansas, 1974).

27
61. Geiger, F. E., and Williams, D., Dielectric Constants of Soils at Microwave Frequencies,

National Aeronautics and Space Administration Report NASA-TM-X-65987 (Greenbelt,

Maryland, 1972).

62. Ulaby, F. T., Radar Measurements of Soil Moisture Content, The University of Kansas

Space Technology Laboratories Report No. CRES 177-35 (Lawrence, Kansas, 1973).

63. Kubo, J. Shin Dojo Suibun Sobuteiho (Momin Ho No Jisshi) , Agricultural Meteorology

(Japan), Vol. 8, pp. 108-110 (1953); translation, A New Method for the Soil Moisture

Measurement (MOMIN 's Method), National Aeronautics and Space Administration Technical

Translation No. TT F-14-394 (Washington, DC, 1972).

64. Blanchard, M. B., Greeley, R. , and Goettelman, R., Use of Visible, Near-Infrared, and

Thermal Infrared Remote Sensing to Study Soil Moisture, National Aeronautics and

Space Administration Technical Memorandum No. TMX-62,343 (1974).

65. Standard Method of Laboratory Determination of Moisture Content of Soil, American

Society for Testing and Materials Designation: D2216-71.

66. Casagrande, L., The Application of Electro-osmosis to Practical Problems in Foundations

and Earthworks, Building Research Technical Paper No. 30 (London, 1947).

67. Casagrande, L., Practical Aspects of Electro-osmosis in Foundation Engineering, Vol.,

pp. 217-233, Pan-American Conference on Soil Mechanics and Foundation Engineering (1959).

68. Casagrande, L. , Electro-osmosis in Soils, Geotechnique, Vol. I (No. 3), pp. 159-177

(1949).

69. Gray, D. H. and Mitchell, J. K,, Fundamental Aspects of Electro-osmosis in Soils,

Journal of the Soil Mechanics and Foundations Division (Proceedings of the American

Society of Civil Engineers), Vol. 93 (No. SM6) , pp. 209-236 (1967).

70. Esrig, M. I., Pore Pressure, Consolidation and Electrokinetics, Journal of the Soil

Mechanics Foundations Division (Proceedings of the American Society of Civil Engineers)

Vol. 94 (No. SM4), pp. 899-921 (1968).

71. Bjerrum, L., Moum, J., and Eide, 0., Application of Electro-osmosis to a Foundation

Problem in a Norwegian Quick Clay, Geotechnique, Vol. XVII (No. 3), pp. 214-235 (1967).

72. Long, R. P., and Zimmie, T. F., Application of Electro-osmosis to Marginal Soils, The

University of Connecticut Civil Engineering Department Report No. JHR 73-62 (Storrs,

Connecticut, 1973).

73. Esrig, M. I., Laboratory Investigation of Electrokinetic Treatment of Consolidated Soils,

Cornell University School of Civil Engineering Contract Report No. 3-73 (1967).

74. Gray, D. H., Electrochemical Hardening of Clay Soils, Geotechnique, Vol. 20 (No. 1),

pp. 81-93 (1970).

75. Youell, R. F., An Electrolytic Method for Producing Chloride-Like Substances from

Montmorillonite, Clay Mineral Bulletin, Vol. 9, pp. 191-195 (1959).

76. Winterborn, H. F., State-of-the-Art Survey Soil-Stabilization - Volume 1, U.S. Naval

Air Engineering Center Report No. NAEC-ENG-7469 (Philadelphia, Pennsylvania, 1968);

available from National Technical Information Service, No. AD-846-786.

28
77. Li, M. C, Effect of Heat on Physico-chemical Properties of Soils as Related to Engineering Behavior, Highway Research Board Research Record, No. 52 (1964).

78. Davidson, S., and White, J., Effect of Heat on Clays and Its Bearing on the Life of

Clay Bonds, Journal of the Foundry Trade, Vol. 95, pp. 165-194 and 235-245 (1953).

79. Winkler, E. M. , Influence of Sun Heat on Clays, Soil Science, Vol. 82 (No. 3), pp. 193-

200 (1956).

80. Design, Siting and Construction of Low-Cost Housing and Community Buildings to Better

Withstand Earthquakes and Windstorms, National Bureau of Standards Building Science

Series 48 (1974)

81. Haapala, K. V., Stabilizing and Restoring Old Adobe Structures in California, Newsletter

of the National Association of Restoration Specialists (Murphy, California, June 1972).

82. Culver, C. G., Lew, H. S., Hart, C. and Pinkham, C. W., Natural Hazards Evaluation of

Existing Buildings, National Bureau of Standards Building Science Series 61 (1975).

83. Boynton, R. S., Chemistry and Technology of Lime and Limestone, Interscience Publishers

(New York, 1967)

84. Stabilized Earth Walls Surface Finishes, Colonial Building Notes No. 14, Building

Research Station (Garston, England, 1953).

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J., International Housing Activities Staff, U.S. Housing and Home Finance Agency (1952).

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of the State, Colorado Farm Bulletin, pp. 13-15 (July-September 1943).

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Preparation and the Effects of Soil Microorganisms on Their Decomposition, Soil Science.

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88. Ensminger, L. E., and Gieseking, J. E., Resistance of Clay-Adsorbed Proteins to Proteolytic

Hydrolysis, Soil Science, Vol. 53, pp. 205-209 (1942).

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Air Engineering Center Report No. NAEC-ENG-7468 (Philadelphia, Pennsylvania, 1970).

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Technical Information Service, No. PB-242-556.

29
92. Hurley, C. H. , and Thornburn, T. T., Sodium Silicate Stabilization of Soils - A Review

of the Literature, Illinois Highway Report No. 47 (1970); available from National

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30
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN -934

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Preservation of Historic Adobe Structures - A Status Report

5. Publication Date February 1977

6. Performing Organization Code

7. AUTHOR(S)

James R. Clifton

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

U.S. Department of the Interior

National Park Service

Washington, DC 20240

13. Type of Report & Period Covered Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less [actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The physicochemical and mechanical properties of adobe soils and building materials,

and the technology of preserving historic adobe structures have been critically

reviewed. In most cases, the deterioration of adobe structures can be directly

or indirectly correlated with the presence of excess moisture. Therefore, the

successful preservation of most historic adobe structures depends largely on effectively protecting these structures from water. This review indicates that the

technology of preserving adobe structures needs further development to ensure the

longevity of the structures.

Areas in which research is needed have been identified and include: (1) the development

of standard methods to characterize the composition and physical properties of adobe

soils, and the mechanical properties of adobe brick; (2) nondestructive methods to measure the water contents of and watermovement in adobe structures; and (3) the

evaluation of the effectiveness of different types of preservation materials and

methods.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Adobe building materials; adobe soil; mechanical properties; moisture determination;

preservation technology.

18. AVAILABILITY jf£ Unlimited

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by Executive Order 11717 (38 FR 12315, dated May 11, 1973) and Part 6 of Title 15 CFR (Code of Federal

Regulations).

NBS Interagency Reports (NBSIR)—A special series of interim or final reports on work performed by NBS for

outside sponsors (both government and non-government). In general, initial distribution is handled by the sponsor; public distribution is by the National Technical Information Services (Springfield, Va. 22161) in paper copy or microfiche form.

BIBLIOGRAPHIC SUBSCRIPTION SERVICES

The following current-awareness and literature-survey

bibliographies are issued periodically by the Bureau:

Cryogenic Data Center Current Awareness Service. A

literature survey issued biweekly. Annual subscription: Domestic, $20.00; Foreign, $25.00.

Liquified Natural Gas. A literature survey issued quarterly. Annual subscription: $20.00.

Superconducting Devices and Materials. A literature survey issued quarterly. Annual subscription: $20.00.

Send subscription orders and remittances for the pre- ceding bibliographic services to National Bureau of Standards, Cryogenic Data Center (275.02) Boulder,

Colorado 80302.
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National Buraau of Standards

Washington, O.C. 20234

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Penalty for Private Use, S300

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