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Q NBS TECHNICAL NOTE 996

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

753

.996

179

I?
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards' was established by an act of Congress March 3, 1901. The

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The National Bureau of Standards was reorganized, effective April 9, 1978.
:Issal -"JL » 5 1979

Mechanical Properties of Adobe

James R. Clifton

Frankie L. Davis

Center for Building Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

Prepared for: U.S. Department of the Interior National Park Service Washington, D.C. 20240

^'^fAU Of

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

Issued May 1979
National Bureau of Standards Technical Note 996

Nat. Bur. Stand. (U.S.), Tech. Note 996, 45 pages (May 1979) CODEN: NBTNAE

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1979

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

Stock No. 003-003-02066-4 Price $2.25

(Add 25 percent additional for other than U.S. mailing)

6: U.S. GOVERNMENT PRINTING OFFICE : 1979 O—281-067 (126)
ABSTRACT

Relationships between the compressive strength and creep, and the

moisture contents of adobe were investigated. Moisture was found

to have a deleterious effect on these mechanical properties of

adobe, its severity increasing with increasingly higher relative

humidities and higher moisture contents. It was concluded that

rain and gound water would have a greater deleterious effect on

the mechanical properties of adobe than high relative humidities.

The physicochemical properties of adobe, mix proportions, drying

conditions, and shrinkage of specimens were also found to

influence the compressive strength of adobe. Procedures for preparing, curing and testing of adobe specimens are given.

A nondestructive test method, based on measuring the penetration

resistance of adobe, was found to give reliable predictions of

the compressive strength and moisture content of adobe specimens.

Key Words: Adobe; adobe soil; compressive strength; creep;

flexural strength; mechanical properties; moisture;

non-destructive testing; preservation.

iii
Table of Contents

Page

ABSTRACT iii

1. Introduction 1

1.1 Background and Purpose 1

* 1.2 Project Scope 1

2. Test Specimens and Testing Procedures 1

2.1 Materials 1

2.2 Amount of Mixing Water 2

2.3 Preparation of Test Specimens 4

2.4 Measurement of Compressive and Flexural Strengths . . 5

2.5 Measurement of Moisture Contents 5

3. Compressive and Flexural Strengths of Dried Adobe .... 5

3.1 Maximum Compressive Strengths 6

3.2 Flexural Strengths 6

3.3 Effect of Drying Conditions on Compressive Strength . 6

4. Effect of Moisture on Compressive Strength of Adobe ... 9

4.1 Effect of Absorption of Rain and Ground Water .... 9

4.1.1 Method of Testing 9

4.1.2 Discussion of Results 10

4.1.3 Example of Application of Compressive Strength

Versus Moisture Relationships ..... 10

^•2 Exposure of Adobe to a Wide Range of Relative

Humidities 12

4.2.1 Method of Testing 12

4.2 2 Discussion of Results ..... 12

4.3 Effect of Wet and Dry Cycles on Compressive

Strength 23

4.3.1 Method of Testing 23

4.3.2 Discussion of Results 23

4.4 Implications of Compressive Strength and Moisture

Relationships 25

5. Creep of Adobe 26

5.1 Test Method 26

5.2 Discussion of Creep Results 30

Iv
Table of Contents (Continued)

Page

6. Penetration Testing of Adobe 33

6.1 Test Method 33

6.2 Results and Discussion 33

7. Summary and Conclusions 36

8. References 38

r1. INTRODUCTION

The National Bureau of Standards has been carrying out a project for

the National Park Service on the preservation of historic abode structures. Part of this project is concerned with the mechanical

properties of abode and the effect of moisture.

1.1 BACKGROUND AND PURPOSE

In a review of abode preservation technology, Clifton [l]^noted that

most of the mechanical properties of abode are not well-characterized.

Most of the past work has been concerned with measuring the ultimate

compressive strength of abode [2-6], and with determining the effects

of different soils, mix proportions and specimen sizes on compressive

strength [2-4, 7]. Little information has been reported on the creep

of either wet or dry abode, or on relationships between compressive

strength and the moisture content of abode. The present study was undertaken to obtain needed information on the mechanical properties

of abode and to develop relationships between moisture and these

mechanical properties.

1.2 PROJECT SCOPE

Three main areas were addressed in the investigation, which are (1)

methods for preparing and testing abode specimens, (2) the mechanical

properties of abode and the effects of moisture on their values, and

(3) nondestructive testing of abode. Standard methods have not been

established for the preparation and testing of abode specimens.

Therefore, the procedures used at the National Bureau of Standards'

(NBS) laboratory are given in detail in Section 2, The mechanical

properties of abode which were characterized are compresive and flexural strengths, and creep (Sections 3-5). A nondestructive method

developed for predicting the compressive strength and moisture content of abode is described in Section 6.

2. TEST SPECIMENS AND TESTING PROCEDURES

2.1 MATERIALS

Abode materials were obtained from Escalante Ruin and Tumacacori

National Monument, which are located in Arizona. Escalante Ruin is

the remains of a prehistoric Indian farming community, constructed

from earth, located along the Gila River. This site was occupied for

several centuries between 900 to 1500 A.D. The abode material from

Escalante Ruin was taken from the wall of a dwelling unit. Tumacacori

National Monument consists of a mission church, San Jose de Tumacacori,

and service buildings. These structures were constructed from abode

by Franciscan priests in the earlv 19th century. The existing ^

- Numbers in brackets refer to references at the end of the report.
buildings at Tamacacorl are being preserved in their present condition

and abode samples in the needed amount could not be removed from them.

Test material, therefore, was taken from the grounds of the site.

The abode soils were shipped to the NBS facilities at Gaithersburg,

Maryland, in several 55 gallon (200 litre) drums. Upon arrival, all

of the material from each site was blended. Then the soils were fractionated using a 1/4 inch (4.75 mm) sieve. The material passing

through the 1/4 in (4.75 mm) sieve was used in the preparation of test

specimens

The physical properties (Table 1) of the test materials were measured

following the methods given in Reference 8. The mineralogical compositions of the test materials have been previously reported [9] and are summarized in Table 1.

The soil from the Tumacacori area is reicher in silt (26%) and clay

(24%) than that found- in abode samples from the church at Tumacacori

(silt, 8 to 12%; clay, 8 to 12%) [9]. This is greater than that found

in the range characteristic of duable abode (20 to 30% combined silt

and clay) [1]. Sand was added to the soil to simulate the abode in the

Tumacacori church and to determine the effect of reducing the silt and

clay contents on compressive strength. Graded sand, meeting the

specifications of ASTM C-109 [10] was added to and blended with the

Tumacacori soil on an equal weight basis. The silt and clay contents

of this mixture were 18% and 31%, and this mixture is termed "Tumacacori

and Sand" in this report.

2.2 AMOUNT OF MIXING WATER

The amount of water needed to make a workable paste and wellconsolidated specimens was determined by making trial mixes. In the first

trial mix, the water contents corresponding to the liquid limit was

used. Usually the amount of water found necessary to prepare a workable paste was less than the liquid limit. The probable reason for

this water reduction is as follows. In making the determination of

the liquid limit, soil particles larger than 425 Mm are removed.

However, the abode soils which have been used to make test specimens

in the NBS laboratory contained some particles which were larger than

425 ^m and only those particles larger than 4.75 mm were removed prior

to making specimens. Particles larger than 425 Urn usually absorb much

less water than clay-size particles and the reduction in the amount of

mix water from the liquid limit value was found to be roughly proportional to the percentage of material larger than 425 Mm. For example,

the amount of water used in making Tumacacori and Sand specimens (15%) was roughly one half of the liquid limit of the Tumacacori soil (36.5%).
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mortars [11] to determine the required amount of mix water was

explored. Based on a few flow determinations it appeared that the

flow technique could be used for this purpose. However, the adobe

soils were found to corrode the metal table top and this approach

was not investigated further,

2.3 PREPARATION OF TEST SPECIMENS

Adobe soils and water were usually mixed in an electrically driven

mechanical mixer of the epicyclic type with a capacity of 1.25 gallons (4,73 liters), used for mixing cement paste and mortars. The

mixer met the specifications of ASTM C-305 [12] . When large amounts

of material were required, a 5.0 gallon (18.9 litre) capacity rotary

mixer was used.

The procedure used for preparing adobe soil and water mixtures is as follows:

(1) Place all the soil in the mixing bowl.

(2) Start the mixer and slow add the water over a one minute

period. If the mixer has variable speed control, mix at

low speed,

(3) Mix for two minutes,

(4) Stop the mixer and scrape down into the batch any soil which

may have collected on the side of the bowl,

(5) Finish by mixing for two additional minutes. If the mixer

has speed control, use medium speed.

The method of molding test specimens is similar to that used for preparing cement mortar specimens and is described herein. Immediately

after preparing an adobe soil batch, it was placed into specimen molds

which were previously lightly oiled with 20-30 weight motor oil. Each

mold was held filled with abode and the abode was tamped 12 times with

a hard rubber tamper having a flat tamping face of 1/2 by 1 inch (13

by 25 mm) cross section. Then the mold was filled with a second layer

which was tamped until the abode was well compacted. Finally, the

abode protruding above the top of the mold was removed by drawing the

flat side of a trowel across the top of the mold and the face of the

specimen was leveled with the trowel.

After molding, specimens were exposed to laboratory air (approximately

70°F (21°C) and relative humidity between 30 to 50%) until they were

removed from the molds. Unless early age strength data was desired,

the specimens were allowed to harden for 48 hours before being removed

from the molds. Exploratory studies were undertaken to determine the

effect of specimen size on the measured compressive strength. Specimen
cube sizes of 2 Inch (50 mm), 4 inch (102 mm), and 6 inch (154 mm) were

used. However, the 4 inch (102 mm) and 6 inch (154 ram) cube specimens

dried very slowly in the molds and often did not retain their shape if

removed within a week, after casting. Therefore, 2 inch (50 mm) cube

specimens were used exclusively for compressive strength determinations

and also in creep studies.

2.4 MEASUREMENT OF COMPRESSIVE AND FLEXURAL STRENGTHS

The compressive strength of 2 inch (50 mm) cubes was measured following

the methods of ASTM-C109 [10] . The load was applied at a rate so that

the maximum load was reached between 20 to 80 seconds after start of

loading.

Drying shrinkages of up to 7 percent were measured for abode specimens.

Such a shrinkage results in approximately a 14 percent reduction in the

surface area of a 2 inch (50 mm) cube. Therefore, if the linear dimensions of a specimen are not measured and are assumed to be 2 inch

(50 mm), the calculated compressive strengths could be substantially

less than the actual strength. The dimensions of the loading and bearing

faces of compressive test specimens should be measured and their average

value used in calculating compressive strengths.

The flexural stength of 1.575 by 1.575 by 6.3 inch (40 by 40 by 160 mm)

prism specimens was measured using the methods of ASTM C-348 [14]

.

Measurements of the flexural strengths of abode prisms were not found

to be highly reproducible (Section 3.2) and no corrections for specimen

shrinkage were made.

2.5 MEASUREMENT OF MOISTURE CONTENTS

The moisture contents of abode specimens were measured by the procedure

given in ASTM D-2216 [14] . Specimens were dried in a drying oven at

230 °F (110 °C) to a constant weight. In most cases, constant weight

was obtained in 24 hours. Samples were allowed to cool to room temperature before being weighed. Moisture contents were calculated on the

basis of the dry weight of samples by the following equation:

Moisture contents (%) = Weight of wet soil - Weight of dry soil

Weight of dry soil ^

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3. COMPRESSIVE AND FLEXURAL STRENGTHS OF DRIED ADOBE

The maximum compressive strengths of the three abode specimens and the

flexural stengths of two of the adobe were measured and are given in

this section. In addition, results of an exploratory study on the

effects of drying temperature and age on the compressive strength of

the Escalanate adobe are presented.
3.1 MAXIMUM COMPEIESSIVE STRENGTHS

The maximum compressive strengths measured in the present study for

Tumacacori soil, Tumacacori and Sand and Escalante adobe specimens are

listed in Table 2. Note that the Escalante adobe specimens were conditioned differently than those of the other two adobe systems. The order

of the maximum strengths is Tumacacori soil > Tumacacori and Sands > Escalante. This sequence does not appear to correlate well with the

differences between the physical properties of the adobe systems and

probably can be attributed to differences in the chemical and mlneralogical compositions of the Tumacacori soil and Escalante adobe* The

closeness of the values for the Tumacacori soil and Tumacacori and Sand

specimens suggests that even when the particle size distribution of the

Tumacacori soil was adjusted by adding sand, sufficient clay and silt was still present to form a matrix in which the sand particles of the

Tumacacori and Sand adobe were firmly embedded [1].

3.2 FLEXUEIAL STRENGTHS

The flexural strengths of the Tumacacori soil and Escalante adobe were measured using the methods described in Section 2.5. Specimens were

prepared using water/soil ratio of 0.30 for Tumacacori soil and 0.25

for Escalante adobe. They were stored under laboratory conditions and

tested at 14 days.

The average flexural strength of the Tumacacori soil specimens was 60

psi (0.41 MN/m^) and 34 psi (0.23 MN/m^) for the Escalante adobe.

Their respective standard deviations (S.D.) were 17 psi (0.11 MN/m ) and 7 psi (0.05 MN/m ). These flexual strengths are in the range of

those reported by others [2-4].

3.3 EFFECT OF DRYING CONDITIONS ON COMPRESSIVE STRENGTH

Freshly prepared adobe bricks are usually slowly cured (allowed to

harden) by being exposed to sunlight or to a dry atmosphere at ambient

temperature. An exploratory study was carried out to ascertain the

effects of two drying temperatures and the length of the drying period

on the compressive strength. Escalante adobe was used for this study

and the results are given in Table 3.

An increase in compressive strength with curing age was found for speci- mens cured at 70 °F (21 °C); while the strength of specimens cured at

140 °F (60 °C) first increased and then decreased with age. Both Webb

et al, [5] and Patty [7] also observed that the compressive strength

of earth bricks increased with age when cured at ambient temperatures.

The decrease in strength of specimens cured at 140°F (60°) possible was caused by some phase or mineralogical change in the adobe.
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TABLE 3. EFFECT OF DRYING TEMPERATURE ON COMPRESSIVE STRENGTH OF

ESCALANTE ADOBE SPECIMENS-*-'

AGE COMPRESSIVE STRENGTHS^/ MOISTURE CONTENTS^/

(DAYS) PSI S.D.'^/ (MN/m^) (S.D) PERCENT S.D.

Specimens dried at 70 °F (21 °C)^/

17 2.77 0.20

25 2.50 0.27

22 2.06 0.25

35 Not measured

Specimens dried at 140 °F (60 °C)

1 258 44 2.63 0.16

2 312 18 2.38 0.09

3 281 28 2.19 0.12

4 253 40 1.98 0.17

7 221

14 261

28 286

35 295

1/ Two inch (40 mm) cubes were tested.

2/' Between 3 and 6 specimens were tested at each condition.

3/' Moisture contents measured after specimens were tested in compression.

Specimens were prepared using a water to soil weight ration of 0.25.

' S.D. denotes standard deviation.

Relative humidity in the range of 30 to 50 percent.
This brief study indicates that standard curing conditions must be

established and used if results from the strength testing of adobes

are to be compared.

4. EFFECT OF MOISTURE ON COMPRESSIVE STRENGTH OF ADOBE

Among the major factors thought to be responsible for the deterioration of adobe is the loss of compressive strength caused by the

presence of excess moisture [15], Excess moisture is defined by

the present authors as a moisture content which is greater than

the equilibrium moisture content of sun-dried adobe. The equilibrium contents of sun-dried adobe brick vary with their clay content but usually are in the range of 1 to 3 precent [2,4,5], As

previously discussed (Section 1) the effects of moisture on the

strength of adobe have not been well studied.

The effects of absorption of rain and ground water, exposure to a wide range of relative humidities, and wet and dry cycles on the

compressive strength of adobe were explored.

4,1 EFFECT OF ABSORPTION OF RAIN AND GROUND WATER

Adobe can absorb appreciable quantities of rain and ground water

which is believed to reduced its compressive strength [15], However

the extent of such strength losses have not been reported. Relationships between excess moisture and strength of adobe were investigated

in the present study by measuring the strength of partially dried

specimens, i,e, they contained a portion of their mix water.

4,1,1 Method of Testing

Tumacacori soil and Escalante adobe specimens were prepared using

amounts of mixing water equivalent to their liquid limits 36.5%

and 32,9%, respectively (Table 1), Tumacacori and Sand specimens

were prepared using a water/soil ratio of 0,15, by weight.

The compressive strengths and moisture contents of 2 inch (50 mm)

adobe cubes were periodically measured by starting within 24 hours

after their formation. Specimens were dried under laboratory conditions, 70°F (21°C), relative humidity between 30 and 50%, By this

method, a broad range of moisture contents was obtained. After speci- mens were tested in compression, they were immediately placed in preweighed (tared) jars, which were tightly sealed. Then their moisture

contents were measured by the methods described in Section 2,6.
4.1.2 Discussion of Results

Results are shown in Figure 1 for specimens made from Escalante adobe,

Tumacacori soil, and Tumacacori and Sand. Compressive strength of all

the systems rapidly decreased from their maximum measured values with

increases in moisture content. The highest moisture content plotted

for each material was close to the moisture content at which specimens

began to sag and deform. The moisture region between 2 and 10% is of

particular interest because almost all of the moisture measurements

taken on the adobe of San Jose de Tumacacori Church [16] lie within

this region. Within these general moisture limits, the compressive

strength of Tumacacori soil specimens decreased from 230 psi (1.58

MN/m^) to 94 psi (0.65 MN/m^), a change of 59%, and the strength of

Tumacacori and Sand specimens decreased from 350 psi (2,4 MN/m ) to

53 psi (0.37 MN/m ) , or by 84%. Within the same moisture region the

compressive strength of Escalante adobe specimens decreased from a maximum of 250 psi (1.7 MN/m ) to 90 psi (0.62 MN/m ) , a decrease

of 64%. Both Tumacacori soil and Escalante adobe specimens have small

residual strengths, approximately 30 psi (0,21 MN/m )

, at their respective plastic limits of 20,7% and 21.8%.

4.1.3 Example of Application of Compressive Strength Versus Moisture

Relationships

The impact of moisture on the structural stability of the Church at

Tumacacori can be estimated if both the loading factors and the

relationship between compressive strength and moisture are known.

In a structural analysis of the church, Fattal [17] estimated that the

maximum compressive load on the adobe was around 20 psi (0.14 MN/m ).

If a safety factor of 2.5 is desired then the strength of the adobe

should not be less than 50 psi (0.34 MN/m^).

The Tumacacori and Sand system is fairly representative of the "average

adobe in the church, so that its strength versus moisture relationship

will be used in the present analysis. As previously observed, the

compressive strength of the Tumacacori and Sand adobe was greater than

50 psi (0.34 MN/m ) as long as its moisture content was less than

10%. If a higher safety factor is though to be necessary, the permissible amount of moisture can be obtained from Figure 1. For example,

a safety factor of 5 is equivalent to a strength demand of 100 psi

(0.69 MN/m ). The compressive strength of the Tumacacori and Sand

adobe is above this value as long as its moisture contnet does not

exceed 5%.

The purpose of the preceding analysis was to demonstrate a procedure

for estimating if moisture in an adobe wall is likely to impair its

structural resistance. Application of these results to other adobe

structures should only be done if the adobe materials have the same response to moisture as the materials used in this study.

10
385

330

D TUMACACORi AND SAND

A TUMACACORI

O ESCALANTE

275 —

220

2.65

2.27

1.89

1.51 CN

E

Z — 1.13

— .758

379

MOISTURE, PERCENT

Figure 1. Effect of moisture contents on the compressive strength

of adobe. Compressive strength of adobe moisture contents

were calculated on the basis of the dry weight of samples.

11
4.2 EXPOSURE OF ADOBE TO A WIDE RANGE OF RELATIVE HUMIDITIES

Adobe specimens were exposed to a wide range of humidity conditions

for the purpose of ascertaining the effects on adobe strength of (1)

exposure to different humidities, and (2) prolonged exposure to

constant relative humidities.

4.2.1 Method of Testing

Adobe specimens were prepared using the following water/soil ratios:

0.30 for Tumacacori soil; 0.15 for Tumacacori and sand; and, 0.25

for Escalante adobe.

The specimens were allowed to dry at laboratory conditions for 14

days, then oven-dried at 131 °F (55 °C) for 24 hours. As soon as the specimens had cooled to ambient temperature, they were weighed.

Then the specimens were placed in 4.5 gallon (17 liter) glass containers

which had a diameter of 12 inches (305 mm) and height of 12 inches

(305 mm). The containers were sealed with 0.5 inch (12.7) thick

plastic plates (Figure 2). Constant relative humidities were maintained

In each container by placing in the bottom of the containers one

of the following: a dessicant, a saturated aqueous solution of a suitably chosen salt, or pure water (Table 4). The containers were periodically agitated to ensure that constant relative humidities

were being maintained. Containers were opened only when specimens

were removed for testing. The amount of moisture absorbed and the

compressive strength of specimens were measured after specimens had

been exposed to constant humidities for 60, 120 and 180 days.

4.2.2 Discussion of Results

The change in moisture content and in the compressive strength of adobe

specimens exposed to relative humidities of 0, 10, 56, 79 and 100 percent

are shown in Figures 3-8, Each data point is the average of between

3 to 5 measurements.

An analysis of the moisture gain versus relative humidity curves

(Figures 3-5) indicates that:

(1) Regardless of the exposure time, the Tumacacori soil and

Escalante adobe systems absorbed increasingly larger

amounts of moisture as the relative humidity increased.

In contrast, the Tunacacori and Sand system absorbed

little moisture in the range of to 60% relative humidity,

and only in higher humidity region was an an appreciable

moisture increase measured.

12
(2) The relative affinity for moisture in the region below 80%

relative humidity was roughly: Tumacacori soil > Escalante

adobe > Tumacacori and Sand. This order is in the same sequence as that based on the clay contents of the materials

(Table 1). In the region above 80% relative humidity, the

moisture affinity of the Escalante adobe increased more

rapidly than that of the other systems.

(3) Small moisture fluctuations are evident when the curves obtained after 180 days are compared with those at 120 days.

This indicates that the moisture contents of the adobe speci- mens did not reach complete equilibrium after 120 days and

possible did not even after 180 days of exposure.

13
Figure 2. Adobe specimens exposed to constant relative humidities.

14
TABLE 4. MATERIALS USED FOR OBTAINING CONSTANT HUMIDITIES

% RELATIVE HUMIDITY ^^ MATERIAL AT 68 °F (20 °C)

Anhydrous CaSO^

ZnCl2 1.5 H20^/ 10

Ca(N03)2 4 H2O 56

NaC2H302 3 H20^/ 76

Water 100

' Values taken from:

(1) Lange's Handbook of Chemistry, Editor-John A. Dean, 11th

edition (McGraw-Hill Book Company, N.Y., 1973)

(2) R.H. Stokes and R.A. Robinson, Standard Solutions for

Humidity Control at 25 °C, Industrial and Engineering

Chemistry 4d (9), 2013 (1949)

(3) Handbook of Chemistry and Physics, 50th edj.tion (The

Chemical Rubber Company, Cleveland, 1970)

2/' Saturated aqueous solution.

15
14

13 60 DAYS OF EXPOSURE

D TUMACACORI AND SAND

A TUMACACORI

11 O ESCALANTE

\u

Z

<

o

UJ

I—CO

O

RELATIVE HUMIDITY, PERCENT

Figure 3. Moisture gain of adobe specimens exposed to

different relative humidities for 60 days.

16
14

13 120 DAYS OF EXPOSURE

12 D TUMACACORI AND SAND

11 A TUMACACORI

O ESCALANTE

10

Figure 4,

40 60

RELATIVE HUMIDITY, PERCENT

Moisture gain of adobe specimens exposed to

different relative humidities for 120 days.

100

17
14

13

12

180 DAYS OF EXPOSURE

11

10

n TUMACACORI AND SAND

A TUMACACORI

O ESCALANTE

20

Figure 5.

40 60

RELATIVE HUMIDITY, PERCENT

80 100

Moisture gain of adobe specimens exposed to

different relative humidities for 180 days.

18 I
Comparison of the relationships between compressive strength and

relative humidity (Figures 6-8) for the three abode systems indicates that they have some similar characteristics in their response

to relative humidity. For example, their strengths decreased sharply

between 0-10% relative humidity, a more gradual decrease occurred

between 10 and 60% relative humidity, and then an appreciable decrease

in strength took place when the relative humidity was above 60%.

However, even with a relative humidity as high as 80%, the compressive

strengths of the three adobes were above 100 psi (0.68 MN/m ) . In

addition, it was observed that while the strengths of the specimens

were lower at 120 days of exposure than at 60 days, their strengths

were generally higher after 180 days.

The negative effect of increasingly higher relative humidity on the

compressive strength of adobe is clearly attributable to an increase

in moisture content. However, the adobe system, Tumacacori soil,

which exhibited in greatest affinity for moisture, also had the highest

strength over the complete humidity range. Another peculiarity is the

closeness of strength versus relative humidity curves for the Tumacacori

and Sand and the Escalante adobe systems, which almost coincide at 180

days, despite the noticeable differences in their moisture versus relative humidity curves. Possibly, differences in the clay mineralogy of

the two systems is manifested more in their affinity for moisture than

in their compressive strengths. It does appear that the major factor

controlling the response of the three adobes to relative humidity is

their clay contents. The compressive strengths and moisture affinity

of the adobes roughly increased with higher clay content in the sequence:

Tumacacori soil Escalante adobe tumacacori and Sand.

19
600

60 DAYS OF EXPOSURE

4.13

D TUMACACORI AND SAND

A TUMACACORI

O ESCALANTE

3.44

2.75

2.06

E

Z

1.37

.68

20 40 60 80

RELATIVE HUMIDITY, PERCENT

Figure 6. Effect of relative humidity on compressive

strength of adobe specimens.

100

20
120 DAYS OF EXPOSURE

600 — 4.13

500

COQ.

3-- 400

I—O

zliJ Q£

I—CO^ 300

COCO

UJoc

Q.

o

u

200

100

D TUMACACORI AND SAND

A TUMACACORI

OESCALANTE

3.44

— 2.75

2.06

CNE

Z

1.37

.68

20 40 60

RELATIVE HUMIDITY, PERCENT

80 100

Figure 7, Effect of relative humidity on compressive

strength of adobe specimens.

21
600 180 DAYS OF EXPOSURE 4.13

D TUMACACORI AND SAND

A TUMACACORI

O ESCALANTE

3.44

2.75

CNE

2.06 Z

1.37

.68

20 40 60 80 100

RELATIVE HUMIDITY, PERCENT

Figure 8, Effect of relative humidity on compressive

strength of adobe specimens.

22
4.3 EFFECT OF WET AND DRY CYCLES ON COMPRESSIVE STRENGTH

Adobe walls will usually swell upon becoming damp and shrink upon

drying, which can result in their cracking. It is believed that if

wet and dry cycles continue for several years, adobe walls may

become so badly cracked that they could collapse [15]. In the

present study, the effect of wet and dry cycles were explored by

subjecting specimens to conditions which should accelerate the

cracking process.

4.3.1 Method of Testing

Adobe specimens were prepared using mix water/soil ratios of 0.30

for Tumacacori soil, 0.15 for Tumacacori and Sand, and 0.25 for

Escalante adobe. Specimens were allowed to dry for 7 days under

laboratory conditions, and then dried at 221°F (105°C) for 24 hours

before being subjected to wet and dry cycles. Specimens were exposed

to 122°F (50°C) and 100% relative humidity for 24 hours during the

wet cycling. After completion of a wet cycle, specimens were immediately subjected to a dry cycle by being placed in an oven at

221°F (105°C.) for 24 hours. The drying process completed a wet

and dry cycle.

4.3.2 Discussion of Results

The results (Table 5) clearly indicate that the accelerated wet

and dry cycling process has a significant effect on the compressive

strength of the adobe specimens. After 7 cycles, the Escalante

adobe had a residual strength which was 28% of its original value,

and similarly the Tumacacori soil strength was reduced to 26% of

its original. In addition, extensive cracking was observed in these

specimens. The Tumacacori and Sand system had a greater resistance

to the cycling process. For example, after particles, its strength

was 258 psi (1.78 MN/m ) or 49% of Its original value. Furthermore,

no cracks were observed in these specimens.

The improved behavior of the Tumacacori and Sand system compared to

the Tumacacori soil is probably attributable to the decrease in clay

content caused by the addition of sand to the soil (see Section 2.1).

Several investigators [3,18] have observed that the cracking tendency

of abode bricks with high clay content could be reduced by the

addition of sand.

Conditions used to accelerate the effects of wet and dry cycles

certainly are more severe than those that adobe brick would normally

encounter. However, the test results do suggest that adobe may

deteriorate when subjected to normal wet and dry cycles. Several

years, however, would probably be required for the wetting and

drying process to be manifested in the field.

23
TABLE 5. RESULTS OF WET AND DRY CYCLES ON COMPRESSIVE STRENGTH

COMPRESSIVE STRENGTH

PSI (MN/m^)

T4BER OF

CYCLES^'

TUMACACORI

SOIL

TUMACACORI AND

SAND

ESCALANTE

ADOBE

483 (3.33) 530 (3.65) 303 (2.09)

4 213 (1.47) 337 (2.32) 206 (1.42)

7 124 (0.85) 305 (2.10) 86 ( .59)

11 258 (1.78)

1/ Wet cycle: 24 hours at 122 °F (50 °C) and 100% relative humidity,

Dry Cycle: 24 hours in oven at 221 °F (105 °C).

24
4.4 IMPLICATIONS OF COMPRESSIVE STRENGTH AND MOISTURE RELATIONSHIPS

The exploratory studies described herein indicate that both excess moisture and wetting and drying processes can be detrimental to the

compressive strength of adobe. The magnitude of the effects of

absorption of rain and ground water, exposure to high humidities,

and wetting and drying processes will depend on the severity of the

conditions. Effects of exposure to high relative humidities may be

manifested only over many years. In contrast, penetration of adobe

by rain and ground water can have a more immediate effect. In addition,

rapid accumulation of moisture in adobe would probably cause a rapid

and large expansion, which could be followed by shrinkage cracking

as the adobe dries. Probably, the most favorable condition for adobe,

besides being dry, is on in which its moisture content remains constant

below a critical level. '

The loading on most historic adobe structures is low. For example,

for an adobe wall 20 feet (6.1 m) long constructed from adobe brick

with a dry density of 125 Ib/ft"^ (2000 kg/m"^) [1], the maximum dead

weight load stress is 17 psi (0.12 MN/m ) . If the adobe contains an average of 10 percent moisture, the dead load stress is still only 19

psi (0.13 t^N/m ). It is unlikely that an adobe structure located in

the arid southewestern region of the United States with a good roof

and proper drainage would have and average moisture content as high

as ten percent. Therefore, such an adobe structure constructed with

a good quality adobe brick would probably not collapse solely because

of loss of compressive strength due to excess moisture. In cases where

a roof has deteriorated or never existed and proper drainage does not

exist, loss of compressive strength due to accumulation of moisture

could endanger the structural integrity of adobe structures. Other

major deleterious processes of rain water are undercutting at the

base of walls, slow erosion of the vertical surfaces of walls [1].

Critical moisture level is defined as the moisture content of an adobe at which an wall is no longer structurally safe.

25
5. CREEP OF ADOBE

The creep of a soil describes its stress-strain-time (rheological)

performance [19]. The creep after construction of an adobe wall is

part of the settling compaction process. Settling creep involves particle reorientation and extrusion of air and water from the adobe pores

and voids. After the settling process is completed, the creep of adobe

under normal loads will probably be infinitesimal as long as it is

dry. However, the reentry of moisture into the pores and voids of an adobe could cause further reorientation of particles and swelling of

the clay particles. The creep can again take place, its magnitude

depending on the amount of moisture and the type of clay in the adobe.

The creep properties of adobe have not been well-characterized. Therefore, an exploratory investigation was carried out to provide information

on the creep of adobe and to determine if moisture has an appreciable

effect on creep.

5.1 TEST METHOD

Two inch (25.4 mm) cube adobe specimens were prepared using water/soil

ratios of 0.30 for Tumacacori soil, 0,15 for Tumacacori and Sand, and

0.25 for Escalante adobe. The specimens were allowed to dry in laboratory air for four weeks prior to testing.

A creep specimen with loading assembly attached is illustrated in

Figure 9. Specimens were dead-weight loaded with lead bricks suspended

underneath the specimens. The loading assembly was free to move through

holes drilled in the supporting platform. Creep of the specimens was measured with 1 X 10" inch (2.54 X 10" mm) micrometer dial gages.

The gages were attached to the top of the loading assembly and gage

stems bore on the supporting platform. Creep specimens under test are shown in Figure 10,

Sj-X creep specimens were exposed to a relative humidity close to 100%.

The portion of a loading assembly above the platform was completely

enclosed in a square shaped glass container attached to the platform

with a sealant (Figure 11). A dish containing water was placed under

the glass container. Problems were encountered with this setup because

of corrosion of the dial gages and reliable data was obtained from

only three of the original six specimens.

Specimens were loaded at levels of 21 to 36 psi (.15 to .25 MN/m )

,

which are thought to be close to or slightly higher than those

encountered in most historic adobe structures.

26
z

UJ

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a:

CDQ

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UJ

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UJ

o

a:OQ

Q

<

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Ui UJ UJ

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X

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E

<u

enw

(U

(U

l-l o

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(U x:o

C/1

3

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fa

27
Figure lU. Creep specimens under test,

28
•u

CO a •

en oo

CL 1—1 X

01 o

4-1 COC 0)

0) CO e o

•H iH o o

ex >.

CO -u •H

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29
5.2 DISCUSSION OF CREEP RESULTS

Creep measurement (Table 6) were not closely reproducible but several

trends are discernible. The largest amount of creep took place during

the first three weeks of testing (Figure 12). This early creep

probably can be attributed to compaction of the specimen and possibly

is similar to that taking place in the settling of an adobe structure.

Little creep was measured after about 7-9 weeks. The effect of

increasing the load on creep was more pronounced for the Tumacacori

soil than for the Escalante adobe.

The first 11 specimens listed in Table 6 were exposed to relative

humidities between 30 and 50% at 70°F (21''C), which are higher than

those normally encountered in the arid southwestern region of the

United States. However, during rainstorms, humidities higher than 80%

have been measured at the Tumacacori National Monument [16], The last

three specimens listed in Table 6 were exposed to relative humidities

close to 100%. Creep of those three specimens was more than 200%

greater than the creep of specimens exposed to the lower relative

humidity.

While the creep data presented herein should not be quantitatively

applied to an actual adobe structure, the large effect of moisture

on creep is clearly evident. Furthermore, it can be speculated that

cycles of wetting and drying can lead to progressive creep*

30
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31
Figure 12, Creep of same adobe specimens selected

from table 6. Applied load was 21 psi

(0.14 MN/m^)

32
6. PENETRATION TESTING OF ADOBE

A Method was developed by Winkler [20] for measuring the penetration

resistance of adobe. He attached a needle to the head of a Soiltest

Soil Penetrometer and measured the relative force required to insert

the needle to a prescribed depth in adobe. This device was modified

in the present study, and was used to develop relationships between

the penetration resistance and moisture content and compressive strength

of adobe.

6.1 TEST METHOD

Penetration resistance of the specimens used to construct the curves in

Figure 2 were obtained immediately before their compressive strengths

were measured. A modified penetrometer is shown in Figure 13. The

portion which penetrated the adobe was machined down from the original

0.25 inch (6.3 mm) diameter rod to a cone with a base diameter of 0.063

(1,6 mm) and a height of 0.063 inch (1.6 mm). During testing, only the

force required to completely embed the cone in the adobe was applied.

Penetration values were taken from the sliding scale on the barrel

of the penetrometer. Ten measurements were taken on each specimen, and

the average values were used in constructing the curves shown in Figures

14 and 15.

6.2 RESULTS AND DISCUSSION

Relationships between penetration resistance and moisture content of

the three adobe systems are given in Figure 14. The linear regression

equations were derived by combining the three sets of data. The regression coefficient for the equation relating moisture content to penetration is 0.87 and the standard error is 3%, i.e. M+ 3%, Similar curves

for penetration resistance versus compressive strengths are given in

Figure 15. In this case, the regression coefficient is 0.98 and the

standard error is 21 psi (0.14 MN/m ) . These statistical analyses indicate that the modified penetrometer used in the NBS laboratory can give

reliable predictions of moisture contents and compressive strengths of

adobes. However, new calibration curves should be obtained when testing

adobes which may have physical and chemical properties grossly different

from the three systems tested in the present study. Furthermore,

measuring the penetration resistance of the surface layers of an adobe

may not give results characteristing of the interior of the adobe.

1/ This instrument is identified in this paper in order to adequately

specify the experimental conditions. This identification does not

imply recommendation or endorsement by the National Bureau of

Standards

.

33
Figure 13. Penetrometer used to measure penetration resistance of adobe

34
4.0

o

z

o

<LLI Z

o

I—<

»—

LUz

LUa.

3.0

2.0

n TUMACACGRI AND SAND

A TUMACACORI

O ESCALANTE

REGRESSION EQUATION

m{%)=-5.3 P+ 25.2

S . E.= 3.0 %

MOISTURE, PERCENT

Figure 14. Relationship between penetration

resistance and moisture contents

(M) of adobe.

35
7. SUMMARY AND CONCLUSIONS •

"

In this study, the effects of mix proportions, curing conditions,

relative humidity, water contents, and wet-dry cycles on the compressive strength of adobe were investigated. The flexural strength of one adobe was also measured. In addition, the creep of dry and wet adobe

was characterized.

A non-destructive test method, based on measuring the penetration

resistance, was found to give reliable prediction of the compressive strength and moisture content of adobe. However, for different

types of adobe, new calibration curves should be developed.

Of the experimental factors studied, moisture and wet-dry cycles were found to have the most deleterious effect on the compressive

strength of adobe. Moisture also had a significant effect on the

creep of adobe. For example, creep of wet adobe specimens were

200% greater than the creep of dry specimens. Adobe can become wet

by absorption of rain, absorption of ground water and by absorption

of moisture during periods of high relative humidities. The effects

of these processes and also wet and dry cycles on the mechanical

properties of adobe will depend on the severity of the conditions.

Because of the low relative humidities normally present in the

arid southwestern states, ground water and rain water are probably

the main source for excess moisture in historic adobe structures

located in such areas. Therefore, it appears that providing an abode structure with a water-tight roof and positive ground drainage

will greatly assist in mitigating the most severe processes by which

water reduces the mechanical properties of adobe.

An obvious need exists for the development and establishment of

standard test methods to characterize the important mechanical

properties of adobe. For example it has been found in this study

that the drying conditions and moisture contents affect the compressive strength of adobe specimens. Other important factors, which

no doubt affect compressive strength, include method of preparing

specimens, shrinkage of specimens, and the loading rate. Unless

these and other factors are controlled or measured it will remain

difficult to compare the data from different laboratories, or even

from the same laboratory.

36
.379 .758 1.13 1.51 1.89 2.27 2.65

o

z

a

<

Z

g

I—<

I—

LUZ

UJa.

385

COMPRESSIVE STRENGTH, psi

Figure 15. Relationship between penetration

resistance and compressive strength

(f^') of adobe

37
8. REFERENCES

1. J. R. Clifton, "Preservation of Historic Adobe Structures - A

Status Report," Technical Note 934, National Bureau of Standards

(1977); Available from U. S. Government Printing Office, SD Catalog

No. C13.46: 934.

2. R. H. Clough, "A Qualitative Comparison of Rammed Earth and SunDried Adobe Brick," University of New Mexico Publications in

Engineering No. 4 (University of New Mexico Press, Albuquerque,

1950).

3. T. A. Eyre, "The Physical Properties of Adobe Used as a Building

Material," The University of New Mexico Bulletin, Engineering

Series, Vol. 1 (No. 3) (1935).

4. H. C. Swalen, "Effect of Soil Testure upon the Physical Characteristics of Adobe Brick," Agricultural Experimental Station

Technical Bulletin No. 58 (University of Arizona, Tucson, 1935).

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

Compacted Soil and Soil-Cement Mixtures for Use in Buildings,"

National Building Research Institute Series DR 2 (South African

Council for Scientific and Industrial Research, 1950).

6. H. L. Whittenmore, A. H. Stang, E. Hubbell and R. Dill, "Structural,

Heat-Transfer, and Water-Permeability Properties of Five Earth-Wall

Constructions," Building Materials and Structures Report BMS 78

(National Bureau of Standards, 1941).

7. R. L. Patty, "The Relation of Colloids in Soils to Its Favorable

Use in Pise or Rammed Earth Walls," South Dakota Department of

Agriculture Engineering Bulletin No. 298 (South Dakota State

College, Brookings, South Dakota, 1936).

8. J. R. Clifton, P. W. Brown and C. R. Robbins , "Methods for Characterizing Adobe Building Materials," NBS Technical Note (in Press).

9. P. W. Brown, C. R. Robbins and J. R. Clifton, "Factors Affecting

the Durability of Adobe Structures," NBSIR (in Press).

10. Standard Test Method for Compressive Strength of Hydraulic Cement

Mortars (Using 2-In or 50-MM Cube Specimens), American Society for

Testing and Materials (ASTM) , Designation C109-73.

11. Standard Specification for Flow Table for Use in Test of Hydraulic

Cement, ASTM Designation C230-68.

12. Standard Method for Mechanical Mixing of Hydraulic Cement Pastes

and Mortars of Plastic Consistency, ASTM Designation C-305-65.

38
8. REFERENCES (Continued)

13, Standard Method of Test for Flexural Strength of Hydraulic Cement

Mortars, ASTM Designation C348-72.

14, Standard Method of Laboratory Determination of Moisture Content of

Soil, ASTM Designation D2216-71.

15, A, A, Hammond, "Prolonging the Life of Earth Buildings in the

Tropics," Building Research and Practice, pp 154-164

(May/June 1973),

16, A, Crosby, "Preliminary Report on the Preservation Project at Tumacacori National Monument," U, S, National Park Service, 1977,

17, S, G, Fattal, "Structural Preservation of Historic Monuments at

Casa Grande and Tumacacori," (in Press),

18, F, C, Fenton, "The Use of Earth as a Building Material," Kansas

State College Bulletin, Vol 25 (No, 6) (Manhattan, Kansas, 1941),

19, R, N, Yong and B, P, Warkentin, '"Soil Properties and Behavior

(Elsevier Scientific Publishing Company, Inc, New York, 1975),

20, E, Winkler, Private Communication,

39
NBS-114A (REV. 11-77) I

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS TN 996

2. Gov't Accession

No.

4. TITLE AND SUBTITLE

Mechanical Properties of Adobe

3. Recipient's Accession No.

5. Publication Date May 1979

6. Peifoiming Organization Code

7. AUTHOR(S) James R, Clifton and Frankie L, Davis 8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Taslc/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, D. C. 20240

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-worcl or less factual summary of most sigpificant intotmation. If document includes a si^ificant

bibliography or literature survey, mention it here.) Relationships between the compressive strength and creep, and the moisture contents

of adobe were investigated. Moisture was found to have a deleterious effect on these

mechanical properties of adobe, its severity increasing with increasingly higher

relative humidities and higher moisture contents. It was concluded that rain and

ground water would have a greater deleterious effect on the mechanical properties of

adobe than high relative humidities.

Thephysicochemical properties of adobe, mix proportions, drying conditions, and

shrinkage of specimens were also found to influence the compressive strength of adobe.

Procedures for preparing, curing and testing of adobe specimens are given.

A nondestructive test method, based on measuring the penetration resistance of adobe, was found to give reliable predictions of the compressive strength and moisture content

of adobe specimens.

ii

h

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

Adobe; adobe soil; compressive strength; creep; flexural strength;

mechanical properties; moisture; non-destructive testing; preservation.

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