NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-10-4716 June 22, 195?3 2^95

REVIEW OP DATA ON THE

SHRINKAGE OF CONCRETE MASONRY

By

C. C. Fishburn

At the request of

Office of the Chief of Engineers

Department of the Army

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, or In part, Is prohibited

unless permissio Approved for public release by the rstandards , Washington 25,d,c, Such Director of the National Institute ot » report has been specifically prepared i Standards and Technology (NIST) r report for its own use. on October 9, 2015.

June 25, 1953 10.1

Department of the Army

Office of the Chief of Engineers

(Attn; Mr. H. 3. Zackrison, Chief

engineering Division,

Military Construction)

Washington 25, 0.

Subject; Shrinkage of Concrete Masonry

your file ElGES

Gentlemen;

This refers to your letter of July 30, 1952, regarding a review of reports covering the concrete masonry investigations made by the Research Foundation, University

of Toledo. Six copies of this review are enclosed.

Yery truly yours.

C. C. Fishburn

Materials EngineerStructural Engineering Section

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Contents

page

1. Introduction 1

2. Volume Changes in Concrete Masonry Units 1

2.1 Coefficient of thermal expansion 1

2.2 Drying shrinkage of concrete masonry 2

units

3. Drying Shrinkage of Walls and Panels 4

3.1 Walls built of saturated units 4

3.2 Walls built of dry units 5

3.3 Early tests of wall panels 6

3.4 Walls with joint reinforcement 6

4. Discussion of Tests Data 9

4.1 Limitations on moisture content of 9

the units

4.2 Steel joint reinforcement 10

4.3 Summary and conclusions 13

References 15

Review of Data on the

Shrinkage of Concrete Masonry

1. INTRODUCTION

This report reviews data on the drying shrinkage and the

effects of temperature changes on the shrinkage of concrete

masonry walls and units, and on the shrinkage cracking of restrained concrete masonry walls. Particular attention was

given to the reports, listed below, which were prepared by

the Research Foundation of the University of Toledo, Ohio.

"Volume Changes of Blocks and Wallettes," by J. K. Selden,

et al SJ-3 • "Relation of Shrinkage to Moisture Content in Concrete

Blocks," by G. L. Kalousek, et al \_22 • "Welded Wire Mesh for Horizontal Mortar Joint Reinforcing," by J. K. Selden and W. G. Rohr [ 3] • Pertinent data from other sources also were reviewed.

<* It may be noted that the restrained walls tested at the

Research Foundation were restrained at the ends only, and

those tests did not represent the more common conditions of

restraint, along the base of the concrete masonry wall.

2. VOLUME CHANGES IN CONCRETE MASONRY UNITS

2.1 Coefficient of Thermal Expansion

Values of the coefficients of thermal expansion for concrete masonry units and for two lightweight aggregate concretes were obtained from three sources', [T] , Wendt and Woodworth and Petersen Da and are listed in Table 1.

The data, Table 1, Indicate that the coefficient of thermal

expansion was dependent upon the kind of aggregate and the amount

of cement used in the concrete. A study of data obtained from

the Research Foundation ^_lj indicated that the kind of curing

given the concrete masonry units did not have an important effect

on the coefficient of thermal expansion. Concrete masonry units

made of cinder aggregate concrete had the least thermal expansion; sand and gravel aggregate units had the greatest. The

thermal expansion of the sand and gravel aggregate units was

about double that of the cinder units.

l/ The numbers refer to the references listed at the end of this

report

.
2 . Data on the bars of shale and slag aggregate concretes [5j

are included to indicate the effects of the cement factor on the

coefficient of thermal expansion. It is unlikely that the use of

an air-entraining agent had an important effect on the value of

the thermal coefficient.

In general, the coefficients of thermal expansion were

approximately as listed in the following table:

Kind of aggregate

Sand and gravel

Crushed limestone

Expanded slag

Pumice

Expanded shale

Cinders

Coefficient of

thermal expansion

1<5

6

/F°

5.2

5.0

4.8

4.1

4.0

2 .

2.2 Drying Shrinkage of Concrete Masonry Units

Values of the drying shrinkage, from a saturated condition,

of concrete masonry units was obtained from four different

sources, and are listed in Table 2. Although not complete, the

data include values of the cement factor, compressive strength,

method of curing, method of drying, and the moisture content of

the unit. The moisture contents are the moisture retained in

the units, in terms of the total moisture loss on drying from a saturated condition at 73°F to a dry condition in a ventilated

oven at 220°F , Data on the shrinkage observed from a second

wetting and drying are included in the Table, for some units.

The two most important factors affecting the drying shrinkage of concrete masonry units were the kind of curing of the

units and the kind of aggregates used in them. As may be noted.

Table 2, units cured in high-pressure steam at a pressure of

125 to 140 lb/in. 2 shrank about one-half as much on drying from

a saturated condition as did units cured at atmospheric pressure,

with or without steam. For like methods of curing and drying,

the units containing heavy aggregates such as sand and gravel

shrank less than did units containing lighter aggregates.

In general, there was no great difference between the

shrinkages of similar units cured at atmospheric pressure, with

high- temperature steam, low- temperature steam or in moist air.
3 . As indicated, in Table 2, reference 2, there was a progressive shortening of concrete masonry units with wetting and

drying o The shortening was not as great for high-pressure steamcured units as for units cured by other methods.

The data obtained from the Progress Report of ACI Committee 716, £6] , give the effects on shrinkage of three methods

of drying saturated high-pressure steam-cured units. For the

Rapid Method, the units were dried in a ventilated oven at

220°F. In the British Method (similar to British Standards

No. 824), the units were dried to constant weight over a saturated solution of calcium chloride at 120°F. In the Reference

Method, the units were dried to constant weight over a saturated

solution of sodium hydroxide at 73°?.

The relative humidity over the sodium hydroxide solution

was about 8 percent. The drying conditions to which the units

were subjected in both the British and the Reference Methods

probably equalled or exceeded the most dry conditions that

would be found in service. Even so, the shrinkages obtained

by the Rapid Method of drying were about double those obtained

by either the British or the Reference Methods and are far

greater than the shrinkages to be expected in the field. There

was little difference between theshrinkages of like units dried

by the British and Reference Methods and there was not much

difference between the shrinkage of units dried by those methods

and the shrinkage of similar units dried in air at 73°F and 2^

percent relative humidity. The data, [ 6] , also show that the

shrinkages of high-pressure steam-cured units of like composition and method of drying, but made at different plants, may

have a considerable range in values. These differences in

shrinkage may be attributed, in most part, to differences in

the technique and procedures used to cure the units at the

different plants.

The range and the approximate average values of the

shrinkage of concrete masonry units, showing the effects of

curing and drying methods are listed in the following Tables In

this Table, the units have been divided into two classes, one

for heavy-weight aggregate concrete (such as sand and gravel)

weighing more than 100 lb per cu ft of concrete and the other

for lightweight aggregate concretes weighing 100 lb per cu ft

or less.
4 . Range of Shrinkage Values

A. Units weighing over 100 lb/cu ft of concrete

High-pressure steam cured Other methods of cure

Rapid

Method

of drying

Other

drying

methods

Rapid

Method

of drying

Other

drying

methods

percent percent percent percent

Maximum

Minimum

0.042

.018

0.025

.013

0,033

.027

Average .028 .016 — .030

B. Units weighing over 100 lb or

concrete

less, per cu ft of

High-pressure steam cured Other methods of cure

Rapid

Method

of drying

Other

drying

methods

Rapid

Method

of drying

Other

drying

methods

percent percent percent percent

Maximum

Minimum

0.051

.036

0.032

.018 — 0.072

.040

Average .040 .023 — .051

3. DRYING SHRINKAGE OF MALLS AND PANELS

3.1 Walls built of saturated units

Data on the extensibility of restrained walls and on the

drying shrinkage of unrestrained wall specimens, built of saturated units, at the Research Foundation £l] are listed in Table

3. Nearly 4o walls, 8-in. thick, 5 courses high and 9 ft long,

were dried to moisture equilibrium with air at 73°F and 25

percent relative humidity. About half of the wall specimens

were constructed and supported on small plates and rollers and

were free to shrink without restraint. The other walls were

similarly supported but were restrained from shrinking by steel

frames fastened at the ends of each of the five courses.
The data. Table 3? are erratic and not much dependence maybe placed on the values for extensibility. The data indicate

that the free shrinkage of the unrestrained walls was slightly

greater than the free shrinkage observed for the units. This

may be due, in part, to the drier condition of the walls at the

end of the tests 0 With the exception of the sand and gravel aggregate walls,

the restrained walls cracked at average moisture contents

greater than the minimum observed for the unrestrained walls,

of like aggregates. The restrained walls of sand and gravel

aggregates cracked at moisture contents equal to or less than

those contents observed for the free, unrestrained walls. The

first cracks were said to have occurred in the vertical joint

of a course. As these cracks extended vertically they passed

through the units immediately above and below the vertical

joint. Both the free shrinkage of unrestrained walls and the

extensibility of restrained walls built of high-pressure steamcured units were about 60 percent of the values for the walls

built of high- temperature steam-cured units. This ratio ( 60

percent) corresponds closely with the relation between the

shrinkage of units cured by the two methods.

The ratio of the extensibility of restrained walls to the

free shrinkage of similar walls of like units was highest for

walls built of sand and gravel aggregate blocks and was dependent up©n the moisture content, at first crack in the restrained

walls. The moisture content, when dry, of unrestrained walls

built of high-pressure steam-cured units averaged about 70 percent of the values for similar walls built of high- temperature

steam-cured units. Similarly the moisture content at first

crack in the high-pressure-cured restrained walls was less than

that for the high-temperature-cured restrained walls.

3.2 Walls built of dry units

Data on the free shrinkage and the extensibili ty_of five

walls built of dry units at the Research Foundation, M are

listed in Table 1+. Two aggregates, cinders, and sand and

gravel, and two curing methods, using high pressure and hightemperature steam, were represented. Before building the

walls, the units had been dried to moisture equilibrium in

air at 73°F and 70 percent relative humidity. After construction, the walls were dried to moisture equilibrium with air at

73°P and 25 percent relative humidity.
The data. Table L|_, show that the free shrinkage of the

walls greatly exceeded that of the units. At least some of the

difference may be attributed to a moisture increase of the units

due to their being placed in contact with wet mortar when laid

in the walls. Like those in Table 3, the data in Table l_j. are

erratic and too much dependence should not be placed on the

values for extensibility.

Neither of the two walls built of high-pressure steamcured units cracked on drying, whereas two of those walls built

of high-temperature steam-cured units did crack on drying. It

may be noted that, irrespective of the aggregate, the free

shrinkage of dry high-pressure steam-cured units and walls was

small and averaged less than 0.010 percent.

3.3 Early tests of wall panels

Some measurements of the drying shrinkage of concrete

masonry were made about 1930 by P. M. Woodworth and W. D. M.

Allen. These tests are reported by Allen and p] and by

Woodworth Ld , . Data from the unrestrained shrinkage in the

vertical direction of wall panels composed of nine moisture

cured units, are given in Table 5. The panels were built of

sand and gravel, cinder, and expanded shale units that had been

moist cured in air at 70°F and 100 percent relative humidity

for two days. After curing and prior to placing in the panels,

the units were stored in laboratory air for periods ranging between zero to 88 days. The measurements on the panels were begun within one-half hour after their completion. The tests were

conducted in rooms without humidity control.

The data. Table 5, show the effects, on shrinkage, of drying the units before laying them in the wall. The shrinkage of

panels built of air-dried units were 25 to 35 percent of the

shrinkages of panels built of saturated units. The shrinkages

of the panels built of saturated units were higher than the

shrinkage of unrestrained walls built of high-temperature-cured

units of similar aggregates, see Tables 3 and 5.

3. If Walls with joint reinforcement

The drying shrinkage and the extensibility of some walls

containing joint reinforcement and built of high- temperaturecured units are listed in Table 6. The walls were built and

tested at the Research Foundation Da • Two walls were built
7.

of saturated sand and gravel aggregate units and two walls were

built of saturated expanded shale aggregate units. The walls

were dried to equilibrium with air at 73°E and 25 percent relative humidity. The joint reinforcement consisted of two plain

3/l6-in. diameter cold drawn longitudinal wires held 6 in.

apart with transverse No. 9 cross wires welded at l6-in. centers.

The yield point of "the longitudinal wire was 70,000 lb/in, . One

wall of each aggregate contained reinforcement in, every joint;

the others were reinforced in half of the joints.

The data, Tables 3 and 6, indicate that the use of joint

reinforcement slightly reduced the free shrinkage of the unrestrained walls. The indicated reductions in free shrinkage

are listed in the following $able

Kind of Aggregate Curing

Number of

j oints

reinf or ced

Reduction

in free

shrinkage

Sand and gravel H.T.

percent

50

percent

0

Sand and gravel H.T. 100 6

Expanded shale H.T . 50 7

Expanded shale H.T. 100 11

The extensibility of the restrained walls was not greatly

affected by the use of joint reinforcing. However, the moisture

content at first crack of the reinforced wall of expanded shale

aggregate units was much lower than the moisture content at

first crack of an unreinforced shale aggregate wall. It was

stated that the width and visibility of cracks in walls built

of high-temperature-cured block were reduced by the use of

reinforcing. However, the width of cracks in the reinforced

high- temperature walls was greater than the width of cracks

in unreinforced walls built of high-pressure steam-cured units

and, regardless of curing, in unreinforced walls built of dry

units .With all joints reinforced, the steel area in the joint

reinforcement was equivalent to 0.2/ percent of the minimum

cross sectional area of blocks. Assuming the steel carries

the tensile load at a section where a crack occurs, the steel

stress would be about lj.00 times the average tensile stress on

the section. Since the average tensile stress on the minimum

section at failure of the sand and gravel aggregate block walls

was about 220 lb/in. ^ and since the stress in the expanded

shale wall with 50 percent joint reinforcement was 130 lb/in. 2,

the yield point of the steel (70,000 lb/in.2) would have been
8 . exceeded in at least three of the four restrained walls if there

had been no slip of the mortar on the reinforcement.

Further data, not reported in Table 6, were obtained on

tests of four, high- temperature steam-cured cinder aggregate

walls. The tests were sponsored by Adrian Peerless, Inc.,

Adrian, Michigan, and were made by the Research Foundation

£ 3] . The walls were built of saturated units and were dried

to equilibrium with air at 73°F and 30 percent relative humidity.

Two walls were not reinforced, one wall was reinforced in the

top and the bottom bed joints, and one was reinforced in every

bed joint. The joint reinforcement was Adrian Peerless Crimped

Wire Wall-Lok, consisting of two 3/l6-in. diameter crimped

longitudinal wires with No. 9 cross wires welded at l6-in.

centers. One of the two unreinforced walls was not restrained

against shrinkage. The other three walls were restrained, presumably in a similar manner to the restrained walls previously

described [i] • Illustrations in the report on tests of the high- temperature steam-cured, cinder-aggregate block walls, sponsored by

Adrian Peerless, Inc., showed the importance of adequate joint

reinforcing. The free shrinkage of the unreinforced, unrestrained wall on drying from saturation to equilibrium with

air at 73°B and- 30 percent relative humidity was 0.061 percent,

which agrees well with the 0.069 percent shrinkage of a similar

wall dried to equilibrium in air at 25 percent relative humidity

(Table 3). The unreinforced but restrained wall of this series,

wall C, had a single vertical crack extending through blocks

and head joints. The maximum width of this crack was 0.048 in.

Reinforcement of the top and bottom joints in a restrained wall,

wall B, resulted in a single vertical crack through blocks and

head joints. This crack had a maximum width of 0.028 in. at

the center course and the maximum width of cracks in the reinforced courses was 0.020 in. The total number of cracks in

the restrained walls reinforced in every joint, wall A, was

increased over the number of cracks in the other walls, walls

B and C, but the size of the cracks was reduced, and there was

no vertical crack extending through all five courses. The

maximum width of cracks was 0.008 in. The relative widths

of the maximum-sized cracks in walls A, B, and C, taking wall C

as 100 percent, was respectively 17, 58, and 100 percent. It

was stated that the crimped joint reinforcing was much more

effective in controlling the width of the cracks than the plain

wire reinforcing previously tested.
9.

4- o DISCUSSION OP TEST DATA

1| 0 1 Limitations on moisture content of the units

Specifications for concrete masonry units usually limit the

moisture content of the units on delivery. If the moisture content is to be limited, the question arises as to what the limits

should be. It may be presumed that the drying shrinkage of the

masonry may be held to a practical minimum if the blocks, on

delivery, are dry enough to be in moisture equilibrium with air

at the average temperature and relative humidity to which the

masonry will be exposed. Even so, it is likely that some volume

changes due to changes in moisture content will occur, particularly if there is a high seasonal fluctuation in relative

humidity. Some aid to the solving of this problem may be obtained from the data in Table 7 3 which lists the shrinkage of

masonry units from moisture contents of 100, Lj.0, 30, and 20 percent to equilibrium at different relative humidities. Information on the moisture content of the units when In equilibrium at

different relative humidities is listed in Table 8.

The data in Tables 7 and 8 were obtained from an interpolation of the shrinkage curves contained in test reports of the

Research Foundation, M and [2] . Tfeje concrete masonry units

represented and tested in these two investigations may have been

similar but were not necessarily alike. The values- obtained [_'2j

were for blocks which had been resaturated and dried for the

second time. The values for shrinkage and moisture content were

slightly increased on the second cycle of wetting and drying.

It has been noted (Table 2 , £.6] ) that units of like compositions but made at different plants using high-pressure

steam, show a wide variation in shrinkage values. While complete

data in Tables 7 and 8 were not available for all the different

units represented in the two Research Foundation ]JLj| and \_ 2] , investigations, the data for similar units that can be compared

are In fair agreement and appear to be consistent. The moisture

contents at different relative humidities are consistently

higher for the high-temperature- cured units than for the highpressure-cured units ( see'; Table 8). From Table 7, it may be

noted that at moisture contents of 100, 40, and 30 percent

moisture contents, the shrinkages of the high-pressure steamcured units is consistently less than the shrinkages of the

high- temperature units. This advantage is much lessened at

moisture contents of 20 percent. In general, the shrinkage of

the high- temperature units at 20 percent of moisture content is
10 . approximately equal to the shrinkage of the high-pre ssure units

at 30 percent moisture content. It follows that the rate of

loss in potential shrinkage of the high- temperature units is

greatest at moisture losses from 100- to Lj.0 — and 30- percent

moisture content. It is important, therefore, that any limitations on the moisture content of high-temperature units be

rigidly enforced; wetting of such units just prior to laying

may be inadvisable.

4.2 Steel Joint Reinforcement

In general, the effects of steel joint-reinforcement were

as follows:

g.) Slightly reduce the free shrinkage of unrestrained

walls

b) Increase the number but reduce the maximum width of

cracks in restrained walls.

c) Reduce the number of cracks in the blocks relative

to the number of cracks in the head joints of restrained walls.

These effects were most pronounced when the longitudinal

wires were deformed and had a high bonding efficiency. Information on the maximum width of cracks in restrained walls,

two of which contained crimped wire joint reinforcement, is

given below for 5“ course test walls of high-t emperature cinder

aggregate blocks dried from a saturated condition to equilibrium

at 30 percent relative humidity.

Amount of Total number Maximum width of crack

joint rein- of separate

for cement vertical cracks Vertical joint block

• in. in.

N one 1 0.0i|8 0.040

Top and bottom courses 3 0.028 .020

Every joint 6 0.012 .008

For the wall reinforced in the top and bottom joint only,

the maximum width of crack (0.028 in.) was in the center course.

The width of this crack was nearly 5>0 percent greater than the

maximum width of cracks in the reinforced courses. It follows,

so far as these tests are concerned, that where dependence is

placed on joint reinforcement to reduce crack width, the reinforcement should be placed in every joint or in alternate joints.

It is unlikely that bond beams, spaced 12 to 15 courses apart,

vertically, will prevent the formation of large shrinkage cracks

near midheight between the bond beams in walls without openings.
11

Attempts were made to formulate rational methods for

determining the amount of reinforcement needed in concrete

masonry walls to avoid the likelihood of yielding of the

steel, and for estimating the probable widths of cracks under

various conditions. These methods are needed. If there is

yielding of the reinforcement, it occurs at cracks, permitting them to widen unduly and thereby diminishing the value

of the reinforcement. Provided there is no yielding of the

steel, the widths of the cracks in reinforced masonry depend chiefly upon the slip of the reinforcement in the

masonry. However, the amount of slip depends upon many

factors, and adequate test data on the various factors are

not yet available

.

Although reliable methods for designing reinforcement

can not be formulated until more test data become available, it may be helpful to consider some of the factors.

For simiplicity, it will be assumed that a long wall is prevented from shortening longitudinally by end anchorages and

that It is not restrained otherwise. First consideration

will be limited to the central portion sufficiently remote

from the ends to be unaffected by any Irregularities there.

As shrinkage of the wall material occurs, the wall is subjected to longitudinal tensile stresses. If the shrinkage

proceeds until the longitudinal tensile strength is exceeded, a crack forms. Ordinarily the crack will be

vertical or nearly so. If the wall does not contain longitudinal reinforcement, the cracking would relieve the

stresses completely; in reinforced walls the shrinkage

stresses would be decreased somewhat and the longitudinal

force (summation of the longitudinal stresses in a vertical

plane) at the crack would be borne by the reinforcement.

For the idealized conditions assumed, a single crack

would relieve the stresses In a non-reinf or ced wall, and

additional shrinkage would increase the crack size but

would not produce more cracks. In a reinforced wall,

further shrinkage would tend to increase the tensile

stresses until the tensile strength of the masonry is again

exceeded, if there is no yielding of the reinforcement; and

a second crack forms, and so on. The maximum stresses in

the reinforcement and the masonry occur just before the

appearance of each new crack. Then, as the total tensile

force in the reinforcement equals approximately:
12 . A s f s = Awft , in which

A.

Aw

ft

P

f „

= cross sectional area of steel bars or wires, in

a vertical section of the wall, sq in.

= tensile stress in the reinforcement at a crack, psi

= gross area of wall in a vertical section, sq in. = tensile strength of the masonry (gross area), psi

= ^s , and, solving for f s gives:

Aw

= ft/p 5 in which (1) = ratio of cross sectional area of the reinforcement to that of the masonry wall.

As it is obviously undesirable for the reinforcement to

undergo appreciable yielding, f

g

should not exceed the yield

strength Y of the reinforcement. Therefore one criterion of

design should be fu

p^ _Y

( 2 )

As previously mentioned, the basic data needed for an

analysis of widths of cracks are not available. Therefore, a

pretentious theoretical analysis is not warranted. However,

for the simple case considered in the foregoing it will be

further assumed that the imaginary wall is reinforced with

conventional smooth 3/l6-in. diameter wire in some of the

horizontal joints. The relation between the slip at the

loaded end of such wire and the stress on that end is indicated in plotted data DJ and may be estimated by the empirical

relation p

S = 2x10IT in which

S = slip at a crack, inch, and

f

(3)

is given by equation (1). Substituting from

equation (l) and noting that the maximum width of

the cracks equals twice the slip of reinforcement

at a crack

f r~ W = 2S = t in which

p

2

x 10 11

(]|)

W = maximum width of the cracks, inch.
13

.

For example, assuming a tensile strength (gross area) of the

masonry of 30 lb/in .

2

and reinforcement consisting of two 3/l6 -in.

diameter wires in every joint, the ratio of reinforcement area

to gross cross sectional area of the masonry "p" would be about

0.0009 and the maximum width of crack would be about 0.011 in.

(equation 4)« For reinforcement in every other joint the value

of "p n would be halved and the maximum crack width would be

0.044 in.

4.3 Summary and Conclusions

The coefficients of thermal expansion of concrete masonry

units ranged from about 4-2 by 10.-6 per 1 °F for heavy aggregate

sand and gravel units to about 2.6 by 10~6 per 1 °F for lightweight units made of cinder aggregate.

The drying shrinkage of saturated concrete masonry units

was affected by the kind of aggregates, the kind of curing and

the method, or degree, of drying the units. Units made of concrete weighing more than 100 lb per cu ft (of concrete) shrank

much less than did' units made of concretes weighing 100 lb or

less per cu ft

.

Units cured with high-pressure-steam at 120 to 140 psi

shrank much less on drying than did units cured at atmospheric

pressure by other methods. Other things being equal, there was

little difference between the drying shrinkages of units cured

at atmospheric pressure in air, in low-t emperature (120°F) steam

and in high temperature (170°F) steam.

The drying of saturated units in a ventilated oven at

220°F (Rapid Method of drying) produced a much greater shrinkage

of the units than did drying them in an oven over a saturated

solution of calcium chloride at 122°F (British Method) or drying In a drum over a saturated solution of sodium hydroxide at

73°F (Reference Method) . There was little difference between

the shrinkages of like units dried by the British and the

Reference Methods and there was not much difference between the

shrinkage of units dried by these methods and the shrinkage of

similar units dried in air at 73°F and 24 percent relative

humidity. Regardless of the kind of aggregate, the shrinkage

of high-pressure-steam cured units dried by the Rapid Method

was nearly twice (174 percent) as much as the average shrinkage

of similar units dried by the other three methods. The exposure,

in the Reference Method, to a relative humidity of 8 percent

represents an extreme of drying that would hardly be expected in

service. For specification purposes, the Rapid Method of Measuring shrinkage is a practical one, but it should be remembered
14 . that the shrinkages obtained by this method are far in excess

of the shrinkage that may occur under the most severe service

conditions

Walls restrained at the ends and supported on a series

of rollers cracked when the longitudinal shrinkage exceeded

the extensibility of the masonry. Cracks were formed in restrained walls built of saturated units when the walls were

dried to moisture equilibrium with air at 73°F and 25 percent

relative humidity. Some walls built of units dried to equilibrium at 73°P and 70 percent relative humidity did not crack

on drying to 25 percent relative humidity. The first cr&cks

in restrained walls occurred at the vertical joints between

units. These cracks then extended into the unit 'above and below the cracked head joint. None of the restrained test walls

were restrained along the supporting base and it may be noted

that walls restrained along the base may crack even though the

shrinkage is less than the extensibility of the masonry.

Steel joint reinforcement slighly reduced the free

shrinkage of unrestrained walls, increased the number but

greatly reduced the size of cracks in restrained walls and

tended to reduce the number of cracks in the units. These

effects were most pronounced when the longitudinal reinforcing

wires were crimped. Regardless of how1 a concrete masonry wall

may be restrained against shrinkage and provided the blocks

are laid dry enough to prevent excessive shrinkage, it is possi

ble that the proper use of joint reinforcement will reduce the

size of cracks to a satisfactory minimum.

Drying of sand and gravel aggregate units to a 30 percent

moisture content was roughly equivalent to placing them in

moisture equilibrium with air at 73°F and- about 75 percent

relative humidity. This appears to be a satisfactory and

practical limit for those parts of the country where the mean

annual relative humidity is somewhat below 75 percent. For

example, the mean annual relative humidity at Washington, D. C.

is- about 65 percental Units made of lightweight aggregates contained 20 to 25 percent moisture when in equilibrium at 70 percent relative humidity. For very dry locations and for special

exposure conditions, thp^ blocks may be dried to less than 30

percent of moisture content.
References

"Volume Change Characteristics of Blocks and Wallettes,"

J. K. Selden, et al. Research Foundation, University of

Toledo, Ohio, (April 1952)

.

"Relation of Shrinkage to Moisture Content in Concrete

Blocks," G. L. Kalousek, et al , by Research Foundation,

University of Toledo, Ohio, (April 1952).

"Welded Wire Mesh for Horizontal Joint Reinforcing,"

J. K. Selden and W„ G. Rohr, Research Foundation, University of Toledo, Ohio, (December 1952).

"Tests on Concrete Masonry Units Using Tamping and Vibration Molding Methods," K, F. Wendt and P. F. Woodworth,

American Concrete Institute 36-121 (I 9I4-O).

’iProperties of Some Lightweight Aggregate Concretes With

and Without an Air-Entraining Admixture," Perry H.

Petersen, NBS Report BMS 112.

"Physical Properties of High-Pressure Steam-Cured Concrete Block." Progress Report of ACI Committee 716 - Journal, American Concrete Institute, April 1953> page 7l+5.

"Shrinkage Measurements of Concrete Masonry," W. D. M.

Allen, American Concrete Institute 26, page 699 (1930).

"Shrinkage Measurements of Concrete Block Masonry,"

W. D. M. Allen, American Concrete Institute 28, page 177

(1932)

.

Some* Tests of Concrete Masonry Units Cured with HighPressure Steam, P. M. Woodworth, American Concrete

Institute 26, page 50Lj. (1930).
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the

Research

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2,

Table

1.

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Table

2,
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J.

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