m •

NATIONAL BUREAU OF STANDARDS REPORT

2324

Extensibility of Some Concrete Masonry Units

By

C. C. Fishbum

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1000-10-4716 March 3, 1953 2324

Extensibility of Some Concrete Masonry Units

By

C. C. Fishburn

At the request of

Office of the Chief of Engineers

Department of the Army

The publication, reprlo

unlewpormltilo* laobt 2S.D.C. Suck pormla

eally prepared If that

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

ait, la prohibited

irda, Washington

haa boon apedt>

t for Ita own uae.

EXTENSIBILITY OF SOME CONCRETE MASONRY UNITS

1.

INTRODUCTION

The volume of the concrete in concrete masonry units is

affected by changes in temperature and in moisture content.

The units in masonry walls are more or less restrained against

free shrinkage. Such restraints produce tensile stresses

which may produce "shrinkage cracking" of the masonry. Cracks

at vertical mortar joints usually indicate a bond failure. The

concrete masonry units will crack if the tensile strains

developed in them equal or exceed the extensibility of the units.

(The extensibility of a material is the maximum elongation per

unit of length produced by tensile stress).

2.

SPECIMENS

The concrete masonry units were some of those furnished

by the American Concrete Institute Committee 716 for measurement of shrinkage. The 3-cell, 8- by 8- by 16-in. units were

made by the Concrete Pipe and Products Company, Richmond, Va„,

from expanded shale, cinders, and sand and gravel. The expanded

shale aggregate units were designated as VL , the cinder aggregate units as VC, and the sand and gravel units as VS. The

shrinkage and other physical properties of the units are listed

in Table 1.

When tested, the units were air dry and had been stored

in the laboratory for more than six months.

3.

EXTENSIBILITY TESTS

The tensile strains in the concrete masonry units were observed for two different methods of loading, designated as the

tensile load method and the flexure method. A considerable

number of units were tested before an acceptable method of testing was devised. Despite great care in the preparation of the

units for testing and in assembling the various connecting parts

of the testing apparatus, observations taken during the loading

indicated that many of the specimens either had not been loaded

concentrically or had been subjected to conditions which caused

non-uniform distribution of stress. The data from these tests

are not included in the summaries; those given and discussed

were obtained with seven blocks in the tensile load tests and

nine others tested in flexure.
- 2-

3.1 Tensile Load Method

In the tensile load method the blocks were pulled apart

by applying tensile loads to the end shells. Steel plates,

bedded against the inside faces of the end shells, were connected with the upper and lower heads of the testing machine

by steel cables, balanced levers and shackles, as shown in

Figure 1. The loading method was selected to reduce eccentricity of load application from the testing machine to the specimen and to balance, if possible, the tensile loads applied to

each face shell.

Since it was desirable to have the block fail on a section passing through the center cell and since the face shells

of the block were thickest at the center cell, the end cells

of the block were reinforced. The reinforcement, consisting

of strips of metal plaster lath embedded in calcined gypsum

(Hydrocal), was applied after the blocks were painted with

shellac, to reduce water absorption.

Tensile strains were measured with Baldwin Southwark

SR -ip type A-ll gages of 1-in. length connected with an SR-ip

strain indicator. The gages were placed on the surfaces of

the face shells parallel with the long dimension of the block

and on a section passing through the center cell, as shown in

Figure 2. The gages were similarly located and numbered on all

of the tensile load specimens. Gages 1 and 5 were on the outside faces of the face shells, gages 2 and 6 on the inside

faces, and gages 3> ip, 7, and 8 were on the edges of the face

shell s

.

The load was applied at increments of about 200 lb until

failure occurred and was held constant for 3 to Ip min until a reading was obtained on each gage. The average time required

to complete a test was ipO minutes.

3.2 Flexure Load Method

Each block was supported as a beam with the cells vertical

over a span of 13.5 in. The support lines were under the end

shells and the two load lines, ip-in. apart, were centered on

the cross webs. The steel bearing surfaces along the support

and load lines were bedded in Hydrocal. Those portions of the

face shells enclosing the end cells were reinforced on the

bottom with metal lath and Hydrocal and the method of loading

was as shown in Figure 3.
-3-

An SR -4, type A-ll gage was placed on the bottom edge of

each face shell parallel with the length of the block and between the load lines.

The load was applied in increments of 50 to 500 lb, until

failure occurred, the increments decreasing as the total load

increased. The load was held constant at each increment for

about 1 min, while the observations on the two gages were obtained. The average time needed to complete a test was 15 min.

4 . TEST DATA

4.1 Tensile Load Data

Data obtained from the tensile load tests are listed in

Table 2. It is evident from these data that unequal strains

were observed in the face shells by the tensile loading. While

most of the observed strains were tensile, a gradual increase

in compressive strain was noted at one or more gages on each of

specimens VC-10 and VS-11. The application of the tensile loads

from the bearing plates to the inner faces of the end shells

tended to produce bending in the face shells and rotation of

the joints in the 3-cell frame of the block. It is also probable

that the curing and drying of the blocks caused some warp, stress

and strain in the face shells that may still have been present at

the time of test. These uncertainties, combined with the effects

of reinforcing the end cells, made it inadvisable to attempt an

analysis of the stresses and strains in the specimens.

In the tests, the failure of the face shells occurred at

the center cell and usually near one of the cross webs. The

failure section passed through gage lines in only two of the

seven specimens. The applied loads and the strains observed for

these two specimens VC-6 and VS-12 were greater than those noted

for the other specimens containing like aggregate.

It may be noted, Table 2, that the tensile strain in some

gages reached a maximum at loads less than the maximum loads.

There was no consistent relation between the relative strains

observed for like gage locations on the specimens.

The average maximum tensile strain for all gages on all

blocks was 0.0077 percent. The average maximum strain observed

on the outer face (gages 1 and 5), the inner faces (gages 2 and

6), the bottom edges (gages 3 and 7), and the top edges of the.

blocks were, respectively, 138-., 87-, 74“ and 101 percent of

the average strains. The bottom edges of the face shells were
slightly thicker than the top edges and with the exception of

block VL-10, the face shells were subjected to bending which

tended to place the outer faces in compression. The average

strains observed on the sand and gravel aggregate block were

considerably lower than the strains observed on blocks containing the lightweight aggregates.

L|..2 Flexure Load Data

The failure section in three of the nine flexure test

specimens passed through gage lines. Five specimens failed

at the center cell between the gages and a cross web and one

specimen, VS-114., broke at one of the end cells. It may be

noted from Table 3 that the maximum flexure loads were consistent for each kind of aggregate and did not appear to be

greatly affected by the location of the section of failure.

The tensile strain in some gages reached a maximum at

loads considerably lower than the maximum. It is probable

that the observed strains were somewhat affected by the restraints due to the cross webs, residual strains due to drying

shrinkage and by shearing strains resulting from the high ratio

of depth of beam to length of span. Even so, the data clearly

indicate that the sand and gravel aggregate units, VS, were

stronger in flexure than the units containing lightweight aggregates and the average maximum observed tensile strain for the

VS units was only about 75 percent of that for the other units.

The secant modulus of elasticity was calculated for the

loads corresponding to the maximum observed strain, assuming

that loads were carried only by the face shells and that the

stress distribution on a vertical section of a face shell was

linear

5 . DISCUSSION OF THE DATA

The relation between the values for tensile strength as

determined by the two types of loading are similar to those

for ordinary concretes In that the modulus of rupture was considerably higher than the tensile strength developed by the

tensile load method. The relations between the tensile strength

and the modulus of rupture and the compressive strength of the

blocks are listed in Table Other investigators who measured

the tensile strength of concrete by testing cylindrical specimens report higher tensile strength and higher ratios of TA and
-5-

°f than th,ose in Table l\.. For example, H. F. Gonnerman and

E. C. Shuman?- report values of ^/c averaging about 0.09 and

values of T averaging about 0.55. The tensile strength and the

tensile strength ratios in Table I4

. may have been affected by

residual stresses locked in the blocks and built up during curing

and drying. These effects were greatest in the sand and gravel

blocks, VS, which were the strongest in compression and flexure

but the weakest in tension. A previous investigation on the

shrinkage of the blocks, see Table 1 , indicated a very large expansion of the VS blocks on heating to 110°C (Rapid Method of

drying). These expansions were not observed for the VL and the

VC blocks and were much greater than what would be estimated for

thermal expansion.

Unpublished data obtained from tests of symmetrical specimens of heavy aggregate concrete indicate an extensibility in

tension of 80 to 100 millionths whereas specimens subjected to

bending yield values of 120 to 160 millionths. From Table 2

and Figure 2, it may be noted that the sand and gravel block

VS-12, the only VS block that failed near the gage line, had an

average maximum strain of 8l millionths. The average of the

maximum strains for the VS flexure test blocks was 162 millionths

These values are within the given limits for the unpublished

data referred to above. It should be noted, Table 3 , that the

lightweight aggregate blocks VL and VC had greater extensibility

(and lower moduli of elasticity) than did the heavier VS blocks.

The data in Table 2 may be culled by arbitrarily discarding

those data on blocks which did not fail near the gage lines and

which failed at low loads. By comparing the average of the

maximum tensile strains observed for blocks VL-10, VC-6, and

VS-12, Table 2, with the average flexure strains for similar

blocks, we obtain respective ratios of 0.5l> 0.70, and 0 , 50 . These ratios are in fair agreement with the stress ratio, Vp

,

found by other investigators.

The temperature and the moisture content of concrete

masonry walls is rarely uniform and the masonry is usually subjected to thermal and shrinkage gradients. Similarly, the tensile stresses in the masonry are more likely to be highly flexural

rather than uniform, as in pure tension. Since seasonal temperature changes and drying of the masonry occur more or less

gradually, the resulting stresses, if any, may be somewhat relieved by plastic deformation of the concrete. Bond failures especially if continuous through vertical and horizontal joints,

^"Compression, flexure

Proc. A.S.T.M. 28, 527

and tension tests

, part 2 ( 1928)

of plain concrete it
- 6-

also tend to reduce the tensile stresses in the blocks. Furthermore, a few minor shrinkage cracks may not seriously affect the

structural stability of the walls and their resistance to winddriven rain. For these reasons, it may be assumed that concrete

masonry walls will have a satisfactory resistance to shrinkage

cracking if the tensile stresses do not result in strains amounting to a considerable percentage of the extensibility of the walls

in flexure. For the blocks tested in this investigation the limiting tensile strains assuming an arbitrarily selected limit of 70

percent of the maximum flexural strain would be:

Sand and gravel aggregate blocks (VS) - 110 millionths

Cinder aggregate blocks (VC) - II4.O millionths

Expanded shale aggregate blocks (VL) - l£0 millionths

It may be noted, for blocks VL-10, VC- 6 , and VS-12 ,( tested

by the tensile load method, Table 2) that the limiting strains

listed above are equalled or exceeded on one or more of the gage

lines in each block.
Table

1.

Physical

Properties

of

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Concrete

Masonry

Units.

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Figure 2- Tensile load specimen, after test



THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

The functions of the National Bureau of Standards are set forth in the Act of Congress, March

3, 1901, as amended by Congress in Public Law 619, 1950. These include the development and

maintenance of the national standards of measurement and the provision of means and methods

for making measurements consistent with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,

devices, and structures; advisory services to Government Agencies on scientific and technical

problems; invention and development of devices to serve special needs of the Government; and the

development of standard practices, codes, and specifications. The work includes basic and applied

research, development, engineering, instrumentation, testing, evaluation, calibration services, and

various consultation and information services. A major portion of the Bureau’s work is performed

for other Government Agencies, particularly the Department of Defense and the Atomic Energy

Commission. The scope of activities is suggested by the listing of divisions and sections on the

inside of the front cover.

Reports and Publications

The results of the Bureau’s work take the form of either actual equipment and devices or published papers and reports. Reports are issued to the sponsoring agency of a particular project or program. Published papers appear either in the Bureau’s own series of publications or in the

journals of professional and scientific societies. The Bureau itself publishes three monthly periodicals, available from the Government Printing Office: The Journal of Research, which presents

complete papers reporting technical investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

which provides data for determining the best frequencies to use for radio communications throughout

the world. There are also five series of nonperiodical publications: The Applied Mathematics

Series, Circulars, Handbooks, Building Materials and Structures Reports, and Miscellaneous

Publications.

Information on the Bureau’s publications can be found in NBS Circular 460, Publications of

the National Bureau of Standards ($1.00). Information on calibration services and fees can be

found in NBS Circular 483, Testing by the National Bureau of Standards (25 cents). Both are

available from the Government Printing Office. Inquiries regarding the Bureau’s reports and

publications should be addressed to the Office of Scientific Publications, National Bureau of Standards, Washington 25, D. C.
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