NATIONAL BUREAU OF STANDARDS REPORT

4951

TESTS OF PRESTRESSED CELLULAR SLABS

(Slabs Nos. 5> 7 , 8, and 9)

by

M. Chi and A. F. Kirstein

Report to

Bureau of Yards and Docks

Department of the Navy

<nb|>

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

ms PROJECT NBS REPORT

1001-10-48H December 11, 1956 4951

TESTS OF PRESTRESSED CELLULAR SLABS

(Slabs Nos. 5 , 7 t 8, and 9)

by

M. Chi and A. F, Kirstein

To

Bureau of Yards and Docks

Department of the Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publicatio

unless permiss 25.D.C. Sue

cal ly preparec

Approved tor public release by the 6 or in P

a,t ' is prohibited Director of the National Institute of ofstandards, Washington Standards and Technology (NIST) are P° rthasbeensP

ecifi - on October 9, 2015.

ar repor ' for i,s ow " use‘

TESTS OP PRESTRESSED CELLULAR SLABS

(Slabs No s„ 5, Is 8, and 9)

by

M„ Chi and A. P. Kirstein

Abstract

In the continuing study of the properties of

prestressed cellular slabs* four additional

slabs were tested under concentrated loads.

Slabs No So 5 and 7 were made of unreinforced

NBS blocks* Slab No„ 8 was made of NBS blocks

containing web reinforcement , and Slab No. 9

was made of structural clay units. Deflections and strains were measured and were compared with the computed values. Maximum loadcarrying capacities and crack patterns were

recorded. The data reported herein substantiate the data in NBS Report [j_396, in that

the elastic theory for solid slabs is applicable to slabs of this type. The previously

drawn conclusion was confirmed that reinforcement In the webs of the concrete blocks

is essential to obtain a high load- carrying

capacity for these slabs. The clay tile

blocks used in Slab No. 9 had approximately

twice the compressive strength of concrete

blocks. However* due to the relatively low

tensile strength of the tile, no increase in

load- carrying capacity was observed.

Comparison of grouted and ungrouted slabs made

of precision cast NBS blocks demonstrated the

need for grouting the joints even for units cast

to closest dimensional tolerance obtainable

under laboratory conditions.

1. INTRODUCTION

As a continuation of the study of the properties of slabs

composed of 6= in. cellular blocks, four additional 5~ hy £-ft

slabs were tested In the Structural Engineering Section Laboratories. These slabs were prestressed in two directions and

supported along four edges.
•

.
A previous MBS Report* No, 4396, showed conclusively that

reinforcement in the webs of the blocks and intimate contact

between the blocks served to increase the load-carrying capacity

of this type of slab, With this knowledge at hand the tests

described in this report were planned as follows:

(1) Cellular blocks were cast in precision molds and were

assembled with grouted and ungrouted joints to determine whether closer tolerances on block manufacturing

would afford the intimate contact between blocks

obtained with grout

(2) Reinforcing steel was provided in the webs of the

blocks used to fabricate one of the slabs. This was

done to study the effect of the reinforcing on the

previously observed diagonal tension failures,

(3) Since the manufacture of cellular units of this type

with steel web reinforcement is a difficult process

at best* the thought was entertained that possibly

clay tile would be a more suitable material and would

exhibit better tensile properties. To explore this

possibility one slab of this test series was made of

clay tile units and tested to failure.

2, DESCRIPTION OF TEST SPECIMENS

2,1 Cellular blocks

The cellular concrete blocks were hollow 6-in, cubes having

an opening by 4°5> in, in cross section. There was a 1- by

2-in, elliptical access hole in each web to permit the passage

of the prestressing unit. The principal dimensions of the cellular blocks are shown in figure I,

The concrete blocks used In this test series were of two

types: IJnreinf orced block and block reinforced with a No, 4

gage mild steel wire stirrup in each web, A diagram of the

stirrup used in. the block Is shown in figure 1,

The cellular concrete blocks were made of a mix proportioned

of one part type III cement and three parts of sand by weight

with a water-cement ratio of 0,^7 » This mix had a 7-day compressive strength of approximately 6000 psi as determined by tests

of 2-In, cubes. However* the actual units were moist—cured continuously until they were assembled into slabs so that the compressive strength of the individual units should be expected to

be somewhat greater.

3 ° Additional tests on the concrete were conducted to determine the Young’s modulus and Poisson’s ratio® Axial compression

tests were made on columns of three blocks that were grouted

together in such a fashion as to make the* direction of loading

coincide with the axis of the cello Sonic modulus tests were

also performed on 0®7P>- by 0 . 80- by 6-in® strips that were cut

from the cellular blocks „ The results obtained by both methods

indicated an average Young's modulus of 4 x 10° psi with a variation of ± 10 percento Poisson’s ratio was found to be

approximately 0®lp>»

Three -unit -gang molds that were constructed to very close

tolerances and had polished surfaces were used to form all of

the concrete blocks ® Due to this method of manufacture, the

exterior dimensions of the blocks were well within 0*01 in® of

the nominal values and the texture of the surface of the blocks

was excellent o Upon close examination of these surfaces, it

was also found that there were virtually no shrinkage cracks®

The cellular c'lay=»tile blocks were made of a high grade

vitrified clay by the Federal Seaboard Terra Gotta Corporation®

The principal dimensions of the tile blocks were approximately

the same as those of the concrete blocks except that the tile

had circular instead of elliptical holes in the webs® The tile

blocks had a good texture but the overall dimensions varied as

much as l/lp in® A sketch of a typical tile block is shown in

figure 1,

Several of the clay tile were tested in axial compression

with the load applied in the direction of the cell® The average

compressive strength was approximately 12,000 psi® The typical

stress-strain relationship for this material was essentially

linear up to the failure load and yielded a Young’s modulus of

Lj.o-4 x 10° psi and a Poisson’s ratio of 0®15>® Even though the

tile exhibited a compressive strength that was approximately

twice that of the concrete, the tensile strength was presumably

not much greater than that of concrete®

2®2 Pre stressing steel

A new type of steel bar known a3 ”E la stuff” was used in

this test series® Tensile tests of this material indicated a stress-strain relationship that was essentially linear up to

9p>,000 psi, and exhibited a Young's modulus of approximately

30 x 10° psi® The yield strength of the bar was found to be

120,000 psi as determined by the 0®2 percent offset method and

the tensile strength was found to be 133?000 psi. Although

"Elastuff” bars are made of cold-worked high carbon steel,

they are fairly ductile and can be machined easily.

2.3 Test specimens

The nominal dimensions of each slab containing 100 cellular

blocks were 5 ft by 5 ft by 6 in, The blocks were arranged so

that the axis through the open ends of one block was perpendicular to the axis of each adjacent blocko The holes in the webs

of each block were arranged so as to enable the pre stressing

tendons to be staggered with respect to the midplane of the slab*

Thus* the resultant prestressing force produced an axial compression in two directions through the slab*

The slabs in this test series were numbered 7» 8* and 9.

Slabs Nos. 5 and 7 were made of unreinforced NBS blocks, with

ungrouted and grouted joints, respectively. Slab No. 8 was made

of reinforced NBS blocks; the joints in this slab were grouted

by dipping the ends of each block in a neat cement paste before

they were assembled. Slab No. 9 was made of clay tile blocks

and was fabricated in the same fashion as was Slab No. 3? which

was described in NBS Report 4396. This slab was constructed

in two stages. First, the blocks were assembled into ft beams

with about l/8 in. mortar joints. Then the beams were assembled

together with sufficient thicknesses of mortar joints to form a square slab.

All of the slabs gave a good general appearance, and all

of the blocks appeared to be in intimate contact with one

another. However, Slab No. 9 did not remain in its true plane

after it was prestressed. This warping was presumably due to

the relatively poorer tolerances of the tile blocks.

2.4 Prestressing procedure

The tensioning force was applied to the pre stressing bars

by means of a hydraulic jack equipped with a suitable adapter

bar. This adapter bar was equipped with bonded wire gages and

calibrated so that it could be used as a dynamometer to determine the amount of prestressing that was applied to the tendons.

The calibration curve for the adapter is shown in figure 2.

Approximately one half of the prestress was applied in small

increments by tightening the anchorage nuts with a wrench. The

remaining prestressing force was applied by means of the hydraulic jack. This final stage of the prestressing was accomplished

by using a suitable sequence of stressing the tendons so that no

unduly large differences in strains would be induced in the

blocks

.

The total amount of prestress in each bar of Slabs Nos.

7, 8, and 9 was 12,400 pounds. This force was sufficient to

produce the specified 1000 psi stress in the blocks. It was
' -

'
believed that the load-carrying capacity of the slabs could be

increased by increasing the amount of prestressing. In order to

verify this hypothesis a prestress of 1500 psi was proposed for

Slab No. 6. However, supplementary tests reported in NBS Report

No. 4^13 indicated that the prestressing force necessary to produce a 1^00 psi stress in the slab would severely damage the

blocks. Therefore , the test of Slab No. 6 was dropped from

this series, and the slab was disassembled.

3. TESTING PROCEDURE

3.1 Test setup

Slabs Nos. 5? 7s 8? and 9 were tested to failure in a 6 0 0, 000* lb capacity hydraulic testing machine. The specimens spanned 54 in. and were simply supported on four edges by

1-In. square aluminum bars. The aluminum supports rested on a frame made of 6- by 12-in. reinforced concrete members that were

supported by six 6-in. diameter cylindrical concrete columns.

All bearing surfaces were set firmly with high-strength plaster

to insure intimate contact between individual members. This

setup placed the test specimen about 5 ft above the testing

machine platen and permitted visual observation of the bottom

face of the specimen during the test.

3.2 Instrumentation

The deflection measurements of Slabs Nos. 5* 7s and 8 were

made by 0.001-in. micrometer dial gages attached to steel bars

supported by wooden frames. These frames were attached to the

testing machine. There were 20 deflection gages disposed over

the top surface of the specimens. Figure 3 shows the symmetrical arrangement of these gages. The deflection measurements

of Slab No. 9 were made in a different manner. The dial gages

were attached to steel angles which rested on the slab directly

over the supports. Thus, the dial gage readily gave directly

the deflections with reference to the edges of the slab. The

general setup of test of Slab No. 9 is shown in figure 4°

The variation of strain in eight prestressing bars was

measured by means of bonded wire strain gages. Each of the

eight bars was instrumented with two gages that were connected

in series to eliminate the effects of any bending that might

take place. Four of these bars ran through the slab in one

direction, and the other four were perpendicular to them. The

locations of the instrumented bars are shown in figure 3°

Figure 3 also shows the location of eight bonded wire

strain gages that were applied to the concrete. Four of these

gages were on the top surface of the slab and four were on the

bottom.

6 .

«

3.3 Test procedure

The test specimens were loaded at the center of the top

surface through a 6- by 12= by 12 -in* concrete loading block.

The load was applied to the slabs in increments of 5000 and 2500

pounds 9 and gage readings were made for each increment until the

maximum load was reached*

4. TEST DATA

4*1 Deflections of slabs

Figure 5 shows the experimental and theoretical relationship

between the applied load and center deflection of the slabs* The

theoretical calculations were based on the average Young's modulus

of 4 x 10° psio The deflections of the slabs were measured with

reference to the datum plane at the centers of the supported

edges s and the deflection profiles were obtained by averaging the

deflections along two rows of dial gages disposed symmetrically

along two lines adjacent to the loading block. Figures 6 through

8 show these profiles for the various increments of load applied

to these slabs.

Since the supports did not restrict the upward movement of

the slab, the twisting moments induced by the applied load caused

a "curl-up" action to take place at the corners of the slabs.

The amount of curl-up was measured by the upward movement with

respect to the datum plane as described in the preceding paragraph. Figure 9 shows the experimentally determined relationship

between the applied load and the vertical displacement upward

caused by the twisting moments.

4.2 Variation of prestressing force

After the slabs were fully prestressed the relaxation of

the prestressing tendons was observed. These observations are

graphically presented in figure 10. The clay tile slab exhibited

an average loss of strain in the tendons of 4 percent at three

days and 8 percent at seven days* while the tendons in the concrete slabs lost 8 percent of their strain at three days and 13

percent at 7 days. It Is evident from these data that the loss

of strain in the tendons is not only a function of time but also

depends on the material being prestressed. Since the concrete

blocks were moist-cured until they were assembled and prestressed,

a portion of the loss of strain in the tendons can be attributed

to shrinkage of the concrete. However, the shrinkage of the

clay tile slab can be assumed to be negligible.

During the load tests, the maximum loss of prestress in

the tendons of all the slabs was approximately 2 percent of the

total initial strain. However, Slab No. 8 , which carried the
.
7 *

largest load of all, registered a gain of almost I4. percent of

the original strain in those tendons that were located 3/4 in.

below the mid-depth of the slabc Table 1 contains a tabulation

of the strain variations in the prestressing tendons with re**

spect to the applied loads.

4.3 Concrete strains

The average strains in the concrete corresponding to a prestress of 1000 psi for Slabs Nos. 5 and 7 were 300 x 10~& and

200 x 10“6 in./in. respectively. These data were not available

for Slabs Nos. 8 and 9.

The loss of strain in the concrete due to shrinkage and

creep was measured for the ungrouted Slab No. 5? and the results

are presented graphically in figure 11. The average loss in

strain for a six-day period was 37 percent of the original

strain.

Figures 12 and 13 show the relationship between the applied load and the strain in the concrete.

4.4 Load-carrying capacity

Table 2 lists the maximum loads sustained by the slabs in

this series along with the type of blocks and joints used in

the construction of the slabs.

4*5 Crack patterns

Figures 14 through 19 show the crack patterns on the top

and bottom surfaces of Slabs Nos. 5* 7? 8 , and 9* The cracks

in the bottom surface of slab No. 8 were more or less circular

arcs that developed some distance from the loading block.

This crack pattern very closely resembled that of a similar

slab (No. 4 ) that was illustrated in figure 10 of NBS Report

4396 .

The crack patterns in the webs of the unreinforced blocks

along various sections through the slabs as indicated in figure

20 are shown in figures 21 through 25* These cracks were very

wide and extensive , and their direction was usually parallel

to the shells of the slabs. It Is apparent from the crack

pattern in the webs of the reinforced blocks shown in figure

25 that the cracks were confined to relatively few blocks, and

upon further inspection after the test, the cracks were found

to be narrower than those in the unreinforced blocks.
.

.
8 *

£® DISCUSSION

The comparison of the load-carrying capacities of Slabs Nos 0 5 and 7 leads to the conclusion that slabs of this type require

a more intimate contact between the cellular blocks than can be

obtained by using precision blocks with a dimensional tolerance

of 0®01 inches® Therefore, it appears that grouted joints between

the blocks are required to obtain the intimate contact necessary

to develop the full strength of these slabs®

If it is assumed that eight blocks bordering the loaded area

resist the punching shear, and if the shear resistance of the

flanges of the blocks Is neglected, then only the webs of each

of the eight blocks resist the shear,. Calculations based on

these assumptions show that the average vertical shear at the

ultimate load sustained by Slab No® 7 was approximately 5>00 psi®

When this shear stress was combined with the corresponding flexural stress, the principal tensile stress at the critical section

was found to be 4^0 psi, which approximated the tensile strength

of the concrete used to cast the blocks® Furthermore, If it is

assumed that the clay tile in Slab No® 9 had the same tensile

strength as the concrete blocks in Slab No® 7> then it can be

stated that the full strength of both slabs was developed®

Using the same assumptions as before, the average vertical

shear in the webs of the eight blocks bordering the loaded area

in Slab No® 8 at the maximum load was found to be 740 psi® It

is important to note that this value of shear corresponds to a nominal diagonal tensile stress of 740 psi® This theoretical

tensile stress could not have been attained if the webs had not

been reinforced with mild steel stirrups®

The extensive horizontal cracks in the block having unreinforced webs were attributed to the bursting effect of the pre~

stressing force® This effect was noted in auxiliary tests and

reported in the NBS Report No® 4^13

Since no compression failures occurred in these slabs, it

appears that the compressive strength of the concrete Is adequate

for/ slabs of this type. Therefore, the larger compressive

strength of the clay tile was not used to advantage because the

diagonal tension failure of the webs precluded all other modes

of failure. Furthermore, its brittle nature resulted in the

formation of long, abrupt cracks In the webs due to the diagonal

tension and the bursting effect of the prestressing force (see

figure 26)

.

A more suitable material for use in this type of

slab would be one that has a compressive strength comparable to

that of concrete, but having a greater extensibility and a greater tensile strength.
.
9.

6, CONCLUSIONS

The foregoing discussion can be summarized in the following conclusions:

(1) Slabs of this type behave like solid slabs in

flexure and shear,,

(2) All of the slabs reported herein failed in

diagonal tension*

(3) T° improve the load-carrying capacity of these

slabs it is necessary to provide suitable reinforcement in the block to increase their

resistance to diagonal tension failures* An

increase in the compressive strength of the

material cannot be utilized as long as there

is no commensurate increase in the tensile

strength,

(4) A suitable joint material must be provided

between the blocks to insure intimate contact*

Table 1„ Strain increments in prestressing bars

due to applied loads

* Slab 1 'No, » Load

kips ? Strain increments. 10-6 in./in.. in bars Nos,

!

f 1 » 2 f 3 ’ 4

1 5 ' 6 » 71 8 »

2.5 1 -1 -2 -3 10 5 3

5 -6 4 -7 -1 -4 3 -2 8

7*5 -7 4

-8 1 -13 13 -5 9

10 -6 8 -9 1 -14 7 -6 6

i 5 -8 0 -9 "3 -27 13 -18 12

2.5 -2 ”4 "13 -6 -6 -2 0 -1

5 “4 -5 -2 -8 8 -i 5

7*5 l 0 -7 0 -12 8 =5 0

7 10 ”4 -6 -15 1 -11 8 ~3 5

15 -4 -6 -17 -5 -19 9 -li 4

20 -2 -21 2 -17 16 -13 8

22 „5 -9 -12 -26 "5 -21 17 -16 6

25 -8 -5 -21 2 -24 17 -14 10

2.5 0 0 0 -3 2 -1 2

5 0 1 -1 "5 7 -4 2

7*5 -1 Ti© 0 -1 -7 7 -8 4

10 -1 50 0 -1 -7 9 -9 5

15 -1 s -1 ~3 -l4 12 -12 li

20 -1 cd -2 "3 -18 18 -18 13

8 22*5 -1 ~3 -5 -22 19 -20 17

25 0 cd -3 -3 -24 25 -20 20

27.5 -1 -2 -2 -26 27 -21 22

30 1 0

bO -2 -l -26 30 -23 28

32.5 2 cd -1 -1 -28 36 -27 31

35 2

CO 0 l -27 40 -28 39

37.5 5 0 2 -26 49 "27 49

2 c 5 -1 0 0 2 -22 0 -10 “3

5 0 0 -3 2 -5o -1 -11 "7

7*5 -4 -2 -2 -2 -26 "3 -10 =7

10 -4 -2 -4 -2 -20 -8 -9 -7

15 -6 -4 -6 "3 -36 “9 -4 -10

20 -9 -12 -10 "9 +20 -10 -l -15

25 -24 -24 -23 -19 +42 -3 +10 -19

Table 2 0 Schedule of slabs

8 Slab ! Type of 5 Type of T" Prestress ! Maximum 8

8 No* block ? joint 8 T load 8

! I

I 5

t

5 UBS, unreinj

* Ungrouted

8

8 psi

lTUOO

* lb

» 187000

8

8

u

tf

J forced 8

I

! Grouted

f

y

y

f

8 7

' NBS, unrein- 8 1,000 1 27 , 27s

8

! s forced 8 8 8 8

f 8 8 8

>y. 8 8

! 8 ' NBS s rein- ' Grouted 8 $1,000 f 39,850 8

8 r forced 8 8 8 8

r ? 8 8 J 8

y 9

f Structural s Grouted 8 1,000 * 26,850 t

t * clay 8 8 8 8

j 8 8 8 8 8

X. 1 •' \

'—•75 CHAMFER _ 3

" 4

/

*\

1 -fcnoj

1 UNREINFORCED

. 6

"

BLOCK

— NO. 4 GAGE STEEL

REINFORCED BLOCK

WIRE

NORMAL AREA

MOMENT OF INERTIA

SECTION MODULUS

EQUIVALENT DEPTH

12.4 IN

2

68.2 IN

4

22.8 IN 3

4.77 IN

6

" 6

II

SOME BLOCKS

HAD HOLES IN THIS POSITION

TO PERMIT THE

STAGGERING OF

PRESTRESSiNG

TENDONS.

STRUCTURAL CLAY BLOCK

FIGURE I. CELLULAR BLOCKS

FIGURE 2

1000

x

10'

legend:

GAGES I THROUGH 20 INDICATE DIAL GAGES

B I THROUGH B8 INDICATE GAGES ON BARS

CI-C4 INDICATE SR-4 GAGES ON BOTTOM SURFACE OF SLAB

C5 - C8 " " » " TOP " " "

FIGURE 3. LOCATION OF DIAL GAGES AND STRAIN GAGES.


%> r s-'
FIGURE 5. COMPUTED AND OBSERVED CENTER DEFLECTIONS

IN SLABS NOS. 5, 7, 8 AND 9.

FIGURE 6. DEFLECTION PROFILES FOR SLABS NOS. 5 AND 7

DEFLECTION,

IN.

DEFLECTION,

IN.

FIGURE 7. DEFLECTION PROFILE FOR SLAB NO. 8.

DEFLECTION

FIGURE 8. DEFLECTION PROFILE FOR SLAB NO. 9.

DEFLECTION,

IN.

FIGURE 9 . EXPERIMENTAL RELATIONSHIP BETWEEN APPLIED

LOAD AND VERTICAL MOVEMENT OF CORNERS.

SLAB

NO.

UJ

a:3

O

Li_

FIGURES 10 8 II.

NUMBER

OF

DAYS

AFTER

PRESTRESSING

LOSS

OF

PRESTRESS

IN

CONCRETE

OF

SLAB

NO.

5

PRIOR

TO

TEST.

(£

io

r* Z

<

ir

H

C/)

LxJ >

C/)

(/)

LlI a:

CL

s

o

o

FIGURE 12

FIGURE

12.

RELATIONSHIP

BETWEEN

APPLIED

LOAD

AND

COMPRESSIVE

STRAIN

IN

CONCRETE

.

SLAB

NO.

FIGURE 13

FIGURE

13.

RELATIONSHIP

BETWEEN

APPLIED

LOAD

AND

TENSILE

STRAIN

IN

CONCRETE.

N

A

TOP VIEW AFTER FAILURE

(BOTTOM SURFACE INTACT)

FIGURE 14. VIEW OF TOP SURFACE OF SLAB NO. 5

AFTER TEST.

SECTION IO-IO

N

TOP VIEW

(BOTTOM INTACT)

FIGURE 15. VIEW OF TOP SURFACE OF SLAB NO. 7

AFTER TEST.

— H

/

r 1 1

1 r .

/

1

•w J

BOTTOM VIEW

’//////, s

^PUSHED/

< DOWN /

/> 3" O

TOP VIEW

FIGURE 16. VIEWS OF SLAB NO. 8 AFTER TEST

note:

SOLID LINE INDICATES CRACKS ON BOTTOM SURFACE

DOTTED LINE INDICATES CRACKS ON TOP SURFACE

N

/ /

BROKE

( BOl

:n out

noM) -

r

i

j

'

MORTAR CHIPPED

ALL ALONG

^ JOINT (TOP) —

' /

\ ^

r

\

1

1

\ BROKEN OUT

1 (BOTTOM )—

K\

Vx X

FIGURE 17. VIEWS OF SLAB NO. 9 AFTER TEST

Fig

e

18.

Top

view

of

Slab

No.

9

after

test.
cv—


N

A

10

t

10

9 9

}

8

t 8

t

7

t 7I

6

t H

''////

// ///

/////

W/a 6

t

5 mm

// / //

5

4

i 4

t

3

L_

3

t

2 2

1* KEY DIAGRAM

FIGURE 20 DIAGRAM SHOWING LOCATION OF SECTIONS

OF SLABS IN FIGURES 21 THROUGH 25.

SECTION 4-4

FIGURE 21 . CRACK PATTERN IN SECTIONS OF SLAB NO. 5.

0 La—

j

J 1-

f "I

i \-

| | j_0 1 1—~

i nr

SECTION l-l

SECTION 2-2

SECTION 3-3

SECTION 7-7

FIGURE 22. CRACK PATTERN IN SECTIONS OF SLAB NO. 7.

(CONTINUED IN FIGURE 15).
. - * . ’
0 0

SECTION 3-3

SECTION 4-4

0

4 0

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FIGURE 24. CRACK PATTERN IN SECTIONS OF SLAB NO. 9.

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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 ($1.25) and its Supplement ($0.75), available from the Superintendent of Documents, Government Printing Office. Washington 25, D. C.

Inquiries regarding the Bureau’s reports should be addressed to the Office of Technical Information, National Bureau of Standards, Washington 25, D. C.