NATIONAL BUREAU OF STANDARDS REPORT

5212

TESTS OP PRESTRESSED CELLULAR SLABS

(Slabs Nos. 10 through 19)

by

Arthur P. Klrstein and Michael Chi

Report to

Bureau of Yards and Docks

Department of the Navy

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

1001-10-4811 April 8, 1957 5212

TESTS OP PRESTRESSED CELLULAR SLABS

(Slabs Nos. 10 through 19)

by

Arthur P. Kirstein and Michael Chi

To

Bureau of Yards and Docks

Department of the Navy

IMPORTANT NOTICE

NATIONAL BUREAU OF ST

intended for use within the

to additional evaluation and r listing of this Report, either the Office of the Director, Na

however, by the Government

to reproduce additional copie

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

rogress accounting documents

mally published it is subjected

eproduction, or open-literature

ion is obtained in writing from Such permission is not needed,

)repared if that agency wishes <NB^

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

TESTS OP PRESTRESSED CELLULAR SLABS

(Slabs Nos. 10 through 19)

by

Arthur F, Kirsteln and Michael Chi

Abstract

In the continuing study of the properties

of prestressed cellular slabs, ten additional slabs were tested under concentrated

loads. All of the slabs were simply sup-=

ported on two edges except Slab No. l6 which

was simply supported on four edges. Deflections, strains, crack patterns, and maximum

load- carrying capacities were recorded. The

data reported herein substantiate the conclusions drawn in NBS Reports 4396 and l|95l*

Since the primary mode of failure is diagonal tension, the load- carrying capacity of

the slabs can only be Increased by suitable

reinforcement of the webs of the concrete

blocks or the use of a material for the construction of the blocks which has a higher

tensile strength.

1. INTRODUCTION

As a continuation of the study of the properties of slabs

composed of 6-ln. cellular blocks, ten additional 5~ by 5-ft

slabs were tested in the Structural Engineering Section Laboratories. These slabs were pmstressed in two directions with

1000 psi in the longitudinal direction and either 500 or 1000

psl in the transverse direction. The slabs were simply supported by either two or four supports, and the arrangement of

the blocks was altered along with the use of various jointing

materials

The test results from these ten slabs substantiate the

conclusions drawn in NBS Reports 4396 and 4951- A complete

summary of the deflections and maximum load- carrying capacities

2 ,

of all of the slabs in this testing program is presented and

discussed, along with the test results from Slabs Nos. 10

through 19.

2o DESCRIPTION OP TEST SPECIMENS

2.1 Cellular blocks

The cellular concrete blocks were hollow 6-in. cubes

having an opening 4*5 by 4»5 in. in cross section. There

was 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 1.

All of the blocks used in this test series were of the

type designated as NBS blocks, that is, they were unreinforced and cast in three-unit gang molds. These polished

steel molds were constructed to very close tolerances which

produced blocks of superior quality, and the exterior dimensions of the blocks were within 0.01 in. of the nominal

value s

The 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 0.57. This mix had a 7-day compressive

strength of approximately 6000 psi as determined by tests of

2-ln. 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.

Additional tests of 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 cell. Sonic

modulus tests were also performed on 0,75“ 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 percent. Poisson’s

ratio was found to be approximately 0,l5.
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2.2 Prestressing steel

The prestressing units In the test slabs were 5/8 In,

’’Stressteel" bars. The anchorage for these bars consisted

of hexagonal nuts 1 l/l| In. long which bore on 5 3/4“

5 3/4“ 3/4”in. anchor plates.

The tensile strength of unthreaded ”Stressteel” bars

was found to be 163,000 psl, whereas threaded bars supported

by fully tightened nuts developed a tensile strength of

152.000 psl. The yield strength of the steel, determined by

the ’’offset" method (offset = 0.2/) was 142,000 psl. The

stre s S“ strain characteristic was a straight line up to

65.000 psl, giving a Young’s modulus of 30 x lO^ psl; the

secant modulus at 100,000 psl was 28,2 x 10^ psl. The reduction In area at the point of fracture was 35 percent,

2,3 Description of prestressed slabs

The nominal dimensions of each slab containing 100 cellular blocks were 5 Tt by 5 Dt by 6 In, The arrangements of the

blocks In the slabs of this test series are described as

"crisscross" and ’’aligned". The crisscross assemblies contained blocks that were arranged so that the axis through the

open ends of one block was perpendicular to the axis of each

adjacent block, while the aligned assemblies were arranged so that the axes through the open ends of all blocks were parallel. Both assembly methods arranged the holes In the webs

of each block In such a fashion to permit the prestressing

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 10 through

19 . All of the slabs were constructed with the unreinforced

NBS blocks. The principal variables In this test series were

the method of support, amount of prestressing, arrangement

of blocks, and jointing material. A complete summary of these

variable factors will be made later In the text along with the

maximum load- carrying capacity of each slab for comparison

with the first eight slabs of this testing program reported

In NBS Reports 4396 and 4951.
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The tensioning force was applied to the prestressing

bars by means of a hydraulic jacking rig that was equipped

with a dynamometer to determine the amount of prestressing

that was applied to the tendons. The calibration curve for

the dynamometer is shown in figure 2.

Approximately one=half of the prestress was applied to

the slab in small Increments by tightening the anchorage

nuts with a wrench. The remaining prestressing force was

applied by means of the hydraulic jacking rig. This final

stage of the prestressing operation was accomplished by

using a suitable sequence of stressing the tendons so that

no unduly large differences in strain would be induced in

the blocks.

3. TESTING PROCEDURE

3.1 Test setup

A 600, 000-lb capacity hydraulic testing machine was

used to test the slabs. The specimens were simply supported

on ^hr-ln. spans by 1-in, square aluminum bars that were

attached to the steel frames resting on the testing machine

platen. To ensure intimate contact between individual members, all bearing surfaces were set firmly with high-strength

plaster. All of the slabs were simply supported on two

edges as described above except Slab No. 16 which was simply

supported on four edges. The four-edged support required

additional steel frames and aluminum support bars, otherwise

the setup was identical to that of the two-edged support.

3.2 Instrumentation

The deflection measurements of all the slabs except Slab

No. 16 were made with 0.001-in. micrometer dial gages that

were attached to steel angles. These steel angles rested on

the slab directly over the supports, thus placing the datum

plane at the supports. Figure 3 shows the symmetrical

arrangement of these gages. The deflection measurements of

Slab No. 16 were made in a different manner to accommodate

the measurement of the ” curl-up” action of the corners that

is typical of slabs supported on four edges. In this case
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the dial gages were attached to a steel frame that was supported by the columns of the testing machine. Figure i| shows

the location of the dial gages on Slab No. 16.

The strains on the top and bottom surfaces of the slabs

were measured with electrical resistance wire gages of the

A-11 type. The locations of these gages are shown in figures

3 and 4*

3.3 Test procedure

The slabs were loaded at the center of the top surface

through a 6- by 12~ by 12-in. concrete loading block. The

load was applied in increments of 1000, 2000 and 2^00 pounds,

and gage readings were made for each increment until the maximum load was reached.

4 . TEST DATA

4.1 Deflection and load- carrying capacity of slabs

Figure 5 shows the observed relationship between the

apnlled load and center deflection of Slabs No. 10 through

19 . The reciprocal of the slope of the linear or elastic

portions of these curves can be considered to be a measure

of the deflection sensitivity of the slabs, and is expressed

in terms of micro inches of deflection per pound of applied

load {yCCin, /I'b) , Table 1 shows a comparison of the deflection sensitivities and the maximum load- carrying capacities of all of the

slabs of this testing program.

The typical "curl-up” of the corners of slabs simply

supported on four edges was observed for Slab No. I 6 and is

shown in figure 6. Slab No, I 6 was the only slab reported

herein that was supported on four edges.

4.2 Concrete strains

The relationship between the applied load and the

strains in the concrete for Slabs Nos.lO through 19 are

shown in figures 7» 8, 9, and 10, The legends at the top

of figures 7> 8, and 9 Indicate the positions of the strain

gages on the concrete and Identify the curves as top longitudinal, top transverse, bottom longitudinal, and bottom
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transverse. Figure 10 shows the load-strain relationship for

Slab No, 16 which was simply supported on four edges. Therefore, the direction of strain in the concrete cannot be considered longitudinal or transverse. Since the strain gages

were applied to Slab No. l6 in the same locations as indicated

by the legends in figures ?, 8, and 9? the average of the

gages on the top and bottom are presented in figure 10,

i|,3 Crack patterns

The crack patterns observed in these ten slabs are all

very nearly alike as only NBS unreinforced blocks were used

in the construction of the slabs. A typical crack pattern is

Illustrated in figure 11 for various sections through the

slabs. The location and direction of the cracks indicate

that the primary failure was diagonal tension, and possibly

a secondary failure was due to the bursting effect of the prestressing force.

DISCUSSION

For the purpose of comparison, table 1 of this report includes the pertinent Information obtained from the previously

tested slabs (Nos. 2, 3> 4> 7> 8, and 9) along with the

ten additional slabs which form the subject of this report.

Upon examination of table 1 it will be seen that the

slabs are grouped according to the method of support, arrangement of blocks, amount of prestress, jointing material, and

the type of block used. Group I shows a great Improvetaent in

the deflection sensitivity of Slab No. 5 over that of No. 2.

This is apparently due to the Improved dimensional tolerances

of the NBS blocks, as no jointing material was used in the

fabrication of these slabs. However, attention is directed

to Group II where the deflection sensitivities do not vary

appreciably for the slabs constructed from Preload, NBS, and

clay tile block. This, of course, is due to the fact that

the grout jointing material furnishes the intimate contact

necessary between individual blocks regardless of their dimensional tolerances.

The slabs having polyester resin joints exhibited values

of deflection sensitivity comparable to the slabs containing

grout or neat cement joints, but the slabs containing polyester
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load. This may be due to an actual tensile relnTor cement

of the block webs by the resin.

The relative magnitude of the deflection sensitivities

of the individual slabs in Groups IV, V, and VI indicate the

effect of the arrangement of blocks and the amount of prestress on the rigidity of the slabs.

In general, the slabs exhibiting high values of deflection sensitivity deviate from a linear relationship on the

load-deflection curves at low loads. This can be observed

for Slabs Nos. 10, 12 , and 17 in figure 5.

The erratic relationship between the applied load and

strain in the concrete blocks shown in figure 7 may be attributed to the lack of Intimate contact between the individual

blocks in the slabs. Intimate contact between the blocks

in Slabs Nos. 10 , 12 , and 17 was not attained because no

jointing material was used and the dimensional tolerance of

the blocks permitted slight individual motion caused by

points of stress concentration.

Figure 8 shows the relationship between applied load

and strain in the concrete for Slabs Nos. 11 , 13 , and l8.

These slabs are simply supported on two edges and have neat

cement joints between the blocks. For other particulars of

the slab construction, see table 1, The relationships between

the applied load and strain in concrete for Slabs Nos. ll|, l5,

and 19 , are shown in figure 9 ; these slabs had polyester resin

joints. Attention is directed to the fact that figures 8 and

9 do not display the erratic relationship exhibited in figure

7 .

The crack patterns observed throughout this series of

slab tests indicate that diagonal tension is the mode of

failure that limits the load- carrying capacity of the slabs.

To increase the load- carrying capacity of slabs of this type,

one of two things could be done--either the webs of the concrete blocks must be heavily reinforced with steel, or another

material could be used in the construction of the blocks. The

first alternative, while effective, would most likely entail

considerable expense in producing quantities of web reinforced blocks. Therefore, a search for an existing material

or the development of a new material with a more suitable

tensile strength might be a more fruitful endeavor.
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• 'k. ' •

. C-.-** (SaJ^rjM

• i

' ' p -. ^

1., ', - H rj r .fr,-5d .iaairocl'

.•: - : '.r ( .^,. .^ai.7;, 'u i T-jj I" • w 0 <; .JU'

' .A ;;• v'_ 44.*' .b^f, v .laXjt/-):

' ,

. . A s'l't- .i'.’.- t- uf •*f'?.f I'li

'

'. A. ‘a.^i.j nJ 5- - e ‘ i8‘i

• <?) .« . f . i: 5

/ '4 0 i-lrf

^ -H I is ;o iiiont jo.l ^v wl)

/ ' tf »*.' infitm rfdsne*f;lc

i

. ^ ?
8 ,

6. CONCLUSIONS

The foregoing discussion can be summarized in the following conclusions:

1. All of the slabs reported herein failed in

diagonal tension.

2. A suitable joint material must be provided between the blocks to ensure intimate contact.

3. To Improve the load- carrying capacity of these

slabs, it is necessary to either heavily reinforce the webs of the blocks with steel or use

a material with a more suitable tensile strength

for the construction of the blocks to increase

their resistance to diagonal tension failures.
. • • I

if fi-OB U' erfT „ >: • .wS

i Ciivte 'vf f 030,6 s^if-

^r; ;''7'"!> (i:, - I- ,

X£'aog;^ib

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2^',) ,v' J' erJj

• c ^ - el li 3

1 ecrf 'Ta'tA'I- ,v,r

"^’tnoo A'"!* 'K>‘i

' 'm' *

m
Table

1„

Summary

of

Slab

Construction

and

Test

Results

- - " - U-

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pH H H H > > ft O ft 1 ft fH > Ph j ft

hi

j

1 *T ^ ^ 1 to- to- C- oj

to.' to- to-. to. <r- c- to- toBlocks

reinforced

with

l/l|

in,

stirrups.

FIGURE I. NOMINAL DIMENSIONS OF NBS

UNREINFORCED BLOCKS.

u>

IO

Sdl>< ‘30a0d

FIGURE 2

STRAIN

0

1 ”io

=G

=C

=c

=c

<

=G

LA iLJl ft A

B2 B3 B4

5

- 0

'

A A

2 3 4

c-e“

A

C-l

C-5

o =7 P

C-3

C-7

BLOCK X

f o

»C-4

C-8

9 10 II 12

>

>

>

>

legend:

GAGES I THROUGH 12 INDICATE DIAL GAGES

Bl 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 ON

SLABS SIMPLY SUPPORTED ON TWO EDGES.
1' '* r- ' *

’ :f^;.

, ..r,.

-»T t

•y<Hr «r

‘ * •• ' >.- .11-. -V|M<"-i >—

^

:

i: :

' ^ ‘ ^ i ^5, ‘i^/ Ui!

•

r

I.-— ( — , iiiiiln ^

»,

,Td./

U >1 T-tr»

iy.: *>

j

igi; .t«y^ijj^ .i IW

r^ W - V<^', „ Vi -*^|'

'^V 4^:

S .Ull*!)l -jftil

_ O^T -|pMi ^ - i

,

...ju^^ &.: .mMM

bfa . ^xf‘. fMwi

\iiu'' .*}' 1 iT..;

lO'lf Jr.iUi
N

legend:

GAGES I THROUGH 20 INDICATE DIAL GAGES '

Bl THROUGH B8 INDICATE GAGES ON BARS

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

C5 - C8 " " " •' TOP '' " "

FIGURE 4. LOCATION OF DIAL GAGES AND STRAIN GAGES

ON SLAB NO. 16. (SIMPLY SUPPORTED ON FOUR EDGES).

FIGURt

5.

OBSERVED

RELATIONSHIP

BETWEEN

APPLIED

LOAD

AND

CENTER

DEFLECTION

OF

SLABS.
t

I

li

I

t

fi

I

. r

4

*

i. 1
30

25

20

15

10

5

0

) 20 40 60 80 100x10

VERTICAL DISPLACEMENT OF CORNERS, IN.

-4

6. VERTICAL DISPLACEMENT OF THE CORNERS DUE

TO THE "CURL UP" ACTION OF SLAB NO. 16.

o

(/5

LU

O

(/)

LlI

O

U)

(/) LlI CC

CL5

O

o

o

(/)

LlI

Q." O

o

<0O

<

q:

C/)

UJ

hLiJ oro

o

o

sdi>i ‘avoi aanddv

FIGURE

7.

OBSERVED

RELATIONSHIP

BETWEEN

APPLIED

LOAD

AND

CONCRETE

STRAIN.

(NO

JOINTING

MATERIAL)

CL CL o o

UJ <

CO z wz o < z

IT

O

t- _l I I o o

CD

m

O

O

(fi

LlI

cn

(/) UJ

a.

0.ooo

(/) z

UJ

»-

2:o

(/) CD

UJ

q:

CL2o

(j

CO

2:UJ

a:oo \<

q:

H”

CO

LU

hUJ

cnooo

FIGURE

8.

OBSERVED

RELATIONSHIP

BETWEEN

APPLIED

LOAD

AND

CONCRETE

STRAIN.

(BONDED

WITH

NEAT

CEMENT)

UJ <

Vi z

sicn t

o

< z

I I o p

0. CL o o

I- 1- o o

CD m

CD CD

sdi>i ‘avon a3iiddv

FIGURE

9.

OBSERVED

RELATIONSHIP

BETWEEN

APPLIED

LOAD

AND

CONCRETE

STRAIN.

(BONDED

WITH

POLYESTER

RESIN)

APPLIED

LOAD,

KIPS

FIGURE 10. OBSERVED RELATIONSHIP BETWEEN APPLIED LOAD AND

CONCRETE STRAIN. (BONDED WITH POLYESTER RESIN)

FIGURE II. TYPICAL CRACK PATTERNS AT VARIOUS

SECTIONS OF SLAB.

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

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or program. Published papers appear either in the Bureau’s own series of publications or in the

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Information on tbe 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 tbe Superintendent of Documents, 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.