NATIONAL BUREAU OF STANDARDS REPORT

5314

TEST OF A PRESTRESSED CELLULAR SLAB

by

Arthur F. Kirstein

Report to

Bureau of Yards and Docks

Department of the Wavy

<JNBS>

U. S. DEPARTMENT OF COMMERCE
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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1001-10-4811 June 6, 1957 £314

TEST OF A PRESTRESSED CELLULAR SLAB

by

Arthur F. Kirstein

To

Bureau of Yards and Docks

Department of the Navy

IMPORTANT NOTICE

NATIONAL BUREAU OF S intended for use within th

to additional evaluation am

listing of this Report, eithe the Office of the Director, however, by the Governmei

to reproduce additional co|

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

progress accounting documents

ormally published it is subjected

j, reproduction, or open-literature

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

y prepared if that agency wishes

<^JBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

TEST OP A PRESTRESSED CELLULAR SLAB

by

Arthur P, Kirstein

Abstract

As a continuation of the study of the

properties of prestressed cellular slabs,

a nine celled unit of cement-asbestos was

developed, and a slab composed of 16 of

these units was tested. Since concrete

cannot in itself develop high enough tensile strengths to withstand the diagonal

tension encountered in slabs of this kind,

cement-asbestos was used. The test results appear favorable as the load-carrying capacity of the cement-asbestos slab

was found to be over 60 percent higher than

comparable concrete slabs. It is of further

interest that no evidence of diagonal tension cracking was observed in this slab

which failed by compression. This mode of

failure indicates that a still greater increase in maximum load- carrying capacity

and cracking load can be realized by increasing the thickness of the flanges of

the units and the amount of prestress applied to the slab.

1 . INTRODUCTION

Since the study of the properties of prestressed cellular slabs was initiated, a series of nineteen 5- by 5-ft

slabs have been tested. The conclusions drawn from the work

presented in NBS Reports Nos. 4396, I|i3b3» 495>1» and 5212,

were that the units must be bonded together to attain intimate contact between units and that the webs of the units

must be able to resist higher tensile stresses if the loadcarrying capacity of the slab is to be increased.

2 .

A previous attempt to increase the load- carrying capacity

of the prestressed concrete block slabs was made by reinforcing the concrete block webs with steel to prevent the typical

diagonal tension failure. The increase in load- carrying capacity was appreciable, but the blocks were difficult to manufacture in any sizeable quantity. Therefore, it was found

necessary to consider materials other than concrete for use in

manufacturing these slabs. Prestressed beam tests were made

to compare concrete units having reinforced webs with units of

identical size and shape made of cement-asbestos. These tests

indicated that the cement-asbestos beams carried up to percent more load than the beams with reinforced concrete units.

This report is concerned with the prestressed slab test that

was made to verify these findings.

The basic unit was redesigned with interlocking lugs to

aid in the distribution of stress and to facilitate proper

assembly in the field. This unit contains nine cells and four

interlocking lugs and the nominal dimensions are 6- by 15- by

l£ in. It is believed that these units can be mass produced

efficiently and shipped to the fabrication site as individual

units.

The prestressed cement-asbestos slab tests reported herein indicates that the load- carrying capacity was increased by

more than 60 percent over that of comparable concrete slabs,

and that a further increase is possible inasmuch as no evidence

of diagonal tension cracking was observed in any of the webs.

2. DESCRIPTION OF TEST SPECIMENS

2.1 Cellular blocks

As mentioned previously, the basic cellular unit was redesigned to have outside nominal dimensions of 6- by 1 $- by

l5-in. The unit containing nine cells and four interlocking

lugs is shown in figure 1. Upon examination of figure 1, it

is readily apparent that the slots of the unit and interlocking lugs of the adjoining units make the field assembly practically fool-proof.

The basic components that are used to fabricate the typical unit are shown in figure 2. The members are cemented together with epoxy resin to form the nine-cell unit. Figure 3> a photograph of the partially fabricated unit, shows how the

unit is assembled in an ”egg-crate M fashion. Attention is
. '
3 .

directed to the location of the circular holes in the webs.

These holes are arranged so that the prestressing tendons

can be staggered above and below the midplane of the block

to produce a resultant prestressing force at midplane in

both directions. By cutting an oval hole in the webs these

units could be made interchangeable.

Figure I|. is a detailed drawing of the components of

the typical cellular unit. Upon comparing figure 4 with

figure 2 it can be seen that an alteration of the interlocking lugs was made. This alteration was believed necessary

because of the cutting technique employed.

The physical properties of the cement-asbestos used in

this investigation were determined by tensile, compressive,

and sonic tests. The tensile tests indicated that the

cement-asbestos could carry an ultimate stress of 1390 psi

in direct tension with a modulus of elasticity of 1.82 x 10°

psi, while the compressive tests indicated an ultimate stress

of 4890 psi in direct compression with a modulus of elasticity of 2.05 x 10° psi. The sonic tests gave a modulus of

elasticity of 1.95 x 10° psi with a Poisson’s Ratio of 0.30.

2.2 Jointing material

Care was taken to select a resin for use in bonding the

cellular units together so that a premature failure of the

slab would not result from a cement failure. The first test

performed was a comparative test between a polyester resin

and an epoxy resin. Cement joints of polyester and epoxy

resin were made by butting pieces of cement-asbestos together

and cementing the 3/4“ by 1 13/l6-in. butted cross-sections.

Direct tension tests of these cemented joints revealed that

the polyester resin had an ultimate tensile stress of only

432 psi, while the epoxy resin had a tensile strength of

948 psi.

From the test results reported above, it was evident that

the epoxy resin with its tensile strength more than twice that

of polyester resin was a more suitable cement. Further testing of the epoxy resin was carried out to determine the type

and proportions of fillers necessary to make a strong workable

cement. Ignited AI0O3 and a finely divided siliceous mineral

filler were selected for testing. The final result of this

testing was a cement of 100 parts epoxy resin (Epon 828), 10

parts of die thylenetriamine , and 5 parts of the finely divided

siliceous filler by weight. This particular mixture was
*I
4 . stronger than cement-asbestos in pure shear and had the consistency of petroleum jelly with a pot life of 53 minutes.

This mixture had good workability yet would not run off of a vertical surface of cement-asbestos. 2.3

Prestressing steel

The steel prestressing tendons used in the cellular

cement-asbestos slab were 0.75 in. diameter "Elastuff" bars.

Tensile tests of this material indicated a stress-strain relationship that was essentially linear up to 70,000 psi, and

exhibited a Young’s Modulus of 28.6 x 10° psi. The yield

strength of the bar was found to be 105,000 psi as determined

by the 0.2 percent offset method and the tensile strength was

found to be 125,500 psi. The tensile stress-strain curve for

this material is shown in figure 5. Although the ’’Elastuff"

bars are made of cold- worked high carbon steel, they are

fairly ductile and can be machined easily.

2.4

Description of prestressed slab

The 5- by 5-ft slab was made of 16 units that were

bonded together with epoxy resin, and eight 0.75 in. diameter

"Elastuff” steel bars were used to apply the 1000 psi prestress in both directions. Figure 6 shows the arrangement of

the units in the slab and the placement of the prestressing

tendons. The end rows of cells were filled with concrete to

distribute the prestressing force over the entire area of the

cement-asbestos in the direction of the span, while the concrete anchorage blocks were used to distribute the prestress

in the transverse direction. The slab was assembled without

the top flanges in place so that the assembly could be viewed

(Figure 6), but the slab can be assembled as easily with the

flanges in place.

2.5

Prestressing procedure

Approximately one-third 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 a 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. The

hydraulic jacking rig 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 7.

5.

3. 3.1TESTING PROCEDURE

Test setup

The slab was simply supported on two edges over a 54~in * span by 1-in. square aluminum bars that were attached to the

steel frames resting on the testing machine platen. All

bearing surfaces were set firmly with high-strength plaster

to obtain intimate contact between individual members. The

load was applied to the center of the slab through a 12- by

12-in. concrete loading block 6-in. thick.

3.2

Instrumentation

The deflection measurements of the slab were made with

0.001-in. micrometer dial gages that were attached to steel

angles. These angles rested on the top surface of the slab

directly over the supports, thus placing the datum plane at

the supports. Figure 8 shows the test setup with the dial

gages in place, and the A-l type bonded wire strain gages

connected to the strain measuring equipment. The exact locations of the gages are shown on the Instrumentation Diagram

in figure 9.

3.3

Test procedure

The slab was 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 2500 and 5000 lb, 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 slab

Figure 10 shows the observed relationship between the

applied load and center deflection of Slab No. T-l. The reciprocal of the slope of the linear or elastic portion of this

curve can be considered to be a measure of the deflection

sensitivity of the slab, and is expressed in terms of micro

inches of deflection per pound of applied load iyU. in. /lb) . This factor is useful in comparing the performance of similar

slabs.
!

,
6

Table 1 shows a comparison of the deflection sensitivities

and the maximum load- carrying capacities of the cement-asbestos

Slab No. T-l and the comparable concrete slabs summarized in

NBS Report 5212.

ip. 2 Strain in cement-asbestos

The relationships between the applied load and the longitudinal strains in the top and bottom of the cement-asbestos

slab are shown in figure 11. The transverse strains were of

minor importance because the slab was simply supported along

two edges.

Ip. 3 Crack patterns

The first audible crack occurred at a load of 21,500 lb

This crack was formed in the epoxy resin on the bottom surface

and propagated transversely across the mid-span of the slab.

The formation of this crack was evidenced by the change in

the relationship between the applied load and the center deflection shown in figure 10.

The transverse crack was the only crack that formed during

the test until the upper flange crushed at the maximum load of

1|5>6 00 lb. When the prestressing tendons were removed after

the test the slab broke in half, and upon further investigation

of the slab it was discovered that no diagonal tension cracks

had formed in the webs. The slab was then broken into quarters

to make a more thorough inspection of the webs, but no diagonal

tension cracks could be found. Figure 12 is a photograph of

the broken ends of two of the quarters that clearly indicates

that the webs are still intact.

5. DISCUSSION AND SUMMARY OF RESULTS

Upon examination of Table 1, it is immediately apparent

that Slab No. T-l carried over 60 percent more load than any

of the comparable concrete slabs. This increase in the loadcarrying capacity coupled with the fact that Slab No. T-l

failed in compression bears out the conclusion from a previous

report that the webs of the units must be strengthened in tension to increase the load-carrying capacity.

I
. \ 1
7 .

Since Slab No. T-l failed in compression, the conclusions

drawn in previous reports about the need for a jointing material to ensure intimate contact between units and eliminate

local stress concentrations is again substantiated.

The deflection sensitivity of the cement-asbestos slab

was greater than those of the concrete slabs, but that was

expected from the lower modulus material. The greater deflection of the slab displays the relative toughness of the cementasbestos as compared to the concrete.

It is believed that the cracking load of 21,500 i 1q can

be increased by increasing the prestress from lOOOto 1^00 or

2000 psi. However, it is not known how this increase in prestress will effect the maximum load- carrying capacity of the

slab, since the ultimate load was reached when the upper

flanges failed in compre ssion„ Possibly it may be necessary

to increase the thickness of the flanges if greater prestressing forcesr are used.

Although the cement-asbestos material exhibits good tensile properties, little is known of its creep properties.

Therefore, it would be necessary to conduct research over a period of time to ascertain whether cement-asbestos would be

a suitable material to be used in prestressed units.

Table 1. Comparison of Test Results from CementAsbestos Slab and Comparable Concrete Slabs

Slab

No.

1 *

’ Method ’ Arrangement

’ of ’ of

f support* blocks

f »

1

t

t Jointing

> material

t

1

’ Type

’ of

’ block

t

—

i

7 Deflection

sensi ti-

’ vi ty

t

t

t Maximum

t load

i

t t

r t

t

i

t

« J /

in. /lb

\ kips

10 ’2 edge ’Criss-cross

t i

’ None

!

’ NBS

i

' 1.57

t

’ 17.0

t 11 ’2 edge ’Criss-cross

» t ’Neat cement

I

’ NBS

t

7 1.27 ’ 26.25

t 19 ’2 edge ’Criss-cross

t i

t t

’Polyester

’ resin

i

’ NBS

t

t

’ 1.39

t

i

’ 28.0

t

i T-l ’2 edge ’Interlocked

i i

t t

i t

« t

'Epoxy resin

t

!

T

’9 unit

’ cement-

’ asbestos

t

t

’ 2.31

1

t

t

I

7 4^.6

i

i

t

Note: All slabs were tested over a 54"in * span, and contained

a 1000 psi prestress in both directions.
>* '

.
FIGURE

I.

NINE-CELL

CEMENT

-

ASBESTOS

UNIT



FIGURE

3.

PARTIALLY

ASSEMBLED

UNIT

Q

IdO

-I

COo

z

if)

IdO

UJ

Q.

CLIdO

X

Id UJ

if) <

Id5

<

if)

NUMBER REO'D. —

PIECE Q Id li_

NUMBER REO'D. CVJ <\J —

PIECE < CD O

cn2

UJ

O ^

o

o

ZZ UJ < 2 orQ i,

<

I-UJQ

note:

slots

on

pieces

a,b,

c

a

o

are

2^

deep.

120

FIGURE 6. SLAB NO. T - I WITHOUT TOP FLANGES IN PLACE

FIGURE

^

N

1

©- ©-

i <f>

(2)_

|||P //////////

_(4)

©-

6 8

| (l)

5

©- 0-

4'- 6“

GAGES © THROUGH @ ARE DIAL GAGES

GAGES (I) THROUGH (4) ARE SR-4 GAGES ON BOTTOM SURFACE OF SLAB.

GAGES 5 » 8

" " " " TOP " "

FIGURE 9. INSTRUMENTATION DIAGRAM FOR SLAB NO. T-l

LOAD

45

to

OL

.2 .3 .4 .5 .6 .7 .8

CENTER DEFLECTION, IN.

FIGURE 10. RELATIONSHIP BETWEEN LOAD AND CENTER

DEFLECTION OF SLAB NO. T-l .

BOTTOM TOP

400 200 0

TENSILE

200 400 600 800 1000

COMPRESSIVE

1200 1400x10

STRAIN IN CEMENT - ASBESTOS

FIGURE II. LONGITUDINAL STRAIN ON TOP AND

BOTTOM SURFACE OF SLAB.

VIEW

OF

UNDAMAGED

WEBS

IN

CENTER

SECTION

U. S. DEPARTMENT of commerce

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

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

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