NATIONAL BUREAU OF STANDARDS REPORT

5825

FLEXURAL TESTS OF PRESTRESSED CELLULAR SLABS

by

A. F. Kirstein

Report to

Bureau of Yards and Docks

Department of the Navy

<NBS>

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

1001-10-4811 March 27, 1958 5825

FLEXURAL TESTS OF PRESTRESSED CELLULAR SLABS

A. F. Kirstein

To

Bureau of Yards and Docks

Department of the Navy

IMPORTANT NOTICE

NATIONAL BUREAU OF S’ Intended for use within the

to additional evaluation and

listing of this Report, either

the Office of the Director, N

however, by the Governmenl

to reproduce additional cop

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 201 5.

progress accounting documents

brmally published It is subjected

?, reproduction, or open-literature

ission Is obtained in writing from Such permission is not needed,

y prepared if that agency wishes

U. S. DEPARTMENT OF COMMERCE
" : ;.. . : .*?:

.. ... *

-•V
FLEXURAL TESTS OF PRESTRESSED CELLULAR SLABS

By

A. F. Kirstein

To further the investigation of the properties of

prestressed cellular slabs, two specially designed specimens

were subjected to pure bending. These slabs were made of

unreinforced NBS blocks, and were prestressed to 1000 psi

in both directions. Deflections, strains, crack patterns,

and maximum bending moments were recorded. Both slabs exhibited abrupt compression failures in the top flange of

the blocks. A comparison with previously tested slabs subjected to a concentrated load at midspan failing in shear

shows that the slabs reported herein failed at bending

moments 39 and 63 percent greater than the maximum moments

resisted by those slabs that were supported along two and

four edges respectively. This indicated that a sizeable

increase in load-carrying capacity could be realized in the

previously tested slabs by increasing the resistance of the

prestressed slabs to diagonal tension and punching shear.

1. INTRODUCTION

In the previous tests carried out in the investigation of properties of prestressed cellular slabs, the

specimens were 5- by £-ft slabs supported either on four

or two edges and subjected to a concentrated load at the

center. The information developed in these tests served

as the basis for modification of the design of the cellular blocks and the method of jointing which resulted in

substantial increases in the shear resistance of these slabs.

In the current phase of the study of these slabs,

the object is to determine the flexural strength of prestressed assemblies. The specimens selected for these tests

were 2- by 10-ft slabs loaded at two points so as to have a section 5 feet long subjected to pure bending. The results

obtained in these tests will serve to plan future flexural

tests and also shear tests of specimens in which the ratio

of shear span to depth of slab will be a variable.
.
2

2. DESCRIPTION OF TEST SPECIMENS

2.1 Cellular Blocks

The cellular concrete blocks used in this investigation were the regular unreinforced NBS blocks that were

hollow six inch cubes having an opening lj.,5 by l\.S in. in

cross section. There was a 1- by 2-in. elliptical access

hole in each web to permit the passage of the prestressing

tendon. The principal dimensions of the cellular blocks

are shown in figure 1.

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 in. cubes. However, the actual units were

moist-cured for a considerably longer period of time and

then placed in dry storages before they were assembled into

slabs. Therefore, the compressive strength of the concrete

in the individual units should be expected to be somewhat

greater

The Young's modulus and Poisson^s ratio of the concrete were determined in previous tests -of the concrete

units and other specimens. Axial compression tests and

sonic modulus tests 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 0.15.

2.2 Prestressing Steel

"Elastuff" steel prestressing tendons were used in

the slabs. Tensile tests of this material indicated a stress-strain relationship that was essentially linear up

to 95)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 coldworked high carbon steel, they are fairly ductile and can

be machined easily.

-3 -

2.3 Description of Prestressed Slabs

The overall dimensions of slabs were 2 ft by 10 ft

3 in. ( fig. 4)

The nominal dimensions of the portion of each slab

which was in pure bending and which contained 40 cellular

blocks, were 2 ft by 5 ft by 6 in. The blocks were arranged

in a crisscross pattern so that the axis through the open

ends of one block was perpendicular to the axis of each

adjacent block. The holes in the webs of the block were

arranged to permit the longitudinal prestressing tendon to

be located along the mid-plane of the slab, and the transverse tendons were staggered above and below the mid-plane.

Thus, the resultant prestressing force produced an axial

compression of 1000 psi in two directions through the slab.

There were two slabs tested in this investigation.

Both slabs were constructed as specified above, except that.

Slab No. F-l had epoxy resin and Slab No. P-2 had neat

cement paste as jointing materials.

Both slabs were equipped with concrete end spans

in the longitudinal direction that withstood the shearing

forces of the load and served as anchorage blocks. With

the use of these end spans and the loading technique described later it was possible to subject the test section

of the specimen to pure flexure.

2.4 Prestressing Procedure

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.

Approximd^ly 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. 3.1TESTING PROCEDURE

Test Setup

A 600, OOO-lb capacity hydraulic testing machine was

used to test the slabs. The specimens were simply supported

by rockers that rested on the testing machine platen, and

the loads were applied through a knife edge at one end of

the test section and rollers at the other end. The loads

were distributed across the end of the concrete shear spans

with stiffened I sections, and all loads and reactions were

transmitted to the specimen through steel plates that were

firmly set in plaster. Figure 3 is a photograph of one of

the slabs and the test setup.

3.2

Instrumentation

The deflection measurements of these slabs were made

with 0 . 001 -in. micrometer dial gages that were attached to

steel angles resting onthe slab directly over the joints

between the test section and the end shear spans, thus

placing the datum plane on the top surface of the specimen

and at the ends of the test section. Figure 4 shows the

symmetrical arrangement of these gages.

The strains in the concrete on the top and bottom

surfaces of the slabs were measured with bonded electrical

resistance wire gages of the A_

type. The location of

these gages are also shown in figure 4 * Bonded electrical resistance wire gages of the A_

type were also placed at the center of the two prestressing

tendons indicated in figure 4 to determine the strains in

the steel due to the applied load.

3.3

Test Procedure

Both specimens were loaded as shown in figures 3

and 4* ^e load was applied In increments of 1000 lb, and

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

- 5 - 4.1 4. TEST DATA

Deflection and Load-carrying Capacity of Slabs

Slab No. P-1 carried a maximum applied load of

16,000 lb and Slab No. P-2 carried a maximum of 154700 lb

which, correspond, respectively, to total bending moments

of 213.6 and 210 in-kips when the dead load is considered.

During the test, signs of distress were noted in both slabs

at loads considerably below the maximum. At about 130 inkips of moment, audible cracking of the webs in Slab No.

P-1 was noted, and shortly after that the epoxy joints on

the tensile surface of the slab cracked open. Slab No. P-2

also cracked at about 130 in-kips, but the neat cement

joints opened at the same time as the cracking occurred in

the webs. Some crushing of the top surface of Slab No. P-2

was noted at about 200 in-kips. However, it continued to

carry load up to 210 in-kips when it failed very abruptly.

It appears that no strength advantage is gained from using

the stronger epoxy resin as a jointing material. Both

slabs failed by flexural compression.

The relationship between deflections and applied

moments is shown in figure 5.

4.2

Concrete and Steel Strains

The concrete strains on the tensile and compressive

surfaces of the slabs are plotted against the total bending

moment in figure 6. The open circles represent data from

Slab No. P-1, and the solid circles represent data from

Slab No. P-2.

The relationship between the total bending moment

and the steel strains are given for both slabs in figure 7.

4.3

Crack patterns

The crack patterns exhibited by these two slabs were

quite different from those observed in the previous tests

where the slabs failed in diagonal tension or punching shear.

The cracks that formed in the webs were for the most part

horizontal as shown in figure 8. The shaded and missing

portions of the blocks indicate the location of the compression failure.

6

The more widespread cracking shown in the webs of

the blocks in Slab No. P-1 is probably due to the use of a quick setting plaster in the joint between the test section

and the concrete end spans. It is believed that the rapid

setting of the plaster resulted in nonuniform contact between the shear span and the test section of slab. A

slower setting neat cement paste was used for these joints

in Slab No. P-2, which produced a more uniform joint.

5. DISCUSSION

As mentioned in the preceding section both specimens carried approximately the same maximum bending moment,

and showed signs of distress prior to failure at approximately the same stage of loading. Thus, it would appear

that no real advantage was derived from the use of the

stronger epoxy resin. However, it was also pointed out that

the use of a quick setting plaster as a jointing material

between the test section and the end shear spans of Slab

No. P-1 having epoxy resin joints was the cause of the widespread premature cracking of the block webs in that slab.

This widespread cracking shows up in the relationships between total bending moment and deflection, concrete strain,

and steel strain as seen in figures 5* 6, and 7 respectively.

It may have prematurely transferred the ultimate compressive

stress to the top flanges of the blocks, thus causing a premature failure.

It is important to note that the apparent lack of

rigidity shown in Slab No. P-1 can be traced to the imperfect plaster joints. The concrete and steel strains

also reflect this action in greater compressive strains in

the concrete and greater tensile strains in the steel.

Since both the epoxy resin and neat cement jointing materials are fairly brittle and the cracks in the

joints formed at about 60 percent of the maximum moment,

it is believed that greater usable strengths may be obtained in marine structures if a tough and more extensible

joint material is used.
,

'

'
- 7 - SUMMARY

The preceding work can be summarized as follows:

1. Both slabs failed at approximately the same load in

abrupt compression failures of the concrete in the

compressive flanges of the blocks.

2. Both slabs showed signs of distress prior to final

failure at approximately 60 percent of the maximum

load when cracks began to form.

3. There is a possibility that the use of a tougher and

more extensible jointing material might increase the

maximum usable strength of a marine or other structures where corrosion of the steel is a problem.

1}.. The apparent lack of rigidity of Slab No. P-1 was

attributed to the use of a quick setting plaster as

a jointing material between the test section and the

concrete end shear spans, which might have resulted

in an imperfect contact between the two portions of

the slab.

USCOMM-NBS-DC

FIGURE I NOMINAL DIMENSIONS OF NBS

UNREINFORCED BLOCKS.

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SLAB NO. F - I

FIGURE 8. CRACK PATTERNS

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Asti ii. Director THE NATIONAL BUREAU OF STANDARDS

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