S?cfo

NATIONAL BUREAU OF STANDARDS REPORT

5990

STRENGTH OP JOINTS IN UNREINPORCED

CELLULAR CONCRETE BEAMS

Arthur P. Kirstein and Harvey M. Shirley

Report to

Bureau of Yards and Docks

Department of the Navy

<|5b§>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
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 back 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.
NATIONAL BUREAU OF STANDARDS REPORT

MBS PROJECT MBS REPORT

1001-12-4811 July 31, 1958 5990

STRENGTH OF JOINTS IN UNREINFORCED

CELLULAR CONCRETE BEAMS

by

Arthur F. Kirsteln and Harvey M. Shirley

To

Bureau of Yards and Docks

Department of the Navy

IMPORTANT NOTICE

NATIONAL BUREAU OF

Intended for use within

to additional evaluation

listing of this Report, elt the Office of the Dlrecto however, by the Governr

to reproduce additional

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

or progress accounting documents

is formally published It Is subjected

itlng, reproduction, or open-literature srmleslon Is obtained In writing from

. C. Such permission Is not needed,

ically prepared If that agency wishes

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

STRENGTH OP JOINTS IN UNREINFORCED

CELLULAR CONCRETE BEAMS

By

Arthur F. Kirstein and Harvey M. Shirley

Load tests were conducted on a number of unreinforced

beams containing cellular concrete blocks to determine the

strength of the joints between the cellular units. The

units were reinforced and were bonded together with various

mixtures of epoxy and polyester resins. Deflections, strains,

and load- carrying capacities were recorded. It was found

that most joint failures could be attributed to the failure

of the thin layer of cement paste adjacent to the adhesive

in the joint, and only a few joint failures were due to a weakness in the resin.

1. INTRODUCTION

At the request of the Bureau of Yards and Docks, the

Structural Engineering Section initiated an investigation to

determine the strength of adhesive in the joints of cellular

concrete beams containing no longitudinal reinforcement.

The investigation was conducted to determine the cause

of apparent joint failures in a series of prestressed cellular concrete slabs, and to give an indication of the amount

of handling that an unstressed unit can withstand in a prestressing yard. The scope of this study was extended to include epoxy resin jointing materials that were activated by

various different amines and to include a polyester resin as

a jointing material.
2. DESCRIPTION OF TEST SPECIMENS

2.1 Cellular blocks

The cellular concrete blocks used in this investigation

were the regular NBS blocks reinforced with welded wire fabric

being used in the current investigation of the properties of

prestressed cellular concrete slabs. These blocks were hollow

6-in. cubes having an opening lj.,5- by 4. 5-in. in cross section.

There were 1- by 2-in. elliptical access holes in the webs to

permit the passage of prestressing tendons when used in a prestressed assembly. These blocks were cast in steel three-unit

gang molds that were constructed to very close tolerances,

thus forming the blocks to exterior dimensions which were within 0.01-in. of the nominal values. The principal dimensions

of these 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 from tests of

2-in. cubes. However, the actual units were moist-cured for a considerably longer period of time, and then were placed in

dry storage before being assembled Into beams. Therefore, the

compressive strength of the concrete in the individual units

should be expected to be somewhat greater.

2 .
The Young’s modulus and Poisson’s ration 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 Ip x 10^ psi

with variations of ± 10 percent. Poisson’s ratio was found

to be approximately 0 . 15 . 2.2 Description of beam specimens

There were nine specimens included in this investigation

and each was composed of six reinforced NBS blocks. Specimens

A, B, C, and D contained reinforced NBS blocks as cast, and

specimens Ap , A2 * Bp, Dp and Ep , contained similar NBS blocks

that were ground to remove the layer of cement paste from

the surfaces which formed the joint.

The letters in the specimen designation denote the type

of resin and activator used to bond the blocks together. The

subscripts indicate that the blocks used in the beams had the

layer of cement paste ground from the surfaces that formed

the joint. The type and mixture of the resin and the letter

designation are listed below.

Type A - 200 grams Epon 828 " w 13 grams Cab-o-sil“ „ 16 grams Diethylenetriamine"

Type B - 200 grams Epon 828

12 grams Cab-o-_sil

16 grams DMP-30''

3 .
Type C - 200 grams Epon 828

12 grams Cab-o-sil

16 grams DMP-10’’

Type D - 200 grams Epon 828

12 grams Cab-o-sil

8 grams DMP- 30

8 grams DMP-10

Type E - 200 grams Selectron 5119”

12 grams Cab-o-sil

2 grams Lupersol DDM“

3. TESTING- PROCEDURE

3-1 Test setup

All of the specimens were tested to failure in a hydraulic

testing machine. They were simply supported over a 30.5-in.

span and two equal concentrated load were applied as shown in

figure 2. All loads and reactions were transmitted to the

specimen through 1- by 2- by 6-in. steel bearing plates, and

the load and reaction devices were designed to eliminate eccentric or torsional loading.

3.2 Instrumentation

The deflection measurements of the specimens were made

with two 0.001-in. micrometer dial gages which were placed at

midspan on both sides of the specimen. Longitudinal strain

readings were made with an electrical resistance wire strain

gage of type A-3 which was bonded to the center of the tensile

surface of the specimen across the midspan joint.

''"List of manufacturers or sources are given in the Appendix.
3.3 Test procedure

The specimens were loaded continuously as shown in figure

2 at a rate of 600 lb per min, and simultaneous readings of

strain and deflection were made at 200 lb intervals.

4. TEST DATA

4.1 Load- carrying capacity

Table 1 lists the maximum loads sustained by the specimens along with the computed maximum stresses at the extreme

fibers and descriptions of the types of failure exhibited by

each specimen. Figure 3 is a photograph of Beam Ap after

failure. This type of failure was also exhibited by Beam A£

which was a companion to Beam Ap . The failure of these two

beams was typified by a sudden drop of load after the maximum

load was reached, and the beams continued to deflect at lower

loads

This action was due to the yielding of the No. 1%-ga

wires of the 1- by 1-in. welded wire fabric. All of the other

specimens that failed at or near the center joint exhibited an

abrupt failure that permitted the two halves to fall to the

testing machine platen.

4-2 Deflection

The relationship between the computed stress in the extreme fibers at midspan and the observed deflection at midspan

can be seen in figure 4 along with a theoretical relationship

5 .
between these variables. The theoretical curve was computed

with the assumption that the modulus of elasticity was equal

to i| x 10^ psi, and that the presence of the reinforcement

could be neglected in computing the moment of inertia of the

cross section.

4.3 Concrete strains

The relationship between the computed stress in the

extreme fiber at midspan and the observed strain in the concrete on the tensile surface of the specimen at midspan is

shown in figure 5. A theoretical relationship between these

variables is also shown. The theoretical curve was computed

using the assumptions listed above.

DISCUSSION

As can be seen in Table l y the type of failure of specimens A, B, C, and D points directly to a weakness in the

cement paste that covers the surface of cast blocks of this

type. Therefore, another series of specimens were ^fabricated

and tested with blocks having the cement paste ground from

surfaces that formed the joints. It should be noted that the

type "C" adhesive was dropped from the second series as the

DMP-10 activator was too slow. The adhesive in Beam C while

partially set was still tacky after 7 days of curing. Since

this slow rate of curing at room temperature was deemed unsuitable for this purpose, testing of specimens containing

this resin was discontinued.

6 .
Upon comparing the test results shown in figure 4 it 1 3

noted that there is fairly good agreement between the theoretical stress-deflection curve and the test data. There is

also good agreement between theory and test results indicated

in figure 5 except for Specimens Ap and Ap. The irregularity

in the curves or these two specimens is probably due to the

fact that the blocks rather than the center joints failed.

This was quite evident in Specimen Ap , as the crack shown in

figure 3 appeared simultaneously with the decrease in tensile

strain indicated between 400 and 450 psi in figure 5.

It is evident from the results in Table 1 that companion

Specimens Ap and Ap exhibited the highest load carrying capacity of the specimens tested.. Furthermore, it should be noted

that the epoxy resin mixture used to bond these specimens together was identical to the mixture used in the shear and

flexural tests of prestressed cellular concrete slabs that

were reported in UBS Reports 5714 and 5825.

The DMP-30 used to activate the epoxy resin of Specimens

B and Bp produced a resin with a pot life of only 15 min and

the resin exhibited brittle characteristics under flexural

tests. Therefore, epoxy resin using this activator was found

to be unsuitable for bonding these concrete blocks together.

7 .
As mentioned previously, the DMP-10 used to activate the resin

in Specimen C was found unsuitable because it cured too slowly.

The mixture of DMP- 30 and DMP-10 used to activate the epoxy

resin of Specimens D and Dp produced a resin with a fairly

short pot life which took a long time to cure.

The polyester resin used in fabricating Specimen Ep was

identical to the polyester resin used to bond together some of

the prestressed cellular concrete slabs that were reported in

NBS Report 5212, The resin appeared to be fairly strong as it

did not fail under load test as indicated in Table 1. However,

it was not as workable as the epoxy resin used to bond Specimens

A, Ap, and A2 , When considering the handling and moving of unstressed

bonded units similar to those bonded with the type A epoxy

resin, it would appear to be desirable to limit the extreme

fiber stress to 2^0 psi or to one half the modulus of rupture

of the concrete* whichever is smaller.

8 .
6. SUMMARY

The foregoing discussion can be summarized as follows:

1. The apparent failure of the resin joints reported in

NBS Report No. 5825 can be attributed to the failure of the

cement paste surfaces of the concrete blocks that formed the

joints in the prestressed slabs.

2. Epoxy and polyester joints can be made to resist more

flexural stress than the reinforced concrete units used in this

investigation.

3. Unstressed assemblies similar to those tested in this

investigation can be handled and moved if the handling stresses

are limited to a reasonable value.

9 .
APPENDIX

The following is a list of materials used in preparing

the adhesives used in these tests and their sources:

Epon 828

Cab-o-sil -

DMP-10)

DMP- 30

)

epoxy resin

Shell Chemical Corp.

380 Madison Ave

.

New York 17, N. Y.

Finely divided silica

Godfrey L. Cabot, Inc.

Mineral and Chemical Division

77 Franklin Street

Bo ston 10 , Mass

.

Dimethylaminomethyl phenols

Rohm and Hass Co.

Washington Square

Philadelphia 5, Pa.

Diethylene triamine

Chemical Rubber Co.

86l6 Georgia Ave.

Silver Spring, Md.

Selectron £119 - polyester resin

Pittsburg Plate Glass Co.

One Gateway Center

Pittsburgh, Pa.

Lupersol-DDM - 60$ Methyl ethyl ketone peroxide in

dimethyl phthalate

Novadel-Agene Corp.

Lucidol Division

1740 Military Rd.

Buffalo, N. Y.

10 . USCOMM-NBS-DC
Table 1. Load- carrying capacity and type of failure of specimens.

Specimen

’ Max imum

1 Load

!

» lb

1 Modulus

« of

’ Rupture

’ ps i

A

t

* 1890

t

t 471

t

B

!

1 1300

t

T

' 324

| C * 760

T

» 189

1 D » 1200

1

|

t 299

t

|

A

i

t

’ 2300

t

' 573

1

A

2

t

» 2260

1

t

1 363

t b

i

t

* 2080

t

T

' 518

t h

1 1980

T

• 493

t E

i

« 1922

t

t

• 479

t

t

Type of Failure

Cement paste and small pieces of

coarse aggregate pulled out of

surface of concrete.

Cement paste pulled out of surface

of concrete.

Adhesive failure in center joint.

Primary failure in adhesive in

center joint; small portion of cement paste pulled out of surface

of concrete.

Crack formed in block under loadpoint - joints intact.

Crack formed in block under loadpoint - joints intact.

Brittle fracture of adhesive in

center joint.

Adhesive failure in center joint.

Concrete failed at center joint.
:
l“X l"X I5/I5GAUGE MESH

FIG. I - NOMINAL DIMENSIONS OF NBS

REINFORCED BLOCKS.

FIG.

2-

ARRANGEMENT

FOR

BEAM

LOADING

TESTS

FIG.

3

-PHOTOGRAPH

OF

BEAM

A,

AFTER

FAILURE

009

|sd *NVdS —Q I I/M IV 839ld 3W381X3 Nl SS381S Q3indlAI00

0.01

DEFLECTION

OF

SPECIMENS

AT

MID-SPAN,

in.

FIG.

4

-

RELATIONSHIP

BETWEEN

COMPUTED

STRESS

AND

DEFLECTION

009

!Sd

4 NVdS~0IW IV H39I3 3lN3aiX3 Nl SS3U1S Q3indWOO

<

t

hif)

Q

Z

<

CO

CO

UJ

tr

i

CO

Q

LU

h-3

CL

o

o

UJ

UJ

£

H

LU

CD

O.

X

CO

o

I-

<

UJ

CL

if)

6

U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director

THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

D. C., and its major laboratories in Boulder, Colo., is suggested in the following listing of the

divisions and sections engaged in technical work. In general, each section carries out specialized

research, development, and engineering in the field indicated by its title. A brief description of

the activities, and of the resultant publications, appears on the inside front cover.

WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instruments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Engine

Fuels. Free Radicals Research.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State

Physics. Electron Physics. Atomic Physics. Neutron Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic Chemistry. Electrodepqsition. Molecular Structure and Properties of Gases.

Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metal Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories, Enameled Metals. Concreting Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Air Conditioning, Heating,

and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat

Transfer.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering. Mathematical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques. Digital

Circuitry. Digital Systems. Analog Systems. Application Engineering.

* Office of Basic Instrumentation. • Office of Weights and Measures.

BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services. Sun-Earth Relationships. VHF Research.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

Navigation Systems. Radio Noise. Tropospheric Measurements. Tropospheric Analysis.

Radio Systems Application Engineering. Radio Meteorology.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High

Frequency Impedance Standards. Calibration Center. Microwave Physics. Microwave Circuit

Standards.
(