NATIONAL BUREAU OF STANDARDS REPORT

3665

PROPERTIES OP CONCRETE IN FLEXURE

AT HIGH RATES OP LOADING

(Progress Report)

by

D. Watstein and T. W. Reichard

Report to

Bureau of Yards and Docks

Department of the Navy

<NBS>

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

The scope of activities of the National Bureau of Standards is suggested in the following listing of

the divisions and sections engaged in technical work. In general, each section is engaged in specialized research, development, and engineering in the field indicated by its title. A brief description

of the activities, and of the resultant reports and publications, appears on the inside of the back

cover of this report.

Electricity. Resistance and Reactance Measurements. Electrical Instruments. Magnetic

Measurements. Electrochemistry.

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

Technology. Length. Engineering Metrology.

Heat and Power. Temperature Measurements. Thermodynamics. Cryogenic Physics. Engines

and Lubrication. Engine Fuels. Cryogenic Engineering.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Measurements. Infrared Spectros- copy. Nuclear Physics. Radioactivity. X-Ray. Betatron. Nucleonic Instrumentation. Radiological Equipment. Atomic Energy Commission Radiation Instruments Branch.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry.

Inorganic Chemistry. Electrodeposition. Gas Chemistry. Physical Chemistry. Thermochemistry.

Spectrochemistry. Pure Substances.

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

and Scale. Capacity, Density, and Fluid Meters. Combustion Control.

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

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

Mineral Products. Porcelain and Pottery . Glass. Refractories. Enameled Metals. Concreting

Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Heating and Air Conditioning. Floor, Roof, and Wall Coverings. Codes and Specifications.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering.

Electronics. Engineering Electronics. Electron Tubes. Electronic Computers. Electronic

Instrumentation. Process Technology.

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

Services. Frequency' Utilization Research. Tropospheric Propagation Research. High Frequency

Standards. Microwave Standards.

^Office of Basic Instrumentation ^Office of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT mS REPORT

1001-10-1000 September 9, 1954- 366^

PROPERTIES OF CONCRETE IN FLEXURE

AT HIGH RATES OF LOADING

(Progress Report)

by

D. Wat stein and T. W. Reichard

To

Bureau of Yards and Docks

Department of the Navy

<NBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, ref

unless permission Is

25, D, C. Such peri

cal ly prepared If tt

a i r- i ,

• , , ,, In part, is prohibited Approved for public release by the „ . „ , _ T • . T „

sndards, Washington Director of the National Institute of , ,, port has been specln- Standards and Technology (NIST) port for lts own use . on October 9, 201 5.

PROPERTIES OF CONCRETE IN FLEXURE

AT HIGH RATES OP LOADING

(Progress Report)

by

Do Watstein and T. W. Reichard

Abstract

Two methods of applying impulsive loads to

flexural concrete specimens were investigated„ One of these utilized a drop-hammer

machine and an attempt was made to produce

a triangular load pulse by means of suitable buffers. The second method of producing the required load pulse utilized a spring

loaded device, whose energy was transferred

to the specimen in a prescribed time. Exploratory tests of a third method are in

progress

.

1. INTRODUCTION

The object of the current study sponsored by the Bureau

of Yards and Dooks is to investigate the effect of dynamic

loading on the physical properties of plain concrete in flexure o This project is a continuation of a previous study in

which the effect of rate of straining on the compressive

strength of concrete was investigated.

The work in the current investigation has been confined

thus far to the development of a device suitable for applying

a load pulse of controlled duration and magnitude. The load

pulse was to be triangular in shape, with the rise time being

about 0.1 of total duration of the pulse.
.

’
2 , Two methods of producing the required load pulse are described. One of these utilizes a drop-hammer machine and the

other a spring loaded device. It is planned to explore a third

method of producing a dynamic load before undertaking a systematic study of properties of concrete.

2. DESCRIPTION OP TEST SPECIMENS

The preliminary tests in which various loading devices

and techniques were evaluated were made with plain concrete

beams I4 - by 9 -in. in cross-section and I4

.l4.-in. long 0 The beams

were tested on 2I4- , 30 - and 30 -in. spans.

The static compressive strength of the concrete, as determined with 6 - by 12-in. cylinders, averaged about 7500 psi . It

has been planned to use two different mixes of concrete in

this investigation. The two mixes, classified as "weak" and

"strong” concretes, were to have compressive strengths of

about 2500 and 6500 psi, respectively.

3. METHOD OP TESTING

3.1 Drop-Hammer Machine

1 /

The drop-hammer used in a previous investigation produced a load pulse in tests of concrete cylinders whose shape

1/ Effect of straining rate on the compressive strength and

elastic properties of concrete, Journal of the American

Concrete Institute , April 1953.
. .

• -

.
3 . was determined by the elastic properties of the buffer interposed between the drop-hammer and the test specimen. In the

first phase of the investigation it was decided to utilize the

existing impact machine to obtain a triangular load pulse by

means of suitable buffers. It was hoped that a material could

be found which would be sufficiently inelastic to dissipate

the energy of the drop-hammer and thus produce a load pulse

having the required triangular shape.

Various materials including modeling clay and sand were

tried with varying degrees of success. A fine dry sand in a piston type buffer gave the most promising results as shown

by the approximately triangular load pulse in figure lb. The

use of the sand buffer was discontinued, however, since the

duration of impact produced with it was too short and the

magnitude of the peak value of the force could not be controlled with adequate accuracy.

The device used in conjunction with the sand buffer to

limit the peak value of the applied load consisted of an

attachment which interrupted the travel of the hammer at a prescribed instant and served to produce a relatively long decay period in the load pulse.

3.2 Spring Loaded Impact Machine

This device was developed in an effort to improve control

over the duration and magnitude of the load pulse. The device

is illustrated in figures 2 and 3. It consists basically of

ka steel coil spring which is compressed by the hydraulic ram

of a 50,000 lb capacity testing machine. The compressed spring

bears against the hydraulic ram at its upper end, while its

lower end bears on an auxiliary crosshead where it is locked

by a trigger mechanism. When the trigger is released, the loading shaft which supports the spring is shot down against the

test specimen. The test specimen supported by the lower crosshead of the machine is brought up to bear against the shaft

just prior to release of the trigger. It will be noted that

a dynamometer and a suitable buffer are interposed between the

test beam and the loading shaft.

The trigger mechanism is illustrated in figure I4

. . It consists essentially of a pair of 1 -in. steel bearing balls spring

loaded into a groove cut in the loading shaft. The groove in

the shaft was sufficiently deep to permit nearly half of the

steel balls to interlock with the shaft. In this manner, a slight pressure on the balls was sufficient to lock the shaft

in position while it carried a large load. A simple lever

system was used to relieve the pressure on the steel balls and

release the loading shaft.

Some needed changes in the design of the device became

apparent as the work progressed, but the basic ideas of this

device have been proven sound. The device, as used now, is

5.

designed for a maximum load of 5000 lb which is sufficient

for the flexure tests in progress. Figure la shows a typical test record obtained from the dynamometer using this

machine „ The rise time is about O.OOlj. sec and the decay

time about 0.016 sec.

]+. INSTRUMENTATION

The instrumentation used in recording the applied

loads and strains in the concrete beams was essentially

the same as used previously in the "hard" impact tests of

compressive strength made in the drop-hammer machine. (See

Ref. 1 in Introduction). Slight modifications were made

in the preamplifiers to improve their low frequency response

and an automatic six-step calibrator (Figure 5) was constructed. A highly filtered, voltage regulated power supply

was tried out as a gage circuit voltage source, but due to

rapid line voltage fluctuations the idea was abandoned, A

block diagram of the instruments as used at present is shown

in figure 6.

At the present time a change in instrumentation is contemplated. An amplifying system reliable down to d.c. is

necessary to accurately amplify the long decay portion of

the required load pulse. A system using a carrier frequency

of at least 10 kc is the probable answer.
: .'.V.

.

.

.

l”.

'

'
Fig. la TEST DATA FROM SPRING LOADED MACHINE

Fig. lb LOAD HJLSB FROM DHCP HAMMER USING SAND BUFFER

'ig. Ic SIX STEP CALIBRATION SIGNAL SUPER-IMPOSED ON LOAD PULSE



Fig.

3

SPRING

LOADED

IMPACT

TESTING

MACHINE

SYMMETRICAL

ABOUT THIS I CENTER LINE

SPRING SEAT

3" 0 SHAFT

T

BRAKE

TRIGGER

MECHANISM

TRIGGER ASSEMBLY (^-) FOR SPRING LOADED IMPACT MACHINE

FIG. 4

3

0.

ir>

SIX

STEP

CALIBRATING

DEVICE

cc

too

UINSTRUMENTATION

FOR

FLEXURAL

IMPACT

TESTS

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. 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.
li 

%

)

)

:

b

c

\

)

)