NATIONAL BUREAU OF STANDARDS REPORT

6116

SOME EXPERIENCE GAINED FROM THE USE OF HUMIDITY SENSING

ELEMENTS TO INDICATE THE DRYING OF FIRE TEST SPECIMENS

by

J. V. Ryan

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 technit'ai investigations; the Technical News Bulletin, which presents

summary and preliminary reports on work in progress; and Basic Radio Propagation Predictions,

whi(‘h 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 (.'SI. 25) and its Supplement (10.75), available from the Superintendent of Documents, Government Printing Office, Washington 25, D. C.

Inquiries regarding the Bureau’s reports shmdd be addressed to the Office of Technical Information, National Bureau of Standards, Washington 25, D. G.
NATIOS^AL BUREAU OF STANDARDS REPORT

MSS PROiECT NS3S REPORT

1002 - 12-1029 August 12, 195^ 6116

SOME EXPERIENCE GAINED FROM THE USE OF HUMIDITY SENSING

ELEMENTS TO INDICATE THE DRYING OF FIRE TEST SPECIMENS

by

J. V. Ryan

f^AUONAL BUREAU Of

intended for use within 1 9c sdditiona! evaluation a Sistlng of Ihit Report, eiti the tMRce of the Dlrsttor howexer, by the Governm

to reproduce additional c

IMPORTANf NOfICE

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST) on October 9, 201 5.

OiT progress accounting documents

is formally published it is subjected

iting, reproduction, or open-literature irmission Is obtained in writing from

. C. Such permission is not needed,

caily prepared If that agency wishes

<CNBS>

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
I V. .^ ^- *v *

TJ

..’^5 -sC-fliAQsSAT?? yAi^aS^ JilWOfTAH

>«

. 5^ t

r:^ii%m :i

I i --?rM

_fc

'^Wjl

'* : I '.Jlij; ',!

M

. a • ; .V ' ' 4 ^

'

' » ,J V‘^. 44-^1

si/rfitii' ^ii »iii,.,. ••<i<,

1 , V =! -

' y^n ii fu, . ^}^, ,f ii, ^ *

• ^ 'S' ''‘••'i- >-!,»«( #^7444^

••' . u«J(*(<S i >4 fli ''»'?; j|ff'

.•'. f- iT-O-: 4i. i'at< J

• airj' ,Ai

iC? r»|^5«W .,2 J

caKf'u;^^Tv; P&153

. -S&'i!

-V> 4K^ :

5:5

t , iltr,S-i
SOME EXPERIENCE GAINED FROM THE USE OF HUMIDITY

SENSING ELEMENTS TO INDICATE THE DRYING OF FIRE TEST

SPECIMENS

ABSTRACT

The presence of water in materials exposed ro fire is

known to affect their fire endurance performanceo

Because of this, use has been made of humidity sensing

elements as an aid in determining when test specimens

have approached moisture equilibrium with the ambient.

The data have indicated that certain procedures may

introduce errors. The procedures followed are reviewed

and recommendations made.

1. INTRODUCTION

In order that the results of fire endurance tests may be as

representative as feasible, the specimens constructed especially

for the test must be as closely representative as practicable of

similar constructions in buildings after several years of use.

For concrete, plaster, masonry and wooden constructions, this

makes it necessary that the testing laboratory be concerned, in

addition to other factors, with the state-of -dryness of the specimen.

Every attempt should be made to allov; the specimen to reach moisture

equilibrium with the ambient.

For many materials, data on moisture content as related to

the relative humidity of the ambient indicate a large change of the

former from saturation down to equilibrium with air of relative

humidity near 80 percent, and a comparatively small change from

there down to equilibrium with air at about 20 percent. Therefore,

equilibrium with air at or below 70 percent relative humidity

should provide a reasonable indication that the specimen is quite

close to its ultimate moisture content.

The National Bureau of Standards’ Fire Protection Section

has used hygrometer elements in attempts to measure the relative

humidities of air believed to be in moisture equilibrium with

various parts of specimens. Two types of hygrometer elements have

been employed, dye-spot cards and electrical humidity elements.

They have been used in arrangements whereby air in contact with the

surface or the interior of the specimen has been isolated from the

general atmosphere by vapor barriers. It was assumed that the air

thus isolated would come to moisture equilibrium with the specimen

material, at a relative humidity indio-ative of the moisture content

of the intjier. Since the state of ..dryness was of primary concern,

rather than the exact moisture content, it was not necess'ary to have
V'* b- imxi *

* '>-7

r;d.u:c^A

1%*^ ]r V- ' I •> ’ r.i I if. : ;

\ ;. 'i rr.utl'

' -tlv >' 1; ' .* ^

I- i't^.f: ^ -r-. (I

‘ •

1

'

I •' .>

'"?'

;

1 J

'.1 . ' /-‘fM i

.

•:

• T * »'.o-Ti.% ^':.lsii ' --'i'i- .•. a\

:.vr^ .,-:stvi> : t,. u

,'

I Mm , I

1 *i*J l£>

- '•

. L .’’3 ^tj' J - LI>’.

I, ,v.T'' r' , .’7L. ru:

«'

1 i- •./• •i-’

_ '.'. ' i • '

.

'! ^ * ' I \ I _ '. ti- -•

.

' i£

' -V i 1. :' •' "T VJ ^''

?. •;•» ? . -^Mia -n.. 4..f'

' 1 r

’ 'i. V 5; ... I

' i' '

' /J'isi'Wj. ^ f \

f > r '., V J- !•• '•

; ..T(.;if«- • ImM' ,

' ',i 'V

.' '^ 1 Ir j Z .-1 [ i':>1

.= ! t '1 »;• 'I 'U iU tii;

J

I 1 ^

r T

- f.n ' i- 1 ', 'T.rL* !’• «•'; f . Iv^iS

;'. p u. .''.’ju ?ai;cj.»j'.i

;,'.ia.tt, 1 . l'i«Tt/a 1*IT< ..;i .^1

...••< .Ti * j .G

,

i 1

1

* eic .'5

- Xft 1

'_ • ' >'T .

t:f. ’r, .?•:

•. •.

I *

' u , iljr' ill t9tUt,.-

,1 ..f- . ':-.-4’jf '*AT -^0 zott^iol

Vi*' -

' 'li’-r. '.'1

*tv ' i' .'V'

i.

'
2

accurate calibration charts of moisture content versus relative

humidity for the particular concretes, plasters, etc.

Data obtained from some specimens have indicated that the

geometries of the vapor barrier systems have been^such that

moisture equilibrium was not reached rapidly. This report outlines the various physical arrangements employed and the experiences

obtained.

2. MEASUREMENT SYSTEMS

The measurement systems consisted of a hygrometer, the test

specimen, and an assembly to trap air against some part of the

specimen, either the exterior, or an interior surface at the open

end of a tube.

2.1 Exterior Measurements

The vapor barrier used vjas a transparent polyethylene

plastic sheet, the lateral dimensions of which were large compared

to the thickness of the specimen, or that part of the specimen

that might have excess moisture content. The hygrometers were of

a commercial type, each consisting of six dye-spots on a piece of

blotting paper. By means of color changes they provided an indication of relative humidity in the range from 20 to "over

bo percent." The hygrometer was placed between the specimen

surface and the vapor barrier. On smooth surfaces the barrier was

secured around its periphery by plastic electrician's tape. This

tape was not satisfactory on rough concrete. No attempt was made

to find a suitable peripheral seal for such surfaces. This method

was used with plastered floors and columns, primarily, in which the

plaster and lath were backed by air trapped between the floor joists

or between the flanges of the steel column. This trapped air being

in moisture equilibrium with the back surface of the lath and plaster,

had a similar effect to that of a vapor barrier on this same surface.

2.1 Interior Measurements

In massive or monolithic constructions, the previous method

was impracticable and another method was used to determine the dryness at or near the centers of concrete sections. The vapor barriers

were pipes or tubes ranging from l/l6 to 1-in. inside diameter and

from b in. to several feet in length. The tubes of less than 7/^ in.

inside diameter were fitted to short pipe nipples of that or greater

diameter, into which the hygrometer elements fitted; larger pipes

required no extra fitting as an adapter. The end for the insertion

of hygrometer element was closed with a pipe cap; the other end was

left open (except for a fine screen or gauze to keep concrete out

of the tube) and placed in the form before the concrete was -poured.

The eD.-^ictrical elements were spidered to stiff wire leads",' in turn

soldered to Kovar sealed connectors in the pipe caps. These

elements, with caps, were used for tubes over the entire diameter

range. The dye-spot cards, or strips cut therefrom, were used in

tubes of 5/lb-in. or greater inside diameter.

3. RESULTS

It should be kept in mind that equilibrium within the closed

measuring system is not necessarily indicative of equilibrium

between the specimen and the laboratory atmosphere. Usually, the

former is reached more rapidly than the latter. The following

paragraphs summarize data on the times required for the establishment of equilibrium within the measuring systems. For some systems,

the time lag was of the same order of magnitude as that expected

for the specimen, thereby limiting the usefulness of the method.

3.1 Exterior Measurements

The results were very good for smooth surfaced materials, such

as plaster, having a vapor barrier behind, (or trapped air as mentioned in section 2.1. ) However, it was difficult to obtain a satisfactory peripheral seal to any but the best concrete surfaces. The

time lag, for thin plaster applications, between placing the element

and reaching equilibrium was as little as an hour for fairly wet

specimens, and probably not over a day for drier specimens, although

it was not always obvious at that time that equilibrium had been

established.

3.2 Interior Measurements

The data from electrical elements 1-1/2 in. from the open ends

of 1-in. inside diameter pipes, placed in concrete, indicated that

moisture equilibrium had been reached in from two to six hours

after placing the element within the tube. Data from electrical

elements in fittings at the closed ends of long small diameter tubes

indicated that equilibrium was not attained for much longer periods.

In some instances, three months or more were required to reach

equilibrium.

Tests were made in which the electrical elements, bare and in

pipe fittings, were brought to equilibrium with air at about

15 percent relative humidity and then exposed to the atmosphere of

a laboratory room controlled to about 50 percent relative humidity.

The times required for the elements to come to equilibrium with the
i t Air

:']f?i::\ .r ^eh-:^c

'L' 'bU- > 1-jr^T^ -TO

n

C 1 ti . I .»'qo>f ‘^t, -i‘.:'"";w j‘:\l/.

•

' \

1

‘^' .-’.i, .w-jj" -itir , --.t Y,i. -'.rv' !, ^

' . ' i ;3;t

'
- 4 -

latter atmosphere were: 1) bare elements, 2 to 3 min; 2) elements

1-1/2 in. from open ends of 9-1/2 in. long 1-in. inside diameter

pipes, about 1-1/4 to 2 hr; 3) elements in 1-in. nipples with

fittings to reduce to 5/l6-in. tubing, about 6 hr; 4) with nipple

and fittings, plus 38-in. of 5/l6-in. tubing, about 2 days.

The data from the dye-spot elements did not show as great a

spread of times required for the system to come to equilibrium.

This was due, presumably, to the fact that they were used in large

diameter tubes of comparatively short length, for the most part.

One application of these elements was in reinforced concrete

T-beams of 11-3/4-in. stem width. The data indicated that the concretes at the centers of the beams were still wet two years after

casting.

4. DISCUSSION AND COMMENTS

The exterior measurement procedure is well suited to direct

readings but requires a fairly smooth surface. The dimensions of

the vapor barrier should be large enough compared to the thickness

of the specimen so that edge effects due to lateral migration of

moisture are minimized. However, the area covered by the vapor

barrier must be small compared to the total surface area in order

not to restrict, excessively, the free drying of the specimen.

The pipe or tube method permits measurements at remote

locations. However, the use of long tubes of comparatively small

diameter seems likely to result in a significant time lag.

Comparison of the data from such systems having the open ends in

concrete with those from similar systems having the open ends in

air shows that much, but not all, of the lag may be attributed to

the concrete. This is so because water vapor migrates through

concrete more slowly than thr.ough air.

In addition to lag, the use of very long, thin tubes may lead

to erroneously low readings. V'hen the ratio of the volume of air

trapped to the area of the open end (the concrete area from which

moisture is taken) is quite large, the concrete near the open end

may be dried out significantly in the process of providing the

moisture necessary to bring the trapped air to equilibrium. This

equilibrium humidity would not be representative of the relatively

wet concrete at the center, but of the somewhat drier concrete at

or near the normal surface.

Experience has shown that tubes of small diameter may be

subject to. mechanical damage that reduces the effective diameter

or closes the tube completely. This damage may occur during the

pouring of the concrete or during later handling of the specimen.

- 5 -

The particular dye-spot elements used were adversely affected

by the extremely high humidities experienced shortly after specimens were cast. If the card was subject to very high humidities,

the dyes ran and were no longer reliable. No attempt was made to

determine the effect of very high humidities on the electrical

elements; they were, however, of a type which makes use of an ion

exchange resin which is claimed by the manufacturer to be less

subject to damage resulting from moisture condensation than other

units using hygroscopic salts.

5. RECOMMENDATIONS

The vapor barrier for exterior measurements should be large

compared to the specimen' s thickness but small compared to its

surface area.

Individual dye-spot elements should be discarded if the dyespots have run, due to very high humidities.

The exterior measurement system is of doubtful value for

thick concrete specimens.

The pijDe used for interior measurements should be as short

as possible and of sufficient diameter to permit insertion of the

hygrometer element if this is compatible with the specimen dimensions.

The pipes should be capped whether or not an element is in the

pipe, to minimize localized drying of the material adjacent to the

open end.

Relative humidity of 70 percent, for air in moisture equilibrium

with the specimen, should be low enough to indicate satisfactory

dryness. Because of possible lag in the system, however, the data

should have established a clear trend of decreasing humidity.

REFERENCE

Some of the general principles of this type of measurement have

previously been described in the following reference:

"A Method for Determining the Moisture Condition of Hardened

Concrete in Terms of Relative Humidity" by C. A. Menzel,

Proc ASTM V 55, 1955-

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 Instru- ments. 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. Electrodeposition. 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 Basie 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.
1)J)