NATIONAL BUREAU OF STANDARDS REPORT

3368

THE FIRE RESISTANCE OF PRESTRESSED CONCRETE BEAMS

by

J. V. Ryan

Report to

Bureau of Yards and Docks

Department of the Navy

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
U. S. DEPARTMENT OF COMMERCE

Sinclair \X'eeks, Secretary NATIONAL BUREAU OF STANDARDS

A. \ . Astin, Director

THE NATIONAL BUREAU OF STANDARDS

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^Office of Basic Instrumentation 0( fflice of Weights and Measures.
NATIONAL BUREAU OF STANDARDS REPORT

NSS FIS©JE€T ms REPORT

1002-30-4822 June 1, 1954 3368

THE FIRE RESISTANCE OF PRESTRESSED CONCRETE BEAMS

by

J . V . Ryan

To

Bureau of Yards and Docks

Department of the Navy

Project Order No. 61705

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

The publication, reprl part, is prohibited

unless permission is ot APProvcd for public release by thedardSiWashington 25,D,C. Such perml Director of the National Institute ot r t has been specifically prepared if thai Standards and Technology (NIST) >rt for its own use, on October 9, 201 5.

The Fire Resistance of Prestressed Concrete Beams

by

J. V. Ryan

Abstract

Six fire-endurance tests of prestressed concrete

beams were made to supplement the results frcm

an extensive program in England which was designed to investigate the fire resistances of

large beams by determining the fire resistance

of geometrically similar scale models at various scales. Of the six beams tested here, five

were at 4/5 scale and the sixth at l/2 scale.

Variables included beam section, concrete cover

to the prestressed cable, and the use of lightweight insulating concrete as an outer shell.

The test results reported provide the information on the behavior of prestressed concrete

during fire exposure and tend to confirm the

scale relationship previously observed with

smaller specimens during fire tests in England.

1. INTRODUCTION

The use of prestressed concrete construction has become

increasingly popular during recent years, largely because of

the reduction in the amount of steel and concrete necessary

to provide the required design strength and load capacity.

This results in lower dead loads and conservation of building

materials

With the more frequent use of prestressed construction

the fire resistance behavior of such constructions became important. A research program to study this behavior was therefore started at the British Fire Research Station, Elstree, in

cooperation with the Building Research Station. A survey of

prestressed concrete applications in large buildings led to

the selection of a 20 ft span beam as the basic unit to be investigated. Because the size of the floor testing furnace

would not permit tests of 20 ft long beams, the program was

carried out on geometrically similar small scale beams in the

belief that a relationship between fire resistance and scale

would be established by which the fire resistances of larger

scale beams could be predicted,, Tests were made on beams of

three different design types at scales of l/4, 3/8, and l/2

of the 20 ft basic unit under various conditions of loading

and restraint.

The results of the tests seemed to indicate a linear

relationship between geometric scale and fire resistance for

T-section beams with a single cable of prestressed wires, designated type A, and it appeared desirable to extend the

range of information by obtaining data on such beams at larger

scales. The National Bureau of Standards offered to perform

tests on a series of beams of larger size because of its interest both in prestressed concrete construction and information on the value of tests of small models for determining

what performance may be expected from large structures. The

six tests described herein were undertaken in cooperation

with the Bureau of Yards and Docks, U. S. Navy. The test

specimens were built at the Building Research Station in

England and shipped to this country by the Fire Research Stati on

.

2. TEST SPECIMENS

Each of the six test specimens was different from all

the others in some respect, but they had points of similarity.

Three were type A beams (T-section, single cable) one of l/2

scale, for the purpose of correlation with the tests at the

British Fire Research Station, one of 4/5 scale, and the third

4/5 scale with 1-in. protection of reinforced vermiculite concrete on bottom and sides of the beam and t her under side of the

top slab. A fourth beam was type D, 4/5 scale, similar to

type A, but having a greater concrete cover to the cable. The

fifth and sixth beams were type E, 4/5 scale, identical to

type A except that type E had no top slab. The maximum compressive stresses in a fully loaded T-section beam (types A

and D) occur at the slab's upper surface, which is not exposed to the fire. The E type beam was introduced in order

that the zone of maximum compressive stresses, located at the

top of the rectangular section, would be within the furnace

and therefore experience more severe fire exposure. One of

the type E beams had 1 in. of vermiculite concrete on bottom

and sides.

2 .

'

The beams were designated by National Bureau of Standards

fire test numbers and by a three-part system of notation used

by the British, which consisted of type letter, scale fraction, and thickness of vermiculite concrete protection in

inches. For example, A:l/2:0 indicated an A type beam of l/2

scale with no (0?in.) vermiculite concrete protection. The

test numbers and beam notations are given in table 1.

Each of the beams was of uniform cross section over the

center half of the span, increased in depth by bottom haunches

to near each end, increased in width immediately beyond the

haunches, and were of constant depth and width at each end

bearing. The haunches were incorporated in the design to

allow the use of straight prestressed cable cavities and still

provide the greater bottom cover near the ends that would be

found in beams in which a curved cable and cavity were used.

The ends of the prestressed cable and their anchorages,

which extended beyond the ends of the beam proper, were encased in concrete. The dimensions of the various test specimens are given in figure 1. The prestressed cable in the l/2

scale beam consisted of 32 steel wires of 0 o 103 in. diameter

(12 ga British Standard Gage) while the cables in the ip/5- scale beams each consisted of 24 steel wires of 0.2 in.

diameter. The wires were of cold drawn steel having tensile

strengths as shown in table 3. The vermiculite concrete protection had been cast in 1-in. thick slabs with 1 l/2-in.

hexagonal mesh O.OI4.O in. diam (No. 19 B.S. Ga) wire fabric at

the mi d- thickness , cut to the desired sizes and shapes, and

placed to line the molds of the beams to be so protected. The

gravel aggregate concrete for the beam proper was then cast

into the lined mold, forming a direct bond with the vermiculite

concrete slabs. The pattern of protective slabs, and the joints

between, is shown in figure 2.

The

slabs of

valley

.

selected

28 days, when tested

strengths of 6-

concrete aggregate for the beams, including the top

the T-sections, was a river gravel from the Thames

The mix proportions and water cement ratio were

to give a compressive strength of 6000 ib/in. 2 at

in the form of 6-in. cubes. (The

by 12-in. cylinders may be estimated by multiplying these values by 0.85, giving about 5100 lb/in/). For

the A:l/2:0 beam the proportions by weightwere appr oximateljr

1 part of rapid hardening portland cement, 1 3/4 parts of sand

(3/l6 in. max size), 2 5/8 parts of gravel (3/l6 to 3/8-in.

size). For the 4/5 scale specimens, the proportions by weight

were 1 part of rapid hardening portland cement, 1 l/2 parts of

sand (3/l6 in. max),ipart of gravel ( 3/16 to 3/8 in. size),

and 2 parts of coarser gravel (3/8- to 3/4 in. size). The

water-cement ratio by weight was O .48 for all specimens. A

3 .

petrographic survey of a sample of gravel recovered from one

beam after test indicated the composition to be 95 to 98

percent chert (or flint). This gravel is one of the most

common concrete aggregates in use in Britain; the particular

gravel selected was a fair representative of its class.

The rectangular portions of the T-beams were cast first,

with cas tellati ons along the top. The rectangular eliemeht

was stressed seven or eight days after casting, when the

cube strength of the concrete was at least 4500 lb/in. 2, the

initial stress in the cable being l45>600 lb/in. 2 (65 long

tons/in. 2), A grout of rapid hardening portland cement and

water, in the ratio of 0.4/1 by weight, was injected into

the cable duct when tensioning was completed. The slabs for

the A and D type beams were then cast on the castellated

upper faces of the beams. During the period between completion of manufacture and shipment from England the specimens were stored in the open. During the 2 l/2 to 3/1/2 month

period between arrival at the National Bureau of Standards

and testing, they were stored indoors at approximately 70° F

and 50 percent relative humidity.

3. TEST METHOD AND EQUIPMENT

The beams were subjected to tests under the provisions

of British Standard Ip 76 , Fire Tests of Building Materials

and Structures, with additional data taken to provide the

basis for comparison with ASTM E119-50, Standard Methods of

Fire Tests of Building Construction and Materials. The former requires that a beam continue to support the test load

(representing the assumed dead load plus 1 l/2 times the design load) while exposed to furnace fires controlled to produce temperatures as close as possible to: 1000° F (5/8° C)

at 5 min, 1300° F ( 704 ° C) at 10 min, 1550° F (8/3° C) at 30

min, 1700° F (927° C) at 1 hr, l850° F (1010° C ) at 2 hr,

2050° F (1121° C) at 4 hr, and 2200° F (1204° C) at 6 hr.

The furnace temperatures were measured by 12 chr omel-alumel

thermocouples of 18 ga wire encased in ceramic insulators

but with the junctions and about 1 in. of the wires bare.

Six other thermocouples were located in the concrete and six

on the wires of the prestressed cable. The former were 19 ga

chr omel-alumel while the latter were steel-constantan, employing an individual wire of the cable as half of each thermocouple. For those beams (tests 334 and 338) with vermiculite

concrete protection, other thermocouples of 19 ga chromelalumel wire were located at the inner face of the vermiculite

concrete. In addition, 12 thermocouples in wrought iron pipes

(per ASTM E119-50) were distributed throughout the furnace to

provide comparative data. However, the temperatures measured by the bare thermocouples were used as the indication

of proper furnace control in each test. The locations of

the various thermocouples in the beams are indicated in

figure 3.

Each beam was placed in the floor test furnace, in special supports prepared to receive it. The beam was located

mid-way between the furnace sides with the underside of the

top slab level with the undersides of the other concrete slabs

used to fill out the remainder of the furnace opening. A gap

of about 1 in. was left between these slabs and the test specimen, which gap was filled with mineral wool insulation. Separate slabs were cast to be placed on the type E beams. These

slabs were made about 5-in. deep except at the center and

quarter points where glass fiber inserts, 2- by 5- by 32 in.

kept the top slabs from being rigid and continuous. Full

length reinforcing bars tied the slab sections and inserts together without rigidity. This was done to provide a flexible

method of loading the beam and closing the furnace opening.

The beams were. loaded at four points, the l/8, 3/8, 5/8 and 7/8

points of span, by means of the mechanism shown in figure 4*

The loads applied to the various beams, as well as other data,

are given in table 1. The computed stresses in the concrete,

before and after loading, were as given in table 2. The beams

were tested under conditions described as loaded and unrestrained against longitudinal expansion.

4. RESULTS

The* results of the tests are given as logs of test observations, plots of the temperatures recorded, and representative photographs.

4.1 Test 331-A:l/2:0 beam

During the first 1 hr 10 min the beam showed no heat

effects other than reddening of the edges and some deflection;

at 1 hr 25 min, cracks across the beam were observed near

center span and the deflections were increasing rapidly;

1 hr 29 min, crack across soffit, deflection nearly 4 in.,

load removed; 1 hr 37 min, gas off.

The temperatures indicated by the steel cable' thermocouples changed rather erratically during the test. However,

the furnace temperature control was- fairly good, and the temperatures at the various locations in the beam rose in smooth

curves, as shown in figure 5-

5 .
.
Table

1.

Summary

of

Results i mi •H I d o rO Tj

00 mi Ph

a o -p i

i d 0 O d i—l 0 o d mi mi mi d O i—1 d d •>0 P H mi 0 d —1 U H 0 d 0 Tj rH mi 0s 0 0 i—1 i—1

*H mi

d *tH

d *H r—1 •> > p 0 Odd •H r—1 o d OCO «H o a d &o a a ‘h CJM3 i—1 1 t>s X a W 0d do?) 0 £ 0 d 0 >

•H

O 0 a a m

•• •• ••AA • A

a 0 £ d

X 0 d d

EhH

g

•H 0 d •• •• i—i a ad -p > o da 0 o 3 0 i—1 aacd Eh a do

•Ha

a

d

a CO

d o 0 d a P

midoP • • • • • • • • • A a 0 CO fc>o_p 0 p d a

hO • d

CO «H . JkjH « f- • d

PQ

gs\ i—1

Hi oa d £‘ £ ° d

pp 0 w a a ca m • PQ 0 o

-pd

CDod

CDaao

ip co M3 MO o p • • A e A A p co 1 1 CO CO CO co O o CO CO co i—1 1—1

M3 CM CM co 1—1 o i—1 P MO CO p i—i P O M3 P CO a i—1 i—1 1—1 O co CO CO 1—1 a i—1

M3 CO co

a i—i

M3 o P a

p MO 1—1 X 1 0 O© CO 0 0 0 mi 0 CD a d £ a M i—i 0 © £ is! £ a a o d a o a w d d o d C3 © a © d d d O o 0 d 0 d P a PQ ° a o a d PQ a PQ rQ o O M3 CM

CM at CO o MO co • A A • • • • • A A A A 1 1 CO CM a 1—1

at

Oo

at

•n -d"

C\J

CO

CO

cm

CO

CMPh

PQO• • CMi—I

CO

CO

Oo

CM

•n CO

MO

CO

CM

CM

at" CO

Pi

PQO* m MO\-d- • • P

CMco

co

O

vO

•n co MOa

MO

CM

co

CM

PL, PQO• A MO>0 c

co

coco

OO

vO

an CO MO

CMvO

CMO

co

PQ

PQ

MO\at• •

at

coco

O

CM

ON vO

CM

CM sO

CM

CMCO

PQ

PQO• • MO\at« • W

MO

co

co oo

CM

on M3

CMPvD

CM

co

co

PQ

PQ

MO

-d-« •

M3

CO

CO 6

1/

System

of

notation

introduced

by

B.R.S.:

Letter

refers

to

beam

type;

fraction

to

scale,

final

number

to

thickness

of

vermiculite

concrete

in

inches.

2/

Defined

as

ratio

of

area

under

curve

of

average

furnace

temperatures

to

area

under

Standard

Time-Temperature

Curve,

each

area

above

20°C

(68°F)

base

line.

Table

2.

Applied

loads

for

the

different

beams

and

estimated

stresses

in

the

concrete

OJ -P • •H d ft s

•H

d ftd \ 0 O 0 d •H 0 d 1 1 1—1

ft 4-5 ft • • ft <3 O

-P CCS •• •* •• •• 0 d 0 0 i d

-P •H ft

O i—1 o 0 d

ft

d P d bOd o d d d o 4-5 ft d ft 0 ft < O p ffl hOd -p ft 0 1 o

-P 0 d d -P 0 0 d d d d 0 d d *H ft

-P 0 jZ) 0 0 0 -P d ft d o <3 ft d d O i—| o 40 0 o

40 ft 0 O

-p

40

d •H

•H ft e d ft d 0 O 0 0 •H 0 d 0 i—1 0 ft 40 ft 0 ft <3 O d d • # •• •••• -p d 0 0 ! d

•H ft oft 0 o 0 d > d 40 d hod

•H

d ci

d 0 o 40 ft d ft 0 1—1 <J O 40 « ©d -p ft a 0 1 O s 0 d o -p 0 0 d o d d d 0 d ft ft d d fb 0 0 0 o • • o • • •• • -p ft d a 0 ft d & PQ o i—I •H 40 0

-P0 40 ft w <3j O d0

•H

ft

d

ft

d

ft o

<$ ft01 1 do

cn • *0 i d , ft o °

>ift 0 ft E-4 O 0 ft o o o o

CO CO o CO

ir\ 1A

d LA1 o O o o

CO OJ CO pf

CO 00 p± CO

00 o o o

CO OJ o 1 CO CO CO 1 o o o 1 OJ o CO 1 C\J OJ co 1 1—1 1—1 o o o o

cO CO

d o

vO to o— 0- 1—1

ft 1—1 o o o o 1—1

CO OJ 0- - 1—1 o o o 1 1—1

00 pf o o o

so CO (A I o o o o o o o o

p± vO 00 00

•N •\ r\

a

CO CO

d

1A 1A 00

C\J 1A LA LA \ \ 1—1 Pt pf -d" c <u o 7

The day after the test, the beam had recovered some of

the deflection and a reloading test was made „ The beam failed

under a load of 10. 4 long tons, almost the same as the test

load. Following the reloading test, when the specimen was

broken up, it was found that all of the wires in the prestressed

cable were unbroken but the steel-cons tantan junctions were

rusty, possibly accounting for the observed variations in cable

temperatures. The conditions of the beam after the fire test

and after the reloading test are shown in figures 6 and 7.

4.2 Test 332-D:l+/5:0

During the first hour the beam showed no heat effects other

than reddening, some deflection and emission of steam; at 1 hr

10 min, two small pop- outs and four short cracks in beam; 1 hr

13 min, crack 1 to 3-in. from bottom of beam runs full length,

about l/8-in. wide; 1 hr 30 min, cracks wider; 1 hr ip 3 min,

considerable concrete fell from bottom and one side of south

haunch, about 2-in. deep; 1 hr 54 min, cracks along beam up to

2-

in. wide, fall of concrete from north haunch; 1 hr 56 min,

concrete down from 3 to 4 ft of south haunch and beam, 2- to

3-

in. deep; 3 hr 31 min, considerable concrete fell from west

face of beam near center of span; 3 hr 38 min, concrete fell

from full width of beam, north half, 3-in. deep, pressure in

loading system unsteady; 3 hr 40 min, beam broke and collapsed;

3 hr 4l min, gas off.

The furnace . control was fairly good, the average temperature having been near or slightly above the Standard curve

throughout the test. The temperatures at the various locations

in the beam rose to about 212° F (100°C) and leveled off for

periods ranging from 10 min to 1 hr 10 min before continuing

upward, apparently indicating time required to eliminate moisture.

The cable temperatures behaved in a similar manner but remained

near 212° F about 1 3/4 hr. The temperature curves are shown

in figure 8. . After the test, the beam and slab were found to have

broken in the middle, and the bottom concrete had broken off

to expose the prestressed wires from haunch to haunch. The

breaks in the broken wires were not all in one location but . were divided into two locations, as shown in figure 9. All the

wires were broken.

8 .
I '
4.3 Test 333-A:l;/5:0

At 12 min, water was dripping from the beam at center

span; 15 min, four or five puddles of water, each 3 to 10 in.

across, on top of slab near each quarter point; 20 min, two

transverse hairline cracks in top slab near each quarter

point, water or steam therefrom bubbling into covering puddle;

34 min, water from beneath steel plates on top slab at two

loading points; £3 min, short longitudinal crack 1 in. from

bottom of east side of south haunch; 1 hr 23 min, eight transverse cracks in top slab ranged from hairline to l/l6-in.

wide; 1 hr 32 min, 1 ft long crack 1 in. from bottom of east

face near center span; 2 hrs, cracks in beam l/8- to l/4-in.

wide; 2 hr 4 min, momentary drop of oil pressure in loading

system; 2 hr 9 min, concrete fell from bottom of beam along

north half, including most of haunch, in depth of 2- to 3-in.

over full width, temporary large drop in oil pressure; 2 hr

27 min, some wires of prestressed cable exposed, apparently

since 2 hr 9 min, concrete fallen from bottom edges in south

half; 2 hr 36 min, load failure as beam settled down quickly;

2 hr 38 min, gas off.

The furnace control was good, the average temperature

having been near that specified in the Standard Time-Temperature Curve throughout the test. The temperatures indicated

by the six thermocouples in the concrete rose smoothly during

the first 2 hr but those by two jumped quickly thereafter.

The temperatures in the prestressed cable behaved in a similar

manner, as shown in figure 10.

On the day following the test the beam had recovered

partially from the maximum deflection and the beam and slab

were cracked in several locations. They were cracked through

or broken at center span. Concrete had fallen from the

bottom to expose the lower side of the grouted cable duct

over the north half including most of the haunch, and to expose much of the cable in the south, again including most of

the haunch. Fifteen of the 24 wires in the cable were broken.

Their locations are shown in figure 11. The condition of the

beam is shown in figure 12.

4.1+ Test 3 34 “A : li/5 i 1

At 11 min there appeared to be a lq-in. diam pop-out in

the vermiculite concrete; 47 min, a piece of vermiculite concrete, 4

_ i n * long by l/2-in. wide, hanging down near south

end; 1 hr 49 min, full thickness of vermiculite concrete fell

from bottom of beam at south end for 2 ft; 1 hr 59 min to

2 hr 1 min, repeated loud popping *as concrete spalled

9 .

violently along 1 to 2 ft of beam and haunch in north, dislodging the vermiculite concrete; 2 hr 30 min, a few moist

spots on top of slab, each 10 to 12-in. diam; 4 hr, loud

thump and repeated popping as slab spalls away all but top

1 l/2 in. of depth along 2 ft of east edge north of center;

5 hr 39 min, loud metallic sound; 6 hr 2 min, test stopped,

gas off.

The furnace control was good, the average temperatures

being a little low only near the end of the test, and the

temperatures in the beam and cable rose gradually and

smoothly throughout the test, as shown in figure 13. Thermocouple Pp became inoperative at about 4 l/4 hr and Pl had

been found to be inoperative in a pre-test check.

Upon being unloaded and cooling over the weekend, the

beam recovered about one-third of the total deflection.

The vermiculite concrete slabs were thoroughly cracked and

the joints betwesn had opened. The beam had spalled at

each end and the slab in the north half on the east side,

as shown in figure 14 . The spalling had involved, or been

caused by, splitting of the aggregate rather than loss of

bond between aggregate and cement. This is shown in figure

15. The beam was subjected to a reloading test and failed

under a load of about 65,180 lb ( 29.1 long tons), exclusive

of the weight of the beam and slab. The beam and slab broke

at the worst of the spall in the slab, about 2 ft north Of

center, exposing the cable from that location to the north

end of the exposed length, as shown in figure 16. After

the reloading test it was found that none of the prestressed

wires was broken.

4.5 Test 335-E :4/5:0

During the first hour, the beam showed no fire effects

other than reddening and some deflection; 1 hr 19 min, two

full length cracks on east side of the beam, each about

l/8-in. wide, one 1 in. and the other 2 l/2 in. from bottom;

1 hr 24 min, at least 1 in. of concrete fell from bottom of

beam, full width along central 8 ft; 1 hr 43 min, more concrete fell from north end; 1 hr 48 min, more concrete fell

from south; 1 hr 55 min, pressure in loading system fell;

1 hr 55 l/2 min, beam collapsed; 1 hr 58 min, gas off.

The furnace control was fairly good, the average temperature being a little low during most of the test. The

temperatures in the concrete and cable rose smoothly at first

but jumped after 1 l/2 hr as shown in figure 17, probably due

to cracking. Three of the thermocouples in the concrete became inoperative before the end of the test.

10 .

The beam had broken at three places, roughly the center and the quarter points. The cable was exposed over the

central lip to 15 ft but only two wires were broken, as indicated in figure 11, The concrete of the beam was much

more "burned out" than in previous tests, and could be

crumbled in the hand except that from the ends which had been

in the protected supports.

4- „ 6 Test 336-E : l)./5 :

No fire effects other than reddening and deflection

were observed until 5l min, when the joints between vermiculite concrete slabs appeared to have opened about l/4 in,;

1 hr 28 min, joints slightly wider, possibly l/2 in.; 1 hr

50 l/2 min to 1 hr 55 min, repeated loud popping and spalling

of beam in north until vermiculite concrete slabs and some

of beam down from bottom of north haunch and adjoining 1 ft

of central section; 3 dr 30 min, no change since 2 hr except

some loosened vermiculite concrete fell; 4 hr 29 min, loud

thump, temporary drop in oil pressure; 4 hr 39 l/2 min, beam

"sat down" in slow collapse; 4 hr !pl min, gas off.

The furnace control was good, the average tanperature

having been very near that specified by the Standard TimeTemperature Curve throughout the test. The temperatures in

the concrete and cable rose smoothly and gradually, except

that indicated by thermocouple Pp which jumped at about 4 hr,

as shown in figure 18 , After test, the beam was found to have broken at the

center of the span and a considerable amount of concrete was

off the north half, exposing the cable. Twelve of the 24

wires in the cable were broken, the breaks having been about

4 ft north of where the beam broke. A large piece of concrete

had sheared off the top, as shown in figure 19. The spalled

areas showed shearing of the aggregate, as in the test of the

A: 4/5:1 beam.

5. DISCUSSION

5.1 Size of Beam

The fire endurance of the A:l/2;0 beam (test 331) observed in this series of tests was about 91 percent of that

of a similar beam in the previous British tests, the latter

beam having been aged about one month longer, and was about

93 percent of that of a duplicate beam made and tested in

England at the same time as the beam in test 331. This

agreement was considered to be close enough to permit comparisons between the results obtained in this series and

ll f

those from the British Fire Research Station. The slight

differences between the fire endurances may have resulted

from differences in temperature and humidity conditions during aging.

The results obtained for the A:lp/5:0 beam (test 333)

were about 10 percent lower than those anticipated from an

extension of the linear relationship tentatively based on

the results from England. As the beams tested by the Fire

Research Station were of l/lq, 3/8 and l/2 scales and those

tested at the National Bureau of Standards of l/2 and lq/£

scales (A:l/2:0 and A:iq/5:0), predictions of the fire

endurance of a full scale beam involve an extrapolation

equal to 36 percent of the total range of available test

data. If it is assumed that the fire endurance is proportional to a linear dimension of the geometrically similar

beams, the predicted value for a full-scale beam A:1:0 would

depend somewhat upon the data used In making the extrapolation. Considering the data for the beams A:l/]q:0, A:3/8:0

and A:l/2:0 originally tested by the Fire Research Station

in England, the predicted value is about 3 hr lq2 min; using

only the data for the beams A:l/2:0 and A :!}./£ :0 tested in

Washington, it is 3 hr 20 min; and by giving equal weight to

all available data, it is about 3 hr 2]q min.

5.2 Concrete Cover to Cable

On the A and E type beams, the thickness of concrete on

each side of the cable cavity was slightly less than that

below, the difference having been l/8 in. on the l/2 scale

and about l/lO in. on the ]q/5 scale. However, on the lq/5

scale E type beam, the concrete thickness was about l/lO in.

greater on the sides than the bottom. This leaves some

question as to which thickness to consider; the bottom in

all cases, the side, or whichever is least on each specimen.

The orientation of the cable in the beam requires consideration of how extensive a "face” it presents to the heat penetrating the concrete. Since both sides of each beam were

exposed, the sum of the two sides of the cable made a greater

"face” than the bottom In all cases. However, the bottom

concrete was subject to exposure from three sides and often

showed evidence of greater heat damage than the sides, as

evidenced by cracks, crumbling, and spalling. Theref ore , the

thickness of concrete below the cable appears to have been

more significant than that on each side, so long as they were

as nearly equal as in these test beams. The results obtained

in the three tests of unprotected T-section beams (tests 331,

332, 333 ), when plotted as fire resistance vs thickness of

concrete below the cable, gave points approximating a straight

line. Interpolation on this line for the Iq- in. thickness expected on a full scale type A beam gives 3 Hr 20 min as the

12 .

predicted fire resistance for such a beam. Extrapolation

along a line based on the results of the two A type beams,

alone, would predict a resistance more nearly 3 3/4 dr . However, the lower value of 3 hr 20 min, having been determined

by consideration of one more data point and by interpolation

rather than extrapolation, should be more nearly correct.

The cracking and crumbling of the concrete mentioned

above would become increasingly important at the larger

scales. It would appear desirable and might prove necessary

to Incorporate some reinforcement in the concrete, such as a

cage or basket of wire mesh located about 1 in. beneath the

concrete surface. Such reinforcement should prevent loss

of concrete.

5.3 Insulation

The 1-in. thick reinforced vermiculite concrete insulation used in two tests proved very effective in extending the

period of fire resistance. The unprotected A;4/5‘0 beam had

failed at 2 hr 36 min whereas the insulated A:4/5:l beam

withstood the fire exposure during the maximum period defined

in BS476, 6 hours, without failure and, after cooling, retained strength to support a load up to 13, 000 lb greater

than that applied during the fire test. The ratio of fire

resistance periods for the protected beam compared to the unprotected was 2.32/1. However, since the A:4/5:l beam had

not failed at the end of the test, this ratio would have been

higher if the beam could have been tested to failure. Very

similar results were obtained in the tests of the E:4/5*0 and

E:4/5:l beams (tests 335 and 336), respectively. In this

case, however, both beams broke. The fire resistance of the

former was 1 hr 56 min and of the latter was 4 hr 40 min.

The ratio of fire resistance for the E:4/5:l to that for the

E:4/5:0 was 2.42/1 of the same order as that for the A beams.

It is believed that about 1 l/4-in. thick vermiculite concrete would be required on full scale beams in order to give

the same ratio between the fire resistances of protected and

unprotected beams.

The only violent spalling observed in this series of

tests occurred in the two tests of beams with insulation of

1-in. reinforced vermiculite concrete. Each of these beams

spalled explosively along the haunches and ends of the

central section. Spalling near one end occurred in two of

the four British tests of A-type beams with 1 in. of vermiculite concrete insulation but did not occur in any other

tests, either with greater thicknesses of the same type of

insulation or with no insulation. * The spalling might have

13 .

been caused by stresses resulting from excessive local heating through cracks along the joints between adjacent vermiculite-concrete slabs at the transition from central section to

haunch. It is also possible that the vermiculite concrete

may have retarded the escape of excess moisture from the beams

during curing. Previous observations during fire tests of

dense concrete having high moisture content have indicated

greater spalling than that observed during- tests of more

thoroughly dried specimens. Neither of the foregoing possible explanations is considered completely satisfactory; they

are suggested, however, as possibilities.

5. If Shape of Section

The difference between A and D type beams was largely

one of thickness of concrete around the cable, as discussed

in section 5.2. That between the A and E beams, however, • was in the top slab’s contact with the beam. Since the cover

to the prestressed cable was the same in each type, this factor should not enter Into any difference In fire resistance.

The intimate contact between beam and slab in the A type was

such that the top slab would dissipate heat from the concrete

and assist in supporting the load. This assisting effect was

of greatest importance late in the fire exposure as the concrete at the bottom of the beam lost strength. Loss of

strength to a given depth in the concrete should require

about the same time in each type but would represent a larger fraction of the total load bearing section in the

type E beam than in the type A, thereby leading to the

earlier failure of the former.

5.5 Cable Temperature

Of the six thermocouples in the prestressed cable, four

were located at the center of the span. Although the temperatures of the prestressed wires measured during the tests

may not have been the maxima over the full spans of the wires

due to the small number of thermocouples and their concentration, the wire temperatures at which failure should be expected can be predicted from the results of a study made in

England. The English data on the strength- temperature relationship for prestressed wire used in their beams indicates

a 50 percent loss of strength at about 750° F (about 1+00° C) . Similar data 1/ on steel for convent! onally reinf orced concrete

l/ Mechanical Properties of Metals and Alloys, National

Bureau of Standards Circular Cl+1+7, Dec. l,19l|3> pp.252-

251+

.

111 .
/

I

>

.
shows a 50 percent loss of strength at 1000 to 1060°P

(about 54 0 to 570° C). Thus if beams of the two types of

construction are fabricated using a factor of safety of two

in their designs and equal concrete cover, it would be expected that failure would occur in the prestressed beam at

an earlier time during fire tests of the two beams.

The average and median temperatures of the prestressed

wires measured at the time of failure in each test are given

in table 3 as degrees F above the predicted 750° F.

Table 3. Temperatures at Failure,

degrees F above 750° F

Beam Average Median

A: 1/2:0 -140 -142

A: 4/5^0 + 38 -124

D : Ip/5:0 +319 + 322

E : Ip/5:0 + 11 -277

A: 4/5:1 -52* - 95*

E:i*./S:l -239 -248

*»* Temperature at end of 6 hr exposure without

failure

.

The fact that these temperatures were lower than expected in

most cases indicates that the wires in the prestressed cables

were heated to failure at locations somewhat removed from

the central location of the thermocouples. Examination of

the beams after the tests showed that most of the breaks and

drawnout sections in the wires were at least 1 ft from the

thermocouple junctions.

5.6 Deflections

The curves of deflections versus time in figure 20 show

large early deflections followed by a steady state period

for the unprotected beams, but a slow more or less uniform

increase of deflection for the insulated beams. This effect

is believed to be one of differential thermal expansion. As

the concrete at the beam soffit, and the bottom of the slab

of an A or D beam, is heated rapidly at the start of the

fire exposure it expands while the concrete in the interior

of the beam and the top of the slab remains cool and does

not expand. In these beams of high strength concrete, a bowing toward the hot surface resulted. For the E:l|/5:0

15 .

beam, the effect was of reduced magnitude since the separate

top slab would not offer much restraint against the expansion

of the upper surface of the beam. The vermi culi te-concre te

insulation prevented the rapid heating of the beam soffit,

thereby diminishing the tendency for large deflections after

the initial fire exposure.

6. SUMMARY

These tests provide data which, coupled with the data

from tests at the British Fire Research Station, provide the

basis for a linear relationship between fire resistance and

scale of geometrically similar A type beams by which the fire

resistance of full scale A beams may be predicted. This relationship may be expressed as:

Rp = 218 S - 14

where Rp is fire resistance in minutes and S is scale. For

an A:1:0 beam (S = l) , this formula gives a predicted Rp of

3 hr 2l| min. In view of the range of data, S should not be

taken greater than unity. The D and E type beams were

tested only at 4/5 scale so no relationship may be proposed.

The thickness of the concrete cover over the reinforcing cable is one of the several variables which affect the

fire resistances of prestressed beams. The results of these

tests indicate it may be one of the most important. The

D: 4/5:0 beam had 4*4 in. cover and a fire resistance of

3 hr 40 min, the A:4/5:0 beam with 3.0 in. cover had a fire

resistance of 2 hr 36 min, and the A;l/2:0 beam with 2 in.

cover had a fire resistance of 1 hr 29 min. An A:1:0 beam,

with 4“i n . cover would be between the first two and would

be expected to have a fire resistance of 3 hr 20 min on the

basis of cover thickness. With the thick covers of 3 in.

and more it would appear advisable to include some light reinforcement such as wire mesh not more than 1 in. from the

concrete surface to prevent the loss of concrete during fire

exposure

.

The use of vermiculite concrete well bonded to the

structural concrete, as when used as a form liner for the

latter, will greatly increase the fire resistance which may

be expected from a given size beam. The use of 1 in. of

vermiculite concrete on 4/5 scale A and E beams gave 2 l/3

to 2 l/2 times the fire resistance of the unprotected beams

and might be expected to give at least 2 times the fire resistance when used on A:1 and E:l. beams. Although beams so

protected showbd a tendency to spall violently, which was

not shown by the other beams, this tendency is more than

offset by the increased fire resistance.

16.

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17

Table

5.

Results

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18

L

* F r r

^

SIDE ELEVATION

TYPES A SD TYPE E

SECTIONS THROUGH CENTER

OF SPANS

DIMENSIONS OF TEST SPECIMENS

IN INCHES

TYPE - A A D E SCALE

% % %

L - 126 201.6 201.6 201.6 F

- 7 11.2 11.2 11.2 G

~

6 9.6 9.6 9.6 H

~ 20 32 32 32 J

- 60 96 96 96 s

- 20 32 32 -

A - 3 4.8 4.8 -

B - 5 8 12 8 C - 10 16 16 16 D - 9 14.4 15.4 14.4 E - II 17.6 17.6 17.6 R - 3 4.5 5.5 4.5 X - 1.25 2.16 2.95 2.16 Y - 2 2.95 2.16 2.95

FIG. I DIMENSIONS OF TEST BEAMS

1 1.2“ 11.2"

1 LfJ

0)2 CD 11

SIDE

BOTTOM

DIMENSIONS FOR INSULATION ON PROTECTED BEAMS

JOINTS BETWEEN '£>“ VERMICULITE CONCRETE SLABS WERE 1^" TO WIDE— GROUT FILLED

FIG. 2 ARRANGEMENT OF VERMICULITE- CONCRETE SLABS ON INSULATED BEAM (E>4/5 : 1)

TYPE A \

BEAM

-4- CONCRETE - Cl TO C6 AT CENTER OF BEAM

® CABLE-WI TO W4 AT CENTER OF BEAM W5 S W6 4 FT FROM CENTER OF BEAM # INSULATION - PI TO P4 FOR TYPEV PI TO P3

FOR TYPE"E" ALL AT CENTER OF BEAM

-{•CONCRETE - Cl TO C6 AT CENTER OF BEAM

• CABLE-WI TO W4 AT CENTER OF BEAM W5 8 W6 2^ FT. FROM CENTER OF BEAM

TYPE 0 % BEAM

-{-CONCRETE- Cl TO C6 AT CENTER OF BEAM

® CABLE-WI T0W4 ATCENTEROF BEAM wsa W6 4 FT FROM CENTER OF BEAM

FIG. 3 THERMOCOUPLE LOCATIONS IN BEAMS
.
FIG. 4 LOADING APPARATUS IN PLACE BEFORE TEST

"TIME, HRS.

FIG. 5 TEMPERATURES OBSERVED DURING TEST 331

TEMPERATURE,

FIG. 6 A : l/2 : 0 BEAM AFTER FIRE TEST 331

FIG. 7 A ; l/2 0 BEAM AFTER FIRE TEST 331 AND RELOADING TEST

TIME, HRS.

FIG. 8 TEMPERATURES OBSERVED DURING TEST 332 . TEST STOPPED

BEFORE ALL 3HR.40MIN. TEMPERATURE OBSERVATIONS COMPLETED

FIG. 9, D‘4/5'0 BEAM AFTER FIRE TEST 332 , SHOWING BREAKS IN SLAB

BEAM, AND CABLE, THE LATTER GROUPED IN TWO LOCATIONS.

TEMPERATURE,

TIME, HRS.

FIG. 10 TEMPERATURES OBSERVED DURING TEST 333

TEST 331 - 1/2 SCALE TYPE A - NONE BROKEN

TEST 332 - 4/5 SCALE TYPE D -ALL BROKEN

TEST 334-4/5 SCALE TYPE A WITH VERMICULITE CONCRETE-NONE BROKEN

X- BROKEN

333

4/5 SCALE TYPE A

0- UNBROKEN

335

4/5 SCALE TYPE E

0 0 0 0

X 0 0 0

X X X X

X X X X

X 0 X X X X 0 X

X 0 0 0

0 0 0 0

0 00 0

0 0 0 0 0 0 0 0

0 0 0 X

336

4/5 SCALE TYPEE

WITH

VERMICULITE CONCRETE

X 0 00

X X 0 X

X X 0 X

X X XX

0 0 0 0

0 0 0 X

FIG. LOCATIONS OF BROKEN AND UNBROKEN PRESTRESSED WIRES

TEMPERATURE,

FIG. 12 A-4/5: 0 BEAM AFTER FIRE TEST 333

1800

1400 o

-- 1000 — 800

.= -400

FIG. 13 TEMPERATURES OBSERVED DURING FIRE TEST 334

TEMPERATURE,

FIG. 14 A: 4/5: l BEAM AFTER FIRE TEST 334

FIG. 15 REPRESENTATIVE SPALLED AREA (SLAB) FROM TEST OF A= f

I BEAM

FIG. 16 NORTH HALF OF A- 4/51 BEAM AFTER FIRE TEST 334

AND RELOADING TEST

TIME, HRS.

FIG. 17 TEMPERATURES OBSERVED DURING TEST 335

0 12 3 4 5 TIME, HRS.

FIG. 18 TEMPERATURES OBSERVED DURING TEST 336

FIG. 19 E'4/5'l BEAM FROM ABOVE AFTER FIRE TEST 336

SHOWING SHEAR FAILURE AT TOP

2200

2000

-1800

1600

Li 1400 o

1200

1000

800

600

400

200

TEMPERATURE,

TIME ,HRS.

FIG. 20 DEFLECTIONS AT CENTERS OF BEAMS OBSERVED DURING TESTS, ALL DEFLECTIONS DOWNWARD

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