Thermal properties of fibre-reinforced concrete at elevated temperatures

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temperatures

Ser

I

THl

I R427

1+1

I no. 683

National Research Conseil national Council Canada de recherches Canada

Institute for lnstitut de Research in recherche en Construction construction

Thermal Properties of Fibre-Reinforced

Concrete

at

Elevated Temperatures

by T.T. Lie and V.K.R. Kodur

Internal Report No. 683

Date of issue: April 1995

This is an ~nternal report of the Institute for Research in Construction. Although not intended for general distnDutlon. it may be cited as a reference in otner publications.

THERMAL PROPERTIES OF FLBRE-REINFORCED CONCRETE AT

ELEVATED TEMPERATURES

T.T. Lie and V.K.R. Kodur

ABSTRACT

Experimental studies were canied out to determine the thermal properties of fibre- reinforced concrete at elevated temperatures. The effect of steel-fibres on thermal

conductivity, thermal expansion, specific heat and mass loss of fibre-reinforced concrete at elevated temperatures was investigated. Test data indicate that the steel-fibre-reinforced concrete, under elevated temperatures, exhibits thermal properties that are slightly

different from those of plain concrete. These studies were canied out as part of a research program aimed at developing methods capable of predicting the fire resistance of fibre- reinforced concrete columns.

THERMAL PROPERTIES OF FIBREREINFORCED CONCRETE AT ELEVATED TEMPERATURES

T.T. Lie and V.K.R. Kodur INTRODUCTION

~nvesti~ations'~.~ have shown that beneficial effects can be obtained by adding discontinuous discrete steel-fibres to plain concrete. The use of fibre-reinforced concrete, although limited at the present time, is likely to substantially increase once the

construction industry becomes aware of these benefits and has access to proper design guidelines.

The use of steel-fibres as reinforcement in plain concrete enhances the tensile strength of the composite system, reduces the amount of cracking under serviceability conditions and improves resistance to material deterioration as a result of fatigue, impact, shrinkage and thermal stresses. As a result, fibre-reinforced concrete can be used to improve the performance of structural members such as columns, floors on grade and oavements. and to reduce the section thickness of the members. Fibre-reinforced concrete

+

is also used in applications such as slabs in nuclear reactors, gravity dams and large furnace supports where thermal stresses could be significant.

In the past, the performance of structural members at elevated temperatures, in paaicular at temperatures encountered in fire, could only be determined by testing. Over the years, however, methods have been developed for the calculation of the fire resistance of various structural These calculation methods are far less costly and time consuming than testing. However, to perform these calculations, knowledge of the

thermal properties at elevated temperatures of the materials used to construct the members is required.

Previous in~esti~ations'.~ have concentrated on the pryerties of fibre-reinforced concrete under ambient conditions, and very little information is available on its thermal properties at elevated temperatures. The present study was undertaken to determine the thermal properties of fibre-reinforced concrete under the latter conditions.

The study was carried out as part of a research project on the fire resistance performance of concrete-filled hollow steel sections, which included the use of fibre- reinforced concrete filling. The study was conducted at the National Fire Laboratory

(NFL)

of the Institute for Research in Construction, National Research Council of Canada, with the support of the Canadian Steel Construction Council and the American Iron and Steel Institute.

THERMAL PROPERTIES OF FIBREREINFORCED CONCRETE

To be able to predict the fire resistance of a structure, the temperatures in the structure must be determined. For such calculations, knowledge of the thermal properties at elevated temperatures of the materials that comprise the structure is requised. Whereas these properties have been established for various commonly-used concretes , this is not the case with steel-fibre-reinforced concretes. In the investigations discussed in this report, the relevant thermal properties of various fibre-reinforced concretes at elevated temperatures were measured. These properties included thermal conductivity, specific heat, thermal expansion and mass loss of the various concretes at elevated temperatures.

EXPERIMENTAL DETAILS

Three types of concrete specimens, namely, NRCl, NRC2 and NRC3, were investigated. The NRCl specimens were made with siliceous aggregate, while the NRC2 and NRC3 specimens were made with carbonate aggregate. The NRCl and NRC3

specimens were reinforced with steel-fibres.

Three batches of concrete were made for preparing the three types of specimens. The concrete was designed to produce a 28day compressive strength of 35 MPa. In all three batches, general purpose portland cement for construction of concrete structures was used. The concrete mix in Batch 1 was made with siliceous stone aggregate while the mix in Batches 2 and 3 was made with carbonate stone aggregate. The fine aggregate for all three batches consisted of*silica-based sand. In order to improve workability, a

superplasticizer (Mifity 150 ) was added to all three batches and a retarding admixture (Mulco TCDA 727 ) to Batch 1. Pre-mixed concrete, including cement, coarse aggregate, sand, water and retarding admixtures, were supplied by Dufferin Concrete, Ottawa.

RIBTEC* steel-fibres of the XOREX' type1', supplied by Ribbon Technology Coxporation, were used as reinforcement in Batches 1 and 3. XOREX is a mild carbon steel with tensile strength of approximately 960 MPa. A typical XOREX steel-fibre is shown in Fig. 1. The cormgated shape of these fibres provides a strong mechanical bond to the concrete. The fibres, which were 50 mm in length with 0.9 mm equivalent

diameter, had an aspect ratio of 57. The mass percentage of steel-fibres in Batches 1 and

3 was 1.77 and 1.76, respectively.

The mix proportions, together with concrete and steel data for the three batches, are given in Table 1. The steel-fibres were added to the fresh concrete and mixed for about 2 minutes to ensure uniform dispersion. Vibrators were used to consolidate the concrete.

From each batch of concrete, the following specimens were made: 8 cylinders of 150

mm

diameter and 300 mm length

3bricksof200mmx100mmx50mm

Compression tests on the cylinders were conducted for each of the samples at 28 days after the pouring of the concrete. The 28-day cylinder compressive strengths are given in Table 1. The bricks were used for determining the thermal properties of the concretes.

The test specimens for the determination of the thermal conductivity and the thermal expansion were prepared by cutting the bricks to appropriate sizes. Specimens for the determination of the specific heat and mass loss were obtained by grinding a portion of the bricks.

' Certain commercial products are identified in this paper in order to adequately specify the experimental procedure. In no case does such identification imply recommendations or endorsement by the National Rescarch Council, nor does it imply lhat the product or material identified is the ben available for the purpose.

THERMAL CONDUCTIVITY

The thermal,lconductivity of the concretes was measured using a non-steady state conduction method

.

In this method, the temperature rise of a heating wire placed between two specimens is measured. The thermal conductivity of the specimen is

determined according to the following relation:

where:

k

=

the thermal conductivity of the specimen (W/m°C)

9 = the heat generated by the wire (Vim)

t,, $2 = time since heating switch is on (rnin)

TI, T, = temperature of hot wire at times tl and t2 (OC)

A schematic diagram of the system is shown in Figure 2(a). In this system, the thermocouple measures the temperature rise of the heating wire. This temperature rise is automatically converted to the thermal conductivity of the specimen according to Eq. (1) and displayed on the meter. A typical arrangement of specimen, heating wire and

thermocouple is shown in Fig. 2(b).

The fibre-reinforced specimens consisted of two bricks of 100

x

200 x 50 mrn in size. For the measurement of the thermal conductivity at elevated temperatures, the specimens were placed in a furnace chamber and exposed to heating until the desired temperature was reached, at which time the heating wire was activated and the thermal conductivity measured. The thermal conductivity of the three concretes was measured at room temperature, 50°C, 75OC, 100°C and, subsequently, at 100°C temperature intervals

up to 1000°C. Two tests were conducted for each concrete type, and the average value obtained in these tests was regarded as the representative value of the thermal conductivity of the concrete.

The thermal conductivity for the three types of concretes is shown in Fig. 3, where tabulated values and plots of the t h e m 1 conductivity of the three concretes are given as a function of temperature. The thermal conductivity for all three concrete types decreases with increase in temperature up to approximately 400°C. Above this temperature, the thermal conductivity was nearly constant.

The addition of steel-fibres increases the thermal conductivity of concrete at temperatures up to about 700°C, with the maximum increase at room temperature. This increase in thermal conductivity can be attributed to the fact that the thermal conductivity of steel is about 50 times higher than that of concrete. Previous inve~ti~ations'~ have shown that the thermal conductivity of the aggregates pximarily determines the thermal conductivity of the concrete. In the case of fibre-reinforced concrete, the steel-fibres also contribute to the thermal conductivity.

The thermal conductivity of fibre-reinforced siliceous concrete is higher than that of the carbonate concrete throughout the temDerature range investigated. This is due to the higher clystallinity of the siEceous aggregates as compared to &at of the carbonate aggregate. The higher the cystallinity, the higher the thermal conductivity and the rate of its decrease with temperature*.

SPECIFIC HEAT

The specific heat, at constant pressure, was measured using a ~ u ~ o n t ' Differential Scanning Calorimeter (DSC) for temperatures up to 600°C. The DSC measurements were carried out on the concretes without the steel-fibres, which were removed from the samples. The specific heat of the steel was taken inio account by applying the additive theorem", in which the known specific heat of steel was added to that of the concrete. The tests were conducted with a sample size of 30-40 mg in a nitrogen atmosphere and a heating rate of 10°C/min.

For measuring the specific heat above 600°C, a ~ u ~ o n t ' high temperature

Differential Thermal Analyzer (DTA) was used. Two runs of specific heat measurements were canied out: one with powdered sapphire ( A L 0 3 ) as the calibration material and one with the sample. The experiments were carried out in a stepwise heating program. In this program, a period of constant material temperature, during which the measurements were made, was followed by a period of temperature rise, and this was followed again by a period of constant material temperature in which measurements were made. This program was maintained in the 400-1000°C temperature range. The heating rate was 10°C/min. The specific heat of the sample, which is proportional to the temperature difference measured by the DTA, was calculated by comparing the temperature difference measured during the test of the samples with that of the powdered sapphire.

The specific heat of the three types of concrete is shown in Fig. 4, where tabulated I

values and plots of the specific heat of the three concretes are given as a function of the

I

temperature. For all three types of concrete, the specific heat shows a peak at

I

temperatures near 100°C and 425°C. The first increase is caused by evagoration of free

water and the second by removal of crystal water from the cement paste

.

In these temperature regions, most of the heat supplied to the concrete is used for the removal of water and only a small amount is available for raising the temperature of the material. As a consequence, the specific heat increases substantially in these temperature regions.

The specific heat of concrete is also affected by other physicochemical processes that occur in the cement paste as well as in the aggregates at temperatures above 500°C. The increase in specific heat for the fibre-reinforced siliceous aggregate concrete, at about 550°C, can be attributed to the presence of quartz, which transforms in this temperature region. The steep increase in specific heat, for both the plain and the fibre-reinforced carbonate concretes, at about 750°C is due to the presence of d~lomite in the aggregate, which disassociates and absorbs heat in this temperature region

.

The prcscnce of stcel generally decrcascs the specific hcat of the fibre-reinforced concrete. cxccnt in the temDerature region near 750°C. This can be amibuted to the fact that the specific heat of steil is generay lower than that of the concrete except in the temperatye region near 750°C, where the specific heat of steel peaks and exceeds that of concrete

.

However, the influence of the steel on the specific heat of the concrete is very small and insignificant in the temperature range examined. The specific heat of fibre- reinforced siliceous aggregate concrete is generally higher than that of plain and fibre- reinforced carbonate aggregate concrete.

THERMAL

EXPANSION

The thermal expansion of the concretes was measured using a Theta' dilatory apparatus with a computer-controlled heating and data acquisition system. The specimens were approximately 40 mm long and had square cross sections of 10 to 12

mm

The specimens were heated at a rate of 10°C/min temperature rise, measured on the surface of

the specimen. Two tests were performed on each concrete type and the average of the thermal expansions, measured in these tests, was taken as the thermal expansion of the concrete.

In Fig. 5, the thermal expansions for the three concrete types are shown as a function of the concrete temperature. For the fibre-reinforced siliceous aggregate concrete, the thermal expansion increases with temperature up to about 600°C and then remains constant. The considerable enhancement of the thermal expansion near 550°C can be attributed to transformation of quartz in the siliceous aggregate. A comparison of the thermal expansion of fibre-reinforced siliceous concrete with that of plain siliceous concrete15 indicates that the steel-fibres somewhat increase the thermal expansion.

The plain and the fibre-reinforced carbonate concrete have similar thermal

expansions up to a temperature of about 850°C. Above 850°C, the thermal expansion of the plain concrete declines somewhat, due to further dehydration and shrinkage of the concrete", whereas the expansion of fibre-reinforced carbonate concrete increases steeply with temperature. This steep increase with temperature can be attributed to the presence of the steel-fibres, which continue to expand at an increasing rate.

MASS LOSS

The mass loss for the three concrete types was measured by means of a ~ u ~ o n t ' 9900 Thermomavimetric Analvzer (TGA). The mass loss measurements were carried out on the concreres without the &el-fibres, which were removed from the concrete. The mass of the steel was taken into account by adding the known mass of the steel to that of the concrete.

The specimens, which had a mass between 30 and 60 mg, were placed in a nitrogcn atmosphcrc and hcatcd at a rate of 10°C/min.

The test data from the TGA are presented in Fig. 6 in the form of

thermogravime+t curves for the three types of concretes examined in this study. Previous studies have indicated that the type of aggregate has strong influence on the mass loss and, therefore, on the density of the concrete at elevated temperatures. The mass loss for all three concrete types is vely small until about 600°C, where it is about 3% of the original mass. Between 600°C and 800°C, the mass of plain and fibre-reinforced carbonate aggregate concrete drops considerably with the temperature. Above 800°C, the mass loss again decreases slowly with temperature. The curves and the tabulated values in the figure show that the influence of the steel-fibres on the mass loss is vely small and insignificant in the whole temperature range examined. In the case of fibre-reinforced siliceous aggregate concrete, the mass loss remains insignificant even after 600°C.

SUMMARY AND CONCLUSIONS

Experimental and theoretical studies were carried out to investigate the influence of steel-fibre reinforcement on the thermal behaviour of concrete at elevated temperatures. Two types of fibre-reinforced concrete, namely, a siliceous and a carbonate aggregate concrete were studied, and the thermal behaviour of these concretes was compared to each other and to that of a plain carbonate aggregate concrete. The results and conclusions can be summarized as follows:

1. The thermal conductivity of steel-fibre-reinforced concrete decreases with increasing temperature. For carbonate aggregate concrete, it is higher than that of plain

with increasing temperature up to approximately 700°C. At this temperature, the thermal conductivity of the plain concrete exceeds that of the fibre-reinforced carbonate concrete. However, above 1500°C, the differences are about 10% or less and not significant.

2. The specific heat of steel-fibre-reinforced carbonate concrete is slightly less than that of plain concrete in the temperature range 0-1000°C, except for the temperature region near 750°C, where the specific heat of fibre-reinforced carbonate aggregate concrete slightly exceeds the plain concrete. The influence of steel-fibre reinforcement on the specific heat of the concrete is very small, however, and negligible in the whole temperature range investigated. The specific heat of siliceous aggregate concrete is higher than that of carbonate aggregate concrete up to a temperature of approximately 750°C, where the specific heat of the carbonate aggregate concrete increases steeply, and considerably exceeds that of the siliceous aggregate concrete.

3. The thermal expansion of concrete is not significantly affected by the presence of steel-fibre reinforcement at temperatures up to approximately 800°C. Above this temperature, the thermal expansion of the fibre-reinforced concrete increases considerably above that of the plain concrete.

4. The mass loss of concrete is not significantly affected by the presence of steel-fibre reinforcement in the investigated temperature range of 0-1000°C.

5. Overall, steel-fibre-reinforced concrete, at elevated temperatures, exhibits thermal properties that are similar to those of plain concrete.

REFERENCES

1. _{Karneda Y., Koizumi, H., Akiham, S., Suenga, T. and Banno, T., Studies and}

applications of steel fiber reinforced concrete, Report No. 3 1. Kajima Institute of Construction Technology, Tokyo, Japan, October 1979,46 pp.

2. Swarny, R.N. and Lankard, D.R,. Some practical applications of steel-fibre- reinforced concrete, Institution of Civil Engineers, Proceedings, Part 1, Vol. 56, A u ~ . 1974, pp. 235-256.

3. Craig, R.J., Structural applications of reinforced fibrous concrete, Concrete International, Dec. 1984, pp. 28-32.

4. Lie, T.T., Editor, Structural Fire Protection: Manual of Practice, ASCE Manual and

Reports on Engineering Practice No. 78, American Society of Civil Engineers, New

York, NY, 1992,241 pp.

5. Friedman, R., An international survey of computer models for fire and smoke,

Journal of Fire Protection Engineering, Vol. 4, No. 1, 1992, pp. 81-91.

6. Sullivan, P.J.E., Terro, M.J. and Moms, W.A., Critical review of fire-dedicated thermal and structural computer programs, Journal of Applied Fire Science, Vol. 3, NO. 2, 1993-94, pp. 113-135.

7. Cook, D.J. and Uher, C., The thermal conductivity of fibre-reinforced concrete, Cement and Concrete Research, Pergomon Press, Vol. 4, No. 4, 1974, pp. 497-509.

8. Testing and test methods of fibre cement composites, RILEM Symposium 1978

Proceedings, The Construction Press, Lancaster, UK, 545 pp.

9. Schneider, U., Editor, Properties of materials at high temperatures

-

concrete, REEM, Deparhnent of Civil Engineering, Kassel University, Kassel, Germany, 1985, 108 pp.

10. Carbon steel-fibres for concrete reinforcement, Ribbon Technology Corporation, Gahanna, OH, 6 pp.

1 1. Thermal conductivity meter (TC-31), Instruction Manual, Kyoto Electronics

Manufacturing Co. Ltd., Tokyo, Japan, 21 pp.

12. Harmathy, T.Z., Thermal properties of concrete at elevated temperatures, ASTM

Journal of Materials, Vol. 5, No. 1, Mar. 1970, pp. 47-74.

13. Lie, T.T., Fire and Buildings, Applied Science Publishers Ltd., London, 1972, 276 pp.

14. Lie, T.T. and Allen, D.E., Calculation of the fire resistance of reinforced concrete columns, National Research Council of Canada, Division of Building Research, NRCC 14047, Ottawa, 1972,44 pp.

15. Hu, X.F., Lie, T.T., Polomark, G.M. and MacLaurin, J.W., Thennal properties of building materials at elevated temperatures, Internal Report No. 643, Institute for Research in Construction, National Research Council of Canada, Ontario, 1993, 54 PP.

16. Harmathy, T.Z. and Allen, L.W., Thermal properties of concrete of selected masonry unit concretes, ACI Journal, Vol. 70, No. 2, Feb. 1973, pp. 132-142.

8

UNIQUE CRESCENT CROSS -ON

Figure

1.

Typical XOREX steel-fibre reinforcement used in the

concrete mix

i

Power aourca tor

I

I

(a) Block diagram for measuring Thermal Conductivity

(b) Arrangement of sample. heating wire. and thermocouple

Figure 2. Schematic diagram and sample arrangement for thermal conductivity test

I I 1 I I I I I I Fibre reinforced siliceous concrete

-..-.

Fibre reinforced carbonate concrete

...

_{Plain carbonate concrete}

_-

-

....

-

..-*.-..

----

-

...

::+

...

..

-

7

..-..

-..--.

T I I 1 I I 1 1 I 1

Temperature

("C)

Figure 3. Thermal conductivity for various concrete types as a

function of temperature

0

0 100 200 300 400 500 600 700 800 900 1000

Temperature

PC)

I I I I I I I I I

-

Fibre reinforced siliceous concrete

-

-

_-..-.*

Fibre reinforced carbonate concrete

..*...

_{Plain carbonate concrete}

_-

-

-

-

-

-

-

-

-..--.---

-

I 1 I 1 1 1 I 1 I 7

Figure

4.

Specific heat for various concrete types

as

a

function

of

temperature

I I I t 1 I I I I

Fibre reinforced siliceous concrete

-..-.*

-

_..-*....-.-

Fibre reinforced carbonate concrete

Plain carbonate concrete

-

/"

-

..

/-+

-

.(...--

....,

-

-

I I I 1 I I I 1 I 0 100 200 300 400 500 600 700 800 900 1000

Temperature

("C)

Figure 5. Thermal expansion

for

various concrete types as a

function of temperature

65 0 100 200 300 400 500 600 700 800 900 1000

Temperature

("C)

I I I I I I 1 I I

-

-

..-.a

-.-.

-

_*.:a

-

.. ..

-

_..

_{. .}

-

-:\

-

:.

_.

_.

-

:\

-

Fibre reinforced siliceous concrete

..

.

.

-

-..-..

_{Fibre reinforced carbonate concrete}

:\

_:.

..-.--...

-

Plain carbonate concrete

..

3

.

.

-

+

- .

-

-.

-.

-

_{..-.--._-..._.}

_-

I 1 I I I I 1 1 1

Figure

6.

Mass

loss for various concrete types

as a