Stress-strain curves for high strength concrete at elevated temperatures. Cheng, F-P.; Kodur, V.K.R.; Wang, T-C.

Transcription

1 Stress-strain curves fr high strength cncrete at elevated temperatures Cheng, F-P.; Kdur, V.K.R.; Wang, T-C. NRCC A versin f this dcument is published in / Une versin de ce dcument se truve dans : Jurnal f Materials in Civil Engineering, v. 16, n. 1, January/February 2004, pp

2 STRESS-STRAIN CURVES FOR HIGH STRENGTH CONCRETE AT ELEVATED TEMPERATURES By Fu-Ping Cheng 1, V.K.R. Kdur 2 and Tien-Chih Wang 3 Abstract: The effects f high temperature n strength and stress-strain relatinship f high strength cncrete (HSC) were investigated. Stress-strain curve tests were cnducted at varius temperatures (20, 100, 200, 400, 600 and 800 C) fr fur types f HSC. The variables cnsidered in the experimental study included cncrete strength, type f aggregate, and the additin f steel fibers. Results frm stress-strain curve tests shw that plain HSC exhibits brittle prperties belw 600 C, and ductility abve 600 C. HSC with steel fibers exhibits ductility fr temperatures ver 400 C. The cmpressive strength f HSC decreases by abut a quarter f its rm temperature strength within the range f C. The strength further decreases with the increase f temperature and reaches abut a quarter f its initial strength at 800 C. The strain at peak lading increases with temperature; frm at rm temperature t 0.02 at 800 C. Further, the increase in strains fr carbnate aggregate HSC is larger than that fr siliceus aggregate HSC. Key wrds: fire resistance, high strength cncrete, high temperature behavir, stress-strain curves INTRODUCTION In recent years, high strength cncrete (HSC) is becming an attractive alternative t traditinal nrmal strength cncrete (NSC). Cncretes f strength in excess f 70 MPa are ften used in a wide range f applicatins. With the increased use f HSC, cncern has develped regarding the behavir f such cncrete in fire. 1 Assciate Prf., Department f Civil Engineering, Natinal Chia Tung University, HsinChu, Taiwan. 2 Senir Research Officer, Inst. fr Res. in Cnstr., Natinal Research Cuncil, Ottawa, ON, Canada. 3 Ph. D. Student, Department f Civil Engineering, Natinal Chia Tung University, Hsin Chu, Taiwan. 1

3 The high strength in HSC is ften btained, by reducing the amunt f water, thrugh the use f special admixtures that als imprve the wrkability. Hwever, the lwer water-cement rati leads t lwer prsity that makes HSC mre brittle and makes it have less fire endurance at elevated temperatures as cmpared t NSC. Hence, ne f the main cncerns fr the usage f HSC in fire resistance applicatins is its perfrmance in fire cnditins. Preliminary studies (Furumura et al. 1993; Hsu and Hsu 1994; Khury 1992; Lankard et al. 1971) have shwn that there are well-defined differences between the prperties f HSC and NSC at elevated temperatures. In rder t understand and eventually predict the perfrmance f HSC structural members, the material prperties that determine the behavir f the member at elevated temperatures must be knwn. The behavir f a structural member expsed t fire is dependent, in part, n the thermal and mechanical prperties f the material f which the member is cmpsed (Castill and Durrani 1990; Furumura et al. 1993). In the past, the fire resistance f structural members culd be determined nly by testing. In recent years, hwever, the use f numerical methds fr the calculatin f the fire resistance f varius structural members is gaining wide acceptance. These calculatin methds are far less cstly and time cnsuming. Hwever, fr the use f these calculatin methds, the material prperties at elevated temperatures are required. One f the basic mechanical prperties that is required fr predictin f structural perfrmance under fire cnditins is the stress-strain relatinship. Fr develping stress-strain curves f HSC at elevated temperature, a jint research prject between Natinal Chia Tung University (NCTU), Taiwan, and the Natinal Research Cuncil Canada (NRCC) is currently in prgress. As part f this prject, detailed experimental studies were undertaken n fur different types f HSC. Bth plain HSC and steel fiber reinfrced cncrete HSC were cnsidered in the study. This is because the additin f steel fibers enhances the mechanical behavir f HSC at elevated temperature and significantly imprves the ductility f cncrete (Diederichs et al. 1989; Hsu and Hsu 1994; Lie and Kdur 1996). Als, tw types f cmmnly used aggregates, siliceus and carbnate, were cnsidered in the tests since the type f aggregate influences the fire perfrmance and the shape f the stress-strain curves. Detailed results frm the experimental study are presented in this paper. 2

4 RESEARCH SIGNIFICANCE Fire represents ne f the mre severe expsure cnditins and hence prvisins f apprpriate fire resistance fr structural members are majr safety requirements in building design. The fire resistance f structural members is dependent n the thermal and mechanical prperties, at elevated temperatures, f the materials f which the members are cmpsed. One f the key prperties needed fr the assessment f the verall respnse f structures under fire cnditins is the stress-strain relatinships. Whereas the stress-strain curves at elevated temperatures have been established fr varius types f NSC, this is nt the case with HSC, specifically HSC with steel fibers (Lie and Kdur 1996). Temperature is ne f the main factrs that influence the strength, as well as the stress-strain relatinships, f cncrete. The present study was undertaken t investigate the effect f high temperature n the strength and stress-strain relatinship fr HSC. The variables cnsidered in the experimental study included cncrete strength, type f aggregate and the additin f steel fibers, since these variables have shwn t be f significant influence in the fire endurance f high strength cncrete (Kdur 2000). Stress-strain curve tests were cnducted at varius temperatures (20, 100, 200, 400, 600 and 800 C) fr tw types f HSC; plain HSC and steel fiber-reinfrced HSC; made with siliceus and carbnate aggregate. Cncrete strengths in the range f MPa were cnsidered. The data, n mechanical prperties, presented in this paper can be used t develp stress-strain relatinships, as a functin f temperature, fr HSC. These relatinships can be used as input t numerical mdels, which in turn can be used t determine the behavir f HSC structural members at high temperatures. The develpment f cmputer prgrams fr the calculatin f the fire resistance f HSC clumns is in prgress at NRCC (Kdur, 2001). EXPERIMENTAL PROGRAM Test Specimens Fur types f HSC specimens were investigated, namely specimens made with siliceus and carbnate aggregate cncrete; with and withut steel fiber reinfrcement. The 100 mm diameter x 200 mm height specimens were prepared by NRCC, and made in fur batches; TWN1, TWN2, TWN3 and TWN4. Each batch cnsists f ne mix prprtin. The TWN1 was made f siliceus aggregate and the TWN2 was made f carbnate aggregate. The TWN3 and TWN4 were the 3

5 cunterpart f TWN1 and TWN2, but with the additin f steel fibers. The strength f the specimens at 28 days was in the range 75 MPa t 84 MPa. Granite was used fr siliceus aggregate and limestne was used fr carbnate aggregate. In all batches, general purpse Prtland cement fr cnstructin f cncrete structures was used. The fine aggregate fr all fur batches cnsisted f silica-based sand. In rder t imprve wrkability, superplasticizer and retarding admixture were added t all fur batches f cncrete mix. Steel-fibers f typical crrugated type were used as reinfrcement in TWN3 and TWN4 batches f cncrete mix. The steel-fiber is a mild carbnaceus steel with tensile strength f apprximately 960 MPa. A typical steel-fiber is shwn in Figure 1. The crrugated shape f these fibers prvided a strng mechanical bnd t the cncrete t resist shear stress. The fibers, which were 50 mm in length with a 0.9 mm equivalent diameter, had an aspect rati f 57. Superplasticizer was added t the cncrete, half in the plant and half at the site befre the fabricatin f specimens, t imprve wrkability. The mix prprtins f fur batches are given in Table 1. Frm each batch f cncrete, the fllwing specimens were made: 40 small cylinders f 100 mm diameter and 200 mm length. 10 large cylinders f 150 mm diameter and 300 mm length. T measure representative temperature, thermcuples were installed in tw small cylinders (100 x 200 mm) frm each batch f cncrete. Tw thermcuples were installed at mid-height f the cylinder; ne n the center axis and the ther at quarter depth (25 mm frm surface) f the cylinder, as shwn in Figure 2. The specimens were de-mlded ne day after casting, then saked under water fr seven days and, subsequently, cured in a climate rm at 50% relative humidity and 20 C temperature. The cmpressin tests were cnducted n large cylinders (150 x 300 mm) at 7 days, 28 days and 91 days, respectively, fr each type f cncrete. The cmpressive strengths f cncrete are given in Table 1. 4

6 Afterwards, 100 mm diameter cylinders f each cncrete type were shipped t NCTU, Taiwan, fr undertaking experimental studies. These specimens were stred in the NCTU labratry in an atmsphere f apprximately 50% relative humidity at 20 C. Test Apparatus The tests were perfrmed in a clsed-lp serv-cntrl 220 Kip (985 kn) hydraulic pump actuatr with a lading frame and an electric furnace. Figure 3 shws the test apparatus, (lading frame with furnace), while Figure 4 shws the schematic f the test set up. Tw special cylindrical nickel-based ally attachments were used in the tp and bttm f the specimen t transmit lad frm the actuatr t the specimen at high temperature. One cncrete cylinder f 150 mm diameter and 300 mm height, made f high strength cncrete, was placed n the tp f the ally attachment as a heat insulatr t prtect the test equipment frm extreme temperature. Tw blankets were welded t the bttm f the upper attachment and the ther tw were welded n the tp f the bttm attachment, respectively, fr setting up LVDT t measure the relative defrmatin f the specimen. Tw half-spherical discs were attached n the bttm f the lwer attachment and the tp f the cncrete heat insulatr fr a unifrm lad distributin. The clearance between the ally attachment and the furnace was filled with a ceramic fiber insulatr t limit the lss f heat thrugh air circulatin. The specimens were encased in a wire mesh, made f nickel-based ally, t prtect the interir f the furnace frm explsive spalling f HSC. T accunt fr the effect f defrmatin frm the attachments and the test equipment, the defrmatin f the specimen and the displacements f the actuatr were calculated by averaging the difference f tw pairs f LVDT. The average defrmatin, avg, was calculated as fllws (see Figure 4): ave = ( ) + ( ) up1 lw1 2 up2 lw2 The lad was applied, at a cnstant displacement rate, using an autmated cmputer cntrlled system. The lad and defrmatin f specimens were recrded cntinuusly fr the duratin f each test. Experimental Prcedure Fr each type f cncrete and target test temperature, three specimens were tested and the 5

7 average f these results was used as the final result. The target test temperatures were determined t be 20 (rm temperature), 100, 200, 400, 600 and 800 C, respectively. All tests were cnducted under ht cnditins and n residual strength tests were carried ut. Befre undertaking stress-strain curve tests, the temperature f the furnace was calibrated against the specimen temperatures. T facilitate this, the cncrete cylinders, with thermcuples, were heated in the furnace and the furnace temperatures were measured when the center f the specimens reached the specified target temperature. These temperatures were used as the representative highest furnace temperature fr the fllwing stress-strain tests. The cylindrical specimens were heated, withut any lad, at a cnstant heating rate f 2 C/min in the furnace chamber until the temperature at the center f the specimen reaches the target temperature. Then the furnace temperature was drpped t the desired target temperature and was kept cnstant fr 60 minutes t attain a steady state at the center f the specimen (Figure 5). Frm the calibratin f the furnace, the cnservative time needed t reach the desired target temperature at the center f the specimen was fund t be 70, 135, 230, 325 and 435 minutes, crrespnding t 100, 200, 400, 600 and 800 C, respectively. During the heating perid, misture in the test specimens was allwed t escape freely. Fr generating data in the descending part f the stress-strain curve f cncrete, a strain cntrl technique was adpted. After the specimen reached the steady state, it was laded under uniaxial cmpressin at a rate f (machine crsshead mvement) 0.05%/min (0.1 mm/min) until the specimen culd n lnger sustain the axial lad. During the tests, lad and displacement f specimens were measured. The lad was measured frm the MTS system and the displacement was measured frm the average f tw pairs f LVDT. RESULTS AND DISCUSSION Cmpressive Strength The variatin f the cmpressive strength rati with temperature is shwn in Figure 6 fr fur types f HSC. The cmpressive strength rati is defined as the rati f the maximum cmpressive strength, at a specified temperature, t the maximum cmpressive strength at rm temperature (riginal strength). The cmpressive strength was calculated as the average f three cylinder tests. In all fur types f HSC, cmpressive strength decreases with temperature. Initially, as the 6

8 temperature increased t 100 C, the strength decreased cmpared t the riginal strength (Khury 1992). With further increase in temperature, the specimens recvered part f its strength at 200 C. The strength at 200 C, is abut 80% f the riginal strength (at 20 C). In the temperature range f 400 C t 800 C, the strength drps sharply, reaching t a lw level f 45 and 20 percent, f initial strength, at 600 C and 800 C, respectively. The misture cntent has a significant bearing n the strength f cncrete in the temperature range frm 20 C t 200 C. It is believed that water in cncrete sftens the cement gel, r attenuates the surface frces between gel particles, thus reducing the strength (Lankard et al. 1971). The slight increase in cncrete strength assciated with a further increase in temperature (between 100 and 200 C) is attributed t the general stiffening f the cement gel, r the increase in surface frces between gel particles, due t the remval f absrbed misture. The temperature at which absrbed water is remved and the strength begins t increase depends n the prsity f the cncrete. Abve 400 C, all fur types f HSC lse their strength at a faster rate. At these temperatures, the dehydratin f the cement paste results in its gradual disintegratin. Since the paste tends t shrink and aggregate expands at high temperature, (differential thermal expansin at temperatures abve 100 C), the bnd between the aggregate and the paste is weakened, thus reducing the strength f the cncrete. The presence f steel fiber has little influence n cmpressive strength. The HSC withut steel fiber has a slightly higher cmpressive strength belw 400 C fr bth aggregate types. Hwever, the ductility f HSC with steel fibers increases at temperatures abve 400 C. Overall, HSC at elevated temperatures lses a significant amunt f its cmpressive strength abve 400 C and attains a strength lss f abut 75% at 800 C. The lss f strength in the temperature range f C is marginal and C is minimal, but slightly higher than that in nrmal strength cncrete (Lie 1992). Elastic Mdulus f HSC under High Temperature The elastic mdulus defined as the rati f elastic mdulus (taken as the tangent t the 7

9 stress-strain curve at the rigin) at a specified temperature t that at rm temperature, is shwn in Figure 7 as a functin f temperature. Up t abut 400 C the elastic mdulus f all fur types f HSC decreases in a similar fashin, reaching t abut 50% f its initial values. The extent f decrease in elastic mdulus f steel fiber reinfrced HSC is much higher than that f plain HSC in the C range. But, the aggregate type des nt influence the variatin f mdulus f elasticity f HSC with temperature. Stress-strain Relatinship The stress-strain plts fr HSC, made frm siliceus aggregate and carbnate aggregate, at elevated temperatures are shwn in Figure 8 and Figure 9, respectively. The stress rati, defined as the rati f the cmpressive strength at specified temperature (f c ) t the cmpressive strength at rm temperature (f c ). The stress-strain curves fr their cunterparts with steel fiber reinfrcement are shwn in Figure 10 and Figure 11, respectively. The strain crrespnding t peak strength did nt significantly change up t abut 400 C fr all fur types f HSC. Abve this temperature the strains, crrespnding t peak strength, increased cnsiderably. The strains attained, crrespnding t peak strength at 600 C and 800 C were twice and seven times the strain at rm temperature. Fr all fur types f HSC the strain crrespnding t peak strength is almst the same up t abut 600 C. These results suggest that the steel fiber reinfrced HSC exhibits better ductility characteristics than plain HSC at elevated temperatures. It can be seen frm Figures 8 and 9, that the aggregate type has an effect n the ultimate strain attained in HSC at elevated temperatures. The strain attained, crrespnding t peak strength, in HSC specimens with carbnate aggregate was 40% larger than thse made frm siliceus aggregate. It can be seen in Figures 10 and 11, that a similar trend exists fr steel fiber reinfrced HSC. Failure Mde HSC specimens withut steel fiber shwed a very brittle type f failure at temperatures less than 600 C. It was impssible t define the cmplete stress-strain curves, especially in the descending prtin. The specimens failed sn after reaching their peak strength. The failure surfaces are "neat surfaces" and pass thrugh the "brken aggregate". The end prtin f the failed specimens resembled "duble cne pattern" (at the tp and bttm), as can be seen in Figure 12. 8

10 When expsed t a temperature f 800 C, cncrete specimens withut steel fibers exhibited a gradual failure and cmplete stress-strain curves culd be defined. These specimens failed in an irregular pattern (Figure 13). The specimens with and withut steel fibers exhibited similar failure mde, up t abut 400 C. Due t bnding, vertical cracks that appeared in the brken specimens were held tgether by steel fibers (Figure 14). With further increase f temperature, the specimens failed gradually. In these specimens, the tests culd be easily carried ut (t trace the descending prtin f the stress-strain curve) abve 600 C (Figure 15). Since HSC is a brittle material, it fails sn after crack prpagatin is initiated. If the number f vids is increased, the prpagatin f cracks ccurs mre gradually, resulting in less brittle failure. When temperature increases, the pre water is remved and mre vids appear. The ductility f specimens increased with increased temperatures. CONCLUSIONS Based n the results btained in this study, and within the limitatins f the test parameters, the fllwing cnclusins can be drawn: 1. HSC, lses a significant amunt f its cmpressive strength abve 400 C and attains a strength lss f abut 75% at 800 C. The change f strength in the temperature range f C is marginal. 2. The additin f steel fibers in HSC imprves the ductility at elevated temperature. Steel fiber reinfrced HSC exhibits better ductility characteristics than plain HSC at elevated temperatures. 3. The presence f steel fiber has little influence n the variatin f mdulus f elasticity with temperature. The type f aggregate has mderate influence n the variatin f the elastic mdulus with temperature. 4. The aggregate type has an effect n the ultimate strain attained in HSC expsed t elevated temperatures; with carbnate aggregate HSC attaining larger strain as cmpared t siliceus aggregate HSC. 9

11 REFERENCES 1. Castill, C. and Durrani, A.J. (1990). "Effect f Transient High Temperature n High-Strength Cncrete." ACI Materials Jurnal, Title N.87-M7, January-February, Diederichs, U., Jumppanen, U.M. and Penttala, V. (1989). "Behavir f high strength cncrete at high Temperatures." Reprt N. 92, pp. 76, Helsinki University f Technlgy, Helsinki, Finland. 3. Fukujir, Furumura, Take, Ave and Yasuji, Shinhara (1993). "Mechanical Prperties f High Strength Cncrete at High Temperatures." Reprt f Research Labratry f Engineering Materials, Tky Institute f Technlgy, Japan. 4. Hsu, L.S. and Hsu, C.T. (1994). "Stress-Strain Behavir f Steel-Fiber High-Strength Cncrete under Cmpressin." ACI Structural Jurnal, Title N.91-S44, V.91, N.4, July-August, Khury, G.A. (1992). "Cmpressive Strength f Cncrete at High Temperatures." Magazine f Cncrete Research, Kdur, V.K.R., (2000). Spalling in high strength cncrete expsed t fire cncerns, causes, critical parameters and cures. Prceedings: ASCE Structures Cngress, 1-8, Philadelphia, U.S.A. 7. Kdur, V.K.R. and Sultan, M.A., (1998). "Structural behaviur f high strength cncrete clumns expsed t fire." Prceedings: Internatinal Sympsium n High Perfrmance and Reactive Pwder Cncrete, Vl. 4, , Sherbrke, Quebec. 8. Kdur V.K.R., Cheng F.P., Wang T.C. and Sultan M.A. (2003). "Effect f Strength and Fiber Reinfrcement n the Fire Resistance f High Strength Cncrete Clumns." Jurnal f Structural Engineering, ASCE, V 129, N2, Lankard, D.R., Birkimer, D.L., Fndfriest, F.F. and Synder, M.J. (1971). "Effects f Misture Cntent n the Structure Prperties f Prtland Cement Cncrete Expsed t Temperatures up t 500 F." Temperature and Cncrete, SP-25, American Cncrete Institute, Detrit,

12 10. Lie, T.T. Editr (1992). Structural fire prtectin: Manual f Practice. ASCE Manuals and Reprts n Engineering Practice N. 78, pp. 241, American Sciety f Civil Engineers, New Yrk, NY. 11. Lie, T.T. and Kdur, V.K.R. (1996). "Thermal and Mechanical Prperties f Steel-Fiber-Reinfrced Cncrete at Elevated Temperatures." Canadian Jurnal f Civil Engineering, Vl. 23, Phan, L.T. (1996). "Fire Perfrmance f High-Strength Cncrete." A Reprt f the State-f-the-Art, Natinal Institute f Standards and Technlgy, pp. 105, Gaithersburg, MD. 13. Schnelder, U. (1985). "Prperties f materials at high temperatures Cncrete." Reprt, Published by Department f Civil Engineering, Kassel University. 11

13 Table 1 Mix prprtins f high strength cncrete specimens Cncrete mix Batch 1 Batch 2 Batch 3 Batch 4 Specimen TWN 1 TWN 2 TWN 3 TWN 4 Cement (kg/cu. m) Carse aggregate (kg/cu. m) Fine aggregate (kg/cu. m) Silica Fume (kg/cu. m) Water (kg/cu. m) Steel fiber (kg/cu. m) Water reducing/retarding admixture (ml) Superplasticizer ( L ) Water-binder rati Cmpressive strength 7 days days (MPa) 91 days

14 Table 2 Calibratin f furnace temperatures with specimen temperatures Specimen Type Time (minutes) t attain target temperature Temperature C TWN TWN TWN TWN

15 List f Figure Captins Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Typical steel fiber Typical test specimen with thermcuples Test equipment Lading frame with furnace Schematic f test setup fr stress-strain curve tests The sketch map fr typical heating prcess Nrmalized cmpressive strength f HSC as a functin f temperature Nrmalized elastic mdulus f HSC as a functin f temperature Stress-strain curves fr siliceus aggregate HSC withut steel fiber Stress-strain curves fr carbnate aggregate HSC withut steel fiber Figure 10 Stress-strain curves fr siliceus aggregate HSC with steel fiber Figure 11 Stress-strain curves fr carbnate aggregate HSC with steel fiber Figure 12 Failure mde f typical HSC specimen (withut steel fiber) at 100 C Figure 13 Failure mde f typical HSC specimen (withut steel fiber) at 800 C Figure 14 Failure mde f typical specimen (with steel fiber) at 100 C Figure 15 Failure mde f typical specimen (with steel fiber) at 800 C 14

16 Fig. 1 Typical steel fiber 15

17 Fig. 2 Typical test specimen with thermcuples 16

18 Fig. 3 Test equipment - Lading frame with furnace 17

19 Fig. 4 Schematic f test setup fr stress-strain curve tests 18

20 Target temperature Temperature ( C) Heating rate: 2 C/min Furnace temperature TC1 TC2 center Needed time t reach the desired temperature Time (min) 60 min Test start Fig. 5 The sketch map fr typical heating prcess 19

21 1.2 Nrmalized Cmpressive strength Siliceus Aggregate HSC Carbnate Aggregate HSC Siliceus Aggregate HSC with Steel Fiber Carbnate Aggregate HSC with Steel Fiber Temperature ( C) Fig. 6 Nrmalized cmpressive strength f HSC as a functin f temperature 20

22 1.2 Nrmalized Elastic Mdulus Siliceus Aggregate HSC Carbnate Aggregate HS C Siliceus Aggregate HSC with Steel Fiber Carbnate Aggregate HS C with Steel Fiber Temperature ( C) Fig. 7 Nrmalized elastic mdulus f HSC as a functin f temperature 21

23 1.2 Stress Rati ( fc/f'c ) C 100 C 200 C 400 C 600 C 800 C Strain Fig. 8 Stress-strain curves fr siliceus aggregate HSC withut steel fiber 22

24 1.2 Stress Rati ( fc/f'c ) C 100 C 200 C 400 C 600 C 800 C Strain Fig. 9 Stress-strain curves fr carbnate aggregate HSC withut steel fiber 23

25 1.2 Stress Rati ( fc/f'c ) C 100 C 200 C 400 C 600 C 800 C Strain Fig. 10 Stress-strain curves fr siliceus aggregate HSC with steel fiber 24

26 1.2 Stress Rati ( fc/f'c ) C 100 C 200 C 400 C 600 C 800 C Strain Fig. 11 Stress-strain curves fr carbnate aggregate HSC with steel fiber 25

27 Fig. 12 Failure Mde f typical HSC specimen (withut steel fiber) at 100 C 26

28 Fig. 13 Failure Mde f typical HSC specimen (withut steel fiber) at 800 C 27

29 Fig. 14 Failure Mde f typical specimen (with steel fiber) at 100 C 28

30 Fig. 15 Failure Mde f typical specimen (with steel fiber) at 800 C 29