Ž .Construction and Building Materials 14 2000 261]266

Effect of high temperature and cooling regimes on thecompressive strength and pore properties of high

performance concrete

Sammy Yin Nin Chana,U, Xin Luob, Wei Sunb

aWong & Cheng Consulting Engineers Ltd., 608 Bank Centre, 636 Nathan Road, Kowloon, Hong KongbSoutheast Uni ersity, Nanjing 210096, PR China

Received 15 November 1999; received in revised form 24 April 2000; accepted 22 May 2000

Abstract

Ž . Ž .This paper describes the behavior of high performance concrete HPC , compared with normal strength concrete NSC , afterŽ . Ž .subject to different high temperatures 800 and 11008C and cooling regimes gradual and rapid cooling . Deterioration of

compressive strength of the concrete was measured. The results obtained showed that the strength of both the HPC and NSCreduced sharply after their exposure to high temperatures. Thermal shock due to rapid cooling caused a bit more deterioration instrength than in the case of gradual cooling without thermal shock. However, thermal shock did not significantly increase the

Ž .spalling of HPC. Mercury intrusion porosimetry MIP tests were carried out to measure changes in the pore size distribution inthe concrete. Test results showed that the pore volume in the HPC increased much more than that in the NSC. A significantchange in the cumulative pore volume was observed and the difference in cumulative pore volume between the two coolingregimes was less after subject to the peak temperature of 11008C when compared with that of 8008C peak temperature. Q 2000Elsevier Science Ltd. All rights reserved.

Keywords: High performance concrete; High temperature; Compressive strength; Pore

1. Introduction

Concrete has been the leading construction materialŽ .for nearly a century. High performance concrete HPC

with high strength or high durability is gradually replac-ing the normal strength concrete, especially in struc-tures exposed to severe environment. The advantagesof HPC result from the improvement of internal struc-ture of the material as compared with that of the

w xnormal concrete 1 . The dense microstructure of HPCensures a high strength and a very low permeability.

U Corresponding author. Tel.: q86-2866-3011; fax: q86-25-771-2719.

Ž .E-mail address: sunwei@seu.edu.cn W. Sun .

The low permeability is probably essential to obtaingood durability in severe exposure conditions wherethere are aggressive agents such as chloride, sulfate,etc. However, the dense microstructure of HPC seemsto be a disadvantage in the situation where the HPC isexposed to fire. Recent fire test results show that thereis a great difference between the properties of HPCand normal concrete after being subjected to high

w xtemperature 2]4 . It was observed that HPC, espe-Ž .cially high strength concrete HSC is susceptible to

spalling, or even explosive spalling when subject tow xrapid temperature rise such as in the case of a fire 5,6 .

Experimental studies have also shown that spalling ofHPC is influenced by many complicated factors, suchas rate of temperature elevation, mineral constituentsof aggregates, thermally induced mechanical stress,

0950-0618r00r$ - see front matter Q 2000 Elsevier Science Ltd. All rights reserved.Ž .PII: S 0 9 5 0 - 0 6 1 8 0 0 0 0 0 3 1 - 3

( )S.Y. Chan et al. r Construction and Building Materials 14 2000 261]266262

density of concrete, moisture content, etc. Althoughthe failure mechanisms of HPC spalling have not beensufficiently understood, there are two mechanismswhich are considered as the direct causes of spalling ofHPC. One is the thermal stress induced by the rapid

w xtemperature rise or thermal shock 7 ; the other is thewater vapor, which may cause high pore vapor pressurew x8 . The properties of this binder are, therefore, criticalto the HPC. In this study, three grades of HPC and oneNSC were tested for their compressive strength afterexposure to different high temperatures and coolingregimes. The variation in the microstructures of boththe HPC and NSC were tested by means of mercury

Ž .intrusion porosimetry MIP .

2. Materials and test methods

Ordinary Portland cement conforming to BS12:1991,crushed granite with a maximum size of 20 and 10 mm,superplasticizer based on naphthalene sulfonates com-plying with BS5075 part3:1985, silica fume, pulverized

Ž .fly ash PFA meeting the requirements of BS3892part1:1982, as well as steel and polypropylene fiberswere adopted in this study. The chemical compositionsand physical properties of cement, silica fume and flyash are given in Table 1.

The length and aspect ratio are 25 mm and 60,respectively, for the steel fiber, 19 mm and 360, respec-tively, for the polypropylene fiber. The mix proportionsare summarized in Table 2, in which series NP is NSCand series HS, HF1S, and HF2S are HPC.

The specimens in high temperature test were 100mm cubes. For each group, three specimens were testedand averaged. The specimens were cast and demouldedafter 24-h of moist curing in mould. Afterwards, theywere stored in a water tank with fresh water at 20"58Cuntil testing. After 3-month curing, the specimens satu-rated with water were taken out of the tank, wiped of

Table 1Ž .Chemical analysis percentage by mass and properties of cement,

silica fume and PFA

Cement Silica fume PFA

SiO 20.7 94.0 44.92CaO 64.4 0.3 5.7Al O 5.4 0.3 35.42 3Fe O 2.3 0.8 4.92 3MgO 0.9 0.4 1.2Na O 0.13 1.0 1.02K O 0.4 r 1.02SO 2.4 0.2 0.73L.O.I. 0.97 2.8 5.6

Specific gravity 3.1 2.2 2.12 4Ž .Specific surface m rkg 355 2.0=10 528

Table 23Ž .Mix proportions for HPC and NSC kgrm

Series NP HS HF1 HF2S

Cement 263 358 358 358Water 210 176 176 176Sand 590 540 540 540

Ž .Coarse 10 mm 420 385 385 385Ž .Aggre. 20 mm 830 770 770 770

Silica fume r 55 55 55PFA 88 138 138 138Steel fiber r r 78 rPolymer fiber r r r 1.8

3Ž .Superplasticizer lrm r 9 9 9

water on the specimen surfaces, and immediately placedinto a computer-controlled electric furnace for per-forming high temperature tests. The temperature inthe furnace was elevated at a rate of 5]78Crmin. Twopeak temperatures, i.e. 800 and 11008C were chosen.After the peak temperature was reached, it was main-tained for 1 h, and then the specimens were allowed tocool down. Two cooling regimes were chosen. One isgradual cooling, i.e. natural cooling at a rate of2]38Crmin in the furnace until the room temperatureŽ .20"58C was reached. The other is rapid cooling, i.e.

Žthe hot specimens at the peak temperature 800 or.11008C were taken out of the furnace and dipped into

Ž .a water tank until the ambient temperature 20"58Cwas reached. Compressive strength test was carried outafter the cubic specimens were cooled to an ambienttemperature. The samples for MIP test were carefullycored from the specimens and precaution was taken toensure no coarse aggregates were included. Afterwards,the samples were immediately immersed in absolutealcohol to prevent further hydration. In the MIP test,the maximum pressure of the porosimeter was 207Nrmm2, covering the pore diameter range from ap-proximately 0.006 to 360 mm. Values of 1408 and 0.485Nrm were adopted for the contact angle and mercurysurface tension, respectively. The samples were dried atapproximately 1058C before the MIP test.

3. Test results and discussion

As shown in Fig. 1, compressive strength of the HPCdecreased more sharply than that of the NSC. Controlsample referred to the standard cured specimen thatwas tested at 90 days and not subjected to high temper-atures. When temperature was elevated to 8008C, theresidual strength percentage of the HPC was 26]34%for the gradual cooling and 22]28% for the rapid

Žcooling. When the peak temperature was 11008C Fig..2 , the residual strength percentage of HPC was 8]12%

for gradual cooling and 8]10% for rapid cooling. Itappeared that the cooling regimes had a minor effect

( )S.Y. Chan et al. r Construction and Building Materials 14 2000 261]266 263

Ž .Fig. 1. Compressive strength at 90 days before and after exposureto 8008C.

on the residual compressive strength. It is also notedthat effect of cooling rate was less pronounced forconcrete exposed to higher peak temperatures.

Incorporation of steel fiber in HPC aimed to con-strain the volume change of concrete due to the rapid

Ž .temperature change elevating or cooling so as toreduce the initiation and propagation of micro-defectsin concrete material. A recent study showed that steelfiber performed well in concrete when exposed to high

w xtemperatures 9 . For example, after 8008C and furnaceŽ .cooling, HS without steel fiber had the residual

Žstrength percentage of 26% while HF1S with steel.fiber had the residual percentage of 34%. At room

temperature the compressive strength of HF1S was17% higher than that of HS while after 8008C andfurnace cooling the residual strength of the former was55% higher than that of the latter. Thus, steel fibercould reduce the deterioration of concrete to an extentwhen subjected to high temperature.

Spalling, especially explosive spalling, when subjectto fire or rapid temperature rise is a major disadvan-

w xtage of HPC 10,11 . Spalling of HPC is a complex

Ž .Fig. 2. Compressive strength at 90 days before and after exposureto 11008C.

phenomenon that requires detailed investigation, forexample, spalling does not occur in every specimen ofthe same batch under identical conditions. Despite thisinconsistency, it is believed that spalling occurs underthe combination of certain conditions. Addition ofpolypropylene fiber burned off and creating channelsaimed to release water vapor pressure developed within

w xthe HPC 5,8,12 . The release of the water vapor pres-sure significantly reduces the spalling tendency of HPC.However, no spalling in both NSC and HPC specimenshappened on heating. This was owing to the heating

Ž .rate that the furnace could reach 5]78Crmin , whichwas not high enough to cause spalling.

On the other hand, from the viewpoint of mechanics,spalling results from the non-uniformity of stress dis-tribution in material. As mentioned above, there aretwo major mechanisms which directly contribute to thenon-uniform stress distribution. Rapid cooling test inwhich the HPC specimens were subjected to water

Ž .cooling at the peak temperatures 800 and 11008Caimed to verify the effect of thermal stress on thespalling of HPC. However, no spalling in both NSC andHPC specimens happened, which denoted, to a greatextent, that thermal gradient or thermal shock was notthe primary reason for the spalling of HPC. Besides,comparing the test results, thermal shock seemed tohave slight effect only on the residual strength of HPC.

It is necessary to assess the residual strength afterfire. This arises from the fact that the presence of fiberimproves strength, especially tensile or flexural strengththat may be considered in the design. The voids andmicrocracks left after the polymer fibers are vaporizedmight have a marked decrease in strength. However,the addition of polypropylene fiber did not cause asignificant decrease in residual strength compared withthe case without adding of fibers.

Pore structure significantly influences the strength ofcement paste, which is critical to the performance ofthe concrete as a whole. Especially, a coarsening of thepore size at elevated temperatures could result in a

w xstrength reduction 13 . Total porosity increases in anon-linear way with an increase in temperature, mainlyon account of the processes of the decomposition ofhydration products. For example, cement gel begins to

Ž .dehydrate at approximately 1808C, Ca OH in cement2paste decomposes at approximately 5008C, and so on. Itwas also found that the increase in total porosity ofcement paste in the temperature range 300]5008C,mainly due to an increase in the percentage of poreswith diameters greater than 500 nm, may be caused by

w xthe formation of microcracks 14 . Figs. 3 and 4 give thechanges of porosity measured by MIP both before andafter being subjected to 800 and 11008C, respectively.Control sample referred to the standard cured speci-men that was tested at 90 days and not subjected tohigh temperatures. After subjected to the peak temper-

( )S.Y. Chan et al. r Construction and Building Materials 14 2000 261]266264

Fig. 3. Porosity before and after being subjected to 8008C.

ature of 8008C, comparing with the original porosity ofcontrol sample, the porosity of NSC was increased by94 and 56% for gradual and rapid cooling, respectively,while HPC was increased by 170]315% for gradualcooling and 100]177% for rapid cooling. After 11008Cthe porosity of NSC increased by 17% for both gradualand rapid cooling, while HPC increased by 57]192%for both the cooling regimes. It shows that HPC had amarked larger increase rate in pore volume than NSC.

Fig. 4. Porosity before and after being subjected to 11008C.

This was an important reason that was responsible forthe more severe deterioration of strength in HPC.Rapid cooling in water caused a denser pore structurein concrete than gradual cooling. This could be at-tributed to the re-hydration of the constituents result-ing from decomposing of concrete at high temperaturesw x13 .

Fig. 5 gives the changes in the cumulative porevolume distribution for the different grades of concrete

Fig. 5. Cumulative pore volume before and after high temperatures.

( )S.Y. Chan et al. r Construction and Building Materials 14 2000 261]266 265

subject to different temperatures and different coolingregimes. It can be observed that the effect of differentcooling regimes was less pronounced for specimenssubjected to 11008C than that for 8008C.

It is noticeable that the total porosity in the concreteafter being subjected to 11008C was smaller than thatafter being exposed to 8008C, however, the residualstrength of the former was much lower than that of thelatter. That test result did not seem to be consistent

w xwith many models 15,16 , which quantitatively denotedthat the smaller of the porosity, the higher strength ofconcrete. This could be attributed to the sinteringeffect, i.e. melting or fusion of cement paste and aggre-gate takes place in concrete at very high temperature.Composition and microstructure of the concrete afterexperiencing the sintering was quite different from thatbefore sintering. Thus, it is not adequate to assess thestrength remain by means of the porosity after theconcrete is subjected to very high temperature, e.g.11008C.

4. Conclusions

1. When subjected to the peak temperature of 8008C,the residual compressive strength of HPC droppedto approximately 26]34% of the original after thegradual cooling process and 22]28% after the rapidcooling; when subjected to 11008C, the residualstrength of HPC reduced to approximately 8]12%for gradual cooling and 8]10% for rapid cooling.

2. Steel fiber could reduce the deterioration of con-crete to an extent. Polypropylene fiber used toprevent spalling of HPC did not lead to a markeddegradation in residual strength even if it evap-orated at high temperatures. Thermal shock due torapid cooling caused slightly more of a deteriora-tion in strength than in the case of gradual coolingwithout thermal shock. However, thermal shock didnot increase the spalling of HPC.

3. The pore volume of the HPC increased at a largerrate than that of the NSC after being subjected tothe same high temperatures. This coarsening effectin pore structure was responsible for the moresevere deterioration in mechanical properties ofHPC when compared with NSC.

4. A significant change of the cumulative pore volumedistribution was observed after the specimens weresubjected to high temperatures and the differenceafter the two different cooling regimes at 11008Cwhen compared with that at 8008C. It is not ade-quate to assess the strength remain by means of

the porosity after the concrete is subjected to veryhigh temperature due to sintering effect.

Acknowledgements

The authors would like to thank the Hong KongPolytechnic University for supporting the researchwork. This research was also part of a project financedby the Chinese National Nature Science Foundation.The support from the State Key Laboratory of MineralDeposit Research, Nanjing University is also ac-knowledged.

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