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ICCBT 2008 - A - (019) pp217-220

ICCBT2008

Effect of Elevated Temperature on the Concrete CompressiveStrength

H. A. M. Bishr, Sana'a University, YEMEN

ABSTRACT

This work aims to show the effect of elevated temperature on the residual compressivestrength of concrete made with available ordinary Portland cement, crushed basalt aggregate, sand and silica fume added in the form of a dry powder as a percentage of thecementitious material. Variables of this study are temperatures (20 to 900 o C), different Silica

fume contents (0% to 15%). The samples were cured in water tanks for 28 days prior toheating. After heating of the samples in the oven to the desired temperatures allowed to coolto the room temperatures and tested under compression. The average of the three readingsobtained is used to represent the condition of the experimental test. The study showed that the

compressive strength of silica fume concrete and normal concrete decreases when exposed tohigh temperatures where the silica fume concrete is more sensitive to elevated temperaturesthan normal concrete. The residual compressive strength ranged between (62 81%) of theoriginal value around 900 o C.

Keyword: Compressive strength, High temperature, Silica fume

*Correspondence Authr: H. A. M. Bishr, Sana'a University, Yemen

http://www.uniten.edu.my/newhome/content_list.asp?contentid=4017

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Effect of elevated temperature on the concrete compressive strength

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1. INTRODUCTION

Concrete with some additives has been used in special structures, such as nuclear reactors andstorage tanks for crude oil and hot waters, whereas concrete subjected to elevated temperature

[1]. Silica fume is being strongly suggested as admixture to improve the properties of concretein general and its durability in particular. It is used as a partial replacement or an addition tocement to modify both the physical structure and the chemical environment of concrete.

Silica fume concrete is used in structures under normal temperature conditions. Properties of Silica fume concrete under ambient temperature conditions have received considerableresearch attention [2-4]. [7] studied the effect of additives on the concrete properties subjectedto elevated temperatures using two different local aggregates. However, the behavior of Silicafume concrete at elevated temperature has not sufficiently been covered. It is essential tounderstand the residual compressive strength of Silica fume concrete.

2. EXPERIMENTAL WORK

To perform the objectives of this study, basalt aggregate concrete was mixed according toASTM C192. The specifications of the specimens, concrete proportions and testing conditionsare as follows:

Cube specimens with dimensions 100*100*100 mm 3.Cement : Ordinary Portland Cement 350 kg/ m 3 Coarse aggregate: Basalt aggregate (crushed stones)Fine aggregate : Local sandWater : 175 Liters (W/C = 0.50)Silica fume ratio: 0, 5, 7, 10, 15 %Temperatures : 20, 150, 300, 500, 700, 900 o C.Curing age : 28 days

The silica fume used in this study was sprinkled by hand in several layers with the cement andaggregates to ensure a reasonable uniform distribution prior to water addition. For each testingcondition, three specimens were used and the averages of the three readings were obtained.The total number of cube specimens was 90. The specimens were cured in water tanks prior toheating, after 28 days of curing, the specimens were taken out of tanks and placed in the

electric oven, its temperature capacity is 1200o

C. Specimens were left in the oven for 4 hoursto achieve a uniform temperature distribution across them. After that, specimens were allowedto cool in the oven for 20 hours, a total of 24 hours of heating and cooling past the curing age.A loading rate of 3 kN/s was used to get the residual compressive strength of concrete.

3. RESULTS AND DISCUSSION

The measured values of the residual compressive strength for the various mixes at elevatedtemperatures are shown in figure 1. Also, the ratio of the residual compressive strength of concrete at elevated temperatures to that at ambient temperature is shown in table 1.

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H. A. M. Bishr

ICCBT 2008 - A - (019) pp217-220 219

It is clear that the compressive strength of concrete decreases at elevated temperature exceptaround 300 o C where a considerable increase in the compressive strength is observed. This ismay be due to the evaporation of the free water content which accelerates the hydration andhence increases the compressive strength till 300 o C. For temperatures higher than 300 o C, the

strength of Silica fume concrete starts to decrease. According to [5] this decrease is attributedto the fact that chemically-bound water starts to disintegrate and evaporate at this stage. Thecompressive strength of concrete with 15% silica fume after four hours of exposure at 150,300, 500, 700, 900 o C was 102, 118, 94, 57, and 19 % of its unheated strength respectively,while the compressive strength of blended cement concrete was 103, 112, 95, 78, and 38% of the original unheated value which means that silica fume concrete is sensitive to hightemperatures.

Silica fume concrete samples start to lose the integrity above 500 o C, where the compressivestrength of the specimen is reduced to around 25% of the original value at 900 o C. [6] reportedthat the increase in the compressive strength of concrete heated at 200 o C was as a result of theinternal autoclaving formed in cement paste. The temperature range between 100 to 300 o C isthe most favorable for the formation of such conditions because the steam is liberated mostintensively during this range and additional hydration of cement grains results. The resultsfrom [7] are obtained using two different local aggregates indicate similar trends of decreasingstrength with high temperatures of SFC. The peak value of the strength was around 400 o Cand spalling noticed for some specimens with low water-cement ratio. It should be pointed outthat the change in the results between this research and [7] was due to the difference in theexperimental details such as mix proportions and testing conditions.

4. CONCLUSIONS

The following conclusions can be drawn from this study:1. The compressive strength of concrete with or without silica fume decreases with

increasing temperature, the peak value in the ratio of the compressive strength at hightemperature to that at ambient temperature is observed around 300 o C. This peak valuecould be attributed to the evaporation of free water inside the concrete.

2. Silica fume concrete is more sensitive to high temperatures than blended cementconcrete where the poor performance of silica fume concrete, exposed to elevatedtemperature, compared to plain concrete can be attributed to the effect of vapor

pressure built-up inside the concrete causing expansion and cracking because of the

highly dense structure.3. For all mixes, the compressive strength was found to increase after four hours of exposure to an elevated temperature up to 300 o C. An obvious reduction in thecompressive strength was observed after exposure to 700 o C, increasing thetemperature up to 900 o C causes serious deterioration where the decreasing ratio in thecompressive strength reached to 81% of the unheated strength.

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Table (1): The ratio of the residual compressive strength of concrete at elevated temperatures to that atambient temperature

9007005003001502015% SF0.190.570.941.181.021

10% SF0.220.710.891.221.11 7% SF0.290.70.931.211.0515% SF0.260.740.911.131.0110% SF0.380.780.951.121.031

Figure 1. Effect of temperature on the concrete strength

REFERENCES

[1]. Felicetti, R. and Gambarova, P. G.,"Effect of high temperature on the residual compressivestrength of high strength siliceous concrete" ACI Materials Journal, Vol. 95, No. 4, July-August1998, pp. 395-406.

[2]. Mehta, P.K.," Pozzolanic and cementitious by-production concrete another look" ACI Sp-114,1989.

[3]. Anon, F.,"How microsilica improves concrete", Concrete Construction, Vol. 30, No. 4, pp. 327-329.

[4]. Carlson, M., Hope, R. and Pedersen, J.,"Use of condensed silica fume in concrete", ACIPublication No. SP 91-50, pp. 1013-1030.

[5]. Khoury, G.A., The effect of elevated temperature on concrete. Ph.D. Thesis University of London, London, England.

[6]. Piasta, J., Sawicz, Z. and Rvdzinski, l., "Changes in the structure of hardened cement p aste dueto high temperature", Materials and Structure Journal, pp. 100-109, 1984.

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