Concrete Suitable for Higher Temperature

J. Prasad, Associate Professor; A.K. Ahuja, Associate Professor, and M. Shahiq Khan, Deptt. of Civil Engg., Indian Instt of Technology (IITR), Roorkee.

Concrete structures need repair and rehabilitation many a time as they get damaged due to excessive stresses due to unforeseen forces caused by man-made or natural disasters. But there are few situations in practice, where concrete is expected to show signs of damage such as cracks during its service period itself. If precautions are not taken while designing or constructing such concrete structures, they may also demand repair and rehabilitation work. However, with careful design and construction such situations can be avoided. Present paper enlists and recommends various prevention measures which should be incorporated while designing a concrete mix to be used for structural elements which will be subjected to high temperature exposure, thus causing damage during its design life.

Introduction

Pozzolanic concretes are used extensively throughout the world; the oil, gas, nuclear and power industries are among the major users. The applications of such concretes are increasing day by day due to their superior structural performance, environmental friendliness, and energy-conserving implications [1]. Although the concrete in such structures is always provided with adequate insulation, nevertheless, it is difficult to inspect the insulation and there is always a possibility of its deterioration. Also, problems with the cooling system in nuclear power plants may lead to hazardous situations where the concrete gets overheated. The exposure of concrete to elevated temperatures has a direct effect on its compressive, tensile and flexural strength [2, 3]. One of the main reasons for loss of strength in concrete at elevated temperature is the formation of cracks between cement paste and aggregate because of thermal incompatibility between the two ingredients [3]. Present paper reports the results of a study on compressive strength of pozzolanic concrete as well as ordinary concrete after being heated to elevated temperature up to 200⁰C. Since cyclic temperature changes occur in nuclear reactors and can lead to further decrease in concrete strength [4], the experimental program is expanded to also include the effect of cyclic heating on the strength of concrete.

Experimental Programme

The effect of high temperature on the compressive strength of concrete has been thoroughly explored in the present study. The strength of ordinary and pozzolanic concretes incorpo– rating silica fume and ground granulated blast furnace slag at 200⁰C temperature for heating cycle ranging between 0 to 28 days are compared. For this purpose, different concrete mixes are obtained mixing blast furnace slag and silica fume with 43-Grade Ordinary Portland Cement (OPC). To start with, various tests are conducted on cement to determine its physical properties as per recommendations of IS: 4031-1999 and the test results are listed in Table 1.

While making concrete, water– cement ratio is maintained as 0.45 by weight and cement content as 396 kg/m³ in all the cases. Various concrete mixes are prepared by replacing Ordinary Portland Cement (OPC) 43-Grade with silica fume and ground granulated blast furnace slag (GGBFS) at a 5%, 10% and 20% by weight of cement.


GGBFS is processed from granulated slag, a cementitious material that consists of same constituents, which are also present in cement. It has more than 90% glass content or calcium alumino silicates. The physical and chemical properties of GGBFS and micro-silica are given in Table 2.


The physical and chemical characteristics of both micro-silica and GGBFS satisfy the requirement as given in IS: 15388-2003 for micro-silica and IS: 12089-1987 for GGBFS.

The cube specimens of 150 mm size are cast in steel moulds in three layers and compacted on a vibrating table. A total of seventy–two cubes are prepared. The specimens are kept in the moulds for 24 hours, after which they are de-moulded and placed in a water curing tank for 28 days. The specimens are then dried and heated in an electric oven with a maximum temperature of 200⁰C for 7, 14, 21 and 28 heating cycles and tested under compression to failure after cooling. Each heating cycle consists of a heating period of 8 hours after the oven temperature reaches 200⁰C and subsequent cooling period of 12 hours as shown in Figure 1.

Experimental Results

To understand the behavior of heated concrete specimens under varying conditions considered in the present investigation, the results obtained are represented in graphical form as given below.

Compressive Strength of Silica Fume Concrete

The failure load under compression resisted by an average of the test concrete cubes in the same batch is considered for the calculation of compressive strength. The compressive strength of tested specimens is shown plotted against the heating cycles in Figure 2. It is observed that silica fume concrete gives better result in comparison to plain concrete for all heating cycles. It is also noticed that the maximum compressive strength occurs at 7- day heating cycles for all cases of concrete. Compressive strength of 10% and 20% silica fume concrete (on an average) is 38% and 28% respectively higher than plain concrete. It is due to the fact that higher percentage of silica fumes with same water–cement ratio increases porosity inside the concrete and also porosity increases with increasing temperature. Hence, it is better to use lower percentage of silica fume i.e. around 10% only to achieve better compressive strength when concrete is exposed to heating cycles with highest temperature of around 200⁰C.

Compressive Strength of GGBFS Concrete

As in the case of silica fume concrete, ground granulated blast furnace slag (GGBFS) concrete also gives better result in comparison to ordinary concrete for all heating cycles as shown in Figure 3.

It is observed that the compressive strength of ordinary and 20% GGBFS concrete is maximum at 7-day heating cycles after which it decreases, whereas in 5% and 10% GGBFS concrete, strength increases as the heating cycle increases. Compressive strength of all percentage slag concretes are almost equal at 7-day heating cycles with their values around 21% higher than that of ordinary concrete. At 28-days heating cycle, whereas 5% and 10% GGBFS concretes result in 32% and 36% higher strength respectively, compressive strength of 20% ground granulated blast furnace slag concrete is only around 10% more than that of plain concrete. Hence, 10% GGBFS is the optimum quantity of slag for replacement of cement by weight for concrete which is to be exposed to elevated temperature up to 200⁰C for different cycles.

Comparison of Strength of Concrete

Figure 4 compares the performance of 10% pozzolanic concrete with ordinary concrete. Comparison for20% replacement of cement by pozzolanic material by weight is shown in Figure 5. It is observed that whereas both types of pozzolanic concrete show better compressive strength than ordinary concrete, silica fume concrete shows better results than slag concrete.

Conclusions

The following conclusions are drawn from the experimental investigation reported here in.

1. The pozzolanic concretes show better performance at elevated temperature for different heating cycles than ordinary concrete.
2. Compressive strength of 10% and 20% silica fume concrete on an average is 38% and 28% respectively higher than plain concrete.
3. Compressive strength of all percentage slag concretes are almost equal at 7-day heating cycles with their values around 21% higher than that of ordinary concrete.
4. At 28-days heating cycle, whereas 5% and 10% GGBFS concretes result in 32% and 36% higher strength respectively, compressive strength of 20% ground granulated blast furnace slag concrete is only around 10% more than that of plain concrete.
5. Both silica fume based and ground granulated blast furnace slag based concretes can safely be used for construction of structural elements which are expected to be subjected to higher temperature during their service life. Whereas both types of pozzolanic concrete show better compressive strength than ordinary concrete, silica fume concrete shows better results than slag concrete.

References

• Mehta, P.K., “Advancements in Concrete Technology,” Concr. Int. 96(4) (1999), pp. 69-76.
• Blundel, R., Diamond, C. and Browne, R., “The Properties of Concrete Subjected to Elevated
• Temperatures,” Construction Industry Research and Information Association, Report No. 9, London, (1976), pp. 78-87.
• Ghosh, S. and Nasser, K.W., “Effect of High Temperature and Pressure on Strength and Elasticity of Lignite Flyash and Silica Fume Concrete,” ACI Material Journal, V. 93, No.1 (1996), pp. 51-60.
• Bertero, V. and Polivka, M., “Influence of Thermal Exposure on Mechanical Characteristics of Concrete,” Proceedings of International Seminar on Concrete for Nuclear Reactors. ACI SP-34 (1972), pp. 505-531.
• India Standards, IS: 4031-1999, “Indian Standard Methods of Physical Tests for Hydraulic Cement,” Bureau of India Standards, New Delhi.
• India Standards, IS: 8112-1989, “Specification for 43 grade Ordinary Portland Cement,” Bureau of India Standards, New Delhi.
• India Standards, IS: 1 2 0 8 9 - 1 9 8 7 , “Specification for Granulated Slag for Manufacture of Portland Slag Cement,” Bureau of India Standards, New Delhi.