Materials and Structures/Matdriaux et Constructions, Vol. 29, December 1996, pp 616-619

Comparison between high strength concrete and normal strength concrete subjected to high temperature

Sammy Y. N. Chan t, Gai-fei Peng 1 and John K. W. Chan 2 (1) Hong Kong Polytechnic University, Hung Horn, Kowloon, Hong Kong

(2) Hong Kong University, Pokfulam, Hong Kong

A B S T R A C T

Two normal strength concretes and three high strength concretes, with 28-day compressive strengths of 28, 47, 76, 79 and 94 MPa respectively, were used to compare the effect of high temperatures on high strength concrete and normal strength concrete. After being heated to a series of maximum temperatures at 400, 600, 800, 1000 and 1200~ and maintained for i hour, their compressive strengths and tensile splitting strengths were determined. The pore size distribution of hardened cement paste in high strength concrete and normal strength concrete was also investigated. Results show that high strength concrete lost its mechanical strength in a manner similar to or slightly better than that of NSC. The range between 400 and 800~ was critical to the strength loss of concrete with a large percentage of loss of strength. Microstructural study carried out revealed that high temperatures have a coarsening effect on the microstructure of both of high strength concrete and normal strength concrete.

RI~SUMI~

On a compar{ l'effet des hautes temperatures sur deux b~tons de re'sistance ordinaire et trois b~tons de haute r&is- tance, want des re'sistances a la compression a 28 jours de 28, 47, 76, 79 et 94 MPa, respectivement. Ces b&ons ont ~t~ chauff~s selon une ser~e" ' de temperatures" maximales de 400, 600, 800, 1 000 et 1 200~ et maintenus pendant une heure ; ensuite, on a d~termin~ leurs r&is- tames a la compression et leur r&istance en traction. On a 8galement {tudi~ la r~partition du diamftre des pores des pdtes de ciment durci des b~tons de haute r&istance et de re'sistance ordinaire. Les b&ons de haute re'sistance mon- trent une perte de re'sistance me'canique similaire, ou lSg&e- ment moindre, aux b&ons de r&istance ordinaire. La gamme des temperatures entre 400 et 800~ s'est av&{ critique en ce qui concerne les plus forts pourcentages de perte de re'sistance du b~ton. Une ~tude de la microstructure a montr~ que les hautes temperatures ont pour effet de rendre plus grossifre la microstructure des b~tons de haute r&istance et des b~tons de r&istance ordinaire.

1. INTRODUCTION

Fire remains one of the serious potential risks to most buildings and structures. The extensive use of concrete as a structural material has led to the demand to fully understand the effect of fire on concrete. Generally, concrete is thought to have good fire resistance [1-3]. With the further development and use of high strength concrete (HSC), however, some doubts about its fire resistance have emerged [4].

Although a great deal of research has been conducted on the fire resistance of concrete, since HSC is a relatively new type of concrete, knowledge about the performance of HSC subjected to fire is limited in comparison with that of normal strength concrete (NSC). In this body of research, HSC was found to be prone to spalling under high temperature in some cases [4, 5, 9]. The possible reason might be that the dense, hardened cement paste keeps the moisture vapor from escaping under high tem- perature. Considerable pore pressure is therefore estab-

lished. Naturally, the spalling problem led to suspicion regarding the behavior of HSC under fire conditions. Apart from spalling, whether HSC suffers in terms of mechanical strength to a greater degree than NSC in the presence of fire is also very important to ensuring that HSC can be used safely in buildings.

Strictly speaking, the term "fire resistance" applies to the behavior of structural elements rather than to an individual material forming part of the elements. However, one of the major factors determining the fire resistance of a concrete structure is the capacity of the concrete material to insulate heat from reaching the embedded reinforcement and to withstand heat and the subsequent action of water and cooling without unduly losing strength and without explosive spalling. To inves- tigate the effect of high temperatures and to obtain nec- essary information for evaluanng the structural safety and establishing reparation methods, the residual strength and properties of HSC that has been exposed to high temperatures should be deterlmned.

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Y.N Chan, Peng, K.W. Chan

Table I - Mix proportion and compressive strength of five types of concrete Mix Proportion (kg/m3) W/C Compressive Strength (MPa)

Concrete Mix Aggregate Cement Sand

200mm lOmm

806 403

806 403

785 393

834 417

872 436

Water 28 day 90 day

NSC-1 330 645 219 0.66 28 37

NSC-2 360 645 205 0.57 47 57

HSC-1 550 478 190 0.35 76 84

HSC-2 550 437 173 0.31 79 91

HSC-3 550 433 152 0.28 94 118

1400

G" lOOO ..

g 800 v

600

40o

200

0 0 120 240 360 480 600

Timo (rain)

Fig. 1 - Experimental temperature-t ime curve and standard curve r e c o m m e n d e d in BS476:Part 20:1987.

In this paper, five concrete specimens including HSC and NSC were prepared to compare the behaviour of HSC and NSC under high temperatures. Their compres- sive strength and tensile splitting strength were deter- mined, and samples were also selected to identify the pore size distribution of their hardened cement paste.

2. EXPERIMENTAL DETAILS

Concrete specimens of 100-mm cubes and 100-mm diameter cylinders were prepared using ordinary Portland cement (OPC), crushed granite of 10 mm and 20 mm in size, sand and a superplasticizer. The OPC used in the experiment complied with the requirements ofBS12:1991. The mix proportion of the concretes and the 28-day compressive strength are given in Table 1.

After demoulding at one day of age, the specimens were cured in 20~ water for 27 days and then cured in an environmental chamber under a controlled condition of 20~ temperature and 75% RH until testing.

At an age of 90 days, the specimens were heated in an electrical furnace up to 400, 600, 800, 1000 and 1200~ and the maximum temperature was maintained for 1 hour. The initial heating rate was set at 5~ per minute, and the rate was lowered to 2.5~ per minute and then 1.7~ per minute after reaching 600~ and 900~ respectively. The time-temperature curve of the furnace, as well as the standard curve as r e c o m m e n d e d in

BS476:Part20:1987, are given in Fig. 1. After the specimens were allowed to cool naturally to

room temperature, the compressive strength and tensile split t ing strength were de te rmined according to BS1881:Part 120:1983 and BS1881:Part117:1983, respec- tively. Six specimens were prepared for each reported test result.

The pore size distribution was developed by using a mercury intrusion porosimeter (MIP), which has a pres- sure measurement range from 0.01 MPa (1.5 psi) to 207 MPa (30,000 psi). The contact angle was selected at 141 ~ , so the measurable pore size range is 0.007 to 144 gm. The samples, in the form of pellets of about 5 mm in size, were made of hardened cement paste retrieved from the specimens by sawing.

3. RESULTS AND DISCUSSION

The behavior of concrete exposed to high temperatures is a result of many competing factors. Generally, for con- crete matured with an increase in exposed temperature, concrete gradually loses its mechanical strength.

The results of the residual compressive strength of concrete after exposure at 400, 600, 800, 1000 and 1200~ are shown in Table 2 and Figure 2. The rela- tionship between compressive strength and exposure temperature was found to be similar to that reported previously [5, 6]. From the viewpoint of strength loss, there were three temperature ranges, 20 to 400~ 400 to 800~ and 800 to 1200~ where the strength loss of concrete was markedly different. Up to 400~ only a small part of the original strength was lost, between 1% and 10% for HSC and about 15% for NSC. The severe compressive strength loss occurred mainly within the 400 to 800~ range. This may be regarded as a common feature for OPC concrete, because the hardened cement paste, which is the main source of the concrete's strength, undergoes the dehydration of C-S-H gel and loses its cementing ability under these conditions [2, 7, 8]. For the temperature range between 400 and 600~ HSC performed better, since it maintained a higher percent- age value of its residual strength, as shown in Table 2.

Above 800~ only a small part of the original com- pressive strength was left, about 9 to 20%. In this case, both the HSC and NSC were structurally damaged.

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Materials and Structures/Mat~riaux et Constructions, Vol. 29, December 1996

Table 2 - Residual compressive strength of concrete exposed to different temperatures

Residual Compressive Strength (MPa) Concrete Mix Maximum Temperature

20oc 400oC 600oC 800~ 1000~ 1200~

NSC-1 38 (100%)* 28 (75%)* 15 (39%)* 5.6 (15%)* 3.2 (9.0%)* 5.8 (16%)*

NSC-2 57 (100%)* 52 (92%)* 39 (69%)* 29 (52%)* 13 (24%)* 6.8 (12%)*

HSC-1 84 (100%)* 84 (100%)* 46 (55%)* 24 (24%)* 8.1 (9.6%)* 10 (12%)*

HSC-2 91 (100%)* 83 (91%)* 46 (51%)* 27 (30%)* 8.9 (9.8%)* 9.1 (10%)*

HSC-3 118 (100%)* 117 (99%)* 29 (25%)* 61 (52%)* 12 (11%)* 13(11%)* *: Percentage strength.

residual

120

100

80

60 m

"~ 40

20 8

0

~ = ~ ~ ~ HSC-1

20 400 600 800 1000 1200 Temperature (Deg C)

Fig. 2 - Compressive s t rength o f five types o f concrete subjected to a variety o f temperatures.

7

5

4

r 1 .__.-----'4k " - ' - ' - - ~ A

0 37 5'7 84 NSC-I

--0---20 C

~ 6 0 0 C

--I----1200 C

NSC-2 HSC- 1 HSC.-2 Concrete strength (MPa) and type

HSC.3

Fig. 3 - Tensile splitting s t rength o f five types o f concrete subjected to a variety o f temperatures.

Some products of sintering reaction between hardened cement paste and aggregates reported [10, 111 were found in the present experimental study within some specimens of HSC and NSC, but the concrete strength as shown in Fig. 2 was only slightly increased by such sintering reaction after exposure to 1200~ temperature.

The results of the tensile splitting strength are shown in Fig. 3. The sharp loss of tensile splitting strength for both HSC and NSC subjected to high temperatures is clearly different from the more gradual loss of compres- sive strength. This is because many micro- and macro- cracks were produced in the specimens due to the ther- mal incompatibility [2, 12, 13] within the concrete. Generally, tensile strength is more sensitive to such cracks than is compressive strength.

0.2

0.15

o 0.1

0.05

0

A 20 Deg C

100 10 I 0.1 0.01

Pore diameter (gin)

(A) Pore size distribution of HCP in NSC-I

0.06

0.05 v

o 0.04

-~ 0.03

.~_ 0.02

~_ 0.01

D o

~ 2 0 DcgC

----D--- 600 DegC

100 10 1 0.1 0.01

Pore diameter (gm)

(B) Pore size distribution of HCP in HSC-1

0.04 ~ 20 Deg C

.~ 0.03 ----IN---600 DegC

3 0.02

o.01

o 0 - 100 10 1 0.1 0.01

Pore diameter (gin)

(C) Pore size distribution of HCP in HSC-3

Fig. 4 - Pore size distr ibution o f hardened cement paste (HCP) before and after exposure to 600~ temper taure .

In this experimental investigation, some specimens of HSC and NSC encountered spalling damage during heating even at a lower temperature, such as 400 or 500~ But there was no evidence that HSC was more

618

Y.N Chan, Peng, K.W. Chan

prone to spalling than NSC. Research on the topic of spalling is in progress and will be reported later.

The pore size distribution of hardened cement paste of concretes NSC-1, HSC-1 and HSC-3 was deter- mined using a mercury porosimeter. The results, given in Fig. 4(A), (B) and (C), confirm the coarsening effect of high temperatures on the pore structure reported pre- viously [8, 12, 14]. The coarsening effect increased the porosity of hardened cement paste, as shown in Fig. 5(A). The increased porosity or the increased cumula- tive volume of pores larger than 0.1 gm, which influ- enced the strength of hardened cement paste [14], was one of the factors besides the cracking and dehydration of C-S-H gel within the concrete that have contributed to the strength loss of concrete. It is apparent from this figure that the cumulative pore volume of hardened cement paste in concrete subjected to an elevated tem- perature increases with decreasing concrete strength.

Meanwhile, the cumulative volume of pores in the range greater than 1.3 gm, which should be responsible for the permeability of hardened cement paste [15], was also increased after exposure to 600~ as shown in Fig. 5(B). Therefore, high temperatures have reduced the permeability-related durability of HSC and NSC as well as their mechanical strength. This deterioration was more severe for NSC.

4. CONCLUSIONS

The mechanical strength of HSC decreased in a simi- lar manner to that of NSC when subjected to high tem- peratures of up to 1200~ Under the condition of elec- trical heating, there was no special danger of spalling for HSC although the hardened cement paste within it was much denser than in NSC.

High temperatures can be divided into three ranges in terms of effect on concrete strength loss, namely 20 to 400~ 400 to 800~ and above 800~ In the range of 20 to 400~ HSC maintained its original strength while NSC lost its strength slightly. In the range of 400 to 800~ both HSC and NSC lost most of their original strength, especially at temperatures above 600~ This range, within which the unavoidable dehydration of C- S-H gel in OPC paste occurred to a greater degree than that within 20 to 400~ may be regarded as critical to the strength loss of concrete. Above 800~ only a small portion of the original strength was left for both HSC and NSC. Therefore, it is only within the range of 400 to 800~ that research efforts can be carried out to reduce the strength loss and improve the thermal behav- ior of concrete. Like NSC, HSC lost its tensile splitting strength more sharply than its compressive strength, even at temperatures below 600~

Under high temperatures, HSC and NSC experi- enced a change in pore size distribution, called the "coarsening effect". This effect is believed to be one of the reasons for the strength loss at temperatures below 600~ This effect also reduced the permeability and hence the durability of concrete which has been exposed to high temperatures.

Fig. 5 - Cumulative pore volume of HCP in concrete before and after exposure to 600~ temperature.

REFERENCES

[1] Lea, F.M., 'The Chemistry of Cement and Concrete' (Edward Arnold Ltd., London, UK, 1983) 656.

[2] Metha, P.K., 'Concrete: structure, properties and materials' (Prentice-Hall, Inc., USA, 1986) 129-132.

[3] Curwell, S. et al., 'Building and Health' (RIBA Publishers Ltd., London, UK, 1990) 156.

[4] Jahren, P.A., 'Fire Resistance of High Strength/Dense Concrete with Particular Reference to the Use of Condensed Silica Fume - A Review', in 'Fly Ash, Silica Fume, Slag and Natural Pozzolans in Concrete' , Proceedings of the Third International Conference, ACI SP-114, Detroit, USA, 1989, 1013-1049.

[5] Castillo, C. and Durrani, A.J., 'Effect of transient high tempera- ture on high-strength concrete', ACI Materials Journal, January- February 1990, 47-53.

[6] Sarshar, R. and Khoury, G.A., 'Material and environmental fac- tors influencing the compressive strength of unsealed cement paste and concrete at high temperatures', Magazine of Concrete Research, 45 (162) (1993) 51-61.

[7] Mindess, S. and Young, J.F., 'Concrete' (Prentice-Hall. Inc., Englewood Cliffs, New Jersey, USA, 1981) 530.

[8] Crook, D.N. and Murray, M.J., 'Regain of strength after firing of concrete', Magazine of Concrete Research 22 (72) (1970) 149-154.

[9] Schneider, U., 'Fire Resistance of High Performance Concrete', Proceedings of the RILEM International Workshop. Vienna, Austria, February 1994, 237- 242.

[10] Lydon, F.D., 'Development in Concrete Technology-I' (Applied Science Publishers Ltd., London, UK, 1979) 37-38.

[11] Khoury, G.A., 'Compressive strength of concrete at high tem- perature: A reassessment', Magazine of Concrete Research, 44 (161) (1992) 291-309.

[12] Tanaka, H., et al., 'Properties after being heated and re-hydra- tion of hardened cement paste', Cement and Concrete (Japan), April, 1983, 34-40.

[13] Woods, H., 'Durability of Concrete Construction' (ACI & The Iowa State University Press, USA, 1968) 149-154.

[14] Rostasy, R,.S., Weiss, R. and Wiedemann, G., 'Changes of pore structure of cement mortar due to temperatures', Cement and Concrete Research 10 (1980) 151.

[15] Neville, M., 'Properties of Concrete' (Pitman Publishing Limited, London, UK, 1981) 32.

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