FIRE AND MATERIALS

Fire Mater. 2009; 33:79–88

Published online 21 November 2008 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/fam.987

Effect of elevated temperatures and cooling regimes on normal

strength concrete

A. Ferhat Bingöl∗, † and Rüstem Gül

Civil Engineering Department, Atatürk University, Erzurum, Turkey

SUMMARY
The compressive strength of normal strength concrete at elevated temperatures up to 700◦ C and the effect
of cooling regimes were investigated and compared in this study. Thus, two different mixture groups with
initial strengths of 20 and 35 MPa were produced by using river sand, normal aggregate and portland
cement. Thirteen different temperature values were chosen from 50 to 700◦ C. The specimens were heated
for 3 h at each temperature. After heating, concretes were cooled to room temperature either in water rapidly
or in laboratory conditions gradually. The residual strengths were determined by an axial compressive
strength test. Strength and unit weight losses were compared with the initial values. Throughout this
study, ASTM and Turkish Standards were used. It was observed that concrete properties deteriorated with
the heat; however, a small increase in strength was observed from 50 to 100◦ C. Strength loss was more
significant on the specimens rapidly cooled in water. Both concrete mixtures lost a significant part of their
initial strength when the temperature reached 700◦ C. Copyright q 2008 John Wiley & Sons, Ltd.

Received 25 December 2007; Accepted 8 November 2008

KEY WORDS: concrete; elevated temperatures; fire; compressive strength; cooling regimes

1. INTRODUCTION

Concrete is a composite material that consists essentially of a binding medium within which are
embedded particles of aggregate, usually a combination of fine aggregate and coarse aggregate.
In portland cement (PC) concrete; the binder is a mixture of cement and water [1]. Concrete has
been the leading construction material for a century and its strength is a frequently investigated
property because it gives a good indication of the overall quality of the concrete and is relatively
easy to measure, particularly in uniaxial compression [2, 3].
Concrete may be exposed to elevated temperatures during a fire or when it is near to furnaces
and reactors. Its mechanical properties such as strength, modulus of elasticity and volume defor-
mation decrease remarkably and this results in structural quality deterioration of concrete [4–8].

∗ Correspondence to: A. Ferhat Bingöl, Civil Engineering Department, Atatürk University, Erzurum, Turkey.
†
E-mail: afbingol@atauni.edu.tr

Copyright q 2008 John Wiley & Sons, Ltd.

80 A. F. BİNGÖL AND R. GÜL

Fire represents one of the most severe risks to buildings and structures. Since the work of the
research pioneers, Lea and Stradling who in the 1920s investigated the influence of high temper-
atures on concrete strength, a number of research studies related to the fire resistance of concrete
have been carried out. Initially, research was focused on the chemical and physical changes within
the concrete, such as the decomposition of calcium hydroxide (Ca(OH)2 ), the incompatibility
at the aggregate–cement paste boundary and the crystal transformation of quartz (SiO2 ) [9]. During
the last decade, there has been extensive research on the fire performance of concrete and most
studies put emphasis on aspects such as types of aggregate, addition of fibers, heating rate and
maximum temperature level, methods of testing, etc. [10].
Unless large temperature differentials develop (as in rapid heating), the compressive strength
of concrete at elevated temperatures is usually maintained up to 300◦ C. However, above this
temperature, significant decreases can be anticipated. The magnitudes of the decreases depend on
the nature of the aggregate and the initial moisture content of the specimen. The changes in strength
have been attributed to a combination of decomposition of the hydrated pastes, deterioration of
the aggregates and the thermal incompatibilities between paste and aggregate leading to stress
concentrations and microcracking [11].
When exposed to high temperature, the chemical composition and physical structure of the
concrete change considerably. The dehydration, such as the release of chemically bound water
from the calcium silicate hydrate, becomes significant above about 110◦ C. The dehydration of the
hydrated calcium silicate and the thermal expansion of the aggregate increase internal stresses and
from 300◦ C microcracks are induced through the material. [Ca(OH)2 ], which is one of the most
important compounds in cement paste, dissociates at around 530◦ C resulting in the shrinkage of
concrete [4].
Fire resistance of concrete is affected by many factors, including constituent materials such
as the type of aggregate and cement used in its composition, sizes of structure members and
moisture content of concrete and the environmental factors: rate of heating, maximum temperature
attained, duration of exposure at the maximum temperature, method of cooling after the maximum
temperature is reached and the level of applied load [10, 12]. The influence of elevated temperatures
on the mechanical properties of concrete is important for fire resistance studies. Heat-resistant
materials are increasingly being used for structural purposes. The need for such building materials
is particularly great in chemical and metallurgical industries and for the thermal shielding of
nuclear power plants. In such installations structural members may be subjected to sustained and
cyclic thermal exposures at the lower heat levels, at which the use of refractory materials is not
essential. Concrete generally resists the effects of high temperatures, but in some cases it is aimed
to produce concrete which is more resistant to fire [13].
An assessment of the degree of deterioration of the concrete structure after exposure to high
temperatures can help engineers decide whether a structure can be repaired rather than required to
be demolished [14].
Early studies on the effects of high temperature on the mechanical properties of concrete
primarily used normal strength concrete specimens. In recent years different concrete specimens,
including high-strength concrete [2, 5, 15–17], fiber-reinforced concrete [9, 18–20], lightweight
aggregate concrete [13, 21] were used in high-temperature tests. In the high-strength concrete
studies explosive spalling was the main problem. The lowest temperature at which explosive
spalling occurred was reported to be about 300◦ C and the highest was about 650◦ C in different
papers. Also it is mentioned that lightweight aggregate concretes show better performance than
the normal strength and high-strength concretes [13, 21, 22].

Copyright q 2008 John Wiley & Sons, Ltd. Fire Mater. 2009; 33:79–88
DOI: 10.1002/fam
 EFFECT OF ELEVATED TEMPERATURES AND COOLING REGIMES 81

In this experimental investigation, the effect of elevated temperatures and cooling regimes
after heating process on the compressive strength of concrete mixtures produced by different
water/cement (w/c) ratios was extensively examined. Normal strength concrete is the most
commonly used concrete in practice, in Turkey. That is why normal strength mix groups were
chosen in the study.

2. MATERIALS AND METHODS

ASTM Type I, PC, from Aşkale Cement Factory in Erzurum, Turkey was used in this study. Sixteen
millimeter maximum size natural aggregate was obtained from Altunkent region in Erzincan,
Turkey. The chemical composition of PC is summarized in Table I, physical and mechanical
properties of PC are given in Table II and the properties of aggregate are shown in Table III. Two
groups of normal strength concrete samples are produced with the initial compressive strengths of
20 and 35 MPa. The mix ratios were calculated according to strength by using Graf formula and
the full details of these mixes are given in Table IV.
The concrete mixes were prepared in a laboratory mixer with a capacity of 60 dm3 . For each
mixture, six samples of 100 mm diameter and 200 mm height cylinders were prepared. Hand
compaction was used. The samples were placed in a water tank at 23±2◦ C, 24 h after the casting
and cured until the 27th day. At the end of 28 days of water curing the specimens were dried and unit

Table I. Chemical components of PC.

Component
SiO2 21.15
Fe2 O3 3.44
Al2 O3 5.34
CaO 56.30
MgO 3.01
SO3 2.73
K2 O 0.66
Na2 O 0.31
Cl 0.021
Undetermined 0.55
Free CaO 0.25
LOI 6.51

Table II. Physical and mechanical properties of PC.

Specific gravity (g/cm3 ) 2.99

Setting time start (h) 3.01
Setting time end (h) 3.59
Volume expansion (Le Chatelier, mm) 2
Compressive strength (MPa)
2 days 20.6
7 days 35.0
28 days 46.5

Copyright q 2008 John Wiley & Sons, Ltd. Fire Mater. 2009; 33:79–88
DOI: 10.1002/fam
82 A. F. BİNGÖL AND R. GÜL

Table III. Properties of aggregate.

Aggregate size Specific gravity Water absorption (%)
0–2 2.26 6.48
2–4 2.41 3.69
4–8 2.59 2.92
8–16 2.61 2.24

Table IV. Mix proportions of groups.

I (C20) II (C35)
Mix group Volume Weight Volume Weight
Cement 103.00 307.97 131.75 393.93
Water 194.00 194.00 194.00 194.00
Air 10 — 10 —
Agg. 0–2 138.60 334.03 132.85 320.17
Agg. 2–4 138.60 360.36 132.85 345.41
Agg. 4–8 138.60 358.97 132.85 344.08
Agg. 8–16 277.20 723.49 265.70 693.48

weights are determined. The dried samples are heated to different temperatures, then three samples
from each group were cooled in water and three samples were cooled in laboratory conditions
after heating. After heating and cooling processes, specimens were tested for compressive strength
in accordance with ASTM C 39.
The effect of high temperatures on the compressive strength of the produced concretes was
investigated. The temperature values were chosen as 50, 100, 150, 200, 250, 300, 350, 400,
450, 500, 600 and 700◦ C in this study. Unheated specimens were also produced and tested for
comparison with the heated specimens. A laboratory-type furnace, with a temperature range of
1000◦ C, was used for the heating process. The heating rate of the furnace was 12–20◦ C/min.
After heating the specimens up to the reference temperature, the furnace temperature was kept
constant for 3 h. Heated specimens were cooled to room temperature either in ambient laboratory
conditions or in a water tank and the effect of cooling regimes was investigated.

3. RESULTS AND DISCUSSION

3.1. Fresh and hardened concrete unit weight

The unit weights of fresh concrete specimens are given in Table V. The hardened concrete samples
were tested, after 28 days, for unit weight in accordance with ASTM C 332. The unit weight of
hardened concrete samples was measured both before and after heating to target temperatures. The
results are shown graphically in Figures 1 and 2 for C20 and C35, respectively.
Unit weights decrease as temperature increases. There is weight reduction of the specimens
caused by released water. Because of the released bound water from the cement paste, air voids
occur in concrete. The structural integrity of the specimens is deteriorated as confirmed by a

Copyright q 2008 John Wiley & Sons, Ltd. Fire Mater. 2009; 33:79–88
DOI: 10.1002/fam
 EFFECT OF ELEVATED TEMPERATURES AND COOLING REGIMES 83

Table V. Test results of fresh concrete.

Initial strength Theoretical slump Measured slump Theoretical unit weight Measured unit weight
(MPa) (cm) (cm) (kg/m3 ) (kg/m3 )
20 12±3 15.4 2278.82 2361.81
35 7±3 7.8 2291.07 2374.12

2400

2350

2300
Unit Weight (kg/m )
3

2250

2200

2150 Fresh Concrete

Before Heating
2100 After Heating

2050
0 100 200 300 400 500 600 700 800
Temperature (ºC)

Figure 1. Fresh and hardened unit weights of C20.

2400

2350
Unit Weight (kg/m )

2300
3

2250

2200
Fresh Concrete
Before Heating
2150
After Heating

2100
0 100 200 300 400 500 600 700 800
Temperature (ºC)

Figure 2. Fresh and hardened unit weights of C35.

reduction in weight at any temperature elevation. Reduction in the weight confirms mass loss of
concrete material and the growth of the air voids portion [5]. For both concrete types up to 400◦ C
the reduction in unit weight are at 4% levels. At 400◦ C, this ratio is 6% for both concretes. The
maximum losses are recorded at 600 and 700◦ C. At these temperatures weight losses are 9, 70%
for both mixes. Sancak and Şimşek [23] reported unit weight losses for normal strength concrete
in their experimental study, as 5 and 8% for 400 and 800◦ C, respectively. The results of this study
are in agreement with these values.

Copyright q 2008 John Wiley & Sons, Ltd. Fire Mater. 2009; 33:79–88
DOI: 10.1002/fam
84 A. F. BİNGÖL AND R. GÜL

3.2. Compressive strength of concrete exposed to elevated temperatures

As mentioned before, the change of mechanical properties of concrete subjected to high temperature
is influenced by many factors. Generally, an increase in exposed temperature causes concrete to
gradually lose its mechanical strength.
The compressive strengths of C20 and C35 concrete groups subjected to elevated temperatures
in a range of 50–700◦ C and cooled by different methods in this experimental study are shown in
Figures 3 and 4, respectively. The percentage change in compressive strengths from the effect of
elevated temperatures is also presented in Table VI and discussed below.
When the temperature is 50◦ C the compressive strength of C20 concrete specimens showed an
increase for both cooling regimes. Increase in compressive strength is more for air-cooled specimens
than the water-cooled specimens. For 100◦ C, compressive strength of air-cooled specimens of C20
is more than the initial strength, but water-cooled specimens’ strength began to decrease. After

30

Air Cooled
Compressive Strength (MPa)

25
Water Cooled

20

15
R2 = 0.9111

10
R2 = 0.9224
5

0
0 200 400 600
Temperature (ºC)

Figure 3. Residual compressive strength of C20 concrete specimens.

45

40 Air Cooled
Compressive Strength (MPa)

Water Cooled
35

30

25 2
R = 0.9153
20

15
2
10 R = 0.9501

0
0 100 200 300 400 500 600 700
Temperature (ºC)

Figure 4. Residual compressive strength of C35 concrete specimens.

Copyright q 2008 John Wiley & Sons, Ltd. Fire Mater. 2009; 33:79–88
DOI: 10.1002/fam
 EFFECT OF ELEVATED TEMPERATURES AND COOLING REGIMES 85

Table VI. Compressive strength changes of concretes after exposure to high temperatures (%).
C20 C 35
Temperature (◦ C) Cooled on air Cooled in water Cooled on air Cooled in water
23 0 0 0 0
50 +13 +8 −5 −6
100 +20 −5 −9 −13
150 −1 −6 −9 −18
200 −16 −23 −19 −21
250 −19 −26 −20 −33
300 −24 −32 −16 −21
350 −19 −28 −19 −30
400 −31 −58 −20 −43
450 −34 −48 −24 −38
500 −49 −56 −31 −46
600 −50 −60 −43 −63
700 −62 −68 −55 −68

100◦ C, both air-cooled and water-cooled concretes’ compressive strengths can be determined less
than the unheated specimens for C20. The highest strength loss is observed at 700◦ C. For this
temperature the loss amount is 62% for air-cooled concretes, while this value is 68% for the
water-cooled specimens at the same temperature.
The effect of elevated temperatures on the C35 specimens acted in a similar manner as the C20
specimens. However, no strength increase was recorded for this group at any temperature value.
The strength loss of this group is 5% of the initial compressive strength for 50◦ C at air-cooled
concretes while a 6% reduction is seen for water-cooled specimens at the same temperature. As
temperature increased strength decreased. Moreover, no significant strength loss was observed
between 200 and 400◦ C for the air-cooled specimens. Similarly Dias et al. [24] pointed out that the
original strength of concrete is maintained until 300◦ C. Also, Abrams [25] reported that concretes
did not start losing strength until 300◦ C, irrespective of the aggregate type. When the temperature
reached 700◦ C the strength losses for air-cooled and water-cooled C35 specimens were determined
to be 55 and 68%, respectively.
It was observed that both of the mix groups lost a significant part of their initial strengths when
the temperature reached 700◦ C and none of the groups could retain its structural properties. As the
temperature increased, micro and macro cracks and color changes are observed on the specimens.
The cracks were in macro level at 700◦ C. Figure 5 represents the cracks on the specimens heated
up to 700◦ C.
In conclusion; the concrete specimens, which were heated to 50–700◦ C and kept at these
temperatures for 3 h lost some of their initial compressive strength with the increase in temperature,
but less strength loss was observed in the groups that was cooled in air gradually. The fire is
generally extinguished by water and CaO turns into [Ca(OH)2 ] causing cracking and crumbling
of concrete. Therefore, the effects of high temperatures are generally visible in the form of surface
cracking and spalling [4]. The crack development and separation between aggregate and cement
paste may take place on an appreciable scale during cooling. The thermal stresses induced by
temperature gradients in a cooling period cause severe cracks after the first heating. The behavior
of heated cement paste in the post cooling period is a function of the absorption of moisture from

Copyright q 2008 John Wiley & Sons, Ltd. Fire Mater. 2009; 33:79–88
DOI: 10.1002/fam
86 A. F. BİNGÖL AND R. GÜL

Figure 5. Cracks on the concrete surface after exposure to 700◦ C.

1
Relative Magnitude of Com. Str.

0.5
Mindess's Curve
Log. (C20 air cooled)
Log. (C35 air cooled)
Log. (C20 water cooled)
Log. (C35 water cooled)

0
0 200 400 600 800 1000 1200
Temperature (°C)

Figure 6. Comparison of the test results with Mindess’s curve.

the surrounding medium. Considerable loss and gain in strength have been reported by different
researchers. The strength loss is generally attributed to rehydration of lime accompanied by an
increase in volume [8]. The reason for more reduction in compressive strength for water-cooled
specimens may be the thermal shock. Thermal shock due to rapid cooling causes a bit more
deterioration in strength than in the case of gradual cooling without thermal shock [2].
All other researchers pointed out that strength decreases with the increase in the temperature.
Mindess et al. [11] offer a curve for compressive strength–temperature relationship in their book.
The test results of this study are compared with this curve in Figure 6, graphically. In comparison
to Mindess’s findings, the results from this study show a more severe loss in strength.

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DOI: 10.1002/fam
 EFFECT OF ELEVATED TEMPERATURES AND COOLING REGIMES 87

On the other hand Neville [26] indicated that the effect of increase in temperature on the strength
of concrete is small and somewhat irregular below 250◦ C, but above 300◦ C a definite loss of
strength takes place. Similar results are obtained in this study.

4. CONCLUSIONS

The results obtained from this investigation indicate the following.

High temperatures can be divided into distinct ranges in terms of effect on concrete strength. In
the range of 50.100◦ C, an increase in strength was observed in C20 and a slight decrease observed
in C35 concretes. Until 400◦ C, air-cooled concrete maintained 80% of original strength, while an
average loss of 30% strength was observed in water-cooled specimens. After 400◦ C, both types
of concrete lost their strength rapidly and the strength loss was more in water-cooled specimens.
The mechanical strength of C20 decreased in a similar manner to that of C35 when subjected to
high temperatures. Moreover, air-cooled specimens maintained a greater proportion of their relative
residual compressive strength than the water-cooled specimens. The highest losses in strength for
both concrete types are recorded in the specimens that are heated to 700◦ C and then cooled in
water. Both concretes could keep only 32% of their initial strengths in these groups. No explosive
spalling was observed during the tests. But insignificant crumbling was observed on the specimens
heated over 500◦ C. This effect was more evident in the water-cooled specimens.

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