コンクリート工学年次論文集，Vol.32，No.1，2010- Technical Paper -INVESTIGATION ON THE RECOVERY OF COMPRESSIVE STRENGTH
OF CONCRETE EXPOSED TO HIGH TEMPERATURE
Qingtao LI*1, Zhuguo LI*2

ABSTRACTTo develop a new repairing technology of fire-damaged concrete, the strength recovery of concrete after
exposed to high temperature was investigated. The effects of temperature, cooling method, performancemodifying agent (PMA), and re-curing time on the compressive strength and its recovery were discussed.
The test results indicate that the strength of heated concrete decreased with raising the heating
temperature for any cooling method, and was smaller when the heated concrete was cooled by water
than in the air. The embrocation of PMA on the heated concrete could accelerate the strength recovery.
Keywords: concrete, compressive strength, recovery, cooling method, high temperature,
performance-modifying agent

1. INTRODUCTION
The compressive strength of concrete deteriorates
when it is subjected to high temperature as in a fire
because of the changes in the chemical composition of
cement paste and in the physical properties ofaggregates.
The mechanical properties of concrete exposed toelevated temperatures have been widely studied. Some
investigations have been focused on the effects ofheating temperature, exposure time, cooling methods
and loading conditions [1-6]. In spite of differing in theproperties of concrete below 300 ℃ , there is a
uniformity of opinion concerning a decrease incompressive strength above 300 ℃ . Then a rapid
reduction of about 40% at 500-600 ℃ [5]. At thetemperature of 600 ℃ , the compressive strength of
concrete is reduced to 50% of its normal strength [3].
At 800℃, the reduction is from 74% to 97% [2]. The
reduction is very sharp beyond 800℃. The relative
strengths of concrete after exposure to 1000℃ and
1200℃ were measured as 13% and 6% [4]. Metin
Husem [1] and Chan [6] studied the influence ofcooling methods on the compressive strength, andpresented that concrete strength decreases withincreasing temperature and the type of cooling affects
the residual compressive strength remarkably. The
decrease of compressive strength is greater cooled inwater than that cooled in the air.
Experimental studies have been performed toinvestigate the strength recovery of the concrete
exposed to high temperature [7-9]. Ichise et al. [8]observed that the compressive strength of high-strength
concrete subjected to temperatures of up to 500℃
recovered up to 80% at the room temperature with
underwater curing. Matsudo [9] studied the recovery of
the ultra-high strength concrete of which the strength is*1*2

above 100N/mm2, and found that the recovery ofcompressive strength after two years is negligible,
compared to concrete with high water-binder ratio.
The fire-damaged concrete has to be strengthened
and repaired in order to lessen the cost of rebuilding theconcrete structure. The usual repairing methods ofstructural concrete include resin injection, polymer
wraps, and so on. Mihashi developed the new methodthat the crack repairing agents were installed as corematerials in shell bodies and previously embedded inconcrete [10]. Nishiwaki developed the concrete thathas the self-healing function, attributed to embedded
heating device and repair agent that is protected byheat-plasticity film and is hardened when heated [11].
In spite that the strength of the fire-damaged
concrete can regain naturally, the recovery extent islittle. There is a need to improve the strength recovery.
The studies on self-repairing concrete were mostly
about the adhesives, such as resin and repair agent
applied to crack repairing. It can’t be generalized toapply to fire-damaged concrete because the fire-damage
not only includes cracks. We do not yet find an
investigation in the technical literature on how to
improve the strength of fire-damaged concrete.
In this study, we investigated the extent andinfluencing factors of the compressive strength
recovery of concrete exposed to high temperature insuccession of the research of carbonation resistancerecovery [12]. The concrete specimens were exposed todifferent high temperatures for 150min, and then cooledin the air or by water jet. The compressive strength test
was carried out after the heated concrete specimens
were cured in the air at ambient temperature in thelaboratory for 0 day, 28 days, 60 days, and 90 days. We
also examined the improvement in the strength
recovery when applying the PMA to the fire-damaged
concrete for developing a new repairing technology.

College student, Graduate School of Science and Engineering, Yamaguchi University, M. E., JCI Member
Associate Prof., Graduate School of Science and Engineering, Yamaguchi University, Dr. E., JCI Member-1163-

2.1 Materials Used and Preparation of Specimens
The cement used in this study was ordinary
portland cement, of which the specific gravity is 3160
kg/m3. Coarse aggregate of crushed stone with amaximum size of 20mm was used. The density atsaturated surface dry condition, and water absorption
ratio of coarse aggregate were 2730 kg/m3, and 0.40%,
respectively. Fine aggregate was sea sand with densityof 2590 kg/m3 at saturated surface dry condition, water
absorption ratio of 1.60%, and fineness modulus of
2.57. Water reducing admixture air-entraining (AE)agent was also used in a dosage of 1.1% by mass of
cement. Polypropylene fiber with a length of 12 mm
was used at 0.2% per 1m3 concrete to improve thespalling resistance of concrete during heating.
Mix proportions of concrete were 1.0(cement) :0.50 (water) : 2.42 (sand) : 2.78 (coarse aggregate) by
mass (Table 1). Unit weight of cement is 346 kg/m3.
216 cylindrical specimens with diameter of 100mm andlength of 200mm were produced.
The specimens were cast, and demoulded after 24hours of moist curing in mould. Afterwards, thespecimens were cured in water of 20±3℃ for 28 days,
and then moved to a room to cure at ambient
temperature naturally for 12 months until they areheated. The compressive strength of the concrete at 28
days and 12 months were 40MPa, and 51MPa,
respectively.

concrete.
In order to develop a technology to improve theproperties of heated concrete, after the heatedspecimens were cooled to room temperature, some ofthem were embrocated with PMA on their surfaces,
using a brush. The surfaces of the specimens werefirstly moistened with water before embrocating thePMA, then were sprinkled with water about 1 lit/m2
after the PMA was dried naturally. The totally used
PMA was about 0.3 kg/m2. The specimens with theapplication of the PMA were cured with sprinklingwater about 1 lit/m2 twice every day for 7 days.
3. RESULTS AND DISCUSSION
3.1 Compressive Strength of heated concrete
The test results of the compressive strength of theconcrete specimens exposed to different temperaturesare shown in Fig. 1. It is observed that the compressive
strength decreased obviously with the increase of theheating temperature. The temperature ranges can bedivided to two stages at 450℃.
The compressive strength of concrete cooled in
the air and by water jet after exposed to hightemperature of 200℃, decreased only 8% and 10% of
its initial value, respectively.
Compressive strength (MPa)

2. EXPERIMENTAL PROGRAM

Table 1 Mix proportions of Concrete
Unit weight (kg/m3)
Cement Water
Sand
CoarseAdmixture
aggregate
346
173
837
962
3.8
2.2 Heating and Cooling Regimes
After they were cured for 12 months, the concrete
specimens were heated up to different temperatures byan electric furnace, in which the temperature wascontrolled to follow the ISO834 standard firetemperature curve. The heating temperatures weremonitored by thermocouple set up in the electric
furnace. In the same time, the inside temperatures of theconcrete specimens were monitored by thermocouples
to ensure the inside to be heated to the desired
temperatures too. After heated at each predeterminedhigh temperature level for 2.5 hours, the concrete
specimens were taken out from the electric furnace andcooled down to room temperature in the air or by waterjet for 15 minutes.
2.3 Embrocation of PMA
The PMA, being viscous liquid, is usually used tostrengthen and repair cracks of concrete to lower its
water permeability. The PMA which is a silicate system
material would react with Ca(OH)2 and othercement-like ingredients in hardened concrete to form
calcium silicate hydrate when the water exists. The
hydration product fills into the pores and cracks in the

60
50
40
30
20
Cooled in the air
Cooled by water jet

10
0
0

200
400
600
800
Temperature (℃)
Fig.1 Variation of compressive strength withtemperature (right after cooled)

Otherwise, at the temperatures of 300℃ and
450 ℃ , the compressive strength reduced rapidly,compared to the temperature of 200℃. After heated at
300℃, the strength loss of concrete was 17.69% when
the concrete were cooled in the air, and the strength loss
was 20.0% when the concrete were cooled by water jet.
Up to 450℃, the reduction of compressive strength was
27.55% when the concrete were cooled in the air, andthe reduction was 33.0% when the concrete werecooled by water jet. High temperatures of 300℃ and
450℃ leaded to a great decrease in the compressive
strength. It may be considered that with increasing
temperature, all of the bound water is removed up, andthe decomposition of hydrates and the destruction of
the gel structure occur, which result in a weak pore
structure in the concrete.
With further increasing the heating temperature
up to 600 ℃ and 750 ℃ , the loss of compressive-1164-

strength became more significant. For the concretecooled in air, the reduction of compressive strength was
52.82% after heated at temperature of 600℃. When theconcrete was cooled in air, the strength loss was
68.26% after heated at temperature of 750℃. The
strength loss was 67.0% when the concrete was cooledby water jet after heated at 600℃. Up to 750℃, thereduction of compressive strength was 82.0% when theconcrete was cooled by water jet. It is clearly that there
was an extremely great decrease in the compressive
strength, compared to the strength loss at temperatureof 25℃, 200℃ , 300℃ and 450℃. This is probably
because of an increase in the decomposition of calcite
and connection of cracks and gas channels in large
quantities. Therefore, we can conclude that the higher
the heating temperature, the greater the strength
reduction.

46.93
45.74

40

Coole in air
Coole by water jet
36.95
34.04

30

24.06

20

16.98 16.19
9.31

10
0
200

Compressive strength (MPa)

41.98
40.68

Compressive strength (MPa)50

300 450 600
Temperature (℃)

50.17
48.38 46.84
44.48

40
30

Cooled by water jet
41.03
38.43
24.03
20.84

20

48.98
46.76 45.07
41.87

Cooled in air
Cooled by water jet
37.48
34.46

40
30

23.00

20.70

20

14.91
14.16

10
0

300 450 600 750
Temperature (℃)(b) Elapsed time after cooled: 28 days
60
Cooled in air(a) Elapsed time after cooled: 0 day
60
Cooled in air

50

50

200

750

Compressive strength (MPa)Compressive strength (MPa)

3.2 Effect of Cooling Method on Strength Recovery
Fig. 2 shows the variation of the compressive
strength of the concrete cooled by different methods
after exposed to high temperatures.
As shown in Fig. 2(a), when the heating
temperatures were 200 ℃ , 300 ℃ , and 450 ℃ , thewater-cooling reduced the compressive strength of
concrete slightly, compared to the air-cooling. However,
the effect of cooling method became remarkable if theconcretes were exposed to 600℃ and 750℃, and thewater-cooling caused a bit more deterioration incompressive strength of concrete than in the case of air

cooling.
It is clear that the compressive strength of theheated concretes was not greatly affected by the coolingmethod when the temperature was not higher than
450℃. The recovery of compressive strength was not
influenced clearly by cooling method when the
re-curing time was 0 days or 28 days, and the strengthof concrete cooled by water jet was smaller than that of
the concrete cooled in air. However, the effect ofcooling method was remarkable after the concrete were
re-cured for 60 days and 90 days when the concrete
were exposed to 600℃ and 750℃, the water-coolingcaused a greater recovery in the strength, compared tothe air-cooling.
The result mentioned above, which the reductionin the compressive strength right after cooled down toroom temperature by water was greater than that cooledin the air, is attributed to that sudden cooling produces
many cracks in the concrete.
As shown in Fig. 2, with the increase in the
re-curing time, the compressive strength of the heatedconcrete cooled by water was close to that of theconcrete cooled in the air, when the temperatures were
not over 450℃. At the temperature of 200℃ and
300℃, most of the free water and part of the boundwater have escaped from the concrete in form of vapor
exclusive of portlandite. At 450 ℃ , part of theportlandite dehydrated and lost the dehydration water.
However, the degree of compressive strength recovery
in case of water-cooling was greater than in case of

14.68
13.13

10

50
40

50.57
49.22 49.69
44.00

Cooled by water jet

41.39
40.56

30
20

20.44

22.30
14.17
13.24

10

0
200 300 450 600 750
300 450 600 750
Temperature (℃)
Temperature (℃)(c) Elapsed time after cooled: 60 days(d) Elapsed time after cooled: 90 days
Fig.2 Effect of cooling method on the strength recovery
0200-1165-

air-cooling, when the concrete were heated attemperatures of 600℃ and 750℃.
At 600℃the portlandite dehydrated to CaO andwater, and lost the dehydration water. At 750℃, thecalcite decomposes to generate CaO and CO2[13]. Thus,
if cooled by water, the dehydrated cement paste would
rehydrate, and the CaO would participate in rehydration
process with water to form new portlandite, which
repair the cracks and improve the pore structure in
fire-damaged concrete.

50

48.98 49.02
45.72
45.07

40

Without application of PMA
With application of PMA

37.48

Compressive strength (MPa)Compressive strength (MPa)

3.3 Effect of PMA Embrocation on Strength Recovery
Fig. 3 shows the effect of PMA embrocation on
the strength recovery of heated concrete. As shown inthis figure, the use of the PMA improved significantly

the compressive strength of the concrete subjected tohigh temperature. When the concrete were exposed tothe temperature in a range of 200-750 ℃ , thecompressive strengths of the specimens embrocated the
PMA were substantially larger than those without using
the PMA. The higher the heating temperature, thegreater the recovery degree of compressive strength of
concrete using the PMA. In other words, when theconcrete was heated below the temperature of 750℃,
the PMA embrocation speeds up more greatly thestrength recovery of hard-damaged concrete than that ofslight-damaged concrete. The application of the PMA
significantly increased the compressive strength of theconcrete specimens subjected to high temperature.
As observed from Fig.3 (e), Fig.3 (f), when the

39.01

28.38

30

23.00
20.40

20

14.91

10
200

300
450
600
Temperature (℃)

42.58
41.03
35.91

30

24.11
20.84

20

13.13

10
200

300

450

600

40

36.95
34.46

30
23.40
20.70

20

16.48
14.16

10

300 450 600 750
Temperature (℃)(b) Cooled by water jet, 28 days after cooled

Without application of PMA
With application of PMA

40

Without application of PMA
With application of PMA

43.68
41.87

Compressive strength (MPa)Compressive strength (MPa)

50

50.88
50.17
48.16
46.84

47.27
46.76

200

750

(a) Cooled in air, 28 days after cooled

60

50

60
50
40

Without application of PMA
With application of PMA
49.02
48.38
46.37
44.48
39.81
38.43
28.84

30

24.03
18.90
14.68

20
10
200

750

300

450

600

750

Temperature (℃)(d) Cooled by water jet, 60 days after cooledCompressive strength (MPa)Compressive strength (MPa)Temperature (℃)(c) Cooled in the air, 60 days after cooled
60
5052.10 52.30
50.57 49.69

Without application of PMA
With application of PMA
45.07
41.39

40

34.47

30
20.44

20

19.81
13.24

10
200

300

450

600

750

60
50

47.68
44.00
40.56

40

41.98

29.53

30

22.30
15.42
14.17

20
10
200

Temperature(℃)

Without application of PMA
With application of PMA

50.37
49.22

300

450

600

750

Temperature(℃)(e) Cooled in the air, 90 days after cooled(f) Cooled by water jet, 90 days after cooled
Fig.3 Effects of PMA embrocation on the strength recovery-1166-

concrete exposed to high temperatures below 600℃
increased gradually with re-curing time. However, theconcrete almost did not show an increase in the strength
after suffered to high temperature of 750℃.
The concrete, which was subjected to thetemperature of 200℃, then cooled in the air, had 92%of the initial compressive strength, then became to be
96.04%, 98.37% and 99.15% after re-cured for 28 days,
60 days and 90 days, respectively. Also the compressive
strengths of the heated concrete, which wereembrocated with PMA after air-cooled, were 96.12%,
99.76%, and 102.15% of its initial value, after placing
28 days, 60 days, and 90 days, respectively.
However, it can be seen from Fig. 4 that thestrength of concrete, cooled in the air and without using
the PMA, decreased with the elapsed time after
exposing to temperatures of 600℃ or 750℃. Moreover,the compressive strengths of the specimens heated at
60

50
40
30
Cooled in air, without PMA
Cooled in air, with PMA
Cooled by water, without PMA
Cooled by water, with PMA

20
10
0

20
40
60
80 100Elapsed time after cooled (days)(b) 300℃

60

Cooled in air, without PMA
Cooled in air, with PMA
Cooled by water, without PMA
Cooled by water, with PMA

50
40
30
20
10
0
0

40
30
20
10

Cooled in air, without PMA
Cooled in air, with PMA
Cooled by water, without PMA
Cooled by water, with PMA

0

60
50
40
30
20
10

Cooled in air, without PMA
Cooled in air, with PMA
Cooled by water, without PMA
Cooled by water, with PMA

0
0
20
40
60
80 100Elapsed time after cooled (days)(c) 450℃

Compressive strength (MPa)

0

50

0
20
40
60
80 100Elapsed time after cooled (days)(a) 200℃

Compressive strength (MPa)60

Compressive strength (MPa)Compressive strength (MPa)

3.4 Time-dependent of the Strength Recovery
The strength recovery of heated concrete withelapsed re-curing time is demonstrated in Fig. 4. It canbe seen that the increase of the compressive strengthwith the elapsed time was dependent on the extent ofhigh temperature-damage. The compressive strength ofCompressive strength(MPa)concrete was exposed to high temperatures of 200℃,
300℃, 450℃, 600℃ and 750℃, the recoveries in thecompressive strength of the concretes embrocated the
PMA after cooled in air, being 1.53MPa, 2.61MPa,
3.68MPa, 14.03MPa and 6.57MPa, respectively, werehigher than those of the concretes without using the
PMA. Moreover, if exposed to 600℃, the strengths of
the air-cooled and the water-cooled concrete appliedwith PMA, which recovered to 67.59% and 57.90% of
the initial strength respectively, were much greater thanthat of the concrete without applying PMA only
recovered to 40.07% and 43.73% of the initial value.
The effect of PMA on the recovery ofcompressive strength of concrete exposed to 600℃ is
more obviously than that the case of 200℃. This result
may be attributed to the permeation of the PMA and therehydration. The cracks of the concrete exposed to
200℃ were fewer than that of the concrete heated up
to 600℃, at which the decomposition of portlandite andcalcite is in a greater degree. Much the PMA permeatedinto the damaged concrete through much the cracks andpores, thus greatly improved the compressive strength
through much rehydration of the decomposed
portlandite and calcite.

60
50
40

Cooled in air, without PMA
Cooled in air, with PMA
Cooled by water, without PMA
Cooled by water, with PMA

30
20
10
0

0
20
40
60
80 10020
40
60
80 100Elapsedtime
after
cooled(days)
Elapsed time after cooled (days)(d) 600℃(e) 750℃
Fig.4 Effect of re-curing time after heated on the strength recovery-1167-

750℃ and then cooled by water jet were lower than at
600℃, and they can’t recover naturally as the case of
600℃ though the water was supplied outside.
The strength recovery of the heated concrete withthe re-curing time, which was embrocated with PMA
after cooled by water or in the air, was more rapid thanthat of the concrete without using the PMA. That is, thePMA would be used to improve the recovery of thecompressive strength of fire-damaged concrete.
3. CONCLUSIONSIn this study, in order to develop a repairing
technology for fire-damaged concrete, the compressive
strength and its recovery of heated concrete were
investigated under different heating temperatures,cooling methods, and re-curing time. And the effect ofembrocating the performance-modifying agent on thestrength recovery was examined in detail. The obtained
conclusions are as follows.
(1) The compressive strength of concretedecreased with raising the heating temperature. When
exposed to 750℃ and cooled by water jet, it was only
18% of its initial strength.
(2) The reduction in the compressive strength of
concrete, which was heated and then cooled in air, wassmaller than that of the concrete cooled by water when
the concrete was re-cured for 0 days and 28 days. Whenthe concrete were re-cured for 60 days and 90 days
after the concrete were exposed to 600℃ and 750℃,
the water-cooling caused a greater recovery in thestrength, compared to the air-cooling.
(3) The PMA embrocation can greatly improve thestrength recovery of the fire-damaged concrete exposed
to temperatures below 750℃. If exposed to 600℃,
the strengths of the air-cooled and the water-cooled
concrete applied with PMA, which recovered to
67.59% and 57.90% of the initial strength respectively,
were much greater than that of the concrete without
applying PMA only recovered to 40.07% and 43.73%of the initial value.
(4) The recovery degree of the compressive
strength varies with re-curing time after heated at thetemperature below 450℃. The compressive strength
increased obviously with the re-curing time. However,the compressive strength of the concrete heated at
600℃ and 750℃ decreased with the elapsed timeafter air-cooled.
As a further research work, we would like tochange the embrocating method of the PMA to improve
the strength recovery of fire-damaged concrete more
greatly.
ACKNOWLEDGEMENT
The authors are grateful to China ScholarshipCouncil for providing a generous support to the author.
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Metin Husem: The Effects of High Temperature-1168-