In China, accidental fires are known to occur during construction, causing concrete to be exposed to high temperatures when it is at an early age (i.e. ‘‘young’’). In this paper, compressive and splitting tensile strengths of concretes cured for different periods and exposed to high temperatures were obtained. The effects of the duration of curing, maximum temperature and the type of cooling on the strengths of concrete were investigated. Experimental results indicate that after exposure to high temperatures up to 800 1C, early-age concrete that has been cured for a certain period can regain 80% of the compressive strength of the control sample of concrete. The 3-day-cured early-age concrete was observed to recover the most strength. The type of cooling also affects the level of recovery of compressive and splitting tensile strength. For early-age concrete, the relative recovered strengths of specimens cooled by sprayed water are higher than those of specimens cooled in air when exposed to temperatures below 800 1C, while the changes for 28-day concrete are the converse. When the maximum temperature exceeds 800 1C, the relative strength values of all specimens cooled by water spray are lower than those of specimens cooled in air.
Trang 1Experimental study of mechanical properties of normal-strength concrete
exposed to high temperatures at an early age
Department of Civil Engineering, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai 200240, PR China
a r t i c l e i n f o
Article history:
Received 6 June 2008
Received in revised form
17 June 2009
Accepted 19 June 2009
Keywords:
Compressive strength
Concrete
Curing
High temperature
Mechanical properties
a b s t r a c t
In China, accidental fires are known to occur during construction, causing concrete to be exposed to high temperatures when it is at an early age (i.e ‘‘young’’) In this paper, compressive and splitting tensile strengths of concretes cured for different periods and exposed to high temperatures were obtained The effects of the duration of curing, maximum temperature and the type of cooling on the strengths of concrete were investigated Experimental results indicate that after exposure to high temperatures up to
800 1C, early-age concrete that has been cured for a certain period can regain 80% of the compressive strength of the control sample of concrete The 3-day-cured early-age concrete was observed to recover the most strength The type of cooling also affects the level of recovery of compressive and splitting tensile strength For early-age concrete, the relative recovered strengths of specimens cooled by sprayed water are higher than those of specimens cooled in air when exposed to temperatures below 800 1C, while the changes for 28-day concrete are the converse When the maximum temperature exceeds
800 1C, the relative strength values of all specimens cooled by water spray are lower than those of specimens cooled in air
&2009 Elsevier Ltd All rights reserved
1 Introduction
Fire has become one of the greatest threats to buildings Being
a primary construction material, the properties of concrete after
exposure to high temperatures have gained a great deal of
Although concrete is generally believed to be a fireproof material,
studies have shown extensive damage and even catastrophic
of concrete at high temperature degrade mainly because of
two relevant mechanisms: mechanical and physico-chemical
the temperature reaches about 300 1C, the interlayer calcium
silicate hydrate (C–S–H) water and some of the combined water
from C–S–H and sulfoaluminate hydrates will evaporate Calcium
compounds in cement paste, dissociates at around 530 1C,
resulting in the shrinkage of concrete The chemical changes in
the aggregates occur at temperatures above 600 1C The
mechan-ical degradation of concrete takes place due to the formation of
cracks and microcracks They appear in concrete because of
self-equilibrating stresses (caused by incompatible thermal strains) when concrete is rapidly heated up or cooled down and the
the factors that influence the strength of concrete at high temperature can be divided into two groups: material properties and environmental factors Some research has studied the effect of material properties, such as properties of the aggregate, cement paste, aggregate–cement paste bond and their thermal compat-ibility with each other, on the resistance of concrete In addition, heat resistance of concrete is affected by environmental factors such as heating rate, duration of exposure to maximum tempera-ture, cooling rate, loading conditions and moisture regime
In China, large fire accidents occur every year during construction Normally, in these situations the concrete in the field is at its early age (i.e ‘‘young’’) The inner structure and chemical composition of early-age concrete are different from that
in service due to uncompleted hydration at an early age There are few or no studies in the technical literature to determine how these early-age concretes behave after exposure to high tempera-tures and whether they are able to recover to the design strength after certain curing Thus, an experimental study on the strength properties of early-age concrete after exposure to high tempera-ture was carried out in this paper The compressive and splitting tensile strengths of concrete after different curing ages, where the specimens were kept in the curing room for another 28 days after exposure to high temperatures, were examined The effects of
Fire Safety Journal
0379-7112/$ - see front matter & 2009 Elsevier Ltd All rights reserved.
Corresponding author Tel.: +86 21 54705110.
E-mail address: hntchen@sjtu.edu.cn (B Chen).
Trang 2properties of cement and type C fly ash (FA) produced by the
siliceous river sand (fineness modulus of 2.8) and nature granite
crushed stone (maximum size of 19 mm) were used as aggregates
In this paper, the designed compressive strength of concrete
is 35 MPa at 28 days, which is a typical value for concrete for
most civil engineering structures in China The designations,
proportions and properties of the concrete mixtures are given in
Table 2
2.2 Test procedure
casting, the specimens were left in the molds for 24 h at room
temperature of 20 1C After demolding, the specimens were placed
in a curing room at a temperature of 20 1C and relative humidity of
ages, i.e 1, 3, 7, 14 and 28 days Before testing, the surface water of
the specimens was wiped off with a damp cloth All samples were
heated in an electrical resistance furnace at a rate of 10 1C/min,
which was controlled by a computer The heating rate of 10 1C/min
simulate the fire accidents, in which the temperature rises rapidly
method After the cooling period, the specimens were placed into the curing room at a temperature of 20 1C and relative humidity of
specimens were taken out for compressive and splitting tensile strength tests The compressive strength was measured by a testing machine of 2000 kN capacity at a loading rate of 2.5 kN/s The splitting tensile strength test was conducted on cubes as per ASTM C 496-89 Six companion specimens were tested for each property, and average values were recorded
3 Experimental results and analysis 3.1 Testing phenomena
During the testing, different curing-age concrete samples heated to different temperatures showed different phenomena When the temperature increased at the rate of 10 1C/min, some amounts of water mist appeared near the entrance of the furnace before the temperature reached 200 1C Meanwhile, the amount
of water mist decreased with increasing curing age For those concrete specimens with only 1 and 3 days of curing, large amounts of white vapor appeared around the entrance of the furnace The main reason is that the cement inside the early-age concrete is not fully hydrated and most of the water is in a free state When the temperature reached 400 1C, for concrete samples with curing age over 7 days, a great deal of moisture began to evaporate, which resulted from dehydrations of C–S–H and AFt White water mist was emitted from the entrance of the oven and thickened when peak temperature was maintained After the temperature exceeded 400 1C, higher temperature led to thicker water mist release When maintaining a peak temperature of
600 1C for 50 min, water mist around the entrance of the oven almost dissipated, indicating the completion of the dehydrations
of C–S–H and AFt
The damage to the concrete after being subjected to high temperatures can be roughly evaluated by observing the concrete surface Thus, assessment of fire-damaged concrete usually starts with visual observation of color change, cracking and spalling of
exposure to 200 and 400 1C temperatures The color and appearance of those specimens was the same between those cooled in air and ones cooled by water spray The concrete started
to crack when the temperature increased to 600 1C, but the effect was not significant at that temperature level At this temperature, the color of the specimens cooled in air became white, while those cooled by water spray became yellow The cracks became very pronounced at 800 1C and extensively increased at 900 1C The color of the specimens became white and even red in some cases,
specimens disappeared, especially those concrete specimens with only 1, 3 and 7 days of prior curing
Table 1
Physical, chemical and mechanical properties of cement and fly ash.
Chemical composition (%)
Physical and mechanical properties of cement
Compressive strength (MPa) of cement
Table 2
Concrete mix design.
Portland
cement
(kg/m 3
)
Fly ash
(kg/
m 3
)
Coarse aggregate (kg/m 3 )
Sand (kg/m 3 ) Water (kg/
m 3 )
Super-plasticizer (kg/m 3 )
Slump (mm)
Trang 33.2 Strengths
The cooled specimens were placed into a curing room for
another 28-day curing and then removed for strength tests
Compressive and splitting tensile strengths of air-cooled and
sprayed-water-cooled concrete specimens heated to 20 1C (control
3.2.1 Compressive strength
The relative recovered compressive strength values of
air-cooled and sprayed-water-air-cooled concrete specimens are shown
inFigs 3 and 4, respectively The relative recovered strength was
calculated as the percent retained concrete strength with respect
to the strength of the unheated specimen at an age of 28 days
concrete specimens except for those initially aged for 1 day
increased by about 10–15% at 200 1C in comparison with the
control specimens The strength gain under 200 1C can be partially
evaporation of free water, which leads to greater van der Waal’s forces as a result of the cement gel layers moving closer to each
concrete is rather gradual, residual moisture in concrete allowed for accelerated hydration at the early stage of heating the concrete
to high temperatures Further hydration of cementitious materials
is another important cause of the hardening of hydrated cement paste For concrete specimens aged for 1 day, the strength was too low to be damaged by high temperature because of the unhydrated cement materials After heating up to 800 1C, the recovered compressive strengths of early-age concrete specimens, except those aged for 1 day, were much higher than that of specimens aged for 28 days The early-age concrete specimens, except ones aged for 1 day, showed a negligible strength loss at this temperature level, which indicates that the strength of early-age concrete can recover after certain curing The main reason for this is the existence of some unhydrated cement grains in the early-age concrete specimens Those unhydrated cement grains
Fig 1 Specimens after exposure to (a) 200 1C and (b) 400 1C at a curing age of 3 days.
Fig 2 Specimens after exposure to (a) 600 1C, (b) 800 1C and (c) 1000 1C at a curing age of 3 days.
Table 3
Compressive and splitting tensile strengths of all mixtures.
Curing age (day) Compressive strength (MPa) Splitting tensile strength (MPa)
20 1C 200 1C 400 1C 600 1C 800 1C 1000 1C 20 1C 200 1C 400 1C 600 1C 800 1C 1000 1C Type of
cooling
Type of cooling
Type of cooling
Type of cooling
Type of cooling
Type of cooling
Type of cooling
Type of cooling
Type of cooling
Type of cooling Air Water Air Water Air Water Air Water Air Water Air Water Air Water Air Water Air Water Air Water
1 – 33.0 34.5 31.0 34.1 28.5 33.0 16.0 16.8 8.5 6.5 – 5.1 5.3 4.8 5.0 4.6 4.7 2.5 2.4 1.0 0.6
3 – 39.5 42.6 34.0 38.2 33.5 37.2 26.5 27.2 13.0 7.0 – 6.0 6.15 5.8 6.0 5.2 5.4 4.3 4.4 1.1 0.8
7 – 37.5 41.0 33.5 37.1 33.0 35.6 25.0 26.1 11.5 8.0 – 5.8 5.8 5.5 5.6 5.1 5.2 4.2 4.3 1.2 0.9
14 – 37.0 40.7 33.0 35.6 31.5 33.1 24.5 25.3 10.0 8.5 – 5.7 5.8 5.3 5.4 5.0 5.1 4.1 4.2 1.2 1.0
28 35.0 36.5 34.5 29.5 26.5 26.5 23.0 16.5 15.0 13.5 9.0 5.4 5.6 5.4 5.0 4.9 4.0 4.0 2.6 2.4 1.2 1.0
Trang 4continue to hydrate and gain strength after exposure to high
temperature and then being kept in a curing room for some time
After heating to temperatures below 800 1C, the relative recovered
compressive strength tended to increase with increasing curing age,
with the order being 3 days47 days414 days 428 days41 day For
concrete specimens aged for 3 days, the hydration may be
approximately 45% complete, and the specimens were strong
enough to resist the high temperature With increasing cure time,
the amount of unhydrated cement grains decreases Therefore,
the recovered strength of concrete specimens aged for 3 days was
the highest However, when the temperature was elevated to 1000 1C,
the recovered strength of concrete specimens increased depending
on their curing age The relative recovered compressive strength
values for 1-, 3-, 7-, 14- and 28-day specimens were found to be 24%,
37%, 33%, 28% and 38%, respectively Thus, 28-day specimens revealed
showed that the decomposition of hydration products and the
destruction of C–S–H gel generally occur at temperatures above
800 1C At the same time, chemical changes occur in the aggregates
Therefore, the concrete specimens cannot revert to their original
strength after being kept in a curing room following exposure to
above 800 1C At these temperatures, it is more reasonable to refer to
residual strength rather than to recovered strength
Relative recovered compressive strength of water-cooled
the effects of curing age and maximum temperature on the
recovered strength of concrete specimens cooled by water spray
1000 1C, the recovered strengths of all specimens after air-cooling were higher than those of specimens cooled by water spray Previous research indicated that the residual compressive strength values of water-cooled concrete specimens are smaller than those of air-cooled specimens at all temperatures, which is
result can be attributed to the formation of microcracks due to large thermal gradients occurring within the concrete (thermal shock) and to the increment in the degree of water saturation of the specimen However, for early-age concrete, some of the cement grains remained unhydrated, and the loss was only from the free water when exposed to high temperatures less than
800 1C The sprayed water compensated for the evaporated free water and would take part in the chemical reaction At the same time, the high temperature of the specimens accelerated the hydration reaction Even though a large number of microcracks and damage appeared due to the thermal gradients
0
0
Temperature (°C)
Fig 3 Relative recovered compressive strength of air-cooled concrete after 28-day
curing.
0
0
20
40
60
80
100
120
1 d
3 d
7 d
14 d
28 d
Temperature (°C)
Fig 4 Relative recovered compressive strength of sprayed–water-cooled concrete
after 28-day curing.
0 0 20 40 60 80 100 120
1 d
3 d
7 d
14 d
28 d
Temperature (°C)
Fig 5 Relative splitting tensile strength of air-cooled concrete after 28-day curing.
0 0 20 40 60 80 100 120
Temperature (°C)
1 d
3 d
7 d
14 d
28 d
Fig 6 Relative splitting tensile strength of sprayed-water-cooled concrete after
Trang 5within concrete specimens, those microcracks and damage can be
healed and strength recovered to the reference strength after
certain curing
3.2.2 Splitting tensile strength
The relative splitting tensile strength values of air-cooled and
and 7, respectively From the figures, it can be seen that the trends
in recovered splitting tensile strength were similar to those in
compressive strength However, the deteriorating effect of
elevated temperatures on splitting tensile strength of concrete
specimens was more severe than that on compressive strength
The main reason is that the existence of cracks caused by high
temperature reduces the effective cross-sectional area, and the
existence of tensile stress causes expansion of the cracks Due to
this, microcracks that form at elevated temperatures are more
destructive on splitting tensile strength than on compressive
strength[22,23]
Fig 6shows the variation of relative recovered splitting tensile
strength for air-cooled concrete specimens It was shown that the
recovered splitting tensile strengths of specimens aged for 3 and 7
days were higher than that of the control specimens, while those
of the specimens aged for 1 and 28 days were much lower than
that of the control specimens, in the case of temperature exposure
to below 400 1C When the maximum temperature reached 600
and 800 1C, the recovered splitting tensile strength of early-age
concrete was much higher than that of concrete aged for 28 days
For the maximum temperature of 1000 1C, the recovered splitting
tensile strength of 28-day concrete was the highest, with a value
of only 20% This indicates that the hydration products and
aggregates in concrete have been decomposed and that the curing
recovered splitting tensile strengths for concrete specimens
cooled by sprayed water Similarly to concrete specimens cooled
in air, it also indicates that the recovered strength of early-age
concrete was much higher than that of 28-day concrete after
heating up to 800 1C However, the values of concrete specimen
cooled by water spray were higher than those of air-cooled
specimens, with the exception of 28-day ones When the
maximum temperature reaches 1000 1C, for all specimens, the
recovered strength of air-cooled specimens was higher than that
of water-sprayed ones
4 Conclusions
(1) For early-age concrete, 80–90% of its initial strength can be
recovered when kept in a curing room for another 28 days
after exposure to high temperatures up to 800 1C The recovered strength can be higher than that of the control specimen when the maximum temperature is only 200 or
400 1C
(2) Among the different early-age concretes exposed to high temperatures less than 800 1C, the recovered strength of the 3-day-cured specimens was the highest The order of the recovered strengths of the high-temperature exposed speci-mens was 3 days47 days414 days41 day
(3) When the maximum temperature reached 1000 1C, the strength of early-age concrete specimens could not be recovered after curing, and the recovered strength was lower than that of concrete aged for 28 days
(4) Compared with the compressive strengths of the elevated-temperature exposed specimens, a greater decrease was shown in the splitting tensile strength due to the more destructive microcrack and brittle microstructure formation that resulted from the tensile stress
(5) The recovered mechanical properties of concretes are notice-ably affected by the cooling method In the case of maximum temperature being below 800 1C, for early-age concrete, the recovered strength of specimens cooled by sprayed water was higher than that of specimens cooled by air, while for concrete specimens aged for 28 days, the converse is true In the case of the maximum temperature being above 1000 1C, the recov-ered strength of all specimens cooled by air was higher than that of water-sprayed specimens
Acknowledgements This research work was financially supported by the National Natural Science Foundation of China, Grant no 50708059 The authors also wish to acknowledge Shanghai Jiaotong University for sponsoring the research program
References
[1] H.L Malhotra, The effects of temperature on the compressive strength concrete, Magn Concr Res 8 (1) (1956) 85–94.
[2] C.A Menzel, Tests of the fire resistance and thermal properties of solid concrete slabs and their significance, Proc Am Soc Testing Mater vol 43 (1943) 1099–1153.
[3] Metin Husem, The effects of high temperature on compressive and flexural strengths of ordinary and high-perforce concrete, Fire Saf J 4 (1) (2006) 155–163.
[4] U Schneider, Concrete at high temperatures—a general review, Fire Saf J 13 (1) (1988) 55–68.
[5] G.A Khoury, Compressive strength of concrete at high temperatures: a
0 20 40 60 80 100 120 140
In air 1 d
By water 1d
In air 3 d
By water 3 d
In air 7 d
By water 7 d
In air 14 d
By water 14 d
In air 28 d
By water 28 d
Temperature (°C)
Fig 7 Effect of cooling method on the relative splitting strength of different curing-age concrete after exposure to high temperature.
Trang 6strength concrete, J Mater Civ Eng (ASCE) 15 (2) (2003) 101–107.
[14] Z.P Bazant, M.F Kaplan, Concrete at high temperatures, Material Properties
and Mathematical Models, Longman Group, Essex, 1996.
[15] Serdar Aydm, Bulent Baradan, Effect of pumice and fly ash incorporation on
high temperature resistance of cement based mortars, Cem Concr Res 37
(2007) 988–995.
[16] G.F Peng, W.W Yang, J Zhao, Y.F Liu, S.H Bian, L.H Zhao, Explosive spalling
and residual mechanical properties of fiber-toughened high-performance
compressive strength of unsealed cement paste and concrete at high temperatures, Magn Concr Res 45 (162) (1993) 51–61.
[22] Y Xu, Y.L Wong, C.S Poon, M Anson, Influence of PFA on cracking of concrete and cement paste after exposure to high temperatures, Cem Concr Res 33 (12) (2005) 2009–2016.
[23] M Li, C Qian, W Sun, Mechanical properties of high-strength concrete after fire, Cem Concr Res 34 (2004) 1001–1005.