Experimental study of mechanical properties of normal-strength concrete exposed to high temperatures at an early age

Experimental study of mechanical properties of normal-strength concreteexposed to high temperatures at an early age Department of Civil Engineering, Shanghai Jiaotong University, 800 Don

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Experimental 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

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

Corresponding author Tel.: +86 21 54705110.

E-mail address: hntchen@sjtu.edu.cn (B Chen).

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properties 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)

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3.2 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 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

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continue 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

Temperature (°C)

Fig 3 Relative recovered compressive strength of air-cooled concrete after 28-day

curing.

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

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within 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

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