Mechanical properties of high-strength concrete subjected to
high temperature by stresd test
Gyu-Yong KIM, Young-Sun KIM, Tae-Gyu LEE
Division of Architecture Engineering, Chungnam National University, Daejeon, Korea
这边风景
Received 2 March 2009; accepted 30 May 2009
Abstract: Recently, the effects of high temperature on compressive strength and elastic modulus of high strength concrete were experimentally investigated. The prent study is aimed to study the effect of elevated temperatures ranging from 20 ℃ to 700 ℃ on the material mechanical properties of high-strength concrete of 40, 60 and 80 MPa grade. During the strength test, the specimens are subjected to a 25% of ultimate compressive strength at room temperature and sustained during heating, and whe
n the target temperature is reached, the specimens are loaded to failure. The tests were conducted at various temperatures (20−700 ℃) for concretes made with W/B ratios of 46%, 32% and 25%, respectively. The results show that the relative values of compressive strength and elastic modulus decrea with increasing compressive strength grade of specimen.
Key words: high-strength concrete; stresd test; high temperature
1 Introduction
Concrete is a widely ud construction material in all the modern concrete structures becau of its high compressive strength, good durability and plasticity. High-strength concrete can be achieved by using state-of-art additives such as mineral and chemical admixtures, which can reduce the water requirement and also improve the workability.
In recent years, for that reason the u of high-strength concrete has become increasingly popular. It feasibles technically and economically to produce ready-mixed high-strength concrete using conventional methods and materials. Concretes of strength in excess of 40 MPa are typically obtained by using special admixtures, which can reduce the water requirement and also improve the workability. The limit of 40 MPa is, therefore, well accepted as a reasonable value to differentiate hig
h-strength concrete from normal strength concrete[1−2].
As the u of high-strength concrete becomes common, the risk of exposing it to high temperatures also increas. The behavior of high-strength concrete under elevated temperatures differs from that of normal-strength concrete. The two main differences between high-strength concrete and normal-strength concrete are the relative strength loss in the temperature range from 100 ℃ to 400 ℃ and the occurrence of explosive spalling in high-strength concrete specimens in the temperature range from 500 ℃ to 700 ℃. To be able to predict the respon of structures employing high-strength concrete during and after exposure to high temperature, it is esntial that the strength and deformation properties of high-strength concrete subjected to high temperatures should be clearly understood[3−6]. The prent paper, which focus on the strength and deformation properties of high-strength concrete, is a part of major rearch program of the Chungnam National University in Korea that aims to study the mechanical properties of high-strength concrete under elevated temperature.
因循守旧的意思Three types of test are commonly ud to study the effect of transient high temperature on the stress-strain properties of concrete under axial compression. 1) Stresd test where a fraction of the ultimate compressive strength at room temperature is applied and sustained during heating and, wh
en the target temperature is reached, the specimens are loaded to failure; 2) Unstresd test where the specimens are heated under no
Foundation item: The Korea Rearch Foundation Grant and Brain Korea 21-2th (BK21-2th) funded by the Korean government (MOEHRD, Basic Rearch Promotion Fund) (KRF-2007-314-D00271)
Corresponding author: Young-Sun KIM; Tel: +82-42-821-7731; E-mail: kellery@cnu.ac.kr
Gyu-Yong KIM, et al/Trans. Nonferrous Met. Soc. China 19(2009) s128−s133 s129
initial stress and loaded to failure at the desired elevated temperature; 3) Unstresd residual strength test where the specimens are heated without any preload, cooled down to room temperature, and then loaded to failure[1−2].
淡淡女人香
炎立消片In the prent study, the specimens were heated under 25% of the ultimate compressive strength at room temperature and loaded to failure in hot state after the desired heat treatment[7]. 2 Rearch significance and objective
With the increasing application of high-strength concrete in different concrete structures of high-ri, the risk of exposing it to elevated temperatures increas significantly. In order to asss the structur
al safety of such structures after a fire, it is important that the effect of exposure to high temperature on strength and deformation characteristics of high-strength concrete should be well understood. This paper provides a part of an ongoing study on the mechanical properties of (ultra) high-strength concrete under utmost temperature conditions.
The main objective of the prent study is to compare the variation of stress-strain relationship of high-strength concrete with temperature. The test specimens that are cylinders with 100 mm in diameter and 200 mm in height were subjected to temperatures ranging from 100 ℃ to 700 ℃ at 100 ℃ increments, and their mechanical properties were compared with tho obtained at room temperature (23−25 ℃).
3 Experimental program
To study the effect of transient high temperature on the strength and deformation characteristics of high- strength concrete, the test specimens of high-strength concrete with nominal strength of 40, 60 and 80 MPa were subjected to temperatures up to 700 ℃ and loaded to failure under axial compression. For each type of concrete, the specimens were tested under stresd conditions. In stress tests, the specimens were preloaded to 25% of their ultimate compressive strength at room temperature.
High-strength concrete were made from type ⅠPortland cement, natural a sand, and crushed granitic
gravel. Owing to the low W/C ratio adopted, the superplasticizer was ud to increa the workability. A commercially available sulfonated naphthalene formaldehyde-type superplasticizer was ud in Mix Ⅰ and Mix Ⅱ, and the polycarboxylic-acid type superplasticizer was ud in Mix Ⅲ to obtain high-strength concrete. The properties of the ud materials and the mix proportion are given in Tables 1 and 2, respectively.
Table 1 Properties of materials
Material Physical property Cement OPC, density: 3.15 g/cm 3 Fine aggregate
Sea sand, density: 2.61g/cm 3,
absorption: 0.97%
Coar aggregate (W/B46, 32%) Crushed granitic aggregate, size: 25 mm,Density: 2.67 g/cm 3, absorption: 0.9% Coar aggregate (W/B 25%) Crushed granitic aggregate, size: 20 mm,Density: 2.64 g/cm 3, absorption: 0.9%
Fly ash Density: 2.2 g/cm 3, brain: 3 090 cm 2/g Silica fume
Density: 2.2 g/cm 3, brain: 230 000 cm 2/g
The specimens, as shown in Fig. 1, were demolded one day after casting, then soaked under water for 7-day and, subquently, cured in a climate room at relative humidity of 50% and temperature of 20 ℃ for a period of 113−150 days; the specimens for 28-day compressive strength test were soaked under water for 28-day and, subquently, then the compressive strength test were conducted[8−10].
3.1 Test tup and temperature control
The tests were performed in a clod-loop rvo- controlled 4 600 kN hydraulic testing machine equipped with an electric furnace, as shown in Fig.2. Special cylindrical carbon-bad alloy attachments were designed to transmit load from the frame to the specimen at high temperature. A continuously circulation water-cooling system was ud to protect the instrumentation and to avoid heating the testing frame. The specimens were encad in heat transmission jig made with stainless-steel to heat the whole specimen and to restrain the explosive failure of high-strength concrete specimen, as shown in Fig.3. During the tests, the load and displacement of specimens we
re measured. The load
Table 2 Mix proportion of concrete Unit content/(kg·m −3)熙字组词
AD/%CW No. (W/B)/ % FA rep./ % SF rep./ % (s/a)/ % Water/ (kg·m −3) C FA SF S G SP. Mix Ⅰ Mix Ⅱ Mix Ⅲ
46 32 25
10 15 15
− − 7
第二种人46.4 40.0 36.0
176 170 165
344
452 515
38 80 99
− − 46
793 634 537
919 955 972
0.6 1.4 2.0
Gyu-Yong KIM, et al/Trans. Nonferrous Met. Soc. China 19(2009) s128−s133
s130
Fig.1 Experimental program
Fig.2 Loading and heating machine
Fig.3 Heat transmission jig
was measured by the MTS system and the displacement was measured by the average of two pairs of LVDT [11−13].
It required a total of three specimens for each test at a given temperature and on average of the test results. To measure reprentative temperature and to control furnace temperature, three Type-K Chromel-alumel thermocouples, 0.91 mm thick, were installed in all testing specimens. Two thermocouples were installed at
top-bottom height (10 mm from top and bottom) of the cylinder and one thermocouple was installed at mid-height; all the thermocouples were installed at 5 mm in depth from surface of the cylinder, as shown in Fig.4. The heating temperature of furnace was controlled from electric heater by voltage feedback-type thyristor regulator system.
3.2 Testing procedure
For each t of tests at a given temperature, three specimens from the same batch were also tested at room temperature. The target temperatures varied from 100 ℃ to 700 ℃ at 100 ℃ increment. As shown in Fig.5, the rate of heating for all specimens carry out 0.77 ℃/min, 30 min sustenance per 50 ℃ incread by RELEM TC 129-MHT and preceding experiment[8, 12]. In the stress tests, 25% of the ultimate compressive strength at room temperature was applied to the specimens and sustained during the heating period. After the temperature reached the steady state, the load was incread at the prescribed
Gyu-Yong KIM, et al/Trans. Nonferrous Met. Soc. China 19(2009) s128−s133 s131
rate until the specimen failed. The specimens from the all batch of mixed concrete were tested under the stresd condition. However, three specimens of each batch for thermal strain and SEM test wer
e tested under the unstresd condition. The control specimens were tested at room temperature in unstresd state on the day of the high temperature tests. The average compressive strength of the control specimens was 49.3 MPa for Mix Ⅰ at 113 days, 78.8 MPa for Mix Ⅱ at 139 days, and 99.3MPa for Mix Ⅲ at 140 days, as shown in Table 3.
Fig.4 Location and numbering of thermocouples in specimen
Fig.5 Testing method ud in this study (heating velocity: 0.77 ℃/min, heating cycle: 50 ℃/cycle)
4 Results and discussion
4.1 Effect of temperature on compressive strength
The variation of the compressive strength ratio with temperature is shown in Figs.6 and 7 for three types of high-strength concrete. Each point in the figure reprents an average of the maximum compressive strength of three specimens normalized with respect to the average maximum compressive strength at room temperature. The change in the strength of high-strength concrete specimens appears to follow a common trend. Initially, as the temperature incread to 100 ℃, the strength decread compared with the room-temperature strength. The strength at 100 ℃ is about 80% of the room-temperature strength. With further increa in temperature, the specimens recovered the strength loss and of 90%−110% of the room-temperature strength. In the temperature range from 400 ℃ to 700 ℃, the strength drops sharply, reaching to a low level of 60%
Fig.6 Variation of compressive strength with temperature
Fig.7 Compressive strength with C/W
Table 3 Compressive strength and water content
Average compressive strength/MPa General U.T.M
Load and heat machine
No.
f ck /MPa
Compresiive strength of 28 days/MPa
Curing time/d
Water curing
Dry air curing
Dry air curing
Water content/%Mix Ⅰ 40 40.5 113 49.3 42.0 41.0 2.0 Mix Ⅱ 60
68.4 139 81.2 78.8 76.0 2.1 Mix Ⅲ 80
82.2
140
98.7
99.3
93.9
2.0
Gyu-Yong KIM, et al/Trans. Nonferrous Met. Soc. China 19(2009) s128−s133 s132
and 45% of initial strength, at 600 ℃ and 700 ℃, respectively.
The moisture content has a significant bearing on the strength of concrete in the temperature range from 20 ℃ to 200 ℃. It is believed that water in concrete softens the cement gel, or attenuates the surface forces between gel particles, thus reducing the strength[14−18].
The slight increa in concrete strength associated with a further increa of temperature (between 100 ℃and 200 ℃) is attributed to the general stiffening of the cement gel and the increa in surface forces between gel particles, due to the removal of absorbed moisture. The temperature at which absorbed water is removed and the strength begins to increa depends on the porosity of the concrete.
Above 400 ℃, all three types of high-strength concrete lo their strength at a faster rate. At the temperatures, the dehydration of the cement paste results in its gradual disintegration. Since the paste tends to shrink and aggregate expands at high temperature (differential thermal expansion at temperatures above 100 ℃), the bond between the aggregate and the paste is weakened, thus reducing the strength of the concrete. SEM analysis of 40 MPa strength is shown in Fig.8.
Fig.8 SEM analysis of MixⅠ: (a) Cracks of cement paste at 100 ℃; (b) Production of hydrate at 300 ℃
4.2 Elastic modulus of high strength concrete under
high temperature大刺鳅
The elastic modulus, defined as the ratio of the elastic modulus (taken as the tangent to the stress-strain curve at the origin) at a specified temperature to that at room temperature, is shown in Fig.9 as a function of temperature. As the temperature incread to 100 ℃, the elastic modulus decread compared with the room- temperature elastic modulus. The elastic modulus at 100 ℃ is about 80%−90% of the room-temperature strength. With further increa in temperature, the specimens recover the elastic modulus loss and are 85%−100% of room temperature elastic modulus. Up to about 600 ℃ the elastic modulus of all three types of high-strength concrete decread in a similar fashion, reaching to about 50% of its initial values. In the temperature range of 100−400 ℃, as the dehydration progresd and the bond between materials was gradually lost, the modulus of elasticity decread to about 20%−35% of the value at room temperature. The effect of high temperature on the load-deformation behavior of high-strength concretes is shown in Fig.10.
Fig.9 Variation of elastic modulus with increa in temperature
惊蛰的习俗
5 Conclusions
1) When exposing at 100 ℃, the high-strength concrete showed a 20% loss of compressive strength. As the strength of concrete incread, the loss of strength from exposure to high temperature also incread.
2) After an initial loss of strength, the high-strength concrete recovered its strength between 200 and 300 ℃, reaching a maximum value of 8%−13% above the room temperature strength. As the strength of concrete incread, the recovery point of strength from exposure to high temperature also incread.
3) The high-strength concrete los a significant amount of its compressive strength above 400 ℃ and attains a strength loss of about 55% at 700 ℃. The change of strength in the temperature range of 100−400 ℃ is marginal.
4) The elastic modulus of the high-strength concrete decread by 10%−20% when exposing in the temperature range of 100−300 ℃. At 700 ℃, the elastic modulus was only 45%−50% of the value at room temperature.
