Viscoelastic creep of high-temperature concrete Page: 4 of 13
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P. A. Pfeiffer
-3-
H 5/2
2. Numerical Studies
Narechal [9] conducted tests on creep properties of concrete subjected to different tem-
perature. One of the tests performed was on concrete that had been dried at 105*C for a
period of one month. The specimen size was a 7 x 7 x 28 cm prism and was cured for one year
at 20*C and 1002 relative humidity. A number of tests were done at 20, 70, 105, 150, 250 and
400*C with a uniaxial load of 10 MPa. Loading was Imposed after the concrete was dried at
105*C; the test temperature was applied slowly (0.25'C/hour) and once the test temperature
was reached 1t was held for two weeks; next the axial load of 10 MPa (approximately 202 of
the compressive strength at 20*0 was applied. The results of the creep test are shown In
F1g. 1, with the creep deflection difference between 100 days and ten days (Acr(100) -
Acr(io)] versus test temperature. Material properties of the dried concrete at 20*C are:
Young's Modulus, E * 27500 MPa (4,0 x 10® ps1), and Poisson's ratio, v = 0,18.
The problem was analyzed using one ax1symmetric finite element to approximate the test
specimen. Since the concrete was dried at 105*C, 1t was assumed that only the function
will be needed because the other functions, gfw/and fw. Involve moisture changes. By fit-
ting the creep data for 20*C (reference temperature for which ^ = 1) the constants In Eq.
(2), were Eg = 8.0 x 10® ps1 (55200 MPa), = 10.69, t' * 400 days, m = 0.33, a = 0.05 and n
= 0.09. The best fit of Marechal's data was found by setting Ua/R = 4000*K for ^ (Ua =
activation energy of creep, R * gas constant); the results are shown 1n Fig. 1. The value of
Ua/R = 3700*K was estimated by Marechal [9] 1n h1s analysis.
Anderberg and Thelanderson [6] performed creep tests on concrete subjected to a con-
stant, stabilized temperature. A constant load was applied after the actual test temperature
was reached and sustained for three hours 1n order to measure creep. The cylindrical test
specimen size was 7.5 cm 1n diameter and 15.0 cm long, see Fig. 2; 1t was cured 21 weeks at
20*C and 652 relative humidity. Numerous tests were done at 20, 130, 20C, 300, and 400*C
with a uniaxial compressive load of 9.65 MPa (approximately 232 of compressive strength at
20*0. The temperature was applied at 5’C/min until the desired temperature was reached and
then held constant for a total combined time of three hours. For each test temperature, un-
loaded comparison specimens were tested to obtain the volume change (e.g. shrinkage) at the
same temperature. There was no observed measurable deformation on these specimens during the
time period corresponding to the creep test (three hours). The results of the creep test are
shown 1n Fig. 2, as the creep strain at three hours versus test temperature. Material prop-
erties of the concrete at 20*C are: Young's Modulus, E = 2580C MPa (3.742 x 10® psD, and
Poisson's ratio, v = 0.18.
This problem was also analyzed using one axisymmetrlc element. According to the test
data the water content, w, varied with temperature as follows: 20*C, w = 70 kg/m3; 130*C, w
= 15 kg/m3; 200*C, 300*C and 400’C, w = 0 kg/m3. Because the water content is temperature
dependent, the moisture effect on creep, fw, must be taken Into account. Since no shrinkage
was observed during the tests, the water content, w, was assumed constant at each temperature
level. The values of wj and w0 (dried) were set as follows: wj = w (at 20*C) = 70 kg/m3 and
Wq = w (at 200*C through 400*0 = 0 xg/m3. Also, since the water contents are assumed con-
stant, the effect of variable moisture content, g(w), on creep is not needed. The double
power law in Eq. (2) was calibrated for the creep data at reference temperature, Tg = 20*C
and Initial water content, wj = 70 kg/m3. This resulted 1n the following constants; Eg = 6.0
x 10® psi (41400 MPa), * 5.788, t' = 150 days, m = 0.33, a = 0.05 and n = 0.1215.
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Pfeiffer, P. A.; Marchertas, A. H. & Bazant, Z. P. Viscoelastic creep of high-temperature concrete, article, January 1, 1985; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc1092988/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.