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13 2022
UIF-2022: 8.2 SCIENCE AND INNOVATION
ISSN: 2181-3337 INTERNATIONAL SCIENTIFIC JOURNAL №4
DETERMINATION OF THE BOUNDARY OF THE LINEAR CREEP OF EXPANDED CLAY CONCRETE DURING COMPRESSION Chorikul Raupov
Professor of Tashkent State Transport University Ganisher Malikov
Doctoral Student of Tashkent State Transport University Javlon Zokirov
Master's student of Tashkent State Transport University https://doi.org/10.5281/zenodo.6981518
Abstract. The paper presents the results of experimental data on creep and shrinkage, and the kinetics of the change in the creep measure of expanded clay concrete over time under axial compression at different levels of long-term load (0.2 - 0.7)Rb. Empirical formulas convenient for practical application for determining creep deformation and shrinkage of expanded clay concrete are proposed.
Keywords: expanded clay concrete, strength, modulus of elasticity, long-term load, creep, shrinkage, creep measure.
ОПРЕДЕЛЕНИЕ ГРАНИЦЫ ЛИНЕЙНОЙ ПОЛЗУЧЕСТИ КЕРАМЗИТОБЕТОНА
ПРИ СЖАТИИ
Аннотация. В работе представлены результаты экспериментальных данных по ползучести и усадке, а также кинетика изменения меры ползучести керамзитобетона во времени при осевом сжатии при различных уровнях длительной нагрузки (0,2-0,7)Rb. Предложены удобные для практического применения эмпирические формулы для определения деформации ползучести и усадки керамзитобетона.
Ключевые слова: керамзитобетон, прочность, модуль упругости, длительная нагрузка, ползучесть, усадка, мера ползучести.
INTRODUCTION
Rational design of prestressed structures made of expanded clay concrete can be ensured only if such important characteristics as shrinkage and creep during compression are properly taken into account [1-4].
In the operating conditions of the structure, when concrete is constantly subjected to alternating moistening and drying, the creep deformation increases, i.e. the results of laboratory tests do not allow to accurately determine the real creep deformation value is noted in [5, 6]. A higher temperature leads to an increase in the initial creep rate compared to concrete tested at normal (room) temperature. The creep deformations of concrete are very significant and should always be taken into account in addition to the initial elastic deformations.
MATERIALS AND METHODS
The composition of expanded clay concrete is given in Tables 1 and 2. As a large aggregate, expanded clay gravel of the Tashkent plant of two fractions: 5-10 and 10-20 mm in a ratio of 40:60. The cement of the Navoi cement Plant was used as Portland cement, and the sand of the Tashkent quarry was used as quartz sand.
Table 1
Composition of expanded clay concrete (CC)
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UIF-2022: 8.2 SCIENCE AND INNOVATION
ISSN: 2181-3337 INTERNATIONAL SCIENTIFIC JOURNAL №4
Actual consumption of materials per m3 of concrete cement, kg 427
sand, kg 629
expanded clay, kg (l) 414(727)
W/C (water/cement ratio) 0,49
Ta
Characteristics of expanded clay concrete
Bulk density of dry expanded clay concrete, kg/m3 1760
Cubic strength, R, MPa 33.0
Prism strength, Rb, MPa 28.4
Prism strength factor, Rb/R 0.86
Initial modulus of elasticity, Eb, Pa 15.4
Note: Specimens were tested at the age of 28 days. Prisms dimensions - 150x150x600 mm, cubes ribs - 150 mm. The cone slump of concrete mix - 1...2 cm.
Loading of samples at different stress levels was carried out in spring and lever installations with a maximum force of 210 kN, respectively. During testing, the prism samples with a size of 70x70x280 mm were installed with 30 cm thick metal base plates with ball joints glued to the ends. When loading with high load levels, one sample was installed in the installation (Fig. 1, a), and two samples were installed with low load levels (Fig. 1, b).
When installing two prisms, metal plates with a thickness of 20 mm were laid between
them.
6)
a)
Fig. 1. Testing of samples under prolonged compression
In order to eliminate the error associated with the nonadditivity of shrinkage and creep, the samples were waterproofing on the sides before loading with a layer of paraffin 2-3 mm thick and two layers of polyethylene film with sizing the seams with insulating tape.
Before the samples were loaded with a long-acting load, their twins were tested on a press according to a similar scheme with a short-term load action before destruction. At the same time, the cubes were tested.
The test of samples loaded under load at (0.20 - 0.70)Rb levels lasted for 210 days, and then they were unloaded and the deformation effects were measured for 56 days. After 56 days after unloading, the main and control samples were brought to destruction under the short-term action of the compressive load and the elastic modulus and compressive strength were determined to determine the value of the function m(t,zj) = R'/R = E'/E (where R'/R = E'/E is the strength and modulus of elasticity, respectively, of the main and control samples), taking into account the influence of the previous loading of the material on the short-term
2022
UIF-2022: 8.2 SCIENCE AND INNOVATION
ISSN: 2181-3337 INTERNATIONAL SCIENTIFIC JOURNAL №4
strength and modulus of elasticity.
RESULTS
The kinetics of changes in the creep measure of expanded clay concrete over time during axial compression are shown in Fig. 2, and shrinkage deformations are shown in Fig. 3. The growth curves of the creep measure of the samples have the greatest rise in the initial period after the application of the load (30 days), in the future there is a slight decrease in the growth of the creep measure with a tendency to stabilization. Up to stresses of 0.4Rb, creep deformations of expanded clay concrete under compression develop almost in a conditional linear region. At 0.5Rb, creep deformations clearly do not develop according to a linear law, it can be assumed that in the case of Ol/Rb > 0,4, microcracks begin to appear in the samples.
Cb(t;ti)-m5, MPtr'
eoW.U
& 0 100 200 300 41>t) 500
f fI-T,), day's
0 30 100 150 200 250 300 350
Fig.2. Kinetics of the change in the creep Fig. 3. Kinetics of changes in shrinkage
measure of expanded clay concrete in time for deformation of expanded clay concrete axial compression at different levels of long-term over time
load
When analyzing the nature of changes in creep deformations, a simpler method was used to identify the nonlinearity of their development. Figure 4 shows the graphical dependences of the creep limit strain Sx,p (^,11), determined according to GOST 24544-97, on the magnitude of the relative stresses of long-term compression o/Rb. As follows from this graph, in expanded clay concrete up to stresses of 0.4Rb, creep deformations develop almost in a linear region. The results obtained are in good agreement with the data of [7].
In [7], the dependence is proposed to determine the relative deformation of the nonlinear creep of expanded clay concrete:
£*,P(t; Ti) = (°bi + aaj3l)Cb(t;x1) (5)
where aM is the effective voltage in MPa; a - numerical coefficient determined from experience;
Cb(t;r1) is a measure of the linear creep of expanded clay concrete, which can be determined by the dependence proposed in [13].
For the numerical solution of equation (5), it is necessary to determine the values of the limit of short-term strength under axial compression and tension and the creep measures Cb(t; vi), Cbt(t; vi) and its change in time, the values of the elastic modulus and the functions m(t; vi).
The values of the measure of linear creep of the mortar part of expanded clay concrete under axial tension (if there are no direct experimental data) are determined by the formula (6)
CM(t; Ti) = Ti )cb(t; *i); (6)
Where
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UIF-2022: 8.2 SCIENCE AND INNOVATION
ISSN: 2181-3337 INTERNATIONAL SCIENTIFIC JOURNAL №4
r ^ _ 70R2(Ti-2)+13000Rb-14000^n-5
Lh(t; Ti) —-3/2-10
(177Rö-1700)RöT3/2
+
0,2Rb+15r1-0,2Rb+100
Rb
[1
(7)
e-y(t-Tl)]10-5;
y1 = 0,015 + 18/Rb; the second multiplier in the formula (8) is taken into account when n > 16 days.
(8)
A(t; t 1) — a +
b(t; Ti)
(9)
Based on experimental data for expanded clay concrete of dense structure (on quartz
sand):
a = 2.15 a = 3.40 a = 2.25 a = 2.00
b = 0.63 b = -0.077 b = -0.03 b = 0
at 0 < (t-n) < 2 days. at 2 < (t-n) < 15 days. at 15 < (t-n) < 100 days. at (t-n) > 100 days.
The values of the function m(t;zi), taking into account the effect of a prolonged load on the change in the surface energy of the material in the crack zone for expanded clay concrete, are calculated by the formula (10)
m(t ;t1) = a1 + b1lg(t — t1), (10)
where a1 = 1.091; b1 = 0.035 at n < 28 days.
ai = 1.033; bi = 0.029 at n > 28 days. The limiting values of the relative creep deformation and shrinkage of expanded clay concrete were determined by the method of constructing regression lines of the form
(t — Ti) /eXiP(t;r1) = [A+B(t — Ti)]10-5, (11)
(according to GOST 24544-97), allowing to linearize curves describing creep deformations and shrinkage over time.
The numerical values of the coefficients A and B of equation (11) adopted to describe the curves of change in shrinkage deformation and creep during compression are given in Table 3.
Table 4 shows the results of a study of the creep deformation of expanded clay concrete under axial compression. As can be seen from the data obtained, the limiting value of creep deformations depended on the intensity of stress and increased as the level of long-term stress increased. It can be seen that the creep deformation was 75-90% of the ultimate creep deformation.
Table 3
Results of statistical processing of tests (value of coefficients A and B)
:q )-I0-!
Figure 4. Change in creep limit deformations depending on the level of long-term load
oi/Rb A B
0,2 1,329 0,053
0,3 0,745 0,032
0,4 0,463 0,023
0,5 0,667 0,014
0,6 0,513 0,011
0,7 0,295 0,009
shrinkage 1,976 0,017
t
i
2022
UIF-2022: 8.2 SCIENCE AND INNOVATION
ISSN: 2181-3337 INTERNATIONAL SCIENTIFIC JOURNAL №4
Table 4
Results of the study of creep deformation of expanded clay concrete under axial compression
Cl/Rb Deformation
elastic creep Limit
by the time the long-term load is applied for 210 days creep deformation measure of creep
when compressing
0,2 38 18 19 2.76
0,3 57 28 31 3.01
0,4 76 40 44 3.21
0,5 95 64 74 4.31
0,6 114 79 96 4.42
0,7 133 97 109 4.62
The results of determining the values of the functions (Table. 5) show that at stresses (0.2-0.4)Rb, the increase in strength and modulus of elasticity under compression was 6-15% and 1-5%, respectively, which is confirmed by the conclusions [7]. The hardening zone is shifted towards low stresses (0.2-0.4)Rb.
Table 5
The effect of previous loading on the strength and modulus of elasticity of concrete (Above the line - strength data, below the line - modulus of elasticity)
The value of the function m(t;x_1) at the intensity of the preceding stresses
0,2Rb 0,3Rb 0,4Rb 0,5Rb 0,6Rb 0,7Rb
1,13/1,03 1,09/1,05 1,06/0,97 0,90/0,78 0,90/0,78 0,88/0,76
DISCUSSION
The increase in the strength of compressed concrete can be explained by the removal of internal, redistribution of stresses between structural elements, which in turn equalizes the stress field due to the ongoing physico-chemical processes of hardening of cement stone, as well as some compaction of the concrete structure. High stresses (a/Rb > 0,5) caused mainly a decrease in strength and modulus of elasticity, respectively, to 10-12% and 22-24%. (Large values correspond to large voltages). The decrease in the strength and modulus of elasticity of samples that have undergone prolonged compression can be explained by the fact that in the case of a/Rb > 0,5, microcracks begin to appear in the samples.
Jointly analyzing the relationship between the creep limit deformations and the relative level of long-term stress (Fig. 4), we can reasonably conclude that the creep of expanded clay concrete at a loading level not exceeding a/Rb = 0,5 can be considered conditionally linear, and the highest level of long-term stress, at which the short-term strength and modulus of elasticity of the main samples, those that have undergone prolonged compression do not differ from those for control twin samples m(t; rx) = 1.
CONCLUSIONS
2022
UIF-2022: 8.2 SCIENCE AND INNOVATION
ISSN: 2181-3337 INTERNATIONAL SCIENTIFIC JOURNAL №4
1. Empirical formulas for determining and describing the relative deformation of the nonlinear creep of expanded clay concrete (5), convenient for practical application, are proposed.
2. It is experimentally proved that the creep of expanded clay concrete under compression at a loading level of ц < 0,5 is within the conditional linear dependence.
3. It was revealed that the previous effect of a prolonged compressive load of low intensity (ц < = 0,5) increases the strength and elastic modulus of expanded clay concrete by 615% and 1-5%, respectively, and of high intensity (ц < = 0,5) - reduces to 10-12% and 22-24%, respectively.
REFERENCES
1. Raupov, C., Karimova, A., Zokirov, F., & Khakimova, Y. (2021). Experimental and theoretical assessment of the long-term strength of lightweight concrete and its components under compression and tension, taking into account the macrostructure of the material. In E3S Web of Conferences (Vol. 264, p. 02024). EDP Sciences.
2. Ashrabov A.A., Raupov Ch.S. Work of lightweight concrete beams in view of a descending branch of the diagram. International Conference held in Malaysia, the collection of scientific researches, 2002. pp 139-142.
3. Ashrabov A.A., Raupov Ch.S. The normalization of long-lived durability of lightweight concrete at monoaxial stressing. International Conference held in Malaysia. The collection of scientific researches, 2002. pp/ 134-138.
4. Raupov Ch.S. Expanded clay concrete for transport construction: Monograph / Tashkent: Tamaddun, 2020. - 356 p.
5. The mechanism of concrete creep and the factors affecting it (review). https://elibrary.ru/item.asp?id=23032537
6. Brooks J.J., Neville A.M. Creep and Shrinkage of Concrete as Affected by Admixtures and Cement Replacement Materials. Как воздействуют добавки и наполнители на ползучесть и усадку бетона. Creep and Shrinkage of Concrete: Effect of Materials and Environment, ACI SP 135, American Concrete Institute, Detroit, 1992, pp.19-36.
7. Bakradze D.D. Investigation of the long-term strength of lightweight concrete. Diss...Candidate of Technical Sciences: - Tbilisi, I9885. - 140 p.
