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ЗмістE03 =7,29 10 MPa, Puasson coefficient
Fig. 1. Scheme of endurance test of samples
Endurance of centrally pressed concrete elements joined by acrylic glues AT dynamic loads of different frequencies
Shutenko L., Zolotov M., Garbuz A.
Kharkov National Academy of Municipal Economy
61002, Ukraine, Kharkov, Revolution st., 12
It is common knowledge that construction structures are exposed to few cycles and repeated loads when being maintained. The use of acrylic glues is expected to decrease labor expenditure on restoration of bearing capacity of construction structures. Nowadays, due to the lack of the data concerning structures endurance joined by acrylic glues in conditions of cyclic loading it is very difficult to determine bearing capacity of structures.
The aim of the investigation is to study the influence of the frequency of repeated loading on endurance of centrally pressed glued elements.
The investigation of endurance limit and deformation properties of glued concrete prism-samples with the size 100x100x400 mm under repeated loading was carried out at frequencies = 6,7…7 Hz (400…420 vibr/min), = 4,2 Hz (250…260 vibr/min) and =0,1 Hz (5…6 vibr/min). The samples were made of concrete of B12.5 and B25 and the composition of acrylic glue for joining concrete samples – 1:1:1.5 (polymer, monomer, quartz sand).
Concrete cubes and prisms (with the size 100x100x100, 100x100x200 and 100x100x400mm) were made in the laboratory. The process of samples hardening in metal moulds took 14 days in wet conditions, then the samples were taken out and kept in wet conditions for 28 days.
Glued samples were made of two kinds: of concrete prisms (with the size 100x100x200mm) with smooth surface of concrete and obtained by breaking concrete prisms (100x100x400mm) into equal parts.
Smooth and broken surfaces of prisms were joined by acrylic glue. The thickness of a glue joint was = 3 mm, = 6 mm, = 9 mm.
Acrylic glue has high physical and mechanic strength characteristics: strength at compression – 75,72 MPa, at tension – 18,46 MPa, elasticity modulus – ^ 03 =7,29 103 MPa, Puasson coefficient = 0,282.
Strength and deformation characteristics of concrete and glued samples were determined according to standard methods under static short-term loading using hydraulic press PG-100. The age of concrete was 120 days at the moment of testing.
The results of standard tests under static short-term loading showed that prism strength of the whole and glued prisms is practically the same. Therefore, a glue layer and its thickness do not influence the strength, they influence the numerical values of elasticity moduli Em of ultimate and longitudinal strains.
hanic characteristics. Three glued samples were tested simultaneously and the speed of ultrasonic spreading in longitudinal direction differed not more than by ±5%. Using ultrasonic test before endurance test the beginning of crack formation in concrete has been determined. In this case the time of ultrasonic spreading was measured at each stage of loading equal to 0,1Rb to the point of sample destruction.
First, as it has been pointed out in the works of O. Berger and A. Rozhkov, a slight increase of speed of longitudinal wave spreading occurred, i.e. a material thickening, and then when the load increased the speed of ultrasonic spreading decreased, concrete destruction was observed.
The diagram of changes in the time of ultrasonic spreading in samples of concrete of B25 class is shown in fig. 2.
At endurance test the upper level of load is taken from the moment of micro-cracks formation. Taking the upper level of load for glued concrete samples of B12.5 class and B25 class (accordingly b max = 0,485Rb and b max = 0,503Rb) allows to reach the base number of repeated loading n 2 106.
Endurance test was carried out in the following regime.
irst the samples were loaded statically with the constant speed 0,25…0,3 MPa/sec. to specified value of stress σ and the first reading an instrument was made. Then the load increased with the same speed to specified value of stress b min. After reading an instrument a pulser was switched on and for some period of time the samples were in conditions of repeated loading with variable characteristic of a cycle , approaching specified value gradually.
+t [mcs] 0,8 0,6 0,4 0,2 t0 -0,2 -t [mcs]
Fig.2. Changing the time increase of ultrasonic spreading when loading concrete prisms to the point of destruction
Measurement of longitudinal and cross deformation was carried out first in 50 thousand cycles of loading and after 500 thousand cycles of loads – in each 250 thousand cycles (with the frequencies 6,7…7 and 4,2 Hz). At the frequency 0,1 Hz measurements were carried out first in 1…2 thousand and then in 5…10 thousand cycles of repeated loading. The values of maximum and minimum stresses were measured accordingly in the range from 0,47 to 0,68Rb and a cycle characteristic ρ = 0,11…0,2.
Prism strength of glued samples was less than mean quantity of concrete strength Rb, obtained as a result of their testing. It is indicative of partial destruction of material and is confirmed by ultrasonic tests.
The destruction of glued samples was characterized by vertical cracks, occurring on the ends of samples. The number of cycles n, which a sample endured to the full destruction, depended on the frequency of repeated loading. As the number of loading cycles increased cracks appeared along a sample, their width reached 0,5 mm, after that flaking of “splits” took place and a sample was destructed along the vertical crack in inclined plane. Mention should be made that cracks occurred in concrete only in one half of a sample and did not cross the glue level irrespective of its thickness. The number of cycles n, causing destruction of glued concrete prisms 0,1 Hz, was in the range from 2500 to 2 105, and = 7…4,2 Hz accordingly in the range from 1 10 to 5,8 106.
Endurance curves were obtained as a result of experimental data using correlation method. Correlative dependence obtained (Ky = βn-) shows proportional dependence of the value of relative endurance limit of glued concrete elements on the number of load cycles in logarithmic coordinates. The expression Ky = 1,08n-0,05 has been obtained for endurance curve of concrete at a high frequency of repeated loading 7…4,2 Hz, at a low frequency 0,1 Hz – Ky = 0,88n-0,05 (at = -0,05).
The investigation showed the influence of multi-repeated loading on concrete endurance. When the frequency of repeated loading changes from = 7…4,2 Hz 0,1 Hz the relative limit of concrete endurance decreases, on the average, 20%.
For one and the same endurance limit base number of cycles n of repeated loading at the frequency = 4,2 Hz and 0,1 Hz differs more than by an order. When the base number of loading cycles n = 2 106 relative limit of concrete endurance at the frequency = 0,1 Hz is equal to 0,425, it corresponds to lower limit of micro-crack formation of studied concrete classes.
0 10 102 103 104 105 106 107 lg n
Fig. 3. Endurance curves of concrete: 1 – Ky = 1,08n-0,05, 2 – Ky = 0,88n-0,05
It has been determined that a class of concrete does not influence relative limit of endurance of glued concrete elements. The main factors, determining endurance limit of glued elements are: the level of maximum stresses, characteristics of a cycle, frequency of load application, scale factor and the type of grain concrete filler.
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