Материал: Russian Journal of Building Construction and Architecture

Russian Journal of Building Construction and Architecture

The main feature of light concrete based on porous fillers is that it is almost impossible to design precise compositions of a concrete mix as strength and deformative characteristics of porous fillers vary significantly and thus the strength of concrete using different fillers will range considerably as well [13, 15]. Therefore the strength of concrete and its density will not be directly dependent on one another, i.e. for lower densities a higher axial compressive strength can be obtained as a result of using other fillers or additives [13]. Note that in order to obtain a construction lightweight concrete as a fine filler dense sand (river, silica or pit) but not porous one (e.g., claydite sand) is used. It is because the use of a fine porous filler causes a significant reduction in the concrete strength [6, 13, 14, 16].

One of the parameters that characterizes the operation of concrete under load is formation and development of microcracks. Particularly, it is very important to determine the upper boundary of microcrack formation (a so-called critical loading level) as its increase suggests that the third stage of the stress-strain has started (the destruction stage). This means that not only has a strength limit been reached and other operational characteristics have degraded but that there has been a change in some other parameters as well. E.g., in [19] it is shown that chloride resistance decreases dramatically as a critical level is reached.

The boundaries of microand macrocracks in lightweight concrete on any fillers are significantly higher than in normal density concrete [12, 13, 17]. The higher the porosity is, the higher the boundary of microcrack formation is [1]. There are some reasons for that. The cohesion of a cement matrix with a large porous filler is a lot higher than that with a dense one as there is no clear boundary between the filler and cement rock [13, 14, 16, 18] as seen in the photos taken with an electronic microscope [13, 18]. The first microcracks are generally formed along the contact of a large filler and cement matrix [24]. In [21, 23] it is also noted that around grains of a large filler the porosity of a cement rock is smaller compared to regular concrete due to a high water absorption resulting in cement rocks of lightweight concrete having a higher strength and durability than those in heavyweight ones.

Determining limits of microcracking (the lower and upper ones) is based on empirical dependencies and a system of particular coordinates allowing for various parameters. Traditionally for regular concrete formulas using a common logarithm set forth by O. Ya. Berg [2] are used that came under a lot of criticism in scientific literature as they are only applicable for concrete with dense medium-strength fillers [1, 3, 4, 7]. In [7] dependencies using exponents are suggested while [11] presents a finite element method for large-scale modeling of microcracking and concrete failure. In [22] it is noted that in order to determine when the

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first cracks are going to appear in ferroconcrete beams made of any lightweight concrete, most existing calculation methods do not yield a correct results and a correction coefficient has thus to be introduced.

1.Objectives and tasks. The objective of the research is to develop a calculation method for relative loads that correspond with the lower and upper limits of microcracking for claydite concrete made of a local raw material.

The objectives of the study are to consider the influence of the density of claydite concrete on the ranges of limits of microcracking and to provide their accordance with Eurocode 2.

2.Characteristics of the experimental samples. In order to prepare the experimental samples in the form of cubes, prisms and cylinders local materials were employed:

–– claydite gravel with the fraction of 5—10 mm with the apparent density of 382 kg/m3, relative strength in the cylinder 2.68 МPа (manufacturer — Ltd. “Claydite Gravel Plant”, Novolukoml);

–– claydite gravel with the fraction of 10—20 mm with the apparent density 326 kg/m3, relative density in the cylinder 1.86 МPа (manufacturer — Ltd. “Claydite Gravel Plant”, Novolukoml);

–– claydite crushed stone with the fraction 5—10 mm with the apparent density 585 kg/m3, relative strength in the cylinder 10.26 МPа (manufacturer — Petrikovsky Claydite Plant Ltd. “Gomel Integrated House-Building Factory”);

–– claydite sand with the fraction of 0—4 mm with the apparent density 432 kg/m3, relative density in the cylinder 4.58 МPа (manufacturer — Ltd. “Claydite Gravel Plant”, Novolukoml);

–– natural pit sand with the apparent density 1580 kg/m3;

–– М500 Portland cement with the activity 49.0 МPа, with the normal density index of 25—28 % (manufacturer — Ltd. “Belorussian Cement Plant”).

The composition of the claydite concrete mixes was selected according to [8]. The composition of the concrete mix is detailed in Table 1.

The concrete mix was prepared manually in a laboratory setting. Inventory metal assem- bly-disassembly forms were utilized for making the samples. The experimental samples

were stored in natural temperature and humidity conditions of solidification (t = 20 ± 2 °С, humidity is 90—95 %). The samples were covered with a bagging fabric and were regularly moisturized for 7 days.

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Russian Journal of Building Construction and Architecture

Таble 1

Characteristics of the experimental claydite concrete samples

Note: *binder is Portland cement М500.

3. Determining the limits for microcracking. The limits for microcracking for the experimental samples were determined using the graph method according to the results of the experiments. The lower level of microcracking η0crc is determined by the second derivative from the dependence “loading level –– Poisson coefficient”. The upper level of microcracking ηvcrc (level

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corresponding with macrocracks) is identified by means of designing the dependence “strain level –– volumetric deformation) using the averaged experimental data.

The relative microcracking limits (the lower and upper one) can be given by the formulas (1),

(2) respectively [9, 20]:

where fc is the strength of concrete, МPа; fc,0 is a value of the concrete strength, fc,0 = 1 МPа. In [9, 20] it was experimentally found that the ratio η0crc/ηvcrc for concrete of each type remains stable (i.e. changes in a small range) and for the above types of concrete it can be assumed that:

––η0crc/ηvcrc ≈ 0.67 for normal density concrete on dense fillers;

––η0crc/ηvcrc ≈ 0.70 for steel fibre concrete;

––η0crc/ηvcrc ≈ 0.73 for concrete using metallurgical slag as a fine filler (metallurgical slag concrete);

––η0crc/ηvcrc ≈ 0.60 for claydite concrete.

The empirical coefficient kcrc was introduced which is based on the value of η0crc/ηvcrc and can be used for designing and testing of concrete and ferroconcrete structures:

In [20] it is noted that for normal concrete, steel fibre concrete, metallurgic slag concrete the coefficient kc1 ≈ 1 and for claydite concrete kc1 ≈ 1.2. I.e. for claydite concrete kcrc ≈ 0.72. However, as was previously shown, for lightweight concrete overall and claydite concrete in particular, the following is typical [13, 14]: for the same strength the density of a concrete matrix the applications might be fundamentally different due to the use of different fillers. Therefore the coefficient kc1 should be accepted considering the density of a material and once there is enough amount of correct experimental data, a formula for analytical calculations should be deduced.

According to the EN 1992 [5], the density of lightweight concrete is considered with the parameter ( /2200) where ρ is a calculation density of lightweight concrete accepted based on the density class [5, Table 11.1]. This parameter is actually a relative upper limit of the density of lightweight concrete of a corresponding class.

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Russian Journal of Building Construction and Architecture

Based on the experimental data using the method of linear approximation, the dependence for calculating the coefficient kc1 depending on the parameter ( /2200) was introduced:

where ρ is in kg/m3.

4. Comparison of the experimental and calculation data. Table 2 shows the comparison of the experimental and calculation values of relative limits of microcracking of claydite concrete. The calculation values are obtained according to the suggested method.

The calculations in Table 2 suggest that the method is in good agreement with the experimental data.

Таble 2

Comparison of the experimental and theoretical values of the lower η0crc and upper ηvcrc limits of microcracking

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