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Strength of concrete for bending, stretching and splitting
The strength of concrete for bending is determined on samples-prisms of a square section of 100 × 100, 150 × 150 or 200 × 200 mm, four times the size of the section, i.е. respectively 400, 600 and 800 mm.
The sample-prism (Fig. 11.5) is placed in a horizontal position on two symmetrically placed hinged supports, fixed on the bottom plate of the press. One of the supports is movable, the other is fixed. The distance between the supports (test span) is three times the size of the prism section, i.e., 1 = 3a. On the prism, two hinged supports are installed from above, also symmetrical with respect to the middle and located one from the other at a distance equal to the size of the section: a = 1/3.
Fig. 11.5. The device for testing concrete for bending:
1 - the ice rink; 2 - swinging cylindrical hinge; 3 - the spherical hinge; 4 - traverse
The pillars are laid with a steel traverse, in the center of the upper face of which the ball joint is fixed. Through the hinge, the load P from the top plate of the press is transferred to the traverse, and from it through the supports to the test prism in the form of two concentrated forces, each equal to P / 2 and applied at a distance of 1/3 from each other and from the prism supports.
It should be ensured that the prism itself on the supports and supports are supported densely across the entire width, all supports are perpendicular to the axis of the test specimen, and the axes of the prism and the traverse are in the same vertical plane.
In bending tests, the strength of concrete is calculated as the arithmetic mean of the values of RpH for all specimens in this series, the strength of which is not more than 15%, and the failure occurred in the middle third of the test span. When testing a prism measuring 200 × 200 × 800 mm, the "reference" strength (strength for a sample measuring 150 × 150 × 600 mm) is determined by multiplying the values obtained by a factor of 1.0, and for a prism measuring 100 × 100 × 400 mm by 0, 95.
The tensile strength of concrete is determined in two ways: direct (axial tension test) and indirect; (splitting test).
Axial stretching is tested with samples of square section with thickenings to the ends, the so-called "eight" (Figure 11.6).
When stretched, the sample is broken (torn) in the middle, thinner working part, which can have a section of 100 × 100, 150 × 150 or 200 × 200 mm. In the extreme thickened parts, the cross-section is respectively 150 × 150, 250 × 250 or 360 × 360 mm. The length of the working part of the sample is three times, and the total length of the sample is seven times larger than the size of the working section. In the thickened parts there are reinforcing and mounting loops made of steel with a diameter of 6 mm, protruding beyond the ends of the sample and intended for fastening in a tensile machine.
Splitting is tested with the same cubes or cylinders as in the compression test (cubes should have 14 mm wide chamfers on two opposite edges). Samples are installed in the press according to the scheme (Fig. 11.7). The cube is supported by an edge so that the compression force is directed along the axis, and the cylinder rests on the generatrix (the compression force is directed along the diameter).
Figure 11.6. Sample-eight for testing concrete for tension
Fig. 11.7. Schemes of concrete testing for splitting:
a - samples-cubes; b - sample-cylinders; 1 - sample; 2 - half-cylinder; 3 - press plate
Analysis of the experimental data shows that the strength for axial tension is less than the tensile strength at bending. In conventional heavy concrete, the ratio of the stretching value at bending Rp.w to the axial tension Rp varies within rather wide limits
The increase in the tensile strength at bending is due to the plastic extensibility of the concrete before it breaks. The ratio between the deformations at the time of the break of the beam and at the moment when the concrete reaches the stress RP.H can serve as a measure of the increase in the extensibility of concrete due to its plastic properties. If the experiment is carried out very quickly, plastic extensibility may not manifest itself in full measure. The slower the load on the beam, the more favorable are the conditions for the development of plastic deformations and the less is the bending moment. Therefore, in order to obtain comparable results, the speed of testing the samples should be the same.
According to GOST, the load on the sample during the test should increase continuously and evenly at a rate of 0.5 + 0.2 kg / cm2 per second until the sample is destroyed. The rate of loading of concrete is decisive for its plastic extensibility. In this connection, the ratio between the total deformation corresponding to the moment of destruction of the beam and the deformation at which a crack appears (curve a-c) varies from 1.5 to 3.
Consider the process of deformation of concrete bending beam until its destruction (Figure 24).
AE Golikov tested samples measuring 15x15x X60 cm, prepared from concrete of grade 800. The beams were loaded with two weights, which were applied in the thirds of the span. Deformations were measured by strain gauges of resistance, pasted in the stretched and compressed zones.
The tensile strength of concrete in bending, according to GOST, is determined on beam samples with a section of 20x20, 15x15x10x10cl and length of 80, 60 and 40 cm respectively. For the standard, a beam measuring 15 X 15x60 cm should be taken (its dimensions are chosen on the basis of the same considerations that were taken when choosing the dimensions of the compression test specimens). The load on the beams is applied in the thirds of the span. It is created by two equal concentrated forces, which make up half of the total load. In this case, in the zone of pure bending, the moment is kept constant, and the transverse force is zero.
To establish the correlation function # pi = f (R), data were used on high-strength and conventional concretes.
The flexural stretching was determined on the beam samples, applying two equal concentrated forces, making up half the total load P. The forces applied in the thirds of the span worked uniformly along the entire width of the beam. The span of the beams corresponded to a threefold size of its height. Beams of various sizes were used, both in cross-section and along the length in the experiments, with a cross section of 17.5x25 cm and a span of 75 cm.
The results of statistical processing of the data shown in Fig. 25, allowed us to establish a general correlation between £ p and u for concrete grades from 100 to 1200, which is expressed by curve (a).
The curves apb coincide quite closely, so Rp.w can be determined by the formula (III. 3). This formula differs from formula (II 1.2) only by the coefficient. For the whole range of strengths from 100 to 1200 the average value, unlike that taken according to GOST for concrete grades up to 600 / CP = 1,7.
As can be seen from Fig. 25, dependences (III.2) and (III.Z) are mainly due to the strength of the concrete.
In the factory laboratories, where it is practically impossible to determine the temporary resistance of the concrete to tensile axial and when bending is practically impossible, the method of splitting cubes or cylinders is also used. Stretching of concrete during splitting Rv.p is expedient for determining on samples-cubes or cylinders.
In order to take into account the degree of reduction in the strength of concrete during its Fretting, an attenuation coefficient is introduced into the generally accepted formula for estimating the expansion during splitting, which for heavy concrete is assumed to be equal to: Kc = 1.1.
In parallel with the studies of high-strength concrete on axial and bending tensile, samples-cubes measuring 15x15x15 cm for splitting were tested. They were made from the same mixtures and by the same methods as the samples tested for axial and flexural stretching. In addition, samples-cylinders with a diameter of 15 cm, a length of 30 cm and samples-cubes of 10 X10 X10 cm in size from 3 to 360 days and with a strength of 100-1050 kg / cm2 were tested.
A sufficiently high value of the correlation coefficient r = 0.926 makes it possible to regard this dependence as stable.
Considering the dependencies (III.2), (III.3) and (III.6), we come to the conclusion that they all have the structure of the Fourier formula and differ only in the coefficients. The main factor affecting the tensile strength of concrete grades 100-1200 is the compressive strength.
The size and shape of the samples, the age of the concrete, as well as the composition of the concrete mix when using fractionated aggregates in them, the effectiveness of the methods of laying concrete affect less significantly the tensile strength.
In addition to the usual strength of concrete for compression, which is used to determine the class and grade of concrete, the material has one more characteristic of tensile strength. This is directly the opposite characteristic of compressive strength, indicating the ability of the mixture to maintain integrity when performing a tear force.
There are several types of tensile strength of concrete:
- Bending tensile
- On axial tension
- Stretching during splitting
From a practical point of view, the tensile strength of concrete shows the ability of the product to withstand various strain loads while maintaining the geometry of the structure unchanged. In addition, it shows the stability of the material to the destructive effect of temperature changes, and is also taken into account when calculating the load-carrying capacity of structures, in the first place - beams.
The most significant is the strength of concrete for tension during the construction of concrete road surfaces, in particular - runways at aerodromes, since it depends on the bearing capacity of the roadway. For these purposes, heavy concrete is used, on which marks are set for tensile strength.
Strength indices of concrete for tension
These marks are determined in laboratory conditions, similar to compressive strength grades, but under other experimental conditions. Beams with a length of 400-800 mm and sections of 100x100, 150x150 or 200x200 are used (that is, the section dimensions should be 4 times smaller than the length). The beam is mounted on the support and a force is applied at two points in the thirds of the span. At each point an equal force equal to 50% of the total load is applied. The force is increased until the beam breaks at the point of bending. As a result, the received indicator is taken for the concrete grade for stretching. In practice, there are grades from M5 to M50 in terms of tensile strength.
An important indicator is the ratio of compressive strength and tensile strength. As a rule, for most concrete mixtures, the compressive strength significantly exceeds the tensile strength, but with an increase in the strength of concrete, this gap gradually decreases. It is also important to take into account that for different concrete compositions the ratio between the tensile strength at bending and with axial tension is also significantly different.
Since the concrete itself is not a material showing high tensile strengths in both bending and axial tension, reinforcement is used. The introduction of metal reinforcement in the concrete structure provides an increase in the plasticity and elasticity of the product, radically increasing the tensile strength.
For heavy concrete used in the construction of roads and aerodromes, concrete grades of tensile strength in bending are determined, which are determined by testing square beams. The beam is tested with the application of forces in 1/3 of the span.
The tensile strength at bending RK3r (MPa) is calculated by the formula
The strength of concrete in bending is several times less than its compressive strength. Stretch marks of concrete for bending: M5, Ml 0, Ml Y] M20, M25, MZO, M35, M40, M45, M50
The strength of concrete in bending depends on the same factors as the strength of concrete under compression, however, in this case, the el- evate dependences are obtained by others. The ratio increases with increasing strength of concrete. In practice, it is usually difficult to achieve the strength of concrete when bending more than 6 MPa.
Volga, the exact dependence of the strength of concrete in bending on the quality of cement is obtained if it takes into account the activity of cement for bending, whale is determined in accordance with GOST 310.4-81. In this case, one can use in the calculations the formula
As the age of concrete increases, its flexural and tensile strength increases more slowly than compressive strength, and the ratio decreases.
Prismatic Strength of Concrete
Prismatic strength is understood as a temporary resistance to axial compression of a prism with a ratio of the height of the prism to the size of the side of the square equal to 4. Samples of a prismatic shape for which the influence of frictional forces is less than for cubes, with the same cross-section shows a lower compressive strength. In real constructions, the stressed state of concrete approaches the tensile state of the prisms. Therefore, to calculate the designs for axial compression, the prismatic strength of concrete is assumed, its value has the maximum value for instantaneous loading. With such a ratio of N / b, the influence of the supporting plates of the press in the middle part of the prism (fracture site), as well as the flexibility of the concrete sample, is practically unaffected. At the same time, it is meant that the reference prisms gained strength under normal conditions for 28 days and that the loading conditions meet the requirements of GOST.
Prismatic strength is approximately 0.75 cubic strength for a class of concrete B25 and higher and 0.8 for a class of concrete below B25.
Uniformity of concrete
Uniformity of concrete in strength and other properties is an important factor in the reliability of concrete and reinforced concrete structures.
Estimated concrete resistances according to the current norms for design of structures make up only about half of the design strength values, since it is necessary to focus not on average indicators, but on the statistically probable minimum strength of concrete, whose quality is subject to random fluctuations.
Increasing the uniformity of concrete opens the possibility of its more effective use at the required security of its assigned parameters.
Uniformity of concrete along the strength, along with other factors, depends on the content and quality of the fillers used, especially if any of the properties of the concrete limit the production of concrete of the required strength.
When trying to get high-strength concrete on smooth rolled gravel, the weak point is the contact of the cement stone with the aggregate, and the more there is in the concrete of the aggregate, the less is the strength of the concrete. In this case, the inaccuracy of the dosing and the uneven distribution of the aggregate over the volume of concrete will reduce the uniformity of the concrete in strength and the more significant the higher the design strength of the concrete.
If the properties of the aggregate ensure proper adherence to the cement stone in concrete, and the strength of the aggregate is sufficiently high in accordance with condition (4.6), then the possible fluctuations in the content of such aggregate in concrete, as follows from the foregoing, have relatively little impact on the strength of the concrete and its variability.
Finally, if the strength of the aggregate is insufficient to produce concrete of the required strength, then the fluctuations in the content and the heterogeneity of the aggregate can very sharply reduce the uniformity of the concrete.
Therefore, the uniformity of concrete is usually associated with its strength, although the available experimental data are often inconsistent. For a long time it was believed that the higher the strength of concrete, the higher its uniformity. This was explained by the increase in the culture of production, the strengthening of technological control. However, subsequent studies (AE Desova, VA Voznesensky) showed that high-strength concretes, on the contrary, have less uniformity. The latter also corresponds to the ideas arising from the above analysis of the influence of aggregates on the strength of concrete.
According to GOST 10268-80, the ultimate strength of rock fillers for heavy concrete should exceed the design strength of concrete not less than 1.5 times if the latter is below 30 MPa and not less than 2 times if it is 30 MPa and higher . However, here we mean the average tensile strength from the test results of five control rock samples for compression or two samples of crushed stone for crushing according to GOST 8269-76. If the initial rock is non-homogeneous in strength, the minimum statistically probable aggregate strength may be much lower than the average. It is not excluded that it will be lower than required by formula (4.6) and even below the design strength of concrete, and the probability of this increases with the design strength of concrete.
Uniformity of light concretes in addition to general technological factors, depends on how well the field of application of a particular porous aggregate is chosen. The ratio of the concrete strength and the strength of the aggregate in the concrete is important, the latter being evaluated not only integrally by average indices, but also by the homogeneity characteristic. If the given concrete strength limit exceeds the minimum statistically probable value of the aggregate tensile strength, and even more so the average value of the concrete, the uniformity of the concrete decreases.
It is not uncommon to obtain lightweight concrete of the highest possible strength without taking into account the fact that with Re\u003e R3 the increase in concrete strength is accompanied by a decrease in its uniformity, so the design resistance can not be increased without the risk of reducing the usual safety factor of structures. Hence, in addition to the foregoing, higher demands are placed on the strength of aggregates for concrete and their uniformity.
The increase in the homogeneity of the aggregates, i.e., the approximation of the minimum statistically probable limit of strength to the average, is just as important as the increase in the average tensile strength. Therefore, in subsequent chapters, recommendations are given on how to increase the uniformity of aggregates by enrichment methods.
For lightweight heat-insulating and structural-heat-insulating concretes, uniformity with respect to heat conductivity. Given the relationship of thermal conductivity to the density of concrete, usually to simplify the problem determine the uniformity of concrete in density, and calculate not the minimum but the maximum statistically probable density of concrete.
The stability of all the parameters of the quality of concrete is affected by the uniformity of the fillers used also in terms of moisture content, grain size, grain form, etc.
Since the highly developed cement industry of the USSR ensures the stability of the quality of cement, and the mechanization and automation of the processes of preparation and laying of the concrete mix allow to provide the required technological parameters, the heterogeneity of the aggregates remains a significant obstacle to increasing the uniformity of the concrete. It is because of the heterogeneity of the aggregates that in the main it is necessary to increase the safety factor, using only half the potential of concrete.
In the scientific and technical literature the concept of homogeneity of concrete has recently expanded. In addition to characterizing the variability of the test results of individual concrete samples, the concept of structural homogeneity is introduced as characteristics of the variability of strength, deformative and other properties in the bulk of the sample. In this aspect, the distribution between the structural components of concrete internal stresses from external load, shrinkage, temperature, examples of which are described above, is considered. Fine-grained concrete is structurally more homogeneous than concrete with a large aggregate, which in some cases gives it certain advantages. Concrete on porous aggregates, whose properties are close to the properties of cement stone, are structurally more homogeneous than ordinary heavy concrete.