Method of determining frost resistance of stone

FIELD: physics.

SUBSTANCE: stone sample saturated with water is frozen to a specified temperature. The sample is unfrozen and deformation is measured. Before freezing, the threshold load which accounts for long-term compression strength of the sample is measured using a nondestructive method. After several thermal cycles, residual deformation of the unfrozen sample is measured in the direction perpendicular the previous compression. A least residual deformation is achieved through periodical compression of the sample in the current direction from zero to a load which exceeds the threshold by not more than a third. The number of thermal cycles required for reducing long-term strength to the required value is determined by repeating these operations.

EFFECT: reduced labour input and increased efficiency.

1 tbl

 

The invention relates to methods for evaluation of long-term strength non-destructive method and is intended to clarify the brand F frost resistance of natural and artificial stones (bricks, concrete), i.e. the number of standard cycles of freezing and thawing (e.g., from +20 to -20°C for 4 hours)needed to reduce the tensile strength of the R samples, saturated with water, by an amount specified by the standard, in particular for 5 or 15% (ΔR/R=0,05 0,15...).

For basic (primary) adopted method [GOST - 10060.1-95], including the production of several samples of samples, each of size n, and the saturation of the samples with water, the initial values of R when the compression of the first sample, thermal Cycling of the other, the destruction of the compression of these samples after different number N of cycles and definitions as the mark F number of cycles needed to reduce R within the limits specified by the standard. Significant scatter of the values of R stone (coefficient of variation of 15...20%) at constant conditions of manufacture and testing of samples leads to a large scatter of sample meansand require a substantial amount of samples (n=25...50) to prove the significance of the change ΔR/R=0,05 0,15...as a result of thermal Cycling. Even with a small value of F≈50 total time thermal Cycling to exceed 6 days. That is they way the main disadvantage of the basic ways - complexity and low efficiency.

Closest to predlagaemom way [GOST - 10060.3-95], which consists in the manufacture of samples, saturated with water, the measurement of relative deformation of the sample (relative change δ volume) during the first freezing to the normalized temperature and thermal Cycling of samples, finding their mark F, and the dependence of F on δ for a particular type of stone and the use of this relationship in the future to determine F by δ when the first freezing of the sample.

The prototype disadvantages - complexity and low efficiency due to the need to investigate the dependence of F(δ) for each type of stone or binder.

The purpose of the invention is to reduce the complexity and increase efficiency.

Goal reach the fact that, as in the prototype water-rich sample is frozen up to the standard temperature, defrost and measure its deformation. But unlike prototype

- before freezing non-destructive method to determine when the compression threshold load for the water-saturated sample, consistent with its long-term strength;

after thawing of the sample is measured residual strain in the direction perpendicular to the preceding compression;

- achieve no less residual deformation cyclizes the m stationary compression of the sample in the same direction from zero to the load, exceeding the threshold is not more than one third;

by repeating these operations, determine the number of cycles required to reduce the limit R long-term strength to the desired value.

The use of non-destructive testing R allows you to identify the brand F test for one sample that has already dramatically reduces the complexity. However, such control must have high accuracy (1...3%) and not to increase the damage of the material, which leads to the choice R of the long-term strength - see patent No. 2305281. Other distinctive operations aimed at replacing more cycles operating cycles mechanical loading equivalent for insertion of damage, for the measure was adopted as the amount of permanent deformation. When replacing cycles mechanical stress must exceed a threshold of not more than 1/3. In this case, the crack shear in compression will not move into the cracks of separation, parallel compression, leading to fragmentation of the sample.

To validate the proposed method implemented in parallel with the basic method of determining the frost resistance [GOST - 10060.1-95] on samples with an edge 15 cm, made from a mixture of Portland cement (1 weight part) type 400, sand (2 concerns), crushed granite 5...20 mm (4.5 century'clock) and water (0,6 concerns). From one batch produced 108 samples fo the mA which differed from the cube with an edge 15 cm, however, what is the point of intersection of two diagonals formed opposite faces (reference point) distance from each other farther (up to 1.5 mm)than the remaining points of these edges that correspond to each other. The samples were verdeli 28 days in water at room temperature, and then 60 days in moist sand at 18...26°C. During the implementation of the basic method, the distance l between the reference points of each sample (100 PCs) was measured using instrumental microscope before and after freeze-thaw durability testing. Residual strain ε was calculated as Δℓ/l. The distance change Δℓ between reference points when implementing the proposed method on 8 samples were led at 20±2°C before and after thermal Cycling using brackets with a variable base and with a measuring cylinder and dial type (intercept 1 μm). Threshold load L0water-saturated sample was determined by recording acoustic emission (AE) with the help of the device AF-15 under cyclic loading and unloading of the sample to zero. In the first cycle, the load L brought up to 11 tons; in the absence of AE at the end of unloading the value L was increased by 5% and so on until until at the end of the load did not arise AE. For L0took an average L of the last two cycles. Thermal Cycling mentioned 8 samples from +20 to -20°C was performed 10 times in a row (decades), and then measured the residual fact deformation is s ℓΔ T. Then the sample was loaded mechanically to load 1,33R 1...2 min and unloaded. Cycles were repeated until such time as the residual strain Δℓ between reference points are not exceeded ΔℓT(12 to 19 cycles). Then spent the next decade of cycles, etc. to reduce ΔR/R=0,12 0,18.... The results of the tests are shown in the table, where the following notation: i is the number of decade of thermal Cycling; ΔεTi= ΔℓT/l - increment relative residual deformation after the first decade of thermal Cycling; Ri- long-term strength before the i-th decade thermal Cycling;i=ΔεiTi- the equivalence of mechanical and thermal loading; Δεi- increment relative residual deformation during mechanical Cycling after the first decade of cycles;- the amount of cycles and equivalent mechanical loading

(each of 12...19 mechanical cycles). As can be seen from the table, ΔR/R=0,142 when ∑=102...107, and an average of 105. With this in mind, the results of the 50 samples, saturated with water, was thermosensible 105 times (from 20 to -20°C), then for each of them measured residual strain Δℓ and destructive method defined temporal resistance Ri. For Rithe average value of status is vilo and standard deviationfor residual strain εiaverageseemed equal 1448×10-5, Sε=77·10-5,For the other 50 samples without thermal Cycling received=34.8 MPa, S=Of 5.92. As for sample meansthe confidence interval relationslies in the range 0,12 0,20...that is not contrary to the result obtained using the proposed method (0,142) and proves its correctness. The same conclusion holds for the residual strain.

The proposed method is compared with the reference [GOST - 10060.1-95] reduces the amount of testing procedure and increases their efficiency.

The method for determining the frost resistance of stone, which consists in the fact that water-rich sample of stone frozen up to the standard temperature, defrost and measure the deformation, characterized in that before freezing non-destructive method to determine the threshold load that meets the long-term strength of the sample during compression, after a number of cycles measured residual strain thawed sample in the direction perpendicular to the preceding compression, achieve no less residual deformation of cyclic the mere compression of the sample in the same direction from zero to the load, exceeding the threshold is not more than a third, repeating these operations, determine the number of cycles needed to reduce the long-term strength to the desired values.



 

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