抗折强度与抗压强度的换算关系（Conversion relationship between folding strength and compressive strength）Technical properties of ordinary concrete (middle part)
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切术The performance of hardened concrete Strength of concrete
Strength is the most important properties of hardened concrete, and the other properties of concrete and strength all have close relations, the strength of the concrete mixture ratio design, construction control and main technical indexes for evaluation of the quality inspection. The strength of concrete is mainly compressive strength, folding strength, tensile strength and shear strength. The compressive strength value is the largest and the most important strength index.
1. Concrete cube compressive strength and strength grade. According to the test methods of normal concrete mechanical properties (gbj81-85), the standard size of the cube specimen is 150mm x 150m
m x 150mm. Standard curing conditions for 20 + / - 3 ℃ temperature, relative humidity above 90%; The standard age is 28 days. The compressive strength measured under the above conditions is called concrete cube compressive strength. The test and calculation methods are shown in the test section.
疟疾疫苗According to specification for design of concrete structures
(GB50010-2002), the strength grade of concrete is determined according to the cube compressive strength standard values, and the concrete cube compressive strength standard value refers to the standard method to make maintenance of side length is 150 mm cube specimens, 28 days in age with standard method of compressive strength with 95% reliability. Concrete structure of reinforced concrete is divided into C15, C20, C25, C30, C35, C40, C45, C50, C55, C60, C65, C70, C75, C80 and 14 levels. According to the concrete quality control standards
异硫氰酸酯
热看网(GB50164-1992) regulation, strength grade is expressed symbols C and the corresponding standard value, ordinary concrete is divided into C7.5, C10, C15, C20, C25, C30, C35, C40, C45, C50, C55 and C60 a total of 12 strength grade. For example, C30 indicates that the standard value of the cube compressive strength is 30MPa, that is, the probability of the concrete cube compressive strength is more than 95 %.
The classification of concrete strength grade is mainly for the convenience of design and construction acceptance. The selection of strength grade is based on the importance of buildings, structural parts and load conditions. The following principles are generally available:
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(1) the structure or non-permanent structure of the floor, foundation, floor and force of ordinary buildings shall be selected from C7.5 ~ C15.
(2) beam, board, column, stair and roof of ordinary buildings shall be selected from C20 ~ C30.
(3) high-rise buildings, large-span structures, prestressed
concrete and special structures should be selected with more than C30 concrete.
2. Axial compressive strength. The axial compressive strength is also known as prism compressive strength.
Because the actual structures (such as beams and columns) are mostly prismatic structures, it is more practical to use prismatic specimen strength. It is a prismatic specimen with 150mm x 150mm x (300 ~ 450) mm, which is obtained by standard maintenance to 28 days. The axial compressive strength of the same material is less than cube strength, and its ratio is approximately equal to 0.7 ~
0.8. This is because the compressive strength test, the specimen in the top and bottom friction under the constraint of two pieces of steel plate, the lateral deformation is restricted, namely the "hoop effect" the impact of height is about 0.866 times that of the specimen length, as shown in figure 4-8. Therefore, the test pieces of the cube are limited by the hoop effect and the measured intensity is relatively high. The intermediate region of the prismatic specimen is not affected by the "ring effect", it is a pure pressure zone, and the measured intensity is relatively low. When the steel pressure plate and the specimens are lubricated, the friction resistance decreases, the hoop effect weakens, and the compressive strength of the cube tends to be equal to the compressive strength of the prism.
FIG. 4-8 the constraint effect of the steel pressure plate on the specimen
3. Tensile strength. The tensile strength of concrete is very
small, only 1/10 ~ 1/20 of compressive strength, and the higher the strength of concrete, the smaller the ratio. For this reason, in the design of reinforced concrete structure, it is generally not considered to bear the tension, but through the configuration of steel reinforcement, by the reinforcement to bear the tension of the structure. But the tensile strength of concrete crack resistance plays an important r
ole, it is a crack in the design of structure the main indicators of width and crack spacing calculation control, and resistance to cracking due to shrinkage and temperature deformation and main indicators.
Axial tensile test is used to test the tensile strength of concrete, the result of the load is not easy to axis alignment deviation, and fixture in local damage due to stress concentration often occur, so the test is very difficult, the accuracy of the test value is lower, so popular at home and abroad by splitting method of indirect determination of the tensile strength of concrete, namely, splitting tensile strength.
Splitting tensile test of standard specimen size of the cube side length 150 mm, the two relative to the center line line load applied, the specimen in the vertical plane are blah stress, as shown in figure 4-9.
FIG. 4-9 schematic diagram of the split tensile test device
The tensile stress can be calculated by elastic theory. The calculation formula is as follows:
(4 to 8)
Type:
- concrete splitting tensile strength (MPa);
P - damage load (N);
A - specimen splitting area (mm2).
It not only greatly simplifies the test process, but also can accurately reflect the tensile strength of concrete. The experimental results show that the axial tensile strength is lower than the tensile strength, and the ratio of the two is about 0.8 ~ 0.9. In the absence of test data, the tensile strength can be calculated by the following equation:
(9) 4 -
4. Anti-folding strength. The compressive strength is the main strength index of the road surface or airport road surface with cement concrete. According to the regulation of highway cement concrete pavement design (jtj012-94), the strength of concrete concrete calculated with different traffic volume grades is as table 4-14. The conversion relationship between compressive strength and compressive strength of road cement concrete is discussed in table 4-1
5.
Table 4-14 pavement cement concrete to calculate the tensile strength
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