In this test, 12 specimens of C15 and C35 two kinds of strength concrete surface layers were designed. The thickness of the test piece surface layer was further divided into 30mm, 40mm and 50mm3 types to compare the surface layer strength and the surface layer thickness respectively. Pulling and impact of shear capacity. The standard values ​​of the measured cubic strength of the concrete and the standard values ​​of the axial compressive strength are shown.
Concrete strength grade measured cubic strength standard value fcu, k / (Nmm-2) axial compressive strength fck, k / (Nm-2) pull test and shear test are carried out on a universal testing machine, through the universal testing machine The compass reads the data. When performing the drawing test, first screw an elongated screw with an external thread into the bolt hole, and test the expansion resistance of the expansion sleeve by pulling the screw with a universal testing machine. The shear test is to test the shear performance of the expansion bolt by transmitting the shear force applied by the universal testing machine to the pre-installed expansion bolt through a rigid steel ingot.
In the failure characteristic drawing test, the final failure mode is that the expansion bolt embedded in the CS plate is directly pulled out from the concrete. Observation of the damaged test piece revealed that the concrete portion around the bolt hole on the surface of the surface layer was pulled away. The expansion bolt exerts a pressing force on the hole wall through the expansion of the expansion sleeve to transmit the pulling load. During the pull-up process, the expansion sleeve applies a symmetric and uniform annular tension <2> to the concrete of the hole wall, so that the concrete near the hole wall It is in the state of two-axis tension-compression stress. With the increase of the pulling force, the tensile strength of the concrete under the two-axis tension-pressure state is also reduced <3>. When the concrete strength grade is not high, the surface concrete first breaks in the direction of the main tensile stress <4 >, the expansion bolts are easily pulled out directly. Through experiments, it is found that even if the surface concrete grade is C35, the expansion bolts will be directly pulled out.
In the shear test, the final failure mode was that the concrete pressed by the bolt was crushed. Due to the high strength of the expansion bolt, the bolt does not undergo shear damage. It can be determined that the bearing wall failure <5> is the main reason for the loss of shear bearing capacity of the expansion bolt.
The test results are compared and compared with the same thickness and bearing capacity shear failure characteristics test data on different strength surface layers. It can be seen that with the increase of concrete strength, the uplift and shear capacity of expansion bolts are obviously improved. This is because the strength of the expansion bolt is high, and both the pull test and the shear test lose the bearing capacity due to the failure of the surface concrete, so the strength of the surface concrete is one of the factors affecting the pullout and shear capacity.
Comparing the bearing strength test data of the same strength and different thickness concrete surface layer, it is found that the expansion bolts on the three thickness surface layers have the same tensile and shear bearing capacity, and are not affected by the thickness of the surface layer. Analysis of the reasons, this paper believes that the CS board concrete surface layer is thin, but because the surface layer is embedded with steel Wire Mesh, and together with the oblique wire in the sandwich layer to form a space grid, so that the upper and lower layers of the CS board Concrete has better overall workability when subjected to force, and is different from single-layer concrete surface layer alone. Therefore, in the CS plate with a thickness of 30 mm, even if the expansion bolt penetrates the surface layer, as long as the expansion of the expansion tube is ensured to generate friction, the tensile and shear bearing capacity is not lower than that of the expansion bolts in other thickness layers.
Through the above analysis, the thickness of the surface layer has no effect on the bearing capacity test data. Therefore, the test data on the same strength concrete surface layer can be analyzed and analyzed. Through analysis, it is found that all the bearing capacity test data are greater than 0.75 times the average value of bearing capacity, so the average value of 0.75 times of bearing capacity is taken as the allowable value of the tensile and shear capacity of the CS plate.
The shear bearing capacity of the expansion bolt in the CS plate is found to be crushed according to the damage of the test piece, and the concrete on the pressed side of the hole wall has been crushed, and the expansion bolt has no strength damage. Therefore, the shear capacity is determined by the compressive strength of the surface concrete. Since the actual bearing stress distribution in the bolt hole is difficult to determine, it is assumed that the concrete hole wall stress is evenly distributed at the same buried depth for the simplified calculation. When the shear bearing capacity reaches the limit, the wall of the pressure side is only partially crushed in the buried depth range, and the degree of damage of the hole wall is related to the concrete grade.
Therefore, it is suggested that the following formula can be used to simplify the calculation of shear capacity. V=Conclusion 1) When the commonly used 8@30 expansion bolts are used as fasteners on the CS board, the allowable values ​​of pullout and shear capacity can fully meet the daily use of residential buildings. 2) Since the thickness of the surface layer of the CS plate has no effect on the pull-out and shear ultimate bearing capacity of the expansion bolt.
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