In order to study the effect of the external carbon fiber composite sheets on the shear strength of the specimens, the 12 specimens are rectangular beams with no abdominal reinforcement, and the bottom of the beam is matched with 2 18 longitudinal bars, and the size of the specimen is 2600 * 150 x 300.

4 groups of 12 low strength concrete beams were designed and fabricated in batches. Considering the influence of floor slab and convenient construction operation, a reinforcing scheme of externally bonded carbon fiber U shaped cloth is adopted, and the strip is cut off at the 50mm position from the top of the beam. The first groups of specimens were pasted with the same amount of U shaped cloth strip to change the anchorage way of the U shaped cloth end. Keeping the spacing between strips unchanged, the second groups of specimens were treated with 2 cloth strips as additional anchorage measures. Third groups of specimens were set up with steel layering and bolts to change the width of U shaped cloth. The fourth groups of specimens change the strength of concrete. In order to study the effect of the external carbon fiber composite sheets on the shear strength of the specimens, the 12 specimens are rectangular beams with no abdominal reinforcement, and the bottom of the beam is matched with 2 18 longitudinal bars, and the size of the specimen is 2600 * 150 x 300.

Test device

Two point vertical symmetrical loading is carried out on a concrete beam top through a 200kN hydraulic jack and loading beam. Each specimen is applied incrementally according to the increment of 2kN per level. When the vertical displacement of the beam span is suddenly increased or the maximum width of the beam end slanting crack is close to 01 3mm, while the data are collected continuously, the deformation continues to increase slowly until the specimen is destroyed.

Summary

1)

The shear strength of low strength concrete beams can be obviously improved by adding enough width of carbon fiber U – shaped strip and pressure strip, and peeling is the main factor affecting the effect of shear reinforcement.

2)

The contribution of the reinforcement to the shear bearing capacity of the beam is mainly determined by the tensile force transmitted by the fiber cloth on the premise that the concrete strength and the section size ensure that the low strength beam does not have baroclinic failure. The fracture mechanics analysis shows that if the length of the fiber is short, the tensile force will be determined by the product of the paste area and the tensile strength of the concrete. If the length of the fiber is sufficient, the tensile force will be determined by the product of the width of the material, the effective bond length and the product of the tensile strength of the concrete.

3)

Considering the effect of the effective tension of the fiber bar and the additional pulling force of the bar, the formula of the shear bearing capacity of the reinforced beam reinforced by the U – shaped bar perpendicular to the axis of the beam is proposed. The calculated value is in good agreement with the experimental value of the final peel failure specimen and slightly below the experimental value. The formula can be used for shear strengthening design of low strength concrete beams.

4)

Because the concrete strength of the test beam is less than 12MPa, the fracture energy constant is 012. The design formula is only limited to determine the shear capacity of the U – shaped low strength concrete beam. It is still to be studied whether the results can be used in the shear reinforcement design of the ordinary strength concrete beams.