Experimental Study on Crack Characteristics of Damaged Reinforced Concrete Beams strengthened with CFRP

Lect. Dr. Ayad S. Adi

College of Engineering / University of Anbar

The bending tests of 2 control beams and 6 damaged beams strengthened with CFRP (DBSC) were carried out to study the effects of adhesive layer thickness, number of CFRP layers and reinforcement ratio on crack characteristics of DBSC. The test results show that the crack development of the DBSC is accelerated and the crack resistance of the DBSC is reduced compared with the intact beam. With the increase of the thickness of the adhesive layer, the crack development of the DBSC is slowed down and the crack resistance is improved. Increasing the number of CFRP layers is helpful to restrain the crack development of DBSC under high load level. The increase of reinforcement ratio can significantly improve the crack resistance of DBSC.

 Experimental program:

Standard cube specimen and standard prism were poured with the same batch of concrete poured into the test beam, and were cured synchronously with the test beam for 28d. Based on the measured data, the standard value of concrete’s compressive strength is 39.6MPa, the standard value of axial tensile strength is 2.49MPa, and the elastic modulus is 4 2.72 10´ MPa. The mechanical property parameters of the steel bars are calculated according to Code for Design of Concrete Structures (GB 50010-2010). Toray UT70-30 CFRP and Sikadur 330CN two-component epoxy carbon cloth impregnated adhesive are used. The mechanical properties of the materials are CEAEE 2021 by FRP, and FRP inhibited the crack development of the reinforced beam. The crack width and spacing of the reinforced beam were smaller than the corresponding the unreinforced beam.

During the test, the hydraulic jack with the maximum lifting mass of 20 t was used to apply the load. The jack was arranged in the position of the middle line of the distribution beam, and the distance between the two fulcrums of the distribution beam was 700 mm. The load of the test beam was controlled before the yield of the longitudinal bars, and the load of each stage was 2 kN. The mid-span displacement of the test beam was used to control the loading after the yield of the longitudinal bars, and the displacement of each stage was 1 mm. After the completion of loading at each stage, the load was held for 2 min, and the test data were collected after the load and deformation were stabilized. The test loading device is shown in Figure:

When the load reached 9.7 kN, the bottom of DB-1 beam cracked, and three initial cracks appeared near the loading point and mid-span section. With the increase of load, several main cracks appeared successively among the initial cracks. The width and length of each crack developed with the increase of load, and the growth of the crack near the loading point and mid-span section developed the fastest. There were few new cracks in this stage and the crack spacing was large. After the longitudinal bars yielded, the new cracks no longer appeared, and the developing speed of the main crack was accelerated.
Finally, the crushing of the concrete compression zone marked the end of loading. At this time, the main crack developed to 4/5 of beam height, and there were several penetrating cracks at the bottom of the beam.

Conclusion
The bending tests of 2 control beams and 6 DBSC were carried out to study the effects of adhesive layer thickness, number of CFRP layers and reinforcement ratio on crack characteristics of DBSC. The following conclusion can be drawn.

1- Compared with the intact beams, the growth speed of cracks of DBSC is accelerated and the crack.

2- With the increase of the thickness of the adhesive layer, the crack development of the DBSC slows down, and the average spacing and width of the crack decrease.

3- Increasing the number of CFRP layers has little effect on the crack resistance of the beams under low load, but it is helpful to restrain the crack development of the beams under high load.

4- The increase of reinforcement ratio can obviously delay the crack development and enhance the crack resistance of the reinforced beam.