Concrete Durability and Steel Corrosion is by far the biggest durability issue for reinforced concrete structures, although other deterioration mechanisms will lead to attack of the concrete itself, he notes, such as freeze-thaw scaling, moisture, acid or sulfate attack, thermal cracking, shrinkage from drying, impact, erosion, and wear.
In our experiments, 3 concrete square columns and 5 RC square columns strengthened with HPFL and BSP were built. The dimensions and reinforcement bars are shown in Figure 1, and the strengthening design of the column is shown in Figure 2. RG-JS polymer mortar, Araldite XH130AB concrete interface bound rubber, bisphenol A-type modified epoxy resin, and amine curing agent were used in this study. The diameter of the steel strand for reinforcement was 3.2 mm, and its cross-sectional area was 5.1 mm2 and elastic modulus was 1.16 × 105 MPa. The size of the angle was 25 mm × 25 mm × 3 mm, and its cross-sectional area was 143.2 mm2 and elastic modulus was 2.10 × 105 MPa, similar to those of steel. The yield strength of steel and the ultimate strength are shown in Table 1, and the cube strength of concrete and mortar is shown in Table 2.
The classification of strengthened columns and the main test results are shown in Table 2. Hydraulic press of 2000 kN was used in the axially compressed experiment. The loading rules were as follows: 60 kN was added per level in initial loading steps; then, 120 kN was added after cracking; finally, the load was decreased back to 60 kN as soon as the load reached 80% of the ultimate strength. As shown in Table 2, the ultimate bearing capacity for the strengthened column Z1 increased by 22.13% compared with column Z02. However, the range of bearing capacity improvement was small, because the mortar strength was only 13.80 MPa after 2 days of curing, considering the early strength properties of mortar. The ultimate bearing capacity for the strengthened columns Z2–Z4 increased by 35%–45% after 5 days of curing; their mortar strength was 32.61 MPa and was close to the concrete strength. The ultimate bearing capacity for the strengthened columns Z5-Z6 increased by 48%–52% compared with unreinforced column Z01. Thus, the strengthening effect was significant. The results indicated that the ultimate bearing capacity for the strengthened column increased with the increase of the ratio of reinforcement and the strength of the polymer mortar, whereas the ductility decreased to some extent.
The load-deformation curve of the strengthened column is shown in Figure 3. The break time of the unreinforced column Z01 was short, the ductility and deformation capacity were poor, and the characteristic of brittle failure was obvious. Given the reinforcement skeleton, the ultimate axial compressive capacity for column Z02 increased by 12.96%, the ductility increased obviously, and the descending part of the load-deformation curve was smooth. Therefore, the ultimate bearing capacity, ductility, and deformation capacity for the strengthened column all increased. This increase was due to the improvement of the compressive strength of the core concrete; such improvement was caused by the active constraints of the lateral deformation in the core concrete by the reinforced layer.
The typical failure modes of the strengthened columns are shown in Figure 4. Cracks formed mainly along the bolts in the vertical direction of the columns when columns were damaged, and the mortar and concrete were crushed and bulged outward in the middle of the columns. Moreover, all the steel strands in the middle part of the angle were cut off, and the four corner angles tended to be stripped because of the loss of the steel strand constraints. The specimens broke suddenly as a result of the sudden unloading of the steel strands. The load of the strengthened columns decreased rapidly at the later steps of loading, which indicated poor member ductility. This characteristic was also shown on the load-deformation curves. Therefore, to improve the late ductility and deformation capacity for the strengthened columns, the sudden cutting off of the steel strands was slowed by increasing the chamfer radius of external angle and sticking steel plates in the column edge after chamfering.
Discussion of Influence Factors of Bearing Capacity of Strengthened Column
Given the use of reinforcement materials, including high-strength steel strand, angle bar, and high-performance mortar, and the use of shear bolts in the angle bars, the integrity and interoperability between the reinforced layer and the original column were improved. In addition, the reinforced layer restrained the core concrete effectively. Accordingly, the compressive strength of core concrete was improved. The bearing capacity for strengthened columns was affected by the following factors.
Bearing Capacity of the Original Column
The ultimate bearing capacity for the strengthened columns increased evidently with the increase of the concrete strength of the original component. Thus, controlling the range of the concrete strength of the columns strengthened with HPFL and BSP is necessary. Concrete Durability
Moreover, the ultimate bearing capacity for the strengthened columns increased with the increase of the longitudinal reinforcement ratio and stirrup reinforcement ratio of the original component. The value is in between the maximum and the minimum reinforcement ratio values.
Constraint Area of the Reinforced Layer
According to existing experimental conclusions, the constraints were uniformly distributed and equal in all directions for circular cross section columns under lateral confinement, and the strengthening effect was satisfied. By contrast, the strengthening effect on the cross section was not superior to that on the circular cross section, because the lateral confinement led to obvious stress concentrations in the four corners of the rectangular columns and the stress was comparatively feeble in the middle of each side. Given the differences in length-to-width ratios, the lateral restraints of concrete were also different apparently. Angles were arranged in the four corners of the component; hence, if the width of the angle was larger, then the confinement area under the lateral passive restriction in the corner of the original member would be larger. The areas of the weak constraint region in the center of the sides decreased relatively. Thus, the triaxial compressive area of the cross section increased relatively.
Capacity of the Strengthened Columns
Traditional theory of confined concrete considered that the lateral expansion deformation of the core concrete under the axial compression caused a horizontal bending in the straight-line segment of the stirrup; the constraint of core concrete was small because of the small bending rigidity of the stirrup. However, the stiffness was large and the deformation was small in the intersecting parts of the stirrup; the utmost restraint was formed by the resultant vertical force of stirrups, which acted on the diagonal of the core concrete. The restraint caused by the steel strands was passed through the angle to the strengthened column, as shown in Figure 5. Hence, the strong constraint region of the strengthened column was radically different from that of traditional theories. Steel strand formed the restraint stress on core concrete through angle, and the strong constraint regions A and B were also formed. Meanwhile, the flexural stiffness of the steel strand was nearly zero, and region C was the weak constraint region of the strengthened column.
Calculation Equation of Axial Bearing Capacity
The calculation of axial bearing capacity was based on the loads carried by different parts of the strengthened column. Concrete Durability
Bearing Capacity of Reinforced Mortar Layer N1
The mortar layer was equipped with the characteristics of high early strength, which supported load as a part of the reinforced structure; and the thickness of the layer of the column strengthened with HPFL and BSP was approximately 25 mm. The mortar layer could not contact the beam and slab on the top of columns closely under gravity. Thus, the reinforced layer could not fully and effectively bear the pressure transferred from the beam and slab. This study and another test study indicated that the compressive strain of the mortar on the angle surface was much larger than that among angles because of the shear connector on the angle surface. In particular, the former was 70%–80% of the peak strain of the mortar, and the latter was merely around 30% of the peak strain.