The Effect of Cross Sectional Properties of Concrete Column on the Repair Material: A Review

The utilization of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) as a material for repairing concrete structures holds immense promise and potential. UHPFRC usage in concrete has been a significant rise in the interest surrounding the enhancement and renovation of concrete structures, particularly focusing on columns. UHPFRC has showcased remarkable bonding potency with concrete, along with an impressive capacity to renew the load-bearing capability of compromised reinforced concrete columns. However, the limited application of UHPRFC as a repair material for concrete columns is primarily attributable to a lack of comprehension regarding the influence of shape and size effects, particularly as they pertain to square and rectangular columns. In this paper, the factors behind the impact of cross-sectional properties on the effectiveness of UHPFRC as a repair material is presented. Therefore, it can be adopted by researchers to explore the UHPFRC potential in structural rehabilitation applications especially for concrete columns. It is believed that the relationship between the cross-sectional area, aspect ratio, and corner radius ratio of the column and repair material is a vital parameter affecting the performance of repaired columns.


Introduction
Throughout the years, concrete structures exhibit various faults caused by a range of variables; by clearly identifying the diagnostic and root causes of these defects, there is a greater likelihood of preventing them through the selection of the most appropriate repair solutions.Over the past three decades, various academics have presented a variety of strengthen and repairing methods for Reinforced Concrete (RC) column [1].Most prevalent repair methods presented included: steel jacketing, steel reinforced concrete jacketing as well as Fibre Reinforced Polymer (FRP) wrap and plate [2].In many nations, the annual cost of construction activities associated with the rehabilitation of concrete facilities is in the billions of dollars.The industry struggles to identify a repair material that increases structural integrity and serviceability while taking into account the damage, cost, time, and practicability.1303 (2024) 012015 IOP Publishing doi:10.1088/1755-1315/1303/1/012015 2 FRP, which is gaining the confidence of civil engineers, is being investigated as a method for repairing RC structures [3,4].Many of the previous studies used FRP as jacketing material, but the main problem is the weak bonding strength between substrate and FRP, which can prevent the steel stirrups from reaching their yield strength at the maximum stress [5,6].
The usage of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) in strengthening and repairing works was investigated in various research works [7,8].UHPFRC has huge potential to be used as repair material.UHPFRC jacket was employed to enhance the efficiency and load-bearing capability of members in RC structures and it was found that it is a viable option for reinforcing degraded structures [9,10].Because of its numerous benefits, UHPFRC has been examined for its potential as a strengthening material.Multiple investigations have indicated that using UHPFRC jackets as a strengthening technique is effective, as it enhances the RC column's ultimate load capacity, ductility, and energy absorption.However, to deepen our understanding of UHPFRC jacketing behaviour and assess its effectiveness, further studies are required [11].UHPFRC has the capability to enhance the stiffness of structural members and prolong the occurrence of isolated fractures, thereby enhancing the resilience and longevity of RC structures [12,13].
Due to the variety of benefits of the UHPFRC, the numbers of research are growing, but mostly on the strengthening RC beam with main study parameter of the steel fibre content, repair material thickness and the mechanical properties of the UHPFRC.However, the studies on the effectiveness of UHPFRC as repair material for RC column are limited and desperately required.
The cross-sectional properties including the cross-sectional area of the column, the aspect ratio and side to corner radius ratio of column cross-section play significant role on the performance of jacketing materials.It was found that columns' original load carrying capacities can be restored using UHPFRC.As can be seen from this, UHPFRC outperforms FRP, which can only recover the original strength of circular columns [11].
Previous researcher has studied the effects of the cross-sectional properties of normal RC column, they name it, column size-effect law, also known as Bazant's law, and it is a concept proposed by Zdeněk P. Bažant, according to Bazant's law, the compressive strength of a slender structural column decreases as its size or slenderness ratio increases.This effect is due to the increased influence of imperfections, such as initial geometric deviations or material heterogeneity, this phenomenon shall result in a reduction of load-carrying capacity [14].Another, research has demonstrated that the crosssectional properties have notable implications for the restored concrete columns.Test outcomes revealed that the type of RC column cross-sections such as circular, square and rectangular are significantly affect the maximum load of FRP wrapped RC columns [15].
Concrete columns might necessitate strengthening as a remedial approach to address issues such as deterioration, seismic activity, deficient design, and construction errors.Additionally, it was found that in comparison with rectangular column, the square columns strengthened with Fabric Reinforced Cementitious Matrix (FRCM) and Carbon Fibre-Reinforced Polymers (CFRP) can support greater maximum load [16,17].As the aspect ratio increases, the maximum load of rectangular columns decreases [18].Moreover, an increase in column dimension shall reduce the load carrying capacity and make the RC column less ductile [19].Similar observation was also made for circular concrete columns surrounded by steel tubing [20].
However, due to lack of knowledge on the influence of cross-sectional shape effect of RC column and the impact of various cross-sectional properties of columns, including aspect ratio, dimension, and the ratio of column width to corner radius, restricts the utilization of UHPRFC repair to concrete columns.This knowledge is vital for employing UHPFRC as a jacketing material to reinforce damaged concrete columns.

Column Shape Effect on Confinement of UHPFRC
It is well known that FRP are very effective for circular column but less effective for square and rectangular columns.Researchers have discovered that shape modified square and rectangular column and strengthened with FRP has resulted better performance in terms of load carrying capacity, compressive strain and energy absorption [21].
The effectiveness of high-strength fibre-reinforced concrete (HSFRC), CFRP and GFRP in strengthening RC columns with various cross-sectional shape and loading conditions was investigated [22,23].They found that the confinement strength of CFRP on the square and rectangular columns are less efficient and lead to the drop of their ultimate load carry capacity.Another study that focusing on the effectiveness of CFRP as jacketing material reported that CFRP is very effective for circular column but ineffective for square and rectangular columns.Moreover, the square member confined with CFRP exhibited a slightly lower flexural capacity compared to its unconfined counterpart However, there is a scarcity of comprehensive research investigating the effect of cross-sectional shape on the effectiveness of UHPFRC confinement for RC columns.This insufficiency highlights the necessity for additional investigation in this particular domain.

The Effect of Jacketing Thickness on Column
The RC column cross-sectional area is directly proportionate to its load carrying capacity.In general, larger cross-sectional areas result in greater load-bearing capacities [15].This means that a larger cross-sectional area typically results in a greater load-carrying capacity due to the greater volume of material available to resist applied loads.
The thickness of UHPFRC jacket plays a vital role on the mechanical performance of the strengthened rectangular concrete column.The load carrying capacity, stiffness and toughness of concrete column increase as the thickness increases.In contrast, the ultimate displacement reduces as the thickness rises [24].
The confinement of UHPFRC for circular column directly proportional to the tensile strength and thickness of the jacketing material.On the other hand, the radius of the circular column inversely proportional to the confinement pressure [11].
Based on the abovementioned studies, they clearly show that the thickness of UHPFRC jacket critically influences the mechanical properties of strengthened column.However, a systemic study is required to obtain insight into the effect of UHPFRC jacket thickness on the various cross-sections of concrete column.

The Effect of Column Aspect Ratio
Aspect ratio is an additional crucial factor that affects the performance of columns and the repaired material.In general, the aspect ratio represents the proportional relationship between its height and width of the column's cross-section, a greater aspect ratio indicates that the columns are wider, while a lower aspect ratio indicates that the columns are narrower as shown in Figure 1.The confinement of jacketing material is less effective in square and rectangular column compared to circular column due to non-uniform axial compressive stress distribution exist in non-circular column.This is caused by the arching phenomena which is critically influenced by the arch height and the arch height is directly proportional to the aspect ratio [25].
An experimental study reported that all type of FRP wrap namely CFRP, Glass Fibre Reinforced Polymer (GFRP) and Aramid Fibre Reinforced Polymer (AFRP) exhibit similar pattern in which the load carrying capacity decrease with the increase of aspect ratio [26].The increase of the stiffness of the FRP by applying double or triple plies can improve the load carrying capacity of the FRP wrapped column but the effect of aspect ratio remains unchanged [27].However, the increase of aspect ratio shall reduce the effect of size and edge sharpness [25].
The above studies show that the aspect ratio plays a vital role in the performance of FRP as jacketing material, however the knowledge of aspect ratio for UHPFRC jacketing is still limited and more studies are required.

The Effect of Corner Radius Ratio
Corner radius ratio, also known as fillet radius, is the relationship between the radius of the corner fillet and the section's thickness as shown in Figure 2. In RC columns, the corner radius ratio influences stress distribution and can have an effect on the column's overall strength.

Figure 2. Side to Corner Radius Ratio
In the research about the stress-strain behaviour of FRP-confined square column sections, the corner radius ratio was identified as an important factor affecting the confinement of these columns.Their results demonstrated the significance of the parameter of radius ratio in determining the efficacy of FRP confinement [28]."A separate research investigation was undertaken to examine the notable rupture strain of FRP composites used for the confinement of square concrete columns with varying corner radii, spanning from 0 to 75 mm.The ultimate strength of FRP-confined specimens with substantial rupture strain increased as the corner radius grew [29].Furthermore, the compressive performance of 29 rectangular specimens filled with concrete and FRP was assessed under various compression loading scenarios, divided into five sets according to the corner radius (0, 15, 30, 45, and 66.5mm).With the augmentation of the corner radius, there is an improvement in the linear slopes that ascend or descend after reaching peak points in the stress-strain curve during the third phase of the column's stress-strain behaviour.Consequently, a stress-strain curve that originally descends might transition into an ascending pattern [18].A declining curve characterized by a larger corner radius exhibits a slower reduction rate and higher stiffness after reaching the peak point compared to an ascending curve.Hence, it can be concluded that the ultimate strength exhibits an upward trend with the increment of corner radius, considering a constant FRP confinement level [30].
In addition, rectangular columns with varied corner radius (5, 15, 25, and 38 mm) are encased in CFRP composites.Based on [31], it has been observed that the ultimate strength and ductility of CFRPconfined rectangular columns can be enhanced by increasing the corner radius.The corner radius ratio affecting the efficacy of a steel-reinforced concrete jacketing system.
The performance of low-strength concrete prisms confined with FRCM composites was investigated, the researchers discovered a distinguishable influence of corner radius ratio on FRCM jackets, means the compressive strength increases as the corner radius increases.Moreover, the peak values of strain and cracks in FRCM jackets predominantly concentrated around the sharp corners of thespecimens, as observed in reference [32].
Based on above research works it shows that the corners radius ratio has a significant effect on the performance of jacketing material.Therefore, explicit research exploring the impact of corner radius ratio on the performance of UHPFRC jacketed columns is limited, indicating a need for further study in this area.

Conclusion
This paper offers a comprehensive review and assessment of the existing understanding regarding the impact of the cross-sectional properties on the performance of column repaired with UHPFRC.On the basis of current knowledge, a clearer vision of research parameters that affecting the effectiveness of the jacketing are presented.Based on the above discussion, the following conclusions can be drawn: • The relationship between the cross-sectional area of column and repair material is a vital parameter affecting the performance of repaired columns, influencing their load-bearing capacity and their mechanical behaviour under various loading conditions.• The thickness of UHPFRC jacket is critical and further study is required to get insight into the effect of thickness on the various concrete column cross-section.• The aspect ratio is an essential parameter that significantly impacts the performance of repaired columns.Higher aspect ratios generally correspond to lower mechanical properties, including flexural strength and confinement effectiveness.• Larger corner radius ratio generally leads to improved confinement effectiveness and load-bearing capacity.
The roles of these parameters can improve the design and use to retrofit columns with UHPFRC in construction projects.Additional research is required to completely comprehend the intricate interplay between these elements and their collective effect on the behaviour and performance of columns repaired with UHPFRC.

Figure 1 .
Figure 1.Aspect Ratio of Column