Rehabilitation of overload-damaged reinforced concrete columns using ultra-high-performance ﬁ ber-reinforced concrete

: One of the problems experienced by reinforced concrete (RC) structures, whether a mistake at the design phase or a change in building use, is an overload. The goal of this study is to determine whether ultra-high-performance ﬁ ber-reinforced concrete (UHPFRC) is e ﬀ ective in repairing damaged concrete columns utilizing 30 mm thin concrete jacketing made of di ﬀ erent steel ﬁ bers at various contents with varied aspect ratios (28, 37, and 45). Nine pieces of 500 mm long RC column specimens with a cross-sectional size of 150 mm × 150 mm were cast as part of the experimental program. By loading these columns with roughly 90% of their actual ultimate axial load capacities, damage was caused, and the columns were subsequently strengthened and rebuilt using UHPFRC jacketing materials. The results demonstrated that when steel ﬁ ber content increased, so did the mechanical qualities, such as compressive and tensile strengths. Rehab materials that contained 0.5% steel ﬁ ber recovered 138% of their maximum load-bearing capacities, while materials that contained 1.5% steel ﬁ ber recovered 164% of their original capacities, which were roughly 1.38 – 1.64 times that of the unjacketed reference column. Additionally, it was observed that employing steel ﬁ ber in UHPFRC contents in these approaches resulted in the ultimate displacement being improved from 2.91 to 5.53 mm at 1.5% of steel ﬁ ber content, which is an increase of almost double that of control specimens.


Introduction
Concrete is widely used worldwide and considered a common material in different sectors of the construction industry. Because of its properties such as durability and performance, it can reach up to several decades of building service life through fulfilling safety requirements of design either for concrete mix materials or structural design of the building. However, the concrete often suffers damage due to ignorance or non-compliance of building codes in the design phase, or unexpected environmental conditions that affect the safety of reinforced concrete (RC) [1]. For clarification, RC structures are damaged when exposed to severe environmental conditions, such as corrosion, fire, and natural disasters. Additionally, accidental mistakes in the design or changes in building usage which reach their intended design life causing overloading are considered as the main factors of damage that lead to failure to meet the functional requirements of the service life, thereby leading to loss in the bearing capacity of the principal components of structure building, such as beams, columns, and slabs [1,2]. If attention is not paid to these causes, then structures could fail to carry their design load and disasters could occur especially to the most important structural elements, i.e., columns, which are widely used in residential and highrise buildings, leading to a partial or complete collapse of the structure because it is only the members that transmit the total vertical loads of buildings to the soil. Thus, if the damage occurs by overload, the columns lose their strength and stiffness by significantly degrading and causing the deformation of the steel bar and will appear as cracks in the surface. The cracks would then allow corrosive factors such as salt water and high humidity to penetrate and reach the steel reinforcement, and failure could occur [3]. Therefore, repair and rehabilitation are necessary in case of noticeably damaged cracks on the concrete surface to confirm that loads are further carried and transmitted to the soil [4]. Thus, rehabilitation for damaged structures is considered a challenging activity that experts could face during the repair process. According to Yaqub and Bailey [5], repairing and reusing the affected concrete structure is more economical than demolishing and constructing a new building, because the damaged RC columns have superficial damage in the concrete that appear as slight cracks without damage to the reinforcement and their tie [1]. Previous studies have suggested several solutions for the rehabilitation depending on the type of structure and loading. Strengthening method represents one of the most popular methods used in the rehabilitation of damaged columns [6]. Several techniques can be applied based on the degree of structural damage, such as injections, removal and replacement, or jacketing. To strengthen RC column damage by jacketing, which is commonly used, three principal techniques are used: concrete jacketing, steel jacketing, and composite jacketing [2,7]. Tayeh et al. [1] studied the repairing and strengthening of columns by jacketing with normal strength concrete (NSC) and ultrahigh-performance fiber-reinforced self-compacting concrete (UHPFRSCC) as jacketing materials. Results revealed that the ultimate load-carrying capacities and axial displacements had significantly increased in both materials with respect to the unjacketed reference column. Contrastingly, studies found that UHPFRSCC is more effective due to the use of steel fibers than using NSC as a jacketing material for repairing and strengthening RC columns. Elsamny et al. [8] investigated 37 specimens under various eccentricities from e/t = 0-25%, and they were split into three groups, 5 specimens were evaluated under various eccentricities as part of group 1 control columns, and 16 samples from group 2 were reinforced using varying numbers of steel wire mesh plies. Sixteen specimens from group 3 were strengthened with a sandwich of varied numbers of steel wire mesh plies and external vertical steel bars 3Ø8 in compression side and all groups were jacketed with cement mortar. The test findings demonstrated that employing the wire mesh jacketing approach increases the load carrying capacity by up to 23%, while adopting a sandwich wrapping system technology that consists of external vertical steel bars in compression side and steel wire mesh delivers a 54% improvement in load carrying capability. Composite jacketing is considered to be a modern technology that uses ultra-high-performance fiber-reinforced concrete (UHPFRC), which has been developed to increase the service life of a RC structure at a more effective cost. Furthermore, damage to the RC structure would improve the structural durability and load-bearing capacity of the structure by using UHPFRC, which was the most recent material used due to its capability to carry or sustain a higher load even with the presence of crack on the concrete. The UHPFRC consist of cement, sand, a high amount of silica fume, water, superplasticizer (SP), and steel fiber, which make these mixtures stronger and ductile [9]. Abbas et al. [9] indicated that UHPFRC is considered a new cementitious material because it contains fibers that are distributed in a homogeneous way and lead to an increase in the properties of concrete by improving the compressive strength, tensile strength, toughness, ductility, and durability of a concrete structure. Additionally, this technique has been developed to increase the service life of a RC structure that has been damaged and retained back in service. Brühwiler et al. [10] indicated that UHPFRC material is a favorable repair method due to its ease of placement on damaged areas. Plizzari et al. [11] conducted a study on the applications of a high-performance fiber-RC jacket with 170 MPa compressive strength to strengthen RC short columns. The result shows that improved durability marks an advancement in the concrete industry and leads to sustainable and economic buildings with resistance against all kinds of corrosion. Tayeh et al. [1] conducted an experimental study on the application of thin concrete jacketing by applying two jacketing materials, which were 25 and 35 mm thick, to repair and strengthen the damaged columns exposed to approximately 90% of their actual ultimate axial load capacities. Two groups were used in the experiment: group 1 with material consisting of NSC with a maximum aggregate size of 4.75 mm and steel reinforcement and group 2 containing ultra-high-performance fiber reinforced with steel reinforcement. The results showed that group 2 had a significant increase in ultimate load capacity by around three times than that of the unjacketed reference column and 1.86 times than that of group 1. Nowadays, the low cost and fast execution time of repairing techniques is desired, which should be identified and recognized. Hence, the current research studied UHPFRC for repairing and strengthening of damaged square RC columns that reach 90% of their actual axial capacity without failure by applying three contents of steel fiber (0, 0.5, and 1.5%) at fixed jacketing thickness with three different aspect ratios of steel fiber (28, 37, and 45). In addition, the impact of steel fiber content on axial displacement and maximum load carrying capability was investigated. The outcomes were contrasted with those of the reference columns.

Experimental program
The current experiment aims to investigate the UHPFRC using various contents of steel fiber at different aspect ratios (28, 37, and 45) as composite repair material for RC column damage caused by overload. By bonding this material to all four sides of a damaged column area at fixed jacketing thickness (30 mm), we can strengthen and restore damaged columns for further load-carrying capacity and axial displacement.

Column preparation
This research includes the fabrication of nine column specimens that are repaired and strengthened by applying jacketing using UHPFRC, which were damaged after being exposed to an overload. According to ACI 318 code requirements [12], all tested column specimens are designed with not less than 1% of longitudinal reinforcement ratio. Nine square column specimens are designed to act as normal columns and are exposed to an overload. Columns tested with cross-sectional square of 150 mm × 150 mm and a height of 500 mm with 4Ø12 mm longitudinal steel reinforcement and 6Ø8 mm use as link bars spaced at 75 mm center to center ties, as shown in Figure 1.

Mix design for columns
The concrete mix comprises sand with a relative density of 3.15, fineness modulus of 3.4, and absorption of 1%, natural crushed coarse aggregate (maximum size 14 mm), and ordinary Portland cement (Type 1). The mix proportion was designed to achieve a target compressive strength of 30 MPa at 28 days accordance to Building Research Establishment Method [13]. Concrete mix proportions as reported by Mohd Zahid et al. [14] were used for reinforcement, which consist of 550 kg/m 3 cement, 250 kg/m 3 water, 745 kg/m 3 sand, and 805 kg/m 3 coarse aggregate as shown in Table 1. The mix() were used to cast the columns and 100 mm cube edge were prepared, cured, and tested cubes. The cube-shaped columns were cured in water for 28 days and then kept at room temperature until testing as shown in Figure 2. These mixtures used for specimens act as normal columns until exposed to an overload test.

Mix design for repair material (UHPFRC)
The mix proportion of UHPFRC for repair material consists of three mixtures depending on the percentage of fiber as shown in Table 2. Sodium naphthalene sulfonate (SNF) is included in this mixture to help concrete to be more workable and enhance the strength of the mixture by increasing the strength by around 20-60% and enhancing their durability in their service life [15]. Silica fume is one component in the UHPFRC mixture with its content limited to 6% due to its effect on the rheology property of UHPFRC, and the compressive strength of UHPFRC decreases when silica fume content exceeds a certain value (>10%) [16]. SP, conplast SP430, was used with a specific gravity of 1.06 for improving the workability of the concrete. Figure 3 shows that all the damaged columns were repaired by UHPFRC   with 30 mm thickness jacket with different contents of micro copper-coated steel fibers (MSFs) of 0, 0.5, and 1.5% and at different aspect ratios of 28, 37, and 45, respectively, as shown in Table 3. Concrete cube was also cast from the reinforcement and repair mix to monitor concrete compressive strength at each mix.

Test set-up and instrumentation columns before and after repair
Column specimens were tested using a Universal Testing Machine with a load capacity of 200 tons before and after repair, as shown in Figure 4a and b. To prepare specimens for the test, the top surface of the column was smoothed using a Gipson layer before testing. To distribute the axial load into the column surfaces, two steel plates were placed on the upper and lower surfaces of the columns. Table 4 illustrates four groups of the columns that have been divided depending on the testing variables. The concrete type (normal), jacket strengthening (UHPFRC and MSFs 0%), jacket strengthening (MSFs 0.5% and MSFs with composite aspect ratio of 28, 37, and 45), and jacket strengthening (MSFs 1.5% and MSFs with composite aspect ratio of 28, 37, and 45). A total of nine RC columns were tested   under concentric axial compression load with approximately 90% of their actual axial capacity at loading rate of 0.5 kN until the appearance of hairline cracks without reaching failure. Three best values out of nine non-repaired columns were chosen as a control sample depending on their strength and displacement and grouped into RC. All repaired specimens with UHPFRC were tested under axial compression with a displacement rate of 1 mm/min as recommended by Yaqub and Bailey [17], until crack appeared and failure occurred in the specimen. The average value of the three column specimens was considered. During the loading process, the axial load and vertical displacement data were recorded through a computerized system connected to the machine and then the vertical load-displacement curve was plotted which led to obtained three important mechanical properties of RC column i.e., ultimate load, displacement, and ductility.

Results and discussion
The properties of the normal concrete mixes and UHPFRC were investigated. Compressive strength of all specimens was obtained by testing three standard test cubes (100 mm × 100 mm × 100 mm) at 28 days. Figure 5 shows the average of compressive strength of normal and UHPFRC. On the testing date of the cubes of Mix0, the average compressive strength was 37.3 MPa which was lower when compared to all the mixtures of UHPFRC. As the MSFs increased in Mix2 and Mix3 by 0.5 and 1.5, the value of compressive strength increased by 14.51% for H2 (0.5%) fiber contents and 44.82% for H3 (1.5%), respectively, when compared to the batch of H1 with non-fiber (0%). Also, the compressive strength of UHPFRC in the Mix1 with non-fiber (0%) was increased by 47.72% compared to the normal concrete (Mix0). Previous studies reported that using steel fiber enhanced the concrete properties [18]. Moreover, according to Denarié [19], the addition of a large amount of steel fibers led to extremely high compressive strength.
In contrast, compressive strength is not the main feature of UHPFRC for judging it as a good material for rehabilitation of the damaged columns. There are other features, for instance, tensile strength, which is considered the most important feature of UHPFRC because tensile strength in cementitious repair material avoids or reduces shrinkage deformations. In this study, dogbone-shaped specimens were used to obtain tensile strength by testing all mixtures at 28 days, as shown in Figure 6.
The average tensile strength of the three UHPFRC ranged from 2.41 to 9.28 MPa, as shown in Figure 7. The tensile strength is crucial for cementitious repair material Type of Mix because the existing structure is restrained to more or less large shrinkage deformations, which means it needs high tensile strength [20,21]. In this study, as seen, the increase in steel fiber content led to an increase in the tensile strength, which is similar to the findings from previous studies [20]. UHPFRC (H3) with a 1.5% fiber content had the highest tensile strength. Table 5 indicates that the H1 and H2 jacketed column specimens show an increase in the average ultimate loadcarrying capacity by 1.13 and 1.38 times than those of the RC-controlled columns, respectively. The average ultimate load-carrying capacity of H3 increased by 1.64 times than those of the RC-controlled columns. Under axial compression loading, the fiber orientation of the square cross-section column at the corner is in tension, whereas it behaves primarily in flexure conditions at the jacketing side due to the stresses produced from the column corner to the middle zone, which leads to an increased loading capacity [22]. Average load-displacement diagram, as shown in Figure 8, was plotted for all jacketed columns and control specimens. H3 jacketed column specimens gained almost a double increase at 90.03% in axial displacements at rupture with respect to the RC reference columns. As shown in Table 5, the axial displacements of the H1 and H2 jacketed column specimens increased by 73.88 and 78.35%, respectively,    compared to the RC reference columns. Furthermore, the axial displacements of H3 give indications that 1.5% of steel fiber is appropriate for UHPFRC. Furthermore, the fibers in UHPFRC play a role in increasing the ductility of the columns, as shown in Figure 9, from 5.45 to 10.9%, compared with the control specimens, which affected the number of cracks and the crack width opening as observed. Hence, repairing with UHPFRC as a modern technique would restore the load-bearing capacity of the concrete, and these reflected on bearing capacity of rising up their figures [23].

Conclusion
The outcomes of the experimental study using UHPFRC of jacketing materials for rehabilitation of damaged columns, which was observed, can be summarized as follows: -All the columns strengthened by UHPFRC showed higher maximum load capacities than the unstrengthened columns (RC), while increasing the MSFs content increased the load capacities. -The columns strengthened with UHPFRC jacket consisting of composite of different aspect ratios of steel fiber showed higher displacement than the unstrengthened columns and the corresponding columns with higher MSFs content, which then increased the column ductility afterward -There are substantial improvements in concrete compressive strength from H1 to H3 at 47.72-113.94% of its original value (RC), and the ultimate load-carrying capacities of the square columns were strengthened by 13-63% after applying UHPFRC jacketing procedures.   Rehabilitation of overload-damaged reinforced concrete columns using UHPFRC  7