An experimental investigation on the ultimate strength of epoxy-repaired braced partial infilled RC frames

Abstract

Due to earthquake, buildings are damaged partially or completely. Particularly, structures with soft storey are mostly affected. In general, such damaged structures are repaired and reused. In this regard, an experimental investigation was planned and conducted on models of single bay, single storey of partial concrete infilled reinforced concrete (RC) frames up to collapse with corner, central, and diagonal steel bracings. Such collapsed frames were repaired with epoxy resin and retested. The initiative was to identify the behavior, extent of restored ultimate strength, and deflection of epoxy-retrofitted frames in comparison with the braced RC frames. The performance of such frames has been considered only for lateral loads. In comparison with bare RC frames, epoxy-repaired partial infilled frames have significant increase in lateral load capacity. Central bracing is more effective than corner and diagonal bracing. For the same load, epoxy-repaired frames have comparable deflection than similar braced frames.

Introduction

Nowadays, soft storeys are common at the parking level in multistoried buildings. There is elimination of infill walls in bottom storey, whereas the storeys above are filled with partition or shear walls. Such frames have less capacity to bear lateral loads. Based on an analytical investigation, Arshad and Alok (2008) had studied seismic performance and potential seismic damage of masonry infilled reinforced concrete (RC) framed building with soft storey. It was observed that if the storey is partially infilled (in comparison with that with no infills), it decreases the storey drift and deformations in the column and reduces the related damage to the overall frame. Partially infilled RC frames were used as these were one of the best solutions to overcome the lateral strength problem for frames with soft storey. Many prototypes as well as models of reinforced cement concrete (RCC) frames have been tested by Benjamin and Williams (1958, 1959) with plain and reinforced concrete infill walls. It was concluded that there was no scale effect, i.e., test can be performed on any scale model; results of scale models were found to be consistent with the prototype.

Based on the study of Popov and Bertero (1975), it was found that the effectiveness of the epoxy repair is limited by the access to the joints surrounded by transverse beams and floor slab. This limitation can possibly be overcome by further advances in the vacuum impregnation technique. The effectiveness of vacuum impregnation epoxy inlet port techniques was studied by French et al. (1990) to repair interior joints of beams and columns that were moderately damaged due to inadequate anchorage of continuous bars of beam. It was concluded that vacuum impregnation is an effective means of repairing large regions of damage with fewer reachable sides.

The effects of joint reinforcement arrangement were studied by Karayannis et al. (1998) on the efficiency of epoxy repair by pressure injection. Eleven of the tested one-way exterior joint specimens were repaired by epoxy injection and then retested. On the local retrofit of RC members, Mahmoud (2005) had investigated with the application of a new high performance fiber-reinforced composite material by using a suitable epoxy adhesive. This technique was used to enhance the strength and ductility compared with other methods of local retrofit such as steel plates and FRP laminates. It was reviewed that the global retrofit of RC frames using direct internal steel bracing can increase the yield and strength capacities and reduces the global displacements of the frames.

For repairing and restoring superior strength, Bhikshma et al. (2010) repaired RCC beams with epoxy resin material (EexpacreteSne1), and the flexural strength increased significantly up to about 15% for concrete beams compared to other epoxy resin materials. Deflections were lesser in reinforced concrete beams with epoxy resin compared to conventional concrete beams. The stress distribution for the relative stiffness has been studied by Kanakambara (2011) with aluminum frame and araldite AY103 with a hardener HY 951 as the infill material. The mutual interaction of the frame and the infill plays an important role in controlling the stiffness and strength of the infilled frame.

Considering all these factors, steel-braced RC frames with concrete partial infill, which is one of the suggested possible solutions for soft storeys, were tested up to collapse. These collapsed frames were repaired by epoxy resin and were retested under lateral loads to understand the behavior and contribution of retrofitted structure. This study consists of tests on 14 models of bare, braced, and infilled frames subjected to lateral loads as shown in Table 1. For main reinforcement and bracings, high-grade steel bars and mild steel square bars are used for frames.

Table 1 Description of various frames

The present work is predominantly experimental oriented, and experiments have been performed on models up to failure. Though studies have been carried out on single bay, single storey frames for infilled frames but the same can be incorporated for multi bay, multistoried framed structures. For each frame, two models were tested and average value is considered for experimental loads and deflections.

Methods

Experimental setup

RC portal frame of single bay, single storey with a welded base plate of 10 mm thick was mounted on a supporting girder and rigidly bolted with four bolts of 20 mm diameter. Horizontal load is applied to RC frame through column of reaction frame with the help of a jack. The models tested of each category are mentioned in Table 1. The details regarding dimensions, position of proving ring, loading jack, and dial gauge are highlighted in Figure 1. The frame consists of two columns of height of 400 mm and a beam with a span of 600 mm. The size of the column is 60 mm × 100 mm, and for the beam, it is 100 mm × 100 mm. For measurement of load, proving ring of capacity of 10 kN was attached for bare frames and a hydraulic jack of 500 kN was utilized for rest of the frames. Dial gauge of range of 20 mm was used to measure the horizontal displacement at the beam level. The steel skeleton of reinforcement is shown in Figure 2.

Figure 1
figure1

Experimental setup.

Figure 2
figure2

Reinforcement details of frames.

After the testing up to collapse, frames R5, R6, and R7 were repaired by filling cracks (Figure 3) with epoxy. Epoxy is a thermosetting polymer formed from the reaction of an epoxy ‘resin’ with polyamine ‘hardener’. Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives. Loose concrete is removed, and dirt or debris is cleaned to open up cracks or holes in the concrete before applying epoxy. The concrete is cleaned with a pH neutral cleaner using scrub brush to remove any remaining dirt from the damaged area. It was rinsed it off with clean water and allowed to dry before applying epoxy. The most accurate method of proportioning is the use of preproportioned units supplied by the manufacturer. If such packaging is not available, the components may be mixed in the ratios established manually by laboratory tests. Though the ratio for mix proportion of resin, araldite GY 257, and hardener, Aradur HY 140, was taken as 1:0.5 as suggested by the manufacturer, it was manually checked for suitability of the initial and final setting time. The properties of resin and hardener are mentioned in Table 2.

Figure 3
figure3

Crack patterns for different epoxy-repaired concrete infilled RC frames.

Table 2 The properties of epoxy resin and hardener

Materials for models

The following materials were used for the frame and epoxy:

  • For main reinforcement, ∅8 mm was used; for ties and stirrups, ∅6 mm was used for the RC frames. For bracings, 10 mm square bars of mild steel was used.

  • Ordinary Portland cement of 53 grades, river sand, and coarse aggregate of 12 mm in the ratio of 1:1.5:3 were used for concrete. Cubes of size 150 mm × 150 mm × 150 mm were cast and tested to obtain the compressive strength after 28 days. The partial infills of thickness 50 mm made up with concrete.

Test procedure

The RC frames were cast, and after curing, these were mounted on the reaction frame. The bolts were fully tightened to ensure the fixity of supports. The alignment of jack was checked along the beam axis. The initial reading on the proving ring and the dial gauge was recorded. The application of horizontal load was with the help of a screw/hydraulic jack, and horizontal displacement was noted down from dial gauge. The load was applied at a uniform rate. The loads and the deflections were recorded at regular intervals for each test setup. The load was applied continuously till it remains constant for a particular time on the loading gauge and then moves in a reverse order. This is called as plastic state condition. The collapse load corresponding to this stage was recorded as an ultimate load.

Results

While conducting the experiments with bare RC frames, precautions were taken to keep the proving ring at its position as it was trying to lift itself. During application of load crack formation and its propagation at different load, levels were recorded. The final collapse modes were photographed for full details. The behavior of frames has been studied with parameters such as the following:

  • Partial infill system - bare frames and different types of braced partial infilled frames

  • Epoxy-repaired cement concrete infill frames with similar braced frames

  • Strength, stiffness, and deformation of frames

The compressive strength for concrete was observed to be 24.2 N/mm2. For mix proportion of resin and hardener with ratio of 1:0.5, the initial and final setting times were 30 and 90 min, respectively. The comparison of the ultimate load of RC frames R5, R6, and R7 with epoxy-repaired frames R8, R9, and R10 as mentioned in Table 3 shows a decrease of 16.6%, 24.42%, and 11.7%, respectively in lateral load capacity. Figure 4 shows load vs deflection curve for bare RC frame. It can be concluded from Figures 5, 6, and 7 that epoxy-repaired frames have reduced amount of deflection for the similar value of load in comparison with braced frames. The percentage decrease in stiffness for epoxy-repaired frames from similar braced infilled RC frame is 9.09%, 24.42%, and 27.08%, correspondingly.

Figure 4
figure4

Load vs deflection graph for bare RC frame R4.

Figure 5
figure5

Load vs deflection graph for R5 and R8.

Figure 6
figure6

Load vs deflection graph for R6 and R9.

Table 3 Comparison of ultimate load and stiffness for various frames
Figure 7
figure7

Load vs deflection graph for R7 and R10.

Discussions

The behavior of partial infilled braced RC frames subjected to racking load was studied with different patterns of bracings such as corner, central, and diagonal. Load was applied by a screw jack and measured on proving ring of capacity 10 kN on bare frames for precise results. As the load carrying capacity of other frames was greater than 10 kN, the rest of the frames were tested with a hydraulic jack of 500 kN. The partial infilled braced RC frames were tested up to collapse. Afterwards, the major cracks were filled with cement slurry and curing is done for 7 days. The ratio for mix proportion of resin and hardener was taken as 1:0.5 as suggested by the manufacturer and manually checked for suitability of the initial and final setting time. The cracks within the deficient RC frames were filled with epoxy by grouting and pouring and using brush and allowed to harden for 24 h before testing. Safe handling of epoxy was accomplished by using disposable gloves and working in a well-ventilated area with the use of safety eye glasses. The repaired frames were tested by using the same procedure. In general, concrete material crushed at the corners with major cracks developed along tension column, and predominately, sway mechanism can be observed in Figures 8, 9, and 10 for different frames. The stiffness of epoxy-repaired frames was compared with the similar braced infilled frames. In order to have comparative similarity for calculation of stiffness, the ultimate load of epoxy-repaired frame was considered for similar frames. Though full corner braced infilled system shows better results than that of the other two systems, practically, it is difficult to implement for soft storey frames as it would hinder the movement of users around the space and thus central bracing system is more effective with moderate ultimate strength. Epoxy can be used as a binding material which has been widely used for patching or repairing surface defects of different types of concrete structures. It forms a good bond with old concrete surface and rebar which is one of the prime requirement of a good repair work. Epoxy-repaired partial infill frames have significant lateral load carrying capacity and the deflection is under control in comparison with bare RC frames.

Figure 8
figure8

Epoxy repaired, with partial concrete infill corner-braced frame.

Figure 9
figure9

Epoxy repaired, with partial concrete infill central-braced frame.

Figure 10
figure10

Epoxy repaired, with concrete infill diagonal-braced frame.

Conclusions

The purpose of this study was to evaluate the strength of epoxy-repaired RC frames in comparison with those studied by previous researchers, as well as to add to the database of strengthening test results in order to lead to changes or acceptance in design codes and standards. To study the ultimate load of two bare, six partially infilled, and epoxy-repaired partially infilled, RC frames were constructed and tested up to collapse.

Based on the results of the investigation, the following findings and conclusions are presented for epoxy-retrofitted frames:

  • If the infill is stronger than the frame, the failure mode corresponds to sway mechanism with major tension cracks along tension column, and for braced RC infilled frames, possible plastic hinge locations are at column-beam junction and bottom of column.

  • The initial and final setting time for epoxy is 30 and 90 min with ratio of 1:0.5 for resin and hardener, respectively. Safe handling of epoxy can be accomplished by using disposable gloves and eye glasses and working in a well-ventilated area.

  • Based on a comparison with the braced infilled RC frames, epoxy-retrofitted frames have shown a decrease of 16.6%, 21.42%, and 11.7% in lateral load capacity. It specifies that the strength is restored for deficient frames up to a significant level.

  • From load vs deflection curves, it is concluded that epoxy-repaired frames have maximum deflection under control in comparison to bare RC frames.

  • Practically, the partially infilled center-braced system may be a viable solution which may not affect architectural or interior function with moderate strength than that of the corner and diagonal bracing partially infilled system for soft storey frames.

  • The percentage decrease in stiffness is 9.09%, 24.4%, and 27.08% for epoxy-repaired frames from similar braced infilled RC frames. It specifies that the frames are stiffened up to considerable level after epoxy application.

However, with the limited number of tests (two for each frame) carried on various frames, the researchers recommend further testing to increase the database for epoxy-retrofitted soft storey RC frames.

Authors’ information

SKDD has done his M.Tech. in the field of structural engineering from IIT Mumbai in 1991. He is an associate professor in SSVPS COE Dhule and doing his Ph.D. in the field of structural engineering. He has published papers in the field of structural engineering and building materials. SYK is a Ph.D. and is a professor in K. K. Wagh COE Nasik and has published papers in the field of structural and hydraulics engineering.

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Correspondence to Shailendra kumar Damodar Dubey.

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The authors declare no competing interests.

Authors’ contributions

SKDD prepared the experimental model, carried out the experiments, and drafted the anuscript. SYK had given the idea, participated in designing the experimental model, helped in the interpretation of experimental output, and also helped in drafting the manuscript. Both authors read and approved the final manuscript.

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Dubey, S.k.D., Kute, S. An experimental investigation on the ultimate strength of epoxy-repaired braced partial infilled RC frames. Int J Adv Struct Eng 6, 3 (2014). https://doi.org/10.1186/2008-6695-6-3

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Keywords

  • Central braced frame
  • Epoxy resin
  • Lateral load
  • Soft storey
  • Retrofitted frame