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Structural performance of a novel technique for repairing reinforced concrete slabs subjected to corrosion damage

H.P. Sooriyaarachchi1, Anuradha Abeynayake1 1Department of Civil and Environmental

Faculty of Engineering, University of Ruhuna Hapugala, Galle.

SRI LANKA

E-mail: harsha@cee.ruh.ac.lk

Abstract: Corrosion of reinforcement is one of the main durability concerns in costal structures. For number of reasons, due to both design and construction, slabs elements are corroded earlier than other structural elements. Common method of restoring the corrosion damages of reinforced concrete flexural elements has been found not only highly technically demanding but also less effective to repair the tension phase of flexural elements. This paper presents the performance of repair slab elements using a repair technique developed around the conventional slab casting. In the instance of heavy corrosion or expected excessive loading of the existing slab, the technique allows introducing new reinforcements or layers of new reinforcement beneath the existing slab. It is found that the static performance of the repaired slab elements under the proposed method of repair is better than the conventional repair technique. Keywords: Reinforced concrete, corrosion damage, Repair techniques, Static performance.

1 INTRODUCTION

Corrosion of reinforced concrete is by far the main durability issue in costal structures. Due to number of reasons, it is considered that the slab elements are more vulnerable to corrosion damages compared to other structural elements (i.e. beams and column elements). Due to relative small moments in slab elements, reinforce requirement to resist ultimate limit state loading is often found to be considerably low which leads engineers to use ever decreasing slab thicknesses for slab elements. This leads to excessive deflect and cracking of slabs at serviceability limit state giving rise to number of durability issues. It is also a common practice to reduce the cover in slab reinforcement as means to increase the lever arm and thereby reduce reinforcement requirement. All of such practices often lead to less protection to reinforcement. In terms of environmental exposure, slabs, due to its significantly larger external exposure dimensions to volume ratio compeered to other structural elements, have the highest exposure to the environmental conditions. In addition slabs are subjected to wetting and drying more often than the other structural elements. All of which leads slabs more vulnerable to chloride ingress and corrosion damage compares to other structural elements. Figure 1 below show example of slab corrosion while other structural elements are still in good health and spalling off repair mortar from slab due to not attending to proper details (1).

(a) Slab corrosion (b) Spalling of concrete after slab repair after corrosion

Figure 1 Slab Corrosion before the beam

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Spalling of concrete layers and cracking concrete around reinforcement bars are common signs of corrosion of reinforcement in RC structures. This is caused by the expansive rust forming on the reinforcement and pressure exerted by it on concrete. Initial rust formation around reinforcement can lead to better structural performance of reinforced concrete temporarily due to improved bond between concrete and reinforcement. However, this behaviour is short-lived as more rust will crack and displace concrete around the reinforcement bars. It is now found that the structural behaviour of exposed reinforcement with spalling cover is not one of composite action but one of tied arch behaviour (2, 3, and 4). Figure 2 shows the strain pattern at various sections along an exposed beam with clear evidence of stress reversal close to the support. Stress reversal of exposed beams are further confirmed by the occasional tension cracks emanating from top of the beam, what normally considered as the compression phase of the beam as depicted in inserts of Figure 2.

1280mm

450mm

1800mm

Figure 2 Strain pattern of exposed beam clearly show the stress reversal close to the support and

inserts evidence of tension cracking close to support.

Conventional method of structural repair of reinforced concrete involves shot-creating repair mortar into a cavity created after removing the concrete surrounded by the corroded reinforcement. Figure 3 shows different steps in conventional method of mechanical repair simulated in preparing test specimens to evaluate performance of conventional method of mechanical repair which involves; removing lose concrete around the reinforcement and creating a cavity with repair extending at least 20mm beyond the reinforcement level ( which is in this experimental purposes is simulated by beams cast with a cavity and exposed reinforcement), exposing aggregate to create good bond between the parent concrete and repair mortar (which in this experimental study is conducted by water blasting), applying a layer of primer onto the prepared surface and pump repair mortar into the cavity. The properly performed repair is expected to restore the structural action and recover the moment carrying capacity of the corroded elements. However, this is largely depending upon the stress transfer between the repair and parent concrete. When the repair is properly done it is expected that the structure restore its load carrying capacity. Conventional method of repair of corrosion damage is often considered highly technically demanding and costly method of repair for corrosion damage elements.

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2 SIGNIFICANCE OF THE STUDY

This study suggests an alternative repair technique for corroded reinforced concrete slabs. The main advantage of the method is that it often seen as extension of current practice of slab casting with only exception being the use of self compacting concrete against normal concrete. The method involves use of shuttering arrangement very similar to the one that is used for new construction. This shuttering arrangement shall be placed after the removal of lose concrete around reinforcement and after application of any primer or bonding material on the surface of the parent concrete coming into contact with the repair concrete. After shuttering arrangement is completed, the self compacting concrete is poured on to the cavity created by the shuttering though holes drilled trough the existing slab. The technique allows any surface preparation and application of bonding agents in the existing concrete strata and replacement or addition of new reinforcement layers beneath the existing corroded slab. Rigorous static test conducted on the repaired test specimen under different shear stress intensities suggest that the new technique of repair is as effective as the conventional method in restoring the structural behaviour.

3 METHODOLOGY

There are number of variables that can influence the performance of repaired beams. Repair length, repair depth, properties of the repair material, surface treatment, force intensity experienced at the interface, bond strength between the repair and old concrete are all considered factors influencing the performance of repair (5,6). For the longevity of the repairs, it is recommended that repair depth shall be at least 20mm beyond the tensile reinforcement level as it would be otherwise difficult to ensure the durability after repair (6). Depths not covering the reinforcement level can easily lead to corrosion of the reinforcement and delaminating of repair mortar from the parent concrete with short period after repair (see Figure 1 for evidence). Differential shrinkage, strength and stiffness properties of repair mortar are all considered important parameters in determining material for repair. It is considered that shrinkage compensated mortar with stiffness +- 10 GPa compare with the parent concrete is more appropriate for repair (5, 6). In this investigation repair beams were cast in two stages to simulate the concrete repair under the new technique. In the first stage beams were cast keeping repair dimensions empty by filling it with packing material. In the second stage that is after 28 days packing material was strapped and typical shuttering arrangement was placed around the cavity to receive self compacting concrete. Self

3(a) Prepare repair

surfaces using water

blasting

3(b) Complete the

required shuttering

work for the repair

3(c) Application of

primer to the prepared

surface before being

repaired

3(d) Shotcrete the

repair mortar to the

cavity

Figure 3 Procedure for conventional shotcrete repairing for corrosion damage

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compacting concrete was then poured into to cavity through a cores drilled though the compression face of the slab. Figure 4 shows the specimen preparation done according to the proposed method of slab repair.

4(a) Drilling through the specimen 4(b) Placement of shutters 4(c) Finished specimen

Figure 4 Different stages of sample preparation under the proposed slab repair technique.

Table 1. Specimens tested under the experimental program to evaluate the performance of the repair

technique.

650mm

60mm

1600mm

900mm

1600mm

60mm

R/42/80/6.0 R/42/80/4.5

650mm

1000mm

60mm

900mm

1000mm

60mm

R/42/50/6.0 R/42/50/4.5

650mm

60mm

1600mm

900mm

1600mm

60mm

R/69/80/6.0 R/69/80/4.5

650mm

1000mm

60mm

900mm

1000mm

60mm

R/69/50/6.0 R/69/50/4.5

4(b)

4(c)

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Repair length, concrete grade and shear arm length in load application are considered variable in this experimental investigation to evaluate the performance of the new repair technique. Two concrete grades, Grade 40 and Grade 70 self compacting concrete are used as repair mortar while 50% and 80% repair lengths are used to find the influence of repair length. In addition, in order to critically evaluate the structural integrity of the repair under possible worse loading combinations, the specimens were subjected to different shear force intensities by changing the shear arm of the two point bending test setup (7). Two shear arms 4.5 and 6 times effective depth are used for testing specimens after initial investigation of control specimens (Beams without repair) suggested no shear failure in the control specimen (specimen without repair) for the considered beam dimensions up to 4.5 shear arm to effective depth ratio. Table 1 show the schematic diagrams of specimens tested under this experimental investigation. Reinforcement detail and the two point testing set ups used for beam are shown in Figure 5. For simple reference purposes 4 letters/Digit name was given to identify the beams. First letter of the name is given to denote the study R to denote the current study and C to denote the comparison study. Digits following the English letter denote the compressive strength of the repair mortar which is followed by the length of repair expressed as a percentage of the total length of the beam. Final digits denote the shear arm to depth ratio of the beam testing. For example R/42/80/6.0 refer to the beam tested under the current study repaired with mortar strength 42 over a 80 percentage of the total length tested with shear arm to depth ratio of 6.0. The static test results of this investigation is compared with the results obtained from test specimens cast using conversional techniques with the state of the art technologies used in mechanical repair. In the referred study (3) the surfaces of the specimens in the parent concrete that are coming into contact with the repair material were water blasted to expose aggregate before bonding agents are applied as depicted in Figure 3. In evaluation of the new repair technique under the current experimental investigation, neither the bonding surfaces had any treatment nor were the surfaces applied with any bonding agent (primers).

150mm

900mm

2000mm1900mm

2T12-1

5 R6-2-80 2T12-1250mm

20mm

(a) Reinforcement detail and 1st loading arrangement of the current study (Shear arm 4.5xd) (b)

150mm

550mm

2000mm

1900mm

2T12-1

5 R6-2-802T12-1

20mm

250mm

(c) Reinforcement detail and 2nd loading arrangement of the current study (Shear arm 6.0xd)

2T12-1

23 R6-2-80

150mm

1800mm1700mm

2T12-1

450mm

20mm

(d) Reinforcement detail and loading arrangement of the conventional repair (Shear arm 6.0xd)

Figure 5 Reinforcement detail and the loading arrangements of the two studies

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Furthermore, in the referred study, test specimens were cast with shear links as recommended as the minimum requirement by BS8110 for beam elements where as beams tested under this experimental investigation are provided with no shear links. It is considered that both the surface preparations, bonding agents and the provision of shear links greatly influence the bond performance and integrity of the repair that the presence or absence will have a direct influence on the performance of the repair technique.Although the beams tested in comparison study underwent four-point bending, only shear arm to depth ratio of 6 is used in the comparison study where as in the current investigation both shear arm to depth ratios 6 ratio 4.5 (intense shear loading) were used. Under these circumstances the specimens in this experimental investigation are considered to have gone through rigorous assessment compared to the study taken for the comparison purposes. Reinforcement details and loading details of the two studies are shown in Figure 5.

4 RESULTS

Figure 6 (a) and Figure 6(b) below shows the load-deflection behaviour of the beams tested under static loading for shear arm to depth ratio (av/d ) for av/d=6 and av/d=4.5 respectively compared with control beam tested under same loading condition. All repaired beams tested under av/d=6 had better performance compared to control beam (composite beam without repair) and there were little evidence of delaminating of the repair from the parent concrete. Although most of the beams under shear arm to depth ratio 4.5 showed some signs of shear cracking and delaminating of the repair from the parent concrete all beams have shown yielding prior to substantial shear and delaminating failures. Most of beams tested at av/d=4.5 showed significant delaminating progressed into shear cracking. However except in the case of R/69/80/4.5 for all other beams despite shear cracking final failure bending with enough evidence of compression failure at the top of the beam in the constant bending zone. Beam R80/69/4.5 is the only beam under av/d=4.5 that did not show bending failure and the only beam failed without appreciable deformation after yielding. Similar to the comparison study beams tested under this has shown similar crack patterns to control with most of the crack stopping at interface between parent concrete with significant crack opening at the penetrating cracks. Table 2 summarizes the key parameters of the beams tested in the current study under the two av/d ratios.

0

5

10

15

20

25

30

35

40

45

0 10 20 30 40 50 60 70

Loa

d(k

N)

Deflection (mm)

control6.0

R/42/50/6.0

R/69/50/6.0

R/69/80/6.0

R/42/80/6.0

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70

Lo

ad

(

kN

)

Deflection (mm)

control beam 4.5

R/69/50/4.5

R/42/80/4.5

R/69/80/4.5

R/42/50/4.5

av/d=6 av/d=4.5

Figure 6 Load deformation relations ship of the beams tested under different shear arm (av) depth (d)ratio

in the current study.

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Table 2 Summary of results under current study for different shear arm to depth ratio

av/d=6 av/d=4.5

Beam no Yield

point

Ultimate

load Failure mode Beam no

Yield

point

Ultimate

load Failure mode

Control 30.20 32.80 Bending failure Control 50.10 52.10 Bending Failure

R/42/80/6.0 31.80 35.27 Bending failure R/42/80/4.5 44.00 50.73 Bending failure

R/69/80/6.0 37.03 42.84 Bending failure R/69/80/4.5 45.50 46.02 Shear failure

R/42/50/6.0 34.90 35.10 Bending failure R/42/50/4.5 47.20 49.80 Bending failure

R/69/50/6.0 31.21 32.82 Bending failure R/69/50/4.5 45.27 48.63 Bending failure

5 DISCUSSION AND CONCLUSION

Table 3 compares the ultimate flexural capacity of the repaired elements with control elements for the two repair methods. Comparison was limited only for shear arm to depth ratio of 6.0. Results suggest that the novel repair technique outperform conventional method at av/d=6. Given the fact that, those beams repaired under novel method have minimum precautions against delaminating of the repair from the parent concrete in terms of surface preparation and the provision of shear links, it is safe to conclude that the novel repair technique is significantly better than conventional shot-crete repair in restoring the static performance of repair beams. It is noted that the repair beams under the novel technique demonstrate the crack propagation patterns similar to beams with conventional repair characterized by crack trapping and bifurcations at the interface between the repair and parent concrete and substantially lesser number of cracking making its way to the parent concrete. Similar crack patterns in the conventional method of repair has resulted poor fatigue performance of the repaired beams. As it is through these penetrating cracks that most of the deformations are mobilized there is direct relation of bar straining and crack number. Lesser cracking means more strain in the bar at crack locations and therefore poor fatigue performance. Given similarities in cracking between the conventional repair method and new technique, it will be interesting to find how beams under new repair technique would fare under cyclic loading. It is expected that those beams under novel repair technique would also have subdued fatigue performance compared with their control beams. Further research is required to find the fatigue performance of the repaired beams under the new repair technique.

Table 3 Comparison of two technique of repair

Current study

Convention method of repair

Beams in the

current study

Max

load(k N)

Max. load of beam

Max. Load of

Control beam

Beams in the

comparison

study

Max.

load(k

N)

Max. Load of beam

Max. Load of

Control beam

Control 32.5 1.00 Control 36.13 1.00

R/69/50/6.0 34.89 1.07 C/A/40/6.0 34.65 0.96

R/42/50/6.0 32.36 1.00 C/B /40/6.0 35.32 0.98

R/69/80/6.0 42.84 1.32 C/A/80/6.0 32.47 0.90

R/42/80/6.0 35.27 1.09 C/B/80/6.0 32.75 0.91

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6 REFERENCES

Soorriyaarachchi H.P, De Silva G.S.Y. ‘Conditional report of the main building of the maternity ward complex, Mahamodara Teaching Hospital, Galle, Dept Civil Engineering, University of Ruhuna, 2010. Sooriyaarachchi H.P. “Static and Fatigue Performance of Repaired Reinforced concrete Beams”, ENGINEER, Institute of Engineers, Sri Lanka, Volume xxxxv, No. 04, October 2012, pp. 01-12. Crains, J., and Zhao, Z., “Behaviour of concrete beams with exposed reinforcement”, Proc. Institution Civil Engineers, Structures & Buildings, V.99, May 1993, pp.141-154 Roof, M., and Lin, Z. , “Behavior of concrete beams with exposed main reinforcement”, Proceeding Institute of Civil Engineers, Structures & Buildings, V.122, February 1997, pp. 35-51. Emberson, N.K., and Mays, G.C., “Significance of property mismatch in patch repair concrete, Part I: Properties of repair system,” Magazine of Concrete Research, V.42, No.152, Sept.1990, pp. 147-160. ICRI, Technical guideline committee of ICRI, “Guide for surface preparation for the repair of deteriorated concrete resulting from reinforcing steel corrosion,” International Concrete Repair Institute, Guideline No. 03730, January 1996. Kani, G.N.J., “The riddle of shear and its solution,” proceedings, ACI Journal , V.61, No.4 April 1964, pp.441-467.