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The use of FRP for Shear strengthening of RC beams

| March 14, 2015

Introduction
The shear capacity of most old structures fall below the current requirements. This maybe due to the increasing load, increased flexural reinforcement without shear reinforcement, or due to original design specifications not accurately reflecting the shear capacity. Reinforced concrete beams used in bridges and other structures therefore require adequate shear strengthening for improving the overall structural integrity and strength. Over the last two decades, CFRP’s are finding increasing application in providing structural reinforcement for RC structures. FRP’s have better weight to strength ratio and are easy to handle onsite. Besides being extremely useful for flexural strengthening, CFRP wrappings are also finding extensive application for shear strengthening in RC beams. CFRP’s also constitute a good prestressing material (Yasumori et.al, 2006). Several approaches to shear strengthening RC beams are available and these include externally bonded CFRP’s, wrapping prestressed CFRP sheets around concrete beams, sprayed GFRP coatings and deep embedment of FRP bars. The choice among these techniques is based on the demands of the particular structural requirement. Compared with steel based reinforcement FRP has several advantages. Its low density which is less than 1/4th of steel, Lower Young’s modulus and high tensile strength make it an ideal choice of reinforcement material for RC structures. (Kobraei et.al, 2011) Furthermore, CFRP’s are very resistant to corrosion and hence are highly suitable for structural reinforcement even in extreme environmental conditions. This paper will discuss the Shear strengthening aspect of CFRP’s with the help of case studies and compare it with other alternative techniques.

AIM
This paper aims to study the shear strengthening potential of FRP for RC constructions. The study also briefly considers alternative materials that provide shear strengthening and includes a general comparison with the chosen material.

Methodology
This is a secondary research that makes use of both real life and experimental case studies that use FRP and other alternative materials for shear strengthening of reinforced concrete structures. Relevant material for the study is gathered through industrial reports, fact files and journal articles and the shear strengthening potential of the selected material is discussed. A final write up is done comparing the effectiveness of FRP and the other chosen material in terms of their shear strengthening potential.

Case Study 1: CFRP based Reinforcement and shear strengthening of Tenthill Creeks Bridge
The Tenthill creeks bridge in Queensland, Australia, is a 3 span reinforced concrete bridge originally constructed in 1970. The bridge constructed along the Gatton Helidon road has a total length of 82.15m and width of 8.6m. There are 12 prestressed beams each 27.38m extending over three spans. Two headstocks and abutments support these beams. @!ac The Queensland department of main roads (QDMR) performed a thorough field inspection and review of the original design documents to assess the structural capacity of the bridge which comes under the functional class 3 of the Austroads code classifications. This classification implies that the bridge should not be subject to load restrictions and must be open for free access for heavy vehicles. However, onsite inspections revealed that the headstock which supports the RC beams was deteriorating and that it had both flexural (.6mm) as well as shear cracks (2.8mm) . Concrete bridges are vulnerable to corrosion due to chlorination. CFRP reinforcements help to minimize chlorination induced corrosion by controlling chloride diffusion. (Berver et al. 2001). The existence of these cracks necessitated immediate reinforcement and shear strengthening of the bridge. The following image clearly shows the shear cracks in the headstock. _P Fig 3: Visible Shear cracks in the headstock of the Tenthill Bridge. (Nezamian & Setunge, 2007)
After the structural analysis, the strengthening target was calculated based on the load requirements in accordance with AS3600 (Sep 2002). The ultimate bending moment and shear force was calculated based on the traffic load and dead load of the bridge. The engineers calculated the ultimate positive bending moment as 5,320 kN m and the shear force as 2,520 kN. The serviceability bending moment was calculated as 2,526 kN m and the serviceability shear force was put at 1,797 kN. The engineers decided to use FRP composites for both flexural as well as shear strengthening of the bridge. The design guidelines for externally bonded FRP composite reinforcement as listed in the European FIB 14 report as well as the American ACI guidelines for concrete strengthening were followed. The following table shows the ACI specifications for material compliance .

_+{}#@__The researchers also took into consideration the possibility of FRP debonding and in accordance with the ACI guideline a limit of strain level in the FRP laminates that would help prevent delamination was calculated. The following equations indicate the various parameters such as Km= bond dependent coefficient, εcu- ultimate strain and Ef = elastic modulus, tf= thickness of the FRP strips and ε0, the initial strain before strengthening and εcu which is the ultimate strain. #############!@#u

has rendered the Tenthill bridge open for usage of heavy vehicles without any sort of load regulations. This study is a good example of how CFRP reinforcements and shear strengthening could be usefully implemented to retrofit old concrete highway structures whose load capacities need to be improved to fit in with the demands of the current volume of traffic and load levels.
Case Study 2: Shear Strengthening of the Grondals Railway Bridge in Sweden

Located along the railway line south of Stockholm, the Grondals Bridge is a relatively new structure constructed in the year 2000. The bridge has three spans with a main span that extends up to 120m while the two smaller spans are each 70m. The bridge is used for routine railway commuter traffic. Within a year following its construction and operation the Grondals bridge stared developing visible signs of shear stress. Structural inspections revealed several small cracks on the webs of the hollow box girder structure. The BRO 94 and BBK 94 codes of the Swedish construction regulations permit the construction of bridges using very thin steel webs. Though the actual cause for the development of cracks was not found immediately, the engineers knew that the dominant permanent load of the structure and the resultant tensile stress would have crossed the tensile strength of the concrete structure leading to crack formation. Furthermore, the engineers also felt that the environmental tensile stress could have added to the problem. The longitudinal crack formation along the bridge indicates the areas of maximum principle tensile stress. The width of the crack region was between .1 to. 3 mm in most cases while some cracks were even wider at .4 to .5 mm.Lmx321+__.

In the above figure the readings from the various LVDT monitoring devices indicate that there is not much difference in measurements between the readings observed at the beginning of 2003 compared with those measurements recorded for 2004. The largest crack measurement is only approximately .06mm which is well within acceptable limits. This suggests that post reinforcement, the cracks are not progressively worsening indicating the effectiveness of the CFRP intervention. It is also clear from the above image that there is a periodic fluctuation in the cracks suggesting that they are opening and closing due to the changes in environmental temperature.

Conclusion
CFRP reinforcement for the Grondals railway bridge in Sweden has proved that it is possible to fix design specific issues that were neglected during the original construction and prevent further damage to the structures. The data gathered from traditional LVDT monitors and the more expensive FOS monitors seem to confirm that temperature rather than traffic load has a significant effect on crack displacement. Also the numerical computations and graphical interpretations of the data gathered from both the sensors indicate that the crack displacement measured over a year post intervention is insignificant and the largest crack was less than .06mm. These data confirm that reinforcement has contained the progression of the cracks by distributing the forces along the entire length of the laminate.

Case Study 3: Trenchard Street Multistorey car park (CFRP reinforcement improves Shear Stiffness)

The Trenchard multistory parking facility is operated by the Bristol city council. This eleven storey parking facility was designed and built in 1966. This structure has downstand beams of reinforced concrete with longitudinal grid spacing of 7.5m and transverse spacing of 10.9 m. Also all the edges of the structure have 3m cantilever beams. The 225mm thick slabs taper down to 150mm at the edge of the cantilevers. Inspections revealed several defects due to variable cover in the slabs and reduced moment capacity and shear strength. Structure inspectors wanted the top surface of the cantilever beams to be strengthened. Since the vehicles would be running directly on the surface that is to be reinforced, near surface mounting option was considered as the most apt as it helps to improve both the shear as well as flexural strength of the structure. The engineers conducted a more detailed surface survey to identify the concrete substrate suitability and to repair any surface damage.

For the near surface mounting technique, the engineers selected the best available reinforcement material and the MBT Galileo bar was chosen. The following specifications are the material properties of the chosen MBT Galileo bar.

1. Young’s Modulus = 130 kN/mm2
2. Ultimate tensile Capacity 2300 N/mm2
3. Ultimate tensile strain 0.0126
4. Fibre type carbon
5. Resin matrix type epoxy
6. Diameter of bar 7.5 mm
7. Surface treatment peel-ply (Parkman et.al, 2005)

The engineers calculated the reinforcement design requirements as per the TR55 specifications pertaining to surface mounted strips. The surface preparation and anchorage design were carefully considered to be according to the recommendations provided in the TR55 guideline for near surface mounted reinforcements. The design created was to use 7mm bars fitted in 20 mm chases arranged at 250mm centre across the slab.

Surface preparation is essential before CFRP could be wrapped. Diamond sawing was used as the method for surface preparation. The chases were then vacuum cleaned and a primer coating was applied. The adhesive was then applied on top and the CFRP bars were then pressed on top of the adhesive. In order to ensure that there are no air locks the engineers used adhesive to completely cover the chases. An injection gun was used for this purpose as it helps to provide a uniform and smooth delivery of the adhesive material. The following illustration shows surface preparation.14e

Fig 12: Surface Preparation before reinforcement with CFRP bars. (Parkman et.al, 2005,pg 14)

Since this project happened to be the very first NSM FRP reinforcement in the UK, the designers preferred to do load testing both prior to and after the strengthening. For this purpose two of the cantilever beams were load tested with two large water bags that were gradually loaded to 1.25 times the service loading levels. The deflections were simultaneously monitored along the different areas of the cantilever beam and the adjacent bays. These load testing procedures were well documented with instruction to discontinue the testing if any abnormal results were obtained. Overall the testing was progressively repeated over a period of 10 hours.

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The acceptable load deflection criteria as measured by the deflections at the cantilever tips under service load conditions were arrived at during the design stage. Data from the load testing prior to and after the reinforcement suggested a 30% increase in the stiffness of the structure. This implies that the strength applied to the cantilever beam was now carrying the load. This case study was the first near surface mounted FRP reinforcement in the UK. The results from the load testing conducted before and after CFRP bar reinforcement clearly improved the shear strength as well as the flexural strength of the cantilevers.

Case Study 4: Ealing Road Bridge (CFRP strips for Shear strengthening)
The Ealing road bridge over the Grand Union Canal in North London is a concrete portal frame bridge. This bridge was built in 1924 and has a skew span of 13.68 m. The bridge is over a towpath with frequent boating activity and carries both vehicular and pedestrian loads. The design of the bridge includes a main slab of 190mm thickness supported by ten downstand beams each measuring 1070mm by 370 mm. The original specifications of the bridge was 17 tonnes as per the BD21 and it was felt necessary to increase the load capacity to 40 tonnes (BD21/97). Since this is a very old bridge there were already reinforcement operations that were done to the original structure. The details of these reinforcements were gathered from the existing design drawings. q

Structural inspections of the bridge revealed deficits in the deck beams and the slabs and these were spreading transversely and to the top of the abutments. This bridge was located on a very busy route and the only access route across the canal. Furthermore the boating routes could not be affected as the British waterways were using it for maintenance purposes. Also the bridge carried high voltage current, and fiber optic telecommunication cables along the towpath. The existence of a Bus terminus near the bridge meant that frequent plying of busses across the bridge. Since alternative routes were atleast 5 miles away it was impossible to stop traffic in the bridge during the reinforcement operations. Initially a complete replacement of the deck was proposed but since there were severe operational constraints it was ruled out.
Since the bridge had some significant damage it was decided to combine structural reinforcement for flexural and shear strengthening using CFRP in combination with additional concrete strengthening. To improve shear sagging it was proposed to use pultruded CFRP strips as a reinforcement for the main beams. Shear strengthening near the abutments was proposed by using in situ lay-up fabrics. Unidirectional carbon fiber fabrics were chosen for this purpose. Also in situ lay-up fabrics were proposed for the flexural strengthening of the deck. The engineers also planned to use new concrete to strengthen the concrete walls near the abutments. The soffit was tap tested while chloride and carbonation testing were performed to identify the damage of the concrete. The investigators also noted that the water loading pressure could also contribute to the stress on the structure and hence they proposed to build a drainage system in the wing walls to manage this.

The following material properties formed the basis of the reinforcement design. This project was concerned with boosting both the flexural strength as well as the shear strength of the bridge . The material specifications for flexural strengthening are
1.Young’s modulus, E, at least 150kN/mm2
2. Tensile strength at least 2000N/mm2
3. Tensile strain 1.5%

For the shear strengthening the choice was to use a unidirectional carbon fiber fabric.
1.Young’s modulus, E at least 230kN/mm2
2. Tensile strength of at least 3000N/mm2
3. Ultimate tensile strain 1.5%
The following partial factors were included into the design considerations.

Fabrics for shear
Material type factor – 1.4 (carbon)
Young’s modulus factor – 1.1

Strips for flexure
Material type factor – 1.4 (carbon)
Method of manufacture factor – 1.1 (pultruded plates)
Young’s Modulus factor – 1.1 (Parkman et.al, 2005)

The engineers used 100 mm wide and 1.2 mm thick CFRP strip Sika CarboDur S. The use of single size strip was preferred to ease the design calculations and wherever required 2 or more layers of strips were used. Similarly unidirectional 600mm wide and 0.13 mm thick carbon fabric was chosen. For the shear strengthening requirements the engineers used a single ply of the unidirectional fabric along each face of the beam. Since U shaped CFRP fabrics are known to be very effective for shear strengthening the engineers adopted the same approach. In the areas where new concrete was required the engineers used fast hardening concrete.
Surface Preparation
Surface preparation was done using the common grit blasting method. Since the area beneath the bridge was a tow path it was necessary to contain the dust and grit. To ensure proper surface preparation surface pull off tests using 50mm dollies that were bonded along the surface were used. The values from the pull off tests were in the range of 2.9 to 5.1 N/mm2 m which is well above the expected standard of 1.5N/mm2.

Application of Composite
After surface preparation the adhesive was applied both to the surface as well as the composite layers. The following illustration shows the application of the composite material c

Conclusion
The Ealing bridge was strengthened successfully using CFRP strips as well as unidirectional fabrics. Also conventional concrete was used to add strength to areas of the bridge that were particularly damaged. Thorough inspection of the bridge provided the engineers the necessary design specifications to correctly assess the reinforcement requirements.

Case Study 5: Steel Fiber Reinforcement (Improved shear resistance)
Besides FRP which is fast becoming the popular choice of shear and flexural reinforcement there are also other materials that are used for reinforcement of concrete structures. Traditional steel stirrups and the more recent stainless steel reinforcement are some alternatives. Furthermore there is also a growing interest in the application of steel fibers as a shear strengthening mechanism. It has been well established that randomly oriented fibers offer post cracking tensile resistance to concrete structures (Parra-Montesinos, 2006). The enhanced shear resistance offered by fiber reinforcement is attributed to the ability of fibers to transfer the tensile stress across the cracks and consequently increases the aggregate interlocking. In other words, the use of fiber reinforcement reduces the crack spacing and could even totally eliminate shear size effect without the use of stirrups. (Wight & MacGregor 2008)
This experimental case study by Hai et.al (2010) focused on observing the shear behavior of steel fiber reinforced concrete beams.(SFRC) For this purpose the researchers tested 28 beams by gradually increasing the load and the shear response. The engineers involved in this study grouped the beams into tow different categories based on their depth. Series B 18 represented beams with depth less than 455 mm and series B27 with beams of depth of 685mm. Overall there were 8 pairs of the B18 beams and 4 pairs of the B27 beams. For B18 group a pair of beams without stirrups or reinforcement with fibers, were tested S18 (0). Similarly for the series F B27 also one beam was tested without stirrups. B27(0). This allowed the researchers to compare the behavior of the regular RC beams with that of the SFRC beams. (Hai et.al, 2010)

All the beams in this study were constructed with concrete with compressive strength of 41 MPa (6000 psi). Three types of steel fibers were tested in this study with type 1 30mm , type 2 60mm and type 3 and made of high tensile fiber. The tensile strength of type 1 and type 2 fibers were 1100 MPa [160 ksi]), while that of the type 3 was 2300 MPa [330 ksi]). The longitudinal reinforcement ratio ρ was calculated using the ‘area of tension steel, As’, depth of the beam d and the width b. Since longitudinal reinforcement affects the compression zone it impacts the shear resistance of the beam. Strain gauges placed at different regions of the beam were used to measure the strain while linear potentiometers were used to measure the deflection of the beams. To explore the strain field ‘infra red optical position tracking system’ was used.
The behavior of the beams were evaluated based on crack distribution, ‘average shear stress load’ vs displacement , ultimate strength , failure mode etc. p

RC beams without any reinforcement exhibited single crack and brittle shear failure. Compared to this all the SFRC beams had atleast two diagonal cracks. Also when stirrups were used the cracking pattern was further improved in the SFRC beams comapared to the RC beams. While multiple diagonal cracks were witnessed in SFRC beams the spacing between the cracks was more in beams with more depth, with 0.4d being the average spacing between cracks. The researchers also studied the shear stress and the displacement response for the beams. The following graph illustrates this.5o

Fig 18: Shear Stress vs Deflection (Hai et.al, 2010)
The longitudinal reinforcement influenced the dispalcement in the SFRC beams. All the SFRC beams exhibited an increase in shear strength of upto 40% compared to the RC beams without reinforcement. Upto the failure point the shear stress vs displacement was a linear progression. The engineers noticed that steel fiber usage led to the development of multiple diagonal cracks. They also noticed that all the SFRC beams developed a clear widening in atleast one of the cracks before shear collapse. Almost all the SFRC beams in the test exhibited a shear stress of atleast 0.38√fc′ MPa (4.6√fc′ psi). The graph below shows the average shear stress with respect to the ‘fiber volume fraction’. 0

Fig 19: Average Shear stress VS. Fiber volume fraction (Hai et.al, 2010)
All the SFRC beams showed increasing transverse strain when the laod applied was increased over 623 kN (140 kips). This is attributed to the formation of the diagonal cracks across the beam. At a load of 710 kN (160 kips) the shear stress was 0.43√fc′ MPa [5.2√fc′ psi]), the formation of the wide crack indicated the worsening of the transverse strain. This is the critical point where shear failure occurs.s

Fig 20: Vertical Strain vs Load and beam Depth (Hai et.al, 2010)
Conclusion
This study proved that the use of Steel fibers at a volume fraction of .75% or 1% led to an increase in shear strength. The study also proved that the shear strengthening provided by conventional steel stirrups could be obtained by the use of steel fibers at .75% volume fraction. Also it was noticed that fibers of 60 mm length provided improved inclined crack opening before shear failure. This is witnessed as greater width in the diagonal crack before failure and could be useful as a warning. Overall the use of steel fiber for reinforcing concrete beams resulted in a 40% improvement in shear strength.
Conclusion
The application of CFRP composites as a reinforcement material for concrete structures is becoming a popular choice among construction engineers. Adhesively bonded CFRP’s improve the shear strength and the flexural strength of structures. CFRP wrappings are particularly useful for shear strengthening of RC beams. The high costs of CFRP composites are however a significant factor but these cost considerations are easily offset by the ease of use. In particular, the light weight of CFRP fibers make it easy for construction engineers to complete CFRP reinforcements as opposed to the heavy steel based reinforcements. This superior weight to strength ratio of CFRP makes it an ideal choice of Flexural and shear strengthening material for RC structures. Particularly in quake prone areas the use of CFRP is highly useful in improving ductility of the structure. Another attractive feature of CFRP is its corrosion resistance behavior which implies longer life for the structure and lower maintenance costs. However, CFRP wrapping sometimes suffer from debonding and peel off.

The case studies discussed above clearly indicate the usefulness of CFRP based reinforcement of RC structures to improve shear strengthening and the load capacity of the structures. The anticipated decrease in price of CFRP composites in the future would help to further the usage of CFRP as the choice material for concrete reinforcements. Also, for antique structures that do not tolerate any structural deformation or do not permit any changes to the existing structure simple CFRP wrappings are the ideal choice.

Along with carbon the use of GFRP’s and Steel fibers are also becoming popular for structural reinforcement. Steel fiber’s are very effective and are comparable to CFRP’s in improving the shear strength of reinforced concretes. Conventional carbon steel support incurs high maintenance costs due to corrosion related maintenance costs. However, Stainless steel fibers are corrosion free and are cost effective in the long term. Steel fibers achieve good bonding with the substrate. Studies have shown that SFRC’s behave similarly to CFRP strengthened RC beams with comparable yield, maximum load, deflection ductility, etc. In fact, Steel fibers provide better fatigue performance compared to CFRP’s. Both the materials provide similar stiffness degradation. The experimental case study that was discussed in this paper showed a 40% improvement in the shear strength of the RC beams that were reinforced by steel fibers confirming that steel fibers offer matching shear strengthening compared with CFRP’s. Shear strengthening of concrete structures using CFRP’s or SF’s provide an excellent choice for construction engineers to improve the load capacity and the stiffness of the structures for which total reconstruction would be an economically unviable option.

References

1) Abolghasem Nezamian & Sujeeva Setunge (Oct, 2007), Case study of application of FRP composites in strengthening the reinforced concrete headstock of a Bridge structure, Journal of Composites for Construction, Vol 2, no 5.

2) Berver, E. W., Fowler, D. W., and King, J. J (2001), Corrosion in FRP-wrapped concrete members, Structural Faults and Repair 2001.

3) B Taljsten & A.Hejll, (2007), CFRP strengthening and monitoring of the Grondals Bridge in Sweden, Journal of Composites for Construction, Vol 11, Iss 2.
4) H Yasumori, Y. Hamada, A Kobayashi & K.Kuzume (2006), Strengthening and application of prestressing CFRP plate for Concrete structures, Concrete Journal, Vol 44. No 10
5) Mohsen Kobraei, Mohd Zamin Jumaat and Payam Shafigh, (2011), An experimental study on shear reinforcement in RC beams using CFRP bars, Science Research Essays, Vol. 6(16), pp. 3447-3460.
6) Parkman, A., Tony Gee and Partners. (2005). Maunders Road Bridge –CFRP plates bonded to cast iron. Classification and Assessment of Composite Material Systems for use in the Civil Infrastructure Technical Report Num 8. (3), 17-24.
7) Parra-Montesinos, G. J., 2006, “Shear Strength of Beams with Deformed Steel Fibers,” Concrete International, V. 28, No. 11, Nov., pp. 57-66.
8) Wight, J. K., and MacGregor, J. G., 2008, Reinforced Concrete:Mechanics and Design, fifth edition, Pearson Prentice Hall, Upper Saddle River, NJ, pp. 247-248.

9) Hai H Dinh, Gustavo J Parra-Montesinos & James K Wight, (Oct 2010), Shear Behaviour of Steel fiber reinforced Concrete beams without stirrup reinforcement, ACI Structural Journal, Vol 107 . no 5.

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