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Physics Procedia 55 (2014) 10 - 16
Eighth International Conference on Material Sciences (CSM8-ISM5)
CFRP mechanical anchorage for externally strengthened
RC beams under flexure
Alnadher Alia, Jamal Abdallaa*, Rami Hawileha, Khaled Galala
a Civil Engineering Department, American University of Sharjah, Sharjah, UAE
Abstract
De-bonding of carbon fiber reinforced polymers (CFRP) sheets and plates from the concrete substrate is one of the major reasons behind premature failures of beams that are externally strengthened with such CFRP materials. To delay or prevent de-bonding and therefore enhancing the load carrying capacity of strengthened beams, several anchorage systems were developed and used. This paper investigates the use of CFRP mechanical anchorage of CFRP sheets and plates used to externally strengthen reinforced concrete beams under flexure. The pin-and-fan shape CFRP anchor, which is custom-made from typical rolled fiber sheets and bundles of loose fiber is used. Several reinforced concrete beams were casted and tested in standard four-point bending scheme to study the effectiveness of this anchorage system. The beams were externally strengthened in flexure with bonded CFRP sheets and plates and then fastened to the soffit of the beams' using various patterns of CFRP anchors. It is observed that the CFRP plates begins to separate from the beams as soon as de-bonding occurs in specimens without CFRP anchors, while in beams with CFRP anchors de-bonding was delayed leading to increase in the load carrying capacity over the un-anchored strengthened beams.
©2014ElsevierB.V.Thisisanopenaccess articleunderthe CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of CSM8-ISM5
Keywords: CFRP mechanical anchors, CFRP strengthening, de-bonding, beam ductility
1. Introduction
Strengthening of existing reinforced concrete (RC) and prestressed concrete (PC) structures may become necessary as a result of several possible factors, such as insufficient design or construction errors, deterioration due to corrosion of the embedded reinforcement and exposure to environmental effects, need for increase in structural capacity and seismic retrofit. Moreover, aging and resulting deterioration of the infrastructure is nowadays a major challenge for the civil engineering community and with time larger
Corresponding author: Tel.: 97165152959; fax: 97165152979. E-mail address: jabdalla@aus.edu (Jamal Abdalla)
1875-3892 © 2014 Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/3.0/).
Peer-review under responsibility of the Organizing Committee of CSM8-ISM5 doi: 10.1016/j.phpro.2014.07.002
number of concrete structures are expected to continue to deteriorate due to several factors. The use of Carbon Fiber Reinforced Polymer (CFRP) composites for external strengthening of different structures has been accepted as a practical and viable technology in materials and structural engineering [1, 2, 3, 4]. CFRP materials gained popularity in repairing and strengthening of existing structures as well as in the construction of new structures. The advantage of CFRP over other materials is its high strength-to-weight ratio, non-corrosiveness, electro-magnetic neutrality and its versatility where it can be successfully utilized for strengthening and rehabilitation of reinforced concrete, masonry as well as timber structures. Externally bonded CFRP sheets/plates have been effectively utilized to enhance flexural and/or shear capacity of RC beams, stiffness of RC, PC and timber beams, and to provide confinement to RC and masonry columns and to strengthen masonry walls subjected to out-of-plane as well as in-plane loading. One of the main challenges to the use of externally bonded CFRP in repairing and strengthening applications of RC beams under flexure is its susceptibility to de-bonding without achieving its full capacity. To delay or prevent de-bonding of CFRP from the concrete substrate and consequently enhance its load carrying capacity, different anchorage systems including a CFRP mechanical anchorage system have been used [5-10]. The pin-and-fan shape CFRP anchor, which is custom-made from typical rolled fiber sheets and bundles of loose fiber is used in this study. Such anchors are particularly appropriate for fastening FRP composites to a variety of structural elements with different shapes. The pin-part of the anchor is inserted into pre-drilled holes in the concrete while the fan (comb) ends spread over and rest on the surface of the FRP sheet or plate and are adhesively bonded to it.
Several reinforced concrete beams were tested in standard four-point bending scheme to study the effectiveness of this anchorage system. The beams were externally strengthened in flexure with bonded CFRP sheets and fastened using different arrangements of CFRP anchors. The load-midspan deflection responses of the tested beams were plotted and compared. The results were compared with an un-strengthened control beam and other control beams strengthened with CFRP with different patterns and without anchorages. The study presented the load-deflection response of the tested specimens. It is observed that the CFRP plates begins to separate from the beams as soon as de-bonding occurs in specimens without anchors, while in beams with closely-spaced anchors de-bonding was delayed considerably leading to a considerable increase in the load carrying capacity over the un-anchored strengthened beams. It is concluded that the developed anchorage system is promising, yet further investigation is needed to determine and quantify the exact number, size and location of such anchors in order to fully utilize the capacity of the bonded FRP sheets and plates.
2. Experimental Program
2.1 Tested Specimens
A total of sixteen reinforced concrete (RC) beams were designed and cast in one batch using self-consolidating concrete (SCC). Result of only four beams will be presented in this study. Figure 1 shows the beam dimensions, reinforcement details and cross section details of the beam specimens. The beams are loaded as shown in Figure 2 and tested in four-point bending to failure. The clear span of the beams was 1690 mm and the two loading points were 560 mm apart located symmetrically about the beam's mid-span location.. The shear span of the tested beams was 565 mm. Figure 2 shows the location of the loading points. The Control Beams (CB1 and CB2) was left un-strengthened, while the remaining fourteen beams were tested to investigate different FRP sheets and plates strengthening schemes with several anchors configurations simulating an FRP-strengthened structure. Some beams were strengthened by CFRP sheets (fabric) used with epoxy and the drilled holes are 10 mm in diameter and 40 mm in depth while others were strengthened with CFRP laminates (plates) used with epoxy and the drilled holes are 10 mm in diameter and 80 mm in depth. The amount of CFRP reinforcement ratios and styles (short-length, full-length and short-length with U-wrap) varied from one beam to another and the number and spacing of anchors and anchors' arrangements were also varied.
75 mm #10 mm bars
# 8 mm bars
90 mm I /
10 stirrups @80 mm = 800 m
10 stirrups @80 mm = 800 mm
2#10 mm
■1840 mm
Section a - a
Figure 1. Details of the longitudinal and traverse reinforcement details of the tested beams
75 minn
Figure 2 Details of tested beams location of four point loadings 2.2 Materials
Self-Consolidating Concrete (SCC) of grade 45 was used to cast the specimens. The SCC consisted of 20 mm maximum aggregate size, GGBS, mircosilica and w/c ratio of 0.36. Sixteen 100*200 mm cylinders were prepared and cured under the same conditions as the test beams. The curing consisted of moist curing for the first three days and air-curing for the subsequent 28-days and then they were tested for compressive and tensile strength. The average measured compressive strength of the concrete was 44.6 MPa while the average tensile strength was 4.27 MPa. Uniaxial coupon tensile tests were conducted
to measure the modulus of elasticity and yield strength for the 8 mm deformed tensile steel reinforcement (3 samples) measuring 193 GPa and 618 MPa respectively while for the 10 mm deformed tensile steel reinforcement (3 samples), it measured 182 GPa and 621 MPa, respectively.
2.3 Strengthening and test procedures
The strengthening procedure of the tested beams included surface preparation by grinding, application of a priming adhesive layer, bonding of the CFRP laminates and sheets and insertion of the CFRP anchors in the drilled holes. Prior to bonding, special consideration was given to the surface preparation. Uniform mechanical grinding was employed to remove the outer weak surface of the concrete until the aggregates were exposed. Afterwards, the surface of the beams was cleaned with water and then compressed air was used to remove any loose particles. Holes to insert CFRP anchors were drilled using a small-job electrical driller at the proper spacing, according to the designed arrangements and to the appropriate depth for each specimen. Also, a pressurized air blower and a small steel grinder were used to clean and remove dust and any fine or loose particles from inside the holes. Once the surface was prepared to the required standard, the epoxy resins were mixed in accordance with the manufacturer's instructions until the mixture had a uniform color.
CFRP sheets and plates were cut exactly according to the specified dimensions and placed over the beam specimens. After filling the holes approximately half-way with epoxy resin and applying an initial layer of epoxy resin to the surface of the RC beams, the CFRP anchors were inserted into the holes and the splayed CFRP anchors were then rolled in the fibers' longitudinal direction using an air removal roller and by hand to remove any trapped air bubbles and to impregnate the fibers firmly. Application was followed by a high solids saturant onto which the CFRP sheet was applied. After waiting for some time to allow the saturant to impregnate into the carbon fiber sheet and into the embedded CFRP anchors, a second application of epoxy resin layer was then applied to the top of the CFRP sheet and the splayed CFRP anchor fibers to ensure that the two elements are bonded together during curing. It was followed by hand rolling to remove any trapped air bubbles from inside. Adhesive was cured at room temperature for at least 7 days under laboratory conditions before testing. Similar strengthening procedure was carried out for all strengthened specimens. Figure 3 shows a typical configuration of two CFRP anchors for sheets and plates covering 40% of the shear span.
-420 mm -
■1840 mm--1000 mm-
-420 mm
- Holes of CFRP anchors
Figure 3 Details of the configuration of the CFRP anchors of tested beams
120 mm
The beams were loaded monotonically using a digitally controlled INSTRON 8806 Universal Testing Machine (UTM) that has a capacity of 2500 kN. The loading rate was relatively slow to simulate static loading condition.
3. Test results and discussion
Figure 4 shows the load deflection responses for the beams strengthened with sheets and plates with and without CFRP anchors along with the unstrengthened control beam specimen. It is observed that beams with anchors have larger ultimate loads than beams without CFRP anchors. The ductility indices of the tested beam specimens were calculated using values extracted from the load-deflection curves. Ductility was evaluated on a quantitative basis by computing ductility indices in terms of mid-span deflection at ultimate loads to that at yielding of the tension steel. The deflection ductility index at ultimate load (^A,uit) was computed as the ratio of deflection at ultimate load to the deflection at yield of tension steel and is given by equation (1).
MAuUlt =ir (1)
Where 5y is the mid-span deflection at yield load (mm) and 5u is the mid-span deflection at ultimate (peak) load (mm). Table 1 provides a summary of the obtained results. It is observed from Table 1 that the control beam has the largest ductility while beams strengthened with sheets and plates anchored by CFRP anchored have the smallest ductility. It can be concluded that the attained ductility at ultimate loads of the strengthened beams, with and without the developed CFRP anchorages system, is significantly lower than that of the control beam. The use of anchors also increased the ultimate load carrying capacity of beams as shown in Table 1 and Figure 4.
Figure 4 Load-deflection response of beams strengthened with CFRP sheet or plate
Table 1 - Summary of results for tested specimens
Groups Pu (kN) 8y at PY (mm) Su at Pu (mm) M'A, ult
Control Beam without CFRP sheet 67.98 5.41 30.33 5.61
Beam with CFRP Sheet 73.01 5.20 25.34 4.87
Beam with CFRP Sheet + two anchors 80.15 6.29 7.34 1.17
Beams with CFRP Plate 65.02 3.24 4.74 1.46
Beams with CFRP Plate + two anchors 78.28 3.97 4.74 1.19
4. Conclusion
This paper presents the effect of using CFRP mechanical anchorages on the flexural behavior of externally strengthened RC beams with CFRP sheets and plates and measured in terms of load carrying capacity and ductility of the beam specimens. It can be concluded from this study that:
• The beams with anchors generally have higher delamination load than their companion beams without anchors, but the true significance of the observed increase can be only measured by proper statistical analysis using more samples.
• The control beam specimen has the largest ductility while the beams strengthened with sheets and plates anchored by CFRP anchored have the smallest ductility.
• The anchors did not significantly contribute to the flexural stiffness of the beam specimens.
Acknowledgement
The support for the research presented in this paper has been provided by the American University of Sharjah. The support is gratefully acknowledged. We also want to acknowledge the assistance of Eng. Arshi Faridi in the instrumentation and testing operations. His help is highly appreciated. The views and conclusions, expressed or implied, in this document are those of the authors and should not be interpreted as those of the American University of Sharjah.
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