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Contents lists available atScienceDirect

Composite Structures

journal homepage:www.elsevier.com/locate/compstruct

Experimental investigation of shear capacity and damage analysis of thinned

end prefabricated concrete purlins strengthened by CFRP composite

Lokman Gemi

a,⁎

, Ceyhun Aksoylu

b

,

Şakir Yazman

c

, Yasin Onuralp Özk

ılıç

d

, Musa Hakan Arslan

b

aMeram Vocational School, Necmettin Erbakan University, Konya 42000, Turkey

bFaculty of Engineering and Natural Sciences, Department of Civil Engineering, Konya Technical University, Konya 42130, Turkey cIlgın Vocational School, Selçuk University, Konya 42615, Turkey

dFaculty of Engineering and Architecture, Department of Civil Engineering, Necmettin Erbakan University, Konya 42000, Turkey

A R T I C L E I N F O

Keywords:

Carbonfiber reinforced polymer (CFRP) Composite concrete Damage analysis Prefabricated structures Purlins Strengthening A B S T R A C T

Prefabricated structures supported with purlins are exposed to numerous damages due to the excessive snow loadings as vertical loadings. The thinned regions of the purlins are responsible with the failure of the structure since the shear cracks usually initiate at these regions and propagate along with the purlins, and as a result, a total collapse may occur. In this study, carbonfiber reinforced polymer (CFRP) composites with four different configurations (P2–P5) were employed for strengthening prefabricated purlins in order to increase the strength of

the purlin against shear damage generated under vertical loading. The load carrying capacities and damage patterns of the purlins were compared. The failure of the reference purlin (P1) was occurred as a shear damage at

the thinned regions before reaching its bending capacity. However, the failure characteristic of the CFRP re-inforced purlins was dominated by the bending damage and the vertical loading capacity of the purlins were increased up to 59% depends on the CFRP wrapping. Damage analysis of the CFRP composite was also per-formed. Various damage modes of the structure such as cover separation, air voids, delamination, debonding, fiber bundles breakage, matrix cracks, fiber bundles debonding, fiber breakage and buckling were observed and explained thoroughly.

1. Introduction

In Turkey, many types and species prefabricated structures, which can be classified as moment resisting and hinged connections and multiple and consisting of multi-storey prefabricated panels, are being built. Depends on the application, structural elements with pre-stressing may be preferred in these types of structures. It is known that most of the prefabricated industrial buildings in Turkey are formed by a single storey frame with hinged connections which its columns arefixed to the foundation. Purlins with thinned ends and long span lengths are pre-ferred in order to reduce storey height of the beam or beam-to-column hinged connections in the prefabricated buildings. Since shear force becomes critical the thinned ends due to the decrease of effective area at these regions under vertical load, necessary precautions should be taken. Hence, TS9967[1]and TS3233[2]standards are used for the design of thinned ended prestressed prefabricated purlins. Although the earthquake provision which came into force at 2018 in Turkey[3]has a section related to prefabricated buildings, no sanction is available for detailing of the purlin ends. Even though TS9967 and TS3233 are not

currently in the force, these provisions are still being used by the building sector due to the fact that no alternative provision is available. Even though purlins are manufactured according to the provisions, it is observed that when the roof system is quite wide, the thinned ended regions of the purlins have been damaged due to excessive snow accumulation. It can be said that transformation of the accumulated snow on the roof into ice with advancing time is the governor of purlin failure and there are no other loads over the snow loads in the TS498 [4] provision. Therefore, sudden and brittle damage occurs in the thinned ended region of the purlins and this leads to a partial or total collapse of the roof system. Naturally, this situation can cause an in-ability to use in the long term of the industrial building and also can cause death and/or property damage.

Prefabricated purlins are extremely delicate elements in terms of their size. These purlins with usually 7–8 m span lengths, are limited with 30 cm of height. As a result, these purlins are preferred to produce with pre-stressing. These elements are placed on the top of roof beams with the help of thinned ends. Since the cross-section decreases gra-dually through the end of the purlin, important damages and collapses

https://doi.org/10.1016/j.compstruct.2019.111399

Received 19 May 2019; Received in revised form 27 July 2019; Accepted 9 September 2019

Corresponding author.

E-mail address:lgemi@erbakan.edu.tr(L. Gemi).

Composite Structures 229 (2019) 111399

Available online 10 September 2019

0263-8223/ © 2019 Elsevier Ltd. All rights reserved.

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may occur, especially under snow loading. Nonetheless, the deflection occurred at purlins due to wide openings leads to deformations beyond the elasticity and leads to cracks. Numerous analytical and experi-mental studies have been conducted to investigate the effects of change of the cross-section to the shear capacity in the reinforced concrete elements[5–18]. These studies have been performed to limit the shear damage at the thinned ended region of the purlin by modifying the orientation of the steel reinforcements.

Prefabricated structure designers seek to prevent brittle shear failure occurred at the thinned ended region of the beams without changing its size of the beam. This can be achieved by either different design of the precast beams during manufacturing or a number of strengthening apparatus to be applied outside of the precast beam. The main motivation of this study is to investigate the effectiveness of CFRP which will be used to delay or prevent shear failure occurred at the thinned ended region. Carbon fiber reinforced polymer (CFRP) com-posites has become a building material used in the construction in-dustry[19,20]. CFRP having a silk appearance andfineness is a new generation strengthening method that is about 10–14 times more strength than steel and its weight is one-fifth of the weight of the steel [21]. CFRP is used to reinforce damaged elements to achieve their in-itial strength or used with steel plates as an alternative strengthening method [22–35]. Furthermore, different studies have related to the columns with CFRP strengthening under cyclic horizontal and axial loading been carried out and especially parameters such as axial load, column size, steel tube size in the column have been examined in the literature[36]. Besides, there are different studies on the behavior of concrete wrapped by FRP under normal force[37–39].

Especially, employing CFRP and glass fiber reinforced polymer is highly prevalent to prevent damage to conventional reinforced concrete beams[40–60]. It is a practical and effective technique to apply CFRP material together with epoxy to the tension zone of the beam for strengthening traditional reinforced concrete beams[61–63]. Studies that applyingfiber-reinforced polymers externally to the thinned ended region of the purlin beams increases shear capacity of the end region are available in the literature[64–66]. However, CFRP is an alternative reinforcement that is required to be implemented with extreme caution to establish effective load transfer between the structural member and the CFRP reinforcement by means of adhesive bonding. Firstly, the beam surface should be prepared for CFRP material. For this purpose, the surface of the application area should be cleaned from oil, burr, paint and plaster to achieve clean and dry exposed concrete. Then, after employing the prepared epoxy to carbon fiber and exposed concrete, carbon fiber is placed at the desired position and it is applied by pressing slightly to the direction of the fibers with the help of con-solidation roller. Settling of carbonfiber without stretching and pot is the most important stage of the result of the application. In the practice, one of the important points to be considered is that at least 100 mm overlapping should be employed at the joint.

In this study, the thinned ended regions accepted as the Achilles’ heel of the prefabricated purlin beams and numerous damages have been reported due to snow loading, were reinforced by four different configurations of CFRP. The effects of the form of CFRP application, the place of application on the beam and properties of the material on the mechanical performance under vertical loading were examined in the study. The results were compared to reference prefabricated purlin in terms of the shear capacity, rigidity, ductility and energy absorption capacity and damage analysis of CFRP was performed.

2. Material and method

2.1. Preparation of the thinned ended purlins

Thinned ended pre-stressed prefabricated purlins were manu-factured by Yardımcı Prefabricated Building Components Inc. located in Konya (Fig. 1). Although the cross-section of purlins was selected to be

same completely with the application, the length of purlins was selected shorter. The length of purlins was designed as 320 cm. The aim of this is providing shear damage before the bending damage and investigating the performance of the thinned end region.

The compressive strength of the 28-day concrete cylinder used in the production of the purlins was obtained as 30 MPa. The reinforce-ments used according to TS9967 in the purlins are B420c type 2Ф9 + 1Ф8, 2Ф8 suspension reinforcements and a 8 ½ inch strand. Fig. 2shows the reinforcements and crack formation mechanism of the thinned ended region of the purlins designed according to TS9967.

The shear loading dominated crack formations can be classified as follows: (1) the crack formed from the intersection point of the thinned ended by the tensile effect directly at bending, (2) forming diagonal bending and shear from the starting point at thinned ended region, (3) forming diagonal crack before reaching the thinned ended region, (4) diagonal cracks occurred at the thinned ended region.

Asreinforcement is recommended in TS9967 to prevent (1) and (3)

cracks, Ahreinforcement is recommended to prevent (2) crack, Av

re-inforcement is recommended to prevent (4) crack, Ashand Ashare

re-commended to prevent (1) and (2) cracks. 2.2. Composing composite purlins

In this study, 400 gr/m2carbonfiber with [ ± 45] lay-up sequence was used. F-1564 epoxy and F-3486–3487 hardener (100:34 by weight) were used as the matrix. The effect of fiber orientation on laminated composite beams and plates are presented by many studies[67–71]. Carbonfiber fabrics used in the application were cut according to the design and weighed. The resin and hardener were weighed as equal amount of thefiber and mixed with mechanical mixer for 5 min. The prepared fabrics were laid on the CFRP production table and the epoxy resin was impregnated. A sponge roller and a plastic spatula were used to improve the impregnation of the resin to avoid dry zones without damagingfiber fabrics. Epoxy adhesive was also applied on the pre-fabricated purlins whose application surfaces previously had prepared with the help of sponge roller. The impregnated CFRP fabrics, were laid on the prefabricated purlins in accordance with the design, and the application was made by hand lay-up. The CFRP reinforced purlins were allowed to cure for three days at room temperature (23 °C). The stages of preparation and implementation of CFRP material to the purlins are given inFig. 3.

3. Results and discussions

The experiments of the purlins under vertical load were carried out at Konya Technical University, Faculty of Engineering and Natural Sciences, Civil Engineering Department, Earthquake Research Laboratory. Since it was aimed to observe shear damage at the purlins, the experiments were conducted by selecting the ratio a/(H-d′) (shear span/effective depth) as 2.4. As known from the literature if this ratio is equal or greater than 3, it leads to bending plus shear damage or bending damage instead of shear damage. The required load and dis-placement readings were recorded using 8 channel data acquisition system. In the experiments, the vertical load measurements were ob-tained from load cell and displacements were obob-tained from linear variable displacement transducer (LVDT). Capacity of the load cell used in the experiments was 300 kN. Test setup is given inFig. 4. All the specimens were manufactured according to TS9967. In the experi-ments, reference specimen (P1), two ends fully wrapped specimen (P2),

two ends half wrapped specimen (P3), two ends half wrapped plus

bending zone wrapped specimen (P4) and two ends fully wrapped plus

bending zone wrapped specimen (P5) of the totallyfive purlin

speci-mens were tested.

The P1specimen used in the experiments represents the reference

specimen which was not strengthened by CFRP. This is thefirst spe-cimen in which the shear damage occurred at the thinned end region.

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The CFRP strengthening were applied to prevent the occurrence of this damage. Thefirst type of strengthening (P2) consists of wrapping both

ends of the thinned purlin beam completely with composite material. During the wrapping of the specimens, at least 20 cm overlapping was applied above the beam considering the wrapping conditions specified for thefiber polymers in the TBEC-2018 provision. While full wrapping was applied at P2specimen, half wrapping was applied to the thinned

end region of P3specimen. The reason of this was to observe the effect

of the material quantity to the behavior. Since the damage location of the P3specimen shifted to the bending zone, it was aimed to increase

ductility of the specimen by applying composite material to the lower part of the beam (bending zone) at the P4 specimen. However, the

composite material placed in the bending zone in the P4specimen

in-creased the bending capacity of the beam and caused damage to occur

in the thinned end region. Finally, in order to prevent damage occurred at the thinned end region of the P4sample, the P5sample was wrapped

fully at the thinned ended region and also wrapped at bending zone. However, it was observed that the damage was again occurred at the thinned ended region since the bending capacity was much higher than shear capacity. As a result of the strengthening, strengthening method which was applied to the specimen P3can be used for strengthening the

purlins because of its ductile behavior and low use of composite ma-terial.

The thinned ended regions of the test specimens were shown in Table 1. In the experiments, vertical monotonic loading was applied with 10 kN increments until yielding occurred in the specimen. After the yielding, loading was continued with displacement-controlled loading. The initial stiffness, ductility, strength and energy absorption Fig. 1. Preparation of the purlin beams a) preparation of reinforcements details according to the designed types of the specimen b) checking the suitability of the prepared reinforcement according to TS9967 c) placing reinforcement by lubricating molds d) performing pre-stressing afterfinal inspection.

Fig. 2. The details of thinned ended region according to TS9967.

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capacity for each specimen were obtained from load-displacement curves. InFig. 5, the calculations of the relevant parameters are shown schematically on a load-displacement graph.Fig. 5a. describes how the first stiffness of a specimen is calculated. It can be taken as the slope of the linear section of load-displacement curve. Ductility value given in Fig. 5b is defined as the ratio of the highest displacement to yielding displacement. The ultimate displacement was accepted as the dis-placement value at the point where the maximum vertical load carrying capacity of the element fell to 85% [72]. The value of yielding dis-placement is defined by the secant drawn from between the point of origin and the point where the maximum vertical load carrying capacity reach 85% of its maximum value and the peak displacement value at the intersection point of the line extending horizontally from the maximum vertical load level with this secant line is considered as the yielding displacement. The energy dissipation capacity given inFig. 5c is a function of the vertical displacement that a purlin will make under the effects of the vertical load. For all purlin specimens, this value is obtained by calculating total area in the vertical load-vertical dis-placement plot. The strength value of a purlin element is shown sche-matically inFig. 5d. The maximum load value of each specimen is taken as the load value at which the specimen reaches the load carrying ca-pacity during the experiment.

The damage at the test specimens P1, P4 and P5occurred in the

thinned ended region of the purlins as expected. However, the increase in the shear capacity of the end region is P5> P4> P1, respectively.

In addition, the damages at the P2and P3specimens were not occurred

in the thinned ended region but shear and bending behavior were oc-curred at approximately 2H (twice the height of purlin) away from the support by shifting the damage location. Therefore, a ductile behavior was observed due to bending. This ductile behavior has become more evident in the P3specimen. The wrapping of the specimens and the

quality of workmanship with CFRP play an important role in the initial stiffness of the test specimens as can be seen inTable 1. The load his-tories of the elements are very important in the formation of these values. Since all test specimens were carried out in a load-controlled loading, all specimens were subjected to the same load levels. CFRP strengthening increased the energy absorption capacity of the test specimens. Vertical load-vertical displacement curves of the specimens are given in theFig. 6. It is seen that a significant increase in the load carrying capacity of the test specimens has been achieved as a result of the CFRP strengthening. The values calculated fromFig. 5are given in Table 1separately for each experiment.

The experimental results are examined in the context of reinforced concrete mechanics. When the damage and crack mechanism of the Fig. 3. The stages of the CFRP applications.

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Fig. 4. Test setup and data acquisition system.

Table 1 Results summary.

Test specimens Configuration Initial stiffness value

(kN/mm)

Ductility (mm/mm)

Max strength (kN)

Energy absorption capacity (kN·mm)

P1

Reference specimen

14.39 1.54 83.51 983.41

P2

Two ends fully wrapped specimen

22.02 2.17 123.00 2722. 47

P3

Two ends half wrapped specimen

20.54 3.17 116.40 5252.48

P4

Two ends half wrapped plus bending zone wrapped specimen

11.05 1.67 118.85 3124.00

P5

Two ends fully wrapped plus bending zone wrapped specimen

15.05 2.08 132.49 5696.76

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reference specimen is investigated, it is seen that crack number 2 given in TS 9967 was formed in this purlin. This situation shows that the suspension reinforcement proposed by the provision for the crack at the thinned ended region of the specimen manufactured according to TS 9967 may be insufficient for the capacity design. The other cracks mentioned in TS 9967 have not occurred.

It was observed that the damage shifted approximately 2H away from the thinned ended region at the fully wrapped P2and the half

wrapped P3. This situation delayed shear damage by taking advantages

of bending capacity of the purlin. Hence, shear capacity of P2and P3

purlins increased by 47% and 39% respectively. As a result, it was seen that the ductile plateau developed significantly in the P3specimen. It

was observed that the ductility value of the P3purlin is increased by

106% according to purlin performance values given inTable 1. Failure was occurred due to rupture of the tension reinforcement by reaching energy consumption at the P2and the P3specimens. The shear capacity

gain of the P2was higher than the P3and semi-brittle behavior was

observed. The P3specimen reached to ideal ductile behavior. This

re-sults show that using CFRP more than the required increased shear capacity besides the ductile plateau cannot reach at the desired level due to the fact that after the CFRP strengthened cross section reaches the capacity, unwrapped part of purlin cannot meet this capacity. The CFRP used at tension zone in the P4 and the P5 specimens has

sig-nificantly increased to especially the load carrying capacity of the P5

specimen.

When the initial stiffness of the specimens is compared, it is seen that the P2has approximately equal stiffness with the P5and the P3has

approximately equal stiffness with the P4. The underlying reason of this

is that the amount of composite used on the cross-sectional surface is similar.

Fig. 5. The parameters used for comparison of the specimens.

Fig. 6. Comparative results of test specimens.

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4. Damage analysis of composites concrete

Many studies are present to investigate design and application fa-cilities provided by composite materials since it had been commenced to utilize in the construction sector. The characteristic properties of composites are high strength or stiffness to weight ratio, light weight, high resistance to corrosion, and high resistance to impact damage [70]. The determination of the damage modes formed by the behavior of the composite material after the design and the application will be the basis for the subsequent works in the composite applications in the construction sector. Many studies on damage analysis of the composite materials commonly used in other engineeringfields are available in the literature [73–76]. A number of studies are carry out based on concrete and composite concrete and composite were investigated as

macrostructure and microstructure after the experiments which were designed by taking into account of the main damage modes (delami-nation, matrix cracks,fiber breakage, debonding and buckling). These damages are illustrated on the macrostructure and microstructure photographs and the damage modes are explained in detail.

4.1. CFRP macro damage analysis in retrofit application

There are destructive and non-destructive analysis methods in the literature[77–79]. The damages occurred in each specimen are given in Fig. 7. Both front and back photographs are given in same line in order to compare the damages among itself. The damage photographs are given in same column to compare the damage occurred infive speci-mens. The specimens strengthened by CFRP composite were examined Fig. 7. Damages occurred at the thinned end region.

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in detail in order to determine the damages and the damage modes. Due to the damages occurred at reinforcement and concrete in the tension zone of the fully wrapped P2and the half wrapped P3, no

da-mage was raised at CFRP. However, matrix cracking sounds have

started in CFRP around 86 kN at the fully wrapped P2 during the

loading. Partial separation between concrete and CFRP interface, and minor cracks starting from tension zone in the concrete have been ob-served around 103 kN. Although the failure was occurred at 123 kN, no Fig. 8. Micro cracks occurred at the thinned ended region of the P2specimen.

Fig. 9. Concrete cover separation and concrete damage at the thinned region of P4specimen.

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damage at CFRP was observed. After the failure, the damages occurred in concrete were investigated by cutting the wrapped CFRP. A micro crack (Fig. 8) was observed at the thinned ended region under the full wrapped CFRP. The micro crack formed during the loading could not progress further due to the effect of CFRP and changed the area of damage in the purlin. Matrix cracks and concrete cover separation failures occurred in the P2were not observed in the half wrapped P3

owing to complete adhesion of CFRP application to the thinned ended region.

The damages were occurred at the thinned ended region at the half wrapped P4 and the full wrapped P5 which are wrapped along the

bending zone. CFRP application strengthened the bending zone con-siderably and this leaded to change the damage location. Matrix cracks at bending zone have started to be heard around 90 kN in P4specimen.

Minor cracks have started to form in concrete around 100 kN. Matrix crack sounds were increased by continued loading and failure occurred by collapsing of concrete at the thinned ended region around 118 kN. It was seen that a serious concrete cover separation damage was occurred between CFRP and concrete due to the failure caused by 45° shear damage in concrete when the photo givenFig. 9is investigated. When CFRP is investigated, it was seen thatfiber breakage or delamination were not occurred.

Epoxy crack sounds started to come around 70 kN at the fully wrapped P5specimen. It was seen that concrete cover separation

da-mage between CFRP and concrete started at 104 kN. The load reduction to 93 kN due to concrete cover separation is a sign of the damage in-itiated at concrete under CFRP. It was observed that concrete cover separation and concrete damage increased considerably by continued load at 118 kN. It was seen that failure was occurred approximately at 132.5 kN as a result of brittle failure of concrete with CFRP application and 45° shear damage. When the damage was investigated, it was realized that the adhesion between CFRP and concrete removed and CFRP still continued to carry loads after concrete damage. When the damage at CFRP was investigated in detail, it was seen that the fracture occurred due to fiber breakage of +45° fiber lamina at shear zone. Delamination,fiber breakage, buckling due to bending were observed at CFRP composite (Fig. 10).

Matrix crack sounds and concrete cover separation sounds were observed at undamaged right thinned ended region of the P5specimen

during the experiments. Therefore, the undamaged CPRP given in Fig. 11was cut and the damages under CFRP were examined. After the damage investigation, it was seen that micro cracks normal to the di-rection of tension were formed and propagated. These cracks occurred in concrete were limited and prevented to develop macro cracks due to Fig. 10. Failure and damage modes at the thinned ended region of the P5specimen.

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the effects of confinement by wrapped CFRP. It was observed that in-terface between CFRP and concrete were broken at the same thinned ended region near the tensile zone and concrete cover separation da-mage has occurred. When the collection of concrete cover separation damage is examined, it can be interpreted that CFRP has concrete parts and this proves that interface adhesion has done successfully. It was seen that the interface crack did not propagate to the lowest part of tension zone and remained on side surfaces with 7 cm.

4.2. CFRP micro damage analysis and damage modes in retrofit application In this present paper, micro damage analysis of damaged composite materials which were taken from the specimens after the experiments was executed. As a result of the analysis, damage modes especially occurred in the CFRP composite and concrete cover separation damage was examined. The samples for micro damage analysis were taken from the P5which all damage modes were observed. The samples which were

taken from the side surfaces and the bottom of the purlins were in-vestigated atFig. 12and the damages which lead to buckling at over-lapping region of the upper sides of the purlins were investigated at Fig. 13.

4.2.1. Concrete cover separation

When the CFRP samples separated from concrete is examined, concrete core particles on the CFRP is seen and air voids are observed in areas where adhesion did not materialize. The concrete core particles on the samples is an evidence of the sufficient adhesion between the CFRP and the concrete.

4.2.2. Air voids

Although the resin was applied as coating on the concrete core before application, air voids and non-adhesive regions due to voids at concrete were observed. The carbon fiber bundles seen in air voids (Fig. 12a) is an evidence that successful process of wetting out thefibers were performed to prepare CFRP.

4.2.3. Delamination

Delamination between +45° and −45° CFRP layers is seen at Fig. 12b. Delamination was formed as a result of tensile loads at shear region. A results of damage occurred in the−45° CFRP which resist tensile loads,fiber bundles debonding have been formed in the direc-tion offibers. This damage has continued throughout the cutting axis. 4.2.4. Fiber bundles breakage

This damage type is occurred as a result of the rupture of thefiber bundles in the CFRP layers which carry tensile loads. In the formation of this damage, it is observed that the matrix cracks occurred in the direction offiber configuration (Fig. 12c) and debonding occurred in fiber-matrix interface (Fig. 12d) are effective. It is observed that the damages caused due to especially buckling and bending are more ir-regular and larger (Fig. 13).

4.2.5. Matrix cracks

When the samples taken from the regions where concrete cover separation occurred are investigated, matrix cracks in the direction of fibers are observed (Fig. 12c). These cracks are occurred as a result of sliding between tensile loads perpendicular to thefiber bundles and resultant force occurred during experiments. It has been observed that the matrix cracks parallel to the tensile loads propagate through the region offiber bundle breakage.

4.2.6. Fiber bundles debonding

Micro structure taken from inner and outer surfaces of the samples which were taken from near the region offiber bundle breakage is depicted inFig. 12d. Before the CFRP has been damaged, separation occurs between thefiber bundles due to varying loads during the test, especially after the concrete damage has occurred. This separation is commenced to occur at the region of the carbon fibers stitched by polyester yarns. Debonding which propagates with the increase of loading are combined to form matrix cracks.

4.2.7. Debonding

It was observed that the separations between carbonfibers in the Fig. 11. Damage analysis of undamaged full wrapping right region of P5specimen.

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fiber bundles were formed when the region of the fiber bundles breakages (Figs.12b and13) is closely investigated. It is seen that de-bonding occurred as a result of these separations forms parallel to the fiber direction. It is seen that tensile load perpendicular to the fiber and shear loads parallel to thefibers leads debonding.

4.2.8. Fiber breakage

When the region of thefiber bundles breakage (Fig. 13b) is closely examined, it is observed thatfiber bundles breakages are occurred in the form of brittle fracture by rupturing fibers due to the fact that carbonfibers cannot carry tensile loads parallel to the fibers. Beginning

offiber breakage in CFRP can be considered as the beginning of the failure. The formation offiber damage, which starts in the form of brittle fractures, results in a sudden and severe failure.

4.2.9. Buckling

Buckling occurs at the top of the beam owing to the effects of compression in the CFRP during loading and failure. The joint is formed by overlapping the CFRP at the top of the beam. The damages at the region without overlapping and the region with overlapping due to buckling is investigated inFig. 13a and b, respectively. Delamination, debonding andfiber breakage were observed as a result of buckling. Fig. 12. Micro structure damage analysis of the CFRP samples taken from the side surfaces and the bottom surfaces of the P5specimen after the experiment.

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5. Summary of results

In this study, especially in the prefabricated industrial structures which are preferred frequently in Turkey and similar countries, vertical load capacity of purlins and other structural performances resulted from CFRP strengthening in different configurations to prevent shear damage in the thinned ended region of purlins were compared and following results are achieved:

It was observed that the P1purlin which is not strengthened and

designed according to TS-9967 deplete its energy due to sudden shear damage. This situation showed that if necessary precaution is not taken, sudden failure may happen. Since shear failure occurs suddenly and without showing any warnings, total collapses, which cannot be prevented, take place at the roof plane in the industrial structures. Snow load and snow accumulation can be more critical than other loads especially in wide span prefabricated industrial structures and cause damage or collapse of structures. When the damage and crack mechanism of the P1purlin was examined, it is

seen that the crack number 2 given in TS9967 was formed in this purlin. The other cracks mentioned in TS9967 was not observed.

Using CFRP in the P2and the P3purlins whose bending zones do not

have CFRP increased shear capacity. Therefore, it was observed that using CFRP delayed shear damage of the purlin. This situation showed that the purlin had reached its bending capacity before shear failure and significant deformation was occur in tension re-inforcement. As a result, it was noticed that the ductile plateau developed considerably especially in the P3specimen.

Ductility capacity of the P4purlin having CFRP at bending region

had not formed as much as ductility capacity of the P3. This

situa-tion shows that using reinforcement or CFRP more than requirement in tension zone leads to brittle behavior. The engineers must act according to this fact in their design.

The amount of adhesion area between polymer and reinforced concrete or the amount of beam interface area is much more im-portant than the amount of polymer used in CFRP application. High interface area and not using CFRP in bending zone in the P3

spe-cimen increased the shear capacity of the purlin and also cause the purlin to have ideal energy dissipation mechanism by acting ductile.

The purlins were exposed to loadings for only short duration. Therefore, creep deformation depending on the time and damages that may occur due to creep were not investigated in this study.

Even though applied load was below the capacity of the purlin, the damage at the end region of the purlin were observed. The reason of this situation is that long-term loading triggers creep damage in concrete. For this reason, the performance of the detail of the end region of the purlins under permanent loads may be evaluated in further studies.

6. Conclusion

At the end of the study, it is seen that the hypothesis proposed at the beginning of the study was supported by the results. It is observed in the experiments that the thinned ends of the prefabricated reinforced concrete purlins are very vulnerable to shear damage. In this case, the behavior in these regions must be improved and the purlin beam should be able to reach its bending capacity. It is seen that CFRP material used in variousfields especially in structural engineering both prevents shear cracks at thinned ends of industrial building purlins examined in this study and increases the load carrying capacity of the purlins. This study also reveals that the form of CFRP application has a significant effect on purlin capacity and damage mechanism.

It is preferred that the energy consumption is formed in a ductile at the reinforced concrete mechanics. It is seen that the most suitable wrapping will be the P3. Only difference between the P3and the P4is

the use of CFRP at bending zone at the P4. In spite of this, ductility

capacity of the P3was not formed as much as the P4does. This shows

that using CFRP or reinforcement more than the requirement in the tension zone leads to brittle behavior.

Rather than the amount of polymer used in CFRP application, how much of the reinforced concrete elements are effectively adhered or interface area of beam are very important. The high interface area and the fact that CFRP was not used at the P3 both increased shear capacity and leads to ideal energy consumption of the purlin by ductile behavior. Energy dissipation capacity of the P2, P3, P4and P5increased by 177%,

434%, 217% and 479% compared to reference specimen, respectively. The damage modes occurred in the purlins strengthened by CFRP can be sorted as follow: concrete cover separation, air voids, delami-nation,fiber bundles breakage, matrix cracks, fiber bundles debonding, debonding,fiber breakage and buckling.

For future studies, the effects of ply sequences, the number of CFRP plies and material properties of CFRP to the behavior of the purlins should be examined. Furthermore, apart from composite strengthening, changing concrete and reinforcement materials, rearranging Fig. 13. Micro structure damage analysis of the CFRP samples taken from buckling region of the upper surface of the P5specimen after the experiment.

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reinforcement layout or pre-stressing level should be investigated in order to improve shear capacity of the purlin.

Declaration of Competing Interest We have no conflict of interest to declare. Acknowledgement

The authors would like to thank Yardımcı Prefabricated Building Components Inc. for the production of prefabricated specimens and also thank laboratory technician Yüksel Çiftçi who works with dedication in the experiments.

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Şekil

Fig. 2. The details of thinned ended region according to TS9967.
Fig. 4. Test setup and data acquisition system.
Fig. 5. The parameters used for comparison of the specimens.
Fig. 9. Concrete cover separation and concrete damage at the thinned region of P 4 specimen.
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