Keywords

1 Introduction

Throughout the world, the CFRP laminates are widely utilized to reinforce/retrofit currently-standing RC structural members. Usually, the externally installed laminates of CFRP are utilized to externally reinforce/mend RC beams, whether in flexure or shear. Experimentally, it has been found that the delamination of the externally reinforced-with-CFRP concrete beams encountered de-bonding failure, resulting from the detachment of CFRP from concrete at the concrete-CFRP interface [1,2,3]. The CFRP materials have had the potential to pioneer the reinforcing/repairing businesses because they are highly strong in tension, although this strength is not used in full because of delamination of CFRP, the brittleness in the tensile performance, and overloading the mended/ reinforced RC beams [4].

The practitioners in the field of construction employed externally installed CFRP laminates to additionally reinforce/mend RC members in flexure and shear. Generally, the efficacy of employing CFRP depends completely on the way this material is attached to the concrete. Nevertheless, a great number of reports mentioned that the reinforced-with-CFRP structures usually fail in de-bonding mode, where the CFRP material detaches prematurely from concrete [5]. Many researchers have experimentally proven that the externally installed CFRP laminates much enhanced the RC structural strength in shear, flexure, or both [6,7,8]. The problem of CFRP’s detachment is an ongoing issue, particularly at locations with high stress or at the corners of CFRP, even when the laminates are adhered to recommended-by-manufacturer substances [9,10,11,12,13]. In case of overloading a building or removing one of its pillars, the building becomes deficient and needs to be fixed to retrieve its original strengths, whether in flexure, shear, or both. At this point, it must be assured that the reinforcing processes, in shear and flexure, should be performed at the same time in order to have a ductile form of failure [14, 15].

RC beams and other structures may encounter deformation, or even damage, when they are exposed to extremely high temperatures; as this kind of temperature causes a degradation in many of the mechanical qualities of concrete and reinforcing steel, which redistributes the developed stresses within the beam [16]. Researchers have concluded that utilizing external laminates of CFRP to strengthen/repair damaged-by-heat beams regained, to some extent, the beams’ capacity of flexural strength and enhanced their performance in shear. The laminates of CFRP have gained good acceptance, and are favored to be used more than steel, due to their: resistance-to-corrosion, simple erection and handling, durability, high strength, cost-effectiveness, great strength-weight ratio, and capability of withstanding harsh environments [1, 17]. In order to judge the effectiveness of a material in reinforcing and/or retrofitting damaged-by-heat structures, the following factors must be taken into consideration: type utilized [18]; capability of resisting extreme temperatures [19]; the employed method of analysis [20, 21]; ability to resist energy integrity [22], the utilized method of anchorage [23]; conditions of heating [24]; type and size of employed fibers and severity of damage [25,26,27]; in addition to the reinforced-with-CFRP bridges’ factors of safety [28].

Some concrete buildings are often prone to frequent heating-cooling cycles which degrade substantially, the quality of concrete within structures; thus, it is imperative to take such cycles when planning to design buildings like these, such as platforms for launching rockets; and nuclear plants for producing power [29, 30]. It is stipulated that concrete stays intact and preserves its qualities when exposed to a temperature below 300 °C; otherwise, it begins to lose its intactness when the exposure temperature exceeds 500 °C. Further, concrete is subject to drastic alterations in its qualities when it is prone to a fire attack which is distinguished rabidly. The reason for this is that the temperature of the core of concrete is way different from the temperature at the surface; this temperature variation produces tensile stresses on the concrete’s surface which result in many incompatible expansion-retraction movements in the cementitious paste from one side and aggregates from the other side. In consequence, cracks emerge within the concrete’s surface. The magnitude of damage within concrete structural elements is governed by several factors, for example: size of structure, type of cement, utilized type of aggregates, content of moisture within concrete, frequency and times that a structure is exposed to high levels of heat, the way used for cooling, and the highest level of exposure temperature [31].

2 Methodology and Used Materials

2.1 Constructing the Specimens

A total of eight concrete beam specimens have been built, having: a cross-section of (150 × 200) mm, and a length of (1100) mm total. The beam specimens were examined as they were subject to a four points loading, as simply supported (Fig. 1). The bottom side of the specimens was strengthened with two (\(\varnothing \)10) steel bars, while the upper side was strengthened with two (\(\varnothing \)10) steel bars (Fig. 1). Loading was applied using concentrated loading applied by a hydraulic jacket of 2000 kN capacity and beams were supported at 50 mm from each end.

Fig. 1.
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Specimens test setup, reinforcement, dimensions, and instrumentations.

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Strengthening was done using 50 mm width CFRP strips positioned internally on the top of the bottom main reinforcement with their strain values measured using an internally positioned strain gauge within the beam tensile zone, as presented in Fig. 2. The strengthening steel bars were supported and fixed in position by stirrups having a diameter of 8 mm placed at 50 mm away in the shear’s span. Moreover, each beam specimen was equipped, at the flexural (bottom) side’s bars, with a CFRP strip; these strips had the same width (of 50 mm) but were different lengths, namely: 400 mm, 600 mm, and 800 mm (Fig. 2). The qualities and the designations of the experimental beam specimens are elaborated, in summary, in Table 1.

2.2 Properties of the Materials

To make the experimental work more accurate, all of the specimens were built utilizing the same concrete mix which complied with the ACI design code of mixtures [32]. The concrete mix was made of ordinary type I Portland cement (422 kg/m3), fine aggregates (crushed) (621 kg/m3), coarse aggregates (crushed) (706 kg/m3), and tap (fresh) water (147.6 kg/m3); this mix was designed to achieve 28-day cylindrical strengths of 4.31 MPa in tension and 50 MPa in compression, at 23 °C, and also to have an 80 mm slump. As for the reinforcing bars of steel, their type was grade 60 and had a strength of yielding reaching 420 MPa. As for the sheets of CFRP, they were SikaWrap® −300 °C, made by Sika; while the adhesive’s type was (Sikadur®-330), respectively, made by SIKA Fabric thickness of 0.167 mm (based on fiber content); Fiber Density of 1.82 g/cm3; Tensile Modulus of 230,000 N/mm2; Tensile Strength of 4,000 N/mm2; Elongation break of 1.67%.

Fig. 2.
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CFRP strips internal attachment at the beam’s bottom tensile zone (width = 50 mm).

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2.3 Mixing of Specimens and Heat Treatment

The process went as follows: the starting step was soaking the inner side, of the 0.15 m3-capacity tilting drum mixer, with water. Then, upon running, the mixer was fed with the full quantity of coarse aggregates and some amount of the water already used for soaking. Next, the mixer was fed, in a step-by-step manner, with the ingredients of cement, fine aggregates, and water. Later, the last amount of water and a super-plasticizing material were put in the mixer. The last step was to mix the materials for five minutes before they were poured into (150 × 200 × 1100 mm) wooden molds for molding; then, they were put for compaction utilizing an electrical vibrator. After twenty-four hours of molding, the beam specimens were moved out of the molds, and they were immersed for twenty-eight days in a tank filled with lime water for curing.

The heat treatment process was performed by utilizing an easy-to-control automatic electrical furnace (Fig. 3). The specimens were heated at temperatures of 150 °C, 250 °C, and 500 °C for 120 min. Then, the heated specimens were allowed to self-cool inside the furnace.

Table 1. The details and results tested beams.
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Fig. 3.
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The furnace time-temperature timetable.

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2.4 Test Setup and Instrumentation

As previously mentioned, the whole beam specimens were simply supported and exposed to s four-point loading (Fig. 1). To make sure that the specimens would encounter a shear mode of failure in the shear’s span, the shear’s span-to-depth ratio (a/d) was made 1.7. A special actuator was employed to exert loading while controlling the servo, utilizing a special system for the acquisition of data. As for instrumentation, an LVDT (a contraction for linear variable displacement transducer) was employed to record the value of deflection at the middle of the span. Additionally, two strain gauges were employed to record the values of strain at the ends of the beam; these gauges were mounted on the at-the-center CFRP strip’s sides (Fig. 1).

3 Results and Discussion

3.1 Mode of Failure

To analyze the beam specimens’ flexural behavior more precisely, the cracks’ appearance was recorded at different values of load steps. The modes of failure encountered by the control (with no reinforcement) beam are shown in Fig. 4, and those encountered by the reinforced beam specimens are in Fig. 5. The control beam witnessed its initial flexural crack at its mid-span; this crack was succeeded by more cracks in other places. Later, the beam encountered a de-bonding mode of failure located at the externally-bonded sheet of CFRP.

Fig. 4.
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The effect of temperature on the failure mode of the control specimen.

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Fig. 5.
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The effect of CFRP length on failure mode.

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Figure 5 showed that the length of the CFRP sheet governed the intensity of cracks, their length, and the way they were distributed. Figure 5 indicated that the cracks’ intensity enhanced when the exposure temperature was raised, while Fig. 5 indicated that the propagation of cracks enhanced when the length of the CFRP was increased; this could be because of the CFRP sheet’s capability of bridging the emerged cracks by availing adequate length. This is possibly confirmed as the internally installed CFRP sheets stop unexpected sheets’ detachment; this is evidence that the internally bonded sheets of CFRP have improved the structural behavior by seizing flexural cracking.

3.2 Failure Loads and Corresponding Deflection

Table 1 demonstrates the ultimate values of the beam specimens’ strength and resultant deflection. The values of failure load and resultant deflection were modified as per the ones obtained from the control beam specimen (Fig. 6). As for the internally strengthened with CFRP, utilizing CFRP sheets with lengths of 400 mm, 600 mm, and 800 mm raised the ultimate value of strength by 21%, 48%, and 77%, in respective order; these values were 1.97 times the values gotten from the externally-strengthened beam specimens (Fig. 6(a)); while the same CFRP sheets enhanced the values of the resultant deflection by 24%, 45%, and 67%, corresponding to 2.22 times the values gotten from the externally-reinforced specimens (Fig. 6(b)). As for the temperature’s influence on the failure load (Fig. 6(a)) (compared with the beam specimens at 23 °C), the load of failure lessened: by 19% at 150 °C, by 34% at 250 °C, and by 49% at 500 °C.

Fig. 6.
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The ultimate load and deflection behavior.

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3.3 Load-Deflection Behavior

Figure 7, which elucidates graphically the beam specimens’ load-deflection behavior, indicates that the graph has two parts: the before-cracking part, represented by a straight line, while the second part represents the after-cracking, where the slope noticeably changed post-emergence of cracks in flexure. The reinforced-with-CFRP beam specimens, whether internally or externally, had a better capacity for load carrying than the control (without reinforcement) beam specimen. Further, the load-deflection graphs also indicated when the area of bonding was raised, the beams’ behavior was much improved; to put it differently, the beam specimens strengthened with an 800-mm-long sheet of CFRP showed a better behavior than those strengthened with a 400-mm-long sheet of CFRP. The outcomes of a conducted comparison analysis indicated that the strengthened-with-CFRP beam specimens demonstrated better behavior than the control specimen, in regard to stiffness, at-ultimate strength, and at-ultimate deflection. Moreover, the latter characteristics improved more when the CFRP sheet’s length was raised. Furthermore, at a given length of the CFRP sheet, the internally strengthened beam specimens had better levels of stiffness, at-ultimate strength, and at-ultimate deflection, in comparison with the externally-reinforced specimens.

Fig. 7.
figure 7

Load-deflection curves.

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4 Conclusions

In light of the achieved outcomes, the next conclusions have been extracted:

  1. 1)

    Internally bonded sheets proved to be highly effective because it was simple to adopt and economical because neither anchoring nor adhering was needed; the reinforcing sheets could be handled and shaped easily, and they could be attached using regular tape at the upper and lower surfaces of steel. It should be affirmed that the CFRP sheets could not be out of position while casting the concrete.

  2. 2)

    The research paper in hand proved that utilizing the internally attached sheets of CFRP was viable and feasible in reinforcing/mending damaged-by-heat RC beams and improving their performance because this method eliminated the failure in de-bonding.

  3. 3)

    When subjected to temperatures less than 150 °C and higher than 250 °C, the RC beams encountered a reduction in stiffness and shear capacity; also, the concrete encountered numerous cracks with no spalling.