Repair of heat-damaged SCC cantilever beams using SNSM CFRP strips

Structures 24 (2020) 151–162

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Repair of heat-damaged SCC cantilever beams using SNSM CFRP strips a,⁎

b

Ahmed M. Ashteyat , Rami Haddad , Yasmeen T. Obaidat a b

b

T

Civil Engineering Department, University of Jordan, Amman, Jordan Civil Engineering Department, Jordan University of Science and Technology, P.O. Box 3030, 22110 Irbid, Jordan

ARTICLE INFO

ABSTRACT

Keywords: Self-compacting concrete Heat-damage Repair Strengthening SNSM CFRP Efficiency

The potential of using side near surface mounted (SNSM) fiber reinforced polymer (FRP) for strengthening/ repairing cantilever self-compacting concrete (SCC) beams was investigated. A total of twelve L-shaped reinforced SCC prototypes with a cantilever component (150x150x750 mm) were cast then cured for 28 days. Eight specimens were heated in an electric furnace at 400 °C and 500 °C for two hours with remaining four SCC specimens left at laboratory air, as controls. Control and heat-damaged cantilever beams were strengthened/ repaired using single SNSM-CFRP strips, inserted in two parallel longitudinal man-made side grooves, located at 25 or 60 mm from the beams' top tension fiber. Another set of beams were strengthened/repaired using double strips, inserted in same grooves at 60 mm from the beams' top fiber or in duplicate grooves separated by 10 mm. The mechanical behavior of the SCC cantilever beams was evaluated under one-point static loading acting on the fee-end of the beams. Measurements of deflection at the beams' free-end and strain in SNSM CFRP strips were recorded as load was increased. SNSM CFRP strips' number and location, as well as the exposure temperature significantly impact mechanical performance of the present SCC cantilever beam. Inserting two SNSM CFRP strips in side grooves with spacing imparted the best contribution to mechanical response. The use of SNSM CFRP single strips at 25 mm helps averting concrete cover peeling-off prior to flexural failure as observed in the remaining repaired/strengthened SCC cantilever beams.

1. Introduction Concrete is a primary structural material with many substantiated advantages, such as availability and low cost of its ingredients, and adequate long-term strength and durability. Self-compacting concrete (SCC) has been proposed and used in Japan and Europe since more that 20 years as an alternative to conventional concrete due to its many advantages such as higher durability, better finish surfaces, less tendency for segregation, and lower labor cost [1–3]. When exposed to elevated temperatures, structures, constructed using SCC, undergo higher damage extent as compared to that of those made using traditional concrete [4]. Therefore, the load capacity and durability of SCC structural elements are significantly reduced proportional to the temperature level and duration [1–6]. In the temperature range of 600 °C to 900 °C, the degradation of SCC elements may be demonstrated either in sever reductions in SCC compressive strength, noticeable tendency for spalling, and reduction in reinforcing steel yield strength [7–10]. Distortion of reinforcing steel at the upper temperature spectrum of this range becomes highly possible especially in SCC elements with an inadequate concrete cover. Furthermore, the bond strength between re-

⁎

inforcing steel and surrounding concrete is crucially reduced under exposure temperature > 200 °C [11–12]. As a result, the structural stability of fire-damaged SCC structures is compromised unless proper retrofitting measures are followed. In the past few decades, the use of carbon fiber-reinforced polymer (CFRP) as a strengthening material received a lot of attention, [13–23]. Use of CFRP for repair works can effectively overcome the limitations of conventional repair techniques such as steel jacketing and concrete enlargement, [24–26]. For example, CFRP is more durable than plates and sections of steel when used to repair structures at aggressive environments. High strength-to-weight ratio, and fatigue resistance, and ease of handling, and availability in different lengths or shapes made CFRP material a favorable repair material worldwide [14–16]. CFRP composites are either externally bonded (EB) to the concrete elements or near surface mounted (NSM) in grooves, created within the concrete cover of flexural elements [27–29]. The EB method of application is problematic with regard to the tendency of CFRP composites to detach from concrete elements under relatively low external loads. The vulnerability of EB CFRP composites to weathering or fire attack complicates further its functionality, [30–33]. On contrast, near surface

Corresponding author. E-mail address: [email protected] (A.M. Ashteyat).

https://doi.org/10.1016/j.istruc.2020.01.005 Received 1 July 2019; Received in revised form 21 December 2019; Accepted 6 January 2020 2352-0124/ © 2020 Institution of Structural Engineers. Published by Elsevier Ltd. All rights reserved.

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2.2. Material properties

mounted (NSM) CFRP composites show excellent bond within grooved concrete and resilience to humidity effect, wear loads, fire attack and sunlight [34–38]. The potential of using NSM CFRP strips for shear and flexural strengthening of beam elements was intensively studied over the past 10 years or so [13–23,39–43]. Major findings indicate that bond characteristics are dependent upon key factors such bond length, grooves toughness, dimension and filler type, strips number and spacing, and depth concrete cover. Few studies have tackled the potential of repairing heat-damaged reinforced concrete beams using NSM-CFRP, [44–46]. Jadou et al. [45] repaired reinforced concrete beams, heatdamaged at 600 and 700 °C for two hours, using NSM CFRP strips. Upon implementation of the proposed repair technique, the load capacity and the stiffness lost upon heating of beams were partial to fully regained. In another work, Haddad and Almomani [46] studied the potential of recovering the flexural performance of thermally damaged concrete beams using different configurations of NSM CFRP strips. Heat-damaged and repaired beams showed improved load capacity and toughness, yet experienced reductions in ductility and toughness as compared to control ones. End-cover separation failure mode was observed for both strengthened as well as repaired beams. The application of NSM-CFRP strips as a repair technique requires a concrete cover of not less than 25 mm, necessary edge distance, and adequate spacing between grooves [15]. Unfortunately, flexural elements repaired/strengthened with this technique tend to show concrete cover peeling-off before NSM CFRP strips develop it ultimate strain capacity [15,46]. This presents a limitation on the scale of its application, worldwide. Therefore, the idea of mounting CFRP strips in grooves created on the flexural members' side covers recently immerged [47–48]. The proposed side near surface mounted (SNSM) CFRP strengthening technique significantly enhanced flexural load capacity, stiffness and ductility of strengthened beams tangibly as compared to those of the control ones. Also, the serviceability loads were increased drastically when the proposed technique was applied. Application of NSM CFRP strips to cantilever beams is complicated as compared to that when applied to simply supported or continuous ones. This is attributed to the difficulty in extending the strips beyond the critical stress zone for the required development length because of obstructing columns, [47–48]. To overcome this problem it is proposed to insert SNSM CFRP strips in longitudinal grooves, which are created on both sides of the cantilever beam with extensions to the column's side. In this paper the contribution of straight profiles of SNSM CFRP strips to recovering the load capacity of intact and heat-damaged selfcompacting concrete cantilever beam is investigated. Load-displacement response, and strain in SNSM CFRP strips were evaluated with cracking development and failure modes monitored for intact cantilever and heat-damaged (at 400 °C and 500 °C) beams before and after strengthening/repairing.

2.2.1. Concrete Ordinary Portland cement (Type I), coarse aggregate with maximum aggregate size of 12.5 mm, a mixture of fine aggregate and silica sand at (60% fine limestone and 40% silica sand) were used in preparing a selfcompacting concrete mixture for casting all specimens. The mixture was designed at a water-to-cement ratio of 0.45 according to the rational mix design method, [50]. A commercial superplasticizer (Structuro 520) was added at a content of at 1% (by cement mass) to achieve the required consistency for the SCC mixture. The proportions of cement, water, coarse aggregate, fine aggregate, and silica sand were 450, 203, 850, 446, and 298 kg/m3, respectively. 2.2.2. Reinforcing steel The mechanical properties of different sizes of steel bars used in reinforcement of present SCC specimens were experimentally evaluated at room temperature and after the reinforcing bars were exposed to 400 and 500 °C for two hours. Results are summarized in Table 1. As can be noticed, both the post-heating yield and ultimate strengths of the reinforcing steel showed considerable losses especially after being exposed to 500 °C. For example, the corresponding reduction in yield strength ranged from 24 to 39% for the different bar sizes used in this work. These reductions along with those of concrete compressive strength shape the magnitude of reduction in the loading capacity of the present post-heated structural specimens as discussed later on. 2.2.3. NSM CFRP strips and adhesive SIKA brand NSM-CFRP strips were used in the present study for strengthening /repairing self-compacting reinforced intact and heatdamaged concrete cantilever beams. Their physical, geometrical, and mechanical properties, as provided by manufacturer, as summarized in Table 2. Sikadur adhesive epoxy of two parts was used to bond NSMCFRP strips to SCC. Its mechanical properties, as obtained from manufacturer, are listed in Table 3. 2.3. Preparation of specimens The different batches of SCC mixtures used in preparing the present test specimens were mixed using a tilting drum mixer of 0.15 m3 capacity. The volume of each batch was sufficient for casting two specimens, and three standard cylinders (100x200 mm). The fresh properties of SCC mixtures were evaluated using slump-flow test, V-funnel test, and L-Box according to EFNARC [51]. These averaged at 700 mm, 9 s, and 0.85, hence meet specified limits stipulated at 550–850 mm, 5–12 s, and 0.80–1.00, respectively [51]. Wooden molds of 20 mm thickness were used to cast the test specimens, as shown in Fig. 2a. The wooden molds were oiled before the steel cages were placed inside, as shown in Fig. 2b. Sufficient concrete covers at the bottom and sides of all specimens at 25 mm was maintained using proper spacers. SCC was poured in three layers before compacted and their top surface finished smooth using a trowel, as shown in Fig. 2c. Twenty four hours later, specimens were demolded and covered with wet burlap for another 27 days before transferred to an open environment awaiting heat-treatment and the following strengthening works.

2. Experimental programs 2.1. Test specimens A total of twelve cantilever SCC beams were cast using ordinary strength SCC beams. The beams were structurally designed according to ACI 318 to ensure flexural failure, [49]. Specimens consisted from extruded beams (150 × 150 mm) which are connected at their end to column (150 × 250 mm) and footing components, as shown in Fig. 1.The cantilever beams were reinforced by 2Φ14 mm at the bottom and 2Φ10 mm at the top with steel stirrups of Φ 8 mm spaced at 50-mm (center to center). The columns were reinforced using 4Φ 14 mm with Φ8-mm ties spaced at 150 mm (center to center). The footings were reinforced with 4Φ 14 mm deformed bars and Φ10-mm stirrups, as illustrated in Fig. 1.

2.4. Heat treatment After 28 days of curing, eight reinforced cantilever beams were placed in pairs in an electric furnace. Heating time and temperature were controlled using a control panel attached to the furnace. The beams were exposed to 400 °C and 500 °C for two hours before allowed cooling down inside the furnace until next day. The heat-treatment adopted is depicted in Fig. 3. 152

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Fig.1. Geometry and steel reinforcement details for specimens. Table 1 Pre and post-heating mechanical properties of the steel bars.

Table 3 Properties of adhesive resin (Sikadur).

T

Bar size (mm)

Yield stress (MPa)

Ultimate stress (MPa)

Description/Value

Property

23 °C

8 10 14 8 10 14 8 10 14

390 441 450 306 401 419 239 333 339

570 641 682 412 537 571 335 445 482

Gray 1.8 g/cm3 (approx.) 0.04% 58 N/mm2 at 3 days 25 N/mm2 at 7 days 18 N/mm2 at 7 days 21 N/mm2 at 7 days > 4 N/mm2 at 1 day (concrete fracture) 10000 N/mm2

Color Mixed density at 25 °C Shrinkage Compressive strength Flexural strength Tensile strength Shear strength Bond to concrete E-Modulus

400 °C 500 °C

Table 2 Properties of NSM-CFRP strips. NSM CFRP 3

1.6 g/cm 15 mm 2.5 mm 37.5 mm2 3100 N / mm2 165 N/ mm2 > 1.7% (nominal)

ratio of 3 of A to 1 of B. After that, the epoxy blend was filled into the grooves before the NSM CFRP strip (already cut to desired lengths) were pressed in these grooves and coated with another layer of the epoxy. Finally, the epoxy allowed curing for seven days to achieve the required strength.

Fiber type Fiber density Strip width Strip thickness Cross sectional area Mean tensile strength Tensile E-modulus Strain at break

2.6. Configuration of repair using NSM- CFRP strips Different NSM-CFRP retrofitting configurations were proposed for flexural strengthening/ repair of heat-damaged beams using NSM CFRP strips. Those were inserted in side grooves created on the negative moment side with extensions to the column element. A letter-number description was adopted to define control, heat-damaged, control and strengthened; and heat- damaged and repaired specimens. The designation of control and heat-damaged specimens comprised from a letter and a number designates the treatment temperature. For strengthened and repaired specimens five-letter-number designation was used. The first letter (S or R) referred to strengthening or repair, the following number designates exposure temperature, and the remaining third, fourth, and fifth numbers reflects the depth of groove, number of strips, and the distance between the strip(s) and the top surface, respectively. Different specimens are defined in Table 4 along with schematics of repair/strengthening configurations.

2.5. Strengthening and repair process Control and heat-damaged specimens were strengthened/repaired using NSM-CFRP strips inserted in grooves, which were created at the side of the beam parallel to the direction of the main flexural reinforcement. The procedure for strengthening/repair of the cantilever SCC beams is illustrated in Fig. 4. First, grooves width and length were located on the beam surface using a paper tape and a pencil. Then, grooves at a depth of 20 mm and 8 mm width were made using an electrical saw machine before cleaned using a vacuum machine to remove dust accumulated inside. Components (A and B) of Sikadur epoxy were blended according to the instructions by the manufacturer at a 153

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Fig. 2. Cantilever beam specimens.

Fig. 3. The time temperature schedule for the beams.

Fig. 4. Creating a grooves on the tension side of the beam; inserting CFRP strip; beam after strengthening (right).

2.7. Test setup

allowed an ideal simulation of rigid joint behavior between the column and the cantilever without any tilting or overturning in the specimen. A linear variable differential transducer (LVDT) was placed at 50 mm from the free end of the cantilever beam to acquire deflection readings under loading effect. All data were collected electronically by a data acquisition system and analyzed to obtain the load–deflection diagrams. In addition, three strain gauges of 5 mm length were adhered

Load testing was performed using a steel frame; especially fabricated to support the specimens. The cantilevers beams were tested under one point bending using a hydraulic testing machine. Special support made of hardened steel plates with thickness of 20 mm were used to provide a full fixation of the footing of the specimen. This 154

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Table 4 Detailing of repair configuration applied. Specimen Designation

Temp.

Number of strip

Distance from top surface

Groove spacing

S-23–25-1

23

1

25

None

S-23–60-1

23

1

60

None

S-23–25-2–35

23

2

25

40

R-400–25-1

400

1

25

None

R-400–60-1

400

1

60

None

R-400–60-2–0

400

2

60

0

Schematic of repair configuration

(continued on next page)

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Table 4 (continued) Specimen Designation

Temp.

Number of strip

Distance from top surface

Groove spacing

R-400–25-2–35

400

2

60

40

R-500–25-1

500

1

25

None

R-500–60-2–0

500

2

60

0

along NSM CFRP strips to measure strain, as shown in Fig. 5.

Schematic of repair configuration

sides of the beams (S-23–25-2–35) was the most efficient in boosting load capacity. Slight differences in mechanical responses are noticed for beams strengthened using singles strips, side-inserted at distances of 25 mm (S-23–25-1) and 60 mm (S-23–60-1) from the top of the concrete beams: corresponding grooves are located at distances of 0.5 mm above and 3 mm below top steel reinforcement, respectively. The insignificant impact of strips location upon the mechanical response is explained as follows. The benefit of inserting the SNSM CFRP strips at a shallow distance of 25 mm from the tension side with regard to the moment resisting arm for the strips was undermined by the premature concrete cover separation. On the other hand, inserting single strips at a relatively large distance of 60 mm although helped delaying concrete cover separation, had resulted in a reduced effective depth for the strips to compression side. The contradicting effects related to the location of inserts resulted in close mechanical behavior for the two repair cases described above. Nevertheless, it should be warned that such a conclusion cannot be generalized for other beams having different geometries and reinforcement ratios. The improvement in the mechanical performance upon insertion of SNSM CFRP strips in two different parallel grooves suggest that doubling of the reinforcement ratio of SNSM CFRP strips is beneficial only when such reinforcement is distributed in separate grooves. In this case, concrete cover separation can be delayed without undermining the contribution of the additional reinforcement. This argument is supported by the discussions relevant to the repair of SCC specimens, pre-damaged at 500 °C. The efficiency of using SNSM-CFRP strips to recover structural performance of the present cantilever beams, damaged by exposure to 400 °C and 500 °C for 2 h, can be understood by referring to Figs. 8 and 9, respectively. The curves indicate that the repair efficiency was the best when two separated NSM CFRP strips were side-inserted at

3. Results and discussion 3.1. Damage of SCC beams under elevated temperature Exposure of SCC cantilever beams to elevated temperatures resulted in sever concrete cracking and loss of strength in their concrete. The compressive strength as evaluated using concrete cylinders from different batches of concrete stood at 35 for control specimens as compared to 20 and 15 MPa for those exposed to 400 °C and 500 °C for 2 h with percentage losses of 50 and 75%, respectively. Furthermore, exposure of cantilever specimens to high temperatures caused tangible concrete surface cracking proportional to temperature level without any distortion in steel reinforcement in any part of the specimens. 3.2. Load-Deflection response of beams Fig. 6 shows load versus deflection for control and heat-damaged beams. As can be deduced, exposure of concrete beams to elevated temperatures of 400 °C and 500 °C impacts negatively their mechanical properties except ductility which shows a noticeable enhancement. The curves show small bi-linear segments at relatively low to moderate loads before experience non-linearity thereafter. As can be deduced, exposure of the present specimens to the above temperatures had introduced significant changes in their mechanical response; especially in the load capacity and ductility as discussed in the following section. Fig. 7 shows load–deflection curve for control and strengthened beams using 1 or 2 SNSM CFRP strips. As may be expected, the use of two separate SNSM CFRP strips inserted in the two opposite tension 156

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Fig. 5. Loading configuration of present test specimens.

Fig. 7. Load- deflection curve for control beam and strengthened beams.

Fig. 6. Load versus deflection for control and heat-damaged beams.

3.3. Characteristics of load–deflection diagram

distances of 25 and 35 mm) from the top of heat-damaged beams. There beams are designated as (R-400–25-2–35) and (S-23–25-2–35) in Table 4. SNSM CFRP contribute less to mechanical response when inserted as double strips in same groove at 60 mm, (R-400–60-2–0) or (R500–60-2–0), as single strips at 25 mm, (R-400–25-1) or (R-500–25-1), or as single strips at 60 mm, (R-400–60-1). All curves corresponding to strengthened/repaired beams showed bi-linear segment at low to moderate loads before experienced non-linearity thereafter.

In order to understand the significance of the proposed strengthening /repair techniques on the overall structural performance of retrofitted cantilever beams, the characteristics of different load–deflection diagrams of Figs. 7-9 were obtained and summarized in Table 5 along with their residuals computed with respect to the characteristics of beams without repair. Those included ultimate load capacity, toughness, deflection at failure, and rotational ductility. The toughness 157

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Fig. 8. Load- deflection curve for beams damaged by exposure to 400 °C.

Fig. 9. Load- deflection curve for beams damaged by exposure to 500 °C. Table 5 Mechanical characteristics for control and CFRP strengthened beams. Failure Mode

DI

Δmax (mm)

TS J

ULC (kN)

Beam

GN

FF

1.42 100 1.98 100 2.24 100 1.72 121 1.09 77 1.62 114 1.36 69 1.04 53 2.11 107 1.28 65 1.16 52 1.35 60

17.10 100 19.88 100 26.97 100 31.10 182 22.30 130 40.50 237 28.23 142 23.93 120 48.70 245 28.24 142 24.93 92 24.40 90

170.15 100 140.55 100 130.00 100 449.40 264 281.67 166 684.74 402 273.40 195 273.94 195 548.48 390 338.88 241 264.76 204 264.66 204

19.95 100* 4.85 100 10.95 100 29.26 147 28.02 140 36.11 181 23.60 159 25.83 174 34.52 232 27.40 185 21.02 192 24.95 228

C23

#1

FF FF CP CP CP CP CP CP CP CP CP

Fig. 10. Strain induced in SNSM-CFRP strip versus distance from face of column for (a) intact; and (b) heat-damaged (at 400 °C) beams.

C400 C500 S-23–25-1

#2

S-23–60-1 S-23–25-2–35 R-400–25-1

#3

R-400–60-1 R-400–25-2–35

Fig. 11. Strain induced in SNSM-CFRP strip versus distance from face of column for heat-damaged beams (at 400 and 500 °C).

R-400–60-2–0 R-500–25-1

#4

percentages for load capacity, flexural toughness, and ductility index at (400 °C and 500 °C) were about (74 and 55%), (83 and 76%), and (139 and 158%), respectively. As can be noticed, the deflection at failure showed similar trend to that of rotational ductility with respect to the impact of exposure temperature. These behaviors were expected in light of the reductions in strengths for reinforcing steel and concrete comprising the present specimens. Their softening was reflected in an enhanced ductility performance of the heat-damaged beams. Strengthening of intact beams using one or two SNSM CFRP strips was very efficient in promoting load capacity and toughness by as high as 81 and 302%, respectively. Of course, using double strips (S-23–25-2–35) imparted better benefit towards improving the latter characteristics. Ductility was improved for the strengthening case involving single strips position at 25 mm from the top tension side of the beam (S-23–25-1).

R-500–60-2–0

*, Residual property with respect to unrepaired specimens; ULC, ultimate load capacity; Δmax; Maximum Deflection; DI, Ductility Index; TS, Toughness; FF, Flexural Failure; CP, Concrete peeling off.

was computed as the area underneath the load deflection diagram, whereas the ductility index was computed as the ratio of deflection at failure to that corresponding to reinforcing steel yielding. Results of Table 5 show a significant reduction in load capacity, a slight reduction in flexural toughness and an increase in ductility index of heat damaged specimens as compared to intact one. The residual 158

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Fig.12. Crack pattern of control and heat damaged beams.

Beams exposed to 400 °C were repaired using four different techniques, represented in inserting single SNSM-CFRP strips at distances of 25 and 60 mm from the top tension surface (R-400–25-1), and (R400–60-1), respectively, double separated (R-400–25-2–35) or double attached (R-400–60-2–0) SNSM-CFRP strips on both sides the beams. The findings indicate significant improvements in load capacity and toughness when different techniques are applied to heat-damaged beams with best performance achieved for beam (R-400–25-2–35) at

residual of (232 and 390%) followed, in sequence, by beams (R400–60-2–0), (R-400–60-1) and (R-400–25-1) at residual of (185 and 241%), (174 and 195%) and (159 and 195%), respectively. Furthermore, the different repair configurations impart improvements to deflection at failure ranging from 120 to 245%. The ductility index is improved only for beam (R-400–25-2–35); other repaired beams demonstrated clear reduction in their ductility. In general, the benefit from repair is greater for beams, heat-

Fig. 13. Failure modes.

159

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Fig. 13. (continued)

damaged at 500 °C, than that of those at 400 °C; repaired using similar schemes with SNSM CFRP strips. The residuals for load capacity and ductility index are higher for the former beams, strengthened by inserting single SNSM CFRP strips at a distance of 25 mm, (R-500–25-1), or double SNSM CFRP strips in same groove at a distance of 60 mm, (R500–60-2–0), from top tension surface. The residuals for toughness and ductility in terms of deflection for beams, heat-damaged at 500 °C and repaired by inserting single SNSM CFRP strips at a distance of 25 mm, (R-500–25-1), is higher than that of those heat-damaged at 400 °C and repaired using same scheme, (R-400–25-1). This is not true when double SNSM CFRP strips are inserted in same groove of heat-damaged beams at a distance of 60 mm as higher corresponding residuals are noticed for beam (R-400–60-2–0) than for (R-500–60-2–0).

3.5. Failure modes and Crack pattern The cracking pattern for the reinforced SCC cantilever beams exposed to 400 and 500 °C for 2 h is shown in Fig. 12. The cracks are pronounced at 400 °C yet their intensity increased as temperature is raised to 500 °C. Cracking observed at the lower exposure temperature is referred to vapor water pressure escaping SCC's pore system, incompatible expansions and contractions between aggregate and cement as well as between SCC and reinforcing steel. As temperature is raised to 500 °C, more damage is generated in SCC due to full and partial decomposition of CH and C-S-H, respectively [4,7,10]. Failure of control and heat-damaged beams is caused by yielding of steel reinforcement that is accompanied by relatively large deflections. The picture of Fig. 13(a) shows a typical representation of cracks formation and propagation leading to failure. Cracks initiate at the maximum moment zone in the vicinity of columns then propagate further towards the lower compression zone with load increase until failure. Different failure modes are noticed for strengthened/repaired beams, as shown in Fig. 13(b-i). For these beams, segmented tension cracks appeared first at the beam-column interface before more tension cracks appeared at the two parallel side surfaces close to the critical moment section and later extended to compression zone. Prior to failure, cantilever beams, repaired by inserting single SNSM-CFRP strips at 25 and 60 mm from their top fiber, showed concrete cover peeling which led ultimately to a sudden failure at the critical moment section, as shown in the relevant photos of Fig. 13. Horizontal and inclined cracking was also noticed at the column surface at the failure point, as shown in Fig. 13(b-i). Same failure sequence and trend was noticed for strengthening/retrofitting cases involved inserting two CFRP strips in same or different grooves across the beams depth, as shown in Fig. 13(g-i). Steel yielding is recognized based on the excessive deflection noticed at the moment of failure. A summary of failure mode

3.4. SNSM- CFRP strain The percentage strain in NSM CFRP composites at failure relative to ultimate stain capacity of the composites reflects the efficiency of repair. Fig. 10 shows comparison between strain induced in SNSM-CFRP strip inserted in intact and heat-damaged beams. The results indicate higher induced strains in SNSM CFRP used to repair the latter beams especially at the critical section, located at the face of the column. Since concrete is significantly weakened in heat-damaged beams, as indicated by the reduction of compressive strength by 42%, SNSM CFRP strips accommodate higher proportions of flexural stresses than that of NSM CFRP strips inserted in intact ones of similar configurations. Fig. 11 indicates that the strain induced in SNSM CFRP strips was higher when used to repair SCC reinforced beams, damaged at higher temperature of 500 °C. These results confirm previous discussions related to repair efficiency as evaluated using residual characteristic of different strengthened/repaired SCC beams. 160

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for different cantilever beams is provided in the last column of Table 5.

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4. Conclusions Based upon the study results, the following conclusions can be outlined: o Use of side near surface mounted (SNSM) CFRP is proved in this work as a practical solution that overcomes the development length problem associated with repair/strengthening of reinforced concrete cantilever beams. o Doubling the reinforcement ratio of SNSM-CFRP strips is effective in promoting the structural performance of present strengthened beams only when such reinforcement is inserted into two separated grooves rather a single groove, created at the tension side of present SCC cantilever beams. o The benefit of repair using different schemes of SNSM CFRP strips is increased as heat induced damage is increased. This is substantiated based upon the residuals for load capacity, ductility, and toughness as well as the strain measurements in SNSM CFRP strips at failure. o All strengthened/repaired specimens with single NSM-CFRP strips, inserted at a distance of 25 mm from the top fiber of the present SCC beams, showed typical flexural failure mode. However, inserting double or single strips (at 60 mm) in grooves at the tension side of intact or heat-damaged SCC beams resulted in peeling-off concrete cover prior to reinforcing steel yielding. Declaration of competing interest There is no conflict of interest Acknowledgments The authors wish to acknowledge the financial support provided by the Ministry of Higher Education and Research / Scientific Research and Innovation Support Fund, via a research grant, (Eng/2/8/2015). References [1] Karahan O, Özbay E, Hossain KMA, Lachemi M, Atiş CD. Fresh, Mechanical, Transport and Durability Properties of Self-Consolidating Rubberized Concrete. ACI Mat J. 2013;109:413–20. [2] Dinakar P, Babu KG, Santhanam M. Durability properties of high volume fly ash self compacting concretes. Cem Concr Compos 2008;30(10):880–6. https://doi.org/10. 1016/j.cemconcomp.2008.06.011. [3] Yung WH, Yung LC, Hua LH. A study of the durability properties of waste tire rubber applied to self-compacting concrete. Constr Build Mater 2013;41:665–72. https://doi.org/10.1016/j.conbuildmat.2012.11.019. [4] George, C., Hoff etal. 2000. Elevated temperature effects HSC residual strength. Conc. Int., 41-47. [5] Haddad N, Al-Mekhlafy A Ashteyat. Repair of heat-damaged reinforced concrete slabs using fibrous composite materials. Constr Build Mater 2011;25:1213–21. https://doi.org/10.1016/j.conbuildmat.2010.09.033. [6] Leonardi A, Meda ZR. Fire-damaged RC members repaired with high performance fibre-reinforced jacket. Strain 2011;47:28–35. [7] Zhai Y, Deng Z, Li N, Xu R. Study on compressive mechanical capability of concrete after high temperature exposure and thermo-damage constitutive model. Constr Build Mater 2014;68:777–82. https://doi.org/10.1016/j.conbuildmat.2014.06.052. [8] Wu Y, Wu B. Residual compressive strength and freeze–thaw resistance of ordinary concrete after high temperature. Constr Build Mater 2014;54:596–604. https://doi. org/10.1016/j.conbuildmat.2013.12.089. [9] Yue Z, Zichen D, Nan L, Ryi Y. Study on compressive mechanical capability of concrete after high temperature exposure and thermo-damage constitutive model. Constr Build Mater 2014;68:777–82. https://doi.org/10.1016/j.conbuildmat.2014. 06.052. [10] Zhiwu Li Xu, Jinyu Erlei Bai. Static and dynamic mechanical properties of concrete after high temperature exposure. Mater Sci Eng, A 2012;544:27–32. https://doi. org/10.1016/j.msea.2012.02.058. [11] Al-Nimry H, Haddad R, Afram S, Abdel-Halim M. Effectiveness of advanced composites in repairing heat-damaged RC columns. Mater Str 2013;46(11):1843–60. https://doi.org/10.1617/s11527-013-0022-8. [12] Neves IC, Rodrigues JPC, Loureiro ADP. Mechanical properties of reinforcing and prestressing steels after heating. J Mater Civil Eng 1996;8(4):189–94. https://doi. org/10.1061/(ASCE)0899-1561(1996) 8:4(189).

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