polymers Article
Preloading Effect on Strengthening Efficiency of RC Beams Strengthened with Non- and Pretensioned NSM Strips Renata Kotynia *
ID
and Marta Przygocka
Department of Concrete Structures, Lodz University of Technology, 90-924 Lodz, Poland;
[email protected] * Correspondence:
[email protected]; Tel.: +48-42-631-38-70 Received: 2 January 2018; Accepted: 31 January 2018; Published: 3 February 2018
Abstract: The near surface mounted (NSM) technique has been shown to be one of the most promising methods for upgrading reinforced concrete (RC) structures. Many tests carried out on RC members strengthened in flexure with NSM fiber-reinforced polymer (FRP) systems have demonstrated greater strengthening efficiency than the use of externally-bonded (EB) FRP laminates. Strengthening with simultaneous pretensioning of the FRP results in improvements in the serviceability limit state (SLS) conditions, including the increased cracking moment and decreased deflections. The objective of the reported experimental program, which consisted of two series of RC beams strengthened in flexure with NSM CFRP strips, was to investigate the influence of a number of parameters on the strengthening efficiency. The test program focused on an analysis of the effects of preloading on the strengthening efficiency which has been investigated very rarely despite being one of the most important parameters to be taken into account in strengthening design. Two preloading levels were considered: the beam self-weight only, which corresponded to stresses on the internal longitudinal reinforcement of 25% and 14% of the yield stress (depending on a steel reinforcement ratio), and the self-weight with the additional superimposed load, corresponding to 60% of the yield strength of the unstrengthened beam and a deflection equal to the allowable deflection at the SLS. The influence of the longitudinal steel reinforcement ratio was also considered in this study. To reflect the variability seen in existing structures, test specimens were varied by using different steel bar diameters. Finally, the impact of the composite reinforcement ratio and the number of pretensioned FRP strips was considered. Specimens were divided into two series based on their strengthening configuration: series “A” were strengthened with one pretensioned and two non-pretensioned carbon FRP (CFRP) strips, while series “B” were strengthened with two pretensioned strips. Experimental tests illustrated promising results at ultimate and serviceability limit state conditions. A significant gain of the load bearing capacity, in the range between 56% and 135% compared to the unstrengthened beams, was obtained. Tensile rupture of the NSM CFRP strips was achieved, confirming full utilization of the material. Keywords: flexural strengthening; NSM; CFRP; pretensioning; preloading; load bearing capacity; serviceability limit state
1. Introduction There are two primary methods of strengthening reinforced concrete (RC) members in flexure with fiber reinforced polymer (FRP) materials: laminates externally bonded (EB) to the tensile surface of the concrete and the near surface mounted (NSM) method in which narrow, typically carbon FRP (CFRP) strips or bars are embedded into slots made in the concrete cover. Although, EB FRP strengthening has been widely used, it has poor strengthening efficiency (in terms of utilizing the FRP material strength) Polymers 2018, 10, 145; doi:10.3390/polym10020145
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due primarily to premature debonding from the concrete surface. Pretensioning EB FRP increases the efficiency of the system by increasing the FRP strain that is attained prior to debonding, but does not mitigate debonding failure. The NSM method was proposed to mitigate FRP debonding and allow the full utilization of FRP material strength. Strengthening with FRP materials that are pretensioned prior to bonding is an efficient technique to increase the load bearing capacity of the RC members and to improve performance at the serviceability limit state (SLS), specifically increasing the cracking moment and reducing deflections under service loads. Pretensioning the FRP effectively reduces existing crack widths, delays their further development, decreases existing deflections, and relieves stress in the internal reinforcement. The influence of many parameters on the strengthening efficiency of the NSM method have been investigated, including the CFRP pretensioning strain, internal reinforcement ratio, concrete strength, and the CFRP reinforcement length and elastic modulus [1–13]. Review of the available literature on flexural strengthening of RC members with passive and pretensioned NSM materials concludes that the strengthening efficiency depends on a number of factors, including the internal steel reinforcement ratio, the pretensioning limit, defined as the level of CFRP pretension as a ratio of the ultimate CFRP strain, and the pretensioning method. Although the CFRP pretensioning limit affects the serviceability of FRP-strengthened structures, it has no influence on the increase in load-carrying capacity attributed to the CFRP. From a review of published research, an increase of the load bearing capacity is affected by the existing longitudinal steel reinforcement ratio [2]. The increase in the capacity of prestressed RC members ranges from 11.5% for specimens with an internal steel reinforcement ratio of ρs = 1.75% [4] to 152% for specimens with a very low internal reinforcement ratio of ρs = 0.35% [5]. An increase in the steel reinforcement ratio results in a decrease in the degree of strengthening that can be achieved. Another investigated parameter was the pretensioning level of the CFRP laminates [2,3,5–11]. The maximum pretensioning level studied was 60% of the tensile strength of the CFRP [2,5]. However such a high pretensioning level did not lead to an increase in load capacity in comparison to specimens pretensioned to 40% of the CFRP tensile strength. Rather, the higher pretension led to a decrease in the ultimate load capacity due to premature rupture of the CFRP strips. The most effective pretensioning level of NSM CFRP strips was determined to fall in the range of 20% and 30% of the tensile strength [5]. The greatest benefit of the NSM pretensioning technique is in preventing premature debonding of the FRP from the concrete surface [6], which is the dominant limit state for EB FRP laminates. Although preloading is one of the most important parameters to be taken into account in strengthening existing RC structures, this issue has been investigated very rarely. To the authors’ knowledge, no research results have been published on the effects of initially preloaded RC members strengthened with pretensioned NSM FRP strips. In the experimental tests presented in this paper a novel system for pretensioning narrow NSM CFRP strips embedded in the concrete cover was used [14]. The aim of the experimental program was to evaluate the strengthening efficiency of the pretensioned NSM method. The study focused on the influence of the internal steel reinforcement ratio and the number of pretensioned strips used on the strengthening effectiveness. The presented research also presents an analysis of the effect of preloading on strengthening efficiency. 2. Experimental Program The experimental program comprised six 500 × 220 mm2 rectangular RC beams having a simply-supported span of 6000 mm. Shear reinforcement consisted of 8-mm-diameter steel stirrups with 150-mm spacing. The beams were divided into two series: A and B, depending on the number of NSM CFRP strips. Single-span simply-supported beams were tested in static six point loading with load points spaced at 1200 mm across the span, as shown in Figure 1. Loading was introduced using two 100 kN hydraulic jacks and transferred to the beams through a steel spreader beam.
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Each specimen was tested to failure. Specimens were labelled as follows: NSM12 and NSM16— beams reinforced with 12 and 16 mm diameter longitudinal steel bars, respectively; A or B—Beam of Each specimen was tested to failure. Specimens were labelled as follows: NSM12 and series A or series B; L—External preloading of the beam prior to strengthening. NSM16—beams reinforced with 12 and 16 mm diameter longitudinal steel bars, respectively; A or Series A consisted of two beams (NSM12A and NSM16A) strengthened with a combination of B—Beam of series A or series B; L—External preloading of the beam prior to strengthening. one pretensioned and two passive CFRP strips. Series B included four specimens (NSM12B, NSM16B, Series A consisted of two beams (NSM12A and NSM16A) strengthened with a combination of NSM12B-L and NSM16B-L), each strengthened with two pretensioned CFRP strips. The internal one pretensioned and two passive CFRP strips. Series B included four specimens (NSM12B, NSM16B, tensile steel reinforcement consisted of four steel bars with a nominal diameter of 12 mm (ρs = 0.49%) NSM12B-L and NSM16B-L), each strengthened with two pretensioned CFRP strips. The internal tensile in beams labelled NSM12, and four bars with a diameter of 16 mm (ρs = 0.87%) in beams labelled steel reinforcement consisted of four steel bars with a nominal diameter of 12 mm (ρs = 0.49%) in NSM16. beams labelled NSM12, and four bars with a diameter of 16 mm (ρs = 0.87%) in beams labelled NSM16.
Figure 1. Static schemes, specimens’ cross-sections, and strengthening configurations. Figure 1. Static schemes, specimens’ cross-sections, and strengthening configurations.
Beams were strengthened with rectangular CFRP strips 2.5 mm wide and 15 mm high, resulting werereinforcement strengthened with strips 2.5 mm wide and 15 mm high, resulting in in a Beams composite ratiorectangular equal to ρCFRP f = 0.10% and ρf = 0.07%, in series A and series B, arespectively. composite reinforcement ratio equal to ρ = 0.10% and ρf = 0.07%, in series A and control series B,torespectively. The pretensioning force wasf applied to the CFRP strip with strain εfp = 0.006, The pretensioning force was applied to the CFRP strip with strain control to ε = 0.006, corresponding fp corresponding to 33% of the ultimate tensile strength of the CFRP (0.33ffu). to 33% ofbeams the ultimate tensile strength of the CFRP (0.33f fu ).levels. The first was the self-weight of the The were strengthened under two preloading The beams were strengthened under two preloading levels. The firststeel wasofthe self-weight of the beam only, corresponding to the stresses in the longitudinal reinforcing 25% and 14% of the beamstrength only, corresponding to the stresses in the(M longitudinal reinforcing steel of 25% and 14% of the yield in the unstrengthened members u0) for the beams NSM12 and NSM16, respectively. yield strength inpreloading the unstrengthened members (M ) for of thethe beams NSM16, respectively. The maximum level, corresponding tou060% steelNSM12 yield inand a unstrengthened beam The maximum preloading level, corresponding to 60% of the steel yield in a unstrengthened beam (Mu0), was applied using the self-weight and external preloading using the loading pattern shown in (M applied using the self-weight and external preloading u0 ), was Figure 1. The resulting moment associated with preloading, Mp, isusing giventhe in loading Table 1. pattern shown in Figure 1. The resulting moment associated with preloading, Mp , is given in Table 1.
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Table 1. Summary of tested beams, investigated parameters, and strengthening configuration. Specimen
Tensile Steel Reinforcement
Number of CFRP Strips
Number of Pretensioned CFRP Strips
M u0 1 [kNm]
Mp 2 [kNm]
NSM12A NSM16A NSM12B NSM16B NSM12B-L NSM16B-L
4φ12 4φ16 4φ12 4φ16 4φ12 4φ16
3 3 2 2 2 2
1 1 2 2 2 2
46.5 84.7 46.5 84.7 46.5 84.7
13.5 15.0 13.5 15.0 27.9 50.9
1 M —Analytically-determined moment bearing u0 bending moment, 3 Mp /Mu0 —Preloading ratio.
M p /M u0 [%]
3
25 14 25 14 60 60
capacity of the unstrengthened specimen, 2 Mp —Preloading
The beams were fabricated using commercially-supplied class C50/60 concrete. The average compressive strength of concrete and the modulus of elasticity (Ecm ) were defined at the day of testing using uniaxial compression tests on 150 × 300 mm2 cylinders (f ck ) and 150 mm cube samples (f c,cube ) [15], while the tensile strength (f ct,sp ) was determined based on the splitting test for 150 mm cube samples [16]. All material test specimens were cured under the same conditions as the tested beams. Concrete properties are reported in Table 2. The tensile characteristics of longitudinal steel bars are presented in Table 2 [17]. The average experimentally-determined strength characteristics of CFRP strips [18]: tensile strength (f fu ), elastic modulus (Ef ), and ultimate strain (εfu ) are also reported in Table 2. Strength properties according to the manufacturer are also shown Table 2. Table 2. Strength characteristics of used materials. Series A Material
Property
Unit
NSM12 8
Steel
Concrete
CFRP
Es fy fu f ck f c,cube f ct,sp Ecm Ef f fu εfu
[GPa] [MPa] [MPa] [MPa] [MPa] [MPa] [GPa] [GPa] [MPa] [–]
12
186.1 191.3 416.2 539.6 734.1 627.5 46.0 44.9 3.95 25.3
Series B NSM16
8
16
NSM12 8
12
NSM16 8
16
196.5 198.0 205.5 214.0 205.5 204.9 555.8 595.0 554.9 563.4 554.9 578.3 646.0 672.0 608.9 651.7 608.9 693.8 53.9 51.0 52.0 59.5 60.0 60.1 4.30 4.5 4.1 24.0 25.8 24.3 170.4 (manufacturer reports 160) 2551 (manufacturer reports 2800) 0.0136 (manufacturer reports 0.017)
The beams were strengthened with CFRP laminates bonded into slots with a commerciallyavailable two-component (3:1) epoxy adhesive intended for the purpose. In order to affect preload, the strengthening process was undertaken in the test frame with the beam in its correct position; that is, the NSM strips were installed overhead. The beams were strengthened under loading (consisting of the dead load and/or external loading as indicated in Table 1). Prior to strengthening, two (series B) or three (series A) slots were cut using a diamond saw blade. Each slot was 6 mm wide and 19 mm deep. To accommodate the pretensioning hardware, the slots prepared for the pretensioned laminates terminated with a 110 mm × 300 mm rectangular region having a depth of 19 mm (Figure 2).
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Two passive CFRP Twostrips passive CFRP strips
Gap in concrete cover of Gap in concrete
One pretensioned
of 2 110 × cover 310 mm
One pretensioned CFRP strip
110 × 310 mm2
CFRP strip
(a) (a)
Embedded Embedded steel bolts
CFRP strip CFRP groove strip inside inside groove
steel bolts
Gaps for NSM Gaps for NSM prestressing system prestressing system (110 × 330mm2) (110 × 330mm2)
Embedded
Embedded
steel bolts
Steel frame
steel bolts
Steel frame
(b) (b) Figure (a) beams beamsseries seriesA; A;and and(b) (b)beams beams series Figure2.2.Preparation Preparationof ofstrengthening: strengthening: (a) series B. B. Figure 2. Preparation of strengthening: (a) beams series A; and (b) beams series B.
After installed on on the thebottom bottomofofthe thebeam beam NSM Afterpreparing preparingthe theslots, slots, steel steel bolts bolts were installed forfor thethe NSM After preparing the slots, steel bolts were installed on the bottom of the beam for the NSM pretensioning mounted in in the thepretensioning pretensioningsystem system and hydraulic pretensioningsystem. system.The TheCFRP CFRP strips strips were were mounted and hydraulic pretensioning system. CFRP strips were mounted thethe pretensioning system and jacks jackswere weremounted mountedThe both pretensioning pretensioning frames strip’s (Figure 3).hydraulic jacks atat both framesinat at the strip’s ends ends (Figure 3).The Theinitial initial were mounted at both pretensioning frames at the strip’s ends (Figure 3). The initial pretensioning pretensioning force was strain-controlled, intended to be equal to ε fp = 0.006, corresponding to 33% of of pretensioning force was strain-controlled, intended to be equal to εfp = 0.006, corresponding to 33% force was strain-controlled, CFRP tensilestrength. strength. intended to be equal to εfp = 0.006, corresponding to 33% of the CFRP thethe CFRP tensile tensile strength.
(a) (a)
(b)
(b)
Figure 3. NSM pretensioning system for CFRP strips: (a) top view (against the beam soffit); and (b) Figure NSM pretensioning system for for CFRP strips: (a) top the beam (b) bottom Figure 3. 3. view. NSM pretensioning system CFRP strips: (a)view top (against view (against thesoffit); beamand soffit);
bottom and (b)view. bottom view.
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Beams of of series series A A were were strengthened strengthened with with one one pretensioned pretensioned CFRP CFRP strip. strip. The The middle middle slot slot was was Beams Beams of series A were strengthened with one pretensioned CFRP strip. The middle slot was filled with adhesive and the CFRP strip was embedded into the slot. A 300 mm length at each of the filled with adhesive and the CFRP strip was embedded into the slot. A 300 mm length at each of the filled with adhesive and the CFRP strip was embedded into the slot. A 300 mm length at each of slots was cured at 90 °C for 45 min. The remainder of the strip was not subject to accelerated cure. As slots was cured at 90 °C for 45 min. The remainder of the strip was not subject to accelerated cure. As ◦ was cured at 90cured, C forthe 45 pretensioning min. The remainder of the strip was not subject to accelerated the slots anchoring adhesive cured, the pretensioning force was was reduced by approximately approximately 50%. After After the anchoring adhesive force reduced by 50%. cure. As the anchoring adhesive cured, the pretensioning force was reduced by approximately 50%. pretensioning of the middle CFRP strip (Figure 1), it was anchored to clamps mounted in the gaps pretensioning of the middle CFRP strip (Figure 1), it was anchored to clamps mounted in the gaps After pretensioning of the middle CFRP strip (Figure 1), it was anchored to clamps mounted in the gaps made in the concrete cover for an additional 12 h prior to complete release. Releasing the pretension made in the concrete cover for an additional 12 h prior to complete release. Releasing the pretension madein inthis the incremental concrete cover for anwas additional 12 to h prior tothe complete the pretension force in this incremental manner was intended to ensure the properrelease. transferReleasing of the the tensile tensile force from from force manner intended ensure proper transfer of force the CFRP strip to concrete. Two non-pretensioned CFRP strips were bonded into the remaining slots. force in this incremental manner was intended to ensure the proper transfer of the tensile force from the CFRP strip to concrete. Two non-pretensioned CFRP strips were bonded into the remaining slots. The combination of active and and passive strengthening wasstrips used due due tobonded the difficulty difficulty ofremaining using the the NSM NSM the CFRP strip to of concrete. Twopassive non-pretensioned CFRP wereto into the slots. The combination active strengthening was used the of using pretensioning system for all three strips due to the lack of space on the bottom surface of the The combination of active and passive strengthening was used due to the difficulty of using the NSM pretensioning system for all three strips due to the lack of space on the bottom surface of the specimens. NSM spacing of 160 mm is required to effectively use the pretensioning system for pretensioning system for all three strips due to the lack of space on the bottom surface of the specimens. specimens. NSM spacing of 160 mm is required to effectively use the pretensioning system for adjacent strips.of 160 mm is required to effectively use the pretensioning system for adjacent strips. NSM spacing adjacent strips. In the beamsofof of series B, strengthened strengthened with two pretensioned pretensioned CFRP the strips, the strips were the series B, strengthened withwith two pretensioned CFRP strips, strips were installed In thebeams beams series B, two CFRP strips, the strips were installed and pretensioned one after another. The pretensioning process proceeded in the same and pretensioned one after another. The pretensioning process proceeded in the same manner as installed and pretensioned one after another. The pretensioning process proceeded in the same manner asfor described for series series A. A. described series A.for manner as described To monitor the strain distribution along the the full To monitor the strain distribution along full length length of of the the CFRP CFRP strip, strip, twelve twelve strain strain gauges gauges in Figure Vertical displacements of the were on each strip at at the shown in Figure 4. 4. Vertical the beams beams were mounted mounted on each strip the locations locations shown displacements of nine 50 mm stroke linear variable differential transformers (LVDTs) (Figure 5a). were recorded with linear variable differential transformers (LVDTs) (Figure 5a). were recorded with nine 50 mm stroke linear variable differential transformers (LVDTs) (Figure 5a). were with horizontal LVDTs The concrete strains measured arranged over 300 mm gauge lengths The concrete strains were measured with horizontal LVDTs arranged over 300 mm gauge lengths gauges in in the the tensile tensile zone zone and and 5–10 5–10 mm mm stroke stroke gauges gauges in in the the compressive compressive zone), (13–20 mm mm stroke stroke gauges compressive zone), zone), (13–20 as shown in Figure 5b. as shown in Figure 5b.
side; B—Lateral side). Figure 4. 4. Strain Strain gauges gauges on on CFRP CFRP strips strips (A—Bottom (A—Bottom and and lateral lateral side; Figure B—Lateral side).
(a) (a)
(b) (b)
Figure 5. Location Location of LVDT gauges: (a) (a) vertical vertical displacement; displacement; and and (b) (b) horizontal horizontal concrete concrete strain. strain. Figure 5. Location of of LVDT LVDT gauges: Figure
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3. Test Results 3. Test Results 3.1. Failure Modes 3.1. Failure Modes All beams failed in flexure due to rupture of both pretensioned and passive NSM CFRP strips All beams failed in flexure due to rupture of both pretensioned and passive NSM CFRP strips preceeded by sounds of gradually-breaking carbon fibers (Figure 6a). This failure mode confirmed preceeded by sounds of gradually-breaking carbon fibers (Figure 6a). This failure mode confirmed the full utilization of the CFRP tensile strength, in both pretensioned and passive strips. The single the full utilization of the CFRP tensile strength, in both pretensioned and passive strips. The single pretensioned strip in series A ruptured first in both members (NSM12A and NSM16A). Further load pretensioned strip in series A ruptured first in both members (NSM12A and NSM16A). Further load increase led to the rupture of the remaining two passive strips. Concrete crushing in the compression increase led to the rupture of the remaining two passive strips. Concrete crushing in the compression zone occurred in beam NSM16A (having a higher reinforcement ratio), as shown in Figure 6b. zone occurred in beam NSM16A (having a higher reinforcement ratio), as shown in Figure 6b.
(a)
(b) Figure6.6.Failure Failureofofthe the specimens in series A: bottom (a) bottom of NSM12A with rupture of strips; CFRP Figure specimens in series A: (a) viewview of NSM12A with rupture of CFRP strips; and (b) the top view of NSM16A with concrete crushing. and (b) the top view of NSM16A with concrete crushing.
3.2. Cracking Pattern 3.2. Cracking Pattern Concrete cracking in all tested beams was monitored during the full range of loading. Concrete cracking in all tested beams was monitored during the full range of loading. Irrespective Irrespective of preloading level, crack patterns were similar in all beams, as seen in Figure 7. of preloading level, crack patterns were similar in all beams, as seen in Figure 7. Differences in Differences in the cracking pattern after failure were caused by the different composite reinforcement the cracking pattern after failure were caused by the different composite reinforcement ratio of ratio of each series and by the number of pretensioned CFRP strips. The beams strengthened with each series and by the number of pretensioned CFRP strips. The beams strengthened with two two pretensioned laminates exhibited fewer cracks than the beams strengthened with one pretensioned laminates exhibited fewer cracks than the beams strengthened with one pretensioned pretensioned and two non-pretensioned CFRP strips. No longitudinal cracks in the concrete along and two non-pretensioned CFRP strips. No longitudinal cracks in the concrete along the slot, at the the slot, at the level of the bottom steel reinforcement, or at the epoxy-concrete interface were level of the bottom steel reinforcement, or at the epoxy-concrete interface were observed, confirming observed, confirming the very good bond behavior of the NSM CFRP strips to concrete through the the very good bond behavior of the NSM CFRP strips to concrete through the full range of loading. full range of loading. The “fishbone” cracking pattern on the bottom surface of the beams, typical for The “fishbone” cracking pattern on the bottom surface of the beams, typical for NSM strengthening [19], NSM strengthening [19], was observed in all beams (Figure 8a,b). This cracking pattern confirms was observed in all beams (Figure 8a,b). This cracking pattern confirms progressive tensile stress progressive tensile stress transfer from the pretensioned strips to the concrete substrate. transfer from the pretensioned strips to the concrete substrate.
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Figure 7. Cracking pattern of specimens with the higher reinforcement ratio (NSM16). Figure 7. Cracking pattern of specimens with the higher reinforcement ratio (NSM16).
(b)
(a)
Figure 8.8.Cracking Crackingpattern pattern on bottom the bottom surface of of beams B: (a) and NSM12B; and (b) Figure on the surface of beams seriesofB: series (a) NSM12B; (b) NSM12B-L. NSM12B-L.
4. Discussion of the Test Results 4. Discussion of the Test Results Analysis of strengthening efficiency for all beams was performed for three loading levels Analysis of strengthening efficiency for all beams was performed for three loading levels corresponding to concrete cracking, steel yielding, and ultimate load. Values of the bending moment corresponding to concrete cracking, steel yielding, and ultimate load. Values of the bending moment of the unstrengthened the strengthened beams, corresponding to these levels, are summarized in of the unstrengthened the strengthened beams, corresponding to these levels, are summarized in Table 3. Bending moments for the unstrengthened beams were calculated using a nonlinear analytical Table 3. Bending moments for the unstrengthened beams were calculated using a nonlinear analytical model, which considers normal stresses in the cross-section, experimental stress-strain characteristics model, which considers normal stresses in the cross-section, experimental stress-strain characteristics of concrete and steel using the plane sections principle, and accounting for tension stiffening in the full of concrete and steel using the plane sections principle, and accounting for tension stiffening in the range of loading. Details of the model were described and applied for similar RC beams in [19]. full range of loading. Details of the model were described and applied for similar RC beams in [19]. The strengthening ratio was determined as a percentage of the difference between the ultimate The strengthening ratio was determined as a percentage of the difference between the ultimate bending moment of the strengthened (Mu ) and unstrengthened (Mu0 ) beam to the ultimate bending bending moment of the strengthened (Muu) and unstrengthened (Mu0 u0) beam to the ultimate bending moment of the unstrengthened member (Mu0 ) as follows: moment of the unstrengthened member (Mu0 u0) as follows: ∆MuΔM = 100 × (M MM )/M uu = 100 × (M uu − )/Mu0 u0 u − u0u0 u0 u0
(1) (1)
Table 3. Summary of test results. Table 3. Summary of test results. Specimen Specimen Specimen
Failure M /M u0 M M Mcrcrcr Failure /M M Failure MppM /Mpu0 u0 cr0 cr0 Mcr0 M Mode [kNm] [kNm] [kNm] mode [%] mode [%][%] [kNm] [kNm] [kNm] R 25 13.0 22.0 R 11R 1 25 25 13.0 22.0 13.0 22.0 R 25 13.0 26.0 RR 25 25 13.0 26.0 13.0 26.0
NSM12A NSM12A NSM12A NSM12B NSM12B NSM12B NSM12BNSM12BNSM12B-L R RR L L NSM16A R + CC 2 2 NSM16A NSM16A R ++ CC CC NSM16B R R2 NSM16B R NSM16B RR NSM16B-L NSM16BNSM16BR R L L
60 60 60
13.0 13.0 13.0
14 14 14 14 14 14 60
15.0 15.0 15.0 15.0 15.0 15.0 15.0
1
60 60 11
-- 27.3 27.3 27.3 29.0 29.0 29.0 -
∆Mcrcrcr ΔM ΔM [%] [%] [%] 69.2 69.2 69.2 100.0 100.0 100.0
M M y0 My0 y0 [kNm] [kNm] [kNm] 46.5 46.5 46.5 49.5 49.5 49.5
--82.0 82.0 82.0 93.3 93.3 93.3 -
49.5 49.5 49.5
85.2
85.2 85.2 83.0 83.0 83.0 83.0
M M Myyy [kNm] [kNm] [kNm]
∆M ΔM ΔMyyy [%] [%] [%]
70.0 70.0 70.0 61.5 61.5 61.5 58.5 58.5 58.5 100.0 100.0 100.0 98.0 98.0 98.0 100.1
50.5 50.5 50.5 24.0 24.0 24.0 18.0 18.0 18.0 18.0 18.0 18.0 18.1 18.1 18.1 21.0
46.7 46.7 46.7 49.7 49.7 49.7 49.7 49.7 49.7 86.0 86.0 86.0 83.3 83.3 83.3 83.3
21.0 21.0
83.3 83.3
15.0 --of CFRP, 2-- CC—Concrete 83.0 100.1 15.0 83.0 100.1 R—Rupture crushing.
R—Rupture of CFRP, 22 CC—Concrete crushing.
MM M uM ∆MΔM u0 u uu u0 M Muu ΔM u0 [kNm] [kNm] [kNm] [%] [kNm] [kNm] [kNm] [%] [%] 110.2 135.0 110.2 135.0 135.0 110.2 97.0 95.2 97.0 95.2 97.0 95.2 98.0 97.0 98.0 97.0 98.0 97.0 146.9 70.8 146.9 70.8 146.9 70.8 130.0 56.1 130.0 56.1 130.0 59.7 56.1 133.0 133.0 133.0
59.7 59.7
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Test results confirmed a common opinion about dependence of the strengthening efficiency on the internal steel reinforcement ratio. Beams with a lower steel reinforcement ratio (NSM12) demonstrated the greatest increase of load bearing capacity: ranging from 97% and 135%, while the beams with the higher reinforcement ratio (NSM16) exhibited only about one-half improvement: ranging from 56% to 70.8%. The research revealed a very interesting result with respect to the preloading effect, which had an insignificant influence on the load bearing capacity. This conclusion is very promising for practical applications of NSM strengthening even for structures having relatively high dead load to residual capacity ratios. The increase in the cracking moment was more affected by the number of pretensioned CFRP strips than by a total number of the NSM strips. This was confirmed by an increase in the cracking moment of 69.2% for beam NSM12A (strengthened with one pretensioned strip and two non-pretensioned ones) and 100% for the beam NSM12B (strengthened with two pretensioned strips). However, for the beams with the higher internal reinforcement ratio this increase was lower and equal to 82% and 93.3% for the beam NSM16A and NSM16B (Table 3). The steel yielding moment was sensitive to the total number of strips only in the beams with a lower reinforcement ratio reaching increases of the steel yielding moment of 50.5% and 24%, respectively, for the beams strengthened with three (NSM12A) and two CFRP strips (NSM12B) relative to the analytically-determined value of yield capacity. No influence of the number of strips on the steel yielding bending moment was observed in beams NSM16A and NSM16B, which confirms the influence of the internal steel reinforcement ratio on the steel yielding moment (Table 3). In the specimens of series A, the CFRP strains after pretensioning stabilized at values of 0.0054 and 0.0061, respectively, for beams NSM12A and NSM16A (Table 4). While in the specimens strengthened with two pretensioned CFRP strips (series B), the CFRP pretensioning strains stabilized when they reached values of 0.0046 and 0.0054, respectively, for beams with lower (NSM12B) and higher (NSM16B) internal reinforcement ratios. The ultimate strains in the CFRP strips in the beams of series A reached values of 0.0186 and 0.0171 in the pretensioned CFRP strips and between 0.0149 and 0.0179 in the non-pretensioned CFRP strips. Similarly, in the specimens of series B the maximum strains in the tests ranged between 0.0147 and 0.0183. The CFRP strips in the beams of series A deformed uniformly across the section, as shown by strain measurements on all three CFRP strips. Irrespective of the internal steel reinforcement ratio (NSM12, NSM16) or type of the CFRP strip (pretensioned (CFRP1), non-pretensioned (CFRP2, CFRP3)), the slopes of the CFRP strain curves as functions of the bending moment were very similar (Figure 9). The offset of the curve for CFRP1 results from the pretensioning of this strip to 0.0054 and 0.0061 in beams NSM12A and NSM16A, respectively. From the comparison of strain curves of pretensioned (CFRP1) and non-pretensioned (CFRP2, CFRP3) strips the moment when the pretensioned strip ruptured, resulting in stress redistribution to the remaining two strips took over is evident (see the curves of CFRP2 and CFRP3 with an abrupt drop of the bending moment at strains equal to 0.0149 and 0.0164, respectively, in beam NSM12A, and at 0.0179 and 0.0177 in beam NSM16A). The strain curves of the pretensioned CFRP strips in the beams of series B, strengthened under only self-weight (NSM12B) demonstrated uniform deformation of both strips (Figure 10a), while the corresponding strain curves in the beam preloaded before strengthening (NSM12B-L) shows a difference in the CFRP strains (Figure 10b). The vertical translation of the curves in Figures 10 and 11 represents the bending moment caused by preloading the specimens. The difference in strains seen in Figure 10b was caused by the step-wise bonding the pretension strips. After installation of the first strip, the second was pretensioned and bonded resulting in an additional prestress applied to the beam and a resulting decrease in the pretension of the first strip. This difference in the beam NSM16B-L is reduced when the internal steel starts to yield (Figure 11b). However, in the beam NSM12B-L the difference in strain curves between strips remains for the full range of loading. The difference between NSM12B-L and NSM16B-L is due to the relative contribution of the CFRP strips in resisting the tensile
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the tensile forces: the CFRP contribution in beam NSM12B-L, having a lower internal reinforcement the tensile forces: the CFRP contribution in beam NSM12B-L, lower internal reinforcement forces: the CFRP contribution in beam NSM12B-L, having ahaving lower ainternal reinforcement ratio, is ratio, is greater than that of beam NSM16B-L. ratio,than is greater than thatNSM16B-L. of beam NSM16B-L. greater that of beam
100 100
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CFRP1 CFRP1 CFRP2 CFRP2 CFRP3 CFRP3 5.0 10.0 15.0 20.0 5.0 10.0 15.0 20.0 CFRP strain, εfs (‰) CFRP strain, εfs (‰)
(a) (a)
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Figure 9. Bending moment vs. CFRP strain curves in beams of series A: (a) NSM12A; and (b) NSM16A. Figure 9. Bending moment vs. CFRP strain curves in beams of series A: (a) NSM12A; and (b) NSM16A. Figure 9. Bending moment vs. CFRP strain curves in beams of series A: (a) NSM12A; and (b) NSM16A. 100 100
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10.0 15.0 10.0 15.0 CFRP strain, εfs (‰) CFRP strain, εfs (‰)
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Figure 10. Bending moment vs. CFRP strain curves of series B: (a) NSM12B; and (b) NSM12B-L. Figure 10.10. Bending series B: B:(a) (a)NSM12B; NSM12B;and and(b) (b)NSM12B-L. NSM12B-L. Figure Bendingmoment momentvs. vs.CFRP CFRPstrain strain curves curves of series
Pretensioning the strips resulted in a camber (negative vertical displacement) occurring during Pretensioning thestrips stripsresulted resultedin inaacamber camber (negative (negative vertical displacement) occurring during vertical displacement) occurring during thePretensioning pretensioningthe process. Values of camber resulting from pretensioning (vp), bending moment the pretensioning process. Values of camber resulting from pretensioning (v p), bending moment theMpretensioning process. Values of camber resulting from pretensioninglimit (vp ),state bending (L/200), corresponding to the allowable deflection based on the serviceability (va = 30moment mm), M(L/200), corresponding to the allowable deflection based on the serviceability limit state (va = 30 mm), M(L/200) , corresponding to the allowable based onmoment, the serviceability state (vain = Table 30 mm), and the deflection corresponding to thedeflection ultimate bending vM(max), are limit summarized and the deflection corresponding to the ultimate bending moment, vM(max), are summarized in Table and correspondingwith to the ultimate bendingstrip moment, v A) differed , are summarized in Table 4. 4. the Thedeflection beams strengthened one pretensioned (series in their internal 4. The beams strengthened with one pretensioned strip (seriesM(max) A) differed in their internal reinforcement ratio, butwith exhibited a camber equal to 1.9 and 1.7 respectively, for beams NSM12A The beams strengthened one pretensioned strip (series A) mm, differed in their internal reinforcement reinforcement ratio, but exhibited a camber equal to 1.9 and 1.7 mm, respectively, for beams NSM12A andbut NSM16B. Asaexpected, beams strengthened with two pretensioned strips exhibited ratio, exhibited camber equal to 1.9 and 1.7 mm, respectively, for beams NSM12A and greater NSM16B. and NSM16B. As expected, beams strengthened with two pretensioned strips exhibited greater camber: 6.4 mm for beam NSM12B and 3.6 mm for beam NSM16B. The beams preloaded As camber: expected, strengthened withand two3.6 pretensioned strips exhibited 6.4before mm for 6.4beams mm for beam NSM12B mm for beam NSM16B. Thegreater beams camber: preloaded before strengthening were cracked and, therefore, exhibited a reduced stiffness, which led to greater camber beam NSM12B and 3.6 mm for beam NSM16B. The beams preloaded before strengthening were cracked strengthening were cracked and, therefore, exhibited a reduced stiffness, which led to greater camber following pretensioning: 7.7 and 5.2 mm, respectively, forgreater beams NSM12B-L and NSM16B-L (Table7.7 and, therefore, exhibited a reduced stiffness, which led to following pretensioning: following pretensioning: 7.7 and 5.2 mm, respectively, for beamscamber NSM12B-L and NSM16B-L (Table 4). and4).5.2 mm, respectively, for beams NSM12B-L and NSM16B-L (Table 4).
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140 NSM16B
NSM16B-L 120
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15.0
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CFRP strain, εfs (‰)
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(b)
Figure 11. Bending moment vs. CFRP strain curves of series B: (a) NSM16B; and (b) NSM16B-L. Figure 11. Bending moment vs. CFRP strain curves of series B: (a) NSM16B; and (b) NSM16B-L. Table 4. Summary of CFRP strain and deflections after pretensioning. Table 4. Summary of CFRP strain and deflections after pretensioning. Specimen
Preloading
NSM12A Preloading 0.25Mu0 Specimen NSM16A NSM12B NSM12A NSM12B-L NSM16A NSM16B NSM12B NSM16B-L
0.14Mu0 0.25Mu0 0.25Mu0 0.60Mu0 0.14Mu0 0.14Mu0 0.25M u0 u0 0.60M
CFRP strain after pretensioning εfp [‰] σfp [MPa] CFRP Strain after Pretensioning 5.4
εfp [‰] 6.1 4.6 4.7 5.4 5.1 4.3 6.1 5.4 5.4 4.6 5.2 4.7 6.1
864
σfp [MPa] 976 736 752 864 816 688 976 864 704 736 752 826 973
vp [mm] vp −1.9 [mm] −1.7 −6.4 −1.9 −7.7 − 1.7 −3.6 − 6.4 −5.2
M(L/200) ΔM(L/200) vM(max) [kNm] [%] [mm] v M ∆M 41.0(L/200) 70.0 (L/200) 258M(max) [mm] [kNm] [%] 51.0 44.0 245 44.0 83.0 256 41.0 70.0 258 26.0 250 51.0 44.0 245 60.0 69.0 230 44.0 83.0 256 35.0 236
NSM12B-L 0.60Mu0 5.1 4.3 816 688 −7.7 NSM16B 0.14Mu0 5.4 5.4 864 704 −3.6 A comparison of vertical displacements for the beams with NSM16B-L 0.60Mu0 5.2 6.1 826 973 −5.2lower
26.0 250 60.0 69.0 230 and higher reinforcement 35.0 236
ratios are shown in Figure 12a,b, respectively. The vertical translation of the curves in Figure 12 represents the bending moment caused by preloading the specimens. The curves of the beams A comparison of vertical displacements forsimilar the beams with and higher reinforcement ratios pretensioned under only self-weight are very before thelower steel yields, however, they diverge areafter shown in Figure 12a,b, respectively. The vertical translation of the curves in Figure 12 represents steel yielding. The similar behavior before yield illustrates that the presence of NSM CFRP,the bending moment causedhas by little preloading thethe specimens. of the beams pretensioned under regardless of amount, effect on uncrackedThe andcurves partially-cracked (i.e., pre steel yield) only self-weight are very similar before the however, they divergethe after steel yielding. stiffness of the cross-sections. Following the steel yieldyields, of the internal reinforcement, CFRP provides allsimilar tensilebehavior stiffness and, therefore, beam stiffness is expected greater forregardless the beam strengthened The before yield illustrates that the presence to of be NSM CFRP, of amount, has with three rather than two, CFRP strips (NSM12B). The ultimate capacity is similarly little effect on(NSM12A), the uncracked and partially-cracked (i.e., pre steel yield) stiffness of the cross-sections. affected:the beam NSM12B at areinforcement, lower ultimatethe load thanprovides beam NSM12A. Similar observations are Following yield of the failed internal CFRP all tensile stiffness and, therefore, made from the load-displacement curves of the displacement for the beams reinforced with a higher beam stiffness is expected to be greater for the beam strengthened with three (NSM12A), rather than reinforcement (NSM16). greatercapacity externalispreloading did not influence the flexural two, CFRP stripsratio (NSM12B). TheThe ultimate similarly affected: beam NSM12B failedbehavior at a lower of the tested specimens (NSM12B-L and NSM16B-L). ultimate load than beam NSM12A. Similar observations are made from the load-displacement curves Following a decrease in the existing deflections due to prestressing in the specimens of the displacement for the beams reinforced with a higher reinforcement ratio (NSM16).NSM12B-L The greater and NSM16B-L (vp in Table 4), the slope of the bending moment vs. deflection curves is the same as external preloading did not influence the flexural behavior of the tested specimens (NSM12B-L and those of the beams strengthened under only self-weight. In Figure 12, a dashed vertical line shows NSM16B-L). the allowable deflection at the service limits state (L/200 = 30 mm). The increase in the bending Following a decrease in the existing deflections due to prestressing in the specimens NSM12B-L moment corresponding to the allowable deflection ranged from 70% and 83% for beams NSM12A and NSM16B-L (vp in Table 4), the slope of the bending moment vs. deflection curves is the same as and NSM12B, and from 44% and 69% for beams NSM16B and NSM16A. This confirms a beneficial those of the beams strengthened under only self-weight. In Figure 12, a dashed vertical line shows influence of prestressing on the overall flexural behavior and stiffness of the beams initially preloaded theunder allowable deflection atThe thedisplacement service limitsvs. state (L/200 = 30 mm). The in the bending only self-weight. bending moment curves forincrease both NSM12B-L and moment corresponding to the allowable deflection ranged from 70% and 83% for beams and NSM16B-L beams confirm the beneficial effect of prestressing, which is a very promisingNSM12A conclusion NSM12B, and from 44% and beams NSM16B confirms a beneficial influence for practical application of 69% NSMfor strengthening evenand for NSM16A. structures This having relatively high dead load of to prestressing on the overall flexural behavior and stiffness of the beams initially preloaded under residual capacity ratios. only self-weight. The displacement vs. bending moment curves for both NSM12B-L and NSM16B-L beams confirm the beneficial effect of prestressing, which is a very promising conclusion for practical
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application of NSM strengthening even for structures having relatively high dead load to residual Polymers 2018, 10, 45 12 of 14 capacity ratios. 120 vSLS=30mm 100
Moment, M (kNm)
80 150% My0 124% My0 60
118% My0
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My0=46.5 170% MSLS
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60% My0
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NSM16A NSM16B NSM16B-L
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Vertical displacement, v (mm) (b)
Figure 12.12.Bending curves:(a) (a)Series SeriesA;A;and and Series Figure Bendingmoment momentvs. vs.vertical vertical displacement displacement curves: (b)(b) Series B. B.
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5. Conclusions This study provided several important conclusions concerning the factors influencing the efficiency of pretensioned CFRP strips for strengthening reinforced concrete members:
•
• •
•
The high efficiency of the pretensioning technique for flexural strengthening with NSM CFRP was confirmed by high strengthening ratios, ranging from 56% to 135%. The strengthening ratio that may be achieved is shown to be inversely proportional to the existing internal steel reinforcement ratio. A different preloading history for beams NSM12B-L and NSM16B-L did not affect the flexural behavior of these specimens during subsequent loading. Pretensioning of the CFRP reinforcement enhances the beam performance at the serviceability limit state. This was demonstrated by an increase of bending moment corresponding to the allowable deflection of 70%. This also applies to members having large preload prior to strengthening. Moreover, it is the overall composite reinforcement ratio, rather than number of pretensioned strips, that effects the final increase in load bearing capacity. The results of the tests indicated high efficiency of the novel pretensioning system invented by the authors, which makes it possible to apply NSM CFRP without any anchorage systems.
6. Patents The research disseminate the Patent P.407898, 14 April 2014. Acknowledgments: The authors wish to acknowledge the research grant “Innovative Measures and Effective Methods of Improving Safety and Sustainability of Construction Works and Transport Infrastructure in Sustainable Development Strategy”, co-financed by the European Regional Development Fund within the Operational Program Innovative Economy. Author Contributions: Renata Kotynia and Marta Przygocka conceived and designed the experiments, performed the experiments, analyzed the data, contributed reagents/materials/analysis tools, and wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.
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Rezazadeh, M.; Costa, I.; Barros, J. Influence of prestress level on NSM CFRP laminates for the flexural strengthening of RC beams. Compos. Struct. 2014, 116, 489–500. [CrossRef] Lee, H.Y.; Jung, W.T.; Chung, W. Flexural strengthening of reinforced concrete beams with pre-stressed near surface mounted CFRP systems. Compos. Struct. 2017, 163, 1–12. [CrossRef] Al-Saadia, N.T.K.; Mohammeda, A.; Al-Mahaidia, R. Bond performance of NSM CFRP strips embedded in concrete using direct pull-out testing with cementitious adhesive made with graphene oxide. Constr. Build. Mater. 2018, 162, 523–533. [CrossRef] Kotynia, R.; Lasek, K. Anchoring and pretensioning system for the plates in particular near surface mounded composite strips. PŁ Patent P.407898, 14 April 2014. Testing Hardened Concrete. Compressive Strength of Test Specimens; PN-EN 12390-3; Polish Committee for Standardization: Warszawa, Poland, 2011. Testing Hardened Concrete. Tensile Splitting Strength of Test Specimens; PN-EN 12390-6; Polish Committee for Standardization: Warszawa, Poland, 2011. Steel for the Reinforcement and Prestressing of Concrete—Test Methods—Part 1: Reinforcing Bars, Wire Rod and Wire; PN-EN ISO 15630-1; Polish Committee for Standardization: Warszawa, Poland, 2014. Guide Test Methods for Fiber-Reinforced Polymers (FRPs) for Reinforcing and Strengthening Concrete Structures; ACI Committee 440; American Concrete Institute: Farmington Hills, MI, USA, 2004. Kotynia, R.; Lasek, K.; Staskiewicz, M. Flexural behavior of preloaded (RC) slabs strengthened with prestressed CFRP laminates. J. Compos. Constr. 2013, 18, 1–11. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
