Experimental tests on existing RC beams strengthened in flexure and retrofitted for shear by CFRP in presence of negative moments
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Abstract
The shear strength of reinforced concrete beams extracted from existing buildings often reveals insufficient transversal steel reinforcement, mainly due to design or construction defects or increased design load requirements. FRP wrapping is one of the best solutions to improve beam shear strength as the retrofitting intervention is fast and the cost is modest. Design codes provide clear indication about the retrofitting design of simply supported beams, while the case of a beam with negative moments at the end is not considered, although this is in the case of a beam in a framed structure. One of the main uncertainties lies in the effectiveness of the FRP U sheet anchorage behavior in the area of negative bending moments with cracked concrete. This may limit the shear strength of the retrofitted beam. In this study, two beams extracted from an existing building constructed in the 1930s in Rome and retrofitted by carbon fiberreinforced polymer (CFRP) U strips placed at beam ends, where also negative bending moments were present, and have been evaluated with experimental tests at the laboratory of the Department of Architecture of Roma Tre University. Beam steel and concrete characteristics were evaluated by means of different tests. The experimental results are discussed considering the final results in terms of maximum shear resistance in the presence of negative bending moments. Load deflections at different points along the beam, shearCFRP deformation along the reinforcement strips and the damage state for different load levels, are presented. The importance of avoiding possible fragile mechanisms in the sections retrofitted with FRP is clearly shown.
Keywords
Existing building RC beams FRP Experimental test Strengthening End anchorageIntroduction
Recent earthquakes which hit Italy Between August and October 2016 (Fiorentino et al. 2017) demonstrated the high vulnerability of the Italian building stock. This vulnerability is evident for masonry and for reinforced concrete (RC) structures which were designed and built using old design criteria that heavily underestimated the seismic actions.
The use of FRP materials can be a valid solution for the retrofitting of existing RC buildings and bridges (Albanesi et al. 2008, 2009; Lavorato and Nuti 2010, 2011, 2015; Lavorato et al. 2010, 2015, 2017a; Marano et al. 2017). Many existing RC beams have insufficient shear reinforcement, due to design or construction defects or increased design load requirements. It is worth noting that infill panels can interact with the structural elements and influence their shear strength (Fiore et al. 2016).
The beam retrofitting with carbon fiberreinforced polymer (CFRP) sheets or strips is a valid solution to increase the shear strength. The retrofitting intervention with CFRP is fast and the cost is modest. International and National Guidelines and Codes (CNRDT 200 R1/2013 2013; Fib 2001; Italian Council of Public Works 2009) give design prescriptions for shear retrofitting of beams by CFRP which are essentially based on exhaustive experimental data for simply supported beams only.
Key example tests from the literature have been conducted by Pellegrino and Modena (2000), Adhikary and Mutsuyoshi (2004), Carolin and Taljsten (2005), Zhang and Hsu (2005), Barros and Dias (2006), Guadagnini et al. (2006), Monti and Liotta (2007), Bousselham and Chaallal (2008) and Garcez et al. (2008).
In those cases, the new CFRP shear reinforcements are placed, where moments are negligible and anchored in the compressed upper part of the section in absence of concrete cracking.
RC frames have beams in which the shear forces are combined with negative bending moments at supports with consequent concrete cracking on the upper part of the section (in tension), where the CFRP anchorages are placed. As a consequence, specific tests are required to validate CFRP anchorage behavior.
Few studies deal with this issue and new research efforts seem useful. In the scientific literature, Khalifa and Nanni (2000) tested nine continuous twospan beams with different CFRP amounts and wrapping schemes highlighting that the Uwrap CFRP reinforcement does not seem to reduce cracks, even if an overall increase of the shear strength is observed.
In the present paper, the behavior of existing RC beams reinforced for shear by CFRP U strips placed in negative moment sections and anchored under the slab was investigated by experimental tests.
An existing RC building in Rome, dated around year 1930, had to be upgraded to withstand seismic actions and vertical loads according to the Italian Design Code (OPCM 2003; NTC 2008). Although for very extended buildings, as the case study presented herein, there could be spatial variability of the strong motion (Trifunac and Todorowska 1997) which should be taken into account by means of appropriate methods (Lavorato et al. 2017b), this was not taken into account in the retrofitting of this particular building. New RC shear walls and slabs had to be inserted and the application of CFRP wrapping on existing columns and Ushape strips on beams was necessary for confinement and shear strength, after the construction of an additional slab which increased bending resistance and consequently shear demands.
The beams were integrated with a new cantilever at one support to reproduce, in the lab, the negative moments and shear due to adequate vertical concentrated loads.
The original removed beams
Geometrical properties and steel reinforcement
Geometrical properties and steel reinforcement of beams TM1 and TM2 (mm)
Material properties
Material properties for beams TM1 and TM2
Concrete  Reinforcement  

f_{cm} (MPa)  Dev. St  Specimen label  ø (mm)  σ_{y} (MPa)  σ_{r} (MPa)  A_{gt} (%)  
TM1  16.38  4.78  Transversal steel  FV_8  8  354.13  472.72  19.64 
FV_12  12  347.61  494.25  9.25  
TM2  18.16  –  Longitudinal steel  FV_24  24  265.82  404.87  23.20 
Interventions on the original beams
The intervention consisted in the addition of a cantilever and retrofitting as in the real frame structure building, i.e., for bending and shear. The cantilever was added at one end of the beams to apply negative moments by means of a monodirectional force actuator (“Additional RC cantilever”). Therefore, the two beams have a stress condition which is similar to the real building, with simultaneous presence of shear and negative moment at the support.
The beam TM1 was tested twice. The first time the beam (labelled STM1A) showed a fragile failure due to collapse of the compressed concrete at the support, and in the added upper slab. Subsequently, the same beam (labelled STM1B) was tested again after adding a new cantilever on the undamaged opposite end with a proper reinforcement in bending for the new slab and the new cantilever and a proper distribution of the connectors between the new and the existing beam slab.
The second beam TM2 was retrofitted, taking into account the previous experience on beam TM1 and tested as STM2.
Additional RC cantilever
The cantilever lengths L2 are equal to 1.82 m for the beam STM1A and 1.53 m for the beams STM1B and STM2. These differences in the test geometry are due to practical needs for the construction of the cantilevers in the lab, as they were built in different dates. Other differences are in the steel arrangement to improve the reinforcement configuration on the base of the experience acquired during the test on the first beam STM1A.
Added RC cantilevers: concrete and reinforcement mechanical properties, section geometries

longitudinal upper rebars: 4ø24 sidewelded to the original 4ø24 upper rebars of TM1 beam;

longitudinal lower rebars: 2ø12 anchored chemically for a length of 250 mm, drilled in the original beam;

stirrups: ø12/150 mm in the web,

longitudinal bars 4ø12 and stirrups ø8/150 mm in the flange.

longitudinal upper rebars: 4ø18 buttwelded to the upper reinforcement of the existing beams: 4ø24;

longitudinal lower rebars: 2ø18 buttwelded to the original lower reinforcement of the existing beam: 2ø24;

stirrups ø10/200 mm in the web;

longitudinal bars 4ø12 and stirrups ø10/200 mm in the cantilever flange.
The need of a different arrangement in the bottom cantilever reinforcement with respect to STM1A arose after the test of STM1A. In fact, there was failure of bottom concrete of the original TM1 beam which crashed, where cantilever bottom bars, compressed, ended at 250 mm from the support. It was, therefore, decided the buttwelding of the cantilever bottom reinforcement to the bottom reinforcement of the original beam, while larger diameter bars were used, ø18 instead of ø12, to reduce compression side failure probability. The steel reinforcement of the added cantilever was of the typical B450C type (characteristic value of steel yield strength f_{yk} = 450 MPa).
The longitudinal ø18 bars and the ø24 rebars of the original beams are more or less equivalent in terms of strength, because the yielding strength of B450C steel type is higher with respect to the one of the original smooth rebars. On the other hand, the smaller diameter simplified the buttwelding connection.
Flexural strengthening
In beam STM1A, the connection between the beam and the additional slab is simply straight vertical rebars ø12/200 mm, see Fig. 3, top and Fig. 4, left, limited to part of the zones of the additional longitudinal reinforcement. These latter consisted in 2 + 2 ø20, which was limited to the cantilever zone and to the final part of the slab opposite to the cantilever.
The additional slab reinforcement consisted of electrowelded mesh (ø8/200 × 200 mm) disposed along the entire RC slab between the lower (2 ø20) and the upper (2 ø20) longitudinal rebar layers.
STM1B is obtained from the same beam TM1 after testing STM1A, removing the slab, the cantilever, adding a cantilever on the opposite side and a new slab, with better reinforcement details. For the beam STM1B, the new RC slab had two sets of connectors (ø12/125 mm) with buttwelded steel plate and Ushape rebars (as stirrups, ø12/125 mm) anchored chemically to connect the new RC slab to the existing one. The connectors, the slab main longitudinal reinforcement (6 ø16), and two layers of electrowelded net (ø6/200 × 200 mm, below and above the longitudinal reinforcement) are disposed along the whole length of the beam (Figs. 3, bottom, Fig. 4 right).
Retrofitted beams STM1A, STM1B, and STM2: mechanical properties of concrete and reinforcement of the new RC slab
Concrete  Steel reinforcement  

R_{cm} (MPa)  Italian class or experimental properties  
RC slab of retrofitted beams  
STM1A  61.70  Connectors  B450C  
ø (mm)  σ_{r} (MPa)  A_{gt} (%)  
Electrowelded net  8  587.49  8.56  
Long. rebars  20  594.55  12.97  
STM1B STM2  31.22  Connectors  B450C  
Electrowelded net  B450C  
Long. rebars  B450C 
 1.
Anchorage of two sets of connectors ø12/200 mm at the beam extrados in correspondence of each beam supports (points A and B, Fig. 2); these connectors were realized with simple rebar segments.
 2.
Placing of 2 ø20 longitudinal rebars in correspondence of each beam supports (points A and B, Fig. 2).
 3.
Placing of one layer of electrowelded net ø8/200 × 200 mm above the longitudinal rebars.
 4.
Placing of another 2ø20 longitudinal rebars in correspondence of each supports (points A and B, Fig. 2) on the electrowelded net.
 5.
Casting of lightweight concrete Leca CLS 1800^{®} (http://www.leca.it).

Anchorage of two lines of connectors ø12/125 mm along the entire beam extrados; each connector free end has circular buttwelded plate ø30 with thickness equal to 5 mm.

Placing of one layer of electrowelded net ø6/200 × 200 mm along the entire slab.

Placing of 6ø16 longitudinal bars along the entire slab.

Placing of another layer of electrowelded net ø6/200 × 200 mm along the entire slab.

Chemical anchorage of stirrups (Ushaped rebars) ø12/125 mm along the entire beam extrados.

Casting of lightweight concrete class C28/35.
Shear strengthening
The compressed concrete strut contribution (V_{Rd,c}) and the transversal steel contribution (V_{Rd,s}) considering a concrete shear crack angle equal to 45°. They result 522 and 80–150 kN (depending on possible stirrups’ diameter; see "The original removed beams"), respectively.
The applied CFRP shear reinforcement for each beam specimen is made of eight CFRP U strips (Fig. 6, left) applied along the beam ends for a length of 140 cm starting from both the beam supports. Each U strip is composed by two sheets which cross at 45° on the intrados of the beam (Fig. 6, right). The CFRP U strips are made of commercial product Carbostru^{®} (https://www.interbausrl.it) UDHM 400 (weight of 400 g/m^{2}, tensile strength of 3000 MPa, elastic modulus of 390 GPa, and equivalent thickness t_{f} = 0.225 mm according to the data from technical datasheet). The CFRP strips have a width w_{f} = 100 mm, the spacing of the strips is p_{f} = 212 mm, and the strips distance is p_{f}′ = 300 mm (Fig. 6).
 1.
Mechanical removal of the external grout for a length of 140 cm from the supports.
 2.
Leveling of the concrete surface.
 3.
Regularization of the surface by plaster.
 4.
Application of a thixotropic plaster (epoxy resin, Sikadur30^{®}, https://ita.sika.com/), having high strength and low shrinkage on the concrete surface.
 5.
Application of the CFRP strips using the epoxy resin Carbostru^{®} RS85 (https://www.interbausrl.it).
CFRP coupon geometry and mechanical properties width (b), thickness (s), fiber area (A_{fib}), composite area (A_{com}), failure force (F), maximum deformation (ε_{max}), fiber stress (σ_{rfib}), composite stress (σ_{rcom}), fiber elastic modulus (E_{fib}), and composite elastic modulus (E_{com})
Specimen  b (mm)  s (mm)  A_{fib} (mm^{2})  A_{com} (mm^{2})  F (kN)  ε_{max} (%)  σ_{rfib} (MPa)  σ_{rcom} (MPa)  E_{fib} (GPa)  E_{com} (GPa) 

T_01  16.30  4.1  10.76  66.2  28.24  0.7846  2625.02  426.73  335  54 
T_02  17.70  3.7  11.68  66.0  28.11  0.6660  2406.61  425.83  361  64 
T_03  18.45  3.7  12.18  68.8  44.78  0.8435  3677.66  650.74  436  77 
The effective design strength of FRP (f_{fed}) is assumed equal to 541 Mpa, the concrete tensile strength (f_{ctm}) is equal to 2 MPa (a conservative value), and the concrete compressive strength is equal to 16.6 MPa. The concrete strut angle is equal to 45° according to Italian Code prescription and the safety coefficients γ_{f,d}, γ_{c,} and γ_{R,d} are assumed equal to 1. The resulting CFRP reinforcement theoretical shear strength [V_{Rd,f}, Eq. (2)] results equal to 116 kN.
The two resulting shear strength values for the beam (V_{Rd,s} + V_{Rd,f}) considering the two possible stirrups’ arrangements (see “The original removed beams”) and so the two possible values of the stirrups’ shear contributions (V_{Rd,s} = 80–150) results equal to 196–266 kN. The large scatter for the stirrups’ shear contributions depends on the possible stirrup diameter. The stirrups’ rebar diameter is difficult to evaluate by nondestructive tests.
Tests: set up for loadings and applied load histories
Two set of tests are presented: the first one was carried out after the construction of the cantilever to evaluate the stiffness of the beam in the elastic field before retrofitting and the second one was performed until failure to check the ultimate behavior of the retrofitted beams. The latter was performed with special attention on the behavior of the CFRP strip contribution to beam shear strength at support B, where the CFRP reinforcement was anchored in a zone with negative bending moment. Some details about the tests are reported in Nuti et al. (2010, 2014).
The simply supported beam with cantilever is an isostatic load scheme, and therefore, internal forces, bending moment, and shear depend on the position and intensity of the external applied loads only, as shown in Fig. 2. Two loads have been applied: F1 on the beam span and F2 on the cantilever end. They are increased in steps to obtain at each loading step the desired values of shear and moment at support B.
Experimental setup for loading and measurements

National Instruments DAQ station SCXI 1001 with DAQ moduli SCXI 1314/1520.

National Instruments DAQ board 6281 (18 bit).

LabView acquisition program.

24 potentiometers Penny–Giles on the two beam sides (10 with stroke ± 25.0 mm and 14 with stroke ± 50.0 mm); 12 potentiometers for the beam vertical displacements along the span at 0.2–0.4–0.6 L (load F1 application point)—0.8 L from the simple support without cantilever and along the cantilever at 0.5 L2—L2 (load F2 application point) (Fig. 7); four potentiometers for the beam support vertical displacements; and eight potentiometers for the beam shear deformations (diagonal direction) beside the CFRP strips

18 strain gauges (10 mm grid) placed at 45° on the CFRP strips f4, f3, f2, and f1 (Fig. 7) near support B (six for strip and three for each side).
Load steps for elastic tests before retrofitting
Theoretical bending and shear strength of the beams before retrofitting (assuming 8 mm for stirrups’ diameter)
Beam  M_{uc} (kN m)  M_{ub} (kN m)  V_{u} (kN m) 

TM1  254  248  89 
TM2  251  243  87 
Loading steps for the external forces F1 and F2 (F_{1hyp}: desired applied; F_{1ef}: effectively applied load) and corresponding internal forces (shear T, bending moment M) at beam support B and C in the elastic tests before retrofitting
Step  F_{1hyp} (kN)  TM1  TM2  

F_{1ef} (kN)  F_{2} (kN)  T_{Bsx} (kN)  M_{C} (kN × m)  M_{B} (kN × m)  F_{1ef} (kN)  F_{2} (kN)  T_{Bsx} (kN)  M_{C} (kN × m)  M_{B} (kN × m)  
1  20.00  23.00  0.00  − 13.73  23.35  0.00  32.00  0.00  − 19.11  32.49  0.00 
2  20.00  29.50  9.00  − 22.32  18.10  − 19.85  36.00  9.00  − 26.20  24.70  − 19.85 
3  40.00  52.50  9.00  − 36.05  41.45  − 19.85  45.00  9.00  − 31.57  33.83  − 19.85 
4  40.00  58.00  18.00  − 44.04  35.18  − 39.69  51.00  18.00  − 39.86  28.07  − 39.69 
5  60.00  64.50  18.00  − 47.92  41.78  − 39.69  60.00  18.00  − 45.23  37.21  − 39.69 
6  60.00  73.00  27.00  − 57.70  38.56  − 59.54  66.00  27.00  − 53.52  31.45  − 59.54 
7  80.00  79.00  27.00  − 61.28  44.65  − 59.54  80.00  27.00  − 61.88  45.66  − 59.54 
8  80.00  88.00  36.00  − 71.36  41.93  − 79.38  87.00  36.00  − 70.76  40.92  − 79.38 
Load steps for failure tests on the retrofitted beams
Theoretical bending resistance at midspan and bending and shear resistance at left of support B after retrofitting
Beam  M_{uC} (kN m)  M_{uB} (kN m)  V_{u} (kN) 

TM1  291  457  215 
Loading steps differ among the two elastic tests before retrofitting and tests to failure after retrofitting, because the geometry of the loads was different. Small difference in the geometries of beam STM1A and the couple STM1B, STM2, existed as well, therefore, the loading steps were slightly different.
The goal of the failure test for each beam is the evaluation of the bending and shear strength on the left side of beam support B.
STM1A loading steps and corresponding internal forces in the elastic tests before retrofitting
Step  \(F_{1}^{\text{hyp}}\) (kN)  TM1  

\(F_{1}^{\text{ef}}\) (kN)  F_{2} (kN)  T_{Bsx} (kN)  T_{Bsx}/T_{Bu} (%)  M_{C} (kN × m)  M_{C}/M_{Cu} (%)  M_{B} (kN × m)  M_{B}/M_{Bu} (%)  
1  20.00  24.40  0.00  − 14.28  6.60  26.00  8.93  0.00  0.00 
2  20.00  35.90  20.00  − 30.49  14.18  13.90  4.77  − 41.60  9.10 
3  50.00  47.90  20.00  − 37.52  17.45  26.68  9.17  − 41.60  9.10 
4  50.00  61.90  40.00  − 55.19  25.67  17.25  5.93  − 83.20  18.21 
5  80.00  79.00  40.00  − 65.20  30.32  35.46  12.18  − 83.20  18.21 
6  80.00  92.50  60.00  − 82.58  38.41  25.50  8.76  − 124.80  27.31 
7  110.00  108.60  60.00  − 92.01  42.80  42.65  14.66  − 124.80  27.31 
8  110.00  123.60  80.00  − 110.26  51.28  34.28  11.78  − 166.40  36.41 
9  145.00  143.70  80.00  − 122.03  56.76  55.69  19.14  − 166.40  36.41 
10  145.00  160.50  100.00  − 141.34  65.74  49.24  16.92  − 208.00  45.51 
11  180.00  178.50  100.00  − 151.88  70.64  68.42  23.51  − 208.00  45.51 
12  180.00  201.00  118.00  − 173.58  80.73  70.47  24.22  − 245.44  53.71 
12a  –  219.40  111.92  − 181.47  84.40  97.48  33.50  − 232.79  50.94 
12b  –  239.50  73.30  − 174.94  81.36  165.92  57.02  − 152.46  33.36 
The last load steps were N. 11 with F1 = 180 kN and F2 = 100 kN and N. 12 with F1 = 201 and F2 = 118 kN. One can see that in steps N. 12A and 12B, we could increase F1, but not F2 due to bending failure of the support.
STM2 loading steps and corresponding internal forces in the elastic tests before retrofitting
Step  \(F_{1}^{\text{hyp}}\) (kN)  TM2  

\(F_{1}^{\text{ef}}\) (kN)  F_{2} (kN)  T_{Bsx} (kN)  T_{Bsx}/T_{Bu} (%)  M_{C} (kN × m)  M_{C}/M_{Cu} (%)  M_{B} (kN × m)  M_{B}/M_{Bu} (%)  
1  50.00  50.71  0.00  − 33.04  16.04  50.55  15.12  0.00  0.00 
2  50.00  57.33  15.00  − 45.55  22.11  33.69  10.08  − 36.00  8.29 
3  100.00  102.09  15.00  − 74.71  36.27  78.31  23.43  − 36.00  8.29 
4  100.00  110.37  30.00  − 88.30  42.87  63.11  18.88  − 72.00  16.59 
5  150.00  152.78  30.00  − 115.93  56.28  105.38  31.53  − 72.00  16.59 
6  150.00  171.58  60.00  − 144.58  70.19  77.21  23.10  − 144.00  33.18 
7  200.00  201.80  60.00  − 164.27  79.74  107.33  32.12  − 144.00  33.18 
8  200.00  218.58  85.00  − 188.87  91.68  84.97  25.43  − 204.00  47.00 
9  250.00  251.39  85.00  − 210.24  102.06  117.67  35.21  − 204.00  47.00 
10  250.00  254.64  100.00  − 220.56  107.07  97.46  29.16  − 240.00  55.30 
11  300.00  301.56  100.00  − 251.13  121.91  144.23  43.16  − 240.00  55.30 
12  300.00  302.31  115.00  − 259.82  126.13  121.52  36.36  − 276.00  63.59 
13  350.00  351.24  115.00  − 291.70  141.60  170.30  50.96  − 276.00  63.59 
14  350.00  357.52  130.00  − 303.99  147.57  153.10  45.81  − 312.00  71.89 
15  400.00  401.71  130.00  − 332.78  161.54  197.15  58.99  − 312.00  71.89 
16  400.00  400.09  145.00  − 339.92  165.01  172.08  51.49  − 348.00  80.18 
17  450.00  450.85  145.00  − 372.99  181.06  222.68  66.63  − 348.00  80.18 
18  450.00  456.71  165.00  − 387.74  188.22  197.25  59.02  − 396.00  91.24 
19  500.00  500.84  165.00  − 416.49  202.18  241.23  72.18  − 396.00  91.24 
19a  500.00  444.38  165.00  − 379.71  184.33  184.96  55.34  − 396.00  91.24 
20  500.00  452.30  176.40  − 391.10  189.86  175.03  52.37  − 423.36  97.55 
21  –  454.78  132.00  − 368.44  178.86  246.92  73.88  − 316.80  73.00 
Experimental results
The experimental results about the elastic and failure tests described in §4 are discussed in terms of: (1) behavior (deflections) of the nonretrofitted beams during the elastic tests and (2) behavior (deflections and CFRP reinforcement deformations) of the retrofitted beams during the failure tests.
The latter test results are discussed to evaluate the real CFRP contribution to the shear strength of the retrofitted beam in term of: the ratio between the theoretical and the experimental beam shear strength and the additional contribution of the CFRP shear reinforcement to the total beam shear strength, the socalled CFRP efficiency.
Preliminary elastic tests on the no retrofitted beams with the added cantilever
Displacements under F1 at point C in Fig. 11 are from 35% (SMT1) to 25% (SMT2) larger than those calculated with the gross section without considering the cracking state before the test.
Failure test on beam STM1A failure happened in a premature manner at a shear T_{Bsx} = 174 kN with a negative moment of 245 kN m. We remember that the goal shear was 215 kN and bending moment of 457 kN m.
The experimental behavior of the specimen STM1A enhanced the ineffectiveness at failure of the new concrete slab due to the absence of efficient connectors and abrupt interruption of longitudinal reinforcement. The additional slab lifted from the beam near support B at a load F1 = 150 kN, as the connector added for the slab had no anchoring device at the end and loosed vertical connection with concrete.
Beam deflections
This should have been foreseen as the section is strongly reinforced in tension with few compressed steel badly anchored (See Fig. 3) .
CFRP deformations
Failure mechanism
Failure was evident as various fragile mechanism arose. The additional slab detached, lifting upward from the original beam, near support B, as the connectors were not adequately anchored at their top. The longitudinal new slab rebars were interrupted all together in the slab causing, at the interruption section, the rupture of the slab in tension, while the slab lifted from the original beam (see Fig. 15). Flexural reinforcement was not effective near the support B. The new cantilever had bottom reinforcement anchored in the old beam with a short superposition. This detail caused the total inefficiency of bottom reinforcement which rendered the section sensible to brittle failure and caused rupture by crushing of compressed concrete at support B with buckling of the longitudinal rebars and detachment of the CFRP reinforcement (Zhou et al. 2014, 2015). For that reason, the shear retrofitting was not effective.

evident vertical cracks on the original beam flange;

detachment and lifting of the new RC slab (max lifting = 6 cm);

concrete crushing along the beam sides at the support B near the connection with the cantilever with primer (on which CFRP strips are applied) damage.
Finally, wide diagonal shear cracks appeared along the original beam flange. The collapse occurred when the shear T_{B} was 181.47 kN and the moment M_{B} was 232.79 kN m (F1 = 219.4 kN, F2 = 111.92 kN), as displayed in Fig. 15. These actions values were smaller than the design ones, in particular the shear of about 10–20%. The segment of the beam from the support A to the section of the applied load F1 presented no significant damage at beam failure. This permitted to reuse of the beam TM1 with a new cantilever starting from A.
Failure test on beam STM1B and STM2
Beam deflections
Beam STM1B, already tested as STM1A, showed plateau strength only (450 kN) without initial peak value (500 kN) like in STM2. This was probably due to the damage cumulated in STM1A beam failure test. The total displacement was, however, large in both, with values of about 15–20 mm at 0.4 and 0.6 L.
Beam STM2 had a peak strength at the displacement of 7.5 mm at 0.4 and 0.6 L, after a couple of mm the force reduced to about 400 kN. This value remained constant to failure with 33–43 mm of displacement.
The beam collapse resulted plastic as a good design has to guarantee.
CFRP deformations
The maximum experimental CFRP deformations were in range of 1.0‰ (in one point 1.5‰, strip 4) for STM1B (Fig. 17) and in the same range 1.0‰—(but maximum of 2.0‰ but in strip 2) for STM2 (Fig. 18). These values were significantly smaller than the one assumed during the CFRP reinforcement design (5‰).
Failure of the CFRP reinforcement was due to debonding of the strips starting from a shear value of 375–400 kN for STM1B and STM2 beam, respectively. These shear values were greater than the retrofitting objective; therefore, the intervention proved to be effective. In fact, strips had a maximum strain of 1‰, sufficient to develop the additional shear contribution required.
The plain anchorage of the strips guaranteed the design shear strength even if it was applied, at least partially, on the cracked concrete zone.
This is an important conclusion as CFRP plain anchorages are simpler to realize in situ, but international and national guidelines and code about CFRP reinforcement, in case of complex load scheme (different from simple supported beam) do not give detailed information about how to calculate and realize CFRP anchorages on cracked concrete.
In beam STM1B, the greatest deformation had been measured at the middle level of the strip. Differently, the STM2 top and middle strip deformation were similar at the beginning, but then the middle deformations were smaller that the top ones. Top and middle deformations were again very similar at the end of the test. In both beams, the goal internal shear force was overtaken.
The final failure of shear reinforcement happened for debonding, probably due to strong compression deformations in concrete, together with tension in the strip. The simple evaluation of local strain in the strip does not allow to draw comprehensive conclusions concerning effectiveness. However, if we assume that a crack passes through strips 2 and 3, we may evaluate the contribution of the strips 2 and 3 with the measured maximum deformation ε = 0.001, each strip of two layers, the vertical CFRP strip contribution in STM1B is V = 115 kN. This is enough to guarantee the needed shear resistance.
Failure mechanism
The shear strengths of the two beam specimens STM1B and STM2 were greater than the design one; therefore, it was proved that the retrofitting of the beam with CFRP shear reinforcement was the exact solution. In fact, the collapse of beam occurred when the shear T_{B} and bending moment M_{B} at the support B were 384.86 kN and 396.00 kN m (F1 = 450 kN and F2 = 165 kN) or 416.49 kN and 396.00 kN m (F1 = 500 kN, F2 = 165 kN) for the beam STM1B and STM2, respectively.

one diagonal concrete crack from the original beam slab, in the section, where F1 was applied, to the CFRP strip f4 and debonding phenomena on f4 strip when the shear T_{B} was equal to about 251 kN and 210 kN for STM1B and STM2, respectively (F1 = 300kN and F2 = 100kN for STM1B, F1 = 250 kN and F2 = 85 kN for STM2). Some vertical cracks were evident on the slab near the support B in case of STM2;

new diagonal concrete cracks on the two sides of the original beam slab and web and large CFRP local deformations when the shear in section B was equal to about 305 kN for each beam;

diagonal cracks widening and evident debonding of the CFRP strip at the top section near the support B when the shear was equal to about 373 kN.
Conclusions
Two beams (TM1 and TM2) extracted from an existing RC building located in Rome and dated around 1930 have been retrofitted in bending by adding an RC slab and reinforced in shear with CFRP strips, using the same provisions adopted in the real building. The retrofitted beams have been tested until failure to investigate the shear strength of CFRPreinforced concrete when submitted to a stress state typical of a beam of a frame structure, in the presence of negative bending and shear. After the realization of the cantilever, to create negative bending and shear at one end of the span, the retrofitting intervention consisted of two subsequent solutions. The beam TM1 had been first retrofitted as STM1A specimen, but retrofitting provisions revealed unsatisfactory, due to premature fragile bending failure. A second retrofitting detailing was than applied to beam TM2, named STM2 beam specimen, and to a new intervention on TM1 on the opposite site, named STM1B specimen. This second retrofitting detailing proved to be effective and was adopted.
The main conclusion of this study from the experimental evidence is that the shear strengthening of RC beams by externally bonded CFRP sheets can be effective in providing additional shear resistance to existing members also for the analyzed loading scheme. The negative bending at the end of the cantilever, with the adopted provisions, does not render ineffective the shear reinforcement by U strips of CFRP. When shear and negative bending develop simultaneously, shear cracks start from the beam extrados, supporting the FRP debonding if it is not adequately anchored. However, the simple CFRP anchorages used for the retrofitted beams are resulted sufficient to guarantee the necessary shear strength improvement. In the first test on the beam STM1A, debonding happened but for the crushing of compressed concrete at support B and not for cracking of the concrete in the tension zone of the section, where the CFRP anchorages are applied. This failure could be easily avoided with a proper design of the compressed reinforcement. Each section of the retrofitted beam should show a ductile behavior until failure after the retrofitting intervention as crushing of compressed concrete and rebar buckling can produce debonding of the CFRP shear reinforcement. This should be a serious concern in concrete frame retrofitting, and becomes even of larger consequences if CFRP U strips are adopted.
STM1A had some further inadequate detailing which reduced both the overall and the local ductility, related to flexural behavior or anchoring of connection between the new slab and the old beam. In the case at hand, the presence of not wellengineered solutions reduced the foreseen shear resistance to at least 50%.
The second interventions tested in lab with the realization of the specimen STM1B and STM2, with the simple modification of some construction and retrofitting detailing, avoided brittle flexural failure and the detachment of the new slab assuring the target shear and bending strength. These beams showed more than doubled experimental shear resistances with respect to the one of the STM1A beam. It was showed that, even in the presence of negative bending, the use of U CFRP strips, without any particular anchorage provision, can solve possible needs of retrofitting. It seems obvious that the only real proof is a test like the one here presented.
Finally, this paper provides original experimental results, because the tested beams, removed from an existing building, give an effective evaluation of the strengthening intervention as in realframed structures, where the shear forces are combined with negative moments at supports. The obtained experimental results significantly improve the database of fullsize beams, available from literature, with flexural strengthening and shear FRP reinforcements.
Notes
Acknowledgements
The authors wish to express their gratitude to Interbau s.r.l. for providing CFRP sheet and adhesive for the specimens and their realization. They also thank the Italian Ministry of Public Works, office of Rome of 2010, who permitted and supported the experimental investigation. Finally, the writers would also like to express their appreciation for the technical support given by the staff of the PRiSMa Laboratory (Proof Testing and Research on Structures and Materials) of Roma Tre University, with special reference to Dr. Lorena Sguerri who carefully set up and carried out the tests.
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