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GFRP-to-concrete joints subjected to moisture, salt fog and temperature cycles
Composites: Part B 55 (2013) 374–385
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Bond–slip on CFRP/GFRP-to-concrete joints subjected to moisture, salt fog and temperature cycles Manuel A.G. Silva, Hugo C. Biscaia ⇑, Rui Marreiros UNIC, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history: Received 18 January 2013 Received in revised form 27 May 2013 Accepted 9 June 2013 Available online 20 June 2013 Keywords: A. Laminates B. Debonding B. Interface B. Environmental degradation Concrete
a b s t r a c t Research on bond between composite and concrete on beams externally reinforced with fiber reinforced polymers (ERBs) has generated many publications, but uncertainties remain. The issue of the long term behavior of those joints, especially the effect that severe and prolonged environmental actions may induce, justifies the search for additional data and recommendations to avoid premature debonding and failures. The present paper devotes attention to the effects of environmental aging on the constitutive bond-slip curves. Aging was imposed on an accelerated manner and several actions used to degrade the behavior of the joints evaluated by mechanical testing. Prismatic blocks of reinforced concrete linked on the upper side by a stainless steel hinge and externally bonded by a continuous strip of carbon or glass fiber reinforced polymer (CFRP or GFRP) adherent to the soffit were made so as to fit in commercially available laboratory climatic chambers. After aging, they were tested till failure under four point loading. Aging conditions imposed on the CFRP beams were (i) cycles of high-low relative humidity, (ii) salt fog cycles, and, on the GFRP beams, (iii) salt fog cycles, (iv) dry/wet cycles (water with 5.0% weight of NaCl), (v) total immersion in salt water, and (vi) freeze/thaw cycles. The results of the experimental program enabled the proposal of bond-slip laws that take into account the aging of the beams. They also showed that salt fog cycles were more severe in the case of CFRP, while freeze/thaw cycles were more degrading on bond of GFRP-to-concrete. The salt water effects on the GFRP beams appeared to be beneficial, most likely by improving the tensile strength of concrete. Numerical modeling certified by the obtained experimental data is presented that allows more general estimates of the environmental effects. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction The accurate estimate of the behavior of the FRP-to-concrete interfaces is essential to predict the performance of structural members with external reinforcement of FRP and many authors have devoted their work to the characterization and modeling of the response of such structures under external loading. However, long term and severe environmental conditions may cause premature failure by degrading bond of those joints, or decrease the load capacity and durability of the structures, and require further attention. Namely, the effectiveness of the strengthening depends on bond between the composite and concrete substrate and aging may deteriorate it. The study of these aspects is the major scope of the work, specifically geared to establish the influence of different types of aging on the bond–slip constitutive relationships. The exposure of externally bonded composites to cycles of temperature is known to cause microcracking with increased ⇑ Corresponding author. Tel.: +351 21 2948580; fax: +351 21 2948398. E-mail address: [email protected] (H.C. Biscaia).
absorption of water [1] and crystallization. The same type of degradation may result from differential thermal expansion of the interfacing materials creating stresses, damage and relative changes of stiffness [2]. A short survey of research on thermal effects can be found e.g. in [3]. The effects of temperature cycles on the composites have also been recently described by Nordone et al. based on changes of tensile strength of CFRP and GFRP coupons [4]. Data obtained on GFRP showed a slight decrease of tensile strength for increasing temperatures and a reduction of mechanical properties of CFRP for ‘‘extreme’’ temperatures. Nevertheless, changes due to freeze–thaw cycles were non-significant contradicting other experimental work that reported degradation of the bond strength between concrete and FRP after aging caused by freeze–thaw cycles [5]. Mukhopadhyaya et al. [6] also studied the effects of freeze– thaw cycles on specimens made of two different types of concrete and concluded that there was a slight decrease of strength for the 37.1 MPa concrete and a more than triple average rise of 10.7% for the specimens fabricated with 48.6 MPa concrete. The decrease of bond stresses was, respectively, 12.6% and 24.7%, while the bond
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transfer length increased in both cases, confirming the known importance of the type of concrete, namely its tensile strength, in addition to composition and permeability. The failure mechanisms found were essentially associated with adhesive rupture of the specimens at the interface. This latter conclusion contradicts more recent findings that showed mostly cohesive type of failures in the layer of concrete adjacent to the adhesive (e.g. [7]) perhaps because the tensile ultimate stress of the adhesive used by the authors was low. Synergistic effects of freeze–thaw cycling combined with effects of a sodium chloride solution and externally applied load are reported by Sun et al. [8]. The results show more severe effects on the specimens subjected to freeze–thaw cycles in the salt water solution than those in water. The influence of wet/dry cycles has also been studied in connection with bond failures since moisture sorption modifies the mechanical properties of the epoxy matrix, causing the lowering of the glass transition temperature Tg. The mechanisms of migration of water into epoxy resins have been established [9]. Migration of water into the laminates and in the interface degrade the strength of the structures [10–13]. A much more comprehensive listing of contributions could be presented, but the bulk of all available and important work has not addressed the modifications of the bond–slip constitutive laws by environmental aging, an important topic with scarce data. Such is the major objective of this work and the experimental program and results follow. 2. Experimental program The experimental program is the result of the work required for completion of two master theses and consisted essentially on
subjecting small reinforced concrete beams, with external reinforcement of CFRP or GFRP, to pre-defined types of aging and testing the beams till failure, at prescribed ages. The results provided the evolution of the load carrying capacities, and the failure modes and allowed the study of the degradation of bond between the strengthening composites and concrete. The geometry of the beam specimens is shown in Fig. 1a. Two RC blocks (total length L = 620 mm, height h = 150 mm, width w = 100 mm) were connected by a stainless steel hinge at midspan, with a two layered FRP strip 80 mm wide symmetrically bonded to the soffit for 520 mm. The beams were dimensioned so that failure would take place at the interface region defined by the composite, the adhesive and the adjacent layers of concrete. Epoxy resin used to bond the composite to the concrete was of the same type as the epoxy resin used in the impregnation of the FRP composite. The concrete surface to be bonded to the composite was treated by dry sand blasting so as to create an adequate level of roughness, and cleaned before the application of the adhesive. 2.1. Materials characterization The beams reinforced with CFRP and GFRP, henceforth designated as EB-CFRP and EB-GFRP, respectively, were made at different times. Two concrete mixes resulted and the tests to characterize them mechanically followed standardized procedures [14]. The concrete mix for EB-CFRP had an average cube compressive strength at 28 days fcm = 26.1 MPa, whereas the concrete of the beams externally reinforced with GFRP (EB-GFRP) had a cube strength, at the same age, fcm = 34.9 MPa. The corresponding average tensile strength (fctm) for each case was calculated as proposed in EC2 [15] leading to fctm = 2.1 MPa for EB-CFRP and 2.7 MPa for the case of the EB-GFRP beams.
RC block
Metallic hinge CFRP or GFRP
(a)
Rv Rh Metallic hinge
FFRP
(b) Fig. 1. (a) Geometry of specimens to be aged and mechanically tested and (b) ‘‘Half-beam’’ free body diagram for 4 point bending test.
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2.2. Selected environmental conditions
Steel: S400 Coating: 10mm
The conditions imposed on the specimens were basically periodic, with each cycle lasting 24 h, as shown in Fig. 3. In addition to the conditions represented in Fig. 3, total immersion in a solution of deionized water with NaCl was also imposed. Briefly, the imposed aging procedures can be described in five categories:
2Ø3 Stirrups: Ø3 @50mm 2Ø3
Fig. 2. Steel reinforcements of the beams.
Mild steel rods of £3 mm diameter were used. Four rods were placed longitudinally, two in the compression zone and two in the tensile zone of the beam blocks. The stirrups were spaced 5 cm apart. Fig. 2 shows the scheme assumed for the steel reinforcements. The mechanical properties of the steel were obtained by uniaxial tensile tests according to [16] that led to fsy = 450 MPa and yielding strain esy = 0.23%. The composite strips were applied by the wet layup technique in two layers. Coupons of 25 250 mm were cut from plates. The nominal thicknesses, per layer, were used as indicated by the suppliers i.e. 0.165 mm for CFRP and 1.270 mm for GFRP. The tensile strength was evaluated in agreement with ASTM D3039/ D3039M [17]. The tests were made in a universal Zwick tensile machine with a load capacity of 50 kN. The constitutive laws were elastic and linear showing very brittle failure. The ultimate tensile strain and strength obtained by testing the CFRP flat coupons were efm = 1.54%, ffm = 3937 MPa while the GFRP flat coupons led to efm = 2.46%, and ffm = 500.2 MPa. The corresponding elastic moduli were found as 241,000 MPa and 20,390 MPa.
(i) Hygrothermal cycles – 12 h exposed to 20% of relative humidity (RH) followed by 12 h at 90% RH and a constant temperature of 40 °C. (ii) Salt fog cycles consisting of 8 h of spray followed by 16 h of drying at constant 35 °C. (iii) Dry/wet cycles of salt water, 12 h at each state, with 50 g of sodium chloride (NaCl) per liter of water. Freeze/thaw cycles were defined by the imposition of 10 °C for 12 h followed by another 12 h at +10 °C. The transition from one temperature to the other was imposed at the rate of 1.5 °C/min, according to a default programmed rate of the chamber. (iv) Total immersion in salt water with 50 g of sodium chloride (NaCl) per liter of water. At three time stages of each aging, 3000 h, 6000 h and 10,000 h for CFRP and 1000 h, 5000 h and 10,000 h for GFRP, specimens were removed from the chambers and their load carrying capacity was tested as described in the sequel. The results obtained permitted the study of the time evolution of the performance of the beams and the comparison with that obtained for the reference specimens (tested at 0 h).
Fig. 3. Aging cycles considered: hygrothermal or moisture, salt fog, dry-wet and temperature.
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2.3. Bending tests Prismatic blocks of reinforced concrete linked in the compression zone by a stainless steel hinge and externally bounded by a two-layered continuous strip of carbon or glass fiber reinforced polymer (CFRP or GFRP), 80 mm wide and 250 mm long, adherent to the soffit were tested. Before bonding the FRP composite to the concrete surface, dry sand blasting was applied. The beams were simply supported at two points, distant 560 mm. Two equal and symmetric vertical forces were imposed on each specimen at a distance of 235 mm from the nearest support and loading was increased in a static way until failure of the beams. The strains in the CFRP and GFRP composites were measured with strain gauges spaced at 40 mm. Two load cells were placed underneath both supports and the loads were applied with a hydraulic jack. One Linear Variable Displacement Transducer (LVDT) measured the vertical displacement of the beam at midspan. Thirty beams were tested in bending and their identification and the type of environmental conditions subjecting each one of them is shown in Table 1. The condition of vanishing moment at the hinge leads to the traction force FFRP acting on the composite in terms of the applied load P, Fig. 1b.
F FRP ¼
1 ha P 2 b
ð1Þ
with a = 45 mm, b = 280 mm and h = 133 mm. Nonlinear bond–slip curves were calculated from the strain vs. slip (e–s) relationship defined at the loaded end of the FRP as recommended by Ueda and Dai [18]. The relationship e–s was approximated by a smooth curve given by
eðsÞ ¼ emax ð1 ens Þ=ð1 ensult Þ ¼ emax ð1 ens=sult Þ for s < sult
ð2Þ
where emax is the maximum measured strain in the FRP composite is a coefficient obtained the loaded end, sult is the ultimate slip and n Table 1 Designation of the tested specimens and type of environmental exposure. Composite
Specimen
Aging actions
Hours of exposure
CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP
REF-C-01 REF-C-02 RH-C-01 RH-C-02 RH-C-03 RH-C-04 RH-C-05 RH-C-06 SF-C-01 SF-C-02 SF-C-03 SF-C-04 SF-C-05 SF-C-06 REF-G-01 REF-G-02 REF-G-03 SF-G-01 SF-G-02 SF-G-03 DW-G-01 DW-G-02 DW-G-03 FT-G-01 FT-G-02 FT-G-03 TI-G-01 TI-G-02 TI-G-03
Reference Reference Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Salt fog cycles Salt fog cycles Salt fog cycles Salt fog cycles Salt fog cycles Salt fog cycles Reference Reference Reference Salt fog cycles Salt fog cycles Salt fog cycles Dry/wet cycles Dry/wet cycles Dry/wet cycles Freeze/thaw cycles Freeze/thaw cycles Freeze/thaw cycles Total immersion Total immersion Total immersion
0 0 3000 3000 6000 6000 10,000 10,000 3000 3000 6000 6000 10,000 10,000 0 0 0 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000
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to fit e(s) to the experimental results by the least square procedure minimizing the sum of the squares of the difference between are given by approximate and experimental values, emax and n
emax ¼ emax =ð1 ensult Þ ¼ emax =ð1 en Þ
ð3Þ
¼ nsult n
ð4Þ
The truncation of the interval of application of (2) at s = sult appears physically justifiable, since the expression respects the zero deformation for no slip and the maximum for the ultimate slip. , as well as a dimensionThe choice of a dimensionless parameter n less maximum deformation emax , both defined from a quantity obtainable from experimental data, sult allows easier generalization of results obtained. Variation of those parameters with aging and material characteristics will be shown in the sequel. The shear stress s(s) is given by
sðsÞ ¼ Ef tf
de ds ds dx
ð5Þ
where Ef and tf are the Young modulus and the thickness of the FRP composite, respectively defines the bond–slip curve. Approximating the strain by e = ds/dx, the bond–slip curve can be written
sðsÞ ¼ 4 smax ðens e2ns Þ
ð6Þ
Designating as maximum slip smax the slip that corresponds to the maximum value of bond stress (smax) it is
smax ¼
n e2max Ef t f 4
ð7:aÞ
lnð2Þ n
ð7:bÞ
and
smax ¼
The average fracture energy is given by the area below the average bond–slip curve
GF ¼
e2max Ef tf 2
ð8Þ
The nonlinear character of the slip distribution can be approximately obtained from the solution of the Boundary Value Problem (BVP) posed by the governing equation and conditions below (e.g. [19]), 2
sðsÞ ¼0 Ef t f
ð9:aÞ
ds F ¼ efjx¼0 ¼ dx Af Ef
ð9:bÞ
ds ¼ efjx¼L ¼ 0 eff dx
ð9:cÞ
d sðxÞ 2
dx
The nonlinearity of s(s) has originated the consideration of several linear and nonlinear types of constitutive bond–slip curves to approximate s(s), each leading to approximate slip and stress distributions as described e.g. in [20]. The maximum load Fmax transmitted to the FRP plate is reached when Leff is attained and, designating by emax the strain measured at the pulling end, it is
F max ¼ emax bf t f Ef
ð10Þ
3. Reference beams – experimental results The results obtained with tests on control beams not subjected to artificial aging serve for measuring the degradation experienced
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Fig. 4. EB-CFRP beam REF-C-01. View prior to failure and view of CFRP strip after failure.
TI-G-02
REF-G-02
Fig. 5. Surface failure for EB-GFRP beams REF-G-02 and TI-G-02.
by the aged beams. Fig. 4 shows the aspect of reference beam EBCFRP (REF-C-01) after failure, displaying a cohesive type of failure inside the concrete layer next to the adhesive-concrete contact surface. Fig. 5 shows the aspect of beams REF-G-02 and TI-G-02 after failure. In the case of the EB-GFRP reference beam the type of failure is similar to that exhibited by REF-C-01, whereas for the total immersion case, TI-G-02, failure was ‘‘less cohesive’’. The bond–slip laws were obtained for each specimen according to the methodology described in Section 2.3. The corresponding average bond–slip curves obtained for the reference beams, for both CFRP and GFRP, are shown in Fig. 6, and were obtained by averaging the individual curves that define the gray shade. The values obtained for CFRP and GFRP were, respectively, ¼ 3:103; e ¼ 0:481 sult = 0.737 mm, n and sult = 0.733 mm, ¼ 2:778; e ¼ 0:597. n In the case of EB-CFRP beams the maximum bond stress smax is larger than in the case of EB-GFRP beams while the maximum slip (smax) is smaller. The post-peak branches of both composites have similar shapes and the ultimate slips sult are almost equal. The effective length has important physical meaning and design consequences and several proposals can be found in the literature on how to estimate it from the characteristics of the component materials. Nakaba et al. [19] approximated Leff by the distance between the points at which the bond stresses are 10% of the maximum bond stress (smax). A more direct approach is given in fib bulletin 14 [21], that recommends
Leff
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ef t f ¼ c2 fctm
ð11Þ
with c2 = 2.0 for CFRP beams. For interfaces of GFRP-to-concrete the code does not specify c2 and the choice c2 = 0.8 was made in this study based in published results [21], though this value requires further experimental data and possible rectification.The closed form solution of the boundary value problem for linear bond–slip laws leads to [20]:
Leff
sffiffiffiffiffiffiffiffiffiffiffiffi Ef t f ¼ j sult GF
ð12Þ
where j is a parameter that depends on the bond–slip law. In the pffiffiffi case of the rigid-plastic bond–slip law jp¼ffiffiffi 2; for the linear ascending law j can bepapproximated by 2 2; for the rigid-linear ffiffiffi softening law j ¼ p 2=4. In the latter case, the effective bond length is 344 mm for CFRP and 295 mm for GFRP. Finally, the maximum value Fmax of the force FFRP sustained by the composite was obtained applying the definition: the bonded length was successively increased until the force reached a maximum value, utilizing (3) as the constitutive relation and limiting sult to 0.74 mm. The results are plotted in Fig. 7. The effective length obtained for EB-CFRP and EB-GFRP is approximately 325 mm and 275 mm, in the reference cases.
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379
Fig. 6. Bond–slip and fracture energy curves for the reference beams.
Fig. 7. Calculation of effective length for both EB-CFRP and EB-GFRP reference beams from the definition.
4. Aged beams Table 2 Major bond parameters for the reference beams.
smax (MPa)
smax (mm)
sult (mm)
GF (N/mm)
1.9 1.6
– –
– –
– –
0.17 0.18
0.74 0.73
0.91 0.82
Model associated with rigid-linear softening CFRP 276 1.9 – GFRP 242 1.6 –
0.74 0.73
0.91 0.82
Criterion from Nakaba et al. [17] CFRP 370 1.9 GFRP 350 1.6
0.74 0.73
0.91 0.82
Composite
Leff (mm)
Fib bulletin 14 CFRP 142 GFRP 155
Model associated with Eq. (3) CFRP 325 1.9 GFRP 275 1.6
0.17 0.18
The major bond parameters for the reference beams are summarized in Table 2.
The results of the bending tests for aged specimens are summarized next. The evolution of the parameters like the maximum bond stress smax and the corresponding slip smax, the ultimate slip sult and the fracture energy GF along the different aging processes is shown in Tables 3 and 4 for EB-CFRP and EB-GFRP beams, respectively. 4.1. CFRP The comparison of the values reached on the tests after relative humidity or hygrothermal cycles and salt fog cycles with those of the reference specimens, on EB-CFRP beams, is possible consulting Table 3. The maximum bond stress smax experienced a significant increment while the maximum slip (smax) fracture energy GF and maximum strain in the composite eFRP,max = emax decreased with the duration of exposure. The maximum load Fmax transmitted to the FRP plate was calculated according to
F FRP ¼ eFRP bf t f Ef
ð13Þ
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Table 3 Bond characterization for the EB-CFRP beams. Aging
Exposure (h)
Fmax (kN)
sult(mm)
emax (%)
¼ nSult n
smax (MPa)
smax (mm)
GF (N/mm)
Reference Hygroth. Hygroth. Hygroth. Salt fog Salt fog Salt fog
0 3000 6000 10,000 3000 6000 10,000
39.8 32.2 34.1 39.3 37.6 32.8 37.0
0.737 0.555 0.569 0.646 0.754 0.577 0.579
0.48 0.371 0.394 0.393 0.443 0.371 0.423
3.103 6.288 4.580 4.813 5.112 5.805 4.800
2.09 3.36 2.61 2.49 2.80 3.00 3.20
0.165 0.066 0.089 0.096 0.105 0.071 0.086
0.83 0.63 0.64 0.67 0.83 0.61 0.77
Table 4 Bond characterization for the EB-GFRP beams. Aging
Exposure (h)
Fmax (kN)
sult (mm)
emax (%)
¼ nSult n
smax (MPa)
smax (mm)
GF (N/mm)
Reference Salt fog Salt fog Salt fog Dry/wet Dry/wet Dry/wet Freeze/thaw Freeze/thaw Freeze/thaw Total immersion Total immersion Total immersion
0 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000
25.4 26.8 26.4 24.8 33.3 33.3 33.4 25.3 23.0 22.0 33.8 35.5 36.3
0.733 0.776 0.778 0.873 1.016 1.162 0.937 0.886 0.679 0.696 0.790 1.044 0.969
0.60 0.69 0.66 0.61 0.74 0.74 0.75 0.55 0.53 0.51 0.75 0.79 0.81
2.778 2.995 4.831 7.621 4.399 4.787 3.757 7.256 2.960 2.735 4.708 5.001 3.857
1.77 2.12 2.23 3.03 2.06 2.03 2.19 2.21 1.36 1.24 2.28 2.65 2.22
0.183 0.180 0.112 0.079 0.160 0.168 0.173 0.085 0.159 0.177 0.116 0.145 0.174
0.72 0.89 0.70 0.69 0.91 0.95 1.00 0.54 0.50 0.48 0.74 1.08 1.03
Fig. 8. Bond–slip curves at CFRP-to-concrete interface and different stages of aging for cycles of moisture and salt fog.
where emax is the strain measured at the pulling end of the FRP plate. Fig. 8 shows the bond–slip curves of CFRP-to-concrete, drawn for 3000, 6000 and 10,000 h of exposure to the varying moisture and to the salt fog cycles. The effects of salt fog are easier to characterize, showing typically an increase of maximum shear for smaller slip, and a decrease of the ultimate slip and fracture energy, with regard to the reference case. The hygrothermal cycles defined by the 20% and 90% limiting cases of relative humidity (RH) caused a high increase of smax at 3000 h, a change that has not been satisfactorily interpreted. Otherwise the observed trends are similar to those present after salt fog cycles. At 10,000 h, the curves obtained for salt fog cycles caused nearly a 9% decrease of emax against 15% for the moisture cycles. The same less significant decrease by the salt fog cycles was observed for the fracture energy. The results also showed that the maximum load transmitted to the CFRP plate decreased in agreement with its lower fracture energy.
4.2. GFRP Table 4 displays the effects of salt fog, dry/wet and freeze–thaw cycles, as well as total immersion on beams externally reinforced with GFRP. The values show an increment of the maximum bond stress smax observed for all aging environments except for the freeze/thaw cycles. In the latter case, at 10,000 h, a reduction of 30.8% was found for smax. The maximum strain in the composite eFRP,max decreased also for freeze/thaw cycles (from a negligible 2% to 14%) and increased about 30% for tidal-like dry/wet cycles and 32–41% for total immersion in salt water. In terms of the fracture energy GF, the freeze/thaw cycles caused a reduction of more than 30% throughout the entire duration of this type of aging. At 10,000 h, the fracture energy showed significant increases for dry/wet cycles (above 38%) and total immersion (above 49%), both in salt water, while it decreased 5% for salt fog cycles. Fig. 9 displays the bond–slip curves for GFRP-to-concrete interfaces for each stage of exposure and aging by salt fog, dry/wet and freeze–thaw cycles, and total immersion in salt water.
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381
Fig. 9. Bond–slip curves for EB-GFRP joints at different stages of aging and salt fog, dry/wet and freeze–thaw cycles, and constant immersion in salt water.
The changes of the bond–slip curves appear to indicate, for some types of aging, a modification of the relative stiffness by comparison with the reference cases, as mentioned in [22]. The shear stiffness of the bond layer, estimated by the ratio of smax/smax, appears as significantly increased at 10,000 h for salt-fog cycles, while not much affected for dry/wet cycles and total immersion in salt water. For freeze–thaw cycles that stiffness actually decreases. Fig. 10 graphically summarizes the evolution of maximum load transmitted to the FRP plate (FFRP,max), maximum bond stress (smax), maximum slip (smax) and fracture energy (GF) for all cases, normalized by the values for the corresponding reference specimens. All environmental conditions that include NaCl show smax/smaxRef > 1. Only dry/wet cycles for EB-GFRP did not cause effective loss of load capacity measured. Comparing with maximum slip (smax) obtained at 0 h of aging, all specimens exhibited lower slips after aging. Freeze–thaw caused decrease of all the indicators shown in the diagrams.
described in the sequel, respectively for CFRP and GFRP. It is noticed that, in the future, care must be exercised to make sure that eventual dependence on strength of concrete, surface treatment or desired duration of structure may need to be considered. Results from studies on type of surface treatment seem to indicate that, although important in itself, the treatment alone is not decisive for the quantification of the degradation of bond [23]. From the numerical solution of the problem governed by Eq. (9) and for different types of bond–slip curves (the thickest curves in Figs. 11 and 12), the maximum load transmitted to the FRP plate was determined and compared with the lower value obtained experimentally for each environmental condition studied. The results are depicted in Fig. 13 and show good agreement with those obtained experimentally, with deviations smaller than 20% except for dry/wet cycles on GFRP where the deviation reached 20.3%. 5.2. Environmental reduction coefficients
5. Influence of aging on bond – recommendations
The quantification of environmental reduction factors aE is submitted hereafter. Firstly, an expression is submitted for the maximum shearing stress after environmental aging,
5.1. Proposed design bond–slip curves
smax;ENV ¼ a1
The study aims at the definition, for each environmental condition, of a bond–slip curve associated with minimum fracture energy found for each combined case of type of composite and type of environmental condition, as well as the ensuing determination of bond related parameters e.g. the maximum load transmitted to the FRP plate (FFRP,max). Figs. 12 and 13 show the aged bond–slip curves that correspond to selected values of the parameters as
where GF,ENV is the ‘‘aged’’ fracture energy; n = 4.2 mm1 for CFRPto-concrete interfaces and n = 3.8 mm1 for GFRP-to-concrete interfaces; and a1 is a corrective coefficient that adjusts the value of maximum bond stress to that determined for the reference condition, Tables 3 and 4. A coefficient a2 that approximates the fracture energy after aging GF,ENV to the reference GF obtained at 0 h is given by
n GF;ENV 2
ð14Þ
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Fig. 10. Relative evolution of: (a) maximum load transmitted to the FRP plate; (b) maximum bond stress; (c) maximum slip; and (d) fracture energy.
Fig. 11. Proposed bond–slip curves for aged EB-CFRP joints for cycles of relative humidity and salt fog.
GF;ENV ¼ a2
e2FRP;max Ef tf 2
¼ a2 GF
ð15Þ
Considering that the average experimental maximum strains for CFRP and GFRP were about 30% and 26% of the rupture strain from tensile tests (efm), preliminary values are suggested for a1 and a2 and displayed in Table 5. In addition, the maximum strain in the FRP plate eFRP,max can be written as
eFRP;max ¼ uREF efm
ð16Þ
and in the case of CFRP-to-concrete interfaces uREF = 0.30 and in the case of GFRP-to-concrete interfaces uREF = 0.26.The slip (smax) for
the maximum extension in the FRP (eFRP,max) for each aged bond– slip curve, can be expressed as
smax ¼
lnð2Þ
ð18Þ
a1 n
Maximum strains in FRP plate after aging, can be also obtained independently of the shape of the bond–slip curve [20] from the fracture energy of the reference specimens according to:
eENV FRP;max ¼ aE
sffiffiffiffiffiffiffiffiffiffiffiffi 2GF Ef t f
where aE is an environmental parameter defined as:
ð19Þ
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Fig. 12. Proposed bond–slip curves for aged EB-GFRP joints and salt fog, dry/wet and freeze–thaw cycles, and constant immersion in salt water.
Table 5 Values proposed for the coefficients a1 and a2.
Fig. 13. Comparison between the maximum loads transmitted to the FRP plate estimated from the proposal and from the experiments.
Fig. 14. Comparison between the effective bond lengths obtained from Eq. (22) with j = 1.32 and from the numerical analysis, Eq. (9).
Interface
Aggressive environment
Coefficient, a1
Coefficient, a2
CFRP-to-concrete
Reference Hygrothermal cycles Salt fog cycles
1.00 1.95 2.20
1.00 0.82 0.80
GFRP-to-concrete
Reference Salt fog cycles Dry/wet cycles Freeze/thaw cycles Total immersion
1.00 1.45 1.14 1.03 1.49
1.00 0.95 1.09 0.84 0.99
aE ¼
eENV FRP;max eFRP;max
ð20Þ
and is quantified in Table 6. The results presented in Table 6 attempt at comparing the environmental reduction factors proposed (aE) for beams externally bonded with CFRP with those recommended by ACI 440 [24] for degradation of CFRP laminates. The GFRP-to-concrete interfaces advise environmental reduction factors higher than 0.75 which is the upper limit recommended for GFRPto-concrete interfaces. To a certain extent, the tabulated values show that the degree of degradation of the carrying capacity of the structural member is relatively close to that observed for FRP composite laminates. The results found for the GFRP-to-concrete interfaces subjected to actions involving salt, are thought to be due to (i) improvement of the concrete strength; or/and to (ii) a time of exposure not long enough to cause more severe degradation. This aspect of converting the results obtained into conclusions valid for longer exposures is being treated based on Arrhenius equation (e.g. [25]), but no definitive results have yet been obtained. The maximum load transmitted to the FRP plate taking into account the environmental effects can be calculated by
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Table 6 Comparison between the values of the environmental reduction factors found in current study (aE) and those given in ACI 440 (CE) for laminates [24]. Interfaces
Environment
Proposed reduction factor aE for structures
ACI reduction factor CE for laminates
CFRP-to-concrete
Reference Hygrothermal cycles Salt fog cycles
1.00 0.81
1.00 0.85–0.95
GFRP-to-concrete
Reference Salt fog cycles Dry/wet cycles Freeze/thaw cycles Total immersion
1.00 0.97 1.00 0.80
Table 8 Evolution of the effective bond length (Leff) for the EB-GFRP-to-concrete joints. Composite
Type of aging
GFRP
Reference Salt fog cycles Salt fog cycles Salt fog cycles Dry/wet cycles Dry/wet cycles Dry/wet cycles Freeze/ thaw cycles Freeze/ thaw cycles Freeze/ thaw cycles Total immersion Total immersion Total immersion
0.81 1.00 0.50–0.75
0.95
Table 7 Evolution of the effective bond length (Leff) for the EB-CFRP-to-concrete joints. Composite
CFRP
F ENV FRP;max
Type of aging
Reference Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Salt fog cycles Salt fog cycles Salt fog cycles
fib (mm)
Numerical results (Eq. (9)) (mm)
Rigid-linear softening model – Eq. (12) (mm)
0 3000
370 170
325 200
276 180
6000
230
275
209
10,000
245
275
228
3000 6000 10,000
245 195 205
300 225 225
231 195 190
Hours of exposure
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ aE bf 2GF Ef t f
Rigid-linear softening model – Eq. (12) (mm)
0 1000
350 300
275 275
242 228
5000
220
250
213
10,000
160
175
192
1000
275
325
254
5000
285
350
273
10,000
285
300
239
1000
250
250
226
5000
355
325
267
10,000
400
350
287
1000
320
275
212
5000
230
300
226
10,000
280
300
241
Interface
Aggressive environment
Coefficient kREF
CFRP-to-concrete
Reference Hygrothermal cycles Salt fog cycles
1.00 0.87 1.02
GFRP-to-concrete
Reference Salt fog cycles Dry/wet cycles Freeze/thaw cycles Total immersion
1.00 1.19 1.58 1.21 1.42
ð21Þ
The influence of the different environmental conditions on the effective bonded length (Leff) was studied numerically utilizing the bond–slip curves reproduced in Section 5.1 to find the bond stresses along the composite. In this sub-section it is examined the formulation of the nonlinear problem and its solution for successive lengths of composite bonded to concrete until a length is attained beyond which the traction force cannot be balanced by the generated shear stress field (as in Section 3). The numerical results obtained from the bond–slip law based on the Ueda and Dai proposal [18] (synthetically resumed by Eq. (9)) showed that the effective bond lengths for the CFRP-toconcrete joints are smaller after aging than before, as displayed in Table 7. Differently, in the aged GFRP-to-concrete interfaces, it can be noticed that only the salt fog cycles had smaller effective bond length than the reference situation while the dry/wet and freeze/ thaw cycles led to higher effective bond lengths, as shown in Table 8. The environmental effects on Leff can be calculated conjugating Eqs. (12), (15), and (16) introducing a factor kREF.
Leff
Numerical analysis – Eq. (9) (mm)
Table 9 Values proposed for the coefficient kREF.
5.3. Changes on effective bond length
sffiffiffiffiffi 2 kREF sult ¼ j a2 /REF efm
fib (mm)
Hours of exposure
ð22Þ
where kREF is given by the relation between the maximum ultimate slip obtained at the different stages of exposure and the ultimate
slip, given in Table 9 and j = 1.32 according to the recommendation made in [20].
kREF ¼
sENV ult;max sult
ð23Þ
The ultimate slip sult can be set equal to 0.740 mm from the reference beam tests with CFRP and GFRP composites. In the cases of aged beams, the ultimate slips (sult) are adjusted by the factor kREF. Eq. (22) with j = 1.32 for CFRP-to-concrete interfaces subjected to salt fog cycles, overestimated the effective bond length by 13.7% when compared with the most unfavourable results obtained from the numerical analysis based on Ueda and Dai [18] bond–slip law, as shown in Table 8. For the aged GFRP-to-concrete interfaces, the freeze/thaw cycles caused the highest deviation (65 mm) found between the numerical analysis and Eq. (22) for j = 1.32. Fig. 14 shows the comparison between the effective bond lengths calculated from Eq. (22) and the numerical analysis, again for j = 1.32.
6. Conclusions The experimental study on the degradation of CFRP and GFRP-toconcrete bond–slip laws due to several accelerated environmental
M.A.G. Silva et al. / Composites: Part B 55 (2013) 374–385
actions on beams externally strengthened with FRP plates bonded to their soffit, made on simply supported small beams connected at mid-span by a stainless steel hinge, led to conclusions summarized below. – The approximation used for the stress/strain-slip constitutive curves based solely on the maximum extension measured in the FRP composite predicted satisfactorily the performance of the joints. – CFRP-to-concrete interfaces showed an increment of the maximum bond stresses smax attained at a shorter slip smax. – The post-peak stage of the bond–slip curve decays more rapidly than for the reference beams, determines lower fracture energy (GF) and reduced carrying load capacity, decreasing the maximum load transmitted to the CFRP aged plate. – Salt water immersion of EB-GFRP systems tended to increase the maximum bond stress (smax) associated to a corresponding shorter slip smax, and did not impose significant degradation to the GFRP-to-concrete interface. – Dry/wet cycles with salt water, actually, improved the performance of the systems. Contrarily, the freeze/thaw cycles imposed a loss of maximum bond stress. The significant loss of fracture energy (GF) for the specimens subjected to the freeze/thaw cycles advises an environmental reduction factor of 0.80. – Effective lengths Leff obtained are longer than those proposed by most codes. – The environmental effects reduced Leff on the CFRP beams, but, in general, increased it for GFRP reinforced beams. – Bond–slip curves that approximate the effects of aging through dimensionless parameters, calibrated by experimental data, are suggested and were applied for comparison and estimates of results. – A complementary study was made of the effects on effective bonded length showing that it decreased, in general, except for EB-GFRP joints subjected to 5000 and 10,000 h of exposure to freeze/thaw cycles when the effective bond length increased. – For the EB-CFRP both the hygrothermal and the salt fog cycles caused an initial loss of load carrying capacity essentially recovered at 10,000 h as well as a decay on the fracture energy GF which can be explained by a post-curing effect. – Environmental reduction factors are tentatively indicated, though a larger number of tests, as well as a more accurate estimate of the correlation between accelerated tests and real performance, require further studies and caution.
Acknowledgment The authors are grateful to Fundação para a Ciência e Tecnologia for partial financing of the work under Project PTDC/ECM/100538/ 2008.
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References [1] Dutta PK, Hui D. Low-temperature and freeze-thaw durability of thick composites. Compos Part B Eng 1996;27(3–4):371–9. [2] Bisby LA, Green MF. Resistance to freezing and thawing of fiber-reinforced polymer-concrete bond. ACI Struct J 2002;99(2):215–23. [3] Silva MAG, Biscaia H, Chastre C. Influence of temperature cycles on bond between GFRP and concrete. ACI Struct J 2013 [in press]. [4] Nardone F, Ludovico M, Basalo FJC, Prota A, Nanni A. Tensile behavior of epoxy based FRP composites under extreme service conditions. Compos Part B Eng 2012;43(3):1468–74. [5] Ren H, Hu A, Zhao G. Freeze-thaw resistance behavior of bonded joints between FRP and concrete. J Dalian Univ Technol 2003;43(4):495–9. [6] Mukhopadhyaya P, Swamy RN, Lynsdale CJ. Influence of aggressive exposure conditions on the behaviour of adhesive bonded concrete–GFRP joints. J Constr Build Mater 1998;12(8):427–46. [7] Silva MAG, Biscaia H. Degradation of bond between FRP and RC beams. Compos Struct 2008;85(2):166–74. [8] Sun W, Mu R, Luo X, Miao CW. Effect of chloride salt, freeze – thaw cycling and externally applied load on the performance of the concrete. Cem Concr Res 2002;32(20):1859–64. [9] Zhou J, Lucas JP. Hygrothermal effects of epoxy resin. Part I: The nature of water in epoxy. Polymer 1999;40(20):5505–12. [10] Thomason JL. The interface region in glass fibre-reinforced epoxy resin composites: 2. Water absorption, voids and the interface. Composites 1995;26(7):477–85. [11] Silva MAG. Aging of GFRP laminates and confinement of concrete columns. Compos Struct 2007;79(1):97–106. [12] Au Ching, Büyüköztürk Oral. Peel and shear fracture characterization of debonding in FRP plated concrete affected by moisture source. J Compos Constr 2006;10(1):35–47. [13] Dai Jian-Guo, Yokota Hiroshi, Iwanami Mitsuyasu, Kato Ema. Experimental investigation of the influence of moisture on the bond behavior of FRP to concrete interfaces. J Compos Constr 2010;14(6):834–44. [14] EN 12390-3:2009 with corrigenda EN 12390-3:2009/AC 2011:08 of 31.08.11. [15] Eurocode 2 (EC2). Design of concrete structures – general rules and rules for buildings. EN 1992-1-1, December, 2004. [16] NP-EN 10002-1. Materiais metálicos – ensaio de tracção. Comité Europeu de Normalização; 1990, [in portuguese]. [17] ASTM D3039/D3039M-08. Standard test method for tensile properties of polymer matrix composite materials 2008. [18] Ueda T, Dai JG. Interface bond between FRP sheets and concrete substrates: properties, numerical modeling and roles in member behavior. Progr Struct Eng Mater 2005;7(1):27–43. [19] Nakaba K, Kanakubo T, Furuta T, Yoshizawa H. Bond behavior between fiberreinforced polymer laminates and concrete. ACI Struct 2001;98(3):359–67. [20] Biscaia H, Chastre C, Silva MAG. Linear and nonlinear analysis of bond–slip models for interfaces between FRP composites and concrete. Compos Part B Eng 2013;45(1):1554–68. [21] Fédération Internationale du Béton (FIB). Bulletin d’information no. 14, externally bonded FRP reinforcement for RC structures, July, 2001. [22] Ueda T, Dai JG. Interface of fiber reinforced polymer laminates externally bonded to concrete substrate: From test methods to bond modeling. BBFS2005, Hong Kong, 7–9, December, 2005. [23] Lucas D, Biscaia H, Silva MAG, Chastre C. Factores que influenciam o desempenho da ligação GFRP/betão. In: Proceedings Encontro Nacional Betão Estrutural BE2012, FEUP, 24–26, October, 2012, [in Portuguese]. [24] ACI Committee 440. Guide for design and construction of externally bonded FRP systems for strengthening concrete structures. American Concrete Institute, ACI 440.2R-02, Farmington Hills, MI, USA; 2002. [25] Dejke V. Durability FRP reinforcement in concrete – Literature review and experiments. Thesis for the degree of licentiate of engineering. Department of Building Materials of Chalmers University of Tlogy, Göteborg, Sweden; 2001.
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Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Bond–slip on CFRP/GFRP-to-concrete joints subjected to moisture, salt fog and temperature cycles Manuel A.G. Silva, Hugo C. Biscaia ⇑, Rui Marreiros UNIC, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
a r t i c l e
i n f o
Article history: Received 18 January 2013 Received in revised form 27 May 2013 Accepted 9 June 2013 Available online 20 June 2013 Keywords: A. Laminates B. Debonding B. Interface B. Environmental degradation Concrete
a b s t r a c t Research on bond between composite and concrete on beams externally reinforced with fiber reinforced polymers (ERBs) has generated many publications, but uncertainties remain. The issue of the long term behavior of those joints, especially the effect that severe and prolonged environmental actions may induce, justifies the search for additional data and recommendations to avoid premature debonding and failures. The present paper devotes attention to the effects of environmental aging on the constitutive bond-slip curves. Aging was imposed on an accelerated manner and several actions used to degrade the behavior of the joints evaluated by mechanical testing. Prismatic blocks of reinforced concrete linked on the upper side by a stainless steel hinge and externally bonded by a continuous strip of carbon or glass fiber reinforced polymer (CFRP or GFRP) adherent to the soffit were made so as to fit in commercially available laboratory climatic chambers. After aging, they were tested till failure under four point loading. Aging conditions imposed on the CFRP beams were (i) cycles of high-low relative humidity, (ii) salt fog cycles, and, on the GFRP beams, (iii) salt fog cycles, (iv) dry/wet cycles (water with 5.0% weight of NaCl), (v) total immersion in salt water, and (vi) freeze/thaw cycles. The results of the experimental program enabled the proposal of bond-slip laws that take into account the aging of the beams. They also showed that salt fog cycles were more severe in the case of CFRP, while freeze/thaw cycles were more degrading on bond of GFRP-to-concrete. The salt water effects on the GFRP beams appeared to be beneficial, most likely by improving the tensile strength of concrete. Numerical modeling certified by the obtained experimental data is presented that allows more general estimates of the environmental effects. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction The accurate estimate of the behavior of the FRP-to-concrete interfaces is essential to predict the performance of structural members with external reinforcement of FRP and many authors have devoted their work to the characterization and modeling of the response of such structures under external loading. However, long term and severe environmental conditions may cause premature failure by degrading bond of those joints, or decrease the load capacity and durability of the structures, and require further attention. Namely, the effectiveness of the strengthening depends on bond between the composite and concrete substrate and aging may deteriorate it. The study of these aspects is the major scope of the work, specifically geared to establish the influence of different types of aging on the bond–slip constitutive relationships. The exposure of externally bonded composites to cycles of temperature is known to cause microcracking with increased ⇑ Corresponding author. Tel.: +351 21 2948580; fax: +351 21 2948398. E-mail address: [email protected] (H.C. Biscaia).
absorption of water [1] and crystallization. The same type of degradation may result from differential thermal expansion of the interfacing materials creating stresses, damage and relative changes of stiffness [2]. A short survey of research on thermal effects can be found e.g. in [3]. The effects of temperature cycles on the composites have also been recently described by Nordone et al. based on changes of tensile strength of CFRP and GFRP coupons [4]. Data obtained on GFRP showed a slight decrease of tensile strength for increasing temperatures and a reduction of mechanical properties of CFRP for ‘‘extreme’’ temperatures. Nevertheless, changes due to freeze–thaw cycles were non-significant contradicting other experimental work that reported degradation of the bond strength between concrete and FRP after aging caused by freeze–thaw cycles [5]. Mukhopadhyaya et al. [6] also studied the effects of freeze– thaw cycles on specimens made of two different types of concrete and concluded that there was a slight decrease of strength for the 37.1 MPa concrete and a more than triple average rise of 10.7% for the specimens fabricated with 48.6 MPa concrete. The decrease of bond stresses was, respectively, 12.6% and 24.7%, while the bond
1359-8368/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.06.015
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transfer length increased in both cases, confirming the known importance of the type of concrete, namely its tensile strength, in addition to composition and permeability. The failure mechanisms found were essentially associated with adhesive rupture of the specimens at the interface. This latter conclusion contradicts more recent findings that showed mostly cohesive type of failures in the layer of concrete adjacent to the adhesive (e.g. [7]) perhaps because the tensile ultimate stress of the adhesive used by the authors was low. Synergistic effects of freeze–thaw cycling combined with effects of a sodium chloride solution and externally applied load are reported by Sun et al. [8]. The results show more severe effects on the specimens subjected to freeze–thaw cycles in the salt water solution than those in water. The influence of wet/dry cycles has also been studied in connection with bond failures since moisture sorption modifies the mechanical properties of the epoxy matrix, causing the lowering of the glass transition temperature Tg. The mechanisms of migration of water into epoxy resins have been established [9]. Migration of water into the laminates and in the interface degrade the strength of the structures [10–13]. A much more comprehensive listing of contributions could be presented, but the bulk of all available and important work has not addressed the modifications of the bond–slip constitutive laws by environmental aging, an important topic with scarce data. Such is the major objective of this work and the experimental program and results follow. 2. Experimental program The experimental program is the result of the work required for completion of two master theses and consisted essentially on
subjecting small reinforced concrete beams, with external reinforcement of CFRP or GFRP, to pre-defined types of aging and testing the beams till failure, at prescribed ages. The results provided the evolution of the load carrying capacities, and the failure modes and allowed the study of the degradation of bond between the strengthening composites and concrete. The geometry of the beam specimens is shown in Fig. 1a. Two RC blocks (total length L = 620 mm, height h = 150 mm, width w = 100 mm) were connected by a stainless steel hinge at midspan, with a two layered FRP strip 80 mm wide symmetrically bonded to the soffit for 520 mm. The beams were dimensioned so that failure would take place at the interface region defined by the composite, the adhesive and the adjacent layers of concrete. Epoxy resin used to bond the composite to the concrete was of the same type as the epoxy resin used in the impregnation of the FRP composite. The concrete surface to be bonded to the composite was treated by dry sand blasting so as to create an adequate level of roughness, and cleaned before the application of the adhesive. 2.1. Materials characterization The beams reinforced with CFRP and GFRP, henceforth designated as EB-CFRP and EB-GFRP, respectively, were made at different times. Two concrete mixes resulted and the tests to characterize them mechanically followed standardized procedures [14]. The concrete mix for EB-CFRP had an average cube compressive strength at 28 days fcm = 26.1 MPa, whereas the concrete of the beams externally reinforced with GFRP (EB-GFRP) had a cube strength, at the same age, fcm = 34.9 MPa. The corresponding average tensile strength (fctm) for each case was calculated as proposed in EC2 [15] leading to fctm = 2.1 MPa for EB-CFRP and 2.7 MPa for the case of the EB-GFRP beams.
RC block
Metallic hinge CFRP or GFRP
(a)
Rv Rh Metallic hinge
FFRP
(b) Fig. 1. (a) Geometry of specimens to be aged and mechanically tested and (b) ‘‘Half-beam’’ free body diagram for 4 point bending test.
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2.2. Selected environmental conditions
Steel: S400 Coating: 10mm
The conditions imposed on the specimens were basically periodic, with each cycle lasting 24 h, as shown in Fig. 3. In addition to the conditions represented in Fig. 3, total immersion in a solution of deionized water with NaCl was also imposed. Briefly, the imposed aging procedures can be described in five categories:
2Ø3 Stirrups: Ø3 @50mm 2Ø3
Fig. 2. Steel reinforcements of the beams.
Mild steel rods of £3 mm diameter were used. Four rods were placed longitudinally, two in the compression zone and two in the tensile zone of the beam blocks. The stirrups were spaced 5 cm apart. Fig. 2 shows the scheme assumed for the steel reinforcements. The mechanical properties of the steel were obtained by uniaxial tensile tests according to [16] that led to fsy = 450 MPa and yielding strain esy = 0.23%. The composite strips were applied by the wet layup technique in two layers. Coupons of 25 250 mm were cut from plates. The nominal thicknesses, per layer, were used as indicated by the suppliers i.e. 0.165 mm for CFRP and 1.270 mm for GFRP. The tensile strength was evaluated in agreement with ASTM D3039/ D3039M [17]. The tests were made in a universal Zwick tensile machine with a load capacity of 50 kN. The constitutive laws were elastic and linear showing very brittle failure. The ultimate tensile strain and strength obtained by testing the CFRP flat coupons were efm = 1.54%, ffm = 3937 MPa while the GFRP flat coupons led to efm = 2.46%, and ffm = 500.2 MPa. The corresponding elastic moduli were found as 241,000 MPa and 20,390 MPa.
(i) Hygrothermal cycles – 12 h exposed to 20% of relative humidity (RH) followed by 12 h at 90% RH and a constant temperature of 40 °C. (ii) Salt fog cycles consisting of 8 h of spray followed by 16 h of drying at constant 35 °C. (iii) Dry/wet cycles of salt water, 12 h at each state, with 50 g of sodium chloride (NaCl) per liter of water. Freeze/thaw cycles were defined by the imposition of 10 °C for 12 h followed by another 12 h at +10 °C. The transition from one temperature to the other was imposed at the rate of 1.5 °C/min, according to a default programmed rate of the chamber. (iv) Total immersion in salt water with 50 g of sodium chloride (NaCl) per liter of water. At three time stages of each aging, 3000 h, 6000 h and 10,000 h for CFRP and 1000 h, 5000 h and 10,000 h for GFRP, specimens were removed from the chambers and their load carrying capacity was tested as described in the sequel. The results obtained permitted the study of the time evolution of the performance of the beams and the comparison with that obtained for the reference specimens (tested at 0 h).
Fig. 3. Aging cycles considered: hygrothermal or moisture, salt fog, dry-wet and temperature.
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2.3. Bending tests Prismatic blocks of reinforced concrete linked in the compression zone by a stainless steel hinge and externally bounded by a two-layered continuous strip of carbon or glass fiber reinforced polymer (CFRP or GFRP), 80 mm wide and 250 mm long, adherent to the soffit were tested. Before bonding the FRP composite to the concrete surface, dry sand blasting was applied. The beams were simply supported at two points, distant 560 mm. Two equal and symmetric vertical forces were imposed on each specimen at a distance of 235 mm from the nearest support and loading was increased in a static way until failure of the beams. The strains in the CFRP and GFRP composites were measured with strain gauges spaced at 40 mm. Two load cells were placed underneath both supports and the loads were applied with a hydraulic jack. One Linear Variable Displacement Transducer (LVDT) measured the vertical displacement of the beam at midspan. Thirty beams were tested in bending and their identification and the type of environmental conditions subjecting each one of them is shown in Table 1. The condition of vanishing moment at the hinge leads to the traction force FFRP acting on the composite in terms of the applied load P, Fig. 1b.
F FRP ¼
1 ha P 2 b
ð1Þ
with a = 45 mm, b = 280 mm and h = 133 mm. Nonlinear bond–slip curves were calculated from the strain vs. slip (e–s) relationship defined at the loaded end of the FRP as recommended by Ueda and Dai [18]. The relationship e–s was approximated by a smooth curve given by
eðsÞ ¼ emax ð1 ens Þ=ð1 ensult Þ ¼ emax ð1 ens=sult Þ for s < sult
ð2Þ
where emax is the maximum measured strain in the FRP composite is a coefficient obtained the loaded end, sult is the ultimate slip and n Table 1 Designation of the tested specimens and type of environmental exposure. Composite
Specimen
Aging actions
Hours of exposure
CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP CFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP GFRP
REF-C-01 REF-C-02 RH-C-01 RH-C-02 RH-C-03 RH-C-04 RH-C-05 RH-C-06 SF-C-01 SF-C-02 SF-C-03 SF-C-04 SF-C-05 SF-C-06 REF-G-01 REF-G-02 REF-G-03 SF-G-01 SF-G-02 SF-G-03 DW-G-01 DW-G-02 DW-G-03 FT-G-01 FT-G-02 FT-G-03 TI-G-01 TI-G-02 TI-G-03
Reference Reference Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Salt fog cycles Salt fog cycles Salt fog cycles Salt fog cycles Salt fog cycles Salt fog cycles Reference Reference Reference Salt fog cycles Salt fog cycles Salt fog cycles Dry/wet cycles Dry/wet cycles Dry/wet cycles Freeze/thaw cycles Freeze/thaw cycles Freeze/thaw cycles Total immersion Total immersion Total immersion
0 0 3000 3000 6000 6000 10,000 10,000 3000 3000 6000 6000 10,000 10,000 0 0 0 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000
377
to fit e(s) to the experimental results by the least square procedure minimizing the sum of the squares of the difference between are given by approximate and experimental values, emax and n
emax ¼ emax =ð1 ensult Þ ¼ emax =ð1 en Þ
ð3Þ
¼ nsult n
ð4Þ
The truncation of the interval of application of (2) at s = sult appears physically justifiable, since the expression respects the zero deformation for no slip and the maximum for the ultimate slip. , as well as a dimensionThe choice of a dimensionless parameter n less maximum deformation emax , both defined from a quantity obtainable from experimental data, sult allows easier generalization of results obtained. Variation of those parameters with aging and material characteristics will be shown in the sequel. The shear stress s(s) is given by
sðsÞ ¼ Ef tf
de ds ds dx
ð5Þ
where Ef and tf are the Young modulus and the thickness of the FRP composite, respectively defines the bond–slip curve. Approximating the strain by e = ds/dx, the bond–slip curve can be written
sðsÞ ¼ 4 smax ðens e2ns Þ
ð6Þ
Designating as maximum slip smax the slip that corresponds to the maximum value of bond stress (smax) it is
smax ¼
n e2max Ef t f 4
ð7:aÞ
lnð2Þ n
ð7:bÞ
and
smax ¼
The average fracture energy is given by the area below the average bond–slip curve
GF ¼
e2max Ef tf 2
ð8Þ
The nonlinear character of the slip distribution can be approximately obtained from the solution of the Boundary Value Problem (BVP) posed by the governing equation and conditions below (e.g. [19]), 2
sðsÞ ¼0 Ef t f
ð9:aÞ
ds F ¼ efjx¼0 ¼ dx Af Ef
ð9:bÞ
ds ¼ efjx¼L ¼ 0 eff dx
ð9:cÞ
d sðxÞ 2
dx
The nonlinearity of s(s) has originated the consideration of several linear and nonlinear types of constitutive bond–slip curves to approximate s(s), each leading to approximate slip and stress distributions as described e.g. in [20]. The maximum load Fmax transmitted to the FRP plate is reached when Leff is attained and, designating by emax the strain measured at the pulling end, it is
F max ¼ emax bf t f Ef
ð10Þ
3. Reference beams – experimental results The results obtained with tests on control beams not subjected to artificial aging serve for measuring the degradation experienced
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Fig. 4. EB-CFRP beam REF-C-01. View prior to failure and view of CFRP strip after failure.
TI-G-02
REF-G-02
Fig. 5. Surface failure for EB-GFRP beams REF-G-02 and TI-G-02.
by the aged beams. Fig. 4 shows the aspect of reference beam EBCFRP (REF-C-01) after failure, displaying a cohesive type of failure inside the concrete layer next to the adhesive-concrete contact surface. Fig. 5 shows the aspect of beams REF-G-02 and TI-G-02 after failure. In the case of the EB-GFRP reference beam the type of failure is similar to that exhibited by REF-C-01, whereas for the total immersion case, TI-G-02, failure was ‘‘less cohesive’’. The bond–slip laws were obtained for each specimen according to the methodology described in Section 2.3. The corresponding average bond–slip curves obtained for the reference beams, for both CFRP and GFRP, are shown in Fig. 6, and were obtained by averaging the individual curves that define the gray shade. The values obtained for CFRP and GFRP were, respectively, ¼ 3:103; e ¼ 0:481 sult = 0.737 mm, n and sult = 0.733 mm, ¼ 2:778; e ¼ 0:597. n In the case of EB-CFRP beams the maximum bond stress smax is larger than in the case of EB-GFRP beams while the maximum slip (smax) is smaller. The post-peak branches of both composites have similar shapes and the ultimate slips sult are almost equal. The effective length has important physical meaning and design consequences and several proposals can be found in the literature on how to estimate it from the characteristics of the component materials. Nakaba et al. [19] approximated Leff by the distance between the points at which the bond stresses are 10% of the maximum bond stress (smax). A more direct approach is given in fib bulletin 14 [21], that recommends
Leff
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ef t f ¼ c2 fctm
ð11Þ
with c2 = 2.0 for CFRP beams. For interfaces of GFRP-to-concrete the code does not specify c2 and the choice c2 = 0.8 was made in this study based in published results [21], though this value requires further experimental data and possible rectification.The closed form solution of the boundary value problem for linear bond–slip laws leads to [20]:
Leff
sffiffiffiffiffiffiffiffiffiffiffiffi Ef t f ¼ j sult GF
ð12Þ
where j is a parameter that depends on the bond–slip law. In the pffiffiffi case of the rigid-plastic bond–slip law jp¼ffiffiffi 2; for the linear ascending law j can bepapproximated by 2 2; for the rigid-linear ffiffiffi softening law j ¼ p 2=4. In the latter case, the effective bond length is 344 mm for CFRP and 295 mm for GFRP. Finally, the maximum value Fmax of the force FFRP sustained by the composite was obtained applying the definition: the bonded length was successively increased until the force reached a maximum value, utilizing (3) as the constitutive relation and limiting sult to 0.74 mm. The results are plotted in Fig. 7. The effective length obtained for EB-CFRP and EB-GFRP is approximately 325 mm and 275 mm, in the reference cases.
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Fig. 6. Bond–slip and fracture energy curves for the reference beams.
Fig. 7. Calculation of effective length for both EB-CFRP and EB-GFRP reference beams from the definition.
4. Aged beams Table 2 Major bond parameters for the reference beams.
smax (MPa)
smax (mm)
sult (mm)
GF (N/mm)
1.9 1.6
– –
– –
– –
0.17 0.18
0.74 0.73
0.91 0.82
Model associated with rigid-linear softening CFRP 276 1.9 – GFRP 242 1.6 –
0.74 0.73
0.91 0.82
Criterion from Nakaba et al. [17] CFRP 370 1.9 GFRP 350 1.6
0.74 0.73
0.91 0.82
Composite
Leff (mm)
Fib bulletin 14 CFRP 142 GFRP 155
Model associated with Eq. (3) CFRP 325 1.9 GFRP 275 1.6
0.17 0.18
The major bond parameters for the reference beams are summarized in Table 2.
The results of the bending tests for aged specimens are summarized next. The evolution of the parameters like the maximum bond stress smax and the corresponding slip smax, the ultimate slip sult and the fracture energy GF along the different aging processes is shown in Tables 3 and 4 for EB-CFRP and EB-GFRP beams, respectively. 4.1. CFRP The comparison of the values reached on the tests after relative humidity or hygrothermal cycles and salt fog cycles with those of the reference specimens, on EB-CFRP beams, is possible consulting Table 3. The maximum bond stress smax experienced a significant increment while the maximum slip (smax) fracture energy GF and maximum strain in the composite eFRP,max = emax decreased with the duration of exposure. The maximum load Fmax transmitted to the FRP plate was calculated according to
F FRP ¼ eFRP bf t f Ef
ð13Þ
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Table 3 Bond characterization for the EB-CFRP beams. Aging
Exposure (h)
Fmax (kN)
sult(mm)
emax (%)
¼ nSult n
smax (MPa)
smax (mm)
GF (N/mm)
Reference Hygroth. Hygroth. Hygroth. Salt fog Salt fog Salt fog
0 3000 6000 10,000 3000 6000 10,000
39.8 32.2 34.1 39.3 37.6 32.8 37.0
0.737 0.555 0.569 0.646 0.754 0.577 0.579
0.48 0.371 0.394 0.393 0.443 0.371 0.423
3.103 6.288 4.580 4.813 5.112 5.805 4.800
2.09 3.36 2.61 2.49 2.80 3.00 3.20
0.165 0.066 0.089 0.096 0.105 0.071 0.086
0.83 0.63 0.64 0.67 0.83 0.61 0.77
Table 4 Bond characterization for the EB-GFRP beams. Aging
Exposure (h)
Fmax (kN)
sult (mm)
emax (%)
¼ nSult n
smax (MPa)
smax (mm)
GF (N/mm)
Reference Salt fog Salt fog Salt fog Dry/wet Dry/wet Dry/wet Freeze/thaw Freeze/thaw Freeze/thaw Total immersion Total immersion Total immersion
0 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000 1000 5000 10,000
25.4 26.8 26.4 24.8 33.3 33.3 33.4 25.3 23.0 22.0 33.8 35.5 36.3
0.733 0.776 0.778 0.873 1.016 1.162 0.937 0.886 0.679 0.696 0.790 1.044 0.969
0.60 0.69 0.66 0.61 0.74 0.74 0.75 0.55 0.53 0.51 0.75 0.79 0.81
2.778 2.995 4.831 7.621 4.399 4.787 3.757 7.256 2.960 2.735 4.708 5.001 3.857
1.77 2.12 2.23 3.03 2.06 2.03 2.19 2.21 1.36 1.24 2.28 2.65 2.22
0.183 0.180 0.112 0.079 0.160 0.168 0.173 0.085 0.159 0.177 0.116 0.145 0.174
0.72 0.89 0.70 0.69 0.91 0.95 1.00 0.54 0.50 0.48 0.74 1.08 1.03
Fig. 8. Bond–slip curves at CFRP-to-concrete interface and different stages of aging for cycles of moisture and salt fog.
where emax is the strain measured at the pulling end of the FRP plate. Fig. 8 shows the bond–slip curves of CFRP-to-concrete, drawn for 3000, 6000 and 10,000 h of exposure to the varying moisture and to the salt fog cycles. The effects of salt fog are easier to characterize, showing typically an increase of maximum shear for smaller slip, and a decrease of the ultimate slip and fracture energy, with regard to the reference case. The hygrothermal cycles defined by the 20% and 90% limiting cases of relative humidity (RH) caused a high increase of smax at 3000 h, a change that has not been satisfactorily interpreted. Otherwise the observed trends are similar to those present after salt fog cycles. At 10,000 h, the curves obtained for salt fog cycles caused nearly a 9% decrease of emax against 15% for the moisture cycles. The same less significant decrease by the salt fog cycles was observed for the fracture energy. The results also showed that the maximum load transmitted to the CFRP plate decreased in agreement with its lower fracture energy.
4.2. GFRP Table 4 displays the effects of salt fog, dry/wet and freeze–thaw cycles, as well as total immersion on beams externally reinforced with GFRP. The values show an increment of the maximum bond stress smax observed for all aging environments except for the freeze/thaw cycles. In the latter case, at 10,000 h, a reduction of 30.8% was found for smax. The maximum strain in the composite eFRP,max decreased also for freeze/thaw cycles (from a negligible 2% to 14%) and increased about 30% for tidal-like dry/wet cycles and 32–41% for total immersion in salt water. In terms of the fracture energy GF, the freeze/thaw cycles caused a reduction of more than 30% throughout the entire duration of this type of aging. At 10,000 h, the fracture energy showed significant increases for dry/wet cycles (above 38%) and total immersion (above 49%), both in salt water, while it decreased 5% for salt fog cycles. Fig. 9 displays the bond–slip curves for GFRP-to-concrete interfaces for each stage of exposure and aging by salt fog, dry/wet and freeze–thaw cycles, and total immersion in salt water.
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381
Fig. 9. Bond–slip curves for EB-GFRP joints at different stages of aging and salt fog, dry/wet and freeze–thaw cycles, and constant immersion in salt water.
The changes of the bond–slip curves appear to indicate, for some types of aging, a modification of the relative stiffness by comparison with the reference cases, as mentioned in [22]. The shear stiffness of the bond layer, estimated by the ratio of smax/smax, appears as significantly increased at 10,000 h for salt-fog cycles, while not much affected for dry/wet cycles and total immersion in salt water. For freeze–thaw cycles that stiffness actually decreases. Fig. 10 graphically summarizes the evolution of maximum load transmitted to the FRP plate (FFRP,max), maximum bond stress (smax), maximum slip (smax) and fracture energy (GF) for all cases, normalized by the values for the corresponding reference specimens. All environmental conditions that include NaCl show smax/smaxRef > 1. Only dry/wet cycles for EB-GFRP did not cause effective loss of load capacity measured. Comparing with maximum slip (smax) obtained at 0 h of aging, all specimens exhibited lower slips after aging. Freeze–thaw caused decrease of all the indicators shown in the diagrams.
described in the sequel, respectively for CFRP and GFRP. It is noticed that, in the future, care must be exercised to make sure that eventual dependence on strength of concrete, surface treatment or desired duration of structure may need to be considered. Results from studies on type of surface treatment seem to indicate that, although important in itself, the treatment alone is not decisive for the quantification of the degradation of bond [23]. From the numerical solution of the problem governed by Eq. (9) and for different types of bond–slip curves (the thickest curves in Figs. 11 and 12), the maximum load transmitted to the FRP plate was determined and compared with the lower value obtained experimentally for each environmental condition studied. The results are depicted in Fig. 13 and show good agreement with those obtained experimentally, with deviations smaller than 20% except for dry/wet cycles on GFRP where the deviation reached 20.3%. 5.2. Environmental reduction coefficients
5. Influence of aging on bond – recommendations
The quantification of environmental reduction factors aE is submitted hereafter. Firstly, an expression is submitted for the maximum shearing stress after environmental aging,
5.1. Proposed design bond–slip curves
smax;ENV ¼ a1
The study aims at the definition, for each environmental condition, of a bond–slip curve associated with minimum fracture energy found for each combined case of type of composite and type of environmental condition, as well as the ensuing determination of bond related parameters e.g. the maximum load transmitted to the FRP plate (FFRP,max). Figs. 12 and 13 show the aged bond–slip curves that correspond to selected values of the parameters as
where GF,ENV is the ‘‘aged’’ fracture energy; n = 4.2 mm1 for CFRPto-concrete interfaces and n = 3.8 mm1 for GFRP-to-concrete interfaces; and a1 is a corrective coefficient that adjusts the value of maximum bond stress to that determined for the reference condition, Tables 3 and 4. A coefficient a2 that approximates the fracture energy after aging GF,ENV to the reference GF obtained at 0 h is given by
n GF;ENV 2
ð14Þ
382
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Fig. 10. Relative evolution of: (a) maximum load transmitted to the FRP plate; (b) maximum bond stress; (c) maximum slip; and (d) fracture energy.
Fig. 11. Proposed bond–slip curves for aged EB-CFRP joints for cycles of relative humidity and salt fog.
GF;ENV ¼ a2
e2FRP;max Ef tf 2
¼ a2 GF
ð15Þ
Considering that the average experimental maximum strains for CFRP and GFRP were about 30% and 26% of the rupture strain from tensile tests (efm), preliminary values are suggested for a1 and a2 and displayed in Table 5. In addition, the maximum strain in the FRP plate eFRP,max can be written as
eFRP;max ¼ uREF efm
ð16Þ
and in the case of CFRP-to-concrete interfaces uREF = 0.30 and in the case of GFRP-to-concrete interfaces uREF = 0.26.The slip (smax) for
the maximum extension in the FRP (eFRP,max) for each aged bond– slip curve, can be expressed as
smax ¼
lnð2Þ
ð18Þ
a1 n
Maximum strains in FRP plate after aging, can be also obtained independently of the shape of the bond–slip curve [20] from the fracture energy of the reference specimens according to:
eENV FRP;max ¼ aE
sffiffiffiffiffiffiffiffiffiffiffiffi 2GF Ef t f
where aE is an environmental parameter defined as:
ð19Þ
383
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Fig. 12. Proposed bond–slip curves for aged EB-GFRP joints and salt fog, dry/wet and freeze–thaw cycles, and constant immersion in salt water.
Table 5 Values proposed for the coefficients a1 and a2.
Fig. 13. Comparison between the maximum loads transmitted to the FRP plate estimated from the proposal and from the experiments.
Fig. 14. Comparison between the effective bond lengths obtained from Eq. (22) with j = 1.32 and from the numerical analysis, Eq. (9).
Interface
Aggressive environment
Coefficient, a1
Coefficient, a2
CFRP-to-concrete
Reference Hygrothermal cycles Salt fog cycles
1.00 1.95 2.20
1.00 0.82 0.80
GFRP-to-concrete
Reference Salt fog cycles Dry/wet cycles Freeze/thaw cycles Total immersion
1.00 1.45 1.14 1.03 1.49
1.00 0.95 1.09 0.84 0.99
aE ¼
eENV FRP;max eFRP;max
ð20Þ
and is quantified in Table 6. The results presented in Table 6 attempt at comparing the environmental reduction factors proposed (aE) for beams externally bonded with CFRP with those recommended by ACI 440 [24] for degradation of CFRP laminates. The GFRP-to-concrete interfaces advise environmental reduction factors higher than 0.75 which is the upper limit recommended for GFRPto-concrete interfaces. To a certain extent, the tabulated values show that the degree of degradation of the carrying capacity of the structural member is relatively close to that observed for FRP composite laminates. The results found for the GFRP-to-concrete interfaces subjected to actions involving salt, are thought to be due to (i) improvement of the concrete strength; or/and to (ii) a time of exposure not long enough to cause more severe degradation. This aspect of converting the results obtained into conclusions valid for longer exposures is being treated based on Arrhenius equation (e.g. [25]), but no definitive results have yet been obtained. The maximum load transmitted to the FRP plate taking into account the environmental effects can be calculated by
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Table 6 Comparison between the values of the environmental reduction factors found in current study (aE) and those given in ACI 440 (CE) for laminates [24]. Interfaces
Environment
Proposed reduction factor aE for structures
ACI reduction factor CE for laminates
CFRP-to-concrete
Reference Hygrothermal cycles Salt fog cycles
1.00 0.81
1.00 0.85–0.95
GFRP-to-concrete
Reference Salt fog cycles Dry/wet cycles Freeze/thaw cycles Total immersion
1.00 0.97 1.00 0.80
Table 8 Evolution of the effective bond length (Leff) for the EB-GFRP-to-concrete joints. Composite
Type of aging
GFRP
Reference Salt fog cycles Salt fog cycles Salt fog cycles Dry/wet cycles Dry/wet cycles Dry/wet cycles Freeze/ thaw cycles Freeze/ thaw cycles Freeze/ thaw cycles Total immersion Total immersion Total immersion
0.81 1.00 0.50–0.75
0.95
Table 7 Evolution of the effective bond length (Leff) for the EB-CFRP-to-concrete joints. Composite
CFRP
F ENV FRP;max
Type of aging
Reference Hygrothermal cycles Hygrothermal cycles Hygrothermal cycles Salt fog cycles Salt fog cycles Salt fog cycles
fib (mm)
Numerical results (Eq. (9)) (mm)
Rigid-linear softening model – Eq. (12) (mm)
0 3000
370 170
325 200
276 180
6000
230
275
209
10,000
245
275
228
3000 6000 10,000
245 195 205
300 225 225
231 195 190
Hours of exposure
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ aE bf 2GF Ef t f
Rigid-linear softening model – Eq. (12) (mm)
0 1000
350 300
275 275
242 228
5000
220
250
213
10,000
160
175
192
1000
275
325
254
5000
285
350
273
10,000
285
300
239
1000
250
250
226
5000
355
325
267
10,000
400
350
287
1000
320
275
212
5000
230
300
226
10,000
280
300
241
Interface
Aggressive environment
Coefficient kREF
CFRP-to-concrete
Reference Hygrothermal cycles Salt fog cycles
1.00 0.87 1.02
GFRP-to-concrete
Reference Salt fog cycles Dry/wet cycles Freeze/thaw cycles Total immersion
1.00 1.19 1.58 1.21 1.42
ð21Þ
The influence of the different environmental conditions on the effective bonded length (Leff) was studied numerically utilizing the bond–slip curves reproduced in Section 5.1 to find the bond stresses along the composite. In this sub-section it is examined the formulation of the nonlinear problem and its solution for successive lengths of composite bonded to concrete until a length is attained beyond which the traction force cannot be balanced by the generated shear stress field (as in Section 3). The numerical results obtained from the bond–slip law based on the Ueda and Dai proposal [18] (synthetically resumed by Eq. (9)) showed that the effective bond lengths for the CFRP-toconcrete joints are smaller after aging than before, as displayed in Table 7. Differently, in the aged GFRP-to-concrete interfaces, it can be noticed that only the salt fog cycles had smaller effective bond length than the reference situation while the dry/wet and freeze/ thaw cycles led to higher effective bond lengths, as shown in Table 8. The environmental effects on Leff can be calculated conjugating Eqs. (12), (15), and (16) introducing a factor kREF.
Leff
Numerical analysis – Eq. (9) (mm)
Table 9 Values proposed for the coefficient kREF.
5.3. Changes on effective bond length
sffiffiffiffiffi 2 kREF sult ¼ j a2 /REF efm
fib (mm)
Hours of exposure
ð22Þ
where kREF is given by the relation between the maximum ultimate slip obtained at the different stages of exposure and the ultimate
slip, given in Table 9 and j = 1.32 according to the recommendation made in [20].
kREF ¼
sENV ult;max sult
ð23Þ
The ultimate slip sult can be set equal to 0.740 mm from the reference beam tests with CFRP and GFRP composites. In the cases of aged beams, the ultimate slips (sult) are adjusted by the factor kREF. Eq. (22) with j = 1.32 for CFRP-to-concrete interfaces subjected to salt fog cycles, overestimated the effective bond length by 13.7% when compared with the most unfavourable results obtained from the numerical analysis based on Ueda and Dai [18] bond–slip law, as shown in Table 8. For the aged GFRP-to-concrete interfaces, the freeze/thaw cycles caused the highest deviation (65 mm) found between the numerical analysis and Eq. (22) for j = 1.32. Fig. 14 shows the comparison between the effective bond lengths calculated from Eq. (22) and the numerical analysis, again for j = 1.32.
6. Conclusions The experimental study on the degradation of CFRP and GFRP-toconcrete bond–slip laws due to several accelerated environmental
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actions on beams externally strengthened with FRP plates bonded to their soffit, made on simply supported small beams connected at mid-span by a stainless steel hinge, led to conclusions summarized below. – The approximation used for the stress/strain-slip constitutive curves based solely on the maximum extension measured in the FRP composite predicted satisfactorily the performance of the joints. – CFRP-to-concrete interfaces showed an increment of the maximum bond stresses smax attained at a shorter slip smax. – The post-peak stage of the bond–slip curve decays more rapidly than for the reference beams, determines lower fracture energy (GF) and reduced carrying load capacity, decreasing the maximum load transmitted to the CFRP aged plate. – Salt water immersion of EB-GFRP systems tended to increase the maximum bond stress (smax) associated to a corresponding shorter slip smax, and did not impose significant degradation to the GFRP-to-concrete interface. – Dry/wet cycles with salt water, actually, improved the performance of the systems. Contrarily, the freeze/thaw cycles imposed a loss of maximum bond stress. The significant loss of fracture energy (GF) for the specimens subjected to the freeze/thaw cycles advises an environmental reduction factor of 0.80. – Effective lengths Leff obtained are longer than those proposed by most codes. – The environmental effects reduced Leff on the CFRP beams, but, in general, increased it for GFRP reinforced beams. – Bond–slip curves that approximate the effects of aging through dimensionless parameters, calibrated by experimental data, are suggested and were applied for comparison and estimates of results. – A complementary study was made of the effects on effective bonded length showing that it decreased, in general, except for EB-GFRP joints subjected to 5000 and 10,000 h of exposure to freeze/thaw cycles when the effective bond length increased. – For the EB-CFRP both the hygrothermal and the salt fog cycles caused an initial loss of load carrying capacity essentially recovered at 10,000 h as well as a decay on the fracture energy GF which can be explained by a post-curing effect. – Environmental reduction factors are tentatively indicated, though a larger number of tests, as well as a more accurate estimate of the correlation between accelerated tests and real performance, require further studies and caution.
Acknowledgment The authors are grateful to Fundação para a Ciência e Tecnologia for partial financing of the work under Project PTDC/ECM/100538/ 2008.
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