Composites in Construction 2005 – Third International Conference Lyon, France, July 11 – 13, 2005

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STRUCTURAL BEHAVIOUR OF GFRP-CONCRETE HYBRID BEAMS

J.R. Correia, F.A. Branco, J.G. Ferreira Civil Engineering Department, Instituto Superior Técnico (IST), Technical University of Lisbon

Av. Rovisco Pais, Lisboa, 1049-001 Lisboa, Portugal jcorreia@civil.ist.utl.pt fbranco@civil.ist.utl.pt joaof@civil.ist.utl.pt

ABSTRACT: The design of glass fibre reinforced polymer (GFRP) pultruded beams is usually governed by deformability or instability phenomena. To obtain a better use of the material properties, GFRP profiles can be connected to concrete elements with several advantages, associated with the global stiffness and strength of the structural elements. This leads to solutions particularly useful for rehabilitation of old floors or even new construction slabs. This paper presents the results of an experimental research developed to characterize the flexural behaviour of a GFRP-concrete hybrid solution. Shear connection tests were first performed on GFRP I-profiles, connected to concrete with stainless steel bolts. The results of those tests were then used to design simply supported GFRP-concrete hybrid beams that were tested in bending. The flexural behaviour of the GFRP-concrete hybrid beams is discussed, with particular relevance to the effect of the interconnection slip. The overall behaviour of the hybrid beams is compared with that corresponding to a simple GFRP I-profile beam, demonstrating the structural advantages of this new hybrid constructive solution.

1. INTRODUCTION

Glass fibre reinforced polymer (GFRP) pultruded profiles present several advantages when compared with traditional materials. However, low elastic modulus leads to structural designs that are usually governed by instability phenomena and deformability, rather than by strength limitations. Moreover, the low elasticity to shear moduli ratio may also result in a significant contribution of shear to the total deformation, especially in less slender beams. These aspects, and the associated limited use of the material’s ultimate strength, as well as the high costs of these elements, may explain the fact that the use of GFRP profiles in new structures is still limited to a few demonstration projects. The alternative use of GFRP pultruded profiles in GFRP-concrete hybrid structural elements presents however a very interesting potential, either for rehabilitation (particularly in the substitution of existing old wooden floors), or for new constructions. In fact, there are several structural advantages with the connection of GFRP pultruded profiles to concrete compression elements, namely the increase of the flexural stiffness, reducing the structure’s deformability, and the increase of the structure’s strength capacity, making a better use of the GFRP profiles and preventing the buckling phenomena. Previous experiments with different GFRP-concrete elements have been reported by Saadatmanesh and Ehsani [1], Snow [2], Fam and Rizkalla [3], Fam et al [4], Deskovic et al [5] and Seible et al [6]. This paper presents the results of an experimental research concerning the flexural behaviour of a new GFRP-concrete hybrid solution. Shear connection tests were first performed on GFRP I-profiles, made of E-glass fibres and polyester resin, connected to concrete with stainless steel bolts. The results of those shear tests were then used to design simply supported GFRP-concrete hybrid beams that were experimentally tested in bending. The flexural behaviour of the hybrid beams is discussed, regarding both service and ultimate behaviour, with particular relevance to the effect of the interconnection slip. The overall behaviour of the hybrid beams is compared with that of a simple GFRP I-profile, demonstrating the technical advantages of this new constructive solution.

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2. EXPERIMENTAL PROGRAM

The experimental program to study the flexural behaviour of GFRP-concrete hybrid beams was developed in two phases. The first phase considered several shear connection tests performed on GFRP profiles connected to concrete with steel bolts. The results were then used to design the shear connection and the geometry of a 4.00 m span hybrid beam (HB1). This beam was tested and the results were used to study its bending behaviour. A second test phase was then developed to evaluate the effects of the slippage in the interface between the materials, the effective width of the concrete slab and the shear contribution to deformation. Here, the same GFRP profile connected to concrete, was tested, but in a beam with only 1.80 m span (HB2), subjected to a situation with significant shear deformation. Tests were also performed in a simple GFRP beam identical to the one of first phase to evaluate the material properties and the structural effects of the composite section. 2.1 Materials

The GFRP I-profile used in the experimental program was produced by pultrusion and is made of a polyester matrix reinforced with E-glass fibres (62% in weight, determined from burn-off tests). Coupons cut from the original profile were used for extensive mechanical characterization regarding tensile, compressive, flexural and shear properties [7, 8]. The longitudinal flexural modulus (Ep = 38.4 GPa) and the shear modulus (Gp = 3.58 GPa) were determined in a full-scale test performed on a profile similar to that used in the hybrid beam HB1 (phase I). Three different ready-mixed concrete compositions (mixes C1, C2 and C3) were used in the shear tests and the average compressive strength and the Young’s modulus were experimentally determined (table 1). M8 and M10 steel bolts, with a class resistance of 8.8 (ultimate shear strength of 480 MPa), were also used as shear connectors in the shear tests. 2.2 Shear connection tests

Three shear connection tests were performed to analyze the behaviour of the connection between the GFRP profile and concrete with different compositions, using shear connection systems with different bolt diameters (details are given in table 1). The two shear connection systems tested in the phase I of the experimental program (SCS1 and SCS2) allowed for the definition of the steel bolts diameter and the longitudinal spacing between the shear connectors of the GFRP-concrete hybrid beam HB1. In the phase II of the experimental program, the shear connection test (SCS3) allowed to determine the connection’s flexibility and the ultimate strength of hybrid beam HB2. In all shear connection tests, the flanges of a segment of the GFRP profile used in the hybrid beams were connected to two 20 cm concrete cubes with steel bolts (M8 in SCS1 and M10 in SCS2 and SCS3) - figure 1. The profile was then loaded in compression until failure. During the tests the load was monotonically applied until separation of the materials occurred. Load-relative displacement curves are presented in figure 2 and complete results are summarized in table 1, where the connections stiffnesses were estimated according to the definition given by Johnson and May [9].

Table 1 – Shear connection tests. Shear connection

system Concrete

mix fc

(MPa)Ec

(GPa) φφφφbolts (mm)

K(kN/mm)

Fs,max (kN)

Failure mode

SCS1 C1 43.3 33.9 8 50.33 67.46 Bolts (shear) SCS2 C2 39.9 32.5 10 80.73 157.10 Bolts (shear) SCS3 C3 36.9 30.9 10 50.25 108.25 Concrete (compression)

3

Concrete

GFRPProfile

Steel Bolt

Concrete

GFRPProfile

Concrete

Steel Bolt

Concrete

020406080

100120140160180

0 2 4 6 8

Average relative displacement (mm)

Loa

d(k

N)

SCS2

SCS1

SCS3

Figure 1 – Geometry of the test specimen (mm): Plan (top) and section (bottom).

Figure 2 – Load-average relative displacement behaviour in the shear connection tests.

In shear connection test SCS1, connection stiffness was estimated as 50.3 kN/mm and materials separation occurred suddenly due to shear failure of the bolts at a maximum load of only 67.5 kN. In shear connection test SCS2, used to design hybrid beam HB1 connectors, the connection stiffness was estimated as 80.7 kN/mm (40.4 kN/mm per flange), and collapse occurred also due to shear failure of the bolts, at a maximum load of 157.1 kN. Nevertheless, when designing the spacing between shear connectors for hybrid beam HB1, a maximum load of 120 kN (60 kN per flange) was adopted because during the test, for higher loads, the deformation increased highly. The ovalization of the holes and the high compression stress in the flange, in front of the bolts (estimated as 400.8 MPa), suggested that failure of the flanges was imminent. In shear connection test SCS3, failure occurred due to concrete crushing, for a maximum load of 108.3 kN and the stiffness was estimated as 50.3 kN/mm. As a result of the shear connection tests, a connection system with M10 bolts was adopted, as it maximized the ultimate strength and stiffness. Shear connection tests also proved the advantages of using high grade concrete. 2.3 Fabrication and experimental setup for GFRP-concrete hybrid beams

2.3.1 Fabrication and properties The hybrid beam HB1 (phase I), with a total length of 4.80 m, was tested in bending in a 4.00 m span, under one point load at midspan. The hybrid beam HB2 (phase II), was tested in bending (1.80 m span), under two point loads 0.64 m apart and centred in the midspan. The cross-section’s dimensions for both hybrid beams were selected, in order to obtain a concrete compressive failure with the neutral axis lying in the concrete slab) and regarding the preferential application of this type of structural elements, namely in rehabilitation and strengthening of concrete slabs. Consequently, a concrete slab with 100 mm height and 400 mm width was adopted, longitudinally reinforced on the top and bottom of the section with constructive 2φ6 mm (fsu = 681 MPa) + 2φ8 mm (fsu = 652 MPa) steel bars (1.57 cm2), and transversally with φ6 mm // 0.10 m shear stirrups (5.65 cm2/m) with a 15 mm cover. Considering the results obtained in the shear connection test and the maximum compressive load of the concrete slab, a 12.5 cm longitudinal spacing between shear connectors was adopted. Steel bolts, identical to those used in the shear connection tests, were placed on holes previously drilled on the top flange, and manually tightened. Slabs were then cast against the beam top flange. Figure 3 shows different phases of the fabrication of hybrid beam HB1. Design equations were developed based on the classical hypothesis for the analysis of composite elements (complete results are reported in [7]), in order to estimate the properties of the hybrid section and beams, for both full and partial interaction situations. When the flexibility of the shear connection is considered (partial interaction), a slip strain (εslip) occurs on the interface between the two materials of the hybrid beam, corresponding to the difference between concrete and GFRP strains at the interface. Therefore, a reduction of the flexural stiffness occurs, and its magnitude depends on the connection’s flexibility. In the limit, when there is no shear interaction, the profile and the concrete layer work separately and the hybrid beam’s stiffness corresponds to the sum of the stiffnesses of each isolated element.

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The flexural stiffness for full shear interaction (after concrete cracking), the neutral axis depth (before and after concrete cracking) and the ultimate bending moment for partial shear connection of the equivalent hybrid section, were estimated based on the experimental mechanical properties, and using those equations: EIeq

GFRP = 4554 kN.m2; X,el = 64.8 mm (before cracking); X,el = 56.1 mm (after cracking); Mu = 174.7 kN.m for hybrid beam HB1. The ratios between midspan bending deflection with partial shear interaction (δp) and full shear interaction (δf) were also estimated for both hybrid beams, using equations developed based on investigations presented by Knowles [10] and Girhammar and Gopu [11]: (δp/δf)HB1 = 1.13; (δp/δf)HB2 = 1.66. The global deformation of GFRP-concrete hybrid beams results from the sum of the deflection due to bending, considering the connection flexibility, and the deflection due to shear. Here, a conservative hypothesis was assumed, considering that shear is carried only by the profile’s web. Under these hypotheses, the maximum deflection of simply supported GFRP-concrete hybrid beams, submitted to a point load P applied at any point along its span, can be computed by the equation (1), where K1 and K2 are factors which depend on the load’s position and are given by elasticity theory (for a load applied at midspan, K1 = 48 and K2 = 4).

wp2f

pGFRPeq1

3

AGKLP

EIKLP

×××+

δ

δ×

××=δ (1)

2.3.2 Experimental setup and instrumentation Both beam end supports allowed for free rotation and one of them also allowed for sliding. Load was applied (at midspan of hybrid beam HB1 and in two points 64 cm apart for hybrid beam HB2) using hydraulic jacks (figure 4). The load was measured by load cells. Displacement transducers and strain gauges were placed at several cross sections of hybrid beams HB1 (figure 5) and HB2 (figure 6).

Figure 3 – Preparation of the hybrid beam HB1. Figure 4 – Flexural test setup – HB1.

ε7

S3 S1 S2

ε5 ε6ε8

ε3 ε4

ε1 ε2

ε10ε9

ε11 ε12

δ1

εεε

ε

εε

ε

ε

ε

εε

ε ε

ε

ε

ε ε

ε

ε

ε

ε

δ1

S2S3

F1

S1S5

εε

ε εε

ε ε ε

ε

εεε

εεε

εε

ε

F2

S6 S4

Figure 5 – HB1 instrumentation (cm): frontal view (top), section S2 (bottom left) and frontal view of

section S3 (bottom right).

Figure 6 – HB2 instrumentation (cm): frontal view (top), section S1 (bottom left) and frontal view of

section S2 (bottom right).

Displacement transducer δ1 was used in section S1 of both hybrid beams to measure deflections at midspan. In hybrid beam HB1, strain gauges ε1 to ε8 were attached to section S2, to assess longitudinal strains throughout the depth of the section and strain gauges ε9 to ε12 were attached to section S3, forming a rosette at the middle depth of the profile, to assess the profile’s web

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distortion. In hybrid beam HB2, longitudinal strains were measured in section S1 with 21 strain gauges, placed in the concrete slab (ε1 to ε8), in the steel reinforcement (ε9 to ε12) and in the GFRP profile (ε13 to ε21). To study the shear distribution between the concrete slab and the GFRP profile’s web, 18 strain gauges (ε22 to ε39) were attached in section S2 forming three rosettes in each element. In the extremity sections of hybrid beam HB2, two stiffeners were attached on each side of the web in order to prevent web compressive failure. Tests were performed 28 days after concreting. 2.4 Hybrid beam HB1 - Results and discussion

2.4.1 Deformation and ultimate strength Two load controlled cycles were performed and figure 7 shows the corresponding load-deflection at midspan curves. Midspan deflection computed with equation (1) is also presented indicating an excellent agreement with experimental results. In the same figure a curve is also shown (GFRP beam), corresponding to the bending test in an identical simple GFRP I-beam. In this test, lateral-torsional buckling was prevented and the beam failed suddenly by local buckling of the compression top flange.

In the first cycle, load was monotonically applied to hybrid beam HB1 until compressive cracking of the concrete layer appeared. The beam was then unloaded from a load of 178.4 kN, very close to the expected analytical value of the ultimate load (174.7 kN). Comparing with the behaviour of the simple GFRP beam, the following structural improvements of the GFRP-concrete hybrid beam can be referred: (i) The flexural stiffness (after concrete cracking) increased about 350%; (ii) The flexural strength increased about 300%; (iii) The GFRP beam failed (due to local buckling), for a longitudinal maximum stress of 269 MPa while the hybrid beam failed for a longitudinal maximum stress in the GFRP profile of 386 MPa, corresponding to a 40% increase in using GFRP profile’s strength capacity; (iv) Regarding first load cycle, GFRP-concrete hybrid beam presented a pseudo-ductile behaviour, considering the loss of stiffness observed prior to failure. In the second cycle, the load was also applied monotonically and while the compressive failure region was progressing along the beam’s length and depth, final failure occurred suddenly, at a load of 182.0 kN. This was associated to interlaminar shear of the web’s profile, around 1 cm above its middle depth, alongside the beam’s length, causing the complete separation of the beam in two parts. Interlaminar failure was followed by delamination and transverse bending of the web (figure 8).

0255075

100125150175200

0 20 40 60 80 100 120

Midsplan deflection (mm)

Load

(kN

) Hybrid beamfirst cycle

Hybrid beamsecond cycle

GFRP beam

Model

Figure 7 – Load versus midspan deflection for

HB1. Figure 8 – Failure of HB1 - Concrete crushing and

interlaminar shear failure.

2.4.2 Composite action Experimental longitudinal strains for section S2 are shown in figure 9 for different values of the bending moment. Slip strain values at the interface between the materials were estimated assuming that Bernoulli’s hypothesis is applicable for each element separately. Figure 10 illustrates the comparison between the experimentally obtained slip strain values in section S2 and those predicted with the model developed by Knowles [10], both as a function of the bending moment.

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0,00

0,05

0,10

0,15

0,20

0,25

0,30

-2000 0 2000 4000 6000 8000Axial strain (µµµµstrains)

Sect

ion

dept

h(m

)

εεεε,slip

M=70M=80M=90M=100M=110M=120

M=10M=20M=30M=40M=50M=60

0200

400600800

100012001400

0 25 50 75 100 125Moment (kN.m)

ε,ε, ε,ε,sl

ip( µµ µµ

stra

in)

Experimental Model Figure 9 – Longitudinal strains evolution with

moment (kN.m) - HB1. Figure 10 – Slip strain versus moment - HB1.

Although estimations of slip strain values were obtained from only two measurements in each element of hybrid beam HB1, results demonstrate that slippage occurred in the interface between the two materials, and it increased with the value of the moment. Although experimental results present non-linear behaviour (arising due to several factors namely, initial friction, non-continuous/discrete connection, materials non-linearities, etc.), their values present the same order of magnitude of the analytical ones. Figure 11 shows the neutral axis height (Z = H – X) in section S2 as a function of the moment. Experimental values show a very good agreement with the calculations for the neutral axis height before and after cracking. 2.4.3 Shear distribution Results obtained from strain gauges ε9 to ε12, attached to the profile’s web (at middle depth) in section S3 were used to estimate the shear distribution between the GFRP profile and the concrete slab (figure 12), assuming that in the GFRP profile, shear is entirely carried by the web and the shear stress distribution across the web’s depth is approximately uniform. Results show that for service loads, GFRP profile absorbs more than 50% of shear and prior to failure, due to concrete’s non-linear behaviour and crushing in midspan section, this value is almost 70% of the total shear.

0,20

0,22

0,24

0,26

0,28

0,30

0 25 50 75 100 125

Moment (kN.m)

Neu

tral

axis

heig

ht,Z

(m)

Experimental Model before cracking Model after cracking

0102030405060708090

100

0 50 100 150 200

Load (kN)

Shea

rca

rrie

dby

GFR

P(%

)

Load cycle 1

Load cycle 2

Figure 11 – Neutral axis versus moment - HB1. Figure 12 – Shear carried by GFRP versus load -

HB1. 2.5 Hybrid beam HB2 - Results and discussion

2.5.1 Deformation and ultimate strength Figure 13 shows the load-deflection at midspan curve for hybrid beam HB2. Midspan deflection predicted with equation (1) is also presented indicating a good agreement with experimental results, especially for load values up to about 60 kN, as for higher loads differences between the two curves increase. Comparing with deflection predictions for hybrid beam HB1, model seems to underestimate deflections of hybrid beam HB2. Most likely, as discussed further on, this difference is due to the fact that in hybrid beam HB2 sections do not remain plane after deformation, i.e. section warping occurs and therefore Bernoulli’s hypotheses is not verified due to the higher shear. Failure occurred, without warning, for a total load of 296.2 kN, due to shear of the GFRP profile, which occurred in the web-top flange junction (figure 14).

7

050

100150200250300350

0 10 20 30 40

Midspan deflection (mm)

Tota

lloa

d(k

N)

Experimental Model

Figure 13 – Total load versus midspan deflection - HB2.

Figure 14 – Shear failure of HB2 on the web-top flange junction.

2.5.2 Composite action Longitudinal strains measured in section S1 (midspan) are shown in figure 15, and the results are plotted as a function of the section depth in figure 16 for different values of the bending moment. Taking into consideration the similarity of the readings of strain gauges (ε1, ε2 and ε3), (ε4, ε9 and ε10) and (ε6, ε11 and ε12), placed at different positions throughout the concrete slab depth, it is possible to conclude that the entire width of the concrete section is effective. This result is in agreement with the limit value suggested in Eurocode 4 for the effective width of the concrete slab (beff) in simply supported composite structural elements (bc/2 = 0.20 m < beff = L/8 + b0 = 0.275 m). It is also possible to conclude from figure 16 that a slip strain develops in the interface between the two materials. Further, it is quite clear from that figure that the hybrid section, and particularly the GFRP profile, does not remain plane after deformation. Regarding axial strains throughout the GFRP profile’s depth, it can be seen that warping occurs. Figure 17 shows the neutral axis height (Z = H – X) in section S1 as a function of the moment. Once again, experimental values show a good agreement with the calculations for the neutral axis depth before and after cracking.

0

50

100

150

200

250

300

350

-2000 0 2000 4000 6000

Strain (µµµµstrains)

Tot

allo

ad(k

N)

3, 2, 1 4, 9, 105

13, 14, 16, 156, 12, 11

17 18 19 20, 21

0

50

100

150

200

250

300

-2000 0 2000 4000 6000

Axial strain (µµµµstrains)

Sect

ion

dept

h(m

m) M=50

M=60M=70M=80

M=10M=20M=30M=40

Figure 15 – Longitudinal strains as a function of the moment (kN.m) - HB2.

Figure 16 – Longitudinal strain versus section depth as a function of the moment - HB2.

2.5.3 Shear distribution Results obtained from rosette strain gauges attached to section S2 were used to estimate the shear distribution between the GFRP profile and the concrete slab (figure 18). Again, it was assumed that in the GFRP profile, shear is entirely carried by the web. Results are in good agreement with those presented for hybrid beam HB1, showing that profile’s web absorbs almost 80% of the total shear at ultimate load, beginning with values around 40%.

8

0,20

0,22

0,24

0,26

0,28

0,30

0 20 40 60 80 100

Moment (kN.m)

Neu

tral

axis

heig

ht,Z

(m)

Experimental Model after cracking Mode before cracking

0102030405060708090

100

0 100 200 300

Total load (kN)

Shea

rca

rrie

dby

GFR

P(%

)

Figure 17 – Neutral axis versus moment for hybrid beam HB2.

Figure 18 – Shear carried by GFRP versus load for hybrid beam HB2.

3. CONCLUSIONS

This paper presents the analysis and the experimental study of a structural solution, which combines GFRP pultruded profiles and concrete elements in GFRP-concrete hybrid structural elements. The following main conclusions can be addressed:

• GFRP-concrete hybrid beams are a viable structural solution that can be used in repair, strengthening, or even in new construction, presenting reasonable stiffness, high strength and low self-weight.

• Comparing with the behaviour of simple GFRP profiles, hybrid beams show a considerable stiffness and strength increase, with a better use of the profiles’ properties.

• The ultimate strength and deflections of GFRP-hybrid beams can be predicted with a good precision with the proposed methods of analysis where shear deformation and interconnecting slip must be considered.

4. REFERENCES

[1] Saadatmanesh H, Ehsani MR. RC Beams Strengthened with GFRP Plates. I: Experimental Study. Journal of Structural Engineering, ASCE 1991; 117(11): 3417-3433. [2] Snow RK. Encapsulation: Protecting Concrete Piles in Marine Environments. Concrete International, ACI 1999; 21(12): 33-38. [3] Fam AZ, Rizkalla SH. Flexural Behavior of Concrete-Filled Fiber-Reinforced Polymer Circular Tubes. Journal of Composites for Construction, ASCE 2002; 6(2): 123-131. [4] Fam, AZ, Schnerch DA, Rizkalla SH. Rectangular FRP Tubes Filles with Concrete for Beam and Column Applications. Fibre-Reinforced Polymer Reinforcement for Concrete Structures, Proceedings of the 6th Int. Symp. on FRP Reinf. for Conc. Struct. (FRPPCS-6), Singapore, July 2003. p. 685-694. [5] Deskovic N, Triantafillou T, Meier U. Innovative Design of FRP Combined with Concrete: Short-Term Behaviour. Journal of Structural Engineering, ASCE 1995; 121(7): 1069-1078. [6] Seible F, Karbhari VM, Burgueño R. Kings Stormwater Channel and I-5/Gilman Bridges, USA. Structural Engineering International, IABSE 1999; 9(4): 250-253. [7] Correia JR. Glass Fibre Reinforced Polymer (GFRP) Pultruded Profiles. Structural Behaviour of GFRP-Concrete Hybrid Beams. MSc Thesis, Instituto Superior Técnico, 2004 (in Portuguese). [8] Branco FA, Ferreira J, Correia JR. The use of GRC and GFRP-concrete beams in bridge decks. FRP Composites in Bridge Design and Civil Engineering, COBRAE Conference Proceedings, Porto, November 2003. [9] Johnson RP, May IM. Partial-interaction design of composite beams. The Structural Engineer 1975; 53(8): 305-311. [10] Knowles PR. Composite Steel and Construction, Butterworths, 194 p. [11] Girhammar UA, Gopu KA. Composite Beam-Columns with Interlayer Slip – Exact Analysis. Journal of Structural Engineering, ASCE 1993; 119(4): 1265-1282.