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ADHESIVELY BONDED COMPOSITE REINFORCEMENTS FOR STEEL STRUCTURES: DURABILITY OF THE STRESS TRANSFER

Sylvain CHATAIGNER LUNAM Université – IFSTTAR - SOA F-44341 BOUGUENAIS France [email protected]* Marc QUIERTANT, Karim BENZARTI Université Paris-Est – IFSTTAR - SOA, MAT F-75732 PARIS France [email protected], [email protected] Arnaud GAGNON, Christophe AUBAGNAC DL Autun – CETE Lyon Bd de l’Industrie, BP 141, 71405 AUTUN France [email protected], [email protected]

Abstract Nowadays, externally bonded composites are currently used for reinforced concrete structures repair or reinforcement. Such a technique allows increasing the service life of concrete bridges. In the case of steel bridges, applications of this technique are not significant. This may be due to the similarity of both materials rigidity, but also to the lack of confidence of structure owners in using structural adhesive bonding as assembly technique. In the scope of a research work led by the IFSTTAR (Institut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux) interested in the repair of steel structures, we have investigated composite to steel bonded assemblies. This article presents some of the realized investigations. It first introduces the field of use of the technique and its advantages. Then, characterization of stress transfer through adhesive bonding is presented, and some accelerated ageing and pseudo-fatigue tests are described. Though more work is needed on the topic, obtained results allows considering the technique as suitable for steel structures.

Keywords: composite material, durability, reinforcement, steel structures, structural adhesive bonding.

1. Introduction Strengthening and repair of concrete structures using adhesively bonded composite reinforcement has become current. This implies the application to an existing structure of a light material whose mechanical properties are higher than the concrete one (in terms of stiffness and tensile capacity) using adhesive bonding. These materials known as composite materials are based on two parts: most often, for civil engineering applications, an epoxy matrix, and carbon fibres [1]. When compared with steel, mechanical performances of composite materials seem to be less interesting. Yet, for several issues and in regard with the evolution of the technology (application technique, adhesive material quality, high modulus composite materials availability), the technique could be also increasingly applied to steel structures.

In the literature, several studies reported such applications [2], [3], [4], [5], [6]. In the case of old structures, the method is particularly interesting as the implied steel materials are often

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non-weldable. Besides, the use of adhesively bonded composite materials causes only a small weight increase of the structure and the technique seems to be advantageous for fatigue purposes [7]. In France, the use of structural adhesive bonding on steel substrate has been investigated in [8] in the case of steel/concrete structures. Yet, several uncertainties remain regarding the long term performance of this kind of assembly. As a result, in the design codes, safety coefficients are disposed of in order to take into account environmental durability. There is a lack of experimental research on this topic.

This study was led to get first results on environmental durability of composite to steel adhesively bonded joints and on its pseudo-fatigue (5,000cycles) behaviour. In a first part, the characterization test for stress transfer is presented. Then, some results concerning an ageing under severe hygrothermal conditions are presented. Finally, some pseudo-fatigue investigations are described and commented.

2. Stress transfer characterization In the case of adhesive bonding on steel substrate, the application process quality is momentous and will strongly affect the properties of the realized joint. It will condition the adhesive forces and the quality of the resin curing. In order to characterize the resulting stress transfer, a test set-up has been adapted and several series led to check results variability. The influences of both the bonded length and surface roughness on short term properties have been investigated.

2.1 Test procedure

The test is similar to a single lap shear test used to characterize composite to concrete bonded joints in [9] and [10]. It can be noted that during the aforementioned investigations, preliminary studies had shown that in both cases, composite to concrete and composite to steel, it was possible to model the assembly using a bilinear cohesive zone.

For this study, the sample geometry was adapted to steel plates of 300 × 200 × 16 mm3. In order to compensate the induced flexure moment, a specific system was applied on the end of the sample before testing (Figure 1). During the test, conical grips allow the application of a force in the axis of the composite reinforcement, and thus induces shear within the adhesive layer. The displacement rate of the grips is constant during the test at 6 µm/s.

Figure 1. Single lap shear test setting

Induced flexure compensation Force

application

Adhesive

Composi te material

Steel substrate

Appl iedforce

Flexure compensation

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After the test, the failure mode is observed indicating the “weak” link of the assembly, and the ultimate capacity is registered. If needed, local strain information may be recorded using bonded strain gages.

2.2 Influence of the bonded length

First series of test were done to check that the characteristic anchorage length was smaller than 70 mm as described in [10]. Four samples were fabricated. Surface preparation was done using mechanical abrasion and degreasing. Carbon-fiber-reinforced polymer (CFRP) plates (Young’s modulus of 165 GPa, 50 mm wide, 1.4 mm thick) were bonded following data sheet recommendations. Two samples were bonded along more than 200 mm (superior or equal to the classic anchorage length on concrete adherend), and the two others were bonded along 70 mm. For all the samples, the bonded joint started 10 mm far from the end of the plate (Figure 2).

For all the samples, failure modes were cohesive within the adhesive layer. Concerning the ultimate capacity, the results for samples 1, 2, 3 and 4 are respectively 36.78 kN, 36.2 kN, 37.81 kN, and 36.51 kN. The four values are close, indicating that the characteristic anchorage length is below 70 mm. Besides, the results prove that the test set-up has satisfactory repeatability characteristics.

Figure 2. Scheme of the four tested samples to analyse the influence of the bonded length

2.3 Influence of surface roughness

Different adhesion theories exist to explain adhesive bonding (mechanical, chemical, physico-chemical) [11]. One of them relies on the existence of surface roughness to explain bonding. To check the validity of such a theory in this case, it was decided to study the influence of surface roughness on the ultimate capacity of the joint in shear.

All the samples were subjected to typical on site surface preparations used for steel structures in the case of painting operations: a shot blasting with three different surface roughnesses and a state degree SA3 according to [12]. The obtained roughnesses were assessed using a roughness meter: 7 µm (Thin: Samples F1a and F1b); 10 µm (Average: Samples M1a and M1b), 13 µm (Thick: G1a and G1b).

After blasting, surfaces were dusted and degreased, and CFRP reinforcements (Young’s modulus of 160 GPa, 50 mm wide, 1.2 mm thick) were bonded according to the technical data sheet recommendations. The bonded length was 85 mm (slightly higher than 70 mm) and tests were carried out 4 days after reinforcement applications.

For all the samples and thus all the studied roughnesses, the failure modes were cohesive within the resin (Figure 3). As far as ultimate capacities are concerned, no clear difference has

70 mm

70 mm

200 mm

10 mm

50 mm

Sample 1 Sample 2

Sample 3

Sample 4

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been detected. The capacity for samples F1a, F1b, M1a, M1b, G1a and G1b are respectively 34.03 kN, 37.03 kN, 31.73 kN, 35.84 kN, 35.19 kN, 33.02 kN. Once again, it can be noted that the repeatability of the test is rather good.

Figure 3. Photos of the failure modes for the thin roughness, the average roughness and the thick roughness (left to right)

3. Environmental ageing consequences on stress transfer Following these preliminary investigations, similar samples were placed in ageing conditions to study the assembly durability.

3.1 Ageing and test procedures

The adopted ageing procedure is similar to the one adopted in [13], i.e. a storage at 40°C and more than 95 % RH. As it was wished to be in the worst conditions, no treatment was applied to protect the samples against rusting. The storage last during 22 months and samples were tested continuously during this period.

3.2 Results in terms of failure profiles and ultimate capacity

For all the samples, failure modes were cohesive within the adhesive layer (Figure 4). Though, the whole steel adherend is rusted, the bonded surface does not show any degradation. Epoxy resins are indeed closed to the typical treatment used to protect steel structures from rusting.

As far as ultimate capacity is concerned, no clear evolution has been remarked during ageing (Figure 4: each dot corresponds to one test). It must be highlighted that this result concerns the studied geometry and that it may be possible as noted in [13] that stress transfer may evolve with ageing without changing ultimate capacity value.

Figure 4. Photos of the failure modes after 8 and 22 months ageing, and evolution of the ultimate capacity

0

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0 200 400 600 800

Ageing period, in days

Ulti

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e ca

paci

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n kN

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3.3 Strain gage measures

In order to study more particularly stress transfer along the lap joint in the case of an aged sample, two strain gages were placed on top of the composite at respectively 15 and 50 mm for the loaded end of the adhesive joint. The gage placed at 15 mm is close to the end where shear stresses are concentrated [10]. To study the behaviour of the adhesive layer, load-unload paths were then conducted (0-10 kN; 0-20 kN; 0-30 kN) [14].

The obtained evolutions for both gages are plotted in figure 5. It can be noted that the gage placed at 50 mm from the edge is not much deformed. The shear stress transfer occurs consequently mainly within the first 50 mm of the lap length. As far as the gage placed at 15 mm is concerned, it can be remarked that the load-unload paths induce residual strain within the adhesive layer. The adhesive ageing seems thus to have lead to plastification of the resin. As noted in [13], this affects the local stress transfer but does not necessarily affect ultimate capacity.

0

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15

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30

35

0 0,0002 0,0004 0,0006 0,0008 0,001 0,0012 0,0014

Measured strain

App

lied

forc

e, in

kN

Gage 15 mmGage 50 mm

Figure 5. Evolution of the gage measurement during the load-unload paths

4. Fatigue of the assemblies In order to study the influence of pseudo-fatigue (5,000 cycles) on the durability of the studied adhesive joints, a specific test setting has been designed as the existing single lap shear test machine was not adapted for thousands of cycles loading paths.

4.1 Test procedure

In order to apply the shear stress to the assembly, it was decided to use a four point bending test (Figure 6). This bending test allows obtaining between the applied forces a zone with no shear forces and thus no additional shear stress. This is thus close to the single lap shear test. In the case of the bending test, the applied force is limited by steel plasticity. For our samples (16 mm thick steel plates and classic steel), the pseudo-fatigue test has been performed at range representing 25 % of the ultimate capacity of the bonded joint in shear (single lap shear test). The bending cycles were done between 10 and 110 kN at 5 kN/s.

Each sample has been provided with additional support wedges so that the charge is not

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applied on the CFRP layer (Figure 7), and one gage was bonded on each composite plate (in the middle in the direction of the reinforcement) so that strain level may be assessed, and thus force level. This allowed us to check as well the symmetry of the test setting.

Figure 6. Scheme of the four point bending test

Figure 7. Photos of the instrumented sample and the test setting

4.2 Realized investigations

Four samples were tested under the single lap shear test previously presented (Figure 1) after 5,000 cycles of four point bending test (Figure 7) (Plate AC and BC). Plate AC (samples AC1 and AC2) had been stored during 22 months in laboratory conditions (20°C and 60 %RH). Plate BC (samples BC1 and BC2) had been stored during 22 months in accelerated ageing conditions (40°C and more than 95 %RH). For each of these two plates, a companion reference one with similar ageing but not previously subjected to pseudo-fatigue was tested by the single lap shear test (plate ANC and BNC for respectively AC and BC).

Strain gage allowed checking the symmetry of the test setting. They also measure that axial stress within the composite reinforcement that reached 150 MPa corresponding to almost 25% of the ultimate capacity in the single lap shear test of the assembly. Measured strains did not evolve significantly during the tests except at the beginning where a small increase has been observed. This may be due to a small heating of the system at the beginning of the test.

After the 5,000 bending cycles, the single lap shear tests were performed. The results are given in table 1. All the failure modes were cohesive within the resin layer indicating no degradation of adhesion forces. No clear evolution of the ultimate capacities has been noticed concerning either hygrothermal ageing or pseudo-fatigue investigations.

Composite plate

Steel plateEpoxy resin

300 mm

85 mm60 mm

P P

Strain gages

Support Support wedges

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Table 1. Ultimate capacities of the samples for the fatigue investigations

Plate Storage conditions Fatigue Sample Ultimate capacity (kN)

AC1 37,93 AC Laboratory Yes

AC2 37,09

ANC1 36,85 ANC Laboratory No

ANC2 40,65

BC1 33,19 BC 40°C and more than 95% RH Yes

BC2 34,72

BNCa 34,73 BNC 40°C and more than 95% RH No

BNCb 35,97

5. Conclusion The presented study has presented the adaptation of a single lap shear test usually used to characterize stress transfer between composite reinforcement bonded to concrete to the composite to steel adherend case. The test repeatability proved to be rather good.

This set-up allowed us to study the influence of the bonded lap length and the surface roughness on the ultimate capacity of steel to composite bonded joints. As far as the lap length is concerned, it has been checked that the characteristic anchorage length was below 70 mm as previously exposed in [10]. As far as the surface roughness is concerned, for the studied cases, it had no clear influence on the ultimate capacities. The shot blasting method currently used before painting operations of steel structures seems to create sufficient adhesion forces.

The other part of the article was dedicated to the study of the environmental and pseudo-fatigue durability of such adhesive joints. In the case of the environmental durability, an accelerated ageing procedure was used (storage within 40°C and more than 95 % RH). It was shown that 22 months ageing did not affect the ultimate capacity though strain gage measures showed that the adhesive plasticized as demonstrated in previous studies [13] [14]. Bonded reinforcement may even protect steel from moisture.

In the last part, pseudo-fatigue investigations were conducted (5,000 cycles). A specific test setting relying on a four point flexure test was designed to fatigue samples before testing them using the single lap shear test device. For the studied stress level (25 % of ultimate capacity), no evolution has been observed. Though additional studies should be conducted to check these results after more cycles, or under sustained stress, these former results seem to demonstrate that adhesive bonding may be a reliable and durable assembly technique when properly implemented.

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6. References [1] AFGC (Association Française du Génie Civil), Réparation et renforcement des

structures en béton au moyen des matériaux composites – Recommandations provisoires, Documents scientifiques et techniques, Février 2011.

[2] MOY S.S.J., Guidelines FRP Composites: Life extension and strengthening of metallic structures, ICE Design and practice guide, Thomas Telford, 2001.

[3] CADEI J.M.C., STRATFORD T.J., HOLLOWAY L.C., DUCKETT W.G., Strengthening metallic structures using externally bonded fibre-reinforced polymers, CIRIA Design guide, C595, London, ISBN 0-86017-595-2, 2004.

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[5] NATIONAL RESEARCH COUNCIL, Guidelines for the design and construction of externally bonded FRP systems for strengthening existing structures – Metallic Structures – Preliminary study, CNR DT 202/2005, Rome, 2007.

[6] SCHNERCH D., DAWOOD M., RIZKALLA S., SUMNER E., “Proposed design guidelines for strengthening of steel bridges with FRP materials”, Construction and building materials, Vol. 21, 2007, pp. 1001-1010.

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[8] SI LARBI A., FERRIER E., JURKIEWIEZ B., HAMELIN P., “Static behaviour of steel concrete beam connected by bonding”, Engineering structures, Vol. 29, 2007, pp. 1034-1042.

[9] CHATAIGNER S., CARON J.F., BENZARTI K., QUIERTANT M., AUBAGNAC C., “Characterization of FRP-to-concrete bonded interface”, European Journal of Environmental and Civil Engineering, Vol. 13, No.9, 2009, pp. 1073-1082.

[10] CHATAIGNER S., CARON J.F., BENZARTI K., QUIERTANT M., AUBAGNAC C., “Use of a single lap shear test to characterize composite-to-concrete or composite-to-steel bonded interfaces”, Construction and building materials, Vol. 25, 2011, pp. 468-478.

[11] BRUNEAUX M.A ., Durabilité des assemblages collés, PhD Thesis, ENPC, Marne La Vallée, 2004.

[12] NF EN ISO 8503, Préparation des subjectiles d’acier avant application de peintures et de produits assimilés – Caractéristiques de rugosité des subjectiles d’acier décapés, Juillet 1995.

[13] BENZARTI K., CHATAIGNER S., QUIERTANT M., MARTY C., AUBAGNAC C., “Accelerated ageing behaviour of the adhesive bond between concrete specimens and CFRP overlays”, Construction and Building Materials, Vol. 25, 2011, pp. 523-538.

[14] BENZARTI K., QUIERTANT M., AUBAGNAC C., CHATAIGNER S., NISHIZAKI I., KATO Y.. “Durability of CFRP strengthened concrete structures under accelerated or environmental ageing conditions” , International Conference on Concrete Repair, Rehabilitation and Retrofitting (ICCRRR 2008)” - Abstract 421-422 (full paper on CD-Proceedings 1187-1193). Cape Town, South Africa, 24-26 November 2008.

[15] CHATAIGNER S., CARON J.F., DUONG V.A., DIAZ DIAZ A., “Experimental and numerical investigation of shear strain along an elasto-plastic bonded lap joint”, Construction and building materials, Vol. 25, 2011, pp. 432-441.