MODIFIED CEMENT-BASED ADHESIVE FOR NEAR-SURFACE MOUNTED CFRP STRENGTHENING SYSTEM

Transcription

1 Fourth Asia-Pacific Conference on FRP in Structures (APFIS 13) December 13, Melbourne, Australia 13 International Institute for FRP in Construction MODIFIED CEMENT-BASED ADHESIVE FOR NEAR-SURFACE MOUNTED CFRP STRENGTHENING SYSTEM A. Al-Abdwais 1, R. Al-Mahaidi, K. Abdouka 3 1,, 3 Civil Engineering Department, Swinburne University of Technology, Australia 1 alabdwais@swin.edu.au, ralmahaidi@swin.edu.au, 3 kabdouka@swin.edu.au ABSTRACT The near-surface mounted (NSM) strengthening system has emerged as a promising technology to increase the strength of concrete for flexure, shear and torsion. Strengthening by this technique has been applied to a large number of concrete structures worldwide using epoxy adhesives. Due to the rapid deterioration of the mechanical properties of epoxy-based polymer matrix with elevated temperature, and the hazards from toxic fumes, replacing this polymer with a new cement-based bonding agent is essential to enhance the performance of structures in high-temperature environments and reduce the environmental and health hazards. The application of cement-based adhesive has been studied for externally-bonded techniques using CFRP textile. Although, the good bond properties has been achieved, debonding was the critical failure between fiber and concrete substrate. To exploit the advantages of NSM technique, a new modification of cement-based adhesive has been achieved to meet the requirements of NSM application system. To assess the efficiency of this adhesive, an experimental investigation of pull-out testing using a single-lap shear test set-up was conducted to study the bond characteristics between CFRP textile and concret. Four different mixes were used in this investigation. The test results show the efficiency of the NSM technique using modified cement adhesive and its superior bond properties compared to externally-bonded CFRP. KEYWORDS Modified cement-based adhesive, NSM, Strengthening, FRP INTRODUCTION In the civil engineering construction field, the need for the rehabilitation and strengthening of reinforced concrete and steel structures has become a critical issue around the world. This is due to the ageing of structures, overload exposure due to increased service loads, design faults, updating of existing codes, and deterioration due to exposure to aggressive environmental conditions. These factors make structures more vulnerable to sudden catastrophic failure. Civil engineering researchers have recognised that FRP composites with their excellent mechanical properties are vital for the strengthening and rehabilitation of infrastructure projects. Although epoxy adhesive has taken the major role as a bonding agent in this technique as an effective bonding agent for strengthening of structures for both external and NSM techniques, it has some disadvantages. These include hazardous toxic fumes, skin irritation, moisture impermeability and flammability, and the rapid deterioration of strength with elevated temperature higher than the glass transition T g (-7) C (Gamage, and Wong, ), (fib Bulletin 1, 1).which is a critical issue in high-temperature climates. Therefore, replacing the epoxy with cement-based adhesive as bonding agent is essential to improve the performance at high temperature, to lower the cost of material, reduce the toxic hazards to workers, and minimize the environmental impact of toxic fume emission with curing (Taljsten and Blanksvard, 7), (Kolsch, 199). Attempts to use cement-based adhesive for strengthening applications have been carried out only for externally-bonded FPR. Polymer-modified mortar has been applied as a cement-based adhesive, achieving 3-5% improvements in ultimate strength compared to unstrengthened samples (Badanoiu, and Holmgren, 3), (Wiberg, 3), (Bousias, and Fardis, 7), (Taljsten and Blanksvard, 7). Further studies have attempted to improve the density and properties of mortar by adding silica fume (Taljsten and Blanksvard, 7), (Wu and Sun, 5). Excellent bond properties have been achieved in the latest reasearch into cement based-adhesive for externally-

2 bonded CFRP textile by (Hashimi and Al-Mahaidi, 1), who reported MPa of bond stress and an increase 5-3% in ultimate capacity compared to unstrengthened specimens. However, debonding of fibre from the concrete surface was the critical failure in the external bond, which reduced the resulting ultimate bond strength. NSM is a promising new technology for the strengthenimg of concrete structures. The advantages of the NSM technique include better bond properties, no delamination of fibre at the ends being expected to occur, and less preparation of the concrete surface. Despite the excellent bond properties using epoxy resin, no detailed investigations of the NSM technique using cement-based adhesive have been reported. This is due to the low viscosity of cement paste, which causes it to flow away from the groove surface, making it difficult to apply in practice for NSM techniques. The main objective of this investigation is to develop a modified cement-based adhesive which works efficiently for NSM practical applications at normal and elevated temperature environments. DEVELOPMENT OF NEW CEMENT-BASED ADHESIVE FOR NSM APPLICATION In order to efficiently utilise the improvement in bonding properties between FRP reinforcement and concrete substrate using the NSM technique compared to the externally-bonded FRP strengthening technique, a modified cement-based adhesive has been developed for NSM strengthening systems as an alternative to epoxy adhesive. In the case of NSM with thin slots, it is difficult to pour the adhesive and in existing structures the strengthening is mainly applied to the bottom or side surfaces, Therefore, special physical properties of the adhesive required for NSM application including: high viscosity to provide stability of the cement paste on the surface without dropping or flowing away. slipperiness to allow ease of penetration of the fibre through the adhesive in the groove by sliding the adhesive around fibre surface. suitable open-ended time period for application. good bond properties to improve the load-carrying capacity. Following trials with different mixes, it has been found that mixing the cement-based adhesive developed by (Hashimi and Al-Mahaidi, ) with MBRACE primer and changing the superplasticizer ratio can significantly improve the viscosity of the adhesive to be consistent with implementation requirements. Adding primer improves the physical properties with high viscosity to avoid dropping or flowing away and allowing the paste to slide around the fibre to enable ease penetration of fiber through the adhesive. With this development, the adhesive can be applied for velrtical surfaces and work more efficiently for NSM strengthening in practical applications. EXPERIMENTAL PROGRAM T he experimental program involved testing sixteen specimens to study the modification of cement paste and identify the best bond properties with different mix designs and the workable ages of cement paste (pot-life). Table 1 presents four different mix designs with different ratios of superplasticizer and primer. The required level of viscosity depends on the primer and superplasticizer ratios. FRP textile strips (three weaves) were embedded in slits cut in the longitudinal direction of the concrete prism mm wide and 1 mm deep. The bond length was fixed at 5 mm for all specimens. The bonding started 5 mm away from edge of the prism to avoid concentration of stresses and premature edge failure. Figure 1 illustrates the specimen details and groove dimensions. The fibres were bonded in the concrete slit using modified cement-based adhesive. Specimen details are shown in Tables. The mechanical properties of matrials are presented in Table 3.The compressive strength represents the average values of three specimens of mm diameter of concrete cylinders and 5 mm diameter of Mortar cylinder tested according to AS 1.9. The tensile strength of concrete was calculated according to AS1.11. Splitting tensile test was conducted to find the tensile strength of mortar according AS1.. The CFRP textile was supplied by (Fortress). The cross-section dimensions are 1.5 mm thickness and.33 mm wide. The tensile test with strain gages used to find the mechanical properties the fibre.

3 Table 1 Different mix ratios of mortars (grams) Mix type C a MC b W c F d SF e SP f P g Mortar Mix-M Mortar Mix-M Mortar Mix-M Mortar Mix-M M1, M, M3, M: Mi x Type 1,, 3, a: Ordinary Portland cement b: Micro-cement c: Water d: Filler (Silica G) e: Silica fume f: Superplasticizer (Viscocret-5-5) g: Primer (Mbrace primer part A and B) a) Specimen details (b) Section A-A of specimen Figure 1 Specimen details and cross-section

4 Table Specimen details Specimen No of Bond length Fiber area Time to fix the fiber disgnation specimens (mm) (3 weaves) (mm ) after mixing (minutes) TC5-M ,,, 3 TC5-M 5.5 5,,, 3 TC5-M ,,, 3 TC5-M 5.5 5,,, 3 TC5: FRP textile-cement-5 mm bond le ngth, M: mix type Table 3 Mechanical properties of m aterials Material Comprassive strength (MPa) Tensile strength (MPa) Modulus of ElasticityMPa Concrete , Mortar M. 5, FRP Textile , SPECIMEN PREPARATION AND TEST SET-UP To evaluate the efficiency of the modified cement-based adhesive to bond fibre and concrete substrate, pull-out tests using a single-lap shear test set-up were used. The specimens of concrete prisms were designed to fit the set-up. After days of curing of the concrete prisms a slit was cut on the surface of each one in a longitudinal direction using a saw, as shown in (Figure.a). CFRP textile consisting of weaves of carbon fiber longitudinally and kelvar in the transverse direction (Figure.b) was applied by placing a thin layer of modified cement adhesive on the surface, ensuring the whole surface of the fibre was covered by adhesive. The groove was filled with the adhesive using a steel blade. The fibre was inserted into the groove to the required depth, then the surface was levelled. The fibre was extended out of each specimen by mm to fix the aluminum plates for gripping. After 1 days of curing, the aluminum plates were glued using Araldite epoxy adhesive, which has high bonding strength to avoid slipping of grips during the test. Figure 3 illustrates the single-lap shear test set-up. (a ) Slit in concrete prism (b) FRP textile c) Mortar in the groove (d) Mortar on the Fiber Figure Specimen preparation

5 Figure 3 Single-lap shear test set-up EXPERIMENTAL RESULTS AND DISCUSSION In all specimens, the bond length was fixed at 5 mm to identify the effect of different mix ratios on bond stress. The test results showed a significant difference of the modified cement-based adhesive as bonding agent between CFRP textile and concrete substrate. Table shows the test results. Table Test results (Ultimate load) (kn) (a) (b) (c) 5 min min min Specimen designation TC5-M1 TC5-M TC5-M3 TC5-M (d) 3 min In the TC5-M1 series of specimens with Mix M1, four specimens were tested. It can be seen that there is about a % drop in bond after 3 minutes of pot-life time. The maximum ultimate load achieved by these specimens was 9.7 kn. Figure shows the force-displacement relationships. The failure mode was the debonding at interfacial zone between the adhesive and the fibre associated with cracks and peeling of adhesive at the surface, as plotted in Figure 5. 1 TC5 M1 a TC5 M1 b TC5 M1 c TC5 M1 d Displacement (mm) Figure Force-displacement relationship of series TC5-M1

6 Figure 5 Failure mode of specimens TC5-M1 In the TC5-M series, four specimens were tested. The maximum ultimate load achieved by the first specimen after 5 minutes was.5 kn. The drop in strength within 3 minuts of time was more than %. The failure mode in all specimens was similar to that of series 1. Figures present the force-displacement curves for these four specimens. 1 TC5 M a TC5 M b TC5 M c TC5 M d Displacement (mm) Figure Force-displacement relationship of series TC5-M In the TC5-M3 series, four specimens were tested. The maximum force achieved by the five specimens was 11.5 kn. It can be seen that the decrease in strength was about 1%, which is less than the previous two series, as illustrated in Figure 7. The mode of failure was the interfacial cracking between adhesive and fibre showed that longitudinal cracks propagated along the adhesive surface as presented in figure. 1 TC5 M3 a TC5 M3 b TC5 M3 c TC5 M3 d Displacement (mm) Figure 7 Force-displacement relationship of series TC5-M3

7 Figure Failure mode of specimens TC5-M3 In the TC5-M series, fo ur specimens were tested. The maximum force was 11.3 kn. The decrease in strength was only 3% within minutes and of 1% between to 3 minutes. The average load value of the first three specimens in minutes was 11. kn. Figure 9 shows the load-displacement curve for all four specimens. The failure mode was similar to that of specimen TC5-M TC5 M a TC5 M b TC5 M c TC5 M d 1 Displacement (mm) Figure 9 Force-displacement relationship of series TC5-M The average bond stress with 5 mm bond length is about MPa. The value was simply calculated by dividing the average load of the first three specimens ( ie excluding specimen with 3 minutes pot life) of Mix M by fiber surface area bonded with adhesive. To evaluate the test results, Figure illustrates the load versus time (pot life). It can be seen that M has the best performance within minutes and provides better bond properties compared to the other mixes M1 M M3 M Time (min) Figure Load-time (pot life) relationship for different mixes

8 CONCLUSION The aim of this investigation was to develop an efficient cement-based adhesive for the NSM CFRP strengthening technique. It can be concluded that excellent bond properties can be achieved using modified cement-based adhesive and it works efficiently as a bonding agent with the following findings: The best results were achieved by M with inconsiderable difference of strength within minutes of time. It is therefore recommended to use M with minutes of pot-life as a cement bonding agent for strengthening with NSM CFRP textile. Strengthening with NSM using modified cement adhesive is about.5 times more efficient than the externally-bonded CFRP. The average bond stress was about MPa compared to the externallybonded CFRP textile reported by (Hashimi and Al-Mahaidi, 1) which achieved only MPa. The failure mode was the interfacial zone betwen fibre and adhesive associated with longitudinal cracks in the adhesive surface for all specimens. The results showed a considerable ductile behaviour in all specimens. REFERENCES Badanoiu, A. and Holmgren, J. (3), "Cementations composites reinforced with continuous carbon fibers for strengthening of concrete structures", Cement and Concrete Composites 5(3): Bournas, D., Lontou, P., Trianfillou, T. and Papanicolau, C. (7). Textile-reinforced mortar versus fiber reinforced polymerconfinement in reinforced concrete columns, ACI Structural Journal, (): pp.7-7. Bousia s, S., Spathis, A.-L and Fardis, M.N. (7). Seismic retrofitting of columns with lap spliced smooth bars through FRP or concrete jackets,journal of Earthquake Engineering,. 11(5): pp fib Bulletin 1, (1). Externally bonded FRP reinforcement for RC structures. Gamag e, J.C.P.H., Al-Mahaidi, R. and Wong, M.B. (). Bond characteristics of CFRP plated concrete members under elevated temperatures, Composite Structures, 75(1 ): pp Hashemi, S., Al-Mahaidi, R. () Cement Based Bonding Material for FRP, Proceedings of the eleventh Int. Inorganic-Based Fiber Composites Conference, 5-7 November, Madrid - Span Hashemi, S. and Al-Mahaidi R. (1). Experimental and finite element analysis of flexural behavior of FRP-strengthened RC beams using cement-based adhesives, Construction and Building Materials, (1):pp Kolsch, H., (199). Carbon fibre cement matrix (CFCM) system for masonry strengthening,journal of Composites for Construction, : pp Taljsten, B. and Blanksvard, T. (7). Mineral-based bonding of carbon FRP to strengthen structures,journal of Composites for Construction, 11(): pp concrete Wiberg, A. (3). Strengthening of Concrete Beams Using Cementitious Carbon Fiber Composites, Stockholm, Sweden, KTH Royal Institute of Technology, PhD thesis? Wu, H.C. and Sun,(5). Fiber reinforced cement based composite sheets for structural retrofit, Bond Behaviour of FRP in Structures: Proceedings of the International Symposium (BBFS 5): pp