On the mechanical behaviour of FRCM composites

Transcription

1 On the mechanical behaviour of FRCM composites Luigi Ascione 1, Carlo Poggi 2, Marco Savoia 3 1 Department of Civil Engineering, University of Salerno, Italy l.ascione@unisa.it 2 Structural Engineering Department of the Polytechnic of Milan, Italy carlo.poggi@polimi.it 3 Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna, Italy marco.savoia@unibo.it Keywords: FRCM, Tensile strength, Pull-off strength. SUMMARY. In addition to the now-classic FRP composite materials (Fibre-Reinforced Polymer), made with long glass, carbon or aramid fibres embedded in polymer matrices (such as epoxy resins), the use of FRCM composites (Fabric-Reinforced Cementitious Matrix) is becoming more and more popular. The inorganic matrix presents numerous advantages compared to the organic one of FRPs, especially for applications to masonry constructions, given its greater compatibility with this type of substrate. In spite of their widespread application, the constitutive laws and failure mechanisms of these materials have not been adequately studied. Consequently, to date there are no agreed upon criteria for qualification of FRCM. This paper will present the results of an experimental investigation carried out in the laboratories of the Universities of Bologna, Milan and Salerno on different FRCM products available on the Italian market in order to characterize their behavior via uniaxial tensile and pull-off tests. 1 INTRODUCTION Nowadays the strengthening of existing reinforced concrete or masonry buildings is often carried out using innovative materials and/or non-conventional techniques. The strengthening with FRP (Fibre-Reinforced Polymer) materials, made of pre-impregnated fabrics or pultruted plates, the plating with steel elements, the confinement with reinforced light concrete and the prestress with steel bars are the most common examples of these techniques. Recently, the use of FRCM (Fabric-Reinforced Cementitious Matrix) is also going popularity. Such composites are obtained by matching a fibre mesh with an inorganic matrix composed of cement or lime mortar. Generally, the mesh is composed of fibres in the dry state or impregnated in polymer resins. The innovative fibres include those of PBO (Poliparafenilenbenzobisoxazolo), basalt and steel. In the following a brief description of the production technology of the above mentioned types of fibre is given. The rapid spread of FRCM materials is due to their better properties compared with FRP materials: they have good mechanical performances, excellent resistance to high temperature and fire, good vapor permeability and they can be applied on wet surfaces [1-13]. Strengthening

2 existing buildings with FRCM was become increasingly frequent, especially for the repair of buildings damaged by earthquake, such as the buildings damaged by the recent earthquake in Abruzzo and Emilia. However in spite of their widespread application diffusion, the FRCM materials mechanical behavior and failure mechanisms have not been adequately investigated with particular luck of knowledge regarding debonding mechanism and stresses at FRCM-substrate interface. Consequently, there are not universally agreed upon criteria for the qualification of these materials, which causes problems of acceptance in the manufacturing, design and quality control of structural strengthening techniques with these materials. The present paper deals with the mechanical characterization of these materials. Specifically, the first results of an experimental investigation carried out by the Universities of Bologna, Milan and Salerno for characterizing the uniaxial tensile constitutive law, as well as the debonding from the support, of some FRCM products are presented here. The first of such topics is particularly complex due to the low tensile strength of the inorganic matrix and the interaction between the fibre mesh and the matrix in the cracked state. Furthermore, the debonding phenomenon is different from that of FRP reinforcement. The study has been carried out on different products available on the Italian market. It has allowed the authors to draw up specific Guidelines on the classification of these types of reinforcement, recently commissioned by the Ministry of Infrastructure and Transportation (MIT) to a Working Group. The guideline will provide useful information as well as recommendations on the use of the FRCM materials involving a wide class of repair and strengthening. A similar guideline has recently been licensed in the USA by the ICC Evaluation Service [14]. 2 REINFORCEMENT FIBRES Some characteristic properties of the newest types of fibre used as reinforcement for FRCM, such as PBO, basalt and high-strength steel are described here. The PBO fibres represent a recent innovation in the field of structural strengthening. They were first produced in 1988 by the TOYOBO Co. with the commercial name of Pbo Zylon and were utilized for flak jackets. In the last few years, several tests on this type of fibre have been carried out with the aim of using them in Civil Engineering. The PBO fibers exhibit higher toughness, modulus of elasticity, resistance to abrasion, cutting, UV, fire and heat than aramid fibres. Furthermore, they are characterized by excellent stability and very low absorption in a humid environment (0.6%). Against the above advantages, the PBO fibers are soft and very flexible, they have very low specific weight and they are expensive. Their chemical structure (Fig. 1) makes the PBO fibers particularly suitable as reinforcement in combination with cement matrix. On the other hand, basalt fibres have better mechanical and physical properties than the other mineral fibers, like the carbon and glass fibres. They typically have a diameter between 9 µm and 13 µm and they are an excellent substitute for asbestos fibers because their diameter exceeds the limit of breathability (about 5 µm). Moreover, they are excellent thermal and acoustic insulation, they retain their mechanical properties even at high temperatures and are very chemically stable (both in acidic and alkaline environment). The latter property makes them particularly attractive as reinforcement in FRCM.

3 Figure 1- Chemical structure of PBO fibres. Unlike the carbon fibres and aramid fibres, which have a crystalline structure, basalt fibres are amorphous. Their use in Civil Engineering started in 1995; currently the largest producers are Russia, China and the United States of America. Figure 2 shows a mesh made of basalt fibers. Figure 2- Basalt fibers net. The steel wires, used in the production of FRCM, are designated with the acronym UHTSS, which stands for Ultra High Tensile Strength Steel. This kind of steel is derived from pearlitic steel (with carbon contents between 0.8% and 0.95%) and is produced using an innovative production process. The tensile strength of the wires is very high: MPa. The diameter of a single fibre can vary between 0.20 mm and 0.48 mm but they are collected in the form of small strands, whose diameter is in the range of 0.89 mm to 1.2 mm (Fig. 3). The strands are assembled in the form of a mesh to be embedded in the inorganic matrix. Figure 3- Strand of steel fibres.

4 The inorganic matrix of FRCM presents many advantages compared to that of FRP materials, especially for applications to masonry structures, due to its better compatibility with this type of substrate. On the other hand, comparing it to the polymer matrixes, a disadvantages is its lower ultimate strain value. 3 EXPERIMENTAL PROGRAM: UNIAXIAL TESTS Generally, the uniaxial tensile stress-strain relationship of FRCM materials is characterized by a multi-linear curve consisting of three segments (Fig. 4), corresponding to the un-cracked (phase A), cracking (phase B) and cracked (phase C) phases, respectively. Figure 4- FRCM material constitutive law. The main mechanical properties of interest are: tensile strength, ultimate tensile strain, moduli of elasticity in tension in the un-cracked, cracking and cracked phases and the transition point between the phases A and B. The modulus of elasticity in the cracked phase equals that of the fibres. The tests have been carried out under displacement control on samples of FRCM material with rectangular cross section, whose width has been chosen in such a way as to include at least 3 strands of the fibers in the test direction (warp or weft). The samples have been tested after a maturation period of 14 days under room conditions (20 C and 50% RH). To avoid localized cracks in the zones close to the gripping jaws of the tensile machine, two GFRP plates were glued with araldite on both sides of the ends of samples. The tensile strength of the samples was referred to the dry reinforcement area. During the test the values of the load, the crosshead displacement and deformations relative to a suitable gage length were recorded. After the un-cracked phase, characterized by a straight line in the load-displacement and load-deformation graphs, was surpassed the test was continued through subsequent cycles of loading and unloading. At the end of each stage of the unloading the residual deformation was measured. Starting from this value, according to the number of cracks over the gage length, the average value of the cracks opening was computed. The idea of the Working Group is to assume the tensile strength of the samples equal to the value of the stress corresponding to a conventional average opening of the cracks in the range 0.25mm mm. This conventional value is still object of discussion. In the phase A the speed of the movement of crosshead of the testing machine was 0.2mm/min.

5 In phase B the average strain of the sample was increased by not more than 4.0x10-4 compared to the peak value of the previous cycle, while in step C of not more than 4.0x10-3. The graph of Figure 5 shows the stress-strain trend exhibited by samples of a FRCM composite produced by RUREDIL with the cement matrix and the mesh of PBO (X Mesh Gold). The tests were carried out at the Polytechnic of Milan. The samples sizes were 40mm x 10mm x 400mm and were reinforced with 1.68mm 2 of dry fibers of PBO. The graph shows the trend of the cyclic test, which is compared with the average of monotone tests performed on three different samples (dashed curve) and with the monotone test that has most similar trend. The free length of the samples, measured between the jaws of the testing machine, was 240mm. Mean values of tensile stress obtained by monotonic tests Tensile stress [MPa] Reference monotonic test Loading-unloading test Tensile strain [mm/mm] Figure 5- Uniaxial tests on FRCM with PBO fibres. In Figures 6-7 are shown the experimental results exhibited by four samples of a FRCM material produced by KERAKOLL. The samples were made of GeoLite matrix and steel HarwireTM fibers (GeoSteel G600 or G2000). The tests were performed at the University of Salerno following both a monotonic and a cyclic path composed of loading unloading branches. The samples sizes were 40mm x 10mm x 400mm. The free length of the samples, measured between the jaws of the testing machine, was 230mm. All specimens exhibited a non-linear behavior up to failure affected by several cracks within the matrix. The tensile strength of the specimens was attained when the ultimate load of the steel strands was reached. Samples with Geosteel G600 (0.844mm 2 /cm steel reinforcement) and Geosteel G2000 (2.535mm 2 /cm steel reinforcement) were considered and compared. The following average opening of cracks was observed at collapse: mm (sample reinforced with GeoSteel 600 collapse load : 9.8kN ); mm (sample reinforced with GeoSteel 2000 collapse load: 24.9kN ).

6 2000 Tensile stress [MPa] Monotonic test Loading-unloading test Tensile strain [mm/mm] Figure 6 - Uniaxial tests on FRCM with Geolite and GeoSteel G Tensile stress [MPa] Monotonic test Loading-unloading test Tensile strain [mm/mm] Figure 7 - Uniaxial tests on FRCM with Geolite and GeoSteel G EXPERIMENTAL PROGRAM: PULL-OFF TESTS The static scheme adopted for the test set-up, shown in Figure 8, was kind of a push-pull test. The tests were performed under displacement control with a speed of 0.2mm/min. During each test, the pull-off force, applied by means of an electromechanical actuator; the displacement of the initial

7 section of the reinforcement with respect to the substrate; the longitudinal deformation, measured by the test instrumentation Digital Image Correlation (DIC) were recorded. Figure 8- Pull-off test set-up. Figures show the results of some tests carried out at the University of Bologna. They relate, respectively, to two specimens reinforced with carbon fibers (Betontex RC170-TH12), glued to a C20/25 concrete substrate (Fig.9), and three samples reinforced with glass fibers (Betontex RV320-AR), glued to UNI masonry bricks substrate (Fig.10). The tested FRCM materials were provided by the ARDEA Projects and Systems SRL. Figure 9- Concrete support. Figure 10- Masonry bricks support. The dimensions of the concrete substrate were 200mm x 150mm x 600mm and the reinforcement was applied on the surface having width of 150mm after removing the external grout by sandblasting. The reinforcement had dimension of 40mm x 10mm x 400mm. The masonry specimens were made with the following dimensions: width of the face with reinforcement 200mm; length 400mm. The dimensions of the FRCM reinforcement were the same as in the tensile tests. In Figure 11 are shown the results of one of the tests on the concrete substrate: the value of axial load that caused the failure of the reinforcement (Fig. 13) was 4.94kN. The results of the two tests were very close to each other. In Figure 12 are shown the results of one of the tests on the masonry bricks: the associated debonding load (Fig. 14) was 3.07kN. Also in this case the results of the three tests were very close to each other.

8 Figure 11- Betontex RC170-TH12. Figure 12- Betontex RV320-AR. Figure 13- Reinforcement failure. Figure 14- Specimen after debonding: (A) masonry brick surface ; (B) FRCM surface. 5 CONCLUSIONS The current experimental investigation made it possible to establish appropriate standard for performing uniaxial tensile and pull-off from substrate tests for a FRCM reinforcement systems qualification. The results obtained will help speed up the qualification process that is now starting in our country by encouraging the introductionn of necessary and useful procedures on the use of these materials in a wide class of repair and retrofitt interventions. References [1] Triantafillou, T.C., Papanicolaou, C. G., Zissimopoulos, P., Laourdekis, T., Concrete confinement with textile-reinforced mortar jackets ; ACI Structural Journal, 103, no.1, (2006). [2] Triantafillou, T.C., Papanicolaou, C. G., Shear strengthening of reinforced concrete members with Textile reinforced Mortar (TRM) jackets, Materials and Structures, 39, (2006).

9 [3] Di Tommaso, A. Focacci, F., Mantegazza, G., PBO-FRCM composites to strengthen r.c. beams: mechanics of adhesion and efficiency, Proc. Fourth International Conference on FRP Composites in Civil Engineering (CICE 2008), Zurich, Switzerland, (2008). [4] Ombres, L., Flexural analysis of reinforced concrete beams strengthened with a cement based high strength composite material, Composite Structures, 94, no. 1, (2011). [5] D'Ambrisi, A., Focacci, F., Caporale, A. Strengthening of masonry-unreinforced concrete railway bridges with PBO-FRCM materials Composite Structures, 102, pp (2013). [6] D'Ambrisi, A., Feo, L., Focacci, F. Experimental and analytical investigation on bond between Carbon-FRCM materials and masonry Composites Part B: Engineering, 46, (2013). [7] Trapko, T. Stress-strain model for FRCM confined concrete elements Composites Part B: Engineering, 45 (1), (2013). [8] D'Ambrisi, A., Feo, L., Focacci, F. Experimental analysis on bond between PBO-FRCM strengthening materials and concrete Composites Part B: Engineering, 44 (1), (2013). [9] D'Ambrisi, A., Feo, L., Focacci, F. Bond-slip relations for PBO-FRCM materials externally bonded to concrete Composites Part B: Engineering, 43 (8), (2012). [10] Mantegazza, G., Gatti, A., Barbieri, A. Fiber reinforced cementitious matrix (FRCM)- advanced composite material and emerging technology for retrofitting concrete and masonry buildings. Proceedings of the 3rd International Conference on Bridge Maintenance, Safety and Management - Bridge Maintenance, Safety, Management, Life-Cycle Performance and Cost, (2006) [11] Prota, A., Marcari, G., Fabbrocino, G., Manfredi, G., Aldea, C. Experimental in-plane behavior of tuff masonry strengthened with cementitious matrix-grid composites Journal of Composites for Construction, 10 (3), art. no QCC, (2006). [12] Mazzotti C., Savoia M., Ferracuti B. A new single-shear set-up for stable delamination tests on FRP-concrete joints. Construction and Building Materials, vol. 23(4), p , ISSN: (2009). [13] Mazzotti C., Savoia M., Ferracuti B. An Experimental Study on Delamination of FRP Plates Bonded to Concrete. Construction and Building Materials, vol. 22, p , ISSN: (2008). [14] ICC Evaluation Service, Acceptance criteria for masonry and concrete strengthening using Fabric-Reinforced Cementitious Matrix (FRCM) composite systems, AC434, Draft August 1, 2011.