International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. I, ICTRC 235 EXPERIMENTAL STUDY OF THE FLEXURAL BEHAVIOUR OF LOW PERFORMANCE RC BEAMS STRENGTHENED WITH TEXTILE REINFORCED MORTAR P. Larrinaga (1), J. T. San-José (1, 2), D. García (1), L. Garmendia (1), J. Díez (1) (1) LABEIN-Tecnalia, Spain (2) Universidad del País Vasco-University of the Basque Country, Spain ABSTRACT: During the last years several projects and studies have improved the knowledge about Textile Reinforced Mortar technology. TRM has already been used in strengthening masonry structural elements such as walls or arches and even concrete pillars through confinement, situations where the use of fibro plastic (FRP) composites have presented some drawbacks. One of these disadvantages could be the incompatibility with low performance concrete. These concretes are a consequence of aggressive environments, use of inadequate materials or lack of knowledge in general. The adherence features achieved by organic binders does not suit with poor concrete surfaces and could cause the debonding of FRP laminates or the pulling off concrete substrate when interface forces reach critical values. Besides, organic binders develop problems in moist or chemical aggressive environments showing an adherence shortage. Following this scope, this paper presents an experimental campaign developed to analyse the effect of TRM in flexural retrofitting of 14 beams made of low compressive strength concrete (17 MPa). Carbon, basalt and steel fibres have been used as different core materials for TRM. 1 INTRODUCTION There is an increasing need to strengthen or upgrade the existing housing state. It is estimated that about 16% of housing (128.700) needs rehabilitation in the Basque Country, an area with 2 million inhabitants [GV09]. There may be several causes for that necessity, such us changes in functionality, damages caused by mechanical actions and environmental effects, more stringent design requirements, original design or construction mistakes, and the age of the concrete. A considerable amount of structures were erected when the knowledge of the material was still limited and the durability issues were not considered in constructions standards. For example, Spanish concrete regulation attached appropriate and extensive control of materials and durability chapters for the first time in 1973 and 1998 respectively. For these reasons, there are generations of low quality concrete structures which do not suit nowadays standards, neither mechanical characteristics, nor durability exigencies. Low compressive strength, steel reinforcement corrosion, use of plain steel rebars, high porosity and decayed coverings are some of the features of ancient concrete. The use of fiber Reinforced Polymer (FRP) supposed a revolution in the rehabilitation area. Traditional materials were substituted by a light, durable and easy to use material which achieved good results. However, FRP solutions presented some drawbacks when it was used with ancient or low performance concrete, especially in flexural strengthening. The high adherence achieved by organic binders allows the development of high shear forces in the interface between concrete and the composite. The condition of the covering and the low mechanical characteristics of concrete cause FRP debonding and concrete cover ripping
Transcript
International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. I, ICTRC 235
EXPERIMENTAL STUDY OF THE FLEXURAL BEHAVIOUR OF LOW PERFORMANCE RC BEAMS STRENGTHENED WITH TEXTILE REINFORCED MORTAR
P. Larrinaga (1), J. T. San-José (1, 2), D. García (1), L. Garmendia (1), J. Díez (1)
(1) LABEIN-Tecnalia, Spain
(2) Universidad del País Vasco-University of the Basque Country, Spain
ABSTRACT: During the last years several projects and studies have improved the knowledge about Textile Reinforced Mortar technology. TRM has already been used in strengthening masonry structural elements such as walls or arches and even concrete pillars through confinement, situations where the use of fibro plastic (FRP) composites have presented some drawbacks. One of these disadvantages could be the incompatibility with low performance concrete. These concretes are a consequence of aggressive environments, use of inadequate materials or lack of knowledge in general. The adherence features achieved by organic binders does not suit with poor concrete surfaces and could cause the debonding of FRP laminates or the pulling off concrete substrate when interface forces reach critical values. Besides, organic binders develop problems in moist or chemical aggressive environments showing an adherence shortage. Following this scope, this paper presents an experimental campaign developed to analyse the effect of TRM in flexural retrofitting of 14 beams made of low compressive strength concrete (17 MPa). Carbon, basalt and steel fibres have been used as different core materials for TRM.
1 INTRODUCTION
There is an increasing need to strengthen or upgrade the existing housing state. It is estimated that about 16% of housing (128.700) needs rehabilitation in the Basque Country, an area with 2 million inhabitants [GV09]. There may be several causes for that necessity, such us changes in functionality, damages caused by mechanical actions and environmental effects, more stringent design requirements, original design or construction mistakes, and the age of the concrete. A considerable amount of structures were erected when the knowledge of the material was still limited and the durability issues were not considered in constructions standards. For example, Spanish concrete regulation attached appropriate and extensive control of materials and durability chapters for the first time in 1973 and 1998 respectively. For these reasons, there are generations of low quality concrete structures which do not suit nowadays standards, neither mechanical characteristics, nor durability exigencies. Low compressive strength, steel reinforcement corrosion, use of plain steel rebars, high porosity and decayed coverings are some of the features of ancient concrete.
The use of fiber Reinforced Polymer (FRP) supposed a revolution in the rehabilitation area. Traditional materials were substituted by a light, durable and easy to use material which achieved good results. However, FRP solutions presented some drawbacks when it was used with ancient or low performance concrete, especially in flexural strengthening. The high adherence achieved by organic binders allows the development of high shear forces in the interface between concrete and the composite. The condition of the covering and the low mechanical characteristics of concrete cause FRP debonding and concrete cover ripping
236 LARRINAGA, SAN-JOSÉ, ET AL.: Experimental Study of the Flexural Behaviour of Low Performance
[Esf06]. This failure not only involves the loss of the strengthening action, also an important damage to the element.
Innovative composites made of cementitious matrix reinforced by a core of textile could be an alternative solution on strengthening RC elements subjected to flexural forces. The behaviour of this technological system, Textile Reinforced Mortar (TRM) is investigated and discussed in this paper.
2 CONCRETE AND TRM
The research on the use of textile meshes as reinforcement of cementitious products started in the early 1980s. Nevertheless, developments in this material progressed in the late 1990s. During the last years the research community has considered the use of textiles as reinforcement core of cement-based products, in new constructions or in rehabilitation. Chopped fibers are used in concrete mixtures to improve the mechanical properties and the production of precast concrete elements reinforced with textile fabrics has been recently investigated [Heg06]. In addition, Textile Reinforced Mortar has focused the efforts of numerous research projects in the rehabilitation field, especially in masonry [Fae09, Gar09] and concrete elements. Fire protection, easiness to install, low price of mortar, cheap matrix, compatibility with moist surfaces, vapour permeability and no hazardous emanations are some of the features which present cementitious composites as an attractive alternative to the use of FRP solutions. Studies on the use of TRM in the upgrading of concrete elements have been limited. Recent works in confinement [Bou07], shear [Tri06] and flexural strengthening [Tom07] demonstrates the effectiveness of Textile Reinforced Mortar system applied to concrete structures.
The reinforced mechanisms of fibers in a cementitious matrix are different from those of polymeric matrices. Drawbacks presented by FRP are mainly related to the use of organic binders, a material more ductile than carbon or glass fibers. On the other hand, the low strain in tension of the cement mortar contrasts with the achieved by the fibers. For this reason, the matrix breaks longer before the fabric and the composite becomes effective when the matrix has cracked and the fibers are acting as bridges transmitting forces over the crack and managing a better distribution of the forces [Pel02]. On the other hand, the adherence between concrete and the cementitious matrix is lower than the achieved with organic binders. The combination of these two features, formation of cracks and lower adherence, is indicated for strengthening ancient concretes. The number of stress concentration points is reduced and the risk of concrete cover ripping is avoided.
The main aim of this experimental study was to present an overview of the behaviour of TRM flexural strengthening in low performance RC third scale beams, the influence of the different textile fabrics used as reinforcement core and the failure mechanism. This structural aspect was investigated during the campaign and new strengthening systems were designed looking for an improvement of the Textile Reinforced Mortar effect.
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3 EXPERIMENTAL PROCEDURES
3.1 Concrete beams
For this experimental study fourteen rectangular beams were cast with dimensions 150mm x 150mm x 1500mm. A low performance concrete was requested to a supplier in order to simulate the effect of ancient concrete described in the Introduction. Table 3.1 shows details about the concrete mixture.
Table 3.1. Characteristics of concrete mixture
Characteristics Content [kg/m3] Unit weight 2270 Water 120 Cement 210 Sand, limestone 0/5mm 1200 Aggregate, limestone 12/20mm 750 Super plasticizer 1.3 Cement type Portland type II 42.5 Water/cement ratio 0.57 Slump (mm) 100 Table 3.2. Concrete and steel reinforcement characteristics
Concrete Steel Stirrup spacing Tension Steel Comp. Steel fc (MPa) fc (MPa) (mm) As (mm2) A’s (mm2)
17 567 10 39.2 39.2 Beams were cast in plywood formworks which were previously impregnated with release agents to avoid the adhesion between the fresh mixture and the wood. Concrete was vibrated to consolidate the mixture compacting and prevent voids presence. Specimens were removed from the formworks after 48 hours and sprayed with water every day during two weeks. Concrete compressive strength was determined with Ø150mm x 300mm cylinders tested after 28-day curing (UNE-83303/84). In addition, steel reinforcement was also characterized. Characteristics of concrete and steel are presented in Table 3.2, whereas details of the beam and test setup are shown in the Fig. 3.1, all dimensions are in mm.
238 LARRINAGA, SAN-JOSÉ, ET AL.: Experimental Study of the Flexural Behaviour of Low Performance
Fig. 3.1. Test beam details and setup. The right end shows the anchorage device
3.2 TRM materials
Three different materials were used as Textile Reinforced Mortar fabric core: basalt, carbon and steel. In the following lines these materials are presented:
a) Basalt fabric is a grid with fibers disposed along two orthogonal directions. The cell dimensions are 20mm x 20mm.
b) Carbon grid has similar shape as basalt. The distance between rovings is 8mm and the cell’s dimensions are 4mm x 4mm. See Figure 3.2.
c) Low-Density Steel fabric. Hardwire fibers are disposed in one direction. The textile is conformed by a polypropylene net, forcing steel fibers to spread in the longitudinal direction. The distribution is 1,57 wire/cm. In this paper this material is called LD-Steel.
d) Medium-Density Steel fabric, MD-Steel, has the same features than the previous one, the only difference is the density of wires per cm. 4,71 wire/cm.
Fig. 3.2. Basalt, carbon, LD-Steel and MD-Steel fabric
Mechanical characteristics of fabrics, as given by the producer, are summarized in Table 3.3. Table 3.3. Fibers materials mechanical properties
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The cementitious mortar used as matrix material contains less than 4% of organic resins. After 28-day curing, 40mm x 40mm x 160mm prisms were tested to determine mortar mechanical properties. Compressive strength is 19,8MPa while tensile flexural strength is 7,2MPa.
3.3 Mechanical anchorage
Debonding failure of the TRM reinforcement was expected in some beams. Mechanical anchor devices were installed to avoid that problem. For this purpose, steel plates of 150mm x 270mm x 5mm were designed. Each couple of plates was fixed to the concrete surface by three bolts M16. Fig. 3.1 presents the anchor device installed in the right end of the beam.
3.4 Specimen strengthening
Tensile surface of each beam was roughened in order to achieve an optimum adherence with the strengthening composite. As concrete would absorb water from the mortar, it was necessary to humidify the rough surface before TRM was applied.
Specimens strengthened with basalt contained two fabric layers within the TRM whereas only one ply of the rest of materials was applied in the reinforcement. The strengthening steps are described in the following lines. After humidify the surface, a first lay (4-5mm) of mortar was applied with a trowel. The next step was to install the fabric into the matrix. To ensure that the fibers were completely immersed in the mortar two tools were used. Firstly, a roller was applied to stretch smoothly the fabric and prevent waves. Secondly, the use of a brush spread the mortar over all the fabric and assured the matrix soak in the fiber net. The last step is to cover TRM core with another mortar layer. In the cases with steel plates were used the first mortar layer had to have the same thickness, 5mm. Steel plates were placed 150mm from the ends of the beam (see Figure 3.1). The bolts were tightened with the same force using a dynamometric wrench.
Fig. 3.3. Strengthening steps
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The system used to number the specimens is as follows: “r” refers to control beams, “b” to basalt textile, “c” means that carbon was the used material, “ls” and “ms” define the specimens strengthened with LD-Steel and MD-Steel respectively. Where mechanical anchors are used an “a” accompanies the label in the end, e.g. “lsa”.
3.5 Test setup
The beams were placed in a compressive loading frame (100kN electro-hydraulic programmable universal testing machine) for the four-point flexural test. The load was applied under a displacement rate of 3μm/s. Three Linear Variable Differential Transducers (LVDT) were mounted at mid-span and under load points to obtain an accurate measure of the deflection of the constant bending moment zone. A MGC-Plus data logger was connected to the three LVDTs and to the load cell for collecting the real-time loading and deflection in the same hardware. Data was recorded with a frequency of 1 Hz.
4 TEST RESULTS AND DISCUSSION
4.1 Failure modes
The control beams failed due to yielding of the longitudinal steel reinforcement. All strengthened beams exhibited a change in crack pattern from widely spaced cracks to many smaller cracks at a closer spacing. This effect is desirable, the deflection is limited, the forces are more distributed in the external reinforcement and the durability, directly related with the width of cracks, is improved. Specimens can be classified among three different failure modes.
a) Steel yielding and TRM fracture. Fibers broke when their elongation limit was reached, at that moment the steel, which was yielding, could not bear the applied load. It was observed in beams strengthened with basalt and carbon TRM. (Figure 4.1.a).
b) Debonding at intermediate flexural crack. Fig 4.1.b. Delamination in concrete-matrix interface started from the zone with maximum bending moment and spread in both directions. The debonding caused sudden plastic failure of the steel rebars due to the loss of the reinforcement. Specimens strengthened with LD-Steel suffered this failure mode.
c) Debonding at outermost crack. Delamination appeared at the end of the composite reinforcement and moved forward the mid-pan as it is observed in Figure 4.1.c. This failure mode happened with MD-Steel TRM specimens. After TRM debonding, beams continued bearing load for a while till the longitudinal rebars collapse.
Mechanical anchors were used in order to avoid the brittle failure of the TRM debonding. Their effect was substantial and entailed different failure mechanisms.
d) In anchored LD-Steel TRM beams the failure mode was the same than in case a). Steel fibers broke and this fact caused the collapse of the whole beams. See Figure 4.2.a.
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e) Beams strengthened with MD-Steel TRM anchored at the ends presented a new failure mode. The steel longitudinal rebars broke and the TRM, debonded from the concrete, started to slip between the steel plates and acted as a kind of tie rod. (Fig. 4.2.b)
a b c
Figure 4.1. Failure modes
a b Fig 4.2. Failure modes
4.2 Load-carrying capacity
Table 4.1 shows a summary of the experimental campaign. Flexural bearing capacity, deflection and failure mechanisms are represented for each specimen.
The addition of TRM reinforcement achieved gain of bending capacity in all the specimens. While basalt and carbon increased the maximum moment capacity by 40,5% and 48,5% respectively the beams strengthened with steel fibers gained between 94% and 148% compared to the control beams. The deflection values of strengthened with carbon TRM beams was not increased. No anchored MD-Steel beams suffered sudden loss of bearing capacity before the ultimate deflection which was achieved in the control beams. Basalt and LD-Steel TRM contributed to increase the deformability of the beams, especially LD-Steel.
242 LARRINAGA, SAN-JOSÉ, ET AL.: Experimental Study of the Flexural Behaviour of Low Performance
Table 4.1. Test campaign summary
Moment [kN·m] Deflection [mm] Beam
Maximum Rate Ultimate Maximum Rate Ultimate Failure Mode 1
Figs. 4.3-4.6 show moment vs. deflection at mid-span for all the specimens tested in this campaign. All the graphics have the same scale to compare directly the results and only one control beams is represented, r-1. Each curve consists of three parts. The first one finishes when the first crack appears in the beam. The second represents the behaviour of the specimen until the steel longitudinal rebars starts to yield. The last part shows the behaviour of the beam during the yielding of the rebars and it finishes when the collapse happened. The end of the curve is influenced by the different failure mechanisms.
Fig. 4.3. Basalt strengthened series Fig. 4.4. Carbon strengthened series
International RILEM Conference on Material Science – MATSCI, Aachen 2010 – Vol. I, ICTRC 243
Fig. 4.5. LD-Steel strengthened series Fig. 4.6. MD-Steel strengthened series
Curves complement the result given in Table 4.1. Basalt and carbon achieved a similar improvement of bear capacity, but the increase was obtained by carbon TRM with less deformation than basalt series. Another interesting remark is the effect of anchors with steel fibers. In the case of beams strengthened with LD-Steel TRM, there was not a considerable gain in terms of flexural strength. However, the deformability of TRM anchored beams was increased a 40% compared to the specimens with no steel plates. On the other hand, the difference between TRM anchored and no-anchored was more substantial in MD-Steel due to the brittleness of the failure mode, 25% in ultimate flexural strength and 152% in ultimate mid-span deflection. Figure 4.6 also shows the behaviour of medium density steel TRM after the failure of the steel rebars. Fibers acted as a tie rod achieving a residual bearing capacity. Graphics also give information about serviceability state; Fig. 4.7presents the results amplified in the serviceability deflection. The different slopes of the branches show the influence of the different elastic modulus presented in Table 3.3.
Fig. 4.7. Serviceability state
5 CONCLUSIONS AND FUTURE ACTIONS
Test results of 14 beams have been presented. TRM composite resulted effective for strengthening low performance third-scale RC beams. Under flexural loads the strengthened beams increased their bearing capacity and showed different failure modes. The failure mechanisms related to loss of strengthening action (debonding) depended on the concrete-matrix shear strength or the tensile strength of the fabrics. Interface failure was caused by the high force transmitted to the TRM reinforcement and it is responsible of brittle collapse.
Steel fabrics, basalt and carbon fabrics immersed in cementitious mortar broke at the same moment the beam collapsed. Specimens strengthened with steel TRM suffered debonding of the inorganic reinforcement in concrete-matrix interface. This drawback was counteracted
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with the use of steel plates as mechanical anchors at the ends of the TRM strengthening. The effect was desirable, the brittle failure was prevented and the bearing capacity and the deformability were increased.
Similarly materials used in this project were characterized in laboratory in order to develop an analytical model. Another future action is to test Textile Reinforced Mortar in full-scale beams (4 m.)
ACKNOWLEDGMENT
The authors would thank to Orion Reparaciones Estructurales and Bikain for the support given to this project. The financial contribution of the Basque Government (GV-Emaitek 2008), the Regional Biscay Government (Birgaitek Project, DFB-7/12/TK/2009/10) and the Foundation Iñaki Goenaga with its scholarship programme is also acknowledged by the authors. Finally, it is necessary to underline the collaboration of Francisco Bengoetxea, Josu Lucena and Francisco Alonso.
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[Gar09] García, D,: Experimental and numerical analysis of stone masonry walls strengthened with advanced composite materials. PhD. (2009). Universidad del País Vasco. University of the Basque Country.
[Tri06] Triantafillou, T,; Papanicolau, C,: Shear strengthening of reinforced concrete members with textile reinforced mortar (TRM) jackets. Materials and Structures. (2006). 39:93-103.
[Tom07] Di Tomasso, A,; Focacci, F,; Mantegazza, G,; Garri, A,: FCRM versus FRP composites to strengthen RC beams a comparative analysis. FRPRCS-8. University of Patras. (2008). 636-638.
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