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Procedía Engineering 193 (2017) 289 - 296
Procedía Engineering
www.elsevier.com/locate/procedia
International Conference on Analytical Models and New Concepts in Concrete and Masonry
Structures AMCM'2017
Features of SRP tapes against CFRP composites used for strengthening of concrete structures
Rafal Krzywona*, Marcin Gorskia, Szymon Dawczynski a
aSilesian University of Technology, Department of Structural Engineering, Akademicka 5, Gliwice 44-100, Poland
Abstract
SRP (Steel Reinforced Polymer) tapes used as a reinforcement of externally bonded composite strengthening are representing a relatively new material, competitive to popular organic fibers. SRP tapes are based on ultra-high strength steel wires formed in cords and assembled into a fabric. Through advanced treatment, the mechanical properties of SRP steel are similar to other high carbon cold drawn steels used in construction (for example prestressing steel): strength exceed 2000 MPa, there is no perfect plasticity at yield stress level. Almost linear stress-strain relationship makes SRP steel competitive to carbon fibers. Also flexibility and weight ratio of the composite overlay is a little worse than FRP strip. The paper shows results of the laboratory test of beams strengthened with SRP and CFRP laminates. Features of both materials were compared in the aspects of mechanical properties, strengthening effectiveness, delamination, longitudinal behavior, application process and shaping easiness. The greatest noticed advantage of SRP composites is better bond performance which has a beneficial effect on strengthening efficiency. Improved ductility ensure better cooperation with the concrete surface and thus increase the delamination strain and limits probability of brittle rupture. Among the disadvantages there can be found greater weight and limited assortment of appropriate adhesives on the market, nevertheless SRP composites represent interesting alternative for the most popular carbon fibre strips and sheets.
© 2017 The Authors.PublishedbyElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).
Peer-reviewunder responsibilityof the scientificcommittee of the International Conference on Analytical Models and New Concepts in Concrete and Masonry Structures
Keywords: SRP composites; strengthening of structures
* Corresponding author. Tel.: +48-32-2372262; fax: +48-32-2372288. E-mail address: rafal.krzywon@polsl.pl
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the scientific committee of the International Conference on Analytical Models and New Concepts in Concrete and
Masonry Structures
doi: 10.1016/j.proeng.2017.06.216
1. Introduction
Among the materials used in construction especially composites based on organic fibers are considered as the strongest materials with the greatest potential of use as strengthening of structure. They are increasingly used in various branches of construction engineering including geotechnical, wooden, masonry and concrete structures. These materials are perfectly suited for structural strengthening. They are both strong and light and thanks to that easy to handle. As laminates they are difficult to shape, but adhered in wet lay-up process they could be ideally matched to the structure. Described advantages caused, that composites began to replace structural steel, which is not only weaker, but also several times heavier. Of course, the industry is able to deliver a steel with a much better strength parameters, but it is definitely more expensive due to the high contents of rare chemical elements and must be subjected to complex treatment processes. In a slightly larger scale such type of steel is used for many years in construction engineering to prestress the concrete structures. It has a carbon content not greater than 1%, its mechanical properties are generally improved by addition of manganese, nickel and chromium alloying elements. Some specific properties as hardness, brittleness reduction stress relieving are obtained by proper heat treatment. Ultimate strength properties can be achieved by cold working of steel. Usually this is drawing of wires through a series of dies, with progressive reduction in the diameter of wire at each die.
2. Mechanical properties of the UHTS steel and SRP tapes
Presented in this paper steel cords used in production of SRP composites are in fact manufactured by one of the largest tire companies and then merged into the tapes, rebranded and distributed by producer [1]. It should be underlined, that steel cords used for the tire reinforcement belong to the strongest industrial materials [2]. With the tensile strength reaching 4000 MPa they are inferior to the strongest carbon fibers only, as eg Toray T1000 [3].
SRP steel cords have a structure of pearlite steel consisting of lamellar ferrite and cementite strengthened by patenting and drawing. The work hardening ratio of eutectoid pearlite steel is greater than ferrite single phase that has low amounts of carbon, or dual phase steel comprising ferrite and martensite [2]. As shown in Figure 1a, thanks to that phenomena a small drawing strain is able to produce most effective growth of tensile strength.
Fig. 1. a) Effectiveness of strengthening by drawing eutectoid pearlite steel, ferrite-martensite dual phase steel and ferrite single phase steel [2]; b) Comparison of strengthening by patenting and wire drawing in the tensile strength of steel wires [4].
Primarily the ratio of strengthening by drawing determines their superior strength properties. It exceeds 60% and steel is drawn down to very fine wire diameters of about 0.20 ^ 0.35 mm. It is this feature that distinguishes mentioned steel from other high-strength structural steel. Figure 1b shows the comparison of strengthening efficiency by patenting and drawing in the strength of thick wire.
Ultra-fine diameter allows steel to be transformed into the specific microstructure of pearlite. During the process of drawing, the grains, or single crystals of steel pearlite microstructure are oriented in the drawing direction. It causes increase of strength in the direction of wire drawing (axial direction) while strengthening in radial direction is smaller.
Theoretical tensile strength of pearlite steel is estimated at approximately 10 GPa [2] and limited by deformation ability of cementite platelets and lamellar spacing. In practice intensive drawing process leads to a reduction in ductility thus wire could break even being drawn. That phenomena is caused by non-uniform deformations of wire section during drawing (plastic deformation of outer layer is usually greater than that in center what promotes friction and danger of delamination). SRP wire diameters of about 0.20 ^ 0.35 mm become a compromise between high strength and abundant ductility.
The mechanical properties of reinforced composites, inter alia, depend on the cooperation of the reinforcing fibers with the surrounding matrix. The smooth surface of the individual steel wire and lack of rigidity does not ensure sufficient adhesion, which should occur to prevent an undesirable slip of wires from the composite matrix. Significant improvement in grip could be easily achieved by twisting wires into strands. A disadvantage of twisting is a slight deterioration in mechanical properties in comparison to straight wires. This can be limited by introduction of straight wire core inside twisted wires. In this way, actually produced 3X2 strands are constructed, comprising three straight and two wires wrapped (Fig. 2a).
Fig. 2. a) The most popular 3X2 cord type; b) SRP tapes with densities of 12 and c) 20 strands per inch.
Stress strain relationship of UHTS SRP steel is characteristic for high tensile steels with high amount of carbon. Destruction occurs rather unexpectedly, there is no perfect plasticity at yield stress level and ideal elasticity to the moment of rupture. Figure 3 shows results of tensile tests of laminates based on two types of SRP tapes containing 12 and 20 strands per inch. In both cases inelastic behavior of SRP laminate may be described with the use of parabolic function [5]. Noticeable, slight decrease of the modulus of elasticity differs SRP from the majority of composites based on the organic fibers, characterized by constancy of this feature in the whole range of stress.
3. Laboratory tests of strengthening efficiency
Described in this chapter experimental program was aimed at evaluating the effect of SRP and CFRP types of external flexural strengthening on RC beams. The results will be compared with outcomes of similar studies conducted in other scientific centers [7-11].
3X2-12 cord type
4000 3500 3000 2500 2000 1500 1000 500 0
rc7 Oh
St rain [%]
3X2-20 cord type
4000 3500 3000 2500 2000 1500 1000 500 0
rc? PH
St rain [%]
Fig. 3. Stress-strain relationship for SRP composites.
The study included cases of beams externally strengthened with three types of overlays. In addition to the non-strengthened reference beam, seven beams were strengthened using one layer of SRP 3X2-20, four with 2 layers of SRP 3X2-12 tape, three beams with CFRP 200/2000 strips 60x1.4mm and two beams with CFRP carbon sheet 640400. Part of beams were overwrapped in the end zone to enhance the efficiency of anchorage. Finally a total number of 17 full scale reinforced concrete beams were tested as simple supported members over a clear span of 2.8 m, according to a four points bending scheme (Fig. 4). All beams had a rectangular cross section of 0.3 x 0.2 m. The mean value of the concrete compressive strength was 44.7 MPa. More detailed description of test is given in [6].
Fig. 4. Test set up.
3.1. Ultimate force and failure model
The experimental program proved the good effectiveness of both CFRP and SRP external reinforcements. All strengthened beams achieved a relevant flexural strength increase compared with the reference one. Particularly, the beams strengthened with CFRP sheets attained a mean load increase of about 39% and strips 43% while the beams with one layer of SRP tape had a load increase in the range of 48-52% (not overwrapped and overwrapped, respectively). Best performance was observed for beams strengthened with two layers of SRP 3X2-12 (average 57%), but this type of strengthening was characterized by a very large dispersion of results (from 47% to 70%). Generally it could be stated that strengthening effects are similar, but referring these results to the effective area of reinforcement (cross section area related with tensile strength) differences in the strengthening effectiveness are growing. Theoretical tensile force which breaks provided CFRP strip is equal to 216 kN, CFRP sheet 167 kN (77% when compared to 60x1,4 CFRP strip), one layer of SRP 3X2-20 155 kN (72%) and two layers of 3X2-12 190 kN (respectively 88%). Referring those values to the mentioned above effectiveness of the strengthening with CFRP strips equal to 43%, relative effectiveness for CFRP sheet will increase to 80%, while for one layer of SRP tape to 205-211% and for two layers of SRP 67-89%.
Reasons of described differences can be found by comparing the models of destruction. Only beams reinforced with CFRP sheet failed due to rupture of carbon fibers. For other strengthened beams, the failure occurred due to the detachment of the external reinforcement; in particular, debonding started at one of the flexural cracks. Furthermore, one beam strengthened with two layers of SRP failed by delamination of the outer layer of the laminate. In this single case, delamination was initiated in the anchoring zone by the end shear stresses. That shows the difficulty of ensuring the proper adhesion of the SRP laminate having increased thickness and explains worse relative strengthening efficiency.
Interestingly, there was no beneficial effect of provided overwrapping in the anchoring zones. Analyzing the failure mode it should be noted, that almost all of the specimens were destroyed in delamination initiated in the area of constant bending (midspan zone). Delamination initiated in the midspan zone, running towards the supports caused separation of the bottom part of overwrapping and eject the laminate. Stiffness of the overwrapping across the fibers was too small to realize its anchoring role and finally delamination provoked the detachment of overwrapping.
3.2. Delamination strain
Analyzing the change in strain of the composite during the test, it should be stated that for SRP and CFRP laminates they are very similar, but delamination of carbon strip occurs earlier. This observation is contrary to the majority of already reported tests (similar results in part of the study received only Minaugh [7]). It should be emphasized, however, that most of these investigations concerned the CFRP sheets rather than strips.
The received delamination strain in the SRP strengthened beams was greater than 6 %o, and up to 8 %o. These values are consistent with Wobbe [8] and Balsamo studies [9]. Described differences may result from the form of delamination. Delamination of models strengthened with CFRP strips occurred in the adhesive, while SRP tapes debonded in the surrounding concrete. All surfaces before application of the laminate were prepared in the same manner by epoxy resin impregnation. The most likely cause of better bond performance of SRP tape is its width, and therefore more than twice the area of contact with concrete surface thus shear stress in the adhesive can be significantly reduced.
3.3. Load - deflection
For both CFRP type beams and SRP beams effective area of external reinforcement and also its axial stiffness were different. As a consequence, the beams strengthened with CFRP strips showed a stiffer behavior compared to beams reinforced with SRP tapes and CFRP sheets. Under the cracking load difference is not visible, also the cracking level is similar and appears in the range of 14-15 kNm. Over that value, mentioned difference increases.
The external reinforcement allowed to delay the yielding of the internal steel reinforcement. Visible in Figure 5 change of the slope of stress strain relationship starts from 120 kN. It is 10 kN more than reference beam, without strengthening.
load - deflection
120 100 80 60 40 20 0 -20
reference RC CFRP strip CFRP sheet SRP 3X2-20 SRP 2x 3x2-12
deflection [mm]
Fig. 5. Measured laminate strains at midspan and deflections.
Another important aspect in terms of behaviour of strengthened element is deformability and ductility. In comparison to reference, not strengthened specimen, external reinforcement enables reduction of the deflection. As mentioned earlier, only CFRP sheet was broken by rupture of fibres. Failure occurred suddenly, before the yield of internal reinforcement. CFRP sheet most effectively reduces deflection, what is associated with application of high modulus carbon fibre type. Modules of elasticity of CFRP strips and SRP tapes were similar. This explains the similar growth of deflection of beams strengthened with that reinforcement. The variance in this case appears after the yield of internal steel and is the result of the difference in the effective area of strengthening. Post-yield growth of deflection more effectively provides SRP tape, however, this is not the result of better ductility of SRP steel, but better bond performance of that kind of reinforcement. This is confirmed by the analysis of laminate strains. At the moment of failure they are far from the ultimate value (~8%o vs 18%o).
3.4. Crack pattern
As mentioned earlier, there was no specific impact of external reinforcement at cracking moment. What could be expected, was that the greatest difference between crack widths was visible in comparison to reference beam (without strengthening). Due to the additional tension stiffening effect provided by the external composite material bonded to the bottom surface, process of crack opening occurred more slowly. Characteristic was a little denser crack pattern in case of SRP strengthening, but crack widths were comparable for all investigated load levels. That phenomenon is related to lower flexural stiffness and thus greater curvature and strain in the tensile zone of SRP strengthened beams. At load level causing the yield of internal reinforcement, appearance of new cracks was observed rather than accelerated opening of existing ones.
Phenomenon of better cracks distribution, their smaller spacing and width was also reported by Ceroni and Pecce [10, 11] and Balsamo et all [9]. Despite the greater deformability, better distribution of cracks was observed for SRP strengthened beams. It could be explained by to the greater width of the SRP tape compared to CFRP strip.
3.5. Long-term behavior
Two beams were tested under long-term load, first strengthened with CFRP strip and second with one layer of SRP 3X2-20. Total applied load was equal to 67.7 kN and was maintained for two years. Geometry, load scheme, measured values were the same as previously described. Figure 6 shows the deflection increase in time. Course of curves are very similar, but, as can be expected, because of differences in axial stiffness of CFRP strip and SRP laminate, SRP strengthened beam has a larger deformation. Those difference also relates to crack image. The average crack spacing for beam SRP is 110mm, while CFRP beam CFRPLT 140mm respectively [12]. Crack opening in both cases does not exceed 0.1 mm. Range of crack zone and crack pattern has not changed during the observed period. This may prove relatively small creep deformation in the area of composite overlays.
Fig. 6. Measured laminate strains at midspan and deflections.
4. Applicability
Application method of SRP tapes corresponds with general principles of „wet lay-up" method used for other composites, but adhesives used are denser and similar to those allowed for strips rather than sheets or mats. On the properly prepared base the thin bonding layer made of epoxy adhesive is applied and the SRP tape is placed. After the initial bonding on the mat surface the adhesive is rolled up to its full saturation. In order to improve the adhesion of any finishing or protective coverings (paints, plasters) last layer of adhesive may contain admixture of coarse sand.
In the construction industry the main area of use of SRP composites as well as FRP materials is the strengthening of concrete and masonry structures. They can be used for strengthening slabs, beams, columns, nodes in frame structures, walls, wall panels [13]. Due to the diameter of the steel strands, incomparably greater than the diameter of the organic fibers, shaping the SRP tapes to fit the structure is difficult but possible as opposed to CFRP strips. Bending radii smaller than 20 cm requires prior preparation. On the construction site it is possible to use portable hand-held or hydraulic bending machines. Bending itself is relatively simple and does not constitute a substantial executive complications, however, requires a prior adjustment to the strengthened structure. It is difficult in situations where it is necessary to wrap structures around, such as the helical reinforcement of the column -alternatively then can be used a single wrap with an overlap on one side of the element, which unfortunately results in a bit increased material consumption. It should be noted that a similar problem occurs in the case of CFRP L- or C-shape plates.
4. Conclusions
The main outcomes of the presented bending tests of externally strengthened RC beams can be summarized as follows:
Achieved the failure load increase compared to the reference beams was very significant for all the strengthened beams (about 39% and 70% for beams strengthened with CFRP sheet and two layers of steel fabric, respectively).
SRP system based on UHTS steel fabric showed a good strengthening efficiency comparable with the traditional one made of CFRP strip. Despite the lower axial capacity, the failure load was 4-21% higher. A lower effective consumption of material means also competitive cost of strengthening application.
Failure strains of SRP composite are higher than those measured for CFRP strips. It proves better material utilization, presumably caused by the improved bond performance.
Deflections of SRP strengthened beams were higher, due to the lesser axial stiffness. For the same reason, the width of flexural cracks was similar despite their smaller spacing for SRP strengthened beams.
SRP type reinforcement proves to be very competitive to other externally bonded strengthenings. Its popularization seems to be a matter of time and marketing activities of manufacturer.
Acknowledgements
Research program partly supported by the National Science Centre of Poland, grant no 1231/B/T02/2011/40.
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