Materials and Structures (2008)41: 879-890 DOI10.1617/s11527-00792914 ORIGINAL ARTICLE Determination of tensile strength of glass fiber straps Zorislav Soric Josip Galic Tatjana Rukavina Received: 19 January 2007Accepted: 19 July 2007/Published online: 20 September 2007 C RILEM 2007 Abstract Glass fibers straps have been used for E01-0.3% Modulus of elasticity of fabric strengthening of masonry and concrete structures in determined by the increase of the ing tensile strength of dry glass fiber straps as well as E. -4s8 0.3% as recommended by ASMe and the last decade. Recently, their use has become specimens strain, aF, between 0.1% greater. This paper describes the research of measur modulus of el sticity of fabric determined straps that were made of glass fibers and epoxy by the increase of the specimen's strain, coating. The effect of strap widths, the effect of F, between 0.1% and 0.5%o loading speeds and the effect of epoxy coating placed Fn Force at failure, [kNI on fiber straps, on tensile strength of straps have been fmax Tensile strength of fabrics, [kN/ analyzed. Differences of tension strengths of fiber fr Tensile strength of longitudinally straps with and without epoxy coating are shown riented FRP composite Tensile fiber strength Keywords Glass fibers. Epoxy coating tensile matrix strength Straps. Fabric. Composites Strengthening fab Tensile strength of fabrics, IMPal 1-0.5% Secant stiffness of fabric Notations L Strap area Mean value B Strap width Standard deviation Modulus of elasticity of longitudinally xsVV Volume of matrix in oriented FRP composites Volume of fibers in composit Efb Modulus of elasticity of fibers EFma Strain of strap at failure E Modulus of elasticity of matrix Coefficient of variation Ve Volume ratio of fibers in composite Z Soric(<).J Galic. T Rukavina Faculty of Civil Engineering, University of Zagreb, olume ratio of matrix in composite Kaciceva 26, 10000 Zagreb. Croatia e-mail: soric@ grad.hr J. Galic 1 Introduction T Ruk FRP products are polymer composite materials comprising fibers, and polymer matrix, thus the name
ORIGINAL ARTICLE Determination of tensile strength of glass fiber straps Zorislav Soric Æ Josip Galic Æ Tatjana Rukavina Received: 19 January 2007 / Accepted: 19 July 2007 / Published online: 20 September 2007 RILEM 2007 Abstract Glass fibers straps have been used for strengthening of masonry and concrete structures in the last decade. Recently, their use has become greater. This paper describes the research of measuring tensile strength of dry glass fiber straps as well as straps that were made of glass fibers and epoxy coating. The effect of strap widths, the effect of loading speeds and the effect of epoxy coating placed on fiber straps, on tensile strength of straps have been analyzed. Differences of tension strengths of fiber straps with and without epoxy coating are shown. Keywords Glass fibers Epoxy coating Straps Fabric Composites Strengthening Notations A Strap area B Strap width Ef Modulus of elasticity of longitudinally oriented FRP composites Efib Modulus of elasticity of fibers Em Modulus of elasticity of matrix E0.1–0.3% Modulus of elasticity of fabric determined by the increase of the specimen’s strain, eF, between 0.1% and 0.3% as recommended by ASTM [3] E0.1–0.5% modulus of elasticity of fabric determined by the increase of the specimen’s strain, eF, between 0.1% and 0.5% Fmax Force at failure, [kN] fmax Tensile strength of fabrics, [kN/m] ff Tensile strength of longitudinally oriented FRP composite ffib Tensile fiber strength fm tensile matrix strength ffab Tensile strength of fabrics, [MPa] Jmax 0.1–0.5% Secant stiffness of fabrics L Strap length x Mean value s Standard deviation Vm Volume of matrix in composite Vfib Volume of fibers in composite eFmax Strain of strap at failure m Coefficient of variation vfib Volume ratio of fibers in composite: vfib ¼ Vfib VfibþVm vm Volume ratio of matrix in composite: vm ¼ Vm VfibþVm 1 Introduction FRP products are polymer composite materials, comprising fibers, and polymer matrix, thus the name Z. Soric (&) J. Galic T. Rukavina Faculty of Civil Engineering, University of Zagreb, Kaciceva 26, 10000 Zagreb, Croatia e-mail: soric@grad.hr J. Galic e-mail: jgalic@grad.hr T. Rukavina e-mail: rukavina@grad.hr Materials and Structures (2008) 41:879–890 DOI 10.1617/s11527-007-9291-4
880 Materials and Structures(2008)41: 879-890 Fiber reinforced Polymers or abbreviated FRP. Due temperature boundary between the areas of different to their properties, small mass, and high tensile mechanical properties of the same polymer. At this ally used in military and temperature, fibers will not be damaged, but the bond aviation industries. The manufactures were seeking strength between the matrix and fibers will decrease, new applications for their FRP product and the as well as the bond between the matrix and the possibility of massive use in civil engineering surrounding material. The most commonly used industry appeared. The basic idea was to produce a coating is epoxy resin. The usual properties of the material that would satisfy the growing needs for cold curing epoxy adhesive, concrete and steel used strengthening of structural elements and replace the in civil engineering, are shown, for comparison, in steel reinforcement of concrete and/or masonry Table 1 [1]. structures in aggressive environment. It is well known that aggressive environment can accelerate the corrosion of steel that was placed as reinforce 2.2 Fibers ment in concrete or masonry elements. Polymer composites are used in civil engineering and are Polymer composites have acronyms depending on the made of Aramid Fibers(AF), Carbon Fibers(CF)and fibers they are reinforced with: GFRP(glass fibers), Glass Fibers(GF). All these fibers possess high AFRP(aramid fibers), or CFRP(carbon fibers). The tensile strength. Polymer glue bounds fibers and such fibers increase the strength and stiffness of the com posite are shaped in moulds, applying pressure, composites. In order to create composites, one of which results in: AFRP, CFRP, and GFRP products. three types of fibers can be used: (1) short fibers There are two basic components of FRP products: (50 mm), oriented in all directions;(2) longitudinally fibers and polymer matrix. In polymer composite oriented, interwoven, straps of long fibers; and (3) fiber content is between 35 and 70 vol%o long fibers in bundles or fabric woven from bundles. The fibers are 5-20 um (10 m) thick and thei physical and mechanical properties vary depending 2 Components of polymer composites on fiber's type, [1, 2]. Carbon fibers are manufactured by controlled oxidation, carbonization, and graphiti- 2.1 Polymer matrix zation comparison with the other types of The matrix is made of different polymer resins, fibers have a higher strength and modulus of elastic selection of which depends on the type of fiber that ity, as well as better corrosion, fatigue, and creep should be bounded. Polymer matrix bounds fibers resistance Glass fibers are produced by protrusion of into composites and ensures the right position and a mixture of melted quartz sand, china clay, lime direction of fibers. At the same time the matrix stone, and colemanite, through small apertures at the protects fibers from environmental damage and mixture temperature of 1, 600oC. Th partially also from the mechanical damage. The later cooled. Different types of glass fibers are hatrix is also responsible for the transfer of load manufactured. Most often used glass fibers in com- between fibers, which implies a need for a good bond posites are: A-glasses (alkaline), E-glasse between fibers and the matrix. Strength and defor- (electrical), and C-glasses (chemical) fibers. The mation of composites are determined by the content A-glass fibers contain high proportion of boric acid of fibers and polymer matrix in the composites and and aluminates, thus, they are sensitive to alkaline he quality of bond between the matrix and the fibers. corrosion. E-glasses are more resistant to alkaline An important property of a synthetic resin is water agents and are significantly stronger and stiffer resistance, especially in extremely damp environ- C-glasses are highly resistant to alkaline agents ments. Thermal properties of the matrix determine E-glass fibers are most commonly used fibers due to he resistance of the composite, because the matrix their acceptable price and good technical properties has got much smaller thermal resistance and stability Composites with the glass fibers are heavier and of than fibers. The main property of the matrix is smaller strength and modulus of elasticity, but they Tg.the glass transition temperature, i.e., the are several times less expensive than the composites
Fiber Reinforced Polymers or abbreviated FRP. Due to their properties, small mass, and high tensile strength, they were initially used in military and aviation industries. The manufactures were seeking new applications for their FRP product and the possibility of massive use in civil engineering industry appeared. The basic idea was to produce a material that would satisfy the growing needs for strengthening of structural elements and replace the steel reinforcement of concrete and/or masonry structures in aggressive environment. It is well known that aggressive environment can accelerate the corrosion of steel that was placed as reinforcement in concrete or masonry elements. Polymer composites are used in civil engineering and are made of Aramid Fibers (AF), Carbon Fibers (CF) and Glass Fibers (GF). All these fibers possess high tensile strength. Polymer glue bounds fibers and such composite are shaped in moulds, applying pressure, which results in: AFRP, CFRP, and GFRP products. There are two basic components of FRP products: fibers and polymer matrix. In polymer composites, fiber content is between 35 and 70 vol%. 2 Components of polymer composites 2.1 Polymer matrix The matrix is made of different polymer resins, selection of which depends on the type of fiber that should be bounded. Polymer matrix bounds fibers into composites and ensures the right position and direction of fibers. At the same time the matrix protects fibers from environmental damage and partially also from the mechanical damage. The matrix is also responsible for the transfer of load between fibers, which implies a need for a good bond between fibers and the matrix. Strength and deformation of composites are determined by the content of fibers and polymer matrix in the composites and the quality of bond between the matrix and the fibers. An important property of a synthetic resin is water resistance, especially in extremely damp environments. Thermal properties of the matrix determine the resistance of the composite, because the matrix has got much smaller thermal resistance and stability than fibers. The main property of the matrix is Tg, ‘‘the glass transition temperature,’’ i.e., the temperature boundary between the areas of different mechanical properties of the same polymer. At this temperature, fibers will not be damaged, but the bond strength between the matrix and fibers will decrease, as well as the bond between the matrix and the surrounding material. The most commonly used coating is epoxy resin. The usual properties of the cold curing epoxy adhesive, concrete and steel used in civil engineering, are shown, for comparison, in Table 1 [1]. 2.2 Fibers Polymer composites have acronyms depending on the fibers they are reinforced with: GFRP (glass fibers), AFRP (aramid fibers), or CFRP (carbon fibers). The fibers increase the strength and stiffness of the composites. In order to create composites, one of three types of fibers can be used: (1) short fibers (50 mm), oriented in all directions; (2) longitudinally oriented, interwoven, straps of long fibers; and (3) long fibers in bundles or fabric woven from bundles. The fibers are 5–20 lm (106 m) thick and their physical and mechanical properties vary depending on fiber’s type, [1, 2]. Carbon fibers are manufactured by controlled oxidation, carbonization, and graphitization of organic materials rich in carbon. In comparison with the other types of fibers, carbon fibers have a higher strength and modulus of elasticity, as well as better corrosion, fatigue, and creep resistance. Glass fibers are produced by protrusion of a mixture of melted quartz sand, china clay, limestone, and colemanite, through small apertures at the mixture temperature of 1,600C. Those fibers are later cooled. Different types of glass fibers are manufactured. Most often used glass fibers in composites are: A-glasses (alkaline), E-glasses (electrical), and C-glasses (chemical) fibers. The A-glass fibers contain high proportion of boric acid and aluminates, thus, they are sensitive to alkaline corrosion. E-glasses are more resistant to alkaline agents and are significantly stronger and stiffer. C-glasses are highly resistant to alkaline agents. E-glass fibers are most commonly used fibers due to their acceptable price and good technical properties. Composites with the glass fibers are heavier and of smaller strength and modulus of elasticity, but they are several times less expensive than the composites 880 Materials and Structures (2008) 41:879–890
Materials and Structures (2008)41: 879-890 Table 1 Comparison of typical properties for epoxy adhesives, concrete, and steel [ll Property(at20°C Cold-curing epoxy adhesive Concrete Density(kg/m) 1,100-1,700 2, 7.800 Modulus of elasticity (GPa) 0.520 Shear modulus(GPa) 0.2-8 80 Tensile strength(MPa) 200-600 25-150 200.600 Tensile strain at break (%o 0.015 Coefficient of thermal expansion(10/C) 25-100 ll-13 0=15 Glass transition temperature-Tg(C) 45-80 with carbon fibers. Due to the sensitivity of glass to classified under two product types:(1)Prefabricated alkaline corrosion, the matrix material must protect FRP composite elements in form of wires, bars, glass fibers in composite. Aramid is a synthetic lamellas(Fig 1a); and (2) Dry, one or two direction polymer of high specific strength. Aramid fibers are oriented interwoven fiber fabric(Fig. la, b)set in flame resistant, but not UV-resistant Composites of epoxy resin during the strengthening process aramid fibers are of high impact strength and they absorb and disperse impact energy. Typical prope ties of all three-fiber types are shown in Table 2 [1]. 3.1 Prefabricated FRP products in form of rods and lamellas 3 Polymer composite products in civil engineering Composite products that are made of fibers and Polymer composites are used in civil engineering in matrix are first produced and then incorporated into a various forms: as wires, bars, or rods for reinforce- structure(e.g, FRP bars), or glued with adhesive glue ment, cables and ropes for pre-stressing and post- on the prepared surface(as lamellas). They are ready stressing, fabrics, laminate, straps and various types for use products, similar (in shape and appearance)to of sandwich panels. Polymer composites can be steel products. Their properties for certain types of Table 2 Typical properties of fibers [1I Material Modulus of elasticity Tensile strength Ultimate tensile strain MPa) High strengt 215-235 3,500-4,800 l4-2.0 215-235 3,5006000 15-2.3 High modulus 350-500 2,500-3,100 Ultra high 500-700 2,100-2,400 Glass fibers 70 1,900-3,000 3.0-4.5 3,500-4,800 Low modulus 70-80 3,500-4,100 4.3-5.0 High modulus l15-130 3,500-4,000 5-3.5
with carbon fibers. Due to the sensitivity of glass to alkaline corrosion, the matrix material must protect glass fibers in composite. Aramid is a synthetic polymer of high specific strength. Aramid fibers are flame resistant, but not UV-resistant. Composites of aramid fibers are of high impact strength and they absorb and disperse impact energy. Typical properties of all three-fiber types are shown in Table 2 [1]. 3 Polymer composite products in civil engineering Polymer composites are used in civil engineering in various forms: as wires, bars, or rods for reinforcement, cables and ropes for pre-stressing and poststressing, fabrics, laminate, straps and various types of sandwich panels. Polymer composites can be classified under two product types: (1) Prefabricated FRP composite elements in form of wires, bars, lamellas (Fig. 1a); and (2) Dry, one or two direction oriented interwoven fiber fabric (Fig. 1a, b) set in epoxy resin during the strengthening process. 3.1 Prefabricated FRP products in form of rods and lamellas Composite products that are made of fibers and matrix are first produced and then incorporated into a structure (e.g., FRP bars), or glued with adhesive glue on the prepared surface (as lamellas). They are ready for use products, similar (in shape and appearance) to steel products. Their properties for certain types of Table 1 Comparison of typical properties for epoxy adhesives, concrete, and steel [1] Property (at 20C) Cold-curing epoxy adhesive Concrete Mild steel Density (kg/m3 ) 1,100–1,700 2,350 7,800 Modulus of elasticity (GPa) 0.5–20 20–50 205 Shear modulus (GPa) 0.2–8 8–21 80 Poisson’s ratio 0.3–0.4 0.2 0.3 Tensile strength (MPa) 9–30 1–4 200–600 Shear strength (MPa) 10–30 2–5 200–600 Compressive strength (MPa) 55–110 25–150 200–600 Tensile strain at break (%) 0.5–5 0.015 25 Coefficient of thermal expansion (106 /C) 25–100 11–13 10–15 Glass transition temperature—Tg (C) 45–80 – – Table 2 Typical properties of fibers [1] Material Modulus of elasticity (GPa) Tensile strength (MPa) Ultimate tensile strain (%) Carbon fibers High strength 215–235 3,500–4,800 1.4–2.0 Ultra high strength 215–235 3,500–6,000 1.5–2.3 High modulus 350–500 2,500–3,100 0.5–0.9 Ultra high modulus 500–700 2,100–2,400 0.2–0.4 Glass fibers A 70 1,900–3,000 3.0–4.5 E 85–90 3,500–4,800 4.5–5.5 Aramid fibers Low modulus 70–80 3,500–4,100 4.3–5.0 High modulus 115–130 3,500–4,000 2.5–3.5 Materials and Structures (2008) 41:879–890 881
Materials and Structures(2008)41: 879-890 Fig 1 (a) FRP products bars. lamellas. fabric and b)glass fiber fabric that as tested fibers depend on the volume ratio of fibers in the and dimension is based on the overall cross section of composite. The usual fiber content of fibers in the composI ite product(fiber matrix) lamellas is between 35 and 70 vol%o. The modulus of elasticity and tensile strength of final composites 3.2 Dry, one-way or two-way oriented can be calculated from the volume ratio of fibers and interwoven fibers matrix using Eqs. (1) and (2)[1] E=Eb·wib+Em·Vm (1) These products come to the market in the form of f=f·Vb+fm·v (2) without the matrix. In the process of strengthening where straps, or fabric should be applied on cleaned and even (leveled) element surface that has previously Er= modulus of elasticity of longitudinally ori ented FRP composites; been coated with a fresh synthetic glue of a specific quality. The straps or fabric should then be pressed Efib modulus of elasticity of fibers: Em=modulus of elasticity of matrix: toward treated surface, causing the glue to penetrate between fibers. Since the composite product, fibers Vtb= volume ratio of fibers in composit matrix,emerges only after the strap or fabric Vm= volume ratio of matrix in composite ously been treated with the glue, the volume proportion of the glue (matrix) significantly varies Vm volume of matrix in composite and it is very difficult to control thickness of such Vib= volume of fibers in composite composite. Therefore, the analysis of strengthenin fr= tensile strength of longitudinally oriented FRP that is oriented to the cross section area of the composite composite product is difficult to estimate. Most of the fib tensile strength of fibers; fm= tensile strength of matrix researchers recommend analysis that is based only on the properties of the fibers, i. e, their tensile strength, Equations (1)and(2) do not always match the which neglects the influence of the matrix. That kind experimental results, so it is necessary to determinate, of analysis underestimates the effectiveness of for special types of lamellas and rods, the real strengthening because the tests that were carried ou modulus of elasticity and the real tensile strength. So and the results of which will be shown later in this far,the standard for FRP product verification does not paper show that epoxy glue significantly increases the exist,but verification for plastic products is usually strength of composite product. It should be pointed made according to one of the two standards: either out that the tensile strength of fabric is smaller than ASTM D 3039/D 3039M(ASTM 1995)[3], or en the sum of strengths of all fibers In specimens, some ISO527-5(sO1997)[4 of the fibers were initially stretched more than others The real properties of the composites should be and thus they broke earler The fabric strength, fab, btained by experiment and those values will later be which could be expressed as the first part of the eq used for further analysis. The analysis of resistance (2), was tested according to the standards [1,3, 4]
fibers depend on the volume ratio of fibers in the composite. The usual fiber content of fibers in lamellas is between 35 and 70 vol%. The modulus of elasticity and tensile strength of final composites can be calculated from the volume ratio of fibers and matrix using Eqs. (1) and (2) [1]. Ef ¼ Efib mfib þ Em mm ð1Þ ff ¼ ffib mfib þ fm mm ð2Þ where: Ef = modulus of elasticity of longitudinally oriented FRP composites; Efib = modulus of elasticity of fibers; Em = modulus of elasticity of matrix; vfib = volume ratio of fibers in composite: vfib ¼ Vfib VfibþVm ; vm = volume ratio of matrix in composite: vm ¼ Vm VfibþVm ; Vm = volume of matrix in composite; Vfib = volume of fibers in composite; ff = tensile strength of longitudinally oriented FRP composite; ffib = tensile strength of fibers; fm = tensile strength of matrix; Equations (1) and (2) do not always match the experimental results, so it is necessary to determinate, for special types of lamellas and rods, the real modulus of elasticity and the real tensile strength. So far, the standard for FRP product verification does not exist, but verification for plastic products is usually made according to one of the two standards: either ASTM D 3039/D 3039M (ASTM 1995) [3], or EN ISO 527-5 (ISO 1997) [4]. The real properties of the composites should be obtained by experiment and those values will later be used for further analysis. The analysis of resistance and dimension is based on the overall cross section of the composite product (fiber + matrix). 3.2 Dry, one-way or two-way oriented interwoven fibers These products come to the market in the form of fiber straps and fabrics. They consist of fibers only, without the matrix. In the process of strengthening, straps, or fabric should be applied on cleaned and even (leveled) element surface that has previously been coated with a fresh synthetic glue of a specific quality. The straps or fabric should then be pressed toward treated surface, causing the glue to penetrate between fibers. Since the composite product, fibers + matrix, emerges only after the strap or fabric is pressed toward the element surface that has previously been treated with the glue, the volume proportion of the glue (matrix) significantly varies and it is very difficult to control thickness of such composite. Therefore, the analysis of strengthening that is oriented to the cross section area of the composite product is difficult to estimate. Most of the researchers recommend analysis that is based only on the properties of the fibers, i.e., their tensile strength, which neglects the influence of the matrix. That kind of analysis underestimates the effectiveness of strengthening because the tests that were carried out and the results of which will be shown later in this paper show that epoxy glue significantly increases the strength of composite product. It should be pointed out that the tensile strength of fabric is smaller than the sum of strengths of all fibers. In specimens, some of the fibers were initially stretched more than others and thus they broke earlier. The fabric strength, ffab, which could be expressed as the first part of the Eq. (2), was tested according to the standards [1, 3, 4]. Fig. 1 (a) FRP products: bars, lamellas, fabric and (b) glass fiber fabric that was tested 882 Materials and Structures (2008) 41:879–890
Materials and Structures (2008)41: 879-890 of the glass fiber fabric was Mapewrap G UNI-AX 900/60, provided by Italian manufacturer Mapei [7]. The glue Mape Wrap 31 that was used as matrix was produced by Mapei too. The testing of tensile strength was performed by adapting standards described in [3, 4] nd the procedure given by the standard of testing the tensile strength and secant stiffness of geotextiles [5]. All specimens were tested by the use of rubber jaws Zwick Z100 testing machine Standards [ 3, 4, prescribe testing of the FRP Fig. 2 Measuring the thickness of glass fiber fabric fabric without matrix. However. since there are no The fabric strength is expressed as a product of rigid specimens the application of these standards to tensile strength of fibers fib, and a volume ratio of fabric is very demanding. In order to determine fibers in composite vib different influences on tensile strength, and modulus of elasticity of glass fiber straps, four types of testin Geotextile is similar product to the fiber fabrics. were performed. These four types of testing are The tensile strength and secant stiffness of geotextiles (a) Influence of specimen width on the tensile could be estimated according to trength and the modulus of elasticity of glass (1996)[5]. This standard [5], recommended calcula- fiber straps tion of tensile strength and secant stiffness (b) Influence of deformation speed on the tensile geotextiles both expressed in kilonewtons per meter, strength and the modulus of elasticity of glass ( kN/m). The average thickness of fabric, that was measured in this research on 15 specimens(see (c) Influence of strengthening at strap grip areas on ig. 2), according to EN ISo 964-1 [6]was the tensile strength of glass fiber straps 0.636 mm. Measuring of thickness of fabric accord-(d) Influence of epoxy glue on the tensile strength ing to the standard for geotextiles was chosen because of glass fiber straps of the similarity of products. According to standard [6] the stress on textile surface, during measuring of 5 Influence of specimen width, on the tensile fabric's thickness. was 200 kPa. strength and the modulus of elasticity of glass fiber straps 4 Determination of tensile strength of glass fiber straps and their composites by testi The first specimen testing has measured the influence of specimen width on the tensile strength and the For the sake of testing tensile strength, narrow straps modulus of elasticity of glass fiber straps. The were cut out from the 60 cm wide fabric, which has standard requirements [3] for specimens are shown Table 3 Tensile specimen geometry recommendation [3] Fiber orientation idth b length Thickness Tab length Tab thickness Angle of /(mm) a(mm) c(mr d(mm) Tabx(° 90° unidirectional 175 2.0 Random-discontinuous 2.5
The fabric strength is expressed as a product of tensile strength of fibers ffib, and a volume ratio of fibers in composite vfib. ffab ¼ ffib mfib ð3Þ Geotextile is similar product to the fiber fabrics. The tensile strength and secant stiffness of geotextiles could be estimated according to EN ISO 10319 (1996) [5]. This standard [5], recommended calculation of tensile strength and secant stiffness of geotextiles both expressed in kilonewtons per meter, (kN/m). The average thickness of fabric, that was measured in this research on 15 specimens (see Fig. 2), according to EN ISO 964-1 [6] was 0.636 mm. Measuring of thickness of fabric according to the standard for geotextiles was chosen because of the similarity of products. According to standard [6] the stress on textile surface, during measuring of fabric’s thickness, was 200 kPa. 4 Determination of tensile strength of glass fiber straps and their composites by testing For the sake of testing tensile strength, narrow straps were cut out from the 60 cm wide fabric, which has had unidirectional fiber orientation. The type of the glass fiber fabric was Mapewrap G UNI-AX 900/60, provided by Italian manufacturer Mapei [7]. The glue MapeWrap 31 that was used as matrix was produced by Mapei too. The testing of tensile strength was performed by adapting standards described in [3, 4], but also taking into account recommendations and the procedure given by the standard of testing the tensile strength and secant stiffness of geotextiles [5]. All specimens were tested by the use of rubber jaws in Zwick Z100 testing machine. Standards [3, 4], prescribe testing of the FRP composites, lamellas, i.e., real composites, and not fabric without matrix. However, since there are no rigid specimens the application of these standards to fabric is very demanding. In order to determine different influences on tensile strength, and modulus of elasticity of glass fiber straps, four types of testing were performed. These four types of testing are: (a) Influence of specimen width on the tensile strength and the modulus of elasticity of glass fiber straps, (b) Influence of deformation speed on the tensile strength and the modulus of elasticity of glass fiber straps, (c) Influence of strengthening at strap grip areas on the tensile strength of glass fiber straps, (d) Influence of epoxy glue on the tensile strength of glass fiber straps. 5 Influence of specimen width, on the tensile strength and the modulus of elasticity of glass fiber straps The first specimen testing has measured the influence of specimen width on the tensile strength and the modulus of elasticity of glass fiber straps. The standard requirements [3] for specimens are shown Fig. 2 Measuring the thickness of glass fiber fabric Table 3 Tensile specimen geometry recommendation [3] Fiber orientation Width b (mm) Overall length l (mm) Thickness a (mm) Tab length c (mm) Tab thickness d (mm) Angle of Tab a () 0 unidirectional 15 250 1.0 56 1.5 0 or 90 90 unidirectional 25 175 2.0 25 1.5 90 Balanced and symmetric 25 250 2.5 – – Random—discontinuous 25 250 2.5 – – Materials and Structures (2008) 41:879–890 883