Chapter 3. Experimental campaign
3.2.1. Composite profile
Since one of the purposes of the experimental campaign was to develop a better cost-effective hybrid FRP-concrete structural solution, design began with choosing an off-the-shelf glass fiber-reinforced polymer (GFRP) pultruded profile with high performance characteristics and qualities. The composite beams selected were manufactured by GDP S.A. in France and supplied by Composites ate S.L. from Spain.
During a previous small-scale comparative experimental study it was found that a hybrid beam design with features similar to common steel-concrete beams, made of an I-shaped profile and a reinforced concrete slab, displayed the best results among the hybrid designs trialed. Therefore, the composite shape chosen for the real-scale experimental campaign was an IPE 120 profile classified as structural and with the following nominal dimensions: 120 mm in height, 60 mm in flange widths and a thickness of the web and flanges of 8 mm, as seen in Figure 3.1. It is noted that the FRP profile is stockier than its steel IPE 120 counterpart, especially in the web. The transition between the flanges and the web had a 5 mm fillet radius. Profiles came in batches of 3 meters (Figure 3.2), and were later adjusted to the desired testing length.
Figure 3.1: Cross-section geometry of the GFRP IPE 120
pultruded profile.
Figure 3.2: GFRP profiles used in the experimental
The composite profiles were made from a PR500 grade thermosetting unsaturated polyester matrix with basic formulation, reinforced with E-glass fibers. A visual inspection of the transverse section shown in Figure 3.3 reveals a well-structured, highly inhomogeneous symmetrical assembly of unidirectional fibers which act as longitudinal reinforcement, and non-woven continuous strand mats (CSM) disposed on the contour of the shape and at the center plane of the web which perform the role of shear, transverse reinforcement. The mats were made of the same type of fibers but arranged in a multidirectional pattern. The exterior CSM was covered with a thin polymeric, transparent surface veil that provides mechanical and chemical protection.
Figure 3.3: Internal macrostructure of the GFRP profile: (a) cross-section layers (emphasized colors);
(b) fiber rovings; (c) non-woven continuous strand mats (CSM).
The anisotropic nature of the composite material was further analyzed with help from the Electron Microscopy Service of UPC, under a JEOL scanning electron microscope (model JSM-5610). A sample of the profile’s flange was extracted manually, peeled of its veil and examined. The pictures revealing the composite’s microstructure are shown in Figure 3.4. Both unidirectional and multidirectional fibers were visible as well as fragments of the polyester resin. The diameter of a single glass fiber appeared to be around 30 μm.
Figure 3.4: Internal microstructure of the GFRP profile: (a) flange sample; (b) unidirectional fibers;
(c) multidirectional fibers (CSM); (d) a single glass fiber covered in resin.
A sample of the web-flange junction of the pultruded profile was also analyzed under the microscope. The images presented in Figure 3.5 highlight the dichotomy of the structure at the mid- plane level, where the white continuous multidirectional fibers are positioned alongside the grey unidirectional fiber rovings. The cross-section of the longitudinal fibers, although transparent, appears grey due to the way light is reflected inside of them. The deepest transverse scan also exposed the existence of microscopic structural pores that arise during fabrication and curing that may possibly weaken the profile. Under all these circumstances, the web-flange junction transition area appears to be a region susceptible to failure, as it was confirmed by the full-scale experimental tests.
Figure 3.5: Internal microstructure of the GFRP web-flange region: (a) junction sample;
(b) lengthwise view; (c) transverse view; (d) structural pores.
The main physical, mechanical and electrical properties of the pultruded GFRP bars and profiles given by the manufacturer are summarized in Table 3.1. The reported values are suggested to be taken only as a guide.
Table 3.1: Declared properties of the GFRP PR500 pultruded shapes.
Property Bars Profiles Units Testing method
Physical
Reinforcement ratting in weight 70-80 50-65 % EN ISO 1172 Apparent density 2.0 1.8 kg/dm3 EN ISO 1183-1 Barcol hardness 45/50 45/50 EN 59 Water absorption 1.50 1.50 % in weight EN ISO 62 Coefficient of linear thermal expansion 5.4∙10-6 9∙10-6 /ºC ISO 11359-2 Thermal conductivity 0.3 0.15 W/K∙m ASTM C117
Mechanical
Tensile strength 690 207 MPa EN ISO 527-4 Modulus of elasticity 41.4 17.2 GPa EN ISO 527-4 Flexural strength 690 207 MPa EN ISO 14125 Shear strength 35 35 MPa EN ISO 14130 Compressive strength 414 276 MPa EN ISO 14126
Electrical
Dielectric strength 2360 984 kV/m ASTM D149 Resistivity 1012 1012 Ω∙m CEI 60093 Arc resistance 120 120 s ASTM D495
Due to the lay-up configuration of the profile in webs and flanges, results differ between coupon tests and full-section tests. In addition, it is not possible to predict any of the values from data obtained from a different test mode or test direction. According to the spreadsheet, the glass fiber-reinforced profiles meet the minimum structural requirements of grade E17 indicated by EN 13706-3:2002 [16].
Regarding the thermal properties, the manufacturer states that the pultruded shapes can be used without any restriction between -20 °C and +80 °C. For harsher conditions, below -20 °C and between +80 °C / +200 °C, special formulations are required. The coefficient of thermal expansion of the GFRP profiles lies between 8-10∙10-6/K, which is ideally similar to the coefficient of reinforced concrete
The PR500 grade profiles utilized in the investigation have a thermal endurance class “B” (130 °C) and a limited oxygen index in the axial direction of 30-35%, and in the transverse direction of 25 to 30% (NFT 51-071). Their flame resistance to an incandescent filament during 30 seconds at 960 °C (NFT 20 455 20 455) suggests an extinction time of less than 5 seconds. In what concerns the chemical resistance of the GFRP profiles, the details provided are given in Table 3.2. Higher environmental protection could be achieved by using a vinyl ester matrix in the composition.
Table 3.2: Declared chemical resistance of the GFRP PR500 pultruded shapes.
Chemical resistance Grade Resistance to acids Very resistant Resistance to bases Resistant Organic solvents Not recommended Chlorinated solvents Not recommended Sea water Very resistant Petrol/Diesel oil Very resistant Industrial detergents Excellent resistance Weathering Excellent resistance
The composite profiles selected for the tests exhibit a linearly-elastic behavior and are especially recommended for applications that involve either cyclical mechanical stresses or vibrations, or repeated impacts. Bending tests performed by the producer indicate a 10% slight decrease in the elastic modulus after 500.000 cycles under imposed elongation at 80% ultimate stress, with a frequency of 10 Hz.
Given the fact that the mechanical properties declared by the manufacturer were incomplete, have an informative nature and could have been adjusted with safety coefficients, in the first stage of the research the pultruded FRP product was subjected to an extensive campaign of characterization tests. Some of the mechanical properties were evaluated in both axial and transverse directions of the composite shape due to the transverse isotropy (i.e., a particular case of orthotropic materials which possess a plane of symmetry).
Before the mechanical characterization tests, the density of the profiles was reevaluated by weighing and measuring five specimens. The determined apparent density was found to be 1.93 kg/dm3, higher
than the corresponding value prescribed in Table 3.1.
The flexural, tensile, compressive, shear and full section properties were obtained by the author, from the experimental tests illustrated in Figure 3.6, following relevant standardized principles and methods specified in CEN, ISO and ASTM International standards. Appendix A of the current document contains the detailed reports of the characterization tests – including scope, principles, testing procedures, results and observations – of over 40 specimens.
Figure 3.6: GFRP profile material characterization tests: (a) flexure; (b) tension; (c) in-plane compression;
(d) interlaminar shear; (e) in-plane shear; (f) full section effective moduli.
Flexural properties were obtained by deflecting simply-supported coupons in a three-point bending configuration setup, at a constant rate until they fractured. During the procedure, the force applied to the specimens and the bottom longitudinal strains were measured. In the tensile tests, a specimen was extended along its major longitudinal axis at a constant speed until it ruptured. The load sustained by the coupon and the lengthwise and crosswise elongations were measured. For the compressive trials, an axial force was applied to the unsupported length of a rectangular specimen held in a loading fixture while the applied load and axial strain were recorded. The loading fixture was designed by the author based on the recommendations presented in the informative Annex C of ISO 14126:1999, and served further for the in-plane shear tests. The standard’s informative annex references similar compressive fixtures from ISO 8515:1991 and ASD-STAN prEN 2850.
The interlaminar shear strength was determined straightforward using the short-beam method, in which a bar of rectangular cross-section is loaded over a small test span as a simple beam in flexure so that interlaminar failure occurs in the matrix layer. In exchange, determining the in-plane shear strength proved to be more contentious, requiring an adaptation of the ASTM D 3846 method suggested in [10]. Basically, in this case, the strength is defined as the shear stress at rupture in which the plane of fracture is located along the longitudinal axis of a specimen, between two centrally positioned notches machined halfway through its thickness on opposing faces.
Lastly, in order to determine the flexural moduli, pultruded profile specimens were repeatedly loaded in an elastic manner as simple beams in three-point flexure, over a number of different decreasing span lengths. Because the bending and shear contributions to the overall beam deflection vary with each test span, the elastic moduli can be obtained using a linear regression analysis of the bending equation.
In the course of the iterative procedure, the force applied to the specimen and the resulting deflection were measured. Table 3.3 summarizes the main results of the mechanical properties post-processed in Appendix A, together with their corresponding standards for determination.
Table 3.3: Experimentally determined mechanical properties of the GFRP profile (average ± standard deviation values).
Mechanical property Value Units Testing method
Flexural
EN ISO 14125:1998 [155] Ultimate strain 2.10 ± 0.05 %
Strength 734 ± 39 MPa Modulus of elasticity 35.0 ± 2.1 GPa
Tensile EN ISO 527-1:2012 [156] EN ISO 527-4:1997 [157] Ultimate strain 1.37 ± 0.11 % Strength 520 ± 27 MPa Poisson’s ratio a 0.27 ± 0.02 Modulus of elasticity 38.0 ± 1.4 GPa
Compressive - lengthwise
EN ISO 14126:1999 [158] Ultimate strain 1.02 ± 0.11 %
Strength 406 ± 30 MPa Modulus of elasticity 40.6 ± 1.8 GPa
Compressive - crosswise
Ultimate strain 1.60 ± 0.13 % Strength 115 ± 3 MPa Modulus of elasticity 10.8 ± 0.5 GPa
Shear
Apparent interlaminar strength 31.1 ± 0.7 MPa EN ISO 14130:1997 [159] In-plane strength b 49.0 ± 4.7 MPa ASTM D 3846-08 [160]
Full-section moduli
EN 13706-2:2002 [15] Effective flexural modulus 39.1 ± 0.14 GPa
Effective shear modulus 3.98 ± 0.26 GPa a determined for the axial-transversal case.
b coupons rotated 90°.
A few observations are important to be made regarding the experimental characterization. The method for determining the flexural properties and interlaminar shear strength are not appropriate for the determination of design parameters although they may be used instead for screening materials or quality-control tests. As such, the evaluation of the flexural modulus of elasticity does not account for the shear contribution to deformation and thus the resulting value is less than in reality. Nevertheless, the standard suggests various test span/specimen dimension ratios that minimize this effect and inhibit the development of an interlaminar shear failure. Secondly, the interlaminar shear strength is not an absolute value due to the fact that the shear stress distribution in this case is notably different than the parabolic distribution described by the elasticity theory in cross-sections sufficiently distanced from the supports and the load-application areas. For this reason the term “apparent interlaminar shear strength”
is used. Although manufacturers usually report the interlaminar shear strength of composite shapes, the in-plane shear strength is generally not included due to the difficulty of its evaluation. Considering that the in-plane shear strength can be much greater than the interlaminar shear strength [161], evaluating the shear capacity of a fiber-reinforced pultruded profile solely on the latter property will yield very conservative results. There is currently no European standard that deals with this matter for composites made from multidirectional fibers or combinations of continuous and multidirectional fiber systems. Opposed to using the method specified in ASTM D 3846, other authors have evaluated the in-plane shear strength using tensile tests on double-lap joints [48,162] or the 10° off-axis test [163–165]. The specimen preparation in both cases is fairly complex and the latter method is only suitable for continuous aligned fibers. The 10° off-axis method tends to underestimate the ultimate shear strength and strain due to the combination of transverse tensile and shear stresses [166]. Consequently, there is an important need that has to be addressed, to develop a standardized European testing method to effectively evaluate the in-plane shear strength of FRP pultruded profiles.