DEVELOPMENT AND APPLICATION OF HIGH PERFORMANCE HYBRID FIBER REINFORCED CONCRETE

Transcription

1 DEVELOPMENT AND APPLICATION OF HIGH PERFORMANCE HYBRID FIBER REINFORCED CONCRETE N. Banthia, C. Yan, and V. Bindiganavile The University of British Columbia, Canada Abstract With the objective of developing very high performance fiber reinforced concrete composites, the combined use of macro and micro steel fibers in the same mix was investigated. Three types of macro fibers (crimped with a crescent section, crimped with a circular section, and twin-cone with a circular section) at a fixed dosage rate of 40 kg/m 3 were investigated. These composites were then further reinforced with steel micro-fibers at dosage rates of 1 and 2 % by volume to produce the hybrid fiber composites. It was seen that hybrid fiber composites were not only stronger in compression, but also depicted greater strength and energy absorption capability in flexure. The paper also describes a repair project at a parking garage in Vancouver where such hybrid fiber composites were employed. 1. Introduction Significant progress has been made in the last several decades towards understanding the various micro and macro reinforcing mechanisms in fiber reinforced concrete (1-2). Looking back, one of the prominent achievements has been the development of highly promising micro-fiber reinforced cement composites (3). It has been shown that with the use of very fine fibers (called micro-fibers) with lengths less than 10 mm and diameters less than about 80 microns, one can achieve a fiber-fiber spacing less than about 100 microns. This prevents an unsupported length between fibers from becoming unstable, and thus impedes a coalescence of micro-cracks. This, in turn, increases the tensile strengths of composite over and above that of the plain matrix, and allows the composite to carry a greater tensile load with minimal sub-critical crack growth and per-peak nonlinearity in the constitutive response curve. This was not possible in the traditional fiber reinforced composites where cracks rapidly acquired large unsupported lengths between fibers and precipitated a tensile failure at stresses no higher than the strengths of the plain matrix. 471

2 One concept far less examined is that of improving the properties of fiber reinforced concrete by using two or more different kinds of fibers in the same mix. It is expected that fibers, if properly chosen, will develop a synergistic response. It is imperative, however, that the fibers combined have different reinforcing mechanisms and provide different property enhancements. Macro and micro-fibers, as described above, are thus very good candidates for hybridization as they operate at two different levels within the composite. When properly developed and optimized, these composites will depict high toughness, impact resistance and improved durability and will be suitable for a whole host of new applications. The purpose of the study reported here was to further validate this hypothesis and develop composites with very attractive properties. 2. Laboratory Study Materials, Mixes, Specimens and Testing A high strength mix was reinforced with three types of 60 mm long macro-fibers at 40 kg/cu.mt. Of the three macro-fibers, two were crimped [ X with a crescent section, and C with a 1 mmφ circular section] and the third had conical ends ( T, twin-cone, 1 mm φ). The micro-fibers used were 4.5 mm long and 80 µm in diameter, and were investigated at 1 and 2% by volume. The various composite investigated and the notation used are given in Table 1. The fibers are shown in Figure 1. Macro-fiber Micro-fiber Figure 1: Macro and Micro-fibres used in the field study 472

3 From each mix, four cylinders (100 φ x 200 mm) and four beams (100 mm x 100 mm x 350 mm) were cast in Plexiglas moulds. These were demoulded 24 h later and cured in lime saturated water for at least 28 days before testing. The cylinders were tested in compression as per ASTM C-39. The compressive strengths for the various mixes are given in Table 1 where the standard deviations are also reported. Beams (100 mm x 100 mm x 350 mm ) were tested in four-point bending as per ASTM C using a closed-loop test assembly as shown in Figure 2. Notice that a Yoke was placed around the specimens to account for spurious beam displacements (arising from crushing at the load point and support points and the general deformations of the loading frame) such that only the net displacements were noted. Two LVDTs mounted on either side of the beam recorded this net displacement, and an averaged signal from the two LVDTs was used as feed back control signal to the servo-valve. The analysis of the curves followed the JSCE SF-4 procedure (4) and the proposed PCSm method (5). This is due to a lack of confidence in the analysis proposed in ASTM C1018 and the subjectivity in placing the point of first crack on the curve. Results from Laboratory Testing Compressive and flexural strengths are given in Table 1 and plotted in Figure 3. The load-displacement curves from beam tests were further analyzed to obtain the JSCE flexural toughness factors (in stress units) to a net mid-span displacement of 2 mm (span/150), and these values are also given in Table 1. The flexural toughness factors are plotted in Figure 3. The PCS m values are plotted in Figures 4, 5 and 6, respectively, for the X, C and T fibers. It may be mentioned that the shape of the PCS m curves emulates that of the actual load-displacement curve. Table 1. Mixes and Average Properties Type of kg/m 3 Micro- Fiber (%) Compressive Strength (MPa) Flexural Strength (MPa) Flexural Toughness Factor (MPa) Mix Identifica tion Control Control Crimped X00 Crescent X01 Section (X) X02 Crimped Circular Section (C) Twin-Cone Circular Section (T) C C C T T T02 473

4 Figure 2: Set-up for the closed-loop testing. From Figure 3, notice that there is a general increase in the compressive strength due to macro-fiber addition, and a further increase occurred due to the addition of micro-fibers. The flexural strengths (also shown in Figure 3), on the other hand, are not affected by macro-fiber addition, but an increase due to micro-fiber addition may be noted. From a flexural toughness and energy absorption points of view, it may be noticed in Figures 3, 4, 5 and 6 that, in general, having a hybrid combination of macro-steel and micro-steel fibers in the same mix is more effective than having macro-steel fiber alone. One can also say that these improvements occur only beyond 1% of micro-fiber by volume and specifically at 2% by volume. Such hybrid composites will resist fracture better, demonstrate a superior fatigue endurance, impact resistance, and long-term durability. 474

5 85 80 f c (MPa) MOR & FTF (MPa) H (Plain) HX 00 HX 01 HX 02 HC 00 HC 01 HC 02 HT 00 HT 01 HT 02 FTF (SF-4) MOR f'c FTF MOR f'c H (Plain) HX 00 HX 01 HX 02 HC 00 HC 01 HC 02 HT 00 HT 01 HT 02 FTF Figure 3: Compressive strength, flexural strength and flexural toughness of the various hybrid mixes. 3. Field Work A severely damaged parking garage in a high-rise building in Downtown Vancouver was identified for the application of the hybrid composites. This was the first known application of hybrid composites involving both macro and micro steel fibers. The parking level of this high-rise building was constructed three decades ago and had 475

6 developed significant cracking. At some places, the steel reinforcing bars were fully exposed and were susceptible to environmental attack. Some also showed signs of severe corrosion MOR HX 00 HX 01 HX 02 PCSm dpeak L/1000 L/750 L/600 L/400 L/300 L/200 L/150 L/m L=300 mm Figure 4: Graphical representation of PCSm vs. beam deflection (Xorex macro-fiber) MOR HC 00 HC 01 HC 02 PCSm dpeak L/1000 L/750 L/600 L/400 L/m L/300 L/200 L/150 L=300 mm Figure 5: Graphical representation of PCSm vs. beam deflection (Crimped micro-fibres) 476

7 MOR HT 00 HT 01 HT 02 PCSm dpeak L/1000 L/750 L/600 L/400 L/300 L/200 L/150 L/m L=300 mm Figure 6: Graphical representation of PCSm vs. beam deflection (Twin-cone macrofibre) A new, more cost-effective hybrid mix containing both macro and micro steel fibers was developed for the rehabilitation project. The final mix carried a 0.5% by volume of a 30 mm long flattened-end macro-fiber and a 0.5% by volume of a 4.5 mm long, 80 micron diameter micro-fiber. Beams (100 mm x100 mm x 300) were cast for flexural toughness tests (ASTM C1018) and the results of these tests are given in Figure 7. It was clear that having micro-fibers increases not only the peak load but also the energy absorption capacity. Load(kN) Beams cast with hybrid steel fiber reinforced concrete Beams cast with steel macrofiber reinforced concrete Deflection(mm) Figure 7: Flexural response of macro-fiber and hybrid fiber reinforced concrete 477

8 For the actual placement, the 200 mm floor of the parking garage was first prepared by removing the top layer of concrete down to the underside of the rebar using a jackhammer. Next, the area was cleaned by chipping the surface of the existing concrete using a shot blaster. This also removed any rust formed around the rebars. Since some of the rebar had undergone severe corrosion, new rebars were tied to the existing ones. Figure 8 shows the prepared surface ready to receive the fiber concrete overlay. About 1.6 m 3 of hybrid fiber reinforced concrete was prepared to which both macro and micro steel fibers were added. Concrete was poured using a bobcat loader (Figure 9) and finished in a traditional manner (Figure 10). The hybrid mix needed no greater effort to work with, level and finish compared to ordinary concrete. Surface after sandblasting Figure 8: Surface ready to receive the hybrid concrete overlay 478

9 Figure 9: Placing concrete with a bobcat loader Figure 10: Leveling and finishing fiber concrete 479

10 4. Conclusions One may conclude that very high performance cement-based composites can be obtained by using a hybrid combination of macro and micro steel-fiber reinforcement. These composites have promising applications. One such application involving repair of a parking garage was successfully carried out as described. 5. References 1. Rossi, P., Acker, P. and Malier, Y., Effect of Steel Fibers at Two Stages: The Material and the Structure, Materials and Structures, 20 (1987), pp Chanvillard, G., Aitcin, P.C., Micromechanical Modeling of the Pull-Out Behavior of Wiredrawn Steel Fibers from Cementitious Matrices, Materials Research Society, Proceedings, Volume 211, 1994, pp Banthia, N. and Sheng, J., Fracture Toughness of Micro-Fiber Reinforced Cements, Cement and Concrete. Composites., 18, 1996, pp Japan Society of Civil Engineers. Standard, SF-4, Method of Test for Flexural Strength and Flexural Toughness of Fiber Reinforced Concrete, 1984, pp Banthia, N. et al, Test Methods for Flexural Toughness Characterization of Fiber Reinforced Concrete: Some Concerns and a Proposition, ACI Materials Journal, 92(1),