DURABILITY ON THE FRACTURE PARAMETERS OF CRACK-REPAIRED HIGH PERFORMANCE FIBER REINFORCED CEMENTITIOUS COMPOSITES

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1 DURABILITY ON THE FRACTURE PARAMETERS OF CRACK-REPAIRED HIGH PERFORMANCE FIBER REINFORCED CEMENTITIOUS COMPOSITES Yoshinori Kitsutaka and Masaki Tamura Faculty of Engineering, Tokyo Metropolitan University, Japan Abstract An important issue related to maintenance of the domestic stock of concrete structures is the evaluation of the degree of performance recovery and subsequent longterm performance retention brought about by repair. Many papers pointed out the effectiveness to use the High Performance Fiber Reinforced Cementitious Composites (HPFRCC) for improving the durability of structure. However, few studies have been conducted on the evaluation of mechanical performance retention and the crack propagation resistance of repaired HPFRCC under deterioration action. In this study, the performances on the recovery of fracture toughness and on the effectiveness of durability maintenance for crack-repaired HPFRCC were investigated. Affects of the air content, matrix strength, fiber properties, crack repairing and the freezing and thawing durability test conditions on fracture properties were evaluated by using tension softening evaluation system. The specimen of non-fiber concrete on which the crack was repaired by resin showed extremely low toughness by the freezing and thawing test. Toughness and durability performance of specimen were improved by containing air, vinylon fiber, because of the effect of reducing the stress by inner water pressure. Information on improving fracture toughness and effectiveness of durability maintenance of crackrepaired HPFRCC was obtained. 1. INTRODUCTION Many researches have been conducted in recent years regarding subjects related to the High Performance Fiber Reinforced Cementitious Composites (HPFRCC). Among such subjects, enhancement of the ductility of concrete by fiber reinforcement is of paramount importance from the aspect of improving the safety performance and durability of

2 concrete members. However there are a few researches on the durability of HPFRCC especially regarding the performances on the recovery and the maintainability of fracture toughness of crack-repaired HPFRCC. In this study, the durability of crack repaired HPFRCC was experimentally investigated. Various cracked concrete samples were repaired by epoxy resin injection and repaired samples were subjected to a repeated freezing and thawing test in order to investigate their toughness retention. Affects of the air content, matrix strength, fiber properties, crack repairing and the freezing and thawing test conditions on fracture properties were evaluated by using tension softening evaluation system. 2. OUTLINE OF EXPERIMENT 2.1 Materials used for experiments The materials are given in Table 1. Table 2 gives the factors and levels of experiment. Table 3 gives the basic mixture proportions of concrete samples. Polyvinyl alcohol (PVA) fibers were selected as short fibers to be blended in concrete for its excellent bonding with cement paste and energy absorbing capability, as well as its light weight and low price. To investigate the effects of the fiber lengths (Lm and Sm) and fiber contents (, 1 and 2%), four (4) types of fiber phases were prepared. As well, two (2) levels of concrete strengths (high strength with a W/C of 3% and normal strength with a W/C of 6%) were selected, each with the mortar phase composition being kept constant regardless of the volumetric fiber content. Table 1: Materials Materials Mark Contents Cement ormal Portland C Density: 3.16(g/cm 3 ) Fine Density:2.58(g/cm 3 ) andstone S aggregate Absorption:1.71(%), Mas diameter :5.(mm) Fiber Vinyron V Density 1.3(g/cm 3 ), Diameter 4 (µm) Injection resin Epoxy E Length S:12(mm), Length L:3(mm) JIS A 624 Low viscosity type(13±2 (mpas) Tensile strength >2(N/mm 2 ) Table 2: Factors and levels of experiment Factors Levels(sign) Water cement ratio (%) 6(N), 3(H) Fiber length (mm) 12(Sm), 3(Lm) Containing fiber Volume (%) (), 1(1), 2 (2) Crack repairing of concrete Non-repaired (-N), Repaired (-R) Freezing and thawing test (cycle) (c), 6(6c), 12(12c)

3 Table 3: The basic mixture proportions of concrete W/C Unit weight (kg/m 3 ) Type (%) Water Cement Fine Agg. Additives (C x %) H N Remarks: These mixtures contain no fibers 2.2 Fabrication of specimens Table 4 gives the types and properties of specimens. A 2-liter biaxial forced action mixer was used for mixing. After dry-mixing materials other than fibers for 6 sec, water was added and further mixing of the components was continued for 9 sec. Fibers were then added in small portions to achieve a uniform dispersion during the mixing process that lasted a total of 5 min. As-mixed concrete samples were subjected to air content and unit weight testing in accordance with JIS A 1116 and flow testing in accordance with JIS R 521. Three compression specimens (1 mm diameter, 2 mm height) and three toughness specimens (1 by 1 by 12 mm) were fabricated from each sample, demolded at an age of 2 days, and subjected to standard-curing. Table 4: Basic properties of specimens Fiber Air W/C Mark Volume Contents (%) (%) (%) H N Flow Value (mm) Unit Weight (kg/l) Compressive Strength (N/mm 2 ) Sm Sm Lm Lm Sm Sm Lm Lm Fracture toughness test Figure 1 shows a sketch of a wedge-splitting type of fracture toughness test. Original and repaired concretes were subjected to wedge splitting tests 1) for fracture toughness. A wedge was inserted in the notch in the center of the top surface with a ligament depth of 5 mm to induce tension failure, while measuring the load-crack-mouth-opening displacement (L-CMOD) curve. In order to measure stable L-CMOD curves, loading rates of.4 mm/min and.2 mm/min were applied to original and repaired concretes, respectively.

4 [Front] P [Side] Guide Device for Wedge Splitting 15 Clip Gauge P Tension Tension Cut Length5mm Test Piece (1mm x 1mm x 12mm) Figure 1: Wedge-splitting type fracture toughness test 2.4 Test procedure Fracture toughness test began at an age of 28 days for original concretes. Repaired concretes were prepared by epoxy-repair of cracks of original concretes after wedge testing. These were air-cured at 2 C and 6% R.H. until an age of 6 months and then exposed to the specified number of freezing and thawing cycles (, 6, and 12) before being subjected to the same wedge testing (Figure 2). Figure 2: Test procedure 2.5 Fracture toughness evaluation The fracture parameters were calculated from the tension-softening diagram (TSD) determined from the L-CMOD curve by poly-linear approximation analysis 2). A TSD is an index of the relationship between the cohesive stress and the crack opening displacement (COD) when the progress of fracture is represented by a cohesive force model. Also, the area under the TSD to a CMOD of δu =.5 mm was defined as the effective fracture energy, G F u, for evaluation in terms of energy (Eq. (1)) 3).

5 u δu G F = σ ( δ ) dδ (1) where, δ : crack opening displacement [mm], σ(δ): cohesive stress [MPa]. 2.6 Crack repairing An epoxy resin with a low viscosity suitable for fine cracks, to which normal resin is not readily applicable, was used for repairing cracks. The repair was carried out as follows: a silicone sealant was applied to both sides and ends of the cracks (to prevent spilling of resin when it is applied in the crack opening). After confirming that the sealant had completely hardened, the epoxy resin was then injected at the top of the notch using a syringe. By carrying out the injection process twice, repaired specimens were produced with openings completely filled with epoxy resin. 2.7 Freezing and thawing tests Freezing and thawing tests for evaluating durability were conducted in accordance with JIS A 1148 (Method A). Three repaired specimens for each type of concrete were subjected to 3-hour cycles of freezing and thawing. At the end of, 6, and 12 cycles, each specimen was again subjected to fracture toughness testing. Also, the mass loss ratio was measured at the end of every 3 cycles to further estimate the degree of deterioration and evaluate the durability. 3. RESULTS AND DISCUSSIONS 3.1 L-CMOD curve Figures 3 (a to d) show typical measured L-CMOD curves for fiber reinforced concretes. In regard to the long fiber series (Lm, 3 mm) in Figs. 3(a) and 3(b), the repair increased the peak load 1.5 times, while improving the toughness. This tendency is noticeable in Lm2 specimens with a high fiber content. As shown in Fig. 2(c), the load-bearing capacities of repaired concretes subjected to freezing and thawing (6 cycles) tend to be lower than those of repaired concretes. However, Lm2 specimens with a high fiber content show strain-hardening type deformation, indicating their high toughness. The peak loads of all specimens in the short fiber series (Sm, 12 mm) in Figs. 3(c) and 3(d) tend to increase similarly to Lm series, but these specimens are found to show no marked improvement in terms of toughness. Note that the properties of all types of concrete samples inferred from the L-CMOD curves were confirmed as similar tendencies in TSDs.

6 Figure 3 : Examples of measured L-CMOD curves for fiber reinforced concretes 3.2 Initial cohesive stress Initial cohesive stress, which refers to the cohesive stress when the crack opening displacement is mm on the tension-softening diagram, is a value for the assessment of cracking strength. Figure 4 (a and b) shows the initial cohesive stress ratio related to the fiber content. Figures 4(a) and 4(b) focusing on the fiber length (Lm and Sm) compare the effects of the fiber length, fiber content, repair, and freezing and thawing action with respect to the initial cohesive stress of the original concretes with no fibers (N- and H-). The initial cohesive stress ratios of repaired specimens without freezing and thawing action (-Rc) are found to increase to 1.5 to 2 times those of the original concretes regardless of the fiber content. After being subjected to freezing and thawing action (6 and 12 cycles), however, the values substantially decrease similarly to L-CMOD curves regardless of the fiber content. It is therefore inferred that it is difficult to retain the initial cohesive force, a force that serves to resist freezing and thawing action, by means of crack repair using epoxy resin.

7 a)normal Strengh Strength 3 b)hign High Strengh Strength N- Standard (1time=3.4MPa) Lm-N Sm-N Lm-R6c Sm-R6c Lm-ROc Sm-Rc 1 2 Fiber Volume(%) 2 1 H- Standard (1time=6.7MPa) Lm-N Sm-N Sm+Lm- R(6+12)c 1 2 Fiber Volume(%) Lm-ROc Sm-Rc Figure 4 : Initial cohesive stress ratio related to the fiber content Figure 5 (a and b) shows the initial cohesive stress ratio related to the number of freezing and thawing cycles. These results show the strong effects of mortar strength, fiber length and fiber content on the decreasing tendency of the initial cohesive stress ratio as the number of cycles increases. 4 a)normal Strength 4 b)hign Strengh b) High Strength 3 2 N-Lm2 N-Sm2 N-Lm1 N-Sm1 N- 3 2 H-Lm2 H-Sm2 H-Lm1 H-Sm1 H- 1 1 N- Standard (1time=3.4MPa) Normal 6 12 Cycle H- Standard (1time=6.7MPa) Normal 6 12 Cycle Figure 5 : Initial cohesive stress ratio related to the freezing and thawing cycles 3.3 Effective fracture energy Figure 6 (a and b) shows the relationship between the effective fracture energy ratio (G F u ratio) and the fiber content. In these figures focusing on the fiber length, fiber

8 content, repair, and freezing thawing action are compared in the form of G F u with respect to the effective fracture energy of the original concretes with no fibers (N- and H-) similarly to the initial cohesive stress ratio. The G F u ratios of repaired specimens without freezing and thawing action (-Rc) are found to be extremely higher than those of the original concretes, being nearly 2 times higher at the maximum with long fibers (Lm2) at a fiber content of 2%. This tendency is more significant with high strength concrete. This indicates the high potential for energy absorption of fibers in concrete. Energy absorption is also recognized accordingly with the fiber content even after freezing and thawing cycles. It can therefore be said that the toughness recovered by repair was retained through repeated freezing and thawing a)normal Normal Strengh Strength Lm-N 2 15 b)high High Strengh Strength Lm-Rc Lm-N 1 5 Sm-N Sm-Rc N- Standard (1time=68N/m) Lm-Rc Lm-R6Oc Sm-R6c 1 2 Fiber Volume(%) 1 5 H- Standard (1time=87N/m) Sm-ROc Sm-N Lm-R12c Lm-R6c Sm-R12c Sm-R6c 1 2 Fiber Volume(%) Figure 6: Relationship between the effective fracture energy ratio (G F u ratio) and fiber content Figure 7 (a and b) shows the relationship between the freezing and thawing cycles and the G F u ratio. The fracture energy-retaining effect through the increasing number of cycles is found to be related to the strength, fiber length, and fiber content, and the values tend to decrease moderately when compared with the reductions in the initial cohesive stress ratio.

9 a)normal Normal Strengh Strength Lm2(c),Sm1(6c) could not be calculated N- Standard (1time=68N/m) N-Lm2 N-Sm2 N-Sm1 N-Lm1 N- Normal 6 12 Cycle b)high High Strengh Strength H- Standard (1time=87N/m) H-Lm2 H-Sm2 H-Lm1 H-Sm1 H- Normal 6 12 Cycle Figure 7: Relationship between the number of freezing and thawing cycles and G F u ratio 4. CONCLUSIONS The following has been elucidated from this study: (1) Strength recovery by repair is satisfactory regardless of the inclusion of fibers. Toughness is also significantly improved by the addition of fibers. The degree of improvement to toughness becomes greater as the volumetric content of fibers and fiber length increases. (2) Repair of concrete improves its initial cohesive stress regardless of the presence or absence of fibers, and certain improving effects are accordingly anticipated with the fiber content. (3) Inclusion of fibers drastically improves the effective fracture energy of repaired concrete and strong improving effects are anticipated accordingly with the fiber content. (4) When exposed to freezing and thawing action, the durability of repaired concrete is found to decrease concomitant with losses in its strength but its toughness is affected by the presence of fibers. A high content of long fibers produces a toughnessretaining effect. ACKNOWLEDGEMENTS This study was conducted as part of Tokyo Metropolitan University the 21 st Century COE Program for Development of Technologies for Activation and Renewal of Building Stocks in Megalopolis. The authors express their gratitude to Kuraray Co., Ltd. for providing samples for the experiments and to Mr. Arai Kenji for performing experiments.

10 REFERENCES [1] RILEM AAC , Determination of the specific fracture energy and strain softening of AAC, RILEM Technical Recommendations for the Testing and Use of Construction Materials, pp [2] Kitsutaka, Y. 1997, Fracture parameters by poly-linear tension softening analysis, Journal of Engineering Mechanics, ASCE, 123[5], pp [3] Kitsutaka, Y., Kamimura, K. & Nakamura, S. 1994, Evaluation of aggregate properties on tension softening behavior of high strength concrete. High Performance Concrete, ACI SP149-4, pp.711-pp.727