EXPERIMENTAL STUDY ON CRACK-BRIDGING ABILITY OF ECC FOR REPAIR UNDER TRAIN LOADING

Transcription

1 EXPERIMENTAL STUDY ON CRACK-BRIDGING ABILITY OF ECC FOR REPAIR UNDER TRAIN LOADING Hiroshi Inaguma 1, Masaki Seki 1, Kumiko Suda 2 and Keitetsu Rokugo 3 (1) Technology Research and Development Dept., Central Japan Railway Company, Japan (2) Civil Eng. Dept., Kajima Technical Research Institute, Kajima Co., Ltd, Japan (3) Dept. of Civil Engineering, Gifu University, Japan Abstract Cracks on the beams of railway viaducts tend to move under active loadings of trains passing over them, which can be deleterious in terms of the ingress of detrimental factors or can be a trigger of spallings of the cover concrete. In this study the crack-bridging ability of ECC (Engineered Cementitios Composite), which is one of the HPFRCC materials with tensile strain hardening performance generating smaller cracks due to their multiple crack characteristic, is investigated as a repair material. In the experiment after an introduction of several cracks by a static loading for two RC beam specimens one of which has ECC applied on its bottom surface, fatigue tests of 17 million cycles are conducted giving the cracks such a small amplitude of movement as can be observed on real viaduct beams when trains pass on them. As a result, it is found that ECC has an effectiveness of lowering the growth and amplitude of the crack width on the base concrete under fatigue loadings. The results also show that little crack dispersion is performed with only one or two small cracks generating on the ECC surface at the openings of the base concrete cracks, however the amplitude of the movement of the new cracks width on the ECC surface is small (less than 0.02mm). Keywords: railway reinforced concrete bridge, Tokaido Shinkansen, crack switching width, carbonation alleviation countermeasure, ECC, fatigue test 1. INTRODUCTION Approximately 40 years have passed since the inauguration of the Tokaido Shinkansen in October 1964, which is the first high speed railway to be operated in Japan. For the maintenance of reinforced concrete bridges (hereafter abbreviated as RC

2 Overhang slab Beam Column Fig.1 Railway RC viaduct Fig.2 Surface coating Part name Characteristic of member Required performance of the surface coating (A) Beam Mainly support train loading Crack-bridging ability (B) Center slab Thin and easily affected by water from above Water permeablity (C) Overhang slab Often located above roads Prevention of falling on concrete fragments Viaduct (C) (A) (B) Fig.3 Standard for each member of viaducts bridges) of the Tokaido Shinkansen, carbonation induced corrosion of the embedded reinforcement bars has been considered to be one of the most important factors that affect the long-term durability of the structures. After the priority for the repair had been argued taking into consideration the existing volume, cover depth, and compression strength for each type of structures, surface coating work to alleviate the further ongoing of carbonation has first been carried out for RC viaducts (hereafter abbreviated as RC viaducts, shown in Fig.1 since A picture of the viaducts with this treatment is shown in Fig.2 and the required material for each type of three members is shown in Fig.3 with its characteristics. Organic surface coating (Fig.4) has been applied for this purpose to three types of members of a viaduct, beams, slabs, and cantilevers respectively with different materials according to the required performance for these

3 Table 1 Material properties of concrete used Compression strength Young's modulus Tensile strength [MPa] [GPa] [MPa] Table 2 Material properties of steel bars used steel bar Yield strength Young's modulus Tensile strength [MPa] [GPa] [MPa] D D Fig.4 Railway RC girder bridge three types of members. However, as railway RC viaducts have a characteristic that their cracks move under the passage of trains that pass on them, it has so far been observed that the crack-bridging ability of the coatings applied is not sometimes sufficient to follow the movement of the cracks resulting in damage on the coatings due to fatigue. Since the existence of cracks may accelerate the ingress of deleterious factors or even can cause spallings, once the coatings are found to be broken they need to be repaired. With high performance in crack dispersion, high performance fiber reinforced cement composites [1] (ECC:Engineered Cementitious Composite), which has been worked out recently and can be sprayed, is expected to be a surface protective material which is able to follow the movement of base concrete cracks. This study examines the effectiveness of ECC [2] to railway RC bridges for alleviating carbonation process, taking advantage of its multiple crack characteristics, small crack settlement and the tensile strain hardening characteristics. In this study the following three characteristics of ECC are investigated: 1) Carbonation alleviation of base concrete 2) Crack dispersion performance under train loadings 3) Long-term durability of crack bridging ability against the fatigue caused by train loadings In this paper, experiment was carried out for the purposes 2) and 3) mentioned above, where ECC was sprayed on the bottom surface of an RC beam specimen that has statically introduced bending cracks and then fatigue tests of 17 million cycles were conducted under a small amplitude of stress. This experiment revealed several characteristics of ECC application under fatigue loadings: crack dispersion, crackbridging ability, and stress redistribution subsequent to crack dispersion, which are described in later sections.

4 2. EXPERIMENT OUTLINE 2.1 Dimension of specimens Two beam specimens were prepared in same dimension, one of which was sprayed with ECC at the bottom surface (Specimen No.2) and the other of which a control specimen. These specimens modeled real railway RC girders (Fig.4), which were produced to well simulate the condition of the cracks on real structures by a careful selection of the diameter of steel bars and the interval of stirrups. The material property of concrete used for the specimens is shown in Table 1, the material property of the steel in Table 2, and the schematic bar arrangement in Fig.5. An RC girder was chosen to for the model here because it has a longer span and a longer interval in stirrup than a viaduct, which could lead to less congested crack occurrence and a larger movement in cracks. 2.2 Experimental method The experiment flow is shown in Fig.6. After two specimens were produced and bending cracks were introduced by static loadings, ECC was sprayed on the bottom surface of one of the specimens and fatigue tests of 17 million cycles were carried out for both specimens under an amplitude equivalent to the movement of cracks on real concrete structures observed during the passage of trains. Unit : mm D13 Loading position D32 (a) Specimen No D13 10@70 2@300 10@70 2,000 Loading position Plate t=20mm D32 (b) Specimen No ECC t=10mm 10@70 2@300 10@70 2,000 Fig.5 Bar arrangement in RC beam specimens Plate t=20mm

5 Specimen No.1 Specimen No.2 (a) Static loading test The purposes of the static loading test were to create cracks Production of specimen Static loading test Production of specimen Surface treatment Static loading test whose width is equivalent to that observed on actual bridges and to find the stress range which was to be used in the subsequent fatigue tests. First the bending crack width was measured during static loading tests by means of π- shaped displacement gages Sprayed ECC installed over cracks on the part Fatigue test Fatigue test of the bottom surface of the specimens where constant bending moment was applied. Both specimens were loaded on Fig.6 Experiment flow four supports until the maximum bending crack width reached 0.4mm, aiming at leaving residual cracks of 0.2mm width on the specimens. After unloading, the specimens were loaded again for finding the upper limit load that was to be used in the subsequent fatigue test. A load was selected as the upper limit when at least one crack opened by 0.025mm additionally after 1kN loading. Since there was a fear of an impact occurring when the specimen was detached from the servo pulsar at the moment of fatigue tests, the lower limit was chosen as 1KN so that the detachment would not take place. The method of setting the upper limit load mentioned above was based on on-site measurement, which showed that the movement of cracks due to the passage of trains was 10% of the static crack width of 0.2mm. Then the movement of the crack, 0.02mm, was multiplied by 1.25, a safety factor to obtain 0.025mm. (b) Application of ECC After the static loadings, ECC of a 10mm thickness was sprayed on the bottom surface of No.2 specimen. The material property of ECC is shown in Table 3. 12mm long PVA fiber with a diameter of 0.04mm and a tensile strength of 1,600 MPa was mixed with ECC with a volume fraction of 2.1%. Tensile property was measured by a single-axis tensile test [1] using dumbbell shape specimens. Before spraying, the bottom Initial cracking strength Young's modulus Table 3 material properties of ECC sprayed Tensile yield strength Tensile strength End tensile strain Bond strength Unit weight [MPa] [GPa] [MPa] [MPa] [%] [MPa] [kg/m 3 ] ,816

6 surface of the beam specimens was sandblasted to secure good adhesion. Curing chemical was sprayed before curing taking into account the practical application. The reason for setting ECC thickness as 10mm was follows; based on the report [3], spraying ECC could be well performed at the thickness of 5-20mm and ECC of this thickness could disperse a crack on the base concrete to 2-3 smaller cracks at the interface. Then the thickness desired was decided taking into consideration the variant workmanship and the uneven surface condition. (c) Fatigue test Fatigue tests of 17 million cycles, equivalent to 20 years exposure to passing Shinkansen trains loadings, were carried out with the lower limit load of 1kN and the upper limit load which had been obtained from static loading tests. The deflection of the specimens, strain of the reinforcement and the concrete, crack width and crack distribution were monitored during the tests. The crack width was measured at 3 points on the specimen No.1 and 5 on the specimen No.2, where the largest crack width had been observed during the static loadings. The photograph of this test is shown in Fig.7. Load(kN) Specimen No.1 Specimen No.2 No.1 No Displacement(mm) Fig.7 View of Fatigue test Fig.8 Load-Displacement profile of static loading test 3. RESULTS AND DISCUSSIONS 3.1 Static loading test Fig.8 shows the relationship between the applied load and the displacement at the span center. The load applied when the maximum crack width reached 0.4mm was 50kN for the specimen No.1 and 62kN for the specimen No.2. As a result of re-loading after unloading, it was found that the load that needs to open the crack by 0.025mm after the loading of 1kN was 6.7kN for the specimen No.1 and 7.6kN for the specimen No.2. These values were subsequently used for upper limit load for the each fatigue test.

7 L C L R L CL R L Side of specimen No.1 C L R L Side of specimen No.2 LC R Basal plane of specimen Basal plane of specimen Fig.9 Crack map after 17 million cycles W C1 W C2 Unit : mm W C1 : Crack width of the base concrete surface at the minimum loading W C2 : Crack width of the base concrete surface at the maximum loading W E1 : Crack width of ECC at the minimum loading W E2 : Crack width of ECC at the maximum loading Crack amplitude : W E2 - W E1, W C2 - W C1 W E2 W E1 Ratio (%) = (W E2 - W E1 ) / (W C2 - W C1 ) Fig.10 Schematic index of crack-bridging ability 3.2 Performance of crack dispersiveness in fatigue test Fig.9 shows the condition of the cracks of the specimens after 17 million cyclic loadings. There was not much significant difference in cracking conditions observed between the both specimens except that the specimen No.2 had relatively more cracks than the other. In this experiment only a few cracks in the ECC were observed to generate along a crack on the base concrete surface. Multiple-cracks [4] spreading in triangle shape from a crack on the concrete surface, which are often found at a large deformation process, were not observed this time. The crack width of ECC generating under a large deformation process is usually 0.1mm, however, in this experiment the crack width was observed only 0.025mm. This can be mainly because that ECC had a small strain due to the small level of the loading resulting in little dispersion of cracks in ECC. However, in the specimen No.2 the ECC cover did not show any detachment or spallings after the test. 3.3 Performance of crack-bridging ability against fatigue When ECC is chosen as a surface protective material, a key factor is to know the level of the movement of cracks in ECC during the passage of trains. In this paper crackbridging ability of ECC is evaluated by the ratio of the movement of cracks in ECC to that on the base concrete. The movement of the cracks in ECC was calculated by the two widths of the cracks observed with a microscope at the upper and lower limit loadings which was carried out every 2 million cycles after 4 million cycles. On the other hand it was obtained by means of π-shaped displacement gages for the cracks on the base concrete. Fig.11 shows the transition of the amplitude of ECC cracks. The amplitude is prone to increase according to the increase of cycles. Next, Fig.12 shows the ratio of the amplitude of ECC cracks to concrete cracks. The amplitude of ECC cracks is prone to

8 approach that of concrete cracks. When the test was finished at 17 million cycles, the ratio was 89%. It is assumed that creep of PVA caused the progress of the amplitude of ECC cracks, however, it was 0.02mm at the maximum and considered to be small. L C L R 6L 3L 1L 3R 7R Crack amplitude (mm) L 3L 1L 3R 7R Ratio (%) Measurement position is 7R Cycle( 10 4 ) Fig.11 Transition of crack amplitude of ECC Cycle( 10 4 ) Fig.12 Ratio of ECC crack amplitude to that on base concrete 3.4 Tensile stress distribution after the crack dispersion The characteristic of tensile stress distribution after the dispersion of ECC was evaluated from the transition of the deflection and the amplitude of cracks of both specimens. (a) Transition of the deflection The relationship between fatigue cycle and displacement at the center of the span for the both specimens is shown in Fig.13. In this experiment the range of the load for the fatigue test was derived from the maximum crack width and its amplitude by static loadings and it was larger for the specimen No.2 than for No.1, which might cause a larger deflection in the former specimen. On the other hand after 9 million cycles the deflection rate in the specimen No.1 was found to exceed that in the other specimen. At the end of the experiment the rate in the specimen No.2 was smaller than the other by approximately 20%, which indicates the tensile reinforcement effect by ECC. In addition, the upper limit load applied to the specimen No.2 was larger than that in the specimen of No.1 by 14%. This hard condition to which the specimen No.2 was exposed may indicate that spraying ECC of a 10mm thickness can lower much more displacement. (b) TRANSITION OF THE AMPLITUDE OF THE CRACK OPENING ON THE BASE CONCRETE SURFACE The transition of the amplitude of the crack opening on the base concrete surface at the maximum loading is shown in Fig.14. The initial amplitude of the crack movement was 0.025mm on the specimen No.1, and it was 0.018mm on the other specimen (28%

9 smaller). After 17 million cycles it increased to approximately 0.031mm on the specimen No.1, however, it remained almost the same on the other specimen with the final amplitude of 0.019mm. It can be said that ECC has an effectiveness of distributing tensile stress in this experiment though this is not always in the case of the beams of real structures since the height of the beams used in this experiment was smaller than that of real ones Displacement (mm) specimen No.1 No.1 No.2 specimen No Cycle ( 10 4 ) Fig.13 Transition of the deflection at the center of span Crack amplitude (mm) 0.03 No No specimen No.1 specimen No Cycle ( 10 4 ) Fig.14 Transition of crack amplitude 4. CONCLUSIONS Fatigue tests of 17 million cycles with a small amplitude assuming the actual fatigue load under the passage of the Shinkansen trains were carried out using two RC beam specimens which had cracks introduced by static loadings and one of which had ECC sprayed on the bottom surface. The results obtained are as follows; Under such load level as is caused by train loadings, cracks were not well dispersed in ECC resulting in only one or two cracks being generated in ECC from the crack openings on the base concrete. Even in the case of spraying a 10mm thickness of ECC, no delamination or spalling of the cover were not observed under the fatigue application that is equivalent to 20 years exposure to the loadings of passing Shinkansen trains. The crack width in ECC is prone to increase according to the increasing cycles, and the amplitude of the crack movement in ECC reached 89% of that on the base concrete. Compared to the control specimen, the one with 10mm thick sprayed ECC had a 20% smaller deflection and avoided the amplitude of the crack movement from becoming larger, which indicates an effectiveness of a tensile stress distribution brought by an ECC application. REFERENCES [1] Tetsushi KANDA, Tadashi SAITO, Noboru SAKATA, FUNDAMENTAL PROPERTIES OF DIRECT SPRAYED ECC, Proceedings of the JCI International Workshop on Ductile Fiber Reinforced Cementitious Composites (DFRCC) Application and Evaluation -, pp , October 2002

10 [2] Li, V. C., From micromechanics to structural engineering the design of cementitious composites for civil engineering applications, Journal of Structural Mechanics and Earthquake Engineering, JSCE, Vol.10, No.2, pp.37-48, 1993 [3] Minoru KUNIEDA, Hiroshi INAGUMA, Junji MASUKAWA, Keitetsu ROKUGO, Fundamental Study on Surface Protection Repair System Using Sprayed Engineered Cementitious Composites, Proceedings of the Concrete Structures Scenarios, JSMS Vol.4, October 2004 [4] Minoru KUNIEDA, Ryodai MIYATA, Toshiro KAMADA, Keitetsu ROKUGO, Effect of ductility of patch repair material on crack distribution within repaired members, Proceedings of the Japan Concrete Institute, Vol.25, No.1, pp , July 2003