Patching repair with ECC on cracked concrete surface

Patching repair with ECC on cracked concrete surface K. Rokugo 1, M. Kunieda 2 and S. C. Lim 3 1 Gifu University, Japan 2 Nagoya University, Japan 3 Deros Co.,Ltd, Japan Abstract Performance requirements of the patch repair material are strength, size stability, shieldability, durability, bond property with the substrate concrete, ductility and so forth. Various kinds of patch repair materials with short fibers have been developed and used. Engineered Cementitious Composites (ECC) which provides multiple cracking and strain hardening behavior is also one of the attractive materials for a patching repair. It promotes the narrow crack width that gives resistance to penetration of substances such as chloride ions, water and so forth, and tensile capacity and anti-spalling performance to repaired structures. However, the conditions to obtain these advantages of the ECC should be clarified in its application. In order to apply the ECC to patching repair of concrete structures with cracks, this paper discusses the performance required for patch repair materials for concrete structures with existing cracks. Numerical and experimental studies on patching repair using the ECC or ductile repair materials are briefly introduced focusing on evaluation of zero-span elongation. It shows that existing cracks of substrate concrete induce a localized crack of patch repair material itself. On the other hand, it can be clarified that unbonded region around the existing cracks imparts crack distribution to the repaired specimens with the ECC. A trial application on patching repair of wall structure with existing cracks induced by ASR is conducted. In the observation at 12months after the repair, it is clarified that the applied repair material such as the ECC reduces the crack width on surface. Keywords: ECC, patching repair, trial application, zero-span elongation, ASR. Keitetsu Rokugo Dept. of Civil Engineering, Gifu University 1-1 Yanagido Gifu, 51-1193 Japan Email: rk@cc.gifu-u.ac.jp Tel: +81-58-293-2417

1. Introduction Patching repair is one of the repair methods for deteriorated concrete structures. Performance requirements of the patch repair materials are strength, size stability, shieldability, durability, bond property with the substrate concrete, and so forth. In some cases, anti-spalling performance may be required for patch repair materials. Various kinds of short fibers, such as steel and polyvinyl alcohol (vinylon) fibers, are currently included in patch repair materials to improve their toughness or to obtain their viscosity. Many experiments have been conducted to determine the mechanical properties of members repaired by patch repair techniques. Engineered Cementitious Composites (ECC), which provides strain-hardening behavior in tensile loading, has been developed by Li [1], and its applicability to patching repair (overlay systems) has also been investigated [2-4]. This paper discusses the performance required for the patch repair materials with higher ductility, through introducing previous investigation [4]. This paper also introduces a trial application using the ECC as a patch repair material, to establish design concepts and construction methods. 2. Performance Required for Patch Repair Materials 2.1 Evaluation of Patch Repair Materials Focusing on Zero-span Elongation ECC used in patching repair provides: -narrow crack width that gives resistance against penetration of substances such as chloride ions, water and so forth, and -tensile capacity and anti-spalling performance to repaired structures. Usually, the performance of the ECC or other ductile materials is confirmed though tensile or bending tests with no restraint along the length of the test specimen. In applications of surface coating repair materials (paint type), such as epoxy and acrylic rubber, evaluation should not be performed only with free film elongation tests, as shown in Fig. 1. In a deteriorated concrete structure having cracks, the deformation of the members increases as cracks open wider. It means that localized fracture occurs on a surface repair material near the cracks in substrate concrete. So, patch repair materials are also required to follow the crack-opening motion under so-called zero-span elongation. Coating material Coating material Localized fracture Substrate Crack opening (a) Free film (b) Zero-span Figure 1. Schematic Illustrations of zero-span elongation in case of surface coating materials

Treated surface Repair material placement Treated surface Repair material placement Unbonded region (length: 5mm) Substrate concrete (1x1x4mm) Substrate concrete (1x1x4mm) Substrate concrete (1x1x4mm) Substrate concrete (1x1x4mm) (a) Repaired specimens with artificial crack (b) Repaired specimens with unbonded area Figure 2. Procedures to make the specimens with an artificial crack 6 Tensile stress(mpa) 5 4 3 2 1 5mm 3mm.1.2.3 Strain Figure 3. Uniaxial tensile test by dumbbell-shaped specimens P/2 P/2 Substrate concrete 1 Repair material 3 Unit: mm : Measurement point by pi-type disp. transducer 3 1 3 Figure 4. Loading method for the specimens with an artificial crack In the next section, the experimental and numerical approach to demonstrate that cracks in the substrate induce localized cracks within the patch repair material itself will be introduced, and the importance of evaluating the toughness of patch repair materials under zero-span elongation will be presented. 2.2 Experimental Investigations [4] Two types of specimens were adopted to confirm the localized cracking behavior due to the conditions of the bond between the substrate and the patch repair material. As shown in Fig. 2(a), two beams each having a size of 1x1x4mm were carefully positioned with no clearance between them. The patch repair material with a thickness of 3mm was placed over these beams. In addition, some specimens were made with a partially unbonded region above the artificial crack by applying a 5-mm wide gum tape prior to placing the repair material, to investigate the effects of bond between substrate concrete and the patch repair material on the cracking behavior of the

2 2 15 15 Load(kN) 1 Load(kN) 1 5 Without unbonded region With unbonded region 1 2 3 4 Displacement(mm) 5 Without unbonded region With unbonded region.1.2.3.4.5.6 Strain (a) Load-displacement curves (b) Load-strain curves Figure 5. Test results Artificial crack in substrate concrete Cracks in patch repair material Artificial crack in substrate concrete Cracks in patch repair material Unbonded region Cracked area (a) Without unbonded region Figure 6. Crack patterns Cracked area (b) With unbonded region repair material itself (Fig. 2(b)). The ECC [4] with polyethylene fiber (12x1-6 m in diameter, 12 mm in length, 1.5% in volume fraction) was placed on the concrete as patch repair material. Uniaxial tensile tests were conducted on the ECC using dumbbell-shaped specimens to evaluate its tensile properties, as shown in Fig. 3. Figure 4 shows the specimens with the patched surface down on the tension side. Figure 5(a) shows the load-displacement curves obtained from the tests on the repaired specimens. Most specimens failed after reaching a displacement of 2 to 3mm. The unbonded region scarcely affected the shapes of the load-displacement curves of repaired specimens, except for the initial stiffness. Figure 5(b) shows the load-strain curves measured at the specimen bottom. The strain was measured at the specimen bottom by pi-type transducers (measurement length: 5mm), which is positioned across the artificial crack plane, as shown in Fig. 4. In the specimens with the unbonded region, the specimens fractured after generating a strain of approximately 5% on the bottom of the repaired specimens. The large strain can be attributed to the large area of the ECC that can contribute to deformation without being restrained by the substrate concrete.

Displacement Figure 7. Discretized repaired specimen Figure 6 shows typical crack patterns (emphasized) on the repaired specimens without and with unbonded region, respectively. Diagonal cracks occurred within the ECC near the specimen center, and this set of cracks has an overall triangular shape. The length of the cracked part at the bottom was approximately 3 to 5mm along the beam axis. In specimens having an unbonded region, the length of the crack site on the bottom of the ECC was longer than that of fully bonded specimens. 2.3 Numerical Investigations [4] Parametric studies subject to the effect of material ductility or boundary conditions on distributed cracking were conducted. Three kinds of ultimate strains (Eu) of repair materials (i.e. Eu=.1,.3 and.6) were adopted, with strain-hardening shape stress-strain curves in tension. An initial crack occurs at stress of 4.MPa, and then stress increases up to 5.MPa, with increasing of strain values. The discretized model of the repaired specimen is shown in Fig. 7. Rigid-Body-Spring Networks (RBSN)[5] were used to model the repaired specimens. For comparison, a FRC model (that does not include strain-hardening) was also analyzed. It was based on a tensile strength of 5.MPa and fracture energy of 2.kN/m. The calculated strains at the bottom of the repaired specimens, within the measurement length of 5mm, are shown in Fig. 8. The calculated maximum strains in Fig. 8(a) were smaller than the maximum strains of the original stress-strain relations (input values). This is because the localized fracture within the patch repair material was constrained by the substrate concrete. However, the maximum strains obtained from the repaired specimen with an unbonded region were larger than those without one, in all cases. The geometry of the repaired part in patching repair can induce localized fracture in patch repair material itself. However, more detailed modeling are needed to simulate quantitatively. Crack patterns for each repaired specimen, just before the sudden drop in load, are shown in Fig. 9. Here, the portrayed cracks have a crack width larger than.1mm, calculated by integrating the crack strain over the crack band width. In the case of maximum strain of.1, only a single crack is formed within the patch repair material. The material having the maximum strain of.1 may provide large deformation capacity and many cracks in an ordinary tensile test. So, the cracking behavior in patching repair was different from that in ordinary material tests with no constraint. In the case of maximum strain of.3, there was a set of cracks with narrow crack width, and the overall shape of the cracked zone was triangular. As shown in Fig. 9(c), the unbonded region exhibited uniformly distributed cracks within the patch repair material, which also agrees well with the experimental results.

Load(kN) 15 1 5 εeu=.1 u εeu=.3 u εeu=.6 u FRC model Load(kN) 15 1 5 εeu=.1 u εeu=.3 u εeu=.6 u FRC model.1.2.3 Strain at specimen bottom (a) Without unbonded region Figure 8. Analytical load-strain curves.2.4.6.8.1 Stain Strain at at specimen bottom (b) With unbonded region (a) Eu=.1, Disp.=.5mm (b) Eu=.3, Disp.=.8mm (c) Unbonded, Eu=.1, Disp.=1.2mm Figure 9. Analytical crack patterns From these experimental and numerical investigations, it is clarified that evaluating the material properties under the zero-span elongation is important in the case of patching repair. 3. Trial Application on Patching Repair of Wall Structure The structure for patching repair is a concrete gravity retaining wall measuring 18m in width and 5m in height constructed in the mid 197s, and is located on the premises of the Gifu Prefectural Tono Water Service Office. The cracks on concrete surface induced by Alkali Silica Reaction (ASR) were recognized on this wall, and repaired through crack injection and overlay techniques in 1994. However, cracks occurred again on the surface before 22, as shown in Fig. 1 (a). The total residual expansion by ASR was.5 to.11% by accelerated expansion testing in accordance with JCI-DD2. So, the main purpose of this patching repair is not strengthening but repair for prevention of occurring surface cracks within repair material. The trial application applying the ECC to patching repair was carried out in the middle of April 23 3.1 Conditions of Repair In the trial application, the wall was divided into ten blocks (1.8m in width and 5m in height), and different repair was applied to each of nine blocks. These were combinations of three repair materials, two steel reinforcement types, and sealing on cracking, as shown in Fig. 11 and Table 1. Wall surface was treated to be rough by a water-jet technique to a depth of a few millimeters. On

Close-up (a) Before the repair Figure 1. Concrete retaining wall for the patching repair 16m (b) After the repair 3m No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9 Mat. C No.1 None 2m Surface coating repair (paint type) Figure 11. Division for construction (each block size: 1.8m in width, 5m in height) Blocks 4 and 8, cracks were sealed with a polyurethane sealant (3mm in width, 5mm in thickness) to make an unbonded region that imparts surface crack distribution to the repaired structures, as described in previous section. Two days after the repair of the blocks, an acrylic surface coating material (paint type) was applied to an area of 2m from the bottom throughout the blocks, to prevent penetration of water from outside. As given in Table 1, repair material A was a premixed polymer cement mortar contains both PVA (vinylon) and high strength polyethylene fibers with a total volume fraction of 1.5%. Repair material B is a premixed ECC contains high strength PVA fiber with a fiber content of 2.1% by volume. Repair material C is an ordinary cement-type mortar without a fiber. On blocks involving steel reinforcement, welded bar mesh and expanded metal were placed 1 mm off the substrate surface of the wall. The welded bar mesh was made of D6 and SD295 rebars welded into a grid with 1 mm intervals. An expanded metal with a mesh size of 75 by 23 mm (spec: XS- 82) was selected. Repair material A was mixed using a new geared mixer having a capacity of 32 liters and pumped with a snake pump for spraying. Repair materials B and C were produced using a hobart mixer having a capacity of 12 liters and pumped with a squeeze pump. The sprayed thickness was 5 to 7 mm. Figure 12 sequentially presents the repairing procedures in this application. As shown in Fig. 1 (b), all of the three repair materials formed smooth finished surfaces. Tensile test results of those materials, which are obtained from uniaxial direct tensile

Table 1. Conditions for the trial application Unbonded region at crack 1 Welded bar mesh None 2 Expanded metal None 3 None None Repair materials Block No. Reinforcement Repair material A Fiber: PVA+ High strength PE Volume fraction of fiber: 1.5% Matrix: Premixed polymer cement mortar 4 None Present Repair material B 5 Welded bar mesh None Fiber: High strength PVA 6 Expanded metal None Volume fraction of fiber: 2.1% 7 None None Matrix: Premixed cement mortar 8 None Present Repair material C Fiber: None Matrix: Premixed cement mortar 9 Welded bar mesh None No repair 1 None None (a) Surface treatment by water-jet (b) Settlement of reinforcements (c) Unbonded region at cracks Figure 12. Procedure in this repair (d) Spraying tests (cross section of the specimens: 75x2mm) at 1 and 3months [6], are shown in Fig. 13. Repair materials A and B provide maximum strain of.1% and.4%, respectively. 3.2 Effects of the Repair Visual observation of cracks and measurement of crack width using a crack scale were carried out 1, 2, 3, 7, 1, and 12 months after the repair. At the end of the 12 month period after the repair, a crack map of the areas from 1m below to 1m above the 2m high boundary line between areas with and without surface coating was produced by image analysis, while measuring the crack width and length. Figure 14 shows the pre-existing cracks before the repair and newly developed cracks after

4 Stress(MPa) 3 2 1 (a) Repair material A(1month) (b) Repair material B(1month) (c) Repair material C(1month) 4 Stress(MPa) 3 2 1.1.2.3.4.5 Strain(%) (d) Repair material A(3months) Figure 13. Tensile stress-strain curves of each repair material the repair shown in thick and thin lines, respectively. On Block 9 to which repair material C was applied, fine cracks began to appear 1 month after application, and a vertical crack.3 mm in width was observed 3 months after application. Map crack was observed over the entire block 1 months after application, with the maximum crack width being around.2 mm. On the other hand, no crack was observed until 7 months after the repair on blocks to which repair materials A and B were applied. It seems that lower temperature due to change of season (winter) induces shrinkage cracks. A large number of fine cracks with a width of up to.5 mm occurred on all blocks (Blocks 1 to 8) by 1 months after the repair. It was clarified that the performance to reduce a crack width is provided by these ductile repair materials (materials A and B). Cracks were randomly distributed on blocks with no steel reinforcement (Blocks 3, 4, 7, and 8) and those reinforced with expanded metal (Blocks 2 and 6). On blocks reinforced with welded bar mesh, however, the crack patterns resembled the shape of bar mesh with both of repair materials A and B. This agrees with the tendency of cracking at intervals similar to the bar spacing (1 mm) of welded bar mesh observed in the tension tests on specimens reinforced with welded bar mesh, obtained from other investigation [6]. In the observation by 12 months after the repair, there was no efficiency of unbonded region on cracking behaviors. And also, there is no crack in the repaired part with the acrylic surface coating. Further continuative monitoring should be needed. 4. Summary.1.2.3.4.5.1.2 Stain(%) Strain(%) (e) Repair material B(3months) (f) Repair material C(3months) This paper discussed the performance required for patch repair materials for concrete structures with existing cracks. Numerical and experimental studies on patching repair using the ECC or ductile repair materials were briefly introduced focusing on evaluation of zero-span elongation. They showed that the existing cracks of substrate induce localized fracture of patch repair

Cracks of substrate concrete (before repair) Cracks of patch repair materials (after repair) No.1 (re-bar) No.2 (expand) No.3 No.4 (unbond) No.5 (re-bar) Figure 14. Crack pattern (12 month after repair) No.6 No.7 (expand) No.8 (unbond) No.1 No.9 No Mat. C repair (re-bar) materials. And then, a trial application on patching repair of wall structure deteriorated by ASR was conducted. The applied repair material such as the ECC reduced the crack width on surface. However, the effect of unbonded region on cracking behavior cannot be observed at 12 months after the repair yet. Continuative monitoring will be carried out. 5. Acknowledgement This trial application was supported by the member of Research Committee on Application of ECC to Repair of Concrete Wall Surface. The authors would like to thank them for their frequent support and discussions. 6. References 1. Li, V. C., 1993. From micromechanics to structural engineering - the design of cementitious composites for civil engineering applications, Journal of Structural Mechanics and Earthquake Engineering, JSCE, 1(2): 37-48. 2. Lim, Y. M. and Li, V. C., 1997. Durable repair of aged infrastructures using trapping mechanism of engineered cementitious composites, Journal of Cement and Concrete Composites, Elsevier, 19(4): 373-385. 3. Lim, Y. M. et al., Is ductility important for repair application?, Proceedings of JCI International Workshop on DFRCC, October-22, Japan Concrete Institute, Takayama, 22, pp.199-28. 4. Kunieda, M. et al., Localized fracture of repair material in patch repair systems, Proceedings of FRAMCOS-5, April-24, IA-FraMCoS, Colorado, 24, pp.765-772. 5. Bolander, J. and Saito, S., 1998. Fracture analysis using spring networks with random geometry, Engineering Fracture Mechanics, 61(5-6): 569-591. 6. Rokugo, K. et al., Tensile behavior of ECC members with or without reinforcement, Proceedings of HPFRCC in Structural Applications, May-25 (to be published).