• Journal of the Korea Concrete Institute
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• v.24 no.2
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• pp.111-120
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• 2012

This paper is a report about lap splice performance of rebar embedded in the strain-hardening cement-based composites (SHCCs) under monotonic and repeated tension loading. Ten mix proportions of cement-based composites such as SHCCs and normal concrete were investigated. The study parameters are comprised of (1) types of reinforcing fibers (polyethylene and steel fiber), (2) replacement levels of expansive admixture (EXA, 0% and 10%), and (3) compressive strength (30 and 100 MPa) of cement-based composites. Lap splice lengths (ld) of rebars in SHCC materials and normal concrete were 60% and 100% of splice length calculated by code requirements for structural concrete, respectively. Test results indicated that SHCCs materials can lead to enhancements in the lap splice performance of embedded rebar. All of the fiber reinforcement conditions (PE-SHCC and PESF-SHCC) considered in this study produced considerable improvements in the tensile strength, cracking behavior, and bond strength of lap-spliced rebar. Furthermore, adding EXA to SHCC matrix improved the tensile lap splice performance of rebar in SHCC materials. However, for controlling crack behavior, the performance of PE-SHCC was better than that of PESF-SHCC due to its mechanical properties. This study demonstrated an effective approach for reducing required development length of lap spliced rebar by using SHCC materials.

• Journal of the Korea Concrete Institute
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• v.22 no.4
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• pp.575-583
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• 2010

This research investigated the influence of the number of twist on single fiber pullout behavior of Twisted steel (T-) fiber and tensile behavior of high performance cementitious composites reinforced with the (T-) fibers (HPFRCC). Micromechanical pullout model for T- fibers has been applied to analytically investigate the influence of various fiber parameters including the number of twist on single fiber pullout behavior; and, to optimize the number of twist to generate larger pullout energy during fiber pullout without fiber breakage. In addition, an experimental program including single fiber pullout and tensile tests has been performed to investigate the influence of twist ratio experimentally. Two types of T- fiber with different twisted ratios, T(L)- fiber (6ribs/30 mm) and T(H)- fiber (18ribs/30 mm), were tested. T(L)- fiber produced higher equivalent bond strength (larger pullout energy) although T(H)- fiber produced higher pullout stress during pullout since T(H)- fiber showed fiber breakage during pullout. Tensile test results confirmed that T(L)- fiber in high strength mortar generates better tensile performance of HPFRCC, e.g., load carrying capacity, strain capacity and multiple micro-cracking behavior.

• Structural Engineering and Mechanics
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• v.82 no.1
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• pp.121-131
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• 2022

The main goal of this study is to prepare a program for analyzing High Strength Steel Fibrous Reinforced Concrete (HSSFRC) slabs and predict the response and strength of the slab instead of preparing a prototype and testing it in the laboratory. For this purpose, new equations are proposed to represent the material properties of High Strength Steel Fibrous Reinforced Concrete. The proposed equations obtained from performing regression analysis on many experimental results using statistical programs. The finite element method is adopted for non-linear analysis of the slabs. The eight-node "Serendipity element" (3 DoF) is chosen to represent the concrete. The layered approach is adopted for concrete elements and the steel reinforcement is represented by a smeared layer. The compression properties of the concrete are modeled by a work hardening plasticity approach and the yield condition is determined depending on the first two stress invariants. A tensile strength criterion is adopted in order to estimate the cracks propagation. many experimental results for testing slabs are compared with the numerical results of the present study and a good agreement is achieved regarding load-deflection curves and crack pattern. The response of the load deflection curve is slightly stiff at the beginning because the creep effect is not considered in this study and for assuming perfect bond between the steel reinforcement and the concrete, however, a great agreement is achieved between the ultimate load from the present study and experimental results. For the models of the tension stiffening and cracked shear modulus, the value of Bg and Bt (Where Bg and Bt are the curvature factor for the cracked shear modulus and tension stiffening models respectively) equal to 0.005 give good results compared with experimental result.

• Advances in concrete construction
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• v.15 no.6
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• pp.359-376
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• 2023

Ultra-high-performance concrete (UHPC) has become an attractive cast-in-place repairing material for existing engineering structures. The present study aims to investigate age-dependent high-early-strength UHPC (HESUHPC) material properties (i.e., compressive strength, elastic modulus, flexural strength, and tensile strength) as well as interfacial shear properties of HESUHPC-normal strength concrete (NSC) composites cured at different season temperatures (i.e., summer, autumn, and winter). The typical temperatures were kept for at least seven days in different seasons from weather forecasting to guarantee an approximately consistent curing and testing condition (i.e., temperature and relative humidity) for specimens at different ages. The HESUHPC material properties are tested through standardized testing methods, and the interfacial bond performance is tested through a bi-surface shear testing method. The test results quantify the positive development of HESUHPC material properties at the early age, and the increasing amplitude decreases from summer to winter. Three-day mechanical properties in winter (with the lowest curing temperature) still gain more than 60% of the 28-day mechanical properties, and the impact of season temperatures becomes small at the later age. The HESUHPC shrinkage mainly occurs at the early age, and the final shrinkage value is not significant. The HESUHPC-NSC interface exhibits sound shear performance, the interface in most specimens does not fail, and most interfacial shear strengths are higher than the NSC-NSC composite. The HESUHPC-NSC composites at the shear failure do not exhibit a large relative slip and present a significant brittleness at the failure. The typical failures are characterized by thin-layer NSC debonding near the interface, and NSC pure shear failure. Two load-slip development patterns, and two types of main crack location are identified for the HESUHPC-NSC composites tested in different ages and seasons. In addition, shear capacity of the HESUHPC-NSC composite develops rapidly at the early age, and the increasing amplitude decreases as the season temperature decreases. This study will promote the HESUHPC application in practical engineering as a cast-in-place repairing material subjected to different natural environments.

• Structural Engineering and Mechanics
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• v.30 no.6
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• pp.713-732
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• 2008

Serviceability and durability of the concrete members can be seriously affected by the corrosion of steel rebar. Carbonation front and or chloride ingress can destroy the passive film on rebar and may set the corrosion (oxidation process). Depending on the level of oxidation (expansive corrosion products/rust) damage to the cover concrete takes place in the form of expansion, cracking and spalling or delamination. This makes the concrete unable to develop forces through bond and also become unprotected against further degradation from corrosion; and thus marks the end of service life for corrosion-affected structures. This paper presents an analytical model that predicts the weight loss of steel rebar and the corresponding time from onset of corrosion for the known corrosion rate and thus can be used for the determination of time to cover cracking in corrosion affected RC member. This model uses fully the thick-walled cylinder approach. The gradual crack propagation in radial directions (from inside) is considered when the circumferential tensile stresses at the inner surface of intact concrete have reached the tensile strength of concrete. The analysis is done separately with and without considering the stiffness of reinforcing steel and rust combine along with the assumption of zero residual strength of cracked concrete. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel rebar. The capability of the model to produce the experimental trends is demonstrated by comparing the model's predictions with the results of experimental data published in the literature. The effect of considering the corroded reinforcing steel bar stiffness is demonstrated. A sensitivity analysis has also been carried out to show the influence of the various parameters. It has been found that material properties and their inter-relations significantly influence weight loss of rebar. Time to cover cracking from onset of corrosion for the same weight loss is influenced by corrosion rate and state of oxidation of corrosion product formed. Time to cover cracking from onset of corrosion is useful in making certain decisions pertaining to inspection, repair, rehabilitation, replacement and demolition of RC member/structure in corrosive environment.

• Journal of the Korean Applied Science and Technology
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• v.25 no.4
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• pp.459-468
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• 2008

Methyl methacrylate(MMA) was grafted onto microcrystalline cellulose(MCC) with ceric ammonium nitrate(CAN) as a redox initiator at the various conditions. The cellulose triacetate(CTA) composite films added MCC and MMA-grafted MCC powders were prepared on a glass plate. The graft yield(GY) and graft efficiency(GE) of the grafted MCC were calculated with the simple equations by the weight balance method. The double bond of C=O on the grafted MCC surfaces was confirmed by the fourier transform infrared spectroscopy with attenuated total reflection(FT-IT ATR) spectrophotometer. After grafting, the degree of crystallinity of cellulose powders was decresed by judging from x-ray diffraction(XRD) data. Scanning electron microscope(SEM) photos showed the only solvent and CAN solution could change the roughness of MCC powders and the effect of powder dispersions in composite matrix. The tensile strength of MCC/CTA composite films was decreased with increase of MCC powder contents. When 5% grafted MCC was added, the tensile strength of grafted MCC/CTA composite films was increased from 82.3 MPa to 97.2 MPa. The thermal property of powders was also analyzed by the thermogravimetric analysis(TGA).

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.3
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• pp.153-159
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• 2014

This research reviewed the joint and duration characteristics depending on the surface treatment condition and lap spliced length in the PET film jointing method using the high hardness liquid material. As a result, the corona discharge treatment was improved to the contact angle, joint tensile strength, and joint peel resistance compared to non-treatment. Particularly, a surface treatment E (Corona discharge + Primer + PU bond + Polyester fabric) turned out to the best, and especially when the lap spliced length is longer than 15mm, stable joint performance was secured under the long term deterioration treatment of 16 weeks. Thus, the joint is considered to be applicable as the water-proof material.

• Journal of Conservation Science
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• v.31 no.2
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• pp.115-123
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• 2015

This study aims to increase the transparency of the natural adhesives made with lacquer and glue. For the purpose, samples were prepared by mixing urushiol with glue in varied proportion and the characteristics and adhesive properties were investigated. By adding glue on urushiol, IR spectra of the natural adhesives became similar to that of glue as the N-H band related with protein of glue increased, while that of methylene C-H bond related to urushiol decreased. Samples were dried within a day and maintained a bright color without blackening by oxidation during the curing process. The natural adhesives with urushiol and glue showed various range of viscosity and tensile shear strength as Cemedine C or Epoxy resin according to mixing ratio. In addition, the sample of mixing ratio of 6:4 showed bright and transparency in appearance and tensile shear strength similar to that of Araldite AY103-1/HY956 for earthenware layer.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.909-912
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• 2008

Fiber is an important ingredient in strain-hardening cementitious composite (SHCC), which can control fracture of cementitious composite by bridging action. The properties of reinforcing fiber, as tensile strength, aspect ratio and elastic modulus, have great effect on the fracture behavior of SHCC. But SHCC has serious problem as drying shrinkage because silica powder is used to make SHCC in order to improve bond strength between reinforcing fibers and cement matrix. Therefore, curing method (period and temperature) is very important for SHCC to show high tensile performance. a variety of experiments have being performed to access the performance of SHCC recently. This research emphasis is on the mechanical properties of SHCC made in Polyvinyl alcohol (PVA), Polyethylene (PE) fibers and steel cord (SC), and how curing method affects the composite property, and ultimately its strain-hardening performance.

• Journal of the Korea institute for structural maintenance and inspection
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• v.16 no.1
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• pp.110-119
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• 2012

Fiber reinforced polymer (FRP) reinforcing rebar with non-corrosive property is suggested as an alternative replacement to steel reinforcing rebar due to its enhanced durability and non-corrosive characteristics. Currently, a limited number of glass fiber reinforced polymer rebar (GFRP) are sold commercially due to their high cost, relatively low performances, and brittle failure characteristics. Therefore, the performance enhancements and cost reduction of GFRP rebar are needed to increase its applications in construction fields. The intent of this study is to develop high performance GFRP rebar by improving its tensile and shear properties. Also, in order to reduce manufacturing costs, factors such as material composition and manufacturing process were evaluated to improve manufacturing efficiency. Finally a GFRP rebar with enhanced material properties and less expensive than the GFRP rebar currently sold in the market was manufactured and evaluated for its application possibility in construction fields.