• Journal of the Korea institute for structural maintenance and inspection
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• v.12 no.2
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• pp.67-74
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• 2008

This paper provides the test results of bonding behavior of the interface between concrete substrate and carbon fiber in the rehabilitation method applying carbon fiber with epoxy based resin adhesive. The difference in each components was gradually increased subjected to the repetition of temperature variation, regardless of the strength of the substrate concrete, while the ultrasonic interface between each component occurred. An increase in difference of the temperature resulted in a decrease in bond strength of each component. Associated failure mode was shown to be interfacial failure and substrate concrete failure. No remarkable changes were found in the deformation and ultrasonic velocity of each component until the four cycles of the dry and moisture test. Hence, the moisture condition may not affect the bonding behavior of each component. After the repetition of dry and moisture test, corresponding bond strength was reduced to 40% of that before test. For the effect of freeze and thaw test, the cycle of freeze and thaw within 4 cycles resulted in debonding of each component.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2013.11a
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• pp.38-39
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• 2013

In this study, in regard to fiber reinforced mortar mixing steel fiber and 4types of organic fiber, impact test was carried out. Because to predict fracture reduction performance with flexural, tensile strength when types of fiber were different as impact reduction performance of concrete is closely related with toughness such as flexural strength, tensile strength and fracture energy etc. As a result, enhancement of toughness by fiber reinforcement controls the spall of rear. On the other hand in case of steel fiber relatively turned up high toughness in appropriate load compared with organic fiber but in same mixing rate, impact reduction performance by projectile showed low performance due to few number of an individual of mixing.

• Journal of the Korean Society of Safety
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• v.18 no.2
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• pp.79-85
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• 2003

In this study, the behavior characteristics of specimen strengthened with CFRP plate were analyzed according to the change of temperature. CFRP plate itself has a good resistance at the high temperature, but epoxy used as a adhesive is lost its bonding strength at the relatively low temperature. Therefore, this study carries out experiment on the beams slot-bonded with CFRP plates in order to maintain the successful bonding strength of epoxy at high temperature. It is presented that the range of glass transition temperature is 60-8$0^{\circ}C$ and RC beams slot-bonded with CFRP plate shows more increasing resistance and failure load than that of interface bonded at the high temperature.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2022.04a
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• pp.257-258
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• 2022

In this study, as a part of the study to alleviate the durability degradation due to low tensile performance and cracking of concrete, two kinds of catechol functional chitosan (Cat-Chit) were replaced at a ratio of 5, 10, 15, 20% as a mixed water substitute to evaluate the fluidility and strength characteristics of mortar.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2021.05a
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• pp.129-130
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• 2021

In order to solve the problem of low early-age compressive strength of high volume fly ash concrete. This paper studies the effect of 2% sodium sulfate (Na₂SO₄) as a chemical activator on the paste with 40% fly ash content and a water-binder ratio of 0.30. The results indicate that the addition of Na₂SO₄ can effectively improve the early-age compressive strength of the fly ash-cement system, and the strength improvement rate on the first day reached nearly 70%. In addition, calorimetric analysis reveals that the incorporation of Na₂SO₄ promotes the early hydration of cement and fly ash, increases the cumulative hydration heat and delays the heat peak of the aluminum phase.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.4C
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• pp.283-290
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• 2006

Concrete has advantages in fire situations as it is non-combustible and has low thermal conductivity. However, concrete that is not designed against fire can experience significant explosive spalling from the build-up of pore pressures and internal tensile stresses when heated. In this study, the performance of wet-mixed high strength sprayed polymer mortar for fire resistance of tunnel system was evaluated by experimentally and numerically. The fire test was performed in fire resistance(electric) furnace according to RABT(Richtlinien fur die Ausstatung und den Betrieb von $Stra{\beta}entunneln$) time heating temperature curve, so as to evaluate the temperature distribution with cover thickness of wet-mixed high strength sprayed polymer mortar for fire resistance of tunnel system. Based on experimental results and numerical analysis, the proper cover thickness of wet-mixed high strength sprayed polymer mortar determined the more than 4cm.

• Steel and Composite Structures
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• v.52 no.2
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• pp.145-163
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• 2024

This paper presents six novel hybrid machine learning (ML) models that combine support vector machines (SVM), Decision Tree (DT), Random Forest (RF), Gradient Boosting (GB), extreme gradient boosting (XGB), and categorical gradient boosting (CGB) with the Harris Hawks Optimization (HHO) algorithm. These models, namely HHO-SVM, HHO-DT, HHO-RF, HHO-GB, HHO-XGB, and HHO-CGB, are designed to predict the ultimate strength of both rectangular and circular reinforced concrete (RC) columns. The prediction models are established using a comprehensive database consisting of 325 experimental data for rectangular columns and 172 experimental data for circular columns. The ML model hyperparameters are optimized through a combination of cross-validation technique and the HHO. The performance of the hybrid ML models is evaluated and compared using various metrics, ultimately identifying the HHO-CGB model as the top-performing model for predicting the ultimate shear strength of both rectangular and circular RC columns. The mean R-value and mean a20-index are relatively high, reaching 0.991 and 0.959, respectively, while the mean absolute error and root mean square error are low (10.302 kN and 27.954 kN, respectively). Another comparison is conducted with four existing formulas to further validate the efficiency of the proposed HHO-CGB model. The Shapely Additive Explanations method is applied to analyze the contribution of each variable to the output within the HHO-CGB model, providing insights into the local and global influence of variables. The analysis reveals that the depth of the column, length of the column, and axial loading exert the most significant influence on the ultimate shear strength of RC columns. A user-friendly graphical interface tool is then developed based on the HHO-CGB to facilitate practical and cost-effective usage.

• Journal of the Korea Institute of Building Construction
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• v.10 no.4
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• pp.11-20
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• 2010

This study carried out a Mock-up test to apply Low-heat Cement (CF) that is adjusted to a fineness of $3,000\;{\pm}\;200\;cm^2/g$ by substituting Coarse particle Cement (CC) and fly ash with ordinary Portland Cement (OPC), then applied it on-site. The result of the test is as follows. The Mock-up test showed that the amount of admixture in CF increased SP agent and AE agent slightly more compared to OPC, while temperature history showed that the highest temperature of CF was around $6{\sim}10^{\circ}C$ lower than that of OPC. Compressive strength in CF was low compared to that of OPC, but the strength width became narrow at the age of 28 days, which is not considered to be significant. In on-site application, slump, air content and chloride content all satisfied the target values, while the temperature history showed that the highest temperature in the center by each cast was about $34^{\circ}C$ in the first cast, $42^{\circ}C$ in the second cast, and $39^{\circ}C$ in the third cast. Compressive strength of specimen for strut management showed low value compared to standard curing, but its strength was reduced at the age of 28 days.

• International Journal of Highway Engineering
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• v.19 no.6
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• pp.37-46
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• 2017

PURPOSES : A composite pavement utilizes both an asphalt surface and a concrete base. Typically, a concrete base layer provides structural capacity, while an asphalt surface layer provides smoothness and riding quality. This pavement type can be used in conjunction with rollercompacted concrete (RCC) pavement as a base layer due to its fast construction, economic efficiency, and structural performance. However, the service life and functionality of composite pavement may be reduced due to interfacial bond failure. Therefore, adequate interfacial bonding between the asphalt surface and the concrete base is essential to achieving monolithic behavior. The purpose of this study is to investigate the bond characteristics at the interface between asphalt (HMA; hot-mixed asphalt) and the RCC base. METHODS : This study was performed to determine the optimal type and application rate of tack coat material for RCC-base composite pavement. In addition, the core size effect, temperature condition, and bonding failure shape were analyzed to investigate the bonding characteristics at the interface between the RCC base and HMA surface. To evaluate the bond strength, a pull-off test was performed using different diameters of specimens such as 50 mm and 100 mm. Tack coat materials such as RSC-4 and BD-Coat were applied in amounts of 0.3, 0.5, 0.7, 0.9, and $1.1l/m^2$ to determine the optimal application rate. In order to evaluate the bond strength characteristics with temperature changes, a pull-off test was carried out at -15, 0, 20, and $40^{\circ}C$. In addition, the bond failure shapes were analyzed using an image analysis program after the pull-off tests were completed. RESULTS : The test results indicated that the optimal application rate of RSC-4 and BD-Coat were $0.8l/m^2$, $0.9l/m^2$, respectively. The core size effect was determined to be negligible because the bond strengths were similar in specimens with diameters of 50 mm and 100 mm. The bond strengths of RSC-4 and BD-Coat were found to decrease significantly when the temperature increased. As a result of the bonding failure shape in low-temperature conditions such as -15, 0, and $20^{\circ}C$, it was found that most of the debonding occurred at the interface between the tack coat and RCC surface. On the other hand, the interface between the HMA and tack coat was weaker than that between the tack coat and RCC at a high temperature of $40^{\circ}C$. CONCLUSIONS : This study suggested an optimal application rate of tack coat materials to apply to RCC-base composite pavement. The bond strengths at high temperatures were significantly lower than the required bond (tensile) strength of 0.4 MPa. It was known that the temperature was a critical factor affecting the bond strength at the interface of the RCC-base composite pavement.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.3
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• pp.1059-1065
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• 2010

Very-early strength latex-modified concrete (VES-LMC) was developed for the purpose of fast-track overlay of a concrete bridge deck under heavy traffic, concentrated on the workability, durability, and strength gain so that it can be opened to the traffic only three hours after its placement. The mixture of VES-LMC might accompany very high heat of hydration at early-age because of its inherent rapid hardening property and could have susceptibility to autogenous shrinkage because of its relatively low water-cement ratio. This study evaluated the effect of the latex-cement ratio(L/C) both of the constant and variable slumps on the autogenous shrinkage of VES-LMC by carrying out simple temperature rise test and early-age shrinkage experiment. Test results are as follows: The latex contributes on the enhancement of the concrete durability but has little effect on its hydration and the accompanied heat of hydration in VES-LMC. Autogenous shrinkage increased with the increase in latex-cement ratio at variable slumps and its pattern followed regularly a logarithmic increase. However, the influence of water-cement ratio and latex-cement ratios for the test specimens at constant slump on early-age autogenous shrinkage property was found to be minor due to the simultaneous effect of the two experimental variables.