• Journal of the Korea Concrete Institute
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• v.25 no.1
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• pp.73-80
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• 2013

Polymer cement slurry (PCS) made from organic polymer dispersion and cement has good adhesion to steel, waterproofness and acid resistance due to being of polymer films formed in cement slurry. The purpose of this study is to evaluate the mix design conditions at early curing age of PCS-coating material effected on improvement in bond strength of coated rebar. The test pieces are prepared with two types of polymer dispersions such as St/BA and EVA, four polymer-cement ratios, two types of cement, four coating thicknesses and three curing ages, and tested for the bond strength test. From the test results, in general, bond strength of PCS-coated rebar is better than that of uncoated rebar and epoxy-coated rebar. It is also high bond strength at curing ages of 7-day or less, and coating thicknesses of $75{\mu}m$ and $100{\mu}m$. The maximum bond strength of PCS-coated rebar at curing age of 3-hour is almost same as that of curing age of 1-day and 7-day. The maximum bond strength of PCS-coated rebar with ultra high-early strength cement and St/BA at polymer-cement ratio of 80%, and coating thickness of $100{\mu}m$ is about 1.52 and 1.58 times respectively, the strength of uncoated rebar and epoxy-coated rebar. It is apparent that the curing age, coating thickness, type of polymer and cement are very important factors to improve the bond strength of PCS-coated rebar to cement concrete. We can have basic information that PCS-coated rebar with curing age at 3-hour and coating thickness of $100{\mu}m$ can replace epoxy-coated rebar.

• Journal of the Korea Institute of Building Construction
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• v.12 no.4
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• pp.369-376
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• 2012

In this study, to reduce the hydration heat velocity (HHV) of high-strength mass concrete at early ages, phase change materials (PCM) that could absorb hydration heat were applied, and the changes in autogenous shrinkage were investigated, as well as the relationship between the hydration temperature and autogenous shrinkage. The acceleration of the cement hydration process by the PCM leads to an early setting and a higher development of the compressive strength and elastic modulus of concrete at very early ages. The function of PCM could be worked below the original melting point due to the eutectic effect, while the hydration temperature and HHV of high-strength mass concrete can be decreased through the use of the PCM. A close relationship was found between the hydration temperature and autogenous shrinkage: the higher the HHV, the greater the ultimate autogenous shrinkage.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.645-648
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• 2008

In this study, to evaluate the correlation between hydration heat and autogenous shrinkage of high strength concrete in early age, statistical method present numerically hydration heat and autogenous shrinkage was studied. First of all, hydration heating velocity and autogenous shrinking velocity as quantitative coefficients which represent the main properties of hydration heat and autogenous shrinkage were proposed. Two coefficients were calculated by statistical analysis and were equal with the regression coefficient. To verify the validity of the proposed statistical analysis method, data of hydration heat and autogenous shrinkage gathered by a real experiment were analyzed by it. In results, properties of hydration heat and autogenous shrinkage of high strength concrete in early age were analyzed quantitatively. Also evaluation and comparison of the correlation between hydration heat and autogenous shrinkage with numerical value were possible.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.29 no.4A
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• pp.385-393
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• 2009

This study grasped the relationship between relative humidity in concrete and concrete shrinkage followed by pre-absorbed water of porous lightweight aggregates through measurements of concrete shrinkage and humidity and comparisons with established research results. It was showed that shrinkage reduction effect of lightweight concrete is 36% at 7 days early ages and 25% at 180 days long-term ages when water-binder ratio is 0.3. It also showed that shrinkage reduction effect is 19% at 7 days and 16% at 180 days when water-binder ratio is 0.4 and 37%, 32% when water-binder ratio is 0.5. The moisture supply effect of lightweight aggregates was remarkable at early age within 7~10 days irrespective of water-binder ratio. In case of waterbinder ratio is 0.3, the relationship between shrinkage and internal humidity of concrete has been underestimated regardless of applied existing model type and in case of water-binder ratio is 0.4, 0.5, measurement values are relatively similar with existing model equations. Finally this study did regression analyses about the relation among the humidity change and the shrinkage strain as a high-degree polynomial and derived parameters that can connect moisture movement analysis with differential shrinkage analysis in case of considering relative humidity at the time by moisture movement analysis of concrete.

• Journal of the Korea Concrete Institute
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• v.17 no.3 s.87
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• pp.313-322
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• 2005

Recently, as the remodeling market gradually substitute for new construction market and safety diagnosis for reconstruction apartment become a matter of principal Interest, it is demanded that scientific diagnosis and evaluation for existing concrete structure state. And it is increasing that the significance for reliability of data which is used for estimating the concrete compressive strength by nondestructive test. As a result, it is found that different proposal to material age and hitting angle is good to improving the reliability of presumption of concrete compressive strength in the linear hitting rebound test hammer. And for the reason that mutual relation between the compressive strength and rebound degree is highest in linear hitting rebound test hammer 25mm in all portion according to early md middle material age and hitting angle except the early material age $-45^{\circ}$, analysis showed that linear hitting rebound test hammer is more reliable than existing schmidt hammer in presumption of concrete compressive strength.

• Proceedings of the Korea Concrete Institute Conference
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• 2009.05a
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• pp.89-90
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• 2009

FBG temperature sensor and strain sensor has been used to monitoring shrinkage and temperature of concrete retaining wall in construction site in its casting early age. The test results indicate that this monitoring method is a practical method for monitoring concrete at very early age. The monitoring technique used in this research could be extended to monitor shrinkage and temperature for mass concrete structure.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.05a
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• pp.55-56
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• 2023

Recently, large and high-rise buildings are increasing, and accordingly, concrete weight reduction is required. Lightweight aggregate concrete can provide economic feasibility and large space, but safety can be reduced due to problems such as low strength and poor durability. Since the development of such low strength of concrete is important in the early construction stage, it is necessary to evaluate the vertical formwork demolding period at the early age. The correlation was analyzed by measuring the compressive strength and ultrasonic pulse velocity. As a result, the ultrasonic pulse rates of normal and lightweight aggregate concrete at the time of 5 MPa expression, which is the time of vertical mold deformation, were 3.07 km/s and 2.77 km/s for W/B 41, and 2.89 km/s and 2.73 km/s for W/B 33.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2023.05a
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• pp.409-410
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• 2023

In this study, the compressive strength and ultrasonic pulse velocity were evaluated according to the elapsed time on concrete mixed with normal and lightweight aggregates at early age. For evaluation in various strength ranges, the design compressive strength was set to 30, 45, and 60MPa and evaluated. As a result of the experiment, the compressive strength of concrete mixed with lightweight aggregates developed 5MPa earlier compared to normal aggregate concrete, and the UPV showed a similar tendency.

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.5
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• pp.19-26
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• 2014

Concrete guide rail and median barrier are an attached RC member, however they are vulnerable to cracking due to slip form construction and large surface of member. In this study, causes and pattern of cracking are analyzed through assessment and NDT (Non-Destructive Technique) evaluation for concrete guide rail and median barrier on highway structure. For this work, analysis on drying shrinkage and hydration heat are performed considering installation period, and plastic shrinkage is also analyzed considering their environmental conditions. From the evaluation, plastic settlement around steel location, drying/ plastic shrinkage, and aggregate segregation are inferred to be the main causes of cracking in the structures. The crack causes and patterns are schematized and techniques of crack-control are suggested. Furthermore concrete guide rail/ median barrier in the bridge on the sea are vulnerable to cracking at early age so that special attentions should be paid at the stages of material selection and construction.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.641-644
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• 2008

The strength of concrete is developed by cement hydration reaction influenced by the circumferential temperatures. In this study, therefore, the experiments are conducted and evaluated about the characteristics as changes of early concrete placing temperature and curing temperature to understand the effects of the temperature which influences concrete properties. The results of the experiments changing the early concrete placing temperature in 5$^{\circ}C$ and 10$^{\circ}C$ are followed. In case of conducting standard concrete curing, early compressive strength development rate of the concrete which had low placing temperature was low, but it was shown that early compressive strength development rate was not affected by low placing temperature in age 28 days of concrete. In case of conducting outdoor curing in winter, early compressive strength development rate of the concrete which had high placing temperature was high in all test specimens. As a results, early compressive strength development of concrete was influenced by temperature of early concrete, but after aging 28 days of concrete, it was influenced by curing temperature rather than temperature of early concrete.