• Journal of the Korea Institute of Building Construction
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• v.15 no.1
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• pp.25-33
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• 2015

In this study, initial crack index was evaluated by FEM analysis to find the crack propagation from hydration heat in precast concrete. As results, as the usage of hardening accelerator increased, initial compressive strength increased and setting time was shortened. Additionally, as amounts of hardening accelerators increased, the central temperature of concrete increased and the time to reach the highest temperature was shortened. It was demonstrated that the hardening accelerators accelerated the hydration reaction of cement, and caused the increase of hydration heat within the short period of time. Furthermore, the crack index for evaluating the heat level was performed by FEM. As results, there was no problem about the cracks, despite of the growth of initial high hydration heat. This is because of the increased tensile strength that is large enough to sustain the thermally induced-stress.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2006.11a
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• pp.5-8
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• 2006

This study investigates the field application, Songdo the # 1st WORLD, on zero-crack construction of the fundamental mass concrete using double bubble sheets and applying low heat mixture. Experimental results of hydration heat analysis showed that crack modulus of concrete incorporating 20% of blast furnace slag cement was 1.0 in 120 hours, representing 50% probability of crack occurrence, thus requiring additional measures. As for a curing method, a specimen insulating two layers of vinyl chloride+double bubble sheets exhibited only $16.5^{\circ}C$ difference between upper and lower sections, and it also showed favorable workability as well as competitive economic side. Therefore it was determined to use it for curing method in this field. For the curing results of practical field, using 2 layers of vinyl chloride+double bubble sheets and applying low heat mixture on the fundamental mass concrete in 3A residential building exhibited less than only $15^{\circ}C$ difference between surface and center section of that in 5 days elapse and less than $20^{\circ}C$ in 9 days. This means that the crack by hydration heat was prevented, and other fields structures also resisted the plastic shrinkage by insulating the vinyl, sinking crack by second temping, even drying shrinkage by the mixture of low unit water. Therefore the crack on the placement has not been found so far, since the construction was started before 6 month.

• Journal of the Korea Concrete Institute
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• v.11 no.6
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• pp.3-12
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• 1999

The larger, loftier and more highly strengthened the recent structures become, the greater attention is paid to the problem of thermal crack occurrence associate with hydration heat. As one of methods to solve the problem, a care has been taken to the improvement of construction such as the application of pre-cooling or pipe-cooling, adjustment of concrete block size, concrete placement timing, joint arrangement and so on. But it is expected that a proper selection of cement shall additionally contribute to the control of thermal cracks. In this study, thus, we selected 4 types of cements such as Type V for anti-sulphate, blast furnace cements (slag content of 45% and 65% respectively)and ternary blended low heat cement, and carried out mock-up tests. In every assigned time, temperatures and thermal stresses were measured and calculated from raw data. As a result of measurement, it was found that the magnitude of hydration heat is in order of blast furnace slag cement. Type V and ternary blended low heat cement. Results of thermal stresses were same as the order of temperature. In addition, thermal stresses calculated from the data of strain gauges showed almost similar to those measured from effective stress gauges only when strain values were adjusted properly in accordance with initial time of stress appearance. Theoretical results agreed well with the measured values comparatively, but showed slight differences. It is inferred that these differences shall be reduced if more tests capable of evaluating thermal characteristics of concrete are carried out.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.04a
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• pp.677-680
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• 2008

The surface of particles was energetically modified by inter-grinding OPC and BFS in vibration mill for improvement of the early strength and low-heat evolution of concretes. BFS was pre-grinding in ball-mill to 2535(BS2) and 3245 $cm^2/g$(BS3), in blaine surface area. The inter-grinding time in vibration mill was changed from 10 minutes to 30 minutes. And Mixing ration of BFS to OPC was changed in 60, 70, 80%. After inter-grinding, the change of specific surface area, particle size distribution, hydration heat of cement and compressive strength of mortar were measured. As the result of comparison test with LHC, it was found that the mixture and inter-grinding time satisfying the value of over 100% of compressive strength for 7 days and under 170J/g of heat of hydration for 72 hours. and it was confirmed that the possibility of low heat slag cement utilizing blast furnace slag(BS2, BS3) with the low fineness in high volumes.

• Proceedings of the Korea Concrete Institute Conference
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• 1998.04a
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• pp.199-204
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• 1998

To analysis the failure character of Low-Heat concrete which is used to prevent the thermal crack caused by hydration heat, static loading test was performed by this test method, "Determination of the Fracture Energy of Motar and Concrete by Means of Three-Point Band Tests on Notched Beam" (suggested by RILEM 50-FMC Committe). This study compared and analysised the fracture energy of Mode I (opening mode), the most general pattern in the view of water-cemente ratio(W/C), compressive strength and age of Ordinary Portland Concrete and Low-Heat Concrete under the same mixture. The test results show that the case of Ordinary Portland Concrete and Low-Heat Concrete, low Water-Cemente ratio(W/C) cause the increase of fracture energy, and high failure-strength decrease failure-deflection, and the fracture energy of Low-Heat Concrete is similar to Ordinary Portland Concrete as the age increase. increase.

• Proceedings of the Korea Concrete Institute Conference
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• 1996.10a
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• pp.133-139
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• 1996

As construction technology advances, most of civil engineering structures are becoming larger and taller. Therefore, high strength concrete is necessary for them. For high strength concrete, it needs a large amount of unit cement content and low water-cement ratio inevitably, so that a large amount of heat occurs in concrete. The thermal cracks make the durability and quality of concrete structures become worse, result from temperature rise and thermal stress due to heat of hydration. In this study, the proposal of using ground granulated blast furnace slag, fly ash and chemical admixtures was investigated to decrease the temperature rise of concrete.

• Proceedings of the Korea Concrete Institute Conference
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• 1997.04a
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• pp.357-364
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• 1997

In this experiment high-strength concrete of good workability is made by means of either replacing unit cement quantity to fly-ash by 20% or using low heat cement to reduce the internal hydration heat suggested as the fault of concrete. Concrete is poured through ready mixed concrete to test the ability of applying to the substantial works and is carried out the test of flowing and core strength and record of temperature of imitating specimens. It is capable to apply high-strength concrete replaced to fly-adh 20% to the substantial works through the experiment

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.385-390
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• 2001

The thermal crack in mass concrete is mainly due to the difference of concrete temperature, which is generated by hydration heat of cement. As the thickness of mat foundation increases, the difference of temperature becomes bigger. The purpose of this study is to estimate the optimum placing depth. The temperature of real mat foundation was observed and the thermal analysis by Finite Element Method was executed. Finally, the crack index according to the placing depth was estimated.

• Proceedings of the SAREK Conference
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• 2005.06a
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• pp.128-128
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• 2005

• Journal of the Korea institute for structural maintenance and inspection
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• v.18 no.1
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• pp.84-92
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• 2014

The purpose of study is to analyze mechanism with early high portland cement and hardening accelerator. As the result, it was concluded that hardening accelerator makes accelerates appearance of $Ca(OH)_2$ through experiment using TG-DTA when it hydrates with cement. On the result of compressive strength, as increasing the amount of hardening accelerator used, early compressive strength was improved. Also, as a result of hydration heat, hardening accelerator accelerates hydration of $C_3S$ that is cement's component. On the result of XRD's analyzation, hydration product for each age could be check and it was shown that as increasing the amount of hardening accelerator used, peak point of hydration product was recorded high. As the result of SEM, appearance of C-S-H was shown as the amount of $Ca(OH)_2$'s appearance and each age according to additive contents of hardening accelerator. Therefore hardening accelerator used on this study is effective on getting early compressive strength.