• Journal of the Korean Recycled Construction Resources Institute
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• v.9 no.1
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• pp.66-74
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• 2021

The objective of this study is to evaluate the practical application potential and limitations of the modified fly ash(MFA) by vibration grinding as a partial replacement of ordinary portland cement(OPC). The test parameters investigated were the replacement level of fly ash(FA) and FA for OPC, varying from 10% to 40%, and curing temperatures of 5, 20, and 40℃. The various characteristics(including slump, air content, bleeding, setting time, compressive strength development, and hydration products) of MFA concrete were measured and then compared with those of the concrete with conventional FA. Test resul ts showed that the MFA prefers to FA in reducing the bl eeding of fresh concrete and enhancing the compressive strength gain at an early age. The compressive strength ratios between MFA and FA concrete specimens at an age of 1 day were 135%, 146%, and 111% at the curing temperatures of 5, 20, and 40℃, respectively. The corresponding ratios at an age of 28 days were approximately 110%, regardless of the curing temperatures. The X-ray diffraction analysis also revealed less calcium hydroxide products in MFA pastes than in FA pastes.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2020.06a
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• pp.206-207
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• 2020

Accurate estimation of concrete strength development at early ages is a critical factor to secure structural stability as well as to speed up the construction process. The temperature generated from the heat of hydration is considered as a key parameter in predicting the early age strength. Conventionally, concrete temperature has been measured by temperature sensors installed inside concrete. However, considering the measurement on building structures with multiple floors, this method requires reinstallation and repositioning of hardware such as sensors, data loggers and routers for data transfer. This makes the temperature monitoring work cumbersome and inefficient. Concrete temperature monitoring by using thermal remote sensing can be an effective alternative to supplement those shortcomings. In this study, image processing was carried out through K-means clustering technique, which is a unsupervised learning method, and the classification results were analyzed accordingly. In the future, research will be conducted on how to automatically recognize concrete among various objects by using deep learning techniques.

• Journal of the Korea Concrete Institute
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• v.16 no.3 s.81
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• pp.327-334
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• 2004

In this paper, method for the determination of removal time of the side forms in high strength concrete are discussed using the estimation model of compressive strength development, the development of bond strength and rebound number of P type Schmidt hammer in order to review the validity of existing regulation as to side form removal and offer effective quality control method. According to the results, as W/B increases by $10\%$, the setting time is shortened by about 2 hours. In the scope of the paper, required time to gain 8MPa of compressive strength is determined about 17 ${\~}$20 hours of age and $21{\~}25^{\circ}D{\cdot}D$ of maturity. Bond strength between form and concrete shows the highest value around final setting time, but decreases drastically after that. Amount of concrete sticking on the form is large before setting completed, but after that, its amount shows decline tendency. The rebound value test with P type schmidt hammer can be started faster by 2${\~}$3 hours than compressive strength test. It is also confirmed that the removal of forms is possible when the rebound value of P type schmidt hammer is more than 32. It is found from the results that existing regulation regarding removal time of the side form of high strength concrete provided in KCI needs no revision because required time to gain the strength provided in KCI has no adverse effect on strength development at early age and surface condition during stripping the side form. Effective procedure to decide the removal time of side form can be performed by applying P type Schmidt hammer.

• Journal of the Korean Society of Safety
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• v.24 no.6
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• pp.103-110
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• 2009

Since the mechanical properties of cement-based materials are time-dependent due to the prolonged cement hydration process, those of fiber reinforced concrete(FRC) may also be time-dependent. Toughness is one of important properties of FRC. Therefore, it should be investigated toughness development of FRCs with curing ages to fully understand the time-dependent characteristics of FRCs. To this end, the effect of curing ages on flexural toughness development of steel fiber reinforced concrete is studied. Three point bending test with notched beam specimen was adapted for this study. Hooked-end steel fiber(DRAMIX 40/30) was used as a fiber ingredient to investigate w/c ratio and fiber volume fraction effect on toughness development during curing. Three different water-cement ratios(0.44, 0.5 and 0.6) and fiber volume fractions(0%, 0.5% and 1%) were used as influence factors. Each mixture specimens were tested at five different ages, 0.5, 1, 3, 7 and 28 days. The study shows that flexure toughness development with age is quite different than other concrete material properties such as compressive strength. The study also shows that the toughness development trend correlates more closely to water/cement ratio than to fiber volume fraction.

• Journal of the Korea Concrete Institute
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• v.23 no.2
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• pp.219-224
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• 2011

In this study, a piezoelectric smart sensor that can be embedded inside of concrete structures is developed to investigate the early stage of concrete curing. A waterproof coating is used to protect the piezoelectric sensor from moistures of concrete mixture. Also, a mortar case is utilized to encapsulate the sensor to protect it from impact loads. To estimate the strength of concrete, a self-sense guided-wave actuated sensing technique is applied. In the guided wave, its velocity is varied according to the mechanical properties of concrete such as modulus of elasticity. Because modulus of elasticity directly affects the strength of concrete, the guidedwave's velocity also affects the concrete strength development. To verify the feasibility of using the proposed approach, the smart sensor was embedded into a 100MPa concrete cylinder and the self-sense guided wave is continuously measured throughout the curing process. The measurements showed that the propagation time (TOF) of the measured guided waves gradually decreased as the curing age increased. Especially, at the early age of the curing process, the variation of the TOF was very significant. Furthermore, the results showed that there is a linear relationship between the TOF of the self-sense guided waves and the strength of concrete existed. It is safe to conclude that the proposed approach can be used very effectively in monitoring of the strength development of high strength concrete structures.

• Journal of the Korea Institute of Building Construction
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• v.19 no.1
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• pp.59-66
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• 2019

In this study, the effects of blast furnance slag cement and frost-resistant accelerator on shrinkage properties and shrinkage properties of mortar were examined. As a result, the addition of the frost-resistant accelerator to both OPC and BB has a small effect on the flash properties of mortar and the compressive strength increases from the early ages. In addition, when a frost-resistant accelerator is used in excess of the standard usage amount, it is necessary to examine the relationship of the expansion behavior at the early age, especially, between the compressive strength development and the expansion property. And it was confirmed that the addition of the frost-resistant accelerator tended to increase the shrinkage of mortar using the OPC and BB. With the addition of the frost-resistant accelerator, the amount of pores with a diameter of under the 30nm, especially, the amount of pores with a diameter of 20 to 30nm and the amount of pores with an ink-bottle decrease, and the shrinkage increases. And it is considered that a change in the amount this range of pores has a large effect on the shrinkage property.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.3
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• pp.79-87
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• 2011

Concrete incorporating high volume blast-furnace slag placed in cold weather regions might be in danger of initial frost damage because dependently on the mix proportions, the setting and the hardening would be remarkably delayed. Therefore, this study investigated to effect of the degree of frost on the strength development and the resistance to freezing and thawing of the concrete incorporating blast-furnace slag when being subjected to freeze at early age. As the experimental results, the concrete incorporating blast-furnace slag attacked by initial frost damage showed the remarkable reduction of both the compressive strength development and the resistance to freezing and thawing. Especially, the resistance to freezing-thawing of the concrete incorporating high volume blast-furnace slag became much lower than that of the normal concrete.

• Proceedings of the Korea Concrete Institute Conference
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• 2008.11a
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• pp.617-620
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• 2008

When fresh concrete is exposed to the cold weather, the concrete may happen frost damage because of freezing at early ages and the strength development may be go down. Therefore, this paper is intend to investigate the properties and availabilities of cold weather concrete using anti-freeze based formate-acetate for ensure of excellent concrete quality on cold temperature. According to test result for concrete in anti-freeze agent, strength properties in sub-zero temperature increases in comparison with concrete of non anti-freeze agent without decline of strength until later age. When anti-freezing agent is added by 3.0% to binder, it gives good effects on the performance of the fresh and harden concrete.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2017.05a
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• pp.230-231
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• 2017

The addition of a limestone filler(LF) to fill into the voids between cement and aggregate particles can reduce the cementitious paste volume. This paper aim to evaluate the influence of LF contents on the hydration kinetics and compressive strength. Hydration kinetics were evaluate using heat of hydration, ignition loss and thermal analysis. The heat of hydration was measured using Isothermal Calorimetry. The degree of hydration was measured using ignition loss. Hydration product analysis was carried out by Thermal Gravimetric and Differential Thermal Analysis. The results show that the addition of LF reduces not only the initial setting time and heat of hydration peak, also degree of hydration and rate of strength development at early age increase with the addition of LF. It can be concluded the LF fills the pore between cement particles due to formation of carboaluminate, which may accelerate the setting of cement pastes.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2012.11a
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• pp.47-49
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• 2012

Under this study, the characteristics of concrete intensity condition following the curing method under the extremely low temperature environment have been contemplated, and as a result, in the event of insulation + heat cable curing, the intensity and accumulated temperature accomplishment period is required for two times of requiring initial frost damage prevention than the case of heating + heat insulation curing method due to the insufficient calories supplied in general.