• 한국전산구조공학회:학술대회논문집
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• 한국전산구조공학회 2006년도 정기 학술대회 논문집
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• pp.97-103
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• 2006

A modified one-sided measurement technique is proposed for Rayleigh wave (R-wave) velocity measurement in concrete. The scattering from heterogeneity may affect the waveforms of R-waves in concrete, which may make the R-waves dispersive. Conventional one-sided techniques do not consider the scattering dispersion of R-waves in concrete. In this study, the maximum energy arrival concept is adopted to determine the wave velocity by employing its continuous wavelet transform. Experimental study was performed to show the effectiveness of the proposed method. The present method is applied to monitor the strength development of early-age concrete. A series of experiments were performed on early-age concrete specimens with various curing conditions. Results reveal that the proposed method can be effectively used to measure the R-wave velocity in concrete structures and to monitor the strength development of early-age concrete.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 춘계 학술발표회 논문집(II)
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• pp.469-472
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• 2006

Amount used of admixture fare is increasing for concrete economic efficiency and ability security. Security of economic efficiency and ability can expect by use of this admixture fare but is displaying a lot of problems on early age strength hold by hydrate delay relatively. Specially, in the case of construction site, concrete strength can speak that interrelation is high with mold removal of forms time. Therefore, is economical and need examination of plan that can secure robber within 3 days using admixture fare such as fly ash and blast furnace slage differential speech to secure function. In this study, adding liquid admixture within 1% of a binding agent quantity to examine these problem with physical characteristic after hardening ago specially, strength change at standard and air dry curing of observed change in priority. Air dried and water curing total strength enhancement effects appeared by thing which is in case of add test result liquid admixture by below 1% and strength deputy by passage of age could know is not big.

• 한국환경과학회지
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• 제19권1호
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• pp.105-114
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• 2010

The objective of the paper is to experimentally investigate the compressive strength of the concrete incorporating fly ash. Ordinary Portland cement(OPC). Water to binder ratio(W/B) ranging from 30% to 60% and curing temperature ranging from $-10^{\circ}{\sim}65^{\circ}C$ were also adopted for experimental parameters. Fly ash was replaced by 30% of cement contents. According to the results, strength development of concrete contained with fly ash is lower than that of plain concrete in low temperature at early age and maturity. In high curing temperature, the concrete with fly ash has higher strength development than that of low temperature regardless of the elapse of age and maturity. Fly ash can have much effect on the strength development of concrete at the condition of mass concrete, hot weather concreting and the concrete products for the steam curing.

• Nuclear Engineering and Technology
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• 제44권5호
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• pp.523-534
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• 2012

The objective of this study was to experimentally investigate the effect of heat curing methods on the temperature history and strength development of slab concrete exposed to $-10^{\circ}C$. The goal was to determine proper heat curing methods for the protection of nuclear power plant structures against early-age frost damage under adverse (cold) conditions. Two types of methods were studied: heat insulation alone and in combination with a heating cable. For heat curing with heat insulation alone, either sawdust or a double layer bubble sheet (2-BS) was applied. For curing with a combination of heat insulation and a heating cable, an embedded heating cable was used with either a sawdust cover, a 2-BS cover, or a quadruple layer bubble sheet (4-BS) cover. Seven different slab specimens with dimensions of $1200{\times}600{\times}200$ mm and a design strength of 27 MPa were fabricated and cured at $-10^{\circ}C$ for 7 d. The application of sawdust and 2-BS allowed the concrete temperature to fall below $0^{\circ}C$ within 40 h after exposure to $-10^{\circ}C$, and then, the temperature dropped to $-10^{\circ}C$ and remained there for 7 d owing to insufficient thermal resistance. However, the combination of a heating cable plus sawdust or 2-BS maintained the concrete temperature around $5^{\circ}C$ for 7 d. Moreover, the combination of the heating cable and 4-BS maintained the concrete temperature around $10^{\circ}C$ for 7 d. This was due to the continuous heat supply from the heating cable and the prevention of heat loss by the 4-BS. For maturity development, which is an index of early-age frost damage, the application of heat insulation materials alone did not allow the concrete to meet the minimum maturity required to protect against early-age frost damage after 7 d, owing to poor thermal resistance. However, the combination of the heating cable and the heat insulating materials allowed the concrete to attain the minimum maturity level after just 3 d. In the case of strength development, the heat insulation materials alone were insufficient to achieve the minimum 7-d strength required to prevent early-age frost damage. However, the combination of a heating cable and heat insulating materials met both the minimum 7-d strength and the 28-d design strength owing to the heat supply and thermal resistance. Therefore, it is believed that by combining a heating cable and 4-BS, concrete exposed to $-10^{\circ}C$ can be effectively protected from early-age frost damage and can attain the required 28-d compressive strength.

• 콘크리트학회논문집
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• 제12권6호
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• pp.67-74
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• 2000

The usage of nondestructive testing on early-aged concrete leads to enhacned safty and allows effective scheduling of construction, thus making it possible to maximize the time and cost efficiencies. In this study, a reliable nondestructive strength evaluation method for early-aged concrete using the longitudinal wave velocity is proposed. Compression tests were performed to examine factors influencing the velocity-strength relationship of concrete, such as water-cement (w/c) ratio, fine aggregate ratio, curing temperature, and curing condition. The test results show that a change in the w/c ratio and curing temperature has minor effect on the velocity-strength relationship/ However, curing condition significantly influences the velocity-strength relationship of early-aged concrete. Moreover, the longitudinal wave velocity increases with decreasing fine aggregate ratio. It is concluded from this study that the strength evaluation of early-age concrete can be achieved by a nonlinear equation which considers the effects of curing condition and fine aggregate ratio.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2021년도 가을 학술논문 발표대회
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• pp.103-104
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• 2021

This study attempted to confirm the possibility of estimating condensation time and initial compressive strength with five types of estimation needles in the existing Durometer D type. In order to determine the surface finishing operation time and develop a method for estimating the initial age compression strength, an estimation needle capable of complexly measuring the estimation time and the initial age compression strength based on the Durometer D type was derived as 1.5, 2.0mm.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2009년도 춘계 학술논문 발표대회 학계
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• pp.53-56
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• 2009

This study reviewed the characteristics of concrete made of performance improving mixture materials based on KOH as a means to resolve the problems of initial quality reduction that result in concretes with blast furnace slag powder. Summarizing the results, first as the characteristics of fresh concrete, liquidity was found to reduce in general with increased BS substitution ratio. Objective range of liquidity was not satisfied in all mixes according to the use of performance improving mixture materials. Air capacity was satisfied to the objective range in all mixes. As the characteristics of hardened concrete, while compressive strength showed a decreasing trend with increasing BS substitution ratio at early age, increasing trend was shown by the plain with increasing BS substitution ratio at later age. On the other hand, K1 and K2 were only effective among mixture materials at early age, but K1F30 showed excellent strength at both early and later ages.

• 콘크리트학회논문집
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• 제22권2호
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• pp.237-245
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• 2010

최근 장스팬 구조물의 건설이 증가하면서 슬래브의 시공도중 처짐 문제가 중요한 이슈 중의 하나로 대두되고 있다. 특히, 건축물의 경우 골조공기를 단축하기 위하여 콘크리트의 소요강도가 발현되기 이전에 거푸집을 조기 탈형함으로써 구조물의 품질저하를 초래하고 있다. 이러한 시공하자를 없애고 공기단축을 실현하기 위해서는 콘크리트의 조기 재령에서의 강도 및 강성 발현에 관한 연구를 수행하여 재령에 따른 강도 및 탄성계수 발현 모델의 구축이 필요하다. 기존의 설계기준에서 제시하고 있는 콘크리트 모델은 미성숙단계인 조기재령 콘크리트의 역학적 특성을 제대로 반영하지 못한다는 연구결과들이 있다. 따라서, 이 연구에서는 조기 재령에서의 재령에 따른 콘크리트의 재료실험을 수행하였다. 실험 결과를 바탕으로 기존 국내외 설계기준에서 제시하고 있는 강도 및 탄성계수 제안식의 적합성을 평가하며, 최종적으로 조기재령에서의 콘크리트 강도와 강성간의 발현 관계 모델을 구축함으로써, 향후 공기단축 공법의 개발 및 슬래브 처짐에 따른 하자를 방지할 수 있는 초기 재료 모델을 제시하도록 하였다. 재료실험은 재령 1, 3, 7, 14, 28일에 실시하였으며, 총 159개의 실린더형 공시체의 실험 결과를 분석하여 콘크리트의 조기재령에서 압축강도와 탄성계수 간의 상관관계를 제안하였다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2005년도 봄학술 발표회 논문집(II)
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• pp.309-312
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• 2005

Admixture compounds for shortening setting time and accelerating early strength development of EVA polymer powder-modified mortars were made by mixing various quick setting agents. As a result, the quick setting agents contribute to strength development of the mortars in the early curing age of 168h or less. In the viewpoint of early strength development of EVA polymer powder-modified mortars, an quick setting agent content of 20$\%$ is recommended. Early strength of EVA polymer powder-modified mortars expresses the excellent strength with 5$\%$, 10$\%$ of rates of polymer mixing. The rate of polymer mixing was able to be adjusted and flexural strength which is a predeterminded initial strength was also able to satisfy 3MPa(s).

• Computers and Concrete
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• 제20권2호
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• pp.215-227
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• 2017

A finite element model (FEM) for predicting early-age behavior of reinforced concrete (RC) bridge deck slabs with fiber-reinforced polymer (FRP) bars is presented. In this model, the shrinkage profile of concrete accounted for the effect of surrounding conditions including air flow. The results of the model were verified against the experimental test results, published by the authors. The model was verified for cracking pattern, crack width and spacing, and reinforcement strains in the vicinity of the crack using different types and ratios of longitudinal reinforcement. The FEM was able to predict the experimental results within 6 to 10% error. The verified model was utilized to conduct a parametric study investigating the effect of four key parameters including reinforcement spacing, concrete cover, FRP bar type, and concrete compressive strength on the behavior of FRP-RC bridge deck slabs subjected to restrained shrinkage at early-age. It is concluded that a reinforcement ratio of 0.45% carbon FRP (CFRP) can control the early-age crack width and reinforcement strain in CFRP-RC members subjected to restrained shrinkage. Also, the results indicate that changing the bond-slippage characteristics (sand-coated and ribbed bars) or concrete cover had an insignificant effect on the early-age crack behavior of FRP-RC bridge deck slabs subjected to shrinkage. However, reducing bar spacing and concrete strength resulted in a decrease in crack width and reinforcement strain.