• International Journal of Concrete Structures and Materials
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• 제6권4호
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• pp.231-237
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• 2012

Recently, it has been difficult to get natural sand for concrete due to an insufficient supply in Korea. Crushed sand was thought as a substitute and previous research has been focused on low fluidity and normal compressive strength (24-30 MPa). Study on high performance concrete using crushed sand is hardly found in Korea. In this study it was investigated that the effect of the crushed sand on fluidity and compressive strength properties of high performance concrete. Blending crushed sand (FM: 3.98) produced in Namyangju, Kyunggido and sea sand (FM: 2.80) produced in Asan bay in Chungnam. The final FMs of fine aggregate were 3.50, 3.23, and 3.08. W/B was set as 0.25 to get high performance. With the test results an analysis of relationship was performed using a statistical program. It was shown that strength property of concrete using crushed aggregate at the very early age or after specific time was mainly affected by strength development properties of binders instead of the crushed sand.

• 한국건설순환자원학회논문집
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• 제2권1호
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• pp.66-73
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• 2014

Currently many researches have been performed for enhancing durability of concrete. Rice husk ash has several advantages like early strength of concrete and dense pore structure. A calcium silicate hydrate (CSH) gel around the cement particles due to pozzolanic reaction of rice husk can increase the strength of concrete against cracking. Very limitedly a systematic and detailed investigation on the corrosion performance of rice husk ash and silica fume blended concrete is performed. A realistic approach has been made through compressive strength, bond strength, and split tensile strength etc. Corrosion performance was also evaluated rapid chloride ion penetration test (RCPT) and impressed voltage test, and the results were discussed in the paper.

• Journal of the Korean Wood Science and Technology
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• 제24권1호
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• pp.81-86
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• 1996

This study was performed to evaluate the reduction in bending properties of radiata pine sapwood associated with incipient brown-rot decay. Decayed bending specimens by Tyromyces palustris and Gloeophyllum trabeum for varoious periods were tested destructively. Brown-rot decay by T. palustris and G. trabeum caused serious reduction in bending properties at very early stages of decay, with about 30 percent decrease in bending strength observed for only 1~2 percent weight loss. In general, the reduction in bending properties caused by T. palustris was somewhat greater than that by G. trabeum. Work to maximum load was reduced most severely and rapidly from the onset of decay, while modulus of elasticity showed a much more moderate rate of reduction. Modulus of rupture was affected by decay to a greater extent than was modulus of elasticity. Since a relatively strong correlation between weight loss and bending strength was observed, the residual strength of decayed wood can be predicted by weight loss due to decay. The results of this study indicate that very early stages of brown-rot decay reduce the bending strength significantly. Thus, it is recommended that all load-bearing members in wooden structures, especially those that are periodically wetted, should be inspected regularly to prevent a sudden failure even though there are no definite signs of decay.

• Smart Structures and Systems
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• 제31권1호
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• pp.1-11
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• 2023

Assessment of the compressive strength of concrete plays a major role during formwork removal and in the prestressing process. In concrete, temperature changes occur due to hydration which is an influencing factor that decides the compressive strength of concrete. Many methods are available to find the compressive strength of concrete, but the maturity method has the advantage of prognosticating strength without destruction. The temperature-time factor is found using a LM35 temperature sensor through the IoT technique. An experimental investigation was carried out with 56 concrete cubes, where 35 cubes were for obtaining the compressive strength of concrete using a universal testing machine while 21 concrete cubes monitored concrete's temperature by embedding a temperature sensor in each grade of M25, M30, M35, and M40 concrete. The mathematical prediction model equation was developed based on the temperature-time factor during the early age compressive strength on the 1^st, 2^nd, 3^rd and 7^th days in the M25, M30, M35, and M40 grades of concrete with their temperature. The 14^th, 21^st and 28^th day's compressive strength was predicted with the mathematical predicted equation and compared with conventional results which fall within a 2% difference. The compressive strength of concrete at any desired age (day) before reaching 28 days results in the discovery of the prediction coefficient. Comparative analysis of the results found by the predicted mathematical model show that, it was very close to the results of the conventional method.

• Structural Engineering and Mechanics
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• 제45권6호
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• pp.837-851
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• 2013

In construction industry, strength is a primary criterion in selecting a concrete for a particular application. The concrete used for construction gains strength over a long period of time after pouring the concrete. The characteristic strength of concrete is defined as the compressive strength of a sample that has been aged for 28 days. Neither waiting for 28 days for such a test would serve the rapidity of construction, nor would neglecting it serve the quality control process on concrete in large construction sites. Therefore, rapid and reliable prediction of the strength of concrete would be of great significance. On this backdrop, the method is proposed to establish a predictive relationship between properties and proportions of ingredients of concrete, compaction factor, weight of concrete cubes and strength of concrete whereby the strength of concrete can be predicted at early age. Multiple regression analysis was carried out for predicting the compressive strength of concrete containing Portland Pozolana cement using statistical analysis for the concrete data obtained from the experimental work done in this study. The multiple linear regression models yielded fairly good correlation coefficient for the prediction of compressive strength for 7, 28 and 40 days curing. The results indicate that the proposed regression models are effectively capable of evaluating the compressive strength of the concrete containing Portaland Pozolana Cement. The derived formulas are very simple, straightforward and provide an effective analysis tool accessible to practicing engineers.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2005년도 추계 학술발표회 제17권2호
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• pp.781-784
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• 2005

Concrete structures often present volumetrical changes particularly due to thermal and moisture related shrinkages. Volumetric instability is detrimental to the performance and durability of concrete structures because structural elements are usually restrained. These restrained shrinkages develope tensile stresses which often results in cracking in combination with the low fracture resistance of concrete. Early-age defects in high-performance concrete due to thermal and autogenous deformation shorten the life cycle of concrete structures. Thus, it is necessary to examine the behavior .of early-age concrete at the stages of design and construction. The purpose of this study was to propose a shrinkage models of VES-LMC (very-early strength latex-modified concrete) at early-age considering thermal deformation and autogenous shrinkage.

• 한국방재학회 논문집
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• 제10권2호
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• pp.47-53
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• 2010

초속경 LMC(Very-Early Strength Latex-Modified Concrete ; VES-LMC)는 작업성, 강도발현 및 장기 내구성 측면에서 콘크리트 타설 후 3시간 이내에 교통개방을 가능하게 할 목적으로 개발되었고, 교량바닥판 긴급보수 공사에 유용하게 사용되고 있다. 그러나 초속경 LMC에서 라텍스의 사용이 콘크리트의 내구성은 향상시키지만 다량의 갇힌 공기를 양산한다는 단점이 있다. 따라서 취약한 공극구조를 개선하기 위해서는 특별한 방법이 필요하며, 본 논문에서는 울트라파인 플라이 애시가 사용되었다. UFFA VES-LMC의 공기량은 재령 1일에 포졸란 반응의 대부분이 발생하여 재령 1일 공기량이 경화전 공기량보다 저하되었다. 또한 재령 28일 공기량은 재령 1일 공기량의 공극구조와 동일하게 나타나, 포졸란 반응에 의해 콘크리트의 전체 공기량이 감소함을 확인하였다. 수산화칼슘의 첨가는 200 ${\mu}m$이하 크기의 연행공기를 현저하게 증가시키고, UFFA를 15%이상 사용하면 간격계수를 200 ${\mu}m$ 이하까지 확보할 수 있어 VES-LMC의 공극구조 개선에 매우 효과적인 것으로 나타났다.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1999년도 봄 학술발표회 논문집(I)
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• pp.631-637
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• 1999

Effect of excessive addition of water reducing agent was examined by concrete tests. Water reducing agents of lignin or naphtalene base were used in the experiment. Setting of concrete was retarded according to the increase of dosage of water reducing agent and was extremely delayed at the 3 times or more use of lignin base agent and near 5 times or more use of naphtalene base agent respectively. When water reducing agent was used more than 6 times of standard dosage, early strength of concrete was very low and the strength reduction was very severe at all test ages in the concrete using lignin base agent.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2006년도 춘계 학술발표회 논문집(II)
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• pp.545-548
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• 2006

This study has been carried out to examine the properties of concrete according to replacement ratio and curing method of fly ash, in order to increase utilization of it. As the result of experiments, the 7 days of early age strength presented around 20MPa, up to 20% of replacement ratio, which is almost the same strength as non-replacement. However, when the replacement ratio was 30%, the strength was decreased to 16MPa, as 20% reduction compared to the non-replacement condition. In 365 days of long term aging, the strength was 5% higher, up to 20% of the replacement ratio, due to the pozzolanic reaction of fly ash. When the replacement ratio was 30%, it presented similar strength development as the non-replacement condition. Steam curing and autoclave curing increased the short age strength, regardless of the replacement ratio of fly ash; however, they don't have an effect on increasing the 365 days of long term strength. Water curing showed high strength development after 28 days, 51.81MPa, which is around 30% higher than air curing, 38.9MPa, steam curing, 38.6MPa, and autoclave curing, 39MPa. Therefore, water curing was examined as one of the very effective curing methods for developing long term strength of concrete.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2001년도 가을 학술발표회 논문집
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• pp.665-668
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• 2001

Generally, according to rising of atmospheric temperature, a consistency of concrete decreases, and a slump property of concrete is changed to be large. Also, in the strength development of concrete, the strength development rate of long-term age(28day) in comparison to strength of early age(7day) and the absolute compressive strength decreases. Accordingly, in this study, experiments about quality evaluation of concrete utilizing Ordinary Portland Cement is carried out. As a result of experiments, there were a conspicuous change in slump of concrete due to temperature increase. In conclusion, the rising of atmospheric temperature was very important factor in affecting the quality fluctuation of hot weather concrete.