• Title/Summary/Keyword: Early-Age Concrete

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NDT Determination of Cement Mortar Compressive Strength Using SASW Technique

  • Cho, Young-Sang
    • KCI Concrete Journal
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    • v.13 no.2
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    • pp.10-18
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    • 2001
  • The spectral analysis of surface waves (SASW) method, which is an in-situ seismic technique, has mainly been developed and used for many years to determine the stiffness profile of layered media (such as asphalt concrete and layered soils) in an infinite half-space. This paper presents a modified experimental technique for nondestructive evaluation of in-place cement mortar compressive strength in single-layer concrete slabs of rather a finite thickness through a correlation to surface wave velocity. This correlation can be used in the quality control of early age cement mortar structures and in evaluating the integrity of structural members where the infinite half space condition is not met. In the proposed SASW field test, the surface of the structural members is subjected to an impact, using a 12 mm steel ball, to generate surface wave energy at various frequencies. Two accelerometer receivers detect the energy transmitted through the medium. By digitizing the analog receiver outputs, and recording the signals for spectral analysis, surface wave velocities can be identified. Modifications to the SASW method includes the reduction of boundary reflections as adopted on the surface waves before the point where the reflected compression waves reach the receivers. In this study, the correlation between the surface wave velocity and the compressive strength of cement mortar is developed using one 36"x36"x4"(91.44$\times$91.44$\times$91.44 cm) cement mortar slab of 2,000 psi (140 kgf/$\textrm{cm}^2$) and two 36"x36"x4"(91.44$\times$91.44$\times$91.44 cm) cement mortar slabs of 3,000 psi (210 kgf/$\textrm{cm}^2$).

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Characteristics of Compressive Strength of Concrete due to Form Curing Condition (거푸집 양생 조건에 따른 콘크리트의 압축강도 특성)

  • Kim, Kyoungnam;Park, Sangyeol;Moon, Kyoungtae;Shim, Jaeyong
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.38 no.1
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    • pp.19-28
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    • 2018
  • The time for form removal is an important factor for ensuring the safety and economical efficiency of concrete structures, because it affects the quality, period, and cost of construction. Although local specifications suggest the form curing time, there is a problem of low quality of concrete due to early removing of form. This is because they do not fully understand effect of curing condition, and they want to shorten construction period in the field. Therefore, this research evaluates the effect of curing condition according to the time for form removal by testing specimen. As a result, the concrete compressive strength at the age of 28 days decreased about 40% in the condition of form removal after 12 hours, while the strength in the condition of form removal after 28 days decreased about 7%. Finally, this paper suggests an estimating equation for the concrete compressive strength due to the time for form removal considering various curing temperatures as equivalent ages. The proposed equation can be used in the field for evaluating the strength after form removal.

Application Properties of Slag Concrete in Winter Season (슬래그 활용 콘크리트의 동절기 적용 성능 평가)

  • Yoo, Jo-Hyeong;Kim, Woo-Jae
    • Journal of the Korea institute for structural maintenance and inspection
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    • v.21 no.6
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    • pp.52-58
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    • 2017
  • Concrete made with ground granulated blast-furnace slag(GGBS) has many advantage, including improved durability, workability and economic benefits. GGBS concrete is that its strength development is considerably slower under standard $20^{\circ}C$ curing conditions than that of portland cement concrete, although the ultimate strength is higher for same water-binder ratio. GGBS is not therefore used in application where high early age strength is required. In this study, to overcome the limitation of the initial strength decrease due to the use of slag, the slag substitution rate was changed to 30% under the low temperature curing temperature condition and the slag used concrete composition with the same or higher strength performance as OPC(Ordinary Portland Cement).

Maturity-Based Model for Concrete Compressive Strength with Different Supplementary Cementitious Materials (혼화재 치환율을 고려한 성숙도 기반의 콘크리트 압축강도 평가 모델)

  • Mun, Jae-Sung;Yang, Keun-Hyeok;Jeon, Yong-Soo
    • Journal of the Korea institute for structural maintenance and inspection
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    • v.18 no.6
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    • pp.82-89
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    • 2014
  • The purpose of this study is to propose a simple model to evaluate the compressive strength development of concrete with various supplementary cementitious materials (SCMs) and cured under different temperatures. For the generalization of the model, the ACI 209 parabola equation was modified based on the maturity function and then experimental constants A and B and 28-day compressive strength were determined from the regression analysis using a total of 265 data-sets compiled from the available literature. To verify the proposed model, concrete specimens classified into 3 Groups were prepared according to the SCM level as a partial replacement of cement and curing temperature. The analysis of existing data clearly revealed that the 28-day compressive strength decreases when the curing temperature is higher and/or lower than the reference curing temperature ($20^{\circ}C$). Furthermore, test results showed that the compressive strength development of concrete cured under $20^{\circ}C$ until an early age of 3 days was marginally affected by the curing temperature afterward. The proposed model accurately predicts the compressive strength development of concrete tested, indicating that the mean and standard deviation of the ratios between predictions and experiments are 1.00 and 0.08, respectively.

Synthesis and Mechanical Properties of Alkali-Activated Slag Concretes (무시멘트 알칼리 활성 고로슬래그 콘크리트의 배합에 따른 재료 역학적 특성)

  • Song, Jin-Kyu;Lee, Kang-Seok;Han, Sun-Ae;Kim, Young-In
    • Proceedings of the Korea Concrete Institute Conference
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    • 2008.04a
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    • pp.1005-1008
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    • 2008
  • The purpose of this study is to estimate basic mechanical properties of alkali-activated concretes based on GGBS(Ground Granulated Blast Furnace Slag). In this study, various mix ratios of alkali activated concretes based on sodium silicate and GGBS were set to evaluate concrete's compressive strengths and strains on the basis of results of existing alkali-activated cements and preliminary concrete tests, which were already performed by authors [Ref. 1]. Compressive strengths of concretes of ages 1, 3, 7, 28, 56 and 91 days were tested and investigated, respectively, and at early ages (< 7days) alkali-activated slag concrete (AASC) showed a high strength development, compared to that of Ordinary Portland Cement (OPC). A compressive strengths of AASC at age-3days range between 18 and 24 MPa, while those of OPC range 12 and 15 MPa. The stress-strain curve after maximum stress, on the other hand, is approximately reached at a compressive strain between 0.002 and 0.0025, which mechanical property is very similar to that of OPC.

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Field Measurement and Analysis of Post-Tensioned Prestressed Concrete Pavement Behavior under Tensioning (현장실험을 통한 포스트텐션드 프리스트레스트 콘크리트 포장의 긴장 시 거동분석)

  • Park, Hee-Beom;Kim, Seong-Min;Kim, Dong-Ho
    • International Journal of Highway Engineering
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    • v.11 no.1
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    • pp.247-256
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    • 2009
  • This research was conducted to analyze the behavior of PTCP (Post-Tensioned prestressed Concrete Pavement) under tensioning by performing field tests when the experimental PTCP slab was being constructed. The displacements in the slab under the environmental loading and tensioning were measured using temperature measurement sensors and displacement transducers. Tensioning was performed three times and appropriateness of tensioning could be determined by investigating the relationship between temperature and displacement, behavior of transverse crack, and daily change in displacement. The results of this study showed that under the first tensioning at very early age, large displacements were observed only near the joints because of the friction between slab and underlying layer and concrete inelasticity. Under consecutive tensioning, displacements were clearly observed all over the slab, but still affected by the friction. In addition, appropriate tensioning ensured the one-slab behavior of the PTCP slab even though cracks existed.

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Finite element modeling of RC columns made of inferior concrete mix strengthened with CFRP sheets

  • Khaled A. Alawi, Al-Sodani;Muhammad Kalimur ,Rahman;Mohammed A., Al-Osta;Omar S. Baghabra, Al-Amoudi
    • Earthquakes and Structures
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    • v.23 no.5
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    • pp.403-417
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    • 2022
  • Reinforced concrete (RC) structures with low-strength RC columns are rampant in several countries, especially those constructed during the early 1960s and 1970s. The weakness of these structures due to overloading or some natural disasters such as earthquakes and building age effects are some of the main reasons to collapse, particularly with the scarcity of data on the impact of aspect ratio and corner radius on the confinement effectiveness. Hence, it is crucial to investigate if these columns (with different aspect ratios) can be made safe by strengthening them with carbon fiber-reinforced polymers (CFRP) sheets. Therefore, experimental and numerical studies of CFRP-strengthened low-strength reinforced concrete short rectangular, square, and circular columns were studied. In this investigation, a total of 6 columns divided into three sets were evaluated. The first set had two circular cross-sectional columns, the second set had two square cross-section columns, and the third set has two rectangular cross-section columns. Furthermore, FEM validation has been conducted for some of the experimental results obtained from the literature. The experimental results revealed that the confinement equations for RC columns as per both CSA and ACI codes could give incorrect results for low-strength concrete. The control specimen (unstrengthened ones) displayed that both ACI and CSA equations overestimate the ultimate strength of low-strength RC columns by order of extent. For strengthened columns with CFRP, the code equations of CSA and ACI code overestimate the maximum strength by around 6 to 13% and 23 to 29%, respectively, depending on the cross-section of the column (i.e., square, rectangular, or circular). Results of finite element models (FEMs) showed that increasing the layer number of new commonly CFRP type (B) from one to 3 for circular columns can increase the column's ultimate loads by around eight times compared to unjacketed columns. However, in the case of strengthened square and rectangular columns with CFRP, the increase of the ultimate loads of columns can reach up to six times and two times, respectively.

A Study on the Physical Properties of Recycled Aggregates Using Concrete of Changing Waste Pottery Blain Fineness (폐도자기 분말도 변화에 따른 순환골재 사용 콘크리트의 물리적 특성에 관한 연구)

  • Ryu, Hyun-Gi;Park, Jeong-Min;Joung, Jae-Ho;Kim, Eui-Chang;Yoon, Seung-Joe
    • Journal of the Korean Recycled Construction Resources Institute
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    • v.6 no.2
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    • pp.119-127
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    • 2011
  • Objective of this study is to identify properties on strength increase of hardened concrete and fluidization of non-hardened concrete using waste ceramics generated by construction waste, which is a type of industrial waste, and by ceramics, which is a clay plastic, during its production process, and determine length change ratio caused by drying shrinkage during substitution of recycle aggregate and waste ceramics, and whether they can be used as concrete compounds. Slump of non-hardened concrete exhibited the best fluidization and formability at recycled aggregate's replacement ratio of 60% driven by higher substitution ratio of recycled aggregate and waste ceramics while air content met the KS requirement when substitution ratio of waste ceramics was $4,000cm^2/g$. Compressive strength of hardened concrete exceeded the requirements at early age and standard age and temperature dropped by roughly $6{\sim}10^{\circ}C$ less than the standard at maximum temperature in adiabatic temperature increase, which will hopefully result in stronger durability.

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A Study on the Development of Strength Prediction Model and Strength Control for Construction Field by Maturity Method (적산온도 방법에 의한 강도예측모델 개발 및 건설생산현장에서의 강도관리에 관한 연구)

  • Kim, Moo-Han;Jang, Jong-Ho;Nam, Jae-Hyun;Khil, Bae-Su;Kang, Suk-Pyo
    • Journal of the Korea Concrete Institute
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    • v.15 no.1
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    • pp.87-94
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    • 2003
  • Construction plan and strength control have limitations in construction production field because it is difficult to predict the form removal strength and development of specified concrete strength. However, we can have reasonable construction plan and strength control if prediction of concrete strength is available. In this study, firstly, the newly proposed strength prediction model with maturity method was compared with the logistic model to test the adaptability. Secondly, the determination of time of form removal was verified through the new strength prediction model. As the results, it is found that investigation of the activation energy that are used to calculate equivalent age is necessary, and new strength prediction model was proved to be more accurate in the strength prediction than logistic model in the early age. Moreover, the use of new model was more reasonable because it has low SSE and high decisive factor. If we adopt new strength prediction model at construction field, we can expect the reduced period of work through the reduced time of form removal.

An Experimental Study on the Influence of the Qualities of Ordinary Portland Cement on the Flowability of High Flow Concrete (보통 포틀랜드 시멘트 품질이 고유동 콘크리트의 유동 특성에 미치는 영향에 관한 연구)

  • Choi, Sung-Woo;Jo, Hyun-Tae;Ryu, Deug-Hyun;Kim, Gyu-Yong
    • Journal of the Korea Concrete Institute
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    • v.24 no.1
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    • pp.37-44
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    • 2012
  • Recently, due to developments in construction technology, the use of high-performance concrete became popular. High-performance concrete when compared to the ordinary concrete can better satisfy required performances by using mineral admixture and superplasticizer. Various studies on the effect of admixture materials on the quality of high-performance concrete have been reported. But there exist limited number of reported results on the effect of cement qualities, which is the most important constituent material in concrete. Therefore, in this study, the relationship between the quality of cement and the flowability of high flowing concrete is investigated. Qualities of domestically produced cement were identified, and then the influence of the qualities of cement on the flowability of high flowing concrete is evaluated. The result showed that the dosage of required superplasticizer was dependent on cement fineness, to brain, free-CaO, and interstitial phase, which all trigger initial hydration process of cement. Particularly, the results showed that fineness of cement has a high impact on the dosage of the superplasticizer. For strength property of concrete, the dosage of superplasticizer had a significant effect on the early age strength, but had negligible effect in the long term strength.