• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 2003년도 봄 학술발표회 논문집
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• pp.669-674
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• 2003

The purpose of this study is to identify the diffusion coefficients of carbon dioxide for various concrete mixtures. The test results indicate that the diffusion coefficient increases with the increase of water-cement ratio. The diffusion coefficient decreases with the increase of relative humidity at the same water-cement ratio. The diffusion of carbon dioxide reached the steady-state within about five hours after exposure. It was found that the diffusion coefficient of cement paste is larger then that of concrete or mortar. The quantitative values of diffusivity of carbon dioxide in this study will allow more realistic assessment of carbonation depth in concrete structures.

• Safety and Health at Work
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• 제3권3호
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• pp.235-240
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• 2012

The objectives are to compare the airborne asbestos concentrations resulted from mitering of abestos cement roof sheets by a high-speed motor and a hand saw, and to monitor whether other workers near the test sites are vulnerable to the fibers exceeding the occupational exposure limit. Four test cases were carried out and altogether 7 personal and 4 area air samples were collected. The NIOSH method 7400 was employed for the air samplings and analysis. Using the phase contrast microscopy, fiber counting was conducted under Rule A. The study showed that the fiber concentration medians for personal air samples gathered from the two tools were 4.11 fibers/cc (ranged: 1.33-12.41 fibers/cc) and 0.13 fibers/cc (ranged: 0.01-5.00 fibers/cc) respectively. The median for the area samples was 0.59 fibers/cc (ranged: 0.14-3.32 fibers/cc). Comparing each study case, the concentration level caused by the high-speed motor saw was more than twice that of the hand saw. According to the area samples, the workers nearby the test site are at risk from high exposure to asbestos.

• 한국농공학회지
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• 제18권4호
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• pp.4226-4231
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• 1976

The objective of this study is to investigate the characteristics of the concrete mixture and the plastic shrinkage of concrete exposed to a rapid evaporation immediately after placement, and to measure the shringkage rate according to the cement-mixture. Drying of concrete was conducted under a controlled chamber in which the temperature was 36${\pm}$2$^{\circ}C$, the relative humidity 45${\pm}$2%, and the wind velocity 4${\pm}$1m/sec. All of the concrete mo1ds were made to have good workability as 6${\pm}$0.5cm in slump. The results obtained are as follows; 1) The evaporation rate was 3.36kg/$\textrm{km}^2$, 2.98kg/$\textrm{km}^2$ and 2.91kg/$\textrm{km}^2$ respectively for each concretemixtures 365kg, 335kg and 317kg after 5 hour exposure, and the evaporation was measured at all 55∼57% after 2 hours and additional in 10% on after 5 hours exposure. 2) The weight of the concrete were was reduced to 3.0%, 2.6% and 2.4% respectively for each of them. 3) The shrinkage of concrete was 0.87 mm/m, 0.56mm/m and 0.31 mm/m after 5 hour exposure, and the shringkage of concrete was measured at 84%, 59% and 45% after 2 hours and it in 1∼5% after 5 hour exposure and then stopped. 4) A recommendation for good concrete is that the evaporation should be minimized during 2 hours after placement of concrete, if possible.

• International Journal of Concrete Structures and Materials
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• 제9권2호
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• pp.241-254
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• 2015

A lot of reinforced concrete (RC) structures in Syria went out of service after a few years of construction. This was mainly due to reinforcement corrosion or chemical attack on concrete. The use of blended cements is growing rapidly in the construction industry due to economical, ecological and technical benefits. Syria is relatively rich in scoria. In the study, mortar/concrete specimens were produced with seven types of cement: one plain Portland cement (control) and six blended cements with replacement levels ranging from 10 to 35 %. Rapid chloride penetration test was carried in accordance with ASTM C 1202 after two curing times of 28 and 90 days. The effect on the resistance of concrete against damage caused by corrosion of the embedded steel has been investigated using an accelerated corrosion test by impressing a constant anodic potential. The variation of current with time and time to failure of RC specimens were determined at 28 and 90 days curing. In addition, effects of aggressive acidic environments on mortars were investigated through 100 days of exposure to 5 % $H_2SO_4$, 10 % HCl, 5 % $HNO_3$ and 10 % $CH_3COOH$ solutions. Evaluation of sulfate resistance of mortars was also performed by immersing in 5 % $Na_2SO_4$ solution for 52 weeks. Test results reveal that the resistance to chloride penetration of concrete improves substantially with the increase of replacement level, and the concretes containing scoria based-blended cements, especially CEM II/B-P, exhibited corrosion initiation periods several times longer than the control mix. Further, an increase in scoria addition improves the acid resistance of mortar, especially in the early days of exposure, whereas after a long period of continuous exposure all specimens show the same behavior against the acid attack. According to results of sulfate resistance, CEM II/B-P can be used instead of SRPC in sulfate-bearing environments.

• 한국콘크리트학회:학술대회논문집
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• 한국콘크리트학회 1996년도 봄 학술발표회 논문집
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• pp.1-6
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• 1996

Cellulose fiber reinforced cement composites manufactured by the slurry-dewatering process have found broad applications in thin cement products as replacement for asbestos cement. This paper focuses on the durability characteristics of these composites under different aging conditions. The effects of wetting-drying and freezing-thaw cycles, carbonation, and exposure to hot and humid environments on the structure and properties of cellulose fiber-cement composites were investigated. The predominant mechanisms of aging in the composites were identified through investigation of structure-property relationships. Measures to control these aging mechanisms were diversed and evaluated. Refined cellulose fiber-cement composites are shown to possess excellent durability characteristics under the effects of various aging processes.

• 한국건축시공학회:학술대회논문집
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• 한국건축시공학회 2013년도 추계 학술논문 발표대회
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• pp.154-155
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• 2013

The aim of this work is to have a better knowledge of reactions that take place in a cement paste, blast furnace slag mixed cement paste and fly ash mixed cement paste and know about the change in chemical components exposed to elevated temperature. The results show that the dehydration reactions appeared differently in the each admixture mixed cement paste and can be used as tracers for determining the temperature history of concrete after a fire exposure.

• Advances in concrete construction
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• 제7권3호
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• pp.175-181
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• 2019

Low-calcium fly ash (LCFA) were used to prepare cement/LCFA specimens in this study. The basic physical properties including water demand, fluidity, setting time, soundness and drying shrinkage of cement/LCFA paste were investigated. The effects of curing time, immersion time and wet-dry cycles in 3% $Na_2SO_4$ solution on the compressive strength and the microstructures of specimens were also discussed. The results show that LCFA increases the water demand, setting time, soundness of cement paste samples. 50% and 60% LCFA replacement ratio decrease the drying shrinkage of hardened cement paste. The compressive strength of plain cement specimens decreases at the later immersion stage in 3% $Na_2SO_4$ solution. The addition of LCFA can decrease this strength reduction of cement specimens. For all specimens with LCFA, the compressive strength increases with increasing immersion time. During the wet-dry cycles, the compressive strength of plain cement specimens decreases with increasing wet-dry cycles. However, the pores in the specimens with 30% and 40% LCFA at early ages could be large enough for the crystal of sodium sulfate, which leads to the compressive strength increase with the increase of wet-dry cycles in 3% $Na_2SO_4$ solution. The microstructures of cement/LCFA specimens are in good agreement with the compressive strength.

• Corrosion Science and Technology
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• 제3권6호
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• pp.245-250
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• 2004

A series of classical G109 type concrete specimens was exposed to cyclic wet and dry ponding with 15 w/o NaCl solution for approximately five years. Mix design variables included 1) three cement alkalinities (EqA of 0.97, 0.52, and 0.36) and 2) three water-cement ratios (0.50, 0.41, and 0.37). To determine the corrosion initiation time, corrosion potential and macro-cell current between top and bottom bars were monitored. Subsequent to corrosion initiation, specimens were autopsied and visually inspected. Concrete powder samples were collected from top rebar trace and chloride concentration was measured. Also, time-to-corrosion, $T_i$, for specimens of the individual mix designs was represented using Weibull analysis. Time-to-corrosion was a distributed parameter; and because of this, corrosion initiation of four identical specimens for each mix varied, often over a relatively wide range. Specimens fabricated using the lowest water cement ratio and the highest alkalinity cement exhibited the longest time-to-corrosion initiation and the highest chloride threshold levels. Time-to-corrosion did not increase monotonically with cement alkalinity, however, presumably as a consequence of relatively high $Cl^-$ binding in the lower pore water pH range. The chloride threshold level, $Cl_{th}$, increased with increasing $T_i$ and, consequently, was greatest for the highest cement alkalinity specimens.

• Computers and Concrete
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• 제10권4호
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• pp.391-407
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• 2012

Utilization of supplementary cementing materials (SCM) by the cement industry, as a highly promising solution of sustainable cement development aiming to reduce carbon dioxide emissions, necessitates a more thorough evaluation of these types of materials on concrete durability. In this study a comparative assessment of the effect of SCM on concrete durability, of every cement type as defined in the European Standard EN 197-1 is taking place, using a software tool, based on proven predictive models (according to performance-related methods for assessing durability) developed and wide-validated for the estimation of concrete service life when designing for durability under harsh environments. The effect of Type II additives (fly ash, silica fume) on CEM I type of cement, as well as the effect of every Portland-composite type of cement (and others) are evaluated in terms of their performance in carbonation and chloride exposure, for a service life of 50 years. The main aim is to portray a unified and comprehensive evaluation of the efficiency of SCM in order to create the basis for future consideration of more types of cement to enter the production line in industry.

• Advances in concrete construction
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• 제5권6호
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• pp.671-683
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• 2017

Commonly used concrete in general, consists of cement, fine aggregate, coarse aggregate and water. Natural river sand is the most commonly used material as fine aggregate in concrete. One of the important requirements of concrete is that it should be durable under certain conditions of exposure. The durability of concrete is defined as its ability to resist weathering action, chemical attack or any other process of deterioration. Durable concrete will retain its original form, quality and serviceability when exposed to its environment. Deterioration can occur in various forms such as alkali aggregate expansion, freeze-thaw expansion, salt scaling by de-icing salts, shrinkage, attack on the reinforcement due to carbonation, sulphate attack on exposure to ground water, sea water attack and corrosion caused by salts. Addition of admixtures may control these effects. In this paper, an attempt has been made to replace part of fine aggregate by tailing material and part of cement by fly ash to improve the durability of concrete. The various durability tests performed were chemical attack tests such as sulphate attack, chloride attack and acid attack test and water absorption test. The concrete blend with 35% Tailing Material (TM) in place of river sand and 20% Fly Ash (FA) in place of OPC, has exhibited higher durability characteristics.