• Advances in concrete construction
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• v.7 no.3
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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.

• Journal of the Society of Naval Architects of Korea
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• v.48 no.4
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• pp.330-335
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• 2011

This paper presents estimation of average compressive strengths of three types of stiffened panels under lateral pressure and axial compression based on simplified formulas from CSRs and nonlinear FEAs. FEA scenarios are prepared based on the slenderness ratios of the stiffened panels used for in-service vessels. The seven step lateral pressures by 1bar increment are imposed on FE models assuming maximum 30m water height. The number of FEAs for FB-, AB-, and TB-stiffened panels is totally 189 times. FEA results show that existence of pressure can evolves significant reduction of ultimate strengths, meanwhile CSR formulas do not take into account the lateral pressure effect. Lateral pressure acting on the stiffened panel with higher column slenderness ratio more reduces the ultimate strengths than those with smaller column slenderness ratio. A new concept of relative average compressive strain energy instead of the ultimate strength is introduced in order to rationally compare the average compressive strength through complete compressive straining regime. The differences of the ultimate strengths between CSR formulas and FEA results are relatively small for FB- and AB-stiffened panels, but larger discrepancies of relative average compressive strain energies are shown.

• Journal of the Korea Institute of Building Construction
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• v.19 no.1
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• pp.1-7
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• 2019

This study evaluated the mechanical properties of the alpha-calcium sulfate hemihydrate replaced concrete to reduce the cracking in a box culvert power transmission. After setting the replacement ratio of alpha-calcium sulfate hemihydrate at 0, 6, 9, 12, and 15%, the setting time, compressive strength, and drying shrinkage were measured and the microstructure and crystal structure were analyzed. As a result, it was confirmed that as the replacement ratio of alpha-calcium sulfate hemihydrate increased, the setting time decreased and the compressive strength declined. On the other hand, when the alpha-calcium sulfate hemihydrate was set with 15% of replacement ratio, about 60% reduction in the drying shrinkage was shown compared to that of ordinary Portland cement. Therefore, it is thought that when the concrete replacing the alpha-calcium sulfate hemihydrate is applied to a box culvert power transmission, the cracking reduction performance will be improved, and the improvement of compressive strength will be required.

• Journal of the Korea institute for structural maintenance and inspection
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• v.24 no.5
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• pp.86-94
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• 2020

When the Superabsorbent Polymer (SAP) is added into concrete, the slump decreases rapidly, deteriorating the workability, the internal curing effect can be obtained through the water absorption and discharge process, and the internal voids of the concrete are increased. In this study, the effects of internal curing and voids were evaluated by evaluating the compressive strength, freeze-thaw resistance, and chloride penetration resistance of SAP-adding concrete that secured workability using a water reducing agent. Also, the internal curing effect of SAP was evaluated by dividing the curing conditions of concrete into water curing and sealed curing. From the result, as the SAP adding ratio increased, the amount of water reducing agent increased, and as for the compressive strength, the SAP adding ratio of 1.5% showed the greatest compressive strength. In particular, in the case of sealed curing showed higher compressive strength than the water curing. It is considered that the compressive strength increased due to the reduction of the effective water-cement ratio and the internal curing effect. Adding 1.0~1.5% of SAP improved the freeze-thaw resistance similar to the case of adding the AE agent, and the addition of more than 1.0% of SAP improved the chloride penetration resistance. The optimal adding ratio of SAP is 1.5%, and the adding ratio of 2.0% or more adversely affects the compressive strength and freeze-thaw resistance.

• Journal of the Korea Institute of Building Construction
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• v.9 no.6
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• pp.113-119
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• 2009

This study is to investigate the fundamental and fireproof qualities of high strength concrete corresponding to changes in the curing factors and the PP fiber ratio. The results were as follows. For the fundamental characteristics of concrete, the fluidity was reduced in proportion to the increase in the PP fiber ratio. The compressive strength was somewhat reduced according to an increase in the PP fiber ratio. However, it had the high strength scope of more than 60 MPa at 7 days and of more than 90 MPa at 28 days. On the spalling mechanism followed by changes of the water content ratio, spalling was prevented in all combinations, except the specimen without PP fiber and subjected to 3.0% of moisture contents. When spalling was prevented at that time, the residual compressive strength ratio was 22%~41% and the mass reduction ratio was 5%~7%, which was relatively favorable. As the spalling mechanism corresponds to changes in the curing method, spalling was prevented in concrete with a PP fiber mixing ratio of more than 0.05% in the event of standard curing, and in concrete with a PP fiber mixing ratio of more than 0.10% in the case of steam curing and autoclave curing. In these cases, when spalling was prevented, the residual compressive strength ratio was 23~42% and the mass reduction ratio was 7~11%. In these results, the ease of spalling prevention in high strength concrete was inversely proportional to the water content ratio. Depending on the curing method, spalling was prevented in concrete with over 0.05% PP fiber with standard curing and in concrete with over 0.1% PP fiber with steam curing and autoclave curing.

• Geomechanics and Engineering
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• v.12 no.6
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• pp.935-948
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• 2017

An experimental program was conducted to investigate the effects of the static compaction pressure, cement content, water/cement ratio, and curing time on unconfined compressive strength (UCS) of the cement treated sand. UCS were conducted on samples prepared with 4 different cement/sand ratios and were compacted under the lowest and highest static pressures (8 MPa and 40 MPa). Each sample was cured for 7 and 28 days to observe the impact of curing time on UCS of cement treated samples. Results of the study showed the unconfined compressive strength of sand increased as the cement content (5% to 10%) of the cement-sand mixture and compaction pressure (8 MPa to 40 MPa) increased. UCS of sand soil increased 30% to 800% when cement content was increased from 2.5% to 10%. Impact of compaction pressure on UCS decreased with a reduction in cement contents. On the other hand, it was observed that as the water content the cement-sand mixture increased, the unconfined compressive strength showed tendency to decrease regardless of compaction pressure and cement content. When the curing time was extended from 7 days to 28 days, the unconfined compressive strengths of almost all the samples increased approximately by 2 or 3 times.

• Magazine of the Korean Society of Agricultural Engineers
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• v.44 no.3
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• pp.85-91
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• 2002

This study is performed to evaluate characteristics of plantable concrete using waste stone and stone dust. The test result shows that the void ratio is decreased as the size of waste stone smaller and the content of stone dust increased. The strength of neutralized plantable concrete is decreased by approximately 4∼5% than that of the normal plantable concrete. The reduction effect of pH value is achieved by chemical treatment. Also, the plant is grown well with increase of the void ratio and size of waste stone.

• Resources Recycling
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• v.19 no.4
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• pp.35-40
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• 2010

Existing porous concrete has problems with reduction of strength due to freezing and thawing and exfoliation of aggregate at joints. In this study, a method for increasing strength and durability of porous concrete by using fine aggregate, silica-fume and high-range water-reducing agent was proposed by laboratory tests. Mixing ratio between silica-fume (10%) and fine aggregate (0%, 7%, 15%) was selected as a major test factor, and laboratory tests for compressive strength, flexural strength, permeability coefficient, porosity, freezing and thawing were conducted. Compressive strength and flexural strength were increased as the mixing ratio of fine aggregate was increased. However, permeability and freezing-thawing resistance were decreased due to reduction of porosity. Therefore, the ratio of fine aggregate should be limited to increase strength and durability of the porous concrete, while the mixing ratio of silica-fume should be over 10%.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2020.06a
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• pp.88-89
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• 2020

Research is underway to incorporate water-repellent agents inside mortars to improve the durability of concrete. Therefore, in this study, the mechanical properties and absorption rate were evaluated by adding a hybrid water repellent in which a liquid and a solid were mixed at a constant ratio.As a result of the experiment, the compressive strength of the mortar added with the hybrid water repellent showed a strength reduction of about 5% than the compressive strength of the OPC, and the overall water absorption was lower than that of the water repellent used alone.

• Proceedings of the Korean Institute of Building Construction Conference
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• 2015.05a
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• pp.30-31
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• 2015

Usually, carbonation of concrete causes pH reduction and corrosion of steel, it leads to decrease of durability. However, CaCO₃, as results of reaction with hydrates products and CO₂, can contribute to improvement of compressive strength. Based on this theory, using carbonation depth, the researches about CO₂ absorption of plain concrete and concrete containing CO₂ reactive materials has been performed. But, the researches has limitation about using one material, therefore, for this study, considering various CO₂ reactive materials, experiment has been proceeded. With water to binder ratio 50%, after initial curing for 2days, accelerated carbonation was performed for 28days, and carbonation depth and compressive strength were measured. As results of carbonation depth, specimen containing desulfurized slag, zeolite showed the highest CO₂ absorption, in case of compressive strength, specimens with MgO were indicated as highest compressive strength.