• Journal of the Korea institute for structural maintenance and inspection
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• v.25 no.4
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• pp.56-64
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• 2021

Leakage due to deterioration and damage is one of the major causes of volume change by freezing and thawing, and it leads micro-cracking and surface scaling in concrete structures. The deterioration of damaged concrete accelerates with the chloride attack. Thus, in the detailed guidelines for facility performance evaluation (2020), the quality of cover concrete and the freeze-thaw (FT) repetition cycle were newly suggested for concrete durability assessment. The quality of cover concrete should be evaluated by the rebound hammer test and the FT repetition cycle should be also considered in the deterioration environmental assessment. This study suggested the application of fast dynamic based nonlinear ultrasound method to monitor initial micro-scale damage under freezing and thawing environment. Concrete specimens were fabricated with different water-cement ratios (40%, 60%) and air contents (1.5% and 3.0%). The compressive strength, rebound number, relative dynamic modulus, and nonlinear ultrasound were measured with different FT cycles. The scanning electron microscopy was also performed to investigate the micro-scale FT damage. As a result, both the rebound number and the relative dynamic modulus had difficulty to detect early damage but the proposed method showed a potential to detect initial micro-scale damage and predict the FT resistance performance of concrete.

• Journal of the Korean Geotechnical Society
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• v.25 no.10
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• pp.17-30
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• 2009

Underground construction such as tunneling can induce damages on the surrounding rock mass, due to the stress concentration of in situ stresses and excessive energy input during construction sequence, such as blasting. The developed damage on the rock mass can have substantial influence on the mechanical and hydraulic behaviors of the rock masses around a tunnel. In this study, investigation on the generation of damage around an opening in a jointed rock model under biaxial compression condition was conducted. The joint dip angles employed are 30, 45, and 60 degrees to the horizontal, and the synthetic rock mass was made using early strength cement and water. From the biaxial compression test, initiation and propagation of tensile cracks at norm to the joint angle were found. The propagated tensile cracks eventually developed rock blocks, which were dislodged from the rock mass. Furthermore, the propagation process of the tensile cracks varies with joint angle: lower joint angle model shows more stable and progressive tensile crack propagation. The development of the tensile crack can be explained under the hypothesis that the rock segment encompassed by the joint set is subjected to the developing moment, which can be induced by the geometric irregularity around the opening in the rock model. The experiment results were simulated by using discrete element method PFC 2D. From the simulation, as has been observed from the test, a rock mass with lower joint angle produces wider damage region and rock block by tensile cracks. In addition, a rock model with lower joint angle shows progressive tensile cracks generation around the opening from the investigation of the interacted tensile cracks.

• Journal of the Korea Concrete Institute
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• v.18 no.3 s.93
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• pp.331-338
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• 2006

This paper presents the experimental results on volumetric changes in ordinary portland cement concrete made with various water-to-cement ratios(W/C's) ranging from 0.32 to 0.50 and cured in low different conditions. Curing regimes employed in this work were designed to exhibit autogenous and drying shrinkage as well as swelling of concrete. The concrete avoided any moist evaporation(Regime f showed only autogenous shrinkage and the lower the W/C, the feater the autogenous shrinkage. The concrete exposed to air drying conditions at $20{\pm}1^{\circ}C$ and $60{\pm}3%$ RH after 6-day water curing at $20{\pm}1^{\circ}C$(Regime II) swelled and then started to shrink. The maximum swelling value of concrete developed in water curing was between 15 and $40{\pm}10^{-6}$, and the greatest total shrinkage(autogenous+drying shrinkage) was obtained for the mixture made with W/C of 0.32. The concrete let to air drying conditions(Regime III) showed greater total shrinkage compared to the concrete cured in Regime II. The concrete exposed to air drying condition after 6-day sealed curing(Regime IV) exhibited slightly smaller total shrinkage than that of the concrete cured in Regime III. Net drying shrinkage that can be derived from the results of Regime I, III, and IV increased as the W/C increased despite of similar total shrinkage. This result indicated that drying shrinkage governs total shrinkage of high-W/C concretes. In other words, a portion of autogenous shrinkage in total shrinkage increased in low-W/C concretes. Therefore, it should be controlled in terms of cracking potential. Finally, total shrinkage of high-strength and high-performance concrete made with low W/C can be effectively reduced by appropriate early moisture curing.