해저 콘크리트 구조물의 신뢰성 이론에 의한 시간 의존적 내구수명 평가

국문 초록 (Abstract)

본 연구에서는 철근 콘크리트 터널 구조물을 해상 대기중 비래염분이 침투하는 터널 내벽과 해수에 항시 접촉하는 터널 외벽으로 구분하여, 몬테카를로 시뮬레이션에 의해 철근 부식 개시 확률을 예측하였다. 염해관련 변수의 변동성을 평가하기 위하여 염소이온 확산계수, 표면 염소이온농도, 피복두께, 임계 염소이온농도를 실제 실험 및 문헌 조사를 통해 확률특성을 구하였다. 그 결과 염소이온 확산계수의 평균치는 3.77x10^(-12) ㎡/s 이었으며, 대상 부재인 터널 내벽과 외벽의 피복두께는 각각 45.5mm, 94.7mm으로 조사되었고, 임계 염소이온농도의 평균은 결합재 단위중량당 0.69%이었다. 각 변수의 확률적 특성에 근거하여 노출기간에 따른 철근위치에서의 염소이온 농도 분포를 구하였다. 재령이 증가할수록 침투 염소이온 농도의 평균값은 증가하며, 변동계수는 감소하게 됨을 알 수 있었다. 또한 확률론적 염해 해석기법을 적용하여 콘크리트 터널 내벽과 외벽에 대해 내구수명 및 부식개시 확률을 평가하였다. 염소이온 침투의 시간의존성을 고려하지 않은 경우 터널 내벽과 외벽에 대해 각각 8년, 12년의 내구수명이 도출되었으나, 시간의존적 모델에서는 178년, 283년의 내구수명이 계산되어 구조물의 설계내구수명(100년)을 만족하고 있음을 보였다. 또한, 시간의존성을 고려하지 않은 경우 100년에서의 부식 개시 확률은 터널 내벽과 외벽에 대해 각각 59.5, 95.5%였으며, 시간의존성 모델에서는 2.9, 0.2%로 계산되었다. 따라서 구조물의 과다설계를 방지하고 보다 합리적인 내구수명 설계 및 평가를 위해서는 염소이온 확산의 시간의존성을 고려하여야 한다. 마지막으로 본 연구에서 문헌 조사를 통해 구한 부식 발생 임계 농도를 현재 콘크리트 관련 기준에 제시한 값과 비교하여 분석하였다.

다국어 초록 (Multilingual Abstract)

This study predicted the probability of corrosion initiation of reinforced concrete tunnel boxes structures using the Monte Carlo Simulation. For the inner wall and outer wall in the tunnel boxes, exposed to airborne chloride ion and seawater directly respectively, statistical values of parameters like diffusion coefficient D, surface chloride content CS, cover depth c, and the chloride threshold level C_(lim) were examined from experiment or literature review. Their average values accounted for 3.77x10^(-12)㎡/s, 3.0% by weight of cement, 94.7mm and 45.5mm for outer wall and inner wall, respectively, and 0.69% by weight of cement for D, CS, c, and C_(lim), respectively. With these parametric values, the distribution of chloride contents at rebar with time and the probability of corrosion initiation of the tunnel boxes, inner wall and outer wall, was examined by considering time dependency of chloride transport. From the examination, the histogram of chloride contents at rebar is closer to a gamma distribution, and the mean value increases with time, while the coefficient of variance decreases with time. It was found that the probability of corrosion initiation and the time to corrosion were dependent on the time dependency of chloride transport. Time independent model predicted time to corrosion initiation of inner wall and outer wall as 8 and 12 years, respectively, while 178 and 283 years of time to corrosion was calculated by time dependent model for inner wall and outer wall, respectively. For time independent model, the probability of corrosion at 100 years of exposure for inner wall and outer wall was ranged 59.5 and 95.5%, respectively, while time dependent model indicated 2.9 and 0.2% of the probability corrosion, respectively. Finally, impact of C_(lim), including values specified in current codes, on the probability of corrosion initiation and corrosion free life is discussed.

참고문헌 (Reference)

1 한국콘크리트학회, "해양환경에서 염해를 받는 철근콘크리트조구조물의 유지관리 지침(안)" 특수환경 콘크리트 위원회 2004

2 건설교통부, "콘크리트 표준시방서 유지관리편"

3 건설교통부, "콘크리트 표준시방서 내구성편"

4 日本土木學會, "コンクリート標準示方書 [構造性能照査編]"

5 Page, C. L, "The influence of different cements on chloride-induced corrosion of reinforcing steel" 16 (16): 79-86, 1986

6 Song, H.-W, "The influence of chemistry of chloride ions in cement matrix on corrosion of steel" 107 (107): 332-339, 2010

7 Bamforth,P.B, "The derivation of input data for modelling chloride ingress from eight-years UK coastal exposure trials" 51 (51): 87-96, 1999

8 Kawamura, C, "Study on the penetration of chloride ions into concrete based on investigation of railway structures" 66 (66): 193-204, 2005

9 British Standard 8110 Part 1, "Structural Use of Concrete - Code of Practice for Design and Construction" British Standards Institute 1985

10 Tikalsky, P. J, "Statistical variations in chloride diffusion in concrete bridges" 102 (102): 481-486, 2005

11 Song, H.-W, "Probability-based service life prediction of fixed linkbridge exposed to a marine environment" 25-28, 2007

12 Song, H.-W, "Probabilistic assessment to predict the time to corrosion of steel inreinforced concrete tunnel box exposed to sea water" 23 (23): 3270-3278, 2009

13 DuraCrete, "Probabilistic Performance Based Durability Design of Concrete Structures, Final Technical Report" European Brite-Euram Programme 2000

14 Pack, S.-W, "Prediction of time dependent chloride transport in concrete structures exposed to a marine environment" 4 (4): 302-312, 2010

15 Collepardi, M, "Penetration of chloride ions into cement pastes and concrete" 55 (55): 534-535, 1972

16 CEB-FIP, "Model Code for Service Life Design, the International Federation for Structural Concrete (fib)" Task Group 5.6 2006

17 Page, C. L, "Mechanism of corrosion protection in reinforced concrete marine structure" 258 (258): 514-515, 1975

18 Thomas, M. D. A, "Marine performance of PFAconcrete" 43 (43): 171-185, 1991

19 Thomas, M. D. A, "Life-365 Manual"

20 Bamforth, P. B, "Factors influencing chloride ingress into marine structures" Economic and Durable Construction through Excellence, E & FN Spon, 1105-1118, 1993

21 Song, H.-W, "Durability evaluation of concrete structures exposed to marine environment focusing on a chloride build-up on concrete surface" 1-9, 2006

22 Gjørv, O. E, "Durability and service life of concrete structures" 1-16, 2002

23 ACI Committee 222, "Corrosion of metals in concrete" Manual of Concrete Practice, Part 3, American Concrete Institute 1994

24 NT BUILD 492, "Concrete, Mortar and Cement Based Repair Materials: Chloride Migration Coefficient from Non-steady State Migration Experiments" 1999

25 Weyers, R. E, "Concrete BridgeProtection and Rehabilitation: Chemical and Physical Techniques. Service Life Estimates, Strategic Highway Research Program" National Research Council 1994

26 Ann, K. Y, "Chloride threshold level for corrosion of steel in concrete" 49 (49): 4113-4133, 2007

27 Mustafa, M. A, "Atmospheric chloride penetration into concrete in semitropical marine environment" 24 (24): 661-670, 1994