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Accelerated Simulation of Chloride Ingress Into Concrete Under Dryingв

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Accelerated Simulation of Chloride Ingress Into Concrete Under Dryingв Construction and Building Materials 93 (2015) 205–213 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat Accelerated simulation of chloride ingress into concrete under drying–wetting alternation condition chloride environment ⇑ Zhiwu Yu a,b, Ying Chen a,b, Peng Liu a,b,c, , Weilun Wang c a School of Civil Engineering, Central South University, 22 Shaoshan Road, Changsha 410075, China b National Engineering Laboratory for High Speed Railway Construction, Central South University, Changsha 410075, China c School of Civil Engineering, Shenzhen University, 3688 Nanhai Road, Shenzhen 518060, China highlights We investigate the chloride ingress into concrete under various environments. Acceleration factor is defined to represent environmental effect on chloride ingress. We propose the chloride diffusion coefficient model coupled with environment factors. Environmental conditions and concrete properties affect the acceleration factor. Numerical simulations and tests of chloride ingress are conducted. article info abstract Article history: In this study, the chloride diffusion coefficient model coupled with environmental factors is proposed to Received 6 April 2015 describe chloride ingress into concrete. The S curve model is also presented to fit the error function. Received in revised form 3 May 2015 Simultaneously, field test and artificial simulated environment experiments are conducted. Moreover, Accepted 11 May 2015 some typical RC structures are used as examples to investigate the application of the aforementioned models in forecasting service life. The results show that the chloride diffusion in concrete can be described by an equivalent chloride diffusion coefficient model proposed in this study, and the measured Keywords: data of chloride content in concrete correspond with the fitted profiles. Under natural and artificial sim- Concrete ulated environments, the differences of chloride ingress into concrete are in terms of surface chloride Chloride Diffusion coefficient content, convective zone depth, and diffusion coefficient. Numerical simulation indicates that the accel- Acceleration factor eration factor can be considered as a key parameter to determine the correlation of chloride ingress into Accelerated simulation concrete under various environments. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction modeling of chloride transport or diffusion coefficient coupled with enhanced moisture conductivity, exposure time, and environ- Chloride attack is considered one of the most important factors mental factors in concrete exposed to marine environment. In that affect the service life of reinforced concrete (RC) structures addition, numerical modeling to predict the service life of RC struc- [1,2], and the degradation of RC because of chloride penetration tures exposed to chloride environments has been conducted has been a serious problem in civil engineering for many years [3,10,11]. Subsequently, a series of studies has been conducted to [3]. Substantial research has been conducted by experts all over address the influence of cracking on the penetration resistance or the world [4], such as Saetta and Nielsen et al. [5,6] who investi- permeability of concrete [12–14]. Although existing research has gated chloride diffusion into partially saturated concrete and pro- made astounding advances in the development of civil engineer- posed the chloride diffusion coefficient model. Song et al. [7] ing, deficiencies and divergence exist in test procedures, corrosion predicted the time to corrosion of steel in a reinforced concrete mechanics, and theoretical models, particularly with regard to the tunnel box exposed to seawater. Some studies [3,8,9] noted the interaction between the natural and accelerated tests. The accurate and short-term forecast of the service life of RC becomes a signifi- cant problem. ⇑ Corresponding author at: School of Civil Engineering, Central South University, Two approaches, namely, real test (or natural environment test) 22 Shaoshan Road, Changsha 410075, China. Tel.: +86 15116277646. and accelerated experiment, have been employed to investigate E-mail address: [email protected] (P. Liu). http://dx.doi.org/10.1016/j.conbuildmat.2015.05.090 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved. 206 Z. Yu et al. / Construction and Building Materials 93 (2015) 205–213 the influence of chloride on RC degradation. However, these two 2.2. Equivalent chloride diffusion coefficient in concrete coupled with approaches have limitations that are difficult to overcome; the environmental factors accelerated experiment frequently adopts electric current to achieve acceleration, which results in inconsistency of RC degrada- Chloride diffusion coefficient in concrete is a key parameter for tion with real cases [15–17], and the real test often consumes sig- the durability of marine RC structures, which characterizes the nificant amounts of time, manpower, and physical resource. To velocity of chloride penetrating into concrete. Environmental fac- overcome these deficiencies, the accelerated simulation test [or tors such as temperature (T) and relative humidity (RH) can make artificial simulated environment (ASE)] with advantages of reliable the chloride diffusion in concrete dominant; thus, these factors are results, good correlation, and obvious acceleration effect, has been significant in investigating their influence on the diffusion coeffi- improved. For example, the similarity of the chloride ingress in cient of chloride in concrete. In this study, the equivalent chloride concrete under natural and artificial simulated environments was diffusion coefficient De is assumed as a function of temperature, determined [18], and a statistical treatment was proposed to relative humidity, and exposure time. The coefficient is formulated establish a deterministic service life model of concrete structures as: in marine environments [19]. However, the researchers have not D ¼ D f f f ; ð4Þ reached a consensus on the major issues in the criteria for test pro- e app t T RH cedures and experimental parameters. where fT, fRH and ft indicate the influence coefficient of temperature, The goal of this study is to propose the accelerated simulation of RH and exposure time on the chloride diffusion coefficient, chloride ingress into a concrete under chloride environment. Based respectively. on theoretical analysis, the chloride diffusion coefficient coupled Existing research [21] proposes that the influence of RH on the with environmental factors is modeled. Moreover, the acceleration chloride diffusion coefficient can be written as: factor model between the natural and artificial simulated environ- "# À1 ments is established, thereby providing a novel approach to evalu- 1 À h 4 f RH ¼ 1 þ ; ð5Þ ate the durability and forecast the service life of RC structures 1 À hc under natural and artificial simulated environments. where h represents relative humidity in concrete, and hc represents the critical relative humidity with a recommended value of 0.75. 2. Theoretical analysis Temperature has a double effect on chloride diffusion [22]. Based on Nernst–Einstein equation [5], the influence of tem- 2.1. Chloride diffusion in concrete perature on the chloride diffusion coefficient can be written as: Chloride transport can occur in concrete through several mech- q 1 À 1 T0 T1 anisms, including diffusion, absorption, migration, f T ¼ðT1=T0Þe ; ð6Þ pressure-induced flow, and wick action. Diffusion is the primary mechanism of chloride transport in a concrete under chloride envi- where T0 and T1 are reference temperature and service environmen- ronment with no applied electric field and stable moisture condi- tal temperature, respectively; q represents activation energy of tion of pore structure in concrete [17]. For a one-dimensional to hydration divided by gas constant, which relates to semi-infinite medium, Fick’s second law of diffusion is widely used water-to-cement ratio (W/C) such that if the W/C is 0.4, 0.5, and to evaluate the behavior of chloride transport in concrete [1,5,20], 0.6, then the value of q is equal to 6000, 5450, and 3850 K, respec- as written in Eq. (1). Given a constant diffusion coefficient, an ana- tively. The linear interpolation and extrapolation methods can be lytical solution of Eq. (1) can be expressed by Eq. (2) with proper used to determine the q value for different W/C. The diffusion coefficient is time-dependent because the process initial and boundary conditions (i.e., C(0,t) = Cs, C(1,t) = C0, C(x,0) = C0): of cement hydration results in connection and condensation of concrete pore structures [1]. The influence factor of time can be @C @ @C ¼ Dapp ; ð1Þ expressed as a decay function of the following equation: @t @x @x ( m ðtR=tÞ ; t 6 30 years "# ! f ¼ ; ð7Þ t ðt =t Þm: t > 30 years x À Dx R lim Cðx; tÞ¼C0 þðCs À C0Þ 1 À erf pffiffiffiffiffiffiffiffiffiffiffi ; ð2Þ 2 Dappt where the limit time of the equation, tlim, is 30 years. tR and t are the reference time and actual exposure time, respectively. m represents where Dapp is the apparent chloride diffusion coefficient in concrete; the rate of diffusion coefficient decay and depends on the content of C(x, t) represents the chloride content as a function of position x and fly ash and slag, whose value is 0.25 for ordinary concrete and time t; Cs and C0 are the surface and initial chloride contents, reaches 0.6 for concrete with admixture. respectively; t is the exposure time; x is the depth from the concrete surface to the test position;
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