Advances in Materials Science and Engineering

Volume 2017, Article ID 3186371, 10 pages

https://doi.org/10.1155/2017/3186371

## Estimation of Corrosion-Free Life for Concrete Containing Ground Granulated Blast-Furnace Slag under a Chloride-Bearing Environment

Department of Civil and Environmental Engineering, Hanyang University, Ansan 15588, Republic of Korea

Correspondence should be addressed to Ki Yong Ann; rk.ca.gnaynah@nnak

Received 15 March 2017; Accepted 6 June 2017; Published 24 July 2017

Academic Editor: Xiao-Jian Gao

Copyright © 2017 Sung In Hong and Ki Yong Ann. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

The rate of chloride transport by diffusion in concrete containing ground granulated blast-furnace slag (GGBS) was mathematically estimated to predict the corrosion-free service life of concrete structures exposed to seawater environment. As a factor to corrosiveness of steel embedment, replacement ratio of GGBS was selected, accounting for 25 and 50% to total binder. As a result, it was found that an increase in the GGBS content resulted in an increase in the chloride binding capacity, which would give rise to a lower chloride diffusion rate, thereby reducing the risk of chloride-induced corrosion. When it comes to the sensitivity of parameters to service life, the effective diffusivity showed a marginal influence on serviceability, irrespective of GGBS contents while surface chloride content and critical threshold concentration revealed more crucial factors to long term chloride diffusion. As the GGBS replacement increased, the variation in service life has become less influential with changing parameters. Substantially, GGBS concrete at high replacement ratio enhanced the service life due to a combination of dense pore structure and enhanced chloride binding capacity.

#### 1. Introduction

Steel corrosion in concrete mainly occurs by dissolution of passive film (i.e., thin layer of iron oxide) which is formed in the high alkaline pore solution and can protect the steel from corrosion [1]. When a certain amount of chloride ions reaches the steel embedment, pit nucleation on the steel surface accompanies a pH drop in the concrete pore solution to initiate the corrosion process [2]. Then, volume expansion of corrosion products induces cracking of cover concrete, together with a reduction of steel area, leading to structural failure.

To mitigate the corrosion process in terms of chloride transport and corrosiveness, lots of attention has been drawn to development of durable concrete using supplementary cements such as GGBS, silica fume, and fly ash. Among them, GGBS concrete is known to be durable material in terms of dense pore structure and chemically improved resistance to chloride penetration. GGBS has high alumina content typically within 8–18% [3], compared with OPC which enables binding chloride ions penetrating into concrete at much higher level. Though the initial development of strength is generally weaker, the effect of latent hydration promotes durability of the material after enough curing period. This is especially dominant in using high replacement ratio, about 55–60% by binder mass [4]. Moreover, efficiencies to reduce cost for Portland cement usage and generation of CO_{2} from the cement production [4] are also valid in using GGBS concrete.

Despite these potential feasibilities in GGBS concrete, serviceability assessment for GGBS concrete as to long term chloride attack has been rarely conducted. For example, dealing with chloride diffusion in GGBS concrete for 30 years of chloride exposure shows substantial reduction of chloride diffusivity with time [5]. Though time dependent chloride diffusivity was considered, the effect of chloride binding capacity on the rate of chloride diffusion was not reflected in this analysis. This might give rise to unrealistic chloride profile which would otherwise lead to the concentration dependent rate of diffusion if chloride binding capacity is considered [6–8].

This study concerns chloride diffusion in GGBS concrete depending on its replacement ratio where chloride binding capacity and chloride diffusivity are affected. Determination of chloride binding capacity was achieved by the Freundlich isotherm with experimentally obtained data to represent the nonlinear relation of chloride phases during diffusion process. Additionally, the rate of chloride adsorption was considered based on mass transfer theory [9], representing nonequilibrium state of chloride phase transition, in terms of resistance to chloride binding. Accordingly, the concentration dependent chloride binding capacity and chloride diffusivity were achieved and then chloride diffusion at nonsteady state was determined to predict chloride diffusion in GGBS concrete.

#### 2. Transient Diffusion Model

To evaluate durability of GGBS concrete against chloride-induced corrosion, chloride diffusion under nonsteady state was modeled. As for the chloride phases in concrete, this paper considers free chloride, defined as a mobile phase in pore solution, bound chloride defined as immobilized phase adsorbed in hydrates, and total chloride for the sum of the free and bound chlorides. The free chloride is only one phase that diffuses through concrete media under continuous adsorption process. For the mixes, GGBS based blended cements, of which replacement ratios are 0, 25, and 50% by total binder mass with 0.5 of water-to-binder ratio (W/B), were maintained.

##### 2.1. Governing Equation

In this study, chloride diffusion was considered as a main driving force through concrete media, assuming that the electrical interruption with other ionic species and moisture gradient are absent in concrete pore system. Hence, chloride transport in concrete submerged in a saline environment can be described as a one-dimensional transient diffusion such that where is the free chloride concentration (percentage by binder mass), is the time (sec), is the penetration distance (m), and is the apparent diffusion coefficient (m^{2}/s). The apparent diffusivity, incorporating effects of pore structure and chloride binding on the rate of chloride penetration, can be written aswhere is the effective diffusion coefficient (m^{2}/s) and is the bound chloride concentration (percentage by binder mass). The derivative term, , in (2) corresponds to the binding capacity [7] which in turn leads to the concentration dependent diffusivity.

##### 2.2. Chloride Binding

Distribution of chlorides in concrete during the diffusion process is continuously affected by chloride reaction with hydrates. As for nonlinear relation between free and bound chlorides, the present study adopted Freundlich isotherm, describing equilibrium state of chlorides in concrete such that where and are the regression parameters from Freundlich isotherm and is the free chloride concentration at interface between pore solution and solid surface (percentage by binder mass). To consider the concentration dependent rate of chloride adsorption, transition of in relatively unstable pore solution (i.e., agitated solvent) into in relatively stable state of pore solution directly contacted with the surface of hydrate was described using the mass transfer theory [9]. Accordingly, the amount of adsorbed chlorides at given unit time can be expressed as where is the overall mass transfer coefficient (1/s) that specifies velocity of mass transfer from bulk pore solution into adsorption site of hydrates. When the is relatively smaller than that of interface (i.e., ), the desorption process occurs in the system.

##### 2.3. Mathematical Treatment

To predict the chloride diffusion in concrete in the presence of chloride binding, the governing equation described in (1) was modified using (2)–(4) such that Due to nonlinearity in partial differential form (i.e., (5)), explicit finite difference method with 0.001 m and 1.0 day as the step sizes of distance and time, respectively, was used to solve it. The derivative terms for time and distance were expressed as forward difference and central difference, respectively. Then, those chloride concentrations (i.e., and ) were calculated by applying the mass balance equation to (5) such that where is the total chloride (percentage by binder mass). Iterative procedure was carried out on (5) and (6) until relative change of solutions in a unit time increment is achieved below 10^{−6}. As for the boundary condition, the external chloride concentration submitted to the concrete surface was set by 0.5 M of sodium chloride, aiming to mimic that of seawater together with initially zero concentration in concrete. Thus, it is possible to implement numerical solutions for free, bound, and total chloride concentrations along the concrete depth with time.

##### 2.4. Determination of Input Parameters

For chloride diffusion analysis using concrete mixes varying with GGBS replacements, an experimental work for chloride binding test was conducted. Fabrication of samples was carried out on cement paste with 0.5 W/B in which nine levels of sodium chloride content (0.1–3.0% by cement) were initially mixed. Then, water curing under °C was taken on all the samples for 56 days. After that, about 5 g of powder was collected from fragments of cement pastes and was mixed with water at 50°C, followed by stirring about 30 min until equilibrium state is reached in the solution. Then, the concentration of chloride in solution, filtered with filtering paper, was measured by potentiometric titration against silver nitrate. Thus, free chloride concentrations were determined directly by this procedure and thus bound chloride concentrations were obtained by subtracting free one from initial total chloride content. Hence, it is possible to obtain binding isotherm constants, and , by regression analysis using Freundlich model (3). As for the mass transfer coefficient, , it was set by 7.58 × 10^{−7} s^{−1} for all the cases due to marginal difference observed within the values from related analysis [10, 11].

To determine the surface chloride content, mercury intrusion porosimetry was carried out on mortar samples with OPC, 25% and 50% GGBS. The samples for the test were made by adjusting a mix proportion as 0.5 W/B and 2.12 of sand-to-binder ratio. After water immersion for 56 days, fragments from the samples were obtained and dried in oven to evacuate most of pore solution inside the materials. At constant contact angle (130°) for mercury intrusion and extrusion, the pressure from the device was gradually increased up to maximum pressure (228 MPa). After the test, total intrusion of mercury for given samples was obtained. Then, using the values and information of unit binder mass, it is possible to obtain water filled porosity, defined by volume of pore solution per mass of binder. Those parameters including literature values of effective diffusivity for GGBS concrete [12] which are comparable with mixtures considered in this study are given in Table 1. As increasing the GGBS contents, the values of effective diffusivity were reduced due to denser pore structure while the values of water filled porosity appear to be inconsistent with the replacement ratio. This might be due to lower density of GGBS mortar at same unit mass of binder applied as compared with OPC mortar. Moreover, the latent hydration in GGBS gives rise to lower connectivity of pore network for effective diffusion path [13], which is more dominant in the case of higher GGBS replacement.