The chloride diffusion coefficient is the most important parameter when predicting chloride ingress in concrete. This paper proposed a model for calculating the chloride diffusion coefficient of steel fiber reinforced concrete (SFRC). Considering the concrete structures in service are usually subjected to external loads, the effect of bending load was discussed and expressed with a stress factor
Chlorideinduced rebar corrosion is one of the major forms of environmental attack to reinforced concrete, which may lead to reduction in the strength, serviceability, and esthetics of the concrete structures [
Some work has been carried out over the past few decades to investigate the chloride transport property of SFRC [
The chloride diffusion coefficient is the most important parameter when predicting the chloride ingress in concrete. The intention of this paper is to present a model for predicting the chloride diffusion coefficient of SFRC under bending load. Besides, the effect of steel fibers on the chloride transport in concrete was investigated with bulk diffusion tests. The chloride diffusion coefficients of SFRC under compression and tension were determined and discussed.
The mechanism of chloride transport into concrete structures is a rather complicated process [
The chloride diffusion coefficient of concrete is not constant but decreases over time due to the continuous hydration [
The analytical solution of Fick’s second law can only be derived under the assumption that the chloride diffusion coefficient is constant. Therefore, mathematical treatment needs to be done to the timedependent chloride diffusion coefficient to get an average coefficient, which is defined as apparent chloride diffusion coefficient
Strictly speaking, the apparent diffusion coefficient
If the age factor
SFRC consists of cement paste, aggregate, and steel fibers. Compared with cement paste, the aggregate and steel fibers can be considered as impermeable. The chloride diffusion coefficient of SFRC at time
The presence of aggregate and steel fibers leads to formation of interfacial transition zone (ITZ) between the paste and the aggregate/fibers in SFRC. Due to its higher porosity and bigger pore size, ITZ has much higher diffusivity than paste. As a result, the presence of ITZ increases the rate of chloride transport in concrete. On the other hand, the presence of aggregate increases the tortuosity of chloride’s transport path in concrete and consequently decreases the chloride diffusivity. Moreover, steel fibers can restrain the crack initiation and propagation, and the high relative surface area of fibers can adsorb some chlorides and retard the chloride ingress. It requires a lot of work to study these effects of aggregate and fibers on chloride transport in SFRC. For simplicity, in this study, the factor
Cracks caused by mechanical loads create easy path for chloride transport and speed up the chloride ingress in concrete, which frequently occurs in field applications [
There are several models predicting the chloride diffusion coefficient of cement paste [
Sun [
Mixture proportions of the concrete used in this study are given in Table
Mixture proportions.
Cement (kg/m^{3})  FA (kg/m^{3})  Sand (kg/m^{3})  Gravel (kg/m^{3})  Water (kg/m^{3}) 


 

C50  315  135  663  1179  158  0.35  —  0.687 
C501  370  158  779  917  185  0.35  1  0.644 
C501.5  379  163  799  834  189  0.35  1.5  0.625 
The mixture was cast into 500 mm × 130 mm × 100 mm molds. After 24 hours curing in molds, the specimens were demolded and cured in standard condition (20°C, RH > 95%) till the age of 90 days.
Chloride bulk diffusion tests were performed on specimens according to NT Build 443 [
Loading device for the chloride bulk diffusion test.
Flexural strength and bending load (at a stress level of 0.5) at 90 days.
C50  C501  C501.5  

Flexural strength (MPa)  5.87  8.69  9.16 
Bending load in bulk diffusion tests (kN)  13.7  18.9  19.8 
After 60 days, 90 days, and 150 days immersion, respectively, specimens were taken out to determine the watersoluble chloride contents, namely, free chloride in concrete according to JTJ 27098 [
Based on (
Results of bestfit apparent chloride diffusion coefficients. Immersion time: (a) 60 d, (b) 90 d, and (c) 150 d.
For unstressed concrete, there was a slight decreasing tendency in diffusion coefficients with the increase of steel fiber fraction. Roque et al. [
The effects of stress on
On the other hand, due to the superior resistance of steel fibers to crack initiation and propagation, the apparent chloride diffusion coefficient
When the SFRC specimens were under compression,
The age factor
Given
Results of chloride diffusion coefficient
Immersion time (days)  C50  C501  C501.5  

No stress  Under tension  Under compression  No stress  Under tension  Under compression  No stress  Under tension  Under compression  
60  3.47  5.29  3.57  3.36  3.57  2.98  3.14  3.44  2.80 
90  3.51  5.25  3.44  3.30  3.78  3.01  3.10  3.18  2.77 
150  3.48  5.21  3.58  3.44  3.69  2.67  3.02  3.23  2.04 
Average of 
3.49  5.25  3.53  3.37  3.68  2.89  3.08  3.28  2.54 
The stress factor
The total volume fraction of aggregate and steel fibers was calculated with the mix proportions and is given in Table
Results of




No stress  Under tension  Under compression  
C50  1.75  1  1.50  1.01 
C501  1.49  1  1.09  0.86 
C501.5  1.29  1  1.06  0.82 
As seen in Table
This paper proposed a model for calculating the chloride diffusion coefficient of SFRC under bending load. The chloride bulk diffusion tests were performed on SFRC and plain concrete specimens to investigate the chloride resistance of SFRC under load and determine some factors in the model. The results obtained are summarized below.
The chloride resistance of unstressed SFRC was slightly better compared with unstressed plain concrete. For concrete under bending load at a stress level of 0.5, the apparent chloride diffusion coefficient of SFRC in the tensile zone was about 40% lower than that of plain concrete. The compressive stress seemed to have no apparent effect on the chloride transport in plain concrete. On the other hand, the chloride transport in SFRC was a little retarded by compressive stress. Overall, the addition of steel fibers could improve the chloride resistance of concrete and consequently prolong the service life of reinforced concrete in chloride environment.
The chloride diffusion coefficient
Considering the time dependence of chloride diffusion coefficient, it is apparent that chloride diffusion coefficient
The authors declare that they have no conflicts of interest.
This work was supported by a technical research program from NV Bekaert SA of Belgium, the National Natural Science Foundation of China (Grant nos. 1378113, 51438003, and 51278098), and a Plan of Six Peak Talents in Jiangsu Province (Grant no. JZ004) and was partly supported by a National Basic Research Program of China (973 Program with Grant no. 2015CB655102).