- Open Access
Effect of Wet Curing Duration on Long-Term Performance of Concrete in Tidal Zone of Marine Environment
© The Author(s) 2015
- Received: 6 May 2015
- Accepted: 2 November 2015
- Published: 24 November 2015
A proper initial curing is a very simple and inexpensive alternative to improve concrete cover quality and accordingly extend the service life of reinforced concrete structures exposed to aggressive species. A current study investigates the effect of wet curing duration on chloride penetration in plain and blended cement concretes which subjected to tidal exposure condition in south of Iran for 5 years. The results show that wet curing extension preserves concrete against high rate of chloride penetration at early ages and decreases the difference between initial and long-term diffusion coefficients due to improvement of concrete cover quality. But, as the length of exposure period to marine environment increased the effects of initial wet curing became less pronounced. Furthermore, a relationship is developed between wet curing time and diffusion coefficient at early ages and the effect of curing length on time-to-corrosion initiation of concrete is addressed.
- silica fume
- service life
The chloride-induced corrosion of the embedded steel has become the most common cause of loss of integrity and failure in concrete structures and infrastructures placed in the marine environment (Swamy 1988; Neville 2000; Radlińska et al. 2014; Pritzl et al. 2014; Ghassemzadeh et al. 2011). Hence, the chloride permeability has been recognized to be a critical intrinsic property of the concrete (Guneyisi et al. 2005, 2009), and a lot of research has been conducted to enhance concrete resistance to chloride permeability (Shekarchi et al. 2009).
From durability point of view, concrete cover quality plays significant role in blocking of aggressive substance ingress such as chloride ions into the reinforced concrete (Thomas 1991; Bonavetti et al. 2000). There are several methods to improve the quality of the concrete cover such as use of supplementary cementitious materials, reduction in water-to-cementitious materials ratio (w/cm), and appropriate initial curing regimes (Neville and Brooks 1990; Ghassemzadeh et al. 2010). Although it is a very simple and inexpensive procedure, proper initial curing, prior to exposure to marine environment, has an important influence on improving concrete cover quality so that the concrete acting as a fine barrier to the access of aggressive species and accordingly extend the service life of reinforced concrete structures exposed to chloride (Alizadeh et al. 2008; Khatib and Mangat 2002; Khatib 2014; Radlinski and Olek 2015).
The objective of curing is considered by the duration of providing concrete with sufficient humidity and appropriate temperature conditions to reduce the loss of moisture to ensure the progress of hydration reactions causing the filling and segmentation of capillary voids by hydrated compounds (Guneyisi et al. 2005, 2009). On the contrary, drying of concrete particularly at the concrete surface, caused by a poor curing regime, leads to a restricted hydration and thus higher porosity and permeability in the surface layers which form covers for the reinforcement protections (Mangat and Limbachiya 1999; Khanzadeh-Moradllo et al. 2009).
The matter would be more critical in the case of concrete containing silica fume replacement because the pozzolanic reaction is, in general, very sensitive to the curing procedure (Toutanji and Bayasi 1999; Atis et al. 2005). According to the ACI 308 Recommended Practice (ACI Committee 308 1998), the curing period should be extended to 14 days when the cement contains supplementary cementitious materials, owing to the slow hydration reactions between supplementary cementitious materials and the calcium hydroxide. In addition, curing condition also could be an important parameter in controlling durability of the reinforced concrete in a harsh condition of the marine tidal zone prior to exposing to sea water, where the concrete cover is subjected to wetting–drying cycles.
A considerable volume of research has been conducted on different curing regimes and related effects on concrete properties. However, the effect of curing conditions on the chloride penetration into the concrete in real field condition at long term has not been well studied. Also, despite the importance of this object in Persian Gulf region, which is one of the high aggressive environments in the world, a few investigations were conducted in this region in long-term (Neville 2000; Shekarchi et al. 2009; Khanzadeh-Moradllo et al. 2012). In this regard, a comprehensive effort is accomplished in Construction Material Institute (CMI) to examine the short and long-term effect of curing regimes on durability of concretes located in Persian Gulf marine environment. The objective of this study is to investigate the effect of wet curing duration on chloride penetration in plain and blended cement concretes with 7.5 % silica fume which subjected to tidal exposure condition in Persian Gulf for 5 years.
2.1 Materials and Mixture Proportions
Chemical properties of binders.
Oxide composition % by mass
Details of the concrete mixtures.
Silica fume (kg/m3)
Coarse aggregate (kg/m3)
Fine aggregate (kg/m3)
The concrete mix proportions used in this study were not specifically chosen to meet the durability requirements given in Iranian National Code for concrete durability in Persian Gulf for the conditions of exposure used, but to provide concretes which would undergo a measurable amount of chloride penetration and deterioration in the short exposure periods used. Therefore, the results of this study may not be generalized for concrete made with a low w/cm. In addition, silica fume content of 7.5 % by weight of cement was used, because a previous study (Shekarchi et al. 2009) showed that there is an optimum silica fume content of 7.5 % by weight of cement beyond which additional silica fume does not produce additional benefits in line with the additional costs.
2.2 Casting and Curing of Concrete Specimens
Properties of fresh and hardened concrete.
Air content (%)
Compressive strength (MPa)
2.3 Exposure Condition
Next, the investigated prism specimens which were located in Bandar-Abbas coast in south of Iran are sealed on four sides using epoxy polyurethane coating to ensure one-dimensional diffusion. The performance of this type of coating has been confirmed by previous studies (Khanzadeh-Moradllo et al. 2012). Specimens were then subjected to tidal zone exposure condition in Persian Gulf for the entire period of investigation (60 months). Tidal exposure was situated at the about 2.2 m from sea level, so that concrete specimens were in contact with sea water for 12 h per day then they were exposed to dry condition (air) for rest of the day, simulating the tidal zone condition.
Concentration of various ions in seawater at Persian Gulf.
2.4 Sampling and Testing
In the laboratory, a nominal 45 mm diameter core is taken from each slice to provide chloride concentration profiles. Each core is grinded in eight increments from the finished surface to an estimated depth of chloride penetration. The method used to estimate the chloride penetration depth was according to the procedure described by NordTest NT Build 492, which involves measuring the depth of color change of a freshly cut concrete surface in the direction of the chloride flow using 0.1 M AgNO3 aqueous solution. Fine particles for chloride analysis are collected using a profile grinder parallel to the exposed surface according to NordTest NT Build 443 method with the accuracy of 0.5 mm at eight different depths. The first 1 mm fine particles are not included in calculations as it might be affected by actions such as washout, etc. The profile grinder and a grind hole are cleaned between depth increments to reduce the possibility of cross-contamination of samples from different depths. For each sample of concrete, fine particles are collected, the depth below the exposed surface is calculated as the average of six uniformly distributed measurements using a slide caliper. The fine particles from each layer is collected and pulverized so that all the material will pass a 850-µm (No. 20) sieve. At each depths, a sample having a mass of approximately 10 g is selected to the nearest 0.01 g and then analyzed for acid-soluble chloride content by the potentiometric titration of chloride with silver nitrate according with ASTM C 1152, and ASTM C 114, part 19. The cross-sectional area of a 45 mm diameter core is large enough to represent the concrete so that there is no need to be concerned about variations from sample to sample due to varying aggregate contents.
2.5 Chloride Diffusion Coefficient (D c ) and Surface Chloride Content (C s ) Calculation
Fick’s second law for one-dimensional diffusion, as shown in Eq. (1), is a special case of a more generalized model of diffusion where concrete is assumed to be a homogenous material; chloride concentration at the exposure surface is considered constant; no chemical or physical binding between the diffusing species and material occurs; and the effect of co-existing ions is constant. In other words, these limitations of analysis may be neglected as measured data are used for comparison purposes within the same set of exposure conditions. Also, the effect of other mechanisms of chloride ion penetration such as a capillary suction or sorption mechanism is not considered in this study.
Using a computer statistical analysis program, the nonlinear regression is carried out on the experimental data and by curve fitting of solutions of Fick’s second law of diffusion, the values of D c and C s in the Eq. (2) are determined. The curve fitting has been done in such a way that the chloride profiles are fitted where the correlation between the measured and fitted profiles has a maximum. Curve fitting has been performed in accordance with a procedure described in NordTest NT Build 443 and resulted in two regression parameters; Namely a diffusion coefficient and surface chloride content. For each specimen, at the time of testing, a single measurement of chloride concentration at each specified depth has been done and the diffusion coefficient and surface chloride build-up have been calculated accordingly.
3.1 Chloride Profiles at Varying Exposure Time
It is obvious from chloride concentration profiles during exposure time (Fig. 2) that the wet curing effect is time-dependent and its influence on chloride resistance diminishes in long-term irrespective of the concrete mixture. Further analysis is provided in following sections based on calculated diffusion coefficient and surface chloride content.
3.2 Chloride Diffusion Coefficient (D c )
According to Fig. 3, the differences in value of diffusion coefficient in various ages, especially between early and long-term ages, were notable in short-term curing regimes and non-curing, but this is not the case in 27 days curing. Indeed, as the curing time increases, the decreasing rate of diffusion coefficient upon time reduces while the microstructure of concrete improves and the ingress of chloride ions into the concrete diminishes in early ages. This reduction in early age diffusion preserves the concrete against high rate of chloride penetration at early ages. So the initiation time of reinforcement corrosion will delay and the service life of the structure will increase.
As it is shown in Fig. 3, with comparison of silica fume and plain specimens, it is found that the silica fume specimens have lower diffusion coefficient in all curing times which confirms that concretes containing silica fume exhibit improved chloride penetration resistance compared to those of plain Portland cement concretes.
Modeling the chloride ion diffusivity versus exposure time.
Based on Table 5, good correlation between Dc and exposure time is observed for all NPC and SFC samples with regression coefficients varying from 0.78 to 0.99. The model introduced in Eq. (3) can be employed to estimate the variation of chloride diffusion coefficient with time for different curing regimes. The fitted equation has also been incorporated in estimating the time-to-corrosion initiation which will be discussed in following sections.
3.3 Relationship Between Diffusion Coefficient and Curing Time
Based on Fig. 4, there is no distinct relationship between k curing and t curing in long-term (36 and 60 months) and some of the curing regimes are not any more effective in reducing the diffusion coefficient. A “k curing = 1” is considered as a efficiency boundary of curing regimes, where the wet cured sample acts similar to no-cured sample. According to results in long-term ages, a 27 days wet curing is the only curing regime which preserves its efficiency in reducing diffusion coefficient in both of NPC and SFC mixtures. As mentioned, this might be due to the curing effects of the seawater masking initial differences.
The results of long-term ages also imply that there is a slight increase in diffusion coefficient from a 0-D curing condition to the 6-D and 3-D in NPC and SFC samples (k curing > 1). William F. Perenchio observed the same trend between initial curing period and long-term drying shrinkage (Perenchio 1997). According to Perenchio’s suggestion (Perenchio 1997), it is possible that there is a pessimum initial curing time with respect to drying shrinkage or other parameters which produces the greatest value for that parameter. This higher drying shrinkage might cause microcracks and respectively an increase in diffusion coefficient. More work is needed to further understand this behavior.
Curing factor for different curing conditions.
k c,cl * (NPC)
k c,cl (SFC)
k c,cl (The European Union 2000)
1 day wet curing
3 days wet curing
7 days wet curing**
28 days wet curing**
3.4 The Influence of the Curing Time on Surface Chloride Content (C s )
The time-dependent characteristic of the chloride content at a concrete surface is another significant parameter in predicting the chloride ingress at the depth of steel and the concrete structures service life (Ann et al. 2009). Therefore, the influence of curing conditions on surface chloride content is addressed in current study.
Modeling the surface chloride concentration versus exposure time.
3.5 The Influence of the Wet Curing Time on Time-to-Corrosion Initiation of Concrete Structures
According to results, it seems that longer wet curing (27 days) provides high quality skin layer in concrete surface, and accordingly it plays significant role as a barrier to concrete inner depths in controlling chloride ions penetration into the concrete specimens. These results are for the materials, mixtures, environmental conditions, and specifications used on this study.
A wet curing extension decreases difference between initial and long-term diffusion coefficients due to improvement of concrete cover quality and blocking the ingress of aggressive substance in initial ages. This reduction in early age diffusion preserves concrete against high rate of chloride penetration at early ages.
As the length of exposure period to marine environment increased the effects of initial wet curing became less pronounced. This might be due to the curing effects of the seawater which compensate for the differences observed in early age diffusion coefficient due to the duration of initial wet curing. In long-term ages, a 27 days wet curing is the only curing regime which preserves its efficiency in reducing diffusion coefficient in both of NPC and SFC mixtures.
A power functional relationship is derived between curing factor (k curing = D t /D 0, where D 0 is the diffusion coefficient of no-cured concrete, D t is the diffusion coefficient of wet cured specimen) and time of wet curing (t curing ) at early ages.
The general trend of surface chloride shows its increment as the time goes on. The SFC specimens have more surface chloride content in comparison with NPC specimens in early ages presumably due to the higher level of chloride binding and sorptivity. But as the time goes, the rate of surface chloride content increment is lower in SFC specimens in contrast to NPC specimens.
Both plain and silica fume specimens show that 27 days wet curing causes tangible increase in time-to-corrosion initiation and service life of concrete structures. It seems that no difference is observable between the time-to-corrosion initiation values of the curing times less than 6 days in both silica fume and plain specimens based on time dependent results from field. It might happen because of the availability of continuous capillary pores in concrete specimens (w/c of 0.50) with less amount of wet curing.
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