A site study of durability indexes for concrete in marine conditions

The paper describes an investigation into the validity of durability index tests when used in a site situation and to evaluate the effectiveness of site curing methods. Concretes manufactured and cured under site conditions in a warm, humid coastal environment were investigated using recently developed durability index tests.Three blended binders (GGBS, FA and CSF) were used to cast a series of wall and slab elements. The elements were cured using practical site methods currently employed in the industry. Cores were extracted at early (28-day) and later (120-day) ages and used to determine the durability index properties.The results indicated that it is possible to manufacture, place and cure site concrete to achieve acceptable durability properties. Full wet curing proved to be the most effective method, as expected. While variation in potential durability properties existed at early age (28 days) between site and wet cured samples, at a later age (120 days) the variations had reduced such that, in practical and general terms, the different site curing methods were virtually indistinguishable. It was clear that environmental curing continues after 28 days provided climatic conditions are conducive for this to occur. Thus, on-going environmental curing largely governs the potential durability. To develop a concrete durability performance specification, it is imperative that a system is developed to quantify the effects of the environment on potential durability.     

Effects of W/B ratios and fly ash finenesses on chloride diffusion coefficient of concrete in marine environment

The objectives of this investigation were to study the effect of W/B ratios and fly ash finenesses on chloride diffusion coefficient (D_c) of concrete under marine environment. Original and classified fly ashes were used as a partial replacement of Portland cement type I at 0%, 15%, 25%, 35%, and 50% by weight of binder. Water to binder ratios (W/B) were varied as 0.45, 0.55, and 0.65. Concrete cube specimens of 200 mm were cast and removed from the molds after casting 1 day and then cured in fresh water for 27 days. After that, the specimens were placed to the tidal zone of marine environment in the Gulf of Thailand. Subsequently, the specimens were tested for chloride penetration profile after being exposed to the tidal zone for 2, 3, 4, and 5 years. The regression analysis of investigated data was carried out and Fick’s second law of diffusion was applied to calculate the chloride diffusion coefficient (D_c) and chloride concentration at concrete surface (C_o) based on one-dimensional analysis. The results showed that D_c of all concrete mixtures decreased with an exposure time and the decrease of W/B ratio resulted in the decrease of D_c. When the W/B ratio of concrete was reduced, the decrease of D_c in cement concrete was higher than that of the fly ash concrete. The use of fly ash with high fineness clearly reduced the rate of chloride ingress into concrete. In addition, fly ash with high fineness has more effective on reducing of D_c in concrete with higher W/B ratio than that with lower W/B ratio.     

Long-term chloride diffusion in silica fume concrete in harsh marine climates

Supplementary cementitious materials such as silica fume are typically necessary for producing high performance concrete for marine environments in hot regions, such as the Persian Gulf. Silica fume use generally improves the strength and/or durability properties of the concrete. This paper investigates the effects of silica fume on various properties of concrete specimens that were exposed to Persian Gulf conditions. Samples were taken at the ages of 3, 9 and 36 months and analyzed to determine the chloride diffusion coefficient. The results show that partial cement replacement with up to 7.5% silica fume reduces the diffusion coefficient, whereas for higher replacement rates the diffusion coefficient does not decrease significantly. Also time-dependent chloride diffusion and compressive strength of concrete containing silica fume are investigated.     

Determination of diffusion coefficient of chloride in concrete: an electrochemical impedance spectroscopic approach

For predicting the service life of concrete structures in marine environment, diffusion of chloride (D) is an important parameter. Electro-migration tests and ponding tests are two techniques conventionally adopted, however they are destructive in nature. EIS (Electrochemical impedance spectroscopy) being non-destructive appears a promising technique to arrive at ‘D_R’ (D from EIS) in situ in structures. The D_R of ordinary Portland cement concrete (OPC) was compared with that of Portland pozzolana cement concrete (PPC). The effect of curing on D_R was analyzed. The splash zone condition was created by subjecting the specimens to alternate wetting and drying cycles. At the end of 28 days of curing, the D_R of PPC concrete is only 66.7% of that obtained in OPC concrete. A linear correlation was established between D_R and the porosity of the concrete_. Due to pozzolanic reaction, the rate of pore refinement is faster in PPC concrete compared to OPC concrete. In M25-PPC concrete at the end of 28 days of curing, the pore size is decreased to 14.6% of that obtained at the end of 3 days of curing. The reduction of pore size by densification of pore structure due to pozzolanic reaction reduces the D_R value in PPC concrete. In 30 MPa concrete the D_R under wet cycle is 3 times higher than in dry cycle, which implied that corrosion is initiated 3 times faster in concrete exposed to the splash zone condition.     

Methodology of calculating required chloride diffusion coefficient for intended service life as function of concrete cover in reinforced marine structures

For marine reinforced concrete (RC) structures, chloride initiated corrosion of reinforcement is generally accepted as the service life limiting degradation mechanism. A methodology is described for how the maximum required chloride diffusion coefficient (D) of a concrete for achieving an intended service life (t) can simply be calculated as a function of concrete cover thickness (x) over the reinforcement by D = constant·x²/t.The principle is based on the usual mathematical solution to Fick's 2nd law of diffusion. The constant is broken down into 3 factors; chloride concentration factor, aging factor and temperature factor, in which input parameters have to be selected. These factors are calculated for a range of selected input parameters illustrating their sensitivity. The largest uncertainty lays in proper selection of the aging factor of the diffusion coefficient. Some concrete recipes satisfying estimated chloride diffusion coefficients are proposed as part of the methodology.     

On the corrosion risk presented by chloride bound in concrete

In previous work the influence of the solid phases of cement hydration on the pore solution chemistry during corrosion initiation has been discussed. It was noted that, because a fall in local pH is necessary for stable pits to develop on the passive steel, much of the chloride bound in concrete may participate in the process of corrosion initiation. At least two phases in hydrated ordinary Portland cement (OPC) will release such bound chloride before the pH falls to 11. In this work, these studies have been extended to include OPC blended with 10% calcium aluminate cement (CAC) and sulphate resisting Portland cement (SRPC). Evidence of a third phase that releases bound chloride was uncovered. Once again the data confirms that most of the bound chloride will be released by a relatively small reduction in pH. The release of chloride at such a high pH value compared to that required to sustain passive film breakdown suggests that the corrosion risk presented by bound chloride may be very similar to that presented by free chloride in concrete.     

Prediction of long-term chloride diffusion in silica fume concrete in a marine environment

Chloride-induced corrosion is the main factor in determining the durability and service life of the reinforced concrete structures exposed to marine environments. Recognition of chloride diffusion phenomenon in concrete and developing a prediction model that can estimate the service life of the concrete structures subject to long-term exposure is vital for aggressive marine environments. The present study focuses on developing such a prediction model of chloride diffusion coefficient for silica fume concrete under long-term exposure to a durability site located in the southern region of Iran. All investigations are based on 16 concrete mix designs containing silica fume with variable water-to-binder ratios exposed to sea water for maximum period of 60 months. This empirical model is developed by applying regression analysis based on Fick’s second law on the experimental results and is compared with previous studies in this area. This comparison indicates that the predicted chloride diffusion coefficient level is within a ±25% error margin in the specimens. The results indicate that reducing the water-to-binder ratio and adding the silica fume to a dosage of 10% reduces the chloride diffusion coefficient in concrete. This study also confirms that the chloride diffusion coefficient increases with temperature and decreases over time.     

Kirchhoff transformation analysis for determining time/depth dependent chloride diffusion coefficient in concrete

This article uses the Kirchhoff transformation method to solve a non-steady one-dimensional diffusion equation when the apparent diffusion coefficient is expressed as a function of time, depth, and concentration of chloride for concrete exposed to chloride environment. The analytical results obtained by the proposed method, which are coincided with those calculated from the Boltzmann–Matano methodology under specific condition, can be used to conveniently predict the chloride diffusion process physically and chemically so that the traditional natural diffusion test to obtain time/depth dependent apparent diffusion coefficient may be greatly simplified. Two new simplified methods to effectively process the experimental results from the natural diffusion test are proposed: one is called the long-specimen-at-one-specific-time method using fewer specimens at one time and the other the short-specimen-at-long-elapsed-time method using more specimens at various service times. Two numerical examples are provided to illustrate the application of these two proposed methods.     

Effect of silica fume addition and repeated loading on chloride diffusion coefficient of concrete

In this paper, the effect of silica fume (SF) on the chloride diffusion coefficient of concrete subjected to repeated loading was examined. Portland cement was replaced by 5 and 10 % SF. Five cycles repeated loadings were applied to concrete specimens, the maximum loadings were 40 and 80 % of the axial cylinder compressive strength ( $ f_{\text{c}}^{\prime } $ ), respectively. The diffusion coefficients were calculated from the steady state in the chloride migration test using the Nernst-Planck equation. The service life of concrete in chloride environment was predicted by Life-365 model. The results indicate that the diffusion coefficients of concrete containing 5 and 10 % SF replacements are lower than that of the control concrete at the age of 28 days. This trend increases with the increase of SF replacement. Five cycles repeated loading at 40 % $ f_{\text{c}}^{\prime } $ or 80 % $ f_{\text{c}}^{\prime } $ increase the diffusion coefficients (D ₂₈) for all mixes investigated in this study. However, the effect of 80 % $ f_{\text{c}}^{\prime } $ on D ₂₈ of concrete with 10 % SF is significantly lower than that of the control concrete without SF. Compared with the control concrete without SF, 10 % SF replacements increase the service life of concrete by more than 10 times.     

Determination of the concrete chloride diffusion coefficient based on an electrochemical test and an optimization model

The chloride diffusion coefficient is an indication of the capacity of any type of concrete to resist chloride penetration and is used to predict the service life of reinforced concrete structures. Its determination has been subject of intense study in the last 50 years and today there is no complete agreement in the methodologies used. In addition, its experimental determination is time and recourse consuming. As alternative, this article presents a new methodology to find the fundamental properties of concrete including the diffusion coefficients. From the measurement of the electrical properties of concrete during a migration test and using and integrated physical–neural network model the chloride related properties can be found. Results of experiments of normal diffusion were compared with the results of the new methodology and gave encouraging results.     

Strength and durability of lightweight concrete in hot marine exposure conditions

The paper presents results on strength development and durability of 35 and 50 MPa total lightweight and 50 MPa normal weight concretes exposed to hot marine exposure conditions for a period of two years. An initial water curing of 7 days and subsequent seaside exposure is more beneficial for the strength development of light-weight concrete than it is for normal weight concrete. One day initial curing and subsequent seaside exposure was not very conductive for the strength development of both lightweight and normal weight concretes. The water penetrability of total lightweight concretes was found higher than the normal weight concrete under all the initial curing conditions and on subsequent exposure to the hot-marine environment. In the same vein, the depth of carbonation for the total lightweight concretes was more than that of the normal weght concrete. On the overall, the results suggest that the higher the water penetrability of a given concrete, the more is the penetration of the damaging species like carbon dioxide, sulphate and chloride ions into a concrete.     

Lattice modeling of chloride diffusion in sound and cracked concrete

Reinforced concrete structures are frequently exposed to aggressive environmental conditions. Most notably, chloride ions from sea water or de-icing salts are potentially harmful since they promote corrosion of steel reinforcement. Concrete cover of sufficient quality and depth can ensure protection of the steel reinforcement. However, it is necessary to study the effects of material heterogeneity and cracking on chloride ingress in concrete. This is done herein by proposing a three-dimensional lattice model capable of simulating chloride transport in saturated sound and cracked concrete. Means of computationally determining transport properties of individual phases in heterogeneous concrete (aggregate, mortar, and interface), knowing the concrete composition and its averaged transport properties, are presented and discussed. Based on numerical experimentation and available literature, a relation between the effective diffusion coefficient of cracked lattice elements and the crack width was adopted. The proposed model is coupled with a lattice fracture model to enable simulation of chloride ingress in cracked concrete. The model was validated on data from the literature, showing good agreement with experimental results.     

Pseudo-coating model for predicting chloride diffusion into surface-coated concrete in tidal zone: Time-dependent approach

There are three fundamental physical types for repairs by surface treatment on concrete, i.e., coating, penetrant, and sealer. Due to the inconsistency in using the penetrant-based model in a previous study, three limitations occur. To avoid the limitations, this paper proposes a simple pseudo-coating model embedded in a Fick-based time-dependent approach for modeling the performance of surface-coated concrete in the tidal zone subjected to chloride attack. In addition, the time-dependent regression equation is also developed and combined in computation. To validate the approach, the chloride diffusion predicted by the developed approach is compared to that measured in the previous study. From the study, it is found that the nonlinear and linear surface chloride functions are suitable for uncoated and coated concrete, respectively. The sensitivity analysis shows that the chloride content in concrete is most sensitive to the time-dependent parameter in the surface chloride functions. The surface coating can either decrease or increase initial surface chloride, and the deterioration rate of the surface coating depends on coating materials. By considering the change of the surface chloride after surface treatment, two criteria are proposed to assess the lifetime of coating materials, i.e., equivalent amount of surface chloride, and equivalent instantaneous slope of surface chloride. And, the lifetime is calculated and compared to other literatures. The service life extension of concrete structures with different surface coatings is also compared. The benefit of combining the repair by surface coating with the durability design by increasing concrete cover is also shown.     

Chloride penetration into concrete in marine environment-Part II: Prediction of long term chloride penetration

The development of chloride penetration models is essential for the assessment of the service life of concrete structures exposed to marine environment. Simple models derived from Fick's second law of diffusion are at present the best way to predict the chloride penetration in practical situations. However these models need to be calibrated with experimental results. This paper presents an experimental study where the parameters used in the penetration model were calibrated to allow the prediction of long term chloride content in concrete. The results showed that both the concrete cover and concrete quality requirements stated in the present codes need to be increased so that an acceptable service life can be achieved.     

Chloride penetration into concrete in marine environment—Part I: Main parameters affecting chloride penetration

The durability of concrete structures exposed to marine environment depends mainly on the ability of concrete to resist to chloride ingress. This complex phenomenon depends on many parameters related to the concrete properties and to the micro-environmental characteristics. This paper presents the results of an experimental study where fifty four 1.0×0.5×0.12 m concrete panels were exposed to marine environment for three to five years. A study of three concrete mixes in five different exposure conditions was conducted and the results show that the chloride penetration is strongly dependent on both the concrete quality and the exposure conditions. Both the diffusion coefficientsD and the surface chloride concentrationCs derived from the chloride profiles show a clear time dependence. This effect has serious implications for long term predictions of chloride penetration, therefore if constant values ofD andCs are assumed gross errors can be made.     

Resistance of concrete with blast-furnace slag against chlorides, investigated by comparing chloride profiles after migration and diffusion

The effect of partial replacement of ordinary Portland cement (OPC) by blast-furnace slag (BFS) on concrete’s resistance to chlorides was examined by comparing chloride profiles and colour change boundaries after non-steady state migration and natural diffusion tests, cfr. NT Build 492 and NT Build 443, respectively. Chloride profiles were obtained by means of potentiometric titration and the colour change boundaries by spraying 0.1 M AgNO₃-solution. One OPC mixture and three BFS mixtures, with cement replacement levels of 50, 70 and 85 %, were tested. It was found that, compared to OPC concrete, migration coefficients of 50 and 70 % BFS concrete are decreased with 32 and 57 %, respectively. Diffusion coefficients are reduced by 66 and 50 %, respectively. Nevertheless, the chloride binding capacity of BFS concrete, determined from the difference between total and free chlorides, was lower than for OPC concrete. The results for 85 % BFS concrete differ from the results for 50 and 70 % BFS concrete, since higher migration and diffusion coefficients are found. Besides, free chloride concentrations at the colour change boundary (ccb), obtained after spraying with AgNO₃-solution, amounted to 0.30 mol/l on average. There was no clear relation between the free chloride content at ccb and the BFS content.     

Analysis of diffusion coefficient distributions in humic and fulvic acids by means of diffusion ordered NMR spectroscopy

The use of the computer program CONTIN to analyze pulsed-field gradient NMR (PFG-NMR) data for several standard humic and fulvic acids is described. An advantage of PFG-NMR analysis is that integration of different spectral regions provides a picture of how the diffusion coefficients vary with functional group composition for a given sample. Using prior knowledge of the sample and the principle of parsimony, CONTIN approximates a solution to the inverse Laplace transform applied to the decay of peak intensity with gradient area in the PFG-NMR experiment. Thus, a continuous distribution of diffusion coefficients is resolved for the polydisperse humic and fulvic acids. The results of the CONTIN analyses are in the form of a distribution function and a two-dimensional DOSY plot. The 2D DOSY spectrum displays chemical shifts along one axis and diffusion coefficients along the other, while a number-average diffusion coefficient, D(N), a weight-average diffusion coefficient, D(W), and a most probable diffusion coefficient, D(P), are realized from the diffusion coefficient distribution. For all spectral regions of each humic sample, D(W) was greater than D(N), which in turn was greater than or equal to the D(P), suggesting that the diffusion coefficient distribution is weighted toward smaller, more rapidly diffusing molecules. Polydispersities, estimated from the ratio D(W)/D(N), were less than the reported M(W)/M(N) values for similar humic substances. Thus, the D(W)/D(N) ratio obtained by CONTIN analysis of PFG-NMR data can be at least a qualitative, and at best a semiquantitative, indication of the polydispersity of the humic sample, but should not be used as a quantitative measure of polydispersity.     

Computational results of a model for chloride ingress in concrete including convection, drying-wetting cycles and carbonation

Over the past decades a considerable research effort has been attributed to the modelling of chloride ingress into reinforced concrete in order to predict its durability. Traditional models are based on the error-function. In the present paper computational results of an advanced model are presented, which takes environmental temperature and humidity fluctuations, chloride binding, diffusion and convection, as well as carbonation effects into account. These qualifications make the model particularly suitable for simulating drying-wetting cycles, an example of which is included. The good performance of the moisture sub-model is demonstrated in an example concerning drying cement paste. Another example deals with a simple submersion case. The examples are based on laboratory and real-life experiments with documented concrete or paste compositions and environmental conditions, as well as the eventual measured chloride profiles. In general, the computational results are in good agreement with the measurements. The paper concludes with a comparison between simulation results of the present model and the error-function model for the case of cyclic drying-wetting exposure. It appeared that the implicit negligence of the moisture fluctuations by the error-function model, forced that model to deploy strongly deviating material characteristics to reach agreement with the measured chloride profile.     

Influence of long-term chloride diffusion in concrete and the resulting corrosion of reinforcement on the serviceability of RC beams

This paper presents the chloride penetration and the effect of chloride ingress on the serviceability of reinforced concrete (RC) beams. A series of experimental studies were carried out on beams with various corroded ages up to 28 years. The chloride content in different locations were tested during various periods. Different states influencing the serviceability of the corroded beams were investigated, including the maximum width of the corrosion-induced cracks of the concrete cover, the mid-span deflection of the beams under the service load and their load-bearing capacity. Based on the results available from this programme, the service life of corroded beams was predicted by the corrosion process of the reinforcement. The results showed that the chloride corrosion could significantly deteriorate the serviceability of the beams. The current criteria concerning the chloride content at the level of the reinforcement of the concrete beams and the maximum width of the corrosion-induced cracks appear to be very conservative.