Toward a probabilistic design of reinforced concrete durability: application to a marine environment

This work aims to propose a complete design methodology for concrete durability problems based on a probabilistic method including an original chloride diffusion modelling compatible with the new performance-based approaches. This is illustrated by the study of a concrete immersed in sea water. The physical and chemical processes that lead to the corrosion of the concrete reinforcement bars are presented first. A chloride penetration model based on Fick’s second law is proposed. Next, the durability modelling parameters chosen among the durability indicators (Baroghel-Bouny, Concrete design for structures with predefined service life—durability control with respect to reinforcement corrosion and alkali–silica reaction, 2004; Alexander et al., Mater Struct 41:921–936, 2008) are assessed. They depend on the concrete formulation and the chemical composition of the cement. These indicators are characterized by their statistical distributions, which are realistically specified from a wide literature review. The whole probabilistic modelling is included in a Bayesian network so that it can be easily updated to include new experimental data. The evaluation of the time dependant corrosion risk is estimated for two types of cement: CEM I and CEM I with silica fume. The result shows the effect on the Lind–Hasofer reliability index of the type of cement, the concrete quality and the design options. The quality is integrated through the mean value and the standard deviation of the modelling parameters. The method could be used either directly for cover design or for semi-probabilistic design code calibration.     

Some effects of cement and curing upon carbonation and reinforcement corrosion in concrete

Experimental data are presented to illustrate the effects of cement type and curing upon the depth of carbonation and reinforcement corrosion in cover concrete after exposure for 18 months at 20°C and 60% relative humidity. Three curing periods (1, 3 and 28-days) and 17 cements, with various proportions of granulated blastfurnace slag or limestone, were used to make concretes, at 0.59 water/cement ratio, with 28 day strengths in the range 26 to 46 MPa. The depth of carbonation after 18 months was 64% greater than after 6 months and was affected more by cement type than by curing. The depth of carbonation increased when Portland cement clinker was replaced by 19% or more of limestone or granulated blastfurnace slag. The depth of carbonation after 18 months correlated better with the air permeability of cover concrete, initial weight loss (an indicator of moisture diffusion rate in cover concrete) or the cube strength 8 days after the end of curing than it did with 28-day cube strength. The rate of reinforcement corrosion increased steeply when the carbonation front approached the reinforcing steel, and it was still increasing after the carbonation front had completely passed the reinforcement. For a given unneutralised remainder (i.e. cover depth minus the depth of carbonation), curing had little effect upon the rate of corrosion but higher rates were observed when the cement contained granulated blastfurnace slag. The results were broadly consistent with a simple engineering strategy in which the rate of carbonation was related to the air permeability of cover concrete, and the rate of any subsequent reinforcement corrosion was largely dependent upon moisture conditions, without any obvious influence of the cover depth or the permeability of the cover concrete. The results also suggested that estimation of the rate of reinforcement corrosion could be improved by taking account of the cement type and treating the unneutralised remainder as a variable.     

Investigations on the influence of oxygen on corrosion of steel in concrete—Part 2

In Part I of this article, potentiostatic tests on reinforced concrete specimens under different environmental conditions have been described. The results of these tests have shown that it is possible to define four cases related to the humidity, in which different dominant parameters influence the corrosion rate for the reinforcement, discussed in detail in Part I. The main result is that the diffusion of oxygen is only a significant limiting factor for the corrosion rate when the concrete around the reinforcement is water-saturated and most of the oxygen within the concrete near the reinforcement has been consumed by the cathodic reaction of the corrosion process. As a consequence, the corrosion rate is only influenced by oxygen diffusion through the concrete cover in structures which are submerged or exposed to long term or cyclic water application, thereby causing water saturation of the concrete around the reinforcement for periods of several weeks. In Part II of this article, the determination of cathodic current-density/potential curves is described. Using the results of these tests and a simplified equivalent electric circuit model for the corrosion process, it is possible to calculate the influence of oxygen diffusion on the corrosion rate of the reinforcement quantitatively for defined conditions. One interesting consequence of this work is that the thickness of the concrete cover does not influence the corrosion rate by reducing the oxygen diffusion rate significantly when the concrete is not water-saturated. However, a sufficiently thick concrete cover is essential due to several other reasons (ingress of chlorides, carbonation, etc.).     

Engineering performance of a new siloxane-based corrosion inhibitor

This paper presents an evaluation of a new non-toxic corrosion inhibitor on selected engineering properties of concrete mixes with different cementitious materials following a corrosion and durability study on concrete samples. Corrosion inhibitors consist of powders or solutions which are added to concrete when mixed to prevent or delay corrosion of steel by their reaction with ferrous ions to form a stable and passive ferric oxide film on the steel surface. The new inhibitor functions slightly differently and its corrosion inhibition effect is due to the formation of a siloxane coating on the steel surface. Therefore, the performance of the new inhibitor in concrete mixes manufactured with CEM I, PFA and GGBS cements was compared against a well known and established corrosion inhibitor on the market, namely calcium nitrite in terms of their effect on workability (measured in terms of slump), compressive strength, freeze–thaw durability and macro-cell corrosion. The results from this experimental programme have demonstrated that the new inhibitor is effective in reducing or slowing down corrosion. In addition, it was found that CEM I concrete containing the new inhibitor was less penetrable to chlorides than that without. A similar set of results was obtained for the freeze–thaw resistance, but the compressive strength was found to decrease with the addition of the new inhibitor. In the case of concretes containing PFA and GGBS, the new inhibitor was found to be less effective. Further, long-term investigations are recommended to assess the effectiveness over time.     

阻锈剂对混凝土钢筋氯盐腐蚀行为的影响

过去对混凝土腐蚀的研究,其腐蚀液与实际的混凝土孔溶液有较大的差异。采用水泥提取液模拟混凝土孔溶液,通过自腐蚀电位、动电位极化和电化学阻抗谱研究了Ca(NO2)2及商用阻锈剂NNMI和NNRI对混凝土中钢筋氯盐腐蚀行为的影响。结果表明:3种阻锈剂均能有效降低钢筋受Cl-腐蚀的风险,减小腐蚀速率,并能不同程度地提高钢筋腐蚀的临界Cl-浓度;Ca(NO2)2为阳极型阻锈剂,当[NO2-]/[Cl-]为0.62～1.47时,阻锈效果最好,高达98.2%,NNMI和NNRI为以抑制阴极反应为主的综合型阻锈剂,在推荐掺量下,当环境中Cl-浓度为0.03 mol/L左右时,阻锈效率分别可达到85.0%和87.1%。因此,应根据腐蚀环境状况,选用不同的阻锈剂。     

Influence of curing conditions on durability of alkali-resistant glass fibres in cement matrix

Glass fibres in concrete material often increase the flexural strength. However, these fibres when in contact with cement are altered by alkali reactions due to the presence of portlandite. This study presents the results of investigation to show the effect of curing conditions on the durability of alkali-resistant glass fibres in cement matrix. Test results show that even alkali resistant fibres treated with zirconium oxide present the same degradation phenomenon. They also show that the nature of the cement has a large influence on the protection of the fibres: the Portland CEM II is less damaging than the CEM I. The substitutions of a part of cement by silica fume gave no substantial improvements to the mechanical strength of the glass fibre reinforced cement (GFRC). However, the observed microstructures in the samples show that the degradation is weakened with the addition of silica fumes. The analytical techniques used in this study are scanning electron microscope (SEM) and X-ray diffraction.     

Investigations on the influence of oxygen on corrosion of steel in concrete—Part I

The main objective of the research programme presented in the two parts of this article is determining the influence of oxygen diffusion through the concrete cover, on the corrosion process of the reinforcement in concrete structures. Altogether, 66 so-called “concrete corrosion cells”, i. e. reinforced concrete specimens with locally separated anodically and cathodically acting steel bars and different concrete compositions, have been used for the laboratory tests. Two testing methods have been used to investigate the influence of oxygen diffusion on the cathodic reaction under different environmental conditions: potentiostatic tests (presented in part I) and the determination of cathodic current density/potential curves (presented in part II). Using the results of these tests and a simplified equivalent electric circuit model for the corrosion process, it was possible to calculate the influence of oxygen diffusion on the corrosion rate of the reinforcement under defined conditions. The results of the laboratory tests and calculations show that the diffusion of oxygen is a significant limiting factor for the corrosion rate only when the concrete around the reinforcement is water saturated and most of the oxygen within the concrete near the reinforcement surface has been consumed by the cathodic reaction of the corrosion process. As a consequence, the corrosion rate is influenced by oxygen diffusion only through the concrete cover in structures which are submerged or exposed to long-term or cyclic water application that causes water saturation of the concrete for periods of several weeks. In the case of common outdoor structures being exposed to rain and not submerged or constantly water saturated due to other reasons, no reduction of the corrosion rate induced by limited oxygen diffusion is therefore to be expected.     

Long term properties under marine exposure of steel fibre reinforced concrete containing pfa

The paper presents results on the long-term mechanical properties and durability under marine exposure of a steel fibre reinforced concrete (SFRC) mix containing pulverized fuel ash (PFA) which was developed for marine applications. The mix was of proportions by weight of PFA:OPC:fine aggregate:coarse aggregate of 0.26:0.74:1.51:0.84 with a water/(OPC+PFA) ratio of 0.4. The resulting cement content of the mix was 435 kg m⁻³. Theconcrete was reinforced with low-carbon steel, corrosion-resistant (galvanized) or melt-extract (stainless) steel fibres. Prism specimens were cured in the tidal zone at Aberdeen beach, under wet-dry cycles of sea-water spray in the laboratory, in a water-tank in the laboratory and in the laboratory air. The specimens were cured for up to 1200 marine cycles of exposure (640 days) and were tested at regular intervals of age. The paper presents results on long-term compressive strength, flexural strength and energy absorption capacity as measured from the load-deflection curves. The state of corrosion of fibres is also described. The results indicate that fibres embedded within concrete remain free from corrosion under marine exposure. In the case of fibres exposed at the concrete surface during casting, extensive corrosion occurs in low-carbon steel fibres, isolated rust spots appear in corrosion-resistant fibres and no corrosion is evident in melt-extract fibres. This corrosion, however, remains a surface phenomenon and does not penetrate the concrete. The long-term mechanical properties indicate no deterioration due to possible corrosion. In general the compressive strength of concrete increases significantly with fibre reinforcement.Increases in flexural strength and post-cracking ductility due to fibre reinforcement are of the order normally expected of SFRC.     

A study on reinforcement corrosion and related properties of plain and blended cement concretes under different curing conditions

This paper presents the results of an experimental investigation on the steel reinforcement corrosion, electrical resistivity, and compressive strength of concretes. Concretes having two different water–cement ratios (0.65 and 0.45) and two different cement contents (300 and 400 kg/m³) were produced by using a plain and four different blended portland cements. Concrete specimens were subjected to three different curing procedures (uncontrolled, controlled, and wet curing). The effect of using plain or blended cements on the resistance of concrete against damage caused by corrosion of the embedded reinforcement has been investigated using an accelerated impressed voltage setup. The resistivity of the cover concrete has been measured non-destructively by placing electrodes on concrete surface. The compressive strength, electrical resistivity, and corrosion resistance of the concretes were determined at different ages up to 180 days. The results of the tests indicated that the wet curing was essential to achieve higher strength and durability characteristics for both plain and especially blended cement concretes. The concretes, which received inadequate (uncontrolled) curing, exhibited poor performance in terms of strength and corrosion resistance.     

Effect of polypropylene fibre reinforced mortars on steel reinforcement corrosion induced by carbonation

Low amounts of polypropylene fibres are added to concrete as a secondary reinforcement to control cracking. Whether this addition might improve the corrosion resistance of the concrete reinforcement by increasing the resistance to carbonation, via reducing penetrating paths, is the subject addressed in the present paper. Crack control by the fibres in plastic state mortars and crack evolution with time have been studied. Furthermore, the influence of crack width on the steel bar corrosion induced by carbonation has also been monitored. Circular specimens made of mortar have been employed in the experimental phase of the study, using a water/cement ratio of 0.50 and cement/sand ratio of 2/1. The polypropylene fibre content was 0% and 0.5% by volume. Low modulus polypropylene fibres may control the crack width in specimens subjected to inadequate curing conditions. No relationship between crack width and time for corrosion initiation has been observed. However, a beneficial effect of fibre addition on the corrosion rate was found.     

Improved method of electrical resistance—a suitable technique for checking the state of concrete reinforcement

The paper describes an improved method of electrical resistance for checking the state of concrete reinforcement and its corrosion. The method is based on the phenomena of the weakening of the cross-section of reinforcement and the increase of its electrical resistance as a result of corrosion. The method was improved by developing a so-called ‘corrosion sensor’. This sensor is embedded in cement composite test specimens or in the concrete structure and enables one to check the condition of the reinforcement. The structure of the sensor excludes disturbing effects during the measurement (e.g. influence of temperature changes of the environment) and increases the sensitivity of the method and the reliability of the test results. This significantly improves this experimental and test technique. The results of the verification of the improved method are given.     

Effect of Permeable Crystalline Material on Steel Reinforcement Corrosion of Concrete

Permeable crystalline materialcan permeate into pores and cracks of concrete and catalyze the reaction between Ca(OH) 2and unhydrated cement to generate a great quantity needle non-soluble crystals, which can stop up the pores and cracks of concrete, and increase the impermeability of concrete. This paper reported the results of a study conducted to evaluate steel reinforcement corrosion of concrete specimens uncoated and coated with permeable crystalline material as well as mixed with the permeable crystalline material. The properties evaluated for corrosion test were water impermeability, water absorption, compressive strength and potential. The results of water impermeability, water absorption, compressive strength clearly showed that the permeable crystalline material could prohibit water, any soluble salts and moisture from penetrating the concrete to cause corrosion, leaking, and other problems, and it did increase the compressive strength, which was favorable for protection of corrosion of reinforcing steel. Moreover, it was concluded from the potential-time curve that the steel reinforcement of uncoated specimen was in the state of activation whereas that of other specimens coated and mixed with the permeable crystalline material was in the state of inactivation. Above all, it was indicated that the permeable crystalline materialis very effective to protect the steel reinforcement of concrete from corrosion.     

Chloride corrosion of steel fibre reinforcement in cement mortar

This paper presents an experimental study on the corrosion resistance of steel fibres and steel bar reinforcement in cement mortar. The mortar matrix incorporated various amounts of calcium chloride from 2 to 10%, and the rate of corrosion was monitored by the electrode potential method. The structure of the mortar and the steel surfaces were examined by scanning electron microscopy. The results showed that the addition of calcium chloride modified the microstructure of the mortar matrix, both its water absorption capacity and its porosity increased with increasing amounts of calcium chloride. The electric potential measurements showed that while the bar reinforcement displayed corrosion at 2% calcium chloride, the fibres did not indicate any harmful corrosion until the chloride content was 6%. Chloride admixtures added to concrete may thus be less harmful to steel in steel fibre concrete than in reinforced concrete.     

A model for a long-term diffusion of multispecies in concrete based on ion–cement-hydrate interaction

A model for simulating a long-term diffusion in concrete under submerged conditions has been developed. The model focuses on the interaction between cement hydrates and electrolyte solution adopting the physical properties of concrete as alternative parameters for estimating long-term diffusion. The model was verified by the large variety of long-term experimental data involving verification of cement hydrates, porosity properties, pH value, element distribution, and chloride penetration for sulphate-resistant Portland cement (SRPC) concretes. Evaluating impacts of hundreds of years’ exposure on concrete durability, a simulation was also performed for an exposure time of 500 years. The results confirmed the importance of ion–cement-hydrate interaction in the evaluation of a long-term diffusion of harmful substances such as chlorides into concrete. The simulation results show also that the solid phases of the SRPC concrete mix that was used are stable in the long-term. The threshold concentration of chlorides which may initiate reinforcement corrosion defined as Cl^?/OH^? ratio could be exceeded in concrete after moderate exposure period under the conditions investigated with typical protective concrete layers. Using of case-specific values for a threshold chloride content and evaluating the method used for estimating the initiation time of reinforcement corrosion are recommended in the structures with a long-design service life.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

Effectiveness of corrosion inhibitors in contaminated concrete

Four types of corrosion inhibitors (calcium nitrite at two dosages, calcium nitrate at three dosages and two organic inhibitors at their recommended dosages) were evaluated at five different levels of contamination, i.e., 0.8% chloride; 0.8% chloride plus 1.5% SO₃; seawater; brackish water; and unwashed aggregates. Concrete specimens were used to assess the effect of corrosion inhibitors on the compressive strength of concrete and reinforcement corrosion. The results indicated that the corrosion inhibitors investigated in this study did not adversely affect the compressive strength of concrete. Furthermore, calcium nitrite was efficient in delaying the initiation of reinforcement corrosion in the concrete specimens contaminated with chloride, while both calcium nitrite and calcium nitrate mitigated the corrosive effects of chloride plus sulfate salts or sea water. In the concrete specimens prepared with brackish water or unwashed aggregates, all the inhibitors were effective in reducing the rate of reinforcement corrosion. The type and dosage of corrosion inhibitor were observed to be dependent on the nature and level of contamination.     

Surface settlement of cementitious-based materials determined by oedometer testing

The oedometer test is commonly used in soil mechanics to evaluate consolidation of clays and other compressible soils. A research project was undertaken to evaluate the suitability of such test to assess surface settlement of freshly mixed cementitious-based materials. Three series of concrete-equivalent-mortar (CEM) mixtures made with different cement contents, water-to-cementitious materials ratio (w/cm), silica fume, and viscosity-modifying admixture (VMA) were tested. Validation of test results was performed by casting concrete mixtures in a PVC column having 600-mm height and 200-mm diameter. Test results have shown that CEM settlements determined using the oedometer test as well as concrete settlements determined using the 600-mm PVC column are affected quite similarly by the mixture composition. For example, for given consistency, settlement decreases with the increase in cement content, decrease in w/cm, and incorporation of increased concentrations of silica fume or VMA. Statistical analysis was then performed to propose regression models for predicting maximum settlement of concrete through the determination of CEM characteristics.     

Properties and performance of silane: blended cement systems

The paper presents the results of a study dealing with the performance of water repellents on hardened blended cement pastes. Since on the European market Portland cement does not play the dominant role anymore and due to the new national and European policies concerning Greenhouse Gases and sustainability, cement manufacturers produce more and more blended cements (CEM II–CEM V). Nevertheless, the majority of experience concerning the efficacy of water repellents is gained from Portland cement; therefore knowledge in regard to the interactions of blended cement with water repellent agent is minimal. Two silane-based products were applied on ‘fresh’ and carbonated cement substrates containing limestone, fly ash, slag and trass, and were investigated in terms of their functionality. The evaluation of the treatments’ performance and effectiveness were assessed using various laboratory measurements. Hydrophobicity, water absorption, colour changes and the penetration depth of silanes into the substrate were evaluated before and after artificial aging experiments. Moreover, the outdoor weathering test was performed to shed light on treated surface appearance in a ‘real’ outdoor environment. The results showed that surface wettability was independent on water ingress or colour variations, especially for cement specimens artificially aged by accelerated carbonation. Cement pastes containing slag and trass seemed to more distinctly affect the water repellents’ surface performance.     

Chloride redistribution and reinforcement corrosion in the interfacial region between substrate and repair concrete—a laboratory study

One of the most common deterioration mechanisms in concrete structures is reinforcement corrosion caused by chlorides. An often used repair strategy is to remove the damaged concrete and sometimes also undamaged concrete and replace with a repair concrete. The chloride contaminated undamaged concrete and the repair concrete have to be compatible in order to achieve a durable system. This laboratory study has investigated 13-year-old reinforced concrete specimens with both substrate concrete with mixed-in chlorides and an initially chloride free repair concrete. The main objective was to study chloride transport from the contaminated substrate concrete into the repair concrete and establish chloride profiles across the interfacial region and interfacial zone between the two materials. Another objective was to evaluate the location of reinforcement corrosion in the interfacial zone, in the substrate concrete and in the repair concrete. The main results from this laboratory investigation show that reinforcement corrosion occurs in and near the interfacial zone between chloride contaminated and repair concrete. It was found that the corrosion occurs in local areas with passive steel areas between, i.e. macrocell corrosion. The chlorides are transported from the contaminated substrate concrete into the repair concrete. This investigation indicates that there is a risk for reinforcement corrosion around a patch repair when the substrate concrete has chloride contents exceeding 1.0 wt% by weight of cement.     

Corrosion resistance of steel embedded in sulfoaluminate-based binders

The use of new binders for structural concrete raises questions about the durability of reinforced concrete structures. Calcium sulfoaluminate cement (CSA) has become in recent years an environmentally-friendly alternative to ordinary Portland cement (OPC), but the protection capacity of CSA concrete in relation to the corrosion of embedded steel reinforcement needs to be studied. This paper describes a study on the corrosion behaviour of carbon steel embedded in concrete made with CSA-based binders. It is shown that the pore solution of sulfoaluminate concrete was sufficiently alkaline to passivate embedded carbon steel rebars. Compared to traditional Portland and Portland-limestone cements, CSA cement led to higher carbonation rate of concrete, but to lower corrosion rate of steel in carbonated concrete, likely due to the higher electrical resistivity. Corrosion rate was negligible up to 95% RH at 20 °C. Blending of CSA and OPC cements improved steel passivity and concrete resistance to carbonation.