The effect of silane-based hydrophobic admixture on corrosion of reinforcing steel in concrete

The influence of a hydrophobic admixture based on silane on the corrosion resistance of steel reinforcement in concrete was studied. Sound or deliberately pre-cracked concrete specimens were manufactured with w/c of 0.45 and 0.80, both in the presence and in the absence of silane. The specimens were fully immersed in a 3.5% NaCl aqueous solution.The results, in terms of electrochemical measurements, visual observations, and weight loss measurements of steel reinforcement, show that silane blocked corrosion process in uncracked concrete specimens. On the other hand, in cracked concrete specimens, corrosion of steel reinforcements was unexpectedly more severe in hydrophobic specimens rather than in the corresponding not hydrophobic ones. It is believed that oxygen, which is needed to feed the corrosion process, diffuses faster in a gaseous phase through the open concrete porosity in the hydrophobic concrete, whereas in concrete without silane, oxygen diffuses much more slowly through the water filling the pores of the saturated concrete.     

Corrosion inhibitors for chlorides induced corrosion in reinforced concrete structures

Reinforcements corrosion is the most important cause of premature failure on reinforced concrete structures. Phenomena promoting corrosion are the ingress of chlorides and the reaction of atmospheric CO₂ with cement paste. Aim of this paper is the investigation on the effectiveness of three organic commercial inhibitors in preventing carbon steel chlorides induced corrosion in concrete, since there is not yet a clear knowledge on the real effectiveness of these products. Inhibitors were added to the concrete mixture in dosage suggested by the manufacturers. Chlorides were added in the concrete mixture or penetrated from outside by “ponding” cycles with a 3.5% sodium chloride solution. The effectiveness of the inhibitors has been evaluated by long-term rebar corrosion monitoring in reinforced concrete and by rebar visual inspection after three years tests. Also solution tests were performed in order to verify the effectiveness of inhibition. Results give information about corrosion prevention ability of analysed commercial inhibitors.     

Probabilistic evaluation of initiation time in RC bridge beams with load-induced cracks exposed to de-icing salts

In this study, a reliability-based method for predicting the initiation time of reinforced concrete bridge beams with load-induced cracks exposed to de-icing salts is presented. A practical model for predicting the diffusion coefficient of chloride ingress into load-induced cracked concrete is proposed. Probabilistic information about uncertainties related to the surface chloride content and the threshold chloride concentration has been estimated from a wide review of previous experimental or statistical studies. Probabilistic analysis to estimate the time to corrosion initiation with/without considering the effect of the load-induced cracks on the chloride ingress into concrete has been carried out. Results of the analysis demonstrate the importance of considering the effect of the load-induced cracks for correct prediction of corrosion initiation in RC bridge beams exposed to chlorides.     

Corrosion process and structural performance of a 17 year old reinforced concrete beam stored in chloride environment

The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel-concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel-concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.     

Use of no-fines concrete as a building material: Strength,durability properties and corrosion protection of embedded steel

Although no-fines concrete has been generally used for paving applications, it could be considered in other non-structural applications, such as reinforced panels, thanks to its acoustic, thermal and permeability properties. In this work, mechanical, durability-related properties and the protection provided by no-fines concrete to embedded steel against carbonation-induced corrosion have been investigated on mixtures with compressive strength in the range 7–30 MPa. Additional protections, such as a mixed-in hydrophobic admixture, the coating of cement paste on the reinforcing bar or the use of galvanized or stainless steel bars, are also considered. Results show that, although no-fines concrete is susceptible to fast carbonation and it cannot provide long-term passivation to embedded steel, it may prevent corrosion in elements exposed to the atmosphere and sheltered by rain. In case of frequent contact with water, additional protections are required, preferably based on the use of corrosion resistant bars.     

Experimental study on bond behavior between corrosion-cracked reinforced concrete and CFRP sheets

Bond behavior between corrosion damaged reinforced concrete and carbon fiber reinforced concrete polymer (CFRP) sheets was experimentally investigated. Forty ordinary strength concrete blocks (150 × 150 × 200 mm) were reinforced at one side across the 200-mm-dimension using three conventional ?12 mm steel bars at a spacing of (30, 40, and 50 mm) at a concrete clear cover of 15 mm. Thirty blocks were subjected to a cyclic treatment in 3% chloride solution until corrosion initiated and resulted in three different global cracking widths of up to 0.90 mm. Both control and corrosion damaged blocks were attached to CFRP sheets over their steel reinforced zone at bond lengths and widths ranging from (90 to 150 mm) and (50 to 150 mm), respectively, with CFRP bond length-to-bar spacing ratio kept constant at 1/3. Near-end pull-off tests were carried out using a special setup, mounted on a Universal Testing machine. Corrosion cracking caused significant reductions in bond strength, and slippage at ultimate stress at (41 and 68%), respectively. Other bond characteristics such as stress at first slippage, and bond stiffness and toughness were reduced, as well, by as high as (83, 44 and 67%) of those of control specimens, respectively. Corrosion cracks were more detrimental for smaller bond length and width values; especially after first and second corrosion stages, where bond failure was categorized by concrete skin peeling-off.     

Corrosion effects on pullout behavior of hooked steel fibers in self-compacting concrete

Fiber reinforced concrete structures are subjected to chloride and carbonation penetration that could initiate corrosion of steel fibers, with eventual pernicious consequences in terms of structural and durability performance. Cracks in concrete are known to hasten initiation of steel corrosion in reinforced concrete structures. The investigation of the impact of cracks on the corrosion initiation and the associated interfacial damage between concrete and steel fibers is important for understanding the mechanical behavior of steel fiber reinforced concrete.In the present work, with the aim of studying the corrosion action on the mechanical behavior of cracked Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC), an experimental program was performed to characterize the corrosion of hooked-end steel fibers and to assess the fiber pullout behavior in cracked concrete, previously subject to the action of corrosion by exposure to aggressive chloride environment.     

The corrosion pattern of reinforcement and its influence on serviceability of reinforced concrete members in chloride environment

This paper deals with two corroded reinforcement concrete beams, which have been stored under sustained load in a chloride environment for 14 and 23 years respectively. The evolution of corrosion pattern of reinforcement and its influence on serviceability are studied. In chloride-induced corrosion process, corrosion cracking affects significantly the corrosion pattern. During the corrosion cracking initiation period, only local pitting corrosion occurs. At early stage of cracking propagation, localized pitting corrosion is still predominant as cracks widths are very small and cracks are not interconnected, but a general corrosion slowly develops as the cracks widen. At late cracking stage, interconnected cracking with wide width develops along large parts of the beam leading to a general corrosion pattern. Macrocells and microcells concepts are used for the interpretation of the results.Mechanical experiments and corrosion simulation tests are performed to clarify the influence of this corrosion pattern evolution on the serviceability of the beams (deflection increase). Experimental results show that, when the corrosion is localized (early cracking stage), the steel–concrete bond loss is the main factor affecting the beams serviceability. The local cross-section loss resulting from pitting attack does not significantly influence the deflection of the beam. When corrosion is generalized (late cracking stage), as the steel–concrete bond is already lost, the generalized steel cross-section reduction becomes the main factor affecting the beams serviceability. But, at this stage, the deflection increase is slower due to the low general corrosion rate.     

格尔木盐湖地区内掺阻锈剂对钢筋混凝土耐久性影响

阻锈剂常用来缓解混凝土中钢筋锈蚀问题,而阻锈剂种类是否对混凝土耐久性有一定影响尚需要进行进一步研究。因此分别制备内掺阻锈剂Ca(NO₂)₂(又称CAN)和C₄H₁₁NO(又称CN)混凝土,并对两组试件在格尔木盐湖地区进行现场暴露试验。每3个月对试件进行电化学试验研究、超声波检测、质量检测,以混凝土中钢筋锈蚀程度、混凝土中裂缝的发展、混凝土的质量损失三方面对其耐久性的影响进行分析。研究表明,两组试件经过180 d现场暴露,其内部钢筋达到了低锈蚀状态,经过810 d后的现场暴露,A组试件中钢筋就达到中等锈蚀状态,而B组钢筋依然处于低锈蚀状态。经过810 d的现场暴露,A、B组试件的相对质量评价参数ω₁值分别为0.42、0.46,相对动弹性模量评价参数ω₂值分别为0.47、0.53,自暴露180 d后,同时期A组试件的ω₁和ω₂值小于B组试件。在810 d时A、B组试件的表面最大裂缝分别达到了0.15 mm、0.11 mm左右。阻锈剂CN对混凝土耐久性的提升效果要好于CAN。     

Chloride-induced corrosion products of steel in cracked-concrete subjected to different loading conditions

This project focused on examining the composition and distribution of chloride-induced corrosion products at the rebar-concrete interfaces and on crack surfaces in reinforced ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) subjected to different loading conditions. The results indicated that, regardless of the type of loading, there was a larger distribution of corrosion products along the rebar surface in the HPC than in the OPCC. Also, dynamic loading caused a greater detachment of the aggregate-paste bond in OPCC than static loading. The opening and closing of the cracks in salt solution under dynamic load forced corrosion products to flow from the rebar-concrete interface into the cracks in both OPCC and HPC. As a result, corrosion products diffused from the crack into the cement paste in the dynamically loaded OPCC but remained in the cracks in the dynamically loaded HPC, where they induced branched cracks. The mill-scale on the rebar was also evaluated before and after being embedded in concrete and was observed to be porous allowing ingress of species from the cement paste.     

基于不同胶凝材料钢筋混凝土耐腐蚀性试验研究

针对混凝土结构中钢筋的锈蚀问题,以普通硅酸盐水泥和镁水泥作为胶凝材料,制作了钢筋裸露和涂有环氧树脂涂层的4种钢筋混凝土试件,将其浸泡在氯盐、硫酸盐及镁盐的耦合溶液中,利用电化学方法得到极化曲线和交流阻抗谱,通过塔菲尔外推法进行非线性最小二乘法拟合计算得到腐蚀电流密度、腐蚀电位、腐蚀速率等相关评价指标来衡量混凝土中钢筋锈蚀情况.结果表明:长期浸泡在耦合盐溶液中,普通硅酸盐水泥和镁水泥钢筋混凝土试件都发生了较严重的腐蚀,普通硅酸盐水泥的抗腐蚀性优于镁水泥;在钢筋表面涂有GHT涂层可极大地降低钢筋的腐蚀速率;镁水泥GHT涂层混凝土试件的抗腐蚀性能劣于硅酸盐GHT涂层混凝土试件但优于硅酸盐裸露钢筋混凝土.     

Application of electrochemical techniques for the control of cracks and steel corrosion in concrete

Electrochemical techniques are becoming increasingly popular as a means of rehabilitating deteriorated reinforced concrete. In this paper we explore the possibility of using an electrodeposition technique for repairs of land-based cracked concrete structures and investigate the effect of this technique on various concrete properties. Concrete specimens reinforced with steel rebars were used as test specimens in this study. The specimens were first precracked by exposing rebars to chloride-induced corrosion, and then immersed in a solution of zinc sulfate and subjected to a constant d.c. current. The current was applied between the reinforcing steel in the concrete and an external electrode immersed in the solution for eight weeks. The degree of crack closure, water permeability, condition of reinforcing steel, and chloride concentration were investigated in order to assess the effectiveness of this method as a potential rehabilitation technique. The results indicate that the formation of electrodeposits caused the closure of cracks and led to improvement of the concrete watertightness. In addition, the investigation shows that the application of the electrical current increases the passivity of steel in concrete.     

Surface corrosion of steel fibre reinforced concrete

Corrosion of SFRC (Steel Fibre Reinforced Concrete) in an adverse environment, less harmful as compared to corrosion of steel reinforced concrete, is often considered to be of minor importance, however it exists. It can affect the fibres bridging the cracks and then decrease the strength of the concerned structures. As well, it results in the appearance of corrosion spots at exposed surfaces. Then the damaging effect is no longer expressed in terms of resistance, it is only aesthetic. It is especially undesirable in prefabricated structures.The work developed in this paper focuses on this second point: surface corrosion. Fibre reinforced concrete prisms have been subjected to cycles of salt fog (1 week) and drying (1 week). The results obtained demonstrate that with high W/C ratio (0.78), all the fibres which are embedded in concrete less than 1 mm are susceptible to give corrosion spots at the surface. When W/C is reduced to about 0.5, the minimum necessary cover to prevent surface corrosion drops to 1/10 mm or 2/10 mm and further decrease of W/C does not bring extra significant benefit. This result is in agreement with the analysis, by mercury intrusion, of the skin concrete porosity. It confirms a sharp change in the pore diameters when W/C is decreased from 0.78 to 0.48 and a quasi stability when it is varied from 0.48 to 0.36.     

Monitoring reinforcement corrosion and corrosion-induced cracking using non-destructive x-ray attenuation measurements

To test the applicability of the x-ray attenuation method to monitor corrosion products as well as the formation and propagation of cracks in cementitious materials, reinforced mortar samples were tested under accelerated corrosion conditions. Experimental results demonstrate x-ray attenuation measurements can track time-dependent development of corrosion products and the subsequent initiation and propagation of corrosion-induced cracks. Also, x-ray attenuation measurements allowed determination of the actual concentration of the corrosion products averaged through the specimen thickness. The total mass loss of steel, obtained by the x-ray attenuation method, was found to be in very good agreement with the mass loss obtained by gravimetric method as well as Faraday's law. Results of the presented experimental approach provide pertinent information for the further development and verification of numerical tools simulating corrosion-induced damage in reinforced concrete.     

Oxygen diffusion through hydrophobic cement-based materials

The oxygen diffusion coefficient through hydrophobic cement-based materials fully immersed in water was determined by potentiostatic measurements on concrete and by the use of a diffusion cell on cement pastes and mortars. The obtained results show that very high oxygen diffusion occurs through cement paste, mortar and concrete made with hydrophobic admixture as opposed to negligible diffusion through the reference cement matrix without admixture. Moreover, the oxygen diffusion coefficients measured through hydrophobic cement matrices immersed in water were comparable with those reported in literature for unsaturated cement materials in air. These experimental results appear to confirm that oxygen dissolved in water directly diffuses as a gaseous phase through the empty pores of a hydrophobic cement matrix. This could explain the severe corrosion of steel reinforcement embedded in cracked hydrophobic concrete immersed in an aqueous chloride solution observed in a previous work.     

Interaction between corrosion crack width and steel loss in RC beams corroded under load

This paper presents results and discussions on an experimental study conducted to relate the rate of widening of corrosion cracks with the pattern of corrosion cracks as well as the level of steel corrosion for RC beams (153 × 254 × 3000 mm) that were corroded whilst subjected to varying levels of sustained loads. Steel corrosion was limited to the tensile reinforcement and to a length of 700 mm at the centre of the beams. The rate of widening of corrosion cracks as well as strains on uncracked faces of RC beams was constantly monitored during the corrosion process, along the corrosion region and along other potential cracking faces of beams using a demec gauge. The distribution of the gravimetric mass loss of steel along the corrosion region was measured at the end of the corrosion process. The results obtained showed that: the rate of widening of each corrosion crack is dependent on the overall pattern of the cracks whilst the rate of corrosion is independent of the pattern of corrosion cracks. A mass loss of steel of 1% was found to induce a corrosion crack width of about 0.04 mm.     

A migrating corrosion inhibitor evaluated in concrete containing various contents of admixed chlorides

The performance of a surface-applied migrating corrosion inhibitor (MCI) based on an alkylaminoalcohol was evaluated on concrete specimens containing reinforcing steel bar (rebar) segments. Two water/cement ratios (w/c's), various chloride contents and two exposure conditions were investigated. The inhibiting efficiency was followed over a period of 1000 days by means of parameters such as corrosion potential, corrosion current and electrical resistance. Results show that when concrete is exposed to the marine environment, the inhibitor is able to reduce the corrosion rate (CR) only when the initial chloride content is below 0.16 wt.% relative to cement content. Efficiency increases as the w/c increases. There is no beneficial effect when the initial chloride content is greater than 0.43%. When concrete is immersed in a saline solution, no beneficial effect associated to the use of the inhibitor could be appreciated, regardless of w/c or initial chloride content in concrete.     

Cover cracking of reinforced concrete due to rebar corrosion induced by chloride penetration

Cracking of concrete cover due to corrosion induced expansion of steel rebar is one of the major causes of the deterioration of reinforced concrete (RC) structures exposed to marine environments and de-icing salts.This paper presents two models that deal with the chloride-induced corrosion and subsequent cracking of concrete cover in RC structures. The former analyses the chloride diffusion within partially saturated concrete. A comprehensive model is developed through the governing equations of moisture, heat and chloride-ion flow. Nonlinearity of diffusion coefficients, chloride binding isotherms and convection phenomena are also highlighted. The latter describes the internal cracking around the bar due to expansive pressures as corrosion of the reinforcing bar progresses. Once a certain chloride concentration threshold is reached in the area surrounding the bar, oxidation of steel begins and oxide products are generated, which occupy much greater volume than the original steel consumed by corrosion. An embedded cohesive crack model is applied for cracking simulation.Both models are incorporated in the same finite element program. The models are chained, though not explicitly coupled, at first instance. Comparisons with experimental results are carried out, with reasonably good agreements being obtained. The work is a step forward for the integration of the two traditional phases (initiation and propagation) widely used in the literature and usually analysed separately. The estimation of the service life of the structure needs to evaluate the associated time for each one.     

Resistance of metallic substrates protected by an organic coating containing aluminum powder

The corrosion behavior of an epoxy primer containing aluminum powder (10 vol.%) applied on carbon steel and on galvanized steel was examined by electrochemical impedance spectroscopy (EIS). The data show that this coating is more protective when applied onto carbon steel substrates, and that on galvanized steel thicker coatings allow to achieve similar protection levels as those obtained for carbon steel. These effects are probably due to aluminum pigments providing a cathodic protection of the substrate, and to the resulting products precipitating inside the pores of the polymeric coating. Three stages can be distinguished during exposure of the coated specimens. Upon immersion of the coated samples in the test solution, a pre-saturated stage is observed. After a certain period of immersion, which strongly depends on the thickness of the applied coating, a saturation stage is reached in which an effective protection of the metallic substrate against corrosion is achieved. Finally, at sufficiently long exposure times, swelling through the coating eventually leads to the detachment of the coating.     

Prediction of reinforcement corrosion using corrosion induced cracks width in corroded reinforced concrete beams

This paper studies the evolution of reinforcement corrosion in comparison to corrosion crack width in a highly corroded reinforced concrete beam. Cracking and corrosion maps of the beam were drawn and steel reinforcement was recovered from the beam to observe the corrosion pattern and to measure the loss of mass of steel reinforcement. Maximum steel cross-section loss of the main reinforcement and average steel cross-section loss between stirrups were plotted against the crack width. The experimental results were compared with existing models proposed by Rodriguez et al., Vidal et al. and Zhang et al. Time prediction models for a given opening threshold are also compared to experimental results. Steel cross-section loss for stirrups was also measured and was plotted against the crack width. It was observed that steel cross-section loss in the stirrups had no relationship with the crack width of longitudinal corrosion cracks.