Palliative Repairs for Reinforced Concrete: A Best Value Approach Aston University 6 March 2002 1 Electro-Osmosis & Moisture Control Treating the cause, not the symptoms. Paul Lambert Materials & Corrosion Engineering Mott MacDonald Altrincham Introduction: The environment provided by good quality concrete for the embedded steel reinforcement is one of high alkalinity (generally >pH 13), produced by the hydroxides of sodium, potassium and calcium released during the various hydration reactions. In addition, the bulk of surrounding concrete acts as a physical barrier to most of the substances that may lead to degradation of the reinforcement. Provided this environment is maintained, the steel remains passive and any small breaks in the stable protective oxide film are soon repaired. However, if the alkalinity of the surroundings is reduced, for example by reaction with atmospheric carbon dioxide (carbonation), or if depassivating chloride ions are made available at the surface of the steel then corrosion may be initiated, resulting in loss of steel section and spalling of cover concrete. For the consideration of how design or modification of reinforced concrete structures to create a benign environment can influence durability, it is essential to understand how exposure conditions influence the following factors:- The rate of breakdown of protection and initiation of corrosion or other deterioration. The rate of corrosion or deterioration. Secondary deterioration processes. For example, carbonation develops much more in dry, internal conditions than in wet, near saturated conditions. Once carbonation has reached the steel, corrosion rates are negligible when dry and most rapid when near saturation or subject to wet and dry cycling. Once corrosion has developed sufficiently to crack the cover, corrosion is accelerated. Spalling of the cracked cover by frost action further accelerates the deterioration. Palliative Repairs for Reinforced Concrete: A Best Value Approach Aston University 6 March 2002 2 In the case of chloride induced corrosion, chloride ion ingress can only occur through the liquid phase of the concrete. The maximum rate of ingress occurs during cycles of wetting and drying. Chloride induced corrosion is a distinct function of temperature, humidity and cycles of wetting and drying. For alkali aggregate reaction (AAR), the most damaging condition is from 95% RH to near saturation. Total immersion may leach alkali and ameliorate the reaction, while at 80% RH AAR proceeds very slowly until re-wetting occurs. AAR is effectively dormant below 75% RH. Alkali Aggregate Reaction requires moisture to progress. Improved durability can be achieved by resisting the ingress of aggressive substances through the use of 'high quality' concrete. A thick cover depth of low permeability concrete will greatly assist in preventing depassivation of the reinforcement, however structural factors may limit the maximum cover. The composition of the cementitious matrix is clearly important and the use of cement replacement materials such as fly ash, blast furnace slag and microsilica can also be beneficial, particularly when improving the resistance to chloride ion ingress. The corrosion rate of steel in concrete is dependant upon the presence of an ionically conductive aqueous phase in contact with the steel, the existence of anodic and cathodic sites on the metal surface in contact with this electrolyte and the availability of oxygen to enable the reactions to proceed. This indicates an additional route to restricting corrosion and hence improving durability through control of one or more of the reactants. Palliative Repairs for Reinforced Concrete: A Best Value Approach Aston University 6 March 2002 3 While it is possible to stifle corrosion by restricting oxygen availability, the amount of oxygen required to maintain corrosion is very small and may be difficult to limit other than by complete immersion in water. Alternatively, reinforcement corrosion may be controlled by limiting water access and encouraging the concrete to dry, the associated increase in electrical resistance limiting the magnitude of corrosion currents. Moisture and Structural Degradation: The control or exclusion of high moisture levels represents one of the oldest and most widely used methods of protecting and extending the service life of structural materials. The passivating effect of the alkaline environment generated and maintained in Portland cement-based reinforced concrete will generally overcome the need to maintain a dry environment. However, as discussed above, a number of mechanisms are commonly encountered where unrestricted access to moisture can result in damage to the concrete, the reinforcement or both. The following factors all link the level of free moisture to the degradation of steel reinforced concrete: Corrosion Rate of Steel The relative humidity of an environment has a profound effect on the rate of corrosion of steel. There is a critical level of relative humidity below which corrosion does not occur and often secondary and tertiary levels above which the corrosion rate increases significantly. In the case of steel, corrosion commences at a slow rate at approximately 60% RH, the rate increases at 75-80% RH and again at 90%. Contamination of the environment has the tendency to reduce the relative humidity at which corrosion is initiated. Concrete Resistivity The rate of corrosion of steel in concrete is strongly affected by the resistivity of the concrete as this determines the degree of current flow. Resistivity is effectively a measure of the resistance of concrete to the passage of current. When the resistivity is high, >12 k .cm, the resistance to current flow is high and the rate of corrosion is minimal. Conversely, when the resistivity is low, <5 k .cm, the resistance to current flow is low and the rate of corrosion is high. The relative humidity of the concrete has a significant effect on its resistivity. As the relative humidity/moisture content of the concrete increases the resistivity of the concrete decreases. Thus as indicated for atmospheric conditions, increasing the relative humidity of the concrete has a tendency to increase the rate of corrosion of the steel. Palliative Repairs for Reinforced Concrete: A Best Value Approach Aston University 6 March 2002 4 Increasing the concentration of chloride ions also reduces the resistivity, and it may therefore be assumed, that for a given relative humidity corrosion rates increase with increasing chloride content. Variation in concrete age and composition can effect the degree to which changes in relative humidity alters resistivity, but the general trends remain constant. Rate of Carbonation The rate of passage of carbon dioxide into concrete is controlled, in part, by the permeability of the concrete, with high permeabilities resulting in more rapid carbonation. It has been shown that the optimum relative humidity for carbonation lies in the range 50-70%. Below these values the reaction is stifled by lack of moisture, and above these values the passage of carbon dioxide into the concrete through the pore structure is impeded. Where reducing relative humidity is employed as a means of controlling corrosion rates, it may be advisable to apply a protective anti-carbonation coating to minimise the risk of increased levels of carbonation, particularly with highly porous concretes and low covers. Once carbonation has reached the steel, corrosion rates are negligible at 50% RH but most rapid at 95% RH. Wetting and Drying Concrete exposed to the atmosphere is subjected to cycles of wetting and drying which tend to affect the cover concrete, most significantly where the corrosion process is active. Repetitive cycles of wetting and drying have a significant effect on the rate of corrosion, typically resulting in higher corrosion rates than those encountered under steady state conditions. Moisture Control in Reinforced Concrete: The durability of reinforced concrete can be significantly affected by its relative humidity. It has been shown that there is a limiting level of relative humidity, below which corrosion will not occur. For steel in concrete this appears to be in the range 60-70% RH and is affected by factors such as the level of chloride contamination. As relative humidity increases so does the rate of corrosion, although at full saturation the corrosion rate is significantly reduced as moisture fills the concrete pores and acts as a barrier to oxygen transfer. Controlling the relative humidity of reinforced concrete should therefore provide an effective means of controlling reinforcement corrosion, particularly where the removal or exclusion of excess moisture also removes or prevents the ingress of potentially aggressive species. a range of mostly passive approaches are: Palliative Repairs for Reinforced Concrete: A Best Value Approach Aston University 6 March 2002 5 Design The importance of design with respect to more recent reinforced concrete structures can often be best illustrated by observing where bad of illconsidered design has lead to loss of durability. Coatings & Surface Treatments As most of the moisture and other mobile species that influence the durability of reinforced concrete must cross the boundary between substrate and atmosphere, the application of coatings and surface treatments can be highly effective at limiting or preventing degradation. Cladding Carefully designed ventilated cladding should be more durable and can be more effective than coatings and surface treatments in reducing moisture ingress and shielding from chlorides. Electro-osmosis Electro-osmosis has been applied to the movement of moisture through porous materials, most notable masonry, for a considerable time and with varying and generally disappointing results. More recently, an electro-osmosis system has been specifically developed to control moisture levels in reinforced concrete by the application of controlled low voltage DC pulses. The electronically-controlled system is designed to operate at maximum efficiency while avoiding problems of stray-current corrosion to buried steel. Additionally, the design, installation and monitoring of the system is significantly simpler than for existing electro-chemical remediation techniques. The system is capable of reducing moisture levels in concrete to between 60% and 70% RH, and maintaining this level irrespective of external weather conditions. The technique is equally applicable to new and existing structures, although new concrete should be fully cured prior to energising the system. An additional benefit to the removal of excess free moisture is the associated reduction in dissolved salts, particularly chloride, present within the pore solution of the concrete, with the overall effect of reducing chloride ions to below critical levels with respect to chloride-induced corrosion. The combination of reduced chloride ion concentration and controlled relative humidity can result in a significant reduction in corrosion rate to low or negligible values. The continuing operation of an appropriately designed and installed system can prevent or control further ingress of moisture and associated dissolved salts, which may be aggressive to either the concrete (eg. sulphates) or the reinforcement (predominantly chlorides). Palliative Repairs for Reinforced Concrete: A Best Value Approach Aston University 6 March 2002 6 Additionally, the system is designed to negatively polarise the reinforcement resulting in a degree of cathodic protection, helping to reduce the corrosion risk of embedded steel during the transition period from high to low relative humidity (typically several months), and providing additional protection throughout the life of the installation. The system has been specifically assessed for possible side effects resulting from its operation, such as may be encountered with other remedial techniques. No evidence has been found to indicate significant risks of bond strength reduction, excess alkali generation, hydrogen evolution or stray current corrosion of adjacent discontinuous steel. Full-scale electro-osmosis trial. Conclusions: The reduction of free moisture levels, as measured by the relative humidity, is a well-established method of preventing or controlling corrosion. Research over the past 60 years indicates that at around 60% to 70% RH the corrosion of steel all but ceases. In addition, limiting the ingress and availability of excess water can be beneficial in the prevention or control of many other forms of degradation associated with reinforced concrete.