Browsing by Author "Otieno, Mike"

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    Open Access
    Corrosion propagation in cracked and uncracked concrete
    (2008) Otieno, Mike; Alexander, Mark Gavin; Beushausen, Hans-Dieter
    Thesis (M.Sc. (Civil Engineering))--University of Cape Town, 2008. Includes bibliographical references.
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    Discrete sacrificial anodes and their use in service life extension of chloride contaminated reinforced concrete structures
    (2012) Arito, Philemon; Beushausen, Hans-Dieter; Alexander, Mark G.; Otieno, Mike
    There is an urgent need for economic and technically sound concrete repair technologies in South Africa as the number of deteriorating RC structures needing repair is increasing. Currently, the South African concrete repair industry mainly relies on the application of patch repairs and corrosion inhibitors while other potentially successful methods such as sacrificial anodes are rarely applied. Sacrificial anode cathodic protection (SACP) systems use metals that are higher than steel in the electrochemical series. These metals corrode preferentially to steel thereby supplying electrons to the cathodic steel surface. Common SACP systems include discrete zinc anodes installed in slots or drilled holes, non -structural jacket encasements, and overlays made from expanded zinc mesh or any other convenient form. The effectiveness of discrete SACP systems in service life extension of chloride contaminated RC slabs has been investigated in this study. Similarly, the extent to which discrete sacrificial anodes can be used to extend the service life of corroding chloride contaminated RC structures has been assessed through an extensive review of literature a s well as laboratory experiments on corroding chloride contaminated RC slabs . This study evaluates the performance of discrete SACP systems with respect to binder type, corrosion rate and the level of chloride contamination with an objective of developing guidelines for rational concrete repair when using SACP systems to repair RC structures that are deteriorating from corrosion of reinforcing steel in South Africa. Eight slabs were investigated. Four slabs were cast using 100% PC CEM 1, 42.5N while the other four were cast using a blend of 70/30 PC CEM 1, 42.5N/FA. Each slab comprised two portions: one half of each slab was cast using concrete that was admixed with 0.6% chloride by mass of binder while the other half was cast using concrete that was admixed with 1.8% chloride by mass of binder. The corrosion in the slabs was induced and accelerated using direct anodic current. The SACP system that was used in this study comprised discrete zinc disks put in a cylinder of a proprietary high alkaline mortar. The se anodes were installed in 1 0 0 x 10 0 x 60 mm deep cavities that had been made in concrete. The anodes were connected to the reinforcement cage using tie wires and thereafter backfilled with a repair mortar. The cavities in which the anodes were installed were arranged in a square grid of 450 x 450 mm. The corrosion rate and half-cell potential of the steel as well as the resistivity of concrete in the slabs were monitored over a duration of five month s. The discrete SACP system that has been investigated in this study can increase the service life of chloride contaminated RC structures that are deteriorating as a result of rebar corrosion. The discrete anodes reduced the corrosion rate of steel in the test slabs. The percentage reduction in the average corrosion rate was higher within the slabs that were cast using 100% PC CEM 1, 42.5N than in the slabs that were cast using a blend of 70/30 PC CEM 1, 42.5N/FA. The reduction in average corrosion rate within the slabs cast using 100% PC CEM 1, 42.5N ranged between 45-95%. Similarly, the reduction in the average corrosion rate of within the slabs that were cast using a blend of 70/30 PC CEM, 42.5N 1/ FA ranged between 54-75%. Throughout the test specimens, the portions of the slabs that was admixed with 0.6% chloride by mass of binder exhibited superior performance. These portions (admixed with 0.6% chloride by mass of binder) experienced the greatest reduction in average corrosion rate as well as the greatest shift in average half -cell potential towards values that are more negative. Finally, in order to achieve the long-term objective of increasing the service life of RC structures that are deteriorating as a result of rebar corrosion, lo ng-term studies ought to be undertaken to ascertain, with surety, the effectiveness of discrete SACP systems in corroding structures.
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    Effect of oxygen availability on the corrosion rate of reinforced concrete in marine exposure zones: inference from site and lab studies
    (2019) Moore, Amy; Beushausen, Hans-Dieter; Otieno, Mike
    Corrosion of steel in reinforced concrete structures is a large problem facing engineers today. The marine environment is considered to be the most severe owing to the high levels of chlorides available, and so structures located here are particularly vulnerable to chloride attack and chloride induced corrosion of steel. In order to intervene and address the concern of premature deterioration in the marine environment, design guidelines and frameworks have been developed and implemented. For example, the Eurocode (EN 206, 2013) provides three class designations in order to predict the severity of potential steel corrosion. The three exposure zones, namely i) structures exposed to airborne salts but not in direct contact with sea water, ii) submerged structures, and iii) structures in the tidal, splash and spray zones are given here in order of increasing assumed probability of corrosion. However, it has been found through condition assessments of structures along the Atlantic Ocean and Indian Ocean coasts of Southern Africa that structures in the tidal zone generally show no signs of corrosion damage despite having high levels of chlorides at the depth of reinforcement. Often, on a structure where both zones have the same cover to reinforcing, the splash and spray zone will show significant damage while the tidal zone shows no signs of reinforcement corrosion. These findings challenge the existing idea that the tidal zone can be characterised as the most severe exposure zone. In order to further understand the mechanisms of reinforcement corrosion within different marine environmental conditions, a total of 36 corrosion cells were manufactured with dimensions of 120 × 122 × 380 mm and placed in simulated marine environmental conditions. These three environmental conditions included submerged, cyclic wetting and drying, and periodic splashing. In order to simulate the submerged environment, the cover surface of the specimens were permanently saturated with a 5% NaCl solution while for the splash zone concrete specimens were sprayed with 5% NaCl solution every second day. The tidal zone attempted to simulate natural tidal conditions by exposing the corrosion cells to 12 hour cycles consisting of 6 hours wetting and 6 hours drying All 36 corrosion cells were connected to a data logger where the voltage was measured weekly across a resistor of 100Ω. Ohm’s law was then used to determine the current flowing through the circuit. The concrete cover depth was varied in the corrosion cells (10, 20 and 30 mm) as well as the w/b ratio (0,5 and 0,8). The corrosion current resulting from these corrosion cells were used to infer relationships between all 3 parameters (cover depth, w/b ratio, and exposure conditions) and their influence on the corrosion current. Three companion moisture specimens were cast per mix in order to establish moisture profiles in the different exposure zones. From these specimens, the relative humidity of the concrete at different cover depths and w/b ratio’s could be determined for corrosion cells located in differentexposure conditions. Companion cubes were also cast with the corrosion cells in order to determine the durability index values. The results of the experiments indicate that the exposure condition has a very large impact on the availability of oxygen, and hence the corrosion rate. High relative humidity (or moisture content) in concrete stifles the supply of oxygen to the steel, and hence prevents active corrosion. For concrete submerged and partially or completely saturated with water, oxygen accessibility can become limited at the steel surface. The results obtained conform to the widely accepted principle that submerged concrete is less vulnerable to corrosion as a result of insufficient oxygen supply at the reinforcing steel. For concrete exposed to a 6 hour cycle of wetting and drying, the corrosion rate was significantly lower at higher cover depths due to the short cycle durations, the pores surrounding the steel were still partially or fully saturated with water. As a result, oxygen diffused slower through the cover layer. Consequently, where the concrete in the tidal zone has drying times of about 6 hours and a cover depth exceeding 30 mm, the steel will be deprived of oxygen, and corrosion will be stifled. This means that the concrete in the tidal zone would theoretically perform as if it were permanently submerged (provided sufficient cover depth exists). Specimens exposed to a splash environment performed as expected. A low moisture content showed that oxygen is able to diffuse through the pore system and facilitate corrosion in the specimens. This is owing to the pores around the steel not being saturated with moisture, and oxygen being readily available to participate in the cathodic reaction. In the case of these specimens, the electrical resistivity of the specimens was found to be the main limiting factor in controlling the corrosion rate (and not cathodic control as with tidal and submerged specimens). As a result, it is recommended that the current SANS exposure classification be broadened to include concrete exposed to cyclic wetting and drying (i.e. tidal zone exposure) a separate category, and not be classified as having the same severity as the splash zone. The application of these research findings is in infrastructure that is primarily exposed to only tidal and submerged marine conditions.
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    Open Access
    Improvement of the approaches to monitoring of reinforced concrete bridges subjected to chloride-induced reinforcement corrosion in bridge management systems
    (2022) Simon, Jaziitha; Beushausen, Hans; Otieno, Mike
    Reinforcement corrosion in reinforced concrete (RC) bridges has undeniably become the main cause of civil infrastructure durability problems in many countries. The steel reinforcing in concrete corrode when certain conditions (e.g., carbonation, chlorides, insufficient cover and moisture) are met. Even though RC bridges are built to provide a service over a specified period, their serviceability changes over time due to gradual deterioration. In the marine environment, this deterioration is mainly caused by the ingress of chlorides in concrete, which results in chlorideinduced steel corrosion. Thus, effective maintenance and management are needed to ensure the functionality of bridges throughout their designed service life. Bridge management systems (BMSs) are effective means for managing bridges throughout their design life. BMSs require the collection of data related to the condition of the bridge for decision making. South Africa uses the Struman BMS to assess and prioritise bridges for maintenance and repair. The system solely relies on visual inspection as a basis for the identification of deterioration processes and bridge condition rating. However, visual inspections do not allow detection of rebar corrosion until damage has occurred; and once the damage has occurred, it becomes very costly to repair. Thus, this research focuses on identifying available reinforcement corrosion monitoring methods that can be used for condition assessment to supplement visual inspections in the Struman BMS. The research involved reviewing different BMSs with the emphasis on their structure, application, and assessment methodology. The review of various BMSs used in the USA, Canada, UK and Southern Africa shows that while BMSs differ, they all assess the risk of failure and prioritise bridges for repair within limited budgets. Focusing on the Struman BMS, shortcomings with respect to the monitoring of reinforcement corrosion were identified. It was found that the condition ratings of RC bridges are based on visible defects assessed during visual inspections, which is still the dominant method used for bridge inspection. There exists a lack of integration of appropriate monitoring systems in the BMS, especially with regards to the monitoring of reinforcement corrosion. Available corrosion monitoring methods were critically reviewed based on their principles, applications, and technical aspects. These corrosion monitoring methods were categorised into visual inspections, Non-Destructive Testing (NDT), and remote monitoring methods. It became evident that monitoring technologies are continuously evolving and that the progress achieved to date is promising. However, further development is needed when using these methods in the BMSs for condition assessment, particularly in the continuous monitoring of bridges. The use of these modern technologies will allow earlier detection of problems and hence support sound maintenance and damage prevention. This is expected to simultaneously reduce the costs needed for maintenance and repair. In addition, modern monitoring methods have the potential to enhance the speed and scope of condition assessments, to provide reliable and wide-ranging data, and to reduce traffic interruptions when taking measurements. An integrated approach is proposed based on applying corrosion monitoring methods as a supplement to visual inspections in the Struman BMS. Integrating these methods with the existing approach to bridge management is expected to compensate for the limitations of the Struman BMS and enhance its capability. The proposed approach involves identifying bridges that need to be assessed (new or existing bridges). Selecting monitoring methods for new bridges based on the needs and the type of data that need to be collected from bridges, and existing bridges based on the past assessment data and visual inspection. Monitoring is proposed to incorporate periodic and permanent monitoring. The former includes NDT measurements using any selected methods, and the latter includes installing sensor systems in new and existing bridges. Condition ratings are defined for monitored parameters, including corrosion risk (from corrosion potentials and electrical resistivity), corrosion rate, pulse velocity, defective areas (from the presence of cracks and delamination), moisture content and visual inspection. Other corrosion-related parameters considered include chloride content, cover depth measurement and monitoring time to corrosion onset. The obtained data from visual inspection and corrosion monitoring methods are then holistically analysed and interpreted to obtain integrated condition ratings, which are used to rate the condition of the bridge and its various RC elements. Consequently, this approach is expected to assist in prioritising critical bridges for repair and maintenance. Lastly, recommendations are provided related to a comprehensive study on the implementation of available reinforcement corrosion monitoring methods for the monitoring of in-service bridges. The findings of this research also need to be taken forward by an experienced bridge inspector to help develop practical guidelines for applying corrosion monitoring methods in the Struman BMS.
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    An investigation into the relationship between surface concrete resistivity and chloride conductivity test
    (2014) Mutale, Lombe; Beushausen, Hans-Dieter; Alexander, Mark Gavin; Otieno, Mike
    The chloride conductivity test (CCT) is a South African-developed Durability Index (DI) test used for the evaluation and quantification of the quality of concrete cover. It is also used as an input parameter for service life prediction (SLP) of RC structures in the marine environment, using a modified version of Crank’s solution to Fick’s second law of diffusion. The surface concrete resistivity test is an electrochemical test that has a good correlation with the concrete chloride diffusion process. The surface concrete resistivity test is used as a quick way to determine the durability of concrete. The purpose of this study was to provide an in-depth literature review on surface concrete resistivity and investigate its use for the design and prediction of durability in RC structures. The study also compared and contrasted surface concrete resistivity with CCT, using results from previous work, in terms of their application in the design of RC structures. Thereafter, the study investigated the relationship between surface concrete resistivity and CCT. The study was carried out by comparing laboratory and field surface concrete resistivity with CCT results. Then, corrosion initiation periods (CIP’s) and diffusion coefficients were estimated using Mackechnie’s (1996) CCT SLP model as well as surface concrete resistivity models by Andrade (2004) and Baroghel-Bouny et al. (2009). Input parameters for the model such as surface concrete resistivity and CCT results were based on measurements from a previous study. It was found that moisture gradients, chloride contamination and temperature gradients easily influence concrete electrical resistivity testing done in-situ. The input parameters for the surface concrete resistivity models were restricted to laboratory results. The analysis revealed that for blended cement concrete, w/b ratio has a greater influence on chloride conductivity than binder type. It was observed that using a different binder type has a greater influence on surface concrete resistivity at a high w/b ratio than a low one. In addition, decreasing the w/b ratio for GGBS concrete is more effective than decreasing it for FA concrete in influencing the surface concrete resistivity. Andrade’s surface concrete resistivity SLP model resulted in similar CIP values as the CCT model for CEM I only concrete and slag concrete at 0.40 w/b. A constant links the diffusion coefficients from surface concrete resistivity and CCT models. An inverse relationship was found between surface concrete resistivity and the diffusion coefficient calculated from the CCT SLP model.