Deterioration of RC structures due to chloride-induced corrosion of steel is a common problem in marine environments. Chloride ingress in concrete compromises the durability of RC structures located in marine environments by rendering the embedded steel susceptible to corrosion propagation. Durability is defined as ‘the ability of a RC structure to withstand its design service conditions for its design life without significant deterioration’. The consequence of loss of durability is that a lot of resources will be spent on maintaining and repairing corrosion-affected RC structures; these resources can better be invested on new much-needed infrastructure or diverted to other well-deserving sectors of the economy such as manufacturing, education, healthcare and housing, and ensuring food security. For a developing country like Kenya where a lot of infrastructural development such as the LAPSSET (Lamu Port – Southern Sudan – Ethiopia Transport) project, and the Mombasa–Nairobi Standard Gauge Railway project, is currently taking place, it is imperative that measures are taken to ensure that the capital investments on these developments are worthwhile for the intended duration of time. In the case of marine infrastructure (including ports and harbours, and residential and commercial buildings), proper durability design, specification and testing is mandatory if this is to be achieved. In a recent publication in the Kenya Engineer, I presented compelling reasons why durability of concrete structures in general should be taken seriously – especially by engineers. This paper focuses on chloride-induced corrosion of steel in concrete.
Corrosion of steel in concrete – brief overview
The service life of a corrosion-affected RC is sub-divided into two main phases viz initiation and propagation phases. For marine RC structures, once the steel has been rendered liable to corrosion due to accumulation of sufficient chlorides at the steel level during the initiation phase, the presence corrosion-sustaining agents (O2 and H2O) allows for corrosion propagation to proceed. In the propagation phase, electrochemical anodic and cathodic reactions that take place on the steel surface result in dissolution of steel into voluminous corrosion products (e.g. Fe2O3). If left unabated, corrosion can lead to several inter-related negative consequences including, but not limited to, cover cracking, delamination and spalling of concrete cover, loss of steel cross-section area, degradation of steel-concrete interface bond, and ultimately collapse [3]. Both corrosion initiation and propagation are affected by several inter-related factors such as binder type, cover depth and condition (cracked or uncracked), temperature, steel-concrete interface, concrete penetrability/quality, relative humidity and concrete resistivity.
Mitigating corrosion of steel in marine RC structures
The need to mitigate steel corrosion in marine RC structures is driven not only by durability concerns but also by sustainable development demands such as reduction in the amount of plain Portland cement used in concrete, increasing the use of supplementary cementitious materials such as fly ash, silica fume, slag and even calcined agricultural wastes (e.g. rice husk ash, corn cob ash, etc), and construction of durable structures that require little or no maintenance; these measures aim to minimize the high carbon footprint of the cement and concrete construction industry. In order to do this, a fundamental understanding of the steel corrosion process in concrete and the factors involved is necessary. This understanding should be common among the different stakeholders in the cement and concrete industry – contractors, engineers, cement producers, readymix concrete producers, statutory/regulatory bodies, admixture suppliers and tertiary institutions. The following sections discuss ways to ensure that durability of RC in the marine environment is achieved.
Application of performance-based durability design and specification approaches
In the recent years, the design for durability in concrete structures has evolved from a purely prescriptive (also referred to as deemed-to-satisfy) approach to a hybrid approach comprising of a combination of both the prescriptive and performance-based approaches. Prescriptive approaches are of the recipe type, setting limits on water-to-binder ratio, cement type, compressive strength, etc for different exposure classes, and fail to capture the material performance concept of a structure in a given environment. They also do not take into account the effects of in-situ handling of concrete such as compaction and curing which impact on the durability of concrete. On the other hand, performance-based approaches are based on quantitative prediction for durability from exposure conditions and measured material parameters. Suitable test methods are therefore required; a typical example is the suite of durability index tests used in South Africa [8]. In applying performance-based durability design and specification, engineers need to appreciate that concrete durability is an interaction between the exposure environment and concrete system (quality, composition and condition).
Use of blended cements
Binder type plays an important role in the durability of marine RC structures [9]. It influences durability-related aspects such as pore solution chemistry, microstructure, early heat development and penetrability of concrete [5]. The use of blended cements therefore contributes greatly to enhancing the durability of marine concrete structures. Furthermore, it also contributes to limiting the negative environmental impact of using plain Portland cement. It is therefore advisable to always specify the use of blended cements for marine RC structures. Blended cements contain supplementary cementitious materials such as fly ash, slag, silica fume and metakaolin. In Kenya, the use of pozzolanic cements which are available in abundance (mainly from volcanic ash) should be sustained but other supplementary cementitious materials should be actively sought. This should be guided by research in our local institutions of higher learning.
Careful quantification of the aggressivity of the exposure environment
As already mentioned, the aggressiveness of the exposure environment is a key factor to consider in durability design and specification. In the marine environment where chloride ingress is the major deleterious substance w.r.t. RC durability, the severity of the exposure environment is classified according to the risk of corrosion initiation and the potential rate of steel corrosion. The tidal and splash zones are considered to be most aggressive while the submerged zones are least aggressive w.r.t. steel corrosion [10]. Knowledge of the location of the RC structure in the marine environment is therefore important because it informs the durability design and specification in terms of aspects such as cover depth and concrete quality depending on the expected service life. At the Kenyan coastal region (e.g. Mombasa, Lamu, Malindi and Kilifi), for example, the high relative humidity (~74%) and high ambient temperatures (~21–31 °C) exacerbate the risk of steel corrosion in concrete and point to the need for engineers to take more stringent measures in the design and specification of concrete in such environments. The EN-206-1 [11] design code can be adapted for Kenyan conditions (end even standardized by KEBS) and used by engineers to classify the aggressivity of the marine environment.
Specification of adequate cover depth
Concrete cover to the steel acts as a physical and chemical barrier to delay the time to corrosion initiation and subsequent propagation. Specification of adequate cover depth is therefore important for marine RC structures. Adequate cover depth can be informed by using suitable service life prediction models. In the case of chloride-induced corrosion, Fickian-based and corrosion rate prediction models can be used. However, even in the absence of these models, some durability design codes (e.g. EN 206–1 [11]) can provide guidance. Typically, a cover depth of at least 40–45 mm is recommended for marine RC structures. Proper reinforcing steel detailing, strict site supervision and provision of good quality and dense cover blocks, of the correct thickness and placed on the reinforcement cage at spacings close enough to ensure that the cage does not move during concreting are also necessary.
Good site construction practices
Achieving the desired concrete quality is not just a matter of careful selection of concrete mix ingredients and accurate quantification of the exposure environment. Proper handling of fresh concrete on site plays an important role in determining its final properties in the hardened state [9]. On site, resident engineers and contractors should give attention to adequate compaction and sufficient curing. This is important because chloride ingress in marine concrete is sensitive to its penetrability which is directly affected by both the degree of compaction and method and duration of curing. In addition to compaction and curing, it is important that measures are taken to ensure that the specified minimum cover depth is achieved during construction, e.g. using cover block of the correct thickness.
Testing of concrete
In a performance-based durability design and specification, it is important to use robust and reliable tests to assess the quality of the concrete before and after construction. In marine concrete, tests are required to quantify the penetrability of the concrete to chloride ingress. The South African chloride conductivity test is a typical example of such a test [12] is used. Such a test can easily be adopted for use in Kenya to assess concrete before and after construction. In addition to testing the quality of concrete, it is important that tests are carried out to check if the specified minimum cover depth has been achieved. This can easily be done using readily available cover meters in the market.
Dealing with non-compliance
Inasmuch as steps can be put in place to ensure that the concrete used in marine RC structures meets the desired quantifiable durability requirements, we must acknowledge that cases will inevitably arise where the as-cast concrete doesn’t comply with the engineer’s specifications. This requires careful decision-making on the part of the engineer which should be guided by a sound understanding of the various remedial actions available to try and restore the RC structure to its desired durability potential. In the case of chloride-induced corrosion, the use of suitable surface coatings (e.g. Silanes and Siloxanes) is usually recommended but the additional life-cycle cost associated with this should be taken into account.
Closing remarks
The fundamental requirements of marine RC structures are similar to other types of concrete structures i.e. structural robustness and reliability, adequate serviceability, and excellent durability. However, for marine concrete structures, durability considerations largely govern the choice of constituent materials, with strength being a secondary but not unimportant consideration. This paper has provided (i) a general overview of durability of RC structures in the marine environment by focusing on the main deterioration mechanism in these structures (i.e. chloride-induced steel corrosion), and (ii) an overview of the ways in which the design, specification and construction of durable marine concrete structures can be achieved. The paper was written with the aim of creating awareness to the different stakeholders in the Kenyan cement and concrete construction industry on the need to take durability design, specification, construction and testing as important aspects to consider especially for new infrastructural developments in the country in general. Engineers play central roles (design, specification and on-site supervision) in ensuring the RC structures are not only structurally sound but are also serviceable and durable to withstand their in-service exposure environments. As a way forward, this paper advocates for the implementation of performance-based durability design and specification of RC structures. Even though this paper focused on marine RC structures, the approach can be extended to cover other concrete structures and for different deterioration mechanisms, e.g. soft-water attack, abrasion, acid attack, etc. By doing this, the capital investments currently being undertaken by the national and county governments will be worthwhile and contribute positively towards attaining a sustainable economy. It should go without mention that engineers should always remember to uphold high ethical standards – guided by the Institution of Engineers of Kenya’s (IEK) Code of Professional Conduct [13] – in implementing ideas and concepts the presented in this paper.
Finally, it is also important that (i) regulatory / professional bodies such as the Engineers Board of Kenya (EBK) and the IEK are actively involved in ensuring that engineers adopt and incorporate durability designs and specifications in their day to day work (e.g. through conferences, workshops and seminars targeted towards updating practicing engineers with new and practical concrete practices), and (ii) educators and researchers, working closely with IEK and EBK, make learners aware of the performance-based durability design and specification approaches. These steps will encourage both ‘new’ and ‘experienced’ engineers to ensure that durable RC structures are designed and constructed.
