Abstract

Concrete contributes to not only the strength of a reinforced concrete (RC) structure but also, to a large extent, its long-term durability performance in a given exposure environment. It is common practice in Kenya that, in the majority of cases,focus in placed mainly on strength.Durability is seldom taken into account even though it is a vital property of concrete if the design life of the RC structure is to be realized and/or extended while mitigating and minimizing, respectively, deterioration and maintenance and repair costs. This trend is partly, due to lack of appreciation of the importance of the contribution of durability towards achieving the structure’s design service life, and partly due to insufficient knowledge on how to incorporate concrete durability in the design of RC structures.Concrete durability should be given the seriousness it deserves both at the design and construction stages of a RC structure. This should be the case, at least, for RC structures in harsh exposure environments e.g. at the Kenyan coastline where chloride-induced steel reinforcement corrosion can significantly shorten the design life of the structure if concrete durability aspects are not incorporated into the project. This paper puts forward a case for the need to consistently incorporate durability in the design and construction stages in order to ensure that RC structures meet their design life. 

Keywords:Concrete;cement;durability; sustainability

1           Introduction

Concreteis currently the most commonly used construction material not only in Kenya but all over the world. The current global consumption stands at over 20 billion metric tonsper annum[1], with increasing rates of consumption in developing countries especially in Africa.Over the years, a lot of practical research developments have been made in the field of cement and concrete use– innovative cements, concretes and design approaches have been developed and are being used. However, the suitability of these developments are not being actively explored and fully exploited in Kenya’s concrete construction industry, especially with respect to concrete durability. The new developments relate not only to the adoption of new cement and concrete standards but also to the drive towards the adoption of performance-based specification approaches to optimize onstrength, durability and sustainability requirements.

In the recent past, there has been a general increase in cement consumption in Kenya (see Fig. 1) which has seen the number of cement manufacturers double from three to six – East Africa Portland Cement, Bamburi Cement, Athi River Mining, Mombasa Cement, Savannah Cement and National Cement; a seventh cement company ‘Dangote Cement’ has already been awarded an operating licence and is expected to enter the Kenyan market in 2021[2]. The increase in the number of cement manufacturing companies is consistent with the recent and ongoing boom in the real estate sector (urbanization) and large infrastructural investments such as the LAPSSET (Lamu Port – Southern Sudan – Ethiopia Transport) project, and the Mombasa–Nairobi Standard Gauge Railway project. It is without doubt that we will continue to experience this increase in cement consumption in the coming years as long as there is a political goodwill, and the county and national governments are committed toinvesting in large infrastructural projects. In fact, CW Group[3], a leading global research firm that tracks cement production and consumption, projects that Kenya’s cement per capita consumption will continue to increase exponentially.

However, for these RC infrastructural developments to be long-term investments as they are intended to (or maximize on the return on investment from both economic and functional viewpoints), it is important that civil engineers, especially those dealing directly with their design and supervision during construction, ensure that they are designed and constructed to not only be strong enough to carry the expected service loads but is also be durable enough to resist deterioration such as reinforcement corrosion.It is the author’s view that if the measures proposed and discussed in this paper are adoptedand implemented correctly, less capital investment will be spent in the near future in maintaining, repairing and retrofitting the structures being built now; this capital can better be spent on investing in new much needed infrastructure. This is capital that, if proper concrete technology is applied, can be diverted to other well-deserving sectors of the economy such as education and health.

2           The need to consider concrete durability

In the context of this paper, durability is defined as ‘the ability of a RC structure to withstand its design service conditions for its design life without significant deterioration’. Concrete was in the past perceived as a material whose main contribution to the structural system is compressive strength – this is undoubtedly not the case now! However, many civil practicing engineers working with concrete have been reluctant to adopt new concrete technology practices; it is still a common practice for RC structural design engineers to specify only the minimum compressive strength grade e.g. 25 MPa, 30 MPa, etc. If any, very little consideration is given to the concrete durability requirements of the RC structure with respect to the intended service life vis-à-visthe aggressivity of the exposure environment e.g. coastal region, inland environment, etc. It is seldom that in addition to strength, other concrete parameters such as permeability, sorptivity and diffusion coefficient are specified. The consequence of this is thatconcrete durability has been neglected as an important parameter contributing to the service life of the RC structure. In some cases, even when durability is considered important, procedures and experience to incorporate it in the design process is, respectively, seldom lacking and limited.

 

Fouraspects that needto be taken into account for durability to be achieved in RC structures include the following [6]:

  • selection of materials: proper selection of materials (cement type and content, water-to-cement (w/c) ratio, aggregate type, etc) at the design stage should be done with regard to foreseen in-service exposure conditions;
  • testing: appropriate test methods and interpretation of results are critical;
  • responsibilities: the roles of the engineer/owner/client, contractor and concrete supplier should be well stipulated and understood. These will be covered briefly later in this publication;
  • cost: ensured durability may require higher initial costs, although this is not inevitable. These costs should be incorporated in the tender and cost schedules, either implicitly or explicitly, or both.

A few salient and practical factors will be discussed in the following sections.

2.1          Contribution of cement to concrete durability

Cement is an important constituent of concrete both from strength and durability viewpoints. It is important that the contribution of cement to the durability of concrete is understood by the engineer. For example, it is improper to specify plain Portland cement, PC, (commonly known as ordinary Portland cement) for a structure that will be situated in a coastal region such as Lamu or Mombasa due to its established history of poor durability performance i.e. high penetrability of PC concretes (even with low w/c ratios and proper curing)[7].In such a case, blended cements should be used.Currently, the following types of cements available in the Kenyan market (produced by the six manufacturers mentioned previously) amidst other imported cements:CEM I (plain PC), CEM IIA-LL (Portland limestone cement) andCEM II/B-P (Portland pozzolanic cement).These cements should all comply with Kenyan [8] and/or international standards[9]. From a durability point of view, it is obvious that the concrete engineer (or the contractor) has very little cement options to select from. It is therefore important that the durability performance of these cements and cement blends is studied and well documented so that practicing engineers can make informed decisions when specifying concrete for use in a project.

It is also important to strongly discredit the misconception that high cement content results in more durable concrete i.e. high quality. First, from a sustainability point of view, this route should be avoided so that the amount of clinker used in the manufacture of PC is minimized in order to minimize CO2 emissions which contribute towards global warming. Secondly, high cement content may have detrimental effects such as increases heat of hydration and hence (internal cracking) – this is important especially for large concrete structures such as concrete dams and deep beams.

Finally, concrete durability should be viewed more as a function of cement type than cement content. It is also advisable not to specify minimum cement content for concrete as this places a ceiling to innovation on the part of the contractor or concrete supplier.

2.2          Consideration of exposure environment

Concrete durability is usually defined with respect to the exposure environment of a RC structure – see the definition that was presented earlier. It is therefore important that the expected in-service exposure environment of a RC structure is taken into consideration when specifying durability-related parameters such as cement type, concrete cover depth to reinforcement and concrete quality (i.e. penetrability). It is important to underscore that the latter parameter, concrete quality, should be seen as a function of cement type (not cement content), water-to-cement ratio, aggregate properties (e.g. grading), compaction, cover depth and curing. Among other exposure environments in Kenya, the following need specialist specification of concrete to ensure desired durability performance is achieved:

  • Coastal regions: The main problem in these areas is usually corrosion of the embedded reinforcing steel in the concrete due to ingress of chlorides i.e. chloride-induced corrosion. This is a more severe type of steel corrosion (with respect to rate of deterioration) than carbonation-induced corrosion which is common in inland (and industrial) regions e.g. in car parks. Its prevention and/or mitigation requires use of highly impenetrable concrete, proper curing and provision of adequate concrete cover to the steel. These parameters are important considering that in coastal regions such as Mombasa, the availability of airborne chlorides coupled with the high ambient temperatures and relative humidity are conduciveto sustain the propagation reinforcement corrosion once it is initiated. In such areas, the limitation of crack widths is also vital as their presence accelerates the rate of steel reinforcement corrosion[10].
  • Concrete sewer pipes: The use of concrete to manufacture sewer pipes is common but requires the use of special cements (and aggregates) such as calcium aluminate cements (CACs) as opposed to using plain PCwhich leads to significant reduction in the life of the sewer pipes due to corrosion (or decomposition) of the concrete (mainly the cement paste). The process involvesconversion of hydrogen sulphide gas (from the wastewater) to sulphuric acid by anaerobic bacteria (on the surface of the sewer pipe) to sulphuric acid which corrodes the concrete[11].

It is important to bring to the attention of the reader the durability exposure classesin the European Standard EN 206-1 [12]that are meant to assist the engineer in assessing the severity of an exposure environment and hence specify concrete durability parameters (quality, cover depth, cement type, etc) appropriately; this type of approach to durability design is referred to as the performance-based durability design and specification. However, these exposure classes are used as a general guide and should,if necessary, be modified to meet specific or general Kenyan exposure environments.

Table 1: Exposure classes according to EN 206-1 [12]

Class designation Description of the environment
X0 No risk of reinforcement corrosion or attack
XC Reinforcement corrosion induced by carbonation
Sub-classes: XC1, XC2, XC3, XC4
XD Reinforcement corrosion induced by chlorides other than from sea water
Sub-classes: XD1, XD2, XD3
XS Reinforcement corrosion induced by chlorides from sea water
Sub-classes: XS1, XS2, XS3
XF Freeze/thaw attack with or without de-icing agents
Sub-classes: XF1, XF2, XF3, XF4
XA Chemical attack
Sub-classes: XA1, XA2, XA3

 

2.3          Contribution of concrete cover to durability

Concrete cover to reinforcing steel acts as a physical protective barrier to the ingress of deleterious species such as chlorides, sulphates, moisture, oxygen and carbon dioxide. These species are detrimental in the sense that they can either facilitate the initiation of chemical deterioration processes such as alkali silica reaction and reinforcement corrosion or sustain the propagation of such processes. Therefore, the quality, depth and condition (cracked or uncracked) of the concrete cover are important to achieving durability of RC structures. As a general guide, a relatively low concrete quality should be accompanied by high cover depth (within some limits) and vice versa. However, in reality, it is not always a straightforward process because other factors such as cost and the presence of cracks have to be taken into consideration.

In day to day practice, concrete cover depth, quality and condition are seldom given in-depth consideration before being specified; a common practice among structural concrete engineers is to adopt a universal cover depth, and in some cases, crack width (especially at the design stage) regardless of the RC structure’s exposure environment. This trend should be reversed and more emphasis placed on not only specifying appropriate concrete cover depth, quality and condition but also ensuring that the contractor and concrete supplier adhere to the laid down specifications.

2.4          Tertiary education and continuous professional development

To achieve concrete durability, both civil engineering scholars and practicing engineers need adequate knowledge of concrete as a construction material which, as already mentioned, not only provides strength to the structural system but also, in a large proportion, contributes to the RC structure attaining its intended service life. More emphasis should be placed on the teaching of [modern] concrete technology in our tertiary institutions. At the same time, practicing civil engineers dealing with concrete should constantly update their knowledge in this field; this is the intension of continuous professional education/development (as articulated in the Engineers Act 2011[13]) and should therefore be exploited. This will require the input of academics in our tertiary institutions to work closely with the Institution of Engineers of Kenya (IEK) and the Engineers’ Board of Kenya (EBK) to develop conferences, workshops and seminars targeted towards updating practicing engineers with new and practical concrete practices.

2.5          Appropriate testing methods

Potential durability can be inferred by measuring the resistance of the cover layer to transport mechanisms (e.g. permeation, absorption and diffusion) by which deleterious substances (e.g. carbon dioxide, chlorides, sulphates, etc) penetrate concrete.Without appropriate or standardized durability tests, it is not possible to check if durability is achieved or not. These tests can be used in both the selection of concrete mix materials and proportions (through trial mixes) and to quantify the actual durability of concrete in the as-built RC structure. Currently, the most commonly used test to check conformity of concrete in Kenya is compressive strength test. However, as already mentioned, concrete strength is in general not synonymous to concrete quality (i.e. penetrability). It is therefore imperative that suitable test methods are used to test for durability conformance. At this stage, it is advisable to adopt the durability test methods that are already being used internationally to achieve this e.g. the South African durability index test methods[14-16]. The use of standard durability tests also asserts confidence on the engineer/client/owner that the contractor will supply concrete with the specified durability properties. With such tests in place, engineers can then focus on specifying the desired concrete durability properties (e.g. sorptivity, chloride resistance, permeability, etc) as opposed to prescribing concrete mix ingredients, and strength. It also gives the contractor room (within the specified boundaries) for innovation with respect to developing concrete mixes that meet the engineer’s/owner’s/client’s strength and durability specifications.

3           The role of the cement manufacturing companies

As already mentioned, the Kenyan market has limited cement types available to the concrete engineer to choose from. It is the author’s assumption that, from a cement manufacturer’s viewpoint, the development of new cement types is usually motivated by the need to maximize cement sales profit while at the same time minimizing carbon emissions during cement manufacture to meet legislative requirements regarding environmental protection (i.e. the Environmental Management and Co-ordination Act No. 8 of 1999[17]). Cement manufacturing companies should be seen by the concrete engineer more as partners than just producers/suppliers of cement. A healthy working partnership between the existing cement manufacturing companies and institutions of higher learning should be formed and nurtured to encourage and facilitate market-driven basic and applied research into the improvement and development of cements that can satisfy local needs. This should also involve exploring the use of locally available materials such as concrete (rubble) waste or shredded rubber tyres as aggregates. In short, the cement companies in Kenya should be ready to invest generously in cement and concrete research in tertiary institutions.

4           Responsibilities of the engineer, contractor and supplier

In order achieve concrete durability in the Kenyan concrete construction industry, it is imperative to clearly define the responsibilities of the various players in the concrete production process and use chain– engineer/client/owner, contractor and concrete supplier. This is important not only to ensure each player clearly understands their roles but also for arbitration purposes, if any.These roles should be agreed upon and clearly defined in the contract tender documents. In the following sections, a few of the salient responsibilities of these players will be outlined.

4.1          Responsibilities of the concrete engineer

The concrete engineer should be responsible for, among other duties:

  1. defining the relevant environmental exposure class for the RC structure.
  2. establishing the performance criteria for concrete depending on the expected environmental exposure conditions during placement of the concrete and when the RC structure is in service.
  3. stating any other concrete properties that may be required to meet the desired durability performance e.g. permeability, sorptivity, chloride resistance, etc.
  4. preparing the technical specification stating the performance criteria together with the teststo be used for checking conformity in the as-built RC structure.
  5. conductingin-situ quality assurance to ascertain on the that the performance criteria have been met.
  6. clearly state the steps to be taken if the specified concrete durability performance requirements are not achieved e.g. repair options, financial penalties depending on the degree of conformity to the specifications, etc.

4.2          Responsibilities of the contractor

The contractor:

  1. is responsible for procuring concrete and related materials and incorporating them into the structure in a manner that meets the desired (engineer’s) performance requirements.
  2. needs to be aware of the durability conformance test programme prior to bidding. If necessary, pre-qualification concrete testing can be carried out and results included in the project bid.
  3. should conduct appropriate and sufficient quality control to demonstrate that the durability performance requirements have been met.
  4. must be aware of and share the responsibility for handling, constructability, curing concrete and scheduling issues that influence the in-place (fresh and hardened) concrete properties.
  5. shouldclearly detail in their bid how they intend to meet the durability performance requirements part of the bid e.g. placement methods, protection, curing, etc.

4.3          Responsibilities of the concrete supplier/producer

The concrete supplier/producer should:

  1. at the request of the engineer, provide documentation to the satisfaction of the engineer demonstrating that the proposed concrete will achieve the required strength and durability performance targets.
  2. certify that the concrete mix design to be used satisfies the engineer’s durability performance standard.Pre-qualification concrete testing may be required to achieve this.
  3. certify that the production and delivery of concrete will meet the engineer’s durability requirements.
  4. implement a quality control plan to ensure that the engineer’s and contractor’s concrete performance requirements will be met.
  5. provide documentation verifying that they meet industry certification requirements, if required.

It is also important to state that, with appropriate conformity test methods in place, the concrete supplier and contractor should be flexible enough to choose suitable combinations of materials, concrete mixtures and construction techniques to meet the desired performance criteria.

5           Concluding remarks

The objective of this paper was to raise awareness among the concrete users in Kenya’s construction industry about the importance of taking concrete durability into account during design and construction. It was motivated by the current ongoing huge infrastructural investments being undertaken in Kenya which will without doubt utilize concrete as the main construction material. It is highlighted in the paper that for these investments to achieve their design life with minimum maintenance and repair costs, concrete durability needs to be incorporated into their design and construction. Parameters such as, among others, thickness of the concrete cover to steel reinforcement, condition of the concrete cover (cracked or uncracked) and cement type should be clearly specified by the engineer and implemented by both the contractor and concrete supplier/producer. In summary, the contribution of concrete to not only the strength of the structure but also its long-term durability should be appreciated by engineers working with this material, and appropriate measures such as the ones proposed and discussed in this paper taken to ensure desired durability targets are met. Finally, even though not explicitly covered in this paper, sustainability of concrete as a construction material should also not be ignored in the quest to achieve durability.

References

[1]        Mehta, P. K. & Meryman, H. (2009) Tools for reducing carbon emissions due to cement consumption. Structure Magazine, March, Structural Engineering Institute (ASCE, Sustainability Committee), pp. 11-15.

[2]        Dangote (2017) Dangote cement now delays Kenya entry to 2021. https://www.businessdailyafrica.com, Accessed on 4th January, 2018.

[3]        CW-Group, www.cwgrp.com.

[4]        KNBS (Kenya National Burueau of Statistics) Cement production and consumption. 1 pp (Accessed on 4th January 20118).

[5]        Global-cement (2017) http://www.globalcement.com, 1 pp (Accessed on 4th January 20118).

[6]        Alexander, M. G. & Beushausen, H. (2007) Performance-based durability design and specification in South Africa. Proceedings of the International Concrete Conference and Exhibition (ICCX – Concrete Awareness), 14-16 February, 2007, Cape Town, South Africa.

[7]        Neville, A. M. (2011) Properties of Concrete (5th Edition). Pearson Education Limited, Edinburg Gate, Harlow, Essex CM20 2JE, England, ISBN: 978-0-273-75580-7.

[8]        KS-EAS-18-1 (2005) Cement – Part 1: Composition, specification and conformity criteria for common cements (Kenya Bureau of Standards).

[9]        ENV-197-1 (2000) Cement – Part 1: Composition, specifications and conformity criteria for common cements. European Standard.

[10]      Otieno, M. B., Alexander, M. G. & Beushausen, H.-D. (2010) Corrosion in cracked and uncracked concrete – influence of crack width, concrete quality and crack re-opening. Magazine of Concrete Research, Vol. 62(6), pp. 393-404.

[11]      Fattuhi, N. I. & Hughes, B. P. (1988) The performance of cement paste and concrete subjected to sulphuric acid attack. Cement and Concrete Research, Vol. 18(4), pp. 545-553.

[12]      BS-EN-206 (2013) Concrete – Part 1: Specification, performance, production and conformity, European Standard.

[13]      Laws-of-Kenya (2012) The Engineers Act (No. 43 of 2011). Kenya Gazette Supplement, The Government Printer.

[14]      SANS-3001-CO3-1 (2015) Civil engineering test methods: Part CO3-1: Concrete durability index testing – Preparation of test specimens. South African Bureau of Standards  – Standards Division, Pretoria, South Africa, ISBN 978-0-626-32799-6.

[15]      SANS-3001-CO3-2 (2015) Civil engineering test methods: Part CO3-2: Concrete durability index testing – Oxygen permeability test. South African Bureau of Standards  – Standards Division, Pretoria, South Africa, ISBN 978-0-626-32800-9.

[16]      SANS-3001-CO3-3 (2015) Civil engineering test methods: Part CO3-3: Concrete durability index testing – Chloride conductivity test. South African Bureau of Standards  – Standards Division, Pretoria, South Africa, ISBN 978-0-626-32801-6.

[17]      NEMA (1999 (ammended 2015)) National Environmental Management Authority. https://www.nema.go.ke.

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Mike Otieno
Mike Otieno is a Senior Lecturer in the School of Civil and Environmental Engineering at the University of Witwatersrand in South Africa. He holds a First Class Honours Bachelors degree in civil engineering from the University of Nairobi, and Masters and PhD degrees in civil engineering from the University of Cape Town. He also holds a Postgradiate Diploma in Edication (in Higher Education, with distinction) from the University of the Witwatersrand, Johannesburg, and is registered with the Engineers Board of Kenya. His research interests are in the field of concrete durability, service life prediction and repair and rehabilitation of reinforced concrete structures.

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