Eco-efficient design of concrete repair and rehabilitation

Eco-efficient design of concrete repair and rehabilitation

21

Rachel Muigai University of Johannesburg, Johannesburg, South Africa

21.1

Introduction

From the mid-20th Century and into the 21st century, there has been an increased uptake of concrete as a structural material. The worldwide consumption of concrete is estimated to have increased from 6.4 billion m3 in 1997 (Aı¨tcin, 2000) to about 8 billion m3 in 2009 (CEMBUREAU, 2009), a 25% increase in only 12 years. This volume will continue to increase, particularly in developing countries, due to an exponential increase in population growth, urbanization and economic growth (Scheubel and Nachtwey, 1997; Humphreys and Mahasenan, 2002). While concrete production continues to grow and contribute towards economic development around the world, evidence suggests, however, that this growth is also associated with escalating impacts on the environment and society. Firstly, cement production and aggregate extraction and processing may lead to loss of arable/forest land coupled with the loss of biodiversity, waste generation, and resource depletion. (Uher, 1999; Alexander and Mindess, 2006; Cheng et al., 2006). Secondly, quarrying, construction, and repair activities may affect society negatively, due to the noise and air pollution that arise during blasting at quarry/construction sites, transportation of materials, and repair activities which also lead to user inconveniences. Thirdly, cement—the key constituent in concrete—is energy-intensive and accounts for 5%
8% of global anthropogenic CO2 emissions (WBCSD, 2002; Damtoft et al., 2008), as well as significant levels of SOx, NOx, particulate matter, and other pollutants (USEPA, 1999). Lastly, concrete produces massive amounts of inert waste through construction, repair, rehabilitation, and demolition activities. It is clear that if no action is taken, an increase in concrete production with time will cause an escalation of concrete’s environmental damage through depletion of the natural resource base, and pollution. Sustainable development is a key concept that is seen as contributory towards solutions to environmental degradation, as well as economic and social conditions that have an influence on the environment. The concept requires interdisciplinary efforts to deal with environmental problems, such as natural resource depletion, pollution, and loss of biodiversity caused by economic activities and social conditions, which include both poverty and affluence. The term “sustainable development” has a wide range of definitions, though it is commonly defined as “. . .development that meets the needs of the present without compromising the ability of future generations to meet their own needs” (WCED, Brundtland Commission, 1987). Eco-efficient Repair and Rehabilitation of Concrete Infrastructures. DOI: http://dx.doi.org/10.1016/B978-0-08-102181-1.00021-6 © 2018 Elsevier Ltd. All rights reserved.

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This definition has been purposefully put in a general manner to allow the involvement of a multiplicity of persons, institutions, governments and practitioners—including civil engineers—to include the sustainable development concept in decision making. Transference of the concept of sustainable development to the concrete construction industry gives rise to the associated concept of “eco-efficient concrete structures.” Furthermore, concrete is invariably used in construction, implying some form of structure or infrastructural expression. However, there remains a lack of definition of the term “eco-efficient concrete structure” to allow for its operationalization. One of the contributions of this study is to give a working definition of the term “eco-efficient concrete structure.” This study first reviews sustainable development principles that can be applied to the concrete construction industry. Sustainable development principles range from those given by international organizations such as the United Nations, to ones given by environmental groups and individuals. Finally, a comprehensive definition of an “eco-efficient concrete structure” is given, based on the underlying principles of sustainable development. The responsibility of achieving an eco-efficient concrete structure lies on the industry stakeholders, including the material producers, and those involved in the design and specification, construction, maintenance and repair, and the demolition and recycling of a structure. There is a potential area(s) of the life-cycle of the structure within which each of the industry stakeholders can influence. Of importance and focus of this study is the potential of structural engineers in reducing the environmental impacts of concrete structures through selecting repair and rehabilitation systems which consume less natural raw materials and induce less CO2 emissions, while providing the same reliability with a much longer durability. As is evidenced in current design practice, the practitioner in structural engineering is increasingly required by the client to synthesize a solution, which includes sustainability requirements of the structure as a whole. However, in structural design there is a lack of grasp of materials aspects, and environmental aspects of construction. Hence, the main contribution of this study will be to develop a framework for the design of reinforced concrete (RC) structures which structural and materials engineers can use to consistently and rationally consider “sustainability” in their repair and rehabilitation systems design.

21.2

Eco-efficient design of concrete structures

21.2.1 Background The sustainable development concept can be applied to the concrete construction industry attempting to ensure that activities within the industry are carried out within the ecological capacity of the earth. While there is little consensus about the definition for “sustainable development,” there are certain commonly accepted principles and practices that can nonetheless be used to guide sustainable development.

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“Principle,” as defined by the Oxford Dictionary refers to “a fundamental truth or a general doctrine that is used as a basis for reasoning or action.” Sustainable development principles range from those of international organizations such as the United Nations, as detailed in international agreements such as Agenda 21 (1992)— agreed upon at the United Nations Conference on Environment and Development held in 1992; to those put forward by environmental groups and individuals, such as the “cradle-to-cradle” principles for sustainable design, which were formulated by Michael Braungart and William McDonough (Braungart and McDonough, 2002). This set of principles can be used to operationalize the concept of sustainable development on different scales and levels, from the government level when passing legislation and formulating policies, to practitioners at local level institutions or businesses during decision making. This study reviews sustainable development principles that can be applied to the concrete construction industry in general. These principles are detailed in the subsequent sections and include: (1) The circular materials design model; (2) Dematerialization; (3) Increased production and operational efficiency; and (4) Durability design. The circular materials model draws from the “cradle-to-cradle” approach for sustainable design (Braungart and McDonough, 2002). The model advocates for the design of materials and products in ways that can relieve the environmental burden from waste disposal and also reduce the extraction of virgin materials. Dematerialization is defined as the reduction of the quantities of materials needed to serve an economic function, or the decline over time in the mass of materials used in industrial end products (Wernick et al., 1996 as cited in Kibert et al., 2002). This implies delivering the same performance with less volume of raw materials, hence minimizing the generation of wastes and eliminating problems associated with waste disposal (Peng et al., 1997). Improving the production and operational efficiency of a product can lower the energy requirements and emissions associated with its production and functioning. Durability design, in the context of construction, refers to the ability of a structure to resist deterioration during its intended service life. Suffice to say, products designed for durability will lead to the conservation of resources that could have otherwise been expended in repair actions. In summary, these set of principles can be used to operationalize the concept of sustainable development in the concrete construction industry. As such, a definition of an “eco-efficient concrete structure” is important, as it would ideally incorporate the various sustainable development principles and assist in defining the roles of the key players in the concrete construction industry, establishing their potential in contributing towards the overall sustainability of the industry.

21.2.2 Circular materials design model Some anthropogenic activities which impact negatively on the physical environment, such as resource depletion, can be attributed to the current economic system model which follows a linear structure (Doppelt, 2003) as illustrated in Fig. 21.1.

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Biosphere (Land, Air, Water)

Resources (Raw materials, Energy)

Quarrying and transportation of raw materials to processing plant

Processing of raw materials

Transportation (to batching plant and/or construction site)

Discharge (Solid waste, Emissions)

Construction (Mixing, casting, and curing of concrete on site or in a precast plant)

Use phase (Repair/ Maintenance)

Demolition

Transportation to landfill/ dumpsite of demolished materials and structural elements

Figure 21.1 Linear model applied in the life-cycle of concrete construction material. Source: Adapted from Turner, K.R., Pearce, D., Bateman, I., 1993. Environmental Economics: An elementary introduction. The John Hoplins University Press, Baltimore.

This economic system model views the production of products and services as a linear progression, from extraction of materials to their final disposal into landfills. Using the linear model, natural resources are extracted from the physical environment and refined into raw or constituent materials that are then remanufactured into consumer products, based mainly on cost and time-efficiency considerations, and occasionally on quality. The latter point in this case refers to construction management skills that control the workmanship and curing of a concrete structure. Such construction practices determine partly whether additional materials will be consumed for repair and maintenance during the structure’s service life. Notwithstanding, high quality construction may result in overconsumption of resources (materials and labor costs). Thus, a balance must be struck between the three bases of the economic systems model: time, cost, and quality, in order to avoid over- or under-consumption of resources. The linear model makes an assumption of infinite natural resources and so may lead to their over-exploitation and inequality in resource distribution between current and future generations. In addition, the linear model gives little thought, if any, to the discharge of waste products and emissions to the biosphere. In particular, the generation of construction and demolition waste (C&DW) has increased substantially in recent decades. C&DW refers to the nonhazardous waste resulting from the construction, remodeling, repair and demolition of structures (Macozoma, 2006). Developed and developing countries both generate considerable amounts of C&DW, which are mostly disposed of in landfill sites. For example, South Africa produces approximately 4 million tons/year of C&DW (Department of Environmental Affairs (DEA), 2012). Of the total C&DW generated per year, only 16% is recycled and the rest is disposed of by land (landfill sites, illegal dumps, or backfills) (DEA, 2012). Australia generates 13.7 million tonnes of C&DW per year, 81% of which is concrete waste, whereas Japan generates only 0.75 million tonnes of C&DW annually, of which 98% is recycled (Tam, 2009). In the global setting, approximately 1 billion tons of C&DW are generated yearly (Katz, 2004). Growing demands for resource conservation and recycling due to scarcity of landfill capacity or sites, present considerable challenges not only to the concrete construction industry but to all large solid waste emitting industries. These challenges can be partially addressed through the adoption of the circular model, illustrated in Fig. 21.2. The circular model is a biomimetic (life-imitating) approach that

Eco-efficient design of concrete repair and rehabilitation

Construction (Mixing, casting, and curing of concrete and concrete elements on site or in a precast plant)

Transportation (to batching plant and/or construction site)

Na tur al inp reso ut urc e

Re-use

Manufacture/ Reprocessing of recycled materials

595

Use phase (Maintenance and repair)

Rec

Disassembly (Demolition and sorting of rubble) ycli

ng

Transportation of demolished materials and structural elements

Figure 21.2 Circular (or closed-loop) model applied in the life-cycle of concrete construction material. Source: Adapted from: Allenby, B.R., 1992. Industrial ecology: the materials scientist in an environmentally constrained world. MRS Bull. 17(3), 46
51 (Allenby, 1992).

borrows from ecosystem cycles which operate off solar energy, and allow for the flow of energy and matter from the physical environment, and the release of wastes back to the physical environment. The circular model is also referred to as the “cradle-to-cradle” approach to design (Braungart and McDonough, 2002). The model encourages the designer to rethink ways the design product can relieve the environmental burden from waste disposal and also reduce the extraction of virgin materials. Following the circular model, economic activities including construction aim at utilizing wastes produced from all production processes as substitutes for natural resources and as inputs to construction activities. Waste management mechanisms that are available in a circular model include the biodegradation, reuse and/or recycling of wastes. Biodegradation of wastes, as explained in Braungart and McDonough (2002), involves the design of materials for the purpose of biodegradation and the absence of toxic substances after their useful life. A product can also be designed for adaptive reuse. In this case, the reuse of a structural component or material can be achieved if the structural engineer considers beforehand the possible changes in use of the structure, and designs the structure for adaptability. For example, a building may be designed to have a flat slab that avoids the use of beams to make it adaptable to functions in the future other than the one for which it was originally designed. Furthermore, C&DW can be recycled using two different processes (Calkins, 2009): (1) Up-cycling—which occurs when C&DW is remanufactured to produce value added products, e.g., the use of demolished waste for cement manufacture (Schepper et al., 2013) or as aggregates in concrete (Hansen, 1992; Olorunsogo and Padayachee, 2002; Kutegeza and Alexander, 2004). (2) Downcycling—which occurs when a material is used in low-grade applications, due to its low durability or strength properties, and since demolished concrete has a lower quality compared to natural aggregates (NA) due to mortar and cement paste which remains adhered after the recycling process (Marinkovic et al., 2010). Consequently,

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recycled aggregates are currently used in the construction of road base and subbase layers instead of concrete production for high strength applications. Recycling promotes resource conservation and creates value in the economy by reducing the input of virgin raw materials, reducing the need for landfills, and increasing use of labor through sorting of demolished waste on site or at dump sites. In summary, a circular model limits the use of virgin materials for economic activity and also minimizes the use of the environment as a sink for discharged solids and emissions. The adoption of such a model in economic activities requires product developers to design products that facilitate recycling, both within the economy and via natural ecosystem cycles (biodegradability) (Daly, 1990). Through the use of a circular model, the concrete practitioner is able to take on a life-cycle perspective to the design of a concrete structure. This is a conscious process that requires the designer to plan the life-cycle flow of resources and wastes of a structure.

21.2.3 Dematerialization Concrete construction is marked by activities related to the quarrying and processing of raw materials, which consist largely of NA. NA are nonrenewable as their geological processes of formation take a long time (millions of years) and their continuous and increased consumption decreases their reserves. Currently, high-grade reserves of the earth’s NA have been exploited in construction activities to a point where the availability of NA is now scarce, if not practically unrealizable in some regions or countries, particularly in urban areas. As a result, materials are transported for long distances, and this in turn elevates the energy consumed and construction project costs, both leading to a number of environmental problems such as greenhouse gas (GHG) emissions and resource depletion. Environmental concerns over the excessive mining of NA compared to other aggregate types, such as recycled aggregates, can be addressed by changing raw material consumption patterns in concrete construction through dematerialization. The application of dematerialization in concrete construction can be partially achieved through the use of recycled concrete aggregates and through the structural optimization of a structural component to reduce the volume of materials used, which in turn leads to a reduction in pollution generation.

21.2.4 Increased production and operational efficiency Improved efficiency in all the manufacturing processes of a product, including the extraction of raw materials for its production and processing of these materials, can lower the energy requirements and emissions associated with their production. For example, current solutions to aid in reducing the 5%
8% global anthropogenic CO2 emissions from cement manufacture (WBCSD, 2002; Damtoft et al., 2008) include improving the efficiency of cement kilns. Optimizing kiln processes and plant efficiencies during cement production results in the reduction of CO2 emissions and also brings down the cost of production. Modern cement kilns should

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use the dry process of raw materials, as opposed to the wet process. The former refers to the process whereby raw materials are first ground and heated before being fed into the kiln, whereas in the wet process, the raw materials are crushed, ground, and mixed as slurry. The most efficient dry-process kilns use approximately 2.9 GJ of energy per ton of clinker (http://www.energyefficiencyasia.org/docs/industrysectorscementdraftMay05. pdf). Wet-process kilns are more energy-intensive and can consume more than twice the amount used by dry process kilns (Gartner, 2004). However, there is a thermodynamic limit where it is not possible to increase production efficiency and hence limit GHG emissions. Further reductions in energy used in materials production can then be achieved through the substitution of renewable energy sources for fossil fuels. For example, waste tyres can help reduce the amount of coal energy used in cement kilns. In addition, the use of blended cements can improve the efficiency of concrete production. Blended cements are produced by inter-grinding Portland cement clinker (Clinker is the main product of Portland cement manufacture and is generated by heating raw materials (limestone, iron ore, and aluminosilicates such as clay) together at temperatures of about 1400
1500 C.) with supplementary cementitious materials (SCMs) or by blending Portland cement with SCMs such as fly ash from coal combustion in electricity-producing plants or blast furnace slag from iron-making plants. The use of blended cements reduces the amount of clinker that needs to be produced, also lowers the GHG emissions, and diverts wastes from landfills, as SCMs are by-products of other industries that would otherwise have been disposed. In addition to the production efficiency considerations above, there are operational efficiency considerations that are specific to the type of structure, e.g., civil engineering structure or building, and relate to the use-phase of the structure. This includes the use of efficient heating and ventilating systems in buildings so as to reduce their operational energy. In addition, the thermal performance of alternative construction materials should not be ignored by the designer, as they have a role to play in reducing the operation energy losses in heating and/or cooling a building. In particular, concrete has a higher thermal mass than other building materials, and its use in the fabric of a building can reduce the cooling and/or heating energy needs of a building.

21.2.5 Durability design Durability refers to the ability of a structure to resist deterioration during its intended service life. Construction products are marked by a long life-time and can consume large resources in their life-cycle if they do not have adequate durability. The durability design of concrete structures is concerned with ensuring the ability of concrete, inter alia, to resist the penetration of aggressive agents during its intended service life. Most approaches to concrete durability design and specification rely on the socalled “prescriptive method,” i.e., the design and specification “rules” are intended to provide for durability by prescribing limiting values for material properties and proportions, depending on the environmental conditions and life span of the structure.

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Eco-efficient Repair and Rehabilitation of Concrete Infrastructures

The specified parameters are usually the concrete cover to reinforcement, 28-day compressive strength, maximum water-cement (w/c) ratio, and minimum cement content. For example, design standards such as BS 8500-1: 2006, give the limiting values of the concrete cover to be provided to all reinforcement, 28-day compressive strength and cement content in order to achieve a durable concrete for a range of w/c ratios. Beside the fact that these requirements can sometimes be mutually contradictory, this approach does not explicitly address rational, quantitative durability design, nor does it address sustainability issues. Regarding this latter point, prescriptive specifications are generally restricted to conventional materials and do not have the flexibility to address “new” and marginal concrete materials such as recycled and site-derived materials. These materials may, in certain circumstances, be adequately durable but also bring reductions in raw material resource use, and also possible cost savings. Furthermore, by using the prescriptive method there is a danger of overspecification, since the prescriptive approach is inherently conservative and results in resource waste. Lastly, the approach assumes that the as-built quality of concrete is what has been specified, without the means to check actual as-built quality. It also does not account for variability in as-built quality that may occur due to material variability and variable site practices, including poor workmanship and inadequate curing in as-built quality. Such practices may result in poor quality concrete which will require additional repair and maintenance during the structure’s service life, resulting in additional unanticipated material consumption, social disruptions, and costs. Thus, the present prescriptive approach to durability specifications should be phased out, since it contributes, in many cases, directly to un-sustainability. On the other hand, performance-based approaches to durability design, such as those put forth by the fib (2010) (fe´de´ration international du beton) Model Code for Service Life Design, are specifically intended to limit the environmental consequences on the structure, to defined acceptable levels or targets during the structure’s service life. The approach advocates the use of service life prediction models that quantify environmental deterioration and provide an output in terms of the required material quality. From this requirement, the designer is left with the choice of selecting a suitable material (conventional, new or marginal) that will meet the requirements within the predefined acceptable level. The specified material quality is then verified on site, using suitable tests that characterize that quality. Considerations of sustainability of concrete structures should consequently relate to service life and performance requirements of the structure, in which durability considerations are embedded.

21.2.6 Definition of an “eco-efficient concrete structure” Based on the principles of sustainable development given in the previous sections, a definition of an “eco-efficient concrete structure” is suggested here as (Muigai, 2014): a concrete structure that is designed to meet case-specific needs of the users, that minimizes life-cycle costs and environmental impacts through (i) use of efficient production, construction operational, repair and rehabilitation technologies (ii) selection of materials that have a minimal negative environmental impact and

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which give optimized properties for long-term durability (iii) selection of an appropriate structural layout and form, and optimized volume, and (iv) is designed for deconstruction and recycling

The definition of an “eco-efficient concrete structure” assists in establishing the roles of the key players in the concrete construction industry, in a bid to show their potential in contributing towards the overall environmental performance of the cement and concrete industry. This information is summarized in Fig. 21.3. The key players in the industry consist of the material producers and the design and construction team which includes: the architect, structural and materials engineer, geotechnical engineer, mechanical and electrical engineer, the quantity surveyor, the project manager and the contractor and the client. Fig. 21.3 shows the potential areas in which these key players can influence (in order to improve) the overall environmental performance of the industry. For example, from Fig. 21.3, it is noted that the structural and materials engineer is faced with a major role in ascertaining the probable service life of the repaired structure and in selecting appropriate materials and methods for concrete repair and rehabilitation, which not only contribute to the durability performance of a concrete structure but additionally reduce the recurring environmental impacts due to deterioration.

Design phase: Best design practice: • Project environmental impact assessment • Structural layout based on aesthetic quality, constructability, environmental impact and cost • Life cycle design optimization of material properties (e.g., durability, strength) and section dimensions • Design for deconstruction

Design Team

Materials production phase: Efficient production technology:

Geotechnical engineer

Structural and materials engineer

Construction phase: Efficient concrete technology: • Noise reduction • Quality control

Project manager Architect

Quantity surveyor

Mechanical and Electrical engineer

Construction and repair team Material manufacturers

Material suppliers

Contractors

Client

Use phase: Energy efficiency: • Buildings: • Passive design: Building orientation for natural lighting and ventilation • Renewable energy sources for HVAC • Bridge • Riding comfort and noise reduction • Lighting devices

Repair phase: Best design practice:

Deconstruction/Disassembly phase: Best deconstruction technology

• Environmental impact assessment of repair systems • Selection of repair system based on aesthetic quality, compatibility, environmental impact, and cost

Figure 21.3 Roles of the key players in the concrete construction industry.

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21.2.7 Proposed framework for the eco-efficient design of concrete repair methods and materials 21.2.7.1 Introduction Concrete repairs may be broadly classified as “nonstructural” or cosmetic repairs, in which stress-carrying is not a major consideration for the repair, and “structural” repairs where the repair is required to carry stress (Morgan, 1996). Both types of concrete repairs can be carried out using protective surface treatments and/or patch repairs, among other repair methods. Protective surface treatments mainly use polymeric resins based on epoxy, silicone, acrylics, polyurethanes, or polymethacrylate. Patch repairs are mortars that can be grouped into three categories (Emberson and Mays, 1990): (1) resin mortars; (2) plain cementitious mortars; or (3) polymer-modified cementitious mortars. In addition to ensuring the structural performance of a repair system, the practitioner in structural engineering is increasingly required by the client to synthesize a solution, which includes sustainability requirements of the structure as a whole. This can be made possible through the development of a framework for design which structural and materials engineers can use to consistently and rationally consider “sustainability” in their designs. The current key driver to the design of eco-efficient repair and rehabilitation methods has been the need to minimize resource use of natural resources and GHG emissions over the life-cycle of concrete. This involves GHG-emission reduction through the design of durable repaired structures, by ensuring adequate resistance to environmental effects and providing adequate structural capacity and safety under the expected loading. The durability of the repaired structure depends on the interactions with the service environment, in which penetration of deleterious substances is highly significant. The ingress of various ions, liquids, and gases from the environment is responsible for the deterioration of the original concrete and/or repair material directly or indirectly (Basheer et al., 2001). When the loading and environmental conditions to which a repaired structure would be exposed to during its remaining service life are well defined, it is the responsibility of the Engineer to tailor the design of eco-efficient repair methods and materials. An eco-efficient repair method/material is one that attains specified performance levels in terms of strength, durability, costs, and carbon footprint to respond to the design requirements.

21.2.7.2 Design framework The concept of designing for eco-efficient concrete repairs, calls for the design team to adopt a different approach to thinking about the decisions regarding the choice of repair materials and their long-term effects on the environment and cost. To facilitate this process, a framework for design is proposed as shown in Fig. 21.4 that consists of key criteria that should be taken into consideration for concrete repair design. These are: 1. A set of quantifiable design parameters and variables—consisting of the repair material constituents and concrete hardened properties that have an influence on the life-cycle sustainability of concrete. The framework is limited to quantifiable parameters and variables. However, there are other qualitative parameters that have an influence on the overall sustainability of concrete, e.g., construction site practices such as curing, compaction,

A framework towards the design of ecoefficient concrete repair methods Functional design requirements • • • •

Remaining design service life Repair and Rehabilitation Service environment Structural loading conditions

– – – –

Client’s requirements EN 1504 EN 206-1: 2000 EN 1991-1

Optimization process Design variables Material design variables

Quantifiable design requirements

•

Repair material composition

(The value of each design requirement is dependent on the choice of design variables)

• • • • •

Durability – e.g., diffusivity of corrosive agents [m2/s] Tensile strength [MPa] Elastic modulus of repair material [GPa] Thermal expansion of repair material Creep and shrinkage of repair material

Sustainability requirements • Environmental impacts due to repair [GWP100] • Repair costs [Rands]

Sustainability design performance measures for rc structures

•

Material properties • • • •

Repair material type Diffusion coefficient Thermal conductivity Others

No: Select new values for Design variables

Structural and material requirements (Verifiable using laboratory test methods)

Design Target

(Integrated measure expressed as a function of the design requirements)

(Verifiable using limit-state theory) OR • Life-cycle environmental impacts/m3/MPa • Life-cycle costs/m3 /MPa

Life-cycle Inventory database Repair methods database • GWP100, energy or exergy of different repair methods • Economic costs for different repair options • Social costs/benefits arising from repair activities

The overall objective is to ensure that the functional design requirements have been met for a given set of design variables

Figure 21.4 Proposed framework for the eco-efficient design of concrete repair methods and materials.

Decision making

Yes

Life-cycle environmental impacts/m3 Life-cycle costs/m3

Environmental impacts, and financial and social costs of repair

• •

Design output Repair material Type and composition

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and good workmanship. These qualitative factors also play a major role in the long-term structural performance of the repaired concrete. They are excluded from this study as they cannot be quantified in physical units. Suffice to say, the best practice in these aspects is necessary to realize eco-efficient concrete structures. 2. Performance measures—that consist of quantitative indicators that allow for the selection of appropriate design variables and parameters. 3. A database—of alternative materials for concrete repair methods and end-of-life strategies for concrete, and their associated unit environmental life-cycle impacts and costs/benefits to the user and owner of the structure.

21.2.7.3 Description of the design framework The framework (Fig. 21.4) consists of the following processes: 1. A set of functional design requirements of an RC structure, as specified by the client and/ or the design codes and standards. For example, European Standards EN 1504 gives guidelines for the repair and protection of RC structures. Other functional requirements include the remaining service-life of the structure, which may be specified by the client/owner of the structure. To ensure that the remaining design service life is met, the designer should take into consideration the service environment of the structure and classify it based on the requirements of EN 206-1: 2013. In essence, the designer is required to establish the environmental actions, i.e., those chemical and physical actions to which the RC structure is exposed and that result in deterioration of the concrete or reinforcement. Deterioration of RC results from reinforcement corrosion, alkali
silica reaction, chemical attack, leaching by nonbasic (and nonalkaline) solutions, and high temperatures generated in case of fire (EN 1992-1-1: 2004). The main environmental action on an RC structure is frequently related to corrosion caused by ingress of chlorides or CO2 gas. 2. The functional design requirements are translated into measurable design requirements which consist of structural and material requirements, and sustainability performance requirement in terms of the life-cycle environmental impact of the repaired structure. The latter requires knowledge of the quality of the material at the end of its service life. 3. A set of measurable design variables which have an influence on the sustainability of RC structures. 4. The design requirements have different measurement units. Consideration should be given to the selection of a suitable integrated unit for comparing the performance of different materials with respect to the design requirements. The selected integrated performance measure is expressed as a function of the design requirements. 5. A reliable database of the unit environmental impact and costs of repair materials and construction activities. The current study includes the global warming potential (GWP100) (kg CO2-eq) metric as a measure of the environmental impact and the unit costs are expressed in Rands. 6. The design verification is an optimization process seeking to ensure that the selected design variables satisfy the performance requirements and result in minimum life-cycle environmental/cost impact. This assessment involves a limit-state approach. 7. The outputs of the framework are the optimal repair material properties for the selection of eco-efficient repair concrete methods and materials.

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Figure 21.5 Design example of a simply-supported RC beam. Where d (mm) is the effective depth of the beam; b (mm) is width of the beam; l (m) 5 6 m is the span of the beam; Ø (mm) is the diameter of the tension steel (Ast) and is assumed to be 25 mm; xmin (mm) is the minimum cover to reinforcement; and h (mm) is the total height of the beam; b, h, xmin, and Ast are the structural design variables in the optimization problem. Asc and Asv are taken as nominal reinforcement.

21.2.8 Case study The design framework guides a designer in finding the geometry and materials specifications for structural components that result in the lowest environmental impact while meeting design requirements for serviceability and safety. To demonstrate the application of the proposed framework to design, the study uses a hypothetical simplified RC beam idealized in Fig. 21.5. The span (l) of the RC beam is determined a priori at the conceptual phase of design and is indicated in Fig. 21.5. While the example below is ideally simplistic, it should be noted that more complex geometries and shapes could easily be adopted for further refinement. The hypothetical RC beam is assumed to be part of a structure located in a marine environment. For this example, it is assumed that the environmental condition corresponds to the exposure class XS1 in EN 206-1:2013. For the XS1 marine zone, the concrete is exposed to airborne salts, but not in direct contact with sea water. The design service life of the structure is specified as 30 years. In addition to its self-weight, the RC beam is expected to support uniformly distributed loads of: 30 kN/m live load; 60 kN/m dead load.

21.2.8.1 Design variables This study utilizes a vector of design variables [X] represented as:
 ½X 5 fx1 ; x2 ; x3 ; x4 ; x5 ; x6 ; x7 g 5 b; h; xmin ; Ast ; w=b; Mw ; Ma

(21.1)

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where b (mm) is the width of the beam; h (mm) is the overall depth of the beam; xmin (mm) is the minimum concrete cover; Ast (mm2) is the area of tension steel reinforcement; w/b (-) is the water/binder ratio of the concrete design mix (which depends on the choice of the binder system); Mw (kg/m3) is the water content; and Ma (kg/m3) is the mass of fine and coarse aggregates. In addition, there are response variables describing the resultant material properties represented by a vector [Y] as follows:

 ½Y 5 y1 ; y2 5 Do ; fck

(21.2)

where, Do (m2/s) is the chloride diffusion coefficient of the concrete; and fck (MPa) is the concrete characteristic compressive strength. The two variables are dependent on: the binder system, w/b ratio, and aggregate/binder ratio (Papadakis et al., 1996; Papadakis and Tsimas, 2002).

21.2.8.2 Design parameters Other than the design variables, there are particular design parameters which have an influence on the sustainability of concrete. These design parameters relate to: (1) The span (l); (2) Reinforcing steel bar diameter; and (3) Unit environmental impacts of the constituent materials indicated in units of greenhouse warming potential (GWP100) (kg CO2-eq).

21.2.8.3 Objective function The optimization problem in this study considers the nonlinear objective function f ðX; YÞ(Eq. (21.3)) that represents the life-cycle environmental impacts of the structural component as a function of selected design variables. The aim is to select a vector of material variables, [X] and [Y] that gives the minimum environmental impact for the concrete section. Minimize f ðX; YÞ

(21.3)

where, X and Y represents vectors of design variables (see [X] and [Y] in Eq. 21.1 and Eq. 21.2, respectively); f(X,Y) represents the environmental impact per-unit length of the structural component. Eq. (21.4) gives the expanded form of f(X,Y), and includes the environmental impacts of concrete and steel. The environmental impacts of the formwork and placing of concrete have been excluded as these are assumed to be similar for all the concrete mixes to be compared.       φ f ðX; Y Þ 5 ρs As Envsteel 1 b d 1 1 xmin 2 As Envconcrete 2

(21.4)

Eco-efficient design of concrete repair and rehabilitation

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where, X,Y

:

ρs (kg/m3) As (mm2)

: :

b (mm) d (mm) φ (mm) xmin (mm) Envsteel (kg CO2-eq/ton) Envconcrete (kg CO2-eq/m3)

: : : : : :

Vector of material design variables (in Eqs. (21.1) and (21.2)) which optimize the value of the objective function Density of steel Reinforcement area for a unit length of beam (consists of tension steel—Ast, nominal compression steel—Asc, and nominal shear links—Asv). As is a function of φ and the number of steel bars in the beam section Width of the concrete component Effective depth of the concrete component Diameter of the steel reinforcement Minimum concrete cover to reinforcing steel Unit environmental impact of steel per unit mass Unit environmental impact of concrete per unit volume as given by Eq. (21.5)

Eq. (21.4) gives the quantified cradle-to-gate environmental impacts of materials in the RC beam (i.e., the concrete and steel) in units of kg CO2-eq per unit length of the beam. The cradle-to-gate environmental impacts cover all material processing activities until the factory gate. Further, Eq. (21.5) gives the environmental impact of concrete per unit volume (kg CO2-eq/m3), needed for Eq. (21.4), and is computed as a function of its compressive strength. 2

3 2 0 3 1 1 f f Envbinder ck ck Envconcrete 5 4Mw @ 1 aA 2 kMP 5Envbinder 1 4Mw @ 1 aA 2 M c 5 KB KB k 2 0 3 1 2:65 4 @ fck 1 2650Enva 2 Mw 1 aA 2 kMP 5Enva RDbinder KB 3 2 0

7 6 2 0 3 1 7 6 6 2:65 4 @ fck Env M 1 kM a c P 7 6 5 A 17 2 2:6560 2 Mw 1 a 2 Mc 7Enva RDbinder KB k 7 6 fck 4@ 1 aA 5 KB 3 2 7 6 7 6 6 Mc 1 kMP 7 6 17 2 2:65wa Enva 1 60 7Envw 1 MAdm EnvAdm 7 6 fck 4@ 1 aA 5 KB (21.5)

606

Eco-efficient Repair and Rehabilitation of Concrete Infrastructures

where, KB (MPa)

:

fck (MPa) a (-)

: :

wa (%) k (-)

: :

Mc (kg/m3) Mw (kg/m3) MP (kg/m3)

: : :

MAdm (kg/m3) RDbinder (-) Enva (kg CO2-eq/ton) Envbinder (kg CO2-eq/ton) EnvAdm (kg CO2-eq/ton) Envw (kg CO2-eq/ 1000 L)

: : : : : :

Bolomey coefficient that depends on the aggregate and cement type, and is assumed to be 21.3 MPa for all concrete types characteristic compressive strength of concrete, at 28-days a parameter that depends on the time and curing of the concrete and is estimated as 0.5 for fck at 28 days (Papadakis and Tsimas, 2002) air content in fresh concrete efficiency factor of the respective supplementary cementitious material mass of Portland cement per cubic meter of concrete mass of water per cubic meter of concrete mass of supplementary cementitious materials per cubic meter of concrete. This is expressed as a percentage of the mass of Portland cement, e.g., 30% Mc mass of admixture (superplasticizer) per cubic meter of concrete relative density of the binder environmental impact of aggregates per unit mass environmental impact of the binder per unit mass environmental impact of admixture (superplasticizer) per unit mass environmental impact of water per 1000 liter

The form of Eq. (21.5) was chosen so as to be able to quantify the variables involved in the problem, rather than using purely empirical relationships. The parameters in Eq. (21.5) represent the mix-design composition and the resultant concrete property (compressive strength) which have an influence on the life-cycle environmental performance of concrete.

21.2.8.4 Design constraints The design constraints relate to the ultimate limit states (ULS) and serviceability limit states (SLS) for RC given in EN 1992-1-1: 2004. The constraints include: (1) the ultimate-limit state of bending resistance of the structural component; (2) the durability of the structural component in its service environment; (3) deflection in the member due to service loads. In addition, the optimization problem includes two side constraints: (1) the upper and lower boundaries of the area of steel reinforcement; and (2) the upper and lower boundaries of the geometry of the crosssection of the structural member.

21.2.8.5 Results and discussion The member section and material design involves the selection of optimum crosssection dimensions and concrete mix-design properties for the RC beam. This is

Eco-efficient design of concrete repair and rehabilitation

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achieved by optimizing the objective function (Eq. (21.4)), subject to the given design constraints. Due to the nonlinear nature of the objective and constraint functions, the optimization problem is solved using a nonlinear programming technique based on the generalized reduced-gradient optimization algorithm (Drud, 1994).

21.2.8.6 Optimized concrete mix-design and structural geometry for the RC beam The following procedure is used to evaluate and select the optimum concrete mixdesign, material properties, and geometry of the RC beam: 1. A set of commonly used binder types is selected. In general, this selection will depend on binder types available in the locality of concern, and a measure of judgement. The binder types chosen for the present example are: CEM I 52.5 N, CEM II/B-V 42.5 N, CEM II/A-V 52.5 N, CEM III/A-S 42.5 N, and CEM I 52.5 N with a superplasticizing admixture. The corresponding compositions of the binder types are indicated in the third row of Table 21.1. 2. A common concrete grade of C30/37 was selected for all concrete made using the four binder types. 3. Using MATLABs software, a generalized, reduced-gradient optimization algorithm was written and used to solve the optimization problem, to give optimized values for the design variables and response variables represented by Eq. (21.1) and Eq. (21.2), respectively. 4. A comparative analysis of the optimum design variables for the different binder types was then carried out.

Using the optimization procedure given above, the optimum design variables for the different concretes made using the four binder types with similar binder contents are given in Table 21.1. Also in Table 21.1, it can be seen that the water content is selected to vary with binder type in order to give a constant slump for all concrete mixes. From Table 21.1, it can be seen that concrete made using CEM III/A-S 42.5 N has the lowest environmental impact compared to other concrete types. This is followed by CEM II/B-V 42.5, CEM II/A-V 52.5, CEM I 52.5 N with admixture, and finally CEM I 52.5 N. In comparison to conventional, prescriptive-based design, the minimum cover depth design provisions for CEM III/A-S 42.5 N and CEM II/B-V 42.5 concretes for the optimized RC beam are lower than the recommended value of 40 mm by BS 8500-1: 2006 at a w/b ratio of 0.4, due to better chloride resisting properties of these binders. The latter comparison shows that the design provisions by current design codes are conservative for certain binder types. The latter comparison also shows that the design provisions by current design codes are conservative for certain binder types. Note: in this case the comparison is not strictly correct, since BS8500-1:2006 allows for a 50-year design life, while the example given was for a 30-year design life. The principles however still apply, noting that codes normally prescribe

Table 21.1

Optimized material and structural design variables for a C30/37 RC beam Optimized solutions

Variables [X] and [Y] Mbinder (Mass of binder) Mwater (Mass of water) w/b ratio Ma (Mass of aggregates) b (width) d (effective depth) xmin (minimum cover depth) h (overall depth) nb (number of 25 Ø bars) Ast, required Diffusion coefficient (Do) f(X,Y)

Mix I

Mix II

Mix III

Mix IV

Mix V

CEM II/A-V 52.5 N (20%:80% FA:PC)

CEM III/A-S 42.5 N (50%:50% GGBS:PC)

CEM I 52.5 N (100% PC)

Units

CEM II/B-V 42.5 N (30%:70% FA:PC)

CEM I 52.5 N (100% PC with admixture)

(kg/m3)

425

350

450

370

320

(kg/m3)

170

175

180

185

160

(2) (kg/m3)

0.4 1805

0.5 1750

0.4 1780

0.5 1848

0.5 1848

(mm) (mm) (mm)

170 680 20

185 730 40

175 685 25

180 720 75

180 720 75

(mm) (mm)

715 6

785 5

740 6

810 5

810 5

(mm2) m2/s

2792 6.7 3 10213

2489 3.0 3 10212

2676 1.3 3 10212

2477 4.5 3 10212

2477 4.5 3 10212

(kg CO2 2 eq/m)

66

67

60

78

69

Eco-efficient design of concrete repair and rehabilitation

609

minimum cover values. However, the proposed method can be used to modify code values in selected cases. Generally, the following deductions can be made from Table 21.1: 1. It is important to select an appropriate binder content for a binder system, and vice-versa, as the choice of binder system is based on its environmental impact at a particular binder content. 2. The use of SCMs allows the designer to select optimized values of concrete cover and hence leads to reduced cross-sectional dimensions, which translates to reduced volume of materials. 3. The use of a chemical admixture in concrete made using CEM I 52.5 results in a 12% reduction in the unit environmental impact of concrete, despite the relatively high unit environmental impact of superplasticizers. This shows that the use of chemical admixtures is beneficial to the environment as it leads to resource conservation i.e. chemical admixtures lead to a reduction in the water content of the mix-design and hence the binder content in order to maintain the original w/b ratio.

In conclusion, the optimization enables the selection of the optimum section dimensions, binder content, and type and strength of binder that meet the required performance in terms of characteristic compressive strength (fck) and durability requirements of concrete, and in addition minimizes the environmental impact.

21.3

Conclusions

Increased concrete repair activities due to durability failure are associated with escalating impacts on the environment and society worldwide. Concrete repair and rehabilitation activities contribute to natural resource depletion and produce massive amounts of CO2 emissions and inert waste. In addition, the repair activities affect society negatively due to noise and air pollution and lead to user inconveniences. It is clear that if no action is taken, an increase in concrete repair activities with time will cause an escalation of concrete’s environmental damage through depletion of the natural resource base, and pollution. Engineers have a role to play in designing eco-efficient repair systems that attain specified performance levels in terms of strength, durability, costs, and carbon footprint to respond to the design requirements.

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702. Turner, K.R., Pearce, D., Bateman, I., 1993. Environmental Economics: An elementary introduction. The John Hoplins University Press, Baltimore. Uher, T.E., 1999. Absolute indicators of sustainable construction. Royal Institution of Chartered Surveyors (RICS) series1999. United Nations Conference on Environment and Development (UNCED), 1992. Agenda 21. USEPA, 1999. United States Environmental Protection Agency. WBCSD, 2002. World Business Council on Sustainable Development, 2002. WCED (World Commission on Environment and Development), 1987. Our Common Future. Oxford University Press, Walton Street, Oxford. Wernick, I.K., Herman, R., Govind, S., Aususbel, J.H., 1996. Materialization and dematerialization: measures and trends. Daedalus. 25, 171
198. As cited in: Kibert, CJ., Sendzimir, J., Guy, G.B., 2002. Construction Ecology: Nature as the Basis for Green Buildings, London and New York, Spon Press.

Further reading EN 1991. Eurocode 1: Actions on Structures. EN 1990-1, 2002. Basis of Structural Design. European Committee for Standardization (CEN). Fulton’s Concrete Technology 2001. In: Addis, B.J., Owens, G., (Eds.), Midrand: Portland Cement Institute, pp.135.