Keywords

1 Introduction

Concrete is an extensively utilized building material, second only to water in terms of consumption (Gagg 2014) with an estimated annual usage of over 30 billion tonnes (Monteiro, Miller & Horvath 2017). Ancient concrete structures, such as the Ponte de Gard in France, attest to the extraordinary long-term performance of this material. Concrete functioned remarkably well for load-bearing structures with rounded curves, such as domes and arches, and this classic concept is being recreated today with modern 3D-printing technology, such as the Striatus 3D-printed bridge in Venice, Rome.

Steel reinforcement in concrete enabled the development of diverse structural components capable of withstanding compression and tension stresses. However, if R.C. is not effectively handled, steel members may corrode, severely damage the structure, and shorten its service life.

There has been a growth in knowledge and understanding of durability in recent years. An extensive study on this topic was conducted between 1970 and 1990, resulting in a wealth of knowledge (Noyce & Crevello 2016). This is due to the economic effect of shorter structure life spans on a country’s budget due to costly repairs, disruption of important daily activities, and a decrease in sustainability.(Beushausen et al. 2021). As a result, efforts have been carried out in most design codes and standards to include requirements for providing robust and durable RC structures.

This paper compares and critically review the durability design approach and provisions of three international codes: The British Code BS8500-1, Eurocode EN1992:1-1 and ISO13823 standard, it suggests improvements to these provisions that may be a beneficial start for these codes’ update. The study revealed that the BS 8500-1 and EN2 are similar, they included provisions to guide the design engineer to produce durable structures, they adopted a prescriptive approach, which sets requirements for material compositions and quantities, techniques, and test methods that are assigned according to the environmental exposure category. On the other hand, the ISO 13823 International Standard has a different approach, even though it does not identify design procedures for durability, it establishes a sturdy framework by defining a process of concrete damage due to environmental effect.

2 Design for Durability

The process of designing and constructing buildings and/or infrastructure to meet design lifetime and required standards in severe conditions is known as design for durability. Designing for durability is a difficult task, it is a multilevel process that incorporates various factors, including design for the entire structure, structural elements, and structural materials. The project’s service life must be established at the initiation of the project, and the design phase must outline the exposure conditions, and member features such as concrete cover, materials, mix design, and curing procedures, as well as build a monitoring and maintenance plan. Finally, workmanship and material quality must be considered throughout the project’s execution stage. To do so, clear guidelines must be available for design experts to use to direct their design process toward long-lasting structures. Douglas Hooton (2019) and Grković & Folić (2015) stated that current building codes have generally focused on structural capacity and serviceability for life safety considerations, but many do not fully address durability design. From the scholars’ conclusions, we can understand the importance of providing clear guidelines in the design codes to create a comprehensive design addressing the concrete structures’ durability along with capacity and serviceability design.

3 Design for Durability in the Arab Gulf

The Gulf region’s environment has an impact on concrete structures because it is one of the world’s most aggressive exposures for the durability of reinforced concrete structures, which degrade faster unless special precautions are taken due to high ambient temperature, low relative humidity, salt-contaminated dust, seawater, and underground salts.( Mehta 2003). Raouf (2012) mentioned that this is justified by the variable low rainfall, high evaporation rates, and frequent droughts that characterize the region.

To overcome the harsh environmental effect on concrete durability, multiple studies took place in different Gulf countries. To begin with, Ghous Sohail et al. (2020) examined how well newly created high-performance concretes (HPC) and ultra-high-performance concretes (UHPC) withstand over time. Al Nuaimi et al. (2021) assessed the durability performance of RC beams reinforced with CFRP composites performance after being exposed to sunshine and salt water.

According to the above scholars’ outcomes, the adaptation of advanced materials and supplementary cementitious materials in the concrete mix, chosen at precise amounts and suitable to the exposure, together with correct curing, can overcome durability limiting factors in the Gulf’s severe environment.

The Concrete Society’s guide “Guide for the Construction of Concrete Buildings on the Arabian Peninsula” includes details on the durability design of RC structures. A four-step durability design approach for RC structures has been devised based on the fib principles (Demis & Papadakis 2019).

4 Comparison for Durability Design Provisions in International Codes and Standards

4.1 Durability Design Approach

BS 8500-1, EN 1992.1.1 have a similar approach for durability design requirements, they provide provisions for the environmental effect and recommend limits for critical values that directly impact concrete structure durability and methods for durability verification with variations among them of the details and given values. On the other hand, ISO 13823 has a different approach, it provides the limit state method and comprehensive guidance of the process of structure deterioration, starting from the environmental influence (exposures) that turns into agents throughout a transfer mechanism that will exert action and result in environmental effect.

4.2 Exposure Conditions

Both BS 8500-1 and EN 1992.1-1 codes based their exposure conditions on EN 206 code, consequently, they have the same exposure conditions except for the category of chemical attack due to seawater that is mentioned in the British standard but not in Eurocode. On the other hand, ISO 13823 listed 11 influences that have the same concept of exposure categories, and no subdivision due to severity is provided, nonetheless, the standard dictates that after assigning the deterioration assessment by experience, modelling and testing, the severity of the environment can be decided.

Freezing and Thawing

Freezing and thawing cycles are very detrimental for concrete structures (see Fig. 1), the British and Eurocode focus for each class on the water saturation concentration and the usage of de-icing agents, they have two main categories, moderate and high-water saturation, each one is subdivided to two classes, with and without the usage of de-icing agents. In contrast, ISO 13823 doesn’t categorize freezing and thawing as the compared codes, it is listed as an agent due to the influence of temperature and it must be considered in the durability design process.

Fig. 1.
figure 1

A concrete bridge parapet wall exhibits severe deterioration caused by repeated freezing and thawing cycles (Susca 2006)

Full size image

Chemical and Sulphate Attack

EN 1992.1-1 included chemical attacks from natural soil or groundwater, three classes are given as per the aggressiveness of the environment, XA1, XA2, and XA3. Although the BS 8500-1 has similarity to the Eurocode, and such exposures don’t exist in the UK, more comprehensive exposure classes are specified, six site exposure DS (design sulphate attack) classes depending on magnesium and sulphate amounts, and ACEC (aggressive chemical environment for concrete) exposure classes based on acidity and water mobility. Adding to that is the exposure category chemical attack from seawater which is not included in the Eurocode (Al-Haddad, Jokhio & Tair 2023).

From the ISO 13823 aspect, the chemical and sulphate agents act as an influence from the soil and ground containment. The effect of these agents depends on their amount, combination, and soil type. The standard then mentions that special concrete types and mortar mixes can be used to limit their effect. No specific concentrations are given or designated classes. (See Fig. 2).

Fig. 2.
figure 2

A structure subjected to sulphate attack (Sadanandam Anupoju 2009).

Full size image

In Contact with Water

Despite the detrimental effect of water in contact with structures, the British standard and Eurocode didn’t provide a direct exposure category for it. Additionally, the analysed ISO standard contained water exposure (agent) in all its states (liquid, gas and solid) and with its different mechanisms to react with different contaminants and impact the structure’s durability. The Eurocode mentioned it as part of a physical attack arising from water penetration.

Reinforcement Corrosion

For this exposure condition that is very damaging for the structures and if not repaired might lead to performance loss (see Fig. 3), the Eurocode and the British standard are more compendious than the ISO standards. The Eurocode and British standard grouped corrosion conditions according to the source of the agent that is initiating this reaction. Moreover, the British standard included a combination of exposures, as corrosion might be initiated with freezing and thawing, these exposures might occur simultaneously, these exposures are XC1 and XF3, XC2 and XF3 and or ACEC exposure.

Corrosion in the ISO 13823 standard is mentioned in different sections and for different metals, corrosion as per the standard might be initiated by chemical compatibility, electrical current in the ground and the presence of moisture and chloride under the effect of multiple environmental actions such as the environmental atmosphere, marine structures, type of soil, cracked concrete and masonry.

Fig. 3.
figure 3

Corrosion in concrete structures (Murari et al. 2021).

Full size image

Other Exposure Conditions

The Eurocode and ISO standard mentioned other exposure conditions that the British standard didn’t include, the Eurocode includes them as indirect aggressive actions that might have a form of chemical attack, that arises from the function of the structure such as liquid storage, or solutions of acids or sulphate salts (EN 206-1, ISO 9690), chlorides contained in the concrete (EN 206-1), or alkali-aggregate reactions (EN 206-1, National Standards). Or the form of physical attack, arising from temperature change, abrasion, and water penetration (EN 206-1).

For the ISO, the inclusion of solar radiation such as UV and IR radiation, chemical incompatibility and biological agents are adapted as deteriorating agents for concrete structures.

5 Discussion and Critical Review of Durability Requirements

Structure durability is not a new phenomenon anymore; it has been addressed in the last decade, and several studies and research have been completed to further our understanding of it. As a result, it must be considered during all phases of the project to ensure that its impact on the structures is minimal.

Factors influencing durability could be from the concrete system itself or the environment, as reflected in the studied design codes as exposure conditions and allocated values or design parameters that should not be exceeded or allowed levels of some elements such as chloride in the material constituents of concrete.

As the study shows, the BS8500-1 standard is the most comprehensive and detailed among the compared documents, the Eurocode and ISO standard is also satisfying in covering durability design.

The studied existing exposure conditions are established to develop a prescriptive durability design approach rather than performance-based methods, alternative hybrid requirements enable the design expert to choose the best durability design approach and specification format for the overall project goals. Prescriptive requirements are beneficial to provide a minimum expected durability performance without the need for extensive concrete mixture design development. Performance requirements are effective means of optimizing concrete mixtures for durability, especially as concrete technology evolves.

Although the British and Euro standards are detailed about exposure conditions, some aspects need to be revised. To begin with, sulphate attack exposure, several parameters need to be included in the Eurocode that directly affect the severity of this attack: the source of sulphates, the frequency of water exposure, the type of cation associated with the sulphates. Hence, the type of sulphates must be included in the exposure conditions as each of them attacks concrete differently. Lastly is the chloride presence as it limits sulphate ingress.

The ISO standard covers water exposure but not the EC2 or British standards. Water can serve as an agent or as a transporter for other substances that can harm the structure, such as chlorides and sulphates. The ISO standard is more consistent in addressing such exposure and emphasizes a very critical impact: acidic rain, which is destructive to concrete.

Alkali-Silica reaction is another detrimental effect of water which the studied codes need to include to exposure conditions, the British standard allocated the obligation of avoiding it to the provider, whereas the ISO standards mentioned it as a degrading agent. The Eurocode related it as a cause of the chemical attack. This could be explained by the factors that need to combine to start the reaction, where alkali is primarily from the materials making up the concrete and moisture is from the environment.

For corrosion exposure, class XC1 includes two scenarios which relate to the carbonation mechanism and corrosion induced by carbonation. XC1(wet) exposures can be neglected and XC1 (dry) is the same as X0 conditions, thereby, this category needs to be revised as carbonations requires humidity levels range 35%–75%.

For corrosion-induced carbonation, XC classes, a further exposure class should be introduced to encompass places with significant CO2 concentrations including tunnels, parking lots, and industrial locations. This will make it easier for designers to account for relative humidity and CO2 levels when creating durable designs.

Consideration for the notification provided from the amendment of the Norwegian Annex to EN206–1, on which EC2 and BS are based, for corrosion caused by chlorides from sources other than seawater, XD3 class. In this update, it is stated that exposure to XD3 indoor parking garages could have more severe effects than exposure to XD3 outdoors on a highway structure. This is due to the impact of salt slurry precipitation, followed by wetting/drying, on the slab surfaces of indoor parking garages, as well as the subsequent gradual increase in surface salt concentrations.

Steel corrosion for structures exposed to the maritime environment may be caused by the presence of chlorides that are either dissolved in water or airborne. Since numerous factors affect such chloride ingresses, such as the distance of the structure from the sea, ambient relative humidity, topographical conditions, wind features such as its speed, direction, and frequency, as well as the formation of salt-laden fog and mist, the exposure class that needs to be revised in this case is XS1. More information should be provided regarding the term “airborne”. In light of South Africa’s experience, they realized that, in contrast to other nations, chloride ingress began within 30 kms of the sea when there was a sufficient mix of onshore wind and relative humidity.

6 Proposal for the Improvement of Exposure Classes in the British and EC2 Code

Based on the comparison and critical review, the following exposure categories are indicated as a starting point for this area’s development in Tables 1, 2 and 3. For carbonation-induced corrosion, exposure class XC1 can be omitted as, exposure class XC4 is proposed to be added to consider areas with a high concentration of CO2 (see Table 1), XD4 class is added for chloride-induced corrosion apart from seawater to address indoor areas (see Table 2), and classes XS1a,b for chloride-induced corrosion from seawater to be assigned by several factors that vary from area to another (see Table 3). Additionally, the terms “Severe” and “very severe” are replaced with more relative terms “Major” and “Severe”, respectively.

Table 1. Proposed exposure classes for carbonation-induced corrosion.
Full size table
Table 2. Proposed exposure classes for chloride-induced corrosion apart from seawater.
Full size table
Table 3. Proposed exposure classes for chloride-induced corrosion from seawater.
Full size table

7 Conclusions and Recommendations

This study investigated the durability approach and design provisions for concrete durability design in international codes, it provides a beneficial review of the situation on durability design in international codes and standards. The outcomes can be used as a starting point for amendment.

For the durability design approach, the British and Eurocode codes have a prescriptive approach, this can be adequate for conventional structures, however, for complicated or modern structures, a performance approach is required. This study recommends the utilization of hybrid specifications as each of them has its benefits depending on the type of structures.

For Exposure conditions, the BS8500-1 and EC2 have similar approaches whereas ISO 13823 follows a different approach, the three studied documents are comprehensive, with the British standard being the most detailed among all, nonetheless, more clarification is required for some terms and more classes can be added to include the most probable environmental situations that might affect concrete durability. Therefore, this study recommends considering the tables proposed in Sect. 5 for the improvement of exposure conditions.

Further research is recommended for assigning durability parameter values for the proposed additional exposure classes, the provided values in the studied documents are acceptable and adequate to lower the penetration of harmful elements to the concrete.