The durability of concrete can be considered as part of the structural design and is related to the number of years the structure is designed to last.
The durability of concrete is discussed in this article from two aspects.
- Shelf life requirements
- Durability issues
Shelf life requirements
The expectation when considering durability requirements in reinforced concrete construction is that the structure will continue to exist for a long period of time without significantly losing the condition it was in at the time of construction. T Durability of structures is discussed in terms of the expected service life of a structure.
Durability measurements are carried out specifying material requirements and based on the loading conditions to which the structure will be subjected.
According to construction regulations, durability is determined in relation to loading conditions. Furthermore, the selection of structural class for the selection of reinforcing cover is done based on some other factors besides durability in the design according to Eurocode 2. All building codes follow similar procedures in determining durability.
When selecting durability, the following aspects, among others, are taken into account.
Coverage for reinforcements
The roof design and selection process play a crucial role in evaluating the durability of reinforced concrete structures. Reinforcement corrosion is mainly due to inadequate coverage. Inadequate construction practices and failure to determine the required reinforcement coverage can result in this inadequacy.
Depending on the type of structure and the location where construction will be carried out, the designer must select the appropriate coverage for the reinforcement in accordance with construction standards. When selecting coverage for reinforcement, Eurocode takes into account the following three main factors.
- Requirements for bond reinforcements
- Requirements for the durability of reinforcements
- Tolerances for design deviations.
According to the Eurocode 2 guidelines regarding the above factors, the coverage section can be prepared. For more information on the reinforcement cover section, see the article “ Nominal reinforcement cover according to Eurocode ”
Limiting the formation of cracks in concrete
Due to regional differences, cracks form in every building in the world. It is almost impossible to prevent cracks in structural elements, although they can be controlled to a point where they do not cause damage, even under severe environmental conditions. Typically, the width and depth of the crack cause durability issues.
There are two methods that can be used to build fissures
- Method of limiting stress
- Limitation of the crack width method
Method of limiting stress
This method limits the stress on the armature. By keeping the reinforcement tension at a lower level, the tension in the concrete is also reduced. There is more room for stress to build up due to thermal and other effects that cause cracks in the concrete. Additionally, lower stresses in concrete do not increase the size of surface cracks that can occur as the concrete hardens.
Since less tension is maintained in the reinforcement, a larger area of reinforcement must be provided to accommodate the same bending moment. Although this increases construction costs, it is the best method for minimizing cracks.
Crack Width Limitation Method
The crack width is limited to the specified level based on the type of structure and according to the design code used. The crack width limitation method is not as expensive as the limit stress method. However, it presents a higher risk compared to the limit voltage method.
According to BS 8007 – Design of concrete structures for the retention of aqueous liquids – the crack width should be limited to 0.2mm or 0.1mm depending on the following conditions.
In addition to the above, BS 5400 also specified crack width based on exposure conditions. The following table is taken from BS5400 and can also be used when selecting a suitable covering for reinforcement.
Strength class and water-cement ratio
The strength class of concrete plays an important role in its durability. The structures are classified according to Eurocode 2, Table 4.3N and according to this classification, different resistance classes are recommended depending on the exposure conditions. The water-cement ratio is directly related to the strength of concrete. Therefore, selecting the correct resistance class is essential when evaluating the durability of concrete structures.
BS 8007 used to construct structures for retaining aqueous fluids, suggests the acceptable water-cement ratio and the cement content to be used. The following information was extracted from the code.
- The maximum cement content must be 325 kg/m 3 . A maximum water-cement ratio of 0.55 for ordinary Portland cement and when Portland cement made from pulverized fuel ash or a combination of ordinary Portland cement and pulverized fuel ash is used, the water-cement ratio should be 0.50.
- The characteristic breaking strength of concrete must not be less than 35 N/mm. 2 and concrete is rated 35A.
- For reinforced concrete, the cement content should not exceed 400 kg/m. 3 of normal Portland cement or cement with ggbs or 450 kg/m 3 in which cement with PFA is used. For prestressed concrete, the maximum cement content can be increased to 500 kg/m3. 3 or 550 kg/m 3 respectively.
For more information about cement and its additives, see the article “ Cement and cement additives ” you could read.
Durability problems in concrete structures
Durability-related issues are generally irreversible when they arise. Some of the problems can't even be solved, you can just watch them get worse. Therefore, it is important to recognize this problem at the right time and take precautionary measures. Durability problems can be attributed to the following main causes.
Reinforcement corrosion
The corrosion rate of reinforcement depends on the loading conditions. If cracks are present or if cracks are provided with most of the reinforcement, deterioration of the reinforcement is inevitable. However, properly constructed structures will not face this problem if they are built according to the designers' specifications and following correct construction practices.
As an example of the corrosion rate of a structure built near the sea or in contact with seawater, the Steel Piling Group suggests the following values, assuming a constant corrosion rate.
Depending on the loading condition, the expected corrosion depth can be calculated from the above values, depending on the planned service life of the structure. Furthermore, additional thickness, especially for structural steel, can be considered as corrosion tolerance in the design phase. For example, if the project requires a thickness of 20 mm, you can choose a thickness of 22 mm. Selection depends solely on the designer's competence and experience.
Corrosion of the reinforcement or structural steel of a structure affects it in different ways. Furthermore, it directly or indirectly affects the other problems listed above such as durability problems. Therefore, it is extremely important to consider these issues at the design stage and take corrective measures to avoid them. Article “ Chemical attack on concrete ” and Nominal reinforcement coverage according to Eurocode “Discuss in more detail the causes of corrosion and methods to prevent corrosion of reinforcements.”
Concrete deterioration
Concrete deteriorates over time as it is exposed to the environment and subjected to stress. Concrete deterioration can be discussed under the following categories, which are not included in the above list of durability issues. In the design phase, the necessary measures must be taken to avoid or minimize the occurrence of these problems.
For more information on alkali-aggregate reaction and cracking in concrete, see the articles “ Chemical attack on concrete ” and “ Cracks in concrete ” could be referred.
Structural errors
Structural components may fail due to inadequate design strengths, resulting in loss of element capabilities. Structures are designed for a service life based on the selected structural class. Based on Eurocode there are six structural classes from S1 to S6. The article “ Structural classes according to Eurocode ” Discuss the six different classes that can be considered in design.
If the structure is unable to fulfill its useful life due to failures caused by durability issues, the design or changes in exposure conditions must be carefully considered. Necessary actions must be taken to resolve the problems. Exposure conditions and environmental conditions may change over time.
Structural systems need to be regularly evaluated based on their importance, especially when exposed to challenging environmental conditions over a long period of time. For example, a structure built in ports is exposed to seawater for a long period of time. This can lead to deterioration of the concrete, as shown in the figure below.
The photo above is clear proof that the structure requires attention from civil engineers to continue being used. It can be concluded that it is the responsibility of civil engineers at client organizations to take care of this matter in a timely manner after delivery of the structures. Lack of attention to durability aspects can lead to structural defects.
Concrete cracking and spalling
There are several articles on this subject on the Internet Concrete cracks for various reasons. For more information, you can refer to the article and other related articles.
When concrete fails, the concrete separates and falls or remains as concrete separate from the primary structure. Concrete spalling may be due to corrosion of the reinforcement and the resulting increase in volume around the reinforcement bars. Due to the additional tensile stress created by the increase in volume, the concrete begins to separate.
If this problem is not given enough attention, it can even cause structural damage. Timely intervention and use of correct repair methods can prevent corrosion of reinforcement and spalling of concrete.
The framework cannot be used
Durability issues cause structures to become unusable even without their flaws. The collapse of one or more parts of structures makes access difficult, especially in building structures. For example, the building cannot be occupied due to fragmentation of the structural component. Additionally, some areas of the building cannot be used.
As shown in the figure, there are many cases where structures cannot be used due to durability issues. Furthermore, the population's fear of the appearance of cracks means people do not enter buildings. The same scenario applies to other types of structures.
Regular maintenance and its costs
Durability issues cause many problems for the owner or clients who own the building. You need to take care of the building in a timely manner and carry out regular inspections to see if it requires the attention of a structural engineer. Depending on the condition of the building, maintenance work may be carried out upon recommendation from the structural engineer. Furthermore, durability problems in steel structures are very common and require periodic maintenance.
There are costs associated with any type of repair or maintenance work. These costs represent additional costs for the customer that they must bear during the repair.
