Assoc. Dr. Hasan YILDIRIM
Istanbul Technical University Faculty of Civil Engineering Construction Materials
Group Faculty Member
1- Introduction
From the time of construction structural components and building materials are influenced by the mechanical, physical and chemical factors that are exerted from the outside of the building, losing their initial characteristics and incurring damage over time. Such factors should be taken into consideration in order to improve the performance of structures. The durability of structures cannot only be ensured by accurate support system selection, project design, and construction. In addition, measures that ensure the structure has a long lifespan and is deemed as “durable” in an acceptable period of time should be taken and the structure should complete its useful life while requiring the minimum amount of maintenance.
Conducting evaluations according to the current conditions and estimating the service lifespan for extending the life of structures has great importance for designers, appliers, and users. Therefore, in recent years, comprehensive studies have been conducted both globally and nationally with regards to the durability of building materials that are exposed to external environmental factors. Thus, the necessity for always taking durability into consideration in design is being emphasized. For instance, various environmental impacts were defined in the standard TS EN 206-1 “Concrete-Chapter 1: Characteristic, Performance, Production and Suitability”, which ready-mixed concrete companies are required to follow, and limit values concerning the parameters (max. water/cement ratio, min. cement dosage, min. strength class) to be used in concrete design against such factors were stated. Also, the fact that the 5th National Concrete Congress, recently held in Turkey, was themed around “Durability of Concrete” emphasizes the importance of the durability concept. The durability concept has become prominent in the recent studies of the American Concrete Institute.
As mentioned above, limitations in maximum water/cement ratio and minimum cement dosing significantly influence the strength and durability of concrete. In general, concrete design according to environmental factors, in other words, durability, is carried out according to these two parameters. The extent to which the limitations concerning such parameters in concrete can be realized (in other words, maximum water/cement ratio according to environmental impact class may not be higher than a specific value and cement dosing may not be less than the stipulated minimum value) directly depends on the components of concrete and its conformity with the standards.
Corrosive effect:
When the subject is regarded in terms of reinforced concrete or pre-stressed concrete structures, it is important to remember that steel is embedded in concrete. Exposed to multiple environmental factors, such elements are influenced by the marine environment directly or consequentially, since nearly 78% of the earth is covered by seas. Corrosion formed as the result of these factors can be eliminated by ensuring the physical and chemical protection of the reinforcement through a properly designed, impermeable, high-quality concrete. Physical protection is realized by preventing penetration of harmful matters to the reinforcement, and chemical protection is realized by creating a high pH environment. Despite this positive characteristic of concrete, due to mistakes in application corrosion is considered as the most important factor determining the service life of reinforced concrete structures today. Corrosion both weakens the reinforcement section and also causes severe damage to concrete.
Chlorine effect:
Chlorine ions are accepted as the most harmful matter in terms of reinforcement corrosion. Seawater or saline underground waters coming into contact with concrete, and ice thawing salts from industrial salt production/processing facilities are among important chlorine sources. In marine structures that are exposed to repetitive wetting-drying effect, chloride ions penetrate the concrete and remain inside due to the evaporation of water. Chlorine concentration increases with an increased number of recurrences. In this case, chlorine quantities higher than the ion concentration in seawater can accumulate in concrete. Also, very fine seawater particles (consequently, chlorines) sprayed by the sea can be carried over significant distances and adhere to the surface of concrete.
In developed countries, reinforced concrete structures are designed to have a minimum service life of 100-150 years. By taking certain measures, it is possible to achieve the targeted service lifespan according to the performance values expected from concrete. The impermeability characteristics of concrete in terms of water, gases, and ions are the most important factors for protection. The first condition for the production of impermeable and highly durable concrete is to minimize the water/cement ratio to the lowest possible level. The use of superplasticizers ensures that this phenomenon is no longer a problem. The water/cement ratio can easily be decreased in high performance concretes produced with these admixtures and the current technology allows production of pumpable concretes with a 28 day durability of 100 MPa.
Pozzolanic materials:
Mineral admixtures provide significant benefits in addition to superplasticizers for minimizing the void ratio. Such materials (silica fume, fly ash, blast furnace slag, and natural pozzolans), which have pozzolanic characteristics, improve impermeability by ensuring the formation of additional binder. On the other hand, these materials provide a filler effect and allow production of a void-free structure. The use of such materials in marine structures is particularly recommended due to these advantages offered.
C3A content in cement:
The chemical structure of cement, particularly its C3A content, is an important parameter in terms of resistance of concrete to sulfate. Therefore, American standards define cements with 8% or less C3A content as fairly resistant to sulfate and cements with 5% and less as highly resistant to sulfate.
Studies carried out indicate that ettringite, which is formed as a result of the reaction of the C3A component in cement with sulfates, does not cause expansion in an environment where chlorine ions are present and dissolve in seawater. Past studies also show that the C3A component, which is harmful in terms of sulfate effect, also offers the cement chlorine binding characteristic, thus reducing the risk of chlorine ions’ impact on the reinforcement. Since the chlorine permeability of cements with low C3A content is high, the solution should be produced by considering the corrosion problem of the reinforcement in this case. Otherwise, while an attempt is made to protect the structure or structural component against the effect of sulfate, it can be rendered nonfunctional by losing its function in unexpectedly short times due to the corrosion of the reinforcement. Therefore, the use of sulfate-resistant cements with very low C3A content in marine environment is not preferred. A very widespread and misleading view, which considers sulfate as the only parameter that ensures the durability of structures in marine environment, has also been formed in our country. However, as addressed above, the dominant factor affecting the service life of marine structures is not the sulfate effect, but the corrosion of reinforcements. Detailed explanations concerning the inaccuracy of this perception, which has long been adopted in our country, are mentioned in the following chapters.
Seawater allows sulfate to affect concrete due to its high sulfate ion concentration. Although the use of sulfate-resistant cement is recommended against the effects of sulfate, when there is a combined chemical attack, such as in the case of seawater, the use of sulfate-resistant cement with low C3A content cannot be a correct solution in terms of the corrosion of reinforcement bars. In diagram F.1 of TS EN 206-1 “Concrete-Chapter 1: Characteristic, Performance, Production and Suitability” standard, the requirement for the use of a special cement with regard to corrosion risk (XS1, XS2, and XS3 impact classes) caused by seawater-based chlorine is not addressed. Only the criteria for max. water/cement ratio, min. cement dosage, and minimum durability class are provided. Also in the same standard, no obligation for the use of Sulfate Resistant Cement of XA1 class for hazardous chemical environments (XA1, XA2, and XA3 impact class) is stipulated, and obligation for the use of Sulfate Resistant Cement in XA2 and XA3 classes is related with the condition that the dominant factor in hazardous chemical impact is sulfate (SO4). Additionally, it is specified that “Cements resistant to sulfate must be used on XA1 impact class in case of chemical impact with sulfate (excluding seawaters)” and it is emphasized that sulfate-resistant cement may be used only in cases where a high level of chemical attack (XA2 and XA3 impact classes) is present in regions other than seawaters. Thereby, our national TS EN 206 standard specifies no obligation for the use of sulfate-resistant cement in marine structures.
Considered as the largest concrete association in the world, and having prepared the most comprehensive committee reports relating to concrete, the American Concrete Institute (ACI) has indicated in Article 2.5.2 of its ACI 357R-84 report that the corrosion of reinforcements can be prevented if C3A is no lower than 4% and that it will demonstrate resistance against sulfate if it is no lower than 10%. Also, according to Article 4.3.1 of Report 201.2R, in cases where cement with C3A content in the range of 5-8% is used, the formation of cracks caused by corrosion is reported to be less than cement with a C3A content of less than 5%.
Again, another study addressed research where cements with C3A content of 0.6%, 8.8%, and 14.1% were used in an environment affected by seawater and revealed that the most durable concrete was the one prepared with a cement with a C3A content of 8.8%, rather than the sulfate-resistant cement. A further study emphasized that fly ash offers resistance to concrete against chlorine-based corrosion and that fly ash was able to consume free ions that cause corrosion, thanks to its capability of binding chlorine ions.
2- The Use Of Blended Cement And Fly Ash In Marine Structures
Natural and artificial pozzolanic materials, such as trass, fly ash, silica fume, and blast furnace slag, the use of which has been brought to the agenda in terms of technical, environmental, and economical advantages in line with the advancements in concrete technology, are included in concrete production either during cement production or by being directly added to the concrete mixture. In addition to calcium silicate hydrate-CSH-(main binder) structure, which is formed in the cement as a result of its reaction with water and resistance to chemical effects, calcium hydroxide (CaOH2) form, which is not resistant to such effects and has a weaker structure, is generated. With the use of pozzolanic material either in blended cement or in concrete, the structure of calcium hydroxide transforms into CSH structure and additional binder is formed, thus, resistance against chemical effects increases. Also, pore structure is improved with the use of such materials, concrete voids are minimized, and impermeability is improved (Figure 1).
Figure 1. Change in the pore structure when using concrete with/without fly ash
When unblended cement or a non-pozzolanic material is used in concrete, the calcium hydroxide structure cannot turn into a CSH structure and resistance against chemical effects is respectively low.
With their very advantageous use against multiple environmental factors, cements and concretes that contain pozzolanic material can also be used in marine structures too. Elements found in seawater, such as sulfate and chlorine, which are damaging to concrete and reinforcements, can be eliminated through production of impermeable concrete. Impermeable concrete can be obtained with the use of low water/cement ratio (high concrete class) or blended cement. In a study that was carried out, it was revealed that the capillary water permeability of concretes produced with sulfate-resistant cements was higher than that of concretes produced with other pozzolanic cements. This case is more evident in lower concrete classes particularly of C25 class.
Although cements with low C3A content are recommended against the effects of sulfate, such judgment is not all that valid since the presence of seawater and chlorine ions has been detected. The C3A component in cement has the potential of binding free chlorine ions, causing reinforcement corrosion. The binding of chlorine ions (formation of a compound with stable structure, Friedel’s salt) means that chlorine ions leading to reinforcement damage are consumed. Therefore, the use of a cement with low C3A content will result in some of the chlorine ions remaining free and will lead to problems with the reinforcement. Despite this, the decrease of the C3A component, due to it being bound with chlorine ions, or its total consumption, will also ensure high resistance against the effects of sulfate, too. In the study shown in Figure 2, the chlorine permeability of concrete produced with low C3A cement is shown for concretes produced with different cements.
As shown in Figure 2, in concrete produced using Portland cement, the chlorine permeability decreases after concrete class C35. The low chlorine permeability in class C40 concrete with Portland cement is still three times higher than that of concrete produced using cement with slag.
Also, as shown in Figure 2, the chlorine permeability of concretes of higher classes produced partially with fly ash and regular Portland cement is decreased by half, compared to concrete produced only with Portland cement. This case has not occurred in concrete class C25. In concrete class C40, concretes with fly ash and slag have demonstrated similar permeability.
Figure 2. Effects of cement types on the quick chlorine permeability in concrete
The quick chlorine permeability in concretes containing sulfate-resistant cement is six times higher than the chlorine permeability of concretes produced with class C25, C30, and C35 cement with slag, and such permeability is three times higher than class C40 concrete. Sulfate-resistant cements are planned for structures to be constructed near the sea and included in the specifications of many construction projects in Turkey. However, the effect of chlorine is generally ignored. The results of these tests clearly show that sulfate-resistant cements demonstrate a performance close to Portland cement in environments that contain chlorine.
Figure 3 shows the result of a study concerning the development of corrosion in relation with the C3A content of concretes with/without fly ash. As it can be seen, corrosion is initiated within a much shorter time in concretes that do not employ fly ash. It can be said that this judgment can be applicable for blended cements since they demonstrate pozzolanic properties.
Figure 3. Change of corrosion initiation period in concretes with/without fly ash
3- Conclusion:
When evaluated in terms of either the chlorine effect, sulfate effect, or many other environmental effects, the main goal against such harmful effects is to produce impermeable concrete in a more general sense, rather than optimize the values in material compositions at a small scale. This is because, durability, in other words, the ability to withstand environmental factors throughout service life, is a function of permeability. The penetration of other harmful ions through water in an impermeable manner also presents difficulties. Such kinds of concretes can be made with the use of blended cement, fly ash, and slag or other pozzolanic materials as indicated before.
Sulfate-resistant cements produced in compliance with the TS 10157 standard provide high resistance to structural components under conditions where sulfate attack is present. Sulfate attack in concrete can be soil or underground water-based. As explained above, the use of sulfate-resistant cement in environments such as seawater, where combined effects are present, is unsuitable. However, sulfate-resistant cement demonstrates superior performance against simple sulfate effects produced by soil and underground water. As with all durability issues, the resistance of concrete against sulfate depends highly on its impermeability. All parameters affecting the permeability of concrete, such as material properties, mixture ratios, cracking conditions, compression, the curing of fresh concrete, etc. also influence sulfate resistance indirectly.
The exclusive use of sulfate-resistant cement to prevent chlorine permeability in concretes is insufficient. Concretes with low water/cement ratio should also be designed. Depending on the case, the use of cements or concretes with pozzolan will prove beneficial. The sulfate resistance of cements with cements with pozzolan will also be helpful. It has been seen that the pozzolan and admixtures mentioned above are beneficial in terms of alkali aggregate reaction.
In conclusion, it has been established and is interpreted that the use of pozzolan (fly ash, slag, etc.) in concrete would be beneficial and should be required in terms of the impact of harmful environments on concrete.
References:
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15. E.G. Aydın, H.Yıldırım, “ The effect of mineral admixtures and micronized calcite on alkali silica reaction expansions in the usage of aggregates from different origin”, International Journal of Physical Sciences , 5996-6011 pp., 2012.
16. Swamy, R.N., 1992, Alkali-Aggregate Reactions in Concrete: Material and Structural Implications, Sciences in Concrete Technology, Energy,Mines and Resources, Ottawa, Canada,533-581
17. Swamy, R.N., 1988, Expansion of Concrete due to Alkali-Silica Reaction, ACI Materials Journal,V.85,No.1,33-40.
18. Mullick,A.K.,1988,Distress in a Concrete Gravity Dam due to Alkali Silica Reaciton, Int.J.of Cement Composites and Lightweight Concrete,V.10,No.4,225-232.
19. West, G., 1996, Alkali-aggregate reaction in concrete roads and bridges