003. Durability of Concrete_Updated.pptx

Durability is one of the most essential properties of concrete. It refers to the ability of concrete to withstand the environmental conditions to which it is exposed, without losing its strength, serviceability, or appearance over time. While compressive strength determines how much load a structure can carry, durability ensures how long that strength lasts in real-world exposure.

In civil engineering practice, a concrete structure may be subjected to various aggressive agents such as moisture, temperature variations, chemical attacks, corrosion of reinforcement, freezing and thawing cycles, and abrasion. If a concrete mix is designed only for strength without considering these factors, it may deteriorate prematurely. Thus, the durability of concrete is critical for the long-term sustainability, safety, and economy of any construction.

2. Definition of Durability

According to the Indian Standard IS 456:2000, the durability of concrete is defined as “the ability of concrete to resist weathering action, chemical attack, abrasion, or any other process of deterioration while maintaining its desired engineering properties.”

In simple terms, durable concrete is that which performs satisfactorily during its intended life span under the environmental conditions to which it is exposed.

3. Factors Affecting Durability of Concrete

Durability of concrete depends on several factors, broadly classified into internal and external factors.

3.1 Internal Factors

Water-Cement Ratio (w/c):
A low water-cement ratio is crucial for dense and less permeable concrete. High w/c ratio increases porosity, allowing harmful agents like chlorides and sulphates to penetrate easily.

Cement Quality and Type:
The chemical composition of cement affects resistance to sulphate attack, alkali-aggregate reaction, and carbonation. For example, sulphate-resisting Portland cement is used in sulphate-rich environments.

Aggregate Quality:
Aggregates should be sound, clean, and free from deleterious substances. Porous or reactive aggregates can significantly reduce concrete durability.

Mix Design:
Proper proportioning of cement, water, and aggregates ensures minimal voids, higher density, and better performance under environmental stresses.

Compaction and Curing:
Improper compaction leaves voids that increase permeability. Inadequate curing leads to incomplete hydration, surface cracking, and loss of strength.

Admixtures:
Use of mineral admixtures such as fly ash, silica fume, and slag can improve durability by reducing permeability and refining pore structure.

3.2 External Factors

Environmental Conditions:
Exposure to alternate wetting and drying, freezing and thawing, and aggressive chemical environments accelerate deterioration.

Chemical Attack:
Sulphates, chlorides, and acids in soil or groundwater can cause expansion, cracking, and loss of mass in concrete.

Temperature Variations:
Rapid temperature changes induce thermal stresses and micro-cracks.

Mechanical Abrasion a

1. Modern Construction Materials | CE 6601 Course Instructor Dr. Chikkam Ramakrishna Balaji Mail Id: [email protected]

2. Mid Semester Examination 5b Supplementary Question How to proceed with trial mixes and give your comments on cast concrete cubes their number of sets, curing pattern, strength and limits?

3. Unit 8 Durability of Concrete Factors influencing durability Chemical effects on concrete Carbonation Sulphate attack Chloride attack

4. Durability of Concrete It is defined as the ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration. Features of a durable concrete are: Will retain its original form, quality and serviceability when exposed to its environment Should perform satisfactorily in the working environment during its anticipated exposure conditions during service Should not disintegrate or show sign of wearing under adverse conditions

5. Causes of Lack of Durability The main factors which stand out are the external factors or the environment and secondly internal factors or causes within the concrete. Normally, concrete fails due to a combination of these factors. The environmental factors could be quantified as physical, chemical and mechanical which could be collectively summarised as : Weathering (Physical cause) Occurrence of extreme temperatures; fire and frost effect (Physical cause) Attack by natural or industrial liquids like sulphate, sea water, acids, oils and sewage (Chemical cause) Exposure conditions (Chemical cause) Abrasion and cavitation (Mechanical cause) The internal factors are the, Alkali- aggregate reaction, Difference in thermal properties of aggregate and cement paste, and Permeability of concrete.

6. Physical Cause - Freeze Thaw Cycle

7. Chemical action Sulphate attack Carbonation Chloride attack Effect of sea water

8. Sulphate Attack Common occurrence in natural or industrial situations Sources of sulphates: Ground water & Soils containing Calcium, Sodium, Potassium & Magnesium Ammonium sulphate mostly available in agricultural lands Decay of organic matters in marshy land, shallow lakes often leads to the formation of H 2 S, transforms into sulphuric acid by bacterial action Water used in cooling towers Solid sulphates do not attack concrete severely but when the chemicals are in solution, they find entry into porous concrete and react with hydrated cement products Magnesium sulphate causes maximum damage to concrete. A characteristic whitish appearance is the indication of sulphate attack

9. Sulphate attack – Mechanism The sulphates of various bases react with cement in different ways. Sodium, potassium and ammonium sulphates first react with Ca(OH) 2 to form gypsum which in turn reacts with hydrated calcium aluminates to form calcium - sulpho - aluminate. This chemical reaction is as below : Ca(OH) 2 + Na 2 SO 4 10 H 2 O CaSO 4 . 2 H 2 O + 2 NaOH + 8 H 2 O 3 CaO Al 2 O 3 . 19H 2 O + 3(CaSO 4 . 2H 2 O ) + 16 H 2 O 3 CaO . 2Al 2 O 3 3CaSO 4 . 31H 2 O The extent to which the first stage reaction proceeds depends on the ambient condition and concentration of sulphates. Gypsum (CaSO 4 ) also forms Calcium Sulpho aluminate directly as per the second stage of reaction. Magnesium sulphate has much more active action as it reacts not only with calcium hydroxide and hydrated calcium aluminates as shown above, but also decomposes the hydrated calcium silicates completely. In reaction form this can be written as : 3 CaO 2SiO 2 aq + 3 MgSO 4 . 7 H 2 O CaSO 4 .2H 2 O + 3 Mg(OH) 2 + 2 SiO 2 .aq

10. Sulphate attack A saturated solution of magnesium sulphate can cause serious damage to concrete with higher water cement ratio , in a relatively short time. While low water cement ratio concrete can withstand this attack for 2 to 3 years. Further, the rate of attack also depends upon the rate at which the sulphate consumed in the reaction is replenished. This situation could be visualized by you, when you could imagine a structure retaining sulphate bearing water on one side. In a sea structure this could happen due to alternate wetting and drying as a result of tidal variation or spraying

11. Methods of controlling sulphate attack Use of sulphate resisting cement Quality concrete – Dense, impermeable, Must have low w/c ratio Use of air- entrainment – About 6%, increases workability, no segregation/bleeding Use of pozzolana – Improves impermeability, removes calcium hydroxide and reduces susceptibility of concrete to attack by Magnesium sulphate High pressure steam curing – Reduces C 3 AH 6 and removes calcium hydroxide

12. Carbonation Process by which carbon dioxide from the air penetrates into concrete and reacts with calcium hydroxide to form calcium carbonates CO 2 by itself is not reactive. But in the presence of moisture, it changes to dilute carbonic acid which attacks the concrete and reduces alkalinity of concrete CO 2 + Ca(OH) 2 > CaCO 3 + H 2 O pH of pore water in hardened concrete is 12.5- 13.5. Steel is placed in this highly alkaline condition & thus doesn’t corrode. This condition is called as Passivation.

13. Carbonation Carbon dioxide diffuses into concrete through pore system & forms mild acids. Reduction of pH Alkalinity reduced to less than pH 9 Passive layer on steel lost when pH drops below 9. Protective layer around the steel is destroyed and it gets exposed to corrosion

14. Carbonation

15. Carbonation time (years) for various depths of cover and w/c ratios

16. Measurement of depth of Carbonation Freshly broken surface of concrete is treated with Phenophthalein in diluted alcohol. If Ca(OH) 2 is unaffected by CO 2 , then the colour of concrete turns to pink If concrete is carbonated, it will remain uncoloured Colour pink shows the presence of Ca(OH) 2 that may have less carbonated The colour pink will show even up to a pH value of 9.5

17. Chloride Attack One of the most important aspects for consideration related to durability of concrete It is the primary cause of corrosion of reinforcement (Over 40% of failure of structures) The protective passive layer around steel is lost by chlorides in the presence of water and oxygen Sulphates attack the concrete Chlorides attack the steel reinforcements

18. Chloride Attack Chlorides break down passive film when they reach steel Sets up alternate anodes and cathodes forming Corrosion Cells. Concrete acts as an electrolyte Causes pitting corrosion

19. Formation of Cathodes and Anodes Corrosion of steel is an electrochemical process involving two key areas: Anode:  Where iron (Fe) loses electrons and oxidizes: This area is often where chloride concentration is high, causing breakdown of the passive layer and localized pitting. Cathode:  Where electrons are consumed, typically by oxygen reacting with water: The steel surface doesn't corrode uniformly; instead, anodic and cathodic areas can form next to each other and switch based on local chemistry, oxygen availability, moisture, and chloride concentration. The steel acts as the conductor, concrete is the electrolyte, anodic dissolution releases iron ions, and at the cathode, oxygen is reduced creating hydroxide ions.  

20. Chloride Attack

21. Chloride Attack

22. Chloride Attack

23. Corrosion Control Metallurgical Methods Corrosion inhibitors Coatings to reinforcement Cathodic protection Coatings to concrete Design and detailing Coatings to reinforcement – Conventional approach

24. Leaching

25. Exposure Conditions

26. Alkali- Aggregate Reaction Aggregates are not completely inert; Have reactive silica Factors responsible: Reactive type of aggregate High alkali content in cement Availability of moisture Optimum temperature conditions

27. Alkali Silica Reaction

28. Permeability Cement paste consists of C-S- H gel, Ca(OH) 2 and water filled/empty capillary cavities Gel pores (28%) are so small that hardly any water can pass through under normal conditions. Hence gel pores do not contribute to permeability of cement paste. Permeability of gel pores = 7 × 10 - 16 m/s (1/100 th of cement paste) Extent & Size of capillary cavities depend on the W/C ratio Capillary cavities resulting from low W/C ratio will get filled by hydration products of cement Unduly large cavities resulting from higher W/C ratio will remain as unsegmented cavities – Responsible for Permeability of cement paste

29. Permeability

30. Permeability – Causes & Factors responsible Causes: Formation of micro- cracks due to long- term drying shrinkage. Rupture of bond between Interface of paste and aggregate due to differential thermal stresses. Cracks generated through higher structural stresses. Existence of entrapped air due to insufficient compaction. Volume change in concrete on account of various reasons.

31. Ways to improve Impermeability By use of pozzolanic materials, the leaking of calcium hydroxide is prevented and hence porosity is controlled. By air-entrainment up to 6%. High pressure steam cured concrete exhibits better impermeability due to coarser gel and hence lower drying shrinkage.

32. Types and Causes of Concrete Cracks

33. Types and Causes of Concrete Cracks

34. Factors contributing to Cracks in Concrete Plastic Shrinkage cracks Settlement cracks Bleeding Delayed Curing Constructional effects Early frost damage Unsound Materials Shrinkage Drying shrinkage Thermal shrinkage Coefficient of thermal expansion

35. Deterioration of concrete - Abrasion

36. Deterioration of concrete - Erosion

37. Erosion

38. Deterioration of concrete - Cavitation

39. Efflorescence

40. Effect of some Materials on Durability Action of Mineral oils Action of Organic Acids Vegetables and Animal oils and fats Action of sugar on concrete Action of sewage

41. Different Concretes Lightweight Concrete Aerated/Foam Concrete High Performance/Strength Concrete Self- Compacting Concrete Fiber Reinforced Concrete No- Fines/ pervious Concrete Ferrocement Shotcrete Self Healing Concrete Geopolymer Concrete Polymer Concrete Roller Compacted Concrete Silica fume Concrete Ternary blend Concrete Smart Concrete 3D Printed Concrete Smart Concrete Glass Concrete

42. Guidelines Presentation Duration and Q&A Total Time: 12 minutes per presentation Q&A: 3 minutes for audience/discussion Number of Slides Recommended: 8–10 slides is ideal for a focused 12-minute talk Supplementary Materials Images, Tables/Charts, Short video (optional), Sample calculations or mix design: (optional, if applicable)

43. Required Slide Contents 1. Title Slide:  Topic name, student name, roll no., date 2. Introduction:  Definition and historical background 3. Composition and Materials:  Key constituents, admixtures 4. Manufacturing/Preparation Method:  Main features and process 5. Properties:  Fresh and hardened states, mechanical and durability aspects 6. Advantages & Limitations:  Practical engineering pros and cons 7. Applications:  Real-world usage, types of structures suited 8. Case Study / Example Projects:  At least one practical example from literature or recent projects 9. Future Developments / Innovations:  New trends, ongoing research 10. Conclusion:  Summary and takeaways

44. Reference Books "Concrete Technology" by Neville & Brooks "Concrete: Microstructure, Properties, and Materials" by P.K. Mehta, Paulo J.M. Monteiro "Advanced Concrete Technology" by Zongjin Li Journals Cement and Concrete Research Journal of Materials in Civil Engineering National/international codes, model codes for referenced material types 

45. Criteria Weight (%) Description Technical Content & Depth 30 Accuracy, clarity, relevance, depth of analysis, completeness of technical information Organization & Structure 15 Logical flow, balanced coverage of topics, clear transitions between sections Visual Aids 15 Quality, relevance, clarity of slides, charts, tables, images (should aid understanding) Delivery & Communication 20 Confidence, clarity, engagement, pace, eye contact, speech quality Handling Questions (Q&A) 10 Ability to answer questions, demonstrate subject knowledge, composure during Q&A References & Citations 5 Accurate referencing, use of credible books/journals/web sources, proper citation style Overall Impact & Timeliness 5 Stayed within time limit, presentation effectiveness, originality, and overall impression