Properties of Fresh and Hardened Concrete

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Properties of Fresh (Workability)
Hardened Concrete (Workability,Permiablitiy,Durablility)
Thermal properties
Micro-cracking of concrete
Mix Design
Rheology
Causes of Damage of Concrete

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  • Properties of Fresh and Hardened Concrete

    1. 1. Unit -2,RGPV PROPERTIES OF FRESH AND HARDENED CONCRETE Rishabh Lala VI SEM Rajiv Gandhi Technological University,Bhopal
    2. 2. Syllabus Covered (Left Click to enter ) • Properties of Fresh (Workability) • Hardened Concrete (Workability,Permiablitiy,Durablility) • Thermal properties • Micro-cracking of concrete • Mix Design • Rheology • Causes of Damage of Concrete
    3. 3. Introduction: The potential strength and durability of concrete of a given mix proportion is very dependent on the degree of its compaction. It is vital, therefore, that the consistency of the mix be such that the concrete can be transported, placed, and finished sufficiently early enough to attain the expected strength and durability. Properties of Fresh Concrete: Significance: The first 48 hours are very important for the performance of the concrete structure. It controls the long-term behavior, influence f'c (ultimate strength), Ec (elastic modulus), creep, and durability. Properties of Fresh Concrete : Properties at Early Ages : • Workability • Slump Loss • Segregation/Bleeding • Plastic Shrinkage • Time of Set • Temperature
    4. 4. Workability : Definition: Effort required to manipulate a concrete mixture with a minimum of segregation. It is not a fundamental property of concrete. I) consistency (slump)-- easy to flow II) cohesiveness --tendency to bleed and segregate Slump Test: Slump test is a test conducting before concrete to be used for casting. The purpose of slump test Is to determine the water content in concrete and its workability Consistency: Consistency or fluidity of concrete is an important component of workability and refers in a way to the wetness of the concrete. However, it must not be assumed that the wetter the mix the more workable it is. If a mix is too wet, segregation may occur with resulting honeycomb, excessive bleeding, and sand streaking on the formed surfaces. On the other hand, if a mix is too dry it may be difficult to place and compact, and segregation may occur because of lack of cohesiveness and plasticity of the paste.
    5. 5. PROPERTIES OF HARDENED CONCRETE  The principal properties of hardened concrete which are of practical importance can be listed as: 1. 2. 3. 4. Strength Permeability & durability Shrinkage & creep deformations Response to temperature variations Of these compressive strength is the most important property of concrete. Because;
    6. 6. PROPERTIES OF HARDENED CONCRETE Of the abovementioned hardened properties compressive strength is one of the most important property that is often required, simply because; 1. Concrete is used for compressive loads 2. Compressive strength is easily obtained 3. It is a good measure of all the other properties.
    7. 7. STRENGTH OF CONCRETE  The strength of a concrete specimen prepared, cured and tested under specified conditions at a given age depends on: 1. w/c ratio 2. Degree of compaction
    8. 8. COMPRESSIVE STRENGTH  Compressive Strength is determined by loading properly prepared and cured cubic, cylindrical or prismatic specimens under compression.
    9. 9. COMPRESSIVE STRENGTH • Cubic: 15x15x15 cm Cubic specimens are crushed after rotating them 90 to decrease the amount of friction caused by the rough finishing. • Cylinder: h/D=2 with h=15 To decrease the amount of friction, capping of the rough casting surface is performed.
    10. 10. PERMEABILITY OF CONCRETE  Permeability is important because: 1. 2. 3. The penetration of some aggresive solution may result in leaching out of Ca(OH)2 which adversely affects the durability of concrete. The moisture penetration depends on permeability & if concrete becomes saturated it is more liable to frost-action. In some structural members permeability itself is of importance, such as, dams, water retaining tanks.
    11. 11. DURABILITY A durable concrete is the one which will withstand in a satisfactory degree, the effects of service conditions to which it will be subjected. Factors Affecting Durability:  External → Environmental  Internal → Permeability, Characteristics of ingredients, Air-Void System...
    12. 12. Structure of “un-damaged” Concrete Macrostructure Microstructure
    13. 13. Structure of “damaged” Concrete Macrostructure Visible cracks in hcp and aggregates due to volume changes (to understand cause of cracks, microstructure should be examined) Microstructure    Alkali-silica reaction: Reaction product forms at TZ and expands Frost action: Water freezes in capillary pores and expands Sulfate attack: reaction products form in hcp and expand
    14. 14. LEACHING & EFFLORESCENCE  When water penetrates into concrete, it dissolves the non-hydraulic CH (and various salts, sulfates and carbonates of Na, K, Ca)  C-S-H and CH is produced upon hydration of C3S and C2S  These salts are taken outside of concrete by water and leave a salt deposit.
    15. 15. SULFATE ATTACK  Ground water in clayey soils containing alkali sulfates may affect concrete.  These solutions attack CH to produce gypsum. Later, gypsum and calcium alumina sulfates together with water react to form “ettringite”.  Formation of ettringite is hardened cement paste or concrete leads to volume expansion thus cracking.  Moreover, Magnesium sulfate may lead to the decomposition of the C-S-H gel.
    16. 16. SULFATE ATTACK  Seawater contains some amount of Na and Mg Sulfates. However, these sulfates do not cause severe deleterious expansion/cracking because both gypsum and ettringite are soluble in solutions containing the Cl ion. However, problem with seawater is the frequent wetting/drying and corrosion of reinforcing steel in concrete.  To reduce the sulfate attack 1. Use low w/c ratio→ reduced permeability & porosity 2. Use proper cement → reduced C3A and C3S 3. Use pozzolans → they use up some of the CH to produce C-S-H
    17. 17. CORROSION  Electrochemical reactions in the steel rebars of a R/C structure results in corrosion products which have larger volumes than original steel.  Thus this volume expansion causes cracks in R/C. In fact, steel is protected by a thin film provided by concrete against corrosion. However, that shield is broken by CO2 of air or the Cl- ions.
    18. 18. FREEZING AND THAWING  Water when freezes expands in volume. This will cause internal hydraulic pressure and cracks the concrete.  To prevent the concrete from this distress air-entraining admixtures are used to produce air-entrained concrete.
    19. 19. MIX DESIGN  Mix design is the process of selecting suitable ingredients of concrete & determining their relative quantities with the objective of producing as economically as possible concrete of certain minimum properties such as workability, strength & durability.  So, basic considerations in a mix design is cost & min. properties.
    20. 20. RHEOLOGY OF CONCRETE • Concrete's and mortar's workability is related to the rheological properties of the fresh cement paste. The mechanical properties of hardened concrete increase if less water is used in the concrete mix design, however reducing the water-to-cement ratio may decrease the ease of mixing and application. To avoid these undesired effects, superplasticizers are typically added to decrease the apparent yield stress and the viscosity of the fresh paste. Their addition highly improves concrete and mortar properties • Rheology measurements on concrete indicate that it is reasonable to approximate the concrete flow behaviour using a Bingham model.

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