(Aktu Btech) Concrete Technology Important Unit-5 Specific Concretes

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Q1. Discuss the guidelines to selection of materials for HSC.

Ans.

• 1. The maximum size of concrete chosen should be modest in order to achieve the greater goal compressive strength of concrete, which will result in more dense, compact concrete with a lower void ratio.
• 2. A good coarse aggregate with a maximum size ranging from 20 to 28 mm can provide compressive strengths of up to 70 MPa.
• 3. An aggregate with a maximum size of 10 to 20 mm should be utilized to create 100 MPa of compressive strength.
• 4. Concrete with compressive strengths of above 125 MPa has been made using coarse aggregate with a maximum size of 10 to 14 mm.
• 5. Employing additional cementitious ingredients, like fly ash from blast furnaces and natural pozzolanas, not only lowers the cost of producing concrete but also solves the slump loss issue.
• 6. The decrease in 12 or 24 hour strength that is deemed acceptable, given climatic conditions or the minimal strength required, frequently determines the optimal substitute level.
• 7. The majority of concrete mixtures contain silica fume even though it is typically not necessary for compressive strengths under 70 MPa.

Q2. What are the various methods to achieve high strength in concrete ? Also discuss their applications.

Ans. A. Methods to Achieve High Strength in Concrete: Following are the special methods to achieve high strength in concrete:

• 1. Seeding: This entails a tiny amount of freshly mixed concrete being mixed with Portland cement that has been finely powdered and fully hydrated. This approach might not be very effective.
• 2. Revibration: Controlled revibration improves concrete strength by removing all flaws such bleeding, water accumulation, plastic shrinkage, and continuous capillary channels. 
• 3. High Speed Slurry Mixing: The cement water mixture must first be prepared in advance before being combined with aggregate to create concrete.
• 4. Use of Admixtures: Use of water reducing agents are known to produce increased compressive strength. 
• 5. Sulphur Impregnation: By adding sulphur to porous low strength concrete, satisfactory high strength concrete has been developed. The concrete with sulphur in it has a strength of up to 58 MPa.
• 6. Inhibition of Cracks: The strength will be greater if the spread of cracks is prevented. The strength of these types of concrete cubes has reached 105 MPa.

B. Applications of High Strength Concrete: 

• 1. Engineering projects requiring concrete components that must withstand high compressive loads must use high strength concrete.
• 2. Construction of high-rise structures frequently involves the use of high strength concrete.
• 3. It has been utilised in foundations, shear walls, columns (particularly on lower levels where the weights are higher).
• 4. Bridge applications also occasionally make use of high strengths.
• 5. On occasion, high strength concrete is used to build highway bridges.
• 6. Column size is reduced when HSC is used in column section.
• 7. The amount of steel needed for the same column is reduced when HSC is used.
• 8. The usage of HSC enhances the floor area available for rental purposes in high rise buildings.
• 9. Using HSC in bridges minimizes the amount of beams needed to support the slab.

Q3. What is the necessity fibre reinforced concrete and explain briefly the factors affecting properties of fiber reinforced concrete.

Ans. Necessity of Fiber Reinforced Concrete: 

• 1. It improves the concrete’s tensile strength.
• 2. It lessens the intrinsic porosity of gel, which includes air and water gaps.
• 3. It makes the concrete more resilient.
• 4. In contrast to most resins, fibers like glass and graphite have high creep resistance.
• 5. The differential deformations of the reinforcement and the concrete are kept to a minimum.
• 6. It has been acknowledged that the insertion of small, evenly distributed fibres will significantly enhance the static and dynamic properties of concrete and act as a crack arrestor.

Factors Affecting the Properties of FRC: Following are the factors affecting the properties of fiber reinforced concrete:

• 1. Volume of Fiber: 
  • i. Low volume fraction (less than 1%): Used in slabs and pavement with large exposed surfaces that are prone to excessive shrinkage cracking.
  • ii. Moderate volume portion (between 1 and 2%): Used in shotcrete building methods and in constructions that need to be more resistant to delamination, spalling, and fatigue.
  • iii. High volume fraction (> 2 G): Used in making high performance fiber reinforced composites.
• 2. Aspect Ratio of Fiber: 
  • i. It is defined as the fiber length to diameter ratio (L/d).
  • ii. As the aspect ratio rises up to 75, relative strength and toughness increase.
  • iii. Strength and toughness decrease if the aspect ratio reaches 75.
• 3. Orientation of Fibers: More tensile strength and toughness were supplied by fibers oriented parallel to the applied load than by randomly dispersed or perpendicular fibers.  
• 4. Relative Fiber Matrix:
  • i. For effective stress transfer, the matrix’s modulus of elasticity must be lower than that of the fibers.
  • ii. High modulus fibers (Steel, Glass, and Carbon) contribute strength and stiffness, whereas low modulus fibers (Nylon, Polypropylene) absorb more energy.
• 5. Workability and Compaction of Concrete: The workability is significantly reduced by the addition of steel fiber. The consolidation of fresh mix is negatively impacted by this condition. Concrete cannot be compacted by continuous external vibration.
• 6. Size of Coarse Aggregate: To prevent noticeably weakening of the composite, the coarse aggregate maximum size should be limited to 10 mm. Fibers also serve as aggregate.
• 7. Mixing: Concrete with fiber reinforcement must be mixed under strict circumstances to prevent fiber balling, segregation, and generally problematic uniform mixing of the constituents.

Q4. What are the advantages and disadvantages of using fiber reinforced concrete?

Ans. Advantages of FRC: Following are the advantages of FRC: 

• 1. Once the peak load has been attained, FRC has adequate plasticity to undergo significant deformation.
• 2. The structure can be formed into crooked or thin sheets.
• 3. Greater flexural strength, based on the rate of addition.
• 4. More durable concrete mixes made with traditional materials.
• 5. Less labor-intensive to lay, cast, spray, and place than rebar.
• 6. They have the perfect aspect ratio, which is great for young children’s performance.
• 7. Requires no minimum cover and neither rusts nor corrodes.
• 8. High elastic modulus for long-term reinforcing that works even in cemented concrete.

Disadvantages of FRC: Following are the disadvantages of FRC:

• 1.  High cost of materials. 
• 2. Greater reduction of workability. 
• 3. The flexural strength of concrete is typically not increased by fibers, hence they cannot take the role of moment-resisting or structural steel reinforcement.

Q5. Define ferro cement. What are the advantages and disadvantages of ferro cement ?

Ans. Ferro Cement: It is a kind of thin wall reinforced concrete that is typically made of hydraulic cement mortar reinforced with relatively thin, continuously layered wire mesh.

Advantages of Ferro-Cement: 

• 1. Low maintenance costs. 
• 2. Good impermeability. 
• 3. Good fire resistance 
• 4. Very appropriate for developing countries; labour intensive. 
• 5. Flexibility in cutting, drilling and jointing. 
• 6. Suitability for pre-casting. 
• 7. 20% savings on materials and cost. 
• 8. It can be moulded into practically any shape and has a wide range of applications.
• 9. Lightweight construction, self-weight reduction, and simple processes demand the least amount of specialized effort.
• 10. Elimination of expensive form work to increase efficiency and speed.
• 11. Construction of any constructions only requires a few basic hand tools.
• 12. Buildings have a higher strength to weight ratio than R.C.C. and are very waterproof.

Q6. What are the major differences between ferro cement and reinforced concrete ?

Ans. Following are the properties of ferro cement over reinforced concrete:

• 1. There is uniform distribution of reinforcement.
• 2. Both orientations receive reinforcement.
• 3. A segment that is thinner than RCC.
• 4. Ferro cement has the ability to be homogenous and isotropic in two directions.
• 5. Ferro cement typically has a high rupture modulus and strong tensile strength. Its compressive strength and tensile strength may be on the same scale.
• 6. In both tension and compression, as well as in both directions, ferro cement often has a high reinforcement ratio.
• 7. Compared to reinforced concrete, ferro cement has a one to two orders of magnitude larger specific surface of reinforcement.
• 8. Compared to reinforced concrete, ferro cement is more extensible.
• 9. In general, the crack widths are relatively tiny. It has strong resilience when exposed to diverse environmental conditions.
• 10. When compared to reinforced concrete, ferro cement has better punching shear and impact resistance.

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