PRELIM Flashcards
shet
Is a stone-like material obtained by permitting a carefully proportioned mixture of cement, sand, and gravel or other aggregate and water to hardened in forms of shape and dimensions of the desired structure.
Concrete
Cement and water interact chemically to land the aggregate particles into a solid mass.
Concrete
What are the special aggregates
Lightweight Aggregate
Heavyweight Aggregate
What are the example of admixtures
Plasticizers
Airentraining Agents
Silica Pume
Fly Ash
The process of controlling conditions after placement of concrete
Curing
The resulting combination of two materials when reinforcement is completely surrounded by the hardened concrete mass, it forms an integral part of the member.
Reinforced Concrete
Concrete in which effective internal stress are induced artificially, usually by means of tensioned steel, prior to loading the structure
Prestressed Concrete
A technique for introducing stresses instructures during the process of construction, so that a more favorable state of stress will prevail under service loads
Prestressing
A stretched element in a concrete (usually made of steel) different from reinforcement which is without initial prestressing
Tendon
-This is a method of prestressing by which the tendons are tensioned before concrete is placed
Pre-Tensioning
-This is a method of prestressing by which tendons are tensioned after the concrete has harded.
Post-tensioning
-The tendons, usually wire or strands, are stretched between bulkheads or abutments and the concrete is poured around them
-When the concrete has hardened and reached the required strength, the tendons are released from the anchorages and the prestress is transferred to the concrete by bond
Pre-tensioning
-Concrete is cast, incorporating sheats for the tendons to pass
-When the concrete has gained the required strength, the tendons are tensioned against the steel bearing plate cast into the ends of the member
-The prestress is transfered to the concrete mechanical anchorage
-The space between the tendons and sheats is usually grouted on completion of the tensioning operation
Post-tensioning
Advantage of prestressed over reinforced concrete about cracks
- Free from cracks under service loads and hence better appearance and durability
Materials for prestressed concrete steel
- High tensile steel wire or strand complying BS 5896:1980
- Allow steel bars complying BS 4486:1980
- Concrete
- 30 to 60 MPa
- Water with cement ratio 0:30 - 0.45 usually plasticizers is added to improve workability of concrete
- 28 days curing
- Strength transfer should not be less than 25 MPa in 7 days
The support of the beam
Bearing
Tendon bonded/not bonded throughout its length to the surrounding concrete
Bonded and Unbonded Tendon
A group of tendons
Cables
The upward deflection of a prestressed concrete beam when it is prestressed
Camber
Circular prestressing refers to prestressing in round structures like tanks; prestressing in all other members is termed linear
Circular and Linear Prestressing
The portion of the prestressed concrete beam surrounding the anchorage of post-tensioned tendons
End block
Degree of prestress applied to concrete in which no tension (some tension and usually some flexural cracking) is permitted in the concrete under working loads
Full and partial prestressing
The transferring of prestress to concrete
Transfer
Classification of concrete structure where fully prestressed concrete structures in which there is no flexural tensile stress allowed under service conditions (working load) and very limited flexural tensile stress is allowed at transfer
Class 1
Classification of concrete structure where prestressed concrete structures in which flexural tensile stress is allowed but not limited so that no visible cracking may occur under service conditions
Class 2
Prestressed concrete structures are generally design with reference to the limit states of allowable stress and deflection of the service loads using elastic theory, and adequate ultimate strength theory
Limit States Requirements
Classification of concrete structure where prestressed concrete structures in which cracking is allowed to occur under service conditions, but the width of cracks is limited to 0.1mm for members in very severe environments and not exceeding to 0.2mm for all members
Class 3
The maximum stress that can be safely applied to a structure
Allowable Stress or Allowable Strength
2 design considerations at transfer
a. Flexural Stresses
b. Upward deflection (camber)
2 design considerations under service condition
a. flexural stresses
b. downward deflection
2 design considerations under failure condition
a. Flexural strength
b. Shear strength
Stress limitations in prestressed concrete in prestressing tendons
- The jacking force should not normally exceed 75% of the characteristic strength of the tendon
- The maximum initial prestress at transfer should not normally exceed 70% of the characteristic strength of the tendon
Stress limitations in prestressed concrete at concrete for Allowable compressive stresses at transfer
At extreme fiber = 0.5fci
For near uniform distribution of prestress = 0.4fci
where fci is the concrete strength at transfer
Stress limitations in prestressed concrete at concrete for Allowable tensile stress at transfer
Class 1 members - 1.0 N/mm² (solely due to prestressing force)
Class 2 & Class 3 members
-Pre-tensioned members 0.45√fci
-Post-tensioned memvers 0.36√fci
where fci is the concrete strength at transfer
Stress limitations in prestressed concrete at concrete for allowable compressive stress under service conditions
In bending = 0.33fcu
In direct compression = 0.25fcu
where fcu is the characteristic strength of concrete
Stress limitations in prestressed concrete at concrete for allowable tensile stress under service condition
Class 1 members
- no tensile stress
Class 2 members
-Pre tensioned: 0.45√fcu
-post tensioned 0.36√fcu
Value of Grade 30,40,50,60 of pre tensioned
Pretensioned
Grade 30 = —-
Grade 40 = 2.9
Grade 50 = 3.2
Grade 60 = 3.5
Value of Grade 30,40,50,60 of post-tensioned
Grade 30 = 2.1
Grade 40 = 2.3
Grade 50 = 2.6
Grade 60 = 2.8
Advantage of prestressed over reinforced concrete about shear resistance
-Greater shear resistance and hence thinner web is possible, reduces dead weight, especially in long span structures
Advantage of prestressed over reinforced concrete about rigidity
-Greater rigidity due to effectiveness of the whole uncracked section, hence smaller deflection
Advantage of prestressed over reinforced concrete about load
-Lighter structure: hence saving headroom or smaller load on foundation, usually suitable for long span structures
Advantage of prestressed over reinforced concrete about stress
-Test-proofing of materials for many structures, during prestressing, both the steel and the concrete are subjected to the highest stress that will exist in the structures during their life of service
Material for prestressed concrete steel of high tensire wire or strand
High tensile steel wire or strand complying BS 5896:1980
Material for prestressed concrete steel of alloy steel bars
Allow steel bars complying BS 4486:1980
Material for prestressed concrete steel of concrete
- 30 to 60 MPa
- Water with cement ratio 0:30 - 0.45 usually plasticizers is added to improve workability of concrete
- 28 days curing
- Strength transfer should not be less than 25 MPa in 7 days
