Precast Concrete Structures, Second Edition
Kim S. Elliott
Preference :
What makes precast concrete different from other forms of concrete construction? Whether
concrete is precast, that is statically reinforced or pretension (prestressed), is not always
apparent. It is only when we consider the role concrete will play in developing structural
characteristics that its precast nature becomes significant. The most obvious definition for
precast concrete is that it is concrete which has been prepared for casting, cast and cured in
a location which is not its final destination. The distance travelled from the casting site may
only be a few meters, where on-site precasting methods are used to avoid expensive haulage
(or VAT in some countries), or maybe thousands of kilometers, in the case of high-value-
added products where manufacturing and haulage costs are low. The grit basted architectural precast concrete in Figure 1.1 was manufactured 600 km from the site, whereas the
precast concrete columns, beams and walls traveled less than
60 m; wall panels have been stack-cast in layers between sheets of polythene adjacent to the
final building.
What really distinguishes precast concrete from cast in situ is its stress and strain
response to external (load-induced) and internal (autogenous volumetric changes) effects.
These are collectively known as ‘actions’ in the Eurocodes, and those mainly applicable
to precast concrete structures are the ‘keynote’ code EC0 (BS EN 1990 2002), the loading
or ‘actions’ code EC1 (BS EN 1991-1-1 2002) and the ‘concrete design’ code EC2 (BS EN
1992-1-1 2004).
A precast concrete element is, by definition, of a finite size and must therefore be joined
to other elements to form a complete structure. A simple bearing ledge or corbel will suffice,
as shown in Figure 1.3. But when thermal shrinkage or load-induced strains cause volu-
metric changes (and shortening or lengthening), the two precast elements try to move apart.
Interface friction at the mating surface prevents movement, but in doing so
creates a force F = μR which is capable of splitting both elements unless the section was
suitably reinforced. Figure 1.5a shows an example of where frictional forces
due to relative, unreinforced movement between precast slabs and beams caused spalling in
the beam. In other cases, spurious positive bending moments due to the restraint of relative
movement or end rotation have caused cracking in the soffit of slabs, or at a beam-to-column
corbel connection.
Flexural rotations of the suspended element (the beam) reduce the mating length lb (bearing
length), creating a stress concentration until local crushing at the top of the pillar (the column)
occurs, unless a bearing pad is used to prevent stress concentration. If the bearing is narrow, dispersal of stress from the interior to the exterior of the pillar causes lateral
tensile strain, leading to bursting of the concrete at some distance below the bearing unless the
section is suitably reinforced.
concrete is precast, that is statically reinforced or pretension (prestressed), is not always
apparent. It is only when we consider the role concrete will play in developing structural
characteristics that its precast nature becomes significant. The most obvious definition for
precast concrete is that it is concrete which has been prepared for casting, cast and cured in
a location which is not its final destination. The distance travelled from the casting site may
only be a few meters, where on-site precasting methods are used to avoid expensive haulage
(or VAT in some countries), or maybe thousands of kilometers, in the case of high-value-
added products where manufacturing and haulage costs are low. The grit basted architectural precast concrete in Figure 1.1 was manufactured 600 km from the site, whereas the
precast concrete columns, beams and walls traveled less than
60 m; wall panels have been stack-cast in layers between sheets of polythene adjacent to the
final building.
What really distinguishes precast concrete from cast in situ is its stress and strain
response to external (load-induced) and internal (autogenous volumetric changes) effects.
These are collectively known as ‘actions’ in the Eurocodes, and those mainly applicable
to precast concrete structures are the ‘keynote’ code EC0 (BS EN 1990 2002), the loading
or ‘actions’ code EC1 (BS EN 1991-1-1 2002) and the ‘concrete design’ code EC2 (BS EN
1992-1-1 2004).
A precast concrete element is, by definition, of a finite size and must therefore be joined
to other elements to form a complete structure. A simple bearing ledge or corbel will suffice,
as shown in Figure 1.3. But when thermal shrinkage or load-induced strains cause volu-
metric changes (and shortening or lengthening), the two precast elements try to move apart.
Interface friction at the mating surface prevents movement, but in doing so
creates a force F = μR which is capable of splitting both elements unless the section was
suitably reinforced. Figure 1.5a shows an example of where frictional forces
due to relative, unreinforced movement between precast slabs and beams caused spalling in
the beam. In other cases, spurious positive bending moments due to the restraint of relative
movement or end rotation have caused cracking in the soffit of slabs, or at a beam-to-column
corbel connection.
Flexural rotations of the suspended element (the beam) reduce the mating length lb (bearing
length), creating a stress concentration until local crushing at the top of the pillar (the column)
occurs, unless a bearing pad is used to prevent stress concentration. If the bearing is narrow, dispersal of stress from the interior to the exterior of the pillar causes lateral
tensile strain, leading to bursting of the concrete at some distance below the bearing unless the
section is suitably reinforced.
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| Precast Concrete Structures, Second Edition |
Content :
- What is precast concrete
- Materials used in precast structures
- Precast frame analysis
- Precast concrete floors
- Precast concrete beams
- Precast concrete columns
- Shear walls
- Horizontal floor diaphragms
- Joints and connections
- Beam and column connections
- Ties in precast concrete structures
- Design exercise for 10-story precast skeletal frame
Download Precast Concrete Structures, Second Edition free PDF
2 comments
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