We cannot deny the advancement in technology in terms of building establishments. In the past four decades, it has evolved drastically. And precast concrete contributed the most in precast slabs.
Several engineers and architects suggest using precast concrete. It is because it has a high-quality structural and architectural product.
Moreover, it is fabricated in a wide range of shapes and sizes and contributes to a long life span of buildings. It has massive structural and economic benefits. And they are as follows:
Structural strength and economic contribution:
You can make a slab that will help you make substantial savings in materials. To do it, you may combine a precast slab compositely with an in-situ concrete topping for producing the slab. Precast slabs serve as formwork, and thus, besides offering you savings in material, you also save in formwork.
Cross-sectional patterns for structural beams not only maximize strength but also minimizes material quantity. The best part is that it results in structural and economic efficiency.
If you can combine a precast concrete beam with a cast-in-place slab compositely for a rectangular beam, you can make substantial savings on reinforcing steel and concrete.
Design a better structurally efficient geometry cross-section. It will reduce the material quantity even further. Additionally, it will result in more savings. It offers you immediate economic benefits.
Benefits of shoring:
Shoring the slab or precast beam causes lesser temporary bending moments in elements. It contributes mainly to the structural benefits. The cast-in-place slab’s wet weight causes a significant reduction in the steel and concrete quantity needed in the precast element.
For example, shoring each of the members of precast slabs or beam’s midspan can result in a uniform camber from one member to the other.
Shoring supports the precast units. And thus, it reduces joint rotation and deflection that occurs during composite deck placement.
Moreover, combining initial cambering with shoring up to desired levels can offset anticipated deflections in the long term.
Combining the precast beams and slabs with in situ concrete will work fine for developing the shear resistance and desirable bending movements.
The steel area in the beams having typical loading conditions and spans resists vertical beam shear. It is mainly because of the specific design.
The steel’s area is generally adequate for developing the needful horizontal shear stress between the in-situ slab and the precast beam.
According to testing and research in terms of both cyclic and static loading, the nominal ultimate horizontal shear stress for broom finish contact surface without shear ties is 115 psi.
The broom-finished precast slabs do not require shear ties before the composite concrete’s application.
Extending the reinforcing steel from the precast units into the reinforced in-situ concrete is the most practical and versatile method.
It reduces the sensitivity to precast concrete dimensional tolerance. And also contributes to structural safety, monolithic action, and continuity throughout the framing system at all connections.
Besides providing a virtually “fail-safe” connection, it alleviates the close precision that is usually needed in erection operations and member dimensions.
A negative moment reinforcing steel besides developing continuity also serves as a mechanism to create high compressive forces.
Among all the benefits of precast, the best one is that it develops shear friction and better resistance if joint to vertical shear. And also reduces the bearing area’s importance between the precast slabs or beam and the supporting wall or girder.
In the seismic areas, the moment reversal is prevented in the joints. Thus, the bottom steel’s extension from the precast slab or beam is clamped or welded together for developing the necessary positive movement.
This mechanism provides a safe joint even if the precast slab does not hold on to any part of the wall. The structure will still be adequate, as the extended diagonal bars support the vertical shear load.
This particular mechanism develops high compressive forces and provides sheer friction resistance additionally.
For joining the vertical structural elements like wall panels and columns, mechanical steel couplers are used widely. Especially for connecting reinforcing steel bars in precast concrete, it is functional. It is also effective for connecting horizontal precast units.
Embedding and grouting the couplers into the precast units by injecting them from the exterior can be done for precast columns. It results in a continuous reinforcing steel splice having no pockets for patching during erection.
Moment redistribution: Precast Slabs
The precast units must be designed in terms of repetition and uniformity in their fabrication. It accommodates loadings and deck spans that vary in a limited range. It will then have a consistent area of positive moment reinforcing steel.
Moreover, adjusting the remainder of the total required moments is possible by varying the negative movement steel area in the topping area or composite slab.
There is the redistribution of calculated bending movements, which the plastic yielding of the harmful reinforcing steel transmits.
Structural Precast Slabs Systems:
Research indicates that designing and constructing multi-story precast building frames can potentially resist even the severe seismic forces. Technology has made it possible to achieve seismic-resistant and monolithically-acting structures through any convenient framing pattern.
Over the last few decades, composite precast frame systems have successfully constructed high and low-rise buildings in both non-seismic and seismic areas.
This concept is considered the most successful and suitable for structural framing in even the most severe seismic areas. It is because the system can produce robust strength, ductility, monolithic action, and stiffness.
It provides all these uniformly in the overall frame of the building for safely resisting several vertical and horizontal ground accelerations during earthquakes.
Establishing buildings and other structures is possible with both economy and speed, with precast components. Significant labor efficiency can be achieved through mass-production techniques that are factory-controlled.
High quality in both material and artistry can also be achieved. Integration of the precast units for making building frames plays an integral role and enhances the strength in every aspect.