BT5: Cast-in-place, Pre-cast and Pre-stressed Concrete

Cast-in-place Concrete

Concrete deposited, formed, cured, and finished in its final position on site as part of a structure.

Cast-in-situ Concrete

Another term for cast-in-place concrete.

Ready-mixed Concrete

Concrete proportioned and mixed off the project site and delivered to the site, usually in a truck agitator.

Common applications of Cast-in-place Concrete

Foundations, slabs-on-ground, walls, beams, columns, floors, roofs, bridges, pavements, and other infrastructure.

Concrete Wash Water

Wash water from concrete operations that can be collected, returned to the ready-mix plant, and recycled.

Extra Ready-mix Concrete

Excess concrete that may be returned to the plant, recycled, used for jersey barriers or retaining wall blocks, or washed to recover coarse aggregate.

Proper Placement Rate

Concrete should not be placed more rapidly than it can be spread, struck off, and consolidated.

Consolidation

The process of compacting fresh concrete to mold it within forms around embedded items and reinforcement while eliminating stone pockets, honeycombing, and entrapped air.

Vibration

The most widely used method for consolidating concrete.

Self-compacting Concrete

Concrete that can flow and consolidate under its own weight without requiring vibration.

Curing

The process of maintaining satisfactory moisture and temperature after concrete placement so it develops adequate strength and durability.

Curing Compounds

Compounds or surface treatments that prevent rapid moisture loss from the concrete surface and aid curing.

Strikeoff / Screeding

The process of removing excess concrete and bringing the exposed surface to the proper contour and elevation.

Self-compacting High-performance Concrete

A concrete mixture that eliminates vibration activities during placement.

Traditional Forms

A cast-in-place wall-forming technique using temporary forms that can be removed and reused after curing.

Tunnel Forms

A forming system in which walls, floors, and ceilings are cast at the same time.

Why are Tunnel Forms suitable for repetitive housing?

They are suited to multi-family and attached housing with repeated room dimensions, allowing multiple units to be created in a single pour.

How do Tunnel Forms speed up construction?

Heaters inside the forms speed curing so the forms can be stripped and repositioned the next day.

Precast Concrete

Concrete cast in a reusable mold or form, cured in a controlled environment, transported to the site, and lifted into place.

Pre-fabricated Concrete

Another term used in the module for precast concrete.

Why is Precast Concrete suitable for mass production?

It uses industrial production methods that allow many buildings or components to be produced quickly and at relatively low cost.

Large-panel Systems

Precast multistory structures composed of large wall and floor concrete panels connected vertically and horizontally to enclose room spaces.

Frame Systems

Precast systems constructed using linear elements or spatial beam-column subassemblages.

Why are linear precast elements generally preferred?

Spatial elements have greater difficulties in forming, handling, and erecting.

Slab-column Systems with Walls

Precast systems where shear walls resist lateral loads while the slab-column structure mainly resists gravity loads.

Two main Slab-column Systems with Walls

Lift-slab system with walls and pre-stressed slab-column system.

Mixed Systems

Precast structural systems that combine different system types.

Flat Plate

A concrete slab of uniform thickness reinforced in two or more directions and supported directly by columns without beams or girders.

Flat Plate suitable spans and loads

Short to medium spans with relatively light live loads.

Why does shear govern Flat Plate thickness?

Flat plates have no column capitals or drop panels, giving them lower shear capacity.

Advantages of Flat Plate

Inexpensive formwork, exposed ceilings, minimum thickness, fast erection, and flexible column location.

Disadvantages of Flat Plate

Excess concrete for longer spans, low shear capacity, and greater deflections.

Typical uses of Flat Plate

Hotels, motels, dormitories, condominiums, and hospitals.

Flat Slab

A flat plate thickened at column supports with column capitals and drop panels to increase shear strength and moment-resisting capacity.

When is a Flat Slab suitable?

For heavily loaded spans.

When is a Flat Slab especially economical?

When live load exceeds 150 lb/ft² (150 psf).

Advantages of Flat Slab

Economical for loads greater than 150 psf.

Disadvantage of Flat Slab

Formwork is costly.

Typical uses of Flat Slab

Warehouses, industrial structures, and parking structures.

Flat Plate vs Flat Slab

A flat plate has uniform thickness and no column capitals or drop panels; a flat slab has thickened areas at column supports to increase shear and moment capacity.

Ribbed Slab

A reinforced concrete slab cast integrally with closely spaced joists supported by parallel beams.

Joist Slab

Another name for ribbed slab.

How are Ribbed Slabs designed?

As a series of parallel T-beams.

When are Ribbed Slabs economical?

For medium spans with light to medium live loads.

Distribution Rib

A rib perpendicular to the joists that distributes possible load concentrations over a larger area.

When is one Distribution Rib required?

For spans between 20 and 30 ft (6–9 m).

When are two Distribution Ribs required?

For spans over 30 ft.

Joist Band

A broad, shallow supporting beam for a ribbed slab with the same depth as the joists.

Pan

A reusable metal or fiberglass mold used to form a ribbed slab.

Advantages of Ribbed Slab

Suitable for medium to long spans, lightweight, accommodates service holes, and can provide an architectural/passive-cooling ceiling profile.

Disadvantages of Ribbed Slab

Higher formwork cost, slightly greater floor thickness, and slower construction.

Waffle Slab

A two-way concrete slab reinforced by ribs in two directions.

Why can Waffle Slabs carry heavier loads and longer spans than Flat Slabs?

Their two-way ribs create a deeper structural grid with greater structural capacity.

Dome

A square metal or fiberglass pan used to form the ribs of a waffle slab.

Advantages of Waffle Slab

Longer two-way spans, attractive exposed ceilings, and heavy load capacity.

Disadvantages of Waffle Slab

Higher formwork cost and greater use of concrete and steel than a joist slab.

Typical uses of Waffle Slab

Buildings requiring longer spans and exposed ceiling structures, including prominent buildings and structures similar to those suited to flat slabs.

Column spacing for Waffle Slabs

Column spacing should be multiples of pan spacing to maintain uniformity of drop panels.

Lift Slab

A construction method in which floor and roof slabs are cast at ground level and then lifted into position using hydraulic jacks.

How are Lift Slabs cast?

They are cast on the ground with plastic sheets between the slabs.

Purpose of embedded steel shear heads in Lift-slab construction

They are embedded around columns and leave holes that facilitate lifting.

How are Lift Slabs raised?

Hydraulic jacks lift the matured slabs to their final positions.

Main disadvantage of Lift Slab

High cost, complexity, unreliability, and limited suitability for different building forms.

Slip Form

A construction method for large towers or bridges in which moving forms continuously rise as previously poured concrete hardens behind them.

How does Vertical Slipform work?

Formwork is continuously raised vertically by hydraulic jacks.

What does Vertical Slipform require from concrete?

A balance between early strength gain and workability.

Applications of Slip Form

Core walls in high-rise structures such as lift shafts, stair shafts, and towers.

Advantage of Slipform regarding cranes

The system is largely self-contained and requires little crane time during construction.

Composite Construction Method

A method of using two materials together so each performs to its best advantage.

Steel-concrete Composite Construction

Steel beams and a concrete slab are connected so they act together to resist loads, with the slab effectively serving as a cover plate.

Structural advantage of Composite Construction

A lighter steel section can be used.

Architectural benefits of Composite Construction

Longer spans, thinner slabs, more slender columns, and greater design opportunities.

Economical benefits of Composite Construction

Reduced building height, longer spans with column-free rooms, and additional storeys within the same total building height.

Time benefits of Composite Construction

Faster construction, earlier completion, lower financing costs, and earlier building use.

Functional benefit of Composite Construction

Concrete can provide fire protection to the steel.

Service and flexibility benefits of Composite Construction

Structures can be adapted or modified during their life, with services accommodated in ceilings, false floors, or coffers.

Spanstress™ Floor System

Precast fabrications made from rebars and concrete based on calculated span and load; also known as Spanstress precast connectors.

What is Spanstress™ used for?

It is commonly used for earthquake-proofing, including flooring for pedestrian and overpass bridges.

C-Joist

A precast, prestressed concrete product used for floor and roof slab systems.

C-Joist concrete strength

5,500 psi.

How is a C-Joist reinforced?

With pre-tensioned tendons.

What is the basic function of a C-Joist?

It acts as a concrete joist supporting suspended flooring and forms part of a slab system.

C-Joist shoring requirement for spans over 5 m

Quarter spacing of shoring is required.

C-Joist shoring requirement for spans above 3 m to 5 m

Only midspan shoring is required.

C-Joist shoring requirement for spans below 3 m

No shoring is required.

How long must C-Joist shoring remain?

Not less than seven days.

Pre-stressed Concrete

Concrete reinforced by pre-tensioning or post-tensioning high-strength steel tendons within their elastic limit to actively resist service loads.

What does prestressing do to the concrete cross section?

Tensile stresses in the tendons are transferred to the concrete, placing the flexural member's cross section in compression.

What does prestressing allow a concrete member to do?

Deflect less, carry greater loads, or span greater distances than a conventionally reinforced member of similar size and weight.

Three ways prestressing can be accomplished

Pre-tensioned concrete, bonded post-tensioned concrete, and unbonded post-tensioned concrete.

Pre-tensioning

Prestressing a concrete member by tensioning reinforcing tendons before concrete is cast.

How is Pre-tensioning performed?

Tendons are stretched between abutments, concrete is cast and cured around them, then the tendons are cut and their tensile stress is transferred to the concrete through bond stresses.

Post-tensioning

Prestressing a concrete member by tensioning reinforcing tendons after the concrete has set.

How is Post-tensioning performed?

Unstressed tendons are placed in sheaths before casting; after curing, tendons are tensioned and anchored at the jacking end.

Sheath

A tube that encases tendons in a post-tensioned member to prevent bonding to concrete during placement.

Bonded Post-tensioning

Post-tensioning in which tendons are bonded to surrounding concrete by injecting grout into the spaces around the strands.

Unbonded Post-tensioning

Post-tensioning in which the spaces around the tendons are not grouted, allowing the tendons to move relative to the surrounding concrete.

Applications of Prestressed Concrete

Floors in high-rise buildings and concrete chambers in nuclear reactors.

Common use of Unbonded Post-tensioning

Parking garages, where tendons can function as barrier cables.

Advantages of Prestressed Concrete over Regular Reinforced Concrete

Crack control, lower construction costs, thinner slabs, and fewer joints.