BT5 Module 2: Cast-in-place, Pre-cast and Pre-stressed Concrete

Cast-in-place concrete (Cast-in-situ concrete)

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

How is cast-in-place concrete transported and placed

It is transported in an unhardened state primarily as ready-mix and placed in forms

ready mixed concrete

Proportioned and mixed off the project site

How is ready mixed concrete delivered to the site

Delivered in a truck agitator or non-agitating truck

Specialized paving equipment

used to mix and spread concrete for pavement

Slab-on-ground and foundations because of its long-term durability and structural support

Cast-in-place concrete is the material of choice for what elements

Beams columns floors walls and roofs

Cast-in-place concrete is used in all types of buildings for structural support as what elements

Formwork reinforcing steel bars and the placement curing and finishing of the concrete

What three key sets of activities are performed within the process of cast-in-place reinforced concrete construction

Cast-in-place concrete

What material is used for most foundations and slabs-on-ground

Cast-in-place concrete

What material is used for walls beams columns floors and roofs

Cast-in-place concrete

What material is used for large portions of bridges pavements and other infrastructure

How is concrete ordered and handled during installation

Ordered and placed as needed and does not need to be trimmed or cut after installation

How is wash water handled in concrete operations

Frequently recycled using trucks equipped with devices that collect wash water and return it to the drum where it can be returned to the ready mixed concrete plant for recycling

What happens to extra concrete

Returned to the ready-mix plant where it is recycled or used to make jersey barriers or retaining wall blocks or washed to recycle the coarse aggregate

Special set retarding admixtures

What can be added to returned concrete to allow for storage and future use

Thermal mass

What improves energy performance of concrete when appropriately insulated

3 in. or more in thickness

Concrete forms an air barrier when it reaches what thickness

Light or natural colored material

What type of material helps reduce the heat island effect

Not more rapidly than it can be spread struck off and consolidated

How fast should concrete be placed

Continuously as near as possible to its final position

How should concrete be deposited

Compacts fresh concrete to mold it within the forms around embedded items and reinforcement and eliminates stone pockets honeycombing and entrapped air

What does consolidation do to fresh concrete

Vibration

What is the most widely used method for consolidating concrete

Self-compacting concrete (self-consolidating concrete)

What concrete is able to flow and consolidate under its own weight and requires no vibration

Curing process

What is the process required after concrete is placed to develop adequate strength and durability through satisfactory moisture content and temperature

Curing compounds or other surface treatments

What prevents the rapid loss of moisture from the surface of concrete and aids in the curing process

Exposed concrete surfaces usually the top surface

What concrete surfaces generally require finishing if they will be visible

Driveways pavements sidewalks floors slabs and other flatwork

What are examples of flatwork that may require finishing

Various colors and textures such as exposed aggregate or a pattern-stamped surface

What are finishing options for exposed concrete surfaces

Strikeoff or screeding

What operation removes excess concrete and evens out the exposed surface to the proper contour and elevation

Talon 2 360 Rebar Crosstie system

What is a machine which automatically wires together reinforcing steel bars

Self-compacting high-performance concrete

What is a novel mixture of standard concrete materials which eliminates the vibration activities of the placement subprocess

Traditional Forms

What traditional concrete forming technique uses temporary forms typically made of aluminum with rigid foam insulation placed inside or between forms held by non-conductive ties

Removed and re-used many times with a minimum of maintenance

What happens to traditional forms once the concrete has cured

Tunnel Forms

What forming system casts walls floors and ceilings at the same time

Special heaters suspended inside the forms speed the curing process actually baking the concrete

How do tunnel forms speed up the curing process

Allows forms to be stripped and repositioned the next day

What is the benefit of the enhanced curing process in tunnel forms

Multi-family buildings and attached housing where room dimensions are repeated

What building types are tunnel forms best suited to

Pre-cast Concrete (Pre-Fabricated)

What form of construction involves casting concrete in a reusable mould or form which is cured in a controlled environment transported to the construction site and lifted into place

Industrial methods based on mass production in order to build a large number of buildings in a small amount of time at low costs

How is pre-cast concrete manufactured

Large-panel systems

What category of precast system refers to multistory structures composed of large wall and floor concrete panels connected in vertical and horizontal directions

Frame systems

What category of precast system can be constructed using either linear elements or spatial beam-column sub-assemblages

Linear elements

Which elements are generally preferred in precast frame systems due to difficulties associated with spatial elements

At the beam-column junctions

The use of linear elements in precast frames generally means placing connecting faces where

On corbels at the columns for ease of construction

How can beams be seated in precast linear frame systems

Slab-column systems with walls

Which precast system relies on shear walls to sustain lateral load effects while the slab-column structure resists mainly gravity loads

Lift-slab system with walls and Pre-stressed slab-column system

What are the two main systems in the slab-column systems with walls category

Mixed systems

What is the category of precast system that combines different structural approaches

FLAT PLATE

What is a concrete slab of uniform thickness reinforced in two or more directions and supported directly by columns without beams or girders

Short to medium spans with relatively light live loads

What spans and loads are flat plates suitable for

Shear

What governs the thickness of a flat plate since there are no column capitals or drop panels

Inexpensive formwork ceilings may be exposed minimum thickness fast erection flexible column location

What are the advantages of a flat plate

Excess concrete for longer spans low shear capacity greater deflections

What are the disadvantages of a flat plate

Hotels motels dormitories condominiums hospitals

What applications are appropriate for flat plates

Moderate spans

A flat plate is best for what spans because it is the most economical floor system and has the lowest structural thickness

Spandrel beams

What may be necessary in flat plate systems to avoid penetrations near columns

Minimum thickness especially when post-tensioned

Why does flat plate concrete construction permit more stories to be fitted into a given building height than any other system

The underside of the floor plate

What can serve as the finish ceiling in many building types using flat plate construction

With flat plate shoring still in place while concrete is being poured for upper-floor columns and plates

When can exterior precast concrete panels be attached on lower floors in flat plate construction

A hole is left in a section of each plate to allow passage of the hoist and is filled later

What is done when crane hoists are used to lift concrete buckets or equipment through flat plates

Positions of steel angles for welding to spandrels require greater exactitude and positioning spandrels for framing/glass requires greatest care

How do tolerances change across flat plate envelope systems

Ceiling height power and communications poles in furred-out areas around columns and in corridor partition walls

How can workstations in unpartitioned interior offices be serviced unobtrusively in flat plate systems

Usable floor area in the perimeter can be maximized

Why is the central core placed farthest from perimeter zones in flat plate layouts

Moved against an unfenestrated wall

Where can the central core be moved on constrained urban sites

FLAT SLAB

What is a flat plate thickened at its column supports with column capitals and drop panels to increase its shear strength and moment-resisting capacity

Heavily loaded spans

What types of spans are flat slabs suitable for

Exceeds 150 lb per sq. ft.

Flat slab scheme is by far the most economical when live load reaches what level

Economical design for loads greater than 150 psf

What is the advantage of a flat slab

Formwork is costly

What is the disadvantage of a flat slab

Warehouses industrial structures parking structures

What applications are appropriate for flat slabs

RIBBED SLAB (joist slab)

What is a reinforced concrete slab cast integrally with a series of closely spaced joists which in turn are supported by a parallel set of beams

Medium spans with light to medium live loads

Ribbed slabs are designed as a series of parallel t-beams and are economical for what spans and loads

Distribution rib

What is a rib formed perpendicular to the joists of a ribbed slab to distribute possible load concentrations over a larger area

One distribution rib

How many distribution ribs are required for ribbed slab spans between 20 and 30 ft. (6 and 9 m)

Two distribution ribs

How many distribution ribs are required for ribbed slab spans over 30 ft.

Joist band

What is a broad shallow supporting beam for a ribbed slab that is economical to form since its depth is the same as that of the joists

Pan

What is a reusable metal or fiberglass mold used in forming a ribbed slab available in standard 20- and 30-inch widths and a variety of depths

Tapered endform

What is a pan used to form thickened joist ends for a greater shear resistance in a ribbed slab

Medium to long spans lightweight holes in topping easily accommodated large holes can be accommodated profile may be expressed architecturally or used for heat transfer in passive cooling

What are the advantages of a ribbed slab

Higher formwork costs than for other slab systems slightly greater floor thicknesses slower

What are the disadvantages of a ribbed slab

WAFFLE SLAB

What is a two-way concrete slab reinforced by ribs in two directions

Flat slabs

Waffle slabs are able to carry heavier loads and span longer distances than what slab type

Omitting dome forms in selected areas

How can supporting beams and drop panels be formed in a waffle slab

Rectangular grid of deep concrete beams running at right angles

Describe the under-surface of a waffle slab

30 feet on center

How are columns typically spaced in a waffle slab system

Areas requiring isolation from low-frequency vibrations and low floor deflection

Where is a waffle slab system used

Dome

What is a square metal or fiberglass pan used in forming the ribs of a waffle slab available in standard 19- and 30-in. widths

Longer two-way spans attractive exposed ceilings heavy load capacity

What are the advantages of a waffle slab

Formwork costs more and uses more concrete and steel than a joist slab

What are the disadvantages of a waffle slab

Prominent buildings with exposed ceiling structure and same types as flat slab but with longer spans

What applications are appropriate for waffle slabs

Multiples of pan spacing to ensure uniformity of drop panels at each column

How should column spacing be determined for waffle slabs

Diamond-shaped square or rectangular

What shapes can drop panels in a waffle slab be

LIFT SLAB

What method of concrete construction involves floor and roof slabs being cast on or at ground level and lifted into position using jacks

Plastic sheets

What is placed between slabs when they are cast on the ground in lift slab construction

To facilitate the lifting process to their final positions

Why are steel shear heads embedded in concrete around columns in lift slab construction

Hydraulic jacks

What equipment lifts the matured slabs to their final position in lift slab construction

Ground floor

What serves as the initial soffit form in lift slab construction

High cost complexity unreliability and unsuitability to a variety of building forms

What are the limitations and drawbacks of most known lift-slab methods

Unstable condition of the structure during construction

What condition during construction threatens the use of the lift-slab method

SLIP FORM

What construction method builds large towers or bridges from concrete using a moving form that moves along the project as previously poured concrete hardens behind it

Early 20th century for building silos and grain elevators

When was the slipform technique first in use and for what structures