Site Cast Concrete

Tiltup Construction

Floor Framing

Roof Framing

What is tilt-up construction?

The building's walls are poured directly at the jobsite in large slabs of concrete called "tilt-up panels" or "tiltwall panels.These panels are then raised into position around the building's perimeter forming the exterior walls.

Casting Surface

Tilt-up panels are usually cast with the outside face down against the casting bed. This allows the use of form liners, rustication strips,

or other highlighting schemes on the outside or architectural face of the panel.

Face-down casting also places the lifting insert holes on the inside face of the panel, where they are easier to patch after the panels are erected.

Usually the concrete floor slab of the building is used for casting tilt-up panels.

Casting Surface

When the concrete floor slab is used as a casting platform, it must be on a constant plane and smoothly troweled. Smoothness is important because the panel

will reflect any imperfection in the slab. A smooth surface also helps prevent a

mechanical bond between the panel and floor slab when lifting.

Tilt up Forms

One of the advantages of tilt-up concrete wall construction is the savings in form material and labor.

Panel edge forms are commonly made of dressed timber and nailed to the floor by using two double-headed form nails wedged into a drilled hole in the form and the floor.

Variations, such as putting a wood dowel in the drilled hole in the floor and nailing to it, are also used. These drilled holes in the floor can be patched

easily with an epoxy after form removal.

Tilting and Erecting Panels

Tilt-up panels are normally tilted and erected by using inserts embedded in the panel face that are attached to the rigging and a crane. This is called a face lift.

Pickup points have embedded inserts Panels may be erected by a crane operating on the

floor or from outside the building. When the crane is operating outside the building, the

operator will be unable to see the lift points as the panel is being erected This is called a blind pick.

Tilting and Bracing

Bracing Panels

Prior to tilting, temporary braces are attached to the panel.

Once erected, the tilt-up panels must be temporarily braced against wind and other lateral forces until all final structural connections are completed.

Telescoping pipe braces are commonly used for this application.

The temporary braces will remain in place until the roof diaphragm is erected and all structural connections completed.

Roof & Floor Framing Systems

Site Cast Concrete Floors

One-Way Solid Slab

One-Way Ribbed Slab

One-Way Ribbed Slab

Broadened Joist Ends

Two Way Flat Slab

A two-way floor system spans between beam on square bays. The deep spandrel beams and wide columns

become part of the exterior walls. Two-way floor framing allows for irregularity in

the placement of columns.

Two-Way Flat Slab

Two-Way Flat Plate

Two-Way Flat Slab

The use of a two-way flat plate in this Chicago apartment building make cantilevered slab edges and balconies easy.

Two-Way Slab & Beam

Two-Way Waffle Slab

Waffle Slab Formwork

Waffle Slab Formwork

Prestressed concrete

Prestressed concrete

Is reinforced by Pre-tensioning or Post-tensioning high-strength steel tendons .

The tensile stresses in the tendons are transferred to the concrete, placing the entire cross section of the flexural member in compression.

The resulting compressive stresses counteract the tensile bending stresses from the applied load, enabling the prestressed member to carry a greater load

Prestressing Techniques

Pre-tensioning is accomplished in a precastingplant before arriving at the job site. Post-tensioning is usually performed at the

building site, especially when the structural units are too large to transport from factory to site.

Pretensioning

Prestresses a concrete member by stretching the reinforcing tendons before the concrete is cast.

Pretensioning

Pretensioning (cont)

Post-tensioning

Post-tensioning

Is a method of reinforcing (strengthening) concrete or other materials with high- strength steel strands or bars, typically referred to as tendons.

Applications include office and apartment buildings, parking structures, slabs- on- ground, bridges, sports stadiums.

In many cases, post- tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements.

Rebar vs Tendons

Rebar is what is called passive reinforcement however; it does not carry any force until the concrete has already deflected enough to crack.

Post- tensioning tendons, on the other hand, are considered Active reinforcing. Because it is prestressed, the steel is effective as

reinforcement even though the concrete may not be cracked.

Post- tensioned structures can be designed to have minimal deflection and cracking, even under full load.

Post-tensioning

Done almost entirely on the job site High-strength steel strands (tendons) are covered with

a steel or plastic tube to prevent them from bonding with the concrete

The prestressing of a concrete member by tensioning the reinforcing tendons after the concrete has set.

Tendon" is defined as a complete assembly consisting of the anchorages, the prestressing strand or bar, the sheathing or duct and any grout or corrosion- inhibiting coating (grease) surrounding the prestressing steel.

Post-tensioning

Post-tensioning

ADVANTAGES/APPLICATIONS

Allows longer clear spans, thinner slabs, fewer beams Thinner slabs mean less concrete is required. Post- tensioning can thus allow a significant reduction

in building weight versus a conventional concrete building with the same number of floors.

Post- tensioning is the system of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations.

Post-tensioned prestressed slab

Types of Post-Tensioning

There are two main types of post-tensioning: Unbonded Bonded (grouted)

Post-tensioning Process

Abutments are not needed. Tendons may be left unbonded or if in a steel

tube bonded with a injected grout. Most are unbonded.

Bonded post-tension systems

More commonly used in bridges, both in the superstructure (the roadway) and in cable- stayed bridges, the cable- stays.

In buildings, they are typically only used in heavily loaded beams such as transfer girders and landscaped plaza decks where the large number of strands required makes them more economical.

Bonded post-tension systems

Typically, a cased hole is drilled into the side of the excavation, the hillside or the tunnel wall.

A tendon is inserted into the casing and then the casing is grouted.

Once the grout has reached sufficient strength, the tendon is stressed.

In slope and tunnel wall stabilization, the anchors hold loose soil and rock together; in excavations they hold the wood lagging and steel piles in place.

Unbonded tendon

Is one in which the prestressing steel is not actually bonded to the concrete that surrounds it except at the anchorages.

The most common unbonded systems are monostrand(single strand) tendons, which are used in slabs and beams for buildings, parking structures and slabs- on-ground.

A monostrand tendon consists of a seven- wire strand that is coated with a corrosion- inhibiting grease and encased in an extruded plastic protective sheathing.

Unbonded tendon (cont)

The anchorage consists of an iron casting and a conical, two- piece wedge which grips the strand.

In bonded systems, two or more strands are inserted into a metal or plastic duct that is embedded in the concrete.

The strands are stressed with a large, multi- strand jack and anchored in a common anchorage device.

The duct is then filled with a cementitious grout that provides corrosion protection to the strand and bonds the tendon to the concrete surrounding the duct.

Construction of Post-Tensioning

In building and slab- on- ground construction, unbondedtendons are typically prefabricated at a plant and delivered to the construction site, ready to install.

The tendons are laid out in the forms in accordance with installation drawings that indicate how they are to be spaced, what their profile (height above the form) should be, and where they are to be stressed.

After the concrete is placed and has reached its required strength, usually between 3000 and 3500 psi ( pounds per square inch), the tendons are stressed and anchored. The fact the tendons are kept in a permanently stressed

(elongated) state causes a compressive force to act on the concrete.

Anchorages in Post-Tensioning

Anchorages are a critical element, particularly in unbonded systems. After the concrete has cured and obtained the

necessary strength, the wedges are inserted inside the anchor casting and the strand is stressed.

When the jack releases the strand, the strand retracts slightly and pulls the wedges into the anchor.

This creates a tight lock on the strand. The wedges thus maintain the applied force in the tendon and transfer it to the surrounding concrete.

Post-Tension Anchorage

Hydraulic Jack (Ram)

Smaller Hydraulic Jack

Post-tensioning

Is Post-tensioned members tend to shorten over time due to elastic compression, shrinkage, and creep. Adjoining elements that would be affected by

this movement should be constructed after the post-tensioning process is completed and be isolated from the post-tensioned members with expansion joints.

PRECAST CONCRETE FRAMING SYSTEMS

Pretensioned Framing Systems

Types of Precast Concrete

Types of Precast Concrete

Types of Precast Concrete

Double & Triple Tees

Double & Triple Tees are primarily used for applications that require exceptionally long spans such as parking decks, bridges, swimming pools, gymnasiums and industrial building walls.

The pieces are cast in forms rather than extruded by a machine.

Double and triple tees exceed fire codes and require virtually no maintenance.

The tee sizes vary from 8' - 12' wide and 20" - 40" deep.

Double Tee Construction

Double Tee Installation

Double Tee Floor

Double Tee Detail

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Inverted T

Double Tee Wall Parking Structure

Hollowcore slabs

Hollowcore slab

A hollowcore slab is a precast, prestressed concrete member with continuous voids provided to reduce weight and, therefore, cost and, as a side benefit, to use for concealed electrical or mechanical runs.

Primarily used as floor or roof deck systems, hollowcore slabs also have applications as wall panels, spandrel members and bridge deck units.

Hollowcore slab

In most cases, the slabs are cast on long line beds, normally 300 ft to 600 ft long.

Slabs are then sawcut to the appropriate length for the intended project.

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Hollowcore Slab

Hollowcore Slabs

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Methods of Manufacturing

Two basic manufacturing methods are used for the production of hollowcore slabs. Dry cast or extrusion system Stationary, fixed forms

Dry cast or extrusion system

A very low slump concrete is forced through the machine.

The cores are formed with augers or tubes with the concrete being compacted around the cores.

Stationary, fixed forms

Higher slump concrete used. Sides are formed either with stationary, fixed forms or

with forms attached to the machine with the sides being slip formed.

The cores in the normal slump, or wet cast, systems are formed with either lightweight aggregate fed through tubes attached to the casting machine, pneumatic tubes anchored in a fixed form or long tubes attached to the casting machine which slip form the cores.

Hollowcore Sizes

Hollowcore Size

Advantages to Hollowcore Slabs

Hollowcore slabs are most widely known for providing economical, efficient floor and roof systems.

The top surface can be prepared for the installation of a floor covering by installing non-structural fill concretes ranging from 1/2 inch to 2 inches thick by casting a composite structural concrete topping.

The underside can be used as a finished ceiling as installed, by painting, or by applying an acoustical spray.

Advantages to Hollowcore Slabs

When properly coordinated for alignment, the voids in a hollowcore slab may be used for electrical or mechanical runs. For example, routing of a lighting circuit through

the cores can allow fixtures in an exposed slab ceiling without unsightly surface mounted conduit.

Slabs used as the heated mass in a passive solar application can be detailed to distribute the heated air through the cores.

Structural applications

Hollowcore slab provides the efficiency of a prestressed member for load capacity, span range, and deflection control.

In addition, a basic diaphragm is provided for resisting lateral loads by the grouted slab assembly provided proper connections and details exist.

Other applications

Fire resistance Depending on thickness and strand cover,

ratings up to a 4 hour endurance can be achieved.

Excellent sound transmission characteristics associated with concrete. Weather resistant

HOLLOWCORE DETAILS

Structural Connections

Hollowcore NonBearing Detail

Hollowcore Bearing Detail

Hollowcore Bearing Detail

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Hollowcore Walls

Spandeck is a patented garage floor system which is produced in Salt Lake City, Utah.

Residential Construction

Two-Foot Widths Offer Clear Spans Up to 36' Without

Additional Bearing

Residential Construction