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Transcript of PRECAST CONCRETE
Precast Concrete Elements
Procedures for erecting precast concrete units and Types of connections used.
Precast Concrete Columns
- For structures of five storeys or less, each column will normally be continuous to the full height of the building. For structures greater than five storeys two or more columns are spliced together.
Fabrigaras, Rex Marvin R.
Gallardo, Elijah Van Claude V.
Garcia, Anna Maryette R.
Garma, Patrick John D.
Hernandez, Rayette Clyde M.
Advantages and Disadvantages
Good quality control
Entire building can be precast-walls, floors,beams,etc
Particular effects of form and colour
Rapid construction on site
Prestressing is easily done
Very heavy members
Camber in beams and slabs
Very small margin for error
Connections may be difficult
Somewhat limited building design flexibility
Because panel size is limited, precast concrete can not be used for two- way structural systems.
Economics of scale demand regularly shaped buildings.
Need for repetition of forms will affect building design.
•Joints between panels are often expensive and complicated.
Skilled workmanship is required in the application of the panel on site.
Cranes are required to lift panels.
Precast concrete is a form of construction, where concrete is cast in a reusable mould or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place.
Topics to be discussed:
Major group of Precast Concrete
Advantages and Limitations of Precast Concrete and Concrete Units
Typical sizes of precast concrete beams, columns, girders and wall panels
Procedures for erecting precast concrete units and Types of connections used.
Precast System Members
- Columns are provided with necessary supports for the ends of the precast beams
Precast Beams and Girders
- Reinforced or pre-stressed concrete beams in a precast concrete frame are designed for the specified loadings and support conditions.
- Precast wall panels are high strength units which can be quickly installed with little or no site preparation.
A flat piece of concrete, put on the walls or columns of a structure. It serves as a walking surface but may also serve as a load bearing member, as in slab homes.
Other Precast Structures
(Stairs, Balconies, etc.)
Types of Precast Columns:
1. Edge Columns -symmetrical in one direction
2. Internal columns - symmetrical in all directions.
3. Corner columns - not symmetrical at all.
Precast Concrete Beams
Precast concrete beams can be produced in different profiles:
Precast Concrete Wall Panels
There are three basic precast concrete wall panels:
Solid Wall Panel
- Solid Wall panels are typically used as interior partition walls and can be either load or non-load bearing. These panels can weigh from 75 to 100 psf.
- A Solid Wall System is a non-cavity, pre-insulated, monolithic
structural panel system that can be installed on any building.
Thin shell Wall Panel
- Thinshell is made of a 2¼" thick concrete skin attached to a metal stud frame system that achieves the beauty of precast mixes and can do so at a lower cost and lighter weight than traditional precast.
-Thin-shell precast is a hybrid precast panel combining cold-formed steel framing with as little as 2" of concrete producing a composite system superior to any conventional product on the market.
Insulated Sandwich Wall Panel
- Insulated Sandwich wall panels typically include 2 inches or more of rigid insulation between two wythes of concrete. It also provides high energy efficiency, meeting the continuous insulation requirements as well as an interior concrete wall that can be painted and used as the interior finished surface, avoiding the need for furring strips and dry wall. Precast concrete's high thermal mass also minimizes energy consumption naturally.
- A minimum thickness of 25 mm is recommended with no limitation on maximum thickness.
Precast Concrete Slabs
- Hollowcore slabs are used in a wide range of buildings as floor/roof components. The precast concrete slabs are cut to length for each project. Hollowcore is frequently used in residential, commercial, industrial, institutional and other applications.
- The cores can function as cable/services ducts and significantly reduce the self-weight of the slabs, maximising structural efficiency. Units are available in standard 1200mm widths and in depths of 150mm to 500mm.
- Hollowcore slabs have excellent long-span capabilities, achieving a maximum capacity of 2.3 KN/sq.m over a 16 metre span.
Solid Wide slab floor
- referred to as "plate flooring" which is generally used in residential developments. Wideslab flooring contains an internal mesh/strand which facilitates notching and the forming of opes.
- 2 Distinct kinds of Solid Wide Slab:
In situ screeded slabs
- 2400mm wide and in depths of 65mm to 200mm deep with spans of up to 7.5m. These slabs are generally for upper floors and have smooth self-finishing soffits.
- 150mm to 200mm deep are delivered self-finished for ground floor applications. These slabs are particularly suited to poor ground conditions or where ëcut and fillí is required.
Double-Tee Floor Slabs
- Double-Tee floor units are produced in standard widths of 2400mm and in depths of between 250mm and 1000mm. The system offers greater structural capacity at longer spans than hollowcore or wideslabs, but often requires a deeper floor zone.
- The Double-Tee system is the only system which offers a solution for spans over 16m.
Advantages of Precast Beams and Columns
Precast Prestressed Beams & Columns provide unlimited flexibility in design, shape and application.
They are extremely durable compared to alternative building materials. Beams & Columns are produced indoors with high-strength concrete creating a quality, strong and durable product with no need for extra fireproofing.
Beams & Columns work well with other precast components such as Wall Panels, Hollowcore and Double Tees to form a Total Precast application.
Precast Prestressed Beams & Columns provide a clean, finished look for the structural component of the building.
Advantages of Precast Concrete Walls
Precast panels can be produced in virtually any color and a widevariety of finishes
Strength gradually increases over time.
Because precast panels are manufactured well in advance of installation, they are ready for transportation to the job site at a moment’s notice.
It is nontoxic, environmentally safe and composed of natural materials.
Precast products are designed snd manufactured for simple connection to reduce installation time.
Advantages of Precast Concrete Slabs
Precast floors can be designed to act compositely with the structure of the building to reduce frame sizes: e.g. main support beams supporting solid slab and screed can be designed as T-beams.
Prestressed composite floors can be tied-in to the main structure and are therefore particularly suited to buildings where progressive collapse is a consideration.
Precast floor slabs together with the structural screed provide a structural deck with full diaphragm action where required in multi-storey buildings.
Types of Connection
- A bolted connection often constitutes the weakest link in the design; in many cases, the bolted connection can be responsible for determining the overall reliability and safety of an entire system.
- Bolted Connections simplify and speed-up the erection operation, because the connection is positive immediately. Final alignment and adjustment can be made later without tying up crane time. Bolting should be in accordance with the erection drawings, using material specified by the designer.
- Welded connections are the most common and typical connection used in the erection of precast concrete.
- The connections are usually made by placing a loose plate between two structural steel plates that are embedded both in the cast-in-place or the precast concrete panel and welded together.
Dowel/Anchor Bolt Connections
- the strength of dowels in tension or shear depends on dowel diameter, embedded length and the bond developed. Good practice is to provide sufficient embedment to develop the full dowel strength
- Threaded anchor bolts and rebar anchor dowels that protrude from the foundation are the critical first connection to precast members. Usually, this work is performed by a subcontractor not responsible to the erector. It is important that these items be placed accurately in both plan and vertical alignment.
Connections for Precast Floor Slabs
Precast Building System
Large Panel Systems
- The designation “large-panel system” refers to multistory structures composed of large wall and floor concrete panels connected in the vertical and horizontal directions so that the wall panels enclose appropriate spaces for the rooms within a building.
- Three Basic Configurations of Large Panel Systems
Cross-wall system -The main walls that resist gravity and lateral loads are placed in the short direction of the building.
Longitudinal-wall system - The walls resisting gravity and lateral loads are placed in the longitudinal direction
Two-way system -The walls are placed in both directions.
- Precast frames can be constructed using either linear elements or spatial beam-column subassemblages.
- Precast beam-column subassemblages have the advantage that the connecting faces between the subassemblages can be placed away from the critical frame regions; however, linear elements are generally preferred because of the difficulties associated with forming, handling, and erecting spatial elements.
Slab-Column Systems with Shear Walls
- These systems rely on shear walls to sustain lateral load effects, whereas the slab-column structure resists mainly gravity loads.
- Two main systems:
Lift-slab system with walls
Prestressed slab-column system
Manufacturing Precast Units
Production of reinforced cages and main connections:
The precast factory often has specialist workshops for the manufacture and maintenance of moulds, and for the production of jig-built reinforcing cages and connections.
Assembly of Molds:
The reinforced cage is positioned in the partly assembled mould, then the remaining mould section is completed.
Mix being poured
Carefully specified concrete is placed into the mould. Many precast works now employ computer controlled batching plants.
Compaction of concrete using poker vibrator
To ensure that optimum density is obtained and that specified strengths are achieved, concrete is placed and compacted using high-frequency external vibrators or pokers.
Precast concrete being moved to the storage area:
Once an appropriate strength has been reached, the precast units are moved to the storage area. Units are usually handled within hours of casting as part of the rapid production cycle.
The product exhibits a high degree of dimensional accuracy and quality of finish. Economies of production are achieved through the repetitive and automated process.
Storage of high-quality units in works area
The finished precast components are stacked on clean battens or plastic pads positioned to suit the design of the component. Care is taken to keep the stacks vertical and to ensure that battens are placed directly above one another within the stack.
Transport to site
The components are delivered to site in a pre-determined sequence to ensure that hardened concrete are ready for instant erection.
Erection at site
The components are erected straight from the lorry. This leads to faster erection times with reduced on-site activity.
Manufacturing of Precast Cement
By producing precast concrete in a controlled environment (typically referred to as a precast plant), the precast concrete is afforded the opportunity to properly cure and be closely monitored by plant employees
The raw material consumption is similar for similar qualities of concrete, whether the production takes place in a factory, at a ready-mix plant or at a building site.
Precast molds for the casting of precast concrete products are usually are made of steel or wood. However, they may be constructed of any material that remains stable during casting and is able to withstand the abuse of preparation, installation of steel reinforcements, oiling, curing, stripping and reuse.
Woods is commonly used for the construction of forms expected to have a limited use; that is, forms built for the casting of fairly unique concrete products.
Concrete is used for casting intricate shapes.
Styrofoams, fiber glass or any paperboards are used for special surface effects.
Steel forms are the most commonly used for the production of repetitive precast unit forms.
The form surface is coated with a release agent to keep the fresh concrete from bonding to the form. Form release agents usually have a kerosene or paraffin base and are applied to the form by swabbing or spraying before the reinforcing steel is placed.
Swabbing is the hand application and spreading of the agent on the interior surface of the form with the use of brooms, mops, rags or brushes.
Most spray application of form oil is hand-pumped, hand-carried sprayers.
The reinforcing operation includes the fabrication and placement of steel reinforcement into the product forms.
There are four basic types of steel reinforcements:
-Plant fabricated cage and mats
-Prefabricated wire, loops, bars, rods, and welded wire mats
-Steel plates and rolled sections
-Prestressing steel strand
Concretes may be batched and mixed at the product plants or purchased from a ready-mix concrete producers.
The correctly weighed proportions of cement, aggregates, and water are charged into a drum mixer, pan mixer, or a transit mix truck.
Two most commonly used types of mixers used in the industry:
- Drum Mixer
- Pan Mixer
Concrete delivery and Casting:
Casting of small products can be a manual operation; a quantity of mix is made available to the operator, who trowels or shovels in into a prepared mold around the reinforcements.
For casting larger pieces, the concrete is delivered from a truck, a concrete bucket, or a concrete buggy moved to the location where the concrete is used.
Wet finishing includes tasks such as screeding, floating, troweling, patching, rubbing, and cleaning surfaces not in contact with the forms.
Three fundamental factors in all methods of curing concrete are time, temperature, and moisture.
The curing of many larger precast products is accelerated by radiant heat, steam, hot water, or hot oil.
In curing, the products remain in the steam shed for 2-8 hours before being removed and the form stripped.
Steam and hot water for accelerated curing procedures are usually generated at the plant by a low-pressure boiler and piped to designated locations prior to release into the curing shed. Occasionally, a chemical curing compound is sprayed on the finished surface of the concrete.
The inner form is rigged to the crane and removed -- a task made more difficult because the product frequently adheres to the form surfaces.
In some instance, striking the form with a hard rubber mallet is sufficient to jar the concrete loose.
In others, a hoist is used to shake the form and/or pull the form free.
Flat products are removed from the mold by means of a vacuum lifter. Girders, beams, or any other large vertical products may have side forms to be removed before the product is freed from the forms.
Materials handling operations are performed throughout all phases of the production of precast concrete products. In fact, throughout the industry, manual handling is necessary to perform many of the task required in the production process including:
- Building and locating from and
- Placing and tying reinforcing steel
- Hand batching concrete
- Transporting moist concrete by
- Form stripping
- Housekeeping and general cleanup
- Handling and moving products
Industry makes use of a variety of mechanical devices to assist in handling and movement operations of heavy precast concrete products such as hoists, cranes and forklifts.
- Floor slab connection essentially consists of a compression joint in the bearing surface. The bearing length should be atleast 75mm to avoid spalling of the edges.
- To prevent this short of failure occurring, the slab must be tied to the beam.
Connections on Hollow Core Slabs
- The problem with hollow core slabs is that there is no projecting reinforcement in the slab, so a frictional tie is required. The tops of some of the cores are removed and are made ready to receive the site bars which are projecting form the beam. Finally the gap at the ends of the slab and the beam is filed so that there is a positive tie between the two components.
Connections in Double Tee Slabs
- The tie force in the double tee slabs is provided in the temporary situation by welding two plates together. This tie prevents the beam and the slab separating during the in-situ concrete screeding operation.
- First, a rubber bearing is placed at the support point. When the slab is in position, a welded connection is made. Reinforcement is placed onto the top of the slab. This should lap with the projecting bars. Finally, in-situ concrete is poured.
Other Examples of Connections:
Column to Column Connection
Beam to Column Connection
Beam to Slab Joints Connection
Wall to Slab Connection
Precast concrete structure consisting of solid wall panels and hollow core slabs
Installation of Precast Structures
1. Setting Out
1.1 Set reference line and offset line
1.2 Provide level pads for setting the level of elements.
1.3 For precast external wall/column, fix the compressible form or bracker rod on the outer perimeters of wall.
2. Lifting and Installation
2.1 Lift and rig the panel to it designated location with the use of ropes.
2.2 Adjust the panel to position and secure it with the diagonal props.
3. Grouting Work
3.1 Prepare and apply non-shrink mortar to seal the gaps along the bottom edge of the inner side of the panel.
3.2 For corrugated pipe sleeve or splice sleeve connection, prepare and pour non-shrink grout or proprietary grout into the pipe inlets provided.
3.3 Keep the installed panel undisturbed for at least 24 hours.
4. Joint Casting and sealing
4.1 For panels with cast in-situ joints, install the joints rebars as required.
4.2 Set up forms for the casting of the vertical joint.
4.3 Carry out concrete casting
4.4 Remove forms after sufficient concrete strength has been achieved.
4.5 For joints between facade walls or between external columns with beams or wall elements, approved sealant and grout will be installed at later stage.
4.6 For panel with welded connection, place the connecting plate between the panels and carry out welding as per design requirement.
1. Setting Out
1.1 Set the reference line and offset line to determine the required alignment and level of the precast slab/beam elements during installations.
2. Lifting and Installation
2.1 Put temporary props to support the precast slab/beam elements.
2.2 Lift and rig the elements to designated location with the use of wire ropes.
2.3 Align and check level to suit the required setting out before placement of precast members to final position.
3. Casting of joints
3.1 For components with cast in-situ joints, place and lap the rebars as required.
3.2 Set up the formwork for the casting of the joint.
3.3 Carry out concrete casting.
3.4 Remove forms after sufficient concrete strength has been achieved.
Manufacturing precast Cement and Concrete units
Lift Slab Systems
- partially precast in plant (pillars)/ partially precast on-site (slabs).
- one or more storey high pillars (max 5).
- up to 30 storey high constructions.
- Special designed joints and temporary joints.
- slabs are casted on the ground (one on top of the other) - then lifted with crane or special elevators
Advantages and Limitations of Precast Concrete Units
Inherent Fire Properties
Health & Safety
Reduced Construction Programme
Greater Project Control
Larger Clear Spans
Very Heavy Members
Joints between panels
Cranes are required to lift panels
Need Bracing during on-site erection of structure
Cannot be used in two-way structural system
Limited building flexibility
Very small margin of error