Precast Concrete

Precast Concrete

Structural precast concrete units are used to form the structural system of a building. These include columns, beams, girders, floor and roof slabs, and exterior and interior wall panels. They are cast under factory – controlled conditions and moved to the job site.

How is precast concrete made?Precast concrete is made in a factory, where a dedicated batch plant

produces a specially designed concrete for precast products such as structural beams, columns and double tees, architectural cladding, and wall systems. Aggregates usually come from nearby quarries, and cement and other ingredients are often supplied by local manufacturers.

The mixed concrete is placed into a form around reinforcement and, often, prestressing strands that provide load-resisting camber to the finished precast concrete member. After the member is cured, the precast concrete product is stripped from the form and moved to the precaster s yard for finishing and storage prior to shipping to the jobsite.

Precast Concrete is a construction product produced by casting concrete in a reusable mould or "form" which is then cured in a controlled environment, transported to the construction site and lifted into place. In contrast, standard concrete is poured into site-specific forms and cured on site. Precast stone is distinguished from precast concrete by using a fine aggregate in the mixture so the final product approaches the appearance of naturally occurring rock or stone.

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. Utilizing a Precast Concrete system offers many potential advantages over site casting of concrete. The production process for Precast Concrete is performed on ground level which helps with safety throughout a project. There is a greater control of the quality of materials and workmanship in a precast plant rather than on a construction site. Financially, the forms used in a precast plant may be reused hundreds to thousands of times before they have to be replaced which allow cost of formwork per unit to be lower than for site-cast production.

The concept of precast (also known as “prefabricated”) construction includes those buildings where the majority of structural components are standardized and produced in plants in a location away from the building, and then transported to the site for assembly. These components are manufactured by industrial methods based on mass production in order to build a large number of buildings in a short time at low cost. The main features of this construction process are as follows:

• The division and specialization of the human workforce• The use of tools, machinery, and other equipment, usually

automated, in the production of standard, interchangeable parts and products

This type of construction requires a restructuring of the entire conventional construction process to enable interaction between the design phase and production planning in order to improve and speed up the construction. One of the key premises for achieving that objective is to design buildings with a regular configuration in plan and elevation.

In general, precast building systems are more economical when compared to conventional multifamily residential construction (apartment buildings) in many countries.

Precast Concrete Products

The following is a sampling of the numerous products that utilize precast/prestressed concrete. While this is not a complete list, the majority of precast/prestressed products can fall under one or more of the following categories:

Agricultural Products Precast concrete products can withstand the most extreme weather

conditions and will hold up for many decades of constant usage. Products include bunker silos, cattle feed bunks, cattle grid, agricultural fencing, H-bunks, J-bunks, livestock slats, livestock watering troughs, etc...

Building and Site Amenities Precast concrete building components and site amenities are used

architecturally as cladding, trim products, accessories, and curtain walls. Structural applications of precast concrete include foundations, beams, floors, walls and other structural components. It is essential that each structural component be design and tested to withstand both the tensile and compressive loads that the member will be subjected to over its lifespan.

Retaining Walls Precast concrete provides the manufacturers with the ability to

produce a wide range of engineered earth retaining systems. Products include: commercial retaining wall, residential retaining walls, sea walls, mechanically stabilized earth (MSE) panels,

modular block systems, segmental retaining walls, etc. Retaining walls have 5 different types which include: gravity retaining wall, semi gravity retaining wall, cantilever retaining wall, counter fort retaining wall, and buttress retaining wall.

Sanitary and Storm water Sanitary and Storm water management products are structures

designed for underground installation that have been specifically engineered for the treatment and removal of pollutants from sanitary and storm water run-off. These precast concrete products include storm water detention vaults, catch basins and manholes.

Transportation and Traffic Related Products Precast concrete transportation products are used in the

construction, safety and site protection of road, airport and railroad transportation systems. Products include: box culverts, 3-sided culverts, bridge systems, railroad crossings, railroad ties, sound walls/barriers, Jersey barriers, tunnel segments and other transportation products. Used to make underpasses, surface-passes and pedestrian subways, so that traffic in cities is disturbed for less amount of time.

Utility Structures For communications, electrical, gas or steam systems, precast

concrete utility structures protect the vital connections and controls for utility distribution. Precast concrete is nontoxic and environmentally safe. Products include: hand holes, hollow core products, light pole bases, meter boxes, panel vaults, pull boxes, telecommunications structures, transformer pads, transformer vaults, trenches, utility buildings, utility vaults, utility poles, controlled environment vaults (CEVs) and other utility structures.

Water and Wastewater Products Precast water and wastewater products hold or contain water, oil or

other liquids for the purpose of further processing into non-contaminating liquids and soil products. Products include: aeration

systems, distribution boxes, dosing tanks, dry wells, grease interceptors, leaching pits, sand-oil/oil-water interceptors, septic tanks, water/sewage storage tanks, wet wells, fire cisterns and other water & wastewater products.

Precast Concrete Units

Precast units are cast in beds, which are permanent forms made of

metal, wood, or fibreglass. The quality of the surface of the form produces the finished surface of the precast unit.

It can be classified into two major groups – precast structural concrete units and precast architectural units.

Typical precast structural units include floor and roof slabs, beams, girders, columns, and wall panels.

Precast architectural concrete units include wall panels and other features. These may or may not be structural.

Advantages and Disadvantages of Precast Concrete UnitsPrecast construction is widely used because it affords a number of

advantages when compared with cast – in – place concrete.

Advantages:1. Casting takes place in computer – controlled facilities where an

experienced crew produces units under close supervision;2. The control of materials, mixing, and placing the concrete is rigid,

producing higher quality concrete. Samples of the concrete are taken regularly and tested per ASTM standards. Typically, 5000 psi (35 MN/m3) concrete is used;

3. Units are finished and cured under carefully controlled conditions the concrete is mechanically vibrated in the form and cured. To speed up production, the units are covered (autoclaved);

4. Forms are on the ground, making the placing of steel reinforcing, casting, and curing faster and easier;

5. The forms are used over and over, reducing these costs;6. Generally, precast units can be smaller and lighter than cast – in –

place units for the same span and loads;7. Members can be cast and stored until needed, speeding up the

erection process. They are erected with a crane, much the same as structural steel;

8. Inclement weather does not slow down precast construction as easily as it does cast – in – place jobs.

After the units have achieved adequate strength, they are delivered to the job site, usually by truck. A limiting factor is that their size is determined by what can be moved by the truck or railroad. Highway regulations limit load sizes and weights. Extra – large precast units can be cast by moving the forms to the job and casting them on the site.

Limitations:1. Very heavy members;  2. Camber in beams and slabs;  3. Very small margin for error ;4. Connections may be difficult;5. Need bracing during on-site erection of structure;

6. Somewhat limited building design flexibility.

Nonprestressed and Prestressed Precast Concrete Units

Nonprestressed units are cast in molds in a plant, then cured and shipped to the job site. They are reinforced in the same manner as cast – in – place concrete and are not under tension forces. Some large or unusual beams, girders, and many columns and lintels are cast this way

.Prestressed concrete units have

stresses introduced before they are placed under a load. There are two types:

Pretensioned and Posttensioned.

a) Pretensioning is used to produce units that are standardized in size or when enough identical units are needed to make it economically possible to cast them.

b) Posttensioning involves applying stresses to the concrete unit after it has been cast and hardened. It is used primarily with cast – in – palce concrete but finds some use with precast concrete units.

A comparison of nonprestressed and prestressed concrete shows that the nonprestressed unit has no camber (arch) and deflects slightly under load, but the pretensioned member has some camber due to the tension in the strands. When under a load, a prestensioned member tends to move

toward a level position. the stressed strands produce high compressive tresses in the lower part of the unit and a tensile stress in the upper part. As the unit is loaded these stresses are reduced.

Precast Concrete Slabs

The major types of slabs used for floor and roof construction are solid flat slabs, tongue – and – groove planks, channel slabs,

hollow core, double tees, and single tees. Some are prestressed and others precast with untensioned and reinforcing.

Solid slabs and channel slabs are used for short spans and minimum slab depth.

Hollow core slabs are used for spans ranging up to 40 ft. in length and are 6 to 12 in. thick.

Double tees and single tees are used to span the longest distances.

Tees are cast with a rough or smooth surface on top. The rough surface is used when a concrete topping is to be applied over them. Structural continuity across the units can e provided by placing steel reinforcing bars in the topping.

Normal and light weight concrete are used to cast these units. Lightweight concrete is more expensive than normal weight, but it reduced the weight on the structure. Since double tees do not require temporary support to prevent tipping, they are easier and more economical to erect than single tees.

Precast Concrete Columns

Precast columns are usually combined with precast concrete beams, forming a post – and – beam structural framework. Most precast concrete columns are reinforced conventionally. If they are prestressed, this is mainly to reduce stresses on the column during transportation and handling.

Beams and Girders

Precast prestressed concrete beams and girders can be used in any building in which precast construction is desired. They are made in several standard shapes, and special designs can be

produced. The L – shaped and inverted tee beams provide a bearing surface for precast floor units, such as single and double tee units. This allow lower

overall height than does resting floor and roof units on top of rectangular beams.

Precast concrete Wall Panels

Precast concrete wall panels may be prestressed or conventionally reinforced. Their design varies considerably. They may be used in connection with a precast concrete framing system, cast – in – place concrete, or a steel framing system.

The designation “large-panel system” refers to multi-storey 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. These panels form a box-like structure.

Both vertical and horizontal panels resist gravity load. Wall panels are usually one story high. Horizontal floor and roof panels span either as one-way or two-way slabs. When properly joined together, these horizontal elements act as diaphragms that transfer the lateral loads to the walls.

Depending on the wall layout, there are three basic configurations of large-panel buildings:

• 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; usually, there is only one longitudinal wall, except for the system with two longitudinal walls.

• Two-way system. The walls are placed in both directions.

Types of connections

The types of connections include bolt connections, welded connections, posttensioned connections, and doweled connections.

a) Bolt connections speed up erection and allow final alignment to be made after the crane has placed the member.

b) Welded connections are strong and easy to make on the site. These are made before the unit is released by the hoisting device. They should be made following the details on the erection drawings, which include the size, type, length of the weld, type of electrode, and required temperatures.

c) Posttensioned connections are made using either bonded or unbounded tendons. Bonded tendons are installed in holes cast in the member and are bonded to the member after tensioning by filling the hole with grout. Unbounded tendons are not grouted in place ut are coated with an organic cao==oating to prevent corrosion.

d) Doweled connections use reinforcing bars grouted into dowel holes in the precast member. The strength of the connection depends on the dowel diameter, the depth it is in the member, and the bond developed by the grout.

Grout, Mortar, and Drypack

Grouts and mortars are used to transfer loads between members. They are also used to fill noncritical voids. Load – bearing grouts should be nonshrinking, and grouts manufactured for this specific purpose should be used.

Drypack describes a method of placing grout. The dry grout material has only enough water added to produce a stiff granular mix, which is packed into an opening.

Erecting Precast Concrete

1. Planning After the structural design work

has been completed and reviewed, the plans go to the company that will cast the unit. Here they plan the work using standard moulds when possible and building special moulds as needed.

2. Transporting units to the site Deliveries of precast members

are usually made by truck. This requires the shipper to do some

careful planning. The distance of the job, the type, size, and weight of the member, the type of vehicle required the weather, road conditions, and other such factors. Must be considered.

3. Rigging Refers to the equipment, cables, slings, and other hoisting

equipment used to lift the precast units into place. This is the work of specialists.

Prestressed members must never be lifted by their center only, and they should not be lifted or stored upside down. They must be kept in the upright position in which they will be installed. If lifting devices are not in the member, it can usually be safely lifted by slings placed near each end.

4. Placing Precast Units

The foundations and footings are cast – in – place and carefully located. Tolerances are rather tight, and assembly will halt if tolerances have not been carefully maintained.

The structure may be built using precast columns and beams. The columns are lifted and placed with a crane, then plumbed and grouted with drypack grout.

Columns are joined by beams and the floor decking is set in place. Floor decking often is hollow core units.

5. Framing Plan The specific way these are drawn

varies but the information shown is consistent. Single or double tees also are often used for floor decking. Double tees are preferred to single tees because they do not require bracing during erection.

Some buildings use precast structural walls instead of columns. These are erected on foundations and braced until members abutting them in place.

If precast exterior wall panels are used to enclose the building, they

are lifted in place and secured to the structural frame.