PRECAST CONCRETE



Precast Concrete

In 1950, the completion of the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania,US signaled the beginning of the precast concrete industry in North America. Virtually unknown in the United States until the construction of this prestressed concrete bridge, today  precast concrete structures, including bridges, are common place every where  in the world.

Now even the Precast concrete is widely used in low- and mid-rise buildings also. The concrete provides superior fire resistance and sound control for the individual units also.

Precast concrete is also a popular material for constructing buildings. The walls of the building can be manufactured while the on-site foundations are being built, providing significant time savings and resulting in early occupancy.

The speed and ease with which precast structures can be built has helped make precast a popular building material for parking structures. Precast concrete allows efficient, economical construction in all weather conditions and provides the long clear spans and open spaces needed in parking structures. For stadiums and arenas, seating units and concrete steps can be mass produced according to specifications, providing fast installation and long lasting service. In addition, pedestrian ramps, concession stands, and dressing room areas can all be framed and constructed with precast concrete.

The smooth surfaces produced with precast concrete and the ability of precast, prestressed concrete to span long distances makes precast suitable for use in manufacturing and storage structures. Additional applications for precast concrete include piles and deck for railroad and highway bridges, railway crossties, educational institutions, commercial buildings such as shopping malls, and public buildings including hospitals, libraries, and airport terminals.

A benefit of precast concrete is that the product is created in ideal manufacturing conditions. Although some products are cast outdoors, especially in temperate climates, many precast plants operate indoors where the climate can be fully controlled.

There are two types of precast products. Standard products such as beams, decks, and railroad ties are shaped in one way and used repeatedly. The other type of product is a specialty product, designed especially for the building, bridge, or structure where it will be used. Most precast companies have their own carpentry shops where skilled workers create forms for the many specialty-precast products available. Architectural concrete is often cast specially for each new project.  

The forms, whether standard or specialty, are well oiled. Concrete is placed in the forms and allowed to cure. After curing, the product is carefully lifted from the form and taken to a yard for further curing before it is shipped to the project site. The form is then carefully cleaned and prepared for the next batch of concrete.

Advantages of Precast Concrete in Constructing Highways & Bridges:

Prefabrication of any structure component off-site during Highway construction (or reconstruction) offers major time and user cost savings in comparison with the traditional cast-in-place methods of construction. Precast prestressed road pavements’ technology offers dramatic increase in durability, while substantially decreasing construction time and cost. Precasting bridge parts and elements offsite is also very beneficial as bridges are generally among the most expensive objects constructed. It also brings substantial safety advantages, lowers disruption to traffic and increases overall convenience for the road users.

But this technologically, economically and environmentally advantageous approach also has its opponents – prevailingly from the contractors’ side due to the inability to create every project unique and so with higher possible mark-up. The objective is to analyze and appraise the advantages and benefits of the innovative prefabrication approach in contrast to traditional cast-in-place construction method, and to come up with a set of conclusions and recommendations for the general practice. The current aging highway infrastructure  in general is being used by increasing volumes of road traffic. For example, the volume of traffic on the most important Highways has almost doubled between the years 2010 and 2017, it is steadily becoming a reality in every country across the world.

Advantages of precasting over cast-in-place:

Since its use dating back to the ancient days, precast concrete technology has developed considerably - from the composition of the concrete mix (by adding steel to increase tensile strength) to new ways of casting and curing. The main advantages of precast concrete over cast-in-place (placement of pavement or object in-situ) are as follows:

Precast pavement panels or bridge elements can be cast and cured in a controlled environment at a precast plant, providing greater control over consistency of the concrete mix, procedures of vibration and proper curing. Precast objects reduce or eliminate curling and air-entrainment problems that are common with conventional concrete paving. Importantly, precast objects are also prestressed during their plant production and post-tensioned during their installation, which is not possible for the cast-in-place.

Prestressing in the plant significantly improves performance (most importantly the tensile strength) by inducing compressive stresses in the panels. This effectively prevents cracking and strengthens them for better onsite manipulation, preventing them from bending and torqueing. This durability cuts maintenance costs over the life of the roadway and user costs through reduced repair cycles.

As proved by the testing and suggested  post-tensioned
  panels provide the same design life as thicker conventional concrete pavements, using thinner slabs, which means that a 200mm post-tensioned pavement would have the same life as that of a 355mm conventional pavement. Post-tensioning also increases durability by minimizing or even eliminating cracking as it ties the individual panels together, promoting load transfer between the panels. Additionally, post-tensioning not only reduces the required pavement thickness, but also greatly increases durability, lessening or even preventing cracking in the pavement. This increases the life of the pavement with reduced maintenance.

Precast concrete pavement systems:

In precast concrete pavement construction, adjacent panels are assembled sequentially and tied together onsite through post-tensioning or cast-in-load transfer systems. Precast concrete pavement systems can be used for single-lane replacements, multiple-lane replacement (an additional lane may be needed to accommodate materials and equipment) or full-width road construction . Using the precast concrete parts for the road’s pavement speeds up the construction time two to three times against the traditional cast-in-place, if well organized. Such a pavement can be used by vehicles the very next day after installation as no further curing of the newly laid road surface is necessary.

Accelerated Bridge Construction:

Precast technology is applicable for both replacement/rehabilitation of existing bridges and construction of new ones. Accelerated Bridge Construction (ABC) concentrates on the innovative planning, subordinates design to the technology, uses resources and techniques to accelerate bridge construction and maintenance without compromising their safety.

As with the precast pavements, it is mainly about cutting construction time and increasing durability using higher quality factory-made concrete. So, it makes sense to precast most of the bridge elements such as the pile, pier column, pier cap, beam, deck and barrier/railing as well, i.e. all but the column’s footing, which is usually cast-in-place. The construction cost depends on the quantities, which being high, turn the economics in favor of prefabrication, unification and standardization. Regarding user costs, which might easily exceed those of construction if limiting the high traffic areas for a long time, precast is a clear winner.

Further:

  • Faster pace of construction to cut time of traffic flow limitation
  • Higher and stable quality due to better controlled production process in a dedicated facility and benefits of pre- and post-tensioning
  • Long-term durability due to higher quality materials used
  • Less frequent maintenance of critical parts of the highway pavements and bridges
  • Economy of scale due to mass production of all standardized elements (mainly pavement panels and bridge elements).

Source:SRINIVAS KASA

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