DESIGN

REASONS FOR THE CHOICE OF HOLLOW CORE FLOORS


There are many reasons why hollow core slabs have met with such a warm reception and have spread to all continents. It can rightly be defined the most "cosmopolitan" of prefabricated components in the building industry worldwide.
Among the many advantages they offer, three are especially important:

Reasons for the choice of hollow core floors

Hollow core slabs are produced in well-equipped, up-to-date plants using advanced technologies requiring little labour. They are produced on casting beds, usally steel, and made with slipform machines or by extrusion. Concrete batching plants with automatic control of weights and the water/cement ratio and, almost universally, equipment for the hot curing of concrete in controlled conditions of temperature and humidity are the other essential components. Thus, the production of hollow core slab floors has always been accompanied by continuous quality control.

Technically, this means that:
- concretes are made with selected aggregates and with controlled grain-size curves which are particularly constant in time, with a low water-cement ratio, well-compacted and with high physical and mechanical characteristics;
- prestressing tendons possess certified strengths and characteristics of relaxation and constantly controlled concrete cover, and are thus well protected from aggressive outside elements and fire. The compactness of the concrete, the low water/cement ratio and the integral prestressing of the section, besides inhibiting cracking, also greatly slow down the velocity of concrete carbonation, thus assuring durability and allowing its use even in highly aggressive environments so long as standard concrete cover is assured.

The class of concrete also guarantees a high elasticity modulus, equal to at least 1.3 - 1.5 times that of concrete normally cast in situ. It follows that installed floors are quite rigid and show very slight elastic-deflection under loads applied during inspection.
For this reason it is possible to install thinner slabs for the same span and loads compared to floors that are similar but not entirely prefabricated and prestressed.


No ends of steel reinforcement protrude from prestressed hollow core slabs for connection to surrounding structures in cast concrete.

Such indispensable connecting reinforcement is inserted in situ in the longitudinal joins or in specially provided open cores at the ends, of suitable number and lengths, for joining the set in row slabs.

These efficacious connections with adjoining structures, which make the entire floor monolithic, allow the use of hollow core slabs in all structural applications, even in seismic areas, and together with all kinds of bearing structures, whether cast in situ, precast or steel. 


Economic advantages

There is a substantial reduction in building times and thus large savings in machinery and labour.

In fact, labour is kept to a minimum at all stages of production, stocking, transport, erection and completion of the finished floor at the site.

This very low incidence of labour provides the user with a substantial economic advantage, but requires the producer to make large capital investments and employ qualified personnel, since the entire manufacturing process is characterized by a very high technological content so as to guarantee high yield in a continuous cycle and at the same time maintain the high quality standards required by product codes.

Versatility in application


 

The positive union of hollow core slab floor and reinforced concrete beam cast in such a way as to englobe the slab ends, led to unexpected developments in applications and to the generalized use of hollow core floors in all kinds of buildings.

Up to the 1970s hollow core floors were used almost exclusively with the simple support on steel beams, precast concrete beams and bearing walls. They were often used as the simple covering of prefabricated industrial sheds.


Today, hollow core slabs of large depths (even one meter) allow construction of floors with spans up to 20 metres under industrial loads, no longer with simple support, but with restraints of structural continuity and even perfectly fixed ends. Further advantages of these slabs come from the possibility of their use as a clear span between beams cast in situ having the same depth as the floor.


These possible applications have favoured the adoption of hollow core floors in underground construction works where it is of primary importance for the structure to be monolithic.




The great versatility of hollow core slabs allows their use not only as floors, but also as walls of tanks for hydraulic plants, as earth retaining walls in civil and road works and efllcaceously as external and bearing walls for civil and  industrial buldings of all heights.

Numerous examples of multistorey buildings advantageously erected with such bearing walls demonstrate that all the possible uses of this very special precast element still have not been exploited fully. Its development worldwide must therefore be considered as still at the beginning; the future will certainly see its use in applications that have not yet been conceived.


Hollow core slab floors in a multistorey
underground parking garage

The hollow core walls of a water
treatment tank


Hollow core bearing walls and floors in a 
multistorey residential building


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