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Updated: Mar 28, 2021

Deadweight! That is the problem! Everybody knows that concrete is quite heavy (how much? Find it out in my post “Concrete weight”) let´s say way over 2000 kg/m³…so what to do when it comes about raising an existing structure that maybe is even a bit old, or refurbish slabs and roof tops in renovations and restorations of historical buildings, or realizing cantilever balcony and so on?

The answer is “Structural light weight concrete”. Their weight ranges from 1600 kg/m³ to 2000 kg/m³ with compression strength Fck from 16 N/mm² to 45 N/mm² (Rck 18-50 N/mm²).

This particularly light weight concrete is obtained replacing the gravels (and sometimes even the sands) with structural expanded clay.

This special kind of aggregate is produced by burning clays at about 1200 °C transforming it in clay clinker. The result is an aggregate with an almost sphere shape constitutes by closed and vitrified cells. Despite its high rigidity and non-deformability, it is a particularly light material, chemically inert, refractory to high temperatures and with interesting thermal resistance values.

It is possible to design several mixes with different workability, up to S.C.C. (please read my posts “Self compacting concrete” and “Self compacting concrete checks and tips”); different Fck/Rck values, different exposition class (more on “Concrete durability”). Structural light weight concrete is for sure the solution when it comes about weight reduction, but there are pros and cons to evaluate.

Some pros:

  • Weight reduction around 30% (that´s a big help also for constructions in seismic zones)

  • Lesser thermal conductivity

  • Superior fire resistance

  • Better sound absorption

  • Better behavior under freeze and defreeze circumstances

  • Better durability to carbon dioxide and marine mist aggression

Some cons:

  • Lesser ductility in comparison with regular concrete

  • Lesser bending and tensile strength (around 10-20%)

  • Not possible to have low value of permeability

  • The higher the strength value, the higher the weight

It is good to remember that structural light weight concrete is one of the best allied in case of new buildings sitting on soils with poor bearing capacity, and whenever the building site is located in an area where the road load is limited.

Just a little tip: when compacting structural light weight concrete, remember that vibration propagates lesser than in regular concrete, so never indulge in vibration rather be fast and immerge the vibrating needle more often to have a good result without bleeding and/or segregation (more details on my posts “Concrete compaction”, “Concrete segregation”, "Concrete bleeding" and “Concrete quality is a team work”).

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