Lightweight Concrete

Lightweight Concrete is a generic term that covers many types of concrete that are light by nature or manmade and typically range in density from 20 pcf to 120 pcf. The two most common ways to make a concrete mix lighter are by the introduction of lightweight aggregates such as shale, volcanic rock or pumice in place of standard solid rock aggregate and by introducing air into the concrete mix, commonly referred to as “air entrainment”. The most common type used in residential construction is air-entrained meaning air is introduced into cement slurry and agitated resulting in a honey-comb matrix of concrete and air voids. These Lightweight Concrete mixes typically produce approximately 1000 psi and were used extensively in the 1960s,70s and well into the 80s for fire and sound control in wood frame multi-family apartment homes, offices, medical facilities, and many other types of structures. This generic product was made using a foaming agent (soap type surfactant material) which is injected under pressure through a nozzle to create “foam” (microscopic air cells). The foam is introduced into a cement mixer and displaces aggregates and increases the volume of the concrete without adding weight, which creates lightweight concrete.

Although Lightweight Concrete was the standby product for floor underlayments 30 years ago, it also came with a host of issues. Building paper is stapled to the subfloor and expansion joints are installed off of the outside corners of framed walls to help control the direction of cracking as the product is pumped into place and finished. As the concrete cures it shrinks and cracks appear. The building paper helps to plug small holes and acts as a bond breaker between the concrete and the plywood to reduce cracking when shrinkage occurs. Slab curling is also expected at the expansion joints as the top dries quicker than the bottom. Although there were many drawbacks, overall Lightweight Concrete was a great new innovation for wood frame construction sound and fire control in the 1960s. Lightweight Concrete never was developed or intended for use as a thermal mass for radiant heating systems. Imagine ¾” tubing with only ¾” lightweight concrete cover over the tubes - as the lightweight shrinks the tubes act as the expansion joints and cracking develops. The air or honey comb matrix within the Lightweight Concrete has insulating properties which result in higher operating costs, less efficient heating systems and potentially a non warrantable substrate for finish floor goods.