Autoclaved Aerated Concrete

By Robert L. Nelson, Ronald E. Barnett, P. E.

Autoclaved Aerated Concrete (AAC) is a masonry building material suitable for small residential construction, multi-story buildings, and commercial and industrial buildings. Although AAC has been used as a building material for over 60 years in most industrialized nations, after brief trials in the 1950's and 1970's it has yet to gain popular acceptance in the United States and Canada.

It was developed by the Swedish architect Johan Axel Eriksson and was patented in 1924. Eriksson was looking for a building material which had the properties of wood - good thermal insulation, solid structure, easy to work with and handle - but without the disadvantages of combustibility, decay, and termite damage. At that time there were only small, heavy building blocks with poor insulation properties. He succeeded in producing a highly cellular, lightweight material from sand, cement, lime, and water.

The natural materials--sand, lime, and water--can be found in almost unlimited quantities throughout the world. These raw materials are then processed to provide a building material with a large number of air pores - aerated concrete. And it is precisely these pores plus the solid structure of calcium silicate hydrates which, while having a low material weight, give AAC its exceptional product properties: excellent thermal insulation against high and low temperatures, ease of workability and handling, universal loadbearing and non-loadbearing applications, non-allergenic, and efficient construction.

AAC is an environmentally-friendly and energy-conserving material. Pollutants or hazardous wastes are not generated in the manufacturing process and precious raw materials are conserved. When unused, cured AAC is available, it can be pulverized into a powder and substituted for the sand constituent in the beginning mixture, thus minimizing waste. AAC can even be recycled, for example, as an oil absorbing granular material, cat litter, or as a flue gas purification product. Since AAC does not contain any toxic substances, it is not harmful to the ground water or vegetation.

Products manufactured with AAC come in many shapes and sizes. Building blocks, solid reinforced lintels, "U-shaped" lintels, non-loadbearing wall panels, loadbearing wall panels, and reinforced floor and roof panels are the standard components produced. These components are used to construct residential, commercial, industrial, and public buildings. The individual components are uniform in size, shape and function, and extremely versatile in their use, in their ability to be shaped and worked with, and in their ability to be assembled into the desired building configuration.

As with any building product, the material characteristics and properties must be known and documented before it can be widely used. Since AAC provides so many different features to a building, research and testing is currently underway in many diverse areas. Material properties of AAC, such as strength characteristics, fire resistance, thermal behavior, and sound transmission reduction are well known since extensive worldwide research has been conducted over many decades. However, research is underway in the United States to confirm and correlate those results into local building codes based on American standards and tests.

Currently, research to determine and confirm many properties such as flexural and shear strengths of various wall construction assemblies, compressive strength of prisms, flexural bond strength and absorption are ongoing at the University of Texas at Arlington under the direction of John H. Matthys, Ph.D., P. E., and at Robert L. Nelson & Associates, Inc. Construction Materials Laboratory in Schaumburg, Illinois under the direction of Robert L. Nelson.

A Southern Building Code Congress International approved laboratory has performed fire rating tests in accordance with ASTM E119 for various wall assemblies. More testing and research is ongoing as it relates to the use of AAC as fire partition assemblies in commercial and industrial applications.

The Florida Solar Energy Center in Cocoa, Florida under the direction of Dr. Subrato Chandra, P. E., is currently performing studies to determine the thermal characteristics of AAC and how they relate to the energy codes in the United States. The material characteristics of AAC provide "integral" insulation throughout the material which is unlike conventional products with "added" insulation. Provisions for the correct inclusion of AAC into the energy codes is a critical part of this research.

Acoustical testing is also currently being performed to determine Sound Transmission Coefficients (STC) for wall assemblies such as single and double wythe AAC walls. It has been found in Europe that AAC provides a sound insulation value of 7 dB greater than other solid building materials of the same weight per surface area. Research currently being conducted will determine the STC ratings for several typical wall assembly installations.

AAC has been found to perform very well even during seismic events. AAC has been successfully used in many countries prone to severe earthquakes such as Japan and Mexico since the low material weight of AAC during a seismic event minimizes the forces developed by the inertia of the building mass. Research has started in the United States to determine the seismic characteristics of AAC as they relate to the national and local building codes.

As can be seen, the research and testing of AAC in the United States must be diverse and is only in its infancy. Additional research and testing, both short and long term, will continue for many years to come in order to fully understand the many beneficial properties of Autoclaved Aerated Concrete.