Moisture content

Shortly after manufacture, the moisture content is usually in the range of 25 - 40 percent by mass of the dry material, the lower the density the greater the initial moisture content. With time, however, the material gradually dries out and an equilibrium moisture content of 4 to 6 percent by mass is reached in about 1 to 2 years. Measurements of moisture in typical ACCOA AAC structures in service indicated that the moisture content in 90 percent of the cases was less than 5 percent by mass.

Pore structure

The pore structure of ACCOA AAC has a direct effect on the physical properties of the material such as the strength, thermal conductivity, and freeze-thaw resistance. Pores present in ACCOA AAC can be divided according to size into micro-pores (< 1 mm) and macro-pores (0.1 to 1.0 mm), or what are referred to as structural pores and artificial pores respectively. Of the total porosity or pore volume, approximately 35 percent is micro-pores and the remainder 65 percent is macropores.

Freezing and thawing

Due to the pore structure of the material, ACCOA AAC has good resistance to freezing and thawing as demonstrated by field performance of untreated (uncoated) ACCOA AAC in severe environments. The macro-pores, which are spherical and large in size, are almost closed and not connected. These pores possess negligible capillary suction. Capillary suction takes place only through the micro-pores. Hence the rate of water penetration through ACCOA AAC is very low.

Shrinkage

Loss of moisture in ACCOA AAC results in drying shrinkage. The average shrinkage for ACCOA AAC is 200x10-6 as compared to 500x10-6 for ordinary concrete. The lower shrinkage for ACCOA AAC is attributed mainly to the high pressure steam curing (autoclaving) that the fresh material is subjected to during production.

Air Tightness

Even with normal construction procedures, ACCOA AAC buildings have excellent air tightness qualities. Tests on ACCOA AAC buildings measured 1 to 2.5 air changes per hour at a reference pressure of 50 Pa. This compares favorably with other types of construction, such as wood frame or concrete block, where normally the average air change rate at 50 Pa is about 7.

Thermal expansion

The coefficient of thermal expansion of ACCOA AAC is about 8x10-6/°C, which is about 20 percent less than that of normal weight concrete. Due to the thermal insulation properties of ACCOA AAC, it is necessary in calculations of thermal effects to account for the time lag between air temperatures and concrete temperatures. The ACCOA AAC unit will not simply elongate or contract under temperature changes but will exhibit a camber towards the side with increasing temperature.