AAC autoclaved aerated concrete fire resistance

Autoclaved aerated concrete does not support combustion and does not spread fire. According to the manufacturer of autoclaved aerated concrete "H+H" autoclaved aerated concrete 20 cm wall can work as fire-resistance wall. Stability of autoclaved aerated concrete to fire depends on AAC density.2 Replacement of portland cement to pozzolan cement (with additions of milled volcanic minerals, caolinitic clay or fuel ash)increases fire resistance of autoclaved aerated concrete.3 Heat tests of autoclaved aerated pozzolan cement based concrete demonstrated no blocks deformations, cracking, melting, falling or sliding at temperatures up to 750°C. Heat tests of autoclaved aerated portland cement based concrete showed surface block cracks at temperature 700°C.4

The autoclaved aerated concrete blocks homeowner should be aware of AAC compressive and splitting tensile strength. What will happen to the AAC compressive strength after the real house fire?
Let's refer to the results of an experimental study on the residual compressive and splitting tensile strength of autoclaved aerated concrete (AAC) subjected to high temperatures. Fire resistance test was done to find out effects of different burning temperatures on the srength properties of autoclaved aerated concrete block samples. For this purpose AAC 50mm cubes were tested at six different temperatures by using an electrical furnace. After fire resistance test, compressive strength test was done to detect effects of fire on strength properties of AAC samples.4

Table #1

Fig #1: Heat test AAC color changes (Image courtesy of S. Somi, 2011).

After 30 minutes heating procedure at a temperature of 100°C no changes were observed on the appearance of AAC blocks and also no reduction in weight was detected. On the
other hand average compressive strength of blocks after fire test was about 2.074MPa which shows a slight decrease comparing to compressive strength of dry state.
After 30 minutes heating procedure with temperature of 300°C no changes were observed on the appearance of AAC blocks but slight reduction in weight of blocks was detected. On the other hand average compressive strength of blocks after fire test was about 1.809 MPa which shows more decrease in compressive strength comparing to lower temperature heated samples.
After 30 minutes heating procedure at the temperature of 500°C, color of AAC blocks was observed to become darker, and slight reduction in weight of blocks was also detected. On the other hand, average compressive strength of blocks after fire test was about 1.65 MPa which shows more decrease in compressive strength compared to lower temperature heated samples.
After 30 minutes heating procedure at the temperature of 700°C color of AAC blocks was observed to become darker, also reduction in weight of blocks was detected. On the other hand average compressive strength of blocks after fire test reduced to 1.43 MPa which shows more decrease in compressive strength comparing to lower temperature heated samples.
After 30 minutes heating procedure with temperature of 900°C, changes in color from grey to light grey for AAC blocks, became more obvious and in addition to reduction in weight of blocks cracks also appeared on surface of blocks. On the other hand average compressive strength of blocks after fire test reduced to 1.23 MPa which shows more decrease in compressive strength comparing to lower temperature heated samples.
After 30 minutes heating procedure with temperature of 1000°C, color of AAC blocks became bright white and in addition to 0.006 kg reduction in weight of blocks, number of cracks on the surface also increased. On the other hand no compressive strength was obtained for these blocks after heating procedure at a temperature of 1000°C.

These findings are generally consistent with the results of fire resistance of autoclaved aerated concrete, conducted by the University of Technology in Bangkok (Thailand)6

Table #2

In this study, heated AAC blocks found a slight increase in the strength while heated up to 400°C. At the higher temperatures the autoclaved aerated concrete compressive and splitting strength falled down to zero (decrease in AAC strength 6 times or more) at temperatures above 800°C. Loss of strength of autoclaved aerated concrete at high temperatures occurs also in fiber reinforced autoclaved aerated concrete blocks and blocks with perlite replacement.

So can the fire damage and destroy autoclaved aerated concrete walls house?

To answer this question we need to know what the temperature are reached during a fire inside and outside buildings.

Table #3 "Standard fire"* temperature dynamics.
(According to the ISO 834 standards )7

*Standard fire - the empirical model used in the evaluation of fire resistance of structural elements of buildings.

In a real outdoor fire the equilibrium temperature (due to heat loss to the environment) of about 680 ° C is set.
In indoor real fire without gas exchange with the outdoor atmosphere and without the presence of gasoline and other combustibles temperature reaches 800-900°C.
In indoor fire with the gas exchange to outdoor atmosphere, or where high energy combustible materials are present temperature above 1000°C can be reached. The maximum temperature of an open fire for combustible gases is 1200 - 1350°C, for the combustible liquid is 1100-1300°C and for solid combustible organic material is 1100-1250°C.
Extinguishing a fire with water leads to rapid cooling the heated AAC. That cooling may lead to additional strength loss as compared to the cooling of the heated concrete blocks under normal atmospheric conditions.8

Thus, after a fire in the house made of autoclaved aerated concrete be sure to make an examination of the AAC wall blocks strength, which determines the carrying capacity of the aerated concrete wall by the material strength.

For comparison, the ordinary dense concrete can be deformed (with the formation of through-holes or through-cracks) after 5 - 20 minutes of indoor fire (with temperature range 400-700°C)9 In thin-walled reinforced concrete structures (40 - 200 mm thickness), hight heat leads to the formation of through holes and cracks. In walls thicker than 200 mm, overheating leads to spalling of the concrete layers, and formation of cracks.