Unfired clay bricks

Andrew Heath

Unfired clay bricks offer a cost-effective form form of construction of very low environmental impact. Dr Andew Heath of the University of Bath provides an overview of the technology and application.


Introduction


Also known as earth masonry, unfired clay brickwork is constructed using earth materials (possibly with some additives). Earth masonry is not “fired” like conventional bricks, but the masonry units are air dried after manufacture to reduce shrinkage and improve strength. In some traditional forms of earth construction (e.g. cob or rammed earth), monolithic (solid) walls are constructed, but unfired clay bricks are similar to other masonry systems where there the units (“bricks”) are bonded together with mortar and possibly covered with a finishing system (paint or render).

Traditional forms of unfired clay bricks (cob blocks, adobe and mudbricks) are generally made by hand and as a result, have variable dimensions and other properties. Traditional earth masonry has thick walls (often over 300mm thick) as the mortar provides low bond strength and the thick walls have sufficient mass to keep themselves stable against lateral loads in dwellings.

Because of the environmental and financial cost of using materials in construction, it is preferable to reduce the wall thickness to approximately 100mm for internal partitions (the standard thickness for fired clay bricks and concrete blockwork). Thinner walls also reduce the structural loading and increase available space inside buildings.

Modern unfired clay brickwork uses units manufactured to accurate tolerances using a commercial extrusion or pressing system to provide a consistent, high quality product. This enables rapid, cost effective, 100mm thick walls with low environmental impact to be constructed. In most cases, modern unfired clay bricks are produced in commercial fired brick manufacturing plants using similar materials to fired bricks, but without putting the bricks through the firing process. This significantly reduces the energy used in manufacture and previous research has indicated unfired bricks have 14% of the embodied energy of fired bricks and 25% of the embodied energy of concrete blocks. In Germany, some fired brick plants have moved to making only modern earth masonry and associated products.

 

Control of internal environment

Unfired clay brickwork has been shown to provide passive environmental control in buildings through buffering of the temperature in the building (through the provision of thermal mass), and through buffering relative humidity by absorbing moisture from the air at high humidity, and releasing it at low humidity. Buffering of temperature and humidity will normally reduce the energy required to operate buildings. To enable buffering of relative humidity, a specialist vapour-permeable render and paint are required. Gypsum plasterboard and non-permeable paints should not be used with unfired brickwork as they could lead to premature failure through build-up of water in the masonry.

The amount of moisture that will be absorbed by the walls in a 4 x 4 x2.4m high room with a 100mm wall thickness is illustrated in the figure below. As shown, the unfired brickwork can absorb significantly more moisture from the air than either concrete blockwork or fired brick masonry.

clay brick humidity

Moisture buffering capacity of earth masonry

Empirical evidence has show that unfired clay brickwork can buffer humidity to medium humidity levels (40-65% relative humidity), but further research is required to confirm and model this effect. If this is confirmed, it could have positive implications for occupant health: “The incidence of absenteeism or respiratory infections was found to be lower among people working or living in environments with mid-range versus low or high relative humidities.” (Arundel et. al. Indirect health effects of relative humidity in indoor environments. Environ Health Perspect. March; 65 pp 351-361 (1986)).

 

Strength of unfired clay brickwork

The compressive strength of unfired clay brickwork is much more complicated than for blockwork or fired clay bricks and no single strength value can be assigned. The strength of unfired brickwork is dependent on the material properties, the dimensions of the wall and the water content. The material property that influences the masonry strength more than any other is the clay content in the masonry units.

As the water content in the masonry units is increased, the strength decreases and it is therefore important to keep the masonry dry once constructed through appropriate detailing, such as provision of a fired masonry or blockwork plinth to prevent accidental wetting from spills. Further information on detailing is available in the books listed at the end of this factsheet. The water content will normally be highest during construction (from application of wet mortar and render), and will then stabilise to a lower level (stronger masonry) during use.

After construction and in the absence of any accidental wetting (through appropriate detailing), the water content will be controlled by the relative humidity in the air, resulting in the relationship in the figure below. It is worth noting that the humidity must be maintained at the level for a considerable period of time (a number of weeks) before the water content will stabilise throughout the masonry. Boiling a kettle or having a shower will have negligible effect on the strength of the masonry.

clay bricks and relative humidity

Effect of relative humidity on strength

As shown in the figure, even in the extreme range likely to be experienced (30% to 97.5% relative humidity), there is only a small change in strength. Long-term monitoring of a house constructed with unfired clay brickwork in Dalguise, Scotland showed that the relative humidity in the house remained between 40% and 65% throughout the year, even in the bathroom where a shower was used. Under this change in relative humidity the strength will change by approximately 12% for the earth masonry with high clay content, and only 8% for the low clay content.

The strength of unfired brickwork is normally lower than fired clay bricks or concrete blockwork, and 100mm thick unfired claly brick walls are currently not recommended for high load structural applications. Increasing the wall thickness will open the possibility for structural use of unfired brickwork.

 

Mortars for unfired clay brickwork

As the wall thickness decreases, the mortar must bond more to the masonry units to provide sufficient structural strength against lateral loads (pushing horizontally against the wall). The effect of wall thickness on required bond strength can be determined by a structural engineer, but it can be calculated that a 300mm thick wall with almost no bond strength (traditional earth masonry) can support the same load as a 100mm thick wall with a bond strength of approximately 0.2N/mm2. The bond strength of different mortars with modern earth masonry is shown in the figure below. This figure includes clay/sand and lime mortars used for traditional earth masonry and a cement/sand mortar used with fired bricks.



Bond strength with different mortars

As shown, the mortars used for traditional earth masonry do not provide the bond strength required to construct 100mm thin walls using modern earth masonry. The use of a preformulated sodium silicate/clay/sand mix does, however, provide the required strength and provides a bond strength similar to cement mortars with fired bricks. The preformulated sodium silicate mortar has less than 10% of the embodied CO2 than typical cement based mortars but does not perform as well at high water contents. These high water contents can be avoided through appropriate detailing.

An alternative to a sodium silicate based mortars is to tie 100mm thick modern earth masonry to a timber or other frame to provide the required lateral load capacity. This will provide the environmental benefits of earth masonry (thermal mass and humidity buffering) to a timber framed building.

 

Bricks, blockwork or unfired clay bricks - which is best?

There is no simple answer to which is best as the different materials are suited to different applications. Some point to consider are:

Pro

Unfired brickwork generally has lower embodied energy and is easier to recycle and dispose of at end of use than blockwork or fired clay masonry

Pro

Unfired brickwork has the ability to absorb more moisture from the air than blockwork or fired brick masonry, and therefore provides better passive humidity control

Con

Unfired brickwork does not have the same moisture resistance as blockwork or fired clay masonry, and detailing should ensure it is kept dry during and after construction

Con

Unfired brickwork generally has a lower strength than blockwork or fired clay masonry, and it is currently not recommended to use thin-walled earth masonry in high-load structural applications. It will also not support as high a load from fixings as fired

Further information

Earth masonry: Design and construction guidelines. - Morton, T. ISBN 9781860819780, BRE Press, (2008)

Building with Earth: Design and Technology of a Sustainable Architecture. Minke, G. Birkhauser, Basel, Germany, (2006).


• UK earth building association ( www.arc-architects.com/research/EBUK.htm )
• German earth building association ( www.dachverband-lehm.de/index_gb.html )
• UK Brick Development Association ( www.brick.org.uk )

 

 

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