Can unfired clay bricks offer a low-energy alternative in masonry construction? Dr Ali Arasteh of the BDA reports on the initial findings of a research project led by the University of Bath.
Unfired clay bricks were first used in Mesopotamia around the third millennium BC although the use of 'earth masonry' predates this by far. Almost a third of the world's inhabitants live in houses made of earth and in Germany alone there are more than two million buildings in which earth has been used as a construction material.
The use of earth masonry declined after the end of the nineteenth century with the development of new construction materials. However, the level of energy use associated with the production of masonry units has created interest among manufacturers, researchers, architects and engineers to develop further the properties of earth masonry or unfired clay units. Furthermore the ability of unfired clay to act as a buffer in controlling the humidity and temperature changes that typically occur within dwellings, together with its low environmental impact, makes it an attractive construction material in dealing with the environmental challenges that lie ahead.
The Brick Development Association and three of its member companies, Ibstock Brick, Hanson Building Products and Errol Brick Company signed up as partners to a technology research programme funded by the Department of Trade & Industry in November 2006. The project leader is the University of Bath and the other partners are Lime Technology and ARC Architects. This feature reports on the project and its findings to date.
The project began by collating information from the project partners and obtaining brick samples in order to develop an evaluation plan to establish the material properties.
Most of the materials used in the manufacture of unfired clay units are low-plasticity clays with low sulfate and chloride levels and a large variation in the organic content. Due to differences in the shrinkage characteristics of clay, dimensional measurements showed considerable variation. Linear shrinkage was found to be a function of ambient moisture and could vary from around 6.5 per cent to just over 10 per cent. The net bulk density of the units was around 2000kg/m3. The compressive strengths of the units were determined in accordance with EN772-1:2000 on 18 samples, four moulded and the rest extruded, and values of 1.5N/mm2 to 2.25N/mm2 were recorded.
Compressive strength is a function of moisture content; the higher the moisture content the lower the strength (see diagram below).
• There is a trend towards increasing compressive strength with increasing clay content.
• Moisture content affects the expansion or shrinkage of unfired clay.
• Denser unfired clay bricks exhibit greater length change than less dense bricks.
It should be noted that the expected equilibrium moisture content of unfired clay units is 3-5 per cent in a domestic environment.
The next step was to look at masonry made using different mortars. Past experience had shown that some brick/mortar combinations worked well and some did not, and this was mainly due to differences in the surface characteristics of different bricks. Different mortars, including some provided by the unit manufacturers, were tested and although some worked with different bricks the average bond strength was less than 0.1N/mm2 (values less than about 0.2N/mm2 are considered low for structural applications). Some thought was given to the use of mechanical fixings but this idea was abandoned for reasons of practicality, cost and lack of harmony with the concept of earth buildings. The findings included:
• Lime mortars do not achieve 0.2N/mm2.
• Lignosulfate mortars have a tendency to deteriorate with time, causing cracking at brick/mortar interface.
• Lime/starch and lime/casein were not fully satisfactory in terms of strength and long-term performance.
• Initial tests on sodium silicate mortars proved encouraging.
– With high contents, bond was stronger than flexural tensile strength at 14 days.
– Further tests with 8% concentration showed that bond strengths of about 0.2N/mm2 at seven days were achievable.
Compressive strength tests to BS EN 1015-1
Mean value: 2.49N/mm2
Characteristic value: 2.07N/mm2
Compressive strength: 2.99N/mm2
Flextural strength tests to BS EN 1052-2
Tests perpendicular to bedjoints
Mean value: 0.43N/mm2
Characteristic value: 0.38N/mm2
Tests parallel to bedjoints
Mean value: 0.57N/mm2
Characteristic value: 0.44N/mm2
Bond strength at 28 days
Mean value: 0.435N/mm2
Characteristic value: 0.375N/mm2
It is worth noting that the variation in compressive strength is remarkably low and in other tests quite acceptable. This results in a 'low' standard deviation which is very beneficial for low-strength construction materials. Furthermore the compressive strength of the wall is close to the compressive strength of the units, which proves the suitability of sodium silicate mortar; it's role is to bond the units and not to affect the strength disproportionately.
Tests were carried out on some walls over a gauge length of 200mm, with horizontal and vertical changes measured (see diagram below). Shrinkage values in the two directions follow the same trend and magnitude and appear to have stabilised at about 0.37 per cent after a period of two months. Around half of 'final' shrinkage occurs one day after construction and therefore the magnitude of shrinkage after construction is only half as high. For example, a 2.4 metre high wall of unfired brick would be expected to shrink by about 9mm; half of this would have occurred in the first day.
Clay plasters led the development of the market for commercial mass-produced clay products in Germany over the last 15 years. As well as aesthetics there are technical benefits such as the regulation of internal air humidity, an important factor when upgrading the environmental performance of many twentieth century buildings. The use of clay plasters in the UK has been limited to expensive imported materials from mainland Europe. Recent research at the University of Bath in collaboration with ARC has shown that many dry ground clays currently used in brick manufacture in the UK can be easily adapted to form workable clay plasters, with perhaps 10 per cent having the natural qualities of fine grading, workability and colour needed to produce a high quality commercial material.
The findings to date have demonstrated that unfired clay units can be used in the construction of domestic loadbearing walls as well as partitions. Such walls will effectively control internal moisture and humidity and provide a healthier living environment. Their thermal mass can effectively store and release heat out of phase with the outside temperatures, thus providing more comfortable indoor temperatures. And they have a low carbon footprint.
This article has been reproduced with the kind permission of the Brick Development Association. It first appeared in the Spring 2009 edition of the ''I'.
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