HPZ-Walls - High-performance exterior brick walls without additional thermal insulation

Status

Completed (February 2023)

Starting point, content and results

One quarter of all heat flows between the interior and exterior of a building passes through the exterior wall. This illustrates the importance of the exterior wall on the energy demand of a building. Driven by legal regulations on "energy saving and thermal insulation", external thermal insulation composite systems have become established. Although these fulfil the required thermal insulation properties, they cause air temperatures in neighbourhoods to rise in summer, they hinder the wall from extracting solar thermal energy during the heating season and they have to be disposed of as hazardous materials at the end of their relatively short service life. The monolithic, minerally plastered brick wall, on the other hand, can meet both the sustainability requirements and the high thermal requirements of urban residential and office buildings as long as the architecture, the masonry and the brick are optimised for this purpose. The thermally optimised, highly porous bricks currently on the market are weak. They cannot be used to realise the architecture of choice, namely multi-storey housing. The question now arises as to whether it is possible to combine the required strength properties and the high demands on thermal behaviour in one brick.

DI Andreas Petermann addressed this question as part of his master's thesis. He proved theoretically that the problem can be solved with a new layout of the air chambers (patented under the name TRALAM by Horst Gamerith) in combination with a "medium-weight" clay. Based on a case study of an 8-storey residential building, he showed that the load-bearing capacity requirements of a 50cm TRALAM masonry are achieved for a building height of 22m, the heating requirement according to the U-value method corresponds to that of a composite thermal insulation system and furthermore, 10% additional solar energy can be gained via the non-insulated opaque outer surface. It is obvious that an increase of the load-bearing capacity without changing the fired clay strength, more clay has to be fired and consumed. Thus, all the associated characteristic values of the ecological footprint become worse but only those of the production's footprint. In return, the high-performance masonry requires only one third of the building ground and one third of the roof area to create the same living space, no insulation material is used in 8-storey buildings, the service life is significantly extended, and it brings solar energy gains in winter as well as a reduction of overheating in summer.

In order to be able to transfer these promising, but currently still purely theoretical results to reality, comprehensive, experimental verification is necessary. As a first step, the exploratory project presented here aims at answering the question of the industrial manufacturability of such bricks. The production-related sticking point is the reduction of the air gap thickness of the bricks from usually no less than 8 mm to 4 mm. This is necessary for the new air chamber layout.

With the help of the experience of the Austrian brick industry, represented by the "Forschungsverein Steine-Keramik", the most suitable clay available in Austria was sought in this project. The tests on some clays delivered and the evaluation of the same were carried out at the Ziegelforschungsinstitut Essen. All tests regarding the production of the bricks were carried out on quarter bricks for financal reasons. For the subsequent tests to determine the most important properties of the fired test bricks, the quarter bricks were glued together to form whole bricks, if necessary. Finally, the properties determined in this way, such as compressive strength, modulus of elasticity, masonry strength, thermal resistance, natural frequencies and sound reduction index, were used to assess the quality of the manufactured test bricks.

The production of an extrusion nozzle for bricks with 4mm air gaps turned out to be unproblematic. Likewise, it turned out that extruding the bricks was not a problem if the plasticity of the clay mixture was adjusted appropriately. However, undesirable transverse cracks appeared in a large number of test bricks. These cracks are only due to the non-homogeneous compaction, which is due to the overloading of the laboratory extruder. Such cracks are not expected if an industrial set-up is used. The drying process had to be slightly adjusted to accommodate the higher airflow resistance in the 4 mm chambers. The new brick geometry requires a preheating phase in the firing process that is extended by approx. 5 hours.

The fired body and thus also the test bricks have an unexpectedly high bulk density, which is reflected in a very high compressive strength but unfortunately also in a high thermal conductivity. The conductivity of fired material given in the literature, which represents the starting point for the theoretical development of TRALAM masonry, could not be confirmed experimentally. In contrast, the tests showed significantly higher strengths than expected based on the state of the art and science. The compressive strength on the single brick is approx. 30% higher than targeted. Even the experiment on a masonry body, whose individual bricks showed transverse cracks that will not be present in the final product, showed a very homogeneous fracture pattern and an unexpectedly high compressive strength. The dynamic tests on the single brick confirm the predicted quality of the TRALAM brick with regard to its soundproofing performance.

Since the project shows that the industrial production of bricks with very thin air gaps is unproblematic, it makes sense to tackle all the further steps that were already planned before this exploratory project. The focus of the follow-up project is to carry out a one-year, large-scale field test in one of the test houses at the TU-Graz with all the necessary theoretical preparation and evaluation work. In this test, the actual thermal energy balance of the insulation-free TRALAM exterior wall is to be determined by measurement and compared with the demands of the Austrian standard. It must be mentioned that the standard gives preferential treatment to composite thermal insulation systems due to the use of physically incomplete calculation models. In this large-scale test, whole bricks must be used in any case, which requires close cooperation with the brick industry. Since it has not yet been possible to achieve all the desired brick properties in the exploratory project, further research on how these can be achieved is needed before prototypical production of the whole bricks will start. There are three screws to be turned: (i) changes in the clay mixture (reduction of bulk density and thermal conductivity); (ii) utilisation of the observed strength reserves: the strength needed can also be achieved with less clay content, which goes hand in hand with a thermally improvement of the brick; (iii) if the currently very fine grid size of the brick bond is extended, a large potential for thermal improvement opens up due to relaxed boundary conditions for the air chamber layout.

Project management

Graz University of Technology / Laboratory for Structural Engineering

Project or cooperation partners

• Forschungsverein Steine – Keramik
• Architekt Horst Gamerith, Em.Univ.-Prof. Dipl.-Ing. Dr.techn.

Graz University of Technology
Laboratory for Structural Engineering
Priv.-Doz. Dipl.-Ing. Dr.techn. Bernhard Freytag
Inffeldgasse 24
A-8010 Graz
Tel.: +43 (316) 873 7050
E-Mail: freytag@tugraz.at
Web: www.lki.tugraz.at