DW² - Thermal improvements on diaphragm walls

Status

ongoing

Starting point / Motivation

Due to growing requirements regarding the thermal properties of buildings and structures an increasing demand for thermally enhanced construction elements can be noticed. For high-rise buildings, this issue is being addressed by means of innovative design of external wall elements (e.g. wood panels) or improvements in the field of sustainable planning of city districts and urban areas. For underground construction, such as underground car parking or below ground structures, similar developments are not to be noticed at the moment.

This is owed to the structural design and execution methods especially when piles or diaphragm walls are used. In addition to this, the subsequent installation of insulation layers is not feasible in most cases since large parts of the structures underground are not accessible. Therefore, improving the thermal properties of underground construction elements is a challenging task.

Content and Goals

Nevertheless, the use of high-performance and thermally improved concrete for geotechnical construction elements is a promising approach in this field. Using foam concrete for diaphragm walls – which has a significantly lower thermal conductivity compared to classic concrete - could offer an increased insulation effect and therefore lead to thermally improved underground construction elements.

Nevertheless, the material needs to be adapted in order to fulfil geotechnical requirements related to the acting earth pressure and the presence of bentonite during the construction process. Furthermore, the influence of foam concrete on the bearing capacity and the serviceability of such structural elements and its effects on the environment have to be observed. Especially the impact on the surrounding ground-water and the use of resources necessary for the erection have to be considered.

Methods

As already mentioned, a main part of this project involves testing and validating the proposed concept for a thermally enhanced diaphragm wall in order to ensure the practical feasibility of this new construction element.

First, existing mixtures for foam concrete are adjusted and improved under geotechnical boundary conditions. A validation of these results is performed by means of laboratory tests. Secondly, a series of laboratory and field tests is performed using the improved foam concrete mixtures and the behaviour in combination with bentonite (used as support for diaphragm walls) is observed.

Furthermore, the influence of a thermally enhanced diaphragm wall on the heat transfer in the soil body and its impact on the ground water is investigated based on numerical simulations. In addition to the geotechnical and hydrogeological observations the sustainability of the foam concrete and its application for diaphragm walls is investigated.

Expected results

The project aims to apply the developed innovations and improvements to other fields of research such as underground thermal energy storages. Especially hybrid thermal energy storages such as thermal earth-water-storage basins could be improved by using thermally enhanced diaphragm walls to reduce heat losses.

Furthermore, the project results could be implemented in the future development of urban thermal energy storages and therefore lead to ecological and resource-saving inner-city cooling and heating solutions for districts.

Project management

Graz University of Technology - Institute of Soil Mechanics, Foundation Engineering and Computational Geotechnics

Project or cooperation partners

• Graz University of Technology - Institute of Structural Concrete
• Graz University of Technology - Working Group Sustainable Construction
• University of Graz - Institute for Earth Sciences

Graz University of Technology
Institute of Soil Mechanics, Foundation Engineering and Computational Geotechnics
Matthias J. Rebhan
Rechbauerstraße 12
A-8010 Graz
Tel.: +43 (316) 873 6738
E-mail: rebhan@tugraz.at
Web: www.soil.tugraz.at