Motivation and research question

The consistent reduction of CO₂ emissions in the construction sector is one of the most pressing challenges of our time. Earthen construction methods such as rammed earth and clay masonry can contribute to this reduction due to their comparatively low emissions during production. When used as load-bearing wall elements, in terms of mass they represent a relevant alternative to conventional mineral construction materials such as concrete and fired clay bricks. Rammed earth structures, characterized by the layer-wise placement and mechanical compaction of a mixture of clay and aggregates (gravel) within formwork, enables the construction of monolithic wall elements either directly on site or as large-format prefabricated components. However, knowledge of the mechanical behaviour of rammed earth, especially its long-term mechanical behaviour (shrinkage and creep), remains limited. This knowledge gap significantly hinders its integration into standard engineering design and planning processes. This research project therefore investigates the suitability of fiber optic sensing for deformation measurement in rammed earth elements, with a view toward future long-term monitoring applications.

Initial situation

Clay is characterized by easy workability, local availability, and the ability to provide an excellent indoor climate. Its highly favourable energy and environmental balance (ÖKOBAUDAT according to EN 15804+A2) makes it a promising alternative for the construction industry. The use of clay as a building material is not only ecologically but also economically advantageous: clay can be sourced from excavation material that would otherwise require disposal, thereby significantly strengthening circular economy concepts. Without the addition of binders such as lime or cement, clay remains fully recyclable and enables a sustainable, resource-efficient construction method.

Despite these advantages, earthen construction is currently only used sporadically. Established design and calculation frameworks for load-bearing earthen structures are largely lacking. Existing buildings are predominantly based on empirical craftsmanship rather than engineering-based approaches. Structural behaviour is strongly influenced by moisture distribution, degree of compaction, and climatic boundary conditions. A key knowledge gap concerns the time-dependent deformation behaviour, particularly shrinkage and creep, which substantially limits the broader application of the material.

Project contents and objectives

Against this background, this research project addresses the central question of whether distributed fiber optic sensing (DFOS) is suitable as a non-destructive, continuous measurement method for capturing the deformation behaviour of rammed earth walls under moisture and temperature variations as well as mechanical loading. The aim is to establish a basis for long-term monitoring of real, load-bearing structural elements.

For robust engineering-based modelling, comprehensive datasets on time-dependent deformation behaviour are still missing. Distributed fiber optic sensing offers considerable potential in this regard; however, its application in rammed earth structures has not yet been systematically investigated.

Methodological procedure

Distributed fiber optic sensing enables continuous, high-resolution monitoring of deformations along the entire sensor length. This allows the early detection of local changes and very small deformations in the micrometer range, such as crack initiation. Due to their small dimensions and low weight, sensor fibers and cables do not influence the material properties of the specimen and are therefore well suited for non-destructive measurement applications. To date, fiber optic sensors have mainly been applied in concrete structures, on steel surfaces, and in engineering geodesy.

The project comprises (i) experimental test series on rammed earth specimens of different sizes, including small-scale specimens to determine material parameters (compressive strength, elastic modulus, flexural tensile strength, drying shrinkage); (ii) an assessment of whether DFOS is suitable for capturing load-dependent material behaviour (creep); and (iii) an evaluation of its applicability for in-situ measurements on full-scale, load-bearing rammed earth components.

Fiber optic sensor cables were installed both by embedding them during compaction and by bonding them into grooves on the specimen surface. Particular focus was placed on the bond between the sensor cable, adhesive, and rammed earth. Measurement results were validated using conventional reference methods (displacement transducers and digital 3D image correlation).

The theoretical part of the project integrates the experimentally obtained material parameters into a finite element (FE) model to realistically simulate the load-bearing and deformation behaviour of rammed earth elements under static loading.

Results and conclusions

The experimental investigations demonstrate that fiber optic sensing is generally suitable for capturing deformation behaviour in rammed earth. Both embedded and surface-bonded sensors provided reproducible measurement data. The strains measured with DFOS correlated well with the reference measurements. The results further indicate that the adhesive used for surface-bonded sensor cables is critical for measurement quality. At the same time, the bond between embedded sensor cables and rammed earth was found to be insufficiently reliable to ensure accurate strain measurements.

The numerical model realistically simulated the load test of a rammed earth vault and confirmed that experimentally derived material parameters can be effectively transferred into numerical simulations.

Future research should further investigate the applicability of fiber optic sensing for large-scale tests and in-situ measurements in rammed earth structures, particularly to enable continuous, non-destructive monitoring of long-term effects caused by moisture and temperature variations (shrinkage) as well as sustained mechanical loading (creep).

The results obtained in this study form the basis for a follow-up project aimed at developing a practice-oriented design guideline for load-bearing rammed earth walls, which is based on the semi-probabilistic safety concept of the Eurocode.

Project management

Graz University of Technology - Institute of Structural Design

Project or cooperation partners

• Graz University of Technology - Institute of Engineering Geodesy and Measurement Systems
• Graz University of Technology - Laboratory for Structural Engineering

Graz University of Technology – Institute of Structural Design
Assoc.Prof. Dipl.-Ing. Dr.nat.techn. Andreas Trummer
Technikerstraße 4/4
A-8010 Graz
Tel.: +43 (316) 873 6211
E-mail: andreas.trummer@tugraz.at
Web: www.ite.tugraz.at