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Structural performances of earthen building materials. A comparison between different typologies
(2011)
Traditional construction techniques utilizing earthen materials are often seen as inferior compared to modern ways of building. Structural earthen elements are perceived as vulnerable towards environmental influences (moisture, frost) and in particular towards the load scenarios during earthquakes. In comparison to the recent advances in research on stone and brick masonry, knowledge on the structural performance of earthen building construction is limited and scattered. Consequently the confidence in the performance of these earthen buildings constructed in the traditional techniques during earthquakes is fairly low. The research presented here aims to make a comparison of mechanical behavior between different earth masonry material typologies, consisting of earth block masonry, rammed earth and cob. The paper has been developed in the framework of a larger research program called NIKER. BAM and other seventeen research partners from the Mediterrean area are jointly involved to develop and validate innovative materials and technologies for the systemic improvement of the seismic behavior of Cultural Heritage assets
The paper has been developed in the framework of a larger EC‐research program called NIKER, in which BAM and other seventeen research partners from the Mediterrean area are jointly involved. It aims to develop and validate innovative materials and technologies for the systemic improvement of the seismic behavior of Cultural Heritage assets. The death tolls brought about by recent catastrophes in
developing countries where many inhabitants lived in earthen dwellings (Gujarat, India 2001, Bam, Iran 2003 and Concepción, Chile 2010 Earthquakes, Tamil Nadu 2004 Tsunami) have brought about research studies aimed at improving earth construction in terms of strength, seismic resistance and speed of construction.
In comparison to the recent advances in research on stone and brick masonry, knowledge on the material properties and failure mechanisms of earthen Building construction is limited and scattered.
The research presented here by aims making a comparison of mechanical behaviour between different earth masonry material typologies, consisting of earth block masonry, rammed earth and cob.
Today, building is a complicated commercial process characterised by legalised agreements between all participants. For commercial building projects, all materials and constructions have to conform to building standards and regulations. Material properties must be reproducible in standardised test procedures. This also applies for the control of the quality of production of earth building materials and constructions. Building conservation is a part of this process. Generally, national building standards have to reflect the current situation of building materials / construction systems of that country for which they have been developed.
The goal of the present study is to assess the feasibility to develop a first reliable database of materials parameters for Earth Block Masonry (EBM). The database is crucial when defining the materials safety factors. In the first part an experimental campaign of compressive tests were carried out on two types of earth block and two types of earth mortar. The results showed that the mean variation of the compressive strength was remarkably less than expected. This low variation is related to a production with high quality standards of the materials employed. In the second part a partial safety factor for EBM under uniaxial compression was determined through the reliability method. The results proved the reliability of a common calculation method for EBM based on partial safety factors following the current standards.
Earth, timber, fibre boards and insulation materials based on wooden and other natural fibres offer a variety of properties beneficial for eco innovative constructions that are able to improve the energy and resource efficiency of buildings.
Due to their porosity, natural building materials are vapour active and are able to buffer moisture. In combination with highly insulated and airtight but vapour permeable building envelopes, modern earth-timber constructions provide stable indoor humidity levels and can therefore be naturally ventilated while achieving highest energy efficiency standards. Experimental evidence suggests that monitored pilot buildings in Berlin do show healthy indoor air humidity levels (around 50%) in wintertime, while mechanically ventilated buildings demonstrate significantly lower values (around 25%), which have to be considered as uncomfortable and unhealthy.
The application of building materials being poor in chemical emissions, particularly volatile organic compounds (VOC) and radon, improves the indoor air quality further, so that intermittent ventilation twice a day will be sufficient to provide healthy indoor air quality. The air quality in critical rooms (e.g. small bedrooms), demonstrating a smaller air volume, should be monitored if appropriate ratios of room size to occupancy level cannot be realised.
Through night time ventilation in summer, vapour active earth-timber constructions provide evaporative cooling (humidity adsorption at night time and desorption during the day). As a result, indoor temperatures of earth-timber buildings range around 8 °C below the outside temperature peak, when an appropriate glazing ratio is reflected. The EU funded research project H-house is investigating various construction materials regarding water vapour adsorption as well as emission and absorption of harmful substances. Based on this investigation new wall constructions are designed to provide a healthier indoor environment.
This study presents diagnostic techniques to assess the structural vulnerability of earthen buildings. Medieval structures at Ambel (near Zaragoza, Spain), once a preceptory or monastic house belonging to the Military Orders, provide a useful case study. After more than a thousand years of construction, failure and repair from the 10th century to the present day, Ambel preceptory today is characterised by marked inhomogeneities: construction materials and structural typologies are juxtaposed, with structural discontinuities at the interfaces between construction phases. This paper argues that, while static analysis is an essential prerequisite before a suitable maintenance program can be defined, no evaluation of the structural behaviour of any historic building can ignore an understanding of the building’s history.
Both in developed and developing countries, modern building materials tend to be preferred to traditional earthen construction. Reasons include low durability, inadequate performance under seismic loading and, in developing countries, a wish to replace what is perceived as 'poor' with what is perceived as 'rich'. In an age when building permissions and construction standards are a must, particularly in seismic areas, even when an owner is willing to build in earth, construction approval needs to be granted: we rely on values and standards to build, but the necessary data on material properties and structural performance of earthen building techniques is scarce if compared to the abundance of data for other materials (clay brick masonry, concrete, steel) available to the engineer. At the same time, traditional builders' skills, knowledge and confidence in earthen building techniques are decreasing if not disappearing. A wallette testing campaign was thus carried out with the aim of filling this knowledge gap. Prior to the wallette campaign, material properties, including composition and physical-mechanical parameters, were determined. Compression and diagonal compression (shear) tests were then performed, and a basic analysis of the mechanical behaviour of structural elements built in cob is provided in relation to earth block (adobe) masonry and rammed earth elements. Cob, shown to have low compressive resistance, has a relatively ductile post-peak behaviour if compared to earth block masonry specimens which, as expected, show a marked brittle behaviour. In terms of shear strength, cob performs relatively well in view of its low compressive strength. The study is part of our work within the framework of the ongoing project NIKER funded by the European Commission dealing with improving the structural performance of Cultural Heritage assets in order to limit earthquake hazards.
The main aim of the research was the development of a first valid database for material parameters of earth block masonry (EBM) with particular regard to statistical characteristics. A solid database is needed for the determination of the materials partial safety factor. Therefore, compressive strength tests were carried out with two types of earth blocks and two types of prefabricated earth mortar.
The evaluation has shown that the mean variation of the compressive strength was remarkably less than expected, which indicates high quality standards of the components earth block and mortar with regard to industrial production. Using the reliability method, a partial safety factor for EBM subjected to compression was determined on the basis of these test results. The findings have shown that a common calculation method for EBM based on partial safety factors following the valid masonry construction standard is feasible.