A prospective Study on the Evolution of Airtightness in 41 low energy Dwellings

Airtightness of the building envelope is an important parameter affecting the performance of (low energy) buildings. In case the airtightness is effectively measured, this is typically only done once as part of the commissioning of the construction work. Several factors could affect the evolution of the airtightness of the envelope after the building is constructed. In this work, follow-up airtightness tests have been carried out to investigate the evolution of the performance in the interval of 0.5 up to 12 years compared to the original pressurisation test. The results on 41 low-energy dwellings indicate that the airtightness is indeed not a fixed value over time. Of the 41 buildings, 29 display an increased air permeability resulting in an increase of up to 200% in relative terms or up to 1.36 ACH50 (air changes per hour at 50 Pa pressure difference [h-1]). Conversely, four of the buildings in the dataset show a significant improvement of the airtightness; resulting in a decrease of air leakage of up to -1.19 ACH50. Analysis of the data shows that on average the air permeability at 50 Pa pressure difference increased by 38%, but with great variation depending multiple factors such as initial airtightness value and construction type. This corresponds to an average increase of the specific air permeability of the building envelope of 0.15 m³/(h·m²). Most of the buildings under analysis are low energy buildings or passive houses which were very airtight at time of construction. Despite the observed evolution in air permeability, many buildings under investigation can still be considered sufficiently airtight a few years after initial construction.

Effect of the Loading Rate on Failure of Composite Pressure Vessel

A multi-scale model has been successfully applied to the simulation of the effects of pressurisation rate on damage accumulation in carbon fibre/epoxy plates and composite pressure vessels. The results of the simulations agree with experimental results and reveal that the point at which the structures become unstable in a monotonic pressurisation test depends on the speed of loading. The faster the loading rate the higher the applied stress at which the composite structure becomes unstable. The mechanism which governs this behaviour is seen to be the viscoelastic nature of the matrix material through which stresses are transferred from broken to neighbouring intact fibres. At loading rates that allow greater relaxation of the resin around fibre breaks neighbouring fibres are subjected to increased loads over a significantly greater length, leading to further earlier breaks.

Predicting Hoop Tendon Behaviour in Pre-Stressed Concrete Containment Vessel

This paper is based on the experimental and numerical analysis work carried out as part of an international round robin aimed to predict the limit load of the ¼ scale Pre-stressed Concrete Containment Vessel (PCCV) which was tested at Sandia National Laboratories (SNL) in USA. The design pressure, Pd, for the PCCV was 0.39MPa. Pressurisation test was conducted causing global collapse of the PCCV structure, which occurred at the pressure of 1.423MPa (3.65 Pd). Displacements, loads and strains were monitored at 55 standard locations giving a unique opportunity to assess the accuracy and reliability in predicting failure modes and limit loads of PCCV structures. To simulate the inelastic response of the structure with extensive concrete cracking requires specialist numerical models and detailed geometric representation of the main structural features. One of the most important structural features was the prestressed hoop tendon system. The paper presents a brief explanation of the test, the instrumentation used to monitor the tendon behaviour and describes the analytical models employed to predict the tendon behaviour.