Corrosion induced cracking modelled by a coupled transport-structural approach

Transport of corrosion products into pores and cracks in concrete must be considered when predicting corrosion induced cracking in reinforced concrete structures, since this transport significantly delays the onset of cracking and spalling by reducing the amount of radial displacement imposed on the concrete at the steel/concrete interface. We aim to model this process by means of a coupled transport-structural approach, whereby the transport of corrosion products is determined by a pressure gradient generated by the confined volumetric expansion due to the transformation of steel into corrosion products. This pressure driven transport was studied by using both an axisymmetric thick-walled cylinder model and a network approach. The network approach was then applied to corrosion induced cracking experiments reported in the literature

Constitutive and Numerical Modelling of Unsaturated Soils

The thesis focuses on three different areas: development of constitutive models for unsaturated soils, improvement of the finite element code "Compass" for coupled flow-deformation analysis involving unsaturated soils and application of the improved code to the simulation of pressuremeter tests in unsaturated soils. On the constitutive side, a unique relationship is proposed between degree of saturation, suction and specific volume, by introducing dependency on specific volume in the simplified van Genuchten [48] equation. This is a significant improvement over the common assumption of a state surface expression for degree of saturation. If combined with an elasto-plastic stress-strain model predicting the variation of specific volume, the proposed relationship is capable of reproducing irreversible changes of degree of saturation and changes of degree of saturation experimentally observed during shearing. Predictions show very good agreement with experimental results from tests on compacted Speswhite Kaolin published in the literature. On the numerical side, a number of changes to the code "Compass" have been performed. The new relationship for degree of saturation is implemented in the code and the implementation is validated against three benchmark problems. Use of the new relationship for degree of saturation results in significantly different predictions to those obtained if a conventional state surface expression for degree of saturation is used (as present in the original code). Implementation of the water and air continuity equations in "Compass" has been corrected by expressing these equations in terms of flux velocities relative to the soil skeleton. This is the form in which the equations should be expressed if they are to be combined with Darcy's law for liquid and gas flows. The simulation of a notional laboratory test shows that the incorrect combination of Darcy's law with absolute flux velocities, as present in the original code, causes significant errors. The convergency algorithm at constitutive level employed in the code has been corrected by introducing residual flux terms in the two flow equations, analogous to residual forces in the equilibrium equation. These terms must be taken into account if a convergency algorithm for an elasto-plastic stress-strain model is used and the relationship assumed for variation of degree of saturation involves any dependency on net stresses. A numerical study of a notional laboratory test shows that omission of residual flux terms results in substantial errors and may cause failure to converge. The plane-strain formulation of code "Compass" has been corrected by imposing the condition of nullity only on the out-of-plane component of the total strain rate vector instead of the out-of-plane component of each single contribution to the total strain rate, as was done in the original code. Such inconsistency, due to the history of development of finite element programs, also appears in other examples published in the literature. Numerical simulations of two types of bi-axial tests show that significantly different results are generally predicted by the correct and incorrect formulations, and also provide an explanation why this type of error was difficult to detect in codes implementing traditional models for saturated soils. The potential of the enhanced version of code "Compass" for analysing boundary value problems is demonstrated by simulations of pressuremeter tests in unsaturated soil. This study also provides some initial insight into the interpretation of pressuremeter tests in unsaturated soil, by simulating tests at different loading rates in a normally consolidated soil. The mechanical behaviour of the soil is represented by the elasto-plastic model of Alonso, Gens and Josa [1] while the variation of degree of saturation is modelled by the new relationship proposed in the thesis. The entire range of loading rates, from undrained to fully drained (with respect to liquid), is simulated. Relatively small changes of suction are predicted even in the fastest test and the computed cavity pressure-cavity strain relationships are all very similar regardless of loading rate. It may therefore be possible to model even rapid pressuremeter tests in unsaturated soils as a drained (constant suction) process. Further work is required to investigate the generality of this conclusion

Assessing the performance of earth building materials: a review of recent developments

After being almost abandoned in Europe at the end of the Second World War, raw earth is currently regaining the interest of civil engineers and architects worldwide. Raw earth (unfired earth) displays very interesting thermo\u2010hygro\u2010mechanical properties, which can contribute to the reduction of the environmental impact of buildings not only during construction but also during service life. Nevertheless, one of the main reasons preventing dissemination of raw earth into mainstream construction practice is the lack of commonly agreed protocols for assessing engineering performance. In this context, the RILEM Technical Committee 274\u2010TCE is critically examining current experimental procedures to propose appropriate testing methods that could be adopted as standards. The present paper summarizes the main challenges faced by the committee and describes some of the existing procedures for measuring the engineering properties of earth materials. The main issue identified by the committee is that laboratory protocols do not accurately reproduce field conditions. The representativeness of laboratory samples is also questionable due, for example, to different degrees of material homogeneity with respect to the field. Finally, the paper identifies some possible routes to reduce the discrepancies between laboratory testing and field conditions in relation to the thermo\u2010hygro\u2010mechanical characterization of earth materials

Modelling unsaturated soil behaviour during normal consolidation and at critical state

An analysis of results from published laboratory tests on Jossigny silt and Barcelona clayey silt is presented to confirm the existence of a unique capillary bonding function linking the quotient between unsaturated and saturated void ratio, at the same mean average skeleton stress, to a single capillary bonding scalar variable. The analysis confirms that the same capillary bonding function applies to both normally consolidated and critical stress states. The above two experimental sets with the addition of further published data for Speswhite Kaolin are also used to study the relationship between unsaturated critical shear strength, mean average skeleton stress and capillary bonding variable. The results of such analysis are assessed in the light of a similar modelling framework proposed in the literature

Comparison of high capacity tensiometer designs for long-term suction measurements

This paper investigates the long-term measurement of negative (tensile) pore-water pressures in soils by using high capacity tensiometers (HCTs). Seven different HCT prototypes were designed and manufactured by using different porous filters, pressure transducers, water reservoirs and protective casings. The ability of these prototypes to record negative pore-water pressures over long times was initially assessed by a series of measurements on small clay samples equalised at different suction levels. These tests were followed by two larger scale experiments in which four HCT prototypes were simultaneously installed inside a lysimeter filled with a sandy soil alongside two standard dielectric permittivity sensors measuring suction and water content, respectively. In one experiment, the soil was left to dry until all four HCTs cavitated while, in another test, the soil was allowed to dry up to an intermediate level of suction before triggering a rainfall, after which the soil was left to dry again until all four HCTs cavitated. In each lysimeter experiment, the readings of the HCTs were reasonably consistent, which suggests that sensor design has little effect on the accuracy of measurements. These experiments also indicated that HCTs are more accurate and exhibit a faster response than standard dielectric permittivity sensors. Moreover, the HCTs incorporating a small water reservoir showed a greater ability to sustain suction over long period of time without cavitating

On a 2D hydro-mechanical lattice approach for modelling hydraulic fracture

A 2D lattice approach to describe hydraulic fracturing is presented. The interaction of fluid pressure and mechanical response is described by Biot's theory. The lattice model is applied to the analysis of a thick-walled cylinder, for which an analytical solution for the elastic response is derived. The numerical results obtained with the lattice model agree well with the analytical solution. Furthermore, the coupled lattice approach is applied to the fracture analysis of the thick-walled cylinder. It is shown that the proposed lattice approach provides results that are independent of the mesh size. Moreover, a strong geometrical size effect on nominal strength is observed which lies between analytically derived lower and upper bounds. This size effect decreases with increasing Biot's coefficient

A microstructural insight into the hygro-mechanical behaviour of a stabilised hypercompacted earth

The use of raw earth as construction material can save embodied and operational energy because of low processing costs and passive regulation of indoor ambient conditions. Raw earth must however be mechanically and/or chemically stabilised to enhance stiffness, strength and water durability. In this work, stiffness and strength are enhanced by compacting raw earth to very high pressures up to 100 MPa while water durability is improved by using alkaline solutions and silicon based admixtures. The effect of these stabilisation methods on hygro-mechanical behaviour is explored and interpreted in terms of the microstructural features of the material. Stiffness and strength are defined at different humidity levels by unconfined compression tests while the moisture buffering capacity is measured by humidification/desiccation cycles as prescribed by the norm ISO 24353 (Hygrothermal performance of building materials and products determination of moisture adsorption/desorption properties in response to humidity variation. International Organization for Standardization, Geneva, 2008). As for the microstructural characterisation, different tests (i.e. X-ray diffractometry, Infrared Spectroscopy, Mercury Intrusion Porosimetry, Nitrogen Adsorption) are performed to analyse the effect of stabilisation on material fabric and mineralogy. Results indicate that the use of alkaline activators and silicon based admixtures significantly improves water durability while preserving good mechanical and moisture buffering properties. Similarly, the compaction to very high pressures results in high levels of stiffness and strength, which are comparable to those of standard masonry bricks. This macroscopic behaviour is then linked to the microscopic observations to clarify the mechanisms through which stabilisation affects the properties of raw earth at different scales

On the choice of stress–strain variables for unsaturated soils and its effect on plastic flow

The net stress plus suction and the average skeleton stress plus modified suction are two alternative sets of energetically consistent stress variables for modelling the hydro-mechanical behaviour of unsaturated soils. When used in conjunction with their work-conjugate strains, both sets of stress variables correctly calculate the first-order term of the hydro-mechanical work input into a soil element subjected to infinitesimal changes of deformation and water content. They therefore also correctly calculate the increment of internal energy along a given stress–strain path, that is the integral of the first-order term of the infinitesimal work input. This paper shows, however, that the above two sets of stress variables lead to different values of the second-order term of the hydro-mechanical work input. They are therefore no longer equivalent with respect to other aspects of material behaviour governed by the second-order work such as the flow rule of elasto-plastic models. The flow rule assumes the normality between plastic strains and equipotential surfaces defined in the conjugate stress–strain space. This normality is however lost when an elasto-plastic model originally formulated in terms of net stress plus suction is recast in terms of average skeleton stress plus modified suction (or vice versa) by means of standard mapping relationships between stress variables. To restore normality in both stress spaces, it is necessary to impose specific forms of elastic and plastic behaviour

Optimization of bricks production by earth hypercompaction prior to firing

This paper presents an innovative method for the production of masonry bricks, which combines earth compaction and quick firing at low temperatures. Earth bricks were manufactured according to three different methods, i.e. extrusion, standard Proctor compaction and hypercompaction to 100 MPa. All bricks were fired inside an electrical furnace by rising the temperature at a quick rate of about 9 °C per minute to 280, 455, 640, 825 and 1000 °C, after which the furnace was turned off and left to cool to the atmosphere with the brick inside it. These firing temperatures and times are significantly lower than those employed for the manufacture of commercial bricks, which are typically exposed to a maximum of 1100 °C for at least 10 hours (Brick Industry Association, 2006). A testing campaign was performed to investigate the effect of quick firing on the porosity, strength, water durability and moisture buffering capacity of the different bricks. Quick firing of hypercompacted bricks at moderate temperatures, between 455 and 640 °C, is enough to attain very high levels of compressive strength, between 29 and 34 MPa, with a good to excellent moisture buffering capacity. These properties are better than those of commercially available bricks. The strength of hypercompacted bricks further increases to 53 MPa, a value similar to that of high-strength concrete, after quick firing at 825 °C. Earth densification prior to thermal treatment therefore improves material performance while enabling a significant reduction of firing temperatures and times compared to current bricks production methods

Building the UPPA high capacity tensiometer

High capacity tensiometers (HCTs) are sensors capable of directly measuring tensile pore water pressure (suction) in soils. HCTs are typically composed of a casing that encapsulates a high air entry value ceramic filter, a water reservoir and a pressure sensing element. Since the creation of the first HCT by Ridley and Burland in 1993 at Imperial College London, HCTs have been almost exclusively built and used in academic research. The limited use in industrial applications can be explained by a lack of unsaturated soil mechanics knowledge among engineering practitioners but also by the technical difficulties associated to the direct measurement of tensile water pressures beyond the cavitation limit of -100kPa. In this paper, we present the recent design and manufacture of a new HCT at the Université de Pau et des Pays de l’Adour (UPPA) in France. Different prototypes were tried by changing the main components of the device including the type of ceramic filter, pressure transducer and geometry of the external casing. In particular, two ceramic filters of distinct porosity, three pressure transducers with distinct materials/geometries and four casing designs were tested