Investigating geogrid-reinforced ballast: Experimental pull-out tests and discrete element modelling

AbstractThis paper presents an evaluation of the interlocking behaviour of geogrid-reinforced railway ballast. Experimental large box pull-out tests were conducted to examine the interaction between ballast and a biaxial geogrid. The discrete element method (DEM) was then used to model the interaction between the ballast and the geogrid by simulating large box pull-out tests and comparing the findings with the experimental results. Four different shapes of clumps were used to represent each ballast particle in order to obtain an acceptable shape for modelling the railway ballast. The DEM simulation results were shown to provide good predictions of the pull-out resistance and to examine the effect of clump shape on both the pull-out resistance and the distribution of contact forces. Therefore, the calibrated geogrid model and the 8-ball tetrahedral clumps, used as ballast particles, hold much promise for investigating the interaction between geogrids and ballast, and thus, optimising performance

Microstructural characteristics, porosity and strength development in ceramic-laterized concrete

Interfacial bonding between constituent materials and pore sizes in a concrete matrix are major contributors to enhancing the strength of concrete. In a bid to examine how this phenomenon affects a laterized concrete, this study explored the relationship between the morphological changes, porosity, phase change, compressive, and split tensile strength development in a ceramic-laterized concrete. Varying proportions of ceramic aggregates, sorted from construction and demolition wastes, and lateritic soil were used as substitutes for natural aggregates. Strength properties of the concrete specimens were evaluated after 7, 14, 28 and 91 days curing, but morphological features, using secondary electron mode, were examined only at 7 and 28 days on cured specimens, using Scanning electron microscope (SEM). From all the mixes, selected samples with higher 28 day crushing strength, and the reference mix, were further characterized with more advanced analysis techniques, using the mercury intrusion porosimetry (MIP), thermogravimetric analysis (TGA), X-ray Diffractometer, and SEM (backscatter electron mode-for assessment of the interfacial transition properties between aggregates and paste). The reference mix yielded higher mechanical properties than the concrete containing secondary aggregates, this was traced to be as a result of higher peaks of hydration minerals of the concrete, coupled with its low tortuosity and compactness. However, a laterized concrete mix containing both 90% of ceramic fine and 10% of laterite as fine aggregate provided the optimal strength out of all the modified mixes. Although, the strength reduction was about 9% when compared with the reference case, however, this reduction in strength is acceptable, and does not compromise the use of these alternative aggregates in structural concrete

Design of road foundations

SIGLEAvailable from British Library Document Supply Centre- DSC:D82590 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

An Evaluation of the Springbox test for Unbound Materials

The Springbox is a relatively new piece of equipment, designed to generate realistic values of modulus for use in pavement design. The specimen, which can be of soil, granular or lightly stabilized material, is a 17cm cube, confined by steel sides, two of which are spring-loaded and free to move during the test. Repeated load is applied vertically through a full-face loading plate. Detailed descriptions are given elsewhere, but this paper will include an overview of the equipment. However, the principal content of the paper concerns an evaluation of the true meaning of the test based both on comparisons between Springbox data and on finite element analysis (FE) of the stress conditions within the specimen. The comparative test data are obtained on three different granular materials (Clay soil, Sub-Base and Cement Treated Base). The FE analysis was performed using experimentally obtained values of resilient modulus and Poisson’s ratio and comparing the simulation with the real test in terms of longitudinal and vertical strain in order to assess the FE model. Reference is then made to the FE analysis of the equipment in order to explain the material’s local behaviour inside the specimen with respect to the global mechanical behaviour, which is evaluated during the test by means of point measurements, which are, therefore, assumed to be representative of the stress and deformation state of the material. The model is able to take into account the effect of friction between the steel sides and the material (cf = 0.4), which can highly affect the interpretation of results. Moreover, a FE parametric study was carried out under different static and dynamic load conditions on unbound materials with different characteristics. Finally, conclusions are drawn regarding the potential use of the equipment in generating values of stiffness modulus for design

A study into the use of crumb rubber in railway ballast

Ballasted track is the most common form of construction used in railway transportation due to a number of benefits in comparison with other solutions such as slab track. However, the degradation of the ballast particles and the layer settlement lead to important maintenance costs. Thus, diverse research has been carried out to develop new materials with the aim of increasing the service life of the track. To this end, the present paper focuses on the use of crumb rubber (from end-of-life tires) as elastic aggregates mixed with ballast particles, which could reduce ballast degradation and consumption of natural aggregates. At the same time, an abundant waste source is reused and the use of raw binders (proposed technique employed to bond elastic particles to ballast particles) is not necessary, which potentially reduces costs and consumption of raw materials. For this reason, the influence of different percentages of crumb rubber was studied in the laboratory using a ballast box. In addition, once the optimal quantity of rubber had been determined, its effect on ballast behaviour under high stress level was analysed. Results show that the use of 10% of crumb rubber (by volume) could reduce ballast degradation and at the same time as the capacity of the ballast layer to dissipate energy is increased and its stiffness is reduced. Additionally, based on the present laboratory study, the track settlement could be reduced with 10% rubber particles used as elastic aggregates