Characterization of the Fouled Ballast Layer in the Substructure of a 19th Century Railway Track Under Renewal

The purpose of these field and lab studies undertaken during rehabilitation work being done on an ancient railway line was to characterize a layer of ballast fouled with soil found in the track substructure. The field studies included the characterization of the thickness, grain size distribution and void ratio of the fouled ballast layer, as well as a large number of plate load tests, both on the fouled ballast layer and on the subgrade. The resilient behaviour of the fouled ballast was evaluated in the lab by cyclic triaxial tests on large size reconstituted specimens with distinct fouling indexes (different grain size distribution) and distinct humidity states (dry or wet). The results obtained were used as support for the decision to maintain the fouled ballast layer under the new sub-ballast in a number of stretches of the renewed line.     

Effects of fines and water contents on the mechanical behavior of interlayer soil in ancient railway sub-structure

In the ancient railway sub-structure in France, after years of operation, the inter-penetration of fine particles of sub-grade and ballast has created a new layer referred to as the interlayer. As it was naturally formed, the fines content and water content of the interlayer vary considerably. In this study, the effects of the fines and water contents on the mechanical behavior of interlayer soil were investigated by carrying out large-scale monotonic and cyclic triaxial tests. The results of the monotonic triaxial tests show that adding more fines in the interlayer soil does not significantly change the shear strength in the dry condition (water content w=4% and 6%), but drastically decreases the shear strength parameters (friction angle and cohesion) in the nearly saturated condition (w=12%). The cyclic triaxial tests were performed at various deviator stress levels. By considering the permanent axial strain at the end of application of each stress level, it was found that the higher the fines content in the nearly saturated condition (w=12%), the larger the permanent axial strain. In the case of lower water content (w=4% and 6%), the opposite trend was identified: adding fines decreases the permanent axial strain.     

Effect of different types of wetting fluids on the behaviour of expansive soil during wetting and drying

This paper presents the results of an experimental investigation into the mechanical behaviour of an expansive soil during wetting and drying cycles. The experimental tests were conducted in a modified oedometer under two different surcharge pressures (10 and 20 kPa). During the tests, the samples were inundated with different types of wetting fluids (distilled water, saline water and acidic water). The volumetric deformation, void ratio and water content of the samples were determined during cycles of wetting and drying. The results show that the swelling potential increases with an increasing number of wetting and drying cycles. The effect of the distilled water on the swelling potential is not the same as that of the saline water or the acidic water, particularly for different surcharge pressures. The variations in void ratio and water content show that, at the equilibrium condition, the wetting and drying paths converge to nearly an S-shaped curve. This curve consists of a linear portion and two curved portions, and the majority of the deformation is located between the saturation curves of 90% and 40%.     

Large-scale triaxial tests of dense gravel material at low confining pressures

The results of a series of large-scale triaxial tests performed on dense, prismatic gravel specimens, with a height of 50 cm and a cross-section of 23 cm×23 cm, are described. The specimens were prepared at a density equal to approximately 95% of the maximum density at the optimum moisture content. Deformations were measured locally using vertical and horizontal local deformation transducers. Stress conditions with selected levels of very low confining pressure were used to simulate specific conditions in the case of road and railway embankments. Particular attention was paid to the bedding error at the top and the bottom ends of the specimens, and to fixing transducers onto the membrane to be used under low confining pressure. The confining pressure was applied by vacuum and varied from 10 kPa to 75 kPa. Unsaturated specimens were tested under drained triaxial compression using monotonic and cyclic loading with frequencies in the range of 0.5–5 Hz. The effects of a large number of load cycles and of specimen preloading were investigated.     

Self-ignition of natural fuels: Can wildfires of carbon-rich soil start by self-heating?

Carbon-rich soils, like histosols or gelisols, cover more than 3% of the Earth's land surface, and store roughly three times more carbon than the Earth's forests. Carbon-rich soils are reactive porous materials, prone to smouldering combustion if the inert and moisture contents are low enough. An example of soil combustion happens in peatlands, where smouldering wildfires are common in both boreal and tropical regions. This work focuses on understanding soil ignition by self-heating, which is due to spontaneous exothermic reactions in the presence of oxygen under certain thermal conditions. We investigate the effect of soil inorganic content by creating under controlled conditions soil samples with inorganic content (IC) ranging from 3% to 86% of dry weight: we use sand as a surrogate of inorganic matter and peat as a surrogate of organic matter. This range is very wide and covers all IC values of known carbon-rich soils on Earth. The experimental results show that self-heating ignition in different soil types is possible, even with the 86% inorganic content, but the tendency to ignite decreases quickly with increasing IC. We report a clear increase in ambient temperature required for ignition as the IC increases. Combining results from 39 thermostatically-controlled oven experiments, totalling 401 h of heating time, with the Frank-Kamenetskii theory of ignition, the lumped chemical kinetic and thermal parameters are determined. We then use these parameters to upscale the laboratory experiments to soil layers of different thicknesses for a range of ambient temperatures ranging from 0 °C to 40 °C. The analysis predicts the critical soil layer thicknesses in nature for self-ignition at various possible environmental temperatures. For example, at 40 °C a soil layer of 3% inorganic content can be ignited through self-heating if it is thicker than 8.8 m, but at 86% IC the layer has to be 1.8 km thick, which is impossible to find in nature. We estimate that the critical IC for a ambient temperature of 40 °C and soil thickness of 50 m is 68%. Because those are extreme values of temperature and thickness, no self-heating ignition of soil can be expected above the 68% threshold of inorganic content. This is the first in-depth experimental quantification of soil self-heating and shows that indeed it is possible that wildfires are initiated by self-heating in some soil types and conditions.     

Prediction of bituminous mixture fatigue life based on accumulated strain

This research was aimed to predict the number of cycles that cause fracture of hot-mix asphalt (HMA) based on the number of cycles at which the slope of accumulated strain switched from decreasing to increasing mode. In addition, the effect of aggregate gradation and temperature on fatigue behaviors of HMA were evaluated.HMA specimens were prepared at optimum asphalt content using the Marshall mix design procedure. The specimens were prepared using crushed limestone aggregate, 60/70 penetration asphalt, and three different aggregate gradations with maximum nominal aggregate size of 12.5, 19.0, and 25.0 mm. Five magnitudes of load (1.5, 2.0, 2.5, 3.0, and 3.5 kN) were evaluated for their effect on fatigue behavior.Constant stress fatigue tests were performed using the Universal Testing Machine (UTM) at 25 °C. Other temperatures (10, 45, and 60 °C) were evaluated at a load of 3.5 kN.The tests results indicated that the slope of accumulated strain continued to decrease until the number of loading cycles approached 44% of the number of cycles that caused fracture of the HMA. Also, the initial stiffness of asphalt mixtures was found to increase as the magnitude of the load applied increased and as the aggregate gradation maximum nominal size decreased.     

Resilient behavior of compacted subgrade soils under the repeated triaxial test

Subgrade soils are very important materials to support highways. Resilient modulus (M_r) has been used for characterizing stress-strain behavior of subgrades subjected to repeated traffic loadings. Recently the repeated triaxial test procedure has been upgraded through AASHTO T 307. Since the testing procedure is still complex, the testing has not been widely implemented in practice. In order to evaluate resilient behavior of compacted subgrades soils, the repeated triaxial test and the unconfined compressive test as well as some fundamental property tests were conducted. In this study, the applicability of a simplified procedure with a confining pressure of 13.8 kPa and deviator stresses of 13.8, 27.6, 41.4, 55.2, 69 and 103.4 kPa was investigated on the typical sandy–silty–clay and silty–clay subgrade soils encountered in Indiana. The results obtained from the simplified procedure are comparable with those obtained from AASHTO T 307 which calls for 15 combinations of stresses. This shows the simplified procedure to be feasible and effective for design purpose. Some soils compacted wet of optimum moisture content showed an excessive permanent deformation. This phenomenon was investigated by the comparison of the unconfined compressive test and the repeated triaxial test results. For soils exhibiting excessive permanent deformation, use of deformed length is desirable for more accurate calculation of M_r. Usually the soils compacted dry of optimum shows the largest M_r for sandy–silty–clay soils due to capillary suction, but it is not necessarily true for silty–clay soils. A predictive model to estimate regression coefficients k₁, k₂, and k₃ using 11 soil variables obtained from the soil property tests and the standard Proctor compaction tests was developed. The predicted regression coefficients compare well with measured ones.     

Stiffness and strength parameters for hardening soil model of soft and stiff Bangkok clays

A comprehensive set of experimental data on Bangkok subsoils from oedometer and triaxial tests are analysed in this paper in order to determine the stiffness and strength parameters for Hardening Soil Model. The parameters determined are the Mohr–Coulomb effective stress strength parameters together with the stiffness parameters; tangent stiffness for primary oedometer loading, secant stiffness in undrained and drained triaxial tests, unloading/reloading stiffness and the power for stress level dependency of stiffness. The oedometer data are obtained from three different Bangkok soil layers: soft clay at 6–8 m depths; medium clay at 12–14 m depths; and stiff clay at 15.5–18 m depths. The triaxial tests data are carried out for soft and stiff clays at depths of 5.5–6 m and of 16–18 m under both undrained and drained conditions, respectively. Finally, two sets of parameters for soft and stiff Bangkok clays are numerically calibrated against undrained and drained triaxial results using PLAXIS finite element software.     

Silica sol for rock grouting: Laboratory testing of strength,fracture behaviour and hydraulic conductivity

A recently introduced non-cementitious grout silica sol is a refined product of colloidal silica, where the particle sizes have been reduced to between 5 and 100 nm. Laboratory tests were performed to determine the behaviour of silica sol as a permeation grout in hard rock. The tests have involved methods such as fall-cone, unconfined compression, triaxial, and oedometer tests. Samples were tested at different time intervals and in different storage environments. Results showed that the initial strength of silica sol, a few kPa, increases over time. Silica sol has a ductile behaviour during the first few days and then becomes elastic–plastic. Its hydraulic conductivity ranges from 10^?10 to 10^?11 m/s. When immersed in water, silica sol hardens and a thin layer of weaker strength is formed at the surface. However, this layer only extends a couple of millimetres into the sample; beyond that the silica sol is not affected, rendering breakdown by erosion a negligible risk. The conclusions are: (1) the strength obtained in silica sol after hardening is sufficient to withstand most grouting conditions; (2) when sufficiently confined, silica sol is able to withstand loading and unloading cycles; (3) a pH environment of around 11 does not appreciably change the strength of the silica sol; (4) silica sol is a material with low risk of failure under blasting vibrations; and (5) due to its low hydraulic conductivity, silica sol can be compared to low permeable clays.     

Preparation of eco-friendly construction bricks from hematite tailings

With the objective of reducing the negative impacts on environment and utilizing the secondary resource of tailings, the possibility of making construction bricks by using the hematite tailings from western Hubei province of China was investigated. Besides hematite tailings, the additives of clay and fly ash were added to the raw materials to improve the brick quality. Through the process of mixing, forming, drying and firing, the bricks were produced. The optimum conditions were found to be that the hematite tailings content were as high as 84%, forming water content and forming pressure were respectively in the range of 12.5–15% and 20–25 MPa, and the suitable firing temperature was ranged from 980 to 1030 °C for 2 h. Under these conditions, the mechanical strength and water absorption of the reddish fired specimens were 20.03–22.92 MPa and 16.54–17.93%, respectively, and the other physical properties and durability were well conformed to Chinese Fired Common Bricks Standard (GB/T5101-2003). The phases and morphologies of the green tailings and fired specimen were characterized by XRD and SEM. The results showed that the main mineral phases of the product were hematite, quartz, anorthite and tridymite, which were principally responsible for the mechanical strength of bricks.     

Rutting evaluation of hydrated lime and SBS modified asphalt mixtures for laboratory and field compacted samples

The purpose of this study is to evaluate permanent deformation for hydrated lime and SBS modified asphalt mixtures. Control (C), 2% hydrated lime (2L), 5% SBS polymer mixtures and 2%hydrated lime–5%SBS (2L5SBS) mixtures were prepared. The Laboratoire Central des Ponts et Chaussées (LCPC) wheel tracker, also known as French Rutting Tester were realized with two different stages. Same LCPC slabs were produced. Original LCPC compactors and also field cylinder were used separately. LCPC rutting values were determined with left and right wheel loadings. Also averages were obtained with calculation. Repeated creep tests were used for these mixtures and permanent deformations were plotted for two different moisture conditioning that water immersion and freeze and thaw cycles. Diameter samples (100 mm and 150 mm) were studied in repeated creep tests. In the result that LCPC tracking values were compared with repeated creep tests in terms of sample diameters. LCPC wheel-tracking test results show that 2L5SBS mixtures reveal utmost performance according to the other mixtures types. Polymer modification increased rutting resistance of lime modified ones. Both original LCPC compactor and field cylinder compaction showed resemble results. 150 mm samples showed highest correlation (higher than R² = 0.80) between LCPC test and repeated creep test for different compaction types and different moisture conditionings.     

Frost attack resistance and steel bar corrosion of antiwashout underwater concrete containing mineral admixtures

This study aims to evaluate frost durability and steel-bar corrosion in antiwashout-underwater concrete, which has been neglected to date. To achieve this goal, repeated freezing and thawing and accelerated steel-bar corrosion tests have been performed for three types of antiwashout-underwater concrete specimens.The results of repeated freezing and thawing test reveal that adding mineral admixtures has little effects on frost durability because of the large and uneven entrapped-air imprisoned by the cellulose-type antiwashout-underwater admixture. Slight improvement of frost durability was observed through the action of air-entrained (AE) agent in the case of SG50 which presented an air content of 6 ± 0.5%.Measurement results using the half-cell potential showed that, among the entire specimens, steel-bar in Control specimen manufactured under artificial seawater was the first one that exceeded the threshold value, −350 mV proposed by ASTM C 876, at 14 cycles, where the corresponding corrosion current density and concentration of water soluble chloride were measured as 0.3 μA/cm² and 0.258%, respectively. For the other specimens, potential values became below −350 mV later than 18 cycles.     

Modeling mechanical performance of lightweight concrete containing silica fume exposed to high temperature using genetic programming

In this study, the mechanical performance of lightweight concrete exposed to high temperature has been modeled using genetic programming. The mixes incorporating 0%, 10%, 20% and 30% silica fumes were prepared. Two different cement contents (400 and 500 kg/m³) were used in this study. After being heated to temperatures of 20 °C, 200 °C, 400 °C and 800 °C, respectively, the compressive and splitting tensile strength of lightweight concrete was tested. Empirical genetic programming based equations for compressive and splitting tensile strength were obtained in terms of temperature (T), cement content (C), silica fume content (SF), pumice aggregate content (A), water/cement ratio (W/C) and super plasticizer content (SP). Proposed genetic programming based equations are observed to be quite accurate as compared to experimental results.     

Shear strength of RC beams subjected to cyclic thermal loading

The experiments were performed for assessing the influence of cyclic thermal loading on the shear strength of reinforced concrete (RC) beam specimens. One hundred eleven RC beams of 100 × 150 × 1200 mm size reinforced in tension zone with two bars of 8, 10 and 12 mm diameters were tested under four point loading. The beams were subjected to a number of thermal cycles varying from 7 to 28 cycles with peak temperature taken as 100, 200 and 300 °C. The effects of thermal cycles on the crack pattern, failure mechanism, first crack load and the shear strength of beams have been discussed. The shear strength of the beams has been found to increase by up to 10% at lower temperature cycles of 100 and 200 °C but reduces by up to 14% at higher temperature (300 °C) depending on the severity of thermal loading. The results of study emphasize the need for developing appropriate guidelines for the design of RC structural elements used in comparatively high temperature environment with cyclic thermal loading conditions.     

Effect of fiber type and content on bleeding of steel fiber reinforced concrete

In general, the addition of fibers in concrete mix significantly improves many of the engineering properties of concrete. On the other hand, steel fibers reduce the workability of concrete. This paper presents the effect of steel-fiber length (aspect ratio) and content on bleeding of steel fiber reinforced concrete (SFRC). Two different steel fiber types (both is hooked-end) were used at a ratio of 0% (control), 0.3%, 0.64%, 1% and 1.3% by volume. Slump, Ve-Be test, air content and unit weight were determined experimentally. Specimens were poured in the standard moulds and the bleeding water content was measured 30 min, 60 min, 90 min, 120 min, 150 min and 180 min after starting the test. The results indicated that the workability of concrete significantly reduced as the fiber dosage rate increases. This was assessed through standard slump test and Ve-Be consistometer test. The bleeding water content was increased by increase of the fiber volume fraction and fiber aspect ratio according to experimental results. Also, a bleeding coefficient value for SFRC made with and without steel fiber was proposed as a result of this study.     

Effect of red mud addition on the rheological behaviour and on hardened state characteristics of cement mortars

This paper reports on the use of red mud (RM) in mortars, applying design of experiments. Portland cement was replaced up to 50 wt.% RM, adjusting the relative amount of water (34–38 wt.%) in order to get mortars with suitable workability as defined by rheometry and flow table measurements. Temperature of hydration, compressive strength and water absorption were also determined. RM decreases the workability and increases the torque, but causes lower impact than water variation. The effect on initial yield stress depends on water content. Mortars with similar spread on table show different behaviour along the rheology test. Values of spread on table follow a quadratic model and RM exhibited an interactive effect with water. RM did not change the hydration process, but above 20% the maximum temperature decreases. The reduction of compressive strength is not constant and depends on the water added. Its variation also follows a quadratic model.     

Laboratory study and interpretation of mechanical behavior of frozen clay through state concept

Towards the development of a mechanical model that can be part of multi-physical analysis of frozen soils, a program of systematic frozen-unfrozen parallel triaxial tests at different temperatures and strain rates was conducted. The mechanical behavior of the reconstituted high-plasticity clay samples was investigated and interpreted through a state concept based on Ladanyi and Morel’s (1990) postulate on the unique relationship between the inter-particle “effective” stress and the strain path. The Critical State Lines (CSLs) for clay specimens frozen undrained were mapped by referring to the shear behavior of unfrozen specimens sharing the same strain history. With other conditions set identical, the shear strength linearly increased with a decrease in the temperature for the range from ?10 °C to ?2 °C, and log-linearly increased with an increase in the strain rate for the range from 0.001%/min to 0.1%/min. Direct comparison of the strain-rate effects between frozen and unfrozen specimens with identical strain paths and states in the soil skeleton clearly indicates that the viscoplasticity derives from that of pore ice. A conceptual interpretative framework invoking temperature- and strain rate-dependent state bounding surfaces and CSLs was proposed to describe the behavior of frozen soils under steady and non-steady temperature and strain rate. The above observations of the behavioral features of frozen and unfrozen soils, with further experimental work, are expected to lead to the construction of a unified framework for describing the behavior under both states and the transition between them.     

Modeling with ANN and effect of pumice aggregate and air entrainment on the freeze–thaw durabilities of HSC

The objective of this work is to calculate the compressive strength, ultrasound pulse velocity (UPV), relative dynamic modulus of elasticity (RDME) and porosity induced into concrete during freezing and thawing. Freeze–thaw durability of concrete is of great importance to hydraulic structures in cold areas. In this paper, freezing of pore solution in concrete exposed to a freeze–thaw cycle is studied by following the change of concrete some mechanical and physical properties with freezing temperatures. The effects of pumice aggregate (PA) ratios on the high strength concrete (HSC) properties were studied at 28 days. PA replacements of fine aggregate (0–2 mm) were used: 10%, 20%, and 30%. The properties examined included compressive strength, UPV and RDME properties of HSC. Results showed that compressive strength, UPV and RDME of samples were decreased with increase in PA ratios. Test results revealed that HSC was still durable after 100, 200 and 300 cycles of freezing and thawing in accordance with ASTM C666. After 300 cycles, HSC showed a reduction in compressive strength between 6% and 21%, and reduction in RDME up to 16%. For 300 cycles, the porosity was increased up to 12% for HSC with PA. In this paper, feed-forward artificial neural networks (ANNs) techniques are used to model the relative change in compressive strength and relative change in UPV in cyclic thermal loading. Then genetic algorithms are applied in order to determine optimum mix proportions subjected to 300 thermal cycling.     

Multiaxial tensile–compressive strengths and failure criterion of plain high-performance concrete before and after high temperatures

Multiaxial tensile–compressive tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-performance concrete (HPC) at all kinds of stress ratios after exposure to normal and high temperatures of 20, 200, 300, 400, 500, and 600 °C, using a large static–dynamic true triaxial machine. Friction-reducing pads were three layers of plastic membrane with glycerine in-between for the compressive loading plane; the tensile loading planes of concrete samples were processed by attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of specimens and the direction of the crack were observed and described, respectively. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures, stress ratios, and stress states on the triaxial strengths of HPC after exposure to high temperatures were also analyzed respectively. The experimental results showed that the uniaxial compressive strength of plain HPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength depend on brittleness–stiffness of HPC after different high temperatures besides the stress states and stress ratios. On this basis, the formula of a new failure criterion with the temperature parameters under multiaxial tensile–compressive stress states for plain HPC is proposed. This study is helpful to reveal the multiaxial mechanical properties of HPC structure enduring high temperatures, and provides the experimental and theory foundations (testing data and correlated formula) for fire-resistant structural design, and for structural safety assessment and maintenance after fire.