Non-newtonian viscosity of polymer-modified bitumens

The viscosity of road bitumen is a rational, physical property which plays an important role in the technological and quality assessment of a binder. On the other hand, it is believed that the viscosity might substitute for the penetration of a bitumen as a classification property in the appropriate standards. Measurement of the bitumen’s viscosity at lower temperatures, e.g. 60°C, creates some problems caused by the non-Newtonian character of the material. This is observed when dealing with hard bitumens and particularly with polymer-modified bitumens (PmBs). This paper presents viscosity measurements of three PmBs with a rotational viscometer. Regression analysis of the results was conducted according to the four-parameter Cross model, which enabled calculation of the absolute viscosity (or zero shear-rate viscosity). The computed flow curves according to the Cross model were compared with those according to the power-law model frequently applied for road bitumens. The calculated absolute viscosity values together with the conventional test results were applied to the Heukelom bitumen test data chart.     

Effect of ageing and temperature on the fatigue behaviour of bitumens

Bitumen ageing plays a significant role in determining the resistance of asphalt mixes to fatigue cracking. Regardless of the type of ageing (oxidation during manufacture or during the service life), hardening effects increase the risk of cracking. The objective of this work is to examine the combined effect of the loss of volatiles and oxidation produced during ageing on the fatigue behaviour of the bitumen. To this end, different types of bitumen were subjected to accelerated ageing in the laboratory, simulating long-term ageing (RTFOT + PAV). They were then subjected to traditional tests (penetration, softening point, Fraass fragility point, dynamic viscosity, etc.), Dynamic Shear Rheometer tests (frequency and temperature sweep), and the EBADE test (a fatigue strain sweep test at different temperatures). Different temperatures have been used to evaluate the effect of visco-elastic phenomena on aged binder fatigue. The results showed that, in terms of their response to ageing, modified binders show a higher rate of variation in their general properties than conventional binders. In addition, it was shown that temperature plays an important role in the impact of ageing on the fatigue response of bituminous binders, and in the same way, in the mechanical response of these materials.     

Relationship between absolute viscosity of polymer-modified bitumens and rutting resistance of pavement

New polymer-modified bituminous binders require verification of the relationship between binder properties and road pavement performance, formerly found for conventional road bitumens. The paper deals with one particular problem of polymer-bitumen (PB) viscosity and bituminous mixture rutting resistance. Non-Newtonian, shear-dependent behaviour of PB in a viscosity meassurement has been characterized by a new equation proposed by the author. This equation enables calculation of the absolute (zero shear rate) viscosity of PB and is applied for evaluation of test results at 60and 90°C. The absolute viscosity of PB at 60°C has been correlated with the bituminous mixture's rutting resistance at 45°C. Correlation with other rheological properties of PB is also examined by means of multiple regression analysis.     

Effect of thermal ageing on creep and oxidation behaviour of Type 316H stainless steel

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO₂ environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.     

Attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy of oxidized polymer-modified bitumens

Oxidative age hardening of bitumen results in increasing fatigue susceptibility of bituminous mixtures, thus reducing the service life of asphalt pavements. Polymer additives to bitumen have been shown to improve its viscoelastic properties and, in some cases, reduce the level of bitumen hardening. Fourier transform infrared (FT-IR) spectroscopy enables evaluation of oxidation levels in bitumen by measuring the concentration of oxygen-containing chemical functionalities. This paper summarizes the results of the investigation of oxidative age hardening of polymer-modified bitumens (PMB) caused by accelerated aging in laboratory conditions. The PMB samples are prepared with different concentrations of styrene-butadiene-based co-polymers. Next, the PMB samples are aged using standard procedures that employ air blowing at 163 °C for 85 min followed by conditioning the samples at 100 °C and 2.1 MPa pressure for 20 to 48 hours. The resultant changes in their chemical composition are evaluated by portable attenuated total reflection (ATR) spectrometer. Measurements of ketone, sulfoxide, and hydroxyl content in PMB samples indicated similar oxidation pathways to those of non-modified bitumens. In addition, no evidence of polymer degradation due to accelerated aging of PMB was found in this study.     

Low temperature fracture properties of polymer-modified asphalts relationships with the morphology

A methodology for studying the relationships between fracture behavior and morphology of polymer-modified asphalts used as binders was developed by using the linear elastic fracture mechanics (LEFM) method and confocal laser scanning and environmental and cryo-scanning electron microscopies. Different types of polymers were used as modifiers: (i) copolymers from ethylene and methyl acrylate (EMA), butyl acrylate (EBA) or, vinyl acetate (EVA); (ii) diblock or star-shape triblock styrene-butadiene copolymers (SB or SBS^*). The 4 to 6 wt. % blends display an heterogeneous structure with a polymer-rich dispersed phase based on the initial polymer swollen by the aromatic fractions of the asphalt. The fracture toughness of the blends is higher than for the neat asphalt even if KIc of blends remains low compared to usual polymer blends due to the brittleness of the asphalt matrix. The fracture behavior which is strongly dependent on the nature of the polymer is discussed from the toughening mechanisms given for the filled polymers and the polymer blends. The EBA, SB, and SBS-based blends compared to the EMA and EVA-based ones display a higher KIc due to the elastomeric behavior of the polymer phase leading to a more efficient energy dissipation during crack propagation. The sample prepared with 4% crosslinked SB (Styrelf) and the corresponding physical blend (non-crosslinked) display the better fracture properties.     

Effect of precipitate type and morphology on high-temperature creep and internal stress in Al-Li based alloys

The tensile creep of a series of aluminium-lithium-based alloys, two binary alloys containing precipitate, and the 2090 alloy containing and T₁ precipitate, has been studied over a range of stresses at 150°C. In some cases the internal stress developed during creep has been determined using the strain transient dip test. The results have been compared with similar data previously obtained for the 8090 alloy containing and S precipitates. The solid solution alloy and the binary alloy containing shearable particles exhibited normal Class II behaviour, with the development of sub-grains and a stress dependence of the creep rate given by a single stress exponent,n, between 4 and 5 at all applied stresses. The alloys containing particles not easily sheared by dislocations, coarse , S and T₁, exhibited similar stress dependencies of the creep rate at low stresses but exhibited large values ofn, between 18 and 35 at high stresses. The internal stress, _i, in these alloys was found to be approximately constant at high stresses possibly due to partial shearing of the coarse , T₁, and the S on sub-boundaries. The stress dependence of the minimum creep rate, , could be represented at all applied stresses, _a, by , where (_a–_i) is the effective stress driving dislocations during creep, andn is a single stress exponent of between 5 and 6 for all applied stresses. The internal stress, which increases with applied stress, at least at a low applied stress, arises from inhomogeneity of plastic deformation, due to hard sub-boundaries or hard particles which are Orowan looped. These two types of contribution to the internal stress are of similar magnitude in the alloys containing coarse and T₁ but the majority of the internal stress in the 8090 alloy may arise as a result of the hardening of sub-boundaries by the S precipitate.     

The recovery of mechano-sorptive creep strains

Mechano-sorptive strain was generated in strips of rectangular section stressed in torsion with the humidity cycled between 38% and 84%. After a number of humidity cycles the specimen was unloaded, the applied stress was reversed and further humidity cycles were imposed. All the initial strain was recovered and reversed strain was developed. Tests were carried out at two maximum shear stress levels, 1.12 MPa and 5.05 MPa. After seven humidity cycles at the higher stress a large mechano-sorptive strain of about 0.06 was developed, which was nearly nine times the initial elastic strain. During the development of this strain no significant change of the shear modulus occurred, from which it is concluded that the large MS strain produces no permanent change of the features of the cell wall structure that determine the shear modulus. There are close parallels between the mechanism generating the MS strains and that responsible for the large plastic strains reported by Keckes et al. in wet compression wood loaded in tension. In both cases it is suggested that the shear strains arise from the breaking and reforming of hydrogen bonds between parallel polymer chains.     

Effect of hygrothermal ageing on morphology and indentation modulus of injection moulded nylon 6/organoclay nanocomposites

The effect of water immersion on the morphology and indentation modulus of injection moulded nylon 6 and its organoclay nanocomposites was investigated. XRD analysis showed that at 70 °C water promoted further crystallization in the nylon matrix and aided the γ- to α-crystal phase transition in the skin region. However, the presence of organoclay deterred this transformation. The combined actions of water and heat (70 °C) did not further degrade nylon 6 and its nanocomposites compared to water ageing at room temperature (25 °C). In fact, there was relative enhancement of the indentation moduli owing to the beneficial morphological changes induced in the nylon matrix. The largest improvements were found in the skin region of the injection moulded bars.     

Creep and creep recovery of cast aluminum alloys

Constant load uniaxial creep tests were performed on four aluminum alloys (designated M4032-2, 332, 332RR, and 333) at stresses of 31.5 MPa, 56.5 MPa, and 73 MPa and temperatures of 220°C and 260°C. Of the four materials, M4032-2 had the greatest resistance to creep, while 332RR alloy had the least. In addition to creep, the creep recovery phase was observed as well. It was found that, even for short loading periods, much of the time-dependent strain was not recoverable for all of the materials studied. Hardening was observed to occur in each of the alloys, resulting in a reduced creep rate on subsequent loadings. A constitutive equation for creep and recovery incorporating both stress and temperature dependence was developed for each of the alloys tested based on a viscous-viscoelastic model.     

Part 8: Steady-state creep and creep recovery for service and accident conditions

RILEM Technical CommitteesRILEM TC 129-MHT: Test Methods for Mechanical Properties of Concrete at High Temperatures Recommendations

Part 8: Steady-state creep and creep recovery for service and accident conditions     

The effects of extensional creep and creep recovery on the dynamic properties of an unfilled and filled crosslinked polyester resin

Changes in the dynamic properties of unfilled and filled cured polyester resins during extensional creep were measured with the aid of a modified torsion pendulum. Glass beads were used as filler. The decrease in the real shear modulus during creep is attributed mainly to a debonding process. Dewetting-rewetting processes reflected by modulus changes, were discussed on the basis of a theory of Takahashiet al. Voigt-Kelvin rheological models were used to characterize the modulus and the creep compliance recoveries after load release.     

A creep apparatus to explore the quenching and ageing phenomena of PVC films

A creep apparatus has been constructed for anin situ determination of length and length change. Using this apparatus, the creep behaviour of PVC thin films associated with quenching and ageing was studied. The more severe the quench through the glass transition temperature (T _g), the greater is the instantaneous elastic deformation and the subsequent creep behaviour. As ageing proceeds, the quenched films gradually lose the ductility incurred by quenching. These results agree well with the well-known phenomena of physical ageing. Thus, the changes reflecting molecular mobilities due to quenching and ageing can be properly monitored by such a creep apparatus.     

Negative creep and recovery during high-temperature creep of MgO single crystals at low stresses

High-temperature creep equipment with very high precision has been used to measure the creep of MgO single crystals above 1948 K and stresses lower than 4 MPa. A transition in exponent,n, from 3 at stresses higher than 2 MPa to almost unity at lower stress region was observed. Since in a single crystal deformation can only occur by the generation and movement of dislocations, the transition in stress exponent from high to low stress region cannot be interpreted in terms of a change from dislocation to diffusional creep processes. Decreasing the stress by a small amount during steady-state creep resulted in an incubation period of zero creep rate before creep commenced at lower stress. However, large stress reduction led to a period of negative creep during which the dislocation substructure coarsens and the subgrain cell boundaries straighten. On the basis of dislocation substructure studies, it is proposed that the kinetics of backflow are thought to be based on the local network refinement caused by the reverse movement of dislocations and that recovery is necessary before further movement of dislocation can occur. It is shown that the network theory proposed by Davis and Wilshire can satisfactorily account for all stress reduction observed during forward creep.     

Steady state creep and creep recovery behaviours of pre-aging Al–Si alloys

The creep and creep recovery of pre-aging Al–1 wt.%Si and Al–1 wt.%Si–0.1 wt.%Zr–0.1 wt.%Ti alloys have been investigated at room temperature under different constant stresses. The aging temperature dependence of steady creep rate, _st, and the recovery strain rate, π, show that under the same test conditions first alloy yields creep or creep recovery rates much higher as compared with those of second alloy. The stress exponent n was found to change from 2.5 to 7.43 and 4.57 to 11.99 for two alloys, respectively, characterizing dislocation slipping mechanism. The activation energies of steady state creep of the two alloys were found to be 78.4 kJ/mol and 32.8 kJ/mol for Al–Si and Al–Si–Zr–Ti alloys, respectively. The microstructure of the samples studied was investigated by optical and transmission electron microscopy (TEM).     

Effect of ageing on critical cut length and morphology of fracture surface in tensile rupture of natural rubber

Variation of tensile strength with flaw sizes has been studied both for unaged and aged natural rubber (NR) gum vulcanizates (aged up to 150° C). A precut of varying lengths is given at the centre of the tensile specimens. The morphology of the fracture surfaces has also been reported. A critical cut length is observed for NR vulcanizates. There is an increase in the critical cut length (l _c) on ageing. The sharp fall of tensile strength at the critical cut length, however, gradually diminishes. On prolonged ageing, no critical cut length is observed. A mathematical model has been made to explain the behaviour of the critical cut length of NR with ageing time/temperature. Scanning electron microscopic studies support the prediction of Thomas, that there is a change in the mechanism of rupture above thel _c. Below thel _c. it is a cut growth process and fracture is originated from natural flaws/nicks and proceeds towards the precut at the centre. However, for samples with precut greater than thel _c, the fracture is mainly a tearing phenomena initiating from the given precut. A quantative correlation between the tensile strength and the distance between crack lines/tear lines has been found.     

The variation of beta phase morphology after creep and negative creep for duplex titanium alloys

The creep resistance of SP700 (Ti–4.5Al–3V–2Mo–2Fe) is superior to Ti–6–4 (Ti–6Al–4V) at 500 °C under a constant load corresponding to an initial stress of 100 MPa. The β phase grains in the SP700 alloy prefer to orient along the loading axis in contrast to the Ti–6–4 alloy. The grain growth occurs during the stress drop incubation period. The observation of different amounts of negative creep/anelasticity upon loading is closely associated with the difference in the amount of grain/subgrain coarsening.     

Direct methods of analysing the structural effects of linear creep of ageing concrete

Starting from the Boltzmann principle of superposition, and incorporating various different creep functions, governing relationships for concrete are developed in the form of differential equations. These equations are compared in terms of the ease with which they enable direct solutions to be found for structural problems involving the creep of concrete. Two of the formulations, rate of flow and the improved Dischinger method, are judged to be the best compromised between reality and simplicity.