The rheological behaviour of the dynamically stressed concrete

Summary In the field of concrete dynamics the knowledge so far available has been too disjointed and scanty. To enable a sound analytical arrangement of rheological bonds, we have therefore carried out a vast experimental work, devised to allow simultaneous and cross controls of the several quantities in play: velocity of strains, amplitude of stress excursions, stress gradients, average stresses, physical state, metallic reinforcement, etc. This investigation has given us a general, yet detailed, picture of the dynamic behaviour of concrete, or non-fissured reinforced concrete, clearing up some aspects and correlations that until now seemed to be confused, or did not appear at all. The adoption of special techniques to reveal parasitic energy dissipations, has further shown us how easily certain behaviour in a dynamic system can be confused, and perhaps it has at times happened, for a peculiarity of the material.

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Résumé Il appara?t que les connaissances que nous possédons sur le comportement dynamique du béton sont encore insuffisantes et trop fragmentaires. Afin d’obtenir une représentation analytique correcte des liaisons rhéologiques, il a été entrepris un important travail expérimental con?u pour fournir le contr?le simultané et combiné des différents facteurs en jeu: vitesse des déformations, amplitude des variations de contrainte, gradients des contraintes, contraintes moyennes, état physique, armatures métalliques, etc.. Cette étude nous a donné une vue générale, et cependant détaillée, du comportement dynamique du béton comme du béton armé exempt de fissures, mettant en lumière ainsi certains aspects et corrélations qui jusqu’alors semblaient être obscurs ou qui n’apparaissaient pas. L’adoption de techniques spéciales pour révéler les pertes d’énergie parasites a, en outre, montré la facilité avec laquelle certains phénomènes dans un système dynamique pouvaient se dérober à l’observation, ce qui a pu peut-être aussi, se produire parfois, pour des caractéristiques du matériau.

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This Paper, in concise form, was presented at the “2° Convegno Nazionale del Gruppo di Studio C.N.R. sulla Ingegneria Sismica”, Camogli (Genova), Nov. 1966. Work carried out under grant of the C.N.R.     

The rheological behaviour of concentrated dispersions of graphene oxide

The rheological behaviour of concentrated aqueous dispersions of graphene oxide (GO) was studied as a model system and then compared to those of GO in poly(methyl methacrylate) (PMMA). Dynamic and steady shear tests were conducted using a parallel plate rheometer. The aqueous system behaved as a reversibly flocculated dispersion with linear viscoelastic regions (LVR) extending up to strains of 10 %. Dynamic frequency sweeps conducted within the LVR showed a classic strong-gel spectrum for high concentrations. Under steady shear, the dispersions shear-thinned up to a Peclet number (Pe) <1, followed by a power law at higher Pe. The dispersions were thixotropic and recovered their structure after 60 min rest. The change in rheological properties of the PMMA upon the addition of the GO was less pronounced possibly due to the absence of hydrogen bonding; a relatively small increase in viscosity was found, which is encouraging for the melt processing of graphene composites.     

The effect of fly ashes on the rheological behaviour of cement pastes

A study of the behaviour of fresh cement pastes under shear in a rotational viscometer of the Rheotest-2 type, especially designed for this purpose, is described. It is shown that an addition of fly ash to Portland clinker influences the change of flow behaviour of cement pastes from thixotropic flow to thixotropic-dilatant flow. Moreover, introduction of fly ashes without decreasing the amount of cement brings about an increase in the basic rheological characteristics of cement pastes, growth intensity being dependent on volume fraction of solid and percentage of addition.     

The effects of shearing on the rheological behaviour of thixotropic bentonite muds

The bentonite muds, widely used in civil engineering and in the oil industry, have a Binghamian and thixotropic behaviour that impedes their flow. It has been shown that vibrations, whatever may be their characteristics, are ineffective for improving this flow. Techniques aimed at applying large and slow shearing actions are no better: the thixotropy reduces considerably the size of the flow zone in the vicinity of the shearing surface, even with low concentrations of mud. It is now obvious that if one wishes to cause a large zone of bentonite mud to flow, it is necessary to reduce the thixotropy with the help of appropriate additives.     

The effects of vibrations on the rheological behaviour of bentonite muds. Comparison with cement pastes

The bentonite muds, widely used in civil engineering and in the oil industry, have a binghamian and thixotropic behaviour, that impedes their flow (pumping, displacement by a slurry, etc). A mechanical vibration able to reduce this rheological character, as it does with fresh cement pastes, would improve the flow. Unfortunately vibrations, whatever their frequency may be (from 0 to 3000 Hz) and acceleration (from 1 to 55 times the gravity one) reduce neither the yield-value nor the thixotropic activity of the muds. This results from the structure of the solid phase which is not very resistant but particularly deformable, contrary to the fresh cement pastes: vibration amplitude is not enough to break the bonds between bentonite particles.     

The effect of wall-slip formation on the rheological behaviour of lead-free solder pastes

Wall-slip plays an important role in the flow behaviour of solder paste materials. The wall-slip arises due to the various attractive and repulsive forces acting between the solder particles and the walls of the measuring geometry. These interactions could lead to the presence of a thin liquid layer adjacent to the wall, which causes slippage. The aim of this study is to investigate the influence of the solder paste formulation on wall-slip formation and its effect on the printability of these pastes material. A wall-slip model is utilised to calculate the true viscosity and slip velocity for the lead-free solder pastes samples used in this study. The difference in the measured viscosity and the true viscosity could indicate wall-slip formation between the solder pastes and the parallel plate. Sample P1 showed a higher slip velocity compared to sample P2. The slip velocity calculated for the solder pastes could be used as a performance indicator to understand the paste release characteristics in the stencil printing process.     

Testing bone substitutes in a small animal model of revision arthroplasty

This study evaluated a modification of the rat-pin model to enable testing of bone substitute materials. The model was characterized using the ceramic, -tricalcium phosphate (TCP) as a filler. A 1 mm wide, 3.6 mm deep defect was created around a stainless steel (SS) implant in the proximal tibia of a rat. This defect was filled with a ceramic powder. Large particles (90–312m) of TCP were mixed with Gelfoam® to form a paste which was then molded around the proximal end of either an uncoated SS pin or a pin coated with hydroxyapatite (HA). The pin with its ceramic collar was then implanted into the proximal tibia of 16 male Sprague Dawley rats. Two animals with coated implants and two with uncoated implants were sacrificed at 3, 6, 14 and 26 weeks. Longitudinal sections of each tibia were stained with toluidine blue and labeled for tartrate resistant acid phosphatase (TRAP). There was initial fibrous tissue interposition around the implants which was completely remodeled around the HA coated pins but which persisted in apposition to the SS pins. The remodeling process peaked at 3 weeks around the HA coated pins and at 6 weeks around the uncoated implants. There was little remodeling around either implant by 26 weeks. There was considerable residual TCP present which was well tolerated as the particles were often encased in bone. The model has several characteristics of revision arthroplasty and the results demonstrate the suitability of this model for testing bone substitutes.An erratum to this article can be found at     

Cohesion aspects in rheological behaviour of fresh cement mortars

The cohesion of fresh cement mortars was measured with a device currently used for shear test on clays, the Laboratory Vane. The effect of the water/cement ratio and the specific surface of the cement on cohesion was examined. The influence of the addition of a superfluidizer based on melaminic resins on the cohesion, flow and compressive strength of the mortars was investigated.

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Résumé Dans cet article on considère une composante du seuil de cisaillement des mortiers de ciment à l'état frais, la cohésion, mesurée avec un appareil normalement employé pour essayer le comportement au cisaillement des argiles, le ≪Laboratory Vane≫ On examine l'influence de la teneur en eau du mortier et de la surface specifique du ciment sur la cohésion, en déterminant l'existence d'un intervalle de rapports eau/ciment critiques (0,46–0,48), indépendant de la surface spécifique du ciment, où la cohésion du mortier est maximale et sa valeur est une fonction de la surface spécifique du ciment. Le rapport eau/ciment critique représente la limite inférieure du champ du comportement plastique du mortier et peut être défini comme la limite de plasticité. L'addition aux mortiers de ciment d'un adjuvant superplastifiant à base de résines de mélamine provoque le glissement de la limite de plasticité vers des rapports eau/ciment plus bas, au-dessous de 0,40 pour une addition de 5% en poids de ciment. On examine l'influence de l'addition d'adjuvant en différents pourcentages sur l'écoulement, la cohésion et la résistance mécanique dans un mortier plastique.

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Influence of powder characteristics on the rheological behaviour of hydroxyapatite slurries

A study of the influence of several dispersants (nature and concentration), of the calcination temperature and of the concentration of hydroxyapatite (HAP), on the rheological behaviour of HAP slurries, made it possible to obtain low viscosity slurries containing 73 wt% HAP and 3.5 wt% dispersant. The calcination of the HAP powder affected both the morphology and the chemical nature of the surface of particles. This last effect was clearly shown by the decrease of the zeta potential with increasing calcination temperature. The optimal calcination temperature was chosen beyond 800 °C. Parts sintered at 1200 °C for 30 min were obtained with a density higher than 96% of theoretical by casting optimized suspensions.     

Histomorphological, histomorphometrical and biomechanical analysis of ceramic bone substitutes in a weight-bearing animal model

It was the purpose of this investigation to prove the biomechanical properties, the osteoconductive capacity and the degradation rate of tricalcium phosphate ( TCP), a neutralized glass ceramics (GB9N) and a composite material (GB9N+copolymers). In a weight-bearing animal model six substitutes each were implanted in the medial tibial head of the right lower leg of adult Merino-sheep in a standardized surgical technique. After nine months the implants were harvested and prepared for histomorphological and histomorphometrical investigations (undecalcified Masson Goldner staining). For additional biomechanical testing of the specimens, non-operated bone blocks from the contralateral tibia as well as native implants served as controls. No significant differences for the maximum fracture load as well as for the yield strength were detected between harvested specimens and bone blocks from the contralateral tibia. However there were marked differences to ceramics that were not implanted. All substitutes showed osteoconduction, leading to a continuous ingrowth of new formed bone. However in the composite material soft tissue could be identified within the scaffold and there were signs of ongoing bone remodeling, nine months after implantation. The bone per tissue volume of -TCP in conjunction to new bone (=percentage of trabecular bone volume plus percentage of residual substitute) was higher than for GB9N and the composite material. Nine months after implantation the percentage of residual -TCP was 48%, it was 32% for GB9N and 28% for the composite.The intention of further studies should be to accelerate the degradation rates of substitutes and to improve biomechanical properties of implants by either modifying the chemical composition or combining materials with agents as, e.g. growth factors.     

Tensile behaviour of nonwoven structures: comparison with experimental results

Nonwoven structures have been recently explored for numerous novel applications ranging from composites to scaffolds. The tensile property of nonwovens is a pre-requisite and indeed, one of the main parameters to determine their performance for such applications. In the first part, a modified micromechanical model describing the tensile behaviour of thermally bonded nonwovens was proposed by incorporating the effect of fibre re-orientation during the deformation (Rawal et al., J Mater Sci 45:2274, 2010). In this study, an attempt has been made to compare the theoretical and experimental stress–strain curves of thermally bonded and spunbonded nonwoven structures. These theoretical findings have been obtained from the most popular analytical tensile models of nonwovens available in the literature in addition to our modified tensile model. Poisson’s ratio has also been determined experimentally in order to predict the stress–strain behaviour of nonwoven, and its relationship with longitudinal strain has clearly distinguished between the randomly and preferentially orientated types of structures. In thermally bonded nonwovens, the tensile strength in various test directions is computed through pull-out stress and a comparison is made with the experimental results.     

增容剂对热塑性淀粉/聚丙烯复合材料的力学及流变加工性能的影响

先通过甘油对淀粉进行塑化制备热塑性淀粉(TPS),然后以马来酸酐接枝聚丙烯(MA-g-PP)作为增容剂将聚丙烯(PP)与TPS共混,制成TPS/PP复合材料。通过电子拉力机、热重法、转矩流变仪、挤出毛细管流变仪以及熔融指数仪,研究了不同含量的MAg-PP对TPS/PP性能的影响。结果表明,MA-g-PP的添加可有效提高材料的力学和加工性能,综合生产成本和增容效果考虑,当MA-g-PP的添加量为PP/TPS总量的4%(质量分数)时,材料具有较高的力学性能和较佳的加工性能。     

Synthesis,extrusion and rheological behaviour of PU/HA composites for biomedical applications

Biostable polyurethane/hydroxyapatite (PU/HA) composites with potential application as bone replacement materials were synthesized in bulk and processed in a screw extruder. The polyurethanes (PU) were prepared by reacting an aliphatic diisocyanate, 4-methylene-bis-diisocyanate (MDI), with poly-(ε-caprolactone) (PCL) diols and polytetramethylene oxide (PTMO) of different molecular weights, extended with 1, 4-butanediol (BDO). Glass-transition temperatures were measured by differential scanning calorimetry (DSC). The specific PU groups were assessed by total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR). The effects of polymer chemistry and filler content on the rheological behaviour were studied by oscillatory rheometry. Polymers with larger chain lengths showed higher viscosity and, for identical chain lengths, polyether urethanes seem to have higher viscosities than polyester based urethanes. A lubricating effect was found for composites containing 50% weight of filler, whereas at higher filler contents a solid-like behaviour was measured. Polymer chemistry seems to be affected by ageing but not so by the presence of filler. Ageing is characterized by a decrease in the concentration of hydrogen bonds involving between urethane linkages.     

Consequences of competitive adsorption between polymers on the rheological behaviour of cement pastes

Self Compacting Concretes (SCC) are characterized by their high fluidity so they can be placed in sections with congested reinforcements and in restricted areas without vibration. Furthermore, SCC cement paste has to be viscous enough to avoid segregation and to maintain the stability of the suspension until the onset of hardening. To fulfil these rheological requirements, mix design engineers combine use of superplasticizers and viscosity agents. The mechanism of action of these chemical admixtures is very sensitive to their adsorption. The blending of these polymers generates a competitive adsorption on surface sites of cement particles, which influences their performances. For a better understanding of competitive adsorption, we measure here both the amount of adsorbed polymers and its consequences on the rheological behaviour of the system in terms of yield stress and plastic viscosity. Our results suggest that the competitive adsorption prevents some of the polymer molecules from adsorbing, thereby moderating the performances of adsorbing polymers and enhancing the effects of polymers potentially in solution on the rheological properties of cement paste.     

Influence of rheological behaviour on extrusion parameters during non-continuous extrusion of multi-base propellants

During the production of multi-base propellants, in order to identify both the optimal and safety conditions to be used at processing these rubber-like compounds, its rheological behaviour is considered as a key factor. Thus, an analysis of their rheological moduli at the end of the mixing stage and prior to the non-continuous extrusion (ram extrusion) is required so as to predict the optimum parameters to be assumed for both the mixing and extrusion processes. The rheological analysis of this multi-base propellant is carried out by means of a DHR-1 rotational rheometer of TA Instruments, and the complex modulus is proposed as a generalized indicator of the rheological response of these special materials. The complex interrelations among the main parameters of the extrusion process (steady extrusion pressure and extrusion ram velocity) together with the rheological properties of these multi-base propellants, will serve to deduce useful technical guidelines for improving the manufacturing system productivity, safety processing conditions and final product quality.     

Dependence of the rheological behaviour of electrostatically stabilized alumina slurries on pH and solid loading

Alumina slurries with different solid loadings were prepared using aqueous solutions of hydrochloric acid the concentrations of which were predetermined as suitable for dispersing media; their rheological behaviour was investigated with respect to the pH of the slurry. In regions of low pH (below 3.5), the viscosity of the slurries gradually increased with decreasing pH of the slurry, irrespective of the amount of solid loading. Near the isoelectric point of alumina (about pH 8–8.5), the viscosities of the slurries were high and dependent on the solid loading. The pH range which exhibits the regions with low viscosity narrows on increasing the solid loading. These results were interpreted using an interparticle potential energy diagram for the slurry system. © 1998 Kluwer Academic Publishers     

Effect of polymeric thickeners on pigment coatings: Adsorption, rheological behaviour and surface structures

Polymeric thickeners play a vital role in controlling water retention in paper coating processes. In this work, the interactions between various polymer water retention agents (i.e., thickeners) and coating pigments were investigated, in an attempt to improve the understanding of the thickening mechanisms. Adsorption isotherms of the polymeric thickeners, up to four types of commercial thickeners based mainly on copolymers of acrylate and acrylic acid, onto mineral pigments (clay pigment and ground calcium carbonate, GCC) were determined. It was found that the adsorption behaviour was strongly affected by the surface structure of the pigment. All the selected thickeners showed greater affinity towards clay pigment than calcium carbonate. The influence of the selected thickeners on coating mixtures behaviour at low shear rate was also investigated. In coating mixture based on clay, it was found that HASE (hydrophobically modified alkali swellable emulsion) thickener that has the highest adsorption led to more effective thickening. All the selected thickeners showed moderate effect on the viscosity of the coating mixture based on GCC. The presence of thickeners also produced the coated paper with smooth and homogeneous surfaces, revealed by SEM observation.     

Shrinkage behaviour of superplasticised RCCP and its relationship with internal temperature

Shrinkage properties of roller-compacted concrete pavement (RCCP) and its relationship with internal temperature variations in superplasticised RCCP were studied. The control and superplasticised RCCPs containing different amounts of sodium naphthalene sulphonate superplasticiser between 1 and 4% by weight of cement were compacted at their optimum water content. The short- and long-term shrinkage and internal temperature variations were monitored in isolated and non-isolated conditions. Micro-structural analysis was implemented using the electron scanning microscope and X-ray diffraction methods. The initial internal temperature of superplasticised RCCP during the first hours of setting was higher than the control mix. A dormant period of low internal temperature was found for superplasticised mixes which started after about 7–13 and 6–10 h in non-isolated and isolated conditions, respectively. For non-isolated condition, the dormant period lasted about 3 days. Under isolated condition, similar trend was found after 4–8 days. During the first day of curing, the total and drying shrinkages of the control RCCP were considerably higher than the superplasticised mixes. On the contrary, the long-term monitoring indicated higher total and drying shrinkages in superplasticised mixes. Based on statistical analysis, the relationship between the autogenous shrinkage and area under the internal temperature diagram was significant.