D. J. Hannant

THE MECHANISM OF CREEP IN CONCRETE

RésuméL’étude qu’on décrit ici est une tentative d’élucider les mécanismes physiques fondamentaux qui déterminent le fluage dans la pâte de ciment et, en particulier, d’apporter une preuve expérimentale qui indique laquelle des nombreuses théories du fluage peut être considérée comme étant correcte. Il a été procédé par séchage à l’étuve, chargement, puis resaturation de la pâte de ciment et des éprouvettes de béton à différentes températures. Un seul type d’essai a permis de reconnaître clairement les mécanismes de fluage proposés. Cet essai indique que le cisaillement ou le glissement les unes sur les autres des particules de gèle sub-microscopiques lubrifiées par l’eau doit être un facteur prédominant. Les théories sur les dimensions des espaces critiques dans le gèle de ciment, qui provoquent une instabilité dimensionnelle dans les ouvrages en béton, ont été confirmées avec des liquides organiques aux molécules de diverses dimensions.SummaryAn attempt has been made to investigate the fundamental physical mechanisms which cause creep in cement paste and in particular, to provide experimental evidence which will indicate which of the many theories of creep is most likely to be correct.The method used was to oven dry, load, and then resaturate cement paste and concrete specimens at different temperatures. Only one type of test clearly differentiated between proposed creep mechanisms. This test indicated that the shear or sliding of submicroscopic gel particles over one another, lubricated by water, must be a predominant factor.Existing theories regarding the size of the critical spaces in the cement gel which cause dimensional instability in concrete structures have been confirmed by the use of organic liquids of varying molecular size.

Brittle matrices reinforced with polyalkene films of varying elastic moduli

The inclusion of polyalkene films of different moduli in a cement-based matrix has shown the benefits to be gained, in terms of increased stress at a given strain, from the use of films of high elastic modulus. Further, the concept of load-bearing cracks is used to explain the transition region between the limit of proportionality and the bend-over point on the tensile stress-strain curve, which is found to exist with high film modulus composites. This transition region could be an important factor affecting the choice of film to be used in a commercial composite.

The effects of age up to 18 years under various exposure conditions on the tensile properties of a polypropylene fibre reinforced cement composites

An 18 year test programme has been completed on the tensile stress-strain performance of cement composites reinforced with two types of polypropylene fibrillated networks. Three storage conditions have been used —natural weathering storage inside in laboratory air, and under water storage. Some composites have been precracked before exposure and others left uncracked before testing. The parameters measured included the elastic modulus and cracking stress of the matrix, complete tensile stress-strain curves and crack distribution in the composite, and the effects of the different weathering conditions on the tensile strength, strain to failure and bond strength of the polypropylene reinforcement.In general terms, it was shown that where the fibre volume remained above the critical fibre volume, the strength, ductility and toughness of the composite was maintained regardless of exposure conditions for very long periods of time. In particular, the material has remained ductile, with a failure strain in excess of 5% after 18 years under water which is unusual for fibre reinforced cement composites.RésuméUn programme d’étude d’une durée de 18 ans vient de s’achever sur le fonctionnement en tension des composites ciment renforcés avec deux types de réseaux de polypropylène fibrillé. Trois conditions de conservation ont été utilisées—vieillissement naturel en plein air, conservation ambiante dans le laboratoire, et conservation sous l’eau. Quelques-uns des composites ont été pré-fissurés avant d’être exposés et d’autres sont restés non fissurés jusqu’à l’essai.Les paramètres messurés ont compris le module élastique et la résistance en traction de la matrice, des courbes de contrainte-déformation et la distribution des fissures dans le composite, et les effets de diverses conditions de vieillissement sur la résistance en traction, la déformation à la rupture et la force d’adhérence de l’amature en polypropylène. Il a pu être démontré que lorsque le volume de fibres dépasse le volume critique, la résistance, la ductilité et la dureté du composite ont été maintenues quelles que soient les conditions de l’exposition pendant de très longues périodes. En particulier, le matériau est resté ductile, avec une déformation à la rupture augmentée de 5% après 18 ans sous l’eau, ce qui est rare pour des composites ciment renforcés de fibres.

Hydraulic transport of solids in pipes: a simple method for the prediction of pressure drop

Suspensions such as grouting slurries, concrete and dredge muds exhibit a complex flow behaviour in pipes which is not fully understood. This flow in cement systems is often complicated by the time-dependent behaviour of the material and the apparent slippage at the wall of the pipe. Most of the methods which have been developed to predict the flow behaviour of thick suspensions are very complex and are frequently inaccurate. In this paper a simple method is discussed which can be used to predict the pressure drop in pipes for time-dependent or time-independent materials with a yield stress. The method is based on the assumption that the sheared layer in the pipe can be simulated by a similar layer in the coasial cylinder viscometer, and the properties of the layer are only dependent on plug speed in both systems. The implication of this assumption is that the shear stresses at the surface of the pipe and at the surface of the inner cylinder of the coaxial viscometer are independent of the pipe diameter and the diameter of the cylinders. The method was tested against the authors’ own data and by other data from the literature.ResumeDes suspensions telles que coulis, bétons et boues de drague montrent des caractéristiques complexes d’écoulement en conduit qui ne sont pas encore bien comprises et qui souvent se compliquent dans les systèmes cimentaires par le comportement du matériau en fonction du temps et le glissement apparent à la paroi du conduit. La plupart des méthodes développées pour la prédiction du comportement à l’écoulement de suspensions épaisses sont très complexes et souvent inexactes. On considère ici une méthode simple permettant de prédire la perte de charge en fonction du temps ou non à un seuil de plasticité. Cette méthode s’appuie sur l’hypothèse qu’on peut simuler la couche cisaillée dans le conduit par une couche similaire dans le viscosimètre à cylindres biaxiaux, et que les propriétés de la couche dépendent uniquement de la rapidité de colmatage. Cette hypothèse implique que des contraintes de cisaillement à la surface du conduit et du cylindre intérieur du viscosimètre coaxial sont indépendantes du diamètre du conduit et de celui des cylindres. La méthode a été essayée avec les données personnelles des auteurs et d’autres données tirées de la littérature.

Accelerated ageing of cement sheets containing polypropylene networks

Oven ageing experiments are described which have been used to accelerate the thermal oxidation of networks made of polypropylene films when embedded in cement mortar. The composite is intended to be used in approximately 6 mm thick sheets as an alternative to asbestos cement, the required in-service lifetime for which may be more than 30 years. The accelerated ageing data were obtained from direct tensile tests on the composite, the predictive criteria being strength of the film and strain to failure of the composite. It is shown that 0.25% of commercial antioxidants by weight of the film should give resistance to degradation of the films for well in excess of the required lifetimes when contained in the composite. The performance of the networks with these antioxidants at the lowest test temperature of 80‡ C is so good that the tests at this temperature will probably take more than five years to complete, and therefore the data presented mostly relate to temperatures between 100 and 140‡ C.

Filament fracture within glass fibre strands in hybrid fibre cement composites

In the study of hybrid fibre cement composites containing continuous polypropylene fibres and glass fibres, it is important to know the fracture behaviour of the glass fibre strand in order to minimise the discrepancies between experiment and theory. A new technique of light transmission through the glass fibres has been developed in order to obtain independent information about the failure of individual glass filaments within a strand. The technique gave quantitative results showing that in the hybrid composite, about 80% of the glass filaments were broken somewhere in the strands before the maximum stress in the composite was reached. This was in contrast to the composite reinforced with glass fibres alone where only about 30% of the filaments were fractured before the ultimate stress. The fractures of the glass filaments in the hybrid composite were more evenly distributed than in the singly reinforced composite which enabled greater strains to be achieved in the hybrid composite at the maximum stress.