Failure characteristics of glass-reinforced plastic pipe and pipe assembly

In the examination of glass-reinforced plastic (GRP) pipes and components after failure the fracture is often so catastrophic that much of the evidence is destroyed or hidden. When a leak failure occurred during a hydraulic pressure test on a GRP pipe bend the opportunity was taken to examine the bend wall in detail at a number of localities. They included, not only the region where the leak was apparent but also at areas where strain and displacement measurements had indicated the possibility of failure arising. The 250 mm diameter pipe specimen tested consisted of a right-angled pipe bend with flanged, straight lengths each attached by a butt joint. The resin matrix was polyester and the reinforcement was E-glass in the form of chopped strand mat of four layers weighing 2·4 kg/m².

Although the failure pressure was entirely satisfactory from a design point of view, three important areas of weakness were discovered. They were associated with procedures for butt jointing and for moulding of the bend unit. The resulting micrographs were of sufficient quality to act as comparators for examination of possible service failures.     

Correlation between structure and compressive strength in a reticulated glass-reinforced hydroxyapatite foam

Glass-reinforced hydroxyapatite (HA) foams were produced using reticulated foam technology using a polyurethane template with two different pore size distributions. The mechanical properties were evaluated and the structure analyzed through density measurements, image analysis, X-ray diffraction (XRD) and scanning electron microscopy (SEM). For the mechanical properties, the use of a glass significantly improved the ultimate compressive strength (UCS) as did the use of a second coating. All the samples tested showed the classic three regions characteristic of an elastic brittle foam. From the density measurements, after application of a correction to compensate for the closed porosity, the bulk and apparent density showed a 1 : 1 correlation. When relative bulk density was plotted against UCS, a non-linear relationship was found characteristic of an isotropic open celled material. It was found by image analysis that the pore size distribution did not change and there was no degradation of the macrostructure when replicating the ceramic from the initial polyurethane template during processing. However, the pore size distributions did shift to a lower size by about 0.5 mm due to the firing process. The ceramic foams were found to exhibit mechanical properties typical of isotropic open cellular foams.     

The influence of electron beam irradiation on the shear bond strength of glass-reinforced frameworks and veneer composites

The bond between glass fibre framework and veneer composite can be achieved by silane coupling agents or by monomers that penetrate into a polymer network. However, it has been clinically demonstrated that his bond can fail. This study investigated whether electron beam irradiation improved the bond strength of fibre-frameworks and veneer composite with and without additional coupling agents.     

The interfacial zone and bond strength between aggregates and cement pastes incorporating high volumes of fly ash

The weak transition zone between aggregate and cement paste controls many important properties of concrete. A number of studies dealing with interfacial zone are available in the literature for normal concrete and concrete containing silica fume. High-volume fly ash concrete for structural applications was developed at CANMET in the 1980s, but to date there has been no information available for interfacial zone in high-volume fly ash concrete.In this paper, the orientation index and mean size of Ca(OH)₂ crystals in the aggregate-paste interfacial zone were determined by the X-ray diffractometer. The bond strength between the aggregate and paste was also investigated. It was found that, at the age of 28 days, there was no obvious transition zone between the aggregate and cement paste incorporating high volumes of fly ash. The higher the paste strength, the higher is the bond strength.     

Attempts to improve the bond between cement paste and aggregate

Because there are many coatings on the surface of coarse aggregate particles that reduce the concrete strengths through the reduction of the bond between cement paste and aggregate, a laboratory work is presented whose objective was to find such aggregate coating that improves the concrete strength. To achieve this, coarse aggregate particles were coated with a thin epoxy layer, with or without an adhesive promoter, in which either fine sand particles, or unhydrated Portland cement particles were embedded to make the surface rougher, or chemically more reactive, or both. After the hardening of the epoxy, concrete was made with the coated aggregate and concrete strengths were tested. The test results obtained show that the attempts to increase strength were unsuccessful because even the most effective coating, that with unhydrated Portland cement, produced only small increases in strength.     

Dynamic bond strength between CFRP sheet and steel

The efficiency of the application of adhesively-bonded CFRP materials to rehabilitate and/or strengthen steel structures is completely dependent on the bond between the CFRP composites and the steel. As the repaired and/or upgraded structural elements are likely to be exposed to impact tensile loads during service, this necessitates the importance of understanding the bond strength between CFRP materials and steel under dynamic loadings. This paper presents the experimental procedure, the equipment and the test results for double strap joints at four speeds of loading to highlight the effect of loading rate on the bond strength. Two different CFRP layouts with various bond lengths were examined. It was found that the loading rate has significant influence on bond strength and failure modes, although it has little influence on the effective bond length.     

Evaluating the effect of shrinkage on the effective thermal conductivity of a glass-reinforced plastic

Results are presented from experimental-theoretical determination of the temperature dependence of the effective thermal conductivity of a glass-reinforced plastic on the basis of solution of the inverse problem.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 6, pp. 1001–1008, June, 1989.     

Determination of bond strength in glass fibre reinforced cement using petrography and image analysis

The reinforcement in glass fibre reinforced cement (grc) is not present as discrete fibres, but as strands of about 200 filaments each. This configuration greatly complicates determination of the perimeter of the reinforcing elements, a crucial parameter in bond strength determination. Previous investigators have attempted to quantify strand perimeters but their methods have always involved at least one subjective step and are prone to operator bias. This paper describes an objective method of determining strand perimeters using digital analysis of images captured from petrological thin sections. The measured perimeters were found to be sensitive to the threshold value chosen for the analysis, and consideration of perimeter vs. threshold value curves eliminated subjectivity involved in the analysis. The perimeter value obtained, together with microscopic analysis of the crack patterns produced during tensile testing, were used for calculating bond parameters for different cement matrices. The development of bond with both ageing time and temperature were also studied. The method uses fundamental image analysis concepts and as such is readily adaptable to solve conceptually similar problems in a wide range of materials.     

Effect of heat-aging on the thermal properties of glass-reinforced epoxy plastic

Experimental studies of the thermophysical properties of epoxy-based glass-reinforced plastics have been conducted in the range of 10°–400°K. The appearance of secondary anisotropy in polymer has been observed after repeated cooling of the material.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 29, No. 6, pp. 1057–1062, December, 1975.     

Effects of transition zone densification on fiber/cement paste bond strength improvement

This paper investigates the effectiveness of transition zone densification on the fiber-cement paste system. By controlling the water: cement (w:c) ratio and the condensed silica fume content, environmental scanning electron microscopy studies confirm that transition zone densification can be achieved in all brass, steel, and polyethylene fiber-cement systems. However, single fiber pullout tests indicate that densification only enhances the brass-cement paste interface bond strength and not the other systems. Further microscopy investigation of the surface of fibers peeled off from a composite fracture surface and of the groove left by the fiber on the cement paste suggests that bond failure for the brass-cement system is of a cohesive type, whereas bond failure for the other two systems is of an adhesive type. It is concluded that the transition zone densification technique should be effective in fiber-cement systems in which bond strength is controlled by cohesive failure of the transition zone material.     

On the correlation between porosity and strength in high-alumina cement mortars

The evolution in time of the strength and porosity of mortars made with the same type of high-alumina cement and two different types of aggregate, calcareous and siliceous, is studied, and the effects of aggregate composition on both parameters are analysed. It is found that between strength and porosity a linear correlation exists. The effects of pore diameter on such correlation is investigated and it is ascertained that the best correlation can be obtained for total cumulative porosity. The results show that this correlation can be useful for the determination of compressive strengthin situ.

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Résumé On étudie l’évolution dans le temps de la résistance et de la porosité de mortiers confectionnés avec le même type de ciment alumineux et deux types différents de granulats, calcaire et siliceux, et on analyse les effets de la composition du granulat sur les deux paramètres. On constate qu’il existe une corrélation linéaire entre la résistance et la porosité. On examine les effets du diamètre des pores sur une telle corrélation et on établit qu’on peut obtenir la meilleure corrélation pour la porosité totale. Les résultats montrent que cette corrélation peut servir pour déterminer la résistance à la compressionin situ.

     

Relation between cement composition and compressive strength of pure pastes

In Europe, cement paste compressive strength models are frequently described by a power law similar to that of Féret. The French LCPC has also adopted this mathematical approach in its recent concrete mixture proportioning model. However the coefficient k and exponent b of the adopted power law were calibrated with only one Portland cement clinker, starting from pure pastes with various concentrations. These parameters do not take into account the chemical properties of the clinker. The purpose of this work was to check if the clinker chemical nature had an influence on the mechanical performances of the pastes (at 28 days). Complementary tests with eight cements coming from six different cement plants were then carried out. The mathematical treatment of the results made it possible to connect coefficient k to the silica content of the clinkers and more particularly to their C₃S rates. It is linked to the strength of hydrated cement paste. Thus, the suggested approach significantly improves the accuracy of paste strength calculation. These results can be used to calculate concrete strength.