Flexural cement

Singh, S.P. Kaushik, S.K. (2003). Fatigue Strength of Steel Fibre Reinforced Concrete in Flexure, Cement and Concrete Composites, Vol. 25, pp. 779-786, ISSN 0958-9465. 

Singh, S. P., Kaushik, S. K. (2003) Fatigue strength of steel fibre reinforced concrete in flexure, Cement and Concrete Composites, 25 779-86. 

The reinforcing capacity of asbestos fibers in a cement matrix constitutes another key criteria for the evaluation of asbestos fibers. This property is assessed by preparing samples of asbestos —cement composites which, after a standard curing period, are tested for flexural resistance. The measured mpture modub are converted into a parameter referred to as the fiber strength unit (FSU) (34). 

Figure 22 Influence of fiber content on flexural strength and fracture toughness of (O) softwood-cement composites and ( ) hardwood-cement composites (air-cured) [78].

Table 14 Influence of Humidity on Flexural Strength and Fracture Toughness of Cellulose Fiber Reinforced Cements [78]...

Flexural strength and fracture toughness are clinically more significant than compressive strength. The flexural strength of a glass-ionomer cement can reach 39 MPa after 24 hours (Pearson Atkinson, 1991) which is a much higher value than that attained by any dental silicate cement. 

However, these values are less than those recorded for composite resins used in dentistry. Goldman (1985) reports values of 29 to 49 MPa for anterior composite resins and Lloyd Adamson (1987) values of 76 to 125 MPa for posterior composite resins. A typical amalgam has a flexural strength of 6 MPa (Lloyd Adamson, 1987) (Table 5.16). However, the flexural strengths of some glass-ionomer cements increase with time and values as high as 59 MPa (after 3 months) and 70 MPa (after 7 days) have been reported (Pearson Atkinson, 1991). 

These low values for flexural strength and fracture toughness compared with the values for composite resins and dental amalgams make the glass-ionomer cement less suitable than these materials in high-stress situations. 

The use of reinforcing fillers was examined by Seed Wilson (1980). An alumina-fibre cement had a flexural strength of 44 MPa, while one reinforced by carbon fibre had a flexural strength of 53 MPa. Metal reinforcement has also been examined. Seed Wilson (1980) found that a cement reinforced with silver-tin alloy had a flexural strength of 40 MPa. 

Recently, Oldfield Ellis (1991) have examined the reinforcement of glass-ionomer cement with alumina (Safil) and carbon fibres. The introduction of only small amounts of carbon fibres (5% to 7-5% by volume) into cements based on MP4 and G-338 glasses was found to increase considerably both the elastic modulus and flexural strength. There was an increase in work of fracture attributable to fibre pull-out. A modulus as high as 12-5 GPa has been attained with the addition of 12% by voliune of fibre into MP4 glass (Bailey et al, 1991). Results using alumina fibre were less promising as there was no fibre pull-out because of the brittle nature of alumina fibres which fractured under load. 

Very recently, Williams, Billington Pearson (1992) have examined the effect of reinforcement by silver or silver-tin alloy on the mechanical properties of three glass-ionomer cements. Measurements of compressive, flexural, tensile (measured by the diametral compressive procedure) and shell strength are given in Table 5.17. These results show that the effect of reinforcement varies from cement to cement but, in general, increases it. 

The effect on cement C is particularly dramatic and the flexural strength of cement C is exceptionally high. In part, this is to be attributed to the high powder/liquid ratio. These results are to be compared with the flexural strengths of early polyalkenoate cements which were c. 10 MPa. 

Pearson, G. J. Atkinson, A. S. (1991). Long-term flexural strength of glass-ionomer cements. Biomaterials, 12, 658-60. 

Prosser, H. J., Powis, D. R. Wilson, A. D. (1986). Glass-ionomer cements of improved flexural strength. Journal of Dental Research, 65,... 

Little information is available on other tests of strength. Isolated measurements give a flexural strength of 24-5 MPa (0ilo, 1988) and a tensile strength of 13-6 MPa (Kent Wilson, 1971). These values lie within the range of those recorded for glass polyalkenoate cement. Translucency is easily achieved as values for the inverse property of opacity show (Table 6.10). 

Although these cements have high compressive strength, their low flexural and tensile strengths coupled with brittleness and lack of toughness makes them suitable only for low-stress anterior (front teeth) restorations. 

In addition to spectrosopic studies of the setting chemistry of AB cements, numerous mechanical tests have been used to measure properties of the set materials. This latter group has included determination of compressive and flexural strengths, translucency, electrical conductivity and permittivity. The present chapter describes each of these techniques in outline, and shows how they have been applied. Results obtained using these techniques are described in earlier chapters which deal more thoroughly with each individual type of AB cement. 

AB cements tend to be essentially brittle materials. This means that when subjected to mechanical loading, they tend to rupture suddenly with minimal deformation. There are a number of different types of strength which have been identified and have been determined for AB cements. These include compressive, tensile and flexural strengths. Which one is determined depends on the direction in which the fracturing force is applied. For full characterization, it is necessary to evaluate all of these parameters for a given material no one of them can be regarded as the sole criterion of strength. 

The compressive strength of AB cements used in dentistry has been widely studied (Wilson McLean, 1988). It is the method, for example, specified in the British Standard on dental cements. However, there is concern that the result is less clinically relevant than the evaluation of flexural strength. Moreover, the latter is more discriminating (Prosser et al., 1984). Despite this, compressive strength has been used to indicate clinical acceptability phosphate-bonded cements with low compressive strength tend to be unsatisfactory in other respects such as durability, and... 

The presence of the cement hydrate/polymer comatrix in LMM and LMC confers superior properties, such as high tensile and flexural strengths, excellent adhesion, high waterproofhess, high abrasion resistance and good chemical resistance, when compared to ordinary cement mortar and concrete. The degree of these improvements however depends on polymer type, polymer-cement ratio, water-cement ratio, air content and curing conditions. Some of the properties affected by these factors are discussed below [87, 88, 93-95]. 

Most latex-modified mortars and concretes have good adhesion to most substrates (tile, stone, brick, steel and aged concrete) compared to conventional mortar and concrete. In general, bond strength in tension and flexure increases with an increase in the polymer-cement ratio,... 

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