Property mismatch

Possible concerns arise over the potential property mismatch between the repair material and the substrate concrete. Short-term problems may arise because the repair material contracts on curing relative to the surrounding concrete. With resin based mortars the 

To illustrate the effect of modulus mismatch on structural performance consider the symmetrically repaired reinforced concrete prism shown in Fig. 6.4 and loaded axially in tension. The concrete has a modulus of elasticity in tension of 25 kN/mm. For material C ( t = 14 kN/mm ) the elastic stress induced in the concrete at mid-height of the prism will be 2.5 times that in the repair material at the same position. Conversely for material D ( , = 43 kN/mm ) the elastic stress carried by the repair material is now 1.5 times that 

It can also be seen from Table 6.1 that the coefficients of thermal expansion/contraction of resin-based repair systems may be two or three times that of the substrate concrete. If the repair is carried out at, say, 10 °C then a rise in ambient temperature to 25 °C will potentially induce a compressive strain in the repair material of the 

A further major consideration in the repair of structural concrete elements is the decision as to whether to relieve the element of load whilst the repair process is being performed. If the application of live or imposed load is prohibited during the process of concrete removal but dead load is not temporarily resisted by propping then there will be a redistribution of dead load stresses within the residual section. Further, the patch repair material will not subsequently contribute to the support of dead load. Alternatively, if both dead and live loads are supported by temporary props whilst the repair is carried out then the reapplication of dead load will result in subsequent creep of the repair material under the sustained stress, whilst any cyclic live loads will induce fatigue loading on the repaired section. A third possibility, for example in the case of columns 

The effect of a sustained compressive stress of 10 N/mm on the creep strains induced by 160 mm long by 40 mm square prisms of four different generic forms of repair material is shown in Fig. 6.6. The range of ultimate creep strains varies from over 4500 mierostrain with one resin-based system down to less than 400 mierostrain with some modified eementitious systems. These strains inelude the effect of any curing shrinkage/expansion which may also be occurring during the period under load. 

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At the microscopic scale, mechanical stress can also develop due to the property-mismatch of the electrode and the membrane. The porous electrode is typically much weaker than the membrane and has lower hygrothermal expansion coefficient than the membrane hence it can develop mud-cracks and delaminates from the membrane after RH cycling. This will perturb the local electrochemical processes and results in non-uniform decay of membranes. 

The main bottleneck in this case will be in relaxation on the Y-Z plane (perpendicular to gradient), since there will be the most property mismatch and restricted movement of the parts of the whole specimen. Interesting, that relative anisotropy in elastic module between X and Y/Z components is not large (2-3%), but depends on temperature and anisotropy coefficient in a complicated way (Fig.4a and b). On the other hand, differences in values of thermal conductivity between X and Y/Z are almost the same for different anisotropy, but change strongly with the temperature (Fig.4c and d). The results for Y/Z-plane could be summarised in Table 1. 

These calculations show sources and values of the possible properties mismatch in properties of the graded joints. For instance, such large differences and anisotropy in thermal conductivity confirm that heat flow in non-steady conditions would affect the temperature distribution in the joint quite significantly. In this case additional thermal stresses could be generated by internal gradients of temperature. 

Figure 5 shows the peak values of the axial and radial residual stresses for different types of proHles. As expected, the peak stresses depend strongly on the gradient profile, but they are always larger in the non-graded material as a result of the large property mismatch. 

The potential for further extensive development in ceramic-fiber composites is thus dependent upon the production of a wide array of fiber compositions as a significant existing problem is of property mismatch between the fiber and matrix. It has, therefore, been emphasized that the challenge is to either obtain new fibers with improved thermoxidative stability in comparison to the limited number currently available, or to suitably modify existing fiber materials. 

Emberson, N.K. and Mays G.C. Polymer mortars for the repair of structural concrete the significance of property mismatch. The Production, Performance and Potential of Polymers in Concrete (Ed. Staynes, B. W.), Fifth International Congress on Polymers in Concrete, ICPIC 87, Brighton, September 1987, pp. 335-42. 

Weber and Newman [68] developed one-dimensional model to study the stresses development in the fuel cell. They showed that hygro-thermal stresses might be an important reason for membrane failure, and the mechanieal stresses might be particularly important in systems that are non-isothermal. However, their model is one-dimensional and does not include the effects of material property mismatch among PEM, GDL, and bipolar plates. 

The results of fracture tests of adhesive bonds are almost never independent of the experimental geometry because the presence of the interface with its discontinuity in elastic properties ensures that the stress field at the interface depends on both the external loading and the elastic properties mismatch as discussed in chapters on hard adhesives. However, soft adhesives have the added complication to dissipate energy, not only in a restricted plastic zone near the interface, but over a large volume, often the entire volume of the sample. This means that there is a very strong coupling between the boundary conditions of the test (thickness of the layer, size of the probe and stiffness of the probe) and the observed deformation mechanisms. 

The simplicity of this plastic model is that all the parameter that is required isp , which can be determined through routine static indentation testing, as also demonstrated for various ductile projectiles (steels and brass) impacted on silicon nitride targets [10]. However, pertinent experimental techniques should be sought to determine more accurately the related dynamic patameters such as impact force, stresses, deformation, duration of impact, coefficient of restitution, and stress wave propagations, etc. Frictional constraint by property mismatch between projectiles and targets of dissimilar materials needs to be taken into account in some cases. 

During the last 30 years, advances in material science have led to the development of synthetic materials that have unique properties for medical applications. Metals, ceramics, polymers, composites are the main classes of synthetic biomaterials. Metals and their alloys have been used in various forms as implants and for hard tissue repair (e.g., dental implants, joint replacement, fracture plates, screws, pins). They are mechanically strong, tough and ductile. They can be readily fabricated and sterilised. However, they may corrode in the biological media, their densities are high and their mechanical properties mismatch with bone, which may result undesirable destruction of the surrounding hard tissues. 

Emberson, N.K., and Mays, G.C., Polymer Mortars for the Repair of Structural Concrete The Significance of Property Mismatch., Proceedings of 5th International Congress Polymers in Concrete, Brighton Polytechnic, September 1987, p335-34l. 

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