Initial concrete stress

Where F jAc = initial concrete stress from prestress... 

Another aspect of epoxy resin mortar floorings which needs careful attention is that their coefficients of thermal expansion are approximately three times that of concrete. This, coupled with the relative low thermal conductivity of epoxy mortar, can cause stresses to be induced at the resin mortar/concrete interface under conditions of thermal shock (e.g. thermal cleaning), resulting in break-up of the flooring due to initial failure in the concrete. Two approaches have been tried to overcome this problem ... 

Concrete calculations carried out via formula (10) for different values of constant have shown that it reflects the behavior of flow curves quite really. However, a series doubt remains for such a system (with a yield stress) it is not obvious how to determine the initial Newtonian viscosity (is it necessary to determine it and does it exist ). 

A comprehensive list of behavioral phenomena and physical attributes affecting the strength and stability of steel frames is compiled in White 1991. Some of the items listed include initial imperfections, residual stresses, initial strains, construction sequence, effects of simultaneous axial force, shear and moment on section capacities, P-dclta effect, local buckling and spread of inelastic zones in members. A similar list of items could be compiled for reinforced concrete and other structural materials. It is clear that a comprehensive advanced analysts can become quite... 

Sulphur concretes appear well suited for use in environments corrosive to Portland cement concretes. The extensive work by the U.S. Bureau of Mines shows their material performs admirably in such environments. When used as a lining, the initial stress-strain behaviour will allow the material to adapt to the main structural element and relieve internal stresses without cracking. Corrosion resistance will be maintained by the material thereafter, even though the stress strain behaviour alters. On its own, the material retains sufficient strength to withstand typical loads involved in this type of application (eg. liquid container). Sudicrete has not been tested much in this area, although laboratory tests show similar promise. 

There are some paste properties which influence the behaviour of fresh concrete. The workability of concrete mixture is the function of the rheological properties of paste which depend on the fine fraction content in cement. The yield stress value, being the measure of concrete mixture consistency at final stage of mixing, is increasing with the rise of cement specific surface area (Fig. 5.12). The initial setting time is significantly shorter in the case of this cement property. 

If, however, the expansive stresses generated initially are higher than needed just to outbalance the tensile stresses caused by drying, then the hydrated material remains imder expansive stresses even after it has dried completely. Such stresses may attain a magnitude of up to 8 MPa. To preverrt excessive expansion and crack formation tmder these conditions the concrete body must be restrained or reirrforced by a srritable reinforcement. Binders of this kind are called self-stressing cements. They may be... 

Note that the initial anchor bolt tension does not increase the maximum bolt tension caused by wind or earthquake. This initial tension will only clamp the base ring to the concrete. Both are in equal compression until the external load is applied. The external load reduces the compression in the concrete before additional load is applied to the bolts. After the external load overcomes aU the compression in the concrete, the stress in the bolt wiU increase to the value it would have been, had there been no initial tension. 

By virtue of its basic size and the relative thinness of its component make-up a liner requires continuous structural support in all directions in order to withstand the loading from thermal and pressure conditions in a reactor and still remain Impermeable. Present liners are therefore usually firmly attached to and fully supported by the concrete structure of the PCRV. Due to this positive attachment, the liner is subjected to the stresses and strains Imparted to it by the PCRV during initial prestressing and, later, by movements of the PCRV due to the operating condition of the reactor. To ensure even distribution of these movements across the full size of the liner, attachment of the liner to the concrete is achieved by the embedment of a combination of steel sections and a multitude of studs or hook anchors. See figure 9. 

In following the movements of the vessel during initial prestressing the stress in the liner approaches and, in some cases, reaches yield stress. During operation, when temperature has been applied, and in late life, when concrete creep has taken place, the stress in the liner is certainly yield stress. 

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