Stress cement

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... 

Figure 21.2 Volume changes and stresses in OPC, shrinkage-compensated and self-stressing cement pastes due to hydration and drying.

Mikhailov, V.V. (1960) Stressing cement and the mechanism self-stressing concrete regulation, in Proceedings 4th ICCC, Washington, Vol. 2, pp. 927-955. 

A fuelling machine located on top of the reactor block carries out on-line refuelling in the vertical channels. The reactor is confined within a double containment to minimize ground release of radioactivity. The inner primary containment is built of pre-stressed cement concrete whereas the outer secondary containment is built of reinforced cement concrete. 

Mikhailov, V. V., Stressing Cement and the Mechanism of Self-stressing Concrete Regulation, Proc. 4 Int. Symp. on Chem. Cement, 2 927-955 (1960)... 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

In 1971 a metal-backed polyethylene acetabular cup was introduced. This cup provided an eccentric socket which was replaceable, leaving the metal and replacing only the polyethylene. Because of the success of this component, metal-backed high density polyethylene (HDPE) liner is standard for prosthetic acetabular components. Research confirms that metal-backing reduces the peak stresses in the bone cement, and that HDPE forms a successfiil articulating surface for the prosthetic joint. 

Over time a large variety of materials have been used, including ivory, stainless steel, chromium—cobalt, and ceramics for the acetabular component. None proved sufficient. The implant material composition must provide a smooth surface for joint articulation, withstand hip joint stresses from normal loads, and the substance must disperse stress evenly to the cement and surrounding bone. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

Vitahium FHS ahoy is a cobalt—chromium—molybdenum ahoy having a high modulus of elasticity. This ahoy is also a preferred material. When combiaed with a properly designed stem, the properties of this ahoy provide protection for the cement mantle by decreasing proximal cement stress. This ahoy also exhibits high yields and tensile strength, is corrosion resistant, and biocompatible. Composites used ia orthopedics include carbon—carbon, carbon—epoxy, hydroxyapatite, ceramics, etc. 

Type IV (Low Heat of Hydration). Type IV is used where the rate and amount of heat generated from hydration have to be minimised, ie, large dams. Compared to Type I, Type IV Pordand cement has only about 40 to 60% of the heat of hydration during the tirst seven days and cures at a slower rate. In large stmctures such as dams where the heat of hydration cannot be readily released from the core of the stmcture, the concrete may cure at an elevated temperature, and thermal stresses can build up in the stmcture because of nonuniform cooling that weakens the stmcture. U.S. production of Type IV Pordand cement is less than 1%. 

Fig. 20.8. The stress-strain curve for cement or concrete in compression. Cracking starts at about half the ultimate strength.

Tube and shell heat exchangers, small distillation columns, reactors, valves, pumps and other items are available in impregnated grapliite. Graphite can be joined only by cementing, which embrittles on aging. It is prone to mechanical damage, particularly when subjected to tensile stresses. 

M. Goland and E. Relssner, The Stresses In Cemented Joints, Journal of Applied Mechanics, March 1944, pp. A-17-A-27. 

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