ness and anti-roll action and to con- struct thin and stiff corrugated fire and pressure bulkheads. It also allows improvements in the area of interac tion between scantlings and the shell of the hull, and in deck buckling con- ditions and stress concentrations around all openings.
Further, it enables one to easily integrate two basically similar mate- rials; reinforced concrete and ferroce- ment in ballasting and in prestressed concrete pressure vessels for the inevit- able marine atomic propulsion. Con- crete is an ideal medium for prestress- ing, and ferrocement, even at the highest steel dispertions achieved experimentally so far, is still very much concrete in its volume. A state of partial or high hull precompression
would achieve the desired reduction of Ferrocement hull built in Canada by the shot-crete process. Courtesy of Prof. Lachance longitudinal wave moments and of the
consequent material stresses.
Prestressing through the greater stiffness that this would confer on the hull, would probably also enable the currently enforced length to beam ratios to be exceeded, thus achieving greater hydrodynamic efficiencies and lower propulsion costs.
Prestressing methods could also be elegantly combined with production and design requirements. Using pre- fabricated units both transverse and longitudinal framing would be possi- ble, and the necessary thickening in the anchorage zones of the longitu- dinal cables could also cater for the greater "slamming" stresses in the for- ward areas of the ship.
Older vessels could also be recondi- tioned and readapted for fresh tasks,
by the combined use of a protective Close-up of reinforcement showing polythene clad mould underneath
"gunnited" outside membrane and of inside prestressing tendons.
In order to allow for the cumula- tive effect of fatigue, creep, shrinkage, and other factors, a proportion of the total tendons could be replacable, peri- odically inspected and, if needs be, re- tensioned. To cater for corrosion in these, the tendons could be carried in oil filled transparent plastic sheaths. The majority of the tendons would be protected by normal methods of grouting an enclosed duct.
Role of steel
If a steel bar embedded in concrete is subjected to a force along its length, this force will transfer itself to the adjacent concrete by means of adhe- sion of concrete and steel, simply called bond.
At the surface of contact between the two materials, this force will pro- duce very considerable stresses which fluctuate depending on concrete uni- formity and compaction, richness of the mix, duration and type of loading and the surface conditions of the steel bar.
Concrete in the immediate vicinity of the reinforcement is consequently able to undergo very high deformation which rapidly falls off to zero away from the bar surface.
In a reinforced concrete section undergoing tension, the limit of con- tinuous cooperation between concrete and steel is quickly overstepped when cracking occurs. In contrast, a fer- rocement section subjected to the same load, will extend this cooperation for much longer due to the greater subdi-
vision of reinforcement. As well as this, the same characteristics will en- sure that when the cracking stress is reached, the crack widths will remain appreciably smaller and more numer
ous.
At the same time, unless a certain limiting stress is overstepped, the re- moval of the load will ensure the closing up of the cracks to the extent of becoming virtually invisible to the naked eye.
The extent of this cracking is directly related to the amount of steel present and its subdivision, which is to say the diameter and the spacing. It is related also to the type of steel, the richness of the mix, its uniformity and methods of its preparation.
In most cases so far, the main mesh reinforcing had been originally destin
Far East BUILDER, July 1971
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