July_1971 — Page 33

Продольный разраз

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Typical Russian prestressed ferrocement industrial roof unit of 16 m span. Prestressing cable runs through entire length of unit

Superior

In comparison with steel, ferroce- ment hulls will exhibit better technical features in the following areas:

1. corrosion resistance and overall durability;

2. watertightness;

3. spark proofing;

4. localization of damage and post- impact behaviour;

5. repairs;

6. sound insulation and vibration damping;

7. thermal insulation and fire proofing;

8. shaping facility, variation of hull thickness and prestressing facilities.

The better corrosion and durability characteristics of well-made concrete hulls are undisputed facts, backed by many examples, some dating well back in time. The earliest concrete rowing boat (still in existence in a museum) dates back to the 1850's.

A point which needs more em- phasis here as it is important in con- sideration of depreciation costs is that well-made ferrocement hulls will go on increasing in strength with the passage of time. This quality is not shared by any metal, timber or plastic product used in ship construction.

Watertightness as evidenced by an absence of bilge water has also been a welcome and surprising feature in fer- rocement boats. Provided the ferroce- ment panels were well made, numer- ous U.S.S.R. tests maintained up to 1,6 N/mm2 (230 psi) hydrostatic pres

Far East BUILDER, July 1971

sures on only 20 mm thicknesses without water percolation.

Much depends on the quality of the mix, the selection of its consti- tuents, placement etc., but both prac tice and laboratory confirm that normal "water-heads" can be safely supported by minimal ferrocement thicknesses.

Watertightness is also closely con- nected with jointing which is likely to be far more "monolithic" in a ferroce- ment hull than in a steel one · even when the former is assembled of pre- fabricated elements. Spark proofing, so important for the conveyance of inflammables and explosives is de- finitely superior in all concrete vessels. This explains the comparative popu- larity of concrete barges as ammuni- tion lighters, dynamite carriers and even L.P.G. carriers.

Localized damage and post-impact characteristics of ferrocement consti- tute perhaps the most valuable pro- perty of the material, for unlike steel this material is not beset by crack pro- pagation, residual stresses, notch ducti- lity or brittle fracture problems.

Ferrocement vessels are no more unsinkable or damage-free than other kinds of construction, but if the im- pact has been relatively moderate fer- rocement in contrast to other mate- rials will not shatter completely or be ripped apart. What may happen is local and irretrievable deformation, even bad fissuration, but the material will still hold together thanks to the action

of the multitude of small wires. The damage will be localised to one place

and its presence will not, as in steel, endanger the safety of the entire hull through progressive failure.

A partial failure will cause the in- gress of the water into the hull to be greatly retarded by the same multitude of wires and fissured concrete still clinging to the wires. At the same time, the egress of any liquid inside the hull is similarly retarded, an im- portant consideration in terms of oil pollution and the associated costs.

Repairs can often be carried out at sea, using aggregate and cement, and modern epoxy resins.

Sound insulation and vibration damping are better in concrete vessels due to a considerably lowered modu- lus of elasticity - in comparison with steel and greater thicknesses. But in a ferrocement hull this does not neces sarily mean more weight since ferroce- ment is some 32-times lighter than steel.

Thermal insulation is also better in ferrocement vessels than in steel. The conductivity is in fact around 1/6th of steel figures. Apart from the superior comfort levels that this produces, there is an absence of "hull sweating" an important feature for certain bulk cargos in the tropics. Moreover, not only does this facilitate and cheapen refrigeration but it also highlights the fire protective qualities of this medium of construction.

Concrete has long been a fire pro- tective material; indeed certain kinds are used for refractive furnace linings where they can be subjected to very high temperatures.

However, the ordinary bar-rein- forced concrete will sometimes spall when rapidly cooled by water after a fire heat-build-up. Ferrocement, on the other hand, with its multitude of finely dispersed mesh, will not only lessen the sudden spalling, but will also reduce the distortion of the hull through the uniformity of reinforce- ment throughout its section. The pro- tective value is still enhanced when water is present on one side of the material.

The facility to shape, the ease of variation of hull thickness and the pos- sibilities and relevance of prestressing are more pronounced and better com- bined in ferrocement than in any other shipping material. Shaping and thick- nessing enable one for instance to introduce longtitudinal folds for stiff-

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