MATERIALS USED IN FERROCEMENT CONSTRUCTION
The ideal rod to use is a cold worked high yield (CWHY) rod to British Standard (B.S) 4461 although both plain bars and deformed bars may be used. The latter may be more difficult to use when welded by inexperienced people.
The requirement of steel grade and minimum yield point will depend on the construction method employed, but will have sufficient tensile and yield strength and ductility and other properties essential for good construction.
All reinforcement should be free from contamination, grease and millscale. While not objected to, light corrosion should be wire brushed to remove the rust.
For use in the hull shell, deck floors, bulkheads and stiffening webs or girders; ideally should be a semi-bright hard-drawn (SBHD) rod of 6 and 8 mm diameter for concrete reinforcement to BS 4482. Mild steel rod to BS 15 may also be used but in practice will require closer frame spacing or support to prevent being bent out of shape during construction and/or distortion by welding.
The ideal mesh is a 13 x 13 mm x 19 gauge (1 mm) welded mesh to BS 4482. Although meshes of 18-22 gauge can be used, 19 gauge will prove to be the best from a practical point of view. In colder, less humid climates, it may be used ungalvanised; in semi-tropical/tropical it will need to be galvanised.
A specification for welded mesh: Initially rod used in the manufacture of welded wire mesh is a low carbon content (0.15% max by weight) rimming steel with the carbon concentrated in the centre. The rod used in the manufacture of the wire mesh is hard-drawn from 'X' size down to 19 gauge (1 mm), hot rolled (perhaps copper washed) passed through stearate soap and then welded. It is then passed through the galvanising process. (The rod has a very low silicon value.)
A cautionary note with reference to using galvanised materials in the construction before casting is applicable at this stage. The reason for care is that it may be possible to build in a fault into the hull through the interaction of the zinc on the mesh and the remaining steel, in the moisture of the setting mortar, causing hydrogen bubbles between the mortar and the steel, and reducing bond between the mortar and steel.
Galvanised mesh which has been exposed to weather for some time prior to use may have less effect on the structure.
As a precaution, by adding 300 parts per million by weight added to the casting water, of chromium trioxide (chromic oxide) the problem can be negated. Chromium trioxide must be handled with caution as it is highly toxic to the skin and especially to the eyes. The crystals must be kept from moisture until they are actually added to the mixing water (approximately 66 g to 200 litres.)
Other mesh types which may be used are hexagonal mesh and, to a lesser extent, woven square mesh. Classification societies may need evidence of how the alternate meshes are used, in what direction they lay, and the combination of meshes that can or may be used.
Using expanded mesh in certain forms of construction may also be acceptable but is rarely employed other than in a construction using moulds.
Staples and lacing wire are best made from 1.6 mm or 16 gauge plain degreased annealed mild steel lacing wire. This can be purchased in 25 kg reels and cut on a suitable jig to provide either 30 mm or 40 mm length staples to suit single and double rod construction, with the legs of the staples set at a width to suit the size of mesh being used.
The normal gauge of welding rod required will be 10 swg or 3.25 mm, although small quantities of other gauges will also be used. Welding rods are of general purpose, all angle type, and should be cared for in compliance with the manufacturers advice and good practice.
The cement to use is usually ordinary Portland. However, a rapid hardening Portland cement may be used in cold climates. Sometimes a sulphate resistant Portland cement is used, either wholly or in part mixed with ordinary Portland against sulphate attack, although as most vessels are protected by marine paints and antifouling, its use is hardly necessary. If the cement is used with admixtures, care should be exercised in compatibility. All cements are to be to BS 12 or equivalent local standard.
In some underdeveloped countries it may be necessary to obtain a certificate of the materials composition and date of manufacture, where there is the likelihood of low quality and, perhaps, adulteration between point of manufacture and delivery. This may mean that the cement has to be picked up by the yard's transport and a reliable person checks there is no problem in delivery.
Ideally the cement will be no more than three weeks old and delivery accomplished two to three days before use.
Other cements may be considered providing they offer adequate strength, density, and uniform consistency.
The importance of good, clean, well graded sand, cannot be over emphasized if one is to make the high grade impervious mortar required for boatbuilding.
The sand will be of a silicious nature and conform to the grading envelope shown in Fig. No. 6. The sand is not to contain sulphates, pyrites, or other chemically active substances in such amounts that the mix is harmed. If sea sand is used, it is to be washed free of any saline compositions. (It is always preferable to use non-saline river sand.) The sand should not contain loose clay or clay which adheres or covers the sand grains. The sand is not to contain humic acid or organic materials in quantities that may be detrimental. Preferably, the sand should be 'sharp' and not contain non-crystalline minerals.
The sand should be stored in as dry a place as possible and so that water content is evenly balanced. The sand should be protected against pollution.
There is such a wide range of admixtures available today that one cannot make recommendation without first testing those chosen. If they are to be employed, care and discretion should be exercised at all times.
Three main criteria should be considered when applying admixtures:
a) Is the strength of the mortar increased or decreased?
b) The effect of the admixture on the steel reinforcement.
c) Practicality of use on site and supervision of exact mixing quantities.
Jointing compounds
With the advent of new methods for joining concrete, far more jointing compounds are available for making 'wet' joints in cement structures. It is always best to carry out tests on the potential system before use.
For joints in hull construction and repair, a two component epoxy polysulphide resin gives an excellent bond. If this is not obtainable, a cement grout applied judiciously will give better results than some PVA glues that are commonly available to the building construction industry.
Mixing water should comply with the requirements of BS 3148. Water should be potable, clean, and free from harmful salts or foreign materials which may impair the strength and resistance of the mortar.
BS 3148 gives details of testing water for concrete by comparing the properties of concrete made with any particular sample of water with those of an otherwise similar concrete made with distilled water; therefore the tests will usually be performed in a laboratory.