July_1971 — Page 36

cover to the steel wires since to achieve optimum effect in bending, steel could be as close a practicable to the surface.

The three basic ingredients of aggregate, water and cement must be carefully chosen and tested in order to meet the following requirements:

1. highest durability and protection of steel,

2. highest compactability,

3. highest compressive and tensile strength with the least creep, shrinkage and absorption.

theless substantial numbers of success- ful boats have been built using this process which should not prevent us from thinking of its improvement.

One possible way is by gunniting which would shorten the time of con- creting, employ only a small number of skilled operatives, and ensure a greater hull rigidity and consequently its fairness through the use of a male mould. The necessarily slight ferroce- ment thickness, the very reason for the existance of the material, constitutes a possible source of mishaps in the tran

These requirements are all inter- sitional phase until the material be- dependent.

comes well established in its own right. Paradoxically this well may be because reinforced concrete is today widely accepted and familiar as a construction medium.

In practice we get comparatively rich and uniform mixes, ranging from 0.5 to 3.0 parts of fine aggregate to 1 part by weight of cement.

The water-cement ratios also range widely from around 0.25 to 0.5, al- though the higher values give mixes which prove less durable. For durabi- lity the smallest praticable w/c ratios should be employed, as well as water reducing additives which lower the mixing water content and retard the setting in wet and warm climates.

Manufacturing techniques

Broadly speaking, there are two main techniques; "in-situ" and "pre- fabrication". The "in-situ" methods have the merit of simplicity through their absence of shuttering, which explains their popularity with amateur boat builders and fishing industries of developing nations. In this last respect FAO of the United Nations alone has been helping to date in Thailand, Uganda, Senegal and the U.A.R. Fish- ing vessels have also been produed commercially using the in-situ methods in U.K.; Canada, U.S.A., Italy and Australasia.

Unfortunately the manual applica- tion of concrete in this method is both lengthy and tiresome even on small boats and the "cold joints" which arise whenever work has been broken off constitute a potential scource of lea- kage. The work is usually carried on inside a covered shed, which adds to the cost as the previously mentioned pipe frames are often hung from the rafters of this shed to facilitate con- creting of the bottom.

The "in-situ" process is also labour intensive and its final product relies to a large degree on the conscientiousness and strength of the operatives who can get rather tired at the end of a con- tinuous long concrete pour. Never-

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Thus the liberties which may be taken with the latter, a much more robust sized material, if extended to the very thin ferrocement construc- tion, could prove dangerous.

For this reason the writer is more than a little uneasy when he reads of amateur boat builders producing ferro- cement in their own backyards. Safe construction requires specialist know- ledge and is best left in experienced hands, otherwise ferrocement will quickly fall into disrepute.

Concrete cover to the very fine reinforcement would also appear to be a potential durability problem, espe- cially in the hand applied (in-situ) con- creting. Theoretically sufficient cover would be between 0.5 to 2.5 mm, in practice this can be reduced to nothing with poor supervision.

This underlines the importance of supervision and of good workmanship. At the same time it is indicative of the protective role of the dense alkaline concrete environment, since few cases of serious distress have been reported so far. (We are talking of distress which could be directly attributable to corrosion of exposed fine diameter steel.) In fact, provided the surround- ing concrete was sound, the corrosion would usually only be superficial.

The position regarding reinforce- ment in marine uses is less clear and several often conflicting opinions conflicting opinions exist. The calculational practice which appears to be followed in Eastern Europe seems more based on static Europe seems more based on static R.C. application, which is hardly ideal when thinking of hull impact, percola- tion, hull fairness and production, or even of the quasi-homogeneous action which is possible with this material.

Some research results speak nega- tively regarding galvanising of the mesh, others condemn woven mesh, others still, chicken wires as a means of reinforcing. In the first case the rea- son for this is the apparent destruction through galvanic action of the im portant primary covering layer of con- crete which envelopes the steel wire. This can only take place if an absor- bant concrete is cast, i.e. a concrete which will allow moisture to penetrate and set up a potential between the zinc sheathing and the highly alkaline environment of the cement.

Regarding the woven meshes in comparison with welded, the more complete tieing of the latter prevents the local uninhibited vibration of the wires when placed close to the vibrat- ing scource. Such a vibration in the loosely connected woven mesh dis- turbs the concrete locally and weakens it, rendering it more absorbent and creating a path for moisture penetra- tion and corrosion in the very place where maximum protection is called for.

Turning to concrete again, since its material costs when compared to steel are much lower in ferrocement than in R.C., richer mixes may well become popular. In broad comparison; in ordinary R.C. the steel costs and con- crete costs will be more or less equal, in ferrocement steel costs alone can be 80% and more of the total. Con- sequently, an increased cement con- sumption represents a smaller amount of the overall costs in ferrocement as compared to ordinary R.C. A richer mix however, can appreciably raise the strength and durability characteristics, and in some cases even lead to a lower- ing of the thickness to be employed.

In ferrocement the concrete part of the material is both more variable, and more capable of improvement at a lesser cost than the steel part. As a re- sult of this it is possible, starting out with the same quantities of cement and aggregate, to improve the crushing strengths and other characteristics by simple methods such as preparatory mixing, additives, selection of particle size to limit interference and finer cements.

If however, one adds to these simple methods pre-heating of water and aggregates, vibro grinding of aggre gates and cements to great finenesses, careful moist curing, very low w/c ratios and sophisticated forms of com. paction by pressure, it is quite clear

Far East BUILDER, July 1971