Website Banner. John Monash: Engineering enterprise prior to World War 1.

[Introduction] [Main JM Index] [People Index.] [Abbreviations.]

Concrete Construction
with special reference to reinforced concrete

"Paper read by Lieut.-Colonel J. Monash, M. Inst. C.E.
at the Royal Victorian Institute of Architects, 23rd June, 1908.

Cement concrete is a material which, whatever its physical advantages in comparison with burnt clay products or natural stone or timber, is - bulk for bulk - more costly than these latter for purposes of general construction, as distinguished from surface decoration. Wherever, therefore, in building design, stability or strength are to be attained by the employment of mass and weight, cement concrete cannot compete, as to first cost, with other materials available to the architect. For this reason, concrete construction has, until recent years, been confined in building practice, to mass foundations, or as a mere filling in floors and the like -where its production involved special advantages; but it was of a generally poor texture, and played only a small part in dealing with stresses.

The introduction of the principle of reinforcing the concrete with steel has changed all that. Today cement concrete occupies a place a long way in front of every other known material in the composition of every kind of engineering structure, and in most building structures of the industrial type.

This revolution in building practice rests partly upon a recognition of the superior advantages of a plastic, monolithic construction - in regard both to strength, rigidity, durability, and fire resistance - and partly upon the enormous economies both in actual cost of building and in land values which can be effected by its use.

Doubtless, constructional advantages alone would have failed to stimulate progress in this direction, if economic advantages had been absent. It is therefore of interest to consider briefly upon what the economy of modern concrete construction depends.

First and foremost comes the entire change in the point of view as to the underlying principles of design rendered possible by the characteristics of this new material. Older practice was to achieve strength and stability by mere bulk and weight. Ancient structures such as the Pyramids may be regarded as extreme examples of this type. Modern practice aims at getting rid of bulk and weight in every possible direction. To do so judiciously - far from detracting in any way from strength - operates to actually increase the strength of the building. The weight of the structure itself bears, usually, a high ratio to the external loads and forces which it is called upon to resist, and consequently, a reduction in the dead weight greatly lessens the total stress duty which it has to perform.

The most striking features of this new method of design are the employment of rigid, monolithic horizontal diaphragms forming the floors and roofs, and the abandonment of the heavy exterior walls. These walls are relegated to their true function of weather screens, and become in fact mere curtains to keep out the rain and to carry the surface decoration. While these curtain walls operate, of course, to stiffen the construction, yet in the best practice any advantages they may render in this regard are ignored.

The next most important factor is the comprehensive application to building structures of the same careful and minute processes of stress calculation as have been in vogue for engineering structures for half a century. Design by 'rule of thumb' gives place to careful and exact determination of the stresses to be met in every separate member of the structure, from the footings to the parapet. This procedure carries a step further the central idea of getting rid of unnecessary bulk and hence useless weight and cost, in every part of the building.

Lastly, the principle of reinforcing cement concrete with steel aims at getting the maximum strength value out of a given quantity of these materials. If this be done in accordance with correct scientific principles, it is no exaggeration to say that no single material, or combination of materials known to us to-day will afford so great a strength for so low a cost; quite apart from the durability and fire-resisting quality of the resulting construction.

It may be of interest to you to consider briefly the characteristics of up-to-date practice in the design and construction of concrete in combination with steel. The following observations, so far as concerns the concrete, apply largely also to steel-frame cased in concrete - although this latter form is in no sense reinforced concrete as scientifically understood; inasmuch as the steel frame must be designed so carry the whole of the stresses independently of the concrete, which is, in this case, used purely as a fire-protective covering.

Firstly as to the cement concrete, its texture, its manipulation and its ingredients, it has to be affirmed, with a great deal of emphasis, that to achieve success and economy, the aim must be so produce a material altogether superior to and widely different from the cement concrete hitherto commonly employed in foundations, backing to masonry, filling on arches and the like. Unless and until this is clearly recognised and the reasons are fully understood, there will be considerable danger of an entire failure to accomplish safe as well as economic results. So vital is this point, and so necessary is it that all the building professions and trades should recognise that cement concrete for constructional work is a scientifically designed material requiring specially careful selection and manipulation in its production, that it would be eminently desirable if some entirely new name for it could be used, so that builders and clerks of works should be under no misapprehension as to the entire distinction in its character from ordinary foundation concrete or the like.

Inasmuch as the designer of a reinforced concrete structure bases its dimensions and thickness of its several parts wholly upon the known experimentally determined strengths (in compression, or in shear, or in adhesion) of the particular composition of concrete which he decides to specify - it is just as important that his directions as to materials, their character, their granular texture, their proportions, their mode of mixing, and the percentage of water should be accurately and faithfully realised upon the work, as it is, say, to put in the proper number of rivets in a steel girder joint, or the proper scantlings of timber in a roof truss. If through ignorance or carelessness, or worse, this be not done the factor of safety relied upon may easily be cut into by one half or more. It is this consideration, and the difficulty of coping with it, and the many disastrous failures which have ensued, that has necessitated the almost universal employment in Europe and America of specialist builders who are required to prove their experience and to accept, under stringent guarantees, the full responsibility for the safety of the work and who are able to bring skilled, trained and experienced labour and supervision to the task.

Cement concrete for this class of work should have the following properties:- The cement should be not less than one-fourth in bulk of the finished bulk of the work; the sand used should be not less than one-half of the bulk of the coarser aggregates used; the coarse aggregates should be of the hardest and toughest materials procurable, and should not exceed 3/4 inch in gauge; all ingredients should be of unquestionable purity; the mixing, particularly in the dry state, should be most intimate and thorough; and the percentage of water used should be at least 10 per cent., or ample to produce a thoroughly 'wet mixture'.

Warning. This simplistic approach to water content was superseded within a decade. It is included here for its historical interest.

For materials available in the city of Melbourne an outside limit of strength for important structural parts would be:- One part of cement, two parts of clean coarse sand, and four parts of 3/4 inch gauge clean bluestone screenings. (Here the RVIA Editor switches temporarily to reported speech) In his practice, he habitually used concrete stronger, and often much stronger than this; for the reason that rich concrete pays, and pays well. The compressive and shearing strength of concrete increases in very much higher ratio than the proportion of cement used. A 1:2:3 concrete is 50 per cent, stronger than a 1:3:6 concrete, but costs nothing like 50 per cent more; consequently by employing a more costly concrete the savings in bulk which can be effected actually result in substantial economies in the work as a whole. To indicate, in passing, how little the meaning of rich concrete is yet understood here, he might say that, at present values, the mere cost of the raw materials landed on the job without labour of any kind would, for a concrete such as he advocated, cost over 30 shillings per cubic yard; which is a price considered to be normally ample to cover the cost of bulk concrete in place, including all labour, all boxing, and builder's profit.

Another heresy which may be alluded to is the employment of coke-breeze, in the belief that this will lessen the dead loading. That is a fallacy. This material fails to answer to the condition that the aggregates most be hard and tough. It is neither, and can generally be crushed with the fingers. Consequently it permits of nothing like the same working stresses as would first class screenings concrete, with the result that breeze concrete, for equivalent strength, has to be designed so much thicker that the advantages of its lesser specific gravity are more than lost in its increased bulk. The lightest floor or girder is that made with the hardest, heaviest and richest concrete.

Moreover, coke breeze can hardly ever be procured free from sulphur and other chemical contents, which have a most serious effect in setting up rapid corrosion in the embedded steel. That this is no mere speculative evil is demonstrated by the fact that most recent building ordinances in Europe and America have absolutely prohibited the use of coke breeze or furnace slag in reinforced concrete work, and for the very reason here indicated.

Brief allusions may now be made to the reinforcing metal - almost universally mild steel. The functions of the embedded steel are wholly to take up tensile stresses or shearing stresses due to flexure. It plays only a negligible part in dealing with compressive stresses. The extent to which the steel can perform its allotted functions to best advantage depends firstly upon its cross-sectional area, that is to say, its weight, and secondly upon its being assigned the correct position within the mass. Both these aspects are worthy of some examination.

At the present time the building professions in this community are being bewildered by a multiplicity of patented contrivances, such as the Ransome twisted bar, the Kahn bar, the indented bar, the Thatcher bar, expanded steel, and several different forms of wire-woven, or wire-welded fabrics. It would be invidious to express any public opinion as to the comparative merits or demerits of any one of these. It is only fair to say, however, that they are all useful and efficient reinforcements, if used in the proper positions, and in appropriate quantities. But that is the whole point. The proper quantity must be used and if in any given case the stresses demand that there shall be, say, ten square inches of steel effective to resist the tensions or shears, then by no conceivable procedure in giving the steel any special shape can the work required be done by less than ten square inches of steel. The whole question therefore resolves itself into one of cost, namely, in what form, and under what circumstances, can the requisite cross section be supplied at the least cost? Inasmuch as plain round bars of commerce cost, in this community, less than one-third of the cost of many of these special forms of manufactured reinforcement, it is difficult to see how their employment could be justified upon economic grounds, without seriously exceeding the safe stress intensities laid down by public authority.

Again, as to the positioning of the steel in the concrete, it is matter for a good deal of surprise to note how slowly the fundamental principles of correct design are permeating the building community. It is not uncommon, of course, to find among laymen a vague impression that in some mysterious way the mere presence of the steel within the concrete lends to the latter greatly added strength, somewhat in the fashion that sugar is used to sweeten tea; but it is quite astonishing to discover among people whose business is building construction a strange ineptitude for intuitively grasping correct ideas. Over and over again one meets cases where steel bars have been placed in beams, near the top, or near the neutral axis, or impartially distributed over the cross section and again, cases where beams have contained bars which have not been long enough to extend into the bearings. Of course such beams inevitably break when they get their load. The most common fault, however, is a sublimely innocent disregard of all shearing stresses.

There are even professional men who flippantly refer to reinforced concrete as 'only ordinary concrete with a few bits of steel stuck in here and there! - nothing special about it!' Now, that is just the frame of mind which attunes best to an ignorant public prejudice against specialisation in industry on the part of people who are only too apt to confuse specialisation with monopoly. This is therefore the mental attitude which involves the greatest danger to the future of reinforced concrete.

In the event of the failure of a reinforced concrete construction the public will be unable to discriminate, and will attribute to this method of construction faults which lie at the door either of incorrect design or improper execution or both.

On an occasion like the present it is appropriate, therefore, to point out very strongly that in regard to the quantity and positioning of the steel within the concrete, precisely the same extreme faithfulness and accuracy of execution, formerly alluded to, are as essential to successful construction as is the case in the construction of a reliable piece of mechanism, while the consequences of error are even more disastrous.

Regarding reinforced concrete design, generally, the day has gone by when conflicting theories have held the stage. The ablest engineers and mathematicians have discussed every phase of this question to a finish, and the results have been embodied in rules and regulations authenticated by State authority and adopted by the leading scientific bodies. With few and trifling variations, accepted principles are on almost identical lines. Perhaps the best and most complete code of rules is that adopted early last year by the Royal Institute of British Architects. These are proposed to be embodied in the new Melbourne building regulations, and may he commended to this Institute as a safe guide to sound design in this field of construction.

Lastly, in the limits of so short a paper it has not been possible to do more than touch lightly upon a few phases of current importance, (Editor again switches to reported speech.) but he had ventured to place before them views upon what may be regarded as the principal aspects of the subject open to controversy, in the hope that this course may lead to a discussion which will be of value and interest to the Institute.

As a member of a kindred Institute he desired to place on record his thanks for their invitation to address them that evening, and for their very patient hearing.

Subsequent to the reading of the paper the author replied at considerable length to a number of questions submitted by members present.

Regarding the mixing of concrete for constructional work of the type discussed in the paper, he strongly advocated the 'wet' mixture, not only in the interests of effective consolidation, but also to secure the full development of the adhesion between the steel and the concrete.

The chemical characteristics of the two materials were discussed in detail, and it was shewn that the protection of the steel against corrosion depended not upon mere physical covering but upon well-established chemical affinities.

In response to a request by several members, the author commented lengthily upon some of the principal clauses in the proposed new Melbourne building regulations, as adopted from the rules for design authorised by the Royal Institute of British Architects and by the aid of diagrams illustrated the effect of shearing stresses, and the methods employed for providing for same in reinforced concrete design. At the request of the President some classical instances of the satisfactory behaviour of reinforced concrete in large conflagrations - as at Baltimore and San Francisco - were detailed; and the physical and thermal principles involved in these fire-protective qualities were explained.

After the reading of the paper and the replies to many questions, a vote of thanks to Lieut.-Col. Monash for his able address was proposed by Mr. F. J. Davies (F.), who invited members of the Institute to visit the British Australian Tobacco Co.'s premises in Melbourne, where the largest span of reinforced concrete beam in the city was just about being completed by Lieut.-Col. Monash, who was also constructing some large beams over shop fronts in Elizabeth Street.

Mr. A. M. Henderson (F.), in seconding the motion, asked whether any information could be given respecting the use of slag from the public destructors in reinforced concrete work?

[This subject will be discussed at next meeting - Ed. 'Proceedings'.]

The President introduced Mr. John Gibson, of the 'Emu' Cement Works, who had hoped to take part in the paper on 'Cement Tests'. Mr. Gibson thought the public would be satisfied with the report of the Cement Board when published, as the interests of the users had been carefully looked after, while no hardship would be inflicted on the manufacturers of the best material. As President of the Society of Chemical Industry, his body applied science to the practical purposes which architects, as artists, embodied in the many forms of buildings they were instructed to build.

The vote of thanks having been carried by acclamation the proceedings terminated."

Other papers by Monash on this web site.

[Top.]