AACS Proceedings of the 17th Annual Convention
In speaking on the subject of concrete construction one is met with the condition that in some phases the principles are very simple, while in others they are in the highest degree technical. To discuss the subject as its importance demands requires not only a thoroughly trained engineer, but one with a wider experience than falls to the lot of a cemetery superintendent. It is left for me, therefore, to treat the subject from the kindergarten point of view only and in a way which will have little that is new or of interest to those of you who have had any considerable experience in handling this material. There is, however, one distinct advantage to be had from the reading of these papers--the discussion which follows. So many who are adverse to reading formal papers are willing to give their experiences, with the result that almost any paper attains its end by leading others to express themselves.
Concrete as a material for foundations you are all familiar with and have doubtless handled under the condition when natural or Rosendale cement sold at double its present price and the cost of Portland cement forbade its use for any except very special purposes. Investigation and experience have now given us data by which to work with some degree of certainty and the rapid multiplication of plants manufacturing this cement leads engineers to believe that in the near future the retail price in the vicinity of New York will be not far from a dollar a barrel. Under ordinary circumstances this hope would be reasonable, considering the large variety of materials from which that article can be manufactured and the abundance in which it is found over a wide territory, together with the fact that the production as estimated by Edwin C. Eckel, Asso. Am. Soc. C. E. of the US Geological Survey, increased from a little over 3,500,000 bbls. in 1898 to almost 17,000,000 in 1902. The variation in price during these four years has not, however, been such as to lead us to place much dependence on an immediate reduction. Nevertheless, under present conditions and at current rates Portland cement as its principal ingredient insures a concrete at so Iowa cost as to make its use and application the most important in the engineering department of the cemetery, and as so much of our labor is of an unclassified character (I advisedly abstain from calling it unskilled), concrete is found to be a very satisfactory substitute for structures which, made of other material, would require the help of outside tradesmen.
We have come to regard this material almost solely from the point of view of its adaptability for constructing foundations, but it is not by any means a substitute for ordinary masonry under every condition. The old-fashioned foundation of field boulders, into which a very wet mixture of one part natural cement and two parts clean, sharp sand is poured, is probably the, cheapest form of foundation which can be made for cemetery purposes, but whatever style of masonry this may be, or by whatever name it is called, it is not concrete and its use is limited to medium-sized foundations. As the unbroken stones make poor contact with the other ingredients it is not strong enough for the foundations of narrow headstones, being unreliable should another opening afterwards necessitate the removal of the adjoining soil. For very heavy superstructures it is altogether out of the question.
There is a form of rubble concrete coming largely into use which is cheaper than ordinary concrete and like that material, needs no very skilled workmen to put it in place. Pieces of quarried or broken stone limited only in size by economy in handling are placed either on beds of ordinary mortar or of concrete; the spaces are then filled and the whole brought to a level by thoroughly ramming concrete of the ordinary character into these spaces, or if the foundation or wall is narrow, wet mixed concrete is placed in very crudely constructed molds and spalls of stone are driven hard into the mixture. With the same care in selecting clean stone which must be used in making concrete, a foundation of this character should cost about 15 to 20 percent less than concrete made of broken stone. There is a decided saving in the cost of crushing the stone, in the forms needed and in the amount of cement used, while the labor need not cost more.
All such structures may be depended upon to resist the crushing weight or overturning. As soon, however; as we have to deal with a tensile strain or have an exposed surface, or one which requires stone cut or shaped by hand, we have in concrete an economical substitute almost unlimited in its adaptability. Without considering concrete as a substitute for masonry foundations, or its application under conditions where compression of the material alone need be taken into account, there are cases in the older cemeteries in which the interments are so close together as to prevent the foundations for headstones and monuments being built in the usual way, or where the physical conditions require the foundation to be arched over the graves and in many newer cemeteries financial considerations make concrete an admirable substitute for cut stone in, structural work, such as entrance gate posts, boundary fences and the like. Now, as with all of these there is another force besides the crushing strength of the material to be taken into consideration, the application of my opening remarks as to the difficulty of treating this question before a mixed company by anyone not in active engineering practice will be apparent, as I have to note a very elementary question--that of the properties of a beam. Good practice requires not only that a structure have ample strength for the purpose intended, but the question of cost as well as of the weight of the individual member requires that it be not over large. With the information now at hand for anyone, the old rule of thumb method of making it big enough has no place in the practices of today. In this connection the US Government, various technical schools and some individuals interested in the subject have made tests of the strength of different materials and the laws governing their action. To illustrate the result, let us suppose a beam of white pine of the size and shape of figure "A".
It has been found that this timber has a breaking strength of 450 lbs, for each 1 foot long by 1 inch square, or rather that may be assumed to be a safe load for it to carry. Understand I am not now speaking of the bending or deflection of the timber or of shearing. For the present purpose it is not necessary to go into these, but to consider the breaking strain alone. The strength of a beam is inverse in proportion to its length. That is, one 5 feet long, but otherwise of the same dimensions, will have double the strength of one 10 feet long. Their strength is directly proportioned as to the width. If one is of the same length and depth as the one illustrated, but 8 inches wide, it will have twice the strength and last the strength of a beam is directly proportioned as the square of its depth; that is, if one is 20 inches deep, but otherwise equal, it will be four times as strong as one 10 inches deep. Therefore, the strength of the beam “A” loaded in the center will be 4" x 10" x 450=18,000 lbs.
When a beam is loaded beyond its strength the result will be as shown by figure "B"; the lower portion will be torn apart, while the upper half will be crushed together, so that the efficiency of any beam depends not only on its ability to resist compression or having its particles forced together, but on its tensile strength or its ability to prevent them from rupture.
Keeping this somewhat elementary and dry digression in view, let us return to the properties of concrete. Made of one part Portland cement, two parts sharp sand and four parts crushed stone, concrete one year old has a crushing strength of 75 to 125 tons per square foot, but its tensile strength, or ability to resist pulling apart, is only about 1-10 as much, and as the strength of a chain is limited to its weakest link, the strength of concrete without reinforcement is limited to such an extent by its low tensile strength as to render it of little value when in any case it takes on the functions of a beam. Fortunately a way has been found through the addition of steel bars by which the necessary strength can be supplied. Bars of steel having an elastic limit of 30,000 to 80,000 lbs. per square inch and an ultimate strength of nearly twice as much can be imbedded in the concrete in such position as with a very limited amount of metal, to offset the weakness of the other material under tension. A beam is thus formed, which, being itself light, does not add unnecessary weight to the structure, which has great strength for its size, which is practically fireproof and which can be constructed at a very low cost. By reference to figure "A" you will note that by adding to the depth of a beam its strength is increased in proportion to the square of its depth and that the rupture takes place on the lower side; therefore the steel is placed as near the lower edge as possible. If, however, it is placed so as to be exposed, no amount of painting or other care will prevent it from decay. The chemical action of lime and concrete on steel as a preservative is well known. Iron, clamps have been uncovered in the Parthenon in a perfect state of preservation, and numerous investigations of modern work reveal the same conditions. A very important discovery was recently made at Berlin in the examination of some bars forming a part of the shore protection of the river Spree. These were imbedded in concrete at various depths, and it was found that all covered with ⅜ of an inch of concrete were in good preservation. It is safe, therefore, to say that the steel bars should not be less than ½ inch from the lower side of the concrete. In practice they are placed above the lower edge at from 3 to 5% of the total depth of the beam. The manufacturers of the Thacher bars advise placing them so that the distance from their center to the outside of the concrete be not less than one and one-half times the diameter of the bar. The amount of steel varies from 0-3 to 2-0 percent of the cross section of the concrete. Those who are interested and desire to obtain an analysis of the proportions between the steel and concrete and the moment of resistance of reinforced concrete steel beams will find a very simple diagram for that purpose prepared by Mr. J. W. Schaub, M. A. M., Soc. C. E. of Chicago, published in No. 18 vol. xlix, Engineering News.
There is one much debated phase of this question regarding which I do not feel justified in making any positive statement, namely, what is the best method of securing contact between the steel and concrete. The cement, sand and broken stone where properly compounded become homogeneous, the steel if in the shape of a plain bar holds its place by friction only. An engineer of considerable experience gave his opinion lately that turning down the ends of the bar and so anchoring it gave all needed strength. While this is often done it is a rather primitive and unsafe way as the whole result is dependent on the compressive strength of the concrete at the ends, and unless an unduly large factor of safety is used is hardly to be considered as exact work. For work of this sort such as floors and walls, steel wire netting, or expanded metal is used.
The three principal systems now in use for beams or deep work are the Thacher by which a round bar is at intervals rolled into ovals, the projections thus formed preventing the bars from slipping. Another is that used by the Expanded Metal Co., in which the bar is corrugated. That most generally in use is the Ransome bar. By this process a mild steel bar is twisted cold until it looks like a screw. You all know the extraordinary hold a screw has in wood as compared with say an ordinary wire nail. Such a bar, embedded in cement, is not only proof against any strain less than it ultimate tensile strength but by the twisting process its strength is doubled and as a bar with any defect is broken in the machine, there is the great gain that instead of being content with testing sample bars a practical test of every bar is thus made.
There is the limitation in concrete construction that the work is not able to carry the weight intended until some time after it is in place. A beam of wood, steel or stone is able to take the load at once. Concrete cannot do this and moreover when first put in place it is but a mass of wet cement, sand and stone incapable of supporting itself and open to the further objection of requiring a framework of wood both to support and mould it to shape. If workmen would bear this in mind we would probably hear less of the failure or cracking of floors. The details of the construction of these supports or false work as it is called, I hardly need enter into; it can be made without difficulty by any good Yankee carpenter.
I have spoken so far of this material as a beam, but that is by way of illustration. The same principle underlies anything which supports weight, a sidewalk, a floor, a wall, a roof or a bridge. Some of these have been constructed with concrete alone; by reinforcing it they can be made stronger with less material and at less cost. Before an audience of so much practical experience as this, I need hardly stop to explain its application to these. Reinforced concrete will in the opinion of many engineers soon supersede the steel frame business blocks now so frequently seen, but even should it not attain so general a use, it is so easy of construction with no other help than that to be found in every cemetery, it can be constructed so quickly and can be adapted to such a variety of uses that it will be of advantage to the management of every cemetery to investigate. We so often need some profitable employment for our workmen during the winter months and have here a substitute for boundary posts and other work of a like character which can be prepared in a barn or shed kept at the proper temperature. We cannot all have boulder walls as at Swan Point, indeed some of us cannot even afford stone posts for the main gateway, but this material gives a satisfactory and permanent substitute at low cost. Posts can be made of reinforced concrete on foundations, say 15 feet apart and a steel fence or a solid wall of reinforced concrete carried on these foundations without other support. It can be used as piles when wood is unsatisfactory. Numerous bridges are being built of it. It is beyond question the best material for floors as its very general use in high class buildings demonstrates. You will remember seeing last year its use for this purpose in the Chapel and Administration building at Mount Auburn Cemetery, as well as for the floor over the boiler room of the greenhouse where wood was dangerous and needed frequent renewal. Buildings of any character can be made of it and a little taste and ingenuity will suggest relief and effect by the use of brick, stone or other trimmings. Toilet rooms except at the gate and near the boundary line of a cemetery are objectionable. With this material they can be built underground at the junction of avenues or other suitable places. In cemeteries with abrupt grades where the wash in rainstorms makes ruts, it is cheaper to build concrete paths than to make repairs.
To those desiring to look up the literature of this question I would suggests Gillmore's "Limes, Hydraulic Cement and Mortars," Cumming's "American Cements", Butler's "Portland Cements", Newman's "Notes on Concrete and Work in Cement". The Ransome Concrete Machinery Co. of New York, the St. Louis Expanded Metal Co. and the Concrete Steel Engineering Co. of New York all issue pamphlets which not only explain their respective systems but give formulae of exceptional value. Above all, so fast does experience and consequent information increase on this subject that every cemetery superintendent should subscribe to some technical paper like the "Engineering News," to which journal I am indebted for much of the data here given.
These and kindred engineering, subjects should have the close and constant attention of every man in charge of a cemetery. They are the practical things which make for the comfort and convenience of every lot owner and they are much closer in contact with the pocket nerve of the corporation than is the ornamentation of the grounds. And while I have no desire to be ill natured, I must confess that as a general rule they are a trifle neglected. Planting and grading are very important and the whole question of the adornment of cemeteries is a pleasant and absorbing one. But while we tumble over each other to get into a discussion of a "Yellow Evergreen" we should also give our attention to some of the more prosaic things.
From the publication:
AACS - Proceedings of the 17th Annual Convention
Held at Rochester, NY
September 8, 9 and 10, 1903