THE COLLIERY GUARDIAN AND JOURNAL OF THE COAL AND IRON TRADES. Vol. CV. FRIDAY APRIL 4, 1913. No. 2727. PROPS AND BEAMS IN MINES.* By S. M. Dtxon, M.A., M.Sc., M.Inst.C.E., Professor of Civil Engineering at Birmingham University. Reinforced concrete is very little used underground, and doubtless there are economic reasons for this, but the use is growing, and on many occasions when I have been testing columns and beams reinforced in the usual way, mining engineers who were present at the tests have said that doubtless within a few years the use of rein- forced concrete in mines will become more general. The use of reinforced concrete is becoming general for pit- head works, such as bunkers and hoppers, and in a few mines in Great Britain it is used on a large scale below ground, some of the principal pits being Baggeridge, near Dudley; Sneyd, near Stoke-on-Trent; and Tunnel pit, Haunchford, near Nuneaton. Plain concrete is not used in mines to the extent which engineers who work above ground would anticipate. Of course, one reason for this is that time is an all-important quantity in mining, and the methods in use at the present, which give the full width of the roads immediately they are timbered, seem preferable to those methods which need temporary support, blocking the narrow roads for considerable periods. But by the collaboration between the reinforced concrete specialist and the mining engineer special means will be found to meet the special require- ments, and so an immense field opened for the use of reinforced concrete. To a very limited extent, do I anticipate that beams and posts of reinforced concrete will be used in mining as substitutes for timber, but after a complete study of the possibilities it ought to be possible to design some really efficient substitute for timber in many cases. Besides, with our present limited capacity in testing machines in this country it is satisfactory to have for testing purposes numbers of structures of full size as constructed. The results obtained from experiments on 8 ft. beams and posts will have a direct bearing on the posts and beams which might be used in mining work. It is only in main roads and permanent work in mines that it is anticipated there will be any extensive use of reinforced concrete work. The strength of the timbers used in mining is fully discussed in a paper by Prof. Louis,* and he states that the strength of larch props is 3,360 lb. per square inch, and Scotch fir is 2,688 lb. per square inch when the timber in each case is thoroughly seasoned, dry and also fully grown, while the strength of wet timber was from 40 to 50 per cent. less. These experiments were made with ordinary pit props 4 ft. long, and Prof. Louis states that the prop has the same strength, whatever its length, under the ordinary con- ditions of practice. It is evident that frequently in mines the props and beams will be wet, and therefore, according to these figures, the existing strength of a larch bar should not be assumed to be more than 1,7801b. per square inch, which will certainly be less than a well made reinforced concrete post four months old. Besides, in a mine dry rot quickly attacks timber, unless constantly wet. The cost of timber for mine work is continually increasing, the price of foreign timber having risen as much as 45 per cent, during the last five years. For many reasons, therefore, substitutes have to be found for timber for main roads where the work is to be of a permanent character, and we find that the use of steel joists is very usual. It is evident that reinforced concrete slabs with rock packing, or concrete filling if excessive loads are expected, might be most effectively used in conjunction with joists. When reinforced concrete slabs between steel joints are suggested for the roof and walls, it is natural to think of reinforced concrete beams and posts or rein- forced concrete -[--sections. Experimental work will be useful, as these structures will most probably be used * From a paper read before the Concrete Institute, March 27, 1913. j t Louis, Trans. I Min.E., xv., 1898. | in positions where the maximum load has to be supported by the structures as soon as they are in place. No doubt one of the chief reasons why concrete has not been used in mines is that it takes a con- siderable time to attain its ultimate strength. It is possible, therefore, that a method of supporting main roads in which can be used posts, beams and slabs, manufactured and stored for at least six months, will be found economical in some cases. In arranging a preliminary set of experiments on the strength of beams and posts of reinforced concrete for mine work, it was decided to compare the results obtained from specimens of three different cross sections for the post—viz., square 8 in. by 8 in., round 8 in. diameter and triangular 5 in. side—and also to vary the reinforcement both in percentage and in method of arranging it. The cross-sections of the beams were (a) 8 in. by 12 in., (b) triangular 8 in. side. If the beams are to be made and stored before use, and afterwards transported to the mines, it is evident that they must have some reinforcement near the compression side for safety in transit, and it may be ultimately more economical to use some particular shape and similar reinforcement both for props and beams, the lack of economy in design being more than counterbalanced by economy in manufacture and the advantage of having one form of structure only in the mine. In making the test specimens, wooden forms were used for the square and triangular sections and steel forms for the circular sections. The beams were filled horizontally and the columns vertically. The beams of circular cross-section have not yet been made. All the concrete used was of the same composition, and was made under as similar conditions as possible. The proportions selected were 1:2:5. The sand was from Leighton Buzzard, and the gravel was well screened between 1 in. and J in. Tests of the materials used were made from time to time, and special efforts were made to mix the concrete and ram the moulds under the normal conditions obtaining in practice. The concrete was mixed wet—that is, with about 8 per cent, of water —experiments having been previously made on the effect of varying the amount of water in this concrete. The results are given in Table I. Table I — Influence of Amount Water in Strength of Concrete* Average compression Per cent, of water. 6 Age in days. 38 strength. Lb. per sq. in. 1,247 6 64 1,543 6 121 1,350 8 37 791 8 66 1,253 8 132 1,260 10 35 390 10 ;.. 64 750 10 130 1,010 These results seem important, since in most reinforced work wet mixtures are required, and till the concrete is over two months old the consequent diminution in strength is very serious. The cement was slow setting and uniform in quality. It was supplied by the makers in barrels, and each barrel tested according to the British standard specifications, the strength being— Lb. per square inch at 7 days .... 437 „ „ 28 „ ...... 635 and the time of setting three and a-quarter hours. The steel used had a tensile strength of 28 to 30 tons per square inch and a yield point at 20 to 21 tons per per square inch. Modulus of elasticity, 30‘8 by 10® lb. per square inch. All the tests on the beams and posts were carried out at the age of two months. About 100 cubes of concrete 6 in. by 6 in. by 6 in., made the same time as the beams, were tested at two months, and gave an average compressive strength of 1,6001b. per square inch; and another series of these blocks were made in order to determine the increase of strength with age for this concrete, the results being given in the following table:— * Dixon and Villiers, Inst. C.E., clxxxv. Increase of strength of concrete, 1:2:5 (lin. gravel) with age. Blocks 6 in. by 6 in. by 6 in., mixed with about 8 per cent, of water. These results seem very low when compared with the records given from tests on concrete blocks made at Vyrnwy, and also with the high-compression stresses given from American tests. But, as is well known, the strength of any mixture can always be increased by carefully selecting and grading the gravel or broken stone, and then having the maximum size of stone as large as possible. On the other hand, wet mixtures with small gravel (which is therefore of nearly uniform size), such as are suitable for reinforced work, both to obtain good surfaces and to obtain the absence of voids round the reinforcement, tend always to small compres- sive strengths. In any case, hundreds of tests on cubes of 6 in. side of concrete made of the same materials, in the same proportions and under the same conditions, confirm these results, while greater strengths, with the same proportions, have been obtained under the condi- tions specified above. But it has been considered that the method of mixing adopted for the beams and props gives results more like those usually obtained in prac- tice, and this opinion is confirmed by results obtained in practice. During the last five years I have carried out many tests on 6 in. cubes of various mixtures, made and filled by workmen engaged on reinforced concrete buildings in the Midlands, and in no single instance has a strength of 2,000 lb. per square inch been obtained when the age was under two months. The gravel, which was fairly uniform in size, passed a 1 in. sieve. The voids were 36 per cent. The following figures show its analysis :— Per cent. Under 1 in. and above f in........ 17 2 „ f „ ., | .......... 44 2 » 8 >» :♦ TF >» ........ 38’0 „ t3b ,, ......................... 6’6 10000 The gravel was not washed, since preliminary experi- I ments failed to show a noticeable increase in the | strength of the concrete by so doing, and it was desired i also to have the conditions as like as possible to those | usually obtaining in practice. The forms were removed three days after casting, and the concrete was not allowed to dry for six weeks. Tests of Beams. In testing the 12 in. by 8 in. beams the supports were 7 ft. 6 in. apart; the load was applied at two points 3 ft. 6 in. centre to centre in the method usually adopted. Table II. shows the result of the preliminary tests. Table II.—Tests of Reinforced Beams, 8 ft. x 1 ft. x 8 in. 00 fl 0) S ’5 Reinforcement. No. 0. 0Q Kind. o o Per cent. Breaking load. Equiv. uniformly distrib. load. Tons per lineal foot. Method of failure. 1 .. 2 .. 3 .. 3 ...2 straight £ in. ( 2 straight | in. I 1 bent up i in. i" s 0 45 .. 0 67 ... . 10 . 13 3 .. .. 3 V 1 straight £ in. ( 2 bent up | in. ”1 straight £ in. top \ i 1 0 67 .. . 11 4 .. .. 3 1 straight | in. 1-34 ... . 14 bottom j 4 bent up £ in. J ... Tension ... Diag.tension C Plain & diag. (. tension ... Diag. tension Tests of Posts. In making the posts for these preliminary experiments only one rod was used for reinforcement in each case, and the rods were only 7 ft. 8 in. long, thus leaving 2 in. of concrete at each end, so that the rod did not come in contact with the crushing plate of the machine. The only reason for reinforcing the posts is to make them portable, as it was considered that the crushing strength of the concrete would be ample for it to work, provided that it could be developed. It was anticipated that