USE OF REINFORCED CONCRETE ON SMALL RESERVOIRS AND TANKS
Special Point* in Their Construction from this Material—Water Tightness Depends on Strength of Mixture and Proper Mixing
WHILE the following article treats the subject of reinforced concrete from the British standpoint there will be many points of interest to the American Water Works end of it. As Mr. Johnstone-Taylor points out, much depends in the use of reinforced concrete as in massed concrete in the strength of the mixture and in mixing and placing of it. Mr. Johnstone-Taylor’s paper is worthy of careful reading.
The advances made in both the study and practice of reinforced concrete for structures of all kinds demanding lightness and strength have now rendered it possible to adopt the system with confidence in connection with small reservoirs, tanks, elevated and otherwise, filter beds, etc., in connection with water and other hydraulic works. The advantages of the system are several. There’ is at once, of course, a considerable saving of material as against mass concrete, and while although cast iron and pressed steel plates provide an easy and suitable means of building small elevated tanks, in addition to the more ordinary method of steel plate construction, these materials are not always easy to obtain especially in places abroad, and may often have to be lined with cement to prevent corrosion, while the maintenance costs in the shape of painting, etc., have to be reckoned with. In many instances reinforced concrete provides a means of overcoming most or all of these difficulties.
What the System Is
In a tank the metal would be disposed near the internal (water) surface of the walls. Thus these walls, in place of being built as one thick mass as they would be jn ordinary concrete would be formed of quite thin material 3 to 8 thick or so, sometimes supported on the outside by buttresses at intervals, all duly reinforced with steel and tied together.
This reinforcement would be erected according to plan, forms would be placed round it, and the concrete carefully rammed into these forms. The floor, if not laid on firm ground, would be constructed in the form of thin slabs, beams and cross beams, laid in a similar manner to the walls, and in the case of an elevated tank the whole could be supported on a scries of pillars also reinforced.
To give a general idea of the class of work under consideration, Fig. 1 is given showing a section of a typical elevated tank 16 feet in diameter, while Fig. 2 is a view of what a tank placed on the ground would appear like, the bottom in this case being on soft ground is also reinforced. This may be taken as a typical section of a tank capable of retaining water to a depth of 15 feet and is an actual example of a reservoir erected in Scotland its size being 192
feet square. ……,
If the walls of this tank had been built of mass concrete they would have been over six feet thick at the base for safety. Reservoir walls may be built on this system to withstand greater heads, but for the class of work under consideration, 15 feet is about the maximum height likely to be demanded. It will be noted that the walls of the tank in question are self-supporting without buttresses.
When reinforced concrete is used for water retaining structures, there are several points that become important when it means getting a watertight structure and keeping it watertight. Of course, concrete tanks of any sort may be made watertight by means of a bitumastic lining, but this is a very expensive procedure when properly done and useless when not and it is just the necessity for this lining which should be guarded against. Another point to bear in mind is that once a tank starts to leak practically nothing will stop it, palliatives may be applied but they are so much waste of time and money.
The general theory of design of reinforced concrete is based upon the assumption that concrete offers no resistance to tensile stress, and as its strength in tension is neglected, it is considered safe to assume that steel rods embedded therein may be stressed up to anything between 16,000 pounds and 20,000 pounds per square inch without detriment to the structure.
It is generally assumed that before the lower figure is reached, the concrete surrounding the steel must have cracked. This cracking is very minute and often invisible and while permissable in ordinary work is inadmissable in hydraulic work.
The design of such work should not allow of the steel being stressed to more than 10,000 pounds per square inch, while the usual assumption that tensile strain in the concrete may be ignored, may not, to say the least, be taken too lightly. The net tensile stress must be kept within the limits which experience has shown will yield watertightness, and this must not exceed ten per cent, of its ultimate strength when 30 days old. The latter does not exceed 180 pounds per square inch for 4-2-1 concrete, which is a good average strength for this class of work. These precautions necessitate, of course, extra reinforcement, and consequently, extra cost, and if the job is to be done at a competitive price by a contractor in the reinforced concrete business, these points have to be watched, or else leaks will occur, which it may be stated are much more troublesome in reinforced concrete than in plain, due to the peculiar nature of the former.
The circular form for elevated tanks is to be preferred for many reasons. The first is appearance, the second is a possible saving in cost over the rectangular form. Tanks of this class have been erected in various places, many of considerable size. The circular form is not confined to elevated tanks, but is more usually employed for them than for tanks at or under ground level.
When circular tanks are designed for hoop tension, as they usually are, stresses will be set up in the steel and concrete due to the reduction of volume of the latter on setting. If the work is properly designed and carried out this will not affect its strength but cracking will^ occur. This cracking can be localized by means of contraction joints. These latter would ordinarily be placed in the vertical plane at which each section of the work would be stopped each day. There would thus be a series of vertical butt joints, of the form shown in Fig. 3, a suitable distance apart being 20 feet. Thus there would be three such joints in a tank 20 feet in diameter. For quite small tanks they would be unnecessary.
Some engineers do not consider them necessary in any tanks filled within three months after completion and kept filled as the water is considered as having sufficient swelling effect on the concrete to cause the cracks to take up, but they are undoubtedly a wise precaution, especially if the tank is subjected to extremes of temperature. Small circular tanks are usually made continuous with the floor and reinforced thereto, but the larger ones more often have the wall isolated by means of an elastic joint. The former conditions increase the risk of cracks and set up peculiar stresses in the wall demanding extra reinforcement. This, however, is a matter for the designer. The planes of stoppage in flat slab walls may also necessitate contraction joints. The most effective form is a vertical circular groove of about one-third the mean thickness of the wall as shown in Fig. 4. In fact, contraction joints may be generally considered useful in tank work of this class as localizing the points of cracking, and their small extra expense is usually well warranted.
As before stated, the main object of these notes is to show how tanks may be made watertight without the necessity of lining. Many engineers are of the opinion that unlined concrete tanks of any kind will leak, no matter how well the work is done. Great claims are made for many so-called waterproofing compounds.
The basis of most of these is alum, and while in most cases they are not injurious to the concrete their value is problematical.
In the main, watertightness depends upon the strength of the mixture, quality of cement and aggregate and especially on the efficiency of the mixing and placing thereof.
The better the materials, the stronger the mixture, and the better and more carefully it is mixed and placed the greater will be the chance of the final structure being watertight, but the more expensive will it be. Consequently, if the work is let by contract, at anything approaching a cut figure there will be a grave risk of faulty work.
Economy in this direction is therefore likely to be false. Nothing is worse than a leaky tank, nothing harder to rectify, while an asphaltic lining will more than offset any saving Obtained by other means. Experiments carried out in England at the National Physical Laboratory showed that even rich concrete was porous under a head of 20 feet, but the degree of porosity decreased after several weeks, not so much due to supposed silting up of the pores as to swelling of the concrete. It would therefore, appear that all concrete was porous. On the other hand, impermeability does not depend so much upon the richness of the mixture as upon the proper proportioning of the sand and aggregate. The obvious thing to do would seem at first to be to plaster the inside with a very rich mixture, say 1-1 cement and sand.
It is a most unsatisfactory procedure in _ practice, such lining invariable breaking away from the main body of the concrete.
The now recognized procedure is to cast a rich face on the concrete itself. That is to say, the base concrete may be a 4-2-1 strength and the water face say 1 1/2 inches thick of 2-1-1 strength. The latter is placed first against the timber sheeting and a temporary sheet iron plate resting against the reinforcement. The base concrete is then rammed in immediately after, and the iron plate withdrawn, the two sections being then thoroughly puddled together.
Economy of Reinforced Concrete
From what has been said it will be rightly inferred that the whole idea underlying the use of steel in concrete is to reduce the total cost of the work as compared with mass concrete or steel and iron, and in the latter case maintenance costs.
It is well proved that marked economy can be effected in many instances, and the system has much to recommend it and, although comparatively new, has stood the test now of several years. The effects of frost, however, on exposed tanks (that is not finally covered with earth) must not be lost sight of, and it may be a determining factor in severe climates. It must also be remembered that when a tank is at ground level and covered or banked up with earth, these banks resting against the wall will have a supporting effect which may be taken into account in reducing the section of the wall, the amount of reinforcement, and its total cost.
The type of tank shown in Fig. 2 could be done at 50 per cent, of the cost of mass concrete of similar strength, if banked the saving would be more marked still.