The use of concrete in engineering is exemplified by dams, bridges, tunnels, roads and buildings all over the world. The manufacture of Portland cement, which is used in making the concrete, has become a vast industry in which many spectacular processes are involved
REINFORCED CONCRETE PILES used for the foundations of the great electricity generating station at Barking, Essex. Reinforced concrete piles made from a material known as high-alumina cement can be driven less than two days after moulding. The reinforcement takes the form of steel rods or mesh embedded in the concrete.
TOWERING high in the valley, linking the hilltops on either side, the vast man-made cliff curves back upon a load that Nature herself would prefer to release as a deep roaring flood sweeping down to the sea. Such is a barrage, built to impound river or lake that mounts ever higher until millions of tons of water press on the wall that man has thrown across the valley. That great wall, a marvel of twentieth-century engineering, owes its strength to the finest of powder, the cement that bonds great quantities of loose material and makes of it concrete — artificial rock.
Cements of various kinds were known hundreds of years ago, and to-day cement concrete may be said to be the most important material of the civil engineer, just as steel reigns supreme in mechanical engineering throughout the world.
Portland cement may be produced from any suitable material which contains the necessary constituents, lime, silica and alumina. The raw materials are generally carbonate of lime — in the form of chalk or limestone — and silica and alumina — in the form of clay or shale. For the works at the mouths of the Thames and Medway, the principal producing centre in England, chalk and clay, of which there are abundant local supplies, are used. It seems almost incredible that two substances of this nature should yield a powder which, mixed with water, will set rock-hard in a few hours and then remain impervious to water for all time.
The chemical changes undergone by the raw material in cement manufacture are somewhat complex, but briefly the reason for the “setting” and subsequent hardening of the product is due to the action of water. In the first instance we have rock in the form of limestone, with more rock in the form of clay or mud formed by the action of water. All water is driven off during the calcination process (described below) and then, on the addition of water once again to the finely ground cement, the product again turns into rock. There are two main theories in connexion with the setting of cement. One theory is attributed to the scientist Le Chatelier, who submitted that the addition of water to cement produced crystals that interlocked and, on the evaporation of moisture, produced a compact strong mass.
A later theory, propounded by W. Michaelis, holds that the set cement becomes a colloidal gelatinous mass that hardens as it dries and shrinks.
Portland cement is yet another of Great Britain’s contributions to engineering, but due credit must be given to the work of the Greeks and Romans many centuries ago.
CONCRETE MIXING PLANT used during the building of the Boulder Dam, one of the largest concrete structures ever built. Altogether, including subsidiary works, about 4,000,000 cubic yards of concrete were required. This amount involved the consumption of 755,360 tons of cement and the use of many mixing plants. Compressed air was used to force cement from the silos through pipes to the containers of each mixing plant. In one instance the pipe was 5,600 feet long (over a mile), and the cement was delivered to the container at the rate of 76 tons an hour.
Greek and Roman builders used a mortar made from finely ground volcanic deposits mixed with lime and sand. The Greeks used volcanic tuff from the island of Thera (now Santorin). This substance, now known as Santorin earth, is still used in the Mediterranean for building purposes. The Romans made use of the volcanic tuff round the Bay of Naples, and the best of such material was found near Pozzuoli: hence the name “pozzolana” for this class of mineral matter. Pozzolana and Rhenish volcanic tuffs known as “trass” are still used for cements to-day.
Considerable use was made of such materials by the Romans. An outstanding example is the famous Pantheon, a temple built in A.D. 120-130 by the Emperor Hadrian, in Rome. The walls of the building, 20 feet thick, are made of tuff and pozzolana, and are faced with brick. The great dome, which has a diameter of 142 ft. 6 in., is cast in a solid mass made from pumice and pozzolana. The semi-fluid mixture was poured into moulds formed by shuttering, or wooden boards, exactly as in modern practice. Mortar walls built by the Romans on the Italian coast are still in existence, despite centuries of battering by the sea. Roman mortar was introduced into Roman Britain. Later, a form of concrete was used by the Saxons and the famous stronghold of Corfe Castle, Dorset, was built of concrete.
In 1756 John Smeaton was called upon to build a lighthouse on the Eddystone Rock, after the destruction of the earlier building by fire. Smeaton carried out numerous experiments with different limestones in his search for a mortar that would resist the action of water. Finally he cemented the stones of his lighthouse with mortar prepared from blue lias hydraulic lime and pozzolana. The importance of hydraulic or clayey lime was thus appreciated for the first time.
Early in the nineteenth century other experimenters prepared artificial hydraulic lime by calcining (roasting) a mixture of limestone (chalk) and clay. Chief among these pioneers were L. J. Vicat, James Frost and Major-General Sir C. W. Pasley, the military engineer. Frost established a factory at Northfleet, Kent, which was later acquired by the firm of Francis and White. This was the forerunner of other great cement works that now stand along the estuary of the Thames.
General Pasley began his researches on cement making in 1826 when he was appointed to the Military School at Chatham. He had a plentiful supply of chalk, but in place of clay he used brick loam. His first experiments were unsuccessful but were resumed at the request of a brother officer. The stock of loam had become exhausted and Pasley ordered his assistant to obtain some clay. By a fortunate chance the assistant procured some of the blue Medway clay used by the Army Engineers for securing fuses in military mining practice.
WASH MILLS in a cement factory. A wash mill consists of a large pit, lined with bricks or concrete, in the wall of which there are gratings through which the raw materials pass when they have been sufficiently reduced. Above each pit is a bridge which carries the upper end of a vertical post, pivoted in the centre of the pit floor. Radial arms carrying water sprinklers project from this shaft, and the shaft is rotated until the contents of the mill form a creamy mud or slurry.
SLURRY MIXERS, in which rotating arms and a compressed-air agitating device keep the material in an adequately mixed condition. After the mixing of the raw materials has been completed, the slurry is pumped into kiln feeders for calcining, the next stage of the process.
The results of the renewed experiments, using the blue clay, were so satisfactory that work was continued. By 1830 Pasley had produced a good material. To-day the importance of cement-concrete in military engineering needs no emphasis. Concrete roads, trenches, dugouts, gun emplacements, the German “pill-boxes” of the war of 1914-18, the blockships of Zeebrugge — all are instances of concrete as a vital necessity in war.
Credit for the invention of modern Portland cement is, however, generally given to Joseph Aspdin, a Leeds bricklayer. He named the material Portland cement because of its fancied resemblance, after having set, to the famous stone quarried at Portland, Dorset. Joseph Aspdin, who took out his first patent on October 21, 1824, used a hard limestone which was calcined and crushed into powder. This was mixed with clay and ground into a fine slurry (fluid mixture) with water.
When dry, the mixture was broken into lumps and calcined in a kiln until all carbonic acid had been expelled. The clinker thus formed was then crushed and ground into powder ready for use as Portland cement. Aspdin inferred that heat applied to the clay yielded a material similar to volcanic earth, but the temperature used was inadequate for the best results.
In the early days of cement making it was thought necessary to pick out from the kiln all material that showed signs of beginning to vitrify. Thus the material now known to be the most valuable content of the kiln was wasted. Later, however, greater heat was used in the firing process and Portland cement was used by Marc Isambard Brunel on the resumption of work on the Thames Tunnel in 1836.
The introduction of high-temperature firing was claimed by Isaac Charles Johnson, who built a cement works at Rochester, on the Medway, in 1851. Johnson died in 1911 at the age of 100, Joseph Aspdin went to Germany, where he established cement factories in 1856, and his son William continued the manufacture of Portland cement on the banks of the Thames.
To-day the cement works on the Thames and the Medway are among the most important in Great Britain and are well-known landmarks for the shipping that serves the Port of London. These works have their raw materials close at hand — the chalk all round them and the clay in the form of river mud right at their door. Cement manufacture is now a vast industry with great works throughout the world.
Soft clay and crumbling chalk form the basis of an extremely hard artificial rock through a remarkable change in properties brought about in the process used in the manufacture of Portland cement. The properties of the cement itself can also be changed within limits by variation in the proportions of raw materials used and by the addition of other substances. One of the most important developments in cement manufacture is the increase in the rate of hardening that has been achieved in recent years by fine grinding and slight alteration in composition.
Wet and Dry Processes
The chemical constituents of Portland cement are lime (60 to 64 per cent), silica (20 to 24 per cent), alumina (6 to 10 per cent) and iron oxide (3 to 5 per cent). In addition there are small quantities of sulphuric anhydride, magnesia and alkalis. The bulk of the lime, in the form of carbonate, is contained in the chalk. Most of the silica and alumina is in the mud or clay. Oxide of iron also is contained in the mud. Marls, a mixture of chalk and clay and shales, are used also in the manufacture of Portland cement.
HUGE CONCRETE MIXER at Elm Park Estate, near Romford, Essex. Cement, sand, water, and an “aggregate” of broken stone or washed gravel are poured into the container. This has a drum fitted with blades and is driven by a stationary engine. When the mixing is complete the wet concrete is poured into barrows or skips from a discharge chute.
There are now two different processes available to the cement manufacturer. The wet process involves the grinding and mixing of the raw materials in a wet condition. This process was used originally for the treatment of chalk and river mud, but is now applied also to harder limestones and shales. The wet process is almost exclusive to Great Britain and to new plants in the United States, where, however, the alternative dry method was at one time popular to cope with raw materials in the form of hard rocks. At many cement factories neighbouring chalk pits are linked with the works by a number of contractors’ narrow-gauge railways on which wagonloads of chalk are hauled to the wash mills by steam or internal combustion locomotives. Close to the wash mills is a landing-stage, to which are towed barges laden with mud dredged from the lower reaches of the river. A wash mill consists of a large circular pit lined with bricks or concrete. In the wall of the pit are a number of gratings through which the raw materials pass when they have been sufficiently reduced. About ⅜ in. is allowed between the bars of the gratings.
Above the pit is a bridge that carries a power shaft, gearing and the upper end of a vertical spindle or post, pivoted in the centre of the pit floor. Six radial arms, joined together at their outer ends, project from the vertical post and each of them carries an iron rake or harrow. The chalk and mud (or clay) are fed into the mill in the correct proportions and the arms are set revolving. Attached to the arms are water sprinklers that are turned on until the contents of the mill form a creamy liquid or slurry. Any flints that may be in the chalk fall to the bottom of the mill and are removed periodically. The slurry is not yet in a sufficiently fine condition, and it is therefore passed through another wash mill fitted with finer gratings. Sometimes a centrifugal screening mill, a revolving perforated cylinder, is used for the secondary stage of the wash-mill process. A third method is to pass the slurry through a tube mill, which comprises a rotating steel cylinder containing a number of steel balls. These tumble over with the contents of the mill and reduce the material by a crushing and rolling action.
Calcining the Mixture
Tube mills are sometimes divided into two, three or four compartments containing steel balls of varying size. Their action is continuous, and material is fed into one end and delivered in a finished condition at the other. Large tube mills are used when the raw materials consist of hard limestone and shale; the water is then added direct to the mill to form the slurry. When clay is used with hard limestone the soft material is fed to the tube mill mixed with the water.
Slurry from the washing or tube mills is pumped to huge basins in which rotating arms and a compressed-air agitating device maintain the material in an adequately mixed condition. At this point samples of the slurry are analysed to ensure that it contains the correct proportion of lime. Adjustments are then made as necessary, generally by the use of two mixing basins, one containing a mixture with an excess of lime, the other with a deficiency of lime.
The mechanical mixing of the raw materials has at this stage been completed and the slurry is pumped up into kiln feeders ready for the next process in manufacture. This is the calcining by which the materials undergo a chemical change and assume some of the remarkable properties of Portland cement. Calcining in the manufacture of cement is one of the most spectacular of industrial processes, and it is carried out in a special kiln of enormous size.
A modern cement kiln is of the rotary type, and comprises a huge steel cylinder, between 9 and 12 feet in diameter, lined with firebrick or other refractory material. The kiln is generally from 200 to 250 feet long, although some of these great tubes are 500 feet in length. The kiln is fitted with a number of steel bands or tyres which rest on special roller bearings mounted on large masses of concrete. Encircling the cylinder there is also a toothed ring to which is geared the power supply; this is derived from steam engines or from an electric motor. One end of the kiln, smaller in diameter than the other, is raised to provide a downward slope of about 1 in 90.
The upper end of the kiln leads into the base flue of an immense chimney, towering high above the works and visible for miles. At the lower and larger end of the kiln is a shield through which protrudes a special type of nozzle. This is the burner through which pulverized coal is blown by a powerful rotary fan. The issuing spray of extremely fine coal dust is ignited and is swept as a giant torch of white-hot flame from one end of the huge tube to the other. The slurry is fed through a pipe into the top end of the kiln and, as the great cylinder slowly revolves, the water leaves the wet material in the form of steam. The dry particles of mixed chalk and clay gravitate towards the hottest part of the kiln, where they form particles of clinker that vary in diameter from ⅛ in. to ¾ in. The white-hot clinker continues its journey towards the lower end of the kiln, from which it emerges to “clinker coolers”. In some kilns the clinker falls through an opening in the bottom of the burner shield into a chute leading into another revolving, but fireless inclined steel cylinder. From this cylinder the cooled clinker is delivered by a conveyer to the grinding machinery.
Later types of rotary kiln are provided at the lower end with a number of open-ended cylinders. These cylinders are arranged round the periphery of the kiln and are parallel to it. Openings in the kiln walls provide outlets for the clinker to the cylinders, which are fitted with loose chains. As the kiln with its cooling cylinders revolves, the clinkers fall through the openings, strike the chains in a continuous cascade and finally drop on to conveyer belts on the way to the grinding mills. Air is blown through the cylinders to assist the cooling process and the resulting hot air is used as a blast for the pulverized coal burner. The coal dust used in the kiln is ground in special mills to the consistency of the finest flour.
The grinding of clinker in a cement works is of the utmost importance, and at this stage in manufacture an important addition is made to the materials. The substance added during the grinding process is gypsum (from which plaster of Paris is made). Gypsum has the property of retarding the setting quality of the finished cement. If this course were not adopted the product, because of the action of the rotary kiln, would set too quickly for efficient use.
ROTARY KILNS in which cement slurry is calcined. These huge steel cylinders may have diameters varying between 9 and 12 feet and their length may be as great as 250 feet or more. Pulverized coal is blown through a burner at one end of the kiln. The spray is ignited, and a giant torch of flame sweeps through the kiln. As the great cylinder revolves the water leaves the slurry, in the form of steam. The dry particles, forming white-hot clinker, emerge from the lower end of the kiln, after which they are cooled.
Grinding mills and their methods of use vary somewhat in different factories, but their efficiency attains a high standard, as on this depends the quality of the finished cement. When other factors in the manufacture of cement are equal the strength of the material may be regarded as dependent on the degree of fineness attained in grinding.
Many different types of grinding machine are used to reduce the clinker to a fine powder. One device in general use, especially for the finishing process, is the four-stage tube mill. Formerly cement was ground between horizontal millstones similar to those used for grinding wheat. For the first half-century in the history of cement this was the only method used in the reduction of the clinker to a fine powder.
The use of millstones gave place to grinding in edge-runner mills, generally consisting of four heavy rollers, about 5 feet in diameter and 18 in. wide, driven round on a circular path in a pit or container. In later machines of the edge-runner type the path and the rollers were tilted inwards to enable the rollers to exert a certain amount of centrifugal force as they revolved. Pressure on the rollers was obtained in later edge-runner mills by the use of a hydraulic ram. These mills were used extensively in the cement works on the Thames and the Medway. The mills comprised a set of three rollers, 2 feet in diameter and 10 in. wide, carried in a heavy cast-iron frame and revolving on a cast-iron bedplate. On top of the machine was a powerful hydraulic ram that applied pressure to the rollers.
Stored in Concrete Silos
Another type of grinding machine that has proved of great value in the cement industry is the Griffin mill, an ingenious device that originated in the United States. The Griffin mill may be compared with a huge bell in which the clapper or tongue rolls round the interior of the “lip” and crushes the material between the two by centrifugal force, combined with a gyroscopic action. The base of the machine, which alone may weigh some 7 tons, consists of a drum with a tapered mouth pointing upwards. Near the bottom of the drum is a massive ring or die, with an internal diameter of 40 in.
Above the drum rises a bell-shaped framework, from which is suspended a vertical steel shaft. At the lower end of the shaft is a 24-in. horizontal roll, on the underside of which project numerous stirrers. The top of the shaft is fitted in to a universal joint contained within driving pulleys at the head of the framework. As the pulleys are driven round, the shaft revolves and swings outwards so that the roll presses heavily against the ring in the drum. The rolling action grinds the clinker into powder and, when this has become fine enough, it passes through a sieve surrounding the ring. This type of mill is also used extensively for grinding coal into powder for use as pulverized fuel.
The tube mill, however, is the most extensively used grinding machine in modern cement manufacture. In some ways a tube mill resembles a rotary kiln, but it is much smaller and has no firing arrangements. The cylinder is placed horizontally and at one end is a hollow trunnion that serves as a bearing. A hopper leads into the trunnion, and to this the clinker is fed either by a vibrating tray or by a revolving disk. The material falls on to the disk in the form of a cone, which is pared off by an adjustable scraper. The first compartment of the machine, at the feeding end, is lined with a perforated steel plate on which roll a number of steel balls as the mill revolves.
As the material is crushed it falls through the perforations on to a sieve that automatically returns oversize material to the centre of the mill for regrinding. The particles that are small enough to pass through the sieve enter the next compartment in the mill for further grinding by another set of steel balls. The grinding is repeated in a third or fourth compartment, according to the type of mill in use. The exterior of the mill is cooled by a spray of water while grinding is in progress.
CEMENT FACTORIES at Greenhithe, Kent. Chalk and clay, of which there are abundant local supplies, are fed into wash mills from barges and steam navvies. The circular tanks in the foreground are slurry mixers similar to those illustrated on above.
The finished cement is discharged from the tube mills and taken by elevators and conveyers to hoppers that feed into enormous reinforced concrete storage bins or silos, generally circular in section and coned top and bottom. A silo may hold as much as 1,500 tons of cement. Beneath the row of silos is a screw conveyer which removes the lowest layers of the stored cement. The conveyer carries the cement forward to another elevator in the packing shed, generally arranged alongside a railway siding and lorry loading platform. The cement is weighed out automatically into paper bags or sacks ready for transport.
The most careful tests are made of the finished product to ensure that it shall fulfil all the requirements demanded for constructional purposes. The most important quality of cement is soundness, that is, it must not disintegrate when mixed with water and allowed to set. Tests have been devised to ascertain the soundness of cement without waiting for it to harden naturally. In one test a freshly mixed pat of cement is subjected to a moist heat until set and is then immersed for some hours in warm water. The cement is thus artificially aged within twenty-four hours, and any bad qualities are brought to notice.
Cement must also be tested for fineness, and for this a weighed, sample is passed through a sieve or series of sieves made of brass wire gauze with a given number of meshes to the inch. The testing of cement to ascertain its tensile strength is particularly interesting. The primary function of the test is to find out the strength developed in the mixed cement in a given period of time. Such tests, taken at intervals, also indicate the degree of strength likely to be attained by a given batch of material.
For the purpose of the test a sample of cement is mixed with water and moulded into briquettes of standard size and shape. The briquettes are 1 in. thick, 1¾ in. wide and 3 in. long, pointed at either end and “waisted”, hour-glass fashion. The narrowest part of the waisted portion is 1 in. wide, so that its sectional area is 1 square inch.
After a predetermined interval following the mixing, a briquette is placed between the jaws, top and bottom, of a cement testing machine. This consists, in its simplest form, of a steelyard, fitted with jaws at the “load” end. A weight on the graduated steel bar is moved along, away from the pivot, until the briquette breaks. The reading on the steel arm then gives the breaking strain in pounds.
Another strength test imposed on cement is that of compression. A cement block, generally from 2 in. to 6 in. cube, is subjected to pressure in a hydraulic machine until it begins to crush. The pressure of the hydraulic ram is known and from this the resistance of the cement block is readily calculated. In engineering, cement is used primarily in the making of concrete, the application of which is exemplified in enormous dams and barrages, docks, bridges, tunnels, roads and buildings of every kind.
Concrete is a mechanical mixture of cement, sand, water and an “aggregate” of broken stone or washed gravel. Mixing is generally done by a portable machine in which a revolving drum is driven by a petrol or heavy oil engine. Mixing machines vary considerably in their design and capacity. The smaller mixers are generally of about 5 cubic feet capacity, and are driven by 1½ horse-power engines. Larger machines have a capacity of 16 cubic feet and engines of 9 horse-power.
REINFORCED CONCRETE is extensively used for road building. The steel mesh on to which the concrete is poured makes the finished material immensely strong. The illustration shows men with a concrete pump at work on Riverside Drive, New York, on the bank of the Hudson River.
Mixing machines are mounted on a framework or chassis of steel girders carried on four steel or cast-iron wheels. Common to both types is a revolving drum driven by an internal combustion engine. In the smaller machines the drum is pear-shaped, and is encircled by a toothed ring which engages with a pinion driven by the engine. Inside the container are a number of blades that assist the mixing process. The mixture of cement, sand and aggregate is shovelled into the upturned mouth of the container, the requisite amount of water is poured in from buckets and the machine is then set in motion. When the contents have been thoroughly mixed, the drum is tipped bodily to one side by the action of a lever. The wet concrete mixture is poured into wheelbarrows, or into the special skips used for hoisting it to the top of buildings under construction.
In the larger machines the chassis is surmounted by a steel framework within which a cylindrical drum revolves, with its axis horizontal, on rollers. The watertight drum is of welded sheet steel, and is fitted inside with a number of buckets and blades to ensure a rapid mixing of the contents. In addition to the steel bands that rest oil the rollers, the drum is encircled by a toothed ring which carries a driving chain from the engine. Above the drum is a water tank fitted with an ingenious device that enables a predetermined quantity to be fed into the container during the mixing process.
A special feature of this type of mixer is the loading hopper at one side of the machine. This consists of a large spoon-shaped steel box, mounted on a girder framework. The concrete mixture is shovelled in the correct proportions into the hopper, and the framework is then raised so that the material is shot into the mixing drums. At the opposite end of the drum is a discharge chute leading from the interior mixing buckets to a position from which the wet concrete can be deposited into barrows or skips. In some concrete mixing machines the loading hoppers are used in conjunction with pear-shaped drums that tip on the opposite side of the machine when discharging their contents. The wet concrete is poured into moulds or shuttering. Although concrete has extremely high compressive strength, it is weak in tension. To strengthen it, steel reinforcement in the form of rods or mesh is embedded in it.
In recent years numerous special cements that produce an exceedingly quick-hardening concrete have appeared on the market. In addition to the rapidly-hardening Portland cement, another material known as high-alumina cement, which is even more rapid in the hardening process, is now produced. The basis of this cement is generally bauxite, a mineral composed mainly of alumina (aluminium oxide), which is fused with lime. Reinforced concrete piles made from this type of cement can be driven in less than two days after moulding. Shuttering can be removed from quick-setting concrete in less than six hours, and roads laid with it can be used within twenty-four hours. Among other special cements is the pure white variety, free from oxide of iron. This cement is widely used for facings to structures and for pre-cast concrete units such as posts, including railway signal posts, and for making kerbstones. Other cements are made containing small quantities of colouring material. They are used for all types of decorative facings and finishes.
Cement pipes are made by the centrifugal process, in which the liquid cement is introduced into a mould. The mould is then revolved and the cement is flung outwards by centrifugal force to form a lining inside. When set, the tubular lining is removed as a completed pipe.
The cement gun resembles the apparatus used for spray painting but on a much larger scale. The dry-mixed cement mortar is forced by compressed air through a flexible pipe to the gun, from whose nozzle the mixture is sprayed by compressed air. The water for mixing at the nozzle is supplied by a second pipe line. The cement gun is used for applying stucco to walls, for coating steel work with cement and for cementing in inaccessible positions.
Concrete can be pumped through a pipe line of considerable length for distribution to the moulds or shuttering used in building construction. Similar methods are used in the construction of reinforced concrete “rafts”, for the reclamation of land on the banks of rivers and on the shores of lakes. A notable instance of the use of concrete pumped from a distance occurred in the building of the famous Riverside Drive on the bank of the Hudson River at New York.
CONCRETE POURING PLANT in use during the building of Unilever House, Blackfriars, London. Concrete mixed on the ground is raised by an elevator and poured down the channel of a distributing arm which can be swung over a wide area.