Introduction to Water Treatment

Water treatment is required for surface waters and some ground waters for drinking purposes. Water treatments involves the removal of pollutants generated from different sources and to produce water that is pure and suitable for human consumption without causing any long term or short-term adverse health effects. Many aquifers and isolated surface waters are high in water quality and may be pumped from the supply and transmission network directly to any number of end uses, including human consumption, irrigation, industrial processes or fire control. However, clean water sources are the exception in many parts of the world, particularly regions where the population is dense or where there is heavy agricultural use. In these places, the water supply must receive varying degrees of treatment before distribution.

The available raw water must be treated and purified before they can be supplied to the public for their domestic, industrial or any other uses. The extent of treatment required to be given to the particular water depends upon the characteristics and quality of the available water and also upon the quality requirements for the intended use. Raw water may contain suspended, colloidal and dissolved impurities. The purpose of water treatments is to remove all those impurities which are objectionable either from taste and odour perspective or from public health perspective.

Impurities enter water as it moves through the atmosphere, across the earth’s surface and between soil particles in the ground. These background levels of impurities are often supplemented by human activities. Chemicals from industrial discharges and pathogenic organisms of human origin, if allowed to enter the water distribution system, may cause health problems. Excessive silt and other solids may make water aesthetically unpleasant and unsightly. Heavy metal pollution, including lead, zinc and copper may be caused by corrosion of the very pipes that carry water from its source to the consumer.

The method and degree of water treatment are important considerations for environmental engineers. Generally, the characteristics of raw water determine the treatment method. Most public water systems are relied on for drinking water as well as for industrial consumption and firefighting, so that human consumption, the highest use of the water, defines the degree of treatment. The flow chart of the process occurring in conventional water treatment plant is shown below in Fig.1.

Fig.1 Process of Water Treatment

A typical water treatment plant is diagrammed in Fig.2. It is designed to remove odour, colour, and turbidity as well as bacteria and other contaminants. Raw water entering a treatment plant usually has significant turbidity caused by colloidal clay and silt particles. These particles carry an electrostatic charge that keeps them in continual motion and prevents them from colliding and sticking together. Chemicals like alum (aluminium sulphate) are added to the water both to neutralize the particles electrically and to aid in making them “sticky” so that they can coalesce and form large particles called flocs. This process is called coagulation.

Fig.2 Diagram of a Typical Water Treatment Facility

For surface water, following are the treatment processes that are generally adopted.

1) Screening

This is adopted to remove all the floating matter from surface waters. It is generally provided at the intake point

2) Aeration

This is adopted to remove objectionable tastes and colour and also to remove the dissolved gases such as carbon-dioxide, hydrogen sulphide etc. The iron and manganese present in water also oxidized to some extent. This process is optional and is not adopted in cases where water does not contain objectionable taste and odour.

3) Sedimentation with or without Coagulants

The purpose of sedimentation is to remove the suspended impurities. With the help of plain sedimentation, silt, sand etc. can be removed. However, with the help of sedimentation with coagulants, very fine suspended particles and some bacteria can be removed.

4) Filtration

The process of filtration forms the most important stage in the purification of water. Filtration removes very fine suspended impurities and colloidal impurities that may have escaped the sedimentation tanks. In addition to this, the micro-organisms present in the water are largely removed.

5) Disinfection

It is carried out to eliminate or reduce to a safe minimum limit, the remaining microorganisms and to prevent the contamination of water during its transit from the treatment plant to the place of its consumption

6) Miscellaneous Processes

These include water softening, desalination, removal of iron, manganese and other harmful constituents.

Objectives of Water Treatment

• To remove the dissolved gases and colour of water.
• To remove the unpleasant and objectionable tastes and odours from the water.
• To kill all the pathogenic organisms which are harmful to the human health.
• To make water fit for domestic use such as cooking, washing and various industrial purposes as dyeing, steam generation etc.
• To eradicate the contaminants that are contained in water as found in nature.
• To control the impurities from scale formation.
• Pure water quality is required to minimize the corrosion, radiation levels and fouling of heat transfer surfaces in reactor facility systems.
• To prove safe potable water to the public.
• To reduce the physical, chemical and biological contaminants in water.
• To eliminate the tuberculation and corrosive properties of water which affects the conduits and pipes.

Location of Treatment Plant

The water treatment plants should be located as near to the towns as possible. If the source of water supply is tube well the treatment plant should be located in the central part of the town, so that purified water may reach the public as early as possible. If the city is very large to which water cannot be supplied from one tube well, the city should be divided into zones and a separate tube well with necessary treatment plants should be provided for each zone. If the source of water is river or reservoir, the treatment plant should be located as near the town as possible preferably in the central place. The following points should be kept in mind while giving the layout of any treatment plant.

• All the plants should be located in order of sequence, so that water from one process should directly go into next process.
• If possible, all the plants should be located at such elevations that water should flow from one plant to next under its force of gravity only.
• All the treatment units should be arranged in such a way that minimum area is required, it will also insure economy in its cost.
• Sufficient area should be occupied for future extension in the beginning.
• Staff quarters and office should also be provided near the treatment plants, so that operators can watch the plants easily.
• The site of treatment plant should be very neat and give very good aesthetic appearance.

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Transportation of Water

The term conveyance or transportation of water refers to taking of water from source to purification plants and from treatment plant to consumers. Water supply system broadly involves transportation of water from the sources to the area of consumption, through free flow channels or conduits or pressure mains. Depending on the topography of the land, conveyance may be in free flow and/or pressure conduits. Transmission of water accounts for an appreciable part of the capital outlay and hence careful consideration for the economics is called for before deciding on the best mode of conveyance. Care should be taken so that there is no possibility of pollution from surrounding areas.

If the source is at higher level than the treatment plant, the water can flow under gravity, automatically. Similarly, after necessary purification of water, it has to be conveyed to the consumers. Therefore, for conveyance of water some sort of devices or structures is required. The arrangement may be in the form of open channels, aqueducts, tunnels or pipes.

1) Open Channels

In any water supply systems, raw water from source to treatment plants may be carried in open channels. These can be constructed by cutting in high grounds and banking low grounds. Economical sections of open channels are generally trapezoidal while rectangular sections prove economical when rock cutting is involved. The channels are to be properly lined to prevent seepage. These kind of channels need to be taken along the gradient and therefore the initial cost and maintenance cost may be high. While open channels are not recommended for conveyance of treated water, they may be adopted for conveying raw water. If these kind of channels are unlined, they have to be run with limited velocity of flow so that it does not affect scouring. As water flows only due to gravitational force a longitudinal uniform slope should be given. The velocity of water in channels should not exceed the permissible limit, otherwise scouring will start in the bed and water will get dirty. In channels there is always loss of water by seepage and evaporation.

2) Aqueducts

The term aqueduct is usually restricted to closed conduits made up of masonry. These can be used for conveyance of water from source to treatment plant or for distribution. Aqueducts normally run half to two-third full at required capacity of supply in most circumstances. In ancient times, rectangular aqueducts were most commonly used, but these days circular or horse-shoe shaped ones are more common. Masonry aqueducts unless reinforced with steel, are usually constructed in horse-shoe cross-section. This cross-section has good hydraulic properties and resists earth pressure well. It is economical and easy to build.

3) Tunnels

Tunnels are also like aqueducts. This is also a gravity conduit, in which water flows under gravitational force. But sometimes, water flows under pressure and in such cases, these are called pressure tunnels. Tunnels which are not under pressure are usually constructed in horse-shoe shape. But if they convey water under pressure, circular cross section is the best. In pressure tunnels, the depth of cover is generally such that the weight of overlying material overcomes the bursting pressure. Tunnels are used to convey water into the cities from outside sources. Tunnels should be water- tight and there should be no loss of water.

4) Pipes

Pipe is a circular closed conduit used to convey water from one point to another, under gravity or under pressure. Usually pipes follow the profile of the ground surface closely. If pipes do not run full, they are called to flowing under gravity. But flow under gravity is possible only if the pipe is given a definite longitudinal slope. Pipes running full will be said to be running under pressure. Water is under pressure always and hence the pipe material and the fixture should withstand stresses due to the internal pressure, vacuum pressure, when the pipes are empty it has to withstand water hammer, when the valves are closed it has to withstand temperature stresses. Pipes are mostly made up of materials like cast iron, wrought iron, RCC, asbestos cement, plastic, timber etc.

Requirements of Pipe Material

• It should be capable of withstanding internal and external pressure
• It should facilitate easy joints
• It should be available in all sizes, transport and erection should be easy
• It should be durable
• It should not react with water to alter its quality
• Cost of pipes should be less
• Frictional head loss should be minimum
• The damaged units should be replaced easily

a) Cast Iron Pipes

Cast iron pipes are used in majority of water conveyance mains because of centuries of satisfactory experience with it. Cast iron pipe is resistant to corrosion and accordingly long lived; its life may be over 100 years.

Advantages

• Cast iron pipes are of moderate cost
• Their jointing is easier
• They are resistant to corrosion
• They have long life

Disadvantages

• They are heavier and hence uneconomical when their diameter is more than 120 cm
• They cannot be used for pressures greater than 7 kg/cm².
• They are fragile

b) Wrought Iron Pipes

Wrought iron pipes are manufactured by rolling flat plates of the wrought iron to the proper diameter and welding the edges. Such pipes are much lighter than the cast iron pipes and can be more easily cut, threaded and worked. These pipes are stronger than cast iron pipes and it can withstand higher pressure. They look much neater, but are much costlier. They corrode quickly and hence are used principally for installation within buildings. These pipes are usually protected by coating them with a thin film of molten zinc. Such coated pipes are known as galvanized iron pipes and they are commonly jointed by screwed and socketed joints.

c) Galvanised Iron (GI) Pipes

Gl pipes are highly suitable for distribution system. They are available in light (yellow colour code), medium (blue colour code) and heavy grades (red colour code) depending on the thickness of pipe used. Normally, medium grade pipes (wall thickness 2.6-4.8 mm) are used for water supply system. It is cheap in cost, light in weight and easy to join. It is usually affected by acidic or alkaline water. Gl pipes can be used in non-corrosive water with pH value greater than 6.5. Gl pipes are normally joined with lead putty on threaded end.

Fig. 1 GI Pipes

d) Mild Steel Pipes

Mild steel pipes are durable and can resist high internal water pressure and highly suitable for long distance high pressure piping. It is flexible to lay in certain curves. The number of joints are less as they are available in longer length. It is light in weight, easy to transport and the damage in transportation is minimum. These pipes are prone to rust and require higher maintenance. It requires more time for repairs and not very suitable for distribution piping. These are available in diameter of 150-250 mm for water supply and cut lengths of 4 - 7 m (2.6-4.5 mm wall thickness).

Fig. 2 Mild Steel Pipes

e) Cement Concrete Pipes

Cement concrete pipes may be either plain or reinforced and are best made by the spinning process. They may be either precast or may be cast-in-situ. Transportation costs are much reduced if pipes are cast in situ. Concrete pipes have low maintenance in resistant to corrosion and particularly suitable to soft and acidic waters. They however can withstand high pressure if reinforced. The plain cement concrete pipes are used for heads up to 7 m while reinforced cement concrete pipes are normally used for head upto 60 m.

Advantages

• They are more suitable to resist the external loads and loads due to backfilling.
• The maintenance cost is low.
• The inside surface of pipes can be made smooth, thus reducing the frictional losses.
• The problem of corrosion is not there.
• Pipes can be cast at site and hence the transportation problems are reduced.
• Due to their heavy weight, the problem of floatation is not there when they are empty.

Disadvantages

• Unreinforced pipes are liable to tensile cracks and they cannot withstand high pressure.
• The tendency of leakage is not ruled out as a result of its porosity and shrinkage cracks.
• It is very difficult to repair them.
• Precast pipes are very heavy and it is difficult to transport them.

f) Poly Vinyl Chloride (PVC Un-plasticised) Pipes

These are cheap in cost and light in weight. It is economical in laying and jointing and are rigid pipes. It is highly durable and suitable for distribution network. These pipes are free from corrosion, tough against chemical attack and good electric insulation. It is highly suitable for distribution piping and branch pipes. The disadvantage is that it is less resistance to heat and direct exposure to sun. Hence, not very suitable for piping above the ground. PVC pipes weigh only 1/5^th of steel pipes of same diameter.

Fig. 3 PVC Pipes

g) HDPE (High Density Polyethylene) Pipes

These are light in weight and flexible than PVC pipes. HDPE pipes are black in colour. These are suitable for underground piping and can withstand movement of heavy traffic. It allows free flowing of water and highly durable and suitable for distribution network. These are free from corrosion and has good electric insulation. It is useful for water conveyance as they do not constitute toxic hazard and does not support microbial growth.

Fig. 4 HDPE Pipes

h) Ductile Iron Pipes

Ductile Iron pipes are better version of cast iron pipes with better tensile strength. These pipes are prepared using centrifugal cast process. DI pipes have high impact resistance, high wear and tear resistance, high tensile strength, ductility and good internal and external corrosion resistance. These pipes are provided with cement mortar lining on inside surface which provides smooth surface and is suitable for providing chemical and physical barriers to water. Such pipes reduce water contamination. The outer coating of such pipes is done with bituminous or Zinc paint. DI pressure pipes are available in range from 80-1000 mm diameter in lengths from 5.5-6 m. They are about 30 percent lighter than conventional cast iron pipes. DI pipes have lower pumping cost due to lower frictional resistance.

Fig. 5 DI Pipes

i) Asbestos Cement Pipe

These pipes are composed of asbestos fibre and Portland cement combined under pressure into dense homogenous structure. These are available in large variation from 50 to 600mm.

j) Wood Pipe

These pipes are built of staves of wood held together by steel bands. Wood is less durable for pipe material and pipe must be constantly full of water to prevent crackdown due to alternate wet and dry conditions.

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Intake Structure for Water Supply

Intakes are the structures used for admitting water from the surface sources (i.e., river, reservoir or lake) and conveying it further to the treatment plant. The basic function of the intake structure is to help in safely withdrawing water from the source over predetermined pool levels and then to discharge this water into the withdrawal conduit (normally called intake conduit), through which it flows up to water treatment plant. It essentially consists of opening, grating or strainer through which the raw water from river, canal or reservoir and is carried to a sump well by means of conduits. Water from the sump well is pumped through the rising mains to the treatment plants. Generally, an intake is a masonry or concrete structure with an aim of providing relatively clean water, free from pollution, sand and objectionable floating material. Its main purpose is to provide calm and still water conditions, so that comparatively purer water may be collected from the source. If intake well has to withstand the effects of severe forces which may be due to striking of high water currents, it may be made from reinforced cement concrete. Intake consists of the following.

• Conduit with protective works
• Screens at open ends
• Gates and Valves to regulate flow

Site for Location of Intake

While selecting a site for location of intakes, the following points should be taken into account.

• As far as possible, the site should be near the treatment plant so that the cost of conveying water to the city is less.
• The intake must be located in the purer zone of the source to draw best quality water from the source, thereby reducing load on the treatment plant.
• The intake must never be located at the downstream or in the vicinity of the point of disposal of wastewater.
• The site should be such as to permit greater withdrawal of water, if required at a future date.
• The intake must be located at a place from where it can draw water even during the driest period of the year.
• The intake site should remain easily accessible during floods and should not get flooded. Moreover, the flood waters should not be concentrated in the vicinity of the intake.
• Heavy water currents should not strike the intake directly.
• Site should be well connected by good type or roads
• Site should not be located in navigation channels, the reason being water in such channels are generally polluted.
• As far as possible, the site should be located on the upstream side of the watercourse.
• The intake should be so located that good foundation conditions are prevalent and the possibility of scouring is minimal.
• The site should be selected in such a manner that there is ample scope for further expansion.

Design of Intake

An intake should be designed keeping in mind the following considerations.

• Intake should be sufficiently heavy so that it may not start floating due to up thrust of water.
• All the forces which are expected to work on intake should be carefully analysed and intake should be designed to withstand all these forces such as heavy currents, floating materials, submerged bodies, ice pressure etc.
• The foundation of the intake should be taken sufficiently deep to avoid overturning.
• It should have sufficient self-weight so that it does not float by upthrust of water.
• Strainers in the form of wire mesh should be provided on all the intake inlets to avoid entry of large floating objects.
• Intake should be of such size and so located that sufficient quantity of water can be obtained from the intake in all circumstances.

Types of Intakes

1) Submerged Intake

Submerged intake is the one which is constructed entirely under water. Such an intake is commonly used to obtain supply from a lake. An exposed intake is in the form of a well or tower constructed near the bank of a river or in some cases even away from the river banks. Exposed intakes are more common due to ease in its operation. A wet intake is that type of intake tower in which the water level is practically the same as the water level of the sources of supply. Such an intake is sometimes known as jack well and is most commonly used. In the case of dry intake, there is no water in the water tower. Water enters through entry point directly into the conveying pipes. The dry tower is simply used for the operation of valves etc.

2) Reservoir Intake

There are large variations in discharge of all the rivers during monsoon and winter. The discharge of some rivers in summer remains sufficient to meet up the demand, but some rivers dry up partly or fully and cannot meet the hot weather demand. In such cases reservoirs are constructed by constructing weirs or dams across the rivers. Reservoir intakes is mostly used to draw the water from earthen dam reservoir. When the flow in the river is not guaranteed throughout the year a dam is constructed across it to store water in the reservoir so formed. The reservoir intakes are practically similar to the river intake, except that these are located near the upstream face of the dam where maximum depth of water is available.

It essentially consists of an intake tower constructed on the slope of the dam at such place from where intake can draw sufficient quantity of water even in the driest period. Intake pipes are fixed at different levels, so as to draw water near the surface in all variations of water level. These all inlet pipes are connected to one vertical pipe inside the intake well. Screens are provided at the mouth of all intakes to prevent the entrance of floating and suspended matter in them. The water which enters the vertical pipe is taken to the other side of the dam by means of an outlet pipe. At the top of the intake tower, sluice valves are provided to control the flow of water.

Fig. 1 Reservoir Intake

3) River Intake

Water from the river is always drawn from the upstream side, because it is free from the contamination caused by the disposal of sewage or industrial waste water disposal in it. It is circular masonry tower of 4 to 7 m in diameter constructed along the bank of the river at such place from where required quantity of water can be obtained even in the dry period. They are either located sufficiently inside the river so that demands of water are met with in all the seasons of the year or they may be located near the river bank where a sufficient depth of water is available. The water enters in the lower portion of the intake known as sump-well from penstocks.

The penstocks are fitted with screens to check the entry of floating solids and are placed on the downstream side so that water free from most of the suspended solids may only enter the jack well. Number of penstock openings are provided in the intake tower to admit water at different levels. The opening and closing of penstock valves is done with the help of wheels provided at the pump house floor. Sometimes, an approach channel is constructed and water is led to the intake tower. If the water level in the river is low, a weir may be constructed across it to raise the water level and divert it to the intake tower.

Fig. 2 River Intake

4) Lake Intake

Lake intakes are similar to reservoir intakes if the depth of the water near the banks is reasonable. For obtaining water from lakes mostly submersible intakes are used. These intakes are constructed in the bed of the lake below the slow water level so as to draw water in dry season also. It essentially consists of a pipe laid in the bed of the river. One end of which is in the middle of the lake is fitted with bell mouth opening covered with a mesh and protected by timber or concrete crib. The water enters in the pipe through the bell mouth opening and flows under gravity to the bank where it is collected in a sump-well and then pumped to the treatment plants for necessary treatment. If one pipe is not sufficient two or more pipes may be laid to get the required quantity of water.

Fig. 3 Lake Intake

Advantages of lake intake

• No obstruction to the navigation.
• No danger from floating bodies.
• No trouble due to ice.

5) Canal Intake

Canal intake is a very simple structure constructed on the bank. Sometimes, the source of water supply to a small town may be an irrigation canal passing near the town. The canal intake essentially consists of concrete or masonry intake chamber of rectangular shape, admitting water through a coarse screen. A fine screen is provided over the bell mouth entry of the outlet pipe. The bell mouth entry is located below the expected low water level in the canal. Water may flow from outlet pipe under gravity if the filter house is situated at a lower elevation. Otherwise, the outlet pipe may serve as suction pipe and the pump house may be located on or near the canal bank. The outlet pipe carries the water to the other side of the canal bank from where it is taken to the treatment plants. One sluice valve which is operated by a wheel from the top of the masonry chamber is provided to control the flow of water in the pipe. The intake chamber is so constructed that is does not offer any appreciable resistance to normal flow in the canal. Otherwise, the intake chamber is located inside the canal bank.

Fig. 4 Canal Intake

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Batching of Concrete

The measurement of materials for making concrete is known as batching. It is the process of measurement of specified quantities of cement, aggregates, water and admixture, i.e., ingredients of concrete in correct proportion. There are two methods of batching.

1) Volume Batching

Volume batching is not a good method for proportioning the material because of the difficulty it offers to measure granular material in terms of volume. Volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. The amount of solid granular material in a cubic metre is an indefinite quantity. Because of this, for quality concrete material have to be measured by weight only. However, for unimportant concrete or for any small job, concrete may be batched by volume.

Cement is always measured by weight. It is never measured in volume. Generally, for each batch mix, one bag of cement is used. The volume of one bag of cement is taken as thirty-five (35) litres. Gauge boxes are used for measuring the fine and coarse aggregates. The typical sketch of a gauge box is shown in Fig.1. The volume of the box is made equal to the volume of one bag of cement i.e., 35 litres or multiple thereof. The gauge boxes are made comparatively deeper with narrow surface rather than shallow with wider surface to facilitate easy estimation of top level. Sometimes bottomless gauge-boxes are used. This should be avoided. Correction to the effect of bulking should be made to cater for bulking of fine aggregate, when the fine aggregate is moist and volume batching is adopted.

Fig.1 Gauge Box

Gauge boxes are generally called ‘farmas’. They can be made of timber or steel plates. In a major site it is recommended to have the following gauge boxes at site to cater for change in mix design or bulking of sand. The volume of each gauge box is clearly marked with paint on the external surface.

Water is measured either in kg or litres as may be convenient. In this case, the two units are same, as the density of water is one kg per litre. The quantity of water required is a product of water/cement ratio and the weight of cement; consider an example, if the water/cement ratio of 0.5 is specified, the quantity of mixing water required per bag of cement is 0.5 x 50.00 = 25 kg or 25 litres. The quantity is inclusive of any surface moisture present in the aggregate.

2) Weigh Batching

Weigh batching is the correct method of measuring the materials. For important concrete, weigh batching system should be adopted. Use of weight system in batching, facilitates accuracy, flexibility and simplicity. Different types of weigh batchers are available and the particular type to be used depends upon the nature of the job. Large weigh batching plants have automatic weighing equipment. The use of this automatic equipment for batching is one of sophistication and requires qualified and experienced engineers. In this, further complication will come to adjust water content to cater for the moisture content in the aggregate. In smaller works, the weighing arrangement consists of two weighing buckets, each connected through a system of levers to spring-loaded dials which indicate the load. The weighing buckets are mounted on a central spindle about which they rotate. Thus one can be loaded while the other is being discharged into the mixer skip. A simple spring balance or the common platform weighing machines also can be used for small jobs.

On large work sites, the weigh bucket type of weighing equipment is used. This fed from a large overhead storage hopper and it discharges by gravity, straight into the mixer. The weighing is done through a lever-arm system and two interlinked beams and jockey weights. The required quantity of coarse aggregate is weighed, having only the lower beam in operation. After balancing, by turning the smaller lever, to the left of the beam, the two beams are interlinked and the fine aggregate is added until they both balance. The final balance is indicated by the pointer on the scale to the right of the beams. Discharge is through the swivel gate at the bottom.

Fig. 2 Weigh Batcher

Automatic batching plants are available in small or large capacity. In this, the operator has only to press one or two buttons to put into motion the weighing of all the different materials, the flow of each being cut off when the correct weight is reached. In their most advanced forms, automatic plants are electrically operated on a punched card system. This type of plant is particularly only suitable for the production of ready-mixed concrete in which very frequent changes in mix proportion have to be made to meet the varying requirements of different customers. In some of the recent automatic weigh batching equipment, recorders are fitted which record graphically the weight of each material, delivered to each batch. They are meant to record and check the actual and designed proportions.

Aggregate weighing machines require regular attention if they are to maintain their accuracy. Check calibrations should always be made by adding weights in the hopper equal to the full weight of the aggregate in the batch. The error found is adjusted from time to time. In small jobs, cement is often not weighed; it is added in bags assuming the weight of the bag as 50 kg. In reality, though the cement bag is made of 50 kg at the factory, due to transportation, handling at a number of places, it loses some cement, particularly, when jute bags are used. In fact, the weight of a cement bag at the site is considerably less. Sometimes, the loss of weight becomes more than 5 kg. This is one of the sources of error in volume batching and also in weigh batching, when the cement is not actually weighed. But in important major concreting jobs, cement is also actually weighed and the exact proportion as designed is maintained.

Components of a Batching Plant

• Aggregate bins for various types of aggregates
• Feeding mechanisms such as scrappers, conveyors or hoists etc. to transfer aggregate to scales (balances)
• Balance and measuring system
• Cement silos and a conveyor screw or bucket conveyor
• The storage tank for water and water measuring system
• Dispenser for chemical (liquid) admixture

Measurement of Water

When weigh batching is adopted, the measurement of water must be done accurately. Addition of water by graduated bucket in terms of litres will not be accurate enough for the reason of spillage of water etc. It is usual to have the water measured in a horizontal tank or vertical tank fitted to the mixer. These tanks are filled up after every batch. The filling is so designed to have a control to admit any desired quantity of water. Sometimes, water meters are fitted in the main water supply to the mixer from which the exact quantity of water can be let into the mixer.

In modern batching plants sophisticated automatic microprocessor controlled weigh batching arrangements which not only accurately measures the constituent materials, but also the moisture content of aggregates. Moisture content is automatically measured by sensor probes and corrective action is taken to deduct that much quantity of water contained in sand from the total quantity of water.

Fig. 3 Cans for Measuring Water

(Ref : Concrete Technology – M S Shetty)

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Stages of Concrete Production – Brief Note

Production of quality concrete requires meticulous care exercised at every stage of manufacture of concrete. It is interesting to note that the ingredients of good concrete and bad concrete are the same. If meticulous care is not exercised and good rules are not observed, the resultant concrete is going to be of bad quality. With the same material if intense care is taken to exercise control at every stage, it will result in good concrete. Therefore, it is necessary to know what are the good rules to be followed in each stage of manufacture of concrete for producing good quality concrete.

Production of concrete involves two distinct activities. One is related to ‘material’ and the other to ‘processes’. The material part is generally taken care by everybody, but the involved processes in the production of concrete are often neglected. Therefore, the ‘process’ is responsible for good or bad quality of concrete. If we take care of processes, the quality of concrete will be improved automatically without incurring any extra expenditure as the major expenditure has already been made in procurement of material. In order to ensure the quality, it is very important to have a knowledge of each and every process. The various process involved in concrete production are as given below.

1) Proportioning/Batching

It is the relative quantity of each ingredient to make the desired concrete. It is decided based upon the calculations of mix-design. The proportioning should be such that the resultant mass should be compact with minimum voids and the required strength should be achieved.

(For more information visit - https://www.civilengineeringencyclopedia.com/2024/09/batching-of-concrete.html )

2) Mixing

The purpose of proper mixing is to ensure that mass should become homogeneous, uniform in colour and uniform in consistency. There are two types of mixing that are adopted in the field i.e. hand mixing and machine mixing.

3) Transportation

Transportation of concrete is an important activity in the production of concrete. The time taken in transit should be a design parameter as it depends on the initial setting time as well as the requirement of workability at the destination. The method of transportation adopted at site should be decided in advance so that suitable admixtures can be decided.

4) Placing

It is not enough that a concrete mix correctly designed, batched, mixed and transported, it is of utmost importance that the concrete must be placed in systematic manner to yield optimum results.

5) Compaction

Compaction is a process of expelling the entrapped air. If we don’t expel this air, it will result into honeycombing and reduced strength. It has been found from the experimental studies that 1% air in the concrete approximately reduces the strength by 6%. There are two methods of compaction adopted in the field such as hand compaction and mechanical compaction.

6) Curing

Curing is a procedure of promoting the hydration of cement for development of concrete strength and controlling the temperature. As a result of curing, we can achieve higher strength and reduced permeability which is very vital for the long term strength or durability.

7) Finishing

Finishing operation is the last operation in making concrete. Finishing in real sense does not apply to all concrete operations. For a beam concreting, finishing may not be applicable, whereas for the concrete road pavement, airfield pavement or for the flooring of a domestic building, careful finishing is of great importance. Concrete is often dubbed as a drab material, incapable of offering pleasant architectural appearance and finish. This shortcoming of concrete is being rectified and concretes these days are made to exhibit pleasant surface finishes. Particularly, many types of prefabricated concrete panels used as floor slab or wall unit are made in such a way as to give very attractive architectural affect. Even concrete claddings are made to give attractive look.

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Setting Time of Concrete

The initial setting is defined as the loss of plasticity or the onset of rigidity (stiffening or consolidating) in fresh concrete. The final setting is defined as the onset point of strength. It is different from hardening, which describes the development of useful and measurable strength. Setting precedes hardening, although both are controlled by the continuing hydration of the cement. Setting time of cement is found out by a standard Vicat apparatus in laboratory conditions. Setting time, both initial and final indicate the quality of cement.

Setting time of concrete differs widely from setting time of cement. The setting time of concrete depends upon the w/c ratio, temperature conditions, type of cement, use of mineral admixture and use of plasticizers in particular retarding plasticizer. The setting parameter of concrete is more of practical significance for site engineers than setting time of cement. For cement paste, it uses the samples made of the water amount needed for consistency. For concrete, it uses the sieved mortar from a concrete with different water/cement or water/binder ratios. Moreover, for cement paste, it measures the penetration depth of the Vicat needle, 1 mm in diameter, under a constant weight. For concrete, it measures the resistance of the mortar to a rod under an action of the load.

The setting time of concrete is found by pentrometer test. This method of test is covered by IS 8142 of 1976 and ASTM C – 403. The procedure given below may also be applied to prepared mortar and grouts. The apparatus consists of a container which should have minimum lateral dimension of 150 mm and minimum depth of 150 mm. There are six penetration needles with bearing areas of 645, 323, 161, 65, 32 and 16 mm². Each needle stem is scribed circumferentially at a distance of 25 mm from the bearing area.

A device is provided to measure the force required to cause penetration of the needle. The test procedure involves the collection of representative sample of concrete in sufficient quantity and sieve it through 4.75 mm sieve and the resulting mortar is filled in the container. Compact the mortar by rodding, tapping, rocking or by vibrating. Level the surface and keep it covered to prevent the loss of moisture. Remove bleeding water, if any, by means of pipette. Insert a needle of appropriate size, depending upon the degree of setting of the mortar in the following manner.

Bring the bearing surface of needle in contact with the mortar surface. Gradually and uniformly apply a vertical force downwards on the apparatus until the needle penetrates to a depth of 25 ± 1.5 mm, as indicated by the scribe mark. The time taken to penetrate 25 mm depth could be about 10 seconds. Record the force required to produce 25 mm penetration and the time of inserting from the time water is added to cement. Calculate the penetration resistance by dividing the recorded force by the bearing area of the needle. This is the penetration resistance. For the subsequent penetration avoid the area where the mortar has been disturbed. The clear distance should be two times the diameter of the bearing area. Needle is inserted at least 25 mm away from the wall of container. A setup for concrete setting time measurement is shown in Fig. 1.

Fig. 1 Measurement Setup of Concrete Mixture Setting Time

Plot a graph of penetration resistance as ordinate and elapsed time as abscissa. Not less than six penetration resistance determination is made. Continue the tests until one penetration resistance of at least 27.6 MPa is reached. Connect the various point by a smooth curve. From penetration resistance equal to 3.5 MPa, draw a horizontal line. The point of intersection of this with the smooth curve, is read on the x-axis which gives the initial setting time. Similarly, a horizontal line is drawn from the penetration resistance of 27.6 MPa and point it cuts the smooth curve is read on the x-axis which gives the final set. A typical graph is shown in Fig. 2.

Fig. 2 Penetration Resistance – Time Graph

Fig. 3 Needle with different bearing area

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Bleeding

Bleeding is a form of local concentration of water in some special positions in concrete, usually the bottom of the coarse aggregates, the bottom of the reinforcement and the top surface of the concrete member as shown in Fig.1. During placing and compaction, some of water in the mix tends to rise to the surface of freshly placed concrete. This is caused by the inability of the solid constituents of the mix to hold all the mixing water when they settle downward due to the lighter density of water. Bleeding can be expressed quantitatively as the total settlement (reduction in height) per unit height of concrete, and bleeding capacity as the amount (in volume or weight) of water that rises to the surface of freshly placed concrete.

As a result of bleeding, an interface between aggregates and bulk cement paste is formed and the top of every lift (layer of concrete placed) may become too wet. If the water is trapped by the superimposed concrete, a porous and weak layer of nondurable concrete may result. If the bleeding water is remixed during the finishing process of the surface, a weak wearing surface can be formed. This can be avoided by delaying the finishing operations until the bleeding water has evaporated and also by the use of wood floats and avoidance of overworking the surface. On the other hand, if evaporation of water from the surface of the concrete is faster than the bleeding rate, plastic shrinkage cracking may be generated.

Fig.1 Bleeding Phenomenon

Bleeding is sometimes referred as water gain. It is a particular form of segregation, in which some of the water from the concrete comes out to the surface of the concrete, being of the lowest specific gravity among all the ingredients of concrete. Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. In thin members like roof slab or road slabs and when concrete is placed in sunny weather show excessive bleeding. Due to bleeding, water comes up and accumulates at the surface. Sometimes, along with this water, certain quantity of cement also comes to the surface. When the surface is worked up with the trowel and floats, the aggregate goes down and the cement and water come up to the top surface. This formation of cement paste at the surface is known as Laitance.

While the mixing water is in the process of coming up, it may be intercepted by aggregates. The bleeding water is likely to accumulate below the aggregate. This accumulation of water creates water voids and reduces the bond between the aggregates and the paste. The above aspect is more pronounced in the case of flaky aggregate. Similarly, the water that accumulates below the reinforcing bars, particularly below the cranked bars, reduces the bond between the reinforcement and the concrete. The poor bond between the aggregate and the paste or the reinforcement and the paste due to bleeding can be remedied by revibration of concrete. The formation of laitance and the consequent bad effect can be reduced by delayed finishing operations.

Bleeding rate increases with time up to about one hour or so and thereafter the rate decreases but continues more or less till the final setting time of cement. Bleeding is an inherent phenomenon in concrete. All the same, it can be reduced by proper proportioning and uniform and complete mixing. Use of finely divided pozzolanic materials reduces bleeding by creating a longer path for the water to traverse. The use of air-entraining agent is very effective in reducing the bleeding. It is also reported that the bleeding can be reduced by the use of finer cement or cement with low alkali content. Rich mixes are less susceptible to bleeding than lean mixes.

The bleeding is not completely harmful if the rate of evaporation of water from the surface is equal to the rate of bleeding. Removal of water, after it had played its role in providing workability, from the body of concrete by way of bleeding will do good to the concrete. Early bleeding when the concrete mass is fully plastic, may not cause much harm, because concrete being in a fully plastic condition at that stage, will get subsided and compacted. It is the delayed bleeding, when the concrete has lost its plasticity, that causes undue harm to the concrete. Controlled revibration may be adopted to overcome the bad effect of bleeding.

Bleeding presents a very serious problem when Slip Form Paver is used for construction of concrete pavements. If too much of bleeding water accumulates on the surface of pavement slab, the bleeding water flows out over the unsupported sides which causes collapsing of sides. Bleeding becomes a major consideration in such situations. In the pavement construction finishing is done by texturing or brooming. Bleeding water delays the texturing and application of curing compounds.

Method of Test for Bleeding of Concrete

This method covers determination of relative quantity of mixing water that will bleed from a sample of freshly mixed concrete. A cylindrical container of approximately 0.01 m³ capacity, having an inside diameter of 250 mm and inside height of 280 mm is used. A tamping bar similar to the one used for slump test is used. A pipette for drawing off free water from the surface, a graduated jar of 100 cm³ capacity is required for test. A sample of freshly mixed concrete is obtained. The concrete is filled in 50 mm layer for a depth of 250 ± 3 mm (5 layers) and each layer is tamped by giving strokes and the top surface is made smooth by trowelling.

The test specimen is weighed and the weight of the concrete is noted. Knowing the total water content in 1 m3 of concrete quantity of water in the cylindrical container is also calculated. The cylindrical container is kept in a level surface free from vibration at a temperature of 27°C ± 2°C and it is covered with a lid. Water accumulated at top is drawn by means of pipette at 10 minutes interval for the first 40 minutes and at 30 minutes interval subsequently till bleeding ceases. To facilitate collection of bleeding water the container may be slightly tilted. All the bleeding water collected in a jar.

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Segregation of Concrete

Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. If a sample of concrete exhibits a tendency for separation such as coarse aggregate from the rest of the ingredients, then, that sample is said to be showing the tendency for segregation. Such concrete is not only going to be weak; lack of homogeneity is also going to induce all undesirable properties in the hardened concrete. There are considerable differences in the sizes and specific gravities of the constituent ingredients of concrete. Therefore, it is natural that the materials show a tendency to fall apart. Segregation may be of three types.

1. Coarse aggregate separating out or settling down from the rest of the matrix.
2. Paste separating away from coarse aggregate.
3. Water separating out from the rest of the material being a material of lowest specific gravity.

Cohesiveness is an important characteristic of the workability and a proper cohesiveness can ensure concrete to hold all the ingredients in a homogeneous way without any concentration of a single component and even after the full compaction is achieved. An obvious separation of different constituents in concrete is called segregation. Thus, segregation can be defined as concentration of individual constituents of a heterogeneous (nonuniform) mixture so that their distribution is no longer uniform. In the case of concrete, it is the differences in the size and weight of particles (and sometimes in the specific gravity of the mix constituents) that are the primary causes of segregation, but the extent can be controlled by the concrete proportion, choice of suitable grading, and care in handling.

A well-made concrete, taking into consideration of various parameters such as grading, size, shape and surface texture of aggregate with optimum quantity of water make a cohesive mix. Such concrete will not exhibit any tendency for segregation. The cohesive and fatty characteristics of matrix do not allow the aggregate to fall apart, at the same time, the matrix itself is sufficiently contained by the aggregate. Similarly, water also does not find it easy to move out freely from the rest of the ingredients. The conditions favourable for segregation are the badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates.

Fig. 1 Segregation of Concrete Mixture

The conditions favorable for segregation are given below.

1. Badly proportioned mix where sufficient matrix is not there to bind and contain the aggregates.
2. Insufficiently mixed concrete with excess water content.
3. Dropping of concrete from heights as in the case of placing concrete in column concreting.
4. When concrete is discharged from a badly designed mixer or from a mixer with worn out blades.
5. Conveyance of concrete by conveyor belts, wheel barrow, long distance haul by dumper, long lift by skip and hoist are the other situations promoting segregation of concrete.

Vibration of concrete is one of the important methods of compaction. It should be remembered that only comparatively dry mix should be vibrated. If too wet a mix is excessively vibrated, it is likely that the concrete gets segregated. It should also be remembered that vibration is continued just for required time for optimum results. If the vibration is continued for a long time, particularly, in too wet a mix, it is likely to result in segregation of concrete due to settlement of coarse aggregate in matrix.

In the recent time we use concrete with very high slump particularly in RMC. The slump value required at the batching point may be in the order of 150 mm and at the pumping point the slump may be around 100 mm. At both these points cubes are cast. One has to take care to compact the cube mould with these high slump concrete. If sufficient care and understanding of concrete is not exercised, the concrete in the cube mould may get segregated and show low strength. Similarly, care must be taken in the compaction of such concrete in actual structures to avoid segregation.

While finishing concrete floors or pavement, with a view to achieve a smooth surface, masons are likely to work too much with the trowel, float or tamping rule immediately on placing concrete. This immediate working on the concrete on placing, without any time interval, is likely to press the coarse aggregate down, which results in the movement of excess of matrix or paste to the surface. Segregation caused on this account, impairs the homogeneity and serviceability of concrete. The excess mortar at the top causes plastic shrinkage cracks. The tendency for segregation can be remedied by correctly proportioning the mix, by proper handling, transporting, placing, compacting and finishing. At any stage, if segregation is observed, remixing for a short time would make the concrete again homogeneous. As mentioned earlier, a cohesive mix would reduce the tendency for segregation. For this reason, use of certain workability agents and pozzolanic materials greatly help in reducing segregation. The use of air-entraining agent appreciably reduces segregation. Segregation is difficult to measure quantitatively, but it can be easily observed at the time of concreting operation. The pattern of subsidence of concrete in slump test or the pattern of spread in the flow test gives a fair idea of the quality of concrete with respect to segregation.

Fig. 2 Segregation

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