Measurement Scale and Protractor

Measurement Scale

Scales are used for measurement of lengths and distances on the drawing. Scales are made of wood, steel, ivory, celluloid, plastic or card board. Stainless-steel scales are more durable. Scales may be flat or of triangular cross-section. 15 cm long and 2 cm wide or 30 cm long and 3 cm wide flat scales are in common use. They are usually about 1 mm thick. Scales of greater thickness have their longer edges bevelled. This helps in marking measurements from the scale to the drawing paper accurately. Both the longer edges of the scales are marked with divisions of centimetres, which are sub-divided into millimetres. Least count of such scale is generally 1 mm. The scale is used to transfer the true or relative dimensions of an object to the drawing. It is placed with its edge on the line on which measurements are to be marked and looking from exactly above the required division, the marking is done with a fine pencil point. The scale should never be used as a straight-edge for drawing lines. Parallax error will be nil or least while using thin/tapered edge scales.

A number of kinds of scales are available for various types of engineering design. Scale is a measuring stick, graduated on both its edges with different divisions to represent the corresponding actual distances of ground according to some fixed proportions. Such scales are known as RF scales or representative fraction scales. Such scales facilitate rapid marking off distances on drawing. Scales of various representative fractions (RFs) are available in the market e.g., 1:1, 1:2, 1:5, 1:10, 1:20, 1:25, 1:50, 1:100, 1:200, 1:500, 1:1000, etc. the proportion 1:50 means that 50 units on the ground is represented by 1 unit in the drawing. The card-board scales are available in a set of eight scales. They are designated from M7 to M8 as shown in Table 1.

Table 1 Scale Set Designation

Designation	Scale
M1	1:1
	1:2
M2	1:2.5
	1:5
M3	1:10
	1:20
M4	1:50
	1:100
M5	1:200
	1:500
M6	1:300
	1:600
M7	1:400
	1:800
M8	1:1000
	1:2000

Fig. 1 Scale

Protractor

Protractors are used for constructing and measuring angles. Protractors are generally semi-circular in shape. The base diameter of the semicircle serves as the straight edge. A semi-circular protractor can measure and construct angles of any measure between 0⁰ to 180⁰. Least count of the protractor is generally 1⁰. Protractor is made of wood, tin or celluloid. Protractors of transparent celluloid are in common use. The protractor is used to draw or measure such angles that cannot be drawn with set-squares. A circle can be divided into any number of equal parts by means of the protractor.

Fig. 2 Protractor

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Drafting Machine / Mini-Drafter

Mini-Drafter is a device used to draw parallel or inclined lines very effectively with ease. It is mounted on the top left corner of the drawing board by means of a clamping mechanism which is an integral part of the device. Mini-drafter is an instrument which combines together all the features of a T-square, set squares, scales and protractor and hence is a unified instrument capable of performing the functions of all these instruments put together. One end of the mini-drafter is clamped by means of a screw, to the distant longer edge of the drawing board. At its other end, an adjustable head having protractor markings is fitted. Two blades of transparent celluloid accurately set at right angles to each other are attached to the head.

The machine has a mechanism which keeps the two blades always parallel to their original position, irrespective of their position on the board. The blades have scales marked on them and are used as straight edges. The blades may be set at any desired angle with the help of the protractor markings. The L-shaped scale which is graduated in millimeters acts as the working edge of the mini-drafter. The L-Shaped scale also has a degree scale for angle measurement. The working edge can be moved to any desired location on the drawing board. In some machines, the blades are removable and hence a variety of scales can be used. Thus, by means of this machine, horizontal, vertical or inclined parallel lines of desired lengths can be drawn anywhere on the sheet with considerable ease and saving of time. Drafting machines are common among the college students and draughtsman.

Fig.1 Mini-Drafter

The drafting in the machine come in different sizes and a pattern called ‘Pantagraph’ type. The protractor head has a spring loaded clutch relieving handle, which rotates and locks at 150 intervals automatically. For setting any angle other than multiples of 150, the clutch spring is released and by rotating the centre knob, the zero line is set to the required angle and the friction clutch knob is tightened. It is capable of rotating 1800, thereby any angle can be set. They can be reversed with the help of dovetail slide fitting. There is a fine adjusting mechanism on the drafting head to set the scale parallel to the edge of the board. The scales also can be adjusted if there is any error in measuring between them. The angle of the drafting board can be adjusted by pedal operating system.

Procedure for Clamping Mini-Drafter

Set the protractor head with reference mark indexing zero degree, then fix the clamp of the mini-drafter at the top left corner either along the top horizontal edge of the board or along the left vertical edge of the board. With the drawing sheet placed underneath the scales of the mini-drafter, fix the drawing sheet to the drawing board with the scales of the mini drafter aligned either with the vertical or the horizontal borderlines of the drawing sheet.

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Compass and Divider

Compass

Compass is used for drawing circles and arcs of required diameter. It consists of two metal legs hinged together at its upper end by means of joint known as knee joint. An adjustable or fixed needle is fitted on to the end of one of the legs whereas the other leg is provided with an attachment which can be fitted with a pencil lid or pencil depending upon the nature of attachment. There are different types of compass available in the market depending upon their sizes, such as

i) Large Size Compass

ii) Small Size Compass

Circles up to 120 mm diameters are drawn by keeping the legs of compass straight. For drawing circles more than 150 mm radius, a lengthening bar is used. It is advisable to keep the needle end about 1mm long compared to that of pencil end so that while drawing circles, when the needle end is pressed it goes inside the drawing sheet by a small distance (approximately 1mm). Curves drawn with the compass should be of the same darkness as that of the straight lines. It is difficult to exert the same amount of pressure on the lead in the compass as on a pencil. It is, therefore, desirable to use slightly softer variety of lead about one grade lower in the compass than the pencil used for drawing straight lines, to maintain uniform darkness in all the lines. As a rule, while drawing concentric circles, small circles should be drawn first before the centre hole gets worn.

Fig. 1 Compass

1) Large-Size Compass with Interchangeable Pencil and Pen Legs

It consists of two legs hinged together at its upper end. A pointed needle is fitted at the lower end of one leg, while a pencil lead is inserted at the end of the other leg. The lower part of the pencil leg is detachable and it can be interchanged with a similar piece containing an inking pen. Both the legs are provided with knee joints. Circles upto about 120 mm diameter can be drawn with the legs of the compass kept straight. For drawing larger circles, both the legs should be bent at the knee joints so that they are perpendicular to the surface of the paper.

To draw a circle, adjust the opening of the legs of the compass to the required radius. Hold the compass with the thumb and the first two fingers of the right hand and place the needle point lightly on the centre, with the help of the left hand. Bring the pencil point down on the paper and swing the compass about the needle-leg with a twist of the thumb and the two fingers, in clockwise direction, until the circle is completed. The compass should be kept slightly inclined in the direction of its rotation. While drawing concentric circles, beginning should be made with the smallest circle.

Fig. 2 Large-Size Compass with Interchangeable Pencil and Pen Legs

2) Lengthening Bar

Circles of more than 150 mm radius are drawn with the aid of the lengthening bar. The lower part of the pencil leg is detached and the lengthening bar is inserted in its place. The detached part is then fitted at the end of the lengthening bar, thus increasing the length of the pencil leg. It is often necessary to guide the pencil leg with the other hand, while drawing large circles.

Fig. 3 Lengthening Bar

3) Small Bow Compass

For drawing small circles and arcs of less than 25 mm radius and particularly, when a large number of small circles of the same diameter are to be drawn, small bow compass is used.

Fig. 4 Small Bow Compass

Divider

Dividers are used to transfer lengths to the drawings either from scales or from the drawing itself. Dividers are similar to the compass and are made in square, flat and round forms. However, unlike compass, a divider is provided with two needles on both the legs. Similar to the compasses, two sizes of dividers are used in technical drawings. One large divider and the other small spring bow divider. They are very convenient for setting-off points at equal distances around a given point or along a given line. The dividers are used for the following purpose.

1. To divide curved or straight lines into desired number of equal parts,
2. To transfer dimensions from one part of the drawing to another part
3. To set-off given distances from the scale to the drawing.

Fig. 5 Divider

1) Large-Size Divider

It is used to transfer dimensions and dividing lines into a number of equal parts. The divider has two legs hinged at the upper end and is provided with steel pins at both the lower ends, but it does not have the knee joints. In most of the instrument boxes, a needle attachment is also provided which can be interchanged with the pencil part of the compass, thus converting it into a divider.

2) Small Bow Divider

The small bow divider is adjusted by a nut and is very convenient for marking minute divisions and large number of short equal distances.

Bow Instruments

Bow pencil and bow pen compass are used for drawing circles of approximately 25 mm radius. Bow divider is used for marking or dividing smaller spaces. There are two types

(i) Integral legs with spring action

(ii) Two legs held with a curved spring on top with handle on it.

Bow instruments may have adjusting wheel and nut. To draw circles, it is better to mark the required distance separately and set the instruments and check. Then only the circles or arcs should be drawn on the drawing. Adjustment should be made with the thumb and middle finger. The instrument is manipulated by twisting the knurled head between the thumb and finger.

Fig. 6 Bow Instruments

Drop Spring Bow Pencil and Pen

Drop spring bow pencil and pen are designed for drawing multiple identical small circles. Example: Rivet holes, Drilled/reamed holes etc. The central pin is made to move freely up and down through the tube attached to the pen or pencil unit. It is used by holding the knurled head of the tube between thumb and middle finger while the index finger is placed on the top of the pin. The pin point is placed on the centre point of the circle to be drawn and pencil or pen is lowered until it touches paper. The instrument is turned clockwise and the circle is drawn.

Fig. 6 Drop Spring Bow Instruments

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Sources of Water

Sources from which water is available for water supply schemes can conveniently be classified into the following two categories according to their proximity to the ground surface.

1. Surface Sources

2. Sub-surface Sources or Underground Sources

Surface Water

Water that gets collected on the surface of the ground or top layer of a body of water is called surface water. In this type of source, the surface runoff is available for water supply schemes. Usual forms of surface sources are as follows.

1) Lakes and Streams

A natural lake represents a large body of water within land with impervious bed. Hence, it may be used as source of water supply scheme for nearby localities. The quantity of runoff that goes to the lake should be accurately determined and it should be seen that it is at least equal to the expected demand of locality. Similar is the case with streams which are formed by the surface runoff. It is found that the flow of water in streams is quite ample in rainy season. But it becomes less and less in hot season and sometimes the stream may even become absolutely dry.

The catchment area of lakes and streams is very small and hence, the quantity of water available from them is also very low. Hence, lakes and streams are not considered as principal sources of water supply schemes for the large cities. But they can be adopted as sources of water supply schemes for hilly areas and small towns. The water which is available from lakes and streams is generally free from undesirable impurities and can therefore be safely used for drinking purposes.

2) Ponds

A natural large sized depression formed within the surface of the earth, when gets filled with water is known as pond. A pond is a man-made body of standing water smaller than a lake. Thus ponds are formed due to excessive digging of ground for the construction of roads, houses, etc. and they are filled up with water in rainy season. The quantity of water in pond is very small and it contains many impurities. A pond cannot be adopted as a source of water supply and its water can only be used for washing of clothes or animals only.

3) Rivers

Since the dawn of civilization, the ancient man settled on the banks of river, drank river water and ate fish caught from river water and sailed down rivers to find out unknown lands. Large rivers constitute the principal source of water supply schemes for many cities. Some rivers are perennial while others are non-perennial. The former rivers are snow fed and hence, water flows in such rivers for all the seasons. The latter type of rivers dries in summer either wholly or partly and in monsoon, heavy flood visits them. For such types of rivers, it is desirable to store the excess water of flood in monsoons by constructing dams across such rivers. This stored water may then be used in summer.

In order to ascertain the quantity of water available from the river, the discharges at various periods of the year are taken and recorded. The observations over a number of years serve as a good guide for estimating the quantity of water available from the river in any particular period of the year. Generally, the quantity of water available from non-perennial rivers is variable throughout the year and it is likely to fall down in hot season when demand of water is maximum. It becomes therefore essential to augment such source of water supply by some other sources so as to make the water supply scheme successful.

The quality of surface water obtained from rivers is not reliable. It contains silt and suspended impurities. When completely or partly treated sewage is being discharged into the river at some upstream point, the river water is to be suspected for high contamination. The river water requires to be properly analyzed as regards to the contents of disease bacteria, harmful impurities, etc. The presence of all such undesirable elements in river water requires an exhaustive treatment of water before it can be make fit for drinking purposes. It should however be noted that the quality of river water is subject to the widest variations because it depends on various uncertain factors such as character of the catchment area, the discharges of sewage and industrial wastes, climatic conditions, season of the year, etc. The character of the water differs not only with each individual river, but also at many points along the course of the same river. It is usually found that the quality of river water at its head is good, but it goes on deteriorating as the river proceeds along its course. The chief use points to be considered in investigating a river supply of water are as follows.

• Adequacy of storage of purified water so as not to disturb the distribution system during periods of fold when the river water is turbid
• Efficiency of the subsequent stages of purification system adopted
• General nature of river, the rate of flow and the distance between the sources of pollution and the intake of the water
• Relative proportions of the polluting matter and the flow of river when at its minimum.

4) Storage Reservoirs

An artificial lake formed by the construction of dam across a valley is termed as a storage reservoir. It essentially consists of the following three parts

• A dam to hold water
• A spillway to allow the excess water to flow and
• A gate chamber containing necessary valves for regulating the flow of water

At present, this is rather the chief source of water supply schemes for very big cities. The multi-purpose reservoirs also make provisions for other uses in addition to water supply such as irrigation and power generation.

5) Oceans

Normally, it is not used as water supply source.

Underground Sources

In this type of source, the water that has percolated into the ground is brought on the surface. The entrance of rain water or melted snow into the ground is referred to as infiltration. The movement of water after entrance is called percolation. It is observed that the surface of earth consists of alternate courses of pervious and impervious strata. The pervious layers are those through which water can easily pass while it is not possible for water to go through an impervious layer. The pervious layers are known as aquifers or water-bearing strata. If aquifer consists of sand and gravel strata, it gives good supply of drinking water. The aquifer of limestone strata can supply good amount of drinking water, provided there is presence of cracks or fissures in it.

Forms of Underground Sources

Following are the four forms in which underground sources are found.

1) Infiltration Galleries

An infiltration gallery is a horizontal or nearly horizontal tunnel which is constructed through water bearing strata. It is sometimes referred to as horizontal well. Horizontal tunnels constructed at shallow depth (3-5m). The gallery is usually constructed of brick walls with slab roof. The gallery obtains its water from water bearing strata by various porous drain pipes. These pipes are covered with gravel, pebble, etc. so as to prevent the entry of very fine material into the pipe.

The gallery is laid at a slope and the water collected in the gallery is led to a sump from where it is pumped and supplied to consumers after proper treatment. The manholes are provided along the infiltration gallery for the purposes of cleaning and inspection. The infiltration galleries are useful as sources of water supply when ground water is available in sufficient quantity just below ground level or so. The galleries are usually constructed at depth of about 5 to 10 metres from the ground level.

Fig. 1 Infiltration Gallery

2) Infiltration Wells

In order to collect large quantities of water, infiltration wells are sunk in series in the banks of river. The wells are closed at top and open at bottom. They are constructed of brick masonry with open joints. Various infiltration wells are connected by porous pipes to a sum well, called jack well.

3) Springs

Natural outflow of ground water at the earth’s surface forms a spring. A pervious layer sandwiched between two impervious layers form a spring. Springs are capable of supplying very small amount of water and is not recommended for water supplies. Good developed springs can sometimes use for water supply source for small towns, especially in hilly areas. The two types of springs are

a) Gravity Springs – when the ground water raises high and water overflows through the sides of a natural valley or depression.

b) Surface Springs – sometimes an impervious obstruction (or) stratum supporting the underground storages become inclined causing water table to go up and get exposed to the ground surfaces.

c) Artesian Springs – when water comes out of pressure it’s called artesian springs. Since water comes out of pressure; they are able to provide higher yields and may be considered as source of water supply.

Fig. 2 Spring

4) Open Well

Well is a vertical structure dug in ground for the purpose of bringing ground water to the earth’s surface. Open wells have comparatively large diameters and lower discharges. Usually they have discharge of 20 m³/hr but if constructed by efficient planning it gives discharge of 200-300 m³/hr. They are constructed of diameter of about 1-10 m and have depth of about 2-20m. They are constructed by digging therefore they are also known as dug wells. It can put 8 to 10 cm diameter bore hole in the center of the well, to extract additional water. Types of open wells are given below.

a) Shallow Open Well - These are the wells resting on the water bearing strata and gets their supplies from the surrounding materials.

b) Deep Open Well - These are the wells resting on the impervious layer known as mota layer beneath which lies water bearing pervious layer and gets their supply from this layer.

c) Kachha wells - These type of wells is only constructed when water table is high as these type of wells sometimes collapses.

d) Driven Well - This is a shallow well, constructed by driving a casting pipe of 2.5cm – 15cm in diameter. The bottom end of casting pipe is pointed known as well point or drive point. The discharge of these wells is very small and these are suitable for domestic purpose only.

5) Tube Well

A tube well is a long pipe sunk in ground intercepting one or more water bearing strata. As compared to open well their diameter is less about 80-600 mm. It is deep as 70 to 300m and tap more than one aquifer. Types of tube wells are given below.

a) Shallow Tube Well - These are the tube which has depth limited to 30 meters and maximum have discharge of 20 m³/hr.

b) Deep Tube Well - These are the tube wells which have maximum depth of about 600 m and may give discharge more than 800 m³/hr.

c) Strainer type Tube Well - These is most commonly used tube such that in general a tube well means strainer tube well. In this type of well, a strainer which a wire mesh with small openings is wrapped around the main pipe which also has large openings such that area of opening in strainer and main pipe remains same. Annual space is left between two strainers so that the open area of pipe perforations is not reduced. The type of flow is radial.

Fig. 3 Strainer type Tube Well

d) Cavity type Tube Well - A cavity type tube well consists of a pipe sunk in ground up to the hard clay layer. It draws water from the bottom of well. In initial stages fine sand is also pumped with water and in such manner a cavity is formed at the bottom so the water enters from the aquifer into the well through this cavity.

Fig. 4 Cavity type Tube Well

e) Slotted type Tube Well - If the geological formation of the earth strata is such that; sufficient number of water bearing stratum are not available for the construction of strainer tube well even up to the depth of 100m, slotted type tube wells are constructed.

Fig. 5 Slotted type Tube Well

f) Perforated type Tube Well - When the water table is very near to the ground or the tube wells are required for obtaining water for short duration only, these types of tube wells are used. In these tube wells the pipes are made perforated by drilling holes in them.

Fig. 6 Perforated type Tube Well

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Chemical Oxygen Demand (COD)

One problem with the BOD test is that it takes 5 days to run. If the organic compounds were oxidized chemically instead of biologically, the test could be shortened considerably. Such oxidation can be accomplished with the chemical oxygen demand (COD) test. Because nearly all organic compounds are oxidized in the COD test, while only some are decomposed during the BOD test, COD results are always higher than BOD results. The common compounds which cause COD to be higher than BOD include sulphides, sulphites, thiosulfates and chlorides. One example of this is wood pulping waste, in which compounds such as cellulose are easily oxidized chemically (high COD) but are very slow to decompose biologically (low BOD).

This test is carried out on the sewage to determine the extent of readily oxidizable organic matter, which is of two types.

1. Organic matter which can be biologically oxidized is called biologically active.
2. Organic matter which cannot be oxidized biologically is called biologically inactive.

COD gives the oxygen required for the complete oxidation of both biodegradable and non-biodegradable matter. COD is a measure of the oxygen equivalent of the organic matter content of a sample that is susceptible to oxidation by a strong chemical oxidant. It is an indirect method to measure the amount of organic compounds in water. It is expressed in milligrams per liter (mg/L), which indicates the mass of oxygen consumed per liter of solution.

The standard COD test uses a mixture of potassium dichromate and sulphuric acid to oxidize the organic matter (HCOH), with silver (Ag+) added as a catalyst. A simplified example of this reaction is illustrated below, using dichromate (Cr₂0₇^2-) and hydrogen ions (H+). A sample is refluxed in strongly acidic solution with a known excess of potassium dichromate (K₂Cr₂O₇) for 2-3 hours. After digestion, the remaining unreduced K₂Cr₂O₇ is titrated with ferrous ammonium sulphate to determine the amount of K₂Cr₂O₇ consumed. Then, the oxidizable matter is calculated in terms of oxygen equivalent. This procedure is applicable to COD values between 40 and 400 mg/L.

A known amount of a solution of K₂Cr₂O₇ in moderately concentrated sulphuric acid is added to a measured amount of sample and the mixture is boiled in air. In this reaction, the oxidizing agent, hexavalent chromium (Cr^VI), is reduced to trivalent chromium (Cr^III). After boiling, the remaining Cr^VI is titrated against a reducing agent, usually ferrous ammonium sulphate. The difference between the initial amount of Cr^VI added to the sample and the Cr^VI remaining after the organic matter has been oxidized is proportional to the chemical oxygen demand.

Total Organic Carbon

Since the ultimate oxidation of organic carbon is to CO₂, the total combustion of a sample yields some information about the potential oxygen demand in an effluent sample. A common application of total organic carbon testing is to assess the potential for creating disinfection by-products. Disinfection by-products are the result of halogens (e.g., bromine, chlorine) or ozone interacting with naturally occurring organic carbon compounds during the drinking water disinfection process. For example, trihalomethane, a carcinogen, is created when halogens displace three hydrogen ions on methane. Water that is high in total organic carbon has a greater potential to develop disinfection by-products. Some of the organics can be removed by adding levels of treatment specific for organic carbon absorption. It is usually not economically feasible to remove all naturally occurring organics from finished drinking water.

Total organic carbon is measured by oxidizing the organic carbon to CO₂ and H₂0 and measuring the CO₂ gas using an infrared carbon analyser. The oxidation is done by direct injection of the sample into a high-temperature (680-950°C) combustion chamber or by placing a sample into a vial containing an oxidizing agent such as potassium persulfate, sealing and heating the sample to complete the oxidation, then measuring the C02 using the carbon analyser.

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Biochemical Oxygen Demand (BOD)

The rate of oxygen use is commonly referred to as Biochemical Oxygen Demand (BOD). Biochemical Oxygen Demand is a chemical procedure for determining how fast biological organisms use up oxygen in a body of water. It is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it is considered as an indication of the quality of a water source. Biochemical oxygen demand is not a specific pollutant, but rather a measure of the amount of oxygen required by bacteria and other microorganisms engaged in stabilizing decomposable organic matter over a specified period of time. The BOD test is often used to estimate the impacts of effluents that contain large amounts of biodegradable organics such as that from food processing plants and feedlots, municipal wastewater treatment facilities and pulp mills. A high oxygen demand indicates the potential for developing a dissolved oxygen sag as the microbiota oxidize the organic matter in the effluent. A very low oxygen demand indicates either clean water or the presence of a toxic or non-degradable matter.

The BOD test was first used in the late 1800s by the Royal Commission on Sewage Disposal as a measure of the amount of organic pollution in British rivers. At that time, the test was standardized to run for 5 days at 18.3⁰C. These numbers were chosen because none of the British rivers had headwater-to-sea travel times greater than 5 days and the average summer temperature for the rivers was 18.3⁰C. Accordingly, this should reveal the "worst case" oxygen demand in any British river. The BOD incubation temperature was later rounded to 20⁰C, but the 5-day test period remains the current as standard.

In its simplest version, the 5-day BOD test begins by placing water or effluent samples into two standard 60 - or 300 - mL BOD bottles (Fig.1). One sample is analysed immediately to measure the initial dissolved oxygen concentration in the effluent, often using a Winkler titration. The second BOD bottle is sealed and stored at 20°C in the dark. (The samples are stored in the dark to avoid photosynthetic oxygen generation). After 5 days the amount of dissolved oxygen remaining in the sample is measured. The difference between the initial and ending oxygen concentrations is the BOD.

Fig.1 Biochemical Oxygen Demand (BOD) Bottle

BOD is used in water quality management and assessment, ecology and environmental science. BOD is not an accurate quantitative test, although it is considered as an indication of the quality of a water source. It is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20°C or 3 days of incubation at 27°C. The BOD test must be inhibited to prevent oxidation of ammonia. If the inhibitor is not added, the BOD will be between 10% and 40% higher than can be accounted for by carbonaceous oxidation.

Stages of Decomposition in the BOD test

There are two stages of decomposition in the BOD test: a carbonaceous stage and a nitrogenous stage. The carbonaceous stage represents oxygen demand involved in the conversion of organic carbon to carbon dioxide. The second stage or the nitrogenous stage represents a combined carbonaceous plus nitrogenous demand, when organic nitrogen, ammonia and nitrite are converted to nitrate. Nitrogenous oxygen demand generally begins after about 6 days. Under some conditions, if ammonia, nitrite and nitrifying bacteria are present, nitrification can occur in less than 5 days. In this case, a chemical compound that prevents nitrification is added to the sample if the intent is to measure only the carbonaceous demand. The results are reported as carbonaceous BOD (CBOD) or as CBOD5 when a nitrification inhibitor is used.

BOD – Dilution Method

BOD is the amount of oxygen (Dissolved Oxygen (DO)) required for the biological decomposition of organic matter. The oxygen consumed is related to the amount of biodegradable organics. When organic substances are broken down in water, oxygen is consumed

              Organic Carbon + O₂ → CO2

Where, organic carbon in human waste includes protein, carbohydrates, fats, etc. Measure of BOD = Initial oxygen- Final Oxygen after (5 days at 20 °C) or (3 days at 27 °C). Two standard 300 mL BOD bottles are filled completely with wastewater. The bottles are sealed. Oxygen content (DO) of one bottle is determined immediately. The other bottle is incubated at 20°C for 5 days or (or at 27 °C for 3 days) in total darkness to prevent algal growth. After which its oxygen content is again measured. The difference between the two DO values is the amount of oxygen consumed by micro-organisms during 5 days and is reported as BOD5.

Where, DOi and DOf are initial and final DO concentrations of the diluted sample, respectively. P is called as dilution factor and it is the ratio of sample volume (volume of wastewater) to total volume (wastewater plus dilution water). 

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Water Quality - Biological Characteristics

The examination of water for the presence of bacteria is very important. The bacteria are very small organisms and it is not possible to detect them by microscopes. Hence, they are detected by circumstantial evidences or chemical reactions. The growth of bacteria takes place by cell division and there are various classifications of bacteria depending upon their shapes, oxygen requirements and effects on mankind. The last classification is important for the water supply engineer from the view point of public health. The bacteria may be harmless to mankind or harmful to mankind. The former category is known as non-pathogenic bacteria and the latter category is known as pathogenic bacteria. It is not possible to isolate pathogenic bacteria with the help of laboratory instruments. Their chances of presence in a sample of water are increased in relation to the amount of non-pathogenic bacteria present in the sample of water. The combined group of pathogenic and non-pathogenic bacteria is designated by Bacillus coli or B-coli group. This group of bacteria is present in the intestines of all living warm-blooded animals.

Water polluted by sewage contain one or more species of disease producing pathogenic bacteria. Pathogenic organisms cause water borne diseases and many non-pathogenic bacteria such as E.Coli, a member of coliform group, also live in the intestinal tract of human beings. Coliform itself is not a harmful group but it has more resistance to adverse condition than any other group. So, if it is ensured to minimize the number of coliforms, the harmful species will be very less. So, coliform group serves as indicator of contamination of water with sewage and presence of pathogens.

Many of the bacteria found in water are derived from air, soil and vegetation. Some of these are able to multiply and continue their existence while the remaining die out in due course of time. The selective medium that promote the growth of particular bacteria and inbuilt the growth of other organisms is used in the lab to detect the presence of the required bacteria, usually coliform bacteria. Following are the two standard bacteriological tests for bacteriological examination of water.

1. Total count or Agar plate count test

2. B-coli test

1) Total Count or Agar Plate Count Test

In this test, bacteria are cultivated on specially prepared medium of agar for different dilutions of sample of water with sterilized water. In this method total number of bacteria presents in a millitre of water is counted. 1 ml of sample water is diluted in 99ml of sterilized water and 1ml of dilute water is mixed with 10ml of agar of gelatine. The diluted sample is placed in an incubator for 24 hours at 37°C or for 48 hours at 20°C. After the sample will be taken out from the incubator and colonies of bacteria are counted by means of microscope. Drinking water should not have more than 10 coliforms/100ml.

2) B-coli Test

Sometimes this is also called as E–coli test. This test is divided into the following three parts.

1. Presumptive test

2. Confirmed test

3. Completed test

The presumptive test is based on the ability of coliform group to ferment the lactose broth and producing gas. The confirmed test consists of growing cultures of coliform bacteria on media which suppress the growth of other organisms. The completed test is based on the ability of the culture grown in the confirmed test to again ferment the lactose broth.

a) Presumptive Test

Following procedure is adopted in this test.

1. The definite amounts of diluted samples of water are taken in multiples of ten, such as 0.1 cc, 1.0 cc, 10 cc, etc.
2. The water is diluted in standard fermentation tubes containing lactose broth.
3. The tube is maintained at a temperature of 37°C for a period of 48 hours.
4. If gas is seen in the tube after this period is over, it indicates presence of B-coli group and the result of test is treated as positive. If reverse is the case, it indicates the absence of B-coli group and the result of test is treated as negative.
5. A negative result of presumptive test indicates that water is fit for drinking.

b) Confirmed Test

A small portion of lactose broth showing positive presumptive test is carefully transferred to another fermentation tube containing brilliant green lactose bile. If gas is seen in the tube after 48 hours, the result is considered positive and the completed test becomes essential.

c) Completed Test

This test is made by introducing or inoculating bacterial colonies into lactose broth fermentation tubes and agar tubes. The incubation is carried out at 37°C for 24 to 48 hours. If gas is seen after this period, it indicates positive result and further detailed tests are carried out to detect the particular type of bacteria present in water. The absence of gas indicates negative result and water is considered safe for drinking.

B-coli Index

This is an index or number which represents approximately the number of B-coli per cc of sample of water under consideration. The presumptive tests are carried out with different dilution ratios of the sample of water with sterilized water. A number of tests is carried out for each proportion and percentage of positive results is recorded. The difference between successive percentages is worked out and it is multiplied by the reciprocal of quantity of solution. The sum of such values indicates B-coli index. For potable water, B-coli index should be preferably less than 3 and it should not exceed 10 in any case.

M.P.N. Test (Most Probable Number)

It is the number which represents the bacterial density which is most likely to be present. The detection of bacteria by mixing different dilutions of a sample of water with fructose broth and keeping it in the incubator at 37°C for 48hours. The presence of acid or carbon dioxide gas in the test tube will indicate the presence of B-coli. After this the standard statistical tables (Maccardy’s) are referred and the “Most Probable Number” (MPN) of B-coli per 100ml of water are determined. For drinking water, the M.P.N. should not be more than 2.

Membrane Filter Technique

Now a days, a new technique of finding out the B-coli is developed which is called ‘Membrane Filter Technique”. This is a very simple method. In this method the sample of water is filtered through a sterilized membrane of special design due to which all the bacteria are retained on the membrane. The member is then put in contact of culture medium called M-Endo’s medium in the incubator for 24 hours at 37°C. The membrane after incubating is taken out and the colonies of bacteria are counted by means of microscope.

Coliform Index

Coliforms are the rod, shaped, non-pathogenic bacteria whose presence or absence in water indicates the presence or absence of faecal pollution. The total coliform group consists of members whose normal habitat is the lower portion of intestines of humans and warm and cold- blooded animals and soil. Some members which are not found in soil and vegetation constitute about 96% of all the coliforms of human faecal. Such members are called faecal coliforms. The total coliform group is widely used as an indicator organism of choice for drinking water. Escherichia coli (E-Coli) is the predominant member of the faecal coliform group. It is used to measure coliform bacteria present in water sample.

Water Borne Diseases

When water contains certain harmful and disease producing matter, it may lead to many diseases on being consumed by healthy persons. World health organization has observed that 80% of communicable diseases that are transmitted through water. The diseases like cholera, gastroenteritis, typhoid, diarrhoea, polio, hepatitis (Jaundice), Leptospirosis, Dracontiasis are caused by bacteria. Excess of fluorides present in water (above 1.5 mg/litre) cause diseases like dental fluorosis and skeletal fluorosis. This is a permanent irreversible disease that weakens the bone structure. The patient becomes immobile and bedridden.

Types of Water Borne Diseases

1) Common Cold and Flu

The disease that catches people across all of the age lines. When a person will get wet and may start constant sneezing, throat and fever are the severe symptoms of common cold and flu. It can be prevented by not getting in rain.

2) Dengue

The very common disease during rainy seasons. The virus is spread by the Aedes mosquito. The symptoms include high fever, pain in joints and muscles, vomiting, bleeding from nose, gums and even under skin due to haemorrhagic fever. It can be prevented by staying away from mosquitoes and clean the surroundings so that the mosquitoes don’t multiply.

3) Chikungunya

It is an another mosquito transmitted disease. The virus is spread by the Aedes Aegyptus mosquito. The symptoms include fever, swelling and stiffness of joints, muscular pain, headache, fatigue and nausea. It can be prevented by protecting from mosquito bites.

4) Cholera

It spreads through contaminated food, water and poor hygienic conditions. The symptoms include diarrhoea, vomiting, low blood pressure, dry mouth etc. It can be prevented by keeping drinking boiled water and maintain personal hygiene.

5) Typhoid Fever

The disease that spreads during the monsoon season. This disease is spread through contaminated food and water. The symptoms include prolonged fever, abdominal pain and headache. It can be prevented by getting a vaccination in advance and getting high intake of fluid to prevent dehydration.

Prevention of Waterborne Disease

• Improve quality and quantity of drinking at source, at the tap or in the storage vessel.
• Interrupt routes of spreading mosquitos by emptying accumulated water sources.
• Use chlorinated water
• Change hygiene behaviour, like hand washing
• Proper use of latrines
• Careful disposal of all waste products
• Proper maintenance of water supply, sanitation systems, pumps and wells
• Good food hygiene - wash before eating, protect from flies
• Improved immunizations practices, especially rotavirus
• Develop or enhance public health surveillance system
• Faster responses to emergent and dangerous pandemic strains of pathogenic infections
• Health education programs across the country

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Water Quality - Chemical Characteristics

1) pH value of Water/Hydrogen Ion Concentration

The acidity or alkalinity of water is measured in terms of its pH value or H-ion concentration. pH value is the logarithm of reciprocal of hydrogen ion activity in moles per litre. Depending upon the nature of dissolved salts and minerals, water may be acidic or alkaline. When acids or alkalis are dissolved in water, they dissociate into electrically charged hydrogen and hydroxyl radicals, respectively. pH of natural water is generally in the range of 6 - 8. Industrial wastes may be strongly acidic or basic and their effect on pH value of receiving water depends on the buffering capacity of receiving water. It is desirable to maintain pH value of water very close to 7. The acidic water causes tuberculation and the alkaline water causes incrustation. For potable water, the pH value should be between 7 and 8.50. Following are the two methods which are employed to measure the pH value of water.

a) Electrometric Method

In this method, potentiometer is used to measure the electrical pressure exerted by positively charged H-ions. The pH value is then correspondingly expressed.

b) Colourimetric Method

In this method, chemical reagents are added to water and the colour produced is compared with standard colours of known pH values. A set of sealed tubes containing coloured waters of known pH values is kept in the laboratory for ready reference. This test is simple and hence, it is commonly carried out in public health laboratories. The usual indicators are Benzol yellow, Methyl red, Bromphenol blue, etc., for acidic range and Thymol blue, Phenol red, Tolyl red, etc. for alkaline range.

2) Chlorides

The chloride contents, especially of sodium chloride or salt, are worked out for a sample of water. The excess presence of sodium chloride indicates pollution of water due to sewage, minerals, etc. and is dangerous and unfit for use. The water has lower contents of salt than sewage due to the fact that salt consumed in food is excreted by body. For potable water, the highest desirable level of chloride content is 200 mg/litre and its maximum permissible level is 600 mg per litre.

The natural waters near the mines and sea dissolve sodium chloride and also presence of chlorides may be due to mixing of saline water and sewage in the water. Sodium chloride is the main substance in chloride water. The natural water near the mines and sea has dissolved sodium chloride. Similarly, the presence of chlorides may be due to the mixing of saline water and sewage in the water. Excess of chlorides is considered as dangerous and makes the water unfit for many uses. Chloride content is determined by titrating the wastewater with silver nitrate and potassium chromate. Appearance of reddish colour confirms presence of chlorides in water. The measurement of chloride contents is carried out as follows.

1. 50 cc of sample of water is taken by pipette in a porcelain dish.
2. Two or three drops of potassium chromate solution are added to the sample of water.
3. The chloride content is then determined by titrating with standard solution of silver nitrate.
4. The silver reacts first with all chlorides and silver chloride thus formed then reacts with potassium chromate.
5. The silver chromate appears as reddish precipitate and the amount of silver nitrate required to produce such reddish precipitate determines the amount of chlorides present in water.

3) Dissolved Gases

The water contains various gases from its contact with the atmosphere and ground surfaces. The usual gases are nitrogen, methane, carbon dioxide and oxygen. In addition, water may contain some amount of hydrogen sulphide and ammonia depending upon the pH and anaerobic/aerobic condition of water. The contents of these dissolved gases in a sample of water are suitably worked out. The methane concentration is to be studied for its explosive property. The hydrogen sulphide gives disagreeable odour to water even if its amount is very small. The carbon dioxide content indicates biological activities, causes corrosion, increases solubility of many minerals in water and gives taste to water. Dissolved oxygen is necessary for sustenance of aquatic life in water and to keep it fresh. Oxygen in the dissolved state is obtained from atmosphere and pure natural surface water is usually saturated with it. The simple test to determine the amount of dissolved oxygen present in a sample of water is to expose water for 4 hours at a temperature of 27°C with 10% acid solution of potassium permanganate. The quantity of oxygen absorbed can then be calculated. This amount, for potable water, should be about 5 to 10 ppm.

4) Hardness

The hardness or soap-destroying power of water is of two types – temporary hardness and permanent hardness. The temporary hardness is also known as carbonate hardness and it is mainly due to the presence of bicarbonates of calcium and magnesium. It can be removed by boiling or by adding lime to the water. The permanent hardness is also known as non-carbonate hardness and it is due to the presence of sulphates, chlorides and nitrates of calcium and magnesium. It cannot be removed by simply boiling the water. It requires special treatment of water softening.

Total hardness = Carbonate hardness or alkalinity + Non carbonate hardness

The excess hardness of water is undesirable because of various reasons such as it causes more consumption of soap, affects the working of dyeing system, provides scales on boilers, causes corrosion and incrustation of pipes, makes food tasteless, etc. Presence of hardness in water prevents the lathering of the soap during cleaning of clothes, etc.

Hardness is usually expressed in mg of calcium carbonate per litre of water. Hardness is generally determined by Versenate Method. In this method, the water is titrated against EDTA salt solution using Eriochrome Black T as indicator solution. While titrating, colour changes from wine red to blue. In general, under a normal range of pH values, water with hardness up to 75 mg/L are considered as soft and those with 200 mg/L and above are considered as hard. In between, the water is considered as moderately hard. Underground water is generally harder than the surface water, as they have more opportunity to come in contact with minerals. For boiler feed water and for efficient cloth washing, etc., the water must be soft. However, for drinking purposes, water with hardness below 75 mg/L is generally tasteless and hence, the prescribed hardness limit for drinking ranges between 75 to 150 mg/L.

5) Alkalinity

The alkalinity is the capacity of a given sample to neutralize a standard solution of acid. The alkalinity is due to the presence of bicarbonate (HCO₃), carbonate (CO₃) or hydroxide (OH). The determination of alkalinity is very useful in waters and wastes because it provides buffering to resist changes in pH value. The alkalinity is usually divided into the following two parts. Total alkalinity i.e. above pH 4.5 and Caustic alkalinity i.e. above pH 8.2. The alkalinity is measured by volumetric analysis. The commonly adopted two indicators are given below.

1. Phenolphthalein : Pink above pH 8.5 and colourless below pH 8.2
2. Methyl Orange : Red below pH 4.5 and yellow orange above pH 4.5

6) Acidity

Acidity is a measure of the capacity of water to neutralise bases. Acidity is the sum of all titrable acid present in the water sample. Strong mineral acids, weak acids such as carbonic acid, acetic acid present in the water sample contributes to acidity of the water. Usually dissolved carbon dioxide (CO₂) is the major acidic component present in the unpolluted surface waters. The volume of standard alkali required to titrate a specific volume of the sample to pH 8.3 is called phenolphthalein acidity (Total Acidity). The volume of standard alkali required to titrate a specific volume of the water sample (wastewater and highly polluted water) to pH 3.7 is called methyl orange acidity (Mineral Acidity).

Acidity interferes in the treatment of water. Carbon dioxide is of important considerations in determining whether removal by aeration or simple neutralisation with lime /lime soda ash or NaOH will be chosen as the water treatment method. The size of the equipment, chemical requirements, storage spaces and cost of the treatment all depends on the carbon dioxide present. Aquatic life is affected by high water acidity. The organisms present are prone to death with low pH of water. High acidity water is not used for construction purposes. Especially in reinforced concrete construction due to the corrosive nature of high acidity water. Water containing mineral acidity is not fit for drinking purposes. Industrial wastewaters containing high mineral acidity is must be neutralized before they are subjected to biological treatment or direct discharge to water sources.

7) Total Solids

Total solids include suspended and dissolved solids. In this test, the amounts of dissolved and suspended matter present in water are determined separately and then added together to get the total amount of solids present in water. The highest desirable level of total solids is 500 mg/litre and its maximum permissible level is 1500 mg/litre. For measuring suspended solids, water is filtered through a fine filter and dry material retained on the filter is weighed. The filtered water is evaporated and weight of residue that remains on evaporation represents the amount of dissolved solids in water. Total solids can also be considered as the sum of organic and inorganic solids. Amount of inorganic solids can be determined by fusing the residue of total solids in a muffle-furnace and weighing the fused residue. Amount of organic solids is the difference between the amount of inorganic and total solid.

8) Nitrogen and its Compounds

The nitrogen is present in water in the following four forms. The presence of nitrogen in the water indicates the presence of organic maters in the water. 

• Free ammonia 
• Albuminoidal ammonia 
• Nitrites 
• Nitrates

The amount of free ammonia in potable water should not exceed 0.15 ppm and that of albuminoidal ammonia should not exceed 0.3 ppm. The term albuminoidal ammonia is used to represent the quantity of nitrogen present in water before decomposition of organic matter has started. The presence of nitrites indicates that the organic matter present in water is not fully oxidized or in other words, it indicates an intermediate oxidation stage. The amount of nitrites in potable water should be nil.

The presence of the nitrites in the water, due to partly oxidized organic matters, is very dangerous. Therefore, in no case nitrites should be allowed in the water. For potable water, the highest desirable level of nitrates is 45 mg per litre. The free ammonia is measured by simply boiling the water. The ammonia gas is then liberated. The albuminoidal ammonia is measured by adding strong alkaline solution of potassium permanganate to water and then boiling it. The ammonia gas is then liberated. The nitrites and nitrates are converted chemically into ammonia and then measured by comparison with standard colours. The nitrites are rapidly and easily converted to nitrates by the full oxidation of the organic matters. The presence of nitrates is not so harmful. But nitrates > 45 mg/L can cause “mathemoglobinemia” disease to the children.

9) Chlorine

Dissolved free chlorine is never found in natural waters. It is present in the treated water resulting from disinfection with chlorine. The chlorine remains as residual in treated water for the sake of safety against pathogenic bacteria. Residual chlorine is determined by the starch-iodide test. In starch-iodide test, potassium iodide and starch solutions are added to the sample of water due to which blue colour is formed. This blue colour is then removed by titrating with sodium thiosuplhate solution and the quantity of chloride is calculated. On the addition of ortho-iodine solution if yellow colour is formed, it indicates the presence of residual chlorine in the water. The intensity of this yellow colour is compared with standard colours to determine the quantity of residual chlorine. The residual chlorine should remain between 0.5 to 0.2 mg/L in the water so that it remains safe against pathogenic bacteria.

10) Iron and Manganese

These are generally found in ground water. The presence of iron and manganese in water makes it brownish red in colour. Presence of these elements leads to the growth of micro-organism and corrodes the water pipes. Iron and manganese also causes taste and odour in the water. The quantity of iron and manganese is determined by colorimetric methods.

11) Lead and Arsenic

These are not usually found in natural waters. But sometimes lead is mixed up in water from lead pipes or from tanks lined with lead paint when water moves through them. These are poisonous and dangerous to the health of public. The presence of lead and arsenic is detected by means of chemical tests.

Water contains various types of minerals and metals such as, copper, barium, cadmium, selenium, fluoride etc. Arsenic and selenium are poisonous; therefore, they must be removed totally. Human lungs are affected by the presence of high quantity of copper in the water. Fewer cavities in the teeth will be formed due to excessive presence of fluoride in water. The quantity of the metals and other substances can be done indirectly by colorimetric methods using UV-visible spectrophotometer or directly by the use of sophisticated instruments such as Atomic Absorption Spectrophotometer (AAS), Atomic Emission Spectrophotometer (AES), Inductively Coupled Mass Spectrophotometer (ICP-MS) etc.

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Characteristics of Concrete

Advantages of Concrete

a) Economical

Concrete is the most inexpensive and the most readily available material in the world. The cost of production of concrete is low compared with other engineered construction materials. The three major components in concrete are water, aggregate and cement. Compared with steel, plastics and polymers, these components are the most inexpensive and are available in every corner of the world. This enables concrete to be produced worldwide at very low cost for local markets, thus avoiding the transport expenses necessary for most other materials.

b) Ambient Temperature-Hardened Material

Because cement is a low-temperature bonded inorganic material and its reaction occurs at room temperature, concrete can gain its strength at ambient temperature. No high temperature is needed.

c) Ability to be Cast

Fresh concrete is flowable like a liquid and hence can be poured into various formworks to form different desired shapes and sizes right on a construction site. Hence, concrete can be cast into many different configurations.

d) Energy Efficient

Compared with steel, the energy consumption of concrete production is low. The energy required to produce plain concrete is only 450–750 kWh/ton and that of reinforced concrete is 800–3200 kWh/ton, while structural steel requires 8000 kWh/ton or more to make.

e) Excellent Resistance to Water

Unlike wood (timber) and steel, concrete can be hardened in water and can withstand the action of water without serious deterioration, which makes concrete an ideal material for building structures to control, store and transport water, such as pipelines, dams and submarine structures. Contrary to popular belief, water is not deleterious to concrete, even to reinforced concrete; it is the chemicals dissolved in water, such as chlorides, sulphates and carbon dioxide, that cause deterioration of concrete structures.

f) High-Temperature Resistance

Concrete conducts heat slowly and is able to store considerable quantities of heat from the environment. Moreover, the main hydrate that provides binding to aggregates in concrete, calcium silicate hydrate (C–S–H), will not be completely dehydrated until 910^oC. Thus, concrete can withstand high temperatures much better than wood and steel. Even in a fire, a concrete structure can withstand heat for 2–6 hours, leaving sufficient time for people to be rescued. This is why concrete is frequently used to build up protective layers for a steel structure.

g) Ability to Consume Waste

With the development of industry, more and more by-products or waste has been generated, causing a serious environmental pollution problem. To solve the problem, people have to find a way to consume such wastes. It has been found that many industrial wastes can be recycled as a substitute (replacement) for cement or aggregate, such as fly ash, slag (GGBFS = ground granulated blast-furnaces slag), waste glass and ground vehicle tires in concrete. Production of concrete with the incorporation of industrial waste not only provides an effective way to protect our environment, but also leads to better performance of a concrete structure. Due to the large amount of concrete produced annually, it is possible to completely consume most of industry waste in the world, provided that suitable techniques for individual waste incorporation are available.

h) Ability to Work with Reinforcing Steel

Concrete has a similar value to steel for the coefficient of thermal expansion (steel 1.2 × 10⁻⁵; concrete 1.0 – 1.5 × 10⁻⁵). Concrete produces a good protection to steel due to existence of CH and other alkalis (this is for normal conditions). Therefore, while steel bars provide the necessary tensile strength, concrete provides a perfect environment for the steel, acting as a physical barrier to the ingress of aggressive species and giving chemical protection in a highly alkaline environment (pH value is about 13.5), in which black steel is readily passivated.

i) Less Maintenance Required

Under normal conditions, concrete structures do not need coating or painting as protection for weathering, while for a steel or wooden structure, it is necessary. Moreover, the coatings and paintings have to be replaced few years. Thus, the maintenance cost for concrete structures is much lower than that for steel or wooden structures.

Limitations of Concrete

a) Quasi-Brittle Failure Mode

The failure mode of materials can be classified into three categories: brittle failure, quasi-brittle failure and ductile failure, as shown in Fig. 1. Glass is a typical brittle material. It will break as soon as its tension strength is reached. Materials exhibiting a strain-softening behaviour (Fig. 1-b) are called quasi-brittle materials. Both brittle and quasi-brittle materials fail suddenly without giving a large deformation as a warning sign. Ductile failure is a failure with a large deformation that serves as a warning before collapse, such as low-carbon steel. Concrete is a type of quasi-brittle material with low fracture toughness. Usually, concrete has to be used with steel bars to form so-called reinforced concrete, in which steel bars are used to carry tension and the concrete compression loads. Moreover, concrete can provide a structure with excellent stability. Reinforced concrete is realized as the second generation of concrete.

Fig.1 Three Failure Modes of Materials

b) Low Tensile Strength

Concrete has different values in compression and tension strength. Its tension strength is only about 1/10 of its compressive strength for normal-strength concrete or lower for high-strength concrete. To improve the tensile strength of concrete, fiber-reinforced concrete and polymer concrete have been developed.

c) Low Toughness (Ductility)

Toughness is usually defined as the ability of a material to consume energy. Toughness can be evaluated by the area of a load–displacement curve. Compared to steel, concrete has very low toughness, with a value only about 1/50 to 1/100 of that of steel, as shown in Fig. 2. Adding fibers is a good way to improve the toughness of concrete.

Fig.2 Toughness of Steel and Concrete

d) Low Specific Strength (Strength/Density Ratio)

For normal-strength concrete, the specific strength is less than 20, while for steel it is about 40. There are two ways to increase concrete specific strength: one is to reduce its density and the other is to increase its strength. Hence, lightweight concrete and high-strength concrete have been developed.

e) Formwork is Needed

Fresh concrete is in a liquid state and needs formwork to hold its shape and to support its weight. Formwork can be made of steel or wood. The formwork is expensive because it is labour intensive and time-consuming. To improve efficiency, precast techniques have been developed.

f) Long Curing Time

The design index for concrete strength is the 28-day compression strength. Hence, full strength development needs a month at ambient temperature. The improvement measure to reduce the curing period is steam curing or microwave curing.

g) Working with Cracks

Even for reinforced concrete structure members, the tension side has a concrete cover to protect the steel bars. Due to the low tensile strength, the concrete cover cracks. To solve the crack problem, prestressed concrete is developed and it is also realized as a third-generation concrete. Most reinforced concrete structures have existing cracks on their tension sides while carrying the service load.

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