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.

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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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Stream Flow

Flow Characteristics of a Stream

The flow characteristics of a stream depend upon (i) the intensity and duration of rainfall besides spatial and temporal distribution of the rainfall, (ii) shape, soil, vegetation, slope and drainage network of the catchment basin and (iii) climatic factors influencing evapotranspiration. Based on the characteristics of yearly hydrograph (graphical plot of discharge versus time in chronological order), one can classify streams into the following three types.

i) Perennial streams which have some flow, at all times of a year due to considerable amount of base flow into the stream during dry periods of the year. The stream bed is lower than the ground water table in the adjoining aquifer (i.e., water bearing strata which is capable of storing and yielding large quantity of water).

Fig. 1 Temporal Variation of Discharge in Perennial Streams

ii) Intermittent streams have limited contribution from the ground water and that too during the wet season only when the ground water table is above the stream bed and there is base flow contributing to the stream flow. Excepting for some occasional storm that can produce short duration flow, such streams remain dry for most of the dry season periods of a year.

Fig. 2. Temporal Variation of Discharge in Intermittent Streams

iii) Ephemeral streams do not have any contribution from the base flow. The annual hydrograph of such a stream shows series of short duration hydrographs indicating flash flows in response to the storm and the stream turning dry soon after the end of the storm. Such streams, generally found in arid zones and it do not have well defined channels.

Fig. 3 Temporal Variation of Discharge in Ephemeral Streams

Streams are also classified as effluent (streams receiving water from ground water storage) and influent (streams contributing water to the ground water storage) streams. Effluent streams are usually perennial while the influent streams generally remain dry during long periods of dry spell.

Graphical Representation of Stream Flow

The stream flow data are usually recorded in tabular form. For analyzing these data, one has to prepare graphical plots of the stream flow data such as hydrograph, flow-duration curve, flow-mass curve or simply mass curve etc. Hydrograph is a graphical plot between discharge (on y-axis) and the corresponding time (days or months or even hours).

Flow-Mass Curves

Flow-mass curve or runoff-mass curve or inflow mass curve or simply mass curve is cumulative flow volume ‘V’ versus time curve. The mass curve ordinate V (m³ or ha.m or cumec-day) at any time t (in days or weeks or months) is given as

where, t0 is the time at the beginning of the curve.

The mass curve is an integral (i.e., summation) curve of a given hydrograph. Also, slope of the mass curve at any point on the plot i.e., dV/dt equals the rate of stream flow (i.e., stream discharge) at that time. Mass curve is always a rising curve or horizontal (when there is no inflow or runoff added into the stream) and is a useful means by which one can calculate storage capacity of a reservoir to meet specified demand as well as safe yield of a reservoir of given capacity.

Fig. 4 Reservoir Capacity from Mass-Flow Curve

Slope of the cumulative demand curve (usually a line since the demand rate is generally constant) is the demand rate which is known. The reservoir is assumed to be full at the beginning of a dry period (i.e., when the withdrawal or demand rate exceeds the rate of inflow into the reservoir) such as A in Fig. 4. Draw line AD (i.e., demand line) such that it is tangential to the mass curve at A and has a slope of the demand rate, between A and B (where there is maximum difference between the demand line and the mass curve) the demand is larger than the inflow (supply) rate and the reservoir storage would deplete. Between B and D, the supply rate is higher than the demand rate and the reservoir would get refilled. The maximum difference in the ordinates of the demand line and mass curve between A and D (i.e., BC) represents the volume of water required as storage in the reservoir to meet the demand from the time the reservoir was full i.e., A in Fig. 4. If the mass curve is for a large time period, there may be more than one such duration of dry periods which obtain the storages required for those durations (EH and IL). The largest of these storages (BC, FG and JK) is the required storage capacity of the reservoir to be provided on the stream in order to meet the demand.

For determining the safe yield of (or maintainable demand by) a reservoir of given capacity one needs to draw tangents from the apex points (A, E and I of Fig. 4) such that the maximum difference between the tangent and the mass curve equals the given capacity of the reservoir. The slopes of these tangents equals to the safe yield for the relevant dry period. The smallest slope of these slopes is the firm dependable yield of the reservoir. It should be noted that a reservoir gets refilled only if the demand line intersects the mass curve. Non-intersection of the demand line with the mass curve indicates inflow which is insufficient to meet the given demand. Also, the vertical difference between points D and E represents the spilled volume of water over the spillway.

The losses from reservoir (such as due to evaporation and seepage into the ground or leakage) in a known duration can either be included in the demand rates or deducted from inflow rates. In practice, demand rates for irrigation, power generation or water supply vary with time. For such situations, mass curve of demand is superposed over the flow-mass curve with proper matching of time. If the reservoir is full at the first intersection of the two curves, the maximum intercept between the two curves represents the required storage capacity of the reservoir to meet the variable demand.

Flow-Duration Curve

Flow-duration curve (or discharge-frequency curve) of a stream is a graphical plot of stream discharge against the corresponding per cent of time the stream discharge was equaled or exceeded. The flow-duration curve describes the variability of the stream flow and is useful for

1. Determining dependable flow which information is required for planning of water resources and hydropower projects
2. Designing a drainage system
3. Flood control studies

For preparing a flow-duration curve, the stream flow data (individual values or range of values) are arranged in a descending order of stream discharges. If the number of such discharges is very large, one can use range of values as class intervals. Percentage probability Pp of any flow (or class value) magnitude Q being equaled or exceeded is given as

in which ‘m’ is the order number of the discharge (or class value) and N is the number of data points in the list. The discharge Q is plotted against Pp to get the yield flow-duration curve.

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Runoff

Runoff can be defined as the portion of the precipitation that makes its way towards rivers or oceans etc., as surface or subsurface flow. Portion which is not absorbed by the deep strata. Runoff occurs only when the rate of precipitation exceeds the rate at which water may infiltrate into the soil. Precipitation (or rainfall), after satisfying the requirements of evapotranspiration, interception, infiltration into the ground and detention storage, drains off or flows off from a catchment basin as an overland flow (or surface runoff which includes precipitation falling on the stream system too) into a stream channel. Some part of the infiltrating water moves laterally through the upper layers of the soil and returns to the ground surface as interflow or subsurface runoff at some place away from the point of infiltration into the soil. Part of the infiltrating water percolates deep into the ground and joins the ground water storage. When water table intersects the stream channels of the catchment basin, some ground water may reach the surface or join the stream as ground water runoff, also called base flow or dry-weather flow. Thus, the runoff from a catchment includes surface runoff, subsurface runoff and base flow.

The surface runoff starts soon after the precipitation and is the first to join the stream flow. Subsurface runoff is slower and joins the stream later. Depending upon the time taken by the subsurface runoff between the infiltration and joining the stream channel, it may be termed as prompt subsurface runoff or delayed subsurface runoff. The groundwater runoff is the slowest in joining the stream channel but, is responsible in maintaining low flows in the stream during dry season. Based on the time interval between the precipitation and runoff, the runoff is categorized as direct runoff (that enters the stream immediately after precipitation i.e., surface runoff, subsurface runoff) and base flow (i.e., ground water runoff). Runoff is the response of a catchment to the precipitation reflecting the combined effects of the nature of precipitation, other climatic characteristics of the region and the physiographic characteristics of the catchment basin.

Type, intensity, duration and areal distribution of precipitation over the catchment are the chief characteristics of the precipitation that affect the stream flow. Precipitation in the form of rainfall is quicker to appear as stream flow than when it is in the form of snow. For the surface runoff to start, the intensity of rainfall (or precipitation) must exceed the infiltration capacity of the soil which decreases with the increase in the duration of rainfall. It is obvious that a longer duration rainfall may produce higher runoff even if the intensity of rainfall is less but exceeding the infiltration capacity of the soil. Heavy rainfalls in the downstream region of the catchment will cause rapid rise in the stream levels and early peaking of the discharge. A rare occurrence of uniformly distributed rainfall may result in increased infiltration and therefore, increased subsurface runoff and base flow resulting in slow rise in levels and delayed peaking of the discharge. Likewise, antecedent higher soil moisture conditions at the time of precipitation would hasten the rise in the stream levels. Other climatic characteristics influencing the runoff are temperature, wind velocity and relative humidity. These characteristics affect the evapotranspiration and thus influence the availability of the precipitation for runoff.

Types of Runoff

1) Surface Runoff

Portion of rainfall (after all losses such as interception, infiltration, depression storage etc. are met) that enters streams immediately after occurring rainfall. After laps of few time, overland flow joins streams; sometime termed prompt runoff (as very quickly enters streams).

2) Subsurface Runoff

Amount of rainfall first enter into soil and then flows laterally towards stream without joining water table. It also takes little time to reach stream.

3) Base Flow

It is a delayed flow. Water that meets the groundwater table and join the stream or ocean. It is very slow movement and take months or years to reach streams.

Factors affecting runoff

• Climatic factors
• Rain and snow fall
• Duration of rainfall
• Rainfall distribution
• Direction of prevailing wind
• Other climatic factors like Temperature, wind velocity, relative humidity, annual rainfall etc.

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Interpretation of Precipitation Data

Precipitation data must be checked for the continuity and consistency before they are analysed for any significant purpose. This is essential when it is suspected that the gauge site (or its surroundings) might have changed appreciably during the period for which the average is being computed.

Estimation of Missing Data

The continuity of a record of precipitation data may have been broken with missing data due to several reasons such as damage (or fault) in a rain gauge during a certain period. The missing data is estimated using the rainfall data of the neighbouring rain gauge stations. The missing annual precipitation Px at a station x is related to the annual precipitation values, P1, P2, P3 ...... Pm and normal annual precipitation, N1, N2, N3 ...... Nm at the neighbouring M stations 1, 2, 3, … M respectively. The normal precipitation (for a particular duration) is the mean value of rainfall on a particular day or in a month or year over a specified 30-year period.

The 30-year normals are computed every decade. The term normal annual precipitation at any station is, therefore, the mean of annual precipitations at that station based on 30-year record.

The missing annual precipitation Px is simply given as

If the normal annual precipitations at various stations are within about 10% of the normal annual precipitation at station x i.e., Nx. Otherwise, one uses the normal ratio method which gives

This method works well when the precipitation regimes of the neighbouring stations and the station x are similar (or almost the same).

Multiple linear regression (amongst precipitation data of M stations and the station x, excluding the unknown missing data of station x and the concurrent (or corresponding) data of the neighbouring M stations) will yield an equation of the form

The regression method allows for some weighting of the stations and adjusts, to some extent, for departures from the assumption of the normal ratio method.

Test for Consistency of Precipitation Data

Changes in relevant conditions of a rain gauge (such as gauge location, exposure, instrumentation or observation techniques and surroundings) may cause a relative change in the precipitation catchment of the rain gauge. The consistency of the precipitation data of such rain gauges needs to be examined. Double-mass analysis, also termed double-mass curve technique, compares the accumulated annual or seasonal precipitation at a given station with the concurrent accumulated values of mean precipitation for a group of the surrounding stations (i.e., base stations). Since the past response is to be related to the present conditions, the data (accumulated precipitation of the station x, i.e., ΣPx and the accumulated values of the average of the group of the base stations, i.e., ΣPav) are usually assembled in reverse chronological order. Values of ΣPx are plotted against ΣPav for the concurrent time periods and is given in Fig. 1. A definite break in the slope of the resulting plot points to the inconsistency of the data indicating a change in the precipitation regime of the station x. The precipitation values at station x at and beyond the period of change is corrected using the relation,

Where,

     Pcx = corrected value of precipitation at station x at any time t

     Px = original recorded value of precipitation at station x at time t.

     Sc = corrected slope of the double-mass curve

     Sa = original slope of the curve

Fig. 1 Double-Mass Curve

Thus, the older records of station x have been corrected so as to be consistent with the new precipitation regime of the station x.

Presentation of Precipitation Data

Precipitation (or rainfall) data are presented as either a mass curve of rainfall (accumulated precipitation v/s time plotted in chronological order, Fig. 2) or a hyetograph (rainfall intensity v/s time). Mass curves of rainfall provide the information on the duration and magnitude of a storm. Intensities of rainfall at a given time can be estimated by measuring the slope of the curve at the specified time. The hyetograph derived from the mass curve is usually represented as a chart. The area of a hyetograph represents the total precipitation received during the period.

Depth – Area - Duration (DAD) Analysis

Depth-area-duration (DAD) curves are plots of accumulated average precipitation versus area for different durations of a storm period. Depth-area-duration analysis of a storm is performed to estimate the maximum amounts of precipitation for different durations and over different areas. A storm of certain duration over a specified basin area seldom results in uniform rainfall depth over the entire specified area. The difference between the maximum rainfall depth over an area P0 and its average rainfall depth (P-bar) for a given storm, i.e., (P0 – P-bar) increases with increase in the basin area and decreases with increase in the storm duration. The depth-area-duration curve is obtained as given in the following example figure.

Fig. 2 Example Figure for DAD Curves

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Average Depth of Precipitation Over an Area

The information on the average depth of precipitation (or rainfall) over a specified area on either the storm basis or seasonal basis or annual basis is often required in several types of hydrologic problems. The depth of rainfall measured by a rain gauge is valid for that rain gauge station and in its immediate vicinity. Over a large area like watershed (or catchment) of a stream, there will be several such stations and the average depth of rainfall over the entire area can be estimated by one of the following methods.

1) Arithmetic Mean Method

This is the simplest method in which average depth of rainfall is obtained by obtaining the sum of the depths of rainfall (say P1, P2, P3, P4 .... Pn) measured at stations 1, 2, 3…. n and dividing the sum by the total number of stations i.e. n. Thus, 

This method is suitable if the rain gauge stations are uniformly distributed over the entire area and the rainfall variation in the area is not large.

2) Theissen Polygon Method

The Theissen polygon method takes into account the non-uniform distribution of the gauges by assigning a weightage factor for each rain gauge. In this method, the enitre area is divided into number of triangular areas by joining adjacent rain gauge stations with straight lines, as shown in Fig. 1 (a and b). If a bisector is drawn on each of the lines joining adjacent rain gauge stations, there will be number of polygons and each polygon, within itself, will have only one rain gauge station. Assuming that rainfall Pi recorded at any station ‘i’ is representative rainfall of the area Ai of the polygon i within which rain gauge station is located, the weighted average depth of rainfall P-bar for the given area is given as

Here, Ai/ A is termed the weightage factor for i^th  rain gauge.

Fig. 1 Areal averaging of precipitation a) Rain gauge network, b) Theissen polygons, c) Isohyets

This method is, obviously, better than the arithmetic mean method since it assigns some weightage to all rain gauge stations on area basis. Also, the rain gauge stations outside the catchment can also be used effectively. Once the weightage factors for all the rain gauge stations are computed, the calculation of the average rainfall depth P-bar is relatively easy for a given network of stations.

While drawing Theissen polygons, one should first join all the outermost rain gauge stations. Thereafter, the remaining stations should be connected suitably to form quadrilaterals. The shorter diagonals of all these quadrilaterals are then drawn. The sides of all these triangles are then bisected and thus, Theissen polygons for all rain gauge stations are obtained.

3) Isohyetal Method

An isohyet is a contour of equal rainfall. Knowing the depths of rainfall at each rain gauge station of an area and assuming linear variation of rainfall between any two adjacent stations, one can draw a smooth curve passing through all points indicating the same value of rainfall, Fig. 1 (c). The area between two adjacent isohyets is measured with the help of a planimeter.

The average depth of rainfall P-bar for the entire area A is given as

Since this method considers actual spatial variation of rainfall, it is considered as the best method for computing average depth of rainfall.

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Measurement of Precipitation - Rain Gauges

Rainfall may be measured by a network of rain gauges which may either be of non-recording or recording type. Rainfall can be measured with a weather radar also.

Non-Recording Rain gauge

The non-recording rain gauge used in India is the Symon’s rain gauge (Fig. 1). It consists of a funnel with a circular rim of 12.7 cm diameter and a glass bottle as a receiver. The cylindrical metal casing is fixed vertically to the masonry foundation with the level rim 30.5 cm above the ground surface. The rain falling into the funnel is collected in the receiver and is measured in a special measuring glass graduated in mm of rainfall; when full it can measure 1.25 cm of rain. The rainfall is measured every day at 08.30 hours IST. The funnel shank is inserted in a bottle which receives the rainwater. The water collected in the bottle is measured by pouring it into a measuring cylinder which gives the depth of rainfall in mm.

Fig. 1 Symon’s Rain Gauge

During heavy rains, it must be measured three or four times in the day in which the receiver fill and overflow, but the last measurement should be at 08.30 hours IST and the sum total of all the measurements during the previous 24 hours entered as the rainfall of the day in the register. Usually, rainfall measurements are made at 08.30 hr IST and sometimes at 17.30 hr IST also. Thus the non-recording or the Symon’s rain gauge gives only the total depth of rainfall for the previous 24 hours (i.e., daily rainfall) and does not give the intensity and duration of rainfall during different time intervals of the day. It is often desirable to protect the gauge from being damaged by cattle and for this purpose a barbed wire fence may be erected around it.

Recording Rain Gauge

This is also called self-recording, automatic or integrating rain gauge. This type of rain gauge has an automatic mechanical arrangement consisting of a clockwork, a drum with a graph paper fixed around it and a pencil point, which draws the mass curve of rainfall. From this mass curve, the depth of rainfall in a given time, the rate or intensity of rainfall at any instant during a storm and time of onset and cessation of rainfall can be determined. The gauge is installed on a concrete or masonry platform 45 cm square in the observatory enclosure by the side of the ordinary rain gauge at a distance of 2-3 m from it. The gauge is so installed that the rim of the funnel is horizontal and at a height of exactly 75 cm above ground surface. The self-recording rain gauge is generally used in conjunction with an ordinary rain gauge exposed close by, for use as standard, by means of which the readings of the recording rain gauge can be checked and if necessary adjusted. There are three types of recording rain gauges—tipping bucket gauge, weighing gauge and float gauge.

1) Tipping Bucket Rain Gauge

This consists of a cylindrical receiver 30 cm diameter with a funnel inside (Fig. 2). Just below the funnel a pair of tipping buckets is pivoted such that when one of the bucket receives a rainfall of 0.25 mm it tips and empties into a tank below, while the other bucket takes its position and the process is repeated. The tipping of the bucket actuates on electric circuit which causes a pen to move on a chart wrapped round a drum which revolves by a clock mechanism. This type of rain gauge cannot record snowfall.

Fig. 2 Tipping Bucket Rain Gauge

2) Weighing Type Rain Gauge

In this type of rain-gauge, when a certain weight of rainfall is collected in a tank, which rests on a spring-lever balance, it makes a pen to move on a chart wrapped round a clock driven drum (Fig. 3). The rotation of the drum sets the time scale while the vertical motion of the pen records the cumulative precipitation. The record thus gives the accumulation of rainfall with time.

Fig. 3 Weighing Type Rain Gauge

3) Float Type Rain Gauge

In this type, as the rain is collected in a float chamber, the float moves up which makes a pen to move on a chart wrapped round a clock driven drum. When the float chamber fills up, the water siphons out automatically through a siphon tube kept in an interconnected siphon chamber. The float rises with the rise of water level in the chamber and its movement is recorded on a chart through a suitable mechanism. The clockwork revolves the drum once in 24 hours. The clock mechanism needs rewinding once in a week when the chart wrapped round the drum is also replaced. A siphon arrangement is also provided to empty the chamber quickly whenever it becomes full. The weighing and float type rain gauges can store a moderate snow fall which the operator can weigh or melt and record the equivalent depth of rain.

Fig. 4 Float Type Rain Gauge

Bureau of Indian Standards has laid down the following guidelines for selecting the site for rain gauges (IS : 4897-1968).

• The rain gauge shall be placed on a level ground, not upon a slope or a terrace and never upon a wall or roof.
• On no account, the rain gauge shall be placed on a slope such that the ground falls away steeply in the direction of the prevailing wind.
• The distance of the rain gauge from any object shall not be less than twice the height of the object above the rim of the gauge.
• Great care shall be taken at mountain and coast stations so that the gauges are not unduly exposed to the sweep of the wind. A belt of trees or a wall on the side of the prevailing wind at a distance exceeding twice its height shall form an efficient shelter.
• In hills where it is difficult to find a level space, the site for the rain gauge shall be chosen where it is best shielded from high winds and where the wind does not cause eddies.
• The location of the gauge should not be changed without taking suitable precautions. Description of the site and surroundings should be made a matter of record.

Radar Measurement of Precipitation

In regions of difficult and inaccessible terrains, precipitation can be measured (within about 10% accuracy of the rain gauge measurements) with the help of a radar (radio detecting and ranging). A radar transmits a pulse of electromagnetic waves as a beam in a direction depending upon the position of the movable antenna. The wave travelling at a speed of light is partially reflected by cloud or precipitation particles and returns to the radar where it is received by the same antenna. The display of the magnitude of energy of the returned wave on the radarscope (i.e., radar screen) is called an echo and its brightness is termed echo intensity. The duration between the transmission of the pulse and appearance of the echo on the radarscope is a measure of the distance (i.e., range) of the target from the radar.
Direction of the target with respect to the radar is decided by the orientation of the antenna at the time the target signal is received. The echo is seen in polar coordinates. If there is no target (i.e., cloud or precipitation particles), the screen is dimly illuminated. A small target would appear as a bright point whereas an extended target (such as a rain shower) would appear as a bright patch. The radarscope being divided as per the coordinate system; the position of the target can be estimated. By having a proper calibration between the echo intensity and rainfall (or its intensity), one can estimate the rainfall (or rainfall intensity).

Satellite Measurement of Precipitation

It is a common experience that gauge network for measuring precipitation in a large and inaccessible area (such as in desert and hilly regions) is generally inadequate and non-existent in oceans. The satellite observation is the only effective way for continuous monitoring of precipitation events over a large or inaccessible area. Use of the meterological satellites for weather and water balance studies is, therefore, continuously increasing.

In satellite measurements, the precipitation is estimated by correlating the satellite- derived data and observed rainfall data. These relationships can be developed for a part of electromagnetic spectrum using cloud life history or cloud indexing approach. The first approach uses data from geo-stationary satellites that produce data at every half an hour interval. The second approach, based on cloud classification, does not require a series of consecutive observations of the same cloud system. Microwave remote sensing techniques that can directly monitor the rainfall characteristics have great potential in rainfall measurement.

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Types of Supports for Plane Structures

Supports are used to attach structures to the ground or other bodies, thereby restricting their movements under the action of applied loads. The loads tend to move the structures; but supports prevent the movements by exerting opposing forces or reactions to neutralize the effects of loads, thereby keeping the structures in equilibrium. The type of reaction a support exerts on a structure depends on the type of supporting device used and the type of movement it prevents. A support that prevents translation of the structure in a particular direction exerts a reaction force on the structure in that direction. Similarly, a support that prevents rotation of the structure about a particular axis exerts a reaction couple on the structure about that axis. The type of supports commonly used for plane structures are given below.

1) Fixed Support

A fixed support is the most rigid type of support or connection. It constrains the member in all translations and rotations (i.e. it cannot move or rotate in any direction). The easiest example of a fixed support would be a pole or column in concrete. The pole cannot twist, rotate or displace; it is basically restricted in all its movements at this connection. Fixed supports are extremely beneficial when you can only use a single support. The fixed support provides all the constraints necessary to ensure the structure is static. It is most widely used as the only support for a cantilever. The fixed support is also called rigid support. It provides greater stability to the structure as compared with all other supports. To provide good stability to the structure, at least one rigid support should be provided. Beam fixed in the wall is a good example of fixed support.

Fig. 1 Fixed Support

2) Pinned Support (Hinged Support)

The hinged support is also called pinned support. A pinned support is a very common type of support and is most commonly compared to a hinge in civil engineering. Like a hinge, a pinned support allows rotation to occur but no translation (i.e. it resists horizontal and vertical forces but not a moment). The horizontal and vertical components of the reaction can be determined using the equation of equilibrium. Pinned supports can be used in trusses. By linking multiple members joined by hinge connections, the members will push against each other; inducing an axial force within the member. The benefit of this is that the members contain no internal moment forces and can be designed according to their axial force only. Hinge support reduces sensitivity to an earthquake.

Fig. 2 Pinned Support

3) Roller Support

It is a support that is free to rotate and translate along the surface on which they rest. The surface on which the roller supports are installed may be horizontal, vertical and inclined to any angle. Roller supports can resist a vertical force but not a horizontal force. The roller supports has only one reaction, this reaction acts perpendicular to the surface and away from it. A roller support or connection is free to move horizontally as there is nothing constraining it. The most common use of a roller support is in a bridge. In civil engineering, a bridge will typically contain a roller support at one end to account for vertical displacement and expansion from changes in temperature. This is required to prevent the expansion causing damage to a pinned support. This type of support does not resist any horizontal forces. This obviously has limitations in itself as it means the structure will require another support to resist this type of force.

Fig. 3 Roller Support

4) Simple Support

A simple support is basically just where the member rests on an external structure. They are quite similar to roller supports in the sense that they are able to restrain vertical forces but not horizontal forces. The member simply rests on an external structure to which the force is transferred to. An example is a plank of wood resting on two concrete blocks. The plank can support any downward (vertical) force but if you apply a horizontal force, the plank will simply slide off the concrete blocks.

Fig. 4 Simple Support

5) Rocker Support

Rocker support is similar to roller support. It also resists vertical force and allows horizontal translation and rotation. But in this case, horizontal movement is due to the curved surface provided at the bottom as shown in Fig. 5. So, the amount of horizontal movement is limited in this case.

Fig. 5 Rocker Support

6) Link Support

A link has two hinges, one at each end. The link is supported and allows rotation and translation perpendicular to the direction of the link only. It does not allow translation in the direction of the link. It has a single linear resultant force component in the direction of the link which can be resolved into vertical and horizontal components. In other words, the reaction force of a link is in the direction of the link, along its longitudinal axis.

Fig. 6 Link Support

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