Rock and Classification of Rocks

Rock can be defined as a compact, semi-hard to hard mass of natural material composed of one or more minerals. The rocks that are encountered at the surface of the earth or beneath are commonly classified into three groups according to their modes of origin. They are igneous, sedimentary and metamorphic rocks. The mineral grains that form the solid phase of a soil aggregate are the product of rock weathering. The size of the individual grains varies over a wide range. Many of the physical properties of soil are dictated by the size, shape and chemical composition of the grains. Fig.1 shows a diagram of the formation cycle of different types of rock and the processes associated with them. This is called the rock cycle.

Fig. 1 Rock Cycle

Classification of Rocks

Rocks used for engineering works may be classified in the following three ways.

1) Geological

2) Physical

3) Chemical

Fig. 2 Classification of Rocks

I) Geological Classification of Rocks

Based on their origin of formation rocks are classified into three main groups - Igneous, sedimentary and metamorphic rocks.

1) Igneous Rock

Igneous rocks are considered to be the primary rocks formed by the cooling of molten magma or by the recrystallization of older rocks under heat and pressure great enough to render them fluid. Igneous rocks are formed by the solidification of molten magma ejected from deep within the earth’s mantle. After ejection by either fissure eruption or volcanic eruption, some of the molten magma cools on the surface of the earth. They have been formed on or at various depths below the earth surface. There are two main classes of igneous rocks. They are

1. Extrusive (poured out at the surface), and

2. Intrusive (large rock masses which have not been formed in contact with the atmosphere).

Intrusive rocks formed in the past may be exposed at the surface as a result of the continuous process of erosion of the materials that once covered them.

Initially both classes of rocks were in a molten state. Their present state results directly from the way in which they solidified. Due to violent volcanic eruptions in the past, some of the molten materials were emitted into the atmosphere with gaseous extrusions. These cooled quickly and eventually fell on the earth's surface as volcanic ash and dust. Extrusive rocks are distinguished by their glass like structure. Intrusive rocks, cooling and solidifying at great depths and under pressure containing entrapped gases, are wholly crystalline in texture. Such rocks occur in masses of great extent, often going to unknown depths. Table 1 shows the mineral composition of igneous rocks.

Table 1 Mineral Composition of Igneous Rocks

Mineral	Percentage
Quartz	12-20
Feldspar	50-60
Ca, Fe and Mg, Silicates	14-17
Micas	4-8
Others	7-8

Feldspars are the most common rock minerals, which account for the abundance of clays derived from the feldspars on the earth's surface. Quartz comes next in order of frequency. Most sands are composed of quartz.

Some of the important rocks that belong to the igneous group are granite and basalt. Granite is primarily composed of feldspar, quartz and mica and possesses a massive structure. Basalt is a dark coloured fine grained rock. It is characterized by the predominance of plagioclase, the presence of considerable amounts of pyroxene and some olivine and the absence of quartz. The colour varies from dark grey to black. Both granite and basalt are used as building stones. Generally igneous rocks are strong and durable.

Example: Basalt, Trap, Andesite, Rhyolite, Diorite, Granite

2) Sedimentary Rock

When the products of the disintegration and decomposition of any rock type are transported, redeposited and partly or fully consolidated or cemented into a new rock type, the resulting material is classified as a sedimentary rock. Due to weathering action of water, wind and frost; existing rocks disintegrates. The disintegrated material is carried by wind and water. Flowing water deposits its suspended materials at some points of obstacles to its flow. These deposited layers of materials get consolidated under pressure and by heat. Chemical agents also contribute to the cementing of the deposits. The rocks thus formed are more uniform, fine grained and compact in their nature.

The sedimentary rocks generally formed in quite definitely arranged beds or strata, which can be seen to have been horizontal at one time although sometimes displaced through angles up to 90 degrees. Sedimentary rocks are generally classified on the basis of grain size, texture and structure. From an engineering point of view, the most important rocks that belong to the group are sandstones, limestones and shales.

The deposits of gravel, sand, silt and clay formed by weathering may become compacted by overburden pressure and cemented by agents like iron oxide, calcite, dolomite and quartz. Cementing agents are generally carried in solution by groundwater. They fill the spaces between particles and form sedimentary rock. Rocks formed in this way are called detrital sedimentary rocks. All detrital rocks have a clastic texture. Sedimentary rock also can be formed by chemical processes. Rocks of this type are classified as chemical sedimentary rock. These rocks can have clastic or non-clastic texture. Table 2 shows some examples of chemical sedimentary rock.

Table 2 Chemical Composition of Sedimentary Rock

Composition	Rock
Calcite (CaCO3)	Limestone
Halite (NaCl)	Rock salt
Dolomite [CaMg(CO3)]	Dolomite
Gypsum (CaSO4 - 2H2O)	Gypsum

Limestone is formed mostly of calcium carbonate deposited either by organisms or by an inorganic process. Most limestone has a clastic texture; however, non-clastic textures also are found commonly. Chalk is a sedimentary rock made in part from biochemically derived calcite, which are skeletal fragments of microscopic plants and animals. Dolomite is formed either by chemical deposition of mixed carbonates or by the reaction of magnesium in water with limestone. Gypsum and anhydrite result from the precipitation of soluble CaSO₄ due to evaporation of ocean water. They belong to a class of rocks generally referred to as evaporites. Rock salt (NaCl) is another example of an evaporite that originates from the salt deposits of seawater. Sedimentary rock may undergo weathering to form sediments or may be subjected to the process of metamorphism to become metamorphic rock.

Example: Limestone, Sandstone, Dolomite and Slate

3) Metamorphic Rock

Metamorphism is the process of changing the composition and texture of rocks (without melting) by heat and pressure. During metamorphism, new minerals are formed and mineral grains are sheared to give a foliated texture to metamorphic rock. Rocks formed by the complete or incomplete recrystallization of igneous or sedimentary rocks by high temperatures, high pressures and/or high shearing stresses are metamorphic rocks. The rocks so produced may display features varying from complete and distinct foliation of a crystalline structure to a fine fragmentary partially crystalline state caused by direct compressive stress, including also the cementation of sediment particles by siliceous matter. Metamorphic rocks formed without intense shear action have a massive structure. Some of the important rocks that belong to this group are gneiss, schist, slate and marble.

Gneiss is a metamorphic rock derived from high grade regional metamorphism of igneous rocks, such as granite, gabbro and diorite. Low grade metamorphism of shales and mudstones results in slate. Slate is a dark coloured, platy rock with extremely fine texture and easy cleavage. Because of this easy cleavage, slate is split into very thin sheets and used as a roofing material. The clay minerals in the shale become chlorite and mica by heat; hence, slate is composed primarily of mica flakes and chlorite. Phyllite is a metamorphic rock, which is derived from slate with further metamorphism being subjected to heat greater than 250 to 300°C. Schist is a type of metamorphic rock derived from several igneous, sedimentary and low grade metamorphic rocks with a well foliated texture and visible flakes of platy and micaceous minerals.

Metamorphic rock generally contains large quantities of quartz and feldspar as well. Marble is formed from calcite and dolomite by recrystallization. The mineral grains in marble are larger than those present in the original rock. Green marbles are coloured by hornblends, serpentine, or talc. Black marbles contain bituminous material and brown marbles contain iron oxide and limonite. Quartzite is a metamorphic rock formed from quartz rich sandstones. Silica enters into the void spaces between the quartz and sand grains and acts as a cementing agent. Quartzite is one of the hardest rocks. Under extreme heat and pressure, metamorphic rocks may melt to form magma and the cycle is repeated.

Example : Due to metamorphic action granite becomes gneiss, trap and basalt change to schist and laterite, lime stone changes to marble, sand stone becomes quartzite and mud stone becomes slate.

II) Physical Classification of Rocks

Based on the structure, the rocks may be classified as:

1) Stratified Rocks

These rocks are having layered structure. They possess planes of stratification or cleavage. They can be easily split along these planes.

Example : Sand stones, lime stones, slate etc.

Fig. 3 Stratified Rock

2) Unstratified Rocks

These rocks are not stratified. They possess crystalline and compact grains. They cannot be split in to thin slab.

Example : Granite, trap, marble etc.

Fig. 4 Unstratified Rock

3) Foliated Rocks

These rocks have a tendency to split along a definite direction only. The direction need not be parallel to each other as in case of stratified rocks. This type of structure is very common in case of metamorphic rocks.

Fig. 5 Foliated Rock

III) Chemical Classification of Rocks

On the basis of their chemical composition rocks are classified as:

1) Silicious Rocks

The main content of these rocks is silica. They are hard and durable.

Example : Granite, trap, sand stones etc.

2) Argillaceous Rocks

The main constituent of these rocks is argil i.e., clay. These stones are hard and durable but they are brittle. They cannot withstand shock.

Example : Slate and laterite

3) Calcareous Rocks

The main constituent of these rocks is calcium carbonate. Limestone is a calcareous rock of sedimentary origin while marble is a calcareous rock of metamorphic origin.

Example : Limestone, marble, kankar, dolomite and gravel

Rock Minerals

It is essential to examine the properties of the rock forming minerals since all soils are derived through the disintegration or decomposition of some parent rock. A 'mineral' is a natural inorganic substance of a definite structure and chemical composition. Some of the very important physical properties of minerals are crystal form, colour, hardness, cleavage, luster, fracture and specific gravity. Out of these only two, specific gravity and hardness are of foundation engineering interest. The specific gravity of the minerals affects the specific gravity of soils derived from them. The specific gravity of most rock and soil forming minerals varies from 2.50 (some feldspars) and 2.65 (quartz) to 3.5 (augite or olivine). Gypsum has a smaller value of 2.3 and salt (NaCl) has 2.1. Some iron minerals may have higher values, for instance, magnetite has 5.2. It is reported that about 95 percent of the known part of the lithosphere consists of igneous rocks and only 5 percent of sedimentary rocks. Soil formation is mostly due to the disintegration of igneous rock which may be termed as a parent rock.

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Stages of Highway Development

Highway design is only one element in the overall highway development process. Historically, detailed design occurs in the middle of the process, linking the preceding phases of planning and project development with the subsequent phases of right-of-way acquisition, construction and maintenance. Now, it is during the first three stages planning, project development and design. The designers and communities, working together can have the greatest impact on the final design features of the project. In fact, the flexibility available for highway design during the detailed design phase is limited a great deal by the decisions made at the earlier stages of planning and project development.

Objectives of Highway Planning

Planning is considered as a pre requisite before attempting any development programme in the present era. This is particularly true for any engineering project, as planning is the basic requirement for any new project or an expansion programme. Thus there is a need for planned development of the road network and the links. Highway planning is of great importance when the funds available are limited whereas the total requirement is much higher. In developing countries like India it is important to utilize the available funds in the best possible manner by resorting to best planning principles. The objects of highway planning are briefly given below.

• To plan overall road network for efficient and safe traffic operation, but at minimum cost. Here the costs of construction, maintenance and resurfacing or strengthening of pavement layers and the vehicle operation cost are to be given due consideration.
• To arrive at the road system and the lengths of different categories of roads which could provide maximum utility and could be constructed within the available resources during the plan period under consideration.
• To divide the overall plan into phases and to decide priorities.
• To fix date-wise priorities for development of each road link based on utility as the main criterion for phasing the road development programme.
• To plan for future requirements and improvements of roads in view of anticipated developments.
• To work out suitable financing system phases of highway planning.

Highway planning includes the following phases.

1. Assessment of road length requirement for an area (it may be a district, state or the whole country)
2. Preparation of master plan showing the phasing of plan in five year plans or annual plans. (In order to plan the road system in the selected region, state or country, different studies and surveys are to be carried out to collect the data required. The data collected are to be processed and analyzed to arrive at the best possible road network and to arrive at the optimum length of the road system.)

Stages of Highway Development

Although the names may vary by State, the five basic stages in the highway development process are: planning, project development (preliminary design), final design, right of way and construction. After construction is completed, ongoing operation and maintenance activities continue throughout the life of the facility.

1) Planning

The initial definition of the need for any highway or bridge improvement project takes place during the planning stage. This problem definition occurs at the State, regional or local level, depending on the scale of the proposed improvement. This is the key time to get the public involved and provide input into the decision making process. The problems identified usually fall into one or more of the following four categories.

1. The existing physical structure needs major repair/replacement (structure repair).
2. Existing or projected future travel demands exceed available capacity and access to transportation and mobility need to be increased (capacity).
3. The route is experiencing an inordinate number of safety and accident problems that can only be resolved through physical and geometric changes (safety).
4. Developmental pressures along the route make a re-examination of the number, location and physical design of access points necessary (access).

Factors to Consider During Planning

It is important to look ahead during the planning stage and consider the potential impact that a proposed facility or improvement may have while the project is still in the conceptual phase. During planning, key decisions are made that will affect and limit the design options in subsequent phases. The important factors to be considered in planning include the following.

• Physical character
• Cost
• Safety
• Capacity
• Environmental quality
• Historic and scenic characteristics
• Multimodal consideration
• Other factors

2) Project Development

After a project has been planned and programmed for implementation, it moves into the project development phase. At this stage, the environmental analysis intensifies. The level of environmental review varies widely, depending on the scale and impact of the project. It can range from a multiyear effort to prepare an Environmental Impact Statement (a comprehensive document that analyses the potential impact of proposed alternatives) to a modest environmental review completed in a matter of weeks. Regardless of the level of detail or duration, the product of the project development process generally includes a description of the location and major design features of the recommended project that is to be further designed and constructed, while continually trying to avoid, minimize and mitigate environmental impact.

3) Final Design

After a preferred alternative has been selected and the project description agreed upon as stated in the environmental document, a project can move into the final design stage. The product of this stage is a complete set of plans, specifications and estimates of required quantities of materials ready for the solicitation of construction bids and subsequent construction. Depending on the scale and complexity of the project, the final design process may take from a few months to several years. The following concepts are important considerations of design and it includes the following.

a) Developing a Concept

A design concept gives the project a focus and helps to move it toward a specific direction. There are many elements in a highway and each involves a number of separate but interrelated design decisions. Integrating all these elements to achieve a common goal or concept helps the designer in making design decisions. Some of the many elements of highway design are

• Number and width of travel lanes, median type and width and shoulders
• Traffic barriers
• Overpasses/bridges
• Horizontal and vertical alignment and affiliated landscape

b) Considering Scale

People driving in a car see the world at a much different scale than people walking on the street. This large discrepancy in the design scale for a car versus the design scale for people has changed the overall planning of our communities. The design element with the greatest effect on the scale of the roadway is its width or cross section. The cross section can include a clear zone, shoulder, parking lanes, travel lanes and/or median. The wider the overall roadway, the larger its scale; however, there are some design techniques that can help to reduce the perceived width and thus, the perceived scale of the roadway. Limiting the width of pavement or breaking up the pavement is one option. In some instances, four lane roadways may look less imposing by designing a grass or planted median in the centre.

c) Detailing the Design

Particularly during the final design phase, it is the details associated with the project are important. Employing a multidisciplinary design team ensures that important design details are considered and those they are compatible with community values. Often it is the details of the project that are most recognizable to the public. A multidisciplinary design team can produce an aesthetic and functional product when the members work together and are flexible in applying guidelines.

4) Right-of-way

Once the final designs have been prepared and needed right-of-way is purchased, construction bid packages are made available, a contractor is selected and construction is initiated. During the right-of-way acquisition and construction stages, minor adjustments in the design may be necessary; therefore, there should be continuous involvement of the design team throughout these stages.

5) Construction and Maintenance

Construction may be simple or complex and may require a few months to several years. Once construction has been completed, the facility is ready to begin its normal sequence of operations and maintenance. Even after the completion of construction, the character of a road can be changed by inappropriate maintenance actions. For example, the replacement of sections of guardrail damaged or destroyed in crashes commonly utilizes whatever spare guardrail sections may be available to the local highway maintenance personnel at the time.

Highway Route Surveys and Location

To determine the geometric features of road design, the following surveys must be conducted after the necessity of the road is decided. A variety of survey and investigations have to be carried out by Road engineers and multidiscipline persons.

1) Transport Planning Surveys

• Traffic Surveys
• Highway inventories
• Pavement Deterioration Study
• Accident study

2) Alignment and Route location surveys Desk study Reconnaissance Preliminary Survey Final location survey

3) Drainage Studies

4) Soil Survey

• Surface run-off - Hydrologic and hydraulic
• Subsurface drainage - Ground water & Seepage
• Cross–drainage - Location and waterway area required for the cross-drainage structures.

5) Pavement Design investigation, Soil property and strength, Material Survey

6) Desk study

7) Site Reconnaissance

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Horizontal Distance Measurement

The task of determining the horizontal distances between two existing points and of setting a new point at a specified distance from some other fixed position are fundamental surveying operations. The surveyor must select the appropriate equipment and apply suitable field procedures in order to determine or set and mark distances with the required degree of accuracy. Depending on the specific application and the required accuracy, one of several methods may be used to determine horizontal distance. The most common methods include pacing, taping, and EDM (Electronic Distance Measurement). Taping has been the traditional surveying method for horizontal distance measurement for many years. It is a direct and relatively slow procedure, which requires manual skill on the part of the surveyors.

One of the basic measurements in surveying is the determination of the distance between two points on the earth’s surface for use in fixing position, set out and in scaling. Usually spatial distance is measured. In plane surveying, the distances measured are reduced to their equivalent horizontal distance either by the procedures used to make the measurement or by applying numerical corrections for the slope distance (spatial distance). The method to be employed in measuring distance depends on the required accuracy of the measurement and this in turn depends on purpose for which the measurement is intended. Horizontal distances may be determined by many methods.

Rough Distance Measurement

In certain surveying applications, only a rough approximation of distance is necessary; a method called pacing or the use of a simple measuring wheels, may be sufficient in these instances. 

Example - Locating topographic features during the preliminary reconnaissance of a building site, searching for the property corners etc.

1) Pacing

Pacing consists of counting the number of steps or paces in a required distance. This is used to provide distance estimates when no measuring device is available or precision is not required. Distances obtained by pacing are sufficiently accurate for many purposes in surveying. Pacing is also used to validate survey work and eliminate any taping blunders. Measuring your pace length requires a measured 100 feet distance. You then walk this distance and count the number of steps. It is best to repeat the process four times and average the results. In this method, distances can be measured with an accuracy of about 1:100 by pacing.

Distance = Unit Pace × Number of Paces

Fig. 1 Distance Measurement by Pacing

It is possible to adjust your pace to an even three feet, but this should usually be avoided. It is very difficult to maintain an unnatural pace length over a long distance. Accurate pacing is done by using your natural pace, even if it is an uneven length such as 2.6 feet. It is difficult to maintain an even pace when going uphill or downhill. Using your natural pace will make this easier.

Another error can occur if you are not consistent in starting with either the heel or toe of your shoe. If you place your toe at the start point, then also measure the end point with your toe. Starting with the heel and ending with the toe is a common mistake. Some surveyors prefer to count strides. A stride is two steps or paces. This reduces the counting but often requires using part of a stride to determine the total distance. Pacing is a valuable skill for surveyors. It requires some practice and concentration.

2) Odometer

Vehicle odometers are helpful in determining long distances such as for layout or checking vision at intersections. Precision of 1/20 is reasonable. Based on diameter of tires (no of revolutions x wheel diameter), this method gives a fairly reliable result provided a check is done periodically on a known length. During each measurement a constant tyre pressure has to be maintained.

3) Measuring Wheel

A simple measuring wheel mounted on a rod can be used to determine distances, by pushing the rod and rolling the wheel along the line to be measured. An attached device called an odometer serves to count the number of turns of the wheels. From the known circumference of the wheel and the number of revolutions, distances for reconnaissance can be determined with relative accuracy of about 1:200. This device is particularly useful for rough measurement of distance along curved lines. It is commonly used to record distances such as curb length or paving quantities and can also be helpful for determining distances along a curve.

Distance = Odometer Reading x Circumference of the Wheel (𝜋D)

Where D is the diameter of the measuring wheel

Fig. 2 Measuring Wheel

4) Tape

This method involves direct measurement of distances with a tape or chain. Steel tapes are most commonly used. It is available in lengths varying from 15m to 100m. Measuring horizontal distances with a tape is simple in theory, but in actual practice, it is not as easy as it appears at first glance. It takes skill and experience for a surveyor to be able to tape a distance with a relative accuracy between 1:3000 and 1:5000, which is generally acceptable range for most preliminary surveys.

5) Tachometry

Distance can be measured indirectly by optical surveying instruments like theodolite. The method is quite rapid and sufficiently accurate for many types of surveying operations.

6) Electronic Distance Measurement (EDM)

These are indirect distance measuring instruments that work using the invariant velocity of light or electromagnetic waves in vacuum. They have high degree of accuracy and are effectively used for long distances for modern surveying operations. This quickly provides very precise measurements but requires experienced personnel and relatively expensive equipment.

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K-Slump Tester

The new apparatus called “K-Slump Tester” can be used to measure the slump directly in one minute after the tester is inserted in the fresh concrete to the level of the floater disc. This tester can also be used to measure the relative workability. The apparatus comprises of the following four principal parts.

1) A chrome plated steel tube with external and internal diameters of 1.9 and 1.6 cm respectively. The tube is 25 cm long and its lower part is used to make the test. The length of this part is 15.5 cm which includes the solid cone that facilitates inserting the tube into the concrete. Two types of openings are provided in this part. Four rectangular slots 5.1 cm long and 0.8 cm wide and 22 round holes 0.64 cm in diameter; all these openings are distributed uniformly in the lower part as shown in Fig 2.

2) A disc floater 6 cm in diameter and 0.24 cm in thickness which divides the tube into two parts; the upper part serves as a handle and the lower one is for testing. The disc serves also to prevent the tester from sinking into the concrete beyond the preselected level.

3) A hollow plastic rod 1.3 cm in diameter and 25 cm long which contains a graduated scale in centimeters. This rod can move freely inside the tube and can be used to measure the height of mortar that flows into the tube and stays there. The rod is plugged at each end with a plastic cap to prevent concrete or any other material from seeping inside.

4) An aluminium cap 3 cm diameter and 2.25 cm long which has a little hole and a screw that can be used to set and adjust the reference zero of the apparatus. There is also in the upper part of the tube, a small pin which is used to support the measuring rod at the beginning of the test. The total weight of the apparatus is 226 g.

Fig. 1 K Slump Test

Test Procedure

• Wet the tester with water and shake off the excess.
• Raise the measuring rod, tilt slightly and let it rest on the pin located inside the tester.
• Insert the tester on the levelled surface of concrete vertically down until the disc floater rests at the surface of the concrete. Do not rotate while inserting or removing the tester.
• After 60 seconds, lower the measuring rod slowly until it rests on the surface of the concrete that has entered the tube and read the K-Slump directly on the scale of the measuring rod.
• Raise the measuring rod again and let it rest on its pin.
• Remove the tester from the concrete vertically up and again lower the measuring rod slowly till it touches the surface of the concrete retained in the tube and read workability (W) directly on the scale of the measuring rod.

Fig. 2 K Slump Tester

In the concrete industry, the slump test is still the most widely used test to control the consistency of concrete mixtures, even though there are some questions about its significance and its effectiveness. Several apparatuses have been proposed to replace or supplement the slump cone, but in general they have proved to be rich in theory and poor in practice. Their use is still limited mainly to research work in laboratories. The K-slump apparatus is very simple, practical and economical to use, both in the field and the laboratory. It has proven that it has a good correlation with the slump cone. The K-slump tester can be used to measure slump in one minute in cylinders, pails, buckets, wheel-barrows, slabs or any other desired location where the fresh concrete is placed.

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Measurement of Workability - Slump Test

A concrete is said to be workable if it can be easily mixed, placed, compacted and finished. A workable concrete should not show any segregation or bleeding. Segregation is said to occur when coarse aggregate tries to separate out from the finer material and a concentration of coarse aggregate at one place occurs. This results in large voids, less durability and strength. Bleeding of concrete is said to occur when excess water comes up at the surface of concrete. This causes small pores through the mass of concrete and is undesirable.

There is no universally accepted test method that can directly measure the workability of concrete. The difficulty in measuring the mechanical work defined in terms of workability, the composite nature of the fresh concrete and the dependence of the workability on the type and method of construction makes it impossible to develop a well-accepted test method to measure workability. The most widely used test, which mainly measures the consistency of concrete, is called the slump test. For the same purpose, the second test in order of importance is the Vebe test, which is more meaningful for mixtures with low consistency. The third test is the compacting factor test, which attempts to evaluate the compactability characteristic of a concrete mixture. The fourth test method is the ball penetration test that is related to the mechanical work.

Slump Test

Unsupported fresh concrete flows to the sides and a sinking in height takes place. This vertical settlement is known as slump. The slump is a measure indicating the consistency or workability of cement concrete. It gives an idea of water content needed for concrete to be used for different works. To measure the slump value, the fresh concrete is filled into a mould of specified shape and dimensions and the settlement or slump is measured when supporting mould is removed. The slump increases as water content is increased. For different works different slump values have been recommended.

Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work where nominal maximum size of aggregates does not exceed 40 mm. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability and it always representative of the placability of the concrete. It is conveniently used as a control test and gives an indication of the uniformity of concrete from batch to batch. Repeated batches of the same mix, brought to the same slump, will have the same water content and water cement ratio, provided the weights of aggregate, cement and admixtures are uniform and aggregate grading is within acceptable limits. Additional information on workability and quality of concrete can be obtained by observing the manner in which concrete slumps. Quality of concrete can also be further assessed by giving a few tappings or blows by tamping rod to the base plate. The deformation shows the characteristics of concrete with respect to tendency for segregation.

Tools and Apparatus Used for Slump Test (Equipments)

• Standard slump cone (100 mm top diameter x 200 mm bottom diameter x 300 mm high)
• Small scoop
• Bullet-nosed rod (600 mm long x 16 mm diameter)
• Rule
• Slump plate (500 mm x 500 mm)

(The thickness of the metallic sheet for the mould should not be thinner than 1.6 mm.)

Fig. 1 Slump Testing Equipment

Fig. 2 Slump Cone

Procedure

• Clean the internal surface of the mould thoroughly and place it on a smooth horizontal, rigid and non-absorbent surface, such as of a metal plate.
• Consider a W-C ratio of 0.5 to 0.6 and design mix of proportion about 1:2:4 (It is presumed that a mix is designed already for the test). Weigh the quantity of cement, sand, aggregate and water correctly. Mix thoroughly. Use this freshly prepared concrete for the test.
• Fill the mould to about one fourth of its height with concrete. While filling, hold the mould firmly in position
• Tamp the layer with the round end of the tamping rod with 25 strokes disturbing the strokes uniformly over the cross section.
• Fill the mould further in 3 layers each time by 1/4^th height and tamping evenly each layer as above. After completion of rodding of the topmost layer strike of the concrete with a trowel or tamping bar, level with the top of mould.
• Lift the mould vertically slowly and remove it.
• The concrete will subside. Measure the height of the specimen of concrete after subsidence.
• The slump of concrete is the subsidence, i.e. difference in original height and height up to the topmost point of the subsided concrete in millimeters.

Fig. 3 Slump Test

The pattern of slump indicates the characteristics of concrete in addition to the slump value. If the concrete slumps evenly it is called true slump. If one half of the cone slides down, it is called shear slump. In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence. Shear slump also indicates that the concrete is non-cohesive and shows the characteristic of segregation. It is seen that the slump test gives fairly good consistent results for a plastic mix. This test is not sensitive for a stiff mix. In case of dry mix, no variation can be detected between mixes of different workability. In the case of rich mixes, the value is often satisfactory, their slump being sensitive to variations in workability. IS 456 - 2000 suggests that in the “very low” category of workability where strict control is necessary, for example, Pavement Quality Concrete (PQC) measurement of workability by determination of compacting factor will be more appropriate than slump and a value of 0.75 to 0.80 compacting factor is suggested.

Fig. 4 Types of Slump 

The above IS also suggests that in the “very high” category of workability, measurement of workability by determination of “flow” by flow test will be more appropriate. However, in a lean mix with a tendency of harshness a true slump can easily change to shear slump. In such case, the tests should be repeated. Despite many limitations, the slump test is very useful on site to check day-to-day or hour to- hour variation in the quality of mix. An increase in slump, may mean for instance that the moisture content of the aggregate has suddenly increased or there has been sudden change in the grading of aggregate. The slump test gives warning to correct the causes for change of slump value. Table 1 shows the nominal slump value for different degrees of workability.

Table 1 Nominal Slump Value for Different Degrees of Workability

Sl.No.	Slump Value (in mm)	Degree of Workability
1	0 – 25	Very Low
2	25 - 50	Low
3	50 – 100	Medium
4	100 - 175	High

The Bureau of Indian standards, in the past, generally adopted compacting factor test values for denoting workability. Even in the IS:10262 of 1982 dealing with recommended guide line for Concrete Mix Design, adopted compacting factor for denoting workability. But now in the revision of IS:456 – 2000, the code has reverted back to slump value to denote the workability rather than compacting factor. It shows that slump test has more practical utility than the other tests for workability.

Although, slump test is popular due to the simplicity of apparatus used and simple procedure, the simplicity is also often allowing a wide variability and many time it could not provide true guide to workability. For example, a harsh mix cannot be said to have same workability as one with a large proportion of sand even though they may have the same slump.

Factors Affecting Slump Test

• Material properties like chemical composition, fineness, particle size distribution, moisture content and temperature of cementitious materials
• Size, texture, combined grading, cleanliness and moisture content of the aggregates
• Chemical admixtures dosage, type, combination, interaction, sequence of addition and its effectiveness,
• Air content of concrete
• Concrete batching, mixing and transporting methods and equipment
• Temperature of the concrete
• Sampling of concrete, slump testing technique and the condition of test equipment
• The amount of free water in the concrete
• Time since mixing of concrete at the time of testing

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Surface Tension

Surface tension is a force per unit length (or surface energy per unit area) acting in the plane of the interface between the plane of the liquid and any other substance. It is also an extra energy that the molecules at the interface have as compared to molecules in the interior. Due to molecular attraction, liquids possess certain properties such as cohesion and adhesion. Cohesion means inter molecular attraction between molecules of the same liquid. That means it is a tendency of the liquid to remain as one assemblage of particles. Adhesion means attraction between the molecules of a liquid and the molecules of a solid boundary surface in contact with the liquid. The property of cohesion enables a liquid to resist tensile stress, while adhesion enables it to stick to another body. Surface tension is due to cohesion between liquid particles at the surface, whereas capillarity is due to both cohesion and adhesion.

A liquid molecule on the interior of the liquid body has other molecules on all sides of it, so that the forces of attraction are in equilibrium and the molecule is equally attracted on all the sides, as a molecule at point A shown in Fig.1. On the other hand, a liquid molecule at the surface of the liquid, (i.e., at the interface between a liquid and a gas) as at point B, does not have any liquid molecule above it, and consequently there is a net downward force on the molecule due to the attraction of the molecules below it. This force on the molecules at the liquid surface, is normal to the liquid surface. Apparently owing to the attraction of liquid molecules below the surface, a film or a special layer seems to form on the liquid at the surface, which is in tension and small loads can be supported over it. For example, a small needle placed gently upon the water surface will not sink but will be supported by the tension at the water surface.

Fig. 1 Inter molecular forces near a liquid surface

The property of the liquid surface film to exert a tension is called the surface tension. It is denoted by ‘σ’ (Greek ‘sigma’) and it is the force required to maintain unit length of the film in equilibrium. In SI units, surface tension is expressed in N/m. In the metric gravitational system of units, it is expressed in kg(f)/cm or kg(f)/m. As surface tension is directly dependent upon inter molecular cohesive forces, its magnitude for all liquids decreases as the temperature rises. It is also dependent on the fluid in contact with the liquid surface; thus surface tensions are usually quoted in contact with air. The surface tension of water in contact with air varies from 0.0736 N/m [or 0.0075 kg (f)/m] at 19°C to 0.0589 N/m [or 0.006 kg (f)/m] at 100°C.

Surface tension has a role in capillary action, or capillarity, in which liquids climb up narrow tubes or narrow gaps between surfaces. Capillarity, which is important in the flow of water through soils as well as in flows in the human body, is the result of free surface forces and fluid-solid attractive forces. In space travel, where the pull of gravity is small, capillarity causes liquids to crawl out of open containers. Therefore, space travelers must drink with special straws that clamp shut when not in use to prevent snacks from climbing up the straw and floating freely throughout the cabin.

Surface forces are important in a wide variety of technical applications, including the breakup of jets, processes involving thin films and foams. Wicking, the drawing of fluid up into a fabric or wick as in a candle or away from the body as in the design of exercise clothing, is another process that works by capillary action. The opposite effect, waterproofing, is a manipulation of surface forces to prevent wicking. Surface tension causes striking effects that are exploited to make engaging fountain displays. In soap and water solutions, for example, variation of the concentration of the solute can cause the surface tension to vary, which in turn causes flow.

Flow driven by surface tension gradients called the Marangoni effect which stabilizes soap bubbles, among other effects. The emerging field of micromechanics creates machinery that works on nearly molecular size scales. The properties of any liquids involved in micro machines are dominated by interfacial forces. Interfacial forces are not always important, however, even when a large amount of free surface is present. In an ocean, wave motion depends on viscous forces and gravity forces, but the contribution of surface tension forces to the momentum balance in oceanic flows is negligible.

Fig. 2 Surface forces cause the curvature of interfaces in small tubes, which is called the meniscus effect

Fig. 3 Unbalanced forces at the free surface of a fluid must be accounted for by including the surface tension in fluid models. The surface tension is the tension per unit length present in an imaginary stretched film coincident with the free surface

Fig. 4 The legs of the water strider make impressions on the water surface as it walks across the free surface. The free surface acts like a membrane under tension that supports the insect

Fig. 5 Soap bubbles are composed of thin fluid layers sandwiched between two free surfaces. Surfactant molecules occupy the free surfaces and reduce the surface tension of the bubble surface compared to the surface tension of pure water. If an external force deforms or inflates the bubble, more surface is generated, reducing the concentration of surfactant molecules at the bubble surfaces. Lower surfactant concentration implies higher surface tension, however, and this locally higher surface tension pulls fluid into the thinning layer, stabilizing the film and preventing bubble rupture

Whenever a liquid is in contact with other liquids or gases, or in this case a gas/solid surface, an interface develops that acts like a stretched elastic membrane, creating surface tension. There are two features to this membrane: the contact angle ′θ′, and the magnitude of the surface tension, ′σ′ (N/m or lbf/ft). Both of these depend on the type of liquid and the type of solid surface (or other liquid or gas) with which it shares an interface. The examples of surface tension effects arise when you are able to place a needle on a water surface and, similarly, when small water insects are able to walk on the surface of the water.

For a soap bubble in air, surface tension acts on both inside and outside interfaces between the soap film and air along the curved bubble surface. Surface tension also leads to the phenomena of capillary. In engineering, probably the most important effect of surface tension is the creation of a curved meniscus that appears in manometers or barometers, leading to a (usually unwanted) capillary rise (or depression).

                                  (a) A wetted surface                                           (b) A non-wetted surface

Fig. 6 Surface tension effects on water droplets

The effect of surface tension is illustrated in the case of a droplet as well as a liquid jet. When a droplet is separated initially from the surface of the main body of liquid, then due to surface tension there is a net inward force exerted over the entire surface of the droplet which causes the surface of the droplet to contract from all the sides and results in increasing the internal pressure within the droplet. The contraction of the droplet continues till the inward force due to surface tension is in balance with the internal pressure and the droplet forms into sphere which is the shape for minimum surface area. The internal pressure within a jet of liquid is also increased due to surface tension. The internal pressure intensity within a droplet and a jet of liquid in excess of the outside pressure intensity may be determined by the expressions derived below.

i) Pressure Intensity Inside a Droplet

Consider a spherical droplet of radius ‘r’ having internal pressure intensity ‘p’ in excess of the outside pressure intensity. If the droplet is cut into two halves, then the forces acting on one half will be those due to pressure intensity p on the projected area (πr²) and the tensile force due to surface tension ‘σ’ acting around the circumference (2πr). These two forces will be equal and opposite for equilibrium and hence we have

This equation indicates that the internal pressure intensity increases with the decrease in the size of droplet.

ii) Pressure Intensity Inside a Soap Bubble

A spherical soap bubble has two surfaces in contact with air, one inside and the other outside, each one of which contributes the same amount of tensile force due to surface tension. As such on a hemispherical section of a soap bubble of radius r the tensile force due to surface tension is equal to 2σ (2πr). The pressure force acting on the hemispherical section of the soap bubble is same as in the case of a droplet and it is equal to p (πr²). Thus equating these two forces for equilibrium, we have

iii) Pressure Intensity Inside a Liquid Jet

Consider a jet of liquid of radius ‘r’, length ‘l’ and having internal pressure intensity ‘p’ in excess of the outside pressure intensity. If the jet is cut into two halves, then the forces acting on one half will be those due to pressure intensity p on the projected area (2rl) and the tensile force due to surface tension ‘σ’ acting along the two sides. These two forces will be equal and opposite for equilibrium and hence we have

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Viscosity of a Fluid

It is defined as the internal resistance offered by one layer of fluid to the adjacent layer. In case of liquids, main reason of the viscosity is molecular bonding or cohesion. In case of gases main reason of viscosity is molecular collision. In case of liquids, due to increase in temperature the viscosity will decrease due to breaking of cohesive bonds. In case of gases, the viscosity will increase with temperature because of molecular collision increases. All fluids offer resistance to any force tending to cause one layer to move over another. Viscosity is the fluid property responsible for this resistance. Since relative motion between layers requires the application of shearing forces, that is, forces parallel to the surfaces over which they act, the resisting forces must be in exactly the opposite direction to the applied shear forces and so they too are parallel to the surfaces.

It is a matter of common experience that, under particular conditions, one fluid offers greater resistance to flow than another. Such liquids as tar, treacle and glycerine cannot be rapidly poured or easily stirred and are commonly spoken of as thick; on the other hand, thin liquids such as water, petrol and paraffin flow much more readily. (Lubricating oils with small viscosity are sometimes referred to as light, and those with large viscosity as heavy; but viscosity is not related to density). Gases as well as liquids have viscosity, although the viscosity of gases is less evident in everyday life.

Quantitative Definition of Viscosity

Consider two plates sufficiently large (so that edge conditions may be neglected) placed a small distance Y apart, the space between them being filled with fluid as shown in Fig.1. The lower plate is assumed to be at rest, while the upper one is moved parallel to it with a velocity ‘V’ by the application of a force ‘F’, corresponding to area ‘A’, of the moving plate in contact with the fluid. Particles of the fluid in contact with each plate will adhere to it and if the distance Y and velocity V are not too great, the velocity v at a distance y from the lower plate will vary uniformly from zero at the lower plate which is at rest, to V at the upper moving plate. Experiments show that for a large variety of fluids,

Fig.1 Fluid motion between two parallel plates

It may be seen from similar triangles in Fig.1 that the ratio V/Y can be replaced by the velocity gradient (dv/dy), which is the rate of angular deformation of the fluid.

If a constant of proportionality 'μ' (Greek ‘mu’) be introduced, the shear stress 'τ' (Greek ‘tau’) equal to (F/A) between any two thin sheets of fluid may be expressed as 

This equation is called Newton’s law of viscosity, it states that, for the straight and parallel motion of a given fluid, the tangential stress between two adjoining layers is proportional to the velocity gradient in a direction perpendicular to the layers.

In the transposed form, it serves to define the proportionality constant. which is called the coefficient of viscosity, or the dynamic viscosity (since it involves force), or simply viscosity of the fluid. Thus the dynamic viscosity μ, may be defined as the shear stress required to produce unit rate of angular deformation. In SI units μ is expressed in N.s/m², or kg/m.s. The dynamic viscosity μ is a property of the fluid and a scalar quantity.

In the metric gravitational system of units, μ is expressed in kg(f)-sec/m². In the metric absolute system of units μ is expressed in dyne-sec/m² or gm(mass)/cm-sec which is also called ‘poise’ after Poiseuille. The ‘centipoise’ is one hundredth of a poise. The numerical conversion from one system to another is as follows.

1 Ns/m² = 10 poise

In many problems involving viscosity, there frequently appears a term dynamic viscosity ‘μ’ divided by mass density ‘ρ’. The ratio of the dynamic viscosity μ and the mass density ρ is known as Kinematic viscosity and is denoted by the symbol ‘υ’ (Greek ‘nu’) so that

On analyzing the dimensions of the kinematic viscosity it will be observed that it involves only the magnitudes of length and time. The name kinematic viscosity has been given to the ratio (μ/ρ) because kinematics is defined as the study of motion without regard to the cause of the motion and hence it is concerned with length and time only.

In SI units υ is expressed in m²/s. In the metric system of units υ is expressed in cm²/sec or m²/sec. The unit cm²/sec is termed as ‘stoke’ after G.G. Stokes and its one-hundredth part is called ‘centistoke’. In the English system of units it is expressed in ft2/sec. The numerical conversion from one system to another is as follows.

1 m²/s = 10⁴ stokes

The dynamic viscosity μ of either a liquid or a gas is practically independent of the pressure for the range that is ordinarily encountered in practice. However, it varies widely with temperature. For gases, viscosity increases with increase in temperature while for liquids it decreases with increase in temperature. This is so because of their fundamentally different intermolecular characteristics. In liquids the viscosity is governed by the cohesive forces between the molecules of the liquid, whereas in gases the molecular activity plays a dominant role. The kinematic viscosity of liquids and of gases at a given pressure, is essentially a function of temperature.

Common fluids such as air, water, glycerine, kerosene etc., follow Newton’s law of viscosity. There are certain fluids which, however, do not follow Newton’s law of viscosity. Accordingly, fluids may be classified as Newtonian fluids and non-Newtonian fluids. In a Newtonian fluid there is a linear relation between the magnitude of shear stress and the resulting rate of deformation i.e., the constant of proportionality μ in the equation does not change with rate of deformation. In a non-Newtonian fluid there is a non-linear relation between the magnitude of the applied shear stress and the rate of angular deformation. In the case of a plastic substance which is a non-Newtonian fluid an initial yield stress is to be exceeded to cause a continuous deformation. An ideal plastic has a definite yield stress and a constant linear relation between shear stress and the rate of angular deformation. A thixotropic substance, which is a non-Newtonian fluid, has a non-linear relationship between the shear stress and the rate of angular deformation, beyond an initial yield stress. The printer’s ink is an example of a thixotropic liquid.

Fig. 2 Variation of shear stress with velocity gradient

An ideal fluid is defined as that having zero viscosity or in other words shear stress is always zero regardless of the motion of the fluid. Thus an ideal fluid is represented by the horizontal axis (τ = 0) in Fig. 2, which gives a diagrammatic representation of the Newtonian, non-Newtonian, plastic, thixotropic and ideal fluids. A true elastic solid may be represented by the vertical axis of the diagram. The fluids with which engineers most often have to deal are Newtonian, that is, their viscosity is not dependent on the rate of angular deformation, and the term ‘fluid-mechanics’ generally refers only to Newtonian fluids. The study of non-Newtonian fluids is termed as ‘rheology’.

Causes of Viscosity

For one possible cause of viscosity we may consider is the forces of attraction between molecules. Yet there is evidently also some other explanation, because gases have by no means negligible viscosity although their molecules are in general so far apart that no appreciable inter-molecular force exists. The individual molecules of a fluid are continuously in motion and this motion makes possible a process of exchange of momentum between different layers of the fluid.

In gases this interchange of molecules forms the principal cause of viscosity and the kinetic theory of gases (which deals with the random motions of the molecules) allows the predictions – borne out by experimental observations is that

1. The viscosity of a gas is independent of its pressure (except at very high or very low pressure) 
2. Because of the molecular motion increases with a rise of temperature, the viscosity also increases with a rise of temperature (unless the gas is so highly compressed that the kinetic theory is invalid).

The process of momentum exchange also occurs in liquids. There is, however, a second mechanism at play. The molecules of a liquid are sufficiently close together for there to be appreciable forces between them. Relative movement of layers in a liquid modifies these inter-molecular forces, thereby causing a net shear force which resists the relative movement. Consequently, the viscosity of a liquid is the resultant of two mechanisms, each of which depends on temperature, and so the variation of viscosity with temperature is much more complex than for a gas. The viscosity of nearly all liquids decreases with rise of temperature, but the rate of decrease also falls. Except at very high pressures, however, the viscosity of a liquid is independent of pressure.

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