Batching of Concrete

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

1) Volume Batching

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

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

Fig.1 Gauge Box

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

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

2) Weigh Batching

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

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

Fig. 2 Weigh Batcher

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

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

Components of a Batching Plant

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

Measurement of Water

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

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

Fig. 3 Cans for Measuring Water

(Ref : Concrete Technology – M S Shetty)

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Stages of Concrete Production – Brief Note

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

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

1) Proportioning/Batching

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

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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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Characteristics of Concrete

Advantages of Concrete

a) Economical

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

b) Ambient Temperature-Hardened Material

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

c) Ability to be Cast

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

d) Energy Efficient

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

e) Excellent Resistance to Water

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

f) High-Temperature Resistance

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

g) Ability to Consume Waste

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

h) Ability to Work with Reinforcing Steel

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

i) Less Maintenance Required

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

Limitations of Concrete

a) Quasi-Brittle Failure Mode

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

Fig.1 Three Failure Modes of Materials

b) Low Tensile Strength

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

c) Low Toughness (Ductility)

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

Fig.2 Toughness of Steel and Concrete

d) Low Specific Strength (Strength/Density Ratio)

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

e) Formwork is Needed

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

f) Long Curing Time

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

g) Working with Cracks

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

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Flow Table Test

This is a laboratory test, which gives an indication of the quality of concrete with respect to consistency, cohesiveness and the proneness to segregation. This test is as per IS: 5512 – 1983. In this test, a standard mass of concrete is subjected to jolting. The spread or the flow of the concrete is measured and this flow is related to workability.

Flow Table Apparatus

The flow table top is constructed from a flat metal of minimum thickness 1.5 mm. The top is in plan 700 mm x 700 mm. The centre of the table is marked with a cross, the lines which run parallel to and out to the edges of the plate and with a central circle 200 mm in diameter. The front of the flow table top is provided with a lifting handle and the total mass of the flow table top is about 16 ± 1 kg. The flow table top is hinged to a base frame using externally mounted hinges in such a way that no aggregate can become trapped easily between the hinges or hinged surfaces.

Fig.1 Flow Table Apparatus

The front of the base frame shall extend a minimum 120 mm beyond the flow table top in order to provide a top board. An upper stop similar to that is provided on each side of the table so that the lower front edge of the table can only be lifted 40 ± 1mm. The lower front edge of the flow table top is provided with two hard rigid stops which transfer the load to the base frame. The base frame is so constructed that this load is then transferred directly to the surface on which the flow table is placed so that there is minimal tendency for the flow table top to bounce when allowed to fall.

Accessory Apparatus

Mould

The mould is made of metal readily not attacked by cement paste or liable to rust and of minimum thickness 1.5 mm. The interior of the mould is smooth and free from projections, such as protruding rivets and is free from dents. The mould shall be in the form of a hollow frustum of a cone having the internal dimensions as shown in Fig. 2. The base and the top is open and parallel to each other and at right angles to the axis of the cone. The mould is provided with two metal foot pieces at the bottom and two handles above them.

Fig. 2 Mould for Flow Test

Tamping Bar

The tamping bar is made of a suitable hardwood.

The table top is cleaned of all gritty material and is wetted. The mould is kept on the centre of the table, firmly held and is filled in two layers. Each layer is rodded 25 times with a tamping rod 1.6 cm in diameter and 61 cm long rounded at the lower tamping end. After the top layer is rodded evenly, the excess of concrete which has overflowed the mould is removed. The mould is lifted vertically upward and the concrete stands on its own without support. The table is then raised and dropped 12.5 mm 15 times in about 15 seconds. The diameter of the spread concrete is measured in about 6 directions to the nearest 5 mm and the average spread is noted. The flow of concrete is the percentage increase in the average diameter of the spread concrete over the base diameter of the mould.

The value could range anything from 0 to 150 per cent. A close look at the pattern of spread of concrete can also give a good indication of the characteristics of concrete such as tendency for segregation. As well as getting an accurate measurement of the workability of the concrete, the flow test gives an indication of the cohesion. A mix that is prone to segregation will produce a noncircular pool of concrete. Cement paste may be seen separating from the aggregate. If the mix is prone to bleeding, a ring of clear water may form after a few minutes.

Procedure

1. The table is made level and properly supported. Before commencing the test, the table-top and inner surface of the mould is wiped with a damp cloth.
2. The 700 mm square flow table is hinged to a rigid base, provided with a stop that allows the far end to be raised by 40 mm.
3. A cone, similar to that used for slump testing but truncated, is filled with concrete in two layers.
4. Each layer is tamped 10 times with a special wooden bar and the concrete of the upper layer finished off level with the top of the cone. Any excess is cleaned off the outside of the cone.
5. The cone is then raised allowing the concrete to flow out and spread out a little on the flow table.
6. The table top is then raised until it meets the stop and allowed to drop freely 15 times.
7. This causes the concrete to spread further, in a roughly circular shape.
8. The diameter of the concrete spread shall then be measured in two directions, parallel to the table edges.
9. The flow diameter is the average of the maximum diameter of the pool of concrete and the diameter at right angles.

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Vee-Bee Consistometer Test

This is a good laboratory test to measure indirectly the workability of concrete as per IS: 1199– 1959. The test equipment, which was developed by Swedish Engineer V. Bahrner, is shown in Fig.1. It consists of a vibrating table, a cylindrical pan, a slump cone and a glass or plastic disk attached to a free-moving rod, which serves as a reference endpoint. The cone is placed in the pan. After it is filled with concrete and any excessive concrete is struck off, the cone is removed. Then, the disk is brought into a position on top of the concrete cone and the vibrating table is set in motion and simultaneously a stop watch started. The vibration is continued till such a time as the conical shape of the concrete disappears and the concrete assumes a cylindrical shape. This can be judged by observing the glass disc from the top for disappearance of transparency. Immediately when the concrete fully assumes a cylindrical shape, the stop watch is switched off. The time required for the shape of concrete to change from slump cone shape to cylindrical shape in seconds is known as Vee Bee Degree. This method is very suitable for very dry concrete whose slump value cannot be measured by slump test but the vibration is too vigorous for concrete with a slump greater than about 50 mm.

The Vee bee test is a good laboratory test, particularly for very dry mixes. This is in contrast to the compacting factor test where error may be introduced by the tendency of some dry mixes to stick in the hoppers. The Vee bee test also has the additional advantage that the treatment of concrete during the test is comparatively closely related to the method of placing in practice. Moreover, the cohesiveness of concrete can be easily distinguished by Vee bee test through the observation of distribution of the coarse aggregate after vibration. Table 1 shows the relationship between workability and Vee Bee values.

Fig.1 Vee bee Test Setup

Procedure

1. Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water.
2. Pour the concrete into the slump cone with the help of the funnel fitted to the stand.
3. Remove the slump mould and rotate the stand so that transparent disc touches the top of the concrete.
4. Start the vibrator on which cylindrical container is placed.
5. Due to vibrating action, the concrete starts remoulding and occupying the cylindrical container. Continue vibrating the cylinder till concrete surface becomes horizontal.
6. The time required for complete remoulding in seconds is the required measure of the workability and it is expressed as number of Vee-bee seconds.

Table 1 Relationship between Workability and Vee Bee Test Results

Workability Description	Vee-bee Time in Seconds
Extremely dry	32 – 18
Very stiff	18 – 10
Stiff	10 – 5
Stiff plastic	5 – 3
Plastic	3 – 0
Flowing	-

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Kelly Ball Test

This is a simple field test consisting of the measurement of the indentation made by 15 cm diameter metal hemisphere weighing 13.6 kg connected to a handle with a ruler, when freely placed on fresh concrete. ASTM C360 covers the Kelly ball penetration test. The hammer is fixed on a box container through a pin. When taking measurements, the box is placed on the top of the concrete to be tested with the surface of the hammer touching the concrete. When the pin is removed, the hammer will sink into the fresh concrete by its own weight. The depth of the hammer penetration can be read from the ruler and is used as an index of workability. A concrete with higher consistency leads to a deeper ball penetration. The penetration test is usually very quick and can be done on site, right in the formwork, provided it is wide enough. The ratio of slump value to penetration depth is from 1.3 to 2.0.

The test has been devised by Kelly and hence known as Kelly Ball Test. This has not been covered by Indian Standards Specification. The advantages of this test is that it can be performed on the concrete placed in site and it is claimed that this test can be performed faster with a greater precision than slump test. The disadvantages are that it requires a large sample of concrete and it cannot be used when the concrete is placed in thin section. The minimum depth of concrete must be at least 20 cm and the minimum distance from the centre of the ball to nearest edge of the concrete is 23 cm. The surface of the concrete is struck off level, avoiding excess working, the ball is lowered gradually on the surface of the concrete. The depth of penetration is read immediately on the stem to the nearest 6 mm. The test can be performed in about 15 seconds and it gives much more consistent results than Slump Test.

Fig. 1 Kelly Ball Apparatus

Test Procedure

• The concrete which is presumed to be tested should be poured into a container such as a buggy or actually in the form which should be up to a depth of 200mm (20cm). Then once the concrete is poured the top surface should be levelled.
• On the surface of the concrete the Kelly ball apparatus should be placed. The handles of the hemisphere (ball) should be placed in such a way that the frame touches the surface of the concrete. Away from the containers end the minimum lateral dimension of frame should be around 230 mm (23 cm).
• Then once it is done, the handle should be released slowly and the ball should be allowed to penetrate through the concrete by its own weight.
• Once the ball (hemisphere) is relieved, the penetration depth of the ball will be signified on the scale.
• Then the reading of the graduated scale should be noted down. (in which the penetration is showed).

The same procedure should be repeated at different portions for at least three times in the container and the then the average values of these readings should be taken down.

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Compaction Factor Test

The compacting factor test is designed primarily for use in the laboratory but it can also be used in the field. It is more precise and sensitive than the slump test and is particularly useful for concrete mixes of very low workability as are normally used when concrete is to be compacted by vibration. Such dry concrete is insensitive to slump test. The compacting factor test has been developed at the Road Research Laboratory U.K. and it is claimed that it is one of the most efficient tests for measuring the workability of concrete. This test works on the principle of determining the degree of compaction achieved by a standard amount of work done by allowing the concrete to fall through a standard height.

The degree of compaction, called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to density of same concrete fully compacted. The sample of concrete to be tested is placed in the upper hopper up to the brim. The trap-door is opened so that the concrete falls into the lower hopper. Then the trap-door of the lower hopper is opened and the concrete is allowed to fall into the cylinder. In the case of a dry-mix, it is likely that the concrete may not fall on opening the trap-door. In such a case, a slight poking by a rod may be required to set the concrete in motion. The excess concrete remaining above the top level of the cylinder is then cut off with the help of plane blades supplied with the apparatus. The outside of the cylinder is wiped clean. The concrete is filled up exactly up to the top level of the cylinder. It is weighed to the nearest 10 grams. This weight is known as ―Weight of partially compacted concrete.

The cylinder is emptied and then refilled with the concrete from the same sample in layers approximately 5 cm deep. The layers are heavily rammed or preferably vibrated so as to obtain full compaction. The top surface of the fully compacted concrete is then carefully struck off level with the top of the cylinder and weighed to the nearest 10 gm. This weight is known as Weight of fully compacted concrete. Usually, the range of compaction factor is from 0.78 to 0.95 and concrete with high fluidity has a higher compaction factor.

Theory

This test is adopted to determine workability of concrete where nominal size of aggregate does not exceed 40 mm and it is as per IS: 1199 – 1959. It is based on the definition, that workability is that property of concrete, which determines the amount of work required to produce full compaction. The test consists essentially of applying a standard amount of work to standard quantity of concrete and measuring the resulting compaction. The compaction factor is defined as the ratio of the weight of partially compacted concrete to the weight of fully compacted concrete. It shall be stated to the nearest second decimal place. The relationship between degree of workability and compaction factor are given below.

Table 1 Relationship between Degree of workability and Compaction Factor

Degree of workability	Compaction Factor
Very Low	0.75- 0.80
Low	0.80- 0.85
Medium	0.85- 0.92
High	> 0.92

Compaction factor test is more sensitive and precise than slump test and is particularly useful for concrete mixes of very low workability. Such concrete may show zero to very low slump value. Also, compaction factor (C.F.) test is able to indicate small variations in workability over a wide range. Compaction factor test proves the fact that with increase in the size of coarse aggregate, the workability will decrease. However, compaction factor test has certain limitations. When maximum size of aggregate is large as compare with mean particle size; the drop into bottom container will produce segregation and give unreliable comparison with other mixes of smaller maximum aggregate sizes. Moreover, the method of introducing concrete into mould bears no relationship to any of the more common methods of placing and compacting high concrete.

Compaction Factor Test Apparatus

Compaction factor test apparatus consists of two conical hoppers, A and B, mounted vertically above a cylindrical mould C. The upper hopper A has internal dimensions as: top diameter 250 mm; bottom diameter 125 mm and height 225 mm. The lower hopper B has internal dimensions as: top diameter 225 mm; bottom diameter 125 mm and height 225 mm. The cylinder has internal dimensions as: 150 mm diameter and 300 mm height. The distances between bottom of upper hopper and top of lower hopper, and bottom of lower hopper and top of cylinder are 200 mm in each case. The lower ends of the hoppers are fitted with quick release flap doors. The hoppers and cylinders are rigid in construction and rigidly mounted on a frame. These hoppers and cylinder are rigid and easily detachable from the frame. Other instruments used for this test consists of trowels, hand scoop (15.2 cm long), a rod of steel or other suitable material (1.6 cm diameter, 61 cm long rounded at one end) and a balance.

Fig.1 Compaction Factor Test Apparatus

Procedure

1. Prepare a concrete mix for testing workability. 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.
2. Place the concrete into the upper hopper up to its brim.
3. Open the trapdoor of the upper hopper. The concrete will fall into the lower hopper.
4. Open the trapdoor of the lower hopper, so that concrete falls into the cylinder below.
5. Remove the excess concrete above the level of the top of the cylinder; clean the outside of the cylinder.
6. Weigh the concrete in the cylinder. This weight of concrete is the "weight of partially compacted concrete", (W1).
7. Empty the cylinder and refill with concrete in layers, compacting each layer well (or the same may be vibrated for full compaction). Top surface may be struck off level.
8. Find cut weight of the concrete in the fully compacted state. This weight is the “Weight of fully compacted concrete" (W2).

The degree of compaction, called the compacting factor is measured by the density ratio i.e., the ratio of the density actually achieved in the test to density of same concrete fully compacted.

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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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