The cement concrete is a mixture of cement, sand, pebbles or crushed rock and water, which, when placed in the skeleton of forms and allowed to cure, becomes hard like a stone.
The cement concrete has attained the status of a major building material in all branches of modern construction because of the following reasons:
(i) It can be readily moulded into durable structural items of various sizes and shapes at practically no considerable labour expenditure.
(ii) It is possible to control the properties of cement concrete within a wide range by using appropriate ingredients and by applying special processing techniques – mechanical, chemical and physical.
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(iii) It is possible to mechanise completely its preparation and placing processes.
(iv) It possesses adequate plasticity for the mechanical working.
Properties of Cement Concrete:
The cement concrete possesses the following important properties:
(i) It has a high compressive strength.
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(ii) It is free from corrosion and there is no appreciable effect of atmospheric agents on it.
(iii) It hardens with age and the process of hardening continues for a long time after the concrete has attained sufficient strength. It is this property of cement concrete which gives it a distinct place among the building materials.
(iv) It is proved to be more economical than steel. This is due to the fact that sand and pebbles or crushed rock, forming the bulk of cement concrete, to the extent of about 80 to 90%, are usually available at moderate cost. The formwork, which is of steel or timber, can be used over and over again or for other purposes after it is removed.
(v) It binds rapidly with steel and as it is weak in tension, the steel reinforcement is placed in cement concrete at suitable places to take up the tensile stresses. This is termed as the Reinforced Cement Concrete or simply R.C.C.
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(vi) Under the following two conditions, it has a tendency to shrink:
(a) There is initial shrinkage of cement concrete which is mainly due to the loss of water through forms, absorption by surfaces of forms, etc.
(b) The shrinkage of cement concrete occurs as it hardens. This tendency of cement concrete can be minimized by proper curing of concrete.
(vii) It has a tendency to be porous. This is due to the presence of voids which are formed during and after its placing.
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The two precautions necessary to avoid this tendency are as follows:
(a) There should be proper grading and consolidating of the aggregates.
(b) The minimum water-cement ratio should be adopted.
(viii) It forms a hard surface, capable of resisting abrasion.
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(ix) It should be remembered that apart from other materials, the concrete comes to the site in the form of raw materials only. Its final strength and quality depend entirely on local conditions and persons handling it. However the materials of which concrete is composed may be subjected to rigid specifications.
Grading of Aggregates:
In order to obtain concrete of denser quality, the fine and coarse aggregates are properly graded. The grading of fine aggregates is expressed in terms of BIS test sieves nos. 480, 240, 120, 60, 30 and 15.
The grading of fine aggregates has a marked effect on the uniformity, workability and finishing qualities of concrete. Table 8-3 shows the grading limits for fine aggregates.
The grading of coarse aggregates may be varied through wider limits than those of sand without appreciable effect on the workability of concrete.
In general, it can be stated that it is difficult to provide satisfactory grading of coarse aggregates than of sand.
Requirements of a Good Aggregate:
Following are the desirable properties or requirements of a good aggregate:
(1) Adhesion
(2) Cementation
(3) Durability
(4) Hardness
(5) Shape
(6) Strength
(7) Toughness
(1) Adhesion:
The aggregates which are to be used for the construction should have less affinity with water as compared with the binding material. If this quality is absent in the aggregate, it will lead to the separation of bituminous or cement coating in the presence of water.
(2) Cementation:
The binding quality of the aggregate depends on its ability to form its own binding material under different loading so as to make the rough broken stone pieces grip together to resist displacement.
(3) Durability:
The durability of an aggregate indicates its resistance to the action of weather and is largely dependent upon its petrological composition. The metal is subjected to the oxidizing influence of air and rain water. It is therefore desirable that the aggregate should possess sufficient soundness to resist the action of weather and age so that the life of the structure made with it may be prolonged.
(4) Hardness:
The aggregates should be reasonably hard to offer resistance to the actions of abrasion and attrition. The aggregates are always subjected to the constant rubbing action. It is known as abrasion and it will be increased due to the presence of abrasive material like sand between the exposed top surface and the tyres of moving vehicles.
The abrasive action is very severe for roads which are used by the steel tyred vehicles. The mutual rubbing of stones is known as attrition and it may also cause a little wear in the aggregates.
(5) Shape:
The shapes of aggregates may be rounded, cubical, angular, flaky or elongated. The flaky and elongated particles possess less strength and durability and their use in the construction should be avoided as far as possible. The rounded particles are preferred in cement concrete construction.
But they are unsuitable in W.B.M. construction, bituminous construction and in granular base course because their stability due to interlocking is less. The angular particles are preferred in such types of construction.
(6) Strength:
The aggregates should be sufficiently strong to withstand the stresses developed due to the wheel loads of the traffic. This property is especially desirable for the road aggregates which are to be used in top layers of the pavements. Thus, the wearing course of road should be composed of aggregates which possess enough strength in addition to enough resistance to crushing.
(7) Toughness:
The toughness of an aggregate is that property which enables the aggregate to resist fracture when struck with a hammer and it is necessary in a metal to withstand the impact blows caused by traffic. The magnitude of impact is governed by the roughness of surface, speed of the vehicle and other vehicular characteristics. It is desirable that the aggregate is reasonably tough.
Water-Cement Ratio:
The water in concrete has to perform the following two functions:
(i) The water enters into chemical action with cement and this action causes the setting and hardening of concrete.
(ii) The water lubricates the aggregates and it facilitates the passage of cement through voids of aggregates. This means that water makes the concrete workable.
It is found theoretically that water required for these two functions is about 0.50 to 0.60 times the weight of cement. This ratio of the amount of water to the amount of cement by weight is termed as the water-cement ratio and the strength and quality of concrete primarily depend upon this ratio.
The quantity of water is usually expressed in litres per bag of cement and hence the water-cement ratio reduces to the quantity of water required in litres per kg of cement as 1 litre of water weighs 1 kg. For instance, if water required for 1 bag of cement is 30 litres, the water-cement ratio is equal to 30/50 = 0.60.
The important points to be observed in connection with the water-cement ratio are as follows:
(i) The minimum quantity of water should be used to have reasonable degree of workability. The excess water occupies space in concrete and on evaporation, the voids are created in concrete. Thus the excess water affects considerably the strength and durability of concrete. In general, it may be stated that addition of one extra litre of water to the concrete of one bag of cement will reduce its strength by about 1.47 N/mm2 In other words, the strength of concrete is inversely proportional to the water-cement ratio.
(ii) The water-cement ratio for structures which are exposed to weather should be carefully decided. For instance, for structures which are regularly wetting and drying, the water-cement ratio by weight should be 0.45 and 0.55 for thin sections and mass concrete respectively. For structures which are continuously under water, the water-cement ratio by weight should be 0.55 and 0.65 for thin sections and mass concrete respectively.
(iii) Some rules-of-thumb are developed for deciding the quantity of water in concrete. The two such rules are mentioned below. The rules are for ordinary concrete and they assume that the materials are non-absorbent and dry.
(a) Weight of water = 28% of the weight of the cement + 4% of the weight of total aggregate.
(b) Weight of water = 30% of the weight of the cement + 5% of the weight of total aggregate.
Table 8-8 shows the strength of concrete with various water-cement ratios.
Workability:
The term workability is used to describe the ease or difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity. However this gives a very loose description of this vital property of concrete which also depends on the means of compaction available.
For instance, the workability suitable for mass concrete is not necessarily sufficient for thin, inaccessible or heavily reinforced sections. The compaction is achieved either by ramming or by vibrating.
The workability, as a physical property of concrete alone irrespective of a particular type of construction, can be defined as the amount of useful internal work necessary to produce full compaction.
If the concrete mixture is too wet, the coarse aggregates settle at the bottom of concrete mass and the resulting concrete becomes of non-uniform composition. On the other hand, if the concrete mixture is too dry, it will be difficult to handle and place it in position. Both these conflicting conditions should be correlated by proportioning carefully various components of concrete mixture.
The important facts in connection with workability are as follows:
(i) If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.
(ii) If the strength of concrete is not to be affected, the degree of workability can be obtained:
(a) By slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet; and
(b) By adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry.
(iii) A concrete mixture for one work may prove to be too stiff or too wet for another work. For instance, the stiff concrete mixture will be required in case of vibrated concrete work while wet concrete mixture will be required for thin sections containing reinforcing bars.
(iv) The workability of concrete is affected mainly by water content, water- cement ratio and aggregate-cement ratio.
(v) The workability of concrete is also affected by the grading, shape, texture and maximum size of the coarse aggregates to be used in the mixture.
In order to measure the workability of concrete mixture, the various tests are developed. The tests such as flow test and compacting test are used in great extent in laboratory. The slump test, which is commonly used in the field, is briefly described.
It should however be remembered that numerous attempts have been made to correlate workability with some easily determinable physical measurement. But none of these tests is fully satisfactory although they may provide useful information within a range of variation in workability. At the same time, the slump test does not measure the workability of concrete. It is simply useful in detecting variations in the uniformity of a mix of given nominal proportions.
Slump Test:
The standard slump cone, as shown in fig. 8-6, is placed on the ground. The operator holds the cone firmly by standing on the foot pieces.
The cone is filled with about one-fourth portion and then rammed with a rod which is provided with bullet nose at the lower end.
The diameter of the rod is 16 mm and its length is 60 mm. The strokes to be given for ramming vary from 20 to 30.
The remaining portion of the cone is filled in with similar layers and then the top of concrete surface is struck off so that the cone is completely full of concrete. The cone is then gradually raised vertically and removed.
The concrete is allowed to subside and then the height of concrete is measured. The slump of concrete is obtained by deducting height of concrete after subsidence from 30 cm.
Table 8-9 shows the recommended slumps of concrete for various types of concrete.
Table 8-10 shows the classification of concrete-mixes on the basis of slump.
Following are the advantages of slump test:
(i) It grants the facility to easily detect the difference in water content of successive batches of concrete of the same identical mix.
(ii) The apparatus is cheap, portable and convenient to be used at site.
Following are the limitations of slump test:
(i) As such, there is no direct relationship between the workability and the value of slump.
(ii) It is not suitable for a concrete in which maximum size of the aggregate exceeds 40 mm.
(iii) There are chances of many shapes of slump to occur and it is difficult to decide which is the correct value.
(iv) The slump occurs only in case of plastic mixes. It does not occur in case of dry mixes.
Estimating Yield of Concrete:
The amount of concrete formed from a concrete mix depends on various factors such as water-cement ratio, size of aggregates, compaction, etc. But a rule-of-thumb, as given below, may be used to find out the approximate yield of concrete from a given concrete mix.
If the proportion of concrete is a:b:c i.e., if a parts of cement, b parts of sand and c parts of coarse aggregates are mixed by volume, the resulting concrete will have a volume of 2/3 (a + b + c).
The yield of concrete can be determined more accurately by considering the absolute volumes of various components of concrete plus the volume of entrapped air. In well compacted concrete, the volume of entrapped air is less than 1 per cent and therefore, it can be neglected.
Let w, a, b and c be absolute volumes of water, cement, fine aggregate and coarse aggregate respectively. Then, w + a + b + c = 1. The value of absolute volume can be obtained by the relation –
Importance of Bulking of Sand:
The important facts in connection with the bulking of sand are as follows:
(i) When moisture content is increased by adding more water, the sand particles pack near each other and the amount of bulking of sand is decreased. Thus, the dry sand and the sand completely flooded with water have practically the same volume.
(ii) The coarse aggregate is little affected by the moisture content.
(iii) One of the reasons of adopting proportioning by weight is the bulking of sand as proportioning by weight avoids the difficulty due to the bulking of sand.
(iv) The bulking of sand should be taken into account when volumetric proportioning of the aggregates is adopted. Otherwise, less quantity of concrete per bag of cement will be produced, which naturally will increase the cost of concrete. Also, there will be less quantity of fine aggregate in the concrete mix which may make the concrete difficult to place.
Let the bulking of sand be 25%. Then, if the concrete is of proportion 1.2:4, the actual volume of sand to be used will be 1.25 x 2 = 2.50 instead of 2 per unit volume of cement. If this correction is not applied, the actual dry sand in the concrete will be 1/1.25 x 2 = 1.60 instead of 2 per unit volume cement.
The proportion of concrete will then be 1:1.60:4. This indicates that less quantity of concrete will be produced and in most of the cases, there will not be enough quantity of fine aggregate to give a workable mix.
Mixing the Materials of Concrete:
The process of rolling, folding and spreading of particles is known as the mixing of concrete.
The materials of concrete should be mixed thoroughly so that there is uniform distribution of materials in the mass of concrete. The thorough mixing also ensures that cement water paste completely covers the surfaces of aggregates. The mixing of materials of concrete can be done either with hand or with the help of a machine.
(1) Hand Mixing:
For hand mixing, the materials are stacked on a water-tight platform, which may be either of wood, brick or steel.
The materials should be thoroughly mixed, at least three times, in dry condition before water is added. The prepared mix should be consumed in 30 minutes after adding water. The mixing by hand is allowed in case of small works or unimportant works where small quantity of concrete is required. For important works, if hand mixing is to be adopted, it is advisable to use 10 per cent more cement than specified.
(2) Machine Mixing:
For machine mixing, all the materials of concrete including water are collected in a revolving drum and then the drum is rotated for a certain period. The resulting mix is then taken out of the drum.
The features of machine mixing are as follows:
(i) It is found that mixing the materials of concrete with the help of machines is more efficient and it produces concrete of better quality in a short time.
(ii) The mixers of various types and capacities are available in the market. They may either be of tilting type or non-tilting type. They are generally provided with power-operated loading hoppers. For small works, a mixer capable of producing concrete of one bag of cement, is used. For works such as roads, aerodromes, dams, etc., special types of mixers are used. Fig. 8-7 shows a typical concrete mixer.
(iii) The water should enter the mixer at the same time or before the other materials are placed. This ensures even distribution of water.
(iv) The concrete mixer should be thoroughly washed and cleaned after use. If this precaution is not taken, the cakes of hardened concrete will be formed inside the mixer. These cakes are not only difficult to remove at a later stage, but they considerably affect the efficiency of the mixer.
(v) The inside portion of the mixer should be inspected carefully at regular intervals. The damaged or broken blades should be replaced.
(vi) The time of mixing the materials in the mixer and the speed of the mixer are very important factors in deciding the strength of concrete which is formed. The mixing time should be at least one minute and preferably two minutes. The mixer should be rotated at a speed as recommended by the manufacturers of the mixer.
(vii) The concrete discharged by the mixer should be consumed within 30 minutes.
Transporting and Placing of Concrete:
The concrete, as it comes out of the mixer or as it is ready for use on the platform, is to be transported and placed on the formwork. The type of equipment to be used for transport of concrete depends on the nature of work, height above ground level and distance between the points of preparation and placing of concrete.
For ordinary building works, the human ladder is formed and concrete is conveyed in pans from hand to hand. For important works, the various mechanical devices such as dumpers, truck mixers, buckets, chutes, belt conveyors, pumps, hoist, etc. may be used.
The two important precautions necessary in the transportation of concrete are as follows:
(i) The concrete should be transported in such a way that there is no segregation of the aggregates.
(ii) Under no circumstances, the water should be added to the concrete during its passage from mixer to the formwork.
The precautions to be taken during the placing of concrete are as follows:
(i) The formwork or the surface which is to receive the fresh concrete should be properly cleaned prepared and well-watered.
(ii) It is desirable to deposit concrete as near as practicable to its final position.
(iii) The large quantities of concrete should not be deposited at a time. Otherwise the concrete will start to flow along the formwork and consequently the resulting concrete will not have uniform composition.
(iv) The concrete should be dropped vertically from a reasonable height. For vertical laying of concrete, care should be taken to use stiff mix. Otherwise the bleeding of concrete through cracks in forms will take place. The term bleeding is used to mean the diffusion or running of concrete through formwork.
(v) The concrete should be deposited in horizontal layers of about 150 mm height. For mass concrete, the layers may be of 400 mm to 500 mm height. The accumulation of excess water in the upper layers is known as the laitance and it should be prevented by using shallow layers with stiff mix or by putting dry batches of concrete to absorb the excess water.
(vi) As far as possible, the concrete should be placed in single thickness. In case of deep sections, the concrete should be placed in successive horizontal layers and proper care should be taken to develop enough bond between successive layers.
(vii) The concrete should be thoroughly worked around the reinforcement and tapped in such a way that no honeycombed surface appears on removal of the formwork. The term honeycomb is used to mean comb or mass of waxy cells formed by bees in which they store their honey. Hence, if this precaution is not taken, the concrete surface so formed would have a honeycomb like surface.
(viii) The concrete should be placed on the formwork as soon as possible. But in no case, it should be placed after 30 minutes of its preparation.
(ix) During placing, it should be seen that all edges and corners of concrete surface remain unbroken, sharp and straight in line.
(x) The placing of concrete should be carried out uninterrupted between predetermined construction joints.
Consolidation of Concrete:
The term consolidation of concrete is used to mean the compaction between aggregate and aggregate; between aggregate and reinforcement; and between aggregate and forms.
The main aim of consolidation of concrete is to eliminate air bubbles and thus to give maximum density to the concrete. An intimate contact between concrete and reinforcement is ensured by proper consolidation.
The importance of consolidation of concrete can be seen from the fact that a presence of 5% of voids reduces 30% strength of concrete.
The difference between voids and pores may be noted. The voids are the gaps between two individual particles. The pores represent the openings within the individual particles. The process of consolidation of concrete can be carried out either with hand or with the help of vibrators.
For unimportant works, the consolidation of concrete is carried out by hand methods which include ramming, tamping, spading and slicing with suitable tools.
The hand methods require use of a fairly wet concrete. It should however be remembered that wherever feasible, the hand compaction should be preferred because the use of vibrator may lead to the segregation of the aggregates. As a matter of fact, the concrete mixes which can be hand-compacted should not be compacted by the use of vibrators.
Vibrators Used to Compact Concrete:
These are the mechanical devices which are used to compact concrete in the formwork.
The advantages of vibrators over hand methods are as follows:
(i) It is possible by means of vibrators to make a harsh and stiff concrete mix, with a slump of about 40 mm or less, workable.
(ii) The quality of concrete can be improved by use of vibrators as less water will be required or in other way, economy can be achieved by adopting a leaner mix when vibrators are used.
(iii) The use of vibrators results in the reduction of consolidation time. Hence the vibrators are used where the rapid progress of work is of great importance.
(iv) With the help of vibrators, it is possible to deposit concrete in small openings or places where it will be difficult to deposit concrete by hand methods.
Following are the four types of vibrators:
(1) Internal or immersion vibrators
(2) Surface vibrators
(3) Form or shutter vibrators
(4) Vibrating tables.
(1) Internal or Immersion Vibrators:
These vibrators consist of a steel tube which is inserted in fresh concrete. This steel tube is called the poker and it is connected to an electric motor or a petrol engine through a flexible tube. They are available in sizes varying from 40 mm to 100 mm diameters and the size is decided by keeping in mind the spacing between reinforcing bars in concrete. The frequency of vibration is about 3000 to 6000 r.p.m.
The poker vibrates while it is being inserted. The internal vibrators should be inserted and withdrawn slowly and they should be operated continuously while they are being withdrawn. Otherwise holes will be formed inside the concrete.
The vibrator can be placed vertically or at a slight inclination not exceeding 10° to the vertical with a view to avoid flow of concrete due to vibration into the mould and consequent scope of segregation. Hence skilled and experienced men should handle internal vibrators. These vibrators are more efficient than other types of vibrators and hence they are most commonly used.
(2) Surface Vibrators:
These vibrators are mounted on platform or screeds. They are used to finish concrete surfaces such as bridge floors, road slabs, station platform, etc. These vibrators are found to be more effective for compacting very dry concrete mixes because the vibration acts in the same direction of gravity and the concrete is compacted in a confined zone.
These vibrators also cause movement of fine material to the top and it aids in finishing operations. However the movement of excess fine material at top will not be desirable for plastic mixes as the wearing resistance of such fine material is very low.
(3) Form or Shutter Vibrators:
These vibrators are attached to the formwork and external centering of walls, columns, etc. The vibrating action is conveyed to the concrete through the formwork during transmission of vibrations. Hence, they are not generally used. But they are very much helpful for concrete sections which are too thin for the use of internal vibrators.
These vibrators require more power because of loss of some power in vibrating the rigid shutters. They are also heavy and hence they cannot be clamped at as many points as possible for uniform compaction of concrete. The compaction by these vibrators is found to be effective only upto a distance of about 450 mm from the face of the formwork.
(4) Vibrating Tables:
These are in the form of a rigidly built steel platform mounted on flexible springs and they are operated by electromagnetic action or electric motors. They are found to be very effective in compacting stiff and harsh concrete mixes and hence they are invariably used in the preparation of pre-cast structural products in factories and test specimens in laboratories.
The tables are vibrated either mechanically or by placing the springs under the supports of tables. The frequency of vibration varies from 3000 to 7200 vibrations per minute. The two parameters of vibrations are frequency and time and they are related as follows –
It means that the frequency is inversely proportional to the time of vibration. In other words, if frequency is more, the consolidation of concrete will be achieved in less time and vice versa.