2.6 CEMENT CONCRETE “i dy = Ceaser cotta eiitae esha ay crushed Ged and ater -ment, Sand) ld and water which when placed in the skeleton of forms and alist becomes Ward such a3 TRE. Concrete has attained the status of a major building material in all branches of modern construction and hence itis necessary to know the properties and uses of concrete. 2.6.1 Properties of Concrete It has a high compressive strength and its strength depends on the proportion in which cement, sand, stones and water are riixed. 2. Itis free from corrosion and there is no appreciable effect of atmospheric agents on it. 3. It hardens withvage ang the process of hardening continues for a long time after the concrete has attained sufficient strength. 4, As it is weak in tension, steel einforcement is placed in it to take up the tensile stresses. This is termed as ‘Reinforced Cement Concrete’. a 5. Itshrinks in the initial stage due to loss of water through forms. The shrinkage of cement concrete occurs as it hardens. 6. It has a tendency to be porous. This is due to the presence of voids which are formed during and after its placing. a 7. It forms a hard surface, capable of resisting abrasion.) 2 Uses of Concrete Concrete can be made impermeable by usind hydrophobic cemen)) This is used for “Tene construction of RCC flat-xaof slabs._ 2. Coloured concrete is used for ornamental finishes in buildings, park lanes, separating lines of road surfaces, underground pedestrian crossings, ett. 3. Light weight concrete is used in multi-storeyed constructions.© 2 Pi 4 Basic Civil Engineering (No-fine3)zoncrete is one in which sand is eliminated. This can be used for cst, Situ external load bearing walls of single arid multi-storey houses, retaining walls damp-proofing material, et 5. Concrete is mainly used in floors, roof slabs, columns, beams, lintels, foundation, and in precast constructions 6. This used in massive structures, such as dams and bridges. 7. Concrete is used in the construction of roads, runways, playgrounds, water tanks and chimneys. 8. Ibis ased in the construction of sleepers in railways. estressed conerel) isa relatively new type of concrete which is used in many ‘onstructions particularly in the construction of bridges. 10. Concrete trusses)are also used in factory constructions. 11. Concrete is used in the construction of bunkers, silos; etc. gp finds place nthe consracon of mice reactors esse of igh sing 1B capacity for the radioactivity. ae ‘conomical shell construction are possible with the use of concrete. 9.6.3 Reinforced Concrete ain concrete is very weak in tension and cannot be used in the construction of lintels, roof slabs, beams, etc. in which the bottom fibres of them are subjected to tensile stresses. Ai igure 2.1 explains how a loaded beam or a slab is subjected to a flexural action when it is laid over an opening known as span. The top portion is compressed while the bottom P ortion is stretched. As concrete withstands compression but not tension, steel rods are embedded in the bottom portion to withstand the tension. A combination of concrete and steel is known as reinforced cement concrete and is widely used in various situations. —— span + Span (2) Apiain concrote member (b)Aroetrced concreta member suteced tenure sitjected o toxure 2.1 flexure actionSe eens ania, aa Reinforcing bars are avai tilable from mm diamet et 2.6.4 Advantages of Reinforced Concrete 1. Reinforced concrete is a versatile bui members of any shape,” “stile building material and can be used for casting . Ithas good resistance to fire, te temperature and weathering actions. . RCC construction iseasy and fask a ape component materials used for preparing RCC are easily available Monolithic construction is possible with the us increases the stabilit and rigidity of the stact geese With the se of RCC. This increases the stability 6. RCCis tough and durable. 7. Maintenance of RCC construction is very cheap. 8. With proper cover, RCC can be made free from rusting and corrosion. 2.6.5 Types of Concrete 1. Light-weight concrete, One ofthe disadvantages of normal concrete is the high self-weight which has a density of 2200 to 2600 kg/m?. This heavy self-weight causes heavy load and increases the haulage and handling costs. In order to make an economical concrete, attempts were made in the past to reduce the self weight of concrete, As a result the light weight concrete was developed whose density varies from 300-1850 kg/m?, ‘Advantages of light-weight concrete (@) Ithas low density. (©) Ithas low thermal conductivity. (6) Itlowers haulage and handling costs. Types of tigh ight concrete Or Eo eh wea (b) Aerated concrete (©) No-fine concrete in | mineral aggregate by cellular Light-weight aggregate concrete By replacing the usual 7 rorous orightwelght aggregate lightweight nggegte concrete ca be produced ght weight aggregate can be classified into two categories namely natural and artificial light- weight aggregate. Natural light-weight aggregates are (i) Pumice i) Diatomite2.26, Bosie iil Engineering (i (iv) Volcanic cinders (v) Saw dust (i) Rice husk Artificial light-weight aggregates are (i) Artificial cinders (ii) Foamed slag (ii) Bloated clay (iv) Sintered flyash (©) Aerated concrete By introducing gas or air bubbles in mortar, aerated concrete can be Produced. This concrete is a mixture of water, cement and finely crushed sand with air o¢ g2s introducing agents. ‘There are several ways in which aerated concrete can be manufactured. One important way is by the formation of gas or air bubbles using finely powdered metal (usually aluminium powder). Chemical reaction takes place in the concrete and finally large quantity of hydrogen gas is liberated which gives the cellular structure (©) We-Fine concrete By omitting sand fraction from the aggregate, no-fine concrete can be Produced. This concrete is made up of only single-sized aggregate of size passing of 20mm and retained on 10 mum coarse aggregate, cement and water. The single sized aggregate makes a good no-fine concrete, which in addition gives large voids and hence is light in weight Italso offers an architecturally attractive look. Out of the three main groups of light-weight concrete, the light-weight aggregate concrete and aerated concrete are more often used than the no-fine concrete 2. High-density concrete The concrete whose unit weight ranges from about 3360-3840 ig/itrand hit abou 0 Per cent higher than the unit weight of normal conerete is known as high-density concrete The high-density concrete is mainly used in the construction of radioactive shields High-density concrete is made by using such a heavy-weight aggregate whose specific gravity Is more than 35. The aggregates used in this type of concrete should be-clean, strong, inert and relatively free from deleterious material. Normally bare, znagetite, demonite are used to make high-density concrete. To produce high density and high strength concrete, itis necessary to control the water-cement ratio, correct admintte and vibrators for good compaction, 3. Polymer Cancite Air voids and water voids are present in the conventional concrete due to improper compaction, high water-cement ratio and some other causes, Duc to compaction, these voids are founed and the strength ofthe concrete is naturally vedused There are number of methods available to reduce the air voids but none of these maths could really help to reduce the water voids. The impregnation of monomer and subsequent polymerisation is the latest technique adopted to reduce the inherent porosity of the‘ypee of polymer concrete 1, Polymer impregnated ¢ 2. Polymer Cement Concrete 3. Polymer Concrete 4. Partially impregnated ands, ‘The following are the monore n°? Aled polymer concrete caren 1. Methyl melthacryate (MaMa) Maly Wd in polymer concrete 2. Styrene 3. Acrylonitrite 4, tbutyl styrene flexural strength of concrete and gives pene compressive strength, tensile strength, high esistance to sulphate ann eyes conerete a high eeze haw restance and 0 Applications of polymer impregnated concrete 1, Prefabricated structural elements Prestressed concrete Marine works L. Desalination plants Nuclear power plants Sewage works—pipes and disposal works For water proofing of structures, 8. Industrial applications 4, Fibreseinforced concrete Plain concrete possesses a very low tensile strength, limited ductility and little resistance to cracking. Due to its poor tensile strength, internal microcracks are present in concrete which leads to brittle fracture. To improve the tensile strength of concrete one of the method used is that of the conventional reinforced stee! bars and the other way is by introducing fibres in the concrete and thereby increasing the inherent tensile strength of concrete. In order to reduce the microcracks, addition of small, closely spaced and uniformly dispersed fibres are used. These fibres act as crack arrester and substantially improve its static and dynamic properties. This type of concrete is known as Fibre Reinforced Concrete (FRC). Some of the fibres used are steel fibres. polypropylene, nylons, asbestos, coir, glass and carbon. The property of concrete may vary depending upon the type, diameter, length and volume of fibres. ‘Steal fibre is one of the most commonly used fibre. Most of the times round fibres are used The diameter of such fibres may vary from 0.25-0.75 mm, The use of steel fibres may improve the flexural, impact and fatigue strength of concretebasic Cl Engineering 2. LIy, FRC are used in airfield, road pavemeng, Aplications firesenfrcedcancrete NOTE Ceplsive resistant Structres, retract industrial floorings, bridge decks, cans like pipes, boats, beams, staircasy Iinings, ete It can also be used in pre-cast products like Pips set. It can al past pro steps, wall panels, roof panels, manhole covers, 2.6.6 Testing of Fresh and Hardened Concrete is i te resting of fiesh concrete Fresh concrete or plastic concrete is a fres nate 2 Tetig of es nats Spe The oat important property of fesh conc its workability, ane Workabitity The term worksbility is used to describe the ease or difficulty with ae the Concrete is handled, transported and placed between the forms with minimum loss o homogeneity. However, this gives very loose description ofthis vital property of conc which also depends on the means of compaction available. For instance, the workability Suitable for mass concrete isnot necessarily sufficient for thin, inaccessible or heaviy reinforced sections, The compaction is achieved ether by ramming or vibrating, The ‘workability, a8 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 conerete mixture is too wet, the coarse aggregates settle at the bottom of the concrete mass and the resulting concrete has a non-uniform composition. On the other hand, f the concrete mixture iS 100 dry, it will be difficult to handle and place it in position. To correlate these two conflicting conditions proportions of various components ‘of concrete mixture should be carefully decided. The important facts in connection with workability areas follows: 1, Ifmore water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability. | 2. Ifthe strength of concrete is not tobe affected then, the degree of workability can | be obtained in following ways: {@) by slightly changing the proportions of fine and coarse aggregates, in case the | concrete mixture is too wet | (i) by adding a small quantity of water cement paste in the proportion of original © ©" mix, in case the concrete mixture is too dry | 3. A concrete mixture for one work may prove to be too stiff or too wet for another | ‘work. For instance, siffconcrete mixture will be required in case of vibrated concrele ork while wet concrete misture wil be required for thin sections containing | 4, The workability of concrete is affected mainly ._. fatio and aggregate-cement ratio. 5, The workability of concrete is also affected by the grading, shape, texture and maximum size ofthe coarse aggregates used inthe mture, by water content, water—cementG hi In order to measure the ees workal rests such as flow test and compara Of Concrete mixture, various tests are developed. are used m oaean which is commonly used in th remembered that numerous at easily determinable physical me within a range of variation in the same time, the slump test useful in detecting variations Fes 8 Used mostly in laboratory, The sum net, tempts nity described below It should however, be jebeen made to concise wortabiitr three ‘ough they may provide useful information 00m ‘200m (2) The cone filed wih concrete paste () The setled paste ater te cone removed Fig, 22 _ Slump test ‘The standard slump cone, as shown in Fig. 22 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 ofthe cone is filled in with Similar layers and then the top of concrete surface is struck off such thatthe cone is 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 the height of concrete after ‘subsidence from 30 cm. Table .2 shows the recommended slumps of concrete for various types of concrete and Table 2.3 shows the classification of concrete mixes on the basis of slump, 2. Testing of Hardened Concrete 1 ieee ‘Strength It may be defined as the maximum compressive load that can be fee re seme unit are, It has been shown that with special care ne sonal inant canta inka to Beas Toads ashigh as N/a oreven snore: In practi: honveves2.30, Basic Gvil Engineering ‘concrete with compressive strength between 10-50 N/mm* can be easily made on the site for common type of construction. Table 2.2, Recommended slumps of concrete ‘EN. Tye ofeoneree a Concrete for oad constuction 200.40 mn and walls that are horizontal 401050 mm. 1 2 Concrete for tops of curbs, parapets, 3. Concrete fr canal linings Retinn 4 Concrete for arch and side walls of tunnels 90 to 100 mem 5. Normal RCC work wisn 6 Mass concrete 251050 mm 2.__Conerete tobe vibrated 10t025 mm Table 2.3, lassifcation of concrete mixes ‘Samp ‘ature of concrete mix Noslump ‘Stilfand extra stiff mix From 101030mm Poorly mobile nix From 40 to 150mm Mobile mix (Over 180.mm, Cast mix ‘The compressive strength, also called the crushing strength, of concrete is determined by loading axially cube shaped (or cylindrical shaped) specimens made out of the concrete. ‘The tests are carried out 3 days, 7 days and 28 days after the casting of the samples. Itis the 28 days compressive strength which is taken as a standard value for concrete of a particular batch. thas been observed that the compressive (crushing) strength of concrete is influenced by a very large number of factors. The most important of these factors are the following (i) Types of cement The composition, quality and age of the cement used in making concrete influences its strength. Thus, cement that has been stored for considerable time ‘make concrete of lower strength despite all the other factors being the same. Cement with higher proportions of tri-calcium silicates produce concrete that show higher strengths, at least in earlier stages. Similarly, finer the particle size of the cement, higher is the ultimate compressive strength. (i) Nature of aggregates Sand and coarse aggregates are the other two essential components of concrete. A good bond between cement and the aggregate is possible only when the latter have sharp edges, clean surfaces and rough texture. Smooth and rounded aggregates result in comparatively poor bonds. Similarly, the aggregates used in concrete making should have in themselves, good compressive strength. For example, if chalk (very soft limestone) is used in making concrete instead of massive limestone, the resulting concrete will be weak in compressive strength because of the poor strength of the aggregate.G vil En Engineering ond Moterots Water-cement ratio The coy a a) mpres: water-cement ratio (other things bein etenBlN de src er ng APSE engl wh ng comp! fs 'sth on proper compactinn he resulting concrete wil give maximum 1 eft ang sh TESA tach ope cing a to the extent of 40 per cent or (0) Weather conditions The same concy extremely cold, dry and hot, may devetes to incomplete hydration of the cement in the eae (i) Admixtures Certain admixtures a a specific purposes. It hasbeen observ increase the compressive strength, however, affect the compre ‘on water-cement ratio, placed in different weather conditions like evelop different strength values. The cause is related re added to the concrete at the mixing stage for some that cerain admires especialy elm code ~ er admixtures (eg. air entraining agents) sive strength adversely proper conols are not mainsined (i) Methods of preparation Improper mixing of the con. Soya ne Ng eens imucntnip atone sera geen eee es Ae wen prev ce eeoe incompetent worker, however, may spoil the entire wor eing given the concrete mix The vids lt i he concrete on compaction a3 cus hve 8 profound influence on the strength of the concrete. 2. Tensile strength Plain concrete (without steel reinforcement) is quite weak in tensile strength which may vary from 1/8 to 1/20 of the ultimate compressive strength. It is primarily for this reason that steel bars (reinforcement) are introduced into the concrete at the laying stage so as to get a concrete which is very strong in compression as wel as in tension. In plain concrete, tensile strength depends toa great extenton the same factors as the compressive strength does. Tensile strength of concrete becomes an important property when it isto be used in toad making and runways. It is determined by using indirect methods. Th one of auch methods, itis derived from the flexural strength tests. n these test, a beam of contrate is east in standard dimensions depending upon the nominal size of the aggregate, The beam is properly cured and tested after 25 days. Is simply supported Hee meal equally ooded ats one-third span points from both suppor laure The bending moments, obviously, induce compressive stresses at the top and ee Sree tS ottean of beam. The beam fails in tension. Modulus of rupture or Rexural Strengths then calculate by wsing the wal beam formu ven below: faye i—— - where {fe= flexural strength F = maximum applied load L = distance between supports b= breadth ' a =depth i In the second indirect method, called the split cylinder method as shown in Fig. 23, cylinder of specified dimensions is made to fail under tension by applying compressive Toad across the diameter. This is termed as splitting tensile strength. The testing machine is adjusted to distribute the load along the entire length of the cylinder. From the load at failure, tensile strength is calculated using the following relationship. | 2? Sad where fi = splitting tensile strength in N/mm? = maximum applied load in N 1 = length ofthe cylinder (mm) 4 = diameter of the cylinder (mm) | —§ Test samale © unitate pa $2.3 pind esi or te ethe t Git Engineering and Moerais 3 For approximate use, tensile st fits (ube) compressive stanga B of concrete may be taken beeen 10-12 percent 4. Nondestructive tests for concrete ommon requirement of in-siti i Estimation of concrete or member strength is the most ‘methods can be used to provide unfortunately, none of the available (@) Rebound test method. (b) Ultrasonic method (2) Rebound test method febound test equipment and operation rebound test hammer in the late on this test. Figure 2.4 shows the! less than two kg, ‘The spring-controlled hammer mass slides on a plunger within a tubular housing, The plunger rotates against a spring when pressed against a concrete surface and this spring, 'sautomatically released when tensioned, causing the hammer mass to impact against the concrete through the plunger, When the spring controlled mass rebounds it takes with it «rider which slides along a scale and is visible through a small window in the side of the casing. The rider can be held in position on the scale by depressing the locking button. The equipment is very simple to use. The scale reading is known as the rebound number and isan arbitrary measure as it depends on the energy and the mass used. With this number, the compressive strength of concrete can be obtained from the graph attached with the instrument, ASwiss engineer, EmstSchmidt, developed a practicable 1940s for the fist time and modern versions are based. basic features ofa typical type of hammer which weighs Impact Winn aleaso andecale eon $e. 24 Typical rebound hommer B24 typical rebound hommerBasie Civil Engineering 34, Procedure The reading is very sensitive to local variations of the concrete, especial aggregate particles. dees tt aactace ile Grete necessary to take several readings each test location and to find their average between 9 and 25 readings taken over an are, rot exceeding 300 mm with the impact points not less than 20 mm from each other or from the edge. The use of grid to locate these points reduce operator bias. The surface must jy smooth, clean and dry and properly formed. In case rough surfaces are unavoidable they should be rubbed smooth with carborundum stone. (6) Uttrsonic method In ultrasonic pulse velocity method, the time of travel of an ultrasonic pulse, passing through the concrete to be tested is measured. This is shown in Fig. 25. The pulse generator circuit consists of an electronic circuit for generating pulse, and also a transducer which transforms these electronic pulses into mechanical energy having vibration frequencies in the range of 15-50 kHz. The time of travel betweey initial onset and the reception of the pulse is measured electronically. The path length between transducer when divided by the time of travel, gives the average velocity of wave propagation. With the velocity of wave propagation, the quality and compressive strength of concrete can be obtained as per the classification in Table 2.4. Concrete Cube t R Taosnitor ier pote pre fig. Direct transmission of ultrasonic waves Table 24, duality gradings for conete Woy hs Clifton (Gua, Coral se coapresion acai Na? “and above 33080 cn Bee 301035 ‘Medium aoe 20t025 3.0and below ee