VIEWS: 599 PAGES: 68 CATEGORY: College POSTED ON: 7/16/2012
A PROJECT REPORT ON CONCRETE MIX DESIGN GUIDE : G.A SURYAWANSHI SUBMITTED BY : SANTOSH LALWANI TUSHAR BARAPATRE PALLAVI DHABARDE ANWAR ASHARF PALLAVI DHAWAD MAYUR RAMTEKE ASHLESHA GANVIR NEHA JANBADE TRUPTI KSHISAGAR APPLIED MECHANICS DEPARTMENT GOVERNMENT POLYTECHNIC, NAGPUR. CERTIFICATE This is to certify that Shri……………………………………………… III year Civil Engineering students studying in the institute has submitted his project work entitled “CONCRETE MIX DESIGN” In partial fulfillment for the award of Diploma in Civil Engineering Which is record of his own work carried out by him under supervision and guidance. Date : Guide : G.A.SURYAWANSHI Lect. in Applied Mechanics Department G.P.Nagpur. PROF. Y.K.CHANADRANA PROF.N.D.NANDANWAR H.O.D PRINCIPAL Applied Mechanics Department Govt.Polytechnic, Govt. Polytechnic, Nagpur. Nagpur. ACKNOLEDGEMENT We take this golden opportunity to express our sense of gratitude and sincere thanks to our guide Prof. G.A SURYAWANSHI.For their valuable guidance. Their ability to motivate capability to induce ,even willingness to solve our difficulties, generous suggestions has faced us to make our project work a unique one and our task easier. The work would not have seen the light of the day without his existence and encouragement. We give sincere thanks to Prof.Y.K CHANDARANA ( Head of Applied Mechanics Dept.) to give us a golden opportunity to prepare such a project. For their direct & indirect help to our project we are also thankful of all other our Staff members for giving their valuable suggestions and their co-operation and help in our project work. Finally our special thanks to all those who directly or indirectly helped in during the courses of our project. PROJECTEES SANTOSH LALWANI TUSHAR BARAPATRE PALLAVI DHABARDE ANWAR ASHARF PALLAVI DHAWAD MAYUR RAMTEKE ASHLESHA GANVIR NEHA JANBADE TRUPTI KSHISAGAR INDEX Sr.No. Unit PageNo. 1. INTRODUCTION 2. PROPERTIES & OF INGREDIENTS OF CONCRETE 3. WATER CEMENT RATIO 4. PRINCIPLES OF CONCRETE MIX DESIGN 5. METHODS OF CONCRETE MIX DESIGN 6. WORKABILITY 7. CURING OF CONCRETE 8. TEST ON CONRETE 9. REFRENCES UNIT NO.1 INTRODUCTION 1 INTRODUCTION 1.1 CONCRETE: Concrete is a composite material form by combination of cement, fine aggregates, coarse aggregates & water together in a suitable proportion. This mix form easily workable paste known as plastic wet or green concrete. After mixing and placing the concrete in position the cement and water undergoes hardening the cement paste In this way, cement binds the aggregates like hard mass, which IS called as concrete. Now a days concrete is used most commonly as a building material because of its high strength and durability at low cost concrete is the only building material which gain strength in presence of water while other decayed_ due to this property of concrete it widely used in modem engineering construction. For economical preparation of the high quality concrete mixes, it is essential to study the design of such concrete mixes by various methods and - ~ ~ adopt one, which IS most economical and durable. For this study of various Ingredients of concrete, fresh concrete, hardened concrete is very essential. 1.2 ADVANT AGES OF CONCRETE: The various advantages of concrete as construction materials are as below :- a) Concrete gains strength in presence of water whereas the materials other than concrete decay when they are exposed to water. b) Insect does not attack concrete. c) Concrete can be molded at any shape at normal temperature and pressure. d) Concrete is fire proof and sound proof e) Concrete has strength. f) Strength of concrete can further increased by reinforcing and prestressing. 1.3 CHARACTERISTICS OF GOOD CONCRETE: A good concrete is obtained by careful selection pf its ingredients, well grading of aggregates, suitable proportioning, adding sufficient water and adopting workmanship Through the freshly prepared concrete remains plastic only temporarily It should fulfill certain characteristics in this state because the quality and the cost of hardened concrete are much affected by the characteristics of freshly prepared concrete. The desired properties of good concrete in plastic and hardened stage are below Properties of plastic concrete: a) Workability:- Concrete in green stage should have good workability so that it can be easily placed and compacted. b) Freedom from segregation: Separations of ingredients of concrete mix so that the concrete mix is no longer in a homogeneous and stable condition is called as segregation good concrete must be free from segregations. c) Freedom from bleeding Separation of slurry from cement and water from concrete mix is called as “Bleeding.” Good concrete should be free from bleeding. Properties of hardened concrete :- a) Strength A good concrete in hardened stage should have good strength. b) Durability A good concrete must be durable enough to resist the effect of weathering agents. C) Impermeability A good concrete should be impervious and airtight 1.4 DISADVENTAGE AND REMEDIES OF CONCRETE: I) Concrete has low tensile strength therefore It cracks under tensile load. Bit this can be removed by using reinforcement or prestressing. 2) Concrete expands and contracts due to temperature variation. To overcome this difficult expansion Joints are provided . 3) Concrete is not totally impervious but it can made Impervious by using water proofing agents or rich mixes. 4) Concrete disintegrates when comes in contact with alkalies or sulphates resisting cement. 1.5 CONCRETE MIX DESIGN: One of the ultimate aims of studying the various properties of materials of concrete. plastic concrete and harden concrete, is to unable the concrete technologist to design a concrete mix for a particular strength. The design of concrete mix is not a simple task and the condition that prevail at the site of work and the condition that are demanded for a particular work for which the mix is designed . The Structural Engineer stipulates the certain minimum strength and the concrete technologist design the concrete mix with the knowledge of the materials, site conditions and standard of supervision available at site to achieve this minimum strength. Further, the site engineer is required to make the concrete at Site, closely following the parameters suggested by the" Mix Designer" to achieve the minimum strength specified by the Structural Engineer. The Site Engineer makes cubes or cylinders sufficient in numbers and tests them to confirm the achievement with respect to minimum specified strength . "Mix design can be defined as the process of selecting suitable Ingredients of concrete and determining there relative proportions with the object of producing concrete of certain minimum strength and durability as economically as possible" UNIT NO.2 PROPERTIES OF INGREDIENTS OF CONCRETE 2 PROPERTIES OF INGRADIENTS OF CONCRETE 2.1 GENERAL: The common Ingredients of concrete are coarse aggregates, fine aggregates and water. By Judicious use of available materials for concrete making and their proportioning, proportioning, concrete mixes are produced to have the desired properties in the fresh and hardened states, as the situations demand. In this section, the physical and chemical properties of materials which Influence the properties and performance of concrete mixes are discussed Materials are also tested to meet the requirements of Mix design . 2.2 CEMENT: Cement is by far the most important constituent of the concrete, in that it forms the binding medium for the discrete ingredients The product manufactured by crushing and burning to an intimate well - proportioned mixture of calcareous and argillaceous material is called as "Cement. " Raw materials used to produce cement are as follows - Sr.No. Chemical Range Common Ingredients % proportion 1 Lime 60 to70 63 2 Silica 17 to 25 22 3 Alumina 3 to 8 6 4 Iron oxide 0.5 to6 3 5 Magnesium oxide 0.4 to 4 2.5 6 Sulphur Trioxide 1 to 3 1.75 7 Alkalies such as 0.2 to 1 0.25 soda & potash 8 Loss on ignition 1 to 2 1.5 2.2.1 Ordinary Portland Cement (OPC) (IS:269-1976) This type of cement is also normal setting cement since its setting is normal as compared to other cement when mixed with water I t is the general-purpose cement suitable for general concrete construction for use in general concrete construction, which requires no special consideration. It should satisfy the requirements as per IS269-1976. The, weight of one bag of OPC is 50 kg and its volume is 35 lit. or 0.035 M2 .One cubic meter of OPC contains 28.8 bags. 2.2.2 Properties of OPC: 1) Fineness :- The residue of OPC should not exceed when sieved through IS-sieve No.9 (90 micron) 2) Soundness :- Its expansion should not be more than 1Omm for untreated cement and 5 mm for aerated cement. 3) Setting time :- Its initial setting time should not be less than 30 minute & final setting time should not be more than 600 minute. 4) Compressive Strength:- after 3 days its compressive strength should not be less than 115 kg/cm3 & 175kg/cm3 After 7 days when ordinary sand is used and 160 kg/cm 3 and 220kg/cm3 respectively when standard sand is used. 2.2.3 Uses: - 1) OPC is used in important structures where great strength is required such as heavy buildings and bridges. 2) It is used in structure subjected to action of water such as foundation, under water reservoir. Water tight floor, Clock yards etc. 3) It is used for cement mortar, plain concrete, and reinforcement concrete, reinforce brickwork . 4) It is used for plastering and pointing It is used for drainage & water supply works. 2.3 AGGREGATES 2.3.0 GENERAL The inert material used in preparation of mortar or concrete such as sand , gravel ,crushed stone etc. it is termed as aggregate. The aggregate are bound together by a cement paste to form a hard concrete mass. They are called inert because , they do not take part in chemical reaction & remains inactive during setting & hardening of cement. Almost 75 % of the volume of concrete is occupied by aggregates , so they affect various properties of concrete .As aggregate are cheaper than cement, it is economical to put in concrete keeping in view the durability .Mostly used aggregates are sand, gravel ,broken bricks, furnace slag are used for plain & fire resisting concrete .But for R.C.C sand, gravel, crush rock are used as aggregates. 2.3.1 REQUUIRMENTS OF GOOD AGGREGATES :- As per IS 383-1975, good aggregate should fulfill the following requirements:- a) It should be chemically inert. b) It should be sufficiently strong. c) It should be sufficiently hard d) It should be sufficiently durable. e) It should be preferably cubical or spherical. f) It should have rough surface. g) It should be free from inherent coating, clay, organic material or any other matter, which is likely to reduce setting of cement. and may reduce strength of concrete. 2.3.2 PROPERTIES OF AGGREGATES:- 1) SURFACE TEXTURE :- Surface texture influences the properties of plastic concrete more than harden concrete . The strength of the bond between aggregate and cement paste depends on surface texture or porosity of the aggregates. Aggregates having rough surface requires more cement and provides better bond. Also smooth aggregates having surface pores develop good bond. Surface texture may of following type - a) Glassy b) Smooth C) Granular d) Rough e) Crystalline f) Honey combed 2) STRENGTH OF AGGREGATE:- Generally, strength of aggregates refers to crushing strength. It is the resistance of aggregates crushing that is compressive forces. Unless the aggregates are stronger than concrete, it is not likely to influence strength of concrete. On the other hand, of aggregates is having low crushing strength its use will adversely affect the strength of concrete 3) SPECIFIC GRA VITY:- As the aggregate contains pores both permeable & impermeable, the meaning of specific gravity has to be carefully defined and hence specific gravity may be of several types a) Apparent specific gravity :- J).0,………… specific gravity:- K) M) Apparent L) N) O) P) Q) R)~1) It is defined as ratio of weight of oven dry aggregates to its absolute excluding all pores. It is also called Absolute specific gravity. b)Bulk specific gravity:- It is defined as the ratio of oven dry aggregates to its absolute volume including the natural pores in the particles It is termed as simply, " Specific Gravity Sand -- 16 kN/m3 Steel -- 78 kN/m3 Bulk specific gravity is generally used for calculation of yield of concrete ,in concluding quantities etc.but the actual value of the specific gravity of aggregates is not a measure of its quality. It is also required in mass concrete structures such as dams ,retaining walls, where minimum density of concrete is essential for the stability of the structure. The majority of natural aggregates have a specific gravity between 2.6 % to 2.7 %, but values less than 2.6 % does not mean that aggregates are of bad quality. However no maximum limits of specific gravity has been specified. . 4) MOISTURE:- Generally termed as surface moisture. It is defined as the difference in weight of aggregates of in saturated surface dry condition (i.e. an aggregate having all pores filled with water but having dry surface) and in moisture condition (i.e The aggregates having all pores filled with water and also having \vet surface) It is a expressed as percentage. 5) GRADING OF AGGREGA TES:- The art of doing grading (Particle sized distribution) of an aggregates as determined by sieve analysis is know as grading of aggregates Sieve analysis is the simple operation of dividing the sample of aggregates into fractions, each consisting of particle of same size The principle of grading is that, the smaller particle will fill up the void between the large particles. This result in most economical use of cement paste for filling the voids and binding together the aggregates In the preparation of concrete Thus proper grading of coarse & fine aggregates In a concrete mix produces a dense concrete with less quantity of cement Concrete of desired strength can economically prepared with various grading of coarse & fine aggregates. The grading of aggregates has a considerable affect on workability. uniformity and finishing qualities of concrete mix which thus ultimately affect the economy, strength, durability and ,1ther properties of concrete .Hence, fine and coarse aggregates to be used in concrete should be well graded. The following IS sieve are generally used for grading of aggregates- with fine mesh wire cloth -- 3.35 mm , 2,36 mm , 1,18 mm, 600 micron, 300 micron , 150 micron, 75 micron. With square hole performed plate- 80 mm, 63 mm. 50 mm, 40mm,31.5 mm,25 mm, 20mm, 16 mm, 12.5 mm, 10 mm, 6.3 mm. 4.75mm. 6) FINENESS MODULUS:- A single factor calculated from the sieve analysis, which gives an idea about fineness or coarseness of an aggregate is know as Fineness modulus (FM.) The fineness modulus is defined as the sum of the cumulative percentage retained on IS sieves, according to the type of aggregates, dividing by 100. The F.M is roughly proportional to the average size of particles in the aggregates i.e. coarser the aggregates more are the F.M. It does not represent the particle size distribution but useful to measure slightly variation in aggregates from same sources .The IS sieve is used to determine F. M. are:- Coarse aggregates -- 80 mm , 40mm, 20mm, 10mm, 4.75mm. Fine aggregates -- 475 mm, 2.36 mm, 1.18 mm, 600 micron, 300 micron, 150 micron, etc. UNIT NO.3 WATER CEMENT RATIO 3. WATER CEMENT RATIO 3.0 GENERAL:- Strength of concrete depends on quantity and quality of its grade. A rich mix provides higher strength not only because of using more cement, but amount of water to be added plays an important role in this respect. Insufficient quantity of water harsh and unworkable mix whereas excess causes segregation and bleeding. Hence, it is necessary to add property of water to allow hydration of cement and makes a workable mix enough to be easily placed inside the form. Definition'- The ratio of volume or weight of water to that of cement in a concrete mix is called as Water-Cement ratio. It is usually expressed in liters of water required per bag (50 Kg of cement) 3.1 DEVELOPMENT OF STRENGTH OF CEMENT COMPOUNDS WITH AGE :- Lower the ratio, greater is the strength of concrete decrease as the water cement ratio, increases. It can be illustrated graphically by plotting the water cement ratio (By weight) as abscissa and compressive strength of 28 days as ordinate. In case water cement ratio is less than 0.45 the curve is seen bending downward This indicate that concrete mix having W/C ratio less than 0.45 by weight is not workable and causes honey combed structures, thus strength of concrete is decreased. Doted curved shows that with good compaction by mechanical vibration the strength of concrete is increased with water cement ratio even less than 0.45, subject to minimum of 0.35. Thus water cement ratio act as a yard stick to obtained concrete of desired strength. In India CRRI (Cement Road Research Institute, New Delhi) charts are gcneral1y used to calculate water cement ratio of concrete. This chart shows relation between water cement ratio and compressive strength. The designed curved for cement concrete mixes in relation to 7 days compressive strength. The curves are very useful for obtaining water cement ratio for trial mixes of concrete. UNIT NO.4 PRINCIPLES OF CONCRETE MIX DESIGN 4.PRINCIPLES OF CONCRETE MIX DESIGN 4.0 BASIC CONSIDERATION :- Design of concrete mixes Involves determination of the proportions of the given constituents, namely cement coarse and fine aggregates, water and admixtures if any, which would produces concrete possessing specified properties both in fresh and harden state with the maximum economy. Workability is specified as Important property of concrete in fresh state, for harden state compressive strength and durability are important. The mix design , therefore, generally carried out for a particular compressive strength of concrete with adequate workability so that fresh concrete can be properly placed and compacted and to achieve required durability . The following are the basic assumptions made in design of plastic concrete mixes or medium strength :- a) The compressive strength of the concrete is governed by the its water cement ratio. b) For a given aggregates characteristics, the workability of concrete is governed by its water content. 4.1 FACTORS IN THE CHOICE OF MIX DESIGN:- Both IS 456--1978 as well as IS 1343--1980 envisage that design of concrete mix be based on following factors :- a) Grade designations. b) Type of cement. c) Maximum normal size of aggregates. d) Minimum water cement ratio. e) Workability f) Minimum cement content 4.2 OUTLINE OF MIX DESIGN PROPORTIONING:-- The various factors for detem1ine the concrete proportions and the step by step procedure for concrete mix design can be represented as in fig. The basic step involved can be summarized as follows. a) Arrive at the mean target strength from the characteristic strength specified and the level of quality control. b) Choose the water cement ratio for mean target strength and check for requirements of durability c) Arrive at the water cement ratio for workability. d) Calculate cement content and check durability. e) Choose the relative proportions of coarse and fine aggregates, from the characteristics of coarse and fine aggregates . f) Arrive at the concrete mix proportions for the first trial mix. g) Conduct trial mixes with suitable adjustment till the final mix composition is arrived. UNIT NO.5 METHODS OF CONCRETE MIX DESIGN 5. METHODS OFCONCRETE MIX DESIGN 5.0 GENERAL :- The mix design methods are being used in different countries are mostly based on empirical relationships. Charts &graphs developed from extensive experiment investigation .Most of them follow the same basic principle include in section 4 & only minor variations exist in different mix design methods in the process of selecting the mix proportions. METHODS OF MIX DESIGN :- Indian Standard ( IS 1062-1982 ) Method . Road Research Laboratory ( RRL ) Method . American concrete Institute ( ACI ) Method . Road Research Laboratory ( RRL ) Method . Department Of Environment (DOE ) Method . Trial & Error Method . Maximum Density Method . Fineness Modulus Method . 5.1 INDIAN STANDARD METHOD OF MIX DESIN GUIDELINES FOR CONCRETE MIX DESIGN ( IS10262-82 ):- Indian Standard Institution has brought out the mix design procedure mainly based on the work done in national laboratory. This method can be applied to both medium & high strength concrete. In case of fly ash cement concretes water content arrived at can be reduced by @3% to 5% & proportion of fine aggregate can be reduced by 2% to 4% . DATA REQUIRED :- DESIGN DATA : a. Grade of concrete (fck ). b. Type of cement . c. Degree of workability . d. Maximum/minimum cement content. e. Water cement ratio . f. Type& Maximum nominal size of aggregates. g. Standard deviation (s ) . h. Type of quality control. TEST DATA FOR MATERIALS:- a. Specific gravity of cement, coarse & fine aggregate. b. Water absorption of coarse & fine aggregate. c. Surface moisture of coarse & fine aggregate. 5.2 STEPS INVOLVED IN THE METHOD :- 1.TARGET MEAN STRENGTH (fck):- ft = fck + K x S Where, ft = Target mean strength. fck = Characteristic strength. K = Statistical value. S = Standard deviation. TABLE NO.5.1 SUGGESTED VALUES OF STANDARD DEVIATION . Grade of Very Good Good Fair concrete 1. 2. 3. 4. M10 2.0 2.3 3.3 M15 2.5 3.5 4.5 M20 3.6 4.6 5.6 M25 4.3 5.3 6.3 M30 5.0 6.0 7.0 M35 5.3 6.3 7.3 M40 5.6 6.6 7.77 M45 6.0 7.0 8.0 M50 6.4 7.4 8.4 M55 6.7 7.7 8.7 M60 6.8 7.8 8.8 2. SELECTION OF WATER CEMENT RATIO:- Various parameters like type of cement & aggregate, max .size of aggregate .Surface texture of aggregate etc. shall influence the strength of concrete, remain a water cement ratio constant, hence it is desirable to establish a relation between concrete strength & Free water cement ratio with material & conditions to be used actually at site . In absence of such relationship the free water cement ratio Corresponding to the target strength may be determine from relationship fig.no.1. If 28days strength of cement is known use of figno.2 may be made for more accurate estimation of water cement ratio .However ,this will need at least 28days for testing of strength of cement, there by delaying whole process by28days. 3. ESTIMATION OF ENTTRAPPED AIR:- The air content is estimated from table 5.2 for the normal maximum size of aggregates used. TABLE NO.5.2 APPROPRIATE ENTRAPPED AIR CONTENT Max.size of aggregate Entrapped air % of volume of concrete 10 3.0 20 2.0 40 1.0 4.SELECTION OF WATER CONTENT & FINE TO TOTAL AGGREGATE RATIO:- The water content and percentage of sand of total aggregate by absolute volume are determined from table no 5.3 for medium strength concrete, this table IS based on the following conditions. 1. Crushed ( angular ) coarser aggregate confirming to IS :383-1970. 2. Fine aggregate of natural sand confirming to grading zone II of table 4 of IS : 383-1970. 3. Workability corresponds to compacting factor of 0.80. Water cement ratio in case of table no. 5.3 is 0.60. for any departure from above mentioned conditions, corrections have to be applied, based on table5.4 , on water content and percentage of sand as percent of total aggregate by absolute volume, determined from table5.3 TABLE 5.4 APPROXIMATE SAND & WATERPER CUBIC METER OF CONCRETE W/C = 0.60 WORKABILITY = 0.80C.F Max.size of aggregate Water content including Sand as % of total surface water per cubic aggregate by absolute meter of concrete volume 10 200 40 20 186 35 40 165 30 TABLE5.4 ADJUSTMENT OF VALUES IN WATER CONTENT AND SAND % FOR OTHER CONDITION Change in condition Adjustment required in Adjustment required in % stipulated for tables water content sand in total aggregate For sand confirming to 0 +1.5 % for zone I grading zone I& zone III - 1.5 % for zone III or zone IV of table -3.0 % for zone VI 4.IS:383-1970 Increase or decrease in +3.0 % 0 value of compacting -3.0 % factor by 0.1 Each 0.05increase or 0 +1 % decrease in water cement -1% ratio For rounded edge -15kg -7 % 5. CALCULATION CEMENT CONTENT:- The cement content per unit volume of concrete may be calculated from free water cement ratio and the quantity of water per unit volume of concrete (cement by mass = water content I water cement ratio) The cement content so calculated shall be checked against minimum cement content for the requirement of durability (Table 5.5 ) and the greater value of two is to be adopted. TABLE 5.5 MINIMUM CEMENT REQUIRED IN CEMENT CONCRETE TO ENSURE DURABILITY UNDER SPECIFIED CONDITION OF EXPOSURE . Exposure Plain Plain concrete Reinforced Reinforced concrete Maximum concrete. concrete. Minimum water content Minimum Maximum cement ( kg ) cement content water content content ( ( kg ) ( kg ) kg) Mild: 220 0.70 250 0.65 For e.g. completely protected against weather or aggressive condition except for a brief period of exposure to normal weather condition during construction Moderate: 250 0.60 290 0.55 For e.g. sheltered from heavy & wind driven rain against freezing whilst saturate with water, concrete soil & concrete continuously under water Severe: 310 0.50 360 0.45 For e.g. exposed to sea water alternate wetting & drying & freezing whilst wet ,subject to heavy condensation or corrosive fumes 6. CALCULATION OF AGGREGATE CONTENT :- Aggregate content can be determined from the following equation : v = [ W + C/Se + 1/p x Fa/Sfa] 1 /1000 -----------( 1 ) Ca = [1- p]/p x Fa x Sca/Sfa ---------------------------( 2 ) Where , V = absolute volume of fresh concrete which is equal to gross volume (m3) minus the volume of entrapped air W = mass of water (kg) per m3of concrete, C = mass of cement (kg) per m3of concrete, Sc = specific gravity of cement P = Ratio of fine aggregate to total aggregate by absolute volume, Fa, ca = Total mass of fine aggregate and coarse aggregate (kg) per m3 of concrete respectively, Sfa, Sca= specific gravity of saturated ,surface dry fine aggregate and coarse aggregate respectively 7. ACTUAL QUANTITIES REQUIRED FOR THE MIX :- It mentioned that above mix proportion has been arrived at on the assumption that aggregates are saturated surface dry. For any deviation than this condition i.e. when aggregate are moist or air dry correction has to be applied uant1ty of mixing water as well as to the aggregate. The calculated mix proportion shall be checked by means of trial batches. Quantities of material for each trial shall be enough for at least three sieve cubes and concrete required to carry out workability test according to IS :1199-1959. Trial mix No. I should be checked for workability and freedom from segregation and bleeding and its finishing property If the measured workability is different from the assumed in the calculation ,a change in the water cement ratio is to be made from table 5.4& the whole mix design has to recalculated keeping the w/c ratio constant .A minor adjustment may be made to improve the finishing quality or freedom from segregations & bleeding .This will comprise trial mix no.2 Now water cement ratio is changed by plus, minus 10 % pre-selected value & mix proportions are recalculated .These will from trial mix no.3 & 4 .Testing for trial mix No.2, 3, 4are done simultaneously .These test normally provide sufficient information ,including the relationship between compressive strength & water cement ratio ,from which the mix proportions for field trials may be arrived. 5.1 1. MIX DESIGN OF M20 GRADE OF CONCRETE BY IS METHOD a. Design Data Required :- 1. Grade of cement (fck ) 20Mpa 2. Maximum size of aggregates 20mm 3. Degree of workability Medium 4. Degree of quality control Fair 5. Type of exposure Moderate b) Test data for materials :- 1. Compressive strength of cement 43N/mm2 2. Specific gravity of cement 3.15 3. i) Sp.Gravity of coarse aggregates 2.85 ii) Sp.Gravity of fine aggregates 2.54 4. Water absorption i) Coarse aggregate 0.3% ii) Fine aggregate 0.2% 5. Free moisture content i) Coarse aggregate Nil ii) Fine aggregate 1.5% c) Target mean strength of concrete :- ft = fck + K.S fck = 20MPa K = 1.65 S = 5.6 Target mean strength = 20 + 1.65 x 5.6 = 29.24N/mm2 d) Selection of water cement ratio :- From fig. 5.1. the water cement ratio required for the target mean strength of 29.24 N/mm2 is 0.5 this is lower the maximum value of Prescribed for “Moderate “ exposure.( see table no.5.5 ) e) Selection of water content & fine to total aggregate ratio :- As grade of concrete is M20 which is less than M35 Therefore , Water content per M3 of concrete = 186lit Sand as % of total Agg. by absolute volume = 35% Contd……. Change in Condition Water Content Adjustment Required % % sand in total aggregate For decrease in water cement ratio by 0.05 ( 0.50 -0.60 ) Nil [-0.1x1]/0.005 -2% For increase in compaction factor By 0.1 ( 0.92 – 0.80 ) +3.6% Nil For sand confirming to zone I Nil +1.5% TOTAL +3.6% -0.5% Therefore , Sand content as percentage of total aggregate by absolute volume = 35 – 0.5 = 34.5% Required water content = 186+3.6 x [186/100] = 192069 lit. f) Determination of cement content :- Water cement ratio = 0.50 Water = 192.69 Cement = 192.69/0.50 = 385.38 kg = 345 kg (Assumed) This cement content is adequate for “Moderate “ exposure condition. g) Determination of coarse & fine aggregates content :- From table 5.2, For the specified maximum size of aggregate of 20mm the amount of entrapped air in the wet concrete is 2% .Taking this into account & applying equation, V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,& V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000 Fine aggregates :- 980 = [192.69 +345/3.15 + 1/0.345 x Fa/2.54] x 1/1000 Fa = 594.55 kg Coarse aggregates :- 980 = [192.69 +345/3.15 + 1/[1-0.345] x Ca/2.85] x 1/1000 Ca = 1264.53 kg The mix proportion thus become, Cement : Water : Sand : Coarse aggregate 345 : 192.69 : 594.55 : 1264.53 OR 1 : 0.50 : 1.72 : 3.66 h) Determination actual quantity required for mix :- Adjustment required for water absorption. 1. Water absorbed by C.A = [Qty x % of saturation] / 100 = [ 1264.53 x 0.3 ] /100 Water absorbed by C.A = 3.79 lit 2. Free water in fine aggregate = [ Qty x %of surface moisture]/100 = [ 594.55 x 1.5 ] /100 Free water in fine aggregate = 8.918 lit Therefore, a) Actual Qty of Cement required = 345kg b) Actual Qty of Water required = 192.69 + 3.79 – 8.918 = 187.56 lit c) Actual Qty of C.A required = 1264.53 -3.79 = 1260.74 kg d) Actual Qty of Sand required = 594.55 + 8.918 = 603.46 kg Therefore, The actual quantity of material required are :- Cement : water : sand : Coarse aggregate 345 : 187.69 : 603.46 : 1260.74 OR 1 : 0.5 : 1.75 : 3.65 The mix should be prepared according to the number of cubes to be prepared from the above data & changing the water cement ratio , other trial mixes should be produced & tested for workability & strength. 5.1 1. MIX DESIGN OF M25 GRADE OF CONCRETE BY IS METHOD b. Design Data Required :- 1. Grade of cement (fck ) 25 Mpa 2. Maximum size of aggregates 20 mm 3. Degree of workability Medium 4. Degree of quality control Fair 5. Type of exposure Moderate f) Test data for materials :- 1. Compressive strength of cement 53 N/mm2 2. Specific gravity of cement 3.15 3. i) Sp.Gravity of coarse aggregates 2.8 ii) Sp.Gravity of fine aggregates 2.5 4. Water absorption i) Coarse aggregate 0.3% ii) Fine aggregate 0.2% 5. Free moisture content i) Coarse aggregate Nil ii) Fine aggregate 1.5% g) Target mean strength of concrete :- ft = fck + K.S fck = 25 MPa K = 1.65 S = 6.3 Target mean strength = 20 + 1.65 x 6.3 = 35.395 N/mm2 h) Selection of water cement ratio :- From fig. 5.1. the water cement ratio required for the target mean strength of 35.395 N/mm2 is 0.47 this is lower the maximum value of Prescribed for “Moderate “ exposure.( see table no.5.5 ) i) Selection of water content & fine to total aggregate ratio :- As grade of concrete is M25 which is less than M35 Therefore , Water content per M3 of concrete = 186lit Sand as % of total Agg. by absolute volume = 35% Contd……. Change in Condition Water Content Adjustment Required % % sand in total aggregate For decrease in water cement ratio by 0.05 ( 0.47 -0.60 ) Nil [-0.13 x 1]/0.005 -2.6 % For increase in compaction factor By 0.1 ( 0.92 – 0.80 ) +3.6% Nil For sand confirming to zone I Nil +1.5% TOTAL +3.6% -1.1 % Therefore , Sand content as percentage of total aggregate by absolute volume = 35 – 1.1 = 33.9 % Required water content = 186+3.6 x [186/100] = 192.69 lit./m3 f) Determination of cement content :- Water cement ratio = 0.47 Water = 192.69 Cement = 192.69/0.47 = 409.99 kg/m3 = 410 kg/m3 (Assumed) This cement content is adequate for “Moderate “ exposure condition. i) Determination of coarse & fine aggregates content :- From table 5.2, For the specified maximum size of aggregate of 20mm the amount of entrapped air in the wet concrete is 2% .Taking this into account & applying equation, V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,& V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000 Fine aggregates :- 980 = [192.69 + 410 / 3.15 + 1 / 0.339 x Fa/2.5] x 1/1000 Fa = 563.6 kg/m3 Coarse aggregates :- 981 = [192.69 +410 / 3.15 + 1 / [1-0.339] x Ca/2.85] x 1/1000 Ca = 1216.96 kg/m3 The mix proportion thus become, Cement : Water : Sand : Coarse aggregate 410 : 192.69 : 563.6 : 1264 OR 1 : 0.50 : 1.72 : 3.66 j) Determination actual quantity required for mix :- Adjustment required for water absorption. 1. Water absorbed by C.A = [Qty x % of saturation] / 100 = [ 1264.53 x 0.3 ] /100 Water absorbed by C.A = 3.79 lit 2. Free water in fine aggregate = [ Qty x %of surface moisture]/100 = [ 594.55 x 1.5 ] /100 Free water in fine aggregate = 8.918 lit Therefore, a) Actual Qty of Cement required = 345kg b) Actual Qty of Water required = 192.69 + 3.79 – 8.918 = 187.56 lit c) Actual Qty of C.A required = 1264.53 -3.79 = 1260.74 kg d) Actual Qty of Sand required = 594.55 + 8.918 = 603.46 kg Therefore, The actual quantity of material required are :- Cement : water : sand : Coarse aggregate 345 : 187.69 : 603.46 : 1260.74 OR 1 : 0.5 : 1.75 : 3.65 The mix should be prepared according to the number of cubes to be prepared from the above data & changing the water cement ratio , other trial mixes should be produced & tested for workability & strength. 5.1 1. MIX DESIGN OF M40 GRADE OF CONCRETE BY IS METHOD c. Design Data Required :- 1. Grade of cement (fck ) 40 Mpa 2. Maximum size of aggregates 20 mm 3. Degree of workability High 4. Degree of quality control Very good 5. Type of exposure Severe j) Test data for materials :- 1. Compressive strength of cement 43N/mm2 2. Specific gravity of cement 3.15 3. i) Sp.Gravity of coarse aggregates 2.97 ii) Sp.Gravity of fine aggregates 2.57 4. Water absorption i) Coarse aggregate 0.3% ii) Fine aggregate 0.2% 5. Free moisture content i) Coarse aggregate Nil ii) Fine aggregate 1.5% k) Target mean strength of concrete :- ft = fck + K.S fck = 40MPa K = 1.65 S = 5.6 Target mean strength = 40 + 1.65 x 5.6 = 49.24 N/mm2 l) Selection of water cement ratio :- From fig. 5.1. the water cement ratio required for the target mean strength of 49.24 N/mm2 is 0.43 this is lower the maximum value of Prescribed for “Severe “ exposure.( see table no.5.5 ) m) Selection of water content & fine to total aggregate ratio :- As grade of concrete is M40 which is higher than M35 Therefore , Water content per M3 of concrete = 180lit Sand as % of total Agg. by absolute volume = 25% Contd……. Change in Condition Water Content Adjustment Required % % sand in total aggregate For decrease in water cement ratio by 0.05 ( 0.43 -0.35 ) Nil [-0.35 x 1]/0.05 +1.6 % For increase in compaction factor By 0.1 ( 0.95 – 0.80 ) + 4.5 % Nil For sand confirming to zone I Nil +1.5% TOTAL +4.5 % +3.1 % Therefore , Sand content as percentage of total aggregate by absolute volume = 25 + 3.1 = 28.100 % Required water content = 180+ 4.5 x [180/100] = 188.1 lit./m3 f) Determination of cement content :- Water cement ratio = 0.43 Water = 188.1 Cement = 188.1/0.43 = 437.44 kg/m3 This cement content is adequate for “Severe “ exposure condition. k) Determination of coarse & fine aggregates content :- From table 5.2, For the specified maximum size of aggregate of 20mm the amount of entrapped air in the wet concrete is 2% .Taking this into account & applying equation, V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,& V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000 Fine aggregates :- 980 = [188.1 + 438/3.15 + 1 / 0.281 x Fa/2.57 ] x 1/1000 Fa = 472 kg/m3 Coarse aggregates :- 980 = [188.1 + 438 / 3.15 + 1 / [1-0.281 ] x Ca/2.97] x 1/1000 Ca = 1389 kg/m3 The mix proportion thus become, Cement : Water : Sand : Coarse aggregate 438 : 188.1 : 472 : 1389 OR 1 : 0.43 : 1.1 : 3.1 l) Determination actual quantity required for mix :- Adjustment required for water absorption. 1. Water absorbed by C.A = [Qty x % of saturation] / 100 = [ 1389 x 0.3 ] /100 Water absorbed by C.A = 4.2 lit 2. Free water in fine aggregate = [ Qty x %of surface moisture]/100 = [ 472 x 1.5 ] /100 Free water in fine aggregate = 7.1 lit Therefore, a) Actual Qty of Cement required = 438 kg/m3 b) Actual Qty of Water required = 188.1 + 4.2 – 7.1 = 186.18 lit/m3 c) Actual Qty of C.A required = 1389 - 4.2 = 1384.8 kg/m3 d) Actual Qty of Sand required = 594.55 + 8.918 = 603.46 kg Therefore, The actual quantity of material required are :- Cement : water : sand : Coarse aggregate 438 : 186.18 : 479 : 1384.8 OR 1 : 0.43 : 1.1 : 3.16 The mix should be prepared according to the number of cubes to be prepared from the above data & changing the water cement ratio , other trial mixes should be produced & tested for workability & strength. 5.1 1. MIX DESIGN OF M45 GRADE OF CONCRETE BY IS METHOD d. Design Data Required :- 1. Grade of cement (fck ) 45Mpa 2. Maximum size of aggregates 20mm 3. Degree of workability High 4. Degree of quality control Very good 5. Type of exposure Severe n) Test data for materials :- 1. Compressive strength of cement 43N/mm2 2. Specific gravity of cement 3.15 3. i) Sp.Gravity of coarse aggregates 2.97 ii) Sp.Gravity of fine aggregates 2.61 4. Water absorption i) Coarse aggregate 0.3% ii) Fine aggregate 0.2% 5. Free moisture content i) Coarse aggregate Nil ii) Fine aggregate 1.5% o) Target mean strength of concrete :- ft = fck + K.S fck = 45MPa K = 1.65 S = 6.0 Target mean strength = 20 + 1.65 x 6.0 = 54.9 N/mm2 p) Selection of water cement ratio :- From fig. 5.1. the water cement ratio required for the target mean strength of 54.9 N/mm2 is 0.45 this is lower the maximum value of Prescribed for “Severe “ exposure.( see table no.5.5 ) q) Selection of water content & fine to total aggregate ratio :- As grade of concrete is M45 which is Higher than M35 Therefore , Water content per M3 of concrete = 180lit Sand as % of total Agg. by absolute volume = 25% Contd……. Change in Condition Water Content Adjustment Required % % sand in total aggregate For decrease in water cement ratio by 0.05 ( 0.45 -0.35 ) Nil [-0.1x1]/0.005 +2% For increase in compaction factor By 0.1 ( 0.95 – 0.80 ) +4.5 % Nil For sand confirming to zone I Nil +1.5% TOTAL +4.5 % +3.5 % Therefore , Sand content as percentage of total aggregate by absolute volume = 25 + 0.45 = 28.5 % Required water content = 180+ 4.5 x [180/100] = 188.1 lit. f) Determination of cement content :- Water cement ratio = 0.42 Water = 188.1 Cement = 188.1 / 0.42 = 447.85 kg/m3 This cement content is adequate for “Severe “ exposure condition. m) Determination of coarse & fine aggregates content :- From table 5.2, For the specified maximum size of aggregate of 20mm the amount of entrapped air in the wet concrete is 2% .Taking this into account & applying equation, V = [ W + C/Sc + 1/P x Fa/Sfa ] x 1/1000,& V = [ W + C/Sc + 1/( 1-P )x Ca/Sca ] x 1/1000 Fine aggregates :- 980 = [188.1 +447.85/3.15 + 1 /0.285 x Fa/2.6] x 1/1000 Fa = 488.30 kg/m3 Coarse aggregates :- 982 = [188.1 +447.85/3.15 + 1/[1-0.285] x Ca/2.97] x 1/1000 Ca = 1402.57 kg/m3 The mix proportion thus become, Cement : Water : Sand : Coarse aggregate 447.85 : 188.1 : 488.30 : 1402.57 OR 1 : 0.42 : 1.10 : 3.13 n) Determination actual quantity required for mix :- Adjustment required for water absorption. 1. Water absorbed by C.A = [Qty x % of saturation] / 100 = [ 1402.57 x 0.3 ] /100 Water absorbed by C.A = 4.2 lit 2. Free water in fine aggregate = [ Qty x %of surface moisture]/100 = [ 488.30 x 1.5 ] /100 Free water in fine aggregate = 7.32 lit Therefore, a) Actual Qty of Cement required = 447.85kg/m3 b) Actual Qty of Water required = 188.1 + 4.2 – 7.32 = 184.98 lit/m3 c) Actual Qty of C.A required = 1402.57 – 4.2 = 1389.37 kg/m3 d) Actual Qty of Sand required = 488.30 + 7.32 = 495.62 kg/m3 Therefore, The actual quantity of material required are :- Cement : water : sand : Coarse aggregate 447.8 : 188.1 : 495.62 : 1398.37 OR 1 : 0.42 : 1.10 : 3.12 The mix should be prepared according to the number of cubes to be prepared from the above data & changing the water cement ratio , other trial mixes should be produced & tested for workability & strength. UNIT NO.6 WORKABILITY 6. WORKABILITY 6.0 GENERAL:- The ease with which the concrete is placed in the formwork and compacted is termed as Workability of concrete. Higher workability is needed for thin and heavily reinforced concrete. The workability of concrete depends on the various properties of its ingredients. 6.1 SEGREGATION: The separation of coarse aggregate from rest of the concrete mix due to difference in the their particle size and specific gravity is called as "SEGREGA TION" It should be prevented under any circumstances, as it is harmful to the concrete properties. Ready mix concrete in which segregation is more should not be used in any case. Using segregated mix results into honey combing, weak and pores layer, surface scaling in the hardened concrete. With cement content less, the segregation is more. The mixes which are not designed properly or which have excess quantity of water are liable to segregation. Segregation is more when the concrete is to be placed under water or pumped. It usually results from the shaking during loading or discharging from considerable height. Segregation indicates poor grading of aggregates. Care should be taken to avoid segregation, however following points should be kept in mind in order to eliminate or minimize segregation. a) The mixes must be correctly designed. b) The water cement ratio must be kept constant. c) The height of fall of concrete must not exceed 3m in any case. d) The concrete operations must me strictly supervised. e) The concrete should be placed from the final position as near as possible. f) The concrete should be mixed until it attains uniform co lour. g) The air-entering agents must be used for reducing segregation. h) The water or sand content or both should be charged for preventing segregation. 6.2 BLEEDING: The appearance of water on the surface of the concrete after compaction is called "BLEEDING" . Bleeding indicates the presence of excess of water in the concrete or deficiency of fine aggregate or too much finishing. This layer is called as "Loitance". It should be removed if new layer of the concrete is to be placed on old one. If the bleeding goes on increasing, the concrete becomes permeable. The aggregate are generally formed in dry state. These have to be washed for removing imparities which increase their moisture content. It varies from time to time depending upon the climatic condition e.g. the aggregates if dry will absorb moisture from the mix and if wet, will have more moisture. Therefore, while calculating the amount of water to be added, consideration must be given to the surface condition of the aggregates. Concrete with fine particles require more water than concrete with coarse particles from the same workability. Following points must be remembered in order to prevent bleeding of concrete:- a) The mix must be carefully and properly designed. b) The water content ratio must be kept constant. c) The richer concrete should be used. d) The air entraining agents should be used. e) The sand used should not be too fine 6.3 HARSHNESS:- The concrete mix which causes difficult in obtaining smooth finish or good contact with formwork is known as "Harsh mix'. It occurs due to excess of middle size particles. It can also be due to deficiency of fine aggregate to fill the voids in the coarse aggregate It can be eliminated with sufficient proportion of mortar till the voids in the coarse aggregates 6.4 FACTORS AFFECTING WORKABILITY:- The various factors affecting workability of concrete are listed as below 1) Water Content :- The workability of any concrete mix mainly depends upon the quantity of water added into the mix. The determination of the correct quantity of water, depends on the shape of the aggregates and size of aggregates, proportioning of ingredients, methods of compaction and climatic conditions. Water plays an important role of lubricants when added into the mix. But on the other hands, if its amount is increased, the strength of the concrete is reduced. When more water is added, it occupies space in concrete. So evaporation takes place from the concrete surface, it leaves behind voids and concrete cracks. It is very essential to limit the water content and should be sufficient enough to produce a workable mix. The quantity of water added influence not only the strength but only the consistency of the concrete. If concrete is to be placed in congested section, then water content is increased to obtain higher degree of workability. An increase in water content must be accompanied by an increase in cement content in order to maintain the strength of concrete. 2) Shape Of The Aggregate: An angular, rough, flaky and elongated aggregate reduces workability. The rounded and smooth aggregate requires comparatively less quantity of water for lubrications and hence they improve the workability of the concrete with same quantity of water. 3) Size Of Aggregates:- Smaller size aggregate offers more surface area than large size aggregates and thus they require more quantity of water for lubrication, hence for same degree of workability less water is required for large size aggregates and they contribute to the improvement of workability. Moreover less quantity of water is used in large size aggregates also reduces quantity of the cement for the given water cement ratio 4) Grading of Aggregates: It plays its vital role when lean concrete mix of high workability is required, for lean concrete mixes the grading should be continuous whereas for rich concrete mixes grading should be coarse. 5) Surface Texture of Aggregate : The aggregate with rough surface requires more cement to produce workable concrete mix and provide a better bond than the aggregate with smooth surface. Thus aggregate with smooth surface will contribute to improvement of workability. 6) Porosity and Absorption of Aggregates: A porous and non saturated aggregate requires more water than the non porous and unsaturated aggregate. For the same degree of workability non porous and saturated aggregate requires less quantity of water. Thus non porous and saturated aggregate contributes to the improvement of workability. UNIT NO.7 CURING OF CONCRETE 7. CURING OF CONCRETE 7.0 GENERAL: Curing is defined as the operation of maintaining the humidity and temperature of freshly placed concrete during some definite period following placing, finishing to assure satisfactory hydration of cement and hardening of cement 7.1 NECESSITY The concrete hardens because of hydration i.e. the chemical reaction between water and cement. The chemical action, which accompanies the setting of concrete, is dependent on the presence of water. Although there is a sufficient water at the time of mixing. It is necessary to ensure that the water is retained to enable the chemical reaction to continue until the concrete is fully hardened. Properties of concrete such as strength, water tightness, durability, wear resistance and volume stability improves with the passage of time. Three gallons of water is approx. required to hydrate one bag of cement If the loss due to evaporation is more from newly placed concrete, the hydration process will stop and concrete will shrinks thus creating tensile stresses at the drying surface. The development of the tensile stresses will result in the formation of plastic shrinkage cracks. Hence curing is necessary and must be done immediately after concrete is placed and compacted. 7.2 METHODS OF CURING: There are various methods of curing. The method to be adopted depends on the nature of work and climatic conditions. Following are the methods of curing the concrete: a) Shading concrete works:- The object of shading concrete work is to prevent evaporation of water from the surface even before setting this is adopted mainly in case of large concrete work such as road slabs. This is essential in dry weather to protect the concrete from the heat, direct sunrays and wind. b) Membrane curing:- The method discussed above comes under moist curing. Another method of curing is to cover the wet concrete surface by a layer of waterproof material, which is kept into contact with the concrete surface for 7 days. This method of curing is done or termed as membrane curing. A membrane will prevent the evaporation of water form the concrete .The membrane can be either solid or liquid. They are known as sealing compounds. This method does not require constant supervision .This method is usually adopted where there is a shortage of water for moist curing. This method is efficient as compared to other moist curing method as rate of hydration is less. Moreover, the strength of concrete cured by membrane curing is less then that of concrete, which is moist cured. When membrane is damaged, the curing is badly affected. c) Steam curing:- By using this method, the strength of the concrete can be increased rapidly This can be used in precast concrete works. In steam curing the temperature of the steam should be restricted to 75 degree C as in the absence of proper humidity the concrete may dry to soon. In case of hot water curing the temperature may be raised to any limit, say 100 degree ,at this temperature the gain of strength is about 70 %.of the 28 days strength after 4 to 5 hours. d) Sprinkling of water:- Sprinkling of water continuously over the concrete surface provides an efficient curing. It is mostly used for curing floor slabs. The concrete should be allowed to set sufficiently before sprinkling is started. On small jobs sprinkling can be done by hand. Vertical and sloping surfaces can be kept wet by sprinkling water on the top surface and allow it to run down between forms and concrete. For this method water requirement is more. e) Pondind method:- This is the best method of curing. It is suitable for curing horizontal surfaces such as floors, roof slabs, roads and airfield pavements. The horizontal top surface of the beam can also be ponded. After placing the concrete, its exposed surface is first covered and removed and small bands of clay and sand, divided into numbers of square and rectangles. The water is filled in bands. The filling of water in this ponds is done twice of or three a day depending upon the atmospheric conditions. CHAPTER NO. 8 TESTS ON CONCRETE COMPRESSIVE STRENGTH :- The compressive strength of concrete is one of the most important and useful property. In most structural applications concrete is employed primarily to resist compressive stress. In those cases where strength in tension or shear is primary importance, the compressive strength is frequently used as a measure of these properties. Therefore the concrete making properties of various ingredients of mix are usually measured in terms of compressive strength. Compressive strength is also used as a qualitative measure for other properties of hardens concrete. No exact qualitative relationship between compressive strength and flexural strength, tensile strength, modulus of elasticity, wear resistance, fire relationship or permeability have been established or nor are they likely to be. However, approximate or statistical relationship in same case has been established and this gives much useful information to the engineers. It should be emphasized that compressive strength gives only an approximate value of these properties and that other tests specially designed to determine these properties should be useful more precise result are required. For instance, the indicated compressive strength increases as the specimen size decrease. The modulus of elasticity in this case does not follow the compressive strength. Concrete containing about 6% of entrapped air is relatively weaker in strength is found to be more durable than the dense and strong concrete Among the materials and mix variables, water cement ratio is the most important parameter governing compressive strength. Beside water cement ratio, the following factors also effect the compressive strength of concrete:- i. The characteristics of cement. ii. The characteristics and properties of aggregates iii. The degree of compaction iv. The efficiency of curing. v. The temperature during the period of curing. vi. The age at the time of compaction. vii. The condition of test. Compressive test is carried out on specimen cubical or cylindrical in shape. Procedure: First of all ingredients are measured as per the design. Then the cement is mixed dry with fine aggregates in concrete mixer until it attains uniform colour. After than the coarse aggregate are mixed. Then the water is added as per the water cement ratio which is derived from the mix design. After this the all ingredients are mixed for five minutes or until the homogeneous is mixture is formed. Simultaneously, the inter surface of the mould is oiled. After this the concrete is placed in moulds in approximately three layers, each layer is compacted twenty five times with 16mm diameter and 60cm long tamping rod. Compaction should be uniform. After the mould is filled complete the moulds are placed on the vibrator. After vibrating the top layer of concrete is struck off with trowel, then the moulds in air for about twenty four hours at a minimum 90% relative humidity and at a temperature of 27oC. After the given period the moulds are opened and the prepared concrete blocks are taken out and place for curing in curing tank. Then the compressive strength test is performed after seven and twenty eight day of curing on compression testing machine. For this the cubes are taken out of curing tank for at least one hour before testing. The loaded indicated by the pointer on the compression machine are recorded. Result:- The results of compressive strength of cubes of M20 grade of concrete for given period of curing are tabulated below: W/C Ratio Curing Load at Compressive Average Period Failure strength compressive (kgf) (N/mm2) strength (N/mm2) 5)