CH-4 Concrete PDF Lecture Note on Construction Materials

ADAMA SCIENCE AND TECHNOLOGY UNIVERSITY SCHOOL OF CIVIL ENGINEERING AND ARCHITECTURE DEPARTMENT OF CIVIL ENGINEERING Course name: Construction Materials Course Code: CEng- 2205 Course Instructors: Zerihun M., Elias L., Melimal W., & Heaven Y...

ADAMA SCIENCE AND TECHNOLOGY UNIVERSITY SCHOOL OF CIVIL ENGINEERING AND ARCHITECTURE DEPARTMENT OF CIVIL ENGINEERING Course name: Construction Materials Course Code: CEng- 2205 Course Instructors: Zerihun M., Elias L., Melimal W., & Heaven Y. Chapter Three: Cementing Materials March , 2023 Adama, Ethiopia Lecture Note on:- Concrete  CHAPTER 4: CONCRETE Introduction Concrete is a product obtained artificially by hardening of the mixture of; binding material (cement), fine aggregate (sand), coarse aggregate (gravel), admixtures in some cases, and water, in predetermined proportions. Since concrete is made from different materials which form different parts, it is known as a composite material. The cement and water form a paste that hardens and bonds the aggregates together. Concrete is often looked upon as “man made rock”. The property of concrete depend on the characteristic of the ingredients and the proportion of the mix. In mix proportioning workability, strength, durability and economy should be taken into consideration. For practical concrete mixes, the ingredients should be so proportioned that the resulting concrete has the following properties. When freshly mixed it is workable enough for economical and easy uniform placement, but not excessively fluid. When hardened it possess strength and durability adequate to the purpose for which it is intended. It involves minimum cost consistent with acceptable quality. Functions of the component material Function of cement: is to react with the water forming a plastic mass when the concrete is fresh and a solid mass when the concrete is hard. Function of water: Enabling the chemical reactions which cause setting and hardening to proceed. And also lubricating the mixture of aggregates and cement in order to facilitate placing. Function of the paste: Fills the voids b/n the particles of the inert aggregates and provides lubrication of the fresh plastic mass and upon hardening, it acts as a binder cementing the particles of aggregate together in a permanent solid mass. It also gives strength and water tightness to the hardened mass. Lecture Note on:- Concrete Page 1 Function of Aggregates: Form the inert mineral filler material which the cement paste binds together and reduce the volume changes resulting from the setting and hardening process and from moisture changes in the paste. Function of Admixtures: To modify the properties of ordinary concrete so as to make it more suitable for any situation and to change one or more properties of fresh or hardened concrete Concrete is a material that literally forms the basis of our modern society. Many of the achievements of our modern civilization have depended on concrete. Concrete is the most widely used construction material in the world. It is estimated that the present consumption of concrete in the world is of the order of 10-12 billion tones every year. Humans consume no material except water in such tremendous quantities. Lecture Note on:- Concrete Page 2 Advantages and disadvantages of concrete Concrete is a versatile construction material, adaptable to a wide variety of uses. Lecture Note on:- Concrete Page 3  4.1 Concrete making materials 4.1.1 Aggregate Aggregates are the important constituents in concrete. Aggregates generally occupy 65- 75% of the volume of concrete. Hence due consideration should be given in their selection and proportioning. Earlier, aggregates were considered as chemically inert materials but now it has been recognised that their physical, thermal and at times chemical properties influence those of the concrete. Basically aggregate serves the following purposes:  Form the inert mineral filler material which the cement paste binds together.  Reduce the volume changes resulting from the setting and hardening process and from moisture changes in the paste.  Provides better durability than hydrated cement paste alone.  Economical advantages. In choosing aggregate for use in particular concrete attention should be given to three important requirements:  Workability when fresh for which the size and gradation of the aggregate should be such that undue labour in mixing and placing will not be required.  Strength and durability when hardened for which the aggregate should:  be stronger than the required concrete strength  contain no impurities which adversely affect strength and durability  not go into undesirable reaction with the cement  be resistant to weathering action Lecture Note on:- Concrete Page 4   Economy of the mixture: the aggregate should be  available from local and easily accessible deposit or quarry  well graded in order to minimize paste, hence cement requirement. Classification of aggregates I. Classification based on source As regards the source aggregates may be natural, artificial or recycled. Natural aggregates are obtained from river beds (sand, gravel) or from quarries (crushed rock). Artificial aggregates are generally obtained from industrial wastes such as the blast furnace slag. Recycled Aggregate – e.g. crushed concrete, clay bricks. Lecture Note on:- Concrete Page 5 II. Classification based on mode of formation Lecture Note on:- Concrete Page 6 III. Classification based on weight Based on weight aggregates are divided into three groups A. Heavy weight aggregates: with specific gravity more than four. These includes steel balls, bronze and other metals used in concrete for radiation. B. Normal weight aggregates: with specific gravity b/n 2.4 and 3.0. E.g. basalt, granite, trachyte, etc. C. Light weight aggregates: with specific gravity less than 2 such as pumice, scoria, diatomite, etc. which are used to make light weight concrete. IV. Classification based on size Aggregate bigger than about 4.75mm in diameter is classified as coarse aggregate (Type CA) and the one smaller as fine aggregate (Type FA). V. Classification based on chemical composition Based on chemical composition aggregates are divided into three groups A. Argillaceous: Composed primarily of aluminum oxide (Al2O3) the chief component of clay. Lecture Note on:- Concrete Page 7  B. Siliceous: Composed primarily of silicon dioxide (Si2O) the principal ingredient of quartz sand. C. Calcareous: composed primarily of calcium carbonate or lime (CaCO3). VI. Classification based on condition Crushed: From quarry - sharp, angular particles, rough surface, good bond strength, low workability. Uncrushed: Uncrushed From river - round shapes, smooth surface, low bonding properties, high workability. Properties of aggregates Grading of aggregate The maximum size of aggregate practicable to handle under a given set of conditions should be used. Using the largest possible maximum size will result in:  Reduction of the cement content, - Reduction in water requirement, and Reduction of drying shrinkage. The maximum aggregate that can be used in any given condition may be limited by the following conditions. - Thickness of section - Spacing of reinforcement - Clear cover - Mixing, placing and handling techniques. Lecture Note on:- Concrete Page 8 One of the most important factors for producing workable concrete is good gradation of aggregate. Good grading implies that a sample of aggregates contains all standard fractions of aggregate in required proportion such that the sample contains minimum voids. A good gradation secures increased economy, higher strength, lower shrinkage and greater durability. The grading or particle size distribution of aggregate is determined by a sieve analysis. Sieve analysis is the name given to the operation of dividing a sample of aggregate into various fractions each consisting of particles of the same size. A sample of aggregate for sieve analysis is first surface dried and then sieved through the series, starting with the largest. The material retained on each sieve after shaking represents the fraction of aggregate coarser than the sieve in question and finer than the sieve above. The summation of the material retained on the sieves divided by 100 is called the Fineness Modulus (FM). It is used as an index to the fineness or coarseness and uniformity of aggregate supplied. Lecture Note on:- Concrete Page 9  Fig: Sieve analysis Standard sieve sizes and square openings Lecture Note on:- Concrete Page 10 Grading requirement for fine and coarse aggregate Ex-1 Sieve Analysis Results for Fine Aggregate (sample size = 500g) Lecture Note on:- Concrete Page 11 Gradation Curve for Fine Aggregate Ex-2 Sieve Analysis Results for Coarse Aggregate (sample size = 5108g) Lecture Note on:- Concrete Page 12 Gradation Curve for Coarse Aggregate Combined Grading of aggregate Aggregate is sometimes analyzed using the combined grading of fine and coarse aggregate together, as they exist in a concrete mixture. The combined gradation can be used to better control workability, pumpability, shrinkage, and other properties of concrete. Lecture Note on:- Concrete Page 13 Particle shape The shape of aggregate is an important characteristic since it affects the workability of concrete. Not only the characteristic of the parent rock, but also the type of crusher used will influence the shape of aggregates. From the standpoint of economy in cement for a given w/c ratio, rounded aggregates are preferable to angular aggregates. Angular aggregates give higher strength and sometimes greater durability as a result of interlocking texture in the hardened concrete. Flat particles in concrete aggregates will have particularly objectionable influence on the workability, cement requirement, strength and durability. In general, excessively flat aggregates make very poor concrete. Particle shape Lecture Note on:- Concrete Page 14 Particle texture Surface texture is the property, which depends upon the relative degree to which particle surfaces are polished or dull, smooth or rough. Surface texture depends on hardness, grain size, pore structure, structure of the rock, etc. Hard, dense, fine-grained materials will generally have smooth structure surfaces. As surface smoothness increases, contact area decreases, hence a highly polished particles will have less bonding area with the matrix than a rough particle of the same volume. Lecture Note on:- Concrete Page 15 Specific gravity The specific gravity of a substance is ratio b/n the weight of the substance and that of the same volume of water. This definition assumes that the substance is solid throughout. In concrete technology distinction is made b/n absolute specific gravity, apparent specific gravity and bulk specific gravity. Absolute specific gravity: is the ratio of the mass of a unit volume of a material (without pores) to the same volume of gas-free distilled water. Apparent specific gravity: is the ratio of the weight in air of a material of given volume (solid matter plus impermeable pores or voids) to the weight in air of an equal volume of distilled water. Apparent specific gravity = A/(A-C) Bulk specific gravity: is defined as the ratio of the weight in air of a given volume of a permeable material (including both its permeable and impermeable voids) to the weight in air of an equal volume of water. Bulk specific gravity = A/(B-C) Lecture Note on:- Concrete Page 16 Bulk specific gravity (SSD basis): is defined as the ratio of the weight in air of a permeable material in a saturated surface dry condition to the weight in air of an equal volume of water. Bulk specific gravity (SSD) = B/(B-C) Where: A= weight of the oven dry sample in air. B= weight of SSD sample in air C= weight of saturated sample in water. In computation of quantities for concrete mixes it is the specific gravity of the SSD aggregates that is always used. In metric units, the specific gravity of a material is numerically equal to its weight in grams per cubic centimeter (sometimes called solid unit weight). Bulk density (unit weight) Bulk density is the weight of the aggregate required to fill a container of a specified unit volume. Volume is occupied by both the aggregates and the voids between the aggregate particles. It is affected by the degree of compaction (voids), aggregate moisture (presence of water), size distribution and shape of particles and how densely the aggregate is packed.  Loose bulk density  Rodded or compact bulk density Lecture Note on:- Concrete Page 17 Example: In order to determine bulk and apparent specific gravity, and absorption of coarse aggregate, 5 kg of sample was brought from the site and the following weights were recorded at different condition. Determine i) Moisture Content, ii) Bulk Specific Gravity at saturated-surface-dry basis, iii) Apparent Specific Gravity, and iv) Absorption Capacity. Solution Let: Weight of oven dry sample in air = A Weight of saturated-surface-dry sample in air = B Weight of saturated sample in water = C i) MC = 100*(Weight of Sample - A)/A = 100* (5000-4944)/4944 = 1.13% ii) Bulk Specific Gravity (SSD) = B/(B-C) = (5029)/(5029-3259) = 2.84 iii) Apparent Specific Gravity = A/(A-C) = 4944/(4944-3259) = 2.93 iv) Absorption Capacity = 100*(B-A)/A = 100*(5029-4944)/4944 = 1.72% voids Lecture Note on:- Concrete Page 18 Represents the amount of air space between individual particles in a mass of coarse or fine aggregates. The difference b/n solid unit weight and the bulk density indicate the amount of voids b/n the particle. Void content affects mortar requirements in mix design; water and mortar requirement tend to increase as aggregate void content increases. Void content between aggregate particles increases with increasing aggregate angularity. Void contents range from 30- 45% for coarse aggregates to about 40-50% for fine aggregates. Total volume of voids can be reduced by using a collection of aggregate sizes. The cement paste requirement for concrete is proportional to the void content of the combined aggregate. Porosity, absorption and surface moisture Porosity: is the ratio of the volume of the pores (small holes in aggregate through which water can go inside) in a particle to its total volume. The porosity of aggregate is important since it affects its bulk specific gravity, permeability and absorption which in Lecture Note on:- Concrete Page 19 turn affect the properties of the resulting concrete. Some of the pores are wholly within the solid, and others are on the surface. As regards the moisture content, the various states in which an aggregate may exist are:  Oven-dry: completely dry  Air dry: dry at the surface, some internal moisture, but less than the amount required to saturate the particle.  Saturated surface dry condition: no free moisture on the particle, but all voids with in the particle filled with water.  Damp or wet: saturated and with free or surface moisture on its surface. Lecture Note on:- Concrete Page 20 Absorbed moisture: Weight of water absorbed by dry aggregate particles in reaching the saturated surface dry condition. No water on the surface of a particle but all the pores are filled with water. Absorption Capacity, (%) = [(WSSD – WOD)/WOD] X 100 Effective absorption Capacity, (%) = [(WSSD – Wair dry)/WOD] X 100 Effective absorption Capacity, (%) = [(Wair dry – WOD)/WOD] X 100 Surface moisture: The moisture that is in excess of absorbed moisture. Surface Moisture ,(%) = [(WWET – WSSD)/WSSD] X 100 Total moisture content: The total amount of water present on the external and internal surfaces of aggregates. Total moisture content = Surface moisture + absorbed moisture Total moisture Content (%) = [(WAGG – WOD)/WOD] X 100 The absorption capacity is the measure of the porosity of an aggregate. Lecture Note on:- Concrete Page 21 In calculating or measuring quantities for concrete mix it is important to know the state at which the aggregate is used.  If it is dry some of the mixing water will be absorbed.  If it is wet, the free moisture will become a part of the mixing water. Because of their small size and weight, sand particles are easily pushed and held apart by surface water thereby increasing the total volume per given weight of sand. This phenomenon is known as bulking. The extent of bulking depends on the fineness of the sand, and its free moisture content. The finer the sand the more pronounced the bulking. The volume of the sand goes on increasing with the increase in moisture content up to a certain limit. The effect of bulking of sand should be considered in mix design, especially when volume batching is adopted. Fig: Bulking due to moisture in fine aggregate Lecture Note on:- Concrete Page 22 Shrinkage of aggregate Aggregates with high absorption capacity may have high shrinkage properties on drying.  Large shrinkage: fine grained sandstones, slate, basalt, trap rock, clay-containing Low shrinkage: quartz, limestone, granite, feldspar Chemical reactivity Chemical reactions involving aggregate can lead to serious deterioration problems in concrete. Certain forms of silica and siliceous material in aggregate (e.g. chert) interact with alkalis released during the hydration of Portland cement. This produces a gel like material which increases in volume in the presence of water causing expansion and cracking of concrete. Factors which promote alkali aggregate reaction:  Reactivity type of aggregate  High alkali content in cement  Availability of moisture  Optimum moisture condition Alkali-aggregate reaction can be controlled:  Selection of non-reactive aggregate  By the use of low alkali cement  By controlling the void space in concrete  By controlling moisture condition and temperature  By the use of corrective admixtures such as pozzolana Lecture Note on:- Concrete Page 23 Testing of aggregates The different types of tests conducted on aggregate for determination of flakiness index, elongation index, inorganic impurities, specific gravity, bulk density and voids, porosity and absorption, bulking of sand, sieve analysis, aggregate crushing value, ten percent value, aggregate impact value and abrasion value. Fine aggregate Production of sand in Ethiopia is very primitive. Sand production sites are not mechanized. The production is done by local people of the area using traditional method of collecting the sand from the river bed by donkey Lecture Note on:- Concrete Page 24 4.1.2 Water Water is an important ingredient of concrete as it actively participates in the chemical reaction with cement. In the production of concrete, water is used for the chemical reaction with cement, workability of concrete, washing aggregate and curing process of concrete. A popular yard stick to the suitability of water for mixing concrete is that, if water is fit for drinking it is fit for making concrete. It should be Free from impurities such as suspended solids, silt, clay, acids, alkalis, organic matters and dissolved salts. Effect of impure water A. For Concrete: These impurities may adversely affect the properties of concrete, e.g. setting time, strength and long term durability. Chloride ions (e.g. from sea water) can accelerate corrosion of reinforcing steel. B. For washing aggregates: It affect strength and durability. Harmful substances deposits on the surface of the particles. C. For curing concrete: No harmful effects but may spoil its appearance. 4.1.3 Admixtures Admixture is a material added to plastic (fresh) concrete or mortar before or during mixing. To change one or more properties of fresh or hardened concrete. When the Lecture Note on:- Concrete Page 25 desired modification of properties of fresh or hardened concrete cannot be achieved by changing the composition of the mix proportion or by using different types of cement. Types of admixtures  Chemical admixtures:  Mineral/Pozzolanic admixtures Other varieties of admixture fall into the specialty category e.g. as a corrosion inhibitor, shrinkage reduction, improve workability, etc. Accelerators CaCl2 (most common). Amount added < 2 % of cement weight. Non-chloride admixtures e.g. calcium nitrate. Application:- it is used in cold-weather condition, rapid removal of formwork or urgent repair work. :When early strength is required. Compressive strength at 3 days at least 25 % higher than normal concrete. Problem: Large dosage of CaCl2 cause severe corrosion to steel reinforcement.  increase heat of hydration and drying shrinkage (One type of concrete cracks). Lecture Note on:- Concrete Page 26 Retarders Delay concrete setting time. Lignin, Borax, Sugar and Hydroxyl acid can be taken as an example, Application: High temperature For large structures and difficult situation (e.g. require longer time for difficult pour) Keep concrete workable during placing (eliminate cold joint) Problem: May reduce the strength of concrete at early age. The admixture is based on lignosulfonate (a by product of wood industry) & hydro-carboxylic (HC) acids. Advantages: Increase strength by reducing quantity of mixing water  To improve the workability of concrete  To attain saving in the cement content Water reducers (Superplasticizer (High range water reducer)) Added in a small dosage with mixing water (~ 0.2% by cement weight). Increase strength by reducing quantity of mixing water. Produce flowing concrete (suitable for difficult placement problem. E.g. tight constricted form work or dense reinforcement bar (rebar) configuration, or where the concrete must be pumped over a long distance. Produce smooth surface concrete and less likely to chip and spall. Water cement ratio (W/C) ~ 0.3 to 0.45 is possible. And it increases cost by 5% BUT savings in labor can be as high as 33%. Lecture Note on:- Concrete Page 27 Air entraining admixtures Liquid chemicals added during mixing to produce microscopic disconnected air bubbles in concrete. It has 20 µm – 200 µm air bubbles in diameter in 0.1-0.2 mm spacing. Admixture from wood resins, petroleum acids, animal & vegetable fats and synthetic detergents. Foaming agents entrain 3% - 10% of air. Advantages : Improve workability  Reduce bleeding & segregation  Increase durability of concrete. Protect concrete from freeze-thaw cycle damage. Disadvantages: Lower compressive strength Mineral admixtures (Supplementary cementing materials) Sources: Natural Pozzolanic materials or industrial by-products.  Added in relatively large quantities in comparison with chemical admixtures.  Replace part of the cement content and can be added during concrete mixing process or grind together with cement (Pozzolana cement / blended cement)  Effect of pozzolanic admixtures on concrete: Lower early strength but higher ultimate strength  Lower heat of hydration and less permeable  More durable – less sulfate attack and Alkali-silica reaction (ASR)  Reduce cost & increase workability Example of pozzolanic admixture: Ash, Fly Slag, Silica Fume, Rice Husk Ash & Palm Oil Fuel Ash Lecture Note on:- Concrete Page 28 Fly ash (pulverized fuel ash) By-product of burning powdered coal. Finer than Portland cement. Consists mainly of small spheres of glass of complex composition involving silica, ferric oxide and alumina. Silica fume By-product of the electric arc furnaces in the silicon metal and ferrosilicon alloy industry. Consists of non-crystalline silica (85% - 90% silicon dioxide) Very fine particles – less than 0.1 µm or 100 times finer than Portland cement. Ground Granulated Blastfurnace Slag (GGBS) Granulated blast furnace slag is the material formed when molten blast furnace slag is rapidly chilled by immersion in water. 4.2 Types of Concrete Concrete works are classified as:  Class I - works under the direction of qualified supervisor  Class II – works with lower level of quality Classification based on density: Based on density, concrete is classified as normal weight, light weight, and heavy weight concrete. Lecture Note on:- Concrete Page 29 The aggregate used in making concrete contribute mainly to its density. Normal weight concrete is produced using natural sand and crushed aggregate. For light weight concrete, either light weight aggregates such as pumice, scoria, diatomite, etc. or pyro-processed aggregates are used. These concretes are used for application in which the load of gravity is to be reduced. Heavy weight concrete is produced using high density aggregate such as hematite or scrap steel pieces. These concretes are used for radiation shielding or increasing the weight of the structure for stability purpose. Classification based on strength Concrete can be classified on the basis of strength as follow: There are also many other special concretes such as fiber reinforced concrete (FRC), self compacted concrete (SCC), roller compacted concrete, etc. Lecture Note on:- Concrete Page 30  Special Types of Concrete 4.3 Process of manufacturing of concrete Production of quality concrete requires thorough care exercised at every stage of manufacture of concrete. The various stages of manufacture of concrete are batching, mixing, transporting, placing, compacting, curing and finishing. A. Batching The measurement of materials for making concrete is known as batching. There are two methods of batching: volume and weight batching. i. Volume batching: Volume batching is not a good method for proportioning the material because of the fact that the quantity of solid materials in a container very much depends on its degree of compaction. Volume of moist sand in a loose condition weighs much less than the same volume of dry compacted sand. Volume batching is used for unimportant concrete or for any small job. Cement is always measured by weight. The volume of one bag of cement is taken as 35 litres. Gauge boxes are used for measuring the fine and coarse aggregates. The volume of the box is made equal to the volume of one bag of cement. Lecture Note on:- Concrete Page 31  ii. Weigh batching: is the correct method of measuring materials. For important concrete, invariably, weigh batching system should be adopted. Use of weigh system in batching, facilitates accuracy, flexibility and simplicity. B. Mixing Thorough mixing of the materials is essential for the production of uniform concrete. The mixing should ensure that the mass becomes homogeneous, uniform in colour, and consistency. There are two methods adopted for mixing concrete. i. Hand mixing ii. Mechanical mixing (machine mixing) i. Hand mixing: is practiced for small scale unimportant concrete works. As the mixing cannot be thorough and efficient, it is desirable to add 10% more cement to cater the inferior concrete produced by this method. Hand mixing should be done over an impervious concrete or brick floor, a wooden water tight platform or steel mixing trough of sufficiently large size to take one bag cement. Spread out the measured quantity of coarse and fine aggregate in alternate layers. Pour the cement on top of it, and mix them dry by shovel, turning the mixture over and over again until uniformity of colour is achieved. This uniform mixture is spread in thickness of about 20cm, and Lecture Note on:- Concrete Page 32  water is sprinkled over the mixture and simultaneously turned over. This process is continued till such time a good uniform, homogeneous concrete is obtained. ii. Machine mixing: obviously gives better and uniform mixes than hand mixing. it is generally preferred and recommended for reinforced concrete work and for medium or large scale mass concrete work. Machine mixing is not only efficient, but also economical, when the quantity of concreted to be produced is large. Many types of mixers are available for mixing concrete. They can be classified as batch-mixers and continuous mixers. Batch mixers produce concrete, batch by batch with time interval. Continuous mixers produce concrete continuously without stoppage till such time the plant is working. Continuous type is used for large works whereas in normal concrete work, it is the batch mixers which are used. Batch mixer may be of pan type or drum type. The drum type may be further classified as tilting, non-tilting, reversing or forced action type. About 25% of the total quantity of water required for mixing, should be introduced to the mixer drum to wet the drum and to prevent any cement sticking. About half the quantity of coarse aggregate is placed in the drum over which about half the quantity of fine aggregate is poured. On that, the full quantity of cement i.e. One bag is poured over which the remaining portion of coarse aggregate and fine aggregate is deposited in sequence. Mixing should continue until the sand particle and all the coarse aggregate are completely coated with thoroughly mixed paste and mortar respectively. Lecture Note on:- Concrete Page 33 Mixing Time Concrete mixers are generally designed to run at a speed of 15-20 revolutions per minute. For proper mixing, about 25-30 revolutions are required in a well-designed mixer. The optimum mixing time depends on the type of the mixer, the condition of the mixer, the speed of rotation, the size of the charge and the nature of the constituent material. The mixing time varies b/n 1-2 minutes. Bigger the capacity of the drum more is the mixing time. The quality of concrete in terms of compressive strength will increase with the increase in the time of mixing. But for mixing time beyond two minutes, the improvement in compressive strength is not very significant. Fig: Effect of mixing time on strength of concrete C. Transporting of concrete Concrete can be transported by a variety of methods and equipment. The precaution to be taken while transporting concrete is that the homogeneity obtained at the time of mixing should be maintained while being transported to the final place of deposition. There are different ways of handling concrete, and the choice will depend mostly on the amount of concrete involved, the size and type of construction, the topography of the job site, the location of the batch plant and the relatively cost. The methods adopted for transportation of concrete are a) Mortar pan b) Wheel barrow, hand cart Lecture Note on:- Concrete Page 34  c) crane, bucket and rope way d) Truck mixers and dumpers e) belt conveyors f) Chute g) skip and hoist h) Transit mixer i) Pump and pipe line j) Helicopter Any method of transportation should protect the concrete from the effects of the weather, should not cause undue segregation by excessive jarring or shaking and should maintain concrete quality. Lecture Note on:- Concrete Page 35  D. Placing of concrete It is of utmost importance that the concrete must be placed in a systematic manner to yield optimum results. As far as placing is concerned, the main objective is to deposit the concrete as close as possible to its final position so that the segregation is avoided and the concrete can be fully compacted. To achieve this objective, the following rules should be borne in mind.  the concrete should be placed in uniform layers, not in large heaps or sloping layers;  the rates of placing and compacting should be equal;  where a good finish and uniform color are required on column and walls, forms should be filled at a rate of at least 2m per hour, avoiding delays (long delays can result in the formation of cold joints);  each layer should be fully compacted before placing the one, and each subsequent layer should be placed whilst the underlying layer is still plastic so that monolithic construction is achieved; Lecture Note on:- Concrete Page 36  collision between concrete and formwork or reinforcement should be avoided. For deep section a long down pipe or termite ensures accuracy of location of the concrete and minimum segregation;  concrete should be placed in a vertical plane. When placing in horizontal or sloping forms, the concrete should be placed vertically against, and not away from the previously placed concrete. Lecture Note on:- Concrete Page 37  E. Compaction Compaction is one of the last, but important steps in concrete making, because the density, strength and durability of the concrete depend so much on it. Compaction of concrete is the process adopted for expelling the entrapped air from the concrete. If this entrapped air is not removed fully, the concrete loses strength considerably. 1 % entrapped air cause 5-6 % reduction in concrete strength. The following methods are adopted for compacting the concrete: a. Hand compaction : Rodding, Ramming and Tamping b. Compaction by vibration, Internal vibrator (needle vibrator), Formwork vibrator (external vibrator), Table vibrator, Platform vibrator, Surface vibrator (screed vibrator) and Vibratory roller c. Compaction by pressure and jolting d. Compaction by spinning New filling shall be vibrated while the concrete is plastic, preferably within one hour. The duration of vibration is dependent on the height of the layer, the size and characteristic of the vibrator, and the workability of the concrete mix. Lecture Note on:- Concrete Page 38  F. Curing of concrete Moisture is necessary for the proper hardening of concrete because the chemical reaction that results in the setting and hardening of the paste takes place only in the presence of water. The loss of water by evaporation from the time the concrete is mixed and placed is usually so rapid that there may not be enough of it left for full hydration and hardening. Excessive loss of water due to evaporation may cause the hydration process to stop all together with a consequent reduced strength development. In addition, if concrete dries out too quickly by exposure to sun and wind, it will shrink. This early and usually rapid shrinkage will result in tensile stresses that will lead to surface cracks. It is important therefore that fresh concrete be kept moist for several days after placing. Curing can be described as keeping the concrete moist and warm enough so that the hydration of cement can continue. The purpose of curing can be summarized as follows: i. Curing is to prevent formation of surface cracks due to rapid loss of water while the concrete is fresh and weak. ii. To assure attainment of strength by providing enough moisture for the hydration of the cement grains throughout the concrete. Curing methods may be divided broadly into four categories: a. Water curing: This is by far the best method of curing as it satisfies all the requirement of curing, namely, promotion of hydration, elimination of shrinkage and absorption of the heat of hydration. Water curing can be done using the following methods; iimmersion, ponding, spraying or fogging and wet covering. b. Membrane curing: Concrete could be covered with membrane which will effectively seal off the evaporation of water from the concrete. c. Application of heat: When concrete is subjected to higher temperature it accelerates the hydration process resulting in faster development of strength. Subjecting the concrete to higher temperature and maintaining the required wetness can be achieved by subjecting the concrete to steam curing. It is most often used in prefabrication of concrete elements. Lecture Note on:- Concrete Page 39  d. Miscellaneous method of curing: Calcium chloride is used either as surface coating or as an admixture. It has been satisfactorily used as curing medium. Calcium chloride being a salt, shows affinity for moisture. The salt, not only absorbs moisture from atmosphere but also retains it at the surface. 4.4 Fresh concrete Having considered the ingredients of concrete, we should now address ourselves to the properties of freshly mixed concrete. Since the long-term properties of hardened concrete; strength, volume stability, and durability are seriously affected by its degree of compaction, it is vital that the consistence or workability of the fresh concrete be such that the concrete can be properly compacted and also that it can be transported, placed, and moulded into the desired shape. For hardened concrete to be of an acceptable quality for a given job, the fresh concrete must satisfy the following requirements:  It must be easily mixed and transported.  It must be uniform throughout a given batch or b/n batches.  It should have flow properties such that it is capable of completely filling the forms for which it was designed. Lecture Note on:- Concrete Page 40   It must have the ability to be compacted fully with out an excessive amount of energy being applied.  It must not segregate during placing and consolidation.  It must be capable of being finished properly, either against the forms or by means of trowelling or other surface treatment. 4.4.1 Properties of fresh concrete Workability ASTM has defined workability as “property determining the effect required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity.” ACI has defined workability as “the property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished. ” Other definitions  The property or group of properties which determines the ease with which a material can be used to give a product of the required properties.  The combined effect of those properties of fresh concrete that determines the amount of work required for placement and compaction that determines the resistance to segregation. Workability comprises three separate properties: i. Compatibility or the ease with which the concrete can be compacted and the air voids be removed. ii. Mobility or the ease with which concrete can flow into moulds, around reinforcing steel and be remoulded. Lecture Note on:- Concrete Page 41  iii. Stability or the ability of concrete to remain a stable, coherent homogeneous mass during handling and vibration with out the constituents segregating. Consistency Consistency is the term used to denote the degree of fluidity or mobility of concrete. The degree of consistency of a concrete mixture can be described as stiff, plastic, and flowing. Note: Every job requires a particular workability. Factors affecting workability of concrete Several factors affect the workability of concrete: A. Water content: Water content in a given volume of concrete, will have significant influences on the workability. The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete. B. Mix proportions: In case of rich concrete with lower aggregate/cement ratio, more paste is available to make the mix more workable. C. Size of aggregate: For a given quantity of water and paste, bigger size of aggregates will give higher workability. Lecture Note on:- Concrete Page 42  D. Shape of aggregates: Angular, elongated or flaky aggregates makes the concrete very harsh when compared to rounded aggregates or cubical shaped aggregates. E. Surface texture: Rough textured aggregates will show poor workability and smooth or glassy textured aggregates will give better workability. F. Grading of aggregates: This is one of the factor which will have maximum influence on workability. The better the grading, the less is the void content and higher the workability. G. Use of admixtures: The plasticizers and superplasticizer greatly improve the workability by many folds. H. Effect of environmental conditions: The workability of the concrete is also affected by the temperature of concrete and therefore, by the ambient temperature. The amount of mixing water required to bring about certain changes in the workability also increases with temperature. I. Effect of time: Fresh concrete loses workability with time mainly because of the loss of moisture due to evaporation. Measurement of workability The following tests are commonly employed to measure workability: Lecture Note on:- Concrete Page 43  A. Slump test: it is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or site work. It is conveniently used as a control test and gives an indication of the uniformity of the concrete from batch to batch. It is not a suitable method for very wet or very dry concrete. It doesn’t measure all factors contributing to workability, nor it is always representative of the placability of concrete. Three types of slumps can be observed:  True slump: the sample slumps evenly all around. These type of slump indicates a well proportioned concrete.  Shear slump: part of the top cone might shear off and slide down an inclined plane. Shear slump indicates that the concrete is non cohesive and shows the characteristic of segregation.  Collapse slump: the cone could completely collapse. Lecture Note on:- Concrete Page 44  Examples of the approximate ranges of slump for different workability. B. Compaction factor test: Is used to determine the degree of compaction achieved by a standard amount of work. Concrete mixture is put in top hopper. Allowed to fall into 2nd hopper then to cylinder. Top of cylinder is struck off. Concrete is weighed. Compared with weight of fully compacted concrete in cylinder. Lecture Note on:- Concrete Page 45  c. Vebe time test: Measures the work (time) needed to compact concrete. Very suitable for very dry concrete whose slump value can not be measured by slump test. The concrete is packed into a cone (similar to slump test). The cone stands within a special cylinder on a platform and lifted. The container is vibrated and the time taken for the concrete to be compacted flat by glass plate is Vebe time. Compared with weight of fully compacted concrete in cylinder. D. Flow test: Gives an indication of the quality of concrete with respect to consistency, cohesiveness and the proneness to segregation. In this test, a standard mass of concrete is subjected to jolting. The spread or flow of the concrete is measured and the flow is related to workability. Bleeding When a newly placed concrete sets and consolidates part of its surplus water appears on the surface when the solids settle through the body of water. This tendency for water to rise in freshly placed concrete is known as bleeding or water gain. It results from the inability of the constituent materials to hold all the mixing water as the relatively heavy solids settle. The rising water tends to carry with it many fine particles which weakens the top portion and in extreme cases form a scum called “laitance” over the surface. Lecture Note on:- Concrete Page 46 Bleeding is predominantly observed in a highly wet mix, badly proportioned and insufficiently mixed concrete. Water while travelling from bottom to top, makes continuous channels. This continuous bleeding channels are often responsible for causing permeability of the concrete structures. Bleeding rate increases with time up to about one hour or so and thereafter the rate decreases but continues more or less till the final setting time of cement. Bleeding is an inherent phenomenon. However, it can be reduced by proper proportioning, uniform and complete mixing, the use of finely divided pozollanic material, the use of air entraining agent, using cement with high C3A content, using cement with lower alkali content and using rich mix. Segregation Segregation can be defined as the separation of the constituent materials of concrete. A good concrete is one in which all the ingredients are properly distributed to make a homogeneous mixture. Segregation may be of three types:  The coarse aggregate separating out or settling down from the rest of the matrix.  The paste separating away from coarse aggregate.  Water separating out from the rest of the material. Problem:  Reduction in concrete strength  Lack of homogeneity is also going to induce all undesirable properties in the hardened concrete  Causes of segregation Lecture Note on:- Concrete Page 47  Badly proportioned mix where sufficient paste is not there to bind and contain the aggregates.  Insufficiently mixed concrete.  High workability (excess water content) or poor grading (excess coarse aggregate).  Dropping fresh concrete from a height  Conveyance of concrete by conveyor belt, wheel barrow, dumper, etc to a long distance.  Excessive or inadequate vibration. 4.4.2 Hardened Concrete Lecture Note on:- Concrete Page 48  The desired characteristics of concrete vary from one construction to the other and as such, they should be considered in relation to the quality required. Heat of hydration and thermal cracks Heat is liberated due to the exothermic chemical reaction between cement and water. In massive structures, the heat cannot be readily released causing high internal temperatures especially during hot weather. As the interior concrete increases in temperature and expands, the surface concrete may be cooling and contracting. If the temperature difference between the surface and the center is too great, thermal crack may occur or if the pour is restrained, crack due to drying shrinkage can occur. Shrinkage and Creep Shrinkage Volume change is one of the most detrimental properties of concrete, which affects the long term strength and durability. The term shrinkage is loosely used to describe the various aspects of volume changes in concrete due to loss of moisture at different stages due to different reasons. Shrinkage can be classified in the following way: i. Plastic Shrinkage Shrinkage of this type manifests itself soon after the concrete is placed in the form while the concrete is still in the plastic state. Plastic shrinkage is considered to be reduction of volume of plastic concrete (typically during first 4 hours after placement). Loss of water by evaporation from the surface of concrete or absorption by the aggregate or subgrade, is believed to be the reason of plastic shrinkage. Plastic shrinkage can be reduced mainly by preventing the rapid loss of water. Lecture Note on:- Concrete Page 49 ii. Drying Shrinkage Shrinkage due to drying of hardened concrete. The drying shrinkage of concrete is analogues to the mechanism of drying of timber specimen. The loss of free water contained in hardened concrete, does not result in any appreciable dimension change. It is the loss of water held in gel pores that cause the change in volume. Drying shrinkage of concrete is affected by:  Unit water content,  Cement content and quantity of the paste,  Composition and fineness of cement,  Type and grading of the aggregate, Size and shape of the concrete mass, and Curing condition. Creep Creep can be defined as the “time dependent” part of the strain resulting from stress. The gradual increase in strain, without increase in stress, with the time is due to creep. From this explanation creep can be defined as the increase in strain under sustained stress. Creep is a very gradual change in length (deformation) which occurs over time when a material is subjected to sustained load. Factors which influence creep are: ♦ Applied stress ♦ W/c ratio ♦ Curing condition ♦ Temperature ♦ Moisture ♦ Cement composition ♦ Chemical admixture ♦ Specimen geometry Lecture Note on:- Concrete Page 50 Strength of concrete Of the various strength properties of concrete it is generally the compressive strength which attracts the greatest interest. Since most concrete structures are designed to resist compressive stress, it is this property which usually prescribed by codes. The strength of concrete primarily depends up on the strength of cement paste. The strength of cement paste increases with cement content and decreases with air and water content. The strength of concrete is affected by a number of factors: A. Effects of water/cement ratio and degree of compaction: The water-cement ratio is the main factor affecting the compressive strength of concrete at all ages. Lower water/cement ratios lead to higher strengths. Every increase of 0.01 in the watercement ratio decreases the strength by 1-1.5 N/mm2. Fig: Relationship b/n strength and water/cement ratio B. Effect of cement: The effect of cement on strength of concrete is dependent both on its type and quantity. The early strength of cement is related to tricalicium silicate Lecture Note on:- Concrete Page 51  (C3S) content – the higher the C3S content relative to the C2S content, more quickly the strength gained after mixing. Higher cement content increases strength and the heat generated. Finer cement causes faster hydration rate, more heat and faster strength development. Storage of cement: The quality of cement stored in bags gradually deteriorates due to hydration. The loses in strength for different periods of storage are 15% in 3 months, 30% in 6 months, and 50% in a year. C. Effect of aggregates : For a constant water cement ratio and the same degree of compaction, the compressive strength of concrete decreases when the specific surface area of the aggregate increases. For the same cement content and degree of compaction, when the quantity of fine is increased, the demand for higher amount of water arises and consequently leads to a weaker concrete. Size: Too large or too fine aggregate decrease strength. Shape and Texture: Crushed or rough surface provides better early strength and similar long term strength as smooth aggregate. Gradation: well grade aggregates insures better strength. Lecture Note on:- Concrete Page 52  Fig: The influence of maximum size of aggregate on compressive strength D. Effect of Age and curing condition: From an age of about 12 hours, the strength of concrete increases rapidly with time. Correlations between strength at different ages are important since they often form the basis of 28 day, or later, strength prediction, by testing at early ages. Best strength is ultimately obtained with concrete that is continuously moist cured. Without moist curing, potential strength may reduce by 50%. Lecture Note on:- Concrete Page 53 E. Effect of compaction: The presence of 1% voids in the mix reduces the strength of concrete by 5%. With improper compaction and 5% voids, a well proportioned concrete of strength 20 N/mm2 would actually exhibit strength of 15 N/mm2 only. Factors Affecting the measured compressive strength are stress Distribution in Specimens, effect of l/d Ratio of cylinder specimen, specimen Geometry, rate of loading, moisture content, temperature at testing, direction of loading and duration of loading. Durability of concrete In practice, concrete is designed and constructed in order to build permanent structures. However, at times, its service life may be markedly reduced by the disintegrating effects of either the environment to which it is exposed or by internal causes within its mass. The durability of concrete is defined as its ability to resist weathering action, chemical attack, abrasion, or any other process of deterioration. Durable concrete will retain its original form, quality, and serviceability when exposed to its environment. The environmental cause may be: Lecture Note on:- Concrete Page 54  Physical: Weathering due to the action of rain and freezing and thawing and also dimensional changes (expansion and contraction) resulting from temperature variations and/or alternate wetting and drying,  Chemical: due to aggressive waters containing sulfates, leaching in hydraulic structures, and chemical corrosion.  Mechanical wear: by abrasion from pedestrian or vehicular use, by wave action in structures along the seashore or erosion from the action of flowing water. Two key factors affecting durability are compressive strength and permeability. Low strength and high permeability decrease durability. Concrete testing: is conducted to ensure that the laboratory mix design was adequate, indicate the statistical variability in the properties of the concrete, reveal problems arising due to inadvertent changes, ensure that all parties in concrete production, do not become careless, important in carrying out quality control and compliance. Types of concrete tests: A. Destructive test  Compression test  Beam bending strength test  Cylinder splitting test  Concrete core test A. Non destructive test  Ultrasonic pulse velocity test  Rebound hammer test (Schmidt hammer test)  Cover-meter test Lecture Note on:- Concrete Page 55  Compression tests Why is compressive strength test the most common of all tests on hardened concrete?  Most of the important properties of concrete are directly related to the compressive strength.  The structural design codes are based mainly on the compressive strength.  The test is easy and relatively inexpensive to carry out.  Most of the quality control and compliance criteria adopt compressive strength. Lecture Note on:- Concrete Page 56 Factors Affecting the Measured compressive strength Specimen Geometry: strength and the variability in strength of concrete decrease as the specimen size increases. Rate of loading: The higher the rate of loading, the higher the measured strength. Temperature at testing: The temperature of the specimen at the time of testing will affect the strength. Moisture content: It has been found that concrete that has been dried shows an increase in strength. Lecture Note on:- Concrete Page 57  4.5 Mix Design Process Introduction The proportioning of concrete mixtures, more commonly referred to as mix design, is a process that consists of two interrelated steps:  Selection of the suitable ingredients (cement, aggregate, water and admixtures) of concrete and  Determining their relative quantities (“proportioning”) to produce, as economically as possible, concrete of the appropriate workability, strength and durability. A. Economy: the cost of concrete is made up of the costs of materials, labor, and equipment. However, except for some special concretes, the costs of labor and equipment are largely independent of the type and quality of concrete produced. It is therefore the material costs that are most important in determining the relative costs of different mix designs. Since cement is much more expensive than aggregate, it is clear that minimizing the cement content is the most important single factor in reducing concrete costs. This can, in general, be done by using the lowest slump that will permit adequate placement, by using the largest practical maximum size of aggregate, by using the optimum ratio of coarse to fine aggregates, where necessary, by using appropriate admixtures. B. Workability: a properly designed mix must be capable of being placed and compacted properly with the equipment available. Finishability must be adequate, and segregation and bleeding should be minimized. As a general rule, the concrete should be supplied at the minimum workability that will permit adequate placement. For concrete without mineral admixtures, the water requirement for workability depends mostly on the characteristics of the aggregate rather than those of the cement. C. Strength and Durability: In general, concrete specification will require a minimum compressive strength. They may also impose limitations on the permissible w/c ratio and minimum cement contents. It is important to ensure that these requirements are not mutually incompatible. Specifications may also require that the concrete meet Lecture Note on:- Concrete Page 58 certain durability requirements, such as resistance to freezing and thawing or chemical attack. Limiting Values: It is obvious to encounter limiting values in many specifications. The limiting values may cover a range of properties; the more usual ones areMinimum compressive strength necessary from structural considerations;  Maximum water/cement ratio and/or minimum cement content and, in certain conditions of exposure, a minimum content of entrained air to give adequate durability;  Maximum cement content to avoid cracking due to the temperature cycle in mass concrete;  Maximum cement content to avoid shrinkage cracking under conditions of exposure to a low humidity; and  Minimum density for gravity dams and similar structures. Mix Design Methods: Some of the prevalent concrete mix design methods are:  ACI: American Concrete Institute Mix Design Method,  DOE: Department of Environment Mix design practice (British),  DIN Mix design Method (German)  IS: Indian Standard Mix Design Method ACI mix design process To the extent possible, selection of concrete proportions should be based on test data or experience with the materials actually to be used. The following information for available materials will be useful; sieve analyses of fine and coarse aggregates, unit weight of coarse aggregate, bulk specific gravities and absorptions of aggregates, specific gravities of Portland cement and other cementitious materials, if used. Optimum combination of coarse aggregates to meet the maximum density grading. Lecture Note on:- Concrete Page 59 Procedure Step-1: Choice of slump If slump is not specified, a value appropriate for the work can be selected from Table-1 Table-1: Recommended slumps for various types of construction Step-2: Choice of maximum size of aggregate Large nominal maximum sizes of well graded aggregates have less voids than smaller sizes. Hence, concretes with the larger-sized aggregates require less mortar per unit volume of concrete. Generally, the nominal maximum size of aggregate should be the largest that is economically available and consistent with dimensions of the structure. Step-3: Estimation of mixing water and air content The quantity of water per unit volume of concrete required to produce a given slump is dependent on the nominal maximum size, particle shape, grading of the aggregates;  the concrete temperature;  the amount of entrained air; and use of chemical admixtures. Table-2 provides estimates of required mixing water for concrete made with various maximum sizes of aggregate, with and without air entrainment. Table-2: Approximate mixing water and air content requirements Lecture Note on:- Concrete Page 60 Step-4: Selection of water-cement or water-cementitious materials ratio Approximate and relatively conservative values for concrete containing Type I Portland cement can be taken from Table-3a. Table-3a: Relationships between water-cement ratio and compressive strength of concrete For severe conditions of exposure, the w/c or w/(c + p) ratio should be kept low even though strength requirements may be met with a higher value. Table-3b gives limiting values. Table-3b: Maximum permissible water-cement ratios for concrete in severe exposure Lecture Note on:- Concrete Page 61 Step-5: Calculation of cement content The amount of cement per unit volume of concrete is fixed by the determinations made in Steps 3 and 4 above. The required cement is equal to the estimated mixing-water content (Step 3) divided by the water-cement ratio (Step 4). If, however, the specification includes a separate minimum limit on cement in addition to requirements for strength and durability, the mixture must be based on whichever criterion leads to the larger amount of cement. Step-6: Estimation of coarse aggregate content Appropriate values for this aggregate volume are given in Table-4. Table-4: Volume of coarse aggregate per unit volume of concrete Step-7: Estimation of fine aggregate content Lecture Note on:- Concrete Page 62 At completion of Step 6, all ingredients of the concrete have been estimated except the fine aggregate. Its quantity is determined by difference. Either of two procedures may be employed: the weight method or the absolute volume method. The weight method: If the weight of the concrete per unit volume is assumed or can be estimated from experience, the required weight of fine aggregate is simply the difference between the weight of fresh concrete and the total weight of the other ingredients. First estimate of weight of fresh concrete can be determined from Table- 5. Table-5: First estimate of weight of fresh concrete If a theoretically exact calculation of fresh concrete weight per m3 is desired, the following formula can be used. Step-8: Adjustment for moisture aggregate The aggregate quantities actually to be weighed out for the concrete must allow for moisture in the aggregates. Generally, the aggregates will be moist and their dry weights should be increased by the percentage of water they contain, both absorbed and surface. Lecture Note on:- Concrete Page 63 The mixing water added to the batch must be reduced by an amount equal to the free moisture contributed by the aggregate -- i.e., total moisture minus absorption. Step-9: Trial batch adjustments The calculated mixture proportions should be checked by means of trial batches prepared and tested in accordance with ASTM C 192 Example-1 Concrete is required for a portion of a structure that will be below ground level in a location where it will not be exposed to severe weathering or sulfate attack. Structural considerations require it to have an average 28-day compressive strength of 24 Mpa (cylindrical). It is determined that under the conditions of placement to be employed, a slump of 75-100 mm should be used. and the coarse aggregate has a nominal maximum size of 37.5 mm and dryrodded mass of 1600 kg/m3.  Other properties of the ingredients are: cement -Type I with specific gravity of 3.15; coarse aggregate - bulk specific gravity 2.68 and absorption 0.5 percent; fine aggregate - bulk specific gravity 2.64, absorption 0.7 percent, and fineness modulus 2.8.  Calculate the weights of all materials that you would use for the first trial mix on a concrete mix. Solution Step 1: The slump is required to be 75 to 100 mm. Step 2: The aggregate to be used has a nominal maximum size of 37.5 mm. Step 3: The concrete will be non-air entrained since the structure is not exposed to severe weathering. From Table-2, the estimated mixing water for a slump of 75 to 100 mm in non-airentrained concrete made with 37.5 mm aggregate is found to be 181 kg/m3. Lecture Note on:- Concrete Page 64 Step 4: The water-cement ratio for non-air entrained concrete with a strength of 24 MPa is found from Table-3a to be 0.62. Step-5: From the information developed in Steps 3 and 4, the required cement content is found to be 181/0.62 = 292 kg/m3. Step 6: The quantity of coarse aggregate is estimated from Table-4. For a fine aggregate having a fineness modulus of 2.8 and a 37.5 mm nominal maximum size of coarse aggregate, the table indicates that 0.71 m3 of coarse aggregate, on a dry-rodded basis, may be used in each cubic meter of concrete. The required dry mass is, therefore, 0.71 x 1600 = 1136 kg. Step 7: The required fine aggregate may be determined on the basis of either mass or absolute volume as shown below: Mass basis: From Table-5, the mass of a cubic meter of non-air-entrained concrete made with aggregate having a nominal maximum size of 37.5 mm is estimated to be 2410 kg. Masses already known are: The mass of fine aggregate is therefore estimated to be 2410 – 1609 = 801Kg Absolute volume basis: With the quantities of cement, water, and coarse aggregate established, and the approximate entrapped air content of 1 percent determined from Table-2 , the sand content can be calculated as follows: Lecture Note on:- Concrete Page 65 Batch masses per cubic meter of concrete calculated on the two bases are compared below: Step 8: Tests indicate total moisture of 2 percent in the coarse aggregate and 6 percent in the fine aggregate. If the trial batch proportions based on assumed concrete mass are used, the adjusted aggregate masses become Absorbed water does not become part of the mixing water and must be excluded from the adjustment in added water. Thus, surface water contributed by the coarse aggregate amounts to 2 - 0.5 = 1.5%; by the fine aggregate 6 - 0.7 = 5.3%. The estimated requirement for added water, therefore, becomes The estimated batch masses for a cubic meter of concrete are: Lecture Note on:- Concrete Page 66 Step 9: For the laboratory trial batch, it is found convenient to scale the masses down to produce 0.02 m3 of concrete. 4.6 Quality Control Lecture Note on:- Concrete Page 67 Concrete is generally produced in batches at the site with the locally available materials of variable characteristics. This results in a variation of strength from batch to batch also with in the batch. The factor controlling this difference for concrete of a given level of strength is called quality control. In other words, quality control is the control of variation in the properties of the mix ingredients and control of the accuracy of all those operations which affect the strength or consistency of concrete. Thus quality control is a production tool to assure that all aspects of materials equipment’s and workmanship are looked after. Concrete quality control spans a number of critical phases:  Production  Handling and stockpiling  Batching and mixing  Sampling and testing  Slump  Air content  Unit weight  Temperature  Transportation and placement Purpose of quality control Quality control is adopted to achieve the following purposes: o To reduces the variation in strength and consistency of concrete produces in different batches. o To ensures the production of uniform material having desirable characteristics. o To ensure desirable quality at every stage of work. o To reduce the maintenance cost. o To expedite the work. 4.6.1 Quality Control in Concrete Construction Quality of concrete construction on site can be accomplished in three distinct stages as follow: Lecture Note on:- Concrete Page 68   Quality control before concreting  Quality control during concreting  Quality control after construction Quality control before concreting This stage of quality control consists of two steps: Checking of specification requirements regarding excavation, forms, reinforcement and embedded fixtures etc. and Control test on concrete ingredients (i.e. on cement, aggregate & water). Cement  Quality of cement is ascertained by making compressive strength tests on cement cubes. However for effective control cement:  Should be tested initially once for each source and subsequently once for every two months.  Should be protected from moisture  Should be retested after 3 months of storage, if long storage in unavoidable  Should be rejected if large lump are found in cement bags.  Cement  Portland cements shall conform to the requirements of AASHTO M85 (ASTM Cl50).  Blended hydraulic cements shall conform to the requirements of AASHTO M240 (ASTM C595) or ASTM Cl157.  Low-alkali cements conforming to the requirements of AASHTO M85 (ASTM Cl50) shall be used when specified in the contract documents or when ordered by engineers as a condition of use for aggregates of limited alkali-silica reactivity. Aggregates  Concrete aggregates should confirm to specified values as per standard specification. Lecture Note on:- Concrete Page 69   The quality of concrete is affected by different physical and mechanical properties of aggregate, i.e. shape, grading, durability, specific gravity and water absorption etc. these properties of aggregated should be tested before using it for concrete production.  The quantity of deleterious materials and organic impurities should also be tested.  Bulking of sand is also an important property in several ways.  It gives wrong results when volume batching is done.  It increases water cement ratio which in turn reduces strength. For effective control aggregates:  Are required to be tested once initially for approval of source  Should subsequently be tested once or twice daily for moisture content & allowance should be made for moisture content of aggregates. Generally, aggregate should be:  Strong, durable and granular,  Hard with minimum abrasion value,  Well graded,  Not containing flaky and elongated materials,  Free from clay and silt,  Not containing organic materials. Water  The quality of water should be checked as specified in respective standard.  Chemical analysis shall be conducted for approval of source.  In case of suspended impurities, it is necessary to store water for some time to allow them to settle down.  In case of doubt concrete cubes made with this water are tested and average 28 days compressive strength of at least three cubes or cylinders or specified size, prepared with water proposed to be used shall not be less than 90% of the average strength of three similar concrete cubes prepared with distilled water. Lecture Note on:- Concrete Page 70   Water used in mixing and curing of concrete shall be subject to approval and shall be reasonably clean and free of oil, salt, acid, alkali, sugar, vegetable, or other injurious substances.  Water shall be tested in accordance with, and shall meet the requirements of AASHTO T26.  Water should be potable quality to use without tests  Where source of water is relatively shallow, an intake shall be enclosed to exclude silt, mud, grass, or other foreign materials. Air-Entraining and Chemical Admixtures  Air-entraining admixtures shall conform to the requirements of AASHTO M154 (ASTM C260).  Chemical admixtures shall conform to the requirements of AASHTO M194 (ASTM494/C494M). Mineral Admixtures Mineral admixtures in concrete shall conform to the following requirements:  Fly ash pozzolans and calcined natural pozzolans - AASHTO M295 (ASTM C618).  Ground granulated blast-furnace slag- AASHTO M302 (ASTM C989)  Silica fume-AASHTO M307 (ASTM C1240). Quality control checklist Lecture Note on:- Concrete Page 71 Quality control during concreting Lecture Note on:- Concrete Page 72 Careful supervision during concrete manufacture is necessary for all concreting operations such as batching, mixing, transporting, laying, compacting and curing. Following precautions should be taken during concreting operation.  The concrete mix should be designed in the laboratory with the materials to be used on site.  As far as possible concrete should be batched by weight.  If weight batching is not possible, then volume batching may be permitted through proper supervision in the presence of engineer.  During mixing the mixer should be charged to its full capacity.  The materials should be fed in proper sequence.  The speed of the mixer should be range from 15 to 20 revolutions per minute.  The mixing time should not be less than 2 minutes in any case.  Segregation should be avoided while unloading the concrete from the mixer.  Workability of concrete is an important property of concrete while concrete is in its fresh state.  Therefore slump test or compaction factor test should be performed to check workability of concrete.  About three tests should be carried out for every 25 m3 of concrete.  Care should be taken so that no segregation takes place during transportation of concrete.  Concrete should not be dropped from a height of more than 1 m. if the drop height exceeds 1 m chutes should be used.  To avoid re handling of concrete it should be placed at its final position as far as possible.  Vibrators should be used for compacting concrete.  The insertion spacing of internal vibrators should not be more than 0.6 m.  It should be drawn out slowly so that no holes remain in the concrete.  The frequency of vibrators should not be less than 7000 cycles/minutes.  Curing should be done for a specified period so that concrete develops requisite strength.  Concrete should be covered with hessian as soon as it becomes hard.  The form work should correspond to final form of the structure.  It should be checked before concreting is started. Lecture Note on:- Concrete Page 73   The inside of the forms should be cleaned and oiled.  The forms should be removed after the specified period.  Concrete should be protected from hot and cold weather at early ages.  Concreting should not be done at temperature below 4.50C and above 400.  In very hot weather water and aggregates should be cooled.  Retarders of approved quality can be used.  In very cold weather water and aggregates should be heated. Accelerators of approved quality can also be used. Quality control checklist Lecture Note on:- Concrete Page 74 Lecture Note on:- Concrete Page 75 Lecture Note on:- Concrete Page 76 Quality control after construction Once the concrete is laid and compacted, compression tests are made on the cubes. The hardened concrete has to be checked for trueness in dimensions, shape and sizes as per design specification. General surface appearance of concrete should also be checked. Dimensions are ascertained by different measurements. Reinforcement should have adequate concrete cover and if the reinforcement is visible in part of a structure, the part should be rejected or necessary actions should be taken accordingly. Concrete strength is normally to be ascertained from cube or cylinder samples tested at 28 days. In case the strength obtained is less than the specific minimum, one or more of following steps may be taken.  Load test and measurement of deflection and / or strain (the quality of the structure can then be ascertained by calculating back the concrete strength)  Cutting cores from the structures and testing them for strength Nondestructive tests like Schmidt rebound hammer or ultrasonic pulse velocity test.  These tests give only a very rough idea and are primarily used to ascertain the uniformity of construction. Lecture Note on:- Concrete Page 77 Quality control checklist Lecture Note on:- Concrete Page 78