aeant Hate ae iiaF va raw MA sod Concrete we Introduction: Concrete is an artificial stone manufactured from a mixture of binding materials and inert materials with water. Conerets nding materials + inert materials + water. Concrete is considered as a chemically combined mass where the inert material acts as filler and the binding material act as a binder. The most common binding materials are cement and lime. The inert materials used in concrete are termed as aggregates. Aggregates are of two types mainly: Fine aggregate: Sand and surki are commonly used as fine aggregate in Bangladesh. Stone screenings, burnt Clays, cinders and fly-ashes are sometimes used as a substitute for sand in making concrete. The fine aggregate should not be larger than 3/16 inch in diameter. 2. Coarse aggregate: Brick khoa (broken bricks), broken stones, gravels, pebbles, clinkers, 3/16 to 2 inch are commonly used as coarse aggregate in Bangladesh. ders etc. of the size of It may be remembered that 3/16 inch is the dividing between fine and coarse aggregate. Qualities of aggregates: ‘The aggregate gives volume to the concrete (at least three quarters of the volume of concrete). The coarse aggregate should be clean, strong, durable and well graded and should be free from impurities and deleterious materials such as salts, coal residue etc. Functions of water in concrete: 1. Towet the surface of the aggregates to develop adhesion because the cement paste adheres quickly and satisfactory to the wet surface of the aggregates than to a dry surface. 2. To prepare a plastic mixture of the various ingredients and to impart workability to concrete to facilitate placing in the desired positions, and 3. Water is also needed for the hydration of the cementing materials to set and harden during the period of curing Classification of concrete: . There are mainly two types of concrete: 1. Lime concrete = lime + surki + khoa + water 2. Cement concrete = cement + sand + khoa or stones + water, 1, Lime concrete:he proportion 1:2:5, unless otherwise thoa or stone is added to it. ixture is obtained, Concrete itis mixed and the concrete Lime conerete consists of lime, surki and khoa or stones in t specified. The lime and surki should be mixed first, and then soaked kh Water is then gradually added and mixing is continued till a workable mi should be mixed in such a quantity that it can be utilized during the day it which has been left overnight should not be used. Concrete is to be laid in layers not exceeding 3 inch in thickness, Each layer should be thoroughly compacted before the next layer is laid, Concrete is to be kept well wetted while setting and at least 7 days after the final layer is completed. Lime concrete is used mainly in foundation and in terrace roofing 2. Cement concrete: It consists of cement, sand and brick khoa or stones of required size in the proportion 1:2:4 or 6, unless otherwise specified. In measuring cement, generally 90 Ibs will be taken as 1 cu ft and hence 1 bag of cement equals 1.25 cu ft. The soaked fine and coarse are mixed first, and cement is. then added and mixed thoroughly. When the coarse aggregate has been soaked thoroughly, the approximate quantity of water is 4.5 gallons for one cu ft of cement in a concrete of 1:2:4. The concrete is to be prepared in small quantities which can be used within 45 minutes and any unused concrete after that time should not be used. The process of curing is to be applied for 28 days to attain its proper strength. Special terms: 1. Segregation Segregation can be defined as separation of the constituent of a heterogeneous mixture so that their distribution is on longer uniform. In case of concrete, it is the difference in the size of particles and in the specific gravity of the mix constituents that are the primary caused of segregation, but its extent can be controlled by the choice of suitable grading and water/ cement ratio, by care in handling transporting and placing of concrete, and also by entrainment. In concrete, there are two forms of segregation. First the coarse particles tend to travel further along a slope or settle more than finer particles. The second form of segregation, occurring particularly in wet mixes, is manifested by the separation of grout (cement plus water) from the imix, With some grading where a lean mix is ysed, the first type of segregation may occur if the mix is too dry; addition of water would improve the cohesion of the mix but when the mix becomes too wet, second type of segregation would take place. 2. Bleeding Bleeding (known also as ‘water gain’) is a form of segregation in which some of the water in the concrete mix tends to rise to the surface of freshly placed concretes. This is caused by the inability of the solid constituent of the mix to hold all of the mixing water when they settle downwards, Bleeding can be expressed quantitatively as the total settlement per unit height of concrete. ‘As a result of bleeding the top of concrete becomes too wet and if the water is trapped by super imposed concrete a porous, weak and non-durable concrete will result. If the bleeding water isremixed during finishing of the top surface, a weak wearing surface will be found. This can be avoided by delaying the finishing operation until the bleeding water has evaporated. On the other hand, if evaporation of water from the surface af the concrete Is faster than the bleeding rate, plastic shrinkage cracking may result. ‘Some of the rising water becomes trapped on the underside of the particles of coarse aggregate or of reinforcement thus creating zones of poor bond, The tendency of bleeding depend largely up to the properties of cement and is also affected by certain chernical factors: there are less bleeding when the cement has a high alkali content, a CsA content or when calcium chloride is added. Rich mixed are less prone to bleeding than lean ones, Reduction in bleeding is obtained by the addition of pozzolona or of an aluminium powder, Air entrainment also effectively reduces bleeding. Physical properties of fine aggregates also affect bleeding Laitance: Laitance is the formation of a crust of squeezed out mortar on the surface of concrete. This is due to bleeding and bad workmanship. This produces a bad or weak concrete. Properties of concrete: ‘The following are the desirable properties of concrete: Strength Elastic properties Fatigue Durability Impermeability Work Strength: Strengths of concrete are of following types: a Compressive strength: ‘Two types of test specimen are used in our country: (I) cubes and (I!) cylinder. The cube specimens of concrete of a desired proportion are cast in steel or cast iron moulds, normally 6 inch cube. The standard cylinder specimen of concrete is 6 inch in diameter and 12 inch high and is cast in a mould generally made of cast iron. The standard practice is to fill the cube mould with concrete in three equal layers. Each layer of concrete is compacted by not less than 35 strokes of 1 inch square steel punner. The top surface. of the cube is to be finished smooth by means of a trowel. Cylindrical specimen are made in a similar way to the cubes but are compacted either in three layers using a 5/8 inch diameter rod or in two layers by means of an immersion of vibrator. After preparation, the cubes and cylindets are stored undisturbed for 24 hours at a temperature of 58°to