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Cement IS 269

cement is used to bind the materials in concrete  The manufacturing process includes Proportioning of raw materials like chalk or limestone,Grinding and burning at 1400-1450OC in rotatory kiln. Due to intense heat the material becomes like spherical balls called clinkers. Gypsum is then added in 3-4%. Cement is formed by grinding the clinkers to required fineness. Tricalcium Silicate (C3S) hardens rapidly and is largely responsible for initial set and early strength. In general, the early strength of portland cement concrete is higher with increased percentages of C3S. Dicalcium Silicate (C2S) hardens slowly and contributes largely to strength increases at ages beyond 7 days. Tricalcium Aluminate (C3A) liberates a large amount of heat during the first few days of hardening and, together with C3S and C2S may somewhat increase the early strength of the hardening cement (this effect being due to the considerable heat of hydration that this compound evolves). It does affect set times. T
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Why do we test concrete on 28th day? why not on 27th day or 29th day?

Before we go to the answer, we have to peep into the history of civil engineering. As we all know civil engineering and concrete technology were developed and practiced in England in ancient times and now US is leading in advancing the technology further. Concrete achieves its maximum material strength over its life time and depreciated by exposure and other environmental conditions. However, engineers with experience observed that about 99% of expected design strength was achieved in about 30 days and also observed that the rate of gaining strength was reducing thereafter making the concrete achieving its 100% design strength in a year or two. The Church was so powerful those days that it was compulsory for every citizen to attend Sunday prayer without fail. The defaulters were punished severely. If they had to test concrete on the 30th day, it may fall on a Sunday. In order to escape the punishment by the Church, the engineers had two choices - they have to test concrete cube o
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Strength of concrete IS 516

Compressive strength Maximum breaking load is given on the cube speciment untill it breaks and no further load can applied. So, compressive strength can be defined as the load which causes failure of specimen per unit cross sectional area in uniaxial compression machine. Strength of specimen is expressed in N/mm2.  Generally in 1 min the load reading should be 315 kN Rate of loading in compressive testing machine should be 14N/mm2/min.By hand the concrete cube should be filled by 3 layers 35 tampings on each layer by rod 16 mm dia. There should not variation of +- 15 % from the average of the three cubes to each individual readings/ strength Core/cylinder strength Its 0.8 x concrete cube strength, The dia of concrete cylinder should be 150 and height will be 300. The height / diameter ratio will be 2. Fluxural strength Its checked in case of road n runways, the flexural strength is checked of a beam casted 150 x 150 mm x 700 mm size. On tensile side if crack occurs greater th
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Standard Deviation in concrete

Standard deviation is a measure of difference of test results from its average. It can be said that it is a measure of deviation from the Quality of concrete from batching of concrete to pouring point where sample cubes are prepared. A standard value is added in charterstic strength of concrete multiplying 1.65 to find out target mean strength while design mix. 30 samples (consisting 3 cubes each) are prepared to establish standard deviation, if data is not avilable values from IS 456 table 8 can be taken as follows : M-10 and M-15: 3.5, For M-20 and M-25 : 4 N/mm2, Above than M -25 : 5.0 N/mm2
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Joints in concrete 456/IRC 58

Concrete joints are provided to compensate thermal expansion and contraction. The structural separation of concrete members to allow independent movement without damage. They are designed to absorb thermal expansion of concrete. They are commonly provided in bridges, slabs, railways. Bitumen Fibre Board, Dura Board Normally structures exceeding 45 m in length are designed with one or more expansion joints. The pre planned cutting grooves are given in concrete where occurrence of cracks are expected. The depth of groove is 25% of total thickness of the concrete member. Grooves are given in a straight line. These are due to shrinkage or temperature variations. Cutting grooves with blade. Providing glass strips panels not exceeding 3 sqm area in flooring When the work cannot be completed in a single day, construction joints are provided in concrete. These are common in big rafts, beams where joint is provided at L/3 Nito bond, Sika latex,Bitumen Fibre Board, Dura Board
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Gel formation in concrete

C3S present in cement when reacts with H2O it forms C-S-H gel and CaOH2 (lime) with heat. This replaces the water and fills the voids in concrete. CaOH2 again reacts with SiO2 (silica) present in admix and again foms gel and fill the voids, hence this reaction continues inside concrete for years and increase its strength. C3S produces 61% of gel and C2S produces 82% of gel. Hydration is a exothermic recation. C3S is  responsible for 7 days strength and C2S is responsible for 28 days strength.
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Curing of concrete

to prevent loss of moisture from concrete it is contineously cured by water for 7 days in general conditions. For hot weather or in dry conditions curing should not be less than 10 days. If mineral admixtures or PPC is being used then curing should be for 14 days. Concrete has enough water inside it to complete hydration but the surface is kept cured to prevent loss of water from inside. Method of curing may be by ponding in water, Sprinkling, Covering with moist hesian cloths, applying curing compound or by steam curing. Curing may effect strength of structure. The inside temp and outside surface temp of concrete should not be differ than 15OC , otherwise cracks may occur in concrete.
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