amardeep singh sohal

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 time...

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...

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 greate...

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

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

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.

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.

Due to high temp and humidity if the rate of moisture loss from concrete raises from eveporation, cracks will appear in concrete. It also occurs due to thermal expansion and heat of hydration in concrete. Hence temp control concrete may be provided with temp below than 25OC while pouring. Shrinkage of concrete may also results in cracks. Other reasons are acid alkali reaction of aggregates, sulphates, chlorides, excess vibration, absorption of water by form work, sub grade, presence of silts in sand etc.

As per IS 456 air contents in concrete should not exceed 5 +-1% for 20 mm aggregate and 4+-1% for 40 mm aggregate concrete. It can be checked theoretically by dividing density of concrete from batching plant to the density as per mix design multiplying by 100. or it can be checked by air pressure method. the decrees in volume in container give the %age of air by giving pressure. However concrete being impermeable there should be 0% air voids ar as taken in mix design.

RAPID CHLORIDE PENETRATION TEST In this test 95 mm dia and 50 mm thick disc is fixed between NaCl and NaOH solution. Then current is passed @ 60 volt for 6 hours and passed current measured in couloumbs its called chloride ion peremiablity. ( actually in this test electrical conductivity of concrete is measured, which gives a corelation to permiablity) generally its limit is 1000 to 1500 coulombs at 28 days, but at 91 days its also permitted.

As per IS 456 table 5 for RCC - M 20 : 0.55, M 25 : 0.50, M 30 : 0.45, M 35 : 0.45, M 40 : 0.40 C2S present in cement requires 21 % of water and C3S requires 24% water for complete reaction in concrete. The C-S-H gel requires 15% water to fill all voids inside concrete. Hence average of (21+24 /2 = 23%) + 15% = 38% water is at least required in concrete. However it may be lowered by using admixtures.

Spraying cement sand mix either in dry form of in wet form by hose with high velocity and pressure at higher elevation surfaces to immediate stick on surface is a shortcrete process. Its also called Gunite. Its used in tunnels or sloped portions. Rapid hardening cement is used.

Its used mainly where concrete comes in contact with water while pouring, 10% cement is incresed in under water concrete. W/c of concrete is max 0.6. Concrete shall contain 10 percent more cement than that required for the same mix placed in the dry to compensate the loss due to wash. Concrete shall not be placed in water having a temperature below 5oC. The temperature of the concrete, when deposited, shall not be less than 16oC, nor more than 40oC. As tremie method of concreting is not under water concreting, there is no need to add 10 percent extra cement.

Self – compacting concrete (SCC) is a fluid mixture, which is suitable for placing difficult conditions and also in congested reinforcement, without vibration.The normal maximum size is generally 16 – 20 mm. however particle size up to 40 mm more have been used in SCC. Other material are Cement 350 kg to 450 kg/cum, Fly ash, Silica, GGBS, VMA, Super plastisizer, water and fibres. The amount of fines less than 0.125 mm is to be considered as powder and is very important for the rheology of the SCC. 10 mm aggregates should not have fines (2.36 passing) more than 5%. 

For nuclear power projects - dome , or where special precautions to be taken for concrete for x rays, radiations etc high density concrete is provided. Limonite, Barite, Magnelite, Iron may be used to achieve density. Density may be higher to normal concrete by 50%. It s density should be 3360 to 3840 kg/cum. Now days high density of 5280 kg is produced by mixing iron.

Plums are stone with min 10 N/mm2 strength. Size of stone should be 200 - 300 mm.it has to be done with 1:4:8 concrete. Gap between stones should not be less than 150 mm. length of stone should not exceed 3 times of its height. It’s a type of masonary where stones are laid in layers between concrete. It’s a replacement of compacted soil.

Any vol of concrete with dimensions large enough to require measures to be taken for generation of heat from hydration of cement and to minimise cracks. In dams the concrete is called mass concrete, there is less importance of strength and more consideration on unit weight.

Concreting in hot weather the temp of concrete in special structures is reduced to 25OC by mixing ice, to reduce chances of creacks. And some times in winters hot water is mixed to raise temp of concrete. Cold Weather Concreting : At the time of placing, it has a temperature of not less than 5oC and that the temperature of the concrete shall be maintained above 4oC until it has thoroughly hardened. Stock-piled aggregate may be heated by the use of dry heat or steam.In general, the temperature of aggregates or water shall not exceed 65oC. Hot Weather Concreting: When depositing concrete in hot weather, precautions shall be taken so that the temperature of wet concrete does not exceed 40oC while placing. Concrete should not be done in temp below 4OC and not more than 40OC of atmosphere.

For thermal insulation purposes some times light weight concrete is provided above roof to minimise temp inside building. Its done by 33 grade cement, mixing of foaming agents in concrete to increase its vol.The roof waterproofing work includes Cast-in-situ light weight concrete (Type – A) as per IS 6598, formed by producing gas or air bubbles in cement slurry or a cement-sand slurry. The material cured under natural conditions. i.e. under ambient temperature. The material shall have a density of 320 Kg/Cu.m. Compressive strength of minimum 0.25 N/mm 2 value of thermal conductivity of maximum 0.7 mw/cm. 50 °C mean temperature. 

Stress/Strain , Ec = 5000 √fck N/mm2

When a concrete member is subjected to a load it undergoes certain deformation. It disappears when load is removed.However if the load is sustained for longer period the deformation continues even the load is not increased. This phenomenon of deformation under constant stress is called creep. creep coff at 7 days loading : 2.2,at 28 days: 1.6 and at 365 days : 1.1

The resistance of concrete to the slipping of reinforcement bars embedded in concrete is called bond strength.The bond strength is determined by pull out test as the unit load require to slippage of 0.25 mm.The load then divided by area of contact between reinforcement bar and concrete. Generally its value is taken 10% of compressive strength.

Concrete subjected to bending and and shear stress is accopanied by tensile and compressive stress. The shear failure are due to resulting diagonal tension. The shear strength is generally 12 to 13 percent of compressive strength.

Tensile strength of concrete is generally 8 to 12 % of compressive strength. However 10% value is taken for calculation purposes.

the approximate value of the total shrinkage strain for design may be taken as 0.000 3

fcr = 0.7 √fck N/mm2

It is determined in compression machine by testing specimen. The ratio of lateral strain to axial strain gives value of Poisson ratio.0.15 to 0.20

The change in length per unit degree Celsius temperature. Its depending upon type of aggregates. It ranges from from 0.6 to 1.3 x 10 -5 degree C A value of 1.2 x 10 -5 degree C to 1.3 x 10 -5 degree C may be taken. The coff of thermal expansion of concrete and steel is nearly same. Hence it is the main reason to provide steel in concrete.

Its also called alkali silica reaction. Alkalise are contained in aggregates. Appearance of Hexagonal type cracks can indicate acid alkali reaction in concrete. It occurs between 10 - 38 degree Celsius temp. cracks can be 0.1 to 10 mm width. Aggregates should alkali free. Alkali in cement should be less than 0.6 % . It can controlled by use of PPC, non reactive aggregates and control of temp , voids in concrete.

Sulphates occur in aggregates ,soil and water. They lead to whitish appearance on concrete surface. Magnesium sulphate is most dangerous to concrete. It leads for formation of sulphuric acid in concrete. It increases the vol of concrete and leads to surface cracks and patches of small size. Cooling towers (NDCT ) contains more sulphates in its water, Thus causes irritation in eyes when stand nearby. SO3 should not exceed 4 % in concrete mix ( including aggregates, water, admix etc) It can be controlled by low w/c ratio, sulphate resisting cement, use of PPC or by steam curing Chlorides leads to rusting in steel. They are present in aggregates, cement , water and admixture. In warm moist conditions there is risk of chloride attack on concrete. A protective layer of oxide is present on reinforcement due to high alkality of concrete. This layer can be destroyed by chloride attack and leads to rusting. Higher pH value ( more than 6 - 10 ) protects concrete from rusting as pH value fal...

In this type of test, compression load is applied to the pile top by means of a hydraulic jack against rolled steel joist or suitable load frame capable of providing reaction and the settlement is recorded by suitably positioned dial gauges.The pile head should be chipped off to natural horizontal plane till sound concrete is met. The projecting reinforcement should be cut off or bent suitably and the top finished smooth and level with plaster of Paris or similar synthetic material where required. A bearing plate with a hole at the centre should be placed on the head of the pile for the jacks to rest.The test should be carried out by applying series of vertical downward incremental load each increment being of about 20 percent of safe load on the pile. The safe load on single pile for the initial test should be least of the following: Two thirds of the final load at which the total displacement attains a value of 12 mm unless otherwise required in a given case on the basis of natur...

RQD is a modified core recovery percentage in which all the pieces of sound core over 10 cm long are counted as recovery, and are expressed as a percentage of the length drilled. The smaller pieces resulting from closer jointing, faulting or weathering are discounted. Rock mass rating is given to rock as per deciding factors given in IS 13365 , 1. Class 2.Classification of rock mass 3.Average stand-up time 4. Cohesion of rock mass (kg/cm2) 5. Angle of internal friction.

This procedure is used to know the modulus of sub grade reaction 'k' value of the road sub grade or runways. Its similar to plate load test in this test. K value may be defined as  pressure of 0.7 kgf/cm2 divided by corresponding settlement. That is when a standard plate of 75 cm dia is subjected to 3100 kgf load k= 0.7/d (kgf/cm2/cm) where d is settlement in cm

This procedure may be adequate for light or less important structures under normal conditions, relevant laboratory tests or field tests are essential in the case of unusual soil types and for all heavy and important structures. Plate load test method used for determination of ultimate bearing capacity of soil in place which assumes that soil strata is reasonably uniform. The load test included in the standard is also used to find modulus sub grade reaction useful in the design of raft foundation -and in the design of pavements. The test results reflect only the character of the soil located within a depth of less than twice the width of the bearing plate. Since the foundations are generally larger than the test plates, the settlement and shear resistance will depend on the properties of a much thicker stratum. Moreover this method does not give the ultimate settlements particularly in case of cohesive soils. Apparatus :Circular or square bearing plates of mild steel, not less than 25...

It is used for determination of site CBR value of under laying layers without disturbing the strata. A standrad cone of 20 mm dia and angle of 60 degree is penetrated into sub layers and 8 kg hammer is dropped from fall of 575 mm The dia of rod attached to cone is 16 mm. Basically penetration in mm is inversely proportional to strength of material. Thus higher CBR = minimum penetration/no of blows. Max to 1.2 mtr depth the strength/CBR can be checked by DCPT. It is used in road pavements, after 30-40 uses the cone has to be replaced.

Coff of uniformity = The ratio D60/D10, where D60 is the particle diameter corresponding to 60 percent finer on the grain size curve and D10, is the particle diameter corresponding to 10 percent finer on the grain-size curve.                                                                                                                                                                                                         Coff of curvature = It is given by Cc =  D302/D10xD60 where D1...

Boulders - above 300 mm, Cobble- 300 to 80 mm, Gravels - 80 mm to 4.75 mm, Sand - 4.75 mm to 75 micron, Silts- 75 micron to 2 micron , Clay below 2 microns if more than 50 % of soil (whole retain on 75 micron IS sieve) is retaining on 4.75 mm IS sieve is called Gravels and denoted by G, and if more than 50 % of soil (whole retain on 75 IS sieve) is retaining on 75 micron IS sieve is called Sandy soil and denoted by S Clay - CL,Silts - ML,Gravel - G,Well graded gravels - GW,Gravel Clays - GC etc As per IS 1498 -  'A' line representing high liquid limit more than 50 - H, modrate liquid limit between 35 to 50 - M, Low liquid limit below 35 - L Soil particles passing 75 micron IS sieve between 5% to 12 % need to detailed studied for their behavior Normally soils having particles more than 12% passing 75 micron IS sieve behaves like cohessive soils, e.g. clay Coarse grained soils having particles less than 5% passing 75 micron IS sieve are cohesion less soils, e.g sands, g...