Que4.14. Explain maturity conception of concrete. Answer Maturity of Concrete 1. The strength of concrete depends on both the period of curing( i.e. age) and temperature during curing, the strength can be imaged as a function of period and temperature of curing. 2. The maturity of concrete is defined as the totality of product of time and temperature. Maturity = ( Time × Temperature) 3. Its units are °C hr or °C days. 4. A sample of concrete cured at 18 °C for 28 days is taken to be completely progressed which is equal to M28 days = 28 × 24( 18 –( – 11)) = 19488 °C hr. 5. The temperature is reckoned from – 11 °C as origin in the calculation of maturity, since hydration continues to take place up to about this temperature. Que4.15. The strength of a sample of completely progressed concrete is set up to be 40MPa. Find the strength of identical concrete at the age of 7 days when cured at an average temperature during day time at 20 °C and night time at 10 °C. Take A = 32, B = 54. Use of strength of concrete at maturity = A B log10 Maturity. Answer Given Strength of progressed concrete = 40 MPa, A = 32 and B = 45 To Find Strength of concrete the age of 7 days. 1. Maturity of concrete at the age of 7 days = ( Time × Temperature) = 7 × 12 ×( 20 –( – 11)) 7 × 12 ×( 10 –( – 11)) = 7 × 12 × 31 7 × 12 × 21 = 4368 °C-h. 2. The chance strength of concrete at maturity of 4368 °C-h. = A B log10 Maturity) 1000 = 32 54 × log10 4368 1000 = 66.5 3. The strength at 7 days = 40.0 × 100 = 26.5 MPa Que4.17. Explain the colorful types of tests for compressive strength and tensile strength of concrete. Answer Following are the colorful test used for determining compressive and tensile strength 1. In accelerated curing compressive strength of a concrete blend is determined by curing concrete cells for about 28 hrs. 2. In accelerated curing temperature of curing water is raised. 3. Carbonation depth under accelerated curing is advanced. In normal curing compressive strength of a concrete blend is determined by curing concrete cells for 28 days. In normal curing temperature of curing water is normal. Carbonation depth under normal curing is lower. 1. Concrete cell Test Concrete specific is determined by characteristics compressive cell strength test of concrete. ii. For cell test two types of samples either cells of 15 cm × 15 cm × 15 cm or 10 cm × 10 cm × 10 cm depending upon the size of total are used. iii. For utmost of the workshop boxy moulds of size 15 cm × 15 cm × 15 cm are generally used. iv. These samples are tested by contraction testing machine after 7 days curing or 28 days curing. cargo should be applied gradationally at the rate of 140 kg/ cm2 per nanosecond till the samples fails. 2. Tensile Strength Test i. The concrete structures are largely vulnerable to tensile cracking and hence the determination of tensile strength of concrete is veritably important. ii. The tensile strength of concrete structures is determined by Split cylinder test. Flexure test. 3. Core Strength Test Cylindrical cores are cut from the finished structure with a rotary slice tool. ii. The core is soaked, limited and tested in contraction to give a measure of the concrete strength in the factual structure. iii. The rate of core height to periphery and the position where the core is taken affect the strength. iv. The strength is smallest at the top face and increases with depth through the element. v. A rate of core height- to- periphery of 2 gives a standard cylinder test. Que4.18. Describe the flexure test and split tensile test of concrete. Answer Flexure Test 1. The guidelines for performing the flexure test is as per BIS 1881 Part 118 1983. 2. Then a concrete ray instance of dimension 15 × 15 × 75 cm is loaded. 3. The span of the ray instance must be three times the depth. 4. As shown in theFig.4.18.1 equal cargo operation is done at one third distance from the end supports. The responses are equal at the support. 5. The nethermost ray fibre gests increase in stress with the increase in cargo operation. 6. The increase of stress is at a rate of0.02 MPa and0.10 MPa. 7. For low strength concrete we make use of low rate and for high strength we use high rate. 8. The theoretical outside tensile stress at the nethermost face at failure is calculated. This is nominated the modulus of rupture. It’s about1.5 times the tensile stress determined by the splitting test. 9. Modulus of rupture is given by, . The test instance employed is 30 cm × 15 cm which is placed over a contraction testing machine. 3. The cargo is applied over the instance diametrically and slightly through the cylinder length till the cylinder undergoes failure. 4. The failure of the cylinder will be along the periphery in perpendicular direction. 5. Between the instance and the lading plates, plywood strips are placed to avoid direct stress due to direct point of operation. 6. The tensile stress formed with the progress of cargo will resolve the cylinder into two halves. The splitting takes place along the perpendicular aeroplane
. This is caused due to the circular tensile stress. 7. Split tensile strength is given by, ft = 2P DL where, ft = Tensile strength. P = Compressive cargo. D = Periphery of cylinder. L = Length of cylinder. Que4.19. Explain the colorful way involved in evaluation of compressive strength of concrete from medication to testing of sample. Answer Following are the step for cell testing 1. cell Casting Measure the dry proportion of constituents( cement, beach and coarse total) as per the design conditions. The constituents should be sufficient enough to cast test cells. ii. Completely mix the dry constituents to gain the invariant admixture. iii. Add design volume of water to the dry proportion( water- cement rate) and mix well to gain invariant texture. iv. Fill the concrete to the mould with the help of vibrator for thorough contraction. Finish the top of the concrete by trowel and tapped well till the cement slurry comes to the top of the cells. 2. Curing i. After some time the mould should be covered with red gunny bag and put unperturbed for 24 hours at a temperature of( 21 ± 2) °C. ii. After 24 hours remove the instance from the mould. iii. Keep the instance submerged under fresh water at 27 °C. The instance should be kept for 7 or 28 days. Every 7 days the water should be renewed. iv. The instance should be removed from the water 30 twinkles previous to the testing. v. The instance should be in dry condition before conducting the testing. vi. The cell weight shouldn’t be lower than8.1 kg. 3. Testing i. Now place the concrete cells into the testing machine( centrally). ii. The cells should be placed rightly on the machine plate( check the circle marks on the machine). Precisely align the instance with the spherically seated plate. iii. The cargo will be applied to the instance axially. iv. Now sluggishly apply the cargo at the rate of 140 kg/ cm2 per nanosecond till the cell collapse. v. The maximum cargo at which the instance breaks is taken as a compressive cargo. 4. computation Compressive Strength of concrete = Maximum compressive cargo Cross sectional area. Que4.20. Describe the non destructive testing of hardened concrete. Answer Non-Destructive Tests on Concrete The mainnon-destructive tests for strength on hardened concrete are as follows 1. Rebound Hammer( Hardness) Test i. The Schmidt hammer is used in the answer hardness test in which a essence hammer held against the concrete is struck by another springdriven essence mass and rebounds. ii. The quantum of answer is recorded on a scale and this gives an suggestion of the concrete strength. iii. The larger the answer number is, the advanced is the concrete strength. 2. Ultrasonic Pulse Velocity Test i. In the ultrasonic palpitation haste test the haste of ultrasonic beats that pass through a concrete section from a transmitter to a receiver is measured. ii. The palpitation haste is identified against strength. iii. The advanced the haste is, the stronger is the concrete. 3. Pull Out Test i. The pull out test will determine the force that’s needed to pull out a sword rod especially shaped from hardened concrete to which the sword was cast. ii. Pulling out of sword is done with a cone of concrete that have a pitch of 45 °. iii. The force needed to pull the concrete out is related with the compressive strength of the concrete. 4. Penetration Resistance Test Penetration resistance tests on concrete offers a means of determining relative strengths of concrete in the same structure or relative strength of different structures. ii. Because of nature of accoutrements , it can not be anticipated to yield absolute values of strength. Que4.21. What are the conditions of non destructive testing of concrete? Also give their advantages and disadvantages. Answer demand of Non Destructive Test Following are the demand of NDT 1. Assessment of being structures in the absence of delineations. 2. Quick assessment of the structure. 3. Quality control of construction, in situ. 4. Determining position of underpinning. 5. position of cracks joints honeycombing. 6. In some cases, it needed to assess of concrete damaged due to fire or any other natural disaster due judge the condition of structure. Advantages Following are the advantages of non destructive testing 1. Access to retired particulars- “ see through walls ”. 2. More examinations with NDT. 3. Rapid and on point accumulation of data. 4. Generally less precious than destructive testing. 5. Gives result without structural damage. Disadvantages Following are the disadvantages of non destructive testing 1. further than one test system may be needed. 2. Environmental conditions may affect or distort results. 3. Construction details and structure factors may affect results. 4. Some conditions can not be determined with a reasonable degree of delicacy without destructive testing. Que4.22. What’s modulus of pliantness of concrete? With the help of stress- strain wind, describe the colorful types of modulus of pliantness? Answer Modulus of Pliantness of Concrete It’s defined as the pitch of the line drawn from a stress of zero to a compressive stress of0.45 fc. Types of Modulus of Pliantness 1. original digression Modulus It’s given by the pitch of a line drawn digression to the stress- strain wind at the origin. It’s used to characterize concrete deviation at veritably low stresses. 2. Tangent Modulus It’s given by the pitch of a line drawn digression to the stress- strain wind at any point on the wind. It’s used to pretend the structure to lading or unloading at different unloading stages. 3. Secant Modulus It’s given by the pitch of a line drawn from the origin to a point on the wind corresponding to a 40 stress of the failure stress. It’s used to pretend the structure during its original lading stage when endless cargo prevail.