TEST ON HARDENED CONCRETE
- 1. ADVANCED CONCRETE TECHNOLOGY 10CV81 UNIT-8 TEST ON HARDENED CONCRETE By Jaswanth J Jayanth V Gowda Nandeesh S
- 2. INTRODUCTION • Testing of hardened concrete plays an important role in controlling and confirming the quality of cement concrete works. • The test methods should be simple, direct and convenient to apply. • The main purpose of testing hardened concrete is to confirm that the concrete used at site has developed the required strength. As the hardening of the concrete takes time, one will not come to know, the actual strength for some time. This is an inherent disadvantage in conventional test.
- 3. • But if the strength of concrete is to be known at an early period, accelerated strength test can be carried out to predict 28 days strength. But mostly when correct materials are used and careful steps are taken at every stage of the work, concrete normally give the required strength.
- 4. Capping specimens and their effects: • Capping is applicable to cylindrical specimen. The ends of all cylindrical specimens that are not plane within 0.05mm are capped. • Caps are made as thin as practicable and care should be taken so that flaw or fracture does not take place, when the specimen is tested. • Capping can be done on completion of casting or a few hours prior to testing of specimen.
- 5. Capping is required to be carried out according to the following methods: 1. Neat cement: • The test cylinders are capped with a thin layer of stiff, neat Portland cement paste after the concrete has set in the moulds. • Capping is done after 4 hours of casting so that concrete in the cylinder undergoes plastic shrinkage and subsides fully. 2. Sulphur: • Just prior to testing, the cylindrical specimens are capped with a sulphur mixture consisting of 1 part of sulphur to 2 or 3 parts of inert filler, such as fire-clay. • The specimens are securely held in a special jig so that the caps formed have a true plane surface.
- 6. 3. Hard plaster: • Just prior to testing, specimens are capped with hard plaster having a compressive strength of at least 42 Mpa cm in an hour. • The caps are formed by means of a glass plate not less than 13 mm in thickness, having a minimum surface dimension at least 25 mm larger than the diameter of the mould.
- 7. Effect of the Height/Diameter ratio on Strength: • Normally, height of the cylinder “h” is made twice the diameter “d”, but sometimes, particularly, when the core is cut from the road pavements or airfield pavements or foundations concrete, it is not always possible to keep the height/diameter ratio of 2:1. • The diameter of the core depends upon the cutting tool, and the height of the core will depend upon the thickness of the concrete member. • If the cut length of the core is too long, it can be trimmed to h/d ratio of 2 before testing. • But if the length is too short, it is necessary to estimate the strength of the same concrete, as if it had been determined on a specimen with h/d ratio equal to 2.
- 8. • Fig 1 below shows the correction factor for height/diameter ratio of a core(IS 516:1959). Fig 1: Influence of h/d ratio on apparent strength of the cylinder
- 9. • Murdock and Kesler found that correlation factor is not constant, instead it depends on the strength level of concrete. • High strength concrete is less affected than the low strength concrete. • Fig 2 shows the influence of h/d ratio on the strength of cylinder for different strength levels. Fig 2: Correction Factor for h/d ratio of core
- 10. Rate of loading: • The specimen is placed in the machine in such a manner that the load is applied to the uppermost surface as cast in the mould, along two lines spaced 20.0 or 13.3 cm apart. • The load is applied without shock and increasing continuously at rate such that extreme fibre stress increases at approximately 0.7 kg/sq cm/min that is, at a rate of loading of 400kg/min for the 15.0 cm specimens and at a rate of 180 kg/min for the 10.0 cm specimens. • The load is increased until the specimen fails, and the maximum load applied to the specimen during the test is recorded.
- 11. • The flexural strength of the specimen is expressed as the modulus of rupture ‘fb’, which is given by: fb=(P*l)/(b*d*d) where b=specimen width measured (cm), d= depth of the specimen at the point of failure(cm), l=length of the span on which specimen is supported(cm), and P=maximum load applied to the specimen(kg).
- 12. DESTRUCTIVE TEST
- 14. • Compression test is the most common test conducted on hardened concrete, partly because it is an easy test to perform, and partly because most of the desirable characteristic properties of concrete are qualitatively related to its compressive strength. • The compression test is carried out on specimens cubical or cylindrical in shape. Prism is also sometimes used, but it is not common in our country.
- 15. • The cube specimen is of the size 15 x 15 x 15 cm and 10 x 10 x10 cm • Metal moulds, preferably steel or cast iron, thick enough to prevent distortion
- 17. Failure Pattern of Cube & Cylinder – Two Pyramid Failure - Ideal
- 18. Failure of Compression Specimen • Due to compression load, the cube or cylinder undergoes lateral expansion owing to the Poisson’s ratio effect. • With friction acting i.e. , under normal conditions of test, the elements within the specimen is subjected to a shearing stress as well as compression
- 20. TENSILE STRENGTH • Tensile strength is one of the basic and important properties of concrete. A knowledge of its value is required for the design of concrete structural elements. • Its value is also used in the design of prestressed concrete structures, liquid retaining structures, roadways and runway slabs. • Direct tensile strength of concrete is difficult to determine; recourse is often taken to the determination of flexural strength or the splitting tensile strength and computing the direct tensile.
- 21. What is split tensile strength test? A method of determining the tensile strength of concrete using a cylinder which splits across the vertical diameter. It is an indirect method of testing tensile strength of concrete.
- 22. Why we are going for split tensile test? • In direct tensile strength test it is impossible to apply true axial load. There will be always some eccentricity present. • Another problem is that stresses induced due to grips. Due to grips there is a tendency for specimen to break at its ends.
- 23. MOULDS Cylinders • The cylindrical mould shall be of 150mm diameter and 300mm height. Similarly the mould and base plate shall be coated with a thin film of mould oil before use, in order to prevent adhesion of the concrete.
- 24. •The load shall be applied without shock and increased continuously at a nominal rate within the range 1.2 N/(mm2/min) to 2.4 N/ (mm2/min). •Record the maximum applied load indicated by the testing machine at failure. Note the type of failure and appearance of fracture.
- 27. Computations: Calculate the splitting tensile strength of the specimen as follows: T = 2P πLd Where: T : splitting tensile strength, kPa P : maximum applied load indicated by testing machine, kN L : Length, m d : diameter, m
- 28. Result : • It is found that the splitting test is closer to the true tensile strength of concrete it gives about 5 to 12% higher value than the direct tensile strength test.
- 29. Advantage of using this method: • Same type and same specimen can also be used for compression test. • It is simple to perform and it gives uniform results than the other tension tests like ring tension test and double punch test.
- 31. • Flexure • The state of being flexed (i.e. being bent) • Flexural strength • It is also known as modulus of rupture, bend strength, or fracture strength, a mechanical parameter for brittle material, is defined as a material's ability to resist deformation under load. • The flexural strength represents the highest stress experienced within the material at its moment of rupture. • When an object formed of a single material, like a wooden beam or a steel rod, is bent, it experiences a range of stresses across its depth.
- 32. • Most materials fail under tensile stress before they fail under compressive stress, so the maximum tensile stress value that can be sustained before the beam or rod fails is its flexural strength • The stress will be at its maximum compressive stress value
- 34. 1
- 36. • The standard size of the specimens are 15 x 15 x 70 cm. Alternatively, if the largest nominal size of the aggregate does not exceed 20 mm, specimens 10 x 10 x 50 cm may be used.
- 37. • Procedure Test specimens are stored in water at a temperature of 24° to 30°C for 48 hours before testing. They are tested immediately on removal from the water whilst they are still in a wet condition. The dimensions of each specimen should be noted before testing. No preparation of the surfaces is required.
- 38. The flexural strength of the specimen is expressed as the modulus of rupture fb fb = Pl/bd2 When ‘ a ’ is greater than 20.0 cm for 15.0 cm specimen or greater than 13.3 cm for a 10.0 cm specimen
- 39. 1) If ‘ a ’ is less than 20.0 cm but greater than 17.0 cm for 15.0 specimen 2) If ‘ a ’ less than 13.3 cm but greater than 11.0 cm for a 10.0 cm specimen 3) If ‘ a ’ less than 17 cm and 11 cm for 15 & 10 cm specimen resp
- 40. Calculation of the flexural strain , Calculation of flexural modulus , D= maximum deflection of the center of the beam, (mm) m = The gradient (i.e., slope) of the initial straight-line portion of the load deflection curve,(P/D), (N/mm)
- 42. NDT-Need & Importance. • To test concrete structure after concrete has hardened. • To test structure without damaging. • To determine whether the structure is suitable for design use. • It can be applied on both old and new structure. • Cost effective. Test available ranges from • SDT(semi destructive test)-causes negligible repairable damages on surface. • NDT(non destructive test)-do not cause any damage to structure.
- 43. Where to use NDT • Determining parameters- density, elastic modulus, strength, surface adsorption. • Location of Cracks/Joints/Honeycombing • Determining position of reinforcement • Quality control of Construction , in situ • Confirming Workmanship
- 44. METHODS • VISUAL TESTING • SCHMIDTS REBOUND HAMMER TEST-surface hardness. • ULTRASONIC PULSE VELOCITY TEST-compressive strength. • PERMEABILITY TEST-flow of water. • HALF CELL ELECTRIC POTENCIAL METHOD-corrosion potential. • COVERMETER TESTING-dia and distance of bars from surface. • RADIO GRAPHIC TESTING-detects voids. • CARBONATION DEPTH MEASUREMENT-detects corrosion. • IMPACT ECO-TESTING. • GROUND PENETRATION RADAR TESTING. • INFRARED THERMOLOGY.
- 45. Qualification/Certification • A person / Organization should have Certification From - ISO – 9712 • IS 1311 -Non Destructive Testing • IS 13311 (PART 1) : 1992-Ultrasonic Pulse Velocity • IS 13311 (PART 2) : 1992-Rebound Hammer Test
- 46. 1 . REBOUND HAMMER TEST • This is used for measuring surface hardness of existing concrete mass which in turn is correlated with the grade of concrete. • calibration curves are available to relate the rebound number with the grade of concrete for the hammer held either horizontal or vertical (down or up) for both dry and wet condition of surface. • Result depends upon– Type and nature of aggregate used, Surface and internal moisture condition, presence of void, Smoothness of surface
- 47. • It can be best used to compare strength of one concrete against another but usually not reliable in determining absolute strength. • Moreover, each hammer varies considerably in performance and require individual calibration 1. Original Schmidt Hammer-Impact direction perpendicular to the surface, Used for concrete/mortar testing,900 g. 2. Silver Schmidt Hammer-Independent of impact direction,Suitable for testing a wide variety of concrete, mortar and rock,600 g. • Quick, simple and in expensive method.
- 48. Components of Hammer and Procedure • Schmidt’s rebound hammer consist of a spring controlled mass that slides on a plunger within a tubular housing. • When plunger is pressed against the surface of the concrete ,the spring controlled mass rebounds . • The extent of such rebound depends upon the surface hardness of concrete.
- 49. Limitation • Accuracy is 25% only. • Results effects by-angle of test, surface smoothness, mix proportions. • Only suitable for closed structured concrete. • Rebound hammer have to be calibrated frequently. • Test results depends on- 1. moisture condition of surface-wet surface produce lower rebound upto 20%. 2. surface carbonation-increases rebound number(carbonated layer should be removed before testing) 3. location of plunger-if plunger is placed on aggregate, high rebound no. is obtained (hence values are taken at several places)
- 50. 2 . Pulse Velocity Method • This is used for measuring the time of travel of pulse of vibrations in ultrasonic ranges, passing through the concrete to judge qualitatively, how good or bad the concrete is. • It can be operated in direct, semi-direct or indirect, i.e. surface mode • The result depends upon- Heterogeneity of concrete within a short length Presence of reinforcing steel or other impurities in concrete • Used to judge uniformity of concrete and to establish acceptance criteria, correlation with strength is possible
- 51. • It can be operated in direct, semi-direct or indirect, i.e. surface mode Direct transmission Semi-direct transmission Indirect or surface transmission • Long waves travel twice fast as other, surface waves are slowest • The time of travel b/w initial onset and reception of the pulse is measured electronically. • The path length b/w transducer divided by time of travel gives avg velocity.
- 52. Factors affecting 1. Smoothness of contact surface under test 2. Influence of path length on pulse velocity 3. Temperature of concrete 4. Moisture condition of concrete 5. Presence of reinforcing steel
- 53. Applications • Determine quality of concrete. > 4500 m/s à Excellent 3500-4500 m/s à Good 3000-3500 m/s à Doubtful 2000-3000 m/s à Poor <2000 m/s à Very poor • Determine the setting characteristic. • Durability study-freeze thaw, acid attack.. • Detects cracks • Measures detoration of concrete due to fire exposure.