Mechanical vibration structure dynamics notes

Dynamics

1. CH-06: Testing of Concrete and Quality Control Presented by : Sujant Jha

2. 6 .1 Various Strength of Concrete Concrete shall bear various types of load depending on its use. Hence, different types of strength of concrete like compressive strength, tensile strength, shear strength, bond strength, etc. are established, and accordingly, the concrete shall be used. However, concrete is normally designed with its 28-days strength. Hence, it is very difficult to test the concrete during the casting to confirm its strength, and perform required adjustment. Generally, samples are casted and cured according to recommended norms and tested.

3. 6.2 Compressive Strength Test Compressive strength is the most important property of concrete and its test is most commonly conducted. It is the property of concrete by which it resists the compressive force acted on it. Normally, concrete is used to resist the compressive force in structures. Compressive strength is generally tested on samples made in cubical or cylindrical shape. It can also be tested on other shapes of concrete also like prisms.

4. 6.2 Compressive Strength Test Test Procedure: Preparation of Moulds : Depending of method, suitable type and size of mould is selected. Normally, the size of mould shall not be less than 3-times the size of coarse aggregate. They shall be rigid, non-absorbent and have smooth internal surface within tolerance of 0.03mm.

5. 6 .2 Compressive Strength Test b. Sampling of Concrete: In lab, concrete is prepared with the same proportion which is expected at construction site, and used for casting the sample. c. Casting and Compaction: The test samples are casted as soon as possible after mixing of concrete. The concrete is filled in mould in layers of 5cm, each layer being compacted. Adequate compaction is required to achieve full compaction without segregation. Compaction can be done by hand using tamping rod or by vibration.

6. 6 .2 Compressive Strength Test d. Removal of Mould and Curing: The sample shall be placed in moist air of at least 90% RH or under moist covering, and temperature of 27 ± 2 o C for 24 ± hour from the time of addition of water. Then, the mould is removed without any damage to concrete and the sample is submerged in clean water until taken out just before test. The water shall be renewed every seven days and its temperature shall be maintained at 27 ± 2 o C. e. Preparation for Testing: The samples are tested on saturated surface dry condition. Hence, the samples are removed from water and surface moisture is wiped of allowed to dry. Test shall be done on smooth surface. However, in cylinder, as the top-surface is not smooth, some sort of capping is done to make it smooth so that load can be applied uniformly. Capping can be done using neat-cement, cement mortar, sulphur or hard-plaster.  

7. 6.2 Compressive Strength Test f. Test of Sample: The sample is placed between plates of testing machine, which increases the load on cube on standard rate [ 0.2 MPa/s] . The failure of sample is characterize by sudden reduction in load on the machine, which gives the ultimate load capacity, which when divided by the loaded area gives the compressive strength.

8. 6.2 Compressive Strength Test Two common types of compressive strength test are: Cube Test Cylinder Test

9. 6.2 Compressive Strength Test A. Concrete Cube Test Most common test for compressive strength of concrete is by making concrete cubes of standard size. The size of standard cube is 150mmx150mmx150mm. However, if maximum size of aggregate does not exceed 20mm, cubes of 100mmx100mmx100mm can be used provided corresponding correction is adopted for observed strength. The concrete cube is applied with an axial compressive force from its smooth faces at a uniform rate of 0.2 MPa/s. The load at which the sample fails is noted. Dividing the failure load by the area of cube on which load is acted gives the ultimate strength of the concrete.

10. 6.2 Compressive Strength Test If the cube of size other than standard size (150mm) is taken, then the observed strength shall be corrected by the following factors: Size effect of cube on compressive strength Cube Size (mm) 100 150 200 300 Relative Strength 1.05 1.00 0.95 0.87

11. 6.2 Compressive Strength Test B. Concrete Cylinder Test Quite often, compressive strength of concrete is also tested by casting concrete cylinder of standard size of 150mm (diameter) x 300mm (height). Since, cylinder are tested from circular faces (top & bottom), the casted top-surface being not smooth, it is difficult to apply load uniformly. For this capping of the surface by strong material (stronger than concrete like neat-cement, rich cement mortar, sulphur or hard-plaster) is necessary to make it plane and smooth.

12. 6.2 Compressive Strength Test B. Concrete Cylinder Test ( contd .) The apparent strength of test cylinder is found to be dependent on its height to diameter ratio (h/d). Due to effect of friction on face, there is some restraints towards ends affecting the true strength. Hence a correction shall be done to determine strength of concrete from cylinder other than standard ratio.

13. 6.2 Compressive Strength Test Comparison of cube test and cylinder test It is found that cylinder specimen gives lower strength compared to cube specimen. Also, cylinder test gives more uniform strength than cube test. It may be because, the cylinder strength is less affected by the friction at loading surface. Hence, cylinder test is preferred in research laboratory, and popular in some developed countries. However being simple, cube test is more popular in construction site, and in countries like Nepal, India, UK, etc. Cylinders are cast and tested in the same position, whereas cubes are cast in one direction and tested from the other direction. In actual structures in the field, the casting and loading is similar to that of the cylinder and not like the cube. As such, cylinder simulates the condition of the actual structural member in the field in respect of direction of load.

14. 6.2 Compressive Strength Test Comparison of cube test and cylinder test The points in favour of the cube specimen are that the shape of the cube resembles the shape of the structural members often met with on the ground. The cube does not require capping, whereas cylinder requires capping. The capping material used in case cylinder may influence to some extent the strength of the cylinder. Cylinder strength = (4/5) * Cube strength However, the ratio is not constant and much affected by strength of concrete. In high strength concrete, the two-values become closer.

15. 6.2 Compressive Strength Test Platen effect Platens are top and bottom plate use for testing concrete cubes and cylinder. Due to compression load, the cubes or cylinder goes lateral expansion owing to the possions ratio effect. The steel plate donot undergro lateral expansion to the same extent that of concrete, this restrict the expansion tendency of concrete in lateral direction. This induces a tangential force between the end surface of the concrete specimen and adjacent steel platens of the testing machine. Due to this the platen restrains the lateral expansion of concrete in the parts of the specimen near its end.

16. 5.2 Compressive Strength Test The platen effect can be reduced by reducing the friction between platens and concrete, this can be achieved by applying grease, graphite or paraffin wax to the top and bottom surface of concrete.

17. 6.3 Tensile Strength Test Concrete is strong in compression but relatively weak in tension. Hence, normally tensile strength of concrete is neglected in design of RCC, however, concrete has to resist some tension. Even plain cement concrete has to resist some tension due to shrinkage, temperature change, and some external forces. There are different methods for tensile strength test. However, each of them have some limitations, and difficult to conduct as well. The most common method of tensile strength test are: Direct Tension Flexural Test Cylinder Splitting Test

18. 6.3 Tensile Strength Test Direct tension It is difficult to test the concrete in direct (uniaxial) tension because of the problem of gripping the specimen satisfactorily and because there must be no eccentricity of the applied load. Therefore, direct tensile strength test is not standardized and rarely used. The most direct way of measuring the tensile strength. Direct tensile strength ( f cd ) = 0.35 f ck Where f ck is compressive strength of concrete.  

19. 6.3 Tensile Strength Test Flexural Test Flexural strength of Concrete, also known as Modulus of rupture, is an indirect measure of the tensile strength of unreinforced concrete. Modulus of rupture can also be defined as the measure of the extreme fibre stresses when a member is subjected to bending. Apart from external loading, tensile stresses can also be caused by warping, corrosion of steel, drying shrinkage and temperature gradient. A concrete road slabs is acted by wheel loads which may cause high tensile stresses due to bending, when there is inadequate sub-grade suppourt. Hence design strength of concrete should be enough to resist such flexural tensile stresses. strength, a mechanical parameter for brittle material, is defined as a ability to resist deformation under load. The flexural strength represents the highest stress experienced within the material at its moment of rupture.

20. 6.3 Tensile Strength Test Flexural Test Equipment & Apparatus Beam mould of size 15 x 15x 70 cm (when size of aggregate is less than or equals to 38 mm) or of size 10 x 10 x 50 cm (when size of aggregate is less than or equals to 19 mm). Tamping bar (40 cm long, weighing 2 kg and tamping section having size of 25 mm x 25 mm)

21. 6.3 Tensile Strength Test Flexural Test Equipment & Apparatus Flexural  test machine– The bed of the testing machine shall be provided with two steel rollers, 38 mm in diameter, on which the specimen is to be supported, and these rollers shall be so mounted that the distance from centre to centre is 60 cm for 15.0 cm specimens or 40 cm for 10.0 cm specimens. In case of centre point loading the load shall be applied through similar roller mounted at the centre points of the supporting span. mechanical 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.

22. 6.3 Tensile Strength Test Flexural Test Equipment & Apparatus I is also known as modulus of rupture , bend In case of third point loading the load shall be applied through two similar rollers mounted at the third points of the supporting span that is, spaced at ‘20cm for 15cm specimen or 13.3 cm for 10cm specimen’ centre to centre. within the material at its moment of rupture.

23. 6.3 Tensile Strength Test Flexural Test Procedure: Prepare the test specimen by filling the concrete into the mould in 3 layers of approximately equal thickness. Tamp each layer 35 times using the tamping bar. Tamping should be distributed uniformly over the entire crossection of the beam mould and throughout the depth of each layer. The specimen stored in water shall be tested immediately on removal from water; whilst they are still wet. The test specimen shall be placed in the machine correctly centered with the longitudinal axis of the specimen at right angles to the rollers. For moulded specimens, the mould filling direction shall be normal to the direction of loading. The load shall be applied at a rate of loading of 400 kg/min for the 15.0 cm specimens and at a rate of 180 kg/min for the 10.0 cm specimens. 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.

24. 6.3 Tensile Strength Test Flexural Test Calculation The Flexural Strength or modulus of rupture ( f b ) is given by f b  = pl/bd 2    (when  a  > 20.0cm for 15.0cm specimen or > 13.3 cm for 10cm specimen) or f b  = 3pa/bd 2    (when  a  < 20.0cm but > 17.0 for 15.0cm specimen or < 13.3 cm but > 11.0cm for 10.0cm specimen.) Where, a = the distance between the line of fracture and the nearer support. b = width of specimen (cm) d = failure point depth (cm) l = supported length (cm) p = max. Load (kg ) 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.

25. 6.3 Tensile Strength Test Flexural Test The value of the modulus of rupture depends on the dimension of the beam and manner of loading. The systems of loading used in finding out the flexural tension are central point loading and third point loading. In the central point loading, maximum fibre stress will come below the point of loading where the bending moment is maximum. f rupture , bend strength , or 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.

26. 6.3 Tensile Strength Test Flexural Test In case of third point loading, the critical crack may appear at any section, not strong enough to resist the stress within the middle third, where the bending moment is maximum. It can be expected that the third point loading will yield a lower value of the modulus of rupture than the centre point loading.

27. 6.3 Tensile Strength Test Cylinder Splitting 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. Test specimens The length of the specimens shall not be less than the diameter and not more than twice the diameter. For testing and comparison of results, unless otherwise specified the specimens shall be cylinder 150 mm in diameter and 300 mm long.

28. 6.3 Tensile Strength Test Cylinder Splitting Test Apparatus: 1. Weights and weighing device. 2. Tools, containers and pans for carrying materials & mixing. 3. A circular cross-sectional rod ( φ l6mm & 600mm length). 4. Testing machine. 5. Three cylinders (φ150mm & 300mm in height).

29. 6.3 Tensile Strength Test Procedure: 1. Prepare three cylindrical concrete specimens. 2. After molding and curing the specimens they can be tested. The cylindrical specimen is placed in a manner that the longitudinal axis is perpendicular to the load. 3. Two strips of nominal thick plywood, free of imperfections, approximately (25mm) wide, and of length equal to or slightly longer than that of the specimen should be provided for each specimen. 4. The bearing strips are placed between the specimen and both upper and lower bearing blocks of the testing machine.

30. 6.3 Tensile Strength Test 5. The load shall be applied without shock and increased continuously at a nominal rate within the range 1.2 N/( mm 2 /min) to 2.4 N/ (mm 2 /min). 6. Record the maximum applied load indicated by the testing machine at failure. Calculate the splitting tensile strength of the specimen as follows: T = Where: T : splitting tensile strength, N/mm2 P : maximum applied load indicated by testing machine, N L : Length, mm d : diameter, mm  

31. 6.3 Tensile Strength Test Advantage of using this method: Same type and same specimen can also be used for compression test. Vertical compressive stress on cylinder Tensile stress on cylinder T =  

32. 6.3 Tensile Strength Test Relation between tensile strength and compressive strength Tensile strength of concrete is approximately 7 to 15 % of compressive strength. Is 456: 2000 provides a standard formula for flexure tensile strength of concrete f cr = 0.7 For normal density concrete, the splitting tensile strength is about 2/3 of flexural tensile strength of concrete.  

33. 6. 3 Bond Strength and its Test Bond strength is resistance to slip of the reinforcing bar embedded in concrete. Bond stress is longitudinal shear stress which acts on the interface of steel and concrete to resist slipping of steel bar from concrete. The bond is due to following resistances: Chemical adhesion between concrete and steel due to cement paste. Frictional resistance is developed at the surface of contact between steel bar circumference and concrete due to shrinkage of concrete which grips the steel bars. Ribs on deformed bars.

34. 6. 3 Bond Strength and its Test Bond strength is determined by pull out test Where D is diameter of steel bar. is the length of embedded steel or development length. T is pull force.   T     As per IS456:2000, the design bond strength of concrete with embedded reinforcements can be taken as follows:

35. 6. 3 Bond Strength and its Test As per IS456:2000, the design bond strength of concrete with embedded reinforcements can be taken as follows

36. 6.3 Shear Strength A shear force is such force that tends to produce sliding failure on a material along a certain plane. The sliding force parallel to that plane per unit area of that plane is called shear stress. And the strength of concrete against shear-failure is called shear strength. Concrete structures often encounter high shear stresses. In most cases, additional reinforcements are provided to resist a portion of shear that cannot be resisted by concrete alone. Direct determination of shear strength is difficult. Shear strength of concrete is approximately taken as 12% of its compressive strength.

37. 6.3 Shear Strength Maximum allowable shear stress for limit state method and working stress method of design as per IS 456: 2000 code is:

38. 6 .4 Variability of Concrete Strength and Acceptance Criteria The strength of concrete varies from batch to batch and even on same batch too. Hence, a value of standard deviation is to be established for mix-design purpose that represents an estimation of deviation of actual strength from the mean strength. For test purpose, random sampling shall be done ensuring that each concrete batch has reasonable chance of being tested. The test shall be spread over entire period of concreting and all mixing units. The frequency of sampling as specified by IS456:200 is as follows: Sn. Quantity of concreting (m3) Minimum no of sample Note: Three test-specimens shall be made from each sample for testing at 28days. Additional samples can be made for 7-day test or at other days. Date, time, weather, temperature and part of structure represented by the sample shall be noted. 1 1 to 5 1 2 6 to 15 2 3 16 to 30 3 4 31 to 50 4 5 Above 51 4 + Qty./50

39. 6 .4 Variability of Concrete Strength and Acceptance Criteria Test Result and Acceptance Criteria: The test result of a sample is an average strength of 3 test-specimens. The individual variation shall not exceed 15% ( +/- ) of average strength. The concrete is liable to be rejected if it is porous or honey-combed or contains other defects. As per IS456:2000, the concrete is deemed to comply with the compressive strength requirements if it satisfies the criteria given in table below. Specified Conc. Grade Mean of group of 4 non-overlapping consecutive test results Individual test results Note: An attempt should be made to obtain results of 30 samples as soon as possible to establish value of standard deviation (SD) M15 >= f ck + 0.825*SD (rounded to 0.5) Whichever is greater >= fck - 3 f ck + 3 M20 or above >= f ck + 0.825*SD (rounded to 0.5) Whichever is greater >= fck – 4 f ck + 4

40. 6 .4 Variability of Concrete Strength and Acceptance Criteria As per amendment 4

41. 6 .4 Variability of Concrete Strength and Acceptance Criteria As per IS456:2000, the concrete is deemed to comply with the flexural strength requirements if: The mean strength determined from any group of four consecutive test results exceed the specified characteristic strength by at least 0.3 N/mm 2 and The strength determined from any test result is not less than specified characteristic strength less 0.3 N/mm 2 .

42. 6. 5 Non destructing Testing of Concrete It is a method of testing existing concrete structure to asses the strength and durability of concrete structures. In NDT without loading the specimen to failure we can measure strength of concrete. Now a days this method has become a part of quality control process. This method help us to investigate crack depth,microcrack and deterioration property of concrete.

43. 6. 5 Non destructing Testing of Concrete Purposes of non destructive testing Estimate the in-situ compressive strength, uniformity, quality and homogeniety. Identitying areas of lower integrity. Monitoring changes in the structure of the concrete. Condition of reinforcement steel with respect to corrosion. Chlorides, sulphates, alkali contents or degree of carbonation. Measurement of elastic modulus. Condition of grouting in prestressing cable ducts.

44. 6. 5 Non destructing Testing of Concrete Different methods of NDT Rebound hammer method Ultrasonic pulse velocity method Penetration method Pull out test method

45. 6. 5 Non destructing Testing of Concrete Rebound hammer method ( Schmidt hammer method) It consist of a spring controlled mass that slides on a plunger within tubular housing. When the plunger of the rebound hammer is pressed against the surface of the concrete, the spring controlled mass rebounds and the extent of such rebounds depends upon the surface hardness of the concrete. The surface hardness and rebound hammer reading can be correlated with compressive strength of concrete. The rebound is read off along a graduated scale and is designated as rebound number or rebound index.

46. 6. 5 Non destructing Testing of Concrete Rebound hammer method ( Schmidt hammer method) It consist of a spring control hammer that slides on a plunger within a tubular housing. When the plunger is pressed against the surface of the concrete, the mass rebound from the plunger. It retracts against the force of the spring. The hammer impacts against the concrete and the spring control mass rebounds, taking the rider with it along the guide scale. By pushing a button, the rider can be held in position to allow the reading to be taken. The distance travelled by the mass, is called the rebound number. It is indicated by the rider moving along a graduated scale .

47. 6. 5 Non destructing Testing of Concrete Procedure The concrete surface should be smooth clean and dry.any loose particle should be rubbed off from the concrete surface with a grinding wheel or stone, before testing. The point of impact of rebound hammer on concrete surface should be at least 20mm away from any edge or shape discontinuity. The rebound hammer should be held at right angles to the surface of concrete member. The test can be conducted horizontally on vertical surface or vertically upward or downward on horizontal surface. Six readings of rebound number is taken at each point of testing and an average of value of the reading is taken as rebound index for the corresponding point of observation on concrete surface.

48. 6. 5 Non destructing Testing of Concrete Rebound hammer method ( Schmidt hammer method) Investigations have shown that there is a general correlation between compressive strength of concrete and rebound number; however, there is a wide degree of disagreement among various research workers regarding the accuracy of estimation of strength from rebound readings. The variation of strength of a properly calibrated hammer may lie between ±15% and ±20%.

49. 6. 5 Non destructing Testing of Concrete Rebound hammer method ( Schmidt hammer method) Limitations Such test are affected by different factors like: Smoothness of the concrete surface Size, shape and rigidity of the specimen Age of the specimen Moisture condition Type and distribution of coarse aggregate Type of cement and mould Carbonation of concrete surface .

50. 6. 5 Non destructing Testing of Concrete Ultrasonic pulse velocity method This method consist of measuring the travel time of a pulse of longitudinal ultrasonic waves that pass through the concrete. The principle of the test is that the velocity of sound in a solid material is a function of square root of its modulus of elasticity E, to its density V = f [ ] 1/2 Where g is acceleration due to gravity.  

51. 6. 5 Non destructing Testing of Concrete Ultrasonic pulse velocity method There are three ways of measuring the pulse velocity through concrete. Direct transmission Semidirect transmission Surface transmission

52. 6. 5 Non destructing Testing of Concrete Pocedure Longitudinal waves with frequencies in the range of 15 to 50 kHz are produced by transmitter. At some length, a receiver is placed which receives the waves. The travel times between the transmission and reception of the waves are measured electronically. The path length between the transmitter and receiver is divided by time of travel which gives the average velocity of wave propagation. V= d/t Pulse velocity (km/hr) Concrete quality Above 4.5 Excellent 3.5-4.5 Good 3-3.5 Medium 2-3 Doubtful Below 2 Very poor