Testing of Hardened Concrete

Part B: Testing of hardened concrete 1. Objectives: The objective of the hardened concrete test was to determine the compressive and indirect tensile strength. On the other hand, this experiment was also used to examine the effect of curing condition on strength of concrete, the influence of specimen shape on compressive strength, the effect of compaction on compressive strength and this experiment was also to examine the effect of increasing water to cement ratio on compressive and in direct tensile strengths of concrete. 2. Procedure (Refer AS1012 for full details) 2. 1 Compressive Strength In this test, standard cylinders and cubes will be subjected to uniaxial compressive loading and the load will be applied gradually at a standard stress rate of 15MPa/min. , up to failure. The maximum applied load is recorded for the determination of the compressive strength. * When testing a cylinder, a hard- rubber cap is needed to achieve uniform loading. * When testing a cube the load is app lied to cast surface and no capping is needed. * Compressive strength of concrete [fc] (MPa) = Maximum Load [P] (N) / Load bearing area [A] (mm2) Load bearing area for cylinder = ? (r^2), where r is the radius of the cylinder Load bearing area for cube = d x d, where d is the cube size 2. 2 In-direct tensile strength (AS 1012:10) In this test, a standard cylinder is subjected to a compressive loading along its length and the cylinder splits in indirect-tension along the diagonal, due to the induced tension (Poisson's effect). It is necessary to use bearing strips between the concrete and the testing machine platens to avoid local crushing. In-direct tensile strength [fst] (MPa) is calculated using the following expression: (MPa) = 2000 x Maximum load P (kN) / ? l (mm) x d (mm) Where d and l are the diameter and length of the cylinder in mm. Testing procedure * Fix the compressometer centrally around the 100mm diameter cylinder. Carefully center the specimen in the testing machine. * Three times gradually load the specimen (15+2 MPa/minutes) to the test load level (40% of the cylinder strength) and unload it. Records need not to be kept during first loading Record the following: 1. Applied load when the deformation is such that the specimen is subjected to a longitudinal strain of 50 microstrain 2. Deformation attained at test load. 3.From these results the following are to be determined: 4. 1 = applied stress at the strain of 50 microstrain 5. 2 = applied stress corresponds to the test load 6. 3 = strain at test load 3. Test Result 3. 1 Compressive Strength – Cylinders (Water cured for 28 days) Specimen No. | Diameter| Height| Weight| Max. Load| cylinder strength (Mpa)| average cylinder strength (Mpa)|   | (mm)| (mm)| (g)| (kN)|   |   | A1. 1| 100. 1| 200| 4138| 569| 72. 3| 71. 6| A1. 2| 100. 1| 200| 4109| 555| 70. 5| | A1. 3| 100. 0| 200| 4125| 566| 72. 1| | B1. 1| 100. 3| 202| 4050| 490| 62. 0| 60. 5| B1. 2| 100. 2| 200| 4025| 463| 58. | | B1. 3 | 100. 1| 200| 4018| 478| 60. 7| | C1. 1| 100. 4| 203| 3995| 345| 43. 6| 45. 5| C1. 2| 99. 7| 204| 3981| 366| 46. 9| | C1. 3| 100. 4| 202| 3978| 365| 46. 1| | D1. 1| 100. 2| 198| 3842| 286| 36. 3| 36. 5| D1. 2| 100. 3| 202| 3833| 277| 35. 1| | D1. 3| 99. 9| 201| 3865| 299| 38. 1| | Table1. Compressive Strength – Cylinders (Water cured for 28 days) Observation: 3. 2 Compressive Strength – Cylinders (Air stored for 28 days) Specimen No. | Diameter| Height| Weight| Max. Load| cylinder strength (Mpa)| average cylinder strength (Mpa)| | (mm)| (mm)| (g)| (kN)| | | A1. 4| 100. 2| 201| 3946| 373| 47. 3| 48. | A1. 5| 100. 2| 200| 3947| 397| 50. 3| | A1. 6| 99. 7| 201| 3954| 383| 49. 1| | B1. 4| 99. 8| 200| 3863| 319| 40. 8| 41. 3| B1. 5| 100. 3| 201| 3890| 334| 42. 3| | B1. 6| 100. 2| 200| 3883| 323| 41. 0| | C1. 4| 100. 0| 202| 3800| 305| 38. 8| 38. 4| C1. 5| 99. 7| 203| 3795| 296| 37. 9| | C1. 6| 100. 2| 202| 3783| 304| 38. 6| | D1. 4| 99. 8| 203| 3738| 193| 24. 7| 25. 7| D1. 5| 100. 1| 202| 3726| 205| 26. 0| | D1. 6| 99. 7| 202| 3717| 205| 26. 3| | Table2. Compressive Strength – Cylinders (Air stored for 28 days) Observation: 3. 3 Indirect Tensile Strength – Cylinders (Water cured for 28 days) Specimen No. Diameter| Length| Weight| Max. Load| cylinder strength (Mpa)| average cylinder strength (Mpa)| | (mm)| (mm)| (g)| (kN)| | | A1. 7| 100. 2| 201| 4151| 151| 19. 1| 20. 1| A1. 8| 100. 1| 201| 4137| 169| 21. 5| | A1. 9| 100. 1| 203| 4166| 155| 19. 7| | B1. 7| 100. 2| 201| 4044| 136| 17. 2| 16. 5| B1. 8| 100. 1| 201| 4022| 129| 16. 4| | B1. 9| 99. 8| 200| 4002| 124| 15. 9| | C1. 7| 100. 2| 202| 3899| 115| 14. 6| 14. 6| C1. 8| 99. 7| 200| 3912| 109| 14. 0| | C1. 9| 99. 9| 201| 3903| 120| 15. 3| | D1. 7| 99. 8| 198| 3861| 96| 12. 3| 12. 3| D1. 8| 100. 1| 200| 3837| 93| 11. 8| | D1. 9| 100. 2| 198| 3859| 102| 12. 9| |Table 3 Indirect Tensile Strength – Cylinders (Water cured for 28 days) Observation 3. 4 Ultrasonic Pulse Velocity  œ Cubes (Water cured for 28 days) Specimen No. | Path Length| Elapsed Time| pulse velocity(km/s)| average velocity(km/s)| cylinder strength (Mpa)| | (mm)| (? sec)| | | | A1. 10| 100. 1| 21. 1| 4. 7| 4. 8| 20. 1| A1. 11| 100. 2| 20. 9| 4. 8| | | A1. 12| 100. 1| 20. 7| 4. 8| | | B1. 10| 100. 0| 21. 2| 4. 7| 4. 7| 16. 5| B1. 11| 100. 1| 21. 4| 4. 7| | | B1. 12| 99. 9| 21. 3| 4. 7| | | C1. 10| 99. 9| 21. 5| 4. 6| 4. 6| 14. 6| C1. 11| 100. 0| 21. 6| 4. 6| | | C1. 12| 100. 1| 21. 6| 4. 6| | | D1. 10| 99. 9| 21. 9| 4. | 4. 5| 12. 3| D1. 11| 100. 2| 22. 0| 4. 6| | | D1. 12| 100. 1| 22. 1| 4. 5| | | Table 4. Ultrasonic Pulse Velocity – Cubes (Water cured for 28 days) 3. 5 Compressive Strength – Cubes (Water cured for 28 days) Specimen No. | Width| Depth| Weight| Max. Load| cylinder strength (Mpa)| average cylinder strength (Mpa)| | (mm)| (mm)| (g)| (kN)| | | A1. 10| 100. 1| 100. 2| 2631| 695| 88. 3| 86. 2| A1. 11| 100. 2| 100. 0| 2625| 677| 85. 9| | A1. 12| 100. 1| 100. 1| 2611 | 664| 84. 4| | B1. 10| 100. 0| 100. 0| 2536| 555| 70. 7| 72. 2| B1. 11| 100. 1| 99. 9| 2548| 567| 72. 0| | B1. 12| 99. 9| 99. 9| 2539| 580| 74. 0| | C1. 10| 99. 9| 100. | 2497| 431| 55. 0| 54. 6| C1. 11| 100. 0| 99. 8| 2484| 420| 53. 5| | C1. 12| 100. 1| 100. 1| 2500| 436| 55. 4| | D1. 10| 99. 9| 100. 0| 2461| 357| 45. 5| 44. 2| D1. 11| 100. 2| 100. 1| 2453| 345| 43. 8| | D1. 12| 100. 1| 100. 0| 2462| 340| 43. 2| | Table 5. Compressive Strength – Cubes (Water cured for 28 days) Observation 4. Presentation of Test Results Materials | Mix A | Mix B | Mix C | Mix D | Cement content (kg/m3) | 16| 16| 16| 16| Free water content (kg/m3) | 6. 4| 7. 2| 8| 8. 8| Free water/cement ratio | 0. 4| 0. 45| 0. 5| 0. 55| Hardened unit weight (kg/m3) | 2614. 03| 2550. 70| 2476. 86| 2467. 41| Cylinder strength (MPa) | 71. 3056253| 60. 4904288| 45. 5208526| 36. 49120537| Indirect tensile strength (MPa) | 20. 10660749| 16. 4968345| 14. 6184321| 12. 34162357| Ultrasonic pulse velocity (km/s) | 4. 791360998| 4. 69489736| 4. 63680017| 4. 548533685| Cube strength (MPa) | 86. 18075386| 72. 236373| 54. 6216624| 44. 16699149| Plot the following graphical relationships and discuss these relationships a) Cylinder compressive strength versus free water-to-cement ratio [water-cured] As seen from the graphical relationship, as the free water content of cement decreases the compressive strength of the concrete specimen will increase.These two properties are inversely proportional to each other. This may be due to the extra water diluting the cement paste mixture which will weaken the bonding between cement paste and aggregates, and hence decreases the compressive strength of the concrete. b) Cylinder compressive strength versus free water-to-cement ratio [air-stored] The ratio between the compressive strength and the free water to cement ratio for the air cured specimens shows a similar trend to that of the water cured i. e. inversely proportional to each other. However it can be obser ved that the compressive strength is lower than that of the water cured specimens.This is due to the superior moisture conditions that the water curing option provides. c) Cylinder compressive strength [water-cured] to cylinder compressive strength [air-stored] ratio versus cylinder strength [water-cured] Comparing the ratio of strengths of water cured concrete and air stored concrete against the strength of just water cured concrete, a difference in strength can be seen. From the graph above, concrete cured in water have higher compressive strength than that of air stored concrete. Therefore, if high strength concrete is needed for construction, it would be important to expose concrete to moist conditions during curing. ) Cylinder indirect tensile strength versus free water-to-cement ratio [water-cured] e) Cylinder indirect tensile strength versus cylinder compressive strength [water-cured] f) Cylinder indirect tensile strength to cylinder compressive strength ratio versus cylinder compressive strength [water-cured] g) Cylinder compressive strength versus ultrasonic pulse velocity [water-cured] h) Cube compressive strength versus free water-to-cement ratio [water-cured] A large free water to cement ratio can cause segregation of aggregates, which leads to uneven distribution of aggregate, strength will vary.This theory can be clearly seen in the graph above. As free water to cement ration increases, compressive strength decreases. i) Cylinder compressive strength versus cube compressive strength [water-cured] – include the theoretical relationship cylinder compressive strength = 0. 80 x cube compressive strength for each mix) As seen from the trend of the results, the cube strength of concrete for a particular mix is always stronger than that of the cylindrical shape. The reason for this result is due to the advantageous geometric properties that a cube precedes over the cylindrical shape.The cubic specimen has anchor points at the corners of the cube which provide greater compressive strength. A general rule states that cylinder strength is about 80% of cube strength. Therefore, it can be stipulated that in construction, members with a square cross – section would have greater compressive strength than that of a cylindrical member. Members with square cross – section would be able to handle futher loads than a same sized cylindrical member. According to our results, the experimental data is quite close to our theoretical data.However, experimental result tends to be slightly higher than theoretical data. Indirect tensile test Apparatus Avery 200Ton concrete test console Bearing strips Dental plaster Procedure Using the same machine as the compressive test, a compressive load is induced along the cylinders length which caused failure along the diagonal direction by tension. Bearing strips are used between the cylinder and testing machine platens which avoids local crushing. Concrete sample was placed between bearing strips which was placed on the undersell testing machine laterally.A constant load was applied to the sample at a rate of 15Mpa/min until the sample fails. Specimen No. | Diameter| Length| Weight| Max. Load| cylinder strength (Mpa)| average cylinder strength (Mpa)|   | (mm)| (mm)| (g)| (kN)|   |   | A1. 7| 100. 2| 201| 4151| 151| 19. 1| 20. 1| A1. 8| 100. 1| 201| 4137| 169| 21. 5| | A1. 9| 100. 1| 203| 4166| 155| 19. 7| | B1. 7| 100. 2| 201| 4044| 136| 17. 2| 16. 5| B1. 8| 100. 1| 201| 4022| 129| 16. 4| | B1. 9| 99. 8| 200| 4002| 124| 15. 9| | C1. 7| 100. 2| 202| 3899| 115| 14. 6| 14. 6| C1. 8| 99. | 200| 3912| 109| 14. 0| | C1. 9| 99. 9| 201| 3903| 120| 15. 3| | D1. 7| 99. 8| 198| 3861| 96| 12. 3| 12. 3| D1. 8| 100. 1| 200| 3837| 93| 11. 8| | D1. 9| 100. 2| 198| 3859| 102| 12. 9| | Indirect Tensile strength – Cylinders (Water cured for 28 days) Non-destructive testing Specimen No. | Path Length| Elapsed Time| pulse velocity(km/s)| average velocity| cylinder strength ( Mpa)|   | (mm)| (? sec)|   |   |   | A1. 10| 100. 1| 21. 1| 4. 7| 4. 8| 71. 6| A1. 11| 100. 2| 20. 9| 4. 8| | | A1. 12| 100. 1| 20. 7| 4. 8| | | B1. 10| 100. 0| 21. | 4. 7| 4. 7| 60. 5| B1. 11| 100. 1| 21. 4| 4. 7| | | B1. 12| 99. 9| 21. 3| 4. 7| | | C1. 10| 99. 9| 21. 5| 4. 6| 4. 6| 45. 5| C1. 11| 100. 0| 21. 6| 4. 6| | | C1. 12| 100. 1| 21. 6| 4. 6| | | D1. 10| 99. 9| 21. 9| 4. 6| 4. 5| 36. 5| D1. 11| 100. 2| 22. 0| 4. 6| | | D1. 12| 100. 1| 22. 1| 4. 5| | | Ultrasonic Pulse Velocity – Cubes (Water cured for 28 days) Cylinder indirect tensile strength versus free water-to-cement ratio [water-cured] The tensile strength of concrete showed a linear relationship with the free water to cement ratio.As the free water to cement ration increased, the tensile strength of concrete decreased. This shows a similar relationship between free water to cement ratio and the compressive strength of concrete as seen in part A and B. It also follows the general trend that an increase in free water to cement ration will decrease the strength of hardened concrete. Cylinder indirect tensile strength to cylinder compressive strength ratio versus cylinder compressive strength [water-cured] From this diagram we are able to observe the relationship between cylinder tensile strength vs. ompressive strength. It is known that concrete is naturally weak in tension, however and increase in the compressive strength will also increase the tensile strength. This is increase the two properties of concrete is due to the lower water to cement ratio which increases the concentration of cement paste providing aggregates to cement paste bonding. This diagram shows the fraction of tensile strength in comparison to its compressive strength of a particular mix. The magnitude is approximately constant for all four mixes, thus showing a concrete tensile strength is an approximate.Cylinder compressive strength versus ultrasonic pulse velocity [water-cured] The above graph shows the relati onship between compressive strength and ultrasonic pulse velocity. The relationship shows that the higher the cylinder strength, the more ultrasonic pulse velocity is produced. This link indicates that with higher concrete strength, the denser the concrete specimen. The transmission time of the pulse travelling through the specimen is much shorter in denser materials. Discussion of all test results Effects of free water content on the properties of fresh concrete (workability and unit weight of concrete).Free water refers to the amount of water that is available for the concrete hydration process after the absorption of the aggregates has been taken into account. If the aggregate is saturated and wet, this will increase the amount of water available for the hydration process, increasing the free water content of the concrete mix. The workability and unit weight of fresh concrete are affected by the free water content as the variables alter the way concrete is utilized and transporte d and a construction site.The most common way of measuring the workability of concrete batch is by measuring the amount of slump in accordance with Australian Standards. A linear relationship between fresh concrete and free water content can be seen from the results of testing fresh concrete. When the free water-content increases, the slump of the concrete batch increases respectively. Lubricating effects can be seen between cement and aggregate particles and thus the more water present in the concrete mix, the easier for particles to slip and slide over each other and therefore increasing the workability.In terms of unit weight, it will decrease with the increase in water content. Cement mainly consists of water, cement and aggregates with respective specific gravities of approximately 1. 00, 3. 15 and 2. 65. With water having the least weight per unit volume, it can be assumed that with an increase in water content in a concrete mixture, the lower unit weight of concrete will be i n the mixture when the mixture is combined as there are lower amount of aggregates and cement which are the heavier elements in a concrete mixture.Lower unit weigh of concrete indicates that lighter concrete mixture for the same volume, and this is beneficial property of fresh concrete as it allows easier pumping around the construction site, however, higher water content can affect the strength of hardened concrete so equilibrium of cost versus benefit must be considered. Effects of free water content on the hardened concrete properties (compressive strength, tensile strength and modulus of elasticity) As shown in the graphs, the compressive strength of concrete generally increases as the free water to cementitious materials ratio is decreased.However as also seen in the graphs the compressive strength of cylindrical shape specimens tends to increase as free water to cementitious materials ratio is decreased at a decreasing rate whilst cube shaped specimens tends to increase in com pressive strength as free water to cementitious materials ratios decreased at an increasing rate. However for cube shaped specimens its compressive strength is relatively higher than that of cylindrical shaped specimens as seen in the graphs.Cylinder strength requires a larger reduction in free water to cementitious materials ratio in order to achieve the same strength as cube strength. A similar behavior can be seen between water cured cylinder specimens and air cured cylinder specimens. Air cured requires a further reduction in free water to cementitious materials ratio to achieve the same strength as water cured. For tensile strength, as the free water to cementitious materials ratio is decreased at a decreasing rate, the strength of the concrete increases.However tensile strength tends to increase less than compressive strength as the free water to cementitious materials ratio is increased. As seen in the graphs also, the modulus of elasticity for concrete tends to increase as t he free water to cement is decreased at an increasing rate. Effects of curing condition on the compressive strength of concrete Curing of concrete is known as the process which encourages cement hydration where an adequate supply of moisture is required to ensure that the rate of hydration of cement is adequate enough to achieve the desired strength for the concrete.Curing allows for continuous hydration of cement where the more days that the concrete is cured the more gain in strength at a decreasing rate there is for the concrete. However this gain in strength will be halted when cement hydration stops, due to the internal relative humidity of the concrete dropping below 80%. Curing of concrete is largely influenced by temperature and humidity, where concrete cured in air after several days of water or moist curing will never reach the strength of concrete that is continuously cured in water.Overall the more days that the concrete is water the more gain in strength there will be f or the concrete. Therefore it is very important to properly cure concrete in order to achieve optimum strength. Effect of specimen shape on compressive strength The compressive strength of concrete is also influenced by the shape of the testing specimen. In general, the compressive strength for cube shape concrete is relatively higher than the compressive strength of cylindrical shape concrete.This is largely due to the fact that in cube shaped concrete, the stress is further away from the uniaxial cracking whilst for cylindrical shaped concrete the stress is near the uniaxial cracking. Discuss your reflection on the importance of the laboratory session After having participated in the laboratory testing of fresh concrete and hardened concrete, we now have an in depth knowledge of the behavior of concrete in civil engineering structures, as well we know the various methods and preparation of producing concrete to achieve a particular goal in terms of strength and durability.The impo rtance of our laboratory class is that it lets us see the practical side of concrete properties where practical properties of concrete may sometimes not match the theoretical methods of concrete. This is quite common. The laboratory classes lets us learn how to prepare and produce concrete in order to achieve a particular goal in terms of strength and durability, as well we are able to see how real life situations and environments can affect the behavior of concrete particularly during curing.This would most likely be an important knowledge to us when we go to work in the real world. The laboratory has also helped gain an in depth knowledge of how to produce the most workable, durable and most economical concrete. Conclusion As highlighted in the report, the factors which influence the performance of concrete include: free water to cement ratio, curing, specimen shape. The amount of free water in concrete is critical given that concrete strength decreases as free water to cement rat io is increased.Curing environment is also an important factor influencing the performance of concrete where an appropriate environment is required to give an adequate supply of moisture is required to ensure that the rate of hydration of cement is adequate enough to achieve the desired strength for the concrete. However in general concrete cured in air after several days of water or moist curing will never reach the strength of concrete that is continuously cured in water. Overall the more days that the concrete is water the more gain in strength there will be for the concrete.The shape of the specimen is also an important factor influencing the performance of the concrete. As observed in the report, cubic strength of concrete is generally higher than that of cylindrical strength. The workability of fresh concrete is also largely influenced by the free water to cement ratio, where the workability of concrete increases as the free water to cement increases REFERENCE Standards Austra lia International Ltd (2010), AS1012. 3. 1-1998:Methods of testing concrete –determination of properties related to the consistency of concrete – slump test, SAI GLOBAL, accessed 24th April 2012 http//www. aiglobal. com. ezproxy. lib. uts. edu. au/online/autologin. asp Standards Australia International Ltd (2010), AS1012. 5 – 1999: Methods of testing concrete – Determination of mass per unit volume of freshly mixed concrete, SAI GLOBAL, accessed 24th April 2012 http//www. saiglobal. com. ezproxy. lib. uts. edu. au/online/autologin. asp Standards Australia International Ltd (2010), AS1012. 10 – 2000: Methods of testing concrete – Determination of tensile strength of concrete cylinders, SAI GLOBAL, accessed 24th April 2012 http//www. aiglobal. com. ezproxy. lib. uts. edu. au/online/autologin. asp Standards Australia International Ltd (2010), AS1012. 9 – 1999: Methods of testing concrete – Determination of the compressive strength of concrete specimens, SAI GLOBAL, accessed 24th April 2012 http//www. saiglobal. com. ezproxy. lib. uts. edu. au/online/autologin. asp Vessalas, K. (2010), 48352: Construction Materials Lecture Notes, University of Technology, Sydney http://www. icar. utexas. edu/publications/105/105 1. pdf viewed on 24/4/2012