Compression and Flexure Test of Concrete LAB REPORT 2020

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1 Compression and Flexure Test of Concrete LAB REPORT 20202 1. Introduction and overview of concrete and the experiment 1.1 Introduction In modern days, concrete is one of the most commonly u... sed materials for construction, such that it have a low cost but high compressive strength. Concrete is a mixture of cement, aggregate and water. Concrete can be divided into different types at which different type of concrete have specified properties for specific usage. After curing, hardening of concrete is undergone which the mechanical properties of the concrete would change with time. It is due the hydration in the water tank. In this lab experiment, regular Portland cement is used to check out the development of strength of concrete from time to time, that is the 7 th day and 28th day after the curing of concrete. Workability of concrete is also an important measure for determining the quality of fresh concrete. As it is important to ensure the concrete mixture suits the specification of materials required that is able to be shaped into desired parts for construction.. For freshly made concrete, several tests are conducted to determine the workability of concrete. Tests include slump test, slump test, V-B test and compacting factor test. Results will be compared with the British Standards in order to check for the workability. On the 7th day and 28th day after curing, the concrete will be conducted severals tests to determine the strength and durability of concrete. Tests include Schmidt Hammer Test, Cube Crushing Test, Ultrasonic Pulse Test, Flexural Strength Test, Equivalent Cube Test and Split Cylinder Test. 1.2 Objectives 1. To study how concrete is made 2. To determine the workability , strength and durability of concrete 3. To study the changes of mechanical properties of hardened concrete on the 7 th and the 28th day3 2. Sieve Analysis of Aggregates 2.1 Objective 1. To determine the grading of the aggregates by sieving the concrete looking for percentage passing of different sizes of sieve. 2. To determine the workability by comparing the result with the British Standard limit (B.S. 1881) 3. To consturct the grading curve of the mixed aggregates 2.2 Apparatus and materials Apparatus: 1. Sieving machines 2. Test sieves 3. Electronic Balance Materials: 1. Fine Aggregates – Crushed Granite rocks with a theoretical maximum size of 5mm 2. Coarse Aggregates – 10mm size aggregates and 20mm size aggregates made of granite rocks 2.3 Procedure 1. 5kg of 20mm aggregate is first weighted then is placed into the sieving machine with different sieve sizes of sieve plates arranged in order according to the British Standard. The sieve plates should be arranged as follow from the top to the bottom: 25mm, 20mm, 10mm, and 5mm. The sieving machine is then turned on. After the machine stops, aggregate retained on each sieve will then be weighted accordingly and recorded. 2. After clear all the aggregate in the sieves, Step 1 is repeated with 2kg of 10mm aggregate using the same set of sieve. 3. 0.5 kg fine aggregates will be sieved using another sieving machine with sieve plates arranged as follow from the top to the bottom: 5mm, 2.36mm, 1.18mm, 600μm, 300μm and 150μm. The sieving machine is then turned on. After the machine stops, aggregate retained on each sieve will then be weighted accordingly and recorded 4. The percentage passing each sieve of the 3 sizes of aggregates is calculated for the plotting of a grading curve.4 5. The grading curve of the combined aggregate will then be deduce. The mixing ration of 20mm aggregate:10mm aggregate:fine aggregate is 2:1:1.5.5 15 2.4 Result 20 mm Aggregates (Total Weight of Sample=5000g) B.S. Weight Retained (g) Weight Passing (g) Percentage Passing (%) B.S. Limits Sieve 25mm 0 4999.4 100 100% 20mm 262.1 4737.9 94.77 85%-100% 10mm 4613 124.9 2.50 0%-25% 5mm 122.9 1.4 0.028 0%-5% Residue 1.4 0 0 -- Total 4999.4 Max. Aggregate Size: 20 mm Particle Size Distribution of 20mm aggregates 120 100 80 Percentage passing (%) 60 40 20 0 0 5 10 20 25 30 Sieve size (mm)6 15 Particle Size Distribution of 10mm aggregates 120 100 80 Percentage passin 60 40 20 0 0 5 10 20 25 30 Sieve size (mm) 10 mm Aggregates(Total Weight of Sample=2000g) B.S. Weight Retained (g) Weight Passing (g) Percentage Passing (%) B.S. Limits Sieve 25mm 0 2000.7 100 100% 20mm 0 2000.7 100 100% 10mm 342.1 1658.6 82.9 85%-10% 5mm 1595.6 63.0 3.15 0%-25% Residue 63.0 0 0 -- Total 2000.7 Max. Aggregate Size: 10 mm g (%)7 50 Fine Aggregates(Total Weight of Sample=500.1g) B.S. Weight Retained (g) Weight Passing (g) Percentage Passing (%) B.S. Limits Sieve 5mm 30.3 469.4 93.94 89%-100% 2.36mm 168.6 300.8 60.20 60%-100% 1.18mm 115.0 185.8 37.18 30%-100% 600μm 71.1 114.7 22.95 15%-100% 300μm 46.8 67.9 13.59 5%-70% 150μm 26.1 41.8 8.37 0%-15% Residue 41.8 0 0 -- Total 499.7 Max. Aggregate Size = 5mm For Combined Aggregates: The combined aggregates is mixed with a ratio of 2:1:1.5 (20mm: 10mm: Fine). Therefore, the percentage passing of different aggregates through different sieve plate should be calculated according to the given ratios. The portion of 20mm aggregate in the combined aggregates = ( 2 (2+1+1.5) ). Particle Size Distribution of Fine aggregates 100 90 80 70 60 Percentage passing (%) 40 30 20 10 0 0 1 2 3 Sieve size (mm) 4 5 68 The portion of 10mm aggregate in the combined aggregates = ( 1 (2+1+1.5) ). The portion of fine aggregate in the combined aggregates = ( 1.5 (2+1+1.5) ). Therefore, the percentage passing of each type of aggregates should multiply with the ratio of their portion in the combined aggreagates. Such that : Combined aggreagates = 20mm aggregate x ( 1 2 (2+1+1.5) ) + 10mm aggregate x (2+1+1.5) ) ¿ + fine aggregate x ( 1.5 (2+1+1.5) ) Combined Aggregates B.S. Sieve 25mm 20mm 10mm 5mm 2.36m m 1.18m m 600μm 300μm 150μm % passing for % passing for 10mm % passing for fine % passing for combined 20mm aggregate aggregate aggregate aggregates 100 100 100 100 94.77 100 100 97.68 2.5 82.9 100 52.87 0.028 3.15 93.94 32.03 0 0 60.2 20.07 0 0 37.18 12.39 0 0 22.95 7.65 0 0 13.59 4.53 0 0 8.37 2.799 Particle Size Distribution of combined aggregates 120 100 80 60 40 20 0 0 5 10 15 20 25 30 Percentage passing (%) Sieve size (mm)10 2.5 Discussion From the results,mostly all samples are within the B.S. Limit given, indicating that the aggregates are suitable for making concrete. In general, the combined aggregates will produce a better concrete than concrete made up of ony 1 type of aggregate. However, in the experiment there are some errors exist, such that it is found that the total weight of each 3 kinds of aggregate measured are not consistant with the original value after the sieving. This problem will be discussed in the part of source of error. 2.6 Precautions 1. All aggregate samples should be kept dry and clean since any water content in the aggregates will increase the mass reading of the aggregates resulting in errors. 2. All the sieve plates and plate for measuring the mass of aggreagate should be kept clean to prevent aggregate leaving from the previous turn. 3. After the sample is processed through the sieve machine, it should be ensured that all aggregate on each sieve plate are transferred onto the tray for weighing. 2.7 Source of error 1. Some aggregates may be lost during the transfer of the aggregates into the sieving machine and electronic balancear, resulting in the total weight of aggregates before and after the experiment has a slight change. 2. Some aggreagates from the previous experiment may be left on the electronic balance and sieving plate may be left, resulting in a inconsisant total weight of aggregates measured. 2.8 Conclusion Aggergates used in the experiment are in good grading such that they are within the B.S. Limits, indicating that they are suitable and good for making concrete. Workabiliby of fresh concrete is not only depends on the quality of the aggregates, but good quality definitely cannot be produced without aggregate of good grading.11 3. Concrete mixing and tests Objective 1. To cast concrete under specific ratio of materials 2. To determine the workability of fresh concrete 3.1 Concrete mixing 3.1.1 Apparatus and materials Apparatus: 1. Concrete mixer 2. Shovel Materials: 1. Coarse aggregates and fine aggregates 2. Cement 3. Water 4. Super plasticizer 3.1.2 Procedure 1. Weight out quantities of aggreagates, cement as mentioned above. 2. Mix the dry materials together first. 3. Add water into the concrete mixer until concrete becomes homogenous and uniform in color. 4. Add super plasticizer to fasten the process. 3.1.3 Mixing proportion 1. The volume of concrete that will be produced is 0.045 m3 . 2. 8 kg of water and 100g of super plasticizers are added. 3. The mixing proportion by mass of Cement: Fine Aggregate: Coarse Aggregate is 16.0:32.5:(24.4 + 24.4) = 1:2.03: (1.525+1.525) 4. 20 mm aggregate to 10 mm aggregate ratio is 1:1. 5. The water:cement ratio is given to be 1:2. 6. Let mass of cement is X kg.12 Material Mass (kg) Density (kg/m3 ) Volume (m3 ) Water 0.5X 1000 0.5X/1000 Cement X 3150 X/3150 20 mm aggregate 1.525X 2650 1.525X/2650 10 mm aggregate 1.525X 2650 1.525X/2650 Fine aggregate 2.03X 2650 2.03X/2650 Total 0.045 The calculated value of X is 16.47. Therfore the quantities of each materials for producing 0.045 m3 of concrete: Material Mass (kg) Water 8.235 Cement 16.47 20 mm aggregate 25.12 10 mm aggregate 25.12 Fine aggregate 33.4313 3.2 Slump Test 3.2.1 Objectives: To determine the workability of fresh concrete by studying the mode of failure after the slump cone which filled with concrete is removed. 3.2.2 Apparatus: 1. Slump cone 2. Shovel 3. Steel rod 3.2.3 Procedures: 1. The slump cone is filled with fresh concrete of 3 equal layers., such that each layer of concrete is tamped for 25 times with a 16mm diameter steel rod before a new layer of concrete is filled. 2. The slump cone is removed through lifting up. Concrete inside drops due to gravity. The drop in height of the concrete cone is measured. 3.2.4 Result: The slump is a shear slump with 110mm height. 3.2.5 Discussion: The table below shows the workability of concrete in slump test under British Standard. Slump (mm) Degree of workablility 0-25 Very low 25-50 Low 50-100 Medium 100-160 High From the experimental result, the slump height is 110 mm, which has a high degree of workability. The accuracy of slump test is not high but it is still frequently used on site. It is due to the simplicity of the test, such that with the simple equipment and little comsumption of time, workability of concrete can be roughly determined. There are some precuations during the conduction of the slump test: 1. The inside surface of the cone and the base should be moistened to reduce friction between the cone inner surface and the concrete, as the friction may has effect on the shape of the slump.14 2. The base of the cone and the lifting cone should be cleaned before filling in fresh concrete in order to prevent leftover concrete to drop accidentally and affect the result. These precuations should be followed to minizie any errors or effects of external factors. 3.2.6 Conclusion: The slump of the concrete sample is 110mm high which indicate that the concrete sample has a high workability.15 3.3 V-B Test 3.3.1 Objectives: To determine the workability by measuring the time required for concrete to transform from a standard cone to a compacted flat cylindrical mass. 3.3.2 Procedures: 1. A glass disc attaching with a swiel arm is placed on top of the slump which has been put into a V-B apparatus from the slump test. 2. An electrical vibrator is placed into the concrete and turned on afterward. When the glass disc rests entirely on the surface such that the concrete are with a shape of cylinder, the time for this is recorded, know as the V-B time. 3.3.3 Apparatus: 1. V-B Container 2. Vibrator 3. Stop-watch 3.3.4 Result: The V-B time is 1.75s. 3.3.5 Discussion: The table below shows the workability of concrete in V-B test under British Standard. V-B time (s) Degree of workablility >20 Extremely low 12-20 Very low 6-12 Low 3-6 Medium >3 High From the experimental result, the V-B time is lower than 3, indicating that the workability of the concrete is high.16 Wet density, wet Wet density, wet 3.4 Compacting Factor Test 3.4.1 Objectives: To determine the workability of fresh concrete by measuring the compaction degree. 3.4.2 Apparatus: 1. Compacting Factors Apparatus 2. Poker Vibrator 3.4.3 Procedures: 1. The upper hopper of the apparatus is filled with concrete. The trap-door is then opened allowing the concrete to fall under gravity into the lower hopper. 2. The trap-door of the lower hopper is opened allowing concrete to fall down again into the cylindrical container at the bottom. 3. The weight of the partially compacted concrete filled in the cylinder is then measured. 4. The concrete inside the cylinder is then compacted and additional concrete is poured to fill in the space. 5. The weight of the cylinder with ully compacted concrete is weighted. 6. The compacting factor is calculated with the equation: weight of partiallycompacted concrete Compacting factor = weight of fully compactedconcrete 7. The wet density of the concrete is calculated with the equation: ρ = weight of fully compacted concrete volumeof fully compactedconcrete 3.4.4 Result: Weight of Cylinder (g) 4251 Weight of partially compacted concrete + cylinder (g) 15496 Weight of partially compacted concrete (g) 11245 Weight of fully compacted concrete + cylinder (g) 16044 Weight of fully compacted concrete (g) 11793 Volume of fully compacted concrete (m3 ) 0.005036 Compacting Factor = weight of partiallycompacted concrete = 11245 =0.954 weight of fully compacted concrete 11793 ρ = weight of fully compacted concrete =2341.6 kg /m 3 volumeof fully compactedconcrete 3.4.5 Discussion: The table below shows the workability of concrete in Compacting Factor Test under British Standard.17 Compacting Factor Degree of workability 0.65-0.70 Extremely low 0.7-0.80 Very low 0.80-0.90 Low 0.90-0.95 Medium 0.95-1.00 High From the experimental result, the compact factor is 0.954, indicating that the concrete has a high degree of workability. 3.4.5 Conclusion of 3 tests on workability Degree of workability Slump test High V-B test High Compacting factor test High The experimental results of three different tests are consistent to each other, such that among the three tests they indicate that the fresh concrete has a high degree of workability.18 4. Casting of Concrete 4.1 Objectives: To make concrete samples for 7-days test and 28-days test for studying the mechanical performances such as strength of concrete in harden state. 4.2 Apparatus: 1. Tamping rod 2. Shovel 3. Six 150 mm cube moulds 4. One 150 mm diameter x300 mm height cylinder mould 5. One 100 mm x 100 mm x 500 mm flexural test specimen mould 4.3 Procedures: 1. The test concrete samples are casted into different shapes with respect to the requirement. a. Cube: Fill with 3 layers which tamping of 40 strokes are applied for each layer. b. Beam: Fill with 2 layers which tamping of 100 strokes are appplied for each layer. c. Cylinder: Fill with 6 layers which tamping of 30 strokes are applied for each layer. 2. A wet sack is covered on the concrete surface when the concrete surface hardens. The demoulding process will be performed on the next day after casting. 3. Strokes are applied for making the concrete more compacted and remove the air bubbles inside. 5. Curing of Concrete 5.1 Objectives: To maintain sufficient humidity in the concrete so as to make sure the rate of hydration between cement and water. 5.2 Apparatus: 1. Spanners 2. Curing Tanks 5.3 Procedures: 1. The test specimens are removed from moulds using spanners on the next day after casting. 2. The specimens are labeled with suitable group number for identification. 3. The specimens are stored into a water curing tank for further testing (7-days, 28-days test).19 6. Testing on Strength As mentioned in the introduction, severals tests will be conducted on the concrete on the 7 th day and 28th day after curing, to determine the strength and durability of concrete. Fot the 7 th day, Schmidt Hammer Test and Cube Crushing Test are conducted. For the 28th day, Schmidt Hammer Test, Cube Crushing Test, Ultrasonic Pulse Test, Flexural Strength Test, Equivalent Cube Test and Split Cylinder Test are conducted. Due to logistic arrangements, our group performed a 14-days and 35-days test instead. 6.1 Schmidt Hammer Test 6.1.1 Objectives: To determine the compressive strength of concrete sample using Schmidt Hammer Test by reference to calibration graph. 6.1.2 Theory: The Schmidt Rebound Hammer is used to determine the compressive strength of a concrete by hitting the concrete and measure the rebound value. The rebound value is depended on the hardness of the concrete and therefore the strength of concrete can be determined by reference to the calibration graph. Calibration graph20 The orientation of the hammer should be exactly perpendicular to the surface of the concrete for accuary of measurement, therefore the orientation of how the person holding the hammer would have great effect on the results. To minimize the error, 8 readings will be taken on different position of both the front and the back side of the cube. The largest 3 and the smallest 3 rebound value will be eliminated from the calculation. 6.1.3 Apparatus: 1. Schmidt Rebound Hammer 2. Concrete cube specimen 6.1.4 Procedures: 1. The plunger of the Schmidt hammer is first pushed perpendicularly to the front surface of the concrete cube. 2. The release button is then pressed which allows the plunger to rebound back to its original position. The rebound value is recorded. 3. Eight rebound values are recorded on both the front side and back side of the cube with the Schmidt hammer respectively. The test should only be carried out on the smooth surface and surface without holes and cracks to avoid inaccuracy. 4. The largest 3 and the smallest 3 rebound values are eliminated. Calculate the average value of the remaining 10 rebound values. 5. Refer to the calibration graph of the Schmidt hammer with the average rebound value to determine the corresponding compressive strength of the concrete cube. 6.1.5 Result: For 14-days test: Cube 1 (No.4) Cube 2 (No.3) Cube 3 (No.5) Front Back Front Back Front Back Rebound value of Schmidt Hammer 38 32 32 40 32 40 38 36 32 40 30 52 36 36 40 40 38 40 38 36 40 38 32 40 38 40 38 40 38 42 32 44 42 36 38 42 44 38 34 38 36 39 42 34 36 40 38 38 Average Reading 37.4 38.6 38.8 Corresponding strength (MPa) 36 38 38 Average compressive strength (MPa) 37.3321 For 35-days test: Cube 1 (No.1) Cube 2 (No.2) Cube 3 (No.6) Front Back Front Back Front Back Rebound value of Schmidt Hammer 38 42 36 40 40 42 36 48 40 38 40 26 46 38 38 40 28 40 36 42 44 40 38 28 40 38 40 40 40 34 38 46 44 42 22 30 44 38 38 40 42 26 44 40 42 40 42 34 Average Reading 40.4 40 36.2 Corresponding strength (MPa) 40 40 34 Average compressive strength (MPa) 38 Percentage change in terms of compressive strength from 14th day to 35th day: 38−37.33 ×100=+1.79 37.33 6.1.6 Discussion: Comparing the results of 14th day with 35th day, there is a slight increase in the compressive strength of concrete, which agree with the fact that fresh concrete would harden with time after casting. 6.1.7 Error: 1. There is human error of holding the hammer in a wrong orientation or do not hit the hammer on the surface perpendicularly, therefore the rebound value is not accurate.22 6.2 Cube Crushing Test 6.2.1 Objectives: To determine the density and compressive strength of the concrete samples through applying loading until failure. 6.2.2 Theory: A concrete sample is placed inside a compressive testing machine to receive increasing loading until failure. The ultimate load will be recorded and the cube strength is determined as following: Compressive Strength= Ultimate loading Crosssectional area of the cube 6.2.3 Apparatus: 1. Compressive testing Machine 2. Electronic Balance 3. Ruler 6.2.4 Procedures: 1. The dimension and the mass of the cube is measured with ruler and electronic balance. 2. The cube is then placed into the compressive testing machine with the mould surface attaching to the plate. 3. Loading is applied until failure. The loading at failure is determined as ultimate loading. 4. Repeat the experiment for the leaving cubes. 6.2.5 Result: For 14-days test, 口 Cube 1 (No.4) Cube 2 (No.3) Cube 3 (No.5) Dimension (m) 0.15x0.15x0.15 0.15x0.15x0.15 0.15x0.15x0.15 Mass (kg) 8.106 7.987 8.110 Density (kg/m3 ) 2401.8 2366.5 2403.0 Ultimate Load (kN) 1115 955 1190 Contact Area (m2 ) 0.15x0.15 0.15x0.15 0.15x0.15 Cube Strength (MPa) 49.56 42.44 52.89 Average Cube Strength 48.3023 (MPa) Average Density (kg/m3 ) 2390.53 For 35-days test, Cube Cube 4 (No.1) Cube 5 (No.2) Cube 6 (No. 6) Dimension (m) 0.15x0.15x0.15 0.15x0.15x0.15 0.15x0.15xo.15 Mass (kg) 8.160 8.040 8.058 Density (kg/m3 ) 2417.8 2382.2 2387.6 Ultimate Load (kN) 1258 1188 1276 Contact Area (m2 ) 0.15x0.15 0.15x0.15 0.15x0.15 Cube Strength (MPa) 55.91 52.80 56.71 Average Cube Strenght (MPa) 55.14 Average Density (kg/m3 ) 2395.87 Percentage increase in compressive cube strength: 55.14−48.30 ×100 =14.16 48.30 6.2.6 Discussion: The density of the cube sample of the 14-days and 35-days are more and less the same, showing that during the hardening of concrete, density of concrete would not undergone great changes and would remain the similar value. The strength of concrete on the 35-days is much greater than that of on the 14-days, as from the result of calculation, the change in strength is about 14%, that agree with the hardering property of concrete such that hardness of concrete would increase with age. Comparing the results with the Schmidt Hammer test, the compressive strength recorded by the cube crushing test is higher than that of the Schmidt Hammer test, it is because great human errors are occurred in the Schmidt Hammer test which greatly affect the results. On the other hand, the calibration curve provided is not very accurate as there are many verisons of calibration curve. Therefore the Schmidt Hammer test is only suitable for quick and easy estimation of the strength of concrete.2425 6 6.3 Flexural Strength Test 6.3.1 Objectives: To determine the modulus of rupture for the concrete sample by applying point load onto 2 points 133mm from each other and the end of the beam. 6.3.2 Theory: Due to the low tensile strength of concrete, resulting in great difficulty in applying a uniaxial tension to a concrete sample. Therefore, an indirect method, which is the Flexural Strenght Test is used to determine the modulus of rupture of concrete. A concrete beam sample is used for testing. Two identical load will be applied onto two points on the beam at which they are 133mm from each other and the ends of the beam. It should besymmetrical about the mid-span. The beam will be bent under a increasing loading until failure. By calculating the maximum bending moment of the beam, the modulus of rupture can be determined. P P Shear Force Diagram 2 2 L P 3 3 P 3 P P 2 2 The maximum bending moment of the beam ¿ PL The modulus of rupture PL PL × d 6 ¿ My = 6 2 = PL 3 2 P 2 226 6.3.3 Apparatus: 1. Flexural Strength testing kit 2. Concrete beam specimen 6.3.4 Procedures: 1. The beam is placed into the flexural strength testing kit. Increasing loading is applied to the beam 2. When the beam fails, record the ultimate load. 6.3.5 Result: Load at which specimen fails (kN) 9.90 Span of the beam (mm) 400 Width of the beam (mm) 100 Depth of the beam (mm) 100 Modulus of Rupture: PL =9.90× 0.4 =3.96 MPa bd2 0.1 x 0.12 Discussion: It is found that the tensile strength of the concrete is about 3.96 MPa. Comparing with the compressive strength found in the cube crushing test, it can be deduced that the compressive strength is much higher than the tensile strength of concrete. Concrete is more able to withstand compressive force instead of tensile stress.27 6.4 Equivalent Cube Test 6.4.1 Objectives: To determine the equivalent cube strength and compare the value with the cube crushing strength value. 6.4.2 Theory: Similar to the cube crushing test, the remain parts of the beam from the Flexural strength test are taken to compressive testing machine to receive increasing loading until failure. The ultimate loading of the remain parts are recorded. The compressive strength of the remaining part is defined as following: Compressive strength= Maximum load crosssectionarea After determining the compressive strength, the results are compared with the results obtained fomr the cube crushing test. 6.4.3 Apparatus: 1. Equivalent cube testing kit 2. The two cube left from the Flexural strength test 6.4.4 Procedures: 1. The remain parts of the beam are placed into the compressive testing machine. 2. The ultimate loading at the point of failure is recorded for further calculation. 6.4.5 Result: Portio n Crushing Load (kN) Cross sectional area (mm2) Equivalent Strength (MPa) 1 544 100x100 54.40 2 546.6 100x100 54.66 Average equivalent cube strength: 54.53 6.4.6 Discussion: The average equivalent cube strength is found to be 54.53 MPa while the cube strength found in the cube crushing test is 55.14 MPa. The two values close. There are slight difference of the two value can be explained as the non uniformity of concrete in the beam and the cube.2829 6.5 Modulus of Elasticity Test 6.5.1 Objectives: 1. To study the stress-strain relationship 2. To determine the modulus of elasticity of the concrete specimen 6.5.2 Theory: To study the stress-strain relastionship, loading of 1/3 of the cube strength is applied to the cube, in order to prevent plastic deformation of the concrete cube. The reading of the extensometer is recorded every 30kN increment of the applied load and the strain can be found in every increment of applied load. A stress-strain curve is drawn with a best-fit line passing through all the points which respresent the modulus of elasticity for concrete. 6.5.3 Apparatus: 1. Extensometer for concrete cylinder 2. Concrete cylinder 6.5.4 Procedures: 1. The cylinder is first capped using a patented capping compound. 2. The cylinder is then subjected to one-third loading of the cube strength. 3. The load will be maintained for 1 minute and will be released afterwards. 4. The same load will then be applied again onto the cylinder with 30 kN increment. 5. The strain of the concrete is measured using the extensometer attached on the cylinder. A reading is taken for every 30kN increment of applied load. 6. The stress-strain curve is plotted. 6.5.5 Result: Diameter of the cylinder (mm) 150 Cross-sectional area (mm2 ) 17671. 5 Distance between the extensometer, L (mm) 150 Maximum Applied Load P: P =P´ × Areaof Cylinder × 1 cylinder cube Area of Cube 330 P =P´ π d 2 × 4 × 1 = π P´ cylinder cube s 2 3 12 cube Pcylinder=324.81 kN Reading Applied Load (kN) Stress (MPa) Meter Reading (mm) Strain, ΔL/L (10-6 ) 1 10 0.566 0 0 2 40 2.264 16 53.333 3 70 3.961 35 116.667 4 100 5.659 55 183.333 5 130 7.356 75 250 6 160 9.054 95 316.667 7 190 10.751 113 376.667 8 220 12.449 132 440 9 250 14.147 152 506.667 10 280 15.845 171 570 11 310 17.542 189 630 Stress (MPa)31 Strain (10-6 )32 Stress-strain Curve 20 18 16 14 12 10 8 6 4 2 0 0 100 200 300 400 500 600 700 f(x) = 0.03x + 0.74 From the stress-strain curve, the modulus of elasticity should be 26.6 GPa.33 6.5.6 Discussion: 28-day compressive strength (MPa) Typical range of a 28-day static modulus of elasticity (GPa) 20 18-30 25 19-31 30 20-32 40 22-34 50 24-36 60 26-38 The crushing cube strength of the concrete sample at 35 day is 55.14 MPa while the modulus of Elasticity is 26.6 GPa. According to the British Standard, the concrete sample is within the typical range. However, the value of modulus of elasticity is quite marginal in the range. It may be due to our test is done on the 35th day, which is much longer than the 28-day, such that the concrete has a longer time to build up more strength. If the test is conducted on the 28th day, the strength of the concrete should be lower, such that the vale of 26.6 GPa of the modulus elasticity would not be marginal in the typical range. 6.5.7 Error: 1. The reading may not be taken exactly on the incement 30Kn interval casuing little error. 2. There are reading error from the extensometer. 3. The best-fit line is only a rough estimation of the modulus of elasticity which may not reflect the exact and acurrate value of the modulus of elasticity.34 6.6 Split Cylinder Test 6.6.1 Objective: To determine the indirect tesile strength. 6.6.2 Theory: Split cylinder test t is an indirect method for determining the tensile strength of concrete. The concrete cylinder is first placed horizontally and loading is applied along the vertical diameter of the cylinder. Two tensile force are induced horizontally in opposite direction, when the tensile force is high enough, the cylinder will then split in two parts along the vertical diameter. 6.6.3 Apparatus: 1. Split Cylinder testing kit 2. Compression testing machine 6.6.4 Procedures: 1. The cylinder to be tested is placed in the compression testing machine while two 3mm x 25mm strips of plywood is placed on the 2 sides of the cylinder. The load are applied through the plywood acting on the vertical diameter of the cylinder. 2. Increase loading until failure of the cylinder. 3. The load at which the specimen fails is recorded. 6.6.5 Result: Load at which specimen fails (kN) 270 Length of the cylinder (mm) 300 Diameter of the cylinder (mm) 150 Induced tensile strength (MPa) 3.82035 6.6.6 Discussion: The induced tenile strength in the split cylinder test is 3.820 MPa. Comparing with the result obtained from the Flexural strength test, which the modulus of rupture is found to be 3.96 MPa. The tensile strength of concrete sample found in the Flexural strength test than that in the Split cylinder test. It is due to the following reasons: 1. The method of compression is different Beam is used in the Flexural strength test while cylinder is used in the Split cylinder test. The method of applying loasding to the specimen of the two tests is different such that the induced tensile stress is not identical in the two test. 2. Shape of the specimen is different The specimen used in the Flexural strength test is a beam shape. When the loading is aplloed to the beam, an induced tensile stress acts outwards, while there is reaction force which help the beam to withstand the tensile stress as shown in the following figure, therefore the the beam can withstand to a larger magnitude of tensile stress. While for the cylinder, there is no outer part like the beam can help to withstand the tenile stress, therefore the cylinder specimen is found to be have a lower tensile strength than that of the beam in the Flexural strength test.36 6.7 Ultrasonic Pulse Test 6.7.1 Objectives: To determine the pulse velocity of the concrete beam. 6.7.2 Theory: Ultrasonic pulse test is a non-destructive test which used for determining the strength and durability of concrete. Ultrasonic pulse is transmitted from one end to another end, while time for the travelling is recorded. Since the pulse velocity depends on the quality of the sample, such that any defects like cracks, air and water voids would hinder the transmiition of ultrasonic pulse, resulting in a lower pulse velocity. Therefore, the pulse velocity is a indicator reflecting the quality of the concrete. 6.7.3 Apparatus: 1. Pulse Generator 2. Transducers 3. Amplifier 4. A device to measure the time interval elapsed between the pulse generated and the receiving transducer 6.7.4 Procedures: 1. Oil the surface of the two ends of the beam specimen. 2. Place the transmitting transducer and the receiving transducer at the two ends of the beam specimen. 3. Measure the transit time for both along the width and length of the beam. 6.7.5 Result: Trial Distance travelled Time for travelling Pulse Velocity 1 500mm 128.4 μs 3.894 kms-1 2 100mm 27.6 μs 3.623 kms-1 Average Pulse Velocity: 3.7585 kms-137 6.7.6 Discussion: Pulse Velocity (km/second) Concrete Quality Above 4.5 Excellent 3.5 – 4.5 Good 3.0 – 3.5 Medium Below 3.0 Doubtful The pulse velocity of the concrete beam is 3.7585 kms-1 . According to British Standard, this concrete beam has a good concrete quality, indicating that there little air and water voids inside the concrete that hinder the transmittion of pulse.38 7. Summary of Test Results Test Objective Result Slump Test Workability 110 mm (High) V-B Test 1.75 s (High) Compacting Factor Test 0.954 (High) Schmidt hammer Test Compressive Strength 14-day 37.33MPa 35-day 38 MPa Cube crushing test 14-day 48.30 MPa 35-day 55.14 Pa Flexural Strength Test Modulus of Rupture 3.96 MPa Equivalent Cube Test Compressive Strength 54.53 MPa Modulus of Elasticity Test Modulus of Elasticity 26.6 GPa Split Cylinder Test Induced Tensile Strength 4.796 MPa Ultrasonic Pulse Test Pulse Velocity 3.7585 kms-139 8. Discussion Q1. In laboratory, the curing of test specimen is carried out prior to the testing of specimens at 7 th day and 28th day. Briefly explain the importance of this procedure. Discuss the possible effect on concrete if no curing is provided. Curing is a process of storing the fresh concrete that is moulded into desired shape into a water tank. Curing is an important procedure since concrete cannot perform hydration for building up the strength without sufficeient water. If the concrete is not stored into a water tank, the water vapour in the air cannot provide enough water for the concrete to conduct hydration, resulting in a low strength of the concrete which is not desirable for construction. Apart from concrete strength, curing also helps in preventing defects such as air voids and cracks in the concrete. Since inadequate water content in the concrete would lead to incomplete hydration, that would produce air voids and cracks inside the concrete. Such defects would greatly reduce the strength of the concrete, resulting in the concrete being not strong enough for construction. Even if more time is provided for building up the strength Ont the other hand, if no curing is provided, the concrete needs more time to develop its strength to a desired level, causing a big problem in construction that it may lead to delay of the project. Even if more time is given for the concrete to build up the strength, it can only reach a much lower strength. Q.2 While mixing a concrete mix with a water/cement (W/C) ratio of 0.55, a student accidently poured in extra water into the mix. Discuss the possible effects on the workability and the outcome on tests performed on such concrete at 7-day and 28-day. Since more water is accidentially added into the concrete mix, the water/cement ratio would get higher. An increase in the water/cement ratio would lead to a higher workability of the fresh concrete due to higher fluidity of the concrete mix, which attribute to the workability of the fresh concrete. For the test results of concrete at 7-day and 28-day, the strength of concrete dedeuced from the tests would be generally smaller. It is due to a higher water/cement ratio would lead to lower ultimate compressive strength of the concrete, as shown by the following graph:40 As shown from the graph, concrete wit higher water/cement ratio would have a lower strength, such that it can withstand to a smaller magnitude of compressive stress and tenile stress. Therefore, for the tests results obtained on the 7-day and 28-day, the strength of concrete, which are the tensile strength and compressive strength, deduced from various tests would be lower. The modulos of elasticity of the concrete would be lower as well. Q.3 An engineer claimed that the compressive strength of a concrete structure is below the specified design compressive strength and the failure of concrete structure would occur anytime. By applying the knowledge learnt in laboratory session, describe the tests that can be applied on the structure to verify the validity of statement made by the engineer. According to the clami of the engineer, the concrete structure would fail anytime, therefore nondestructive test is much more preferable than destructive test to prevent dealing any damage on the structure and fasten the failure of the structure. The Schmidt Hammer Test can be conducted to determine the compressive strength of the concrete structure roughly first. Since the Schmidt Hammer Test is a non-destructive test. It is one of the fastest and simplest way to determine the compressive strength of the concrete. Having a brief idea on the compressive strength of the structure, more accurate tests can be conducted. It may be able to take out some specimens samples form the concrete structure for testing if the result of the Schmidt Hammer Test indicating that the concrete structure is not really close to its limits and almost fail. Q.4 A student proposes to reduce the size of the concrete cubes from each side 150mm to each side 100mm in order to save materials. Discuss on the results of the compression test if such amendment is accepted. Also, discuss briefly in what situation each side 100mm is more applicable than each side 150mm. The results of the compressive strength test would be deduced with a hihger value of strength of the concrete sample, if the concrete sample is reduced of size from each side 150 mm to 100 mm. When concrete has a lower dimension, the water inside the cube is more easy and quicker to penetrate through the cube during curing. The extend of hydration will get greater than concrete cube with a larger size. With a higher extend of hydration, the concrete is able to build up a higher strength during the time Therefore, for a smaller cube, results(compressive strength) deduced fromt the cube compressive test and the equivalent cube test would be larger than that of a larger cube. However, for the Schmidt Hammer Test, the result which is the compressive strength of the specimen would be similar. It is due to the result of this test is highly depends on the surface hardness of the two cubes, such that the extend of the hydration of the two cubes is more or less the same as hydration occurs from the surface of the concrete.41 Using a concrete with a smaller dimension for testing the strength, less loading is required to be applied on the specimen until failure, therefore the time required for determining the ultimate loading is less. In addition, time required for a smaller cube to build up to a desirable strength is less than that with a larger size. Hence, each side 100mm is more applicable than each side 150 mm when time for testing is limited. 9. Conclusion Concrete nowadays is a very important marterial for construction, therefore strict and accurate testing on the mechanical property such as the workability and strength of concrete is necessary for preventing failure of concrete structure. Different tests can be used to determining the property of concrete in different situation, and we can see different limitatios for each of the test. Using the right test method to determined our desirable property is also important. 10. Reference 1. A.M. Neville (1995), Properties of Concrete, Longman Group Ltd., 4th edition 2. Lecture notes of CIVL111 [Show More]

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