A H M 531 C The Civil Engineering Center

Transcription

1 Objectives: Measuring the strength of a concrete mix designed at the lab. The strengths to be measured are: Compressive strength. Tensile strength. Flexural strength. Standards: Compressive strength: G.S : PART 3 : 1970 ASTM C C C Tensile strength ASTM Designation C Flexural strength ASTM : C General Discussion: Strength tests of concrete are important in keeping batches of concrete uniform throughout a job. These tests should be held in a regular pattern rather than once at the beginning. It is important to know that the results obtained from the specimens don t 1Visit for more lab reports and lecture notes!

2 indicate the true picture of the strength of the structure; it is only an approximation of actual strength. As mentioned before the strength tests that are held in the lab are: 1. Compressive strength. 2. Tensile strength. 3. Flexural strength. Test specimens: Three types of specimens are used in the tests: 1. Cube specimens, with dimensions of 15*15*15 cm. 3 of cube specimens are made for the use in the compression test. 2. Cylindrical specimens, with diameter of 15 cm and height of 30 cm. 3 of the cylindrical specimens are made; one for the use in the compressive test and two for the use in the tension splitting test. 3. Beam (prism) specimens with dimensions of 75*15*15 cm. Only one specimen of this type is made for the use in the flexural test. The specimens are removed from the moulds after 24 hours and cured for one week. The specimens are tested at the age of 1 week where two thirds of the strength is obtained. The cube and beam specimens are used directly in the machines of testing, but the cylindrical specimens are capped. The capping operation is done to the cylindrical specimens to insure that the surface is plane and smooth so that the stresses from the testing machine are distributed uniformly. Materials used for the capping process are several but in our work we used a mixture of clay and sulphur which is in melted condition. A special apparatus for capping is used. 2Visit for more lab reports and lecture notes!

3 Compressive test: Axial loading from the device are applied to the specimens (three cube specimens and one cylindrical specimen), where the specimen should be placed at the center of the machine. Compressive from the machine is applied until fracture occurs at the ultimate strength of the specimen. The machine indicator indicates the compressive force of fracture, and from that the compressive strength of the specimen is calculated from the formula: It should be known that the compressive strength of the cube is larger than the strength of the cylindrical specimen. Tensile strength: Direct tensile strength tests are not applicable on concrete, so the tensile strength measurement is done by means of the splitting method. This test is held on the cylindrical specimens (2 cylindrical specimens), where the cylinder is placed in the same machine that is used in the compressive test but this time the cylinder is placed with its axis horizontal. Two pieces of wood are placed between the machine plates and the specimen so that the applied force is uniformly distributed. 3Visit for more lab reports and lecture notes!

4 The reading of the maximum load from the machine is taken at the fracture of the specimen, and from that the tensile strength of the specimen is calculated using the formula: And as we know before the compressive and tensile strengths of concrete are related in that: Flexural strength: It is a measure of the strength that a beam can handle during bending. This test is held on the beam specimen, by applying a concentrated force on one third and two thirds points. The maximum load at failure is read from the device and then the modulus of rupture (A measure to the flexural strength) is calculated using the formula: There is a tolerance for fracture to occur 5% outer of the middle third. 4Visit for more lab reports and lecture notes!

5 Apparatus and Materials: 1- The concrete mix: The mix we used in the tests used the following proportions: Cement / aggregates = 1/6 Water / cement = 50% Cement = 15 kg Coarse = 30 kg Medium = 30 kg Sand = 15 kg Fines = 15 kg Water = 1350 g 2- Testing machines, provided in the lab. The compressive machine used in the compressive test and in the splitting test. And the flexural strength A H machine M 531 used to test the modulus of rupture of the beam, by applying concentrated loads on two specified points The of the beam. Civil Engineering Center 3- Moulds for specimens making 3 cubes, 4 cylinders and 1 prism. 4- Tamping rods: one with a rounded end for the cylinders, and the other with a squared end for the cubes and beam. 5- Capping device: which consists of a circular cavity, where sulphur is placed, and a horizontal right angle to align the cylinder with the cavity. 6- Sulphur for capping 5Visit for more lab reports and lecture notes!

6 Procedures: 1. Making the specimens: The proportions of the concrete mix as specified before are satisfied and the mix is made, the mould are filled in layers with tamping each layer a number of strokes using the suitable tamping rod. As follows: Moulds type # of layers # of strokes Tamping rod type Cubic Three layers 35 Squared end rod Cylindrical Three layers 25 Rounded end rod Prism Two layers 80 Squared end rod After the filling process the moulds are left for 24 hours and then the specimens are removed from the moulds and left for one week which was the age of our test specimens. The specimens are numbered for identification. 2. Capping the cylinder specimens Cylinder specimens shall be capped for the reasons mentioned before. This is done be using the capping device. After filling the cavity with sulphur, the cylinder is immersed in the cavity by aligning it with the right angle, and then removed, one minute later the sulphur is frozen on the surface of the specimen. 3. Compressive strength testing: Three cube specimens and one cylindrical are placed in the compression device. The compression is applied till fracture, and the readings are taken. 4. Tensile strength testing: A cylindrical specimen is placed horizontally and the compression. Two wood pieces are placed between the specimen and the machines plates. Compression is applied till fracture and the reading from the 6Visit for more lab reports and lecture notes!

7 device is taken. 5. Flexural strength testing: The beam is placed in the device in a manner so that the concentrated load acts on one third and two thirds points. The device is turned on and the reading from the device at fracture is taken. Discussion and Conclusion: As seen from the results, the compressive strength of the cubes is larger than it is for the cylinder specimens, that is referred to that the steel platens in the testing machine have larger restraints on cube than cylinders. The compressive strength for the cubes is larger than the compressive strength of the cylinder by 1.1 times which is nearly equal to the predicted value The tension strength for cylindrical specimens is smaller than the compressive strength as predicted, but in 16.3 times (not ten times). For the same concrete mix, the Compressive strength is the larger and the tension strength is the smaller. The failure in the beam occurs in the middle third of it. 7Visit for more lab reports and lecture notes!