Testing of dry/hardened concrete


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compression and tensile test

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Testing of dry/hardened concrete

  1. 1. University of Nairobi Department of Civil and Construction Engineering Name: Owino Elijah Ochieng’ Reg. no: F16/1374/2011 Course Title: Civil Engineering Materials Course Code: FCE 246 Concrete laboratory report: Testing of dry (hardened) concrete.
  2. 2. INRODUCTION Strength of concrete could be defined as the ultimate load that causes failure or its resistance to rapture. Its units are force units divided by area (N/mm2). Strength of concrete is not very directly affected by workability, but by the method of manufacture, water/cement ratio, effective water in the mix, gel/space ratio, aggregate/cement ratio, properties of coarse aggregate, curing and age of the concrete have direct effect on the strength. Most concrete mixes attain over 70% of their strength after 7 days and almost maximum strength after 28 days. OBJECTIVES  To determine the compressive strength of concrete.  To determine the tensile strength of the same concrete. THEORY There are various types of concrete strength, the depending on how the load is applied and the type of loading. These are: Compressive strength – compressive strength of a concrete specimen treated in a standard manner which includes full compaction and wet curing for a specified period give results representing the potential quality of the concrete. The test is performed using either a concrete cube (usually 150mm by 150mm) or cylinder (150mm diameter by 300mm height). Compressive strength obtained from cylinder is 0.8 times that obtained from the cube. There are three types of loading in compression test:  Uniaxial loading – represents the most conservative system and yields the lowest values in compression. There are three types of failure in uniaxial test; tension (splitting) failure, shear (sliding) failure, combined (tension and shear) failure.   Biaxial loading Triaxial loading
  3. 3. In compression test, tangential forces are being developed between the end surfaces of the concrete specimen and the adjacent steel platens of the testing machine. These forces will cause lateral expansion in concrete. The steel platen will restrain the lateral expansion of the concrete in the parts of the specimen near its ends; the degree of restraint exercised depends on the friction actually developed. Break patterns Tensile strength – although concrete is not designed to resist direct tension, the knowledge of tensile strength is of value in estimating the load under which cracking will develop. There are two types of test for strength in tension;  Direct tensile test – is the application of a pure tension force free from eccentricity.  Splitting tension test – is where a concrete cylinder is placed with its axis horizontal between the platens of a testing machine and the load is increased until failure by indirect tension in the form of splitting along the diameter takes place. Flexural strength – is where a plain concrete beam is subjected to flexure using symmetrical two-point loading until failure occurs. PROCEDURE The cubes and cylinders prepared after workability tests were got after curing for 14 days. The cubes were divided into four parts on each of the smooth surfaces. The cube were then put in the compression machine and loaded until it crushes, and the crushing forces recorded. This step was repeated for two more cubes. One of the cylindrical concrete was then placed vertically on the compression machine and the crushing load forces recorded. The mode (mechanism) of crushing was also noted. The remaining cylinder was then placed on the machine horizontally, and the force required to split the cylinder along the centroid measured.
  4. 4. RESULTS Compression test. Cube 1: Force P1 = 430KN Cube 2: Force P2 = 440KN Cube 3: Force P3 = 420KN Average Force P = 430KN = 4.3*105N All the cubes crushed normally. Cylinder 1: Force P = 260KN, mode of crushing was across the cylinder, that is, shearing. Tensile test Cylinder 2: Force P = 145KN CALCUTIONS Cross section area A of the cubes = 150*150 = 22500mm2 Compressive strength 1 = (average force P) / (cross section area A) = 4.30*105/22500 N/mm2 = 19.11 N/mm2 Compressive strength 2(for the cylinder) Cross sectional area A of the cylinder = PI*D2/4 = 3.142*150/4mm2 = 17671.45mm2 Compressive strength 2 = force P/cross sectional area = 2.6*105/17671.56 N/mm2 = 14.71 N/mm2 Tensile strength = 2P/ (PI*D*L) = 2*145000/ (3.142*150*300) N/mm2 = 2.05 N/mm2 DISCUSSION The compressive strength of the cube was found to be 19.11N/m2 while that of the cylinder was 14.71N/m2. The ratio of compressive strength of cylinder to that of the cube is 14.71/19.11, approximately 0.8 as was expected, but this varies from one grade of concrete to the other. The tensile strength was found to be 2.05N/m2, which is approximately 10% of compressive strength of the cube. Since the actual strength of the concrete was 20N/m2 given the concrete mix proportion of 1:1.5:3 (grade M20), the variation could have been due to the friction between the platens of the compression machine and the test specimen. The loss in strength could have also been due to improper mix proportioning; effective water/cement ratio, cement/aggregate ratio, gel space and moisture content during testing. The tensile strength of concrete is very much lower than the compressive strength. This shows that concrete is more brittle and almost non-ductile and thus should be subjected to tensile loading, unless reinforced with steel, thus the knowledge of the behavior under tension is vital.
  5. 5. CONCLUSION This experiment can be considered successful since the aims were achieved. The compressive strength and tensile strength of the concrete were obtained and the relationship between the two were ascertained. It was established that the tensile strength is 10% of the compressive strength thus concrete is weak in tension and strong under compression due to its brittleness. REFERENCES 1. S. K. Duggal, Building Materials, Third revised edition, 2008, New Age International Publishers. 2. A. M. Neville and J. J. Brooks, Concrete Technology, third edition, 2011, Pearson Education Limited.